Thèse présentée pour l’obtention du grade de Docteur ... · Thèse présentée . pour...

Par Sarah LE GUENIC

Thèse présentée pour l’obtention du grade de Docteur de l’UTC

Dehydration in aqueous media

Soutenue le 1er décembre 2015 Spécialité : Génie des Procédés Industriels et du Développement Durable

D2238

Université de Technologie de Compiègne

Ecole Doctorale : Sciences pour l’ingénieur

Thèse présentée par

Sarah LE GUENIC

Pour le titre de Docteur en Génie des Procédés Industriels et du Développement Durable

Dehydration in aqueous media

Thèse dirigée par : Christophe LEN et Claire CEBALLOS

Soutenue le 1er décembre 2015 devant le jury composé de :

François JEROME Directeur de Recherche CNRS, Université de Poitiers Rapporteur

Rafael LUQUE Professeur, Universidad de Cordoba Rapporteur

Florent ALLAIS Professeur, AgroParisTech Examinateur

Stéphane OCTAVE Docteur, Université de Technologie de Compiègne Examinateur

Christophe LEN Professeur, Université de Technologie de Compiègne Directeur de thèse

Claire CEBALLOS Docteur, Ecole Supérieure de Chimie Organique et Minérale

Directrice de thèse

i

Acknowledgements

This PhD work was done in the laboratory Transformations Intégrées de la

Matière Renouvelable (TIMR, EA4297) of the Université de Technologie de Compiègne

under the supervision of Pr. Christophe LEN and Dr. Claire CEBALLOS. I would like to

thank Pr. André PAUSS, director of the laboratory, for giving me the opportunity to do

this PhD. I would also like to thank Georges SANTINI, director of the Ecole Supérieure

de Chimie Organique et Minérale (ESCOM) for letting me work in his school. I also thank

the French Ministère de l’Enseignement Supérieur et de la Recherche for its financial

support.

First of all, I would like to thank the Director and Co-director of my thesis, Pr.

Christophe LEN and Dr. Claire CEBALLOS, for their scientific help, support and

guidance during this PhD. I particularly thank Pr. LEN for his trust and for giving me the

opportunities to present my work at international conferences.

I want to express my gratitude to the members of the jury that participated in my

PhD defense. I would especially like to acknowledge the referees of my thesis, François

JEROME and Rafael LUQUE. Thank you for the time you devoted and the feedback that

you provided.

During my PhD work, I had the opportunity to do a Short Term Scientific Mission

in the team of Pr. Herbert SIXTA, in the Department of Forest Products Technology, in

Aalto University (Helsinki, Finland). I thank the COST program for the grant awarded

and the Pr. SIXTA for his supervision. Thanks to Jean BUFFIERE, Olga ERSHOVA and

Rita HATAKKA from the Biorefineries’ team for their help with the reactor, the HPLC

and in the laboratory.

I would like to thank my colleagues from the OCAT team, with which I had the

pleasure to collaborate: Carole, Claire, Denis, Estelle, Floriane, Frédéric, Gérald,

Gwenaëlle, Muriel and Nicolas T. I particularly thank Nicolas T. and Denis for their many

scientific advices, for their jokes and for the good times we spent together. Thanks to

ii

Frédéric for the time he spent on my manuscript. I thank the interns, who participated on

my project, for their work and implication: Quentin, Fabrice, David and Loïc.

I thank Lysiane, from TIMR, for her help and the whole staff of ESCOM for their

hospitality. Special thanks to France, Anissa, Bernard, Pierre-Yves, Joachim and Nathalie

for the fun talks we had during coffee breaks.

This PhD would not have been the same without the fun and friendly atmosphere

of the lab. Big thanks to Rémi, Nicolas G., Clément, Sophie, Audrey and Christina. Of

course, I particularly thank Hasret and Nico who made this long journey with me, who

suffered and laughed with me, who cheered me up when I was down and supported me

through the important moments. The three PhD Babies have grown up and have become

doctors. Now, one chapter in the story closes, but a whole new adventure begins!

I would also like to thank all the PhD students that I have met during these 3 years

for the nice moments: the RED² team – Christophe, Marina, Benjamin, Fabien, Florian,

Pierre, Pierre, Tifenn, Tony, Rémy and Gaëtan, the Acoustic Boys – Antoine, Romain and

Julien, and the TIMR Girls – Sylène, Maria and Atena.

A special thanks to my friends, Clémentine, Audrey, Marion, Claire, Tania and

Ghania. Even if it was sometimes hard to find a date to see each other, spending time

with you was always a good moment which cheered me up and made me forget (for a

time!) the hard work waiting for me.

Of course, I would like to say a big and loving thanks to my parents and to my

sister Marion and my brother Benjamin. Thank you for your support and your

encouragements, thank you for trying to understand my work and for listening to me

when I needed it. I know that I can always count on you. More broadly, I thank Dany,

Evann, Gaële, Frédérique, Katia and the kids for their support.

And, last but not least: Yorick. Meeting you was one of the greatest things that

happened to me during this PhD. Thank you for your daily support, for comforting me

when I felt bad, for your jokes, for your delicious dishes, and for believing in me.

iii

Table of contents

ACKNOWLEDGEMENTS .................................................................................................. I

TABLE OF CONTENTS ................................................................................................... III

ABBREVIATIONS ......................................................................................................... VII

LIST OF FIGURES .......................................................................................................... IX

LIST OF TABLES ............................................................................................................ XI

LIST OF SCHEMES ....................................................................................................... XII

GENERAL INTRODUCTION ............................................................................................. 1

CHAPTER I ...................................................................................................................... 5

CONTEXT AND LITERATURE REVIEW .......................................................................... 5

1. GREEN CHEMISTRY: DEFINITION AND TOOLS ........................................................................................ 7

a. Green Chemistry: Origin and Twelve Principles ....................................................................... 7

b. Water as a green solvent ............................................................................................................ 10

i. General points on green solvents ........................................................................................................... 10

ii. Sub- and supercritical water .................................................................................................................. 14

c. Alternative activation methods ................................................................................................. 17

i. An overview of the different methods .................................................................................................... 17

ii. Microwave chemistry .............................................................................................................................. 19

2. DEHYDRATION OF POLYHYDROXYLATED COMPOUNDS IN WATER ........................................................ 25

a. Without a catalyst ....................................................................................................................... 25

b. Homogeneous catalysis .............................................................................................................. 26

i. Mineral and organic acids ...................................................................................................................... 26

ii. Metal sulphates ....................................................................................................................................... 27

iii. Metal chlorides ....................................................................................................................................... 29

iv. With the addition of CO2 ....................................................................................................................... 30

c. Heterogeneous catalysis ............................................................................................................. 30

i. Modified silicates and aluminosilicates ................................................................................................. 31

ii. Mixed oxides ........................................................................................................................................... 32

iii. Ion-exchange resins ................................................................................................................................ 33

iv. Solid metal phosphates ........................................................................................................................... 34

d. Enhancing selectivity .................................................................................................................. 34

3. ISSUE AND OBJECTIVES ...................................................................................................................... 36

BIBLIOGRAPHY ........................................................................................................................................... 37

iv

CHAPTER II ................................................................................................................... 43

DEHYDRATION OF 1-PHENYLETHANE-1,2-DIOL IN WATER ...................................... 43

1. INTRODUCTION .................................................................................................................................. 45

a. Phenylacetaldehyde: origin and applications .......................................................................... 45

b. Literature review ........................................................................................................................ 45

c. Mechanism and structure of the by-product ............................................................................ 48

2. DEHYDRATION OF 1-PHENYLETHANE-1,2-DIOL UNDER CONVENTIONAL HEATING ............................... 50

a. Variation of temperature and reaction time ............................................................................ 50

b. Influence of the diol concentration ............................................................................................ 51

c. Conclusion ................................................................................................................................... 52

3. DEHYDRATION OF 1-PHENYLETHANE-1,2-DIOL UNDER MICROWAVE IRRADIATION ............................. 53

a. Optimisation of the phenylacetaldehyde yield in water .......................................................... 53

i. Influence of temperature ........................................................................................................................ 54

ii. Influence of reaction time ...................................................................................................................... 55

iii. Influence of the catalyst nature .............................................................................................................. 56

iv. Conclusion ............................................................................................................................................... 57

b. Introduction of a co-solvent ....................................................................................................... 58

i. Choice of the co-solvent .......................................................................................................................... 58

ii. Influence of the water/co-solvent ratio ................................................................................................. 61

iii. Variation of the catalyst quantity ........................................................................................................... 63

iv. Variation of the reaction time ................................................................................................................ 64

v. Conclusion ............................................................................................................................................... 65

c. Recyclability of the aqueous phase ............................................................................................ 66

d. Application to other alcohols ..................................................................................................... 67

i. 1-phenylethanol and 2-phenylethanol ................................................................................................... 67

ii. 1-phenylpropan-1-ol .............................................................................................................................. 68

iii. Hydrobenzoin ......................................................................................................................................... 69

e. Formation of phenylacetaldehyde from styrene oxide ............................................................ 71

4. CONCLUSION ...................................................................................................................................... 73

BIBLIOGRAPHY ........................................................................................................................................... 76

CHAPTER III ..................................................................................................................79

DEHYDRATION OF D-XYLOSE IN WATER ....................................................................79

1. INTRODUCTION .................................................................................................................................. 81

a. From Biomass... .......................................................................................................................... 81

i. Structure of lignocellulosic biomass ..................................................................................................... 82

ii. Structural diversity of hemicelluloses ................................................................................................... 84

b. ... to Furfural ............................................................................................................................... 87

i. Properties of furfural .............................................................................................................................. 87

ii. Uses and applications of furfural .......................................................................................................... 88

iii. Industrial processes for furfural preparation ....................................................................................... 90

v

c. Mechanistic aspects .................................................................................................................... 91

i. Mechanisms of furfural formation from D-xylose ................................................................................ 91

ii. Side and loss reactions ........................................................................................................................... 94

d. Dehydration of D-xylose to furfural: a brief review of the literature ................................... 95

i. Reactions in batch reactor under conventional heating ....................................................................... 96

ii. Reactions under microwave irradiation ................................................................................................ 99

iii. Reactions in continuous flow reactor................................................................................................... 100

e. Objectives of the Chapter .......................................................................................................... 101

2. DEHYDRATION OF D-XYLOSE UNDER MICROWAVE IRRADIATION WITH HOMOGENEOUS CATALYSIS ... 102

a. Reactions in biphasic medium ................................................................................................. 102

i. Comparison of aqueous and biphasic media ....................................................................................... 102

ii. Choice of the catalyst ............................................................................................................................ 103

iii. Variation of temperature and reaction time ........................................................................................ 105

iv. Kinetic study ......................................................................................................................................... 107

v. Variation of xylose concentration ........................................................................................................ 108

vi. Variation of catalyst quantity coupled with the addition of a salt ...................................................... 109

b. Recyclability of the aqueous phase ........................................................................................... 111

c. Conclusion .................................................................................................................................. 112

3. DEHYDRATION OF D-XYLOSE UNDER MICROWAVE IRRADIATION WITH HETEROGENEOUS CATALYSIS . 114

a. Nafion NR50: properties and applications ............................................................................. 114

b. Reactions in biphasic medium .................................................................................................. 115

i. Preliminary study .................................................................................................................................. 115

ii. Comparison of aqueous and biphasic media ....................................................................................... 116

iii. Variation of Nafion NR50 and salt quantities ...................................................................................... 117

iv. Variation of xylose concentration ........................................................................................................ 118

v. Influence of temperature and reaction time ........................................................................................ 119

c. Mechanism involved and comparison with HCl .................................................................... 120

d. Regeneration and analyses of Nafion NR50 pellets ............................................................... 121

e. Conclusion ................................................................................................................................. 124

4. APPLICATION TO XYLAN AND HEMICELLULOSE SUGARS UNITS .......................................................... 125

a. Dehydration of xylan into furfural .......................................................................................... 125

i. By homogeneous catalysis .................................................................................................................... 125

ii. By heterogeneous catalysis ................................................................................................................... 127

b. Dehydration of other sugars composing hemicelluloses ....................................................... 128

i. From L-arabinose to furfural ............................................................................................................... 129

ii. From D-glucose, D-mannose and D-galactose to 5-HMF ................................................................... 130

c. Conclusion .................................................................................................................................. 131

5. DEHYDRATION OF D-XYLOSE IN A CONTINUOUS FLOW REACTOR ....................................................... 132

a. Reactor and operating conditions ........................................................................................... 132

b. Reactions in water .................................................................................................................... 133

i. Influence of temperature and reaction time ........................................................................................ 134

ii. Influence of the ratio between the inflows........................................................................................... 135

vi

iii. Influence of xylose concentration ........................................................................................................ 136

c. Conclusion ................................................................................................................................. 138

6. CONCLUSIONS .................................................................................................................................. 139

BIBLIOGRAPHY ......................................................................................................................................... 142

GENERAL CONCLUSIONS ............................................................................................ 147

SCIENTIFIC COMMUNICATIONS ................................................................................. 151

RESUME ....................................................................................................................... 155

EXPERIMENTAL SECTION .......................................................................................... 165

1. GENERAL ......................................................................................................................................... 167

a. Chemical products and solvents .............................................................................................. 167

b. Chromatography ...................................................................................................................... 167

i. TLC ........................................................................................................................................................ 167

ii. Flash-chromatography ......................................................................................................................... 167

iii. HPLC ..................................................................................................................................................... 167

c. NMR ........................................................................................................................................... 169

d. SEM-EDX .................................................................................................................................. 169

e. ATR-FTIR .................................................................................................................................. 169

2. CHAPTER II – DEHYDRATION OF 1-PHENYLETHANE-1,2-DIOL .......................................................... 170

a. Thermal heating ........................................................................................................................ 170

i. Equipment ............................................................................................................................................. 170

ii. General procedure ................................................................................................................................ 170

b. Microwave reactor .................................................................................................................... 171

i. Equipment .............................................................................................................................................. 171

ii. Dehydration of 1-phenylethane-1,2-diol .............................................................................................. 172

iii. Applications to other substrates........................................................................................................... 173

c. Identification of reaction products .......................................................................................... 174

3. CHAPTER III - DEHYDRATION OF D-XYLOSE ......................................................................................177

a. Microwave reactor .................................................................................................................... 177

i. Equipment ............................................................................................................................................. 177

ii. General procedures for homogeneous catalysis .................................................................................. 177

iii. General procedures for heterogeneous catalysis ................................................................................. 180

iv. Identification of reaction products ...................................................................................................... 182

b. Continuous flow reactor ........................................................................................................... 182

i. Equipment ............................................................................................................................................. 182

ii. Control of parameters ........................................................................................................................... 184

iii. General procedure ................................................................................................................................ 185

iv. Identification of reaction products ...................................................................................................... 185

APPENDICES ................................................................................................................ 187

vii

Abbreviations

A A

ATR-FTIR

Frequency factor

Attenuated Total Reflectance - Fourier Transform Infrared Spectroscopy

C CPME

[C]org/[C]aq

Cyclopentyl methyl ether

Distribution ratio of a compound C

D DMA

DMF

DMPU

DMSO

N,N-Dimethylacetamide

N,N-Dimethylformamide

1,3-Dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone

Dimethyl sulfoxide

E ε

ε’’

Ea

EDX

EtOAc

Dielectric constant

Dielectric loss

Activation energy

Energy-Dispersive X-ray Spectroscopy

Ethyl acetate

F FAME Fatty Acids Methyl Esters

G GBL

GVL

Gamma-butyrolactone

Gamma-valerolactone

H 5-HMF

HPLC

5-Hydroxymethyl furfural

High Performance Liquid Chromatography

K k

Kw

Kinetic rate constant

Ionic product of water

M MAOS

MEK

Me-THF

MIBK

mol%

MSA

MTBE

MW

Microwave-Assisted Organic Chemistry

Methyl ethyl ketone

2-Methyltetrahydrofuran

Methyl isobutyl ketone

Molar percentage

Methanesulfonic acid

Methyl tert-butyl ether

Microwave

viii

N n/a

NMP

NMR

NOESY

Not available

N-Methyl-2-pyrrolidone

Nuclear Magnetic Resonance

Nuclear Overhauser Effect SpectroscopY

P P

Pc

Pressure

Critical pressure

S SCF

scCO2

scPropane

SCW

SEM

Supercritical Fluids

Supercritical CO2

Supercritical propane

Supercritical Water

Scanning Electron Microscopy

T τ

t

T

tan δ

Tc

THF

TLC

TOF

TON

tR

p-TSA

Residence time

Reaction time

Temperature

Dissipation factor, or loss tangent

Critical temperature

Tetrahydrofuran

Thin Layer Chromatography

Turnover frequency

Turnover number

Retention time

para-Toluenesulfonic acid

V VOC Volatile Organic Compound

W wt% Weight percentage

X X Conversion

Y Y Yield

ix

List of figures

Figure 1: The Twelve Principles of Green Chemistry, from Anastas and Eghbali[3]

. ................................................ 8

Figure 2: The Atom Economy and the E factor. ....................................................................................................... 9

Figure 3: Examples of renewable solvents. ........................................................................................................... 13

Figure 4: Phase diagram of water, from Saka et al.[18]

.......................................................................................... 15

Figure 5: Density, dielectric constant and ion dissociation constant (Kw) of water at 30 MPa as a function of

temperature, from Peterson et al.[19]

................................................................................................................... 15

Figure 6: Two main heating mechanisms under microwave irradiation: (a) dipolar rotation; (b) ionic conduction

mechanism. From Kappe, Dallinger & Murphree.[43]

............................................................................................. 21

Figure 7: The temperature profile after 60 seconds by treatment ........................................................................ 22

Figure 8: Current pros and cons of microwave-assisted synthesis. ....................................................................... 24

Figure 9: By-products obtained during the dehydration of 1-phenylethane-1,2-diol in water[134,135]

................... 47

Figure 10: Comparison of the two possible structures of the by-product – 1H NMR. ............................................ 49

Figure 11: Configuration of the 2,4-diphenylbut-2-enal (3). ................................................................................. 49

Figure 12: Influence of temperature for a) 8h and b) 16h of reaction. ................................................................. 51

Figure 13: Influence of diol concentration. ............................................................................................................ 52

Figure 14: Influence of temperature. .................................................................................................................... 54

Figure 15: Influence of reaction time. ................................................................................................................... 55

Figure 16: Schematic process of a biphasic dehydration of 1-phenylethane-1,2-diol. .......................................... 58

Figure 17: Recyclability of the aqueous phase with a) FeCl3, b) HCl and c) H2SO4................................................. 66

Figure 18: Influence of reaction time on the synthesis of phenylacetaldehyde from styrene oxide. .................... 72

Figure 19: Schematic structure of lignocellulose. The hexagons represent the lignin subunits p-coumaryl alcohol

(H), coniferyl alcohol (G) and sinapyl alcohol (S). From F. Streffer.[145]

................................................................. 82

Figure 20: Cellulose structure. ............................................................................................................................... 83

Figure 21: The three building blocks of lignin. ....................................................................................................... 84

Figure 22: Examples of (a) homoxylans and (b) (arabino)glucuronoxylan. ........................................................... 85

Figure 23: Primary structure of galactomannan. .................................................................................................. 86

Figure 24: Structures of the repeating units of the two main types of xyloglucans. ............................................. 86

Figure 25: Example of β-glucan structure with β-1,4 and β-1,3 bonds. ................................................................ 87

Figure 26: Isomers, tautomers of D-xylose and their distribution in aqueous solution.[160]

.................................. 91

Figure 27: Comparison between aqueous and biphasic media. .......................................................................... 103

Figure 28: Efficacy of different catalysts on the dehydration of xylose. .............................................................. 104

Figure 29: Effect of reaction temperature and time on a) xylose conversion and b) furfural yield. .................... 106

Figure 30: Effect of the initial xylose concentration on xylose conversion and furfural yield. ............................ 109

x

Figure 31: Effect of the combination FeCl3/NaCl on a) xylose conversion and b) furfural yield. ......................... 110

Figure 32: Reusability of the aqueous phase containing FeCl3 and NaCl for the xylose dehydration. ................ 112

Figure 33: Chemical structure of Nafion NR50. ................................................................................................... 114

Figure 34: Images of a) a pristine pellet of Nafion NR50, ................................................................................... 115

Figure 35: Comparison between aqueous and biphasic media. .......................................................................... 116

Figure 36: Effect of the addition of NaCl on xylose conversion and furfural yield ............................................... 117

Figure 37: Effect of the initial xylose concentration on xylose conversion and furfural yield. ............................ 118

Figure 38: Effect of reaction time on furfural yield for different temperatures. ................................................. 119

Figure 39: SEM images of Nafion NR50 (a) before reaction; (b) after a reaction without salt; .......................... 122

Figure 40: Study of the recyclability of the Nafion NR50 pellets after regeneration steps in HCl. ...................... 123

Figure 41: Xylose and furfural yield from xylan in function of ............................................................................. 126

Figure 42: Xylose and furfural yield from xylan as a function of temperature for t=60 min. .............................. 128

Figure 43: Comparison of L-arabinose and D-xylose dehydration to furfural. .................................................... 129

Figure 44: Conversion of D-glucose, D-galactose and D-mannose to 5-HMF. ..................................................... 131

Figure 45: Scheme of the reactor system, adapted from Tolonen et al.[210]

........................................................ 132

Figure 46: Equation for calculating the residence time τ (s) of the reactions. .................................................... 133

Figure 47: Effect of reaction temperature and residence time on ...................................................................... 134

Figure 48: Scheme of the reactor body. .............................................................................................................. 135

Figure 49: Influence of the ratio φ “xylose solution flow”/ “heating water flow” on furfural yield. ................... 136

Figure 50: Influence of initial xylose concentration on a) xylose conversion and b) furfural yield. ..................... 137

Figure 51: Calibration curve of phenylacetaldehyde, HPLC. ................................................................................ 168

Figure 52: Autoclave Parr used for the experiments. .......................................................................................... 170

Figure 53: Microwave apparatus. ....................................................................................................................... 171

Figure 54: Example of power, temperature and pressure profiles during a reaction. ......................................... 171

Figure 55: a) Schematic and b) picture of the reactor system. ............................................................................ 183

Figure 56: Enthalpy calculation for the determination of the temperature of the heating water TH. ................. 184

Figure 57: Enthalpy calculation for the determination of quenching water flow ṁQ. ........................................ 184

Figure 58: Equation for calculating the residence time τ (s) of the reactions. .................................................... 185

xi

List of tables

Table 1: Overall ranking of solvents, adapted from Prat et al.[10]

. ....................................................................... 11

Table 2: Dissipation factors (tan δ) of different solvents (2.45 GHz, 20°C). .......................................................... 21

Table 3: Examples of the effect of metal sulphates on the dehydration of biomass-derived polyols. .................. 28

Table 4: Physical properties of phenylacetaldehyde. ............................................................................................ 45

Table 5: Effect of the salts on the dehydration of 1-phenylethane-1,2-diol under hydrothermal conditions. From

Avola et al.[54]

........................................................................................................................................................ 47

Table 6: Influence of the catalyst nature. .............................................................................................................. 56

Table 7: Choice of the co-solvent. ......................................................................................................................... 59

Table 8: Physical properties of CPME. Data from Watanabe et al.[139]

................................................................. 61

Table 9: Influence of the ratio water/co-solvent. .................................................................................................. 62

Table 10: Variation of the catalyst quantity. ......................................................................................................... 63

Table 11: Variation of reaction time. .................................................................................................................... 64

Table 12: Values of TON and TOF for the three optimized catalysts. .................................................................... 65

Table 13: Dehydration of 1-phenylethanol. ........................................................................................................... 67

Table 14: Dehydration of 2-phenylethanol. ........................................................................................................... 68

Table 15: Dehydration of 1-phenylpropan-1-ol. .................................................................................................... 69

Table 16: Dehydration and pinacol rearrangement of hydrobenzoin. .................................................................. 70

Table 17: Synthesis of phenylacetaldehyde from styrene oxide. ........................................................................... 71

Table 18: Cellulose, hemicelluloses and lignin content of selected biomass (wt%). .............................................. 82

Table 19: Structure and physical properties of furfural. ........................................................................................ 87

Table 20: Examples of furfural production processes. .......................................................................................... 90

Table 21: Examples of studies on the dehydration of xylose using mineral and organic acids. ............................ 96

Table 22: Examples of studies on the dehydration of xylose and hemicelluloses using metal chlorides. ............. 97

Table 23: Examples of studies on the dehydration of xylose and xylan with heterogeneous catalysts. ............... 98

Table 24: Dehydration of xylose, xylan and biomass under microwave irradiation. ............................................. 99

Table 25: Dehydration of xylose and xylan in continuous flow. .......................................................................... 100

Table 26: Kinetic rate constants ki (min-1

) at each experimental temperature. .................................................. 108

Table 27: Frequency factors (Ai, min-1

), activation energies (Eai, kJ.mol-1

) and correlation factors R²................ 108

Table 28: ATR-FTIR absorption peaks of pristine and post-reaction Nafion NR50. ............................................. 123

xii

List of schemes

Scheme 1: Dehydration of sorbitol to isosorbide in solvent-free conditions.[6]

..................................................... 10

Scheme 2: Formation of tetrahydrofuran, THF, from 1,4-butanediol in scCO2.[13]

................................................ 12

Scheme 3: Dehydration of glycerol to acrolein with the addition of sulphuric acid.[63,64]

...................................... 26

Scheme 4: Formation of 5-HMF from fructose in presence of AlCl3 under microwave irradiation.[85]

.................. 29

Scheme 5: Dehydration of cyclohexanol to cyclohexene in presence of carbon dioxide.[88]

.................................. 30

Scheme 6: Formation of methyl ethyl ketone from 2,3-butanediol with modified H-ZMS-5 zeolites.[94]

.............. 31

Scheme 7: Formation of furfural from D-xylose with an ion-exchange resin, Amberlyst 70.[106]

.......................... 33

Scheme 8: Retrosynthetic scheme of phenylacetaldehyde. .................................................................................. 46

Scheme 9: Dehydration of 1-phenylethane-1,2-diol to phenylacetaldehyde. ....................................................... 48

Scheme 10: Aldol condensation from phenylacetaldehyde. .................................................................................. 48

Scheme 11: Proposed action of AlCl3 in the dehydration of 1-phenylethane-1,2-diol. .......................................... 54

Scheme 12: Formation of styrene from 1-phenylethanol and 2-phenylethanol. ................................................... 67

Scheme 13: Formation of 1-phenylpropene from 1-phenylpropan-1-ol. ............................................................... 69

Scheme 14: Dehydration and pinacol rearrangement of hydrobenzoin. .............................................................. 70

Scheme 15: Formation of phenylacetaldehyde from styrene oxide. ..................................................................... 71

Scheme 16: Formation of furfural from hemicelluloses. ....................................................................................... 88

Scheme 17: Examples of chemicals derived from furfural. Adapted from Hoydonckx et al.[150]

........................... 89

Scheme 18: Xylose dehydration mechanism via enolisation, proposed by Marcotullio et al.[70]

........................... 92

Scheme 19: Xylose dehydration mechanism involving both cyclic and acyclic pathways, .................................... 93

Scheme 20: Mechanism of xylose dehydration into furfural proposed by Nimlos et al.[164]

.................................. 94

Scheme 21: Examples of side reactions occurring during the formation of furfural.[93,171]

................................... 95

Scheme 22: Simplified reaction mechanism of furfural formation. ..................................................................... 107

Scheme 23: Cation exchange on the sulfonic acid groups of Nafion NR50. ........................................................ 121

Scheme 24: Dehydration of xylan to furfural via xylose. ..................................................................................... 125

Scheme 25: Dehydration of L-arabinose to furfural. ........................................................................................... 129

Scheme 26: Formation of 5-HMF by the dehydration of D-glucose, D-galactose and D-mannose. .................... 130

General introduction


3

For several years, environmental issues are in the centre of political, economic and

industrial concerns. At the beginning of the 1990s, the first initiative of research to green

chemistry was launched by the U.S. Environmental Protection Agency. Their objective

was to set a framework for the prevention of the pollution linked to chemical industries.

A few years later, the concept of Green Chemistry was developed by the American

chemists Anastas and Warner. Green Chemistry was defined as the ―design of chemical

products and processes to reduce or eliminate the use and generation of hazardous

substances‖. In 1998, they introduced the Twelve Principles of Green Chemistry as a

guiding framework for chemists. These principles contributed to the development and

the popularisation of the concept of Green Chemistry:

1) Prevention.

2) Atom Economy.

3) Less Hazardous Chemical Synthesis.

4) Designing Safer Chemicals.

5) Safer Solvents and Auxiliaries.

6) Design for Energy Efficiency.

7) Use of Renewable Feedstocks.

8) Reduce Derivatives.

9) Catalysis.

10) Design for Degradation.

11) Real-Time Analysis for Pollution Prevention.

12) Inherently Safer Chemicals for Accident Prevention

Currently, one of the main challenges of the chemical industry is to synthesise

molecules with high added value thanks to processes respecting the principles of Green

Chemistry. The aim of this PhD work is to optimise green dehydration methods to form

two target molecules: phenylacetaldehyde and furfural. Several keys issues were

identified in order to design processes greener than the current ones. Experiments will be

performed in water and when co-solvents will be necessary, green or eco-friendly

solvents will be chosen. An activation method alternative to thermal heating, microwave

irradiation, will be applied. A continuous flow reactor was also identified as an

interesting alternative. When possible, reactions will be carried out without a catalyst,

but, when needed, efficient, selective and reusable catalysts will be chosen.


4

In the first Chapter, the context of the thesis will be described by developing the

concept of Green Chemistry and the tools associated. A review of the literature on the

dehydration of polyhydroxylated compounds in water will be discussed.

In Chapter II, the dehydration of 1-phenylethane-1,2-diol to form phenyl-

acetaldehyde will be studied. After a brief introduction, thermal heating and microwave

irradiation will be compared. Then, a procedure will be optimised under microwave

irradiation by varying the nature of the catalyst, temperature, reaction time and initial

reactant concentration. This optimised method will later be applied to other alcohols

with similar structures.

Chapter III will focus on the dehydration of the biosourced D-xylose to produce

furfural. First, an introduction to the reaction will be provided. A description of the

lignocellulosic biomass structure and of the uses and applications of furfural will be

given. The mechanistic aspects of the dehydration of xylose to furfural will be discussed.

A brief state of the art of the reaction will be presented. Secondly, the optimisation of two

methods with either homogeneous or heterogeneous catalysts under microwave

irradiation will be discussed. The influence of reaction time, temperature and proportion

of catalyst will be studied. Then, these two methods will be applied to xylan, a

homopolymer of D-xylose, and to other sugar units. Finally, the formation of furfural

from D-xylose will be studied in a flash continuous flow reactor, without any catalyst.

In the last part, after the concluding remarks, the experimental procedures will be

described, as well as the compounds formed.

CHAPTER I

Context and literature review

Chapter I: Context and literature review

7

1. Green Chemistry: definition and tools

Green Chemistry is a concept which was introduced in the 1990s with the objective of

helping chemists to improve the environmental performance and safety of their chemical

processes. In the first part, the principles of sustainable and green chemistry will be

developed.

Alternative solvents represent one of the main entries in the green chemistry toolkit

and are the subject of an enormous research effort. A description of these green solvents,

and especially sub- and supercritical water, will be given in the second part.

Finally, new activation techniques or the applications of established techniques in

new ways represent important tools in the green chemistry toolkit. Then, the basic ideas

of alternative activation methods such as sonochemistry or mechanochemistry will be

described in the last part. More details will be given on microwave chemistry.

a. Green Chemistry: Origin and Twelve Principles

Green Chemistry is defined as the ―design of chemical products and processes to

reduce or eliminate the use and generation of hazardous substances‖. This definition and

the concept of Green Chemistry were first formulated at the beginning of the 1990s

nearly 30 years ago.[1] In this definition, the concept of ―design‖ is the most important

aspect of Green Chemistry. It induces novelty, planning and systematic conception. To

help the chemists to achieve their goal of sustainability, Paul Anastas and John Warner

introduced, in 1998, the Twelve Principles of Green Chemistry (Figure 1). They are a

guiding framework for the design of new chemical products and processes, applying to all

aspects of the process life-cycle from the raw materials used, to the efficiency and safety

of the transformation, the toxicity and biodegradability of products and reagents used.

Later, Poliakoff and co-workers proposed a mnemonic acronym, PRODUCTIVELY,

which captures the spirit of the Twelve Principles of Green Chemistry in a single slide.[2]


8

1) Prevention. It is better to prevent waste than to treat or clean up waste after it is

formed.

2) Atom Economy. Synthetic methods should be designed to maximize the

incorporation of all materials used in the process into the final product.

3) Less Hazardous Chemical Synthesis. Whenever practicable, synthetic

methodologies should be designed to use and generate substances that pose little

or no toxicity to human health and the environment.

4) Designing Safer Chemicals. Chemical products should be designed to preserve

efficacy of the function while reducing toxicity.

5) Safer Solvents and Auxiliaries. The use of auxiliary substances (e.g. solvents,

separation agents, etc) should be made unnecessary whenever possible and, when

used, innocuous.

6) Design for Energy Efficiency. Energy requirements of chemical processes

should be recognized for their environmental and economic impacts and should be

minimized. If possible, synthetic methods should be conducted at ambient

temperature and pressure.

7) Use of Renewable Feedstocks. A raw material or feedstock should be

renewable rather than depleting whenever technically and economically

practicable.

8) Reduce Derivatives. Unnecessary derivatization (use of blocking groups,

protection/deprotection, temporary modification of physical/chemical processes)

should be minimized or avoided if possible, because such steps require additional

reagents and can generate waste.

9) Catalysis. Catalytic reagents (as selective as possible) are superior to

stoichiometric reagents.

10) Design for Degradation. Chemical products should be designed so that at the

end of their function they break down into innocuous degradation products and do

not persist in the environment.

11) Real-Time Analysis for Pollution Prevention. Analytical methodologies

need to be further developed to allow for real-time, in-process monitoring and

control prior to the formation of hazardous substances.

12) Inherently Safer Chemicals for Accident Prevention. Substances and the

form of a substance used in a chemical process should be chosen to minimize the

potential for chemical accidents, including releases, explosions, and fires.

Figure 1: The Twelve Principles of Green Chemistry, from Anastas and Eghbali[3].


9

Over the years, several tools were introduced to help chemists measure the

environmental impact of their chemical processes. The Atom Economy (AE) or Atom

Efficiency concept, proposed by Trost in 1990, is one of the useful tools available for

designing reactions with minimum waste.[4] It refers to the concept of maximizing the use

of raw materials so that the final product contains the maximum number of atoms from

the reactants. The ideal reaction would incorporate all of the atoms of the reactants. To

sum up, the AE of a reaction is defined as the ratio of the molecular weight of the desired

product over the sum of the molecular weights of all reactants used in the process (Figure

2).

The Atom Economy (AE):

The E factor:

Figure 2: The Atom Economy and the E factor.

The concept of atom economy has been expanded usefully by Sheldon in 1992 by

the introduction of the term ―E factor‖, or Environmental Impact Factor.[5] This metric

can be defined as the actual amount of waste produced in the process (defined as

everything but the desired product) (Figure 2). It takes the chemical yield into account

and includes reagents, solvents losses, and all process aids. This concept is particularly

useful for assessing the environmental impact of manufacturing processes.

Nowadays, despite the vast amount of research into the subject, it seems difficult

to design a process respecting all the Twelve Principles. Therefore, the idea is to try to

respect as many as we can. In this PhD work, we focused on six Principles: Atom

Economy, Less Hazardous Chemical Synthesis, Safer Solvents and Auxiliaries, Use of

Renewable Feedstocks, Reduce Derivatives and Catalysis. For instance, our experiments

were performed with green solvents, in particular water. They were carried out under

microwave irradiation, an activation method alternative to conventional heating.

Efficient and reusable catalysts were preferred. In addition, in Chapter III, the sugar D-

xylose, derived from biomass, was the raw material of the reactions.


10

b. Water as a green solvent

i. General points on green solvents

Solvents are probably the most active area of Green Chemistry research. They

represent an important challenge for Green Chemistry because they often account for the

vast majority of mass wasted in synthesis and processes. Moreover, many conventional

solvents are toxic, flammable and/or corrosive. Their volatility and solubility have

contributed to air, water and land pollution, have increased the risk of workers’ exposure,

and have led to serious accidents. Recovery and reuse, when possible, is often associated

with energy-intensive processes like distillation. Thus, the ideal situation would be to not

use any solvent. Efforts have been made to develop solvent-less conditions thanks,

particularly, to the large amount of research done on heterogeneous catalysts. As a good

example, the ion-exchange resin Amberlyst 35 proved to be efficient for the dehydration

of sorbitol to isosorbide in a solvent-free system under microwave heating with a yield up

to 70% (Scheme 1).[6]

Scheme 1: Dehydration of sorbitol to isosorbide in solvent-free conditions.[6]

However, in industrial processes, solvents are almost unavoidable. Homogeneous

liquid phases are obtained thanks to their ability to dissolve solids and their influence on

the solution viscosity. Furthermore, solvents facilitate mass and heat transfer and they

have a crucial role in separation and purification steps. Thus, chemists needed to develop

more sustainable alternatives than the conventional organic solvents. Two main routes

towards green solvents have been developed: i) the substitution of hazardous solvents

with ―eco-friendly‖ ones that show better EHS (Environmental, Health and Safety)

properties, and ii) the substitution of petro-chemically derived solvents with ―bio-

solvents‖ from renewable resources. The first strategy is based both on the use of safe and

innocuous organic solvents, like acetone or alcohols, and on new generation solvents,

such as ionic liquids and supercritical fluids. The second strategy relies on solvents

produced from biomass such as ethanol by fermentation or glycerol derivatives obtained

from triglycerides.


11

Firstly, in order to substitute problematic solvents with ―eco-friendly‖ ones,

several medicinal companies elaborated solvent selection guides in which the solvents are

classified according to their compliance to the different safety principles:

- workers’ safety (carcinogenicity, mutagenicity, skin and respiration

absorption/sensitisation, toxicity, etc.)

- process safety (flammability, explosiveness, peroxide formation, volatile

organic compound – VOC – formation, etc.)

- environmental safety (eco-toxicity, persistence, water contamination, ozone

depletion potential, etc.).

We can cite for instance the guides of Pfizer in 2008[7], GSK in 2011[8] and Sanofi

in 2014.[9] From a survey of these different selection guides, an overall ranking of

solvents was reported (Table 1).[10] Nevertheless, it has to be kept in mind that the criteria

used corresponded to the needs and constraints of the pharmaceutical industry and may

not fit with the criteria of other fields (academic or other type of industry).

Recommended Water, Ethanol, Isopropyl alcohol, n-Butanol, Ethyl acetate,

Isopropyl acetate, n-Butyl acetate, Anisole, Sulfolane.

Recommended or problematic?

Methanol, tert-Butyl alcohol, Benzyl alcohol, Ethylene glycol,

Acetone, MEK, MIBK, Cyclohexanone, Methyl acetate, Acetic

acid, Diethyl ether.

Problematic Me-THF, Heptanes, Methyl-cyclohexane, Toluene, Xylenes,

Chlorobenzene, Acetonitrile, DMPU, DMSO.

Problematic or hazardous? MTBE, THF, Cyclohexane, DCM, Formic acid, Pyridine.

Hazardous Diisopropyl ether, 1,4-Dioxane, Dimethoxyethane, Pentane,

Hexane, DMF, DMA, NMP, Methoxy-ethanol, Triethylamine.

Highly Hazardous Diethyl ether, Benzene, Chloroform, CCl4, Dichloroethane,

Nitromethane.

Table 1: Overall ranking of solvents, adapted from Prat et al.[10].

Among them, we can focus on water which is probably the greenest alternative

chemists have, beyond using solvent-free systems. It is the most abundant molecule on

the planet and is referred to as a benign ―universal solvent‖. Indeed, water is a highly

polar, cheap and safe solvent which does not induce any hazards, neither for humans nor

for the environment. It is non-flammable and non-explosive. Therefore, water can be a


12

useful solvent for large scale process chemistry. The properties of water have even led to

improved reaction rates and selectivity thanks to the hydrophobic effect, and to an easier

separation since a lot of organic compounds did not dissolve in water. The main

drawbacks of water as solvent are the risk of water contamination that can be energy-

intensive to clean, the risk of deactivation of some catalysts or reagents and the low

solubilisation of organic compounds that can limit the reactions. Surfactants[11], cage

molecules such as cyclodextrin[12], or the use of sub- and supercritical conditions can be

solutions for this last issue.

Other alternatives have been developed in order to replace traditional organic

solvents. For example, the supercritical fluids (SCF) have been extensively studied in the

past decades. Substances enter the supercritical phase above their critical pressures (Pc)

and temperatures (Tc). Common SCF are generated from carbon dioxide, water,

methane, methanol, ethanol or acetone. Carbon dioxide (scCO2) is one of the most widely

used SCF thank to its readily accessible critical point: 304K (31°C) and 72.8 bar. It is a

versatile solvent, safe and easy to handle. Supercritical fluids properties can be tuned by a

simple change in temperature and pressure, and SCF have the advantage of having

simplified experimental procedures: simply degassing the system allows the complete

removal of the solvent. For example, Poliakoff and co-workers reported the formation of

cyclic and acyclic ethers, acetals, and ketals by dehydration of alcohols using solid

catalysts (Deloxan ASP and Amberlyst 15) in supercritical fluids (scCO2 and scPropane)

as shown in Scheme 2.[13]

Scheme 2: Formation of tetrahydrofuran, THF, from 1,4-butanediol in scCO2.[13]

Another example of green solvents would be ionic liquids, pioneered in the last

few decades by Seddon.[14] As their name highlights, ionic liquids, or sometimes called

room temperature ionic liquids, are liquid salts at room temperature. They have no (or

exceedingly low) vapour pressure, so volatile organic reaction products can be separated

easily by distillation or under vacuum. They are thermally stable and can be used over a

wide temperature range compared with conventional solvents, even if some recent

studies have shown that they cannot be considered as non-flammable.[15] Their properties

can be readily adjusted by varying the anion and cation. We can report, for example, the


13

work of Zhang et al. who worked on the dehydration of xylose to furfural in 1-butyl-3-

methylimidazolium chloride with AlCl3. The best yield reached was 85%.[16] However,

ionic liquids have some drawbacks. Their synthesis from substances, that can be toxic

and hazardous, and the presence of perfluorinated counter-anions (PF6- and BF4

-), which

can be released, raise the question of the greenness of these solvents. Besides, the

reaction products often need to be extracted from the ionic liquid phase using standard

organic solvents.

Secondly, the strategy was to develop new solvents derived from renewable

resources to replace the conventional solvents derived from petroleum. The biomass

feedstocks include waste materials, forest products, energy crops (starch crops, sugar

crops, grasses, vegetable oils) and aquatic biomass. The variety of feedstocks can also be

divided into three groups according to their chemical composition: cellulosic biomass,

starch- and sugar-derived biomass and triglyceride-based biomass.[17]

For instance, glycerol, which is a by-product of biodiesel production, is non-toxic

and has promising physical and chemical properties as an alternative solvent (Figure 3).

It has a very high boiling point and negligible vapour pressure and can dissolve many

organic and inorganic compounds. It is poorly miscible with water, some ethers and

hydrocarbons. Therefore, in addition to distilling products from this solvent, simple

extractions with solvents such as ether and ethyl acetate are also possible.

Figure 3: Examples of renewable solvents.

2-Methyltetrahydrofuran (MeTHF) is also a promising renewable solvent

obtained through a two-step hydrogenation of furfural, produced from lignocellulosic

biomass (Figure 3). MeTHF has properties similar to conventional THF, which is used in

many organometallic reactions, and can easily substitute it. Besides, contrary to THF,

MeTHF has the advantage of being immiscible with water, and so provides clean

organic–water phase separations.

In addition to these two examples, many other renewable solvents have already

proven their capacity to substitute hazardous ones: for instance, glycerol derivatives


14

(esters, carbonates, acetals and ketals), gamma-butyrolactone (GBL), gamma-

valerolactone (GVL), fatty acid methyl esters (FAME) or terpene derivatives.

However, even if solvents obtained from renewable sources are technically green

in terms of life-cycle analysis, the majority of them belongs to the group of VOCs and

could be associated with atmospheric pollution and flammability.

In brief, even though the last couple of decades have seen a large development of

these systems (SCF, ionic liquids, renewable solvents, etc) as clean alternatives for

synthesis and catalysis, it also became increasingly clear that no single system will ever

be able to completely replace all conventional solvents as a truly environmentally friendly

alternative. It means that an ideal and universal ―green‖ solvent for all situations does not

exist because there are drawbacks associated with all of these systems, both from the

points of view of applicability and sustainability. Thus, for each situation, a balance has

to be made between the solvent’s physical properties (boiling point, miscibility, solubility

of the target compounds, etc), its green properties (low toxicity, recyclability, etc) and its

possible drawbacks.

ii. Sub- and supercritical water

To combine the green properties of water and supercritical fluids, it was decided

to perform the dehydration of alcohols in sub- and supercritical water.

Supercritical water (SCW) can be defined as water above its critical point (374°C,

22.1 MPa). For subcritical water, the definition is less strict and can describe several

temperature and pressure windows. Other terms referring to subcritical water can be

found in the literature, such as hot compressed water, superheated water or high-

temperature water. The most practical definition denominates subcritical water as

water above its boiling point at ambient pressure (> 100°C and 0.1 MPa) and below its

critical point (374°C at 22.1 MPa). The different phases of water are represented in Figure

4.


15

Figure 4: Phase diagram of water, from Saka et al.[18]

The chemistry in water differs significantly according to the temperature regions.

Besides the sole ―temperature effect,‖ the major changes in physical and chemical

properties of water decisively influence its properties as a solvent. Figure 5 illustrates the

range of property variations that occur when the temperature increases from 0 to 500°C

at 30 MPa.

Figure 5: Density, dielectric constant and ion dissociation constant (Kw) of water at 30 MPa as a function of temperature, from Peterson et al.[19]


16

At temperatures below 300°C, water is fairly incompressible, which means that

pressure has little effect on its physical properties, as long as it is sufficient to maintain

water in its liquid state. In the region near the critical point, water is highly compressible.

For example, the density decreases by nearly two orders of magnitude from liquid-like

(about 800 kg.m-3) to dense gas-like value (about 150 kg.m-3) as the temperature

increases from 300 to 450°C. These changes in density correlate with other macroscopic

properties that reflect changes at the molecular level (solvation power, degree of

hydrogen bonding, polarity, dielectric strength, molecular diffusivity and viscosity).[19]

With increasing temperature, the ion product of water (Kw) increases by three

orders of magnitude from Kw = 10-14 mol2L-2 at 25°C to Kw ≈ 10-11 mol2L-2 at 300°C, thus

providing a source of hydronium and hydroxide ions. This means that water becomes

both a stronger acid and a stronger base as the temperature increases to the subcritical

water range. Thus, reactions typically promoted by either acid or base can be performed

in high-temperature water without the addition of any catalyst. Above 300°C, Kw

decreases again. Close to the supercritical point, the decrease is very sharp and leads to

values below 10-20 mol2L-2 at 380°C. From this data, it can be clearly deduced that in

subcritical water, ionic reactions will be enhanced while supercritical water will promote

radical reactions. In addition, while chemical reactions in supercritical water have mainly

been applied to break up bonds, the milder temperature region of subcritical water allows

bond formation, i.e., synthesis of organic compounds.[20]

The dielectic constant of water ε is also largely influenced by the variation of

temperature and pressure. It decreases from ε=80 at standard conditions of temperature

and pressure to ε=31 at 225°C, P=100 bars, and finally to ε=6 at the critical point. Thus,

water transforms from a polar, highly hydrogen-bonded solvent to behave more typically

as a non-polar solvent like hexane. Consequently, with increasing temperature, the

solubility of ionic molecules like organic salts strongly decreases, whereas the solubility

of hydrophobic molecules increases.[21]

Recently, sub- and supercritical water have been applied intensively in different

fields such as material synthesis[22], waste destruction[23], plastic recycling[24], and

biomass processing[25,26]. Subcritical and supercritical water are suitable solvents for

many chemical reactions such as alkylation reactions, condensations, oxidation or

deprotection reactions.[21,27] Dehydration of alcohols can be performed in sub- and


17

supercritical water in good yields in pure water[28] or with the additions of catalysts such

as metal sulphates.[29] Our team recently worked on the dehydration of glycerol to

acrolein and on the synthesis of quinoline in subcritical water under conventional

heating and microwave irradiation in batch[30] and in continuous flow.[31]

c. Alternative activation methods

Over the last three decades, significant effort has been put into in the development

of environmentally friendly procedures as alternatives to conventional heating. The aim

was to reduce energy consumption and chemical waste (due to inefficient chemical

reactions or extensive purification). More energy-efficient activation methods such as

microwaves, sonochemistry, photochemistry, electrochemistry or mechanochemistry

were developed. First, a brief introduction into each of these clean technologies will be

presented. As microwave irradiation is the alternative activation method applied during

this PhD work, it will be described in more depth in the second paragraph.

i. An overview of the different methods

Mechanochemistry

According to IUPAC, a mechano-chemical reaction is defined as ―a chemical

reaction that is induced by the direct absorption of mechanical energy‖.[32] Nowadays,

mechanochemistry refers to several areas of research: mechanical activation of solids,

mechanical alloying and reactive milling of solids. During the mechanical grinding of two

solids, there are a variety of changes that can take place:

- grinding of the particles to a very small size,

- increase of the surface area,

- formation of defects and dislocations in the crystalline structure,

- chemical reactions.

The enhanced reactivity of chemicals can be explained by efficient mixing and an

important increase of the surface area which induces a better contact between the

reactants, but also by other factors such as increased temperature and pressure reached

when the solids collide.


18

Mechano-chemical methods have the advantage of offering solvent-free (or

minimal solvent) routes to industrial materials and are therefore of great interest in

designing more sustainable processes. The potential to access materials not available by

other methods is also of great interest. In organic chemistry, mechanochemistry finds

various applications including C-C bond formation, amine condensation, syntheses of

heterocycles and fullerene modifications.[33,34]

Sonochemistry

Sonochemistry is the application of ultrasound (20 kHz to 500 MHz) to chemical

reactions and processes. The activation is induced by cavitation, which involves the

creation, growth and collapse of micrometer-sized bubbles that are formed when an

acoustic wave propagates through a liquid. Bubbles collapse in succeeding compression

cycles (shock waves) and generate the energy for chemical and mechanical effects.

Cavitational collapse also produces intense local heating (2000-5000 K) and high

pressures (> 1000 bar). Therefore, sonication of a liquid medium can be thought of as

generating high-energy ―hot spots‖ throughout the system. However, it should be noted

that, in most cases, enhanced yields and reaction rates are due to the mechanical effects

of shock waves (splitting of large molecules into smaller fragments, particle size

reduction, surface cleaning and intensive mixing). Chemical effects of ultra-sound will

occur only if high-energy species, released after the cavitational collapse, act as reaction

intermediates. In these cases, changes in product distribution, switching of reaction

mechanisms or changes in region- or diastereoselectivity may occur.[35]

Sonochemistry includes different research areas such as material synthesis

(preparation of micro- and nano-structured materials for example), environmental

protection by degrading chemical and biological pollutants, and chemical synthesis.

Electrochemistry

Electrochemistry concerns the transfer of charge, by the movement of ions, in

liquid, solid or gaseous phases through which electrochemical transformation of species

is achieved. Electrochemistry plays an important role in the commercial world and can be

used for a wide range of applications: to synthesise chemicals and materials, to extract

and produce metals, to generate power, to analyse compounds, etc.

Therefore, electrochemistry has an important role to play for the development of

greener and more efficient processes thanks to its many advantages. Indeed, electrons


19

are the cheapest, purest and most versatile redox agents able to perform clean and fast

reactions. Thus, stoichiometric and hazardous reagents can be replaced. Water-based

processes and products predominate in electrochemistry so organic solvents are avoided.

New solvents and reaction media such as solid polymers or supercritical fluids can also

be used. Besides, electrochemistry allows mild process conditions and ease of control.

Syntheses are clean by direct oxidation or reduction.[36]

Photochemistry

Photochemistry refers to the chemical effects of light. However, unlike

electrochemistry, microwave irradiation or sonochemistry, photochemistry has long not

been considered as an alternative technology for chemical synthesis. Its potential was

mainly recognized for the clean-up of effluents. Nevertheless, photochemistry has three

main advantages within the context of cleaner chemicals manufacturing. The first

advantage is a reduced use of reagents. Indeed, photons can be regarded as ideal

reagents: they activate reactions without directly generating any by-products as they are

non-material and disappear in the process. Light sources can be switched on and off at

will so photons do not need any storage and do not suffer degradation as conventional

reagents do. The second advantage of photochemistry is the use of lower reaction

temperatures. In a photochemical reaction, the activation energy is supplied by the

photons directly to the molecules that absorb the light. Thus, photoreactions are often

carried out at room temperature. Finally, photochemistry can help to control the

selectivity of some types of reactions such as 2π + 2π cycloaddition or reactions involving

singlet oxygen.

However, nowadays, the exploitation of photochemistry in cleaner chemical

manufacturing is limited by several problems: the need for a specialised processing plant,

the problem of window fouling (deposition of by-product coatings on the lamp

enclosures) that reduces the efficiency of the process or the high cost of protons linked to

the lamp efficiency.[37]

ii. Microwave chemistry

The use of microwave heating in chemical applications emerged originally in 1986

with the work of the groups of Gedye and Giguere.[38,39] They both observed significant

reaction rate enhancement using microwave irradiation. At that time, their equipment


20

was composed of domestic ovens and rudimentary reaction vessels. Over the years,

technology evolved and research-grade instrumentation was introduced, improving the

reproducibility of experiments and limiting the risk of fire and explosion. Then, since the

late 1990s, the number of publications related to microwave-assisted organic synthesis

(MAOS) increased dramatically (up to ≈ 5000). Microwave chemistry can be applied to a

wide range of chemical reactions such as heterocyclic synthesis, transition-metal-

catalysed reactions, N-acylations or esterifications.[40,41] In addition to organic synthesis,

this technology penetrated also other fields such as polymer synthesis, material sciences,

nanotechnology and biochemical processes.

Principle

Microwave irradiation is an electromagnetic radiation in the frequency range of

30 GHz to 300 MHz, which corresponds to wavelengths of 1 cm to 1 m. A large fraction of

the microwave spectrum is reserved for applications in telecommunication and radar

technology. For microwave chemistry, the frequency of 2.45 GHz (corresponding to a

wavelength of 12.2 cm) is used almost exclusively.

Microwave chemistry relies on the ability of the reaction mixture to efficiently

absorb microwave energy, by dielectric loss, and to convert it into heat. Briefly, the

heating mechanism involves two main processes: dipole rotation and ionic conduction.

Dipole rotation refers to the alignment of molecules that have permanent or induced

dipoles with the electric field. At 2.45 GHz, the electric field oscillates 4.9 x 109 times per

second. The oscillation of the molecules generates heat. The second main dissipation

mechanism, ionic conduction, is the migration of dissolved ions with the oscillating

electric field. Heat generation is due to frictional losses that depend on the size, charge

and conductivity of the ions as well as on their interactions with the solvent. The two

heating mechanisms are represented in Figure 6.[42]

Two parameters define the dielectric properties of a substance: (i) the dielectric

constant (ε), describing the ability of molecules to be polarized by the electric field, and

(ii) the dielectric loss (ε’’), indicating the efficiency with which electromagnetic radiation

is converted into heat. The ratio of these two parameters defines the dielectric loss

tangent, or dissipation factor, tan δ = ε’’/ε. This loss factor provides a measure of the

ability of a material to convert electromagnetic energy into heat at a given frequency and

temperature. A reaction medium with a high tan δ is required for a good absorption and,

consequently, an efficient heating.


21

Figure 6: Two main heating mechanisms under microwave irradiation: (a) dipolar rotation; (b) ionic conduction mechanism. From Kappe, Dallinger & Murphree.[43]

In general, solvents used for microwave synthesis can be classified as high (tan δ >

0.5), medium (0.1 < tan δ < 0.5) and low (tan δ < 0.1) microwave absorbing. Generally,

solvents with high dielectric constants, such as water, ethanol or acetonitrile, tend to heat

readily under microwave irradiation. Less polar substances, like aromatic and aliphatic

hydrocarbons, or compounds with no permanent dipole moment (e.g. carbon dioxide,

dioxan, or carbon tetrachloride) are poorly absorbing and are considered as ―microwave

transparent‖. Table 2 shows the classification of common organic solvents according to

their dissipation factor.[40]

Solvent tan δ Solvent tan δ

Ethylene glycol

Ethanol

DMSO

Isopropanol

Methanol

Acetic acid

DMF

1,2-Dichloroethane

1.350

0.941

0.825

0.800

0.659

0.174

0.161

0.127

Water

Chloroform

Acetonitrile

Acetone

Tetrahydrofuran

Dichloromethane

Toluene

Hexane

0.123

0.091

0.062

0.054

0.047

0.042

0.040

0.020

Table 2: Dissipation factors (tan δ) of different solvents (2.45 GHz, 20°C).

Data from Dallinger & Kappe.[40]


22

Advantages/Drawbacks

Traditionally in organic synthesis, elevated temperatures are reached by using an

external heat source such as an oil bath or a sand bath. Heat is transferred to the reaction

medium by conduction and convection. This is a comparatively slow and inefficient

method for transferring energy since it depends on the thermal conductivity of the

materials used and wall effects can be observed (temperature of the reaction vessel

higher than in the reaction medium). In contrast, microwave irradiation raises the

temperature in the whole reaction volume simultaneously (core volumetric heating) by

direct coupling of microwave energy with the molecules (solvents, reagents, catalysts)

present in the reaction medium. As presented in Figure 7, an inverted temperature

gradient can be observed by comparison with conventional heating. The very efficient

internal heating transfer results in minimized wall effects which may lead to diminished

catalyst deactivation for example.[40]

Figure 7: The temperature profile after 60 seconds by treatment

in an oil bath (left) compared to microwave irradiation (right). From J. Schanche.[44]

Temperature scale is in Kelvin. „0‟ on the vertical scale indicates the position of the meniscus.

As seen before, the question of solvent is an important topic in green chemistry.

Solvent-free conditions can be employed while applying microwave irradiation.

Reactions with neat reactants, using solid supports (e.g. silica, montmorillonite K10,

zeolites) or phase-transfer catalysis can be performed using microwave heating. There

are many benefits: avoidance of large volumes of solvent, simplified work-up, reduction


23

of the risks of over-pressure and explosion, recyclability of solid supports (when used).

Besides, when solvents are unavoidable, it is possible to find green alternatives to

hazardous solvents. Indeed, as seen in Table 2, while not all high-absorbing solvents may

be classified as ―green‖, there are many desirable solvents that are excellent microwave-

absorbing solvents.

The reaction rate of microwave-assisted organic reactions is 10- to 1000-fold

faster than conventional synthesis. Besides, rapid heating to the target temperature

inhibits the formation of by-products, leading to greater purity and to yield increase.

From a green perspective, the remarkable improvement in conversion, yield and purity

when using microwave heating is another great advantage. Indeed, these improvements

induce less purification steps (less solvent/silica gel waste), and a smaller amount of

reaction by-products that need to be disposed of.[45]

To summarise, microwave heating can provide the following advantages in

comparison to conventional heating for chemical synthesis (Figure 8): (i) high heating

rates, thus increased reaction rates; (ii) selective heating, if the reaction mixture contains

compounds with different microwave absorbing properties; (iii) higher yields; (iv) better

selectivity due to reduced side reactions; (v) improved reproducibility; (vi) no direct

contact between the heating source and the chemicals; (vii) excellent control of the

reaction parameters; and (viii) automation and high throughput synthesis.

However, in addition to these advantages, microwave chemistry also has some

significant limitations (Figure 8). One of the major drawbacks is the high costs for

microwave reactors, which can easily be in the range of several thousands of Euros. The

short penetration depth of microwave irradiation into the liquid medium limits the size

of the reactors, which is a serious problem for scale-up. Finally, it is difficult to monitor

reactions in situ.[46]

To overcome this scalability issue, the development of new reactors has been a

challenge for engineers in the past few years. The main problem was how to design large-

scale reactors such that the efficient and uniform application of dielectric heating can be

achieved in a way similar to laboratory-scale reactors. Recently, flow reactors had

attracted attention as a solution to these issues. This technology involves pumping a

reaction mixture into a reactor where it undergoes microwave irradiation for a set time,

and is subsequently pumped through a heat exchanger and into a receiving vessel.


24

Figure 8: Current pros and cons of microwave-assisted synthesis.

Microwave Chemistry in Water

At 20°C, water can be considered only as a medium microwave absorbing solvent

with a dissipation factor tan δ of 0.123 (Table 2). However, like many organic solvents,

the tan δ of water is strongly influenced by temperature. As we saw before, the dielectric

constant ε decreases drastically with temperature, from ε=80 at standard conditions of

temperature and pressure to ε=31 at 225°C, P=100 bar, and finally to ε=6 at the critical

point. Thus, the dielectric loss ε’’ and the loss tangent tan δ are also reduced.

Consequently, water can be heated rather effectively by microwave irradiation from room

temperature to 100°C, but it is difficult to superheat water in sealed vessels from 100 to

200°C and very difficult to reach 300°C by microwave heating. In fact, supercritical

water can be considered as transparent to microwave radiation.

Nevertheless, it should be noted that the dissipation factor of water can be

significantly increased by the addition of small amounts of organic salts. Indeed, the

introduction of ions in the solution leads to a marked increase in dielectric heating rates

thanks to the ionic conduction mechanism. Besides, in most cases, the chemicals

(substrates, reagents or catalysts) dissolved in the aqueous reaction mixture are strongly

polar and therefore microwave absorbing, allowing a sufficient heating of the reaction

mixture.[40]

Microwave-Assisted Organic Synthesis

Automation and highthroughput synthesis

Limited scale up+ ‒

Selective heating

High heating rates

Contactless heating

Short reaction times

Improved reproducibility

High yields with less side products

Excellent control of reaction parameters

Difficult in situ monitoring

Expensive setup


25

2. Dehydration of polyhydroxylated compounds in water

The dehydration of polyhydroxylated compounds can be performed in many different

reaction media. Some studies reported the dehydration of polyols in gas phase without

the addition of solvents.[47,48] This reaction can also be performed in organic solvents

(classic or green)[49,50] or in alternative solvents such as ionic liquids[51,52] or supercritical

fluids[13,53]. As we chose to perform our reactions in water, only the examples of

dehydration in water will be reported in this chapter.

a. Without a catalyst

To start, we will focus on the reactions performed without any catalyst. It can be

noticed that reactions are dependent on the temperature region used and so, on the water

properties. For example, Avola et al. performed the dehydration of several activated

alcohols at 180°C under pressure. As expected in the subcritical region, the researchers

did not observe any radical reaction, such as oxidation, and all compounds underwent

dehydration in moderate to high yields. The high concentration of H+ ions, due to the

increase of the self-dissociation constant of water, promotes dehydration but also further

transformations such as pinacol rearrangement and aldolic condensation.[54] Qadariyah

et al. also observed a difference in the type of products formed during the dehydration of

glycerol according to the temperature. Between 200 and 300°C, ionic reactions are

favoured: acrolein and acetaldehyde are the main products, with, nevertheless, limited

yields. At 400°C, in the supercritical region, the radical formation of allyl alcohol is

promoted and reached 96%.[55]

Dehydration of sugars can also be performed without the addition of any catalyst.

From fructose and glucose, moderate temperatures (220°C) allow the formation of the

target molecule, 5-hydroxymethylfurfural (5-HMF), with acceptable yields of 47% and

30% respectively, for a quantitative conversion.[56] The application of higher

temperatures (350-400°C) to glucose produces 5-HMF in a limited yield (8%) while

secondary and degradation products are mainly obtained.[57] The same observations can

be made for the dehydration of xylose. Reactions performed in the subcritical region

leads to the formation of furfural with moderate yields (50% at 220°C after 50 min in a

batch reactor or after 30 min under microwave irradiation)[58,59] whereas reactions

observed close to the critical point and in the supercritical region mainly formed retro-


26

aldol compounds, such as glyceraldehyde and glycoaldehyde due to the degradation of

furfural or xylose.[60,61]

Thus, according to these studies, subcritical water seemed to be a suitable solvent

for the dehydration of polyhydroxylated compounds. Supercritical water seemed to lead

more to secondary products and to favour degradation.

b. Homogeneous catalysis

Secondly, we will focus on dehydration performed with homogeneous catalysis.

Classically, researchers used mineral acids such as H2SO4, HCl and H3PO4. However,

since these acids are very corrosive, alternatives were investigated with the use of organic

acids, soluble metal salts (metal sulphates, metal chlorides) or the addition of CO2.

i. Mineral and organic acids

Sulphuric acid gave good results on the dehydration of polyols. For example, 2,3-

butanediol was successfully dehydrated in methyl ethyl ketone, MEK, (98% of

conversion, 91% of yield) with the addition of H2SO4.[62] This acid had also a positive

effect on the dehydration of glycerol to acrolein. Ramayya et al. and Watanabe et al. both

observed a limited yield of acrolein in pure water. The addition of H2SO4 increased

conversion and yield. Ramayya et al. obtained 55% of conversion and 47% of yield at

350°C with 1 mol% of acid.[63] Watanabe et al. obtained 94% of conversion and 74% of

yield (by-product was acetaldehyde) at 400°C with 10 mol% of acid.[64]

Scheme 3: Dehydration of glycerol to acrolein with the addition of sulphuric acid.[63,64]


27

In industrial processes, hexoses and pentoses were mainly dehydrated by

sulphuric acid. In addition, many articles reported the use of hydrochloric acid for the

dehydration of sugars. Generally, researchers observed limited conversion and yield

when using pure water and concluded that an acidic medium was necessary to perform

the dehydration of sugars. Moreover, according to their studies, the dehydration of

hexoses and pentoses was pH-dependent. A low concentration of H+ in the reaction

medium (pH > 3) was not sufficient to catalyse the dehydration but a pH too acidic (pH <

1) led to the formation of by-products: levulinic and formic acid from hexoses and

humins from both types of sugars. For the dehydration of fructose, a pH of 2-2.5 (HCl or

H2SO4 solutions) seemed to maximise 5-HMF formation.[65–67] A pH more acidic (1-1.5)

was more adequate to dehydrate xylose and xylan.[68]

Alternative acids were tested to avoid the use of H2SO4 or HCl. Asghari et al.

compared the effect of hydrochloric, sulphuric, phosphoric, citric, oxalic, maleic and p-

toluenesulfonic (p-TSA) acids on the dehydration of fructose to 5-HMF. In an interesting

way, they observed that phosphoric acid was more efficient than HCl and H2SO4: 65% of

5-HMF yield against 44 and 40% respectively. p-TSA gave a limited yield of 37%. Maleic

acid led to a good yield of 60% while citric and oxalic acids promoted polymerization

more than 5-HMF formation. Besides, these last three acids tended to degrade under

subcritical conditions.[66] On their side, Yemis et al. compared three strong mineral acids

(hydrochloric, sulphuric, and nitric acids) and three weak organic acids (phosphoric,

acetic, and formic acids) on the dehydration of xylose and xylan to furfural. The furfural

yields obtained from xylose in the presence of HCl, H2SO4, HNO3, H3PO4, CH3COOH and

HCOOH were 59%, 50%, 5%, 43%, 25%, and 37%, respectively.[68] These differences in

furfural yield could be attributed to the different anions generated in the reaction media.

As highlighted by Marcotullio and De Jong, Cl- is the most effective anion favouring the

conversion of xylose and xylan to furfural.[69,70] Good furfural yields can also be obtained

by using formic acid[71] or maleic acid, even though maleic acid tended to degrade into

malic acid when heated[72].

ii. Metal sulphates

Recently, the team of H. Vogel worked on the dehydration of biomass-derived

polyols in sub- and supercritical water.[29,73–76] They mainly focused on the use of

different metal sulphates (Na2SO4, CuSO4, ZnSO4, NiSO4, MgSO4) in order to replace


28

classic mineral acids. They performed their reactions using a continuous flow reactor.

Some examples of their work are presented in Table 3.

Polyol (wt.%) Additive

(ppm)

P

(MPa) T (°C)

Residence

time τ (s)

Conversion

(%)

Yield

(%) Ref.

1,2-Propanediol

5 - 34 360 60 31 20 [75]

5 ZnSO4 (400) 34 360 60 100 90

1,2-Butanediol

5 ZnSO4 (400) 34 340 120 100 70 [74]

Glycerol

1 ZnSO4 (790) 25 360 60 80 45 [73]

meso-Erythritol

5 - 34 360 120 75 48 [75]

5 ZnSO4 (4000) 34 360 120 100 60

Table 3: Examples of the effect of metal sulphates on the dehydration of biomass-derived polyols.

The dehydration of polyols was strongly influenced by the addition of salts. It was

completely inhibited by sodium sulphate while bivalent transition metal cations

increased both the conversion and the selectivity. One possible explanation for this effect

was linked to the acidic properties of the used salts and to a possible pH shift. Depending

on the substrate, copper and zinc sulphates were the best effective additives. However,

copper salts may sometimes be too active and also promoted side reactions.


29

iii. Metal chlorides

As an alternative to corrosive mineral acids, researchers performed dehydration of

polyols in the presence of metal chlorides. Crittendon et al. compared the effects of

MnCl2, CuCl2 and SnCl2 for the dehydration of cyclohexanol to cyclohexene in

supercritical water (375°C). MnCl2 and CuCl2 showed results similar to those obtained

with HCl: an almost quantitative conversion and yields superior to 90% were obtained.[77]

The influence of ZnCl2 was studied by the team of Dai on the dehydration of diethylene

glycol to 1,4-dioxane[78] and by the team of Tagaya on the dehydration of propylene glycol

to 2-methyl-2-pentenal.[79] Both teams observed a limited yield in pure water, which was

increased by the addition of ZnCl2 to 60% for 2-methyl-2-pentenal (300°C, 60 min, 1

wt% of ZnCl2) and 51% for 1,4-dioxane (340°C, 120 min, 0.5 wt% of ZnCl2).

For sugars, the addition of metal chlorides also proved to be efficient. In 2007, for

the first time, Zhao et al. discovered that chromium (II) chlorides were very effective

catalysts for the conversion of glucose to 5-HMF in ionic liquids with a yield near 70%.[80]

This work inspired many studies on metal chlorides for the synthesis of furanic

compounds. Chromium (III), for example, was used by Choudhary et al. to dehydrate

xylose into furfural and led to a yield of 39% in water in combination with HCl.[81] AlCl3

and FeCl3, being abundant and cheap, had received considerable attention for the

production of 5-HMF and furfural[52,82,83] or for the pre-treatment of lignocellulosic

material[84]. If we focus on the dehydrations performed in water, we can cite the work of

De et al. who obtained moderate 5-HMF yields from fructose, glucose and sucrose (54%,

37% and 30% respectively) with the addition of AlCl3 under microwave irradiation

(Scheme 4).[85] Recently, two studies compared the effect of different metal chlorides

(GeCl4, CrCl2, CrCl3, AlCl3, FeCl2, FeCl3, SnCl2, SnCl4, CeCl3, InCl3) on the dehydration of

xylan[86] and of xylose and glucose[87]. Both teams observed a superior effect of SnCl4. The

effects of metal chlorides on the dehydration of xylose will be further discussed in

Chapter III.

Scheme 4: Formation of 5-HMF from fructose in presence of AlCl3 under microwave irradiation.[85]


30

iv. With the addition of CO2

In 2003, Hunter and Savage reported, for the first time, the acceleration of acid-

catalysed reactions in high-temperature liquid water by the addition of carbon dioxide to

the reaction medium. They examined the dehydration of cyclohexanol to form

cyclohexene and obtained doubled yields when carbon dioxide was added to the aqueous

medium (Scheme 5).[88] The basis of this rate enhancement was the reaction between

carbon dioxide and water to yield carbonic acid, which subsequently dissociated. The pH

of the reaction medium increased and dehydration was promoted. The same yield

enhancement was observed for the formation of THF from 1,4-butanediol in high

temperature water.[89]

Scheme 5: Dehydration of cyclohexanol to cyclohexene in presence of carbon dioxide.[88]

For Yamaguchi et al. who worked on the formation of cyclic ethers from polyols,

the addition of CO2 did not lead to a higher yield of product at the end of the reaction but

only enhanced the formation rates to reach the equilibrium yield.[90–92]

The main advantage of an acid solvent composed of water and carbon dioxide is

that it is environmentally-benign: both water and carbon dioxide are non-toxic, and

separation and recycling of these two components are easily performed by

depressurisation after reaction.

c. Heterogeneous catalysis

With comparison to homogeneous catalysis, heterogeneously catalysed processes

have many advantages in the frame of green chemistry: they produce less waste, facilitate

easy separation and recovery of the catalyst, and the elimination of mineral acids makes

the reaction mixture less corrosive. However, finding an active and stable water-tolerant

solid catalyst is still a challenge. Besides, during sugars dehydration, many undesired

reactions take place and form humins, reducing selectivity and deactivating the catalyst.


31

Some examples of heterogeneous catalysts used for dehydrating polyols and sugars are

presented below.

i. Modified silicates and aluminosilicates

Two of the most commonly studied solid acids types are silicates and zeolites

(aluminosilicates), modified or commercial. One of the main advantages of these

materials is their shape selectivity and the possibility to control the pore size. In addition,

the acidity of these materials can be controlled by varying the Si/Al ratio in the matrix.[93]

Zhang et al. studied the effect of the variation of the Si/Al ratio of H-ZSM-5

zeolites and their further modification with boric acid on the dehydration of bio-based

2,3-butanediol to methyl ethyl ketone (MEK). The results showed that a high Si/Al ratio

improved the conversion of 2,3-butanediol with a similar selectivity to MEK (conversion

100% and yield 69% at 300°C). The modification of the zeolite with boric acid allowed

the researchers to obtain the same results but at a temperature of only 180°C (Scheme

6).[94]

Scheme 6: Formation of methyl ethyl ketone from 2,3-butanediol with modified H-ZMS-5 zeolites.[94]

Sulfonic acid and aluminium modified mesoporous shell silica beads were tested

by Jeong et al. on the dehydration of xylose to furfural. Sulfonic acid modified catalysts

gave limited conversion and yield (32% and 19% respectively). Because of their high

Lewis acidity (pKa > -5), the aluminium modified catalysts led to a higher conversion but

to a lower selectivity in furfural. These catalysts were also tested for the dehydration of

glycerol to acrolein and gave good results (conversion 79% and yield 48% with the

sulfonic acid catalyst).[95]

The H-MCM-22 zeolite was an effective and recyclable solid acid catalyst in the

aqueous-phase dehydration of xylose, at 170°C. Up to 54% furfural yield was reached at

97% conversion in water. Besides, this solid acid catalyst was fairly stable (similar

furfural yields were reached in recycling runs; no structural modifications and no

leaching phenomena were detected).[96]


32

You and Park carried out the dehydration of xylose into furfural over different H-

zeolites in a continuous flow. Although all the used H-zeolites had similar surface areas,

amounts of acid sites, and ratios of Brønsted acid sites/Lewis acid sites, the conversion of

D-xylose decreased in the following order: H-Y > H-β > H-mordenite > H-ZSM-5 > H-

ferrierite. For them, this indicated that xylose conversion increased with increasing

channel size of the zeolite. The selectivity of furfural did not follow the same trend: the

selectivity of furfural increased with increasing channel size, to a maximum value at a

channel size of 5.5 Å (corresponding to H-ZSM-5), and then decreased with a further

increase in channel size.[97]

ii. Mixed oxides

Metal oxides, especially ZrO2, TiO2, Fe2O3, and zirconia-containing mixed oxides

modified by sulphate ions have been found to be promising catalysts for a wide variety of

reactions (for example, esterification, isomerisation, alkylation, and cracking). Metal

oxides are utilised both for their acid-base and redox properties and constitute one of the

largest family of catalysts in heterogeneous catalysis.

Akizuki and Oshima studied the activity of a TiO2 catalyst with variable WO3

content on the dehydration of glycerol to acrolein in supercritical water. They observed

that the reaction rate and the selectivity towards acrolein increased with an increase in

WO3 content.[98]

Qi et al. compared the activity of TiO2 (rutile and anatase) and ZrO2 on the

dehydration of hexoses to 5-HMF under microwave irradiation. Rutile TiO2 had no

significant effect on hexoses conversion but anatase TiO2 and ZrO2 led to moderate

results starting from fructose (90% conversion and 41% yield with anatase TiO2) and

limited yields starting from glucose.[99] For Chareonlimkun et al., the presence of TiO2

and SO4–ZrO2 promoted the hydrolysis and dehydration of C5-sugars (xylose), C6-sugars

(glucose), cellulose and lignocellulose to furfural and 5-HMF with less by-products

formation, whereas ZrO2 strongly promoted the isomerisation reaction.[100,101]

Zirconium–tungsten (ZrW) mixed oxides were also relatively active catalysts in

the aqueous phase reaction of xylose, at 170°C. The catalysts led to more than 90%

conversion within 2 h reactions, but furfural yields were less than 35%. Results could be

improved by using mesoporous ZrW with enhanced specific surface area and amount of

accessible acid sites (41% yield at 100% conversion), and furthermore by doping the

inorganic material with aluminium to give mesoporous ZrAlW (51% yield at 98%


33

conversion). Besides, catalyst recycling tests revealed that mesoporous ZrW and ZrAlW

catalysts were fairly stable catalysts under the applied reaction conditions.[102]

Graphene, graphene oxide, sulfonated graphene, and sulfonated graphene oxide

(SGO) were prepared, characterized and tested for the dehydration of xylose to furfural in

water by Lam et al. In particular, SGO was proven to be a rapid and water-tolerant solid

acid catalyst even at very low catalyst loading, maintaining its initial activity after 12

tested repetitions at 200 °C, with an average furfural yield of 61% in comparison to 44%

for the uncatalysed system.[103]

iii. Ion-exchange resins

Ion-exchange resins are sulfonated copolymers of styrene and divinyl benzene in

which sulfonic acid groups act as active sites. Their maximum operating temperature is

usually around 130°C, above which desulfonation of the acid sites starts to deactivate the

catalyst. Ion-exchange resins have nevertheless been observed to work in a stable manner

up to 150°C in the dehydration of monosaccharides.[93]

However, ion-exchange resins were mainly used in organic solvents (DMSO,

acetonitrile, DMF, etc)[49,104,105] and not in pure water. We can report only two

publications which investigate the use of Amberlyst 70 and Nafion SAC-13 for the

dehydration of xylose in water. The first work led to moderate results with Amberlyst 70

(conversion 83% and yield 38%) which were increased by the use of a simultaneous

stripping with nitrogen (conversion 99% and yield 70%, Scheme 7).[106] The second

publication compared the activity of Amberlyst 70 and Nafion SAC-13 to HCl for the

dehydration of xylose under microwave irradiation. Nafion SAC-13 led to lower results

compared to HCl but Amberlyst 70 gave results similar to HCl. However, furfural yield

stayed limited.[107]

Scheme 7: Formation of furfural from D-xylose with an ion-exchange resin, Amberlyst 70.[106]


34

iv. Solid metal phosphates

Metal phosphates have a layered structure consisting of metal atom and

phosphate species planes, and they exhibit both Lewis and Brønsted acidity. They are

known for their ion-exchange and acid properties.[93]

The team of Rusu performed sorbitol dehydration under hydrothermal conditions

with metal (III)- and metal (IV)-phosphates as catalysts. The activity of the metal

phosphates on the conversion of sorbitol and on the selectivity to isosorbide increased as

follows: AlP < ZrP < FeP < LaP < CeP < BP. The presence of boron phosphate in the

reaction medium led to a 100% conversion with an isosorbide selectivity of 70%

(uncatalysed reaction gave 75% sorbitol conversion and 1,4-sorbitan as the main

product). Sorbitol dehydration was favoured by moderate and strong acid sites, and not

promoted by very strong and excessively strong acidity.[108]

Zirconium phosphate obtained by hydrothermal methods using organic amines as

templates had been examined as a solid catalyst for the dehydration reaction of xylose to

furfural in aqueous-phase by Cheng et al. It presented good catalytic performance with

conversions up to 96% and furfural yields up to 52% in short reaction times. Moreover,

the catalyst was easily regenerated by thermal treatment in air and showed quite stable

activity.[109]

d. Enhancing selectivity

As we have seen before, the easiest and most economical solvent for the dehydration

of polyhydroxylated compounds is water. It is a non-toxic, non-flammable, safe and clean

solvent. However, during the dehydration of monosaccharides, water also accelerates

some of the consecutive reactions of furfural and 5-HMF, leading to a decrease in yield.

Therefore, one of the solutions would be to isolate the product continuously from the

catalyst-containing phase as soon as it has been formed, in order to avoid its

consumption in consecutive reactions.

This can be done, for example, by a simultaneous nitrogen stripping of furfural,

transferring the furfural to a gas phase where it cannot react anymore since the catalyst

and intermediates are not volatile.[110] Other approaches included adsorbing the formed

5-HMF from the reaction medium on activated carbon[111] or utilising a supercritical fluid,

such as CO2, to concentrate furfural up to 95 wt%.[112]


35

More classically, the addition of a co-solvent allowed the furan to be transferred from

the aqueous phase into the organic phase. Thus, the product was no longer in contact

with water or with the catalyst, and suffered less side reactions. Toluene, THF or MIBK

were often used as co-solvents[113,114] but recently, greener solvents such as Me-THF or

cyclopentyl methyl ether (CPME) were introduced.[82,115] For example, Choudhary et al.

doubled their furfural yield (from 39% to 76%) by introducing a toluene-phase.[81]


36

3. Issue and objectives

The aim of this PhD work is to perform the dehydration of polyhydroxylated

compounds in order to form target molecules with high added value with the idea to try

to respect the Twelve Principles of Green Chemistry as much as we can. Despite common

sense thinking that it will favour hydrolysis, water is a suitable medium for dehydration

as it was described widely in the literature review. Thus, our reactions will be performed

in sub- and supercritical water. When needed, catalysts will be chosen to be efficient,

selective and recyclable. Several strategies will be tried in order to avoid batch reactions

activated by thermal heating: the use of microwave reactors and the use of continuous

flow reactors.

In Chapter II, we will focus on the dehydration of a petro-sourced molecule, 1-phenyl-

ethane-1,2-diol, in water to form phenylacetaldehyde, a target molecule used in perfume

compositions, in pharmaceuticals or as chemical intermediate.

In Chapter III, the dehydration of biosourced D-xylose, as a model compound for

hemicellulose, will be studied to form furfural, a bio-based platform molecule. The

optimised methods will be extended to xylan and other sugar units present in

hemicellulose.


37

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CHAPTER II

Dehydration of 1-phenylethane-1,2-diol

in water

Chapter II: Dehydration of 1-phenylethane-1,2-diol in water

45

1. Introduction

a. Phenylacetaldehyde: origin and applications

Phenylacetaldehyde (1) is a colourless liquid with a sweet-green odour, similar to

the one of hyacinths and narcissi. It has been identified in many essential oils[116] and as a

volatile constituent of foods such as honey[117], chocolate[118] or buckwheat[119] for

instance. Some of its physical properties are given in Table 4.

Phenylacetaldehyde (1)

CAS number 122-78-1

Molar mass M = 120.15 g/mol

Density d = 1.079 g/mL (25°C)

Melting point mp = - 10°C

Boiling point bp = 195°C

Refractive index nD = 1.535

Table 4: Physical properties of phenylacetaldehyde.

Phenylacetaldehyde is used in perfume compositions, in particular for hyacinth

and rose notes.[120] Further applications include the preparation of pharmaceuticals[121],

insecticides[122], acaricides, disinfectants, and its use as a rate-controlling additive in the

polymerization of polyesters with other monomers. Phenylacetaldehyde is also a useful

intermediate in many organic syntheses. For example, phenylalanine, an intermediate for

the sweetener aspartame, can be obtained by the reaction of phenylacetaldehyde with

ammonia and hydrogen cyanide (via Strecker reaction).[123] Phenylacetic acid can also be

obtained by the oxidation of the aldehyde.

b. Literature review

As represented on Scheme 8, phenylacetaldehyde can be synthesised by different

ways. Firstly, it can be obtained in high yield by vapour-phase isomerisation of styrene

oxide with heterogeneous catalysts such as borosilicate zeolite or magnesium silicate for

instance.[124–127] Styrene oxide isomerisation can also be performed in organic solvents

with the use of micro-organisms[128] or phosphotungstic heteropoly acids.[129]


46

Phenylacetaldehyde can also be synthesized by the direct oxidation of styrene by

hydrogen peroxide in the presence of heterogeneous catalysts, by the catalytic

dehydrogenation of 2-phenylethanol and by the Darzens glycidic ester synthesis from

benzaldehyde and alkyl chloroacetates.[130–132]

Scheme 8: Retrosynthetic scheme of phenylacetaldehyde.

Finally, 1-phenylethane-1,2-diol (2) can be converted into phenylacetaldehyde by

dehydration. In the vapour phase, the reaction is performed in high yield in the presence

of heterogeneous catalysts.[127,133] For example, the use of H-ZSM-5 zeolite, silicalite or

silica gel at 200-300°C led to high conversion and yield (> 96%).[133]

Only three publications report the dehydration of 1-phenylethane-1,2-diol in

water. In the first one, the reaction is performed at reflux and catalysed by mineral acids

(HCl, H2SO4 and H3PO4). Limited yields of phenylacetaldehyde were reached (< 20%)

and a product with a higher boiling point was mainly obtained. According to the

researchers, it could be a dimer of phenylacetaldehyde, 2-benzyl-4-phenyl-1,3-dioxane

(structure on Figure 9). When steam was introduced into the reaction mixture in order to

distil out the product, dimerisation was limited and yield of (1) increased to 70%.[134]


47

Figure 9: By-products obtained during the dehydration of 1-phenylethane-1,2-diol in water[134,135]

Katritzky and co-workers carried out the dehydration of 1-phenylethane-1,2-diol

without a catalyst in cyclohexane and water at 200°C for 6h. In cyclohexane, conversion

was low (14%) while it reached 93% in water. Main products were phenylacetaldehyde

(24%) and an aldol condensation product, 2,4-diphenylbut-3-enal (41%, structure on

Figure 9).[135] Finally, Avola et al. compared the dehydration of the diol in pure water and

in NaCl and Na2SO4 solutions at 180°C for 16h. Their results are presented in Table 5.

The addition of NaCl resulted in increased conversion and yield while the addition of

Na2SO4 drastically reduced the conversion rates (2%).[54]

Solvent Conversion (%) Yields (%)

H2O 43 22 21

1M NaCl 93 29 64

1M Na2SO4 2 1 0

Table 5: Effect of the salts on the dehydration of 1-phenylethane-1,2-diol under hydrothermal conditions. From Avola et al.[54]

However, through these publications, it can be noted that the selectivity towards

phenylacetaldehyde was quite low in water: 26-53% in pure water, 31% in NaCl solution

and less than 20% in acidic solutions. The aldol condensation product (3’) was formed in

high yield. The reactivity of the aldehyde was high and it was difficult to avoid further

reactions to happen. This issue is central in this Chapter II: how to maximise

phenylacetaldehyde yield while limiting the secondary reactions?


48

c. Mechanism and structure of the by-product

The formation of phenylacetaldehyde from 1-phenylethane-1,2-diol is a simple

dehydration. Its mechanism is presented on Scheme 9. The first step involves the

protonation of the secondary alcohol by an acid, followed by loss of water to give a

carbocation. Then, deprotonation of the adjacent carbon atom by a water molecule leads

to the creation of the C=C double bond: an enol is formed. Finally, phenylacetaldehyde is

formed by tautomerisation of the enol.

Scheme 9: Dehydration of 1-phenylethane-1,2-diol to phenylacetaldehyde.

The main by-product is produced by an aldol condensation of the phenyl-

acetaldehyde on itself (Scheme 10). The first step is an aldol reaction. The enol is formed

from the aldehyde by tautomerisation. It attacks a protonated molecule of aldehyde,

leading to the aldol after deprotonation. The aldol then dehydrates to give the

unsaturated carbonyl compound.

Scheme 10: Aldol condensation from phenylacetaldehyde.


49

However, in contrast with previous reports,[54,135] the aldol condensation of

aldehyde (1) led to the formation of the 2,4-diphenylbut-2-enal (3) instead of 2,4-

diphenyl-but-3-enal (3’). The formation of compound (3) is favoured thanks to the

formation of two conjugated double bonds (alkene and aldehyde) which stabilise the

molecule.

The structure of compound (3) was established by 1H and 13C NMR spectroscopy

experiments. The 1H NMR spectrum showed that (i) the H[1] gave one singlet at 9.67

ppm indicating no coupling with another hydrogen atom; (ii) the H[2] gave one triplet at

6.88 ppm; and (iii) the H[3,4] gave one doublet at 3.71 ppm with an integral value for two

hydrogen atoms. The coupling constant J = 7.6 Hz for H[2] and H[3,4] proved that the

CH and CH2 groups were neighbours. This 1H spectrum was not compatible with the

structure (3’) previously described, since (i) a doublet would have been observed for the

H[1]; (ii) H[2] and H[3] would have gave a doublet of doublets; and (iii) the H[3] and

H[4] would have been integrated separately for 1 hydrogen atom each (Figure 10).

Figure 10: Comparison of the two possible structures of the by-product – 1H NMR.

The 13C NMR spectrum confirmed the structure (3) with chemical shifts of 153.47

ppm for the C[2], 138.02 ppm for the C[3] and 35.80 ppm for the C[4]. The comparison

with the literature[136,137] also permitted validation that compound (3) had a 2,4-

disubstituted-but-2-enal structure.

Finally, the configuration Z or E of the enal 3 was assigned thanks to a NOESY

(Nuclear Overhauser Effect SpectroscopY) NMR experiment. NOESY experiment affords

a 2D chemical shift correlation map in which the cross peaks signals connect resonances

from nuclei that are spatially close. In our case, the interaction between protons H[1] and

H[2] indicated their spatial proximity and thus, an E configuration for the condensation

product (Figure 11).

Figure 11: Configuration of the 2,4-diphenylbut-2-enal (3).


50

2. Dehydration of 1-phenylethane-1,2-diol under conventional heating

The first part of the study of the dehydration of 1-phenylethane-1,2-diol (2) into

phenylacetaldehyde (1) has been carried out in a batch reactor under conventional

heating without the addition of any catalyst. The experiments were done in a 316

stainless steel autoclave with an inner volume of 100 mL. The pressure during the

reaction was the autogenous pressure of the solution at reaction temperature and was

between 5 and 35 bar.

The influence of temperature and reaction time on the phenylacetaldehyde yield was

studied. The initial diol concentration was also varied in order to see its influence.

However, because of the reactor alloy, the addition of a catalyst was not tested in order to

avoid its corrosion.

a. Variation of temperature and reaction time

Firstly, the influence of temperature and reaction time was studied: several

temperatures (160, 180, 200, 220 and 240°C) were tested for two reaction times (8 and

16h). Results are presented on Figure 12a (8h) and 12b (16h).

As we can see, diol conversion increased both with temperature and reaction time.

A quantitative conversion was reached after 16h at 200°C and after 8h at 220°C.

However, the phenylacetaldehyde yield was very limited: it was between 9% and 13%

regardless of the time or temperature used. As explained before, phenylacetaldehyde was

very reactive and suffered an aldol condensation to form 2,4-diphenylbut-2-enal (3). The

by-product yield was quite high, reaching 42% after 16h of reaction at 200°C.

Nevertheless, it should be noticed that the formation of 2,4-diphenylbut-2-enal did not

follow a linear trend: the yield increased to a maximum at 200 or 220°C depending on

the reaction time, then decreased if the temperature continued to increase. Since the

conversion was total, it suggested that other products were formed, from

phenylacetaldehyde or from the condensation product, or that degradation occurred due

to the high temperature. Avola et al. observed the formation of 3-phenylnaphtalene in

diluted HCl at 180°C for 16h through an intramolecular Friedel-Crafts addition of the

aldol condensation product.[138] In our case, we had not identified any additional

compounds.


51

Thanks to its properties (increase of ion product Kw, decrease of dielectric

constant), subcritical water was a suitable medium to perform the dehydration of 1-

phenylethane-1,2-diol without the addition of any catalyst. However, subcritical water

also promoted the further condensation of phenylacetaldehyde.

Figure 12: Influence of temperature for a) 8h and b) 16h of reaction.

Autoclave, 1-phenylethane-1,2-diol concentration: 0.5 mol/L.

b. Influence of the diol concentration

In a second time, the influence of the initial 1-phenylethane-1,2-diol concentration

(from 0.25 to 1 mol.L-1) was also studied at 200°C for 16h (Figure 13). Similar conversion

0

10

20

30

40

50

60

70

80

90

100

160 180 200 220 240

%

Temperature (°C)

Diol conversion

Aldehyde yield

By-product yield

a)

0

10

20

30

40

50

60

70

80

90

100

160 180 200 220 240

%

Temperature (°C)

Diol conversion

Aldehyde yield

By-product yield

b)


52

and yields were observed regardless of the diol concentration tested. Thus, the diol

concentration did not seem to have a particular influence on its reactivity.

Figure 13: Influence of diol concentration.

Autoclave, T = 200°C, t = 16h.

c. Conclusion

According to this study, using an autoclave as reactor was not the best option to

produce phenylacetaldehyde in good yield and selectivity. The thermal inertia of the

reactor induced a slow heating rate of the reaction medium: between 1 and 2h were

required to reach the wanted temperature. Then, during this time interval,

phenylacetaldehyde was formed and began to react on itself, thus decreasing yield and

selectivity. Due to the reactor alloy, the use of catalysts was very limited in order to avoid

its corrosion.

0102030405060708090

100

0.25 0.5 1

%

Diol concentration (mol/L)

Diol conversion

Aldehyde yield

By-product yield


53

3. Dehydration of 1-phenylethane-1,2-diol under microwave irradiation

From the results of the study under conventional heating, it appeared to us that the

use of a microwave reactor could be an interesting alternative to autoclave. The fast

heating rate of microwaves and the higher yields and selectivities usually obtained are

ones of the many advantages of the microwave reactors. Thus, using microwave

irradiation could be a tool to improve phenylacetaldehyde (1) yield while limiting the

formation of 2,4-diphenylbut-2-enal (3). In a second part, we will focus on the

dehydration of 1-phenylethane-1,2-diol (2) under microwave irradiation, in water and

later, in biphasic medium.

a. Optimisation of the phenylacetaldehyde yield in water

In a first step, we will try to maximize the phenylacetaldehyde yield in water while

limiting the formation of the aldol condensation product, 2,4-diphenylbut-2-enal, under

microwave irradiation. The first experiment was carried out at 200°C for 30 minutes and

led to a diol conversion and a phenylacetaldehyde yield of only 1%. Thus, a catalyst was

necessary. The Lewis acid AlCl3 was selected as it had already given good results for

promoting dehydration reactions.[85,113]

The dehydration reaction in the presence of AlCl3 in water is believed to be

initiated by the hydrolysis of AlCl3 which formed the cationic species [Al(OH)(H2O)5]2+

and H+. Then, these complexes can coordinate to the hydroxyls groups in order to form a

chelate five-membered ring (Scheme 11). As metal cations draw off the electron density

from the hydroxyl groups, an electron defect is created at the carbons. This means that

the separation of either one of the hydroxyl groups is made easier.[75]


54

Scheme 11: Proposed action of AlCl3 in the dehydration of 1-phenylethane-1,2-diol.

i. Influence of temperature

Temperature is a significant factor in most chemical reactions. In order to see its

influence, eight different temperatures (from 130 to 200°C) were tested on the

dehydration of 1-phenylethane-1,2-diol during 30 minutes in the presence of AlCl3 (20

mol%) in water (Figure 14).

Figure 14: Influence of temperature.

MW, t = 30 min, Catalyst: AlCl3 (20 mol%)

A temperature of 150°C was necessary to have a diol conversion superior to 10%.

Conversion increased with temperature and reached 100% at 180°C. Phenylacetaldehyde

yield increased with temperature to a maximum (55% at 170°C) and then decreased to

0

10

20

30

40

50

60

70

80

90

100

130 140 150 160 170 180 190 200

%

Temperature (°C)

Diol conversion

Aldehyde yield

By-product yield


55

31% at 200°C. If the temperature exceeded 170°C, the aldol condensation product was

formed to the detriment of phenylacetaldehyde. Consequently, the temperature selected

for the rest of the study was 170°C.

ii. Influence of reaction time

Shorter reaction times are one of the many advantages of using microwave

irradiation compared to thermal heating. Microwaves transfer their energy directly to the

reactive species and therefore provide a highly efficient and fast heating of the reaction

medium. Six reaction times, from 10 to 60 minutes, have been compared at 170°C in the

presence of AlCl3 (20 mol%) in water (Figure 15).

Experiments showed that the 1-phenyl-ethane-1,2-diol conversion increased with

reaction time to reach a value of 98% after 60 minutes. Phenylacetaldehyde yield

increased with time to a maximum of 55% after 30 minutes and then decreased if the

reaction was continued. The selectivity towards the aldehyde decreased from 69% at 30

minutes to 49% at 60 minutes. Since the formation of 2,4-diphenylbut-2-enal was

favoured if the reaction time exceeded 30 minutes, this reaction time was selected for

the rest of the study.

Figure 15: Influence of reaction time.

MW, T = 170°C, Catalyst: AlCl3 (20 mol%)

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60

%

Reaction time (min)

Diol conversion

Aldehyde yield

By-product yield


56

iii. Influence of the catalyst nature

The activity of Lewis acids (AlCl3 and FeCl3), mineral Brønsted acids (HCl,

H2SO4), metal sulphates (MnSO4, ZnSO4, FeSO4 and Fe2(SO4)3), salts (NaCl and CaCl2)

and solid acid catalysts (Montmorillonite K10, a clay, and Dowex, 50WX8, an ion-

exchange resin) were compared on the dehydration of 1-phenylethane-1,2-diol. Reactions

were carried out at 170°C for 30 minutes in water. The catalysts quantities were fixed at

20 mol%, except for NaCl and CaCl2 where 1 M solutions were used and solid catalysts

where a mass ratio diol/catalyst of 1 was applied. Results are presented in Table 6.

Entry Catalyst Conversion

(%)

Yield (%)

1 3

1 - 1 1 0

2 AlCl3 80 55 3

3 FeCl3 99 59 6

4 HCl 96 63 4

5 H2SO4 97 62 4

6 MnSO4 1 1 0

7 ZnSO4 3 2 0

8 FeSO4 2 1 0

9 Fe2(SO4)3 54 26 0

10 NaCl a 1 1 0

11 CaCl2 a 4 4 0

12 Dowex 50WX8 b 100 45 13

13 Mont. K10 b 11 8 0

Table 6: Influence of the catalyst nature.

MW, T = 170°C, t = 30 min, Catalyst: 20 mol%. a : Solution 1M, b : Ratio diol/catalyst = 1, wt/wt.

Firstly, in accordance with the literature[62,68,82,85], good phenylacetaldehyde yields

were obtained with Lewis acids and mineral acids (from 55% with AlCl3 to 63% with

HCl). FeCl3 seemed to be a catalyst a little more active than AlCl3, leading to higher

conversion and yields. The two Brønsted acids gave similar results.

In their experiments, Vogel’s group observed a positive effect of metal sulphates,

especially ZnSO4, on the dehydration of biosourced polyols (1,2-propanediol, glycerol,


57

meso-erythritol, etc).[73–75] In our case, the use of metal (II) sulphates led to a very low

conversion and yield, similar to those obtained without catalyst. Only iron (III) sulphate

led to a 26% yield of phenylacetaldehyde for a 54% diol conversion. This difference with

literature can be explained by an important difference in the reaction conditions: Vogel et

coll. carried out their experiments close to the supercritical domain, between 320 and

360°C while our dehydration reaction was performed at 170°C.

Likewise, the use of salts such as NaCl and CaCl2 induced only a limited diol

conversion (< 5%). These results did not follow the observation of Avola et al. For the

same reaction, they observed a diol conversion of 93% and yields of 29% and 64% for the

phenylacetaldehyde and the aldol condensation product respectively, with the use of a 1M

solution of NaCl.[54] The activation method (thermal heating) and thus reaction times

(16h) were different from our conditions and may explain this difference.

Finally, a total conversion was obtained with the ion-exchange resin Dowex

50WX8. Nevertheless, phenylacetaldehyde yield was lower than with AlCl3 (45 instead of

55%) due to the formation of more by-products (13%). On the other hand, the clay

Montmorillonite K10 had a low efficiency on the dehydration of 1-phenylethane-1,2-diol

with limited conversion and yield.

Therefore, it was decided to select the Lewis acids AlCl3 and FeCl3 and the

mineral acids HCl and H2SO4 since they gave the best results.

iv. Conclusion

The optimisation of the phenylacetaldehyde formation in water was done under

microwave irradiation. Best reaction conditions were a temperature of 170°C for a

reaction time of 30 minutes. Mineral acids HCl and H2SO4 gave the highest

phenylacetaldehyde yields (63 and 62% respectively). Nevertheless, Lewis acids AlCl3 and

FeCl3 proved to be a good alternative to these corrosive acids since they led to similar

yields.


58

b. Introduction of a co-solvent

Even if good phenylacetaldehyde yields were obtained in water, its selectivity was

still inferior to 70% because of the formation of the aldol condensation product, 2,4-

diphenylbut-2-enal, and others by-products. To solve this issue, a strategy widely used

for the dehydration of monosaccharides[82,113] was applied: the introduction of a co-

solvent (Figure 16).

Figure 16: Schematic process of a biphasic dehydration of 1-phenylethane-1,2-diol.

In fact, 1-phenylethane-1,2-diol and the catalyst will have a better affinity with the

aqueous phase whereas the partition coefficient of phenylacetaldehyde is in favour of the

organic phase. Thus, in a biphasic system, the dehydration reaction will occur in the

aqueous phase and the phenylacetaldehyde formed will be transferred into the organic

phase and will not be able to react with itself.

i. Choice of the co-solvent

The first step was to choose a co-solvent. The selection criteria was (i) a solvent

not miscible in water; (ii) which can resist the reaction conditions optimised in the first

part of the study (170°C, 30 min of MW); and (iii) was efficient to transfer

phenylacetaldehyde into the organic phase (low condensation product yield). The

volumetric ratio water/co-solvent was first set to 1:1. It had been varied afterwards.

Water

Co-solvent

Extractor

Evaporator

Reaction medium

Aqueous phase

Organic phase


59

Cyclopentyl methyl ether (CPME), ethyl acetate (EtOAc), toluene, methyl-

tetrahydrofuran (Me-THF), tetrahydrofuran (THF), dimethyl carbonate and dioxolane

were tested at 170°C for 30 minutes with AlCl3, FeCl3, HCl and H2SO4 (Table 7).

Entry Catalyst Co-solvent Conversion

(%)

Yield (%)

1 3

1 AlCl3 CPME 64 56 1

2 EtOAc 90 64 22

3 Toluene 80 78 0

4 Me-THF 71 62 9

5 THF 54 47 2

6 FeCl3 CPME 98 86 1

7 EtOAc 93 66 9

8 Toluene 99 88 0

9 Me-THF 88 72 1

10 THF 87 63 5

11 HCl CPME 97 94 1

12 EtOAc 94 79 15

13 Toluene 99 98 0

14 Me-THF 94 83 2

15 THF 91 67 8

16 H2SO4 CPME 98 95 2

17 EtOAc 99 70 25

18 Toluene 99 98 0

19 Me-THF 91 86 1

20 THF 93 70 8

Table 7: Choice of the co-solvent.

MW, T = 170°C, t = 30 min, Catalyst: 20 mol%, Water/Co-solvent, 1:1, v/v

Dimethyl carbonate and dioxolane degraded under the reaction conditions. Thus,

their results were not exploitable and do not appear in Table 7. Ethyl acetate was not an

efficient solvent for the phenylacetaldehyde transfer. Indeed, at the end of the reaction,

only one phase was observed. Thus, condensation product formation was not avoided

and yields between 9 and 25% were obtained. As expected, since THF was partially


60

miscible with water, it did not capture efficiently phenylacetaldehyde which reacted with

itself to form the condensation product with yields up to 8%.

Methyl-tetrahydrofuran, cyclopentyl methyl ether and toluene were efficient co-

solvents since the by-product yields were inferior to 2%, except when Me-THF was

combined with aluminium chloride (9%). Toluene gave the best results. It may be

explained by the presence of the aromatic cycle which may favour the

phenylacetaldehyde transfer into the organic phase. However, toluene is an organic

solvent for which substitution is recommended, because of its environmental impact

notably. Therefore, it would be wiser to opt for Me-THF or CPME which are considered

as green solvents.[10] The use of Me-THF led to lower diol conversion and so lower

phenylacetaldehyde yield than CPME. Thus, we chose CPME as co-solvent for the rest of

the study.

Cyclopentyl methyl ether has many advantages as a solvent. It has (i) a high

hydrophobicity and thus it is very easy to dry, (ii) a suppressed formation of peroxide by-

products, (iii) a low vaporisation energy, (iv) a narrow explosion range and (v) it is

relatively stable under acidic and basic conditions. Its physical properties are

summarised in Table 8. Despite a petro-sourced origin and a high boiling point, CPME is

an eco-friendly solvent considered as an alternative to other ethereal solvents such as

THF, dioxan and 1,2-dimethoxyethane (DME), for a wide range of chemical

reactions.[139,140]

CPME has been successfully used as a co-solvent in the dehydration of pentoses to

furfural in the presence of sulphuric acid. Its use limited the formation of secondary

products, usually formed in water, by transferring furfural into CPME. Xylose conversion

and furfural yield were larger in the presence of CPME: after 60 minutes at 180°C,

furfural yield reached a value close to 60% at 85% xylose conversion, whereas for the

CPME free system, these values were 40 and 75%, respectively.[115]


61

Cyclopentyl methyl ether (CPME)

Density (20 °C) (g/cm3)

Melting point (°C)

Boiling point (°C)

Viscosity (20 °C) (cP)

Surface tension (20 °C) (mN/m)

Vaporization energy (bp) (kcal/kg)

Specific heat (20 °C) (kcal/kg.K)

Refractive index (20 °C)

Dielectric constant (25 °C)

Dipole moment (D)

Azeotropic point with water (°C)

Solubility in water (g/100 g)

Flash point (°C)

Ignition point (°C)

log Pow

Explosion range (vol %)

0.86

<-140

106

0.55

25.17

69.2

0.4346

1.4189

4.76

1.27 (calc)

83a

1.1 (23 °C)

-1

180

1.59

1.1-9.9

a Composition Azeotrope (wt %) = CPME: 83.7, H2O: 16.3

Table 8: Physical properties of CPME. Data from Watanabe et al.[139]

ii. Influence of the water/co-solvent ratio

In their publication, Campos Molina et al. observed variations in conversion and

yield according to the amount of CPME added to the aqueous phase. They varied the

CPME/aqueous phase mass ratio from 0.67 to 2.33 and the best results were obtained for

the ratio 2.33.[115] Thus, it was decided to vary the volumetric ratio water/CPME between

the two phases while maintaining the value of total volume: the proportions 3:1, 2:1, 1:1,

1:2 and 1:3, v/v, were tested. The molar percentage of the four catalysts (AlCl3, FeCl3, HCl

and H2SO4) was set at 20 mol%, the temperature at 170°C and the reaction time at 30

minutes. Results are presented in Table 9.


62

Entry Catalyst Ratio water/co-

solvent, v/v

Conversion

(%)

Yield (%)

1 3

1 AlCl3 3:1 76 63 2

2 2:1 70 66 1

3 1:1 64 56 1

4 1:2 62 59 0

5 1:3 47 45 0

6 FeCl3 3:1 96 81 2

7 2:1 98 81 2

8 1:1 98 86 1

9 1:2 98 85 1

10 1:3 98 85 1

11 HCl 3:1 97 87 2

12 2:1 97 91 1

13 1:1 97 94 1

14 1:2 98 95 1

15 1:3 98 95 1

16 H2SO4 3:1 98 89 3

17 2:1 98 91 2

18 1:1 98 95 2

19 1:2 99 97 1

20 1:3 99 97 1

Table 9: Influence of the ratio water/co-solvent.

MW, T = 170°C, t = 30 min, Catalyst: 20 mol%, Water/CPME, v/v

With FeCl3, HCl or H2SO4, the amount of organic solvent had no influence on the

diol conversion. It was between 96 and 99%. Nevertheless, the more important the

amount of CPME was, the higher the phenylacetaldehyde yield was, reaching 84%, 94%

and 96% with FeCl3, HCl and H2SO4 respectively, with a water/CPME ratio of 1:3 (Table

9, entries 10, 15 and 20).

Among the different experiments, only AlCl3 gave a moderate diol conversion (<

76%) and a moderate phenylacetaldehyde yield (< 66%). Besides, results did not follow

the same trend like for the other catalysts. With AlCl3, the more important the amount of

CPME was, the lower conversion and yield were. Only 45% of aldehyde was obtained with

a water/CPME ratio of 1:3. This decrease may be explained by the possible formation of


63

complexes between the aluminium cation and the ether CPME. In that case, the amount

of free AlCl3 may not be enough to efficiently catalyse the dehydration.

As shown in Table 9, the best results were obtained when the ratio of water/CPME

was 1:2 or 1:3. Moreover, moderate aldehyde yields were obtained when using AlCl3 by

comparison with the other catalysts. Therefore, it was decided that the dehydration of 1-

phenylethanediol-1,2-diol will be done in the presence of FeCl3, HCl or H2SO4 (20 mol%)

in a biphasic system water/CPME, 1:3, v/v at 170°C for 30 minutes under

microwave irradiation.

iii. Variation of the catalyst quantity

Since diol conversions and aldehyde yields were almost total with 20 mol% of

catalyst, we tried to decrease the catalyst quantity. The aim was to see if similar results

could be obtained with a lower quantity of catalysts. Additional experiments were done

with 5 mol% and 10 mol% of catalysts (FeCl3, HCl and H2SO4) at 170°C for 30 minutes in

a biphasic system water/CPME, 1:3, v/v (Table 10).

Entry Catalyst mol% Conversion

(%)

Yield (%)

1 3

1 FeCl3 5 63 55 0

2 10 86 80 0

3 20 98 84 1

4 HCl 5 77 73 0

5 10 90 87 0

6 20 98 94 1

7 H2SO4 5 85 67 0

8 10 95 86 0

9 20 99 96 1

Table 10: Variation of the catalyst quantity.

MW, T = 170°C, t = 30 min, Water/CPME, 1:3, v/v.

The decrease of the catalyst quantity led to a decrease of diol conversion and

aldehyde yield. The latter went from 96 to 86% with H2SO4 and from 94 to 87% with HCl

when the catalyst percentage decreased to 10 mol%. The difference was less important for


64

FeCl3: the yield only decreased from 84 to 80%. Thus, 20 mol% was necessary to obtain

quantitative results.

iv. Variation of the reaction time

Because the optimisation of the reaction time was performed only with AlCl3

(paragraph 3.a.ii.), a new study was reproduced with the three other catalysts (FeCl3, HCl

and H2SO4, 20 mol%) and the reaction conditions optimised so far (170°C, water/CPME,

1:3, v/v). The aim was to reduce the reaction time: four tests were carried out between 5

and 20 minutes for each catalyst (Table 11).

Entry Catalyst Reaction

time (min)

Conversion

(%)

Yield (%)

1 3

1 FeCl3 5 58 43 0

2 10 82 67 0

3 15 92 81 0

4 20 96 84 0

5 30 98 84 1

6 HCl 5 61 60 0

7 10 83 79 0

8 15 94 91 0

9 20 96 93 1

10 30 98 94 1

11 H2SO4 5 70 56 0

12 10 87 77 0

13 15 96 92 0

14 20 98 97 1

15 30 99 96 1

Table 11: Variation of reaction time.

MW, T = 170°C, Catalyst: 20 mol%, Water/CPME , 1:3, v/v.

The decrease of the reaction time from 30 to 20 minutes led to almost no

difference for 1-phenylethane-1,2-diol conversion and phenylacetaldehyde yield for the 3

catalysts (for example, 93% of aldehyde yield instead of 94% with HCl). Afterwards, if we

continue to decrease the reaction time, diol conversion and aldehyde yield began to


65

decrease. Since similar results were obtained, it was decided to choose the shorter

reaction time, 20 minutes, as an optimised value.

v. Conclusion

By comparison with the monophasic method (maximum yield: 63%), the

introduction of a co-solvent allowed us to increase the phenylacetaldehyde yield up to

97% while limiting the formation of the aldol condensation product, 2,4-diphenylbut-2-

enal. The operating conditions optimised under microwave irradiation were:

Temperature: 170°C

Reaction time: 20 minutes

Biphasic medium: water/CPME, 1:3, v/v

Catalysts: FeCl3, HCl, H2SO4

Catalyst quantity: 20 mol%

Best results were obtained with mineral acids (97% of yield with H2SO4 and 93%

with HCl) but FeCl3 seemed to be a good alternative with an 84% yield.

The turnover number (TON), defined as the number of moles of product that a

mole of catalyst can form before becoming inactivated, and the turnover frequency

(TOF), referring to the turnover of the catalyst per unit of time, were calculated for each

catalyst (Table 12).

Entry Catalyst TON TOF (s-1)

1 FeCl3 4.2 3.5 x 10-3

2 HCl 4.6 3.9 x 10-3

3 H2SO4 4.9 4.1 x 10-3

Table 12: Values of TON and TOF for the three optimized catalysts.


66

c. Recyclability of the aqueous phase

In addition to activity and selectivity, one advantage of our system lay in the easy

separation of the aqueous phase, containing the catalyst, and the CPME phase,

containing the product. After a catalytic test, the organic phase could be treated to

recover the aldehyde and the aqueous phase could be reused for a second catalytic cycle

under the same conditions.

For each catalyst, we performed 5 successive reactions with the same catalytic

aqueous phase at 170°C during 20 minutes. The catalytic aqueous phase was recyclable

for up to five cycles even if a progressive decrease in activity was observed for the acid

catalysts FeCl3 and HCl (Figure 17a and 17b). No yield variation was observed with H2SO4

(Figure 17c). Therefore, it seemed that H2SO4 was the best acid catalyst to perform the

dehydration of 1-phenylethane-1,2-diol in water/CPME biphasic system under

microwave irradiation at 170°C for 20 minutes.

Figure 17: Recyclability of the aqueous phase with a) FeCl3, b) HCl and c) H2SO4.

MW, T = 170°C, t = 20 min Catalyst: 20 mol%, Water/CPME , 1:3, v/v.

0

25

50

75

100

1 2 3 4 5

%

Run

By-product yield

Aldehyde yield

a)

0

25

50

75

100

1 2 3 4 5

%

Run

By-product yield

Aldehyde yield

b)

0

50

100

1 2 3 4 5

%

Run

By-product yield

Aldehyde yield

c)


67

d. Application to other alcohols

In order to exemplify our optimised methods, several alcohols of structure similar

to 1-phenylethane-1,2-diol were submitted for dehydration in biphasic and monophasic

media. The dehydration of 1-phenylethanol, 2-phenylethanol, 1-phenyl-propan-1-ol and

hydrobenzoin were compared.

i. 1-phenylethanol and 2-phenylethanol

Dehydration of 1-phenylethanol (4) and 2-phenylethanol (5) led to the formation

of styrene (6) (Scheme 12). The experiments were carried out at 170°C for 20 minutes in

presence of 20 mol% of catalysts (FeCl3, HCl and H2SO4) in a biphasic medium

water/CPME, 1:3, v/v and in water. Results are presented in Table 13 and 14.

Scheme 12: Formation of styrene from 1-phenylethanol and 2-phenylethanol.

Entry Alcohol Catalyst Solvent Conversion

(%)

Styrene

yield (%)

1 1-phenylethanol FeCl3 Water/CPMEa 79 54

2 HCl 74 74

3 H2SO4 70 70

4 FeCl3 Water 99 60

5 HCl 99 45

6 H2SO4 98 40

Table 13: Dehydration of 1-phenylethanol.

MW, T = 170°C, t = 20 min, Catalyseur : 20 mol%. a : ratio Water/CPME = 1:3, v/v


68

The conversion of 1-phenylethanol was higher in water (98% with H2SO4 for

example) than in biphasic medium (70%). However, selectivity towards styrene was

much higher in biphasic medium than in water. With mineral acids for example, no

secondary products were observed in biphasic medium (selectivity of 100%) whereas

styrene selectivity in water did not exceed 45%. Styrene oligomers may be formed during

the reaction, thus decreasing the yield. Therefore, once again, the presence of a co-

solvent proved to be necessary to avoid the formation of by-products.

Entry Alcohol Catalyst Solvent Conversion

(%)

Styrene

yield (%)

1 2-phenylethanol FeCl3 Water/CPMEa 9 0

2 HCl

7 0

3 H2SO4 9 0

4 FeCl3 Water 13 0

5 HCl

12 0

6 H2SO4

13 0

Table 14: Dehydration of 2-phenylethanol.

MW, T = 170°C, t = 20 min, Catalyseur : 20 mol%. a : ratio Water/CPME = 1:3, v/v

By comparison, the dehydration of 2-phenylethanol was very limited under the

reaction conditions, both in monophasic and biphasic media. A conversion inferior to

13% and a styrene yield close to zero were obtained.

The difference of reactivity between the two alcohols can be explained by the

stability of the carbocation formed during the dehydration. The secondary carbocation

obtained from 1-phenylethanol was stabilised by the phenyl group and its formation was

favoured, contrary to the primary one obtained from 2-phenylethanol.

ii. 1-phenylpropan-1-ol

Secondly, the dehydration of 1-phenylpropan-1-ol (7) in order to form 1-

phenylpropene (8 and 8’, Scheme 13) was studied. Like previously, the experiments were

carried out at 170°C for 20 minutes in presence of 20 mol% of catalysts (FeCl3, HCl and

H2SO4) in a biphasic medium water/CPME, 1:3, v/v and in water. Results are presented

in Table 15.


69

Scheme 13: Formation of 1-phenylpropene from 1-phenylpropan-1-ol.

Like it was observed for 1-phenylethanol, the dehydration of 1-phenylpropan-1-ol

led to higher conversion in water than in biphasic medium (95% in water instead of 74%

in water/CPME with FeCl3 for example). However, higher 1-phenylpropene yields were

obtained in biphasic medium than in water. No secondary products were observed in

biphasic medium (selectivity of 100%) whereas 1-phenylpropene selectivity did not

exceed 51% in water. 1-Phenylpropene oligomers may be formed during the reaction in

water, thus decreasing the yield. An NMR analysis showed that the main product was

(E)-1-phenylpropene (94% (E), 6% (Z)). Therefore, the experiments showed that the

presence of a co-solvent was necessary to avoid the formation of by-products.

Entry Catalyst Solvent Conversion

(%)

1-Phenyl-

propene (%)

1 FeCl3 Water/CPMEa

74 73

2 HCl 71 71

3 H2SO4 69 69

4 FeCl3 Water 95 33

5 HCl 93 45

6 H2SO4 92 47

Table 15: Dehydration of 1-phenylpropan-1-ol.

MW, T = 170°C, t = 20 min, Catalyst: 20 mol%. a : ratio Water/CPME = 1:3, v/v

iii. Hydrobenzoin

Finally, the reaction of hydrobenzoin (9) in water was studied: it led to the

formation of deoxybenzoin (10) by dehydration and diphenylacetaldehyde (11) by

pinacol rearrangement (Scheme 14). Like for the other alcohols, the experiments were

carried out at 170°C for 20 minutes in the presence of 20 mol% of catalysts (FeCl3, HCl


70

and H2SO4) in a biphasic medium water/CPME, 1:3, v/v and in water. Results are

presented in Table 16.

Scheme 14: Dehydration and pinacol rearrangement of hydrobenzoin.

The main reaction was the pinacol rearrangement since we observed mostly the

formation of diphenylacetaldehyde. The deoxybenzoin yield did not exceed 8% regardless

of the reaction conditions.

In biphasic medium, hydrobenzoin conversion and diphenylacetaldehyde yield

were moderate. For example, with H2SO4, 62% of hydrobenzoin were converted and 56%

of diphenylacetaldehyde were obtained. In water, hydrobenzoin conversion was

quantitative and the best diphenylacetaldehyde yield was 79%. However, it should be

noticed that selectivity towards diphenylacetaldehyde was higher when using a co-

solvent. Nevertheless, the introduction of CPME was not sufficient to promote only the

pinacol rearrangement since a small amount of deoxybenzoin was still formed.

Entry Catalyst Solvent Conversion

(%)

Yield (%)

Deoxybenzoin Diphenyl-

acetaldehyde

1 FeCl3 Water/CPMEa 53 3 37

2 HCl 57 5 50

3 H2SO4 62 5 56

4 FeCl3 Water 99 6 67

5 HCl 99 8 76

6 H2SO4 100 8 79

Table 16: Dehydration and pinacol rearrangement of hydrobenzoin.

MW, T = 170°C, t = 20 min, Catalyst: 20 mol%. a : ratio Water/CPME = 1:3, v/v


71

e. Formation of phenylacetaldehyde from styrene oxide

Lastly, the formation of phenylacetaldehyde directly from styrene oxide was tried.

Styrene oxide (12) was hydrolysed to form 1-phenylethane-1,2-diol (2) which then

underwent a dehydration to form phenylacetaldehyde (1) (Scheme 15).

Scheme 15: Formation of phenylacetaldehyde from styrene oxide.

The optimised methods were applied to styrene oxide: reactions were carried out

under microwave irradiation at 170°C for 20 minutes in presence of 20 mol% of catalysts

(FeCl3, HCl and H2SO4) in a biphasic medium water/CPME, 1:3, v/v. Results are

presented in Table 17, with the results from 1-phenylethane-1,2-diol for comparison.

Entry Substrate Catalyst Yield (%)

1 3

1 1-Phenylethane-1,2-diol FeCl3 84 0

2 Styrene oxide 86 0

3 1-Phenylethane-1,2-diol HCl 93 1


5 1-Phenylethane-1,2-diol H2SO4 97 0


Table 17: Synthesis of phenylacetaldehyde from styrene oxide.

MW, T = 170°C, t = 20 min, Catalyst: 20 mol%, Water/CPME, 1:3, v/v.

Application of the optimised dehydration methods to styrene oxide produced the

target aldehyde in yields comparable to those obtained from 1-phenylethane-1,2-diol: 86,

95 and 94% of phenylacetaldehyde were produced with FeCl3, HCl and H2SO4

respectively. Thus, additional experiments were performed at 170°C for 5, 10 and 15


72

minutes with H2SO4 (Figure 18). The hydrolysis of styrene oxide to 1-phenylethane-1,2-

diol had a fast reaction rate since all styrene oxide was converted after 5 minutes.

Figure 18: Influence of reaction time on the synthesis of phenylacetaldehyde from styrene oxide.

MW, T = 170°C, Catalyst: H2SO4 (20 mol%), Water/CPME, 1:3, v/v.

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20

%

Reaction time (min)

Styrene oxide conversion

Diol yield

Aldehyde yield

By-product yield


73

4. Conclusion

Phenylacetaldehyde (1) is a molecule used in perfume compositions, for the

preparation of pharmaceuticals, insecticides, etc, or as a useful intermediate in many

organic syntheses. Its synthesis by dehydration of 1-phenylethane-1,2-diol (2) was

studied under thermal heating and under microwave irradiation. The formation of

phenylacetaldehyde can be followed by an aldol condensation leading to the formation of

an unwanted product, 2,4-diphenylbut-2-enal (3). The aim of our work was to produce

phenylacetaldehyde in a good yield while limiting the formation of the condensation

product.

According to our experiments, using an autoclave as reactor was not the best option

to produce phenylacetaldehyde in good yield and selectivity. The slow heating rate of the

reaction medium induced by the thermal inertia of the reactor and the long reaction

times allowed the phenylacetaldehyde to react on itself to form the by-product. Low

phenylacetaldehyde yields and selectivities were obtained. For example, only 9% of

aldehyde was obtained at 200°C after 16h of reaction whereas the yield of condensation

product reached 42%. Because of the reactor alloy, the use of catalysts was very limited in

order to avoid its corrosion.

Therefore, it appeared to us that the use of microwave irradiation could be an

interesting alternative to thermal heating. Among the many advantages of using

microwave irradiation, the fast heating rate and the higher yields and selectivities usually

obtained could help us to improve phenylacetaldehyde yield while limiting the formation

of the by-product.

Firstly, the optimisation of the phenylacetaldehyde formation under microwave

irradiation was done in water. Temperature, reaction time and nature of the catalyst were

varied. Best reaction conditions were found to be a temperature of 170°C for a reaction

time of 30 minutes. Mineral acids HCl and H2SO4 gave the highest phenylacetaldehyde

yields (63 and 62% respectively). Nevertheless, Lewis acids AlCl3 and FeCl3 proved to be

a good alternative to these corrosive acids since they led to similar yields.

However, in water, the selectivity towards phenylacetaldehyde was still inferior to

70% because of the formation of the aldol condensation product. To solve this issue, it

was decided to introduce a co-solvent, a strategy widely used for the dehydration of


74

monosaccharides. Since 1-phenylethane-1,2-diol and the catalyst had a better affinity

with water, the dehydration reaction occurred in the aqueous phase. Then, the

phenylacetaldehyde formed was transferred into the organic phase and was not able to

react with itself into further reactions. First, the co-solvent, cyclopentyl methyl ether

(CPME), was selected and then, the proportions between the aqueous and the organic

phase were varied. The introduction of CPME had proven to be effective. Indeed, by

comparison with the monophasic method, it allowed us to increase the

phenylacetaldehyde yield from 63% up to 97% while limiting the formation of the aldol

condensation product, 2,4-diphenylbut-2-enal, to 1% maximum. The operating

conditions optimised under microwave irradiation were:

Temperature: 170°C



Catalysts: FeCl3, HCl, H2SO4

Catalyst quantity: 20 mol%

Best results were obtained with mineral acids (97% of yield with H2SO4 and 93%

with HCl) but FeCl3 seemed to be a good alternative with a 84% yield.

Moreover, in addition to a higher selectivity, one advantage of our system lay in

the easy separation of the two phases. After the reaction, the organic phase could be

treated to recover the aldehyde and the aqueous phase, containing the catalyst, could be

reused for a second cycle under the same conditions. The reusability of the catalytic

aqueous phase was confirmed by performing 5 successive reactions. It was recyclable for

up to five cycles even if a progressive decrease in activity was observed for the acid

catalysts FeCl3 and HCl. No yield variation was observed with H2SO4.

In order to exemplify our optimised methods, several alcohols of structure similar

to 1-phenylethane-1,2-diol were submitted to dehydration in biphasic and monophasic

media. The dehydration of 1-phenylethanol and 2-phenylethanol to styrene, 1-

phenylpropan-1-ol to 1-phenylpropene and hydrobenzoin to diphenylacetaldehyde

(pinacol rearrangement was favoured) were compared. Except 2-phenylethanol which

was not reactive, all alcohols tested gave the expected products in moderate to good

yields. In general, alcohol conversion was higher in sole water but the yield and


75

selectivity of the target products were higher in biphasic medium. Therefore, as for 1-

phenylethane-1,2-diol, the presence of a co-solvent proved to be necessary to avoid the

formation of by-products.

Finally, application of the optimised dehydration methods to styrene oxide as

alternative substrate produced the target aldehyde in yields comparable to those obtained

from 1-phenylethane-1,2-diol: 86, 95 and 94% of phenylacetaldehyde were produced with

FeCl3, HCl and H2SO4 respectively.

This study led to the publication of two articles in a peer-reviewed journals.[141,142]


76

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CHAPTER III

Dehydration of D-xylose in water

Chapter III: Dehydration of D-xylose in water

81

1. Introduction

The energy needs of the developed world are currently over-dependent on the

utilisation of mineral resources. Although renewable-power technologies, such as wind

and photovoltaics, may in the future have major roles in the production of electricity, the

supply of industrial chemicals currently produced come predominately from oil. In fact,

98% of the 170 chemical compounds produced annually in the USA in volumes exceeding

4.5 x 106 kg are derived from oil and natural gas.[143] Therefore, there is an urgent need to

find alternative feedstocks for the chemical industry.

Biomass and especially lignocellulosic biomass are promising alternatives to fossil

fuels for the production of chemicals. The formation of furfural (13) from C5 sugars

derived from hemicellulose, such as D-xylose (14), is one of the successful examples of

biomass valorisation. The importance and potential of furfural as a renewable chemical is

significant since there is currently no synthetic route for the direct production of this

molecule.

In this introduction, the structure of lignocellulosic biomass and in particular the

diversity of hemicelluloses will be presented. Then, the properties and applications of the

target molecule of the Chapter, furfural (13), will be described, as well as the current

industrial processes leading to its formation. The different mechanisms of its formation

from D-xylose (14) will be discussed and finally, a brief literature review will be

presented.

a. From Biomass...

Biomass is a term for all organic material that comes from plants. It includes

waste materials, forest products, energy crops (starch crops, sugar crops, grasses,

vegetable oils) and aquatic biomass. The variety of feedstocks can also be divided into

three groups according to their chemical composition: lignocellulosic biomass, starch-

and sugar-derived biomass and triglyceride-based biomass. During this PhD work, we

will focus only on lignocellulosic biomass.


82

i. Structure of lignocellulosic biomass

Lignocellulosic biomass mainly consists of three biopolymers which are cellulose,

hemicellulose and lignin. Additionally, low amounts of vegetal oils, proteins, extractable

components and ashes can also be found. Lignocellulosic biomass can be differentiated

into three main families: woody plants, herbaceous plants and waste biomass.[144] The

relative content of each biopolymer can vary significantly depending on the biomass

origin, as shown in Table 18.

Biomass Cellulose (%) Hemicellulose (%) Lignin (%)

Softwood 35-40 25-30 27-30

Hardwood 45-50 20-25 20-25

Wheat straw 33-40 20-25 15-20

Switchgrass 30-50 10-40 5-20

Table 18: Cellulose, hemicelluloses and lignin content of selected biomass (wt%).

Data from P. McKendry.[144]

A schematic structure of lignocellulosic biomass is illustrated in Figure 19.

Figure 19: Schematic structure of lignocellulose. The hexagons represent the lignin subunits p-coumaryl alcohol (H), coniferyl alcohol (G) and sinapyl alcohol (S). From F. Streffer.[145]


83

The cellulose fibers are organised in bundles which are stabilised by hydrogen

bonds. These bundles, called microfibrils, are embedded in a hemicellulosic matrix and

covered by lignin. They have diameters in the range of 10 to 20 nm. These microfibrils

are so tightly packed that neither enzymes nor small molecules, like water, can enter their

complex framework. The microfibrils are usually associated to form macrofibrils which

compose the cell wall.[145]

Cellulose:

Cellulose is the most abundant biomaterial in nature, composing 30 to 50% of

vegetal species. It is a homopolysaccharide composed of D-glucose units linked to each

other via β-1,4-glucosidic bonds (Figure 20). Cellulose is a semi-crystalline polymeric

material containing both crystalline and amorphous regions. The degree of

polymerisation depends on the type of raw material and varies between 100 and 20.000.

The presence of inter- and intra-molecular hydrogen bonds provides a high stability to

the crystalline part of cellulose. Therefore, cellulose is a compound which is very stable

with low reactivity and insoluble in many conventional solvents.[146]

Figure 20: Cellulose structure.

Hemicelluloses:

Hemicelluloses are the second main biopolymer found in nature after cellulose.

Hemicelluloses are hetero-polysaccharides composed of different sugar units such as D-

xylose, D-mannose, D-glucose, D-galactose, and L-arabinose. The acidified form of sugars,

for instance glucuronic acid and galacturonic acid, can also be present. Hemicelluloses

composition varies depending on the vegetal from which they come from even if, in most

cases, D-xylose is present in the largest amount. They can be classified in four main

groups which are xylans, mannans, xyloglucans and β-glucans.[146] Their structure and

composition will be presented in detail in the following paragraph.


84

Lignin:

Lignin is a three-dimensional complex amorphous polymer consisting of

methoxylated phenylpropane structures. It is the dominant aromatic polymer present in

nature. Lignin is composed of the three major phenolic components p-coumaryl alcohol,

coniferyl alcohol and sinapyl alcohol (Figure 21), randomly connected through carbon-

carbon and carbon-oxygen bonds. The structure of lignin is complex and changes greatly

according to biomass source.[147]

Figure 21: The three building blocks of lignin.

Lignin is an essential compound of plant cell walls because it confers strength and

rigidity to the system. It fills the spaces between cellulose and hemicellulose, and acts like

a resin that holds the lignocellulose matrix together. Its cross-linking with the

carbohydrate polymers is mainly responsible of the difficult separation and isolation of

the different lignocellulosic compounds.

ii. Structural diversity of hemicelluloses

Hemicelluloses, the second main component of biomass, are hetero-

polysaccharides composed of different sugar units. Their structure and composition

depend on the vegetal that they come from. In 2005, Ebringerova et al. classified

hemicelluloses in four main groups: (i) xylans, (ii) mannans, (iii) xyloglucans, and (iv) β-

glucans.[148]


85

Xylans:

Xylan-type polysaccharides are the main hemicellulose components of secondary

cell walls constituting about 20–30% of the biomass of hardwoods and herbaceous plants

and up to 50% in some tissues of grasses and cereals. Xylans thus represent a renewable

resource available in large amounts in forestry, agriculture, and in wood, pulp and paper

industries. Xylans are heteropolymers possessing a backbone composed of xylose units

linked by β-glycosydic bonds. They do not possess a regular structure based on repeating

units and their linear chain can be branched by short carbohydrate chains.[148]

Several types of xylans can be differentiated according to the nature of their

substituents:

- Homoxylans which are not substituted (Figure 22a).

- Glucuronoxylans which consist of a linear chain of xylose units branched with

glucuronic acid and its O-methylated derivative.

- Arabinoxylans which are grafted with arabinose substituents.

- (Arabino)glucuronoxylan and (glucurono)arabinoxylan which can be ramified by

the two previous types (Figure 22b).

Figure 22: Examples of (a) homoxylans and (b) (arabino)glucuronoxylan.

Mannans:

Within the mannan-type polysaccharides, two main types can be differentiated: (i)

mannans and (ii) glucomannans. Whereas the backbone of the first type is made up

exclusively of linear chains of β-1,4 linked mannose units, the second type has both

mannose and glucose units linked with β-1,4 bonds. The main chain of these structures

a)

b)


86

can also be substituted by galactose units in different proportions. The resulting

polymers are named galactomannans (Figure 23) and galactoglucomannans. Low-

branched galactomannans are abundant in the cell walls of storage tissues of seeds, such

as guar or tara gum for example. Glucomannans and galactoglucomannans are the major

hemicellulosic components of the secondary cell walls of softwoods.[148]

Figure 23: Primary structure of galactomannan.

Xyloglucans:

Xyloglucans can be found in all higher plants, where they represent a

quantitatively major building material of the primary cell wall. They have a backbone

similar to cellulose, with a main linear chain composed of glucose units linked by β-1,4

bonds, decorated with xylose units. There is some regularity in the distribution of the

xylose side chains, which enables the identification of two major types of xyloglucans in

the plant cells walls, namely XXXG and XXGG (Figure 24). The XXXG type consists of

the repetition of three consecutive glucose units substituted by xylose (X), separated by

an un-substituted glucose (G). In the XXGG type, two un-substituted glucose units

separate two xylosylated glucoses. In both cases, xylose units can also be ramified. The

variety of side chains decorating xyloglucans and their distribution on the backbone are

responsible for the large structural diversity of this polysaccharide. Xyloglucans create

strong bonds with cellulose in the cell walls which negatively affects its extractability.[148]

Figure 24: Structures of the repeating units of the two main types of xyloglucans.


87

β-Glucans:

β-Glucans are hemicellulose components of cereal grains (oat or barley for

example). They are the principal molecules associated with cellulose microfibrils during

cell growth. β-Glucans are unbranched homopolymers of glucose units linked by β-1,4

bonds (≈70%) and β-1,3 bonds (≈30%). Blocks of β-1,4-linkage sequences (cellotriosyl

and cellotetraosyl cellulose-like segments) are separated by single β-1,3-linkages (Figure

25).

Figure 25: Example of β-glucan structure with β-1,4 and β-1,3 bonds.

b. ... to Furfural

i. Properties of furfural

Furfural (2-furaldehyde, 13) is a heterocyclic aldehyde with an almond aromatic

odour. Some of its physical properties are presented in Table 19. It occurs naturally in

many fruits and in tea, coffee, cocoa, etc. Furfural is harmful in contact with skin, toxic by

inhalation or swallowing and irritating to eyes and the respiratory system.

CAS number 98-01-1

Molar mass M = 96.08 g/mol

Density d = 1.1598 g/cm3 (20°C)

Melting point mp = - 37°C

Boiling point bp = 162°C

Refractive index nD20 = 1.526

Table 19: Structure and physical properties of furfural.

β-1,4 bond

β-1,3 bond


88

Furfural is produced from the acid-catalysed dehydration of hemicellulose-rich

biomass in a two-step mechanism. Firstly, the biopolymer is hydrolysed into sugars units,

mainly D-xylose (14) units, which are subsequently dehydrated to form furfural (Scheme

16).

Scheme 16: Formation of furfural from hemicelluloses.

ii. Uses and applications of furfural

Furfural was identified in 2004 as one of the top 30 high-value bio-based

chemicals by the U.S. Department of Energy.[149] It is considered as an excellent and

selective solvent for the refining of lubricating oils and diesel fuels. Furfural can also

serve as a precursor for production of liquid alkanes or as a feedstock to make both

gasoline, diesel or jet fuel.

Furfural can undergo reactions typical for aldehydes like acetalisation, acylation,

aldol and Knoevenagel condensations, reduction to aldohols, reductive amination to

amines, decarbonylation, oxidation to carboxylic acids, and Grignard reactions. Besides,

the furan ring can be subjected to alkylation, hydrogenation, oxidation, halogenations,

and nitration reactions. Due to the electron-withdrawing effect of the carbonyl group, the

furan ring is less susceptible to hydrolytic ring cleavage and Diels-Alder cycloaddition

reactions.

Therefore, furfural is considered to be an attractive platform chemical (Scheme 17)

for the production of a myriad of product types, e.g. solvents (tetrahydrofuran), plastics

(in particular, polyamides), resins via furfuryl alcohol and fuel additives (methyl-

tetrahydrofuran or methylfuran). Of the world production of furfural, 60-70% is

converted to furfuryl alcohol.[150,151]


89

Scheme 17: Examples of chemicals derived from furfural. Adapted from Hoydonckx et al.[150]


90

iii. Industrial processes for furfural preparation

Furfural formation was discovered by Döbereiner in 1821, but it only reached the

stage of industrial production after about one century: in 1921, Quaker Oats started in the

USA furfural industrial production based on the agricultural food residue oat husk.

Currently, about 300–700 Ktons of furfural are produced worldwide annually, the

majority coming from China (> 70%). The other main producers, the Dominican

Republic and South Africa, are significantly smaller in tonnage.[152]

Industrial furfural production largely relied on the batch dehydration of pentosan-

rich biomass using sulphuric acid, with yields typically around 50%, such as the Quaker

Oats process. The product recovery was conventionally accomplished via steam stripping

associated to high energy consumption (in the range of 25–35 tons per ton of furfural

produced). Currently, most of furfural is produced by continuous processes, such as the

Huaxia/Westpro technology used in China. Another promising technology for improving

the energy efficiency and yield of furfural production is the Supra Yield process, which is

utilised for furfural production in Australia.[93,152–154] Some furfural production processes

are briefly described in Table 20.

Company/process Process type

Operating

temperature

(°C)

Catalyst Substrate

Furfural

yield

(% theo.)

Ref.

Quaker Oats Batch/aqueous 153 H2SO4 Oat hulls < 50 [155]

Quaker Oats Continuous/aqueous n/a H2SO4 Bagasse 55 [150]

Huaxia/Westpro Continuous/aqueous 160-165 H2SO4 Corn cobs 35-50 [156]

Vedernikovs Continuous/aqueous 188 H2SO4 Wood chips 75 [157]

Zeitsch/Suprayield® Continuous/aqueous 240 H2SO4 n/a 50-70 [158]

Biofine Continuous/aqueous 190-200 H2SO4 Paper sludge 70 [159]

Table 20: Examples of furfural production processes.


91

c. Mechanistic aspects

The understanding of the mechanism of furfural formation from D-xylose and the

limitation of side and loss reactions are crucial keys to optimise furfural production. The

different mechanisms proposed in the literature and the formation of by-products will be

discussed in this part.

i. Mechanisms of furfural formation from D-xylose

First, it has to be noted that pentoses have two enantiomers (D- and L-) which

exist in pyranose, furanose and linear form. The two cyclic forms also have two tautomers

(α- and β-). These different possible conformations of D-xylose are presented in Figure

26. According to Cui et al., and in the case of xylose in solution, the pyranose form would

be the predominant one, even if all the conformations coexist.[160]

Figure 26: Isomers, tautomers of D-xylose and their distribution in aqueous solution.[160]

β

β

α

α


92

The conversion of xylose to furfural is a difficult reaction, very dependent on the

reaction conditions. Several mechanistic studies have been done among the years but the

mechanism of the formation of furfural from xylose has not been established definitively

yet. Several conformations of the sugar have been studied as different starting points

possible for the reaction, mainly catalysed by acids.

In 1969, Garett was the first to propose a reaction mechanism from the pyranose

form of xylose, which opened during the first step. The mechanism described proposed

two acyclic intermediates which led to the formation of furfural.[161] Later studies also

proposed a mechanism involving the linear form of xylose. Thus, according to O’Neill et

al., the molecule of xylose would open thanks to a proton brought by an acid catalyst, and

then dehydrate to lead to furfural. Then, furfural could rehydrate into an organic acid or

suffer an oligomerisation, leading to many by-products.[162]

Marcotullio et al. have reported studies in which xylose would undergo an

enolisation, the determining step, followed by three dehydrations, each initiated by a

protonation (Scheme 18).[70]

Scheme 18: Xylose dehydration mechanism via enolisation, proposed by Marcotullio et al.[70]

A study published in 2014 by Rasmussen et al. suggested two mechanisms, one

involving the cyclic form of xylose and the other the acyclic form, both starting from the

pyranose form (Scheme 19). The key step here would be the protonation of the hydroxyl

group in position 2 of the xylopyranose, leading to a cyclic mechanism, in competition


93

with the protonation of the oxygen of the pyranose cycle leading to a reaction scheme

involving the linear form.[163]

Scheme 19: Xylose dehydration mechanism involving both cyclic and acyclic pathways,

adapted from Rasmussen et al.[163]

Other works based on quantum chemical calculations and molecular simulations

have been realised. Thus, Nimlos et al. have studied several reaction schemes possible

integrating both linear and cyclic forms of xylose. They proposed the mechanism

represented in Scheme 20, based on the structures and energies of the transition states of

the different chemical intermediates and products. Xylose would initially be under its

pyranose form and would undergo a protonation followed by the loss of a water molecule.

Then, the oxygen of the cycle would attack the carbocation thus formed and two

additional water molecules would be lost, thus leading to the formation of furfural.[164]

Antal et al. and Binder et al. also supposed that the pyranose form is the starting

point of the mechanism whereas Ahmad et al. proposed the xylofuranose as initial

molecule.[165–167]


94

Scheme 20: Mechanism of xylose dehydration into furfural proposed by Nimlos et al.[164]

However, most of the mechanisms proposed involved reaction intermediates

which have not been clearly identified or detected experimentally. Recent studies

introduced an isomer of xylose, xylulose, as the key element of the reaction. Xylose would

isomerise to xylulose which would dehydrate to furfural, by analogy to the dehydration

reaction of glucose to 5-HMF which goes through the formation of its isomer fructose,

more reactive. In that case, the presence of Lewis acids sites to isomerise xylose into

xylulose and Brønsted sites to dehydrate xylulose proved to be necessary.[107,168,169]

Nevertheless, in 2015, thanks to their experiments, Ershova et al. rejected the role of

xylulose as a key intermediate.[170] Kinetic studies and modelling helped them to establish

the role of xylulose along a parallel reaction pathway. For them, the primary reaction

route involved another short-living intermediate which reacted rapidly to the furfural.

ii. Side and loss reactions

Several side reactions can occur during the formation of furfural (Scheme 22). In a

recent publication, Enslow and Bell studied the mechanism of formation of the different

by-products in the presence of Brønsted acids.[171]

At early reaction times, they observed that xylose was consumed at a rate that

exceeded the rate of furfural formation. Thus, it suggested that a part of the xylose was

lost due to self-condensation or reaction with anhydro-xylose intermediates.

When the reaction was continued, furfural yield reached a maximum and then,

usually declined because it underwent secondary reactions. Furfural can react with itself

in a reaction called resinification, a process reported to occur readily at temperatures

below 200°C.[158] It can also undergo condensation reactions with pentose intermediates

via dioxolane-like bridging structures. These furfural oligomers represent the main part

of the solid product formed during reaction, humins. However, direct condensation of

furfural with xylose is unlikely to happen. In parallel, low molecular weight products like

formaldehyde, formic acid, acetaldehyde, crotonaldehyde, lactic acid, dihydroxyacetone,

glyceraldehyde, pyru-valdehyde, acetol, or glycolaldehyde can be formed by the

fragmentation of pentoses or furfural.


95

Scheme 21: Examples of side reactions occurring during the formation of furfural.[93,171]

d. Dehydration of D-xylose to furfural: a brief review of the literature

The different types of catalysts used for the dehydration of xylose to furfural have

already been described in Chapter I. Among the homogeneous catalysts, mineral acids,

organic acids and metal chlorides can be used. The heterogeneous catalysts include

zeolites, ion-exchange resins or mixed oxides for instance. In this part, we chose to

classify the reactions by type of reactor: reactions performed (i) in batch reactor under

conventional heating since it is the type mainly used in industrial processes; (ii) in

microwave reactors as alternative to conventional heating; and finally, (iii) in continuous

flow reactors. This literature review was focused on reactions performed in water and in

water/solvent mixtures (monophasic or biphasic systems).


96

i. Reactions in batch reactor under conventional heating

Classically, since the Quaker Oats process in 1921, sulphuric acid was the acid

most used in industrial processes, leading to a furfural yield between 45 and 50%. Other

studies confirmed its efficiency with good yields obtained (until 83%) in combination

with NaCl and/or the addition of an extracting solvent.[115,172] However, sulphuric acid

may sulfonate the furanic cycle and lead to side products which can decrease furfural

selectivity. Thus, researchers showed interest in other acids, both mineral, like

hydrochloric acid[81,173] or methanesulfonic acid (MSA)[174], and organic like formic[71,175]

or acetic acids.[176] Organic acids possess the advantage to be less corrosive and less toxic

than mineral ones. Results are widely dependent on the experimental conditions

(temperature, reaction time and acid concentration) and furfural yield can reach values

up to 78%. Usually, high temperatures (> 170°C) are associated with short reaction times

(< 60 min) and moderate temperatures allow longer reaction times. Besides, acids are

often associated to an extracting solvent which increases furfural selectivity, and/or to

inorganic salts. Table 21 sums up different results obtained with the use of mineral and

organic acids.

Substrate Solvent Catalyst T(°C) t (min) X (%) Y (%) Ref

Xylose Water H2SO4 160 60 n/a 66 [174]

Xylose Water Formic acid 180 390 95 74 [71]

Xylose Water Formic acid 200 20 90 65 [175]

Xylose Water MSA 180 15 n/a 65 [174]

Xylose Water HCl 145 300 75 29 [81]

Xylose Water/Toluene H2SO4/NaCl n/a 300 n/a 83 [172]

Xylose Water/CPME H2SO4/NaCl 160 20 75 72 [115]

Xylose Water/sec-butylphenol HCl 170 20 98 78 [173]

Xylose Water/Toluene HCl/CrCl3 140 120 96 76 [81]

X= Conversion, Y=Yield

Table 21: Examples of studies on the dehydration of xylose using mineral and organic acids.

At first, metal chlorides were not considered as catalysts by themselves, but only

as promoters associated to classic mineral acids like HCl.[81,172] Nevertheless, as it was

described on Chapter I, metal chlorides such as AlCl3, FeCl3 or SnCl4 proved to be

efficient catalysts for the formation of furfural from xylose, but also from xylan or from


97

raw biomass. For example, a good yield of 77% was obtained from xylose by using FeCl3

combined with NaCl in a biphasic medium water/MeTHF.[82] For Marcotullio et al., the

efficiency of the metal chlorides can be mainly explained by the release of Cl- ions in the

medium which can promote the 1,2-enediol mechanism.[69] Gravitis et al. reported that

the efficacy of the metal cations to catalyse the dehydration of carbohydrates to furans

was proportional to their ionisation potential.[177] Table 22 summarises results obtained

with metal chlorides.


Xylose Acidified water-THF FeCl3 170 20 100 60 [178]

Xylose Acidified water-THF AlCl3 170 10 100 65 [178]

Xylose Water/Butanol SnCl4 140 300 100 77 [87]

Xylose Water/MeTHF FeCl3/NaCl 140 120 n/a 71 [82]

Xylose Water-DMSO SnCl4/LiCl 130 360 n/a 63 [179]

Xylan Water SnCl4 150 120 84 49 [86]

Xylan Water/MeTHF SnCl4 150 120 92 78 [86]

Maple wood Acidified water-THF FeCl3 170 60 n/a 95 [178]

Corn stover Acidified water-THF FeCl3 170 80 n/a 95 [178]

Corncob Water/MeTHF SnCl4 150 120 98 69 [86]

Bagass Water/MeTHF SnCl4 150 120 98 67 [86]


Table 22: Examples of studies on the dehydration of xylose and hemicelluloses using metal chlorides.

In the last 15 years, heterogeneous catalysts were more and more studied for the

formation of furfural from xylose. Compared to homogeneous catalysis, heterogeneous

catalysis has many advantages: the generation of waste is limited, the separation and

recovery of the catalyst is simplified and the elimination of the mineral acids makes the

reaction mixture less corrosive.

The first research dealing with solid acids for the dehydration of xylose to furfural

was the work of Moreau in 1998[180], then followed by many studies on a wide range of

solid catalysts. Reviews published in 2011 by Karinen et al.[93] and in 2014 by

Agirrezabal-Telleria et al.[181] present the majority of the studies carried out for the

production of furfural and 5-HMF by heterogeneous catalysis. Some examples are


98

presented in Table 23. In general, furfural yields were moderate in water (up to 55%),

since the main drawback of many heterogeneous catalysts was their low stability in water

medium. Nevertheless, furfural extraction by a co-solvent or by a gas stream allowed the

yield to increase up to 80%. However, for several heterogeneous catalysts, a long reaction

time (from 4h to 48h) was necessary to achieve good yields.

Substrate Solvent Catalyst T(°C) t X (%) Y (%) Ref

Xylose Water Sulfonated graphene

oxide

200 35 min 83 62 [103]

Xylose Water MSHS-SO3H 190 1h 64 43 [95]

Xylose Water ZrP-HT-Am-C 170 2h 96 52 [109]

Xylose Water HY 180 6h 82 38 [182]

Xylose 20% Acetic acid

solution

HY 180 6h 97 55 [182]

Xylose Water + N2

stripping

Amberlyst 70 175 20 min 99 70 [106]

Xylose Water/Butanol MCM-41 170 3h 97 44 [183]

Xylose Water/CPME Nb2O5 130 6h 100 58 [184]

Xylose Water/Toluene SO42-/SnO2 100 48h 56 31 [185]

Xylose Water/Toluene H-MCM-22 170 48h 98 71 [96]

Xylose Water/Toluene Arenesulfonic SBA-15 160 20h 99 80 [186]

Xylose Water/Toluene SBA-12Nb 160 24h 84 78 [187]

Xylose Water/Toluene SO2-/ZrO2-Al2O3/SBA-15 160 4h 99 53 [188]

Xylose Water/Toluene Mesoporous ZrAlW 170 4h 98 51 [102]

Xylose Water/Toluene Mesoporous NbO2 170 90 min 92 54 [189]

Xylose Water/Toluene HY Faujasite 170 50 min 66 42 [180]

Xylose Water/Toluene Hydroxylated MgF2 180 20h 99 79 [168]

Xylose Water/Toluene NbP 210 1h 98 45 [190]

Xylan Water CrLaCo0.8Cu0.2O3 160 10h 100 21 [191]


Table 23: Examples of studies on the dehydration of xylose and xylan with heterogeneous catalysts.


99

ii. Reactions under microwave irradiation

The main limitation to furfural yield under thermal heating is the formation of

side products by condensation with pentose intermediates or by resinification. In parallel

to the use of an extracting co-solvent, the performance of reactions under microwave

irradiation can be a solution for increasing furfural selectivity. This alternative activation

method offers a rapid and efficient heating which minimises temperature gradients

within the reaction sample. Microwave irradiation can result in accelerated reaction

rates, higher yields and lower amounts of by-products for certain reactions. Table 24

sums up the different studies on the dehydration of xylose, xylan and biomass carried out

under microwave irradiation.


Xylose Water - 200 60 88 49 [59]

Xylose Water HCl 180 20 n/a 59 [68]

Xylose Water H2SO4 180 20 n/a 50 [68]

Xylose Water H3PO4 180 20 n/a 43 [68]

Xylose Water CH3COOH 180 20 n/a 25 [68]

Xylose Water HCOOH 180 20 n/a 37 [68]

Xylose Water Maleic acid 200 28 100 67 [72]

Xylose Water NbP 180 10 83 40 [192]

Xylose Water/MIBK HCl 160 30 52 48 [193]

Xylose Water-THF AlCl3/NaCl 140 45 99 75 [113]

Xylan Water - 200 60 100 15 [59]

Xylan Water HCl 180 20 n/a 54 [68]

Xylan Water-THF AlCl3/NaCl 140 60 100 61 [113]

Corn stover Water Maleic acid 200 15 92 61 [72]

Corn stover Water-THF AlCl3/NaCl 140 60 98 51 [113]

Switchgrass Water Maleic acid 200 15 85 57 [72]

Switchgrass Water-THF AlCl3/NaCl 140 60 88 29 [113]

Pinewood Water Maleic acid 200 15 75 54 [72]

Pinewood Water-THF AlCl3/NaCl 140 60 92 50 [113]

Poplar Water Maleic acid 200 15 81 56 [72]

Poplar Water-THF AlCl3/NaCl 140 60 94 45 [113]


Table 24: Dehydration of xylose, xylan and biomass under microwave irradiation.


100

Moderate to good furfural yields were obtained after short reaction times (< 60

min). Best yield of 75% was obtained by using AlCl3 in combination with NaCl in a

water/THF medium.[113] Microwave irradiation also proved to be an efficient activation

method for dehydrating xylan and raw biomass.

iii. Reactions in continuous flow reactor

Another solution for limiting the formation of side products is the use of a

continuous flow reactor. Indeed, the short residence times associated with this type of

reactors could avoid long contact between reactants and products and thus increase

furfural selectivity. Table 25 presents the studies done using a continuous flow reactor.

Park et al. developed a process which combined the formation of furfural, its separation

from the reaction medium by distillation and the recycling of the effluents. Without any

catalyst, they obtained a selectivity of 79% for a xylose conversion of 66%.[194] The same

recycling system was set up by Lessard et al. They obtained 98% of furfural yield by

reacting an aqueous solution of xylose with a solid catalyst in suspension at an elevated

temperature (260°C) for a short residence time (3 min) in the presence of toluene.[195]

However, a drastic reaction temperature (400°C), even applied for a short residence time

(< 1 s), mainly led to degradation products (glyceraldehyde, glycoaldehyde, etc).[61]

Substrate Solvent Catalyst T(°C) τ (s) X (%) Y (%) Ref

Xylose Water - 400 1 99 4 [61]

Xylose Water - 200 1 67 53 [194]

Xylose Water HCl 190 250 84 69 [69]

Xylose Water HCl/NaCl 200 500 90 81 [69]

Xylose Water H2SO4/KCl 200 n/a 90 77 [70]

Xylose Water H2SO4/KBr 200 n/a 90 80 [70]

Xylose Water H2SO4/KI 200 n/a 91 81 [70]

Xylose Water H-ZSM-5 160 n/a 38 38 [97]

Xylose Water/Toluene Mordenite (H+) 13 260 180 99 98 [195]

Xylose Water/MIBK GaUSY/Amberlyst 36 130 816 98 70 [196]

Xylan Water/MIBK GaUSY/Amberlyst 36 130 816 92 69 [196]


Table 25: Dehydration of xylose and xylan in continuous flow.


101

e. Objectives of the Chapter

Since xylose dehydration had been widely studied in batch reactors under

conventional heating, alternative methods need to be investigated. First, we performed

xylose dehydration under microwave irradiation by using either homogeneous catalysts

(mineral acids and metal chlorides) or a heterogeneous catalyst (ion-exchange resin

Nafion NR50). Then, the optimised methods were applied to xylan and other

hemicellulose monomers. Finally, the dehydration of xylose was studied in a flash

continuous flow reactor without any catalyst.


102

2. Dehydration of D-xylose under microwave irradiation with homogeneous catalysis

In Chapter II, we optimised a dehydration method for the formation of

phenylacetaldehyde from 1-phenylethane-1,2-diol under microwave irradiation. The best

operating conditions were a temperature of 170°C and a reaction time of 20 minutes. The

reaction was performed in a biphasic medium water/CPME, 1:3, v/v with FeCl3, HCl and

H2SO4 as catalysts (20 mol%). High yields and selectivities were obtained (97% of yield

with H2SO4, 93% with HCl, 84% with FeCl3). Therefore, the study on the dehydration of

xylose was started from these optimised conditions.

Firstly, after having checked that a biphasic system was necessary, the efficacy of

different catalysts was compared. Then, reaction time and temperature were optimised

for the selected catalyst. The influence of the xylose concentration and of the catalyst

quantity was studied. Finally, the recyclability of the catalyst-containing aqueous phase

was assessed.

a. Reactions in biphasic medium

i. Comparison of aqueous and biphasic media

Firstly, the reaction conditions optimised for the dehydration of 1-phenyl-ethane-

1,2-diol (T = 170°C, t = 20 min, FeCl3, HCl and H2SO4 as catalysts - 20 mol%) were

applied to the dehydration of xylose. Reactions in water and in the biphasic system

water/CPME, 1:3, v/v were compared (Figure 27).

From the studies reported in the literature[82,113,115,193] and from the previous

experiments in Chapter II, an important increase in xylose conversion and furfural yield

was expected with the introduction of cyclopentyl methyl ether (CPME). As planned,

xylose conversion and furfural yield doubled when performing the reaction in biphasic

medium. With FeCl3 for example, xylose conversion increased from 54 to 99% while

furfural yield rose from 33 to 69%. For the three catalysts, furfural selectivity was a bit

higher when working in a biphasic system but all the values were in the same order of

magnitude (from 61 to 71%). In both cases, the formation of insoluble humins (up to 4

wt% of the initial xylose mass) and soluble humins (dark colouration of the solution, not

quantified) was observed. Therefore, it suggested that the role of CPME was more to


103

drive the equilibrium of the reaction to the formation of furfural by extracting it as soon

as it was formed, than to avoid side reactions.

The repartition of furfural between the two phases was measured: 92% of the

furfural formed can be found in the CPME phase and only 8% in the aqueous phase. The

distribution ratio [C]org/[C]aq of a compound is defined as the ratio of its concentrations

in the two phases in a mixture of two immiscible liquids at equilibrium. In our

experiments, the furfural distribution ratio [F]CPME/[F]aq was equal to 11.5.

Figure 27: Comparison between aqueous and biphasic media.


ii. Choice of the catalyst

The effect of different catalyst types was studied on the dehydration of D-xylose at

170°C for 20 min keeping the amount of catalyst constant (20 mol%) in a biphasic

mixture (water/CPME, 1:3, v/v) under microwave irradiation (Figure 28). Metal

chlorides (LiCl, CuCl, CuCl2, CoCl2, ZnCl2, FeCl2, FeCl3, AlCl3 and CrCl3), iron sulphates

(FeSO4 and Fe2(SO4)3) and mineral acids (HCl, H2SO4 and H3PO4) were compared. As

reference, reaction of xylose without a catalyst gave very low results: only 3% furfural was

formed.

0

10

20

30

40

50

60

70

80

90

100

%

Catalyst

Xylose conversion

Furfural yield

Water Water/CPME, 1:3, v/v


104

Figure 28: Efficacy of different catalysts on the dehydration of xylose.


With LiCl, a furfural yield of only 5% was obtained. Low furfural yields were also

obtained from CuCl, CoCl2, ZnCl2 and FeCl2, with a range of 20–30%, while CuCl2 led to a

better yield of 55% with a xylose conversion of 86%. Trivalent metal chlorides, FeCl3,

AlCl3 and CrCl3, were effective catalysts for converting xylose to furfural, leading to

moderate to good yields of furfural. In general, trivalent chlorides showed a stronger

effect on producing furfural under these reaction conditions, divalent chloride had a

medium effect, and monovalent ones were the weakest, a result similar to Zhang et al.[16]

This ranking can be linked to the proportion of chloride ions released in the aqueous

phase. Indeed, chloride ions played an important role in the mechanism of furfural

formation. As reported earlier, Marcotullio and de Jong showed that halides act as weak

bases assisting the enolisation reaction via proton transfer. They postulated that Cl- ions

promote the formation of the 1,2-enediol from the acyclic form of xylose, and accelerate

the first and second dehydration steps, and then the last intramolecular dehydration and

ring closure leading to furfural.[69,70] The comparison of the results obtained with iron

chlorides (FeCl2 and FeCl3) and iron sulphates (FeSO4 and Fe2(SO4)3 confirmed that Cl-

ions were more effective than SO42- ions for dehydrating xylose.

Among trivalent metal chlorides, FeCl3 was the most effective one, leading to a

xylose conversion of 97% and a furfural yield of 69%. These results were not consistent

with the work of Gravitis et al. who observed a correlation between the activity of the

metal cation and its ionisation potential.[177] According to their theory, the cation Al3+ ion

0

10

20

30

40

50

60

70

80

90

100

%

Catalyst

Xylose conversion

Furfural yield


105

should have a better catalytic performance on the production of furfural than Fe3+ and

Cr3+. Nevertheless, it can be noticed that xylose conversion was total with AlCl3 even if

furfural yield did not exceed 43%. Thus, it suggested that the Lewis acid AlCl3 may have

too strong activity and then promote more side products which decreased furfural

selectivity.

For mineral acids, even if H2SO4 was the most commonly used catalyst

industrially for producing furfural, HCl performed a higher catalytic effect than H2SO4 in

the reaction conditions (yield of 72% instead of 62%). Similar observation was made by

Yemis et al. recently, finding that the furfural yields obtained from xylose in the presence

of HCl and H2SO4 at 180°C were 59% and 50% respectively.[68] H3PO4 only led to a low

furfural yield of 21%. It should be noted that FeCl3 exhibited results similar to HCl, with a

solution less acidic (pH of 1.20 with FeCl3 instead of 0.7 with HCl). Therefore, FeCl3 was

a good alternative to corrosive mineral acids.

iii. Variation of temperature and reaction time

Temperature and reaction time are two important factors with great influence on

the dehydration of D-xylose. Consequently, variations of time periods (5, 10, 15, 20, 30,

45, 60 min) and temperatures (130, 140, 150, 160, 170, 180°C) were tested in order to

determine the optimal conditions (Figure 29). The reactions were performed with 20

mol% of FeCl3 in a biphasic system water/CPME, 1:3, v/v.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60

Xyl

ose

co

nve

rsio

n (

%)

Reaction time (min)

130

140

150

160

170

180

a)

T (°C)


106

Figure 29: Effect of reaction temperature and time on a) xylose conversion and b) furfural yield.

MW, Catalyst: FeCl3, 20 mol%, Water/CPME, 1:3, v/v.

At temperatures lower than 150°C, the conversion of D-xylose and the formation

of furfural were limited. At 180°C, xylose was fully converted after 5 minutes and furfural

was obtained in a 65% yield. If the reaction time was increased, the furfural yield slowly

decreased to reach 44% after 60 minutes. Indeed, the stability of furfural at high

temperature was low and furfural suffered side reactions. For example, Lamminpäa et al.

studied the degradation of furfural in acidic solution and observed a loss of 15% of the

furfural introduced after 60 minutes at 180°C.[175] Enslow and Bell showed that the cross-

condensation between furfural and reaction intermediates was favoured.[171]

Reactions performed at 160 and 170°C both led to a total conversion and a

maximum yield of 69%. However, the reaction rate of xylose dehydration increased with

the temperature. The best results were obtained after 60 minutes at 160°C and after only

20 minutes at 170°C. We chose to work with the shorter reaction time. Thus, the

optimised reaction conditions with FeCl3 were 20 minutes at 170°C.

The same experiments were carried out with AlCl3 as a catalyst. The previous

results suggested that AlCl3 was too active and promoted more side reactions. Thus, it

was supposed that lower temperatures may lead to higher yield. However, even if the

reaction rate was faster with AlCl3 than with FeCl3, furfural yield did not exceed 47%,

regardless of reaction time and temperature.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60

Furf

ura

l yie

ld (

%)

Reaction time (min)

130

140

150

160

170

180

b)

T (°C)


107

iv. Kinetic study

Different reaction kinetic mechanisms leading from xylose to furfural have been

proposed in literature[70,161–164,166], from simple ones to more complicated, but almost all

of them can be expressed in a simplified way, as described in Scheme 22.

Scheme 22: Simplified reaction mechanism of furfural formation.

The variation of the conversion of xylose and furfural with reaction time can be

expressed by the following equations based on the above kinetic model:

(1)

(2)

with the analytical solution as:

(3)

(

) ( ) (4)

where [X], and [F] refer to the molar concentrations of xylose and furfural

respectively.

The equations (3) and (4) were fitted to the experimental results deriving the

kinetic parameters k1, k2 and k3 thanks to a genetic algorithm. The model showed a good

agreement with the experimental measurements, the results are displayed in Table 26.

The apparent activation energies (Table 27) were calculated via Arrhenius plots. Good

correlation factors were obtained except for the determination of Ea3. The apparent

activation energies obtained experimentally have values close to the ones reported in the

literature.

xylose furfural loss reactionsk1

k2

side reactions

k3


108

Entry T (°C) k1 (min-1) k2 (min-1) k3 (min-1)

1 130 3.70 x 10-3 2.60 x 10-3 7.11 x 10-4

2 140 1.10 x 10-2 3.83 x 10-3 8.22 x 10-3

3 150 2.43 x 10-2 1.10 x 10-2 1.63 x 10-3

4 160 5.93 x 10-2 2.64 x 10-2 9.45 x 10-3

5 170 1.55 x 10-1 6.87 x 10-2 8.31 x 10-4

Table 26: Kinetic rate constants ki (min-1) at each experimental temperature.

i Eai (kJ.mol-1) Ai (min-1) R² Eai (kJ.mol-1), literature data

1 135.9 1.56 x 1015 0.978 111.5 [58], 166.2 [170], 123.9 [193]

2 125.4 3.68 x 1013 0.998 143.1 [58], 162.9 [170]

3 89.6 5.01 x 108 0.400 58.8 [58], 72 [170], 67.6 [193]

Table 27: Frequency factors (Ai, min-1), activation energies (Eai, kJ.mol-1) and correlation factors R²

for the kinetic model proposed in Scheme 22.

v. Variation of xylose concentration

The influence of the initial D-xylose concentration (from 0.25 to 4 mol.L-1) on the

xylose conversion and furfural yield was studied in a mixture of water/CPME, 1:3, v/v at

170°C for 20 min under microwave irradiation (Figure 30).

From 0.25 mol.L-1 to 1.25 mol.L-1, xylose conversion and furfural yield increased

with increasing pentose concentration. The highest furfural yield obtained from xylose

was 69% at 1 and 1.25 mol.L-1 with the best selectivity (72%). With a higher xylose

concentration, the yield of furfural decreased together with selectivity. Moreover,

experiments showed that the formation of side products increased with the xylose

concentration. Indeed, the quantity of insoluble humins was multiplied by 10 (from 3% to

30% of the initial xylose mass) when the xylose concentration went from 1.25 to 4 mol/L.

The high pentose concentration favoured the contact between molecules and thus

promoted condensation between pentose intermediates and furfural.[193] The large

amounts of furfural produced from high xylose concentration may also saturate the


109

organic phase and make the furfural extraction no longer efficient. The viscosity of

reaction media also increased with increasing substrate concentration and may have led

to a non-uniform heating in the microwave reactor. Thus, it was decided to continue the

study with a xylose concentration of 1.25 mol.L-1.

Figure 30: Effect of the initial xylose concentration on xylose conversion and furfural yield.

MW, T = 170°C, t = 20 min, Catalyst: FeCl3, 20 mol%, Water/CPME, 1:3, v/v.

vi. Variation of catalyst quantity coupled with the addition of a salt

Several studies in the literature demonstrated the double positive effect of the

addition of a salt such as NaCl on the furfural formation, firstly by salting-out the

reaction product in biphasic systems, and secondly by directly enhancing the furfural

selectivity and rate of formation.[69,82,115,197] Thus, investigation of simultaneous addition

of FeCl3 and NaCl to the reaction mixture was done. In our case, NaCl was selected

because of its accessibility and low price. Moreover, previous studies on biomass

processing showed the potential of using seawater as a solvent.[198] The influence on

xylose conversion and furfural yield was studied by combining several NaCl quantities (0,

0.62, 1.25 and 2.50 mmol) and 5 different percentages of FeCl3 (1, 5, 10, 15 and 20 mol%)

while keeping constant the previous best reaction conditions (water/CPME, 1:3, v/v,

170°C, 20 min) (Figure 31).

0

10

20

30

40

50

60

70

80

90

100

0.25 0.5 0.75 1 1.25 1.5 2 3 4

%

Xylose concentration (mol/L)

Xylose conversion

Furfural yield


110

Figure 31: Effect of the combination FeCl3/NaCl on a) xylose conversion and b) furfural yield.

MW, T = 170°C, t = 20 min, Catalyst: FeCl3, Water/CPME, 1:3, v/v.

In general, xylose conversion increased with the amount of FeCl3 (from 1 mol% to

20 mol%) and with the amount of NaCl (from 0 to 2.50 mmol). The difference of

conversion between reactions performed with or without salt was greater when the

concentration of FeCl3 was low (1, 5 and 10 mol%). With higher concentration of FeCl3

(15 and 20 mol%), the conversion of xylose was in the same order of magnitude with or

without NaCl.

0

10

20

30

40

50

60

70

80

90

100

1 mol% 5 mol% 10 mol% 15 mol% 20 mol%

D-x

ylo

se c

on

vers

ion

(%

)

Amount of FeCl3/xylose

Without

0.62

1.25

2.50

NaCl(mmol)

a)

0

10

20

30

40

50

60

70

80

90

100

1 mol% 5 mol% 10 mol% 15 mol% 20 mol%

Furf

ura

l yie

ld (

%)

Amount of FeCl3/xylose

Without

0.62

1.25

2.50

NaCl(mmol)

b)


111

Concerning the formation of furfural, a positive impact of the salt was obtained at

low proportion of FeCl3 (1 and 5 mol%). At 5 mol% of FeCl3 for example, the furfural yield

reached was 33% without NaCl and doubled with the addition of 2.50 mmol of NaCl.

At higher concentration of FeCl3 (15 and 20 mol%), the addition of NaCl showed

no significant effect on the furfural yields. At 20 mol% of FeCl3, the addition of salt (2.50

mmol) even decreased furfural yield from 71 to 62%.

At intermediate concentration of FeCl3 (10 mol%), the yield increased from 59% to

74% by adding 1.25 mmol of NaCl. 74% was the best furfural yield reached of all

conditions tested. Thus, the addition of 1.25 mmol of NaCl allowed us to decrease the

amount of FeCl3 used by 2 (from 20 to 10 mol%) while reaching a superior yield (74

instead of 69%). These results could be explained by (i) the combined effect of the Cl-

anions and the acidic character of the Fe3+ and (ii) a salting-out effect of NaCl to the

reaction product in the organic phase. Therefore, the experiments showed that the best

results were obtained when the acidic catalyst FeCl3 was used at 10 mol% in the

presence of 1.25 mmol of NaCl in a biphasic mixture water/CPME, 1:3, v/v under

microwave irradiation at 170°C for 20 minutes.

b. Recyclability of the aqueous phase

As for 1-phenylethane-1,2-diol, one advantage of using a biphasic system is the

easy separation of the aqueous phase, containing the catalyst, from the CPME phase,

containing the product. After a catalytic test, the organic phase could be treated to

recover furfural and the aqueous phase could be reused for a second catalytic cycle under

the same conditions. To validate the reusability of the system, we performed 5 successive

reactions with the same catalytic aqueous phase at 170°C during 20 minutes (Figure 32).

After a catalytic run, the reaction medium was separated and the organic CPME

phase was removed. Then, fresh xylose and CPME were added to the recycled aqueous

phase containing FeCl3 (10 mol%) and NaCl (1.25 mmol) without adding a further

portion of catalyst. The catalytic aqueous phase was recyclable for up to four cycles. A low

decrease in activity was observed after the fifth cycle. It can be noticed that the furfural

yield obtained at cycles 2, 3 and 4 were higher than after the first run. These results could

be explained by the presence of xylose remaining from the previous cycle.


112

Figure 32: Reusability of the aqueous phase containing FeCl3 and NaCl for the xylose dehydration.

MW, T = 170°C, t = 20 min, Catalyst: FeCl3 (10 mol%) and NaCl (1 eq), Water/CPME, 1:3, v/v.

c. Conclusion

This study demonstrated an efficient catalytic system using FeCl3 (10 mol%) in

water/CPME to dehydrate D-xylose under microwave irradiation. Substrate

concentration, temperature and reaction time reaction played an important role on the

kinetics of the dehydration of xylose. The operating conditions optimised under

microwave irradiation were:

Temperature: 170°C



Catalyst: FeCl3 (10 mol%)

Additive: NaCl (1.25 mmol, 1 eq)

Initial xylose concentration: 1.25 mol.L-1

A maximum furfural yield of 74% was obtained by using these optimised

conditions. The addition of a salt permitted the reduction of the amount of FeCl3 (10

mol% vs 20 mol%) while slightly increasing the yield. The catalytic system FeCl3/NaCl

proved to be a good alternative to HCl by leading to similar results while limiting the

risks of toxicity and corrosion.

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5

Furf

ura

l yie

ld (

%)

Cycle


113

The turnover number and the turnover frequency were calculated for this catalytic

system: TON = 7.3 and TOF = 6.1 x 10-3 s-1. By comparison, the TOF values obtained by

studies with similar yields under microwave irradiation (67% by Kim et al.[72] and 75% by

Yang et al.[113]) are lower: 1.1 x 10-5 and 6.9 x 10-4 s-1 respectively.

Moreover, the aqueous phase containing the catalytic system FeCl3/NaCl could be

recycled up to 4 times without a loss of activity.

This study led to the publication of an article in a peer-reviewed journal.[199]


114

3. Dehydration of D-xylose under microwave irradiation with heterogeneous catalysis

Compared to homogeneous catalysis, heterogeneous catalysis has many

advantages: less waste is produced and the separation and recovery of the catalyst is

facilitated. However, finding an active and stable water-tolerant solid acid catalyst still

poses a challenge for the production of furfural. This study focused on the use of an ion-

exchange resin, Nafion NR50, for the dehydration of xylose to furfural in a biphasic

system under microwave irradiation.

Firstly, after having checked that a biphasic system was necessary, reaction time

and temperature were optimised. Then, the influence of the quantity of Nafion NR50 and

of salt was studied. The initial xylose concentration was varied. Finally, the recyclability

of the catalyst was investigated.

a. Nafion NR50: properties and applications

Commercialised by the DuPont company, Nafion NR50 is a perfluoroalkane

sulfonic resin (Figure 33) with an acidic capacity of 0.81 meq H+/g, a skeletal density of

2.042 g/cm3, a mean bead diameter of 2350 mm and a maximum operating temperature

of 220°C.[200] Its morphology consists of 3–5 nm clusters of –SO3H-ended perfluoroalkyl

ether side chains, dispersed throughout a hydrophobic semicrystalline tetrafluoroalkyl

ether matrix.[201] One major drawback of this material is its very low surface area (0.02

m2.g−1) which can limit its activity in non-swelling solvents or in gas phase.

Figure 33: Chemical structure of Nafion NR50.


115

Nafion NR50 was used as catalyst for the dehydration of linear alcohols to

ethers.[200,202,203] For example, di-n-hexyl ether was formed from 1-hexanol with a yield of

62% after 6h at 190°C under solvent-free conditions by using Nafion NR50. A high

selectivity of 98% was reached.[202] The ion-exchange resin was also used for the

formation of 5-HMF from fructose in DMF. A quantitative conversion was obtained after

3h at 100°C. However, only 45% of 5-HMF was formed. The researchers explained this

low selectivity by the strong acidity of Nafion NR50 (pKa ≈ 6) which promoted undesired

by-products such as humins.[204] In another study, Nafion NR50 promoted the formation

of levulinic acid from 5-HMF in water with a yield of 54%.[205]

For the dehydration of xylose to furfural, the use of Nafion NR50 has not been

reported yet. However, other types of Nafion were tested. Lam et al. obtained a 60%

furfural yield in DMSO by using Nafion 117. The catalytic activity of the resin remained

unchanged after 15 cycles.[49] Weingarten et al. converted only 25% of the xylose

introduced with Nafion SAC-13 (Nafion-H supported on silica) after 240 min at 160°C

under microwave irradiation.[107]

b. Reactions in biphasic medium

i. Preliminary study

Firstly, the operating conditions optimised for homogeneous catalysis (T=170°C,

water/CPME, 1:3, v/v) were applied to xylose in the presence of Nafion NR50. The resin

quantity was fixed at two pellets (≈ 90 mg). A reaction time of 60 min was necessary to

reach 59% of furfural yield for a quantitative conversion. However, it was noticed that the

Nafion pellets turned to black and their diameter was multiplied by 3. The pellets seemed

to have exploded because of the accumulation of dark solids, humins (Figure 34b).

Figure 34: Images of a) a pristine pellet of Nafion NR50,

b) a pellet after a reaction without salt and c) a pellet after a reaction with salt.

a) b) c)


116

As the addition of NaCl had a positive effect on furfural selectivity when using

homogeneous catalysis, we performed the same reaction with NaCl (with a mass ratio of 1

compared to Nafion). In that case, the pellets kept their initial size even if a dark

colouration also appeared. Moreover, an increase of the furfural yield to 79% was

observed. Consequently, it was decided to continue the study by using a combination

of Nafion NR50 and NaCl.

ii. Comparison of aqueous and biphasic media

In order to check the influence of the co-solvent, the dehydration of xylose was

performed for 60 min at 170°C in aqueous and biphasic media (Figure 35). Two pellets of

Nafion NR50 and 90 mg of NaCl were introduced to the system. As expected and as

already observed with homogeneous catalyst FeCl3, when xylose dehydration was

performed in a monophasic aqueous medium, the yield of furfural did not exceed 38%

and the selectivity was moderate (57 %). With the addition of CPME, results were better:

xylose conversion rose from 68 to 100% while furfural yield doubled, from 38 to 78%.

Therefore, a biphasic system was necessary to obtain good furfural yields and

selectivities.

Figure 35: Comparison between aqueous and biphasic media.

MW, T = 170°C, t = 60 min, Catalyst: Nafion NR50 (2 pellets), NaCl: 90 mg. a: Ratio 1:3, v/v

0102030405060708090

100

Water Water/CPME

%

Solvents

Xylose conversion

Furfural yield

a


117

iii. Variation of Nafion NR50 and salt quantities

The first study done was the optimisation of the quantities of Nafion NR50 and

NaCl. Reactions were performed at 170°C for 60 minutes in a biphasic system

water/CPME, 1:3, v/v. Experiments were done with one (Figure 36a) and two pellets

(Figure 36b) of Nafion NR50 and different masses of salt (from 0 to 90 mg) were added

to the reaction medium (1 pellet of Nafion weights approximately 45 mg).

Figure 36: Effect of the addition of NaCl on xylose conversion and furfural yield

for a) 1 pellet and b) 2 pellets of Nafion NR50.

MW, T = 170°C, t = 60 min, Catalyst: Nafion NR50/NaCl, Water/CPME, 1:3, v/v.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 50 70 90

%

Mass NaCl (mg)

Xylose conversion

Furfural yield

a)

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 50 70 90

%

Mass NaCl (mg)

Xylose conversion

Furfural yield

b)


118

With only one pellet of Nafion NR50, because of the formation of by-products, a

furfural selectivity lower than 50% was obtained without salt. The addition of salt was

thus necessary to increase the selectivity. With salt, good xylose conversion and furfural

yield were reached. A maximum yield of 72% was obtained with one Nafion pellet in

combination with 70 mg of NaCl.

Experiments performed with two pellets of Nafion NR50 led to higher yields but

the increase was less than expected: the yield only increased from 68% with one pellet to

79% with two (with 90 mg of NaCl). With two pellets, xylose conversion and furfural yield

followed the same trend: they increased with the mass of NaCl and the higher furfural

yield was obtained with 90 mg of NaCl. However, it should be noticed that the yields

obtained with 50 and 70 mg were in the same order of magnitude.

Thus, because the best yield was obtained with a combination of two pellets of

Nafion NR50 and 90 mg of NaCl (mass ratio 1/1), these conditions were kept even if

the differences between all the tested conditions were not significant.

iv. Variation of xylose concentration

The influence of the initial D-xylose concentration (from 0.67 to 4 mol.L-1) on the

xylose conversion and furfural yield was studied in a mixture of water/CPME, 1:3, v/v at

170°C for 60 min under microwave irradiation (Figure 37). All the reactions were

performed with the same quantity of catalyst: two pellets of Nafion NR50 and 90 mg of

NaCl.

Figure 37: Effect of the initial xylose concentration on xylose conversion and furfural yield.

MW, T = 170°C, t = 60 min, Catalyst: Nafion NR50 (2 pellets), NaCl: 90 mg, Water/CPME, 1:3, v/v.

0

10

20

30

40

50

60

70

80

90

100

0,67 1 1,33 1,67 2 2,33 2,67 3 3,33 4

%

Xylose concentration (mol/L)

Xylose conversion

Furfural yield

0.67 1 1.33 1.67 2 2.33 2.67 3 3.33 4


119

The highest furfural yield obtained from xylose was 79% at 1 mol.L-1 with the best

selectivity (79%). If the initial xylose concentration was increased, furfural yield and

selectivity decreased slowly, leading to 62% yield with a xylose concentration of 4 mol.L-1.

The high substrate concentration favoured the contact between molecules and thus

promoted condensation between xylose, pentose intermediates and furfural. The large

quantity of furfural formed can also saturate the organic layer and make the extraction of

the furfural from the aqueous phase difficult.

However, it should be noticed that, since all reactions were performed with two

Nafion pellets, more furfural was produced for the same amount of catalyst with high

initial xylose concentrations even if furfural yield was lower.

v. Influence of temperature and reaction time

Finally, as we saw before, temperature and reaction time are two factors with

great influence on the dehydration of D-xylose. Preliminary experiments were carried out

for 60 minutes at 130, 140 and 150°C which led to furfural yields of 4, 14 and 47%

respectively. Thus, a temperature of 160°C was necessary to obtain a furfural yield

superior to 50%. Consequently, variations of time periods (from 5 to 60 min) and

temperatures (160, 170, 180°C) were tested in order to determine the optimal conditions

(Figure 38). The reactions were performed with two pellets of Nafion NR50 and 90 mg of

NaCl in a biphasic system water/CPME, 1:3, v/v.

Figure 38: Effect of reaction time on furfural yield for different temperatures.

MW, Catalyst: Nafion NR50 (2 pellets), NaCl: 90 mg, Water/CPME, 1:3, v/v.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60

Furf

ura

l yie

ld (

%)

Reaction time (min)

160

170

180

T (°C)


120

At 160°C, the formation of furfural was fast between 5 and 20 minutes of reaction:

yield rose from 7 to 50%. Then, if the reaction was continued, furfural yield increased

more slowly: from 50% after 20 minutes to 72% after 60 minutes of reaction. At 170°C,

the rate of furfural formation was higher: only 25 minutes were enough to lead to a

furfural yield of 70%. The highest yield reached was 80% after only 40 minutes. If the

reaction continued, the yield did not increase further.

At 180°C, xylose was fully converted after 10 minutes and furfural was obtained in

a 68% yield. The maximum yield at 180°C was reached after 20 minutes (74%). Then, if

the reaction time was increased, the furfural yield slowly decreased to reach 68% after 60

minutes. As observed with a homogeneous catalyst, the stability of furfural at high

temperature was low and furfural quickly degraded.[175]

The best results (80% of yield for a total conversion) were obtained after 40

minutes at 170°C. Thus, the optimised reaction conditions with Nafion NR50 were 40

minutes at 170°C.

c. Mechanism involved and comparison with HCl

In pure water (without the addition of salt), xylose dehydration was catalysed by

the sulfonic acid groups present on the hydrophilic clusters of the Nafion NR50 beads.

When NaCl was added to the aqueous phase, a cation exchange between the Na+

of the salt and the H+ of the sulfonic acid groups of Nafion occurred and HCl was formed

in the aqueous phase (Scheme 23). Nafion NR50 has a capacity of 0.8 mmol of H+ per

gram. For two pellets, it corresponded to a pH of 1.1 in the vial if the totality of the

protons was released in the medium. The pH of our reaction media reached a value of 1.3

on average which means that a part of the sulfonic groups grafted on the resin remained

protonated.

To verify if the xylose conversion was only catalysed by the HCl released in the

medium or also by the resin, a reaction with an HCl solution at a pH of 1.3 (0.05 mol.L-1)

was performed for 60 minutes at 170°C in water/CPME. A lower furfural yield of 73%

was obtained instead of the 79% reached with Nafion. Besides, a reaction with an HCl

solution with a pH of 1.1 was also performed and the furfural yield was also lower (72%).

Consequently, for us, there is a synergetic effect between the Nafion pellets and the HCl

released in the medium.


121

Scheme 23: Cation exchange on the sulfonic acid groups of Nafion NR50.

To complete our study, Na+ cations were replaced by K+ and Li+ using other

inorganic salts, KCl and LiCl. Because of the variation of the cation size and so to their

relative mobility inside the Nafion pellets, a variation of the release of the protons into

the reaction medium was expected. However, by using the same salt molar ratio, no

variation of pH was recorded and the value of produced furfural was constant with a

value of 79%.

d. Regeneration and analyses of Nafion NR50 pellets

One of the main advantages of using a heterogeneous catalyst is its easy recovery

from the reaction medium and its easy reuse in a following experiment. However, in our

case, when these Nafion NR50 pellets were reused without any treatment, only a 21%

furfural yield was reached. Indeed, the cation exchange between the H+ of the sulfonic

acid groups of Nafion and the Na+ during the first cycle deactivated the catalyst by

reducing the number of available acid sites and the quantity of HCl released in the second

cycle. The deactivation of Nafion 117 in the presence of NaCl was also observed at 150°C

in DMSO by Lam et al. with a decrease in the yield from 58 to 13%.[49]

Therefore, a regeneration step of the catalyst was necessary: a treatment overnight

in an acidic bath (concentrated HCl) regenerated the sulfonic acid groups grafted on the

resins and thus the activity of the catalyst. A second reaction cycle was performed

successfully and led to a furfural yield similar to the first one (81%).

SEM, EDX and ATR-FTIR analyses were performed on Nafion NR50 pellets to

observe variations on their morphology and constitution. A pristine pellet, a pellet

submitted to a reaction without salt, a pellet submitted to a reaction with salt and finally

after a regeneration step in HCl were analysed. SEM pictures are presented in Figure 39,

ATR-FTIR peaks in Table 28 and EDX spectra can be seen in Appendices.


122

Figure 39: SEM images of Nafion NR50 (a) before reaction; (b) after a reaction without salt;

(c) after a reaction with salt; and (d) after regeneration in concentrated HCl solution.

Scale bar = 50 µm.

First, the picture 39a showed the morphology of the pristine Nafion surface. Its

EDX spectrum confirmed the main constituents of Nafion: fluorine, carbon, oxygen and

sulphur. By comparison, as observed during the preliminary study, the surface of the

pellet used in a reaction without NaCl was characterised by a network of cracks of various

sizes. A coating of organic residue can be clearly seen (Figure 39b). This humin

deposition was confirmed by (i) the EDX analysis which showed an enrichment of the

catalyst with carbon and oxygen, and (ii) the ATR-FTIR analysis which exhibited

additional peaks corresponding to C=O and OH bonds (Table 28).

Then, when the reaction was carried out in the presence of NaCl, the recovered

Nafion pellets appeared to remain intact. Nevertheless, even after a washing step, the

surface of the catalyst was entirely coated by NaCl crystals (Figure 39c), observation

confirmed by the EDX analysis.

Finally, the Nafion NR50 pellet could be regenerated in an acidic bath of HCl to

recover its activity. As it can be seen on the picture 398d, the inorganic salt coating was

completely removed by this treatment and the surface structure was similar to the

pristine Nafion surface, observation confirmed by the EDX and ATR-FTIR analyses.

a) b)

c) d)


123

Peak Pristine Nafion

NR50 (cm-1)

Post-reaction Nafion NR50 (cm-1)

Without salt With salt After HCl

regeneration

OH 3200-3600

CO 1364

CF2 1201 1236 1211 1206

CF2 1146 1153 1147 1147

SO 1055 1051 1059 1057

CFRCF3 985 982 982

COC 970 969 965

Table 28: ATR-FTIR absorption peaks of pristine and post-reaction Nafion NR50.

Several consecutive reactions were performed with the same Nafion NR50 pellets.

After each reaction, the resin pellets were recovered from the reaction medium and

regenerated in an acidic HCl bath overnight. The reactions were performed for 60

minutes at 170°C in water/CPME. As shown on the Figure 40, the activity of the Nafion

pellets was kept constant for three consecutive cycles. After the fourth cycle, the furfural

yield began to decrease. This observation can be explained by the gradual deactivation of

the pellets by humin deposition.

Figure 40: Study of the recyclability of the Nafion NR50 pellets after regeneration steps in HCl.

MW, T = 170°C, t = 60 min, Catalyst: Nafion NR50 (2 pellets), NaCl: 90 mg, Water/CPME, 1:3, v/v.

0

20

40

60

80

100

1 2 3 4

Furf

ura

l yie

ld (

%)

Cycle


124

e. Conclusion

This study demonstrated an efficient catalytic system using Nafion NR50 in

water/CPME to dehydrate D-xylose under microwave irradiation. Temperature and

reaction time reaction played an important role on the kinetics of xylose dehydration.

Because of the formation of humins, the addition of NaCl was necessary in order to avoid

the degradation of the Nafion NR50 pellets. A maximum furfural yield of 80% was

obtained under microwave irradiation by using these optimised conditions:

Temperature: 170°C



Catalyst: Nafion NR50 (2 pellets)

Additive: NaCl (90 mg, 1.55 mmol)

Initial xylose concentration: 1 mol.L-1

Because of the addition of NaCl, a cation exchange between the Na+ of the salt and

the H+ of the sulfonic acid groups grafted on Nafion NR50 occurred and HCl was formed

in the aqueous phase. Nevertheless, it was proven that the dehydration reactions were

not catalysed only by the HCl released but also by sulfonic acid groups still partially

protonated grafted on the Nafion pellets. Therefore, there was a synergetic effect between

the Nafion pellets and the HCl released in the medium. Xylose dehydration happened

thanks to the combination of heterogeneous (Nafion NR50) and homogeneous (HCl)

catalysis. The turnover number and the turnover frequency were calculated for this

catalytic system: TON = 11 and TOF = 4.6 x 10-3 s-1.

However, because of the cation exchange between Na+ and H+, the Nafion NR50

pellets lost their activity and a furfural yield of only 21% was reached when the pellets

were reused in a second cycle. A simple treatment in an acidic bath of HCl regenerated

the sulfonic acid groups of the Nafion pellets and allowed their reuse. Three cycles

(dehydration reaction + regeneration step) were performed without any loss of activity.

After the fourth cycle, a low decrease in furfural yield was observed.


125

4. Application to xylan and hemicellulose sugars units

The optimisation of the dehydration methods with the homogeneous catalyst FeCl3

and the heterogeneous catalyst Nafion NR50 were done for the model compound, D-

xylose. To extend the studies, these methods were applied to the homopolymer xylan,

main hemicellulose found in grasses and hardwoods, and to L-arabinose, D-glucose, D-

mannose and D-galactose, other monomers found in hemicelluloses.

a. Dehydration of xylan into furfural

The formation of furfural from xylan (15) followed a two-step mechanism: (i) first,

xylan was hydrolysed to D-xylose monomers (14), and (ii) secondly, xylose was

dehydrated to furfural (13) (Scheme 24).

Scheme 24: Dehydration of xylan to furfural via xylose.

i. By homogeneous catalysis

Our conditions optimised for the dehydration of D-xylose under microwave

irradiation with a homogeneous catalyst (10 mol% FeCl3, NaCl 1.25 mmol, water/CPME,

1:3, v/v, 170°C, 20 min) were applied to the homopolymer xylan (15). However, the

target furfural was produced in only 21% yield and 42% of xylose was still remaining in

the reaction mixture. Thus, it seemed that a longer reaction time was necessary to

perform the two-step mechanism, hydrolysis then dehydration, as observed by Yang et

al.[113] The low solubility of xylan in water may also explain these low results.

Therefore, according to this first result, two new studies were carried out: (i)

variation of reaction time (from 5 to 80 minutes) at 170°C, and (ii) variation of

temperature (from 100 to 200°C) for a reaction time of 20 minutes (Figure 41).


126

Figure 41: Xylose and furfural yield from xylan in function of

a) reaction time at 170°C and of b) temperature for a reaction time of 20 min.

MW, Catalyst: FeCl3 (10 mol%) and NaCl (1 eq), Water/CPME, 1:3, v/v.

At 170°C, after 10 minutes of reaction, 52% of xylose and 12% of furfural were

present in the reaction medium. This means that the hydrolysis of xylan was a fast

reaction. Thus, the limiting step in the formation of furfural was the dehydration of

xylose. The dehydration of xylose required more energy than xylan depolymerisation. A

quantitative conversion of the xylose formed from xylan was obtained after 70 minutes

with a maximum furfural yield of 46% (Figure 41a).

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80

%

Reaction time (min)

Xylose yield

Furfural yield

a)

0

10

20

30

40

50

60

70

80

90

100

100 120 140 160 180 200

%

Temperature (°C)

Xylose yield

Furfural yield

b)


127

When varying the reaction temperature, several observations can be made. A

temperature of 130°C was necessary to obtain a xylose yield superior to 10%. Then, when

the temperature rose from 130°C to 170°C, hydrolysis of xylan into xylose was accelerated

and the sugar released in the media increased. Furfural formation was still limited and

furfural yield exceeded 10% only at a temperature of 170°C. Yield increased from 21% to

52% with an increasing temperature from 170°C to 200°C.

In the literature, similar furfural yields were obtained from xylan under

microwave irradiation. For example, 54% of furfural were obtained by dehydrating xylan

in water in the presence of HCl for 20 min at 180°C.[68] In their experiments, Yang et al.

reached a 61% yield by combining AlCl3 and NaCl in a biphasic system water/THF.[113]

To combine the observations of the two studies, a reaction was performed at

200°C for 70 minutes but no increase of the yield was observed.

We showed that the hydrolysis of xylan to xylose was fast and did seem not to be

the limiting step of the reaction. Results similar to those obtained from pure xylose (74%

of furfural yield) should be expected. Therefore, since lower results were obtained, it

suggested than xylan conversion was not complete and did not liberate all the xylose

units. However, since the analysis of the xylan conversion was not possible, this

hypothesis could not be confirmed.

In regards to our results, two optimised methods for furfural formation from xylan

were developed: 200°C for 20 minutes or 170°C for 70 minutes in a biphasic

system of water/CPME, 1:3, v/v in presence of FeCl3 (10 mol%) and NaCl (1.25 mmol).

ii. By heterogeneous catalysis

Our conditions optimised for the dehydration of D-xylose under microwave

irradiation with a heterogeneous catalyst, Nafion NR50, were also applied to the

homopolymer xylan (15). Temperature was varied between 130 and 190°C for a reaction

time of 60 minutes (Figure 42).

Observations similar to the previous study with homogeneous catalysis can be

made. A temperature of 140°C was necessary to obtain a xylose yield superior to 30%.

Then, at temperatures comprised between 140°C to 160°C, hydrolysis of xylan into xylose

was accelerated and the sugar released in the media increased. However, furfural


128

formation was still limited. A temperature of 170°C was necessary to have a furfural yield

superior to 30%. Yield increased from 30% to 50% with an increasing temperature from

170°C to 190°C. The best furfural yield (50%) obtained from xylan with Nafion NR50 was

in the same order of magnitude than the one obtained with FeCl3 (52%) but was still

moderate compared to maximum yield obtained from pure D-xylose (80%). The same

hypothesis of a non-complete conversion of xylan can be considered.

Figure 42: Xylose and furfural yield from xylan as a function of temperature for t=60 min.

MW, t = 60 min, Catalyst: Nafion NR50 (2 pellets), NaCl: 90 mg, Water/CPME, 1:3, v/v.

Therefore, an optimised method for furfural formation from xylan was developed:

190°C for 60 minutes in a biphasic system of water/CPME, 1:3, v/v in the presence of

Nafion NR50 (2 pellets) and NaCl (90 mg). The dehydration of xylan in the presence of

Nafion NR50 has never been reported before.

b. Dehydration of other sugars composing hemicelluloses

Unlike cellulose only composed of glucose, hemicelluloses are hetero-

polysaccharides composed of different sugar units such as D-xylose, D-mannose, D-

glucose, D-galactose and L-arabinose. Thus, the methods optimised for D-xylose were

applied to these sugar units as model compounds.

0

20

40

60

80

100

130 140 150 160 170 180 190

%

Temperature (°C)

Furfural yield

Xylose yield


129

i. From L-arabinose to furfural

After D-xylose, L-arabinose (16) is the second most abundant pentose in

hemicelluloses. Its dehydration also leads to the formation of furfural (13) (Scheme 25).

Scheme 25: Dehydration of L-arabinose to furfural.

The dehydration of L-arabinose (16) was performed at 170°C for 20 minutes in a

biphasic system water/CPME, 1:3, v/v in the presence of FeCl3 (10 mol%) and NaCl (1.25

mmol). With this method, a furfural yield of 41% and an arabinose conversion of 65%

were achieved (Figure 43). These results were lower than those obtained from xylose

(74% of furfural yield for a total conversion) as it has been observed in literature.[206] It

was reported that arabinose was dehydrating less rapidly than xylose at 170°C.[206,207]

This difference is due to the fact that D-xylose expressed lower activation energy for its

dehydration compared to L-arabinose. The structure of L-arabinose was more stable. The

conversion of arabinose into furfural was reported to be more temperature dependent

and to require high temperatures (above 220°C) to accelerate the process.[208]

Figure 43: Comparison of L-arabinose and D-xylose dehydration to furfural.


0

10

20

30

40

50

60

70

80

90

100

D-Xylose L-Arabinose

%

Pentose

Pentose conversion

Furfural yield


130

ii. From D-glucose, D-mannose and D-galactose to 5-HMF

D-glucose (17), D-galactose (18) and D-mannose (19) are present in the backbone

of several types of hemicelluloses. Their dehydration leads to the formation of 5-

hydroxymethylfurfural, 5-HMF (20) (Scheme 26).

Scheme 26: Formation of 5-HMF by the dehydration of D-glucose, D-galactose and D-mannose.

The dehydration of D-glucose (17), D-galactose (18) and D-mannose (19) was

performed at 170°C for 20 minutes in a biphasic system water/CPME, 1:3, v/v in

presence of FeCl3 (10 mol%) and NaCl (1.25 mmol). Results are presented in Figure 44.

The observed 5-HMF yields were lower than 10%, at conversions of 60–80%, thus

suggesting an important formation of by-products. The vast majority of experimental

studies showed that aldoses, like glucose, were much less efficiently dehydrated to 5-

HMF than ketoses, like fructose. Under classic acidic conditions, 5-HMF yield from

glucose is very low and additional catalysts or alternative systems are required to

optimise the dehydration. The additional catalyst is generally believed to facilitate the

isomerisation of glucose to fructose prior to its dehydration to 5-HMF.[209]

However, in biphasic media, the 5-HMF yields from glucose, galactose or

mannose are expected to be higher. Thus, it suggested that the co-solvent CPME was not

efficient in that case. To prove it, the repartition of 5-HMF between the two phases was

measured: only 52% of the 5-HMF formed was extracted by the CPME and 48% of the 5-


131

HMF stayed in the aqueous phase. The 5-HMF distribution ratio [5-HMF]CPME/[5-

HMF]aq was equal to 1.1, a value 10-fold lower than the distribution ratio of furfural

between CPME and water (11.5). Consequently, the 5-HMF formed was not well extracted

by the CPME and underwent side reactions in the aqueous phase, decreasing its yield.

Figure 44: Conversion of D-glucose, D-galactose and D-mannose to 5-HMF.


c. Conclusion

The application of our two optimised methods (homogeneous and heterogeneous)

to xylan in biphasic medium water/CPME under microwave irradiation led to good

furfural yields. 52% of furfural was obtained after 20 minutes at 200°C with the

combination of FeCl3 (10% mol) and NaCl (1 eq.) and a treatment with Nafion NR50 (2

pellets) and NaCl (90 mg) for 60 minutes at 190°C led to the formation of 50% of

furfural. Even if these results were lower than from pure D-xylose, they were similar to

the results reported in literature. Therefore, we optimised two efficient methods for the

dehydration of xylan. Nevertheless, since the homopolymer xylan does not represent the

total diversity of hemicelluloses, the application of the optimised methods to raw biomass

with a more complex structure should permit to extend their potential.

The extension of the study to sugars units present in hemicelluloses such as L-

arabinose, D-glucose, D-galactose and D-mannose gave expected results. Only the

homogeneous method was applied. L-arabinose dehydration led to the formation of

furfural in a moderate yield of 41%, its high stability making it more difficult to

dehydrate. The dehydration of the aldoses glucose, galactose and mannose gave limited

yields of 5-HMF (< 10%).

0102030405060708090

100

D-Glucose D-Galactose D-Mannose

%

Hexose

Hexose conversion

5-HMF yield


132

5. Dehydration of D-xylose in a continuous flow reactor

Since industrial processes for furfural production are more and more designed in

continuous flow, we focused our work, lastly, in optimising a xylose dehydration method

in sub- and supercritical water in a continuous flow reactor.

This work was a Short Term Scientific Mission (STSM) financed by the COST

program (European Cooperation in Science and Technology) and realised under the

supervision of Pr. Herbert SIXTA in the Department of Forest Products Technology, in

Aalto University (Helsinki, Finland).

a. Reactor and operating conditions

The reactions were performed using a flash continuous flow reactor at 250 bars.

The scheme of the reactor system is represented on Figure 45. The system was composed

of two feeding tanks (one for xylose solution and the other for pure water), a high-

pressure diagram pump with three pump heads, a heat exchanger, a vertical down-flow

reactor made of high-alloy stainless steel and an air-loaded back-pressure regulator. The

volume of the reactor was 390 µL.

Figure 45: Scheme of the reactor system, adapted from Tolonen et al.[210]

Pro

du

ct

Xyl

ose

so

luti

on

Wat

er

Reactor

Heating

Cooling


133

The xylose solution at room temperature was pumped from the xylose feeding

tank into the reactor, where it was instantaneously heated up to reaction temperature by

mixing it with pre-heated supercritical water from the heating water line. The desired

reaction temperature was maintained via heating elements around the reactor body,

which eliminated heat loss. At the bottom of the reactor, the solution was quenched down

by the addition of room-temperature water and then subsequently lowered to room

temperature via a cooling unit. The reaction product was collected into a bottle at the

reactor’s outlet.[210]

The residence time τ was controlled by setting the flow rates of heating water and

xylose solution according to the equation in Figure 46, where Vreactor is the volume of the

reactor part in m3, ṁ is the combined mass flow of xylose solution and heating water in

kg.s-1 and ρwater is the density of water at the reaction temperature and pressure in kg.m-3.

Because of the small volume of the reactor (390 µL), the residence times ranged between

0.25 and 2 s.

ṁ

Figure 46: Equation for calculating the residence time τ (s) of the reactions.

Vreactor (m3), ṁ (kg.s-1), ρwater (kg.m-3)

The reaction temperatures were comprised between 240 and 380°C. They were

controlled by adjusting the temperature and the flow of the heating water line according

to enthalpy balance calculations. Likewise, the amount of quenching water needed to

reach a desired post-quenching temperature was determined by enthalpy balance

calculations. In all calculations, xylose solutions were assumed to behave like pure water

and to have perfect heat transfer. The densities and enthalpies of water as a function of

temperature and pressure were obtained from a web database.[211]

b. Reactions in water

Because of the risks of reactor corrosion, all reactions were performed without the

addition of any catalyst. Nevertheless, the important variations of water properties (ion

product, density, dielectric constant, etc) in the sub- and supercritical domains were

sufficient to promote xylose dehydration.


134

i. Influence of temperature and reaction time

First of all, the influence of temperature and residence time on xylose conversion

and furfural yield was studied. Reaction temperatures were varied from 240 to 380°C for

several residence times (from 0.25 to 2 s). The pressure was kept constant at 250 bars

and the xylose concentration in the reactor was set at 50 mmol.L-1. The results are

displayed in Figure 47.

Figure 47: Effect of reaction temperature and residence time on

a) xylose conversion and b) furfural yield.

Continuous flow, P = 250 bars.

0

10

20

30

40

50

60

70

80

90

100

240 260 280 300 320 340 360 380

Xyl

ose

co

nve

rsio

n (

%)

Temperature (°C)

0.25

0.50

0.75

1

1.50

2

a)

τ (s)

0

1

2

3

4

5

6

7

8

9

10

240 260 280 300 320 340 360 380

Furf

ura

l yie

ld (

%)

Temperature (°C)

0.25

0.50

0.75

1

1.50

2

b)

τ (s)


135

It can be noticed that both xylose conversion and furfural yield increased with the

increase of temperature and residence time. A quantitative conversion (> 90%) was

reached at 360°C with a 2 s residence time. However, as it can be seen in the Figure 47b,

the formation of furfural was very limited. The maximum yield obtained was below 7%.

Thus, the dehydration reaction had a very low selectivity. Moreover, it was observed that

the reaction media became darker and darker when temperature and residence time

increased.

In their work, Aida et al. performed reactions in a similar flash continuous flow

reactor at high temperature (350 and 400°C) and high pressure (40-100 MPa) with very

short reaction times (< 1.5 s). They reported that furfural was formed but in a limited

yield (< 10%) and that the main products were retro-aldol compounds (glyceraldehyde,

pyruvaldehyde, lactic acid, glycoaldehyde, dihydroxyacetone and formaldehyde).[61]

However, in our case, no other compounds were observed in the HPLC analyses despite

the use of two types of detectors (UV and RI).

Furfural degradation experiments were performed between 240 and 380°C with a

residence time of 2 s. However, no conversion of furfural was observed, even at high

temperatures. These results suggested that furfural degradation rate was relatively slow

compared with the short residence time. Thus, it implied that the by-products were

formed only in the presence of xylose by reaction of xylose with itself or with pentose

intermediates, or by the reaction of intermediates with furfural.

ii. Influence of the ratio between the inflows

As described above, the reaction medium was

formed at the top of the reactor by the mixing of xylose

solution at room temperature and pre-heated water

(referred as heating water) (Figure 48). The flows ratio

φ was defined as the ratio between the xylose solution

flow ṁS and the heating water flow ṁH:

Ratio ṁ

ṁ

Figure 48: Scheme of the reactor body.


136

During the first series of experiments, the flows ratio φ was set to 0.67 and the

xylose concentration in the reactor to 50 mmol.L-1. However, with this ratio value, it was

impossible to reach high temperatures (380°C) with the longest residence time (from 1 to

2 s) due to a limitation of the pre-heater. Then, to perform the reactions at 380°C, the

flows ratio φ was changed to 0.33 while keeping constant the total flow at the top of the

reactor (ṁ = ṁS + ṁH) and the xylose concentration in the reactor (50 mmol.L-1). Even if

it should not have an impact on the results, it was observed that furfural yields were

higher than expected. Thus, it seemed that the variation of the flows ratio φ had an

influence on the results even if all the other parameters were kept constant.

To verify this hypothesis, the experiments with the 2 s residence time (from 240 to

380°C) were reproduced with the modified flows ratio φ = 0.33. The comparison of the

results with the two different ratios is shown in Figure 49. Furfural yields doubled with

the 0.33-ratio. The highest yield (12%) was reached at 360°C. Therefore, the ratio φ was

kept to 0.33 for the following studies. However, for now, no explanation was found to

explain these differences.

Figure 49: Influence of the ratio φ “xylose solution flow”/ “heating water flow” on furfural yield.

Continuous flow, P = 250 bars, Residence time τ = 2 s.

iii. Influence of xylose concentration

Finally, the influence of the initial xylose concentration on xylose conversion and

furfural yield was studied. The experiments were carried out at temperatures from

subcritical 240°C to supercritical 380 °C for 2 s, while the pressure was kept constant at

0

2

4

6

8

10

12

14

240 260 280 300 320 340 360 380

Furf

ura

l yie

ld (

%)

Temperature (°C)

0.67

0.33

Ratio ϕ


137

250 bars. The xylose concentration was ranged from 50 to 100 mmol.L-1. The results are

shown on Figure 50. Xylose conversions and furfural yields obtained with xylose

concentration of 50, 75 and 100 mmol.L-1 were in the same order of magnitude.

Therefore, initial xylose concentration did not have a strong influence on the dehydration

of xylose.

Figure 50: Influence of initial xylose concentration on a) xylose conversion and b) furfural yield.

Continuous flow, P = 250 bars, Residence time τ = 2 s.

0

10

20

30

40

50

60

70

80

90

100

240 260 280 300 320 340 360 380

Xyl

ose

co

nve

rsio

n (

%)

Temperature (°C)

50

75

100

a)

[Xylose](mmol/L)

0

2

4

6

8

10

12

14

240 260 280 300 320 340 360 380

Furf

ura

l yie

ld (

%)

Temperature (°C)

50

75

100

b)

[Xylose](mmol/L)


138

The highest furfural yield was 13%, which is a very limited yield. It was obtained

with a reaction time of 2 s at a temperature of 360°C. However, it can be noticed that

furfural yield began to decrease at 380°C. Between these two temperatures, there is the

critical point of water (374°C) and so water changes from the subcritical to the

supercritical state. This leads to a change in the water properties and thus a different

reactivity. For example, ionic reactions are favoured in subcritical water while

supercritical water promotes radical reactions. Therefore, we can suppose that, at 380°C,

furfural was degraded by radical side reactions.

c. Conclusion

The study of the synthesis of furfural in sub- and supercritical water using a flash

continuous flow reactor led to low results. A total conversion of xylose was obtained but a

very limited quantity of furfural was produced (< 15%). Thus, using a flash continuous

flow reactor under high conditions of temperature and pressure was not a selective

solution to produce furfural. The identification of the by-products is a prospect of this

study to understand the mechanisms that took place.


139

6. Conclusions

Furfural (13) is a high value chemical that can be used to produce a wide range of

chemicals and fuels, such as furfuryl alcohol, methyl-tetrahydrofuran or levulinic acid for

instance. It is one of the few chemicals that are currently industrially produced via a

biosourced route since there is no synthetic route for its direct production. Furfural is

obtained by the dehydration of D-xylose (14), the principal component hemicellulose-

rich biomass. However, industrial processes are based on the use of mineral acids

(mainly H2SO4) in batch or continuous reactors at moderate temperature (< 200°C). A

limited formation of furfural (≈ 50%) was obtained because of the formation of many by-

products. The use of mineral acids has drawbacks like a difficult recovery of the catalyst

after the reaction completion or the corrosion of reactors.

In order to find new furfural production methods, two strategies were exploited: (i)

first, the use of microwave irradiation as an alternative to thermal heating, and (ii) the

use of a flash continuous flow reactor at high temperature and pressure. Pure D-xylose

was chosen as a model compound to optimise the reaction conditions but the studies

were also extended to xylan, homopolymer of xylose, and to other sugar units present in

hemicelluloses.

The dehydration of D-xylose under microwave irradiation was based on the method

optimised in Chapter II for 1-phenylethane-1,2-diol. Reactions were performed in a

biphasic medium of water/Cyclopentyl methyl ether (CPME) with a ratio of 1:3, v/v.

CPME proved to be necessary to reach good xylose conversions and furfural yields.

In a first study, homogeneous catalysts were compared and the Lewis acid FeCl3

proved to be the more efficient. Temperature, reaction time and substrate concentration

were varied. Best reaction conditions were found to be a temperature of 170°C for a

reaction time of 20 minutes and an initial xylose concentration of 1.25 mol.L-1. The

addition of a salt, NaCl, permitted to reduce the amount of FeCl3 (10 mol% vs 20 mol%)

while slightly increasing the yield thanks to a double positive effect: (i) firstly by salting-

out furfural in the organic phase, and (ii) secondly by directly enhancing the furfural

selectivity and rate of formation. The catalytic system FeCl3/NaCl was a good alternative

to HCl by leading to similar results while limiting the risks of toxicity and corrosion. A

maximum furfural yield of 74% was obtained by using these optimised conditions:


140

Temperature: 170°C



Catalyst: FeCl3 (10 mol%)

Additive: NaCl (1.25 mmol, 1 eq)

Initial xylose concentration: 1.25 mol.L-1

Moreover, the aqueous phase containing the catalytic system FeCl3/NaCl could be

recycled up to 4 times without a loss of activity.

A second study under microwave irradiation focused on the use of a

heterogeneous catalyst, Nafion NR50, which is an ion-exchange resin grafted with

sulfonic groups. However, because of the formation of humins, Nafion pellets were

degraded and furfural yield was limited (59%). The addition of NaCl proved to be

efficient to avoid the degradation of pellets and to help increasing the furfural yield up to

80%. Temperature and reaction time played an important role on xylose dehydration.

The optimised conditions were:

Temperature: 170°C



Catalyst: Nafion NR50 (2 pellets)

Additive: NaCl (90 mg, 1.55 mmol)

Initial xylose concentration: 1 mol.L-1

However, because of the addition of NaCl, a cation exchange between the Na+ of

the salt and the H+ of the sulfonic acid groups grafted on Nafion NR50 occurred and HCl

was formed in the aqueous phase. Nevertheless, we showed that the dehydration

reactions were catalysed both by the HCl released and by sulfonic acid groups still

partially protonated grafted on the Nafion pellets.

The main drawback of the salt addition was the deactivation of the Nafion NR50

pellets. A furfural yield of only 21% was reached when the pellets were reused in a second

cycle. Nevertheless, a simple treatment in an acidic bath of HCl regenerated the sulfonic


141

acid groups of the Nafion pellets and allowed their reuse for three cycles without any loss

of activity. After the fourth cycle, a low decrease in furfural yield was observed.

Therefore, the first strategy under microwave irradiation led to the optimisation of

two efficient and recyclable methods for the dehydration of D-xylose in a water/CPME

medium. 74% of furfural was formed by using a combination of FeCl3 and NaCl and 80%

of furfural was obtained by performing the reaction with Nafion NR50 and NaCl. To

extend their potential, these two optimised methods were applied to the dehydration of

the homopolymer xylan (15). Xylan dehydration required a higher temperature and/or

longer reaction times than for pure D-xylose dehydration and lower furfural yields were

obtained: 52% of furfural was obtained after 20 minutes at 200°C with the catalytic

system FeCl3/NaCl and a treatment of xylan with Nafion NR50 and NaCl for 60 minutes

at 190°C led to the formation of 50% of furfural. These results were similar to the results

reported in literature from xylan.

The homogeneous method with FeCl3 was also applied to sugar units present in

hemicelluloses such as L-arabinose (16), D-glucose (17), D-galactose (18) and D-mannose

(19). Expected results were obtained. L-Arabinose dehydration led to the formation of

furfural in a moderate yield of 41%, its high stability making it more difficult to

dehydrate. The dehydration of the aldoses glucose, galactose and mannose gave limited

yields of 5-HMF (20) (< 10%), as showed in other studies.

The second strategy of this Chapter was to perform D-xylose dehydration in a flash

continuous flow reactor in sub- and supercritical water, without the addition of a catalyst.

Reactions were performed between 240 and 380°C at 250 bars with short residence

times (≤ 2 s). However, even if total xylose conversions were reached, very limited

furfural yields were obtained (< 15%). The formation of unidentified by-products

decreased drastically the selectivity towards furfural. Thus, using a flash continuous flow

reactor under high conditions of temperature and pressure was not a selective and

efficient solution to produce furfural.


142

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General conclusions

General conclusions

149

The aim of this PhD work was to develop green dehydration methods to form high

added value molecules. Two target molecules were selected: (i) phenylacetaldehyde,

which is used in perfume compositions, for the preparation of pharmaceuticals,

insecticides, etc, or as a chemical intermediate, and (ii) furfural, which can be used as a

selective solvent or as a platform molecule to produce a wide range of chemicals.

Phenylacetaldehyde can be obtained by the dehydration 1-phenylethane-1,2-diol and

furfural is formed thanks to the triple dehydration of D-xylose, the main sugar unit of

hemicellulose. The optimisation of green production processes was focused on several

key points: solvents, activation method and catalysis.

Thanks to its properties, subcritical water proved to be a suitable medium for the

dehydration of polyhydroxylated compounds. However, in both cases, the addition of a

co-solvent was necessary in order to limit the formation of by-products. The eco-friendly

solvent, cyclopentyl methyl ether (CPME), was selected after having compared several

potential co-solvents. The selectivity towards the target molecules doubled when using

CPME as an extracting solvent.

For the two target molecules, microwave irradiation was found to be a good

alternative to thermal heating. The fast and selective heating rate of microwaves helped

us to optimise short reaction times (< 60 minutes) while increasing the selectivity

towards the desired compounds.

In parallel, as an alternative to batch reactions, the dehydration of D-xylose was

also tested in a flash continuous flow reactor in sub- and supercritical water. However,

even if total xylose conversions were reached, very limited furfural yields (< 15%) were

obtained because of the formation of unidentified by-products.

Concerning the catalysts used, the aim was to avoid the use of classic mineral

acids. However, for the dehydration of 1-phenylethan-1,2-diol, sulphuric and hydro-

chloric acids gave the best yields among all the catalysts tested. Nevertheless, the Lewis

acid FeCl3 seemed to be a good alternative, leading to a phenylacetaldehyde yield in the

same order of magnitude. Besides, we showed that the aqueous phase can be reused for

five consecutive reactions without a loss of activity.

For the formation of furfural, two methods were optimised with either

homogeneous or heterogeneous catalysts. First, homogeneous catalysts were compared

and the Lewis acid FeCl3 proved to be the more efficient one. The addition of a salt, NaCl,

permitted to reduce the amount of FeCl3 (10 mol% vs 20 mol%) while slightly increasing

General conclusions

150

the yield. The catalytic system FeCl3/NaCl was a good alternative to HCl by leading to

similar results (best yield: 74%). A second study under microwave irradiation focused on

the use of a heterogeneous catalyst, the ion-exchange resin Nafion NR50. The addition of

NaCl was necessary to avoid the degradation of pellets caused by humin deposition and

to help increasing the furfural yield (up to 80%). The Nafion NR50 pellets can be reused

for three cycles without any loss of activity after a simple treatment in an acidic bath to

regenerate the sulfonic acid groups of the resin.

Thus, three green dehydration methods were optimised during this PhD work,

leading to good yields of the target molecules, phenylacetaldehyde (1) and furfural (13):

These studies were later extended to alcohols of structure similar to 1-

phenylethane-1,2-diol in the first case, and to xylan, homopolymer of D-xylose, and other

sugar units present in hemicelluloses for the second case.

Several prospects can be suggested in order to continue the work undertaken in

this PhD. For the two target molecules, additional research can be done to find

alternative catalysts, preferably heterogeneous ones. In many cases, finding an active and

stable water-tolerant solid catalyst is still a challenge. A second prospect could be the

adaptation of the optimised methods to continuous flow reactor, and in particular to

microwave continuous flow reactors. Finally, in the case of furfural, the application of the

methods to raw biomass could permit to extend their potential.

Scientific communications

Scientific communications

153

This PhD work led to several scientific communications:

Articles in peer-reviewed journals:

- S. Le Guenic, C. Ceballos, C. Len, Catal. Letters. 2015, 145, 1851–1855.

doi:10.1007/s10562-015-1606-4

- S. Le Guenic, F. Delbecq, C. Ceballos, C. Len, J. Mol. Catal. A Chem. 2015, 410, 1–7.

doi:10.1016/j.molcata.2015.08.019

- S. Le Guenic, C. Ceballos, C. Len, J. Mol. Catal. A Chem. 2016, 411, 72-77.

doi:10.1016/j.molcata.2015.10.001

- S. Le Guenic, D. Gergela, C. Ceballos, F. Delbecq, C. Len, Submitted article.

Oral communications in conferences:

- S. Le Guenic, C. Ceballos, C. Len. Etude de la déshydratation d'alcools biosourcés

dans l'eau sub-critique. Colloque Recherche de la Fédération Gay-Lussac "La chimie

et la ville de demain", Paris, December 2013.

- S. Le Guenic, C. Ceballos, C. Len. Dehydration of Polyols in Aqueous Media.

International Symposium of Green Chemistry (ISGC), La Rochelle, May 2015.

Posters in conferences:

- S. Le Guenic, C. Ceballos, C. Len. Towards the synthesis of phenylacetaldehyde in

subcritical water. CABiomass-II, Compiègne, March 2014. (Poster Award)

- S. Le Guenic, C. Ceballos, C. Len. Dehydration of Polyols in Aqueous Media.

International Symposium of Green Chemistry (ISGC), La Rochelle, May 2015.

Résumé

Résumé

157

Depuis quelques dizaines d’années, l’environnement est au centre des

préoccupations politiques, économiques et industrielles. Au début des années 1990,

l’agence américaine pour la protection de l’environnement (U.S. Environmental

Protection Agency) lança la première initiative de recherche en chimie verte. L’objectif

est de donner un cadre à la prévention de la pollution liée aux activités chimiques.

Quelques années plus tard, le concept de Chimie Verte fut développé par les

chimistes américains Anastas et Warner. La Chimie Verte a pour but de concevoir des

produits et des procédés chimiques permettant de réduire ou d'éliminer l'utilisation et la

synthèse de substances dangereuses. Cette définition a été précisée en 1998 par

l’introduction de douze Principes qui ont contribué à l'émergence et à la diffusion du

concept de chimie verte :

1) Prévention

2) Economie

3) Synthèses chimiques moins nocives

4) Conception de produits chimiques plus sécuritaires

5) Solvants et auxiliaires plus sécuritaires

6) Amélioration du rendement énergétique

7) Utilisation de matières premières

8) Réduction de la quantité de produits

9) Catalyse

10) Conception de substances non-persistantes

11) Analyse en temps réel de la lutte contre la pollution

12) Chimie essentiellement sécuritaire afin de prévenir les accidents.

L’un des principaux enjeux actuels de l’industrie chimique est donc la synthèse de

molécules à haute valeur ajoutée grâce à des procédés respectant les principes de la

Chimie Verte. Le but de cette thèse est l’optimisation de méthodes de déshydratation

« vertes » dans le but de former deux molécules cibles : le phénylacétaldéhyde et le

furfural. Plusieurs points-clés ont été identifiés pour concevoir des procédés plus verts

que les procédés actuels : le solvant, la méthode d’activation et le catalyseur.

Résumé

158

Dans un premier Chapitre, le contexte de cette thèse a été décrit en développant le

concept de Chimie Verte et les outils associés. Les différents types de solvants verts

existants (eau, fluides supercritiques, liquides ioniques, biosolvants, etc) ont été décrits.

Les propriétés et avantages de l’eau sub- et supercritique ont été développés en

particulier puisque ce solvant a été choisi comme milieu réactionnel pour les réactions de

déshydratation. Puis, le principe des différentes méthodes d’activation alternatives au

chauffage thermique, telles que la photochimie, l’électrochimie, la méchanochimie ou la

sonochimie, a été décrit. La synthèse assistée par micro-ondes a été plus largement

détaillée. Enfin, un état de l’art de la déshydratation de composés polyhydroxylés dans

l’eau a été réalisé. Plusieurs types de catalyseurs ont été identifiés. Tout d’abord, en

catalyse homogène, la déshydratation de polyols et de sucres peut être réalisée à l’aide

d’acides minéraux (HCl, H2SO4) ou organiques (acides maléique ou acétique par ex.), de

sels de métaux solubles (chlorures de métaux, sulfates de métaux) ou de dioxyde de

carbone. Ensuite, les composés polyhydroxylés peuvent être déshydratés par des

catalyseurs hétérogènes tels que des silicates et aluminosilicates modifiées (zéolithe H-

ZSM-5 par exemple), des oxydes mixtes (TiO2/WO3, ZrO2, etc), des résines échangeuses

d’ions (Amberlyst 70 ou Nafion SAC-13) ou des phosphates de métaux solides (phosphate

de bore par exemple).

Le Chapitre II s’est intéressé à la synthèse du phénylacétaldéhyde par déshydratation

du 1-phényléthane-1,2-diol. Le phénylacétaldéhyde est une molécule qui entre dans la

composition de parfums, dans la préparation de composés pharmaceutiques,

d’insecticides, etc, ou qui est utilisée en tant qu’intermédiaire réactionnel. C’est une

molécule très réactive dont la formation peut être suivie par une condensation aldolique

qui génère un produit secondaire, le 2,4-diphénylbut-2-énal. Le but de cette étude était

de produire du phénylacétaldéhyde de manière efficace et sélective par chauffage

thermique ou par irradiation micro-ondes.

D’après nos expériences, le chauffage thermique s’est révélé ne pas être la meilleure

méthode d’activation. La lente vitesse de réaction due à l’inertie thermique du réacteur

associée aux longs temps de réaction ont permis au phénylacétaldéhyde de réagir sur lui-

même pour former le produit secondaire. De faibles rendements et sélectivités en

phénylacétaldéhyde ont été obtenus. Par exemple, seulement 9% d’aldéhyde ont été

obtenus après 16h à 200°C alors que le rendement en produit de condensation a atteint

42%. De plus, à cause de l’alliage du réacteur, l’utilisation de catalyseurs était limitée afin

d’éviter sa corrosion.

Résumé

159

L’activation par irradiation micro-ondes nous a donc apparu comme une alternative

intéressante au chauffage thermique. Parmi les nombreux avantages de l’utilisation de

micro-ondes, la vitesse de chauffage rapide et les rendements et sélectivités élevés

généralement obtenus nous permettront d’améliorer le rendement en phényl-

acétaldéhyde tout en limitant la formation du produit secondaire.

Tout d’abord, l’optimisation de la formation de phénylacétaldéhyde sous irradiation

micro-ondes a été réalisée dans l’eau. La température, le temps de réaction et la nature

du catalyseur ont été variés. Les meilleures conditions se sont révélées être une

température de 170°C pour une durée de 30 minutes. Les acides minéraux HCl et H2SO4

ont mené aux rendements en phénylacétaldéhyde les plus élevés (63 et 62%

respectivement). Néanmoins, les acides de Lewis AlCl3 et FeCl3 ont prouvé être une

bonne alternative à ces acides corrosifs puisqu’ils mènent à des rendements similaires.

Cependant, dans l’eau, la sélectivité envers le phénylacétaldéhyde reste inférieure à

70% à cause de la formation du produit de condensation aldolique. Pour résoudre ce

problème, il a été décidé d’introduire un co-solvant, une stratégie déjà utilisée pour la

déshydratation des monosaccharides. Puisque le 1-phényléthane-1,2-diol et le catalyseur

ont une meilleure affinité avec l’eau, la réaction de déshydratation aura lieu dans la phase

aqueuse. Puis, le phénylacétaldéhyde formé sera transféré dans la phase organique et ne

pourra plus réagir sur lui-même dans des réactions successives. Dans un premier temps,

le co-solvant, le cyclopentyl méthyl éther (CPME), a été sélectionné, puis les proportions

entre la phase aqueuse et la phase organique ont été variées. L’introduction du CPME a

prouvé son efficacité. En effet, par comparaison avec la méthode dans l’eau seule, cela

nous a permis d’augmenter le rendement en phénylacétaldéhyde de 63% jusqu’à 97%

tout en limitant la formation du produit de condensation aldolique, le 2,4-diphénylbut-2-

énal, à 1% maximum. Les conditions opératoires optimisées sous irradiation micro-ondes

sont les suivantes :

Température : 170°C

Durée de réaction : 20 minutes

Milieu biphasique: eau/CPME, 1:3, v/v

Catalyseurs: FeCl3, HCl, H2SO4

Quantité de catalyseur: 20 mol%

Résumé

160

Les meilleurs résultats ont été obtenus avec les acides minéraux (97% de

rendement avec H2SO4, 93% avec HCl) mais FeCl3 est apparu être une bonne alternative

avec 84% de rendement.

En plus d’une augmentation de la sélectivité, un avantage de notre système repose

sur une séparation facilitée de deux phases. Après réaction, la phase organique peut être

traitée pour récupérer l’aldéhyde, et la phase aqueuse contenant le catalyseur peut être

réutilisée dans un second cycle réactionnel. La recyclabilité de la phase aqueuse a été

confirmée en réalisant 5 réactions successives. La phase aqueuse contenant le catalyseur

a été réutilisable pendant 5 cycles même si une diminution progressive de l’activité a été

observée pour les catalyseurs FeCl3 et HCl. Aucune variation de rendement n’a été

observée avec H2SO4.

Dans le but d’exemplifier nos méthodes optimisées, plusieurs alcools de structure

similaire à celle du 1-phényléthane-1,2-diol ont été soumis à des réactions de

déshydratation dans des milieux biphasique et monophasique. La déshydratation du 1-

phényléthanol et du 2-phényléthanol vers le styrène, du 1-phénylpropan-1-ol vers le 1-

phénylpropène et de l’hydrobenzoïne vers le diphénylacétaldéhyde (le réarrangement

pinacolique était favorisé) ont été comparées. A part le 2-phényléthanol qui n’était pas

réactif, tous les alcools testés ont donné les produits attendus avec des rendements

modérés à élevés. En général, la conversion des alcools était élevée dans l’eau seule mais

les rendements et sélectivités des produits cibles étaient plus élevées en milieu

biphasique. Par conséquent, comme pour le 1-phényléthane-1,2-diol, la présence d’un co-

solvant s’est révélée nécessaire pour éviter la formation de produits secondaires.

Finalement, l’application de la méthode de déshydratation optimisée à l’oxyde de

styrène comme substrat alternatif a produit l’aldéhyde cible avec des rendements

comparables à ceux obtenus à partir de 1-phényléthane-1,2-diol : 86, 95 et 94% de

phénylacétaldéhyde ont été produits avec FeCl3, HCl et H2SO4 respectivement.

Le Chapitre III a été consacré à l’étude de la formation du furfural, composé à

haute valeur ajoutée qui peut être utilisé pour produire une large gamme de produits

chimiques et de carburants, tels que l’alcool furfurylique, le méthyltétrahydrofurane ou

l’acide lévulinique par exemple. C’est l’un des quelques composés chimiques qui sont

actuellement produits industriellement via un procédé biosourcé puisqu’il n’existe pas de

voie de synthèse pour sa production directe. Le furfural est obtenu par la triple

Résumé

161

déshydratation du D-xylose, le principal composant des biomasses riches en

hémicellulose. Cependant, les procédés industriels actuels sont basés sur l’utilisation

d’acides minéraux (principalement H2SO4) dans des réacteurs batch ou continu à des

températures modérées (< 200°C). Une formation limitée de furfural (≈ 50%) est

généralement obtenu à cause de la formation de sous-produits. De plus, l’utilisation

d’acides minéraux pose le problème de récupération du catalyseur après la réaction ou

celui de la corrosion des réacteurs.

Dans le but de concevoir de nouvelles méthodes de production du furfural, deux

stratégies ont été exploitées : (i) tout d’abord, l’utilisation de micro-ondes comme

alternative au chauffage thermique, et (ii) l’utilisation d’un réacteur en flux continu à

haute pression et haute température. Du D-xylose pur a été choisi comme molécule

modèle pour optimiser les conditions réactionnelles mais les études ont aussi été

étendues au xylane, homopolymère du xylose, et à d’autres sucres présents dans les

hémicelluloses.

La déshydratation du D-xylose par irradiation micro-ondes a été basée sur la

méthode optimisée durant le Chapitre II sur le 1-phényléthane-1,2-diol. Les réactions ont

été réalisées dans un milieu biphasique eau/CPME avec un ratio 1:3, v/v. Le CPME a

prouvé être nécessaire pour obtenir de bonnes conversions en xylose et de bons

rendements en furfural.

Dans la première étude, plusieurs catalyseurs homogènes ont été comparés et

l’acide de Lewis FeCl3 a montré l’activité la plus importante. La température, le temps de

réaction et la concentration en substrat ont été variés. Les meilleures conditions de

réactions sont une température de 170°C pour un temps de réaction de 20 minutes et une

concentration initiale de 1,25 mol.L-1. L’addition d’un sel, NaCl, a permis de réduire la

quantité de FeCl3 (10 mol% contre 20 mol%) tout en augmentant légèrement le

rendement grâce à un double effet positif : (i) d’abord en relargant le furfural dans la

phase organique, et (ii) ensuite, en augmentant directement la sélectivité et la vitesse de

formation du furfural. Le système catalytique FeCl3/NaCl a montré être une bonne

alternative à HCl en menant à des résultats similaires tout en limitant les risques de

toxicité et de corrosion. Un rendement maximal de 74% a été obtenu en utilisant les

conditions optimisées décrites ci-dessous. De plus, la phase aqueuse contenant le

système catalytique a pu être recyclée jusqu’à 4 fois sans perte d’activité.

Résumé

162



Milieu biphasique : eau/CPME, 1:3, v/v

Catalyseur : FeCl3 (10 mol%)

Additif : NaCl (1.25 mmol, 1 eq)

Concentration initiale en xylose : 1.25 mol.L-1

Une deuxième étude sous irradiation micro-ondes a porté sur l’utilisation d’un

catalyseur hétérogène, Nafion NR50, qui est une résine échangeuse d’ions greffée avec

des groupements acides sulfoniques. Cependant, à cause de la formation d’humines, les

billes de Nafion ont été dégradées et le rendement en furfural était limité (59%). L’ajout

de NaCl a permis d’éviter la dégradation des billes et d’augmenter le rendement en

furfural jusqu’à 80%. La température et le temps de réaction ont joué un rôle important

sur la déshydratation du xylose : 170°C et 40 minutes de réaction ont été les meilleures

conditions. Les conditions optimisées sont les suivantes :



Milieu biphasique : eau/CPME, 1:3, v/v

Catalyseur: Nafion NR50 (2 pellets)

Additif: NaCl (90 mg, 1.55 mmol)

Concentration initiale en xylose: 1 mol.L-1

Cependant, à cause de l’addition de NaCl, un échange de cations a eu lieu entre les

Na+ du sel et les H+ des acides sulfoniques greffés sur le Nafion NR50 et du HCl a été

formé dans la phase aqueuse. Néanmoins, il a été montré que la déshydratation du xylose

était catalysée à la fois par le HCl relargué et par les groupements acides sulfoniques

toujours partiellement protonés. L’inconvénient principal de l’addition de sel a été la

désactivation des billes de Nafion NR50. Un rendement en furfural de seulement 21% a

été obtenu quand les billes étaient réutilisées dans un second cycle. Néanmoins, un

simple traitement dans un bain d’HCl a permis de régénérer les groupements acides sur

les billes de Nafion et a permis leur réutilisation pour trois cycles sans perte d’activité.

Résumé

163

Pour étendre leur potentiel, les deux méthodes optimisées sous irradiation micro-

ondes ont été appliquées à la déshydratation du xylane, homopolymère du xylose. La

déshydratation du xylane a nécessité des températures plus élevées et/ou des temps de

réaction plus longs que pour le xylose pur, et des rendements plus faibles en furfural ont

été obtenus : 52% de furfural ont été formés après 20 minutes à 200°C avec le système

catalytique FeCl3/NaCl et un traitement du xylane avec du Nafion NR50 et du NaCl

pendant 60 minutes à 190°C a mené à la formation de 50% de furfural. Ces résultats sont

similaires avec ceux reportés dans la littérature.

La méthode par catalyse homogène avec FeCl3 a aussi été appliquée à d’autres

monosaccharides que l’on retrouve dans les hémicelluloses tels que le L-arabinose, le D-

glucose, le D-galactose et le D-mannose. La déshydratation du L-arabinose a mené à la

formation de furfural avec un rendement modéré de 41%, sa stabilité élevée le rendant

moins réactif. La déshydratation des aldoses glucose, galactose et mannose a donné des

rendements limités en 5-HMF (< 10%), suggérant une mauvaise efficacité du CPME.

La seconde stratégie de ce Chapitre était de réaliser la déshydratation du D-xylose

dans un réacteur en flux continu dans l’eau sub- et supercritique sans l’ajout d’un

catalyseur. Les réactions ont été réalisées entre 240 et 380°C à 250 bars pendant des

temps de résidence très courts (< 2 s). Cependant, même si des conversions totales en

xylose ont été atteintes, des rendements très limités en furfural ont été obtenus (< 15%).

La formation de sous-produits non identifiés a entraîné une diminution très importante

de la sélectivité envers le furfural. Par conséquent, l’utilisation d’un réacteur en flux

continu sous des conditions élevées de température et de pression s’est révélée ne pas

être une solution sélective et efficace pour produire du furfural.

Experimental section


167

1. General

a. Chemical products and solvents

Reactants and catalysts were purchased from Acros Organics, Sigma-Aldrich,

Normapur and Carlo Erba. Solvents were purchased from Fisher Scientific and Acros

Organics. All materials were used without further purification. Distilled H2O was used for

preparation of all aqueous solutions.

b. Chromatography

i. TLC

Thin-Layer Chromatography (TLC) was performed on silica gel 60 F254 plates

(Merck). Compounds were eluted with a mixture of ethyl acetate and cyclohexane with

variable proportions and revealed under UV light (λ=254 nm).

ii. Flash-chromatography

Purifications were performed using a flash chromatography system (GRACE,

Reverleris X2) and pre-packed columns (GRACE, Reverleris, Silica 40 µm, 12 g).

Compounds were eluted with a mixture of ethyl acetate and cyclohexane with variable

proportions and revealed under UV light (variable λ) and with an ELSD (Evaporative

Light Scattering) detector.

iii. HPLC

All reactions were monitored by using High-Performance Liquid Chromatography

(HPLC). Two systems were used: the majority of the reactions were analyzed by means of

a Shimadzu HPLC system, and the reactions performed in Aalto University (Chapter III,

part 5.) were analyzed thanks a Dionex Ultimate 3000 HPLC system.


168

The Shimadzu HPLC system was equipped with a Grace Prevail C18 column (250 x

4.6 mm, 5µm), a UV-Vis detector (SDP-M20A, Shimadzu) and a low temperature

evaporative light scattering detector (ELSD-LTII, Shimadzu). The column oven was

set at 40°C. The mobile phase was a mixture of water and MeOH, with a ratio and a

flow varying according to the products analysed.

The Dionex Ultimate 3000 HPLC system was equipped with a HyperRez XP

Carbohydrate Ca2+ column (300 X 7.7 mm) and UV and RI detectors. Sulfuric acid

(0.005 M, 0.8 mL/min) served as eluent. The column was heated at 70°C.

In both cases, reactant and product concentrations were determined using

calibration curves obtained from references samples (synthesised or commercial).

Example of calibration curve: phenylacetaldehyde (1)

HPLC: MeOH/H2O 80/20, 0.5 mL/min, λ=205 nm, tR=7.6 min.

Figure 51: Calibration curve of phenylacetaldehyde, HPLC.


169

c. NMR

1H and 13C NMR spectra and NOESY experiments were recorded on a 400 MHz

Bruker UltraShield 400 MHz/54 mm Ultra long hold. Chemical shifts (d) are quoted in

ppm and are referenced to deuterated chloroform CDCl3 (Carlo-Erba) as an internal

standard (7.26 ppm). Coupling constants (J) are quoted in Hz. The following

abbreviations were used to describe the spectra obtained:

s: singulet

d: doublet

t: triplet

m: multiplet

dd: doublet of doublet

dq: doublet of quadruplet

d. SEM-EDX

SEM–EDX analysis of Nafion pellets was performed on a Quanta FEG 250 (FEI)

equipped with a microanalysis detector for EDX (Brucker). SEM micrographs acquired in

secondary electron mode were obtained at low vacuum, 15 kV of accelerating voltage with

a 10 mm working distance. EDX spectra were collected at 30° angle, 15 kV accelerating

voltage and 10 mm working distance.

e. ATR-FTIR

ATR-FTIR analyses were realised on a Fourier Transform Infrared Spectrometer

(Jasco, FT/IR-400) with an ATR module (Specac, Silver Gate, Ge crystal). Spectra were

acquired for 16 scans between 700 and 4000 cm-1 with a resolution of 4 cm-1.


170

2. Chapter II – Dehydration of 1-phenylethane-1,2-diol

a. Thermal heating

i. Equipment

The experiments were performed in a stirred batch reactor

composed of 316 stainless steel (Parr), with an inner volume of

100 mL (Figure 52). The reactor was heated with an electric

heating collar controlled by a temperature sensor. The pressure

during the reaction was the autogenous pressure of the solution

at reaction temperature.

Figure 52: Autoclave Parr used for the experiments.

ii. General procedure

In a typical experiment, the batch reactor was charged with 25 mmol of 1-

phenylethane-1,2-diol (3.45 g) and 50 mL of water. The reactor was sealed and heated to

the desired temperature under magnetic stirring. After the desired time, the heating was

stopped and the reactor was cooled down to room temperature. The reaction mixture was

extracted three times with ethyl acetate. Then, organic phases were gathered, dried and

the solvent was evaporated by means of a rotary evaporator to a volume of 50 mL. 2 mL

of this solution were taken and diluted with acetonitrile in a 100 mL flask. An aliquot of

the diluted solution was taken (ca. 1.5 mL) and analysed.


171

b. Microwave reactor

i. Equipment

The experiments were performed in a monomode microwave apparatus

AntonPaar Monowave 300 (Figure 53). The vessels used were made of glass and had a

volume of 10 mL. They were sealed with a septum prior to reaction. The program of the

microwave was set as follows: (i) to heat as fast as it can to reach the wanted

temperature, (ii) to keep the temperature for the time wanted and (iii) to cool down the

reaction vessel to 40°C thanks to compressed air.

Temperature in the vessel was measured by

means of an IR sensor. The pressure during the reaction

was the autogenous pressure of the solution at reaction

temperature (generally between 10 and 20 bars) and

was measured by a pressure sensor. At the end of a

reaction, profiles of temperature, pressure and power

measured during the reaction can be obtained by means

of the apparatus software (Figure 54).

Figure 53: Microwave apparatus.

Figure 54: Example of power, temperature and pressure profiles during a reaction.


172

ii. Dehydration of 1-phenylethane-1,2-diol

General procedure for the dehydration of 1-phenylethane-1,2-diol (2) in water:

In a typical experiment, a 10 mL glass vessel was charged with an aqueous

solution (2 g) of 1-phenylethane-1,2-diol (5 wt%, 0.725 mmol) and a catalyst (20 mol%).

The vessel was sealed with a septum, placed in the microwave apparatus (AntonPaar

Monowave 300) and heated to the desired temperature under magnetic stirring (600

rpm) for the desired time. At the end of the reaction, the vessel was cooled down to 40°C.

Then, the reaction mixture was diluted in 100 mL of acetonitrile. An aliquot of the

diluted solution was taken (ca. 1.5 mL) and filtered prior to analysis through a syringe

filter (PTFE, 0.45 µm, VWR).

General procedure for the dehydration of 1-phenylethane-1,2-diol (2) in

biphasic medium:

In a typical experiment, a 10 mL glass vessel was charged with water (0.5 mL),

CPME (1.4 mL), 1-phenylethane-1,2-diol (100 mg, 0.725 mmol) and a catalyst (20 mol%).

The vessel was sealed with a septum, placed in the microwave apparatus (AntonPaar



Then, the reaction mixture was diluted in 100 mL of acetonitrile. An aliquot of the

diluted solution was taken (ca. 1.5 mL) and filtered prior to analysis through a syringe

filter (PTFE, 0.45 µm, VWR).


173

Recycling of the aqueous phase:

The first run of the recycling tests was performed as described in the general

procedure for the dehydration of 1-phenylethane-1,2-diol in biphasic medium. At the end

of the first run, the two phases were separated by using a syringe. The organic phase was

diluted in 100 mL of acetonitrile. An aliquot of the diluted solution was taken and filtered

prior to analysis through a syringe filter (PTFE, 0.45 µm, VWR). The aqueous phase was

placed in another glass vessel, 100 mg of 1-phenylethane-1,2-diol and 1,4 mL of CPME

were added and the reaction was performed like for the first run. These steps were

repeated 4 times. At the end of the fifth cycle, the aqueous phase was also diluted in 100

mL of acetonitrile. An aliquot of the diluted solution was taken and filtered prior to

analysis through a syringe filter (PTFE, 0.45 µm, VWR).

iii. Applications to other substrates

General procedure for the dehydration of other substrates in biphasic medium:

- 1-phenylethanol (4)

- 2-phenylethanol (5)

- 1-phenylpropan-1-ol (7)

- hydrobenzoin (9)

- styrene oxide (12)

In a typical experiment, a 10 mL glass vessel was charged with water (0.5 mL),

CPME (1.4 mL), the reactant (0.725 mmol) and a catalyst (20 mol%). The vessel was

sealed with a septum, placed in the microwave apparatus (AntonPaar Monowave 300)

and heated to the desired temperature under magnetic stirring (600 rpm) for the desired

time. At the end of the reaction, the vessel was cooled down to 40°C. Then, the reaction

mixture was diluted in 100 mL of acetonitrile. An aliquot of the diluted solution was

taken (ca. 1.5 mL) and filtered prior to analysis through a syringe filter (PTFE, 0.45 µm,

VWR).


174

c. Identification of reaction products

Phenylacetaldehyde (1)

Raw formula: C8H8O, M=120.15 g/mol.

Aspect: White solid.

1H NMR (CDCl3, 400 MHz): δ (ppm) 3.70 (d, J = 2.4 Hz, 2 H, H2), 7.18-7.41 (m, 5 H, H4-

H8), 9.75 (t, J = 2.4 Hz, 1 H, H1).

13C NMR (CDCl3, 400 MHz): δ (ppm) 50.5, 127.4, 129.0, 129.6, 131.8, 199.5.


(E)-2,4-Diphenylbut-2-enal (3)


Aspect: Orange viscous liquid.

1H NMR (CDCl3, 400 MHz): δ (ppm) 3.71 (d, J = 7.6 Hz, 2 H, H4), 6.88 (t, J = 7.6 Hz, 1 H,

H3), 7.18-7.48 (m, 10 H, H6-H10 and H12-H16), 9.67 (s, 1 H, H1).

13C NMR (CDCl3, 400 MHz): δ (ppm) 35.8, 126.8, 128.2, 128.4, 128.4, 128.8, 129.4, 132.1,

138.02, 144.1, 153.5, 193.5.

NOESY spectrum in Appendices.



175

Styrene (6)

Raw formula: C8H8, M=104.15 g/mol.

Not isolated, commercial standard used for the calibration curve.


(E)-1-Phenylpropene (8)

Raw formula: C9H10, M=118.18 g/mol.

Aspect: Liquid slightly yellow.

1H NMR (CDCl3, 400 MHz): δ (ppm) 1.90 (dd, J1-3 = 1.6 Hz, J1-2 = 6.8 Hz, 3 H, H1), 6.25

(dq, J1-2 = 6.8 Hz, J2-3 = 16 Hz, 1 H, H2), 6.42 (dd, J1-3 = 1.6 Hz, J2-3 = 16 Hz, 1 H, H3),

7.18-7.35 (m, 5 H, H5-H9).

HPLC: MeOH/H2O 90/10, 0.5 mL/min, λ= 204 nm, tR=10.70 min.

Deoxybenzoin (10)





176

Diphenylacetaldehyde (11)





177

3. Chapter III - Dehydration of D-xylose

a. Microwave reactor

i. Equipment

Like in Chapter II, the experiments were performed in a monomode microwave

apparatus AntonPaar Monowave 300. The vessels used were made of glass and had a

volume of 10 mL. They were sealed with a septum prior to reaction. The program of the

microwave was set as follows: (i) to heat as fast as it can to reach the wanted

temperature, (ii) to keep the temperature for the time wanted and (iii) to cool down the

reaction vessel to 40°C thanks to compressed air.

Temperature in the vessel was measured by means of an IR sensor. The pressure

during the reaction was the autogenous pressure of the solution at reaction temperature

(generally between 10 and 20 bars) and was measured by a pressure sensor.

ii. General procedures for homogeneous catalysis

General procedure for the dehydration of D-xylose (14) in water:

In a typical experiment, a 10 mL glass vessel was charged with water (4 mL), D-

xylose (187.5 mg, 1.25 mmol) and a catalyst (20 mol%). The vessel was sealed with a

septum, placed in the microwave apparatus (AntonPaar Monowave 300) and heated to

the desired temperature under magnetic stirring (600 rpm) for the desired time. At the

end of the reaction, the vessel was cooled down to 40°C. Then, the reaction mixture was

diluted in 100 mL of acetonitrile and filtered prior to analysis through a syringe filter

(PTFE, 0.45 µm, VWR).


178

General procedure for the dehydration of D-xylose (14) in water/CPME medium:

In a typical experiment, a 10 mL glass vessel was charged with water (1 mL),

CPME (3 mL), D-xylose (187.5 mg, 1.25 mmol) and a catalyst (20 mol%). In some

experiments, NaCl was also added to the vial (from 0.62 to 2.5 mmol). The vessel was

sealed with a septum, placed in the microwave apparatus (AntonPaar Monowave 300)

and heated to the desired temperature under magnetic stirring (600 rpm) for the desired

time. At the end of the reaction, the vessel was cooled down to 40°C. Then, the two

phases were separated. The aqueous phase was diluted in 200 mL of distilled water and

filtered prior to analysis through a filter paper (10-20 µm, VWR). The organic phase was

diluted in 200 mL of acetonitrile and filtered prior to analysis through a syringe filter

(PTFE, 0.45 µm, VWR).

Recycling of the aqueous phase:

The first run of the recycling tests was performed as described in the general

procedure for the dehydration of D-xylose in biphasic medium. At the end of the first run,

the two phases were separated by using a syringe. The organic phase was diluted in 200

mL of acetonitrile. An aliquot of the diluted solution was taken and filtered prior to

analysis through a syringe filter (PTFE, 0.45 µm, VWR). The aqueous phase was placed

in another glass vessel, 187.5 mg of D-xylose and 3 mL of CPME were added and the

reaction was performed like for the first run. These steps were repeated 4 times. At the

end of the fifth cycle, the aqueous phase was also diluted in 200 mL of distilled water and

filtered prior to analysis through a filter paper (10-20 µm, VWR).


179

General procedure for the dehydration of xylan (15) in water/CPME medium:


CPME (3 mL), xylan (187.5 mg, 1.25 mmol based on xylose units), FeCl3 (10 mol%) and

NaCl (72.5 mg, 1.25 mmol). The vessel was sealed with a septum, placed in the

microwave apparatus (AntonPaar Monowave 300) and heated to the desired temperature

under magnetic stirring (600 rpm) for the desired time. At the end of the reaction, the

vessel was cooled down to 40°C. Then, the two phases were separated. The aqueous

phase was diluted in 200 mL of distilled water and filtered prior to analysis through a

filter paper (10-20 µm, VWR). The organic phase was diluted in 200 mL of acetonitrile

and filtered prior to analysis through a syringe filter (PTFE, 0.45 µm, VWR).

General procedure for the dehydration of sugars in water/CPME medium:

- L-arabinose (16)

- D-glucose (17)

- D-galactose (18)

- D-mannose (19)


CPME (3 mL), sugar (1.25 mmol), FeCl3 (10 mol%) and NaCl (72.5 mg, 1.25 mmol). The

vessel was sealed with a septum, placed in the microwave apparatus (AntonPaar

Monowave 300) and heated to 170°C under magnetic stirring (600 rpm) for 20 min. At

the end of the reaction, the vessel was cooled down to 40°C. Then, the two phases were

separated. The aqueous phase was diluted in 200 mL of distilled water and filtered prior

to analysis through a filter paper (10-20 µm, VWR). The organic phase was diluted in

200 mL of acetonitrile and filtered prior to analysis through a syringe filter (PTFE, 0.45

µm, VWR).


180

iii. General procedures for heterogeneous catalysis

General procedure for the dehydration of D-xylose (14) in water:

In a typical experiment, a 10 mL glass vessel was charged with water (4 mL), D-

xylose (150 mg, 1 mmol), 2 pellets of Nafion NR50 and NaCl (90 mg, 1.55 mmol). The




Then, the two phases were separated. The aqueous phase was diluted in 200 mL of

distilled water and filtered prior to analysis through a filter paper (10-20 µm, VWR). The

organic phase was diluted in 200 mL of acetonitrile and filtered prior to analysis through

a syringe filter (PTFE, 0.45 µm, VWR).

General procedure for the dehydration of D-xylose (14) in water/CPME medium:


CPME (3 mL), D-xylose (150 mg, 1 mmol), 2 pellets of Nafion NR50 and NaCl (90 mg,

1.55 mmol). The vessel was sealed with a septum, placed in the microwave apparatus

(AntonPaar Monowave 300) and heated to the desired temperature under magnetic

stirring (600 rpm) for the desired time. At the end of the reaction, the vessel was cooled

down to 40°C. Then, the two phases were separated. The aqueous phase was diluted in

200 mL of distilled water and filtered prior to analysis through a filter paper (10-20 µm,

VWR). The organic phase was diluted in 200 mL of acetonitrile and filtered prior to

analysis through a syringe filter (PTFE, 0.45 µm, VWR).


181

Comparison with HCl:

A HCl solution (0,05 mol/L) was prepared by diluting 104 µL of HCl 37% in a 25

mL-flask. Then, a 10 mL glass vessel was charged with 1 mL of the HCl solution (0.05

mol/L), CPME (3 mL), D-xylose (150 mg, 1 mmol) and NaCl (90 mg, 1.55 mmol). The


Monowave 300) and heated to 170°C under magnetic stirring (600 rpm) for 60 min. At

the end of the reaction, the vessel was cooled down to 40°C. Then, the two phases were

separated. The aqueous phase was diluted in 200 mL of distilled water and filtered prior

to analysis through a filter paper (10-20 µm, VWR). The organic phase was diluted in

200 mL of acetonitrile and filtered prior to analysis through a syringe filter (PTFE, 0.45

µm, VWR).

General procedure for the dehydration of xylan (15) in water/CPME medium:


CPME (3 mL), xylan (187.5 mg, 1.25 mmol based on xylose units), 2 pellets of Nafion

NR50 and NaCl (90 mg, 1.55 mmol). The vessel was sealed with a septum, placed in the

microwave apparatus (AntonPaar Monowave 300) and heated to the desired temperature

under magnetic stirring (600 rpm) for 60 min. At the end of the reaction, the vessel was

cooled down to 40°C. Then, the two phases were separated. The aqueous phase was

diluted in 200 mL of distilled water and filtered prior to analysis through a filter paper

(10-20 µm, VWR). The organic phase was diluted in 200 mL of acetonitrile and filtered

prior to analysis through a syringe filter (PTFE, 0.45 µm, VWR).


182

iv. Identification of reaction products

Furfural (13)

Raw formula: C5H4O2, M=96.02 g/mol.



5-HMF (20)




b. Continuous flow reactor

i. Equipment

The bench scale reactor system (Figure 55) featured two feeding tanks, one for

xylose solution and another for pure water. The xylose solution tank was equipped with a

stirrer and an internal circulation pump. A high-pressure diagram pump (model Lewa

EK3) with three pump heads featuring independently-adjustable stroke lengths was used

to pump both water and xylose solution. All three lines were equipped with pulse

dampers. The vertical down-flow reactor was made of high-alloy stainless steel (SAE

designation type 4744) and had a volume of 390 µl. An air-loaded back-pressure

regulator (Tescom 26-1721-24A) was used to control the pressure.


183

Figure 55: a) Schematic and b) picture of the reactor system.

Pro

du

ct

Xyl

ose

so

luti

on

Wat

er

Reactor

Heating

Cooling

Xylose feeding tank

Water feeding tank

Back pressure regulator

Cooling unit Pump withthe three

head pumps

Pre-heater

Pumpcontrol unit

Temperaturecontrol unit

Reactor(protected by a metal box)

a)

b)


184

ii. Control of parameters

Reaction temperature was controlled by adjusting the temperature and the flow of

the heating water line according to enthalpy balance calculations (Figure 56). In this

equation, the values of the enthalpies of the xylose solution hS and the reaction medium

hR were known (TS=room temperature, and TR=temperature wanted for the reaction).

The relation between the xylose solution flow ṁS and the heating water flow ṁH was fixed

(φ=0.67 or 0.33). From this, the enthalpy of the heating water hH was calculated and thus

the temperature TH to set was determined.

ṁ ṁ ṁ ṁ

Figure 56: Enthalpy calculation for the determination of the temperature of the heating water TH.

ṁi (kg.s-1) and hi (J).

Likewise, the amount of quenching water ṁQ needed to reach a desired post-

quenching temperature TQ was determined by enthalpy balance calculations (Figure 57).

In this equation, the values of the three enthalpies were known thanks to the

temperatures (TR=temperature of the reaction, TQ=room temperature, and

TF=temperature after quenching, fixed). Thus, since the xylose solution flow ṁS and the

heating water flow ṁH were fixed, the value of ṁQ can be calculated.

ṁ ṁ ṁ (ṁ ṁ ṁ )

Figure 57: Enthalpy calculation for the determination of quenching water flow ṁQ.

ṁi (kg.s-1) and hi (J).

In all calculations, xylose solutions were assumed to behave like pure water and

to have perfect heat transfer. The densities and enthalpies of water as a function of

temperature and pressure were obtained from a web database.[211]

The residence time τ was controlled by setting the flow rates of heating water and

xylose solution according to the equation of Figure 58, where Vreactor is the volume of the

reactor part in m3, ṁ is the combined mass flow of xylose solution and heating water in

kg.s-1 (ṁ = ṁS + ṁH) and ρwater is the density of water at the reaction temperature and

pressure in kg.m-3.


185

ṁ

Figure 58: Equation for calculating the residence time τ (s) of the reactions.

Vreactor (m3), ṁ (kg.s-1), ρwater (kg.m-3)

iii. General procedure

Prior to start xylose feeding, the system was preheated up to the reaction

temperature using pure water. Then, xylose solution was poured into the xylose feeding

tank. The concentration in the feeding tank was 50 mmol.L-1. The xylose solution at room

temperature was pumped from the xylose feeding tank into the reactor, where it was

instantaneously heated up to reaction temperature by mixing it with pre-heated

supercritical water from the heating water line. The desired reaction temperature was

maintained via heating elements around the reactor body, which eliminated heat losses.

At the bottom of the reactor, the suspension was quenched down via quenching water

and then subsequently lowered to room temperature via a cooler unit. The reaction

medium was recovered and diluted with a dilution factor of 40 prior to analysis.

iv. Identification of reaction products

Furfural (13)



HPLC: Sulfuric acid (0.005 M), 0.8 mL/min, λ=280 nm, tR= 35.6 min.

APPENDICES

189


1. Introduction

NOESY spectrum of 2,4-diphenylbut-2-enal:

190

2. Dehydration of 1-phenylethane-1,2-diol under thermal heating

Values of Figure 12: Influence of temperature for a) 8h and b) 16h of reaction.

Autoclave, 1-phenylethane-1,2-diol concentration: 0.5 mol/L.

Entry t (h) T (°C) Diol (2)

conversion (%) Aldehyde (1)

yield (%) By-product (3)

yield (%)

1 8 160 25 13 0

2

180 33 9 20

3

200 64 10 32

4

220 100 8 33

5

240 100 12 19

6 16 160 23 12 5

7

180 56 12 22

8

200 95 9 42

9

220 100 9 19

10

240 100 13 13

Values of Figure 13: Influence of diol concentration.

Autoclave, T = 200°C, t = 16h.

Entry Concentration

(mol/L) Diol (2)



yield (%)

1 0.25 99 12 39

2 0.5 95 9 42

3 1 96 14 30

191

3. Dehydration of 1-phenylethane-1,2-diol under microwave irradiation

Values of Figure 14: Influence of temperature.

MW, t = 30 min, Catalyst: AlCl3 (20 mol%)

Entry T (°C) Diol (2)



yield (%)

1 130 5 0 0

2 140 6 4 0

3 150 16 13 0

4 160 44 34 0

5 170 80 55 3

6 180 100 45 22

7 190 100 39 28

8 200 100 31 27

Values of Figure 15: Influence of reaction time.

MW, T = 170°C, Catalyst: AlCl3 (20 mol%)

Entry t (min) Diol (2)



yield (%)

1 10 51 36 1

2 20 64 48 1

3 30 80 55 3

4 40 86 53 7

5 50 94 53 12

6 60 98 48 10

192

Values of Figure 17: Recyclability of the aqueous phase with a) FeCl3, b) HCl and c)

H2SO4. MW, T = 170°C, t = 20 min Catalyst: 20 mol%, Water/CPME , 1:3, v/v.

Entry Catalyst Run Diol (2)



yield (%)

1 FeCl3 1 96 84 1

2

2 95 87 0

3

3 91 82 0

4

4 86 76 0

5

5 81 71 0

6 HCl 1 98 93 1

7

2 97 91 0

8

3 95 89 0

9

4 93 83 0

10

5 91 83 0

11 H2SO4 1 98 97 0

12

2 98 98 0

13

3 97 96 0

14

4 98 97 0

15

5 97 96 0

Values of Figure 18: Influence of reaction time on the synthesis of

phenylacetaldehyde from styrene oxide.

MW, T = 170°C, Catalyst: H2SO4 (20 mol%), Water/CPME, 1:3, v/v.

Entry t (min) Styrene oxide (12)

conversion (%) Diol (2)

yield (%) Aldehyde (1)


yield (%)

1 5 100 24 68 0

2 10 100 9 86 0

3 15 100 4 92 0

4 20 100 2 94 0

193


2. Dehydration of D-xylose under microwave irradiation with

homogeneous catalysis

Values of Figure 27: Comparison between aqueous and biphasic media.


Entry Catalyst Solvent Xylose (14)

conversion (%) Furfural (13)

yield (%)

1 FeCl3 Water 54 33

2 HCl

52 34

3 H2SO4

49 32

4 FeCl3 Water/CPME (1:3) 97 69

5 HCl

99 72

6 H2SO4

96 62

Values of Figure 28: Efficacy of different catalysts on the dehydration of xylose.


Entry Catalyst Xylose (14)


yield (%)

1 - 11 3

2 LiCl 16 5

3 CuCl 35 22

4 CuCl2 86 55

5 CoCl2 42 28

6 ZnCl2 43 29

7 FeCl2 43 26

8 FeCl3 97 69

9 AlCl3 100 43

10 CrCl3 99 45

11 HCl 99 72

12 FeSO4 53 22

13 Fe2(SO4)3 65 42

14 H2SO4 96 62

15 H3PO4 32 21

194

Values of Figure 29: Effect of reaction temperature and time on a) xylose

conversion and b) furfural yield.

MW, Catalyst: FeCl3, 20 mol%, Water/CPME, 1:3, v/v.

Xylose (14) conversion (%)

Temperature (°C)

130 140 150 160 170 180

t (m

in)

5 5 15 28 38 69 97

10 6 23 33 55 88 99

15 8 26 41 70 94 99

20 18 31 50 84 97 99

30 22 37 63 92 99 99

45 19 45 79 98 98 99

60 31 50 82 99 100 99

Furfural (13) yield (%)

Temperature (°C)

130 140 150 160 170 180

t (m

in)

5 2 6 14 29 46 65

10 4 11 20 40 63 64

15 6 14 28 48 67 64

20 7 18 35 54 69 63

30 9 22 44 66 68 62

45 13 29 48 68 68 62

60 19 34 59 69 67 44

Values of Figure 30: Effect of the initial xylose concentration on xylose conversion

and furfural yield.

MW, T = 170°C, t = 20 min, Catalyst: FeCl3, 20 mol%, Water/CPME, 1:3, v/v.

Entry Cxylose (mol/L) Xylose (14)


yield (%)

1 0.25 54 39

2 0.5 91 51

3 0.75 94 67

4 1 97 69

5 1.25 96 69

6 1.5 99 68

7 2 100 67

8 3 100 51

9 4 100 34

195

Values of Figure 31: Effect of the combination FeCl3/NaCl on a) xylose conversion

and b) furfural yield.

MW, T = 170°C, t = 20 min, Catalyst: FeCl3, Water/CPME, 1:3, v/v.

Entry FeCl3

(mol%) NaCl

(mmol) Xylose (14)


yield (%)

1 1 - 10 6

2

0.62 33 18

3

1.25 37 23

4

2.50 46 32

5 5 - 49 33

6

0,5 59 46

7

1 68 52

8

2 80 60

9 10 - 82 59

10

0,5 94 70

11

1 100 74

12

2 100 62

13 15 - 93 68

14

0,5 97 72

15

1 100 71

16

2 100 68

17 20 - 97 69

18

0,5 100 69

19

1 100 70

20

2 100 62

Values of Figure 32: Reusability of the aqueous phase containing FeCl3 and NaCl for

the xylose dehydration. MW, T = 170°C, t = 20 min, Catalyst: FeCl3 (10 mol%) and

NaCl (1 eq), Water/CPME, 1:3, v/v.

Entry Run Furfural (13)

yield (%)

1 1 76

2 2 80

3 3 80

4 4 79

5 5 68

196

3. Dehydration of D-xylose under microwave irradiation with

heterogeneous catalysis

Values of Figure 35: Comparison between aqueous and biphasic media.

MW, T = 170°C, t = 60 min, Catalyst: Nafion NR50 (2 pellets), NaCl: 90 mg.

Entry Solvents Xylose (14)


yield (%)

1 Water 68 38

2 Water/CPME 1:3 100 79

Values of Figure 36: Effect of the addition of NaCl on xylose conversion and furfural

yield for a) 1 pellet and b) 2 pellets of Nafion NR50.

MW, T = 170°C, t = 60 min, Catalyst: Nafion NR50/NaCl, Water/CPME, 1:3, v/v.

Entry Nafion NR50

NaCl (mg) Xylose (14)


yield (%)

1 1 pellet 0 79 38

2

10 87 55

3

20 91 62

4

30 93 65

5

50 97 68

6

70 97 72

7

90 96 68

8 2 pellets 0 87 59

9

10 94 67

10

20 95 69

11

30 98 71

12

50 98 77

13

70 98 77

14

90 100 79

197

Values of Figure 37: Effect of the initial xylose concentration on xylose conversion

and furfural yield. MW, T = 170°C, t = 60 min, Catalyst: Nafion NR50 (2 pellets),

NaCl: 90 mg, Water/CPME, 1:3, v/v.

Entry Cxylose

(mol/L) Xylose (14)


yield (%)

1 0,67 92 68

2 1 100 79

3 1,33 100 76

4 1,67 100 75

5 2 100 72

6 2,33 100 74

7 2,67 100 73

8 3 100 72

9 3,33 100 72

10 4 100 62

Values of Figure 38: Effect of reaction time on furfural yield for different

temperatures. MW, Catalyst: Nafion NR50 (2 pellets), NaCl: 90 mg, Water/CPME,

1:3, v/v.

Furfural (13) yield (%)

Temperature (°C)

160 170 180

t (m

in)

5 7 29 35

10 18 44 68

15 37 54 73

20 50 64 74

25 53 70 73

30 55 76 71

35 57 78 69

40 58 80 68

45 60 80 68

50 64 79 68

55 67 79 68

60 72 79 68

198

Values of Figure 40: Study of the recyclability of the Nafion NR50 pellets after

regeneration steps in HCl.

MW, T = 170°C, t = 60 min, Catalyst: Nafion NR50 (2 pellets), NaCl: 90 mg,

Water/CPME, 1:3, v/v.

Entry Run Furfural (13)

yield (%)

1 1 79

2 2 81

3 3 84

4 4 65

EDX analyses of Nafion NR50 pellets:

a) Pristine pellet:

1 2 3 4 5 6 7 8 9 10keV

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5 cps/eV

C

O

F

S

1,00 * 1-

199

b) Pellet after a reaction without NaCl:

c) Pellet after a reaction with NaCl:

1 2 3 4 5 6 7 8 9 10keV

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

cps/eV

C

O

F

S

Al

Si

1,00 * 2-

1 2 3 4 5 6 7 8 9 10keV

0

1

2

3

4

5

6

7

8

cps/eV

C

O

F S

Cl

Na

1,00 * 3-

200

d) Pellet after a reaction with NaCl and a regeneration bath in HCl:

4. Application to xylan and hemicellulose sugar units

Values of Figure 41a: Xylose and furfural yield from xylan in function of reaction

time at 170°C.

MW, T = 170°C, Catalyst: FeCl3 (10 mol%) and NaCl (1 eq), Water/CPME, 1:3, v/v.

Entry t (min) Xylose (14)

yield (%) Furfural (13)

yield (%)

1 10 52 12

2 20 42 21

3 30 33 25

4 40 19 30

5 50 21 39

6 60 18 40

7 70 0 46

8 80 0 40

1 2 3 4 5 6 7 8 9 10keV

0.0

0.5

1.0

1.5

2.0

2.5

cps/eV

C

O

F

S

Cl Na

1,00 * 4-

201

Values of Figure 41b: Xylose and furfural yield from xylan in function of

temperature for a reaction time of 20 min.

MW, t = 20 min, Catalyst: FeCl3 (10 mol%) and NaCl (1 eq), Water/CPME, 1:3, v/v.

Entry T (°C) Xylose (14)


yield (%)

1 100 1 0

2 110 2 0

3 120 6 0

4 130 37 0

5 140 54 1

6 150 58 4

7 160 59 8

8 170 42 21

9 180 23 40

10 190 9 49

11 200 0 52

Values of Figure 42: Xylose and furfural yield from xylan as a function of

temperature for t=60 min. MW, t = 60 min, Catalyst: Nafion NR50 (2 pellets),

NaCl: 90 mg, Water/CPME, 1:3, v/v.

Entry T (°C) Xylose (14)


yield (%)

1 130 0 0

2 140 34 1

3 150 43 3

4 160 43 13

5 170 27 30

6 180 12 44

7 190 0 50

202

Values of Figure 43: Comparison of L-arabinose and D-xylose dehydration to

furfural. MW, T = 170°C, t = 20 min, Catalyst: FeCl3 (10 mol%) and NaCl (1 eq),


Entry Pentose Pentose


yield (%)

1 D-Xylose 100 74

2 L-Arabinose 65 41

Values of Figure 44: Conversion of D-glucose, D-galactose and D-mannose to 5-

HMF. MW, T = 170°C, t = 20 min, Catalyst: FeCl3 (10 mol%) and NaCl (1 eq),


Entry Hexose Hexose

conversion (%) 5-HMF (26)

yield (%)

1 D-Glucose 59 8

2 D-Galactose 59 7

3 D-Mannose 78 9

5. Dehydration of D-xylose in a continuous flow reactor

Values of Figure 47: Effect of reaction temperature and residence time on a) xylose

conversion and b) furfural yield. Continuous flow, P = 250 bars. Ratio φ = 0.67.

Entry T (°C) τ (s) Xylose (14)


yield (%)

1 240 0.25 19 0

2 260

21 0

3 280

22 0

4 300

30 0

5 320

30 0

6 340

32 0

7 360

40 0

203

8 380

55 1

9 240 0.50 14 0

10 260

23 0

11 280

14 0

12 300

15 0

13 320

22 0

14 340

36 1

15 360

57 1

16 380

75 2

17 240 0.75 13 0

18 260

14 0

19 280

22 0

20 300

27 1

21 320

33 1

22 340

43 1

23 360

62 2

24 380

84 2

25 240 1 11 1

26 260

13 0

27 280

21 0

28 300

33 1

29 320

43 2

30 340

59 2

31 360

70 3

32 240 1.5 25 0

33 260

26 0

34 280

28 1

35 300

40 2

36 320

56 3

37 340

69 4

38 360

73 4

39 240 2 15 0

40 260

30 1

41 280

29 1

42 300

41 2

43 320

54 4

44 340

77 5

45 360

92 6

204

Values of Figure 49: Influence of the ratio φ ―xylose solution flow‖/ ―heating water

flow‖ on furfural yield. Continuous flow, P = 250 bars, Residence time τ = 2 s.

Entry T (°C) Ratio ϕ Xylose (14)


yield (%)

1 240 0,67 15 0

2 260

30 1

3 280

29 1

4 300

41 2

5 320

54 4

6 340

77 5

7 360

92 6

8 240 0,33 9 0

9 260

10 1

10 280

22 2

11 300

31 4

12 320

53 8

13 340

77 11

14 360

91 12

15 380

100 9

Values of Figure 50: Influence of initial xylose concentration on a) xylose

conversion and b) furfural yield. Continuous flow, P = 250 bars, Residence time τ =

2 s. Ratio φ = 0.33.

Entry T (°C) Cxylose

(mmol/L) Xylose (14)


yield (%)

1 240 50 9 0

2 260

10 1

3 280

22 2

4 300

31 4

5 320

53 8

6 340

77 11

7 360

91 12

8 380

100 9

9 240 75 1 0

10 260

6 0

11 280

22 1

12 300

40 3

13 320

49 6

205

14 340

71 8

15 360

87 13

16 380

100 12

17 240 100 7 0

18 260

14 1

19 280

14 2

20 300

27 4

21 320

41 8

22 340

72 12

23 360

90 13

24 380

100 12

Résumé :

Les années 1990 ont été marquées par le développement de la Chimie Verte avec les

premiers travaux sur le sujet et l’introduction des Douze Principes. Depuis, le nombre de

recherches sur la Chimie Verte n’a cessé de croître. Ces travaux de thèse portent sur le

développement de méthodes de déshydratation dans le but de former des molécules à

haute valeur ajoutée en utilisant les Douze Principes de la Chimie Verte en tant que ligne

directrice. Deux molécules cibles ont été sélectionnées : (i) le phénylacétaldéhyde, obtenu

par déshydratation du 1-phényléthane-1,2-diol, qui est utilisé dans la composition de

parfums, de médicaments, d’insecticides, etc., ou en tant qu’intermédiaire réactionnel ; et

(ii) le furfural, formé par la triple déshydratation du D-xylose (monomère principal des

hémicelluloses), qui peut être utilisé comme solvant sélectif ou comme molécule

plateforme pour produire une large gamme de composés d’intérêt. Plusieurs points-clés

ont été identifiés pour concevoir des procédés de déshydratation verts: le solvant (l’eau

ou le solvant éco-compatible CPME), la méthode d’activation (utilisation d’irradiation

micro-ondes ou d’un réacteur en flux continu) et le catalyseur (chlorures de métaux ou

résine échangeuse d’ions).

Mots-clés : Chimie Verte ; Eau ; CPME ; Déshydratation ; Phénylacétaldéhyde ; Furfural.

Abstract:

The 1990s have witnessed the development of Green Chemistry with the first researches

on the subject and the introduction of the Twelve Principles. Since then, the number of

scientific works on Green Chemistry has continuously grown. This PhD work focus on the

development of dehydration methods to form high added value molecules by using the

Twelve Principles of Green Chemistry as a guiding framework. Two target molecules

were selected: (i) phenylacetaldehyde, obtained by the dehydration 1-phenylethane-1,2-

diol, which is used in perfume compositions, for the preparation of pharmaceuticals,

insecticides, etc., or as a chemical intermediate; and (ii) furfural, formed thanks to the

triple dehydration of D-xylose (the main sugar unit of hemicellulose), which can be used

as a selective solvent or as a platform molecule to produce a wide range of high-value

chemicals. The optimisation of green production processes was focused on several key

points: solvents (water and the eco-friendly CPME), activation method (microwave

irradiation and continuous flow) and catalysis (metal chlorides and ion-exchange resin).

Key words: Green Chemistry; Water; CPME; Dehydration; Phenylacetaldehyde; Furfural.

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