First of all, I would like to thank to ALLAH the Almighty for the strength, guidance, and blessing until I accomplished my studies. I am grateful to my supervisor, Assoc. Prof. Dr. Ahmad Zuhairi Abdullah. Under his guidance, I was given the freedom to take my research into the areas that interest me the most. Not only he is a valuable information source, but he is also an excellent mentor. Due to my professor, my time at USM has been excellent.

I am thankful to Dean Professor Dr. Azlina Bt. Harun @ Kamaruddin, Dy.

Dean Assoc. Prof. Dr. Mohamad Zailani Bin Abu Bakar, Dy. Dean Assoc. Prof. Dr.

Mohd Azmier Ahmad, Dy. Dean Assoc. Prof. Dr. Lee Keat Teong, Professor Dr.

Bassim H. Hameed and all other persons in the school who encourage me and lead me towards my aim. I greatly appreciate the time and advice of my committee members. Thank you for reading through and editing this document! I am especially grateful to the past and present my research group members. They have been enjoyable to work with and have become life-long friends. I want to acknowledge my colleagues and friends specifically Benoit Faye and Reinhard Eckelt who greatly contributed to my research. I appreciate the laboratory assistance from USM

Engineering campus.

I wouldn’t have been able to make it through my PhD without the help of my friends and family. My parents, parents-in-law and all other family members have supported me with their precious prayers throughout my academic career. I finally made my dream come true! I am delighted to express my deepest affection,

iv appreciation and thanks to my wife, Mrs. Sarah Ayoub, and my beautiful precious daughters, Rameen Ayoub and Farzeen Ayoub, who have always been with me during this period. They encouraged me with their love and understanding during difficult moments and also shared my joy and success all along. The strength of my uncle, K.A.Dost, taught me to fight through even the worst of situations. Special thanks to Dr. Muhammad Faisal Irfan, Dr. Shahid Iqbal, Dr. Muhammad Zubair, Dr.

Javed Akhtar, Mr. Ubaid-Ullah, Mr. Khozema bin Ahmad Ali, Dr. Mutaz S.K, Dr.

Mushtaq Ahmad and his wife Dr. Sahzia Sultana for their friendship and entertainment during long lab days I would also like to thank all other people whom I could not name here, for being near me in these years. All of them have made immense contribution to my study and life by their support, inspiration, understanding and sacrifices and therefore, I sincerely dedicate this piece of work to these people whom I love and cherish for the rest of my life.

Acknowledgements are also extended to the USM for awarding IPS fellowship during whole period of my research. Lastly, but most importantly, I would like to thank Allah (Subhanaho Wa Tala) and our Holy Prophet Muhammad (Peace be upon him) for always keeping an eye over me and blessing me each day.

Muhammad Ayoub,

April 2013.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ______iv

TABLE OF CONTENTS ______vi

LIST OF FIGURES ______xii

LIST OF TABLES ______xvii

LIST OF APPENDICES ______xix

LIST OF SYMBOLS ______xxi

LIST OF ABBREVIATIONS ______xxiii

ABSTRAK ______xxv

ABSTRACT ______xxvii

CHAPTER ONE : INTRODUCTION ______1

1.1 Fuel crisis ______1

1.2 World biodiesel production ______2

1.3 World glycerol production ______4

1.4 World glycerol pricing ______5

1.5 Glycerol applications in industry ______7

1.6 Diglycerol as a valuable product of glycerol ______8

1.6.1 Diglycerol applications ______8

1.6.2 Diglycerol formation ______9

1.7 Glycerol oligomerization for diglycerol production ______10

1.7.1 Homogeneous catalysts ______10

1.7.2 Heterogeneous catalysts ______11

1.7.3 Acid/ base heterogeneous catalysts ______11

1.8 Acidic or basic solid catalysts for glycerol oligomerization ______12

1.9 Modified clay as a catalyst for glycerol oligomerization ______13

1.10 Problem statement ______15

1.11 Objectives ______17

1.12 Scope of study ______18

1.13 Thesis organization ______19

CHAPTER TWO : LITERATURE REVIEW ______22

2.1 Background ______22

2.2 Existing diglycerol synthesis methods ______24

2.2.1 Small scale method ______24

2.2.2 Glycerol derivatization method ______24

2.2.3 Pyrolysis of glycerol method ______25

2.2.4 Industrial epichlorohydrin method ______25

2.2.5 Catalytic reaction method ______26

2.3 Homogeneously catalyzed reaction for selective diglycerol______26

2.3.1 Homogeneous acid catalysts ______27

2.3.2 Homogeneous base catalysts ______28

2.4 Solvent free glycerol etherification over solid catalysts ______31

2.5 Oligomerization of glycerol to diglycerol ______33

2.5.1 Production of undesired diglycerol isomers ______34

2.5.2 Production of the desired diglycerol isomers ______35

vii

2.6 Heterogeneously catalyzed reaction for selective diglycerol ______37

2.6.1 Acidic heterogeneous catalysts ______38

2.6.2 Basic heterogeneous catalysts ______39

2.6.3 Metal oxide ______40

2.6.4 Mixed metal oxides ______41

2.6.5 Microporous base catalysts ______42

2.6.6 Mesoporous base catalysts ______45

2.7 Montmorillonites clay as a solid mesoporous catalyst ______49

2.8 Active metal for glycerol oligomerization to diglycerol ______53

2.8.1 Alkali and alkaline earth metals as an active metal ______53

2.8.2 Lithium compound as an active metal ______54

2.9 Summarize heterogeneous activities for oligomerization ______56

2.10 Effect of operating variables ______59

2.10.1 Effect of reaction temperature ______59

2.10.2 Effect of catalyst loading ______60

2.10.3 Effect of reaction time ______61

2.11 Optimization studies ______62

2.11.1 Response surface methodology (RSM) ______62

2.11.2 Central composite design (CCD) ______64

2.11.3 Model fitting and validation ______65

2.12 Kinetic study for oligomerization ______65

2.13 Catalyst reusability study ______66

CHAPTER THREE : MATERIALS AND METHODOLOGY ______69

viii

3.1 Materials and Chemicals ______69

3.2 Overall experimental flowchart ______72

3.3 Equipment ______73

3.4 Reactor setup for glycerol oligomerization ______74

3.5 Catalyst preparation methods ______76

3.5.1 Preparation of SBA-15 support ______76

3.5.2 Preparation of Li/ USY catalyst ______77

3.5.3 Preparation of Li/ SBA-15 catalyst ______77

3.5.4 Preparation of Li-Mg/ SBA-15 catalyst ______78

3.5.5 Preparation of Li/ MK-10 catalyst ______78

3.6 Characterization of support and catalyst ______79

3.6.1 X-ray diffraction (XRD) ______81

3.6.2 Surface analysis (BET, BJH, Vp, Dp) ______81

3.6.3 Transmission electron microscope (TEM) ______82

3.6.4 Scanning electron microscope (SEM) ______82

3.6.5 Energy dispersive X-ray (EDX) ______83

3.6.6 Basic strengths (H_) ______83

3.6.7 Themogravimetric - Differential thermal analysis (TGA-DTA) ______84

3.6.8 Inductively coupled plasma atomic emission spectroscope (ICP-AES) _ 84

3.6.9 Fourier transformed infrared spectrometer (FTIR) ______85

3.7 Catalytic activity ______85

3.7.1 Oligomerization process ______85

3.7.2 Analysis of reaction product ______86

3.8 Optimization studies ______88

3.9 Kinetic study ______93

3.10 Catalyst reusability study ______95

CHAPTER FOUR : RESULTS AND DISCUSSION ______96

4.1 Homogeneous alkali for glycerol oligomerization to diglycerol ______97

4.1.1 Oligomerization reaction ______98

4.1.2 Performance of different catalysts ______99

4.1.3 Effect of catalyst loading ______102

4.1.4 Effect of reaction time ______105

4.1.5 Effect of reaction temperature ______106

4.1.6 Diglycerol isomer distribution in the product mixture ______107

4.2 Modified zeolite catalyst for glycerol oligomerization ______109

4.2.1 Lithium modified zeolite catalyst (Li/USY) ______110

4.2.2 Characterization of Li/USY catalyst ______111

4.2.3 Activity of Li/USY for glycerol oligomerization ______117

4.3 Stabilized modified SBA-15 catalyst for glycerol oligomerization ______124

4.3.1 Lithium modified SBA-15 catalyst (Li/SBA-15)______124

4.3.2 Instability of SBA-15 to lithium modification ______125

4.3.2.1 Effects on surface characteristics ______126

4.3.2.2 Effects on particle structure ______129

4.3.3 Stabilization of lithium modified mesoporous SBA-15 ______134

4.3.3.1 Characterization of stabilized lithium modified SBA-15 ______135

4.3.3.2 Activity of stable catalyst for glycerol oligomerization ______146

4.4 Lithium modified montmorillonite clay ______152

4.4.1 Characterization of lithium modified clay catalyst ______152

4.4.2 Catalytic activity of the prepared catalyst ______164

4.5 Optimization of diglycerol yield via design of experiments (DOE) ______174

4.5.1 Single response optimization of diglycerol yield ______174

4.5.2 Analysis and model fitting ______175

4.5.3 Effect of process conditions ______181

4.5.3.1 Influence of individual effect ______181

4.5.3.2 Two-dimensional (2-D) interactions between variables ______184

4.5.3.3 Three-dimensional (3-D) interactions between variables ______188

4.5.4 Optimization of process parameters ______192

4.6 Kinetic study for determination of reaction rate and rate parameters ______194

4.7 Reusability of the catalyst ______209

CHAPTER FIVE : CONCLUSIONS AND RECOMMENDATIONS ______217

5.1 Conclusions ______217

5.2 Recommendations ______219

REFERENCES ______221

Appendix A ______237

Appendix B ______239

Appendix C ______241

Appendix D ______244

Appendix E ______245

LIST OF ISI PUBLICATIONS ______248

LIST OF CONFERENCE PRESENTATIONS ______249

LIST OF FIGURES

Page

Figure 1.1: Biodiesel production from vegetable oils and animal fats 2 and their relation with glycerol co-product (Yazdan and Gonzalez, 2007).

Figure 1.2: Production of biodiesel and crude glycerol during 2004- 4 2006 (Rahmat et. al., 2011).

Figure 1.3: Estimated production of crude glycerol in different 5 countries (ABG Inc. Company, 2007).

Figure 1.4: Crude glycerol impact on the cost of biodiesel (Tim, 6 2006).

Figure 1.5: Application of diglycerol and other polyglycerol in food 9 industry.

Figure 2.1: Acidic and basic pathway for the catalytic glycerol 27 conversion to diglycerol (Martin et al., 2012).

Figure 2.2: Glycerol conversion to diglycerol in an acid-catalyzed 28 homogeneous system (Richter et al., 2008).

Figure 2.3: Reaction scheme for glycerol etherification to diglycerol 29 via base-catalyzed reaction (Ruppert et al., 2008).

Figure 2.4: Reactive scheme for glycerol etherification to 32 polyglycerols (Clacens et al., 1998).

Figure 2.5: Cyclic diglycerol isomers and acyclic side-product of 35 glycerol oligomerization in batch mode (Medeiros et al., 2009).

Figure 2.6: Molecular sizes of glycerol, charged glycerol and linear 36 isomers of diglycerol (Krisnandi et al., 2008).

Figure 2.7: Glycerol oligomerization over metal oxides exhibiting 41 both acid and basic sites (Jerome et al., 2008).

Figure 2.8: Zeolite frameworks a) different commonly found cages b) 44 specific Y zeolite (Bekkum et al., 2001).

Figure 2.9: Mesopore and micropore view of mesoporous materials 45 (Tiemann, 2007).

Figure 2.10: Structural model of (a) MCM-41 with cylindrical pore 46 and (b) SBA-15 with hexagonal pore (Khodakov et al.,

xii

2005).

Figure 2.11: A scheme for exchanging Cs inside the porous area of 48 MCM-41 for glycerol oligomerization (Jerome et al., 2008).

Figure 2.12: Structure of 2:1 montmorillonite clay (Sinha Ray and 50 Okamoto, 2003).

Figure 2.13: Reaction scheme for base-catalyzed glycerol 52 oligomerization involving Lewis acidity (Ruppert et al., 2008).

Figure 3.1: Overall experimental works involved in this study. 72

Figure 3.2: Schematic diagram of the semi reactor used in this study. 75

Figure 3.3: Typical GC analysis of product mixture during glycerol 87 conversion at 240 oC.

Figure 4.1: Performance of different catalysts in the oligomerization 100 process measured in terms of glycerol conversion and diglycerol selectivity (temperature: 240 oC, catalyst loading: 2 wt %).

Figure 4.2: Measured pH values of glycerol in the presence of 102 different catalysts prior to glycerol oligomerization reactions.

Figure 4.3: Inﬂuence of LiOH catalyst loading on (a) glycerol 103 conversion and corresponding diglycerol selectivity and, (b) diglycerol yield (Reaction temperature: 240 °C).

Figure 4.4: Effect of reaction temperature on glycerol conversion and 107 corresponding diglycerol selectivity (Catalyst loading:2 wt % of LiOH).

Figure 4.5: Diglycerol isomers distribution in homogenous LiOH 108 catalyzed oligomerization for different reaction time at 240 oC.

Figure 4.6: XRD patterns of (a) USY and (b) Li/USY, with the 112 square marks indicating the FAU structure.

Figure 4.7: Nitrogen adsorption-desorption isotherms for parent USY 113 and lithium modified Li/USY catalyst.

Figure 4.8: Pore size distributions for parent and lithium modified 114 USY catalyst.

xiii

Figure 4.9: SEM images of (a) parent USY and, (b) Li/USY catalyst. 116

Figure 4.10: Glycerol conversion and yield to polyglycerol at 120 different temperature plotted against time for Li/USY catalyst.

Figure 4.11: Diglycerol selectivity and yield during glycerol 122 oligomerization over Li/USY at constant reaction time of 8 h.

Figure 4.12: Diglycerol isomers distribution over Li/USY for 123 different temperatures.

Figure 4.13: Nitrogen adsorption-desorption isotherms for Li/SBA- 127 15 samples with different lithium loadings.

Figure 4.14: Small-angle X-rays scattering (SAXS) patterns of the 130 prepared SBA-15 and lithium loaded SBA-15 catalysts (Li5/SAB-15 and Li10/SBA-15).

Figure 4.15: TEM images of SBA-15 a) before lithium loading and 131 b) after 10 wt % of lithium loading.

Figure 4.16: SEM images at magnification of 5000X a) SBA-15 b) 132 Li5/ SBA-15 and c) Li10 /SBA-15.

Figure 4.17: Reasons for SBA-15 structural collapse after lithium 133 modification.

Figure 4.18: SAXS patterns of the prepared SBA-15 and different 137 bimetallic lithium and magnesium loaded SBA-15 catalysts with different compositions.

Figure 4.19: Nitrogen adsorption-desorption isotherms for the 139 prepared catalysts.

Figure 4.20: Pore size distributions for parent and modified SBA-15. 140

Figure 4.21: SEM images of (a) Li10-Mg10/SBA-15 (b) Li10- 143 Mg20/SBA-15 and (c) Li10-Mg30/SBA-15.

Figure 4.22: TEM images of (a) Li10-Mg10/SBA-15 and (b) Li10- 144 Mg30/SBA-15.

Figure 4.23: Glycerol conversion and yield of diglycerols and 148 polyglycerol over 2 wt % Li10- Mg30/SBA-15 plotted against reaction time at 240 oC.

Figure 4.24: Diglycerol isomers distribution over different catalysts 150

xiv

at 240 oC after 8 h of reaction time.

Figure 4.25: TGA curves recorded for Clay MK-10 and Clay Li/MK- 153 10 catalyst.

Figure 4.26: XRD patterns of (a) Clay MK-10 and (b) Clay Li/MK- 155 10 catalysts.

Figure 4.27: Nitrogen adsorption and desorption isotherms for Clay 158 MK-10 and modified Clay Li/MK-10 catalyst.

Figure 4.28: Pore size distributions for Clay MK-10 and modified 160 Clay Li/MK-10 catalyst.

Figure 4.29: FTIR spectra for (a) MK-10 and (b) Clay Li/MK-10 in 161 the wave number range of 400- 4000/ cm.

Figure 4.30: SEM images for (a) Clay MK-10 and (b) Clay Li/MK- 163 10 catalyst.

Figure 4.31: Glycerol conversion and diglycerol yield of the catalysts 165 via solvent-free oligomerization reaction at 240 °C.

Figure 4.32: Glycerol conversion and selectivity to diglycerol and 168 triglycerol over the Clay Li/MK-10 catalyst at 240 °C.

Figure 4.33: Comparison between diglycerol isomers distribution 169 over homogenous LiOH and Clay Li/MK-10 catalyst.

Figure 4.34: Distribution of linear αα’-, ββ’-, and αβ -diglycerols as a 170 function of reaction time at 240 oC for Clay Li/ MK-10 catalyst.

Figure 4.35: Relation between observed and predicted diglycerol 180 yield.

Figure 4.36: The individual effects of (a) reaction temperature (b) 183 reaction time and (c) weight of catalyst towards diglycerol yield.

Figure 4.37: 2-dimensional plot for the interactive effect between 185 reaction temperature and reaction time (at 2 wt % catalyst amount).

Figure 4.38: 2-dimentional plot for the interactive effect between 186 reaction temperature and catalyst loading (at 8 h).

Figure 4.39: 2-dimentional plot for the interactive effect between 187 reaction time and catalyst loading (at constant 240 oC).

Figure 4.40: Three dimensional and contour plots for the effect of 189 reaction time and temperature on diglycerol yield.

Figure 4.41: Three dimensional and contour plots for the effect of 191 catalyst loading and reaction temperature on diglycerol yield.

Figure 4.42: Effect of reaction temperature on the concentration of 195 glycerol with 2 wt% of catalyst.

Figure 4.43: Nonlinear ﬁtting of relative glycerol concentration 200 against reaction time at 240 oC with 2 wt % of Li/MK- 10 and 50 g of initial glycerol amount.

2 Figure 4.44: Fitting plot of ln(Cg) against reaction time for R value 202 at various temperatures (a) 200 oC, (b) 220 oC, (c) 240 oC and (d) 260 oC.

Figure 4.45: A smooth curve for F=80 % for glycerol conversion via 205 of oligomerization reaction at optimized reaction conditions.

Figure 4.46: A plot for measuring the order of reaction using 206 Fractional Life Method.

Figure 4.47: A plot of ln(Cg) km versus 1/T for measuring the 207 activation energy.

Figure 4.48: Comparison between Cg experimental and Cg calculated 209 versus reaction time at each temperature using the proposed kinetic model equation.

Figure 4.49: XRD patterns for Clay Li/MK-10 catalyst before and 210 after the oligomerization reaction.

Figure 4.50: Diglycerol yield versus run numbers in the glycerol 212 oligomerization process at optimized reaction conditions.

Figure 4.51: Lithium leaching effect on diglycerol yield during the 215 12 h of reaction time.

xvi

LIST OF TABLES

Page

Table 2.1: Basic homogeneous catalysts for glycerol conversion to 30 polyglycerols.

Table 2.2: Activity of different types of heterogeneous catalyst for 57 solvent free glycerol oligomerization to diglycerol.

Table 3.1: List of chemicals and materials for catalyst preparation and 70 characterization.

Table 3.2: List of chemicals and materials used for product analysis. 71

Table 3.3: List of equipment used in the catalyst preparation and 73 product analysis.

Table 3.4: List of glass wares used in the catalyst preparation and 74 product analysis.

Table 3.5: Instrument used for characterization of the catalysts. 80

Table 3.6: Independent variables and levels used for central composite 90 rotatable design.

Table 3.7: Arrangement of the central composite rotatable design 92 (CCRD) for oligomerization process optimization.

Table 4.1: Surface characteristics and basic strengths of USY and 115 Li/USY. Table 4.2: Glycerol conversion to polyglycerol with 2 wt % of catalysts 118 at 240 ᵒC for 8 h reaction time.

Table 4.3: Characteristics of prepared Li/SBA-15 with different lithium 128 loadings.

Table 4.4: Surface properties of the prepared catalysts. 141

Table 4.5: Glycerol oligomerization over 2 wt % catalysts at 240 oC for 147 8 h. Table 4.6: Surface characteristics and basic strength of Clay MK-10 157 and modified Clay Li/MK-10 catalyst.

Table 4.7: Summarize results of prepared catalysts with their properties 173 and activities for glycerol oligomerization to diglycerol (Reaction conditions: 2 wt % catalyst; 240 oC; 8 h).

Table 4.8: The experimental design and corresponding diglycerol 175 yields.

xvii

Table 4.9: Sequential Model Sum of Squares. 176

Table 4.10: Variance (ANOVA) analysis for diglycerol yield. 177

Table 4.11: Statistics of the model to fit for diglycerol yield. 179

Table 4.12: Constraints used to obtain the highest value of diglycerol 193 yield. Table 4.13: Results of validation experiments conducted at optimum 193 conditions as obtained from DOE.

Table 4.14: Rate law equations for zero, first and second order of 199 reactions. Table 4.15: Reaction rate constants and correlation coefficients elicited 203 from the plot of ln (Cg) vs. time.

Table 4.16: The data points for relevant glycerol concentration Cgend 206 with their respective reaction time tF.

Table 4.17: Surface properties of Clay Li/MK-10 catalyst before and 211 after reaction.

Table 4.18: Lithium contents in the final liquid product during the 214 oligomerization process.

xviii

LIST OF APPENDICES Page

APPENDIX A 238

Table A-1: Physical and chemical properties of glycerol 238

Table A-2: Physical and chemical properties of diglycerol 239

APPENDIX B 240

Plate A1: Batch reactor system for oligomerization 240

Plate A2: Batch reactor system used in series 240

Plate A3: Batch glass reactor 240

Plate A4: Dean Stark system for water removal 240

Plate B1: Prepared catalysts 241

Plate B2: Product samples for product analysis 241

Plate C1: GC-FID used for product analysis 241

APPENDIX C 242

Figure C-1-1: Calibration curves for pure glycerol 242

Figure C-1-2: Calibration curves for pure diglycerol 242

Figure C-1-3: Calibration curves for pure triglycerol 242

Figure C-2-1: Standard peak for pure glycerol 243

Figure C-2-2: Standard peak for pure glycerol 243

Figure C-2-3: Standard peak for pure glycerol 243

Figure C-3-1: GC-FID peak for product sample with high 244 value of polyglycerol

Figure C-3-2: GC-FID peak for product sample with high 244 value of αα’-dimer

Figure C-3-3: GC-FID peak for product sample with high 244 value of αβ-dimer

xix

APPENDIX D 245

Silylaltion process for product analysis 245

APPENDIX E 246

Figure E-1: GC-FID analysis signals for product sample 246

Figure E-2: GC-FID analysis external standard report for 246 product sample

Table E-1: Mixture composition of final product 247

LIST OF SYMBOLS

Symbols Descriptions Unit

α primeray - isomer of glycerol Dimensionless

β secondary - isomer of glycerol Dimensionless

α' primeray - isomer of other glycerol Dimensionless

β' secondary - isomer of other glycerol Dimensionless

A Arrhenius factor (pre-exponential factor) Dimensionless

Cg Concentration of glycerrol (mmol/l)

Cgo Concentration of initial glycerol (mmol/l)

CDG Concentration of diglycerol (mmol/l)

Cg calc Simulated data of glycerol concentration (mmol/l)

Cg exp Experimental data of glycerol concentration (mmol/l)

CPG Concentration of polyglycerols (mmol/l)

-dCg/dt Differential of Cg polynomial with respect to (mmol/l.h)

time (t) dX/dt Differential of conversion of glycerol with (mol/h)

respect to time

Df Dilution factor Dimensionless

Ea Activation energy (kJ/mol) k Reaction rate constant (dm3/mol.h) m Order of reaction Dimensionless n Order of reaction Dimensionless

NA Initial numbers of moles (mol)

xxi

P Pressure Pa

Po Pressure Pa

R Gas constant (J/mol.K)

3 -rA Rate of reaction (mol/dm .h)

3 -rg Rate of reaction of diglycerol (mol/dm .h)

T Reaction temperature (oC) t Reaction time (h) tR Retention time for GC (min)

Xg Conversion of glycerol Dimensionless

V Total volume (dm3)

xxii

LIST OF ABBREVIATIONS

a. u Arbitrary unit

AMS Anionic surfactant templated mesoporous silica

ANOVA Analysis of variance

ASTM American Society for Testing and Materials

BET Brunauer-Emmett-Teller

BJH Barrett-Joyner-Halenda

CCD Central composite design

CO2 Carbon dioxide

CCRD Central composite rotatable design

DF Degree of freedom

DOE Design of experiment

EDX Energy dispersive X-ray spectroscopy

EA European standard

FID Flame ionization detector

FTIR Fourier transforms infrared

GC Gas chromatography

HMDS Hexamethyldisilazane

H2O Water

HCl Hydrochloric acid

IEA International energy association

ISO International standard organization

IUPAC International Union of Pure and Applied Chemistry

KOH Potassium hydoxide

Li Lithium

xxiii

LiOH Lithium hydroxide

MCM Mobil composition of matter

MK-10 Montmorillonite K-10 clay

MS Mean square

NaOH Sodium hydroxide

NOX Nitrogen oxides

P123 Pluronic 123

RSM Response Surface Methodology

SAXS Small angle X-ray scattering

SBA Santa-Barbara amorphous

SEM Scanning electron microscope

SS Sum of square

TEM Transmission electron microscope

TEOS Tetraethylorthosilicate

TCP Tri-block copolymer

TMCS Trimethylchlorosilane

TMOS Tetramethylorthosilicate

TMS Transition metal oxide mesoporous molecular sieve

XRD X-ray diffraction

Wt weight

xxiv

SINTESIS DAN PENCIRIAN MANGKIN MESOLIANG TERUBAHSUAI DENGAN LITHIUM UNTUK PROSES OLIGOMERISASI TERPILIH GLISEROL KEPADA DIGLISEROL

ABSTRAK

Industri biodiesel menjana kira-kira 10% gliserol mengikut isipadu sebagai satu produk sampingan yang mempengaruhi keseluruhan industri ini. Kajian ini meneliti suatu proses untuk menukarkan gliserol kepada produk yang mempunyai nilai lebih tinggi iaitu digliserol melalui proses pengoligomeran bermangkinkon bes tanpa pelarut. Keberkesanan pelbagai jenis mangkin homogen alkali (LiOH, NaOH,

KOH dan Na2CO3) dan mangkin heterogen (dengan penyokong yang berbeza iaitu

USY, SBA-15 and montmorillonite K-10 clay) telah disiasat untuk meningkatkan hasil dan kememilihan untuk produk yang diingini. Pemangkin pepejal yang disediakan telah dicirikan untuk mengetahui ciri-ciri tekstur dengan menggunakan

BET, SEM, EDX, TEM, FTIR, TGA, XRD, SAXS, ICP-AES dan penunjuk Hammet untuk kekuatan bes. LiOH mempamerkan aktiviti yang tinggi berdasarkan penukaran gliserol (99%) tetapi dengan kadar kememilihan yang rendah (18%) terhadap digliserol. Li/USY didapati aktif untuk penukaran gliserol (98%) dan hasil polyglycerol (72%) namun kememilihan digliserol masih agak rendah (29%) pada keadaan tindakbalas yang sama. SBA-15 mengalami keruntuhan struktur yang ketara selepas pembebanan alkali. Namum, kestabilannya dikukuhkan melalui salutan magnesium pada tahap yang sesuai dilakukan sebelum litium dimasukkan.

Penukaran glycerol sebanyak 90% dan 59% hasil polyglycerol telah diperhatikan namun kememilihan terhadap digliserol hanyalah sebanyak 14% bagi Li10-

Mg30/SBA-15. Mangkin montmorillonite K-10 yang diubahsuai dengan litium ( Clay

Li/MK-10) telah disediakan dan kecekapannya untuk proses oligomerisasi gliserol kepada digliserol telah disiasat. Penukaran gliserol yang (86%) tinggi dengan

xxv kememilihan yang tinggi terhadap diglycerol (68%) telah diperhatikan di bawah keadaan tindak balas yang sama. Kelakuan tindakbalas untuk semua mangkin yang telah disediakan juga disiasat berdasarkan, pembentukan isomer digliserol (isomer

αα', ββ' dan αβ) dalam campuran produk akhir telah diselidiki. Di antara semua mangkin yang telah disediakan, Li/MK-10 merupakan mangkin yang paling aktif.

Kesan pelbagai keadaan proses termasuk bebanan mangkin, masa dan suhu tindakbalas telah disiasat menggunakan kaedah sambutan permukaan (RSM).

Keadaan optimum yang telah ditemui untuk memberikan hasil digliserol 61% ialah pada suhu tindakbalas 240 oC, 3.35% berat mangkin dan 7 jam masa tindakbalas.

Clay Li/MK-10 juga boleh digunakan semula tetapi penurunan aktiviti sebanyak

30.6% dicatatkan selepas penggunaan pertama. Satu model matematik juga telah berjaya dibangunkan. Tenaga pengaktifan yang rendah iaitu sebanyak 1.550 kJ / mol telah berjaya ditentukan bagi tindakbalas oligomerisasi tersebut. Secara ringkas, Clay

Li/MK-10 merupakan pemangkin yang berkesan untuk proses oligomerisasi gliserol untuk menghasilkan digliserol. Dari aspek industri, penemuan yang telah dicapai melalui kajian ini dapat menyumbang ke arah menggalakkan industri biodiesel melalui penukaran gliserol mentah kapada diglycerol yang bernilai tinggi.

xxvi

SYNTHESIS AND CHARACTERIZATION OF LITHIUM-MODIFIED

MESOPOROUS CATALYST FOR SOLVENT-FREE SELECTIVE

OLIGOMERIZATION OF GLYCEROL TO DIGLYCEROL

ABSTRACT

Biodiesel industry leads to the generation of about 10% glycerol as a co-product, which affects the overall biodiesel economy. This study examines a process for converting glycerol to a higher value product i.e. diglycerol via a solvent free base- catalyzed oligomerization process. The catalytic activity of different homogeneous alkali catalysts (LiOH, NaOH, KOH and Na2CO3) and heterogeneous catalysts (with different supports i.e. USY, SBA-15 and montmorillonite K-10 clay) was investigated and with the aim of improving the yield and selectivity to the desired product. The prepared solid catalysts were characterized to study their textural properties and their surface morphology using BET, SEM, EDX, TEM, FTIR, TGA,

XRD, SAXS, and ICP-AES while Hammet indicator was used for characterizing their basic strength. LiOH exhibited high catalytic activity as indicated by almost complete glycerol conversion (99 %) but with poor selectivity (18%) towards diglycerol. Clay Li/USY was found to be active for glycerol conversion (98 %) with good polyglycerol yield (72 %) but the diglycerol selectivity was still rather low (29

%) under the same reaction conditions. SBA-15 experienced severe structural collapse after the alkali loading but the stability was improved with suitable amount of magnesium coating prior to lithium loading. 90 % glycerol conversion and 59 % polyglycerol yield were observed but the selectivity to diglycerol was observed at only 14 % for the Li10-Mg30/SBA-15 catalyst. Lithium modified montmorillonite K-

10 (Clay Li/MK-10) was then prepared and its activity was investigated. High

xxvii conversion of glycerol (86 %) and high selectivity to diglycerol (68 %) were observed under the same reaction conditions. The reaction behaviors for all prepared catalysts were also investigated based on the formation of diglycerol isomers (αα’,

ββ’ and αβ isomers) in the final product mixture. Among the catalysts prepared, Clay

Li/MK-10 was the most active one. Effect of various process conditions including the catalyst loading, reaction time and temperature were investigated using response surface methodology (RSM). The optimum conditions were found to give 61 % of diglycerol yield at 240 oC of reaction temperature with 3.35 wt % of catalyst and about 7 h of reaction time. Clay Li/MK-10 could also be reused but it experienced an activity drop of about 30.6 % after the first run. A mathematical model was successfully developed for most active catalyst Clay Li/MK-10. Low activation energy of 1.550 kJ/ mol was observed over this catalyst for the glycerol oligomerization reaction. In short, the Clay Li/MK-10 was an effective catalyst for the glycerol oligomerization process to diglycerol production. Industrially, the

ﬁndings attained in this study might contribute towards promoting the biodiesel industry through utilization of its glycerol by-product in the form of highly valuable diglycerol.

xxviii

CHAPTER ONE

INTRODUCTION

1.1 Fuel crisis

Energy demand and its resources are increasing day by day due to the rapid outgrowth of population and urbanization. So, there is a correlation between a country’s living standard and energy consumption. Conventional energy resources like coal, petroleum and natural gas are fulfilling the major energy demand but in the verge of getting exhausted. Unexceptional opportunities have been created in last few years to replace petroleum derived materials with bio-based alternatives due to rapid depletion of fossil fuels and their soaring prices. It has been estimated that the fossil oil sources might be depleted by 2050 (Goyal et al., 2008). It means that limitation in fossil fuel resources and existing world oil capacity is a major issue.

Petroleum is a non-regenerative source of energy and it is also an important resource of the modern society for its roles in areas other than power such as household products, clothing, agriculture, as a basic material for synthetic materials and chemicals. Moreover, the process of obtaining energy from sources other than petroleum can cause atmospheric pollution, and other problem like global warming and acid rain. This has triggered recent interest in alternative source to petroleum- based fuels. An alternative fuel must be technically feasible, economically competitive, environmentally acceptable and readily available (Meher et al., 2004).

Nowadays, fuel crisis has globally flourished the economy in every region, especially the oil consuming countries due to its rapidly decreasing available global stock. In view of this serious situation, biodiesel which comes from 100% renewable resources provides an alternative fuel option for future.

1.2 World biodiesel production

The most common way to produce biodiesel is by transesterifying triglycerols in vegetable oils or animal fats with an alcohol in the presence of alkali or acid catalysts as shown in Figure 1.1. Methanol is the most commonly used alcohol for this process due to its low cost. This process involves the removal of the glycerin from vegetable oil or fat. During this process, methyl esters are separated as the desired product while glycerin is left behind as a by-product. Crude glycerol is normally generated at the rate of one mol for every three mols of methyl esters synthesized.

Approximately, it constitutes about 10 wt% of the total product during bio-fuel production.

Figure 1.1: Biodiesel production from vegetable oils and animal fats and their relation with glycerol co-product (Yazdani and Gonzalez, 2007).

The United States and Europe are the biggest consumers of fuel in the world.

Europe represents 80% of global biodiesel production and consumption but the

United States is increasing its production at a rate faster than Europe. According to a report Brazil is expected to surpass the United States and European biodiesel production by the year 2015 (Xiu et al., 2004). In 2008, the global biodiesel production reached more than 11.1 mill tones while it was below 5 mill tons before

2005 (Chai et al., 2007). The annual consumption of biodiesel alone in the United

States was 15 billion liters in 2006. It has been growing at a rate of 30-50% per year to achieve an annual target of 30 billion liters at the end of year 2012.

A sharp annual increase as high as 28% of biodiesel production in Europe from the year of 2000 has led to biodiesel production of 5 million metric tons

(MTm), The Federal Government of Canada aimed to produce 500 million liters/year of biodiesel by the year 2010 to meet the Kyoto Protocol. In Malaysia, biodiesel production has been targeted to achieve 500,000 tons per year by the year 2010

(Rahmat et al., 2010). At this production capacity of bio-fuel in the world, just in the

US, about 3.5 million gallons of crude glycerol is produced every year. Figure 1.2 shows the increasing trend of production of crude glycerol due to increasing production of biodiesel during the period of 2004 to 2006. Today, with plenty of glycerol stock available in the world market its price and US export have declined.

According to a published report (Omni, 2008), the price of pure glycerol varied from

$0.50 to $1.50/lb and crude glycerol from $ 0.02/ lb to $ 0.15 /lb over the past several year. It was also reported that the surplus amount of crude glycerol will increase up to 1.2 metric tons per year at the end of 2010. The price of glycerol in the market will continue to drop in such an over saturated market and currently the main

3 supply of glycerol coming into the market is from the rapidly growing biodiesel industry.

300

250

200

150

100

Quantity of Biodiesel (millionQuantity gallons) Quantity of Crude Glycerol of (millionCrude Quantity lbs) 0 2004 2005 2006 Year

Crude glycerol (80%) Biodiesel

Figure 1.2: Production of biodiesel and crude glycerol during 2004-2006 (Rahmat et al., 2010).

1.3 World glycerol production

The mass production of biodiesel would entail surplus crude glycerol production and this crude glycerol has the highest purity of about 80–88%. Just in the

United States, this level of bio-fuel production will yield nearly 1.2 million metric tons of crude glycerol as a primary co-product of the biodiesel production process. Figure 1.3 shows the clear estimates crude glycerol production resulting from biodiesel production in different countries. This figure is drawn based on the rates of biodiesel production that were published in a report (ABG, 2007). It is clear from this figure that the estimated production of glycerol would reach 5.8 billion

4 pounds in 2020. This is due to the annual demand of biodiesel that is projected at

66.6 billion pounds in 2020.

Figure 1.3: Estimated production of crude glycerol in different countries (ABG, 2007).

1.4 World glycerol pricing

Worldwide biodiesel industry expansion is practically limited by high capital costs for its refinery and low value by-products like crude glycerol of purity less than

80 % that needs further purification steps to meet the purity of industrial grade glycerol (99% purity). The conventional application and current market of glycerol could not cope with the excess production. However, purification process is costly and the glycerol market is already saturated. According to a report (Higgins, 2002), the biodiesel production cost ranged from $ 0.17 to $ 0.42 per liter over the last decay. However, this price was not constant and continuously decreased over day by

5 day due to purification cost factors. This makes crude glycerol worth very low during production of biodiesel. Thus, the price of crude glycerol that is frequently decline has a direct relation with the biodiesel production cost as shown in Figure 1.4.

0.6

0.5

0.4

0.3

0.2

0.1

Offset to Product Cost ($/gal) to Product Cost Offset 0 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 -0.1 Crude Glycerin Selling Price ($/lb)

Figure 1.4: Crude glycerol impact on the cost of biodiesel (Tim, 2006).

Today, plenty of glycerol stock is available in the world market and its price is declining day by day. The price of glycerol in the market will continue to drop in such an over saturated market and currently the main supply of glycerol into the market is the rapidly growing biodiesel industry.

Basically, the continual high prices of glycerol makes it worthwhile for users to reformulate with alternative materials such as sorbitol and synthetic glycerol, whereas sustained low prices encouraged its use in other applications. The impact of the additional huge quantity of glycerol on its prices is not clear but it is likely that if new uses for glycerol are not found, the glycerol price may drop to a level that justify its use as a burner fuel, which cost is about 5 cents/lb. This also implies that the

6 overproduction of low grade glycerol would impact the viability and economy of biodiesel production (Kolmetz et al., 2005), market price stability of current crude glycerol as well as environmental concerns due to improper disposal of glycerol (da

Silva et al., 2009). The high bio-fuel prices and historically low glycerol prices are two main factors that drive researchers to discover new applications of glycerol and provide an ideal platform for food, chemical and pharmaceutical industries.

1.5 Glycerol applications in industry

Bioavailability of glycerol makes it a green feedstock. It is currently used as a base chemical for the production of plenty of value-added products in chemical industries (Pagliaro et al., 2007). Glycerol is an ideal ingredient in many personal care products, mostly acts in helping to prevent moisture loss while in pharmaceuticals. It provides lubrication and smoothness to many cough syrups and elixirs. For oral care products, glycerol is commonly found in toothpaste, mouthwash and sugar free gum giving a sweet taste without contributing to tooth decay as well as in cosmetics to hold moisture against the skin to prevent dryness (Neumann,

1991).

A wide range of reactions are applicable in these fine chemical industries including dehydration to acrolein, selective oxidation, etherification to fuel- oxygenates, selective hydrogenolysis to yield propanediols, selective transesterification, pyrolysis and gasification, reforming to syngas, polymerisation to oligoglycerols, fermentation to propane-1,3-diol, synthesis of epichlorohydrin, synthesis of fatty esters and conversion of glycerol into glycerol carbonate (Barrault and Jerome, 2008; Jerome et al., 2008; Pagliaro et al., 2007; Zheng et al., 2008). In the current scenario, the biodiesel production is going to skyrocket along with

ﬂooding of crude glycerol. Therefore, intense research in science and industry has been made on the selective production of oligomers to valorize this surplus of glycerol.

1.6 Diglycerol as a valuable product of glycerol

The condensation of two glycerol molecules via etherification reaction yields the simplest oligomer, called diglycerol. The product can be linear, branched or cyclic depending on if the condensation takes place between primary or secondary hydroxyls or an intramolecular condensation is involved (Richter et al., 2008).

Diglycerol, (C6H14O5) is a clear, colorless, viscous, practically odorless and sweet in taste liquid. Likewise, the solubility characteristics of this diglycerol are lying between those of the simple alcohols and glycerol. The boiling point of diglycerol is

205 °C under a pressure of 1.3 mbar. It is soluble easily in water and alcohol at room temperature. Generally, diglycerol of concentration 100 g/ L has pH values between

5- 6.

1.6.1 Diglycerol applications

The oligomers of glycerol, i.e. diglycerol and triglycerol have numerous applications in cosmetic, pharmaceutical and additives in nutrition or lubricants.

Being biocompatible, diglycerol is incorporated in personal care formulas for its mild humectant properties and its ability to enhance fragrance/flavor impact and longevity. The higher viscosity of diglycerol provides greater a body to personal care formulations and could lead to a reduction in the amount of thickening agent needed.

Various Food industries use diglycerol and polyglycerol as emulsifiers in bakery, confectionary, ice-cream, chocolate and margarine products at different percentages

8 as shown in Figure 1.5 (Charles et al., 2003). Diglycerol is also used in the production of fatty acid ester emulsifiers, and is part of food additives. Often, diglycerol is further processed to useful derivatives.

Figure 1.5: Application of diglycerol and other polyglycerol in food industry.

(Charles et al., 2003)

1.6.2 Diglycerol formation

On laboratory-scale, pure diglycerol is formed via direct routes such as the alkailation of dichlorohydrine ether, hydroxylation of diallyl ether with permanganate, hydrolysis of glycidyl ether or hydrolysis of diglycidyl ether via acetone and glycerol transformation into 1,2-0-Isopropylidene glycerol and most feasible route is via glycerol oligomerization or polymerization reaction (Martin and

Richter, 2011). A purely thermal conversion of glycerol without the addition of a catalyst in inert media is another route for formation of diglycerol but much care is

9 required and product is usually of low quality. On industrial scale, diglycerol is produced via basic hydrolysis of epichlorohydrin (Jakobson, 1986). After diglycerol formation, further separation, puriﬁcation and water removal steps are required. The raw diglycerol has to be subjected to a ﬁne distillation for this purpose. The oligomerization/ etherification of glycerol using a catalyst in a reaction system is another route for diglycerol formation. This is uniquely preferable route because it saves two steps (separation and fine distillation) for the purification of diglycerol.

1.7 Glycerol oligomerization for diglycerol production

Glycerol oligomerization has been extensively investigated with or without the use of organic solvents for production of diglycerol. In each case, different homogeneous alkali base catalysts such as carbonates and hydroxides or heterogeneous catalysts including zeolite, mesoporous silica and metal oxides have been applied (Clacens et al., 2002; Jerome et al., 2008; Martin and Richter, 2011).

The use of solvent could create some problems in the production process leading to a more complex overall process. On the other hand, the need to eliminate the solvents from the homogeneous catalysts is a very challenging task (Clacens et al., 1998). In this respect, solvent free etherification process could promise several advantages but limited information is currently available on this mode of glycerol etherification process for selective diglycerol production.

1.7.1 Homogeneous catalysts

Previously, several researchers have reported the different homogeneous industrial processes of glycerol conversion to polyglycerols. Nevertheless, these reactions are usually not sufficiently fast (in terms of glycerol conversion) or do not

10 selectively produce diglycerol apart from difficulties in filtration, neutralization and product purification (Jerome et al., 2008; Martin and Richter, 2011). The main drawback of homogenous catalysts is that the glycerol conversion is not selective to targeted diglycerol but also produces some unwanted products due to secondary reactions (dehydration, oxidation) that deteriorate the product quality.

1.7.2 Heterogeneous catalysts

Heterogeneous catalysts show some advantages such as ease of their separation from reaction mixture and the potential for reusability. However, in terms of glycerol oligomerization, a few disadvantages of some heterogeneous catalysts have been reported by previous researchers (Martin and Richter, 2011). They are usually high cost materials, difficult to be functionalized, have high solubility in polar media, experience leaching of metal clusters from their surfaces, have low surface area (in some cases), as well as in their thermal stability.

1.7.3 Acid/ base heterogeneous catalysts

In the heterogeneous reaction systems, the oligomerization of glycerol is mostly conducted using basic or acidic catalysts. The acidic catalyzed oligomerization which runs via a cationic intermediate efficiently converts glycerol but mostly in the form of unwanted cyclic oligomer. Under this condition, the catalyst deactivates quickly due to the blockage of the internal surface area and the acidic sites by the formed deposits. Another prominent drawback of this category of catalyst is that they lead to the formation of unnecessary acrolein due to acidic catalyzed dehydration reaction (Katryniok et al., 2010; Vaidya and Rodrigues, 2009).

Basic catalyzed oligomerization reaction runs via an anionic, deprotonated glycerol

11 intermediate that reacts with another glycerol molecule forming a dimer with the release of hydroxyl ion (Martin and Richter, 2011). The most promising results of selective glycerol etherification/ oligomerization to diglycerol are usually obtained using basic heterogeneous catalysts.

1.8 Basic solid catalysts for glycerol oligomerization

Generally, the catalysts which have been studied for this reaction are zeolite, mesporous silica mixed metal oxides and metal oxides. The oligromerization of glycerol over zeolite type catalysts has been found to be more active but less selective for diglycerol with short reaction time and also form some higher molecules during reaction attributed to their small microporous structure (Cottin et al., 1998).

Mesoporous catalysts like MCM-41 generally show higher selectivity but with rather low glycerol conversion at slightly high reaction temperature. They also suffer from active component leaching and formation of some higher molecules that block mesopores channels causing a reduced activity of the catalyst (Barrault et al., 2004).

Glycerol oligomerization over alkaline earth metal oxides like CaO shows sufficiently high activity and selectivity but severe leaching of active component and at the end of the reaction can occur so that the reaction is not purely heterogeneous in nature (Ruppert et al., 2008). Mixed oxides like MgAl-Na can show significant conversion and selectivity at slightly low temperature but long reaction time is required due to their basic and hydrotalcite texture properties (García-Sancho et al.,

2011). These deficiencies in catalysts might be produced due to harsh reaction conditions during solvent free reaction for oligomerization process. Therefore, there is still a gap whole for preparation of a solid catalyst for selective conversion of glycerol to diglycerol in the absence of any solvent. The use of clays can be

12 considered another option for this process so that they keep on receiving considerable attention for their ability to overcome these problems.

1.9 Modified clay as a catalyst for glycerol oligomerization

Clay minerals which have a significant potential in chemical processing technologies, are other options to use as basic heterogeneous catalyst for solvent free oligomerization reaction due to their different and interesting properties. In particular, clay minerals have received considerable attention in different organic syntheses because of their environmental compatibility, low cost, operational simplicity and reusability (Adams and Mccabe, 2006). Clays have been widely studied as catalysts due to a number of attractive features such as high porosity and thermal stability as well as exchangeable acidity (Armor, 1994; Shelef, 1995). They are also very effective catalysts for a wide range of reactions, often presenting highly required selective product. The interlayer cations of clay are exchangeable and that allows modifications of the acidic nature of the material to become a basic one by a simple ion-exchange procedure.

Specifically, montmorillonites clay exchanged with metal ions is an efficient catalyst in many organic transformations including etheriﬁcation (Ballantine et al.,

1984) and polymerization reaction (Backhaus et al., 2001b). Montmorillonite clay is a form of smectite group (2:1 layer clay) which is used to produce new inexpensive materials with properties matching those of zeolites (clays are two-dimensional structure where as zeolites are three dimensional). This type of material is prepared by exchanging charge-compensating cations between the clay layers with large inorganic metal hydroxycations.

They have layer structure with significant basal space between interlayer help to pass big molecules of reactant and highly thermal stable due to silica and aluminum structure. They become basic in nature after loading of lithium and calcined at high temperature. Therefore, clay might increase its basic strength and might not be effected its large surface area after alkali loading. They are non- corrosive, environmentally benign and present fewer regeneration problems due to harsh reaction conditions. They are also much easier to be separated from the liquid products and can be designed to give higher activity and selectivity even under harsh reaction conditions.

It is obvious that the design of a catalyst covers all aspects ranging from the choice of the active phases to the method of forming particles. A heterogeneous catalyst is a composite material, characterized by: (a) the relative amounts of different components (active species, physical and/or chemical promoters, and supports); (b) shape; (c) size; (d) pore volume and distribution; (e) surface area.

Suitable promoters are frequently added to achieve adequate performance. They may either modify the catalyst structure, to improve stability, or to enhance the catalytic reactions to give better activity or selectivity. However, the nature of the active species is always the most important factor in heterogeneous catalyst system

(Campanati et al., 2003). Solid heterogeneous catalysts are active as much of their surface is exposed to the reacting molecules (Neurock and Santen, 2006).

Furthermore, on the basis of previous inventions in heterogeneous catalysis, it can be concluded that increases in the catalyst surface area and pore size; it might significantly improve catalytic activity as well as selectivity towards oligomerization process.

1.10 Problem statement

Currently, many types of heterogeneous catalysts such as alkaline metal/ alkaline earth metal oxides, mixed oxides, various alkaline metal compounds supported on alumina, zeolite or mesoporous material have been reported to catalyze oligomerization reactions. However, zeolites and metal support on zeolite present severe limitations when large reactant molecules (glycerol) are involved, especially in the case of liquid-phase reaction systems for synthesis of fine chemicals. This is due to the fact that mass transfer limitations are very severe for microporous solids.

These catalysts have a narrow and uniform micropore size distribution due to their crystallographically defined pore system. In order to overcome the problem, the pursuit of solid base catalyst has been recently focused on mesoporous silica due to very high surface area, uniformity in pore size and high thermal stability which promise great opportunity for application as catalysts and catalytic supports. Most of mesoporous catalysts show high selectivity but very low glycerol conversion with some leaching problem. On the other hand, glycerol oligomerization over alkaline metal or alkaline earth metal oxides or mixed metal oxides show sufficient activity and selectivity but in long reaction time and also involve severe leaching of active component. Therefore, there is still requirement of such solid catalyst which can stabilize itself at these harsh reaction conditions to provide high conversion of glycerol and selective diglycerol over solvent free oligomerization.

It has layer structure with high basal space between interlayer to facilitate the diffusion of large molecules of reactant and product as well as high thermal stability due to presence of silica-alumina structure. It might become more basic in nature after loading with strong alkali (lithium) and calcination it at high temperature. They are non-corrosive, environmentally benign and present fewer regeneration problems

15 under harsh reaction conditions. Therefore, in this study, lithium has been used as active metal component with support montmorillonite clay other than zeolite and

SBA-15 support to improve its catalytic performance for solvent free oligomerization process.

Due to the potential advantages and limited research dedicated to on solvent free oligomerization process using mesoporous solid base catalyst, the main goals of this research project are directed towards the synthesis of the high surface area of mesoporous catalyst and the identification of the final metals which meet specific needs for effectively catalyzing the oligomerization process of glycerol to diglycerol.

Moreover, the process optimization is also required for economization and industrial application. Due to the harsh reaction conditions, catalyst stability and regeneration will be addressed besides other topics such as reaction kinetics and process modeling. The main points of our study that have impact towards society are;

i) The development of basic layered catalyst could be tuned to meet the

specific needs of any catalytic reaction to ensure higher activity,

selectivity and yields towards desired product.

ii) The development of low temperature solvent free reaction system in

chemical processing which can enhance both the chemical reactions and

physical appearance of end product. It may offer the potential for shorter

reaction cycles, cheaper reagents, and less extreme physical conditions,

leading to less expensive and perhaps smaller chemical producing plants.

iii) This present study may serve as a basis for the analysis of metal loaded

layered catalyst (heterogeneously) and reactions of loaded metals with

liquids phase glycerol.

iv) The development of effective heterogeneous catalyst instead of

homogeneous may overcome the industrial problems such as in

recovering the catalyst after the reaction.

v) The knowledge about reaction mechanism and its kinetics study may

helpful for future studies on oligeromerization reaction behavior with

basic layered catalyst and its modification forms.

1.11 Objectives

1) To identify the most suitable and stable catalyst among all lithium modified

with different type of supports (micropore, mesopore, and layered clay) for

solvent free selective oligomerization reaction.

2) To characterize these modified and unmodified form of catalysts in order to

establish the understanding on the correlations between the characteristics of

modified catalyst with the performance in the oligomerization process.

3) To optimize the established solvent free glycerol oligomerization reaction

with the most suitable active catalyst by varying reaction parameters.

4) To perform kinetic study for the optimized glycerol oligomerization reaction

in the presence of most suitable catalyst by finding activation energy and rate

of reaction of the process.

5) To demonstrate the reusability of the most suitable catalyst for the optimized

oligomerization process.

1.12 Scope of study

The aim of this study is to prepare a solid base catalyst for high conversion of glycerol to diglycerol via oligomerization reaction at optimized reaction conditions.

For active component of this catalyst, different alkali (LiOH, KOH, NaOH) are used in the form of homogenous catalyst for prescribed reaction. After finding best alkali

LiOH as an active component for oligomerization reaction, lithium in the form of salt

(10 wt % Li over support) has been impregnated over three different supports for glycerol oligomerization under same reaction conditions. In this study, three different types of support have been chosen including microporous Y Zeolite, mesoporous

SBA-15 and layered material montmorillonite K-10 clay. The lithium salt LiNO3 has been loaded over all supports under same conditions by special impregnation method described in methodology section. These lithium modified and parent supports have been subjected to comprehensive characterization for changes in the characteristics of their structures using available techniques such as; surface analyzer with

Brunauer-Emmett-Teller (BET) surface area measurement, X-ray diffraction (XRD), small angle X-ray scanning (SAXS), inductive couple plasma (ICP-AES) analysis,

Hemmett indicator method for basic strength, energy-dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), and scanning electron microscope (SEM). The investigations on the suitable lithium modified catalyst and its activity for solvent free glycerol oligomerization to diglycerol have also been performed. A simple and economical three-neck glass reactor with Dean

Stark and PID temperature controller system has been established for the oligomerization reaction that provide good quality and reproducibility on the results.

Lithium over montmorillonite K-10 catalyst (Clay Li/MK-10) has been found to be the best catalyst for prescribed reaction. Then reaction parameters were optimized

18 with the help of Design of Experiments (DOE) to get better results at optimum reaction conditions that might be feasible for industrial applications. The product sample has been analyzed by means of gas chromatograph (GC), equipped with a capillary column DB-HT5 and a flame ionization detector (FID). Besides that, a kinetic study for the reaction has been studied by preparing a kinetic model and its validation for best active catalyst using Arrhenius equation with the help of obtaining the activation energy (Ea), reaction rate (-rg) and order of reaction (n). Finally, the best catalyst has been studied for its reusability for up to three runs. It has also been tested for leaching of active component i.e. lithium from the clay support to liquid phase and drawn a relation of this leaching with activity of best catalyst for oligomerization reaction.

1.13 Thesis organization

This thesis consists of five chapters. Each chapter explains the important information of this research.

Chapter One: gives an introduction to the world’s energy crisis, biodiesel demand, glycerol production worldwide, glycerol and its product diglycerol applications. Different catalyst types and their importance for glycerol conversion to diglycerol are also covered. This chapter also presents the problem statement, objectives of the present studies, research scopes and finally the thesis organization.

Chapter Two: focuses the literature review of the three main parts in this study i.e. selective conversion of glycerol to diglycerol, solid base catalyst in oligomerization process and kinetic studies of etherification reaction. This chapter

19 highlights the previous work done on glycerol conversion into diglycerol via different methods and using different types of catalyst. It also provides a brief overview over oligomerization reaction and its products in the form of diglycerol isomers.

Chapter Three: provides details about the experimental methodology and analysis, the materials and chemical reagents used in the present research work, the experimental setup for oligomerization process, experimental method for support and catalyst preparation and modification along with the characterization studies. Details product analysis, optimization studies, performance of catalyst, reusability of the catalyst, and kinetic study are also presented.

Chapter Four: covers all the results and explanation of the findings in the study. This chapter has been organized into seven main sections based on the chronology of the work to provide an ideal flow of information and subsequently an easier understanding of the study. Overall results obtained from this study are discussed and presented in this chapter in the following sequence ; 1) Homogeneous alkali catalyst for glycerol oligomerization to diglycerol, 2) Modified microporous zeolite catalyst for glycerol oligomerization 3) Stabilized modified mesoporous

SBA-15 catalyst for glycerol oligomerization, 4) Modified mesoporous clay catalyst for glycerol oligomerization, 5) Optimization of operating parameters via DOE method, 6) Kinetic study and 7) Reusability study of the catalyst. A logical discussion was made on the prepared catalyst and their characteristics to chose the best catalyst among different types of support (micropore, mesopore, layered clay) for prescribed reaction under given conditions. Subsequently, discuss the

20 optimization of the reaction conditions with the help of DOE and then study the kinetics of the optimized solvent-free glycerol oligomerization reaction in the presence of best catalyst. In the last section of this chapter, the reusability and leaching results for best catalyst were studied and the summary of this section has been given at the end of this chapter.

Chapter Five: gives the conclusions to the findings made in the present study. It also provides some recommendations for future studies based on the results of the present study.

CHAPTER TWO

LITERATURE REVIEW

2.1 Background

The forecasted decline in the production of petroleum fuels and the growing concern about atmospheric greenhouse gas concentrations have necessitated the search for clean, renewable (sustainable), efficient and non expensive alternative fuel. Biodiesel is becoming a key fuel in motor engines if blended in certain portions with petrodiesel (Hasheminejad et al., 2011). It can be readily produced via the transesterification of vegetable oils (edible, non edible or reused) with low alcohols

(methanol or ethanol). Indeed, the inevitable formation of glycerol that accompanies the biodiesel production process is affecting the process economy (Olutoye and

Hameed, 2011; Yuan et al., 2010). Moreover, the growth of the biodiesel industry will result in overproduction of glycerol and create a surplus of this impure product as its production is equivalent to 10 % of the total biodiesel produced (Cardona et al.,

2007; Khayoon and Hameed, 2011).

Glycerol is an abundant carbon-neutral renewable resource for the production of biomaterials as well as source for a variety of chemical intermediates (Garcia,

2011; Rahmat et al., 2010). The physical and chemical properties of glycerol are shown in Appendix A. Unfortunately; biodiesel-derived glycerol is not biocompatible due to its contamination with toxic alcohol (methanol or ethanol).

Therefore, global research is focused on the effective conversion of glycerol to valuable chemicals to ameliorate the economy of the whole biodiesel production process. Recently, many studies have been dedicated to the transformation of this renewable polyol by various catalytic processes (Melero et al., 2012; Rahmat et al.,

2010). This encompasses oxidation process to obtain dihydroxyacetone,

22 glyceraldehyde, glyceric acid, glycolic acid and hydroxypyruvic acid (Augugliaro et al., 2010; Liebminger et al., 2009). It also includes the fermentation process towards

1,3-propanediol production (Tokumoto and Tanaka, 2011), acetylation process with acetic acid to obtain polyglycerol esters (Balaraju et al., 2010; Dosuna-Rodriguez et al., 2011; Goncalves et al., 2008; Khayoon and Hameed, 2011)and acetalisation process with ketones to obtain oxygenated acetals and ketals (da Silva and Mota,

2011; Silva et al., 2010; Umbarkara et al., 2009; Vicente et al., 2010).

Glycerol is also an efficient platform for the synthesis of oxygenated components such as diglycerol, polyglycerols and ployglycerol ethers by means of etherificationor oligomerization process (Arzamendi et al., 2008; Melero et al., 2010;

Melero et al., 2012; Yuan et al., 2011). Diglycerol is produced from catalytic etherification or oligomerization of glycerol with or without use of any solvents. The use of solvent could create some problems in the production process leading to a more complex overall process. In this respect, solvent free etherification process could promise several advantages but limited information is currently available on this mode of glycerol etherification process for diglycerol. Venturing into the possibility of such process is a worthwhile research effort.

This literature review contains information about the preparation methods of diglycerol, glycerol oligomerizaion reaction, and different types of catalyst used for this reaction, behaviors of used catalysts, activity, reusability performance and other knowledge gaps in previous studies. Properties of innovative clay as a catalyst for this reaction, optimization, and kinetic study for glycerol oligomerization reaction are also revived. Finally, conclusive statements about this research work are also given at the end of each section.

2.2 Existing diglycerol synthesis methods

2.2.1 Small scale method

On small-scale, pure diglycerol is produced via direct synthesis routes which are already described by different previous researchers (Behrens and Mieth, 1984;

Jakobson, 1986; Wittcoff et al., 1949). Favorably, diallyl ether is used as a primary reactant. Diallyl ether is accessible by reaction between allyl chloride and allyl alcohol in inert solvents under HCl release. Direct hydroxylation of this product can be performed with peroxyformic acid, CH2O3, or permanganate, at 40 °C under safety precautions for 4.5 h. However, several additional steps are needed for neutralization, filtration, dry and fractional distillation are necessary for isolation of the diglycerol and triglycerol. Isolation of diglycerol prepared by this method can be done using neutralization with e.g. barium hydroxide solution, centrifugation to separate the solid, digestion of the product in absolute ethanol and fractional distillation under reduced pressure.

2.2.2 Glycerol derivatization method

Further routes for manufacturing diglycerol and triglycerol proceed via glycerol derivatization methods (Steinberger, 1989; Kanzler, 2008), using e.g. isopropylidene rests as protecting groups. All described possibilities so far have the disadvantages of either the starting substances are difficult to get or the synthesis requires several intermediate steps, and the conversion produces great amounts of salts as by-products.

2.2.3 Pyrolysis of glycerol method

Generally, the thermal reaction route is performed at a certain temperature under inert atmosphere. For a selective reaction, the glycerol reactant should contain less than 1% of water and should not contain organic impurities. Often, before the use of the diglycerol product for further reactions, a distillation step is required to separate the unconverted glycerol. Reaction temperature, basicity and organic impurities have paramount influence on the glycerol oligomerization (Steinberger,

1989). The temperature window at normal pressure is rather narrow. A purely thermal conversion without the addition of any catalyst initiates at above 200 °C but at a temperature of 290 °C dark, strongly smelling products are also formed. At low temperature (180 °C) even in the presence of alkaline catalyst, a minute formation of diglycerol from glycerol is observed, while the conversion glycerol is usually low.

Care must be taken during the reaction to exclude air from the system. Traces of oxygen will cause the formation of acrolein and other condensation products which can change the color of the final product (Arzamendi et al., 2008).

2.2.4 Industrial epichlorohydrin method

Industrially, the epichlorohydrin route is applied. It is assumed that during basic hydrolysis of epichlorohydrin by NaOH, intermediary glycidol is formed besides glycerol, and it will react with the unconverted epichlorohydrin or glycerol to diglycerol. Further separation and purifications steps are necessary. The residual glycerol has to be separated, then water has to be removed from the raw diglycerol, and, finally, the product has to be subjected to a fine distillation. The specification is given as 90% conversion into diglycerol with some residual glycerol and triglycerol (Miner and Dalton, 1953). The reactions of glycidol or epichlorohydrin

25 with glycerol have something in common that the coupling of OH groups is not confined to the terminal positions but also the middle OH groups of glycerol can be involved as well. This leads to the formation of primary or secondary dimer of glycerol such as αα’, ββ’ or αβ dimer of diglycerol.

2.2.5 Catalytic reaction method

Etherification reaction of glycerol has been extensively investigated either using or without organic solvents; for both cases, different alkali homogeneous catalysts like carbonates and hydroxides or heterogeneous catalysts like zeolite, mesporous silica or metal oxides have been applied (Clacens et al., 2002; Jerome et al., 2008; Martin and Richter, 2011). For catalyzed reaction path, the etherification or oligomerization of glycerol is mostly conducted without the use of any solvent for economical and environmental reasons with basic or acidic catalysts in homogeneous or heterogeneous reaction systems. Hence, preference is given to solvent free catalyzed reaction for conversion of glycerol to polyglycerol.

2.3 Homogeneously catalyzed reaction for selective diglycerol

Conventional methods for diglycerol synthesis with the help of homogenous catalyst are rather difficult as this reaction requires extreme conditions such a high reaction temperature and highly acidic or basic environment (Pagliaro et al., 2007).

The general mechanism of glycerol conversion to diglycerol over acid or base catalyst is shown in Figure 2.1. It is clear from this scheme that there are three possible constitutional isomeric diglycerol dimers named as αα’, αβ and ββ’ according to the position of the oxygen of the OH groups of the glycerol molecule that interact (Steinberger, 1989). In consecutive steps, these dimers may react with

26 the molecules of glycerol to form trimers and even higher linear or cyclic oligomers

(Barrault and Jerome, 2008).

Figure 2.1: Acidic and basic pathway for the catalytic glycerol conversion to diglycerol (Martin et al., 2012).

2.3.1 Homogeneous acid catalysts

The proposed acid homogenous catalyzed glycerol etherification reaction mechanism by Richter and co-workers (Richter et al., 2008) starts with the protonation of one of the glycerol OH groups as shown in Figure 2.2. The acid catalyzed etherification of glycerol in homogeneous phase using H2SO4 as catalyst at

280 °C has been reported by (Medeiros et al., 2009). According to this report, more than 90% of the glycerol was consumed after 2 h of reaction. However, the main products (tri- and tetraglycerol) were obtained at approximately 20 % only. The remaining of 80% were other products that were hardly identified.

Figure 2.2: Glycerol conversion to diglycerol in an acid-catalyzed homogeneous system (Richter et al., 2008).

Reported results suggest that homogeneous acid-catalyzed reaction is generally fast but not selective enough for diglycerol or triglycerol. Thus, main disadvantage of acid-catalyzed glycerol conversion is the occurrence of secondary reactions

(dehydration, oxidation) to result in poor selectivity towards the desired products.

These secondary reactions may also cause the deterioration of the main product quality by changing its color.

2.3.2 Homogeneous base catalysts

The general mechanism for base catalyzed glycerol etherification or etherification reaction is based on deprotonation of a hydroxy group (Ruppert et al.,

2008). Subsequently, the formed alkoxy anion will attack the carbon atom of another glycerol molecule as shown in Figure 2.3. The use of a homogeneous catalyst e.g.,

Na2CO3 usually gives high conversion of glycerol, but relatively low selectivity to diglycerol. Several filtration, purification and neutralization steps are also required to recover pure diglycerol (Barrault et al., 2004). This procedure leaves large amounts of basic aqueous waste, which is not environmentally friendly (Clacens et al., 1998).

In addition, the use of some alkaline homogeneous catalysts (e.g. NaOH at 230 °C) does not selectively to focus diglycerol as only 12.5 % of diglycerol is

28 obtained (Jeromin et al., 1998) and two steps of vacuum distillation are needed to get

98 % diglycerol purity.

Figure 2.3: Reaction scheme for glycerol etherification to diglycerol via base- catalyzed reaction (Ruppert et al., 2008).

Table 2.1 presents a detailed comparison with those reported in previous studies using different basic homogeneous catalysts. The study investigated the catalytic glycerol conversion to polyglycerol using alkaline metals catalysts at reaction temperature of 240~ 260 oC and reaction time 8~ 9 h. More specifically, thses works focused on exploring the catalytic potential of different alkaline metals as an example of homogenous catalysts.

Table 2.1: Basic homogeneous catalysts for glycerol conversion to polyglycerols.

Homo- Reaction Xg Selectivity (%) after 8 h References Catalyst temp; time (%) SDiglyc STriglyc STetraglyc Others

o Na2CO3 240 C; 9 h 76 46 34 13 7 (Cottin et al., 1998) o Na2CO3 260 C; 9 h 96 24 35 22 9 (Charles et al., 2003) o Na2CO3 260 C; 9 h 94 27 31 21 21 (Barrault et al., 2004) o Na2CO3 260 C; 9 h 80 31 28 17 24 (Calatayud et al., 2009) o Na2CO3 220 C; 9 h 80 45 36 - 19 (Ruppert et al., 2008) NaOH 240 oC; 9 h 63 60 32 7 1 (Cottin et al., 1998) o NaHCO3 260 C; 9 h 75 27 12 0 61 (Krisnandi et al., 2008) o CsHCO3 260 C; 9 h 64 23 9.5 2.5 65 (Richter et al., 2008) o Cs2CO3 260 C; 8 h 71 39 19 6 36 (Richter et al., 2008) CsOH 260 oC; 9 h 74 32 21 5 42 (Richter et al., 2008)

Garti and co-researchers (Garti et al., 1981) classified different base catalysts according to their activity in the glycerol etherification. They reported the activity of catalysts for glycerol etherification as following order: K2CO3 > Li2CO3 > Na2CO3 >

KOH > NaOH > CH3ONa > Ca(OH)2 > LiOH > MgCO3 > MgO > CaO > CaCO3.

Basically, it was noted that this ranking was closely associated with the solubility of each base in glycerol for this process. Cottin and coworkers (Cottin et al., 1998) observed that Cs2CO3 was less active than Na2CO3 and K2CO3 at 260 °C for 4–9 h of

30 reaction time. They attributed these ﬁndings to lower solubility of Cs2CO3 under the same reaction conditions. This also provides the evidence that the reaction is catalyzed by the dissolved alkali in a principally homogeneous reaction. There have also been several research works on the optimization and purification of the desired product from this process. However, no one was able to closely control the course of the glycerol etherification reaction to avoid deep reaction products.

It can be concluded from previous research works that catalytic homogeneous routes are not selective to diglycerol production as the activity of the catalyst is directly influenced by the basicity of the catalyst. Therefore, it becomes a difficult task and also costly to purify the product mixture at industrial scale. Hence, to increase the selectivity of the process towards the desired products, many groups of researchers start to focus their work on designing suitable heterogeneous catalysts.

2.4 Solvent free glycerol etherification over solid catalysts

Etherification of glycerol over solid catalyst is an attractive pathway for organic chemists as it directly and more selectively gives access to value-added chemicals. There are a list of products that can be obtained from the etherification of glycerol that have many applications as a result of their anti-inflammatory, antibacterial, antifungal, immunological stimulation, or antitumor properties. These products are also important chemical intermediates for the synthesis of different surfactants as well as oxygenated components such as polyglycerols and glycerol ethers. Glycerol ethers (polyglycerols) are commonly produced from catalytic etherification of glycerol with the use of different solvents. Polyglycerols, especially diglycerol and triglycerol are the main products of glycerol etherification as shown in

Figure 2.4.

Figure 2.4: Reactive scheme for glycerol etherification to polyglycerols (Clacens et al., 1998).

Previously, several researchers have reported different homogeneous industrial processes for glycerol conversion to polyglycerols. Nevertheless, these reactions are usually not sufficiently fast (in terms of glycerol conversion) or do not selectively produce diglycerol apart from difficulties in filtration, neutralization and product purification. On the other hand, the need to eliminate the solvents from the homogeneous catalysts is a very challenging task (Clacens et al., 1998). In the heterogeneous reaction systems, the etherification of glycerol is mostly conducted using basic or acidic catalysts. The acidic catalyzed etherification reaction which runs via cationic intermediates efficiently converts glycerol but mostly to the formation of undesired cyclic oligomer. Under this condition, the catalyst might deactivate rapidly due to the partial blockage of the internal surface area and the acidic sites by the formed deposits. Another prominent drawback of this category of catalysts is that they also involve to formation of unnecessary acrolein due to acid catalyzed dehydration reaction (Katryniok et al., 2010; Vaidya and Rodrigues, 2009).

Base catalyzed etherification reaction runs via anionic intermediates. The deprotonated glycerol intermediate will react with another glycerol molecule forming a dimer with a release of a hydroxyl ion (Martin and Richter, 2011). The most promising results of selective glycerol etherification to diglycerol have been obtained over basic heterogeneous catalysts.

It can be concluded that solvent-free glycerol conversion is a better option for selective diglycerol production via etherification process that involves oligomerization reaction in which two molecules of glycerol produces a dimer i.e. diglycerol in the presence of a catalyst by eliminating one molecule of water.

2.5 Oligomerization of glycerol to diglycerol

Glycerol, in the form of triol is suitable for condensation reactions. The product selectivity is a crucial aspect in glycerol etherification reaction because of its

3 available reactive OH-groups. Due to the reactivity of glycerol, the oligomerization process can lead to dimers, trimers, and higher oligomers as well as some cyclic compounds (Martin and Richter, 2011). In addition to linear polyglycerols, branched polyols as well as oxygenated heterocyclic compounds can also be obtained from cyclization reactions of glycerol and acrolein that is produced by glycerol dehydration. These compounds often show different isomer distributions depending on reaction conditions and the catalyst system used. Diglycerols are mostly obtained from oligomerization of glycerol catalysed by basic and acidic catalysts. However, the dimers may also react with glycerol forming trimers and higher linear or cyclic oligomers (Richter et al., 2008).

2.5.1 Production of undesired diglycerol isomers

In fact, it is rather difficult to obtain pure dimer or trimer fractions as most of the production processes suffer from consecutive reaction in batch operations.

Usually, higher linear and branched oligomers (shown in Figure 2.5) are formed along with some types of cyclic oligomers or and acyclic side-products like ketones, aldehydes or diols of the same molecular composition C6H12O4 as previously reported by Medeiros and co-workers (Medeiros et al., 2009) as shown in Figure 2.5.

A USA based chemical production company, Solvay Chemicals, offers a nearly pure diglycerol product (> 90%). However, it is produced from the reaction between glycerol and epichlorohydrin. In contrast, many commercial diglycerol products of the same average molecular weight contain ca. 40% unreacted glycerol, 15% triglycerol and some higher oligomers and the average molar masses are around 170.

Polyglycerols are products with higher oligomerization degree. Thus, polyglycerols

(i.e. linear or cyclic multiple units of glycerol) may widely differ in composition, depending upon their production process. Some improvements have been made to reduce these cyclization reactions with the help of either solid catalysts such as zeolites (Krisnandi et al., 2008), other mesoporous solids like MCM-41 (Barrault and

Jerome, 2008), alkaline catalysts such as calcium hydroxide (Lemke, 2003), homogeneous catalyst such as sodium carbonate or calcium oxide (Ruppert et al.,

2008).

Figure 2.5: Cyclic diglycerol isomers and acyclic side-product of glycerol oligomerization in batch mode (Medeiros et al., 2009).

2.5.2 Production of the desired diglycerol isomers

The three possible constitutional isomeric diglycerol dimers are named as

αα’, αβ and ββ’ according to the position of the OH groups in the glycerol molecule that interacts as shown in Figure 2.6. The statistical distribution of αα’: αβ : ββ’ dimer product ratios might be 4 : 2 : 1 due to the presumption of the reaction of α- or

β-oxygen of the respective OH-groups of αα’ = 4/7, αβ = 2/7 and ββ’ = 1/7 (Martin and Richter, 2011). The production of the desired diglycerol isomers can be increased by inhibition of undesired cyclic oligomers formation and limitation of higher polyglycerol formation. According to the results of some preliminary density functional theory (DFT) calculations (Krisnandi et al., 2008), the αα’- and αβ- dimers are energetically favored as compared to the ββ’ -dimer. Moreover, for microporous materials, the formation of the smallest diglycerol isomer i.e. αβ -dimer might be favorable instead of other isomers due to their smaller pore size.

Figure 2.6: Molecular sizes of glycerol, charged glycerol and linear isomers of diglycerol (Krisnandi et al., 2008).

Barrault and co-workers (Barrault et al., 2005) stated that most often, no signiﬁcant pore size effects of zeolite catalysts could be expected on the product distribution as the reaction mainly occurred on the external active sites, coupled with limited accessibility of interior sites in the pores. Therefore, it might be considered necessary to employ of catalysts with larger pores e.g. meso porous structured solids to minimize the concentration of active site on the external surface. On the other hand, (Martin and Richter, 2011) claimed that Cs modiﬁed mesoporous MCM-41 solid catalysts had an effect on the product distribution during the batch conversion of glycerol at 260 °C as compared to homogeneous catalysts. They also observed that

36 the porosity of heterogeneous catalyst had an impact on the distribution of the conﬁgurational isomers of linear diglycerol as compared to homogenous Na2CO3 catalyst, (Martin and Richter, 2011).

It could be concluded from the sizes of glycerol and diglycerol that pore entrance size of any type of catalysts should be larger than 0.515 nm to allow access of the glycerol molecule. Moreover, the pore size the material should be even larger to enable diglycerol formation, i.e. at least 0.753 nm to allow the formation of the large ββ’ –diglycerol molecule.

2.6 Heterogeneously catalyzed reaction for selective diglycerol

Heterogeneous catalysis introduces phenomenon that need not be accounted for in homogeneous catalysis such as diffusion and adsorption. They are commonly known as interfacial phenomena. In the last decade, an extensive study has been conducted over heterogeneous catalyst by different group of researchers. Several solid acidic and basic catalysts have been tested by different researchers for oligomerization reaction and in most of the results, it has been found that neither the selectivity nor the reaction rate is improved much but a part of the catalysts is often solubilized during the reaction (Clacens et al., 1998). For the heterogeneously catalyzed reaction path, mostly acidic catalysts like zeolite Beta and basic catalysts like alkaline or alkaline earth metal-exchanged zeolites have been used. However, these solids also tend to lose activity during repeated use (Martin and Richter, 2011).

Although heterogeneous catalysts are generally less active, they exhibit some advantages such as, (i) the catalysts can easily be separated from the reaction mixture, (ii) they can be reused, and (iii) theoretically, the reaction path can be directed towards selected product of condensation. An extensive study has been

37 conducted by a group of researchers (Barrault et al., 2004). They revealed the fact that both types of acidic and basic heterogeneous catalysts could find vast industrial applications on the basis that the selectivity to the desired product could be improved.

2.6.1 Acidic heterogeneous catalysts

The acidic reaction path for oligomerization may start with the formation of a cationic glycerol and afterwards, a proton is liberated again as previously described in Figure 2.6. The acid catalyzed oligomerization has been carried out mostly over zeolites having at least an average pore size of 0.6 nm at 200–250 °C. These zeolites in their protonic form generally exhibit acid strengths comparable to strong mineral acids as reported by a previous researcher (Auroux, 2002). This is due to Brønsted acid sites formed by bridging OH groups between tetrahedrally coordinated Al and

Si in crystalline aluminosilicates. A complete conversion of 200 g of glycerol at 200

°C for 2 h over 20 g acidic zeolites Beta (Si/Al= 50, surface area 750 m2 /g) was reported by (Eshuis et al., 1997). The final product consisted of 30% linear diglycerol, 30% cyclic diglycerol and remaining were found to be higher oligomers at a yield of 120 g yield of polymer products. Moreover, used acidic zeolite Beta

(Si/Al=12, 4 wt %) for conversion of 50 g glycerol at 260 °C, (Cottin et al., 1998).

They achieved 70% of glycerol conversion after 7 h with a 40% of diglycerol and

30% of triglycerol selectivity. The same research group also used zeolites Y (Si/Al =

2.7 and 15) and found that glycerol conversions of 7 and 28% respectively were achieved under the same reaction conditions.

Higher conversion of glycerol by zeolite Y was found with the higher Si/Al molar ratio (15) and it was associated with lower concentration of Brønsted acid sites

38 on the catalyst. However, lower Si/Al molar ratio is also responsible for abundant formation of acrolein which was caused by higher acid strength of the Brønsted acid sites (Cejka et al., 2010; Hensen et al., 2010). The researchers also suggested another reason for this problem that is the solid becomes more hydrophobic with dealumination, and this might inﬂuence the interaction between glycerol and the solid surface (Cottin et al., 1998). They also tried super-acidic organic polymers

(Amberlyst 16, Naﬁon® NM-112) for glycerol oligomerization and found that at nearly complete conversion of glycerol, high percentage (85%) of linear diglycerol was obtained. In batch reactor mode with permanent contact between catalyst, glycerol and the liquid product, the diglycerol content steadily decreased with increasing glycerol conversion. It reaches a value of 35 % at 85% glycerol conversion.

It can be concluded from these results that most of acid catalysts cause the formation of cyclic oligomers instead of linear oligomers. This might be one of the reasons why this reaction system suffers from rather quick deactivation due to the blockage of the internal surface area and the acidic sites by the formed deposits.

Additionally, acid catalyzed glycerol conversion may lead to acrolein formation due to acid catalyzed dehydration reaction. Moreover, the condensation reaction of glycerol catalyzed by acid catalyst mostly produces polyglycerols of high degree of polymerization with low color values.

2.6.2 Basic heterogeneous catalysts

Solid base is an important variety of catalyst which is widely used to synthesize fine chemicals, particularly the purpose of substituting homogeneous base catalysts such as Na2CO3, NaOH and KOH. It is reported that over 1.5 million tons

39 of fine chemicals are produced world-wide every year using homogeneous bases.

However, it worth highlighting that nearly 30 % of the selling price is for the product purification, recovery and waste treatment (Liu et al., 2008). Therefore, solid base catalysis is an area of chemistry that offers an excellent opportunity for exploitation if suitable catalysts and processes can be identified and developed (Hattori, 2001;

Ono, 2003). Base catalyzed oligomerization reaction proceeds via an anionic, deprotonated glycerol intermediate that reacts with another glycerol molecule forming a dimer under releasing a hydroxyl ion as shown in previously described

Figure 2.3. Some research works have already been pursued in this field and they are mainly covered in patents, as well as reported in open literature. Although, there are different types of basic heterogeneous catalyst available, they can categories according to their pore size, structure or surface properties. The details about such types of catalyst are provided as in the following sections.

2.6.3 Metal oxide

Some researchers have also studied the glycerol oligomerization reaction over alkaline earth metal oxides (BaO, SrO, CaO, and MgO) as potential heterogeneous catalysts with high activity. Weckhuysen’s group of research (Ruppert et al., 2008) investigated the oligomerization of glycerol in the presence of alkaline earth metal oxides and compared the results with those obtained with homogenous Na2CO3 catalyst. It was found that glycerol conversion increased with increasing catalyst basicity. It is also revealed in previous studies that the basic strength of solid catalysts plays a major role in the catalyst activity. On the other hand, the oligomerization of glycerol requires high reaction temperatures of about 260 °C to facilitate the elimination of such type of hydroxyl groups to initiate the reaction.

Actually, most of metal oxides exhibit both basic sites for glycerol deprotonation and Lewis acid sites to facilitate the elimination of OH group which helps to propagate free radical during glycerol oligomerization process as shown in

Figure 2.7. Therefore, in particular, CaO, SrO, and BaO metal oxides can show good glycerol conversions the reaction is successfully carried out at low temperature of

220 °C instead of 260 °C with minor side product i.e. acroleine. They were found to be highly efficient with above 80% of glycerol conversion but low selectivity (40%) to diglycerol after 20 h of reaction at 220 °C. It was also observed by the authors that the rate at the beginning of the reaction was quite low and it progressively increased with reaction time due to the fragmentation of the metal oxide particles.

Figure 2.7: Glycerol oligomerization over metal oxides exhibiting both acid and basic sites (Jerome et al., 2008).

2.6.4 Mixed metal oxides

Recently, Garcia-Sancho and co-workers (García-Sancho et al., 2011) reported the oligomerization of glycerol in the presence of hydrotalcite MgAl-mixed oxides at 220 °C without solvent. These solids exhibited basic properties and excellent textural properties, making them suitable base catalyst for such reaction.

The highest conversions (50.7 %) with a maximum diglycerol yield of 43 % were demonstrated by this catalyst. It was also observed that the formation of diglycerol was favored at low conversion. The thermal decomposition of MgAl- hydrotalcites

2− leads to a high surface area Mg(Al)Ox mixed oxide with strong O Lewis basic sites

(Perez-Ramirez et al., 2007). Some other researchers also extensively studied the basic metal oxides derived from layered double hydroxides (LDHs) due to their tunable properties that have allowed them to find potential applications in different catalytic fields (Ruppert et al., 2008; Tichit and Coq, 2003).

Overall, the catalytic activity and selectivity of these MgAl-mixed oxides depend on chemical composition and the conditions of the thermal treatment used to decompose the hydrotalcite precursor. It has been suggested that the optimum Mg/Al ratio for chemical composition might depend on the basic site density and strength required to activate the particular reactant (García-Sancho et al., 2011).

2.6.5 Microporous base catalysts

For application as etherification catalysts, zeolites are largely used as supports for many different active species. Among the family of microporous materials, the best known members are zeolites which are mostly used for this purpose. They have a narrow and uniform micropore size distribution due to their crystallographically defined pore system. However, zeolites present severe limitations when large reactant molecules like glycerol are involved, especially in liquid-phase systems as is frequently the case in the synthesis of fine chemicals. It is due to the fact that mass transfer limitations are very severe for microporous solids

(Taguchi and Schuth, 2005).

The use of zeolites as a solid catalyst for the oligomerization of glycerol has been reported by different groups (Krisnandi et al., 2008; Barrault et al., 1998). They investigated different exchanged zeolites with different pore openings. Different types of zeolites have different type of structure depend upon their prepration under desired conditions. The cages that are commonly found in zeolite frameworks are shown in Figure 2.8 (a) and specific microporous structure of ultra stable zeolite

(USY) is shown in Figure 2.8 (b). No improvement was observed in the diglycerol selectivity with zeolites Na-Mordenite and Na-ZSM5 as compared to the homogeneous catalyst. The productions of diglycerol were recorded at 75% and 20% respectively, at 40% of glycerol conversion. This observable fact was attributed to the narrow pore openings of Na-ZSM5 and Na-Mordenite. Thus, it was difficult to produce diglycerol within the small porous structure. This fact highlights the importance of the external surface area of solid catalysts. In addition, it was found that some zeolites (Na-X, Cs-X) were more selective to diglycerol as they were relatively larger pore openings.

(a) (b)

Figure 2.8: Zeolite frameworks a) different commonly found cages b) specific ultra stable zeolite (USY) (Bekkum et al., 2001).

As a matter of fact, the external surface contribution of microporous materials is less important. Therefore, Na-X and Cs-X zeolites showed more than 90% and

83% selectivity of diglycerol respectively, at 40% glycerol conversion (Krisnandi et al., 2008). However, improvements of more than 15% in the selectivity were found as compared homogeneous catalyst. These improvements were attributed to the presence of pores large enough to allow the formation of diglycerol but not too large to allow further reactions. In the case of Cs-X and Na-X zeolites, Cs-X was found to be relatively more active and selective catalyst. This enhancement was attributed to its stronger basicity as discussed by Barthomeuf (Barthomeuf, 1996). Hence, basicity is another parameter to be manipulated to influence selective glycerol conversion to diglycerol.

2.6.6 Mesoporous base catalysts

Since the discovery of M41S mesoporous materials in 1992, there has been an increasing interest in the design of novel porous materials tailored with various pore organization and dimensions for potential applications in separation, catalysis, chemical sensing, and low dielectric and optical coating (Cheng et al., 2003;

Venkatesan et al., 2005; Zhang et al., 2005). Actually, unique mesoporous materials like MCM-41 and SBA have ordered mesopores as well as disordered interchannel micropores in the mesopore wall as shown in Figure 2.9.

Figure 2.9: Mesopore and micropore view of mesoporous materials (Tiemann, 2007).

MCM-41 materials as see cylindrical structure in Figure 2.9 (a) are usually synthesized in a basic medium in the presence of surfactant cations. The pore diameter of MCM-41 materials may vary from 20 to 100 Å as a function of the alkyl chain length of the surfactant template and synthesis conditions (Khodakov et al.,

2005). Even though this material has a quite high thermal stability, when exposed to high temperature steam and boiling water, it loses its structure (Ooi et al., 2004).

This is caused by their thin silica wall that causes low stability in the presence of water (Cheng et al., 2003; Weitkamp et al., 2001). The collapse of the structure and poor stability in the presence of water represents a serious limitation to their practical application (El-Safty et al., 2005).

Figure 2.10: Structural model of (a) MCM-41 with cylindrical pore and (b) SBA-15 with hexagonal pore (Khodakov et al., 2005).

A research group of Barrault (Barrault et al., 2004) has evaluated both microporous and mesoporous crystalline basic materials in the absence and presence of various promoter elements for further improvement in catalytic glycerol oligomerization reaction. They also functionalized mesoporous silica (MCM-41) with different elements such as Li, La, Na, Cs, and Mg by impregnation and found that the mesoporous silica structure experienced a partial collapse due to the used procedure. They also found that the impregnated elements were not stable on the siliceous surface and leaching occurred during the catalytic reaction. The other type of mesoporous material is SAB-15 which has a stable hexagonal structure as shown in Figure 2.10 (b). This research group recently studied the impregnation of cesium salt over SBA-15 to overcome this problem. The hexagonal structure of SBA-15 was

46 found to be stable. However, leaching of cesium (>25%) was observed during the catalytic process as previously observed with MCM-41 materials. It became difficult to control the selectivity in the reaction with this solid catalyst due to this leaching problem.

As far as catalyst leaching and stability are concerned, Barrault and co- workers (Barrault et al., 2004) succeed to overcome this leaching and stability problem over mesoporous materials with the help of grafted solids. These new grafted solids retained their structure and specific area properties which were not observed for their impregnated mesoporous catalysts. The best catalysts were obtained through the incorporation of La and Mg in the mesostructure of the silico- aluminate material. Both metals possess high basic strength which is an important parameter for the oligomerization reaction. In fact, these catalysts showed glycerol conversions of 94% and 65%, respectively, at 260 °C within 24 h. This basic catalyst incorporated with cesium could be reused with only a slight decrease in its activity. It also confirmed the importance of the catalyst preparation procedure on the catalyst stability. The same process was also applied with a different microporous supports such as zeolite, no significant improvement in the selectivity of diglycerol was observed due to severe pore blocking.

Clacens and co-workers (Clacens et al., 2002) functionalized a MCM-41 solid support by exchanging the proton of silanol groups with cesium to preserve

MCM-41 mesostructure while avoiding the blocking of the pores. The XRD pattern confirmed the pores opening and revealed that they were not blocked by the presence of cesium oxide. The physicochemical and structural analyses of the exchanged catalysts calcined at 550 °C also did not show any destruction or collapse of the structure of the material.

Later on, Jerome and co-workers (Jerome et al., 2008) claimed that their research group succeeded to prepare a special mesoporous catalyst by exchanging Cs into the porous area of MCM-41 exchanged (Cs/MCM-41) using a special procedure as shown in Figure 2.11. This catalyst was found to be more selective than homogeneous catalysts or zeolites leading to over 65% selectivity of the diglycerol at

80% glycerol conversion. In addition, the reused Cs/MCM-41 catalyst showed a similar activity and selectivity to the freshly prepared solid catalyst. On the basis of these findings, it was suggested that the stability of the active species in the porous structure was greatly improved and no significant leaching was observed during the reaction.

Figure 2.11: A scheme for exchanging Cs inside the porous area of MCM-41 for glycerol oligomerization (Jerome et al., 2008).

It is clear from earlier discussion that the stability of the porous structure and mesoporousity are two important factors for solvent free glycerol oligomerization to diglycerol in all categories of heterogeneous catalysts. These are due to harsh reaction conditions created during the solvent free reaction for selectivity to

48 diglycerol. Therefore, recent interest and research focus are directed towards producing a catalyst with larger pore size with high thermal stability to handle the harsh conditions in the oligomerization reaction for selective production of diglycerol. The use of clays is another option which has received considerable attention because of their large pore sizes, but factors such as thermal stability and leaching of loaded metals still need to be considered and improved.

2.7 Montmorillonites clay as a solid mesoporous catalyst

Clay minerals are also microporous layered materials but they have a significant potential in different chemical processing technologies due to their different and interesting set of properties. In particular, montmorillonite clay minerals have received considerable attention in different organic syntheses because of their environmental compatibility low cost, operational simplicity and reusability

(Umberto et al., 2003). They are also very effective catalysts for a wide range of organic reactions, often presenting highly required product selectivity. The interlayer cations of clay are exchangeable to allow modification of the acidic nature of the material to basic one by just a simple ion-exchange procedure.

Well known montmorillonite clay is a hydrated 2:1 layered dioctahedral aluminosilicate of the smectite group as shown in Figure 2.12. It is composed of two tetrahedral (predominantly silicate) sheets which are bonded to either side of an octahedral (predominantly aluminate) sheet. Isomorphous substitution of Mg2+ for the octahedral aluminium, and of Al3+ for the tetrahedral silicon, results in charge deficit. This layer charge is balanced by hydrated exchangeable cations (e.g.,Na+ or

Ca2+) which occupy the surfaces between clay layers, termed the interlayer (Murphy,

2007).

Figure 2.12: Structure of 2:1 montmorillonite clay (Sinha Ray and Okamoto, 2003).

Various aspects of acid-activation of montmorillonite have already been studied in depth and it is known that acid activation results in at least four significant changes in the smectite as follow: (1) exchange of the hydrated interlayer charge compensating cations for H+ and their release into solution; (2) delamination, or loss of layer stacking of individual clay platelets into dis-oriented aggregates; (3) dissolution of the individual clay platelets and release of the constituent cations into solution; and (4) formation of a hydrous, poorly crystalline and highly porous phase

(Fahn, 1979). In general, changes 1 and 2 precede 3 and 4 in time, but harsh acid- treatment can result in all charges occurring simultaneously. The final acid activation products arising from the acid-treatment of smectites are hydrous and poorly crystalline silica that may or may not contain some aluminium (Tkac et al., 1994).

Overall, mesoporous structures are expected to improve mass transfer and catalytic

50 reaction efficiency due to their large pore sizes that warrant good accessibility for large molecules and high mass transfer.

Either in natural or exchanged form montmorillonite clay possesses both

Lewis and Brønsted acid sites. These types of site enable it to function as an efficient catalyst in organic transformations. Montmorillonite clay exchanged with metal ions is an efficient catalyst in many organic transformations such as Diels-Alder reaction

(Reddy et al., 2005), polymerization (Backhaus et al., 2001a), benzylation of aromatics (Cseri et al., 1995), etheriﬁcation (Ballantine et al., 1984) and acetalization reactions (Tateiwa et al., 1995). Montmorillonite K-10 clay is reported to be an effective catalyst in carrying out the esteriﬁcation of carboxylic acids with alcohols

(Kantam et al., 2002) and also in synthesis of amides from carboxylic acids (Srinivas and Das, 2003).

Intercalation of metal oxide clusters generates new materials with specific porous structure containing different incorporated cations and other active sites.

Thus, new possibilities for their applications as adsorbents, catalysts or auxiliary materials in a variety of industrial branches are possible. Due to their strong thermal and hydrothermal stability, pore size and selectivity, they have been initially tested as cracking catalysts of heavy oils (Ding et al., 2001) with very promising results. Later on, they have been tested for many applications in the field of catalysis such as isomerization and skeletal reforming of organic molecules (Moreno et al., 1996), NO reduction by hydrocarbons (Yang and Li, 1996), syngas conversion (Barrault et al.,

1992), alkylation (Horio et al., 1991), Fisher–Tropsch reaction (Yamanaka and

Hattori, 1988), dehydration of glucose to organic acids (Lourvanij and Rorrer, 1994) etc.

Actually, Brønsted acidity is a general feature of the clay structure, while the incorporation of metal oxide mainly introduces Lewis acidity in clay. Moreover,

Lewis acid sites are more thermally stable than Brønsted acid sites. Therefore, if

Lewis acid sites or both Lewis and Brønsted acid sites catalyse a reaction, clay materials with an active metal component would be a good option as a catalyst for glycerol oligomerization reaction to diglycerol according to reaction scheme shown in Figure 2.13.

Figure 2.13: Reaction scheme for base-catalyzed glycerol oligomerization involving Lewis acidity (Ruppert et al., 2008).

Previously, Kraft, (2002) reported the use of saponite catalysts for synthesis of diglycerol via oligomerization of glycerol. Saponite is basically a monoclinicmineral clay of the montmorillonite group having the general formula

Mg3[(OH)2(Si,Al)4O10].(Ca,Na)x(H2O)y. For the purpose of glycerol oligomerization reaction, this Mg saponite clay catalyst was first made acidic by ion exchange with ammonium ions and then calcination. The reaction was performed in a batch process at 250 °C using 1.38 kg of glycerol and 2.5 wt% of the catalyst. It was found that

24% of glycerol was converted to give a product mixture consisting of 17% of linear diglycerol and 7% of other oligomers after 24 h. The selectivity of the linear diglycerol at low conversion of glycerol was as high as 78.5%. It may be due to the

52 space between clay layers that allowed just diglycerol formation and hindered higher oligomers formation.

Hence, due to high basal spacing between montmorillonites clay and exchangeable acidic or basic sites by some modifications in their structure, they become choices as solid basic catalysts for selective glycerol oligomerization to diglycerol. Consequently, active component especially which can help Lewis and

Brønsted acid sites during the oligomerization reaction can play an important role for high activity of clay catalyst and also for selectivity to diglycerol synthesis.

2.8 Active metal for glycerol oligomerization to diglycerol

There are several active metals that can catalyze glycerol oligomerization efficiently. A great variety of active metals such as alkaline-earth metal oxides and hydroxides, alkali metals (Na and Cs) hydroxides or salts supported on mesoporous and microporous catalyst (Jerome et al., 2008) have been investigated to date under different reaction conditions and with variable degrees of success.

2.8.1 Alkali and alkaline earth metals as an active metal

Recently, different alkali and alkaline-earth metals (Na, K, Cs, Ba, Sr, Ca, and Mg) have been used frequently as active metals over different support or in the form of mixed metal oxides in the form of hydrotalcites (García-Sancho et al., 2011).

Some researchers directly use metal oxides like BaO, CaO, SrO and MgO (Ruppert et al., 2008) for this purpose. However, solid bases such as single alkali earth metal oxides (e.g. CaO and MgO) exhibit low specific area and vulnerability to water and carbon dioxide in air, which greatly limit the applicability in industrial scale applications (Duan et al., 2005). The order of activity for glycerol conversion to

53 diglycerol among alkaline earth oxide catalyst has been reported to be BaO > SrO >

CaO > MgO (Ruppert et al., 2008). It is also reported that CaO provides a slow reaction rate and it takes about 24 h to convert glycerol completely. Despite providing higher catalytic activity from these metal oxides, all have a drawback of metal leaching into the liquor solution. Attempts to overcome this problem by changing preparation conditions or by addition of some additives have been made but with limited success.

Alkaline metal-catalyzed oligomerization process is normally used for diglycerol synthesis. Alkaline metal hydroxides and alkoxides which have strong basic strength are the most effective glycerol oligomerization catalysts (Martin and

Richter, 2011). Several recent publications have reported lithium hydroxide or other salts supported on silica or silica-alumina based support as active heterogeneous catalysts for alkylation type reaction (Mitchell et al., 2004; Zhang et al., 1992). In the present study, alkali metal (lithium compounds only) supported on mesoporous material or clay has been chosen as an active metal to catalyze the oligomerization process.

2.8.2 Lithium compound as an active metal

Lithium is the first member element with an atomic number of three and located in the top left corner of the periodic table, is the most electropositive (-3.04

V) versus the standard hydrogen electrode, the lightest (molecular weight = 6.94 g/ mol) and least dense (ρ = 0.53 g/ cm3) of the alkali metal family (Du et al., 2011).

Lithium is an active element, but not as active as the other alkali metals. It reacts slowly with water at room temperature and more rapidly at higher temperatures and reacts with most acids, giving off hydrogen gas. At above 100 °C, lithium reacts with

54 oxygen to form lithium oxide (Li2O) while it does not react with oxygen at room temperature. Lithium metal and its compounds have a great many uses. The important two applications are in the ceramics and glass manufacturing and in the production of aluminum to make the material stronger. Lithium compounds are also used as catalysts in many different industrial processes like synthetic rubber and lithium batteries (Bruce et al., 2008).

Lithium doped magnesium oxide (Li/ MgO) is well established as a catalyst for the oxidative coupling of methane (OCM) and oxidative dehydrogenation of ethane (ODHE) (Swaan et al., 1993). Recently, Corma and co- workers (Corma et al., 2005) reported that calcined Li-Al and Mg-Al layered double hydroxides (LDHs) were able to catalyze the glycerolysis of fatty acid methyl esters to monoglycerides (the reverse of biodiesel synthesis). In the former study the Li-Al catalyst, which was the calcined [Al2Li(OH)6](CO3)0.5•nH2O, was reported to be more active than the Mg-Al material (or MgO) due its higher Lewis basicity (Corma et al., 2005). This calcined catalyst was also used in the synthesis of biodiesel from soybean oil (Shumaker et al., 2007). Recycling studies showed that the catalyst maintained a high level of activity over several cycles, although analyses indicated that a small amount of lithium was leached at from the catalyst. Shumaker and co- workers (Shumaker et al., 2007) also reported that Li-Al catalyst showed higher activity for two transesterification reactions, namely, the reaction between glyceryl tributyrate with methanol, and the reaction of soybean oil with methanol due to strong basic sites.

Consequently, the influence of the metal loading over clay catalyst and its affect on types of support has been investigated as well in the present study. The

55 catalytic activity of each has been compared and physicochemical properties of the catalyst have been elucidated to be correlated with their activities.

2.9 Summarize heterogeneous activities for oligomerization

Table 2.2 summarizes the catalysts and operating conditions that have been studied by other researchers using variety of supports and active components. In this table, most of the researchers show that heterogeneous catalyst can be highly selective to diglycerol by varying the operating parameters such as reaction temperature, weight of catalyst, active component, support and reaction time. It is clear from this table that zeolite type catalysts were more active but less selective for diglycerol with short reaction time as compare to others because of their microporous structure. Due to small pore sizes of matter that of the glycerol molecule (0.52 nm), most reaction occured on the external surface of catalyst that would affect the selectivity to diglycerol as some higher molecules were favorably formed during the reaction (Cottin et al., 1998).

On the other hand, mesoporous MCM-41 showed high selectivity but very low glycerol conversion at a slightly higher reaction temperature 260 °C. These types of catalysts were basic and had high surface area with mesopores openings larger than 2 nm and the reaction mostly occurs inside the pores that will enhance the selectivity to diglycerol. However, the formation of higher molecules can block mesopores channels causing a decrease in the activity of the catalyst (Barrault et al.,

2004). Mixed oxides also showed significant conversion and selectivity at slightly low temperature but only after a long reaction time of 24 h due to their basic and hydrotalcite texture properties (García-Sancho et al., 2011). Montmorillonite

Saponite clay (Mg-Saponite) have also been used by some previous researchers for

56 this purpose but it has been found to be less active as well as less selective for this reaction even after 24 h of reaction due to the acidic behavior of the clay (Kraft,

2002).

Table 2.2: Activities of different types of heterogeneous catalyst for solvent free glycerol oligomerization to diglycerol.

Catalyst Catalyst Reaction Conditions Xg SDG Reference

type (Temp; Time; weight) % %

Metal Oxide BaO 220 °C ; 20 h; 2 % 80 55 (Ruppert et al.,

SrO (Selectivity at 80% 80 52 2008)

CaO Conversion) 60 54

MgO 10 0

Mixed Oxides Na/Mg-Al 220 °C ; 20 h; 2 % 50 80 (García-Sancho

K/Mg-Al 30 76 et al., 2011)

NH3/Mg-Al 20 90

Urea/Mg-Al 15 100

Microporous Na/Mordenite 260 °C ; 9 h; 4 % 39 74 (Clacens et al.,

Na/ Zeolite (Selectivity at 40% 68 83 2002; Cottin et Conversion) Na/ZSM5 17 20 al., 1998; Martin

Cs/ Zeolite 81 90 et al., 2012;

Na/X 260 °C ; 24 h; 2 % 100 25 Martin and

Na/Y 260 °C ; 24 h; 2 % 79 47 Richter, 2011)

Na/Beta 260 °C ; 16 h; 2 % 52 44

Cs/ X 260 °C ; 24 h; 2 % 100 10

Cs/ZSM5 260 °C ; 24 h; 2 % 17 94 260 °C ; 24 h; 5 % K/Al2O3 62 65

Catalyst Catalyst Reaction Conditions Xg SDG Reference

type (Temp; Time; % %

weight)

Mesoporous Al/MCM-41 260 °C ; 8 h; 2 % 21 91 (Barrault et al.,

Li/Al-MCM-41 (Selectivity at 40% 39 78 2004; Clacens et Cs/Al-MCM-41 Conversion) 27 78 al., 2002)

Mg/Al-MCM-41 65 63

La/Al-MCM-41 94 26 260 °C ; 16 h; 2 % Na/MCM-41 85 63

° Cs/MCM-41 260 C ; 20 h; 2 % 80 65

Mn/Mesoporous 240 °C ; 20 h; 2 % 90 30

La/Mesoporous (Selectivity at 90% 75 40 Conversion) Mg/Mesoporous 50 -

Al/Mesoporous 15 0

Saponite clay Mg-Saponite 250 °C ; 48 h; 2 % 24 17 (Kraft, 2002)

*Xg: glycerol conversion; SDG: diglycerol selectivity

There is no reported result found for heterogeneous catalysts using clay material support until now with the exception of Saponite clay which has been reported to be inactive for long reaction time. Thus, clay can be used as the support but first it should be activated in proper way so that its structure in reaction liquor mixture for long reaction time at high reaction temperature. In addition, it should also be taken in account that clay as a support material may easily retain the active components like lithium and it is also important for new composite that it should make such a way that it can used under such harsh conditions. On the other hand, three major factors i.e. temperature, time and catalyst weight are involved to affect the reaction condition of oligomerization process for high activity and selectivity to diglycerol.

2.10 Effect of operating variables

It is also clear from above described Table 2.2 that three operating variables i.e. time, temperature, and weight of catalyst are very important for oligomerization process as well as activity of catalyst for selective conversion of glycerol to diglycerol. These effects of variables are reviewed here one by one.

2.10.1 Effect of reaction temperature

Oligomerization can occur at different temperatures, depending on type of catalyst used. Temperature clearly influences the reaction rate and yield of diglycerol because the intrinsic rate constants are strong functions of temperature (Liu et al.,

2007; Ma and A., 1999; Meher et al., 2004). The oligomerization reaction starts at above 200 °C due to high boiling point of glycerol. The primary advantage of higher temperatures is a shorter reaction time. However, if the reaction temperature exceeds the boiling point of glycerol (> 290 °C), the substance will start to vaporize to form some byproducts. It is also revealed that despite decreasing the reaction time, selectivity to diglycerol deteriorates at high reaction temperature due to rapid glycerol conversion into some undesired products. Therefore reaction temperature is usually restricted to the range of 200 °C to 290 °C.

Ruppert and co-workers (Ruppert et al., 2008) studied the effect of reaction temperature in the range of 220 to 260 oC. The highest conversion of glycerol to diglycerol obtained was 38.9 % at 260 oC. The obtained results showed that higher temperatures resulted in higher reaction rates to produce more diglycerol. However, it also led to the formation of glycerol byproduct. Besides, decreasing the viscosity of the liquor, in also enhances the solubility of metal catalyst into (Ruppert et al., 2008)

59 glycerol liquid phase. The optimum reaction temperature is generally reported to be between 220 to 260 oC.

2.10.2 Effect of catalyst loading

Martin and co-worker (Martin and Richter, 2011) studied the effect of catalyst loading on the conversion of glycerol to diglycerol in the range of 0.2-4.0 % with reference to the weight of glycerol as the solution reactant. They found that when the amount of catalyst was increased from 0.2- 2.0 %, the conversion to diglycerol showed a gradual increase. The conversion of glycerol was very high at 4 wt % of catalyst loading but it was mostly converted other byproducts instead of diglycerol. Without the addition of a catalyst, oligomerization did not occur even at

260 oC. The presence of the supported catalyst significantly increased the reaction rate. High conversion of glycerol to diglycerol was noted at 2 wt % of catalyst loading.

Ricther and co-workers (Richter et al., 2008) compiled some results on etherification reaction rate with different catalysts and they concluded that with more catalyst addition, reaction rate become faster because of the increase in the total number of available active catalytic sites for the reaction. Kim and co-workers (Kim et al., 2004) revealed that by increasing the catalyst loading, in some cases, the slurry

(mixture of catalyst and reactant) became more viscous. This gave rise to a problem of mixing and a demand for higher power consumption for adequate stirring. From the reported literature, the optimum catalyst loading is defined as the loading that will give high glycerol yield and does not contribute to the mixing or coloring problem in the reaction mixture at the same time. Thus, the optimum loading has been further investigated in this study as well.

2.10.3 Effect of reaction time

The result from a study by Martin and co-worker (Martin and Richter, 2011) revealed the correlation between the glycerol conversion and diglycerol yield in the range of 8-24 h of the reaction time. Their results showed that the reaction time needed is mostly influenced by the type of catalyst used and its structure beside the reaction temperature. Some catalysts required short reaction time of less than 12 h like Cs coated MCM-41 (Clacens et al., 2002) and some catalyst gave slow reaction rate so that more than 48 h was needed to complete the conversion of glycerol like saponite clay (Kraft, 2002). On the other hand, glycerol conversion was also found very high within 8h but at comparatively high reaction temperature of 260 oC. In some cases, the reaction rate was found to be very slow and more than 20 h was needed for completion of this reaction when the reaction temperature was reduced from 240 to 220 oC (Ruppert et al., 2008). Similar behavior has been reported by

Martin and co-worker (Martin and Richter, 2011). It can be concluded that reaction time needed is influcened by the reaction temperature, catalyst loading as well as the structure and nature of the catalyst. The optimum reaction time for the production of diglycerol through etherification reaction is usually between 8 to 12 h. Experimental results reported indicated that the conversion increased rapidly in the reaction time range between 1-8 h, and thereafter, became slow due to reduced active sites for glycerol conversion.

Hence, reaction time, temperature and catalyst weight seem to clearly influence the reaction and conversion of glycerol to diglycerol. The contribution of these operating conditions in the oligomerization process has been studied and

61 optimized using a Design of Experiment while the role of each variable and their interactions have been elucidated as well.

2.11 Optimization studies

The DOE (design of experiment) method is considered the most useful technique to optimize complex processes. One may obtain the optimum conditions associated with a specified property by performing much less number of experiments than the conventional single-variable method through this technique. Interaction between statistical significance of the result and variables can also be analyzed at the same time.

2.11.1 Response surface methodology (RSM)

RSM is an experimental strategy for seeking the optimum conditions for a multivariable system, is an efficient technique for optimization (Cho and Zoh, 2007).

Many researchers have used the RSM procedures in different disciplines, for example physics, engineering and chemistry (Benyounis et al., 2008). Vicente and co-workers (Vicente et al., 2007) studied the material balance for the fatty acid methyl ester using factorial design of experiments and a central composite design to evaluate the influence of operating conditions on the process material balance.

Actually, RSM is a collection of statistical and mathematical methods that are useful for the modeling and analyzing engineering problems. In this technique, the main objective is to optimize the response surface that is influenced by various process parameters. RSM also quantifies the relationship between the controllable input parameters and the obtained response surfaces (Montgomery, 2001). The response surface methodology (RSM) is a set of techniques that encompasses (Khuri and

Cornell, 1996; Istadi, 2006):

(i) The designing of a set of experiments for adequate and reliable

measurement of the true mean response of interest;

(ii) The determining of mathematical model with best fits;

(iii) Finding the optimum set of experimental factors that produces maximum

or minimum value of response;

(iv) Representing the direct and interactive effects of process variables on the

bead parameters through two dimensional and three dimensional graphs.

In most RSM problems, the relationship between the response and the variables is unknown. Therefore, the first step in RSM is to approximate the function

(f). Usually, this process employs a low-order polynomial in some region of the independent variables. If the response is well modeled by a linear function of the independent variables, then the approximating function is a first-order model. If there is curvature in the system or in the region of the optimum, then polynomial of higher degree must be used to approximate the response. This is analyzed to locate the optimum, i.e. the set of independent variables so that the partial derivative of the model response with respect to the individual independent variables is equal to zero.

The eventual objective of RSM is to determine the optimum operating conditions for the system, or to determine the region, which satisfies the operating specifications.

Almost all RSM problems utilize one or both of these approximating polynomials

(Khuri and Cornell, 1996).

There are many classes of response surface designs that are useful in practice such as Central Composite Design, 3-level Factorial Design, Hybrid Design,

Pentagonal, Hexagonal and others. In this study, the Central Composite Design has been employed as the design to optimize the conversion of glycerol and selectivity to diglycerol from solvent free oligomerization.

2.11.2 Central composite design (CCD)

Central composite design (CCD) is the most commonly used design for fitting a second order model. This might be used because of its ability to run sequentially. It estimates linear, interaction as well as curvature effects. As the design can provide much information on experimental variable effects and overall experimental error in a minimum number of required runs, it can be considered as a very efficient design. It also has the ability to be used under different experimental regions of interest and also operability because of accessibility of several varieties of

CCD (Montgomery, 2001). The number of tests required for CCD includes the standard 2k factorial with its origin at the center, 2k points fixed axially at a distance, say β, from the center to generate the quadratic terms, and replicate tests at the center; where k is the number of variables. The axial points are chosen such that they allow rotatability, which ensures that the variance of the model prediction is constant at all points equidistant from the design center. Replicates of the test at the center are very important as they provide an independent estimate of the experimental error

(Aslan, 2008). For each experimental factor, the variance is partitioned into components, linear, quadratic and interaction; in order to assess the adequacy of the second order polynomial function and the relative importance or significance of the terms (Istadi, 2006). The response model incorporates:

(i) Linear terms in each of the variables (x1, x2, …, xn)

2 2 2 (ii) Squared terms in each of the variables (x1 , x2 , … xn )

(iii) First order interaction terms for each paired combination (x1x2, x1x3,

…, xn − ixn).

2.11.3 Model fitting and validation

The statistical significance of the full quadratic models predicted is usually evaluated using the analysis of variance (ANOVA). The ANOVA determines which of the factors that significantly affects the response variables in the study, by considering either the F-value or the p-values of the model and of the lack of fit. The

Fisher variance ratio or the F-value explained the ratio between the variations in the data to the pure error variation. The factors studied appear to have a significant effect on the response if a large ratio in the F-value is obtained. The p-value, on the other hand is a level of significance that is used to determine how significant the data at any specified level of significance. The smaller the magnitude of the p-value, the more significant is the corresponding coefficients (Montgomery, 2001; Cao et al.,

2008). On the other hand, the regression models are accepted when the p-value of the model is lower than 0.05 (ideally lower than 0.001) and the lack of fit higher 0.05.

However, if any one of these conditions is not fulfilled, the model is only accepted when the model correlation coefficient, R2 is higher than 0.95, meaning that 95% of the data is explained by the model (Montgomery, 2001). The closer the value of R2 to unity, the better the models fits the actual data (Cao et al., 2008).

2.12 Kinetic study for oligomerization

Several kinetics studies of glycerol conversion to diglycerol via oligomerization reaction have been performed using homogeneous and heterogeneous base catalysts. Richter and co-workers (Richter et al., 2008) studied the kinetics of homogeneous base-catalyzed oligomerization of glycerol based on

65 parameters such as catalyst concentration and temperature. The rate constant, k, for each of the reaction was determined from least square ﬁtting method for glycerol conversion versus reaction time. It was found from previous study (Richter et al.,

2008) that the concentration change of glycerol is best described by a 1st order equation.

It is observed from a report (Dou et al., 2009) that the values of kinetic parameters such as E and A can be different depending on the heating rates. It gives the indication that thermal decomposition of glycerol may depend on the experimental conditions. As the heating rate is increased, the maximum mass loss and/or maximum rate of decomposition shift to higher temperatures. This is attributed to the variations in the rate of heat transfer with the change in the heating rate and the short exposure time to a particular temperature at high heating rates as well as the effect of the kinetics of decomposition. Unfortunately, kinetic data on glycerol oligomerization involving heterogeneous catalysts have yet to be well reported. The numerical methods outlined published by Rotaru research group

(Rotaru et al., 2007) stated that in some pyrolysis experiments of pure compounds, heating rate-independent kinetic parameters might be derived using a small range of small heating rates (e.g. 3, 6 and 9 K/min).

2.13 Catalyst reusability study

All test of deactivation is of great importance in order to prove the industrial application for heterogeneous catalysts. One of the main advantages of homogeneous catalysts is that they cannot be recovered. However, heterogeneous catalysts have the potential to be recovered, regenerated, and reused. Previous researchers (Martin and

Richter, 2011; Ruppert et al., 2008; Clacens et al., 2002) were agreed that the

66 reduction of activity in reused catalyst especially solvent free glycerol oligomerization was because of metal leaching from the support during the reaction.

The leaching of the active metal would reduce the applicability of the catalyst in industrial requirement. Therefore, the reusability of the best catalyst obtained in the experiment will be investigated to evaluate their performance in several cycles of catalytic activity.

According to a previous study by Ruppert et al., (2008), highly active heterogeneous catalysts for selective conversion of glycerol using metal oxides have several problems including leaching of active components and colloidal particles formation during catalytic reaction. These problems cause deactivation of the catalyst so that it is not reusable after the first reaction process. Another problem is with regards to the insufficient stability of the structured solids catalyst which is not confined to zeolites and mesoporous MCM-41, but could also be observed in the case of CaO in colloidal form as catalyst for glycerol conversion at a relatively low reaction temperature (220 ᵒC). They also reported that up to 6 % of Ca were found in the liquid after 20 h of reaction (Ruppert et al., 2008). Thus, a solid catalyst with high activity and stability is yet to be finally developed for this application.

It can be concluded from literature review that selecting heterogeneous catalytic conversion of glycerol into diglycerol is the best and economical route for oligomerization process but some limitations remain in this process. The challenges are low reaction temperature requirement, high conversion of glycerol with maximum selectivity of diglycerol, minimize leaching of active metal from surface of support, reusability of the catalyst, easy separation of catalyst from product and optimum batch time for the catalytic reaction. In such conditions, mesoporous supported alkali or alkaline earth metals catalysts look suitable for this type of

67 process. Therefore more attention should be given to prepare suitable type of catalyst and to optimize the reaction conditions for the oligomerization process towards achieving conversion of glycerol selective to diglycerol.

CHAPTER THREE

MATERIALS AND METHODOLOGY

All experimental works conducted in this study are fully described in this chapter. They consisted of the preparation and characterization of different support materials and their corresponding lithium modified materials prepared under various conditions. This chapter is organized into the following order i.e. materials and chemicals, equipment and facilities, synthesis of the most suitable catalysts for glycerol oligomerization to diglycerol, and catalyst characterizations. Next, catalytic study of those catalysts under different conditions was conducted to demonstrate oligomerization process behaviors and finally the kinetic study. The determination of the optimum values of three variables, (time, temperature, and weight of catalyst) for catalytic study was carried out using design of experiments (DOE) method. The kinetic study for performed reaction which was carried out based on Arrhenius equation involved rate of reaction, order of reaction and rate constant for prescribed reaction is also include in this chapter. The applicability of the catalyst and its reusability and active component leaching was also determined to investigate of their performance in the oligomerization.

3.1 Materials and Chemicals

Different supports like ultra stable zeolite (USY), SBA-15 (mesoporous material) and montmorillonite K-10 clay were used as support materials in the present study and active component lithium (LiOH) was loaded to form active catalysts for glycerol oligomerization reaction. The support USY and montmorillonite K-10 clay were purchased as commercial products while SBA 15

was synthesized in the lab. The materials and chemicals used in this research for

synthesis of catalyst for oligomerization process are listed in Table 3.1 and for

product analysis are listed in Table 3.2 respectively.

Table 3.1: List of chemicals and materials for preparation and characterization of prepared catalysts.

Chemicals Purity Supplier Purpose of use

Pluronic (P123) 99% Sigma-Aldrich Directing agent(SBA-15)

Tetraethylorthosilicate (TEOS) 99% Merck Source of silica(SBA-15)

Hydrochloric acid (HCl) 37% Merck Sol-gel process (SBA-15)

Sodium hydroxide (NaOH) 99% Chemar Metal precursor

Potassium hydroxide(KOH) 99% Chemar Metal precursor

Lithium nitrate (LiNO3) 99% R&M Chem. Metal precursor

Lithium chloride(LiCO3) 99% R&M Chem. Metal precursor

Lithium hydroxide (LiOH) 99% BDH,UK Metal precursor

Montmorillonite K-10 100% Sigma-Aldrich Clay support

Zeolite Y (CBV 600) 99% Zeolyst As a support

Potassium bromide (KBr) 100% R&M Chem. FTIR analysis

Magnesium nitrate hexahydrate 99% Merck Metal precursor

Methanol 100% J.T.Baker Solvent

Table 3.2: List of chemicals and materials used for product analysis.

Chemicals Purity Supplier Purpose of use

Glycerol 99.00% R&M Chem. Reactant

Methanol 100% J.T.Baker Solvent n-hexane 99.99% Merck Solvent

Ethanol 100% Merck Solvent

Pure glycerol 100.00% Solvay Chem. Standard for GC

Diglycerol >90.00% Solvay Chem. Standard for GC

Triglycerol >90.00% Sigma-Aldrich Standard for GC

Hexamethyldisilazane 100% Sigma-Aldrich Silylation for GC

Trimethylchlorosilane 100% Sigma-Aldrich Silylation for GC

Dry Pyridine 100% Fisher Chem. Silylation for GC

Hydrochloric acid 37% Merck Acid for ICP test

Sulfuric acid 32% Merck Acid for ICP test

Nitric acid 35.5% Merck Acid for ICP test

2,4-dinitroaniline - Sigma-Aldrich Layered support

Methyl red - Sigma-Aldrich Hammett indicator

Neutral red - Sigma-Aldrich Hammett indicator

4-nitroaniline - Sigma-Aldrich Hammett indicator

4-chloroaniline - Sigma-Aldrich Hammett indicator

Phenolphthalein - Sigma-Aldrich Hammett indicator

3.2 Overall experimental flowchart

The overall experimental works involved synthesis, characterization, and optimization of active catalyst for glycerol oligomerization to diglycerol. The flow diagram of the experimental works done is shown in Figure 3.1.

Synthesis of basic solid catalyst for oligomerization process

Characterization of modified and unmodified materials with the help of XRD, SEM, EDX, TEM, TGA, Hammett indicator, ICP–AES, BET and FTIR techniques

Product analysis for conversion and selectivity via GC analysis

Invalid catalyst for process Valid catalyst (repeat the choose for further process again) study

Optimization of reaction conditions with the best catalyst for oligomerization process to obtained the highest yield of diglycerol

Mathematical modeling of reaction kinetics of active catalyst for for the highest yield of diglycerol

Study of reusability of the best catalyst and leaching of active component

Figure 3.1: Overall experimental works involved in this study.

3.3 Equipment

There are several equipments used in this study as listed in Table 3.3.

Table 3.3: List of equipment used in the catalyst preparation and product analysis.

Equipment Brand/Model Purpose of use

Analytical Balance Mettler Toledo Weight measurement

Hot plate/ magnetic Favorite HS0707V2 Impregnation process

stirrer

Heating mantle For process of oligomerization in tree-

necked round bottom flask

Heated recirculating Neslab To age the SBA-15 sol-gel solution

waterbath

Furnace Carbolite ELF Catalyst and support preparation

11/68

Vacuum pump Buchi Water Bath Vacuum filtration for SBA-15 powder

B480 recovery

Vacuum rotary Buchi Rotavapor To evaporate solvent from mixture

R114

Oven Memmert UNB Drying of sample

Centrifuge Kubota To separate the catalyst from the product

Gas chromatograph – Agilent Tech. Column : DB-HT5, Agilent

FID 7820A Technologies,USA

Dimension: L: 15m, I.D: 0.32, t: 0.10 μm

Destop pH meter HI 2215-02 To measure pH of solutions

There were different glasswares used in this study but some of the most important ones are listed in the Table 3.4.

Table 3.4: List of some important glasswares used in the catalyst preparation and product analysis.

Glass ware Brand/Model Purpose of use

Three-necked round bottom flask Favorite Glass reactor

Glass reflex system Favorite Water condensing

Dean-Stark system USM Removing water from

system

3.4 Reactor setup for glycerol oligomerization

The experimental rig consisted of a 3-necked glass reactor, PID temperature controller system, N2 gas system, a Dean Stark system and an electric mantle heating coil system with magnetic stirrer as the main part to conduct the glycerol oligomerization process. A schematic diagram of the reactor is depicted in Figure

3.2.

Figure 3.2: Schematic diagram of the semi reactor used in this study.

A 250 ml glass semi reactor equipped with a condenser as shown in

Appendix B was fabricated to carry out the reaction. The reaction mixture (glycerol and catalyst) was placed in the reactor and a PID thermocouple (temperature sensor) was used to measure the temperature inside the glass reactor. The reactor was placed on an electrical mantle system and the temperature of the mixture was fully controlled by PID temperature controller system which was capable of maintaining the desired reaction temperature for the reaction. A continuous flow of N2 gas through reaction mixture was provided during the reaction to provide inert environment in the reaction vessel. A Dean Stark system was connected at central outlet of the reactor to separate water through condenser from the reaction system with the help of continuously flow of the N2 gas and this separated water was removed from system later on.

3.5 Catalyst preparation methods

In the present study, strong alkalis LiOH, NaOH and KOH were used as homogenous catalysts for activation of glycerol oligomerization to diglycerol. These alkalis were used directly as purchased from market without applying any further process. The preparation details of other used heterogeneous catalysts and their supports are given in the following section.

3.5.1 Preparation of SBA-15 support

The chemical composition of commercially purchased support montmorillonite K10 clay is measured by EDX for elemental analysis and results are as follow: SiO2 (43.77 %), Al2O3 (18.57 %), CaO (1.02 %), Na2O (1.03 %), and H2O

(35.61 %). From data given by company, its cation exchange capacity (CEC) is about

80–120 meq/100 g, and it has a surface area of 220–270 m2/g. The support USY was also purchased commercially with molar ratio SiO2/Al2O3 = 5.2 and surface area 660 m2/g. They were used during modification of heterogeneous catalysts without any further purification or additional processes.

As a mesoporous silica support (SBA-15) was synthesized in lab according to a reported method by Zhao and co-workers (Zhao et al., 1998b). Briefly, 4 grams of triblock copolymer P123 (EO20PO70EO20, M=5800, Aldrich) as the templating agent was dissolved in 90 ml of water and 60 ml of 4 M HCl aqueous solution under conditions stirring at 40 °C for 2 h. Next, 8.5 g of tetraethyl orthosilicate (TEOS) was then added to the homogeneous solution and stirred at this temperature for 22 h.

Finally, the mixture was heated to 100 °C and held at this temperature for 24 h under static condition. The prepared sample was recovered by ﬁltration, washed with water

76 and air-dried at room temperature. The removal of the template was carried out at

550 °C in air for 6 h.

3.5.2 Preparation of Li/ USY catalyst

The catalyst Li/USY was prepared by using wet impregnation method. LiOH was used as a lithium salt for this purpose. The required amount of LiOH (1.53 g) for support zeolite Y was first dissolved in a mixture of ethanol and distilled water (1:1) and well mixed by magnetic stir and at room temperature then the desired amount of support USY (2 g) was added this into solution under continuous stirring at room temperature. Then continuously shacked for 12 h and after that left the solution to settle down for 4 h under stationary condition. This solid part was then separated by rotary evaporator at above boiling point of ethanol and water. Then allowed this solid part to dry slowly in an open atmosphere that followed by oven drying at 100 °C overnight. The dried samples were then calcined at 500 °C with very slow heating rate of 1°C/min at start from 35 °C to 400 °C and stay at this temperature for 2 h then increased heating rate of 2°C/min to reached 500 °C for 3 h in air during which the maximum metal salt would decompose into its oxide form.

3.5.3 Preparation of Li/ SBA-15 catalyst

Alkali metal i.e. LiOH was introduced into the support through wet impregnation process. Different amounts of LiOH (namely 5, 10 and 20 wt %) were used to prepare the catalysts. The required amount of LiOH was dissolved in deionized water followed by the addition of the host (SBA-15). After stirring at room temperature for 24 h, the mixture was evaporated at 80 °C and subsequently dried at

100 °C for 4 h. The obtained solid was then calcined at 550 °C for 5 h in air. The

77 resulting SBA-15 materials with 5, 10 and 20 wt % of lithium loading were denoted as Li5/SBA-15, Li10/SBA-15 and Li20/SBA-15, respectively.

3.5.4 Preparation of Li-Mg/ SBA-15 catalyst

Coating of magnesium over SBA-15 was done using wet impregnation method. The required amount of magnesium source i.e. magnesium nitrate was first dissolved in 10 ml of deionised water and 10 ml of ethanol (1:1). Then, 2 g of the prepared SBA-15 support was added and left under stirring for 24 h and allowed to settle for 4 h. The mixture was subsequently dried using a rotary evaporator and then air dried in an oven at 100 °C for 4 h. The prepared samples were then calcined at

550 °C in air for 5 h. The resulting SBA-15 materials with 20 and 30 wt % of magnesium coating were denoted as Mg20/SBA-15 and Mg30/SBA-15, respectively.

Next, lithium was introduced into the prepared SBA-15, Mg20/ SBA-15 and

Mg30/ SBA-15 via wet impregnation using lithium hydroxyl solution. To give 10 % of lithium in the finalized catalyst was first dissolved in deionized water, followed by the addition of the support. After stirring at room temperature for 24 h, the mixture was evaporated at 80 °C and subsequently dried at 100 °C for 4 h. The obtained solid was then calcined at 550 °C for 5h in air. The resulting SBA-15 materials with 10 wt% of lithium loading with 20 and 30 wt % magnesium coating were denoted as

Li10-Mg20/SBA-15 and Li10-Mg30/SBA-15, respectively.

3.5.5 Preparation of Li/ MK-10 catalyst

Li/ MK-10 catalyst was prepared by using wet impregnation method. Lithium hydroxide (LiOH) was used as a lithium salt for this purpose. The required amount of

LiOH (1.53 g) for the support was first dissolved in a mixture of ethanol and distilled

78 water (1:1) and well shacked by magnetic stir and at room temperature then the desired amount of support montmorillonite K-10 (2 g) was added this into solution under continuous stirring at room temperature. Then shaking was continued for 12 h and after that leave the solution to settle down for 4 h under stationary condition.

This solid part was then separated by rotary evaporator at above boiling point of ethanol and water. Then allowed this solid part to dry slowly in an open atmosphere that followed by oven drying at 100 °C overnight. The dried samples were then calcined at 500 °C. Actually, some previous researchers already work on clay and found that clay cannot stable above than 500 °C. Therefore to activate clay catalyst for required process the temperature window was set with maximum 500 °C. Hence, calcine the prepared catalyst at this temperature with very slow heating rate of

1°C/min at start from 35 °C to 400 °C and maintained at this temperature for 2 h then increased at 2°C/min to reach 500 °C for 3 h in air during which the maximum metal salt would decompose into its oxide form.

3.6 Characterization of support and catalyst

All the support materials and corresponding catalysts were subjected to comprehensive characterization tests to study the properties of their surface and structures using appropriate measurement tools. The instruments used for the characterization are briefly summarized in Table 3.5.

Table 3.5: Instrument used for characterization of the catalysts.

Instruments Properties

X-ray diffraction (XRD) Structures of synthesized materials and

of phases present materials

Small angle X-ray Scanning (SAXS) A close view of phases present at low

angle 0- 5ᵒ

Nitrogen adsorption-desorption Specific surface area, pore size and pore isotherms volume

Transmission electron microscope Confirmation of the highly ordered

(TEM) structure of the porous material

Scanning electron microscope (SEM) Surface morphology of prepared

materials at high magnification

Energy-dispersive X-ray (EDX) Quantiﬁcation of the elemental

Spectroscope compositions in samples

Hammett indicators (H_) Basic strengths in a specific range of

materials

Themogravimetric - differential thermal Study the stability of the catalyst with analysis (TGA-DTA) increasing temperature

Inductively coupled plasma atomic Analysis of elemental composition emission spectroscopy (ICP-AES)

Fourier transformed Infrared (FT-IR) Chemical composition (specific function spectrometry group)

3.6.1 X-ray diffraction (XRD)

X-ray diffraction (XRD) patterns of the mesoporous material samples were recorded by small angle X-ray diffraction (SAXRD) using a Bruker D 8 Advance diffractometer. The system was operated using Cu-Kα radiation (λ = 1.5406 Å) and taken in the range of 1-5˚ (2θ) with a step size of 0.01o/ min at 40 kV and 30 mA.

Special glass capillaries (1.0 mm in diameter) were used to hold the powder samples.

The unit cell parameter (ao) and d-spacing were calculated using the analogous formulae and Bragg's law as following,

ao = 2d100/√3 (3.1)

nλ = 2dsinθ (3.2)

Where, n is an integer, λ is the wavelength of incident wave, d is the spacing between the planes in the atomic lattice, ao is cell parameter and θ is the angle between the incident ray and the scattering planes. This SAXRD analysis was done at the School of Physical Sciences, USM. The crystallization phases of the synthesized catalyst before and after impregnation were studied using X-ray diffraction (XRD) method with full range 5-90˚. The analysis was carried out using a

Bruker X-ray diffractometer operated at 40 kW. The diffraction pattern was produced when a crystalline material is irradiated with a collimated beam of X-ray.

The diffraction patterns of the samples were recorded using Cu-Kα radiation and taken in the range of 5-90˚ (2θ) with a step size of 0.03o. This XRD analysis was conducted at the School of Materials and Mineral Resources Engineering, USM.

3.6.2 Surface analysis (BET, BJH, Vp, Dp)

Nitrogen adsorption-desorption measurements were carried out using a surface analyzer Micromeritic ASAP 2020 which is available at the School of

Chemical Engineering, USM. The data obtained for specific surface area were calculated using the BET model (SBET), and the total pore volume (Vp) was determined based on the nitrogen adsorption at a relative pressure of 0.98. The pore size was obtained from the maximum of pore size distribution using the Horvath-

Kawazoe method. The mesopore diameter (Dp) corresponded to the maximum of the pore size distribution micropore was measured using the de Boer method. The measurement of the amount of gas adsorbed over a range of partial pressures at a single temperature results in a graph known as an adsorption isotherm while desorption isotherm was obtained by measuring the quantities of gas desorbed from the sample as a relative pressure was lowered. The nitrogen adsorption-desorption isotherm results were analysed and examined using the physisorption isotherms and hysteresis loops as classified by IUPAC (Buchmeiser, 2003) as a reference.

3.6.3 Transmission electron microscope (TEM)

The presence of hexagonal arrays of mesoporous channels in the SBA-15 was confirmed by TEM images. The images were taken using a Philips CM 12 transmission electron microscope operated at 80 kV which is available at the School of Biological Sciences, USM. The sample of about 0.05 g was first dissolved in a 3 ml of 100 % acetone. Then, the solution was shaken for 30 seconds then the floating powder (light powder) was sucked out slowly with a micropipette and dropped on the grid for the analysis.

3.6.4 Scanning electron microscope (SEM)

Scanning electron microscopy analysis was performed using Zeiss Supra scanning electron microscope equipment operated at 3.00 kV available at the School

82 of Materials and Mineral Resources Engineering, USM. The SEM analysis was performed to investigate and determine the structure and morphology of the surface.

The samples were dried prior to the analysis. To observe differences in the sample structure before and after impregnation and absence or presence of metal precursor on the samples, this analysis was performed.

3.6.5 Energy dispersive X-ray (EDX)

The SEM is equipped with Oxford INCA 400 energy dispersive X-ray (EDX) system with operating voltage in the range of 0.1 kV to 30 kV. The EDX was used to obtain quantitative elemental composition of the catalysts surface. The EDX analysis used Mn Kα as the energy source operated at 15 kV of accelerating voltage, 155 eV resolutions and a takeoff angle of 22.4˚. The analysis was conducted at the School of

Materials and Mineral Resources Engineering, USM.

3.6.6 Basic strengths (H_)

The basic strengths of the materials were determined by using Hammett indicators (H_) according to a published method (Abdullah et al., 2011). About 25 mg of the sample was shaken with 5.0 ml of a solution of Hammett indicators diluted with methanol, and left to equilibrate for 2 h. After the equilibration, the color on the catalyst was noted. The following Hammett indicators were used: methyl red (H_ =

4.8), neutral red (H_ = 6.8), bromthymol blue (H_ = 7.2), phenolphthalein (H_ =

9.3), 2,4-dinitroaniline (H_ = 15.0), and 4-nitroaniline (H_ = 18.4). About 25 mg of sample was shaken with 1 mL of a solution of Hammett indicator diluted in 10 mL methanol. After equilibration (2 h), the suspension was examined for color change.

To measure the amount of basic sites, a titration method was also used as 50 mg of

83 sample was shaken in 10 mL of aqueous HCl (0.05 M) for 24 h and the slurry was separated by means of a centrifuge. The remaining acid in the liquid phase was back titrated with aqueous NaOH (0.01 M).

3.6.7 Themogravimetric - Differential thermal analysis (TGA-DTA)

TGA and DTA analysis were applied to the precursors and support materials corresponding catalysts to determine the thermal stability of support materials or the prepared catalysts. It was also useful to identify the temperature for conversion of the metal precursor to prepare the modified catalysts. This analysis was carried out using a Perkin Elmer STA-6000 system to measure the possible mass loss of the prepared catalyst at temperatures between 100 to 800 ˚C. The sample (ca. 7 mg) was heated from room temperature to 800 ˚C at 10 ˚C/ min. The range of temperature was selected to study the effect of calcination temperature on the mass loss of the prepared catalysts. The DTA plot showed an endothermic peak at 100 ˚C, which could be due the loss of physisorbed water and the TGA plot, shows a weight loss from 100 to 800 ˚C. These peaks were indicative of the transition from the metal precursor to the active component of the catalysts material.

3.6.8 Inductively coupled plasma atomic emission spectroscope (ICP-AES)

The elemental analysis of Li, Mg, Si and Al was done using Perkin

Elmer Optima 3000 DV ICP-AES spectrometer. The analysis was conducted at the

School of Materials and Mineral Resources Engineering, USM. For detection of metal contents in solid catalysts, 0.1 g sample was digested in 10 ml of concentrated

32 % HNO3 and diluted to 10 ml deionized water and then it was shaken it with heating at 50 oC for 2 h. The resulting solution was filtered and then the filtrate was

84 again diluted with deionized water to a final volume of 100 ml. Then, the required amount of sample was introduced in ICP-AES.

3.6.9 Fourier transformed infrared spectrometer (FTIR)

FTIR spectra using KBr pressed disk technique were obtained using a

Perkin–Elmer 1725X Fourier transformed infrared spectrometer. The analysis was conducted at the School of Chemical Engineering, USM. 0.9 mg of sample and 63 mg of KBr were weighted and then ground in an agate mortar for 10 min before making the pellets. The spectra were collected for each measurement over the spectral range of 400–4000 cm−1 with a resolution of 4 cm−1.

3.7 Catalytic activity

3.7.1 Oligomerization process

The activity was studied by means of conversion of glycerol and the selectivity toward diglycerol. The etherification reaction was preceded through polymerization the conversion of glycerol to polyglycerol by removing one water molecule. Glycerol etherification was carried out at 240 oC in a three-neck glass reactor equipped with a PID temperature controller and magnetic stirrer. This semi batch reaction system was conducted at atmospheric pressure under inert media (N2 gas) in the presence of 2 wt% of catalyst. Water that was formed during the reaction was eliminated and collected using a Dean–Stark system. In a typical experiment, the reactor was charged with 50 g of anhydrous glycerol and 1.0 g of catalyst was added.

The reactor was heated to the desired reaction temperature. After each 2 hours, the sample product was withdrawn for GC analysis.

3.7.2 Analysis of reaction product

Samples were collected at every 2 h of time interval (0–12 h) and quantitatively analyzed after silylation using derivatization technique as explained in

Apendix D. Weighted amount (50 mg) of the liquid sample was mixed with carefully dried pyridine (1.5 ml) in a screw-capped septum vial (4 ml). After dissolution, hexamethyldisilazane (HMDS, 0.2 ml) and trimethylchlorosilane (TMCS, 0.1 ml) were added and the mixture was heated to 70 °C for 1 h. An aliquot of the solution

(0.05 ml) was diluted in dried toluene (2 ml). The solution (1 μL) was then injected into a GCD 7820A system (Agilent Technologies) capillary polar column DB-HT5

(L: 15m, I.D: 0.32, t: 0.10 μm, Agilent Technologies) for diglycerol analysis in a temperature-programmed mode (ramp 10°C min−1) from 60 to 250°C. Retention times of the silylated components were to tR=3.205 min, tR=8.632 min and tR=13.104 min recognized to glycerol, diglycerol and tiglycerol respectively as shown in Figure

3.3. This figure also shows the presence of at least three different peaks over diglycerol and triglycerol retention times in GC chromatogram which are attributed to the three linear and branched isomers of diglycerol i.e. ββ’, αβ and αα’, respectively.

Figure 3.3: Typical GC analysis of product mixture during conversion of glycerol at 240 oC.

The calibration of products of glycerol, diglycerol and triglycerol were done by different composition of their relevant standard reagents (99%Glycerol; Sigma-

Aldrich, >90% Diglycerol; Solvay Chemicals, >90% Triglycerol; Sigma-Aldrich).

The data for calibration curve and other product analysis is also attached in Appendix

C. Glycerol conversion (%), diglycerol yield (%) and diglycerol selectivity (%) were calculated using the following equations;

Wt. of glycerol reacted Glycerol conversion (%) = x 100 % ...... (3.3) Wt. of glycerol initially taken

Wt. of diglycerol produced Diglycerol yield (%) = x 100 % ……. (3.4) Wt. of glycerol initially taken

Wt. of diglycerol produced Diglycerol selectivity (%) = x 100 % ……. (3.5) Wt. of converted glycerol

Further, details about glycerol conversion, diglycerol yield and selectivity calculation for product sample through GC-FID standard peaks and ESTD reports are shown in Appendix E.

3.8 Optimization studies

The purpose of this study is to determine the functional relationship for solvent free oligomerization reaction in the presence of clay catalyst (Clay Li/ MK-

10) in regards to three operating variables as identified earlier: reaction temperature

(x1), reaction time (x2) and catalyst loading (x3). Besides, given fixed reactor volume, a change in temperature may result in a change of pressure. Therefore pressure could not be considered as an independent variable. In fact, pressure had no effect on the decomposition compared to temperature and residence time (Daneshvar et al., 2008).

The optimization was done based on the Response Surface Methodology (RSM).

The optimization of developed oligomerization process was done by

Response Surface Methodology (RSM) using Design of Experiment (DOE). This was done based on the reliability of the Response Surface Methodology (RSM) in conjunction with Central composite Rotatable Design (CCRD) using the Design

Expert Software Programme. An advantage of the CCRD lies on its ability of giving a response value when the independent variables were changed in a continuum scale, spanning the boundary limits of the experimentation. Therefore, the optimization was done in regards to three numerical operating factors such as temperature catalyst

88 loading and reaction time. The optimization was conducted for both responses of glycerol conversion as well as diglycerol selectivity.

The first requirement for RSM is the Design of Experiments (DOE) to determine the number of runs that are able to give adequate and reliable measurement for the response of interest (Aslan, 2008). The significant range for each variable was chosen towards the conversion of glycerol as well as for diglycerol selectivity to design the experiment. The Central Composite Rotatable Design

(CCRD) with three variables was carried out in order to obtain the required data for the optimization of the oligomerization process. The ranges of values of each three variables were defined as follows: reaction temperature (a) 200-280 oC, weight of catalyst (b) 0.5-3 wt. % and reaction time (c) 2-12 h.The number of experimental runs was determined from CCRD based on a simple expression as follows: For three variables (n = 3) and five levels [low (-) and high (+)], the total number of experiments was 20 determined by the expression: 2n (23 = 8 factor points) + 2n (2 x

3 = 6 axial points) + 6 (center points: six replications). The 2 axial points on the axis of each design variables were situated at a distance of α = 2 and α = -2. Actually, the default α value is 1.68 for 3 variables but for sake of slightly increasing window of the values of variables we used α= 2. In addion, with default value variable were not comfortable and their values were very close so that their effects on yield was not prominent (temp = 220 and 227, time 2h and 2.39 etc). Therefore α= 2 is used to make window broad and take better effective results of variables on the activity of catalyst. Table

3.6 shows the range value for the independent variables and experimental design levels used for the study.

Table 3.6: Independent variables and levels used for central composite rotatable design.

Variables Symbols Levels

(-α) (-1) 0 (+1) (+α)

o Reaction temperature ( C) x1 200 220 240 260 280

Reaction time (h) x2 2 4 6 8 10

Weight of catalyst (wt %) x3 0 1 2 3 4

A quadratic polynomial equation was developed to predict the response for conversion and selectivity as a function of independent variables and their interaction. Analysis of the experimental data based on selected response surface design is done by fitting the data obtained to the second order polynomial equation for the responses of glycerol yield as shown in equation 3.6.

k k 2   o  ii  iii   iji j   (3.6) i1 i1 i1 ji1

Where;

Y = response of diglycerol yield

β0 = intercept coefficient

βi = linear terms

βii = squared terms

βij = interaction terms

xi and xj = uncoded independent variables

 = the error

Analysis of variance (ANOVA) was applied to estimate the effects of main variables and their potential interaction effects on the biodiesel yield. The student’s t- test was used to determine the statistical significance of the regression coefficients.

Meanwhile, the F-test was used to determine the quadratic second order equation. In this study, the variability in dependent variables was explained by multiple coefficient of determination, R2, and the model was used to predict optimum value and to elucidate the interactions between the specified range factors as suggested by

(Montgomery, 2001). In total, the number of experimental runs was 20. The sequence of the experimental run was randomized to eliminate possible uncontrolled factors that affect the legitimacy of this study. Table 3.7 exhibits the experimental matrix of coded and actual value of the variables in accordance to the DOE procedure. The optimization was carried out in the range of independent variables as given in this table.

Table 3.7: Arrangement of the central composite rotatable design (CCRD) for oligomerization process optimization.

Coded independent value Actual independent value

Temp. Time Catalyst Temp. Time Catalyst Std Run Type °C (x1) h (x2) wt % (x3) °C (x1) h (x2) wt % (x3)

12 1 Axial 0 2 0 240 12 2

8 2 Fact 1 1 1 260 8 3

2 3 Fact 1 -1 -1 260 4 1

9 4 Axial -2 0 0 200 6 2

6 5 Fact 1 -1 1 260 4 3

10 6 Axial 2 0 0 280 6 2

1 7 Fact -1 -1 -1 220 4 1

19 8 Center 0 0 0 240 6 2

20 9 Axial 1 0 0 260 6 2

16 10 Center 0 0 0 240 6 2

15 11 Axial 0 -1 0 240 4 2

5 12 Fact -1 -1 1 220 4 3

18 13 Center 0 0 0 240 6 2

11 14 Axial 0 -2 0 240 2 2

14 15 Axial 0 0 2 240 6 4

7 16 Fact -1 1 1 220 8 3

13 17 Axial 0 0 -2 240 6 0

3 18 Fact -1 1 -1 220 8 1

17 19 Axial 0 1 0 240 8 2

4 20 Fact 1 1 -1 260 8 1

The experimental results obtained were then used to elucidate for regression model fitting and subsequently the statistical analysis to ensure the adequacy of the model in representing the results. After ensuring the validity of the model, the equation was subsequently used to predict the optimum values and to plot the two- dimensional interaction plot, three-dimensional responses surfaces in addition to contour plots to determine the interactions of the parameters. The predicted response value from the model equation was subsequently compared with the response obtained from the experimental work.

3.9 Kinetic study

The kinetic of the oligomerization process using Clay Li/MK-10 catalyst was studied by carrying out the process of different temperatures from 200 to 260 oC. The experiments were carried out at the optimum conditions obtained from the optimization study. The calculations for glycerol and diglycerol concentrations were made based on GC results. The differentiation of a polynomial fit to the concentration-time data method, linear regression, slope curves and standard error linear curves were made using Polymath software, sigma plot version 10 and

Microsoft Excel 2007. The influence of temperature on the kinetic constants was subsequently determined by the well known Arrhenius model with account for the semi batch constant volume reactor system.

For semi batch reactor design equation is

dX N  r V (3.7) A dt A

Where,

NA = Initial numbers of moles (mol)

dX/dt = Differential of conversion of glycerol with respect to time

3 -rA = Rate of reaction (mol/dm .h)

V = Total volume (dm3)

Arrhenius equation for determining activation energy and rate constant:

 E  k(T)  k  Aexp a  (3.8)  RT 

Where,

k = reaction rate constant

T = reaction temperature in Kelvin

Ea = activation energy (kJ/mol)

A = pre-exponential factor or simply prefactor

R = gas constant (8.314 J/mol.K)

Six different methods of analyzing the data collected can be used; the differential method, the integral method, the method half-lives, method of initial rates, and linear and non-linear regression (least squares analysis). The differential and integral methods are used primarily in analyzing batch reactor data (Fogler,

1992). For this purpose in the present study, integral method was used to determined rate constant, Fractional life method for order of reaction and Arrhenius equation for rate of reaction and activation energy. Then put all these values in proposed

Arrhenius model equation and determined its validity with the help of experimental and calculated value of glycerol concentration verses reaction time at each reaction temperature.

3.10 Catalyst reusability study

In order to study the stability of Clay Li/ MK-10 catalyst, the experiment was repeated for up to three times using the same catalyst. The catalyst was first separated from product mixture of polyglycerol sample by first dissolved the liquid in methanol to make it less viscous with continuous shaking and then centrifuge the mixture was for 20 minutes at 4000 rpm. The particles of catalyst were settled down in centrifugal tube and then they were separated from mixture and were washed thoroughly (several times) with methanol and then further dried in an oven for 8 h at

100 oC. This catalyst was then reused for the second cycle of reaction. The same procedure was applied for the subsequent run of experiment. These reused catalysts were further characterized using XRD, FTIR and ICP-AES analysis to study the structural stability and the amount of active metal left from the support after the first, second and third reaction of glycerol oligomerization. In addition, the leaching of active component over the best proposed catalyst to liquid phase was also studied by taking sample of liquor from mixture product and analyzed it for lithium contents presence using ICP-AES analysis. The leaching amount of active component from solid best catalyst was also measured by taking 0.5 g sample from each product liquor which drawn at different reaction time during oligomerization reaction. Then dissolved these samples in 25 ml of 10 % HNO3 and well shaken it and then take required amount of the diluted sample for ICP-AES analysis. A back mathematical calculation was made from ppm amount of lithium using ICP-AES results to its amount in prepared sample for measurement of the lithium content in the form of gram weight.

CHAPTER FOUR

RESULTS AND DISCUSSION

This chapter presents the results obtained from the experimental work done and subsequently interpretation of those results addressing the synthesis and characterization of different types of catalyst for solvent-free glycerol selective oligomerization to diglycerol. This chapter has been organized into seven (7) main sections based on the chronology of the work to provide an ideal flow of information and subsequently an easier understanding of the whole study. This chapter includes results of the prepared catalysts and their characterizations works to demonstrate their thermal stability, porosity, structure, active surface and chemical properties. A logical discussion on these prepared catalyst and their characteristics to identify the most potential catalyst among different types of support (microporous, mesoporous, and layered clay) used for the reaction under given conditions. Then, attempts have been made to elucidate different effects of operating parameters on the activity of prepared catalysts. Next, some discussion will be given on the performance of these catalysts on the basis of reactant conversion and the selective towards the desired product. Subsequently, the optimization of the reaction conditions has been performed with the help of a design of experiment (DOE) method. The optimized catalyst for solvent-free glycerol oligomerization process has then been used in the kinetic study to determined rate constant, order of reaction, rate of reaction and activation energy. Later on these parameters were used to propose and validate the

Arrhenius model equation for prescribed glycerol oligomerization reaction. In the last section of this chapter, different results of the reusability study of the optimized catalyst have been presented. Overall results obtained from this study are discussed and presented according to the following sequence,

1. Homogeneous alkali catalyst for glycerol oligomerization to diglycerol

2. Modified zeolite catalyst for glycerol oligomerization

3. Stabilized modified SBA-15 catalyst for glycerol oligomerization

4. Modified clay catalyst for glycerol oligomerization

5. Design of experiments for process optimization

6. Kinetic study and parameter evaluation

7. Reusability study of the catalyst

4.1 Homogeneous alkali for glycerol oligomerization to diglycerol

Diglycerol and triglycerol are produced from the consecutive condensation of two or three glycerol molecules, respectively. They can be synthesized as linear, branched, or cyclic isomers based on the condensation process which may take place between primary and secondary hydroxyls or even through an intra-molecular condensation (Corma et al., 2007). Actually, homogeneous catalysis is preferred over heterogeneous for the following reasons

 The homogeneous catalysts are usually in solution liquid and easily access to

the reagents so there catalytic activity is improved and milder reaction

conditions can be used.

 As the catalysts is in the solution, heat transfer for highly exothermic or

endothermic reactions is not really a problem

 Reaction mechanisms are better understood.

Homogeneous catalysts usually show some disadvantages such as difficulty in their separation from the reaction mixture without having any possibility for reusability. However, they generally show higher catalytic activity as compared to heterogeneous catalysts. Usually, heterogeneous catalysts require relatively higher

97 temperature and longer reaction time than those needed in the case of homogeneous catalysts. Therefore, some of these heterogamous catalysts cannot even be applied industrially. In addition, the high cost, difficulty to functionalize, solubility in polar media, leaching of metal clusters from their surfaces, low surface area (in some cases) and their poor thermal stability may become the main disadvantages of some heterogeneous catalysts. Hence, as a preliminary study, homogenous catalyst was used as the background study on glycerol oligomerization process.

In this work, the use of different alkali metals (LiOH, NaOH, KOH and

Na2CO3) as homogeneous catalysts for solvent free oligomerization of glycerol has been attempted. This study has been performed to find an applicable, feasible, eco- friendly and cost effective approach to synthesize diglycerol via a controllable homogeneous batch process. The catalytic behaviors of LiOH, NaOH, KOH and

Na2CO3 were evaluated and compared for activity in solvent-free glycerol oligomerization to diglycerol have been evaluated and compared.

4.1.1 Oligomerization reaction

Selective oligomerization of glycerol to polyglycerol using either homogenous or heterogeneous catalysts is an attractive pathway for organic chemists as it directly gives access to value-added chemicals. The products of glycerol oligomerization are diglycerol, triglycerol and tetraglycerol. Among these products, diglycerol is the most favorable one due to its characters as a moisturizer in cosmetics and pharmaceutical industries and more recently as a biodiesel additive.

The reaction involves the conversion of glycerol to diglycerol by removing one water molecule and the subsequent removal leads to the formation of triglycerol and subsequently tetragycerol.

The catalytic activity of the investigated catalysts has been measured based on glycerol conversion and corresponding selectivity to diglycerol during the solvent-less oligomerization of glycerol. The reaction was carried out at 240 oC for up to 8 h. In some cases, leaching of the active phase into the reaction medium and the formation of acrolein at high reaction temperature are the two main drawbacks of heterogeneous catalysts. On the other hand, the use of organic solvents for glycerol oligomerization in the presence of homogeneous catalysts might complicate their separation after the reaction to consequently affect the process economy and products purity. Therefore, the current study addresses a solvent-free process in the presence of alkali metals. Effects of different reaction variables have been particularly investigated. The reproducibility of the obtained results has been checked by repeating some experiments and no more than 7 % error has been observed.

4.1.2 Performance of different catalysts

Figure 4.1 presents the performance of 2 wt. % of four different catalysts used for selective glycerol conversion to diglycerol via etherification reaction at 240 oC. The highest conversion of glycerol (ca. 100 %) was achieved after 6 h in the presence LiOH and NaOH catalysts. However, the application of 2 wt % of NaOH resulted in slightly lower glycerol conversion and diglycerol selectivity than those obtained using 2 wt % LiOH catalyst. This difference became significant after 2 h of reaction, explaining the key role of Li ions in promoting glycerol conversion and enhancing diglycerol formation.

100 100

80 80

60 2%LiOH 60 2% NaOH 2% KOH 2%Na CO 40 2 3 40

Glycerol Conversion (%) Conversion Glycerol

Diglycerol Selectivity (%) Selectivity Diglycerol 20 20

0 0 0 2 4 6 8 Reaction Time (h)

Figure 4.1: Performance of different catalysts in the oligomerization process measured in terms of glycerol conversion and diglycerol selectivity (Reaction temperature: 240 oC, catalyst loading: 2 wt %).

The catalytic activities of NaOH, KOH and Na2CO3 were also examined, and the results revealed that significant glycerol conversion with good diglycerol selectivity were achieved. However, their catalytic activities were found to be lower than that of LiOH. It was also observed that Na2CO3, which was used as reference metal catalyst, presented poor glycerol conversion and diglycerol selectivity compared to those attained using LiOH. Na2CO3 might possess higher catalytic activity than that recorded in this study if applied at reaction temperature higher than

240 oC or longer reaction period than 12 h (Clacens et al., 2002). However, such extreme conditions were not used in this work to avoid significant degradation reactions on the reactants, intermediate and products.

100

Figure 4.2 shows the measured original pH values of glycerol and the mixtures of different alkali catalysts with glycerol which is measured by Destop pH meter as discussed in chapter 3. It is obvious that different catalysts resulted in different pH values after mixing with glycerol. The pH value increased in the order of: LiOH + glycerol > NaOH + glycerol > KOH + glycerol > Na2CO3 + glycerol.

This observation suggested that a mixture with higher pH value performed better during the solvent free glycerol oligomerization. LiOH was the most active catalyst during the selective glycerol conversion to diglycerol and it might be attributed to its highest alkalinity in the reaction mixture. Li has smaller ionic size and higher atomic electronegativity than other metals used in this study. Higher nuclear charge enabled it to show stronger attraction for electrons or protons during the oligomerization reaction. This might interpret the relationship between the atomic characteristics of the metals with their catalytic performance in the glycerol oligomerization reaction.

Actually, higher nuclear charge term was used here because of Lithium smallest atomic radius in the first period of alkaline metals. The atomic radius increases down the graph, therefore, its need more energy to release the proton from nucleus. Since the atomic size of lithium is smaller therefore its nuclear charge is higher. After this preliminary screening test, LiOH was selected for further study on the effects of several important reaction variables.

101

Figure 4.2: Measured pH values of glycerol in the presence of different catalysts prior to glycerol oligomerization reactions.

4.1.3 Effect of catalyst loading

Effect of LiOH catalyst loading was studied using the same reaction conditions as those mentioned in section 4.1.2. The catalyst concentration in the reaction medium was varied in the range of 10.4−83.5 mmole/ liter while the other reaction variables were kept constant. It was observed that different catalyst loadings resulted in different reaction profiles (expressed by glycerol conversion and diglycerol yield) with nonlinear relationship as shown in Figure 4.3(a) and Figure

4.3(b), respectively. The conversion of glycerol attained its maximum level of 100 % after 6 h as the amount of LiOH was increased from 2 to 4 wt %. The highest diglycerol selectivity of about 30 % was achieved after 4 h with 2 wt % of LiOH.

102

100 100 a) 80 80

60 60 0.5 % LiOH 1 % LiOH 2 % LiOH 40 4 % LiOH 40

Glycerol Conversion (%) Conversion Glycerol

Diglycerol Selectivity (%) Selectivity Diglycerol 20 20

0 0 0 2 4 6 8 Reaction time (h)

b) 40 0.5% LiOH 1% LiOH 2% LiOH 4% LiOH 30

Diglycerol Yield (%) Yield Diglycerol

0 2 4 6 8 Reaction Time (h)

Figure 4.3: Inﬂuence of LiOH catalyst loading on (a) glycerol conversion and corresponding diglycerol selectivity and, (b) diglycerol yield at 240 °C.

103

The conversion of glycerol as well as diglycerol selectivity was found to gradually increase as the LiOH amount was increased from 0.5 to 2 wt %. However, further increase beyond 2 wt% resulted in decreasing trends observed in both glycerol conversion and diglycerol selectivity. The same trend of diglycerol yield was observed and the maximum diglycerol yield achieved was 29 %. It was a level at which the highest reaction rate was achieved. Further increase did not bring about the desired effect as the reaction could have been limited by the mass transfer during the reaction. Based on Figure 4.3(a) and Figure 4.3(b), the selectivity and yield of diglycerol at 4 wt % of LiOH loading suddenly decreased after 2 h of reaction and came close to zero value after 6 h. It was already found that complete glycerol conversion was obtained at the same point, low selectivity and yield for diglycerol indicated the subsequent oligomerization reaction beyond diglycerol to form triglycerol and other higher oligomers.

Expectedly, increasing LiOH amount from 0.5 to 2 wt % resulted in improving conversion of glycerol molecules to polymerize into diglycerol molecule by dehydration. Besides promoting over-polymerization reaction, increasing LiOH amount further to 4 wt. % under these reaction conditions could also result in back- scission of diglycerol to glycerol. Martin and Richter, (2011) reported that the interaction between B–OH with glycerol could weaken one of the glycerol OH bonds and enhanced the nucleophilic character of the hydroxyl oxygen. Attack on this polarized glycerol molecule by the hydroxyl group of a second glycerol molecule would simultaneous split off water molecule resulting in diglycerol formation. As 2 wt % of LiOH catalyst showed the best diglycerol yield during the reactions, this loading was selected for the subsequent research work.

104

4.1.4 Effect of reaction time

The influence of reaction time on reaction profiles (glycerol conversion, diglycerol selectivity, and diglycerol yield) was evaluated using 2 wt % LiOH at 240

°C and the results are also shown in the same illustrated Figure 4.3(a) and Figure

4.3(b). It was observed that after 2 h of reaction, the selectivity toward diglycerol attained its highest value of about 30 %, but decreased as the reaction was further prolonged to 8 h. Whilst, glycerol conversion was found to linearly increase with reaction time to 8 h, the diglycerol yield was 29 % after 4 h and behaved like the selectivity to diglycerol. Nevertheless, increasing reaction time further resulted in a decrease in diglycerol yield for all the catalysts used, except for 4 wt % LiOH which showed decreasing diglycerol yield after 2 h.

Indeed, as the etherification reaction was prolonged, more glycerol molecules underwent dehydration or any other form of reaction which resulted in an increase in the conversion of glycerol. Unfortunately, during this conversion, the cleavage of glycerol molecules might not exactly result in polyglycerol form, but in some other forms of by-product such as acrolein due to the double dehydration of glycerol.

These by-products are not desirable in the etherification reaction and might lead to unfavorable products (Clacens et al., 2002). Subsequently, the diglycerol molecules that were formed might be converted into higher glycerol ethers under uncontrolled reaction conditions. This may also happen, in some cases due to the weak formation of two polymerized glycerol molecules (diglycerol isomers) which might not be highly stable under given reaction conditions. Therefore, the selectivity and yield of diglycerol gradually started to decrease after 2 h of reaction time.

105

4.1.5 Effect of reaction temperature

It has been well established that chemical reaction rate is strongly influenced by reaction temperature. Therefore, the influence of reaction temperature in the range of 180 to 260 °C on glycerol oligomerization was investigated. As shown in Figure

4.4, the maximum glycerol conversion after 6 h was 100 % and it was achieved in the presence of 2 wt% LiOH at 240 °C. However, the selectivity to diglycerol at this point was not high (< 20 %) and it decreased with increasing temperature and reaction time. Meanwhile, oligomerization reaction rate was lower at a lower reaction temperature of 240 °C, achieving a glycerol conversion of 23 % (after 8 h) and a diglycerol selectivity of lower than 5 %.

Figure 4.4 shows that glycerol conversion increased uniformly with increasing reaction temperature from 240 to 260 oC. On the contrary, diglycerol selectivity decreased with increasing reaction temperature beyond 240 oC. The observation suggested that reaction temperatures higher than 240 oC might have speeded up the conversion of the remaining glycerol and enhanced the consecutive etherification reaction of diglycerol to higher glycerol oligomers to consequently result in decreasing diglycerol yield. These observations were in good agreement with earlier reported results (Charles et al., 2003; Clacens et al., 1998).

106

100 100 after 2h after 6h after 8h 80 80

60 60

40 40

Glycerol Conversion (%) Conversion Glycerol 20 20 (%) Selectivity Diglycerol

0 0 180 200 220 240 260

Reaction Temperature (oC)

Figure 4.4: Effect of reaction temperature on glycerol conversion and corresponding diglycerol selectivity. (Catalyst loading: 2 wt % of LiOH).

4.1.6 Diglycerol isomer distribution in the product mixture

A minor differences were observed in the diglycerol isomers distribution when the activities of homogenous LiOH catalyst was compared at different reaction time (2 h and 8 h) by keeping the temperature constant at 240 oC as shown in Figure

4.5. This figure suggests that the reactivity of the primary OH group of the first glycerol molecule at the first position of other glycerol molecule (αα’), have some importances in the presence of homogeneous catalysts for oligomerization process. It is clear from this figure that synthesis of the diglycerol isomer αα’ was enhanced during the homogeneously catalyzed oligomerization reaction. It can be seen from the bar chart that the contents of ββ’ and αβ isomers in both cases of reaction times were less than that of αα’ isomer value. This might be a strong indication of the

107 interaction that took place between the activated glycerol molecule and proton provided by LiOH. Martin’s research group (Martin et al., 2012) also observed similar trend of diglycerol isomers distribution during glycerol oligomerization reaction using homogeneous CsHCO3 catalyst. It can be concluded from these observations that in homogeneous catalytic system, the distribution of αα’ isomer was higher than those of ββ’ and αβ isomers during the glycerol oligomerization reaction.

100

80 ' '

  20 ' '

Diglycerol isomers distibution (%) distibution isomers Diglycerol

0 2 h 8 h Reaction Time (h)

Figure 4.5: Diglycerol isomers distribution in homogenous LiOH catalyzed oligomerization at different reaction time at 240 oC.

Overall, solvent free etherification of glycerol to diglycerol in the presence of various alkaline metal precursors as a homogeneous catalyst was successfully investigated and the results were compared with those obtained with Na2CO3 as a

108 reference catalyst. LiOH catalyst showed unique activity for glycerol etherification, achieving complete glycerol conversion with a corresponding 33 % selectivity toward diglycerol as the desired product. The best reaction conditions were 240 oC for the reaction temperature, 2 wt % for the catalyst loading and 6 h for the reaction time. Glycerol conversion and the selectivity to diglycerol were found to be influenced by the atomic size of the metal catalyst as well as the reaction conditions.

For further study, these reaction conditions and lithium precursor as an active component were applied on different supports as heterogeneous catalyst for enhancing the conversion of glycerol as well as the selectivity to diglycerol via solvent-free oligomerization.

4.2 Modified zeolite catalyst for glycerol oligomerization

Solid base catalysts exhibit high activities and selectivities for many kinds of reactions such as; condensation, alkylation, cyclization and isomerization which are carried out using liquid bases as catalysts in industrial applications.

Many of these applications require stoichiometric amounts of the liquid base for conversion of reactants to the selective product. The substitute of these liquid bases to solid base catalysts would make easier separation of catalyst from the product as well as possible reusability of that catalyst. The major advantage of the heterogeneous catalysts over homogeneous is that they can be separated and reutilized rather easily.

In addition, heterogeneous catalysts are simple and cheap compared to homogeneous ones. Despite these advantages, they generally show lower catalytic activity as compared to homogeneous catalysts. Some of them cannot even be applied industrially. Usually, heterogeneous catalysts require relatively higher

109 temperature and longer reaction time than those needed in the case of homogeneous catalysts. In addition, the high cost, difficulty to functionalize, solubility in polar media, leaching of metal clusters from their surfaces, low surface area (in some cases) and their poor thermal stability are the main disadvantages of some heterogeneous catalysts.

Commercially, several types of heterogeneous catalyst are available but they can be categorized into three main groups according to their surface area properties i.e. microporous (< 2 nm), mesoporous (50 > nm > 2) and macroporous (> 50 nm).

Zeolite catalyst belongs to the microporous category. Due to strong catalytic activity of X and Y zeolites, they are commonly used in cracking, hydrocracking, and isomerization of hydrocarbons. The ultra stable zeolite Y catalysts (called herein later as USY) provide vastly superior combinations of strong acid catalytic sites, uniformity of pore structure, and stability, all of which could offer improved selectivity, yield, durability, and production cost of the desire products in those reactions. Hence, miroporous USY was used as a support to the active component i.e. lithium for selective glycerol conversion to diglycerol via solvent-free oligomerization process and the prepared modified catalyst represented as Li/USY in the later on discussion.

4.2.1 Lithium modified zeolite catalyst (Li/USY)

Basic zeolites generally receive significant attention in the catalysis community. These zeolites modified with alkaline metals are potential catalysts due to its unique features. USY is a faujasite molecular sieve with 7.4 Å diameter pores and a three-dimensional pore structure (Taufiqurrahmi; et al., 2011). The basic structural units for this type of zeolites are the sodalite cages, which are arranged so

110 as to form supercages that are large enough to accommodate spheres with 1.2 nm diameter. The primary application for USY is catalytic cracking of petroleum into gasoline range hydrocarbons. Among the all zeolites used in industrial scale, USY are the most widely employed materials (Kim et al., 2008). The oligomerization reaction of glycerol reaction is catalyzed by basic catalyst. Therefore, the incorporation of alkaline active component into zeolite type support might be helpful for this process. Hence, for increasing the basic strength of such zeolite material, first group elements in the periodic table among the best choices as active components and among them, lithium is the most attractive due to its small radii size and high value of alkalinity as proven in section 4.1. Therefore, lithium modified zeolite

(Li/USY) has been prepared for glycerol conversion and polyglycerol production especially with the interest of high selectivity to diglycerol during the oligomerization reaction in the absence of any solvent.

4.2.2 Characterization of Li/USY catalyst

The XRD patterns for USY support and modified Li/USY catalyst sample are shown in Figure 4.6. It can be noticed from the figure that the patterns for both samples are almost identical but peaks intensity of sample after lithium loading decrease a lot. Actually, after strong alkali treatment, the crystalline structure of USY was dissolved and cracked due to strong alkali in the form of Lithium was presence over USY which made up of Si and Al structure. Therefore, after calcinations crystalinity of USY was affected. In addition, these both were found similar to the pattern of a typical Faujasite. This indicates that FAU structure of the sample present in the samples (as indicated by the blank square over the peaks of XRD patterns in

Figure 4.6). Tosheva’s research group (Tosheva et al., 2012) observed similar trend

111 of XRD patterns and claimed that FAU structure was presented in the sample. This means that modified USY successfully sustained its structure after the lithium application. However, there was a clear broadening of the reflections for the modified sample and it was also found to experience a decrease in its peak intensity which might be attributed to some modification in its porous area after the presence of some lithium components.

(b)

Intensity (Cps) Intensity

(a)

10 15 20 25 30 35 40

2 

Figure 4.6: XRD patterns of (a) USY and (b) Li/USY, with the square marks indicating the FAU structure.

The nitrogen adsorption-desorption isotherms of the USY and Li/USY are shown in Figure 4.7. This isotherms figure indicates that it is an excellent micro

112 porous material. These both of samples exhibited type-II isotherms (IUPAC classification), characteristic of microporous materials where N2 uptake increased quickly at low relative pressure (P/P0) due to the adsorption in micropores and external surface. After the monolayer adsorption at low P/P0, the N2 uptake was not constant indicating that the presence of mesopores was insigniﬁcant. The N2 adsorbed volume to reach monolayer of Li/USY was slightly lower than that of USY.

Similar nitrogen adsorption isotherms were reported by (Yao et al., 2008) and

(Schneider et al., 2008) for sodalite and NaY zeolites, respectively.

Figure 4.7: Nitrogen adsorption-desorption isotherms for parent USY and lithium modified Li/USY catalyst.

113

The pore size distribution curves corresponding to nitrogen adsorption- desorption isotherms of the parent USY and lithium modified Li/USY catalyst are shown in Figure 4.8. Almost both of zeolite samples have a sharp peak in the distribution centered around 5-6 Å with the broader distribution between 4-10 Å which matches the size of USY micropore cavities (1.2 nm). The Li/ USY size distribution was also found narrow with a mean value close to USY. Due to these observations it can be suggested that the most of the lithium particles might be located inside the microporous area.

0.25

0.20 USY Li/ USY

0.15

0.10

dV/dw (cm³/g·Å)

0.05

0 5 10 15 20 25 Pore Width (Å)

Figure 4.8: Pore size distributions for parent and lithium modified USY.

The structural properties of both USY and Li/USY were obtained from the nitrogen adsorption-desorption isotherms and they are listed in Table 4.1. From these data, one can observe signiﬁcant decreases in the BET surface area, the micropore

114 area and micropore volume of the lithium modified zeolite sample. Meanwhile, the external surface area was found to increase after this modification. The BET surface area of Li/USY was 531m2/g which slightly decreased from a value of 690 m2/ g of

USY but its external surface area was found to increase compared to that of its parent zeolite i.e. from 72 to 102 m2/ g. The decreases in the micropore area and micropore volume might due to the incorporation of lithium cations which possibly resulted in a certain difficulty in accessing the zeolite pores. The average pore diameter of

Li/USY was found to increase after modification; obviously it was due to the presence of strong alkali which affected the crystallinity of the framework of the parent zeolite. The basic strength of both samples were also measured by means of

Hammet indicators and it was found to increase from 4 < H_ < 9 to 9 < H_ < 15 after lithium modification. It is obviously due to presence of strong alkali inside the pores of USY which caused an increase in its basic strength.

Table 4.1: Surface characteristics and basic strengths of USY and Li/USY catalysts.

Surface properties USY Li/USY

BET surface area (m2/g) 690 531

External surface area (m2/g) 72 102

Micropore surface area (m2/g) 612 429

t-plot pore volume (cm3/g) 0.274 0.261

Average pore size (nm) 0.37 0.95

Basic strength (H_) 4 < H_ < 9 9 < H_ < 15

The microporousity of the corresponding curves were also confirmed from the data of this table in which it is clear that the USY and Li/USY exhibited

115 decreasing micropore surface area from 612 to 429 m2/ g with slightly decrease in volume from 0.27 to 0.26 cm3/ g, respectively. However, the pore size of modified zeolite was found to be increased from 0.37 to 0.95 nm. This might be due to the presence of lithium inside the porous area rather than at the face of porous system.

The decrease in surface area of USY is a usual phenomenon after lithium loading.

The change in pore volumes of both samples was considerably low, suggesting a good dispersion of lithium particles on the surface of the support USY. It can be concluded that lithium directly affect USY surface properties by decreasing its surface area and increasing its average pore size and basic strength after lithium modification.

Scanning electron micrograph (SEM) technique was used to compare the morphology between the parent USY and the prepared Li/USY. Typical micrographs of both samples are shown in Figure 4.9(a) and Figure 4.9(b), respectively. It can be noticed from the figures that the sizes of the parent zeolite crystals were rather uniform with sharp edges but after lithium modification (Li/USY), the morphology of its surface was rougher with angular-shaped crystals. This was expected due to the presence of strong alkali which could affect the crystallinity of parent USY.

Figure 4.9: SEM images of (a) parent USY and, (b) Li/USY catalyst.

116

4.2.3 Activity of Li/USY for glycerol oligomerization

The efficiencies of USY and Li/USY catalyst in glycerol conversion to polyglycerol as well as diglycerol selectivity were compared with those of conventional homogenous catalyst LiOH as presented in Table 4.2. For industrial application of a catalyst high selectivity at high conversion is very important. The prepared Li/USY catalyst showed high activity (98 wt %) for glycerol conversion and polyglycerol yield (72 wt %) after 8 h reaction at 240 ᵒC. For comparison, LiOH gave a glycerol conversion of 99 wt % and a polyglycerol yield of 52 wt % under same reaction conditions. It was found that diglycerol selectivity over modified zeolite also enhanced (29 wt %) comparatively to parent zeolite and homogeneous

LiOH (≤18 wt %). High activity for glycerol conversion and significant selectivity to diglycerol was demonstrated by the prepared modified zeolite catalyst due to the presence of alkali active component (lithium). It is also clear from this table that the selectivity to diglycerol over Li/USY was low compared to triglycerol and tetraglycerol. It can be suggested from the behavior of diglycerol selectivity that it possibly most of reaction might be occurred over the external surface of catalyst instead of the internal surface area. Due to external surface reaction, high conversion of glycerol to diglycerol could be expected to occur on the open area of the catalyst but might be continued the reaction of glycerol with newly formed diglycerol to form higher oligomer. In addition, the reaction inside the pores as internal diffusion of reactant and products could be slow in the micropores of the zeolite.

117

Table 4.2: Glycerol conversion to polyglycerol with 2 wt % of each catalyst at 240 ᵒC for 8 h reaction time.

Parameter LiOH USY Li/USY

Glycerol Conversion (wt %) 99 15 98

Diglycerol Selectivity (wt %) 18 2 29

Triglycerol Selectivity (wt %) 21 7 24

Tetraglycerol Selectivity (wt %) 13 4 19

Others (wt %) 48 67 28

Polyglycerol Yield (wt %) 52 13 72

Actually, on the external surface of the catalyst, it was not easy to control the chemical reaction involving the interaction between active radical with a specific component due to its open free space. It meant, all free to interact with any favored molecule of any size. Thus, with the passage of time, two glycerol molecules etherified into diglycerol and subsequently interacted with another glycerol molecules to form triglycerol the reaction step continued leading to the production of higher oligomers during the etherification reaction. Thus, the shape and size of the pores may be considered critical factors to cause the undesired situation. Basically, the pores structure of the catalyst has a direct influence on the product selectivity and it leads to shape selectivity effect that can allow selected reactions. The comparison between surface properties in Table 4.1 and their activities for selective glycerol conversion in Table 4.2 leads to the conclusion that the basic strength and surface area of Li/USY catalyst was sufficiently high. However, higher oligomers were favored instead of diglycerol. Overall, activity and yield of Li/USY were found to be higher than those of homogenous LiOH under the same reaction conditions.

118

Figure 4.10 shows glycerol conversion and polyglycerol yield demonstrated by 2 wt % of Li/USY catalyst at 240 oC for up to 8 h. For the glycerol conversion plotted against time, an activation period was observed in the first 2 h of reaction with high glycerol conversions of above 40 wt % at 240 and 260 oC while it was below 10 wt % at 220 oC. The glycerol conversion gradually increased with increasing reaction time at temperatures 240 and 260 oC and then reached above 90

% after 8 h at these temperatures. However, at a low temperature of 220 oC it attained a maximum conversion of 45 wt % only after 12 h. It is also evident from this figure that yield to polyglycerol were significantly high at reaction time 6 h at

240 and 260 oC temperatures. It started to decrease with further increase in reaction time in case of higher temperatures (240 and 260 oC). It is also notable that the maximum yield to polyglycerol (72 wt %) at 240 oC was achieved within 8 h after which it gradually decreased and reached less than 50 wt % after 12 h. In addition, polyglycerols production after 6 h at a temperature of 260 oC was higher (67 wt %) than that at 240 oC.

119

100 100 220 oC 240 oC 80 260 oC 80

60 60

40 40

Polyglycerol (%) yield

Glycerol Conversion (%)

20 20 Conversion % - - - - Yield %

0 0 0 2 4 6 8 10 12 Reaction Time (h)

Figure 4.10: Glycerol conversion and yield to polyglycerol at different temperature plotted against time for Li/USY catalyst.

In other words, reaction time curve for polyglycerol production is on a declining trend with increasing reaction temperature from 240 to 260 oC as shown in

Figure 4.10. Meanwhile, Li/USY showed a rather low activity at 220 oC towards polyglycerol production throughout the reaction. The decreasing trend of polyglycerol production after 6 h was attributed to the conversion of small oligomers to higher oligomers and the effect was more noticeable at higher temperature. It might also be possible that some other by-products such as acrolein or non-polymeric compounds to be produced through other side reactions during this period of oligomerization process as previously reported such results (Jerome et al., 2008).

Details about diglycerol selectivity and yield at different temperatures at constant reaction time 8 h is shown in Figure 4.11. It is clear from this figure that the

120 selectivity and yield of diglycerol is not quite sufficient over microporous Li/USY, bearing in mind that glycerol conversion was very high and reached almost 100 % above 240 oC at 8 h of reaction time. The maximum diglycerol yield was found of 28 wt % at 240 oC and 8 h where diglycerol selectivity was noted at 29 wt % with glycerol conversion 98 wt %. The yield of diglycerol was observed decreasing with further increasing reaction time from 8 h to 12 h due to decrease in selectivity of diglycerol. This may be due to the fact that the reaction would not stop after the diglycerol formation and continues to form higher oligomers such as tri- and tetraglycerol. As such, the production of polyglycerol was high enough as proven in

Table 4.2. Another possibility is that the oligomerization reaction might occur mainly on the external surface of Li/USY as its micropores would hinder the diffusion of diglycerol and higher oligomers. Therefore, diglycerol isomers distribution was also studied to find out the reaction behavior on the internal and external surface of the catalyst.

121

100

Glycerol Conversion Diglycerol Selectivity 80 Diglycerol Yield

(%) 40

0 200 210 220 230 240 250 260 Reaction Temperature (oC)

Figure 4.11: Diglycerol selectivity and yield during glycerol oligomerization over Li/USY at a constant reaction time of 8 h.

It can be seen in Figure 4.12 that diglycerol isomer distribution behavior over microporous zeolite Li/USY was close enough to that of homogeneous LiOH. In other words, the formation of primary-primary isomer (αα’) over Li/USY was higher than secondary-primary isomer (αβ) in both cases at 240 oC and 260 oC for 8 h of reaction time. It can be suggested that the reaction might have occurred mainly on the external surface instead of internal pores of zeolite due the large molecular sizes of diglycerol or higher oligomer molecules. It could also be hypothesized that the incorporation of lithium as a highly active component over microporous zeolite could not result in a good catalyst for glycerol oligomerization reaction as this reaction involved the formation of bulky molecules. As a result, the reaction mostly occurred on the external surface area while the internal surface area was not fully utilized.

122

' ' '

 20   ' Diglycerol distibution isomers (%) ' '

LiOH 8 h @ 240 CLi/USY 8 h @ 240 CLi/USY 8 h @ 260 C

Figure 4.12: Diglycerol isomers distribution over Li/USY for different temperatures.

It was concluded that Li/USY catalyst was a sufficiently active solid catalyst for glycerol oligomerization and polyglycerol production as compared to homogenous LiOH. However, this catalyst was only suitable for the production of higher oligomers and its activity was retarded by inaccessibility of its internal micropores by bulky reactant. Thus, the internal pore sizes of the prepared zeolite based catalysts were not sufficiently large to accommodate the reaction of two glycerol molecules leading to the production of diglycerol or higher oligomers. Thus, it is necessary to identify a suitable type of mesoporous material with suitable pore size which can stand the harsh reaction conditions while having the capability to accommodate diglycerol molecules. However, too large pores should be avoided to

123 retard the formation of higher oligomers so that diglycerol can be selectively produced.

4.3 Stabilized modified SBA-15 catalyst for glycerol oligomerization

Porous materials have successful story in heterogeneous catalysis. Both microporous and mesoporous materials have been successfully used in industry for many decades. Both types of material are anticipated to be useful for conversions of higher molecular weight substances. Mesoporous catalysts have attracted considerable interests in recent years. In the present study, an attempt was made to prepare a basic mesoporous catalyst for selective glycerol oligomerization to diglycerol. Here, lithium modified mesoporous SBA-15 was synthesized and subsequently characterized to evaluate the activity in glycerol oligomerization reaction.

4.3.1 Lithium modified SBA-15 catalyst (Li/SBA-15)

SBA-15 has ordered straight mesopores as well as disordered interchannel micropores in the mesopore wall (Yang et al., 2003; Zhang et al., 2006). Since lithium was found to be an active component for glycerol oligomerization to diglycerol, different amounts of LiOH (0-20 wt %) were impregnated into SBA-15.

Recent research focus was directed towards the design of basic mesoporous materials for this type of reaction. The objective was to enhance the catalytic activity in the oligomerization reaction with increasing the basicity of the material. The structure, surface area and basicity of the prepared materials were characterizied using scanning electron microscope (SEM), energy-dispersive X-ray spectroscope (EDX),

124 transmission electron microscope (TEM), small-angle X-ray diffractometer and surface analyzer.

It was observed during the experimental work that the basicity increased with increasing LiOH loading but partial structural collapse of the hexagonal mesopores was detected after lithium loading. It was observed that 20 wt % LiOH loading totally destroyed the porous structure of SBA-15 due to alkali-silica reaction. A new fluffy type material with sharp edge structure resulted. Thus, SBA-15 evidently showed poor resistance against metal hydroxide (LiOH) and its mesoporous structure significantly charged into a highly disordered new structure.

In this work, the discussion on lithium modified SBA-15 catalyst is divided into two parts. The first part (4.3.2) deals with the characteristics and instability of lithium modified SBA-15 while the second part (4.3.3) presents the stabilization of lithium modified SBA-15 via magnesium coating and its activity in selective glycerol oligomerization to diglycerol.

4.3.2 Instability of SBA-15 to lithium modification

There are two main factors which are considered as the obstacles to the successful generation of strong basicity in mesoporous silicas. Firstly, weak host- guest interaction between silica and the base precursors can lead to the difficulty in the decomposition of the base precursors to their basic forms. It has been reported that only a small amount of alkali salt could be decomposed on silica, even when the sample was activated at a high temperature of 600 °C (Sun et al., 2008b). The second factor is the poor resistance of mesoporous silicas against alkali. It is due to the reaction between alkali hydroxides with silica hydroxylsilicates (Sinha Ray, 2003).

This can result in the collapse of the mesoporous structure after the formation of this

125 strongly basic species. The structure, surface area and basicity of the prepared materials have been studied to characterize the structural charges that are caused by the impregnation and the consequent effect in their catalytic behaviors.

4.3.2.1 Effects on surface characteristics

Nitrogen adsorption-desorption isotherms of SBA-15 and SBA-15 with different lithium loadings are given in Figure 4.13. The parent SBA-15 showed an isotherm and hysteresis loop which is generally associated with mesoporous materials (Zhao et al., 1998a). However, for Li5/SBA-15 sample with small amount of lithium loading, the hysteresis loop sharply decreases along with a decrease in surface area and pore volume. The shapes of the curves for samples Li10/SBA-15 and

Li20/SBA-15 are generally agree with the Type I isotherm according to the IUPAC nomenclature (Buchmeiser, 2003) which is a characteristic of microporous materials.

This result was in agreement with data in Table 4.3 that show loss of mesoporousity.

126

Figure 4.13: Nitrogen adsorption-desorption isotherms for Li/SBA-15 samples with different lithium loadings.

In addition, according to further classified shapes of the hysteresis loops, these three metal loaded SBA-15 samples exhibited the type H3 hysteresis loops.

This type of hysteresis is usually shown by aggregates or agglomerates of particle forming slit shaped pores (plates or edged particles like cubes). Thus, in agreement with previous study on titania (Sun et al., 2008a) the incorporation of high loading of alkali metals led to the collapse of some mesostructures leading to a severe drop in porosity. As such, the synthesis of alkali metal loaded porous silica materials through simple wet impregnation was indeed a challenging task.

The surface characteristics based on nitrogen adsorption-desorption isotherms and basic strength of the prepared catalysts are shown in Table 4.3. It is noted that the basic strength of the prepared samples was to be increased after increasing amount of lithium loading from 5 to 20 wt %. The basic strength of SBA-15 was

127 determined to be H_ < 4.8. However, after loading with 20 wt% of lithium, it was noted that the basic strength significantly increased to 9.3 < H_ <15. This observation suggested that alkali metal loading directly involved in the basicity of the materials, regardless of their surface or structural properties. It could be deduced from the sharp decrease in the area of alkali metal loaded samples that the structure of SBA-15 experienced a significant collapse due to the reaction with strong alkali

LiOH. This was also proven by changes in the mesoporous area of these samples as shown in the same table. It was found that the mesoporous area and pore volume also sharply decreased in agreement with the decreasing surface area of these samples.

Obviously, it was an indication of major loss of mesoporous structure of these samples.

Table 4.3: Characteristics of prepared Li/SBA-15 catalyst with different loadings of lithium.

Lithium Basic strength SBET SMeso SMicro VPore Sample (wt %) (H_) (m2/g) (m2/g) (m2/g) (cm3/g)

SBA-15 0 H_ < 4.8 746 699 46 1.10

Li5/SBA-15 5 4.8< H_<6.8 230 155 75 0.52

Li10/SBA-15 10 6.8< H_<9.3 65 45 20 0.25

Li20/SBA-15 20 9.3< H_<15 29 28 1 0.15

*SBET:BET surface area, dPore: pore diameter, VPore: pore volume

Data in the same table prove that the surface area of the prepared SBA-15 samples sharply decreased after loading with alkali metal. These results suggested that major destruction should have occurred with the corresponding loss of porosity and surface area. Clearly, the mesoporous channels in the SBA-15 should have

128 collapsed to form non-porous mass of silica due to the reaction with the alkali metal compound. The solid evidently consisted of small particles that were readily dispersible in water or aqueous solution during basic strength test as the basic strength was found to be increased with increasing metal loading amount.

The collapse of porous structure of SBA-15 was deemed to be directly caused by the alkali-silica reaction between the loaded metal and the silica matrix. This reaction was led to the formation of lithium silicate hydrates. This reaction caused the expansion of the altered aggregates by the formation of a swelling gel of these hydrates. This gel increased in volume with water and exerted an expansive pressure inside the SBA-15 material, causing spalling and loss of its mesoporous structure.

4.3.2.2 Effects on particle structure

The small angle X-rays scattering (SAXS) patterns of prepared SBA-15,

Li5/SBA-15 and Li10/SBA-15 are shown in Figure 4.14. Based on the SAXS patterns, SBA-15 exhibited regular X-rays scattering patterns, with an intense (100) diffraction peak and two well-resolved peaks (110 and 200). These pattern peaks were indexed as 2D hexagonal symmetry as also reported by (Yang et al., 2003).

Compared to the SBA-15 sample, the intensity of the main peak (100) for lithium loaded SBA-15 samples were invisible and other resolved peaks (110 and 200) were found to be very weak and their positions were not clear. This observation suggested that the original mesoporous structure of SBA-15 was not sustained.

129

Figure 4.14: Small-angle X-rays scattering (SAXS) patterns of the prepared SBA-15 and lithium loaded SBA-15 catalysts (Li5/SAB-15 and Li10/SBA-15).

Similar characteristics of the prepared samples were also observed in TEM images. TEM images of sample SBA-15 and Li/SBA-15 prepared by impregnation method with 10 wt % of lithium loadings are shown in Figure 4.15(a) and Figure

4.15(b), respectively. SBA-15 showed well-ordered hexagonal pores with clear mesoporous channels tubing inside the structure. However, with 10 wt % of lithium loading, the ordering of the SBA-15 support was observed to disappear. The black spot in Figure 4.15(b) shows the presence of lithium contents over the demolish structure of SBA-15. It can be concluded that no longer hexagonal structure of SBA-

15 sustained after lithium loading.

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Figure 4.15: TEM images of SBA-15 a) before lithium loading and b) after 10 wt % of lithium loading.

Figure 4.16(a-c) shows SEM images of the prepared SBA-15, Li5/SBA-15 and Li10/SBA-15. In Figure 4.16(a), the microscopic morphology of the prepared

SBA-15 is similar to generally published mesoporous structure of SBA-15 (Rymsa et al., 1999; Thielemann et al., 2011). As observed in Figure 4.3(b), SBA-15 with 5 wt

% lithium loading consisted of a large amount of small spherical particles with spiky structures on the surface, evidently contributed by the loaded metal. When the lithium loading was increased to 10 wt %, the shape of SBA-15 structure had entirely different look with a fluffy structure and sharp edges. No specific SBA-5 morphology can be seen in Figure 4.16(b) and Figure 4.16(c) and they generally showed particles with irregular shape. This was an evidence of mesoporous structure collapse due to the effect of the strong base.

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Figure 4.16: SEM images at magnification of 5000X (a) SBA-15 (b) Li5/ SBA-15 and (c) Li10 /SBA-15.

As reported by different researchers (Kloetstra and van Bekkum, 1997;

Thielemann et al., 2011), mesoporous SBA-15 could have weak resistance to strong alkali. In this work, its structure could be destroyed in the presence of alkali metals.

It is observed in Figure 16(c) that the morphology of 10 wt % lithium loaded over

SBA-15 material was made up of more fluffy structure with large circular ball- shaped particles. Thus, the analysis of TEM and SEM images of the prepared lithium loaded samples suggested that lithium was well-dispersed over the surface of SBA-

15 but its structure was not found stable. It could be concluded that the morphology of SBA-15 structure underwent significant changes after loading with this strong alkali metal. The newly prepared structures of these samples seemed to be fluffier and denser than the parent SBA-15. This type of material was unlikely to act as good basic catalyst due to its non-porous characteristics.

Actually, alkaline metal oxides are often used to improve the basic strength of a solid catalyst. Mesoporous basic catalysts are mostly prepared by impregnation of mesoporous material with alkali metals or alkali earth metals. Some previous researchers (Kloetstra and van Bekkum, 1997) also worked on the preparation of

132 strongly basic mesoporous solid catalyst especially MCM-41 incorporated with cesium oxide. However, the catalyst showed similar behavior of poor stability as cesium oxide could react with the silica host and damaged its mesoporous frame works. Lithium is the strongest alkali metal in the alkali metal group and can be used to generate strong basicity in various porous hosts such as MCM-41 and zeolites but same problem of structural collapse could occurr with other members of the first group elements in the periodic table (alkali metals). The basic reasons for the structural destruction are summarized in Figure 4.17. It can be suggested that the main problem with SBA-15 type support material might be its poor alkali-resistance.

This might be due to direct attack of strong alkali metals on the surface of silica for the formation of metal silicates which causes structural collapse as also suggested by previous researchers (Thielemann et al., 2011).

Figure 4.17: Reasons for SBA-15 structural collapse after lithium modification.

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Increasing severity of mesoporous structure destruction was detected when the SBA-15 was impregnated with increasing lithium loading as confirmed from

SEM images of the prepared sampels. This destruction was ascribed to the chemical reaction between the strong alkali metal and the mesoporous matrices to form metal silicates. The new structure of alkali-loaded SBA-15 was found to be fluffier and lighter than the parent material. The new structure was noted very small micro- and mesoporous area as confirmed from Table 4.3. This lithium modified structure of

SBA-15 was observed entirely different from parent structure of SBA-15. However, the basic strength of the prepared material was found to be increased with increasing lithium loading. Thus, despite having thicker pore walls compared to other mesoporous materials, SBA-15 still showed poor resistance to strong basic metals.

4.3.3 Stabilization of lithium modified mesoporous SBA-15

Heterogeneous catalysts provide some advantages like ease of their separation from reaction mixture and potential for catalyst reusability. However, the catalysts are usually not so active and require high reaction temperature and long reaction time for complete conversion of glycerol. Another, problem with heterogeneous catalysts is their structure stability which often collapse due to chemical reaction between the active component and the support material as discussed in section 4.3.2. Most recent studies for glycerol oligomerization reaction have been focused on the preparation of stable solid basic catalysts synthesized using alkali, alkali-oxides or alkaline earth metal precursors. However, certain degree of structural instability has been reported into many support materials.

Despite considerable basic strength improvement achieved through lithium impregnation over SBA-15, it was found that the mesostructure of SBA-15 was

134 significantly destroyed. Therefore, a more stable solid basic mesoporous catalyst was required to catalyze selective glycerol oligomerization reaction to diglycerol. A new strategy to change the basicity nature of mesoporous silica SBA-15 was attempted by precoating magnesium layer prior to lithium incorporation. Actually, magnesium was used due to its strong stabilizing property and its ability to make good binding with silica host within the mesostructure. Thus, it was thought to be able to may protect the mesostructure by providing a resistant layer between silica and lithium. In other words, the improvement in the resistance of host to lithium and the enhancement of the host guest interaction was the main objective for this investigation. By using this approach, the structural and basic properties of the obtained materials were to be fully characterized. Moreover, higher activity in solvent free etherification of glycerol to produce diglycerol over the prepared catalyst was an expected outcome.

High solvent free glycerol conversion was demonstrated within a short reaction time at a relatively low reaction temperature with significant amount of diglycerol produced.

4.3.3.1 Characterization of stabilized lithium modified SBA-15

Basic and mesopore catalysts from SBA-15 coating with 10, 20 and 30 wt % of magnesium prior to 10 wt % of lithium loading was prepared and subsequently characterized for surface properties and structural stability. Actually, SAXRS, SEM and TEM images results showed that 10% Li loading over SBA-15 have not retained the main peaks at 100, 110 200 showing not presence the hexagonal structure of

SBA-15. In other words Li 10% and obviously all above lithium loading may destroy its structure due to strong alkali reaction with Silicate structure. Therefore, not need to go further surface analysis for above than 10 % of lithium loading and its surface

135 properties.ICP-AES analysis was performed to measure the amount of loaded lithium as active component and amount of magnesium that was coated over the support material to strengthen its resistivity against reaction with lithium. The results showed that 13.4 mmol of lithium per g of support (approximately 10 wt % of lithium) was successfully deposited on the surface of all lithium loaded materials and is denoted with symbol Li10 from here onwards. In addition, 4.5, 7.7 and 11.6 mmol of magnesium per g of support (approximately 10, 20 and 30 wt % of magnesium) were found to be successfully deposited over the coated surfaces of mesoporous SBA-15.

Therefore, here and later on magnesium coated samples are represented as

Mg10/SBA-15, Mg20/SBA-15 and Mg30/SBA-15 while these samples with lithium loading represent as Li10-Mg10/SBA-15, Li10-Mg20/SBA-15 and Li10-Mg30/SBA-15, respectively.

The small angle X-rays scattering (SAXS) analysis of the prepared materials was done and their relative patterns are presented in Figure 4.18. Compared to the

SBA-15, the X-rays scattering patterns with intense (100) diffraction peak and two other well-resolved peaks (110, 200) for Li10-Mg10/SBA-15 and Li10-Mg20/SBA-15 were not observed to indicate that the original mesoporous structure of SBA-15 was totally destroyed. On the other hand, the intensity of these peaks for Mg30/SBA-15 was found to be weak compared to SBA-15 but their positions were very clear showing that original mesoporous structure was sufficient sustained.

In addition, slightly weak and low intensity pattern peaks for Li10-Mg30/SBA-

15 was found similar to SBA-15 pattern peaks which indexed as 2D hexagonal symmetry. These pattern peaks results showed that SBA-15 hexagonal structure retained after increasing amount of magnesium up to 30 wt % prior to 10 wt % of lithium loading. This indicated that the addition of high amount of magnesium (30 wt

136

%) leading in the synthetic SBA-15 did not significantly change the mesoscopic order of SBA-15 so that the composite kept their mesoporous structure of parent

SBA-15. However, the structure was observed to partially collapse on decreasing amount of magnesium coating lower than 30 wt % prior to same amount of lithium loading. This decreasing amount of magnesium might cause some portions of mesopore walls to be exposed to lithium and could not be protected from the reaction to form metal silicates. This led to partial destruction of the mesoporous structure.

Figure 4.18: SAXS patterns of the prepared SBA-15 and different bimetallic lithium and magnesium loaded SBA-15 catalysts with different compositions.

The basic strengths of the prepared catalysts were measured using Hammett indicators and it was found that Li10-Mg20/SBA-15 and Li10-Mg30/SBA-15 achieving

137 maximum basic strength (7.2 < H_ < 18.4) among all prepared catalysts. It was observed that the basic strength of the prepared SBA-15 was improved after coating with magnesium and then further increased by loading of lithium over the magnesium coated SBA-15. The basic strength of prepared catalysts was noted to be in the following order; SBA-15 < Li10/SBA-15 < Mg20/SBA-15 < Mg30/SBA-15 <

Li10-Mg10/SBA-15 < Li10-Mg20/SBA-15 ≈ Li10-Mg30/SBA-15. These observations suggested that combine effect of lithium and magnesium directly involved in the basic strength of the materials but up to a limited coating of magnesium. It was determined that basic strength was increasing with increasing magnesium amount at constant amount of lithium. However, H_ value was noted at same range (H_ ≤ 18.4) for 20 and 30 wt % of magnesium coating. It might be possible that at the basic strength of these prepared catalysts could be increased by further increasing of lithium loading or high coating of magnesium. But it might not be feasible for the characteristics of such prepared materials as a catalyst for glycerol oligomerization reaction which required basic mesoporous catalyst with high porosity.

Figure 4.19 illustrates the nitrogen adsorption-desorption isotherms of the prepared SBA-15, Li10-Mg10/SBA-15, Li10-Mg20/SBA-15, Li10-Mg30/SBA-15 and

Mg30/SBA-15. These isotherms are characterized by a sharp nitrogen uptake at higher relative pressure. SBA-15, Mg30/SBA-15 and Li10-Mg30/SBA-15 exhibited nitrogen adsorption-desorption behaviors that were in agreement with uniform mesoporous ordering, all in terms of type IV with H1 hysteresis loop at high relative pressure. With hysteresis loop of this type, the two branches are almost vertical and nearly parallel. Buchmeiser (Buchmeiser, 2003) also reported such results and suggested that these loops are often associated with porous materials which are known to have very narrow pore size distributions or agglomerates of approximately

138 uniform spheres in fairly regular array. On the other hand, the shape of the curves for

Li10-Mg10/SBA-15 and Li10-Mg20/SBA-15 do not agree with the type IV isotherm but instead, led to the collapse of some mesoporous structures as indicated by a drop in the porosity and exhibited by a narrower hysteresis loop as also reported such type of isotherms by previous researchers (Chris et al., 2003).

Figure 4.19: Nitrogen adsorption-desorption isotherms for the prepared catalysts.

Actually, the isotherm of most of SBA-15 sample exhibit large hysteresis loop of type E with sloping adsorption branch and steep desorption branch. This hysteresis type is due to the presence of different sizes of spheroidal cavities with the same entrance pore diameter (Ooi et al., 2004). However, with hysteresis loop of type H1, the two branches are almost vertical and nearly parallel to each other. Such

139 loops are often associated with porous materials which are known to have narrow pore size distributions. The pore size distributions of prepared catalysts are shown in

Figure 4.20. The figure shows that the average pore size distribution of prepared catalyst is in the range of 5 to 15 nm. It is also clear from this figure that most of pore size distribution curves are agglomerates of approximately uniform pores in fairly regular array. However, the pore size distribution was broader when catalyst SBA-15 was modified with coating magnesium prior to lithium loading. The broader pore size distribution was attributed to the lower structural ordering.

Figure 4.20: Pore size distributions for parent and modified SBA-15 catalysts.

The other surface properties of the prepared materials are tabulated in Table

4.4. A decrease in surface area, pore size and pore volume after coating with magnesium and loading of lithium can be further analyzed based on the data given in this table. The decrease in the BET surface area due to magnesium coating was more pronounced than the decrease in mesopore volume. This decrease might be a

140 result from partial blockage of pore wall micropores. Meanwhile, their relatively large size mesoporisty was also significantly affected by coating and loading of these magnesium and lithium components over SBA-15.

Table 4.4: Surface properties of different prepared catalysts.

SBET SMeso SMicro dPore VPore Catalyst (m2/g) (m2/g) (m2/g) (nm) (cm3/g)

SBA-15 592 560 40 5.35 0.98

Mg20/SBA-15 489 471 5 7.24 0.88

Mg30/SBA-15 456 445 0 7.90 0.85

Li10 -Mg10/SBA-15 90 55 35 - 0.07

Li10-Mg20/SBA-15 29 18 10 - 0.13

Li10-Mg30/SBA-15 371 365 0 5.07 0.45

*SBET:BET surface area, SMeso: mesopore surface area, SMicro: micropore surface area, dPore: pore diameter, VPore: pore volume

It is clear from this table that the BET surface area of SBA-15 sharply decreased from 592 to 90 m2/ g after 10 wt % of magnesium coating and 10 wt % of lithium loading (Li10-Mg10/SBA-15) while the pore volume sharply decreased from

0.98 to 0.07 cm3/ g. Both meso- and micropore surface areas were also found to be significantly altered after this coating and loading process. It was also observed that with the same amount of lithium loading and increasing amount of magnesium in catalyst Li10-Mg20/SBA-15, the surface area and pore volume was further decreased.

Although, further increasing magnesium amount from 20 to 30 wt % in the same catalyst (Li10-Mg30/SBA-15), it showed a significant surface area (higher than half of

141 that of the parent SBA-15) with good retention of porosity. It showed a sustained structure of SBA-15 with high mesoporous area. The surface areas of both

Mg20/SBA-15 and Mg30/SBA-15 without lithium loading were also observed and found to substantially decrease after coating with magnesium but having enough mesoporous area that depicted retention of structure mesoporous of SBA-15.

The significant decrease in the surface area after lithium loading was attributed to partial destruction of mesoporous structure of SBA-15. Previous researchers (Zukal et al., 2008) claimed that hexagonal silica structure of mesoporous material had weak resistance against strong alkali which was also confirmed from the nitrogen adsorption-desorption isotherm in Figure 4.19. It can also be noted that micropores surface area significantly vanished after coating with magnesium. This was ascribed to the partial blockage of inter channels between the straight channels of mesoporous structure. From the data in this table, it is also clear that the pore diameters of the prepared Mg20/SBA-15 and Mg30/SBA-15 were higher than SBA-

15. It was noted that after lithium loading over Mg30/SBA-15, the pore size of this catalyst (Li10-Mg30/SBA-15) became smaller than that of the parent SBA-15 support

(5.35 nm). This observation was attributed to the dispersion of magnesium on the surface of SBA-15 which increase pore size due to its presence inside the mesopores of SBA-15 structure. On the other hand, loading of lithium species on the surface of the magnesium coated on SBA-15 surface decrease again surface area and pore size due to create some hindrance at its presence in mesopore channels.

The other suggested reason behind the decreasing pore size of the coated and loaded materials might be due to the presence of both guest species (lithium and magnesium) that were dispersed inside the channels of the support material leading to increases in the roughness of the pore surface. In fact, the addition of lithium over

142

SBA-15 caused a drastic drop in BET surface area and its porosity even after magnesium coating. This sharp decrease in the surface area and porosity could be explained either by the partial pore blockage micropores or by the partial destruction of the structure of the mesoporous solid. Indeed, the highly basic character of lithium might promote partial destruction of the catalyst walls which were sensitive to strong alkali. However, BET surface area of the catalyst with high magnesium loading up to

30 wt % (Li10-Mg30/SBA-15) was satisfactorily retained and structure was not completely destroyed.

The structure preservation of Li10-Mg30/SBA-15 was also confirmed by SEM images as shown in Figure 4.21 (a-c). It is clearly seen in the images of Figure 4.21

(a) and Figure 4.21 (b) that these samples indicating destroyed structure of SBA-15 as compared to appeared structure as beads form in previous Figure 4.16 (a) which is typical for a mesoporous material of this type. Figure 4.16 (c) shows that Li10-

Mg30/SBA-15 structure satisfactorily retained its beads form close to that of SBA-15 structure. In addition, the SEM images of Li10-Mg10/SBA-15 and Li10-Mg20/SBA-15 showed partial structural distraction with a fluffy form of new structure. It can be concluded that SBA-15 could not sustain its structure even after magnesium coating of 10 and 20 wt % prior to lithium modification. However, it could be satisfactorily retained by applying magnesium coating up to 30 wt % over it prior to loading of lithium as observe in SEM images of their structure.

143

Figure 4.21: SEM images of (a) Li10-Mg10/SBA-15 (b) Li10-Mg20/SBA-15 and (c) Li10-Mg30/SBA-15. For further confirmation of the structural stability of the prepared materials,

TEM images at high magnification were analyzed. Figure 4.22(a) and Figure

4.22(b) shows the TEM images of Li10-Mg10/SBA-15 and Li10-Mg30/SBA-15, respectively. A close hexagonal structure and some straight channels can be seen of the prepared Li10-Mg30/SBA-15 in TEM images Figure 4.22 (b) while a collapse structure of Li10-Mg10/SBA-15 can see in TEM image part (a). These results also verified from low-angle XRD patterns of these samples. It is noted that the silica pellets were actually made up of an array of straight channels of meso size range. It was determined that the decreasing amount of magnesium coating (from 30 to 10 wt

%) prior to constant lithium loading over SBA-15 caused the deterioration of SBA-

15 characteristics.

Figure 4.22: TEM images of (a) Li10-Mg10/SBA-15 and (b) Li10-Mg30/SBA-15.

Actually, the electronegativity of lithium ions as an alkali metal is much lower than that of magnesium ions which is an alkaline earth metal. Thus, the interaction between lithium oxides and the silica support should be much stronger.

144

Such strong interaction may cause some reactions between basic oxides with the silica support to produce some new compounds similar to silicate during activation.

Hence, the mesostructure of the composites material was significantly modiﬁed.

Previous researchers reported that the high electronegativity of cations in alkaline earth metal oxides was responsible for the good preservation of the mesostructure of the resulting composites (Sun et al., 2008b). Based on the periodic table, the electronegativity of silicon ion (1.90 in Pauling unit) is greater than that of magnesium ion (1.31 in Pauling unit). With the decrease in electronegativity, the metal oxygen or silicon oxygen interaction becomes weaker and the coordination ability of lattice oxygen increases (Kumar et al., 2004).

It can be concluded from the characteristics of the prepared samples that an alkali resistant and stable structure of mesoporos SBA-15 could be syntesized with a specific amount of above 30 wt % magnesium coating prior to alkali loading. This magnesium coating might be helpful to avoid direct contact of alkali with silicon structure of SBA-15. As the electronegativity of the magnesium ion is obviously larger than that of the silicon ion, the coordination ability of oxygen in magnesium with the lithium becomes higher. Therefore, coating of the mesoporous silica with magnesium enabled the surface to provide an enhanced host guest interaction and stabilized the mesostructure. Consequently, the mesoporous structure could be satisfactorily preserved by the high coating of magnesium before loading with 10 wt

% of lithium. Subsequently, it can be concluded that the mesoporous structure of

Mg30/SBA-15 was satisfactorily preserved after the 10 wt % lithium loading.

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4.3.3.2 Activity of stable catalyst for glycerol oligomerization

The activity of the prepared catalysts in glycerol conversion to polyglycerols with that of homogenous LiOH for comparison is presented in Table 4.5. Li10/SBA-

15 catalyst showed significant conversion of glycerol (68 wt %) with good polyglycerol yield (45 wt %) while showing low selectivity to diglycerol (32 wt %).

For industrial application, high activity of catalyst with reasonable yield is very important. The selectivity and yield to diglycerol over Li10/SBA-15 was considered rather low level due to its destroyed mesoporous structure as confirmed from Figures

4.16, 4.17 and 4.18. On the other hand, stable Li10-Mg30/SBA15 catalyst was found to be highly active for glycerol conversion and significant yield to polyglycerol was achieved but selectivity to diglycerol was found to be still rather low (compare with repeated result). It was also noticed that the glycerol conversion was high enough (90 wt %) to form polyglycerols with a yield of 59 wt % over this stable catalyst Li10-

o Mg30/SBA15 at 240 C after 8 h. However, diglycerol selectivity and yield were found to be relatively lower at 14 wt % and 12 wt %, respectively. Although homogeneous LiOH showed higher conversion (98 wt %), lower polyglycerol yield

(52 %) was achieved compared to that of stable lithium modified catalyst under the same reaction conditions. Moreover, Li10-Mg30/SBA15 catalyst showed same level of selectivity to products other than polyglycerols (46 wt %) compared with that of homogeneous LiOH at the same temperature for a long reaction time of 8 h.

146

Table 4.5: Glycerol oligomerization over 2 wt % catalysts at 240 oC for 8 h.

Catalyst Glycerol Selectivity (%) DG PG X Di- Tri- Tetra- Others Yield Yield (%) (%) (%)

SBA-15 2 0 1 0 99 0 -

Li10/SBA-15 68 32 7 4 55 22 45

Mg30/SBA15 13 1 4 2 93 0 7

Li10-Mg30/SBA15 90 14 23 22 46 12 59

*LiOH 98 18 21 13 48 17 52

*Homogeneous at same reaction conditions, X: Conversion, DG: Diglycerol, PG: Polyglycerol

The activity in glycerol conversion might increase over Li10-Mg30/SBA15 due to its stable structure with high internal and external surface area of the mesoporous channels to allow good distribution of lithium. The selectivity might decrease in the presence of lithium due to the reduction of pore size after coating with magnesium which is confirmed from Table 4.3. The blockage of some pores after high coating of magnesium might also be responsible to cause a reduction in the selectivity to polyglycerol. The structure and shape of the pores might be considered as important factors to affect the catalytic activity in this reaction. Due to major destruction in the mesoporous structure, the reaction mostly took place on the destroyed surface.

Hence, very low selectivity was attained. This result suggested that the porous structure of catalyst had significant inﬂuence on products selectivity. Martin and co- worker (Martin and Richter, 2011) claimed that the structure and pore size of catalyst are two important factors to affect the glycerol conversion to polyglycerols.

Comparison between surface properties in Table 4.4 and their activities in selective

147 glycerol conversion in Table 4.5 suggests that as the basic strength and surface area of the catalysts increased, glycerol conversion also increased. The exception was for

Mg30-SBA15 which showed low activity even with significantly high bacisity and surface area. This was due to the absence of lithium which served as an active component for glycerol conversion to diglycerol and other polyglycerols.

Figure 4.23 shows glycerol conversion, selectivity and yield to diglycerol as well as yield of polyglycerol over Li10- Mg30/SBA-15 at a fixed reaction temperature

240 oC. In terms of glycerol conversion plotted against time, an activation period was observed in the first 2 h of reaction with very high glycerol conversion level (more than 50% of glycerol conversion). The glycerol conversion gradually increased with increasing reaction time and reached almost 100 % after 12 h. It is evident from this figure that polyglycerol yield was significantly high at a reaction time 6 h and then started to decrease with increasing reaction time and levelled off after 12 h.

100 100

Glycerol conversion 80 Diglycerol selectivity 80 Diglycerol Yield Polyglycerol Yield

60 60

40 40

Selectivity, Yield (%) Yield Selectivity,

Glycerol ConversionGlycerol (%) 20 20

0 0 0 2 4 6 8 10 12 14 Reaction Time (h)

148

Figure 4.23: Glycerol conversion and yield of diglycerols and polyglycerol over 2 o wt % Li10- Mg30/SBA-15 plotted against reaction time at 240 C. On the other hand, the selectivity and yield of diglycerol were found to be very poor (less than 20 wt %) throughout the reaction time. It is also notable that the maximum polyglycerol yield (67 wt %) was achieved at 6 h after that it gradually decreased with increasing reaction time. After 6 h, diglycerol as well as polyglycerols might have been converted to higher oligomers other than triglycerol and tetraglycerol or some other by-products that might be produced through other side reactions. Actually, magnesium is very active basic component for glycerol conversion at high temperature. However, its selectivity towards diglycerol was very poor as major portion of glycerol was converted to some other components through side reactions like dehydration of glycerol to acrolein (Clacens et al., 2002).

The distributions of diglycerol isomers over Li10/SBA-15, Li10-Mg30/SBA-15 and homogenous LiOH catalyst as a reference were compared at a temperature of

240 oC for 8 h of reaction time as shown in Figure 4.24. This figure suggests that the reactivity of all the catalysts was at the primary OH group in first carbon atom position of glycerol (αα’). These results gave some indication on the occurrence of reaction on the external surface of the pores rather than an internal porous area of the heterogeneous catalysts. It can be seen from the bar chart that the ββ’ and αβ isomers distribution in both cases of heterogamous catalyst were less than that of αα’ isomer which was similar to homogenous LiOH. Actually, in the case of Li10/SBA-15, the structure was totally destroyed and reaction was could have occurred in a manner similar to homogenous catalyst.

In the case of Li10-Mg30/SBA-15 catalyst, it was possible that coating of magnesium might have partially blocked the passage of the reactant within the inner channels due to high amount of loading which resisted large molecules of glycerol or

149 diglycerol to travel inside the pores of SBA-15. It might result in the occurrence of glycerol reaction with lithium on the external surface instead of within the pores of the mesoporous materials. Martin and co-worker (Martin and Richter, 2011) also observed similar diglycerol isomers distribution changes during glycerol etherification reaction over homogeneous CsHCO3 catalyst. It can be concluded from these observations that in the case of heterogeneous catalysts i.e. Li10/SBA-15 and

Li10-Mg30/SBA-15, diglycerol isomer distribution clearly to show higher αα’ isomer formation compared to ββ’ and αβ isomers. In other words, the reaction mostly occurred like homogenious LiOH in which glycerol molecules have accessed to react with any other molecule of glycerol or polyglycerol. Hence, it can be suggested that in hetrogeious Li-Mg/SBA-15, most of reaction was occurred on the external surface of the catalyst instead of within the mesoporous structure.

100

80 ' '

60 '

40 

 20 '  Diglycerol isomers distibution (%) distibution isomers Diglycerol ' '

0 LiOH Li10/SBA-15 Li10-Mg30/SBA-15

150

Figure 4.24: Diglycerol isomers distribution over different catalysts at 240 oC after 8 h of reaction time.

These results show that the activity of catalysts for glycerol oligomerization to polyglycerol depends on the nature of the active component incorponted into the stable basic structure of the mesoporous SBA-15. This study demonstrated that lithium deposited on mesoporous SBA-15 was an active catalyst for oligomerization of glycerol. However, the success was rather limited due to the destruction of silica mesostructure. The mesoporous structure of SBA-15 was found to be preserved after coating with a handsome amount of magnesium prior to lithium loading. Li10-

Mg30/SBA-15 was found to be a highly basic catalyst with rather stable structure and large pore size which favored oligomerization reaction for high polyglycerols formation. The prepared catalyst showed significantly high glycerol conversion with considerable amount of polyglycerols in a short reaction time of 6 h. However, it was not suitable to achieve high selectivity to diglycerol even by extending the reaction time up to 12 h.

Hence, a suitable solid catalyst for selective glycerol oligomerization to diglycerol in solvent-free media was still required. The requirements for such catalytic application include having large pores sizes of meso size range, should be able to stand harsh reaction conditions like 240 oC and above 8 h reaction time. Its structure should also be able to sustain under these reaction conditions and it should be easily separated from the final thick polyglycerol product mixture so that this material can be reused for several cycles of reactions.

Clay minerals are considered as another alternative for basic heterogeneous catalysts that have a significant potential in chemical processing technologies due to their different and interesting set of properties. In particular, montmorillonite clay

151 has received considerable attention in different reactions because of their layered structure of meso size, environmental compatibility, low cost, operational simplicity and reusability.

4.4 Lithium modified montmorillonite clay

Clays are among the most commonly used materials as catalysts and absorbents for industrial applications. The thermal stability, meso type layered structure, high surface area and polarity of its structure can help in retaining the ionic species like Li+, K+ or Ca+ which can play a vital role during polymeric reaction as an active component to be incorporated into a suitable support. According to a previous research (Komarneni et al., 2001), modified clay with salt, acid-treated and ion exchanged have been successfully used as efficient catalysts for many transformation reactions. Montmorillonite clay is the most widely investigated clay in catalysis due to its structure that consists of an alumina layer sandwiched between two-silica layers. The clay sheets are arranged in the z-direction to form the structure of montmorillonite

In this study, the activity of montmorillonite K-10 clay intercalated with LiOH was also evaluated for solvent free oligomerization of glycerol to selectively synthesize diglycerol at various reaction conditions. This montmorillonite clay was used due to its highly stable layer structure and availability at on economical cost. This modified clay acted as a solid basic catalyst to enhance oligomerization of glycerol to diglycerol at 240 °C.

4.4.1 Characterization of lithium modified clay catalyst

152

Thermogravimetric analysis (TGA) was conducted to identify the optimum heat-treatment temperature required to prepare the Clay Li/MK-10 catalyst. Figure

4.25 shows TGA curves recorded for the prepared materials. The total weight losses upon heating to 800 ºC for Clay MK-10 and Clay Li/MK-10 were found ca. 15 and

12.5 %, respectively. It is clear from this figure that Clay MK-10 catalyst initially released physically absorbed water (about 9 wt %) below 100 ºC and after that significant weight loss was not observed. A small weight loss (3 wt %) with increasing temperature in TGA of Clay MK-10 between 100 and 200 ºC was due to removal of structural water. In case of Clay Li/MK-10 catalyst, desorption of physically absorbed was started at a temperature of about 100 ºC. It was also noted that the weight losses in Li/Clay MK-10 due to physically absorbed water was less than that of Clay MK-10. This might be happened due to dry process of each sample.

100

0.0 80

-0.5 60

-1.0

Weight loss (%) loss Weight 40

-1.5 (%/min) Weight Derivative 20 Clay MK-10 Clay Li/MK-10

-2.0 200 400 600 800 Temperature (oC)

153

Figure 4.25: TGA curves recorded for ClayMK-10 and Clay Li/MK-10 catalyst.

The two main peaks for weight derivative were observed in Clay Li/MK-10, one was before 100 ºC due to removal of physically absorbed water and other slightly weak peak was noted at about 580 ºC. This peak might be due to dehydration of internal structure of clay in the presence of lithium component. This internal changes might be caused the structure demolish and montmorillonite might be no longer sustained its structure after this temperature. After this region of temperature, no prominent changes were found for both samples and about smooth data were obtained up to 800 ºC. It can be concluded that the structure of prepared

Clay Li/MK-10 might be remained stable in a heat-treatment temperature before 580

ºC. Therefore, it was suggested that the modified clay could be calcined at a temperature below than this value.

The X-ray diffraction patterns of the parent Clay MK-10 and Clay Li/MK-10 are shown in Figure 4.26. A prominent background of XRD patterns for both clay samples between 20° and 30° correspond to amorphous phases. Therefore, the diffraction peaks of these samples suggest the poor crystallinity nature of the clay material. According to Ravi et al., (2012), such peaks as observed at 26.6◦, 45.5◦, and

54.8◦ in this figure are attributed to montmorillonite structure. In the Clay Li/MK-10 system, six new peaks obtained with 2θ values of 18.8◦, 32.2◦, 33.1◦, 37.2◦, 39.6◦, and

48.2◦ corresponding to lithium compounds (#) as suggested by previous researchers

(Serna et al., 1982). These new peaks of modified sample Clay Li/MK-10 also confirm the loading of lithium over Clay MK-10. The peak at 8.9° corresponding to a basal spacing of 9.93 Å is typical to collapsed 2:1 phyllosilicates which match with the diffraction pattern of a mica and phengite-2M1 (p). The same diffraction pattern is detected in Clay Li/MK-10 with decreasing peak intensity. It indicates some

154 changes in the collapse structure of acid treated Clay MK-10 and might be possible that the structure of montmorillonite improved after modification process.

Figure 4.26: XRD patterns of (a) Clay MK-10 and (b) Clay Li/MK-10 catalysts.

Based on a previous study, Cseri and coworkers (Cseri et al., 1995) found that K-10 was an acid treated montmorillonite with a partially destroyed structure.

MK-10 showed a weak and broad peak at 2θ of 5.9° (d = 14.7 Å). It was associated with montmorillonite structure to suggest the presence of clay mineral of smectite group. Moreover, some reflections that matched with the diffraction patterns of albite

(a), phengite peaks (p) and quartz (q) were also observed in both MK-10 and Li/MK-

10. These phengite peaks (p) types of peaks belong to meso and mica type smectite

155 group. These results were in good agreement with the values reported previously for an acid-treated montmorillonite K-10; by (Villegas et al., 2005). It was also observed that after clay modification with lithium (Li/MK-10), these peaks were slightly shifted toward a slightly lower 2θ angle. It might be suggested a small increase in the basal spacing.

It can be concluded that the occurrence of montmorillonite peaks confirm the stable structure of modified clay and new peaks confirm the presence of lithium in this modified Clay Li/MK-10 catalyst. The presence of phengite peaks (p) might be occurred due to presence of some meso or mica pores in the structure formed during lithium modification process. Overall, these results indicated the successful intercalation of lithium into parent Clay MK-10 and the XRD pattern virtually unchanged as compared to that of parent clay to indicate the satisfactory retention of the montmorillonite structure during the preparation of the Li/MK-10 catalyst.

Table 4.6 shows the basic strength, BET surface area and pore volume of

Clay MK-10 and Clay Li/MK-10 after calcination at 450 ºC. As clearly noted, the basic strength of Clay MK-10 increased from H_< 4.6 to 9.3

156

Table 4.6: Surface characteristics and basic strength of Clay MK-10 and modified Clay Li/MK-10 catalysts.

Sample Basic strength SBET Smeso dpore Vpore

(H_ ) (m2/g) (m2/g) (nm) (cm3/g)

Clay MK-10 H_ < 4.6 194 143 5.9 0.37

Clay Li/MK-10 9.3 < H_ < 15 123 97 14.1 0.29

*SBET: BET surface area, SMeso: mesopore surface area, dPore: pore diameter and VPore: pore volume.

On the other hand, pore diameter of this modified clay was observed to increase from 5.9 to 14.1 nm and pore volume was found somewhat decreased from

0.37 to 0.29 cm3/ g. The BET surface areas of Clay MK-10 and modified Clay

Li/MK-10 were recorded as 194 and 123 m2/ g, respectively. The surface area and mesoporousity of modified clay might be decreased due to the partial blockage of some portion of layered structure during lithium modification treatment. The change in pore size and pore volume of the Clay MK-10 after the modification with lithium might be due to some structural changes like some layered defect or overlapping.

According to (Cseri et al., 1995), the dealumination resulting from a stronger acid treatment to clay montmorillonite K-10 led to the creation of large surface areas, located mainly in mesopores while the microporosity was negligible. It is also clear from this table that pore size increased while surface area decreased. It might be due to changes in basal spacing after lithium intercalation as confirmed from XRD results.

Nitrogen adsorption and desorption isotherms for both Clay MK-10 and Clay

Li/MK-10 are shown in Figure 4.27. Both samples revealed type II isotherms which are the characteristic of mesoporous materials (Morris et al., 2008). The hysteresis

157 loop is small and it possesses features reminiscent of both the H3 and H1 type. The adsorption that appears to be retarded at high relative pressure P/P0 to indicate the latter classification is a more valid description. In addition, the hysteresis of H1 type is usually associated with solids consisting of nearly cylindrical channels or agglomerates or compacts of near uniform spheres. In each case, the hysteresis loop is narrow, with almost parallel adsorption and desorption branches. This is suggestive of pores with regular geometry, while the steep desorption behavior shows that the dimensions of the pores were in a narrow range.

Figure 4.27: Nitrogen adsorption and desorption isotherms for Clay MK-10 and modified Clay Li/MK-10.

158

The pore size distribution curves for both Clay MK-10 and Clay Li/MK-10 corresponding to their nitrogen adsorption and desorption isotherms are shown in

Figure 4.28. The P/Po position in the inflection range from 0.5 to 0.9 confirms the mesoporous structural characteristic and the sharpness of the step indicates the uniformity of the mesopore size distribution (Sousa and Sousa, 2006). However, after modification of clay catalyst, the pore size distribution (Figure 4.28) was broader to indicate the partial collapse of some pores into meoporous area. It can also be confirmed from previous Table 4.6 that lithium modification of clay led to decrease in the BET surface area (from 194 to 123 m2/g), pore volume (from 0.37 to

0.29 cm3/g) but huge increase in the pore diameter (from 5.9 to 14.1 nm). These changes were also observed from XRD patterns in Figure 4.26 where some phengite peaks (p) which belong to meso and mica type smectite group (Villegas et al., 2005).

It can be suggested that these changes in montmorillonite clay might be belong to some defect in the structure after lithium modification.

159

0.7

Clay Li/ MK-10 0.6 Clay MK-10

0.5

0.4

0.3

0.2

dV/dlog(D) Pore Volume (cm³/g·Å) 0.1

0 200 400 600 800 1000 Pore Diameter (Å)

Figure 4.28: Pore size distributions for Clay MK-10 and modified Clay Li/MK-10.

Figure 4.29 shows the FTIR spectra obtained for calcined Clay MK-10 and

Clay Li/MK-10 in the wave number range between 400 and 4000 cm-1.

Montmorillonite K-10 clay usually contains mainly Al3+ with some Fe2+/3+ and Mg2+ as octahedral cations and Na+, K+ and Ca2+ as exchangeable interlayer cations. The results showed that the FTIR spectra of modified Clay Li/MK-10 (curve b) is slightly different from that of the parent Clay MK-10 (curve a). The most intensive band at

1035 cm−1 is attributed to Si-O in-plane stretching and 530 cm−1 is due to Si-O bending vibrations. The band at 530 cm−1 shifted towards lower frequency at 524 cm−1 after lithium modification and its intensity increase could be correlated to Si-O-

160

Mg bending vibrations in montmorillonite K-10 due to the presence of lithium. This vibration at band 530 cm−1 in montmorillonite K-10 was also observed by (Muthuvel et al., 2012).

1640 1440 692 1480 915 460 530 1035 3440 b 3623

1320 a 790

4000 3500 3000 2500 2000 1500 1000 500

Wave number (cm- 1)

Figure 4.29: FTIR spectra for (a) MK-10 and (b) Li/MK-10 in the wave number range of 400- 4000 cm-1.

The broad band at 3440 and 1639 cm−1 are the stretching and bending vibrations for the hydroxyl groups of water molecules in the clay. The intensity of these bands was found to decrease in Clay Li/MK-10. The parent Clay MK-10 shows the band at 3623 cm−1 in OH stretching region, which is assigned to hydroxyl groups coordinated to octahedral cations which is due to hydroxyl group bonded with Al3+ cat-ions (Vicente-Rodríguez et al., 1996). The intensity of this band in clay reduced

161 after the intercalation of lithium. It might be due to the removal of octahedral cations causing the loss of water and hydroxyl groups coordinated to them.

In the 700-1500 cm-1 region, the similarities among the smectites are less pronounced. The absorption bands in this region have been assigned to stretching vibrations of Si-O and Si-O-Si bonds, and also to deformation modes of OH groups attached to various ions, e.g., A1+3 and Mg+2 (Bukka and Miller, 1992). Moreover, for the lithium modified clay, two new bands appeared at 1480 cm-1 and 1440 cm−1 which are assigned to deformation of OH group attached to various ions as discussed above. The band at 915 cm−1 has been specifically assigned to OH groups attached to

A1+3 with the vibration of montmorillonite. The bands at 790 and 692 cm−1 show the deformation of quartz admixtures present in the sample. The strong band at 790 cm−1 is assigned to platy form of disordered tridymite and 692 cm−1 for quartz content

(Farmer, 1999). The band at 790 cm−1 entirely changed in clay after lithium modification and showing strong disorder of tridymite while the band at 692 cm−1

(quartz) shows an increase in intensity after lithium treatment that indicated the

-1 disturbance of SiO2 in the clay structure. The band at 460 cm is attributed to the vibration of the double six-member ring, and bending vibration of O–Si–O and Si–

O–M in which M is Al, Li or any other alkali/ alkaline earth metal (Fang et al.,

2012). These results suggested that the structure of Clay MK-10 was not significantly damaged by the modification. However, a little difference between Clay MK-10 and modified Clay Li/MK-10 was observed due to presence of lithium in interlayer of the parent clay after Li intercalation.

Figure 4.30 shows the comparison between SEM images of parent MK-10 and Li/MK-10. This figure reveals that majority of the particles in both samples had a lamellar morphology with a composition similar to 2:1 phyllosilicate that

162 corresponds with the mica phase as observed by XRD. It can be seen that the intercalated sample had mostly similar morphology to that of the parent clay. It consisted of mainly of flake-like particles which could be indirectly attributed to layered structures like smectite clay (Rode et al., 2007). It is also clear that the surface texture of Li/MK-10 was different and somewhat rougher than that of the parent MK-10. Basically, after lithium loading, the boundary between interlayer in this catalyst expand due to increasing distance between tetrahedron sheets and alter the montmorillunite structure in the form of massive and some bulky size of particle.

As a conclusion, layered nature seemed to prevail after the lithium modification of

MK-10.

Figure 4.30: SEM images for (a) Clay MK-10 and (b) Clay Li/MK-10 catalyst.

163

4.4.2 Catalytic activity of the prepared catalyst

The activity of the prepared clay Li/MK-10 catalyst for glycerol oligomerization to diglycerol was determined after through characterization. Figure

4.31 shows the profiles of glycerol conversion, selectivity to diglycerol and yield of diglycerol over parent the Clay MK-10 and modified Clay Li/MK-10 catalyst against reaction time from 2 to 12 h at 240 °C. The acid treated Clay MK-10 was found to be rather inactive for the glycerol oligomerization reaction and showed less than 20% of glycerol conversion after 12 h. The limited amount of diglycerol production during the reaction was demonstrated as indicated by the poor selectivity and yield throughout the reaction with this Clay MK-10. This might be due to the presence of high acidity and the absence of basic sites over this clay. After acid treatment, montmorillonite become strongly acidic in nature and its structure was also partially destroyed as confirmed by surface properties and XRD results. Due to this reason,

Clay MK-10 showed very low activity to selective diglycerol. For Clay Li/MK-10, an activation period was observed in the first 2 h of reaction with considerably low glycerol conversions (about 30 %) as compared to homogenous LiOH (see in Figure

4.3). The glycerol conversion over Clay Li/MK-10 was found to be gradually increased with increasing reaction time and reached 86 wt % after 8 h. It reached almost 100 % conversions after 12 h of reaction time as shown in Figure 4.31. It should be noted that the activation period of homogenous LiOH was found to be very fast and glycerol conversion reached about 96 wt % after 4 h (as can see in Figure

4.3). Meanwhile, Clay Li/MK-10 catalyst achieved about 68 wt % of glycerol conversion at this point.

164

100 Clay MK-10 (Conversion) 100 Clay Li/ MK-10 (Conversion) Clay MK-10 (Yield) Clay Li/MK-10 (Yield) 80 80

60 60

40 40

Diglycerol Yield (%) Yield Diglycerol

Glycerol Conversion (%) Conversion Glycerol

20 20

0 0 0 2 4 6 8 10 12 Reaction Time (h)

Figure 4.31: Glycerol conversion and diglycerol yield of the catalysts via solvent- free oligomerization reaction at 240 °C.

A comparative analysis between homogeneous and prepared Clay Li/MK-10

(Figure 4.3 and 4.31) for glycerol conversion and diglycerol yield clearly shows some important variations in their reaction behaviors. It was noted from this comparison that at the beginning of the reaction, LiOH showed high diglycerol yield

(33 %) but it started to decrease at longer reaction time then reached its minimum level after 8 h. On the other hand, Clay Li/MK-10 showed steadily increasing trend of diglycerol yield to reach its maximum value of 58 wt % after 8 h. The yield slightly decreases with further increase in reaction time till 12 h where the glycerol conversion was noted at maximum level. Finally, the yield of diglycerol dropped to

165

38 wt % after 12 h of reaction. It could be concluded from this comparison that the activity of homogenous LiOH catalyst was very high initially but gradually decreased with increasing reaction time. The activity trend of the prepared Clay

Li/MK-10 was found to increase gradually to 8 h and then slightly decreased at longer reaction times.

Actually, montmorillonite K10 clay is Brønsted acidic material, but it was made basic by treating it with a basic LiOH containing solution. Obviously, due to the change in basic strength of modified lithium clay as shown in Table 4.4 and improvement in its basal spacing as confirmed by XRD results, the activity and selectivity of Clay Li/MK-10 in glycerol oligomerization were improved. This modified clay also had an edge over homogenous LiOH in the sense that lithium quantity used in Clay Li/MK-10 was 5 times less than homogenous one. However, the use of this basic clay catalyst in the oligomerization reaction resulted in the adsorption of glycerol over the surface of the clay which reduced its activity in the selective formation of diglycerol after 12 h. Hence, the overall results for selective glycerol conversion to diglycerol via solvent free oligomerization reaction over Clay

Li/MK-10 catalyst were satisfactory and comparable with previously reported results for this process (Barrault et al., 2004; Martin et al., 2012).

Figure 4.32 shows a more detail view of glycerol conversion selectivities of various products over prepared lithium modified clay. It can be observed from this figure that the selectivity of diglycerol was increased from 37 to about 68 wt % as the glycerol conversion gradually increased from 29 to 86 wt % with increasing reaction time from 2 to 8 h. With further increase in reaction time, the selectivity to diglycerol experienced a gradual decrease while the glycerol conversion increased.

At the reaction time 12 h, maximum glycerol conversion was achieved while the

166 diglycerol selectivity was at 37 wt %. This was around half of its maximum selectivity achieved in 8 h. Triglycerol formation was also observed during the diglycerol production but its presence was noted to be less than 25 % on the base of selectivity. The undesired products, other than diglycerol and triglycerol were found to slightly increase at the beginning of the reaction. However, a decreasing trend was observed with increasing selectivity of diglycerol with increasing reaction time.

Higher glycerol conversion to diglycerol by Clay Li/MK-10 as compared to that of

Clay MK-10 or homogenous LiOH suggested that the pore size with basic strength of the catalyst could signiﬁcantly affect the product selectivity (Martin et al., 2012).

This result also suggested that the reaction might have taken place whithin the pores as well as over the external surface of the clay catalyst. Therefore, with increasing reaction time, the synthesis of diglycerol was gradually increased in the presence of active base component lithium over the porous layer structure of Clay Li/MK-10. It indicated that the pore size of the prepared Clay Li/MK-10 was also an important factor to assure higher diglycerol selectivity.

167

Figure 4.32: Glycerol conversion and selectivity to diglycerol and triglycerol over the Clay Li/MK-10 catalyst at 240 °C.

Some significant differences were observed with the help of diglycerol isomers distributions when homogenous LiOH and heterogeneous Clay Li/MK-10 catalyst were compared as shown in Figure 4.33. This figure shows that the reactivity of the secondary OH group at secondary position of glycerol molecule was affected in the presence of heterogeneous (having mesopores) for synthesis of required diglycerol isomer. It can be seen from the bar-chart that the αβ isomer distribution value is higher than that of αα’ isomer after 8 h over the Clay Li/MK-10 while αα’ isomer value decreases as compared to homogenous LiOH. Similarly, the value of the sum of two isomers (ββ’ + αβ) was found to be much higher than αα’ isomer. This indicated that some new changes in the reaction occurred as compared to the previous study using homogeneous LiOH, modified zeolite and stabilized modified SBA-15 catalysts for the same reaction at the same reaction conditions.

168

100

80 '

60 

40 '

20  ' '

Diglycerol isomers(%) distibution Diglycerol

LiOH @ 240C; 8 h Clay Li/MK-10 @ 240C; 8h

Figure 4.33: Comparison between diglycerol isomers distribution over homogenous LiOH and Clay Li/MK-10 catalyst.

It is suggested that those prepared catalysts which show higher value of αα’- dimer and less value of αβ-dimer or the sum of both (ββ’ and αβ) dimers, in this case most of the reaction occurred on the external surface of catalyst. Therefore, due to the high value of αβ isomer than αα’ in clay catalyst, it can also be proposed that reaction might be happened inside the porous area. The distribution of linear αα’-,

ββ’-, and αβ -diglycerols as a function of reaction time at 240 oC for Clay Li/ MK-10 is shown in Figure 2.34. It is clear from this figure that these isomer distributions also have a change by changing its reaction time. The value of isomer ββ’ was found to be decreased with increasing reaction time from 2 h to 12 h. At the same time αα’-

169 dimers value was found to be increased with increasing reaction time. The major product αβ -dimer distribution was observed relatively constant over the reaction time. Although, its value slightly decreased, this was not found much prominent as compared to other isomer for Clay Li/MK-10 catalyst. However, one thing was cleared that the residence time of the αα’-dimer inside the pore system was not favored its further conversion into higher oligomer.

100

' dimer 80 dimer ' dimer

Diglycerol isomers distribution isomers Diglycerol ( %) 0

2 4 6 8 10 12

Reaction Time (h)

Figure 4.34: Distribution of linear αα’-, ββ’-, and αβ -diglycerols as a function of reaction time at 240 8C for Clay Li/ MK-10 catalyst.

Actually, in the earlier prepared catalysts, most of the reaction occurred on the external catalyst surface. Therefore, their diglycerol isomer distribution suggested the higher formation of αα’ and less formation of αβ during oligomerization process.

In the clay catalyst, the higher value of αβ isomer than αα’ isomer suggested that the reaction might occurred within the internal area of catalyst. It is also clear from these

170 results that isomer distribution changed by changing its reaction time. The value of

αβ was found to decrease with increasing its reaction time. This concluded that with increasing reaction time, some pores could have been blocked by higher oligomers or some side reactions might have initiated to produce mostly higher oligomers on the external surface of catalyst.

It was also noted that the value of the sum of two diglycerol isomers (ββ’ +

αβ) found to be increased than the value of αα’-dimer during given reaction. This also helps the purposed hypothesis that the major reaction upon clay catalyst might be happened inside the porous area not outside of its surface. This might be a strong indication that the reaction took place inside the pores of the solid clay catalyst having mesopores as confirmed from surface properties of modified clay in Table

4.6. Different researchers (Barrault et al., 2004; Ruppert et al., 2008) were also observed similar selectivity changes during glycerol oligomerization reaction over mesoporous catalyst and they correlated these changes with the reaction happening with in the porous area of the catalyst. It can be concluded from these observations that the reaction mostly occurred inside the internal pores of the clay catalyst.

Therefore, purposed hypothesis of diglycerol distribution for reaction occurrence might be considered for oligomerization reaction. Thus, the prepared clay catalyst could enhance the selective synthesis of diglycerol during the glycerol oligomerization reaction.

Higher glycerol conversion to diglycerol by Clay Li/MK-10 catalyst in 12 h suggested that the basic strength and pore size could signiﬁcantly and synergically affects the product selectivity. This result also suggested that the reaction might have take place mostly in the internal pores as well as over the external surface of the catalyst. Therefore, with increasing reaction time, the synthesis of selective

171 diglycerol steadily increased in the presence of active basic lithium component over the porous layer structure of Clay Li/MK-10 catalyst. It indicated that the pore size of the prepared Clay Li/MK-10 catalyst was also an important factor towards achieving higher diglycerol synthesis.

The summarized results obtained in this study for glycerol oligomerization to diglycerol at given conditions are shown in Table 4.7 and their discussion is given as follows. Homogeneous LiOH catalyst showed the best results for selective glycerol conversion to diglycerol with short reaction time 4 h at 240 °C. However, the selectivity starts to decrease after 4 h due to low amount of glycerol remaining in the reaction system and diglycerol started to be converted to new products. Lithium modified microporous material i.e. modified zeolite type catalyst which has high surface area and significant basic strength was sufficiently active for polyglycerol production but less selective for diglycerol due to their microporous structure. The amount of αα’ diglycerol isomers were found higher than αβ -isomer indicating that the reaction was occurred on external surface. This was happened due to small pore sizes of catalyst which was observed slightly larger than that of the glycerol molecule

(0.52 nm) so that high formation of polyglycerol was noted but it was not found selective for diglycerol. Instead, sufficient amount of higher molecules were prepared during the reaction. On the other hand, stabilized modified mesoporous

SBA-15 which has high surface area and high basic strength showed high glycerol conversion but with very low selectivity to diglycerol. The majority of αα’ diglycerol isomers were formed than αβ -isomer indicating that the reaction was occurred on external surface of the catalyst. This might be happened due to presence of magnesium that cloud decreased the size of internal pores and reaction held on external surface with majority formation of higher oligomers.

172

Table 4.7: Summarize results of prepared catalysts with their properties and activities for glycerol oligomerization to diglycerol (Reaction conditions: 2 wt % catalyst; 240 C; 8 h).

Parameters LiOH Li/USY Li-Mg/SBA-15 Li/MK-10

BET surface area (m2/g) - 531 371 123

External surface area (m2/g) - 102 365 97

Micropore surface area (m2/g) - 429 14 - t-plot pore volume (cm3/g) - 0.26 0.73 0.29

Average pore size (nm) - 0.95 3.95 14.1

Basic strength pH > 12 H_< 15 H_< 18.4 H_< 15

αα’ diglycerol isomers (%) 11 14 15 15

αβ diglycerol isomers (%) 17 15 33 55

ββ’ diglycerol isomers (%) 72 71 52 30

Glycerol Conversion (wt %) 99 98 90 86

Diglycerol Selectivity (wt %) 18 29 14 68

DiglyceroYield (wt %) 17.8 28.4 12.6 58.4

The prepared lithium modified montmorillonite K-10 (Clay Li/MK-10) showed high glycerol conversion and selectivity to diglycerol under same reaction conditions as compared to the other catalysts. Actually, montmorillonite clay has layer structure with basal spacing greater than the dimension of glycerol molecule.

The clay became basic but with sustained layered structure after lithium intercalation as a revealed by the characteristics of the prepared Clay Li/MK-10 catalyst. The activity and selectivity to diglycerol of this catalyst was enhanced due to its high basic strength and its layered structure with high pore size. Most of αβ diglycerol isomers were formed than αα’-isomer so that the reaction occurred within this

173 layered structure to form diglycerol and the formation of higher molecules could be significantly retarded.

Consequently, Clay Li/MK-10 catalyst was found most active for solvent free glycerol oligomerization to diglycerol among all prepared catalysts. Therefore, this catalyst was selected for further studies of optimization of oligomerization reaction for diglycerol yield, kinetics of prescribed reaction and reusability of active catalyst.

4.5 Optimization of diglycerol yield via design of experiments (DOE)

The purpose of this study was to find a functional relationship between reaction temperature, reaction time and amount of catalyst used in the reaction, based on the reliability of Response Surface Methodology (RSM) in combination with

Central Composite Rotatable Design (CCRD). The earlier study showed that Clay Li/

MK-10 catalyst was active for selective glycerol oligomerization to diglycerol under solvent-less condition. Therefore, this catalyst was selected for optimization of diglycerol yield using DOE. The glycerol yield ranged from 0 % to 70 %, depending on the experimental conditions. These results were actually better or at least comparable to those obtained with other catalyst used in earlier studies (Barrault et al., 2004). Consequently, the predetermined relationship was used to find out the optimal value to obtain the best response for conversion of glycerol and diglycerol production in the form of diglycerol yield.

4.5.1 Single response optimization of diglycerol yield

The objective of this section was to determine the optimal values of the three mathematical factors i.e. reaction temperature (x1), time (x2) and catalyst loading

(x3). The results obtained from the conducted experiments are summarized in Table

174

4.8. This table shows the experimental design with actual response of diglycerol yields over Clay Li/MK-10 catalyst. In this table, the range of diglycerol yields was found from 1 to 56 %, depending on the experimental conditions. Actually these results were better or at least comparable to those obtained with the previously studied catalysts (Martin and Richter, 2011).

Table 4.8: The experimental design and corresponding diglycerol yield.

Actual independent value Diglycerol Std Run Type Temp. Time Catalyst Yield °C h wt % %

(x1) (x2) (x3) Actual 1 6 Fact 220 2 1 1 2 16 Fact 260 2 1 18 3 17 Fact 220 10 1 26 4 18 Fact 260 10 1 29 5 13 Fact 220 2 3 22 6 5 Fact 260 2 3 34 7 9 Fact 220 10 3 49 8 3 Fact 260 10 3 17 9 7 Axial 200 6 2 1 10 19 Axial 280 6 2 3 11 8 Axial 240 0 2 0 12 20 Axial 240 14 2 26 13 12 Axial 240 6 0 5 14 4 Axial 240 6 4 39 15 10 Axial 260 6 2 56 16 14 Axial 220 6 2 38 17 2 Center 240 6 2 51 18 1 Axial 200 6 2 1 19 15 Axial 240 8 2 70 20 11 Center 240 6 2 54

4.5.2 Analysis and model fitting

The mathematical model for the desired response as a function of selected variables was developed by applying the multiple regressions analysis methods on

175 the experimental data. Based on the input data as given in Table 4.1 and without performing any transformation on the response, the summary output concluded that among the models that were fitted to the response (linear, two factorial interaction, quadratic and cubic polynomial), the quadratic model was statistically significant to represent the results as it could simultaneously satisfy the three (3) variables as shown in Table 4.9.

Table 4.9: Sequential Model Sum of Squares.

Source Sum of Degree of Mean F Value Prob > F Remarks

Squares Freedom Square

Mean 14580 1 14580

Linear 1969.164 3 656.3879 1.532534 0.2445

2FI 705 3 235 0.496923 0.6907

Quadratic 5656.844 3 1885.615 38.40412 < 0.0001 Suggested

Cubic 418.1532 6 69.6922 3.827159 0.1074 Aliased

Residual 72.83962 4 18.20991

Total 23402 20 1170.1

To strengthen this conclusion, the Analysis of Variance (ANOVA), a necessary test procedure for applying the experimental data to verify the adequacy of the model was performed. In addition, it was observed from ANOVA analysis that most of the variables were highly significant to the regression model as suggested by the high F-value. These results are summarized in Table 4.10.

176

Table 4.10: Analysis of Variance (ANOVA) for diglycerol yield.

Sum of Degree of Mean

Source Squares Freedom Square F Value Prob > F Comments

Quadratic 8331.007 9 925.6675 18.85297 < 0.0001 Significant x1 28.43501 1 28.43501 0.579133 0.4642 x2 674.9141 1 674.9141 13.74591 0.0041 Significant x3 841 1 841 17.12856 0.0020 Significant

2 x1 4789.491 1 4789.491 97.54706 < 0.0001 Significant

2 x2 2790.308 1 2790.308 56.82991 < 0.0001 Significant

2 x3 1867.901 1 1867.901 38.04336 0.0001 Significant x1x2 420.5 1 420.5 8.56428 0.0151 x1x3 200 1 200 4.073379 0.0712 x2x3 84.5 1 84.5 1.721003 0.2189

Residual 490.9928 10 49.09928

Lack of Fit 486.4928 8 60.8116 27.02738 0.0362 Not significant

Pure Error 4.5 2 2.25

Cor Total 8822 19

Moreover, It can also be seen in Table 4.10, that based on the F-value, two

2 2 2 factor terms (x1, x2, x3) and three respective quadratic terms (x1 , x3 ,x3 ) with one

interaction factor (x1x2) had the largest effect on the diglycerol yield. Although, three

factors terms (x1, x2, x3) could strongly affect the overall oligomerization process, x1

value was rather insignificant due to experimental error. However, the remaining

terms were at 95% confidence level as indicated by the lowest p-value (<0.05) and

177 the relatively high F-value. In other words, only model terms with values of p-value less than 0.05 were determined to be significant to the model equation. The ‘Lack of

Fit F-value’ of 27.3 implies that it is insignificant relative to the pure error.

Therefore, there was only a 3.62% chance that a ‘Lack of Fit F-value’ this large could occur due to noise. The insignificant lack of fit was good as the primary objective was the model should satisfactorily fit the experimental data.

The R-square value of the model was found to be 0.944 for diglycerol yield as shown in Table 4.11. It means that 0.94 % of the total variation in the results is attributed to the investigated independent variables. Actually, the R-square value is a criterion for evaluation of the appropriateness of the model. In other words, the closer the R-square value to unity, the better the model will be as this will give predicted values which are closer to the actual values for the response. The significance of each term at a specified level of confidence was determined by examining its respective p-value and F-value. In fact, the p-value is actually the smallest level of significance, which could be used to reject the null hypothesis (H0).

Therefore, the smaller the value is, the more significant is its corresponding coefficient and the contribution towards the response variable (Istadi, 2006). In addition, for this model the "Pred R-Squared" of 0.820 is in reasonable agreement with the "Adj R-Squared" of 0.894. Meanwhile, "Adeq Precision" measures the signal to noise ratio and a ratio greater than 4 is usually desirable. The ratio of 13.207 for the prepared new model of diglycerol yield indicates an adequate signal.

Therefore, this model can be used to navigate the design space.

178

Table 4.11: Statistics of the model to fit for diglycerol yield.

Model to Fit R-Squared 0.944345

Adj R-Squared 0.894255

Pred R-Squared 0.820877

Std. Dev. 7.007088

Mean 27

Cofficient of Variance (C.V.) 25.95218

Prediction Error Sum of Squares (PRESS) 1580.226

Adeq Precision 13.20737

Figures 4.35 exhibits the comparison between actual response values as obtained from the experimental runs and the predicted response values based on the quadratic models equation. As can be seen, the predicted values match the observed values reasonably well within the wide ranges of experimental conditions with R-

Squared value of 0.94 in the solvent free glycerol oligomerization over Clay Li/MK-

10 catalyst. The closer the value of R-squared to unity, the better the empirical models fits the experimental data. On the other hand, the smaller the value of R- squared, the lesser will be the relevance of the dependent variables in the model has in explaining the behavior of variations (Cao et al., 2008). These results suggest the applicability and reliability of the equation in representing the reaction over this range of experimental conditions with sufficient degree of accuracy. Thus, it can be used to simulate the reaction. Consequently, the R-Squared value of prepared model for diglycerol yield (0.94) indicates an excellent agreement between the experimental and the predicted response.

179

60 R² = 0.9443

Predicted diglycerol yield (%) yield diglycerol Predicted

0 0 20 40 60 80 Actual diglycerol yield (%)

Figure 4.35: Relation between observed and predicted diglycerol yield.

Further analysis on the results showed that they developed a quadratic equation (in coded units) that could relate diglycerol yield to the parameters studied.

After analyzing the ANOVA data and statistical parameters, the final imperical model in term of coded factors (parameters) for diglycerol yield is given in Equation

4.1.

2 2 2 Y = 59.06 + 1.17x1 + 6.45x2 + 7.25 x3 – 14.5x1 – 11.40x2 – 9.23x3 – 7.25 x1x2

(4.1)

180

The second order response function gives Y as the response for diglycerol yield for the solvent free glycerol oligomerization over Li/MK-10 catalyst, x1 is the coded value of temperature, x2 is the coded value of reaction time and x3 is the coded value of weight of catalyst. The positive sign in front of the term indicates the synergetic effect to increase the yield of a product whereas negative sign indicates antagonistic effect (Kansedo et al., 2009). Therefore, the above model equation indicated that the positive coefficients of x1, x2 and x3 were linear function with the yield of diglycerol.

4.5.3 Effect of process conditions

In this study, the effect of individual variables on the solvent free oligomerization process is discussed based on response surface one factor plot, while the interaction between variables is shown in the form of three-dimensional response surface as well as contour plots.

4.5.3.1 Influence of individual effect

The individual effects of x1, x2 and x3 towards diglycerol yield for solvent free glycerol oligomerization process can be graphically seen in Figure 4.36 (a-c).

These three effects show the positive influence (positive quadratic model) of the terms to the yield of diglycerol as shown in equation 4.1. The parameter with the highest F value would have the most significant effect. Thus, by referring to Table

4.10, the parameters with the most significant effect on the yield of glycerol in descending order were the amount of catalyst followed by reaction time and reaction temperature. It is also clear from this figure that these three individual variables had positive influence on the oligomerization process up to a temperature 240 ºC then it had negative influence due to increasing window temperature above than 240 ºC.

181

This could be justified as an increase in reaction temperature but a limited extend would lead to higher reaction rate; longer reaction time was required to increase the yield of diglycerol; higher amount of catalyst was required to increase the availability of active sites for the reaction the proceed towards product formation

Figure 4.36: The individual effects of (a) reaction temperature towards diglycerol yield.

182

Continue Figure 4.36: The individual effects of (b) reaction time and (c) weight of catalyst towards diglycerol yield.

183

It can be seen in Figure 4.36 (a) that the yield increases steadily with a temperature of 240 ºC. After that, it starts to decrease with the increasing temperature. Heterogeneous catalyst always suffers from a slow mass transfer due to little contact between the catalyst and the liquid reactants to cause low yield of diglycerol at shorter reaction time as shown in Figure 4.36 (b). Therefore the reactants needed longer time to diffuse into the pores of clay that hosted active sites before the reaction could be catalyzed. In addition, at long reaction time (above 8 h), the yield was found to decrease. It might be due to decreasing diglycerol selectivity and formation of higher oligomers or some other unwanted products.

Figure 4.36 (c) shows that, diglycerol yield increased with increasing catalyst loading of up to 2 wt %. After that, increasing weight loading of catalyst not affect significantly. The result clearly suggested that with higher catalyst loading from 1 to

3 wt %, the total number of available active catalytic sites for the reaction also increased to directly result in higher yield of diglycerol at temperature 220 to 240 ºC.

Further increasing temperature, the yield of diglycerol decreased due to some side reaction start to form byproducts. As such and considering, the entire three process variables, the process should be carried out x1, x2 and x3 at high level to maximize the diglycerol yield. However, the interaction effects between the process variables should also be considered as they could give some influences to the yield of diglycerol.

4.5.3.2 Two-dimensional (2-D) interactions between variables

From Table 4.10, it is observed that the significant terms are the interaction between variables, particularly between reaction temperature (x1) and reaction time

(x2). The two dimensional plots for the interaction between reaction temperature (x1)

184 and reaction time (x2) are shown in Figure 4.37. The amount of catalyst (x3) was fixed at 2 wt %. It can be seen that when the reaction temperature was increased from the lower level (200 ºC) to the high level (260 ºC), an increase in the yield of diglycerol resulted initially. However, it starts to decrease and above 240 ºC with reduced reaction time. The increase in the reaction time apparently did not improve the yield at higher reaction temperatures. The diglycerol yield decreased to < 35 % when the reaction time was increased from 2 to 10 h at temperature above 240 ºC.

Thus, the yield of diglycerol reached the highest level of 52 % at intermediate temperature level (240 ºC).

Figure 4.37: 2-dimensional plot for the interactive effect between reaction temperature and reaction time (at 2 wt % catalyst amount).

185

Figure 4.38 shows the two dimensional interactive effect between reaction temperature (x1) and catalyst amount (x3) on the diglycerol yield. The other process variable i.e. the reaction time was kept constant at 8 h. It is generally perceived that higher amount of catalyst used will result in higher yield of diglycerol because of higher availability of active sites. The yield of diglycerol was found to be higher at low temperature with 3 wt % catalyst compared to that with 1 wt % catalyst loading.

However, this trend changed after 240 ºC where increasing catalyst amount resulted in reduced diglycerol yield. This may suggest an interactive effect between the reaction temperature and the catalyst loading. The active site on the catalyst itself might catalyze some new reactions at high reaction temperature to decrease the selectivity of the main product. Subsequently, it was suggested that the reaction should be performed with initial increment for one of these variables at low values of the second variable.

Figure 4.38: 2-dimentional plot for the interactive effect between reaction temperature and catalyst loading (at 8 h).

186

Figure 4.39 shows the two dimensional interactive effect between reaction time (x2) and catalyst amount (x3) on diglycerol yield. The third process variable i.e. the reaction temperature was kept constant at 240 ºC. It is clear from this figure that the trend of diglycerol yield is quite similar upon varying the catalyst loading from 1 to 4 wt % with increasing reaction time. Therefore, this factor could not pose significant effect on the yield of diglycerol as earlier concluded in Table 4.10.

Figure 4.39: 2-dimentional plot for the interactive effect between reaction time and catalyst loading (at 240 ºC).

Overall, 2-D interaction i.e. two dimentional XY-plane between two process variables gave an idea about the trend in diglycerol yield upon changing their values

187 while keeping one variable constant. Therefore, it is of great interest to further characterize the interactions in the form of 3-D i.e. tree dimentional XYZ-plane along with three process variables studied.

4.5.3.3 Three-dimensional (3-D) interactions between variables

The three dimensional and contour plots for the interaction between the reaction temperature and reaction time towards diglycerol yield are shown in Figure

4.40. When the oligomerization reaction was carried out at a lower temperature of

220 ºC, the conversion was found to be less than 25 wt %. However, the diglycerol yield increased (above 55 wt %) with increasing reaction temperature to its central level (240 ºC) while it decreased as the temperature was further increased to its high level (260 ºC). This trend was found to change with changing reaction time (2 to 10 h). At the start of the reaction with low temperature value, the yield was very low and the trend was found to be quite similar with increasing temperature. However, it was more pronounced at longer reaction time (above 6 h). At low temperature, the diglycerol yield was still at low value while with increasing temperature up to the middle level (240 ºC), high yield was and subsequently, it started to decrease with increasing temperature to the higher level of 260 ºC. However, the optimum temperature for the highest yield slightly shifted towards a temperature of 240 ºC as noted in Figure 4.40.

188

Figure 4.40: Three dimensional and contour plots for the effect of reaction time and temperature on diglycerol yield.

The decreasing diglycerol yield at higher reaction temperature was associated with the undesirable condensation of newly formed higher oligomers in the product mixture to produce some higher isomers of diglycerol or other undesired products.

Meanwhile, increasing reaction time positively contributed towards diglycerol yield.

This was actually in accordance with the theory as the oligomerization reaction involves the formation of active glycerol radicals from the neutral glycerol molecules.

The simultaneous dependence of diglycerol yield on the catalyst loading and reaction temperature is shown in Figure 4.41. In the model equation, catalyst loading

(x3) was the major regression variable affecting the response (greatest coefficients),

189 while this variable had a significant interaction with reaction temperature (x1). The regression coefficient of catalyst loading and reaction temperatures are 17.12 and

0.58 for both individual effects, respectively. It is clear from this figure that diglycerol yield steadily increased at low temperature up to the middle level of temperature (240 ºC). Then, this trend started to decrease with further increasing temperature. The same behavior was observed upon increasing the amount of catalyst during that period with increasing temperature. At higher loading (4 wt. %), the diglycerol yield sharply decreased as the reaction temperature was increased to its high level (above 240 ºC) as a result of negative interaction. This is correctly reflected by the negative value (-5.0) of regression coefficients for x1x3 interaction in

Equation 4.2. It is also noted in the comparison study of both Figure 4.40 and

Figure 4.41 that the diglycerol yield moderately increased with increasing catalyst loading, especially at shorter reaction times. The increase was more pronounced at longer reaction time.

190

Figure 4.41: Three dimensional and contour plots for the effect of catalyst loading and reaction temperature on diglycerol yield.

The influence of catalyst loading in increasing the diglycerol yields was attributed to the increase in the amount the active catalytic component that led to improvement in the diglycerol selectivity. Shorter reaction times were required to complete the conversion of glycerol via oligomerization process. However, longer reaction time caused some undesired effect as the already produced diglycerol molecules could be further converted into higher oligomer molecules so that selectivity as well as the yield of diglycerol started to decrease. It can be concluded from above results that the yield of diglycerol increased as the reaction time was increased to its high level (10 h) at 240 ºC after which it started to decrease at higher level of reaction time (10 h) at 240 ºC and above (Clacens et al., 2002) reported that the oligomerization reaction using basic solid catalyst achieved the maximum

191 conversion of glycerol at 260 ºC with long reaction time (above than 12 h) and 2 wt.

% of the catalyst loading. They reported that the selectivity to diglycerol at reaction a time of 8 h was sufficiently high but the overall yield of diglycerol was still rather low due to low conversion of glycerol. In this study, it was found that only 2 wt. % of Clay Li/MK-10 catalyst was needed to achieve the maximum yield of diglycerol.

The optimum reaction temperature was found to be at 240 ºC with the optimum reaction time of 8 h to obtain diglycerol yield of above 55%. The low amount of the mesoporous catalyst needed to achieve the optimum diglycerol yield might be associated with the high surface area of the catalyst. Furthermore, sufficiently large pore sizes facilitated the diffusion of glycerol into its mesopores for the reaction to occur.

4.5.4 Optimization of process parameters

Up to this point, the results revealed that for glycerol oligomerization process, all of the process parameters studied significantly affected the yield of diglycerol.

Thus, the next step is to optimize the process parameter to obtain the highest yield of diglycerol and this effort should take into account not only the individual process parameter but also the interaction between those parameters. The optimization of the diglycerol production process using numerical feature of the Design Expert 6.0.6 software was performed to identify the optimum conditions for achieving the highest yield of diglycerol. Table 4.12 shows the constraints used for process variables and the goal was set as 240 oC for the reaction temperature. It was due to a preliminary result that showed that the maximum diglycerol yield could be obtained at this temperature. Otherwise, the optimum should be determined for the other two variables to give the maximum yield of diglycerol.

192

Table 4.12: Constraints used to obtain the highest value of diglycerol yield.

Name Goal Lower limit Upper limit

o o x1 : Reaction temperature ( C) 240 C - -

x2: Reaction time (h) is in range 2 10

x3 : Catalyst loading (wt.%) is in range 1 3

Diglycerol yield (%) maximize 0 70

Three solutions for the optimum conditions were generated by the software according to the order of suitability. These solutions were then chosen for further process studies to confirm the validity of the statistical experimental strategies with experimental data. The experimental values obtained for diglycerol yield were found to be quite close (within 5.9 % of mean error) to those predicted values using RSM as shown in Table 4.13. The repeated experiments gave an average yield of 60 %.

This meant that the experimental values obtained were reasonably close to the predicted value calculated from the model. It was concluded that the generated model showed reasonable predictability and sufficient accuracy for the diglycerol yield in the ranges experimental of conditions used.

Table 4.13: Results of validation experiments conducted at optimum conditions as obtained from DOE.

Experimental Run x1 x2 x3 Predicted (Actual) value value (%) (%) 1 240 6.94 2.35 61.2 57.6

2 240 6.87 2.39 61.0 59.4

3 240 6.70 2.31 61.1 58.3

193

These results confirm the predictability of the model for the diglycerol yield in the experimental conditions used. For the optimization of process parameters in oligomerization process, it was observed that the optimal set of conditions for the process parameters at 240 oC are given as; 2.35 wt % for the catalyst loading and about 7 h for the reaction time to give 61 % of diglycerol yield. This yield was higher than that reported by previous researchers (Clacens et al., 2002; Ruppert et al., 2008) for the same oligomerization reaction conducted in almost similar conditions. This result showed the potential of the mesoporous clay catalyst to be used in the oligomerization reaction and more future research work should be dedicated to the use of this type of catalyst material.

4.6 Kinetic study for determination of reaction rate and rate parameters

The kinetic study of glycerol conversion to diglycerol was conducted in the presence of the best solid basic clay catalyst (Clay Li/MK-10) under the reaction conditions as; temperature 200 ~ 260 C; time 2 ~ 12 h; 2 wt % catalyst. These parameters were used with various reaction temperatures of 200 ᵒC, 220 ᵒC, 240 ᵒC, and 260 ᵒC at 12 h with 2 wt % catalyst to find out the respective rate of the reaction for this process. The general representation for the oligomerization process by considering only diglycerol formation while ignoring the formation of higher oligomers or is given by Equation (4.2) as follows:

2 Glycerol Diglycerol + H2O (4.2)

The stoichiometry is given by Equation (4.2) with further consecutive reaction of the diglycerols to higher oligomers. The full reaction model consisting of

194 competing-consecutive reaction pathways is not accessible for any analytical solution of the kinetic equations. However, a simpliﬁed approach is possible, taking into account that triglycerol and higher oligoglycerols were not formed within the first 12 h. Then, this reaction is best described by this equation. Figure 4.42 shows the concentration of glycerol (Cg) versus reaction time at different temperatures. It was observed that the value of Cg in the oligomerization reaction decreased with increasing reaction time at all reaction temperatures. It means that glycerol was continuously consumed with the passing of time to form products. In addition, the glycerol concentration was found to decrease more rapidly with increasing reaction time while the reaction temperature was varied from 200 to 260 ᵒC. This rapidly decreasing glycerol concentration was due to its fast conversion into diglycerol or some other byproducts due to an increase in the reaction temperature.

Figure 4.42: Effect of reaction temperature on the concentration of glycerol with 2 wt % of catalyst.

195

From the oligomerization reaction, the rate of reaction per unit time, -rg is expressed as in Equation (4.3):

(4.3)

Where, -rg represents the consumption rate of glycerol and k is usually taken as the global reaction rate constant, which is assumed to obey the Arrhenius equation as given by Equation (2.4). Actually, the reaction rate constant for heterogeneous catalysis can be expressed in terms of apparent rate constant as reported by

(Stamenković et al., 2010):

(4.4)

Where; k = Effective rate constant km = Volumetric glycerol mass transfer coefficient towards catalyst surface

In this equation, km can be expressed as follows:

(4.5)

Where;

ks = glycerol mass transfer coefficient towards catalyst surface active sites

a = specific surface of catalyst

θ = fraction of the catalyst’s available active specific surface

wtcat = weight of catalyst and

V = volume of the reaction mixture

196

As depicted above, kapp takes into account the mass transfer as well as the chemical reaction rate. (Stamenković et al., 2010; Veljković et al., 2009) reported two possible situations for the expression of kapp mentioned in Equation (4.4). The overall reaction process could either be limited by glycerol mass transfer to active sites on the catalyst surface or the chemical reaction between the absorbed molecules of glycerol and active glycerol.

In the present study, fast rate of chemical reaction on the catalyst surface was assumed. In order to perform the kinetic study of the oligomerization reaction, few assumptions are made:

i. Under given reaction conditions, glycerol was considered to be fully

converted into diglycerol and the formation of higher oligomers can be

ignored.

ii. The contribution of homogeneous catalysis was considered negligible due to

showing low leaching rates during the study.

iii. The reaction proceeds with the assumption that the chemical reaction at the

surface of catalyst is much faster than the mass transfer from bulk liquid to

the catalyst so that concentration of glycerol on the surface of the catalyst

could be neglected.

Thus, on the basis of above assumptions, the value of km becomes much smaller than the value of k (km << k). Hence, the above Equation (4.4) can also be written as:

(4.6)

197

Introducing Equation (4.6) into Equation (4.3) generates:

(4.7)

In this equation, ‘Cg’ represents the concentration of glycerol and ‘n’ gives the order of reaction. According to the molecularity of the reaction corresponds to the order of reaction with respect to glycerol in Equation (4.2), a second order rate equation should describe the concentration change of glycerol during the reaction in the batch reactor. Then, this equation can be expressed as:

(4.8)

The solution of Equation (4.8) is

(4.9) where;

Cg = glycerol concentration at time t

Cg0 = initial glycerol concentration at time t=0 and km = reaction rate constant.

For further detail study about the order of reaction, least square ﬁtting was performed for a reaction course according to 0th, 1st and 2nd order of reaction as determined using, integral equations as shown in Table 4.14. A nonlinear ﬁtting of relative glycerol concentration vs. reaction time plot at 240 ᵒC with 2 wt % of catalyst was performed using Sigma plot software and the profiles of different orders

198 of rate equation are shown in Figure 4.43. On the basis of the summarized numerical values and observed minimum correlation coefficients, it was concluded that the Ist or 2nd order equations could not describe the concentration vs. time profile satisfactorily. According to the results of experiments in Figure 4.33, the concentration change of glycerol is best described by a 1st order or pseudo first order equation because experimental values are close to first order of reaction.

Table 4.14: Rate law equations for zero, first and second order of reactions.

Reaction Differential Rate Characteristic Integrated Rate Law Order Law Kinetic Plot d C g C /C vs t Zero - = k C /C = - k t g g0 m g g0 m A straight line d t

d Cg Cg/Cg0 vs t - km t First - = km Cg Cg/Cg0 = e An exponential d t decay curve

d Cg 1 Cg/Cg0 vs t 2 Second - = km Cg Cg/ Cg0 = A deep decay d t 1 + km t Cg0 curve

199

1.0 ReGlycerol Conversion 0th Order ...... 1st Order 0.8 _ _ _ 2nd order

o 0.6

Cg/Cg

0.4

0.2

0.0 0 2 4 6 8 10 12 14 Reaction Time (h)

Figure 4.43: Nonlinear ﬁtting of relative glycerol concentration against reaction time at 240 ᵒC with 2 wt % of Li/MK-10 and 50 g of initial glycerol amount.

It might be suggested that the reaction starts with the attack of basic or an acidic ion (OH-/H+) on the terminal proton to convert the neutral glycerol molecule into a charged form. This charged glycerol species can subsequently attack the second glycerol molecule by eliminating a water molecule. Consequently, the reaction is observed as of pseudo 1st order with respect to the rate-determining reaction of the charged glycerol species.

For further confirmation of the pseudo first order reaction, correlation

2 coefficients R was observed by plotting ln(Cg) against reaction temperature under

200 the same reaction conditions as shown in Figure 4.44. It is clear from the figure that

R2 values vary in the range of 0.92-0.99. All R2 values are found to be above 0.92 to suggest that the data satisfactorily fit the pseudo first order equation. The best straight line for the plot of ln (Cg) versus time is defined as the line gives the highest correlation coefficient (R2) for all the data point obtained from the concentration- time curve for each reaction temperature studied. Hence, pseudo first order polynomial could accurate represent the concentration-time profile for each temperature. Further, rate of reaction for the oligomerization process can be found based on this order of reaction.

201

(a) (b)

2 Figure 4.44: Fitting plot of ln(Cg) against reaction time for R value at various temperatures a) 200 oC, b) 220 oC, c) 240 oC and d) 260 oC.

202

The global reaction rate constant and the correlation coefficient for the straight line obtained are summarized in Table 4.15 for each reaction temperature.

Table 4.15: Reaction rate constants and correlation coefficients elicited from the plot of ln (Cg) vs. time.

o 2 -1 T( C) T(K) km R 1/T (K ) ln km

200 473 0.0243 0.9791 0.002114 -3.71728

220 493 0.0735 0.994 0.002028 -2.61047

240 513 0.3558 0.9250 0.001949 -1.03339

260 533 0.4222 0.9746 0.001876 -0.86228

The integral law for first order as discussed in Table 4.14 can be writing as:

(4.10)

The Equation (4.10) has a form of a straight line i.e.,

(4.11)

It is clear from comparison of Equation (4.10) and (4.11) that the slope ‘m’ corresponds to the rate constant k. This means that, for a first order reaction, a plot of ln(Cg) as a function of time gives a straight line with a slope of ‘–km’. Table 4.15

2 shows the global reaction rate constant ‘km’ and correlation coefficients ‘R ’ extracted from the plot of ln(Cg) versus reaction time ‘t’.

203

From the data in Table 4.15, it is observed that the global reaction rate constant increases with reaction temperature. The increase in global reaction rate constant will result in higher rate of oligomerization reaction at higher reaction temperature. The high values of correlation coefficient (R2 > 0.92) for all straight line drawn suggest that the assumptions used in this study are valid. In order to determine the reaction order “n”, Equation (4.3) is further considered. The integral solution of this equation can be written as:

(4.12)

The value of “n” cannot be found explicitly from Equation (4.12). The solution can only be achieved by trial-and-error method which is rather complicated or by means of the Fractional Life Method. Therefore, the Fractional Life Method

‘tF’ was employed to determine the exact order of reaction (n). This half-life method can be extended to any Fractional Life Method in which the concentration of reactant drops to any fractional value F = Cg/Cgo in time tF. The derivation is a direct extension of the half-life method or:

(4.13)

According to this Fractional Life Method, the order of reaction (1-n) is the slope of a straight line obtained from a plot between log tF versus ln Cg with 80% conversion factor (Athawale and Mathur, 2001). For this purpose, tF value was

204 obtained using the Fractional Life Method with F = 80%. The data points for different concentrations of glycerol and their corresponding reaction time were collected from smooth curve of glycerol conversion over catalyst Li/MK-10 under optimum conditions as shown in Figure 4.45 and Table 4.16.

100

80 Smooth Curve for t (Li/MK-10 at 240 oC) 1.01 1/2

o 60

1.58 2.65

at 240

g 40

3.05 4.25

6.65 7.88 0 0 2 4 6 8 10 12

Time (h)

Figure 4.45: A smooth curve for F=80 % for glycerol conversion via of oligomerization reaction at optimized reaction conditions.

205

Table 4.16: The data points for relevant glycerol concentration Cgend with their respective reaction time tF.

Cg Cgend= 0.8*Cg Time needed tF (s) ln tF sec ln Cg

100 80 3636 3.560624 4.60517

71 56 3852 3.585686 4.26268

45 40 4320 3.635484 3.806662

22 16 4608 3.663512 3.091042

After collecting data points for Cg and tF then a plot between log tF versus ln(Cg) is made to give a linear function as shown in Figure 4.46. The reaction order,

(1- n) is obtained from the slope of this straight line. The slope of the line was found to be -0.07. Therefore, the value of reaction order, n, is calculated to be 1.07 which is quite close to first order reaction. Hence, the use of pseudo first order reaction is also verified through this method.

Figure 4.46: A plot for measuring the order of reaction using Fractional Life Method.

206

In order to obtain the activation energy and frequency factor of the glycerol conversion, Arrhenius Equation (2.3) is used and it is done by determining the value of global reaction rate constant (ln km) verses reciprocal of reaction temperature

(1/T). The plot should again result in a straight line with a slope of ‘–Ea/R’ and intercept is equal to ‘ln A’. Based on the data obtained in Table 4.14, a graph of ‘ln km’ versus ‘1/T’ was plotted as shown in Figure 4.47. The plot was found to obtained a straight line with high value of correlation coefficient (above 0.94) indicating that the global reaction rate constant does obeys the Arrhenius Law.

Figure 4.47: A plot of ln km versus 1/T for measuring the activation energy.

Thus, the slope of the plot would be equivalent to –Ea/R and intercept equivalent to ln A. According to Figure 4.51, intercept value is

ln A = 23.569, so A= 1.72 x 1010 h-1

A= 4.78 x 106 sec-1

The slope of the plot is

207

Ea  = -12864 K R

The value of gas constant R is 8.314 J/mol. K, therefore

Ea = 1550 J/mol

Thus, value of activation energy and frequency factor for the oligomerization reaction are found to be 1550 J mol-1 and 4.78 x106, respectively. With the kinetic parameters obtained, a mathematical model is proposed for the oligomerization process in a batch reactor as given by Equation (4.14):

(4.14)

In order to verify the result, this model was used to predict the concentration-time curve at each reaction temperature. Figure 4.48 shows the comparison between the experimental concentration-time curve and simulated concentration-time curve predicted at various temperatures using the model developed. It was found that the model predicted the concentration-time curve very well with an R2 value of 0.92 and less than 7 % of standard error. The errors are represented as the differences between the calculated glycerol concentrations and the experimental values and they are found to be rather low. Therefore, it can be concluded that model is highly reliable in predicting the kinetics of the glycerol oligomerization process.

208

100

260 oC 80 240 oC 220 oC 200 oC 60

experimental (wt %)

C 20 Aver. value R2 = 0.92 Standard Error = 6.8 %

0 0 20 40 60 80 100

Cg calculated (wt %)

Figure 4.48: Comparison between Cg experimental and Cg calculated versus reaction time at each temperature using the proposed kinetic model equation.

4.7 Reusability of the catalyst

Catalyst reusability is of great technical, economic and ecologic concerns in the practice of chemical processes. Most attention is directed towards the economical and environmental sustainability aspects of the chemical production and it is motivates the scientiﬁc community to develop solid catalysts that are prone to deactivation. The Clay Li/MK-10 catalyst samples were characterize before and after the reaction to study their structure stability, surface area, pore volume and pore size using XRD, ICP-AEC and nitrogen adsorption and desorption methods. A quite similar XRD patterns was found for both clay samples before and after reaction as shown in Figure 4.49. In both catalysts, the basic peaks for montmorillonite structure

209 are observed clearly at between 10° and 40° of 2θ angle corresponding to the amorphous phases of the clay. In addition, there was disappearance of two major peaks at 33° and 38° of 2θ from XRD patterns of catalyst used after reaction. These two peaks were belonging to lithium compound as shown in the fresh catalyst XRD patterns. It may reduce due to lithium leaching as shown in Table 4.17. Although, the intensity of the peaks is noted to reduce significantly for the catalyst after the reaction, overall, it showed a sustained structure like that of the fresh Clay Li/MK-10 sample. In addition, the peaks belonging to lithium component in the fresh catalyst

(see Figure 4.26) are found to be weaker and with lower intensity in case of the catalyst sample after reaction. This might be due to the liquid phase reaction and harsh reaction conditions (12 h reaction at 240 ºC and continuous stirring). Hence, the occurrence of some basic peaks in the catalyst after reaction confirmed the stability of the structure. However, a slight decrease in the intensity of the peaks is also attributed to the incorporation of lithium in the Clay Li/K-10.

Figure 4.49: XRD patterns for Clay Li/MK-10 before and after the oligomerization reaction.

210

The surface properties of the Clay Li/K-10 catalyst before and after the reaction are tabulated in Table 4.17. It can be seen that the surface area, pore size and pore volume of the catalyst significantly decreased after the reaction. However, the surface area and pore size of the catalyst after the reaction was found to be high enough to continue with the second run of the reaction as reusable catalyst. The decrease in surface area and pore size might be a result of the partial blockage of the pores of the catalyst by some substances produced during the reaction or might be due to partial destruction of some portion of the layered structure during long reaction process. Moreover, the lithium contents on the surface of the catalyst were also measured using ICP-AES analysis and its value was also added in the same table. It is clear from this table that the lithium amount slightly decreased after reaction from 9.8 to 6.9 wt %. It was also in agreement with the decreasing intensity of lithium peaks in XRD patterns as shown in Figure 4.43.

Table 4.17: Surface properties of Clay Li/MK-10 catalyst before and after reaction.

Properties Clay Li/MK-10 Clay Li/MK-10

(Before reaction) (After reaction)

BET Surface Area (m2/g) 123 93

Mesoporous Area (m2/g) 97 68

Pore Volume (cm3/g) 18.1 10.3

Pore Size (nm) 0.29 0.24

*Lithium contents (wt %) 9.8 6.9

*Measured using ICP-AES analysis

211

Reusability study of the Li/MK-10 catalyst was carried out at optimum conditions of the oligomerization process for up to three times. Figure 4.50 show the diglycerol yield versus the run numbers. It is clear from this figure that there was a significant reduction in the catalytic activity after each reaction run of the catalyst in the oligomerization process. The diglycerol yield was recorded at 57.4 wt % in the first run and it decreased to 26.8 wt % in the second run. The diglycerol yield experienced further decrease to 16.3 wt % in the third run.

60 57.4 wt %

30 26.8 wt %

20 Diglycerol Yield (wt %) 16.3 wt %

0 1 2 3 Run Number

Figure 4.50: Diglycerol yield versus run numbers in the glycerol oligomerization process at optimized reaction conditions.

212

These results demonstrate that the decrease in catalytic activity between the catalytic run was attributed to the leaching of the active metal lithium from the clay support. This was also confirmed by the characterization results of used catalyst in which a the amount of lithium contant siginificantly decreased after the first reaction process. The lithium amount lost during the reaction might dissolve in the liquid phase of the reaction mixture. According to a previous study by Hunter and co- workers (Hunter et al., 1987), this metal leaching occurred as some species in the process such as the starting material, intermediate, or newly produced components were better ligands, forming a stronger bond with the metal complex than the ligand functionality of the solid support. Otherwise, the modification done to the catalyst after each cyle (filtration, washing and redrying) could also affect the catalytic activity.

It was suggested that the leached active metals would remain in the reaction and contributed to the homogeneous reaction. Actually, the leaching of active component lithium was observed very high as also prove by some previous researchers but this catalyst can be reused up to 2nd cycle. If we need further reusability of this catalyst then we have to regenerate catalyst by socking it in LiOH solution after reusing in oligomerization process. To confirm the leaching phenomenon, the final liquor product samples was collected after the reaction for the first, second and third run.

These final liquor product samples were then subjected to the ICP-AES analyses to determine the amount of lithium content. The ICP-AES results are shown in Table 4.18. The total amount of lithium present in fresh lithium loaded catalyst was measured to be at 9.8 wt %. The amount of lithium in the product liquor was determined to be 25.56 wt % of this fresh lithium loaded catalyst. The amounts of

213 lithium in second and third run were found to be 12.23 and 8.73 wt %, respectively.

These results indicate that active component lithium could partialy dissolve in liquid phase with the passage of reaction time. It was suggested that the lithium component was merely adsorbed over the surface of catalyst rather than forming certain chemical interaction with the catalsyt.

Table 4.18: Lithium contents in the final liquid product during the oligomerization process.

Run number Lithium present in Lithium remove liquid phase from catalyst ICP-AES (ppm) Actual (wt. %) 1 14.3 25.56

2 8.02 12.23

3 5.39 8.73

In other words, the amount of lithium in the used catalyst decreased continuously in the second and third run. This was deemed the main factor to cause a decrease in its activity which was measured in terms of diglycerol yield. The effect of lithium leaching on diglycerol yield was further investigated during the 12 h of reaction time using the Clay Li/MK-10 catalyst as shown in Figure 4.51.

214

25 60 Diglycerol yield Lithium amount 20

40 15

10 20

Diglycerol Yield (wt %)

Lithium Lithium in liquid phase (wt %) 5

0 0 0 2 4 6 8 10 12 14 Reaction Time (h)

Figure 4.51: Lithium leaching effect on diglycerol yield during the 12 h of reaction time.

It can be seen that about 6 % of lithium contant in the fresh clay catalyst material could be found in the liquid phase after 2 h of reaction and these amounts gradually increased with increasing reaction time. It is clear from this figure that the leaching was rather gradual at the beginning of the reaction but it reached at a significantly higher level (above 25 %) after 8 h of reaction. At the same time, the analysis of catalytic activity proved that the diglycerol yield reached its maximum

(58 wt %) at 8 h and then it started to decrease and reached at low level of 37 wt % at

12 h of reaction time. It can be confirmed that there was a direct correlation between the content of active component and the diglycerol yield. It means that at the beginning of the reaction, the lithium component was on the surface of the clay support. At this time, the reaction might take place due to the availability active sites

215 on the clay support. However, with further increase in reaction time after 8 h, some amount of lithium might dissolve in the liquid phase and acted as homogeneously.

Consequently, some undesired products might be formed instead of selective formation of diglycerol molecule. This may be the reason for the decreasing diglycerol yield after 8 h of reaction time.

The obtained result was compared with those reported by other researchers.

(Barrault et al., 2004; Xie et al., 2007) reported that upon the reusability of an alkali based solid catalyst for the first cycle, significant decrease in the catalytic activity was observed. They also attributed the decrease to alkali metal leaching from the solid catalyst to the liquid phase. Ruppert et al., (2008) also reported a remarkable reduction in the content of active alkali metal during glycerol oligomerization process and similar argument was used to explain it. Thus, further research efforts should be dedicated to the leaching problem to minimize the activity loss so that a more successful reutilization of solid catalytic systems in the glycerol oligomerization process can be achieved.

216

CHAPTER FIVE

CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

The present work focused on the solvent-free selective conversion of glycerol to diglycerol via catalytic oligomerization. Different catalysts i.e. homogeneous

LiOH, microporous Li/USY, mesoporous Li10-Mg30/SBA-15 and layered montmorillonite Clay Li/MK-10 were used with enhancing activities for this process.

These catalysts were successfully characterized using different analytical techniques reveal their physical, chemical and structural properties.

Solvent-free glycerol oligomerization to diglycerol in the presence of various alkaline metal precursors as a homogeneous catalyst was successfully investigated.

LiOH catalyst showed good activity for glycerol oligomerization, achieving complete glycerol conversion with a corresponding 33 % selectivity toward diglycerol as the desired product. However, the selectivity started to decrease after 4 h due to low amount of glycerol remaining in the reaction system and diglycerol started to be converted to new products. Lithium modified microporous material i.e. modified zeolite catalyst (Li/USY) which had high surface area and significant basic strength was sufficiently active for polyglycerol production. However, Li/USY was found to be less selective for diglycerol. The reaction mainly occurred on the external surface of the catalyst due to small pore size of its microporous structure.

Mesoporous silica supports, SBA-15 was successfully prepared but severe destruction of the mesoporous structure was detected after loading of lithium. The mesoporous structure of SBA-15 was found to be stable after suitable amount of magnesium coating prior to lithium loading. The stabilized modified mesoporous

SBA-15 (Li10-Mg30/SBA-15) which had high surface area and basic strength showed

217 high glycerol conversion with considerable amount of polyglycerols production but very low selectivity to diglycerol was still observed. The reaction was found to mainly occur on the external surface of the catalyst due to decreasing pore size of the prepared catalyst after magnesium coating.

The prepared lithium modified montmorillonite catalyst (Clay Li/MK-10) showed high glycerol conversion and selectivity to diglycerol under same reaction conditions as compared to the other prepared catalysts. The activity and selectivity to diglycerol of this catalyst were enhanced due to its high basic strength and its layered structure with high pore size and sufficiently large surface area. Most of the reaction occurred within this layered structure to form diglycerol and significantly retarded the formation of higher oligomers. Higher glycerol conversions to diglycerol by Clay

Li/MK-10 catalyst suggested that the basic strength and pore size could signiﬁcantly and synergically affect the product selectivity.

By employing response surface methodology in experiments of the oligomerization process, the interactions between the factors and the results were elucidated. The optimum conditions were found at 240 oC for the reaction temperature, 2.35 wt. % for the catalyst loading and about 7 h for the reaction time to give the best diglycerol yield of 57.6 %. These optimum conditions were used in the subsequent studies to evaluate the performance of the catalyst in the oligomerization of glycerol and the reusability of the catalyst for up to three cycles of reaction.

The Arrhenius equation model was applied to the experimental data obtained from the transesterification studies with different reaction temperatures to determine the kinetic parameters. From the kinetic study, glycerol oligomerization reaction was found to follow a pseudo first order reaction with ‘n’ value of 1.07 and the activation

218 energy was obtained as 1550 J/mol. The predicted values of glycerol concentration from the Arrhenius model correlated well with the experimental values.

Reusability study conducted using the Clay Li/MK-10 catalyst under its optimized reaction conditions. The catalyst was proven to be highly potential for the glycerol oligomerization process showing 57.6 % of diglycerol yield under the optimized reaction conditions. The reusability study of the catalyst for up to three cycles gave 26.8 % of diglycerol yield in the second cycles but significantly dropped in third cycle with only 16.3 %. The study indicated that leaching problems significantly occurred.

5.2 Recommendations

Based on the conclusions above, few recommendations can be drawn for future research work in this area:

1. In the preparation of catalyst, it is suggested to apply some other methods of

modified clay preparation instead of only wet impregnation method. Other

methods for the catalyst preparation in terms of the method for incorporating

metals into the support such as one pot synthesis and ion exchange method

can also be investigated and optimized as well. The catalyst can be calcined

at different temperatures to produce a more stable catalyst, which can avoid

leaching problem.

2. In the present study, only lithium was used to be incorporated into

montmorillonite K-10 clay as the support. It is proposed that the

incorporation of this clay with other types of alkali metals or alkali earth

219

metals to be investigated as well and their activity in the oligomerization

process can be investigated.

3. The leaching problem in heterogeneous catalyst should be further

investigated as this is one of the important criteria that receive little attention

by some researchers.

4. Further in-sight into the mechanism of the oligomerization can be

investigated by determining the distribution of intermediate products and

modeling of the process can be attempt accordingly.

5. Catalyst regeneration and reactivation method can be investified to improve

the activity of the reuse catalyst.

220

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APPENDICES Appendix A

Table A-1: Physical and chemical properties of glycerol

Properties Values Chemical formula CH2OH-CHOH-CH2OH Formula weight 92.09 Form and colour Colourless and liquid Specific gravity 1.260 50/4 Melting point 17.9 oC Boiling point 290 oC Solubility in 100 parts Water Infinity Alcohol Infinity Ether Insoluble Vapor pressure in 760 mmHg 290 oC Heat of fusion at 18.07 oC 47.49 cal/g Viscosity liquid glycerol 100% 10 cP 50% 25 cP Diffusivity in (DL x 105 sq cm/s) i-Amyl alcohol 0.12 Ethanol 0.56 Water 0.94

Specific heat glycerol in 15 oC 30 oC aqueous solution (mol %) (cal/g oC) (cal/g oC)

2.12 0.961 0.960 4.66 0.929 0.924 11.5 0.851 0.841 43.9 0.670 0.672 100 0.555 0.576

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Table A-2: Physical and chemical properties of diglycerol

Physical properties Information Remarks

General appearance Viscous liquid, At temperature: 20 °C ( 68 °F ) colorless and odorless pH 5 - 6 Concentration: 100 g/l Temperature: 20 °C ( 68 °F ) Boiling point/boiling range 205 °C ( 401 °F ) At pressure: 1.3 mbar Flash point 230 °C ( 446 °F ) Combustible material Method: open cup Fire point 264 °C ( 507 °F ) - Autoignition 380 °C (716°F ) - Explosive properties Explosion danger: Vapors may form explosive mixture with air. (Glycerol) Vapor pressure < 1 mbar At temperature: 20 °C ( 68 °F ) Relative density / Density 1.28 At temperature: 25 °C ( 77 °F ) Solubility Soluble in water At temperature: 20 °C ( 68 °F ) and alcohol Viscosity 12,000-13,000 mPa.s At temperature: 25 °C ( 77 °F ) Partition coefficient log Pow/ -1.76 n-octanol / water Refractive index 34 At temperature: 25 °C ( 77 °F ) Heat of dissolution in water -52 J/g At temperature: 25 °C ( 77 °F ) Thermal conductivity 0.28 W/m.K At temperature: 25 °C ( 77 °F ) Thermal expansion 0.00053 At temperature: 20 - 60 °C coefficient (68- 140°F ) Specific heat capacity 2.28 J/g. K At temperature: 25 °C ( 77 °F )

238

Appendix B

Batch reactor system used in this study

Plate A1: Batch reactor system for oligomerization Plate A2: Batch reactor system used in series

Plate A3: Batch glass reactor Plate A4: Dean Stark system for water removal

239

Plate B1: Prepared catalysts Plate B2: Product samples for product analysis

Plate C1: GC-FID used for product analysis

240

Appendix C

GC-FID calibration curves for pure glycerol, diglycerol and triglycerol with different concentrations

Figure C-1-1: Calibration curves for pure glycerol with R2= 0.99997

Figure C-1-2: Calibration curves for pure diglycerol with R2= 0.99997

Figure C-1-3: Calibration curves for pure triglycerol with R2= 0.99997

241

GC-FID Standard peaks for pure glycerol, diglycerol and triglycerol with their retention time tR

Figure C-2-1: Standard peak for pure glycerol with tR= 3.20 minute

Figure C-2-2: Standard peak for pure glycerol with tR= 8.632 minute

Figure C-2-3: Standard peak for pure glycerol with tR= 33.101 minute

242

GC-FID Standard peak for product samples

Figure C-3-1: GC-FID peak for product sample with high value of polyglycerol

Figure C-3-2: GC-FID peak for product sample with high value of αα’-dimer

Figure C-3-3: GC-FID peak for product sample with high value of αβ-dimer

243

Appendix D

Silylation process for product analysis

The reagents and products (glycerol, diglycerol and triglycerol) were analyzed using silylation derivatization technique according to the method used by Clacens and co workers () as follows;

. A product sample amount of 50 mg weighted over balance was mixed with

carefully dried pyridine (1.5 cm3) in a screw-capped septum vial (4 cm3).

. After dissolution, 0.2 cm3 of HMDS (hexamethyldisilazane) and 0.1 cm3 of

TMCS (trimethylchlorosilane) were measured by auto- pipette and added

into this prepared mixture.

. After addition these reagent, the mixture solution was shaken well

vigorously.

. Then, this mixture solution was heated upto 70°C for 1 h in oven and then

cooled down in open air for next 2 h.

. There should be two layers mixture solution after this settle down process

. A portion 0.05 cm3 of top layer of this solution was taken then diluted in

dried toluene of amount 2 cm3.

. Finally, 1 μL of this solution was auto injected into a GC-FID 1800 system

(Agilent Technologies) for GC analysis on a capillary column HP-5 (30

m×0.25 mm×0.25 μm, Agilent Technologies) in a temperature-programmed

mode (ramp 10 K min−1) from 60 to 250 °C.

244

Appendix E

Calculation for the concentration of glycerol, diglycerol and triglycerol at 240 oC for 6 h of reaction over Li/MK-10

Figure E-1: GC-FID analysis signals for product sample

Figure E-2: GC-FID analysis external standard report for product sample

245

Table E-1: According to external standard report, mixture composition of final product

Final products Wt % *Amount in mg Amount in mmol/l

Glycerol 32.3 16.15 0.17926518

Diglycerol 38.5 19.25 0.11584522

Triglycerol 9.8 4.9 0.02039542

Other than polyglycerol 19.4 9.7

Total 100 50

*On the basis of 50 mg sample

Wt. of glycerol reacted Glycerol conversion (%) = x 100 % Wt. of glycerol initially taken

Initial amount of glycerol taken = 50 mg

Final amount of glycerol in product = 16.15 mg

Wt of glycerol reacted = 50 – 16.15 = 33.85 mg

Glycerol Conversion (%) = 33.85*100/50

= 66.7 wt %

Or glycerol Conversion in mg = 0.667 mg

Or glycerol Conversion in mmol/l = 0.007515 mmol/l

Wt. of diglycerol produced Diglycerol yield (%) = x 100 % Wt. of glycerol initially taken

Initial amount of glycerol taken = 50 mg

Final amount of diglycerol in product = 19.25 mg

Wt of glycerol reacted = 50 – 16.15 = 33.85 mg

Diglycerol Yield (%) = 19.25*100/50

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= 38.5 wt %

Or diglycerol Yield in mg = 0.38 mg

Or diglycerol Yield in mmol/l = 0.002287mmol/l

Wt. of polyglycerol (di- + tri-) produced Polyglycerol yield (%) = x 100 % Wt. of glycerol initially taken

Initial amount of glycerol taken = 50 mg

Final amount of polyglycerol (di- + tri-) in product = 24.15 mg

Wt of glycerol reacted = 50 – 16.15 = 33.85 mg

Polyglycerol Yield (%) = 24.15*100/50

= 48.3 wt %

Or diglycerol Yield in mg = 0.48 mg

Wt. of diglycerol produced Diglycerol selectivity (%) = x 100 % Wt. of converted glycerol

Wt. of diglycerol in product = 19.25 mg

Wt of glycerol converted = 50 – 16.15 = 33.85 mg

Diglycerol Selectivity (%) = 19.25*100/35.85

= 53.7 wt %

Or diglycerol Yield in mg = 0.537 mg

Or diglycerol Yield in mmol/l = 0.002235 mmol/l

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LIST OF ISI PUBLICATIONS

(OUTCOME OF THE THESIS)

AYOUB, M. and ABDULLAH, A. Z. 2011. Instability of SBA-15 to Strong Base: Effects of LiOH Impregnation on its Surface Characteristics and Mesoporous Structure. Journal of Applied Sciences, 11, 3510-3514.

AYOUB, M. and ABDULLAH, A. Z. 2012. Critical review on the current scenario and significance of crude glycerol resulting from biodiesel industry towards more sustainable renewable energy industry. Renewable and Sustainable Energy Reviews, 16, 2671-2686.

AYOUB, M. and ABDULLAH, A. Z. 2013. LiOH-modified montmorillonite K-10 as catalyst for selective glycerol etherification to diglycerol. Catalysis Communications, 34, 22-25.

AYOUB, M., KHAYOON, M. S. and ABDULLAH, A. Z. 2012. Synthesis of oxygenated fuel additives via the solventless etherification of glycerol. Bioresource Technology, 112, 308-312.

AYOUB, M. and ABDULLIAH, A. Z. 2013. Diglycerol synthesis via solvent free glycerol etherification over lithium modified clay act as a solid-base catalyst. Chemical Engineering Journal. 225, 784–789.

AYOUB, M. and ABDULLAH, A. Z. 2013. Stabilization of SBA-15 against LiOH loading through coating with MgO. Journal of Applied Sciences, In Press

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LIST OF CONFERENCE PRESENTATIONS

(OUTCOME OF THE THESIS)

1- Muhammad Ayoub*, Ahmad Zuhari Abdullah. Structural effects of strong alkali metal loading over mesoporous SBA-15. 2nd Symposium USM Fellowship Penang, Malaysia. ISBN: 978-967-3940-31-8 2- Muhammad Ayoub*, Ahmad Zuhari Abdullah, Stabilization of SBA-15 against LiOH loading through coating with MgO. 2nd International Conference on Process and advance materials (ICPEAM 2012), Kuala Lumpur, Malaysia. 12-14 Jun 2012, ISBN:978- 983- 2271- 81-9 3- Muhammad Ayoub*, Ahmad Zuhari Abdullah, Characterization of lithium intercalated montmorillonite K-10 clay as a solid basic catalyst. The International Conference on Environment (ICENV 2012), Penang, Malaysia, 11-13 December 2012 4- Muhammad Ayoub*, Ahmad Zuhari Abdullah, Preparation and characterization of lithium modified aluminum pillared clay as a basic solid material. 5th AUN/SEED-Net Regional Conference on Materials & 5th Regional Conference on Natural Resources and Material, 21-23 January 2013, Pinang, Malaysia 5- Muhammad Ayoub, Ahmad Zuhari Abdullah*, Critical review on the significance of crude glycerol resulting from biodiesel industry towards more sustainable renewable fuel production. The International Conference on Environment (ICENV 2012), Penang, Malaysia, 11-13 December 2012 6- Muhammad Ayoub*, Ahmad Zuhari Abdullah, Lithium modified AlPC preparation and analysis for conversion of biodiesel-derived glycerol. Postgraduate Colloquium for Environmental Research (POCER 2013), 28 – 29 June 2013, Genting International Convention Centre (GICC), Malaysia

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