SULFONIC ACID CATALYSTS BASED ON POROUS CARBONS AND POLYMERS

TIAN XIAO NING

NATIONAL UNIVERSITY OF SINGAPORE 2009

SULFONIC ACID CATALYSTS BASED ON POROUS CARBONS AND POLYMERS

TIAN XIAO NING (M.Eng)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009

Acknowledgement

Acknowledgement

I would like to convey my deepest appreciation to my supervisor, Assoc. Prof. Zhao

X. S., George for his constant encouragement, invaluable guidance, patience and understanding throughout the whole period of my PhD candidature. This project had been a tough but enriching experience for me in research. I would like to express my heartfelt thanks to Assoc. Prof. Zhao for his guidance on writing scientific papers including this PhD thesis.

In addition, I want to express my sincerest appreciation to the Department of

Chemical and Biomolecular Engineering for offering me the chance to study at NUS with a scholarship.

It’s my pleasure to work with a group of brilliant, warmhearted and lovely people,

Dr. Su Fabing, Dr. Lv Lu, Dr. Zhou Jinkai, Dr. Li Gang, Dr. Wang Likui, Dr. Bai Peng,

Ms. Lee Fang Yin, Ms. Liu Jiajia, Ms. Zhang Li Li, Ms. Wu Pingping, Mr. Cai

Zhongyu, Mr. Dou Haiqing, and Mr. Zhang Jingtao.

Particular acknowledgement goes to Mr. Chia Phai Ann, Mr. Shang Zhenhua, Dr.

Yuan Zeliang, Mr. Mao Ning, Dr. Rajarathnam D., Madam Chow Pek Jaslyn, Mdm

Fam Hwee Koong Samantha, Ms Lee Chai Keng, Ms Tay Choon Yen, Mr. Toh Keng

Chee, Mr. Chun See Chong, Ms. Ng Ai Mei, Ms. Lum Mei Peng Sharon, and Ms. How

Yoke Leng Doris for their kind supports.

I thank my parents. It is no exaggeration to say that I could not complete the PhD work without their generous help, boundless love, encouragement and support.

i Table of Contents

Table of Contents

Acknowledgement ...... i

Table of Contents ...... ii

Summary ...... v

Nomenclature ...... vii

List of Tables ...... viii

List of Figures ...... ix

Chapter 1. Introduction ...... 1

1.1 Solid sulfonic acid catalysts ...... 1

1.2 Objectives of thesis work ...... 3

1.3 Structure of thesis ...... 4

Chapter 2. Literature Review ...... 7

2.1 Propylsulfonic-modified mesoporous silica ...... 7

2.2 Arenesulfonic-acid modified material ...... 18

2.3 Perfluorosulfonic-acid modified mesopouous material ...... 23

2.4 Organosulfonic-modified periodic mesoporous organosilica ...... 28

2.5 Sulfonic acid-modified carbon ...... 37

2.6 Sulfonic acid-modified resin ...... 43

Chapter 3. Experimental Section ...... 48

3.1 Reagents and apparatus ...... 48

3.2 Preparation of sulfonic acid catalysts ...... 49

ii Table of Contents

3.3 Characterization ...... 53

3.4 Evaluation of conversion for acetic acid ...... 58

Chapter 4. Sulfonated Mesoporous Carbons and Carbon-silica Composites ...... 59

4.1 Introduction ...... 59

4.2 Catalyst preparation ...... 60

4.3 Characterization of sulfonated mesoporous carbons and carbon-silica composites

...... 60

4.4 Catalytic properties in esterification ...... 71

4.5 Summary ...... 73

Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons ...... 74

5.1 Introduction ...... 74

5.2 Catalyst preparation ...... 75

5.3 Characterization of sulfonated mesoporous polymers and carbons ...... 75

5.4 Catalytic properties in esterification ...... 86

5.5 Summary ...... 88

Chapter 6. Sulfonated Polypyrrole and Carbon Nanospheres ...... 90

6.1 Introduction ...... 90

6.2 Catalyst preparation ...... 90

6.3 Characterization of sulfonated polypyrrole and carbon nanospheres ...... 91

6.4 Catalytic properties in esterification ...... 91

6.5 Summary ...... 104

iii Table of Contents

Chapter 7. Sulfonated Polystyrene-Divinylnenzene Spheres ...... 105

7.1 Introduction ...... 105

7.2 Catalyst preparation ...... 106

7.3 Characterization of sulfonated polystyrene spheres ...... 106

7.4 Catalytic properties in esterification ...... 117

7.5 Summary ...... 118

Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated

Polystyrene-Divinylbenzene Spheres ...... 120

8.1 Introduction ...... 120

8.2 Results and Discussion ...... 122

8.3 Summary ...... 133

Chapter 9. Conclusions and Recommendations ...... 134

9.1 Conclusions ...... 134

9.2 Recommendations ...... 137

References ...... 139

Appendix: List of publications ...... 152

iv Summary

Summary

Liquid sulfuric acid is a widely used homogeneous catalyst in many important chemical processes. However, liquid sulfuric acid has a number of problems, such as corrosion, toxicity, and disposal problem. Therefore, solid sulfonic acid catalysts are strongly desired. Over the past few years, solid sulfonic acid materials have been investigated, aimed to replace the liquid sulfuric acid catalyst. The preparation of such solid sulfonic acid materials generally includes functionalization of porous silica, carbon and polymer materials with propylsulfonic acid, arenesulfonic acid, perfluorosulfonic acid and sulfonic acid groups.

Carbon in its chemical allotropes of graphite and diamond occurs in a great variety of species and has been developed to a large number of applications as structural and functional materials. The underlying reason for this unique manifold of species is twofold: (1) the co-ordination chemistry of carbon is flexible in allowing continuous mixtures of C=C and C-C bonding in one structure. This leads to an infinite possibility of 3-dimensional structures (e.g.: carbon nanotubes, graphene, C60) and to continuous tenability of structural and physical properties, (2) carbon accepts foreign elements such as hydrogen, boron, oxygen, nitrogen, and sulfur both on its surfaces and within structural framework. This leads to tunable physical and chemical properties.

Porous carbons such as activated carbons and carbon fibers have long been used as sorbents, catalyst supports and electrode materials because of their unique properties, such as high surface area, good electric conductivity, tunability of surface chemistry, stability against various chemical environments, and low cost. Their high surface area ensures a high density of catalytic active sites when used as catalysts and catalyst supports.

v Summary

In this thesis work, carbon materials were prepared and subsequently sulfonated.

First, mesoporous carbon was prepared using the hard template method. It was found that a high carbonization temperature resulted in the formation of large carbon sheets, unfavorable for the subsequent functionalization of sulfonic acid groups. On the other hand, sulfonated mesoprorous carbon-silica composites exhibited a better catalytic performance than sulfonated mesoporous carbons. Second, mesoporous phenol resins were synthesized by a soft template method and subsequently carbonized to form mesoporous carbons. Sulfonations were conducted on both mesoporous resins and carbons. Temperature was again found to play an important role in both carbonization and sulfonation. Sulfonated mesoporous phenol resins exhibited a higher conversion and stability than sulfonated mesoporous carbons. Third, polypyrrole nanospheres were synthesized and carbonized to carbon nanospheres. Both polypyrrole and carbon nanospheres were sulfonated. It was found that polypyrrole nanospheres were easier to be sulfonated than carbon nanospheres. Fourth, both linear-linked and cross-linked polystyrene spheres were synthesized and sulfonated. Sulfonated cross-linked polystyrene spheres showed a higher conversion and stable recyclability than linear- linked spheres. Finally, the kinetics and mechanism of esterification reaction of methanol with acetic acid over sulfonated cross-linked polystyrene spheres were investigated. The reaction mechanism was experimentally studied and the reaction kinetics in the micro-kinetic region was modeled. The adsorption equilibrium constants of acetic acid, methanol, and water were found to be 0.2, 0.5, and 4.1 L/mol respectively. The initial rate decreased with the increase of water concentration, showing the inhabitation effect of water.

vi Nomenclature

Nomenclature

DI Deionized

C0 Initial concentration (mol/L) ro Initial Reaction Rate (mol/min*L) t Time (min) CVD Chemical vapor deposition CHNS-O Elemental analysis BET Brunauer-Emmett-Teller FESEM Field emission scanning electron microscopy FTIR Fourier Transform Infrared HREM High-Resolution Electron Microsocopy MAS Magic Angle Spinning NMR Nuclear Magnetic Resonance HMS Hexagonal Mesoporous Silica PS Polystyrene SBA Santa Babara SEM Scanning electron microscopy TEM Transmission electron microscopy TEOS Tetraethyl orthosilicate UV Ultraviolet XPS X-ray Photoelectron Spectroscopy XRD X-ray Diffraction

vii List of Tables

List of Tables

Table 3.1 Chemicals used for synthesis of sulfonic acid catalysts.

Table 3.2 Apparatus.

Table 4.1 Texture parameters of samples before and after sulfonation.

Table 4.2 Compositions of samples according to elemental analysis.

Table 4.3 Surface compositions of samples according to XPS analysis.

Table 5.1 Textural parameters of mesoporous resins and carbons.

Table 5.2 The composition of samples analyzed using elemental analysis.

Table 6.1 Elemental compositions of samples analyzed using the CHN-S technique.

Table 6.2 Surface compositions according to XPS analysis and surface areas of samples.

Table 7.1 The acidity titration results.

Table 7.2 Composition of samples analyzed using elemental analysis.

Table 7.3 Decomposition temperatures of samples.

Table 7.4 The preparation parameters and diameter of polymer spheres.

Table 8.1 Initial reaction rate by using catalyst with/without swelling in methanol.

Table 8.2 Initial reaction rate for the determination of apparent reaction orders of acetic acid and methanol in sulfonated cross-linked polystyrene catalyzed esterification at 55ºC.

Table 8.3 Initial reaction rate by using catalyst with/without pre-adsorption in acetic acid.

Table 8.4 Pyridine adsorption experiment.

Table 9.1 Acid density and conversion of acetic acid for prepared catalysts.

viii List of Figures

List of Figures

Figure 2.1 Postoxidative synthesis method for the preparation of silica based sulfonic acid catalysts (Melero et al., 2006).

Figure 2.2 Covalent attachment of alkylsulfonic acid groups to the surface of MCM and HMS molecular sieves, via grafting methods as well as via direct synthesis (Van Rhijn et al., 1998 b).

Figure 2.3 Synthesis of Bisphemol A (Das et al., 2001).

Figure 2.4 In-situ oxidation synthesis strategy for the preparation of organosulfonic- modified mesostructured materials (Melero et al., 2006).

Figure 2.5 Unreacted thiol groups or partially oxidized disulfide species (Perez- Pariente et al., 2003).

Figure 2.6 Prins condensation of styrene with formaldehyde (Reddy et al., 2007).

Figure 2.7 Etherification (Parambadath et al., 2004).

Figure 2.8 Silylation (Parambadath et al., 2004).

Figure 2.9 Sulfonation (Parambadath et al., 2004).

Figure 2.10 Mesoporous silica-perfluorosulfonic-acid materials by co-condensation technique (Macquarrie et al., 2005).

Figure 2.11 Perfluoroalkylsulfonic modified mesoporous materials prepared by the grafting method (Alvaro et al., 2004).

Figure 2.12 Deprotection reaction of benzaldehyde dimethylacetal (Christopher S. Gill, 2007).

Figure 2.13 Preparation of supported perfluoroalkylsulfonic acid (Christopher S. Gill, 2007).

Figure 2.14 Preparation of supported perfluorosulfonic acid (Christopher S. Gill, 2007).

Figure 2.15 (A) Structural model of periodic pore surface attached with propylsulfuric acid groups. (B) TEM image of surfactant-free Ph-MP40 (Qihua Yang, 2002).

Figure 2.16 Schematic illustration of synthetic pathways for organosulfonic-modified periodic mesoporous organosilica (Nakajima et al., 2005).

Figure 2.17 Schematic representation of the two-step synthesis mechanism of PMO acid catalysts (Dube et al., 2008).

ix List of Figures

Figure 2.18 Preparation of perfluoroalkylsulfonic functionalized PMO by the grafting technique (Shen et al., 2008).

Figure 2.19 Pyrolysis of the sugars causes their incomplete carbonization and formation into polycyclic aromatic carbon sheets; sulfuric acid (concentrated or fuming) is used to sulfonate the aromatic rings to produce the catalyst (Toda et al., 2005).

Figure 2.20 Proposed structures of carbonized D-glucose after sulfonation: (A) carbon prepared at 573-723 K; (B) carbon prepared at 873 K. investigated (Okamura et al., 2006).

Figure 2.21 Functionalization of CMK-5 with Sulfonic Acid Groups (Wang et al., 2007).

Figure 4.1 Nitrogen adsorption-desorption isotherms of mesoporous carbons before sulfonation(A), BJH-PSD curves of mesoporous carbons before sulfonation(B), nitrogen adsorption-desorption isotherms of mesoporous carbons after sulfonation(C), BJH-PSD curves of mesoporous carbons after sulfonation(D), nitrogen adsorption-desorption isotherms of sulfonated carbon-silicate composites catalysts (E), BJH-PSD curves of sulfonated carbon-silicate composites catalysts (F).

Figure 4.2 (a) Schematic diagram showing the porous structure of SBA-15 (b) formation of carbon layer inside the SBA-15 channels (c) introduction of –SO3H groups onto carbon layer.

Figure 4.3 The illustration of sucrose infiltrated into template SBA-15 (a)sucrose was not infiltrated into the template pore at all (b-d) the carbon layer increase with the increase of infiltrated sucrose (e) sucrose was fully infiltrated into the pore of the template.

Figure 4.4 XRD patterns of instrument background (a), SMC400(80) (b), SMC500(80) (c), SMC700(80) (d), and SMC900(80) (e).

Figure 4.5 S 2p XPS spectra for SMC400(80) (a), SMC500(80) (b), SMC700(80) (c), SMC900(80) (d), SCS400(80) (e), and SCS400(120) (f).

Figure 4.6 TEM images of SBA-15 silica (a,b), SMC400(80) (c), SMC500(80) (d), SMC700(80) (e) and SMC900(80) (f).

Figure 4.7 TEM images of SCS400(22) (a), SCS400(50) (b), SCS400(80) (c) and SCS400(120) (d).

Figure 4.8 FESEM images of SMC400(80) (a), SMC500(80) (b), SMC700(80) (c) and SMC900(80) (d), SCS400(22) (e), SCS400(50) (f), SCS400(80) (g) and SCS400(120) (h).

Figure 4.9 Conversion of acetic acid over resultant sulfonic acid catalysts.

Figure 5.1 N2 adsorption-desorption isotherms and pore size distribution of sample MC(350).

x List of Figures

Figure 5.2 TG-DTG curves of MP in nitrogen.

Figure 5.3 (A) XRD patterns for (a) MC(500), (b) MC(600) and (c) MC(800); (B)HR-TEM image for MC(800).

Figure 5.4 S2p XPS spectra for (a) MC(350,40), (b) MC(350,100), (c) MC(400,40), and (d) MC(800,40).

Figure 5.5 FT-IR spectra of (a) MC(350), (b) MC(350,100), (c) MC(350,100) after the 4th reaction run, and (d) MC(350,40).

Figure 5.7 FT-IR spectra of (a)MC(800), and (b) MC(800,40)

Figure 5.8 The formation of –SO3H group on (a) polymer framework, (b) carbon framework with small carbon sheets, and (c) carbon framework with big carbon sheets.

Figure 5.9 FESEM images of (a) MC(350), (b) MC(350,100), (c)MC(800,40); HRTEM image of (d) MC(350,100).

Figure 5.10 Catalytic conversion of acetic acid over resultant sulfonic acid catalyst.

Figure 5.11 UV Absorption spectra for resultant catalysts after reaction run.

Figure 6.1 FESEM images of (a) PNs, (b) SPNs(40), (c) SPNs(150), (d) SCPNs(400,40), (e) SCPNs(400,150), (f) SCPNs (900,150), (g) CPNs(400) and (h) CPNs(900).

Figure 6.2 HRTEM images for (a) SCPNs(900, 150), and (b)SCPNs(400,150).

Figure 6.3 XRD patterns of (a) PNs, (b) SPNs(40), (c) SPNs(150), (d) SCPNs(400,40), (e) SCPNs(400,150), and (f) SCPNs(900, 150).

Figure 6.4 (A) N1s XPS spectra (a) PNs, (b) CPNs(400), (c) CPNs(900), and (d) SCPNs(900,150) (B) S2p XPS spectra (a) SPNs(40) and (b) SPNs(40) after the 4th reaction run.

Figure 6.5 FT-IR spectra of (a) PNs, (b) SPNs(40), (c) SPNs 40 after the 4th reaction run, (d) CPNs(400), (e) SCPNs(400,40), and (f) SCPNs(400,40) after the 4th reaction run.

Figure 6.6 FT-IR spectra of (A) SPNs(40), (B) SPNs(40) after the 4th reaction run, (C) SPNs(400,40), and (D) SPNs(400,40) after the 4th reaction run.

Figure 6.7 The formation of –SO3H groups on (a) a pentagonal pyrrole ring, (b) a small carbon sheet and (c) a big carbon sheet.

Figure 6.8 Catalytic conversion of acetic acid over various catalysts.

Figure 6.9 UV-VIS spectra: (A) mixture of pyrrole monomer, methanol, acetic acid and methylacetate; (B) filtrated reaction mixture of SPN(40) after the 1st reaction run; (C) filtrated reaction mixture of SPNs(40) after the 2nd

xi List of Figures

reaction run; (D) filtrated reaction mixture of SPN(40) after the 3rd reaction run; (E) filtrated reaction mixture of SPN(40) after the 4th reaction run; (F) filtrated reaction mixture of CPNs(900,150) after the 1st reaction run.

Figure 7.1 (A) 13C MAS NMR spectra for (a) PS(0.8), (b) SPS(0.8,80), (c) PS(0), and (d) SPS(0,40) (* spinning side bands) (B) Sulfonated line-linked polystyrene (C) Sulfonated cross-linked polystyrene-divinylbenzene.

Figure 7.2 FT-IR spectra of (a)SPS(0.8,40)(b) SPS(0.8,60) (c) SPS(0.8,80) (d) SPS(0.8,100) (e) SPS(0.4,40) (f) SPS(1.2,40) (g) SPS(2.4,40) (h) SPS(0,40) (i) SPS(4.8,40).

Figure 7.3 TEM images of (a) SPS(0.8,40),(b)SPS(2.4,40), (c)SPS(4.8,40),(d) SPS(4.8,40).

Figure 7.4 (A)TG curves of (a)SPS(0,40)(b)SPS(0.4,40) (c) SPS(0.8,40) (d) SPS(0.8,60) (e) SPS(0.8,80) (f) SPS(0.8,100) (g) SPS(1.2,40) (h) SPS(2.4,40) (i) SPS(4.8,40);(B)TG curves of (a)PS(0) (b) PS(0.4) (c) PS(0.8) (d)PS(1.2) (e) PS(2.4) (f) PS(4.8).

Figure 7.5 FESEM images of (a) PS(0)(b) PS(0.4) (c) PS(0.8) (d) PS(1.2) (e) PS(2.4) (f) PS(4.8).

Figure 7.6 Catalytic conversion of acetic acid over resultant sulfonted polymer catalysts.

Figure 8.1 Mechanistic route of acid catalyzed esterification reaction.

Figure 8.2 Initial reaction rate under different reaction stirring speed.

Figure 8.3 Esterification reaction for methanol with acetic acid.

Figue 8.4 Pyridine adsorbed sulfonated cross-linked polystyrene-divinylbenzene spheres catalyzed esterification of acetic acid with methanol at 55ºC.

Figure 8.5 Acetic acid conversion vs time for esterification reaction catalysed by H2SO4 and sulfonated cross-linked polystyrene-divinylbenzene spheres at 55ºC.

Figure 8.6 Water sensitivity of esterification reaction for methanol with acetic acid at 55ºC on (■) sulfonated cross-linked polystyrene-divinylbenzene spheres compared to that on (●) H2SO4.

Figure 8.7 Comparison of experimental data with predicted data derived from the

mathematical model by plot of 1/r0 vs 1/CM0.

Figure 8.8 Comparison of experimental data with predicted data derived from the mathematical model by plot of 1/r0 vs 1/CA0.

Figure 8.9 Dependency of initial reaction rate on the initial water concentration for sulfonate cross-linked polystyrene-divinylbenzene spheres.

xii Chapter 1. Introduction

CHAPTER 1

INTRODUCTION

1.1 Solid sulfonic acid catalysts

Acid catalysts play an important role in many chemical reactions such as Friedel-

Crafts, hydration, esterification, and hydrolysis reactions. Many of these reactions are still carried out by using conventional liquid acid catalysts like H2SO4. Such liquid catalysts create many inevitable problems, such as high toxicity, corrosion, generation of solid wastes, and difficulty in separation and recovery. In comparison, solid acid catalysts have a number of advantages over the liquid ones, such as less corrosion, no or less waste, and easy separation and recovery from the reaction medium. As a result, there has been a great deal of research interest in searching for environmentally friendly solid acid catalysts to replace environmentally unfriendly liquid acid catalysts

(Clark, J. H. and D. J. Macquarrie).

Over the past decade, various solids with sulfonic acid groups (-SO3H) have been reported (Lim et al., 1998; Margolese et al., 2000) since the pioneering work of Van

Rhijn et al.(1998 b), which reported the sulfonation of porous silica materials. -SO3H groups can be introduced on porous silica through two main approaches. One is the post-oxidation method (Lim et al., 1998; Van Rhijn et al., 1998a; Margolese et al.,

2000; Diaz et al., 2001a; Diaz et al., 2001b). In post-oxidation method the supported thiol groups, which were introduced through grafting or co-condensation method, were oxidated by postsynthetical technique. However, the porous structure can not be maintained well after the postoxidation (Margolese et al., 2000). To conquer this drawback, another method named in-situ oxidation method was subsequently developed (Margolese et al., 2000). In in-situ oxidation method the silica precursor,

1 Chapter 1. Introduction organosulfonic precursor, and oxidant were added together into the synthesis process.

And the oxidation of thiol groups concurred with the silica preparation. Sulfonic acid functionalized porous silicas with uniform pores, high surface area and good stability have been found to exhibit excellent catalytic activities in many reactions, such as esterification (Bossaert et al., 1999; Diaz et al., 2001 b), condensation and addition reactions (Das et al., 2001; Shimizu et al., 2005), and alcohol coupling to ethers (Shen et al., 2002).

To tune the acidic strength of sulfonic acid solids, arenesulfonic acid groups were introduced on mesoporous silica materials (Melero et al., 2002; Melero et al., 2004; van Grieken et al., 2005; Wang et al., 2005). The presence of electron-withdrawing species close to the sulfonic group has been found to enhance the acid strength of the acid sites (Harmer et al., 1996; Ledneczki et al., 2005; Jason C. Hicks, 2007).

Organosulfonic-modified periodic mesoporous organosilicas (PMO) have been shown to display a great catalytic performance. Organosulfonic-modified PMO catalyst was first used in the alkylation of phenol with 2-propanol (Yuan et al., 2003).

Subsequently, PMO catalysts were tested in many kinds of chemical reactions, such as condensation (Yang et al., 2004), esterification (Yang et al., 2005), and Friedel-Crafts reaction (Rac et al., 2006).

Carbon-based materials have always attracted much attention in heterogeneous catalysis due to their virtues such as easy modification, high surface area and pore volume, and low cost. By introducing -SO3H groups on carbon, Hara and coworkers

(2004) doscovered a carbon-based solid sulfonic acid catalyst, which displayed a very high catalytic activity (Edward T. Lu, 2005; Okamura et al., 2006). However, these carbon materials possess a low surface area, which is not favorable for some catalytic reactions.

2 Chapter 1. Introduction

1.2 Objectives of thesis work

Organic esters play an important role in the manufacturing of many important chemical products. For an example, methyl acetate is as a volatile low toxicity solvent in glues, paints, and nail polish removers. The most-used method for esters synthesis is direct esterification of carboxylic acids with alcohols in the presence of liquid mineral acid (such as H2SO4). In order to develop solid sulfonic acid catalysts with high catalytic performace, which should be potential replacement candidates for sulfuric acid, several kinds of sulfonic acid catalysts were synthesized in this thesis work. And the relationship between material structure and their catalytic performance was investigated in details as well.

• The hard template method was used to prepare mesoporous carbons with

high surface area and porous structure. Sulfonic acid groups (-SO3H) were

then introduced on the carbon surface by using sulfonation reaction.

Incompletely carbonized carbon-silica composites were prepared. The

composites facilitated the introduction of sulfonic acid groups due to the

presence of small carbon sheets.

• Porous structure can also be introduced into polymer materials by soft

template method. In the preparation of mesoporous phenol resin P123 was

adopted as the template. Mesoporous carbons were obtained through the

carbonization of mesoporous polymer resin. Sulfonic acid catalysts based

on both mesoporous resin and carbon were prepared. The conversion of

acetic acid and recyclability for resultant catalysts were affected by the

catalyst structure, which was formed under different sulfonation and

carbonization temperature.

3 Chapter 1. Introduction

• Polypyrrole nanospheres were prepared and sulfonated to produce polymer

based sulfonic acid catalysts. Through the carbonization process

polypyrrole nanospheres transferred to carbon nanospheres, which were

also sulfonated to prepare carbon based sulfonic acid catalysts. The

conversion of acetic acid and recyclability for sulfonated polypyrrole and

carbon nanospheres were investigated in details to find out the relationship

between catalyst structure and performance.

• Sulfonated polystyrene nanospheres show high acid density. Different kinds

of cross-linked polystyrene-divinylbenzene spheres were prepared. The

amount of added divinylbenzene and sulfonation temperature were tested in

details, which were important factors affected the conversion of acetic acid

and recyclability

• The initial kinetic study of esterification reaction over methanol with acetic

acid catalyzed by sulfonated cross-linked polystyrene-divinylbenzene

spheres was carried out. The apparent reaction order and reaction

mechanism were studied. Furthermore, the kinetic modeling was carried out

by the curve fitting technique. The inhabitation behavior of water for

esterification reaction was also investigated.

1.3 Structure of thesis

The thesis is organized into nine chapters. With a brief introduction and a summary of the objectives of this project in Chapter 1, a detailed literature review on the preparations and applications of various sulfonated solids are discussed in Chapter 2.

The detailed experimental methods and chemicals used are presented in Chapter 3. In

Chapter 4, the preparation, characterization, and catalytic properties of sulfonated

4 Chapter 1. Introduction mesoporous carbon and carbon-silica composites are discussed. The effect of carbonization temperature on the physical, chemical and catalytic properties of the resultant solids is presented. Chapter 5 discusses sulfonated mesoporous resins and carbons, with an emphasis on the influence of sulfonation and carbonization effects on the materials structural and catalytic properties. Chapter 6 describes the esterification reaction over sulfonated polypyrrole and carbon nanospheres. In Chapter 7, the catalytic performance of sulfonated linear-linked and cross-linked polypyrrole spheres is presented. Factors affected acidity and thermal stability of the sulfonated cross- linked polystyrene spheres are discussed. The kinetics and mechanism of esterification of methanol with acetic acid catalyzed by sulfonated cross-linked polystyrene spheres are described in Chapter 8. The main conclusions drawn from the present work and suggestions for future work are presented in Chapter 9.

5 Chapter 2. Literature Review

CHAPTER 2

LITERATURE REVIEW

2.1 Propylsulfonic-modified mesoporous silica

Since mesoporous MCM-41 was first synthesized by Mobil (Kresge et al., 1992), application research on mesoporous silica was tremendously expanded due to it’s good property such as uniform pore sizes, high void volumes and surface areas. Moreover, this physical ability can be tailed by changing synthesis methods. These flexible characters make mesoporous silca as a potential defined catalysts support.

Organosulfonic-modified silica was first reported by Badley and co-workers (Badley and Ford, 1989). Followed by this pioneering work many contributions have been made into this sulfonic-acid-functionalized mesoporous silica catalysts field.

The key precursor in the preparation of propylsulfonic-modified silica is 3- mercaptopropyltrimethoxysilane (MPTMS). This molecule contains an -SH group, a stable propyl spacer and a hydrolisable Si(OMe)3 moiety. MPTMS containing thiol groups were introduced to silica material basically through two main methods. First is grafting methods, including silylation and coating. Second is co-condensation reaction

(direct synthesis) (Dias et al., 2005; Melero et al., 2006). The introduced thiol groups could be oxidized into sulfonic acid groups by postoxidation or in-situ oxidation method.

2.1.1 Postoxidation method

The introduced thiol groups could be oxidized into sulfonic acid groups by using large excess of oxidant (such as hydrogen peroxide, nitric acid). The postoxidative synthesis process is shown in Figure 2.1 (Melero et al., 2006).

7 Chapter 2. Literature Review

Figure 2.1 Postoxidative synthesis method for the preparation of silica based sulfonic acid catalysts (Melero et al., 2006).

Works dealing with the preparation of organosulfonic-modified silica date from

1998, which are based on the covalent attachment of alkylsulfonic acid groups to the surface of MCM and HMS type materials. Pierre A. Jacobs and co-workers first functionalized calcined MCM and HMS samples with propane-thiol groups by reaction of the surface silanols with 3-mercaptopropyltrimethoxysilane (MPTMS) (Figure 2.2)

(Van Rhijn et al., 1998 b). Both grafting and direct reaction methods were adopted in this work. First is the grafting method. Modification comprised the silylation of a vacuum-dried pre-existing MCM support with MPTMS in dry toluene, or the coating of a partially hydrated support with an MPTMS layer (Figure 2.2 routes 2a and b). In grafting processes the surface concentration of organic groups is constrained by the number of reactive surface silanol groups present and by diffusion limitations. These restrictions may be overcome by direct synthesis (Van Rhijn et al., 1998 b). Therefore,

8 Chapter 2. Literature Review

secondary they employed MPTMS and TEOS (Si(OEt)4), which were hydrolyzed together in the presence of an ionic or a non-ionic surfactant (viz. C16NMe3Br and n-

C12-amine), leading to MCM or HMS type materials, respectively.

Figure 2.2 Covalent attachement of alkylsulfonic acid groups to the surface of MCM and HMS molecular sieves, via grafting methods as well as via direct synthesis (Van Rhijn et al., 1998 b).

Furthermore, they enhanced the incorporation of sulfur moieties using a modified grafting procedure (Van Rhijn et al., 1998 a). The surface of mesostructured MCM materials was coated with a cross-linked monolayer of mercaptopropyl-Si groups under well-controlled wet conditions, obtaining an incorporation of up to 4.5 mmol of

S per gram of material in optimal conditions.

Following these pioneering works, the postoxidative synthesis strategy has been expanded to the use of other different surfactants and synthesis conditions. Joaquín

Pérez-Pariente described the synthesis of organosulfonic-modified MCM-41 materials by means of postoxidation of thiolmodified materials, which were synthesized using a mixture of cationic surfactants (cetyltrimethylammonium bromide, C16TAB and dodecyltrimethylammonium bromide, C12TAB) and tetramethoxysilane (TMOS) as the

9 Chapter 2. Literature Review silica source in basic conditions with tetramethylammonium hydroxide (TMAOH) instead of NaOH (Diaz et al., 2001 b). The authors postulated that this novel method allowed the synthesis of highly ordered mesostructured materials in comparison with that using just C16TAB as surfactant, which was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Following the same hypothesis, the cationic C12TAB surfactant was substituted by a neutral surfactant such as n-dodecylamine (Diaz et al., 2001 a). Stucky and co-workers created a thiol- containing mesoporous silica material by the co-condensation of TEOS, MPTMS and employing a triblock copolymer (poly(ethyleneoxide)-poly-(propyleneoxide)- poly(ethyleneoxide), Pluronic 123, EO20-PO70EO20) as template under acidic conditions (Margolese et al., 2000). The resultant materials show acid exchange

+ capacities ranging from 1 to 2 mequiv of H /g of SiO2 and excellent thermal and hydrothermal stabilities.

Organosulfonic-modified porous silica materials prepared in postoxidation method have yielded XRD patterns with lower scattering intensities that indicate relatively poor long-range ordering in comparison to the starting material containing the thiol groups (Lim et al., 1998; Margolese et al., 2000), following a decrease in the surface area and pore volume after oxidation of thiol groups incorporated and reduces the potential application of these catalysts (Van Rhijn et al., 1998 b). The postoxidation method not only needs a large excess of oxidant used in the process but also does not allow quantitative reaction of thiol groups, and in some cases, leaching of sulfur species is clearly evidenced. The presence of un-oxidized sulfur species might have a negative effect on the catalytic performance of these materials.

A new type of sulfonic acid-functionalized monodispersed mesoporous silica spheres (MMSS) were synthesized directly by co-condensation and postoxidation

10 Chapter 2. Literature Review methods (Suzuki et al., 2008). By changing the methanol ratio, organosulfonic- modified MMSS with different particle diameters (390-830nm) and the same mesopore sizes were successfully synthesized. TEM observations revealed that the mesopores were aligned radially from the center towards the outside of the spheres, even in the sulfonic acid-functionalized MMSS. In addition, the catalytic activity of

MMSS in condensation reactions between 2-methylfuran and acetone was much higher than that of other forms of mesoporous silica due to its radially-aligned mesopores.

The catalytic applications of propylsulfonic-modified silica prepared by postoxidation method have been investigated in many fields. Monoglycerides are valuable chemical products with wide application as emulsifiers in food, pharmaceutical, and cosmetic industries. Jacobs et al. reported the synthesis of monolaurin via direct esterification of glycerol with lauric acid over propylsulfonic- acid modified MCM-41 materials (by grafting, coating, and co-condensation strategies), which were prepared by postoxidation method (Bossaert et al., 1999). The resultant propylsulfonic-modified catalysts were far more active than traditional zeolite and commercial sulfonic-acid resins (Amberlyst-15). Moreover, the resultant catalyst was reused, and both conversion and selectivity to monoglyceride remained stable compared with those of the fresh catalyst. Various polyols (1,2-propanediol, 1,3- propanediol, meso-erythritol) and acids (lauric and oleic acid) were tested in their experiment.

Diaz reported the synthesis of propylsulfonic-modified MCM-41 by mixture of surfactant (C16TAB, C12TAB) (Diaz et al., 2001 b). The catalytic performance of catalysts was tested in the esterification of glycerol with fatty acids-oleic and lauric.

The catalysts prepared with mixtures of surfactants are more selective to the monoglycerides than the ones synthesized only with one surfactant (C16TAB) due to

11 Chapter 2. Literature Review the higher order in the channels packing. Moreover, propylsulfonic-modified MCM-41 materials, which were synthesized using mixtures of cationic and neutral surfactants, exhibited an acid conversion of 90% with selectivity to the monoester of 75% after 24 h of the esterification of glycerol with lauric acid reaction (Diaz et al., 2001 a). The selectivity was greatly enhanced compared with propylsulfonic-modified MCM-41 synthesized in the absence of amine. These above works clearly showed that a mixture of surfactants provides propylsulfonic functionialized MCM-41 catalysts with clear improved catalytic properties for the esterification reaction in a comparison with the conventional single-surfactant synthesis process.

Works about the use of propylsulfonic-modified silica materials in condensation and addition reactions have been reported. Bisphenol-A is a very important raw material for the production of epoxy resins and other polymers industrially manufactured through condensation reaction of phenol and acetone using ion-exchange resins such as

Amberlyst (Figure 2.3). However, thermal stability and fouling of the resins are major problems for these catalysts.

Figure 2.3 Synthesis of Bisphemol A (Das et al., 2001).

Debasish Das reported propylsulfonic-modified MCM-41 silica can be an efficient catalyst for the condensation of phenol and acetone at relatively low temperature to synthesize Bisphenol-A with a very high selectivity (Das et al., 2001). A detailed study

12 Chapter 2. Literature Review about propylsulfonic-modified MCM-41 and MCM-48 in the synthesis of p,p’

Bisphenol-A was reported by the same research group (Das et al., 2004). Higher amounts of thiol groups can be incorporated in MCM-48 silicas presumably due to the presence of larger number of surface silanol groups. Propylsulfonic-modified MCM-41 has comparable catalytic activity to that of commercial ion-exchange resin Amberlite-

120, moreover, higher selectivity toward the desired p,p’ isomer. Propylsulfonic- modified MCM-48 are equally effective for selective synthesis of Bisphenol-A. They conclude that at low sulfur loadings oxidation of the thiol precursor to the sulfonic acid active center was almost complete. However, at higher sulfur loadings oxidation was incomplete, sulfide and disulfides C3–S–S–C3 were present along with sulfonic acid groups, which are catalytically inactive lower valent sulfur species.

2.1.2 In-situ oxidation method

Stucky and co-workers used the in-situ oxidation method to create periodic ordered propylsulfonic-modified mesoporous silica with pore sizes up to 70 Å through co- condensation of TEOS and MPTMS, which employed Pluronic 123 (EO20/PO70/EO20) as the templating surfactant in acid medium (Margolese et al., 2000). The in-situ oxidation method profoundly influences the physical and chemical properties of the propylsulfonic-modified mesoporous material relative to that made by postoxidation technique (Figure 2.4). The in-situ oxidation method produces SBA-15 modified materials with greater oxidation efficiency (100% vs 25-77%), with larger more uniform pores, with higher surface areas, and with good long-range order in contrast to postoxidative method. The resultant sulfonic mesoporous silica with acid capacities several times greater than those achieved with postoxidative method and with thermal stabilities to 450 ºC in air.

13 Chapter 2. Literature Review

Figure 2.4 In-situ oxidation synthesis strategy for the preparation of organosulfonic- modified mesostructured materials (Melero et al., 2006).

In postoxidative method unreacted thiol groups or partially oxidized disulfide species can also be found (i.e., organic moieties containing sulfur to sulfur bonds, -R-

S-S-R-) (Diaz et al., 2000; Perez-Pariente et al., 2003). The presence of these disulfide species (Figure 2.5) has been attributed to a nonrandom distribution of S-containing groups during condensation, which seem to cluster upon the surface and are not removed after the subsequent oxidation step.

Figure 2.5 Unreacted thiol groups and partially oxidized disulfide species (Perez- Pariente et al., 2003).

14 Chapter 2. Literature Review

Van Grieken et al. employed nonionic surfactants other than Pluronic 123

(EO20/PO70/EO20), through the in-situ oxidation procedure to prepare propylsulfonic- modified hexagonally mesostructured materials (van Grieken et al., 2002). They tailored the pore size of these sulfonic mesoporous materials from 30Å to 110Å conveniently modifying the synthesis conditions using Pluronic 123 as template and acid conditions.

The catalytic applications for propylsulfonic-modified silica by in-suit oxidation method were explored a lot. Shanks et al. investigated the catalytic performance of propylsulfonic-modified mesoporous silica materials in the esterification of palmitic acid with methanol in oil to produce methyl esters (Mbaraka et al., 2003), which exhibit higher reactivity than commercially available solid acid esterification catalysts.

Tailoring the textural properties of the catalyst structure and tuning the acidity of the active site can enhance the performance of the mesoporous materials. Propylsulfonic- modified SBA-15 afforded higher removal of palmitic acid than sulfonic resins, although less than with the homogeneous H2SO4 catalyst.

Popylsulfonic-modified SBA-15 synthesized through the in-situ oxidation demonstrated significant activity toward biodiesel production under relatively mild conditions with refined and crude vegetable oils as feedstock (Melero et al., 2009). The large surface area and pore diameter of the mesoporous support as well as the moderate acid strength of acid sites are helpful for the remarkable catalytic performance, which are necessary for improve internal diffusion of bulky oil species and to minimize possible deactivation of catalytic sites by strong adsorption of polar byproducts such as water and glycerol.

Propylsulfonic-modified FSM-16 mesoporous silica was investigated in the acetalization of carbonyl compounds with ethylene glycol, which showed a higher rate

15 Chapter 2. Literature Review and 1,3-dioxolane yield than conventional heterogeneous solid acids such as zeolites, montmorillonite K10 clay, silica-alumina, and the sulfonic resin (Amberlyst-15)

(Shimizu et al., 2005). Propylsulfonic-modified FSM exhibits stable recycle catalytic activity with no leaching of sulfonic acid groups. The rate per acid site for sulfonic acid modified FSM catalyst was 10 fold higher than that of the sulfonic-acid resin and large-pore zeolites and 2 orders of magnitude higher than medium-pore size acid zeolites. The high activity of sulfonic acid modified FSM is due to the presence of the strong Brönsted acid sites in the mesopore with a relatively low hydrophilicity, determined by an NH3 adsorption microcalorimetric experiment at 150 ºC, where both reactants can smoothly access the acid sites.

Mesoporous SBA-15 silica functionalized with propylsulfonic acid groups by post oxidation method shows highly active and selective for the condensation of styrene with formaldehyde, furthermore, both the conversion of styrene and the selectivity to

4-phenyl-1,3-dioxane are nearly 100% (Figure 2.6) (Reddy et al., 2007). The resultant mesoporous materials exhibited hexagonal mesoscopic ordering and XRD results indicate that there is no change in the structure after anchoring of -SO3H group.

Sulfonic acid groups anchored to SBA-15 silica pore surfaces are thermally stable, which was tested in boiling water, organic and aqueous solvents under mild conditions.

Moreover no decrease of conversion and selectivity to 4-phenyl-1,3-dioxane was fount in four repeated cycles.

Figure 2.6 Prins condensation of styrene with formaldehyde (Reddy et al., 2007).

16 Chapter 2. Literature Review

Klier et al. employed propylsulfonic-modified SBA-15 materials and used them in the synthesis of unsymmetrical ethers, which possess hexagonal mesostructure with

+ about 74 Å pore size, acid exchange capacities of 1.6 mequiv of H /g of SiO2, surface area of 674 m2/g, and excellent thermal stabilities (Shen et al., 2002). The reaction products after reaction were methyl isobutyl ether (MIBE), secondary ethers such as dimethyl ether (DME), methyl tert-butyl ether (MTBE), and butenes. The resultant propylsulfonic-modified silica exhibited high ether selectivity and higher catalytic activity for ether formation than other inorganic solid acid catalysts.

The same research group further investigated the catalytic performance of propylsulfonic-modified SBA-15 materials in this condensation reaction, demonstrating the mechanistic pathway and reaction intermediates involved in the condensation/dehydration of mixtures of alcohols to form ethers and olefins (Herman et al., 2004). They draw a conclusion that low temperatures and elevated pressures are benefit for enhancement of ether formation. Low reaction pressures help increase the adsorption of isobutanol on the acid sites, which yields predominantly isobutene by dehydration. Following the increase of pressure, adsorption of methanol and production of ethers are enhanced at the expense of isobutene formation.

A novel approach for the synthesis of sulfonic acid functionalized mesoporous silicas via covalent attachment of 2-(3,4 epoxycyclohexyl)ethyltrimethoxysilane or 3- glycidoxypropyltrimethoxysilane followed by reaction with sulfite ions and mild hydrochloric acid is presented by Kapoor et al. (Kapoor et al., 2008). The materials demonstrated outstanding stability and easily recyclable in esterification reaction of acetic acid with benzyl alcohol. These materials exhibit hydrophobic nature along with the crystalline pore wall which are advantageous in increasing reaction rates by

17 Chapter 2. Literature Review altering local water concentration and helpful in adjusting the modest active sites and surface properties.

2.2 Arenesulfonic-acid modified material

The presence of electron-withdrawing species close to the sulfonic group is expected to increase the acid strength of the acid sites compared to other low-electron- withdrawing environments such as methylene groups. Mesostructured materials functionalized with arenesulfonic groups have been reported. Mesoporous zirconium hydroxide, Zr-TMS (zirconium hydroxide with mesostructured framework; TMS, transition metal oxide mesoporous molecular sieves) catalyst has been prepared through the sol–gel method and functionalized with benzyl sulfonic acid (BSA) using post-synthesis route without destroying the mesoporous structure (Parambadath et al.,

2004). The benzyl group anchored Zr-TMS (B-Zr-TMS/_Zr–O–CH2–F) was achieved by etherification reaction of Zr-TMS with benzyl alcohol at 80ºC using cyclohexane as solvent (Figure 2.7, 2.8). Further, B-Zr-TMS was subjected to sulfonation reaction with chlorosulfonic acid (ClSO3H) at 70ºC using chloroform as solvent to yield BSA-

Zr-TMS (_Zr–O–CH2–F–SO3H) (Figure 2.9). Functionalization was carried out by loading the maximum amount of benzyl group over Zr-TMS and varying the concentration of –SO3H. The catalytic activity of the synthesized catalyst was investigated in liquid phase benzoylation of diphenyl ether (DPE) to 4- phenoxybenzophenone (4-PBP) using benzoyl chloride (BC) as benzoylating agent at

160ºC under atmospheric pressure. Sulfonic-modified Zr-TMS catalyst showed DPE conversion of 60% along with 100% selectivity toward 4-PBP after 0.5 h of reaction time starting from an equimolar mixture of DPE and BC. At identical reaction conditions, amorphous sulfonated zirconia yielded only 5% of DPE conversion. The

18 Chapter 2. Literature Review higher activity of the synthesized materials may be attributed to its higher acidity and mesoporous characteristics.

Figure 2.7 Etherification (Parambadath et al., 2004).

Figure 2.8 Silylation (Parambadath et al., 2004).

Figure 2.9 Sulfonation (Parambadath et al., 2004).

19 Chapter 2. Literature Review

Similar with the functionalization of propylsulfonic groups on mesoporous silica materials, Melero and co-workers prepared SBA-15 mesoporous silica functionalized with arenesulfonic acid groups by means of a one-step simple synthesis approach involving co-condensation of tetraethoxysilane (TEOS) and 2-(4- chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS) in the presence of a Pluronic

123 (EO20/PO70/EO20) under acid silica-based catalysis (Melero et al., 2002). The resultant materials show hexagonal mesoscopic order and pores sizes up to 60 Å, with

+ 31 acid exchange capacities of ca. 1.3 mequiv H per SiO2. P MAS NMR measurements of chemically adsorbed triethylphosphine oxide and the catalytic properties confirm the presence of Brönsted acid centers in these mesoporous materials containing arenesulfonic acid groups that are stronger than those found in propylsulfonic- modified SBA-15 and Al-MCM-41.

The introduction of arenesulfonic-acid group gives a chance to tune the strength of acid sites, which may enlarge the potential catalytic applications of this type of material. Shanks et al. reported that the apparent reactivity (defined as the average turnover rate per total number of acid sites) of arenesulfonic-modified SBA-15 materials in the esterification of fatty acids with methanol to produce methyl esters was significantly higher than that shown by Nafion and propylsulfonic-functionalized

SBA-15 and comparable to the homogeneous sulfuric acid (Mbaraka et al., 2003).

Following several examples will illustrate the potential use of the arenesulfonic- modified SBA-15 materials as solid catalyst in the production of fine chemicals which usually demand higher strength of acid sites.

Friedel-Crafts acylation is one of the most important reactions in organic chemistry for synthesizing aromatic ketones, which are important intermediates for the production of fine chemicals. The conventional catalysts are AlCl3, BF3, or HF, which

20 Chapter 2. Literature Review follow serious drawbacks. Melero et al. recently demonstrated the catalytic performance of arenesulfonic-modified SBA-15 materials (Ar-SO3H) for acylation of anisole in the presence of anhydride acetic as acylating agent in solventless conditions

(Melero et al., 2004). Ar-SO3H containing SBA-15 showed higher absolute anisole conversion and specific catalytic activity than that found for conventional acid zeolites and Amberlyst-15. Confinement of the arenesulfonic-acid groups within the mesoporous structure of the SBA-15 material led to enhancement of the activity of the acid sites as compared with the homogeneous catalyst.

The catalytic performance of modified amorphous silica (Shylesh et al., 2004) in the acylation of anisole at 100ºC is much lower than that of arenesulfonic-modified SBA-

15 (Melero et al., 2004), which possesses a significantly lower amount of acid centers.

This comparison confirms the necessary of relatively strong acid centers to carry out the acylation reaction and the benefit of using mesostructured materials as support.

The selective Fries rearrangement of aromatic alcohol esters serves as a valuable synthesis step in the production of industrial pharmaceuticals, dyes, and agrochemicals.

Conventional homogeneous catalysts such as Lewis acids (AlCl3, complexed BF3) or mineral acids (HF or H2SO4) have been widely used in overstoichiometric amounts for this reaction, which involve in many problems such as toxic, corrosive, difficult for recovery. Melero et al. described the catalytic behavior of arenesulfonic-modified

SBA-15 materials in the liquid-phase Fries rearrangement of phenyl acetate using phenol as solvent at 150ºC (Melero et al., 2002). Ar-SO3H SBA-15 catalyst displayed the best performance compared with propylsulfonic-acid SBA-15 and Amberlyst-15.

van Grieken and co-workers also reported the arenesulfonic-modified mesostructured SBA-15 is an active catalyst in the liquid-phase Fries rearrangement of phenyl acetate (van Grieken et al., 2005), which shows high catalytic performance as

21 Chapter 2. Literature Review

compared to H3PW12O40 supported on silica. After 4 h of reaction catalytic activity is dramatically reduced due to catalyst deactivation. However, when dichloromethane is used as solvent over arenesulfonic-modified SBA-15, an enhancement of HAP’s production is clearly evidenced and more important with a slower catalyst deactivation.

The use of dichloromethane for this particular reaction open a new field of research to be further explored with the purpose of obtaining higher yields on HAP’s compounds

(ortho- and para-hydroxyacetophenones, pacetoxyacetophenone). Additionally, the slow decay of activity in presence of dichloromethane may suggest an easy regeneration of the catalyst.

Cheng synthesized arenesulfonic-modified SBA-15, which exhibits great efficiency in catalyzing the Beckmann rearrangement of cyclohexanone oxime to ecaprolactam in the liquid phase (Wang et al., 2005). The oxime conversion and lactam selectivity increase with the amount of arenesulfonic acid groups. They assumed that the oxime rearrangement and hydrolysis were catalyzed on different active sites of –SO3H and

Si–OH, respectively. In comparison with other porous acid catalysts such as SBA-pr-

SO3H, H-ZSM-5, Al-MCM-41 and Al-SBA-15, the mesoporous arenesulfonic- modified SBA-15 showed higher catalytic activity and lactam selectivity due to the strong acid strength.

Dual-functionalized SBA-15 materials with arenesulfonic acid and mercapto groups were synthesized by co-condensation of tetraethylorthosilicate (TEOS) and organosilane precursors in the presence of P123 (EO20PO70EO20) copolymer under acidic condition (Chen et al., 2008). The dual-functionalized mesoporous materials were more efficient than the merely arenesulfonic-modified mesoporous silica or the commercial ion-exchange resin (Amberlyst-15) in catalyzing condensation reaction of phenol and acetone to form p,p’-bisphenol-A. They conclude that the sulfonic acid

22 Chapter 2. Literature Review groups in the mesopores serve as the catalytic active sites, however, the presence of mercapto group markedly increases the reaction rate and p,p’-bisphenol-A selectivity by forming positive charged sulfide intermediates through nucleophilic attack on the protonated acetone and affecting the approach of phenol by steric hindrance. Ordered mesopores is another important matter, which is benefit for the diffusion of the reactants and the accessibility of the acid.

2.3 Perfluorosulfonic-acid modified mesopouous material

Perfluorinated sulfonic-acid resins such as Nafion have been used in a wide range of organic reactions including alkylation, acylation, nitration, etherification, and esterification. Nafion resin is a copolymer of tetrafluoroethene and perfluoro-2-

(fluorosulfonylethoxy)propyl vinyl ether (Harmer et al., 1996). The presence of electron-withdrawing fluorine atoms in the structure significantly increases the acid strength of the terminal sulfonic-acid groups. However, these polymeric catalysts present low surface areas. Supporting Nafion onto mesoporous materials could overcome this limitation. In this way the surface area of the new Nafion in silica composite catalyst is increased and most importantly the accessibility of the acid sites is increased. Therefore, a great of research works have been focused on developing

Nafion-silica composites and tested them in different acid-catalyzed reactions

(Ledneczki et al., 2005; Jason C. Hicks, 2007).

Heidekum et al. (1998) entrapped nano-sized particles of Nafion in a high porous silica matrix, and investigated the catalytic performance over the Fries rearrangement of phenyl acetate, which shows high activity and selectivity compared with conventional zeolites.

23 Chapter 2. Literature Review

A nanocomposite of Nafion resin, in which small (20-60 nm diameter) Nafion resin particles are entrapped by so-gel technique within a porous silica network is tested in the α-Methylstyrene dimerization, benzene propylation and nitration and acylation

(Harmer et al., 1996). BET surface area of this nanocomposite is typically in the range of 150-500 m2/g. The activity per unit weight of Nafion resin has been found to be at least 100 times higher in the composite than the pure polymer. The excellent catalytic performance is attributed to the increased effective surface area and the ease of accessibility to the catalytically active sites in the composite.

Nafion-H SAC-13 solid acid is shown to exhibit outstanding properties as catalyst in formation of tetrahydropyranyl ethers of alcohols and the removal of this protective group, acetalization of carbonyl compounds with ethane-1,2-diol and propane-1,3-diol, and the transformation of aldehydes to 1,1-diacetates (Ledneczki et al., 2005). The catalyst can be reused with practically no loss of activity.

Perfluorosulfonic acid Nafion resin was successfully supported over mesostructured

SBA-15 materials by means of impregnation with different resin contents, which were evaluated in the Friedel-Crafts acylation of anisole (Martinez et al., 2008). The increase of Nafion loadings above 15 wt % does not have obvious enhancement of the catalytic performance. The author concluded that is possibly caused by the loss of the

SBA-15 silica mesoporosity by the dominant deposition of the Nafion resin over the external surface of the silica particles, which inhibits the effective role of the mesostructured silica support. However, a remarkable deactivation was observed by strongly chemisorbed poly-acetylated by-products.

Bringue et al. (2008) also used the impregnation method prepared silica-supported

Nafion catalysts, which were very active and selective for 1-pentanol dehydration. A percentage of about 4% of impregnated Nafion is estimated to be necessary to anchor

24 Chapter 2. Literature Review the polymer on the carrier surface. Acid capacity and 1-pentanol conversion, increased on increasing the initial amount of Nafion.

However, the above Nafion silica composite catalysts still suffer from limited availability of the acid groups due to the imperfect dispersion of the resin within the silica pores. Acid-site accessibility can be enhanced through an alternative grafting procedure. Harmer et al. (1997) described the synthesis of perfluorosulfonic-modified silica via the direct hydrosilylation of CH2=CHCH2(CF2)2O(CF2)2SO2F with

HSi(OEt)3 using a platinum catalyst. The presence of fluorine atoms significantly increases the acid strength of the sulfonic acid group. However, the complexity of the preparation of the organosilane precursor [(OEt)3-Si-(CH2)3-(CF2)2(O)-(CF2)2SO2F] makes this material not very cost effective as compared with the use of MPTMS and

CSPTMS as precursors.

Macquarrie et al. (2005) prepared mesoporous silica-perfluorosulfonic-acid materials by co-condensation of the corresponding silane (precursor of alkylperfluorosulfonic-acid group) and TEOS using ndodecylamine as template

(Figure 2.10). Exhaustive acid extraction of the template using a 0.1 M H2SO4 in 50:50 aqueous ethanol led to the sulfonic acid product. Complete incorporation of silane was obtained, yielding a sulfonic acid loading of 0.2 mmol/g in the final material. The solid was tested in the Friedel-Crafts acylation of anisole with benzoyl chloride in solventless conditions. The catalyst showed a rapid acylation of anisole and provided high selectivity to the para isomer after 24 h of reaction at 100 ºC.

Figure 2.10 Mesoporous silica-perfluorosulfonic-acid materials prepared using the co- condensation technique (Macquarrie et al., 2005).

25 Chapter 2. Literature Review

Alvaro and co-workers (2005) described Nafion-MCM-41 silicas as a promising type of catalyst for acid-catalyzed reactions. The esterification of long-chain fatty acids can be performed satisfactorily at a relatively low temperature with high yields of the desired esters. The perfluoroalkylsulfonic MCM-41 catalyst can be reused after recovery and washing. Furthermore, acylation of anisole can be carried out with good conversion and a very high selectivity for the desired para isomer, although deactivation by diaryl carbocations limits at present the reusability of the system. For both reactions the perfluoroalkylsulfonic MCM-41 catalysts exhibit significantly higher activity than any of the previously reported hybrid inorganic–organic acid catalysts

The same research group synthesized the hybrid organic–inorganic mesoporous materials with terminal perfluorinated sulfonic acid functional groups, which are structurally analogous to Nafion (Alvaro et al., 2004). As shown in Figrue 2.11, the cyclic precursor 1 (1,2,2-trifluoro-2-hydroxy-1-trifluoromethylethane sulfonic acid sultone) reacts with the surface silanol groups of the mesoporous silicas by opening up the sultone ring and forming a covalent bond between the silica framework and the perfluoroalkyl chain with terminal sulfonic acid functional groups. The ratio of 1 to mesoporous silica during the synthesis can be varied over a wide range to get different loadings of perfluoroalkylsulfonic groups. This synthesis strategy allows preparing hybrid organic-inorganic mesoporous silica catalysts functionalized with perfluorosulfonic-acid groups in a single step using a commercial precursor.

26 Chapter 2. Literature Review

Figure 2.11 Perfluoroalkylsulfonic modified mesoporous materials prepared by the grafting method (Alvaro et al., 2004).

Silica-coated magnetic nanoparticle (SiMNP) catalysts exhibited comparable or better activities to other commercially available sulfonic acid catalysts (Amberlyst A-

15 and Nafion) in the deprotection reaction of benzaldehyde dimethylacetal (Figure

2.12) (Christopher S. Gill, 2007). SiMNP catalysts with alkyl-sulfonic acid, phenyl- sulfonic acid, and perfluorosulfonic acid were found to be easily recoverable, recyclable, and surface-bound solid acid catalysts. Catalyst with perfluoroalkylsulfonic acid was observed to act as a source of homogeneous acid under these conditions, and may act as a recyclable, heterogeneous catalyst only under specific, anhydrous conditions. Catalyst with perfluorosulfonic acid generated the highest activity of the magnetic solid acid catalysts, with an activity one half that of triflic acid.

Figure 2.12 Deprotection reaction of benzaldehyde dimethylacetal (Gill, 2007).

Perfluoroalkylsulfonic acid groups are grafted onto silica-coated magnetic nanoparticle (SiMNP), which is shown in Figure 2.13-14 (Gill, 2007). Supporting these acid catalysts on magnetic silica nanoparticles offers a simple recovery method

27 Chapter 2. Literature Review and reuse of these catalysts. The silica coating provides an inert barrier to adverse interactions between surface functionalizations and the metal oxide core.

Figure 2.13 Preparation of supported perfluoroalkylsulfonic acid (Gill, 2007).

Figure 2.14 Preparation of supported perfluorosulfonic acid (Gill, 2007).

2.4 Organosulfonic-modified periodic mesoporous organosilica

Periodic mesoporous organosilicas (PMO) composed by hybrid inorganic-organic frameworks with ordered mesopores were first synthesized in the late 1990s (Inagaki et al., 1999; Melde et al., 1999). The synthesis strategy of PMO is based on the condensation of organosilanes such as (R’O)3-Si-R-Si-(R’O)3 in the presence of the corresponding surfactant, in which the organic moiety (-R-) is covalently attached to two trialkoxysilyl groups (-Si-(R’O)3). PMO exhibited a homogeneous distribution of organic fragments and inorganic oxide within the framework accompanied by highly

28 Chapter 2. Literature Review ordered structures and uniform pore size distributions, which is highly desired in the applications of catalysis. The properties of the framework could be tuned by adopting various kinds of organosilianes containing different organic groups such as ethane

(Melde et al., 1999; Tewodros Asefa, 1999), methane(Tewodros Asefa, 2000), benzene

(Yoshina-Ishii et al., 1999; Inagaki et al., 2002) , thophene(Yoshina-Ishii et al., 1999) and biphenylene(Kapoor et al., 2002). Recently Zhao et al. reported highly ordered ethylene bridged periodic mesoporous organosilica (PMO) materials with ultra-large pore sizes up to 14.7 nm by using triblock copolymer poly(ethylene oxide))106-

(poly(propylene oxide))70-(poly(ethylene oxide))106 (F127) as a template and 1,3,5- trimethylbenzene (TMB) as a swelling agent (Zhou et al., 2007). As compared with mesoporous silica materials synthesized at identical conditions, they found out that different hydrophobicity between precursors, bis(trimethoxysilyl)ethane and tetraethyl orthosilicate, may influence the swelling capacity of F127 micelles with TMB as well as the phase behavior of triblock copolymer F127, resulting in the different physicochemical properties of the final products.

Cho et al. (2009) reported the synthesis of three-dimensional cubic (Im3m) periodic mesoporous organosilicas with aromatic benzene- and thiophene-bridging groups using 1,4-bis(triethoxysilyl)benzene and 2,5-bis(triethoxysilyl)thiophene as organosilica precursors in the presence of a F127 under acidic conditions.

Recently Morell et al. (2008) reported the synthesis of a chiral periodic mesoporous organosilica (PMO) carrying benzylic ether bridging groups. By hydrolysis and condensation of the designed chiral organosilica precursor 1,4-bis- (triethoxysilyl)-2-

(1-methoxyethyl)benzene (BTEMEB) in the presence of the non-ionic oligomeric surfactant Brij76 as supramolecular structure-directing agent under acidic conditions, an ordered mesoporous chiral benzylic ether-bridged hybrid material with a high

29 Chapter 2. Literature Review specific surface area was obtained. The chiral PMO precursor was synthesized in a four-step reaction from 1,4-dibromobenzene as the starting compound. Optical activity measurements was used to detect the presence of the chiral units in the organosilica precursor as well as inside the PMO material

Zhang and co-corkers (2008) synthesized a novel magnetic composite of PMO hollow spheres and Fe3O4 nanocrystals. The particle size, shell wall thickness, and saturation magnetization value of periodic mesoporous organosilica magnetic hollow sphere (PMO–MHS) are tunable by varying the amounts of surfactant and magnetic nanoparticles. The pore surface properties can be adjusted by the co-condensation of

1,2-bis(trimethoxysilyl)ethane (BTME) with functional silane precursor.

PMO materials containing terminal organic groups anchored to the pores have been achieved (Asefa et al., 2001; Burleigh et al., 2001), which provides PMO materials potential applications by introducing other functional groups such as –SO3H. The combination of acidic groups and hydrophobic framework may result in interesting surface properties enhancing diffusion of reactants and products in acid-catalyzed reactions.

Inagaki et al. (2002) reported the synthesis of PMO with crystal-like pore walls having a surface structure with alternating hydrophilic and hydrophobic layers, composed of silica and benzene with a periodicity of 7.6 Å. Through the direct sulfonation reaction using fumed sulfuric acid -SO3H groups were attached onto the pheneylene group located within hydrophobic benzene layers. The resultant sulfonic acid catalysts still preserved meso- and molecular-scale ordered structure and outstanding thermal stability, both of which is benefit for catalytic application.

By co-condensation of 1,4-bis(triethoxysilyl)benzene (BTEB) and 3- mercaptopropyltrimethoxysilane (MPTMS) using a surfactant template in basic

30 Chapter 2. Literature Review conditions the thiol-functionalized benzene-silicas possessing mercaptopropyl (-

C3H6SH) groups were synthesized (Figure 2.15) (Yang et al., 2002). After postoxidation a novel sulfuric acid-functionalized mesoporous benzene-silica materials were prepared. In this kind of functionalized PMO the –SO3H groups were attached on the hydrophilic silica layers. The degree of oxidation was lower than 60%, and the maximum acid capacity observed was 0.7 mmol of H+ per gram of material.

Figure 2.15 (A) Structural model of periodic pore surface attached with propylsulfuric acid groups. (B) TEM image of surfactant-free Ph-MP40 (Yang et al., 2002).

Kaliaguine et al reported the synthesis of alkylsulfonic-acid-bearing ethane-silica

PMOs by the co-condensation of 1,2-bis(trimethoxysilyl)ethane (BTME), 3- mercaptopropyltrimethoxysilane employing cetyltrimethylammonium (CTAC) as template (Hamoudi and Kaliaguine, 2003). Different loadings of functional groups (up to 32mol%) were successfully incorporated into the hybrid mesostructured material, reaching acid capacities up to 0.7 mmol H+ per gram, however, also with poor oxidation efficiency of the thiol groups. They also successfully synthesized periodic mesoporous ethane-silica materials functionalized with arenesulfonic-acid groups via similar approach in the presence of P123 and Brij 76 as surfactants (Hamoudi et al.,

31 Chapter 2. Literature Review

2004). The synthesized materials acid capacity was up to 1.4 mmol of H+ per gram of material.

Kapoor et al. (2003) also reported the synthesis of biphenylenebridged bifunctional hybrid mesoporous materials functionalized with sulfonic-acid functionalities.

Materials were synthesized by co-condensation of 4,4-bis- (triethoxysilyl)biphenyl

(BTEBP) precursor and 3-mercaptopropyltrimethoxysilane (MPTMS) in a basic medium and cationic surfactant followed by an oxidation treatment. The authors claimed that due to the equimolar ratio of phenylene to silica, which provides the possibility to exert an enhanced hydrophobic character in the resulted PMO material.

The materials afforded an acid capacity up to 0.99 mmol of H+ per gram of material.

Yuan et al. (2003) first reported the catalytic results of these hybrid materials. By co-condensation of bis(triethoxysilyl)ethane (BTSE) and 3- mercaptopropyltrimethoxysilane (MPTMS) under basic condition, PMO catalysts were prepared with ethane moieties within the silica framework, functionalized with alkylsulfonic-acid groups. The sulfonic acid modified mesostructured materials (PMO-

SO3H) showed higher activities than ZSM-5 catalysts over the alkylation of phenol with 2-propanol. Compared MCM-41-SO3H the organosulfonic-modified PMO catalyst exhibited more stable activity. The mesoscopic structure of MCM-41-SO3H material was destroyed after reaction, however, the structure of PMO-SO3H did not change. In conclusion, the bridged moieties within the framework yielded materials with excellent catalytic and hydrothermal stability. Followed by this pioneering work catalytic applications of organosulfonic-modified periodic mesoporous organosilicas have been widely investigated.

Yang et al. (2004) synthesized sulfonic acid-functionalized periodic mesoporous organosilicas by co-condensation of (R’O)3Si–R–Si(OR’)3 (R=CH2CH2 and C6H4;

32 Chapter 2. Literature Review

R’=CH3 and C2H5) with 3-mercaptopropyltrimethoxysilane (MPTMS)

3SiCH2CH2CH2SH in the presence of H2O2 using nonionic oligomeric polymer surfactant C18H37(OCH2CH2)10OH in acidic medium. The sulfonic acid functionalities

(SO3H) were generated in situ by oxidation of the propylthiol using H2O2 as oxidant during the synthesis process. The highest acid-exchange capacity was 1.72 H+ mmol/g.

The catalytic properties of the materials were investigated in liquid-phase condensation of phenol with acetone to form Bisphenol A. The resultant sulfonic acid-functionalized mesoporous organosilicas show high catalytic activity.

The catalytic applications of organosulfonic-modified hydrophobic mesoporous benzene–silica with lamellar pore wall structure are reported by Yang et al. (2005 a,b).

The mesoporous benzene–silica attached with propylsulfonic groups to the crystal-like periodic pore walls exhibited the catalytic activity in the esterification of acetic acid with ethanol. The catalytic results show the higher conversion compared to the commercial Nafion-H due to the active role of crystal-like walls and unique surface structure.

PMO with a benzene ring as the rigid unit incorporated in the framework and functionalized with anchored sulfonic acid groups were synthesized by either sol–gel polymerization of 1,4-bis(triethoxysilyl)benzene (BTEB) or co-condensation of BTEB and 3-mercaptopropyltrimethoxysilane (MPTMS) in the presence of octadecyltrimethylammonium bromide surfactant (Rac et al., 2006). Catalytic properties were studied in both gas-phase and liquid-phase reactions. The catalytic activity and stability of the PMO-based samples in the isopropylation of phenol in the gas-phase exceeds markedly those of functionalized mesoporous ordered materials

(MCM-41, HMS and SBA-15). Selectivities in the Fries rearrangement of phenyl acetate over the PMO-based catalysts differ significantly from that of the

33 Chapter 2. Literature Review homogeneous reaction (p-toluenesulfonic acid). The sample with benzenesulfonic acid surface functions exhibits higher activities and different selectivities in the dimerization of 2-phenylpropene and in the rearrangement–aromatization of ketoisophorone as compared to samples functionalized with propylsulfonic acid groups.

The catalytic performance of sulfonated organosilica in the reaction of 1-butanol dehydration was investigated be Kaliaguine et al. (Sow et al., 2005). The higher catalytic activity is associated with the more hydrophobic nature of -SO3H environment in this material. The sulfonic-organosilica material showed higher activity than sulfonic-modified pure silica for the direct production of dibutyl ether from 1- butanol. The authors claimed that the high activity of organosilica materials is related to the hydrophobic character of the ethane-containing silica which reduces the acid site deactivation associated with adsorption of water generated by the reaction.

The surfactant assistant syntheses of organosulfonic-modified periodic mesoporous organosilicas with large pores were prepared by a one-step condensation of tetramethoxysilane (TMOS) with 1,2-bis(trimethoxysilyl)- ethane (BTME) and 3- mercaptopropyltrimethoxysilane (MPTMS) in highly acidic medium was performed in the presence of triblock copolymer Pluronic P123 and inorganic salt as additive (Liu et al., 2005). During the condensation process, thiol (-SH) group was in situ oxidized to sulfonic acid (-SO3H) by hydrogen peroxide. X-ray diffraction studies along with nitrogen and water sorption analyses reveal the formation of stable, highly hydrophobic, and well-ordered hexagonal mesoscopic structures in a wide range of -

CH2CH2- concentrations in the mesoporous framework. Hydrothermal stability of the samples is increased with incorporation of bridging ethane groups to the mesoporous network. The catalytic performance of -SO3H functionalized mesoporous materials was investigated in the esterification of ethanol with acetic acid. And the results

34 Chapter 2. Literature Review demonstrate that the ethane groups incorporated in the mesoporous framework have a positive influence on the catalytic behavior of the materials.

Nakajima et al. (2005) described a novel chemical modification method for preparation of hybrid mesoporous solid acid catalysts with catalytic active sites on the surface of PMO material. The silica framework was functionalized with ethylene moieties through condensation of bis(triethoxysilyl)ethylene in the presence of

Pluronic 123 in acid medium. The ethylene sites were successfully converted to phenylene-sulfonic-acid groups by Diels-Alder reaction with benzocyclobutene followed by sulfonation in concentrated H2SO4 (Figure 2.16). The resultant acid hybrid material possesses an acid capacity of 1.4 mmol of H+ per gram. The catalytic performance of this material was first investigated in the esterification of acetic acid with ethanol and shows higher activity for formation of ethyl acetate than that obtained with niobic acid and comparable to that for Amberlyst-15 and Nafion. Moreover, reuse of the catalysts four times did not evidence loss of activity or leaching of sulfonic-acid species.

Figure 2.16 Schematic illustration of synthetic pathways for organosulfonic-modified periodic mesoporous organosilica (Nakajima et al., 2005).

35 Chapter 2. Literature Review

Dube, D. et al. (2008) reported a new two step functionalization method for ethylene-silica PMO. This ethylene bridged PMO was synthesized using 1,2- bis(triethoxysilyl)ethylene (BTENE) as framework precursor under acidic conditions using Pluronic 123 as surfactant. Ethylene bridges were then arylated with benzene in the presence of AlCl3 and, following by sulfonated with H2SO4 (Figure 2.17). The resulting solid acid catalysts show high acid site density (up to 0.96 mmolH+/g). These new sulfonic acid functionalized PMOs exhibit a high catalytic activity in self- condensation of heptanal, owing to their high density of acid sites and the presence of hydrophobic bridged framework.

Figure 2.17 Schematic representation of the two-step synthesis mechanism of PMO acid catalysts (Dube et al., 2008).

Shen et al. (2008) first reported the preparation of a periodic mesoporous organosilica (PMO) functionalized with perfluoroalkylsulfonic acid groups by reacting the PMO with 1,2,2-trifluoro-2-hydroxy-1-trifluoromethylethane sulfonic acid β-

36 Chapter 2. Literature Review sultone (Figure 2.18). The resulting material exhibits much higher catalytic activity in the alkylation of isobutene by 1-butene than perfluoroalkylsulfonic acid functionalized

SBA-15, alkylsulfonic acid functionalized PMO and zeolite Y.

Figure 2.18 Preparation of perfluoroalkylsulfonic functionalized PMO by the grafting technique (Shen et al., 2008).

2.5 Sulfonic acid-modified carbon

2.5.1 Sulfonic acid-modified nonporous carbon

Normally, it is difficult to introduce large amount of functional groups into carbon materials (D'Amours and Belanger, 2003). However, in 2004 Hara et al. (2004) first reported a new kind of strong carbon protonic acid by incomplete carbonization of sulfoaromatic hydrocarbons (naphthalene). The sulfonation reaction was carried out under high reaction temperature (200-300 ºC). High temperature sulfonation reaction of naphthalene also deduced the incomplete carbonization process. New type of sulfonic acid catalyst composed of small polycyclic aromatic carbon sheets with attached –SO3H groups was prepared through this sulfonation reaction. The resultant carbon-based sulfonic acid catalyst show excellent catalytic performance in esterification and hydrolysis reaction. Moreover, they also proved that sulfonation of incompletely carbonized resin, amorphous glassy carbon, activated carbon, or natural graphite could not produce stable and active sulfonic acid catalysts.

However, this new kind of carbon-based sulfonic acid catalyst is soft and its aromatic molecules are leached out during liquid phase reactions above 100 ºC and its

37 Chapter 2. Literature Review catalytic activity is heavy limited when higher fatty acids are used as reactant. In order to overcome these drawbacks, Toda et al. (2005) employed sucrose, starch or cellulose as the carbon precursor under low temperature carbonization process (400 ºC) to produce the carbon material composed of small polycyclic aromatic carbon sheets.

Amorphous carbon prepared under low temperature carbonization process enhanced the stability of resultant carbon sulfonic acid catalysts greatly. The carbonization and sulfonation was shown in Figure 2.19.

Figure 2.19 Pyrolysis of the sugars causes their incomplete carbonization and formation into polycyclic aromatic carbon sheets; sulfuric acid (concentrated or fuming) is used to sulfonate the aromatic rings to produce the catalyst (Toda et al., 2005).

The effect of the carbonization temperature was also investigated (Okamura et al.,

2006). The amorphous carbon prepared under 550 ºC is composed of larger carbon sheets and is more carbonized than that prepared under 450 ºC and below (Figure 2.20).

Figure 2.20 Proposed structures of carbonized D-glucose after sulfonation: (A) carbon prepared at 300-450 ºC; (B) carbon prepared at 550 ºC. investigated (Okamura et al., 2006).

38 Chapter 2. Literature Review

With trimethylphosphine as the probe, Okamura et al. (2006) employed the 31P-

MAS NMR technique to measure the acid sites of the sulfonated carbon. The authors found that the active centers for the carbon sulfonated at 550 ºC are not fully available for reactants. Because with increasing carbonization temperature, carbon materials become harder and the flexibility of the polycyclic aromatic carbon decrease through plane growth and carbon sheet stacking. Sulfonated amorphous carbon prepared under lower temperature shows better swelling character, which contributes to the high catalytic performance. Another possible reason of the high activity for the amorphous carbon prepared under lower temperature carbonization process is phenolic -OH and -

COOH groups on the catalyst. Normally, these functional groups cannot catalyze the esterification due to the weak acidities, when they attached to such a carbon material, might have strong acidities and contribute to the high catalytic activity (Takagaki et al.,

2006). It is also possible that these functional groups increase hydrophilicity of the carbon material, resulting in the improvement of accessibility for hydrophilic reactants.

This amorphous carbon based sulfonic acid catalysts was used to convert crude vegetable oils into biodiesel in a one-step process, which indicates the potential utility of this material in a wide range of catalytic processes. The outstanding catalytic activity of this material is attributed to the incorporation of reactants into the bulk structure and the high density of strongly acidic -SO3H sites (Nakajima et al., 2007).

The sulfonated amorphous carbon catalysts possess a great number of hydrophilic functional groups, which prevent incorporation of hydrophobic molecules into the bulk.

Therefore hydrophobic acid-catalyzed reactions could proceed only on surfaces with small surface areas, which results in poor or no catalytic activity for the reactions.

Depositing carbon in the mesopores of a mesoporous material such as SBA-15 following the sulfonation of deposited amorphous carbon a SO3H-bearing carbon

39 Chapter 2. Literature Review catalysts with large surface areas available for hydrophobic acid-catalyzed reactions were prepared (Nakajima et al., 2009). This new kind of amorphous carbon sulfonic acid catalyst shows remarkable catalytic activity over the dimerization of α- methylstyrene, which can not be catalyzed by amorphous carbon sulfonic acid catalyst.

The high catalytic performance not only contributed to the high surface area and acid strength but also assigned to the -OH and -COOH groups.

Mo and co-workers (2008) studied the activation and deactivation of amorphous carbon sulfonic acid catalysts. They found out that the poor swelling of amorphous carbon sulfonic acid catalyst in the gas phase acetic acid esterification reaction led to the appearance of an induction period and inferior catalytic performance. In the liquid- phase esterification of acetic acid, catalytic activity of amorphous carbon sulfonic acid catalyst decreased during the initial reaction batch cycles but reached a plateau after several catalytic cycles. Through the elemental analysis, 1H NMR results of the liquid reaction mixtures after reaction, and the decreased activity of the catalysts after extensive solvent washing they concluded that the initial catalyst deactivation was caused by the leaching of polycyclic aromatic hydrocarbons containing –SO3H groups.

2.5.2 Sulfonic acid-modified mesoporous carbon

Ordered mesoporous carbon (OMCs) is another good candidate for sulfonic acid catalyst support due to its high surface, narrow pore size distribution and large pore volume, which would facilitate the high densities of functional groups and ensure good accessibility to active sites. OMCs consisting of three-dimensional regular arrays of uniform mesopores by employing ordered mesoporous silica MCM-48 as the template were first demonstrated independently by two Korean research groups (Lee et al., 1999;

Ryoo et al., 1999). This process includes three main parts (1) polymerization of carbon precursor in the pores of the templates, (2) carbonization at high temperature under

40 Chapter 2. Literature Review nitrogen atmospheres, and (3) subsequent removal of the templates. Subsequently, template synthesis has been successfully employed to prepare various OMCs by using different ordered mesoporous silica (Ryoo et al., 2001; Lee et al., 2004).

Wang and co-workers (2007) functionalized OMCs by covalent attachment of sulfonic acid-containing aryl radicals, which can be produced by homogeneous reduction of diazonium salt by hypophosphorous acid on the carbon surface under mild conditions (see Figure 2.21).

Figure 2.21 Functionalization of CMK-5 with Sulfonic Acid Groups (Wang et al., 2007).

Compared with CMK-5 CMK-5-SO3H can be viewed as a material with a hydrophobic substrate and hydrophilic functional groups. Such amphiphilic properties would allow CMK-5-SO3H to be an efficient solid catalyst in both hydrophobic and hydrophilic environments. The resultant CMK-5-SO3H exhibits high activity for esterification reaction of acetic acid with methanol and good recyclability, which can be attributed to high stability of ordered mesoporous CMK-5 and tight attachment of aryl sulfonic acid group on the substrate through the stable C-C bond.

41 Chapter 2. Literature Review

CMK-5-SO3H also shows high catalytic activity for the production of biodiesel involving long chain molecular. The pore size distribution of CMK-5-SO3H could be tuned by the ageing temperature of the template SBA-15 preparation. The effect of pore size on the catalytic performance demonstrates that the factors favoring the diffusion of large organic molecules contribute to the enhancement of catalytic property (Liu et al., 2008).

Introducing the -SO3H groups onto OMCs through sulfonation reaction using concentrated sulfuric acid (98%) under high temperature (150 ºC) can also prepare the carbon based sulfonic acid catalysts (Tian and Zhao, 2008 b part 1). The resultant catalysts show high activity over esterification reaction for acetic acid and methanol.

The carbonization temperature has great effect on the catalytic activity of final sulfonic acid catalysts due to the pore structure and surface acid groups.

2.5.3 Sulfonic acid-modified carbon nanotubes

Great interest is involved in developing the potential of carbon nanotubes (CNT) for use as a catalyst support that can offer unparalleled flexibility in tailoring catalyst properties because the physical and chemical properties of carbon nanotubes can be dramatically influenced by surface modification. Chemical modification is a common method and is essential for the deposition of catalysts and other species onto nanotube surfaces for nanocatalytic application. Peng and co-worker (2005) sulfonated the CNT using concentrated sulfuric acid under high temperature (250 ºC). The resultant sulfonated CNT exhibit high acid density and thermal stability. And show high catalytic activity through the esterification reaction of methanol with acetic acid.

Single-walled carbon nanotubes (SWCNTs) were also chosen as the sulfonic acid support through the high temperature sulfonation reaction (Yu et al., 2008). High temperature sulfonation reaction is benefit for the amount and stability of attached

42 Chapter 2. Literature Review sulfonic acid groups. The high catalytic activity for sulfonated SWCNTs can be explaind by the strong acidity of sulfonic group and the ability of SWCNTs to support functional groups. Another reason responsible for the enhanced catalytic performance is the excellent dispersibility in solvents, including ethanol, which minimizes the mass transfer resistance in the heterogeneous liquid-solids reaction.

2.6 Sulfonic acid-modified resin

Sulfonic acid-modified resins are widely used as acid catalysts in the production of ethers for the petrochemical industry, esterification, alkylation, alkene and other reactions. Harmer et al. (2001) presented the use of commercially available sulfonic acid-modified resins for a range of industrially important transformations. Sulfonic acid-modified resins (such as Amberlyst) show very high activity in the areas of esterification and etherification. Amberlyst polymer based catalysts involve mostly the use of functionalized styrene divinylbenzene copolymers with different surface properties and porosities. The functional group is generally of the sulfonic acid type.

These types of catalysts are adding to the ever-growing portfolio of highly active solid acid catalysts, which couple both economic and environmental drivers to improve organic transformations within the chemical industry. Studies about the kinetics involved polystyrene sulfonic acid resin catalysts have also been investigated in details.

Rehfinger et al. reported the synthesis of methyl tertiary butyl ether (MTBE) in a liquid phase using a macroporous polystyrene sulfonic acid resin as the catalyst

(Rehfinger and Hoffmann, 1990). Their experimental results concerning the activity and selectivity of the sulfonic acid resin catalyst were in agreement with a shell-core model, according to which almost none of the methanol-containing core produces the by-product diisobutene while only the outer shell forms the main product MTBE.

43 Chapter 2. Literature Review

Vapor-phase addition of ethanol to isobutene has been studied at atmospheric pressure and 53-78.5ºC on resin Amberlyst 15 (Iborra et al., 1992). Four active centers take part in the rate-determining step of the two mechanisms. The first one stems from a mechanism whose rate-determining step is the surface reaction between the ethanol and the isobutene, both adsorbed on one center. The second model is based on a mechanism whose rate-determining step is the surface reaction between the ethanol adsorbed on one center and the isobutene adsorbed on two centers. It was not possible to distinguish between the two models because isobutene adsorbs slightly on

Amberlyst 15.

Dogua and co-worker prepared two sets of resin catalysts (Boz et al., 2004). One set is sulfonated styrene–divinyl benzene cross-linked resin catalysts having different porosities and different hydrogen ion-exchange capacities. Another set is heated commercial Amberlyst-15 having different hydrogen ionexchange capacities.

Activities of these catalysts were compared with the activities of Amberlyst-15 in the etherification of 2M1B and 2M2B with ethanol. It was concluded that hydrogen ion- exchange capacity of acidic ion-exchange resin catalysts was a major factor on the activity in the etherification reactions of isoamylenes with ethanol.

Paakkonen et al. (2003) investigated the reaction rate of ion-exchange resin bead catalysts (A16, A35 and XE586) and a fibrous catalyst (SMOPEX-101). Kinetic modeling results favoured a single-site mechanism for the isomerisation and a dual-site mechanism for the etherification. The activation energy was determined to be 116.7 kJ/mol for the isomerisation of 2M1B to 2M2B, 92.7 kJ/mol for the etherification of

2M1B to tert-amyl methyl ether and 93.0 kJ/mol for the etherification of 2M2B to tert- amyl methyl ether.

44 Chapter 2. Literature Review

Four solid catalysts were used to examine their reactivity toward the synthesis of n- butyl propionate via the esterification of propionic acid and n-butanol, Amberlyst 35,

Amberlyst 39, and HZSM-5 pellets (Liu and Tan, 2001). The experimental results indicated that the Amberlyst resins such as Amberlysts 15wet, 35wet, and 39wet are effective catalysts for this reaction and were found to be better than HZSM-5. A possible mechanism of this reaction was found to follow the Rideal-Eley theory in which an adsorbed and protonated propionic acid molecule reacted with an n-butanol molecule in the bulk. The activation energy of the forward reaction was found to be

14.1 kcal/mol. The activity-based equilibrium constant was found to increase slightly with an increase in temperature, indicating that this reaction is nearly isothermal.

Kinetic studies of the hydrolysis of methyl lactate catalyzed by an acidic cation- exchange resin, Amberlyst 15, is presented by Sagrario Beltran (Sanz et al., 2004). The effects of catalyst loading, temperature, and feed composition were investigated.

Because of the high nonideality of the reaction mixture, the kinetics was expressed in terms of activities. The quasi-homogeneous model was found to represent the esterification and hydrolysis reactions over Amberlyst 15 fairly well.

The same group also described the esterification kinetics of lactic acid with methanol without addition of catalyst and catalyzed by ion-exchange resins (Sanz et al.,

2002). The effect of catalyst type, stirrer speed, catalyst size, catalyst loading, initial reactant ratio, and temperature on reaction kinetics was evaluated. Experimental reaction rates were correlated by models based on homogeneous and heterogeneous

(dual- and single-site mechanisms) approaches. Amberlyst 15 was found to be a good catalyst for this reaction. Internal and external mass transfer was easily avoided. The reaction rate increased with temperature and catalyst loading. The equilibrium conversion increased as the methanol/lactic acid ratio increased. The quasi

45 Chapter 2. Literature Review homogeneous (QH) model represents the experimental data fairly well as expected for a highly polar mixture. The high value of the apparent activation energy supports the assumption that the reaction in the catalyst surface was the rate-controlling step.

Macroreticular resin catalysts, Amberlyst XN-1010 and Amberlyst 15, were used in the hydration of isobutylene (Ihm et al., 1988). The reaction was found to be diffusion- limited, and the apparent reaction rate constants were estimated through a two-phase model, where the internal and external rate constants can be distinguished from each other. It is inferred that the activities of sulfonic acid groups within the microparticles are more active than those on the surface of the microparticles for the present reaction.

The external and internal intrinsic reaction rate constants were estimated, and their activation energies were found to be 14.7 and 19.1 kcal/mol, respectively.

M. Hart and co-workers (2001, 2002) described the preparation of series of macroporous sulfonated poly(styrene-co-divinylbenzene) ion-exchange resins with varying levels of sulfonation. Catalytic activities have been measured in two liquid- phase reactions: the dehydration of 1-hexanol under flow conditions and the hydration of propene. The strength of the acid sites increases as the level of sulfonation is increased the same trend as the catalytic activities. These persulfonated resins, which were sulfonated at levels above one sulfonic acid group per aromatic ring, also show higher thermal stabilities than conventional resins, which were sulfonated at just below one acid group per aromatic ring. Both the increase in acid concentration in the internal solution and in the level of di-sulfonation contributes to an increase in acid strength.

Yadav et al. (2005) described a simple process based on the use of heterogeneous catalysts. The activity of various catalysts such as 20% w/w Cs-DTP/K- 10,

Amberlyst-15, Indion-190 and Indion-130 were studied for the acylation of thioanisole with acetic anhydride to prepare 4-(methylthio)acetophenone. Amberlyst-15 was found

46 Chapter 2. Literature Review to be the best catalyst. The effects of various parameters were studied using this catalyst, and dynamic adsorption studies were performed to get adsorption equilibrium constants for the reactants and products.

47 Chapter 3. Experiment Section

CHAPTER 3

EXPERIMENTAL SECTION

3.1 Reagents and apparatus

Table 3.1 Reagents used for synthesis of sulfonic acid catalysts.

Chemicals Grade Supplier Sucrose 98% Merck Sulfuric acid 98% Merck Hydrochloride acid 37% Merck Decyl alcohol(1-Decanol) 99% Aldrich Dodecyltrimethylammonium 99% Aldrich bromide (DTAB) Pyrrole 98% Aldrich

FeCl3 99% Aldrich PA Phenol 99.5% Panreac Formaldehyde solution 38 wt% Fisher Alfa Styrene 99% Aesar 80% mixture of Divinylbenzene Aldrich isomers Ethanol 99.5% Aldrich Triblock copolymer P123 Mw ~ 5800 Aldrich Tetraethyl orthosilicate 98% Aldrich Ammonia 28% Merck Ethanol 99.5% Aldrich Hydrofluoric acid 48-51% Tyco

48 Chapter 3. Experiment Section

Table 3.2 Apparatus.

Apparatus Model or Specification Manufacturer Hot Plate & Vision KMC-130SH Magnetic stirrer Scientific Hot Plate & MR 3002 Heidolph Magnetic stirrer Stainless steel lined with Autoclave Self-made Teflon Oven Binder FD240 Fisher Scientific Radiometer pH meter Ioncheck 10 Analytical Goldbell Balance AND GF-2000(0.5-2100 g) Weigh-System MonoBloc Balance AB204-S (10 mg-220 g) Weighing Technology Furnace CWF1100 Carbolite Ultrosonicator Transsonic 460/H ACHEMA

3.2 Preparation of sulfonic acid catalysts

3.2.1 Preparation and sulfonation of mesoporous carbon and carbon-silica composite by hard template method

Mesoporous SBA-15 silica template was prepared following the method described by Zhao et al (Zhao et al., 1998). The template was added to a solution containing sucrose (98%, Merck), H2SO4 (98 %, Merck), and H2O with a mass ratio of 1.0 : 0.1 :

4.0. The mixture was stirred at room temperature for 1 h before it was heated in an autoclave at 100 ºC for 6 h followed by at 160 ºC for 6 h. The ratios of the volume of sucrose used in the infiltration step over the total pore volume (measured from physical adsorption data) were 50%, 80%, 120%, respectively. To enhance the infiltration of sucrose, the infiltration process was repeated. The resultant composites were carbonized in a quartz tube under highly pure nitrogen (50 cm3/min) at different

49 Chapter 3. Experiment Section temperatures (400-900 ºC) for 8 h with a heating rate of 2 ºC/min. Upon removal of the template using a HF (46%) solution, mesoporous carbons were obtained. The samples are denoted as MCa(b), where a stands for the carbonization temperature and b stands for the ratio of the volume of sucrose used in the first infiltration over the total pore volume of SBA-15.

By changing the volume ratio of sucrose over the total pore volume of SBA-15, different carbon-silica composites were prepared. The preparation was essentially similar to that of mesoporous carbons as described above apart from that SBA-15 template was not removed and the carbonization temperature was 400 ºC. The resultant carbon-silica composites are designed as CS400(f), where 400 stands for the carbonization temperature and f stands for the volume ratio of sucrose over the total pore volume of SBA-15.

About 0.3 g of a carbon or carbon-silica composite was treated with 70 mL of concentrated H2SO4 (>98%) at 150 ºC for 15 h under Ar atmosphere. Then, the solids were washed using hot water till no sulfate ions were detectable in the filtrate. The resultant sulfonated carbons are denoted as SMCa(b), whereas the resultant sulfonated carbon-silica composites are donated as SCS400(f).

3.2.2 Preparation and sulfonation of mesoporous polymer resin and carbon by soft template method

The synthesis of mesoporous polymers resin was following the soft template method as reported by Zhao et al. (Zhang et al., 2006). First, 2 g of phenol and 7.0 mL of formaldehyde solution (38 wt%) were added in 50 mL of 0.1M NaOH solution. Then the mixture was stirred at 70 ºC for 30 min. A transparent lilac precursor solution was obtained. Second, 8 g of Pluronic P123 (EO20PO70EO20) was dissolved in 50 mL of

50 Chapter 3. Experiment Section water. Third, the prepared lilac precursor solution was added to the above mixture under stirring. The mixture was stirred at 65 ºC for 96 h and then at 70 ºC for 24 h. The final solid was collected by sedimentation and dried in air. The sample is designed as

MP. Carbonization of MP was conducted in a quartz tube under pure nitrogen flow

(99.9995%) at 350 ºC, 400 ºC, 500 ºC, 600 ºC, and 800 ºC respectively for 8 h with a heating rate of 1 ºC/min. The resultant materials were donated as MC(x), where x stands for the carbonization temperature.

About 0.15 g of a solid sample was combined with 50 mL of concentrated H2SO4

(>98%, Merck) at a given temperature. After 4 h, the sample was cooled down to room temperature. Then, 1000 mL of deionized water was added to the mixture to form a black precipitate, which was then washed repeatedly in deionized water (DI water) until sulfate ions were no longer detectable in the washing water. The resultant sulfonated solid was donated as MC(x,y), where y stands for the sulfonation temperature.

3.2.3 Preparation and sulfonation of polypyrrole and carbon nanospheres

In a typical preparation, 4.0 g of decyl alcohol (1-Decanol, 99%, Aldrich) and 120 ml of deionized water were mixed under stirring at room temperature for 40 min. Then,

6.0 g of dodecyltrimethylammonium bromide (DTAB, 99%, Aldrich) was added. After further stirring at 1 ºC for 20 min, the emulsion system was transferred to an ultrasonicator (Elma Transsonic T460/H of capacity 2.75 liters and frequency of 35 kHz). Subsequently, 3.2 g of pyrrole (98%, Aldrich) was added dropwise.

Polymerization was initiated by adding 8.0 g of FeCl3 (99%, Aldrich). After ultrasonication for 3 h, the solid polypyrrole nanoshperes (PNs) were separated by filtration, washed with ethanol to remove the surfactant, and dried in an oven at 60 °C

51 Chapter 3. Experiment Section overnight. Carbonization of the PNs was conducted in a quartz tube under pure nitrogen flow (99.9995%, 50 cm3/min) at a desired temperature for 5 h. The resultant carbonized polypyrrole nanospheres are donated as CPNs(x), where x stands for the carbonization temperature.

0.3 g of PNs or CPNs was heated in 70 ml of concentrated H2SO4 (97%, Merck) at a given temperature (40-150 oC) for 4 h. After cooling down to room temperature, 1000 ml of ethanol was added. The black solid was collected by filtration, washed repeatedly using ethanol till no sulfate ions were detected in the filtrate. The sulfonated

PNs and CPNs are designed as SPNs(y) and SCPNs(x,y), respectively, where y stands for the sulfonation temperature.

3.2.4 Preparation and sulfonation of polystyrene-divinylbenzene spheres

The syntheses of linear-linked (LPSs) and cross-linked (CPSs) polystyrene spheres were synthesized following the method as reported by Zhao et al. (Wang et al., 2006).

A given amount of styrene monomer and divinylbenzene (DVB) were combined with

200 ml of dionized (DI) water. The mixture was stirred at 60 ºC under the protection of nitrogen for 1 h. Then, 0.14 g of potassium presulfate (KPS) dissolved in 50 ml of DI water was added to the mixture under stirring at 60 ºC for 30 h. The final solids were collected by using the centrifuge and dried at 60 ºC in an oven. The amounts of styrene added was 21.4 ml, and DVB added were 0, 0.4, 0.8, 1.2, and 2.4 ml, respectively, leading to different samples donated as PS-DVB(x), where x stands for the volume ratio for DVB/Styrene. When x is 0, LPSs was botained. Otherwise, CPSs were synthesized.

About 3 g of a solid sample was combined with 60 mL of concentrated H2SO4

(>98%, Merck) at a given temperature. After 24 h, the sample was cooled down to

52 Chapter 3. Experiment Section room temperature. Then, 1000 mL of deionized water was added to the mixture, which was then washed repeatedly in deionized water (DI water) until sulfate ions were no longer detectable in the washing water. The resultant sulfonated solids were donated as

SPS-DVB(x,y), where y stands for sulfonation temperature.

3.3 Characterization

3.3.1 Elemental analysis (CHNS-O)

Elemental analysis is a process where a sample of some material is analyzed for its elemental and sometimes isotopic composition. Elemental analysis can be qualitative

(determining what elements are present), and it can be quantitative (determining how much of each are present). For organic chemists, elemental analysis almost always refers to CHNX analysis — the determination of the percentage weights of carbon, hydrogen, nitrogen, and heteroatoms (X) (halogens, sulfur) of a sample. This information is important to help determine the structure of an unknown compound, as well as to help ascertain the structure and purity of a synthesized compound. The elemental analysis data was collected on a CHNS-O Analyzer, FLASH EA 1112 series,

Thermo electron corporation.

3.3.2 Fourier transform infrared spectrophotometer (FT-IR)

FT-IR spectrometers record the interaction of infrared radiation with sample measuring the frequencies at which the sample absorbs the radiation and the intensities of the absorptions. Chemical functional groups are known to absorb light at specific frequencies. Thus the chemical structure can be determined from the frequencies records. Fourier Transform Infrared (FTIR) spectra were collected on a Bio-Rad Fts

135 with a resolution of 4cm-1 in the wavelength range of 400-4000cm-1. A powder

53 Chapter 3. Experiment Section sample was mixed with potassium bromide, KBr (Merk,Lot No. B28 7007 823) in a weight of 1:99, then pellets were formed using a Graseby Specac.

3.3.3 Thermogravimetric analysis

Thermogravimetric Analysis or TGA is a type of testing that is performed on samples to determine changes in weight in relation to change in temperature. Such analysis relies on a high degree of precision in three measurements: weight, temperature, and temperature change. TGA is commonly employed in research and testing to determine characteristics of materials such as polymers, to determine degradation temperatures, absorbed moisture content of materials, the level of inorganic and organic components in materials, decomposition points of explosives, and solvent residues. It is also often used to estimate the corrosion kinetics in high temperature oxidation. Thermogravimetric analysis was conducted on a thermogravimetric analyzer TGA 2050, Thermal Analysis Instrument, USA.

3.3.4 Scanning electron microscopy (SEM)

SEM uses a condensed, accelerated electron beam to focus on a specimen. The electron beam hits the specimen and produces secondary and backscattered electrons.

Secondary electrons are emitted from the sample and collected to create an area map of the secondary emissions. Since the intensity of secondary emission is very dependent on local morphology, the area map is a magnified image of the sample. In this thesis work, SEM images were measured on a JEOL-6700F scanning electron microscope, which was operated at an acceleration voltage of 10kV and filament current of 60 mA.

Before measurement, the samples were stuck onto a double-face conducted tape

54 Chapter 3. Experiment Section mounted on a metal stud and coated with platinum with a sputter coater (JEOL JFC-

1300 Auto fine coater).

3.3.5 UV-Vis spectrophotometer (UV-Vis)

Ultraviolet-visible spectrophotometer (UV-Vis) involves the spectroscopy of photons in the UV-visible region. This means it uses light in the visible and adjacent

(near ultraviolet (UV) and near infrared (NIR)) ranges. The absorption in the visible ranges directly affects the color of the chemicals involved. In this region of the electromagnetic spectrum, molecules undergo electronic transitions. This technique is complementary to fluorescence spectroscopy, in that fluorescence deals with transitions from the excited state to the ground state, while absorption measures transitions from the ground state to the excited state. The UV-Vis reflectance spectra were taken on a Shimadzu UV-1601PC UV-Visible- Spectrophotometer. The collection was made over the wavelength range 400-200 nm.

3.3.6 N2 adsorption/desorption

In N2 adsorption analysis, a sample is exposed to N2 gas of different pressures at a given temperature (usually at -196 oC, the liquid-nitrogen temperature). Increment of pressure results in increased amount of N2 molecules adsorbed on the surface of the sample. The pressure at which adsorption equilibrium is established is measured and the universal gas law is applied to determine the quantity of N2 gas adsorbed. Thus, an adsorption isotherm is obtained. If the pressure is systematically decreased to induce desorption of the adsorbed N2 molecules, a desorption isotherm is obtained. Analysis of the adsorption and desorption isotherms in combination with some physical models yields information about the pore structure of the sample such as surface area, pore

55 Chapter 3. Experiment Section

volume, pore size and surface nature. The N2 sorption/desorption was carried out at 77

K on a NOVA 1200 analyzer. The sample was degassed at 120 ºC for 5 h before the measurement. Specific surface area was calculated by using the multiple-point

Brunauer-Emmett-Teller (BET) model. The pore size distributions were obtained from the analysis of the adsorption branch of the isotherm by the Barrett-Joyner-Halenda

(BJH) method. The total pore volume was obtained from the volume of nitrogen adsorbed at a relative pressure of 0.95.

3.3.7 Magic-angle spinning-nuclear magnetic resonance (MAS-NMR)

Nuclear magnetic resonance (NMR) spectroscopy uses high magnetic fields and radio-frequency pulses to manipulate the spin states of nuclei. The position of NMR peaks reflects the chemical environment and nucleic positions of the atoms within the molecule. Chemical shift is defined as nuclear shielding / applied magnetic field. It is measured relative to a reference compound. Solid-state magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectra were collected on a Bruker DRX400 MHz

FT-NMR spectrometer.

3.3.8 X-ray diffraction (XRD)

X-ray diffraction (XRD) takes advantages of the coherent scattering of x-rays by polycrystalline materials to obtain a wide range of structural information. The x-rays are scattered by each set of lattice planes at a characteristic angle, and the scattered intensity is a function of the atoms, which occupy those planes. The scattering from all the different sets of planes results in a pattern, which is unique to a given compound.

In addition, distortions in the lattice planes due to stress, solid solution, or other effects can be measured. Resultant samples in the thesis were also identified by using X-ray

56 Chapter 3. Experiment Section diffraction (XRD) on a Shimadzu XRD-6000 diffractometer (Cu Kα radiation,

λ=0.15418nm) operated at 40 kV and 30 mA with a scanning speed of 2.00 (deg/min).

3.3.9 X-ray photoelectron spectroscopy (XPS)

When a primary X-ray beam of precisely known energy impinges on sample atoms, inner shell electrons are ejected and the energy of the ejected electrons in measured.

The difference in the energy of the impinging X-ray and the ejected electrons gives the binding energy (Eb) of the electron to the atom. Since this binding energy of the emitted electron depends on the energy of the electronic orbit and the element, it can be used to identify the element involved. Further, the chemical form or environment of the atom affects the binding energy to a considerable extent to give rise to some chemical shift, which can be used to identify the valence of the atom and its exact chemical form. In this experiment, XPS spectra were obtained using AXIS HIS

(Kratos Analytical Ltd., U.K.) with an Al Kα X-ray source (1486.71 eV protons), operated at 15 kV and 10 mA. The pressure in the analysis chamber was maintained below 10-8 torr during each measurement. All spectra were fitted by a software package XPSpeak 4.1 with the subtraction of Shirley (for transition metals) or linear background (for other elements) and a ratio of 0% Lorentzian-Gaussian. In charge-up correction, the calibration of binding energy (BE) of the spectra was referenced to the

C 1s electron bond energy corresponding to graphitic carbon at 284.6 eV.

3.3.10 Transmission electron microscopy (TEM)

Before measurement, the sample was dispersed in ethanol and then dripped and dried on a copper grid. In the operation of TEM, an electron beam is focused on a specimen and part of the electron beam is transmitted. This transmitted portion is

57 Chapter 3. Experiment Section focused by objective lens into an image and the image is passed down through enlarge lenses and a projector lens, being enlarged all the way. In the experiment, the TEM was used to characterize the internal structure of particles and the TEM images were obtained on a JEOL 2010 and JEM 2010F transmission electron microscope operated at an acceleration voltage of 200 kV.

3.4 Evaluation of conversion for acetic acid

The reaction was carried out in a round bottom flask immersed in a silicon oil bath controlled at 55 ºC. Methanol, acetic acid and solid catalyst were added into the flask.

In this batch reaction system, the catalytic performance of various catalysts was compared on the basis of mass. Therefore, the conversion of acetic acid after 5 h of reaction time was detected. Quantitative analysis was carried out after 5 h of reaction using a gas chromatograph (HP 6890 series GC) mass spectrometer (HP 5973 Mass selective detector) with capillary column ( HP 5MS, L 30m, I.D. 0.25mm, Film

0.25um). The quantity of the resultant production concentration was determined by using outside standard method.

58 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

CHAPTER 4

SULFONATED MESOPOROUS CARBONS AND

CARBON-SILICA COMPOSITES

4.1 Introduction

Liquid sulfuric acid used as a catalyst results in a number of problems, such as high toxicity, corrosion, difficulty in separation from the reaction mixture, and the generation of solid sulfate waste. Therefore, an alternative acid catalyst that is recyclable and reusable to replace the liquid sulfuric acid is highly desirable in these environmentally conscious days.

Sulfonic-acid-group-containing resins (e.g., nafion) (Okuhara, 2002), organic/inorganic solid hybrid materials (Yang et al., 2002, 2005 a,b;Cano-Serrano et al., 2003), and sulfonated carbon nanotubes (Peng et al., 2005) show potential methods to the preparation of novel solid acid catalysts. Carbon-based sulfonic acid catalysts with a high catalytic activity were described by Hara and co-workers first (Hara et al.,

2004; Edward T. Lu, 2005; Okamura et al., 2006). In their work, a carbon-based acid catalyst composed of small polycyclic aromatic carbon sheets with attached sulfonic acid groups (-SO3H) was obtained by heating aromatic compounds (e.g., naphthalene) in concentrated sulfuric acid. Lately, they employed sucrose as carbon precursor to prepare carbon-based sulfonic acid catalysts and investigated their catalytic properties

(Edward T. Lu, 2005; Okamura et al., 2006; Takagaki et al., 2006). However, these carbon-based sulfonic acid catalysts are nonporous and possess low surface area, which limited the acid density. Introducing -SO3H groups onto mesoporous carbon materials, which have large surface area, could increase the acid density efficiently.

59 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

Research work focused on this area is limited. In this work, the hard template method

(Johnson et al., 1997; Ryoo et al., 1999; Ryoo et al., 2000; Ryoo et al., 2001; Kyotani et al., 2003; Su et al., 2005 c; Su et al., 2005 d; Su et al., 2005 e; Zhou et al., 2005;

Zhao et al., 2006) was used to prepare mesoporous carbons under different temperature carbonization processes, which were subsequently sulfonated. It was observed that sulfonated mesoporous carbon catalysts prepared under low temperature carbonization process showed non-ordered mesopore structure, lower surface area, however, the best catalytic performance. In order to obtain the well ordered mesopores and higher surface area under low temperature carbonization process, carbon-silica composite sulfonic acid catalysts were prepared by remaining of template, which can enhance the catalytic activity greatly.

4.2 Catalyst preparation

The detailed preparation steps for sulfonated mesoporous carbons and carbon-silica composites were described in Chapter 3.2.1. The resultant sulfonated carbons are denoted as SMCa(b), a stands for carbonization temperature and b stands for the ratio of volume of sucrose used in the first infiltration. And the resultant sulfonated carbon- silica composites are donated as SCS400(f), 400 stands for the carbonization temperature and f stands for the volume ration of sucrose over the total pore volume of template.

4.3 Characterization of sulfonated mesoporous carbons and carbon- silica composites

60 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

4.3.1 Textural properties

The specific surface areas of SMC400(50), SMC400(80) and SMC400(120) increased with the increase of first sucrose infiltration ratio (Table 4.1). However, the pore size distribution decreased with the increase of first sucrose infiltration ratio. In order to maintain the mesopore structure of template well we choose 80% as the first sucrose infiltration ratio for sulfonated mesoporous carbons catalysts (Jun et al., 2000).

The specific surface area of sulfonated mesoporous carbon catalysts increased with the increase of carbonization temperature (Table 4.1). Therefore, high temperature carbonization process is helpful for maintaining the mesopores during sulfonation process.

It can be seen from Figure 4.1A that the isotherms for MC400(80), MC500(80) do not level off below the relative pressure of 0.1, indicating the presence of an appreciable amount of mesopores and the pore size is close to the micropore size

(Kruk and Jaroniec, 2001). The isotherm can be defined between type І and type ІV

(Kruk and Jaroniec, 2001; Su et al., 2005 b). This type of isotherm indicates some degree of broadening of the mesopore size distribution (Kruk and Jaroniec, 2001), which was also revealed in the PSD curves shown in Figure 4.1B. Upon sulfonation, the surface area was decreased (see Table 4.1) and the PSD curve became broader (see

Figure 4.1D), suggesting the destruction of ordered mesopores during the sulfonation process. MC700(80) displays a type ІV isotherm with an H2 hysteresis loop (Figure

4.1A), indicating a mesoporous material. And MC700(80) shows a sharp PSD curve

(Figure 4.1B), which indicates narrow pore size distribution. However, after sulfonation reaction, SMC700(80) exhibits an isotherm between type І and type ІV

(Figure 4.1C), indicating broadening of mesopore size distribution, which can be convinced by the PSD curve for SMC700(80) in Figure 4.1D. Moreover, the surface

61 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites area for SMC700(80) also decreased compared with MC700(80). The isothermals,

PSD, surface area results for MC700(80) and SMC700(80) indicate that ordered mesopore structure of MC700(80) reduced during the sulfonation process. Both

MC900(80) (Figure 4.1A) and SMC900(80) (Figure 4.1C) reveal type ІV isotherms with H2 hysteresis loops. The PSD curves for both MC900(80) (Figure 4.1B) and

SMC900(80) (Figure 4.1D) show a sharp peak, a sign for narrow pore size distribution, indicating the ordered mesopores are remained well for SMC900(80) after sulfonation reaction.

1200 5 (A) (B) MC400 1000 4 MC500 /g.STP)

3 MC700 800 3 MC900 600 2 dV/(logD) 400 MC400 MC500 200 MC700 1 Volume Ads. (cm MC900 0 0 0.0 0.2 0.4 0.6 0.8 1.0 1 10 100 Relative Pressure (P/P ) O Pore size (nm)

3 (C) (D) 800 SMC400 SMC500

/g, STP) SMC700 3 600 2 SMC400 SMC900 SMC500 400 SMC700 dV/(logD) SMC900 1 200

Volume Ads. (cm Volume Ads. 0 0 0.0 0.2 0.4 0.6 0.8 1.0 110100 Relative Pressure (P/P ) O Pore size (nm)

62 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

1200 4 (E) (F) SCS400(22) 1000 SCS400(50) 3 SCS400(80) SCS400(120) /g, STP)

3 800

600 2 dV/(logD) 400 SCS400(22) 1 SCS400(50) 200 SCS400(80)

Volume Ads. (cm Ads. Volume SCS400(120) 0 0 0.0 0.2 0.4 0.6 0.8 1.0 110100 Relative Pressure (P/P ) Pore size (nm) 0

Figure 4.1 Nitrogen adsorption-desorption isotherms of mesoporous carbons before sulfonation(A), BJH-PSD curves of mesoporous carbons before sulfonation(B), nitrogen adsorption-desorption isotherms of mesoporous carbons after sulfonation(C), BJH-PSD curves of mesoporous carbons after sulfonation(D), nitrogen adsorption- desorption isotherms of sulfonated carbon-silicate composites (E), BJH-PSD curves of sulfonated carbon-silicate composites (F).

Table 4.1 Texture parameters of samples before and after sulfonation

Sample S a(m2/g) Sample S a (m2/g) BET BET SMC400(50) 8 MC700(80) 1325

SMC400(120) 375 SMC900(80) 1103 SMC400(80) 276 MC900(80) 1467

MC400(80) 661 SCS400(22) 834 SMC500(80) 289 SCS400(50) 754

MC500(80) 1163 SCS400(80) 519 SMC700(80) 1166 SCS400(120) 506

a BET surface area.

Under the same carbonization temperature the specific surface area of sulfonated carbon-silica composite catalysts are greatly enhanced compared with SMC400(80) due to the remaining of template SBA-15. Figure 4.2a shows the hexagonal porous structure of SBA-15. After the infiltration of sucrose and carbonization the carbon lay was coated inside the SBA-15 channel (Figure 4.2b). Through the sulfonation

63 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

reaction–SO3H groups were introduced onto the coated carbon layer (Figure 4.2c). The specific surface area and pore size distribution of sulfonated mesoporous composite catalysts decreased with the increase of the infiltrated sucrose (Table 4.1). SCS400(22) and SCS400(50) (Figure 4.1E) display type ІV isotherms with H4 hysteresis loops

(Zhao et al., 1998). SCS400(80) and SCS400(120) (Figure 4.1E) reveal type ІV isotherms with H2 hysteresis loops (Zhao et al., 1998). SCS400(22) and SCS400(50)

(Figure 4.1F) have the biomodal pore size distribution. One is much smaller than that of template, however, another is similar with that of template, which indicates that some of the pores may not be infiltrated by sucrose at all (Figure 4.3a). It can be concluded that the infiltration of sucrose for SCS400(22) and SCS400(50) is inhomogeneous. The thermogravimetric analysis results operated under air show that carbon and surface groups content for SCS400(22), SCS400(50), SCS400(80) and

SCS400(120) is 10.6%, 23.4%, 38.5%, and 50.6% respectively, indicating the increase of infiltrated sucrose. This conclusion can also be convinced in Figure 4.1F that the pore size distribution decreased with the increase of infiltrated sucrose continuously, which is caused by the increase of carbon layer thickness (Figure 4.3b-d). In

SCS400(120) the volume of added sucrose over-weights the pore volume of SBA-15, and much more sucrose was infiltrated into the pores of SBA-15 than in other three composites, indicating the most thick carbon layer (Figure 4.3d). In the infiltration process due to the overloading of sucrose, some pores of SBA-15 might be fully infiltrated by sucrose (Figure 4.3e). Therefore the pore size distribution of SCS400(120) is the smallest. SCS400(80) shows the most sharp PSD curve, suggesting the narrow pore size distribution, which indicates that the sucrose infiltrated in SCS400(80) most homogeneously.

64 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

H3OS- H3OS- -SO3H -SO3H H3OS- H3OS- -SO3H -SO3H H3OS- H3OS- -SO H -SO3H 3 H OS- Infiltration of H3OS- 3 H3OS- H3OS- H3OS- Sulfonation -SO3H H3OS- -SO3H H3OS- -SO3H H OS- sucrose 3 -SO3H -SO3H -SO3H H3OS- H3OS- H3OS- -SO3H -SO3H -SO3H Carbonization H3OS- H3OS- H3OS- H3OS- H3OS- -SO H 3 H3OS- -SO3H H3OS- -SO3H -SO3H H3OS- H3OS- -SO3H -SO3H H3OS- H3OS-

Figure 4.2 (a) Schematic diagram showing the porous structure of SBA-15 (b) formation of carbon layer inside the SBA-15 channels (c) introduction of –SO3H groups onto carbon layer.

(a) (b) (c) (d) (e)

Figure 4.3 The illustration of sucrose infiltrated into template SBA-15 (a)sucrose was not infiltrated into the template pore at all (b-d) the carbon layer increase with the increase of infiltrated sucrose (e) sucrose was fully infiltrated into the pore of the template.

4.3.2 Materials structure

Figure 4.4 shows the XRD patterns of the sulfonated carbon samples. The peak at

38.5 and 44.8º for all samples can be characterized as the Al holder background. It can be seen that SMC400(80) possesses a very broad and weak diffraction peak at around

25º, which is the (002) reflection of graphitic carbon, indicating a low crystallinity (Su et al., 2005 d; Okamura et al., 2006). The weak diffraction peak also indicates that

SMC400(80) is composed of small polycyclic aromatic carbon sheets (Okamura et al.,

2006). Except for the (002) diffraction peak, SMC700(80) and SMC900(80) both show another weak diffraction peak at around 43º, corresponding to the (101) diffraction of graphitic carbon (Su et al., 2005 d; Okamura et al., 2006), indicating they are

65 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites composed of larger carbon sheets and are more carbonized than SMC400(80) (Hara et al., 2004). The intensity of these two peaks for SMC900(80) is the strongest, indicating the higher crystalline nature of SMC900(80) (Kyotani et al., 2003), which suggests the growth of carbon sheets with the increase of carbonization temperature.

C(002) C(101) (e)

(d)

(c)

(b) Intensity (a.u.)Intensity

(a)

10 20 30 40 50 60 2 Theta (Degree)

Figure 4.4 XRD patterns of instrument background (a), SMC400(80) (b), SMC500(80) (c), SMC700(80) (d), and SMC900(80) (e).

The XPS spectra for resultant catalysts are shown in Figure 4.5. XPS spectra for both sulfonated mesoporous carbon and carbon-silicate composite catalysts exhibit a single 2p peak of S, which is attributed to -SO3H groups at 168.2ev, which is due to aromatic C atom with -SO3H group (Okamura et al., 2006). As mentioned in XRD part, polycyclic aromatic carbon sheets for SMC400(80) are smaller than those of

SMC700(80) and SMC900(80). Because the –SO3H groups can be attached only on the edge of aromatic carbon sheet (Okamura et al., 2006), therefore small aromatic carbon sheets indicates higher –SO3H densities, suggesting lower temperature carbonization process facilitates the –SO3H group attachment.

66 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

(f)

(e)

(d)

(c) Intensity (b)

(a)

172 168 164 160 156 Binding energy/ev

Figure 4.5 S 2p XPS spectra for SMC400(80) (a), SMC500(80) (b), SMC700(80) (c), SMC900(80) (d), SCS400(80) (e), and SCS400(120) (f).

Therefore from Tabel 4.2 we can see that SMC400(80) shows the highest S/C mass ratio in all resultant sulfonated mesoporous carbons. SMC900(80) shows higher S/C mass ratio than SMC700(80), which implies mesopores is benefit for –SO3H group attachment under higher temperature carbonization process. The S/C data for sulfonated mesoporous carbons obtained by elemental analysis (Table 4.2) is higher than the data obtained by XPS investigation (Table 4.3). Because the XPS technique can only detect sample surface compositions, while elemental analysis can detect sample compositions of the whole bulk. Therefore, it can be assumed that –SO3H groups exist both outside and inside the pores of resultant catalysts. It can be concluded that introduction of mesopores can enhance the acid density of the catalysts.

The C content for sulfonated carbon-silica composites is much lower than that of sulfonated mesoporous carbons. It can be seen in Table 4.2 that the C content for carbon-silica composite catalysts increased with the increase of infiltrated sucrose.

SCS400(80) shows the highest S/C mass ratio among the sulfonated composites

67 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites because of the homogeneous infiltration of sucrose, which increase the specific surface area for coated carbon layer based on same carbon mass. The S mass ratio for

SCS400(22) and SCS400(50) is very low, which is caused by the inhomogeneous infiltration due to the small amount of infiltrated sucrose.

Table 4.2 Compositions of samples according to elemental analysis.

-SO H density Sample C (wt %) S (wt %) S(wt %)/C(wt %) 3 (mmol/g) SMC400(50) 53.5 2.1 0.039 0.66 SMC400(120) 54.9 2.6 0.047 0.81 SMC400(80) 54.5 2.6 0.048 0.81 SMC500(80) 65.6 1.9 0.029 0.59 SMC700(80) 73.6 0.7 0.010 0.22 SMC900(80) 73.0 1.4 0.019 0.44 SCS400(22) 5.8 0.2 0.034 0.06 SCS400(50) 11.3 0.4 0.035 0.13 SCS400(80) 21.7 0.9 0.041 0.28 SCS400(120) 27.1 1.0 0.037 0.31

Table 4.3 Surface compositions of samples according to XPS analysis

Sample C (wt %) S (wt %) S(wt %)/C(wt %) SMC400(50) 75.78 1.45 0.019

SMC400(120) 77.59 1.43 0.018 SMC400(80) 76.68 1.46 0.019

SMC500(80) 75.95 1.72 0.022 SMC700(80) 87.75 0.49 0.0056

SMC900(80) 89.15 1.07 0.012

4.3.3 FESEM and TEM observations

The TEM images for SBA-15 in Figure 4.6 a,b show the pore channel both along the

[001] and [100] directions. The presence of hexagonal mesoporous arrays of SBA-15 is evidenced. The TEM images for SMC400(80), SMC500(80) in Figure 4.6 c,d show

68 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites no obvious ordered pore channel tubes. Pore channel tubes both along [001] and [100] directions can be seen for sample SMC700(80) (Figure 4.6e) and sample SMC900(80)

(Figure 4.6f), suggesting that sulfonated carbons prepared under high carbonization temperatures can hold the ordered pore channel tubes of template better than those prepared at low carbonization temperatures.

(a) (b)

(d) (c)

(f) (e)

Figure 4.6 TEM images of SBA-15 silica (a,b), SMC400(80) (c), SMC500(80) (d), SMC700(80) (e) and SMC900(80) (f).

69 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

The TEM images for sulfonated carbon-silica composite catalysts are shown in

Figure 4.7. It can be seen that the pore channels of template are remained well ordered and the pore size decreases with the increase of infiltrated sucrose.

(a) (b)

(c) (d)

Figure 4.7 TEM images of SCS400(22) (a), SCS400(50) (b), SCS400(80) (c) and SCS400(120) (d).

From the FESEM images we can see that both sulfonated carbons and carbon-silica composites can maintain the morphology of SBA-15 well after the sulfonation reaction

(Figure 4.8).

(b) (a)

70 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

(c) (d)

(e) (f)

(g) (h)

Figure 4.8 FESEM images of SMC400(80) (a), SMC500(80) (b), SMC700(80) (c) and SMC900(80) (d), SCS400(22) (e), SCS400(50) (f), SCS400(80) (g) and SCS400(120) (h).

4.4 Catalytic properties in esterification

The catalytic performance of the resultant sulfonic acid catalysts was evaluated using the esterification reaction of methanol with acetic acid. The reaction was carried out in a three-neck round bottom flask containing 1 mol of methanol, 0.1 mol of acetic acid and 0.2 g of resultant catalyst immersed in silicon oil bath controlled at 55 ºC.

After reaction for 5 h liquid phase was analyzed using a gas chromatograph mass spectrometer. After each reaction run, the catalyst was collected by filtration, then

71 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites wased, dried in an oven. The conversions of acetic acid for all resultant samples are shown in Figure 4.9.

The main factor can affect the conversion of acetic acid for resultant sulfonated mesoporous carbon catalysts is the amount of attached –SO3H group. After sulfonation the carbon catalysts contained -SO3H groups, and all S atoms in the catalysts were contained in -SO3H groups (Hara et al., 2004; Takagaki et al., 2006). Therefore, the higher S mass ratio indicates higher conversion of acetic acid within 5 h reaction time for the same mass of added sulfonic acid catalysts.

Compared with SMC700(80) and SMC900(80), SMC500(80) and SMC400(80) show higher S/C mass ratio, which is caused by lower temperature carbonization procedure. Moreover high temperature carbonization process will deduce rigidity of carbon material and make the sulfonated mesoporous carbon catalysts unable to incorporate large amounts of reactant (Okamura et al., 2006; Nakajima et al., 2007), however, SMC400(80) is more flexible. Therefore among the resultant mesoporous carbon catalysts SMC400(80) shows the highest conversion (Figure 4.9). Compared with SMC700(80), SMC900(80) has higher S content and more mesopores which may facilitate the reactant diffusion. Therefore SMC900(80) exhibits higher acetic acid conversion than SMC700(80).

Under the same temperature carbonization process SCS400(80) shows higher conversion than that of SMC400(80). The S/C ratio for SMC400(80) and SCS400(80) is almost the same, however, the active centers of SCS400(80) was located in the thin carbon lay and the active centers of SMC400(80) was located in the thick carbon wall.

Therefore the remaining template of SCS400(80) may help the reactants to access the active centers of SCS400(80) easier. In another word, the active center efficiency of

SCS400(80) is higher than that of SMC400(80).

72 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

100

80 SCS400(80) SMC400(80)

60 SMC500(80) SMC400(50) SMC400(120) SMC900(80)

40 SCS400(120) SCS400(50) SMC700(80) SCS400(22) Conversion ofConversion acetic acid (%) 20

0

Figure 4.9 Conversion of acetic acid over resultant sulfonic acid catalysts.

4.5 Summary

(1) Sulfonated mesoporous carbon catalysts were prepared under different

temperature carbonization processes. While sulfonated carbon-silicate

composite catalysts with different pore size distribution were also prepared

under the same carbonization temperature.

(2) The sulfonated mesoporous carbon catalysts prepared under lower temperature

carbonization processes show better catalytic performances due to the smaller

size of carbon sheets and more surface acid groups. The catalytic performances

73 Chapter 4. Sulfonated Mesoporous Carbons And Carbon-silica Composites

of sulfonated mesoprous carbons prepared under higher temperature

carbonization procedures show a little increase with the continuous increase of

carbonization temperature due to the existence of large amount of mesopores.

(3) The catalytic performance of SCS400(80) is better than that of SMC400(80),

which may be caused by the increase of active center efficiency.

74 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

CHAPTER 5

SULFONATED MESOPOROUS POLYMER RESINS AND

CARBONS

5.1 Introduction

The prepared sulfonated mesoporous carbon and carbon-silica composite catalysts show good catalytic activity over esterification reaction, however, the recyclability is not good. For example the activity of SC400(80) for the second recycle reaction run decreased to 48.2% and the S/C data decreased to 0.028, which should be due to the leaching of polycyclic aromatic hydrocarbons containing –SO3H groups (Mo et al.,

2008).

Moreover, hard template method for preparing mesoporous carbon is fussy and high-cost, therefore new synthesis method is also desired. The synthesis of mesoporous polymer frameworks using the soft template method has been described (Liang et al.,

2004; Meng et al., 2005; Zhang et al., 2005; Liang and Dai, 2006; Zhang et al., 2006).

Simply by carbonization of mesoporous polymer resin, mesoporous carbon can be obtained. In comparison with the hard template method (Su et al., 2005 d; Zhao et al.,

2006), the soft template method for preparing mesoporous carbon is simpler and more cost-effective. A recent study (Xing et al., 2007) showed that sulfonated mesoporous polymer resin materials can be used as a solid acid catalyst with a good catalytic activity and recyclability. However, the investigation of catalytic activity and recyclability for sulfonic acid catalysts based on both mesoporous polymer resins and carbons is limited.

74 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

In this Chapter, mesoporous polymer resins were synthesized, and subsequently converted to mesoporous carbons. Both the mesoporous polymers and carbons were sulfonated to create surface –SO3H groups. The objective of this work is to compare sulfonated polymers and carbons in terms of their catalytic activity, stability, and recyclability when used as estirification catalysts. Experimental results showed that sulfonated mesoporous polymers exhibited a better catalytic performance and recyclability than sulfonated mesoporous carbons.

5.2 Catalyst preparation

The preparation details of sulfonated porous polymers and carbons were described in Chapter 3.2.2. The resultant sulfonated solid was donated as MC(x,y), where x stands for the carbonization temperature and y stands for the sulfonation temperature.

5.3 Characterization of sulfonated mesoporous polymers and carbons

5.3.1 Textural properties

The N2 adsorption-desorption isotherms of sample MC(350) is shown in Figure 5.1.

The isotherms of MC(350) exhibited typical ΙV curve with clear capillary condensation steps which indicates uniform pore size distribution (about 3.4 nm). However, the adsorption and desorption isotherms of MC(350) are not close at low relative pressure, indicating typical isotherms of polymer material, which is caused by the swell of polymer in condensed nitrogen (Zhang et al., 2005; Zhang et al., 2006). Some kinds of polymer material possess large amounts of void space, which is usually referred to as free volume. As nitrogen is condensed within the free volume, the structure of the polymer relaxes allowing access to voids that are inaccessible at lower relative pressures.

75 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

The textural parameters of resultant mesoporous materials are listed in Table 5.1. It can be seen that under low temperature carbonization process (350 to 500 ºC), the pore size distributions, surface areas, and total pore volumes decreased with the increase in carbonization temperature, which is mainly caused by the shrinkage of framework through thermal treatment process (Zhang et al., 2006). The pore size distribution remains decreasing under high temperature carbonization process (600 to 800 ºC), however, the surface area and total pore volume increase.

200

150 /g, STP) 3

100

50

Adsorbed Volume (cm 110100 Pore size (nm) 0 0.0 0.2 0.4 0.6 0.8 1.0

0 Relative pressure (P/P )

Figure 5.1 N2 adsorption-desorption isotherms and pore size distribution of sample MC(350).

Table 5.1 Textural parameters of mesoporous resins and carbons. a 2 c b 3 SBET (m /g) PSD (nm) Vt (m /g) MC(350) 421 3.4 0.26 MC(400) 314 3.3 0.16 MC(500) 287 3.2 0.17 MC(600) 350 3.2 0.20 MC(800) 514 <2 0.26 a BET surface area. b Total pore volume. c Pore size distribution.

76 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

The variation of textural parameters under high temperature carbonization process is due to two possible reasons. First, thermal treatment will cause the shrinkage of framework both under low and high temperature carbonization process (Zhang et al.,

2006). Therefore, the pore size distribution of resultant samples decreased continuously under high temperature carbonization process. Secondly, more small molecules (CO, CO2 and H2O) were released under high temperature carbonization process. From the TG curve we can see that MP shows large weight losses of about

60% (Figure 6.2) from 350 to 400ºC under nitrogen, indicating the removal of template

P123 (Zhang et al., 2006). About 20% weight loss of MP is from 400 to 800ºC, which indicates the release of small molecules. The release of small molecules may form new micropores (Su et al., 2005 a, b,c), which will cause the increase of surface area and total pore volume. After sulfonation, the surface area and pore size distribution of

MC(350,100) decreased compared with MC(350) due to the damage of pore channel and shrinkage of framework during sulfonation reaction (Su et al., 2005 c).

100

80

60

Wt% 40

20

0 200 400 600 800 0 Temperature ( C)

Figure 5.2 TG-DTG curves of MP in nitrogen.

77 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

The XRD patterns and HRTEM images can convince the existence of carbon framework under high temperature carbonization process. Figure 5.3(A) shows the

XRD patterns of MC(500), MC(600) and MC(800). MC(500), MC(600) and MC(800) have two broad peaks at around 25~35º and 40~45º attributed to (002) and (101) planes of graphitic carbon. These two peaks for MC(800) is a little higher than those of

MC(500) and MC(600), indicating the increase of graphene stacking degree caused by the enhancement of carbonization temperature (Jang et al., 2004). The HR-TEM image of MC(800) (Figure 5.3(B)) displays obvious flat graphene carbon sheets of graphitic nature (Fabing Su, 2005 d), implying the growth of carbon sheets. It can be assumed that compact structure-graphite formed during the high temperature carbonization process.

(A) (002)

(101)

(c) Intensity/(a.u.) (b)

(a)

30 40 50 60 2 Theta (Degree)

(B)

Figure 5.3 (A) XRD patterns for (a) MC(500), (b) MC(600) and (c) MC(800); (B)HR- TEM image for MC(800).

78 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

5.3.2 Compositions

The C/H/O molar ratios of MC(350) is 6.2/4.5/1 confirming it’s phenolic resin polymer frameworks (Table 5.2) (Zhang et al., 2006). MC(500), MC(600) and

MC(800) show lower oxygen content compared with MC(350) and MC(400) due to the release of small molecules. The carbon content for sample MC(500), MC(600) and

MC(800) are about 90%, which is much higher than that of MC(350) and MC(400), implying a carbon material (Zhang et al., 2006). In the carbonization process the polymer framework released small molecules through the pyrolysis reaction and aromatic rings connected together to form carbon sheet (Zhang et al., 2006), through which the framework was transferred from polymer to carbon, consistent with the results of XRD and HRTEM.

It is also seen in Table 5.2 that the polymer-based catalysts possess higher sulfur content than the carbon-based catalysts do. This may be due to two reasons. First, with the increase in carbonization temperature, more aromatic rings connected together to form large carbon sheet, resulting in a decrease in the density of –SO3H groups because –SO3H groups are believed to be attached on the edge of carbon sheets

(Okamura et al., 2006). Second, with the increase in carbonization temperature the carbon sheets grew larger and larger, thus the carbon framework became denser and denser, which deduces more compact graphite structure. And it is hard for compact structure to be sulfonated (Hara et al., 2004; Okamura et al., 2006). However, it can be seen from Table 5.2 that for the carbon samples the S content increased with the increase of carbonization temperature under high carbonization temperatures. This is because of the increase of surface area with increasing carbonization temperature.

79 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

Table 5.2 The composition of samples analyzed using elemental analysis.

-SO3H Sample name C(wt%) O(wt%) H(wt%) S(wt%) density (mmol/g) MC(350) 74.3 21.2 4.5 0 0 MC(350,40) 60.9 30.4 4.2 4.5 1.41 MC(350,40) after the 4th 66.5 26.9 4.2 2.4 0.75 reaction run MC(350,100) 56.9 33.5 3.9 5.7 1.78 MC(350,100) after the 62.1 29.6 4.1 4.2 1.31 4th reaction run MC(400) 79.3 16.2 4.5 0 0 MC(400,40) 71.1 23.3 3.9 1.7 0.53 MC(400,40) after the 4th 74.5 21.1 4.2 0.2 0.063 reaction run MC(500) 88.5 7.8 3.7 0 0 MC(500,40) 79.9 16.2 3.3 0.6 0.19 MC(500,40) after the 4th 83.4 12.7 3.5 0.4 0.13 reaction run MC(600) 91.3 7.2 1.5 0 0 MC(600,40) 76.9 19.6 2.3 1.2 0.38 MC(600,40) after the 4th 82.9 14.9 1.9 0.3 0.94 reaction run MC(800) 89.5 9.9 0.6 0 0 MC(800,40) 78.6 18.3 1.1 2.0 0.63 MC(800,40) after the 4th 80.4 15.5 3.3 0.8 0.25 reaction run

The XPS spectra for samples MC(350,40), MC(350,100), MC(400,40) and

MC(800,40) are shown in Figure 5.4. It can be seen that all samples exhibit a single S2p peak at about 168.8 ev, which is assigned to aromatic carbon atoms in connection with

–SO3H groups (Hara et al., 2004). The XPS data showed the presence of –SO3H groups.

80 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

(d)

(c)

(b)

(a)

171 168 165 Binding energy/ev

Figure 5.4 S2p XPS spectra for (a) MC(350,40), (b) MC(350,100), (c) MC(400,40), and (d) MC(800,40).

Figure 5.5 shows the FTIR spectra of selected samples. The peaks at around 1464 and 1623cm-1 are attributed to C=C stretching vibrations whereas the peaks at around

1209 cm-1 is due to the presence of Ar-OH (Xing et al., 2007). The sulfonated samples,

MC(350,100) and MC(350,40), and sample MC(350,100) after the 4th reaction run all display two new peaks at around 1032 and 610 cm-1 (Figure 5.5 b-d), which are assigned to symmetric stretching of S=O and –OH in –SO3H groups (also see Figure

5.6), respectively (Xing et al., 2007). The S-O stretching band is seen at around 704 cm-1 while the peak at round 630 cm-1 is responsible for the C-S stretching vibration

(Figure 5.6) (Sahin et al., 2004). The peaks attributed to Ar-OH for sample MC(800,40)

(Figure 5.7a) is very weak due to removal of surface functional groups (e.g., Ar-OH and -CH2OH) during high temperature carbonization process. The peaks attributed to

C-S, S=O, and S-O stretching vibrations for sample MC(800,40) (Figure 5.7b) are weak, indicating a low density of –SO3H group.

81 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

(d)

(c)

(b) (%)Transmittance

(a)

500 1000 1500 2000 2500 3000 3500 4000 Wavenumber (cm-1)

Figure 5.5 FT-IR spectra of (a) MC(350), (b) MC(350,100), (c) MC(350,100) after the 4th reaction run, and (d) MC(350,40).

50 (A) (B)

45 40 630cm-1 1032cm-1 40

30 -1 704cm 1209cm-1 35 610cm-1 (%)Transmittance (%)Transmittance 20 30 -1 1464cm 1623cm-1 1200 1400 1600 1800 2000 400 600 800 1000 1200 Wavenumber(cm-1) Wavenumber(cm-1) 40 50 (C) (D)

-1 630cm 40 630cm-1 30 704cm-1 704cm-1 610cm-1 30 -1 -1 610cm 20 1032cm (%)Transmittance (%)Transmittance 1032cm-1 20

400 600 800 1000 1200 400 600 800 1000 1200 -1 -1 Wavenumber(cm ) Wavenumbere(cm ) Figure 5.6 FT-IR spectra of (A)MC(350), (B) MC(350,100), (C) MC(350,100) after the 4th reaction run, and (D) MC(350,40).

82 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

(b)

(a) (%)Trantmittance

1000 2000 3000 4000 Wavenumber (cm-1)

Figure 5.7 FT-IR spectra of (a)MC(800), and (b) MC(800,40).

5.3.3 The formation of –SO3H group

Xing (2007) and Okamura (2006) showed that the –SO3H groups are attached on the aromatic C atoms of both polymer and carbon frameworks. This attachment of –SO3H group on aromatic C atom can also be confirmed by XPS and FT-IR results. In the

XPS spectra, the peak attributing to –SO3H group attached on the aromatic carbon can be seen both for polymer and carbon frameworks. The peaks attributes to S=O and S-O stretching vibrations in –SO3H group can be seen in the FT-IR spectra both for polymer and carbon frameworks. The stretching vibration according to C-S confirmed that –SO3H group is attached on the aromatic C atom both for polymer and carbon framework, which is consistent with the XPS results.

Therefore, it can be assumed that –SO3H group was connected to aromatic C atom of polymer framework which is shown in Figure 5.8(a). As mentioned in XPS and

XRD parts, the surface groups release as small molecules and the aromatic rings fuse together to form carbon sheet under carbonization process, however, the –SO3H group can be attached only on the edge of carbon sheet (Okamura et al., 2006). (Figure

5.8(b)). And the carbon sheet will grow with the increase of carbonization temperature

83 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

(Figure 5.9(c)) (Okamura et al., 2006), which will decrease the density of attached –

SO3H group.

OH OH

HOH2C CH 2 CH2OH

SO H 3 (a) SO3H CH2

SO3H SO3H HO

CH2 CH2OH

HOH2C OH

SO3H SO H 3 CH OH HO 2 SO H 3 HOH2C (b) CH2OH

HO S SO3H 3 OH OH HO SO3H

CH2OH carbonization temperature Increasing

SO3H SO3H

SO H HO3S 3

HO CH2OH (c) HO3S

SO3H OH HO3S SO3H

CH2OH SO3H

SO3H

Figure 5.8 The formation of –SO3H group on (a) polymer framework, (b) carbon framework with small carbon sheets, and (c) carbon framework with big carbon sheets.

84 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

5.3.4 FESEM and TEM observations

The prepared MP is composed of irregular dark red pellets with a diameter about 2-

4mm. After carbonization the pellets turn into dark color and obvious shrinkage of the diameter occurs during the carbonization process. The FESEM images of crashed resultant MC(350), MC(350,100) and MC(800,40) were shown in Figure 5.9. The morphology of MC(350) is similar with MC(350,100) and MC(800,40). It is obvious that carbonization and sulfonation processes do not change the morphology of resultant polymer and carbon materials. The pore channel of MC(350,100) can be seen clearly in Figure 5.9(d).

(a) (b)

(c) (d)

Figure 5.9 FESEM images of (a) MC(350), (b) MC(350,100), (c)MC(800,40); HRTEM image of (d) MC(350,100).

85 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

5.4 Catalytic properties in esterification

Figure 5.10 shows the conversion of acetic acid catalyzed by resultant sulfonated materials after different reaction runs. It is seen that catalyst MC(350,100) showed the highest conversion in the first reaction run. After the fourth reaction run, the sulfur content of MC(350,100) decreased from 5.7 wt% to 4.2 wt%, indicating a stable attachment of –SO3H groups. A nafion resin catalyst has been evaluated under the same reaction conditions. The conversion of acetic acid for the first reaction run was about 98%, which is higher than that of the sulfonated catalysts. However, significant swelling phenomenon was observed for the nafion resin catalyst. The swollen nafion resin was crashed into invisible small parts during stirring. Therefore, the recyclability for the sulfonated polymer is better than that of the nafion resin.

It is also seen from Figure 5.10 that the conversion for all sulfonated catalysts decreased with the increase in reaction times. This may be due to two reasons. One is the hydrolysis of –SO3H groups (Petrus et al., 1981; Aragon et al., 1994). When BaCl2 solution was added to the filtrate after catalytic reaction, white precipitate (BaSO4) was found. The other one is the leaching of aromatic carbon sheets attached with –SO3H groups, similar to the leaching of polycyclic aromatic hydrocarbons (Mo et al., 2008;

Tian et al., 2008 a), which was detected using the UV technique. For the filtrate after catalytic reaction, a UV peak at about 250 nm was found for both polymer and carbon- based catalysts (Figure 5.11). This peak is attributed to the decomposition of catalysts during the catalytic reaction.

Sample MC(800,40) exhibited the highest catalytic activity among the sulfoanted carbon-based materials in the first run due to the high sulfur content. However, the conversion of acetic acid in the second run decreased to 40%. Because the micropores of sample MC(800) partially collapsed during the sulfonation process, the attached

86 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons sulfonic acid groups might be trapped inside the sample. The significant drop of conversion in the second run should thus be due to the leaching of collapsed carbon sheets, which attached with sulfonic acid groups. This was detected using the UV method the results are shown in Figure 5.11. Increasing carbonization temperature enhanced the formation of compact carbon structure, which is difficult to be sulfonated

(Hara et al., 2004; Okamura et al., 2006).

For sample MC(600,40), however, no obvious decrease in acetic acid conversion was observed after the second reaction run, indicating the attached –SO3H groups are stable. The main deactivation reason for sample MC(600,40) is thus due to the hydrolysis of –SO3H groups. While MC(600,40) possessed a higher sulfur content than

MC(500,40) did, the conversion of MC(600,40) is lower than that of MC(500,40).

Therefore, it can be assumed that the hydrolysis of –SO3H groups in MC(600,40) is easier than that in MC(500,40).

100 MC(350,40) MC(350,100) MC(400,40) MC(500,40) 80 MC(600,40) MC(800,40)

60

40 Conversion of acetic acid

20 1234 Recycle times

Figure 5.10 Catalytic conversion of acetic acid over resultant sulfonic acid catalyst.

87 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

1 MC(350,40) after the 1st run 2.0 2 MC(350,40) after the 2nd run 3 MC(600,40) after the 1st run 1 4 MC(600,40) after the 2nd run st 1.5 5 MC(800,40) after the 1 run 6 MC(800,40) after the 2nd run

1.0 3 6 Adsorption (a.u) 5 0.5 2

0.0 4 250 300 350 400 Wavelength (nm)

Figure 5.11 UV Absorption spectra for resultant catalysts after reaction run.

5.5 Summary

(1) Sulfonic acid catalysts based on polymer and carbon frameworks were

prepared. Sulfonic acid catalysts based on polymer framework showed the

highest acetic acid conversion and best recyclability.

(2) The aromatic rings of polymer adhered together to form carbon sheets under

carbonization process and the size of carbon sheet increased with the increase

of carbonization temperature. Therefore the framework transferred from

polymer to carbon under carbonization procedure. Graphite structure can be

prepared under high carbonization temperature such as 800ºC.

(3) Big carbon sheet and compact structure may impede the attachment of –SO3H

group. The density and stability of attached –SO3H group on polymer

88 Chapter 5. Sulfonated Mesoporous Polymer Resins and Carbons

framework is higher than those on carbon framework. Therefore the conversion

and recyclability of sulfonic acid catalysts based on polymer framework are

better than those of sulfonic acid catalysts based on carbon framework.

(4) For the sulfonic acid catalysts based on polymer framework higher sulfonation

temperature is benefit for introducing more stable attached –SO3H groups.

89 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

CHAPTER 6

SULFONATED POLYPYRROLE AND CARBON

NANOSPHERES

6.1 Introduction

Work based on last two chapters show that the amount and stability of sulfonic acid groups (-SO3H) attached on polymer and carbon materials are different. In order to furtherly enhance the amount and stability of acid groups, which are responsible for the catalytic performance, other kinds of polymer and carbon materials were employed in this part of thesis work.

Polypyrrole synthesized by polymerization of pyrrole monomer in the presence of an oxidative catalyst is a technologically important, environmentally stable, and biocompatible conducting polymer (Nagarajan et al.). The ultrasonication method was employed to prepare polypyrrole nanospheres (PNs) and carbonized polypyrrole nanospheres (CPNs) (Wang et al., 2008). Sulfonation of both PNs and CPNs was conducted to prepare sulfonated PNs (SPNs) and CPNs (SCPNs). The catalytic properties of the sulfonated materials were evaluated using the esterification of methanol with acetic acid. It was observed that the catalytic performance of the SPNs is better than that of the SCPNs, and a lower sulfonation temperature is benefit for catalytic activity and recyclability of SPNs.

6.2 Catalyst preparation

The preparation details of sulfonated polypyrrole and carbon nanospheres were described in Chapter 3.2.4. The sulfonated polypyrrole spheres (PNs) and carbon

90 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres spheres (CPNs) are designed as SPNs(y) and SCPNs(x,y), respectively, where x stands for the carbonization temperature and y stands for the sulfonation temperature.

6.3 Characterization of sulfonated polypyrrole and carbon nanospheres

6.3.1 FESEM observations

Figure 6.1 shows the FESEM images of representative samples. It can be seen from

Figure 6.1a that the PNs particles are spherical in shape with an average particle size of around 90 nm in diameter. Upon sulfonation at 40 ºC (Figure 6.1b), the particle surface and size are similar to that before sulfonation. When the sulfonation temperature was increased to 150 ºC (Figure 6.1c) to obtain sample SPNs(150), the particles became smaller with an average diameter of about 73 nm, showing that particle shrinkage occurred during the sulfonation process. The particle shrinkage is due to incomplete carbonization during the sulfonation process (Hara et al., 2004). The particle diameters for samples CPNs(400) and CPNs(900) are about 77 and 74 nm, respectively, smaller than that of PNs but larger than that of SPNs(150). Such shrinkage of PNs during carbonization is a result of the denitrogenation, dehydrogenation, and aromatization in the carbonization process (Jang et al., 2004). However, the particle diameters for

SCPNs(400,40), SCPNs(400,150) and SCPNs(900,150) are similar with that of

CPNs(400) and CPNs(900) (Figure 6.1g and h), indicating that sulfonation of CPNs carbonized in the temperature range of 400-900 oC did not lead obvious particle shrinkage.

91 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure 6.1 FESEM images of (a) PNs, (b) SPNs(40), (c) SPNs(150), (d) SCPNs(400,40), (e) SCPNs(400,150), (f) SCPNs (900,150), (g) CPNs(400) and (h) CPNs(900).

92 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

6.3.2 HRTEM observations

Figure 6.2 shows the HRTEM images of SCPNs(400,150) and SCPNs(900,150).

The increase of graphene stacking degree can be detected by the HRTEM technique.

Graphene sheets can be clearly seen on sample SCPNs(900, 150) (see Figure 6.2a). On contrast, no obvious graphene layer can be found on sample SCPNs(400,150) (Figure

6.2b). The increase of graphene stacking degree indicates that SCPNs(900,150) possesses more compact structure than SCPNs(400,150) due to the higher carbonization temperature. However, the compact structure can be hardly sulfonated and the attached -SO3H groups are unstable (Hara et al., 2004).

(a) (b)

Figure 6.2 HRTEM images for (a) SCPNs(900, 150), and (b)SCPNs(400,150).

The XRD patterns of samples PNs and SPNs(40) shown in Figure 6.3 exhibit a broad peak at around 2θ = 25º, indicating an amorphous structure, consistent with the previous report (Chen et al., 2007). The XRD pattern of sample SPNs(150) (Figure

6.3c) shows a peak at 2θ = 25º with a slightly stronger intensity than that of PNs and

SPNs(40), showing a more compact structure because of higher sulfonation temperature, in agreement with the FESEM observation. The two carbonized sample,

SCPNs(400,40) (Figure 6.3d) and SCPNs(400,150) (Figure 6.3e), possess a broad and low peak at around 2θ = 25º, which is assigned to the (002) reflection of carbon

93 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres material, indicating the low crystalline. The peak for SCPNs(900,150) (Figure 6.3f) is higher than that of SCPNs(400,40) and SCPNs(400,150), indicating the increase of graphene stacking degree caused by the enhancement of carbonization temperature

(Jang et al., 2004).

(f)

(e)

(d)

(c) Intensity (a.u.)

(b)

(a) 10 20 30 40 50 60 70 80 2 Theta (Degree)

Figure 6.3 XRD patterns of (a) PNs, (b) SPNs(40), (c) SPNs(150), (d) SCPNs(400,40), (e) SCPNs(400,150), and (f) SCPNs(900, 150).

6.3.3 XPS analysis

Shown in Figure 6.4A are the N1s XPS spectra of samples PNs, CPNs(400),

CPNs(900), and SCPNs(900,150). The N1s spectrum of sample PNs depicted in Figure

6.3A-a shows one peak at a BE of 399.1 ev, corresponding to the N atoms within the pentagonal pyrrole rings of the Ppy (Wang et al., 2008). However, the N1s spectra of samples CPNs(900) and SCPNs(900,150) display two peaks, one at about 397.8 ev, corresponding to pyridine-type N atoms at the edge of graphene sheets, and the other one at about 400.7 ev, associated with the quaternary N atoms incorporated in graphene sheets (Su et al., 2004; Hou et al., 2005; Su et al., 2006; Wang et al., 2008).

These two peaks for sample CPNs(400) had a little shift compared with that of

CPNs(900) and the peak according to N atoms at the edge of the graphene sheets is weaker than that of CPNs(900), indicating more graphene sheets exist in CPNs(900)

94 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres than in CPNs(400). The above XPS results suggest that during the carbonization process pentagonal pyrrole rings had transferred to hexagon carbon rings, which are the primary building units of graphene sheet. At the same time, the size of graphene sheets increased and much more graphene sheets stacked closely to form a compact structure with the increase of carbonization temperature.

It can be assumed that the graphitization degree increases with the increase in carbonization temperature and the backbone of the resultant material was transformed from polymer to carbon during the carbonization process.

Shown in Figure 6.4B are the S2p XPS spectra for sample SPNs(40) before and after

th the 4 reaction run. Both of the XPS spectra exhibit a single S2p peak at 168 ev, attributed to S atoms in -SO3H (Hara et al., 2004), showing the presence of -SO3H groups in the samples. It can be seen that the S2p peak for SPNs(40) is higher than that

th of SPNs(40) after the 4 reaction run, indicating the decrease of -SO3H groups after the 4th reaction run.

(A) 400.7 (B) 168 (d) 397.8 400.7 397.8 (b) (c) 400.1 168 y ty i

398.4 ntens I Intensit 399.1 (b) (a) (a)

410 405 400 395 175 170 165 160 Binding energy/ev Binding energy/ev

Figure 6.4 (A) N1s XPS spectra (a) PNs, (b) CPNs(400), (c) CPNs(900), and (d) SCPNs(900,150) (B) S2p XPS spectra (a) SPNs(40) and (b) SPNs(40) after the 4th reaction run.

95 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

6.3.4 FT-IR spectra

Figure 6.5 shows the FTIR spectra of samples. The peak at around 1548 cm-1 (also see Figure 6.6) is attributed to the C=C stretching vibration of pyrrole (Sahin et al.,

2004). The peaks at 1708 and 1300 cm-1 are assigned to C=N and C-N vibrations in

Ppy (Sahin et al., 2004). Sample SPNs(40) before and after the 4th reaction run all exhibit a peak at around 1044cm-1 (see Figure 6.6), which is due to the symmetric

O=S=O stretching vibrations, showing the presence of -SO3H groups. The peaks at around 798 and 650cm-1 are responsible for S-O and C-S stretching vibrations, respectively (Sahin et al., 2004). After the 4th reaction runs, catalyst SPNs(40) still possessed these peaks, indicating a good catalyst recyclability of SPNs(40).

For samples CPNs(400), SCPNs(400,40) and SCPNs(400,40) after the 4th reaction run the peak for C=C also can be seen from Figure 6.5 and 6.6. However, the peaks attributed to C=N and C-N are not obvious. The peaks at 1044, 798, and 650 cm-1 attributed to O=S=O, S-O, and C-S, respectively, for sample SCPNs(400,40) can also be seen clearly from Figure 6.5e. However, these three peaks are much weaker for sample SCPNs(400,40) after the 4th reaction run (see Figure 6.5f), indicating the loss of the –SO3H group.

(f)

(e) (d)

(c) (%) Transmittance (b) (a)

1000 2000 3000 4000 Wavenumber (cm-1)

Figure 6.5 FT-IR spectra of (a) PNs, (b) SPNs(40), (c) SPNs 40 after the 4th reaction run, (d) CPNs(400), (e) SCPNs(400,40), (f) SCPNs(400,40) after the 4th reaction run.

96 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

(A) (B)

650cm-1 650cm-1

1548cm-1 -1 (%)Transmittance

1548cm (%) Transmittance 798cm-1 -1 798cm 1044 cm-1 1044cm-1 1000 2000 1000 2000 -1 -1 Wavenumber(cm ) Wavenumber(cm ) (C) (D)

650cm-1 650cm-1

-1 1548cm-1 1548cm

-1 (%)Transmittance -1

(%) Transmittance (%) 798cm 798cm -1 1044cm 1044cm-1 1000 2000 1000 2000 -1 -1 Wavenumber(cm ) Wavenumber(cm )

Figure 6.6 FT-IR spectra of (A) SPNs(40), (B) SPNs(40) after the 4th reaction run, (C) SPNs(400,40), and (D) SPNs(400,40) after the 4th reaction run.

6.3.5 The formation of –SO3H group

Sahin and Okamura have proved that –SO3H groups are attached on the C atom of carbon sheets both on the pentagonal pyrrole rings (Aragon et al., 1994) and hexagon carbon rings (Okamura et al., 2006). This attachment of –SO3H group on C atom can be confirmed by the FTIR spectra. The FTIR spectra of SPNs(40) (Figure 6.5b) and

SCPNs(400,40) (Figure 6.5e) show the S-O and O=S=O stretching vibrations, indicating the existence of –SO3H group. The stretching vibration of C-S for SPNs(40) and SCPNs(400,40) were also detectable, confirming that -SO3H groups were attached on the C atom both on the pentagonal pyrrole rings (Figure 6.7a) and on the hexagon carbon rings (Figure 6.7b).

97 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

As discussed above, the pentatonal pyrrole rings were transferred to the hexagon carbon rings, resulting in the bulk of catalysts transforming from polymer to carbon during the carbonization process. The compact graphene structure appeared under the higher-temperature carbonization process. The -SO3H groups can be attached only on the edge of carbon sheet (Okamura et al., 2006). However, the carbon sheet will grow with the increase in the carbonization temperature as shown in Figure 6.7b and c.

Therefore, the S content decreased with the increase in the carbonization temperature

(Table 6.1).

O O O S O O S O O S OH 4 3 H O 5 2 N 1 H N N N H n H n n H n (a)

OH O S O O O HO O HO S S S OH O O O O S OH O O HO O Increasing carbonization S HO S O temperature O O O S OH S OH O O O O O O S OH OH S HO S S O H O O O O S O O OH O S O OH

(b) (c)

Figure 6.7 The formation of –SO3H groups on (a) a pentagonal pyrrole ring, (b) a small carbon sheet and (c) a big carbon sheet.

98 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

Table 6.1 Elemental compositions of samples analyzed using the CHN-S technique

-SO3H Sample C (wt %) N (wt %) O (wt %) S (wt %) H(wt%) density (mmol/g) PNs 47.0 14.6 35.3 0 2.9 0 SPNs(40) 40.8 13.1 34.9 8.0 3.2 2.5 SPNs(40) after 0.69 47.1 11.6 35.4 2.2 3.7 the 4th run SPNs 55 42.5 13.5 33.7 6.9 3.4 2.16 SPNs(55) after 1.38 47.4 14.3 30.6 4.4 3.3 the 4th run SPNs(70) 42.0 13.2 34.9 6.9 3.0 2.16 SPNs(70) after 1.66 46.0 13.5 32.0 5.3 3.2 the 4th run SPNs(150) 47.3 13.3 34.2 2.6 2.6 0.81 SPNs(150) after 0.34 48.7 13.3 34.3 1.1 2.5 the 4th run CPNs(400) 62.7 17.4 26.4 0 2.5 0 SCPNs(400,40) 54.0 15.0 26.4 2.2 2.4 0.69 SCPNs(400,40) 0.34 55.8 15.3 25.2 1.1 2.6 after the 4th run SCPNs(400,150) 49.8 12.9 32.8 1.8 2.7 0.56 SCPNs(400,150) 0.19 52.3 13.7 30.6 0.6 2.8 after the 4th run CPNs(900) 58.5 4.1 36.4 0 1.0 0 SCPNs(900,150) 72.6 5.5 18.9 2.0 1.0 0.63

6.4 Catalytic properties in esterification

The catalytic properties of both SPNs and SCPNs were evaluated using the reaction of esterification of methanol with acetic acid. The reaction was carried out in a three- neck round bottom flask immersed in a silicon oil bath at 55 ºC. 1.0 mol of methanol,

0.1 mol of acetic acid and 0.2 g of catalyst were added to the flask and the reaction was carried out by 5 h. After each reaction run, the catalyst was collected by filtration, washed, and dried at 60 oC in an oven. The catalytic performances are shown in Figure

6.1.

99 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

According to Okamura et al. (2006), sulfonated carbons contain -SO3H groups, and all S atoms exist in the -SO3H groups. Therefore, based on same mass of added catalyst and defined reaction time, the more the S content and the more stable of -

SO3H groups the catalysts have, the higher conversion of acetic acid and the better the recyclability should obtain. It can be seen from Table 6.1 that the S content for prepared catalysts decreased with the increase of sulfonation and carbonization temperatures. Therefore, the conversion and recyclability for the polymer-based catalysts decreased with the increase in sulfonation temperature, while the conversion and recyclability for the carbon-based catalysts decreased with the increase in both carbonization and sulfonation temperatures.

It is also seen from Table 6.1 that the S content detected using the CHNS method is higher than that measured using the XPS technique (see Table 6.2), indicating that -

SO3H groups were attached on both the surface and bulk of the samples. Comparing the S contents in Table 6.1 and Table 6.2, one can see that after the 4th reaction run the decrease on the bulk S content is almost equal to that of the surface S content expect for sample SPNs(40). Therefore, it can be concluded that the decrease in the S content is mainly caused by the loss in the surface-attached -SO3H groups.

Sample SPNs(40) shows the highest S content, which accounts for the highest conversion (Figure 6.8). While the S content for SPNs(40) after the 4th reaction run decreased greatly, its recyclability was the best among the catalysts studied. On the other hand, although the S contents of samples SPNs(55) and SPNs(70) after the 4th reaction run were higher than that of SPNs(40), their recyclabilities were worse than that of SPNs(40). As discussed above, particle shrinkage occured during the sulfonation and carbonization processes at relatively high temperatures. Those -SO3H groups attached in shrinked bulk spheres are hardly accessible to the reactants

100 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

(Okamura et al., 2006), thus causing the observed bad recyclability. However, the -

SO3H groups attached on both surface and bulk of sample SPNs(40), which has a more flexible structure due to the lower sulfonation temperature, can be easily reached by the reactants.

Table 6.2. Surface compositions according to XPS analysis and surface areas of samples.

a Sample C (wt %) N (wt %) O (wt %) S (wt %) SBET (m2/g) PNs 83.4 7.1 9.5 0 12.4 SPNs(40) 81.0 3.2 13.9 1.9 9.5 SPNs(40) after the 4th run 81.3 4.2 13.0 1.6 16.9 SPNs 55 73.0 7.3 15.6 4.1 13.5 SPNs(55) after the 4th run 81.9 4.1 12.5 1.5 13.7 SPNs(70) 72.9 8.3 14.9 3.9 13.9 SPNs(70) after the 4th run 81.0 4.9 12.1 1.9 22.1 SPNs(150) 81.3 3.8 13.0 1.8 12.9 SPNs(150) after the 4th run 82.7 3.9 13.1 0.3 16.9 CPNs(400) 80.1 11.0 9.0 0 20.3 SCPNs(400,40) 78.7 5.8 13.0 2.5 10.6 SCPNs(400,40) after the 4th run 81.0 6.7 11.4 0.9 18.0 SCPNs(400,150) 78.4 6.8 14.0 0.9 27.6 SCPNs(400,150) after the 4th run 81.4 5.4 13.2 0.1 20.9 CPNs(900) 89.7 3.2 7.1 0 25.4 SCPNs(900,150) 86.9 4.0 8.2 0.9 22.4 a BET surface area.

The carbon-based catalysts, SCPNs(900,150), displayed the lowest S content among all of the CPNs samples, showing it was hard to be sulfonated. While both

CPNs(400,150) and CPNs(900,150) possessed similar bulk and surface S contents (see

Table 6.1), the conversion and recyclability of CPNs(900,150) were worse than that of

CPNs(400,150), indicating the unstable attachment of the -SO3H group on

CPNs(900,150). Therefore, it can be concluded that graphene sheets are hard to be sulfonated and the -SO3H groups attached on graphene sheets are unstable (Hara et al.,

101 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

2004). SCPNs(400,40) showed a better conversion and recyclability than

SCPNs(400,150) because of the higher sulfonation temperature used in preparing the latter.

100 SPNs(40) SPNs(55) 80 SPNs(70) SPNs(150) SCPNs(400,40) 60 SCPNs(400,150) SCPNs(900,150) 40

20

Conversion of acetic aicd(%) 0 1234 Recycle times

Figure 6.8 Catalytic conversion of acetic acid over various catalysts.

From Figure 6.8 we can also see that the conversion decreased with the recycle times. For the polymer-based catalysts there are two main reasons for the deactivation.

One is the hydrolysis of -SO3H groups (Petrus et al., 1981; Aragon et al., 1994), which can be detected by using BaCl2. The hydrolysis of sulfonate groups could be detected by BaSO4 sediment. The reaction mixture of polymer-based catalyst SPNs(40) was filtrated and collected, then followed by adding it into the BaCl2 solution. The white sediment was found in solution, which indicates the leaching of sulfonate groups. The same sediment was also found in the filtrated reaction mixture when using carbon- based SCPNs(900,150) as catalyst. Therefore, it may be convinced that both for polymer- and carbon-based sulfonic acid catalysts, hydrolysis of sulfonate groups is one of the reasons for deactivation.

102 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

The other one is the leaching of polycyclic pentatonal pyrrole rings, similar to the leaching of polycyclic aromatic hydrocarbons (Mo et al., 2008), which can be detected by using the UV technique. The decomposition of catalyst could be detected by UV-

VIS spectra. The mixture of pyrrole monomer, methanol, acetic acid, and methylacetate shows a UV peak at around 253nm. And the filtrated reaction mixtures for SPNs(40) after the 1st, 2nd, 3rd and 4th reaction run show a peak at around 265nm, which had a little shift compared with that of pyrrole monomer mixture (Figure 6.9). It can be assumed that polypyrrole did not decompose into pyrrole monomer. However, the UV spectra of filtrated reaction mixture for SPNs(900,150) after the 1st reaction shows negligible peak, suggesting the trace amount of decomposition of carbon based catalyst.

(B)

(A) Absorption (a.u)

(D) (E) (C) (F) 250 300 350 400 Wavelength (nm)

Figure 6.9 UV-VIS spectra: (A) mixture of pyrrole monomer, methanol, acetic acid and methylacetate; (B) filtrated reaction mixture of SPN(40) after the 1st reaction run; (C) filtrated reaction mixture of SPNs(40) after the 2nd reaction run; (D) filtrated reaction mixture of SPN(40) after the 3rd reaction run; (E) filtrated reaction mixture of SPN(40) after the 4th reaction run; (F) filtrated reaction mixture of CPNs(900,150) after the 1st reaction run.

103 Chapter 6. Sulfonated Polypyrrole And Carbon Nanospheres

For the carbon-based sulfonic acid catalysts, the main reason for the deactivation is the hydrolysis of -SO3H groups. From the data of surface area for resultant sample

(Table 6.2) we can see that the surface area of prepared polymer and carbon based catalysts decreased after sulfonation. And then increased after the 4th reaction run except for sample SCPNs(400,150). Therefore, the pore blockage is not the main reason for deactivation.

6.5 Summary

(1) The sulfonated polypyrrole nanospheres and carbonized polypyrrole

nanospheres were prepared under different kinds of sulfonation and

carbonization temperature.

(2) For the polymer-based solid catalysts, high temperature sulfonation reaction

was an incompletely carbonization process that would deduce compact

structure and this compact structure impeded the reach of reactant to the bulk

active sites. Therefore, SPNs under low sulfonation temperature showed the

best conversion of acetic acid and recyclability.

(3) For carbon-based acid catalysts, the stability of attached -SO3H groups

decreased with the increased of carbonization and sulfonation temperature

since high temperature carbonization process can introduce compact graphene

structure.

(4) The low carbonization and sulfonation temperature may be more helpful in

enhancing the conversion and recyclability of solid acid catalysts derived from

polypyrrole in the esterification reaction of methanol with acetic acid.

104 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

CHAPTER 7

SULFONATED POLYSTYRENE-DIVINYLBENZENE

SPHERES

7.1 Introduction

Several kinds of sulfonic acid catalysts based both on polymer and carbon materials were investigated in the last three chapters. Due to the higher quality and more stability of attached –SO3H groups on polymer based catalysts compared with carbon based catalysts, polymer based catalysts show better catalytic activity and recyclability.

Moreover, introducing sulfonic acid (–SO3H) groups on organic-based materials is more desirable for enhancing the catalytic activity because the organic-based materials in general exhibit a hydrophobic feature (Iimura et al., 2003 a).

Sulfonated polystyrene catalysts show excellent performance in many chemical reactions, due to their high concentrations of acid sites. For example, sulfonic acid catalyst based on polystyrene showed higher yield for the hydrolysis of dodecyl thiolaurate compared several other Brönsted acids (such as sulfuric acid, hydrochloric acid) (Iimura et al., 2003 b). The catalytic activity of alkylated polystyrene based sulfonic acid catalysts were affected by the polymer structure. Sulfonated alkylated polystyrene with long alkyl chain showed best catalytic performance (Iimura et al.,

2003 b). The catalytic activity for hydration of propene using sulfonated polystyrene as catalyst increased with the increase of sulfonation level. And the thermal stability also improved with the enhancement of sulfonation degree (Hart et al., 2001; Hart et al.,

2002). Esterification involved carboxylic acids with alcohols were carried out by using sulfonated alkylated polystyrene as solid acid catalyst (Manabe and Kobayashi, 2002).

105 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

And the highly hydrophobic nature of the resultant polymer based sulfonic acid catalyst played a very important role for activity in the esterification reaction.

In this work, linear-linked polystyrene spheres were synthesized using the emulsifier-free emulsion polymerization method (Wang et al., 2006). By adding divinylbenzene, cross-linked polystyrene-divinylbenzene spheres were also synthesized using the same method. By changing the mass ratio of styrene over divinylbenzene, cross-linked polystyrene-divinylbenzene spheres of different compositions were synthesized. Both the linear-linked polystyrene spheres and cross- linked polystyrene-divinylbenzene spheres were subsequently sulfonated to create –

SO3H groups. It was observed that the sulfonated cross-linked polystyrene- divinylbenzene spheres showed a more stable catalytic activity than the sulfonated line-linked polystyrene spheres did. The sulfonated cross-linked polystyrene- divinylbenzene spheres prepared with a moderate mass ratio of styrene over divinylbenzene and sulfonation temperature exhibited the highest catalytic activity and best recyclability.

7.2 Catalyst preparation

The preparation details for sulfonated polystyrene spheres was described in Chapter

3.2.4. The resultant sulfonated solids were donated as SPS-DVB(x,y), where x stands for the volume ratio of DVB/Styrene and y stands for sulfonation temperature.

7.3 Characterization of sulfonated polystyrene spheres

7.3.1 MAS NMR spectra

Figure 7.1 shows the 13C NMR spectra of cross-linked polystyrene-divinylbenzene and line-linked polystyrene spheres before and after sulfonation reaction. Resonances

106 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres at 41 and 46 ppm for line-linked polystyrene spheres PS(0) were assigned to methylene and methine carbon respectively. At the same time resonances at 146 and 128 ppm were ascribed to non-protonated and protonated aromatic carbons (Martins et al., 2007).

The same chemical shifts could be found for cross-inked polystyrene-divinylbenzene spheres PS(0.8). After sulfonation reaction both sample SPS(0,40) and SPS(0.8,80) show one more 13C NMR signal with chemical shift at about 138 ppm, which was attributed to sulfonated aromatic carbon (Canovas et al., 2006; Martins et al., 2007).

(A)

d

* c *

138

* b *

128 41 146 46 * a * 250 200 150 100 50 0 ppm

S S O O S S O O 3 H 3 H (B) 3 H 3 H

CH-CH CH-CH CH-CH CH-CH 2 2 2 2

107 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

S O 3 H

(C) CH-CH2 CH2-CH CH-CH2 CH-CH2

S S O O S 3 H 3 H O 3 H

CH-CH CH-CH CH-CH2 2 2

S S O O 3 3 H H

Figure 7.1 (A) 13C MAS NMR spectra for (a) PS-DVB(0.04), (b) SPS- DVB(0.04,80), (c) PS-DVB(0), and (d) SPS-DVB(0,40) (*spinning side bands) (B) Sulfonated linear-linked polystyrene (C) Sulfonated cross-linked polystyrene-divinylbenzene.

It is seen that the chemical shifts of SPS-DVB(0,40), assigned to methylene and methine carbon are less obvious compared with PS(0), which indicates that the local structure of line-linked polystyrene spheres is affected by the sulfonation reaction. On the contrary, resonances for methylene, methine and aromatic carbon of cross-linked polystyrene-divinylbenzene spheres SPS-DVB(0.04,80) were slightly influenced by the presence of sulfonic acid group, which indicated the well maintenance of the polymer backbone after sulfonation process.

The –SO3H groups were introduced to line-linked and cross-linked polystyrene spheres by aromatic sulfonation reaction, through which the sulfonic acid groups were attached onto the aromatic carbon rings (Figure 7.1 B, C).

108 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

7.3.2 FT-IR spectra

Figure 7.2 shows the FT-IR spectra of resultant sulfonic acid catalysts. All of them exhibit the same characterized group peaks. The peaks located around 750 and 699cm-1 are originated from the benzene groups of polymer backbone (Bae et al., 2006). The broad characteristic band at 1215cm-1, due to the S=O asymmetirc stretching vibrations of the –SO3H groups that increases in width and intensity with the sulfonic content

(Canovas et al., 2006). At the same time, the brand at 1033cm-1 is assigned to the S=O symmetric stretching vibration in the -SO3H group (Mahdjoub et al., 2005). The peak appeared at 1126cm-1 is corresponding to the vibrations of the sulfonic acid groups attached to the benzene rings (Bae et al., 2006), and at 1007cm-1 corresponding to those of the benzene groups containing the sulfonic acid groups (Bae et al., 2006). The results of FT-IR convince the existence of –SO3H groups on the resultant polymer catalysts, and the attachment of –SO3H groups on the aromatic rings of polymer.

(i)

(h)

(g)

(f)

(e)

(%)Trensmittance (d) (c) (b) (a)

500 1000 1500 2000 Wavenumber(cm-1)

Figure 7.2 FT-IR spectra of (a)SPS-DVB(0.4,40)(b) SPS-DVB(0.04,60) (c) SPS- DVB(0.04,80) (d) SPS-DVB(0.04,100) (e) SPS-DVB(0.02,40) (f) SPS- DVB(0.06,40) (g) SPS-DVB(0.1,40) (h) SPS-DVB(0,40) (i) SPS-DVB(0.2,40).

109 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

7.3.3 Elemental and titration analysis

The elemental compositions of the resultant sulfonic acid catalysts are listed in

Table 7.1. The S content for resultant samples increased with the increase of adding divinylbenzene first, however, decreased with the continuous adding of divinylbenzene.

And the S content also increased with the increase of sulfonation temperature. High S content indicates large amount of attached –SO3H groups and high acid density for sulfonated polymer catalysts. It can be assumed that the by adding of divinylbenzene and increase of sulfonation temperature can facilitate the attachment of –SO3H groups.

Divinylbenzene is more reactive than styrene, which will deduce the variety of cross-link density for the resultant crosslink polymer spheres (Ding et al., 1992). It is possible that the resultant cross-linked polystyrene-divinylbenzene spheres have tightly cross-linked cores, however, more lightly cross-linked or linear-linked polystyrene chains are attached, which causes a core-shell structure (Ding et al., 1992). This can be confirmed by the result of TEM images (Figure 7.3). From Figure 7.3 we can see that the transparency of the outside of spheres is higher than that of the core of the spheres, which indicates the different density. It can be concluded that little amount of divinylbenzene improve the stability of polymer during the sulfonation reaction process and increase the number of attached -SO3H groups. While with the continuous adding of divinylbenzene the number of attached -SO3H groups decreased due to the tighten core-shell structure.

The acid amount determined by titration showed the same trend as the result of elemental analysis. However, the acid amount based on the S content (listed in Table

7.1) is higher than the data investigated by titration (Table 7.2) due to the incompletely

+ ion exchange of the H cation in –SO3H groups.

110 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

Table 7.1 Compositions of samples analyzed using elemental analysis.

Sample C H S O -SO3H (wt%) (wt%) (wt%) (wt%) density (mmol/g) SPS-DVB (0,40) 67.41 6.45 6.63 19.51 2.07

SPS-DVB (0.02,40) 59.77 4.32 15.13 20.78 4.73

SPS-DVB (0.02,40) after the 4th 59.06 5.68 8.88 26.38 reaction run SPS-DVB (0.04,40) 57.98 6.32 10.20 25.5 3.19

SPS-DVB (0.04,40) after the 4th 57.79 6.20 9.65 26.36 reaction run SPS-DVB (0.04,60) 40.61 5.50 15.80 38.09 4.94

SPS-DVB (0.04,60) after the 4th 43.11 5.35 15.06 36.48 reaction run SPS-DVB (0.04,80) 41.10 3.83 15.73 39.34 4.92

SPS-DVB (0.04,80) after the 4th 43.01 5.59 14.93 36.47 reaction run SPS-DVB (0.04,100) 39.64 5.01 15.35 40.00 4.80

SPS-DVB (0.04,100) after the 4th 42.50 4.04 13.07 40.39 reaction run SPS-DVB (0.06,40) 64.77 6.64 7.52 21.07 2.35

SPS-DVB (0.06,40) after the 4th 68.45 6.50 6.82 18.23 reaction run SPS-DVB (0.1,40) 70.27 6.75 6.28 16.70 1.96

SPS-DVB (0.1,40) after the 4th 71.00 6.75 5.96 16.29 reaction run SPS-DVB (0.2,40) 71.47 6.91 6.29 15.33 1.97

SPS-DVB (0.2,40) after the 4th 72.19 6.76 5.60 15.45 reaction run

111 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

(a) (b)

(c) (d)

Figure 7.3 TEM images of (a)SPS-SVB(0.04,40)(b)SPS-DVB(0.1,40)(c)SPS- DVB(0.2,40)(d)SPS-DVB(0.2,40).

Table 7.2 The titration results.

Added NaOH(0.01M) Acid amount (-SO H) Sample 3 sample(g) (ml) (mmol/g) SPS-DVB (0,40) 0.0555 7.55 1.36

SPS-DVB (0.02,40) 0.0522 8.93 1.71

SPS-DVB (0.04,40) 0.0514 11.10 2.16

SPS-DVB (0.04,60) 0.0515 16.70 3.24

SPS-DVB (0.04,80) 0.0519 17.17 3.31

SPS-DVB (0.04,100) 0.0529 17.93 3.39

SPS-DVB (0.06,40) 0.0538 8.20 1.52

SPS-DVB (0.1,40) 0.0529 6.32 1.19

SPS-DVB (0.2,40) 0.0538 6.85 1.27

112 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

7.3.4 TGA analysis

It can be seen in TG curves (Figure 7.4 (A)) that all prepared sulfonated polymer spheres show two transitions of loss in weight in two separate temperature ranges. The first weight loss can be ascribed to the decomposition of -SO3H group (Gao et al.,

2003). However, before sulfonation reaction all of them show only one transition of loss in weight attributing to the degradation of the main polymer chain (Figure 7.4 (B)).

The theoretical weight changes caused by the decomposition of -SO3H group (Table

7.4) are calculated by assuming the decomposition reaction produces SO3 (Gao et al.,

2003). The observed weight loss values for SPS-DVB(0,40) is much higher than that of the theoretical one, which indicates that the main polymer chain degradation occurred in the first temperature range along with the decomposition of -SO3H group.

For SPS-DVB(0.02,40), SPS-DVB(0.04,40), SPS-DVB(0.04,60), SPS-

DVB(0.04,80), and SPS-DVB(0.04,100), the determined weight loss values are much lower than those of the theoretical ones, which suggests that not all attached -SO3H groups have been released when the polymer main chain begins to degrade. For SPS-

DVB(0.06,40), SPS-DVB(0.1,40) and SPS-DVB(0.2,40) the determined weight loss is little lower than those of theoretical ones, implying the degradation of polymer main chain in the first transition range. Compared with SPS0-DVB(0,40), SPS-

DVB(0.02,40)and SPS-DVB(0.04,40) show great difference between theoretical weight loss and determined one, which implies the thermal stability of attached -SO3H groups for cross-linked sulfonated polymer catalysts increased by the adding of divinylbenzene. However, the thermal stability of attached -SO3H groups for SPS-

DVB(0.06,40), SPS-DVB(0.1,40) and SPS-DVB(0.2,40) decreased with the continuous adding of divinylbenzene. SPS-DVB(0.04,60), SPS-DVB(0.04,80) and

SPS-DVB(0.04,100) show the biggest difference between theoretical and determined

113 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

weight loss, suggesting the improvement of -SO3H groups stability with the increase of sulfonation temperature.

(A) (B)

(i) (f) (h) (g) (e)

(f) (e) (d)

(d) (c) Weight/100% Weight/100%

(c) (b) (b)

(a) (a)

200 400 600 800 200 400 600 Temperature(0C) Temperature (0C)

Figure 7.4 (A)TG curves of (a)SPS-DVB(0,40)(b)SPS-DVB(0.02,40) (c) SPS- DVB(0.04,40) (d) SPS-DVB(0.04,60) (e) SPS-DVB(0.04,80) (f) SPS- DVB(0.04,100) (g) SPS-DVB(0.06,40) (h) SPS-DVB(0.1,40) (i) SPS- DVB(0.2,40);(B)TG curves of (a)PS(0) (b) PS(0.02) (c) PS(0.04) (d)PS(0.06) (e) PS(0.1) (f) PS(0.2).

Table 7.3 Decomposition temperatures of samples. Sample Weight change caused Decomposition Degradation by decomposition of – temperature of temperature of SO3H group –SO3H group the main polymer (ºC) chain (ºC) Theoretical Determined

SPS-DVB(0,40) 16.7% 61.7% 200-426 426-660

SPS-DVB(0.02,40) 37.8% 17.6% 200-426 426-900

SPS-DVB (0.04,40) 25.5% 13.6% 200-426 426-700

SPS-DVB (0.04,60) 39.5% 14.8% 200-426 426-900

SPS-DVB (0.04,80) 39.3% 11.6% 200-426 426-900

SPS-DVB (0.04,100) 38.3% 8.2% 200-426 426-800

SPS-DVB (0.06,40) 6.6% 7.3% 200-400 400-850

SPS-DVB (0.1,40) 5.9% 6.8% 200-385 385-680

SPS-DVB (0.2,40) 5.9% 6.7% 200-390 390-700

114 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

7.3.5 FESEM images

Figure 7.5 shows the FESEM images of resultant polymer spheres. It can be seen that the diameter of polymer spheres are not uniform with the adding of divinylbenzene. Table 7.4 listed the preparation parameters and diameter of resultant polymer spheres, which decreased with the increase of divinylbenzene. Because the adding of DVB will cause the varity of cross-link density for resultant spheres, therefore the more tighly cross-linked spheres show the smaller diameter. The spheres diameter for SPS-DVB(0,40) decreased greatly after sulfonation reaction compared with PS(0). However, for other cross-linked polymers spheres the spheres diameter increased a little after sulfonation reaction. It can be assumed that cross-linked polymer spheres swell a little during the sulfonation reaction process.

(a) (b)

(c) (d)

115 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

(e) (f)

Figure 7.5 FESEM images of (a) PS(0)(b) PS(0.02) (c) PS(0.4) (d) PS(0.06) (e) PS(0.1) (f) PS(0.2).

Table 7.4 The preparation parameters and diameter of polymer spheres.

Sulfonation Styrene Amount DVB KPS Diameter Sample temperature (g) (ml) (g) (nm) (ºC) PS(0) 19.6 0 0.14 1518 SPS-DVB 40 205 (0,40) PS(0.02) 19.6 0.4 0.14 603 SPS-DVB 40 628 (0.02,40) PS(0.04) 19.6 0.8 0.14 446 SPS-DVB 40 441 (0.04,40) SPS-DVB 60 500 (0.04,60) SPS-DVB 80 465 (0.04,80) SPS-DVB 100 494 (0.04,100) PS(0.06) 19.6 1.2 0.14 456 SPS-DVB 40 480 (0.06,40) PS(0.1) 19.6 2.4 0.14 405 SPS-DVB 40 450 (0.1,40) PS(0.2) 19.6 4.8 0.14 184 SPS-DVB 40 205 (0.2,40)

116 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

7.4 Catalytic properties in esterification

The catalytic performance for resultant sulfonic acid catalysts was evaluated using the esterification reaction of methanol with acetic acid. The reaction was carried out in a round bottom flask immersed in a silicon oil bath controlled at 55 ºC. 1 mol of methanol, 0.1 mol of acetic acid and 0.2 g of solid catalyst were added into the flask.

After reaction for 5 h, liquid phase was analyzed using a gas chromatograph mass spectrometer. The used catalyst was collected, washed, and dried at 60 ºC for the next reaction cycle.

Figure 7.6 shows the catalytic performance of resultant sulfonic acid catalysts. All sulfonated cross-linked polystyrene-divinylbenzene spheres show stable recyclability after the 4th reaction run. However, the reaction system for SPS-DVB(0,40) became a gel after the 1st reaction run due to the dissolve of SPS-DVB(0,40) in the reaction process. Therefore the stability for sulfonated cross-linked polystyrene-divinylbenzene spheres is much better than that of the sulfonated linear-linked polystyrene spheres during this esterification reaction.

Moreverover, the same mass of commercial strong acid catalyst Amberlyst-15 was also evaluated at the same reaction conditions. However, the conversion of acetic acid for Amberlyst-15 is 58%, which is lower than that of sulfonated cross-linked polystyrene spheres. Therefore, the prepared sulfonic acid catalysts show the better catalytic performance than the commercial one.

The acetic acid conversion for SPS-DVB(0.02,40) and SPS-DVB(0.04,40) increased with the adding of divinylbenzene, while the conversion for SPS-DVB(0.06,40) and

SPS-DVB(0.1,40) decreased with the continuous adding of divinylbenzene. The result of acetic acid conversion is consistent with the result of S content, which is high S content indicating high conversion.

117 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

100

80 SPS-DVB(0.04,40) SPS-DVB(0.04,60) SPS-DVB(0.04,80) SPS-DVB(0.04,100) SPS-DVB(0.02,40)

Conversion of acetic acid (%) SPS-DVB(0.06,40) SPS-DVB(0.1,40) SPS-DVB(0.2,40) 60 1234 Recycle times

Figure 7.6 Catalytic conversion of acetic acid over resultant sulfonted polymer catalysts.

7.5 Summary

(1) With the adding of divinylbenzene different kinds of cross-linked polystyrene

spheres were prepared. Sulfonic acid groups were introduced onto the resultant

polystyrene spheres to produced solid sulfonic acid catalysts.

(2) The thermal stability of sulfonated cross-linked polystyrene-divinylbenzene

spheres increased with the increase of added divinylbenzene and then decrease

with the continuous adding of divinylbenzene. The same trend was shown in

the acidity.

(3) With the continuous adding of divinylbenzene the hard core-shell structure

inhibited the attachment of sulfonic acid groups, which caused the decrease of

acetic acid conversion.

118 Chapter 7. Sulfonated Polystyrene-Divinylbenzene Spheres

(4) The appropriate increase of sulfonation temperature is benefit for the amount

and stability of attached –SO3H group, which is helpful for improvement of the

conversion and recyclability.

119 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres CHAPTER 8

KINETICS AND MECHANISM OF ESTRIFICATION

REACTION OVER SULFONATED POLYSTYRENE-

DIVINYLBENZENE SPHERES

8.1 Introduction

Polystyrene supported sulfonic acid resins are widely used as acid catalysts in many kinds of petrochemical reactions, such as production of ethers (Rehfinger and

Hoffmann, 1990; Iborra et al., 1992; Paakkonen and Krause, 2003; Boz et al., 2004), alkylation (Sharma, 1995), alkene hydration (Ihm et al., 1988; Hart et al., 2001; Hart et al., 2002; Tejero et al., 2002), esterification (Jerabek et al., 1997; Xu and Chuang,

1997; Liu and Tan, 2001; Sanz et al., 2002; Sanz et al., 2004) and other reactions

(Harmer and Sun, 2001; Yadav and Bhagat, 2005). Organic esters attract more attention, which are widely used in the manufacturing of perfumery, flavors, pharmaceuticals, plasticizers, polymerization monomers, emulsifiers in the food and intermediates. The most popular synthesis method is direct esterification of carboxylic acids with alcohols in the presence of acid catalysts. It has been reported that when using these traditional liquid catalyst, the rate limiting step of the reaction is the nucleophilic attack of the alcohol on the protonated carbonyl group of the carboxylic acid (Ronnback et al., 1997; Lotero et al., 2005; Liu et al., 2006). The first mechanism step involves protonation of the carbonyl oxygen on the carboxylic group. Second step is the nucleophilic attack of the alcohol to yield a tetrahedral intermediate. Third step, a proton is lost at one oxygen atom and gained at another to form another intermediate,

120 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres which further loses a molecule of water that gives a protonated ester. In the final step, a proton is transferred to a water molecule to give the ester (Figure 8.1).

OH O OH+ H+ R -OH + 1 R R R C OH OH OH

+ OH + O HO -H2O -H HO OH + OH 2 R R R ROH+ O O O

R1 R1 R1 R1

Figure 8.1 Mechanistic route of acid catalyzed esterification reaction.

Compared with liquid acid catalysts, which cause many inevitable problems such as corrosive, toxic, separation, and solid waste, solid acid catalysts used in esterification reaction are more desired. The reaction mechanisms of esterification reaction catalyzed by different solid acid catalysts were reported. In the esterification of acetic acid with amyl alcohol using acid resin catalysts a dual-site model was concluded (Teo and Saha,

2004). However, for the esterification of hexanoic acid with 1-octanol using SAC-13 yielded an E-R (Eley-Rideal) kinetic model (Nijhuis et al., 2002).

Sulfonated cross-linked polystyrene-divinylbenzene spheres, which contain high acid sites, were prepared in our work. This synthesized sulfonated cross-linked polystyrene-divinylbenzene spheres show high acetic acid conversion and recyclability in the esterification reaction of methanol with acetic acid. Referred to investigation of the initial kinetic study work finished by Liu and co-workers (Liu et al., 2006), the initial reaction rate was investigated in our experiment. Reaction mechanism was also detected by the pyridine adsorbing method. Furthermore the kinetic modeling was

121 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres carried out by the curve fitting technique. The inhibition behavior of water for esterification reaction was also detected.

8.2 Results and Discussion

8.2.1 Effect of polymer swelling and stirring speed

In our experiment, reagents were heated to the desired temperature before the reaction. Once the desired temperature was reached, the esterification reaction was started by adding the catalyst.

Sulfonated cross-linked polystyrene-divinylbenzene spheres are polymer based catalyst, therefore, swelling of the polymer in polar solvent is known to occur (Liu et al., 2006). As we known even slight catalyst swelling could affect catalytic activity because during swelling additional acid sites can be exposed, which will cause unaccurate kinetic measurement. In order to detect whether swelling play a role in initial reaction rate study, sulfonated cross-linked polystyrene-divinylbenzene spheres were presoaked in polar methanol overnight before the reaction, because as we known acetic acid has much less capability to swell the polymeric resin than methanol (Arnett et al., 1986; Liu et al., 2006). And esterification reaction was then initiated by charging the acetic acid. In another hand, the initial reaction rate was investigated by starting the reaction with adding of the catalyst, which is not presoaked in methanol. The initial reaction rate was listed in Table 8.1. And we can see that catalyst swelling due to methanol had a negligible effect on the initial reaction rate.

Table 8.1 Initial reaction rate by using catalyst with/without swelling in methanol

CA0(mol/L) CM0(mol/L) r0(mol/L*min)*100 With swelling in methanol 10 10 1.0 overnight Without swelling in methanol 10 10 1.1

122 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres Reactions under different reaction stirring speed were carried out to access the effect of reaction stirring speed, which was shown in Figure 8.2. Because the particle size of resultant sulfonated cross-linked polystyrene-divinylbenzene spheres is about 465nm.

Therefore the catalyst can be dispersed in the reactant mixtures homogeneously due to the small particle size of catalyst. It can be seen that no obvious change was found in initial reaction rate under different reaction stirring speed, even under statistical reaction condition. In our experiment the catalyst was not prepared in pellets, therefore, the investigation of the effect for particle size and internal mass transfer on initial reaction rate was not involved.

10

5 (mol/L*min)*100 0 r 0 0 400 800 1200 Stirring rate (rpm)

Figure 8.2 Initial reaction rates versus different reaction stirring rate.

8.2.2 Effect of reactant concentration

Apparent reaction orders were determined by varying the concentration of one reactant while fixing that of the other and measuring initial kinetics at 55 ºC. The results are given in Table 8.2. The concentrations of both acetic acid and methanol had positive affects on the reaction rate with increasing reactant concentration. The esterification reaction scheme of methanol with acetic acid was shown in Figure 8.3.

And the reaction power law was listed in 8.1. Using a power law approximation, the apparent reaction orders were determined to be 0.8 for methanol and 1.0 for acetic acid.

123 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres

H H O H H catalyst H C OH + H C C H C O C C H + H2O H OH H H O H

Figure 8.3 Esterification reaction for methanol with acetic acid.

α β r0 = k0CA0CM 0 (8.1)

Table 8.2 Initial reaction rate for the determination of apparent reaction orders of acetic acid and methanol in sulfonated cross-linked polystyrene-divinylbenzene spheres catalyzed esterification at 55ºC.

CA0 (mol/L) CM0 (mol/L) r0 (mol/L*min)*100

10 10 9.8800 3.85 19.23 7.8385 2.17 21.74 5.0935 1.36 22.73 3.2127 15.15 3.03 7.5152 15.82 1.90 5.2373 14.20 4.26 9.6875

8.2.3 Proposed reaction mechanism

In order to detect whether the adsorption/desorption of acetic acid was comparable to or slower than the surface reaction, experiments using sulfonated cross-linked polystyrene-divinylbenzene spheres presaturated with acetic acid before initiating the reaction by charging preheated methanol were carried out (Table 8.3). The results for esterification at 55 ºC showed no improvement in the initial reaction rate compared with experiments using the catalyst with no preadsorbed acetic acid. It is noted that the initial reaction rates determined for these batch runs are smaller than the previous runs.

That is because the amount of solid acid catalyst added for these batches are slightly

124 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres less (~ 1mg less). From this observation, it shows that the rate of reaction is very sensitive to the amount of solid catalyst.

Moreover, the adsorption of methanol is considered. It is based on the consideration that methanol is a strong nuclephile and it also competes for the acid site (Liu et al.,

2006). As mentioned in the polymer swelling part preadsorbed methanol also had a negligible effect on the initial reaction rate. Therefore the surface reaction should be the rate-limiting step due to the rapid reactant adsorption/desorption equilibrium.

Table 8.3 Initial reaction rates with/without pre-adsorption in acetic acid.

CA0(mol/L) CM0(mol/L) r0(mol/L*min)*100 Pre-adsorption in acetic acid 10 10 7.3 overnight Without pre-adsorption in acetic acid 10 10 8.0

Two reactants were involved in our esterification reaction, methanol and acetic acid.

Therefore pyridine adsorbing experiments were conducted to determine whether single active site or dual active sites involved in the esterification reaction. Before each run, the catalyst was first immersed in one reactant-methanol, containing a known amount of pyridine. This immersion lasted overnight to allow complete equilibrated adsorption of the organic base on the acid sites of the catalyst. The reactor was heated to 55 ºC, and then preheated acetic acid was charged into the reactor to initiate the reaction. As shown in Figure 8.4, the initial reaction rates decreased linearly with increasing amounts of pyridine, suggesting that single active site involves in the heterogeneous reaction. Otherwise a non-linear figure should be obtained if dual active sites participated in the esterification reaction.

125 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres Table 8.4 Pyridine adsorption experiment.

-5 CA0(mol/L) CM0( mol /L) CPyridine ( mol /L)*10 r0 ( mol /L*min)*100 10 10 0 9.8800 10 10 3.0 7.8385 10 10 5.7 5.0935 10 10 10.0 3.2127

1.0

0.8

0.6

Initial rate (M/min*10) 0.4

0.0 0.5 1.0 1.5 2.0 2.5 C (M*1000) pyridine

Figue 8.4 Pyridine adsorbed sulfonated cross-linked polystyrene-divinylbenzene spheres catalyzed esterification of acetic acid with methanol at 55ºC.

The mechanism rout of esterification reaction calalyzed by sulfuric acid (Figure 8.1) shows that the oxygen on the carboxylic acid is protonated, thereby activating nucleophilic attach by alcohol. Therefore in our heterogeneous esterification reaction it can be assumed that acetic acid is protonated first, which is attacked by methanol.

From the result of pyridine adsorbing experiment it can be concluded that only single active site was involved in our heterogeneous esterification reaction. Therefore the

126 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres surface reaction should be the nucleophilic attack of protonated acetic acid by methanol forming ester and water. Hence, rapid reactant adsorption/desorption equilibrium with surface reaction as the rate-limiting step is likely the initial reaction mechanistic paths under our reaction conditions, which were shown in 8.2-4.

k1 A+S A S (8.2) k-1 k2 M+S M S (8.3) k-2 k A S + M 3 Product (8.4)

Where A represents acetic acid, M represents methanol, S is a vacant acid site on the solid catalyst surface. A⋅ S and M ⋅ S are molecules adsorbed on the catalytic acid site, respectively.

8.2.4 Modeling the kinetics

Figure 8.5 shows the conversion of acetic acid catalyzed by sulfuric acid and sulfonated cross-linked polystyrene-divinylbenzene spheres at 55 ◦C respectively. The amount of active sites for both sulfuric acid and sulfonated cross-linked polystyrene- divinylbenzene spheres were kept to be the same. It can be seen that the homogeneous acid catalyst shows similar acetic acid conversion as the heterogeneous catalyst within

10 min reaction time. However, after 6 hr the acetic acid conversion for H2SO4 is higher than that of sulfonated cross-linked polystyrene-divinylbenzene spheres. Both catalysts exhibit the same trend for the whole reaction run process.

Initial catalytic activity for sulfuric and sulfonated cross-linked polystyrene- divinylbenzene spheres was calculated in this way:

Product (mol)/[reaction time (min)*amount of active sites (mol)]

• Product: Methylacetate (mol)

127 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres • Reaction time: 10 min for both catalysts

• Amount of active sites: For sulfonated polystyrene the amount of active

sites was calculated based on the titration analysis result, 3.31*10-3mol H+/g

polystyrene; for sulfuric acid the amount of active sites was assumed 1mol

+ H /1mol H2SO4.

Based on the above definition the initial catalytic activity for sulfonated polystyrene spheres and sulfuric acid is 28 min-1 and 27 min-1 respectively.

As we known water is a byproduct of esterification reaction for methanol with acetic acid, which can inhibit the reaction rate. For sulfuric acid catalyzed esterification reaction for methanol with acetic acid the deactivating effect of water was due to the water solvation of the protons. The loss in acid strength of catalytic protons due to water solvation leads to a decrease in the concentration of protonated carboxylic acid, thus inhibiting the formation of esters (Liu et al., 2006). The sulfuric acid and sulfonated cross-linked polystyrene-divinylbenzene spheres had the similar water deactivation profile (Figure 8.6).

30

20

H SO 2 4 10 Sulfonated cross-linked polystyrene-divinylbenzene spheres Conversion (%)

0 0 100 200 300 400 Reaction Time (min)

Figure 8.5 Acetic acid conversion vs time for esterification reaction catalyzed by H2SO4 and sulfonated cross-linked polystyrene-divinylbenzene spheres at 55ºC.

128 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres

Sulfonated cross-linked 1.0 polystyrene-divinylbenzene spheres H SO 2 4 0.8 max

r/r 0.6

0.4

0.2 01234 Water concentration (mol/L)

Figure 8.6 Water sensitivity of esterification reaction for methanol with acetic acid at 55ºC on (■) sulfonated cross-linked polystyrene-divinylbenzene spheres and on (●) H2SO4.

Based on the experimental observed kinetic data, it is proposed that the esterification catalyzed by solid acid catalyst proceeds via a mechanism analogous to the homogeneous catalyzed one. By considering the initial reaction period when reverse hydrolysis is not important, the initial set of mechanistic paths are as mentioned in 8.2-

4:

k1 A+S A S (8.2) k-1

k 2 M+S M S (8.3) k-2

k A S + M 3 Product (8.4)

Using the pseudo-steady-state approximation for the adsorption steps ( A⋅ S and

M ⋅ S ) with surface reaction as the rate limiting step, the initial reaction rate can be expressed as:

129 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres

k3K AC A0CM 0CS 0 r0 = (8.5) K AC A0 + K M CM 0 +1

Where k3 is the surface reaction constant, CA0, CM0 and CS0 are the concentrations of

acetic acid, methanol and the total acid sites, respectively. KA (K A = k1 / k−1 ) and KM

( K M = k2 / k−2 ) are the adsorption equilibrium constants of acetic acid and methanol, respectively.

Using equation 8.5 and the initial kinetic data measured at various initial reactant compositions, a mathematical model was developed by curve fitting method using

Matlab.

Employing least squares fitting algorithm, the kinetic parameters KA and KM are estimated to be 0.2 (L/mol) and 0.5 (L/mol), respectively. Figure 8.7 and Figure 8.8 showed the fitting results by plotting 1/r0 vs 1/CM0 and 1/r0 vs 1/CA0, respectively. As can be seen, the mathematical model derived can successfully predict the initial rate data.

35 exp. data fitting data 30

25

20 1/r0 15

10

5

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 1/CM0

Figure 8.7 Comparison of experimental data with predicted data derived from the mathematical model by plot of 1/r0 vs 1/CM0.

130 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres

40 exp.data 35 fitting data

30

25

20 1/r0

15

10

5

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1/CA0

Figure 8.8 Comparison of experimental data with predicted data derived from the mathematical model by plot of 1/r0 vs 1/CA0.

As discussed earlier, the presence of water can seriously inhibit the catalysis reaction. Hence, the effect of water must be included in the kinetic expression. The initial reaction mechanistic paths with the presence of water are:

k1 A+S A S (8.6) k-1

k2 M+S M S (8.7) k-2

k + 3 A S M Product (8.8)

kW W+S W S (8.9) k-W

where W represents water and W ⋅ S is the adsorbed water. Defining KW = kw / k−w as the adsorption equilibrium constant for water, the rate expression for the presence of initial water is:

131 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres

k3K ACA0CM 0CS 0 r0 = (8.10) K ACA0 + K M CM 0 + KW CW 0 +1

Since KA and KM are determined previously, hence, the only parameter in equation

8.10 is the adsorption equilibrium constant for water KM. Rearrange the above equation

8.10, we get:

K ACA0 + K M CM 0 +1 KW 1/ r0 = + CW 0 (8.11) k3 K ACA0CM 0CS 0 k3K AC A0CM 0CS 0

KW can be easily obtained from the plot of 1/r0 vs Cw0 as 4.1 (L/mol). As can be seen from Figure 8.9, the model can accurately assess the dependency of reaction rate with the presence of initial water, supporting the estimated water inhibition effect on solid acid catalyst.

55

50

45

40

35

30

1/r0 (min/M) 25

20

15

10

5 0 1 2 3 4 5 6 7 8 9 10 Water concentration (Cw0)

Figure 8.9 Dependency of initial reaction rate on the initial water concentration for sulfonated cross-linked polystyrene-divinylbenzene spheres.

132 Chapter 8. Kinetics and Mechanism of Esterification Reaction over Sulfonated Polystyrene-Divinylbenzene Spheres 8.3 Summary

(1) For the esterification reaction of acetic acid with methanol catalyzed by

sulfonated cross-linked polystyrene-divinylbenzene spheres both reactants had

positive affects on the reaction rate with increasing reactant concentration.

Using a power law approximation, the apparent reaction orders were

determined to be 1.0 for methanol and 0.8 for acetic acid.

(2) Initial reaction rates for pre-pyridine adsorbed sulfonated cross-linked

polystyrene-divinylbenzene spheres decreased linearly with increasing amounts

of pyridine, suggesting that single active site involves in the heterogeneous

reaction.

(3) Using the pseudo-steady-state approximation for the adsorption steps with

surface reaction as the rate-limiting step and curve fitting technique, the

adsorption equilibrium constants of acetic acid, methanol, and water are 0.2,

0.5, and 4.1 L/mol respectively.

(4) The initial rate decreased with the increase of added water concentration,

supporting the estimated water inhibition effect on solid acid catalyst, which is

also confirmed by the curve fitting method.

133 Charpter 9. Conclusions and Recommendations

CHAPTER 9

CONCLUSIONS AND RECOMMENDATIONS

9.1 Conclusions

Solid sulfonic acid catalyst is a promising and potential replacement for the liquid sulfuric acid. The subject of this thesis work is to find out sulfonic acid catalysts, which have high acid density and good stability, and investigate the relationship between catalyst material structure and catalytic performance.

In this thesis work several kinds of sulfonic acid catalysts were prepared and the conversion of acetic acid and recyclability over esterification reaction with methanol and acetic acid were evaluated. The acid density and conversion of acetic acid for prepared sulfonic acid catalysts were listed in Table 9.1. For sulfonated mesoporous carbons and carbon-silica composites the conversion of acetic acid is high, but the recyclability for these kinds of acid catalyst is not good due to the leaching of active sites. Polymer-based materials (such as polypyrrole and mesoporous phenol resin) and the corresponding carbon-based materials were chosen in the following part of this thesis work. It was found out that the amount and stability of attached –SO3H groups for polymer-based materials are better than carbon-based materials. As a result, the conversion of acetic acid and recyclability of polymer-based sulfonic acid catalyst is better than carbon-based ones. Moreover, linear-linked and cross-linked polystyrene- divinylbenzene spheres were synthesized. The sulfonated cross-linked polystyrene- divinylbenzene spheres show the highest conversion of acetic acid and stable recyclability, which give us a potential choice for the replacement of sulfuric acid.

134 Charpter 9. Conclusions and Recommendations

Table 9.1 Acid density and conversion of acetic acid for prepared catalysts.

Conversion of acetic Conversion of acetic -SO H density Sample 3 acid for the 1st acid for the 4th (mmol/g) reaction run (%) reaction run (%) SMC400(80) 0.81 83 ~ SCS400(80) 0.28 92 ~ MC(350,100) 1.78 82.6 64.6 MC(500,40) 0.19 47.3 68.3 SPNs(40) 2.5 90.5 23.7 SCPNs(400,40) 0.69 76.6 24.9 SPS-DVB(0.04,80) 4.92 99 93.5

Carbonization and sulfonation temperature, which affect the amount and stability of attached sulfonic acid groups, were investigated. In addition, the transformation of catalysts material structure under sulfonation and carbonization process was discussed.

Furthermore, the initial kinetics of esterification reaction over methanol with acetic acid catalyzed by cross-linked polystyrene spheres were reported. The conclusions that have been drawn from this thesis work are summarized below.

• Sulfonated mesoporous carbons and carbon-silica composites, which possess

smaller size of carbon sheets and sufficient surface acid groups, show higher

acetic acid conversion in esterification reaction of acetic acid with methanol.

And large amount of ordered mesopores is benefit for improving the catalytic

activity. Sulfonation and carbonization processes play important roles on the

performance of resultant catalysts. The catalytic performance of sulfonated

carbon-silicate composite catalysts was greatly increased compared with that of

sulfonated mesoporous carbon catalysts due to the increase of active center

efficiency.

• Sulfonated mesoporous phenol resin catalyst showed the higher acetic acid

conversion and best recyclability compared with mesoporous carbon based

sulfonic acid catalysts. The aromatic rings of polymer adhered together to form

135 Charpter 9. Conclusions and Recommendations

carbon sheets under carbonization process and the size of carbon sheet

increased with the increase of carbonization temperature. Therefore the

framework transferred from polymer to carbon under carbonization procedure.

Graphite structure can be prepared under high carbonization temperature such

as 800ºC. Big carbon sheet and compact structure may impede the attachment

of –SO3H group. The density and stability of attached –SO3H groups on

polymer framework are better than those on carbon framework. Therefore the

conversion and recyclability of sulfonic acid catalysts based on polymer

framework are better than those based on carbon framework. For the sulfonic

acid catalysts based on polymer framework higher sulfonation temperature is

benefit for introducing more stable attached –SO3H groups.

• For sulfonated polypyrrole nanospheres catalyst, high temperature sulfonation

reaction was an incompletely carbonization process that would deduce compact

structure and this compact structure impeded the reach of reactant to the bulk

active sites. Therefore, sulfonated polypyrrole nanospheres (SPNs) under low

sulfonation temperature showed the best conversion and recyclability. For

carbon-based acid catalysts, the stability of attached -SO3H groups decreased

with the increased of carbonization and sulfonation temperature since high

temperature carbonization process can introduce compact graphene structure.

The low carbonization and sulfonation temperature may be more helpful in

enhancing the conversion and recyclability of solid acid catalysts derived from

polypyrrole in the esterification reaction of methanol with acetic acid.

• With the adding of divinylbenzene the catalytic stability for sulfonated cross-

linked polystyrene-divinylbenzene spheres was greatly enhanced compared

with sulfonated line-linked polystyrene spheres. However, with the continuous

136 Charpter 9. Conclusions and Recommendations

adding of divinylbenzene the hard core-shell structure inhibited the attachment

of sulfonic acid groups, which caused the decrease of acetic acid conversion.

The appropriate increase of sulfonation temperature are benefit for the amount

and stability of attached –SO3H groups, which are helpful for improvement of

the catalytic performance.

• For the esterification reaction of acetic acid with methanol catalyzed by

sulfonated cross-linked polystyrene-divinylbenzene spheres both reactants had

positive affects on the reaction rate with increasing reactant concentration. And

single active site was involved in the heterogeneous reaction. Sulfuric acid and

sulfonated cross-linked polystyrene-divinylbenzene spheres exhibit the same

trend for the whole reaction run process and water deactivation profile. And the

mechanistic route involved in esterification reaction catalyzed by sulfonated

cross-linked polystyrene-divinylbenzene spheres resembles that of H2SO4. The

adsorption equilibrium constants of acetic acid, methanol, and water are 0.2,

0.5, and 4.1L/mol respectively. Initial rate decreased with the increase of added

water concentration, which indicates the inhabitation behavior of water.

9.2 Recommendations

The present work suggests that solid sulfonic acid catalysts show quite promising results for the esterification reaction of methanol with acetic acid. The quality and stability of attached –SO3H groups play a significant role for improving activity as well as recyclability over esterification reaction. Compared with polymer based sulfonic acid catalysts carbon material based sulfonic acid catalysts can stand for high temperature reaction, because the carbon material was normally produced by high temperature carbonization process. However, the number of attached –SO3H groups is

137 Charpter 9. Conclusions and Recommendations

limited and the stability of supported –SO3H groups is not good. For polymer based sulfonic acid catalysts the number and stability of attached –SO3H groups are greatly improved. Therefore polymer based solfonic acid catalysts show higher activity and recyclability in the esterification reaction. However, polymer backbones are not suitable for high temperature reaction, which limits their application. Therefore, much work is still need to be conducted to find out high performance sulfonic aicd catalyst material and efficient way for introducing –SO3H groups. Some recommendations are proposed in following section:

• Polymer based sulfonic acid catalysts show high catalytic activity and

recyclability over esterification reaction of methanol with acetic acid.

Therefore polymer based material is a good choice for sulfonic acid catalyst

support. In Chapter 8 we can see that with the addition of divinylbenzene the

catalytic performance of cross-linked polystyrene-divinylbenzene sulfonic acid

catalyst was greatly improved as well as the thermal stability. Therefore

copolymer is a good choice for sulfonic acid catalyst support.

• Carbon based materials possess good thermal stability, which is benefit for

chemical reactions. In our experiment through the same sulfonation method the

amount and stability of attached –SO3H groups for carbon based materials are

not as good as those of polymer based materials. Therefore new methods,

which could introduce –SO3H groups onto carbon based materials more

effectively, are desired.

138 References

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151 Appendix

APPENDIX

List of publications coming from this thesis work

Papers published (or accepted) in international referred journal

(1) Tian XN, Su F and Zhao XS. Sulfonated polypyrrole nanospheres as a solid acid cataslyst. GreenChemistry, 2008, 10, 951-956.

(2) Xiao Ning Tian, X.S. Zhao. Sulfonated mesoporous carbons as a solid sulfonic acid catalyst. Studies in Surface Science and Catalysis, 2008, Volume 174, Part 2, 1347-

1350.

(3) Su F, Zhou Z, Guo W, Liu J, Tian XN and Zhao XS. Template Approaches to

Synthesis of Porous Carbon. Chemistry and Physics of Carbon, Vol. 30, Ed: L. R.

Radovic, 2008, Vol. 30, 63-128;

(4) Liu J, Tian XN, Su F, Lv L and Zhao XS. Hydrogenation of glucose over Ru nanoparticles embedded in templated porous carbon. Australian Journal of Chemistry

2009, in press.

Papers submitted to international referred journal

(5) Tian XN, Zhang Lili, Bai Peng, and Zhao X.S. Sulfonation of mesoporous polymer and carbon. Submitted to Microporous and Mesoporous Materials, 2009.

(6) Tian XN, Zhang Xinhui, Wang Likui, and Zhao X.S. Sulfonated Polystyrene as

Strong sulfonic acid catalyst. Submitted to Industrial & Engineering Chemistry

Research, 2009.

152