· first edition 2014 ©hadi nur 2014 hak cipta terpelihara. tiada dibenarkan mengeluar ulang...

First Edition 2014©HADI NUR 2014

Hak cipta terpelihara. Tiada dibenarkan mengeluar ulang mana-mana bahagianartikel, ilustrasi, dan isi kandungan buku ini dalam apa juga bentuk dan cara apa juasama ada dengan cara elektronik, fotokopi, mekanik, atau cara lain sebelum mendapatizin bertulis daripada Timbalan Naib Canselor (Penyelidikan & Inovasi), UniversitiTeknologi Malaysia, 81310 UTM Johor Bahru, Johor Darul Ta'zim, Malaysia.Perundingan tertakluk kepada perkiraan royalti atau honorarium.

All rights reserved. No part of this publication may be reproduced or transmittedin any form or by any means, electronic or mechanical including photocopying,recording, or any information storage and retrieval system, without permissionin writing from Deputy Vice-Chancellor (Research & Innovation) UniversitiTeknologi Malaysia, 81310 UTM Johor Bahru, Johor Darul Ta'zim, Malaysia.Negotiation is subject to royalty or honorarium estimation.

Perpustakaan Negara Malaysia Cataloguing-in-Publication Data

Particuology of Some Metal Oxides Catalysts / Editors : Hadi NurIncludes IndexISBN 978-983-52-0967-31. Photocatalysis. 2. Catalysis. I. Hadi Nur.541.395

Editor: HADI NURPereka Kulit / Cover Designer: SITI NORULHANA MISKAN

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Faculty of ScienceUNIVERSITI TEKNOLOGI MALAYSIA,

81310 UTM Johor Bahru,

Diterbitkan di Malaysia oleh / Published in Malaysia byPENERBIT UTM PRESS

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x Preface

CONTENTS

List of Contributors Preface

vii ix

Chapter 1 Aligned Titanium Dioxide Catalyst Synthesized under Magnetic Field 1 Sheela Chandren, Nursyafreena Attan and Hadi Nur

Chapter 2 Particuology of Tungsten Oxide as Visible Light-Driven Photocatalyst 15

Leny Yuliati

Chapter 3 Synthesis of Mesoporous Silica Catalyst by a Nanoscopic Template 29 Hendrik O. Lintang

Chapter 4 Particuology of Metal Oxides in Bifunctional Catalyst Design

45

Siew Ling Lee, Jamilah Mohd Ekhsan and Yee Khai Ooi

Index 61

LIST OF CONTRIBUTORS

Hadi Nur Hendrik O. Lintang

Jamilah Mohd Ekhsan Leny Yuliati

Nursyafreena Attan Sheela Chandren

Siew Ling Lee Yee Khai Ooi

Ibnu Sina Institute for Fundamental Science Studies Universiti Teknologi Malaysia

x Preface

PREFACE

This book describes some of the interesting topics in heterogeneous catalysts and photocatalysts. The book reviews four research topics that include the new synthesis strategies in preparation of solid catalysts and photocatalysts. The heterogeneous catalysts and photocatalysts are important due to the decisive advantage of heterogeneous catalysis, such as the easy separation of catalyst and substrates or products just after reaction which makes it possible to avoid additional separation steps post-reaction, such as distillations and other thermally stressing procedures.

All the discussion in this book has the same target that is to synthesize the heterogeneous catalysts and photocatalysts to be excellent in their catalytic activity and selectivity. The scope of this book is divided into four chapters:

Chapter 1: Alligned titanium dioxide catalyst synthesized under magnetic field

Chapter 2: Particuology of tungsten oxide as visible light driven photocatalyst

Chapter 3: Synthesis of mesoporous silica catalyst by a nanoscopic template

Chapter 4: Bifunctional metal oxides catalysts

The authors have tried to portray the scene from the basic idea through the synthesis of the catalysts and photocatalysts by several strategies. All sections give outlooks about the developments to

x Preface

come. Once more, I would like to express my thanks not only to the

authors and co-authors but also to the team at Nanotechnology Research Alliance, Universiti Teknologi Malaysia.

Hadi Nur Ibnu Sina Institute for Fundamental Science Studies Universiti Teknologi Malaysia 2014

1

ALIGNED TITANIUM DIOXIDE

CATALYST SYNTHESIZED

UNDER MAGNETIC FIELD

Sheela Chandren, Nursyafreena Attan and Hadi Nur

1.1 INTRODUCTION

Magnetism is one of the key physical properties of materials and

every material has its own magnetism. Magnetism is divided into

three groups: ferromagnetism, paramagnetism and diamagnetism.

When a ferromagnetic material is placed within a magnetic field,

the magnetic dipoles are aligned to the applied field, thus

expanding the domain walls of the magnetic domains.

Paramagnetic material is attracted to magnetic fields while

diamagnetic material is repulsed by magnetic fields. Lastly,

diamagnetic materials are slightly against the magnetic field and

when the external field is removed, the material does not retain any

magnetic properties.

Materials can be dependent on external magnetic field, where

the magnetic force of the material's electrons can be affected. This

effect is known as Faraday's law of Magnetic Induction. However,

with the presence of an external magnetic field, the material can

respond with very different reaction. This reaction is dependent on

the atomic and molecular structure of the material and also the net

Particuology of Some Metal Oxides Catalyst

2

magnetic field associated with the same atom. Magnetic moment

associated with the original atom has three origins. They are the

movement of electrons, the spin of the electrons and the change in

the motion of electrons caused by the external magnetic field. It is

a dream of chemists and physicists to use this physical property to

control chemical and physical processes.

Magnetic field can be produced from the current flow of

electricity and the intrinsic magnetism of elementary particles,

such as the electron. Moving electric charges give rise to electric

current. A current will produce a magnetic field that will in turn

produce a force on magnetic material, magnet and other currents.

Magnetic field can interact with any atom, electron, nuclei or

molecule with magnetism properties. The magnetism properties are

due to magnetic energy density or magnetic susceptibility of the

materials. Therefore, magnetic field is expected to give effects

towards chemical and physical processes.

The term 'magneto-science' (basic and applied), refers to the

research of magnetic field effects (MFEs) on physical and

chemical phenomena. This chapter demonstrates the magnetic field

effects in the synthesis of well-aligned TiO2 under magnetic field.

1.2 MAGNETIC FIELD

Generally a molecule which has magnetic susceptibility arises

from the magnetic induction of a ring current when the magnetic

field is applied. Thus, the magnetic energy is orientation-dependent

and energetically the molecule undergoes molecular orientation to

the most stable direction. Lots of researchers have explored

numerous methods in order to achieve alignment. Alignment under

magnetic field has an advantage over other methods with respect to

the uniformity, flexibility and range of materials. However, the

energy of a molecule is negligibly small compared with the

thermal energy at room temperature.

Magnetic energy is smaller than thermal energy or electric

energy. The magnetic energy of an electron spin of 1 Bohr

Aligned Titanium Dioxide Catalyst Synthesized under Magnetic

3

magneton in a field of 1 Tesla corresponds to thermal energy of

0.67 K or electric energy of 58 μV. Since the magnetic energy of a

single molecule due to magnetism of paramagnetic and

diamagnetic materials are negligibly small compared with the

thermal energy at room temperature. Consequently, it seems that

magnetic field's effects towards materials do not occur at ordinary

temperatures. Therefore, a very strong magnetic field is needed to

affect materials. However, the usage of strong magnetic field

requires almost impractical temperature. Lately, enormous

progress has been developed towards technology on manufacturing

superconducting magnets.

For many years, scientists have been developing several

methods for structural control of organized molecular assemblies.

Factors like concentration, molar ratio (Hongjuan and Yun, 2012),

charges (Zhangang et al., 2012; Juan et al., 2009) and the structure

(Libing et al., 2012; Matthew et al., 2008; Hyundae et al., 2007) of

molecule play important role during molecular assemblies.

Environment of the system (Matthew et al., 2008) could also affect

the reaction especially temperature (Feifei et al., 2012; Christelle

et al., 2011). Typically, the molecular assemblies were obtained

with the help of templates (Xiaofei et al., 2011; Steven et al.,

2003). One of the greatest advantages of using template-based

synthesis is the precise control and optimization of the lengths and

diameters. However, the use of organic compounds as templates

may require developed techniques of synthetic chemistry where

multiple-step reaction is needed and harmful organic solvents or

toxic substances are normally involved.

Magnetic field is also one of a potential method to align and

orient molecules and domains, because it has an advantage that any

materials, even diamagnetic materials can be aligned by magnetic

fields as long as they have the magnetic anisotropy. It is well

established that diamagnetic assemblies having magnetic

anisotropy will become oriented and rotate in a magnetic field to

achieve the minimum-energy state. The protocols for producing

orientated ordered inorganic-surfactant were reported but only

based on simulation theory. The use of TiO2 as inorganic precursor


4

and organic surfactant, however, has not been reported. Figure 1.1

shows the conceptual model for the alignment of titania and

surfactant under magnetic field.

1.2.1 Molecular Assemblies

The utilization of surfactant-based organized assemblies in

analytical atomic spectroscopy is widely analyzed along several

major factors. The capability of organized medium to improve of

Figure 1.1 The conceptual model for the alignment of titania and

surfactant under magnetic field

atomic spectroscopic methods by favorable treatment of physical

and chemical properties of the sample solution plays important

roles during molecular assemblies. Moreover the extension of

separation mechanisms leads to organized medium. Synergistic

arrangement of liquid chromatography separations and atomic

detectors via the use of vesicular mobile phases is also a key factor


5

in molecular assemblies.

Amphiphilic molecules are molecules possessing both

hydrophobic and hydrophilic groups. Such surfactants display

some fascinating features because of their possibility to self-

associate in water and/or polar solvents. The ultimate structure of

the microscopically-ordered molecular aggregates is formation of

reverse micelles, bilayers, microemulsions and vesicles. These

surfactants aggregate as ‘‘ordered’’ medium because they imitate

the organizational capability of membranes by transporting

reactants collectively in very structured specific

microenvironments. Among the presented surfactant-based

organized assemblies, micelles and vesicles are perhaps the most

remarkable and investigated organized medium.

When dissolved in solvents, amphiphiles, including surfactants,

lipids, and amphiphilic block co-polymers, self-assemble into well-

defined structures with an extensive variety of shapes, such as

spherical and worm-like micelles, vesicles, lamellar sheets,

sponge-phases, nanotubes, networks, disks, toroids, as well as

many intermediate and mutative phases. Exploring their assembled

characteristic is of enormous theoretical and practical value due to

their applications in materials science, bio-engineering, and the

pharmaceutical industry (Honggang et al., 2007).

Micelles are microscopically-organized chemical assemblies

formed by self-aggregation of individual surfactant molecules.

These molecules exist as monomers in much diluted solutions.

However, when the concentration go beyond a certain minimum

‘‘Critical Micellar Concentration’’ (c.m.c.) of the surfactant, the

monomers unite instinctively, producing aggregates of colloidal

dimensions which are known as micelles. As the surfactant

concentration increases higher than the c.m.c., the addition of

monomer leads to the new configuration of micelles, where the

monomer concentration in solution remains fundamentally stable

and approximately equal to the c.m.c. That mean, the micelles are

in an energetic equilibrium with the dissolved monomers of the

surfactant, which continues as more or less stable concentration

after the c.m.c. has been achieved (Alfred et al., 1999).


6

Surfactants are frequently applied for wetting, given that the

accumulation of surfactants can influence the wetting and

spreading performance of a fluid, mainly an extremely polar fluid

like water. The capability of surfactants to control wetting relies on

their self-assembly at the solid-liquid, solid-vapor, and liquid-

vapor interfaces, and the consequent change in the interfacial

energies. These interfacial self-assemblies having numerous

degrees of structural freedom of surfactant molecules lead to

demonstrate rich feature and variation. In bulk aqueous solutions,

surfactant molecules self-assemble into micelles which defend

their hydrophobic tails from the aqueous surroundings (Frank and

Garoff, 1996). The molecular organization of the self-assemblies

and the effects of these organizations on wetting continue as hot

issues of widespread scientific and technological advances.

The addition of surfactant-based organized assemblies might

alter the physical characteristics of density, surface tension;

viscosity of liquid samples and this signify an acknowledged

perspective of using surfactants in atomic spectrometry for

improving sample transport to the atomizer (Alfred et al., 1999). A

lot of forces and interaction play significant role synthesis of

mesostructured materials. Weak non-covalent bonds, for example,

electrostatic interactions, hydrogen bonds, dipole-dipole

interactions, hydrophobic interactions, van der Waals forces, and

π-π stacking between the surfactants and inorganic species are

significant factors.

The self-assembly of organic or inorganic materials are

enormously fascinating and is widely studied nowadays in the

synthetic chemistry field. A synergistic self-assembly of organic or

inorganic materials has been frequently inspired, in order to

achieve the goal of fabricating the extremely ordered

nanostructure. A cathodic electrodeposition of Zn ions in anionic

surfactant solutions has been seriously measured, since it is an

inexpensive and simple method, moreover it has been

demonstrated to be successful for the synergistic self-assembly of a

lamellar-structured hybrid material. This method involved an

organic layer of supported surfactant molecules and an inorganic


7

layer of ZnO or Zn(OH)2 (Hiroyuki et al., 2011).

Mixture of anionic and cationic surfactants is one of the

mesmerizing systems that propose an attractive approach for

creating complex self-assembled nanostructures. Globular

micelles, cylindrical micelles, long thread-like micelle, discs, and

large lamellar sheets have been observed in some of the aqueous

cationic-anionic systems. The molecular self-assemblies in the

cationic surfactant systems are mainly credited to the strong

electrostatic attraction between the oppositely charged headgroups,

which significantly encourages the dense packing of surfactant

molecules in the aggregate. Owing to the important role in the

process of molecular self-assembly, the electrostatic interaction has

been explored in the cationic surfactant systems by varying

surfactant ratio, modifying solvent properties or adding inorganic

salt (Yiyang et al., 2008).

1.2.2 Orientation and Alignment under Magnetic Fields

It is a dream of chemists and physicists to use physical magnetic

properties of materials in controlling chemical and physical

processes. This novel idea was not accepted until recently, except

for the use of ferromagnetism. Ferromagnetic materials were

commonly studied since they demonstrate a strong attraction to

magnetic fields and even after the external field has been

disconnected, they are able to preserve their magnetic properties.

A single molecule or ion scarcely or even hardly go through

magnetic orientation due to its anisotropic magnetic energy which

is negligibly small compared with the thermal energy at room

temperature. On the other hand, aggregates molecules which are

comprised of ordered structures, can go through magnetic

orientation since their anisotropic magnetic energy goes beyond

the thermal energy at room temperature. Various techniques have

been investigated in order to accomplish the alignment. Alignment

under magnetic field has an advantage greater than other methods

due to the homogeneity, flexibility and variety of materials.


8

An example of easy and promising approach for the synthesis of

2D nanostructures of amorphous Fe nanoplatelets is by external

magnetic field induced self-assembly or aggregation without any

templates or surfactants (Jianguo et al., 2012). Next, a Co chains

composed of Co spheres was also carefully manufactured by

scheming the reaction conditions under magnetic fields. The

development of the chain structures might be that magnetic fields

oblige the nanoscale crystals of Co to outline chains (Jun et al.,

2008).

Moreover, a new magnetic approach was reported for

manipulating and orientating nickel nanowires by applying

magnetic fields to orient the nickel nanowires in a head-to-tail

configuration (Monica et al., 2001). Another study has

demonstrated a magnetic alignment with ferromagnetic ends to

accomplish directionality with high achievement for straight and

self-regulating nanowires despite the orientation of the substrate.

One hundred percent magnetic alignment of nanostructures to the

obligatory magnetic fields was accomplished by applying a low

external magnetic field (Carlos and Nosang, 2005).

The effects of magnetic field were also demonstrated in

alignment of multi-walled carbon nanotubes (MWNTs), which has

been demonstrated through deposition of homogeneous layers of

magnetite or maghemite nanoparticles under magnetic field. If a

sufficiently large external magnetic field is applied, the magnetic

moments of the nanoparticles align in corresponding order, and the

consequential dipolar interactions are satisfactorily large to

conquer thermal motion and to reorient the magnetic CNTs

supporting the development of chains of aligned carbon nanotubes

(Miguel et al., 2005).

However, since the magnetic energy of paramagnetic and

diamagnetic materials is very much smaller than the thermal

energy at room temperature, it was believed to be insufficient to

overcome the activation energy associated with chemical and

physical processes. Recent technology on manufacturing

superconducting magnets has shown great progress, thus chemists

and physicists can use strong magnetic field without difficulty.


9

Lately, CNTs that were well-aligned according to the direction of

the magnetic field were obtained under a strong magnetic field of

12 Tesla (Jang and Sakka, 2009).

Since an external magnetic field can be utilized as a promising

technique, the structure of organized molecular assemblies of

surfactants was believed to be aligned by magnetic fields

(Govindachatty and Sumio, 2008; Sumio, 2001). A study shows

that a macroscopically aligned silicate-surfactant liquid crystalline

was created by utilizing their capability to orient in high magnetic

fields.

1.3 TITANIUM DIOXIDE

Titanium dioxide (TiO2) material is currently the most important,

most widespread and most investigated due to its’ low toxicity,

high thermal stability, and broad applicability. As a semiconductor,

titanium dioxide has shown outstanding performance in

photocatalysis (We Jia et al., 2010; Andrew et al., 2011), water-

splitting (Ng et al., 2010) and self-cleaning (Deyong and Mingce,

2011). It is also useful in medical application (Gulaim et al., 2010)

due to its biocompatibility. Moreover, titanium dioxide plays

crucial role in dye-sensitized solar cells (Daesub et al., 2011).

Different shapes and sizes of titanium dioxide were reported to

give different effects in various reactions (Liao and Liao, 2007;

Xinchen et al., 2005). The use of TiO2 as inorganic precursor and

organic surfactant, however, has not been widely explored.

1.3.1 Synthesis of Well-Aligned TiO2 under Magnetic Field

Attan et al. (2012) has successfully shown the synthesis of well-

aligned TiO2 with very high length to diameter ratio using sol-gel

method under magnetic field (up to 9.4 Tesla) with CTAB

surfactant as structure aligning agent.

The authors suggested two possible mechanisms for the

formation of the well-aligned TiO2, shown in Figure 1.2. First, the


10

surfactant was responsible for directing the formation of well-

aligned TiO2 under magnetic field. A slow hydrolysis rate

promoted self-assembly of the surfactant molecule leading to the

optimized interaction between surfactant and TiO2 framework.

Magnetic field can align rod-like micelles in a parallel direction.

Additionally, magnetic field brought forth changes in interfacial

interaction between titanium precursor and surfactant. Pairing up

of electrons from titanium precursor and surfactant was proposed

to occur via (S)-(T) spin conversion mechanism under magnetic

field. The titanium precursor transformed from singlet low-spin to

triplet high-spin when having interaction with surfactant under

magnetic field. Hence, magnetic field controlled the arrangement

of the titanium precursor and surfactant. Concisely, the formation

of well-aligned TiO2 could be demonstrated by the usage of CTAB

surfactant with slow hydrolysis rate under magnetic field. (Attan et

al., 2012).

Figure 1.2 SEM images for well-aligned TiO2 synthesized with

slow hydrolysis (7 days) under strong magnetic field (9.4 Tesla)


11

1.4 CONCLUSION

Some insights into the possible mechanism of the formation of

well-aligned TiO2 were obtained from the following observations.

Well-aligned TiO2 was only obtained in the presence of the CTAB

surfactant, with a slow hydrolysis rate, and under a strong

magnetic field. The surfactant is clearly responsible for directing

the formation of TiO2 under a magnetic field. A slow hydrolysis

rate promotes the self-assembly of the surfactant molecules,

leading to the optimization of the interaction between the

surfactant and the inorganic framework (Khimyak and Klinowski,

2001). In addition, an external magnetic field leads to changes in

the alignment of the surfactant. As a result, owing to the interfacial

interaction between the surfactant and the titanium precursor, the

arrangement of well-aligned TiO2 can be stimulated during theslow

hydrolysis of the titanium precursor. In other words, the formation

of any well-aligned materials can be achieved by the use of the

CTAB surfactant with a slow hydrolysis rate under a strong

magnetic field.This research hopefully generates new perspective

for the application of magnetic field in heterogeneous catalysis and

synthesis of materials.

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Carlos, M. H. and Nosang, V. M. 2005. Magnetic Alignment of

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12

Christelle, V. et al. 2011. Oriented Growth of Zinc(II)

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Particuology of Tungsten Oxide as Visible Light Driven Photocatalyst

15

2

PARTICUOLOGY OF TUNGSTEN

OXIDE AS VISIBLE LIGHT-

DRIVEN PHOTOCATALYST

Leny Yuliati

2.1 INTRODUCTION

Tungsten oxide (WO3) is a semiconductor with band gap of 2.5-2.8

eV, which makes it feasible to absorb visible light and act as one of

the most potential visible light-driven photocatalysts so far. Even

though the WO3 has lower light energy conversion efficiency than

the widely used TiO2, some great advantages on using WO3 have

been recognized, such as the easiness to prepare WO3 with high

purity, strong absorption on both UV and visible light region, and

also the long-term stability under light irradiation in different types

of aqueous electrolytes (Kim et al., 2010). The WO3 also have

good chemical inertness and outstanding photochemical properties

with high chemical stability in aqueous media over wide pH range

(Janáky et al., 2013). These advantageous properties of WO3

motivated many researchers to improve the properties as well as

the performance of WO3 as photocatalyst.

In order to develop good synthesis method and achieve high

photocatalytic activity of WO3, the particuology of WO3 has been

studied intensively in the recent decades. In this chapter, the

Particuology of Some Metal Oxides Catalysts

16

particuology of WO3 is discussed related to the application of WO3

as photocatalyst for various reactions. The reports on the effect of

synthesis conditions on the particle properties, such as crystalline

phase, crystallinity, particle size, surface area, dispersion, and

morphology, as well as the effect of the particle properties on the

photocatalytic performance of WO3 are highlighted.

2.2 CRYSTALLINE PHASE AND CRYSTALLINITY

WO3 can exist in various crystalline phases (Howard et al., 2002).

At room temperature, WO3 usually can be found as monoclinic.

The phase transition from monoclinic to orthorhombic can occur at

high temperature range of 350-720°C. At higher temperature range

of 720-800°C, the orthorhombic WO3 will change to another

monoclinic structure, while at temperature of more than 800°C,

WO3 can be found as tetragonal phase.

In order to study the effect of crystalline phase on the

photocatalytic activity, the commercial WO3 that has orthorhombic

structure was annealed at various temperatures of 650-950°C (Xin

et al., 2009). The annealed samples have three phases, which were

orthorhombic, monoclinic, and tetragonal. The samples loaded

with Pt co-catalyst were tested in the photocatalytic oxygen

evolution from water in the presence of IO3- as the electron

acceptor under visible light irradiation. It was found that the

sample with monoclinic phase exhibited higher photocatalytic

activity than the other phases, suggesting that the WO3 phase is a

decisive factor in the reaction.

Similar result was also reported when WO3 was prepared by a

colloidal crystal templating method (Sadakane et al., 2008). A

mixture of orthorhombic and monoclinic WO3 was obtained when

the calcination temperature was 400°C, while pure monoclinic

WO3 was formed when the calcination temperatures were 500-

700°C. After Pt loading, the prepared WO3 was used as the

photocatalyst in the decomposition of acetic acid. Higher rate of

CO2 evolution was observed on monoclinic WO3, in which the


17

calcination temperature of 600°C gave the highest photocatalytic

activity.

Besides the crystalline phase, crystallinity also plays important

role in the photocatalytic activity. Crystallinity of WO3 can be

altered by changing the tungsten precursors, employing different

synthesis methods, as well as varying the synthesis temperatures.

However, the later usually also results in the changing of other

properties, such as particle size and surface area, that is discussed

in section 2.3.

As for the effect of tungsten precursor, it was reported that three

types of precursors could be used to prepare WO3, which were

ammonium tungstate parapentahydrate, ammonium metatungstate,

and tungstic acid (Bamwenda and Arakawa, 2001). Under the

same synthesis conditions, the level of crystallinity of the obtained

WO3 was found to increase in the order of ammonium tungstate

parapentahydrate ≈ ammonium metatungstate < tungstic acid. On

the other hand, similar order was observed on the activity of the

prepared WO3, in which the activity of the WO3 prepared by

ammonium tungstate parapentahydrate < ammonium metatungstate

< tungstic acid. This result clearly suggested that the tungsten

precursor influenced the crystallinity, which affected the

photocatalytic activity of the WO3 for oxygen evolution under

visible light irradiation. Crystallinity was found to be the important

factor for the reaction since the higher the crystallinity the lower

the concentration of the crystal defect that can act as the charge

recombination center. It was also proposed that the charge

generation, lifetime, mobility, and the charge transfer was more

likely occur on high crystalline material.

The synthesis of WO3 with better crystallinity has been reported

by using the colloidal crystal template of mono-disperse

poly(metyl methacrylate) as compared to the one prepared without

template (Sadakane et al., 2008). After loading with Pt co-catalyst,

the WO3 prepared using template showed enhanced photocatalytic

activity for degradation of acetic acid under visible light

irradiation. It was confirmed that the calcination temperature of

600°C or higher was required to ensure that there was no


18

impurities that can create crystal defects, which may act as

recombination sites.

2.3 PARTICLE SIZE AND SURFACE AREA

Particle size and surface area are important factors that may affect

the performance of WO3 as photocatalyst. During the synthesis

and treatment process, annealing temperature and time can be

altered to give different properties in the particle size and surface

area. Other approaches can also be employed, such as the use of

template to prepare WO3 with different particle sizes and

dispersing WO3 onto support with large surface area.

As for the effect of synthesis temperature, it was reported that

the monoclinic WO3 was successfully prepared by hydrothermal

reaction, followed by calcination at various temperatures from

500-800°C (Hong et al., 2009). The particle sizes of WO3were

found to be varied from 30 to 100 nm, while the surface area was

varied from 2 to 21.3 m2g

-1. The activity of the prepared WO3 was

evaluated for oxygen evolution from water containing AgNO3 as

the sacrificial agent under visible light irradiation for 5 hours.

Figure 2.1 shows the dependence of the particle size and BET

specific surface area on the photocatalytic activity. It was observed

that WO3 with larger particle size but lower surface area gave

higher photocatalytic activity.

Recently, WO3 nanoparticles with different particle sizes but all

are less than 50 nm have been prepared by annealing treatment of

commercial WO3 nanoparticles at 300-600°C (Purwanto et al.,

2011). The increase of the annealing temperature resulted in the

increase of the particle size but decrease of the surface area. After

loading with Pt co-catalyst, the samples were evaluated

forphotocatalytic degradation of amaranth dye using solar light

simulator as the light source.


19

Figure 2.1 Dependence of the amount of oxygen evolved on the

surface area (open circle) and particle size (closed circle) of WO3. The

values shown are taken from Hong et al., 2009

Figure 2.2 Dependence of the initial rate of dye degradation on the

particle size of WO3 nanoparticles. The yellow part shows the

optimum particle size of WO3 as the compromise between surface area

and surface recombination. The values shown are taken from

Purwanto et al., 2011

0 10 20 30 40 500

0.2

0.4

0.6

0.8

Particle size (nm)

Init

ial

rate

(p

pm

/min

)

High surface area,

but high surface

recombination

Low surface

recombination, but

low surface area

OPTIMUM

surface area

and surface

recombination

0 4 8 12 16 200

5

10

15

20

25

0

100

200

300

400

500

600

Amount of produced O2 (mol)

Su

rfac

e ar

ea (

m2g

-1)

Par

ticl

e si

ze (

nm

)


20

It was found that the photocatalytic activity of WO3 was much

depended on the particle size, as shown in Figure 2.2 WO3 with

particle size of 18-26.4 nm showed the highest photocatalytic

activity. The moderate activity was obtained on samples with

particle sizes of 7.3, 9.8, and 42.4 nm, while WO3 with particle

size of 13.4 nm showed the lowest photocatalytic activity. The

reason why there was an optimum range of particle size would be

due to the optimized two parameters of surface area and

recombination process. When the particle size decreased, the

surface area increased, and it would enhance the interfacial

charge-carrier transfer rate. On the other hand, when the particle

size was very small, the surface recombination would be

dominant and this resulted in low photocatalytic rate.

As for the effect of annealing time, it was also reported that

different catalytic activities were observed when commercial WO3

was annealed at 750°C at different annealing times (Xin et al.,

2009). The treated samples, after loaded with Pt co-catalyst, were

tested in the photocatalytic oxygen evolution from water in the

presence of IO3- as the electron acceptor under visible light

irradiation. The activity increased first when the annealing time

increased from 1 to 2 hours and reached the highest activity when

the annealing time was 4 hours. On the other hand, the activity

decreased with longer annealing time of 8 and 16 hours. It was

proposed that the grain size of WO3 increased with the annealing

time, as evidenced by the decrease of the surface area and it

resulted in the decrease in activity. This result again showed that

there was optimum range for the particle size to give the optimum

activity.

The template method can also be employed to prepare WO3

with different sizes. Recently, mesoporous carbon nitride was used

to prepare the WO3 at 700°C (Yuliati et al., 2011). The resulted

WO3 has uniform particle size of 200 nm and it was tested for

photocatalytic removal of salicylic acid under visible light

irradiation. The WO3 was loaded with Pt by photodeposition

method before it was used as the photocatalyst. It was obtained that

the prepared WO3 showed almost two times better photocatalytic


21

activity than the commercial WO3, which has bigger particle size

of 20-100 µm. It was proposed that the higher photocatalytic

activity was mainly due to the particle size effect. The smaller

particle size would result in shorter diffusion length for the

photogenerated electron-hole pairs, which reduced the electron-

hole recombination, thus, gave higher photocatalytic activity.

Since WO3 is generally low surface area material, increasing the

surface area of WO3 in some cases does not give significant benefit

for photocatalytic activity, especially when other factor is found to

be more significant than the factor of surface area. It was reported

that there was no systematic trend in the variation of activity with

the surface area of WO3 when the studies involved the use of

various precursors, which were ammonium metatungstate,

ammonium tungstate parapentahydrate, and tungstic acid

(Bamwenda and Arakawa, 2001). The WO3 with low surface area

showed better photocatalytic activity for oxygen evolution than the

WO3 with high surface area but poor crystallinity. This result

clearly suggested that photocatalytic activity will not only depend

on the surface area.

Conventional method to prepare WO3 usually gives high

crystallinity but low surface area of WO3. Therefore, a synthesis

method to produce both high crystallinity and large surface area is

important. One of the reported approaches is by employing a

colloidal crystal templating method (Sadakane et al., 2008). WO3

prepared without template has size of several micrometers and

showed very low specific surface area (1 m2g

-1). On the other

hand, WO3 prepared by templates with different sizes of 180, 260,

and 490 nm gave high specific surface areas of 21, 15, and 9 m2g

-1.

The larger specific surface area was obtained as a result of the

growth of WO3 single crystals around the macropores. The

photocatalytic activity of the prepared WO3 was evaluated for

decomposition of acetic acid under visible light irradiation. Clear

relationship between specific surface area and photocatalytic

activity was obtained, in which the WO3 sample with larger

specific surface area gave higher photocatalytic activity.

Particle size of WO3 can also be altered by dispersing WO3 on a


22

large surface area of support, such as mesoporous silica (Tanaka et

al., 2010). Two types of silica matrix were synthesized from two

different templates, which were decyltrimethylammonium bromide

and cetyltrimethylammonium chloride, giving mesoporous silica

with pore sizes of 1.7 and 2.4 nm and specific surface areas of 892

and 1210 m2g

-1, respectively. The WO3/mesoporous silica was

prepared by impregnation method using tungsten acid as the

tungsten precursor. After impregnation, the samples were calcined

at 400°C in air for 3 hours. Transmission Electron Microscopy

(TEM) images showed that the size of WO3 formed depended on

the pore size of the matrix mesoporous silica, which was evaluated

as 1.4 and 1.8 nm when the pore size of mesoporous silica was 1.7

and 2.4 nm, respectively. The commercial micro-sized WO3 and

WO3/mesoporous silica were employed as photocatalysts for

decomposition of benzene under UV light irradiation. It was

obtained that the activity was in the order of 1.4 nm-sized WO3>

micro-sized WO3> 1.8 nm-sized WO3. This result suggested that

there is optimum particle size to give optimum photocatalytic

activity. It was proposed that the enhanced performance of WO3

nanoparticles was observed as the result of enhanced reducing

potential, which only occurred when the quantum-confinement

effect resulted in enough band gap of more than 3.0 eV.

2.4 DISPERSION OF ACTIVE SPECIES

Highly dispersed species may behave as an efficient photocatalyst

for some reactions. WO3 has been reported to act as a good

photocatalyst for oxygen evolution from water (Bamwenda and

Arakawa, 2001; Hong et al., 2009; Liu et al., 2012; Xin et al.,

2009).However, due to its positive conduction band,

thermodynamically WO3 would not be able to reduce water to

hydrogen. Recently, unusual activity of WO3 for hydrogen

production was reported when WO3 was dispersed on silica matrix

(Liu et al., 2012). The WO3/SiO2 samples with various contents of

WO3 were prepared by sol-gel method in the presence of citric


23

acid, with calcination temperature of 600°C. The samples were

used in the photocatalytic hydrogen production in the presence of

methanol as the sacrificial agent under UV-visible light irradiation.

While no activity was observed on WO3, all WO3/SiO2 samples

showed activity for the formation of hydrogen. The highest activity

was achieved on 20% WO3/SiO2 and further increase of the WO3

content decreased the photocatalytic activity. The reason for the

high activity was proposed based on the absorption spectra that

showed the presence of two species, which were WO3 with band

gap of 2.6 eV and W-O-Si species with band gap of 3.3 eV. The

presence of W-O-Si resulted in the upshift of conduction band

minimum of WO3 that met the thermodynamical potential for

hydrogen production from water in the presence of sacrificial

agent. This research showed that the highly dispersed WO3 and

their interface W-O-Si like species would be the active sites for the

hydrogen production.

The prepared WO3/SiO2 samples were also evaluated for

oxygen evolution in the presence of Fe3+

sacrificial agent under

visible light irradiation (Liu et al., 2012). It was confirmed that all

WO3/SiO2 samples showed photocatalytic activity, while SiO2 did

not show any photocatalytic activity. The activity of WO3/SiO2

increased with the increase of WO3 content. It was observed that

40% WO3/SiO2 gave similar photocatalytic activity to WO3 and

even 50% WO3/SiO2 showed better photocatalytic activity than the

WO3. This result suggested that introducing WO3 into cheap silica

matric could reduce the amount of WO3, thus, will be a promising

strategy for the real application in the future.

The dispersion of WO3 in mesoporous silica has been also

reported to give enhanced photocatalytic activity for

photodecomposition of benzene (Tanaka et al., 2010). Due to the

small band gap energy of WO3, generally aromatic compounds

could not be completely decomposed on WO3. However,

dispersing WO3 on mesoporous silica resulted in the complete

decomposition of benzene under UV light irradiation. It was

proposed that by dispersing WO3 on mesoporous silica, a blue shift

of absorption edge from 2.58 to 3.05 eV was observed due to the


24

quantum-confinement effect. It was obtained that the widening of

the band gap to more than 3.0 eV gave benefit on increasing the

photocatalytic activity of WO3 under UV light irradiation.

2.5 MORPHOLOGY

Morphology is also one important factor that may affect

photocatalytic activity as different particle shapes can lead to

different photocatalytic activities. The morphology of WO3,

especially the shapes can be controlled by various synthesis

conditions. For example, different morphologies of hydrated WO3

could be obtained when different surfactants were used. Three n-

alkyl chain sodium sulfate surfactants, which were sodium decyl

sulfate, sodium dodecyl and sodium tetradecyl sulfate, resulted in

the formation of nanofibers (50 nm), nanoneedles (60 nm) and

nanoneedles (80 nm), respectively (Salmaoui et al., 2013). Other

factors have also been reported to give different morphology of

WO3, such as the presence of specific inorganic salts (Xu et al.,

2011) and the presence of acid with various concentrations (Yu

and Qi, 2009; Huang et al., 2013).

Some studies showed that WO3 with different morphologies

might give different photocatalytic activity. However, the different

photocatalytic activity usually could not be directly related to the

morphology only, but also some other factors. WO3 nanorods,

WO3 toothpicks, and cubic WO3 can be synthesized by

hydrothermal method in the presence of sodium sulfate, lithium

sulfate, and iron(II) sulfate, respectively (Xu et al., 2011).

However, irregular WO3 was observed when other inorganic salt,

such as sodium chloride, potassium chloride, iron(III) chloride, or

potassium nitride was added. Scanning Electron Microcopy (SEM)

and TEM images showed that the WO3 nanorods have diameters

ranging in 30-70 nm, WO3 toothpicks have lengths of ca. 500 nm,

while the cubic WO3 have width, thickness, and length of 100-500

nm. The appearances of these samples were slightly different to

each other; the nanorods and toothpick gave light-green colour,


25

while the cubic WO3 has bottle-green colour. After loading with Pt

co-catalyst, the WO3 samples were tested for photocatalytic

degradation of acetic acid under visible light irradiation. Among

the samples, the cubic WO3 showed the highest photocatalytic

activity. The high photocatalytic activity was proposed coming

from the synergic effect between the loaded Pt and Fe2O3 that was

originated from the iron(II) sulfate.

Hierarchically flower-like WO3 assemblies were prepared

through a simple hydrothermal method without any templates

using Na2WO4 as the tungsten precursor (Yu and Qi, 2009). The

formation of the flower-like assembly was proposed to follow

three steps, which was self-aggregation via dissolution and

crystallization, self-organization for the oriented attachment, and

ripening and anisotropic growth. The prepared WO3 was found to

be active and stable for degradation of rhodamine B (RhB) under

visible light irradiation. The reasons for high activity were

proposed as the results of the presence of WO3 and hydrated WO3

composites structures, large specific surface area, and the

hierarchically bimodal macro-/mesoprorous structures.

WO3 nanoplates and WO3 flower-like assembly were

successfully prepared by hydrothermal reaction of PbWO4 in the

presence of different concentrations of HNO3, which were 4 and

15 M, respectively (Huang et al., 2013). The WO3 nanoplates has

the size of 50-150 nm with thickness of 25 nm, while the WO3

flower-like assembly has the size of 3-5 nm. Both WO3 samples

showed similar surface area of ca. 13 m2g

-1, and also similar

absorptions with similar estimated band gaps of 2.63 and 2.61 eV

for the nanoplates and flower-like assembly, respectively. In the

photocatalytic degradation of RhB, it was obtained that the WO3

nanoplates and flower-like gave 7.6 and 3.3 times higher

photocatalytic activity than the commercial WO3. It was proposed

that the high photocatalytic activity was due to the larger surface

area on the WO3 samples compared to the commercial one.

Moreover, the enhanced activity of nanoplates was due to the

better crystallinity of the WO3 nanoplates than the WO3 flower-

like assembly.


26

2.6 CONCLUSIONS

WO3 is a potential visible light-driven photocatalyst that can work

as a good photocatalyst for various oxidation reactions, such as

degradation of organic pollutants and oxidation of water to

produce oxygen via half-reaction under visible light irradiation.

Particuology of WO3, such as crystalline phase, crystallinity,

particle size, surface area, dispersion, and morphology, have been

revealed to give influences in the photocatalytic activity of WO3.

In most cases, WO3 with monoclinic structure usually gives the

best activity. WO3 with high crystallinity also tends to give the best

activity. On the other hand, there is an optimum value for particle

size and surface area of WO3 since the particle size would be a

compromise between the surface area and surface recombination.

The dispersed active WO3 species might give unusual property to

WO3, such as ability to reduce water to produce hydrogen via half-

reaction. It also provides one promising strategy to create

supported catalyst that may reduce the amount of WO3, thus,

reducing the cost of the catalyst. While morphology of WO3 seems

to give influences in the photocatalytic activity, most of the studies

have never related the activity solely to the morphology, but also

include other factors, such as crystallinity or surface area. Even

though some research have been made to understand the

particuology of WO3 related to its activity as the photocatalyst,

significant improvements to increase the photocatalytic activity of

WO3 are still highly required. Further studies must be considered

to bring the WO3 into the real application.

REFERENCES

Bamwenda, G. R. and Arakawa, H. 2001. The Visible Light

Induced Photocatalytic Activity of Tungsten Trioxide

Powder. Applied Catalysis A: General, 210(1-2): 181-191.

Hong, S .J. et al. 2009. Size Effects of WO3 Nanocrystals for

Photooxidation of Water in Particulate Supension and


27

Photoelectrochemical Film Systems. International Journal

of Hydrogen Energy, 34(8): 3234-3242.

Howard, C. J., Luca, V. and Knight, K. S. 2002. High-temperature

Phase Transitions in Tungsten Trioxide – the Last Word?

Journal of Physics: Condensed Matter, 14(3): 377- 388.

Huang, J., Xiao, L. and Yang, X. 2013. WO3 Nanoplates,

Hierarchical Flower-like Assemblies and Their

Photocatalytic Properties. Materials Research Bulletin,

48(8): 2782-2785.

Janáky, C. et al. 2013. Tungsten-based Oxide Semiconductors for

Solar Hydrogen Generation. Catalysis Today, 199: 53-64.

Kim, H., Senthil, K. and Yong, K. 2010. Photoelectrochemical and

Photocatalytic Properties of Tungsten Oxide Nanorods

Grown by Thermal Evaporation. Materials Chemistry and

Physics, 120(2-3): 452-455.

Liu, G. et al. 2012. Photocatalytic H2 and O2 Evolution over

Tungsten Oxide Dispersed on Silica. Journal of Catalysis,

293: 61-66.

Purwanto, A. et al. et al. 2011. Role of Particle Size for Platinum-

Loaded Tungsten Oxide Nanoparticles during Dye

Photodegradation Under Solar-simulated Irradiation.

Catalysis Communication, 12(6): 525-529.

Sadakane, M. et al. 2008. Preparation of Nano-structured

Crystalline Tungsten (VI) Oxide and Enhanced

Photocatalytic Activity for Decomposition of Organic

Compounds under Visible Light Irradiation. Chemical

Communications, (48): 6552-6554.

Salmaoui, S. et al. 2013. Hexagonal Hydrated Tungsten Oxide

Nanomaterials: Hydrothermal Synthesis and

Electrochemical Properties. Electrochimica Acta, 108: 634-

643.

Tanaka, D., Oaki, Y. and Imai, H. 2010. Enhanced Photocatalytic

Activity of Quantum-confined Tungsten Trioxide

Nanoparticles in Mesoporous Silica. Chemical

Communications, 46(29): 5286-5288.

Xin, G., Guo, W. and Ma, T. 2009. Effect of Annealing


28

Temperature on the Photocatalytic Activity of WO3 for O2

Evolution, Applied Surface Science, 256(1): 165-169.

Xu, Z. et al. T. 2011. Preparation of Platinum-loaded Cubic

Tungsten Oxide: A Highly Efficient Visible Light-driven

Photocatalyst, Materials Letters, 65(9): 1252-1256.

Yu, J. and Qi, L. 2009. Template-free Fabrication of Hierarchically

Flower-like Tungsten Trioxide Assemblies with Enhanced

Visible-light-driven Photocatalytic Activity. Journal of

Hazardous Materials, 169(1-3): 221-227.

Yuliati, L., Mazalan, M. and Lintang, H. O. 2011. Synthesis of

Tungsten Oxide as Visible Light-driven Photocatalyst for

Removal of Salicylic Acid. Journal of Fundamental

Sciences, 7(1): 62-66.

Synthesis of Mesoporous Silica Catalyst by A Nanoscopic Template

29

3

SYNTHESIS OF MESOPOROUS

SILICA CATALYST BY A

NANOSCOPIC TEMPLATE

Hendrik O. Lintang

3.1 INTRODUCTION

Mesoporous silica material (MSM) is porous silicate with a pore

size from 2 to 50 nm according to International Union of Pure and

Applied Chemistry (IUPAC). Since discovery of ordered MSMs

independently in the early 1990 by Japanese (Yanagisawa et al.,

1990; Inagaki et al., 1993) and American researchers (Kresge et

al.; Beck et al., 1992), these materials have been widely used for

development of adsorbents, molecular sieves, catalysts, insulating

materials and nanometer-scale hosts for optical and electronic

applications (Zhao et al., 1996; Ying et al., 1999; Scott et al.,

2001; Wan et al., 2007; Kresge and Roth 2013). The wide

applications can be possible to be achieved due to its large surface

areas, high thermal and mechanical stability, uniform channel

distribution and control of pore size.

In the development of catalysts, amorphous silica has been

used from long time ago due to its high surface area and low cost.

However, their applications have been limited by irregularity of the

surface and pore structure. To solve these issues, microporous


30

materials such as zeolites can be used owing to the regular

structure. Zeolite as a heterogeneous catalyst can give advantages

in the catalytic reaction by retaining it in reactor or separating it

from the liquid sample, regenerating or reusing, un-dissolving and

minimizing from leaching. However, the surface area and pore of

microporous zeolites are difficult to functionalize and extend (only

around 200 m2g

-1 and pore size less than 2 nm), therefore this

limits the scope of catalytic reactions (Corma 1997).

Recently, utilization of nanoscopic channels of ordered MSMs

can provide improvement in catalytic activity either due to

enhancement of selectivity in a sterically homogeneous

environment or due to higher catalyst turnover from stabilization

of catalysts in the silicate channels (Clark and Macquarrie, 1998;

Brunel, 1999). However, the number of active sites limits the

catalytic activities. In order to increase catalytic activity, active

sites of ordered MSMs can be modified by incorporating

heteroatoms either in the pore wall or on the pore surface (Ying

2000), and by anchoring organic groups onto their surface (Stein et

al., 2000; Melero et al., 2006). For the latter method, ordered

MSMs have been organically functionalized using organic

functional groups attached to condensable silane (organosilanes)

via post-synthetic grafting method (Sayari et al., 2001). This way

can be used to control hydrophobicity with tailored pore size and

surface area according to the catalytic reaction. However, the

problems of pore blocking and non-homogenous distribution of

organosilanes will potentially reduce the catalytic activity

(Hoffmann et al., 2006). Moreover, the silicon-oxygen bond at

external silica surface can be easily cleaved at conditions of

catalytic reactions such as at elevated temperatures and extremes

of pH so that the solid catalyst cannot be reused anymore (Price,

2000). Therefore, synthesis of ordered MSM catalysts by

nanoscopic template effect will be the best methods to solve the

above problems. In this chapter, the resulting mesoporous silica

catalysts prepared from nanoscopic template method will be

highlighted for catalytic oxidation reactions.


31

3.2 NANOSCOPIC TEMPLATE METHOD

Nanoscopic template method is one of synthesis approaches for

preparation of desired nanomaterials using the nanoscopic pore in

a host inorganic material as a template. The nanoscopic pore of

ordered MSMs will template organic functional groups attached to

organosilanes using structure directing agent (SDA). In contrast to

post-synthetic grafting method (Stein et al., 2000; Melero et al.,

2006), Sol-gel one-pot synthesis of ordered MSMs with mono

(Wight and Davis, 2002) and bridged (Yang et al., 2009)

organosilanes in the mixture with surfactant as a SDA can give

high loading and more homogenous distribution of organic

functional groups in the silicate nanochannels. Moreover, the

organic functional groups can be arranged either in the pore or

silica pore wall. In this section, the catalytic activity will be

discussed based on the immobilization of organic functional group

during nanoscopic template of ordered MSMs including their post

modification for specific catalytic reactions such acid catalysis.

3.2.1 Mesoporous Silica Catalysts for Acid Catalytic

Oxidation Reaction

Generally, solid acid catalysts have been utilized as catalyst

materials for petroleum refinery industry using cracking reaction

and for production of fine chemicals using Friedel-Crafts,

esterification, hydration and hydrolysis reactions. Almost all of the

reactions involving water have been found to reduce the

performance of catalysts. By incorporating sulfonic functional

group as an active site in the ordered MSMs, it is expected to solve

the above problems. Moreover, active sulfonic acid sites can

enhance catalytic activity of ordered MSMs due to

increasinghydrophobicity at the functional groups (Yang et al.,

2009). In this section, sulfonic acid functionalized ordered MSMs

would be discussed as acid catalysis for oxidation reaction. In

particular, the discussion will be focused on the method of

incorporation and their post-functionalizations to the performance


32

of the catalysts in several acid catalytic oxidation reactions.

3.2.1.1 Incorporation of Sulfonic Acid Groups in Pore of

Ordered MSMs using Mono Organosilanes

This method has been reported by alkylsulfonic acid functionalized

mesoporous materials using 3-mercaptopropyl-trimethoxysilane

(MPTMS) in the presence of hexadecyltrimethylammonium

bromide (C16TMBr) or n-dodecylamine as SDAs in the one-pot

synthesis of ordered MSMs (Van Rhijn et al., 1998). For removing

the surfactants, the resulting thiol functional groups in ordered

MSMs of Mobile Composition of Matter (MCM)-41 and

Hexagonal Mesoporous Silica (HMS) (MPTMS–MCM-41 and

MPTMS–HMS) were extracted by acid solution or ethanol reflux

to give SH–MCM-41 and SH–HMS. Both thiol (SH) functional

groups of SH–MCM-41 and SH–HMS were oxidized with

hydrogen peroxide (H2O2) to form sulfonic acid (SO3H)

functionalities of SO3H–MCM-41 and SO3H–HMS as shown in

Figure 3.1. The resulting solid acid catalyst of SO3H–MCM-41 and

SO3H–HMS were tested in condensation reaction of 2-methylfuran

with acetone as a solvent to produce bisfurylalkene of 2,2-bis(5-

methylfuryl)propane (DMP) (Figure 3.2). The solid acid catalyst

SO3H–MCM-41 or SO3H–HMS showed catalytic conversions of

85% and 73% with selectivities of 95%. These conversions and

selectivities have been found as the highest performance compared

to the zeolites H–ß (61% and 74%) and H–US–Y (55% and 67%),

Al–MCM-41 (5% and 95%) and SO3H–MCM-41 with silylated

(57% and 92%) due to high polarity and more homogeneous

distribution of the active sites in the SO3H–MCM-41 or SO3H–

HMS.

In esterification of glycerol with fatty acids (lauric acid) to

monoglycerides, the above SO3H–MCM-41 or SO3H–HMS

catalysts were found to be more active compared to other silica

materials (Bossaert et al., 1999).


33

Figure 3.1 Synthesis of solid acid catalysts of ordered MSMs

having SO3H catalytic active sites (SO3H–MCM-41 and SO3H–HMS)

from oxidation of mesoporous silica functionalized thiol groups (SH–

MCM-41 and SH–HMS) with H2O2

Figure 3.2 Condensation reaction of 2-methylfuran and acetone

using solid acid catalysts of SO3H–MCM-41 or SO3H–HMSto give

2,2-bis(5-methylfuryl)propane

The solid acid catalyts showed catalytic conversion up to 53%

while Amberlyst-15, zeolite H-US-Y and grafted method of

SO3H–MCM-41 showed only catalytic conversions of 44%,

36% and 47%, respectively. These results are due to the good

accessibility and distribution of the active sites in ordered

SO3H–MCM-41. In order to study effect of hydrophobicity from

the alkyl functionality on the activity of the sulfonic acid sites,

SH–MCM-41 materials as precursors for SO3H–MCM-41

catalysts were prepared by controlling hydrophobicity using


34

methyl or propyl trimethoxysilane (MTMS or PTMS) in the

mixture of functional groups MPTS with SDAs of hexadecyl,

dodecyland decyl alkyl chains of trimethylammoniumbromides

(C16TABr, C12TABr and C10TABr) (Diaz et al., 2000). They

found that turnover number (TON) of solid acid catalysts with

amount of methyl alkyl group of 1.8 mmol/g (40% methylsilane)

can increase catalytic activity from 2 (without containing

moieties) to 6 mol (1.8 mmol/g of methyl groups) of fatty

acid/(site x time) after 8 h of reaction with lauric acid (Diaz et

al., 2000). In detail, when they used lauric acid in esterification

of glycerol, isolated monoglyceride was 63% in yield with

selectivity of 80%. This result was increased 15% compared to

the previous report by Bossaert et al. in 1999, indicating the

importance of hydrophobicity in this reaction.

3.2.1.2 Incorporation of Sulfonic Acid Groups in Pore of

PMOs using Bridged Organosilanes

Co-condensation method with bridged organosilanes has been

firstly introduced by three research groups to functionalize the

silica framework or wall through two covalent bonds to give

periodic mesoporous organosilica (PMO) materials. The resulting

PMOs will give homogeneous distribution of the functional groups

in the pore walls (Inagaki et al., 1999; Melde et al., 1999; Asefa et

al., 1999). These PMOs were firstly used as solid acid catalysts for

alkylation of phenol with 2-propanol at 150ºC (Yuan et al., 2003).

In detail, thiol functionalized PMO materials (SH–PMO) were

synthesized using bis(triethoxysilyl)ethane (BTSEa) as a main

framework source and MPTMS as a functional group and

octadecyltrimethylammonium chloride (C18TACl) as a SDA. This

SH–PMO was oxidized by H2O2 to give SO3H–PMO-BTSEa as a

solid acid catalyst with the same procedure as shown in Figure 3.1.

The catalytic activities of SO3H–PMO-BTSEa and SO3H–MCM-

41 were almost 60% in 10 h of reaction time due to the large pore

size. In contrast, the catalytic activity of Zeolite Socony Mobil

(ZSM)-5 with Si/Al of 30 was only 25% in 2 h of reaction time.


35

Moreover, it was also found that the catalytic activity of SO3H–

MCM-41 was gradually decreased after 10 h of reaction time while

the catalytic activity of SO3H–PMO-BTSE exhibited similar

performance over a period of 25 h. After the reaction, the acidity of

SO3H–MCM-41 was reduced to half of initial amount, which may

give decrease in catalytic activity. Therefore, the bridged organic

moieties within the framework of PMO materials can potentially

give an excellent catalytic activity as well as hydrothermal

stability.

In order to study the effect of the bridge in the catalytic

reaction, SO3H–PMO solid acid catalysts were prepared by ethane-

(BTSEa) and benzene (BTSB) organosilanes with MPTMS in

acidic media in the presence of H2O2 and Brij 76 as a SDA (Yang

et al., 2004). Both SO3H–PMO-BTSEa and SO3H–PMO-BTSB

catalysts were shown to be efficient catalysts for the condensation

of phenol and acetone to form bisphenol A with the highest

turnover frequency (TOF) of 17.2 (mmol of bisphenol A per mmol

of active site). It was also found that SO3H–PMO-BTSEa showed

higher catalytic activity than SO3H–PMO-BTSB. These

observations indicate larger specific surface areas and average pore

diameters of ethane-bridged SO3H–PMO compared to benzene-

bridged SO3H–PMO organosilicas. These researchers have also

studied the effect of structure on PMOs to the catalytic activity

(Yang et al., 2005) using esterification of acetic acid with ethanol.

The lamellar pore wall structure of SO3H–PMO-BTSB showed

higher catalytic conversion compared to commercial Nafion-H.

However, around 25% of the solid acid catalyst active site was lost

after first cyclic of the reaction due to the weak bonding of

propylsulfonic acid groups to the silicon in the lamellar structure.

Dhepe and co-workers have also reported the performance of

both SO3H–PMO-BTSEa and SO3H–PMO-BTSB solid acid

catalysts (Dhepe et al., 2005). In the hydrolysis of sucrose and

starch to monosaccharides, TOF and conversion of both sulfonated

mesoporous silica materials were higher than that of Amberlyst-15,

Nafion-silica and H-ZSM-5 catalysts. In detail, TOF of SO3H–

PMO-BTSEa was 11.6 and 1.2 while TOF of SO3H–PMO-BTSB


36

was 6.8 and 0.7 in sucrose and starch hydrolysis at 353 and 403 K

of reaction temperature and 4 and 6 h of reaction time,

respectively. In contrast, Amberlyst-15, Nafion-silica and HZSM-5

catalysts can only give 0.5, 3.7 and 0 in sucrose hydrolysis and 0.1,

0.3 and 0 in starch hydrolysis. These results indicate that solid acid

catalysts with ethane moiety bridging SO3H–PMO-BTSEa

exhibited higher TOF than that with benzene moiety SO3H–PMO-

BTSB organosilica due to their hydrophobic properties. High

performance of sulfonated mesoporous organosilicas even

compared to the grafting functionalized catalysts is due tothe

water-toleranceof the catalysts in this hydrolysis.

Sulfonated organosilica materials were used in the Claisen–

Schmidt condensation reaction of aldehydes and ketones to give

chalcone as a product (Shylesh et al., 2007). The SO3H–PMO-

BTSEa showed as active catalysts in their conversions and

selectivities compared to the conventional MCM-41 and an

amorphous silica gel. These results suggest that apart from surface

area, the hydrophobicity of the propyl –SO3H and bridging organic

groups in the pore walls are very important to provide high

catalytic activity. On the other hand, it was found that both the

hydrophobicity of the framework and the presence of organic

moieties in the mesoporous network can provide catalytic

esterification reaction of acetic acid with ethanol as a solvent

(Yang et al., 2005). In this case,the sulfonic acid groups mainly

contributed to the formation of ethyl acetate. They also used the

catalysts in the reverse hydrolysis reaction of cycloacetonate where

they found both the hydrophobic nature and active catalytic sites

are very important factors for increasing product yields.

Recently, local differences in surface hydrophilicities or

hydrophobicities of propyl- and arene-sulfonic acid modified

PMOs have been studied for aqueous-sensitive etherification

reactions of vanillyl alcohol (4-hydroxy-3-methoxybenzylalcohol)

with 1-hexanol to yield 4-hydroxy-3-methoxybenzyl-1-hexyl ether

(Morales et al., 2008). Both SO3H–Pr-PMO-BTSEa prepared from

MPTMS and SO3H–Ar-PMO-BTSEa prepared from 2-(4-chloro-

sulfonylphenyl)-ethyl-trimethoxysilane (CSPTMS) showed


37

significant improvement in catalytic activities up to 54% and 51%

in 15 mins and finally up to 95% in 2 h, respectively compared to

siliceous mesoporous supports (35% for Pr-SO3H silica and 33%

for Ar-SO3H silica in 15 min or around 75-80% in 2 h). The

difference in catalytic activities for SO3H–Pr-PMO-BTSEa and

SO3H–Ar-PMO-BTSEa is attributed to lower hydrophilicity of

propyl alkyl chain compared to the aromatic arene ring. Moreover,

water produced as a by-product of the reaction will co-adsorb at

the nearest sulfonic acid centers in the silica framework, resulting

in partial deactivation of the catalysts due to competition with the

alcohol reactant species. Compared to hydrophilic arenesiloxane

groups in SO3H–Ar-PMO-BTSEa, hydrophobic ethylsiloxane

groups in SO3H–Pr-PMO-BTSEa have been found to minimize

interactions of adsorbed water with the sulfonic acid catalyst sites.

Hence, SO3H–Pr-PMO-BTSEa showed better catalytic activity

than SO3H–Ar-PMO-BTSEa catalyst.

3.2.1.3 Incorporation of Sulfonic Acid Groups in Pore Wall of

PMOs using Bridged Organosilanes

Chemical modification of the bridging organic moieties in pore

wall of PMOs is one of effective approaches to prepare highly

functionalized and controlled chemical environments with uniform

and stable mesopore spacing in solid acid catalysts. Sulfonation of

mesoporous benzene silica SO3H–PMO3D-cub-BTSB with well-

defined 3D cubic structure (Pm3n) synthesized using mixture of

1,4-bis(triallylsilyl)benzene as a functional group and

hexadecyltrimethylammonium chloride (C16TACl) as a SDA

showed an excellent catalytic activity in Friedel–Crafts acylation

of aromatic ether anisol using acetic anhydride as an acylating

agent (Kapoor et al., 2007). This catalytic activity (87.6%) was

higher than other sulfonic acid functionalized MSMs of phenylene-

bridged mesoporous silica with 2D hexagonal (P6mn) structure

SO3H–PMO2D-hex-BTSB (36.1%), sulfonated SBA-1 (Pm3n)

mesoporous silica (26.7%) and sulfonated phenyltrimethoxy silane

(PTMS) grafted SBA-1 (Pm3n). The high catalytic activity


38

indicates that 3D cubic structure of SO3H–PMO3D-cub-BTSB

catalysts has ability to anchor higher concentration of sulfonic acid

sites and easier access of most ofthe available reaction sites in

diffusion of reactants and products during the reaction process.

Recently, in contrast to SO3H–PMO-BTSEa having ethane

bridging in the silica wall, ethylene bridging organosilane PMO in

the pore wall can be used to transform to phenylene sulfonic acid

groups through a two-step chemical modification (Nakajima et al.,

2005). The first step is a Diels–Alder reaction with

benzocyclobutene and the second step is a sulfonation in

concentrated sulfuric acid (H2SO4). In the pinacol-pinacolonere

arrangement reaction, the resulting SO3H–PMO-BTSEe catalyst

exhibited high and stable catalytic activities for formation of 2,3-

dimethyl-1,3-butadiene with conversion of 92%, selectivity of

16.5% and pinacol formation of 83.5% compared to sulfuric acid

(98.4%, 28.3% and 71.7%), heteropolyacids (70.5%, 90.8% and

9.2%) and p-toluene sulfonic acid (42.8%, 74.4% and 255.6%).

These results suggest that the resulting solid acid catalyst will

utilize the presence of electron-withdrawing groups of phenyl

substituents for dispersion of negative charges, stabilization of

anions and increasing of acid strengths.

The PMOs with aryl sulfonic acid groups (SO3H–PMO-

BTSEB) within the framework can be synthesized by sulfonation

of 1,4-diethylenebenzenegroups (PMO-BTSEB) using

chlorosulfonic acid. PMO-BTSEB was firstly prepared using co-

condensation of 1,4-bis(trimethoxysilylethyl)benzene (BTSEB)

with tetramethyl orthosilicate (TMOS) under acidic conditions

using triblock co-polymer Pluronic P123 as the SDA (Li et al.,

2007). The SO3H–PMO-BTSEB solid acid catalysts were used for

esterification of ethanol with different alkyl length of aliphatic

acids (acetic, butyric and hexanoic acids). By increasing length of

the acids, the activity of SO3H–PMO-BTSEB was enhanced with

the increasing amounts of incorporated 1,4-diethyl-enebenzene. It

was also found that PMOs with disulfide moieties bridged in the

pore wall synthesized by co-condensation of bis[3-

(triethoxysilyl)propyl]disulfide (BTPDS) and TMOS in acetic


39

acid–sodium acetate buffer solution (pH 4.4), using nonionic

surfactant P123 as the template can be used as a catalyst. The

disulfide moieties in PMO-BTPDS could be transferred to sulfonic

acid functionality by a simple post-oxidation method using nitrite

acid (HNO3) as an oxidant to give SO3H–PMO-BTPDS (Li et al.,

2008). This SO3H–PMO-BTPDS showed higher yield in the

esterification of aliphatic acid and ethanol with TON of around

80% than conventional heterogeneous solids such as zeolites and

sulfonic acid resin.

Another strategy to form sulfonated catalyst active site is to

attach phenyl ring directly to bridging ethylene group of

organosilanes SO3H–PMO-BTSEe before sulfonation reaction. In

this method, bridging ethylene groups in the silica wall of SO3H–

PMO-BTSEe were reacted by arylation reaction with benzene

using AlCl3 as a catalyst. The phenyl moieties of resulting Ph–

PMO-BTSEe were sulfonated by sulfonic acid to give Ph–SO3H–

PMO-BTSEe (Dube et al., 2008). The Ph–SO3H–PMO-BTSEe

catalysts exhibited a high catalytic activity in self-condensation of

heptanal at 75ºC with conversion of 40%, due to their high density

of acid sites and the presence of hydrophobic character in the

framework. This catalytic activity was shown to be higher than

othermaterials with acid sites in more polar condition such as

SBA-15 (25%) and Amberlyst-15 (5%).

3.3 CONCLUSIONS AND OUTLOOK

It can be concluded that ordered MSMs have been organically

functionalized by nanoscopic template method via one-pot co-

condensation reaction for preparation of solid acid catalysts. In the

catalytic oxidation reaction, the sulfonation of ordered MSMs were

used to create the active sites either in the pore or silica wall

framework. The active sites can be controlled by modifying the

precursors of mono and bridged organosilanes during the template

synthesis of MSMs. Moreover, post-functionalization of bridged

organosilane in the pore wall can be also functionalized by reacting


40

with phenyl ring with or without spacers prior to the sulfonation.

All of the functionalization can produce solid acid catalysts of

ordered MSMs with high catalytic conversions and selectivities

due to the increase of active sites, surface areas and hydrophobic

characters as well as dimension.

Since heteroatom (metal oxides) have been found as a good

solid acid catalyst, it is interesting to combine the above

approaches with metal oxide inside the ordered MSMs. Moreover,

nanoscopic template method can be also used to design specific

heterogeneous solid base catalysts in the oxidation reaction or as

photocatalysts in chemistry and materials science.

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44

Particuology of Metal Oxides in Bifunctional Catalyst Design

45

4

PARTICUOLOGY OF METAL

OXIDES IN BIFUNCTIONAL

CATALYST DESIGN

Siew Ling Lee, Jamilah Mohd Ekhsan and Yee Khai Ooi

4.1 DEVELOPMENT OF BIFUNCTIONAL CATALYST

Over the past few decades, development of effective catalysts is a

challenge in catalysis field. In particular, bifunctional catalyst

which consists of two active sites in a single material (Prasetyoko

et al., 2005). Bifunctional catalyst is referred as bifunctional

chemical species in catalyzed reaction that contains a mechanism

in which both functional groups are involved in the rate-controlling

step.

In industry, many main chemicals are produced through more

than one step reaction. For instance, diols, the important

intermediates in the manufacturing of pharmaceutical, pesticide

and fragrance and also important feedstocks in fine chemical

industry (Ling and Hamdan, 2008; Beller et al., 2004) are currently

produced via a two-step process involving epoxidation of an olefin

with a presence of oxidative catalyst, followed by hydrolysis of the

resulting epoxides using another catalyst possessing Brönsted

acidity. The process of manufacturing is not only time consuming,

but also costly due to involvement of two different reactions in two


46

separate reactors. For that reason, an effective bifunctional catalyst

has gained considerable scientific and industrial interest for a rapid

and cheaper production.

Previously, a bifunctional catalyst was synthesized by

incorporating titanium ion and niobic acid in zeolite molecular-

sieve (Nb/TS-1) via hydrothermal and impregnation method

(Prasetyoko et al., 2005). The catalyst was active in both oxidation

reaction in presence of tetrahedral Ti4+

and acid-catalyzed in

presence of niobic acid. Niobic acid exhibited high catalytic

activity, selectivity and stability for acid-catalyzed reaction

(Nowak and Ziolek, 1999).

Many kind of supports have been widely used in catalyst

preparation such as MCM-41, silica aerogel, fumed silica etc.

Usually, these supports have large surface area and high porosity

which allow well dispersion of active sites and effective diffusion

process (Alcala and Real, 2006). Amongst all, silica aerogels have

received much attention as catalyst support due to the extremely

high surface area, low bulk density, hydrophobicity, optical

transparency, low thermal conductivity and excellent heat

insulation properties (Dorcheh and Abbasi, 2008; Gurav et al.,

2010). Ti-based catalyst could be easily loaded onto and into the

silica aerogel framework without destroying the aerogel structure

via impregnation and sol-gel methods.

Additionally, fumed silica has also been widely used as

catalyst support. This material has good thermal stability and it is

chemically inert (Barthel, 1995). It is usually formed by a random

packet [SiO4]4-

units and it has lower density compared to

crystalline silica (Chai, 2005). SiO2 can also be isomorphously

substituted by some other elements in its structure, making it an

important material used in catalysis design (Astorino et al., 1995).

4.1.1 Surface Acidity on Metal Oxides

Acidic possessions of solid surfaces are interesting aspects of

structure and they are important in the fields of heterogeneous

catalysis (Yazici and Bilgic, 2010). Typically, performance of


47

catalyst can be significantly enriched by improving both quality

and quantity of acidity in the catalyst (Lee et al., 2011; Chandren

et al., 2008; De Pietre et al., 2010). For example, introduction of

anionic species such as sulphates, tungstates and phosphates in

metal oxides catalyst has resulted in high activity in acid-catalyzed

reactions (Ramis et al., 1989).

The surface of a metal oxide consists of ordered arrays of

acid-base centres. The cationic metal centres act as Lewis acid

sites while the anionic oxygen centres act as Lewis bases. Surface

hydroxyl groups are able to serve as Brönsted acid or base sites as

they are able to give up or accept a proton. The surface of most

metal oxides will be, to some extent, hydroxylated under normal

conditions when water vapor is present. The strength and the

amount of Lewis and Brönsted acid-base sites will determine the

catalytic activity of many metal oxides. Due to this there is a great

need to develop standard methods for the characterization of the

strength, concentration, and distribution of surface acid-base sites.

The concepts of Lewis acid-base theory and Brönsted-Lowry

acid-base theory may be applied to surfaces. For metal oxides,

acidity and basicity are dependent on the charge and the radius of

the metal ions as well as the character of the metal oxygen bond.

The bond between oxygen and metal is influenced by the

coordination of metal cations and oxygen anions as well as the

filling of the metal d-orbitals. The surface coordination is

controlled by the face that is exposed and by the surface relaxation.

Structural defects can greatly contribute to the acidity or basicity as

sites of high unsaturation can occur from oxygen or metal ion

vacancies. The catalysis and other properties of the surface are

very condition dependent. The temperature of the surface, defects

and impurities so on are known to have an effect on the behavior

of the surface. The interaction between atoms and small molecules

such as carbon, nitrogen or oxygen and the surface is usually

determined by the interplay between the p-band of the molecule

and the d-band of the transition metal surface.

The catalytic activity for the transition metal oxide catalyst

much depends on the degree of coordinative unsaturation of a


48

surface cation and defect sites. The degree of coordinative

unsaturation of a surface cation measures the number of bonds

involving the cation that have to be broken to form a surface. As

the degree of coordinative unsaturation increases, more bonds are

broken and the metal cation becomes destabilized. The

destabilization of the cation increases the surface Gibbs energy,

which decreases the overall stability and increase in the acid sites.

Defect sites can interfere with the stability of metal oxide surfaces.

Oxides exhibit an abundance of point defect sites. Oxygen and

metal cation vacancies are the most common point defects. The

vacancies are produced by electron bombardment and annealing to

extremely high temperatures. However, oxygen vacancies are more

common and have a greater impact than metal cation vacancies.

Oxygen vacancies cause reduction in between surface cations,

which significantly affect the electronic energy levels. There are

two types of oxygen vacancies, which resulted from either the

removal of a bridging O2-

ions or the removal of an inplane O2-

ion.

Both of these reduced the coordination of the surface cations.

4.2 ROLE AND USAGE OF METAL OXIDES IN

BIFUNCTIONAL CATALYST

Almost all of the metal catalysts are transition metals and the

catalytic behavior is clearly associated with the presence of the d-

orbital. The d-band being narrow and having high density of states

may have some of its states unfilled. The unfilled states are called

holes in the d-band. For catalysis and oxidation the possible strong

surface state of the electronic structure is relevant. Table 1 shows

the various metal oxides catalyst for different industrial

applications.


49

4.3 PARTICUOLOGY OF METAL OXIDES IN

BIFUNCTIONAL CATALYST DESIGN

Different metal oxides including titanium, vanadium, niobium,

chromium, tungsten and others have been proposed for

incorporation in various bifunctional catalyst systems. Transition

metal oxides are compounds composed of oxygen atoms bound to

transition metals. They are commonly utilized for their catalytic

activity and semiconductive properties. Transition metal oxides are

also frequently used as pigments in paints and plastics, most

notably titanium dioxide. Transition metal oxides have a wide

variety of surface structures which affect the surface energy of

these compounds and influence their chemical properties. The

relative acidity and basicity of the atoms present on the surface of

metal oxides is also affected by the coordination of the metal

cation and oxygen anion, which alter the catalytic properties of

these compounds. For this reason, structural defects in transition

metal oxides greatly influence their catalytic properties. The acidic

and basic sites on the surface of metal oxides are commonly

characterized via infrared spectroscopy and calorimetry among

other techniques.

One of the more researched properties of these compounds is

their response to electromagnetic radiation, which makes them

useful catalysts for redox reactions, isotope exchange, specialized

surfaces and a variety of other uses currently being studied.

Considerable efforts have been made for synthesis and

developing a bifunctional catalyst which is potentially active for a

consecutive process. Ti-Al-Beta zeolite is an example of such

catalyst that has demonstrated bifunctionality; oxidative and acid

catalytic activity in consecutive reactions. However, the existing

competition between titanium and aluminium in the isomorphous

substitution of the zeolite framework often results in low

production of epoxide and diol. A bifunctional catalyst of sulfated

zirconia TS-1 having both oxidative site and Brönsted acidity was

reported (Nur et al., 2005).

Tetrahedrally coordinated Ti species in the framework of


50

silicalite was an efficient oxidative site for epoxidation. However,

limited acidity from octahedral zirconia containing sulfate has

restricted the production of diols. On the other hand, silica-titania

aerogel is a promising catalyst for epoxidation due to the high

distribution of Ti4+

species in the catalyst. Unfortunately, these

materials including silica-titania aerogels do not consists of

Brӧnsted acidity, but only Lewis acidity. Thus, there was no

transformation of diols from epoxides by using these materials,

because presence of Brönsted acidity is crucial for the conversion.

Table 1 Various metal oxides catalyst for different industrial

applications

Catalyst Process References

vanadium

oxides

Sulfuric acid synthesis (Contact

process) Roco et al., 2011

iron oxides

on alumina

Ammonia synthesis (Haber-

Bosch process) Roco et al., 2011

unsupported

Pt-Rh gauze

Nitric acid synthesis (Ostwald

process) Roco et al., 2011

Nickel or

K2O

Hydrogen production by Steam

reforming Liu et al., 2011

silver on

alumina,

with many

promotors

Ethylene oxide synthesis Huang et al., 2010

Pt-Rh Hydrogen cyanide synthesis

(Andrussov oxidation)

Thompson et al.,

2011

TiCl3 on

MgCl2

Olefin polymerization Ziegler-

Natta polymerization Valverde et al., 2013

Mo-Co on

alumina

Desulfurization of petroleum

(hydrodesulfurization) Zhang et al., 2010

TS-1 loaded

sulfated

zirconia

Hydroxylation of alkene Nur et al., 2005

Modification via acid treatment is one of the approaches used to

enhance the Brönsted acidity in a catalyst. An enhanced

http://en.wikipedia.org/wiki/Alumina

http://en.wikipedia.org/wiki/Silver

http://en.wikipedia.org/wiki/Alumina

http://en.wikipedia.org/wiki/Titanium_trichloride

http://en.wikipedia.org/wiki/Magnesium_chloride

http://en.wikipedia.org/wiki/Molybdenum

http://en.wikipedia.org/wiki/Molybdenum


51

epoxidation of 1-octene to 1,2-epoxyoctane using sulfated TS-1

catalyst has been reported, which suggested that the local

environment of Ti active sites changed upon interaction with the

SO42-

ions. Hence, creation of Brönsted acid sites in the silica-

titania aerogel is necessary in order for the system to be an

efficient catalyst. In addition, it is desirable to create a catalytic

system that is potentially useful for consecutive reactions which

involve large molecules, contains both the oxidative and Brönsted

acid sites, as well as large specific surface areas and pore size.It

has been demonstrated that sulfated silica-titania aerogel was an

excellent oxidative-acidic bifunctional catalyst in a consecutive

transformation of 1-octene to 1,2-octanediol through the formation

of 1,2-epoxyoctane.

Preparation of solid catalyst via sol-gel method has been

reported by many research groups (Gonzalez et al., 1997; Ueno et

al., 1983). Sol-gel method appears as a simple, inexpensive, and

easy way to synthesize a catalyst. A sol is a stable dispersion of

colloidal particles or polymers in a solvent. The particle may be

amorphous or crystalline. Meanwhile, a gel consists of a three

dimensional continuous network, which enclose a liquid phase.

In the past decades, attachment of titania onto/into silica via

sol-gel method at room temperature had been widely carried out by

the researchers (El-Toni et al., 2006; Lenza and Vasconcelos,

2002). Synthesis of silica via sol-gel method through hydrolysis

process, followed by condensation of metal alkoxide precursor has

resulted in materials of high surface area (Asomoza et al., 1998;

Waseem et al., 2009; Nair et al., 1996). Besides, sol-gel method

allowed better dispersion of catalyst on its support as compared to

other methods.

Since last decade, the interest in the application of niobium

compounds in heterogeneous catalysis is growing. Owing to the

different structures and properties, niobium compounds, especially

niobium oxides, exhibit unique activity, selectivity and stability for

many different catalytic reactions (Ziolek, 2003; Tanabe, 2003). In

general, these compounds can be prepared easily via simple

method using cheap starting niobium compounds, leading to low


52

cost production. Besides, they are also having relatively high

surface area and acidity, making it an excellent catalyst. For

example, hydrated niobium oxide consists of both Lewis and

Brönsted acid sites (Prasetyoko et al., 2008).

4.3.1 Sulphated Silica-Titania Aerogel Bifunctional Catalyst

The dramatic increase in oxidative catalytic activity of sulphated

silica-titania aerogel (SO4/ST) was due to the presence of higher

amount of hydrated, tetrahedral Ti species. Ti-O speciesmay have

converted to tripodal titanium active sites [i.e. Ti(OSi)2(SO3)OH]

to form a more stable hydroperoxide intermediate in SO4/ST.

Besides, the availability of tripodal Ti active site in sulfated

materials evidently improved the activity of epoxidation. Highly

negative sulfur would withdraw electrons from Ti towards the

direction where sulfur is located, hence leading to an increase in

activity of epoxidation as a result of vicinal oxygen atom being

more vulnerable to the attack of π electron clouds of 1-octene. It

has been documented that tetrahedral Ti species is the oxidative

site for the formation of epoxides. Accordingly, tripodal open

lattice site of Ti [i.e. Ti(OSi)3OH] on the surface of silica-titania

aerogel was more active for epoxidation compared to the bipodal

[i.e. Ti(OSi)2(OH)2] and the tetrapodal closed lattice sites [i.e.

Ti(OSi)4]. An excellent activity of SO4/ST for epoxidation of 1-

octene by aqueous H2O2 was due to generation of Brönsted acid

sites as a result of modification with SO42-

ions on the surface of

titania-silica aerogel.

The reactivity of oxo-titanium species in SO4/ST was

generated from the interaction of tetrahedral titanium with aqueous

H2O2. In fact, the adsorption rate of aqueous H2O2 onto the aerogel

surface is believed to be due to the existence of sulphate groups on

the surface. Figure 4.1 shows the generation of Lewis acid and

Brönsted acid sites in the sulphated silica-titania aerogel samples

after the acid treatment (Ling and Hamdan, 2008).


53

Figure 4.1 Proposed scheme showing Lewis acid (LA) and Brönsted

acid sites (BA) in samples ST and SO4/ST at: (a) bipodal (b) tetrapodal

(c) tripodal titanium (Ling and Hamdan, 2008)

4.3.2 Sulfate-Vanadium Treated Silica-Titania Aerogel

Bifunctional Catalyst

After impregnation of 1 wt% vanadium on silica-titania aerogel,

the amount of Lewis acidity sites increased remarkably to 70%,

without formation of any Brönsted acid sites (Lee et al., 2011). It

was claimed that the tetrahedral vanadium probably reacted with

hydrated tetrahedral titanium species or directly with Si-O-Si on

the aerogel, leading to creation of more Lewis acidic sites in

vanadium impregnated silica-titania. As a result, more isolated

titanium species were detected in the catalyst.

As compare to SO4/ST, sulphate-vanadium treated silica-

titania (SO4_V/ST) possesses higher Lewis and Brönsted acid

sites. Since electro negativity of vanadium (1.63) is higher than

that of titanium (1.54), it is believed that more Brönsted acidity

was available with formation of V(OSi)2OH-O-SO3- in which two

protons could be easily released. Similarly, direct interaction

between vanadium and phosphate contributed for Brönsted acidity

generation in silica-titania aerogel, leading to high yield of diol

(Lee et al., 2009). Figure 4.2 illustrates the generation of Lewis

acid sites (LA) after addition of vanadia and formation of Brönsted

acid sites (BA) after sulphuric acid treatment.


54

H2SO4 +

H2O

Figure 4.2 Proposed model of: (a) ST (b) V/ST and (c) SO4_V/ST

showing the formation of Lewis acid (LA) and Brönsted acid (BA) sites

(Lee et al., 2010)

4.3.3 Sulfated Zirconia TS-1 Bifuctional Catalyst

A research on titanium-containing silicalite, TS-1 as catalyst was

reported by Taramasso et al. in 1983 (Taramasso et al., 1983;

Serrano et al., 1995). The combination of isolated tetrahedrally

coordinated titanium in a silicate structure and a hydrophobic as

well as the non-acidic environment is the strength of TS-1 catalyst

(Bellussi and Rigutto, 1994). Besides, the unique catalytic

properties of the material were associated to specific coordination

chemistry of lattice titanium ion. It was stated that the Lewis

acidity would be developed in TiO2-rich region, while Brönsted

acidity would be formed in the SiO2-rich region. However, no

evidence of existence of Brönsted acid sites in TiO2-SiO2 materials

was reported (Hu et al., 2003).

Bifunctional oxidative and acidic catalysts have been

successfully prepared by the dispersion of sulfated zirconia on the

TS-1 (Prasetyoko et al., 2005). The catalysts have oxidative site

due to titanium located in the framework of silicalite, while

octahedral zirconium containing sulfate as Brönsted acidic sites.


55

Figure 4.3 Proposed model of TS-1 loaded with sulphated zirconia as

bifunctional catalyst for consecutive transformation of 1-octene to 1,2-

octanediol through the formation of 1,2-epoxyoctane (Prasetyoko et al.,

2005)

4.3.4 Niobium-Phosphate Impregnated Silica-Titania

Bifunctional Catalyst

Results of the catalytic testing of niobium-phosphate impregnated

silica-titania (PO43–

/Nb/TiO2-SiO2) in consecutive transformation

of 1-octene to 1,2-octanediol through formation of 1,2-

epoxyoctane strongly suggested that Nb2O5 was a more important

oxidative active site compared to tetrahedral Ti species. Besides,

co-existence of Nb2O5 and PO43–

modifiers was important for

Brönsted acidity generation in PO43–

/Nb/TiO2-SiO2. It was

reported that the amount of Brönsted acid created was strongly

dependent on the synthesis method that was greatly affected by the

interfacial interaction between Nb2O5 and PO43–

in the material to

produce Nb—O—PO43–

—H+ bonding. The proposed structure for

the formation of Lewis and Brönsted acid sites in PO43–

/Nb/TiO2-

SiO2 is depicted in Figure 4.4 (Ekhsan, 2013). It was believed that

the acid sites generation was due to the inductive effect of the


56

PO43–

group, the presence of Nb species and also tetrahedral Ti

species on the surface of SiO2.

H3PO

4

+ H2O

Nb

LA

Si Si Si Si Si

O O O O OH

Nb

O

LA LA

Si Si Si Si Si

O O O O

OH

Ti Nb LA

Si Si Si Si

O O OH OH OH

Ti

OH O–

Ti

OH

O O O

O H

H

P P

O O O

O

+

LA BA

BA

O– O

–

O–

Si

Figure 4.4 Proposed model in formation of Lewis acid (LA) and

Brönsted acid (BA) sites (Ekhsan, 2013)

4.4 CONCLUSION AND PERSPECTIVE FOR FUTURE

DIRECTIONS

In short, oxidative-acidic bifunctional catalysts of metal oxides

modified TiO2-SiO2have been a focus in catalysis field for their

high potential use in industry. The researchers have reported on

effect of synthesis method and type of modifier on particulogy of

the resulted materials which subsequently gave great impact to the

catalytic performance. Nevertheless, the creation of two different

active sites in a single solid material remained a challenge to the

researchers in an attempt to further increase the activity and

selectivity of the catalyst. In fact, the amount ratio and the strength

of each oxidative and acidic sites in the catalyst might play


57

important for its selectivity and it is however yet to be explored.

Besides, understanding on how environment affects catalytic

behavior and more precise catalytic mechanisms are also required

for better bifunctional catalyst design.

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62


63

INDEX

A

Amberlyst, 33, 35, 36, 39

Amphiphilic, 5

B

Bifunctional catalyst, 45-46,

48-49,51-53, 55-57

Brönsted acid, 45, 47, 50–56

F

Faraday's law, 1

H

hexadecyltrimethylammonium

bromide, 32

L

Lewis acid, 47, 50, 52-54,56

Lewis base, 47

M

Magnetic field, 1-4, 7-11

Metal alkoxide, 51

3-mercaptopropyl-

trimethoxysilane, 32

propyl trimethoxysilane 34

2-(4-chlorosulfonylphenyl)-

ethyl-trimethoxysilane,

36

Mesoporous silica, 22-23, 29-

33, 35, 37

N

n-Dodecylamine, 32

Nanoparticle, 8, 18-19, 22

Nanoscopic template, 29-31,

39-40

S

Self-assembly, 6-8, 10-11

Silica-titania aerogel, 50–53

Sulfated zirconia, 49-50, 54

Surfactant, 3–11, 24, 31, 33,

39

Anionic, 6 -7

CTAB, 9–11

C16TABr, 34

C12TABr, 34

C10TABr, 34

Cationic, 6-7, 47

T

Titania, 4, 50-53, 55

Titanium dioxide, 9, 49

Tungsten oxide, 15

V

Visible light, 15–18, 20-21,

23, 25-26

W

WO3, 15–26

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