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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
Diatur huruf oleh / Typeset byHADI NUR
Faculty of ScienceUNIVERSITI TEKNOLOGI MALAYSIA,
81310 UTM Johor Bahru,
Diterbitkan di Malaysia oleh / Published in Malaysia byPENERBIT UTM PRESS
UNIVERSITI TEKNOLOGI MALAYSIA,81310 UTM Johor Bahru,
Johor Darul Ta'zim, MALAYSIA.(PENERBIT UTM ahli MAJLIS PENERBITAN ILMIAH MALAYSIA (MAPIM) dan anggota
PERSATUAN PENERBITAN BUKU MALAYSIA (MABOPA) dengan no. keahlian 9101)
Dicetak di Malaysia oleh / Printed in Malaysia byUnivision Press Sdn. Bhd
Lot 47 & Lot 48, Jalan SR 1/9, Seksyen 9 Jalan Serdang Raya, Taman Serdang Raya
43300 Seri Kembangan Selangor, Darul Ehsan, MALAYSIA
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
Particuology of Some Metal Oxides Catalyst
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
Aligned Titanium Dioxide Catalyst Synthesized under Magnetic
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).
Particuology of Some Metal Oxides Catalyst
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
Aligned Titanium Dioxide Catalyst Synthesized under Magnetic
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.
Particuology of Some Metal Oxides Catalyst
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.
Aligned Titanium Dioxide Catalyst Synthesized under Magnetic
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
Particuology of Some Metal Oxides Catalyst
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)
Aligned Titanium Dioxide Catalyst Synthesized under Magnetic
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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Particuology of Some Metal Oxides Catalyst
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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
Particuology of Tungsten Oxide as Visible Light Driven Photocatalyst
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
Particuology of Some Metal Oxides Catalysts
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.
Particuology of Tungsten Oxide as Visible Light Driven Photocatalyst
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
)
Particuology of Some Metal Oxides Catalysts
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
Particuology of Tungsten Oxide as Visible Light Driven Photocatalyst
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
Particuology of Some Metal Oxides Catalysts
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
Particuology of Tungsten Oxide as Visible Light Driven Photocatalyst
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
Particuology of Some Metal Oxides Catalysts
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,
Particuology of Tungsten Oxide as Visible Light Driven Photocatalyst
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.
Particuology of Some Metal Oxides Catalysts
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.
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Particuology of Some Metal Oxides Catalysts
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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
Particuology of Some Metal Oxides Catalysts
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.
Synthesis of Mesoporous Silica Catalyst by A Nanoscopic Template
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
Particuology of Some Metal Oxides Catalysts
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).
Synthesis of Mesoporous Silica Catalyst by A Nanoscopic Template
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
Particuology of Some Metal Oxides Catalysts
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.
Synthesis of Mesoporous Silica Catalyst by A Nanoscopic Template
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
Particuology of Some Metal Oxides Catalysts
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
Synthesis of Mesoporous Silica Catalyst by A Nanoscopic Template
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
Particuology of Some Metal Oxides Catalysts
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
Synthesis of Mesoporous Silica Catalyst by A Nanoscopic Template
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
Particuology of Some Metal Oxides Catalysts
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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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
Particuology of Some Metal Oxides Catalysts
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
Particuology of Metal Oxides in Bifunctional Catalyst Design
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
Particuology of Some Metal Oxides Catalysts
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.
Particuology of Metal Oxides in Bifunctional Catalyst Design
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
Particuology of Some Metal Oxides Catalysts
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
Particuology of Metal Oxides in Bifunctional Catalyst Design
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
Particuology of Some Metal Oxides Catalysts
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).
Particuology of Metal Oxides in Bifunctional Catalyst Design
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.
Particuology of Some Metal Oxides Catalysts
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.
Particuology of Metal Oxides in Bifunctional Catalyst Design
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
Particuology of Some Metal Oxides Catalysts
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
Particuology of Metal Oxides in Bifunctional Catalyst Design
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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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