CHAPTER 1 INTRODUCTION -...
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CHAPTER 1
INTRODUCTION
The word catalyst is a combination of two Greek words “kata” and
“lysein” meaning “loosening down” was first discovered by Berzelius in
1836. In the present days, catalysts market is witnessing rapid growth. There
are many types of catalysts of which environmental catalysts are the biggest
segment accounting for 27%, followed by polymerization catalysts for 22%,
refining catalysts for 21% and petrochemical catalysts for 20%. Basically,
there are two types of classification of catalysts such as homogeneous and
heterogeneous. A homogeneous catalyst (Lewis acid catalyst) is well known
and has been applied in several reactions like Friedel-Crafts alkylation,
acylation, esterification and condensation. But these catalysts lack in
problems such as corrosion, loss of catalysts and above all disrupting the
environment. On the other hand, heterogeneous catalysts can be easily
separated from the reaction mixture and can be reused, non-corrosive in
nature, easy to recover and clean reaction product solution after filtration. As
a result, development of efficient heterogeneous catalysts is interesting and
also useful in various fields. Different classes of materials have been utilized
as heterogeneous catalysts that include zeolites, metal oxides and heteropoly
acids.
1.1 GREEN CHEMISTRY
In the competitive world, chemical industry is one of the most
important manufacturing industries. A great variety of products are
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synthesized in an industry, which can be scaled from simple to complex
(Macquarrie 2000). The major challenge in the present scenario lies in the
development of new methods for clean production of chemicals, which is
termed as green chemistry. Its role is to redesign chemistry with the desired
product from a reaction produced without generating waste. It involves an
extensive range of approaches including invention of new reactions for the
development of new catalysts. The substitution of traditional Lewis and
Brønsted acid catalysts by heterogeneous analogues, i.e., solid acids,
continues to be one of the major research topics in the context of green
chemistry and in the design and application of supported reagent type
catalysts. Catalysts play a vital role in green technologies and can be used to
sustain environmental pollution in two different ways:
· End-of-pipe catalysis: for cleaning of outgoing waste gases
· Process-incorporated catalysis: for improvement or
replacement of existing processes
In the area of catalysis, solid acid catalysts play crucial role which
are found to have many uses as highly selective catalysts in a wide range of
applications.
1.2 POROUS MATERIALS
Porous materials have been intensively studied in respect of
technical applications as catalyst and catalyst support. According to IUPAC
(International Union for Pure and Applied Chemistry), porous materials are
divided into three classes as shown in Table 1.1.
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Table 1.1 Classification of porous materials
Class Pore diameter Example
Microporous <20 Å Zeolites, VPI-5, microporous AlPOs and SAPOs, pillared clays, etc.,
Mesoporous 20-500 Å
MCM-41, MCM-48, MCM-50, mesoporous metal oxides, mesoporous carbon, mesoporous AlPOs, SAPOs, SBA-1, SBA-15, KIT-5, KIT-6, etc.,
Macroporous >500 Å Porous gels, porous glasses, etc.,
In the present scenario, the concept of nanoporous is also being
widely used in the place of mesoporous. Ordered mesoporous silica materials
exhibit tunable pore size, high surface area, pore diameter, pore wall thickness
and pore volume, ease of surface functionalization and controllable
morphology, all these are highly promising properties for numerous
applications. Considerable scientific efforts have been focused on the
preparation, characterization and application of ordered mesoporous silicas
(Ciesla and Schuth 1999, Ying et al 1999, Davis 2002, Taguchi and Schuth
2005, Meynen et al 2009). Many reviews have been published covering
various aspects of ordered mesoporous materials such as their synthesis,
surface modifications, application as host materials and in catalysis. In this
thesis, we will first describe the general methods for the preparation of
ordered mesoporous materials with the emphasis on their applications in
catalysis and focus on exploiting the special features of the ordered
mesoporous materials.
1.3 ZEOLITES
Zeolites are well known micro crystalline porous materials largely
studied and applied in petrochemical industry and more recently in fine
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chemical industry (Corma 1995 and Sen et al 1999). There are more than 40
natural zeolites, of which 20 are of synthetic. The primary structural units of
zeolites, (SiO4)4- and (AlO4)5- tetrahedral, are assembled into secondary
building units (SBU) which may be simple polyhedra such as cubes,
hexagonal prisms or octahedral. Zeolites should be best considered as solid
solvents, when they are used as catalysts for liquid-phase organic reactions
(Derouane 1999 and 2000). The partitioning of the reactants and products are
determined by their nature and relative amount, the type of zeolite,
temperature and other conventional factors. The different behavior can be
observed when zeolites are utilized in the liquid phase and vapor phase
reactions. The majority of the world’s gasoline is also produced by the
fluidized catalytic cracking (FCC) of petroleum using zeolite catalysts.
1.4 MESOPOROUS MATERIALS
A large variety of mesoporous materials with different
mesostructures (two-dimensional (2-D) hexagonal, space group p6mm, three-
dimensional (3-D) hexagonal P63/mmc, 3-D cubic Pm3m, Pm3n, Fd3m,
Fm3m, Im3m, bicontinuous cubic Ia3d, etc.) and compositions (silica, metal
oxides, metal sulfides, metals, polymers and carbons) have been synthesized.
The structures of lamellar, Ia3d, p6mm and Fm3m are shown in Figure 1.1.
The abbreviations with space group for several types of ordered mesoporous
silicas are summarized in Table 1.2.
Figure 1.1 Structures of mesoporous materials
Lamellar Ia3d p6mm Fm3m
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Table 1.2 Mesoporous silicas with their space group
Acronym of ordered mesoporous
silica Significance Structural
symmetry
MCM-41 Mobil Composition of Matter Hexagonal P6mm MCM-48 Mobil Composition of Matter Cubic Ia3d
MCM-50 Mobil Composition of Matter Lamellar SBA-1 Santa Barbara Cubic Pm3n
SBA-15 Santa Barbara Hexagonal P6mm
SBA-16 Santa Barbara Cubic Im3m KIT-6 Korean Advanced Institute of
Science and Technology Cubic Ia3d
KIT-5 Korean Advanced Institute of Science and Technology
Face-Centered-Cubic Fm3m
HMS Hexagonal Mesoporous molecular Sieves
Hexagonal
1.4.1 M41S Family
The first mesoporous material, M41S with a long-range order, was
synthesized by the research group of Mobil Oil company (Kresge et al 1992).
The discovery of a very similar material FSM-16 (formed by recrystallization
of kanemite after ion exchange of Na+ ions for tetraalkyl ammonium ions) by
Inagaki et al (1993) marked the beginning of a new era of well-defined,
periodic mesoporous oxides. Further investigation by the same research group
revealed that the same synthesis, different mesophases could be produced.
MCM-41 forms around rod-like micelle surfactant aggregates. The other
related phases such as MCM-48 and MCM-50 have cubic and lamellar
mesostructure respectively. These three different mesophases of M41S family
are shown in Figure 1.2 (Biz and Occelli 1998). The surfactant to silica ratio
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was the crucial factor in determining the shape of micelle aggregates. Pore
widths of 16 to 100 Ǻ and more can be achieved by choosing appropriate
synthesis conditions and organic templates (Beck et al 1994). Mesoporous
materials are generally synthesized at low temperature (25–100 oC) so that the
condensation reactions are predominantly kinetically controlled. The
mesoporous silica walls in these materials are amorphous on the atomic scale
which means that they are thermodynamically less stable than the metastable
zeolite frameworks.
Hexagonal MCM-41 Cubic MCM-48 Lamellar MCM-50
Figure 1.2 Mesophase structure of M41S family
1.4.2 SBA Family
Apart from M41S family, various SBA series catalysts such as
SBA-1, SBA-11, SBA-12, SBA-15 and SBA-16 were found. A cubic
mesoporous silica structure (SBA-11) with Pm3m diffraction symmetry has
been synthesized in the presence of Brij 52, C16H33(OCH2CH2)10OH
(C16EO10) surfactant species, while a 3-D hexagonal (P63/mmc) mesoporous
silica structure (SBA-12) results when C18EO10 is used. The x-ray diffraction
(XRD) patterns of as-synthesized SBA-11 can be indexed as a cubic
mesophase belonging to Pm3m (221) space group. The XRD patterns of
as-synthesized SBA-12 showed three poorly resolved peaks appear in the 2q
range of 1-2° with d-spacings of 65.7, 63.5 and 58.3 Å and two weak but well
resolved peaks in the 2q range of 2-5° with d-spacings of 24.4 and 21.8 Å.
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The XRD patterns of SBA-16 can be indexed to (110), (200), (211), (220),
(310), (222) and (321) reflections corresponding to a cubic structure (Im3m
space group, unit cell parameter (a) = 176 and 166 Å for as-synthesized and
calcined SBA-16 respectively). The novel mesoporous material SBA-1 has a
cage type structure with open windows (Kim and Ryoo 1999, Kruk et al
1999). It has three dimensional cubic structure (space group pm3n) of uniform
pore size in highly acidic media via S+X-I+ mechanism (S+ is the cationic
surfactant, X- is halogen anion and I+ is cationic silicic acid species). The
powdered XRD patterns of SBA-1 showed three reflections in the 2q range
2 - 3.5° which are indexed to (200), (210) and (211).
The hexagonal mesoporous SBA-15 is one of the most important
ordered mesoporous silica synthesized after MCM-41. The synthesis of
SBA-15 was reported by Zhao et al (1998, 1998a) and Clerc et al (2000).
SBA-15 exhibits a significant amount of disordered micropores and small
mesopores. The volume and size of these complementary pores were found to
be dependent to some extent on the synthesis/aging temperature (Kruk et al
2000). SBA-15 materials were synthesized in acidic media to produce highly
ordered, two-dimensional hexagonal (space group p6mm) mesoporous silica.
Calcination at 500 °C gave porous structures with unusually large interlattice
d-spacing of 74.5 to 320 Ǻ between the plane (100), pore size from 46 to
300 Ǻ, pore volume up to 0.85 and wall thickness from 31 to 64 Ǻ. SBA-15
can be readily prepared with a wide range of uniform pore size and pore wall
thickness at low temperature (35 - 80 °C) using a variety of poly(alkylene
oxide) triblock copolymers and by the addition of organic molecules as
co-solvent. The triblock copolymer species can be recovered for reuse by
solvent extraction with ethanol or removed by heating at 140 °C for 3 h. Both
cases, the product yielded thermally stable in boiling water.
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The low cost and non-toxicity of this surfactant was reported to be
the main advantage. The X-ray patterns of as-synthesized SBA-15 prepared
using EO20PO70EO20 (Pluronic P123) showed four well-resolved peaks that
can be indexed to (100), (110), (200) and (210) diffraction peaks associated
with p6mm hexagonal symmetry. Three additional peaks appeared in the
2q range 2.5-3.5° that can be indexed to (300), (220) and (310) scattering
reflections. SBA-15 has recently attracted much attention due to potential
applications in catalysis and separation processes. Several different synthesis
strategies have been proposed and successfully used to prepare
mesostructures with unique pore-size distribution.
1.4.3 KIT Family
Korean scientists (Kleitz et al 2003) first reported a 3-D cubic
mesoporous material, KIT-6 (KAIST), having large pores with thick wall,
high hydrothermal stability, high surface area and large pore volume. KIT-6
exhibits cubic Ia3d symmetry and its structure consist of the interpenetrating
bicontinuous network of channels such as those in MCM-48. In contrast to
MCM-48, these two intertwined systems of relatively large channels in KIT-6
can also be connected through irregular micropores present in the mesopore
walls analogous to those in SBA-15. The present method has the advantage of
high reproducibility in significantly large quantities. The KIT-6 material
consists uniquely of large ordered domains of pure bicontinuous
mesostructure (Sakamoto et al 2004). KIT-6 silica materials obtained using
the following gel composition: 0.017 P123:1.2 TEOS:1.31 BuOH: x HCl:195
H2O. The unit cell size, calculated from the (211) reflection of the Ia3d phase,
is measured to be 22.5 nm for a calcined material obtained at 373 K and with
TEOS/BuOH = 1.2/1.31, a value much larger than that of MCM-48.
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The nitrogen adsorption-desorption isotherm obtained for calcined
KIT-6 mesoporous Ia3d silica is a type IV with a sharp capillary condensation
step at high relative pressures and H1 hysteresis loop, indicative of large
channel-like pores in a narrow range of size. The material typically
synthesized at 100 °C has a BET surface area of 800 m2/g, high pore volume
reaching 1.05 cm3/g and average pore size of 8.5 nm. The Ia3d phase can be
generated with various ranges of compositions at low HCl concentration in
the presence of butanol. The median pore diameter of materials synthesized at
373 K varies between 6.8 and 8.2 nm, strongly depending on the initial gel
composition. The cubic Ia3d KIT-6 silica can easily be synthesized with
sodium silicate used as silica source instead of TEOS. Varying the
hydrothermal treatment temperature between 35 and 130 °C allows a very
effective tailoring of the mesopore diameters ranging from 4 to 12 nm.
The present synthesis is simple and produces large quantities of high quality cubic Ia3d mesoporous silica KIT-6. The addition of butanol is
decisive for the nature and quality of the final mesophase. At low amount of
butanol (BuOH/P123 in weight < 0.9), a 2-D hexagonal mesophase was obtained. Furthermore, XRD results revealed that the cubic Ia3d phase is
formed via phase transformation mechanism from a lamellar phase appearing
initially after 6 h of reaction at 35 °C. These results indicate that the addition of butanol should be responsible for the preferred swelling of the hydrophobic
volume of the block-copolymer micelles, leading first the formation of
micellar aggregates with decreased curvature (lamellar mesophase). Such a decrease in micelle curvature upon butanol addition was also observed
previously for mesostructured silica obtained with cationic surfactant or high
concentration of block copolymer. Evidently, silicates are loosely condensed at early stages of the formation of the lamellar mesophase. Upon further
reaction at 35 °C or during hydrothermal treatment, condensation increases
progressively in the silicate region, which possibly provokes folding and regular modulation of the silica surface inducing significant changes in
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micelle curvature. Thus, the lamellar mesophase is preferred at low degree of polymerization and evolves into a highly ordered cubic Ia3d mesophase as the
silica condensation proceeds further. Such phase transition resulting from
interplay between silica polymerization and organic packing constraints are well known under basic conditions for silica/cationic surfactant mesophases.
However, no such phenomena have been demonstrated before for syntheses
based on non-ionic triblock copolymers. The structure transformation is determined by the presence of additional reflection below 0.7°, 2 theta angle.
It can be concluded that the two enatiomeric interpenetrating channel
networks forming the gyroid structure are independent from each other in the case of KIT-6 materials synthesized in the temperature range between 308 and
333 K. At higher treatment temperatures, faithful inverse carbon replicas are produced.
Recently, Kim and co-workers (2005) greatly extended the phase
domain for the cubic Ia3d mesoporous silica, whereas the amount of acid,
BuOH and silica source were changed correspondingly and allowed facile synthesis. However, these block copolymer templated Ia3d mesoporous
silicas are still synthesized under strong acidic conditions. Low acid
concentrations favor not only the facile preparation of high-quality mesoporous silicas using block copolymers (Choi et al 2003) but also the
direct incorporation of metal cations into the framework of mesoporous materials under acidic conditions.
Large-pore materials with cubic Ia3d structure could play a
significant role in material science if their preparation is more simplified and widely generalized. Very recently, new synthesis pathways utilizing block
copolymers as structure-directing agents have been proposed for the
preparation of cubic Ia3d materials. The utilization of low acid catalyst concentration condition, closer to thermodynamically favored mesophase
formation, suggested the use of co-solute molecules (hydrotropic molecules)
added to the block copolymer-water system in order to enrich the mesophase
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behavior. Kleitz et al (2003) have first reported a high quality mesoporous KIT-6 synthesized by hydrothermal method with a cubic Ia3d structure in
phase purity using n-butanol as co-solute. The synthesis of cubic Ia3d large
pore mesoporous silica is based on the use of n-butanol in combination with Pluronic P123 (EO20 PO70 EO20) for the structure direction in aqueous solution, at low HCl concentration.
Kleitz et al (2003a) have first synthesized large mesoporous Fm3m silica, designated as KIT-5. They prepared in aqueous solution using
EO106PO70EO106 (Pluronic F127) as structure-directing agent and TEOS as the
silica precursor (ACROS, 98%). High-quality samples were obtained with low HCl concentration of 0.4 - 0.5 M. The ordered mesoporous KIT-5 silica
has spherical cavities arranged in a face-centered-cubic array and connected
through narrow necks. The small angle powder XRD patterns of KIT-5 with (111), (200), (220) and (311) reflections of a face-centered close-packed cubic
lattice with Fm3m symmetry are clearly resolved, and no additional reflections related to 3-D hexagonal intergrowths were observed.
The nitrogen adsorption-desorption isotherm obtained for calcined
mesoporous KIT-5 silica is a type IV with broad H2 hysteresis loop that is
indicative of large uniform cage-like pores. Typical KIT-5 silica synthesized at 373 K has a BET surface area of 715 m2/g, total pore volume of 0.45 cm3/ g
and a cavity diameter of about 8 to 9 nm. Hydrothermal treatment at various
temperatures ranging from 318 to 423 K enabled a really effective tailoring of not only the mesopore diameters, but also the pore aperture size. In the case of
this cubic phase, the exact structure determination was crucial and the assignment to Fm3m symmetry was confirmed by the combined analysis of
the powder XRD patterns and TEM images. The excellent 3-D cubic
mesoscopic order of KIT-5 was also evidenced by TEM. It is important to remark that the images taken along the (110) direction revealed no
intergrowths with 3-D hexagonal phase, which are often reported to occur for other cage-like mesoporous silicas (SBA-2, SBA-12 and FDU-1).
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1.5 FORMATION MECHANISM OF MESOPOROUS MATERIALS
1.5.1 Outline of General Mechanism
A large number of studies have been carried out to investigate the
formation and assembly of mesostructures on the basis of surfactant self-assembly. The initial liquid-crystal template (LCT) mechanism first
proposed by the scientists of Mobil (Kresge et al 1992) is essentially true
because the pathways basically include almost all possibilities. The two main pathways seemed to be effective in the synthesis of ordered mesostructures are illustrated in Figure 1.3. They are
· cooperative self-assembly
· true liquid-crystal templating mechanism
Figure 1.3 Two synthetic strategies of mesoporous materials:
(A) cooperative self-assembly and (B) true liquid-crystal
templating process
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The main characteristic of LCT mechanism is that the liquid
crystalline mesophases or micelles act as templates rather than individual
single molecules or ions. The final material is a silicate skeleton which
contains voids that takes shape of these mesophases. The silicate
condensation is not the dominant factor in the formation of mesoporous
structure. The process may involve two possible mechanistic pathways viz.,
(1) liquid crystal mesophase may form prior to the addition of silicate species
and (2) silicate species added to the reaction mixture may affect the ordering
of the isotropic rod like micelles to the desired liquid crystal phase, i.e.,
hexagonal mesophase. Hence, the mesophase formed is structurally and
morphologically directed by the existing liquid crystal micelles and/or
mesophases.
The influence of alkyl chain length and the addition of mesitylene
on the pore size have been taken as strong evidence for the LCT mechanism,
since this phenomenon is consistent with the well-documented surfactant
chemistry (Winsor 1968). The auxiliary organic species added to the reaction
gel can be solubilized inside the hydrophobic region of micelles, causing an
increase in micelle diameter so as to increase the pore size of MCM-41. The
effect of pore size has been observed by Beck et al (1992). This proposed
LCT mechanism has been further confirmed by many reports (Chen et al
1993, Beck et al 1994). The effect of surfactant to silica molar ratio on the
resultant phase in a simple system containing alkali metal,
tetraethylorthosilicate, water and CTAOH at 100 °C have been studied by
Vartuli et al (1994). According to them, as the surfactant to silica molar ratio
increased from 0.5 to 2, the siliceous products obtained could be classified
into four separate groups: MCM-41 (hexagonal), MCM-48 (cubic), thermally
unstable lamellar phase and the cubic octomer [(CTMA) SiO2.5]8. The data are
in good agreement with the nature and chemistry of surfactants in solution as
mentioned above. There have been a number of models proposed to explain
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the formation of mesoporous materials and to provide rational basis for
various synthetic routes. Recent reports indicate that bicontinuous
body-centered (Ia3d) cubic mesostructured silica with large pores can be
obtained using additives such as inorganic salts and anionic surfactants with
or without a swelling agent (Chen et al 2005). Recently, Kleitz et al (2005)
reported a simple synthesis route for high-quality cubic (Ia3d) silica using
Pluronic P123 and n-butanol at low acid conditions.
The advantage of this synthesis is its high reproducibility and
relatively large range of compositions that produce ordered cubic phase. For a
number of synthesis procedures of bicontinuous cubic phase, it was reported
by Chen et al (2005 and 2006) that variation in the relative amounts of
additives can lead to a transition from 2-D hexagonal to cubic material.
Mesoporous materials are interesting because of their fascinating formation
mechanism. In principle, the formation mechanism can be viewed in three
different scales as follows:
i) The molecular scale, which involves the interaction between
organic and inorganic precursors and silica polymerization
process.
ii) The mesoscopic scale, which involves the development of
micellar structure and onset of long-range order.
iii) The macroscale, which is related to the shape and morphology
of the final product.
It is clear that the processes at the molecular level are the driving
force for the mesoscale structure but the correlation between the two scales is
not explained.
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Many studies that focused on the formation mechanism of various
types of templated mesoporous materials have been summarized in a number
of reviews (Ying et al 1999, Epping and Chmelka 2006, Wan and Zhao 2007).
For dilute systems where the surfactant concentration is low such that liquid
crystalline phases are not preformed, it is generally accepted that the
formation of mesoporous materials occurs in two steps. The initial stage
involves one of the following processes:
· preferable adsorption of silicate ions at the micellar interface,
driven either by charge matching or hydrogen bonding (Huo
et al 1994, Tanev and Pinnavia 1995) or
· the silicate oligomers not adsorbing at the micellar interface but
instead forming siliceous prepolymers that bind surfactant
molecules in a cooperative manner, resulting in the formation
of new silica-surfactant hybrid micellar aggregates (Frasch et al
2000).
Two possibilities that are involved in the next step are
· Silicate adsorption leads to rearrangement of original micellar
morphology, mainly lengthening the micelles followed by
condensation of silicate-covered micelles into ordered or
disordered collapsed phases (this is often referred to as
cooperative self-assembly mechanism) (Firouzi et al 1995).
· Alternatively, silicate adsorption does not change the
morphology of the micelles but rather reduces the intermicellar
repulsion.
This causes aggregation into larger particles and precipitation of
disordered phase, which then may rearrange to form an ordered phase
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(Flodstrom et al 2004). To account for the different phases formed, the
surfactant packing parameter (g = υ/a0l), (Israelchvili et al 1976, Israelchvili
and Wennerstreom 1990) has been used to describe the surfactant
organization in the self-assembly arrays and to predict the resulting
mesostructures (Zhao et al 1998, Ciesla and Schuth 1999) where υ is the chain
volume of the surfactant, a0 is the effective hydrophobic/hydrophilic
interfacial area and l is the kinetic surfactant chain length. The larger g value
results the lowering of aggregate curvature and it can be controlled by
changing a0 through charge matching between the surfactant head group and
the forming silanoate in the case of charged surfactants. For
non-ionic surfactants, like Pluronic, a0 is controlled via the hydration of PEO
groups, which comprise the corona, that serve as effective head group
(Kipkemboi et al 2001). Another way to change g is through the organic chain
packing. The charge-matching is mainly controlled by pH, co-surfactant
concentration and counter anion, whereas the organic packing is influenced
by temperature and organic additives (Lin and Mou 2002).
1.5.2 Formation Mechanism of Large Pore Mesoporous Silica
Poly (alkylene oxide) type block copolymers have been shown to be
particularly versatile as structure-directing agents for the preparation of
ordered large pore (> 5 nm) materials. In principle, the tunable volume ratio
of their hydrophilic/hydrophobic blocks and their specific aggregation
(self-assembly) behavior may provide supramolecular templating properties
with an appreciable degree of control of the resulting porous structures. In
other words, pore topology, pore size and pore connectivity may be tailored as
a function of copolymer concentration, synthesis temperature or volume
fraction of different copolymer blocks (Kipkemboi et al 2001, Matos et al
2003). Structural and textural control is especially desirable for the design of
functional porous solids for applications involving selectively tuned
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adsorption and diffusion and host-guest interactions within elaborated
nanostructured materials. However, the formation of ordered large pore silica
mesophases (Wang et al 2003) still remains a difficult task because of the fast
kinetics of silica condensation in the strongly acidic media employed.
In order to overcome this problem, Choi et al (2003) reported that
low concentration of the acid catalyst permit a more thermodynamic and
easier control of the synthesis of mesoporous silica. It opposed the previous
conditions that usually favored kinetically controlled assembly of inorganic-
organic mesophase. In the presence of polymerizing silica species, the phase
behavior of the triblock copolymers in water could be widely enriched since
slower silica condensation kinetics allow the use of organic co-surfactants to
modify thermodynamically the mesophase behavior. Butanol is used as a
co-surfactant (Armstrong et al 1996, Feng et al 2000) in combination with a
commercially available triblock copolymer (EO106PO70EO106) for the
structure-direction in aqueous solution. In this system, butanol/triblock
copolymer mass ratio only is utilized to direct specifically the formation of
high quality silica mesophases with cubic Fm3m, cubic Im3m or 2-D
hexagonal p6mm structures, with all other synthetic parameters and molar
ratios remaining constant. Excellent control of the phase behavior of highly
ordered large pore mesostructured silica (with the choice of Fm3m, Im3m or
p6mm symmetry) is achieved using a triblock copolymer (Pluronic P123,
EO106PO70EO106) and butanol at low acid concentrations.
1.6 SYNTHESIS STRATEGIES FOR MESOPOROUS
MATERIALS
1.6.1 Nature of Surfactants
A clear homogeneous solution of surfactant in water is required to
get ordered mesostructures. Frequently used surfactants can be classified into
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cationic, anionic and non-ionic. Non-ionic surfactants are available in a wide
variety of chemical structures. They are widely used in industry because of
the attractive characteristics like low price, non-toxicity and biodegradability.
In addition, the self assembling of non-ionic surfactants produces mesophases
with different geometries and arrangements. They become more and more
popular and powerful in the synthesis of mesoporous solids. Attard et al
(1995) employed the liquid-crystalline phases of the non-ionic surfactants
octaethylene glycol monododecyl ether (C12EO8) and octaethylene glycol
monohexadecyl ether (C16EO8) as template in the synthesis of mesoporous
silica. The pore sizes are limited to 3 nm. Other classes of highly ordered
mesoporous materials with uniform pore sizes larger than 5 nm were
synthesized by employing poly (ethylene oxide)-b-poly (propylene oxide)-b-
poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers as templates
under acidic aqueous media. The synthesis that largely promote the
development of mesoporous materials are simple and reproducible. A family
of mesoporous silica materials has been prepared with various mesopore
packing symmetries and well-defined pore connectivity.
This pathway is established on the basis of the interactions between
silicates and surfactants to form inorganic-organic mesostructured
composites. Chen et al (1995) proposed a silicate rod assembly mechanism.
Yuan and Zhou (2001) proposed weak evidence for this mechanism because
they observed a single rod on the edge of samples in different synthetic
periods using TEM. This mechanism is however unconvincing due to the
difficulty of assembling long rods. This is also not as popular as the
cooperative formation mechanism, which was first proposed by Stucky et al
(1994). Detailed investigations on the mesoporous materials have been
focused in the understanding and utilizing the inorganic-organic interactions.
The main synthesis routes, conditions, corresponding surfactants and classical
products are listed in Table 1.3.
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Table 1.3 Synthesis routes for mesoporous materials
Route Interactions Symbols Condition Classical products S+ I- Electrostatic
Coulomb force S+, cationic surfactants I-, anionic silicate species
Basic MCM-41,-48 and -50, SBA-6, -2 and -8, FDU-2, -11 and -13, etc.
S-I+ Electrostatic Coulomb force
S-, anionic surfactants, CnH2n+1COOH, CnH2n+1SO3H, CnH2n+1OSO2H, CnH2n+1OPO2H; I+, transition metal ions such as Al3+
Aqueous mesoporous alumina
S+X-I+ Electrostatic Coulomb force, double layer H bonding
S+, cationic surfactants; I+, silicate species; X-, Cl-, Br-, I-, SO4
2-, NO3-
Acidic SBA-1, SBA-2, SBA-3
S0I0 (N0I0)
H bonding S0, non-ionic surfactants, oligomeric alkyl PEO surfactants and triblock copolymers; N0, organic amines, CnH2n+1NH2, H2NCnH2n+1 NH2; I0, silicate species, aluminate species
Neutral HMS, MSU, disordered worm-like mesoporous silicates
S0H+X-
I+ Electrostatic Coulomb force, double layer H bonding
S0, non-ionic surfactants; I+, silicate species; X-, Cl-, Br-, I-, SO4
2-, NO3-
Acidic, pH < ~2
SBA-n (n=11,12, 15 and 16), FDU-n (n= 1, 5, and 12), KIT-n (n= 5,6)
S+-I- Covalent bond S+, cationic surfactants containing silicate, e.g., C16H33N(CH3)2OSi(OC2H5)3Br; I-, silicate species
Basic mesoporous silica
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Huo et al (1994) proposed four general synthetic routes viz.,
(i) S+I-, (ii) S-I+, (iii) S+X-I+ and (iv) S-X+I- where S+ = surfactant cations,
S- = surfactant anions, I+ = inorganic precursor cations, I- = inorganic
precursor anions, X+ = cationic counterions and X- = anionic counterions. To
yield mesoporous materials, it is important to adjust the chemistry of
surfactant head groups, which can fit the requirement of inorganic
components. Under basic conditions, silicate anions (I-) match with surfactant
cations (S+) through Coulomb forces (S+I-). The assembly of polyacid anions
and surfactant cations to salt-like mesostructures also belongs to S+I-
interaction. Hydrogen-bonding interaction mechanism, namely, S0I0 or N0I0
were proposed by Bagshaw et al (1995) for the preparation of mesoporous
silicates under neutral condition. S0 are neutral amines, N0 are non-ionic
surfactants and I0 are hydrated silicate oligomers from TEOS. It should be
noted that amines and PEO-derived molecules are different. Later on, the
synthesis of mesoporous silica SBA-15 was carried out under strongly acidic
condition using triblock copolymer P123 as template. It is more likely that a
double-layer hydrogen bonding (S0H+X-I+) interactions exist.
1.6.2 Effect of Pore Size
The enlarged surfactant micelles result in large-pore SBA-15, thin
pore walls and low micropore volumes (Galarneau et al 2003, Fulvio et al
2005). The mesopore size of SBA-15 can be easily tuned from 4.6 to 10 nm
and from 9.5 to 11.4 nm by increasing the hydrothermal temperature from
70 to 130 °C and by prolonging the hydrothermal time from 6 h to 4 days,
respectively. Similar results were obtained from mesoporous silicates with
body-centered cubic (Im3m) mesostructure by using F127 as template and
cubic bicontinuous (Ia3d) mesostructure by using triblock copolymer P123 as
template and n-butanol as a co-solute. Increasing the hydrothermal treatment
of SBA- 16 from 45 °C and 1 day to 100 °C and 2 days decreases the wall
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thickness and increases the pore size. The tunable pore size of mesoporous
silica with Ia3d symmetry ranging from 4 to 10 nm could be achieved when
the hydrothermal temperature increases from 65 to 130 °C (Kim et al 2005).
The effect of pore size by varying the method of preparations is given in
Table 1.4.
Table 1.4 Pore size of ordered mesostructures obtained by various
methods
Pore size (nm)
Method
2-5 Surfactants with different chain lengths including long-chain quaternary cationic salts and neutral organoamines
4-7 Long-chain quaternary cationic salts as surfactants and high temperature hydrothermal treatment
5-8 Charged surfactants with the addition of organic swelling agents such as TMB and midchain amines
2-8 Non-ionic surfactants
4-20 Triblock copolymer surfactants
4-11 Secondary synthesis (for example, water-amine post synthesis)
10-27
High molecular weight block copolymers such as PI-b-PEO, PIB-b-PEO and PS-b-PEO triblock copolymers with the addition of swelling agents such as TMB and inorganic salts, low-temperature synthesis
1.6.3 Effect of Synthesis Methods
1.6.3.1 Hydrothermal synthesis
Mesoporous silicates are generally prepared under hydrothermal
condition involving typical sol-gel process. The general procedure includes
several steps. First, a homogeneous solution is obtained by dissolving the
surfactant(s) in a solvent. Water is the most common solvent and medium.
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Silicate precursors are then added into the solution where they undergo
hydrolysis catalyzed by an acid or base and transform to a sol of silicate
oligomers. As a result of the interaction between oligomers and surfactant
micelles, cooperative assembly and aggregation give precipitation from the
gel. During this step, microphase separation and continuous condensation of
silicate oligomers occur. The formation of mesoporous silicate is rapid, only 3
to 5 minutes in cationic surfactant solution, which is reflected by the
precipitation. Hydrothermal treatment is then carried out to induce complete
condensation and solidification and improve the organization. The resultant
product is cooled down to room temperature, filtered, washed and dried.
Mesoporous material is finally obtained after the removal of organic
template(s). Ordered mesoporous silicates are generally synthesized under
basic or acidic condition (Brinker et al 1990). Neutral solutions are unsuitable
to get ordered silicate mesostructures because of too rapid polymerization and
cross-linking rates of silicates to control the surfactant-templating assembly.
In comparison to SBA-15, the previously reported KIT-1, SBA-3 and
MCM-41 mesostructured silicas exhibited comparatively poor hydrothermal
stability.
The main advantages of hydrothermal method are
i) Kinetics of reaction is greatly increased with a small increase
in temperature.
ii) New metastable products can be formed.
iii) Generally single crystals are obtained.
iv) High purity products can be obtained from impure feedstock.
v) No precipitants are needed in many cases and thus the process
is cost effective.
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vi) Pollution is minimized because of the closed system
conditions and reagents can be recycled.
vii) By controlling the hydrothermal temperature and duration of
the treatment, various crystalline products with different
composition, structure and morphology could be obtained.
1.6.3.2 Non-aqueous Synthesis Method
Non-aqueous synthesis is a very convenient method to prepare
ordered mesoporous materials especially mesoporous thin films, membranes,
monoliths and spheres. This method has become more and more powerful.
Most of the synthesis conducted in non-aqueous media adopt the well-known
evaporation induced self-assembly (EISA) process. For the preparation of
mesostructured silica films, TEOS is dissolved in an organic solvent
(normally ethanol, THF or acetonitrile) and prehydrolyzed with
stoichiometric quantities of water (catalyzed by acids such as HCl) at a
temperature of 25-70 °C. Then low polymerized silicate species can randomly
assemble with surfactants. Upon solvent evaporation, the silicate species
further polymerize and condense around the surfactants. The polymerization
rate is gradually increased due to increasing acid concentration during solvent
evaporation. Simultaneously, templating assembly in the concentrated
surfactant solution occurs, resulting in the formation of ordered
mesostructures. This process is very fast and needs only several seconds.
Relatively wide diffraction peaks at 2q of 3 - 5° are detected in the
XRD patterns of SBA-15 samples prepared by using P123 as a template from
the EISA method. Apparently, the mesostructure regularity is quite low.
Crepaldi et al (2003) have reported that the lack of XRD diffraction peaks
may be attributed to the extremely fast formation rate of mesostructure that
causes nonuniform micelles. SBA-15 mesoporous silica synthesized via EISA
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has much larger pore size (9.0 nm) than that obtained from hydrothermal
synthesis (4.6 nm) under similar conditions. Zhao et al (1998) have
successfully synthesized using block copolymers with large PEO segments,
For example, cubic SBA-16 mesostructure can be easily obtained using F127,
F108 or F98 or mixed surfactants.
1.6.4 Effect of Synthesis Conditions
Hydrothermal treatment is one of the most efficient methods to
improve mesoscopic regularity of products (Huo et al 1996, Soler-Illia et al
2002). In this process, temperature is an important parameter to get ordered
materials. After the solution reaction, the mesostructures undergo
reorganization, growth and crystallization during hydrothermal treatment. The
treatment temperature is relatively low, between 80 and 150 °C, in which the
range of 95-100 °C is mostly used. High temperature would result in the
degradation of ordering and decomposition of surfactants, which may direct
the formation of microporous materials. In general, the hydrothermal
temperature is higher when cationic quaternary ammonium salts are used as
templates than in the case of non-ionic surfactants. This phenomenon may be
related to the ordered microdomains of the surfactants and the interactions
between surfactants and silica species. Cationic surfactants (S+) have
comparatively strong Columbic interactions with electronegative silicate
species (I-). Since mesostructures have assembled before the hydrothermal
treatment and the regularity is improved during this process, long treatment is
necessary, ranging from days to weeks. When microwave is involved in this
step, hydrothermal treatment time can be shortened to 2 h or even shorter.
Petitto et al (2005) have reported the hydrolysis and cross-linkage of
inorganic species. High alkaline condition in which MCM-41 is formed with a
low degree of polymerization allow the phase transition from MCM-41 to
MCM-48 during hydrothermal treatment. It is the loosely condensed silicate
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species that facilitate the formation of cubic bicontinuous (Ia3d) phase
through ongoing silica polymerization and enhanced cross-linking. 2-D
hexagonal MCM-41 materials are the usual products in basic CTAB
surfactant systems at room temperature. A direct hydrothermal treatment of
the mother liquor at 110 °C for 3 days can cause mesophase transformation to
3-D cubic bicontinuous MCM-48. It is the easiest way to synthesize MCM-48
when using a low amount of surfactants. Prolonging hydrothermal time at a
certain temperature (e.g., 135 or 140 °C) causes similar continuous phase
transformation from MCM-41 to MCM-48 and to layered mesostructure
(Sayari 2000, Diaz et al 2004, Xia et al 2004). The adsorptive and structural
properties of mesoporous silicates can also be tailored to some degree by
varying hydrothermal treatment time and temperature.
Many non-ionic surfactants have the solubility problem at elevated
temperatures due to phase separation. So the synthesis temperature must be
lower than the cloud-point (CP) value of the surfactant. The common idea to
decrease the synthetic temperature, which reduces the reaction rate and
thereby improves the crystalline regularity. Zhao et al (1998, 1999) have
successfully synthesized SBA-15 by using triblock copolymer P123, with
optimal synthetic temperature of 35-40 °C. This is due to the solubility limit
and critical micelle temperature (CMT) value for the formation of micelles.
The same procedure is done for the synthesis of KIT-6. The reaction
temperature is high when block copolymers with high CMT and CP values
are used. It is found that ordered mesoporous silicates can only be obtained at
temperature higher than 90 °C with triblock copolymer P85 and P65 systems.
1.6.5 Recrystallization
Recrystallization is an efficient method to improve the regularity of
mesoporous materials. However, only a few research groups realize this
method, which is easily confused with the hydrothermal treatment. In fact,
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both processes are largely different. Recrystallization is a procedure in which
as-synthesized powder samples without washing are placed into deionized
water at 100-150 °C for several days. Khushalani et al (1995) and Huo et al
(1996) have reported that the quality (ordering, thermal stability, etc.) can be
improved for most materials, sometimes accompanied with the enlargement
of pore size. This process is quite complicated. Dissolution and crystallization
of silicate species and reorganization of mesostructures may take place. In
comparison with the hydrothermal treatment, reorganization rate in this
process may be slower and more localized for the reason of separated
surfactants and unreacted silicate species. For recrystallization, unwashed
samples are favorable because residues of acid or base catalyst, silicate
oligomers and surfactants could facilitate the reorganization of
mesostructures.
1.6.6 Method of Surfactants Removal
Removal of template plays a vital role in the preparation of
molecular sieves. The most common method to remove template is
calcination owing to easy operation and complete elimination. Organic
surfactants can be totally decomposed or oxidized under oxygen or air
atmosphere. This method is mostly applied in the cases of mesoporous
silicates, aluminosilicates, metal oxides and phosphates. The temperature
programming rate should be low enough to prevent structural collapse caused
by local overheating. Two-step calcination was adopted by the scientists of
Mobil, first 1 h under nitrogen to decompose surfactants and 5 h in air or
oxygen to burn them out (Kresge et al 1992). This complicated procedure was
then simplified. The first calcination step under nitrogen can be substituted by
heating in air with a low rate. As-synthesized SBA-15 materials are heated at
a rate of 1-2 °C/min to 550 °C and kept the same temperature for 4-6 h to
remove triblock copolymer templates. Calcination temperature should be
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lower than the stable temperature of mesoporous materials and higher than
350 °C in order to totally remove PEO-PPO-PEO type surfactants or 550 °C
for long-chain alkyl surfactants. Higher calcination temperature could lead to
low surface area, pore volume and surface hydroxyl groups and high
cross-linking degree of mesoporous materials. But these materials possess
high hydrothermal stability due to high cross-linking degrees.
1.6.7 Effects of Surface Modification
Ordered mesoporous silicas are not often used as catalysts as such.
More frequently, additional catalytic functions are introduced by
incorporation of active sites in the silica walls or by deposition of active
species on the inner surface of the material. The advantage of using ordered
mesoporous solids in catalysis is the relatively large pores which facilitate
mass transfer. The high surface area allows high concentration of active sites
per mass of the material. There are many possible pathways to modify
mesoporous materials when one wants to give them a new catalytic function
as schematically shown in Figure 1.4. Metal ions substituting silicon atoms in
the framework, similar as in zeolites, can act as acid or redox active sites and
may be used for different classes of catalytic reactions. One should bear in
mind that the wall structure of ordered mesoporous silica rather resembles
amorphous silica. Incorporation of other metal centers therefore does not lead
to the formation of defined sites as in zeolites but rather a wide variety of
different sites with different local environment. Therefore, the catalytic
properties of such materials are closer to those of metal substituted
amorphous silica than that of framework substituted zeolites. Interestingly,
these active sites can be constructed either directly or via post synthesis
procedures by multitude of pathways, which means that the properties of
these active sites are variable and controllable depending on the synthetic
procedure.
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Figure 1.4 Schematic sketch of various methods for functionalization of
mesoporous material
1.7 INCORPORATION OF HETEROATOM INTO
MESOSTRUCTURES
Modification of the framework composition is possible by direct
synthesis, i.e. from mixtures containing both silicon and heteroatom to be
incorporated or by post-treatment of an initially prepared silica mesoporous
material. The results of these two different methods are not necessarily
identical. While the direct method typically results in a relatively
homogeneous incorporation of the heteroatom, post synthesis treatment will
primarily modify the wall surface and thus lead to increase in the
concentration of heteroatom on the surface. This method of synthesis of
aluminum modified materials shows enhanced hydrothermal stability (Shen
and Kawi 2002). The increased stability is due to the coating of silica surface
with alumina species, which are less susceptible to hydrothermal degradation.
The nature of reagents used for grafting can strongly influence the properties
Mesoporous Materials (MCM-41,-48, SBA-15 & KIT-6)
High surface area Narrow pore size distribution
Thermal stability
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of resulting materials. Incorporation of aluminum is of special interest in
catalysis, as this result in the formation of Brønsted acidity and ion exchange
sites in the materials. Since O–Al–O angle is less flexible than O–Si–O angle,
Al-MCM-41 materials are commonly less well ordered on the mesoscale and
show a broader pore size distribution than their pure silica analogues (Kresge
et al 1992). Incorporation of aluminium into mesoporous molecular sieves is
of tremendous interest in order to embed catalytic function. The synthesis of
aluminium containing MCM-41 materials has been studied extensively (Jana
et al 2003, Jana et al 2004).
Adsorption of bases such as ammonia or pyridine on Al-MCM-41
allows to determine the strength of acid sites by temperature programmed
desorption (TPD) and FT-IR. One can distinguish between Brønsted and
Lewis acidity and recognize weak and strong acid sites, which are formed
depending on the Si/M ratio and nature of the trivalent element (Al, Fe, Ga).
The local environment around the acid sites in the framework substituted
materials exhibit substantially weaker acidity than zeolites and rather
correspond to amorphous silica–alumina in the number of acid sites and acid
strength distribution. Typically, direct synthesis of aluminum containing
mesoporous silica has both tetrahedrally and octahedrally co-ordinated
aluminum (Schmidt et al 1994). Change of aluminum source can also lead to
the formation of exclusively tetrahedrally co-ordinated aluminum in the
framework. Though aluminum hydroxide (Al(OH)3), aluminum isopropoxide
(Al(OiPr)3) and sodium aluminate (NaAlO2) are used as aluminum source in
the preparation of Al-MCM-41, sodium aluminate offers the strongest
Brønsted acidity. It is also possible to prepare aluminum incorporated
MCM-41, MCM-48, SBA-1, SBA-15, KIT-5 and KIT-6 via direct synthetic
route.
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The Brønsted acid strength of Al, Ga and Fe substituted MCM-48
investigated by NH3-TPD were in the order: Al > Ga > Fe, whereas the Lewis
acid sites showed the order: Ga > Al > Fe. Adding heteroatom to the synthesis
mixture does not only lead to their incorporation but other properties of the
synthesized product are also changed as well (Collart et al 2004).
A comparative study of the acidity of directly synthesized and post-treated
Al-MCM-41 has been published by Chen et al (1999). Al-MCM-41 prepared
directly from Al(OiPr)3 and TEOS in a mixed gel has the highest
concentration of acid sites (Jana et al 2003). Al(OiPr)3 grafted materials show
lower acid site concentration than AlCl3 grafted samples even for similar
Si/Al ratios.
Yue et al (1999) first synthesized aluminium incorporated SBA-15 by
direct hydrothermal synthesis using TEOS and aluminium tri-tert-butoxide as
silicon and aluminium precursors respectively. The octahedrally and
pentagonally coordinated Al species were eliminated while washing the material
in NH4Cl solution, the absence of which was confirmed by 27Al MAS-NMR.
Al-SBA-15 showed higher hydrothermal stability and exhibited higher catalytic
activity in cumene cracking than Al-MCM-41.
Aluminium incorporated mesoporous materials showed great
potentials in moderate acid-catalyzed reactions for large molecules (Armengol
et al 1995, Mokaya and Jones 1997, Pauly et al 1999). However, the resulting
materials have many extra-framework aluminium species. The pH adjusting
method for grafting Al and Ti in SBA-15 materials was reported by Wu et al
(2004). It still remains a challenge to directly synthesize aluminium
substituted SBA-15 materials by standard hydrothermal method.
The difficulties encountered in the direct synthesis of aluminium
substituted mesoporous materials under acidic conditions are due to easy
dissociation of Al–O–Si bond under acidic hydrothermal condition and the
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remarkable difference between the hydrolysis rates of silicon and aluminum
alkoxides (Luan et al 1999, Hernatedez et al 2000). Several strategies have
been used to solve this problem caused by the difference in the reactivity
towards hydrolysis and condensation of silicon and aluminum alkoxides.
Among the strategies are (i) prehydrolysis of alkoxysilanes before the
addition of aluminium alkoxide (Yoldas et al 1988) making the mixed
metaloxane (Si-O-Al) bonds at the stage of the precursors (Lopez et al 1992)
and (ii) decrease the hydrolysis rate of aluminum precursors by complexing
them with chelating agents such as ethyl acetoacetate (Pierre et al 1998).
When aluminum isopropoxide is used as the aluminum source, it
hydrolyzes rapidly to monomeric Al(OH)4- under acidic or basic condition.
Thus, under high acidic condition, aluminum isopropoxide is transformed into
soluble tetrahedrally coordinated Al precursor species that favor the
incorporation of tetrahedral Al into the mesoporous materials (Turova et al
1979 and Janicke et al 1999). The hydrolysis of aluminum isopropoxide is
much faster than tetraethylorthosilicate (TEOS). Tetramethylorthosilicate
(TMOS) hydrolyzes faster than TEOS does, due to steric hindrance at
ethoxide moieties and reduced solvation of the resulting ethanol (Brinker
1988, Brinker et al 1990). The hydrolysis rates are adjusted by two ways, i.e.,
by using fluoride as a catalyst to accelerate the hydrolysis rate of TEOS or by
using TMOS instead of TEOS as silicon precursor. Furthermore, the pH value
of the synthesis solution adjusted by two-step method efficiently avoids the
leaching of framework aluminum under acidic condition. These results
demonstrate that both approaches can yield high quality mesoporous
aluminium substituted SBA-15 materials. The mesoporous Al-SBA-15
materials possess moderate acidity and are potential catalysts for many
catalytic reactions that do not require strong acid sites especially for
Friedel-Crafts alkylation and acylation reactions involving large molecules
(Murugavel and Roesky 1997, Mokaya and Jones 1997, Chiu et al 2004).
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Cerium containing mesoporous materials are not only important but
also an interesting class of materials (Khalil 2007). Cerium containing
MCM-41 materials have shown many catalytic applications, namely, vapor-
phase dehydration of cyclohexanol, hydroxylation of 1-naphthol with aqueous
H2O2 and tert-butyl hydroperoxide, selective acylation, alkylation and
oxidation of cyclohexane and n-heptane oxidation (Kadgaonkar et al 2004).
Cerium incorporated MCM-48 and cerium incorporated SBA-15 exhibited
good catalytic activity and selectivity in the acylation of alcohols, thiols,
phenols and amines (Wangcheng et al 2008, Yao et al 2006).
Similar to zeolites, the incorporation of transition metal ions such as
Ti, V or Mn could isolate these active centers and thus make them highly
efficient. The catalytic behavior is strongly influenced by the nature, the local
environment and the stability of metal introduced and by the hydrophobic
properties of the surface. Incorporation of Ti in mesoporous materials is
generally achieved by direct synthesis procedure, which involves addition of a
titanium source such as titanium ethoxide (Ti(OEt)4) in H2O2 or titanium
isopropoxide (Ti(OiPr)4) in ethanol to the gel for hydrothermal synthesis
(Corma et al 1994, Alba et al 1996). One of the problems in the preparation of
substituted mesoporous silica is the great reactivity differences between the
usual Ti and Si precursor species such as the alkoxides.
The incorporation of other metals into the framework of mesoporous
silica materials has remarkable catalytic and photocatalytic properties. Efforts
have been devoted to study the incorporation of transition metals such as Ti, V,
Mn, Fe, Co, Cr and Mo into the framework of mesoporous MCM-41 and
SBA-15 molecular sieves by direct synthesis. Most of the catalytic applications
of metal substituted MCM-41 have been reviewed by Sayari (1996).
Chen et al (2004) have successfully synthesized titanium substituted
SBA-15 materials by direct synthesis method using TEOS and TiCl3 as
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silicon and titanium source respectively. It was found that when the
concentration of HCl exceeded 1 M in the gel solution, it became difficult to
incorporate titanium into the framework of SBA-15. The titanium content
increased with decreasing activity of the gel solution. When the pH of the gel
solution was 1, titanium could be effectively incorporated with a prerequisite
that the TEOS was prehydrolyzed for 6 h. Under optimized conditions, the
formation of anatase TiO2 could be avoided and Ti-SBA-15 material of high
quality could be obtained. The calcined Ti-SBA-15 materials showed good
catalytic activity in the oxidation of styrene. Ti-substituted SBA-15 materials
have successfully synthesized by Zhang et al (2002) using fluoride to
accelerate the hydrolysis rate of tetramethylorthosilicate (TMOS) to match
that of titanium isopropoxide.
1.7.1 Catalytic Applications of Mesoporous Molecular Sieves
In recent years, the necessity for treating heavier feed stocks as well
as synthesis of large molecules has created a demand for molecular sieves
with in-built acidities, high hydrothermal stability and wide pores. The
disclosure of MCM-41 mesostructures (Kersge et al 1992 and Beck et al
1992) offered such catalytic performance. Kloetstra and van-Bekkum (1995)
exploited alkali containing MCM-41 molecular sieves for base and acid
catalyzed reactions. Armengol et al (1995) studied Friedel-Crafts alkylation
of bulky 2,4-di-t-butyl phenol with cinnamyl alcohol. The yield of principal
benzopyran was higher with MCM-41 than that of commercial zeolite and
sulphuric acid catalysts. This reaction proved that bulky organic compounds
could also diffuse through mesopores, which paved the way for the synthesis
of fine chemicals and pharmaceuticals.
Corma (1997) reviewed the development of ordered mesoporous
materials and their catalytic properties together with their application for a
series of organic reactions of fundamental practical interest. This review
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explains the availability of high surface area and regular porosity of MCM-41
in the preparation of supported metals and bifunctional catalysts. Climent et al
(1998) proposed a new route for the synthesis of a-n-amyl cinnamaldehyde
with high selectivity using low ratio of benzaldehyde/heptanal employing
mesoporous molecular sieve.
Price et al (1998) studied the alkylation of benzene with different
chain length olefins over AlCl3 grafted on two hexagonal mesoporous silica
materials with 16 and 24 Å pore diameters. It was observed that increase in
monoalkylated products with increase in the olefin chain length.
Ce incorporated MCM-41 exhibited high activity for various catalytic
reactions such as acylation of alcohols, vapor-phase dehydration of
cyclohexanol to cyclohexene, hydroxylation of 1-naphthol with peroxides and
alkylation of naphthalene (Laha et al 2002, Kadgaonkar et al 2004).
Jun et al (2000) evaluated the catalytic activity of aluminum
incorporated MCM-48 molecular sieves in the Friedel-Crafts alkylation of
benzene, toluene and m-xylene with benzyl alcohol. Selvam and Dapurkar
(2004) reported the catalytic activity of H-AlMCM-48 catalyst in the tert-
butylation of phenol. H-AlMCM-48 catalyst exhibited enhanced activity. This
could be attributed mainly due to its three-dimensional pore structure in
contrast to one-dimensional H-AlMCM-41. Zhao et al (2001) reported the
catalytic performance of Fe-MCM-48 in phenol hydroxylation reaction. The
active centres of framework isolated Fe3+ are favourable for phenol
hydroxylation and good selectivity towards catechol.
SBA-15 possesses big tubular channels up to 30 nm in diameter. As
SBA-15 possesses greater thickness of pore walls, the hydrothermal stability
is much higher than MCM-41. Gracia et al (2008) reported that mesoporous
Ga-SBA-15, with higher contribution of Lewis acid sites, was highly active
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and selective to monoalkylated products (2- and 4-methyl diphenylmethane)
in the liquid-phase alkylation of toluene with benzyl chloride. Al-SBA-15
materials, with a greater proportion of Brønsted acid sites, exhibited improved
activity in the alkylation of toluene with benzyl alcohol. Al-SBA-15 prepared
by post synthesis grafting displayed high catalytic activity in the alkylation of
phenol with tert-butanol (Shujie et al 2006).
Dubey et al (2006) reported the synthesis of polymer-silica (KIT-6
with cubic symmetry) composite materials through in-situ radical controlled
polymerization (vinylmonomers) inside the silica mesopores for
hydroxylation of phenol. Jermy et al (2008) reported that vanadium
incorporated KIT-6 materials showed excellent catalytic activity in the direct
oxidation of cyclohexane using dilute aqueous H2O2 as the oxidant. Vinu et al
(2008) reported that titanium incorporated KIT-6 materials are better catalyst
for epoxidation of styrene due to the presence of 3-D pore systems. Soni et al
(2009) compared the catalytic activities of Mo, Co-Mo and Ni-Mo supported
KIT-6 material with γ-Al2O3 and SBA-15 supported analogues. They found
that KIT-6 supported catalysts are two to three times more active than γ-Al2O3
and nearly 1.5 to 2 times more active than SBA-15 supported catalysts for
both hydrodesulfurization (HDS) and hydrogenation (HYD) functionalities.
1.7.2 Applications of Mesoporous KIT-6
KIT-6, a mesoporous material has evinced interest in various fields,
a clear distinct application is nanocasting (Ryoo et al 1999). The surface
template is absent and instead of that pore system of ordered mesoporous
silica is used as a hard template. The pores are infiltrated with a carbon
precursor such as sucrose or furfuryl alcohol which is subsequently converted
to carbon by high temperature treatment in an inert atmosphere. Following
this small ordered mesoporous metal could be also prepared by the same way.
The method is already well established to produce carbon based materials and
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ordered mesoporous materials (Johnson et al 1999, Kim et al 2001). Many
scientists recently reported (Laha and Ryoo 2003, Tian et al 2003, Wang et al
2005, Rumplecker et al 2007) the formation of an oxide such as CeO2, Cr2O3
and Co3O4 using silica as a hard template. Ordered mesoporous carbon
(OMC) is synthesized and made it to use as a hard template for the synthesis
of an oxide. The catalytic activity of the cubic (Ia3d) catalysts was slightly
higher than that of 2-D hexagonal. Kleitz et al (2003) reported the preparation
of cubic (Ia3d) silica with very large pores and its replication to the highly
ordered mesoporous carbon.
MCM-48 is known to possess 3-D pore structure (cubic Ia3d) was
first used as inorganic template for the synthesis of a new mesoporous carbon.
Cubic KIT-6 silica was used to manufacture diverse nanostructured porous
metal oxide-based materials. Shen et al (2005) described the use of KIT-6 as a
template for the fabrication of mesoporous RuO2, which seemed to show
interesting catalytic activity for CO oxidation. Very recently, mesostructured
WO3 and CeO2 materials obtained from KIT-6 silica were reported for gas
sensing applications. KIT-6 silicas containing sufficient amount of
complementary pores in their framework walls were employed to create new
types of highly porous and fully integrated functional polymer-inorganic
nanocomposites.
1.7.2.1 Synthesis of ordered mesoporous carbon
The synthesis of ordered mesoporous carbon, denoted as
CMK-1(Carbon Mesostructured by KAIST), using MCM-48 with
bicontinuous cubic Ia3d symmetry as the template was reported by Ryoo et al
(1999). In the field of ordered mesoporous carbon (OMC), the initial research
was mainly focused on the new pore topologies for the synthesis of OMC
(Gierszal et al 2005). The various mesoporous silicate or aluminosilicate
templates with 3-D pore connectivity such as MCM-48, KIT-6 (cubic Ia3d),
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SBA-1 (cubic Pm3n) and SBA-15 (hexagonal p6mm) afforded OMC with
different pore structures. The mesostructures of the resulting OMC were
determined by the structural symmetry of the parent silica template. CMK-2
(Ryoo et al 2001) with cubic Pm3n symmetry was templated from SBA-1
mesoporous silica. On the other hand, hexagonally mesostructured CMK-3
(Jun et al 2000) and CMK-5 (Joo et al 2001) carbon were replicated from
SBA-15. The synthesis of new OMC structures were also successfully
achieved utilizing other ordered mesoporous silica (OMS) templates including
SBA-16 (large pore cubic Im3m), KIT-6 (Kleitz et al 2003 and Kim et al
2005) (large pore cubic Ia3d), HMS (Lee et al 2000) and MCF (Lee et al
2001) silicas.
Ordered mesoporous silicas of different structures such as MCM-48,
SBA-15, SBA-16, KIT-5 and KIT-6 were used as templates to prepare carbon
replicas called CMK-1,-6 CMK-3,-7 and CMK-6,-8. New type of large pore
mesoporous KIT-6 with cubic (Ia3d) structure is composed of two interwoven
mesoporous networks similar to that of MCM-48, but it possesses much large
pore diameter in the range 5-12 nm. Numerous studies have been done on the
carbon replica of OMSs using carbon precursors such as sucrose, furfuryl
alcohol, acenaphthene, mesophase pitch and petroleum pitch with
considerable percentage of oxygen and other elements that are usually
released in the gaseous form during carbonization process. Li et al (2004)
reported an interesting method for fluorination of pitch-based OMCs prepared
using MCM-48 as template.
1.7.2.2 Synthesis of nanocast metal oxides
Tuysuz et al (2008) reported a detailed study on the surface topology of well-known ordered mesoporous KIT-6 and a series of nanocast
Co3O4 and Co3O4/CoFe2O4 composites. Co3O4 that was nanocast from KIT-6
aged at low temperature mainly showed an uncoupled sub-framework while Co3O4 that was prepared from KIT-6 with higher aging temperature showed a
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rather dense structure formed by the two coupled sub-frameworks of KIT-6 mold. The texture parameters of KIT-6 were varied by changing the
hydrothermal synthesis temperature. KIT-6 aged at different temperatures was
used as hard template to prepare ordered mesoporous metal oxide. The wall of KIT-6 with aging temperature of 40 °C appears much denser and more solid,
while the high temperature aged KIT-6 sample has rather open pore structure.
The mesoporous structure is well ordered and in accordance with the reduced symmetry of cubic KIT-6 template. The material is highly ordered and there
is no identification of unstructured bulk phase observed from TEM or SEM image.
1.7.2.3 KIT-6 as a support for photocatalysts
Semiconductor based hetero-structures with desired size,
composition, pore channel and morphology can modulate the properties of materials and find potential applications in biomedicine, photocatalysis and
nanodevices. Recently, environmental problems such as air and water
pollution have provided impetus for sustained fundamental and applied research in the area of environmental remediation. It is believed that
mesoporous semiconductor based hetero-structure photocatalysts are excellent
candidates for degradation of various organic pollutants. It is well known that the fabrication of mesoporous titania with ordered crystalline framework is
still a great challenge because mesoporous framework of TiO2 can easily
collapse during thermal treatment. So a highly stable and ordered mesoporous silica material (KIT-6) was used as a template to fabricate silica supported
Ag–TiO2 photocatalyst. The synthesis of mesoporous Ag-TiO2 using KIT-6 as template was reported by Zhang et al (2009). Ag–TiO2 synthesized using
KIT-6 possesses high BET surface area and large number of ordered pore
channels which facilitate adsorption and transportation of dye molecules, leading to higher photocatalytic activity. In addition, it is found that Ag–TiO2
hetero-structure plays an important role in enhancing the photocatalytic activity.
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1.7.2.4 Application of KIT-6 mesoporous materials as hard templates
Cui et al (2008) have reported a simple one-step impregnating route
to synthesize ordered mesoporous WO3 with a surface area of 86 m2/g by
using cubic Ia3d mesoporous silica (KIT-6) as hard template. The prepared
mesoporous WO3 materials exhibit stable electrochemical catalytic activity
toward hydrogen oxidation, and when mixed with an appropriate amount of
carbon black, the resultant mesostructured WO3/C composites show much
enhanced electrocatalytic activity for hydrogen oxidation. Shon et al (2009)
reported a facile method for the preparation of highly ordered mesoporous
silver using cubic mesoporous silica (KIT-6) with controlled hydrophobicity
as a hard template. It is well-known that the surface properties of supports are
of much importance for the formation of metallic nanoparticles and their
stabilities. The modified the silica surface with hydrophobic methyl groups
decreased the interaction between the silver precursors and pore surfaces,
resulting in the easy aggregation of precursors within the mesopores before
reduction to the metallic domain. Jiao et al (2005) have successful synthesized
of Cr2O3 using cubic mesoporous silica KIT-6. It is of great interest to find
that the crystal orientation of Cr2O3 has a close relation with the symmetry of
the mesopore system in KIT-6. Chromium oxide (Cr2O3) plays an important
role in magnetics and catalysis.
1.8 SCOPE AND OBJECTIVES OF THE PRESENT
INVESTIGATION
Heterogeneous catalysts possess the advantages of ease of recovery,
recycling and are readily amenable to continuous processing. These
advantages of heterogeneous catalysts have received much attention among
the researchers. The discovery of ordered mesoporous KIT-6 has greatly
inspired research interest in various fields including catalysis, adsorption,
separation, sensing, drug delivery, optoelectronics and in the manufacture of
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advanced nanostructured materials due to their large pore dimension
compared to microporous zeolites, M41S family and SBA family.
The non-ionic block copolymers are an interesting class of structure
directing agent whose self assembly characteristics led to kinetically
quenched structures. Block copolymers have the advantage that their ordering
properties can be nearly continuously tuned by adjusting solvent composition,
molecular weight or copolymer architecture. Mesoporous material such as
KIT-6 has large surface area, pore volume, pore diameter and wall thickness.
The synthesis of cubic Ia3d mesoporous KIT-6 is an attractive catalyst in
catalysis research.
The objectives of the present investigation are:
· Hydrothermal synthesis of mesoporous Al-KIT-6 molecular
sieves with Si/Al ratios 20, 30, 40, 50, 100 and 150 using
triblock copolymer as the template and n-butanol as the
co-solute. Tetraethylorthosilicate (TEOS) and aluminium
isopropoxide as the precursors for silicon and aluminium
respectively.
· Hydrothermal synthesis of mesoporous Ce-KIT-6 molecular
sieves with Si/Ce ratios 5, 10, 20, 50, 100 and 150 using
triblock copolymer as the template and n-butanol as the
co-solute. Tetraethylorthosilicate (TEOS) and cerium nitrate
as the sources for silicon and cerium respectively.
· Characterization of Al-KIT-6 using XRD, ICP-AES, FT-IR,
Nitrogen adsorption studies, TG-DTG, pyridine adsorbed
DRIFT-IR, SEM and HR-TEM techniques. Similarly
Ce-KIT-6 materials characterization by XRD, ICP-AES,
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FT-IR, Nitrogen adsorption studies, TG-DTG, DRS-UV-vis,
XPS, SEM and HR-TEM techniques.
· Evaluation of the catalytic activity of Al-KIT-6 molecular
sieves in the vapor phase acylation of phenol. Optimization of
reaction parameters such as temperature, reactant feed ratio
and weight hourly space velocity (WHSV).
· Examination of the catalytic activity of Al-KIT-6 molecular
sieves in the liquid phase acylation of isobutylbenzene and
optimization of reaction parameters such as temperature, feed
ratio, conversion and product selectivity.
· Study of the catalytic activity of Ce-KIT-6 molecular sieves in
the liquid phase oxidation of cyclohexane and hydrogen
peroxide. Optimization of the reaction parameters such as
temperature, feed ratio, conversion and product selectivity.
· Evaluation of the catalytic performance of Ce-KIT-6
molecular sieves in the vapor phase oxidation of cyclohexanol
and optimization of the reaction parameters such as
temperature, flow rate, conversion and product selectivity.
· Correlation of the physicochemical characteristics of the
catalysts and their catalytic activity and selectivity.
· Sustainability study of the catalysts by carrying out time on
stream studies.