CHAPTER 1 INTRODUCTION - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/26569/6/06_chapter1.pdf · oxides, metal sulfides, metals, polymers and carbons) have been synthesized.

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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

20

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

21

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.

22

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.

23

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

24

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

25

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,

26

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

27

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.

28

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

29

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.

30

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

31

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).

32

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

33

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

34

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

35

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

36

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.