1

CHAPTER 1

INTRODUCTION

1.1 CATALYSIS

Catalysis is a process in which acceleration of chemical reaction is

induced in the presence of a material (catalyst) that is chemically unchanged

at the end of the reactionThe phenomenon of reducing the energy requirement

of a chemical process by changing the rate of attainment of equilibrium

through lowering of activation energy is termed as catalysis and the material

as catalyst. However, catalysts do not alter the equilibrium position of a

reaction which is controlled thermodynamically and require high pressures.

Recent estimations revealed that approximately 90% of chemicals ranging

from bulk chemicals to consumer products come into contact with a catalyst

at one stage or another of their manufacturing process. Depending upon their

relative reaction medium catalysts are classified into two basic types,

heterogeneous and homogeneous.

The world wide effort to replace homogeneous acid catalysts by

heterogeneous catalysts in all industries is to control pollution and waste. In

homogeneous type, the catalysts are in the same phase as the substrate and are

uniformly distributed. As the catalyst gets dissolved in the reaction medium

almost all the reactions under homogenous type takes place within the liquid

phase whereas in most cases of heterogeneous system the catalyst used is a

porous solid and the reaction takes place either on its external surface or

surface within the pores of the solid. Heterogeneous catalytic systems, in

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which fluid reactants are reacted over solid acid catalysts, are the most widely

used catalytic processes in the manufacturing industries at present. The

following are the advantages of heterogeneous systemswhen compared to

their homogeneous counterparts:

Minimal pollution, less corrosion and wastes.

High activity, selectivity and suppression of side products.

Shapeselectivity.

Easy removal of product from the reaction mixture and

efficient recycling of the catalyst.

Use of renewable starting materials.

Easy separation of end products.

1.2 CATALYSIS AND GREEN CHEMISTRY

The concept of green chemistry has gained momentum among

researchers both in academic and industries as a tool for achieving

sustainability by promoting innovative chemical technologies that reduce or

eliminate the use or generation of hazardous substances in the design,

manufacture and application of chemical products. Strong legislative

enactments towards controlling discharge of waste products from industries

into the environment and their restrictions in the manufacture, transport,

storage and use of certain hazardous chemicals has sparked the introduction

of cleaner technologies. Realizing the unsustainable consequences of

exceeding the earth’s natural capacity in dealing with the waste and pollution

which society generates, Anastas&Warner (1998) coined a set of twelve

principles as green chemistry. Heterogeneous catalysis is an omnipotent tool

to realize all the twelve principles of green chemistry.

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i) Waste prevention instead of remediation

ii) Atom efficiency

iii) Less hazardous/toxic chemicals

iv) Safer products by design

v) Innocuous solvents and auxiliaries

vi) Energy efficient by design

vii) Preferably renewable raw materials

viii) Shorter synthesis (avoid derivatization)

ix) Catalytic rather than stoichiometric reagents

x) Design products for degradation

xi) Analytical methodologies for pollution prevention

xii) Inherently safer processes

Heterogeneous system is more convincing in controlling

environmental pollution. The two significant factors of heterogeneous

catalysts that influence the environmental impact of cleaner chemical

processes are (1) E-factor, and (2) atom efficiency.

1.2.1 E factor

E factor is an important metric to understand the potential

environmental acceptability of chemical processes. It is defined as the mass

ratio of waste to desired product. The magnitude of the waste problems in

chemical manufacture is readily apparent from the consideration of typical E

factor in various segments of chemical industry (Table 1.1). A high E factor

means more waste and consequently more negative environmental impact.

The ideal E factor is zero. E factor can be calculated using the following

Equation (1.1)

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Kgof secondary productsEfactorKgof desired product

(1.1)

Table 1.1 E factors in various segment of chemical industry

Industry segment Product tonnagea E(kg wasteb/kg product)

Oil refining 106 - 108 < 0.1

Bulk chemicals 104 - 106 < 1to 5

Fine chemicals 102 - 103 5to50Pharmaceuticals 10 - 103 25 to100a Represents annual production volume of a product at one site (lower end

of range) or world-wide (upper end of range) b Defines as everything produced except the desired product (including all

inorganic salts, solvent loss, etc)

For example, the original process of Friedel-Crafts acylation using

AlCl3 had an E factor of about 5 and required a chlorinated hydrocarbon or

nitroaromatic solvent. The new process with zeolite (H-beta)

catalysedFriedel-Crafts acylation, in contrast, has an E factor of < 0.01 and no

solvent is required. The substantially higher E factors in fine chemicals and

pharmaceuticals compared with bulk chemicals is a reflection of more

widespread use of stoichiometric reagent and multi-step synthesis in the

former steps. Thus, replacement of stoichiometric protocols in the fine and

pharmaceutical industries by catalytic methods will help to reduce E factors in

these sectors and thus will help to achieve the goals of green chemistry.

1.2.2 Atom Efficiency or Atom Economy

The concept of atom economy was developed by Trost (1991 and

1995). This is a method of expressing how efficiently a particular reaction

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makes use of the reactant atoms. Thus if all the reactants are completely

incorporated in to the product, the synthetic pathway is said to 100% atom

efficiency because it will not generate any waste. Atom economy of a

particular reaction can be calculated by the following Equation (1.2).

Atom ef iciency =Molecular weight of the product

Sum of the molecular weight of the reactants(1.2)

1.3. HETEROGENEOUS CATALYSIS

Unlike a homogeneous catalytic system, both the catalyst and the

reactants of a heterogeneous system are in different phases such as solid,

liquid or vapour. Solid acids and their salts find important application as

heterogeneous catalysts. The most common are silica, alumina,

aluminosilicates and aluminophosphates. Industrial processes involving

dehydrogenation, oxidation, ammoxidation and polymerization are catalyzed

using metals, metal oxides, clays and zeolites. A landmark in the history of

heterogeneous catalysis was achieved by Fritz Haber in 1970 (Smil

1999)when he prepared large quantities of ammonia from nitrogen and

hydrogen in the presence of Fe2O3 catalyst using a high pressure reactor.

Similarly oxidation with metallic platinum, dehydrogenation with metallic

nickel and Fischer-Tropsch process over cobalt and iron catalysts are other

examples. Lewis and Brönsted acidities in the catalysts are two fundamental

active centres in most of the solid acid catalysts. Chromia is a well known

example of a Lewis solid acid catalyst (Auroux&Gervasini 1990). On the

other hand bulk oxides with loosely bound protons associated with oxide ions

are examples of Brönsted acid catalyst. V2O5 and ZrO2 contain both Lewis

and Brönsted acid sites (Auroux&Gervasini 1990 and Kawai et al 1981).

Yadav et al(1993) and Yadav&Thorat (1996) have reported alkylation of

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toluene with benzyl chloride, benzyl alcohol and benzyl ether over sulfated

zirconia.

1.4 POROUS MATERIALS

Porous materials have widespread applications such as catalysts,

catalyst supports, adsorbents and sensors due to their high thermal,

hydrothermal, mechanical and chemical stabilities as well as high specific

surface area, large specific pore volume and pore diameter. The IUPAC has

recommended specific nomenclature for classification of porous materials

into three groups based on their predominant pore size: microporous (pore

diameter < 2nm), mesoporous (2nm < pore diameter <50 nm) and

macroporous (50 nm < pore diameter). Zeolites, zeotype materials and

activated carbons are examples of microporous materials. M41S family,

mesoporousAlPOs, aero-gels and most recent SBA-1, SBA-15 and KIT-5 are

few examples of mesoporous materials. Examples of macroporous materials

include silica-gel, activated charcoal and CPG (controlled porous glass).

1.5 ZEOLITES

Zeolite is a unique class of oxides, consisting of microporous, crystalline aluminosilicates found in nature or synthesized artificially

(Thomas et al 1999). These materials were discovered in 1756 by the Swedish

mineralogist Axel Frederick Cronstedt, who found that the mineral stilbite

lost significant amount of water when heated. The word zeolite stems from

Greek and means boiling stone. It took almost two centuries before zeolite

received the attention of chemists. Nowadays, new zeolites and associated

materials are still being discovered in laboratories worldwide. Zeolites are

used in various potential applications such as household detergents, desiccants

and toothpaste, whereas their acidity makes them attractive catalysts. In the

middle of 1960s, Raboet al(1966) at Union Carbide demonstrated that

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faujasitic zeolites are very interesting solid acid catalysts. Since then, a wealth

of zeolite-catalyzed reactions of hydrocarbons has been discovered. For

fundamental catalysis they offer the advantage that the crystal structure is

known and that the catalytically active sites are thus well defined. The fact that zeolite possess well-defined pore systems in which the catalytically active

sites are embedded in a defined way gives them some similarity to enzymes.

1.5.1 Structure of Zeolites

Zeolites are crystalline aluminosilicates having three dimensional

framework made up of primary buildings units (PBU) of SiO4 and

AlO4tetrahedra (known as TO4) by sharing a common oxygen atom in their

corners. The PBUs are joined together to form a secondary building unit

(SBU) and twelve such SBUs were identified by Meier & Olson (1987) as

shown in Figure 1.1.These SBUs are arranged in a specific geometrical

pattern to form a definite crystal structure and uniform pore size.

Theoretically thousands of structures can be arrived but only around 160

have been synthesized till today. Out of these, only 40 of them are naturally

occurring zeolites. Zeolites are represented by the following empirical

formula (Breck 1964).

Mx/n [(AlO2)x (SiO2)1-x] . zH2O (1.1)

where M is a cation with valency n, x represents the number of

AlO2tetrahedra and z represents the number of water molecules.

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Figure 1.1 Secondary building units identified in zeolite frameworks

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The presence of extra negative charge in the framework,

compensated by cations especially protons, is the main cause for Brönsted

acidity. Lewis acidity is generated by the formation of trigonally co-ordinated

Al and Si sites by the removal of two hydroxyl groups from the framework

(Uytterhoeven et al 1965). While zeolites are synthesized commercially for

specific uses, many natural zeolites are readily available as minerals from the

earth crust. Their unique properties made tremendous applications in

petrochemical cracking, ion-exchange and in separation and removal of gases

and solvents (Piera et al 1998 and Tomita et al 2004). The other applications

are in agriculture, animal husbandry, construction, etc.

1.5.2 Nomenclature of Zeolites

International Zeolite Association Structure Commission and

IUPAC have assigned structural codes to known natural and synthetic zeolites

(Barrer 1983). Designations consist of three letter abbreviation derived from

the names of species which do not include numbers and characters other than

Roman letters. Some examples are shown in Table 1.2.

Table 1.2 Structure type index as per IUPAC nomenclature

Structure type code

Species Structure type code Species

MFI ZSM-5 LAU Laumonite MOR Mordenite LTA Linde Type A MTN ZSM-39 LTL Linde Type L MTT ZSM-23 MEL ZSM-11 BEA Beta AST ALPO-16 MTW ZSM-22 ATS ALPO-36 AEL ALPO-11 ATT ALPO-33 AFI ALPO-5 CHA Chabazite

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1.5.3 Classification of Zeolites Based on Pore Size

Zeolites exhibit ion-exchange property, extreme thermal stability

and possess channels like pore systems approaching molecular size which

made them attractive for a variety of industrial applications. Barrer (1983)

classified zeolites based on the pore openings(Table 1.3).

Table 1.3 Classification of zeolites based on pore openings

S. No. ClassPore

opening

Pore

diameter (Å) Example

1. Small pore 8 3-4 A, ZK-5

2. Medium pore 10 5-6 ZSM-5, ZSM-11

3. Large pore 12 6-8 X, Y, BEA

4. Ultra large pore 18*, 20# 8-12 VPI-5*, Cloverite#

1.5.4 Unique Properties of Zeolites

The following are the unique and salient properties of zeolites

which made them useful in many areas.

Crystallinity Uniform pore systems

High internal surface area Ion-exchange capabilities

Non-toxic Microporosity

Pore channels or cages High thermal stability

Acidity Environmentally safe

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1.5.5 Applications of Zeolites

Adsorption:In drying, purification and separation,zeolites can absorb up to 25% of their weight in water.

Ion-exchange: Zeolites are builders in washing powder, where

they gradually replaced phosphates to bind calcium. Calcium

and to a lesser extent magnesium in water are exchanged for

sodium in zeolite A. This is the largest application of zeolites

today as they are essentially non-toxic and pose no

environmental risk. Zeolites are also applied in toothpaste, again

to bind calcium and prevent plaque.

Catalysis: Zeolites possess acid sites that are catalytically

active in many hydrocarbon reactions. The pore system allows

molecules that are small enough to enter and hence it affects the

selectivity of reactions by excluding both the participation and

formation of molecules that are too large for the pores.

1.5.6 Molecular Sieves VersusZeolites

McBain (1932) proposed the term molecular sieve. According to

him molecular sieves are materials with the capability of separating

components in a mixture on the basis of molecular size and shape differences.

The two classes of molecular sieves, namely, zeolites and microporous

charcoals were known when McBain formulated his definition. The list now

includes silicates, metalloaluminates, aluminophosphates, silico and

metalloaluminophosphates, mesoporous and macroporous materials in

addition to zeolites. These materials are structurally analogous but differ only

in their elemental composition. Although all the materials stated above are

molecular sieves, only aluminosilicates carry the classical name of zeolites.

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1.5.7 Non-aluminosilicate Molecular Sieves

The preparation of zeolites and zeo-type structures containing

framework components other than aluminium and silicon has become need of

the hour to press forward in the area of new molecular sieves. Zeolites offer

ion-exchange property, high thermal stability, high acidity and shape selective

structural features. However, modification and subsequent improvement of

these properties have served as a driving force for changing the composition

of these microporous materials. Structures with pores larger than the known

12 ring type of zeolite Y (ultra large pore molecular sieves) have not yet been

produced in zeolite types. However, structures containing other composition

offer the possibility that such ultra pores may be realized. Aluminophosphate

(VPI-5) containing 18-membered ring (Davis et al 1988) and Cloverite

containing 22-membered ring pore systems (Merrouche et al 1992) have been

synthesized in non-aluminosilicate system. The change of elemental

composition not only produced ultra large pore materials but also added a new

dimension to find tailor-made molecular sieves. Gallium can easily substitute

aluminium, and germanium for silicon in aluminosilicate system. In addition

to these materials, zeo-type structures crystallized in the presence of organic

cations have been claimed to contain boron, iron, chromium, cobalt, titanium,

zirconium, zinc, beryllium, hafnium, manganese, magnesium, vanadium

and tin.

Since tetravalent germanium crystallizes to form molecular sieve

structures, it is also possible for other tetravalent ions that can occupy

tetrahedral oxide sites to crystallize to form such structures. Based on the

theory formulated by Barrer (1984), titanium could substitute into molecular

sieve structures. Perego et al (1986) synthesized titanium contining ZSM-5

structures. CeAlPO-11 was synthesized by Araujo et al (1997) without

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affecting the structure of AlPO-11. Zahedi-Niaki et al (2000) reported a

comparative study of VAPO-5, -11, -17 and -31 aluminophosphate molecular

sieves.

1.6 GENERAL ASPECTS OF ALUMINOPHOSPHATES

At the onset of 1980s, a novel class of crystalline,

microporousaluminophosphates (AlPOs) was reported by Wilson et al (1982

and 1982a) at the Union Carbide, representing the first family of framework

oxide molecular sieves synthesized without silica. This discovery opened the

door to a new era in open-framework inorganic materials (Cheetham et al

1999). The aluminophosphate molecular sieves known as AlPO4-n(nrefers to

a distinct structure type) were prepared with a wide range of pore sizes by

hydrothermal synthetic technique in the presence of organic amines or

quaternary ammonium cations as templates or structure directing agents

(SDA) (Pastore et al 2005 and Wilson 1991 and 2001). These molecular

sieves are built from strict alternation of AlO4 and PO4tetrahedra. The primary

building units are formed by Al-O-P linkages instead of Si-O-Al or Si-O-Si

bridges of zeolite (Chen et al 1994). The lack of P-O-P and Al-O-Al in these

materials, constraining the structure to be alternate Al and P tetrahedra, limit

the structural building units to only even-numbered rings (Szostak 1989).The

AlPO contains Al3+ and P5+ in tetrahedral position and the resultant

framework is neutral and therefore there are no charge compensating ions as

in zeolites. Thus Brönsted acidity is intrinsic to AlPOs and they are not

suitable for acid catalysis (Pujado et al 1992).

The exciting property of AlPOmaterials is that Al or P can be

replaced by silicon to form SAPO (silicoaluminophosphate) materials resulted

in Brönsted acidity and they can be used as acid catalyst (Gielgens et al

1995). The isomorphous substitution of divalent or trivalent metal ions in

AlPO and SAPO forms MeAPO and MeSAPO respectively (Levi et al 1991

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andMontes et al 1990). Such substitution introduced charge imbalance in the

framework which was balanced by protons, thus generating Brönsted acidity

and offering catalytic activity and ion-exchange capability in these molecular

sieves (Flanigen et al 1988). The overall composition of aluminophosphate

molecular sieves is represented as xR: Al2O3: 1.0 ± 0.2P2O5: yH2Owhere R is

an organic amine or quaternary ammonium ion. The quantities x and y

represent the amount of organics and water respectively that fills the pores of

the crystal as AlPO requires no counter ions (Flanigen et al 1986). The

aluminium to phosphorous ratio of these molecular sieves is always unity.

Aluminophosphate molecular sieves include more than 40

structures. Among these 25 are three dimensional framework structures, of which at least six are two dimensional layered materials and the others are

microporous. Most of the three dimensional structures are novel. AlPO and

SAPO molecular sieves cover a wide range of structure types, some are analogous to certain zeolites such as SAPO-42 (zeolite A structure), SAPO-34

(chabazite structure) and SAPO-37 (faujasite structure). But there is also a

large number of aluminophosphates such as AlPO-5, AlPO-11 or VPI-5, which possess unique structures with no zeolitic analogue (Davis et al 1988).

Aluminophosphate based molecular sieves exhibit excellent thermal and

hydrothermal stability compared to those observed in stable zeolites. Many are thermally stable and resist loss of structure even at 1000 ºC (Wilson et al

1982). Their surface selectivity is mildly hydrophobic. Their general formula

can be expressed as [(AlO2)x(PO2)x]·yH2O indicating that, unlike most

zeolites, aluminophosphate molecular sieves are ordered with Al/P ratio is

always unity. However, in spite of this, aluminophosphate molecular sieves exhibit enhanced structural diversity.

The discovery of open-framework AlPOs has brought some

conceptual breakthrough for traditional microporous compounds, e.g. the

framework elements are not only limited to Al and Si atoms; the upper limit

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of pore size is not only delimited to 12-ring; the primary building units are not

only defined to tetrahedral. The on-going search for new structures

particularly provides some mechanistic clues on the formation of open-

framework materials. Ultimately, the crystallisation mechanism of

microporous materials must be understood in order to rationalise the

synthesized materials with desired structures, compositions and properties.

The discovery of AlPOs also improved the current application areas of

microporous materials(Thomas et al 1999 and Thomas 1999). One of the

important and promising areas of application of AlPOs is in catalysis where

aerial oxidations are possible using linear and cyclic hydrocarbons (Thomas

et al 2001). Selective oxidation reactions are also carried out using AlPOs

(Li et al 2010).The nanosized channels of AlPO-n also present suitable host

systems for the fabrication of advanced functional materials such as nanosized

single walled carbon nanotubes (Wang et al 2000).

1.6.1 NaturalAluminophosphates

The interactions between aluminium and phosphorus oxides to form

stable structures occur to a considerable extent in nature. The nine naturally

occurring neutral aluminophosphate minerals are berlinite (AlPO4), variscite

and metavariscite (AlPO4.2H2O), augelite (Al2PO(OH)3), senegalite

(Al2(OH)3(H2O)(PO4)), wavellite (Al3(OH)3(PO4)2H2O), trolleite

(Al4(PO4)3(OH)3), bolivarite (Al2(PO4)(OH)):4-5H2O) andevansite

(Al3PO4(OH)6 6H2O). To date, at least 200 structure-types of open framework

AlPOs have been identified. These include neutral open framework AlPO4-n

molecular sieves, their isomorphous substitute analogues and anionic AlPOs

framework.

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1.6.2 Synthetic AluminophosphateMolecular Sieves

The researchers at Union Carbide Corporation, USA discovered aluminophosphate materials, a novel class of crystalline microporous solids

that represents the first family of framework oxide molecular sieves

synthesized apart from the well knownaluminosilicates (zeolites) and silica molecular sieves (Flanigen 1976). The periodic table was viewed as potential

scope for new framework compositions and structures. This resulted in the

discovery of aluminophosphate molecular sieves as reported by Wilson et al (1982).

The family of aluminophosphate molecular sieves is microporous

crystalline oxides, many of which contain pores within their framework

structure like zeolites. In aluminophosphate molecular sieves (AlPO4) the

framework sites are occupied by Al3+ or P5+ and the average ionic radius of Al3+ (0.39 Å) and P5+ (0.17 Å) is 0.28 Å, which is very close to the ionic

radius of Si4+ (0.26 Å). The notable feature of AlPO4 composition is the

invariant Al2O3/P2O5 ratio which is in direct contrast to the variable compositions of SiO2/Al2O3 found in zeolite structures. Unlike zeolite

molecular sieves, which contain Al3+ and Si4+ in tetrahedral positions and

exhibit a net negative framework charge, aluminophosphate molecular sieves contain Al3+ and P5+ in tetrahedral position and the resultant framework is

neutral. Structural diversity is observed in AlPO4 materials even though there is only a limited variation in chemical composition.

The linkage of SiO4, AlO4, PO4 and other cationtetrahedra will decide the three dimensional framework shape and final structure type of the material. The structure of aluminophosphate molecular sieves contains 4-, 6-, 8- and 12- rings of alternating AlO4 and PO4tetrahedra. The avoidance of Al-O-Al and P-O-P bonds in aluminophosphate frameworks (Löwenstein’s rule) made their structures contain only even-numbered rings. Therefore zeolitic structures of pentasil family such as ZSM-5 and ZSM-11 were not

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found in AlPOs. Table 1.4 presents five categories of these structures viz., very large pore, large pore, medium pore, small pore and very small pore groups.

1.6.3 Nomenclature

The aluminophosphate materials are classified into (i) binary, (ii) ternary, (iii) quaternary, (iv) quinary and (v) senary. The composition of aluminophosphate molecular sieves depends on the number of elements contained in the cationic framework sites of any given structure. The normalised TO2 formula represents the relative concentration offramework elements in the composition (ElxAlyPz)O2 where El is the incorporated element and x, y and z are the mole fractions of the respective elements in the composition. Acronyms describing the framework composition are shown in Table 1.5 (Flanigen et al 1986).The structure type is indicated by an integer following the compositional acronym, e.g. SAPO-5 is a (Si,Al,P)O2 composition with type 5 structure. The numbering of structure type is arbitrary and bears no relationship to structural numbers used previously in the literature, e.g. ZSM-5. It only identifies structures found in the aluminophosphate based molecular sieves. The same structure number is used for a common structure type with varying framework composition.

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Table 1.4 Structure of aluminophosphate based molecular sieves

Species Structure type Ring size Pore size (Å) Very large pore VPI-5 novel 18 12AlPO-54 novel 18 12

Large pore

AlPO-5 novel 12 8 -36 novel 12 8 -37 faujasite 12 8 -40 novel 12 7 -46 novel 12 7Intermediate pore AlPO-11 novel 10 6.0 -31 novel 10 6.5 -41 novel 10 6.0Small pore AlPO-12 novel 8 4.0 -14 novel 8 4.0 -17 erionite 8 4.3 -18 novel 8 4.3 -34 chabazite 8 4.3 -35 levynite 8 4.3 -44 chabazite-like 8 4.3Very small pore AlPO-16 novel 6 3 -20 sodalite 6 3 -25 novel 6 3 -28 novel 6 3

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Table 1.5 Acronyms for framework composition

TO2, T = Acronym

Al, P AlPO

Si, Al, P SAPO

Me, Al, P MeAPO

Mg, Al, P Zn, Al, P Co, Al, P Mn, Al, P

MAPO ZAPO

CoAPO MnAPO

Me, Al, P, Si Mn, Al, P, Si Mg, Al, P, Si Zn, Al, P, Si Co, Al, P, Si

MeAPSO MnAPSO

MASO ZnAPSO CoAPSO

Other elements El, Al, P El, Al, P, Si

ElAPO ElAPSO

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1.7 STRUCTURAL ASPECTS OF ALUMINOPHOSPHATES

1.7.1 Classification

The open framework AlPOs reported to date comprise a wide range

of structures and compositions. In terms of electrostatic properties and

Al/P ratios of the frameworks, they can be classified into two major

categories viz.,(i) neutral framework AlPO4-n with Al/P = 1 and (ii) anionic

framework AlPOs with Al/P 1.

1.7.2 Neutral Framework

The characters of AlPO4-n include a neutral framework and a

univariant framework composition with Al/P = 1 (Bennett et al 1986).

Subsequent efforts to incorporate other elements led to the formation of

AlPO4-based molecular sieves such as SAPO (S: Si), ElAPO (El: Li, Be, B,

Ga, Ge, As, Ti, etc.), ElAPSO, MeAPO (Me: metal) and MeAPSO. Even

though some of them have not been found pure AlPO4-n counterpart yet, these

structures can be ideally described using a hypothetical AlPO4-n lattice with

alternate Al and P sites as the basis. The AlPO4-based molecular sieves

include 51 unique structure types withextra-large pores (>12-ring), large pores

(12-ring), intermediate pores (10-ring), small pores (8-ring) and very small

pores (6-ring). These structures include 16 zeolite analogues such as chabazite

(AlPO4-n = 34, 44 and 47), erionite (AlPO4-17), faujasite (AlPO4-37),

gismondine (AlPO4-43), levynite (AlPO4-35), linde type A (AlPO4-42),

sodalite (AlPO4-20) and 35 novel structures such as VFI (VPI-5), AEL

(AlPO4-11) and AFI (AlPO4-5). Figure 1.2 illustrates several representative

AlPO4-n molecular sieves with different pore openings and dimensions

including VPI-5 (VFI): 18-ring (1.27 × 1.27 nm), AlPO4-8 (AET): 14-ring

(0.79 x 0.87 nm), AlPO4-5 (AFI): 12-ring (0.73 x 0.73 nm), AlPO4-11 (AEL):

10-ring (0.40 × 0.65 nm), AlPO4-41 (AFO):10-ring (0.43 × 0.70 nm) and

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AlPO4-25 (ATV): 8-ring (0.30 × 0.49 nm). Apart from aluminosilicate or

silica zeolites, AlPO4-based molecular sieves constitute a major class of

zeolitic materials. These AlPO4-based materials are normally stable upon

removal of the occluded template molecules and exhibit excellent thermal

stability up to 1000 ºC. These materials are mildly hydrophilic. The major

structures in the AlPO4-nmolecular sieves are listed in Table 1.6.

Figure 1.2 Representative AlPO4-n molecular sieves with different pore openings

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Table 1.6 Structures of AlPO4-n molecular sieves

n IZAa structure code Pore diameter (nm)

(ring) Very Large Pore

8VPI-5

AET VFI

0.79 × 0.87 (14) 1.21 (18)

Large Pore 536374046

AFI ATS FAU AFR AFS

0.73 (12) 0.75 × 0.65 (12) 0.74 (12) 0.43 × 0.70 (10) 0.64 × 0.62 (12) 0.4 (8)

Intermediate Pore 113141

AEL ATO AFO

0.63 × 0.39 (10) 0.54 (12) 0.43 × 0.70 (10)

Small Pore 17183334353942434447

ERIAEI ATT CHALEV ATN LTA GIS

CHACHA

0.36 × 0.51 (8) 0.38 (8) 0.42 × 0.46 (10) 0.38 (8) 0.36 × 0.48 (8) 0.4 (8) 0.41 (8) 0.31 × 0.45 (8) 0.31 (8) 0.38 (8)

Very Small Pore 162025

AST SOD ATV

(6)(6)0.30 × 0.49 (8)

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1.7.3 Anionic Framework AlPOs

In contrast to neutral framework AlPO4-n with Al/P = 1, most of

the anionic framework AlPOs have Al/P ratio less than unity (Yu &Xu 2003).

The structures of anionic AlPOs comprise three dimensional and low

dimensional frameworks made up of alternate Al-centered polyhedra (AlO4,

AlO5 and AlO6) and P-centered tetrahedra P(Ob)n(Ot)4-n (b = bridging,

t = terminal and n = 1,2,3 and 4) forming diverse stoichiometries. The

existence of terminal P–OH and/or P = O groups or Al(OP)n (n = 5 and 6)

polyhedra results in the deviation of Al/P ratio from unity in the framework.

Their Al/P ratios are found to 1/1, 1/2, 2/3, 3/4, 3/5, 4/5, 5/6, 11/12, 12/13,

13/18 and so on. Their frameworks exhibit fascinating structural architectures.

A notable example is JDF-20 with Al/P = 5/6 (Huo et al 1992), which has the

largest channel ring size of 20 among open framework AlPOs. Anionic

framework AlPOs have also been prepared with diverse low

dimensionalframework structures such as 2D layers with various porous

sheets and sheet stacking sequences and 1D chains which may act as

fundamental building blocks for complex structures. It is significant to note

that within each compositional family a wide variety of structure types have

been observed. For instance, the 2D frameworks with Al/P = 3/4 show diverse

layered structures. Most of the anionic framework AlPOs possess interrupted

open frameworks with terminal P–OH and/or P=O groups. They are unstable

upon removal of the occluded protonated template molecules by calcination.

1.7.4 Bonding Patterns

As that of zeolites, open-framework AlPOs made up of Al–O–P

bonds obey Löwenstein’s (1954) rule with avoidance of Al–O–Al bonds (only

one exceptional case was reported by Huang &Hwu (1999) in a layered AlPO

containing Al–O–Al linkages. The P–O–P bonds do not appear to be stable in

24

these structures. Thus the avoidance of Al–O–Al and P–O–P bonds endows

open-framework AlPOs featured by even-numbered rings.

In interrupted anionic frameworks, a part of Al–O–P linkages are missed and

terminal P–OH and/or P=O bonds are commonly observed that interact with

protonated templating molecules through H-bonds(Yu &Xu 2003). Using the

first-principle quantum chemical techniques, Cora&Catlow (2001)

characterized the bonding properties of crystalline AlPOs and compared them

with isostructural silica-based zeolites. Their calculation results revealed that

silica polymorphs and AlPOs differ in the nature of bonding. The silica

polymorphs consist of covalently bonded SiO4 units while AlPOs are shown

to be of molecular ionic character and comprised of discrete Al3+ and PO43-

ions. The ionicity of AlPO frameworks might be responsible for the major

contrast between the chemistry of AlPOs and that of aluminosilicates relative

to the nature and concentration of dopants that can be introduced into the

frameworks. In AlPOs, ionic substitutional dopants introduce minor

perturbations to the host electric structure and therefore more readily replace

Al in AlPOs than Si in zeolites.

1.8 TOPOLOGICAL CHEMISTRY OF ALUMINOPHOSPHATES

1.8.1 Building Units

The complex structures of open-framework AlPOs can be

understood on the basis of their construction from fundamental building units.

Topologically, the neutral framework AlPO4-n molecular sieves can be

described as four-connected 3D frameworks since Al and P atoms occupy the

4-connected vertices of 3D net. Most of the anionic framework AlPOs can be

described as interrupted frameworks because part of the Al-O-P linkages is

missed. The four connected 3D frameworks, typically for zeolite frameworks,

can be thought to be constructed of finite secondary building units (SBUs).

18 SBUs have been listed for zeolites in the fifth edition of the ATLAS

25

among which 8 occur in four-connected 3D AlPO4-n frameworks

(Figure 1.3). These SBUs are formed by primary building units (PBUs) of

AlO4 and PO4tetrahedra (known as TO4) by sharing a common oxygen atom

in their corners. Different linkages of these tetrahedral units lead to various

sheet topologies. Figure 1.4 shows eight distinct 2D sheet structures. These

SBUs are arranged in a specific geometrical pattern to form a definite crystal

structure and uniform pore size.

Figure 1.3 Secondary building units (SBUs) found in AlPO4-n based

framework

26

Figure 1.4 Eight distinct 2D sheet structures (The SBUs constructing

these sheets are also shown)

27

1.8.2 Al and P Coordinations and Stoichiometries

The structural and compositional richness of AlPOs are attributed to

the diverse coordination of Al and P atoms. The majority of AlPO4-n

molecular sieves are based on a four-connected network of corner sharing

tetrahedra, i.e., AlO4b and PO4b (b = bridging oxygen between Al and P).

There are a number of AlPO4-n with mixed-bonded frameworks containing

five orsix coordinated Al atoms with one or two extraframework oxygen

species such as OH and H2O (Chen et al 1999). For instance, both VPI-5 and

AlPO4-8 contain AlO4b (H2O)2 units; AlPO4-17, -18, -20, -21 and -31 contain

AlO4b(OH) units. By omitting the OH and H2O species, these frameworks can

be idealized as a four-connected framework. Combinations of alternate Al and

P atoms give rise to various framework structures and Al and P

stoichiometries. According to Löwensteinsrule, the number of Al–Ob bonds

must be equal to the number of P–Ob bonds in open framework AlPOs.

Consequently, the correlation of coordination environment of Al and P can be

described in the following equation (1.3) (Yu &Xu 2003).

AlOib AlOib POjb POjbi j

m i n j (1.3)

where i(j) is the number of bridging oxygen coordinated to Al(P), m(n) is the

number of AlOib (POjb) coordination, mAlOib nPOjb = Al/P, i = 3, 4, 5 and 6

corresponding to AlO3b, AlO4b, AlO5b and AlO6b units respectively, j = 1, 2, 3

and 4 corresponding to PO4 units with one, two, three and four bridging

oxygen respectively. Based on this equation, the detailed Al and P

coordination for a given stoichiometry can be enumerated.

28

1.9 TEMPLATING IN THE CONSTRUCTION OF

ALUMINOPHOSPHATES

Open framework AlPOs are synthesized by hydrothermal or

solvothermal crystallization of reactive aluminophosphate gels in the presence

of an organic base as the templating agent (or structure-directing agent) as in

the synthesis of high-silica zeolites. These template species occupy the pores

and cages of the structures and play an important role in directing the

formation of a specific structure.

1.9.1 Types of Templates

A large variety of organic templates can facilitate the synthesis of

open-framework AlPOs. So far, over 100 species have been used successfully

as templates, typically involving quaternary ammonium cations and various

organic amines including primary, secondary, tertiary and cyclic amines, and

alkanolamines. Some stable metal ligand complexes such as Cp2Co2+ and

Co(en)33+ have also been used in the synthesis of AlPO materials. Very

recently, ionic liquids have been used as both solvent and template for the

preparation of SIZ-n type AlPO materials (Cooper et al 2004 and Parnham

et al 2006). The one template multiple structure and multiple template one

structure phenomena are remarkable in open framework AlPOs. For example,

di-n-propylamine (Pr2NH) has been used in the synthesis of at least ten

different AlPO structure types such as AlPO4-11, -31, -39, -41, -43, -46, -47,

-50, H3/MCM-1 and H1/VPI-5/MCM-9, exhibiting low structure specificity.

On the other hand, some structures readily form from many different

templates, e.g., AlPO4-5 is much less template specific and can be synthesized

with more than 25 different templates. Few examples are given in Table 1.7.

Tetrapropylammonium hydroxide (TPAOH) is a typical template for the

synthesis of AlPO4-5, which is stacked in a tripod arrangement with the head

of one TPA ion suspended between three feet of the next TPA ion with a

29

hydroxyl group neatly suspended between them(Bennett et al 1986). As seen

in Figure 1.5, although this tripod arrangement is such a good geometrical fit

with the cylindrical wall, the TPAOH is not a template in the true sense

because of the inconsistency of the three fold molecular symmetry and six

fold channel symmetry.

Table 1.7Templates used for the preparation of specific structure type

Structure type Typical template(s)

AlPO4-5 tetrapropylammonium hydroxide, tripropylamine,triethylamine, etc.

AlPO4-11 dipropylamine, diisopropylamine

AlPO4-14 isopropylamine

AlPO4-17 quinuclidine, piperidine

AlPO4-18 tetraethylammonium hydroxide

AlPO4-20 tetramethylammonium hydroxide

AlPO4-31 dipropylamine

AlPO4-34 tetraethylammonium hydroxide

AlPO4-35 quinuclidine

AlPO4-36 tripropylamine

AlPO4-46 dipropylamine

AlPO4-47 dipropylamine and diethylethanolamine

30

Figure 1.5 Cylindrical channel in AlPO-5 and the stacking of

encapsulated tetrapropylammonium hydroxide species

A few AlPO structures exhibit high template specificity. For

example, AlPO4-20 can be crystallized only with tetramethylammonium

hydroxide (TMAOH). The spherical TMAOH molecule with 0.62 nm

diameter fits neatly into the sodalite cage. In some AlPOs, a mixture of

templates appears to cooperatively direct the formation of structures.For

instance, SAPO-37 is prepared by a mixture of TPAOH and TMAOH.

Structural characterization shows the presence of TMA in the sodalite cages

and TPA in the supercages. In the synthesis of AlPO4-52, both Pr3N and

TEAOH appear to be necessary but only TEAOH is occluded in the structure.

As with organic amine, water can also play an important structure directing

role. A notable example has been seen in VPI-5. Even though VPI-5 is

31

preferentially prepared in the presence of organic amines, the organic species

are not occluded into the extra large 18 ring pores. Instead, water molecules

form an intriguing H-bonded triple helix inside the channel(McCusker et al

1991).

1.9.2 Role of Templating

Templating has been a frequently discussed phenomenon in the

synthesis of zeolites and related open-framework materials (Davis &Lobo

1992 and Zones et al 1996). So far, the relationship between the templating

agents and the structures, usually known as templating effect, is still not fully

understood. The term templating has been frequently used in the context of

synthesizing high silica zeolites. One definition about templating was

described by Lok et al (1983) as the phenomenon occurring during either the

gelation or the nucleation process whereby the organic species organizes

oxide tetrahedra into a particular geometric topology around itself and thus

provides the initial building block for a particular structure type. The gel

chemistry is also essential for the formation of microporous

aluminophosphates. With the addition of organic base, the gel chemistry of

aluminophosphate is altered, and the templating becomes operative only in

the gel with right gel chemistry. Therefore the dual role of organic templates

in the synthesis of open framework AlPOs is evident. It serves the important

role of modifying the gel chemistry, and it also has a structure directing

effect.

The organic template plays at least two additional roles in the

product, i.e., stabilizing voids and balancing the framework charge. By

packing the cages and channels the organic template can increase the overall

thermodynamic stability of the template/lattice composite, so that the

metastablility of the lattice alone is less critical (Wilson 1991 and 2001). The

stabilizing and charge balancing role of organic templates is quite evident in

32

anionic framework AlPOs. After removing the occluded template molecules

by calcination, the anionic frameworks normally collapse. Furthermore, the

templates also determine the stacking sequences of 2D layers. The template

molecules interact with host inorganic network with certain regularity, and

their interaction can be well described based on the interaction between SBUs

and protonated amino groups. This allows the template molecules to be

located with a reasonable success.

Even though a true templating effect, i.e., hand in glove fit between

organic and inorganic lattice, is less pronounced in the synthesis of open

framework AlPOs, it seems that an encapsulated organic species in the void

space of the inorganic host can adopt configuration that conforms best with

the surrounding aluminophosphate framework. For example, AlPO4-12, -21

and -EN3 were synthesized with encapsulated ethylenediamine. It is stabilized

into optical isomers of the gauche form by intramolecular bonding in

AlPO4-12 and -21, while it occurs in AlPO4-EN3 as trans configuration with

N–C–C–N extended along a straight eight-ring channel. The empirical

evidence is that for a template to be successful there must be a good fit

between the guest molecule and the host framework formed. The importance

of template molecules appears not only in its role of structure directing but

also orientating the distribution of Si in the frameworks. Vomscheid et al

(1994) demonstrated the role of template in directing Si distribution in the

lattice of SAPO materials.

In metal-substituted AlPOs, the template molecules also influence the degree of metal ion substitution in the frameworks. Lewis et al (1996) studied the influence of organic templates on the structure and the concentration of framework metal ions in microporousAlPOs. Their calculations demonstrate that the degree of metal ion substitution in the framework is controlled not only by the relative stability of the framework but also by the need to accommodate the structure directing and charge

33

compensating template molecules. Templates with higher charge/size ratios will allow a greater control over the ratio of metal substitution or heteroatom incorporation in the framework.

1.10 ISOMORPHIC SUBSTITUTION

Apart from the structural similarity with zeolites, AlPO molecular sieves exhibit structural diversity due to their neutral framework in contrast to the negatively charged aluminosilicate. Secondly the aluminium atoms in the aluminosilicate framework are always tetrahedrally coordinated as compared to four, five or six coordinated aluminium atoms present in the AlPO framework as mentioned earlier in this chapter. Moreover, they also offer compositional diversity. The Al and/or P ions in the AlPO frameworks can be replaced by another element with similar cation radius and coordination environment. However, elements incorporated in the AlPO framework should possess radius ratios with oxygen, and T-O distances consistent with the applied crystal chemical concept for tetrahedral coordination. Their successful incorporation may be due to flexibility of microporousaluminophosphate framework and to specific interactions with organic template, coupled with mildly acidic gel chemistry used in the synthesis (Flanigen et al 1986).

Thus, the incorporation of silicon in aluminophosphate molecular sieves results silicoaluminophosphate, SAPO-n (Lok et al 1984). The addition of metal cations yield porous metal aluminophosphate, MeAPO-n or metalsilicoalumino phosphate, MeAPSO-n. In SAPO-n materials, silicon substitutes for phosphorous or for aluminium-phosphorous pair whereas metal cations substitute almost exclusively for aluminium. The MeAPO-n and MeAPSO-n materials encompass the characteristics of both zeolites and aluminophosphates which results in their unique catalytic, ion-exchange and adsorbent properties.

Flanigen et al (1988) reported framework incorporation of at least fifteen elements in the framework of aluminophosphate materials. The most

34

important of these cations are Si4+, Co2+, Zn2+, Mn2+, Mg2+, Cr6+, Ti4+ and V5+.It was suggested that successful incorporation of these elements is attributed to flexibility of the microporous framework and specific interactions with organic templates. The charge of the framework is balanced by template molecules in the as-synthesized materials and the charges in the calcined samples are compensated by H+ ions derived from the template. Thus Brönsted acidity generated in the neutral framework is as shown in Scheme 1.1. The strength of Brönsted acidity depends on the electronegativity of the element which is used for the isomorphous substitution.

Scheme 1.1 Generation of Brönsted acid sites in aluminophosphatemolecular sieves

When M2+ ions are substituted in the place of Al3+ ions, two units

of negative charge is generated in which one negative charge is balanced by

2+

Isomorphous substitution of M2+

Neutral AlPO

Isomorphous substitution of M4+

35

the positive charge on the P atom and other negative charge is balanced by H+

ion as shown in Scheme 1.1 (Lohse et al 1995). When M4+ ion substitute P5+

ion results a net negative charge on the Al atom and this is compensated by

protons derived from the template during calcination as shown in Scheme 1.1

(Prakash et al 1994).

1.10.1 Metal Aluminophosphates

Initially, Flanigen et al (1986) reported the incorporation of 13

elements into AlPO-5 including transition metal ions like titanium,

manganese, iron, cobalt and zinc. The incorporation of transition metal ions

into framework sites of aluminophosphate and silicoaluminophosphate

molecular sieves is also of particular interest for the design of novel catalysts.

Paramagnetic metal species are often introduced into the molecular sieves to

generate catalytically reactive species or site. Various pretreatment or

activation procedures are typically used to generate reactive metal ion valence

states which are often paramagnetic. Transition metal ions are incorporated by

three different methods viz., impregnation, ion-exchange and isomorphous

substitution. In the latter method the transition metal ion salt is incorporated

directly into the synthesis mixture. Since the comprehensive papers of

Flanigen et al (1986 and 1988) on aluminophosphates and the periodic table,

many studies have been published, claiming the isomorphous substitution of

transition metal ions into the framework of different structure types

(Hartmann &Kevan 1999).A variety of metals and transition metals can be

incorporated into aluminophosphate structure (Figure 1.6) but actual

incorporation into the tetrahedral framework is difficult to prove.

36

Figure 1.6 Partial periodic table with transition elements introduced intoaluminophosphates and silicoaluminophosphates

1.11 AlPO-5 MOLECULAR SIEVE

AlPO-5 is a microporousaluminophosphate. It consists of alternate

Al and P tetrahedra with cylindrical pores of diameter 0.73 nm. It possesses

hexagonal crystal symmetry with 24 tetrahedral oxide (TO2) units per unit

cell. The novel three dimensional structure of AlPO-5 was determined by

single crystal X-ray method (Bennett et al 1983). It has hexagonal symmetry

with a = 13.72 Å and c = 8.47 Å. It contains one dimensional channels

oriented parallel to the c axis bounded by 12-membered rings. The framework

structure of AlPO-5 is shown in Figure 1.7(Bennett et al 1983).It can be

synthesized with at least 24 different amines and quaternary ammonium

compounds as template. The drawback of AlPO-5 molecular sieve is neutral

and there is no acidity. Adsorption properties of AlPO-5 have been studied by

Stach et al (1986) and Lohse et al (1987) using hydrocarbon and water as

adsorbates.The incorporation of divalent metal ions in the framework of

AlPO-5 creates Brönsted acidity.

Ti V Cr Mn Fe Co Ni Cu Zn Zr Nb Mo Tc Ru Rh Pd Ag CdHf Ta W Re Os Ir Pt Au HgRf Ha

Framework incorporation

claimed

Ion-exchange and impregnation Not Studied

37

(a) (b) (c)

Figure 1.7 Framework topology of AlPO-5 (a) framework structure of AlPO-5, (b) 12-ring channel view along (001) plane and (c) 12-membered ring of AlPO-5

1.12 CATALYTIC APPLICATIONS OF MeAPO-5 MOLECULAR

SIEVES

The use of microporous solid catalysts such as zeolites and related

molecular sieves has an additional benefit in organic synthesis. The highly

precise organisation and discrimination between molecules by molecular

sieves endow them with shape selective properties, reminiscent of enzymatic

catalysts. The incorporation of transition metal ions and complexes into

molecular sieves extends their catalytic scope to redox reactions and a variety

of other transition metal catalysed processes (Sheldon & van Bekkum 2001).

The metal ion substituted aluminophosphate molecular sieves are

interesting as they possess more density of acid sites for catalytic applications.

Lin et al (1993) carried out one-step liquid phase oxidation of cyclohexane over

CoAPO-5 catalyst in the presence of glacial acetic acid as the solvent. Since

the conversion and selectivity reported in this study were moderately good

and hence CoAPO-5 was proved to be a useful catalyst for this reaction.

38

Concepcion et al (1997)carried out oxidative dehydrogenation of ethane over

MgVAPO-5 catalyst. This catalyst is very active and selective in this reaction.

The activity is related to the presence of Mg2+ in the framework of AlPO-5.

The low catalytic activity of VAPO-5 can be related to its lower reducibility.

Although catalysts with high vanadium or magnesium content could be

prepared, their low crystallinity could decrease the number of effective active

sites.

Suresh et al (2004) reported isopropylation of benzene with

2-propanol over alkaline earth metal substituted MeAPO-5 (Me = Mg, Ca, Sr

and Ba) molecular sieves. The selectivity of cumene and benzene conversion

are in the order: Mg >Ca>Sr>BaAPO-5. Among these catalysts,

MgAPO-5 is more active than other catalysts due to the presence of more acid

sites. Dumitriu et al (2002) reported trans-alkylation of toluene with

trimethylbenzenes over various MeAPO-5 catalysts. The activity of

MeAPO-5 catalysts follows the order:

SiAPO>MgAPO>MnAPO>ZnAPO>CoAPO which can be correlated with

acidic properties of the catalysts. The strength of acid sites of the catalyst

influences the competition among various reactions that occur during the

trans-alkylation process. Generally,

trans-alkylation or disproportionation reactions occur on strong acid sites

while isomerisation of xylenes predominates on weak acid sites.

Hentit et al (2007) investigated the alkylation of benzene and other

aromatics over AlPO-5, AlPO-11, FeAPO-5 and FeAPO-11 catalysts using

benzyl chloride as the alkylating agent. Among the catalysts FeAPO-5 and

FeAPO-11 showed both high activity and selectivity due to their pore size and

acidity. The activity of these catalysts for benzylation of different aromatic

compounds is in the following order: benzene > toluene >p-xylene > anisole.

The interesting observation is that this catalyst could be reused in the

39

benzylation of benzene for several times. Hsu and Cheng (1998) reported

pinacol rearrangement over V, Cr, Co, Cu, Ti and Zn substituted AlPO-5,

Fe substituted VPI-5, AlPO-11 and silicalite-1. Among the transition metal

ions substituted in the AFI crystal structure, Fe3+, Cu2+ and Ni2+ showed the

highest pinacol conversion and pinacolone selectivity. The catalytic activity

was found to exhibit no direct correlation with acid strength or amount of acid

sites in the catalysts. Besides, comparison of the catalytic activities of Fe-

substituted molecular sieves of different crystalline structures, the activity

decreased in the order: AlPO-5 > AlPO-l1 > AlPO-8 > VPI-5 > silicalite-1.

Since the catalytic activity is independent of pore diameter, the liquid phase

reaction is considered to proceed mainly on the outer surface of the catalysts.

The hydrophilicity of aluminophosphate surface is in favour of catalysing the

pinacol rearrangement.

Vijayaraghavan& Raj (2004) carried out vapour phase ethylation of

benzene with ethanol over AlPO-5, MgAPO-5, ZnAPO-5 and MnAPO-5.

MnAPO-5 was found to be more active than other catalysts. Although

isomorphic substitution in MnAPO-5 is nearly the same as in MgAPO-5 and

ZnAPO-5, the increased conversion over MnAPO-5 is attributed to the

presence of unpaired electrons in the d-subshell of manganese.

Elangovan&Murugesan (1997) studied the catalytic transformation

of cyclohexanol over AlPO-5, AlPO-11, SAPO-5, SAPO-11, VAPO-5,

VAPO-11, CoAPO-5, CoAPO-11, NiAPO-5, NiAPO-11, ZnAPO-5 and

ZnAPO-11. SAPO-5 and VAPO-5 were found to be more active than other

catalysts because of the presence of more number of acid sites. The product

distribution is influenced by acidity, weight hourly space velocity (WHSV)

and temperature. Kannan et al (1998) reported ethylation of toluene with

ethanol over NiAPO-5, NiAPO-11, ZnAPO-5 and ZnAPO-11 in the vapour

phase. The products formed in this reaction were ethyltoluene, diethylether,

40

benzene and styrene. The conversion was found to be maximumover ZnAPO-

5 at 350 ºC but further increase in temperature decreased the conversion due

to coke formation.

Oxidation of alkylbenzenes is a promising subject in industrial

chemistry. Many bulk chemicals such as terephthalic acid, phenol, benzoic

acid, etc., are manufactured by homogeneous liquid phase oxidation with

oxygen. The large scale liquid phase oxidation is the conversion of

p-xylene into terephthalic acid which is chiefly used as polyethylene

terephthalate polymer material. m-Xylene is also commercially oxidised to

isophthalic acid. Benzoic acid derived from the oxidation of toluene is an

important raw material in the production of various pharmaceuticals and

herbicides. Commercially cumenehydroperoxide and

ethylbenzenehydroperoxide are also manufactured by aerobic oxidation of

isopropyl benzene and ethylbenzene respectively (Ishii &Sakaguchi 2006).

Singh et al (1999) studied the oxidation of ethylbenzene over

MeAPO-11 (Me = Co, Mn or V). The excellent incorporation of metal into

the framework has been achieved by synthesizing MeAPO-11 in the presence

of fluoride ions. In spite of their large crystallite size, MeAPO-11s obtained

from fluoride route are more active in the oxidation of ethylbenzene. The

complete change in the oxidation state of vanadium from lower valence state

(IV) to higher valence state (V) during calcination is observed in VAPO-11.

The redox behaviour of MeAPO-11 has a potential influence on the catalytic

activity during the oxidation of ethylbenzene. VAPO-11, which has

significant redox behaviour, is most active.

Subrahmanyam et al (2002) studied the vapour phase oxidation of

toluene with molecular oxygen over CrAlPO. They reported that CrAlPO

functions both as acid and redox catalyst and observed that in CrAlPO both

acidity (due to Al3+) and redox properties (due to Cr5+/6+) are competing,

41

thus leading to benzene and benzaldehyde respectively. Subrahmanyam et al

(2004) also reported the aerial oxidation of cyclohexane over FeAlPO under

highpressure conditions and reported that the reaction is probably taking place

through radical initiated mechanism. Subrahmanyam et al (2005) studied the

oxidation of toluene over mesoporousVAlPO, and the productsbenzaldehyde

and benzoic acid were obtained when the oxidising agent was 70% TBHP

while with 30% H2O2 cresols were formed. The activity of VAlPO has been

compared with those obtained with other similar porous materials likeV-

MCM-48, V-MCM-41, V-Al-Beta and VS-1.

Potter et al (2012) reported simultaneous framework incorporation

ofheavy metal ions such as Ru(III) and Sn(IV) intoaluminophosphate

architectures and evaluated the catalytic activity in cyclohexene oxidation.

The bimetallic catalyst facilitated synergistic interactions, affordinghigh

degree of selectivity and activity in the catalytic oxidation reactions as

compared with their corresponding transition metal analogues (Co and Ti).

They also reported that heavy metal dopants suchas Ru and Sn in the

framework architecture displayed enhancedcatalytic turnovers compared to

their correspondingtransition metal analogues (such as Co and Ti) in

selectiveoxidation reactions. In particular, the bimetallic analogues ofthe

former exhibit a concomitant enhancement in catalyticactivity when

compared with the corresponding bimetallictransition metal counterparts,

suggesting a synergistic enhancementin catalytic properties.

Raboin et al (2012) reported the grafting of titanium alkoxide over

mesoporousaluminophosphate and evaluated the catalytic activity in the

liquid-phase epoxidation of cyclohexene in the presenceof TBHP. The

catalyst showed 67% selectivity to epoxide formation. Further, they compared

the catalytic properties with Ti grafted SBA-15 catalysts and reported that

both SBA-15 and mesoporousAlPO showed comparable activities and

42

selectivities in cyclohexene oxidation. In addition, Ti-AlPO catalysts

exhibited higher tendency towards allylic oxidationin comparison with similar

Ti-SBA15 catalysts.

Wang et al (2013) synthesized Pt-Co/AlPO-5 catalysts for the

preferential oxidation of CO in H2rich gases. The optimized catalysts were

highly active and selective.CO could be puri ed below 10 ppm in the reaction

temperature range of 110 –125°C under 1% CO, 1%O2, 12.5% CO2, 15%

H2O, 50% H2 in volume and N2 balance at the space velocity of 24,000 ml

gcat1h 1. Pt–Co/CoAPO-5 exhibited the best catalytic performance and Pt–

Co/AlPO-5 was the most active catalystat low reaction temperature, in which

particles of Pt–Co alloy were formed and the particles were highlydispersed

on the surface of the support.

Devika et al (2011) reported the single sited CeAlPO-5 catalyst for

the oxidation of ethylbenzene to acetophenone in air atmosphere. The

selectivity to acetophenone was above 90% at all reaction temperatures.

Devika et al (2012) reported the vapour phase oxidation of diphenylmethane

to benzophenone. They reported that not only active ceriumsite isolation is a

requirement for selective oxidation but also themagnetic field of cerium sites

and free radicals produced duringoxidation were also suggested to play a

major role in theselective oxidation. In addition, free rotation across the

phenyland ethyl carbon bond also key factor forselective oxidation of

diphenylmethane.

Smet et al (1998) reported Pr6O11-MoO3 catalysts for the selective

oxidation of isobutene to methacrolein. The synergism of Pr with Mo played

an important role in the oxidation. Rovira et al (2012) reported the catalytic

activity of ceria-praseodymia nanotubes in the CO oxidation. The

43

incorporation of Pr in CeO2lattice improved the redox properties, and their

nanostructure also aided the catalytic oxidation of CO

Praseodymium incorporated AlPO-5 (PrAlPO-5) with different

(Al+P)/Pr ratios were synthesized under hydrothermal condition in the

presence of fluoride ions. Similarly, iron incorporated AlPO-5 (FeAlPO-5)

with different Al/Fe ratios were also synthesized. It was found that fluoride

ions exhibited several rolessuch as (1) they solubilisealuminium in the

reaction mixture leading to slower nucleation, thus rendering the formation of

dense aluminophosphate phases less favourable, (2) they lead to slow crystal

growth rates yielding crystals of larger size with fewer defects and (3) the

fluoride ions impart a structure-directing and templating effect by interacting

with the framework. In this last role, fluoride ions behave as bidentate ligands

linking two aluminium ions. Consequently, the aluminophosphate framework

requires a cation to balance the charge. Generally, a protonated organic amine

is the counter ion.

General characterization was performed to check the purity and

crystalline nature of the desired phase, its surface area, morphology and

chemical composition. In depth spectroscopic techniques were employed to

understand the accessibility, redox ability, coordination, acidity and oxidation

state of the metal ions in PrAlPO-5 and FeAlPO-5 for specific catalytic

application. The characterization of the samples was performed using XRD,

BET, TEM, SEM and ICP-OES analysis. DRS-UV-Vis,ESR, XPS, 27Al, 31P

MAS NMR and ex-situ pyridine adsorbed IR were used to understand the

nature and surface chemistry of materials.

44

1.13 SCOPE AND OBJECTIVES OF THE PRESENT

INVESTIGATION

Synthesis of fine chemicals using homogeneous catalysts possess

several problems such as difficulty in separation and recovery, disposal of

spent catalyst, formation of undesirable and/or toxic wastes. Efforts have been

made to replace homogeneous catalysts by reusable and easily separable

heterogeneous solid acid catalysts for the synthesis of fine chemicals.

The incorporation of one or more transition metal ions into AlPO

framework has gained considerable importancebecause of their redox

behavior and potential bi-functionality (Lewis and Bronsted acid sites). A

wide array of amines was used as structure directing agents for the

preparation of microporousAlPOs. Among the various AlPOs, AlPO-5 has

been extensively studied structure.AlPO-5 is not template specific,it can be

synthesized using more than one template.

The main objectives of the present investigation are

Hydrothermal synthesis of AlPO-5 in fluoride medium using

aluminiumisopropoxide, orthophosphoric acid and hydrofluoric

acid as the sources for aluminium, phosphorous and fluoride

respectively.

Hydrothermal synthesis of PrAlPO-5 with (Al+P)/Prratios of

25, 50, 75, 100,150 and 200 in fluoride medium using

praseodymium nitrate hexahydrate as the source for

praseodymium. The synthesis procedure was similar to that of

AlPO-5.

45

Calcination of PrAlPO-5((Al+P)/Pr = 25, 50, 75, 100,150 and

200)in air atmosphere at 550 ºC to remove occluded

template.Physico-chemical characterization of the materials

using XRD, SAXS, DRS-UV-vis, BET, SEM, TEM, TPD-

NH3, TPR, FT-IR, ICP-OES, TGA, 27Al and 31P MAS-NMR,

ex-situ pyridine adsorbed IR, ESR and XPS.

Study of liquid phase aerobic oxidation of ethylbenzene over

calcined PrAlPO-5((Al+P)/Pr = 25, 50, 75 and 100) between

60 and 140 °C. Analysis of the product using gas

chromatograph (GC) and gas chromatograph coupled with

mass spectrometer (GC-MS).Study of the influence of

temperature, reaction time, (Al+P)/Pr ratios and substituents on

ethylbenzene conversion and product selectivity.Optimisation

of the reaction parameters for maximum conversion with high

product selectivity. Study of the stability and reusability of the

catalyst.

Study of campholenic aldehyde synthesis from -pinene over

bi-fuctional PrAlPO-5with (Al+P)/Prratios of 75, 100, 150 and

200 catalysts. Analysis of the product using gas chromatograph

(GC) and gas chromatograph coupled with mass spectrometer

(GC-MS). Study of the influence of temperature, reaction time,

solvent and (Al+P)/Pr ratios on -pinene conversion and

campholenic aldehyde selectivity, and optimisation of the

parameters for maximum conversion with high product

selectivity. Study of the stability and reusability of the

catalyst.Separation of the products of the reaction by column

chromatography. Identification of the structure of the isolated

product by 1H-NMR

46

Hydrothermal synthesis of FeAlPO-5 with Al/Fe ratios of 75,

100 and 150 in fluoride medium using iron (III) nitrate

nanohydrate as the source for iron (III).

Calcination of FeAlPO-5(Al/Fe = 75, 100 and 150) in air

atmosphere at 550 ºC to remove occluded template. Physico-

chemical characterization of the materials using XRD, DRS-

UV-vis, BET, SEM, TPD-NH3, ex-situ pyridine adsorbed IR,

ESR, XPS and ICP-OES.

Synthesis of 5-arylidene-2,4-thiazolidinedione over FeAlPO-5.

Study of the influence of temperature, reaction time, solvent

and Al/Fe ratios on 5-benzylidene-2,4-thiazolidinedione

synthesis, and optimization of reaction parameters. The

influence of various substituted benzaldehydes and

heterocyclic aldehydes as substrate in Knovenegal

condensation. The structural identification of isolated product

by 1H-NMR.