CHAPTER-1 - Shodhganga : a reservoir of Indian...

CHAPTER-1

Chapter 1 Introduction

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1.0 General introduction

Chemical technology necessitates a paradigm shift from traditional concepts of process

efficiency, that focus largely on chemical yield, to one that assigns economic value to

eliminating waste at source and avoiding the use of toxic and/or hazardous substances [1-9].

Catalysis is a growing need for environmentally acceptable process to improve sustainability of

the chemical processes in chemical industry due to their eco-friendly nature [1,10,11]. A

catalyst is that which accelerates chemical reactions by opening alternative mechanisms with

lower activation energies [12]. Catalysis play a key role in petrochemical industries and

manufacture of a wide variety of speciality chemicals such as pharmaceuticals, agrochemicals,

and fragrances. The other examples include catalytic cracking, hydrocracking, alkylation,

isomerization, oligomerization, hydration/dehydration, esterification, hydrolysis and a variety

of condensation reactions [13,14]. However, a serious issue concerned with use of

homogeneous catalysts in these processes form a large amount of inorganic salts as by-products

whose disposal is a solemn problem due to the strong environmental consciousness and

stringent regulations. Heterogeneous catalysis is most often used in modern industry that can

provide significant advantages over homogeneous catalysts in terms of stability, corrosion, ease

of handling, recovery, regeneration, and less contamination. They can also be designed to give

increased activity, selectivity, longer catalyst life, and reusability [13,15]. The fast development

of catalyst design, however, made it possible to apply various heterogeneous catalysts in

industrially important organic transformations.

Acid catalysis is one of the most important areas for the application of heterogeneous

catalysis. Industrially a wide variety of solid acid catalysts including zeolites, silica-alumina

mixed oxides, acidic clays, heteropoly acids, sulfated metal oxides, acidic resins were examined

for acid catalyzed reactions [16,17]. High product selectivity can depend on the fine-tuning of

solid acid properties. Among the solid acid catalysts, polyoxometalates familiarly known as

heteropoly acids (HPAs) show appreciable acid catalytic properties that are of practical

importance [18,19]. HPAs exhibit a variety of structures and properties that make them useful

in catalysis, material science and medicine [20]. The HPAs are widely used as acid, redox, and

bifunctional catalysts in homogenous and heterogeneous system because of their high solubility

in polar solvents and fairly high thermal stability in the solid state [18-22]. The pore size and

acidity of the heteropoly acid can be controlled by the counter ions. The tunable redox


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properties of HPAs made them as both heterogeneous and homogeneous catalysts. Hence they

can be used in solution as well as in the solid state [23]. As the acid HPAs are non porous they

need to be supported on a porous solid or altered into microporous insoluble salts [24].

Concerted efforts have been made towards developing a new type of modified heteropoly acids

as heterogeneous catalysts for acid catalyzed reactions such as Friedel-Crafts benzylation

reaction.

1.1 Polyoxometalates

Polyoxometalates (POMs), composed of early transition-metals of group V and VI (V,

Nb, Ta, Mo, W etc.) in their highest oxidation states, are a vast class of metal-oxygen clusters

with a variety of chemical composition and definite structures [25]. Generally, there are two

specific families of POMs, namely, (i) isopolyoxometalates and (ii) heteropolyoxometalates.

Isopolyoxometalates consists of a metal-oxide cluster with only the d0 metal cations (Mo

6+,

W6+

, V5+

etc.) with a general formula [MmOy]n-

. Heteropolyoxometalates are composed of a

metal-oxide framework together with one or more p-, d- or f- block heteroatoms (P, Si, B, Mn,

Co, Gd etc.) often in different oxidation states with general formula [XxMmOy]p-

, where x < m

[26].

Based on the well-defined pioneering work, polyoxometalate chemistry has become one

of the fastest developing areas, in virtue of their fascinating architectures [27,28], excellent

physicochemical properties, including strong Bronsted acidity, fast reversible multielectron

redox transformations, high proton mobility and high solubility in polar solvents and fairly high

thermal stability in the solid state [20]. They find rapid development in various applications

such as optics, electronic materials, magnetic materials, medicine (antiviral, anti-HIV activity),

electronics, and catalysis [18,21,26,29-40]. Since the first polyoxoanion [PMo12O40]3-

was

reported by Berzelius in 1826 [41], they have drawn attention since 1933 when the first X-ray

crystal structure of POM was discovered by Keggin [42]. Most of the researches on POMs

chemistry focused on the two systems: tungsten based POMs (polyoxotungstates) and

molybdenum based POMs (polyoxomolybdates).

1.1.1 Classification of heteropolyoxometalates

Heteropolyoxometalates are with complex structure, which consists of metal-oxygen

octahedral as basic structure unit [21]. Their general formula is [XxMmOy]n-

(x < m). M is

usually Mo or W and to a lesser extent V, Nb or Ta. The heteroatom X, can be one of 64

elements that belong to a various groups of the periodic table except noble gases [43].


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According to the molecular architectures, ratio between polyatom and heteroatom

polyoxometalate structures are classified. Several structures were proposed for heteropoly

acids, the most important three classes of heteropoly ions are shown in Figure 1.1 [44,45].

These are Keggin, Wells-Dawson and Anderson structures. The general chemical formula and

structures of heteropoly ions are listed in Table 1.1.

Figure 1.1 Structure of heteropoly ions (A) Keggin, (B) Wells-Dawson, and (C) Anderson.

Table 1.1 Different types of heteropolyoxometalates

Chemical formula (M = W)

Heteroatom (X)

Structure

[Xn+

M12O40](8-n)-

P5+

, As5+

, Si4+

, Ge4+

1:12 Keggin [41]

[Xn+

M11O39](12-n)-

P5+

, As5+

, Si4+

, Ge4+

1:11 lacunary Keggin

[X25+

M18O62]6-

P5+

, As5+

2:18 Dawson [46]

[Xn+

M6O24]n-

Te6+

, I7+

1:6 Anderson [47]

The important features of each type of heteropoly ions are explained briefly,

i) Keggin heteropoly ion

Keggin type heteropoly anion can be represented as, XnM12O40(8-n)-

, where X = heteroatom. The

ratio between addenda atom and heteroatom for these compounds is 12, (M/X =12). These are

the well-known and widely studied compounds for various applications among which catalytic

applications are most important due to their relatively high thermal stability, redox and acidic

properties. The heteroatom X is usually either P5+

or Si4+

and hardly ever Co3+

, Ge4+

and many

others have been reported to act as heteroatom and M is usually W6+

or Mo6+

.


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In Keggin heteropoly anion a subclass called lacunary Keggin anion which are formed

by the removal of one MO6 octahedron (stoichiometrically one MO unit is lost) from the

Keggin anion with the formation of XM11O39n-

. The formation of lacunary compounds is

obtained by the raising pH of the solution at which the Keggin structure is unstable, and is

hydrolyzed with progressive loss of one or more MO entities.

ii) Wells-Dawson heteropoly anion

General formula of the Wells-Dawson heteropoly anion is X2n+

M18O62(2n-16)-

, (X is P5+

, S6+

,

As5+

; M is W6+

, Mo6+

). The ratio of addenda and heteroatoms for these compounds is 9 (M/X =

9). These HPAs are formed via dimerization of two XM9O34 moieties under increased pH at

which the parent Keggin compound forms the corresponding tri lacunary compounds by the

loss of MO groups [48,49].

iii) Anderson heteropoly anion

The general formula of Anderson HPA compounds is XM6O24n-

(X = Mn4+

, Ni4+

, Pt4+

,

Te6+

; M = Mo6+

, W6+

). The ratio between addenda and heteroatoms is 6 (i.e., M/X = 6). These

compounds possess a hexagonal plane structure composed of a central X ion surrounded by six

octahedral MO6 groups. Each MO6 shares an edge with each of its two neighbouring MO6 and

another edge with the XO6 octahedron [50].

1.1.2 Structure of heteropoly acids

The first heteropoly anion [PMo12O40]3-

was reported in 1826 by Berzelius [41]. Several

conjectures were made on the structure of HPAs, particularly by Alfred Werner followed by

Linus Pauling. Finally a century later in 1933, J.F. Keggin from the University of Manchester

performed powder X-ray diffraction studies on the similar free acid H3PW12O40 and determined

its structure, which is known as the Keggin structure [51].

The primary structure of H3PMo12O40 is the well-established Keggin unit (KU), in

which a central P atom in tetrahedral co-ordination (PO4) is surrounded by 12 metal-oxygen

octahedral (MO6) units [25,41]. The Keggin unit contains a negative three charge which is

neutralized in the acid form by three protons. There are four types of oxygen atoms in a KU

(Figure 1.2). There are 4 central oxygen atoms (Oa), 12 oxygen atoms that bridge two

molybdenum atoms sharing a central oxygen atom (edge-sharing Oc), 12 oxygen atoms that

bridge molybdenum atoms not sharing a central oxygen atom (Ob), and 12 terminal oxygen

atoms (Od) bound to a single molybdenum atom.


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Figure 1.2 Ball and stick representation of Keggin anion (Red: Oxygen, Magenta:

Molybdenum, Yellow: Phosphorus).

The examples of Keggin type heteropoly acids are:

H3PMo12O40 - 12-phosphomolybdic acid

H3PW12O40 - 12-phosphotungstic acid

H4SiMo12O40 - 12-silicomolybdic acid

H4SiW12O40 - 12-silicotungstic acid

These different Keggin HPAs are known based on their central and addenda atoms.

HPAs based on P and Si central atoms and Mo and W as addenda atoms are available. The

Keggin structure is not rigid and has five geometrical rotational isomers, also known as Baker-

Figgis isomers. These ions are known to exist in α form and this term is frequently used in

several applications including heterogeneous catalysis [46]. Although occasionally employed in

heterogeneous catalysis, atleast in part as a result of their instabilities, the β, γ, δ and ε isomers

of the Keggin anion (Figure 1.3) are of some interest.

The β-Keggin structure is identical qualitatively to the α-Keggin unit but with one of the

three edge-shared M3O13 triplets of the latter rotated by 60° [52]. The inter nuclear separations

between the two M atoms in the β form are shorter than those in the α structure and the M-O-M

angles are smaller, which may account for the lower stability of the former structure.

Consequently, columbic repulsions between peripheral metal atoms in the edge sharing should

be more substantial than those in the corner sharing structures, possibly accounting for the

relative stabilities of the α and β forms [53].


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Figure 1.3 Different rotational isomers of the Keggin ion starting from the parent α-isomer.

Red octahedra: MO6 groups, green octahedra: MO6 groups belonging to a rotated triad, green

ball: heteroatom.

1.1.3 Primary, secondary and tertiary structures of heteropoly acids

Generally, heteropoly acids and their salts form ionic crystals composed of heteropoly

anions (primary structure), counter cations (H+, H3O

+,H5O2

+), hydration water, and other

molecules. The hierarchical structure of solid HPAs is important for the understanding of their

catalytic activity and the substructures were denoted as primary, secondary and tertiary [47]. A

schematic model for the microstructure of heteropoly acid is shown in Figure 1.4.

The primary structure is the heteropoly anion itself [PW12O40]3-

i.e., Keggin structure.

The arrangement of primary structure together with counter cations forms the secondary

structure (Eg. Cs2.5H0.5PW12O40). The secondary structure is flexible which depends on the

amount of hydration water, counter cation and heteropoly anion.

The aggregates of secondary structures in three dimensional manner give the tertiary

structure. It explains the formation of solid particles and relates to properties such as particle

size, surface area, pore structure, and distribution of protons in particles [54]. This hierarchical


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structure of HPAs plays a crucial role in understanding and designing these as heterogeneous

catalysts. This may appear a very simple idea, but enormously helps the progress of research in

HPA catalysis.

Figure 1.4 Primary, secondary and tertiary structures; hierarchical structure of heteropoly acids

in the solid state.

1.1.4 Location of protons in Keggin heteropoly acids

Two types of protons have been determined in solid heteropoly acids. They are hydrated

protons [H(H2O)n]+ and non-hydrated protons. The location of protons in heteropoly anions has

been the subject of many studies [55-58]. The hydrated protons possess a high mobility and

they are responsible for extremely high proton conductivity of crystalline heteropoly acid

hydrates. Non-hydrated protons have much less mobility and it has been suggested that they

localize on the peripheral oxygens of the polyanion in one of three positions i.e., on M=O and

bridging oxygens M-O-M (edge sharing and corner sharing) [59]. They may also exist as “free

protons” without a well-defined location. In solid crystalline heteropoly acids, the protons,

hydrated or non-hydrated, take part in the formation of the crystal structure, linking

neighbouring heteropoly anions. In the relatively stable crystalline hexa hydrate


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H3PW12O40.6H2O, bulk proton sites are represented as diaquahydrogen ions (H5O2+), which are

almost planar, quasi-symmetrical hydrogen bonded species that link four neighbouring

heteropoly anions by forming hydrogen bonds with the terminal W=O oxygen as shown in

Figure 1.5 [60,61].

Figure 1.5 Schematic structures of proposed for the states of acidic protons and water in solid

H3PW12O40·nH2O (0 < n < 6).

1.1.5 Properties of heteropoly acids

The general characteristics of heteropoly ions are as following:

i) Heteropoly acids are usually very strong acids.

ii) Possess high molecular weight (oxide clusters with molecular weight ~ 2000).

iii) The lattice energies of heteropoly compounds are low thus the solvation energies of

heteropoly anions are also less. The solubility of the HPAs depends on the solvation

energy of the cation.

iv) In aqueous solution they are stable at lower pH values, but tend to be hydrolyzed at

higher pH values. The stability is elevated in organic media.

v) Heteropoly ions are multielectron oxidants and those are having Mo and V as

polyanions are relatively strong oxidants.

vi) Free acids and their salts with small metal ions (Eg. Na+, Ni

2+, Cu

2+, Co

2+ etc.) are

extremely soluble in water and polar solvents. Salts with large ions (Cs+, Ag

+ and NH4

+)

are insoluble or faintly soluble. The solubility depends up on the water content of the

compounds.


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1.1.6 Acidic properties of heteropoly acids

In aqueous solution HPAs are strong acids. These are stronger than the usual mineral

acids such as H2SO4, HCl, HNO3, etc. and sometimes called as super acids [62]. The protons

present in the secondary structure of HPAs are considered as mobile protons. Thus the high

mobility of these protons made HPAs as super acids. The acid strength of HPAs vary in a wide

range depending on the polyanion structure and its constituent elements (both hetero and

addenda atoms), as well as on the extent of hydration and reduction. Among the Keggin type

HPAs phosphotungstic acid is much stronger and acidity is close to that of super acid [18,21].

The acid strength of crystalline heteropoly acids decrease in the series H3PW12O40 >

H4SiW12O40 > H3PMo12O40 > H4SiMo12O40. This order is same as to that for HPAs in solution

(Table 1.2). The tungsten acids are markedly stronger than molybdenum ones. Although, the

effect of the central atom is not as great as that of the addenda atoms, phosphorus based

heteropoly acids are slightly more acidic than silicon based heteropoly acids.

Table 1.2 Dissociation constants of heteropoly acids in acetone at 25 °C [63].

Acid

pK1

pK2

pK3

H3PW12O40

1.6

3.0

4.0

H3SiW12O40 2.0 3.6 5.3

H3PMo12O40

2.0 3.6 5.3

H4SiMo12O40 2.1 3.9 5.9

H2SO4

6.6

HCl 4.3

HNO3

9.4

Strong Bronsted acidity of HPAs is due to strong polarization of the outer electrons of

surface oxygen (Ot) towards M6+

ions, which results the shift of equilibrium reaction strongly to

the right to create a high concentration of protons in the form of H3O+ [64].

M

O

MH2O

M

O

MH3O+

H


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However, when the HPA is fully dehydrated, the location of the protons is not easily

defined. The terminal (M=Ot) and bridging (M-O-M) atoms of oxygen are the most probable

positions for the protons. The calculation was proved that most energetically favourable site for

the acid proton is a bridging oxygen atom [52].

1.1.7 Catalytic applications of heteropoly acids

The applications of heteropoly compounds are based on their unique properties

including size, mass, electron and proton transfer/storage abilities, thermal stability, and

mobility of lattice oxygen and Bronsted acidity [65]. The heteropoly acids are widely used as

acid, redox, and bifunctional catalysts in homogenous and heterogeneous systems because of

their high solubility in polar solvents and fairly high thermal stability in solid state [19,21]. The

thermal stability of hydrogen forms of heteropoly acids is as follows: H3PW12O40 >

H3PMo12O40 > H4SiMo12O40 > H4SiW12O40. HPAs provide a good basis for the molecular

design of mixed oxide catalysts and they have high capability for practical uses. HPAs are used

as catalysts in solution as well as in the solid state for acid and oxidation reactions [66]. The

reasons for the heteropoly compounds to be the suitable materials for both catalyst design and

practical processes are listed below;

i) Systematic variations of acid and redox properties are possible for catalyst design.

ii) Molecular nature of solid heteropoly compounds originating from heteropoly anion

molecules enable precise design of catalysts and molecular description of catalytic

process.

iii) They can be used as bifunctional catalysts by modification.

iv) HPAs are unique as they exist in a pseudo liquid phase and can act as a phase transfer

catalysts.

v) The soft basicity of the poly anion helps in stabilizing the reaction intermediates.

Acid and redox properties of Keggin type HPAs can be controlled by the substitution of

constituent elements without changing the fundamental structure. The HPAs with Keggin-type

primary structure are polynuclear complexes principally constituted by molybdenum or

tungsten as polyatoms (M) and phosphorus, silicon or germanium as central atom or heteroatom

(X). They could be either multielectron oxidants or strong acids with an acid strength higher

than that of the classical ones.

Many of the known and new HPAs have been used for a variety of reactions, for

instance, formation of carboxylic acids from the corresponding aldehydes, as well as the


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dehydrogenation of alcohols, aldehydes and carboxylic acids to form C=C and C=O bonds [25].

HPAs have been developed and commercialized for oxidation of methacrolein to methacrylic

acid, hydration of olefins such as propene and butene, polymerization of tetrahydrofuran etc.

[66]. A commercial plant for production of ethyl acetate from ethylene by using a silica

supported HPA catalyst with 50 kt/year capacity started in Indonesia, in 1998 [67]. The

bifunctional HPA catalysts can be used for sequential hydroformylation and oxidation of

olefins [68]. HPAs also exhibit homogeneous oxidative catalytic activity such as water splitting

processes and oxygen transfer to alkanes [69]. The oxidation of alkenes and the coupling of

aromatics known as the Wacker process utilizes molybdovanadates as catalysts [70]. Catalysts

containing polyoxomolybdates are widely used in hydrodesulfurization, hydrodenitrification of

fossil fuels [71]. The latest commercial process, the direct oxidation of ethylene to acetic acid

catalyzed by palladium and HPAs produces 100,000 tons per year of the product [18]. Other

applications including ion exchange materials, ion-selective membranes and inorganic resistant

materials have also been reported [72]. It is evident that the research activity on heteropoly

compounds is very high and still growing.

1.1.8 Need for the modification of heteropoly acids

Heteropoly acids have been identified as efficient solid acid and oxidation catalysts.

Bulk form of Keggin heteropoly acids have limited number of catalytic applications since they

have associated with certain drawbacks [73]. Pure heteropoly acids have surface areas as low as

< 10 m2/g, which hinders accessibility to their strong acidic sites. Due to their relatively low

specific surface area, catalytic activity of HPAs is limited in gas solid phase reactions. High

solubility of heteropoly acids in polar solvents such as alcohols, water, and ethers cause them to

be leached during catalytic reactions involving polar molecules as reactants or products.

Furthermore, the thermal stability of pure heteropoly acids are very low and cannot be used in

vapor phase reactions.

In order to overcome these drawbacks and extend their catalytic applications for wide

range applications of these materials need to be modified.

1.1.9 Modification of heteropoly acids

Modification of heteropoly acids can be done in the following ways,

(i) By exchange of HPAs protons with different metal cations

(ii) By supporting HPAs on suitable solid supports


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(iii) By incorporation of various types of transition metal ions like V5+

, Nb5+

, Ta5+

, and Sb3+

etc. into the primary structure of Keggin ion.

1.1.9a Metal exchanged Keggin heteropoly acids

HPAs can be modified by exchanging the protons in their secondary structure

completely or partially with metal ions without affecting the primary structure. Keggin type

metal exchanged heteropoly acids, described by formula M1

xHy-xM2M

312O40, where M

1 = Cs

+,

Rb+ etc. M

2 = P or Si, M

3 = W or Mo, x = 2.5 and y = 3 or 4 if M

2 is P or Si respectively, are

produced by partially exchanging the protons of heteropoly acids. The HPAs modified by this

way can be used as catalysts [21,74,75]. Metal exchanged heteropoly acids can be classified

into two groups [76].

(1) Group A are the HPAs whose protons are exchanged with small metal ions like Na+, Cu

2+.

These group A HPAs posses

low surface area (1-15 m2/g),

high solubility in water,

absorption capability of polar or basic molecules.

(2) Group B are the HPAs with large cations like Cs+, Rb

+ and these exhibit

high surface area (50-200 m2/g),

insoluble in water,

unable to absorb molecules.

Exchange of protons with metal ions would enhance the thermal stability and surface

area of the catalyst. The catalytic behaviour of metal exchanged HPAs has received

considerable interest, because of its multifunctional catalysis [77]. Various metal ions such as

Cu2+

, Fe3+

, Ag+, Al

3+, Sn

4+ etc. have been used to exchange the protons of different Keggin

HPAs [78-82]. The Keggin structure of HPA remains stable even after exchanging of its

protons with different metal ions. The acidic properties of HPAs depend up on the number of

protons exchangeable in the secondary structure of the Keggin ion. It has been reported that

partially exchanged HPAs exhibit high acidity compared to fully exchanged or parent acid due

to high mobility of the protons [83].

Partial exchange of protons of heteropoly acids with large cations such as Cs+, Rb

+,

NH4+, K

+ etc. converts a water-soluble acid with low surface area (<5 m

2/g) into a water-

insoluble acid salt with surface area exceeding 100 m2/g [84,85]. Heteropoly salts are more

stable than the parent acid. However, the relative stabilities depends up on the counter cation


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[60]. The thermal stability varied generally in the order of metal ions Ba2+

, Co2+

< Cu2+

, Ni2+

<

H+, Cd

2+ < Ca

2+, Mn

2+ < Mg

2+ < La

3+, Ce

3+ < NH4

+ < K

+, Tl

+, Cs

+ [18]. The acidic cesium salt

Cs2.5H0.5PW12O40 is more stable than parent H3PW12O40. No decomposition of the salt was

observed even at 500 °C, while the parent acid decomposes at relatively lower temperatures

(300 °C). In the case of metal exchanged HPAs, polar molecules like water and alcohol can

easily enter and exit the bulk, which leads to expanding or contracting the distance between the

Keggin anions in the crystal lattice. While non polar molecules like hydrocarbons cannot enter

into the bulk. Due to this nature of the secondary structure, some reactions may proceed in the

bulk at the state called the pseudo liquid phase. Unlike polar molecules, non polar reactants are

incapable of being absorbed in the bulk of a heteropoly acid. They interact only with the surface

of the catalyst [18].

The formation of certain HPA salts of metal ions (Mn+

) of PW12O403-

(M3/nPW12O40)

exhibit Lewis acidity [82,86-89], originating from the metal cation as electron pair acceptor

apart from the characteristic Bronsted acidity of HPAs.

1.1.9b Supported Keggin heteropoly acids

One of the methods to modify the HPAs is to support them on high surface area acidic

or neutral supports such as silica, zirconia, niobia, titania, tin oxide, ceria, mesoporous materials

(MCM-41, SBA-15) etc. When HPA is supported on acidic support, a strong interaction

between heteropoly acid and oxide surface results in the formation of species such as [≡M-

OH2]n+[H3-nPW12O40]

n-3. Therefore, one can overcome the solubility problem. On the other

hand, support enhance the surface area of active HPA. The number of active sites on the surface

of solid material will also increase along with thermal stability and thereby its catalytic activity.

Basic supports are not suggestible because HPAs readily decompose on basic supports. As the

supported samples are heterogeneous, these can be easily separable from the reaction mixture in

liquid phase applications and stability increases during high temperature gas phase applications.

Keeping this in view, several studies were made on immobilization of HPAs into different

acidic supports and evaluated their physicochemical properties and catalytic applications

[21,74].

1.1.9c Incorporation of metal ion into Keggin heteropoly acids

Another method for the modification of Keggin HPAs is incorporation of transition

metal ions into primary structure of heteropoly anion. Redox properties of the HPAs can be

tuned by incorporating the metal ions such as V5+

, Nb5+

, and Ta5+

into the primary structure of


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the heteropoly anion [90,91]. For example, Keggin type heteropoly tungstate can be represented

as H3PW12O40, which consists of 12 addenda atoms (W) in the primary structure. The

modification of heteropoly tungstate can be done by substitution of other metal ions Mn+

for

one or more W atoms into the Keggin frame work of heteropoly anion. The redox potentials of

metal incorporated HPA catalysts depend up on the number of substituted metal atoms in the

primary structure of the heteropoly anion.

Many studies have been focused on substitution of vanadium in place of Mo or W into

the Keggin structure of heteropoly anion [21,63]. Substitution of V into the primary structure of

heteropoly anion resulted in the change in properties of Keggin HPAs from acid dominated

functionalities to redox nature of the catalyst [92]. However, substitution of more number of

vanadium atoms in primary structure leads to the destabilization of Keggin structure [93].

1.2 Modified Keggin heteropoly acid catalysts for benzylation of aromatics

Friedel-Crafts alkylation reaction is one of the important reactions in synthetic organic

chemistry and is usually carried out by the use of various conventional mineral acids such as

H2SO4, HF, H3PO4, HCl etc. The growing awareness of the unacceptability of these

conventional homogeneous catalysts and the resulting legislation give a major impetus to the

search for greener and cleaner technology. Such cleaner technology could be possible by

making use of solid acid catalysts. Heterogeneous catalysts have applications which include

simplified product isolation, easy recovery, reuse, and reduction in generation of waste as by-

products [94,95]. Among the existing solid acid catalysts, heteropoly acids have been proved to

be alternative to the traditional acid catalysts owing to their tunable acid-base, redox properties

and flexibility in structure. Presently, HPAs are used as catalysts in several industrial processes

[60]. The reactions catalyzed by HPAs in both homogeneous and heterogeneous systems have

been reviewed by many researchers [18,21,96,97]. Application of modified HPAs has

demonstrated high catalytic activity as an acid and oxidation catalyst for a variety of organic

transformations [21,98-100], due to their unique features such as high proton mobility, exhibits

strong Bronsted/Lewis acidity and high oxidation ability.

Satam and co-workers [101] reported liquid phase Friedel-Crafts benzylation of

aromatics with benzyl alcohol over polymer supported TPA catalysts under mild reaction

conditions. Mohan Reddy et al. [102] described the hydroarylation of vinyl arenes using AlTPA

catalyst under solvent free conditions. This catalytic system provides a wide range of substrate

applicability under solvent free conditions. Yadav et al. [103] synthesized mono alkylated


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biphenyl over cesium substituted dodecatungstophosphate acid supported on hexagonal

mesoporous silica. Kamalakar et al. [104] studied the structural features and catalytic activities

of heteropoly acid supported on mesoporous silica and revealed that TPA showed high activity

among TPA, STA, and MPA. High dispersion of HPA on silica with higher surface area

enhanced the benzylation activity. Influence of substituent on benzene ring on rate of

benzylation also studied; Electron donating groups increased the rate of reaction and retarded

by electron-withdrawing groups. Kumber et al. [105] reported TPA supported on titania for

benzylation of phenol with benzyl alcohol. Optimum conversion of benzyl alcohol obtained for

the catalyst with 20 wt% TPA on TiO2 and calcined at 700 °C.

1.3 Aims and objectives

The main aim of present study in the thesis is modification of Keggin type heteropoly

tungstate and understanding the acidic properties of these modified catalysts as environmentally

benign heterogeneous catalysts for the benzylation of anisole/arenes with benzyl

alcohol/dibenzyl ether. The objective is to modify HPAs to generate Lewis acidity along with

its Bronsted acidity. The increase in thermal stability and surface area is also one of the

objective. These modifications may be helpful in providing best alternative to traditional acid

catalysts. The main aims and objectives of the thesis are,

Modification of Keggin heteropoly tungstate to make use them as efficient

heterogeneous acid catalysts.

Generation of Lewis acidity by exchange of protons of tungstophosphoric acid with

different metal ions.

Preparation of supported metal exchanged heteropoly acid catalysts and evaluation of

their catalytic properties for Friedel-Crafts benzylation reaction.

To study the role of exchanged metal and support of HPA catalysts towards the activity

for benzylation of anisole with benzyl alcohol.

Understanding the influence of location of metal in the modified HPA catalysts and

thereby its catalytic properties.

Evaluation of surface and structural properties of modified tungstophosphoric acid

catalysts by using different spectroscopic and non spectroscopic techniques.

Establishment of plausible mechanism involving both Lewis and Bronsted acid sites in

benzylation reaction.

Optimization of reaction parameters using active catalysts for benzylation reaction.


16

1.4 Organization of the thesis

Chapter 1

This chapter deals with the general introduction about the heteropoly compounds and

their significance. A particular prominence has been given to Keggin heteropoly acids and its

modification. A brief introduction to modified Keggin heteropoly acids particularly on their

salts, niobium substituted and supported tungstophosphoric acid catalysts along with their

catalytic applications for benzylation of aromatics is discussed. The aim and objective of the

thesis is discussed.

Chapter 2

It describes the recent literature dealing with the method of preparation of modified

heteropoly acids. Various methods for the preparation of supported, metal exchanged and

incorporated heteropoly acid catalysts are discussed. The study related to physico-chemical

properties of the HPAs by various spectroscopic and non spectroscopic techniques like FT-IR,

Laser Raman, X-ray photoelectron spectroscopy, X-ray diffraction, pyridine adsorbed FT-IR

spectroscopy and temperature programmed desorption of ammonia are also reviewed.

Generation of Lewis acidic sites along with Bronsted acidity in modified HPAs such as

Zn1.2H0.6PW12O40, TPA/SnO2, TPA/MCM-41 etc. are reviewed. Various catalytic systems

reported in literature for liquid phase benzylation of aromatics is reviewed.

Chapter 3

This chapter illustrates the detailed experimental procedure for the preparation of

modified Keggin heteropoly acids. It involved the preparation of various metal exchanged

tungstophosphoric acid, supported TPA, supported metal exchanged TPA catalysts and niobium

incorporated 12-tungstophosphoric acid and supported niobium containing tungstophosphoric

acid catalytic systems. The basic concepts and the experimental procedures of catalyst

characterization techniques are described. The reaction procedure for liquid phase benzylation

of anisole with benzyl alcohol and dibenzyl ether reaction carried is also presented.

Chapter 4

This chapter deals about the modification of 12-tunstophosphoric acid (TPA) by

exchanging the protons of TPA with different metal ions such as Sn2+

, Sm3+

and Hf4+

. Chapter

4 divided into three sections.

Section 4.1 describes the heteropoly tungstate modification by exchanging of its protons with

Sn2+

ions. The catalysts characteristics after modification of TPA with Sn2+

derived by different


17

spectroscopic methods were discussed. The generation of Lewis acidic sites with exchange of

Sn2+

is discussed in detail. Catalytic activity was evaluated for benzylation of anisole with

benzyl alcohol. The effect of Sn content in TPA on catalytic activity is discussed. The catalytic

activity of the catalyst has been explored for benzylation of various aromatics with benzyl

alcohol and substituted benzyl alcohols.

Section 4.2 involves the modification of TPA by exchanging the protons of it with Sm3+

and

characterization of modified catalysts by different spectroscopic and non-spectroscopic

techniques. The catalytic activity was investigated for benzylation reaction. Variation of the

catalytic activity with Sm3+

content also discussed. The thermal stability of SmTPA was studied

by treating the catalyst at various temperatures. Comparison of the catalytic activity of SmTPA

with Cs salt of TPA is also discussed. Various reaction parameters such as effect of reaction

temperature, catalyst weight, and anisole to benzyl alcohol molar ratio are studied to optimize

the reaction conditions.

Section 4.3 deals with the acidic properties of the hafnium exchanged TPA catalysts for

benzylation of anisole with dibenzyl ether. Variation in catalytic activity with Hf content in the

secondary structure of TPA is also studied. Reaction parameters such as effect of temperature,

catalyst weight and anisole to dibenzyl ether ratio also studied. The correlation between

structural features derived from characterization results such as XRD, FT-IR, Raman, pyridine

adsorbed FT-IR, temperature programmed desorption of ammonia and XPS with that of activity

is discussed.

Chapter 5

This chapter describes about the modification of 12-tungstophosphoric acid by supporting on

various metal oxides such as SnO2, TiO2, ZrO2, Nb2O5 etc. This chapter is divided into four

sections.

Section 5.1 describe about the TPA supported on tin oxide catalysts for benzylation of anisole

reaction. The influence of TPA dispersed on the tin oxide towards benzylation reaction is

studied. Structural changes in the catalysts with calcination temperature, optimization of

reaction parameters and catalyst reusability is also studied.

Section 5.2 deals with studies on benzylation of anisole over Al exchanged phosphotungstic

acid supported on titania catalysts. The generation of Lewis acidity in the catalyst also

explained. Influence of AlTPA loading on titania, effect of support and calcination temperature

on the structural characteristics also elucidated.


18

Section 5.3 describes the Ga exchanged TPA supported on zirconia catalysts for benzylation

reaction. The influence of GaTPA loading on zirconia on the conversion of benzyl alcohol is

discussed. Comparison study has been made for TPA/ZrO2 and GaTPA/ZrO2 catalysts in order

to know the influence of Ga present in TPA.

Section 5.4 explores the structural and catalytic evaluation of Cs exchanged TPA supported on

acidic supports such as Nb2O5, ZrO2, SnO2 and TiO2. The effect of support in conversion of

benzyl alcohol is studied. The correlation between structural features derived from

characterization results such as XRD, FT-IR, Raman, pyridine adsorbed FT-IR, temperature

programmed desorption of ammonia and XPS studies with that of activity results is discussed in

detail.

Chapter 6

This chapter deals with the niobium containing TPA catalysts and its activity for

benzylation of anisole reaction. This chapter is divided into two sections.

Section 6.1 deals with the study of location of niobium ion in the primary and secondary

structure of the Keggin type heteropoly tungstate. Catalytic activity was tested for benzylation

of anisole with benzyl alcohol and dibenzyl ether also investigated in order to know the effect

of benzylating agent. Thermal stability of the catalysts is examined by in- situ Raman analysis.

The influence of niobium location in the primary and secondary structure of tungstophosphoric

acid (TPANb1, NbTPA) on catalytic activity and selectivity are discussed. The activity of these

Nb containing TPA catalysts is compared with that of TPA supported on Nb2O5.

Section 6.2 presents silica supported niobium containing catalysts for benzylation of anisole

with benzyl alcohol. Surface and structural changes of the catalyst with calcination temperature

and their influence on the conversion of benzyl alcohol is also studied. Reaction parameters

such as effect of catalyst weight, reaction temperature and anisole to benzyl alcohol molar ratio

are studied to optimize the reaction conditions.

Chapter 7

This chapter discloses the main conclusions drawn from the results obtained in the

present work.


19

1.5 Highlights of the thesis

Keggin type tungstophosphoric acid is modified to tune their acidic properties for the

benzylation reaction.

Tungstophosphoric acid modified by adopting different techniques such as exchanging

the protons of TPA with metal ions, incorporation of metal ions into the primary

structure of the Keggin ion and supporting on acidic supports.

Generation of Lewis acidic sites in 12-tunstophosphoric acid by exchanging the protons

of TPA with metal ions such as Sn2+

, Sm3+

, Hf4+

etc. as heterogeneous catalysts for

liquid phase benzylation of anisole reaction.

Enhancement of acidic properties of 12-tungstophosphoric acid by exchanging the

protons of TPA with metal ions.

Studies on influence of benzylating agent in benzylation of anisole reaction.

Modification of TPA by supported metal exchanged TPA catalysts (AlTPA/TiO2,

GaTPA/ZrO2, TPA-Cs/Support) for benzylation of anisole.

Evaluating the influence of the support on acidic properties of aluminium exchanged

tungstophosphoric acid catalysts.

Role of Lewis acidity in supported metal exchanged tungstophosphoric acid catalysts

for benzylation reaction.

Investigation of role of support for the benzylation of anisole reaction over supported

cesium exchanged tungstophosphoric acid catalysts.

Development of supported 12-tungstophosphoric acid by impregnating on SnO2

catalysts and study of surface, structural feature of the catalysts.

Study of incorporation of niobium ion into the primary and secondary structures of

Keggin ion and evaluation of the acid properties for benzylation of anisole with benzyl

alcohol and dibenzyl ether.

Evaluating the surface structural properties of the silica supported niobium containing

catalysts and their acidic functionalities.


20

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