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CHAPTER 1

INTRODUCTION

Catalysis is one of the most powerful branches of chemistry

successfully applied in chemical industry and has a major impact on the

quality of human life as well as economic development. Several “pushers”

and “pullers” for innovative catalysis have been identified with regard to the

challenges for catalysis research in the 21st century. Among the pushers

appear new approaches to discover potentially new catalytic material,

including innovative preparation methods. Catalysts are considered as

chemical components capable of directing and achieving thermodynamic

equilibrium. Catalyst is a substance that increases the rate at which a chemical

reaction reaches equilibrium without being consumed during the course of

reaction. Catalysis is the word used to describe the action of the catalyst. The

word catalysis was first coined by Berzelius in 1835 to describe a number of

experimental observations such as ammonia decomposition by metals and

decomposition by potassium chlorate by manganese dioxide. Berzelius

suggested that reactions could occur at the surfaces of the solids provided the

latter possessed a catalytic force. Most important advances have been

achieved in the last few decades in the area of catalysis.

The catalysts aid the attainment of chemical equilibrium by

reducing the potential energy barriers in the reaction path. Catalyst in some

form or other is involved in more than 90% of the processes in the petroleum,

petrochemical and fertilizers industries. Catalytic processes are widely

employed in the manufacture of bulk as well as fine chemicals. The

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developments during the last 20 years have been mandated mainly by

considerations related to the abatement and prevention of pollution,

conservation of raw materials and energy, use of alternate methodologies and

productions of more efficient drugs. Catalytic technologies play a key role in

the economic development and growth of the chemical industry due to the

increasing demand for new and green process for the production of many

industrially important chemicals. Catalysis has a wide range of applications in

chemical industry and has major impact on the quality of human life as well

as economic development. The catalytic reactions are generally classified as

homogeneous or heterogeneous, depending on the physical state of the

catalysts. Catalysts are essential, for the present technology for the production

and consumption of fuels, manufacture and processing of chemical feed

stocks and plastics. Today world wide catalysts sales exceed $5.9 billion per

year (Figure 1.1).

Figure 1.1 Chemical catalyst business

Polymerisation 39%

21%

18%

9%

8% 5% 1%

Dehydrogenation Aromatization Organic Synthesis

Oxidation

Hydrogenation

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1.1 HOMOGENEOUS CATALYSIS

Homogeneous catalysis occur when the catalyst and the reactants

are in the same phase. Wacker process is a typical example for homogeneous

catalysis. The industrially important reactions are primarily hydroformylation,

carbonylation and addition of hydrogen cyanide. The manufacture of

sulphuric acid by lead chamber process from SO2 was catalysed by the

metastable intermediate nitrosylsulphuric acid, HNOSO4 in which NO2 acts as

catalyst. Though homogeneous liquid-phase catalytic process and

heterogeneous vapour phase process produce the same product, the factors to

be considered are the relative degree of selectivity in the two processes and

the ease of control to avoid run away reactions or explosions. A review of the

basics of the homogeneous catalysis from the industrial point of view was

given by Parshall (1980). The disadvantages of homogeneous catalysts are,

they may be poisoned and hence its separation from the product in the

reusable form is difficult. Since permanent deactivation is much more

susceptible in homogeneous catalysts, it would be better choice for chemical

industries to opt for heterogeneous catalysts, as they are much less prone to

poisoning.

1.2 HETEROGENEOUS CATALYSIS

Heterogeneous catalysis occurs when the catalyst and the reactants

are in different phase. Clays, zeolites, crystalline microporous

aluminosilicates, mesoporous materials and zeotype catalysts such as

aluminophosphates have been playing a vital role as catalysts in this area. It is

based on the adsorption concept, in which the reactants are activated by

certain active sites on the surface of solid catalyst particles followed by the

interaction of adsorbed species to produce a product and vacating the site for

continuing the catalytic cycle. Heterogeneous catalysis is crucial to chemical

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technology. It has been the basis for most of the commercial processes in

petroleum and the petrochemical industry. In addition to the innovations in

reducing the environment impact associated with production, heterogeneous

catalysts also play a vital role in the development of new technologies in

cleaning the polluting emissions (Armor 1992).

The application of heterogeneous catalysts, especially in fine

chemical production, is expected to grow in future, as they offer several

intrinsic advantages over homogenous catalysis. There are many factors

affecting heterogeneous catalysis and the effectiveness of a particular catalyst

for a given reaction is likely to be the result of combination of factors

including surface area, porosity, acidity-basicity as well as the crystalline or

amorphous nature of the material. Some of the factors include reduced

dimensionality, increased local concentration on the surface, increased surface

area, safe handling, eco friendliness and availability.

1.3 CHOICE OF THE SUPPORT

(i) Naturally occurring materials, diatomaceous silica, activated

carbon, pumice and a variety of clays e.g naturally occuring

kaolin, sepiolite which are often mentioned in the literature

as supports for dispersing metals.

(ii) Standard inorganic supports, include silica, alumina,

magnesia, thoria, zirconia and molecular sieves.

Novel forms of materials of growing interest, are mesoporous

materials, carbon molecular sieves, fullerens, polymers. Recently, anionic

clays (hydrotalcite or layered double hydroxide) are used for dispersing

metals. In the present study, the use of anionic clays, as catalyst support for

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dispersing metals and their use in hydrogenation reaction is described in

detail.

1.4 CLAYS

Clays are among the most common minerals on the earth’s surface

and they are indispensable to our existence. Clays are versatile materials and

hundreds of millions of tons currently find applications in many different

areas. In addition to ceramics, including building materials, clays also are

used as paper coatings, fillings, drilling muds, pharmaceuticals, cat litters, etc.

Furthermore, clays can be used as adsorbents, catalysts or catalyst supports,

ion exchangers, decolorizing agents, etc., depending on their specific

properties. For example, the high surface area and surface polarity of some

clays determine high adsorption and water-retention, capacities. Clays also

play an important role in agriculture, considering that many soils contain large

amounts of clay materials, which determine key soil properties. (structure,

texture, water retention, fertility, etc.) (Schoonheydt et al 1991, Fowden et al

1984). Clays are usually layered compounds and based on the structure and

charges of layers and interlayers they can be classified broadly into two

categories namely,

(i) Smectic type cationic clays have negatively charged

alumino-silicate layers, with small cations in the interlayer

space to balance the charge.

(ii) Hydrotalcite (HT) type anionic clays also known as layered

double hydroxides have positively charged brucite-type

metal hydroxide layers with balancing anions and water

molecules located interstitially.

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1.4.1 Cationic clays

Cationic clays exist in vast quantities in nature and are mainly

prepared starting from the minerals (Grim 1968, Newman 1987) and are

commonly used for many chemical transformations. One example for cationic

clay is commercially modified montmorillonites, K10 or KSF, widely used as

acid catalyst or adsorbent in hydrocarbon cracking. Analogously, “Clayfen”

and “Claycop” are the acronyms for iron (III) or copper (II) nitrate supported

on K10 clay. Cationic clays are mainly obtained from natural materials, that

invariably contain impurities such as quartz, calcite, feldspars, etc., although,

they may also be synthesized, such as for example laponite1 (analog of

hectorite, by Laporte Industries) (Vaccari 1998).

1.4.2 Anionic clays

Anionic clays also known as hydrotalcites are natural or synthetic

lamellar mixed hydroxides with interlayer spaces containing exchangeable

anions (Miyata 1977, Cavani 1991) and different names are used depending

on the composition and polytype form (Drits et al 1987). Anionic clays may

be defined by their chemical composition, basal spacing and stacking

sequence (Vaccari 1999). Anionic clays are usually synthesized by

precipitation methods and are widely used as industrial adsorbents, ion-

exchangers and scavengers in industry and laboratory producing regio and

stereo selective process. Anionic clays based on hydrotalcite-like compounds

have produced sensational success in many organic syntheses in recent years

and represent a puzzle catalyst in many more reactions.

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1.5 FACTORS INFLUENCING THE SYNTEHSIS

Structural variables

Cation size

Value of x [M3+/(M3+ + Mz+)

ratio]

Cation stereochemistry

Cation mixture (nature and

ratio)

Nature of balancing anions

Amount of interlayer water

Crystal morphology and size

Preparation variables

pH

precipitation method

precipitation temperature

Reagent concentration

Aging

Washing and drying

Presence of impurities

1.6 HYDROTALCITE LIKE ANIONIC CLAYS

1.6.1 Historical background

Half way through the nineteenth century, hydrotalcite (HT), a

mineral that can be easily crushed into a white powder similar to talc was

discovered in Snarum, Sweden during 1842. It is a hydroxyl carbonate of

magnesium and aluminium, and occurs in nature as foliated and contorted

plates of masses of the earth’s surface. The exact formula,

Mg6Al2(OH)16CO3·4H2O, was presented by Manasse (1915) Professor of

Minerology at the University of Florence (Italy). The first patent appeared in

1970 referring to a hydrotalcite like structure as an optimal precursor for the

preparation of hydrogenation catalysts (Brocker and Marosi 1970), which led

to the catalytic research into hydrotalcite like compounds in later years. For a

long time, the structure was assumed to exist of consecutive layers of brucite,

Mg(OH)2, and Al(OH)3, as proposed by Feitknecht (1942), to which he gave

the name “doppelschichtstrukturen” (Double sheet hydroxides). Feitknetcht’s

idea, was that the compounds synthesised were constituted by a layer of

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hydroxide of one cation, intercalated with a layer of the second one. This

hypothesis was refuted by Allmann (1970) by means of X-ray analysis of

monocrystals. In fact they concluded that the two cations are localized in the

same layer and only the carbonate ions and water are located in an interlayer

(Cavani et al 1991). The HT structure closely resembles that of brucite, in

which the magnesium cations are octahedrally coordinated by hydroxyl ions,

giving rise to edge-shared layers of octahedra (Figure 1.1a). In HT, part of the

Mg2+ ions is replaced by Al3+ ions resulting in positively charged cation

layers. The compensating negative charge is provided by anions, situated in

the interlayer, which is the space between the brucite-like layers.

1.6.2 Structural features of anionic clays

Hydrotalcite (HT) like compounds can be represented by the

general formula: [Mz+1-x M3+

x(OH)2]b+[An-b/n].mH2O where M=metal;

A=interlayer anion, b=x or 2x-1 for z=2 or 1 and x is given by the ratio M3+/

(M3++ Mz+) respectively. HT like compounds have structures similar to that of

brucite Mg(OH2). The most detailed investigation of hydrotalcites was carried

out by Allmann (1970). Brucite crystallizes in a layer-type lattice as a

consequence of the presence of relatively small positively charged divalent

cations in a close proximity to the non-spherico symmetrical and highly

polarizable OH-anions. Each Mg2+ ion is octahedrally surrounded by six OH-

ions and the successive octahedral share edges to form infinite sheets. These

sheets are stacked on top of each other and are held together by weak

interactions through hydrogen (Trifiro et al 1996, Oswald et al 1977). By

isomorphous replacement of some of octahedrally coordinated Mg2+ cation in

the brucite layer with a cation having higher charge more or less similar in

radius such as Fe3+, Cr3+ and Al3+ (Shannon 1976), and the excess positive

charge produced is compensated by carbonate anions located in the disordered

interlayer domains containing water molecules. The OH sheets may exhibit

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two stacking sequences (Allmann 1970, Frondel 1941), rhombohedral (3R), a

three layer poly type sequence, which is the normal form obtained at high

temperatures. HT structure can accommodate a wide range of variables giving

rise to the possibility of producing tailor made materials synthetically. The

main feature of hydrotalcite like structure are determined by the nature of the

brucite like sheet (Figure 1.2b) by the position of anions and water in the

interlayer spacing and by the type of stacking of the brucite like sheet. The

sheet containing cations are built as in brucite, where the cations randomly

occupy the octahedral holes in the close packed configuration of the OH ions.

The anion and water molecules are randomly located in the interlayer region

via hydrogen bonding and being free to move by breaking the bonds and

forming new ones. The oxygen atoms of the water molecules and of the

carbonate groups are distributed approximately closely around the symmetry

axes that pass through the hydroxyl groups (0.56ºA apart) of the adjacent

brucite like sheet (Cavani et al 1991). These hydroxyls are tied to the

carbonate groups directly or via intermediate H2O through hydrogen bridges

(Allmann 1970). The CO32- groups are situated in the interlayer while H2 is

loosely bound and they can be eliminated without destroying the structure. In

fact layered double hydroxide made with similar divalent and trivalent metal

ions in the hydroxyl carbonate forms represent the family of HTs and HT like

compounds.

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Figure 1.2a Structure of Hydrotalcite (Anionic clays)

Figure 1.2b Schematic representation of brucite layer

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A broad spectrum of divalent and trivalent cations in different

atomic compositions constitute layered double hydroxide with HT like

network. Interlayer anions can be varied which allows the tailoring of desired

catalytic properties in these materials.

1.6.3 Interlayer anions

A variety of interlayer anions can be introduced between the brucite

like sheets having different M2+ and M3+ cations. Generally carbonate is the

preferred interlayer anion and it is held more tenaciously than any other anion

between the adjacent metal hydroxide sheets. The stability of the anions

approximately are in the order of: CO32->>SO4

2-->>OH->F>Cl->Br->NO3->I-

(Miyata 1983). Chloride and nitrate ions are significantly easier to replace

than carbonate ion. The ability of hydrotalcites to scavenge the anionic

species is now being exploited in environmental chemistry and in the new

clean technologies.

1.6.4 Properties of calcined hydrotalcites

The anionic clays based on hydrotalcite like compounds have been

used as such or mainly after calcination. The catalysts obtained by calcination

of Mg-Al hydrotalcites are potential substitutes for the most common bases

such as hydroxides or carbonates of alkaline or alkaline earth metals, salts of

ammonium or amines, among others used in industry (Bastiani et al 2004).

The most interesting properties of the oxides obtained may be summarized as

follows: (Cavani et al 1991, Vaccari 1998).

(i) High Surface area: Hydrotalcite upon calcination upto 723 K

form high surface area (120-300 m2g-1) mixed oxides with

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numerous fine pores and considerable pore volume

(0.1-0.6 cm3g-1).

(ii) Homogenous inter-dispersion : The mixed oxides formed

show homogenous interdispersion of the elements which are

also thermally stable under reducing conditions, with the

formation of very small and stable metal crystallites (with

high metal dispersion).

(iii) Basic Properties: The synergistic effects between the

elements due to the intimate interdispersion, which mainly

favours the development of unusual basic or hydrogenating

properties. The basic properties depend significantly on

composition and calcination temperature. On account of

high basic nature, HTs (mixed oxides from HT) are found to

catalyze various base catalyzed reactions like hydrogenation,

condensation, isomerization etc.

(iv) Memory effect: It is the capacity of the sample obtained by

thermal decomposition of hydrotalcite like precursors,

containing a volatile anion such as CO32- to reconstitute the

original layered structure upon the adsorption of various

anions or on exposure to air.

1.6.5 Generation of basic sites

In general hydrotalcite and HT like compounds are basic

compounds. However, calcination of M-M type mixed oxides produces very

high basicity. The basic sites thus created act as catalytically active centers for

several reactions. The activity of layered double hydroxides (LDHs) or parent

HT like compounds could be attributed to the surface water adherence,

inclusion of other gases and substrate access to the basic centers being

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inhibited. The final basicity of these catalysts and the electronic state of the

metallic phase depend on parameters such as M/M ratio, the nature of

compensating anions and thermal treatments.

1.6.6 Memory effect of hydrotalcite

The term memory effect has been called for the capacity of the

samples obtained by thermal decomposition of HT type precursors. These

samples contain a volatile anion such as carbonate for the reconstruction of

the original hydrotalcite structure upon the adsorption of various anions or

simply upon exposure to the atmospheric air or moisture. The effect is

strongly dependent on calcination temperature (Figure 1.3). Generally, the

first stage of heating, results in the loss of the physisorbed water and heating

from 550 to 723 K leads to the simultaneous loss of hydroxyl and carbonate

groups as water and carbon dioxide respectively.

Figure 1.3 Schematic representation of Memory Effect

Therefore a lamellar microstructure was retained after thermal

decomposition and reconstitution of the HT precursors was thus permitted.

Meixnerite, a natural compound is easily obtained by reconstruction of

Mg(Al)O by memory effect (Tichit and Coq 2003).

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1.6.7 Exchange properties of hydrotalcites

Anionic clays have good anion exchange capacities, although the

true exchange capacity (1.0 ± 1.5 meq/g) (Reichle 1986) is usually much less

than the theoretical one (3.3 meq/g for hydrotalcite). They show a resistance

towards temperature higher than that of anion exchange resins and thus are

used in some high temperature applications, such as treatment of the cooling

water of nuclear reactors. The selectivity in the exchange capacity increases

with increasing anion charge density (Miyata 1983) i.e., anionic clays

strongly prefer multiple charged anions and compounds containing nitrates or

chlorides are the best precursors for exchange reactions. However, very

important is the pH of the solution, which may favor or prevent the exchange

(Bish 1980), and has to be compatible with both the range of stability of the

starting anionic clay and the anion. There is practically no limitation to the

nature of the anions, which can compensate for the positive charge of the

brucite like sheet in the HT like network. One method of anion exchange has

been described by (Bish 1980) using the reaction of dilute mineral acids with

the carbonate form of LDH, the expulsion of carbon dioxide results in anion

exchange according to the equation,

LDH.CO3 + 2HCl LDHCl2 + CO2 + H2O (1.1)

The number of exchangeable anions, which in the structure depends

upon the charge density are carried on the host layer. The readily incorporated

CO32- anion in the preparative stages is held tenaciously within the layer and

is difficult to exchange. The presence of atmospheric CO2 during the

synthesis is therefore highly undesirable for preparing non-carbonate LDHs

(Jones and Chibwe 1986). Water molecules are localized in the interlayer, in

those sites which are not occupied by anions. It is possible to calculate the

maximum amount of water present in the interlayer on the basis of number of

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sites present in the interlayer by assuming a close-packed configuration of

oxygen atoms and substracting the sites occupied by the anions (Miyata

1975).

A wide variety of anions can be incorporated into the network,

which indicates its promising position as exchangers and polyoxometalate

pillared LDHs which are more desirable since they are suitable for developing

materials with unique two-dimensional galleries and zeolite porosites. Such

porous solids can be used as catalysts and photo catalysts.

1.6.8 Structure and surface properties of hydrotalcites

The structure and surface properties of Mg-Al hydrotalcites and of

the resulting mixed oxides depend strongly on the chemical composition and

synthesis procedures (Cavani et al 1991). In catalysis, surface area, intimate

contact between two or more oxide components, and the size of the resulting

metal oxide clusters strongly influence the rate and selectivity of chemical

reactions (Di Cosimo et al 1998). The activity of calcined MgAl hydrotalcites

with low Al content (Mg:Al atomic ratio between 5 and 15) in the double

bond isomerization of 1-pentene shows that the activity depends strongly on

chemical composition and calcination temperature (Shaper et al 1989). The

alkylation of phenol with methanol on Mg-Al calcined hydrotalcites with ratio

3:10 has been studied and concluded that the participation of both acidic and

basic sites is required for alkylation reactions and that surface acid-base

properties are determined by the Al content (Velu and Swamy 1994). The

influence of chemical composition of Mg-Al calcined hydrotalcites on the

dehydrogenation of isopropanol was widely investigated by Corma et al

(1994) and concluded that on calcined MgAl hydrotalcites, the catalytic

activity and selectivity depend on chemical composition.

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1.6.9 Swelling properties of hydrotalcites

Swelling of layered double hydroxides is possible and has been

observed a dependence of basal spacing for hydrotalcite on levels of humidity

and concluded that 7.9A spacing is characteristic of fully dehydrated phase.

Observations by Bish (1980) indicated that the LDH-sulphate and LDH-

chloride could be solvated with glycols, glycerol etc. Swelling of LDHs

depends on several factors similar to those of cationic clays:

a) Dependence on the nature of the exchangeable anion

(i.e., charge, mass, structure, etc.)

b) Nature of the solvent (polarity, molecular dimensions etc.)

c) The charge of the layer

The available data on the swelling properties of layered double

hydroxides is limited when compared to cationic clays.

1.6.10 Applications of hydrotalcites (HTs)

Figure 1.4 shows the commercial and industrial applications of

HTs. HTs have been used for various diverse applications from medicine to

catalysis. Anionic clays are, most importantly, the precursors for the synthesis

of multicomponent catalysts. Because of high thermal stability and

remarkable ion-exchange properties of HTs they have been widely recognized

as ion-exchangers. Hydrotalcites are used as catalysts and catalyst supports:

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(i) HT as base catalysts

Thermally activated hydrotalcites have been successfully applied as

heterogeneous base catalysts for various organic reactions such as,

depolymerisation of paraldehyde (Walvekar and Halgeri 1973), methylation

of phenol and methanol (Velu and Swamy 1994), selective methylation of

aniline (Santhanalakshmi and Thirumalaiswamy 1996). Hydrotalcites are used

in aldol condensation reactions such as, self-aldol condensation of acetone

(Reichle 1984,1985), cross aldol condensation of formaldehyde and acetone

to give methyl vinyl ketone (Suzuki and Ono1988), condensation of

benzaldehyde with ethyl acetoacetate (Corma et al 1992), Claisen-Schmidt

condensation of substituted acetophenones and benzaldehyde for the

production of chalcones and flavanones (Climent et al 1995). Hydrotalcites

are used as base catalysts in Meerwin-Pondorf verley reduction of carbonyl

compounds in liquid phase (Kumbhar 1998a), cyanoethylation of alcohols

(Kumbhar 1998b), liquid phase hydroxylation of phenol with H2O2 over

Zr-containing hydrotalcite (Velu et al 1997), decomposition of nitrous oxide

(Kannan and Swamy 1994), steam reforming of hydrocarbons, methanation of

CO, methanol synthesis, higher alcohol synthesis and Fischer-Tropsch

reaction (Cavani 1991).

(ii) HT as catalyst supports/precursors

Hydrotalcites are found to be suitable supports or precursors for

dispersing noble metals and have been studied for the following reactions,

Aromatization of n-hexane to benzene over Pt/Mg(Al)O (Davis and Derouane

1991), methanol decomposition to synthesis gas over supported Pd catalysts

on Mg-Cr hydrotalcites (Shiozaki et al 1999), partial oxidation of methane

over highly dispersed Rh or Ni supported catalysts from Mg-Al HT as the

precursor (Basile et al 1996), Hydrogenation of acetonitrile over Ni/Mg-Al

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HT (Cabello et al 1997), Synthesis of methyl isobutyl ketone from

hydrogenation of acetone over Mg-AlHT supported Pd or Ni catalysts (Chen

et al 1998), supports of Zeigler – Natta catalysts for ethylene polymerization

or DeSOx additives to FCC catalysts.

Figure 1.4 Applications of anionic clays

1.7 SUPPORTED METAL CATALYSTS

The supported metal catalysts differ from massive metal catalysts,

in that they consist of metal particles, which are to some extent separated

from one another. There are a number of advantages in depositing

catalytically active metals on a support such as alumina, charcoal or silica.

The metal can be highly dispersed throughout the pore system of the support.

As a result a large active metal area is produced relative to the weight of the

metal used, which is especially advantageous in case of precious metals. The

Anionic Clays Anionic Clays

Catalyst - hydrogenation - polymerization - steam reforming

Catalyst Support - Zeiglar –Natta - CeO

- For Pt and Pd

Encapsultated fullerens conductors

Protonic

Adsorbents

- halogen scavenger - PVC stabiliser - waste water treatment

Medicinal - antacid - antipepsin - stabiliser

Industrial - molecular sieves - flame retardent - ion-exchanger - filler

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support can also improve dissipation of reaction heat and retard sintering of

metal crystallites, which results in the loss of active surface and increase

poison resistance, thus promoting a longer catalyst life. These advantages

make supported metal catalysts preferable over bulk metal catalysts for

chemical processing. The support may also have a catalytic role to perform.

For example, in petroleum reforming on Pt/Al2O3, Pt serves as

dehydrogenation site and Al2O3 as isomerisation site. The support may have

strong influence on metal dispersion and active metal surface area available

for the reaction. Hydrogenation activity and selectivity of supported metal

catalyst can be influenced by the choice of support, metal and also the

conditions under which the reaction is carried out. From the economic sense,

it is often more important to achieve highly dispersed catalyst with small

metal content and cheaper metal precursor especially when noble metals such

as Pt, Pd, Ru and Rh are used. Studies on new supported metal catalysts, such

as Pd/hydrotalcite are significant from the point of view of understanding

catalysis and for the development of new polyfunctional catalytic systems and

catalytic process. The influence of catalyst preparation conditions on the

activity, selectivity and the life of a catalyst has led to the development of

new support for metals. The properties of these catalysts depend strongly on

the state and dispersion of the metal component, the nature of the supported

metal and the nature of the support.

1.8 CHOICE OF THE METAL

Metals are classified based on their capacity to chemisorb different

gases. The characteristic feature of transition metals is that they have one or

more unpaired d electrons in the outermost electron shell. The weakly

chemisorbing non transition metals have only s or p valence electrons. It has

been suggested that unpaired electrons are necessary to bring the absorbing

molecule to the surface strongly. This is an empirical way of correlating the

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percentage d-character of the metallic bond with the adsorption or absorption

data (Anderson 1975). There are many reports available in the literature on

the use of metals for various hydrogenation reactions. Among the transition

and noble metals, Platinum and palladium are used extensively for the

hydrogenation reactions. Palladium is most frequently used metal in the

organic synthesis in laboratory scale as well as industrial scale. Many organic

reductions in the laboratory are carried out with carbon supported palladium

catalysts. The reduction of phenol to cyclohexanone is performed with

alumina supported palladium and platinum catalysts (Neri et al 1994) and

hydrotalcite supported palladium catalysts (Narayanan and Krishna 1998).

1.8.1 Palladium

Palladium, first of platinum group metals other than platinum found

in the strange ore platina discovered by Wallson in 1803, has got many unique

and useful properties. These include its selective transformation of hydrogen

and its ability to form workable alloys with more elements than any other

metal. Its catalytic properties are outstanding in many types of reactions

including hydrogenation, isomerization, disproportionation, dehydrogenation

and oxidation reactions (Wise 1968). Palladium is one of the most important

hydrogenation catalysts, and widely employed in industrial processes (Dalla

Betta 1993, Morikawa et al 1991, Moore and Kell 1992) and in basic research

(Stacchiola et al 2001, Shen et al 2001, Surnev et al 2000). Palladium

(Figure 1.5) is very active under ambient conditions and in some reactions

palladium is highly selective in giving a product virtually free of by-products.

Also Pd is unaffected by many chemical reagents. All these properties have

made palladium very effective in many catalytic applications.

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Figure 1.5 Palladium crystal structure: interactively and

non-interactively

Pd has proved to be an excellent catalytic hydrogenation reagent for

most of the functional groups (acids, anhydrides, amides, esters, anilines,

aldehydes, acetylenes, nitriles and aromatic systems etc.,) which can be

reduced under mild conditions. It can be supported on many substances such

as clays, carbon, alumina silica etc. Catalytic hydrogenolysis has got much

importance in synthetic and degradative chemistry. In such type of reductions,

use of palladium as catalyst is frequently met with outstanding success. The

structure of PdCl2 is shown below: (Figure 1.6).

Figure 1.6 Structure of Palladium chloride

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1.8.2 Hydrogen effects in metals with reference to palladium

The interaction between hydrogen and palladium metal has been

attributed due to subsurface bonding of hydrogen (Lagos 1982). According to

his calculations on energy requirements, hydrogen would prefer to bind to

subsurface sites which can facilitate hydrogen diffusion into the bulk of the

metal. According to Van Hove and Hermann (1988) sites between the top of

the first layer and second layer of Pd are the surface sites and the interaction

between these sites with hydrogen is chemisorption in nature. Sites below the

second layer of Pd interstitial site, is a real subsurface site and the interaction

between these interstitial and hydrogen is absorption in nature. The sites much

below these subsurface are responsible for the hydrogen bulk diffusion. The

value of desorption energy for bulk diffusion is ~4.4 kcal/mol, which is

always smaller than the energy of desorption from a surface chemisorbed

state (20 K cal/mol). Hydrogen adsorption on the Pd surface or absorption in

its subsurface is responsible for the formation of PdHx species. The

decomposition of PdHx species during temperature rise occurs ~100C.

The migration of chemisorbed hydrogen atoms from a metal to its

support is called spill over. This is reviewed by Sermon and Bond (1973). The

mobility/diffusion of active hydrogen as atoms or ions on the surface of the

heterogeneous catalysts is an important phenomenon for the activity and

selectivity of these catalysts and for the kinetics and mechanism of the

reactions on them. Studies of tritium radioisotope retention by alumina

supported metal catalysts revealed the ratio of hydrogen atoms to surface

metal atoms in case of Pd is 5.7 (Taylor et al 1968). The ratio of hydrogen

atoms to the total amount of metal atoms is 0.7 and they concluded that the

retention must be attributed to the spillover from the above observation.

Noble metals are generally affected by hydrogen treatment, even at room

temperature which in turn changes the morphology of the metal like

23

crystallinity. It is reported that the hydrogen can be retained in a metallic

catalyst system after reduction and cooling to a reaction temperature and this

hydrogen may affect the catalyst in following ways (Paal and Menon 1983).

(i) Too strongly chemisorbed hydrogen may simply block the

active sites, which are needed for the reactant molecule. For

example in hydrogenolysis reactions, hydrogen is one of the

reactant, its strong adsorption can lead to self-inhibition of

the reaction.

(ii) Hydrogen present in the surface layers of a metal can change

the electronic properties and hence the catalytic performance

of the metal.

(iii) When a metal supported on a carrier like oxide/carbon etc,

the spillover of hydrogen to the support impart unusual

properties to the support.

In following are some of the general considerations on hydrogen

effects in metals and are listed below: (Paal and Menon 1983)

(i) There is certainly more than one type of hydrogen

interacting with surface layers of metals.

(ii) It is possible to conclude the position and form of various

types of hydrogen.

(iii) The energy necessary to remove the hydrogen in most cases

is measured.

When one tries to understand the exact role of hydrogen under

experimental conditions, most of the time, there exists lot of conflicts.

24

Sometimes hydrogen may migrate to the support or may interact with support

or it may reduce the support partially.

1.9 METAL-SUPPORT INTERACTION

The interaction of metal with the support influences the properties

and behaviour of supported metal catalyst in a significant way. The extent of

interaction is a function of the various physico-chemical properties if the

support as well as the dispersed phase in addition to their method of

preparation, activation and the metal content. Factors enhancing the catalyst

support interaction are given below.

(i) High temperature fabrication or use

(ii) Small particle size of all components

(iii) Intimate association of finely divided components

(iv) Compression of components so as to decrease the distance

between intimately associated finely divided particles.

(v) Low melting point of one or more ingredients

(vi) Prolonged exposure to coalescing conditions

(vii) Presence of flux or mineralizers in catalytic components or

in environment where catalyst is used. Minerlaizers are

alkali, alkali sulfates, phosphates, borates or hydrothermal

conditions.

At severe operating conditions, the interaction between the metal

and support becomes pronounced and it can affect the performance of the

catalyst significantly. This is due to the solid state reaction between the metal

and support resulting in the formation of some non-reducible compounds.

25

This was attributed to a strong metal-support interaction (SMSI) (Tauster et al

1978).

1.10 HYDROTALCITE SUPPORTED METAL CATALYSTS

Hydrotalcite supported palladium catalysts are generally employed

for several organic reactions such as hydrogenation, condensation etc. The

structural features of hydrotalcites on metal dispersion, CO adsorption and

phenol hydrogenation activity were studied (Narayanan and Krishna 1998).

Studies on hydrogenation of acetone using metal containing layered double

hydroxide were made in detail by Unnikrishnan and Narayanan (1999).

Calcined hydrotalcite supported palladium catalyst is the most promising

catalyst for selective hydrogenation of phenol to cyclohexanone in a one step

process (Chen et al 1999). Structural influence of hydrotalcite on palladium

dispersion and hydrogenation of phenol was discussed (Narayanan and

Krishna 1997). A comprehensive study on hydrotalcite-based catalysts for the

single-stage liquid-phase synthesis of MIBK from acetone and H2 under mild

conditions was made (Winter et al 2006). Hydrogenation of phenol was

studied over uncalcined and calcined hydrotalcite supported palladium

catalysts and their catalytic activity is compared with that of the conventional

supports such as MgO and -alumina (Narayanan and Krishna 1996).

Sonogoshira coupling reaction was studied using palladium and copper

supported on mixed oxides derived from hydrotalcites (Corma et al 2006).

Palladium and Platinum hydrotalcite-derived Mg–Al mixed oxide catalysts

were used for condensation of acetone and its selective hydrogenation to

MIBK (Nikolopoulos et al 2005). Vapour phase hydrogenation of naphthalene

was investigated in detail (Albertazzi et al 2004) using Pd/Pt catalysts

supported on basic Mg-Al mixed oxide obtained by calcination of hydrotalcite

precursor. Calcined Mg/Al hydrotalcite-supported palladium or nickel

26

catalysts was tested for hydrogenation of acetone to methyl isobutyl ketone

(Chen et al 1998).

Several reports are available on the hydrogenation reactions using

hydrotalcite supported palladium catalyst (Chen et al 1999, Winter et al 2006,

Narayanan and Krishna 1997). Hydrogenation activity and selectivity of

supported metal catalysts can be influenced by the choice of support, metal

and other conditions under which the reaction is carried out. From the

economic sense, it is often more important to achieve highly dispersed

catalyst with small metal content and cheaper metal precursor especially when

using noble metal such as Pt, Pd, Ru and Rh. Studies on hydrotalcite

supported palladium catalyst are significant and are gaining importance from

the point of view of understanding catalysis and for the development of new

polyfunctional catalytic systems and catalytic processes.

1.11 HYDROGENATION REACTION

The French chemist Paul Sabatier is the father of the hydrogenation

process. In 1897 he discovered that the introduction of a trace of nickel as a

catalyst facilitated the addition of hydrogen to molecules of gaseous carbon

compounds. Wilhelm Normann was awarded a patent in Germany in 1902

and in Britain in 1903 for the hydrogenation of liquid oils using hydrogen gas.

Hydrogenation is one of the common organic reactions used in the synthesis

of chemicals. Catalytic hydrogenation is used in the hardening of fats and

production of alcohols from aldehydes, and ketones. Hydrogenation of

nitrobenzene to aniline is an industrially important reaction. Aniline is an

important raw material used for the production of methylene diphenyl

diisocyante (MDI). It is also used as an additive for rubber process,

intermediate dyes and pigments, pesticides and herbicides. About 85% of

global aniline is produced by catalytic hydrogenation of nitrobenzene. Carbon

27

nanotube supported platinum catalyst has been used for the hydrogenation of

nitrobenzene (Li et al 2005). Platinum nanoparticle core-polyaryl ether

trisacetic acid ammonium chloride dendimer shell nanocomposites were

employed for hydrogenation of nitrobenzene to aniline with molecular

hydrogen under mild conditions (Yang et al 2006). Active carbons were used

as supports for palladium in the liquid phase hydrogenation of nitrobenzene to

aniline (Bouchenafa-Saib et al 2005). Hydrogenation of nitrobenzene was

studied over Pt/C catalysts in supercritical carbondioxide and ethanol (Zhao

et al 2004). Polymer anchored metal complex catalyst has been used for the

hydrogenation of nitrobenzene (Patel and Ram 1998). Liquid phase

hydrogenation of nitrobenzene was studied over Pd-B/SiO2 amorphous

catalyst (Yu et al 2000). Three activated carbon supports for palladium are

employed to study the hydrogenation of nitrobenzene in liquid phase

condition (Gelder et al 2002). In the present work vapour phase

hydrogenation of nitrobenzene is studied over hydrotalcite supported

palladium catalysts and a comparative study is also done with commercial

supports such as MgO and -Al2O3.

1.12 SELECTIVE HYDROGENATION REACTION

Selective hydrogenation of , unsaturated ketones, aldehydes and

esters in general is used in perfumery, hardening of fats, drugs and in

synthesis of organic chemical intermediates. The direct catalytic

hydrogenation of unsaturated aldehydes and ketones is however a challenging

proposition. The reduction of C=C bond is thermodynamically more favoured

than that of the C=O bond and hence the reaction becomes difficult in

compounds having both the bonds present in it. Selective hydrogenation of

compounds such as furfural and acetophenone, is difficult since these

molecules has got both C=C and C=O bond present in it. Therefore it is

desirable to find catalysts which will control the intramolecular selectivity by

28

hydrogenation preferentially the C=O group while keeping the olefenic bond

intact (Claus 1998). Chemoselective hydrogenation catalysts are mostly based

on supported metals involving Pt, Ru, Rh and Pd. Hydrogenation of furfural

to furfuryl alcohol is an important reaction in the chemical industries. Furfuryl

alcohol is an important fine chemical in the polymer chemistry. It is widely

used in the production of dark thermostatic resins which have strong resistant

ability against acids, bases and other solvents. It is also used for the

manufacture of liquid resins for galvanic bath tube. Ce promoted Ni-B

amorphous alloy catalyst (Ni-Ce-B) has been employed for the liquid phase

hydrogenation of furfural (Li et al 2004).

Liquid phase selective hydrogenation of furfural to furfuryl alcohol

was studied on Raney nickel catalyst modified by impregnation of salts of

heteropolyacids (Baijun et al 1998). Cu-Ca/SiO2 catalysts were prepared by

impregnation and sol-gel method and their catalytic performance was tested

for the vapor phase hydrogenation of furfural (Wu et al 2005). Furfural

hydrogenation was studied over copper dispersed on three forms of carbon,

such as activated carbon, diamond and graphitized fibres (Rao et al 1999).

Vapor phase hydrogenation of furfural to furfuryl alcohol at atmospheric

pressure over Cu-MgO catalyst prepared by co-precipitation method was

studied (Nagaraja et al 2003). The role of both SMSI and SOOI effects for the

selective hydrogenation of furfural to furfuryl alcohol was demonstrated using

platinum deposited on monolayer supports (Kijenski et al 2002). Polymer

stabilized NiB catalysts were used to test the catalytic activity of furfural

hydrogenation (Liaw et al 2005). Platinum catalysts deposited on monolayer

support containing titania have been tested for the hydrogenation of ,

unsaturated aldehyde (Kijenski and Winiarek 2000). In the present work

vapour phase hydrogenation of furfural is tried over palladium catalysts on

different supports such as hydrotalcite, MgCr, MgAl mixed oxide and MgO.

29

The catalytic activity is compared and well correlated with their characteristic

features.

Selective hydrogenation of , unsaturated ketone is a challenging

task. The catalytic hydrogenation of organic compounds containing a

carbonyl group is important in the synthesis of fine chemicals,

pharmaceuticals, dyes and agrochemicals. An important class of this reaction

is the hydrogenation of acetophenone to 1-phenyl ethanol using supported

metal catalysts. 1-phenyl ethanol finds its application in the manufacture of

perfumery products and pharmaceuticals. The control of hydrogenation

selectivity of a complex molecule containing both C=C and C=O bonds have

been studied rather intensively by several workers. However, the competitive

hydrogenation between phenyl and carbonyl group in one molecule has not

been significantly done. Chen et al (2003) have recently studied the selective

hydrogenation of acetophenone on Pt/SiO2 catalyst under gas-phase

conditions. The reduced Pt/SiO2 containing 4.7 wt.% of Pt prepared by

impregnation gave around 4% conversion and 40-80% selectivity towards

1-phenyl ethanol and 11-39% for cyclohexyl ethanol at room temperature.

Oxidized Pt catalyst provides a lower catalytic activity than the reduced Pt,

but selectivity shifts to 1-phenyl ethanol and ethylbenzene. The hydrogenation

of aromatic ring is almost suppressed by oxygen. The oxidized Pt induces

activation of C=O and hence favouring C=O hydrogenation than C=C

hydrogenation. The adsorption geometry of acetophenone on Pt surface is

assumed to be an important factor in controlling molecular decomposition and

hydrogenation selectivity. Kinetic and reaction mechanism for acetophenone

hydrogenation was proposed by Rajashekharam et al (1999) using Ni

supported on zeolite Y catalyst in the temperature range 353-392 K.

Hydrogenation of acetophenone and its derivatives using monometallic

Ni-supported and bimetallic Ni-Pt supported on zeolite Y catalysts were

studied and the structure activity and structure stability behaviour of various

30

catalysts have been studied (Malyala et al 2000). Liquid phase acetophenone

hydrogenation on Ru/Cr/B catalysts supported on silica was studied

(Casagrande et al 2002). The influence of the nature of the solvent was

studied for the selective hydrogenation of acetophenone on chromium

promoted Raney nickel catalysts and concluded that using 2-proponol-water

and sodium hydroxide as solvent, 1-phenyl ethanol can be obtained with

higher selectivity (Masson et al 1997). The use of oxidized Pd/SiO2 as the

catalyst to rapidly convert acetophenone to ethylbenzene through dehydration

process. The adsorbed oxygen on Pd was expected to have a significant

influence on the reaction selectivity and reaction pathway (Chen and Chen

et al 2004).

1.13 DEHYDROGENATION REACTION

Catalytic dehydrogenation of cyclohexanol to cyclohexanone is an

industrially important reaction for the manufacture of nylon because the two

major raw materials in producing polyamide fiber are caprolactum and adipic

acid, both of which can be obtained from cyclohexanone. Cyclohexanone

plays an important role in chemical and petrochemical industries. It may be

also used as a solvent and as a building block in the synthesis of many organic

compounds used as pharmaceuticals, insecticides etc., The catalytic

dehydrogenation of cyclohexanol has gained much importance in recent

years. Sivaraj et al (1988) reported highly selective dehydrogenation of

cyclohexanol to cyclohexanone over Cu-ZnO-Al2O3 catalysts prepared by a

deposition – precipitation method. The dehydrogenation of cyclohexanol was

studied to test the catalytic activity of these catalysts at 523 K. The activity

and stability of copper oxide and zinc oxide catalysts for the oxidative

dehydrogenation of cyclohexanol to cyclohexanone was studied (Lin et al

1988). From the industrial point of view the production of cyclohexanone is

quite limited, since, the reaction is highly endothermic and the conversion is

31

limited by thermodynamic equilibrium, the selectivity and stability are

drastically affected by increasing the reaction temperature. Therefore different

works are carried out related to catalyst preparations and modifications of

conventional catalysts. Lin et al (1988) developed a Cu-ZnO catalyst

promoted by Na, K, Rb, Cs, Ba and Zr, to enhance cyclohexanone formation

and found that the promoters increases resistance to the presence of water in

the feed and decrease the reaction temperature. According to Jeon and Chung

(1994) copper well dispersed on alkali-doped, silica-support catalysts with

specific preparation conditions are resistant to sintering. Cu/SiO2 catalysts

prepared by electrodless plating procedure indicates that the preparation

method influences significantly the activity and selectivity. A number of

kinetic models are considered for dehydrogenation of cyclohexanol on Cu+

(Fridman et al 2004). They are listed as follows,

(i) Dissociative cyclohexanol adsorption as a first step of a

reaction where cyclohexanol interacts with Cu+ and

hydrogen adsorption occurs on a X adsorption site other than

Cu+.

(ii) Dissociative cyclohexanol adsorption as a first step and

hydrogen recombination in the gas phase.

(iii) Molecular adsorption of cyclohexanol with subsequent

abstraction of the hydroxylic hydrogen as a second step,

with hydrogen adsorption on an X site other than Cu+.

In contrast to the monovalent copper, a number of kinetic models

for dehydrogenation of cyclohexanol on zero valent copper are given. The

most important model considered is as follows:

32

(i) Dissociative adsorption of cyclohexanol on metallic copper

followed by the formation of cyclohexanol alcoholate and

dissociatively adsorbed hydrogen.

(ii) Molecular adsorption of cyclohexanol with subsequent

abstraction of the hydroxylic hydrogen followed by the

formation of cyclohexanol alcoholate and dissociatively

adsorbed hydrogen.

There are several reports available for the dehydrogenation of

cyclohexanol using copper catalysts in both vapor phase and liquid phase

conditions. In literature it is very rare to find the use of palladium based

catalysts for the dehydrogenation of cyclohexanol. Hence, in the present work

vapor phase dehydrogenation of cyclohexanol to cyclohexanone is being tried

using hydrotalcite supported palladium catalyst for the first time.

1.14 COMBINATION REACTION

Catalytic hydrogenation and dehydrogenation reactions are

generally employed in the chemical industries, and such reactions involving

oxygenated compounds are particularly important in the preparation of

pharmaceuticals and fine chemicals. Researchers have challenged today to

develop processes that have minimal detrimental effects on the environment,

and applications of environmental catalysis, are increasing in such diverse

areas as alternative fuels and global pollution clean up and control (Ertl et al

1999). Some of the examples of simultaneous reactions are given below:

(i) 1,4-butanediol dehydrogenation and maleic anhydride

hydrogenation.

(ii) 1,4-butanediol dehydrogenation and furfural hydrogenation.

(iii) Cyclohexanol dehydrogenation and furfural hydrogenation.

33

Coupled catalytic reaction through hydrogen transfer between two

reactants, for example 1,4-butanediol (BDO) and maleic anhydride to produce

one valuable product -butyrolactone is an economic way by energy

utilization and the environmental friendly process. The reaction scheme is

given below in Figure 1.7.

Figure 1.7 Combination reaction of furfural and 1,4-butanediol

Catalytic hydrogenation transfer reactions have widely been

investigated in the past years mostly for the reduction of organic compounds

by using hydrogen donor, which often leads to undesirable products

(Zassinovich et al 1992). In the coupled hydrogen transfer reaction, maleic

anhydride (hydrogen acceptor) is reduced by the hydrogen from 1,4-

butanediol (hydrogen donor) to form the same desired product

-butyrolactone (Figure 1.8). -butyrolactone is currently being manufactured

by the vapour phase hydrogenation of maleic anhydride using reduced copper

chromite catalyst containing some physical and chemical promoters

(Castinglioni et al 1995). The new catalytic process combines reactions of

Maleic anhydride and 1,4-butanediol into one catalytic system to significantly

improve the yield of the catalytic hydrogenation of maleic anhydride, apart

from the better thermal balance and the effective usage of hydrogen through

hydrogen transfer between the two reactants. In addition this new catalytic

process produces one desired product (-butyrolactone) with a substantially

34

increased hydrogenation yield, and the no-hydrogen, operation also simplifies

the technical problem (Zhu et al 2005).

Figure 1.8 Combination reaction of maleic anhydride and

1,4-butanediol

The reaction represents that producing 1 mol of gamma

butyrolactone requires 1 mol of maleic anhydride and 3 mol of hydrogen,

releasing 211 KJ of heat (Messori and Vaccari 1994). Due to the strong

exothermic reaction, the temperature control over this process is very difficult

and this leads to apparent hotspots, in a tubular fixed bed reactor, which very

often causes thermal runaway and lowers the selectivity to desired product,

gamma butyrolactone. Cu based catalytsts are generally employed for the

catalytic dehydrogenation of 1,4-butanediol and maleic anhydride (Tatsumi

et al 1993, 1994). Thus the gamma butyrolactone production by coupling

process eliminates the separate hydrogen preparation procedures leading to

environmental-friendly clean process. From the reaction enthalpies of

simultaneous furfural hydrogenation and 1,4-butanediol dehydrogenation, it is

clear that the coupling process that combine furfural and 1,4-butanediol into

one catalytic process, achieve a better heat balance. The released heat of

reaction can be used for furfural hydrogenation, which drastically decrease the

exothermic heat of the process, leading to a much easier temperature control.

35

In addition to this, the hydrogen released by the dehydrogenation of

1,4-butanediol can be utilized by the hydrogenation of furfural, leading to

more perfect hydrogen mass balance. The detailed study of the coupling

process is given by Yang et al (2004). Recently US, patent by Rao et al

(2006) describes the advantage of combining the hydrogenation and

dehydrogenation reaction over Cu-MgO catalysts to overcome the equilibrium

limitations in the dehydrogenation of cyclohexanol and utilization of the

hydrogen produced in this process to carry out hydrogenation of furfural there

by avoiding the external pumping of hydrogen (Figure 1.9). The exothermic

furfural hydrogenation and endothermic equilibrium constrained cyclohexanol

dehydrogenation, are industrially important reactions. Thus furfural alcohol is

produced from furfural hydrogenation and cyclohexanone is obtained from

cyclohexanol dehydrogenation. The combination reaction in single step over

bifunctional catalysts is an important technology, as an energy efficient

process giving greater yield and hence making it cost effective.

Figure 1.9 Combination reaction of furfural and cyclohexanol

The main aim of the present work is to conduct both the

hydrogenation and dehydrogenation reactions on a single catalyst

simultaneously. In vapour phase reaction conditions, cyclohexanol

dehydrogenation and nitrobenzene hydrogenation takes place simultaneously

to yield cyclohexanone and aniline by using hydrotalcite supported palladium

catalyst. The advantage of this process is that no hydrogen is required for the

hydrogenation reaction because the hydrogen generated in the

36

dehydrogenation reaction is sufficient to promote the hydrogenation reaction

and there is a possibility to shift equilibrium towards the product side.

1.15 ECONOMY OF HYDROGEN

The hydrogen economy describes a system in which our energy

needs are predominantly met by hydrogen, rather than fossil fuels. The

economy would rely on renewable resources in the form of hydrogen gas

water, drastically changing pollution, electrical sources, infrastructure,

engines and international trade, without compromising our quality of life. In

zero emission vehicles like cars and airplanes hydrogen fuel cells are used for

power, rather than petroleum distillates. By considering hydrogen economy,

we are referencing our increasing demand for clean burning fuels that do not

cause air and water pollution nor make us dependent on dwindling energy

sources. It is important to see the ideal of the hydrogen economy as

simultaneously addressing several problems with the current state of

petroleum reliance. It is motivated by a combination of economic and

environmental-friendly process.

1.16 SCOPE OF THE WORK

Hydrotalcite upon calcination leads to the formation of mixed

oxides and it has been the subject of numerous studies especially with respect

to their characterisation. Most part of the heterogeneous catalysis on

hydrotalcites are dealt with catalyst preparation and characterisation. However

catalysis, over these materials are still at its infancy. Most of the studies are

devoted to the electronic effect of the HT support on the metal. The

hydrotalcite supported metals have not been generally well characterised and

the hydrogenation reactions over them have not been correlated with

adsorption properties such as CO or Hydrogen uptake, metal surface area,

37

dispersion and crystallite size. The support plays an important role in the

metal support interaction making the metal available for hydrogenation. With

respect to hydrotalcite supported metal catalysts, such studies are rather

limited. Therefore the preparation of hydrotalcite materials and their use as

support for palladium as well as the characterisation of such catalysts and

their catalytic evaluation for industrially important reaction such as

hydrogenation and dehydrogenation of aromatic compounds are studied.

The present study is motivated by the need to gather information on

the properties of metal supported on HTs, and correlating the structural

properties of HT with respect to metal dispersion and catalytic activity. The

aim of the present investigation is to exploit hydrotalcite materials as supports

for Pd metal. Literature survey has indicated that the hydrotalcite-like

compounds have number of advantages over the conventional base catalysts.

These catalysts are eco-friendly and possess highly desired property such as

memory effect, renumerable surfaces longevity, high selectivity and high

turnover for reactions. The vapour phase reaction can be easily carried out

over these type of catalysts in the temperature range 573-623 K. The present

work aims at the use of these Envirocats as catalysts for specific organic

reactions.

1.17 AIMS AND OBJECTIVES OF THE PRESENT STUDY

The objectives of the present study include

(i) Preparation of support MII-MIII HTs at high and low

supersaturation

(ii) Preparation of support MgCr and MgAl mixed oxide

38

(iii) Preparation of catalysts with different loadings of palladium

(0.5, 1, 2 and 5 wt.%) on hydrotalcite support by wet

impregnation method.

(iv) Preparation of hydrotalcite supported palladium catalyst by

polyol reduction method using ethylene glycol as solvent.

(v) Preparation of palladium catalysts on MgCr and MgAl

mixed oxide.

(vi) Preparation of palladium catalysts on commercial support

such as MgO and -Al2O3.

(vii) Characterisation of the supports by XRD, FT-IR,

TGA/DTA, BET surface area and SEM.

(viii) Characterisation of the palladium catalysts by XRD, FT-IR,

TGA/DTA, BET surface area, UV-Vis DRS, TPR, SEM,

TEM, and XPS.

(ix) CO chemisorption studies to find the dispersion, metal area

and particle size of palladium metal.

(x) Vapour phase hydrogenation of nitrobenzene over high and

low supersaturation hydrotalcite supported palladium

catalysts in the temperature range of 498-573 K.

(xi) Vapour phase hydrogenation of nitrobenzene over different

loadings of palladium on calcined hydrotalcites.

(xii) A comparative study over other conventional supports such

as MgO and -Al2O3 for the hydrogenation of nitrobenzene.

(xiii) Vapour phase dehydrogenation of cyclohexanol to

cyclohexanone in a fixed bed reactor using hydrotalcite

supported palladium catalyst.

39

(xiv) Simultaneous dehydrogenation and hydrogenation of

cyclohexanol and nitrobenzene over hydrotalcite supported

palladium catalysts and its correlation of catalytic activity

with the characterization results.

(xv) Selective hydrogenation of furfural out over palladium

catalysts-A comparative study.

(xvi) Selective hydrogenation of acetophenone to 1-phenyl

ethanol over calcined hydrotalcite supported palladium

catalysts.