Chapter-1

Introduction

An idea that is developed and put into action is more important than an idea that

exists only as an idea.

Buddha

1.1 Introduction to catalysis

The importance and economical significance of catalysis is enormous. More than 80 %

of the present industrial processes established since 1980 in the chemical, petrochemical

and biochemical industries, as well as in the production of polymers, use catalysts [1]. The

development of petroleum fuels led to a vast petrochemicals business which in turn fed a

growth in specialty and performance chemicals. Environmental protection measures such

as automobile exhaust control and purification of gases released from power stations and

industrial plant would be inconceivable without the use of catalysts. Reasons for

widespread use of catalysis is economically and environmentally compelling, as catalytic

process can be carried out under industrially feasible conditions of pressure and

temperature, thus leading to lower operating costs and yield higher products with fewer by-

products compared to non-catalytic processes [2]. In 1991 the catalyst world market

achieved a turnover of about 6 billion dollars, grew to (8 – 9) billion dollars in 1996, and

reached 13 billion dollars in 2008, and the global catalyst demand is forecasted to rise six

percent per year to 17.2 billion dollars in 2014 according to Freedonia Group.

Approximately 24-28 % of produced catalysts were sold to the chemical industry and 38 –

42 % to petrochemical companies including refineries. 28 -32 % of solid catalysts were

used in environmental protection, and 3-5 % in the production of pharmaceuticals [1]. As

we move forward in the new century, the opportunities are created due to the strict

environmental legislation for the use catalysts to meet the new regulatory standards in all

the chemical industries. The market pull is expected to be from growing interests in

“biomass; transformation of biomass as a promising source of raw materials”,

“sustainability; carbon dioxide storage/up grading”, “energy; catalytic water splitting”, and

“emission control; pollution control for vehicles and industrial plants, air/volatile organic

carbon’s/water purification” [3-6].

The term catalyst was conceived by J.J. Berzelius in 1836 with the phrase: “[the]

catalytic force is reflected in the capacity that some substances have, by their mere

presence and not by their own reactivity, to awaken activities that are slumbering in

molecules at a given temperature [4]”. At that time number of catalytic processes was

already known, but the explanation of catalysis was far from clear and of a quite

metaphysical nature. Catalysis obtained an extensive empirical basis after Ostwald (1895),

and he was the first to give the phenomena of catalysis a scientific basis. Ostwald defined

the term catalysis, which is found in the text book to this day: “catalyst is a substance

which, without appearing in the final products, changes the rate of chemical reaction.” His

fundamental work was recognized with the Nobel prize for chemistry in 1909. Several

other Nobel Prizes in chemistry is related to the pioneering work in the field of catalysis.

In 1912, Sabatier received the prize for his work devoted mainly to the hydrogenation of

ethylene and CO over Ni and Co catalysts. Three Nobel prizes in chemistry are closely

related to ammonia synthesis (Haber 1921, Bosch 1931, Gerhard Ertl 2007). John W.

Cornforth received the prize in 1975 for breakthrough work on “stereochemistry of enzyme

catalysis reactions”. Sidney Altman received the prize in 1989 for “Discovery of the

catalytic properties of ribonucleic acid”. The Nobel Prize in 2001 was shared by William S.

Knowles and Ryoji Noyori "for their work on chirally catalysed hydrogenation reactions"

and Barry Sharpless "for his work on chirally catalysed oxidation reactions" [1, 7].

Catalysts can be gases, liquids or solids. Most industrial catalysts are liquids or solids,

whereby the latter react only via their surface [2]. A catalyst accelerates a chemical

reaction. It does so by forming bonds with the reacting molecules, and by allowing these to

react to a product, which detaches from the catalyst, and leaves it unaltered such that it is

available for the next reaction. The role of a catalyst can very well be described by cyclic

catalysis process as shown in Fig. 1.1. For example, consider the catalytic reaction between

two molecules A and B to give a product P. The cycle starts with the bonding of molecules

A and B to the catalyst. A and B then react within this complex to give a product P, which

is also bound to the catalyst. In the final step, P separates from the catalyst, thus leaving the

reaction cycle in its original state.

Fig. 1.1 Catalytic cycle- indicating the sequence of elementary steps.

Further, Fig. 1.2, shows the energy diagram to compare the non-catalytic and the

catalytic reaction to show how catalyst accelerates the reaction. For the non-catalytic

reaction, the figure is simply the familiar way to visualize the Arrhenius equation: the

reaction proceeds when A and B collide with sufficient energy to overcome the activation

barrier. The change in Gibbs free energy between the reactants, A + B, and the product P is

ΔG. The catalytic reaction starts by bonding of the reactants A and B to the catalyst, in a

spontaneous reaction. Hence, the formation of this complex is exothermic, and the free

energy is lowered. The reaction between A and B then follows while they are bound to the

catalyst. This step is associated with activation energy; however, it is significantly lower

than that for the uncatalysed reaction. Finally, the product P separates from the catalyst in

an endothermic step [8].

Fig. 1.2 Potential energy diagram showing the energy barrier for a reaction with and

without catalyst.

1.2 Industrial importance of catalysis

Catalysts have been successfully used in the chemical industry for more than 100 years;

the first major breakthrough in industrial catalysis was the synthesis of ammonia from the

elements, discovered by Haber [9] in 1908, using osmium as catalyst. Laboratory recycles

reactors for the testing of various ammonia catalysts which could be operated at high

pressure and temperature were designed by Bosch [9]. The ammonia synthesis was

commercialized in 1913 by Badische Anilin-und Soda- Fabrik (BASF) as the Haber –

Bosch [9] process. Mittasch [1] at BASF developed and produced iron catalysts for

ammonia production. In 1938 Bergius [9] converted coal to liquid fuel by high-pressure

hydrogenation in the presence of a Fe catalyst. Other highlights of industrial catalysis were

the synthesis of methanol from CO and H2 over ZnO – Cr2O3 and the cracking of heavier

petroleum fractions to gasoline using acid-activated clays, as demonstrated by Houdry [9]

in 1928. The addition of isobutane to C3 – C4 olefins in the presence of AlCl3, leading to

branched C7 – C8 hydrocarbons, components of high quality aviation gasoline, was first

reported by Ipatieff et al. [9] in 1932. This invention led to a commercial process of

Universal Oil Product (UOP). The other eminent development that took place in Germany,

which possesses no natural petroleum resources, was the discovery by Fischer and Tropsch

[9] for the synthesis of hydrocarbons and oxygenated compounds from CO and H2 over an

alkalized iron catalyst. The first plants for the production of hydrocarbons suitable as motor

fuel started up in Germany 1938. After World War II, Fischer-Tropsch synthesis saw its

resurrection in South Africa. Since 1955 Sasol Co. has operated two plants with a capacity

close to 3x106 t/a [9]. Later developments include new highly selective multicomponent

oxide and metallic catalysts, zeolites, and the introduction of homogeneous transition metal

complexes in the chemical industry for all kinds of processes. During and after World War

II numerous catalytic reactions were realized on an industrial scale. Table 1.1 summarizes

examples of catalytic processes representing the current status of the chemical,

petrochemical and biochemical industry as well as the environmental protection.

Year of

Commercialization

Process Catalyst Product

1970-1980

Vapor phase alkylation

(General Electric)

MgO 2,6 Xylenol from alkylation

of phenol with methanol

Carbonylation (Monsanto

process)

Organometallic Rh

complex

Acetic acid from methanol

MTG (Mobil process) Zeolite (ZSM-5) Gasoline from methanol

1981 – 1985

Alkylation (Mobil –

Badger)

Modified zeolite

(ZSM-5)

Ethyl benzene from

ethylene

Selective catalytic

reduction (SCR; stationary

sources)

V Ti (Mo, W) oxides

(monoliths)

Reduction of NOx with NH3

to N2

Esterification (methyl-tert-

butyl ether synthesis)

Mitsui

Cation-exchange

resin

Methyl-tert-butyl ether from

iso-butene with methanol

Oxidation (Sumitomo

Chem., two-step process)

Mo, Bi oxides.

Mo, V, PO

(heteropolyacids) Acrylic acid from propene

Oxidation (Monsanto) Vanadylphosphate

Maleic anhydride from n-

butane

Fluid-bed polymerization

(Unipol) Ziegler – Natta type

Polyethylene and

polypropylene

Hydrocarbon synthesis

(Shell)

Co – (Zr,Ti) – SiO2

Pt – SiO2

Middle distillate from CO

with H2

1986 – 2000

Oxidation with H2O2

(Enichem) Ti silicalite

Hydroquinone and catechol

from phenol

Hydration Enzymes

Acrylamide from

acrylonitrile

Dehydration of 2-

propanolamine (Koei

Chem) ZrO2 Allylamine

Dehydrogenation of C3, C4

alkanes

(Star and Oleflex

processes)

Pt(Sn) – zinc

aluminate,

Pt – Al2O3 C3, C4 olefins

2000 –2010

Catalytic destruction of

N2O fromnitric acid tail

gases (EnviNOxprocess,

Uhde) Fe zeolite Removal of nitrous oxide

HPPO (BASF-Dow,

Degussa-Uhde) Ti silicalite Propylene from propene

Propylene oxide TS-1

Propylene oxide from H2O2

and propylene

Table 1.1 Important catalytic processes commercialized after 1970 [3, 10-14]

1.3 Catalyst Performance

The economics of chemical industry is complex. The price of a catalyst is often a small

fraction of the overall production cost. In crude oil refining processes the catalysts costs

amount to only about 0.1 % of the product value and for petrochemicals this value is about

0.22 % [2]. As a result, the main task of catalyst technology is to look for more efficient

and stable catalysts rather than inexpensive catalysts. For commercial catalysts, it is equally

important to consider the properties such as mechanical strength and thermal stability.

Hence, the successful application of any catalyst on an industrial scale is realized after

intensive research and development studies at laboratory and pilot plant scales. During

these studies, a catalysis scientist looks mainly for catalyst with high activity. A high

activity allows relatively small reactor volumes, short reaction times, and operation under

mild conditions. High selectivity is often more important than high activity. Furthermore, a

catalyst should maintain its activity and selectivity over a period of time, i.e. it should have

sufficient stability [2, 15].

1.3.1 Activity

In industrial practice, activity of a catalyst is defined in terms of productivity, i.e. the

quantity of the product obtained using unit mass of the catalyst in the unit time. One way of

expressing catalytic activity is to multiply the specific reaction rate by the specific surface

of the catalyst. But for most surface reactions the rate expression and the specific rates are

unknown. Hence, percent conversion under a given set of experimental conditions is taken

as a measure of catalytic activity. The high activity leads to fast reaction rates, short

reaction times, and maximum throughput.

1.3.2 Turnover number

Another measure of catalyst activity is the turnover number. The rate of a catalytic

reaction is generally expressed as the number of molecules reacted (or formed) per unit

weight or per unit surface area of the catalyst per second. Since the entire surface does not

take part in the reaction and the reaction occurs only at the active centers, rate should be

more accurately expressed as the number of molecules formed (or reacted) per active site

per second. This is known as turnover number. But it is not easy to estimate the number of

active sites per unit mass of the catalyst. In case of metal catalysts, it is assumed that all the

metal atoms present on the surface are active and turnover number becomes the number of

molecules reacting per surface atom per second.

1.3.3 Selectivity

The selectivity of a reaction is the fraction of the starting material that is converted to

the desired product P. It is expressed by the ratio of the amount of desired product to the

reacted quantity of a reaction partner A, and therefore, gives information about the course

of the reaction. In addition to the desired reaction, parallel and sequential reactions can also

occur, leading to less selectivity for a particular product. Selectivity facilitates maximum

yield, elimination of side products and lowering of purification costs. Thus, it is the most

important target parameter in catalyst development.

1.3.4 Stability

The chemical, thermal and mechanical stability of a catalyst determines its lifetime in

industrial reactors. Catalyst stability is influenced by numerous factors, including

decomposition, coking and poisoning. Catalyst deactivation can be followed by measuring

activity or selectivity as a function of time. Catalysts that lose activity during a process can

often be regenerated before they ultimately have to be replaced. The total catalyst life time

is of crucial importance for the economics of a process.

For a good understanding of catalysis it is crucial to have a good idea of the structure

(both chemical and physical) of a catalyst. The properties of a catalyst can be manipulated

by many process such as active phase (metal, metal oxide; type, morphology), support

(type, texture, chirality), environment of the reaction (solvent, temperature, pressure),

promoters (inorganic, organic, chiral), inhibitors that alters the properties of its surface,

because the nature of the individual sites at the surface is responsible for the activity,

selectivity and stability of the catalyst [2, 15].

1.4 Promoters and poisons in catalysis

1.4.1 Promoters

It is well known that small quantities of certain substances when added to a catalyst

increase its catalytic activity enormously. These substances are called promoters. Promoters

themselves may or may not have catalytic activity. In most cases, there exists an optimum

catalyst to promoter ratio that gives maximum activity. There are four types of promoters:

Structure promoters increase the selectivity by influencing the catalyst surface such

that the number of possible reactions for the adsorbed molecules decreases and a

favored reaction path dominates. They are of major importance since they are

directly involved in the solid-state reaction of the catalytically active metal surface.

Electronic promoters become dispersed in the active phase and influence its

electronic character and therefore the chemical binding of the adsorbate.

Textural promoters inhibit the growth of catalyst particles to form larger, less active

structures during the reaction. Thus they prevent loss of active surface by sintering

and increase the thermal stability of the catalyst.

Catalyst-poison-resistant promoters protect the active phase against poisoning by

impurities, either present in the starting materials or formed in side reactions.

1.4.2 Poisons

Catalyst poisons form strong adsorptive bonds with the catalyst surface, blocking active

centers. Therefore, even very small quantities of catalyst poisons can influence the

adsorption of reactants on the catalyst. The term catalyst poison is usually applied to

foreign materials in the reaction system. Reaction products that diffuse only slowly away

from the catalyst surface and thus disturb the course of the reaction are referred to as

inhibitors [16].

1.5 Types of catalysis

Catalysts can be divided into three major types as heterogeneous catalysts,

homogeneous and biocatalysts. Approximately 80 % of all catalytic processes require

heterogeneous catalysts, 15 % homogeneous catalysts and 5 % biocatalysts [17]. If the

catalyst and reactants or their solution form a common physical phase, then the reactions

called homogeneously catalyzed. Metal salts of organic acids, organometallic complexes

and carbonyls of Co, Fe, and Rh are typical homogeneous catalysts. Examples of

homogeneously catalyzed reactions are oxidation of methanol to acetic acid catalysed by

carbonyls of Fe, Co and especially Rh in the presence of halides and hydroformylation of

olefins to give the corresponding aldehydes [18].

Heterogeneous catalysis involves systems in which catalyst and reactants form separate

physical phases. Typical heterogeneous catalysts are inorganic solids such as metals,

oxides, sulfides and metal salts, but they may also be organic materials such as organic

hydroperoxides, ion exchangers and enzymes. Examples of heterogeneously catalyzed

reactions are vapor phase alkylation of phenol with methanol over magnesium oxide

catalysts and hydrogenation of edible oils on Ni catalysts in the liquid phase, which are

examples of vapor and liquid phase catalysis, respectively [1].

In biocatalysis, enzymes or microorganisms catalyze various biochemical reactions. The

metalloenzymes are organic molecules that almost always have a metal as the active center.

Often the only difference to the industrial homogeneous catalysts is that the metal center is

ligated by one or more proteins, resulting in a relatively high molecular mass. The catalyst

can be immobilized on various carriers such as porous glass, SiO2 and organic polymers.

Enzymes are the driving force for biological reactions. They exhibit remarkable activities

and selectivities. Prominent examples of biochemical reactions practiced in industries

include isomerization of glucose to fructose, important in the production of soft drinks, by

using enzymes such as glucoamylase immobilized on SiO2 and the conversion of

acrylonitrile to acrylamide by cells of coryne bacteria entrapped in a polyacrylamide gel.

The enzyme catalase decomposes hydrogen peroxide 109 times faster than inorganic

catalysts. Biocatalysts have some advantages and disadvantages with respect to other kinds

of catalysts. The major advantage of enzymes, apart from being highly selective and active

is that they function under mild conditions, generally at room temperature in aqueous

solution at pH values near 7. Their disadvantage is that they are sensitive, unstable

molecules which are destroyed by extreme reaction conditions. Enzymes are often

expensive and difficult to obtain in pure form. With the increasing importance of

biotechnological processes, enzyme catalysis field is expected to grow exponentially [2,

19]. In this research, as our objective is to develop and investigate solid catalysts for

industrially important organic transformation, we have focused our discussions on

homogeneous and heterogeneous catalysis in general and heterogeneous catalysis in

particular.

1.5.1 Comparison of homogeneous and heterogeneous catalysis

In homogeneous catalysis, catalyst, starting materials and products are present in the

same phase. Thus, homogeneous catalysts have a higher degree of dispersion than

heterogeneous catalysts since in theory each individual atom can be catalytically active. In

heterogeneous catalysts, phase boundaries are always present between the catalyst and the

reactants and hence only the surface atoms are active [16]. Due to their high degree of

dispersion, homogeneous catalysts exhibit a higher activity per unit mass of metal than

heterogeneous catalysts. The reactants can approach the catalytically active center from any

direction and a reaction at an active center does not block the neighboring centers. This

allows the use of lower catalyst concentrations and milder reaction conditions. The most

prominent feature of homogeneous transition metal catalysts are the high selectivity’s that

can be achieved. Homogeneously catalyzed reactions are controlled mainly by kinetics and

less by material transport, because diffusion of the reactants to the catalyst can occur more

readily. Due to the well-defined reaction site, the mechanism of homogeneous catalysis is

relatively well understood. In contrast, processes occurring in heterogeneous catalysis are

often obscure. Owing to the thermal stability of organometallic complexes in the liquid

phase, industrially realizable homogeneous catalysis is limited to temperatures below 200

ºC. In Table 1.2, some of the characteristic features of homogeneous and heterogeneous

catalysis are listed [2, 16]. The major disadvantages of the homogeneous catalysts with

respect to various parameters are difficult separation of the catalyst from the product, more

complicated processes such as distillation, liquid–liquid extraction, and ion-exchange must

often be used. These problems of separation limit the application on the large-scale.

Homogeneous catalytic processes, therefore, may not be very advantageous either from the

economic or the environmental point of view [2, 15]. Hence, the next level of

sophistication is to design catalytic processes, which lend themselves to facile recovery and

recycling of the catalyst. One way to achieve this is to use solid acid-base catalysis as it is

both economically and ecologically beneficial from the industrial point of view. The solid

acid and base catalysts have many advantages over liquid Brönsted and Lewis-acid and

base catalysts. They are noncorrosive and environmentally benign, presenting fewer

disposal problems. Their repeated use is possible and their separation from liquid/gaseous

products is much easier. Further, technological application can be expanded by designing

fluidized and fixed bed reactors to give higher activity, selectivity and longer catalyst life.

A few drawbacks of heterogeneous catalysis include non-uniform distribution of active

sites and non- uniform strength of the active sites. Hence, some of the active sites cannot be

reached by the reactants and hindered diffusion is often encountered in the heterogeneous

systems. In conclusion, it can be stated that homogeneous and heterogeneous catalysts have

their special characteristics and properties. However, the replacement of soluble Brönsted

and Lewis acids and bases by heterogeneous catalysts, in a wide variety of organic

reactions, continues to attract much attention. One reason for this is that the individual steps

and mechanisms of heterogeneously catalyzed reactions are complex and difficult to

establish. Another is the increasing necessity to produce chemicals in an economic and

environmentally friendly manner [2- 4, 14, 15].

Catalyst properties Homogeneous Heterogeneous

Active centers

Concentration

Selectivity

Diffusion problems

Reaction conditions

Applicability

All metal atoms

Low

High

Practically absent

Mild (50–200 οC)

Limited irreversible reaction

with products

(cluster formation);

poisoning

Only surface atoms

High

Low

Present (mass-transfer-

controlled reaction)

Severe (often >250 οC)

Wide sintering of the metal

crystallites;

poisoning

Structure/stoichiometry

Modification possibilities

Thermal stability

Catalyst separation

Catalyst recycling

Cost of catalyst losses

Defined

High

Low

Sometimes laborious

(Chemical decomposition,

distillation, extraction)

Possible

High

Undefined

Low

High

Fixed-bed: unnecessary

suspension: filtration

Unnecessary (fixed-bed)or

easy(suspension)

Low

Table 1.2 Comparison of homogeneous and heterogeneous catalysts.

1.5.2 The importance of adsorption in heterogeneous catalysis

In heterogeneous catalysis, adsorption of reaction species plays a key role on the

performance of the catalyst and the catalytic reaction mechanism. All surfaces contain

unsaturated bonds and this bond causes the reactant molecules to get attached to the

catalyst surface. The degree of interaction obviously depends on the nature of adsorbate

and the adsorbent. Depending on the nature of interaction, adsorption is classified as either

physical or chemical (called as physisorption and chemisorption respectively) adsorption.

Knowledge of the type of adsorption is useful, since only chemisorbed species act as

intermediate in catalytic reactions [20]. Physisorption is caused by the forces of molecular

interaction, which include dipole and dispersive forces and thus, physisorption is a result of

the same forces that cause condensation and solidification of fluid phases. On the contrary,

chemisorptions involve interaction of electrons of the adsorbate and adsorbent resulting in

the formation of a chemical bond. Often, the differentiation is based on one criterion is not

enough and the use of combination of criteria described in the table can be useful in

deciding the nature of adsorption. Table 1.3 gives a comparison between physical and

chemical adsorption. The information indicates that if the heat of adsorption is very large or

if the adsorption has higher activation energy than the latent heat of evaporation, then the

adsorptions are clearly chemisorptions. Unfortunately, often the heat of adsorption is about

40-50 kJ/mole, it is very difficult to determine whether the adsorption is physical or

chemical. Other criteria, which are helpful in distinguishing between these two types of

adsorption, are electrical conductivity (which changes appreciably upon adsorption) and IR

spectroscopy for identification of surface sites using probe molecules [21].

Parameters Physisorption Chemisorption

Cause Van der Waals forces,

no electron transfer

Covalent/electrostatic forces, electron

transfer

Adsorbents All solids Some solids

Adsorbates All gases below critical

point, intact molecules

Some chemically reactive gases,

dissociation into atoms, ions, radicals

Temperature range

over which

adsorption occurs

Close to condensation

temperature of the

adsorbate

Occurs at a wide range of

temperatures and at

temperatures much above the

condensation temperature.

Heat of adsorption Low, ≈heat of fusion

(ca.10 kJ/mol),always

exothermic

High, ≈ heat of reaction

(80-200 kJ/mol),usually exothermic

Rate of adsorption Rapid, non activated,

reversible

Activated, may be slow and

irreversible

Rate of desorption Activation energy for

desorption equals heat

of adsorption

Activation energy for

desorption may be larger than

heat of adsorption

Surface coverage Multilayers Monolayer

Specificity Non specific Highly specific

Reversibility Highly reversible Often reversible

Applications Determination of

surface area and pore

size

Determination of surface

concentrations and kinetics, rates of

adsorption, determination of active

centers

Table 1.3 Comparisons between physisorption and chemisorption.

1.5.3 Catalytic mechanism

For the catalytic process to take place in heterogeneous catalysis, the starting materials

must be transported to the catalyst. Thus, apart from the actual chemical reaction, diffusion,

adsorption and desorption processes are of importance for the progress of the overall

reaction. The following simple reaction steps 1 to 7 can be expected in the case of a

catalytic gas reaction on a porous catalyst as shown in Fig. 1.3. Step 1: Diffusion of the

starting materials through the boundary layer to the catalyst surface; step 2: Diffusion of the

starting materials into the pores (pore diffusion); step 3: Adsorption of the reactants on the

inner surface of the pores takes place; step 4: Chemical reaction on the catalyst surface;

step 5: Desorption of the products from the catalyst surface; step 6: Diffusion of the

products out of the pores and finally, step 7 involves diffusion of the products away from

the catalyst through the boundary layer and into the gas phase [2,8].

Fig. 1.3 Individual steps of a heterogeneously catalyzed gas-phase reaction (adopted from

[1]).

In heterogeneous catalysis chemisorption of the reactants and products on the catalyst

surface is of central importance, so that the actual chemical reaction (step 4) cannot be

considered independently from steps 3 and 5. Therefore, these steps must be included in the

micro kinetics of the reaction.

Two distinct mechanistic situations are possible in the surface-catalyzed transformation

of reactant species A and B to a product C, (Fig. 1.4):

• The Langmuir – Hinshelwood – Hougen – Watson (LHHW) approach is based on

the Langmuir model describing the surface of a catalyst as an array of equivalent sites

which do not interact either before or after chemisorption. Further, the reaction is said to be

of this type, if both the reactants are adsorbed on the catalyst surface and react with each

other to give product [8].

•Eley-Rideal mechanism where only one of the reactant species is bound on the

catalyst surface and is converted to product when the other impinges upon it from the gas

phase [8].

Fig. 1.4. Surface-catalyzed transformation of reactant species A and B to a product C in

heterogeneously catalyzed processes.

1.6 Solid acid, base and acid-base bifunctional catalysts

1.6.1 Solid acid catalysts

A solid acid may be defined as the one which changes the colour of a basic indicator or

as a solid on which a base is chemically adsorbed. There are two types of acid sites on

surfaces of metal oxides: Lewis acids and Brönsted acids. A solid that is able to donate or

at least partially transfer a proton which becomes associated with surface anions, is said to

Langmuir-Hinshelwood Eley-Rideal

possess Brönsted acidity. A Lewis acid site is one which can accept an electron pair. The

acid strength of a solid acid can be determined by measuring the ability of the surface to

convert an adsorbed neutral base (B) into its conjugate acid (BH+) [22]. Heterogeneous

acid catalysis has attracted much attention due to numerous applications in many areas of

the chemical industry. According to a survey by Tanabe and Holderich, the number of

industrial processes that use solid acids, solid bases, and acid–base bifunctional catalysts

are 103, 10 and 14, respectively [23]. These are extremely useful catalysts in many large

volume applications, especially in the petroleum industry for hydration, alkylation,

isomerization and cracking reactions and in the production of fine and specialty chemicals,

for example, cation-exchange resin (CER) has been commercialized by Mitsui Chemical

for selective hydration of isobutene in mixed C4-fraction to t-butanol as an intermediate for

methyl methacrylate [24] and Sumitaomo Chemical has employed CER for the production

of Methyl-tert-butyl ether (MTBE) by the reaction of iso-butene in mixed C4-fraction with

methanol as the first step of iso-butene separation via MTBE [25]. Zeolite-based acid

catalysts currently play a significant role in petrochemical industries. They find wide

application in vapor-phase and liquid-phase reactions by emphasizing high position-, regio-

, or shape-selectivity mainly in hydrocarbon conversion reactions. Several zeolite catalysts

have been successfully employed for commercial production of valuable chemicals such as

alkylation of toluene with methanol, toluene disproportionation, transalkylation of toluene–

trimethylbenzene and xylene isomerization [26-28]. Solid heteropoly acids (HPA) have

been employed for non-oxidative vapor-phase reaction such as ethyl acetate synthesis.

Showa Denko (Oita, Japan) and British petroleum chemicals have employed vapor-phase

ethylene-esterification to ethyl acetate—Cs-modified PW12-type HPA [29] and SiW12-type

HPA/SiO2 [30].

1.6.2 Solid base catalysts

Solid base catalysts were originally defined as catalysts for which the colour of an acidic

indicator changes when it is chemically adsorbed. Brönsted base is a proton acceptor and

Lewis base is an electron-pair donor. The strength of solid base catalyst can be determined

by measuring the ability of the basic surface to convert an adsorbed acid (BH) into its

conjugate base form (B-) [22]. In contrast to extensive studies on heterogeneous acidic

catalysts, some efforts have been made to the study of heterogeneous basic catalysts. One

of the reasons why the studies of heterogeneous basic catalysts are not as extensive as those

of heterogeneous acidic catalysts seems to be the requirement for severe pretreatment

conditions for generation of basic sites to remove carbon dioxide, water and in some cases,

oxygen [31]. Some of the industrially prominent base catalysed organic transformations

include the following.

i) Preparation of 2, 6-dimethyl phenol by dialkylation of phenol with two molecules

of methanol over magnesium oxide (MgO) based catalyst system commercialized

by General Electric to obtain the high position-selectivity product. Alkylation of

phenol with methanol to form 2,6-xylenol proceeds over MgO catalyst at a high

temperature of 400 ºC [32].

ii) Double bond-isomerization of 2, 3-dimethylbutene-1 to 2, 3-dimetylbutene-2 in the

synthesis of pyrethroid intermediate—Na/NaOH/alumina, Sumitomo [33].

iii) Liquid-phase acrylate/HCHO-condensation to alpha-hydroxymethyl acrylate in

the presence of anion exchange resin, Nippon Shokubai [34].

iv) Methanol-carbonylation with CO to methyl formate in liquid-phase for replacing

alkali alkoxide using strong anionic IER, Mitsubishi Chemical [35].

1.6.3 Solid acid-base catalysts

Some catalysts have both acidic and basic properties and contain suitable acid-base pair

sites. The acid-base catalysts can possess remarkable activity though the strength of their

acidic and basic sites is much weaker than that of acid or base catalysts. For example,

zirconia was found to have both acid-base sites and can act as an acid as well as a base

catalyst. Bi-functional catalysts are used in many reactions, including hydrocracking,

reforming and dewaxing processes. Mitsubishi Chemical has commercialized a process for

hydrogenating aromatic carboxylic acids in vapor-phase to corresponding aromatic

aldehyde over Cr-modified zirconia, which is regarded to be acid/base bifunctional catalyst

[36]. Nippon Shokubai has successfully employed BaO/Cs2O/P2O5/SiO2 (acid/base

bifunctional) catalyst for dehydration of monoethanolamine to ethyleneimine in vapor-

phase (replacing liquid-phase reaction catalyzed by H2SO4) [37].

Thus, it can be seen from the above examples that various solid acid, base and acid-base

systems have been commercialized and the catalysts mainly include hetropolyacids, ion

exchange resins and zeolites. However, the main disadvantage associated with

hetropolyacids is that they are fairly soluble in polar solvents and lose their activity at

higher temperatures by losing structural integrity. To prevent this, there have been some

attempts to immobilize them in silica or activated carbon matrix, which however limits the

accessibility and efficiency of the catalyst [38]. Ion exchange resins pose various problems

like poor thermal stability and low specific surface area [39]. Zeolites, which have been

extensively studied and used as catalysts in many processes, are not sufficiently acidic to

replace liquid-phase systems (HF, H2SO4, BF3) and/or halogen-containing solids (for

example, chlorinated alumina) in processes where lower operating temperature may be

advantageous to obtain the desired product. Some examples are isomerization of alkanes,

isobutane alkylation, aromatic alkylation, olefin oligomerization and a variety of aromatic

acylation processes. Moreover, activities of zeolite materials are much lower than the

conventional homogeneous acids due to pore blocking and hydration [40]. In view of these

reasons, there is an ongoing effort to develop stronger solid acid and base catalyst systems

which are water tolerant, stable at high temperatures and suitable for both liquid and vapor

phase conditions. Metal oxide based catalysts are found to offer several advantages over

zeolite based catalysts. These are active over a wide range of temperatures and more

resistant to thermal excursions.

1.7 Importance of metal oxides

Metal oxides are one of the seminal solid catalysts used for various industrial process

involving dehydrogenation, oxidation, ammoxidation, polymerization and so on. Like the

zeolites and clays, these solids are porous, but the pores are larger and non-uniform. The

pores in metal oxides are void spaces between aggregated primary particles, which are

usually small crystallites of the solid. The pore volume may typically take up about one

half of the volume of the catalyst sample, and the internal surface area is often large [21].

Among oxides, zirconia and magnesia and their various modified forms as catalysts have

been extensively reviewed [41-44]. Zirconia is considered to be amphoteric and magnesia

is generally known to be basic. Modification of these simple oxides have already opened up

new vistas in the field of catalysis and revolutionized the chemical industry, giving rise to

even solid superacids and superbases respectively. Important properties of selected oxides

are discussed below.

1.7.1 Magnesium oxide

Magnesium oxide is one of the well-known basic catalysts and it has simple rock salt

structure, with octahedral coordination of magnesium and oxygen. Catalytic performance

of MgO largely depends on the basic surface character because of extensive electron

transfer from magnesium to oxygen upon MgO formation, the electron rich-oxygen anions

on MgO surfaces act as strong basic, electron-donating sites, while the electron deficient

magnesium cations act as weak acid, electron accepting sites. Besides O2−

sites, hydroxyl

groups also act as basic sites and have been shown to promote basic reactions [45]. The

image of surface acidity on MgO is less clear, because basic properties are predominating

on magnesia. It is assumed that apart from surface magnesium-ions that act as Lewis acidic

sites, magnesia possesses some Brönsted acidity, caused by residual surface hydroxyl

groups [46].

To have basic sites appear on the surface of MgO, pretreatment at high temperatures is

required to remove H2O and CO2 from the surfaces. According to the proposal by Coluccia

and Tench, there exist several Mg–O ion pairs of different coordination numbers on the

surface of MgO catalyst as shown in Fig. 1.5 for completely dehydrated and decabonated

MgO, ion pair of 5-fold-coordinated sites exist on the extended MgO (100) plane, 4-fold-

coordinated sites on the edges between the (100) plane, and 3-fold –coordinated sites exists

on kinks and corners. Among the ion pairs of different coordination numbers, it was also

reported by Coluccia and Tench that the ion pair of 3-fold Mg2+

– 3-fold O2−

(Mg2+

3C - O2-

3C) is most reactive and adsorbs carbon dioxide most strongly. To reveal the ion pair, the

highest pre-treatment temperature is required. As the pre-treatment temperature increases,

the molecules covering the surfaces are successively desorbed according to the strength of

the interaction with the surface sites. The molecules weakly interact with the surfaces are

desorbed at lower pre-treatment temperatures, and those strongly interacting are desorbed

at higher temperatures. The sites that appear on the surfaces by pre-treatment at low

temperatures are suggested to be different from those appearing at high temperatures. At

the same time, the ion pair is most unstable and tends to rearrange easily at high

temperature. The appearance of such highly unsaturated sites by the removal of carbon

dioxide and the elimination by the surface rearrangement compete, which results in the

activity maxima with change in the pre-treatment temperature. Such variations of catalytic

activities with pretreatment temperature as observed for MgO are common to those for

other types of solid base catalysts. It is essential to remove the adsorbed carbon dioxide,

water and in some cases, oxygen from the surfaces to generate basic sites, though proper

pre-treatment temperatures vary with the types of catalysts and reactions [46].

Fig. 1.5 Representation of a surface plane (100) of MgO showing surface imperfections

such as steps and corners which provide sites for ions of low coordination (adopted from

[47]).

Activity of MgO catalysts depends on various parameters such as nature of precursors,

precipitation procedure, concentration of dopants and calcination temperature. Variation in

any of these parameters can substantially influence the catalytic performance (activity and

selectivity) of the resultant MgO catalyst [48-49].

1.7.2 Zirconia – Anion modified

Zirconia has attracted significant interest in the recent past as a catalyst support and as a

base material for the preparation of strong solid acids by surface modification with sulfate,

molybdate or tungstate groups. Zirconia exists either as amorphous, tetragonal, cubic or

monoclinic phases. Amorphous precipitates of Zr(OH)4 transform irreversibly upon thermal

treatment first to the metastable tetragonal phase and then to the monoclinic phase. Zirconia

gives rise to a substantially different interaction between the active phase and the support,

altering the activity and selectivity of the system. Arata and Hino [50] found that when

dopant such as tungstate or molybdate species are dispersed on zirconia supports by

impregnation with a solution of tungstate or molybdate anions and subsequent oxidation

treatments at high temperatures (600 – 800 ºC) leads to the formation of acid sites on the

tungsten oxide/zirconia and molybdate oxide/zirconia catalysts that are stronger than 100 %

sulfuric acid as measured by Hammett indicators (H0 ≤ 14.52). They concluded that

tungsten oxide combines with zirconium oxide to create superacid sites at the time when

zirconia is going through a phase transformation from amorphous to tetragonal. It is known

that anionic dopants create additional electron-deficient regions that increase the Brönsted

acid strength of a metal oxide surface by improving the ability of neighboring hydroxyl

groups to act as proton donors [51]. Based on several physicochemical characterization

results, Iglesia et al. [52] have proposed the surface structure of tungstated zirconia as

shown in Fig. 1.6. Tungsten oxide could exist on the zirconia surface either in the form of

isolated mono-tungstate Fig. 1.6 (a) or as polyoxotungstate clusters as shown in the Fig. 1.6

(b). Activity of these oxides depends on various parameters such as nature of precursors,

precipitation procedure, concentration of dopants and calcinations temperature. Variation in

any of these parameters can drastically affect the resultant catalytic activity of these

materials [53].

a) Isolated mono-tungstate on zirconia support

b) Poly-tungstate cluster on

zirconia support

ZrO2 support ZrO2 support

Fig. 1.6. Schematic surface structures of a) Isolated mono-tungstate and b) Poly-tungstate

growth on monolayer coverage on zirconia.

1.8 Catalyst characterization

Characterization is a central aspect of catalyst development. The elucidation of the

structures, compositions and chemical properties of both the solids used in heterogeneous

catalysis and the study of product and intermediates formed during the reactions is vital for

a better understanding of the relationship between catalyst structure and catalytic

performance. This knowledge is essential to develop more active, selective, durable

catalysts and also to optimize reaction conditions.

In the present investigation the following structural and textural characterization

techniques were used to characterize the prepared catalysts:

i. Elemental analysis

ii. X-ray diffraction (XRD)

iii. Brunauer, Emmett and Teller (BET) surface area

iv. Surface acidity

v. Fourier transformed infrared spectroscopy (FT-IR)

vi. Raman spectroscopy

vii. Scanning electron microscopy (SEM)

viii. Thermal analysis

ix. X-ray photoelectron spectroscopy (XPS)

1.8.1 Elemental analysis

Inductively coupled plasma/optical emission spectrometry (ICP-OES) is a powerful tool

for the estimation of chemical composition of a heterogeneous catalyst. This technique is

commonly employed for the accurate estimation of the chemical composition of

heterogeneous catalysts, as it is important to know the presence and the quantity of trace

elements, additives, poisons in a catalyst. With this technique, aqueous solution of acid

digested samples is injected into a radiofrequency (RF)-induced argon plasma using one of

a variety of nebulizers or sample introduction techniques. The sample mist reaching the

plasma is quickly dried, vaporized and energized through collisional excitation at high

temperature. The atomic emission emanating from the plasma is viewed in either a radial or

axial configuration, collected with a lens or mirror, and imaged onto the entrance slit of a

wavelength selection device. Single element measurements can be performed cost

effectively with a simple monochromator and photomultiplier tube combination and

simultaneous multi-element determinations are performed for up to 70 elements with the

combination of a polychromator and an array detector. The analytical performance of such

systems is competitive with most other inorganic analysis techniques, especially with

regard to sample throughput and sensitivity [54].

1.8.2 X-ray diffraction

In the structural characterization of solid catalysts the most important technique is the

X-ray diffraction. This technique is commonly employed to determine the bulk structure

and composition of heterogeneous catalysts with crystalline structures. It is also used to

estimate the average crystallite or grain size of catalysts [55]. The XRD analysis, typically

involves identification of specific lattice planes that produce peaks at their corresponding

angular positions 2θ, determined by Bragg’s law, 2d sinθ = nλ. Where d is the interplanar

spacing, n is an integer and known as order of diffraction, λ is the x-ray wavelength and θ

is the diffraction angle. Diffraction of the x-ray beam occurs only when Bragg’s law is

satisfied for constructive interference from two lattice planes with a spacing d. The

intensities of the diffracted beams are recorded by the detector and reported in terms of 2θ

angle. Thus, the characteristic patterns associated with individual solids make XRD quite

useful for the identification of the bulk crystalline components of solid catalysts. When

used as a fingerprint technique, patterns are matched by comparison to the standard data

collection by Joint Committee on Powder Diffraction Standards (JCPDS) or International

Centre for Diffraction Data (ICCD) databases.

1.8.3 BET Surface area

In heterogeneous catalysis, the surface area, pore volume and average pore size of

catalysts often play a pivotal role in determining i) the number of active sites available for

catalysis ii) the diffusion rates of reactants and products in and out of these pores and iii)

the deposition of coke and other contaminants. Hence, the determination and control of the

surface areas and porosities of materials are very important in heterogeneous catalysis as

they have a strong influence upon catalytic performance. Most heterogeneous catalysts,

including metal oxides and supported metal catalysts are porous materials with specific

surface areas ranging from 1 to 1000 m2/g. These pores can display fairly complex size

distributions, and can be broadly grouped into three types, namely, micropores (average

pore diameter d < 2 nm), mesopores (2 < d < 50 nm), and macropores (d > 50 nm). The

most common method used to characterize the structural parameters associated with pores

in solids is via the measurement of adsorption–desorption isotherms, that is, of the

adsorption volume of a gas, typically nitrogen, as a function of its partial pressure. Either

single point or multipoint method is used to calculate the surface area. The most widely

used technique for surface area measurement is the BET technique [56]. The BET method

is based upon the Langmuirian physisorption of molecules of precisely known size on the

surface of interest. The monolayer capacity can then be determined and the surface area

extracted by application of the following relationship: P/Va (P0 - P) = 1/Vm C + P (C-1)/Vm

P0 C, where Va is the volume of gas adsorbed at equilibrium pressure, P and P0 is the

saturated vapour pressure of the adsorbate at (say) liquid nitrogen temperature and c is the

isothermal constant. Vm is monolayer volume in mL at STP. By plotting P/Va (P0 - P) vs.

P/P0 and determining Vm from the slope of the resultant straight line in the partial pressure

range of 0.05 to 0.35, the surface area can be calculated. The surface area S of the sample

giving the monolayer adsorbed gas volume Vm (STP) is then calculated from S =

VmAN/M, where A is Avogadro’s number which express the number of gas molecules in a

mole of gas at standard state conditions. M is the molar volume of the gas and N the area of

each adsorbed gas molecule.

1.8.4 Surface acidity

Solid acid catalysts such as sulfated and tungstated zirconia are widely used in many

kinds of chemical reactions including cracking and isomerisation of hydrocarbons,

alkylation of paraffins and aromatics with olefins, transalkylation, disproportionation and

polymerization of olefins. The catalytic activity and selectivity of these reactions are

closely related to both the amount and the strength of the acid sites distributed over the

surface of the catalyst as these reactions are known to occur by means of a carbenium ion

mechanism. The amount of acid sites on a solid surface can be measured by n-butylamine

titration method. The method consists of titration a solid acid suspended in benzene with n-

butylamine, using an indicator or by back titrating the residual n-butylamine with 0.1 N

HClO4 in acetic acid. This method gives the sum of the amounts of both Brönsted and

Lewis acid [57].

1.8.5 Infrared spectroscopy

The vibrational spectroscopy is one of the most widely used techniques for catalyst

characterization. Infrared bands are produced when the electromagnetic radiation in the

infrared region causes a change in the dipole moment (or induced dipole moment) in the

molecules. IR spectra are quite rich in information and can be used to extract or infer both

structural and compositional information on the adsorbate itself as well as on its

coordination on the surface of the catalyst. IR is also used to characterize reaction

intermediates on the catalytic surface faces, often in situ during the course of the reaction.

Several working modes are available for IR spectroscopy studies [58]. The most common

arrangement is transmission, where a thin solid sample is placed between the IR beam and

the detector; this mode works best with weakly absorbing samples. Diffuse reflectance IR

offers an alternative for the study of loose powders, strong scatter, or absorbing particles.

Attenuated total reflection IR is based on the use of the evanescent wave from the surface

of an optical element with trapezoidal or semispherical shape, and works best with samples

in thin films.

Further, identification of surface sites can be carried out by appropriate use of

selected adsorbing probes. For instance, the acid–base properties of specific surface sites

can be tested by recording the ensuing vibrational perturbations and molecular symmetry

lowering of either acidic (CO or CO2) or basic (pyridine and ammonia) adsorbates [58].

Adsorption of pyridine on the surface of solid acids is one of the most frequently applied

methods for the characterization of surface acidity. The use of IR spectroscopy to detect

adsorbed pyridine enables us to distinguish among different acid sites. According to

procedure described by Kung [59], pyridine adsorbed on Brönsted (B) and Lewis (L) acid

sites of a catalyst produces unique bands at 1540 cm−1

and at 1445 cm−1

respectively. This

can be attributed to pyridinium ion alone, as it produces a band in the vicinity of 1540 cm−1

and the appearance of this band in the spectrum is taken as indication of Brönsted acidity.

Coordinately bonded or Lewis pyridine generates a unique band at 1445 cm−1

where the

pyridinium ion does not absorb.

1.8.6 Raman spectroscopy

Raman spectroscopy complements IR data for the characterization of solid catalysts.

This technique has been extensively used for the study of the structure of many solids,

particularly the oxides such as WO3 on ZrO2 [60]. This technique is ideal for the

identification of oxygen species in covalent metal oxides. This is because Raman

spectroscopy directly probes the structure and binding of a metal oxide complex by its

vibrational absorption. A clear distinction can be made with the help of these data between

terminal and bridging oxygen atoms and a correlation can be drawn between the

coordination and bond type of these oxygen sites and their catalytic activity.

1.8.7 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy is a useful technique to probe both the elemental

composition of the surface of catalysts and the oxidation state and electronic environment

of each component [61]. In XPS soft X-rays (200 to 2000 eV) are used and core electronic

levels are examined. Qualitative information is derived from the chemical shifts of the

binding energies of given photoelectrons originating from a specific element on the surface.

In general, binding energies increase with increasing oxidation state and to a lesser extent

with increasing electronegativity of the neighboring atoms. Quantitative information on

elemental composition is obtained from the signal intensities.

1.8.8 Thermal analysis

When a substance is subjected to a programmed heating or cooling it normally

undergoes changes, which may be physical or chemical in nature. The analysis of these

changes recorded as a function of temperature permits the study of composition,

structure, physical and chemical behavior. On the basis of the changes involved, we have

employed thermo-gravimetric analysis (relates to mass changes) and in-house customised

thermal-mass spectrometer system to identify the evolved organic species.

Thermogravimetry is the measure of quantitative changes in mass (mass loss or gain)

occurring in a substance as it undergoes a controlled program of heating as a function of

temperature and/or time. The three modes isothermal, quasi-isothermal and dynamic

thermogravimetry are used for characterizing materials in which dynamic

thermogravimetry is most common [62]. In dynamic thermogravimetry, the sample is

heated in an environment whose temperature is changing in predetermined manner,

preferably at a linear rate. The resulting mass-change versus temperature curve generally

known as thermogram or thermogravimetric curve provides information concerning the

thermal stability and composition of the initial sample. The thermal stability and

composition of any intermediate compounds that may be formed and the composition of

the residue, if any, can also be obtained. It can be used to study any physical (such as

evaporation) or chemical process (such as thermal degradation) that causes a material to

lose volatile gases. To yield useful information with this technique, the sample must

evolve a volatile product. Further, we have studied the evolved gas analysis by coupling

mass spectrometer to the micro-furnace (in-house customised pyrolysis-MS system) to

identity the evolved gases from the sample under investigation as a function of temperature.

1.8.9 Scanning electron microscopy

Electron microscopy is a straight forward technique useful for the determination of

physical characteristics of catalyst particles, such as morphology and size of solid catalysts

[63]. Electron microscopy can be performed in one of two modes — by scanning of a well-

focused electron beam over the surface of the sample, or in a transmission arrangement. In

SEM, the yield of either secondary or back-scattered electrons is recorded as a function of

the position of the primary electron beam, and the contrast of the signal used to determine

the morphology of the surface: the parts facing the detector appear brighter than those

pointing away from the detector. Dedicated SEM instrument scan have resolutions down to

5 nm, but in most cases, SEM is used for imaging catalyst particles and surfaces of

micrometer dimensions. Additional elemental analysis can be added to SEM via energy-

dispersive analysis of the x-rays (EDAX) emitted by the sample.

1.9 Product analysis by gas chromatography/ gas chromatography-mass

spectrometry.

Chromatographic techniques are used to separate mixtures of chemicals into

individual components which then can be individually identified and quantified [64]. The

separation between components is based upon the difference in their partitioning

behavior between stationary and mobile phases. In gas chromatography (GC), the

partitioning behavior has a temperature and column interaction dependence and mixtures

of components can be resolved by passage through a column containing the stationary

phase which may be held isothermally or subjected to a temperature programme. Gas

chromatography typically consists of i) a carrier gas (the mobile phase) which is usually

an inert gas such as helium, argon or nitrogen, a pressure regulator to control the flow

rate of the gas through the chromatograph, ii) an injection port with a syringe needle to

inject the sample. Various injectors can be used such as split/splitless, on-column and

programmable temperature vaporizer. The injection port in split/splitless mode is

maintained at a higher temperature than the boiling point of the sample components. iii) a

column with a stationary phase kept in a heating oven. There are two different general

types of columns, capillary and packed columns. Capillary columns comprise a thin fused

silica coil of around 10-100 m length with the stationary phase coated at the inner surface

and iv) a detector and a signal recorder. Commonly used detectors include flame

ionization (FID), nitrogen phosphorus (NPD), electron capture (ECD), photo ionization

(PID) flame photometric (FPD), electrolytic conductivity (Hall/ELCD), and thermal

conductivity (TCD) detectors. When mass spectrometer is employed as detector, the

instrument is known as GC-MS system and can be used for structural information of all

the components. The temperatures of the column can be programmed to get good

resolution between the peaks of interests; the injector and the detector are usually

controlled independently.

1.10 Scope and objectives of present work

In the last decade there have been continuous efforts not only in universities, but also in

industry towards the design and development of new green catalysts for industrially

important organic transformations to meet the new requirements from both the legislation

and the market. One class of catalysts that has received a lot of attention is anion modified

metal oxides, which show good acidic properties. The focus has been on the development

and application of anion modified zirconia in particular. Also, in recent years, interest in

magnesium oxide, a solid base catalyst, is being strengthened outstandingly as it is found

that some of the industrially important reactions specifically proceed on the heterogeneous

basic catalysts. This has compelled researchers to investigate surface sites together with

elucidation of the reaction mechanisms occurring on the surfaces. A comprehensive

understanding of the surface property is very essential to explore the possibility of

application of solid base catalyst as a potential replacement to solid acid catalysts having

problem of catalyst deactivation by tar-formation. Further, development of rapid analytical

screening techniques is attracting increased attention in recent years for catalyst discovery

and optimization of reaction conditions for a variety of industrially useful organic

transformations. In view of the existing wide scope for the development of catalysts for

industrially useful organic transformations, the following specific objectives were chosen

for the present study.

To develop tunstated zirconia having excellent catalytic properties such as, high

thermal stability, high surface area, well-defined acid-base properties and

correlation of the physico-chemical properties with the catalytic activity for the

synthesis of fine chemicals.

To study the kinetics of the reaction in a batch reactor and estimate the kinetic

parameters using Langmuir–Hinshelwood–Hougen–Watson (L–H–H–W) surface

reaction controlled kinetic model.

To develop and apply test reactions at near operating conditions of actual reactions

to determine the active sites on different MgO catalysts obtained from various

starting materials and to correlate the method of preparation, modification and

surface properties of catalysts with their catalytic activity for the synthesis of fine

chemicals.

To develop a rapid analytical screening system for heterogeneous catalyst discovery

through better understanding of the reaction mechanism and optimization of the

reaction conditions to get good selectivity and conversion by varying reaction

parameters such as molar ratio of reactants, temperature and amount of catalyst

using Response Surface Methodology (RSM) for conversion of reactants and

selectivity of products.

The above subject matter formulates the objectives of the thesis. The thesis has been

organized into six chapters. A brief description of the contents of each chapter is given

below.

1.11 Organisation of subject matter

Chapter 1: The first chapter is dedicated for the general introduction to catalysis, captures

some of the industrially important processes, includes discussion on mode of action of

catalysts with a brief on activity, turnover number, selectivity and stability, significance of

promoters and poisons in catalysis is also included. Definitions of different types of

catalysts, comparison of homogeneous and heterogeneous catalysis and mechanism of

heterogeneous catalysis and discussion on chemisorption and physisorption are described

in detail. It includes details on classification of solid catalysts based on the basis solid acid,

base and acid-base bifunctional surface active sites. A detailed discussion on the

importance of metal oxides, tunstated zirconia and magnesia in particular is also presented.

A brief introduction and application of various physico-chemical techniques like ICP-OES,

XRD, BET, surface acidity, FT-IR, Raman spectroscopy, SEM, Thermal analysis and XPS

to determine bulk and surface properties of the prepared catalysts and a brief description of

the contents of each chapter has been included at the end of the chapter.

Chapter 2: In this chapter, we describe the alkylation of catechol with tert-butyl alcohol in

liquid phase batch mode over tungsten modified zirconia catalyst system. A systematic

study has been made, which includes preparation of catalyst with varying acid strength and

surface area by loading 1, 5, 15, 25 and 50 wt. % tungsten oxide on zirconia, followed by

detailed physico-chemical studies and the effect of various reaction parameters (such as

calcinations temperature, WO3 loading, mole ratio of the reactants, catalyst loading and

temperature) for the liquid phase alkylation of m-cresol with isopropyl alcohol. The effects

of various parameters such as temperature, reactant composition, catalyst loading on

catechol conversion as well as product selectivity were studied. We have made an attempt

to correlate the enormous information collected on the physico-chemical characteristics of

all the catalysts with conversion and selectivity towards catechol and tert-butyl catechol

respectively. It has been observed that acidic and structural features of the catalysts do play

an important role in controlling conversion and selectivity. A mechanism for tert-butylation

of catechol with isopropyl alcohol as alkylating agent on WOx/ZrO2 has been proposed and

L–H–H–W surface reaction controlled kinetic model was used to estimate the kinetic

parameters. The results of the theoretical model were found to fit with the experimentally

observed data reasonably well. The activation energy for tert-butylation of catechol with

isopropyl alcohol was determined from the estimated rate constants obtained at different

temperatures.

Chapter 3: In this chapter we have investigated the application of anion modified zirconia

(WOx/ZrO2) as a potential catalyst for alkylation of m-cresol with isopropyl alcohol. It was

found that both C- and O-alkylation are possible in the case of m-cresol depending on

reaction conditions. The reasons for the observed product distribution are explained and the

effects of various parameters on rates and selectivity’s are discussed. We have also made

an attempt to correlate the physico-chemical properties towards the catalytic activity.

Further, based on the product distribution, a reaction mechanism was proposed and L–H–

H–W surface reaction controlled kinetic model was used to estimate the kinetic parameters.

The results of the theoretical model were found to fit with the experimentally observed data

reasonably well. From the estimated rate constants at different temperatures, the activation

energy for m-cresol alkylation reaction with isopropanol was determined.

Chapter 4: In this chapter, detailed preparation and characterization of MgO, a solid base

catalyst, has been described. MgO catalysts were prepared from different precursors such

as Mg(OH)2, MgCO3 and Mg(OH)2.MgCO3 (Magnesium carbonate hydroxide) under

controlled calcination conditions. Bulk and surface characterization techniques were used

for characterization of the prepared catalysts. Dehydrogenation selectivity in the benzyl

alcohol reaction was used for investigating the acid-base properties of catalysts at the

selected vapor phase reaction conditions. Vapor phase transformation of benzyl alcohol and

alkylation of aniline with various alcohols such as methanol, ethanol and benzyl alcohol is

described in detail. The application of benzyl alcohol to benzaldehyde and toluene test

reaction promises to be a potential tool to study the nature of catalytically active sites on

the surface of different MgO catalysts obtained from various precursors. We have also

made an attempt to correlate the physico-chemical properties of MgO obtained from

different precursors with the selective N-alkylation of aniline with aliphatic and aromatic

alcohols.

Chapter 5: In this chapter, we present in-house designed and fabricated vapor phase pulse

reactor coupled on-line to a GC-MS, and its application as a screening tool for rapid testing

of small amount of heterogeneous catalysts for activity towards selected vapor phase

organic synthesis. The evidences to understand the reaction pathway for the catalytic

hydride reduction of nitrobenzene to aniline using methanol as in-situ hydrogen donor is

discussed. A reaction mechanism has been postulated. Further, we have successfully

applied Design of Experiments (DOE) tool to optimize the functional parameter for

obtaining maximum conversion and selectivity by using methanol as in-situ hydrogen

donor for the reduction of nitrobenzene to aniline.

Chapter 6: This chapter describes the details of the experimental work, results and

discussion concerning vapor phase catalytic hydrogen transfer reduction of nitrobenzene on

an inexpensive catalyst such as MgO, using abundantly available methanol as hydrogen

donor. The catalysts containing ZrO2 and ZnO as dopant on the MgO have been prepared

and all the catalysts were characterized by various physico-chemical techniques. Catalytic

activity studies have been performed using the in-house designed and fabricated pulse

reactor coupled to a GC-MS as an on-line catalyst testing technique. The feed composition

of nitrobenzene, methanol, flow rate and the reaction temperature were optimized to obtain

maximum aniline selectivity. MgO and the doped catalyst were found to give complete

conversion of nitrobenzene, however the aniline selectivity increased in the order MgO <

ZrO2/MgO < ZnO/MgO.

Chapter 7: The summary of the current research work and the scope for the future work is

incorporated in this chapter.

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