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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 byproducts 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
1970-1980
Process
Vapor phase alkylation
(General Electric)
Carbonylation (Monsanto
process)
MTG (Mobil process)
Alkylation (Mobil –
Badger)
Selective catalytic
reduction (SCR; stationary
sources)
Esterification (methyl-tertbutyl ether synthesis)
Mitsui
Oxidation (Sumitomo
Chem., two-step process)
1981 – 1985
1986 – 2000
2000 –2010
Catalyst
MgO
Organometallic Rh
complex
Zeolite (ZSM-5)
Modified zeolite
(ZSM-5)
Product
2,6 Xylenol from alkylation
of phenol with methanol
Acetic acid from methanol
Gasoline from methanol
Ethyl benzene from
ethylene
V Ti (Mo, W) oxides
(monoliths)
Reduction of NOx with NH3
to N2
Cation-exchange
resin
Mo, Bi oxides.
Mo, V, PO
(heteropolyacids)
Methyl-tert-butyl ether from
iso-butene with methanol
Oxidation (Monsanto)
Fluid-bed polymerization
(Unipol)
Hydrocarbon synthesis
(Shell)
Oxidation with H2O2
(Enichem)
Vanadylphosphate
Hydration
Dehydration of 2propanolamine (Koei
Chem)
Dehydrogenation of C3, C4
alkanes
(Star and Oleflex
processes)
Catalytic destruction of
N2O fromnitric acid tail
gases (EnviNOxprocess,
Uhde)
HPPO (BASF-Dow,
Degussa-Uhde)
Enzymes
Acrylic acid from propene
Maleic anhydride from nbutane
Polyethylene and
polypropylene
Middle distillate from CO
with H2
Hydroquinone and catechol
from phenol
Acrylamide from
acrylonitrile
ZrO2
Allylamine
Pt(Sn) – zinc
aluminate,
Pt – Al2O3
C3, C4 olefins
Fe zeolite
Removal of nitrous oxide
Ti silicalite
Propylene oxide
TS-1
Propylene from propene
Propylene oxide from H2O2
and propylene
Ziegler – Natta type
Co – (Zr,Ti) – SiO2
Pt – SiO2
Ti silicalite
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
All metal atoms
Only surface atoms
Concentration
Low
High
Selectivity
High
Low
Diffusion problems
Practically absent
Present
(mass-transfercontrolled reaction)
Reaction conditions
Mild (50–200 οC)
Severe (often >250 οC)
Applicability
Limited irreversible reaction
with products
(cluster formation);
poisoning
Wide sintering of the metal
crystallites;
poisoning
Structure/stoichiometry
Defined
Undefined
Modification possibilities
High
Low
Thermal stability
Low
High
Catalyst separation
Sometimes
laborious Fixed-bed:
unnecessary
(Chemical
decomposition, suspension: filtration
distillation, extraction)
Possible
Unnecessary (fixed-bed)or
easy(suspension)
Catalyst recycling
Cost of catalyst losses
High
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,
Covalent/electrostatic forces, electron
no electron transfer
transfer
Adsorbents
All solids
Some solids
Adsorbates
All gases below critical
Some chemically reactive gases,
point, intact molecules
dissociation into atoms, ions, radicals
Temperature range
Close to condensation
Occurs at a wide range of
over which
temperature of the
temperatures and at
adsorption occurs
adsorbate
temperatures much above the
condensation temperature.
Heat of adsorption
Low, ≈heat of fusion
High, ≈ heat of reaction
(ca.10 kJ/mol),always
(80-200 kJ/mol),usually exothermic
exothermic
Rate of adsorption
Rapid, non activated,
Activated, may be slow and
reversible
irreversible
Activation energy for
Activation energy for
desorption equals heat
desorption may be larger than
of adsorption
heat of adsorption
Surface coverage
Multilayers
Monolayer
Specificity
Non specific
Highly specific
Reversibility
Highly reversible
Often reversible
Applications
Determination of
Determination of surface
surface area and pore
concentrations and kinetics, rates of
size
adsorption, determination of active
Rate of desorption
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].
Langmuir-Hinshelwood
Eley-Rideal
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
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 vaporphase (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-foldcoordinated 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 - O23C)
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].
ZrO2 support
a) Isolated mono-tungstate on
zirconia support
ZrO2 support
b) Poly-tungstate cluster on
zirconia 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 nbutylamine, 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 wellfocused 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 energydispersive 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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