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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. 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