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INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD – 382 481, 08-10 DECEMBER, 2011 1 Characteristics of Oxidation and Oxidative Dehydrogenation Catalysts for Gas Phase Reactions: A Review C.R. Mistry, R.K. Mewada, V.K. Srivastava, R.V. Jasra Abstract—Applications for catalytic selective oxidation and oxidative dehydrogenation for production of various petrochemicals are increasing. However due to series-parallel side reactions in such processes emphasis mainly on higher selectivity of the desired product. Selection and synthesis of suitable catalyst with required composition to achieve the desired selectivity is a very crucial task. For that proper understanding of the catalyst behavior is essential. Each catalytic material has its own characteristics. Behavior of each molecule with that catalytic surface is also going to be completely different. Thus it is highly complex phenomena. Type of promoter, acidity and basicity of the catalysts plays an important role in the reaction mechanism and selectivity of desired product. In this paper various processes based on either oxidation or oxidative dehydrogenation were studied. Compared to dehydrogenation process, oxidative dehydrogenation process offers advantage of complete conversion at low temperatures and high pressures. Substantial improvement in yield of desired product was achieved by optimizing the principal element for the active phase like the presence of molybdenum oxide in FeMo catalyst for Oxidative Dehydrogenation of Methanol to Formaldehyde and Methacrylate from Iso-butylene by using Multi-Component Bismuth Molybdate catalysts. Keywords--Oxidation Catalysts; Oxidative Dehydrogenation Processes; Redox mechanism; Ethyl Benzene to Styrene; Methanol to Formaldehyde; Methyl Methacrylate from IsoButylene I. S INTRODUCTION elective Oxidation of hydro-carbons has been considered to be an important subject for the production of various petro-chemical derivatives. About 50% of the principal chemical products and over 80% of monomers are synthesized by means of at least one stage of selective heterogeneous catalytic oxidation -. The processes that use solid (heterogeneous) catalysts are increasingly replacing the homogeneous type, in order to reduce separation costs and the environmental impact and/or to use new raw materials. Compared with the conventional steam-cracking method of dehydrogenating alkanes to olefins and current catalytic dehydrogenation processes, Oxidative Dehydrogenation could reduce costs, lower greenhouse gas emissions, and save energy. Capital and operational efficiencies are gained by eliminating the need for a furnace and for decoking shutdowns, lowering operating temperatures, lessening material demands, conducting fewer maintenance operations, and using a greater proportion of the alkanes in the olefin conversion process. The processes for oxidative dehydrogenation of alkanes are increasingly more competitive than those for dehydrogenation of alkenes . The action of solid catalysts in oxidation processes had already been noted by the beginning of the Nineteenth century. The first processes to be developed industrially were: oxidation and ammonia oxidation (oxidation in the presence of ammonia) of propylene to produce acrolein and acrylonitrile respectively, oxidation of ethylene to ethylene oxide and the oxidation of aromatics to form anhydrides (maleic and phthalic anhydrides) - A major event in the history of oxidation catalysis was the discovery of bismuth molybdate (Bi+ MoO3) as a selective catalyst for the partial oxidation of propene and also, in a one step operation, for the ammoxidation of propene. The current fourth generation catalysts, containing up to 25 elements, allow yields in excess of 80% to be obtained. The development of new catalysts has brought about a comparable evolution in the type of catalytic reactors used, initially fixed bed, then „bubbling‟ fluid bed and finally „braked‟ fluid bed. There has been an increasingly wider use of alkanes as raw materials, instead of aromatics and alkenes; for example, the synthesis of acrylonitrile from propane instead of propylene and the synthesis of maleic anhydride from n-butane instead of benzene, aimed at reducing costs and/or improving the eco-sustainability of the process. II. DEVELOPMENT OF NEW CLASSES OF CATALYSTS AND PROCESSES Selective catalytic oxidation processes can be divided into three categories. The first relates to oxidation of inorganic molecules (for example, oxidation of ammonia to NO and of H2S by sulphur) . The second class relates to synthesis of basic chemical products (for example, ammonia oxidation of methane by HCN or the partial oxidation of methane by syngas; CO/H2 mixtures). Finally, the third category relates to conversion of hydrocarbons by processing in the liquid phase (principally in the homogeneous phase even if there is a growing interest in the use of heterogeneous catalysts) and processing in the gas phase, which is the most commonly used industrially . INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, „NUiCONE – 2011‟ 2 TABLE 1. PRINCIPAL PROCESSES OF SELECTIVE OXIDATION OF HYDROCARBONS USING SOLID CATALYSTS AND TYPICAL RESULTS OBTAINED (ARPENTINIER ET AL., 2001; CENTI ET AL., 2002 Reagent Principal product Types of catalysts Con. (%) Selectivit y (%) Methanol/air Formaldehyd e Ag on aAl2O3, or FeMo oxides 97-99 91-98 Ethylene/O2/ acetic acid Vinyl acetate Pd-Cu-K on aAl2O3 8-12 92 n-butane/air Maleic anhydride V-P oxides 75-80 67-72 Propylene/ai r /NH3 Acrylonitrile 97-99 75-83 n-butane/air Butenes /butadiene 55-65 93-95 > 97 85-90 97-99 95-98 97-99 81-87 Isobutene/ air Methacrolei n/air o-xylene/air Methacrolein Methacrylic acid Phthalic anhydride Bi-Mo-Fe-CoK supported oxides Bi-Mo-P oxides Bi-Mo-Fe-CoK oxides V-Mo-W oxides Oxides of VP-Cs-Sb on TiO2 TABLE 2. DIFFERENT CLASSES OF GAS PHASE SELECTIVE OXIDATION PROCESSES (ON SOLID CATALYSTS) AND THE RELATIVE INDUSTRIAL REACTIONS (ARPENTINIER ET AL., 2001; CENTI ET AL., 2002) Type of reaction Examples 1) Propylene to Acrolein or Acrylic acid 2) Isobutene to Methacrolein or Methacrylic acid Synthesis of the acids can be carried out in a single Allylic oxidation stage from the alkene, but commercially it is preferred to use two stages for the best possible selectivities 1) Butenes to Butadiene and Isopentenes to Isoprenes Oxidative 2) Methanol to Formaldehyde dehydrogenation 3) Isobutyric acid to Methacrylic acid 4) Ethylbenzene to Styrene 1) Epoxidation of ethylene to Ethylene oxide with Electrophilic O2 insertion of an 2) Direct synthesis of Phenol from Benzene with oxygen atom N2O Epoxidation of ethylene to Ethylene oxide with O2 Synthesis of Vinyl Acetate from Ethylene and Acetic Acetoxylation acid Synthesis of 1,2-dichloroethane from Ethylene and Oxychlorination HCl in the presence of O2 1) Propylene to Acrylonitrile Ammonia 2) Isobutene to Methacrylonitrile oxidation 3) α-Methylstyrene to Atroponitrile Synthesis of anhydrides 1) 2) n-Butane to Maleic anhydride o-Xylene to Phthalic anhydride a) Allylic oxidation: Mixed oxides of transition metals are used as catalyst. These catalysts are capable of selectively extracting hydrogen atom by breaking a C-H bond in the allyl position and if necessary replacing it with an oxygen atom -. These reactions are characterized by a common first stage of allylic oxidation where the extraction of a hydrogen atom in the allyl position gives rise to a chemisorbed p-allylic complex on the transition metal. The nature of the subsequent stages determines the type of reaction and product that is obtained. Oxidation and ammonia oxidation of a side chain of alkyl aromatics in principle follow a similar reaction mechanism, but the interaction of the aromatic ring with the surface is different and therefore different types of catalysts are used, such as vanadium oxides supported on TiO2 or catalysts based on molybdate of Fe-(V, P, K). b) Addition of oxygen to the aromatic nucleus, with ring opening: The electrophilic attack of oxygen on hydrocarbon substrates typically leads to the formation of carbon oxides, however in the case of the oxidation of benzene; selective oxidation to maleic anhydride is obtained . This process, which employs catalysts based on mixed vanadium and molybdenum oxides, has been partially replaced by the synthesis by oxidation of n-butane. Similar catalysts are used in the selective oxidation of polyaromatic compounds. c) Oxidation (or ammonia oxidation) of alkanes: In this case the slow stage is the initial selective activation of the alkane, for example for the concerted extraction of a hydrogen atom by a surface Lewis site (a transition metal) and of a second hydrogen atom by a base site (oxygen atoms) to give an alkene, which is immediately converted into an oxygenated product through oxidation or allylic ammonia oxidation mechanisms . Catalysts with properties which differ from those of catalysts belonging to the Allylic oxidation reaction category are necessary because of the weak interaction of the substrate with the surface, and the activation mechanism. For example, catalysts based on vanadyl pyrophosphate are used for the oxidation of n-butane to maleic anhydride, or those based on vanadium antimonates are used for the ammonia oxidation of propane. Antimony oxide is active in the ammonia oxidation of propylene, but is not able to activate the propane molecule; the addition of V gives the system the capability of oxidizing the alkane. d) Non-classic oxidation mechanisms: Ethyl benzene can be oxidatively dehydrogenated, with high selectivity, to styrene on various catalysts such as oxides and phosphates, but the active phase is constituted by the formation of a thin surface layer of carbon containing the active sites of the reaction . III. MARS-VAN KREVELEN MECHANISM OF SELECTIVE OXIDATION OF HYDROCARBONS ON OXIDE BASED CATALYSTS. The catalysts used for these reactions can be classified on the basis of their characteristic reaction mechanisms. In this mechanism the catalyst undergoes an oxidationreduction cycle during the reaction . In this mechanism a hydro-carbon reactant molecule interacts strongly with the 3 INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD – 382 481, 08-10 DECEMBER, 2011 catalyst at site containing M1n+ during the course of reaction, lattice oxygen is consumed to form products, and the M 1 cation is reduced . The active site and cation M1 are reoxidized by the migration of lattice oxygen from M 2, which in turn is reoxidized by the gas phase oxygen. The M 2 may or may not be distinct from M1. According to Kung the oxidation can be schematically represented as would have existed in the absence of the support . It is clear, therefore, that the support must have surface area characteristics suitable for the reaction of interest. In selective oxidation, where the selectivity in the heavily formation of the partially oxidized product is dependent on the subsequent reactions to undesired products (for example, carbon oxides, which are thermodynamically favoured), a support is needed with a surface area which is not too large. In this way the rate of the undesired secondary reactions, which are also dependent on the time needed by the product to diffuse from the active centre into the gas phase, is limited . In some cases the support serves to alter the characteristics of the intrinsic chemical reactivity of the active phase, through the effects of the interaction between the latter and the support itself. This comes about when the support presents functional groups on its surface which can lead to the formation of chemical bonds with the elements of the active phase, or it takes place as a result of particular crystallographic similarities between the surface and the support. V. Fig.1.Mars-van Krevelen mechanism of selective oxidation of hydrocarbons on oxide based catalysts IV. CHARACTERISTICS OF OXIDATION CATALYSTS Oxidation catalysts belong to a wider class of materials having redox or oxidoreductive type characteristics; systems which catalyse reactions of hydrogenation, dehydrogenation, halogenation and dehalogenation also belong to this class. The most important catalysts in the field of petrochemicals for the oxidation of hydrocarbons for processes carried out in the gas phase. Presence of a transition metal as the principal active component (V, Mo, Cu, Fe, Pd, Pt, Rh, Ag). Often in these cases, a second element is also present which can be transition or post transition (for example, P, Sb or Bi), which contributes to establishing the reactive characteristics of the catalyst . This effect can be explained by the formation of a „mixed oxide‟ (that is, of a specific compound, such as for example Bi2Mo2O9, possibly only on the surface of another oxide, of a solid solution or of an oxide doped with the other element), with reactive characteristics different from those of the single elements, if present in distinct phases. In some cases the element is initially present in a metallic form, but under reaction conditions it can generate the corresponding oxide (or chlorides or oxychlorides). Presence of small quantities of „promoter‟ (or „doping‟) elements can optimize the performance of the principal active elements . The nature of the promoters can vary and they can therefore play different roles in the transformation of the reagents. The active elements and the promoter elements constitute the active phase that is the phase directly involved in the transformation of the reagents into products. Presence of a support (usually silica, alumina or titanium oxide) in the catalyst‟s formulation can fulfil a variety of tasks. A primary task is that of dispersing the active elements, conferring a larger surface area to the active phase compared with what OPTIMIZATION OF THE REDOX CHARACTERISTICS OR OF THE ACIDITY OR BASICITY PROPERTIES OF THE CATALYST. The promoters (or doping agents) can play a fundamental role in the control of these properties. For example, catalysts used for oxidation or allylic ammonia oxidation always contain Mo as the principal element for the active phase, while catalysts for the synthesis of anhydrides or of acids almost always contain V. Promoters with base-type characteristics (alkaline or alkaline earth metal oxides) can reduce the surface acidity of the active phase, with a consequent improvement of selectivity through the suppression of the acid-catalysed reactions (cracking, formation of oligomers of unsaturated compounds) . Promoters with acid-type characteristics can reduce the interaction between the active phase and intermediates of the reaction which have acid-type characteristics, thus favouring their desorption into gas phase and limiting the contribution of the subsequent undesired reactions. VI. REACTION SCHEME The presence of consecutive reactions (typically, combustion reactions of the desired product, or reactions which lead from the reagent to the desired product through the formation of intermediate products with an increasing state of oxidation) involves the use of a catalyst with characteristics such as to limit the contribution of these reactions . This can be achieved not only by control of the intrinsic activity of the catalyst, but also by a modification of the porosity of the active phase. High surface area and porosity values entail effective intra-particle residential times which are much higher than those calculable from the feeding capacity of the reactor, and therefore a significant contribution from the consecutive reactions for a given conversion of the reagent. This can have a considerable influence on the selectivity of the desired product. INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, „NUiCONE – 2011‟ 4 VII. APPLICATIONS 1) Ethyl Benzene to Styrene: Styrene can be produced by two processes: a) Dehydrogenation of Ethyl benzene: b) ∆H° = 84 kJ/mol PRINCIPAL INDUSTRIAL PROCESSES AND RELEVANT ∆H° = 129.4 kJ/mol Oxidative dehydrogenation of Ethyl benzene ∆H° = -126.04 kJ/mol The former process (a), accounts for more than 90% of the worldwide capacity. The catalytic dehydrogenation route, in which the potassium promoted iron oxide catalyst is typically used since 1957, produces most of the St. The process can be run industrially either adiabatically or isothermally over a fixed bed reactor in which the reactants are passed over the catalyst bed employing radial or axial flow . The dehydrogenation reaction of Ethyl benzene is endothermic (∆H = 129.4 kJ/mol) and equilibrium limited. A large amount of super-heated steam is used as the additive to provide heat needed for the reaction, to reduce the partial pressure of ethyl benzene and hydrogen, and to keep the catalyst clean and active. Oxidative dehydrogenation of Ethyl benzene (b) is exothermic reaction and there is no limitation by the equilibrium. This can lower the reaction temperature and feed of super-heated steam is, theoretically not required. Operations nearly under isothermal conditions could aid the yield and conversion . The use of halogen, SO2 and S as oxidants is also possible and good results have been reported. However, high cost of hydrogen acceptors such as halogen and SO2, the corrosion of the apparatus, and the incorporation of hydrogen acceptors in to the products such as sulphides and halides make it difficult to use them in the large scale production. 2) Oxidative dehydrogenation of methanol to formaldehyde: Methanol can be converted into formaldehyde both by direct oxidative dehydrogenation: CH3OH + ½O2 → HCHO + H2O ∆H° = -155 kJ/mol or by dehydrogenation combined with oxidation of the H 2 product: CH3OH → HCHO + H2 H2+ ½O2 → H2O ∆H° = -238 kJ/mol The two processes differ in their operating conditions and type of catalysts. Fig. 2. Schematic of oxide process (Centi et al., 2002) In the first process low concentrations of methanol are used in the feed, in order to avoid the formation of explosive mixtures and to control the temperature of the reaction. Commercial catalysts are based on iron molybdate, but also contain an excess of molybdenum (Fe2 (MoO4)3+MoO3), since the presence of molybdenum oxide is a necessary condition for high selectivity. Typically a ratio of Mo/Fe within the range of 1.5-3.0 is used; occasionally oxides of Co and Cr are added as promoters . Fig. 3.Reaction mechanism of the oxidation of methanol on oxide based catalysts (Centi et al., 2002). The excess of molybdenum is also necessary because the sublimation of the oxide (particularly at the points of greatest overheating) cause the progressive depletion of the Mo in the catalyst and the condensation of MoO3 in the coldest parts of the reactor. This induces, not just the deactivation of the catalyst, but also a progressive increase in pressure loss. Reaction temperatures are typically within the range of 310340°C, with conversions in excess of 98% and selectivity equal to 92-95% at atmospheric pressure. Multi-tubular fixedbed reactors are generally used . A recent development has seen the introduction of a final (postreactor) adiabatic stage. In dehydrogenation combined with partial combustion of H 2 (the overall process turns out to be partially exothermic) under stoichiometric oxygen current is fed in, to operate in the upper region at the limit of flammability -. INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD – 382 481, 08-10 DECEMBER, 2011 5 Due to the thermodynamic limits of dehydrogenation, it is Catalytic dehydrogenation has several draw backs. Thus, necessary to operate at higher reaction temperatures than ODH is very attractive mainly due to the absence of those for oxidative dehydrogenation. Selectivity to thermodynamic limitations (concerning the equilibrium) and formaldehyde of 98-99% is obtained, with the formation of the need for heat transfer, allowing the operation at much the following by-products: dimethylether ((CH 3)2O), whose lower reaction temperatures. An intense research effort has formation is due to the presence of acidic sites in the catalyst; been directed to look for highly efficient catalysts. However, methyl formate (HCOOCH3) obtained through the complexity of the reactions, associated with the large disproportionation of the formaldehyde on basic sites; carbon number of factors that determine their behaviour, has oxides, derived from both parallel and serial reactions. To seriously complicated the task. Moreover, and despite the limit the formation of carbon oxides, rapid cooling of the large number of studies found in the open literature, some reaction products is necessary when they leave the catalyst important aspects, such as the nature of the active sites, the bed. High selectivity is achieved through the optimization of kinetics and mechanism of the reaction, the hydrocarbon the acid-base properties of the catalyst, limitation of the activation process and the factors determine the selectivity, oxidation of the formaldehyde to formic acid (a product are not sufficiently clear. It nevertheless seems evident that which decomposes easily) and control of the redox properties the acid-base surface properties of the mixed oxides are an of the catalyst. important selectivity-determining factor. This has led to the use of promoters, especially alkali or 3) Synthesis of methyl methacrylate: Methyl ester of methacrylic acid, CH2=C (CH3) COOCH3, is alkaline-earth metals for the purpose of modifying the surface used in the production of vinyl polymers. The acetone basicity in the strong favour of products. It is possible that in cyanohydrin process (which consists of a reaction between the near future the combination of both factors, i.e., a highly acetone and HCN, followed by a reaction of acetone efficient and economical reactor configuration with a catalyst cyanohydrin with sulphuric acid and a final hydrolysis of the of excellent performance level, will provide the key for adduct in the presence of methanol) is the most widely used definitive industrial implementation of oxidation processes as for the synthesis of methyl methacrylate and has many an alternative to conventional direct dehydrogenation. disadvantages, linked to the toxicity of HCN and the coformation of high quantities of ammonium sulphate, which is IX. ACKNOWLEDGMENTS generated in the ratio of 2:1 with methyl methacrylate. The This work was supported by Reliance Industries Ltd, alternative process is direct oxidation of isobutene in the gas Vadodara Manufacturing Division (as a part of M.Tech. final phase (with subsequent or integrated stages of esterification); year Project Work). however, this process gives yields and selectivity which are too low to be competitive . X. REFERENCES Alternative methods of synthesis are: a) direct oxidation of  Andrigo P. et al. (1992), “Phenol-acetone process. Cumene oxidation isobutane (a process which is still in the research phase), kinetics and industrial plant simulation”, Chemical Engineering which has the advantage of lower raw material costs and Science, 47, 2511-2516. lower environmental impact; b) oxidation of isobutyric  Benson S.W., Nangia P.S. (1979), “Liquid-phase oxidation of isobutane. 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