Catalysts

substances that change the rate of chemical reactions by repeated intermediate chemical interaction with the reactants but are not part of the composition of the end products. Catalysts are universally distributed in living nature and are widely used in industry. More than 70 percent of all chemical

transformations of substances (more than 90 percent in new productions) are realized with the aid of catalysts. The various catalysts produced by industry are classified according to (1) the type of catalyzed reaction (acid-base, oxidation-reduction), (2) the groups of catalytic processes or the specific features of their industrial use (for example, catalysts for the synthesis of ammonia and cracking of petroleum products, catalysts used in fluidized beds), (3) the nature of the active center (metallic, oxide, sulfide, organometallic, complex), and (4) the method of preparation. Some types of catalysts used in industry are given in Table 1. Protein catalysts, that is, enzymes, assist in effecting metabolic processes in all living organisms.

The most important property of catalysts is specificity of action; that is, each chemical reaction or group of uniform reactions can be accelerated only by very specific catalysts. Catalyst specificity is most clearly manifested in that catalysts can determine the course of the reaction, that is, various products are formed from the same original substances depending on the type of catalyst used. For example, from a mixture of carbon monoxide and hydrogen, using different catalysts it is possible to obtain methane, a mixture of liquid hydrocarbons, high-molecular-weight solid hydrocarbons, mixtures of oxygen-containing compounds of various composition, and methyl or isobutyl alcohols. Selectivity serves as the measure of specificity of catalysts and is estimated by the the ratio of the specific reaction rate to the general transformation rate of the initial substances in the presence of a given catalyst. Catalytic activity is another important indicator of catalytic properties exhibited by substances. It is expressed as the difference in the rates of one particular reaction, measured both in the presence and in the absence of a catalyst, other conditions being equal. Catalytic activity is related to the unit of weight, volume, concentration, or surface of the catalyst. Activity relative to 1 m² of catalyst surface is known as specific catalytic activity. If without a catalyst the reaction practically does not occur, then the measure of activity is taken to be the reaction rate under specific conditions, relative to the unit quantity of the given catalyst. Owing to the specificity of catalysts, it is possible to compare the catalytic activity of substances with respect to only one particular reaction. In applied research, catalyst activity is often expressed as conversion, that is, the quantity of product (or reacted substance) obtained in a unit of time per unit volume of catalyst, and as selectivity, that is, the yield of the specific product in relation to the theoretically possible product.

In addition to activity and selectivity, another operational characteristic of catalysts is stability, which often determines the advisability of using catalysts for one or another industrial process. Industrial catalysts change in the course of time; their activity and selectivity decrease as a result of various side processes—for example, as a result of (1) interaction with impurities during the introduction of the raw material (“poisoning”), (2) the sintering and recrystallization of the catalyst at elevated temperatures or by the effect of the reaction medium (aging), (3) the precipitation of tarry matter and coke on the surface of the catalyst, and (4) the adsorptive reduction of strength (Roebinder effect). Therefore, after a certain period of time catalysts are subjected to special treatment (regeneration) or, if possible, are replaced by new ones. The life of industrial catalysts during continuous processes in equipment with a fixed catalyst bed averages six to 36 months. The most stable catalysts can operate continuously for more than ten years (for example, vanadium catalysts for SO₂ oxidation). Catalysts with a service life of less than one to two months are generally not used in fixed-bed reactors. In the case of such catalysts and those that operate in short cycles with frequent regeneration (for example, aluminosilicate catalysts for cracking, catalysts for hydrocarbon dehydrogenation), it is sometimes advisable to use moving-bed reactors, in particular, reactors with fluidized catalyst beds.

Specific chemical compounds or their mixtures serve as catalysts in homogeneous catalytic processes. In this case, the catalytic properties of the catalysts are wholly determined by their chemical composition and structure. Heterogeneous catalytic processes using solid catalysts in the form of porous grains with a highly developed inner surface are primarily used in industry. The catalytic properties of solid catalysts are dependent on the size of their inner surface and the porous structure as well as on composition and structure. The essential stages of catalytic processes with solid catalysts are the transfer of reactants, products, and heat between the reaction mixture and the external surface of the catalyst grains (external transfer) and the transfer of substances and heat within the porous catalyst grains (internal transfer). The influence of internal mass transfer by diffusion is most frequent in the use of industrial catalysts. When this rate is insufficient, the efficiency of the catalyst decreases and the overall intensity of the process drops. Furthermore, this can lead to a reduction in the yield of unstable intermediate products capable of further transformation on the surface of the catalysts, which in many cases are specific (for example, during partial oxidation of hydrocarbons). The rate of transfer by diffusion within the grains of the catalyst is determined by its porous structure. If the reactants are in the gaseous phase, it is advisable to use catalysts with a maximum developed inner surface and pores measuring approximately 1 X 10 ^-7 m in diameter for slow reactions to ensure the necessary rate of molecular counterdiffu-sion of the reactants and products. For reactions proceeding at an average rate (2–10 kmol/hr per 1 m3 of catalyst), the optimum diameter of pores in the case of a uniform porous structure corresponds to the mean free path of molecules, equaling 1 X 10 ^-7 m at atmospheric pressure and decreasing with increasing pressure. The branched nonuniform porous grain structure is most beneficial in many cases; here fine pores, creating a large inner surface, adjoin the large transfer pores. At atmospheric pressure the transfer from grains of uniform porous structure to those with a branched nonuniform porous structure increases the activity of the unit catalyst volume by a factor of 3–9. Increased understanding of the effect of a porous structure on the activity and selectivity of catalysts, the elaboration of methods of investigating specific catalytic activity and porous structure, and the use of computers for the mathematical modeling of complex processes have created the prerequisites for the transition from empirical to scientifically based methods of developing industrial catalysts.

Various methods are used in the preparation of catalysts: precipitation from solutions, impregnation, mixing (for example, mixed catalysts), and fusion with subsequent washing out of the inactive part (skeletal catalysts). Many catalysts undergo special treatment prior to use, for example, activation, during which the active substance (for example, a metal in a highly dispersed state as a result of the reduction of oxides) is formed and the porous structure is created. The active substance (such as platinum) is supported on the surface of the carrier to stabilize its highly dispersed state or to minimize expense. Different substances that exhibit stability under process conditions, for example, aluminum oxide, silica gel, synthetic and natural silicates, or activated carbon, act as carriers. Carriers can affect catalytic properties and therefore must be selected with care for use with industrial catalysts.

A tendency to pass from single-component catalysts of simple composition to complex multicomponent and polyfunctional catalysts has been observed. The latter reveal surface areas differing according to the nature of the catalytic action. A number of successive chemical transformations are effected on polyfunctional catalysts in one apparatus during a single passage of the reaction mixture. Very often, particularly when the intermediate substance is unstable, a higher yield of the desired product is obtained as compared to separate individual processes in the presence of monofunctional catalysts. Some examples of polyfunctional catalysts are the Lebedev catalyst for the preparation of divinyl from ethyl alcohol and the aluminoplatinum catalyst for the manufacture of high-octane gasoline. Promoted catalysts are being used more frequently and for various purposes. Their activity is significantly improved by the addition of substances (promoters) which, when taken separately, may not exhibit catalytic properties.

Every industrial process requires a specific catalyst with an optimum combination of properties. Therefore a large number of different catalysts are produced, differing in chemical composition, porous structure, and granule size and shape.

World catalyst production totals 500, 000–800, 000 tons per year; approximately 250 basic types of catalysts are manufactured, each type having several varieties. Particular differences are found between catalysts designed for one specific purpose and produced in different countries or by different firms, especially between catalysts used in new processes. A concentration of catalyst production is observed everywhere. Large catalyst plants and workshops are being constructed, which makes it possible to improve the quality of production and to mechanize and automate production; catalysts previously prepared for use within an enterprise have now been put on sale on both the domestic and international markets.