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Significance and scope of industrial catalysis. Essence and types of catalysis

In the chemical industry and related industries (petrochemistry, etc.), more than 90% of existing and newly introduced technologies are catalytic processes. With the use of catalysts, tens of thousands of types of inorganic and organic products are produced, including ammonia, nitric and sulfuric acids, methanol, butadiene, styrene, etc., promising methods for the production of motor fuels, wastewater and gas emissions are being carried out.

Most of the catalytic processes can be organized as continuous, waste-free, low-energy. They are distinguished by high technical and economic indicators, provide a high yield of the target product. The use of catalysts makes it possible to intensify chemical-technological processes, to carry out transformations that cannot be implemented in practice without a catalyst due to the very high energy activation, direct the process in the right direction, regulate the structure and properties of manufactured products (for example, stereospecific catalysts in the production of synthetic rubbers and plastics). Of particular importance is the use of catalysts in reversible exothermic processes, in which an increase in temperature to accelerate the reaction sharply reduces the equilibrium degree of conversion and makes the reaction thermodynamically unresolved. In such processes, the role of catalysts is paramount.

Unlike other factors that intensify a chemical process, a catalyst affects only the rate of a chemical reaction and does not affect thermodynamics, only accelerating the achievement of an equilibrium state.

A catalyst is a substance that changes the rate of a chemical reaction and remains unchanged at the end of the reaction. In this case, the catalyst does not accelerate diffusion processes and only affects the rate of processes occurring in the kinetic region.

Catalytic processes are divided into:

  • homogeneous, in which the reactants and the catalyst constitute one phase;
  • heterogeneous, in which the reactants and the catalyst are in different phases;
  • microheterogeneous, flowing in the liquid phase with the participation of catalysts in the colloidal state;
  • enzymatic occurring in biological systems under the influence of enzymes.

In the chemical industry, the most common are heterogeneous catalytic processes, in which the phase boundary is the surface of a solid catalyst in contact with a gaseous or liquid phase.

Chemical reactions on the catalyst surface are a complex process consisting of several successive elementary stages that differ in chemical and physical nature:

  • diffusion of reagents from the flow to the surface of the catalyst grains (external diffusion stage);
  • diffusion of reactant molecules into the pores of the catalyst (stage of internal diffusion);
  • absorption of reactant molecules on the catalyst surface in the form of physical absorption or chemisorption (activated absorption); the stage of chemisorption consists in the formation of an activated complex of a reagent and a catalyst and determines the specificity of the catalyst in catalytic reactions;
  • surface chemical reaction as a result of rearrangement of an activated complex or interaction of molecules of one adsorbed reagent with molecules of another;
  • desorption of the resulting reaction products from the surface of the catalyst;
  • diffusion of products from the pores of the catalyst to its outer surface (reverse internal diffusion);
  • diffusion of products from the catalyst surface into the flow.

In chemical-technological processes, not individual catalytically active substances are used, but contact masses, representing complex systems, the composition and nature of the components of which should ensure the most efficient and stable flow of the catalytic process. The contact mass consists of a catalytically active substance (catalyst), an activator and a carrier.

The nature of heterogeneous catalysts is very diverse and depends on the type of catalyzed reactions. As catalysts, mainly metals in the free state (platinum, silver, copper, iron) and metal oxides (zinc, chromium, aluminum, molybdenum, vanadium) are used. In those cases when two reactions catalyzed by different substances occur simultaneously in the system, bifunctional catalysts are used, consisting of two corresponding components (for example, zinc oxide and aluminum oxide in the process of dehydration and dehydrogenation of ethanol to butadiene).

Activator (promoter)- called a substance introduced into the contact mass to increase the activity of the catalyst and increase its duration. Activators have a selective effect, so their nature depends on the nature of the catalyst.

Carrier (tribrach)- is called the material on which the catalyst is applied in order to increase its surface, give the mass a porous structure, increase its mechanical strength and reduce the cost of the contact mass. Pumice, asbestos, silica gel, diatomaceous earth, porous ceramics are used as carriers in contact masses.

Contact masses are produced by methods:

  • precipitation of hydroxides and carbonates from salt solutions with subsequent formation and calcination;
  • joint pressing of a mixture of components with a binder;
  • fusion of components;
  • impregnation of the porous carrier with solutions of the catalyst and activator.

Contact masses are molded in the form of granules, tablets or elements of various configurations. Metal catalysts are manufactured and used in the form of fine meshes.

2. Technological characteristics of solid catalysts

The efficiency of using catalysts in industrial heterogeneous catalytic processes essentially depends on their technological characteristics. These include: activity, ignition temperature, selectivity of action, resistance to poisons, porosity, mechanical strength, thermal conductivity, availability and low cost.

1. Catalyst activity (A)- a measure of its accelerating effect in relation to a given chemical reaction. It is defined as the ratio of the rate constants of catalytic and non-catalytic reactions:

For those cases where the catalytic and non-catalytic reactions are of the same order and, therefore, the pre-exponential coefficients in the Arrhenius equation for them are equal (а 1 =а 2), the catalyst activity is determined from (1) as:

By lowering the activation energy of the reaction, the catalyst accelerates it by many orders of magnitude. So, for example, for the reaction:

the activity of the vanadium catalyst used in it, that is, the reaction rate increases hundreds of billions of times.

In most cases, the catalyst also reduces the order of the reaction, and the more so, the higher its activity. So, for example, if the order of the above reaction without a catalyst is 3, then in the presence of a vanadium catalyst it is only 1.8.

2. Catalyst ignition temperature T 3- the minimum temperature at which the process begins to proceed at a rate sufficient for technological purposes. The higher the activity of the catalyst, the lower its ignition temperature, that is:

At a low ignition temperature, the operating interval between T 3 and the process temperature regime is extended, the reactor design is simplified, the heat consumption for heating the reagents is reduced, and the technological regime is stabilized. For exothermic catalytic reactions at a certain value of T 3 the rate of heat release becomes equal to the rate of heat removal (heat consumption for heating the reaction mixture and heat removal with the reaction products). In this case, T 3 represents the minimum temperature at which the process is autothermal.

3. Selectivity (selectivity) catalyst - its ability to selectively accelerate one of the reactions, if several reactions are thermodynamically possible in the system. For a complex parallel reaction proceeding according to the scheme:

and including the reactions A→B and A→C, which are characterized, respectively, by the rate constants k 1 and k 2 and the activation energies E 1 and E 2 , the selectivity in the direction A→B is defined as:

It follows from it that at a given temperature T, it is possible, by selecting a catalyst, to change the difference E 2 - E 1 and, therefore, direct the process towards the formation of the target product.

The selectivity of the catalyst is of great importance in such chemical and technological processes as the oxidation of ammonia in the production of nitric acid, various processes of organic synthesis. Using catalysts, it becomes possible to obtain various target products from common raw materials, for example:

The porosity of the catalyst characterizes its specific surface area and, therefore, affects the contact surface of the catalyst with the reagents. For catalytic processes, the availability of the surface of a solid catalyst for reactants is of great importance, since the larger the contact surface, the higher the rate of their conversion into target products per unit time on the same catalyst.

The porosity of the catalyst is expressed as the ratio of the free volume of pores to the total volume of the catalyst and is characterized by its specific surface, that is, the surface per unit mass or volume of the catalyst. Modern catalysts have a highly developed specific surface, reaching 10–100 m 2 /g.

5. Mechanical strength The contact mass must be such that it does not collapse under the action of its own weight in apparatuses with a fixed catalyst bed and does not wear out in apparatuses with a moving catalyst bed and “KS” apparatuses.

6. Resistance to contact poisons. The practical use of heterogeneous catalytic processes is hampered by the phenomenon of a decrease in catalyst activity during the process. The reasons for this are:

  • decrease in the active surface of the catalyst when dust or reaction products are deposited on it;
  • mechanical destruction of the catalyst;
  • catalyst poisoning with catalytic (contact) poisons

Catalyst poisoning- partial or complete loss of its activity under the influence of a small amount of certain substances - contact poisons. Contact poisons form surface chemical compounds with the activated sites of the catalyst and block them, reducing the activity of the catalyst. For each group of catalysts, there are certain types of contact poisons.

Catalyst poisoning can be reversible, when contact poisons reduce the activity of the catalyst temporarily while they are in the zone of catalysis, and irreversible when the activity of the catalyst is not restored after the removal of contact poisons from the catalysis zone. Contact poisons can be contained in the reagents supplied to the catalytic process, and also formed as by-products in the process itself. Resistance to contact poisons is the most important property industrial catalysts. To extend the service life of contact masses in chemical-technological processes, a stage of thorough purification of reagents from harmful impurities and a catalyst regeneration operation are provided (for example, burning a high-carbon polymer film enveloping catalyst grains in the processes of catalytic cracking, oil products, isomerization and dehydrogenation of organic compounds).

The rapid industrial growth that we are now experiencing would not have been possible without the development of new chemical technologies. To a large extent, this progress is determined by the widespread use of catalysts, with the help of which low-grade raw materials are converted into high-value products. Figuratively speaking, catalyst- this is the philosopher's stone of the modern alchemist, only he does not turn lead into gold, but raw materials into medicines, plastics, chemicals, fuel, fertilizers and other useful products. Perhaps, the very first catalytic process that man has learned to use is fermentation. Recipes for the preparation of alcoholic beverages were known to the Sumerians as early as 3500 BC. See WINE; BEER.

A milestone in the practical application of catalysis became margarine production catalytic hydrogenation of vegetable oil. For the first time, this reaction on an industrial scale was carried out around 1900. And since the 1920s, catalytic methods for obtaining new organic materials especially plastics. The key point was the catalytic production of olefins, nitriles, esters, acids, etc. - "bricks" for the chemical "building" of plastics. The third wave of industrial use of catalytic processes belongs to the 1930s and associated with oil refining. In terms of volume, this production soon left all others far behind. Oil refining consists of several catalytic processes:

cracking,

reforming,

Hydrosulfonation,

Hydrocracking,

isomerization,

Polymerization

Alkylation.

And finally fourth wave in the use of catalysis related to environmental protection. The most famous achievement in this area is creation of a catalytic converter for vehicle exhaust gases. Catalytic converters, which have been installed in cars since 1975, have played a big role in improving air quality and have saved many lives in this way.

About a dozen Nobel Prizes have been awarded for work in the field of catalysis and related fields. The practical significance of catalytic processes is evidenced by the fact that the share nitrogen, which is part of the nitrogen-containing compounds obtained industrially, accounts for about half of all nitrogen that is part of food products. The amount of nitrogen compounds produced naturally is limited, so that the production of dietary protein depends on the amount of nitrogen applied to the soil with fertilizers. It would be impossible to feed even half of humanity without synthetic ammonia, which is produced almost exclusively by catalytic Haber-Bosch process. The scope of catalysts is constantly expanding. It is also important that catalysis can significantly increase the efficiency of previously developed technologies. An example is the improvement of catalytic cracking through the use of zeolites.



Hydrogenation. A large number of catalytic reactions are associated with the activation of a hydrogen atom and some other molecule, leading to their chemical interaction. This process is called hydrogenation and underlies many stages of oil refining and the production of liquid fuels from coal ( Bergius process). The production of aviation gasoline and motor fuel from coal was developed in Germany during World War II, since there are no oil fields in this country. The Bergius process is the direct addition of hydrogen to carbon. Coal is heated under pressure in the presence of hydrogen and a liquid product is obtained, which is then processed into aviation gasoline and motor fuel. Iron oxide is used as a catalyst, as well as catalysts based on tin and molybdenum. During the war, approximately 1,400 tons of liquid fuel per day were obtained at 12 German factories using the Bergius process. Another process, Fischer–Tropsch, consists of two stages. First, the coal is gasified, i.e. carry out its reaction with water vapor and oxygen and get a mixture of hydrogen and carbon oxides. This mixture is converted into liquid fuel using catalysts containing iron or cobalt. With the end of the war, the production of synthetic fuel from coal in Germany was discontinued. As a result of the rise in oil prices that followed the oil embargo of 1973–1974, vigorous efforts were made to develop an economically viable method for producing gasoline from coal. Thus, direct liquefaction of coal can be carried out more efficiently using a two-stage process in which the coal is first contacted with an alumina-cobalt-molybdenum catalyst at a relatively low and then at a higher temperature. The cost of such synthetic gasoline is higher than that obtained from oil.

Ammonia. One of the simplest hydrogenation processes from a chemical point of view is the synthesis of ammonia from hydrogen and nitrogen. Nitrogen is a very inert substance. To break the N–N bond in its molecule, an energy of the order of 200 kcal/mol is required. However, nitrogen binds to the surface of the iron catalyst in the atomic state, and this requires only 20 kcal/mol. Hydrogen bonds with iron even more readily. The synthesis of ammonia proceeds as follows:

This example illustrates the ability of a catalyst to speed up both the forward and reverse reactions equally, i.e. the fact that the catalyst does not change the equilibrium position of the chemical reaction.

Hydrogenation of vegetable oil. One of the most important hydrogenation reactions in practice is the incomplete hydrogenation of vegetable oils to margarine, cooking oil, and other food products. Vegetable oils are obtained from soybeans, cotton seeds and other crops. They include esters, namely triglycerides of fatty acids with varying degrees of unsaturation. Oleic acid CH 3 (CH 2) 7 CH \u003d CH (CH 2) 7 COOH has one double bond C \u003d C, linoleic acid has two and linolenic acid has three. The addition of hydrogen to break this bond prevents the oils from oxidizing (rancidity). This raises their melting point. The hardness of most of the products obtained depends on the degree of hydrogenation. Hydrogenation is carried out in the presence of a finely dispersed nickel powder deposited on a substrate or nickel Raney catalyst in a highly purified hydrogen atmosphere.

Dehydrogenation. Dehydrogenation is also an industrially important catalytic reaction, although the scale of its application is incomparably smaller. With its help, for example, styrene, an important monomer, is obtained. To do this, dehydrogenate ethylbenzene in the presence of a catalyst containing iron oxide; potassium and some structural stabilizer also contribute to the reaction. On an industrial scale, propane, butane and other alkanes are dehydrogenated. Dehydrogenation of butane in the presence of an alumina-chromium catalyst produces butenes and butadiene.

acid catalysis. The catalytic activity of a large class of catalysts is due to their acidic properties. According to I. Bronsted and T. Lowry An acid is a compound that can donate a proton. Strong acids easily donate their protons to bases. The concept of acidity was further developed in the works G. Lewis, who defined an acid as a substance capable of accepting an electron pair from a donor substance with the formation of a covalent bond due to the socialization of this electron pair.

These ideas, together with ideas about reactions with the formation of carbenium ions, helped to understand mechanism of various catalytic reactions, especially those involving hydrocarbons. The strength of an acid can be determined using a set of bases that change color when a proton is added. It turns out that some industrially important catalysts behave like very strong acids. These include the catalyst Friedel-Crafts process, such as HCl–AlCl 2 O 3 (or HAlCl 4), and aluminosilicates. Acid strength is a very important characteristic, since it determines the rate of protonation, a key step in the process of acid catalysis. The activity of catalysts such as aluminosilicates used in oil cracking is determined by the presence of Bronsted and Lewis acids on their surface. Their structure is similar to the structure of silica (silicon dioxide), in which some of the Si 4+ atoms are replaced by Al 3+ atoms. The excess negative charge that arises in this case can be neutralized by the corresponding cations. If the cations are protons, then the aluminosilicate behaves like Bronsted acid:

Activity of acid catalysts conditioned their ability to react with hydrocarbons to form a carbenium ion as an intermediate. Alkylcarbenium ions contain a positively charged carbon atom bonded to three alkyl groups and/or hydrogen atoms. They play an important role as intermediates formed in many reactions involving organic compounds. The mechanism of action of acid catalysts can be illustrated by the example of the isomerization reaction of n-butane to isobutane in the presence of HCl–AlCl 3 or Pt–Cl–Al 2 O 3 . First, a small amount of C 4 H 8 olefin attaches the positively charged hydrogen ion of the acid catalyst to form a tertiary carbenium ion. Then the negatively charged hydride ion H - is split off from n-butane with the formation of isobutane and secondary butylcarbenium ion. The latter, as a result of the rearrangement, turns into a tertiary carbenium ion. This chain can continue with the elimination of the hydride ion from the next n-butane molecule, etc.:

Significantly, tertiary carbenium ions are more stable than primary or secondary ones. As a result, they are mainly present on the catalyst surface, and therefore the main product of butane isomerization is isobutane. Acid catalysts are widely used in oil refining - cracking, alkylation, polymerization and isomerization of hydrocarbons (see also CHEMISTRY AND METHODS OF OIL REFINING).

Installed mechanism of action of carbenium ions playing the role of catalysts in these processes. At the same time, they participate in a number of reactions, including the formation of small molecules by splitting large ones, the combination of molecules (olefin with olefin or olefin with isoparaffin), structural rearrangement by isomerization, the formation of paraffins and aromatic hydrocarbons by hydrogen transfer. One of the latest industrial applications of acid catalysis is the production of leaded fuels by the addition of alcohols to isobutylene or isoamylene. The addition of oxygenated compounds to gasoline reduces the concentration of carbon monoxide in the exhaust gases. Methyl tertiary butyl ether (MTBE) with a blending octane rating of 109 also makes it possible to obtain the high-octane fuel needed to run a high-compression automobile engine without resorting to the introduction of tetraethyl lead into gasoline. The production of fuels with octane numbers 102 and 111 is also organized.

main catalysis. Catalyst activity conditioned their main properties. An old and well-known example of such catalysts is sodium hydroxide, used for the hydrolysis or saponification of fats in the production of soap, and one of recent examples– catalysts used in the production of polyurethane plastics and foams. Urethane is formed by the interaction of alcohol with isocyanate, and this reaction is accelerated in the presence of basic amines. During the reaction, the base is attached to the carbon atom in the isocyanate molecule, as a result of which a negative charge appears on the nitrogen atom and its activity with respect to alcohol increases. A particularly effective catalyst is triethylenediamine. Polyurethane plastics are obtained by reacting diisocyanates with polyols (polyalcohols). When the isocyanate reacts with water, the previously formed urethane decomposes to release CO 2 . When a mixture of polyalcohols and water reacts with diisocyanates, the resulting polyurethane foam foams with gaseous CO 2 .

Dual action catalysts. These catalysts speed up two types of reactions and give better results than passing the reactants in series through two reactors each containing only one type of catalyst. This is due to the fact that the active sites of the double acting catalyst are very close to each other, and the intermediate product formed on one of them immediately turns into the final product on the other. A good result is achieved by combining a hydrogen activating catalyst with a hydrocarbon isomerization promoting catalyst. Hydrogen activation carry out some metals, and the isomerization of hydrocarbons - acids. An effective dual-acting catalyst that is used in oil refining to convert naphtha to gasoline is finely dispersed platinum deposited on acid alumina. Conversion of naphtha components such as methylcyclopentane (ICP), into benzene increases the octane number of gasoline. First ICP dehydrogenates on the platinum part of the catalyst to an olefin with the same carbon backbone; then the olefin passes to the acid part of the catalyst, where it isomerizes to cyclohexene. The latter passes to the platinum part and dehydrogenates to benzene and hydrogen. Dual action catalysts significantly accelerate oil reforming. They are used to isomerize normal paraffins to isoparaffins. The latter, boiling at the same temperatures as gasoline fractions, are valuable because they have a higher octane number compared to straight hydrocarbons. In addition, the conversion of n-butane to isobutane is accompanied by dehydrogenation, contributing to the production of MTBE.

Stereospecific polymerization. An important milestone in the history of catalysis was the discovery of the catalytic polymerization of a-olefins with the formation of stereoregular polymers. Stereospecific polymerization catalysts were discovered by K. Ziegler when he tried to explain the unusual properties of the polymers he obtained. Another chemist, J. Natta, suggested that the uniqueness of Ziegler polymers is determined by their stereoregularity. X-ray diffraction experiments have shown that polymers prepared from propylene in the presence of Ziegler catalysts are highly crystalline and indeed have a stereoregular structure. Natta introduced the terms "isotactic" and "syndiotactic" to describe such ordered structures. In the case where there is no order, the term "atactic" is used:

Stereospecific reaction occurs on the surface solid catalysts containing transition metals of groups IVA-VIII (such as Ti, V, Cr, Zr) in an incompletely oxidized state, and any compound containing carbon or hydrogen, which is associated with a metal from groups I-III. A classic example of such a catalyst is the precipitate formed during the interaction of TiCl 4 and Al(C 2 H 5) 3 in heptane, where titanium is reduced to the trivalent state. This extremely active system catalyzes the polymerization of propylene at normal temperature and pressure.

catalytic oxidation. The use of catalysts to control the chemistry of oxidation processes is of great scientific and practical importance. In some cases, oxidation must be complete, for example, when neutralizing CO and hydrocarbon contaminants in car exhaust gases. However, more often it is necessary that the oxidation be incomplete, for example, in many processes widely used in industry for the conversion of hydrocarbons into valuable intermediate products containing such functional groups as -CHO, -COOH, -C-CO, -CN. In this case, both homogeneous and heterogeneous catalysts are used. An example of a homogeneous catalyst is a transition metal complex that is used to oxidize para-xylene to terephthalic acid, the esters of which are the basis for the production of polyester fibers.

Catalysts for heterogeneous oxidation. These catalysts are usually complex solid oxides. Catalytic oxidation takes place in two stages. First, the oxide oxygen is captured by a hydrocarbon molecule adsorbed on the oxide surface. The hydrocarbon is oxidized and the oxide is reduced. The reduced oxide reacts with oxygen and returns to its original state. Using a vanadium catalyst, phthalic anhydride is obtained by partial oxidation of naphthalene or butane.

Ethylene production by methane dehydrodimerization. The synthesis of ethylene through dehydrodimerization allows natural gas to be converted into more easily transportable hydrocarbons. reaction

2CH 4 + 2O 2 → C 2 H 4 + 2H 2 O

carried out at 850 °C using various catalysts; the best results are obtained with Li-MgO catalyst. Presumably, the reaction proceeds through the formation of a methyl radical by splitting off a hydrogen atom from a methane molecule. Cleavage is carried out by incompletely reduced oxygen, for example, O 2 2–. The methyl radicals in the gas phase recombine to form an ethane molecule and are converted to ethylene during subsequent dehydrogenation. Another example of incomplete oxidation is the conversion of methanol to formaldehyde in the presence of a silver or iron-molybdenum catalyst.

Zeolites. Zeolites make up a special class of heterogeneous catalysts. These are aluminosilicates with an ordered honeycomb structure, the cell size of which is comparable to the size of many organic molecules. They are also called molecular sieves. Of greatest interest are zeolites, the pores of which are formed by rings consisting of 8–12 oxygen ions (Fig. 2). Sometimes the pores overlap, as in the ZSM-5 zeolite (Fig. 3), which is used for the highly specific conversion of methanol to hydrocarbons in the gasoline fraction. Gasoline contains significant amounts of aromatic hydrocarbons and therefore has a high octane number. In New Zealand, for example, one third of all gasoline consumed is obtained using this technology. Methanol is obtained from imported methane.

Picture 2 - The structure of zeolites with large and small pores.

Picture 3 - Zeolite ZSM-5. Schematic representation of the structure in the form of intersecting tubes.

Catalysts that make up the group of Y-zeolites significantly increase the efficiency of catalytic cracking due primarily to their unusual acidic properties. Replacing aluminosilicates with zeolites makes it possible to increase the yield of gasoline by more than 20%. In addition, zeolites are selective with respect to the size of the reacting molecules. Their selectivity is due to the size of the pores through which molecules of only certain sizes and shapes can pass. This applies to both starting materials and reaction products. For example, due to steric constraints, para-xylene is formed more easily than the bulkier ortho and meta isomers. The latter are "locked" in the pores of the zeolite (Fig. 4).

Figure 4 - Scheme explaining the selectivity of zeolites in relation to reagents (a) and products (b).

The use of zeolites has made a real revolution in some industrial technologies - dewaxing gas oil and engine oil, obtaining chemical intermediates for the production of plastics by aromatic alkylation, xylene isomerization, toluene disproportionation and catalytic cracking of oil. Zeolite ZSM-5 is especially effective here.

Dewaxing of petroleum products- extraction of paraffin and ceresin from petroleum products (diesel fuels, oils), as a result of which their quality improves, in particular, the pour point decreases.

Paraffin(German Paraffin, from lat. Parum - little and affinis - related), a mixture of saturated hydrocarbons C 18 -C 35, predominantly. normal structure with a mol. m. 300-400; colorless crystals with t pl. \u003d 45–65 o C, density 0.880–0.915 g / cm 3 (15 o C).

Ceresin(from lat. cera - wax), a mixture of solid hydrocarbons (mainly alkylcyclanes and alkanes), obtained after purification of ozocerite. By density, color (from white to brown), melting point (65-88 ° C) and viscosity, ceresin is similar to wax.

Catalysts and environmental protection. The use of catalysts to reduce air pollution began in the late 1940s. In 1952, A. Hagen-Smith found that hydrocarbons and nitrogen oxides, which are part of exhaust gases, react to light to form oxidants (in particular, ozone), which irritate the eyes and give other undesirable effects. Around the same time, Y. Houdry developed a method for the catalytic purification of exhaust gases by oxidizing CO and hydrocarbons to CO 2 and H 2 O. In 1970, the Clean Air Declaration (revised in 1977, expanded in 1990) was formulated, according to which all new cars , starting with 1975 models, must be equipped with exhaust gas catalytic converters. Norms have been established for the composition of exhaust gases. Since lead compounds added to gasoline poison catalysts, a phase-out program has been adopted. Attention was also drawn to the need to reduce the content of nitrogen oxides. Catalysts have been created specifically for automotive converters, in which active components are deposited on a ceramic substrate with a honeycomb structure, through the cells of which exhaust gases pass. The substrate is covered with a thin layer of metal oxide, such as Al2O3, on which a catalyst is applied - platinum, palladium or rhodium. The content of nitrogen oxides formed during the combustion of natural fuels at thermal power plants can be reduced by adding small amounts of ammonia to the flue gases and passing them through a titanium-vanadium catalyst.

Enzymes. Enzymes are natural catalysts that regulate biochemical processes in a living cell. They participate in the processes of energy exchange, the breakdown of nutrients, biosynthesis reactions. Many complex organic reactions cannot proceed without them. Enzymes function at ordinary temperature and pressure, have very high selectivity and are able to increase the rate of reactions by eight orders of magnitude. Despite these advantages, only about 20 of the 15,000 known enzymes are used on a large scale. Man has been using enzymes for thousands of years to bake bread, produce alcoholic beverages, cheese and vinegar. Now enzymes are also used in industry: in the processing of sugar, in the production of synthetic antibiotics, amino acids and proteins. Proteolytic enzymes that accelerate hydrolysis processes are added to detergents. With the help of Clostridium acetobutylicum bacteria, H. Weizmann carried out the enzymatic conversion of starch into acetone and butyl alcohol. This method of obtaining acetone was widely used in England during the First World War, and during the Second World War, butadiene rubber was made with its help in the USSR. An exceptionally large role was played by the use of enzymes produced by microorganisms for the synthesis of penicillin, as well as streptomycin and vitamin B12. Enzymatically produced ethyl alcohol is widely used as an automotive fuel. In Brazil, more than a third of the approximately 10 million cars run on 96% ethyl alcohol derived from sugar cane, and the rest on a mixture of gasoline and ethyl alcohol (20%). The technology for the production of fuel, which is a mixture of gasoline and alcohol, is well developed in the United States. In 1987, about 4 billion liters of alcohol were obtained from corn kernels, of which approximately 3.2 billion liters were used as fuel. Various applications are also found in the so-called. immobilized enzymes. These enzymes are bound to a solid carrier, such as silica gel, over which the reagents are passed. The advantage of this method is that it ensures efficient contact of the substrates with the enzyme, separation of products and preservation of the enzyme. One example of the industrial use of immobilized enzymes is the isomerization of D-glucose to fructose.

Literature

1. Gates B.K. Chemistry of catalytic processes. M., 1981

2. Boreskov G.K. Catalysis. Questions of theory and practice. Novosibirsk, 1987

3. Gankin V.Yu., Gankin Yu.V. New general theory of catalysis. L., 1991

4. Tokabe K. Catalysts and catalytic processes. M., 1993

5. Collier's Encyclopedia. – Open society. 2000.

The noted features of the phenomena of catalysis, namely, that the catalyst accelerates chemical transformations without consuming energy and practically without consuming the catalyst substance itself, make catalysis an extremely valuable means of carrying out chemical transformations in industry.

Let us dwell briefly only on some catalytic industrial processes. Although the phenomena of catalysis are very widespread in nature and man had to deal with them a long time ago, the widespread use of catalysis in industry began only in the current century.

Catalytic processes are used to produce hydrogen, which serves as a raw material for the synthesis of ammonia and a number of other chemical technology industries. The cheapest source of hydrogen is natural gas. The first stage of hydrogen production includes the interaction of methane with water vapor with the partial addition of oxygen or air at a temperature of 1130-1270 K. Nickel deposited on heat-resistant carriers is used as a catalyst.

As a result of this reaction, along with hydrogen, a significant amount of carbon monoxide is formed. By reacting carbon monoxide with water vapor at lower temperatures, using oxide catalysts, it is possible to oxidize CO to CO2, and hydrogen is formed. Until recently, Fe3O4 with additions of chromium oxide was used as a catalyst for this reaction. Such a catalyst is sufficiently active only at temperatures above 670 K. At this temperature, the equilibrium of the reaction

even with an excess of water vapor, it is significantly shifted to the left and the final reaction mixture contains a significant amount of carbon monoxide, the presence of which sharply reduces the activity of ammonia synthesis catalysts. To remove residual CO, it was necessary to use complex washing of the gas mixture with an ammonia solution of Cu2O under pressure.

Per last years was and found new catalysts for the interaction of carbon monoxide with water vapor, containing copper in the form of various spinels or other oxide compounds. Such catalysts provide a high rate of carbon monoxide conversion already at a temperature of 450-560 K. Due to this, the final content of carbon monoxide can be reduced to tenths of a percent, and the complex washing operation is replaced by a simpler process of converting the remaining carbon monoxide into harmless for catalysts, including including and in the synthesis of ammonia, methane:

This process is also carried out catalytically using nickel catalysts. Thus, the development of a more active catalyst made it possible to significantly simplify the technological scheme.

Another example is the catalytic processes of oil refining. In the 1920s, oil refining was limited to rectification and decomposition when heated to high temperatures, the so-called thermal cracking, without the use of catalysts. It was not until the late 1930s that the first attempts were made to use catalytic processes for oil refining.

The inventor of catalytic cracking, the French engineer Goodry, in a report at the II International Congress on Catalysis, drew attention to the fact that in the American Petroleum Institute review of the state and prospects of the oil refining industry, published in 1935, the word "catalysis" was never mentioned, but through a few years catalytic methods have caused a fundamental transformation of this industry. The efficiency of catalysis turned out to be so significant that in a few years a real technological revolution, which made it possible, based on the use of catalysts, to sharply increase both the yield and the quality of the resulting motor fuels,

Currently, over 80% of oil is processed using catalytic cracking, reforming, hydrogenolysis of sulfur compounds, hydrocracking and other catalytic processes. In table. 2.1 shows the most important modern catalytic processes in oil refining.

Catalytic cracking was previously carried out at temperatures of 670-770 K using synthetic and natural aluminosilicon, silicon-magnesium, aluminum-silicon-zirconium and other acidic catalysts. In recent years, catalysts based on crystalline synthetic zeolites have received wide industrial use. The activity of these catalysts, especially those containing oxides of rare earth elements, is significantly higher than that of amorphous aluminosilicate catalysts.

The use of catalysts makes it possible not only to increase the rate of formation of hydrocarbons of lower molecular weight, but also to increase the yield of valuable fractions compared to thermal cracking.

As a result of the formation of coke-like deposits, the activity of the catalysts rapidly decreases during cracking, but can be completely restored by roasting in an oxygen-containing environment.

The cracking in a fluidized bed of a finely dispersed catalyst has proven to be particularly effective, allowing the catalyst to be easily circulated through the reactor and regenerator.

Catalytic cracking is the most high-tonnage industrial catalytic process. It currently processes over 300 million tons of oil per year, which requires an annual consumption of about 300 thousand tons of catalysts.

Somewhat later, in the 1950s, catalytic reforming began to be widely used in the oil refining industry. Previously, this process was carried out at a temperature of 740-790 K and a pressure of 1.5-4 MPa, using as a catalyst mainly platinum supported on aluminum oxide, treated with hydrogen chloride to increase the acid properties. Currently, the process is carried out at 0.8-1.5 MPa due to the use of new polymetallic catalysts.

During the reforming process, the reactions of dehydrogenation of naphthenes to aromatic hydrocarbons, cyclization of paraffins and olefins, and isomerization of five-membered cyclic hydrocarbons to six-membered ones take place.

Currently, catalytic reforming is used to process more than 200 million tons of oil per year. Its use allowed not only to improve the quality of motor fuel, but also to produce significant amounts of aromatic hydrocarbons for the needs of the chemical industry.

Hydrogen is a valuable by-product of catalytic reforming. The appearance of cheap hydrogen made it possible to widely use the catalytic hydrotreatment of petroleum products containing sulfur, with its release in the form of H2S. Various hydrogenation catalysts can be used for this purpose. The most widely used catalysts are prepared from oxides of cobalt and molybdenum deposited on aluminum oxide. In addition, promising catalysts are the same catalytic compositions, but with the addition of zeolites.

The process conditions depend on the properties of the raw material to be purified, but most often lie within 600–680 K and 3–5 MPa. About 300 million tons of oil products are hydrotreated annually. This process makes it possible to obtain significant amounts of sulfur, facilitates subsequent catalytic oil refining processes, and also reduces atmospheric pollution by exhaust gases during the combustion of motor fuel.

Recently, the hydrocracking process has received significant development, in which cracking, isomerization and hydrotreatment reactions are simultaneously carried out. The use of catalysts makes it possible to carry out this process at 520–740 K, at a pressure of about 5–15 MPa, and to obtain a significant yield of diesel fuel with a high cetane number. As catalysts, tungsten sulfide, mixed tungsten nickel sulfide catalysts on carriers, cobalt-molybdenum catalysts on alumina, with additions of Ni, Pt, Pd and other metals on amorphous or crystalline zeolites are used.

To improve the quality of gasoline, catalytic isomerization processes using platinum and palladium catalysts on various carriers are used.

From the foregoing, we can conclude that catalytic methods currently occupy a leading position in oil refining. Thanks to catalysis, the value of products obtained from oil has been increased several times. Note that this trend continues today. In connection with the rise in oil prices, it becomes extremely important to make the most of all its components. It must be assumed that the growth in the cost of oil will continue, as it will gradually have to move to such sources of oil that present great difficulties for exploitation. Therefore, it is extremely important to increase the degree of extraction of valuable products from oil, which can be achieved by a wider use of advanced catalysts.

It must be admitted that the depth of oil refining is still small, this is due not so much to technical difficulties as to the balance of petroleum products, the bulk of which is boiler fuel. Economically, at least in the long term, it is unprofitable. It is necessary to sharply increase the share of secondary catalytic processes in oil refining. The need for liquid boiler fuel should be compensated by the use of coal.

A more promising possibility of catalytic methods in oil refining is the rejection of the global transformation inherent in modern processes of all complex compounds found in oils. Thus, all sulfur compounds undergo hydrogenolysis with the release of hydrogen sulfide. Meanwhile, many of them are of considerable independent value. The same is true for nitrogen-containing, metal-complex, and many other compounds. It would be very important to isolate these substances or subject them to individual catalytic transformations to obtain valuable products. An example is the production of sulfur-containing extractants such as sulfoxides and sulfones, which are formed during the catalytic oxidation of sulfur compounds contained in oils and boiler fuel. Undoubtedly, this way catalysis will significantly increase the efficiency of oil refining.

The field of application of catalysis continues to expand rapidly, and researchers face new important tasks. In connection with a sharp increase in the cost of oil, extensive developments are underway to obtain liquid fuel from coal. Based on the old catalytic methods used in Germany during the Second World War (Fig. 2.1). New in the hydrogenation method is the extraction of the organic matter of the coal to obtain a heavy oil, which is further subjected to catalytic hydrogenation under pressure. Very promising are also methods for the synthesis of liquid fuel from a gas consisting of carbon monoxide and hydrogen obtained by gasification of coal with water vapor. The Fischer Tropsch method is currently used in South Africa. Its disadvantage is that the resulting gasoline consists mainly of normal paraffins, therefore has a low octane number and requires recycling. The diesel fraction, with good fuel characteristics, has a high pour point, which excludes its use in our country.

More interesting is the route for the synthesis of hydrocarbons, originally proposed by the American company Mobil, through the formation of methanol and its subsequent decomposition on a catalyst containing ultra-high silica zeolite. The synthesis of methanol is carried out at a pressure of 5-10 MPa on an oxide copper-containing catalyst. Dehydration of methanol does not require elevated pressure and proceeds through dimethyl ether with the formation of olefins. Olefins on the same catalyst as a result of the redistribution of hydrogen form a mixture of isoparaffins and aromatic hydrocarbons. The output of the gasoline fraction can be increased to 60-70% with an octane number of 90-95. The diesel fraction under these conditions is about 10% and has good qualities in terms of cetane number and pour point.

It is also possible to obtain hydrocarbons from synthesis gas, bypassing the stage of methanol extraction. The degree of conversion of synthesis gas into methanol is limited by the reversibility of the reaction and in modern plants does not exceed 4% per cycle. The use of polyfunctional catalysts that carry out both the synthesis of methanol and its conversion into hydrocarbons makes it possible to significantly increase the conversion per cycle and significantly simplify the process. This method positively differs from the classical Fischer-Tropsch process by the quality of the resulting gasoline and very low methane formation, but unlike the Mobil process, it requires an increased pressure of 3-5 MPa during its implementation.

The described methods for obtaining hydrocarbons both through methanol and directly using polyfunctional catalysts can be used to produce liquid fuels from natural gas. It is advisable to create such production facilities near large gas fields to facilitate the transport of fuel, since pipelines for moving liquid fuel are much cheaper than for moving gas. In addition, they are useful for providing liquid fuels to many remote areas with gas, to which transport of liquid fuels is difficult.

In the coming years, the use of solid catalysts for fuel combustion will undoubtedly become widespread. Currently, fuel is burned mainly in flare furnaces at a temperature of 1470-1870 K with a low heat efficiency. A method is proposed for burning fuel in a catalytic reactor in a fluidized catalyst bed with simultaneous heat removal for the required purposes. Due to the presence of a catalyst, fuel combustion is carried out quite completely without excess air at a sufficiently low temperature - 670-970 K. The thermal stress of the reaction volume is much higher than in flare furnaces, which allows several times to reduce the size and weight of the installations. The reduced combustion temperature eliminates the formation of harmful nitric oxide. On the basis of catalytic heat generators, small-sized steam boilers, apparatus for heating water, evaporating oil fractions in oil refining processes, for heat treatment, dispersion and activation of solid materials, drying of powder materials, for adsorption-contact drying of grain, agricultural products and materials, can be created. sensitive to overheating, and for other purposes.

A wide area of ​​application of catalysis is the neutralization of emissions from industry and transport. The problem of catalytic combustion of carbon monoxide and most organic compounds in gas emissions from industrial enterprises has already been reliably solved. The problem of catalytic reduction of nitrogen oxides, including selective reduction with ammonia in mixtures containing oxygen, has been fundamentally solved.

The task of neutralizing vehicle exhaust gases is much more difficult due to the difference in the conditions necessary for the reduction of nitrogen oxides and the complete oxidation of organic compounds and carbon monoxide. Significant difficulties are created by the variability of the composition of exhaust gases, which depends on the operating conditions of vehicles. However, catalytic cleaners have been developed that allow almost complete purification of exhaust gases from carbon monoxide and organic compounds and significantly reduce the concentration of nitrogen oxides.

An even more difficult task is catalytic wastewater treatment. Recently, some success has been achieved in the purification of wastewater from some industries from phenols, sulfur compounds and other harmful components by using complexes of certain transition metals as catalysts, as well as complex catalysts fixed on carriers.

Catalytic methods will also receive significant development in solving the food problem. In addition to the production of fertilizers, catalysis will play a significant role in the production of essential amino acids for improving animal feed, herbicides, insecticides and other drugs needed for crop production. Catalysis is the most important method of chemical transformations in industry. Currently, about 80% of all chemical products are produced by catalytic means. This share is growing rapidly as the complexity of chemical transformations mastered by industry. Among new industries, the share of catalytic processes exceeds 90%. The progress of the chemical and other branches of industry largely depends on the development of catalysis. The implementation of many thermodynamically possible and economically profitable processes, obtaining new products, implementing more advanced technological schemes, and using available raw materials are all promising tasks for finding new and improving existing catalysts.

The examples mentioned cover a very small proportion of catalytic processes used in industry. However, some general conclusions clearly follow from them.

1. Catalysis makes it possible to intensify chemical transformations, including those reactions that do not proceed at a noticeable rate without a catalyst.

Catalysts allow you to direct a chemical transformation towards the formation of a specific, desired product from a number of possible ones.

In reactions leading to the formation of high-molecular-weight products, by varying the properties of catalysts, it is possible to control the structure of the resulting substance and, due to this, the properties of the final materials.

Catalysis is a specific phenomenon. There are no substances that would have catalytic properties in a general form. Each reaction must use its own specific catalyst.

Catalysis is one of the most dynamically and rapidly developing areas of science and technology. New catalytic systems are constantly being developed and existing ones are being improved, new catalytic processes are being proposed, their instrumentation is changing, and new physicochemical methods for studying catalysts are being improved and appear. Most of the chemical processes involved in the enterprises of the petrochemical and oil refining complex are catalytic. The development of catalysis and catalytic technologies largely determine the competitiveness of petrochemical products in the market. Therefore, there is an acute issue of the need to train highly qualified specialists in the field of catalysis for petrochemistry.

Catalysis is a specific phenomenon. There are no substances that would have catalytic properties in a general form. Each reaction must use its own specific catalyst.

Application of catalysis in the chemical industry. Catalytic processes are used to produce hydrogen, which serves as a raw material for the synthesis of ammonia and a number of other chemical technology industries. Methane conversion. The cheapest source of hydrogen is natural gas. The first stage of hydrogen production includes the interaction of methane with water vapor with the partial addition of oxygen or air at a temperature of 800–1000°C (reaction 2.1). Nickel supported on heat-resistant alumina carriers (corundum - a-Al 2 O 3) is used as a catalyst.

CH 4 + H 2 O ⇄ 3H 2 + CO (2.1)

CO + H 2 O ⇄ CO 2 + H 2 (2.2)

As a result of this reaction, along with hydrogen, carbon monoxide is formed in a significant amount.

CO conversion. The interaction of carbon monoxide with water vapor is carried out in two stages at decreasing temperature using oxide catalysts (reaction 2.2), while hydrogen is additionally formed. At the first stage, a medium-temperature (435-475°C) iron-chromium catalyst (Fe 3 O 4 with Cr 2 O 3 additives) was used; on the second, a low-temperature (230-280°C) catalyst (a mixture of oxides of aluminium, copper, chromium and zinc). The final content of carbon monoxide, the presence of which sharply reduces the activity of iron catalysts for the synthesis of ammonia, can be reduced to tenths of a percent.

To remove residual CO, it was necessary to apply a complex washing of the gas mixture with an ammonia solution of Cu 2 O at a high pressure of 120-320 atm and a low temperature of 5-20°C.

In practice industrial production purification of gas emissions from CO is carried out by absorption with solutions of Cu-ammonia salts (copper formates and carbonates), which have the ability to form complex compounds with CO. Since formates are not very stable, preference is given to carbonate solutions.

The initial carbonate-ammonia complex of copper has the following composition (kmol / m 3): Cu + - 1.0 - 1.4; Cu 2+ - 0.08 - 0.12; NH 3 - 4.0 - 6.0; CO 2 - 2.4 - 2.6.

The absorption capacity with respect to CO is possessed by monovalent copper salts. Cu 2+ cations, as a rule, do not take part in absorption. However, the concentration of Cu 2+ must be maintained in the solution at least 10 wt. % of Cu + content. The latter helps to prevent the formation of elemental copper deposits, which can clog the pipelines and disrupt the operation of the absorber. The presence of copper carbonate-ammonia complex Cu 2+ in solution shifts the equilibrium of reaction (1) towards the formation of Cu + : Cu 2+ + Cu ⇄ 2 Cu + (1)

The solution of the carbonate-ammonia copper complex used for CO absorption contains 2 CO 3 ; CO 3 ; (NH 4) 2 CO 3; free NH 3 and CO 2 .

The process of absorption of CO by the carbonate-ammonia complex of copper proceeds according to the reaction: + + CO + NH 3 ⇄ + - DH (2)

Simultaneously with CO, CO2 is also absorbed according to the equation:

2 NH 3 + H 2 O + CO 2 ⇄ (NH 4) 2 CO 3 - DH 1 (3)

Metanization. In connection with the development of a new active nickel catalyst, the complex washing operation can be replaced at 250–350°C by a simpler process of converting the carbon monoxide residue into methane, which is inert for the ammonia synthesis catalyst (reaction 2.3):

CO + 3H 2 ⇄ CH 4 + H 2 O (2.3)

Thus, the development of a more active catalyst made it possible to significantly simplify the technological scheme and increase the efficiency of ammonia production.

Application of catalysis in the oil refining industry. The efficiency of the use of catalysis turned out to be so significant that in a few years a genuine technical revolution took place in the oil refining industry, which made it possible to dramatically increase both the yield and the quality of motor fuels obtained based on the use of catalysts.

Currently, over 80% of oil is processed using catalytic processes: cracking, reforming, isomerization and hydrogenation of hydrocarbons, hydrotreatment of oil fractions from sulfur-containing compounds, hydrocracking. Table 2.1 lists the most important modern catalytic processes in oil refining.

Cracking. Catalytic cracking of oil or its fractions is a destructive process carried out at temperatures of 490-540°C on synthetic and natural aluminosilicate catalysts of an acidic nature to produce high-quality gasoline with an octane rating of 98-92, a significant amount of gases containing saturated and unsaturated hydrocarbons C 3 - C 4 , kerosene-gas oil fractions, carbon black and coke.

Octane number (O.ch.) - a conditional indicator of the detonation resistance of light (gasoline, kerosene) motor fuels during combustion in carburetor engines. The reference fuel is isooctane (O.p. = 100), normal heptane (O.p. = 0). The octane number of gasoline is the percentage (by volume) of isooctane in such a mixture of it with n-heptane, which, under standard test conditions on a special single-cylinder engine, detonates in the same way as the tested gasoline.

In recent years, catalysts based on crystalline synthetic zeolites have received wide industrial use. The activity of these catalysts, especially those containing a mixture of oxides of rare earth elements (СеО 2 , La 2 O 3 , Ho 2 O 3 , Dy 2 O 3 and others), is much higher than that of amorphous aluminosilicate catalysts.

The use of catalysts made it possible not only to increase the rate of formation of hydrocarbons of lower molecular weight from naphthenes by 500-4000 times, but also to increase the yield of valuable fractions compared to thermal cracking.

Catalytic cracking is the most high-tonnage industrial catalytic process. It currently processes over 300 million tons of oil per year, which requires an annual consumption of about 300 thousand tons of catalysts.

Reforming. Catalytic reforming is carried out at a temperature of 470-520°C and a pressure of 0.8-1.5 MPa on Pt, Re-catalysts supported on aluminum oxide, treated with hydrogen chloride to increase the acid properties. Reforming is a method of processing petroleum products, mainly gasoline and naphtha fractions of oil (hydrocarbons C 6 -C 9 of three main classes: paraffin, naphthenic and aromatic) in order to obtain high-octane motor gasoline, aromatic hydrocarbons (benzene, toluene, xylene, ethylbenzene) and technical hydrogen. During the reforming process, the reactions of dehydrogenation of naphthenes to aromatic hydrocarbons, cyclization of paraffins and olefins, and isomerization of five-membered cyclic hydrocarbons to six-membered ones take place. Currently, catalytic reforming is used to process more than 200 million tons of oil per year. Its use allowed not only to improve the quality of motor fuel, but also to produce significant amounts of aromatic hydrocarbons for the chemical industry. By-products of catalytic reforming are fuel gas, consisting mainly of methane and ethane, as well as liquefied gas - propane-butane fraction

Hydrotreating of petroleum products. Hydrogen is a valuable by-product of catalytic reforming. The appearance of cheap hydrogen made it possible to widely use the catalytic hydrotreatment of petroleum products from sulfur-, nitrogen- and oxygen-containing compounds, with the formation of easily removable H 2 S, NH 3 and H 2 O, respectively (reactions 2.4 - 2.7):

CS 2 + 4H 2 ⇄ 2H 2 S + CH 4 (2.4)

RSH + H 2 ⇄ H 2 S + RH (2.5)

COS + 4H 2 ⇄ H 2 S + CH 4 + H 2 O (2.6)

RNH + 3/2H 2 ⇄ NH 3 + RH (2.7)

At the same time, hydrogenation of the dienes occurs, which increases the stability of the product. For this purpose, catalysts prepared from oxides of cobalt (2–5 wt.%) and molybdenum (10–19 wt.%) or oxides of nickel and molybdenum deposited on γ-aluminum oxide are most widely used.

Hydrotreating makes it possible to obtain up to 250-300 thousand tons of elemental sulfur per year. To do this, implement the Claus process:

2H 2 S + 3O 2 ⇄ 2SO 2 + 2H 2 O (2.8)

2H 2 S + SO 2 ⇄ 3S + 2H 2 O (2.9)

Part of H 2 S is oxidized by atmospheric oxygen to γ-Al 2 O 3 at 200-250°C (reaction 2.8); the other part of H 2 S reacts with sulfur dioxide to form sulfur (reaction 2.9).

The conditions for hydrotreatment depend on the properties of the raw material to be purified, but most often lie in the range of 330-410°C and 3-5 MPa. About 300 million tons of oil products (gasoline and kerosene fractions, diesel fuel, vacuum distillates, paraffins and oils) are hydrotreated annually. The implementation of the hydrotreatment stage in oil refining has made it possible to prepare raw materials for catalytic reforming (gasolines) and cracking (vacuum distillates), to obtain low-sulphur lighting kerosene and fuel, to improve the quality of products (paraffins and oils), and also has a significant environmental effect, since atmospheric pollution by exhaust gases is reduced. gases from the combustion of motor fuels. The introduction of hydrotreatment made it possible to use high-sulfur oils to obtain petroleum products.

Hydrocracking. Recently, the hydrocracking process has received significant development, in which cracking, isomerization and hydrotreatment reactions are simultaneously carried out. Hydrocracking is a catalytic process of deep conversion of raw materials of various fractional composition in the presence of hydrogen in order to obtain light petroleum products: gasoline, jet and diesel fuel, liquefied gases C 3 -C 4 . The use of polyfunctional catalysts makes it possible to carry out this process at 400–450°C and a pressure of about 5–15 MPa. Tungsten sulfide, mixed tungsten-nickel sulfide catalysts on carriers, cobalt-molybdenum catalysts on alumina, with additions of Ni, Pt, Pd and other metals on amorphous or crystalline zeolites are used as catalysts.

Table 2.1 - Modern catalytic processes of oil refining

Isomerization. To improve the quality of gasoline, add 10-15 wt.% isomerizate with a high octane number. The isomerizate is a mixture of saturated aliphatic (there are no cycles in the molecules) hydrocarbons of isostructure (more than 65 wt.% 2-methylbutane; isohexanes) obtained by isomerization of alkanes (normal saturated paraffins). The raw material for isomerization is a light gasoline fraction of direct distillation of oil, boiling in the range of 62-85°C and containing mainly pentane and hexane, as well as a fraction (75-150°C) obtained by catalytic cracking. The processes of catalytic isomerization proceed in the presence of bifunctional catalysts: platinum or palladium on various acidic carriers (γ-Al 2 O 3 , zeolite) promoted by halogen (Cl, F). Isomerization is the transformation of organic substances into compounds of a different structure (structural isomerism) or with a different arrangement of atoms or groups in space (spatial isomerism) without changing the composition and molecular weight.

Thus, catalytic processes occupy a leading position in oil refining. Thanks to catalysis, the value of products obtained from oil has been increased several times.

A more promising possibility of catalytic methods in oil refining is the rejection of the global transformation inherent in modern processes of all complex compounds found in oils. Thus, all sulfur compounds undergo hydrogenolysis with the release of hydrogen sulfide. Meanwhile, many of them are of considerable independent value. The same is true for nitrogen-containing, metal-complex, and many other compounds. It would be very important to isolate these substances or subject them to individual catalytic transformations to obtain valuable products. An example is the production of sulfur-containing extractants such as sulfoxides and sulfones, which are formed during the catalytic oxidation of sulfur compounds contained in oil and boiler fuel. Undoubtedly, this way catalysis will significantly increase the efficiency of oil refining.

HETEROGENEOUS CATALYSIS

Lecture 2

Over the past decades, the chemical and oil refining industries have grown significantly and have a strong impact on the economies of countries. For example, oil refining is one of the most important sectors of the world economy: annually about 3.5 billion tons of oil worth more than 500 billion US dollars are processed in the world, and the cost of refined products is trillions of dollars. The key basis of the modern chemical and oil refining industry is catalytic technologies and processes.

Development of new catalysts and sorbents, renewal of their range, expansion of their use in different areas determine the technical level and progress of the country's economy. Catalysts and sorbents are high science-intensive products of cross-industry application, because areas of their use include the chemical, petrochemical, oil refining, food, light, metallurgical industries, affecting the environmental aspects of all types of industries.

At present, more than 15% of GDP (gross domestic product) is produced with the use of catalysts in Russia, in the USA this share is more than 50%. In the cost of products, which are created using catalysts, their share is no more than 0.5 - 1.0%, however, it is catalysts that largely determine the final cost and quality of products. Their decisive importance, first of all, affects the resource saving and energy efficiency of large-tonnage industrial basic productions, and the transition from one generation of catalysts to more highly efficient catalytic systems provides a significant reduction in the amount of by-products formed by several times. The total volume of the catalyst market in the world is about 15-20 billion US dollars, with the use of catalysts in different industries, products worth more than 2 trillion dollars are produced. USA. Leading companies are constantly developing new and improving existing catalysts. On average, the range of catalysts on the world market is updated by 15-20%, and the range of foreign catalysts is several times greater than the domestic list.

The use of catalysts is the most effective and economical way to solve environmental problems and rational use of resources by deepening their processing and involving unused by-products.

Share of catalytic processes in strategic important industries industry - production of motor fuels, monomers (ethylene, propylene, butenes, butadiene and others), aromatic compounds - reaches 90%, while the use of domestic catalysts does not exceed 50%. The production of high-quality products that meet international operational and environmental requirements is highly dependent on secondary processes, in which the role of catalysts and their properties are paramount.


The most large-scale production of the chemical and oil refining industries are the production of sulfuric and nitric acid, ammonia, methanol, catalytic cracking, reforming, hydrotreating and others. The catalysts for these processes differ in chemical composition, method of preparation and use, and duration of action.

Thus, for example, cracking catalysts operate under the conditions of a moving bed, moving from the reactor to the regenerator, while severe deformation of the catalyst particles occurs, and it is carried away through the cyclones of the reactor and regenerator. On average, from 250 to 400 tons of catalyst are loaded into the catalytic cracking system at a time, depending on the type of installation, and irretrievable losses range from 0.2 to 1.0 kg per 1 ton of hydrocarbon feedstock. In total, one load of the cracking unit is from 500 to 1000 tons of fresh catalyst.

Hydrotreating catalysts (aluminum-cobalt-molybdenum, aluminum-nickel-molybdenum and other modified catalysts) and reforming (aluminum-platinum and platinum-based polymetallic catalysts) are very different from cracking catalysts (zeolite-containing catalysts). These are long-term catalysts, they are loaded into reactors once every 5-10 years, but the cost of these catalysts is 3-4 times higher.

In this regard, the catalysts are presented with the most high requirements:

They must have high activity in the transformation of the feedstock,

They must be highly selective in the formation of target products, at least 85-90%, because at lower selectivities, the yield of by-products increases greatly, which reduces the profitability of the process,

They must be stable and have a sufficiently high regeneration run and service life.