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In which industries is an industrial catalyst used. Significance and scope of industrial catalysis. Essence and types of catalysis. "Industrial production of catalysts"

Federal Agency for Education

State educational institution

higher professional education

"TOMSK POLYTECHNICAL UNIVERSITY"

Faculty of Chemical Technology

Processes and apparatus of chemical production

Department of Chemical Technology of Fuel

« industrial production catalysts"

Performer: Afanasyeva Yu.I.

Supervisor:

Assistant Chekantsev N.V.

Tomsk - 2010

Catalyst production

The quality of catalysts determines the main indicators of chemical industries using contact masses: product yield, process intensity, duration of continuous operation of reactors. At the same time, the cost of the catalyst, as a rule, is only a fraction of a percent in the cost of the target product. Therefore, their defining characteristic is activity and stability in work. When studying and developing catalyst technology, it is necessary to consider all successive stages of production from the point of view of their influence on the activity and stability of catalysts in operation.

Key points

The production of catalysts includes the following main stages:

Obtaining the initial solid material: in the manufacture of A120s, a gel isolated from a solution, for example, A1(OH)3; in the manufacture of oxide and metal catalysts - acid salts (nitrates, carbonates, acetates, etc.).

Isolation of the catalyst as an independent bulk phase. Excess substances are removed from the source material.

Changes in the composition of the catalyst upon interaction with reagents and under the influence of reaction conditions.

The initial raw materials are usually salts of catalytically active metals, sols, oxides, and natural minerals. The choice of raw materials is determined by the composition of the catalyst, the content of impurities and its price.

Requirements for raw materials: the constancy of chemical and phase compositions, the absence of harmful impurities, the required particle size, the desired humidity, possibly lower cost.

Methods for shaping catalysts and carriers: droplet coagulation, extrusion, tableting, paste spreading, granulating, drying in a spray dryer, material grinding. Forming material by coagulation in a drop and drying in a spray dryer is widely used in the manufacture of precipitated catalysts. Most universal methods are paste extrusion and tableting.

During extrusion, the wet sludge is squeezed out in the form of a cord from a continuously operating screw or hydraulic press. The shape and transverse size of the granules are determined by holes in the forming head of the press. At the exit from the head, the cord of the contact mass is cut with a rotating knife or a stretched string, and the resulting cylinders are picked up by a belt conveyor.

Tableting is carried out under pressure up to 30 MPa, granules are obtained in the form of cylinders, rings, saddles, stars, etc. Talc, graphite, liquid glass, some organic acids and other substances.

Smearing the paste into the holes of the perforated steel plate is possible to granulate sediments of different nature and consistency. The size of the resulting granules is determined by the thickness of the plate and the diameter of the holes. After drying, the granules are knocked out of the plate with a special stamp or squeezed out with compressed air.

Grinding of monolithic catalysts is carried out on crushers and the fraction is separated on vibrating screens or in drum separators. In this case, the particles have an irregular shape, it is observed a large number of waste in the form of fines and dust, but the range of grain sizes obtained can be very wide.

Granulation is mainly used in the manufacture of contact masses by mechanical mixing of the components.

Forming methods affect the specific surface and porous structure of the contact masses, determine the mechanical strength of the granules, making it possible to obtain both very durable materials(during coagulation in a drop, drying in a spray dryer), and low-strength (during tableting, extrusion and grinding).

The method of preparation determines the degree of dispersion of the catalytic component, the shape, porous structure and activity of the contact mass.

A given type of porous structure and specific surface area is obtained by various methods, depending on the nature of the catalyst being produced. In precipitated contact masses, this largely depends on the conditions of precipitation (pH of the medium, concentration of initial solutions, temperature, precipitation rate, precipitation maturation time), washing, and heat treatment. Catalysts obtained by impregnating the active components of a porous carrier retain mainly its secondary structure. With dry mixing of components, porosity is largely determined by the method of molding, the degree of grinding of the initial charge, and the addition of special substances.

The mechanical strength of the granules is achieved by the molding method, heat treatment conditions. The increase in strength is promoted by sintering of primary crystallites, cementation of particles under the influence of special additives, and the use of wear-resistant carriers.

Catalysts intended for operation in a fluidized bed are obtained mainly by depositing active components on strong carriers or by fusing the starting components. Of the deposited contact masses, aluminosilicates, alumogels, and silica gels are most suitable for use under weighing conditions, during the preparation of which the gel is coagulated into strong, smooth spherical granules.

Precipitated contact masses

Approximately 80% of catalysts and carriers are obtained by co-precipitation. It allows varying the porous structure and inner surface within a wide range. The disadvantage is a significant consumption of reagents, a large number Wastewater. Depending on the nature of the precipitate, the contact masses are divided into salt, acid and oxide.

An example of oxide catalysts are iron oxide contacts with various promoters used in the conversion of carbon monoxide with steam. Various silica gels, alumogels, aluminosilicates used for cracking, hydration, dehydration, alkylation and isomerization of hydrocarbons are acid catalysts. In the process of their preparation, when the corresponding solutions are drained, silicic or aluminosilicic acid, aluminum hydroxide precipitates. The formation of salt catalysts is accompanied by salt precipitation, due to which the composition may change in subsequent technological operations.

General technological scheme

Figure 1 shows the production of precipitated contact masses. The specified order of preparation in each case may vary, individual operations may be combined or absent.

Figure 1 - Preparation of catalysts for dry (A) and wet (B) molding methods

Dissolution. In the production of precipitated catalysts, practically pure solid compounds (most often salts in water) are dissolved, the transfer of which into a solution makes it possible to accelerate the subsequent chemical reactions.

Oxide catalysts are obtained from concentrated solutions of the corresponding salts (nitrates, acetates, oxalates, etc.). To prepare solutions of the starting materials, either ready-made crystalline salts are used, or the corresponding oxides, hydroxides, carbonates are dissolved in acids or alkalis.

When the salt interacts with water, hydration occurs, leading to the formation of a hydrated cation, which is further hydrolyzed according to scheme (1):

(1)

The hydrolysis products polymerize into complexes - [Me(OH)m]np+, where n depends on the hydrolysis conditions and the nature of the metal and can vary over a wide range. The depth of hydrolysis of the starting materials affects physical properties precipitation and catalyst properties.

Stirring allows even distribution of solid particles in liquid phase and accelerate dissolution. As the temperature rises, the mass transfer and solubility in water increase, the dissolution rate increases (chlorides, nitrates, ammonium salts). More rapid dissolution is facilitated by grinding the feedstock.

The dissolution is carried out in reactors with stirring, in countercurrent mixers with mechanical movement of the solid material towards the solvent flow.

Precipitation. The process of formation of a solid phase during the pouring of solutions of the initial components. The transition of the dissolved substance to the precipitate occurs through the formation of nuclei of the solid phase and the growth of crystals or the enlargement of gel-like particles with their simultaneous precipitation. Catalytically active forms are thermodynamically unstable states of matter. Crystallization accelerates with decreasing temperature.

The number of nuclei (crystallization centers) n is related to the degree of supersaturation by relation (2):

(2)

where A is the coefficient of proportionality; C is the concentration of the solution, Cp is the concentration of the saturated solution.

The greater the C/Cp supersaturation, the more crystallization centers are formed and the finer and more active the precipitate is. To increase the number of nuclei, concentrated stock solutions should be used. An increase in the temperature and pH of the medium, as well as an increase in the ionic strength of the solution, contribute to a decrease in n. The rate of nucleation also depends on the mechanical action on the solution (mixing, exposure to electric and magnetic fields, etc.).

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 wide application catalysts that turn low-grade raw materials 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 significant milestone in practical application 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 security environment . 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 the total 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 the Second World War, since this country does not have oil fields. 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 received further development in 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- this is a very important characteristic, since the rate of protonation, a key stage in the process of acid catalysis, depends on it. 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 to hydrolyze or saponify fats in the production of soap, and one recent example is the 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 a partially 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–. 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 was formulated (revised in 1977, expanded in 1990), 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 specially 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 associated with 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 the 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.

Most of the reaction processes in the chemical industry proceed with the use of catalysts.

Catalysts can be individual solid, liquid, gaseous substances, as well as their mixtures.

Catalysis is divided into two classes:

1. Homogeneous

2. Heterogeneous

In the first case, the catalyst and the reactants are in the same phase (gas or liquid), in the second case they are in different phases, most often the catalyst is in the solid phase.

Consider the general methods of using catalysts in the chemical industry. Basic requirements for catalysts:

ensure contact of the reagents with the largest surface of the catalyst;

· find optimal conditions for long-term operation of the catalyst (optimal temperature, impossibility of its poisoning or surface contamination, and so on);

· create a convenient catalyst regeneration system.

If all three conditions are met, the catalyst works for decades without a change.

The use of solid catalysts can go according to the following options:

1. the catalyst is placed in the reactor, and there it lies motionless on the grids;

2. the catalyst moves along with the shelves;

3. the catalyst is in the reactor in a finely divided state (almost dust) in a suspended bed (fluidized bed, fluidized bed).

In the first case, when the catalyst is immobile, there are disadvantages: a clean change of the working cycle and regeneration of the catalyst, difficulties in supplying and removing heat in the reaction zone, and complexity in the design of reactors.

In the second case, two devices are used: a contact (reactor) and a regenerator. The catalyst is continuously moved by means of mechanical devices through the contact apparatus, passes from it to the regenerator and returns again to the contact apparatus. In this case, the design of the apparatus is simplified, and the regulation of the process is facilitated.

In the third case (the best one), fine catalyst particles in the reactor are in suspension, which is maintained by the flow of reaction gases. The whole mixture takes on the properties of a boiling liquid, which is why such a process was formerly called a fluid process, and is now called a fluidized or fluidized bed process. Small catalyst particles move along with gases through the reactor from bottom to top. The advantages of this method:

· high developed surface of the catalyst, which is washed by the gas from all sides evenly;

the process is continuous, since the catalyst is easily removed from the reaction sphere, regenerated and returned to the cycle again;

perfect mixing of the reacting gases with each other and with the catalyst is achieved;

· Heat transfer improves sharply, the catalyst quickly exchanges heat with gases, which ensures their uniform heating.

High molecular weight compounds (HMC)

Synthesis of compounds with high molecular weight, i.e. macromolecular compounds is currently one of the main trends in modern chemical science and industry. The great importance of these compounds in our life is well known, which makes it possible to obtain such products as rubber and rubber, plastics, artificial fibers, resins, varnishes and paints, and special oils. In the development of methods for the study and production of IUDs, an outstanding role belongs to Russian chemists. Back in 1859, A.M. Butlerov obtained a formaldehyde polymer, and in 1873 he studied the polymerization of isobutylene. With these works, Butlerov, in the words of academician Arbuzov, “opened the door to the field of macromolecular compounds.” IUDs are obtained by two main methods:

1. Method of polymerization and copolymerization

2. Polycondensation method

polymerization method

Polymerization refers to the reaction of joining together a large number of molecules of the same substance or different substances into one large molecule. The reaction of combining molecules of various substances is called copolymerization. In the very general view the polymerization reaction equation can be represented as follows:

where BUT denotes a monomer molecule, and n- the number of connected molecules, i.e. the degree of polymerization.

Briefly, polymerization is the formation of a polymer from a monomer. Monomer is a term that makes sense only in relation to its polymer. If there is no polymer, there is no monomer. In principle, polymerization is an addition reaction due to the breaking of double bonds in the monomer molecule. However, this reaction is often complicated by isomerization processes with the displacement of double bonds and groups of atoms or the involvement of various other substances (catalysts, growth regulators, emulsifiers, etc.) in the reaction.

There are two main types of polymer chain growth process:

1. Stepwise polymerization - when the connection of molecules is accompanied by the movement of hydrogen atoms or entire groups of atoms. The reaction products at each stage can be isolated.

2. Chain or linear polymerization, when there is no movement of atoms. The products of the initial stages cannot be singled out in isolation; the reaction product is IUD.

An example of a reaction of the first type is the polymerization of isobutylene studied by A.M. Butlerov. The reaction is catalyzed by acids (H 2 S0 4), during which diisobutylenes can be isolated, the hydrogenation of which gives isooctane, or the reaction can be carried out further to obtain polyisobutylene. The reaction mechanism is:

Chain polymerization can proceed by two mechanisms: ionic (catalytic) and radical (initiated).

Radical polymerization is caused (initiated):

Substances capable of decomposing into free radicals under reaction conditions

thermal energy

Irradiation (UV, radiation)

During thermal polymerization, some of the monomer molecules are activated under the influence of elevated temperature and react with each other. Using the example of styrene, this can be imagined as follows

The styrene dimer molecule formed in this way is a biradical particle and due to this it easily attaches other styrene molecules, forming polymeric radicals:

In this case, the polymerization process is a typical valuable reaction: the active site that occurs first (ie the initiation step) causes a long chain of addition reactions (the chain growth step). Thermal initiation generally requires high temperatures at which undesirable side processes occur to achieve sufficient reaction rates. A much more convenient method of initiating the polymerization reaction is by introducing substances capable of generating free radicals into the reaction zone. Such compounds are peroxides, hydroperoxides, and some other compounds. Benzoyl peroxide is often used:

In general: (RCOO) 2 → RCOO+R+CO 2

These free radicals are the active centers that start the process of polymer chain formation. For example, the polymerization of styrene in this case can be represented as follows:

The following styrene molecules are attached to the formed radicals 1 and 2, and so on. (chain growth). In general, these processes can be represented as follows:

Before the formation of a macromolecule. At some stage, chain growth stops (chain termination) and the grown macroradical is converted into a stable polymer macromolecule.

The reason for a broken circuit is:

recombination, i.e. compound, two macroradicals R-(M) n -M+M-(M) n -R→R-(M) n -M-M-(M) n -R;

recombination of a macroradical with some other inactive radical;

the interaction of a macroradical with some other substances;

Isomerization of a macroradical into a stable compound;

spatial difficulties.

In production, the speed of the polymerization process plays a huge role. It is determined both by the nature of the initiator (catalyst) and by the structure of the monomer and the method of carrying out the process.

The polymerization process can be carried out:

in bulk, when a catalyst is added directly to the liquid mass of the monomer and at a certain temperature the accumulation of the polymer occurs, accompanied by thickening and then solidification of the mass;

In solutions

in emulsions;

in the gas phase.

The first method is widely used in the production of synthetic resins. It is convenient for practical implementation, but in this case it is difficult to carry out the removal of heat released during the reaction, mixing is difficult due to the high viscosity, which leads to polymer inhomogeneity. In solution polymerization, the rate is somewhat reduced, and there are difficulties in removing the solvent.

Emulsion polymerization has become widespread, especially in the production of SC. This method consists in the fact that the monomer is distributed among some liquid (most often water), in which it does not dissolve, in the form of tiny droplets, forming a typical emulsion (liquid in a liquid). An initiator is added that is soluble in one or another phase, then substances that give stability to the emulsion - emulsifiers and other unnecessary substances - activators, chain growth regulators, and the mixture is mixed in closed reactors - polymerizers at a certain temperature. Unlike the first two processes, this process takes place in a heterogeneous highly dispersed system. It has been found that under these conditions, the decomposition of the initiator occurs more easily, the processes of chain termination are less developed, and the phenomena of molecular polarity and other phenomena are more affected on the phase interface. All this leads to the fact that the rate of polymerization in emulsions is ten times higher than in a homogeneous medium.

Polymerization conditions are of decisive importance in obtaining polymers with desired properties, since the order of chain growth and the structure of polymer molecules, and hence its physical and technical properties, depend on these conditions. The first emulsion polymerization reaction was carried out by the Russian chemist and engineer Ostromyslensky in 1915. Since then, the issues of emulsion polymerization in our country have been intensively developed by such scientists as B.A. Dogadkin, B.A. Dolgoplosk, P.M. Khomikovsky, S.S. Medvedev, and others.

Basic hydrocarbon polymers are obtained by polymerization of ethylene derivatives. Let's consider some of them.

Polyethylene

Ethylene polymerizes with difficulty, the reaction is carried out at high temperatures (up to 200C) and pressures (up to 1000 atm). Naval Forces and MV 20000 and above are formed.

Polyethylene was first obtained by Gustavson in 1884 by catalytic (AlBr 3) polymerization of ethylene. The PE molecule is a long, zigzag chain of methyl groups.

PE at ordinary temperatures does not dissolve in organic solvents, but at t>80°C it dissolves well in R-Hal and aromatic hydrocarbons. Acids and alkalis do not affect PE. PE is very durable, well processed and welded. Scope - insulating and protective coatings, household products.

During the polymerization of PE in the presence of b / w AlCl 3 as a catalyst at t = 120-200 ° C and P = 100 atm, mixtures of hydrocarbons with branched chains are obtained, which are used after their dilution with acid esters as special lubricating oils.

Polystyrene

The polymerization of styrene is easier than that of ethylene - it is carried out under radical conditions (peroxides). The structure of polystyrene:

Get polymers with MW from 3 to 600 thousand. Higher polymers are solid transparent glassy products. Above 150C, it begins to depolymerize to form styrene. Copolymerization of styrene with butadiene produces styrene-butadiene rubber. Scope - electrical insulator, in technology, in everyday life.

Similar information.

We will buy industrial catalysts in any volume in Rostov-on-Don and the Rostov region.

About catalysts

The catalyst is chemical which helps speed up the reaction. It has wide application in various industries. The main consumers of the catalyst are the oil refining industry, petrochemical, chemical industries, they are successfully used in the field of ecology and environmental protection.

Classification

All manufactured devices are classified:

According to the type of catalysis reaction - acid-base, redox
According to the catalysis process group - ammonia synthesis, oil product cracking
By the nature of the active base used - metal, oxide, sulfide, complex and others
According to the manufacturing method

All catalysts use non-ferrous and precious metals: platinum, aluminum, iron, chromium, nickel, vanadium, cobalt, bismuth, silver, gold and many others.

Industrial catalysts are also homogeneous and heterogeneous. Homogeneous - is in a common phase with the reacting substance. A catalyst that forms its own phase, separated from the reacting substances, is called heterogeneous. Using industrial and other types of catalysts, we can not only save nature from poisonous substances that exist in any industry, but also save raw materials.

Application in industry

The rapid growth of industry, which we are now witnessing, would not have been possible without the development and emergence of new chemical technological processes. To a greater extent, progress is facilitated by the widespread use of catalysts, it is they who help turn low-grade raw materials into high-grade products. The catalyst can be compared to the philosopher's stone, which was believed to turn certain metals into gold. But only catalysts turn raw materials into various drugs, plastics, chemicals, fuels, useful and necessary fertilizers and other useful things.

Application of catalysts

A significant event for the practical use of the catalyst is the start of margarine production by the catalytic hydrogenation of vegetable oils. This was first carried out at the very beginning of the 20th century, and already in the twenties, scientists developed catalytic methods in order to obtain new organic materials. Olefins, nitriles, esters, acids have become a kind of "building blocks" for the production of plastics.

The next wave, when they began to use industrial catalysts, was oil refining. Soon, in this industry, a catalyst was no longer necessary, since these devices are used at all stages of the process, such as:

Cracking
Reforming
Hydrosulfonation
Hydrocracking
Isomerization
Polymerization
Alkylation

In recent years, catalysts have been widely used in the field of environmental protection. The most famous device that helps us save the environment is the exhaust gas catalyst in cars.

The areas of application of neutralizers are constantly expanding, the catalysis reaction makes it possible to improve previously developed technologies. For example, catalytic cracking has been improved through the use of zeolites.

hydrogenation

Basically, catalytic reactions are associated with the fact that a hydrogen atom is activated with some other molecule, which leads to chemical interactions. This process called hydrogenation, and it is he who is the basis for many stages in oil refining, as well as in the production of liquid fuel from coal. During the war, the hydrogenation process was widely used in Germany to produce gasoline for aircraft and fuel for cars from coal, because there is no oil in Germany.

Hydrogenation of vegetable edible oils

Another useful property, which catalysts have in Food Industry- this is the hydrogenation of vegetable oil into margarine, cooking oil, and other food products. In this case, fine nickel powder is applied to the catalyst or substrate.

Dehydrogenation

This chemical reaction of catalysis is used less frequently than hydrogenation, but, nevertheless, it is also important, it helps to obtain styrene, propane, butane, butene.

acid catalysis

The activity of most catalysts is determined by and depends on their acidic properties. It is acidic industrial catalysts that are in most cases used in oil refining to produce paraffins and aromatic hydrocarbons. The latest in the use of catalysts is the production of leaded fuels, as well as high-octane gasolines.

It must be said that there is still no unified cataloging of industrial catalysts. Everything comes from experience. Catalysts are classified based on the following parameters:

Type of catalysis reaction
The nature of the substance that is active
Catalytic process group.

The most complex option is precisely the third one, since it is he who is most focused on modern industry- petrochemical, chemical, oil refining.

History of creation

It is believed that the first use of a catalyst is the production of ethyl ether from alcohol using sulfuric acid as a catalyst. In the 18th century, the discovery of the catalytic action of acid for the saccharification of starch was made. Here, clay and some types of metals were used as a catalyst. But still, the concept of "catalysis" did not yet exist. Only in 1834 was Mitcherlich introduced such a concept as "contact reaction". The name "catalysis" was proposed by Berzelius a year later - in 1835.

The use of platinum metal for oxidation was patented in 1831 by the scientist Phillips, but industrial applications this way did not receive catalysis for a number of reasons (platinum reduced its activity when combined with arsenic and some other toxic substances contained in gases). After they developed a method for cleaning various gases from toxic substances, it became possible to create the first large industrial device. It was put into operation in Russia in 1897 and patented in 1902. Today the most important large enterprises Various industries use industrial "kata", and each process uses its own type of catalyst, which has the optimal combination of properties.

The volume of production of these devices in the world is more than 800 thousand tons per year. Some of the catalysts operate from 6 months to a year, while others have a much longer service life - up to 10-12 years. After the work limit has been exhausted, the catalyst must be properly disposed of.

Our company offers you profitable terms sales at the best prices. Contact us - remember that catalysts contain not only precious metals, but also harmful substances. Do not throw devices in landfills, it is better to save nature, and even a plus to this and get a good amount of money for scrap.

CATALYSIS: APPLICATIONS OF CATALYSIS IN INDUSTRY

To the article CATALYSIS

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, the catalyst is the philosopher's stone of the modern alchemist, only it does not turn lead into gold, but raw materials into medicines, plastics, chemical reagents, fuel, fertilizers and other useful products.

Perhaps the very first catalytic process that man 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 significant milestone in the practical application of catalysis was the production of margarine by catalytic hydrogenation of vegetable oil. For the first time, this reaction on an industrial scale was carried out around 1900. And starting from the 1920s, one after another, catalytic methods were developed for the production of new organic materials, primarily 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 occurs in the 1930s and is 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 and alkylation.

And finally, the fourth wave in the use of catalysis is related to environmental protection. The most famous achievement in this area is the creation of a catalytic converter for automobile 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 of 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 the Haber-Bosch catalytic 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 in 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 (the 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 in 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. For a break N-N connections its molecule requires an energy of the order of 200 kcal/mol. 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 accelerate 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 CH3(CH2)7CH=CH(CH2)7COOH has one C=C double bond, 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 fine powder of nickel deposited on a substrate or Raney nickel 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 capable of donating a proton. Strong acids easily donate their protons to bases. The concept of acidity was further developed in the works of 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 that form carbenium ions, helped to understand the 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 a Friedel-Crafts catalyst such as HCl-AlCl2O3 (or HAlCl4) and aluminosilicates. The strength of the acid 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 that of silica (silicon dioxide), in which some of the Si4+ atoms are replaced by Al3+ 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 a Brønsted acid:

The activity of acid catalysts is determined by their ability to react with hydrocarbons with the formation of a carbenium ion as an intermediate product. 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 of n-butane to isobutane in the presence of HCl-AlCl3 or Pt-Cl-Al2O3. First, a small amount of C4H8 olefin attaches the positively charged hydrogen ion of the acid catalyst to form the tertiary carbenium ion. Then the negatively charged hydride ion H- is cleaved from n-butane to form isobutane and a 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.:

Acid catalysts are widely used in oil refining - cracking, alkylation, polymerization and isomerization of hydrocarbons (see also CHEMISTRY AND METHODS OF OIL REFINING). The mechanism of action of carbenium ions, which play the role of catalysts in these processes, has been established. 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. The activity of catalysts is determined by their basic properties. An old and well-known example of such catalysts is sodium hydroxide used to hydrolyze or saponify fats in the manufacture of soap, and a recent example is the 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 releasing CO2. When a mixture of polyalcohols and water reacts with diisocyanates, the resulting polyurethane foam foams with gaseous CO2.

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. The activation of hydrogen is carried out by some metals, and the isomerization of hydrocarbons by acids. An effective dual-acting catalyst used in oil refining to convert naphtha into gasoline is finely dispersed platinum deposited on acid alumina. The conversion of naphtha components such as methylcyclopentane (MCP) to benzene increases the octane number of gasoline. First, the MCP is dehydrogenated on the platinum part of the catalyst into 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 α-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 coined the terms "isotactic" and "syndiotactic" to describe such ordered structures. In the case where there is no order, the term "atactic" is used:

A stereospecific reaction occurs on the surface of 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 TiCl4 and Al(C2H5)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. More often, however, it is desirable that the oxidation be incomplete, for example in many of the processes widely used in industry for the conversion of hydrocarbons into valuable intermediates containing functional groups such 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 2CH4 + 2O2 ? C2H4 + 2H2O is carried out at 850? With 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, O22-. 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 constitute 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.

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).

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

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, J. Houdry developed a method for catalytic purification of exhaust gases by oxidizing CO and hydrocarbons to CO2 and H2O. In 1970, the Clean Air Declaration (revised in 1977, expanded in 1990) was formulated, requiring all new cars from 1975 models to be equipped with 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 specially 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, for example 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 approx. Of the 15,000 known enzymes, 20 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, approx. 4 billion liters of alcohol, 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 associated with 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 the products, and preservation of the enzyme. One example of the industrial use of immobilized enzymes is the isomerization of D-glucose to fructose.

Collier. Collier's Dictionary. 2012