Examples of industrial processes using catalysts. Application of catalysis in the chemical and oil refining industries

A catalyst is any substance that, without entering the final products of a chemical reaction, changes its rate. A variety of substances can be used as a catalyst, both in pure form and in the form of compounds. Industrial catalysts must have the following properties: high activity and selectivity with respect to a given reaction; high chemical resistance to catalyst poisons; low ignition temperature, wide operating temperature range, thermal stability, increased thermal conductivity; high mechanical strength; be cheap to manufacture.

Considering these requirements for catalysts, we come to the conclusion that they are not individual substances, but are usually mixtures of several substances. The mechanical mixtures that make up the catalyst are called contact masses. The composition of contact masses includes three main parts: catalytically active substances, activators and carriers. Catalytically active substances are the basis of the catalyst. It is they who enter into exchange reactions with atoms and molecules of the component removed from the gas mixture. Pure metals, metal oxides, as well as a large number of a wide variety of chemical elements and compounds, which are chosen empirically, are used as catalytically active substances. Catalysts containing two metals have increased catalytic activity.

Activators (promoters) are substances that increase the activity of catalysts. The activators themselves may not have catalytic properties, but can enhance the effect of catalytically active substances. The effect of activators on the catalysis process has not been fully studied. It is assumed that they react with the catalyst and form compounds that are more active than pure catalytically active substances. A wide variety of substances can be used as activators, the selection of which is also carried out empirically.

Carriers are substances that usually do not themselves have catalytic properties and do not enhance the activity of the catalyst, but serve only as a base on which the catalyst is applied. In some cases, carriers can affect the activity and selectivity of catalysts. The use of carriers makes it possible to change the structure of catalysts and reduce the consumption of catalytically active substances. Inert porous substances with a developed surface (active carbons, silica gels, aluminosilicates, asbestos, pumice, kieselur, zeolites, etc.) are most often used as carriers.

The catalytic activity of a catalyst is a measure of the acceleration of a reaction under the influence of a catalyst; it is the main and main characteristic of a catalyst.

where Uk and U are the rates of the catalytic reaction and the same reaction carried out in the absence of catalysts.

The selectivity of the catalyst shows the ratio of the content of the target product to the amount of all products of the transformation of the starting substance. When sanitary purification of gases, the goal is to transform harmful impurities into neutral substances or substances that are easily released from exhaust gases.

In some cases, the task may be to develop and use universal catalysts that are active not in one, but in several reactions. Such catalysts can be used for complex gas purification, i.e. in the case when not one, but several harmful components are present in the exhaust gases.

A very important characteristic of catalysts is their resistance to the action of various impurities, i.e., to catalytic poisons.

Poisoning refers to the partial or complete loss of catalyst activity under the influence of impurities called contact poisons. For many catalysts, contact poisons include compounds S, H 2 S, CS 2, CO, H 2 0, NO, As, P, Pb, Hg, etc. The poisoning of the catalyst occurs due to the sorption of catalyst poisons on the surface of the catalysts and the blocking of their active centers. If these substances are removed from the surface of the catalyst, it can fully or partially restore its activity. In accordance with this, a distinction is made between reversible and irreversible poisoning of catalysts.

Based on their composition, catalysts are divided into 1) modified; 2) mixed and 3) on carriers.

1) Modified catalysts. A modifier is an addition to the catalyst of a small (up to 10–12 wt. %) amount of another substance that is not catalytically active for a given reaction, but improves certain qualities of the catalyst (heat resistance, strength, poison resistance). If a modifier increases activity, it is a promoter. Based on the nature of their action, promoters are divided into a) electronic, causing deformation of the crystal lattices of the catalyst or changing the work function of electrons in the desired direction. For example, the addition of Cl - to a silver catalyst for methanol oxidation: CH 3 OH ® CH 2 O; b) stabilizing, preventing sintering of the dispersed structure of the catalyst. For example, the promoters Al 2 O 3 and SiO 2 stabilize the primary crystals of the iron catalyst in the synthesis of ammonia: N 2 + 3H 2 ® 2NH 3. On the first day of operation, the crystals are sintered and enlarged from 6 to 20 nm. The supply of free energy at the crystal interface decreases and activity decreases. The introduced promoters, without being restored, melt at the synthesis temperature, and a thin film is wrapped around the crystals, preventing them from sintering. However, both additives have an acidic surface on which the NH 3 molecule is firmly adsorbed, preventing the sorption of nitrogen molecules, and the activity of the catalyst decreases; V) structure-forming, neutralizing acidic centers Al 2 O 3 and SiO 2. For example, K 2 O, CaO and MgO, but their amount should be no more than 4-5 wt.%, since they have a mineralizing effect, i.e. promote sintering of Fe crystals.

2) Mixed catalysts. Mixed catalysts are those that contain several components that are catalytically active for a given reaction, taken in comparable quantities. The activity of such catalysts is not additive, but takes on an extreme value due to the following reasons: the formation of mechanical mixtures with a larger phase boundary, i.e. with a large supply of free energy ( for example, for the reaction HCºHC + H 2 O ® CH 3 -CHO, the catalyst is a mixture of CdO + CaO/P 2 O 5 = 3-4; at a molar ratio £3, high selectivity is observed, but the strength of the catalyst granules is low; at ³4 – high strength of granules, but low selectivity); formation of spinel-type solid solutions(for example, in the V 2 O 5 +MoO 3 oxidation catalyst, the Mo +6 cation is introduced into the vacant positions of the V 2 O 5 crystal lattice. Lattice deformation leads to an increase in the free energy of the system; formation of new, more active catalysts under reaction conditions(for example, for the synthesis of methanol CO + 2H 2 ® CH 3 OH, a zinc chromium catalyst is used:

ZnO + CrO 3 + H 2 O ® ZnCrO 4 × H 2 O

2ZnCrO 4 ×H 2 O + 3H 2 ® + 5H 2 O

The active phase obtained after reduction of the catalyst, which is essentially a new catalyst, is shown in square brackets.

3) Supported catalysts. The carrier determines the shape and size of the granules, the optimal porous structure, strength, heat resistance, and cost reduction. Sometimes increases activity (see ligand field theory). Media classification: synthetic– silica gel, activated carbon, aluminum oxide (g, a), ceramics; natural– pumice, diatomite; by pore volume– porous (more than 10%), non-porous (10% or less); by grain size- large (1-5 mm), small (0.1-1.0 mm), finely dispersed (less than 0.1 mm); by specific surface area– small (less than 1 m 2 /g), medium (1-50 m 2 /g), developed (more than 50 m 2 /g).

The content of the article

CATALYSIS, acceleration of chemical reactions under the influence of small amounts of substances (catalysts), which themselves do not change during the reaction. Catalytic processes play a huge role in our lives. Biological catalysts, called enzymes, are involved in the regulation of biochemical processes. Without catalysts, many industrial processes could not take place.

The most important property of catalysts is selectivity, i.e. the ability to increase the rate of only certain chemical reactions out of many possible ones. This allows reactions that are too slow to be practical under normal conditions and ensures the formation of the desired products.

The use of catalysts contributed to the rapid development of the chemical industry. They are widely used in oil refining, obtaining various products, and creating new materials (for example, plastics), often cheaper than those used before. Approximately 90% of modern chemical production is based on catalytic processes. Catalytic processes play a special role in environmental protection.

Most catalytic reactions are carried out at a certain pressure and temperature by passing the reaction mixture, which is in a gaseous or liquid state, through a reactor filled with catalyst particles. The following concepts are used to describe reaction conditions and product characteristics. Space velocity is the volume of gas or liquid passing through a unit volume of catalyst per unit time. Catalytic activity is the amount of reactants converted by a catalyst into products per unit time. Conversion is the fraction of a substance converted in a given reaction. Selectivity is the ratio of the amount of a particular product to the total amount of products (usually expressed as a percentage). Yield is the ratio of the amount of a given product to the amount of starting material (usually expressed as a percentage). Productivity is the number of reaction products formed per unit volume per unit time.

TYPES OF CATALYST

Catalysts are classified based on the nature of the reaction they accelerate, their chemical composition, or their physical properties. Almost all chemical elements and substances have catalytic properties to one degree or another - on their own or, more often, in various combinations. Based on their physical properties, catalysts are divided into homogeneous and heterogeneous. Heterogeneous catalysts are solid substances that are homogeneous dispersed in the same gas or liquid medium as the reacting substances.

Many heterogeneous catalysts contain metals. Some metals, especially those belonging to group VIII of the periodic table of elements, have catalytic activity on their own; a typical example is platinum. But most metals exhibit catalytic properties when present in compounds; example - alumina (aluminum oxide Al 2 O 3).

An unusual property of many heterogeneous catalysts is their large surface area. They are penetrated by numerous pores, the total area of ​​which sometimes reaches 500 m 2 per 1 g of catalyst. In many cases, oxides with a large surface area serve as a substrate on which metal catalyst particles are deposited in the form of small clusters. This ensures effective interaction of reagents in the gas or liquid phase with the catalytically active metal. A special class of heterogeneous catalysts are zeolites - crystalline minerals of the group of aluminosilicates (compounds of silicon and aluminum). Although many heterogeneous catalysts have a large surface area, they usually have only a small number of active sites, which account for a small portion of the total surface area. Catalysts may lose their activity in the presence of small amounts of chemical compounds called catalyst poisons. These substances bind to active centers, blocking them. Determining the structure of active sites is the subject of intensive research.

Homogeneous catalysts have a different chemical nature - acids (H 2 SO 4 or H 3 PO 4), bases (NaOH), organic amines, metals, most often transition metals (Fe or Rh), in the form of salts, organometallic compounds or carbonyls. Catalysts also include enzymes - protein molecules that regulate biochemical reactions. The active site of some enzymes contains a metal atom (Zn, Cu, Fe or Mo). Metal-containing enzymes catalyze reactions involving small molecules (O 2, CO 2 or N 2). Enzymes have very high activity and selectivity, but they work only under certain conditions, such as those under which reactions occur in living organisms. In industry, the so-called is often used. immobilized enzymes.

HOW CATALYSTS WORK

Energy.

Any chemical reaction can occur only if the reactants overcome the energy barrier, and for this they must acquire a certain energy. As we have already said, the catalytic reaction X ® Y consists of a number of successive stages. Each of them requires energy to take place. E, called activation energy. The change in energy along the reaction coordinate is shown in Fig. 1.

Let us first consider the non-catalytic, “thermal” path. For a reaction to take place, the potential energy of molecules X must exceed the energy barrier E t. The catalytic reaction consists of three stages. The first is the formation of the X-Cat complex. (chemisorption), the activation energy of which is equal to E ads. The second stage is the regrouping of X-Cat. ® Y-Cat. with activation energy E cat, and finally, the third - desorption with activation energy E des; E ads, E cat and E des much less E t. Since the reaction rate depends exponentially on the activation energy, the catalytic reaction proceeds much faster than the thermal reaction at a given temperature.

A catalyst can be likened to a guide who guides climbers (reacting molecules) across a mountain range. He leads one group through the pass and then returns for the next. The path through the pass lies significantly lower than that through the peak (thermal channel of the reaction), and the group makes the transition faster than without a conductor (catalyst). It is even possible that the group would not have been able to overcome the ridge on its own.

Theories of catalysis.

To explain the mechanism of catalytic reactions, three groups of theories have been proposed: geometric, electronic and chemical. In geometric theories, the main attention is paid to the correspondence between the geometric configuration of the atoms of the active centers of the catalyst and the atoms of that part of the reacting molecules that is responsible for binding to the catalyst. Electronic theories are based on the idea that chemisorption is caused by electronic interaction associated with charge transfer, i.e. these theories relate catalytic activity to the electronic properties of the catalyst. Chemical theory views a catalyst as a chemical compound with characteristic properties that forms chemical bonds with reagents, resulting in the formation of an unstable transition complex. After the decomposition of the complex with the release of products, the catalyst returns to its original state. The latter theory is now considered the most adequate.

At the molecular level, a catalytic gas-phase reaction can be represented as follows. One reacting molecule binds to the active site of the catalyst, and the other interacts with it, being directly in the gas phase. An alternative mechanism is also possible: the reacting molecules are adsorbed on neighboring active centers of the catalyst and then interact with each other. Apparently, this is how most catalytic reactions proceed.

Another concept suggests that there is a relationship between the spatial arrangement of atoms on the surface of a catalyst and its catalytic activity. The rate of some catalytic processes, including many hydrogenation reactions, does not depend on the relative position of the catalytically active atoms on the surface; the speed of others, on the contrary, changes significantly with changes in the spatial configuration of surface atoms. An example is the isomerization of neopentane into isopentane and the simultaneous cracking of the latter to isobutane and methane on the surface of a Pt-Al 2 O 3 catalyst.

APPLICATION OF CATALYSIS IN INDUSTRY

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, a catalyst is the philosopher’s stone of a modern alchemist, only it transforms not lead into gold, but raw materials into medicines, plastics, chemicals, fuel, fertilizers and other useful products.

Perhaps the very first catalytic process that man learned to use was fermentation. Recipes for preparing alcoholic beverages were known to the Sumerians as early as 3500 BC. Cm. WINE; BEER.

A significant milestone in the practical application of catalysis was the production of margarine by the catalytic hydrogenation of vegetable oil. This reaction was first carried out on an industrial scale around 1900. And since the 1920s, catalytic methods for producing new organic materials, primarily plastics, have been developed one after another. The key point was the catalytic production of olefins, nitriles, esters, acids, etc. – “bricks” for the chemical “construction” of plastics.

The third wave of industrial use of catalytic processes occurred in the 1930s and was associated with petroleum refining. In terms of volume, this production soon left all others far behind. Petroleum refining consists of several catalytic processes: cracking, reforming, hydrosulfonation, hydrocracking, isomerization, polymerization and alkylation.

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 thus saving many lives.

About a dozen Nobel Prizes have been awarded for work in catalysis and related fields.

The practical importance of catalytic processes is evidenced by the fact that the share of nitrogen included in industrially produced nitrogen-containing compounds accounts for about half of all nitrogen included in food products. The amount of nitrogen compounds produced naturally is limited, so the production of dietary protein depends on the amount of nitrogen added to the soil through fertilizers. It would be impossible to feed half of humanity without synthetic ammonia, which is produced almost exclusively through the Haber-Bosch catalytic process.

The scope of application 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 (Bergius process).

The production of aviation gasoline and motor fuel from coal was developed in Germany during World War II because the country had no oil fields. The Bergius process involves the direct addition of hydrogen to coal. Coal is heated under pressure in the presence of hydrogen to produce a liquid product, 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, 12 factories in Germany produced approximately 1,400 tons of liquid fuel per day using the Bergius process.

Another process, Fischer–Tropsch, consists of two stages. First, the coal is gasified, i.e. They react it with water vapor and oxygen and obtain 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 of 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 aluminum-cobalt-molybdenum catalyst at a relatively low temperature 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 about 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 binds to iron even more readily. Ammonia synthesis proceeds as follows:

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

Hydrogenation of vegetable oil.

One of the most important hydrogenation reactions in practical terms 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 contain esters, namely triglycerides of fatty acids with varying degrees of unsaturation. Oleic acid CH 3 (CH 2) 7 CH=CH(CH 2) 7 COOH has one C=C double bond, linoleic acid has two and linolenic acid has three. The addition of hydrogen to break this bond prevents oils from oxidizing (rancidity). This increases their melting point. The hardness of most resulting products depends on the degree of hydrogenation. Hydrogenation is carried out in the presence of fine nickel powder deposited on a substrate or a Raney nickel catalyst in an atmosphere of highly purified hydrogen.

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, ethylbenzene is dehydrogenated in the presence of a catalyst containing iron oxide; The reaction is also facilitated by potassium and some kind of structural stabilizer. The dehydrogenation of propane, butane and other alkanes is carried out on an industrial scale. Dehydrogenation of butane in the presence of a chromium-alumina catalyst produces butenes and butadiene.

Acid catalysis.

The catalytic activity of a large class of catalysts is determined by their acidic properties. According to I. Brønsted 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 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 produce carbenium ions, helped to understand the mechanism of a variety of catalytic reactions, especially those involving hydrocarbons.

The strength of an acid can be determined by 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 process catalyst, such as HCl-AlCl 2 O 3 (or HAlCl 4), and aluminosilicates. Acid strength is a very important characteristic because it determines the rate of protonation, a key step in the acid catalysis process.

The activity of catalysts such as aluminosilicates, used in oil cracking, is determined by the presence of Brønsted 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 a Brønsted acid:

The activity of acid catalysts is determined by their ability to react with hydrocarbons to form 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 using the example of an isomerization reaction n-butane to isobutane in the presence of HCl-AlCl 3 or Pt-Cl-Al 2 O 3. First, a small amount of the olefin C4H8 attaches to 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 to form isobutane and secondary butylcarbenium ion. The latter, as a result of rearrangement, turns into a tertiary carbenium ion. This chain can continue with the elimination of a hydride ion from the next molecule n-butane, etc.:

It is significant that tertiary carbenium ions are more stable than primary or secondary ones. As a result, they are mainly present on the surface of the catalyst, 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. The mechanism of action of carbenium ions, which play the role of catalysts in these processes, has been established. In doing so, they participate in a number of reactions, including the formation of small molecules by cleavage of large molecules, the combination of molecules (olefin to olefin or olefin to isoparaffin), structural rearrangement by isomerization, and the formation of paraffins and aromatic hydrocarbons by hydrogen transfer.

One of the latest applications of acid catalysis in industry is the production of leaded fuels by adding alcohols to isobutylene or isoamylene. Adding oxygen-containing compounds to gasoline reduces the concentration of carbon monoxide in exhaust gases. Methyl- rubs-butyl ether (MTBE) with an octane mixing number of 109 also makes it possible to obtain high-octane fuel necessary for operating a car engine with a high compression ratio, without introducing tetraethyl lead into gasoline. The production of fuels with octane numbers 102 and 111 has also been organized.

Basic catalysis.

The activity of catalysts is determined by their basic properties. A long-standing and well-known example of such catalysts is sodium hydroxide, used to hydrolyze or saponify fats to make soap, and one recent example is catalysts used in the production of polyurethane plastics and foams. Urethane is formed by the reaction of alcohol with isocyanate, and this reaction is accelerated in the presence of basic amines. During the reaction, a base attaches to the carbon atom in the isocyanate molecule, as a result of which a negative charge appears on the nitrogen atom and its activity towards alcohol increases. Triethylenediamine is a particularly effective catalyst. Polyurethane plastics are produced by reacting diisocyanates with polyols (polyalcohols). When isocyanate reacts with water, the previously formed urethane decomposes, releasing CO 2 . When a mixture of polyalcohols and water interacts with diisocyanates, the resulting polyurethane foam foams with CO 2 gas.

Double acting catalysts.

These catalysts speed up two types of reactions and produce 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 a double-acting catalyst are very close to each other, and the intermediate product formed at one of them is immediately converted into the final product at the other.

A good result is obtained by combining a catalyst that activates hydrogen with a catalyst that promotes the isomerization of hydrocarbons. The activation of hydrogen is carried out by some metals, and the isomerization of hydrocarbons is carried out by acids. An effective dual-acting catalyst used in petroleum refining to convert naphtha into gasoline is finely divided platinum supported on acidic alumina. Converting naphtha constituents such as methylcyclopentane (MCP) to benzene increases the octane number of gasoline. First, MCP is dehydrogenated on the platinum part of the catalyst into an olefin with the same carbon skeleton; the olefin then passes to the acid portion of the catalyst, where it isomerizes to cyclohexene. The latter passes to the platinum part and is dehydrogenated to benzene and hydrogen.

Double-action catalysts significantly accelerate oil reforming. They are used to isomerize normal paraffins into isoparaffins. The latter, boiling at the same temperatures as gasoline fractions, are valuable because they have a higher octane number compared to straight hydrocarbons. Moreover, the transformation n-butane to isobutane is accompanied by dehydrogenation, facilitating the production of MTBE.

Stereospecific polymerization.

An important milestone in the history of catalysis was the discovery of catalytic polymerization a-olefins to form stereoregular polymers. Stereospecific polymerization catalysts were discovered by K. Ziegler when he was trying 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. To describe such ordered structures, Natta introduced the terms “isotactic” and “syndiotactic”. 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), which are in a partially oxidized state, and any compound containing carbon or hydrogen, which is bonded to the metal from groups I–III. A classic example of such a catalyst is the precipitate formed by the interaction of TiCl 4 and Al(C 2 H 5) 3 in heptane, where titanium is reduced to the trivalent state. This exceptionally active system catalyzes the polymerization of propylene at normal temperatures and pressures.

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 automobile exhaust gases. However, more often it is necessary for the oxidation to be incomplete, for example, in many widely used industrial processes for converting hydrocarbons into valuable intermediate products 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, which is used for the oxidation pair-xylene to terephthalic acid, the esters of which serve as the basis for the production of polyester fibers.

Catalysts for heterogeneous oxidation.

These catalysts are usually complex solid oxides. Catalytic oxidation occurs in two stages. First, the oxygen in the oxide is captured by a hydrocarbon molecule adsorbed on the surface of the oxide. In this case, 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 incomplete oxidation of naphthalene or butane.

Production of ethylene by dehydrodimerization of methane.

Ethylene synthesis through dehydrodimerization converts natural gas into more easily transportable hydrocarbons. The reaction 2CH 4 + 2O 2 ® C 2 H 4 + 2H 2 O is carried out at 850 ° C using various catalysts; the best results were obtained with the Li-MgO catalyst. Presumably the reaction proceeds through the formation of a methyl radical by the abstraction of a hydrogen atom from a methane molecule. The elimination 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, during subsequent dehydrogenation, are converted to ethylene. 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 into gasoline fraction hydrocarbons. Gasoline contains significant amounts of aromatic hydrocarbons and therefore has a high octane number. In New Zealand, for example, a third of all gasoline consumed is produced using this technology. Methanol is produced from imported methane.

The 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 gasoline yield by more than 20%.

In addition, zeolites have selectivity regarding the size of the reacting molecules. Their selectivity is determined by 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 restrictions pair-xylene is formed more easily than bulkier ones ortho- And meta-isomers. The latter find themselves “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 engine oil, obtaining chemical intermediates for the production of plastics by alkylation of aromatic compounds, isomerization of xylene, disproportionation of toluene and catalytic cracking of oil. ZSM-5 zeolite 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 contained in exhaust gases react in light to form oxidants (in particular, ozone), which irritate the eyes and give other undesirable effects. Around the same time, Y. Khoudri developed a method for 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 (refined in 1977, expanded in 1990), according to which all new cars , starting with 1975 models, must be equipped with catalytic exhaust converters. Standards for the composition of exhaust gases were established. Because 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 automobile neutralizers, in which active components are applied to a ceramic substrate with a honeycomb structure, through the cells of which exhaust gases pass. The substrate is coated with a thin layer of metal oxide, for example Al 2 O 3, onto which a catalyst - platinum, palladium or rhodium - is applied. The content of nitrogen oxides formed during the combustion of natural fuels in 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 energy exchange processes, breakdown of nutrients, and biosynthesis reactions. Without them, many complex organic reactions cannot occur. Enzymes function at ordinary temperatures and pressures, have very high selectivity, and are capable of increasing reaction rates by eight orders of magnitude. Despite these advantages, only approx. 20 of the 15,000 known enzymes are used on a large scale.

Man has used 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 bacteria Clostridium acetobutylicum H. Weizmann carried out the enzymatic conversion of starch into acetone and butyl alcohol. This method of producing acetone was widely used in England during the First World War, and during the Second World War it was used to produce butadiene rubber in the USSR.

An extremely important role was played by the use of enzymes produced by microorganisms for the synthesis of penicillin, as well as streptomycin and vitamin B 12.

Ethyl alcohol, produced by enzymatic processes, is widely used as automobile fuel. In Brazil, more than a third of about 10 million cars run on 96% ethyl alcohol derived from sugar cane, while the rest run on a mixture of gasoline and ethyl alcohol (20%). The technology for producing fuel, which is a mixture of gasoline and alcohol, has been 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. The so-called also find various applications. immobilized enzymes. These enzymes are bound to a solid support, such as silica gel, over which the reagents are passed. The advantage of this method is that it ensures efficient contact of 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.

TECHNOLOGICAL ASPECTS

Modern technologies cannot be imagined without the use of catalysts. Catalytic reactions can occur at temperatures up to 650° C and pressures of 100 atm or more. This forces new solutions to problems associated with contact between gaseous and solid substances and with the transfer of catalyst particles. For the process to be effective, its modeling must take into account kinetic, thermodynamic and hydrodynamic aspects. Computer modeling is widely used here, as well as new instruments and methods for monitoring technological processes.

In 1960, significant progress was made in ammonia production. The use of a more active catalyst made it possible to lower the temperature of hydrogen production during the decomposition of water vapor, which made it possible to lower the pressure and, therefore, reduce production costs, for example, through the use of cheaper centrifugal compressors. As a result, the cost of ammonia fell by more than half, there was a colossal increase in its production, and in connection with this, an increase in food production, since ammonia is a valuable fertilizer.

Methods.

Research in the field of catalysis is carried out using both traditional and special methods. Radioactive tracers, X-ray, infrared and Raman (Raman) spectroscopy, electron microscopic methods are used; Kinetic measurements are carried out, the influence of methods for preparing catalysts on their activity is studied. Of great importance is the determination of the surface area of ​​the catalyst using the Brunauer–Emmett–Teller method (BET method), based on measuring the physical adsorption of nitrogen at different pressures. To do this, determine the amount of nitrogen required to form a monolayer on the surface of the catalyst, and, knowing the diameter of the N 2 molecule, calculate the total area. In addition to determining the total surface area, chemisorption of different molecules is carried out, which makes it possible to estimate the number of active centers and obtain information about their properties.

Researchers have various methods at their disposal to study the surface structure of catalysts at the atomic level. The EXAFS method allows you to obtain unique information. Among spectroscopic methods, UV, X-ray and Auger photoelectron spectroscopy are increasingly used. Secondary ion mass spectrometry and ion scattering spectroscopy are of great interest. NMR measurements are used to study the nature of catalytic complexes. A scanning tunneling microscope allows you to see the arrangement of atoms on the surface of the catalyst.

PROSPECTS

The scale of catalytic processes in industry is increasing every year. Catalysts are increasingly used to neutralize substances that pollute the environment. The role of catalysts in the production of hydrocarbons and oxygen-containing synthetic fuels from gas and coal is increasing. The creation of fuel cells for the economical conversion of fuel energy into electrical energy seems very promising.

New concepts of catalysis will make it possible to obtain polymeric materials and other products with many valuable properties, improve methods of obtaining energy, and increase food production, in particular by synthesizing proteins from alkanes and ammonia with the help of microorganisms. It may be possible to develop genetically engineered methods for producing enzymes and organometallic compounds that approach natural biological catalysts in their catalytic activity and selectivity.

Literature:

Gates B.K. Chemistry of catalytic processes. M., 1981
Boreskov G.K. Catalysis. Questions of theory and practice. Novosibirsk, 1987
Gankin V.Yu., Gankin Yu.V. New general theory of catalysis. L., 1991
Tokabe K. Catalysts and catalytic processes. M., 1993

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 is the philosopher’s stone of the modern alchemist, only it turns not lead into gold, but raw materials into medicines, plastics, chemicals, fuel, fertilizers and other useful products. Perhaps, the very first catalytic process which man has learned to use is fermentation. Recipes for preparing alcoholic beverages were known to the Sumerians as early as 3500 BC. See WINE; BEER.

A significant milestone in the practical application of catalysis became margarine production catalytic hydrogenation of vegetable oil. This reaction was first carried out on an industrial scale around 1900. And since the 1920s, catalytic methods for producing new organic materials, especially plastics. The key point was the catalytic production of olefins, nitriles, esters, acids, etc. – “bricks” for the chemical “construction” of plastics. The third wave of industrial use of catalytic processes dates back 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 automobile exhaust gases. Catalytic converters, which have been installed in cars since 1975, have played a big role in improving air quality and thus saving many lives.

About a dozen Nobel Prizes have been awarded for work in catalysis and related fields. The practical significance of catalytic processes is evidenced by the fact that the share nitrogen, which is part of industrially produced nitrogen-containing compounds, accounts for about half of all nitrogen included in food products. The amount of nitrogen compounds produced naturally is limited, so the production of dietary protein depends on the amount of nitrogen added to the soil through fertilizers. It would be impossible to feed half of humanity without synthetic ammonia, which is obtained almost exclusively using catalytic Haber–Bosch process. The scope of application 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 because the country had no oil fields. The Bergius process involves the direct addition of hydrogen to coal. Coal is heated under pressure in the presence of hydrogen to produce a liquid product, 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, 12 factories in Germany produced approximately 1,400 tons of liquid fuel per day using the Bergius process. Another process, Fischer–Tropsch, consists of two stages. First, the coal is gasified, i.e. They react it with water vapor and oxygen and obtain 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 of 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 aluminum-cobalt-molybdenum catalyst at a relatively low temperature 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 about 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 binds to iron even more readily. Ammonia synthesis proceeds as follows:

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

Hydrogenation of vegetable oil. One of the most important hydrogenation reactions in practical terms 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 contain esters, namely triglycerides of fatty acids with varying degrees of unsaturation. Oleic acid CH 3 (CH 2) 7 CH=CH(CH 2) 7 COOH has one C=C double bond, linoleic acid has two and linolenic acid has three. The addition of hydrogen to break this bond prevents oils from oxidizing (rancidity). This increases their melting point. The hardness of most resulting products depends on the degree of hydrogenation. Hydrogenation is carried out in the presence of fine nickel powder deposited on a substrate, or nickel Raney catalyst in an atmosphere of highly purified hydrogen.

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, ethylbenzene is dehydrogenated in the presence of a catalyst containing iron oxide; The reaction is also facilitated by potassium and some kind of structural stabilizer. The dehydrogenation of propane, butane and other alkanes is carried out on an industrial scale. Dehydrogenation of butane in the presence of a chromium-alumina catalyst produces butenes and butadiene.

Acid catalysis. The catalytic activity of a large class of catalysts is determined by 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 sharing of this electron pair.

These ideas, together with ideas about reactions producing carbenium ions, helped to understand mechanism of various catalytic reactions, especially those involving hydrocarbons. The strength of an acid can be determined by 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 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 acid catalysis process, 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 is determined their ability to react with hydrocarbons to form 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. 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 the olefin C4H8 attaches to the positively charged hydrogen ion of the acid catalyst to form a tertiary carbenium ion. The negatively charged hydride ion H – is then split off from n-butane to form isobutane and a secondary butylcarbenium ion. The latter, as a result of rearrangement, turns into a tertiary carbenium ion. This chain can continue with the elimination of a hydride ion from the next n-butane molecule, etc.:

It is significant that tertiary carbenium ions are more stable than primary or secondary ones. As a result, they are mainly present on the surface of the catalyst, 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 PROCESSING).

Installed mechanism of action of carbenium ions, playing the role of catalysts in these processes. In doing so, they participate in a number of reactions, including the formation of small molecules by cleavage of large molecules, the combination of molecules (olefin to olefin or olefin to isoparaffin), structural rearrangement by isomerization, and the formation of paraffins and aromatic hydrocarbons by hydrogen transfer. One of the latest applications of acid catalysis in industry is the production of leaded fuels by adding alcohols to isobutylene or isoamylene. Adding oxygen-containing compounds to gasoline reduces the concentration of carbon monoxide in exhaust gases. Methyl tert-butyl ether (MTBE) with an octane mixing number of 109 also makes it possible to obtain the high-octane fuel necessary for running a high-compression automobile engine without introducing tetraethyl lead into gasoline. The production of fuels with octane numbers 102 and 111 has also been organized.

Basic catalysis. Catalyst activity is determined their main properties. A long-standing and well-known example of such catalysts is sodium hydroxide, used to hydrolyze or saponify fats to produce soap, and one of the latest examples are catalysts used in the production of polyurethane plastics and foams. Urethane is formed by the reaction of alcohol with isocyanate, and this reaction is accelerated in the presence of basic amines. During the reaction, a base attaches to the carbon atom in the isocyanate molecule, as a result of which a negative charge appears on the nitrogen atom and its activity towards alcohol increases. Triethylenediamine is a particularly effective catalyst. Polyurethane plastics are produced by reacting diisocyanates with polyols (polyalcohols). When isocyanate reacts with water, the previously formed urethane decomposes, releasing CO 2 . When a mixture of polyalcohols and water interacts with diisocyanates, the resulting polyurethane foam foams with CO 2 gas.

Double acting catalysts. These catalysts speed up two types of reactions and produce 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 obtained by combining a catalyst that activates hydrogen with a catalyst that promotes the isomerization of hydrocarbons. Hydrogen activation Some metals carry out the isomerization of hydrocarbons by acids. An effective dual-action catalyst used in petroleum refining to convert naphtha into gasoline is finely divided platinum supported on acidic alumina. Conversion of naphtha components such as methylcyclopentane (MCP), into benzene increases the octane number of gasoline. At first MCP dehydrogenates on the platinum part of the catalyst into an olefin with the same carbon skeleton; the olefin then passes to the acid portion of the catalyst, where it isomerizes to cyclohexene. The latter passes to the platinum part and is dehydrogenated to benzene and hydrogen. Double-action catalysts significantly accelerate oil reforming. They are used to isomerize normal paraffins into 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, facilitating the production of MTBE.

Stereospecific polymerization. An important milestone in the history of catalysis was the discovery of the catalytic polymerization of a-olefins to form stereoregular polymers. Stereospecific polymerization catalysts were discovered by K. Ziegler when he was trying 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. To describe such ordered structures, Natta introduced the terms “isotactic” and “syndiotactic”. In the case where there is no order, the term “atactic” is used:

A stereospecific reaction occurs on the surface solid catalysts containing transition metals of groups IVA-VIII (such as Ti, V, Cr, Zr), which are 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 by the interaction of TiCl 4 and Al(C 2 H 5) 3 in heptane, where titanium is reduced to the trivalent state. This exceptionally active system catalyzes the polymerization of propylene at normal temperatures and pressures.

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 automobile exhaust gases. However, more often it is necessary for the oxidation to be incomplete, for example, in many widely used industrial processes for converting hydrocarbons into valuable intermediate products 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, which is used to oxidize para-xylene to terephthalic acid, the esters of which form the basis for the production of polyester fibers.

Catalysts for heterogeneous oxidation. These catalysts are usually complex solid oxides. Catalytic oxidation occurs in two stages. First, the oxygen in the oxide is captured by a hydrocarbon molecule adsorbed on the surface of the oxide. In this case, 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 incomplete oxidation of naphthalene or butane.

Production of ethylene by dehydrodimerization of methane. Ethylene synthesis through dehydrodimerization converts natural gas 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 were obtained with the Li-MgO catalyst. Presumably the reaction proceeds through the formation of a methyl radical by the abstraction of a hydrogen atom from a methane molecule. The elimination 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, during subsequent dehydrogenation, are converted to ethylene. 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 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 into gasoline fraction hydrocarbons. Gasoline contains significant amounts of aromatic hydrocarbons and therefore has a high octane number. In New Zealand, for example, a third of all gasoline consumed is produced using this technology. Methanol is produced from imported methane.

Figure 2 – Structure of zeolites with large and small pores.

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

The 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 gasoline yield by more than 20%. In addition, zeolites have selectivity regarding the size of the reacting molecules. Their selectivity is determined by 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 restrictions, para-xylene is formed more easily than the bulkier ortho and meta isomers. The latter find themselves “locked” in the pores of the zeolite (Fig. 4).

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

The use of zeolites has revolutionized some industrial technologies - dewaxing gas oil and machine oil, obtaining chemical intermediates for the production of plastics by alkylation of aromatic compounds, isomerization of xylene, disproportionation of toluene and catalytic cracking of oil. ZSM-5 zeolite 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 Latin Parum - little and affinis - related), a mixture of saturated hydrocarbons C 18 -C 35, mainly. normal structure with a pier. m. 300-400; colorless crystals with t pl. = 45–65 o C, density 0.880–0.915 g/cm 3 (15 o C).

Ceresin(from Latin cera - wax), a mixture of solid hydrocarbons (mainly alkylcyclanes and alkanes), obtained after purification of ozokerite. In terms of 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 contained in exhaust gases react in light to form oxidants (in particular, ozone), which irritate the eyes and give other undesirable effects. Around the same time, Yu. 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 (refined in 1977, expanded in 1990), according to which all new cars , starting with 1975 models, must be equipped with catalytic exhaust converters. Standards for the composition of exhaust gases were established. Because 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 automobile neutralizers, in which active components are applied to a ceramic substrate with a honeycomb structure, through the cells of which exhaust gases pass. The substrate is coated with a thin layer of metal oxide, such as Al2O3, onto which a catalyst - platinum, palladium or rhodium - is applied. The content of nitrogen oxides formed during the combustion of natural fuels in 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 energy exchange processes, breakdown of nutrients, and biosynthesis reactions. Without them, many complex organic reactions cannot occur. Enzymes function at ordinary temperatures and pressures, have very high selectivity, and are capable of increasing reaction rates 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 used 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 the bacteria Clostridium acetobutylicum, H. Weizmann carried out the enzymatic conversion of starch into acetone and butyl alcohol. This method of producing acetone was widely used in England during the First World War, and during the Second World War it was used to produce butadiene rubber in the USSR. An extremely important role was played by the use of enzymes produced by microorganisms for the synthesis of penicillin, as well as streptomycin and vitamin B12. Ethyl alcohol, produced by enzymatic processes, is widely used as automobile fuel. In Brazil, more than a third of the approximately 10 million cars run on 96% ethyl alcohol derived from sugar cane, while the rest run on a mixture of gasoline and ethyl alcohol (20%). The technology for producing fuel, which is a mixture of gasoline and alcohol, has been well developed in the United States. In 1987, about 4 billion liters of alcohol were obtained from corn grains, of which approximately 3.2 billion liters were used as fuel. The so-called also find various applications. immobilized enzymes. These enzymes are bound to a solid support, such as silica gel, over which the reagents are passed. The advantage of this method is that it ensures efficient contact of 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 company AUTOKAT RECYCLE provides the opportunity for companies and individuals to sell these parts that have exhausted their service life for the maximum cost in the shortest possible time. This solution contributes to the benefit of customers and contributes to making a feasible contribution to environmental conservation.

Sales of industrial catalysts

– an excellent solution that allows you to earn a decent amount of money and send the part for “proper” recycling. AUTOKAT RECYCLE is a company specializing in the purchase of these products, which has all the necessary resources to facilitate a highly accurate analysis of the content of valuable elements, thanks to which customers can count on a prompt assessment of the value.

Our company carries out operations in numerous cities of Russia. If there is no point with a convenient location, you can send the product using the services of mail or courier.

The resource of which has been exhausted or if for any reason they have lost their functionality, contact AUTOKAT RECYCLE - a procedure that takes virtually no time. After receiving the product, specialists carry out the necessary analyzes and calculate the payment amount, and if the client is satisfied with them, then an agreement is concluded and money is issued (non-cash and other payment methods are possible).

AUTOKAT RECYCLE clients are guaranteed attractive conditions, not without benefits and convenience. Thus, persons who decide to pass the catalyst can count on the provision of competent advice, high prices, the shortest possible time for tests and payments. On the site, visitors can find out the approximate cost of a product using a special calculator, knowing the approximate content of precious metals in its composition.