The reactions are catalyzed. Catalysis in chemical reactions

Most of the processes underlying chemical technology are catalytic reactions. This is due to the fact that with the introduction of a catalyst, the rate of interaction of substances increases significantly. In this case, manufacturers can reduce costs or obtain more reaction products in the same period of time. That is why much attention is paid to the study of catalysis in the training of technologists. However, this phenomenon also plays an important role in nature. So, special substances regulate the course of biochemical reactions in living organisms, thereby affecting the metabolism.

The concept of catalysis

The essence of this chemical phenomenon is to regulate the rate of transformation of substances using special reagents that can slow down or speed up this process. In this case, one speaks of positive or negative catalysis. There is also the phenomenon of autocatalysis, when one of the intermediate products of a chemical reaction affects the reaction rate. Catalytic processes are diverse, they differ in mechanisms, state of aggregation of compounds and direction.

Substances that slow down chemical interactions are called inhibitors, and those that accelerate catalytic reactions are called catalysts. Both those and others change the reaction rate by means of repeated intermediate interaction with one or more of its participants. At the same time, they are not included in the composition of products and are restored after the end of the cycle of transformation of substances. Therefore, the participation of the catalyst is not reflected in the reaction equation stoichiometrically, but only indicated as a condition for the interaction of substances.

Types of catalytic reactions

According to the state of aggregation of substances taking part in a chemical reaction, there are:

• homogeneous reactions - the reactants, products and catalyst are in the same state of aggregation (phase), while the molecules of the substances are evenly distributed throughout the volume;
• interfacial catalytic reactions - occur at the interface of immiscible liquids, and the role of the catalyst is reduced to the transfer of reagents through it;
• heterogeneous catalytic reactions - in them, the catalyst has a state of aggregation different from the reactants, and it itself is carried out on the interface;
• heterogeneous-homogeneous reactions - are initiated at the interface with the catalyst, and continue in the reaction volume;
• microheterogeneous reactions - small particles of a solid catalyst form micelles over the entire volume of the liquid phase.

There is also redox catalysis, accompanied by a change in the oxidation state of the catalyst upon interaction with reagents. Such transformations are called catalytic oxidation and reduction reactions. The most common in chemical production is the oxidation of sulfur dioxide to trioxide in the production of sulfuric acid.

Types of catalysts

According to the state of aggregation, catalysts are liquid (H 2 SO 4, H 3 RO 4), solid (Pt, V 2 O 5, Al 2 O 3) and gaseous (BF 3).

According to the type of substance, catalysts are classified into:

• metals - can be pure, alloys, solid or deposited on a porous base (Fe, Pt, Ni, Cu);
• metal compounds of the M m E n type - the most common oxides are MgO, Al 2 O 3, MoO 3, etc.;
• acids and bases - used for acid-base catalytic reactions, these can be Lewis acids, Bronsted, etc .;
• metal complexes - this group also includes salts of transition metals, such as PdCl 2 , Ni(CO) 4 ;
• enzymes (they are also enzymes) are biocatalysts that speed up reactions that take place in living organisms.

According to the specifics of the electronic structure, d-catalysts are distinguished, having d-electrons and d-orbitals, as well as s, p-catalysts, the center of which is an element with valence s and p-electrons.

Properties of catalysts

To be used effectively, a rather extensive list of requirements is applied to them, which varies for a particular process. But the most significant are the following two properties of catalysts:

• Specificity lies in the ability of catalysts to influence only one reaction or a series of similar transformations and not affect the rate of others. Thus, platinum is most often used in organic hydrogenation reactions.
• Selectivity is characterized by the ability to accelerate one of several possible parallel reactions, thereby increasing the yield of the most important product.

Catalytic reaction rate

The reason for the acceleration of the interaction of substances is the formation of an active complex with a catalyst, leading to a decrease in the activation energy.

According to the basic postulate of chemical kinetics, the rate of any chemical reaction is directly proportional to the product of the concentrations of the starting substances, which are taken in powers corresponding to their stoichiometric coefficients:

v = k ∙ C A x ∙ C B y ∙ C D z ,

where k is the rate constant of a chemical reaction, numerically equal to the rate of the same reaction, provided that the concentrations of the starting compounds are 1 mol/l.

According to the Arrhenius equation, k depends on the activation energy:

k \u003d A ∙ exp ^ (-E A / RT).

These regularities are also valid for catalytic reactions. This is confirmed by the equation for the ratio of the rate constants:

k K / k = A K /A ∙ exp^((E A -E AK)/RT),

where variables with index K refer to catalytic reactions.

Stages of catalytic reactions

For homogeneous catalytic reactions, two main steps are sufficient:

1. Formation of an activated complex: A + K -> AA.
2. Interaction of the activated complex with other initial substances: AA + B ―> C + K.

In general form, an equation of the form A + B -> C is written.

The mechanism of heterogeneous catalytic reactions is complex. The following six stages are distinguished:

1. Bringing the starting compounds to the catalyst surface.
2. Adsorption of initial reagents by the catalyst surface and formation of an intermediate complex: A + B + K ―> AVK.
3. Activation of the resulting complex: ΑVC ―> ΑVC * .
4. Decomposition of the complex compound, while the products formed are adsorbed by the catalyst: ΑВК * ―> CDK.
5. Desorption of the resulting products by the catalyst surface: CDK ―> C + D + K.
6. Removal of products from the catalyst.

Examples of catalytic reactions

Catalysts are used not only in the chemical industry. Any person in his daily life is faced with various catalytic reactions. This is, for example, the use of hydrogen peroxide in the treatment of wounds. Hydrogen peroxide, when interacting with blood, begins to decompose under the influence of:

2H 2 O 2 -> O 2 + 2H 2 O.

In modern cars, it is equipped with special catalytic chambers that contribute to the decomposition of harmful gaseous substances. For example, platinum or rhodium helps reduce nitrogen oxide pollution, which breaks down to form harmless O 2 and N 2 .

Some toothpastes contain enzymes that break down plaque and food debris.

One of the most common methods for controlling reaction rates is the use of catalysts.

Catalysts- these are substances that actively participate in the intermediate stages of the reaction, change the rate of the overall process, but are found in the reaction products in an unchanged state.

The change in the reaction rate in the presence of catalysts is called catalysis, and the reactions themselves catalytic reactions.

There are two approaches to the classification of catalytic reactions.

1. By the presence of the phase boundary, there are:

– homogeneous catalysis when the reactants, catalyst and reaction products are in the volume of one phase;

– heterogeneous catalysis when the catalyst and reactants with reaction products are in different phases; often the catalyst forms a solid phase, while the reactants and products are in the liquid phase or in the gas phase.

2. By the nature of the change in the reaction rate, it happens:

– positive catalysis, at which the catalyst increases the rate of the reaction;

– negative catalysis (inhibition), at which the catalyst ( inhibitor) slows down the reaction rate;

– autocatalysis when the reaction product plays the role of a catalyst; for example, in the hydrolysis of an ester

CH 3 COOSH 3 + H 2 O CH 3 COOH + CH 3 OH

the acetic acid formed as a result of the reaction splits off a hydrogen ion, which begins to play the role of a catalyst for the hydrolysis reaction. Therefore, at first, a slowly proceeding reaction has an increasingly increasing rate over time.

To explain the mechanism of catalytic reactions, it was proposed intermediate theory. According to this theory, with positive catalysis, the catalyst ( TO) forms an intermediate compound with one of the reagents at a high rate, which also quickly interacts with the second reagent:

A + B D(slowly)

1) A + K AK(fast)

2) AK + B D + K(fast)

Figure 4a shows that the activation energy of the non-catalytic process is much higher than the activation energies of the first and second stages of the catalytic conversion. Thus, with positive catalysis the role of the catalyst is to reduce the activation energy of the reaction.

reaction path a)

Reaction path b) reaction path a)

Figure 4 Energy diagrams of the catalytic reaction (a) and

inhibited reaction (b)

In inhibition reactions, the inhibitor ( I) forms a strong intermediate compound at high speed ( AI), which very slowly turns into a reaction product:

A + B D(slowly)

1) A + IAI(very fast)

2) AI + B D + I(So ​​slow)

Figure 4b shows that the first stage of inhibition, compared to the uninhibited process, has a lower activation energy and proceeds very quickly. At the same time, the activation energy of the second stage of inhibition is much higher than that of the uninhibited reaction. Thus, in inhibited reactions the role of the inhibitor is to increase the activation energy of the reaction.

FEATURES OF ENZYMATIC

catalysis

Enzymes(from lat. fermentum- starter) - biological catalysts present in all biological systems. They carry out the transformation of substances in the body, thereby directing and regulating the metabolism in it. Enzymes are widely used in the food and light industry. By chemical nature, enzymes are a globular protein molecule.

Enzymatic catalysis (biocatalysis)- this is the acceleration of chemical reactions in biological systems by special proteins - enzymes. Enzymatic catalysis is based on the same chemical laws as the basis of conventional chemical catalysis used in chemical production. However, enzymatic catalysis has its own characteristics:

1. Higher Activity in comparison with chemical catalysts (increase in speed by 10 10 - 10 13 times). This is because enzymatic reactions at all stages have very low activation energies (Figure 5).

2. Most enzymes are different specificity of action, so that almost every reaction of the transformation of the reactant ( substrate) into the product is carried out by a special enzyme. There are two theories of the specificity of enzyme action:

1) Fisher's theory(the "key-lock" theory): the enzyme and the substrate should approach each other in terms of spatial structure like a key to its lock;

2) Koshland's theory(the “hands and gloves” theory): the enzyme and the substrate separately may not have spatial forms corresponding to each other, but when approached, their configurations change in such a way that strict spatial correspondence becomes possible.

3. Enzymes tend to inactivation phenomenon- the destruction of the enzyme molecule after interaction with a certain number of substrate molecules. The higher the activity of the enzyme, the faster it is destroyed. The phenomenon of inactivation is explained by Koshland's theory. Indeed, the more active the enzyme, the more intensely it interacts with the substrate, at which the enzyme molecule undergoes significant spatial deformation. Such repeated deformation leads to the rupture of the weakest chemical bonds, that is, to the destruction of the enzyme molecule.

4. Each enzyme contains a protein molecule. One-component are only made up of a protein molecule two-component- from a protein molecule and a non-protein component associated with it (an inorganic ion or a molecule of an organic compound - most often a vitamin molecule or a product of its transformation) - cofactor. The molecular complex of a protein and a cofactor is called holoenzyme, which has the highest catalytic activity. In the composition of the holoenzyme, the protein part is called feron, and the non-protein part agon. A protein component devoid of a cofactor is called apoenzyme, and the cofactor separated from the protein molecule - coenzyme. Apart from the cofactor, the protein molecule has very low activity, and the coenzyme as a catalyst is generally inactive.

5. Most enzymes are regulated, that is, they are able to move from a state of low activity to a state of high activity and vice versa. The mechanism of regulation is a complex system by which the body controls all its functions.

6. Enzymes are very sensitive to the influence of external conditions. They are active in a relatively narrow range of temperatures and pH values.

The mechanism of enzymatic reactions is similar to the mechanism of reactions catalyzed by chemical catalysts:

S + E ES P + E,

that is, at first it is formed very quickly enzyme-substrate complex ES, which can dissociate back into the substrate S and enzyme E, but also slowly transform into the reaction product P. At a constant enzyme concentration, the dependence of the initial rate of substrate conversion v0 from its initial concentration is described Michaelis kinetic equation-Menten:

v0 = ,

Where Km And Vmax– kinetic parameters reflecting the mechanism of enzyme action.

The technique for determining these parameters is based on the use Lineweaver–Burke equations, which is obtained by transforming the Michaelis-Menten equation:

= +

Figure 6 shows the methodology for determining the parameters Km And Vmax. Vmax - is the maximum initial reaction rate at a given enzyme concentration [ E] (Figure 7). Molar activity of the enzyme(a E) is determined by the relation:

which shows the number of substrate molecules converted by one enzyme molecule per unit of time. For example, for the reaction CO 2 + H 2 O H 2 CO 3, catalyzed by the blood enzyme carbonate dehydratase and E \u003d 36 10 6 mol CO 2/ (min∙mol E), that is, 1 enzyme molecule in one minute catalyzes the transformation of 36 million molecules CO 2.

Figure 7 Dependence of the initial rate of the enzymatic reaction on the initial concentration of the substrate

Parameter Km has the meaning of the amount of substrate required to bind half of the available enzyme into the enzyme-substrate complex and achieve half of the maximum rate (Figure 7). That's why Km can be used to assess the specificity of the action of a certain enzyme in relation to a given substrate. For example, for the reaction

monosaccharide + ATP sugar phosphate + ADP,

catalyzed by the enzyme hexokinase, for glucose obtained K m= 8∙10 –6 mol/l, and for allose K m= 8∙10 –3 mol/l. Consequently, the enzyme interacts with glucose more preferentially, since it requires 1000 times less than alloses to achieve the same result.

4. CHEMICAL EQUILIBRIUM

When a chemical equilibrium state is reached, the number of molecules of substances ceases to change and remains constant in time under unchanged external conditions. Chemical equilibrium is characterized by the following features:

1) equality of the rates of direct and reverse reactions;

2) constancy of concentrations (partial pressures) of components under constant external conditions;

3) mobility, that is, the ability to spontaneously recover with small displacements;

4) equilibrium is reached by both forward and reverse reactions.

Consider the energy diagram of a chemical reaction

A + B D(Figure 8). For this reaction:

Figure 8 Energy diagram of a reversible chemical reaction

Consequently, at a given temperature, the forward and reverse reactions have well-defined rate constants. Therefore, in reversible reactions, the kinetic curves have the form shown in Figure 9 A. It can be seen from the figure that after reaching the time t p the concentrations of the components remain unchanged.

according to law at mass action

From figure 9 b It can be seen that after reaching the equilibrium time tp equality of speed is achieved. Then

Where K c= - chemical equilibrium constant determined from the equilibrium concentrations of the components.

Figure 9 Kinetic curves (a) and dependences of the rates of forward and reverse reactions on time (b) for a reversible reaction

In general, for the reaction

mA +nB qD +fE

the equilibrium constant is given by

Thus, K c is a parameter characteristic of the reaction system at a given temperature, which determines the ratio of the concentrations of the components in a state of chemical equilibrium.

If the reaction proceeds in the gas phase, then the partial pressures of the system components are used instead of concentrations. For the above equilibrium reaction, the equilibrium constant, determined from the partial pressures of the components in the equilibrium state, is found as

For ideal gases p i =C i RT. That's why

where - is the change in the number of moles of the components during the reaction.

Values K c And Kp depend on the temperature and on the nature of the components of the reaction systems.

From the Arrhenius equations for the direct and reverse reactions it follows:

ln k pr=ln A pr and ln k arr=ln A arr

Since then

ln K p=ln

Where ΔН pr is the thermal effect of the direct reaction.

It follows from the resulting equation that the dependence Kp has the form of a straight line and for it (Figure 10), whence follows .

For determining ΔH pr analytical method find the value Kp at two different temperatures and carry out calculations using the formula

ΔH pr

Figure 10 Determining the thermal effect of a direct endothermic reaction ( ΔН pr >0)

The last expression is called the integral equation isobars of a chemical reaction. It relates the equilibrium constants at two different temperatures and describes equilibrium systems in which the total pressure remains constant as the temperature changes.

If the volume of the system remains constant when the temperature changes, as, for example, in reactions in solutions, then the relationship between the parameters is expressed through isochore of a chemical reaction

ΔU pr .

Discussing the direction of chemical reactions from the point of view of chemical thermodynamics, it was noted that the system is in a state of chemical equilibrium under the condition ∆G= 0. Based on this position, the equation chemical reaction isotherms, which allows you to determine the sign ∆G and, accordingly, the direction of the chemical reaction under the condition of mixing the components of the reaction system in arbitrary ratios:

ΔG= RT(ln-ln Kp)

Where pA And p B- arbitrary partial pressures of the components obtained by mixing them.

A similar relationship was also proposed for a system whose components are in solution.

For example, for the reaction

mA+nB qD+fE,

the equilibrium of which is established in the liquid phase, the equation of the chemical reaction isotherm has the following form:

ΔG= RT(ln-ln K c)

where are the molar fractions of the components in the solution, which is obtained by mixing an arbitrary amount of substances A, B, D And E.

Balance shift. A change in temperature, concentration, pressure of a system that is in a state of equilibrium, brings it out of balance. But after a certain time, a new equilibrium state is again established in the system, the parameters of which already differ from the initial state. Such a transition of a system from one equilibrium state to another equilibrium state under changing conditions is called a shift in equilibrium. It is used to increase the yield of the target product for those systems that have small equilibrium constants. In addition, the equilibrium shift method can suppress parallel undesirable processes.

But at the same time, it is necessary to keep in mind two factors that do not affect the state of equilibrium. First, the introduction of a catalyst into an equilibrium system does not lead to a shift in equilibrium. The catalyst simultaneously lowers the activation energy of the forward and reverse reactions, which leads to an increase in the rate of both reactions to the same extent. As a result of the use of the catalyst, the equilibrium state is reached in a shorter period of time. Second, in heterogeneous equilibrium systems, the concentrations and partial pressures of insoluble and nonvolatile solids are not included in the expression of the equilibrium constant. For example, for the reaction FeO + CO Fe + CO 2 the equilibrium constant is defined as Kp= .

The effect of temperature. Equations isochores And isobars make it possible to predict the direction of equilibrium shift with temperature change. For example, if the system is in equilibrium and the forward reaction is exothermic (DH pr <0), то при повышении температуры (T 2 >T 1) the inequality K p ,2 K p ,1. This suggests that in the new equilibrium state, the partial pressure of the reaction products will be less, that is, the reaction will shift to the left.

An increase in temperature shifts the equilibrium in the direction of an endothermic reaction, and a decrease in temperature - in the direction of an exothermic reaction.

Thus, the highest yield of products is achieved:

For exothermic reactions at low temperatures;

For endothermic reactions at high temperatures.

Influence of concentration (partial pressure). The equation isotherms allows predicting the direction of equilibrium shift when the concentration of any component of the equilibrium system changes. Let the system be in equilibrium. Then ΔG=0 and the concentrations of the components in the isotherm equation correspond to the equilibrium values ​​and = K c. If part of the reaction products is removed from the system, then a nonequilibrium state arises with the ratio of parameters K c and correspondingly, ΔG< 0. The last inequality is a thermodynamic condition for the spontaneous occurrence of a direct reaction. Consequently, a new equilibrium state is achieved by converting part of the initial reagents into reaction products - by shifting the equilibrium to the right.

An increase in the concentration (partial pressure) of the initial reagents shifts the equilibrium towards the formation of products, and a decrease in their concentration (partial pressure) - towards the reverse transformation of the products into the original ones. An increase in the concentration (partial pressure) of the products shifts the equilibrium in the direction of the reverse reaction, and a decrease in their concentration (partial pressure) in the direction of the direct reaction.

Therefore, to increase the yield of the reaction product, it is necessary to increase the concentrations (partial pressures) of the initial reagents or reduce the concentration (partial pressures) of the products by gradually withdrawing them from the reaction system.

Influence of total system pressure. Let an equilibrium gas-phase system be given mA nB, for which n m, that is, the direct reaction proceeds with an increase in the number of molecules.

According to Dalton's law, pA = p∙y A And p B = p∙yB, Where R- total pressure in the system; r A, r B are the partial pressures of the components; y A , y B are the molar fractions of the components in the gas phase. Then the isotherm equation takes the following form

If at pressure p 1 the system is in equilibrium, then

.

Increasing the pressure to p 2 brings the system out of equilibrium. Because ( p-t) 0, then the following relation of system parameters arises

And ∆G> 0.

This is the thermodynamic condition for the reverse reaction to occur. Consequently, with increasing pressure, a new equilibrium state will arise as a result of the reverse transformation of the product IN to the original connection A, resulting in a decrease in the total number of molecules in the system.

Summarizing the results obtained, the following conclusions can be drawn:

An increase in the total pressure of the system shifts the equilibrium in the direction of the reaction that occurs with a decrease in the number of molecules;

A decrease in the total pressure of the system leads to a shift in equilibrium towards the reaction that proceeds with an increase in the number of molecules.

The generalization of the patterns of influence of all factors on the direction of equilibrium shift leads to a rule called Le Chatelier's principle:

if an external influence is exerted on an equilibrium system (change the temperature, concentration or partial pressures of the components, total pressure), then it will react in such a way that the effect of this influence is weakened.

PHOTOCHEMICAL REACTIONS

Chemical reactions that take place under the influence of light are called photochemical reactions. The most important photochemical reactions include the formation of ozone from molecular oxygen under the action of ultraviolet radiation from the Sun:

O 2 + h O

O + O 2 O 3 + O

The resulting ozone About 3 absorbs ultraviolet rays in the range of 250-260 mmk, which have a detrimental effect on living organisms. Another important photochemical reaction is photosynthesis, which results in the absorption of carbon dioxide from the atmosphere by plants and the release of oxygen. The photochemical decomposition of silver bromide is at the heart of the photographic process.

Photon (radiation quantum) energy ( E) is determined by the relation

E = h

Where h- Planck's constant (h 6.626 10 J∙s); - radiation frequency, s. The wavelength of visible light, infrared and ultraviolet rays lies in the range from 100 nm to 1000 nm, and their energy is from 120 kJ / mol to 1200 kJ / mol. A radiation quantum is absorbed by one single electron of an atom in a molecule, as a result of which this electron passes to a higher energy level. As a result, there are three different consequences of absorbing energy in the form of radiation:

1. An atom or molecule goes into an excited state:

A + h A *

M + h M *

2. Dissociation of a molecule with the formation of atoms or free radicals:

AB + h A + B

3. Education simple or molecular ions by removing one electron:

A + h A + +

AB + h AB + +

All these processes are subject to the following laws.

1. Photochemical reactions can only be caused by that part of the incident radiation that is absorbed by the reacting system ( Grotthuss-Dreper's law).

2. Each absorbed radiation quantum causes the transformation of only one molecule ( Einstein-Stark law).

3. The amount of product formed as a result of a photochemical reaction is proportional to the intensity of the absorbed radiation and the exposure time ( Van't Hoff's law).

The last law can be represented in mathematical form:

m = kt,

Where m is the mass of the photochemically converted substance, g; is the power of the absorbed radiation, i.e. the amount of energy that transfers the luminous flux through a unit area per unit time, J / s; t– irradiation time, s.; k– reaction rate constant, g/J.

During experimental verification of the 1st and 2nd laws, an apparent discrepancy is sometimes observed. Firstly, the number of absorbed quanta is not equal to the number of reacted molecules of the substance, i.e. as if the Einstein-Stark law is violated. Therefore, to characterize photochemical processes, the concept quantum yield, which is equal to the ratio of the number of actually reacted molecules to the number of absorbed quanta. The value varies in the range from 10 -3 to 10 6 . At<1 поглощенная световая энергия частично расходуется на побочные процессы, такие как передача энергии на другие молекулы и самопроизвольное протекание обратного процесса. При >1 leaks in the system chain reaction. In this case, the absorbed radiation quantum causes the appearance of one active particle, which subsequently creates a chain of secondary transformations.

Secondly, some substances do not absorb light in the visible or ultraviolet region, however, they are able to undergo transformation when irradiated. Thus, as it were, Grotgus's law is violated. It turned out that in this case the radiation quantum is absorbed by special substances - photosensitizers, which transfer the absorbed energy to another substance, which undergoes a chemical transformation as a result. Therefore, the violation of the Grotthuss law is only apparent. For example, molecular hydrogen does not absorb light with a wavelength of 253.7 nm. However, when a mixture of mercury vapor and hydrogen is irradiated, the process of dissociation of hydrogen molecules into atoms is observed:

Hg + h Hg*

Hg * + H 2 Hg + H + H

A similar photosensitized process is photosynthesis- synthesis of carbohydrates from carbon monoxide (IV) and water, accompanied by the release of oxygen. The chlorophyll molecule acts as a sensitizer for this photochemical reaction. Moreover, chlorophyll b captures and collects the energy of light radiation. After photoexcitation, it transfers excess energy to the chlorophyll molecule A, which then takes a direct part in the process of photosynthesis.

The total process of photosynthesis is expressed by the reaction:

6CO 2 + 6H 2 O C 6 H 12 O 6 + 6H 2 O, G 0 = 2861.9 kJ/mol

Photosynthesis is a complex redox process that combines photochemical reactions with enzymatic ones. In the mechanism of photosynthesis, two stages are distinguished - light And dark blue. The light stage includes the photochemical reactions themselves and their associated enzymatic reactions, which complete the oxidation of water and form reduced nicotinamide adenine dinucleotide phosphate ( NADPH 2) and adenosine triphosphoric acid ( ATP). In the dark stage NADPH 2 And ATP restore the molecule CO 2 before CH 2 O and then a monosaccharide is formed in a cycle of coupled enzymatic reactions that take place without the participation of a radiation quantum.

SOLUTION PROPERTIES

GENERAL INFORMATION

Solutions called homogeneous(single-phase) systems consisting of solvents, solutes and products of their interaction, the concentrations of which can vary over a wide range.

They can be solid, liquid and gaseous. Processes in biological objects and technological processes in the processing industry of agriculture proceed in aqueous solutions. Therefore, in the future, we restrict ourselves to consideration of only aqueous solutions of various substances.

During dissolution, a uniform distribution of molecules or ions of the dissolved substance occurs in the volume of the solvent. However, dissolution cannot be regarded as a purely physical process of diffusion of one substance into another. This is evidenced by the release of a significant amount of heat when certain substances are dissolved in water ( H2SO4, NaOH and others). It has been established that chemical interactions are possible between solvent molecules and molecules or ions of the solute, accompanied by the breaking of some and the formation of other chemical bonds. This leads to the formation of products of the interaction of the solvent with the solute, which are called solvates, and in aqueous solutions hydrates. The interaction process itself is called solvation or hydration.

Solutions are currently considered physical-chemical systems Occupying in their properties an intermediate position between mechanical mixtures and chemical compounds, and have their characteristic physical and chemical patterns.

The main characteristic of any solution is its concentration. As a rule, the solvent is that component of the solution, which is contained in a relatively large amount and determines its phase state. The physicochemical properties of solutions depend on their concentrations. There are many such dependencies. All of them were obtained on the assumption that the solution is perfect. perfect a solution is called in which:

1) the concentration of the dissolved substance is very low - the mole fraction is less than 0.005;

2) the solute is non-volatile, that is, its molecules cannot leave the liquid phase into the gas phase;

3) there are no forces of interaction between the particles of the solution, that is, the heat of mixing is zero ( H p= 0) and there is no change in the volume of the system ( Vp = 0);

Catamlis- selective acceleration of one of the possible thermodynamically allowed directions of a chemical reaction under the action of a catalyst (s), which repeatedly enters into an intermediate chemical interaction with the participants in the reaction and restores its chemical composition after each cycle of intermediate chemical interactions. The term "catalysis" was introduced in 1835 by the Swedish scientist Jöns Jakob Berzelius.

The phenomenon of catalysis is widespread in nature (most of the processes occurring in living organisms are catalytic) and is widely used in technology (in oil refining and petrochemistry, in the production of sulfuric acid, ammonia, nitric acid, etc.). Most of all industrial reactions are catalytic.

Catalysts Substances that change the rate of chemical reactions are called.

Some catalysts greatly accelerate the reaction - positive catalysis, or just catalysis, others slow down - negative catalysis. Examples of positive catalysis are the production of sulfuric acid, the oxidation of ammonia to nitric acid using a platinum catalyst, etc.

According to the influence on the reaction rate, catalysis is divided into positive (the reaction rate increases) and negative (the reaction rate decreases). In the latter case, an inhibition process takes place, which cannot be considered "negative catalysis", since the inhibitor is consumed during the reaction.

Catalysis can be homogeneous and heterogeneous (contact). In homogeneous catalysis, the catalyst is in the same phase as the reactants, while heterogeneous catalysts differ in phase.

homogeneous catalysis.

An example homogeneous catalysis is the decomposition of hydrogen peroxide in the presence of iodine ions. The reaction proceeds in two stages:

H2 O2+I> H2O+IO, H2O2+io> H2O + O2+ I

In homogeneous catalysis, the action of the catalyst is due to the fact that it interacts with the reactants to form intermediate compounds, which leads to a decrease in the activation energy.

heterogeneous catalysis.

In heterogeneous catalysis, the acceleration of the process usually occurs on the surface of a solid body - the catalyst, so the activity of the catalyst depends on the size and properties of its surface. In practice, the catalyst is usually supported on a solid porous support.

The mechanism of heterogeneous catalysis is more complicated than that of homogeneous catalysis. The mechanism of heterogeneous catalysis includes five stages, all of which are reversible.

• 1. Diffusion of reactants to the surface of a solid
• 2. Physical adsorption on the active sites of the surface of a solid substance of reacting molecules and then their chemisorption
• 3. Chemical reaction between reacting molecules
• 4. Desorption of products from the catalyst surface
• 5. Diffusion of the product from the catalyst surface into the general flow

An example of heterogeneous catalysis is the oxidation of SO 2 to SO 3 on a V 2 O 5 catalyst in the production of sulfuric acid (contact method).

Most catalytic reactions are carried out on porous catalysts, the inner surface of which consists of pores and channels of various sizes and lengths. These pores may be isolated or connected to each other. The main factor determining the rate and nature of the movement of gases in the pores of the catalyst is the pore size. The speed of the free movement of molecules can reach 1000 m/s, and the deceleration of movement in the pores is associated with collisions between gas molecules and with the walls of the pores.

Most catalytic reactions are nonselective, which imposes known limitations on kinetic methods of analysis.

Most catalytic reactions involve several different types of atoms and molecules. Determining the mechanism of the reaction and the nature of the forces acting between these atoms and molecules and between them and the surface is, of course, a difficult task, but it can be simplified by studying the adsorption behavior of one type of atoms or molecules. Such studies have shown that when certain molecules are adsorbed on certain adsorbents, the bond in the molecule is broken and two bonds with the adsorbent appear; in this case, the adsorbed molecule transforms into two adsorbed atoms. This process is a surface chemical reaction, and the formed adsorbed atoms are called chemisorbed atoms. If such a reaction does not occur at sufficiently low temperatures and the adsorbed molecules do not decompose into two adsorbed atoms, then such molecules are called physically adsorbed.

S. I. LEVCHENKOV

PHYSICAL AND COLLOID CHEMISTRY

Abstract of lectures for students of the Faculty of Biology of the Southern Federal University (RSU)

2.3 CATALYTIC PROCESSES

The rate of a chemical reaction at a given temperature is determined by the rate of formation of the activated complex, which, in turn, depends on the value of the activation energy. In many chemical reactions, the structure of the activated complex may include substances that are not stoichiometrically reactants; Obviously, in this case, the value of the activation energy of the process also changes. In the case of the presence of several transition states, the reaction will proceed mainly along the path with the lowest activation barrier.

Catalysis is the phenomenon of changing the rate of a chemical reaction in the presence of substances whose state and quantity remain unchanged after the reaction.

Distinguish positive And negative catalysis (respectively, an increase and decrease in the reaction rate), although often the term "catalysis" means only positive catalysis; negative catalysis is called inhibition.

A substance that is part of the structure of an activated complex, but is not stoichiometrically a reactant, is called a catalyst. All catalysts are characterized by such general properties as specificity and selectivity of action.

Specificity The catalyst lies in its ability to accelerate only one reaction or a group of reactions of the same type and not affect the rate of other reactions. For example, many transition metals (platinum, copper, nickel, iron, etc.) are catalysts for hydrogenation processes; aluminum oxide catalyzes hydration reactions, etc.

Selectivity catalyst - the ability to accelerate one of the parallel reactions possible under given conditions. Due to this, it is possible, using different catalysts, to obtain different products from the same starting materials:

: CO + H 2 ––> CH 3 OH	: C 2 H 5 OH -–> C 2 H 4 + H 2 O
: CO + H 2 -–> CH 4 + H 2 O	: C 2 H 5 OH -–> CH 3 CHO + H 2

The reason for the increase in the reaction rate with positive catalysis is the decrease in the activation energy when the reaction proceeds through the activated complex with the participation of the catalyst (Fig. 2.8).

Since, according to the Arrhenius equation, the rate constant of a chemical reaction is exponentially dependent on the activation energy, a decrease in the latter causes a significant increase in the rate constant. Indeed, if we assume that the pre-exponential factors in the Arrhenius equation (II.32) for catalytic and non-catalytic reactions are close, then for the ratio of rate constants we can write:

If ΔE A = –50 kJ/mol, then the ratio of the rate constants will be 2.7·10 6 times (indeed, in practice, such a decrease in E A increases the reaction rate by approximately 10 5 times).

It should be noted that the presence of a catalyst does not affect the magnitude of the change in the thermodynamic potential as a result of the process and, therefore, no catalyst can make a thermodynamically impossible process spontaneous (of a process whose ΔG (ΔF) is greater than zero). The catalyst does not change the value of the equilibrium constant for reversible reactions; the effect of the catalyst in this case consists only in accelerating the achievement of an equilibrium state.

Depending on the phase state of the reactants and the catalyst, homogeneous and heterogeneous catalysis are distinguished.

Rice. 2.8 Energy diagram of a chemical reaction without a catalyst (1)
and in the presence of a catalyst (2).

2.3.1 Homogeneous catalysis.

Homogeneous catalysis is a catalytic reaction in which the reactants and the catalyst are in the same phase. In the case of homogeneous catalytic processes, the catalyst forms intermediate reactive products with the reagents. Consider some reaction

A + B ––> C

In the presence of a catalyst, two fast steps are carried out, resulting in the formation of particles of the intermediate compound AA and then (via the activated complex AVK #) the final reaction product with catalyst regeneration:

A + K ––> AK

AK + V -–> C + K

An example of such a process is the decomposition of acetaldehyde, the activation energy of which is E A = 190 kJ/mol:

CH 3 CHO -–> CH 4 + CO

In the presence of iodine vapor, this process proceeds in two stages:

CH 3 CHO + I 2 ––> CH 3 I + HI + CO

CH 3 I + HI -–> CH 4 + I 2

The decrease in the activation energy of this reaction in the presence of a catalyst is 54 kJ/mol; in this case, the reaction rate constant increases approximately by a factor of 105. The most common type of homogeneous catalysis is acid catalysis, in which hydrogen ions H + act as a catalyst.

2.3.2 Autocatalysis.

Autocatalysis- the process of catalytic acceleration of a chemical reaction by one of its products. An example is the hydrolysis of esters catalyzed by hydrogen ions. The acid formed during hydrolysis dissociates with the formation of protons, which accelerate the hydrolysis reaction. A feature of the autocatalytic reaction is that this reaction proceeds with a constant increase in the concentration of the catalyst. Therefore, in the initial period of the reaction, its rate increases, and at subsequent stages, as a result of a decrease in the concentration of reagents, the rate begins to decrease; the kinetic curve of the product of an autocatalytic reaction has a characteristic S-shaped form (Fig. 2.9).

Rice. 2.9 Kinetic curve of the autocatalytic reaction product

2.3.3 Heterogeneous catalysis.

heterogeneous catalysis - catalytic reactions occurring at the interface between the phases formed by the catalyst and the reactants. The mechanism of heterogeneous catalytic processes is much more complicated than in the case of homogeneous catalysis. In each heterogeneous catalytic reaction, at least six stages can be distinguished:

1. Diffusion of starting materials to the catalyst surface.

2. Adsorption of starting materials on the surface with the formation of some intermediate compound:

A + B + K -–> AVK

3. Activation of the adsorbed state (the energy required for this is the true activation energy of the process):

AVK ––> AVK #

4. Decomposition of the activated complex with the formation of adsorbed reaction products:

ABK # ––> CDK

5. Desorption of reaction products from the catalyst surface.

СDК ––> С + D + К

6. Diffusion of reaction products from the catalyst surface.

A specific feature of heterocatalytic processes is the ability of the catalyst to be promoted and poisoned.

Promotion– an increase in the activity of the catalyst in the presence of substances that are not themselves catalysts of this process (promoters). For example, for a reaction catalyzed by metallic nickel

CO + H 2 -–> CH 4 + H 2 O

the introduction of a small impurity of cerium into the nickel catalyst leads to a sharp increase in the activity of the catalyst.

Poisoning- a sharp decrease in the activity of the catalyst in the presence of certain substances (so-called catalytic poisons). For example, for the ammonia synthesis reaction (catalyst - sponge iron), the presence of oxygen or sulfur compounds in the reaction mixture causes a sharp decrease in the activity of the iron catalyst; at the same time, the ability of the catalyst to adsorb the initial substances decreases very slightly.

To explain these features of heterogeneous catalytic processes, G. Taylor made the following assumption: not the entire surface of the catalyst is catalytically active, but only some of its sections - the so-called. active centers , which may be various defects in the crystal structure of the catalyst (for example, protrusions or depressions on the surface of the catalyst). At present, there is no unified theory of heterogeneous catalysis. For metal catalysts, a multiplet theory . The main provisions of the multiplet theory are as follows:

1. The active center of the catalyst is a set of a certain number of adsorption centers located on the surface of the catalyst in geometric accordance with the structure of the molecule undergoing transformation.

2. When reacting molecules are adsorbed on the active center, a multiplet complex is formed, as a result of which the bonds are redistributed, leading to the formation of reaction products.

The theory of multiplets is sometimes called the theory of geometric similarity between the active center and reacting molecules. For different reactions, the number of adsorption centers (each of which is identified with a metal atom) in the active center is different - 2, 3, 4, etc. Such active centers are called respectively doublet, triplet, quadruplet, etc. (in the general case, a multiplet, to which the theory owes its name).

For example, according to the theory of multiplets, the dehydrogenation of saturated monohydric alcohols occurs on a doublet, and the dehydrogenation of cyclohexane - on a sextet (Fig. 2.10 - 2.11); The multiplet theory made it possible to relate the catalytic activity of metals to their atomic radius.

Rice. 2.10 Dehydrogenation of alcohols on a doublet

Rice. 2.11 Dehydrogenation of cyclohexane on a sextet

2.3.4 Enzymatic catalysis.

Enzymatic catalysis - catalytic reactions occurring with the participation of enzymes - biological catalysts of protein nature. Enzymatic catalysis has two characteristic features:

1. high activity , which is several orders of magnitude higher than the activity of inorganic catalysts, which is explained by a very significant decrease in the activation energy of the process by enzymes. So, the rate constant of the reaction of decomposition of hydrogen peroxide catalyzed by Fe 2+ ions is 56 s -1 ; the rate constant of the same reaction catalysed by the enzyme catalase is 3.5·10 7 , i.e. the reaction in the presence of the enzyme proceeds a million times faster (the activation energies of the processes are 42 and 7.1 kJ/mol, respectively). The rate constants of urea hydrolysis in the presence of acid and urease differ by thirteen orders of magnitude, amounting to 7.4·10 -7 and 5·10 6 s -1 (the activation energy is 103 and 28 kJ/mol, respectively).

2. High specificity . For example, amylase catalyzes the breakdown of starch, which is a chain of identical glucose units, but does not catalyze the hydrolysis of sucrose, the molecule of which is composed of glucose and fructose fragments.

According to the generally accepted ideas about the mechanism of enzymatic catalysis, substrate S and enzyme F are in equilibrium with a very rapidly formed enzyme-substrate complex FS, which decomposes relatively slowly to the reaction product P with the release of a free enzyme; thus, the stage of decomposition of the enzyme-substrate complex into reaction products is rate-determining (limiting).

F+S<––>FS ––> F+P

The study of the dependence of the rate of the enzymatic reaction on the concentration of the substrate at a constant concentration of the enzyme showed that with an increase in the concentration of the substrate, the reaction rate first increases and then ceases to change (Fig. 2.12) and the dependence of the reaction rate on the concentration of the substrate is described by the following equation:

(II.45)

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 life. Biological catalysts called enzymes are involved in the regulation of biochemical processes. Many industrial processes would not be possible without catalysts.

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. This allows reactions that are too slow under normal conditions to be of practical use, 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, 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.

In 1835, the Swedish chemist J. Berzelius found that in the presence of certain substances, the rate of certain chemical reactions increases significantly. For such substances, he introduced the term "catalyst" (from the Greek.

catalysis- relaxation). According to Berzelius, catalysts have a special ability to weaken the bonds between atoms in the molecules involved in the reaction, thus facilitating their interaction. A great contribution to the development of ideas about the operation of catalysts was made by the German physicochemist W. Ostwald, who in 1880 defined a catalyst as a substance that changes the reaction rate.

According to modern concepts, a catalyst forms a complex with reacting molecules, which is stabilized by chemical bonds. After rearrangement, this complex dissociates to release products and catalyst. For a monomolecular reaction of the transformation of a molecule

X to Y The whole process can be represented as X + Cat. ® X -Cat. ® Y -Cat. ® Y + Cat. The liberated catalyst re-binds with X , and the whole cycle is repeated many times, providing the formation of large quantities of the product - substance Y . Many substances under normal conditions do not enter into a chemical reaction with each other. So, hydrogen and carbon monoxide at room temperature do not interact with each other, since the bond between atoms in a molecule H2 strong enough and does not break when attacked by a molecule CO . Catalyst Brings Molecules Together H2 and CO by forming connections with them. After rearrangement, the catalyst-reactant complex dissociates to form a product containing atoms C, H, and O. Often, when the same substances interact, different products are formed. The catalyst can direct the process along the path most favorable for the formation of a particular product. Consider the reaction between CO and H2 . In the presence of a copper-containing catalyst, methanol is practically the only reaction product:At first, CO and H molecules 2 adsorbed on the catalyst surface. Then the CO molecules form chemical bonds with the catalyst (chemisorption occurs), remaining in the undissociated form. Hydrogen molecules are also chemisorbed on the catalyst surface, but dissociate at the same time. As a result of the rearrangement, the transition complex H-Cat.- CH2OH . After adding an atom H the complex breaks down to release CH 3 OH and catalyst. In the presence of a nickel catalyst, both CO and H 2 are chemisorbed on the surface in a dissociated form, and the Cat.-CH complex is formed 3 . The end products of the reaction are CH 4 and H 2 O:
Most catalytic reactions are carried out at certain pressures and temperatures 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 the reaction conditions and characterize the products. Space velocity - the volume of gas or liquid passing through a unit volume of the catalyst per unit time. Catalytic activity - the amount of reactants converted by the catalyst into products per unit of time. Conversion is the proportion of a substance converted in a given reaction. Selectivity is the ratio of the amount of a certain product to the total amount of products (usually expressed as a percentage). Yield - the ratio of the amount of a given product to the amount of starting material (usually expressed as a percentage). Productivity - the amount of reaction products formed per unit volume per unit time. TYPES OF CATALYSTS Catalysts are classified according to the nature of the reaction they promote, their chemical composition, or their physical properties. Almost all chemical elements and substances have catalytic properties to one degree or another - by themselves or, more often, in various combinations. According to their physical properties, catalysts are divided into homogeneous and heterogeneous. Heterogeneous catalysts are solids that are homogeneous and dispersed in the same gaseous or liquid medium as the reactants.

Many heterogeneous catalysts contain metals. Some metals, especially those related to

VIII group of the periodic system of elements, have catalytic activity by themselves; a typical example is platinum. But most metals exhibit catalytic properties, being in the composition of compounds; example - alumina (alumina 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 efficient interaction of the 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 part of the total surface area. Catalysts can 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 centers is the subject of intense research.

Homogeneous catalysts have different chemical nature - acids (H

2 SO 4 or H 3 RO 4 ), bases (NaOH ), organic amines, metals, most often transitional ( 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 in which reactions occur in living organisms. The industry often uses the so-called. immobilized enzymes. HOW CATALYSTS WORK Energy. Any chemical reaction can proceed 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 series of successive stages. Each one needs energy to run.E called the activation energy. The change in energy along the reaction coordinate is shown in fig. 1.

Consider first the non-catalytic, "thermal" path. For a reaction to take place, the potential energy of the molecules

X must exceed the energy barrierE T . The catalytic reaction consists of three stages. The first is the formation of the X-Cat complex. (chemisorption), the activation energy of which isE ads . The second stage is the X-Cat rearrangement.®Y -Cat. with activation energyE cat , and finally, the third - desorption with activation energyE des; E ads, E kat and E des much smaller E T . Since the reaction rate depends exponentially on the activation energy, the catalytic reaction proceeds much faster than the thermal one at a given temperature.

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

Theories of catalysis. Three groups of theories have been proposed to explain the mechanism of catalytic reactions: 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 due to electronic interaction associated with charge transfer, i.e. these theories relate catalytic activity to the electronic properties of the catalyst. Chemical theory considers a catalyst as a chemical compound with characteristic properties that forms chemical bonds with reactants, 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, while the other interacts with it while being directly in the gas phase. An alternative mechanism is also possible: the reacting molecules are adsorbed on neighboring active sites 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 catalyst surface and its catalytic activity. The rate of some catalytic processes, including many hydrogenation reactions, does not depend on the mutual arrangement of catalytically active atoms on the surface; the speed of others, on the contrary, changes significantly with a change in the spatial configuration of surface atoms. An example is the isomerization of neopentane to isopentane and the simultaneous cracking of the latter to isobutane and methane on the catalyst surface.

Pt-Al 2 O 3 . 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, 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, chemicals, 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.

Cm. 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 since the 1920s, catalytic methods have been developed one after another 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 "construction" 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. To disconnect N-N 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 CH 3 (CH 2) 7 CH \u003d 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 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-AlCl 2 O 3 (or HAlCl 4 ), 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 the structure of silica (silicon dioxide), in which some of the atoms

Si 4+ replaced by atoms Al3+. 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 reactionn -butane to isobutane in the presence of HCl - AlCl 3 or Pt - Cl - Al 2 O 3 . First, a small amount of olefin C 4 H 8 attaches a positively charged hydrogen ion to an acid catalyst to form m tertiary carbenium ion. Then negatively charged hydride ion H - split off from n -butane to form isobutane and secondary butylcarb e no d-ion. Last as a result of rearrangement becomes tertiary carb e ni ion. This chain can continue with the elimination of a hydride ion from the next moleculen- butane, etc.:
essential o that 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). 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-

tert -butyl ether (MTBE) with a blending octane number of 109 also makes it possible to obtain the high-octane fuel required for the operation of an automobile engine with a high compression ratio without resorting to the introduction of tetraethyl lead into gasoline. The production of fuels with octane numbers 102 and 111 is also organized.Basic 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 basicamines. 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 interacts 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.

Combining a hydrogen activating catalyst with a hydrocarbon isomerization promoting catalyst gives a good result. 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 to 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 transformation

n -butane to isobutane is accompanied by dehydrogenation, contributing to the production of MTBE.Stereospecific polymerization. An important milestone in history catalysis came about discovery of catalytic polymerizationa-olefins with the formation stereoregular x polymer ov. To catalysts stereospecific polymerization 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. 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 proceeds on the surface of solid catalysts containing transition metals of the 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 the groups I-III . A classic example of such a catalyst is the precipitate formed during the interaction TiCl 4 and Al(C 2 H 5 ) 3 in heptane, where titanium is reduced to the trivalent state. Thisexceptionally activethe 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 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 oxidizepair -xylene to terephthalic acid, the esters of which serve as the basis for the production of polyester fibers.Heterogeneous oxidation catalysts. 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 is carried out at 850 ° With using various catalysts; best results obtained with catalyst Li - MgO . 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 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 gasoline fraction hydrocarbons. 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. 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 restrictions

pair -xylene is formed more easily than more voluminousortho- 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 engine 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 is especially effective here

ZSM-5. Catalysts and environmental protection. The use of catalysts to reduce air pollution began at the end 19 40s. 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 up to CO 2 and H 2 A. In 1970, the Clean Air Declaration (revised 1977, expanded 1990) was formulated, requiring all new vehicles 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.

Especially for automobile catalytic converters, catalysts have been created 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 coated 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 bacteria

Clostridium acetobutylicum 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 products and preservation of the enzyme. One example of the industrial use of immobilized enzymes is isomerization D -glucose to fructose. TECHNOLOGICAL ASPECTS Modern technologies cannot be imagined without the use of catalysts. Catalytic reactions can proceed at temperatures up to 650° C and pressures of 100 atm or more. This makes it necessary to solve the problems associated with the contact between gaseous and solid substances and with the transfer of catalyst particles in a new way. For the process to be effective, its modeling must take into account the kinetic, thermodynamic and hydrodynamic aspects. Computer modeling is widely used here, as well as new instruments and methods for controlling technological processes.

In the 1960s significant progress was made in the production of ammonia. The use of a more active catalyst made it possible to lower the temperature of hydrogen production during the decomposition of water vapor, due to which it was possible to lower the pressure and, consequently, 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 huge 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 labels, X-ray, infrared and Raman (Raman) spectroscopy, electron microscopy methods are used; kinetic measurements are carried out, the influence of the methods of obtaining catalysts on their activity is studied. Of great importance is the determination of the surface area of ​​the catalyst by the Brunauer-Emmett-Teller method (BET method), based on the measurement of nitrogen physical adsorption at different pressures. To do this, determine the amount of nitrogen required for the formation of a monolayer on the surface of the catalyst, and, knowing the diameter of the molecule N 2 , calculate the total area. In addition to determining the total surface area, chemisorption of various molecules is carried out, which makes it possible to estimate the number of active centers and obtain information about their properties.

Researchers have at their disposal various methods for studying the surface structure of catalysts at the atomic level. Unique information allows you to get a method

EXAFS . Among the spectroscopic methods, UV, X-ray, and Auger photoelectron spectroscopy are increasingly being used. Of great interest is secondary ion mass spectrometry and ion scattering spectroscopy. NMR measurements are used to study the nature of catalytic complexes. The scanning tunneling microscope allows you to see the arrangement of atoms on the surface of the catalyst. PERSPECTIVES The scale of catalytic processes in industry is increasing every year. Catalysts are increasingly being used to neutralize environmental pollutants. The role of catalysts in the production of hydrocarbons and oxygen-containing synthetic fuels from gas and coal is growing. It seems very promising to create fuel cells for the economical conversion of fuel energy into electrical energy.

New concepts of catalysis will make it possible to obtain polymeric materials and other products with many valuable properties, improve energy production methods, 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 the production of 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