What substances catalyze reactions. Catalysis

The rates of chemical reactions can increase dramatically in the presence of various substances that are not reactants and are not part of the reaction products. This remarkable phenomenon is called catalysis(from the Greek "katalysis" - destruction). A substance that increases the rate of a reaction in a mixture is called catalyst. Its amount before and after the reaction remains unchanged. Catalysts do not represent any special class of substances. In various reactions, metals, oxides, acids, salts, and complex compounds can exhibit a catalytic effect. Chemical reactions in living cells proceed under the control of catalytic proteins called enzymes. Catalysis should be considered as a true chemical factor in increasing the rates of chemical reactions, since the catalyst is directly involved in the reaction. Catalysis is often more powerful and less risky in speeding up a reaction than raising the temperature. This is clearly manifested in the example of chemical reactions in living organisms. Reactions, such as the hydrolysis of proteins, which in laboratories have to be carried out with prolonged heating to the boiling point, during digestion proceed without heating at body temperature.

For the first time, the phenomenon of catalysis was observed by the French chemist L. J. Tenard (1777-1857) in 1818. He discovered that oxides of certain metals, when hydrogen peroxide is added to a solution, cause its decomposition. Such an experience is easy to reproduce by adding crystals of potassium permanganate to a 3% hydrogen peroxide solution. Salt KMp0 4 turns into Mn0 2, and oxygen is quickly released from the solution under the action of oxide:

The direct effect of the catalyst on the reaction rate is associated with a decrease in the activation energy. At normal temperature decrease? and by 20 kJ/mol increases the rate constant by approximately 3000 times. downgrade E L may be much stronger. However, the decrease in the activation energy is an external manifestation of the action of the catalyst. The reaction is characterized by a certain value E. v which can only change if the reaction itself changes. Giving the same products, the reaction with the participation of the added substance proceeds along a different path, through different stages and with a different activation energy. If on this new path the activation energy is lower and the reaction is correspondingly faster, then we say that this substance is a catalyst.

The catalyst interacts with one of the reactants, forming some intermediate compound. At one of the subsequent stages of the reaction, the catalyst is regenerated - it leaves the reaction in its original form. Reagents, participating in a catalytic reaction, continue to interact with each other and along a slow path without the participation of a catalyst. Therefore, catalytic reactions belong to a variety of complex reactions called series-parallel. On fig. 11.8 shows the dependence of the rate constant on the concentration of the catalyst. The dependence graph does not pass through zero, since in the absence of a catalyst, the reaction does not stop.

Rice. 11.8.

observable constant k expressed as a sum k u+ & k c(k)

Example 11.5. At a temperature of -500 °C, the oxidation reaction of sulfur oxide (IV)

which is one of the stages of industrial production of sulfuric acid, is very slow. A further increase in temperature is unacceptable, since the equilibrium shifts to the left (exothermic reaction) and the product yield drops too much. But this reaction is accelerated by various catalysts, one of which may be nitric oxide (II). First, the catalyst reacts with oxygen:

and then transfers an oxygen atom to sulfur oxide (IV):

Thus, the final product of the reaction is formed and the catalyst is regenerated. For the reaction, the possibility of flowing along a new path was opened, in which the rate constants increased significantly:

The diagram below shows both pathways of the S0 2 oxidation process. In the absence of a catalyst, the reaction proceeds only along the slow path, and in the presence of a catalyst, along both.

There are two types of catalysis - homogeneous and heterogeneous. In the first case, the catalyst and reagents form a homogeneous system in the form of a gas mixture or solution. An example of sulfur oxide oxidation is homogeneous catalysis. The rate of a homogeneous catalytic reaction depends on both the concentrations of the reactants and the concentration of the catalyst.

In heterogeneous catalysis, the catalyst is a solid in pure form or supported on carrier. For example, platinum as a catalyst can be fixed on asbestos, alumina, etc. Reagent molecules are adsorbed (absorbed) from a gas or solution at specific points on the catalyst surface - active centers and are activated at the same time. After the chemical transformation, the resulting product molecules are desorbed from the catalyst surface. Acts of particle transformation are repeated at active centers. Among other factors, the rate of a heterogeneous catalytic reaction depends on the surface area of the catalytic material.

Heterogeneous catalysis is especially widely used in industry. This is due to the ease of carrying out a continuous catalytic process with the passage of a mixture of reagents through a contact apparatus with a catalyst.

Catalysts act selectively, accelerating a very specific type of reaction or even a single reaction without affecting others. This makes it possible to use catalysts not only to speed up reactions, but also to purposefully convert starting materials into desired products. Methane and water at 450 ° C on the Fe 2 0 3 catalyst are converted into carbon dioxide and hydrogen:

The same substances at 850 °C react on the nickel surface to form carbon monoxide (II) and hydrogen:

Catalysis belongs to those areas of chemistry in which it is not yet possible to make accurate theoretical predictions. All industrial catalysts for the processing of petroleum products, natural gas, ammonia production and many others have been developed on the basis of laborious and lengthy experimental studies.

The ability to control the speed of chemical processes is of inestimable importance in human economic activity. In the industrial production of chemical products, it is usually necessary to increase the rates of technological chemical processes, and in the storage of products, it is required to reduce the rate of decomposition or exposure to oxygen, water, etc. Known substances that can slow down chemical reactions. They're called inhibitors, or negative catalysts. Inhibitors are fundamentally different from real catalysts in that they react with active species (free radicals) that, for one reason or another, arise in a substance or its environment and cause valuable decomposition and oxidation reactions. Inhibitors are gradually consumed, ending their protective action. The most important type of inhibitors are antioxidants, which protect various materials from the effects of oxygen.

It should also be reminded of what cannot be achieved with the help of catalysts. They are capable of accelerating only spontaneous reactions. If the reaction does not proceed spontaneously, then the catalyst will not be able to accelerate it. For example, no catalyst can cause water to decompose into hydrogen and oxygen. This process can be carried out only by electrolysis, while spending electrical work.

Catalysts can also activate unwanted processes. In recent decades, there has been a gradual destruction of the ozone layer of the atmosphere at an altitude of 20-25 km. It is assumed that some substances are involved in the decay of ozone, for example, halogenated hydrocarbons emitted into the atmosphere by industrial enterprises, as well as used for domestic purposes.

In this article, catalytic reactions will be considered. The reader will be introduced to a general idea of catalysts and their effect on the system, and the types of reactions, the features of their course, and much more will be described.

Introduction to catalysis

Before getting acquainted with catalytic reactions, it is worth knowing what catalysis is.

It is a selective process of accelerating a certain thermodynamically allowed direction of reaction that is exposed to a catalyst. It is repeatedly included in the interaction of a chemical nature, and it has an effect on the participants in the reaction. At the end of any cycle of an intermediate nature, the catalyst resumes its original form. The concept of a catalyst was introduced into circulation by J. Barzelius and Jens in 1835.

General information

Catalysis is widely distributed in nature and is widely used by man in the technological industry. The predominant number of all reactions used in industry are catalytic. There is a concept of autocatalysis - a phenomenon in which the accelerator acts as a reaction product or is part of the starting compounds.

All types of chemical interaction of reacting substances are divided into catalytic and non-catalytic reactions. The acceleration of reactions involving catalysts is called positive catalysis. The slowing down of the rate of interaction proceeds with the participation of inhibitors. The reactions are negative-catalytic in nature.

The catalytic reaction is not only a way to increase productivity, but also an opportunity that improves the quality of the resulting product. This is due to the ability of a specially selected substance to accelerate the main reaction and slow down the speed of parallel ones.

Catalytic reactions also reduce the cost of energy, which consumes the apparatus. This is due to the fact that acceleration allows the process to proceed at a lower temperature, which would be required without it.

An example of a catalytic reaction is the production of such valuable things as: nitric acid, hydrogen, ammonia, etc. These processes are most used in the production of aldehydes, phenol, various plastics, resins and rubbers, etc.

Variety of reactions

The essence of catalysis lies in the transfer of the mechanism of the reaction to the most profitable option. This becomes possible due to a decrease in the activation energy.

The catalyst forms a weak chemical bond with a specific reactant molecule. This makes it possible to facilitate the reaction with another reagent. Substances that are catalytic do not affect the shift in chemical equilibrium, since they act reversibly in both directions.

Catalysis is divided into two main types: homogeneous and heterogeneous. A common feature of all interactions of the first type is the presence of a catalyst in a common phase with the reactant of the reaction itself. The second type differs in this point.

Homogeneous catalytic reactions show us that the accelerator, interacting with a certain substance, forms an intermediate compound. This will further reduce the amount of energy required for activation.

Heterogeneous catalysis speeds up the process. As a rule, it flows on the surface of solid bodies. As a result, the capabilities of the catalyst and its activity are determined by the size of the surface and individual properties. A heterogeneous catalytic reaction has a more complex mechanism of operation than a homogeneous one. Its mechanism includes 5 stages, each of which can be reversible.

At the first stage, the diffusion of the interacting reagents to the area of the solid begins, then the adsorption of a physical nature occurs, followed by chemisorption. As a result, the third stage occurs, in which the reaction begins to proceed between the molecules of the reacting substances. At the fourth stage, desorption of the product is observed. At the fifth stage, diffusion of the final substance into the general flows from the catalyst plane takes place.

catalytic materials

There is a concept of a catalyst carrier. It is a material of an inert or low active type, necessary to bring the particle participating in the catalysis phase into a stable state.

Heterogeneous acceleration is necessary to prevent the processes of sintering and agglomeration of active components. In the prevailing number of cases, the amount of carriers exceeds the presence of the deposited component of the active type. The main list of requirements that the carrier must have include a large surface area and porosity, thermal stability, inertness and resistance to mechanical stress.

Chemical base. The chemistry of accelerating the flow of interaction between substances allows us to distinguish two types of substances, namely catalysts and inhibitors. The latter, in turn, slow down the rate of the reaction. One of the types of catalysts are enzymes.

Catalysts do not stoichiometrically interact with the product of the reaction itself and are always regenerated in the end. In modern times, there are many ways to influence the process of molecular activation. However, catalysis is the basis of chemical production.

The nature of catalysts allows them to be divided into homogeneous, heterogeneous, interfacial, enzymatic, and micellar. A chemical reaction with the participation of a catalyst will reduce the energy required for its activation. For example, the non-catalytic decomposition of NH3 to nitrogen and hydrogen would require about 320 kJ/mol. The same reaction, but under the influence of platinum, will reduce this number to 150 kJ/mol.

Hydrogenation process

The predominant number of reactions involving catalysts is based on the activation of a hydrogen atom and a certain molecule, which subsequently leads to an interaction of a chemical nature. This phenomenon is called hydrogenation. It underlies most of the stages of oil refining and the creation of liquid fuel from coal. The production of the latter was opened in Germany, due to the lack of oil deposits in the country. The creation of such a fuel is called the Bergius process. It consists in the direct combination of hydrogen and coal. Coal is subjected to heating under conditions of a certain pressure and the presence of hydrogen. As a result, a liquid type product is formed. The catalysts are iron oxides. But sometimes substances based on metals such as molybdenum and tin are also used.

There is another way to obtain the same fuel, which is called the Fischer-Tropsch process. It consists of two stages. At the first stage, coal is subjected to gasification by treating it with the interaction of water vapor and O 2 . This reaction leads to the formation of a hydrogen mixture and carbon monoxide. Further, with the help of catalysts, the resulting mixture is transferred to the state of liquid fuel.

Relationship between acidity and catalytic capacity

The catalytic reaction is a phenomenon dependent on the acidic properties of the catalyst itself. In accordance with the definition by J. Bronsted, an acid is a substance that can donate protons. A strong acid will easily donate its proton to a base. G. Lewis defined an acid as a substance capable of accepting electron pairs from donor substances and, as a result, forming a covalent bond. These two ideas allowed man to determine the essence of the mechanism of catalysis.

The strength of an acid is determined using sets of bases that can change their color due to the addition of a proton. Some catalytic agents used in industry can behave like extremely strong acids. Their strength determines the rate of protonation, and therefore is a very important characteristic.

The acidic activity of the catalyst is due to its ability to react with hydrocarbons, thus forming an intermediate product - carbenium ion.

Dehydrogenation process

Dehydrogenation is also a catalytic reaction. It is often used in various industries. Despite the fact that catalytic processes based on dehydrogenation are used less frequently than hydrogenation reactions, they nonetheless occupy an important place in human activity. An example of this type of catalytic reaction is the production of styrene, an important monomer. To begin with, dehydrogenation of ethylbenzene occurs with the participation of substances containing iron oxide. Man often uses this phenomenon to dehydrogenate many alkanes.

double action

There are dual-acting catalysts capable of accelerating two types of reaction at once. As a result, they lead to better results, in comparison with passing the reactants in turn through 2 reactors containing only one type of catalyst. This is due to the fact that the active site of the double action accelerator is in a close position with another similar site, as well as with the intermediate product. A good result is achieved, for example, by combining catalysts that activate hydrogen with a substance that allows the process of hydrocarbon isomerization to proceed. Activation is often carried out by metals, and isomerization proceeds with the participation of acids.

Specificity of the main catalytic reactions

The ability and efficiency of the catalyst are also determined by its basic properties. A striking example is sodium hydroxide, which is used during the hydrolysis of fats to produce soap. These types of catalysts are also used in the production of foam and polyurethane sheets. Urethane is obtained during the interaction of alcohol and isocyanate. The acceleration of the reaction occurs when exposed to a certain basic amine. The base is attached to the carbon atom contained in the isocyanate molecule. As a result, the nitrogen atom becomes negatively charged. This leads to an increase in activity in relation to alcohol.

Stereospecific polymerization

Of great historical importance in the history of the study of catalysis is the discovery of olefin polymerization followed by the production of stereoregular polymeric substances. The discovery of catalysts, which are characterized by stereospecific polymerization, belongs to K. Ziegler. Ziegler's work on obtaining polymers interested J. Natta, who suggested that polymer uniqueness should be determined by its stereoregularity. A large number of experiments involving X-rays subjected to diffraction have proven that the polymer obtained from propylene under the influence of a Ziegler catalyst is highly crystalline. The effect of the action is stereoregular in nature.

Reactions of this type take place on the plane of a solid catalyst containing transition metals, such as Ti, Cr, V, Zr. They must be in incomplete oxidation. The equation for the catalytic reaction between interacting TiCl 4 and Al(C 2 H 5) 3 , during which a precipitate is formed, is a vivid example of this. Here, titanium is reduced to a 3-valent state. This type of active system makes it possible to polymerize propylene under normal conditions of temperature and pressure.

Oxidation in a catalytic reaction

Catalytic oxidation reactions are widely used by man, due to the ability of certain substances to regulate the rate of the reaction itself. Some applications require complete oxidation, such as the neutralization of CO and hydrocarbon containing contaminants. However, the vast majority of reactions require incomplete oxidation. This is necessary to obtain industrially valuable, but intermediate products that may contain a specific and important intermediate group: COOH, CN, CHO, C-CO. In this case, a person uses both heterogeneous and monogenic types of catalysts.

Among all substances capable of accelerating the course of chemical reactions, an important place is given to oxides. Mostly in the solid state. The course of oxidation is divided into 2 stages. In the first stage, oxygen oxide is captured by the hydrocarbon molecule of the adsorbed oxide. As a result, the oxide is reduced and the hydrocarbon is oxidized. The renewed oxide interacts with O 2 and returns to its original state.

One of the most rapidly developing areas of organic chemistry in recent years is reactions catalyzed by transition metals. Such processes are widely used both for the synthesis of heterocyclic compounds and for their functionalization. The use of processes catalyzed by transition metals contributed to the development of not only completely new methods, but also made it possible to improve the well-known ones, increase the selectivity and ease of implementation of many processes. Palladium is one of the most important and widely used catalysts in various types of reactions. Nickel is also used as a catalyst, but the range of reactions catalyzed by nickel is much narrower.

In general, heterocyclic compounds enter into palladium-catalyzed reactions similarly to carbocyclic ones. Cycling sulfur and nitrogen atoms rarely interfere with such (homogeneous) palladium-catalyzed processes, although sulfur- and nitrogen-containing molecules are well known to be poisonous to heterogeneous hydrogenation catalysts (metal palladium on charcoal).

Processes catalyzed by palladium are usually carried out using 1-5 mol. % catalyst, and palladium active intermediates are formed in low concentration. Such processes are a sequence of stages, each of which is associated with the participation of an organopalladium compound. The mechanism of processes involving organopalladium intermediates is rather consistent than ionic, and it is unacceptable to compare such reactions with processes that are similar at first glance in "classical" organic chemistry. Curved arrows can be used to facilitate remembering of such processes, in a manner similar to that used in considering cycloaddition reactions, and it is this "explanation" of palladium-catalyzed reactions that will be used in their further discussion.

2012-2019. Chemistry of heterocyclic compounds. Heterocyclic Chemistry.
Rules for determining the main heterocycle: If the cycles have different heteroatoms, then the cycle with a higher serial number of heteroatoms is the main one.

The educational publication, written by famous English scientists, outlines the basic theoretical ideas about the reactivity and methods for the synthesis of various classes of heterocyclic compounds and their individual representatives; the role of heterocyclic compounds in solid state chemistry, biological processes, and the chemistry of semiconductor polymers is shown. Particular attention is paid to the coverage of the latest achievements in this important area of organic chemistry, which is of great importance in medicinal chemistry, pharmacology and biochemistry. According to the completeness and breadth of the material presented, it can be used as a reference and encyclopedic publication.

Introduction

1. General provisions and regularities of catalysis

2. Homogeneous catalysis

3. Acid and base catalysis

4. Homogeneous catalytic reactions catalyzed by complex compounds

5. Enzymatic catalysis

6. Heterogeneous catalysis

Conclusion

List of sources used

Introduction

Catalysis is the phenomenon of a change in the rate of a reaction in the presence of catalysts. Reactions involving catalysts are called catalytic. Substances that increase the rate of a chemical reaction, while remaining unchanged as a result of the overall reaction, are called catalysts.

There are many different types of catalysts and many different mechanisms of action. The catalyst goes through cycles in which it is first bound, then regenerated, bound again, and so on many times. The catalyst allows the reaction to proceed in a different way, and at a faster rate than it does in the absence of a catalyst. The speed can be increased by lowering the activation energy, increasing the pre-exponential factor, or both.

The catalyst simultaneously accelerates both the forward and reverse reactions, so that the equilibrium constant of the overall reaction remains unchanged. If this were not so, then it would be possible to construct a perpetual motion machine using a catalyst to regenerate matter

1. General provisions and regularities of catalysis

Catalysts are divided into homogeneous and heterogeneous. A homogeneous catalyst is in the same phase with the reactants, a heterogeneous one forms an independent phase separated by an interface from the phase in which the reactants are located. Typical homogeneous catalysts are acids and bases. Metals, their oxides and sulfides are used as heterogeneous catalysts.

Reactions of the same type can proceed with both homogeneous and heterogeneous catalysts. Thus, along with acid solutions, solid Al 2 O 3 , TiO 2 , ThO 2 , aluminosilicates, and zeolites with acidic properties are used. Heterogeneous catalysts with basic properties: CaO, BaO, MgO.

Heterogeneous catalysts, as a rule, have a highly developed surface, for which they are distributed on an inert support (silica gel, alumina, activated carbon, etc.).

For each type of reaction, only certain catalysts are effective. In addition to the acid-base ones already mentioned, there are oxidation-reduction catalysts; they are characterized by the presence of a transition metal or its compound (Co +3, V 2 O 5 +, MoO 3). In this case, catalysis is carried out by changing the oxidation state of the transition metal.

Many reactions are carried out with the help of catalysts that act through the coordination of reactants at the atom or ion of the transition metal (Ti, Rh, Ni). Such catalysis is called coordination catalysis.

If the catalyst has chiral properties, then an optically active product is obtained from an optically inactive substrate.

In modern science and technology, systems of several catalysts are often used, each of which accelerates different stages of the reaction. The catalyst can also increase the speed of one of the stages of the catalytic cycle carried out by another catalyst. This is where "catalysis of catalysis" or second-level catalysis takes place.

Enzymes play the role of catalysts in biochemical reactions.

Catalysts must be distinguished from initiators. For example, peroxides break down into free radicals that can initiate radical chain reactions. Initiators are consumed during the reaction, so they cannot be considered catalysts.

Inhibitors are sometimes mistakenly considered negative catalysts. But inhibitors, such as radical chain reactions, react with free radicals and, unlike catalysts, are not preserved. Other inhibitors (catalytic poisons) bind to the catalyst and deactivate it, which is catalysis suppression rather than negative catalysis. Negative catalysis is impossible in principle: it would provide a slower path for the reaction, but the reaction, of course, will go along a faster, in this case, not catalyzed, path.

The catalyst may be one of the reaction products. In this case, the reaction is called autocatalytic, and the phenomenon itself is called autocatalysis. For example, during the oxidation of Fe 2+ with Mn0 4

5Fe 2+ + Mn0 4 - + 8H+ \u003d 5Fe 3+ + Mn 2+ + 4H 2 0

the resulting Mn 2+ ions catalyze the course of the reaction.

Catalytic reactions are extremely common in nature. The most surprising of these are reactions with enzymes, which catalyze many reactions in living organisms. Catalysts are widely used in industry. Production of nitric and sulfuric acids, ammonia, production of synthetic rubber, etc. impossible without catalytic reactions. Catalysts are used in the production of medicinal substances: phenacetin, guaiacol, halogen derivatives of aromatic compounds, etc. Mn(IV), Ni, Co, Fe, A1C1 3 , TeC1 3 oxides are used as catalysts.

There are homogeneous and heterogeneous catalysis, but for any of them the main regularities are as follows:

1. The catalyst actively participates in the elementary act of the reaction, forming either intermediate compounds with one of the participants in the reaction, or an activated complex with all reactants. After each elementary act, it is regenerated and can interact with new molecules of reacting substances.

2. The rate of a catalytic reaction is proportional to the amount of catalyst.

3. The catalyst has selectivity of action. It can change the rate of one reaction and not affect the rate of another.

4. The catalyst allows the reaction to proceed in a different way, and at a faster rate than it does in the absence of a catalyst.

The speed can be increased by lowering the activation energy, increasing the pre-exponential factor, or both. For example, the thermal decomposition of acetaldehyde CH 3 CHO CH 4 + CO is catalyzed by iodine vapor, which causes a decrease in the activation energy by ~55 kJ/mol. This decrease causes an increase in the rate constant by a factor of about 10,000.

5. The catalyst does not affect the position of thermodynamic equilibrium. It equally changes the rate of both forward and reverse reactions.

6. When certain substances, called promoters, are added, the activity of the catalyst increases; the addition of inhibitors reduces the rate of the reaction.

2. Homogeneous catalysis

In homogeneous catalysis, the catalyst is a molecule or ion in a homogeneous solution. In the case of homogeneous catalysis, the catalyst and all reactants form one common phase.

The main assumption of the theory of homogeneous catalysis is the idea that in the course of the reaction unstable intermediate compounds of the catalyst with the reactants are formed, which then decompose with the regeneration of the catalyst:

A + B + K = (A-B-K)* D + K

The rate of this reaction

v=k nc Ac Bc K

is proportional to the catalyst concentration, and the rate constant obeys the Arrhenius equation. This reaction can proceed in two stages:

catalysis homogeneous acid enzymatic heterogeneous

In this case, two cases are possible. In the first stage, the rate of decomposition of the complex into the catalyst and the initial product is much higher than the rate of the second stage, in which the final product is formed. Therefore, the concentration of complexes, which are called Arrhenius complexes in this type of catalysis, is low. In the second case, the rate of decomposition of the complex is commensurate with the rate of the second stage. The concentration of the intermediate complex is significant and stationary. Complexes of this type are called van't Hoff complexes.

The second case, as more typical, will be considered in more detail. Since the intermediate compound AA is in equilibrium with the starting materials, the rates of the direct (v 1) and reverse (v 2) reactions (1) must be equal. Compiling kinetic equations for them, we obtain:

where (With to"-- With AK") is the concentration of the catalyst that did not react; With BUT,With AK"-- equilibrium concentrations of substance A and intermediate compound AA, respectively.

From (2) we find the concentration of the intermediate compound:

The overall rate of the entire process (v) is determined by the rate of the slowest stage, in this case the second. Then

Substituting in (4) the concentration of the intermediate compound (3), we obtain:

Equation (5) indicates the possibility of the existence of two limiting regimes:

In both cases, the reaction rate is directly proportional to the concentration of the catalyst, but the reaction order for the starting materials is different. In the first case, it is equal to two, and in the second - to one. Outside the limiting regimes, the order of the reaction will be fractional.

An example of homogeneous catalysis is the reaction of thermal decomposition of acetaldehyde CH 3 CH 4 + CO, catalyzed by iodine vapor. In the absence of iodine vapor E a=191.0 kJ/mol, in their presence E a= 136.0 kJ/mol. The rate constant increases by a factor of 10,000. This is because the reaction proceeds in two stages:

CH 3 SON + I 2 \u003d CH 3 I + HI + CO

CH 3 I + HI \u003d CH 4 + I 2

The activation energy of each step is less than the activation energy of the non-catalytic reaction.

Homogeneous catalysis includes many acid-base reactions, complex formation reactions, redox reactions, numerous hydrogenation, sulfation reactions, etc.

3. Acid and base catalysis

Acids and bases in many reactions act as a catalyst, i.e., participating in the reaction, they themselves are not consumed (reactions of hydrolysis, alkylation, esterification, etc. There are three types of acid-base catalysis:

1) specific acid (basic) catalysis, in which H + or OH ions serve as a catalyst, respectively;

2) total acid (base) catalysis, which is carried out by any proton donor (acceptor);

3) electrophilic (nucleophilic) catalysis carried out by Lewis acids and bases.

First order rate constant k for the reaction in a buffer solution can be a linear function of [H + ], [OH - ], [HA], [A - ], i.e.:

k \u003d k 0 + k 1 [H+] + k 2 [OH -] + k 3 [ON] + k 4 [A -]

In this expression k 0 -- rate constant of the first order in the absence of all catalytic ions: [H + ], [OH - ], [NA], [A - ], a k t -- catalytic coefficients.

If only the term k 1 [H + ] plays a significant role, then they say that the reaction manifests itself in specific catalysis by hydrogen ions. If a member predominates k 3 [HA], the reaction is said to be subject to general acid catalysis. If the member predominates k 4 [A - ], then the reaction is said to be subject to the action of a common base catalysis.

For specific acid-base catalysis when the rate of the non-catalytic reaction is low (k 0 = 0) can be represented in logarithmic form:

For acidic solutions:

For alkaline solutions:

The equations indicate that, in the case of specific acid-base catalysis, the logarithm of the rate constant depends linearly on the pH of the medium.

The mechanism of the catalytic action of hydrogen ions is that an intermediate compound of a proton and a molecule of the original substance is formed. Due to this process, the chemical bonds present in the initial substance are loosened, the activation energy is reduced, and then the protonated form of BH + decomposes into a reaction product and a catalyst.

4. Homogeneous catalytic reactions catalyzed by complex compounds

The reactions of reduction, hydrogenation, oxidation, isomerization, polymerization under industrial conditions are carried out in the presence of catalysts - complex compounds (metal ions of group VIII of the periodic table Fe, Co, Ni, Ru, as well as Cu, Fg, Hg, Cr, Mn). The essence of the catalytic action is that metal ions act as donors or acceptors of electrons. The chemical interaction between reacting molecules coordinated around the central metal ion is facilitated by polarization of the molecules and a decrease in the energy of individual bonds. The central metal ion is a bridge that facilitates electronic transitions between reacting molecules.

The catalytic activity of a metal ion depends on the binding energy of the ion with the participants in the reaction. If the binding energy is high or low, the metal ion exhibits weak catalytic activity. In the first case, the metal ions are so strongly bound to the reacting molecules that they are removed from the reaction. In the second case, the reacting molecules cannot displace other ligands present in the solution. Coordination-saturated complexes are obtained, which are not active catalysts.

Due to the wide possibilities in regulating the composition of complex catalysts, it became possible to simulate a number of reactions involving enzymes containing ions of group VIII elements.

5. Enzymatic catalysis

Enzymes are the most amazing catalysts. Many reactions in living organisms are associated with them, and therefore they are often called biological catalysts. Enzymatic catalysis is a more complex phenomenon than conventional catalysis. The high organization of enzymatic catalysis processes is determined by the peculiarity of interaction in a living organism, associated with a special combination of the molecular structure of enzymes and substrates, which are called reactants in enzymatic reactions.

Enzymes are proteins, i.e. are made up of amino acids linked by peptide bonds. The enzyme molecule has alternating polar groups COOH, NH 2 , NH, OH, SH, etc., as well as hydrophobic groups. The primary structure of an enzyme is determined by the order of alternation of the various amino acids. As a result of thermal chaotic motion, the enzyme macromolecule bends and coils into loose balls. Intermolecular interaction occurs between individual sections of the polypeptide chain, leading to the formation of hydrogen bonds. The secondary structure of the enzyme appears in the form of a loose medium. For each enzyme, the secondary structure is quite definite. The active catalytic center of the enzyme includes groups that orient the substrate molecules in a certain position. The active center is like a matrix, which can only include a molecule of a certain structure. The mechanism of enzymatic catalysis consists in the interaction of the active sites of the enzyme with the substrate to form an enzyme-substrate complex, which then undergoes several transformations, as a result of which a reaction product appears. Each of the intermediate steps is characterized by a lower activation energy, which contributes to the rapid progress of the reaction. This explains the high activity of enzymes.

Enzymes are divided into classes depending on what type of reaction they catalyze: oxidoreductases (catalyze redox reactions), transferases (catalyze the transfer of chemical groups from one compound to another), hydrolases (catalyze hydrolysis reactions), lyases (break various bonds) , isomerases (carry out isomeric transformations), ligases (catalyze synthesis reactions). As can be seen, enzymes differ in specificity and selectivity. Some catalyze a whole class of reactions of a certain type, some catalyze only one reaction.

Many enzymes contain metal ions (metal enzymes). In metalloenzymes, metal ions form chelate complexes that provide the active structure of the enzyme. Metals with a variable degree of oxidation (Fe, Mn, Cu) participate in redox reactions, carrying out the transfer of electrons to the oxidizing agent. Several dozens of organic compounds are known that perform the functions of hydrogen and electron transfer. They contain derivatives of vitamins.

Heavy metal ions (Ag + , Hg + , Pb 2+) can block the active groups of enzymes.

To assess the action of various enzymes, the concept of molecular activity was introduced, which is determined by the number of substrate molecules that are converted under the action of one enzyme molecule per minute. The most active of the known enzymes is carbonic anhydrase, whose molecular activity is ~36 million molecules per minute.

The rate of a reaction catalyzed by an enzyme is directly proportional to the concentration of the enzyme. At a low substrate concentration, the reaction is of the first order with respect to the substrate. At high concentrations, the reaction rate remains constant and the reaction order becomes zero (the enzyme is completely saturated with the substrate). The reaction rate depends on the temperature and acidity of the medium.

Enzymatic catalysis plays a huge role in all manifestations of life, where we are talking about living beings. To increase the vital activity of the body and improve metabolism, many enzyme preparations have been created that are used as medicines. Enzyme preparations are widely used for violations of the function of the gastrointestinal tract associated with insufficient production of digestive enzymes. So, in some forms of gastritis, pepsin or pancreatin preparations are used. Enzymes are also successfully used in cases where it is necessary to destroy protein formations that have accumulated in large quantities (for burns, purulent wounds, purulent-inflammatory diseases of the lungs, etc.). In these cases, protolytic enzymes are used, leading to rapid hydrolysis of proteins and facilitating the resorption of purulent accumulations. For the treatment of a number of infectious diseases, lysozyme preparations are used, which destroy the membrane of some pathogenic bacteria. Enzymes that dissolve blood clots (blood clots inside blood vessels) are very important. This is plasmin found in the blood; pancreatic enzymes - trypsin and chymotrypsin. On their basis, with various additives, medicinal enzyme preparations have been created - streptokinase, streptase, and others used in medicine.

6. Heterogeneous catalysis

Heterogeneous catalysis is carried out at the interface. The first observed heterogeneous catalytic reaction was carried out by Priestley (1778) dehydration of ethyl alcohol on active clay:

C 2 H 5 OH -- C 2 H 4 + H 2 O

In the first half of the 19th century, a large number of works were devoted to heterogeneous catalysis. Many works have been devoted to the theoretical explanation of the catalytic action of a solid. In the future, the development of the doctrine went both along the path of accumulating experimental data, developing methods for preparing catalysts, discovering and studying new catalytic processes, introducing catalysis into the chemical industry, and along the path of developing the theory of heterogeneous catalysis. However, the success of the theorists was much more modest than the success of the experimenters. And this is no coincidence.

Although there is no fundamental difference between catalytic and non-catalytic processes, both of which obey the laws of chemical kinetics, in both cases the system of reacting substances passes through some special active state, specific features are observed in heterogeneous catalytic reactions. First of all, a solid body appears, on the properties of which all phenomena as a whole essentially depend. Therefore, it is not accidental that the advances in the theory of heterogeneous catalysis are inextricably linked with the development of the theory of solids. Since the process proceeds on the surface, knowledge of the structure of the catalyst surface is decisive for the development of the theory of catalysis. From this follows a close connection between the development of the theory of catalysis and the development of the experimental and theoretical study of adsorption phenomena. The complexity of heterogeneous processes and their inherent specificity lead to the fact that theoretical research in this area has not yet been completed. So far, we can talk about the existence of several theoretical concepts that, in a first approximation, generalize certain experimental facts.

In practice, two types of heterogeneous catalysis are most often encountered:

1) processes, the catalyst of which is in the solid phase, and the reactants are in the liquid phase;

2) processes, the catalyst of which is in the solid phase, and the reactants are in the gas phase. The reaction, as a rule, occurs (and in some multistage processes begins) at the phase boundary, i.e. on the surface of a solid body - a catalyst.

The heterogeneous process can be divided into five stages:

1) transport of reactants to the catalyst surface (diffusion);

2) adsorption of reactants on the catalyst surface;

3) reaction on the surface;

4) desorption of reaction products with the release of the catalyst surface;

5) transport of reaction products into the volume (diffusion).

Depending on the conditions of the process and its characteristics, any of the five stages can be the slowest, and, consequently, the rate of the catalytic process can be limited by any of them. For a comparative assessment of the activity of catalysts, the determining factor is the reaction rate on the surface. Therefore, in those cases where it is important to obtain the value of the activity of the catalyst, they try to conduct the process in such a way that the rate is determined by the second, so-called kinetic stage.

Adsorption and desorption have their own laws. Adsorption is the process of spontaneous change in the concentration of a substance on the interface. The substance on whose surface adsorption takes place is called adsorbent. The adsorbent is called adsorbate. In heterogeneous catalysis, the adsorbent is the catalyst, and the adsorbate is the molecule of the reactant (substrate). Adsorption of the substrate on the catalyst can be carried out due to the forces of interaction arising between the molecules (atoms) of the catalyst located on the surface and the molecules of the substrate (physical adsorption). A chemical interaction (chemical adsorption or chemisorption) can occur between the molecules (atoms) of the catalyst and the molecules of the reactant. As a result of adsorption, the ordering of the system increases, the energy of the system decreases, and the activation energy of the reaction decreases.

For heterogeneous processes, the movement of a substance from the internal volume of a liquid or gas to a solid surface is of particular importance. Mass transfer processes obey the laws of diffusion.

Conclusion

The importance of catalysts and catalytic processes in oil refining and petrochemistry cannot be overestimated. After all, they are the basis of technical progress in the most important areas of meeting the needs of modern human society. The point is, first of all, that oil from various fields usually contains only 5 to 20% of light-boiling fractions corresponding to gasoline. The need for gasoline with the modern development of automobile and air transport is enormous. At the same time, motor fuels distilled directly from oil usually turn out to be of poor quality. The use of catalytic cracking and reforming in combination with other modern processing methods makes it possible to increase the yield of highly active gasolines up to 75% of the oil weight. Motor fuels are also obtained by catalytic hydrogenation of coal using metal catalysts.

Further catalytic processing of hydrocarbons on metal and oxide catalysts makes it possible to obtain intermediate products necessary in the production of consumer goods. Most monomers and polymers derived from them are products of catalytic processes for the processing of hydrocarbons and their derivatives obtained from oil, coal, shale, and natural gas. Catalytic processes play an important role in the production of detergents, dyes for medicinal substances.

The main organic synthesis giving intermediates (and products of organic technology) is based mainly on catalytic reactions. Of great importance in the life of modern society are such products of the chemical industry as sulfuric acid, ammonia and nitric acid. Almost all branches of the national economy consume these substances or other chemical compounds obtained with their help. On their basis, tens of millions of tons of mineral fertilizers are produced, without which it is impossible to increase or even maintain the yield of fields. Hundreds of industries in the chemical, petrochemical, food, light and other industries use sulfuric, nitric acids, ammonia and their derivatives. These compounds are also used in the metallurgical and metalworking industries.

Meanwhile, the large-scale production of sulfuric acid, ammonia, and nitric acid from ammonia became possible only thanks to the discovery of appropriate catalysts and the development of methods for their use.

List of sources used

1) A.P. Belyaev. Physical and colloidal chemistry. M.: GOETAR-Media, 2008

2) I.P. Mukhlenov. Catalyst technology. M.: Bookinist, 2007

3) Chemical encyclopedia. -- M.: Soviet Encyclopedia, 1990.

4) Imyanitov N.S. Systems of several catalysts in metal complex catalysis. // Coordination chemistry. 1984.

Categories

Editor's Choice