The rate of chemical reactions can increase significantly under the influence of substances called catalysts.
The phenomenon of changing the rate of a reaction in the presence of catalysts is called catalysis, and reactions with their participation are called catalytic.
A catalyst is a simple or complex substance that takes part in a chemical reaction and changes its rate, but in the end remains in a chemically unchanged state. If the rate of a chemical reaction increases under the influence of a catalyst, then such catalysis is called positive, and if it decreases, then negative.
Transition metals and their compounds exhibit catalytic properties - oxides, hydroxides, sulfides, amines, amino acids, etc. They are capable of not only significantly accelerating reactions, but also changing their mechanism. For example, during the interaction of carbon monoxide (P) and hydrogen, depending on the nature of the catalyst, different products are formed - methane or methanol.
In the process of oxidation of methane with atmospheric oxygen, in the presence of different catalysts, methanol, formaldehyde or formic acid can be obtained.
Catalysts are widely used in the production of ammonia, sulfuric, nitric, acetic acids, rubber, in oil cracking processes, the synthesis of certain medications, and the like. The reactions of polymerization, hydrogenation and dehydrogenation, the production of alcohols, aldehydes, and carboxylic acids at a rate sufficient for technical needs occur only in the presence of catalysts.
Substances that slow down the rate of chemical reactions are called inhibitors.
Inhibitors are also widely used in technology. Their name is associated with the chemical or biochemical process that they slow down. In particular, substances that reduce the rate of metal corrosion are called corrosion inhibitors, and substances that inhibit any processes of oxidation of various substrates with molecular oxygen - antioxidants.
Catalysts are evaluated according to certain criteria, among which the most important are: activity, specificity, resistance to aging and poisoning.
Activity is determined by the ratio of the rates of catalytic and non-catalytic reactions. The more active a catalyst is, the more it reduces the activation energy of a reaction.
Specificity (selectivity) is the ability of a catalyst to increase the rate of only one reaction.
The activity of catalysts is significantly affected by impurities. Some of them can enhance, while others can slow down the action of catalysts. Substances that do not themselves have catalytic properties, but enhance the action of catalysts, are called promoters or activators.
It is also known that some chemicals have a negative effect on the activity of catalysts, the so-called catalytic poisons. These compounds partially or completely reduce the activity of catalysts.
One of the important types of catalytic processes is enzyme catalysis, which occurs under the influence of protein catalysts: so-called enzymes, or enzymes.
Enzymes as biological catalysts
All chemical processes in the physiological environment of the body (hydrolysis, protolysis, phosphorelation, complex formation, redox reactions) can only occur with the participation of catalysts, which are called enzymes, or enzymes.
Enzymes are protein substances that are produced by the cells of living organisms and significantly increase the speed of biochemical processes.
More than 1,800 enzymes are now known, many of which have been isolated in pure crystalline form. It is believed that a cell contains about 10 thousand molecules of various enzymes, which accelerate over 2 thousand reactions. A quarter of the currently studied enzymes contain ions of different metals and are therefore called metalloenzymes.
Both enzymes and inorganic catalysts obey the general laws of catalysis and are characterized by a number of common features, that is, they:
catalyze only those reactions that are energetically possible;
do not change the direction of reactions;
reduce the activation energy of reactions, thereby accelerating them;
are not consumed during the reaction.
However, enzymes are also characterized by special features that make it possible to distinguish them from conventional inorganic catalysts. These differences are associated with the structural features of enzymes, which are complex macromolecules of a protein nature.
Catalyst– a substance that changes the rate of a chemical reaction without being consumed. There are accelerating and decelerating catalysts.
Catalysis– the phenomenon of changes in reaction rates in the presence of catalysts.
Catalytic reactions– reactions occurring with the participation of catalysts.
If the catalyst is one of the reaction products, then the reaction is called autocatalytic, and the phenomenon itself – autocatalysis.
Inhibitor– a catalyst that slows down a reaction.
An example of positive catalysts is water when aluminum powder reacts with iodine.
Enzymes– biological catalysts of protein nature.
Enzymes are present in all living cells. It is customary to divide enzymes into simple and complex, or single-component or two-component. Simple enzymes consist only of protein, complex enzymes consist of protein and a non-protein part, which is called coenzyme.
Enzymes are characterized by high catalytic activity and selectivity. In terms of catalytic activity, they are significantly superior to inorganic catalysts. For example, 1 mole of catalase at 0 degrees decomposes 200,000 moles of H 2 O 2 in one second, and 1 mole of platinum at 20 degrees decomposes from 10 to 80 moles of hydrogen peroxide in one second.
Such acceleration of the reaction is due to the fact that enzymes sharply reduce the energy barriers to the reaction path. For example, the activation energy for the decomposition reaction of H 2 O 2 under the action of iron (II) ion and catalase molecules is respectively 42 and 7.1 kJ/mol; for hydrolysis of urea with acid and urease – 103 and 28 kJ/mol, respectively.
Enzymes are very specific compared to inorganic catalysts. For example, amylase contained in saliva easily and quickly breaks down starch, but does not catalyze the breakdown of sugar. Urease is exceptionally effective at catalyzing the hydrolysis of urea, but has no effect on its derivatives. This feature of enzymes allows living organisms, having an appropriate set of enzymes, to actively respond to external influences. For example, it has been noted that in stressful situations our body exhibits amazing capabilities. A fact is described when a weak woman lifted a car by the bumper and held it while people arrived in time to free a child who had fallen under it; a person pursued by an angry animal easily overcomes obstacles that are insurmountable for him in his normal state; At important competitions, athletes lose several kilograms in weight during the period of performance.
Everything that has been said about the remarkable properties of enzymes is explained by the fact that selectivity of action (selectivity) and activity are interrelated: the higher the selectivity, the higher its activity. Enzymes have unique selectivity, and therefore their activity is the highest.
Katamliz- 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 intermediate chemical interactions with reaction participants and restores its chemical composition after each cycle of intermediate chemical interactions. The term "catalysis" was introduced in 1835 by the Swedish scientist Jons Jakob Berzelius.
The phenomenon of catalysis is widespread in nature (most 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 simply catalysis, while others slow down - negative catalysis. Examples of positive catalysis include the production of sulfuric acid, the oxidation of ammonia into nitric acid using a platinum catalyst, etc.
Based on their effect on the reaction rate, many sources of catalysis are divided into positive (the reaction rate increases) and negative (the reaction rate decreases). In the latter case, an inhibition process occurs, 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 reaction reagents, while heterogeneous catalysts differ in phase.
Homogeneous catalysis.
Example homogeneous catalysis is the decomposition of hydrogen peroxide in the presence of iodine ions. The reaction occurs in two stages:
H 2 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 reacting substances to form intermediate compounds, this leads to a decrease in activation energy.
Heterogeneous catalysis.
In heterogeneous catalysis, the acceleration of the process usually occurs on the surface of a solid body—the catalyst; therefore, 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 complex 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 centers 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 surface of the catalyst 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 different sizes and lengths. These pores can be isolated or connected to each other. The main factor determining the speed and nature of the movement of gases in the pores of the catalyst is the pore size. The speed of free movement of molecules can reach 1000 m/s, and the inhibition of movement in pores is associated with collisions between gas molecules and with pore walls.
Most catalytic reactions are nonselective, which imposes certain limitations on kinetic analysis methods.
Most catalytic reactions involve several different types of atoms and molecules. Determining the reaction mechanism and the nature of the forces acting between these atoms and molecules and between them and the surface is naturally a complex problem, but it can be simplified by studying the adsorption behavior of one type of atom or molecule. 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 are formed; in this case, the adsorbed molecule transforms into two adsorbed atoms. This process is a surface chemical reaction, and the resulting adsorbed atoms are usually called chemisorbed atoms. If at sufficiently low temperatures such a reaction does not occur and the adsorbed molecules do not disintegrate into two adsorbed atoms, then such molecules are called physically adsorbed.
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Subtitles
Basic principles of catalysis
The catalyst changes the reaction mechanism to an energetically more favorable one, that is, it reduces the activation energy. The catalyst forms an intermediate compound with a molecule of one of the reagents, in which the chemical bonds are weakened. This makes it easier to react with the second reagent. It is important to note that catalysts accelerate reversible reactions in both forward and reverse directions. Therefore, they do not shift the chemical equilibrium.
Types of catalysis
Based on their effect on the reaction rate, many sources of catalysis are divided into positive (the reaction rate increases) and negative (the reaction rate decreases). In the latter case, an inhibition process occurs, which cannot be considered “negative catalysis”, since the inhibitor is consumed during the reaction.
Catalysis happens homogeneous And heterogeneous(contact). In homogeneous catalysis, the catalyst is in the same phase as the reaction reagents, while heterogeneous catalysts differ in phase.
Homogeneous catalysis
An example of homogeneous catalysis is the decomposition of hydrogen peroxide in the presence of iodine ions. The reaction occurs in two stages:
H 2 O 2 + I → H 2 O + IO H 2 O 2 + IO → H 2 O + O 2 + I
In homogeneous catalysis, the action of the catalyst is due to the fact that it interacts with reacting substances to form intermediate compounds, this leads to a decrease in activation energy.
Heterogeneous catalysis
In heterogeneous catalysis, the acceleration of the process usually occurs on the surface of a solid body—the catalyst; therefore, 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 complex than that of homogeneous catalysis. The mechanism of heterogeneous catalysis includes five stages, all of which are reversible.
- Diffusion of reactants to the surface of a solid
- Physical adsorption on the active centers of the surface of a solid substance of reacting molecules and then their chemisorption
- Chemical reaction between reacting molecules
- Desorption of products from the catalyst surface
- Diffusion of 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).
The course of the reaction on the surface of the catalyst can be demonstrated by an experiment in which a platinum plate is heated in the flame of a gas burner, then the flame is extinguished and a gas stream from the burner is released onto the plate, and the plate again becomes red-hot - oxidation of methane occurs on the surface of the metal.
Catalyst carrier
catalyst carrier, otherwise substrate (catalyst) (English carrier or support) - an inert or low-active material that serves to stabilize particles of the active catalytic phase on its surface.
The role of the support in heterogeneous catalysis is to prevent agglomeration or sintering of the active component, which allows maintaining a high contact area between the active substance (see active catalytic phase) and the reactants. The amount of carrier is usually much greater than the amount of active component applied to it. The main requirements for carriers are large surface area and porosity, thermal stability, chemical inertness, and high mechanical strength. In some cases, the carrier affects the properties of the active phase (the “strong metal-carrier interaction” effect). Both natural (clays, pumice, diatomite, asbestos, etc.) and synthetic materials (active carbons, silica gel, aluminosilicates, oxides of aluminum, magnesium, zirconium, etc.) are used as carriers.
Chemistry of catalysis
Chemistry of catalysis studies substances that change the rate of chemical reactions. Substances that slow down reactions are called inhibitors. Enzymes- These are biological catalysts. The catalyst is not in a stoichiometric relationship with the products and is regenerated after each cycle of converting reactants into products. Despite the emergence of new methods of activating molecules (plasma chemistry, radiation and laser effects, and others), catalysis is the basis of chemical production (the relative share of catalytic processes is 80-90%).
The reaction that fed humanity (the solution to the problem of fixed nitrogen) is the Haber-Bosch cycle. Ammonia is produced with a catalyst - porous iron. Occurs at P = 30 MPa and T = 420-500 °C
3H 2 + N 2 = 2NH 3
Hydrogen for the synthesis of NH 3 is obtained by two sequential catalytic processes: the conversion of CH 4 (CH 4 + H 2 O → CO + 3H 2) on Ni − catalysts and the conversion of the resulting carbon monoxide (CO + H 2 O → CO 2 + H 2) . To achieve high degrees of conversion, the last reaction is carried out in two stages: high temperature (315-480 °C) - on Fe - Cr - oxide catalysts and low temperature (200-350 °C) - on Cu - Zn - oxide catalysts. Ammonia is used to produce nitric acid and other nitrogen compounds - from drugs and fertilizers to explosives.
There are different types of catalysis homogeneous, heterogeneous, interfacial, micellar, enzymatic.
Activation energy E There are significantly fewer catalytic reactions than for the same reaction in the absence of a catalyst. For example, for the non-catalytic decomposition of NH 3 into N 2 + H 2 E~ 320 kJ/mol, for the same decomposition in the presence of platinum E~ 150 kJ/mol. Thanks to the reduction E ensures acceleration of catalytic reactions compared to non-catalytic ones.
Application of catalysis in industry
Hydrogenation
A large number of catalytic reactions are associated with the activation of a hydrogen atom and some other molecule, leading to their chemical interaction. This process is called hydrogenation and underlies many stages of oil refining and the production of liquid fuels from coal (Bergius process). The production of aviation gasoline and motor fuel from coal was developed in Germany during World War II because the country had no oil fields. The Bergius process involves the direct addition of hydrogen to coal. Coal is heated under pressure in the presence of hydrogen to produce a liquid product, which is then processed into aviation gasoline and motor fuel. Iron oxide is used as a catalyst, as well as catalysts based on tin and molybdenum. During the war, 12 factories in Germany produced approximately 1,400 tons of liquid fuel per day using the Bergius process. Another process, Fischer-Tropsch, consists of two stages. First, the coal is gasified, that is, it is reacted with water vapor and oxygen to produce a mixture of hydrogen and carbon oxides. This mixture is converted into liquid fuel using catalysts containing iron or cobalt. With the end of the war, the production of synthetic fuel from coal in Germany was discontinued. As a result of the rise in oil prices that followed the oil embargo of 1973–1974, vigorous efforts were made to develop a cost-effective method of producing gasoline from coal. Thus, direct liquefaction of coal can be carried out more efficiently using a two-stage process in which the coal is first contacted with an aluminum-cobalt-molybdenum catalyst at a relatively low temperature and then at a higher temperature. The cost of such synthetic gasoline is higher than that obtained from oil.
Acid catalysis
The catalytic activity of a large class of catalysts is determined by their acidic properties. According to J. Brønsted and T. Lowry, an acid is a compound capable of donating a proton. Strong acids easily donate their protons to bases. The concept of acidity was further developed in the works of G. Lewis, who defined acid as a substance capable of accepting an electron pair from a donor substance with the formation of a covalent bond due to the sharing of this electron pair. These ideas, together with ideas about reactions that produce carbenium ions, helped to understand the mechanism of a variety of catalytic reactions, especially those involving hydrocarbons.
The strength of an acid can be determined by using a set of bases that change color when a proton is added. It turns out that some industrially important catalysts behave like very strong acids. These include a Friedel-Crafts process catalyst, such as HCl-AlCl 2 O 3 (or HAlCl 4), and aluminosilicates. Acid strength is a very important characteristic because it determines the rate of protonation, a key step in the acid catalysis process.
The activity of catalysts such as aluminosilicates used in oil cracking is determined by the presence of Brønsted and Lewis acids on their surface. Their structure is similar to the structure of silica (silicon dioxide), in which some of the Si 4+ atoms are replaced by Al 3+ atoms. The excess negative charge that arises in this case can be neutralized by the corresponding cations.
The activity of acid catalysts is determined by their ability to react with hydrocarbons to form a carbenium ion as an intermediate product. Alkylcarbenium ions contain a positively charged carbon atom bonded to three alkyl groups and/or hydrogen atoms. They play an important role as intermediates formed in many reactions involving organic compounds. The mechanism of action of acid catalysts can be illustrated using the example of an isomerization reaction n-butane to isobutane in the presence of HCl-AlCl 3 or Pt-Cl-Al 2 O 3. First, a small amount of the olefin C4H8 attaches to the positively charged hydrogen ion of the acid catalyst to form a tertiary carbenium ion. Then the negatively charged hydride ion H - is split off from n-butane to form isobutane and secondary butylcarbenium ion. The latter, as a result of rearrangement, turns into a tertiary carbenium ion. This chain can continue with the elimination of a hydride ion from the next molecule n-butane, etc.
It is significant that tertiary carbenium ions are more stable than primary or secondary ones. As a result, they are mainly present on the surface of the catalyst, and therefore the main product of butane isomerization is isobutane.
Acid catalysts are widely used in oil refining - cracking, alkylation, polymerization and isomerization of hydrocarbons. The mechanism of action of carbenium ions, which play the role of catalysts in these processes, has been established. At the same time, they participate in a number of reactions, including the formation of small molecules by cleavage of large ones, the combination of molecules (olefin with olefin or olefin with isoparaffin), structural rearrangement by isomerization, and the formation of paraffins and aromatic hydrocarbons by hydrogen transfer.
One of the latest applications of acid catalysis in industry is the production of leaded fuels by adding alcohols to isobutylene or isoamylene. Adding oxygen-containing compounds to gasoline reduces the concentration of carbon monoxide in exhaust gases. Methyl- rubs-butyl ether (MTBE) with an octane mixing number of 109 also makes it possible to obtain high-octane fuel necessary for operating a car engine with a high compression ratio, without introducing tetraethyl lead into gasoline. The production of fuels with octane numbers 102 and 111 has also been organized.
Hydrogenation of vegetable oil
One of the most important hydrogenation reactions in practical terms is the incomplete hydrogenation of vegetable oils to margarine, cooking oil and other food products. Vegetable oils are obtained from soybeans, cotton seeds and other crops. They contain esters, namely triglycerides of fatty acids with varying degrees of unsaturation. Oleic acid CH 3 (CH 2) 7 CH=CH(CH 2) 7 COOH has one C=C double bond, linoleic acid has two and linolenic acid has three. The addition of hydrogen to break this bond prevents oils from oxidizing (rancidity). This increases their melting point. The hardness of most resulting products depends on the degree of hydrogenation. Hydrogenation is carried out in the presence of fine nickel powder deposited on a substrate or a Raney nickel catalyst in an atmosphere of highly purified hydrogen.
Dehydrogenation
Dehydrogenation is also an industrially important catalytic reaction, although the scale of its application is incomparably smaller. With its help, for example, styrene, an important monomer, is obtained. To do this, ethylbenzene is dehydrogenated in the presence of a catalyst containing iron oxide; The reaction is also facilitated by potassium and some kind of structural stabilizer. The dehydrogenation of propane, butane and other alkanes is carried out on an industrial scale. Dehydrogenation of butane in the presence of a chromium-alumina catalyst produces butenes and butadiene.
Ammonia
One of the simplest hydrogenation processes from a chemical point of view is the synthesis of ammonia from hydrogen and nitrogen. Nitrogen is a very inert substance. To break the N-N bond in its molecule, an energy of about 200 kcal/mol is required. However, nitrogen binds to the surface of the iron catalyst in the atomic state, and this requires only 20 kcal/mol. Hydrogen binds to iron even more readily.
Basic catalysis
The activity of catalysts is determined by their basic properties. A long-standing and well-known example of such catalysts is sodium hydroxide, used to hydrolyze or saponify fats to make soap, and one recent example is catalysts used in the production of polyurethane plastics and foams. Urethane is formed by the reaction of alcohol with isocyanate, and this reaction is accelerated in the presence of basic amines. During the reaction, a base attaches to the carbon atom in the isocyanate molecule, as a result of which a negative charge appears on the nitrogen atom and its activity towards alcohol increases. Triethylenediamine is a particularly effective catalyst. Polyurethane plastics are produced by reacting diisocyanates with polyols (polyalcohols). When isocyanate reacts with water, the previously formed urethane decomposes, releasing CO2. When a mixture of polyalcohols and water interacts with diisocyanates, the resulting polyurethane foam foams with CO 2 gas.
Double acting catalysts
These catalysts speed up two types of reactions and produce better results than passing the reactants in series through two reactors, each containing only one type of catalyst. This is due to the fact that the active sites of a double-acting catalyst are very close to each other, and the intermediate product formed at one of them is immediately converted into the final product at the other. A good result is obtained by combining a catalyst that activates hydrogen with a catalyst that promotes the isomerization of hydrocarbons. The activation of hydrogen is carried out by some metals, and the isomerization of hydrocarbons is carried out by acids. An effective dual-acting catalyst used in petroleum refining to convert naphtha into gasoline is finely divided platinum supported on acidic alumina. Converting naphtha constituents such as methylcyclopentane methylcyclopentane (MCP) into benzene increases the octane number of gasoline. First, MCP is dehydrogenated on the platinum part of the catalyst into an olefin with the same carbon skeleton; the olefin then passes to the acid portion of the catalyst, where it isomerizes to cyclohexene. The latter passes to the platinum part and is dehydrogenated to benzene and hydrogen. Double-action catalysts significantly accelerate oil reforming. They are used to isomerize normal paraffins into isoparaffins. The latter, boiling at the same temperatures as gasoline fractions, are valuable because they have a higher octane number compared to straight hydrocarbons. In addition, the conversion of n-butane to isobutane is accompanied by dehydrogenation, facilitating the production of MTBE.
Stereospecific polymerization
An important milestone in the history of catalysis was the discovery of the catalytic polymerization of olefins with the formation of stereoregular polymers. Stereospecific polymerization catalysts were discovered by K. Ziegler when he was trying to explain the unusual properties of the polymers he obtained. Another chemist, J. Natta, suggested that the uniqueness of Ziegler polymers is determined by their stereoregularity. X-ray diffraction experiments have shown that polymers prepared from propylene in the presence of Ziegler catalysts are highly crystalline and indeed have a stereoregular structure. To describe such ordered structures, Natta introduced the terms “isotactic” and “syndiotactic”. In the case where there is no order, the term “atactic” is used.
A stereospecific reaction occurs on the surface of solid catalysts containing transition metals of groups IVA-VIII (such as Ti, V, Cr, Zr), which are in a partially oxidized state, and any compound containing carbon or hydrogen, which is bonded to the metal from groups I-III. A classic example of such a catalyst is the precipitate formed by the interaction of TiCl 4 and Al(C 2 H 5) 3 in heptane, where titanium is reduced to the trivalent state. This exceptionally active system catalyzes the polymerization of propylene at normal temperatures and pressures.
Catalytic oxidation
The use of catalysts to control the chemistry of oxidation processes is of great scientific and practical importance. In some cases, oxidation must be complete, for example when neutralizing CO and hydrocarbon contaminants in automobile exhaust gases. However, more often it is necessary for the oxidation to be incomplete, for example, in many widely used industrial processes for converting hydrocarbons into valuable intermediate products containing functional groups such as -CHO, -COOH, -C-CO, -CN. In this case, both homogeneous and heterogeneous catalysts are used. An example of a homogeneous catalyst is a transition metal complex, which is used to oxidize para-xylene to terephthalic acid, the esters of which form the basis for the production of polyester fibers.
Production of ethylene by dehydrodimerization of methane
Ethylene synthesis through dehydrodimerization converts natural gas into more easily transportable hydrocarbons. The reaction 2CH 4 + 2O 2 → C 2 H 4 + 2H 2 O is carried out at 850 °C using various catalysts; the best results were obtained with the Li-MgO catalyst. Presumably the reaction proceeds through the formation of a methyl radical by the abstraction of a hydrogen atom from a methane molecule. Elimination is carried out by incompletely reduced oxygen, for example O2−
2. Methyl radicals in the gas phase recombine to form an ethane molecule and, during subsequent dehydrogenation, are converted to ethylene. Another example of incomplete oxidation is the conversion of methanol to formaldehyde in the presence of a silver or iron-molybdenum catalyst.
Catalysts for heterogeneous oxidation
These catalysts are usually complex solid oxides. Catalytic oxidation occurs in two stages. First, the oxygen in the oxide is captured by a hydrocarbon molecule adsorbed on the surface of the oxide. In this case, the hydrocarbon is oxidized, and the oxide is reduced. The reduced oxide reacts with oxygen and returns to its original state. Using a vanadium catalyst, phthalic anhydride is obtained by incomplete oxidation of naphthalene or butane.
Catalysisis the process of changing the rate of a chemical reaction using catalysts– that take part in a chemical reaction, but are not included in the composition of the final products and are not consumed as a result of the reaction.
Some catalysts speed up the reaction ( positive catalysis ), others slow down ( negative catalysis ). Negative catalysis is called inhibition, and catalysts that reduce the rate of a chemical reaction - inhibitors.
There are homogeneous and heterogeneous catalysis.
Homogeneous catalysis.
In homogeneous (homogeneous) catalysis, the reactants and the catalyst are in the same position and there is no interface between them. Example of homogeneous catalysis - oxidation reaction SO 2 And SO 3 in the presence of a catalyst NO(reactants and catalyst are gases).
Heterogeneous catalysis.
In the case of heterogeneous (non-uniform) catalysis, the reactants and the catalyst are in different states of aggregation and there is an interface between them. Typically the catalyst is a solid and the reactants are liquids or gases. Example of heterogeneous catalysis - oxidation NN 3 before NO in the presence Pt(catalyst is a solid substance).
Mechanism of action of catalysts
The effect of positive catalysts is reduced to a decrease in the activation energy of the reaction E a(ref), the effect of inhibitors is the opposite.
Yes, for reaction 2 HI =H2+I 2 E a(ref) =184 kJ/mol. When does this reaction occur in the presence of a catalyst? Au or Pt, That E a(ref) =104 kJ/mol, respectively.
The mechanism of action of the catalyst in homogeneous catalysis is explained by the formation of intermediate compounds between the catalyst and one of the reactants. The intermediate then reacts with a second starting material to produce the reaction product and catalyst in its original form. Since the rate of both intermediate processes is significantly greater than the rate of the direct process, the reaction with the participation of a catalyst proceeds much faster than without it.
For example, the reaction:
SO 2 +1/2O 2 =SO 3 flows very slowly, and if you use a catalyst NO
then reactions NO +1/2О 2 =NO 2 And NO2+SO 2 =SO3+NO proceed quickly.
The mechanism of action of the catalyst in heterogeneous catalysis is different. In this case, the reaction occurs due to adsorption molecules of reacting substances by the surface of the catalyst (the surface of the catalyst is heterogeneous: it has so-called active centers , on which particles of reacting substances are adsorbed.). An increase in the rate of a chemical reaction is achieved mainly by lowering the activation energy of adsorbed molecules, and also, partly, by increasing the concentration of reactants in the places where adsorption occurred.
Catalytic poisons and promoters.
Some substances reduce or completely destroy the activity of the catalyst, such substances are called catalytic poisons. For example, small sulfur impurities (0.1%) completely stop the catalytic effect of the metal catalyst (sponge iron) used in the synthesis of ammonia. Substances that increase the activity of a catalyst are called promoters. For example, the catalytic activity of sponge iron increases significantly with the addition of approximately 2% potassium metaaluminate KAlO2.
Application of catalysts
The action of the catalyst is selective and specific. This means that by using different catalysts, different products can be obtained from the same substances. This is especially true for reactions of organic substances. For example, in the presence of a catalyst AlO3 dehydration of ethyl alcohol occurs in the presence of Cu– dehydrogenation:
Biological catalysts that take part in complex chemical transformations occurring in the body are called enzymes.
Catalysts are widely used in the production of sulfuric acid, ammonia, rubber, plastics and other substances.