Catalysisis the process of changing speed chemical reaction with help 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. Usually the catalyst is solid, and the reacting substances 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.
Catalysts are divided into homogeneous And heterogeneous. A homogeneous catalyst is in the same phase with the reacting substances, a heterogeneous catalyst forms an independent phase, separated by an interface from the phase in which the reacting substances 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 occur 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 carrier (silica gel, aluminum oxide, activated carbon, etc.).
For each type of reaction, only certain catalysts are effective. In addition to those already mentioned acid-base, there are catalysts oxidation-reduction; 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 using catalysts that act through the coordination of reactants at a transition metal atom or ion (Ti, Rh, Ni). This catalysis is called coordination.
If the catalyst has chiral properties, then an optically active product is obtained from an optically inactive substrate.
In modern science and technology they often use multi-catalyst systems, each of which speeds up different stages of the reaction. A catalyst can also increase the rate of one stage of the catalytic cycle carried out by another catalyst. This is where “catalysis of catalysis” takes place, or second level catalysis.
In biochemical reactions, enzymes play the role of catalysts.
Catalysts must be distinguished from initiators. For example, peroxides break down into free radicals, which can initiate radical chain reactions. Initiators are consumed during the reaction, so they cannot be considered catalysts.
catalysis mechanism: 1)fur. stepwise (change in reaction path) 2) associative 3) enzymatic 4) microheterogeneous
Specificity of catalysis lies in the fact that in the presence of a catalyst, the path along which the overall reaction takes place changes, other ones are formed transition states with different activation energies, and therefore the rate of chemistry also changes. reactions.
wood processing requires high costs in production, therefore, catalysts are used that accelerate the chemical transformation process, increase product yield and reduce the emission of harmful substances. advantage of product catalysts are that they do not require large expenditures.
28. Solutions. Processes during the formation of solutions. Ideal and real solutions. Hydrates and solvates.
Solutions- homogeneous (homogeneous) systems, that is, each of the components is distributed in the mass of the other in the form of molecules, atoms or ions
The interaction between a solvent and a solute is called solvation(if the solvent is water - hydration).
The energy characteristic of dissolution is heat of formation solution, considered as the algebraic sum of the thermal effects of all endo- and exothermic stages of the process. The most significant among them are: – heat-absorbing processes- destruction of the crystal lattice, breaking of chemical bonds in molecules; – heat generating processes- formation of products of interaction of a dissolved substance with a solvent (hydrates), etc.
SOLVATES, products of addition of a solvent to dissolved substances. Usually solvates are formed in a solution, but often (when the solution is cooled, the solvent evaporates, etc.) obtained in crystalline form. phase-crystal solvates.
Hydrates are products of the addition of water to inorganic and organic substances
Catalyst classifications
Exist Various types classification of catalysts, which are based on a certain set of properties or characteristics. The most widely used classification is based on the type of substances that are catalysts. It includes the following groups of catalysts:
1. Metals (massive, pure, alloys, skeletal, supported) - heterogeneous catalysts.
For example, group VIII metals (Fe, Ni, Co, Pt, Pd) are used as catalysts for the hydrogenation of unsaturated compounds (mainly alkenes and alkynes). Silver is a catalyst for oxidation and oxidative dehydrogenation (eg, the synthesis of formaldehyde from methanol).
Metal catalysts can be compact, skeletal, or supported.
Pt-mesh (compact metal in the form of intertwined threads) – CT for the oxidation of sulfur dioxide into trioxide (production of sulfuric acid). Or the well-known hydrogenation catalyst - Raney Nickel (this is a skeletal catalyst that is obtained by leaching a nickel-aluminum alloy).
Supported catalysts – palladium on carbon (Pd/C), palladium on aluminum oxide (Pd/Al 2 O 3), etc.
2. Solid binary metal compounds M m E n, where E – O, S, Se, Te, As, P, C, N, Si, B, heterogeneous catalysts. From this group, oxides or chalcogenides of semiconductor-type metals are most often used. Examples are the oxides MgO, ZnO, Fe 2 O 3, Cr 2 O 3, WO 3, MoO 3, V 2 O 5, etc.
Thus, V 2 O 5 is currently used in the synthesis of sulfuric acid.
Catalysts of this type are widely used in hydrogenation processes. For example, in the synthesis of methanol from CO and hydrogen according to the reaction:
CO + H 2 → CH 3 OH
The catalyst is a mixture of oxides:
Cr 2 O 3 +ZnO (I Kt, total gas pressure 250-300 atm, temperature 300-400 0 C), Cr 2 O 3 +ZnO+CuO (II Kt, milder conditions: total pressure 50-100 atm., temperature 220-270 0 C, however, unlike the previous one, the catalyst is extremely sensitive to the presence of sulfur-containing impurities in the source gases).
A mixture of molybdenum, bismuth and phosphorus oxides with the composition MoO 3 ∙Bi 2 O 3 ∙P 2 O 5 – a catalyst for the oxidative ammonolysis of propylene into acrylonitrile:
CH 2 =CH-CH 3 + NH 3 + 1.5 O 2 → CH 2 =CH-CN + 3H 2 O
Molybdenum and tungsten sulfides of the composition Mo x S y +W x S y are good catalysts in the processes of hydrotreating (desulfurization) of petroleum fractions –– in the presence of these catalysts, sulfur-containing substances are reduced organic compounds to elemental sulfur or hydrogen sulfide.
3. Acids and bases (homogeneous and heterogeneous catalysts) - Bronsted protic acids (HA) in aqueous and non-aqueous media, Lewis-Usanovich aprotic acids (BF 3, RI), protic and aprotic centers of solid oxides (aluminum oxides, aluminosilicates), any types of bases (including solid ones - MgO, CaCO 3, ion exchange resins).
Acid-base catalysis reactions occur on such catalysts, namely cracking of petroleum fractions (on aluminosilicates and zeolites), dehydration and hydration, synthesis of amines from alcohols (on Al 2 O 3), esterification of alcohols and acids, condensation of aldehydes and ketones.
3. Metal complexes, including salts (homogeneous and heterogeneous catalysts). It should be noted that transition metal salts are, as a rule, complex compounds.
Consider, for example, the Wacker process (oxidation of ethylene to acetaldehyde), which occurs in the presence of a PdCl 2 /CuCl 2 catalyst. In fact, PdCl 2 is a linear polymer in which the metal atoms are connected by chloride bridges:
In solution in the presence of chloride ions, it depolymerizes to form planar-square anionic palladium complexes
and it is these anionic complexes that are integral part catalyst and participate in the catalytic cycle.
In the hydrogenation of alkynes and alkenes, rhodium phosphine complexes of the composition RhCl(PPh 3) 3 serve as catalysts.
During the carbonylation of methanol into acetic acid(Monsanto process) a mixture of carbonyl complexes Rh(I) and methyl iodide (MeI) is used as a catalyst precursor.
The catalyst for the Reppe reaction (carbonylation of alkynes in the presence of water (hydrocarboxylation), alcohols or amines) is nickel tetracarbonyl of the composition Ni(CO) 4 - a rather labile complex with a tetrahedral structure.
tin – catalytic cracking catalysts. Needs chemical industry– about one third, ecology – a tenth. If the average cost of catalysts is $4 per kilogram, and the average share in the cost is 0.3%, then with the help of catalysts they produce products worth at least $1000 billion. Catalysis saves raw materials and energy, and does not pollute environment. ...
They imply an ordered connection and interaction between the elements of the system, thanks to which new integral properties arise. In a chemical system such as a molecule, it is the specific nature of the interaction of its constituent atoms that determines the properties of the molecule. 3.2 “Triumphal march of organic synthesis” The emergence of structural theory allowed chemists to gain for the first time...
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