Influence of experimental conditions for the hydrogenation of nitriles, dinitriles
At atmospheric pressure, the rate of hydrogenation of dinitriles is very small and the reaction often does not reach the end. Poisoning of the catalyst with reaction products is often observed. Well adsorbed on the surface on the surface of the catalyst, the hydrogenation products lead to a decrease in the process’s speed. To increase the speed of the process and the desorption of products, higher temperature and pressure are used. The increase in temperature in turn contributes to hydrogenolysis and the development of a number of adverse reactions: the release of ammonia, the formation of secondary and tertiary amines /6, 21, 22/, the formation of saturated heterocyclic structures such as piperidine /23/, especially at elevated pressure.
Isophthalonitrile /6/ in methanol at a pressure of 10 MPa begins to recover at 313 K, as the rate and yield of m-xylylenediamine increase (83.6%) with an increase in temperature to 393 K; With further temperature increase, the yield of diamine decreases, the amount of by-products increases. When hydrogenated on a skeleton nickel catalyst, the optimum temperature is lower than on cobalt. In butanol, with an increase in temperature from 373 K to 433 K, the yield of m-xylylenediamine increases from 49 to 87-89%, and the duration of the experiment decreases by a factor of 1.5. A further increase in temperature reduces the yield of m-xylylenediamine. Probably, at high temperatures hydrogenolysis of the resulting diamine occurs.
With increasing temperature, the rate of hydrogenation of acetonitrile, acrylonitrile, capryonitrile, and nitriles of synthetic fatty acids and the degree of conversion increase. At the same time, at a higher value of it, the role of side reactions of the formation of secondary and tertiary amines increases. The increase in temperature by 20 (293-313 K) during the hydrogenation of acrylonitrile on Ni-Ti in 0.1 N NaOH increases the reaction rate by> C = C <bonds in 2.4, and by the C-N- group by a factor of 8, the activation energy is respectively 20 and 38 kJ / mol / 5 /.
The study of the hydrogenation of a number of unsaturated organic compounds under hydrogen pressure showed that the reaction rate is most often directly proportional to pressure, in some cases it increases rapidly, in others it slowly /24, 25/
The ratio of the adsorbed molecules of the hydrogenated substance and hydrogen should largely determine the effect of hydrogen pressure, temperature and other factors on the reaction rate /24/. The pressure primarily affects the concentration of hydrogen on the surface of the catalyst.
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Some papers /5, 24/ show that with increasing hydrogen pressure in the system, the hydrogenation rate increases proportionally to a certain limit, the latter depending on the nature of the catalyst and the hydrogenated compound and on the experimental condition.
The yield of m-xylylenediamine in methanol on cobalt-Raney increases from 55 to 83% in proportion to the increase in pressure from 4 to 8 MPa, a further increase in pressure reflects only the reaction rate /6/. In butanol, increasing the pressure above 10 MPa leads to an increase in the yield of the desired product.
When hydrogenating capryonitrile and synthetic fatty acid fraction C17-C20 / 20 /, the positive pressure effect is more clearly defined can be traced to 5 MPa.
Hydrogenation of acetonitrile /19/ in the presence of Nick. and Ni-Tick. catalysts in alcohol at 313 K in the hydrogen pressure range from 0.5 to 7.0 MPa showed that the reaction rate in the presence of Ni-Ti. The catalyst rises to 6 MPa; When the pressure is raised to 7 MPa, the speed does not change substantially, and the limiting value of the speed is reached at 2 MPa. The order of the reaction, found from the tangent of the angle in the pressure range 0.5-0.6.0 MPa (for Ni-Ti), is 0.4, this value indicates that the reaction involves mainly atomically adsorbed hydrogen (24). Hydrogenation of acetonitrile in alkaline medium showed that with increasing pressure the reaction rate increases from 0.5 to 2 MPa, and in the range of 2-6 MPa remains practically constant. The order of the reaction for hydrogen up to 2 MPa is 0.7, and above it goes to zero. The increase in hydrogen pressure, as a rule, increases the saturation rate of the C = C- and -C = N-bonds of the nitrile.
In the presence of a Ni-Tick catalyst of 0.1 N NaOH, the hydrogenation rate of the ethylenic bond of acrylonitrile increases with increasing hydrogen pressure from 1 to 7 MPa. The rate of hydrogenation of the nitrile group is increased only to 4 MPa. Further increase in hydrogen pressure does not affect the hydrogenation rate; the reaction order for hydrogen varies from 0.5 to zero. The authors found that the value of the limiting pressure of hydrogen varies with the process temperature: the lower the temperature, the sooner the limiting hydrogen pressure is reached /5, 19/.
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During the hydrogenation of various aliphatic nitriles in ethanol under hydrogen pressure at 313 K, it was established that the nitriles, in decreasing reaction rate at the same hydrogen pressure, are arranged in a series:
acetonitrile> acrylonitrile> oleonitrile> stearonitrile
In the same sequence, the value of the limiting pressure of hydrogen decreases depending on the structure of the nitrile /5.19/.
The effect of hydrogen pressure on the kinetics of hydrogenation of unsaturated compounds in solutions /25/ was studied, where along with the study of the process kinetics, the potential of the catalyst was measured. Under hydrogen pressure, hydrogenation proceeds on a surface filled with hydrogen, the potential of the catalyst acquires a more cathodic value. As the pressure increases, the amount of sorbed hydrogen increases, the rate of its reproduction on the surface of the catalyst, and the average binding energy Me-H decreases, therefore, as a rule, the increase in pressure promotes an increase in the reaction rate.
The amount of adsorbed active form of hydrogen determines the reaction mechanism and the selectivity of the process.
In the hydrogenation of unsaturated compounds in solutions, considerable attention is paid to the effect of the nature of the solvent on the rate and mechanism of the reactions /26/.
The solvent serves both to remove heat and to regulate the adsorption of the starting components and reaction products. The nature of the solvent significantly affects not only the adsorption of the reacting components, but also the rate of reproduction of their active forms on the surface. The influence and role of the solvent in the hydrogenation of nitriles have not been studied sufficiently /17, 26/.
The most systematic studies of the influence of the solvent on the kinetics and mechanism of hydrogenation of nitriles belong to the school of D.V. Sokolsky / 5, 6, 19, 27, 28 /.
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When hydrogenating aromatic dinitriles in the absence of ammonia, a sharp difference is observed in the diamine yield, depending on the nature of the solvent /6/. A high yield of amines is observed in alcohols. The best solvent is liquid ammonia, the yield of m-xylylenediamine in which is 94-96%, then aniline and formamide.
When reducing unsaturated nitriles on skeletal nickel, palladium, and platinum catalysts, it was shown that the selectivity of hydrogenation = C = C = and -C = C-bonds can be regulated by the selection of a solvent. Hydrogenation in dimethylformamide significantly reduces the reactivity of the double bonds of nitriles. The high rate of hydrogen addition to the double bond of acrylonitrile is noted in ethanol. At the same time, /5/the ethylenic bond of acrylonitrile with the use of 0.1N NaOH as a solvent, unlike alcohol, is hydrogenated at a strictly constant rate, a sharp kink on the kinetic curve is observed at the end of the hydrogenation = C = C = bond. The nitrile group of the formed propionitrile is hydrogenated with a constant but significantly lower rate. The second characteristic difference between the hydrogenation of acrylonitrile and alkali in comparison with alcohol is the change in the order of the reaction with a = C = C = bond with an increase in the weight of the nitrile.
The mechanism of the reaction and selectivity are directly dependent on the form and binding energy of hydrogen involved in the hydrogenation process. For example, when hydrogenating acetonitrile, it was shown that the specific activity of a nickel-titanium catalyst is 2-4 times higher than the activity of skeletal nickel in alkali, and in alcohol 7-10 times, i.e. for the hydrogenation of acetonitrile, the strongly bound hydrogen of a nickel-titanium catalyst is preferable to hydrogen with a lower binding energy on skeletal nickel. Strengthening the binding energy of Me-N by selecting a solvent gives a positive effect mainly for skeletal nickel. Excessive increase in the binding energy leads to a decrease in the reaction rate /2, 5/.
Various methods /3/ are used to suppress adverse reactions. In the case of noble metal catalysts, acids are introduced into the reaction mixture /27, 28/. The acid promotes a greater yield of primary amines, protonating the primary imino group, thereby making it difficult to further transform. In this case, amines are released as salts. The primary amine is bound to acetic anhydride. When using a Ni, Cr catalyst, sodium acetate, NH3, Ba (OH) 2, NaOH, KOH, quaternary ammonium bases and other basic substances are introduced /1-7, 15-19/.
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Oxides and amines /6/ suppress the formation of secondary amines due to the selective poisoning of the catalyst with respect to the hydrogenolysis reactions leading to their production.
When hydrogenated under mild conditions on inactive non-noble metal catalysts, especially when other nitrile groups (amino-, oxo-, etc., which form complexes with the catalyst are present in the molecule, the reaction may stop at the imine stage.) In the presence of water, alcohol or acetic acid, aldehydes are formed /3/. The classical and generally accepted method for the preparation of primary amines is the method for the catalytic hydrogenation of nitriles and dinitriles in the presence of ammonia ,continuously in liquid ammonia, / 3, 4, 6, 7, 8, / or in a solvent saturated with ammonia / 1-7, 15-18,20 /.
By varying the amount of ammonia in the reaction medium, it is possible to achieve an almost quantitative yield of the primary amine. For example, Schwegler and Adkins / 18 / on hydrogenation of aliphatic and aromatic nitriles (nitrile: ammonia = 1: 4) on skeletal nickel, only primary amines were obtained. On the Cobalt ck ,at 393 K and 10 MPa in a methanol solution of ammonia, the yield of m-xylylenediamine from terephthalonitrile reaches 94.3% /6, 29/.
When hydrogenating nitriles of stearic and oleic acids on a nickel-titanium catalyst under a hydrogen pressure of 3.92 MPa and 353 K, the yield of primary amines was brought to 97% with a nitrile:
ammonia = 1: 4 /5/. A high yield of primary amines from the corresponding nitriles in the presence of ammonia speaks in favor of the aldimine mechanism of the reaction.
In a recent review on the hydrogenation of nitriles, the authors of /30/ analyze the nature and specific features of the action of Ni, Co, and noble metal catalysts, taking into account the effect of the reaction medium. The yield of primary amines increases in the presence of ammonia, a similar result is achieved with an increase in the basicity of the oxide carrier. Of great importance in the formation of the catalyst is the method of transferring the metal to the active phase: preference is given to transferring the nickel to the active form from the corresponding boride. Primary and secondary fatty amines are obtained by hydrogenating the fatty nitriles C8-C22 over reduced nickel borate at 383-423 K, pressure 1.4 MPa for 20-120 min to obtain primary amines and at 443-503 K for 3-20 min for the production of secondary amines. The yield of amines is 94-95% /31/.
Hydrogenation of monocyanipyridines and benzonitrile on Nick, Pt, Pd and Rh-blacks was carried out / 21, 28, 38-42 / at atmospheric pressure in various solvents. Catalysts characterized by a high binding energy with hydrogen are selective upon hydrogenation to amines. Metals having a relatively low binding energy with hydrogen, such as rhodium and nickel, can activate a water molecule in parallel with a high speed, resulting in the simultaneous formation of oxo compounds. By catalytic activity, hydrogenation of monoanepiridines, the following series was obtained:
Pd> Ni> Pt> Rh / 40 /.
The study of nitriles on their reactivity when hydrogenated on Ni, Pt and Pd in various solvents showed that on hydrogen in the nucleus, hydrogen is hydrogenated in platinum in neutral media and in Pt and Pd in acidic solvents. During the hydrogenation, the attack is subjected to a nitrile group. The quantum mechanical calculations performed by the authors showed that the σ-electron charge of the nitrogen atom of the substituent (CN-group in CNPy) decreases in the amine> imine> nitrile series. In this series, the nitrile group has the lowest adsorption capacity. The experimental and calculated data showed that the reaction rate on the paladium is limited by the adsorption of the starting nitrile and the intermediate imine to form a π-complex with the metal atom. On the Pt-mobile, the correlation of the reaction rate with the CN-localization energy is found, the easier the latter, the easier the formation of the σ-complex. There are no distinct correlations to Nick, as it was not detected in Pt and Pd. The reaction proceeds much more complicatedly and is accompanied by the accumulation of significant amounts of aldehyde in the catalyst. The series of cyanopyridines by decreasing the rate and amount of the aldehyde formed are simbatin: 3CNPy> PhCN> 4CNPy> 2CNPy.
Thus, according to the authors of /21/, the formation of the π-complex (weak adsorption) is the limiting stage determining the reactivity of the cyano groups from its position in the pyridine ring on the paladium, and the formation of the σ complex (strong adsorption) on platinum.
According to Sokolsky's ideas (47-49), a balance is established on the surface of the hydrogen-saturated hydrogen between the various forms of hydrogen, which can be expressed by the following scheme:
The mechanism of the reaction and selectivity are directly dependent on the form and binding energy of hydrogen involved in the hydrogenation process.
In the liquid-phase hydrogenation reaction, one of the components of the catalyst system is a solvent [5, 6, 10, 30]. From the correct choice it will depend on the speed of the reaction, its direction, and sometimes the very possibility of carrying out the catalytic process. The molecules of the solvent, like any other chemical, have a direct effect on the adsorption and catalytic properties of the metal, on the behavior of the hydrogenated material and reaction products, which ultimately leads to a change in the kinetic and energy characteristics of the adsorption and interaction of the components on the surface. Therefore, it is understandable why many studies on hydrogenation reactions in the liquid phase are devoted to finding correlations between the reaction rate and some characteristics of the solvent. A great contribution to the creation of the theory of heterogeneous hydrogenation in solutions was made by Kazakhstani catalytic agents /10, 23-25, 27-32/. A number of review works have devoted considerable attention to the consideration, from the theoretical and practical point of view, of the influence of the nature of the solvent and the composition of the solution on the rate and mechanism of the reaction. Very important in solving these issues was the use of electrochemical methods in the study of powdered catalysts, which made it possible to carry out a number of fundamental studies on the basis of the results of which D.V. Sokolsky made deep, general conclusions about the effect of the solvent on the rate and mechanism of the hydrogenation reaction. It is shown that the nature of the solvent determines
1) the binding energy of the atoms of the reacting components on the surface of the catalyst;
2) the distribution coefficient of the hydrogenated material and reaction products between the solution and the catalyst surface;
3) the ratio of the activation rates of hydrogen and the bond's unsaturation and the rate at which they are removed from the surface;
4) adsorption ability of the solvent itself;
5) the solubility of hydrogen in the liquid, the rate of its diffusion at the gas-liquid interface and liquid-solid;
6) The presence in the solution of ions or polar substances that take part in the formation of a double electrical layer on the surface of the catalyst, capable of selective adsorption and, consequently, affect the reaction rate and its direction.
In this chapter of the literature review, some of these issues will be touched upon, which are directly relevant to the study of the kinetics and mechanism of hydrogenation on Group VIII metals.
One of the most important characteristics that determine the activity of a metal catalyst in a hydrogenation reaction is the strength of the metal-hydrogen bond. As established by means of electrochemical methods of investigation, when hydrogenating various compounds in a particular reaction, hydrogen takes part with a certain binding energy Me-H /5, 6, 10, 29, 30/,
Organic compounds that are the objects of the hydrogenation reaction can be divided into 3 to 5 groups by the displacement of the potential of the catalyst when they are adsorbed. So, for example, at Δφ less than 100 mV, it is possible to assume a small adsorption capacity of organic compounds. Such compounds include benzene, cyclohexene, olefins and the nitriles studied by us. In the group of organic compounds possessing an average adsorption capacity (100 <Δφ <200 mV) are acetylene derivatives, some nitro compounds, amines of different structure and so on. There are substances that are hydrogenated, in the absence of adsorbed hydrogen on the surface of the catalyst (200 <Δφ <600 mV).
The state of hydrogen chemisorbed on the catalyst surface depends on the chemical composition of the catalyst, the degree of dispersion, the hydrogen pressure, the temperature, the nature of the solvent, the structure of the double electrical layer, and so on. The most important, determining the state and degree of heterogeneity of hydrogen is the nature of the catalyst itself. In terms of the magnitude of the heterogeneity of hydrogen, platinum, the heat of hydrogen adsorption, is on the first place, on which, depending on the degree of filling, varies from 20 to 4.3 kcal / mol /30, 31/. In rhodium, the surface-adsorption hydrogen is energetically more homogeneous; on this catalyst there is practically no firmly bound form. The heat of adsorption varies in the interval 13.4-4.3 kcal / mol. The main amount of hydrogen of palladium is in its volume and homogeneously. The heat of dissolution of hydrogen in the volume is 10 ± 0.5 kcal / mol. The amount of adsorbed hydrogen is 4-6 times less than that of dissolved hydrogen. The heat of adsorption of hydrogen on palladium varies in a small range (8.4 -13.8 kcal / mol). Skeletal nickel in the state of sorbed hydrogen is between platinum and palladium. It contains both adsorbed and absorbed forms of hydrogen /10, 30, 31/.
he reliability of the heat of hydrogen adsorption, calculated from electrochemical data, was confirmed in recent years by the method of direct measurement by differential heat and microcalorimetric methods. It was shown that the heat of hydrogen adsorption on platinum and rhodium in the gas phase is somewhat greater than in the solution of sulfuric acid. This difference is due to the influence of the pH of the solution and the structure of the double electrical layer. In the gas phase of platinum, the amount of strongly bound hydrogen with a binding energy of 23 kcal / mol is 50% of the total adsorbed hydrogen, whereas in the solution the maximum binding energy of Pt-H is 20 kcal / mol and its amount does not exceed 30-35%. Similar conclusions can be drawn by comparing the results of hydrogen adsorption on rhodium.
Thus, due to the inhomogeneity of the surface of even one and the same metal, the presence of hydrogen, which possesses a whole spectrum of binding energies with the surface, is characteristic.
Another important characteristic of the processes of hydrogenation flowing in the liquid phase is that at the solid-liquid-gas interface there is a double electrical and it consisting of ions, dipoles of atoms or molecules. This layer, the theory of which was created by Frumkin, has a significant effect on the orientation of the molecules of the hydrogenated compound on the surface, the amount of hydrogen adsorbed by the catalyst, and the strength of its bond to the surface.
Since the solvent is adsorbed on the surface of the catalyst, a significant change in the double electric layer occurs, which leads to a thin modification of the surface and causes a change in the catalytic properties of the metal. On this basis, the selection of catalysts for hydrogenation reactions and the conditions for their carrying out can be conducted along the path of finding the optimal catalyst-solvent system.
Most often, water, aliphatic alcohols, dioxane, solutions of mineral and organic acids, alkalis, etc. are used as solvents for catalytic hydrogenation.
In recent years, with the use of electrochemical methods, a huge series of studies was carried out to determine the adsorption of water, alcohols, aldehydes, acids and other ions and compounds. These investigations make it possible to approach with greater accuracy the problem of the structure of the Me-liquid-gas interface.
Water molecules are prone to chemisorption on the surface of the catalyst, which significantly affects the adsorption of the reacting components. Maximum adsorption of water is achieved at potentials of 0.3-0.7 volts. It is also known that the adsorption of hydrogen on platinum does not depend on the presence of chemisorbed water only at potentials less than 0.17 volts. Consequently, from the potential of 0.2v, competition of hydrogen with water molecules over the surface of the catalyst begins.
Of great interest are studies on the adsorption of aliphatic alcohols widely used as solvents. In the writer until recently, there was the notion that limiting alcohols such as methanol can only be physically adsorbed on the surface of the metals of the platinum group and retained at the surface due to van der Waals forces. Indeed, many molecules are adsorbed on the surface of metals physically, however, as it was noted in the works, it is also possible chemisorption of molecules of organic matter, accompanied by their deep destruction.
The mechanism of adsorption of alcohols, including aliphatic ones, on metals of the platinum group has been systematically and thoroughly studied in recent years in the works of N.A. Frumkin, D.V. Sokolsky, G.D. Zakumbaeva. is described in sufficient detail in [10, 35, 36]. It turned out that the chemisorption of saturated alcohols, even from aqueous solutions, is very significant. It should be considered even more when using absolute aliphatic alcohols as solvents in catalytic processes. Depending on the reaction conditions and the nature of the catalyst, the role of solvents in the catalysis process may vary due to their ability to undergo dehydrogenation.
It is shown that the degree of dehydrogenation transformations on metals increases with decreasing binding energy Me-H. In platinum, for example, propyl alcohol is dehydrogenated to propionaldehyde only under conditions where there is no adsorbed hydrogen on the surface, while on iridium and ruthenium even if propyl alcohol undergoes dehydrogenation upon chemisorption on the surface. On rhodium among the products of chemisorption of propanol strongly adsorbed on the surface, it is possible to detect propionic aldehyde, allyl and propargyl alcohols, while in the catalyst these compounds are only trace amounts. On palladium during the chemisorption of propyl alcohol, its hydrogenolysis to propane is observed. This direction of transformation is a characteristic feature of all platinum metals. In reactions where the stages of adsorption and activation of hydrogen on the surface of the catalyst are hampered, the solvent, undergoing destruction, can be a hydrogen donor, i.e. to take a direct part in the catalytic process. However, the behavior of organic solvents at the surface of the catalyst changes sharply in the presence of hydrogen in the gas phase at atmospheric and elevated pressure, i.e. under the conditions of the catalytic hydrogenation reaction: first, the adsorption of organic solvents is sharply reduced, secondly. In most cases, the dissociative mechanism of their adsorption and destruction is suppressed; thirdly, it is necessary to assume that the hydrogenated compounds that have unsaturated π- and σ-bonds in their composition displace solvent molecules. An exception to this rule can be solvents such as dimethylformamide, imines of different structures, which have a free n-pair of electrons.
The results of electrochemical and kinetic measurements show that the reaction rate of catalytic hydrogenation, the rate of reproduction of activated hydrogen on the surface of the catalyst, the relative adsorption of the starting, intermediate and final reaction products depends to a large extent on the pH of the medium.
A continuous decrease in the rate of hydrogenation with increasing pH is observed during the hydrogenation of olefins on platinum. Palladium and rhodium are associated with the weak absorption ability of these compounds, capable of displacing hydrogen from the surface. For strongly adsorbing substances capable of displacing hydrogen and adsorbed cations and anions, on the contrary, the rate of hydrogenation depends on the pH of the solution.
Such dependences as the increase in the rate of hydrogenation with increasing pH, which occurs during the hydrogenation of polyfunctional compounds due to their different orientation on the surface, the passage of the reaction rate through a maximum in solutions with pH 10-11, when aldehydes are subjected to hydrogenation; a sharp decrease in the rate when pH = 7 is reached. This regularity occurs when the benzene nucleus is hydrogenated. It is knitted with the need for a planar orientation of the nucleus and a high degree of symmetry in accordance with A.A. Balandin's multiplet theory.
Similar dependences of the hydrogenation rate on the pH of the solution were obtained in the works.
Thus, by changing the bond strength of hydrogen to the surface, the amount of its adsorption, the pH of the medium should influence the speed of the hydrogenation process as a whole and its individual stages. Consequently, selecting the solvents for the distributed pH, there is the possibility to regulate the course of the hydrogenation reaction, to change its speed, direction, selectivity.
The optimal conditions for the hydrogenation reaction are determined not only by the adsorption of hydrogen on the catalyst surface and the Me-H bond strength, depending also on the adsorption of the hydrogenated material on the catalyst. A very important indicator is the ratio of the reacting components on the catalytic surface. To regulate this ratio, to ensure that it is good for the process, it is also possible to select an appropriate solvent. Naturally, the applicability of a particular solvent is determined by its ability to dissolve the reactants in the ratios convenient for the experimenter and, as shown by the results of numerous studies, a change in the solubility significantly affects the hydrogenation rate. For example, when hydrogenating quinone on platinum, palladium and nickel catalysts, the reaction rate in dioxane and benzene is greater than in ethanol, acetic acid and water, i.e. more in those solvents in which quinone is readily soluble.
A very important factor associated with the use of a solvent is the redistribution of the hydrogenated material between the solution and the surface of the catalyst at the time of the reaction. As was shown by D.V. Sokolsky, the regularities of this phenomenon are of a general nature. If for the strongly adsorbed compounds (containing triple or conjugated double bonds) "pulling" them into the solution from the surface of the catalyst is favorable for increasing the reaction rate. Then for slightly adsorbed compounds a different picture is observed. Thus, in the hydrogenation of methyl acetone on Raney nickel, the cyclohexane rate is much higher than in alcohols. This conclusion can also be confirmed by the data on the hydrogenation of benzahinone on nickel, platinum and palladium catalysts.
In detail, the effect of the nature of the solvent on the rate of hydrogenation has been studied recently in the hydrogenation of unsaturated aliphatic nitriles, furfural and multiple hydrocarbon bonds.
Useful information on the course of hydrogenation of unsaturated compounds is obtained by calculating the ratios of adsorption coefficients. According to the work, for example, the maximum hydrogenation rate of furfural on skeletal nickel is observed in 10% ethanol. Further, to reduce the hydrogenation rate of furfural, water followed by concentrated solutions of ethanol and n-butyl alcohol. Calculation of the ratios of adsorption coefficients showed that the slowing down of the hydrogenation rate in this series of solvents is due to the increasing adsorption of the reaction product.
The action of the solvent on the hydrogenated compounds and reaction products can often be predicted in advance, based on the positions of organic chemistry. For example, by introducing an acid, it is possible to bind the amino group of the starting material or the reduction product and thereby prevent poisoning of the catalyst. The rate of hydrogenation of pyridines also increases substantially in an acidic medium, since in the case of adsorption of these compounds decreases due to an unshared pair of electrons in the nitrogen ring and self-poisoning of the catalyst is inhibited.
The influence of the solvent in the system can also be predicted by determining to which of the four established reaction mechanisms proposed by D.V. Sokolsky, this process belongs. On the other hand, knowledge of the regularities of the action of solvents and other factors for a single catalyst helps to solve the inverse problem of determining the mechanism of hydrogenation and selecting the optimal catalyst [10, 29, 35, 36].
Thus, a far from complete review of data on the effect of solvents on the process of catalytic hydrogenation indicates that their role in deciding on the choice of an active and selective catalyst and optimal process conditions is extremely high.
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