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(hīdrôj`ənā'shən, hī'drəjənā`shən), chemical reaction of a substance with molecular hydrogen, usually in the presence of a catalyst. A common hydrogenation is the hardening of animal fats or vegetable oils to make them solid at room temperature and improve their stability. Hydrogen is added (in the presence of a nickel catalyst) to carbon-carbon double bonds in the unsaturated fatty acid portion of the fat or oil molecule:

Another hydrogenation is the synthesis of methanolmethanol,
 methyl alcohol,
or wood alcohol,
CH3OH, a colorless, flammable liquid that is miscible with water in all proportions. Methanol is a monohydric alcohol. It melts at −97.8°C; and boils at 67°C;.
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 from carbon monoxide. Hydrogenation reactions are important in petroleum refining; production of gasoline by cracking involves destructive hydrogenation (hydrogenolysis), in which large molecules are broken down to smaller ones and reacted with hydrogen. Most hydrogenation reactions are reversible and proceed to favorable equilibria at high pressure and moderate temperature.



a catalytic reaction of the addition of hydrogen to simple substances (elements) or chemical compounds. The reverse reaction, which consists of the splitting off of hydrogen from chemical compounds, is called dehy-drogenation. Hydrogenation and dehydrogenation are important catalytic methods, based on oxidation-reduction reactions, for the synthesis of various organic compounds that are connected by chemical equilibrium. The reversible catalytic conversion of ethyl alcohol into acetaldehyde may serve as an example:

C2H5OH ⇄ CH3CHO + H2

The increase in temperature and decrease in the pressure of H2 promote the formation of acetaldehyde, whereas a decrease in temperature and an increase in the H2 pressure promotes the formation of ethyl alcohol. Such an influence of the conditions is typical of all hydrogenation and dehydrogenation reactions. Many metals (iron, nickel, cobalt, platinum, palladium, and osmium), oxides (NiO, CoO, Cr203, and Mo02), and sulfides (WS2, MoS2, and CrnSm) are catalysts of hydrogenation and dehydrogenation.

hydrogenation and dehydrogenation are widely used in industry. For example, the synthesis of an important product, such as methanol, which is the raw material in many chemical processes and is used as a solvent, is performed by the hydrogenation of carbon monoxide (CO + 2H2 → CH3OH) on zinc-chromium oxide catalysts at 300°-400° C and a hydrogen pressure of 20-30 meganewtons per square meter (MN/m2), or 200-300 kilograms-force per sq cm (kgf/cm2). The use of other catalyst compositions also makes it possible to prepare higher alcohols by this method. The hydrogenation of fats is the basis for the production of margarine. In connection with the development of the production of such materials as kapron and nylon, the hydrogenation method has become widespread in the production of intermediates, such as cyclohexanol from phenol, cyclohexane from benzene, hexamethylenediamine from adiponitrile (on nickel catalysts), and cyclohexylamine from aniline (on cobalt-containing catalysts).

Hydrofining, which consists of hydrogenation on alumina-cobalt-molybdenum or tungsten-nickel catalysts and which leads to the degradation of the organic sulfur compounds and the removal of sulfur in the form of H2S, is of great importance in the refining of fuels produced from sulfur-containing crude petroleum. Another process for refining petroleum products is destructive hydrogenation (on tungsten sulfide and some other catalysts), which leads to an increased yield of light and low-boiling products during the processing of crude petroleum. The hydrogenation of CO on various catalysts leads to the production of gasoline, solid paraffins, or oxygen-containing organic compounds. The synthesis of the inorganic compound ammonia by the reaction of nitrogen with hydrogen under high pressure is also hydrogenation (the hydrogenation of a simple substance).

One of the simplest examples of dehydrogenation is the dehydrogenation of alcohols. A considerable quantity of acetaldehyde is produced by the dehydrogenation of the ethyl alcohol derived from the hydrolysis of cellulose. The dehydrogenation of hydrocarbons is one of the principal reactions occurring on the mixed catalysts during the complex reforming process, which leads to a substantial improvement of the properties of motor fuels. This reaction also makes possible the preparation of various aromatic hydrocarbons from naphthenic and paraffinic hydrocarbons.

Dehydrogenation has found wide application in the production of monomers for the synthesis of rubber and resins. Thus, the paraffinic hydrocarbons butane and isopentane are dehydrogenated at 500°-600° C on catalysts containing chromium oxide to give butylenes and isopentene (iso-amylene), respectively, which are in turn dehydrogenated on complex catalysts to give the diolefins butadiene and iso-prene. The dehydrogenation of alkylated aromatic hydrocarbons—for example, ethylbenzene to styrene and iso-propyl benzene to methylstyrene—has acquired great significance in the production of polymers of styrene and its derivatives.

Broad studies of hydrogenation were begun in 1897-1900 by the schools of P. Sabatier in France and N. D. Zelinskii in Russia. The main principles governing the hydrogenation of mixtures of organic compounds were determined by S. V. Lebedev. In the area of practical applications of hydrogenation, significant achievements were made as early as the first quarter of the 20th century by F. Haber (the synthesis of ammonia), F. Bergius (the hydrogenation of coal), and G. Patart (France; the synthesis of methanol). The dehydrogenation of alcohols was discovered by M. Berthelot in 1886. In 1901, Sabatier observed, among other transformations, the dehydrogenations of hydrocarbons. The dehydrogenation of hydrocarbons was first achieved in pure form by N. D. Zelinskii, who developed a number of selective catalysts. Large contributions to the development of the theory and applications of hydrogenation and dehydrogenation were made by B. A. Kazanskii and A. A. Balandin and their schools.


Lebedev, S. V. Zhizn’ i trudy. Leningrad, 1938.
Dolgov, B. N. Kataliz v organicheskoi khimii, 2nd ed. Leningrad, 1959.
Balandin, A. A. Mul’tipletnaia teoriia kataliza, parts 1-2. Moscow, 1963-64.
Iukel’son, I. I. Tekhnologiia osnovnogo organicheskogo sinteza. Moscow, 1968.
Bond, G. C. Catalysis by Metals. London-New York, 1962.
Rideal, E. Razvitie predstavlenii v oblas ti kataliza. Moscow, 1971. Chapters 6 and 7. (Translated from English.)



(chemical engineering)
Saturation of diolefin impurities in gasolines to form a stable product.
(organic chemistry)
Catalytic reaction of hydrogen with other compounds, usually unsaturated; for example, unsaturated cottonseed oil is hydrogenated to form solid fats.
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