Hydrogenation

Hydrogenation, reaction involving the combination of hydrogen with unsaturated organic compounds (see Hydrocarbons). Unsaturated organic compounds have at least one pair of carbon atoms connected by a double or triple bond. When an unsaturated compound is treated with hydrogen at a suitable temperature and in the presence of a catalyst (see Catalysis), such as finely-divided nickel, platinum, or palladium, the multiple bond between the carbon atoms is broken and a hydrogen atom attaches itself to each carbon atom. For example, when ethylene (C₂H₄) is hydrogenated, the product is ethane (C₂H₆). Hydrogenation is also used with more complicated molecules, yielding a great variety of synthetic products that are important in the laboratory and for industry.

The hydrogenation reaction is applied on a large industrial scale in a number of processes, the most important of which in the United States is the hydrogenation of vegetable oils to produce many edible fats, such as margarine. Oils are esters of fatty acids containing one or more double bond linkages between carbon atoms; solid fats are saturated compounds. To convert the oils, which often have an unpleasant taste and odor, to fats with odor and taste sufficiently innocuous so that they can be used for cooking, the oils are hydrogenated. The reaction is performed at a temperature of about 200° C (about 392° F) in the presence of finely-divided nickel, and under a pressure of 3 to 4 atmospheres of hydrogen. Large volumes of cheap and plentiful vegetable oils, such as soybean and cottonseed oil, are hydrogenated sufficiently to yield creamy solids that resemble lard at room temperatures. Such products are widely used in foods. Low-grade oils, such as fish oils, are hydrogenated and used in the manufacture of soap and wax.

The hydrogenation process is also applied in the production of synthetic gasoline. The Bergius process, named after the German chemist Friedrich Bergius, is used on a large scale in many parts of the world where petroleum resources are low. It utilizes coal and coal tar as a starting material. The coal, mixed with a heavy oil, is ground to a fine paste and heated with hydrogen, under pressure, in the presence of a catalyst composed of metallic sulfides. The resulting oil is further hydrogenated, and a third hydrogenation yields gasoline. One ton of coal yields about 304 liters (about 80 gallons) of gasoline.

The Fischer-Tropsch process, named after its developers, the German chemists Franz Fischer and Hans Tropsch, was used extensively in Germany in the 1930s to produce synthetic petroleum and diesel fuel. It uses a mixture of carbon monoxide and hydrogen gases with a catalyst containing nickel, cobalt, or modified iron. The process is currently used to produce the raw materials for manufacturing synthetic fats and soaps.