dehydrogenations

Dehydrogenation is a class of chemical reactions in which hydrogen atoms are removed from a molecule, typically forming a double bond and releasing molecular hydrogen (H2). In organic chemistry, dehydrogenation contrasts with hydrogenation, which adds H2, and with oxidation, which involves increase in oxidation state and often incorporation of oxygen.

Most dehydrogenations are catalyzed by metals and occur under high temperatures. They are often endothermic and governed by chemical equilibrium, which favors products at higher temperature but requires removal of the produced hydrogen to shift the reaction forward. Heterogeneous catalysts, such as oxides on alumina or other supports, and noble- or transition-metal catalysts (for example platinum, chromium, iron, nickel) are commonly used. Catalyst design aims to minimize side reactions like cracking or coke formation and to achieve high selectivity for the desired unsaturation.

Industrially important dehydrogenations include the conversion of ethylbenzene to styrene (C6H5-CH2-CH3 → C6H5-CH=CH2 + H2), typically carried out on iron-oxide- or chromium-based catalysts at around 600–700 °C with measures to suppress coking. Cyclohexane to benzene (C6H12 → C6H6 + 3 H2) is another major process, using platinum- or chromium-based catalysts. Dehydrogenation of propane to propene (C3H8 → C3H6 + H2) and ethane to ethene (C2H6 → C2H4 + H2) are also practiced, though these reactions require careful temperature control to balance activity, selectivity, and catalyst stability.

Hydrogen generated in dehydrogenation can be recovered for energy or chemical uses, influencing process economics and sustainability.