Dehydrogenation

Dehydrogenation is a chemical reaction that involves the removal of hydrogen from an organic molecule. It is the reverse of hydrogenation. Dehydrogenation is an important reaction because it converts alkanes, which are relatively inert and thus low-valued, to olefins, which are reactive and thus more valuable. Alkenes are precursors to aldehydes, alcohols, polymers, and aromatics.^[1] Dehydrogenation processes are used extensively to produce aromatics and styrene in the petrochemical industry. Such processes are highly endothermic and require temperatures of 500 °C and above.^[1][2] Dehydrogenation also converts saturated fats to unsaturated fats. Enzymes that catalyze dehydrogenation are called dehydrogenases.

Classes of the reaction

A variety of dehydrogenation processes have been described, especially for organic compounds:

• In typical aromatization, six-membered alicyclic rings, e.g. cyclohexene, can be aromatized in the presence of hydrogenation acceptors. The elements sulfur and selenium promote this process. On the laboratory scale, quinones, especially 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone are effective.
• Oxidation of alcohols to ketones or aldehydes can be effected by metal catalysts such as copper chromite. In the Oppenauer oxidation, hydrogen is transferred from an alcohol to an aldehyde or ketone to bring about the oxidation.
• Dehydrogenation of amines to nitriles using a variety of reagents, such as Iodine pentafluoride (IF
  5).
• Dehydrogenation of paraffins and olefins — paraffins such as n-pentane and isopentane can be converted to pentene and isopentene using chromium (III) oxide as a catalyst at 500 °C.

Examples

One of the largest scale dehydrogenation reactions is the production of styrene by dehydrogenation of ethylbenzene. Typical dehydrogenation catalysts are based on iron(III) oxide, promoted by several percent potassium oxide or potassium carbonate.^[3]

C₆H₅CH₂CH₃ → C₆H₅CH=CH₂ + H₂

Formaldehyde is produced industrially by the catalytic oxidation of methanol, which can also be viewed as a dehydrogenation using O₂ as the acceptor. The most common catalysts are silver metal or a mixture of an iron and molybdenum or vanadium oxides. In the commonly used formox process, methanol and oxygen react at ca. 250–400 °C in presence of iron oxide in combination with molybdenum and/or vanadium to produce formaldehyde according to the chemical equation:^[4]

2 CH₃OH + O₂ → 2 CH₂O + 2 H₂O

The importance of catalytic dehydrogenation of paraffin hydrocarbons to olefins has been growing steadily in recent years. Light olefins, such as butenes, are important raw materials for the synthesis of polymers, gasoline additives and various other petrochemical products. The cracking processes especially fluid catalytic cracking and steam cracker produce high-purity mono-olefins, such as 1-butene or butadiene. Despite such processes, currently more research is focused on developing alternatives such as oxidative dehydrogenation (ODH) for two reasons: (1) undesired reactions take place at high temperature leading to coking and catalyst deactivation, making frequent regeneration of the catalyst unavoidable, (2) it consumes a large amount of heat and requires high reaction temperatures. Oxidative dehydrogenation (ODH) of n-butane is an alternative to classical dehydrogenation, steam cracking and fluid catalytic cracking processes.^[5]

• Butane
• Butene
• Cyclohexane
• Propane^[6]

Homogeneous catalysis

Although not of commercial value, dehydrogenation of alkanes can be effected by homogeneous catalysis. Especially active for this reaction are pincer complexes.^[7][8]

References

• Advanced Organic Chemistry, Jerry March, 1162-1173.
• http://www.firstprincipals.com/DeHydrogenation.htm