Inchemistry,dehydrogenation is achemical reaction that involves the removal ofhydrogen, usually from anorganic molecule. It is the reverse ofhydrogenation. Dehydrogenation is important, both as a useful reaction and a serious problem. At its simplest, it is a useful way of convertingalkanes, which are relatively inert and thus low-valued, toolefins, which are reactive and thus more valuable. Alkenes are precursors toaldehydes (R−CH=O),alcohols (R−OH),polymers, andaromatics.[1] As a problematic reaction, the fouling and inactivation of many catalysts arises viacoking, which is the dehydrogenative polymerization of organic substrates.[2]
Enzymes that catalyze dehydrogenation are calleddehydrogenases.
Dehydrogenation processes are used extensively to produce aromatics in thepetrochemical industry. Such processes are highlyendothermic and require temperatures of 500 °C and above.[1][3] Dehydrogenation also convertssaturated fats tounsaturated fats. One of the largest scale dehydrogenation reactions is the production ofstyrene by dehydrogenation ofethylbenzene. Typical dehydrogenation catalysts are based oniron(III) oxide, promoted by several percentpotassium oxide orpotassium carbonate.[4]
Thecracking processes, especially fluid catalytic cracking andsteam cracking, produce high-purity mono-olefins fromparaffins. Typical operating conditions usechromium (III) oxide catalyst at 500 °C. Target products arepropylene, butenes, andisopentane, etc. These simple compounds are important raw materials for the synthesis of polymers and gasoline additives.[citation needed]
Alcohols can be selectively dehydrogenated to give aldehydes. This in employed in the industrial production ofbutanone and is important in the production of certainaroma compounds.
Relative to thermal cracking of alkanes,oxidative dehydrogenation (ODH) is of interest 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) thermal dehydrogenation is expensive as it requires a large amount of heat. Oxidative dehydrogenation (ODH) of n-butane is an alternative to classical dehydrogenation, steam cracking and fluid catalytic cracking processes.[5][6]
Formaldehyde is produced industrially by oxidative dehydrogenation ofmethanol. This reaction can also be viewed as a dehydrogenation usingO2 as the acceptor. The most common catalysts aresilver metal,iron(III) oxide,[7] iron molybdenumoxides [e.g. iron(III)molybdate] with amolybdenum-enriched surface,[8] orvanadiumoxides. In the commonly usedformox process, methanol and oxygen react at ca. 250–400 °C (480–750 °F) in the presence of iron oxide in combination with molybdenum and/or vanadium to produce formaldehyde according to thechemical equation:[9]
A variety of dehydrogenation processes have been described fororganic compounds. These dehydrogenation is of interest in the synthesis of fine organic chemicals.[10] Such reactions often rely on transition metal catalysts.[11][12] Dehydrogenation of unfunctionalized alkanes can be effected byhomogeneous catalysis. Especially active for this reaction arepincer complexes.[13][14]
Dehydrogenation of amines tonitriles can be accomplished using a variety ofreagents, such asiodine pentafluoride (IF
5).[citation needed]
In typicalaromatization, six-memberedalicyclic rings, e.g.cyclohexene, can be aromatized in the presence of hydrogenation acceptors. The elementssulfur andselenium promote this process. On the laboratory scale,quinones, especially2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) are effective.[citation needed]
Thedehydrogenative coupling of silanes has also been developed.[15]
Thedehydrogenation of amine-boranes is related reaction. This process once gained interests for its potential forhydrogen storage.[16]
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