Dehydrogenation
Dehydrogenation is a chemical reaction that involves the removal of hydrogen from a molecule. It is the reverse process of hydrogenation. Dehydrogenation reactions are conducted both on industrial and laboratory scales. Dehydrogenation converts saturated fats to unsaturated fats. Enzymes that catalyze dehydrogenation are called dehydrogenases. Dehydrogenation processes are used extensively to produce styrene in the fine chemicals, oleochemicals, petrochemicals, and detergents industries.
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 sulphur and selenium promote this process. 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.[1]
- 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:[2]
- 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.[3]
References
- Advanced Organic Chemistry, Jerry March, 1162-1173.
- http://www.firstprincipals.com/DeHydrogenation.htm
- ↑ Denis H. James William M. Castor, “Styrene” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
- ↑ Günther Reuss, Walter Disteldorf, Armin Otto Gamer, Albrecht Hilt “Formaldehyde” in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a11_619
- ↑ Ajayi, B. P.; Jermy, B. Rabindran; Ogunronbi, K. E.; Abussaud, B. A.; Al-Khattaf, S. (2013-04-15). "n-Butane dehydrogenation over mono and bimetallic MCM-41 catalysts under oxygen free atmosphere". Catalysis Today. Challenges in Nanoporous and Layered Materials for Catalysis 204: 189–196. doi:10.1016/j.cattod.2012.07.013.
- ↑ "Polypropylene Production via Propane Dehydrogenation part 2, Technology Economics Program". by Intratec, ISBN 978-0615702162, Q3 2012.