In chemistry, dehydrogenation is a chemical reaction that involves the removal of hydrogen, usually from an organic molecule. It is the reverse of hydrogenation. Dehydrogenation is important, both as a useful reaction and a serious problem. At its simplest, it's a useful way of converting alkanes, which are relatively inert and thus low-valued, to olefins, which are reactive and thus more valuable. Alkenes are precursors to aldehydes (R−CH=O), alcohols (R−OH), polymers, and aromatics.[1] As a problematic reaction, the fouling and inactivation of many catalysts arises via coking, which is the dehydrogenative polymerization of organic substrates.[2]
The cracking processes especially fluid catalytic cracking and steam cracker produce high-purity mono-olefins from paraffins. Typical operating conditions use chromium (III) oxide catalyst at 500 °C. Target products are propylene, butenes, and isopentane, etc. These simple compounds are important raw materials for the synthesis of polymers and gasoline additives.[citation needed]
Oxidative dehydrogenation
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 of methanol. This reaction can also be viewed as a dehydrogenation using O2 as the acceptor. The most common catalysts are silver metal, iron(III) oxide,[7] iron molybdenum oxides [e.g. iron(III) molybdate] with a molybdenum-enriched surface,[8] or vanadiumoxides. 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:[9]
CH3OH + O2 → 2 CH2O + 2 H2O
Homogeneous catalytic routes
A variety of dehydrogenation processes have been described for organic 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 by homogeneous catalysis. Especially active for this reaction are pincer complexes.[13][14]
^Denis H. James William M. Castor, "Styrene" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
^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.
^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
^Yeung, Charles S.; Dong, Vy M. (2011). "Catalytic Dehydrogenative Cross-Coupling: Forming Carbon−Carbon Bonds by Oxidizing Two Carbon−Hydrogen Bonds". Chemical Reviews. 111 (3): 1215–1292. doi:10.1021/cr100280d. PMID21391561.
^Dobereiner, Graham E.; Crabtree, Robert H. (2010). "Dehydrogenation as a Substrate-Activating Strategy in Homogeneous Transition-Metal Catalysis". Chemical Reviews. 110 (2): 681–703. doi:10.1021/cr900202j. PMID19938813.
^Choi, Jongwook; MacArthur, Amy H. Roy; Brookhart, Maurice; Goldman, Alan S. (2011). "Dehydrogenation and Related Reactions Catalyzed by Iridium Pincer Complexes". Chemical Reviews. 111 (3): 1761–1779. doi:10.1021/cr1003503. PMID21391566.
^"1". Alkane C-H Activation by Single-Site Metal Catalysis | Pedro J. Pérez | Springer. pp. 1–15.
^Findlater, Michael; Choi, Jongwook; Goldman, Alan S.; Brookhart, Maurice (2012-01-01). Pérez, Pedro J. (ed.). Alkane C-H Activation by Single-Site Metal Catalysis. Catalysis by Metal Complexes. Springer Netherlands. pp. 113–141. doi:10.1007/978-90-481-3698-8_4. ISBN9789048136971.
^Aitken, C.; Harrod, J. F.; Gill, U. S. (1987). "Structural studies of oligosilanes produced by catalytic dehydrogenative coupling of primary organosilanes". Can. J. Chem. 65 (8): 1804–1809. doi:10.1139/v87-303.
^Staubitz, Anne; Robertson, Alasdair P. M.; Manners, Ian (2010). "Ammonia-Borane and Related Compounds as Dihydrogen Sources". Chemical Reviews. 110 (7): 4079–4124. doi:10.1021/cr100088b. PMID20672860.