:Dehydrogenation
{{Short description|Chemical reaction involving the removal of hydrogen}}
{{Hatnote|Compare Deprotonation.}}
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 is 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 ({{chem2|R\sCH\dO}}), alcohols ({{chem2|R\sOH}}), polymers, and aromatics.{{Cite book|title=Industrial Organic Chemicals, Second Edition - Wittcoff - Wiley Online Library|last1=Wittcoff|first1=Harold A.|last2=Reuben|first2=Bryan G.|last3=Plotkin|first3=Jeffrey S.|doi=10.1002/0471651540|year = 2004|isbn = 9780471651543}} As a problematic reaction, the fouling and inactivation of many catalysts arises via coking, which is the dehydrogenative polymerization of organic substrates.{{cite journal |doi=10.1016/S0926-860X(00)00845-0|title=Organic chemistry of coke formation|year=2001|last1=Guisnet|first1=M.|last2=Magnoux|first2=P.|journal=Applied Catalysis A: General|volume=212|issue=1–2|pages=83–96}}
Enzymes that catalyze dehydrogenation are called dehydrogenases.
In metal manufacturing and repairs, dehydrogenation is a thermal treatment which consists in removing the hydrogen absorbed by an object during an electrochemical or chemical process, performed in a specific oven at a temperature of {{convert|180–200|C|F|sigfig=2}} for a minimum time of 2 hours.{{cn|date=November 2024}}
Heterogeneous catalytic routes
=Styrene=
Dehydrogenation processes are used extensively to produce aromatics in the petrochemical industry. Such processes are highly endothermic and require temperatures of 500 °C and above.{{Cite book|url=https://www.springer.com/us/book/9780306472466|title=Survey of Industrial Chemistry {{!}} Philip J. Chenier {{!}} Springer|isbn=9780471651543}} Dehydrogenation also converts saturated fats to unsaturated fats. 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.Denis H. James William M. Castor, "Styrene" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
:{{chem2|C6H5CH2CH3 -> C6H5CH\dCH2 + H2}}
=Other alkenes=
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.{{cn|date=February 2024}}
=Alcohols to aldehydes=
Alcohols can be selectively dehydrogenated to give aldehydes. This in employed in the industrial production of butanone and is important in the production of certain aroma compounds.
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.{{Cite journal|title = n-Butane dehydrogenation over mono and bimetallic MCM-41 catalysts under oxygen free atmosphere|journal = Catalysis Today|date = 2013-04-15|pages = 189–196|volume = 204|series = Challenges in Nanoporous and Layered Materials for Catalysis|doi = 10.1016/j.cattod.2012.07.013|first1 = B. P.|last1 = Ajayi|first2 = B. Rabindran|last2 = Jermy|first3 = K. E.|last3 = Ogunronbi|first4 = B. A.|last4 = Abussaud|first5 = S.|last5 = Al-Khattaf}}{{cite book|url=http://www.slideshare.net/intratec/propylene-production-via-propane-dehydrogenation-part-2|title= Polypropylene Production via Propane Dehydrogenation part 2, Technology Economics Program|publisher=by Intratec |isbn=978-0615702162 |year=2012}}
Formaldehyde is produced industrially by oxidative dehydrogenation of methanol. This reaction can also be viewed as a dehydrogenation using {{chem2|O2}} as the acceptor. The most common catalysts are silver metal, iron(III) oxide,{{Cite journal |last1=Wang |first1=Chien-Tsung |last2=Ro |first2=Shih-Hung |date=2005-05-10 |title=Nanocluster iron oxide-silica aerogel catalysts for methanol partial oxidation |url=https://www.sciencedirect.com/science/article/pii/S0926860X05001523 |journal=Applied Catalysis A: General |language=en |volume=285 |issue=1 |pages=196–204 |doi=10.1016/j.apcata.2005.02.029 |issn=0926-860X|url-access=subscription }} iron molybdenum oxides [e.g. iron(III) molybdate] with a molybdenum-enriched surface,{{Cite journal |last1=Dias |first1=Ana Paula Soares |last2=Montemor |first2=Fátima |author-link2=Maria de Fátima Montemor|last3=Portela |first3=Manuel Farinha |last4=Kiennemann |first4=Alain |date=2015-02-01 |title=The role of the suprastoichiometric molybdenum during methanol to formaldehyde oxidation over Mo–Fe mixed oxides |url=https://www.sciencedirect.com/science/article/pii/S1381116914004774 |journal=Journal of Molecular Catalysis A: Chemical |language=en |volume=397 |pages=93–98 |doi=10.1016/j.molcata.2014.10.022 |issn=1381-1169|url-access=subscription }} or vanadium oxides. In the commonly used formox process, methanol and oxygen react at ca. {{convert|250–400|C|F|sigfig=2}} in the presence of iron oxide in combination with molybdenum and/or vanadium to produce formaldehyde according to the chemical equation:{{cite book | doi = 10.1002/14356007.a11_619 | chapter = Formaldehyde | title = Ullmann's Encyclopedia of Industrial Chemistry | date = 2000 | last1 = Reuss | first1 = Günther | last2 = Disteldorf | first2 = Walter | last3 = Gamer | first3 = Armin Otto | last4 = Hilt | first4 = Albrecht | isbn = 3-527-30673-0 }}
:{{chem2|2 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.{{cite journal |doi=10.1021/cr100280d|title=Catalytic Dehydrogenative Cross-Coupling: Forming Carbon−Carbon Bonds by Oxidizing Two Carbon−Hydrogen Bonds|year=2011|last1=Yeung|first1=Charles S.|last2=Dong|first2=Vy M.|journal=Chemical Reviews|volume=111|issue=3|pages=1215–1292|pmid=21391561}} Such reactions often rely on transition metal catalysts.{{cite journal |doi=10.1021/cr900202j|title=Dehydrogenation as a Substrate-Activating Strategy in Homogeneous Transition-Metal Catalysis|year=2010|last1=Dobereiner|first1=Graham E.|last2=Crabtree|first2=Robert H.|journal=Chemical Reviews|volume=110|issue=2|pages=681–703|pmid=19938813}}{{cite journal |doi=10.1021/cr1003503|title=Dehydrogenation and Related Reactions Catalyzed by Iridium Pincer Complexes|year=2011|last1=Choi|first1=Jongwook|last2=MacArthur|first2=Amy H. Roy|last3=Brookhart|first3=Maurice|last4=Goldman|first4=Alan S.|journal=Chemical Reviews|volume=111|issue=3|pages=1761–1779|pmid=21391566}} Dehydrogenation of unfunctionalized alkanes can be effected by homogeneous catalysis. Especially active for this reaction are pincer complexes.{{Cite book|chapter-url=https://www.springer.com/us/book/9789048136971|title=Alkane C-H Activation by Single-Site Metal Catalysis {{!}} Pedro J. Pérez {{!}} Springer|pages=1–15|chapter=1}}{{Cite book|title=Alkane C-H Activation by Single-Site Metal Catalysis|last1=Findlater|first1=Michael|last2=Choi|first2=Jongwook|last3=Goldman|first3=Alan S.|last4=Brookhart|first4=Maurice|date=2012-01-01|publisher=Springer Netherlands|isbn=9789048136971|editor-last=Pérez|editor-first=Pedro J.|series=Catalysis by Metal Complexes|pages=113–141|language=en|doi=10.1007/978-90-481-3698-8_4}}
Stoichiometric processes
Dehydrogenation of amines to nitriles can be accomplished using a variety of reagents, such as iodine pentafluoride ({{chem|IF|5}}).{{cn|date=February 2024}}
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 (DDQ) are effective.{{cn|date=February 2024}}
Main group hydrides
File:DAB secondary path.png.]]
The dehydrogenative coupling of silanes has also been developed.{{cite journal
| last1 =Aitken | first1 =C.
| last2 =Harrod| first2 =J. F.
| last3 =Gill | first3 =U. S.
| title =Structural studies of oligosilanes produced by catalytic dehydrogenative coupling of primary organosilanes | journal = Can. J. Chem.
| volume =65
| year =1987
| issue =8
| pages =1804–1809 | doi =10.1139/v87-303
}}
:{{chem2|n PhSiH3 -> [PhSiH]_{n} + n H2 }}
The dehydrogenation of amine-boranes is related reaction. This process once gained interests for its potential for hydrogen storage.{{cite journal|doi=10.1021/cr100088b|title=Ammonia-Borane and Related Compounds as Dihydrogen Sources|year=2010|last1=Staubitz|first1=Anne|last2=Robertson|first2=Alasdair P. M.|last3=Manners|first3=Ian|journal=Chemical Reviews|volume=110|issue=7|pages=4079–4124|pmid=20672860}}