Catalytic oxidation

Catalytic oxidation are processes that oxidize compounds using catalysts. Common applications involve oxidation of organic compounds by the oxygen in air. Such processes are conducted on a large scale for the remediation of pollutants, production of valuable chemicals, and the production of energy.[1] In petrochemistry, high-value intermediates such as carboxylic acids, aldehydes, ketones, epoxides, and alcohols are obtained by partial oxidation of alkanes and alkenes with dioxygen. These intermediates are essential to the production of consumer goods. Partial oxidation presents two challenges. The first is that the most favored reaction between oxygen and hydrocarbons is combustion. The second challenge is the considerable difficulty to activate dioxygen, viz. the splitting of the molecule into its constituent atoms, which has an energy barrier of 498 kJ/mol. The usual strategy to activate oxygen in a controlled manner is to use molecular hydrogen or carbon monoxide as sacrificial reductants in presence of a heterogeneous catalyst, such that the activation barrier is lowered to < 10 kJ/mol and hence milder reaction conditions are required.[2]

One of the most challenging selective oxidations, which has been achieved with supported gold catalysts, is the epoxidation of propylene.

An illustrative catalytic oxidation is the conversion of methanol to the more valuable compound formaldehyde using oxygen in air:

2 CH3OH + O2 → 2CH2O + 2 H2O

This conversion is very slow in the absence of catalysts. Typical oxidation catalysts are metal oxides and metal carboxylates.

Examples

Industrially important examples include both inorganic and organic substrates.

SubstrateProcessCatalyst
(homogeneous or heterogeneous		ProductApplication
sulfur dioxidecontact processvanadium pentoxide
(heterogeneous)		sulfuric acidfertilizer production
ammoniaOstwald processplatinum
(heterogeneous)		nitric acidbasic chemicals, TNT
hydrogen sulfideClaus processvanadium pentoxide
(heterogeneous)		sulfurremediation of byproduct of
oil refinery
methane,
ammonia		Andrussow processplatinum
(heterogeneous)		hydrogen cyanidebasic chemicals, gold mining extractant
ethyleneepoxidationmixed Ag oxides
(heterogeneous)		ethylene oxidebasic chemicals, surfactants
cyclohexaneK-A processCo and Mn salts
(homogeneous)		cyclohexanol
cyclohexanone		nylon precursor
ethyleneWacker processPd and Cu salts
(homogeneous)		acetaldehydebasic chemicals
para-xyleneterephthalic acid synthesisMn and Co salts
(homogeneous)		terephthalic acidplastic precursor
propyleneallylic oxidationMo-oxides
(heterogeneous)		acrylic acidplastic precursor
propylene,
ammonia		SOHIO processBi-Mo-oxides
(heterogeneous)		acrylonitrileplastic precursor
methanolFormox processFe-Mo-oxides
(heterogeneous)		formaldehydebasic chemicals, alkyd resins
butaneMaleic anhydride processvanadium phosphates
(heterogeneous)		maleic anhydrideplastics, alkyd resins

Catalysts

Applied catalysis

Oxidation catalysis is conducted by both heterogeneous catalysis and homogeneous catalysis. In the heterogeneous processes, gaseous substrate and oxygen (or air) are passed over solid catalysts. Typical catalysts are platinum, and redox-active oxides of iron, vanadium, and molybdenum. In many cases, catalysts are modified with a host of additives or promoters that enhance rates or selectivities.

Important homogeneous catalysts for the oxidation of organic compounds are carboxylates of cobalt, iron, and manganese. To confer good solubility in the organic solvent, these catalysts are often derived from naphthenic acids and ethylhexanoic acid, which are highly lipophilic. These catalysts initiate radical chain reactions, autoxidation that produce organic radicals that combine with oxygen to give hydroperoxide intermediates. Generally the selectivity of oxidation is determined by bond energies. For example, benzylic C-H bonds are replaced by oxygen faster than aromatic C-H bonds.[3]

Fine chemicals
Main article: Asymmetric catalytic oxidation

Many selective oxidation catalysts have been developed for producing fine chemicals of pharmaceutical or academic interest. Nobel Prize–winning examples are the Sharpless epoxidation and the Sharpless dihydroxylation.

Biological catalysis

Catalytic oxidations are common in biology, especially since aerobic life subsists on the energy of O2 [4] released by oxidation of organic compounds. In contrast to the industrial processes, which are optimized for producing chemical compounds, energy-producing biological oxidations are optimized to produce energy. Many metalloenzymes mediate these reactions.

Fuel cells, etc

Fuel cells rely on oxidation of organic compounds (or hydrogen) using catalysts. Catalytic heaters generate flameless heat from a supply of combustible fuel and oxygen from air as oxidant.

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References
  1. Gerhard Franz, Roger A. Sheldon "Oxidation" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2000 doi:10.1002/14356007.a18_261
  2. Haruta, Masatake (October 2005). "Gold rush". Nature. 437 (7062): 1098–1099. doi:10.1038/4371098a. ISSN 1476-4687. PMID 16237427.
  3. Mario G. Clerici, Marco Ricci and Giorgio Strukul "Formation of C–O Bonds by Oxidation" in Metal-catalysis in Industrial Organic Processes Gian Paolo Chiusoli, Peter M Maitlis, Eds. 2006, RSC. ISBN 978-0-85404-862-5.
  4. Schmidt-Rohr, K. (2020). "Oxygen Is the High-Energy Molecule Powering Complex Multicellular Life: Fundamental Corrections to Traditional Bioenergetics” ACS Omega 5: 2221-2233. http://dx.doi.org/10.1021/acsomega.9b03352


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