Ullmanns 6th:
Industrially, propionic acid is currently produced almost exclusively by three different processes:
1) Carbonylation of ethylene with carbon monoxide and water
2) Oxidation of propanal
3) Direct oxidation of hydrocarbons
Processes such as production of propionic acid as a byproduct in the synthesis of hydroxylammonium salts from 1-nitropropane [108-03-2] [33] (® Hydroxylamine - 4.4. Acid Cleavage of Nitroalkanes), by wood distillation, or by nitric acid oxidation of 1-propanol [62309-51-7] have become obsolete, although the latter gives propionic acid in 90 % yield [34]. Synthesis of propionic acid via the alkali melt of an n- and isopropanol mixture could never compete successfully with other pro-cesses, although propionic acid was obtained with > 98 % yield [35] , [36]. The carbonylation of ethanol [64-17-5] [37] and acetic acid [64-19-7] [38] have also not yet been carried out industrially.
The Koch synthesis has been investigated intensively [39] , [40]. As a carbonylation reaction of ethylene [74-85-1] in a strongly acidic medium, it is a variant of the Reppe synthesis. However, compared with the latter it never achieved much industrial significance and was carried out with little success.
Other possible sources of propionic acid, which are not used industrially for economic reasons, are its formation as a byproduct in the high- and low-pressure carbonylation of methanol [67-56-1] to give acetic acid (2 % formation of propionic acid), the atmospheric oxidation of n-butene [25167-67-3] (4 % formation of propionic acid), and the direct reaction of ethylene, carbon monoxide, and water over noble-metal catalysts [41][42][43][44][45][46][47][48][49].
Processes for making propionic acid accessible specifically by C1- chemistry have yet to be assessed for their future importance. Two-step reactions have been described, in which propionic acid is obtained directly from synthesis gas (20 – 60 bar, 150 – 160 °C, Rh catalysts) [50] , [51]. With the current availability of ethylene and naphtha these processes are, however, not yet competitive.
It is sometimes desirable, to use propionic acid produced by natural methods, particularly for the use of propionic acid in flavors and fragrances. Appropriate microbiological and enzymatic processes have been developed, which are usually based on the anaerobic fermentation of starch or sugars [52][53][54]. However, the expensive production of this "natural" propionic acid limits its use to special areas of application.
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[54] H. Dellweg: Biotechnology, vol. 3, Verlag Chemie, Weinheim 1983, p. 472.