Oxidation of KA Oil with Air

The second step of the KA Oil process can be carried out with oxygen as the oxidant, in place of nitric acid, and catalytic amounts of Co and Mn acetate, at 70-80 °C, in acetic acid solvent 13. The fact that N2O is not generated and that the dedicated HNO3 plant is not necessary (when a large-scale HNO3 plant is not already available) may make the air-based process an alternative to the process currently employed. However, the potential use in industrial applications is still unclear, due to the lower yield achieved than that with nitric acid, and also due to the lower quality of the AA obtained. Moreover, using acetic acid as the solvent leads to severe corrosion problems, particularly in combination with the Mn and Co salts. 

In this reaction it is important to achieve 100% conversion of the reactant, because recycling of unconverted KA Oil complicates the process. Yields to AA reported in most of the patent literature are not higher than 70% at high 01/One conversion, with an overall yield to diacids close to 80%. 

Side-reactions lead to the formation of lighter acids. For instance, monoperoxyadipic acid can decarboxylate to yield the pentanoic acid radical, precursor of the byproduct valeric acid. The same C5 radical may react with. O2 to yield 5-oxopentanoic acid, which is then oxidized to monoperoxyglutaric acid, a precursor of glutaric acid. An analogous mechanism starting from the butanoic acid radical may yield the byproduct succinic acid. Azelaic acid may form by the coupling of radical species (e.g., between the butanoic acid radical and the pentanoic acid radical), whereas the dimerization of the pentanoic acid radical may yield the by-product sebacic acid. 

Researchers from Asahi have ascertained the industrial feasibility of a process based on a Co/Mn catalyst and either pure oxygen at atmospheric pressure or nitrogen-diluted air at 12 atm (to avoid explosion hazards), with water and acetic acid as the solvent [13oj. The authors describe in detail the main features of the process and report a detailed study of the reaction mechanism. 

The combination of a Mn(OAc)2 catalyst and a Co(OAc)2 catalyst (optimum Mn/Co ratio, 1 1) is effective for improving AA selectivity in the liquid-phase oxidation of cyclohexanone with oxygen. With pure oxygen, at atmospheric pressure, a selectivity to AA as high as 77% at total conversion of cyclohexanone is obtained, which is higher than that previously reported in the patent literature. The selectivity to glutaric acid is 12% and that to succinic acid is 2%, to oxoacids 2%, for an overall selectivity to acids of 93%. Selectivity to AA shows a maximum at 70°C. 

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