An Industrial Process for Adipic Acid Production by the Liquid-Phase Oxidation of Cyclohexanone with Molecular Oxygen

Adipic acid is produced industrially by the oxidation of cyclohexanone and cyclohexanol with nitric acid. Dinitrogen oxide (nitrous oxide, N2O), a non-CO2 greenhouse gas, is produced as a by-product in this process. We have investigated an industrial process for the production of adipic acid without the generation of N2O. When acetic acid was used as a solvent and Mn(OAc)2/Co(OAc)2 were used as catalysts under atmospheric pressure of pure O2 at 70 °C, the selectivity of adipic acid was 77% and the combined selectivity of carboxylic acids, which consisted of adipic acid, glutaric acid, and succinic acid, was 93% for the 100% conversion of cyclohexanone. We presume that adipic acid is generated via 6-oxohexanoic acid. Glutaric acid was probably generated via 5-oxopentanoic acid in the reaction path that branched at 6-oxohexanoic acid. When air containing less than 10 vol % of oxygen was used at 1.22 MPa in order to avoid explosion hazards, the selectivity of adipic acid was 78% for a 99.3% conversion of cyclohexanone. We conclude that the present reaction can be used as an industrial process for adipic acid production.