Towards the mechanism of the oxidation of cyclohexylamine by molecular oxygen over alumina-based catalysts

The oxidation of cyclohexylamine with molecular oxygen over a heterogeneous catalyst is an attractive one-step route to prepare a commercially important cyclohexanone oxime. The effect of the reaction conditions on the conversion of cyclohexylamine during its oxidation by molecular oxygen and on the selectivity of cyclohexanone and cyclohexanone oxime formation was investigated over heterogeneous catalysts (alumina and alumina supported silicotungstic acid). The present work is aimed at the enhancement of the cyclohexanone oxime yield, for which the optimized conditions were found as: reaction temperature of 180 °C, LHSV of cyclohexylamine 1 ml g−1 h−1, oxidant 33 % O2 in nitrogen with a total GHSV of 24 ml min−1. Cyclohexanone, formed from cyclohexylamine, reacts with cyclohexylamine to N-cyclohexylidenecyclohexylamine, their Schiff base. The most intense formation of cyclohexanone was observed in the initial period of the catalytic tests. Carbonaceous deposits, responsible for the catalyst deactivation, are formed mainly from the Schiff base; their amount increases by time on stream, and the rate of their formation correlates with the concentration of oxygen in the reaction mixture, as well as with the formation rate of cyclohexanone and the Schiff base. Injection of water to the reactant stream prolongs the catalyst lifetime and reduces the tar formation. However, it deteriorates the catalyst´s activity and selectivity. The supposed mechanism of cyclohexylamine oxidation is discussed.