Aerobic oxidation of cyclohexane by gold-based catalysts: New mechanistic insight by thorough product analysis

The selective oxidation of cyclohexane to cyclohexanone and cyclohexanol with air over Au/Al2O3, Au/TiO2, and Au/SBA-15 catalysts was investigated and compared with the industrial autoxidation process. In contradiction with the literature results, the Au-based catalysts did not exhibit excellent catalytic performance and the combined selectivity toward cyclohexanone and cyclohexanol decreased to ∼70% with the increasing conversion above 5%. In addition, the observed product evolution and by-product evolution were typical for the autoxidation process, although a significant increase in adipic acid and CO2 formation was observed. Additional oxidation experiments containing a radical scavenger completely inhibited the reaction and provided proof that the oxidation follows a radical-chain mechanism instead of a catalytic mechanism. This explains the low selectivity at increasing conversion. This important deviation from the literature results can be clarified by the complicated but necessary product analysis of both the gas and the liquid phases.