Aerobic oxidation of secondary alcohols to ketones catalyzed by cobalt(II)/ZnO in poly(ethylene glycol)/CO2 system

The aerobic oxidation of benzhydrol, 1-phenylethanol and cyclohexanol to corresponding ketones was carried out in poly(ethylene glycol) (PEG-600)/supercritical CO2 (SCCO2) biphasic system using unsupported and supported CoCl2·6H2O as the catalysts. It was demonstrated that CoCl2·6H2O, Co(II)/Al2O3, and Co(II)/ZnO were all active and selective for the reactions. The reactivity of the substrates followed the order benzhydrol > 1-phenylethanol > cyclohexanol. Co(II)/ZnO was most stable and could be reused four times without considerable reduction of activity. The effect of CO2 pressure, reaction time and solvent was further investigated. The Co(II)/ZnO/PEG-600 system is active, selective, cheap, clean, and recyclable for the aerobic oxidation of secondary alcohols.