Selective oxidation of cyclohexane in supercritical carbon dioxide over CoAPO-5 molecular sieves

A series of cobalt-incorporated aluminophosphates (CoAPO-5) with different Co contents were hydrothermally synthesized. The tetrahedral Co2+ in the lattice framework was evidenced by various spectroscopic characterizations, which could be partially oxidized to Co3+ to form redox centers upon calcination. Over CoAPO-5, the selective oxidation of cyclohexane was carried out with CO2 as an additional solvent under supercritical conditions, where the different phase regions were categorized by the critical properties of the nominal reacting mixture (cyclohexane + nitrogen + carbon dioxide + cyclohexanol + cyclohexanone + water) that varies with the reaction extent. The results indicated that CoAPO-5 is an effective catalyst for the selective oxidation of cyclohexane to cyclohexanol and cyclohexanone; in the compressed CO2, the total selectivity of objective products increased and the by-products were suppressed considerably. The conversion of cyclohexane decreased, while the selectivity to cyclohexanol and cyclohexanone increased with the increase of the apparent density.