Liquid-phase cooxidation of cyclohexane and cyclohexanone over supported cerium oxide catalysts

CeO2/gamma-Al2O3 is found to be the best catalyst for the liquid-phase cooxidation of cyclohexane and cyclohexanone among several supported cerium oxide catalysts. By using this catalyst, the conversions of cyclohexane and cyclohexanone exceed those by other supported catalysts under the same reaction conditions. Furthermore, the yield of the desired products surpasses those in our previous studies. In addition, this catalyst is resistant to acidity and is more stable in an acidic medium than other heterogeneous catalysts in our previous works. The induction period, as a characteristic of the free-radical reaction, is not observed if gamma-Al2O3 is used as a catalyst. The epsilon-caprolactone is not formed via a free-radical mechanism but, instead, the Baeyer-Villiger mechanism. Experimental results also indicate that water profoundly influences the elusion of metals from solid catalysts; however, no cerium eludes from the CeO2/gamma-Al2O3 catalyst under the present reaction conditions. The formation of dibasic acids and epsilon-caprolactone is also discussed.