Ce1-xCrxO2- Nanocrystals as Efficient Catalysts for the Selective Oxidation of Cyclohexane to KA Oil at Low Temperature under Ambient Pressure

摘要

The selective oxidation of C-H bonds in saturated hydrocarbons at low temperature with noble-metal-free heterogeneous catalysts is a profound challenge for the catalysis community. Here we report the facile synthesis of Ce1-xCrxO2- (x=0, 0.1, 0.2, 0.3, and 0.4) nanocrystals through a low-temperature hydrothermal method. The catalytic activity of the as-prepared catalysts was evaluated in the selective oxidation of cyclohexane to cyclohexanone/cyclohexanol using H2O2 as an oxidant at room temperature and 45 degrees C. The activity results were correlated with their phase structure, composition, surface areas, and surface element states with the help of X-ray diffraction, N-2 adsorption, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV/Vis spectroscopy. It was found that these Ce0.7Cr0.3O2- nanocrystals are highly active for the selective oxidation of cyclohexane to cyclohexanone/cyclohexanol using H2O2 as an oxidant at low temperature. In the presence of H2O2 at 45 degrees C for 5h using Ce0.7Cr0.3O2- as a catalyst, the conversion of cyclohexane reaches 50.3% and the total selectivity of cyclohexanol/cyclohexanone is 94.4% with relative good reversibility for at least four cycles. Mechanistic investigation results show that the high activity can be attributed to the high Cr6+ content in the Ce1-xCrxO2- solid solutions and the formation of highly oxidative intermediates with H2O2.