Solvent-free selective oxidation of cyclohexane with molecular oxygen over manganese oxides: Effect of the calcination temperature

The effects of calcination temperature on the physicochemical properties of manganese oxide catalysts prepared by a precipitation method were assessed by X-ray diffraction, N2 adsorption-desorption, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, O2 temperature-programmed desorption, and thermogravimetry-differential analysis. The catalytic performance of each of these materials during the selective oxidation of cyclohexane with oxygen in a solvent-free system was subsequently examined. It was found that the MnOx-500 catalyst, calcined at 500 °C, consisted of a Mn2O3 phase in addition to Mn5O8 and Mn3O4 phases and possessed a low surface area. Unlike MnOx-500, the MnOx-400 catalyst prepared at 400 °C was composed solely of Mn3O4 and Mn5O8 and had a higher surface area. The pronounced catalytic activity of this latter material for the oxidation of cyclohexene was determined to result from numerous factors, including a higher concentration of surface adsorbed oxygen, greater quantities of the surface Mn4+ ions that promote oxygen mobility and the extent of O2 adsorption and reducibility on the catalyst. The effects of various reaction conditions on the activity of the MnOx-400 during the oxidation of cyclohexane were also evaluated, such as the reaction temperature, reaction time, and initial oxygen pressure. Following a 4 h reaction at an initial O2 pressure of 0.5 MPa and 140 °C, an 8.0% cyclohexane conversion and 5.0% yield of cyclohexanol and cyclohexanone were achieved over the MnOx-400 catalyst. In contrast, employing MnOx-500 resulted in a 6.1% conversion of cyclohexane and 75% selectivity for cyclohexanol and cyclohexanone. After being recycled through 10 replicate uses, the catalytic activity of the MnOx-400 catalyst was unchanged, demonstrating its good stability.

MnOx exhibited high catalytic activity and excellent stability during cyclohexane oxidation with molecular oxygen. The MnOx phase state, as determined by the calcination temperature, had a crucial effect on its catalytic activity.Figure optionsDownload as PowerPoint slide