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

Abstract

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.