Solvent-free selective oxidation of toluene by oxygen over MnOx/SBA-15 catalysts: Relationship between catalytic behavior and surface structure

Abstract

MnOx/SBA-15 catalysts with different manganese content were studied for the selective oxidation of toluene by oxygen under solvent-free conditions. The catalyst with a manganese content of 6wt% shows high catalytic efficiency and benzoic acid selectivity, whereas the selectivity to benzaldehyde increases remarkably upon increasing manganese content to 17wt%. Several techniques such as N2-physisorption, X-ray diffraction, diffuse reflectance spectra, X-ray photoelectron spectroscopy, Raman spectroscopy and Fourier transform infrared spectroscopy were used to characterize the catalysts. Toluene adsorption investigations over catalysts reveal that the 6wt% MnOx/SBA-15 catalyst has better adsorption capacity. The structure-performance correlation suggests that the well-isolated manganese species with tetrahedral coordination and greater numbers of adsorption sites account for the efficient oxidation of toluene to benzoic acid, whereas the MnOx clusters are less active and depress further oxidation to benzaldehyde and benzoic acid formation. Additionally, reaction temperature, time, catalyst mass, pressure and catalyst reusability were investigated over 6wt% MnOx/SBA-15. A calculated turnover frequency of 532h−1 was obtained in 1h at 180°C. Rate-retarding effects of inhibitors and oxidation took place even without catalyst at 300°C, respectively, indicate a radical chain mechanism.