Effect of manganese oxidative species in as-synthesized K-OMS 2 on the oxidation of benzene

Abstract

The synthesis of manganese-based octahedral molecular sieves of the type K-OMS 2 was designed by central composite design (CCD) method. The effect of aging time and temperature was considered for K-OMS 2 synthesis. Consequently, the synthesis was carried out through hydrothermal process without calcination and the obtained K-OMS 2 was tested for benzene oxidation. The physicochemical properties of as-synthesized K-OMS 2 catalysts were characterized by XRD, BET and SEM. The oxidative species of manganese were identified by analysis of the Mn K-edge XANES spectra. CCD results revealed that the crystallization conditions for K-OMS 2 synthesis were dependent on aging temperature and time. The specific surface area of K-OMS 2 was inversely proportional to the aging temperature. The K-OMS 2 sample with aging time of 21h at aging temperature of 75°C showed the best catalyst for benzene oxidation. This catalyst was a mixture of Mn2O3 and MnO2 with Mn3+/Mn4+ ratio of 0.09 as evaluate by XANES technique. It was discovered that the K-edge energy of Mn in each as-synthesized K-OMS 2 samples was 6551.4eV close to Mn3+ and Mn4+ species. The Mn3+/Mn4+ ratios were found to have a range of 0.09–0.16. However, a decrease in Mn3+/Mn4+ ratio and a corresponding decrease in catalytic activity were encountered at high aging time and temperature. Low aging temperature and long aging time could provide active species of manganese oxides for benzene oxidation.