Aerobic oxidation of primary amines into corresponding nitriles over MnxCe1-xOs catalysts prepared by co-impregnation method

Abstract

Mn-Ce mixed oxides, synthesized by co-impregnation method, have been used in the catalytic oxidation of primary amines to corresponding nitriles. The effects of Mn/Ce molar ratio on the structure and catalytic properties were investigated, and the results showed the MnxCe1-xOs mixed oxides exhibited higher catalytic activity than MnOx or CeO2. It was found that favorable selectivity and the best conversions for benzylic, heterocyclic, and aliphatic primary amines were obtained when the Mn/Ce atomic mole ratio was 2:1 (denoted as Mn0.67Ce0.33Os). These materials have been investigated using XRD, FT-IR, SEM, HRTEM, XPS, DR-UV–vis, H2-TPR, and EPR techniques. XRD results showed that the samples did not appear any diffraction of manganese oxides. SEM images showed that the nanoparticles of the mixed oxides can be uniformly distributed than MnOx or CeO2. HRTEM micrographs exhibited that Mn-Ce mixed oxides were all exposed (111) and (220) planes corresponding to CeO2, and Mn0.67Ce0.33Os catalyst had the highest mutual solubility, thus this catalyst could have the stronger interaction than other Mn-Ce mixed oxides, which was in accordance with XPS, H2-TPR, and EPR analysis. XPS studies confirmed that the highest concentration of Ce3+ was obtained at the surface of Mn0.67Ce0.33Os sample, this situation provides a basis for the formation and stabilization of oxygen vacancies, and it is beneficial for oxidation reaction, which is in consistent with DR-UV–vis results. In addition, XPS data also showed that Mn0.67Ce0.33Os sample provided the most adsorbed oxygen species, this suggested that the mobility and availability of the active oxygen species were enhanced. This conclusion also can be drawn from H2-TPR analysis. EPR studies further supported the formation of Mn0.67Ce0.33Os solid solution. Based on the above analysis, the excellent catalytic performance should be attributed to the formation of a solid solution with the incorporation of Mnx+ into the CeO2 cubic fluorite lattice, this reduces the formation energy of oxygen vacancies and enhances the mobility of active oxygen species from the bulk to the catalyst surface.