Structure-activity relationships of nanoscale MnOx/CeO2 heterostructured catalysts for selective oxidation of amines under eco-friendly conditions

Abstract

The structure-activity properties of MnOx/CeO2 nanorods and MnOx/CeO2 nanoparticles, with the importance of CeO2 morphology have been investigated for the solvent-free oxidation of amines using oxygen as a green oxidant. The physicochemical properties of the samples have been investigated using HRTEM, XRD, Raman, BET, XPS, and FT-IR techniques. HRTEM studies reveal that CeO2 nanorods preferentially expose {110} and {100} crystal planes, while CeO2 nanoparticles expose {111} planes. The addition of manganese to CeO2 supports leads to an enhancement in the concentration of Ce3+ ions and oxygen vacancies, which are more pronounced for the MnOx/CeO2 nanorods as evidenced by Raman and XPS studies. Another striking observation noticed from XPS studies is that MnOx/CeO2 nanorods catalyst exhibits Mn4+, Mn3+, and Mn2+ species, whereas only Mn4+ and Mn3+ are presented in MnOx/CeO2 nanoparticles catalyst. It was found that MnOx/CeO2 nanorods catalyst exhibit a two-fold higher activity for the oxidation of benzylamine with superior selectivity to dibenzylimine (∼99%) compared with that of MnOx/CeO2 nanoparticles catalyst. The MnOx/CeO2 nanorods catalyst was also found to be effective for the oxidation of various amines, and moderate to good product yields were obtained. Novel probable reaction pathways are proposed for solvent-free oxidation of primary and secondary benzylamines over MnOx/CeO2 nanorods catalyst. The presence of surface-active Mn4+/Mn2+ couple and the enhanced defect structure of CeO2 nanorods (i.e., higher numbers of Ce3+ ions and abundant O vacancies) are found to be key factors for the high catalytic efficiency of the MnOx/CeO2 nanorods.