Studies On Catalysts For Homogeneous Selective Oxidation And Heterogeneous Oxidative Carbonylation Of Methane

This dissertation focuses on the studies of homogeneous selective oxidation of methane and ethane catalyzed by OsCl₃ with H₂O₂ in aqueous medium and heterogeneous oxidative carbonylation of methane to methyl acetate over catalysts containing both rhodium and iron phosphate.For the homogeneous selective oxidation of methane or ethane, it is found that OsCl₃ can catalyze the formation of alcohols and aldehydes with H₂O₂ efficiently in aqueous medium. UV-Vis spectroscopy reveals that the active species is Os^â…£ during catalytic reactions. The fact that the oxidation reaction is totally inhibited by a radical scavenger (hydroquinone) suggests that the reaction may proceeded via a radical pathway.For the heterogeneous oxidative carbonylation of methane, it is shown that both rhodium and iron phosphate species play key roles in the conversion of methane with N₂O and CO to methyl acetate. The catalyst prepared by a mixed solution method shows significantly higher performances than that prepared by an impregnation method. Characterizations with XRD, Raman, TEM, DRUV-Vis, N₂-sorption, H₂-TPR, CO-TPR and XPS suggest that a large part of Rh³⁺ ions can be incorporated into the lattice of FePO₄ in the catalyst prepared by the mixed solution method, possibly forming a Rh_xFe_1-xPO₄ solid solution, whereas rhodium species are mainly located on the surface of FePO4 and exist as Rh^â…¢O_x in the catalyst prepared by the impregnation. The two series of catalysts exhibit different reduction behaviors. The rhodium species in catalysts prepared by the impregnation can be reduced more easily. It has been clarified that the metallic rhodium species mainly catalyze the reduction of N₂O by CO, while the ionic rhodium species are responsible for the carbonylation. The neighboring Fe³⁺ and Rh³⁺ dual sites are vital for the activation of methane and the subsequent carbonylation.The supporting of Rh^â…¢ species and FePO₄ onto MCM-41 and SBA-15 by a co-impregnation method can enhance the formation of methyl acetate by one order. The maximum rate and turnover frequency for methyl acetate formation are 696 μmolg^-1h^-1 and 65.2 h^-1 respectively. Characterizations suggest that both rhodium and FePO₄ species are encapsulated into mesoporous channels, possibly forming Rh_xFe_1-xPO₄ clusters which possess high concentration of Fe³⁺ and Rh³⁺ dual sites. On the other hand, no methyl acetate formation has been observed over the catalyst by a two-step impregnation method. In such catalyst, the iron phosphate species and rhodium species exist in the inner and the outer of mesoporous channel, respectively. CO-adsorption FT-IR spctroscopy reveals the formation of Rh^â…  (CO)₂ species. The dissertation proposes a reasonable mechanism for methyl acetae formation.