Characterizations of unsupported and supported rhodium-iron phosphate catalysts effective for oxidative carbonylation of methane

Abstract

Unsupported and MCM-41-supported Rh-doped FePO4 catalysts, which are effective for the oxidative carbonylation of methane directly to methyl acetate in the presence of CO and N2O, have been characterized by CO TPR in combination with XPS and FTIR spectroscopy of CO chemisorption. The CO TPR studies provide further evidence that the location of the Rh species is different in catalysts prepared by different methods. The CO TPR results and the correlation between the dispersion of the Rh species and the catalytic performance support the speculation that the dual sites containing Rh-III and Fe-III connected by phosphate groups are responsible for the conversion of methane to methyl acetate. The CO TPR studies also provide insight into the probable oxidation states of Rh and Fe during the catalytic reaction. We found that the Rh-III and Fe-III sites undergo simultaneous reductions during the reaction on the Rh-FePO4 sample prepared from a mixed aqueous solution and the MCM-41-supported Rh-FePO4 sample prepared by a co-impregnation method, which can catalyze the conversion of methane to methyl acetate efficiently. On the other hand, the sample over which only Rh-III species alone is reduced to Rh-0 can catalyze only the undesirable consumption of N2O via the reduction by CO. FTIR studies of CO chemisorbed on the MCM-41-supported Rh-FePO4 sample prepared by the co-impregnation method showed the formation of a Rh-I geminal dicarbonyl species, i.e., Rh-I(CO)(2), at proper temperatures similar to those for the catalytic formation of methyl acetate. A reaction mechanism via the Rh-I(CO)(2) intermediate with the Fe-II functioning for the activation of N2O to generate an active oxygen species for methane conversion is proposed.