Effects of calcination atmosphere on the structure and performance of MoO₃-SnO₂ catalyst for the oxidation of dimethyl ether at low temperature

MoO₃-SnO₂ catalysts with a Mo/Sn molar ratio of 1:3 was prepared by the co-precipitation method and calcined in different atmospheres (O₂, air, N₂ and H₂); the effect of calcination atmosphere on the catalytic performance of MoO₃-SnO₂ in the oxidation of dimethyl ether (DME) to methyl formate (MF) was investigated. The results show that the MoO₃-SnO₂ catalyst prepared by calcination in O₂ exhibits the highest activity; the conversion of DME reaches 25.10%, with the selectivity of 72.21% to MF. Over the catalyst calcined in H₂, the conversion of DME is only 7.01%, with the selectivity of 75.82% to MF. The activity of the MoO₃-SnO₂ catalysts calcined at different atmospheres follows the order of O₂ > air > N₂ > H₂. The results of XRD, Raman, XPS and ESR characterization indicate the presence of MoO_x domains on the surface of the MoO₃-SnO₂ catalyst with a Mo/Sn molar ratio of 1:3. The terminal Mo=O groups of oligomeric MoO₃ may be the active sites for the methoxy intermediate and the penta-coordinated Mo⁵⁺ species in the Mo-Sn interface may be able to promote the oxidation of DME to MF. Consequently, methoxy species are absorbed on the Mo⁵⁺ species in the Mo-Sn interfaces, which are oxidized to HCHO on the terminal Mo=O groups; after that, the absorbed HCHO may then react with the neighboring absorbed methoxy species, forming MF.