焙烧气氛对二甲醚低温选择氧化MoO3-SnO2催化剂结构及性能的影响

采用共沉淀法制备了Mo/Sn物质的量比为1：3的MoO₃-SnO₂催化剂，考察了焙烧气氛（O₂、air、N₂和H₂）对催化剂结构及二甲醚（DME）低温氧化制甲酸甲酯（MF）性能的影响。结果表明，在O₂中焙烧的催化剂上DME转化率高达25.10%，MF选择性为72.21%，催化剂具有较高的反应活性。而在H₂中焙烧催化剂上DME转化率仅为7.01%，MF选择性为75.82%。不同气氛焙烧催化剂上DME转化率由大到小的顺序：O₂> air> N₂> H₂。采用XRD、Raman、XPS及ESR等对催化剂进行深入表征。结果表明，共沉淀制备Mo1Sn3催化剂中钼物种以高分散MoO_x形式存在。O₂中焙烧催化剂表面Mo=O及存在于Mo-Sn界面处五配位钼氧化物中Mo⁵⁺含量均高于其他三种催化剂，即低聚态MoO_<em>x末端Mo=O可能是反应活性位点之一，五配位钼氧化物中Mo⁵⁺的存在有利于催化剂活性的提高，也有助于MF的生成。结合in suit DRIFTS证实了吸附于Mo⁵⁺上的CH₃O，在催化剂表面Mo=O作用下氧化为HCHO后与另一分子CH₃O耦合为MF。

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.