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

摘要

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 MoOx 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.%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MoOx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MoOx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.