结构原子组成对Keggin型杂多酸铯盐上丙烷选择氧化性能的影响

Effects of Variation of Elemental Compositions of Cesium Salts of Keggin Type Heteropolyacids on Selectively Catalytic Oxidation Performance of Propane

系统地研究了具有不同结构原子组成的Keggin型杂多酸铯盐上丙烷选择氧化性能, 重点考察了H3+n-xCsxPMo12-nVnO40(n=0～4, x＝0～3)系列化合物以及H0.5Cs2.5PW12O40, H1.5Cs2.5PW11VO40和H1.5Cs2.5SiMo12O40样品. 应用BET, UV-Vis, FTIR, XRD, TPR和SEM等手段研究了催化剂的物化性质. V取代Mo增加了H3+nPMo12-nVnO40(n＝0～4)系列杂多酸的酸量和氧化性能. Cs+对质子的取代调变了样品的表面酸性以及氧化还原性, 同时催化剂的热稳定性也随着Cs+取代数的递增而增强, 当Cs+的取代数达到2以上时催化剂的热稳定性较好. Cs+取代对H3+n-xCsxPMo12-nVnO40(n=0～4, x＝0～3)催化剂上丙烷选择氧化性能有重要影响. 随着一级结构中V5+取代数目的增多, 可能的Cs+取代数目相应减少; 而对丙烯酸的选择性则存在一适宜的Cs+取代数目. Keggin型杂多阴离子的结构原子组成及平衡离子的种类和数量决定了其酸性、氧化还原性和热稳定性, 这是影响该类催化剂上丙烷选择氧化性能的首要因素. 研究结果表明, 以P为中心原子, 以Mo和V为配位原子的杂多酸铯盐[x(Cs+)=2.5]具有较好的催化性能.

The selective catalytic oxidation behavior of propane over the cesium salts of heteropoly acids with different elemental compositions were systematically studied. Special attentions were paid to the series of H3＋n-x Csx PMo12-nVnO40 catalysts（n =0—4, x =0—3） as well as the H0.5Cs2.5PW12O40, H1.5Cs2.5PW11VO40 and H1.5Cs2.5SiMo12O40catalysts. The catalysts were characterized by means of BET, UV-Vis diffuse reflectance spectroscopy, FTIR, XRD, TPR and SEM to understand their physico-chemical properties. The substitution of Mo^6＋ with V^5＋ increased the acidity and oxidation ability of the HPAs of H3＋nPMo12-nVnO40（n = 0—4） samples. Substitution of proton with Cs^＋ not only modified the surface acidity, but also affected the redox property as well as the structural stability of the samples, therefore, had a substantial impact on the catalytic performance for the series of H3＋n-x Csx PMo12-nVnO40（n =0—4, x =0—3 ） catalysts. It was found that the structural stability of HPCs increased steadily with increasing the number of cesium cation up to 3. The optimal number of cesium substitution varied along with the number of vanadium atoms existing in the primary structure of heteropoly anions. The appropriate elemental composition of heteropoly anions was found to be the key factor determining the overall catalytic behavior of the Keggin type heteropoly compounds, and the best resuits was achieved over the H1.5Cs2.5PMo11VO40 catalyst. The reaction data collected on various HPCs in this study suggest that the composition of structural atoms of heteropoly anions and the number of balanced cations are closely related to acidity, redox property and catalytic performance of HPCs, and these data also accord well with the theoretical predictions.