The structure of an A2B2O7 pyrochlore compound is open, which permits wide substitutions of the A and B site cations with appropriate radii to tune catalysts with desired active sites. Therefore, this type of composite oxides have attracted much attention in catalysis field over recent years. Because of their high thermal stability, good oxygen ion conductivity and intrinsic alkalinity, A2B2O7 compounds match most of the active site requirements for a good OCM catalyst. When studying OCM on a series of Ln2Ce2O7 compounds, we have testified that the concerted interaction between surface alkaline sites and selective electrophilic oxygen species is the major factor deciding the reaction performance of the catalysts. La2Ce2O7 displays the best activity and selectivity among all the catalysts due to the optimal interaction of the two kinds of surface sites, over which the highest C2 product yield of 16.6% can be achieved even at 750 oC. To gain deeper insight into the structure and reactivity relationship and achieve better catalysts having the potential for industrial applications, we have systematically investigated A2B2O7 compounds with varied A and B site combinations. It has been disclosed that the phase composition of the compounds can be altered with changed rA/rB radius ratio. In detail, if the ratio is above 1.78, a monoclinic layered perovskite phase will be formed; and if the ratio is in the range of 1.46 - 1.78, a stable and ordered cubic pyrochlore crystalline phase will be formed. However, by the decreasing the ratio below 1.46, the ordered cubic pyrochlore phase will eventually become a defective cubic fluorite phase. In the bulk of the latter two, inherent 8a structure oxygen vacancies are present, thus improving the oxygen mobility significantly. In addition, the surface alkalinity can also be adjusted, which is influenced more by the A site than by the B site cations. By combining A and B sites suitably, optimizing the synthesis methods and adding certain additives, A2B2O7 compound catalysts possessing superior low temperature reaction performance for OCM can be obtained. The structure and reactivity relationship of this type of catalysts have been elucidate in this study.

Developing A2B2O7 composite oxide catalysts for low temperature oxidative coupling of methane (OCM): on the relationship between the structure and reactivity

2019 Spring National Meeting

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