SnO2 Based Catalysts with Low-Temperature Performance for Oxidative Coupling of Methane: Insight into the Promotional Effects of Alkali-Metal Oxides

To develop reactive catalysts for oxidative coupling of methane at low temperatures, different alkali-metal oxides are adopted in this study to modify the surface of SnO2. In comparison with the unmodified SnO2, the reaction performance of all of the modified catalysts can be significantly improved, among which lithium oxide shows the best promotional effects, and the optimal catalyst is achieved with a Sn/Li molar ratio of 5:5. With this catalyst, the highest C-2 product yield of 16% is achieved at 750 degrees C. The XPS and CO2-TPD results reveal that for those catalysts with evident enhanced OCM reaction performance, the coexistence of a suitable amount of surface alkaline and electrophilic oxygen sites is indispensable. Furthermore, with the catalysts modified by different amounts of lithium oxide, the C-2 yield at different temperatures is nearly proportional to the amounts of both surface intermediate alkaline sites and electrophilic oxygen species. Therefore, it is concluded that the abundance and the concerted interaction of these two types of surface active sites are the major factors determining the reaction performance of the SnO2 based catalysts. Last, but not least, the optimized catalyst, which has a Sn/Li molar ratio of 5:5 and also contains equal amounts of SnO2 and Li2SnO3 crystalline phases, exhibits much better reaction performance than Mn/Na2WO4/SiO2, the most promising catalyst at present, in the low-temperature region (below 750 degrees C). After all, this may give people some new insight into developing new types of OCM catalysts that can operate at low temperatures and thereby facilitate the industrialization process of this important chemical engineering reaction.