Oxide-supported carbonates reveal a unique descriptor for catalytic performance in the oxidative coupling of methane (OCM)

The oxidative coupling of methane (OCM) is a promising reaction for direct conversion of methane to higher hydrocarbons. The reaction can be performed over oxide-based catalysts with very diverse elemental composition. Yet, despite decades of research, no general common structure-activity relationship has been deduced. Our recent statistical meta-analysis across a wide range of catalyst compositions reported in the literature suggested that only the catalysts combining thermodynamically stable (under reaction conditions) carbonate and thermally stable oxide support exhibit good catalytic performance. Guided by these findings we explore now experimentally correlations between descriptors for structure, stability and decomposition behavior of supported metal carbonates vs. the materials’ respective performance in OCM catalysis. In this study, carbonates of Rb, Cs and Mg were supported on oxides of Sm, Y, Gd, Ce, Sr and Ba, tested in OCM and studied by IR spectroscopy and thermal analysis. From the evaluation of six proposed property-descriptors we derive a statistically robust volcano-type correlation between the onset temperature of carbonate decomposition and the C2 yield, indicating the importance of CO2 adsorption and surface carbonates in selective methane conversion. Moreover, we discuss mechanisms that can account for the observed property-performance correlation across a wide range of OCM catalysts. Carbonate species are suggested to block highly reactive sites during OCM catalysis, which reduces overoxidation and enables the formation of C2 products.