Acid–base properties of the active sites responsible for C2+ and CO2 formation over MO–Sm2O3 (M = Zn, Mg, Ca and Sr) mixed oxides in OCM reaction

The research investigates the quantitative relationship between the surface basicity of MO–Sm2O3 (M = Zn, Mg, Ca and Sr) mixed oxides and catalytic activity for C2+ formation. The amount as well as the evolution of surface carbonate species with nature of MO and reaction temperature were analyzed by XRD, TPD, XPS and IR methods to better understand the reaction mechanism. The concentration of surface basic sites responsible for the formation of C2+ were found to correlate well with the amount of evolved CO2 in the 300–800 °C temperature range. According to the experimental data, the higher was the catalyst basicity in terms of CO2 retaining capacity the better was the efficiency for selectively converting methane to C2+. The turnover frequency (TOF) values were calculated by measuring the total number basic sites retaining reversible CO2 in reaction conditions. The experimental results can be explained in a simplified manner by considering a reaction mechanism in which alkaline active sites play a crucial role in methane selective activation to C2+ whereas the acidic sites, showing little interaction CO2, are responsible for methane combustion. A strategy for improving the performances of OCM catalysts is discussed in light of the experimental results.

Graphical abstractThe behavior of catalytic oxide materials in OCM reaction can be interpreted in term of solid acid–base catalysis: C2+ is formed on the basic sites whereas the acidic sites are responsible for nonselective oxidation of methane to CO2.Figure optionsDownload full-size imageDownload high-quality image (192 K)Download as PowerPoint slideHighlights• CH4 conversion to C2+ correlates with the number of basic sites of oxide catalysts. • C2+ is formed only when surface carbonate decomposes to generate free basic sites. • The amount of available basic sites can calculated from the amount of desorbed CO2. • CH4 is unselectively oxidized to CO2 on the acid sites of oxide catalysts. • The activity of acid sites for CH4 combustion is hindered by basic molecules (NH3).