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 C-2(+) 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 C-2(+) were found to correlate well with the amount of evolved CO2 in the 300-800 degrees 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 C-2(+). 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 C-2(+) 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. (C) 2011 Elsevier B.V. All rights reserved.