Experimental and Theoretical Mechanistic Insights into the Oxidative Coupling of Methane with Soft Oxidants

ABSTRACT BODY: Abstract: The catalytic conversion of methane to high-value molecular building blocks should in principle be possible using abundant soft oxidants that exhibit low thermodynamic driving forces for over-oxidation, thereby increasing the selectivity to valuable C2 products such as ethylene. Previous studies of methane oxidative coupling using gaseous S2 (SOCM) demonstrated ethylene selectivities up to 33% and C2H4/C2H6 ratios of 9-12 over an Fe3O4–derived catalyst at 950 °C. Here we report detailed experimental and theoretical mechanistic studies that examine the SOCM process for CH4 to C2H4 over the Fe3O4-derived catalyst. Kinetic analyses show that ethylene is produced as a primary product in methane activation and likely proceeds via the coupling of CH2 intermediates over Fe-S sites on the sulfided Fe3O4 surface. Hydrocarbon reactivity with S2 increases in the order CH4 < C2H4 < C2H2 < C2H6, while CS2 is found to be unreactive. The reaction rate is found to be first-order in both CH4 and S2, with a CH4/CD4 kinetic isotope effect of 1.86 ± 0.17. The measured (66±8 kJ/mol) and calculated (44 kJ/mol via dimeric S2 sites, 94 kJ/mol for monomeric S sites) apparent activation barriers for methane conversion are significantly lower than the previously reported activation energies for nonoxidative CH4 coupling or for the oxidative coupling of CH4 with O2. The present lower barriers are attributed to the strong binding between both methyl and hydrogen in the transition state and the active sulfur on the catalytic surface. The undesirable CS2 product appears to form predominantly from CH4 over-oxidation via a series of C-H activation and S-addition steps over adsorbed sulfur sites, whereas over-oxidation of the C2 products to CS2 is of minor significance.