Sulfur as a selective ‘soft’ oxidant for catalytic methane conversion probed by experiment and theory

Abstract

Developing efficient catalytic processes to convert methane into useful feedstocks relies critically upon devising new coupling processes that use abundant, thermodynamically 'mild' oxidants together with selective catalysts. We report here on elemental sulfur as a promising 'soft' oxidant for selective methane conversion to ethylene over MoS(2), RuS(2), TiS(2), PdS and Pd/ZrO(2) catalysts. Experiments and density functional theory reveal that methane conversion is directly correlated with surface metal-sulfur bond strengths. Surfaces with weakly bound sulfur are more basic and activate methane C-H bonds more readily. In contrast, experimental and theoretical selectivities scale inversely with surface metal-sulfur bond strengths, and surfaces with the strongest metal-sulfur bonds afford the highest ethylene selectivities. High CH(4)/S ratios, short contact times and the provision of a support maximizes the coupling of CH(x) intermediates and selectivity to ethylene, because these conditions yield surfaces with stronger metal-sulfur bonding (for example, Pd(16)S(7)), which suppresses the over-oxidation of methane.