(743d) Sulfur As a Selective Soft Oxidant in the Catalytic Conversion of Methane to Ethylene over Metal Chalcogenides

Oxidative coupling of methane (OCM) in presence of an oxidant over metal oxide catalysts provides an attractive route for direct conversion of methane to ethylene.¹ The tremendous wealth of previous studies using oxygen as the oxidant have provided important insights into the mechanisms for OCM.² Oxygen, however, is a very strong oxidant and ultimately limits the selectivity in OCM. New coupling processes that use abundant, thermodynamically mild oxidants together with selective catalysts that reduce the thermodynamic driving force for over-oxidation and result in higher ethylene yields would offer exceptional opportunities. We thus report herein on the use of elemental sulfur as a promising soft oxidant for selective conversion of methane to ethylene. We explored the oxidative coupling of methane via sulfur (SOCM) over a range of different metal sulfide catalysts including MoS₂, RuS₂, TiS₂, PdS and Pd/ZrO₂. Experimental results and density functional theory (DFT) calculations reveal that methane conversion is directly correlated with surface metal–sulfur bond strengths. Surfaces with weakly bound sulfur are more basic and activate the C–H bonds of methane more readily. In contrast, ethylene selectivities scale inversely with surface metal–sulfur bond strengths, and surfaces with the strongest metal–sulfur bonds afford the highest ethylene selectivities. The PdS system was found to be unique in that it showed the highest rates and selectivities. More detailed characterization of the catalyst showed that the catalyst was partially reduced under reaction conditions to form Pd₁₆S_7. Theoretical results discussed herein show that this surface exposes both Pd as well as S sites which can more readily activate methane but also promote the C-C bond formation and the production of ethylene. We extend these results to the reactivity of oxides together with sulfur and report C₂ selectivities > 33% for Fe₃O₄. XRD and XPS analyses of Fe₃O₄ indicate that it readily converts to FeS₂. The primary products formed over these FeS₂ catalysts are ethylene, ethane and CS₂. DFT calculations show that ethylene predominantly forms in path similar to that for PdS via CH₂ coupling. The exposed Fe-S site pairs that form on the sulfided Fe₃O₄ catalyst aid in the activation of the C-H bond of methane as well as the production of ethylene. CS₂formation, on the other hand, was found likely to occur over the adsorbed sulfur sites via facile C-H bond activation steps. Ethane formation predominantly occurs via the formation of methyl fragments, their desorption from the sulfide and their recombination in the gas phase.

References:

1) Zavyalova, U., Holena, M., Schlögl, R. & Baerns, M. Statistical analysis of past catalytic
data on oxidative methane coupling for new insights into the composition of high performance
catalysts. ChemCatChem 3, 1935–1947 (2011).

2) Lunsford, J. H. The catalytic oxidative coupling of methane. Angew. Chem. 35, 970-
980 (1995).