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The oxidative coupling of methane to ethylene using gaseous disulfur (2CH₄+ S₂→ C₂H₄+ 2H₂S) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH₄to C₂H₄over an Fe₃O₄-derived FeS₂catalyst achieving a promising ethylene selectivity of 33%. We compare and contrast these results with those for the highly exothermic oxidative coupling of methane (OCM) using O₂(2CH₄+ O₂→ C₂H₄+ 2H₂O). SOCM kinetic/mechanistic analysis, along with density functional theory results, indicate that ethylene is produced as a primary product of methane activation, proceeding predominantly via CH₂coupling over dimeric S–S moieties that bridge Fe surface sites, and to a lesser degree, on heavily sulfided mononuclear sites. In contrast to and unlike OCM, the overoxidized CS₂by-product forms predominantly via CH₄oxidation, rather than from C₂products, through a series of C–H activation and S-addition steps at adsorbed sulfur sites on the FeS₂surface. The experimental rates for methane conversion are first order in both CH₄and S₂, consistent with the involvement of two S sites in the rate-determining methane C–H activation step, with a CD₄/CH₄kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH₄oxidative coupling with O₂. The computed methane activation barrier, rate orders, and kinetic isotope values are consistent with experiment. All evidence indicates that SOCM proceeds via a very different pathway than that of OCM.

 

Co-feeding water leads to a simultaneous attenuation of chain initiation and chain termination rates in HSSZ-13 catalyzed methanol-to-olefins (MTO) conversion. Density functional theory calculations and transient stoichiometric experiments support the plausibility of formaldehyde hydrolysis occurring over zeolitic Brønsted acid sites at MTO-relevant temperatures. A monotonic decrease in MTO chain initiation and termination rates, and a concurrent monotonic increase in total turnovers as a function of water co-feed partial pressure are consistent with the occurrence and mechanistic relevance of formaldehyde hydrolysis effected by co-fed water. Initiation/termination rates and total turnovers normalized by their corresponding values in the absence of water co-feeds at the same temperature show the expected trends as a function of reaction temperature, assuming equilibrium between formaldehyde and methanediol. These results underscore the implications of formaldehyde hydrolysis chemistry when assessing the mechanistic role of water in methanol-to-olefins conversion specifically, and deactivation mechanisms in zeolite-catalyzed hydrocarbon conversion processes more generally.