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The oxidative coupling of methane to ethylene using gaseous disulfur (2CH 4 + S 2 → C 2 H 4 + 2H 2 S) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH 4 to C 2 H 4 over an Fe 3 O 4 -derived FeS 2 catalyst 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 O 2 (2CH 4 + O 2 → C 2 H 4 + 2H 2 O). 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 CH 2 coupling 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 CS 2 by-product forms predominantly via CH 4 oxidation, rather than from C 2 products, through a series of C–H activation and S-addition steps at adsorbed sulfur sites on the FeS 2 surface. The experimental rates for methane conversion are first order in both CH 4 and S 2 , consistent with the involvement of two S sites in the rate-determining methane C–H activation step, with a CD 4 /CH 4 kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH 4 oxidative coupling with O 2 . 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.
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Mechanistic Insights on Coverage‐Dependent Selectivity Limitations in Vinyl Acetate Synthesis
Abstract Developing improved catalysts for sustainable chemical processes often involves understanding atomistic origins of catalytic activity, selectivity, and stability. Using density functional theory and steady‐state kinetic analyses, we probe the elementary steps that form decomposition products that limit selectivity in vinyl acetate (VA) synthesis on Pd surfaces covered with acetate species. Acetate formation and coupling with ethylene control the VA formation catalytic cycle and steady‐state coverage, but acetate and ethylene can separately decompose to form CO2. Both decompositions involve initial C−H activations at acetate vacancies, followed by additional C−H activations and eventual C−O formations and C−C cleavages involving reactions with molecular oxygen. Acetate decomposition paths with non‐oxidative kinetically‐relevant steps exhibit similar free energy barriers to oxidative paths. In contrast, the non‐oxidative ethylene path involving an ethylidyne intermediate exhibits a much lower barrier than paths with oxidative kinetically‐relevant steps. Ethylene decomposition is very facile at low coverages but is more coverage‐sensitive, leading to similar decomposition and VA formation barriers at coverages accessible at steady state, which is consistent with moderate VA selectivity in measurements and ethylene vs. acetate decomposition contributions assessed from regressed kinetic parameters. These insights provide a detailed framework for describing VA synthesis rates and selectivity on metallic catalyst surfaces.
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- Award ID(s):
- 2045675
- PAR ID:
- 10611664
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- ChemSusChem
- Volume:
- 18
- Issue:
- 6
- ISSN:
- 1864-5631
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/cssc.202401911
