“Soft” oxidative coupling of methane to ethylene: Mechanistic insights from combined experiment and theory

Shanfu Liu, Sagar Udyavara, Chi Zhang, Matthias Peter, Tracy L. Lohr, Vinayak P. Dravid, Matthew Neurock, Tobin J. Marks

Research output: Contribution to journalArticlepeer-review

Abstract

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 CH4 to C2H4 over an Fe3O4-derived FeS2 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 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 CH2 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 CS2 by-product forms predominantly via CH4 oxidation, rather than from C2 products, through a series of C-H activation and S-addition steps at adsorbed sulfur sites on the FeS2 surface. The experimental rates for methane conversion are first order in both CH4 and S2, consistent with the involvement of two S sites in the rate-determining methane C-H activation step, with a CD4/CH4 kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH4 oxidative 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.

Original languageEnglish (US)
Article numbere2012666118
JournalProceedings of the National Academy of Sciences of the United States of America
Volume118
Issue number23
DOIs
StatePublished - Jun 8 2021

Bibliographical note

Funding Information:
We thank the Center for Innovative and Strategic Transformation of Alkane Resources (NSF Award EEC-1647722) for funding. We also thank the Minnesota Supercomputing Institute at the University of Minnesota and the Pacific Northwest National Laboratory for computing resources. We thank Profs. F. H. Ribeiro, P. C. Stair, J. M. Notestein, and H. H. Kung and Dr. N. Schweitzer for helpful discussions. The work made use of the Electron Probe Instrumentation Center facility of Northwestern University Atomic and Nanoscale Characterization Experimental Center (NUANCE), which received support from the Materials Research Science and Engineering Center program (NSF Grant DMR-1720139) and the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF Grant ECCS-1542205) at the Northwestern University Materials Research Center. We thank Dr. D. Wang and Prof. E. Weitz for assisting with the Raman spectra, and M.P. thanks the German Research Foundation (DFG) for a postdoctoral fellowship.

Funding Information:
ACKNOWLEDGMENTS. We thank the Center for Innovative and Strategic Transformation of Alkane Resources (NSF Award EEC-1647722) for funding. We also thank the Minnesota Supercomputing Institute at the University of Minnesota and the Pacific Northwest National Laboratory for computing resources. We thank Profs. F. H. Ribeiro, P. C. Stair, J. M. Notestein, and H. H. Kung and Dr. N. Schweitzer for helpful discussions. The work made use of the Electron Probe Instrumentation Center facility of Northwestern University Atomic and Nanoscale Characterization Experimental Center (NUANCE), which received support from the Materials Research Science and Engineering Center program (NSF Grant DMR-1720139) and the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF Grant ECCS-1542205) at the Northwestern University Materials Research Center. We thank Dr. D. Wang and Prof. E. Weitz for assisting with the Raman spectra, and M.P. thanks the German Research Foundation (DFG) for a postdoctoral fellowship.

Publisher Copyright:
© 2021 National Academy of Sciences. All rights reserved.

Keywords

  • Catalysis
  • Kinetics and density functional theory (DFT)
  • Reaction mechanism
  • Sulfur oxidative coupling of methane (SOCM)

PubMed: MeSH publication types

  • Journal Article

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