The catalytic conversion of methane to high-value molecular building blocks should in principle be possible using abundant “soft” oxidants that exhibit less thermodynamic driving forces for over-oxidation than oxygen, thereby potentially biasing the selectivity toward valuable C2 products such as ethylene. Previous studies of methane oxidative coupling using gaseous S2 (SOCM) demonstrated ethylene selectivities as high as 35% and C2H4/C2H6 ratios of 9-12 over an earth-abundant Fe3O4–derived catalyst. Here we report detailed experimental and theoretical mechanistic studies that examine the SOCM process for CH4 to C2H4 over the Fe3O4-derived catalyst. Kinetic analyses show that ethylene is produced as a primary product in methane activation and likely proceeds via the coupling of CH2 intermediates over Fe-S sites on the sulfided Fe3O4 catalyst (a bulk mixture of FeS and Fe11S12). Hydrocarbon reactivity with S2 increases in the order CH4 &lt; C2H4 &lt; C2H2 &lt; C2H6, while CS2 is found to be unreactive. The reaction rate is first-order in both CH4 and S2 (unlike oxidation with O2), with a CH4/CD4 kinetic isotope effect of 1.86 ± 0.17. The measured (66±8 kJ/mol) and calculated (44 kJ/mol via dimeric S2 sites; 94 kJ/mol for monomeric S sites) apparent activation energies for methane conversion are significantly lower than those previously reported for non-oxidative CH4 coupling or for oxidative coupling of CH4 with O2. The present lower barriers are attributed to the strong binding between both methyl and hydrogen in the transition state and the active sulfur on the catalytic surface. The undesirable CS2 product appears to form at different sites than those for oxidative coupling, predominantly from CH4 over-oxidation via a series of C-H activation and S-addition steps, whereas over-oxidation of the C2 products to CS2 is of minor significance.

Oxidative coupling of methane with soft oxidants: Mechanistic insights from combined experiment and theory

Macromolecular Chemistry: The Second Century

[Author Benefits](https://scimeetings.acs.org/?utm_source=pubs_content_marketing&utm_medium=website&utm_campaign=0320_MCF_NPI_Launch_Spring_Homepage&ref=pubs_content_marketing)

[How to use SciMeetings](https://storage.googleapis.com/dl.morressier.com/acs/SciMeetings-ACSSpring2020-HowToGuide_M3020-Final.pdf)

[Frequently Asked Questions](https://storage.googleapis.com/dl.morressier.com/acs/SciMtng_ACSNMSp2020_FAQ_M3120_Final.pdf)

ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.