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Direct conversion of methane into ethylene through the oxidative coupling of methane (OCM) is a technically important reaction. However, conventional co-fed fixed-bed OCM reactors still face serious challenges in conversion and selectivity. In this paper, we apply a finite element model to simulate OCM reaction in a plug-flow CO2/O2transport membrane (CTM) reactor with a directly captured CO2and O2mixture as a soft oxidizer. The CTM is made of three phases: molten carbonate, 20% Sm-doped CeO2, and LiNiO2. The membrane parameters are first validated by CO2/O2flux data obtained from CTM experiments. The OCM reaction is then simulated along the length of tubular plug-flow reactors filled with a La2O3-CaO-modified CeO2catalyst bed, while a mixture of CO2/O2is gradually added through the wall of the tubular membrane. A 12-step OCM kinetic mechanism is considered in the model for the catalyst bed and validated by data obtained from a co-fed fixed-bed reactor. The modeled results indicate a much-improved OCM performance by membrane reactor in terms of C2-yield and CH4conversion rate over the state-of-the-art, co-fed, fixed-bed reactor. The model further reveals that improved performance is fundamentally rooted in the gradual methane conversion with CO2/O2offered by the plug-flow membrane reactor.
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“Soft” oxidative coupling of methane to ethylene: Mechanistic insights from combined experiment and theory
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.
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- Award ID(s):
- 1647722
- PAR ID:
- 10233290
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 118
- Issue:
- 23
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- Article No. e2012666118
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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