Selective Oxidation of Light Alkanes Using Sulfur as a “Soft” Oxidant

LIU, SHANFU

doi:https://doi.org/10.21985/n2-srgm-0q87

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Selective Oxidation of Light Alkanes Using Sulfur as a “Soft” Oxidant

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The discovery of abundant reserves of shale gas over this past decade has reshaped the world’s energy landscape. It has renewed interests in the activation and conversion of methane as well as other light alkanes. While the oxidative coupling of methane (OCM) and oxidative dehydrogenation (ODH) of ethane and propane are examples of the most promising catalytic systems to convert natural gas to olefins efficiently, many OCM and ODH systems are limited in terms of yield as oxygen is a strong oxidant that over-oxidizes the products to CO2. The oxidative coupling of methane using gaseous sulfur as the oxidant (SOCM) proceeds with promising ethylene selectivity. Here we report detailed experimental and theoretical studies that examine the SOCM mechanism for CH4 to C2H4 conversion over an Fe3O4-derived catalyst. The experimental reaction rate for methane conversion is first-order in both CH4 and S2, with a CH4/CD4 kinetic isotope effect of 1.78±0.18. Kinetic analyses, along with density functional theory analysis, show that ethylene is produced as a primary product of methane activation that plausibly proceeds via coupling of CH2 intermediates over Fe-S sites on the sulfided Fe3O4 surface. In contrast, the CS2 by-product forms predominantly via CH4 over-oxidation, rather than from C2 products, via a series of C-H activation and S-addition steps at adsorbed sulfur sites on the FeS surface. We also investigated the concept of sulfur vapor as a less exothermic “soft” oxidant for the conversion of ethane to ethylene. We reported a maximum single-pass ethylene yield of 76% over Fe-based catalyst at 940 °C (90.2% C2H4 selectivity at 820°C). The C2H4 selectivity and yield are on par with some of the most advanced ODH system and surpassing the industrial steam cracking. The reaction rate law is measured to be 1st order in ethane and ½ order in S2 on Fe catalyst at a temperature lower than 700°C. An MVK mechanism was proposed and is in agreement with the experimental observation. Mechanistic insights provided in this report could facilitate future catalyst and process design. Indeed, several modifications to current catalysts and reactors are proposed. Proof-of-concept results obtained here exhibit encouraging potentials.

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