Fundamentals ofLi₂CO₃‑InducedEnhancement of C₂H₄/C₂H₆ Selectivity in Oxidative Coupling of Methane over Mn-Na₂WO₄‑Based Catalysts

Oxidative coupling of methane (OCM) to C₂H₆ and C₂H₄ (C₂-hydrocarbons) is an industrially attractive method for methane valorization. However, its commercialization is hampered by the low selectivity to C₂-hydrocarbons, especially to C₂H₄. In this study, we demonstrate the use of a physical mixture of Li₂CO₃ or LiNO₃ with differently supported catalysts based on the Mn-Na₂WO₄ system for the efficient continuous formation of the target products using O₂ as an oxidant. The selectivity to C₂-hydrocarbons and C₂H₄ of 90.5 and 35.7% or 87.6 and 43.3%, respectively, was obtained at 7.2 or 13.3% CH₄ conversion over the Li₂CO₃-Mn-Na₂WO₄/Siral70 (AlSiO_<i>x</i> with 70 wt % SiO₂) catalyst, while its Li₂CO₃-free counterpart was unselective. Very similar performance is achieved by promoting the support with Li₂CO₃ followed by deposition of the active components. In situ temperature-resolved and ex situ X-ray diffraction characterization studies showed that the promoter reacted completely with the active components and the support to yield highly crystalline materials with a low specific surface area. Such restructuring resulted in a strong improvement in the selectivity to C₂-hydrocarbons due to inhibiting the direct heterogeneous oxidation of methane to carbon oxides. The selectivity changes were rationalized by steady-state isotopic transient kinetic analysis using OCM feeds with ¹⁶O₂ or ¹⁸O₂ and temporal analysis of products. Li₂CO₃-induced phase modifications affect both the concentration and lifetime of surface intermediates leading to CO₂ and CO. The knowledge gained provides guidance for the development of OCM catalysts with improved C₂-selectivity. Moreover, the results obtained demonstrate the potential of our approach to elucidate the selectivity-governing factors relevant for tailored catalyst design and may stimulate future investigations of the development of selective catalysts in various alkane oxidation reactions.