Fundamentals of Li₂CO₃-Induced Enhancement 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 (AlSiOx 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.