Fundamentals of Li2CO3-Induced Enhancement of C2H4/C2H6 Selectivity in Oxidative Coupling of Methane over Mn-Na2WO4-Based Catalysts

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High Resolution Image Download MS PowerPoint Slide Oxidative coupling of methane (OCM) to C 2 H 6 and C 2 H 4 (C 2 -hydrocarbons) is an industrially attractive method for methane valorization. However, its commercialization is hampered by the low selectivity to C 2 -hydrocarbons, especially to C 2 H 4 . In this study, we demonstrate the use of a physical mixture of Li 2 CO 3 or LiNO 3 with differently supported catalysts based on the Mn-Na 2 WO 4 system for the efficient continuous formation of the target products using O 2 as an oxidant. The selectivity to C 2 -hydrocarbons and C 2 H 4 of 90.5 and 35.7% or 87.6 and 43.3%, respectively, was obtained at 7.2 or 13.3% CH 4 conversion over the Li 2 CO 3 -Mn-Na 2 WO 4 /Siral70 (AlSiO x with 70 wt % SiO 2 ) catalyst, while its Li 2 CO 3 -free counterpart was unselective. Very similar performance is achieved by promoting the support with Li 2 CO 3 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 C 2 -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 16 O 2 or 18 O 2 and temporal analysis of products. Li 2 CO 3 -induced phase modifications affect both the concentration and lifetime of surface intermediates leading to CO 2 and CO. The knowledge gained provides guidance for the development of OCM catalysts with improved C 2 -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.