Elucidating the Role of Oxygen Species in Oxidative Coupling of Methane over Supported MnO_x−Na₂WO₄‐containing Catalysts

Abstract

Abstract The present study of oxidative coupling of methane (OCM) over MnO x −Na 2 WO 4 /support catalysts demonstrated that the selectivity to C 2 H 6 and C 2 H 4 (C 2 ‐hydrocarbons) is affected by the kind of support, co‐fed water, and the kind of oxidant (O 2 vs. N 2 O). In addition to previous studies with MnO x −Na 2 WO 4 /SiO 2 , an enhancing water effect was obtained using catalysts based on TiO 2 ‐ or ZrO 2 ‐containing supports. However, a negative effect on methane conversion was established for SiO 2 −Al 2 O 3 ‐supported catalysts. Temporal analysis of products with isotopic tracers suggests that the ability of MnO x −Na 2 WO 4 to generate diatomic adsorbed oxygen species depends on the kind of support and is the key property for the water effect. The strength of the water effect on the activity decreases with an increase in the surface area of working catalysts. The kind of support also affects products selectivity due to its influence on the mobility/releasability of lattice oxygen in supported MnO x −Na 2 WO 4 . Among the prepared catalysts, MnO x −Na 2 WO 4 /TiO 2 was found to be promising for H 2 O‐assisted OCM. The use of N 2 O instead of O 2 further increases the selectivity to C 2 ‐hydrocarbons to 84 % at 6.8 % CH 4 conversion due to the formation of predominantly monoatomic oxygen species from N 2 O that selectively convert CH 4 into C 2 H 6 .