Performance-defining factors of (MnOx)-M2WO4/SiO2 (M = Na, K, Rb, or Cs) catalysts in oxidative coupling of methane

This work unveils the fundamentals relevant for activity and product selectivity in the oxidative coupling of methane (OCM) over the (MnOx)-M2WO4/SiO2 (M = Na, K, Rb, or Cs) catalysts. The presence of the molten phase of Na2WO4 and the ability of the catalysts to release lattice oxygen were not found to be the performance-governing factors. As proven by oxygen isotopic exchange experiments combined with thorough catalytic tests, the performance of both tri- and bimetallic catalysts is defined by the involvement of adsorbed oxygen species formed from gas-phase O2. Lattice oxygen of M2WO4/SiO2 is not capable of converting CH4 to C2H6, while that of MnOx-M2WO4/SiO2 is active for this reaction. The OCM activity of M2WO4/SiO2 decreases with the atomic weight of alkali metal, whereas there is no such effect in the presence of MnOx, which, however, increases the activity. The enhancing effect was explained by improving the active site turnover.