Effect of reaction conditions on the oxidative coupling of methane over doped MnO_x-Na₂WO₄/SiO₂ catalyst

The MnO_x-Na₂WO₄/SiO₂ mixed oxide is a highly promising catalyst for the oxidative coupling of methane to ethylene (OCM). But it seems that its catalytic performance still does not meet industrial requirements. Several additives were reported to promote its activity and selectivity such as Ti, Ce, La, etc. However, contradicting results were obtained in the different studies and the additives were not compared under the same reaction conditions. In this study, we tested several reported additives (Ti, Nb, Ce, and Sn) in OCM, which previously showed promising results, as well as additives (Ge and Fe) that were not examined before. Using these doped catalysts, it is shown that the improved performance and the long-term stability of these catalysts are a product of the specific reaction conditions, i.e. the extent of oxygen conversion and time on stream. We show that overall these catalyst systems are unstable and undergo severe deactivation that is masked if tested under conditions were the O₂ is totally consumed. Finally, by characterizing spent catalysts, several possible deactivation mechanisms are briefly discussed.