Influence of the composition/texture of Mn–Na–W catalysts on the oxidative coupling of methane

Abstract Mn–Na–W silica supported catalysts are highly selective toward ethylene and ethane in the oxidative coupling of methane (OCM) process. However, they present lower activities than La/Sr/CaO catalysts and subsequently a lower ethylene productivity. Optimisation of the Mn–Na–W catalysts has been focused within the range of 2–8 wt% of active elements relative to the support, while silica is partially converted into inert cristobalite. This study focused on the increase of the activity of Mn–Na–W catalysts while maintaining their intrinsically high C 2 selectivity. The increase in the concentration of the active elements with respect to the support revealed that the improvement of the catalyst activity was possible but limited. A high concentration of Mn–Na–W favours the formation of a smooth cristobalite phase and strongly decreased the number of active sites. The silica support was replaced by silica-free supports to limit the formation of cristobalite. The use of SiC as a support led to the formation of cristobalite near the surface of the catalysts but the porous SiC skeleton, which remained non-oxidised at the core of the catalysts, allowed maintaining a high surface area. SiC supported catalysts thus presented higher activities than silica catalysts. The use of α-alumina support led to catalysts with lower performances and revealed that exposed alumina was detrimental for the catalyst performance.