Oxidative coupling of methane (OCM) offers an attractive route to directly convert methane to ethylene, a vital building block in the chemical industry with an expected increase in demand. Mn-Na-W/SiO2 is one of the few promising catalysts for OCM, currently showing 20-40% CH4 conversion with over 60-80% C2 product selectivity for more than 500 h time on stream (TOS). The work here presents a thorough evaluation of the effect of Mn-Na-W/SiO2 catalyst surface parameters on its performance in the oxidative coupling of methane (OCM). To do so, we used microporous dealuminated β-zeolite (Zeo), or mesoporous SBA-15 (SBA) or macroporous fumed silica as precursors for catalyst preparation together with Mn nitrate or Mn acetate and Na2WO4. Catalyst characterization and testing enables us to identify critical surface parameters that govern the activity and C2 selectivity of the Mn-Na-W/SiO2 catalyst. While the current paradigm views the phase-transition of silica to α-cristobalite as the critical step in obtaining dispersed and stable metal sites, we show that the choice of precursors is equally or even more important with respect to tailoring the right catalyst surface properties. Specifically, the Zeo based catalyst is shown to have superior properties which are translated up to four-fold higher activity and enhanced selectivity. We show that a decrease in the size of the Na2WO4 species linearly increases the catalytic activity. This linear dependence correlates well with the number of interfaces between Na2WO4 and Mn2O3 species, which together with other results leads us to single out the interface between Na2WO4 and Mn2O3 as the most probable active site for OCM using this catalyst.

Critical surface parameters for the oxidative coupling of methane over Mn-Na-W/SiO2 catalyst

255th National Meeting (2018)

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