Oxidative coupling of methane on flame-made Mn-Na2WO4/SiO2: Influence of catalyst composition and reaction conditions

• Mn-Na2WO4/SiO2 catalysts of various composition were prepared using flame spray pyrolysis.
• Catalysts containing all metals showed best performance in the oxidative coupling of CH4.
• Transformation of amorphous SiO2 to cristobalite occurred under reaction conditions.
• This transformation lowered the SSA but had only a marginal effect on catalyst performance.
• 1.9%Mn-3%Na2WO4/SiO2 showed optimal performance with 19% yield of C2.

Mn-Na2WO4/SiO2 catalysts of different composition were made in a single step by flame spray pyrolysis, a process that can be scaled up to a production rate in the range of kg/h. The multicomponent catalysts containing 0–5 wt% Mn and 0–6 wt% Na2WO4 were characterized by nitrogen adsorption, XRD, TEM, STEM, EDXS and TPR, and tested in the oxidative coupling of methane (OCM) in a continuous flow microreactor at different reaction conditions (temperature, CH4/O2 feed ratio, space time). As-prepared catalysts showed much higher specific surface area (SSA) and a more homogenous spatial distribution of the constituents than corresponding catalysts prepared by wet-impregnation. Upon exposure to reaction conditions at 810 °C the amorphous SiO2 component gradually transformed to crystalline cristobalite, as shown by in situ XRD. However, the cristobalite formation, which was accompanied by a decrease of the SSA had only a marginal influence on catalyst performance. Conversion and selectivity to C2 compounds (ethene, ethane) increased with Mn content up to 1.9 wt% and remained virtually constant up to 5 wt%, affording a constant C2-yield of 17%. Increasing the Na2WO4 content was most effective up to 1 wt%, while at contents higher than 3 wt% the conversion declined. Only catalysts containing all metal constituents (Mn, W, Na, Si) afforded good catalytic behavior. Flame-derived catalysts with a composition 1.9%Mn-3%Na2WO4/SiO2 exhibited the highest C2-yield of 18.5% at a selectivity of 70%. Both, the flame-made catalyst and a corresponding wet-impregnated reference catalyst showed stable conversion, but different C2-selectivity behavior with time-on-stream, decreasing with the flame-derived catalyst and increasing with the wet-impregnated counterpart. The initial C2-selectivity as well as that after 6 h on stream when a steady-state was achieved was higher for the flame-made catalyst and this at higher conversion, indicating the superior catalytic behavior of this catalyst. The better performance of the flame-derived catalyst is traced back to its more homogeneous spatial distribution of the constituents, which favors their beneficial mutual interaction.

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