(446e) Understanding the Active Sites on Mn/Na2WO4/SiO2 for Oxidative Coupling of Methane (OCM)

With the development of shale gas, natural gas becomes an abundant energy and chemical source for the United States, providing solutions to the energy supply and environmental challenges in a sustainable way.  As a major component in natural gas, methane conversion to liquid fuels and valuable chemicals is very attractive and critically important.  Among various methane conversion routes, oxidative coupling of methane (OCM) provides a direct way to activate methane for ethylene production, which can be further used to produce chemicals and liquid fuels.  Mn/Na₂WO₄/SiO₂ has been reported to give the highest C₂₊ yield (26%) among OCM catalysts (Lunford, J. H. Catal. Today 2000, 63, 165).  To further improve or design more active catalysts, we need to identify the nature of the active sites.  In this context, this study aims to understand the active sites on Mn/Na₂WO₄/SiO₂ for methane activation by doing kinetic measurements and detailed characterizations of the catalyst properties. 

Catalysts (x%Mn/y%W/0.8%Na/SiO₂) with varying Mn (0~2 wt%) and W (0~3.1 wt%) loadings prepared by incipient wetness impregnation were used for the kinetic study.  When changing Mn loading from 0 to 2 wt% (W constant at 3.1 wt%), ethylene formation rate, normalized by catalyst surface area, increased linearly with Mn loading until 0.05 wt%, and did not increase when Mn loading was further increased.  The rate increased continuously with W loading from 0 to 3.1 wt% (Mn constant at 2 wt%).  Ethylene formation rate for the catalyst with only W (3.1%W/0.8%Na/SiO₂) is (0.9×10^-7 mol C₂H₄ (m)^-2 (s)^-1) at 730°C under 40% CH₄, 20% O₂ and balance N₂, 7 times lower compared to the areal rate of 0.05%Mn/3.1%W/0.8%Na/SiO₂.  For the catalyst with only Mn (2%Mn/0.8%Na/ SiO₂), the rate is (0.4×10^-7 mol C₂H₄ (m)^-2 (s)^-1) at 730°C under 40% CH₄, 20% O₂ and balance N₂, 11 times lower compared with 2%Mn/3.1%W/0.8%Na/SiO₂.  These results have shown that ethylene formation rate increased significantly with the addition of the 2^nd metal oxide, indicating more active sites are generated when both Mn and W present on the catalyst surface.