(308h) Single Site Vs Crystalline: Development and Characterization of Mn/W/Na/SiO2 Model Catalysts for the Oxidative Coupling of Methane (OCM)

An abundance of natural gas due to the advancements in exploration and extraction technologies has rejuvenated interest in the direct conversion of natural gas to ethylene via oxidative coupling of methane (OCM). The tri-metal oxide supported Mn/W/Na/SiO2 catalyst has been studied in the past and has shown ~75% C₂ selectivity, ~35% conversion and robust thermal stability under operating conditions. However, improvements need to be made to the catalyst to increase product yield and lower the operating temperature. Most published studies of the tri-metal oxide catalysts before and after OCM typically identify crystalline phases such as Na2WO4, MnWO4, etc. as the active phases. Our in-situ and operando Raman spectroscopy studies of supported Mn/W/Na/SiO2 catalysts during OCM, surprisingly, reveal that the often suggested crystalline phases are not present during reaction and that dispersed surface metal oxide species are present. This suggests that the surface metal oxide species may be the catalytic active sites for OCM by supported Mn/W/Na/SiO₂ catalysts. Developments regarding design and performance of dispersed and crystalline tri-metal oxide catalysts for OCM at/near working conditions will be presented in this work. The focus will be on in-situ, and operando Raman spectroscopic characterization of the model tri-metal oxide Mn/W/Na/SiO₂ catalysts to establish the nature of the surface active sites, their H₂-TPR characteristics, and their structure dependent catalytic performance.