(294h) Oxidative Coupling of Methane (OCM) By Dispersed-Phase Silica-Supported Tungsten Oxide Catalysts

An abundance of natural gas owing to 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 MnO_x-Na₂WO₄/SiO2 catalyst has been investigated in the past, with reported C₂ selectivity of ~70% and CH₄ conversion of ~30%, and robust thermal stability. Owing to a dearth of fundamental studies on structure-function relationships at/near OCM reaction conditions, however, significant improvements in catalyst design and C₂ yields have not been reported in the past three decades.

Most published studies of the catalyst before and after OCM typically identify bulk crystalline phases like Na₂WO₄ as the active phase. Our latest spectroscopic studies of model Na-doped WO_x SiO₂-supported catalysts, surprisingly, reveal that the often-suggested crystalline phase is not present during the OCM reaction conditions 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 in these catalysts. This presentation will focus on recent developments based on careful synthesis and characterization via in situ physical (Raman, IR, and UV–vis) and chemical probe (TPSR and TPR) spectroscopic techniques of model bi-metal oxide catalysts (NaO_x-WO_x/SiO₂) prepared from different Na- and W- precursors. The effects of varying Na/W molar ratio in the model catalysts on the (a) dominant phase (crystalline vs. dispersed), (b) structure, (c) properties, and (d) CH₄ activation will be elucidated.