Elucidating the Effects of Mn Promotion on SiO2-Supported Na-Promoted Tungsten Oxide Catalysts for Oxidative Coupling of Methane (OCM)

Molecular-level understanding of the structure–activity relationships for oxidative coupling of methane (OCM) by the supported Mn2O3–Na2WO4/SiO2 (also written as Mn-Na-WOx/SiO2) catalyst system is currently based on hypotheses presented several decades ago that proposed that Mn–O bonds of the surface MnOx sites are directly involved in activation of CH4 and O2. The current studies, employing in situ Raman, UV–vis, and transient TPSR spectroscopies and steady-state OCM catalytic studies with nonstoichiometric SiO2-supported catalysts, however, reveal that the oligomeric MnOx surface sites and poorly crystalline Mn-WO3 and MnWO4 nanoparticles only play a minor role during OCM and essentially behave as spectator sites. The catalytic active sites responsible for activation of both CH4 and O2 for the formation of C2 products are the isolated, pseudotetrahedral, Na-coordinated WO4 surface sites (Na-WO4) on the SiO2 support. The Na-WO4 surface sites are thermally robust and do not restructure during the OCM reaction. These results indicate the need to critically re-evaluate the role of Mn-promoter for the catalytic OCM reaction, utilizing evidence-based experimental data obtained under OCM reaction conditions with nonstoichiometric catalysts.