Nature and Reactivity of Active Sites in Mn-and Na-promoted WOx/SiO2 Model Catalysts for Oxidative Coupling of Methane (OCM) under Operating Conditions

Abstract The silica (SiO2)-supported, manganese (Mn)- and sodium (Na)-promoted tungsten oxide (WOx) catalyst (Mn-Na2WO4/SiO2; also written as Mn-Na-WOx/SiO2 and Mn2O3-Na2WO4/SiO2) has, for decades, been studied as the most promising catalyst for the production of ethylene (C2H4) via the oxidative coupling of methane (OCM). The moderate activity (~35% CH4 conversion), high C2 selectivity (~80%), and robust thermostability (~1000 hr TOS) has made this catalyst the state-of-the-art catalyst for OCM. However, the techno-economic target for industrial scale application of this OCM catalyst requires that the C2 yield be increased further and that the operating temperature be lowered from the ~800-900oC range. The lack of fundamental understanding of this OCM catalyst system (catalyst structure under working conditions, manganese/sodium promotion mechanism, nature and identity of active phases and sites, and reaction mechanism), however, have hampered the improvement and optimization of this catalyst towards the techno-economic targets. The objectives of the dissertation were to (1) establish the fundamental catalyst structure-activity relationships by application of modern in situ and operando spectroscopy during OCM, combined with kinetic studies and density functional theory (DFT); (2) apply the new fundamental insights to guide rational design of advanced active and selective OCM catalysts functioning at lower temperatures. Various permutations of unpromoted and Na-, Mn- and Na/Mn-promoted SiO2-supported WOx sites were synthesized. The promoters-to W molar ratio was systematically varied to explore their effects on both the structure and properties of the WOx surface sites and the resulting OCM activity and selectivity. The catalysts were characterized in-situ under dehydrated conditions where surface water is not present and under OCM reaction conditions using Raman, UV-vis and IR spectroscopies to determine the molecular and electronic structures of the WOx sites on silica. Experimental findings were complemented with computational insights from VASP DFT calculations to provide additional understanding regarding the structure-activity relationships. Work herein provides the following key pieces of information, for the first time, that will help in the rational design of this OCM catalyst to make it more suitable for industrial application: (1) Dispersed phase WO4 surface sites were identified as the active OCM sites where C-H scission in CH4 takes place. Coordination of these WO4 surface sites to Na-promoter forms Na-WO4 surface sites that are significantly more C2 selective than the unpromoted WO4 surface sites and the crystalline phase Na2WO4. Previously, literature reports had completely missed the presence of these surface sites and had attributed the OCM activity to crystalline phase Na2WO4 which melts above 700oC and hence cannot be the active phase; (2) Mn-promoter was found to be primarily a spectator during OCM, especially at differential conditions where O2 reactant is not limited. Mn-promotion of WO4 surface sites (Mn-WO4) and Mn-promotion of Na-WO4 surface sites (Mn-Na-WO4) does not lead significant changes in the surface kinetics of the parent surface sites. Experimental structural characterization and DFT results further showed that Mn-promotion creates oligomeric MnOx surface sites along with poorly-crystalline nanoparticles (Mn-WO3 and MnWO4), all of which remain spectating during OCM. DFT insights further show that if MnOx oligomers activate CH4, the surface intermediates formed are significantly more stable than those formed from CH4 activation over Na-WO4 surface sites, increasing the likelihood of stable intermediates over-oxidizing to COx due to their higher stability and the resulting slower desorption; (3) the phase of SiO2 support is not relevant in this catalyst and SiO2 is primarily inert. The model catalysts studied herein exhibited ~85% C2 selectivity at ~40% CH4 conversion while the SiO2 was its amorphous phase due to the low Na-promoter concentration. Previously, due to the high Na concentration, researchers found SiO2 to be in its crystalline cristobalite phase as Na induces a phase transformation in SiO2 at elevated temperatures and speculated a critical influence of this crystalline SiO2 phase towards making a more C2 selective OCM catalyst.