Supported MoO3/ZSM-5 is the most promising catalyst for dehydro-aromatization of methane (DHA) due to its near equilibrium conversion. Despite extensive studies of this catalyst system in the past decades, the nature of the initial surface MoOx structure and the catalytic active Mo phases in the supported Mo/ZSM-5 catalyst have been under debate in the literature. To resolve the molecule details of Mo/ZSM-5 catalysts, a systematic in situ molecular spectroscopic study under fully dehydrated conditions and operando spectroscopic study during methane DHA were investigated over a series of supported MoO3/ZSM-5 catalysts with varying Mo loading and Si/Al ratio of the ZSM-5 support.
The anchoring sites, electronic and molecular structure of the initial MoOx phase of dehydrated Mo/ZSM-5 were characterized under in situ condition by DRIFTs, UV-vis, Raman and X-ray absorption spectroscopy. The in situ FTIR results suggested that surface MoOx species prefer to anchor at Brønsted acid sites (Al-(OH)+-Si) but also at external Si-OH and extra framework Al-OH sites at high Mo loading or low framework Al content of ZSM-5. Combined in situ UV-Vis DRS, in situ Raman and in situ XAS results, the surface MoOx species are essentially isolated and fully oxidized species. The absence of any d-d transition in the in situ UV-Vis spectra, the absence of crystalline MoO3 or Mo-O-Mo vibration in the in situ Raman spectra, and the sharp pre-edge characteristic of tetrahedral coordination and the absence of Mo-Mo absorption in the in situ EXAFS spectra confirmed that surface MoOx species are predominate isolated dioxo (O=)2MoO2 species with tetrahedral coordination. Only a small amount of polymeric (MoOx)n species are present on catalysts with high Si content (Si/Al=140) as evident by the presence of weak Mo-Mo absorption in the in situEXAFS spectra.
Operando Raman-MS investigated the relative reactivity of four distinct MoOx species on supported MoO3/ZSM-5 catalysts during methane DHA reaction. The reducibility of the four distinct surface MoOx species decreases in the order of dioxo (O=)2MoO2 on double Al-Al sites >dioxo (O=)2MoO2 on single Al sites >>dioxo (O=)2MoO2 on double Si-Si sites~ mono-oxo O=MoO4 species on extra-framework Al sites. The operando Raman-MS observed that the initial reactivity of methane DHA does not correlate to different MoOx structures but increases with decreasing Si/Al ratio of the ZSM-5 support. The absence of correlation between reactivity and surface MoOx structure might be due to the dynamic structural change of surface MoOx species during methane DHA reaction. The correlation between reactivity and Si/Al ratio suggests that presence of Brønsted acidity is necessary for the catalytic oligomerization of intermediate C2Hxspecies to benzene which is the reason for zeolites being applied to oil cracking.
The in situ EXAFS/XANES during methane DHA reaction condition provides the molecular information of catalytic active Mo phase of supported Mo/ZSM-5 catalysts. In situ EXAFS/XANES demonstrated that the active site is the poorly ordered molybdenum oxycarbide (MoOxCy) nanoparticles with predominate Mo2+ oxidation state with small domain size (~4 Mo atom) rather than crystalline Mo2C phase.
The present study provides a better understanding about the isolated surface MoOx molecular structures of the supported Mo/ZSM-5 catalyst and their transformation to active phases during the methane dehydroaromatization reaction.