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 catalytic active Mo phases in the supported Mo/ZSM-5 catalyst have been under debate in the literature. A systematic in situ and operando spectroscopic study of the active Mo phase 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 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 catalytic active Mo phases during the methane dehydroaromatization reaction.