Implications of the Molybdenum Coordination Environment in MFI Zeolites on Methane Dehydroaromatisation Performance

The structure and activity of Mo/Silicalite-1 (MFI, Si/Al=infinity) were compared to Mo/H-ZSM-5 (MFI, Si/Al=15), a widely studied catalyst for methane dehydroaromatisation (MDA). The anchoring mode of Mo was evaluated by in situ X-ray absorption spectroscopy (XAS) and density functional theory (DFT). The results showed that in Mo/Silicalite-1, calcination leads to dispersion of MoO3 precursor into tetrahedral Mo-oxo species in close proximity to the microporous framework. A weaker interaction of the Mo-oxo species with the Silicalite-1 was determined by XAS and DFT. While both catalysts are active for MDA, Mo/Silicalite-1 undergoes rapid deactivation which was attributed to a faster sintering of Mo species leading to the accumulation of carbon deposits on the zeolite outer surface. The results shed light onto the nature of the Mo structure(s) while evidencing the importance of framework Al in stabilising active Mo species under MDA conditions.