Determination of Molybdenum Species Evolution during Non-Oxidative Dehydroaromatization of Methane and its Implications for Catalytic Performance

Mo/H-ZSM-5 has been studied using a combination of operando X-ray absorption spectroscopy and High Resolution Powder Diffraction in order to study the evolution of Mo species and their location within the zeolite pores. The results indicate that after calcination the majority of the species present are isolated Mo-oxo species, attached to the zeolite framework at the straight channels. During reaction, Mo is first partially carburized to intermediate MoCxOy species. At longer reaction times Mo fully carburizes detaching from the zeolite and aggregates forming initial Mo1.6C3 clusters; this is coincident with maximum benzene production. The Mo1.6C3 clusters are then observed to grow, predominantly on the outer zeolite surface and this appears to be the primary cause of catalyst deactivation. The deactivation is not only due to a decrease in the amount of active Mo surface but also due to a loss in shape-selectivity which leads to an increased carbon deposition at the outer shell of the zeolite crystals and eventually to pore blockage.