Non-oxidative dehydroaromatization of methane over Mo/H-ZSM-5 catalysts: A detailed analysis of the reaction-regeneration cycle

Of several Mo/H-ZSM-5 catalysts (Mo loading = 1, 3, 5, 7 wt%), 5Mo/H-ZSM-5 (5 wt%) exhibited the best catalytic performance for methane conversion and benzene yield for methane dehydroaromatization at 700 °C. It was observed that deactivation of zeolite acid sites precedes deactivation of the Mo2C sites. Increasing the number of accessible Mo carbide sites with minimizing isolated surface acid sites is required for high benzene selectivity and stability. Under oxidative conditions, bifunctional metal-acid sites were reversibly regenerated at 450 °C; at higher temperatures of 550–850 °C, irreversible deactivation was observed. The selective recovery of Brønsted acid sites near Mo sites other than isolated acid sites is sufficient to restore the catalytic activity in terms of benzene formation. Spectroscopic studies revealed that high-temperature oxidative regeneration induced (MoO42−)n oligomerization and subsequent carburization of bulk Mo carbide cluster. The 5Mo/H-ZSM-5 underwent MoO3 sublimation and loss of Brønsted acidity during oxidative regeneration at 850 °C. Reductive regeneration required a temperature higher than 700 °C for coke removal, but resulted in thermal degradation of the catalyst. Theoretical calculations indicated that partial oxidation of coke precursor (i.e. naphthalene) at 450 °C was more favorable than its partial hydrogenation at 850 °C on the Mo clusters in ZSM-5 channel. Overall, oxidative regeneration at 450 °C can maintain high Mo2C dispersion and efficient coke removal during 60 h of methane reaction.