Acid-site control of ZSM-5 zeolites using quaternary ammonium hydroxides for shale gas dehydroaromatization

Abstract

The direct non-oxidative shale gas dehydroaromatization is a promising route for BTX (benzene, toluene, and xylenes) production, offering an effective alternative to conventional crude-oil-based processes. However, it faces crucial challenges such as rapid catalytic deactivation in Mo/ZSM-5 catalysts and limited regenerability due to the formation of external hard coke, resulting from external Mo active sites. This study introduces a hydrothermal post-treatment approach that can selectively control the external Al sites in ZSM-5 zeolites using quaternary ammonium hydroxides, aimed at reducing external Mo anchoring sites. This method successfully reduced the external Al and Mo-site densities in Mo/ZSM-5 catalysts, concurrently introducing mesoporosity, eventually leading to improved catalytic stability, reduced hard coke formation, and enhanced regenerability of spent catalysts. We highlight the significance of the roles of quaternary ammonium cations in the hydrothermal post-treatment, contrasting their effects with the NaOH treatment and analogous processes conducted at lower temperatures.