(683h) Post-Synthetic Approach to Enhance the Catalytic Stability and Regenerability of Mo/ZSM-5 Catalysts for Shale Gas Dehydroaromatization

The non-petroleum production of aromatic compounds (BTX) using shale gas has emerged as a promising alternative to the energy-intensive naphtha reforming process. The ZSM-5 zeolite-supported molybdenum (Mo) oxide catalyst (Mo/ZSM-5) is a highly effective catalyst for the non-oxidative aromatization of shale gas to BTX. However, coke formation under high temperatures above 650°C leads to rapid catalyst deactivation. The formation of graphitic hard coke outside the catalyst particles is particularly problematic as it requires high regeneration temperatures that can sinter Mo-active sites and degrade zeolitic framework, making regenerability crucial for commercialization. In this study, a post-synthetic approach using quaternary ammonium hydroxides (QAHs) as organic bases was proposed to enhance the catalytic lifetime and regenerability of Mo/ZSM-5 by inhibiting the formation of external hard coke during shale gas aromatization. The QAH treatments were performed on ZSM-5 under hydrothermal conditions similar to those used for zeolite crystallization, resulting in the regioselective formation of an Al-depleted region near the surface of ZSM-5 zeolites. The Mo-active sites loaded via the wet-impregnation method exhibited a spatial distribution that strongly correlated with the distribution of framework Al-sites. The Mo/QAH-treated-ZSM-5 catalysts exhibited exceptional shale gas dehydroaromatization performance at 700°C and superior catalytic lifetime compared to untreated Mo/ZSM-5 due to effective suppression of external hard coke formation resulting from the reduced surface active site density (Figure 1a). The QAH-treatment facilitated the formation of soft coke that could be easily removed under milder regeneration conditions, as revealed by thermogravimetric analysis. The Mo/QAH-treated-ZSM-5 catalysts exhibited greater regenerability under oxidative regeneration cycles than the untreated Mo/ZSM-5 catalysts (Figure 1b), owing to the preference for soft coke formation over hard coke formation. This study not only offers a method to produce a coke-resistant catalyst under highly carbonaceous circumstances but also provides detailed insights into the mechanism underlying coke formation during shale gas utilization.