Simple removal of framework aluminum from MWW type zeolites for unprecedented optimal Mo-impregnated catalysts: Systematic elucidation of coke deposition and its impact on methane dehydroaromatization

In this study, we investigated Mo-impregnated H-MCM-22 catalysts (denoted Mo/M) for methane dehydroaromatization (MDA) to produce aromatics such as benzene and toluene (BT). We attempted to improve the performance of the MDA catalysts by reducing the amount of Brönsted acid sites (BAS) of the H-MCM-22 supports via hydrothermal dealumination. Among the prepared catalysts, an optimal hydrothermal treatment (HT) of H-MCM-22 supports at 400 °C, followed by Mo impregnation (denoted Mo/M_400), resulted in a reduced and optimal amount of BAS, along with a comparable Mo distribution to Mo/M. Further, Mo/M_400 enhanced BT formation rates (maximum BT formation rate of 5.23 vs. 4.73 mmol_BT·g⁻¹·h⁻¹ for Mo/M); it appears that dealumination-induced reduction in the quantity of BAS altered their spatial interaction with active Mo species, promoting BT and naphthalene formation. Interestingly, the lifetime of intermediate C₂ (ethane and ethylene) formation was also improved for Mo/M_400. Rigorous coke analyses revealed that the decreased coke content in the aromatic-selective 10-membered-ring (10-MR) pores, as well as the ability of the 12-MR pores to accommodate coke deposits over a longer reaction time, improved the stability of Mo/M_400. Nonetheless, for all catalysts, the deactivations of BAS, and subsequently, the active Mo sites were mainly ascribed to coke deposition. The overall enhancement in MDA performance by Mo/M_400 was attributed to the advantages of the optimally reduced BAS, allowing such performance to surpass those of previously reported Mo-based catalysts.