(676c) Characterization of the Structural Evolution of Mo-MFI for Methane Dehydroaromatization Reaction-Regeneration Cycles

Methane dehydroaromatization (DHA) can be facilitated by molybdenum supported on MFI zeolites (Mo-MFI) with high aromatics selectivity (~80%) [1]. Carbonaceous deposits formed during reaction leads to rapid deactivation, which can be removed by O₂ at relatively high-temperatures (>823 K), but causes structural changes on Mo-MFI leading to irreversible decreases of initial rates with increasing numbers of reaction-regeneration cycles [2,3]. Here, we use a variety of characterization techniques, including H₂ TPR, NH₃ TPD, ²⁷Al NMR and in situ XAS coupled with kinetic measurements to elucidate the structural changes to Mo sites and H⁺ sites on Mo-MFI during successive cycles of reaction-regeneration. Catalysts were prepared by MoO₃ deposition on commercial MFI zeolites (Si/Al ~12), followed by oxidative treatments (823 K) to transform MoO₃ into ion-exchanged Mo species. Forward DHA rates (950 K, 60 CH₄ kPa) were measured using Mo-MFI and decrease systematically with increasing numbers of reaction and regeneration (20 O₂ kPa) cycles. We show that residual H⁺ sites decrease upon exposure to hydrothermal degradation conditions during regeneration because of framework dealumination, and consequently results in the decrease of the fraction of ion-exchanged Mo species, which are the active site precursors for methane DHA. Concurrently, the loss of H⁺ sites on the zeolite support leads to Mo transformation to aluminum molybdate domains during regeneration, as evidenced by ²⁷Al NMR; these Mo species do not carburize under reaction conditions and are inactive for methane DHA. Moreover, we developed a regeneration protocol that allows extending the catalyst lifetime, and synthetic methods using non-commercial zeolites to improve the catalyst stability. This work demonstrates the use of characterization and kinetic measurements to elucidate the fundamental causes of Mo-MFI catalyst deactivation and how such insights provide guidance to developing synthetic and regeneration strategies to extend catalyst lifetime for methane DHA reaction-regeneration cycles.