(188b) Development of Structurally Stable Mo-CHA Catalysts for Methane Dehydroaromatization with High Benzene Selectivity

Molybdenum supported on MFI zeolites (Mo-MFI) facilitates Methane dehydroaromatization (DHA) 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). These regeneration protocols, however, result in decreases to methane DHA rates (per Mo) with each successive cycle because exposure of Mo-MFI to hydrothermal aging conditions leads to the loss of zeolitic H⁺ sites and thus the binding sites for ion-exchanged Mo species that are precursors to active Mo-carbides for methane DHA [2]. Here, we use chabazite (CHA) zeolites as the zeolite support due to its known structural resistance to hydrothermal aging conditions [3] and use H₂ TPR, NH₃ TPD, and HRTEM coupled with kinetic measurements to show that Mo-CHA catalysts remain structurally stable over several (>10) cycles of reaction-regeneration. CHA zeolites were also hydrothermally synthesized by adding organosilane surfactant to reduce crystallite sizes (<10-100 nm) to alleviate diffusion constraints for bulky aromatic products (>0.55 nm) imposed by smaller eight-membered ring (0.38 nm) windows. Catalysts were prepared by MoO₃ deposition on CHA and MFI zeolites (Si/Al ~15), followed by oxidative treatments (823 K) to transform MoO₃ into ion-exchanged Mo species. Initial forward DHA rates (950 K, 60 CH₄ kPa) decrease systematically with increasing numbers of reaction-regeneration (20 O₂ kPa) cycles for Mo-MFI. In contrast, rates remain constant for Mo-CHA and the fraction of ion-exchanged Mo species quantified by H₂ TPR does not decrease upon regeneration. Furthermore, HRTEM imaging shows evidence for mesopore formation and Mo agglomeration in Mo-MFI catalysts after regeneration, while such structural changes are not observed in Mo-CHA. This work provides guidance for the design of Mo-zeolites with improved hydrothermal stability, which is a barrier to the development of a continuous reaction-regeneration process for commercially viable methane DHA strategies.