Structural changes to molybdenum and Brønsted acid sites in MFI zeolites during methane dehydroaromatization reaction-regeneration cycles

Mo-MFI zeolite catalysts for methane dehydroaromatization (DHA) can be regenerated at high temperatures (>773 K) in oxidative environments but undergo irreversible structural degradation that leads to decreasing aromatics yields with increasing numbers of reaction-regeneration cycles. H2 temperature programmed reduction (H2 TPR) was used to quantify the fraction of ion-exchanged Mo sites and MoOx clusters, and NH3 titration methods were used to quantify residual H+ sites, on Mo-MFI materials of different composition and treatment history. These data reveal that oxidative treatments at 823 K preferentially form mononuclear Mo species (i.e., [MoO2]2+) that exchange two proximal H+ sites (per Mo), quantified by Co2+ titration, followed by other Mo species (e.g., [Mo2O5]2+, [MoO2OH]+) that exchange one H+ site (per Mo). Mo species exchange at isolated H+ sites in larger amounts during after oxidative treatment at higher temperatures (973 K), demonstrating the influence of framework Al distribution on extra-framework Mo speciation and dispersion, unifying prior disparate reports of ion-exchanged Mo speciation in MFI zeolites. Forward rates of benzene formation during methane DHA decreased with increasing numbers of reaction (60 kPa CH4, 950 K) and regeneration (20 kPa O2, 823 K) cycles due to decreasing fractions of ion-exchanged Mo species and formation of aluminum molybdate domains, which resulted from the loss of H+ sites due to framework dealumination upon exposure to hydrothermal aging conditions during regeneration. Thus, preserving zeolitic H+ sites is critical for the redispersion of ion-exchanged Mo sites during oxidative regeneration protocols to, in turn, maintain high aromatics yields in subsequent DHA reaction cycles. Physical mixtures of spent Mo-MFI catalysts with H-MFI, which provides an additional reservoir of H+ sites, enables redispersion of aggregated Mo domains into ion-exchanged Mo species and the recovery of initial DHA reaction rates. The complementary suite of experimental methods developed here allows studying active site and catalyst structural changes throughout methane DHA reaction-regeneration cycles and can be used to examine the stability of alternate Mo-zeolite materials and develop more efficacious regeneration strategies to extend catalyst lifetime.