Molybdenum supported on MFI zeolites (Mo-MFI) is able to catalyze methane dehydroaromatization (DHA) to equilibrium conversion with high (~80%) aromatics selectivity, but these materials undergo rapid deactivation. Typical high-temperature (&gt;773 K) oxidative regeneration to remove carbon deposits leads to irreversible deactivation and to various structural changes in Mo species and framework Al over successive reaction-regeneration cycles. Here, we develop characterization methods (H2 TPR, NH3 TPD) and use DHA kinetic measurements to elucidate changes to the number and structure of Brønsted acid sites and Mo species during DHA reaction-regeneration cycles. 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 (IE) Mo species. We show that H2 TPR allows distinguishing and quantifying MoO3 clusters from IE Mo species, and thus monitoring changes in this distribution after each reaction-regeneration cycle. Additionally, residual H+ sites quantification by NH3 TPD allows determining the speciation of exchanged Mo ions, which we show is influenced by the presence of proximal Al atoms (quantified by Co2+ titration) in the MFI lattice. Methane DHA reactions were performed using Mo-MFI zeolites, showing that forward rates of benzene formation (950 K, 60 CH4 kPa) decrease systematically as a function of reaction-regeneration cycles, as the material undergoes structural changes that cause a decrease of IE Mo species and H+ sites due to hydrothermal degradation during the regeneration step. This work combines information from quantitative characterization techniques and kinetic measurements to identify material properties that influence reactivity and stability, providing guidance for new catalyst synthesis and regeneration strategies for methane DHA.

Structural changes to molybdenum and brønsted acid sites on Mo-MFI during methane dehydroaromatization reaction-regeneration cycles

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