(7e) Speciation, Siting, Evolution, and Methane Activation Properties of Mo-Oxide-Impregnated H-ZSM-5 Catalyst Precursors for Methane Dehydroaromatization

Non-oxidative methane dehydroaromatization (MDA), a direct process for converting methane to aromatics, has received substantial research attention in recent decades. Nonetheless, challenges remain to commercialization of the process due to thermodynamic limitations, suboptimal catalyst activity, and rapid catalyst deactivation under process conditions. Additionally, there remains a lack of consensus regarding fundamental details about MDA, such as the nature of the metal oxide catalyst precursors. To improve our understanding of the speciation, siting, and evolution of the catalyst precursor, we pursued computational and experimental investigations, employing a combination of systematic density functional theory (DFT) calculations, automated analysis of extended X-ray absorption fine structure (EXAFS) spectra, and quantitative temperature programmed oxidation (TPO). Through this work, we have evaluated the MoO_x catalyst precursors inside the H-ZSM-5, considering three qualitatively unique motifs of Mo-oxo species consistent with previous literature: Mo monomers with stoichiometry MoO₂OH⁺ and MoO₂²⁺, and Mo dimers, Mo₂O₅²⁺. The electronic structure, local geometry, and thermodynamic stability of each motif were studied using DFT calculations. Coupled with EXAFS (QuantEXAFS) and TPO analysis techniques, we established a more detailed picture of the structure and speciation of the MoO_x catalyst precursors and dependence of their relative distributions on preparation conditions. We have also computationally studied diffusion of MoO_x species within H-ZSM-5. We’ve further modeled the probability of siting Mo oxide species in proximity to one another and investigated the reaction barrier over which MoO_x species condense or disproportionate to form monomeric or dimeric Mo species. Additionally, we have investigated the distance over which two Al can serve as a double Al-atom anchoring site for Mo dimers and commented on the thermodynamic feasibility of formation of dimeric Mo species as a function of the Si/Al ratio. Through this work, we have identified potential viable oxide sitings not conventionally considered in most atomistic modeling of these systems.