(608d) Fundamental Understanding of Structure, Location, and Stability of Zeolite-Supported Molybdenum Oxide Nanostructures for Methane Dehydroaromatization

Methane Dehydroaromatiozation (MDA) is the direct conversion of methane to aromatics and is promising for conversion of methane into valuable liquids, namely benzene. The most investigated catalyst for MDA is Mo supported on H-ZSM-5. Under reaction conditions, catalyst precursor MoO_x species carburize, resulting in Mo-oxycarbides and Mo-carbides. Despite interest in MDA relative to indirect techniques that are not economically viable at the wellhead scale, the commercialization of MDA has been hampered by lack of molecular-level understanding of the process. Although many studies have discussed the structure of the MoO_x catalyst precursor species inside H-ZSM-5 and provided spectroscopic evidence to support their proposed models, no strong consensus has yet been reached and debate remains. Further insights into the location and speciation of MoO_x in H-ZSM-5, especially the structure, siting, and stability, are necessary to fully understand the catalyst precursors and processes fundamental to MDA. To work toward establishing a structure-activity relationship for MDA and rational design of next-generation MDA catalysts, we use density functional theory (DFT) to study three qualitatively unique motifs of MoO_x catalyst precursors: two MoOx “monomers” and one MoOx “dimer”. For each motif, the electronic structure, geometry, anchoring site, and binding energies were investigated at different zeolite T-sites and Al-cation locations. Vibrational frequencies were calculated and compared to the experimental spectral data. We have considered these motifs relative to one another, and we predict that isolated monomer species on single Al sites are preferred relative to dual-site motifs (by 76-154 kJ/mol). We performed population analysis for relative siting of MoO_x monomers inside the zeolite’s channels based on the calculated binding energies. Moreover, based on the binding energies on the double Al sites, it can be assessed whether Mo oxide species are more likely to be observed as monomers or dimers on a basis of thermodynamics.