(2dl) Computational and Experimental Investigation of the Distribution of Mo-Oxide Species in Mo/H-ZSM-5

Research Interests: Computational catalysis, Electronic structure and classical atomistic material simulations, Nanoporous materials

Mo-oxide-impregnated H-ZSM-5, the most extensively studied catalyst for methane dehydroaromatization, has not yet been commercialized due to various challenges, notably rapid deactivation. In spite of the challenges, the potential benefits of Mo/H-ZSM-5 for MDA continue to drive exploration into optimizing its performance. To comprehend the processes underlying MDA including calcination, activation, , and deactivation, it is crucial to begin with the initial stages that play a vital role in establishing the catalyst's structure and composition, which in turn impact the activation and deactivation stages of the reaction. Mo-oxide species, precursors of Mo-oxycarbides and Mo-carbides, have been widely investigated; however, their exact identity, distribution, and location have remained controversial. In this work, employing a combination of computational and experimental investigation, we studied the evolution, speciation, and distribution of Mo-oxide species at the initial stages of MDA. Three qualitatively unique motifs of Mo oxide species, including MoO₂OH⁺, MoO₂²⁺, and Mo₂O₅²⁺ have been studied via Density Fucntional Theory (DFT) calculations. Quantitative temperature-programmed oxidation and automated analysis of extended X-ray absorption fine structure employing a database of DFT-optimized structures provided experimental spectroscopic evidence of the existing Mo-oxide species in samples prepared via incipient wetness impregnation and physical mixing at various Mo loadings. Moreover, the evolution of unique Mo-oxide motifs was studied computationally via the nudged elastic band (NEB) method. We additionally evaluated the possibility of anchoring Mo monomers and dimers to double Al sites that have not been traditionally considered for this purpose, including motifs between ten-member rings inside H-ZSM-5 channels. The study has identified potential oxide sites that were previously overlooked in most atomistic modeling of these systems and further demonstrated their thermodynamic viability. Building upon the obtained results on Mo-oxide speciation, we investigated the distributions of each Mo-oxide species as a function of zeolite acidity and metal loading using thousands of models of Mo/H-ZSM-5 with varying acidity and Mo loading via Monte Carlo methods. Our investigations have revealed that the Mo-oxide precursors of various chemistries may coexist within the zeolite, and the distribution between them may be regulated through catalyst synthesis techniques, zeolite acidity, and metal loading. The insights gained from this analysis can contribute to the rational design of catalysts with potential to exhibit enhanced performance and sustainability.