Ab initio understanding of processes in methane dehydroaromatization

Zeolites are highly effective scaffolds to confine active metal species in the framework and framework-associated positions as well as extra-framework sites. These metal species, encapsulated within zeolite matrices, have been developed into an important class of solid catalysts. The combination of the unique properties of the active metal sites with the intrinsic characteristics of zeolites, such as their high surface area, thermal stability, and shape selectivity, allows metal-in-zeolite catalysts to demonstrate exceptional performance in catalytic reactions. This includes enhanced activity, selectivity, and long-term stability, making them highly advantageous in a wide range of industrial and chemical processes. To establish accurate structure–activity relationships in metal-zeolite catalysts, it is essential to characterize the identity and local structures of dispersed metal cation complexes within zeolite. This task is challenging due to the dispersion of metal sites and the complexity of the zeolite framework. In this work, we employ computational methods to study these systems, with certain sections incorporating experimental characterization to provide additional insights. Our focus is on the Mo/H-ZSM-5 (MFI framework) catalyst, which is used in methane dehydroaromatization (MDA). MDA is a single-step, non-oxidative reaction that converts methane into liquid aromatics, light hydrocarbons, and hydrogen, offering a promising route for converting methane into transportable liquid products, thereby reducing natural gas flaring. During the MDA process, Mo oxide nanostructures undergo reduction and carburization, forming (oxy-)carbidic active centers within the pores of H-ZSM-5. Investigating the true nature of Mo-oxide precursors is a critical first step in understanding the MDA process and establishing rational catalyst design principles for generating more stable, commercially viable catalysts. In our analysis, we conduct a systematic DFT study of various Mo-oxide structures, including MoO2OH+ monomers, MoO22+ monomers, and Mo2O52+ dimers supported on H-ZSM-5. We explore the underlying physics of these structures, their anchoring sites within H-ZSM-5, and their thermodynamic stability. We further integrate our findings with spectroscopic techniques such as temperature- Texas Tech University, Fateme Molajafari, December 2024 viii programmed oxidation and X-ray absorption fine structure analysis, supported by a database of DFT-optimized structures and provide insight into the speciation of Mo-oxide in H-ZSM-5 and their control through tailored preparation methods. To understand the transformations of zeolite-impregnated metal-oxides during catalyst preparation and anchoring, we explore the anchoring and formation processes of distinct MoOx catalyst precursor motifs within H-ZSM-5 and provide insights into the mechanisms of interconversion between various MoOx structures. We develop a statistical modeling approach, informed by the kinetics of these processes, to analyze the distribution of Mo-oxide species within H-ZSM-5. Our theoretical findings are further compared with experimental characterizations performed by our collaborators to enhance the understanding of these systems. Later in the research, we examine the pretreatment and reduction mechanisms of Mo-oxide species in an H2 environment and their impact on CH4 activation. The aim is to advance into the study of reduced Mo species in operational catalysts and investigate the molecular pathways involved in CH4 activation, laying the groundwork for future optimization of the Mo/H-ZSM-5 system for MDA. In this work, we provide ab initio understanding of processes in methane dehydroaromatization, focusing on the evolution and transformation of Mo-oxide species and offering insights into their pretreatment and the initial steps of methane activation on these species.