Designing Metal/Zeolite Catalysts for Methane Dehydroaromatization: Decoupling External Site Effects, Bimetallic Interactions, and Deactivation Pathways

Abstract

Methane, the primary component of natural gas, is an abundant yet underutilized carbon resource that is frequently flared or vented, contributing to greenhouse gas emissions and energy loss. Methane dehydroaromatization (MDA) provides a direct, COx-free route to convert methane into benzene and hydrogen, offering a promising pathway for methane valorization. Mo/ZSM-5 is the most extensively studied catalyst for this reaction, yet its practical application is constrained by rapid deactivation caused by carbon deposition and low product yield. In Mo/ZSM-5 catalysts, the catalytically relevant active phase is widely attributed to Mo carbide or oxycarbide species confined within the zeolite channels. Alongside these desired intrachannel active sites, a fraction of Brønsted acid sites (BAS) and MoOx species can reside on the external surface of the zeolite. These external sites are known to promote unselective reactions, including oligomerization of intermediates and excessive carbon deposition, which accelerate catalyst deactivation. The individual roles of external BAS and external MoOx species remain difficult to distinguish experimentally. In parallel, the incorporation of promoters such as Fe, Ni, or Co into Mo/ZSM-5 has been reported to improve catalytic stability and product selectivity; however, the fundamental origin of these improvements remains unclear, particularly with respect to whether true bimetallic interactions form within the zeolite channels and how they influence active site formation and carbon deposition pathways. This dissertation addresses these challenges by systematically investigating how external site engineering and bimetallic interactions in Mo- and Mo-Fe/ZSM-5 catalysts govern active site formation, catalytic performance, and deactivation pathways, providing a foundation for the rational design of stable MDA catalysts. A key contribution of this work, addressing the unresolved role of external sites, is the decoupling of the individual roles of external metal oxide species and external Brønsted acid sites (BAS). Using selective silylation of ZSM-5 prior to Mo loading and selective extraction of external MoOx, catalysts were synthesized with controlled distributions of metal and acid sites. Catalytic testing, including tandem-bed experiments, revealed that external BAS, while do not activate methane, strongly promote oligomerization of reaction intermediates and products, resulting in decreased benzene selectivity and accelerated formation of hard, graphitic coke. In contrast, external MoOx formed smaller amounts of softer, more disordered carbon and played a comparatively minor role in deactivation. These findings provide a mechanistic basis for targeted external site passivation strategies in Mo/ZSM-5 catalysts. Building on the understanding of external site effects, this dissertation also examines how precursor chemistry influences metal distribution and catalytic performance by introducing Fe2(MoO4)3/ZSM-5 as a bimetallic precursor. Compared with monometallic MoO3/ZSM-5 and mixed MoO3+Fe2O3/ZSM-5 containing equivalent metal loadings, Fe2(MoO4)3/ZSM-5 exhibited superior benzene selectivity and improved catalytic stability. Structural characterization revealed partial segregation of Fe2(MoO4)3 into Fe2O3 and amorphous MoOx during pretreatment, promoting MoOx migration into the zeolite channels while minimizing Mo trapping by external FeOx phases, as observed in MoO3+Fe2O3/ZSM-5 catalysts. Analysis of spent catalysts showed that Fe promotes the formation of carbon nanofibers, contributing to improved catalyst stability. These findings underscore the impact of Mo–Fe interactions originating from the precursor and highlight the importance of understanding Mo–Fe interactions within the zeolite channels. Building on these insights, direct co-impregnation of Mo and Fe into ZSM-5 was employed to promote co-location of both metals within the zeolite channels and to investigate whether true bimetallic active sites can form under MDA conditions. Advanced spectroscopic characterization and DFT modeling collectively confirmed the presence of distinct MoFeOx species anchored within the zeolite channels in fresh catalysts. Upon activation, these bimetallic domains likely evolve into a unique active site, leading to a markedly higher benzene formation rate and lower deactivation rates compared to monometallic Mo/ZSM-5. The enhanced performance is attributed to Mo–Fe synergy, which influences both methane activation pathways and carbon deposition behavior. Together, these studies provide a comprehensive understanding of how external site passivation and bimetallic site formation govern activity, selectivity, and deactivation in Mo- and Mo-Fe/ZSM-5 catalysts. This dissertation offers new design principles for engineering stable, high-performance catalysts for direct methane conversion.