Key Takeaways & Executive Findings
- •• • Methane conversion scales with BAS strength in M-80 zeolites (Al > Ga > Fe), but B-80 achieves exceptional MDA performance via deboronation-generated silanol nests anchoring Mo for monofunctional steps and residual framework boron providing weak BAS activity, demonstrating that weak acidity can outperform moderate acidity in bifunctional synergy. • • Ball milling achieves optimal nanoscale Mo-BAS spacing, increasing methane conversion by 33% and BTX yield by 31% relative to millimeter/micrometer mixtures, establishing spatial confinement as a performance-determining factor in MDA. • • DFT calculations confirm that deprotonation energy (DPE) correlates with experimental acid strength; strong-acid Al-ZSM-5 (DPE = -5.68 eV) significantly lowers the C–H bond dissociation barrier (ΔG = 1.467 eV), while IGMH analysis reveals mixed ionic-covalent character of Mo-O bonds stabilized by charge compensation and geometric compatibility. • • Metal substituents modulate the electronic structure of active centers—not merely serving as anchors—thereby governing CH3 C–H cleavage barriers and overall activity, providing a theoretical basis for rational design of high-performance MDA catalysts.
Abstract
Methane dehydroaromatization (MDA) offers a carbon-neutral route to benzene, toluene, and xylene (BTX), yet the regulatory mechanisms of Brønsted acid site (BAS) strength and spatial proximity to Mo sites remain unresolved. This study systematically tunes BAS strength via isomorphous substitution (Al, Ga, Fe, B) and Mo-BAS proximity in ZSM-5, integrating catalytic evaluations with density functional theory (DFT). Strongly acidic Al-zeolites achieve the highest methane conversion, while weakly acidic B-substituted systems exhibit optimal mono-/bifunctional synergy, outperforming moderate-acid counterparts. DFT reveals that deprotonation energy (DPE) correlates with acid strength; Al-ZSM-5 (DPE = -5.68 eV) lowers the C–H activation barrier (ΔG = 1.467 eV). Spatial proximity analysis shows that nanoscale Mo-BAS distances, achieved via ball milling, enhance methane conversion by 33% and BTX yield by 31% compared to micrometer-scale mixtures, by accelerating intermediate transport and suppressing coke. These findings establish a multi-scale framework linking acid strength, spatial confinement, and electronic modulation, providing actionable guidelines for designing next-generation MDA catalysts.
1. Introduction
The progressive depletion of crude oil and increasing natural gas supply underscore the importance of converting methane (70–90% of natural gas) into light aromatics. Conventional methane conversion technologies—low-temperature oxidation to methanol, oxidative coupling, and steam reforming—suffer from high energy intensity, substantial CO2 emissions, and multi-step processes. Non-oxidative methane dehydroaromatization (MDA) over metal-loaded zeolite catalysts offers inherent advantages of energy efficiency, reduced carbon footprint, and high aromatic selectivity, yet faces challenges of inadequate catalyst activity and stability.
Current MDA research predominantly centers on Mo-loaded acidic ZSM-5 zeolites, but the individual contributions of Brønsted acid site (BAS) strength and spatial proximity to Mo sites remain poorly understood. This study systematically tunes both BAS strength via isomorphous substitution and Mo-BAS spatial proximity, integrating catalytic evaluations with DFT calculations to dissect their roles. The findings establish a multi-scale framework linking acid strength, spatial confinement, and electronic modulation, providing actionable guidelines for designing next-generation MDA catalysts.
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WANG Ce, WEI Lihong, ZHANG Qinghao, ZHANG Hongxiang, SUN Yuewen (2026). Influence of the Distance between Brønsted Acid Sites and Mo Sites in Mo/HZSM-5 on the Mechanism of Methane Dehydroaromatization Performance. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(25)60631-1
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Frequently Asked Questions
How does the spatial distance between Mo and Brønsted acid sites affect methane conversion and BTX yield in MDA?
Nanoscale proximity, achieved via ball milling, increases methane conversion by 33% and BTX yield by 31% compared to micrometer-scale mixtures. This enhancement is attributed to accelerated intermediate transport and suppressed coke formation, as micrometer distances exceed electron interaction and mass transfer limits.
What is the role of Brønsted acid strength in determining the rate-determining step of methane dehydroaromatization?
DFT calculations show that deprotonation energy (DPE) correlates with acid strength. Strong-acid Al-ZSM-5 (DPE = -5.68 eV) lowers the C–H bond dissociation barrier (ΔG = 1.467 eV), facilitating methane activation. Weaker acids (B-substituted) exhibit higher barriers but enable unique bifunctional synergy via silanol nests.
How does isomorphous substitution of Al with Ga, Fe, or B affect the electronic structure of Mo active sites and catalytic performance?
Metal substituents modulate the electronic structure of active centers, not merely serving as anchors. This modulation governs CH3 C–H cleavage barriers and overall activity. Experimentally, methane conversion scales with BAS strength (Al > Ga > Fe), but B-80 achieves exceptional performance through deboronation-generated silanol nests that anchor Mo and residual framework boron providing weak BAS activity.
What are the implications of these findings for the design of industrial MDA catalysts?
The study establishes that both BAS strength and Mo-BAS spatial proximity are critical design parameters. Optimal performance requires nanoscale proximity (achieved via ball milling) and tailored acid strength. B-substituted zeolites offer unique potential for mechanistic studies and may lead to catalysts with improved stability and selectivity, though Al-zeolites deliver the highest activity.
How does the study address the challenge of coke formation in MDA?
Nanoscale Mo-BAS proximity suppresses coke formation by enhancing intermediate transport and reducing residence time of coke precursors. This is evidenced by the 33% increase in methane conversion and 31% increase in BTX yield, which indicate improved catalyst stability and selectivity.
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