Proximity-guided control of framework Al pairs in ZSM-5 for enhancing hydride transfer and aromatics formation in propane–DME co-aromatization

Abstract

This work presents a rational strategy to enhance propane and dimethyl ether (DME) co-aromatization by precisely controlling the spatial proximity of Brønsted acid sites (BAS) in H-ZSM-5. By tuning Na⁺ content during hydrothermal synthesis, the competitive charge balance between Na⁺ and protons is modulated, which favors the formation of Al–O–(Si)₂–O–Al sequences and enables tunable fractions (50–78%) of paired framework aluminum sites, located preferentially at channel intersections. These Al pairs generate adjacent BAS, which accelerate carbocation intermediate formation, as evidenced by intensified mono-alkenyl and aromatic carbocation signals on in situ FTIR spectra. Owing to the synergy of acid sites, the H-Z5-78 catalyst with a higher proportion of paired Al sites exhibited 3.7% higher propane conversion, notably higher than that of the low Al-pair counterpart (2.6%) at 400–500 °C. The DFT study reveals that paired Al sites lower the activation barriers for hydride abstraction and chain-growth steps by 0.16 eV and 0.16 eV, respectively, thus accounting for the promoted intermediate evolution and enhanced catalytic performance. These findings highlight the pronounced effect of the spatial distribution of Al sites on hydride transfer efficiency and intermediate evolution, offering a critical parameter for designing zeolite catalysts with optimized acid site proximity for the co-aromatization of alkanes and oxygenates.