Crafting Zn/nano-H-ZSM-5 catalysts for methanol-to-aromatics conversion: tailoring crystal size and modifying surface electron density

Abstract

The pore size structure and acidity profile are critical parameters influencing the catalytic performance in the methanol-to-aromatics (MTA) reaction. In this study, the carbon deposition behavior and electron migration mechanism of Zn-impregnated nano-H-ZSM-5 catalysts were explored. The results demonstrate that the nanocrystals (60–80 nm) shorten the diffusion path of reactants, enhancing the catalyst's mass transfer ability, optimizing the distribution of acidic sites, and thus slowing down carbon deposition. After ultrasonic impregnation of Zn metal, three active species, Zn²⁺, ZnO, and Zn(OH)⁺, were generated. In DFT calculations, these species exhibited significant electron deficiency and showed strong Zn-Lewis acidity, promoting a more active electron transfer process in the reaction system. Additionally, the nanostructured small crystals provide abundant adsorption sites for Zn reactive species, which readily accept electrons and interact with the π-electron cloud of aromatic precursors, thereby stabilizing carbocations and promoting the activation of C–H and CC bonds, ultimately enhancing the aromatization activity. The 0.5-Zn/NZ5-70 catalyst exhibited an average BTX selectivity of 55.3% after three validation tests, with catalytic activity sustained for 1840 minutes. This study offers valuable insights into the electron migration pathways and mechanistic understanding of nanostructured catalysts following metal impregnation, potentially guiding future catalyst design for improved MTA reaction.