Benefits of H-ZSM-5 zeolite from fluoride-mediated acidic synthesis for liquid-phase conversion of cyclohexanol

The hydrothermal stability of zeolites is essential for their potential applications in biomass conversion, especially in processes involving elevated temperatures alongside the use or generation of H 2 O. In this study, we employed F - ions as mineralizers to synthesize hydrothermally stable ZSM-5 zeolites under acidic conditions. The acidic synthesis system promotes zeolites with fewer silanol-terminated lattice defects (ZSM-5(A)) compared to the traditional basic conditions (ZSM-5(B)), endowing materials with substantially higher structural integrity and hydrophobicity. After 10 days of autoclave treatment at 200 °C in aqueous phase, H-ZSM-5(A) demonstrated nearly unchanged reaction rates in the dehydration of cyclohexanol, while H-ZSM-5(B) lost > 50% of the dehydration activity. Additionally, H-ZSM-5(A) delivered higher initial dehydration rates compared to H-ZSM-5(B). The different measured activation energies further revealed variations in reaction pathways during cyclohexanol dehydration, i.e., the monomer- or dimer-mediated routes depending on the concentration of alcohol molecule within zeolite pores, providing additional evidence for the strengthened hydrophobic nature of H-ZSM-5(A). Beyond this, the zeolite surface properties and the strength of cyclohexanol-zeolite interactions may impose additional transport/adsorption barriers attributed to multi-phase phenomena on the more polar H-ZSM-5(B) zeolite surfaces. More importantly, the hydrothermal treatment did not induce significant desilication and dealumination in H-ZSM-5(A), thereby preserving its active acid sites and ensuring exceptional hydrothermal stability. The present work fundamentally studies the synthesis of hydrothermally stable zeolites in an acidic medium using fluorides and expands the understanding of polar interactions in catalysis, characterized by the dehydration of cyclohexanol, for future application in biomass conversion.