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.
Hu Yaqi,Liu Runze,Lu Kaiyun,Wu Daoning,Gao Pan,Hou Guangjin,Zhang Hongbo,Dai Weili
Hydrated hydronium ions formed in the pores/cages of zeolite catalysts are suggested as the active species that could impact the adsorption of reactant, the stability of intermediates or transition states, and the reaction mechanism in the aqueous phase alcohol dehydration. Nevertheless, under a typical vapor phase reaction condition, the relevant situation is still debatable. Herein, the promotion of water on cyclohexanol (CyOH) adsorption and dehydration under a vapor phase condition was clarified by in situ 1 H MAS NMR, 2D 1 H DQ MAS NMR and 2D 1 H- 27 Al HMQC NMR spectroscopy, systematic kinetic studies, isotope tracing and thermodynamic measurements. The water promoting effects were ascribed to water facilitating the formation of H 3 O + cluster (H 3 O + (H 2 O) n-1 ), and per Brønsted acid site (BAS) could be saturated by about 5 water at ambient temperature. The CyOH dehydration pathway was found to be highly dependent on the water concentrations, and a dimer reaction route with the C-H bond rupture as the kinetically relevant step (KRS) was verified as the main reaction pathway at high water concentrations. All these findings provide the deep insights into how water regulating the zeolite catalyzed dehydration reaction at a molecular level.
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