Evidence of water promotes cyclohexanol adsorption and dehydration over H-ZSM-5 zeolite

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 ¹H MAS NMR, 2D ¹H DQ MAS NMR and 2D ¹H-²⁷Al HMQC NMR spectroscopy, systematic kinetic studies, isotope tracing and thermodynamic measurements. The water promoting effects were ascribed to water facilitating the formation of H₃O⁺ cluster (H₃O⁺(H₂O)_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.