Molecular Origins of Solvent Effects in Cyclohexanol Dehydration on H-BEA Zeolite

Solvents can substantially alter reaction rates by modifying the thermodynamic states of substrates and the free-energy landscapes of elementary steps in acid-catalyzed reactions. Cyclohexanol dehydration within BEA-type zeolite was investigated experimentally and theoretically to elucidate solvent effects on elementary steps. Adsorbed alcohol monomers and dimers were identified in acetone, cyclohexanol, 1,4-dioxane, methanol, and decalin. Hydrated hydronium ions formed in water at Brønsted acid sites (BAS), while solvents interact with protonated cyclohexanol. Kinetic H/D isotope effects (KIE) revealed solvent-dependent rate-determining steps of C–O cleavage (KIE < 2) in acetone, 1,4-dioxane, and decalin, while C–H cleavage (KIE > 2) dominates in water and methanol. Ab initio molecular dynamics simulations showed that acetone induces dissociation of protonated cyclohexanol from BAS to form solvated cyclohexanol-acetone complexes, whereas 1,4-dioxane and decalin leave the alcohol bound to the framework, providing a molecular-level rationale for higher dehydration rates in acetone. Solvation in acetone, 1,4-dioxane, and methanol destabilizes the initial state (excess chemical potential) by 11–13 kJ/mol, compared to 6–7 kJ/mol in water and decalin. The excess chemical potential of the transition state correlates well with the free-energy barrier of the rate-determining step. The excess chemical potential of C–O cleavage transition states in acetone, 1,4-dioxane, and decalin shows greater stabilization (−21 to −11 kJ/mol) than C–H cleavage in water and methanol (−5 to −1 kJ/mol). The combined initial-state and transition-state (de)stabilization yields reaction rates up to 3 orders of magnitude higher in acetone compared to water and decalin.