Abstract
Solvent molecules within zeolite pores provide interactions that influence the stability of reactive intermediates and impact rates and selectivities for catalytic reactions. We show the kinetic and thermodynamic consequences of these interactions and reveal their origins using alkene epoxidations in titanium-substituted *BEA (Ti-BEA) zeolites. Epoxidation turnover rates vary widely among primary n-alkenes (C6–C18) in hydrophilic (Ti-BEA-OH) and hydrophobic (Ti-BEA-F) catalysts in aqueous acetonitrile (CH3CN). Apparent activation enthalpies (ΔHapp‡) and entropies (ΔSapp‡) increase with alkene carbon number in both catalysts; however, the span of ΔHapp‡ values in Ti-BEA-OH (68 kJ mol–1) greatly exceeds that in Ti-BEA-F (18 kJ mol–1). These trends, and commensurate gains in ΔSapp‡, reflect the displacement and reorganization of solvent molecules that scale with the size of transition states and the numbers of solvent molecules stabilized by silanol defects near active sites. Experimental and computational assessments of intrapore solvent composition from 1H NMR, infrared spectroscopy, and grand canonical molecular dynamics (GCMD) simulations show that Ti-BEA-OH uptakes larger quantities of both CH3CN and H2O than Ti-BEA-F. The Born–Haber decomposition of simulated enthalpies of adsorption (ΔHads,epox) for C6–C18 epoxides attributes ΔHads,epox that become more endothermic for larger adsorbates to the displacement of greater numbers of solvent molecules bound to silanol defects into the bulk solvent. A strong correlation between ΔHapp‡ and ΔHads,epox (from GCMD and isothermal titration calorimetry) gives evidence that the disruption of solvent structures provides excess thermodynamic contributions (e.g., Gε) that depend on the solvent composition in the pores, the excluded volume of reactive species, and the density of silanol groups near active sites. Altering Gε values offers opportunities to control selectivities and rates of reactions through the design of extended active site environments.
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