(173v) Role of Surface-Solvent Interactions on Alkene Epoxidation Catalysis within Ti-MFI

Intraporous hydrogen bonded solvent networks stabilize reactive intermediates for 1-hexene (C₆H₁₂) epoxidation with hydrogen peroxide (H₂O₂) within Ti-MFI catalysts. Turnover rates for C₆H₁₂ epoxidation increase by two orders of magnitude as Ti-MFI silanol densities and water concentrations ([H₂O]) increase in methanol solvent under otherwise identical reaction conditions (Figure 1a). These differences do not reflect changes in the reaction mechanism, because turnover rates depend similarly on [C₆H₁₂] and [H₂O₂] at high and low [H­₂O]. Hence, kinetics reflect differences between solvent molecule densities (Figure 1b) and the associated hydrogen bond networks surrounding reactive species bound to Ti-atoms. Apparent activation enthalpy measurements (ΔH_epox^‡) report on specific interactions among surfaces of pores, solvent molecules, and epoxidation transition states (Figure 1c). Values of ΔH_epox^‡ are similar between hydrophilic, silanol dense Ti-MFI-OH and hydrophobic, silanol free Ti-MFI-F catalysts (60 kJ mol^-1) at very low [H₂O] (<5 mM) but change non-monotonically (20 – 90 kJ mol^-1) at higher values (0.05 – 1 M). Pendant silanols form near Ti upon H₂O_{2 ­}activation and may provide similar reaction environments for epoxidation without water present, despite differences in silanol densities. Differences among small [H₂O] lead to a 40 kJ mol^-1 decrease in ΔH_epox^‡ over Ti-MFI-OH (but not over Ti-MFI-F materials) and likely signifies stabilization of transition states by extended hydrogen bond networks of water that nucleate at silanols and produce high intraporous solvent densities. Compensating changes in apparent activation entropies with changes in ΔH_epox^‡ reflect the disruption of hydrogen bond networks by the epoxide transition state within Ti-MFI micropores, which leads to turnover rate increases with [H₂O] and silanol densities. These comparisons imply that intermolecular interactions impact epoxidation catalysis through excess enthalpy and entropy contributions at confined interfaces and show how these factors may be manipulated to change rates by controlling solvent composition and framework silanol densities.