Transition state stabilization depends on solvent identity, pore size, and hydrophilicity for epoxidations in zeolites

Abstract

Ti-silicates activate H2O2 to form Ti-hydroperoxo and Ti-peroxo intermediates that can react with alkenes to form epoxide products. Comparisons of kinetics for 1-octene epoxidation with H2O2 on Ti-BEA and Ti-MFI catalysts with different hydrophilicities in methanol (CH3OH) or acetonitrile (CH3CN) solvents show the significance of the solvent for stabilizing catalytically-relevant species and the complex interdependencies between solvent, catalyst topology, and hydrophilicity. Epoxidation turnover rates are higher in CH3CN than CH3OH for Ti-BEA, but the opposite trend is observed for Ti-MFI. Ti-silicates with greater silanol densities, however, give greater epoxidation turnover rates than their hydrophobic counterparts in both solvents. Kinetic, spectroscopic, and thermodynamic analyses show that differences in turnover rates mainly arise from changes in the stabilization of reactive surface species by solvent mediated interactions, because the mechanism of the reaction and stability of the fluid-phase reactants remain similar in CH3CN and CH3OH. Specifically, apparent activation free energy values (ΔGApp‡) indicate that surface intermediates responsible for alkene epoxidation are stabilized to a greater extent in CH3CN on Ti-BEA and in CH3OH on Ti-MFI. Hydrophilic Ti-silicates present lower ΔGApp‡ values regardless of solvent identity, which suggests that these differences correspond to the number of hydrogen-bonding solvent molecules found near reactive species bound to Ti active sites. Taken together, these findings demonstrate the role of solvent molecules in allowing reactive intermediates to recognize the properties of active sites beyond the length-scale of covalent bonds, which carry implications for epoxidation but also other reactions within solvent-filled pores of microporous materials.