Solvent Mediated Interactions on Alkene Epoxidations in Ti-MFI: Effects of Solvent Identity and Silanol Density

Selective alkene oxidation rates within zeolite poresreflect differencesin contributions from thermodynamic nonidealities introduced by noncovalentinteractions at solid-liquid interfaces and spatial constraintsenforced by the zeolite topology. Epoxidation turnover rates for 1-hexenein Ti-MFI zeolite differ by 1000-fold across 10 combinations of solvents,including alcohols and acetonitrile, over Ti-MFI with distinct silanoldefect densities. Solvent mediated noncovalent interactions persistacross an extended binding environment that influences the stabilityof epoxidation transition states. Apparent activation enthalpies (& UDelta;H (App) (?)) and entropies (& UDelta;S (App) (?)) span 80 kJ mol(-1) and 200 J mol(-1) K-1, respectively,with the most positive values for both appearing in methanol. Hydrophilic(i.e., (SiOH)( x ) defect rich) Ti-MFI-OHcatalysts in methanol give the greatest turnover rates because entropicgains associated with the disruption of hydrogen-bonded networks ofintrapore methanol and water dominate activation free energies. In situ infrared spectra show that the adsorption of 1,2-epoxyhexaneto Ti sites disturbs the equilibrium solvent structure within poresin ways that reflect the density of the hydrogen bond donor and acceptorsnear active sites. This interpretation agrees with measured enthalpiesfor adsorption of 1,2-epoxyhexane to Ti active sites within solventfilled pores that correlate with & UDelta;H (App) (?) across solvent and catalyst combinations. Thedisplacement of solvent molecules and the associated rupture and formationof hydrogen bonds among solvents, pore walls, and nearby reactivemolecules during catalysis and adsorption events incur substantialexcess contributions that govern the reaction rates and barriers.These realizations explain the particular success of alkene epoxidationsin mixtures of H2O2 and methanol over hydrophilicforms of Ti-MFI in industrial processes (e.g., the hydrogen peroxide-propyleneoxide process). These findings demonstrate the gains achieved by appropriatelypairing organic solvents with microporous catalysts to manipulatethe intrinsic kinetics of reactions and the thermodynamics of adsorptionat solid-liquid interfaces.