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

Abstract

Selective alkene oxidation rates within zeolite pores reflect differences in contributions from thermodynamic nonidealities introduced by noncovalent interactions at solid–liquid interfaces and spatial constraints enforced by the zeolite topology. Epoxidation turnover rates for 1-hexene in Ti-MFI zeolite differ by 1000-fold across 10 combinations of solvents, including alcohols and acetonitrile, over Ti-MFI with distinct silanol defect densities. Solvent mediated noncovalent interactions persist across an extended binding environment that influences the stability of epoxidation transition states. Apparent activation enthalpies (ΔHApp⧧) and entropies (ΔSApp⧧) 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-OH catalysts in methanol give the greatest turnover rates because entropic gains associated with the disruption of hydrogen-bonded networks of intrapore methanol and water dominate activation free energies. In situ infrared spectra show that the adsorption of 1,2-epoxyhexane to Ti sites disturbs the equilibrium solvent structure within pores in ways that reflect the density of the hydrogen bond donor and acceptors near active sites. This interpretation agrees with measured enthalpies for adsorption of 1,2-epoxyhexane to Ti active sites within solvent filled pores that correlate with ΔHApp⧧ across solvent and catalyst combinations. The displacement of solvent molecules and the associated rupture and formation of hydrogen bonds among solvents, pore walls, and nearby reactive molecules during catalysis and adsorption events incur substantial excess contributions that govern the reaction rates and barriers. These realizations explain the particular success of alkene epoxidations in mixtures of H2O2 and methanol over hydrophilic forms of Ti-MFI in industrial processes (e.g., the hydrogen peroxide–propylene oxide process). These findings demonstrate the gains achieved by appropriately pairing organic solvents with microporous catalysts to manipulate the intrinsic kinetics of reactions and the thermodynamics of adsorption at solid–liquid interfaces.