(101g) Impact of Zeolite Framework and Solvent Molecules on Vapor-Phase Propylene Epoxidation with Gaseous H2O2

Ti-incorporated zeolites exhibit high rates and selectivities for liquid-phase propylene (C₃H₆) epoxidation with solvents (e.g., methanol (CH₃OH) and acetonitrile (CH₃CN)). However, the effect of interactions between reactants and partially condensed solvents inside confined zeolites on catalytic rates has not been extensively studied. Here, we demonstrate that changing Ti-zeolite frameworks and the solvent density (controlled by capillary condensation of gaseous solvents) induce concomitant changes in vapor-phase C₃H₆ epoxidation rates and activation barriers. These changes depend on the zeolite pore diameters and pore silanol ((SiOH)_x) densities.

We synthesized Ti-zeolite catalysts with varying pore diameters (Ti-MFI, Ti-MWW, Ti-BEA, and Ti-FAU) and (SiOH)_x densities. Infrared spectra reveal that CH₃CN solvent uptake increases by 10 times with pore (SiOH)_x densities. Turnover rates of C₃H₆ epoxidation over (SiOH)_x-dense Ti-zeolites are higher by 2 to 100 times than each defect-free counterpart. Rates among hydrophilic Ti-zeolites increase as a function of pore diameter across Ti-MFI (0.56 nm) to Ti-BEA (0.65 nm), then decrease significantly in Ti-FAU (1.2 nm), while rates over hydrophobic catalysts weakly depend on the pore diameters. Apparent activation enthalpies and entropies increase systematically with the pore diameter in hydrophilic pores, which reflect excess contributions in large pores caused by solvent-(SiOH)_x interactions during the formation of the epoxidation transition states. Infrared spectra show that epoxide adsorption leads to the displacement of intrapore solvent molecules and the disruption of hydrogen bonds between solvent and (SiOH)_x that are more significant in hydrophilic pores. These results demonstrate that the partial displacement of intrapore solvent molecules in hydrophilic pores by transition states for C₃H₆ epoxidation provides an excess entropic benefit responsible for increased rates. Collectively, these comparisons offer insight to the influence of zeolite pore-solvent interactions on C₃H₆ epoxidation and shows these effects depend on zeolite topology, (SiOH)_x densities, and proximity of solvents to active sites.