(101g) Impact of Zeolite Framework and Solvent Molecules on Vapor-Phase Propylene Epoxidation with Gaseous H2O2
AIChE Annual Meeting
2022
2022 Annual Meeting
Catalysis and Reaction Engineering Division
Microporous and Mesoporous Materials III: Structure
Monday, November 14, 2022 - 2:18pm to 2:36pm
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.