Confined spaces induce solvent nonidealities that depend on surface hydrophilicity and affect turnover rates for liquid phase catalysis. Hydrogen bonds (H-bonds) between organic solvent, water, and surfaces stabilize reactive intermediates and transition states within microporous Ti incorporated MFI zeolites. Here, we examine these effects through analysis of linear alkene epoxidation turnover rates with hydrogen peroxide (H2O2) in Ti-MFI zeolites over aqueous methanol (CH3OH) and inspection of infrared spectra of methanol within these materials. Initial epoxidation turnover rate measurements of C6-C10 linear alkenes span two orders of magnitude over Ti-MFI catalysts with high (-OH) and low (-F) densities of silanol defects ((SiOH)x). Turnover rates increase monotonically with alkene chain length over Ti-MFI-OH and are five to fifty times greater across all alkenes than over Ti-MFI-F. Turnover rates further increase by ten with increasing water concentration (0.039 M to 5 M H2O). These differences in epoxidation turnover rates with alkene chain length, water concentration, and (SiOH)x density signify differences in the stabilities of transition states and reactive intermediates caused by changes in the local solvent environment. Under similar kinetic regimes, activation enthalpies (ΔH‡) become more positive with larger alkenes due to the breaking and forming of solvent H-bonds with relatively larger epoxide transition states. Activation entropies (ΔS‡) strongly compensate for changes in ΔH‡ with larger epoxide transition states that enhance solvent reorganization and disrupt nearby molecules within the zeolite pore that coincides with turnover rate increases. Infrared spectra acquired while contacting catalysts with flowing gaseous CH3OH reveal liquid-like CH3OH features that differ between high and low (SiOH)x density catalysts. Hydroxyl stretch features exhibit spectral maxima at lower wavenumbers on Ti-MFI-OH than on Ti-MFI-F by 50 cm-1, indicating that present (SiOH)x defects increase the number of H-bonds among and between CH3OH molecules. Spectra show that CH3OH molecules are disrupted when epoxides are introduced and the magnitude of disruption increases with the number of methylene groups within the epoxide, which agrees with implications from the interpretation of turnover rate and kinetic measurements. These solvent-surface-kinetics relationships necessitate careful catalyst design principles for liquid phase reactions over microporous materials.

Kinetic consequences of solvent disruption during alkene epoxidation with hydrogen peroxide over Ti-MFI zeolites

Where thousands of chemistry professionals meet to share ideas and advance scientific and technical knowledge. Spring 2023 abstracts, posters, and presentations will launch at the end of March.

ACS Spring 2023

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.