As an environmentally benign oxidation process, hydrogen peroxide to propylene oxide (HPPO) has been industrially utilized. Interactions between solvents, reactive species, and active sites inside confined microporous structures have been shown to influence catalytic rates and selectivities, but a fundamental understanding of these interactions remains elusive. Here, we demonstrate that an intentional addition of gaseous acetonitrile (CH3CN) solvent within pores of Ti-incorporated zeolite materials during alkene epoxidation (e.g., propylene (C3H6) and 1-hexene (C6H12)) leads to significant changes in epoxidation rates and activation barriers. These changes depend on the silanol ((SiOH)x) densities and zeolite pore diameters, reflecting the excess contributions of solvent molecules nearby active sites that reorganize during the reaction. Hydrothermal syntheses and post-synthetic modifications of commercial Al-incorporated zeolites lead to various Ti-zeolite materials (Ti-MFI, Ti-BEA, and Ti-FAU) with controlled (SiOH)x densities. In situ FT-IR absorbance spectra of CH3CN adsorption reveal that the number of intrapore solvent molecules varies by a factor of 10 over zeolite materials with different (SiOH)x densities. Turnover rates of C6H12 epoxidation show a 30-fold increase with higher intrapore [CH3CN] over (SiOH)x-dense Ti-BEA, relative to defect-free Ti-BEA with low [CH3CN], implying that pore-solvent interactions influence epoxidation kinetics. Apparent activation enthalpies (ΔH‡) and entropies (ΔS‡) increase by 7 kJ/mol and 40 J/mol K, respectively, by increasing [CH3CN]. These changes in activation barriers demonstrate that the solvent molecules nearby active sites must reorganize to accommodate the formation of the epoxidation transition state, leading to entropic gains that increase rates. In situ FT-IR spectra provide evidence of hydrogen bond disruption and displacement of CH3CN molecules in the pore during epoxide adsorption. We are currently extending this study to C3H6 epoxidation with varying zeolite materials, demonstrating that the pore-solvent interaction affects overall alkene epoxidation reactions. These results will help explain the interactions between reactants, solvents, and pore functions, which significantly affect the rates and selectivities on catalysis, and provide insights for understanding the catalytic system of alkene epoxidations with vaporized H2O2 over confined microporous zeolite catalysts.

Effects of zeolite pore-solvent interaction on gas-phase alkene epoxidation kinetics with vaporized H2O2

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