(521cs) Effect of Condensed Solvent and Its Organization on Vapor-Phase Alkene Epoxidation with H2O2 over Ti-Zeolites

As a successful synthesis method of epoxides, hydrogen peroxide (H₂O₂)-propylene oxide (HPPO) process has gathered interest utilizing Ti-incorporated zeolites and organic solvents. However, solvent structures and interactions between reactive species that affect epoxidation chemistry within confined pores have not been extensively understood. Here, we offer an insight into the role of solvent and its organization on the vapor-phase alkene epoxidation by demonstrating that saturating micropores with gaseous acetonitrile (CH₃CN) leads to significant increases in catalytic rates and selectivities through noncovalent interactions.

We conducted 1-hexene (C₆H₁₂) epoxidation with vaporized H₂O₂ over Ti-BEA with varying intrapore CH₃CN densities in the proximity of active sites. Combined analyses of in situ infrared spectroscopy and dynamic vapor sorption show that zeolite pores spontaneously condense gaseous CH₃CN in the proximity of active sites, and its density increases with pore silanol ((SiOH)_x) densities (“hydrophilicity”) and partial pressures. Turnover rates of C₆H₁₂ epoxidation and the primary product (1,2-epoxyhexane, C₆H₁₂O) selectivity increase monotonically by 20-fold and 2-fold as intrapore CH₃CN density increases, respectively. Changes in rate and selectivity respond more sensitively to CH₃CN within hydrophilic pores than hydrophobic counterparts in a given reaction condition. Apparent activation enthalpy and entropy increase by 11 kJ∙mol^-1 and 48 J∙mol^-1∙K^-1 as a single value function of CH₃CN density (0.4-10 molecules∙(unit cell)^-1) across all Ti-BEA catalysts with varying (SiOH)_x densities. This systematic increase reflects the entropic stabilization that leads to concomitant changes in rates and selectivities, due to the more significant reorganization of CH₃CN while accommodating transition states in CH₃CN-saturated pores than under solvent-free conditions. In situ infrared spectra corroborate the reorganized intrapore CH₃CN in the presence of products (C₆H₁₂O) adsorbed into the pores, and its magnitude depends on pore (SiOH)_x densities. These findings demonstrate the effects of condensed solvents on catalytic rates and selectivities, even without the presence of a saturated liquid-phase.