(176f) Effect of Water Concentration on Rates and Selectivities of Alkene Epoxidations in Ti-BEA Zeolites

Partially replacing organic solvents with a co-solvent like water (H₂O) can increase rates of liquid-phase catalytic reactions. Silanol defects ((SiOH)_x) in zeolites stabilize H₂O clusters within aqueous solvent mixtures, the disruption of which produces excess free energy contributions that influence alkene epoxidations with hydrogen peroxide (H₂O₂). Here, we describe how and why replacing organic solvents with H₂O favors alkene epoxidations over the H₂O₂ decomposition pathway.

As the mole fraction of H₂O increases from 0.002 to 0.8 in acetonitrile, gamma-butyrolactone (GBL), and methanol co-solvents, turnover rates for 1-hexene (C₆H₁₂)epoxidation increase while H₂O₂ decomposition rates decrease. The inverse trends lead to significantly higher epoxidation selectivities at higher H₂O fractions. Rates were normalized by reactant activities to show how changes in the stability of the Ti-OOH MARI and transition state contribute to rate differences. As H₂O fraction increases, activation enthalpies for epoxidation (ΔH_epox^‡) change monotonically over Ti-BEA-F but non-monotonically over Ti-BEA-OH for each co-solvent. ΔH_epox^‡ differences result from changes in the stability of reactive species within the pores of Ti-BEA-OH and Ti-BEA-F. Corresponding measurements of 1,2-epoxyhexane (C₆H₁₂O) adsorption enthalpies with isothermal titration calorimetry correlate to ΔH_epox^‡ measurements, providing further evidence that [H₂O] affects epoxidation transition state stability. Activation enthalpies for H₂O₂ decomposition (ΔH_decomp^‡) show that adding H₂O leads to enthalpic stabilization of the transition state, but the corresponding stabilization of liquid-phase H₂O₂ suppresses decomposition. H₂O molecules stabilize the hydrophilic transition state for H₂O₂ decomposition through hydrogen bonding. In contrast, H₂O likely entropically stabilizes the hydrophobic epoxidation transition state, increasing ΔS_epox^‡ through the disruption of hydrogen bonds between H₂O. These results show that upon adding more H₂O, enthalpic effects suppress H₂O₂ decomposition while entropic effects facilitate epoxidation. Collectively, these findings show that adding H₂O to organic solvents provides opportunities to increase turnover rates and selectivities for desired reaction pathways.