(641h) Effects of Intramolecular Forces & Solvent Mixtures on Epoxidations in Ti-Zeolites

Partially replacing organic solvents with water can lower costs and environmental impact of liquid-phase catalytic reactions. Silanol defects ((SiOH)_x) in zeotype materials stabilize water oligomers within aqueous solvent mixtures. The disruption of these oligomers produces excess free energy contributions that increase rates and selectivities for alkene epoxidations. Here, we show that confined cosolvent structures within Ti-BEA zeolites differ from those in the bulk liquid phase, thereby providing opportunities to stabilize desired transition states.

As the mole fraction of water increases from 0 to 0.9 in acetonitrile, gamma-butyrolactone (GBL), and methanol cosolvents, turnover rates for 1-hexene (C₆H₁₂)epoxidation over Ti-BEA reach maximums that are 2.5, 8, and 20 times greater than with the corresponding neat solvents. Within hydrophobic Ti-BEA-F samples that exclude water from pores, turnover rates increase monotonically with increasing [H₂O], likely due to increases in the thermodynamic activity of liquid phase C₆H₁₂ (from interactions with H₂O­ molecules). In contrast, Ti-BEA-OH stabilizes water within cosolvent structures near Ti active sites. Consequently, turnover rates exhibit complex dependencies on [H₂O] due to changes in the stability of liquid phase reactants and reactive species bound to Ti-atoms. As [H₂O] increases, activation enthalpies (ΔH^‡) for epoxidation change monotonically over Ti-BEA-F but non-monotonically over Ti-BEA-OH for each cosolvent. The liquid phase does not depend on zeolite choice, so ΔH^‡ differences result from changes in the stability of reactive species within the pores, suggesting that solvent molecules arrange differently in Ti-BEA-OH and Ti-BEA-F. Corresponding measurements of 1,2-epoxyhexane adsorption enthalpies with isothermal titration calorimetry correlate to ΔH^‡ measurements, providing further evidence that [H₂O] affects the stability of the epoxidation transition state. Collectively, these findings show that adding water to organic solvents provides opportunities to increase turnover rates for desired reaction pathways while also reducing organic solvent usage.

We gratefully acknowledge support from the Department of Energy (DE-SC0020224).