(349u) Impact of Interactions between Alkyl Chains and Water Networks on Lewis-Acid Catalyzed Epoxidations

Solvent choice affects catalytic reactions within zeolite pores because the pore dimensions require significant changes in solvent structure during the reaction. Hydrophilic zeolites contain silanol defects ((SiOH)_x) that stabilize hydrogen-bonded networks of water molecules. The disruption of water molecules produces excess free energy contributions that increase rates and selectivities for alkene epoxidations. Here, we demonstrate that the silanol density of Ti-BEA pores and alkene chain length affect the degree of solvent disruption needed during epoxidation, thereby altering transition state stability.

Over hydrophobic Ti-BEA-F materials that do not stabilize water networks, epoxidation turnover rates decrease monotonically as chain length increases, with 1-hexene (C₆H₁₂) giving rates 20 times greater than 1-octadecene (C₁₈H₃₆). In contrast, rates over hydrophilic Ti-BEA-OH reach a maximum for C₁₀H₂₀, then decrease by 10 times to C₁₈H₃₆. The rate differences likely do not result from changes in reaction mechanism or mass transfer constraints, but rather changes in transition state stability as quantified by activation enthalpies (ΔH^‡) and entropies (ΔS^‡). ΔH^‡ and ΔS^‡ decrease slightly from C₆H₁₂ to C₁₀H₂₀ over Ti-BEA-F, with respective differences of 3 kJ mol^-1 and 10 J mol^-1 K^-1. However, ΔH^‡ and ΔS^‡ increase systematically with chain length over Ti-BEA-OH, with differences of 68 kJ mol^-1 and 209 J mol^-1 K^-1 between C₆H₁₂ and C₁₈H₃₆. ΔH^‡ and ΔS^‡ likely increase because longer alkyl chains must disrupt more H₂O molecules within Ti-BEA-OH. Corresponding epoxide adsorption enthalpies, measured with isothermal titration calorimetry, increase as chain length increases and correlate to ΔH^‡ measurements over Ti-BEA-OH, providing further evidence that longer alkyl chains disrupt more hydrogen bonds to alter transition state stability. Collectively, these findings show that the entropic gain per enthalpic cost of hydrogen bond disruption varies with alkene chain length and leads to changes in epoxidation rates.

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