Catalytic reactions at solid-liquid interfaces involve complex networks of adsorption, surface reaction, and desorption processes, where each possesses a free energy of reaction and activation that contributes to measured rates and selectivities. The ways in which these free energies depend on the structure of solvating molecules and the topology of the surrounding voids is poorly understood. Here, we demonstrate how the numbers of hydrogen-bonds among intraporous H₂O molecules and distributions of H₂O oligomers depend upon zeolite topology (CDO, MFI, BEA, and FAU) and the density of SiOHnests. We show that the highly correlated motion of H₂O within microporous environments leads to solvation effects that change turnover rates by several orders of magnitude.

Turnover rates for 1-alkene (C₆, C₈, C₁₀) epoxidation are up to 400-fold greater in hydrophilic Ti-zeolites (i.e., which contain large densities of SiOH defects) than within their hydrophobic (i.e., nearly defect-free) counterparts. These changes in rates with SiOH reflect differences in the structure of H₂O molecules within the microporous voids of the Ti-zeolites. Infrared spectra of H₂O within these porous environments reveal distinct populations of H₂O in different hydrogen bonding configurations. These data, paired with MD simulations, suggest that H₂O molecules form bulk-like structures in FAU zeolites, but coalesce into oligomeric chains and clusters within BEA, MFI, and CDO zeolites.

Changes in the excess enthalpies (ΔH_excess^‡) and entropies (ΔS_excess^‡) measured via catalysis strongly correlate with measures of the enthalpic cost and entropic gains of breaking hydrogen bonds within pure water obtained by in situ infrared spectroscopy. These relationships demonstrate that interactions among H₂O molecules within the solvent couple short-range covalent interactions that drive adsorption and catalysis with longer range solvent restructuring. Here, H₂O is responsible for these effects, which suggests judicious choices of zeolite framework and silanol density may be used to increase rates and selectivities for many other catalytic reactions or manipulate adsorption processes, provided that the solvent contains molecules with a proclivity to form hydrogen bonds.

Jun Tan

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