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 intrapore H2O molecules and distributions of H2O oligomers depend upon zeolite topology (MFI, BEA, and FAU) and the density of SiOH nests. We show that the highly correlated motion of H2O, CH3OH, or γ-butyrolactone within microporous environments leads to solvation effects that change turnover rates by many orders of magnitude. Turnover rates for 1-alkene (C6-C18) epoxidation are much greater in Ti-zeolite catalysts that contain significant densities of hydrogen-bonded SiOH than within their defect-free counterparts. These changes in rates and selectivities with SiOH reflect differences in the structure of H2O and other solvent molecules within the microporous voids. 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 (ΔHε,‡) and entropies (ΔSε,‡) 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. Calorimetry reveals similar variations in heats of adsorption, which reflect solvent reorganization induced by 1,2-epoxyalkane adsorption to active sites. These excess contributions scale with the size of transition states and the quantity of solvent disrupted These relationships demonstrate that interactions among intraporous hydrogen-bonded solvent molecules that couple short-range covalent bonding that drive adsorption and catalysis with longer range solvent restructuring.

Catalysis within slippery solvents in small spaces

ACS Fall 2021

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