Molecular interactions at the solid-liquid interface greatly influence the stability of surface-bound intermediates, which has enormous implications for catalysis and adsorption processes; yet, quantitative relationships that describe the influence of proximate confined solvent molecules are not known. Within micropores, solvent molecules assemble into structures that must reorganize to accommodate surface intermediates that form during adsorption and catalysis, which contributes to changes in the free energy of adsorption, reaction, and activation. Here, we compare the rates of 1-octene (C8H16) epoxidation with H2O2 in CH3CN over Ti-substituted zeolite BEA with varying densities of SiOH nests ([(SiOH)4]; 0 – 5 (unit cell)-1) to develop quantitative relationships that describe how confined solvent structures influence the stability of catalytically-relevant surface species. Turnover rates for C8H16 epoxidation with aqueous H2O2 are greater by a factor of 100 on Ti-BEA that contain ~5 (SiOH)4 (unit cell)-1 than pristine Ti-BEA (i.e., ~0 (SiOH)4 (unit cell)-1) in aqueous acetonitrile (39 mM H2O). Among these Ti-BEA, the catalytic differences are not due to differences in the “Lewis acid strength” of the active site, the electronic structure of the active Ti-OOH intermediates, or the mechanism for epoxidation, all of which are indistinguishable among this series of catalysts, shown by heats of 1-octene oxide (C8H16O) adsorption, in situ UV-vis, and kinetic measurements, respectively. Comparisons of apparent activation enthalpies (ΔHE,App‡) and entropies (ΔSE,App‡) for C8H16 epoxidation show that the catalytic differences among Ti-BEA with varying [(SiOH)4] primarily reflect large increases in the excess entropy of the epoxidation transition states that manifest in the disruption of nearby hydrogen-bonded H2O clusters that nucleate at (SiOH)4. These interpretations are supported by liquid-phase calorimetric measurements that show the adsorption enthalpies (ΔHE,Ads) and entropies (ΔSE,Ads) for C8H16O increase linearly with ΔHE,App‡ and ΔSE,App‡, respectively, across all Ti-BEA. These observations and interpretations show that the presence of hydrophobic surface species disrupt hydrogen bonds within proximate solvent structures, which can significantly impact catalysis at the liquid-solid interface.

Interactions between surface species and confined solvent structures within Lewis acid zeolites: (De)stabilization of catalytically-relevant intermediates

2019 Fall National Meeting

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