(119b) Catalysis in Tight Spaces: Confined Solvent Structures Influence Stability of Surface Intermediates during Alkene Epoxidation within Lewis Acid Zeolites

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. Moreover, these interactions become increasingly complex within microporous materials (e.g., zeolites), where the characteristic dimensions of the pores (e.g., <1 nm) approach those of the molecules that react within, because solvent structures that form do not possess the same chemical or physical properties as the bulk. Within voids of molecular dimensions, 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 acetonitrile (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.

The rates of alkene epoxidation are highly dependent on [(SiOH)₄], solvent polarity (e.g., CH₃CN versus CH₃OH), presence of H₂O, and zeolite framework type. For example, 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 low-coverage liquid-phase heats of 1-octene oxide adsorption (C₈H₁₆O; anhydrous), in situ UV-vis and radical clock experiments, and kinetic measurements, respectively. Comparisons of apparent activation enthalpies (ΔH_E,App^‡) and entropies (ΔS_E,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 (ΔH_E,Ads) and entropies (ΔS_E,Ads) for C₈H₁₆O increase linearly with ΔH_E,App^‡ and ΔS_E,App^‡, respectively, across all Ti-BEA. Additionally, infrared spectra of H₂O within Ti-BEA show a significant blue shift in ν(O-H) with a concomitant red shift in the combination libration+bending mode over Ti-BEA that contains ~5 (SiOH)₄(unit cell)^-1 in the presence of C₈H₁₆O, which suggests that hydrophobic surface species disrupt hydrogen bonding in proximate H₂O near active sites. In contrast, within Ti-BEA containing ~0 (SiOH)₄ (unit cell)^-1, there is no change in ν(O-H) or the combination band within intraporous water in the presence of 1-octene oxide, because these materials do not nucleate H₂O clusters near active sites that are influenced by interactions with surface species.

These observations and interpretations show within one combination of zeolite framework (i.e., BEA), solvent (i.e., CH₃CN), and oxidant (i.e., aqueous H₂O₂) that the presence of hydrophobic surface species disrupt hydrogen bonds in proximate solvent structures, which significantly influences the stability of these catalytically-relevant species. However, these observations may not be ubiquitous, as changing the identity of the zeolite framework, solvent, and oxidant each greatly affect the stability of critical surface intermediates. On-going work seeks to combine kinetic, thermodynamic, and spectroscopic measurements to probe the interactions between confined solvent molecules (e.g., CH₃CN, CH₃OH, in the presence and absence of H₂O) and surface species that are present (e.g., Ti-OOR; R = H, t-Bu, cumyl) during the catalytic cycle.