Impact of Specific Interactions among Reactive Surface Intermediates and Confined Water on Epoxidation Catalysis and Adsorption in Lewis Acid Zeolites

Molecular interactions at solid-liquid interfaces greatly influence the stability of surface intermediates central to adsorption and catalysis. These complex interactions include the reorganization of solvent molecules near active sites to accommodate the formation of reactive surface intermediates. The consequences of these interactions and how they depend on the chemical functionality of the extended surface within pores have not been demonstrated in ways that permit the rational use of excess thermodynamic properties in the design of catalytic sites. Here, we show that adsorption enthalpies and entropies for 1,2-epoxyoctane (C₈H₁₆O) increase by 19 kJ mol^-1 and 75 J mol^-1 K^-1, respectively, when the density of silanol nests decrease from ∼5 to 0 (unit cell)^-1 within Ti-substituted zeolite BEA (Ti-BEA) in the presence of trace H₂O. In contrast, these properties are indistinguishable across all Ti-BEA samples under anhydrous conditions, which suggests that H₂O proximate to Ti adsorption sites interacts with bound C₈H₁₆O. In situ infrared spectra of hydrophilic Ti-BEA show that coordination of C₈H₁₆O to framework Ti-sites reduces the extent of hydrogen bonding with and among H₂O molecules, which is reflected by changes in the frequencies of O-H stretching modes and molecular librations. Adsorption of C₈H₁₆O into hydrophobic Ti-BEA, however, does not cause detectable changes in the vibrational spectra of nearby H₂O. The combination of these results, along with values of activation enthalpies and entropies for epoxidation reactions in the same materials, show that the disruption of hydrogen-bonded H₂O near Ti-atoms introduce excess free energies of adsorption that can be manipulated by controlling the number of solid- and liquid-phase hydrogen bond donors and acceptors at interfaces. These findings reveal the complex role of surface moieties on epoxidation reactions in Ti-silicates, show how silanol groups may impact other liquid-phase reactions within zeolites, and provide a basis to understand the manner by which surface chemistry impacts the structure of surrounding solvent molecules.