Hydrophobic voids within titanium silicates have long been considered necessary to achieve high rates and selectivities for alkene epoxidations with H2O2. The catalytic consequences of silanol groups and their stabilization of hydrogen-bonded networks of water (H2O), however, have not been demonstrated in ways that lead to a clear understanding of their importance. We compare turnover rates for 1-octene epoxidation and H2O2 decomposition over a series of Ti-substituted zeolite *BEA (Ti-BEA) that encompasses a wide range of densities of silanol nests ((SiOH)4). The most hydrophilic Ti-BEA gives epoxidation turnover rates that are 100 times larger than those in defect-free Ti-BEA, yet rates of H2O2 decomposition are similar for all (SiOH)4 densities. These differences cause the most hydrophilic Ti-BEA to also give the highest selectivities, which defies conventional wisdom. Spectroscopic, thermodynamic, and kinetic evidence indicate that these catalytic differences are not due to changes in the electronic affinity of the active site, the electronic structure of Ti–OOH intermediates, or the mechanism for epoxidation. Comparisons of apparent activation enthalpies and entropies show that differences in epoxidation rates and selectivities reflect favorable entropy gains produced when epoxidation transition states disrupt hydrogen-bonded H2O clusters anchored to (SiOH)4 near active sites. Transition states for H2O2 decomposition hydrogen bond with H2O in ways similar to Ti–OOH reactive species, such that decomposition becomes insensitive to the presence of (SiOH)4. Collectively, these findings clarify how molecular interactions between reactive species, hydrogen-bonded solvent networks, and polar surfaces can influence rates and selectivities for epoxidation (and other reactions) in zeolite catalysts.

Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO 2 ) irreversibly activate hydrogen peroxide (H 2 O 2 ) to form pools of metal-hydroperoxide (M-OOH) and peroxide (M-(η 2 -O 2 )) species for alkene epoxidation. The product distributions from reactions with Zstilbene, in combination with time-resolved UV−vis spectra of the reaction between H 2 O 2 -activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η 2 -O 2 ) moieties epoxidize substrates on the Nb-and Ta-containing materials. Kinetic measurements of styrene (C 8 H 8 ) epoxidation reveal that these materials first adsorb and then irreversibly activate H 2 O 2 to form pools of interconverting M-OOH and M-(η 2 -O 2 ) intermediates, which then react with styrene or H 2 O 2 to form either styrene oxide or H 2 O 2 decomposition products, respectively. Activation enthalpies (ΔH ⧧ ) for C 8 H 8 epoxidation and H 2 O 2 decomposition decrease linearly with increasing heats of adsorption for pyridine or deuterated acetonitrile coordinated to Lewis acid sites, which suggests that materials with greater electron affinities (i.e., stronger Lewis acids) are more active for C 8 H 8 epoxidation. Values of ΔH ⧧ for C 8 H 8 epoxidation and H 2 O 2 decomposition also decrease linearly with the ligand-to-metal charge-transfer (LMCT) band energies for the reactive intermediates, which is a more relevant measure of the requirements for the active sites in these catalytic cycles. Epoxidation rates depend more strongly on the LMCT band energy than H 2 O 2 decomposition rates, which shows that more electrophilic M-OOH and M-(η 2 -O 2 ) species (i.e., those formed at stronger Lewis acid sites) give both greater rates and greater selectivities for epoxidations. Thermochemical analysis of ΔH ⧧ for C 8 H 8 epoxidation and adsorption enthalpies for C 8 H 8 within the pores of *BEA and SiO 2 reveal that the 0.7 nm pores within M-BEA preferentially stabilize transition states for C 8 H 8 epoxidation with respect to the 5.4 nm pores of M-SiO 2 , while H 2 O 2 decomposition is unaffected by the differences between these pore diameters due to the small Stokes diameter of H 2 O 2 . Thus, the differences in reactivity and selectivity between M-BEA and M-SiO 2 materials is solely attributed to confinement of the transition state and not differences in the identity of the reactive intermediates, mechanism for alkene epoxidation, or intrinsic activation barriers. Consequently, the rates and selectivities for alkene epoxidation reflect at least two orthogonal catalyst design criteriathe electronegativities of the transition metal atoms that determine the electronic structure of the active complex and the mean diameters of the surrounding pores that can selectively stabilize transition states for specific reaction pathways.

Group IV and V framework-substituted zeolites have been used for olefin epoxidation reactions for decades, yet the underlying properties that determine the selectivities and turnover rates of these catalysts have not yet been elucidated. Here, a combination of kinetic, thermodynamic, and in situ spectroscopic measurements show that when group IV (i.e., Ti, Zr, and Hf) or V (i.e., Nb and Ta) transition metals are substituted into zeolite *BEA, the metals that form stronger Lewis acids give greater selectivities and rates for the desired epoxidation pathway and present smaller enthalpic barriers for both epoxidation and HO decomposition reactions. In situ UV-vis spectroscopy shows that these group IV and V materials activate HO to form pools of hydroperoxide, peroxide, and superoxide intermediates. Time-resolved UV-vis measurements and the isomeric distributions of Z-stilbene epoxidation products demonstrate that the active species for epoxidations on group IV and V transition metals are only M-OOH/-(O) and M-(O) species, respectively. Mechanistic interpretations of kinetic data suggest that these group IV and V materials catalyze cyclohexene epoxidation and HO decomposition through largely identical Eley-Rideal mechanisms that involve the irreversible activation of coordinated HO followed by reaction with an olefin or HO. Epoxidation rates and selectivities vary over five- and two-orders of magnitude, respectively, among these catalysts and depend exponentially on the energy for ligand-to-metal charge transfer (LMCT) and the functional Lewis acid strength of the metal centers. Together, these observations show that more electrophilic active-oxygen species (i.e., lower-energy LMCT) are more reactive and selective for epoxidations of electron-rich olefins and explain why Ti-based catalysts have been identified as the most active among early transition metals for these reactions. Further, HO decomposition (the undesirable reaction pathway) possesses a weaker dependence on Lewis acidity than epoxidation, which suggests that the design of catalysts with increased Lewis acid strength will simultaneously increase the reactivity and selectivity of olefin epoxidation.

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.

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