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Dynamic Reorganization and Confinement of Tiᴵⱽ Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes
- Grosso-Giordano, Nicolás A., Hoffman, Adam S., Boubnov, Alexey, Small, David W., Bare, Simon R., Zones, Stacey I., Katz, Alexander
- Journal of the American Chemical Society 2019 v.141 no.17 pp. 7090-7106
- active sites, catalysts, catalytic activity, cyclohexenes, density functional theory, enthalpy, epoxidation reactions, micropores, olefin, silicates, topology
- The effect of dynamic reorganization and confinement of isolated Tiᴵⱽ catalytic centers supported on silicates is investigated for olefin epoxidation. Active sites consist of grafted single-site calixarene–Tiᴵⱽ centers or their calcined counterparts. Their location is synthetically controlled to be either unconfined at terminal T-atom positions (denoted as type-(i)) or within confining 12-MR pockets (denoted as type-(ii); diameter ∼7 Å, volume ∼185 Å³) composed of hemispherical cavities on the external surface of zeotypes with *-SVY topology. Electronic structure calculations (density functional theory) indicate that active sites consist of cooperative assemblies of Tiᴵⱽ centers and silanols. When active sites are located at unconfined type-(i) environments, the rate constants for cyclohexene epoxidation (323 K, 0.05 mM Tiᴵⱽ, 160 mM cyclohexene, 24 mM tert-butyl hydroperoxide) are 9 ± 2 M–² s–¹; whereas within confining type-(ii) 12-MR pockets, there is a ∼5-fold enhancement to 48 ± 8 M–² s–¹. When a mixture of both environments is initially present in the catalyst resting state, the rate constants reflect confining environments exclusively (40 ± 11 M–² s–¹), indicating that dynamic reorganization processes lead to the preferential location of active sites within 12-MR pockets. While activation enthalpies are ΔH‡ₐₚₚ = 43 ± 1 kJ mol–¹ irrespective of active site location, confining environments exhibit diminished entropic barriers (ΔS‡ₐₚₚ = −68 J mol–¹ K–¹ for unconfined type-(i) vs −56 J mol–¹ K–¹ for confining type-(ii)), indicating that confinement leads to more facile association of reactants at active sites to form transition state structures (volume ∼ 225 Å³). These results open new opportunities for controlling reactivity on surfaces through partial confinement on shallow external-surface pockets, which are accessible to molecules that are too bulky to benefit from traditional confinement within micropores.