Main content area

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 calix[4]arene–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 ų) 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 ų). 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.