Dynamic Reorganization and Confinement of Ti IV Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes

• Published in: Journal of the American Chemical Society Vol. 141; no. 17; pp. 7090 - 7106
• Main Authors: Grosso-Giordano, Nicolás A, Hoffman, Adam S, Boubnov, Alexey, Small, David W, Bare, Simon R, Zones, Stacey I, Katz, Alexander
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society (ACS) 01.05.2019
• Subjects: INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.9b02160

The effect of dynamic reorganization and confinement of isolated Ti catalytic centers supported on silicates is investigated for olefin epoxidation. Active sites consist of grafted single-site calix[4]arene-Ti 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 Å ) 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 Ti centers and silanols. When active sites are located at unconfined type-(i) environments, the rate constants for cyclohexene epoxidation (323 K, 0.05 mM Ti , 160 mM cyclohexene, 24 mM tert-butyl hydroperoxide) are 9 ± 2 M s ; whereas within confining type-(ii) 12-MR pockets, there is a ∼5-fold enhancement to 48 ± 8 M s . When a mixture of both environments is initially present in the catalyst resting state, the rate constants reflect confining environments exclusively (40 ± 11 M s ), indicating that dynamic reorganization processes lead to the preferential location of active sites within 12-MR pockets. While activation enthalpies are Δ H = 43 ± 1 kJ mol irrespective of active site location, confining environments exhibit diminished entropic barriers (Δ S = -68 J mol K for unconfined type-(i) vs -56 J mol K for confining type-(ii)), indicating that confinement leads to more facile association of reactants at active sites to form transition state structures (volume ∼ 225 Å ). 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.