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

The effect of dynamic reorganization and confinement of isolated Ti-IV catalytic centers supported on silicates is investigated for olefin epoxidation. Active sites consist of grafted single-site calix[4]arene-Ti-IV 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 similar to 7 angstrom , volume similar to 185 angstrom(3)) 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-IV 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-IV, 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 Delta H-app(double dagger) = 43 +/- 1 kJ mol(-1) irrespective of active site location, confining environments exhibit diminished entropic barriers (Delta S(double dagger)a(pp) = -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 similar to 225 angstrom(3)). These results open new opportunities for controlling reactivity on surfaces through partial confinement on shallow externalsurface pockets, which are accessible to molecules that are too bulky to benefit from traditional confinement within micropores.