Developing two-dimensional solid superacids with enhanced mass transport, extremely high acid strength and superior catalytic performance

Solid acids have been widely used as heterogeneous catalysts in developing green and sustainable chemistry. However, it remains a challenge to improve the mass transport properties and acid strength of solid acids simultaneously. Herein, we report a class of two dimensional (2D) layered hybrid solid acids with outstanding mass transfer and extremely high acid strength by incorporating sulfonated polymers in-between montmorillonite layers. The 2D layered structure and broad distribution of pore sizes allow for highly efficient mass transport of substrate molecules into and out of the solid acids. The acid strength of these solid acids was found to be stronger than that of 100% H₂SO₄, H₃PW₁₂O₄₀ and any other reported solid acids to date, as determined by ¹H and ³¹P solid-state NMR. These 2D solid acids show extraordinary catalytic performance in biomass conversion to fuels, superior to that of H₃PW₁₂O₄₀, HCl and H₂SO₄. Theoretical calculations and control experiments reveal that H-bond based interactions between the polymer and montmorillonite facilitate the unusually high acid strengths found in these samples.

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 ∼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 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 Δ H^‡_app = 43 ± 1 kJ mol^-1 irrespective of active site location, confining environments exhibit diminished entropic barriers (Δ S^‡_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.