Fractal Features of the Catalytic Performances of Bimodal Mesoporous Silica-Supported Organocatalysts Derived from Bipyridine-Proline for Asymmetric Aldol Reaction

Abstract

The three bipyridine-proline chiral ligands (Z) with different structural units as highly active species were successfully introduced on Zn-modified bimodal mesoporous silicas (BMMs). The representative heterogeneous catalysts (ZZnBMMs-n) containing different molar ratios of Z/Zn (5, 20, 50, 100, 200%) were prepared, and their catalytic performances were evaluated in the asymmetric aldol reactions between p-nitrobenzaldehyde and cyclohexanone, showing the satisfactory activities (yields) and good stereoselectivity (dr and ee). Meanwhile, their microstructural features and physicochemical properties were characterized by various methods. Particularly, the fractal evolution, the pair distance distribution function, and the Porod plots of the ZZnBMMs catalysts were demonstrated by SAXS analysis. The results revealed that the interfacial layer thickness and their mass fractal (Dm) value gradually increased with the enhanced molar ratios of Z/Zn, along with the decrease of pore size and densification of volume, suggesting the successful Z-grafting into the mesoporous channels of Zn-modified matrixes. The essential relationships between the fractal features and their catalytic performance (yield, dr and ee) were explored, showing the larger the Dm values, the better catalytic performance. Besides, the uniform distributions of Zn and S elements in ZZnBMMs catalysts and the formation of enamine-imine intermediates during the asymmetric aldol reactions were confirmed.