Fine-crystallized LDHs prepared with SiO2 spheres as highly active solid base catalysts

Abstract

Fine-crystallized layered double hydroxides are prepared via the co-precipitation method with the coexistence of SiO₂ spheres (SiO₂@LDH), and their base catalysis and structural properties are investigated. The as-prepared SiO₂@LDHs exhibit higher base catalysis for the Knoevenagel condensation than conventional LDHs prepared via the co-precipitation method in the absence of SiO₂ spheres. Such an increase in the activity for base catalysis is also observed in various types of SiO₂@LDHs with different metal compositions (M²⁺: Mg²⁺ or Ni²⁺, M³⁺: Al³⁺ or Ga³⁺, and M²⁺/M³⁺: 1 or 3). X-ray diffraction (XRD) measurement suggests that the addition of SiO₂ induces decreases in the LDH crystallite size. The results of transmission electron microscopy – energy dispersive X-ray spectroscopy (TEM-EDS) and ²⁹Si cross polarization magic angle spinning nuclear magnetic resonance (²⁹Si CP-MAS NMR) on SiO₂@Mg–Al LDH suggest that the crystals of Mg–Al LDH are immobilized on the SiO₂ surface through Si–O–Al and Si–O–Mg covalent bonds. According to these results, we conclude that the co-precipitation method in the presence of colloidal spherical SiO₂ seeds, particularly possessing a 40 nm diameter, result in highly-dispersed points of LDH crystal growth on the SiO₂ surface, which lead to the generation of fine-crystallized highly active LDH nanocrystals.