Direct synthesis of highly ordered Co-SBA-15 mesoporous materials by the pH-adjusting approach

Cobalt-substituted SBA-15 (Co-SBA-15) mesoporous materials were directly synthesized under mild conditions (pH > 2.0) by a "pH-adjusting" method using sodium hydroxide. Characterization results, such as powder X-ray diffraction (XRD), N₂ physisorption and high-resolution transmission electron microscopy (HRTEM), showed that the resultant materials exhibit highly ordered hexagonal mesoporous structures, which was strongly correlated to the pH in the synthesis solution. The Co-SBA-15 sample obtained at pH of 3.0 gave the best structure. Solid state magic-angle-spinning nuclear magnetic resonance (MAS NMR) results indicated less condensation under higher pH. In addition, the pH also strongly affected the incorporation of cobalt cations in the silica framework. Diffuse reflectance UV-vis absorption spectra of calcined Co-SBA-15 samples and hydrogen temperature programmed reduction (H₂-TPR) demonstrated that the cobalt ions in the silica framework locate at tetrahedrally coordinated sites. The effect of pH and sodium (NaOH) addition on improving the cobalt ion incorporation and the mesostructure of Co-SBA-15 materials were also discussed.