Synthesis and Characterization of Metal-incorporated Mesoporous Materials

This study is focused on the synthesis and characterization of metal-incorporated mesoporous materials with high crystallinity, including siliceous SBA-15 and zirconia. In the preliminary experiment, it was found that the mesostructured silica materials could be prepared in the acid-free condition. The pore structures were dependent on the concentration of P123 triblock copolymer, salt, co-solvent and amino acid in the mother solution. When the NaCl/Si ratio was fixed, the mesopore was transferred from disordered pore to mesostructured cellular foam by increasing the concentration of P123 from 0.5 to 1.25wt%. When the alcohol was added in the synthesis solution, the mesoporous silica with well-ordered worm pore was obtained. Interestingly, hexagonal P6mm structure of SBA-15 like material could be obtained by using glutamic acid instead of salt in a self-generated acidic condition (pH=3.2). The transformation of pore structures was probably attributed to the change of the hydrophobic to hydrophilic volume and the PPO/PEO interface by varying the concentration of P123, salt, cosolvent and amino acid.
Based on above results, we developed a new method for synthesizing metal-incorporated SBA-15 including Al, Cr, Fe and Zr in a self-generated acidic environment and without the addition of other mineral acids. It was found that the amounts of incorporated metals in SBA-15 were greater than those prepared by traditional acid-synthesis condition. Among the metal species studied, the incorporated amount decreased in the order of Zr(IV) > Fe(III) > Al(III) > Cr(III). The maximum loadings of Zr, Fe, Al and Cr in SBA-15 were 11, 7, 5 and 2 mol%, respectively. The crystallinity of the products was greatly enhanced by the addition of salts in the synthesis gel. Based on the pH value of mother solution and the isoelectric point of silica, synthesis of Zr- and Fe-SBA-15 was likely through the I+X-S+ pathway, whereas Al- and Cr-SBA-15 were through the I+S- pathway.
For the mesoporous zirconia materials, the addition of salts in the synthesis gel was found to increase the crystallinity of the product, but only slightly improve the thermal stability. Partial extraction of the surfactant from the as-made zirconia material before calcination was found to be a key step to maintain the mesostructure of the material up to 600℃ calcination. Impregnation of other metal species on the above material, such as La, Y, and Mo, on mesoporous zirconia was also successfully achieved without significantly losing the mesostructure and the surface area a lot.