Alkali-containing mesoporous (alumino) silicate materials with very high metal loading
Résumé
Mesoporous aluminosilicates with Si/Al ratios ranging from approx. 1 to infini (for a pure-silica compd.) have been synthesized in the presence of sodium and cetyl tri-Me ammonium (CTMA+) cations. All materials are thermally stable and retain their highly ordered hexagonal structure upon removal of surfactant mols. at 800 K. Original procedures have been explored to exchange selectively surfactant and/or sodium cations from as-made materials. Treatments in water exchange exclusively sodium ions, without extg. surfactant mols., while treatments in ethanol remove preferentially org. mols. Combining these two procedures allowed us to exchange completely CTMA+ and Na+ cations and to prep. highly ordered Li+, K+, Cs+ and Ca2+-modified materials with very high alkali metal concns. The stability of the exchanged mesoporous compds. depends on the aluminum content and on the nature of the alkali cation. All materials are particularly stable upon exchange with Li+ and Ca2+ cations. By contrast, the stability of solids exchanged with Cs+ or K+ cations decreases with decreasing the aluminum content. In the particular case of pure silica MCM-41, the hexagonal structure is retained upon exchange of surfactant mols. with Li+ and Ca2+ cations but is severely damaged with other cations. The procedures reported here are not only efficient to introduce a large amt. of cations in mesoporous materials, but they can also be used to remove completely surfactant mols. from MCM-41 under mild conditions.