Alkali-containing mesoporous (alumino) silicate materials with very high metal loading

Abstract

Mesoporous aluminosilicates with Si/Al ratios ranging from approximately 1 to ∞ (for a pure-silica compound) have been synthesized in the presence of sodium and cetyl trimethyl ammonium (CTMA +) cations. All materials are thermally stable and retain their highly ordered hexagonal structure upon removal of surfactant molecules 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 extracting surfactant molecules, whilst treatments in ethanol remove preferentially organic molecules. Combining these two procedures allowed us to exchange completely CTMA + and Na + cations and to prepare highly ordered Li +, K +, Cs + and Ca 2+-modified materials with very high alkali metal concentrations. The stability of the exchanged mesoporous compounds depends on the aluminum content and on the nature of the alkali cation. All materials are particularly stable upon exchange with Li + and Ca 2+ 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 molecules with Li + and Ca 2+ cations but is severely damaged with other cations. The procedures reported here are not only efficient to introduce a large amount of cations in mesoporous materials, but they can also be used to remove completely surfactant molecules from MCM-41 under mild conditions.