Glycometallate surfactants. Part 1: non-aqueous synthesis of mesoporous silica

Abstract

A novel two step procedure for the synthesis of hexagonal mesoporous silica has been developed. The first non-aqueous step involves the use of ethylene glycol both as solvent and chelating alkoxide. In this step a cetyltrimethylammonium glycosilicate(IV), CTA ₂ [Si ₂ (OCH ₂ CH ₂ ) ₅ ], building-block is synthesized under non-aqueous conditions by solubilizing SiO ₂ with NaOH in ethylene glycol in the presence of CTACl. Alternatively, CTA ₂ [Si ₂ (OCH ₂ CH ₂ ) ₅ ] can be formed by reacting sodium glycosilicate(IV), Na ₂ [Si ₂ (OCH ₂ CH ₂ ) ₅ ], with CTACl under non-aqueous conditions in ethylene glycol. The glycosilicate(IV) is a structurally well defined dimeric anion based on trigonal-bipyramidal silicon(IV) containing two bidentate and one bridging monodentate glycolate ligand. In ethylene glycol the glycosilicate(IV) surfactant CTA ₂ [Si ₂ (OCH ₂ CH ₂ ) ₅ ] self-assembles into a lamellar mesophase containing bilayers of cationic CTA ⁺ that are charge-balanced by [Si ₂ (OCH ₂ CH ₂ ) ₅ ] ^2– counter-anions. In the second step of the preparation, controlled hydrolysis of the lamellar glycosilicate(IV) phase with water leads to a well ordered hexagonal mesoporous silica in which the extent of condensation-polymerization of the silica is insufficient to sustain the integrity of the structure when the CTA ⁺ cation is removed from the channels. Structure re-enforcement can however be achieved by various post-treatments of the vacuum dehydrated mesoporous silica that enable the creation of stable silica-based mesoporous materials with a wide range of elemental compositions. In this paper a post-treatment with Si ₂ H ₆ at 100 °C was employed to produce extensively polymerized hexagonal mesoporous silica that is stable to removal of the surfactant. The method described in this paper is a novel approach to the synthesis of stable and structurally well defined mesoporous silica-based materials with a wide range of elemental compositions.