Amino-functionalized mesoporous silica synthesized by an anionic surfactant templating route

Abstract

A “S⁻N⁺∼I⁻ pathway” (S⁻: anionic surfactant, N⁺: cationic amino group and I: inorganic species) for the synthesis of mesoporous silica has been developed by using 3-aminopropyltriethoxysilane (APS) as a co-structure directing agent (CSDA), which can interact with the anionic head group in the surfactant (SDA). Thus synthesized mesoporous silica has been designated as AMS (Anionic-surfactant-templated Mesoporous Silica). Removal of the anionic surfactant by extraction led to the functionalized AMS containing amino groups on the silica surface. Amino-functionalized AMS using 3-aminopropyltriethoxysilane (APS) and lauric acid sodium salt (LAS) as CSDA and SDA, respectively, was synthesized with varying proportions of APS in the silica sources (x-APS-AMS, where x is the proportion of APS in the silica sources, x = 0.1–0.6). In 0.4-APS-AMS, the content of amino groups derived from APS estimated by CHN elemental analysis and the argentometric titration was 2.36 and 2.24 mmol g⁻¹, respectively, suggesting that almost all the aminopropyl moieties were on the surfaces in contrast to the MCM-41 type materials synthesized with a cationic surfactant. Thus obtained amino-functionalized AMS via the anionic surfactant templating route shows a higher adsorption capacity for Co²⁺ cations than amino-functionalized MCM-41 prepared by the direct co-condensation method via a conventional cationic templating route. There was also a marked difference in the activity for the Knoevenagel reaction between amino-functionalized AMS and MCM-41, indicating a significant difference in the state of aminopropyl moieties exposed to the surfaces.