Investigations by fluorescence techniques of the mechanism of formation of silica- and alumina-based MCM-41-type materials

Abstract

Silica- and alumina-based mesostructured MCM-41-type materials were obtained at low temperature in the presence of cationic (cetyltrimethylammonium bromide (CTAB) or chloride) and anionic (sodium dodecylsulfate) surfactants, respectively, by modifying the pH of the corresponding clear precursor solutions.In situ fluorescence techniques were used to study the first steps of formation of these mesostructured solids prior to precipitation. For the silica-based system using CTAB, only a small fraction of micelle-bound bromide ions is exchanged by OH− and/or silicate ions and there is hardly any micelle growth in the presence of these additives and upon lowering the pH to a value close to that where the mesostructured solid precipitates out. A similar result was observed for the alumina-based systems. For both types of systems the micelles remain close to spherical in the precursor solution.