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

Silica- and alumina-based mesostructured MCM-41-type materials were obtaine d at low temperature in the presence of cationic (cetyitrimethylammonium br omide (CTAB) or chloride) and anionic (sodium dodecylsulfate) surfactants, respectively, by modifying the pH of the corresponding clear precursor solu tions. In situ fluorescence techniques were used to study the first steps o f formation of these mesostructured solids prior to precipitation. For the silica-based system using CTAB, only a small fraction of micelle-bound brom ide ions is exchanged by OH and/or silicate ions and there is hardly any mi celle 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 s imilar result was observed for the alumina-based systems. For both types of systems the micelles remain close to spherical in the precursor solution. (C) 2001 Elsevier Science B.V. All rights reserved.