Transformation of layered docosyltrimethyl- and docosyltriethylammonium silicates derived from kanemite into precursors for ordered mesoporous silicas

The transformation of layered silicate-surfactant materials derived from a layered polysilicate kanemite into precursors for ordered mesoporous silicas was investigated. When kanemite was allowed to react with docosyltrimethylammonium (C₂₂TMA) and docosyltriethylammonium (C₂₂TEA) surfactants, layered C₂₂-TMA- and C₂₂TEA-silicate materials were formed. The ²⁹Si MAS NMR spectra of the layered intermediates clearly indicated that fragmented silicate sheets were present in the silicate-surfactant materials. These layered materials can be transformed by acid treatment into mesostructured precursors with three-dimensional (3-d) silicate networks that afford uniform mesopores after the removal of the surfactants. Ordered mesoporous silica with a 2-d hexagonal structure (FSM-16) can be obtained by using C₂₂TEA with a larger headgroup. The results here successfully support the proposed formation mechanism of FSM-16, which consists of assembling of fragmented layered silicate sheets with surfactants and the subsequent structural transformation to the 2-d hexagonal mesophase.