Formation of molecularly ordered layered mesoporous silica via phase transformation of silicate-surfactant composites

We have prepared molecularly ordered layered silicate - surfactant mesophases from the stepwise phase transformation of surfactant-silicate composites. The hexagonal-cubic-lamellar phase transformations were achieved by varying the time allowed for hydrothermal crystallization of simple CTAOH/SiO2/H2O systems at 135 degreesC. At short (3 h) crystallization periods, the hexagonal phase (MCM-41) was obtained, which after 24 h transformed into a cubic (MCM-48) phase stable for up to 4 days. At 6 days a mixture of cubic and lamellar phases was observed, which finally transformed into the molecularly ordered pure lamellar phase after 12 days of hydrothermal synthesis. Molecular ordering, which was probed using powder XRD and silicon-29 solid state NMR, was only observed for the lamellar phase. Crystallization of the silica framework, at long hydrothermal synthesis time, therefore acts as a driving force for the high to low curvature cubic-lamellar transformation. The phase transformations were accompanied by changes in particle morphology and gradual densification of the silica network. The molecular ordering of the layered material was, to some extent, retained after template removal by calcination. The calcined molecularly ordered layered mesoporous silica exhibited structurally well-ordered domains and had relatively high surface area and pore volume.