EFFECT OF SURFACTANT SILICA MOLAR RATIOS ON THE FORMATION OF MESOPOROUS MOLECULAR-SIEVES - INORGANIC MIMICRY OF SURFACTANT LIQUID-CRYSTAL PHASES AND MECHANISTIC IMPLICATIONS

The influence of surfactant/silica molar ratio (Sur/Si) in the synthesis of the mesoporous molecular sieve materials (M41S) was studied in a simple ternary synthesis system containing tetraethylorthosilicate (TEOS), water, and the cetyltrimethylammonium (CTMA) cation at 100-degrees-C. The resulting silicate materials were characterized by X-ray diffraction, Si-29 NMR, and FTIR. As the Sur/Si molar ratio increased from 0.5 to 2, the siliceous products obtained could be classified into four separate groups: MCM-41 (hexagonal), MCM-48 (cubic), thermally unstable M41S, and a molecular species, the cubic octamer [(CTMA)SiO2.5]8. One of the thermally unstable structures has been identified as a lamellar phase. These results are consistent with known micellar phase transformations that occur at various surfactant concentrations and reinforce the concept that liquid-crystal structures serve as templating agents for the formation of M41S type materials.