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 synthes is of the mesoporous molecular sieve materials (M41S) was studied in a simple ternary synthesis system containing tetraethylorthosilicate (T EOS), water, and the cetyltrimethylammonium (CTMA) cation at 100-degre es-C. The resulting silicate materials were characterized by X-ray dif fraction, Si-29 NMR, and FTIR. As the Sur/Si molar ratio increased fro m 0.5 to 2, the siliceous products obtained could be classified into f our separate groups: MCM-41 (hexagonal), MCM-48 (cubic), thermally uns table M41S, and a molecular species, the cubic octamer [(CTMA)SiO2.5]8 . One of the thermally unstable structures has been identified as a la mellar phase. These results are consistent with known micellar phase t ransformations that occur at various surfactant concentrations and rei nforce the concept that liquid-crystal structures serve as templating agents for the formation of M41S type materials.