Surfactant effects on the physical properties of mesoporous silica and silicates

Mesostructured silicas and silicates have been synthesized using hydrogels with molar composition: M:26.0SiO₂:5.2(C₂H₅)₄NOH:7.5CH₃(CH₂)₁₅N(CH₃)₃]₂O:790H₂O, where M=0, Zr(OC₃H₇)₄ or Ti(OC₄H₉)₄. In all preparations, colloidal silica (Ludox) was used as the source of silica. The hydrothermal transformation at 110°C of these gels produced solids with the hexagonal structure typical of MCM-41 type materials. The effects of chain length and surfactant terminal alkyl groups on the properties of mesoporous materials containing Ti or Zr, have been investigated by using different surfactants such as cetyl trimethyl ammonium bromide and chloride, cetyl dimethyl ethyl ammonium bromide, and myristyl trimethyl ammonium bromide. When the surfactant's carbonyl chain decreased to 14 from 16 carbon atoms, a reduction in unit cell dimension and average pore diameter was observed in the mesoporous silicas, titaniumsilicates and zirconiumsilicates under study. Replacement of methyl groups with ethyl groups on the surfactant hydrophobic head, had no measurable effects on crystals' properties. However, a surfactant with a bulky aromatic head group, such as cetyl pyridinium chloride, inhibited crystallization. In general, the use of bromide in place of chloride salts yielded more ordered MCM-41 type crystals. The high thermal stability (to 800°C), surface area (1000–1500 m²/g), pore volume (0.90–1.20 cm³/g) and uniform mesoporosity (with pore diameter in the 2.9 nm–3.6 nm range), of these metalsilicates could be of particular interest in the preparation of catalysts requiring siliceous metal supports.