Crystalline mesoporous silicates from layered precursors

Abstract

Structurally well ordered and crystalline (molecularly ordered) mesoporous silica materials have been prepared from layered silicate precursors. The materials were derived from crystalline layered silicate, Na-RUB-18, by performing secondary hydrothermal synthesis (i.e., restructuring) in the presence of cetyltrimethylammonium (CTA) surfactant molecules. The mesoporous silica materials show retention of molecular ordering (crystallinity) in their framework depending on the method used to remove surfactant molecules. Crystalline RUB-18–surfactant mesophases were first obtained by hydrothermal treatment of Na-RUB-18 in CTA–NaOH–H₂O systems at 150 °C for 48 or 120 h. Benign template removal viasolvent extraction in acidified ethanol retained a significant level of crystallinity or molecular ordering in the resulting mesoporous material. H₂O₂-mediated oxidation of the surfactant at room temperature was also found to be an effective method for generating porous surfactant-free materials with significant crystallinity. Template removal viacalcination generated porous materials that exhibited no crystallinity. The textural properties of the mesoporous materials (surface area: 150–260 m² g⁻¹, and pore volume: 0.2–0.3 cm³ g⁻¹) depend on the mode of surfactant removal, and follow the trend: calcined > extraction > oxidation. The level of mesostructural ordering was not significantly affected by the surfactant removal method. The morphology of the layered silicate, i.e., plate-like particles, was unchanged during the transformation to mesostructured materials.