Synthesis of thermally stable and 2-D hexagonal super-microporous silica from hydrated α-sodium disilicate

Thermally stable and 2-D hexagonal super-microporous silicas were obtained utilizing hydrated α-sodium disilicate over a diverse range of surfactant/Si molar ratios. Silica-based mesostructured materials composed of hydrated α-sodium disilicate and hexadecyltrimethylammonium (C ₁₆TMA) cations were acid-treated, resulting in the formation of 2-D hexagonal mesostructured silica(α)-n (n = C₁₆TMA/Si molar ratio). Although the 2-D hexagonal structure of mesostructured silica(α)-0.2 collapsed at 900°C, those of mesostructured silicas(α)-0.5, -0.7, and -1.0 were retained even after calcination at 1000°C. The retention of the 2-D hexagonal structures was confirmed further by TEM. The higher thermal stability is ascribed to lower Na content in mesostructured silica(α)-n. Higher C₁₆TMA/Si molar ratios lowered the pH values in the reaction suspensions, and then interlayer Na ions in the silicates were further exchanged with hydrated protons. Because of the higher thermal stability, ordered super-microporous silicas can be formed in the present system. The condensation of the silicate frameworks proceeded with the rise of the calcination temperature, leading to a further decrease in pore size. The pore sizes of the products calcined at 1000°C were 1.7-1.8 nm (BJH method), smaller than those reported for ordered mesoporous silicas prepared using C₁₆TMA cations.