Optimum synthesis of mesoporous silica materials from acidic condition

On the basis of the mixtures composed of C_nTMAB/acids/TEOS/ water, the mesoporous materials with well-ordered hexagonal structures can be synthesized under acidic condition (pH < 1.0) with a careful control of the silica condensation. However, the induction period time for the formation of surfactant-silica precipitation varied with the employed acid source and decreased in the series: HNO₃ < HBr < HCl < H ₂SO₄ at the same acid concentration. Increasing the acid concentration not only induced a fast precipitation rate, but also provoked more nuclei seeds formation and reduced the particle size of the mesoporous silica products. A proper aging time is also significant for getting the well-ordered mesoporous materials via acid route, and a longer aging time would induce a gradual decomposition of the mesostructures. For morphology, in acid synthetic medium, various interesting shell-like curved patterns on the surface of mesoporous silica particles were formed. The physical properties (such as d-spacing, pore size, BET surface area and porosity) of the calcined mesoporous silica are almost not affected by the nature of the acid source, but the pore size of the mesoporous materials can be tailored by varying the chain length of the surfactant. Adding the proper amount of ammonia solution for adjusting the pH value between 8-10 can improve the hexagonal mesostructures order of the mesoporous materials while maintaining the original morphology.