Systematic phase control of periodic mesoporous organosilicas using Gemini surfactants

Abstract

Highly ordered periodic mesoporous organosilica (PMO) materials with various mesostructures, including lamellar, bicontinuous cubic Ia3d, 2D hexagonal (P6mm), 3D hexagonal (P6₃/mmc) and cubic Pm3n, have been synthesized using Gemini surfactants with general formulas of [C_nH_2n+1N(CH₃)₂(CH₂)_sN (CH₃)₂C_nH_2n+1]Br₂ (n = 6–18 and s = 3–12, C_n-s-n). The nature of the Gemini surfactant such as alkyl chain length (n) and spacer length (s), and the synthetic conditions such as reaction temperatures and molar compositions are controlling parameters for desired mesostructures. The PMO materials, synthesized at room temperature from C_n-6-n, exhibit phase transition from lamellar to bicontinuous cubic Ia3d, 2D hexagonal, 3D hexagonal and cubic Pm3n as the chain length decreases, whereas only the lamellar and 2D hexagonal PMO materials with different lattice parameters depending on the chain length are obtained at high reaction temperature (373 K). The C_n-8-n and C_n-10-n surfactants also yield 2D hexagonal PMO material in a very wide range of synthetic condition at 373 K. The PMO materials with various mesostructures thus obtained exhibit high BET surface areas in the range of 900–1500 m² g⁻¹ and total pore volumes of about 0.5–1.4 cm³ g⁻¹.