Three-dimensional low symmetry mesoporous silica structures templated from tetra-headgroup rigid bolaform quaternary ammonium surfactant

Two kinds of highly ordered mesoporous silica materials (FDU-11, FDU-13) with novel three-dimensional (3-D) tetragonal and orthorhombic structures were synthesized by using tetra-headgroup rigid bolaform quaternary ammonium surfactant [(CH₃)₃NCH₂CH₂CH ₂N(CH₃)₂CH₂(CH₂) ₁₁OC₆H₄C₆H₄O(CH ₂)₁₁-CH₂N(CH₃)₂CH ₂CH₂CH₂N(CH₃)₃· 4Br] (C_3-12-12-3) as a template under alkaline conditions. High-resolution transmission electron microscopy (HRTEM), small-angle X-ray scattering (SAXS), and X-ray diffraction (XRD) show that mesoporous silica FDU-11 has primitive tetragonal P4/mmm structure with cell parameters a = b = 8.46 nm, c = 5.22 nm, and da ratio = 0.617. N₂ sorption isotherms show that calcined FDU-11 has a high BET surface area of ∼1490 m ²/g, a uniform pore size of ∼2.72 nm, and a pore volume of ∼1.88 cm³/g. Mesoporous silica FDU-13 has primitive orthorhombic Pmmm structure. The cell parameters are a = 9.81, b = 5.67, and c = 3.66 nm. N₂ sorption isotherms show that calcined FDU-13 has a high BET surface area of 1210 m²/g, a uniform mesopore size of ∼1.76 nm, and a large pore volume of ∼1.83 cm³/g. Such low symmetries for 3-D mesostructures (tetragonal and orthorhombic system) have not been observed before even in amphiphilic liquid crystals, which maybe resulted from an oblate aggregation of the bolaform surfactant and its strong electrostatic interaction with inorganic precursor. A probable mechanism has been proposed for the formation of such a 3-D low symmetrical mesostructure. These results will further extend the synthesis of mesoporous materials and may open up new opportunities for their new applications in catalysis, separation, and nanoscience.