Periodic mesoporous organosilicas: mesophase control via binary surfactant mixtures

Abstract

A series of ordered periodic mesoporous organosilicas (PMOs) with cubic and hexagonal symmetry was synthesized from organosilica precursor 1,2-bis(triethoxysilyl)ethane (BTEE) by using cationic surfactants [CH₃(CH₂)₁₇NMe₃]⁺Br⁻ (C₁₈TABr) or [CH₃(CH₂)₁₇NMe₂(CH₂)₃NMe₃]²⁺2Br⁻ (C_18-3-1) or binary mixtures herefrom as structure-directing agents (SDAs). Under the prevailing basic reaction conditions, the commercially available quaternary alkyltrimethylammonium salt C₁₈TABr selectively gave materials PMO[MCM-41]-n with hexagonal symmetry (space group P6mm). Depending on the surfactant concentration, the materials obtained from divalent C_18-3-1 displayed cubic (PMO[SBA-1]-n with space group Pm3n) or disordered hexagonal structure (mixed phase). Fine-tuning of the total amount and molar ratio of surfactant mixtures of C_18-3-1 and C₁₈TABr also accomplished an effective mesophase control of ordered organosilicas PMO[MCM-41]-n and PMO[SBA-1]-n. For a given gel composition, such a mesophase transformation occurred also by addition of inorganic salt KF (hexagonal → cubic) and addition of organic expander molecule mesitylene, TMB (cubic → hexagonal). All of the PMOs were characterized by powder X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen physisorption, FTIR spectroscopy, and ²⁹Si and ¹³C CP MAS NMR spectroscopy. The hexagonal PMOs displayed BET surface areas in the range of 500–1030 m² g⁻¹, pore volumes as large as 1.62 cm³ g⁻¹, and pore diameters ranging from 29 to 52 Å. Materials PMO[SBA-1]-n exhibited BET surface surface areas in the range of 570 to 800 m² g⁻¹, pore volumes between 0.51 and 1.08 cm³ g⁻¹, and pore sizes ranging from 29 to 40 Å. Representative PMO samples indicated silanol group populations in the range of 0.6–0.9 OH per nm² by tetramethyldisilazane silylation (carbon analysis).