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− (C18-3-1) 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 C18-3-1 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 C18-3-1 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).