Periodic mesoporous organosilicas:: mesophase control via binary surfactant mixtures

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)(17)NMe3]Br-+(-)(C(18)TABr) or [CH3(CH2)(17)NMe2(CH2)(3)NMe3](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 C(18)TABr 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 C(18)TABr 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 Si-29 and C-13 CP MAS NMR spectroscopy. The hexagonal PMOs displayed BET surface areas in the range of 500 - 1030 m(2) g(-1), pore volumes as large as 1.62 cm(3) g(-1), and pore diameters ranging from 29 to 52 angstrom. Materials PMO[SBA-1]-n exhibited BET surface surface areas in the range of 570 to 800 m(2) g(-1), pore volumes between 0.51 and 1.08 cm(3) g(-1), and pore sizes ranging from 29 to 40 angstrom. Representative PMO samples indicated silanol group populations in the range of 0.6-0.9 OH per nm(2) by tetramethyldisilazane silylation ( carbon analysis).