Synthesis of pore-enlarged mesoporous organosilicas under basic conditions

Abstract

A series of mesoporous organosilicas (MOs) in the [3.2;11.1] nm pore diameter range has been obtained in basic media by using 1,2-bis(triethoxysilyl)ethane (BTEE) or 1,2-bis(trimethoxysilyl)ethane (BTME) as a silica source, binary surfactant mixtures [CH 3(CH 2) 17NMe 2(CH 2) 3NMe 3] 2+2Br − (C 18-3-1) and [CH 3(CH 2) 15NMe 3] +Br − (C 16TABr) as structure-directing agents (SDAs), and TMB (mesitylene) or TPB (1,3,5-triisopropylbenzene) as swelling agents. Pore size control has been achieved via variation of (i) the binary surfactant ratio, (ii) base (sodium hydroxide) concentration, (iii) reaction time and temperature, and (iv) type of expander molecule. The surface morphology and silanol population of the dehydrated MOs was examined by TMDS (tetramethyldisilazane) silylation. The parent and functionalized MOs were characterized by powder X-ray diffraction, N 2 physisorption, elemental analysis, FTIR, and solid-state ( 1H, 13C, 29Si) NMR spectroscopy. According to their PXRD patterns, the pore-enlarged MO materials do not display any long-range order, however, pore volumes >2 cm 3/g. The type of hysteresis loop exhibited by the N 2 adsorption and desorption isotherm changed remarkably for MOs synthesized at low concentrations of divalent surfactant as well as base, which can be attributed to a change of pore topology. 29Si MAS NMR spectroscopy revealed the prominent T 2 and T 3 peaks at ∼−58 and ∼−64 ppm, respectively, indicative of an intact wall and surface structure. The silyl group coverage was determined as 0.8–1.3 SiHMe 2 groups per nm 2 corresponding to approximately half of that found on purely siliceous periodic mesoporous silicas such as MCM-41 or MCM-48.