Ethylene-bridged mesoporous organosilicas with hexagonal and cubic symmetry

Abstract

A series of ordered periodic mesoporous organosilicas (PMOs) with cubic and hexagonal symmetries were fabricated by using divalent surfactants [CH 3(CH2)15NMe2(CH2) 3NMe3]2+ 2Br- (C16-3-1) or binary surfactant mixtures [CH3(CH2) 15NMe3]+ Br- (C16TABr) and C16-3-1 as structure-directing agents (SDAs) and 1,2-bis(triethoxysilyl)ethane (BTEE) as an organosilica source under various basic conditions. The shape/structure of surfactant, molar ratio of binary surfactant mixtures, and base concentration crucially affect the formation of distinct mesophases. Face-centered cubic Fm3̄m mesoporous organosilicas can be obtained by using various concentrations of divalent surfactant C 16-3-1 or equimolar mixtures of divalent and monocationic surfactants as SDAs under basic conditions. Cubic Pm3̄n or 2D hexagonal p6mm mesophases can be synthesized by changing the molar ratio of the binary surfactant mixtures or the amount of the base NaOH. Use of monocationic C 16TABr instead of C16-3-1 as template produced the hexagonal p6mm mesophase exclusively independent of the amount of the surfactant and the base. In addition, use of trimethylbenzene as expander molecule in the aforementioned binary surfactant template system caused a mesophase transformation from cubic Pm3̄n to p6mm symmetry. All samples were characterized by powder X-ray diffraction (PXRD) analysis and N2 physisorption. The formation of face-centered cubic Fm3̄m, primary cubic Pm3̄n, and hexagonal p6mm PMOs was also confirmed by transmission electron microscopy (TEM), revealing a good long-range ordering with regular arrays. Moreover, variation of the synthesis parameters resulted in a variety of different PMO morphologies, as ascertained by scanning electron microscopy (SEM). FT-IR and solid-state 13Cand 29Si NMR spectroscopy further revealed that the organic groups were uniformly incorporated into the framework. The various BET surface areas of the PMOs range from 470 to 780 m2 g-1, while the pore diameters lie within a 26 to 30 Å range, as derived from N2 physisorption. © 2009 Verlag der Zeitschrift für Naturforschung, Tübingen.