Synthesis of large-pore face-centered-cubic periodic mesoporous organosilicas with unsaturated bridging groups

Abstract

Periodic mesoporous organosilicas (PMOs) with ethenylene (–CHCH–) bridging groups in the framework and large spherical mesopores of tunable size arranged in face-centered-cubic structures (Fm3m symmetry) were synthesized using Pluronic F127 (EO106PO70EO106) as a surfactant and xylene (isomer mixture) as a swelling agent at low temperature (7 °C) under moderately acidic conditions (0.1 M HCl). The resulting PMOs were characterized by small-angle X-ray scattering, nitrogen adsorption, transmission electron microscopy and solid-state 29Si NMR. The unit-cell parameters were enlarged from 31 to 42 nm and the pore diameters increased from 14 nm to as much as 22 nm while increasing the amount of the swelling agent in the reaction mixture, until these two structural parameters reached a plateau. The enlargement was not accompanied by a loss of structural ordering, which is exceptional for not only PMOs, but also other surfactant-templated materials. While the extraction of the surfactant was ineffective in generating readily accessible mesopores for samples prepared with low relative quantities of the swelling agent, extracted samples prepared with higher proportions of the swelling agent exhibited good mesopore accessibility. On the other hand, the calcination under argon at 350 °C afforded well accessible mesopores in all cases, even though the unit-cell shrinkage was significant and a substantial fraction of Si–C bonds was cleaved.