Versatile approach to synthesis of 2-D hexagonal ultra-large-pore periodic mesoporous organosilicas

Abstract

Periodic mesoporous organosilicas (PMOs) with methylene (–CH₂–), ethylene (–CH₂CH₂–), ethenylene (–CHCH–) and phenylene (–C₆H₄–) framework groups were synthesized with 2-dimensional hexagonal structures of very large cylindrical mesopores. A combination of a commercially available triblock copolymer Pluronic P123 (EO₂₀PO₇₀EO₂₀) with a judiciously chosen micelle swelling agent (cyclohexane and 1,3,5-triisopropylbenzene) was used as a micellar template, and the initial step of the synthesis was performed at temperature between 10 and 18 °C, followed by a hydrothermal treatment at 100–150 °C. The PMOs were characterized using small-angle X-ray scattering (SAXS), nitrogen adsorption, transmission electron microscopy, and solid-state ²⁹Si NMR. For all PMO compositions, the formation of 2-D hexagonal structures with (100) interplanar spacing, d₁₀₀, up to 21–26 nm was achieved, which is at least seven nanometres larger than d₁₀₀ reported earlier for any PMO. The nominal (BJH) pore diameters up to 20–27 nm were achieved for the considered compositions of PMOs with 2-D hexagonal ordering, while even larger pore sizes were sometimes attained for disordered or weakly ordered structures. The mesopores exhibited constrictions or narrow entrances that were widened by increasing the hydrothermal treatment temperature. The pore diameter tended to increase as an initial synthesis temperature decreased, allowing for the pore size adjustment, but the useful temperature range depended on the bridging group. The present work suggests that the low-temperature micelle-templated synthesis with judicious selected swelling agents is a general pathway to ultra-large-pore 2-D hexagonal PMOs with both aliphatic and aromatic bridging groups.