Facile Mesophase Control of Periodic Mesoporous Organosilicas under Basic Conditions

The mesophase structure of ordered periodic mesoporous organosilicas (PMOs) can be easily adjusted under basic conditions by variation of the sodium hydroxide concentration. PMOs with hexagonal ( p 6 mm, two-dimensional pore system), cubic ( Pm 3 n or Fm 3 m, three-dimensional cagelike pore system), and disordered hexagonal (MSU-type) symmetry were obtained from 1,2-bis(triethoxysilyl)ethane (BTEE)-derived synthesis gels in the presence of binary surfactant mixtures, [CH 3 (CH 2 ) 15 NMe 3 ] + Br − (C 16 TABr) and [CH 3 (CH 2 ) n NMe 2 (CH 2 ) 3 NMe 3 ] 2+ 2Br − ( n = 15, 17), as structure-directing agents. With an increasing amount of NaOH, mesophase−mesophase transitions from p 6 mm → Pm 3 n → Fm 3 m → distorted cubic → MSU-type (3D-network of “wormholelike” mesopores) were evidenced by powder X-ray diffraction analysis and transmission electron microscopy. Slight variations of the NaOH concentration not only affected the PMO mesophase structure via the charge matching principle but also the PMO morphology. Among others, hexagons with smooth faces, regular decaoctahedrons, and spherical particles were found from scanning electron microscopy. The PMO mesophase was further found to be dependent on the carbon chain length of the surfactant. 13 C and 29 Si magic-angle-spinning NMR spectroscopy revealed that intact ethylene moieties were incorporated uniformly into the framework. According to N 2 physisorption measurements, the PMOs display Brunauer−Emmett−Teller surface areas as high as 870 m 2 /g and cage diameters in the [55,63] Å size range. Finally, the hydrothermal stability of three representative PMOs with cubic Pm 3 n, cubic Fm 3 m, and hexagonal p 6 mm symmetry was investigated in detail.