Model System for Solid-State NMR Study on Co-condensation Behavior of Silicon Precursors in Periodic Mesoporous Organosilicas

Abstract

Ordered periodic mesoporous organosilicas containing different fractions of benzene groups in the silica framework, based on the cubic SBA-1 mesostructure (Pm3̅n mesophase), were synthesized with the direct-synthesis route via co-condensation of tetraethoxysilane (TEOS) and 1,4-bis(triethoxysilyl)benzene (BTEB) under acidic conditions using cetyltriethylammonium bromide as a structure-directing agent. A significantly large amount of TEOS, up to 70 mol % based on silica, can be incorporated into the silica wall without degrading the structural integrity of the materials. By optimization of the synthesis compositions, the resulting materials exhibited much higher surface areas (up to 1210 m2/g) and larger pore volumes (up to 0.64 cm3/g) as compared to the previous analogue, which only exhibited a surface area of 381 m2/g and a pore volume of 0.11 cm3/g. Two-dimensional (2D) 29Si{1H} heteronuclear correlation (HETCOR) NMR spectra, acquired as a function of contact time, provided direct spectroscopic evidence that a single mesophase with various Q (from TEOS) and T silicon species (from BTEB) located randomly within the pore walls via co-condensation of BTEB and TEOS at a molecular level. Such information is often not achievable by other characterization techniques. The 1H−29Si distance information between phenylene protons and nearby T3 silicon atoms obtained from density functional theory calculations is also in good agreement with the observations of 2D 29Si{1H} HETCOR NMR experiments.