Size-Selective Surface Silylation of Cagelike Mesoporous Silica SBA-2 with Disilazane Reagents

Dehydrated, structurally ordered silica materials SBA-1 and SBA-2 were reacted with two differently sized silylamines, tetramethyldisilazane (HN(SiHMe2)2) and tetramethyldiphenyldisilazane (HN(SiMe2Ph)2), at ambient temperature. The intraporous silylation is controlled by the different sizes of the pore entrances/cage windows of these “mesoporous” silica materials. According to nitrogen physisorption, Fourier transform infrared spectroscopy, and carbon elemental analysis, the vast majority of the channels and pore openings, interconnecting the supercages of SBA-2 (P63/mmc), display a diameter larger than the size of a HN(SiHMe2)2 molecule (silylation efficiency, SE, 1.98−2.28 mmol SiHMe2/1 g); however, this is smaller than that of a HN(SiMe2Ph)2 molecule (SE, 0.40−0.63 mmol SiMe2Ph/1 g). The inner surface of SBA-1 (space group Pm3n) is fully accessible to both disilazane reagents giving SE values of 4.03 mmol SiHMe2/1 g and 2.91 mmol SiMe2Ph/1 g. The lower SE value for material SiMe2Ph@SBA-1 reflects the increased steric bulk of the silylating reagent which hampers the silylation of neighbored silanol functional groups.