Tetrafluoroborate salts as site-selective promoters for sol-gel synthesis of mesoporous silica

Tetrafluoroborate ion (BF4-) serves as a powerful and better-behaved promoter than fluoride ion (F-) for hydrolytic condensation of alkoxysilanes, such as tetraethoxy orthosilicate, in aqueous media containing amphiphiles with onium ion headgroups as templates, affording thermally and hydrothermally stable mesoporous silica. According to F-19 NMR spectral profiles, BF4- is localized on a positive-charged micellar surface, thereby allowing a site-selective growth of the silica framework. The resulting porous silica has an ordered hexagonal structure with a well-developed and thick silicate wall. Even without calcination, the condensation with BF4- as the promoter progresses to a large extent to furnish a [Si(OSi-)(4)]/([HOSi(OSi-)(3)] + [(HO)(2)Si(OSi-)(2)]) ratio of 6.2, which is greater than that of mesoporous silica formed without BF4- before (1.5) and even after calcination (3.5) to promote thermal condensation in the solid state.