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

Tetrafluoroborate ion (BF(4)(-)) 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 (19)F NMR spectral profiles, BF(4)(-) 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 BF(4)(-) 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 BF(4)(-) before (1.5) and even after calcination (3.5) to promote thermal condensation in the solid state.