Aqueous silica gels

Pyromellitate, the tetracarboxylate of benzene, forms red crystals of formula [Co2(C6H2 (C00)4)4]-18H20 from aqueous silica gel, and feature infinite chain-like polyanions with [Co(OH2)4[C6H2(COO)4)]2]ra2" stoichiometry where Co is in an octahedral environment of four waters and two traras-disposed carboxylates.436... 

Nassif, N., Coiffier, A., Coradin, T., Roux, C., Livage, J. and Bouvet, O. (2003) Viability of bacteria in hybrid aqueous silica gels. Journal of Sol-Gel Science and Technology, 26, 1141—1144. 

Unique aqueous electrolysis media have been developed and highly efficient electrochemical oxidations of alcohols were accomplished using these systems. Tanaka s group developed the anodic oxidation of alcohols mediated by N-oxyl in an aqueous silica gel disperse system (Figure 12.6) [18]. The system could be successfully applied to the kinetic resolution of sec-alcohols and the enantioselective oxidation of meso-l,4-diols, affording optically active y-lactones (Scheme 12.3). 

Figure 12.6 Anodic oxidation of alcohols mediated by N-oxyl in an aqueous silica gel disperse system.

Tanaka H, Kawakami Y, Goto K, Kiuoboshi M (2001) An aqueous silica gel disperse electrolysis system. N-Oxyl-mediated electrooxidation of alcohols. Tetrahedron Lett 42 445-448... 

As mentioned earlier, even if alcohol is not added to the precursor solution, the hydrolysis and condensation of silicon alkoxides leads to the formation of alcohol as a by-product. The aqueous route does not produce such harmful by-products and should therefore be preferred to the alkoxide route. However, the formation of aqueous silica gels is more difficult to control and very few papers actually report on sol-gel encapsulation using aqueous precursors (Liu, 1999 Bhatia, 2000 Einnie, 2000 Coiffier, 2001). 

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