Silica-phosphate mixed gels have been prepared by the sol-gel process from
tetramethoxysilane and phytic acid (IP6H12). They contain inositol phosphat
e ester units anchored by covalent P-O-X bonds (X = P or Si) as isolated po
lyphytate clusters either embedded or bound in the amorphous network. P-31
NMR of the liquid phases at low temperature was used to monitor the initial
steps of hydrolysis-condensation reactions at the sol stage. Dried gels ha
ve been characterized by C-13, Si-29 and P-31 MAS NMR, powder X-ray diffrac
tion and FTIR. The ability of these gels to sequestrate metallic ions, such
as La3+ and Zn2+, on the open phosphate sites has been investigated. The r
esulting samples undergo structural modifications during the heat treatment
which leads to the formation of silica-lanthanum phosphate and silica-zinc
pyrophosphate mixtures. The specific influence of the inserted cation on t
he structure of the final silica-phosphate obtained at 600 and 900 degrees
C is discussed.