Organically modified SiO2-B2O3 gels displaying a high content of borosiloxane (= B-O-Si ) bonds

Hybrid SiO2-B2O3 gels have been synthesized from cohydrolysis of modified s ilicon alkoxides (R-Si(OE)(3) R = Me, Vi, Et) and triethylborate, B(OEt)(3) . The structure of these gels was investigated by Fourier-transformed infra red (FT-IR) spectroscopy, Si-29 and B-11 solid-state nuclear magnetic reson ance (NMR), X-ray diffraction (XRD), and chemical analysis. The results sho w, for the first time, the possibility of obtaining a SiO2-B2O3 gel in whic h trigonal boron atoms are homogeneously dispersed in the siloxane network via B-O-Si bonds. The identification of such bonds was mainly based on the FT-IR and B-11 MAS NMR results. The formation of the borosiloxane bonds is favored by increasing the pH of hydrolysis water and by the use of Et- or V i-Si(OR)(3) rather than Me-Si(OEt)(3). Some hypotheses to rationalize the e xperimental results are also presented.