Organically Modified SiO₂−B₂O₃ Gels Displaying a High Content of Borosiloxane (B−O−Si⋮) Bonds

Abstract

Hybrid SiO2−B2O3 gels have been synthesized from cohydrolysis of modified silicon alkoxides (R−Si(OE)3 R = Me, Vi, Et) and triethylborate, B(OEt)3. The structure of these gels was investigated by Fourier-transformed infrared (FT-IR) spectroscopy, 29Si and 11B solid-state nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and chemical analysis. The results show, for the first time, the possibility of obtaining a SiO2−B2O3 gel in which 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 11B 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 Vi−Si(OR)3 rather than Me−Si(OEt)3. Some hypotheses to rationalize the experimental results are also presented.