Effect of precursors on lithium containing silicate gels studied by ⁷Li nuclear magnetic resonance

Lithium containing silica gels were prepared by hydrolysis of tetraethylorthosilicate (TEOS) with a methanol-water-LiNO₃ or LiOH solution and by infiltration of silica gel with LiOH or LiNO₃ solution. Gels prepared in this way, dried and heat treated between 200 and 850°C were studied by solid state ⁷Li static and magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. There are dramatic differences in the ⁷Li NMR spectra of the various gels. The lineshape of the NMR spectra for gels prepared from LiOH (both via hydrolysis of TEOS and infiltration of prepared silica gel) is similar to that of crystalline Li₂SiO₃ where the lithium ions are associated with the silica network. The spectra do not change significantly with increasing temperature up to 850°C. For gels prepared from LiNO₃, the ⁷Li NMR spectra indicate the presence of two different Li species; one of the Li species is attributed to hydrated lithium ions trapped in the pores of the gels, the other Li species is due to crystalline LiNO₃. When the LiNO₃ gels are heat treated above 650°C, the lineshape of the NMR spectra becomes similar to that of Li₂SiO₃.