Gel structures containing Al(III)

Monolithic optically clear aluminosilicate and alumina gels can be prepared directly from aluminum nitrate land tetraethoxysilane) dissolved in l-prop anol. The role of aluminum atoms in the formation of the gel networks has b een investigated by Al-27 magic angle spinning nuclear magnetic resonance, infrared spectroscopy, and small-angle X-ray scattering measurements. In th e presence of a cross-linking agent, the Al atoms incorporate in the gel ne twork formed by the agent. The structure of aluminosilicate gels is dominat ed by the silica network; the bonded Al atoms only modify that. Without a c ross-linking agent, the Al atoms are able to form a continuous gel network by itself in one step, directly from a solution. Most of the aluminum atoms bonded in the gel networks are octahedrally coordinated in both aluminosil icate and in alumina wet gels. In aluminosilicate gels the network is shown to have a fractal structure, whereas in the alumina gels a nonfractal netw ork is built up from aggregates.