Gel Structures Containing Al(III)

Abstract

Monolithic optically clear aluminosilicate and alumina gels can be prepared directly from aluminum nitrate (and tetraethoxysilane) dissolved in 1-propanol. The role of aluminum atoms in the formation of the gel networks has been investigated by ²⁷Al magic angle spinning nuclear magnetic resonance, infrared spectroscopy, and small-angle X-ray scattering measurements. In the presence of a cross-linking agent, the Al atoms incorporate in the gel network formed by the agent. The structure of aluminosilicate gels is dominated by the silica network; the bonded Al atoms only modify that. Without a cross-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 aluminosilicate and in alumina wet gels. In aluminosilicate gels the network is shown to have a fractal structure, whereas in the alumina gels a nonfractal network is built up from aggregates.