Studies of Mixed Silica/Alumina and Silica/Boric Oxide Materials

Multinuclear low temperature solution NMR and FTIR has been used to monitor the hydrolysis and polymerisation chemistry involved in the preparation of multicomponent silica/alumina and silica/boric oxide monolithic gels. Pre-hydrolysis of the silicon component (tetraethylorthosilicate) in the presence of low levels of water is an important factor in obtaining transparent materials. In order to obtain high homogeneity and minimise the subsequent precipitation of the fastest hydrolysing component, ethylacetonate (etac) has been used to modify the aluminum alkoxides. Solid state nmr and FTIR studies show that the borosilicate system contains Si-O-B bonds during the early stages of polymerisation but they are absent on final gelation. Thermal treatment to around 500 degrees C is required to generate mixed Si-O-B bonds. Length of prehydrolysis has little affect on the nature of the Si-O-B gel materials but has a significant effect on the chemical nature of Si-O-Al materials. Longer silicon alkoxide prehydrolysis times lead to better defined materials.