New sol-gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors

We have developed a new sol-gel route to synthesize several different trans ition and main-group metal oxide aerogels. The approach is straightforward, inexpensive, versatile, and it produces monolithic microporous materials w ith high surface areas. Specifically, we report the use of epoxides as gela tion agents for the sol-gel synthesis of chromia aerogels and xerogels from simple Cr(III) inorganic salts. The dependence of both gel formation and i ts rate was studied by varying the solvent used, the Cr(III) precursor salt , the epoxide/Cr(III) ratio, as well as the type of epoxide employed. All o f these variables were shown to affect the rate of gel formation and provid e a convenient control of this parameter. Dried chromia aerogels were chara cterized by high-resolution transmission electron microscopy (HRTEM) and ni trogen adsorption/desorption analyses, results of which will be presented. The results presented hcl c show that rigid monolithic metal oxide aerogels can be prepared from solutions of their respective metal ion salts (Fe3+, Al3+, In3+, Ga3+, Zr4+ Hf4+, Ta5+, Nb5+, and W6+), provided the formal oxid ation state of the metal ion is greater. than or equal to +3. Conversely, w hen di-valent transition metal salts are used precipitated solids are the p roducts. (C) 2001 Elsevier Science B.V. All rights reserved.