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

Abstract

We have developed a new sol–gel route to synthesize several different transition and main-group metal oxide aerogels. The approach is straightforward, inexpensive, versatile, and it produces monolithic microporous materials with high surface areas. Specifically, we report the use of epoxides as gelation agents for the sol–gel synthesis of chromia aerogels and xerogels from simple Cr(III) inorganic salts. The dependence of both gel formation and its 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 of these variables were shown to affect the rate of gel formation and provide a convenient control of this parameter. Dried chromia aerogels were characterized by high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption analyses, results of which will be presented. The results presented here show that rigid monolithic metal oxide aerogels can be prepared from solutions of their respective metal ion salts (Fe 3+, Al 3+, In 3+, Ga 3+, Zr 4+, Hf 4+, Ta 5+, Nb 5+, and W 6+), provided the formal oxidation state of the metal ion is greater than or equal to +3. Conversely, when di-valent transition metal salts are used precipitated solids are the products.