Use of epoxides in the sol-gel synthesis of porous iron(III) oxide monoliths from Fe(III) salts

Iron oxide-based porous solids were prepared by a sol-gel process using Fe( III) salts in various solvents. It was observed that the addition of propyl ene oxide to Fe(III) solutions resulted in the formation of transparent red -brown monolithic gels. The resulting gels were converted to either xerogel s by atmospheric drying or aerogels by supercritical extraction with CO2(1) . Some of the dried materials were characterized by nitrogen adsorption and desorption analysis and transmission electron microscopy (TEM). The result s of those analyses indicate that the materials have high surface areas (si milar to 300-400 m(2)/g), pore sizes with mesoporic dimensions (2-23 nm), a nd a microstructure made up of 5-10 nm diameter clusters of iron(III) oxide . The dependence of both gel formation and its rate was studied by varying the epoxide/Fe(III) ratio, the Fe(III) precursor salt, amount of water (H2O /Fe(III)) present, and the solvent employed. All of these variables were sh own to affect the rate of gel formation and provide a convenient control of this parameter. Finally, an investigation of the mechanism of Fe2O3 gel fo rmation was performed. Both pH and nuclear magnetic resonance (NMR) studies suggest that the added epoxide acts as an irreversible proton scavenger th at induces the Fe(III) species to undergo hydrolysis and condensation to fo rm an inorganic iron oxide framework. This method can be extended to prepar e other transition and main-group metal oxide materials.