Synthesis of inorganic aerogels via rapid gelation using chloride precursors

Abstract

This paper describes the synthesis of silica, titania, zirconia and mixed-metal-oxide aerogels by a new facile approach. The corresponding low cost and non-toxic chlorides instead of the well established but more expensive and deleterious alkoxy precursors were used as starting materials. Hydrolysis of the precursors led to wet gels. The strong exothermic reaction in the case of TiCl₄ or SiCl₄ as a gel precursor can be controlled by both cooling and immersing the needle of the syringe in the aqueous solution. Under optimal conditions a rapid formation of silica- and zirconia based hydrogels could be achieved even without additional amounts of an auxiliary agent. It is shown that in all other cases the hydrolysis reaction and gel formation can be controlled by adding agents with crosslinking properties like propylene oxide, dilution with alcohols or starting from water glass. Water glass not only has the function of a cheap network former but also allows the strict adjustment of the pH value via neutralization of the acid forming hydrolysis reactions. Supercritical drying of the wet alkogel in CO₂ delivered aerogels with BET surfaces up to 1390 m² g⁻¹, as determined by nitrogen adsorption measurements. In addition, the characterization includes XRD and SEM. These findings provide new physical insights into the hydrolysis of the gel precursors and lead to a significant reduction in production costs of inorganic aerogels.