Aerogel-like silica powders by combustion of sol-gel derived alcogels

Drying of sol-gel alcogels at ambient conditions leads to shrinkage and cracking due to differential capillary pressure making it difficult to obtain low-density materials. Capillary pressure depends on the surface tension of the liquid, which decreases as the temperature increases and becomes zero at a critical temperature in case of commonly used primary alcohols. Therefore, drying at supercritical conditions is often used to obtain silica aerogels from alcogels. In this case multiple solvent exchange steps are required to remove the residual water beforehand. We propose and show experimentally that the combination of the temperature-dependent behavior of the surface tension of alcohols and the nanoporous nature of sol-gel derived alcogels provides a quick and simple pathway to obtain low-density silica materials without hydrophobization, drying control chemical additives, solvent exchange nor supercritical drying apparatus. We demonstrate that, similarly to the many phenomena that require high pressure to occur in bulk phases being observed in nanopores at ambient pressure, simple combustion of sol-gel derived alcogels can result in pressures and temperatures in gel pores that decrease the surface tension of the alcohol sufficiently to produce granular silica with markedly lower density as compared to xerogels produced at room temperature. Such properties are obtained despite the inevitable high-temperature annealing that accompanies combustion. The method yields SiO2 granules with uniform size (~3 mm), low density (down to 0.358 g/cm3), high porosity (up to 79.2%), average pore size in the range of 9–14 nm and specific surface area of up to 832 m2/g.