chapter  3
14 Pages

Aerogel Synthesis

WithGerard M. Pajonk

Abstract .................................................................................................................... 31 3.1 Introduction .................................................................................................... 31 3.2 The Principles of the Aerogel Process .......................................................... 33

3.2.1 The Solution-to-Sol-to-Gel Step........................................................ 33 3.2.2 The Supercritical Drying Step ........................................................... 36 3.2.3 Some Advantages of the Aero(Gel) Synthesis.................................. 38

3.3 Some Examples of the Synthesis of Catalytic Aerogels .............................. 39 3.4 Conclusion...................................................................................................... 42 Acknowledgment ..................................................................................................... 42 References................................................................................................................ 42

Aerogels are ultra-porous solid materials exhibiting large pore volumes (porosities ≥ 90%), high surface areas (hundreds of m2/g), and low bulk densities. They can be directly prepared as powders, chunks, thin films, or monoliths of many tens of cm2, opaque, translucent, or transparent to light. Despite their low bulk densities, aerogel powders can be fluidized. They are obtained through the combination of sol-gel chemistry and subsequent drying under supercritical conditions with respect to the liquid phase filling their porosities. When the high-temperature supercritical process is applied (with alcohols, acetone, etc.), the catalysts are hydrophobic, and with the low-temperature supercritical method (with CO2) or when they are dried under the form of ambigels, they are hydrophilic. But in every case, they can be made hydrophilic or hydrophobic at will, in a reversible process, through cogelation or derivatization methods with the intervention of a thermal treatment in the first case and with a hydrophobic reactant in the second case. Aerogels are typically nanomaterials. For all these properties, aerogel solids are very interesting candidates for heterogeneous applications.