Synthesis of mesoporous silica foams with hierarchical trimodal pore structures

A new type of mesoporous silica foam having extremely low bulk density (<0.01 g cm^-3), high mesopore volume (>2 cm³ g^-1) and specific surface area (> 1000 m² g^-1) was prepared by the spraying of aqueous siliceous solutions containing alkyltrimethylammonium surfactants under high pH conditions. These remarkable properties were ascribed to macrovoids and two kinds of mesopores ranging around 3 and 40 nm. The macrocellular structure produced with air bubbles was stabilized through a rapid condensation reaction during drying. The framework of the macrovoids contained mesopores with a bimodal distribution derived from the surfactant micelles and the interparticle spaces of silica nanoparticles. The hierarchical self-assembly of surfactant micelles, silica nanoparticles and air bubbles provided the specific architecture with a trimodal pore size distribution.