Meso-cellular silica foams, macro-cellular silica foams and mesoporous solids: a study of emulsion-mediated synthesis

A simple room temperature synthesis of a series of porous silica materials from pure mesoporous solid to meso-cellular foams to macro-cellular foams using an emulsion containing surfactant, water, oil or polystyrene spheres is reported. A mesoporous solid is obtained using either a low oil concentration with slow stirring or synthesis at alkaline pH. Faster stirring at low oil concentration produced meso-cellular silica foam. A series of macro and mesoporous silica composites are obtained with various wall thickness and interconnectivity using intermediate to high oil concentration. An amorphous solid was formed in an acidic pH using polystyrene spheres as droplets whereas an ordered macroporous solid with mesoporous walls was formed at alkaline pH. The formation of such structures is explained based on the rate of various processes, e.g. hydrolysis of silica, precipitation as micelles, condensation of silica, creaming or coalescence of oil or polystyrene.