Salts as structure-directing agents: The hypersaline chemistry of aerogels

Based on recent investigations, the use of inorganic salts is a new powerful route for obtaining high surface area mesoporous materiaIs in a facile one-step approach, also called “salt-templating”. Here, high surface area aerogels were synthesized adding simple inorganic water soluble salts as pore forming agents during the sol-gel process. The model oxide precursor (i.e.,TEOS for silica) hydrolyses in the presence of a salt, e.g. ZnCl2, without using any acid/base catalysts, forming the silica network and producing monoliths with high surface area. The porosity can be controlled by the gel-ageing step (the less the time, the higher the porosity). Two different gel-drying conditions were investigated: room temperature (RT) air drying and CO2 supercritical drying. Supercritically dried silica aerogels reach the impressive BET surface area value of ca. 1400 m2 g-1, but also the simple air-drying procedure produces highly porous monoliths with BET surface areas in the range of 700-800 m2 g-1, thanks to the stabilizing salting-in effect of zinc ions (according to Hofmeister series). Compared to standard sol-gel processes, the herein presented synthesis is extremely simple: it only consists of dissolution of both oxidic precursors and inorganic salts in the selected solvent (i.e. ethanol), gel formation induced by solvent evaporation, salt removal through simple washing with water/ethanol. This way, no organic templates need to be employed and the overall approach is highly efficient and sustainable. In this regard, the salt-templating solution could principally be recovered for following syntheses afterwards, thus reducing by-products. Moreover, the synthesis of highly porous monoliths by RT air drying is of high interest since no special equipment is required, it simplifies the standard procedure and gives access to advanced materials at low cost.