Aerogels Part 1. A Focus on the Most Patented Ultralight, Highly Porous Inorganic Networks and the Plethora of Their Advanced Applications

Aerogels (AGs) are highly porous, low-density, disordered, ultralight macro-scopic materials having immense surface areas. Traditionally synthesized us-ing aqueous sol–gel chemistry, starting by molecular precursors, the nano-particles (NPs) dispersions gelation method is nowadays the most used pro-cedure to obtain AGs with improved crystallinity and broader structural, morphological and compositional complexity. Sol-gel process consists of preparing a solution by hydrolysis of different precursors, followed by gela-tion, ageing and a drying phase, via supercritical, freeze-drying or ambient evaporation. AGs can be classified based on various factors, such as appear-ance, synthetic methods, chemical origin, drying methods, microstructure, etc. Due to their nonpareil characteristics, AGs are completely different from common NPs, thus covering different and more extensive applications. AGs can be applied in supercapacitors, acoustic devices, drug delivery, thermal insulation, catalysis, electrocatalysis, gas absorption, gas separation, organic and inorganic xenobiotics removal from water and air and radionucleotides management. This review provides first an analysis on AGs according to data found in CAS Content Collection. Then, an AGs’ classification based on the chemical origin of their precursors, as well as the different methods existing to prepare AGs and the current optimization strategies have been discussed. Following, focusing on AGs of inorganic origin, silica and metal oxide-based AGs were reviewed, deeply discussing their properties, specific synthesis and possible uses. These classes were chosen based on the evidence that they are the most experimented, patented and marketed AGs. Several related case studies have been reported some of which have been presented in read-er-friendly tables and discussed.