Sol–Gel Processing of Porous Materials

Abstract

Abstract The sol–gel processing of hierarchically porous materials has been reviewed. Especially, a focus was put on monolithic porous gels that can be prepared via sol–gel reaction accompanied by polymerization‐induced phase separation. After continuous development over 20 years, the formation of well‐defined macropores in sol–gel systems in a broad range of chemical compositions has been explored, and the versatility of the method was clearly demonstrated. Most metal oxide gels retain their inherent porosity within the gel framework comprising the continuously macroporous monoliths, so that additional aging treatments under basic and/or hydrothermal conditions worked efficiently to tailor the mesopores essentially independent of the preformed macroframeworks. Although this chapter focuses on demonstrating the formation of hierarchically porous structures, also included are some related topics such as low‐density aerogel‐like materials with unimodal well‐defined porosity. Introduced applications were limited to those proven to work and/or commercialized in separation sciences and biopurification. Further details on basic concepts are best found in one of the reviews. On the materials prepared from ionic precursors (metal salts) in aqueous media, mechanically stable aerogel‐like transparent low‐density hybrid materials, and those convertible to non‐oxide ceramics and carbons, there remain a lot to be explored in the near future from both basic and application viewpoints.