Organic and Carbon Gels Derived from Biosourced Polyphenols

This chapter presents the most recent updates about sol-gel chemistry of phenolic molecules and the corresponding materials: xerogels, cryogels, and aerogels. The structure and properties of the latter, whether in the organic or carbon forms, are detailed and actual and potential applications are reported. After an introduction about plant polyphenols in general, the focus is mainly given to condensed (flavonoid) tannins, shown to be the most relevant raw material for preparing resins, and hence gels. Lignin is considered as well, despite its lower reactivity and its less reproducible character, because of its industrial importance. Details about the nature and the properties of the carbon that can be obtained by pyrolysis of crosslinked polyphenols are also given. Tannin-formaldehyde resins and mixed formulations associating resorcinol, soy protein, lignin, phenol, or surfactant are then discussed in terms of reactivity and ability to produce highly porous gels, depending on the experimental conditions of synthesis (dilution, pH, amount of crosslinker, etc.) and drying (subcritical, super-critical, or lyophilization). The porous structure of those materials is also explained in relation to gelation time and mechanical properties of the corresponding hydrogels. Derived carbons gels, including N-doped, formaldehyde-free materials, and activated carbon gels, are also considered. Mechanical and thermal properties of organic gels, as well as electrochemical properties of carbon gels, are next introduced. Finally, recent developments including one-step microwave synthesis of xerogels, carbon xerogel microspheres having the characteristics of carbon molecular sieves, and elastic gels behaving as rubber springs with tunable elastic properties, all biosourced and tannin-based, are presented.