Tannin Gels and Their Carbon Derivatives: A Review

Abstract

Tannins are one of the most natural, non-toxic, and highly reactive aromatic biomolecules classified as polyphenols. The reactive phenolic compounds present in their chemical structure can be an alternative precursor for the preparation of several polymeric materials for applications in distinct industries: adhesives and coatings, leather tanning, wood protection, wine manufacture, animal feed industries, and recently also in the production of new porous materials (i.e., foams and gels). Among these new polymeric materials synthesized with tannins, organic and carbon gels have shown remarkable textural and physicochemical properties. Thus, this review presents and discusses the available studies on organic and carbon gels produced from tannin feedstock and how their properties are related to the different operating conditions, hence causing their cross-linking reaction mechanisms. Moreover, the steps during tannin gels preparation, such as the gelation and curing processes (under normal or hydrothermal conditions), solvent extraction, and gel drying approaches (i.e., supercritical, subcritical, and freeze-drying) as well as the methods available for their carbonization (i.e., pyrolysis and activation) are presented and discussed. Findings from organic and carbon tannin gels features demonstrate that their physicochemical and textural properties can vary greatly depending on the synthesis parameters, drying conditions, and carbonization methods. Research is still ongoing on the improvement of tannin gels synthesis and properties, but the review evaluates the application of these highly porous materials in multidisciplinary areas of science and engineering, including thermal insulation, contaminant sorption in drinking water and wastewater, and electrochemistry. Finally, the substitution of phenolic materials (i.e., phenol and resorcinol) by tannin in the production of gels could be beneficial to both the bioeconomy and the environment due to its low-cost, bio-based, non-toxic, and non-carcinogenic characteristics.

Energy Storage in Supercapacitors: Focus on Tannin-Derived Carbon Electrodes

“…In general, the production of CGs involves five steps: polymerization of the precursor to form an organic gel, aging, solvent exchange, drying, and carbonization (Grishechko et al, 2016;Ciszewski et al, 2019). CGs are classified, according to the drying technique, as aerogels, xerogels, or cryogels whether supercritical, subcritical or freeze drying, respectively, has been used (Braghiroli et al, 2019). Each step of the synthesis process has an impact on the porous texture of the resultant CGs.…”

“…As for petrochemical precursors, the synthesis of porous materials from tannin usually requires the preparation of a resin by crosslinking with formaldehyde, hexamine, or glutaraldehyde, among other possible aldehydes. Thus, different materials have been synthesized such as foams (Tondi et al, 2009;Lacoste et al, 2013;Jana et al, 2014;Szczurek et al, 2014;Delgado-Sánchez et al, 2018), gels (Szczurek et al, 2011a,b;Amaral-Labat et al, 2013;Braghiroli et al, 2019), microspheres (Grishechko et al, 2016), or polyHIPEs (Szczurek et al, 2013). Furthermore, tannin autocondensation reactions enable its polymerization without having to use crosslinkers, opening the way for the production of OMCs by self-assembly techniques (Schlienger et al, 2012;Braghiroli et al, 2016).…”

Study and characterization of carbon xerogels from tannin-formaldehyde system as catalyst support applications

“…Tannin is natural biopolymer extracted from plants and based on polyphenolic compounds, the condensed flavonoids. These species are reactive with formaldehyde and responsible to the network tridimensional formation of the carbon xerogel [9]. Besides, this biosourced compound is a non-toxic and low-price compared to synthetic phenolics, as resorcinol and phenol, used for the same purpose [6,7,9].The synthesis of xerogels comprises the polycondensation reaction with subsequent drying under subcritical conditions.…”

Perspectives on Tannins

“…Braghiroli et al [ 15 ] extensively reviewed the recent achievements related to the preparation of tannin-based gels and their carbon derivatives. In this review, all crucial aspects of gels preparation (synthesis, drying, and pyrolysis conditions) regarding their final properties were recalled and discussed.…”