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.

Keywords: biopolymer; carbon gel; hydrothermal carbonization; low-cost; organic gel; polyphenolic molecules; pore structure; porous materials; sol-gel; tannin.

Figure 1

Polyflavonoids and carbohydrates linkage.

Figure 2

The four repeating flavonoid units in condensed tannins.

Figure 3

Chemical structure of phlobaphenes.

Figure 4

Ferric tannates [31].

Figure 5

Tridimensional structure of a tetraflavonoid 4,8-linked.

Figure 6

(a) Schematic representation of sol-gel process of tannin gels and their respective SEM images at pH 4 (b) and pH 6 (c). Adapted with permission from reference [61]. Copyright 2013 Elsevier.

Figure 7

Reaction mechanisms of a condensed tannin monomer with formaldehyde resulting in: (a) methylene bridges and (b) methyne ether bridges.

Figure 8

Reactive sites of flavonoid units.

Figure 9

Suggested cross-linking reactions on tannin-soy-formaldehyde gel (a) and ¹³C-NMR spectra of organic gel at pH 6 (b) [54].

Figure 10

Phase diagram of the solvent within the gel structure and the representation of the different drying methods with their respective porous materials, adapted with permission from reference [92].

Figure 11

Tannin-formaldehyde gels prepared with surfactant (a) top view, and without (b) bottom view. Reprinted with permission from reference [98]. Copyright 2011 Elsevier.

Figure 12

Gels prepared under hydrothermal conditions with aqueous tannin solution at low pH (a) (Reprinted with permission from reference [106]. Copyright 2015 Elsevier), with metal salts (b) [113], and with aqueous evaporated aminated tannin (c) [114].

Figure 13

Tannin-formaldehyde carbon aerogels prepared under normal conditions at pH 3.3 (a) and pH 8.3 (b) (Reprinted with permission from reference [77] Copyright 2011 Elsevier); and hydrothermal carbon gels prepared with tannin solution at pH 2 (c) (Reprinted with permission from reference [106] Copyright 2015 Elsevier) and at non-modified pH (4.2) (d) [114].