Advances in tailoring the porosity of tannin-based carbon xerogels

• Cost-effective and eco-friendly carbon xerogels can be synthesised by using tannins.
• Porosity of tannin-based carbon xerogels can be designed by varying reactive ratios.
• The use of sodium dodecyl sulfate as surfactant allows obtaining S-doped carbons.

Usually, carbon xerogels are obtained from resorcinol–formaldehyde organic gels. However, more cost-effective and eco-friendly carbon xerogels can be synthesised by using tannins instead of resorcinol, provided that a suitable surfactant is added to prevent the collapse of the structure. The use of tannin, a natural phenolic compound derived from wood, allows obtaining carbon xerogels with controlled porosity, as the porous properties of these materials can be tailored by an appropriate choice of the synthesis conditions. In this work, tannin–formaldehyde xerogels containing different amounts of surfactant and formaldehyde were synthesised in order to evaluate their effect on the porous structure and chemical composition. It was found that porosity and density depend greatly on the amount of surfactant. The lowest density and highest porosity values −0.34 g/cm3 and 78%, respectively-were obtained by adding 10 wt.% of surfactant. It was also found that S-doped carbon xerogels can be easily synthesized due to the strong affinity between the carbon in the structure and the sulphur from the surfactant. Furthermore, statistical analysis showed that there is interdependence between the effect of formaldehyde and the surfactant, especially in the case of volume and pore size. Hence, the choice of the appropriate surfactant-formaldehyde concentration is essential for controlling the formation of the porous polymeric structure.

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