New tannin–lignin aerogels

Highly porous organic aerogels based on tannin and lignin have been prepared and characterized for the first time. Hydrogels were first described, which were prepared at constant solid weight fraction and constant pH, but with different tannin/lignin and (tannin + lignin)/formaldehyde weight ratios. A phase diagram has been drawn, showing the range of compositions in which nice hydrogels may be obtained. The porosity of the resultant aerogels, dried with supercritical CO2, has been systematically investigated in terms of surface area, macro (pore width > 50 nm), meso (2–50 nm) and microporosity (<2 nm). The impact of the composition on the porous properties was thoroughly discussed and supported by electron microscopy studies. We show how the gradual substitution of tannin by lignin modified the pore size distribution, although the aerogels remained almost purely mesoporous materials. Values of thermal conductivity and mechanical resistance are also given, which are compared with those of much more expensive, non renewable, aerogels derived from resorcinol–formaldehyde.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The first tannin–lignin organic aerogels are reported here for the first time. ► A phase diagram presents the most suitable compositions for getting nice hydrogels. ► All aerogels are almost purely mesoporous (2 nm < pore width < 50 nm). ► More tannin leads to narrower pores, whereas more lignin leads to wider pores.