Activated carbons from walnut wood: microporosity and fractal dimension

Resorcinol-formaldehyde cryogels prepared at different dilution ratios have been activated with phosphoric acid at 450 C and compared with their carbonaceous counterparts obtained by pyrolysis at 900 C. Whereas the latter were, as expected, highly mesoporous carbons, the former cryogels had very different pore textures. Highly diluted cryogels allowed preparation of microporous materials with high surface areas, but activation of initially dense cryogels led to almost non-porous carbons, with much lower surface areas than those obtained by pyrolysis. The optimal acid concentration for activation, corresponding to stoichiometry between molecules of acid and hydroxyl groups, was 2M l−1, and the acid–cryogel contact time also had an optimal value. Such optimization allowed us to achieve surface areas and micropore volumes among the highest ever obtained by activation with H3PO4, close to 2200m2 g−1 and 0.7 cm3 g−1, respectively. Activation of diluted cryogels with a lower acid concentration of 1.2M l−1 led to authentic bimodal activated carbons, having a surface area as high as 1780m2 g−1 and 0.6 cm3 g−1 of microporous volume easily accessible through a widely developed macroporosity.