Comparison of the Dubinin–Radushkevich and Quenched Solid Density Functional Theory approaches for the characterisation of narrow microporosity in activated carbons obtained by chemical activation with KOH or NaOH of Kraft and hydrolytic lignins

The classical DR method and the Quenched Solid Density Functional Theory (QSDFT) approach have been used to analyse N2 at 77 K isotherms determined on activated carbons prepared by alkaline chemical activation of different lignins. The QSDFT pore size distributions are bimodal with a narrow peak below 1 nm and a broad peak from 1 to 2.5–3.5 nm. Deconvolution allows estimation of the volumes and widths of the narrow micropores. These are lower than estimated by the DR analysis as this does not separate micropore and non-micropore adsorption. On the basis of the QSDFT analysis, the optimum conditions for obtaining materials with a high volume of narrow micropores were activation temperatures of 550–650 °C, hydroxide/lignin ratio of 1 and dwell time at the maximum activation temperature of 30 min. KOH was preferable to NaOH as it requires lower temperatures and results in materials with higher narrow micropore volumes. The “best” material obtained, prepared with KOH at 550 °C, had mean micropore width of 0.7 nm and micropore volume of 0.37 cm3 g−1 which compares very favourably with molecular sieve carbons prepared from synthetic polymers. Furthermore, this material was obtained with an activation yield of 32.9%, which is quite high for alkaline chemical activation.