Methodical study of the chemical activation of Kraft lignin with KOH and NaOH

A commercially available Kraft lignin was chemically activated with two alkaline hydroxides, viz NaOH and KOH, using different preparation conditions. The activation was made at various temperatures, hydroxide to lignin mass ratios, activation times, flow rates of inert gas, and heating rates. The resultant activated carbons were characterised in terms of BET surface area, total, micro and meso-pore volumes, average pore width, carbon yield and packing density. The influence of each parameter of the synthesis on the properties of the activated carbons is discussed, and the action of each hydroxide is methodically compared. It is the first time that so many preparation parameters and so many pore texture characteristics are simultaneously considered for two closely related activating agents of the same lignin precursor. Whatever the preparation conditions, it is shown that KOH leads to the most microporous materials, having surface areas and micropore volumes typically 1.5 and 1.2 times higher than those obtained with NaOH, which is in agreement with some early works. However, the surface areas and the micropore volumes obtained in the present study are much higher than in previous studies, up to 3100 m2/g and 1.5 cm3/g, respectively, using KOH. The thorough study of the way each preparation parameter influences the properties of the final materials brings insight into the activation mechanisms. Each time it was possible, the results of lignin chemically activated with hydroxides were compared with those obtained with anthracites: explanations of similarities and differences were systematically looked for.