Development of Porosity upon Chemical Activation of Kraft Lignin with ZnCl₂

Abstract

Chemical activation of kraft lignin with ZnCl2 allows one to obtain high surface area activated carbons with a predominantly microporous structure. Most of porosity develops within the thermal range of 400−500 °C, where an increase of temperature leads to increasing micropore volume values (up to ≈1 cm3/g) and a wider distribution of micropore size. An increasing mesoporositiy is also observed. Increasing the impregnation ratio (ZnCl2/lignin, wt) has a similar effect on the evolution of the porous structure. The mesopore distribution remains within the low-size range even at high impregnation ratios (up to 2.3). At 500 °C and an impregnation ratio of 2.3, where the highest contribution of mesoporosity was observed, the mesopore volume reached a value of 0.59 cm3/g. These conditions allowed us to obtain a BET surface area of 1800 m2/g, including the external or non-microporous surface of 200 m2/g. The porous structure developed suggests a fairly homogeneous distribution of the activating agent favored by the softening and partial fusion which kraft lignin undergoes within the temperature range of 180−280 °C.