Lignin and derivative charcoals: Functional groups involved in the adsorption phenomenon

Abstract

This work focuses on the characterization of raw lignin and its derivative charcoals by infrared spectroscopy. Derivative charcoals are obtained by carbonization of lignin and by activation of chars with or without pretreatment. Carbonization is carried out at 400, 600 and 800°C under nitrogen. The residence time of the carbonaceous residues at above mentioned temperatures is 2 hours. Pre-oxidation of char is made under air at 245°C for 6 hours. The activation is performed under carbon dioxide at 700°C for 30, 45 and 60 minutes. The effects of temperature of carbonization and of the duration of activation on the surface functional groups of coals and activated carbons were examined. The oxygenated groups originally present in the lignin are aliphatic and cyclic ethers, carbonyl, methoxyl and hydroxyl groups of phenolic and primary and secondary aliphatic alcohols. Functional groups undergo a substantial loss when carbonization temperature increases. Carbonization leads to the decrease in aliphaticity and the increase in aromaticity and the development of a plastic phase by crazing. During pre-oxidation and activation, there is a formation of carbonyl groups and appearance of other oxygenated groups. It follows a better resolution of the bands and a decrease in the thermoplastic nature. The best resolution of the bands and the oxygenated functional groups necessary for adsorption are obtained in the case of lignin carbonized at 600°C and activated at 700°C for 60 min. It is also the case of lignin carbonized at 600°C, pre-oxidized at 245°C for 6h then activated at 700°C for 30 min.