New ionic liquid for the dissolution of lignin

Abstract

This work aims to develop a new ionic liquid, used as an aprotic green ä solvent, to dissolve kraft lignin from black liquor. The kraft lignin was extracted through precipitation with carbon dioxide at atmospheric pressure. 1,8-Diazabicyclo[5.4.0]undec-7-ene-based ionic liquids were obtained by quaternization of the nitrogen atom with a hydrogen atom or an alkyl chain. The yields of the synthesis of the ionic liquids varied between 76 and 80%. Dissolving experiments were carried out using the lignin isolated from the black liquor of a kraft process. Up to 20% (w/w) of the lignin can be dissolved in butyl-1,8 diazabicyclo[5.4.0] undec-7-enium chloride ([DBUC4]+[Cl-]), hexyl-1,8 diazabicyclo[5.4.0] undec-7-enium chloride [DBUC6]+[Cl-], and octyl-1,8 diazabicyclo[5.4.0] undec-7-enium chloride [DBUC8]+[Cl-]. The time it takes to dissolve the lignin in these three liquids shows that its solubility is influenced mostly by the nature of the cations. The lignin solubility was reduced in relation to the increased length of the grafted carbon chain. The thermogravimetric analysis (TGA) showed these liquids can be used as lignin solvents from room temperature up to 300 °C (onset of degradation). Steric exclusion chromatography showed a slight decrease (6%) in the molecular weight of the lignin dissolved in these ionic liquids.