Ionic liquid-assisted isolation of lignin from lignocellulose and its esterification with fatty acids

Abstract

Ionic liquids (ILs) have been widely used for lignocellulose fractionation and lignin isolation. However, the effect of IL treatment on lignin structure has been less explored. This study aimed to explore the chemical structure of lignin isolated by widely used imidazolium based ILs and compare it with the well-known milled wood lignin structure (MWL). Four types of ILs were used, and the effects of the treatment conditions on the isolated lignin (ILL) characteristics were evaluated. As the treatment temperature was increased from 60 to 140 °C, the ILL yield increased, whereas the molecular weight and hydroxyl group content decreased. Among the ILs, [EMIM]Ac produced the highest lignin yield (5.28%), and the ILL obtained had the lowest hydroxyl content (1.27 mM/g) and a molecular weight (Mw) of 29,500 g/mol. Esterification of the ILL isolated with [EMIM]Ac was performed using three fatty acid chlorides, octanoyl chloride (C8), lauroyl chloride (C12), and palmitoyl chloride (C16), to extend its applicability. The effects of esterification on the characteristics of ILL were evaluated, and successful esterification was confirmed using Fourier transform infrared and nuclear magnetic resonance spectroscopies.