Effects of hydroxyls on the aggregation behavior of various alkali lignins in an ionic liquid, 1-ethyl-3-methylimidazolium acetate

Abstract

In order to study the effect of hydroxyl in different kinds of industrial alkali lignin (grasses, hardwoods, softwoods) on the aggregation behavior of lignin in ionic liquid, acetylated lignin was prepared by acetylation of lignin. The results of nuclear magnetic resonance spectroscopy (NMR) show that in the process of acetylation about >85% of the phenolic hydroxyl groups reacted, and the hydroxyl group of grass alkali lignin (AL) is more easily acetylated, followed by the softwood. The effects of hydroxyl groups on lignin aggregation were constructed by comparing the differences of particle size, apparent relative molecular weight, scanning electron microscopy (SEM) and transmission electron microscope (TEM) images of AL and acetylated alkali lignin (ACAL). In the process of adding anti-solvent to form microspheres, AL molecules are more likely to aggregate as >1000 nm in particle size with larger intermolecular forces, although the particle size of single molecule is confirmed smaller with only about 0–10 nm. ACALs exists more stretched in the form of single molecule with 10–20 nm particle size because of the reduction of intermolecular and intramolecular forces due to the removal of a large number of hydroxyl groups, the presence of hydroxyl group in ALs causes a significant aggregation and the effects of hydroxyls on the aggregation behavior of lignin increased with the order of aliphatic OH, guaiacyl and catechol OH and condensed phenolic OH. This study provides a theoretical basis for the molecular transformation of lignin and lignin high value utilization.