Efficient separation and physico-chemical characterization of lignin from eucalyptus using ionic liquid–organic solvent and alkaline ethanol solvent

• The presence of organic solvent in IL can increase the yield of lignin.
• IL–organic solvent lignin has a large molecular weight and high polydispersity.
• A small amount of carbohydrate was found associated with the AEL fractions.
• Alkaline ethanol lignin (AEL) has a higher S/G ratio (3.25) than IL lignin (1.20).

Ionic liquid (IL) 1-butyl-3-methylimidazolium acesulfamate ([BMIM]Ace)-organic solvents mixtures were utilized in the dissolution and extraction of lignin from eucalyptus. The resulting carbohydrate-enriched materials were subsequently extracted with 70% ethanol containing 1 M NaOH. The results showed that the yield of alkaline ethanol lignin (AEL, 8.0–23.3%) was relatively higher than that of IL–organic solvents lignin (IOL, 11.5–15.4%, based on the Klason lignin content). The chemical structures of the lignin fractions obtained were characterized by carbohydrate analysis, gel permeation chromatography (GPC), Fourier transform infrared (FT-IR), and 2 D HSQC nuclear magnetic resonance (NMR) spectroscopy. The IOL fractions exhibited a larger molecular weight (Mw, 9050–6590 g/mol) and a higher polydispersity compared to the AEL fractions (Mw, 3820–2440 g/mol) except for IL–dioxane lignin (Mw, 1005 g/mol). Moreover, 2 D NMR spectra of ILL and AEL fractions showed a predominance of β-O-4′ aryl ether linkages (77.4 and 79.0% of total side chains, respectively), followed by β–β′ resinol-type linkages (12.1 and 13.8%), and lower amounts of β-5′ (4.2 and 3.3%) and spirodienone linkages.

In this study, low-cost and environmental benign method using ionic liquid–organic solvents mixtures and alkaline ethanol solvent were effective to separate lignin from eucalyptus. Lignin and carbohydrate-enriched material could be obtained. It was found that the presence of organic solvent in IL can swell up the cell wall to a large content and increase the extraction efficiency of lignin. The IL and organic solvents can be easily recovered and reused with high yield. The results provide valuable insights into the efficient utilization of lignocellulosic biomass.Figure optionsDownload as PowerPoint slide