Abstract
A novel choline-based deep eutectic solvent (DES) with low halogen content—namely choline lactate-lactic acid (CLL)—was synthesized by replacing the chloride anion with lactate anion in choline chloride-lactic acid (CCL). CLL and CCL treatments were conducted at 140 °C for 12 h with hydrogen bond acceptor/hydrogen bond donor =1/10, thereafter composition analysis and characterizations of the lignin extracted by DES treatment (DES lignin) and the solid residue were carried out. The proposed low halogen content DES presented an improved lignin extraction efficiency. The CLL treatment extracted 90.13% of initial lignin from poplar, while CCL extracted 86.02%. In addition, the CLL treatment also provided DES lignin with an improved purity (91.17%), lower molecular weight (Mw/Mn=1805/971 g/mol) and more concentrated distribution (polydispersity index=1.86). The efficient lignin extraction was mainly ascribed to the cleavage of β-O-4 bonds in lignin macromolecule, especially in the guaiacyl units, thereby breaking them into smaller molecules, facilitating the lignin extraction. The replacement of chloride anion allowed CLL acting as a more efficient DES to interact with lignin macromolecules, thus providing lignin with higher uniformity and suitable molecular weight. The low halogen content DES system proposed in present work could benefit the fractionation of biomass, improve the valorization of lignin compounds and facilitate industrial process in the downstream.
Highlights
Lignocellulosic biomass is a promising carbon-based natural alternative to fossil fuel resources due to its abundant availability, low cost and renewable feature [1]
In order to evaluate the efficiency of choline base-deep eutectic solvent (DES) on the lignin extraction from the poplar, choline chloride-lactic acid (CCL) and choline lactate-lactic acid (CLL) at hydrogen bond acceptors (HBA)/hydrogen bond donors (HBD) of 1/10 were prepared
The whiskers regions in the figure indicate the compositions extracted into DES lignin, while black bars are corresponding to compositions remain in the solid residues
Summary
Lignocellulosic biomass is a promising carbon-based natural alternative to fossil fuel resources due to its abundant availability, low cost and renewable feature [1]. Choline carboxylic acid [24,25] and choline amino acid [26,27,28,29,30] were synthesized and reported as effective solvents for lignocellulosic biomass pretreatment These choline-based ionic liquids presented low toxicity and good biodegradability [25,31]. Replacing the chloride anion in choline chloride-lactic acid DES with carboxylate could improve the selectivity during the lignin extraction and prevent the corrosion of process equipment. A novel choline-based DES—namely, choline lactate–lactic acid (CLL)—was synthesized by replacing the chloride anion in choline chloride-lactic acid (CCL) Both CLL and CCL were used to selectively separate lignin from poplar for evaluating the extraction efficiency of CLL and CCL. The purity and structural characterization of lignin were analyzed by gel permeation chromatography (GPC), Fourier-transform infrared (FT-IR) and heteronuclear single quantum coherence nuclear magnetic resonance (HSQC NMR)
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