Abstract
Ionic liquids (ILs) based on 1-butyl-3-methylimidazolium (bmim) cation have proved to be promising solvents for the fractionation of plant biomass with the production of cellulose and lignin. This study deals with the characterization of lignins isolated from coniferous (spruce) wood using [bmim]OAc and [bmim]MeSO4 ionic liquids and their binary mixtures with DMSO (80:20). Molecular weight distributions, functional composition, and structural features of IL lignins were studied by size-exclusion chromatography, NMR spectroscopy (1D and 2D) and atmospheric pressure photoionization high-resolution mass spectrometry. It was shown that the interaction of ILs with lignin leads to significant chemical changes in the biopolymer; a decrease in the degree of polymerization and in the content of free phenolic hydroxyl groups due to alkylation, the disappearance (in the case of [bmim]OAc) of carbonyl groups and a significant destruction of β-O-4 bonds. The chemical reactions between lignin and 1-butyl-3-methylidazolium cation with covalent binding of ionic liquids or products of their decomposition is evidenced by the presence of a large number of nitrogen-containing oligomers in IL lignins.
Highlights
One of the most promising areas of chemical processing of renewable plant feedstock compliant with green chemistry principles is the fractionation of lignocellulosic biomass using ionic liquids (ILs)
Molecular weight distributions of four studied IL lignin preparations (Figure 1) demonstrate unimodality with the molecular weights corresponding to the peak (Mp ) lying in a relatively narrow range of (2–4) × 103 g·mol−1 and being close to this parameter for dioxane lignin (DL) (3 × 103 g·mol−1 )
The difference of IL lignins from dioxane lignin (DL) is in the lack of the highest molecular weight fractions
Summary
One of the most promising areas of chemical processing of renewable plant feedstock compliant with green chemistry principles is the fractionation of lignocellulosic biomass using ionic liquids (ILs). It is known that ILs based on 1-ethyl- and 1-butyl-3-methylimidazolium cations (emim and bmim, respectively) [1,2,3] have an exceptionally high dissolving power in relation to lignin and polysaccharides and a unique ability to fully dissolve wood and other types of lignocellulosic raw materials [4]. ILs are characterized by such valuable properties as non-flammability, low vapor pressure, thermal stability and the possibility of regeneration and reuse in the technological processes. Polysaccharide (cellulose and hemicellulose) and lignin fractions can be extracted from the obtained solutions of plant biomass by fractional sedimentation with water and some other solvents [9]
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