Elucidation of Chemical Structure of Wood Lignin by Dissolving in 1-Butyl-3-methylimidazolium Chloride Ionic Liquid

Abstract

Residual lignin of wood after MWL isolation was dissolved with 1-butyl-3-methylimidazolium chloride ionic liquid. Chemical structure of isolated MWL from poplar wood (Popolus deltiodes) and dissolved residual lignin were characterized with gas chromatography and 1D, 2D NMR techniques. Analytical results showed that Popolus deltoides wood has more 4-(3-hydroxy-1-propenyl)-2-methoxy phenol units (Guayacil lignin) than 4-(3-hydroxy-1-propenyl)-2,6-dimethoxy phenol units (Syringyl moieties). In addition to Guayacil and Syringyl, the extracted ligin is composed of a small amount of p-benzyl alcohol. The residual lignin is mainly composed of condensed structures which are often of carbohydrate complexes. The lignin structural ratios of dissolved wood in ionic liquid are the same as those of dioxan extracted lignin. Anomeric HSQC analysis of lignin showed that the lignin carbohydrate complex is composed of cellulose, xylan, mannan and arabinan sugars. Side-chain region of lignin structure is mainly composed of β-O-4, phenylcoumaran, resinol and spirodienone moieties.

Keywords

• milled wood lignin
• isolation
• chromatography
• natural polymer
• ionic liquid