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Behavior of oxygen-containing groups in grass lignin during dissolution in basic ionic liquids

Liu, Chao, Li, Youming, Hou, Yi
Cellulose 2019 v.26 no.2 pp. 737-749
Fourier transform infrared spectroscopy, acetates, alkalinity, cellulose, grasses, ionic liquids, lignin, molecular weight, pyrolysis, thermogravimetry, wheat
Three basic ionic liquids(BILs), 1-amyl-3methylimidazolium acetate (C5mim][Ac]), 1-butyl-3methylimidazolium acetate ([Bmim][Ac]) and 1-ethyl-3-methylimidazolium acetate ([Emim][Ac]), have been applied for the dissolution of lignin from wheat grass under microwave conditions with the decreased alkalinity order of [Emim][Ac]>[Bmim][Ac]>[C5mim][Ac]. Also the behavior of oxygen-containing groups within lignin in the dissolution of ionic liquids was investigated by elemental analysis, analytical pyrolysis, 2D-HSQC spectra, GPC, TG and FTIR analysis. Results revealed that [Emim][Ac] showed a higher dissolution capacity (≥ 330 g/kg) at 80°C within 30 min under microwave condition with higher alkalinity. In the process of dissolution, ionic liquids mainly attack the oxygen-containing groups and the aryl ether bond in the lignin side chain. The methoxy content reduction, the cleavage of β-O-4’ linkages and the degradation of β–β linkages obviously happened in the dissolution process, resulting in the decrease of the molecular weights. Ionic liquids treatment selectively degraded the G-type lignin fragments with demethoxylation and dehydration reactions occurring during dissolution. The results also indicate that the shorter the alkyl chain of the ionic liquids, the stronger the alkalinity is, the more destructive the oxygen-containing groups in the lignin side chain.