Preparation and structural characterization of lignin micro/nano-particles with ionic liquid treatment by self-assembly

Abstract

In the present research, 1,3-dimethyl-imidazolium dimethyl phosphate ([Mmim][DMP]) ionic liquid was applied to dissolve alkali lignin from wheat grass to prepare for lignin micro/nano-particles. The alkali lignin can be dissolved completely at 80 °C in 30 min under microwave condition with [Mmim][DMP] (solid-liquid ratio of 1:10) which is easy to be recycled. The dissolved lignin can be precipitated by diluting the ionic liquid-lignin solution with water to get lignin micro/nano-particles with diameter ranged from 200 nm to 1.5 µm. Structural analyses with Gel permeation chromatography (GPC), 2D heteronuclear single quantum coherence nuclear magnetic resonance (2D-NMR), Thermogravimetric Analysis (TGA), etc., indicate that a large number of dehydration reactions occurred during dissolution process to result in the generation of unsaturated bonds with higher degree of unsaturation (ω) (from 197–334), lower polydispersity (from 1.49 to 1.12) and lower average molecular weight (from 8634 to 5406 Da). Compared with original alkali lignin, the regenerated alkali lignin as micro/nano-particles showed higher thermal stability. It was also found from the 2D NMR spectra that the cleavages of aryl-ether bonds dramatically happened during the dissolution process, [Mmim][DMP] ionic liquid is more likely to destroy lignin units linked by β–O–4′ bonds to G-type lignin with less steric hindrance, resulting in the least amount of β–O–4′ linkages. In addition, dehydration and demethylation of lignin occurred during the dissolution of ionic liquid.

Keywords

• Nanomaterials
• ionic liquid
• alkali lignin
• dissolution
• micro/nano-particles