Open Access

ARTICLE

Degradation of Alkaline Lignin in the Lactic Acid-Choline Chloride System under Mild Conditions

by Penghui Li^1,2, Zhengwei Jiang², Chi Yang², Jianpeng Ren^1,2, Bo Jiang^1,2, Wenjuan Wu^1,2,*

1 Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing, 210037, China
2 College of Light Industry and Food Engineering, Nanjing Forestry University, Nanjing, 210037, China

* Corresponding Author: Wenjuan Wu. Email:

(This article belongs to the Special Issue: Advances in Catalysis toward the Production of Green Energy and Chemicals)

Journal of Renewable Materials 2023, 11(5), 2233-2248. https://doi.org/10.32604/jrm.2023.025279

Received 03 July 2022; Accepted 03 August 2022; Issue published 13 February 2023

Abstract

Lignin is a natural polymer, second only to cellulose in natural reserves. Degradation is one of the ways to achieve the high-value transformation of lignin. Deep eutectic solvent (DES) thermal degradation of lignin can be used as an excellent green degradation method. This paper introduces the degradation mechanism and effect of the lactic acid-choline chloride DES system in dissolving and degrading alkaline lignin, and the final solvent recovery. It can also be found from the scanning electron microscope (SEM) images that the surface of the degraded solid product is transformed from smooth to disordered. Fourier transform infrared (FTIR) spectroscopy and ¹ H-NMR spectroscopy were used to characterize the changes in lignin functional groups during DES treatment. The results showed that the content of phenolic hydroxyl groups increased after degradation, indicating that the β-O-4 ether bond was broken. The molecular weight of the degraded lignin was observed by gel permeation chromatography (GPC), and the lignin residue with low molecular weight and narrow polydispersity index was obtained. The lowest average molecular weight (Mw) reached 2512 g/mol. The ratio of oxygen to carbon atoms in lignin increased substantially during degradation as measured by X-ray photoelectron spectroscopy (XPS), probably because DES treatment was accompanied by many oxidation reactions, which led to significant structural changes in lignin and a large number of ether bond breakage reactions during the reaction. The main final degradation products are aromatic monomers, vanillin, butyrovanillone, etc.

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