Structural Characterization of Lignin and Its Degradation Products with Spectroscopic Methods

Yao Lu,Xing Fan,Yong-Chao Lu,Xian-Yong Wei,Feng-Jin Xie,Hong-Qin Hu

doi:10.1155/2017/8951658

Abstract

Lignin is highly branched phenolic polymer and accounts 15–30% by weight of lignocellulosic biomass (LCBM). The acceptable molecular structure of lignin is composed with three main constituents linked by different linkages. However, the structure of lignin varies significantly according to the type of LCBM, and the composition of lignin strongly depends on the degradation process. Thus, the elucidation of structural features of lignin is important for the utilization of lignin in high efficient ways. Up to date, degradation of lignin with destructive methods is the main path for the analysis of molecular structure of lignin. Spectroscopic techniques can provide qualitative and quantitative information on functional groups and linkages of constituents in lignin as well as the degradation products. In this review, recent progresses on lignin degradation were presented and compared. Various spectroscopic methods, such as ultraviolet spectroscopy, Fourier-transformed infrared spectroscopy, Raman spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy, for the characterization of structural and compositional features of lignin were summarized. Various NMR techniques, such as 1H, 13C, 19F, and 31P, as well as 2D NMR, were highlighted for the comprehensive investigation of lignin structure. Quantitative 13C NMR and various 2D NMR techniques provide both qualitative and quantitative results on the detailed lignin structure and composition produced from various processes which proved to be ideal methods in practice.

Highlights

The main components of lignocellulosic biomass (LCBM) are cellulose, hemicellulose, and lignin
Cellulose is a polymer of glucose, accounting for 30–50 wt% of dry LCBM; hemicellulose is a mixture of heteropolymers containing various polysaccharides, such as xylan, glucuronoxylan, and glucomannan, accounting for 20–35 wt%; the mainly remaining portion with 15–30 wt% is lignin, which is a multisubstituted phenolic polymer
A switchable ionic liquid (SIL), synthesized from 1,8diazabicyclo[5.4.0]undec-7-ene (DBU), monoethanol amine (MEA), and CO2, named CO2-switched [DBU][MEASIL], was demonstrated to have high ability to extract the interlinked polysaccharide impurities from the sodium lignosulfonate while the linkages and aromatic subunits remain unaffected during the dissolution-recovery cycle

Summary

Introduction

The main components of lignocellulosic biomass (LCBM) are cellulose, hemicellulose, and lignin. The comprehensive elucidation of structural and compositional features of lignin relies on the processes for the degradation and isolation of lignin from LCBM and methods applied in the characterization of the corresponding products [2, 13]. The chemical compositional features of the resulting technical lignins, such as the relative abundance of S/G/H units, the status of side chains, and the contents of functional groups, are highly dependent on the methods and conditions used in degradation processes [17]. The chromatographic techniques coupled to mass spectrometers and high-resolution mass spectrometric techniques were used extensively in the analysis of the bio-oil, biomass, and lignin samples [20,21,22,23,24] These methods concentrate on the detection of individual species basing on the chromatographic separation and high molecular resolution. The degradation processes and instrumental methods involved in the detailed and comprehensive understanding of the lignin structure were prospected

Degradation Processes of Lignin

Spectroscopic Methods

Findings

Conclusions