Study on structural alterations and degradation mechanism of lignin from ozone treated scutched flax tow (SFT)

Ozone is commonly used in lignocellulosic fiber extraction and degumming processes because of its selective reactivity with lignin. However, most studies have focused on the removal of non-cellulosic substances from various lignocellulosic fibers (e.g., wood, pulp, bamboo, and sugarcane bagasse) and the conversion and utilization of the obtained lignin. The lignin degradation mechanism in lignocellulosic fibers during ozone degumming/extraction remains poorly understood. In this study, the structural alterations of milled wood lignin (MWL) isolated from scutched flax tow (SFT) before and after ozone treatment were examined using Fourier transform infrared spectroscopy (FTIR), ultraviolet spectroscopy (UV), gel permeation chromatography (GPC), elemental analysis, thermogravimetric analysis (TGA), pyrolysis-gas chromatography-mass spectroscopy (Py-GC/MS) and nuclear magnetic resonance spectroscopy (NMR). The results showed that the MWL from SFT was primarily composed of syringyl (S) and guaiacyl (G) units. 2D HSQC NMR results revealed that the (3-O-4 ' linkages were dominant in the lignin of SFT. After ozone treatment, some of (3-O-4 ' and (3-(3 linkages and most of (3-5 linkages were damaged. The S-type units further degraded into G-type units in ozone treatment system. Overall, this work provides a detailed analysis and valuable insights into the delignification mechanism of the widely used ozone degumming/extraction process for lignocellulose fibers.