Structural analysis of light-colored separated lignin (lignocresol) and its antioxidant properties

Abstract

The use of lignin is limited by its heterogeneity and complexity. This study was processed using different methods to obtain spruce milled wood lignin (SMWL), spruce kraft lignin (SKL), and spruce lignocresol (SLC) for comparative analysis of the structure and antioxidant activity. SMWL has a complete softwood lignin side-chains structure and lignin carbohydrate complexes. SKL contains fewer ether bonds, while more conjugate structures and condensed structures contribute to the color. However, the α-position of the lignin side chain eliminates most of the hydroxyl and ether bonds (β-O-4/α-OH, phenylcoumaran, and dibenzodioxocine structure) and effectively grafts p-cresol in the phase separation reaction. It not only inhibits the self-condensation of lignin, but also forms the 1,1-diarylpropane unit while protecting β-O-4 linkages from not breaking. Importantly, SLC has few conjugate structures that result in the lightest color among all lignin isolated. Besides, SLC has a high yield and contains trace carbohydrates, indicating that the phase separation method can achieve great amounts of purity separated lignin. The antioxidant activity of lignin was evaluated, results show that 85% of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals were scavenged at the end of 60 min. Owing to its unique color, structural properties, and continuous antioxidant activity, SLC has the potential to manufacture antioxidant cosmetics.