Characterization of an Oligomeric Lignin-Derived UV Absorber Produced from Hardwood Beech

Abstract

The damage caused by ultraviolet (UV) radiation to human skin and various materials, such as solar panels, has been extensively documented. Lignin-derived polymers have demonstrated excellent UV-protective properties; however, the uncertainty of their chemical structures and properties has hindered their further utilization. In this study, hardwood beech was degraded by CuO with the assistance of microwave heating. Several oligomeric lignin fractions were obtained from the degradation products by sequential extraction of the precipitates using diethyl ether, ethyl acetate, and acetone. The ethyl acetate-soluble lignin fraction and acetone-soluble lignin fraction were characterized by UV spectroscopy, heteronuclear single quantum coherence nuclear magnetic resonance (HSQC NMR), and gel permeation chromatography (GPC). The ethyl acetate-soluble lignin fractions exhibited UV absorbance comparable to that of technical lignins. HSQC NMR quantitative analysis results demonstrated an increase of conjugated structures involving aldehyde carboxylic acid, and α-ketone groups, and α, β-conjugated cinnamic acid structure end groups in the acetone-soluble fraction (side chain region: 19.4%; aromatic region: 16.8%) and the ethyl acetate-soluble fraction (side chain region: 28.1%; aromatic region: 22.0%), which may be attributed to the increase of UV absorbance. The average molecular weights of the ethyl acetate- and acetone-soluble lignin fractions were 940 and 1200, respectively. These results highlighted the potential of the oligomeric lignin-derived fractions obtained in this study as ideal natural UV absorbers. Moreover, the characterization method described herein may serve as a standardized protocol for evaluating UV-protective compounds derived from various biomasses through different processing techniques.