Probing the relationship between chemical structure and thermal degradation behavior of acetone fractionated kraft lignin

This study reports the relationships between structure and thermal degradation behavior of industrial softwood and hardwood kraft lignin (KL) after acetone fractionation to obtain acetone soluble (AS) and acetone insoluble (AI) fractions with reduced structural polydispersity. The structure and thermal degradation behavior of AS-KL and AI-KL was examined by gel permeation chromatography, Fourier transform infrared, quantitative carbon-13 nuclear magnetic resonance spectroscopy, and thermogravimetric analysis. The results showed that the AS-KL fractions had reduced apparent molecular weight, lower polydispersity, less native wood lignin side chains, greater aromatic hydroxyl groups, and more condensed structures. In contrast, the AI-KL fractions showed substantially larger apparent molecular weight and polydispersity, as well as more aliphatic hydroxyl groups and native lignin side chains. Consequently, AI-KL samples exhibited greater thermal degradation activation energy than those of AS-KL samples because the former fractions had a larger apparent molecular weight as well as more aliphatic OH groups, which facilitated hydrogen bonding between lignin polymers, improving their thermal stability. This finding suggests that acetone fractionation of KL can be used to examine the relationship between structure and thermal degradation of industrial KL. These results also provide important information on the thermal degradation behavior of acetone fractionated products with relevant chemical and physical properties for a specific application, such as raw materials for lignin valorization.