Preparation of aminated hardwood kraft lignin by Mannich reaction and its structural and thermal characterization

This study investigated the characteristics of aminated kraft lignin (AKL) prepared from the Mannich reaction of hardwood kraft lignin (KL) and diethylenetriamine (DETA) for future application in dye removal. Three aminated samples—AKL-3.5, AKL-7.5, and AKL-15—were prepared using different amounts of DETA (3.5, 7.5, and 15 mmol). The morphology, chemical, and thermal properties of AKLs were investigated through various instrumental analyses and compared with KL. Elemental analysis showed that AKL-15 contained the highest nitrogen content (8%). Based on scanning electron microscopy (SEM), AKLs exhibited a smoother surface compared to KL and aggregated into clusters. Infrared, X-ray photoelectron and 2D heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy confirmed the successful introduction of primary and secondary amines, and C = N functionality into the lignin structure.³¹P NMR indicated a decrease in hydroxyl group contents of guaiacyl and p-hydroxyphenyl units and an increase in C₅ amine-substituted units for AKL-15. Thermal analyses (TGA and DSC) indicated that AKLs were thermally less stable and reduced glass transition temperature (T_g) than KL. These findings contribute to the limited literature on the amination of hardwood lignin, as most amination studies focus on softwood and grass lignins.