Revealing Structural Differences between Alkaline and Kraft Lignins by HSQC NMR

Technical lignins, mostly generated as byproducts from the pulping industry, are highly abundant aromatic sources. However, they are underutilized due to their complexity, as well as structural alteration during chemical pulping processes. In-depth elucidation of technical lignins is becoming essential for their valorization in view of the lack of understanding of technical lignin structures. In this work, alkaline and kraft lignins were prepared and comparatively characterized by 2D HSQC NMR. By applying authenticated reference compounds, the phenylglycerol structures, characteristic of the cleavage of nonphenolic β-aryl ether by soda pulping, in the alkaline lignin were identified and quantified by 2D HSQC NMR. Phenylglycerol structures in alkaline lignin were estimated to be 8–14%, which was much higher than that in kraft lignin. This finding was supported by the results from model studies; i.e., the yield of the phenylglycerol product was 40% that obtained from the soda pulping treatment of a nonphenolic β-aryl ether compound while the yield of that from kraft treatment was lower than 5%. In addition, styryl ether structures from phenolic β-aryl ethers of alkaline lignin were also revealed by 2D NMR. These new findings will benefit the mechanistic understanding of lignin reactions and structural elucidation of technical lignins, which will provide useful information (or new insights) for the development of lignin valorization strategies.