From Lignin Waste to Efficient Catalyst: Illuminating the Impact of Lignin Structure on Catalytic Activity of Cycloaddition Reaction

Lignin, as a byproduct from the paper industry and biorefineries, has been used as a renewable alternative to the conventional catalyst for future industrial application. However, due to the complex structure of lignin, its application has been considerably restricted. Herein, three types of lignin (soda lignin, kraft lignin, and enzymatic hydrolysis lignin) were prepared to illuminate the real impact of lignin interunit linkages and types of hydroxyl groups on the catalytic activity of cycloaddition reaction. Among these lignin/cocatalyst systems, a soda lignin/KI system showed excellent catalytic performance for the cycloaddition reaction at a low KI loading of 0.33 mol % (TON = 273). Particularly, soda lignin with abundant beta-O-4' linkages, higher aliphatic C-a-OH groups, and phenolic hydroxyl groups as well as enriched carboxylic-OH groups could activate epoxides more efficiently and furthermore accelerate the cycloaddition reaction. Moreover, the dimeric intermolecular hydrogen bonding was dominant in soda lignin, which could also account for the higher catalytic activity of cycloaddition reaction. This work highlights the application of the soda lignin in a cycloaddition reaction and opens a new way for the utilization of lignin.