Lignin Functionalization through Chemical Demethylation: Preparation and Tannin-Like Properties of Demethylated Guaiacyl-Type Synthetic Lignins

Demethylation of guaiacyl-type synthetic lignin (GDHP) with three different reagents, 1-dodecanethiol (DSH), hydroiodic acid (HI), and iodocyclohexane (ICH), was investigated as a basic study for lignin functionalization. Demethylation did not proceed efficiently in the reaction with DSH, although cleavage of the lignin side chains and nucleophilic substitution by DSH occurred. Demethylation proceeded efficiently in the reactions with HI and ICH, with significant cleavage of the lignin side chains. Furthermore, a recondensation reaction also occurred in the reaction with ICH. As a result, ICH was found to be the most effective demethylation reagent for increasing the phenolic-OH levels in the synthetic lignin. GDHP demethylated with ICH showed higher tannin-like properties (bovine serum albumin adsorption, 2,2-diphenyl-1-picrylhydrazyl radical scavenging, and iron(III) binding abilities) than nondemethylated GDHP. The latter two abilities especially were significantly improved. In addition, it was found from preliminary experiments that the demethylated GDHP was a useful precursor for lignin-based epoxy resin. These results suggested that chemical demethylation is an effective method for the functionalization of lignin.