Selective Modification of Aliphatic Hydroxy Groups in Lignin Using Ionic Liquid

A facile, sustainable method for the selective modification of aliphatic hydroxy (R-OH) groups in Kraft lignin was developed using an ionic liquid, 1-ethyl-3-methylimidazolium acetate (EmimOAc), as a solvent and catalyst. Selective R-OH modification was achieved by a one-pot, two-step homogeneous reaction: (i) acetylation of R-OH and aromatic OH (Ar-OH) groups with isopropenyl acetate (IPAc) as an acyl donor and (ii) subsequent selective deacetylation of the generated aromatic acetyl (Ar-OAc) groups. In step (i), IPAc reacts rapidly with Ar-OH but slowly with R-OH. The generated Ar-OAc is gradually deacetylated by heating in EmimOAc, whereas the aliphatic acetyl (R-OAc) groups are chemically stable. In step (ii), all R-OH is acetylated by IPAc and Ar-OAc which is a better acyl donor than IPAc, contributing to the rapid acetylation of the remaining R-OH, and selective deacetylation of the residual Ar-OAc is completed by adding a tiny amount of water as a proton source. This two-step reaction resulted in selective R-OH modification (>99%) in Kraft lignin with the remaining being almost all Ar-OH groups (93%). Selectively modified Kraft lignin was obtained with an acceptably high isolated yield (85%) and repeatability (N = 3). Furthermore, despite the lower substitution degree, it exhibited solubility in common solvents, heat-meltability, and thermal stability comparable to completely acetylated Kraft lignin.

Automated summary

A facile and sustainable method using an ionic liquid was developed for selectively modifying aliphatic hydroxy groups in Kraft lignin. The two-step homogeneous reaction resulted in high purity selectively modified Kraft lignin with improved yield and repeatability. The modified Kraft lignin showed good solubility, heat-meltability, and thermal stability comparable to completely acetylated lignin.