Abstract
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.
Highlights
Lignin is the most abundant natural aromatic polymer and a potential renewable source of high-value chemicals and materials for significant applications [1,2]
OH and aromatic OH (Ar–OH) groups revealed that an ionic liquid, ethyl-3-methylimidazolium acetate (EmimOAc), catalyzes the acetylation of both types of OH groups with isopropenyl acetate (IPAc) as an acyl donor and the selective deacetylation of Ar–OAc groups
The selective deacetylation was found to arise from the transesterification of Ar–OAc groups and remaining R–OH groups, and the reaction was faster than the transesterification of R–OH groups with IPAc
Summary
Lignin is the most abundant natural aromatic polymer and a potential renewable source of high-value chemicals and materials for significant applications [1,2]. In its native form, depending on its origin, lignin contains three types of aromatic repeating backbone units—p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) (Scheme 1a)—which have aliphatic hydroxy (R–OH) groups such as primary and secondary alcohols and aromatic hydroxy (Ar–OH) groups. 1-ethyl-3-methylimidazolium acetate (EmimOAc) as catalyst and isopropenyl acetate (IPAc) as an acyl acyl donor and (c) selective deacetylation of Ar-OAc groups. We we assumed thatthat the the selective modification of R-OH groups in Kraft lignin cancan be be. The selective R-OH modification in Kraft lignin groups a one-pot two-step reaction. 3).IfIf such selective withwith acylacyl groups cancan be be realized in in a in one-pot two-step reaction Kraft lignin, it is tivemodification modification improves the solution and thermal processabilities of. Schematic representation of the modification of R-OH groups in Kraft lignin via via
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