Selective substitution of long-acyl groups into alcohols of kraft lignin over transesterification using ionic liquid

Shiori Suzuki,Tadahisa Iwata

doi:10.1186/s10086-021-01985-5

Abstract

Kraft lignin is a valuable aromatic renewable resource that is discharged in large quantities during the kraft pulping process. In this study, kraft lignin derivatives with improved solubility and thermal properties were prepared by a facile chemical modification of the aliphatic hydroxy (R-OH) group in kraft lignin with various ester groups. Kraft lignin was subjected to homogeneous transesterification with vinyl esters as acyl donors using an ionic liquid as a green solvent and catalyst. The selective introduction of acyl groups into the R-OH group was confirmed by nuclear magnetic resonance (NMR) spectroscopy, and it was estimated that approximately 90% of the R-OH group was converted whereas more than 80% of the aromatic hydroxy (Ar-OH) group was retained. The R-OH-selective introduction of long-chain acyl groups of more than six carbons successfully provided superior solubility in common low-boiling solvents, such as chloroform and tetrahydrofuran, and sufficient heat-meltability to be molded into films by hot-pressing. All the kraft lignin derivatives showed high glass transition temperatures of over 100 °C, indicating their potential to be heat-resistant materials. The kraft lignin derivatives, in which only the R-OH group was acylated, retain their inherently rich Ar-OH groups and thus, can be applied as desirable precursors in a wide range of further chemical treatments for functional polymer materials.

Highlights

Kraft lignin is an industrial lignin obtained from the byproduct of the kraft pulping process, which accounts for approximately 85% of the total lignin production in the world [1]
Strategy for development of one‐step selective modification of R‐OH group in kraft lignin In our previous study, a one-pot two-step chemical modification method using Ethyl-3-methylimidazolium acetate (EmimOAc) was proposed for the acetylation of the R-OH group in kraft lignin with remaining the Ar-OH group intact
For developing a more facile method, our focus was on the fact that the reaction rate of acetylation of the Ar-OH group is much faster than that of the R-OH group under the presence of EmimOAc and a limited amount of isopropenyl acetate (IPAc) [16]

Summary

Introduction

Kraft lignin is an industrial lignin obtained from the byproduct (i.e., black liquor) of the kraft pulping process, which accounts for approximately 85% of the total lignin production in the world [1]. The annual production of kraft lignin is approximately 630,000 tons. It is mainly used in the form of combustion for heat recovery, resulting in low-value utilization. High-value utilization of kraft lignin is in demand, there has been no significant progress in industrialization. The major factors that prevent its effective utilization, such as polymeric materials, are: (1) low molecular weight. Despite the difficulties in material application, kraft lignin has an attractive character as an inherently multifunctional polymer having aliphatic hydroxy (R-OH) and carboxy (COOH) groups, but 5–10 times more abundant aromatic hydroxy (Ar-OH) groups than native lignin.

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