Issue 21, 2024

Evaluation on the recovery of lignin from basic [Ch][Lys] systems using low-cost alcohols as anti-solvents under acid-free conditions

Abstract

Delignification of lignocellulose using basic ionic liquids (BILs) such as choline lysinate ([Ch][Lys]) is a promising method due to its high efficiency, mild conditions, and low toxicity. Typically, the following precipitation of lignin by adding acid media makes it challenging to recycle BILs. Herein, we explored a series of low-cost and recyclable alcohols as anti-solvents, including methanol (MeOH), ethanol (EtOH), iso-propanol (i-PrOH), and tert-butanol (t-BuOH), for recovering [Ch][Lys] and precipitating lignin without adding an acid from water-free [Ch][Lys] (case 1) and aqueous [Ch][Lys] (case 2). For case 1, lignin recovery followed the order of EtOH > i-PrOH > t-BuOH (MeOH was not able to recover lignin and [Ch][Lys]), which was negatively correlated with their pKa values, indicating the effect of the inhibited generation of a basic anion (e.g. EtO from EtOH) from –NH2 in [Ch][Lys] on lignin precipitation. t-BuOH showed the highest lignin recovery of 99.7%, ensuring the high purity of the recovered [Ch][Lys] (recovery of 94.7%). Lignin deprotonation and depolymerization were detected. For case 2, t-BuOH also facilitated the recovery of lignin from an aqueous lignin–[Ch][Lys] system with a nearly quantitative lignin recovery, yet with lower [Ch][Lys] recovery of 81.7% and 64.0% at the [Ch][Lys] : water ratios (w/w) of 7 : 3 and 1 : 9, respectively. The lower recovery of [Ch][Lys] might be due to the poor dispersity of lignin solid in t-BuOH, and water also enhanced the deprotonation of lignin, thus making lignin precipitation more difficult. Based on the results, a deprotonation-based lignin dissolution mechanism has been proposed, which also helps to understand lignin dissolution and precipitation in a [Ch][Lys]-based system.

Graphical abstract: Evaluation on the recovery of lignin from basic [Ch][Lys] systems using low-cost alcohols as anti-solvents under acid-free conditions

Supplementary files

Article information

Article type
Paper
Submitted
06 Aug 2024
Accepted
19 Sep 2024
First published
20 Sep 2024

Green Chem., 2024,26, 10950-10959

Evaluation on the recovery of lignin from basic [Ch][Lys] systems using low-cost alcohols as anti-solvents under acid-free conditions

Y. Liu, W. Zhao, Q. Luo, J. Yan and J. Sun, Green Chem., 2024, 26, 10950 DOI: 10.1039/D4GC03898C

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