Effect of Lewis acid catalysts on deep eutectic solvent extraction of elm wood lignin: process efficiency and structural modifications

Abstract In this study, a binary deep eutectic solvent (DES) system composed of choline chloride (ChCl) and formic acid (Fa) was employed, in combination with Lewis acid, which synergistically promoted the separation of lignin from elm wood. The results indicated that the catalytic effect of DES system was most significant after adding AlCl 3 . Under the ChCl-Fa-AlCl 3 condition (ChCl:Fa:AlCl 3 /1:8:0.03, 120 °C, 6 h), the lignin extraction yield reached 72.4 %, significantly higher than the lignin without catalyst system (42.5 %). Furthermore, the purity of all isolated lignin samples were exceeded 89 %. In addition, the focus was on analyzing the structure of the obtained lignin, with an in-depth comparison of the structural differences between lignin produced in both catalyst-free and catalyst-based systems. By using fourier infrared analysis, molecular weight analysis, thermogravimetric analysis, and two-dimensional nuclear magnetic resonance technology, the structure of the obtained lignin were analyzed, and the functional ether bond (β-O-4) and carbon carbon bond (β-β) were quantitatively analyzed. Quantitative analysis revealed that the β-O-4 ether linkage constituted over 50/100Ar of inter-unit linkages in lignin side-chain structure. This work provides a methodological framework for analyzing lignin structural modifications induced by Lewis acid-catalyzed DES systems and underpins potential high-value applications.