Survey of Lignin-Structure Changes and Depolymerization during Ionic Liquid Pretreatment

A detailed study of chemical changes in lignin structure during the ionic liquid (IL) pretreatment process is not only pivotal for understanding and overcoming biomass recalcitrance during IL pretreatment but is also necessary for designing new routes for lignin valorization. Chemical changes in lignin were systematically studied as a function of pretreatment temperature, time, and type of IL used. Kraft lignin was used as the lignin source, and common pretreatment conditions were employed using three different ILs of varying chemical structure in terms of acidic or basic character. The chemical changes in the lignin structure due to IL pretreatment processes were monitored using ¹H-¹³C heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR), ³¹P NMR, elemental analysis, gel permeation chromatography (GPC), and Fourier transform infrared (FT-IR), and the depolymerized products were analyzed using gas chromatography mass spectrometry (GC-MS). Although, with pretreatment in acidic IL, triethylammonium hydrogensulfate ([TEA][HSO₄]) results in the maximum decrease in the β-aryl ether bond, maximum dehydration and recondensation pathways were also evident, with the net process showing a minimum decrease in the molecular weight of regenerated lignin. However, 1-ethyl-3-methylimidazolium acetate ([C₂C₁Im][OAc]) pretreatment yields a smaller decrease in the β-aryl ether content along with minimum evidence of recondensation, resulting in the maximum decrease in the molecular weight. Cholinium lysinate ([Ch][Lys]) pretreatment shows an intermediate result, with moderate depolymerization, dehydration, and recondensation observed. The depolymerization products after IL pretreatment are found to be a function of the pretreatment temperature and the specific chemical nature of the IL used. At higher pretreatment temperature, [Ch][Lys] pretreatment yields guaiacol, [TEA][HSO₄] yields guaiacylacetone, and [C₂C₁Im][OAc] yields both guaiacol and guaiacylacetone as major products. These results clearly indicate that the changes in lignin structure as well as the depolymerized product profile depend on the pretreatment conditions and the nature of the ILs. The insight gained on lignin structure changes and possible depolymerized products during IL pretreatment process would help future lignin valorization efforts in a potential IL-based lignocellulosic biorefinery.