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,...