Investigation of the Lignin Stream Generated during Ionic Liquid Biomass Pretreatment for Renewable Chemicals and Fuels Production

Lignin is a complex biopolymer, which holds together cellulose and hemicelluloses and thus forms a recalcitrant matrix. The ability of certain ionic liquids (ILs) to dissolve cellulose and/or lignin enables the possibility of commercial lignin upgrading and valorization to improve the overall biorefinery economics. However, to date depolymerization mechanisms of lignin in ILs is not well understood. In this study, the changes in the chemical composition of lignin after pretreatment with a biocompatible cholinium lysinate IL were investigated. Three different type of biomass, namely switch grass, eucalyptus and pine were utilized in this study.  The relative changes in the common β-O-4, β-β and β-5 inter-unit linkages of lignin during pretreatment process were investigated using solution state two dimensional (2D) nuclear magnetic resonance (NMR). The changes in the size and molecular weight distribution of lignin were analyzed using size exclusion chromatography (SEC) and Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Lignin model compounds and quantum chemical calculations were employed in order to further elucidate the mechanism of the lignin solvation and depolymerization.