Survey of lignin streams generated during ionic liquid pretreatment of lignocellulosic biomass for renewable chemicals and fuels production

Lignin is a complex three-dimensional amorphous polymer derived from paracoumaryl, coniferyl, and synapyl alcohols, cross-linked together by different a number of linkages. In plant cell walls, lignin fills the spaces between cellulose and hemicellulose, and it acts like a ‘glue’ that holds the lignocellulose matrix together. 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. 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 mass spectrometry. Lignin model compounds and quantum chemical calculations were employed in order to further elucidate the mechanism of the lignin solvation and depolymerization.