Characterization of Lignin Streams during Bionic Liquid-Based Pretreatment from Grass, Hardwood, and Softwood

Delignification as a function of ionic liquid (IL) pretreatment has potential in terms of recovering and converting the fractionated lignin streams to renewable products. Renewable biogenic ionic liquids, or bionic liquids (e.g., cholinium lysinate, ([Ch][Lys])), provide opportunities in terms of effective, economic, and sustainable lignocellulosic biomass pretreatment. We have evaluated [Ch][Lys] pretreatment in terms of sugar and lignin yields for three different feedstocks: switchgrass, eucalyptus, and pine. Four lignin streams isolated during [Ch][Lys] pretreatment and enzymatic hydrolysis were comprehensively analyzed, tracking their changes in physical-chemical structures. We observed changes in major lignin linkages and lignin aromatics units (p-hydroxyphenyl (H), guaiacyl (G), and syringil (S)) that occurred during pretreatment. A compositional analysis of the different process streams and a comprehensive mass balance in conjunction with multiple analytical techniques (nuclear magnetic resonance (NMR), mass spectroscopy, gel permeation chromatography (GPC)) is presented. Qualitative and quantitative analyses indicates that there are significantly more lignin-carbohydrate interactions for G-rich lignin in pine. The lignin removal and extent of lignin depolymerization for switchgrass and eucalyptus were higher than pine and follows the order of switchgrass > eucalyptus > pine. The insights gained from this work contribute to better understanding of physiochemical properties of lignin streams generated during [Ch][Lys] pretreatment, offering a starting point for lignin valorization strategies.