Structure-reactivity relationships governing hydrothermal liquefaction of lignin from co-solvent enhanced lignocellulosic fractionation (CELF)

This study advances the fundamental understanding of the correlation between lignin structure and reactivity during hydrothermal liquefaction (HTL). Five different lignin varieties were obtained using the cosolvent enhanced lignin fraction (CELF) pretreatment; lignin sources were selected to represent a range of lignin types including softwood (pine), hardwood (poplar and maple), and agricultural (bagasse and corn stover) feeds and analyzed using conventional methods and new developed advanced solid-state ¹³C NMR (ssNMR) approaches. Molecular weight, relative quantities of syringol (S), guaiacol (G), and p-hydroxyphenyl monomers, as well as degree of condensation (DC) were measured for the lignin samples, providing a quantitative basis for structure-reactivity relationships. These lignin samples then underwent HTL at 300 °C for 1 h, and the resulting biocrude, water soluble, char, and gas products were quantified. Lignin properties were correlated to HTL yields and biocrude composition, which found feedstock S/G ratio as the strongest predictor of biocrude and char yields, followed closely by DC. Ultrahigh resolution mass spectrometry analysis of the biocrude was used to gain mechanistic insights into lignin reactivity during HTL, finding that biocrude consists of oligomers of lignin subunits composed of 2-5 monomers. The biocrude obtained from high S content lignin is enriched in dimers and trimers, explaining the high biocrude yields obtained from these feeds compared with low S content lignin. These insights significantly advance current understanding of lignin reactivity under hydrothermal conditions.