Investigation of the fate of poplar lignin during autohydrolysis pretreatment to understand the biomass recalcitrance

Lignocellulosic biomass is the most abundant renewable resource for the potential replacement of fossil fuels, though to fully realize this vision, an improved understanding of the chemical structures of its components within the biomass and after bioprocessing is critical. In this study, we investigated the fate of isolated poplar lignin during autohydrolysis pretreatment at different temperatures and subsequently analyzed the structural changes by gel permeation chromatography, ¹³C-¹H HSQC and phosphorylation/ ³¹P NMR. Our results suggested that an increase in temperature and time of autohydrolysis of lignin resulted in an increase in phenolic hydroxyl groups coupled with a decrease in aliphatic hydroxyl groups. This may be attributed to the cleavage of β-O-4 linkages via acidolysis. Molecular weight determination revealed that lignin depolymerization predominates over condensation. Our results also highlight that the cleavage of lignin side-chain units is relatively fast in lignin autohydrolysis compared to the autohydrolysis of biomass. This study provides an enhanced understanding of the fundamental autohydrolysis pretreatment lignin chemistry and will facilitate improved methodology to reduce biomass recalcitrance.