Understanding the chemical transformations of lignin during ionic liquid pretreatment
Unveiling the fundamental chemistry of lignin under ionic liquid (IL) pretreatment will facilitate the understanding of biomass recalcitrance involved in pretreatment processes. To examine in greater detail the chemical transformations of lignin under different IL pretreatment conditions without competing reactions from plant polysaccharides, the IL pretreatment of the isolated poplar alkaline lignin (hardwood lignin) under varying IL pretreatment conditions (i.e., 110–170 °C, 1–16 hours) was performed in an appropriate manner. The structural transformations of the lignin have been investigated by elemental analysis, 2D-HSQC spectra, quantitative 13C-NMR spectra, 31P NMR, and GPC analysis. Results revealed that a decrease of aliphatic OH and an increase in phenolic hydroxyl groups occurred in lignin as the pretreatment proceeded. The increased phenolic OH was mainly as a result of cleavage of β-O-4′ linkages, while the reduced aliphatic OH is probably attributed to the dehydration reaction. The cleavage of β-O-4′ linkages, degradation of β–β′ and β-5′ linkages obviously happened at high temperatures and resulted in the decrease of molecular weights. In addition, IL pretreatment selectively degraded the G-type lignin fractions and the condensation reaction took place more easily at S units than G units. Moreover, the demethoxylation preferentially occurred in G units, especially at higher temperatures. It is believed that investigating the fundamental chemistry of lignin during IL pretreatments would be beneficial to optimize and control the pretreatment process.