(452e) High-Grade Lignin Production: Evolution of Lignin Structure through ?-Valerolactone-Assisted Hydrolysis of Biomass

High-grade lignin, as a renewable material, shows potential value in the products market (e.g., vanillin, bio-based polymers, and carbon fiber). However, the development of high-grade lignin is currently limited by the high cost and small scale of production. γ-Valerolactone (GVL) is a renewable and effective cosolvent to assist biomass hydrolysis and lignin extraction from biomass. Tuning the operating conditions (80–120°C and 1–24 h) can increase the yield of lignin but can also deteriorate its native structure. Our work aims to improve the yield as well as quality of lignin by developing an effective strategy to reach high yields while simultaneously maintaining a high grade of lignin. A variety of techniques, including nuclear magnetic resonance (NMR) spectroscopies (solution- and gel-state ¹H–¹³C 2D HSQC and ³¹P) and gel–permeation chromatography, were applied to characterize the structures of (1) the lignin in virgin biomass, (2) the lignin extracted into the GVL phase, and (3) the lignin remaining in the cellulose-rich solid phase. The crucial β-ether content, the aliphatic hydroxyl group content, and the molecular weight of various lignin fractions indicate that the native structure of lignin remaining in the cellulose-rich solid phase was more similar to native lignin compared to lignin dissolved in the GVL phase under the same conditions. As guided by the above observation, a comparison was carried out between a single-step 90 °C-3 h process and a two-step 90 °C-1.5 h treatment. The results show that the two-step process gave slightly higher lignin yield (31% vs. 29%) with a higher β-ether content (51% vs. 44%). The results demonstrate that multiple cycling of biomass for cosolvent-assisted hydrolysis can improve both yield and quality of lignin. Most importantly, such a strategy can be realized in a large-scale continuous reaction system, which will be conducive to commercialization of high-grade lignin production.