Fractionation and purification of corn stover lignin with hot aqueous ethanol solutions: Phase behavior and molecular properties

Corn stover lignin has been proposed as a sustainable precursor for materials applications, due both to its abundance and low odor compared to kraft lignin. This study investigated the effectiveness of the Aqueous Lignin Purification with Hot Agents (ALPHA) process, not only for fractionating and purifying corn stover lignin, but also for controlling the molecular and thermal properties of the isolated lignin fractions. Pseudo-ternary phase diagrams of lignin with hot aqueous ethanol solutions were mapped out at solvent-to-lignin feed (S/F) ratios of 3:1 and 6:1 at 45°C, and at S/F = 6:1 at 75°C. Unlike previous work with kraft lignins, two regions of liquid–liquid equilibrium (LLE) always existed, one water-rich and the other ethanol-rich, and each was large enough to practice ALPHA. Marked changes in the properties of the lignins were achieved, particularly in the water-rich region of LLE. For example, a lignin fraction isolated in the lignin-rich (LR) phase had double the molecular weight (MW = 43 kDa) and one-tenth the ash content (0.08 wt%) of the feed, with a glass-transition temperature 50°C above that of the feed. With respect to lignins isolated in the solvent-rich (SR) phase, MWs half that of the feed could be isolated. Finally, for the first time since ALPHA was applied to lignins, significant differences in the hydroxyl content of the lignins isolated in the SR and LR phase were observed, even for the same molecular weight.