(434b) Aqueous Lignin Purification with Hot Acids: Cleaning, Fractionating, and Solvating Lignin for Materials Applications

Thies and co-workers [1,2] have discovered that when raw lignins are combined with hot acetic acid–water (AA/H2O) mixtures, two liquid phases are formed: a solvent phase into which are extracted the metal salts and other impurities and an ultrapure, solvated lignin phase that is the desired product. Purities of less than 20 ppm Na and <100 ppm total metals can be achieved in this liquefied lignin phase, as the metal salts readily diffuse out of the solvated lignin and into the solvent. Furthermore, the AA/H2O solvent composition can be “tuned” to partition the raw lignin feed between the solvent and lignin phases according to molecular weight (MW). Finally, because the ultrapure lignin phase is solvated, it has the potential to be directly processed into useful products. No other lignin-treatment method reported in the literature exhibit the ability to simultaneously clean, fractionate, and solvate lignins without washing. We call the process ALPHA, which stands for Aqueous Lignin Purification using Hot Acids. A low-ash, raw Kraft lignin has been purified via ALPHA, dry-spun from the solvated form into lignin fibers, and then processed into the desired final carbon fibers. As a reference and first step, the raw lignin was solvated by ALPHA but was not fractionated or cleaned before conversion into carbon fibers. In succeeding experiments, 50/50, 65/35, and 67/33 AA/H2O solvent mixtures were used to clean, solvate, and fractionate the raw lignin so that a highest 50% MW cut, a highest 25% MW cut, and a highest 10% MW cut were isolated. Conversion of these cuts into carbon fibers showed consistent improvements in fiber properties with increasing number average molecular weight of the lignin.

1. Klett, A. S.; Payne, A. M.; Thies, M. C. Continuous-Flow Process for the Purification and Fractionation of Alkali and Organosolv Lignins. ACS Sustainable Chem. Eng., 2016, 4, 6689-6694.
2. Klett, A. S.; Chappell, P. V.; Thies, M. C. Recovering ultraclean lignins of controlled molecular weight from Kraft black-liquor lignins. Chem. Comm. 2015, 51, 12855-12858.