(266b) Liquid-Liquid Lignin–Solvent Systems: Phase Behavior, Characterization and Applications

Lignin is unique among biopolymers in having significant aromatic character, which makes it potentially useful for a wide range of applications, including electrodeposited coatings and carbon fibers. Unfortunately, most of the commercial-grade lignins available today (primarily Kraft lignins) are too polydisperse and have a high metals and ash content, eliminating them from contention for most high-value applications.

Thies and co-workers [1] have developed a process for simultaneously solvating, fractionating, and purifying the lignin polymer recovered from the biomass by-product streams of pulp-and-paper mills and lignocellulosic biorefineries. Called Aqueous Lignin Purification with Hot Acids, or ALPHA for short, the process involves combining solid lignin with organic solvent–water mixtures to produce two liquid phases: a highly solvated, purified polymer (lignin)-rich phase containing higher molecular weight(MW) lignins and a salty solvent-rich phase containing lower MW lignins. Recently, our group [2] found that carbon fibers spun from purified, fractionated high molecular weight lignin has more than doubled tensile strength and higher tensile modulus than any lignin-derived carbon fiber reported in literature. However, this complex lignin-solvent system is lack of fundamental studies to help understand why ALPHA is so effective for processing lignin.

Liquid–liquid equilibrium (LLE) phase behavior has been measured for two different renewable solvent systems, with the results being presented as ternary phase diagrams depicting all regions of both fluid and solid phase behavior. A number of important thermodynamic properties relevant to the processing of lignin can be obtained from these diagrams, including the size of the LLE region available for polymer processing, and the distribution of lignin between the polymer-rich and solvent-rich phases. The molecular architecture of lignin molecules from each phase were measured both by Gel Permeation Chromatography (GPC) and Static Light Scattering (SLS). Finally, Heteronuclear Single Quantum coherence(HSQC)-NMR was used to measure the functionality changes of ALPHA-processed lignin fractions for investigating proposed reaction mechanisms.

[1] Klett, A. S.; Chappell, P. V.; Thies, M. C., Recovering ultraclean lignins of controlled molecular weight from Kraft black-liquor lignins. Chemical Communications 2015, 51 (64), 12855-12858.

[2] Jin, J.; Klett, A. S.; Ding. J.; Thies, M. C.; Ogale, A. A., Carbon Fibers Derived From Fractionated-Solvated Lignin Precursor with Enhanced Mechanical Performance. in preparation, 2018.