Lignin is one of the most common organic compounds on earth, comprising about 30% of all organic carbon. Lignin is also unique among biopolymers in having significant aromatic character, which makes it potentially attractive for a wide range of uses, including coatings and carbon fibers. Unfortunately, the commercial-grade lignins available today are severely limited in their applications because of their high metals (primarily sodium) and ash content. Thus, less than 0.2% of these 50 million tons/yr of available lignin is being recovered. Clearly, cost-effective separations processes must be devised to dramatically improve the purities of today's commercial lignins, if lignin is to achieve its potential as a biopolymer. Such an ultrapure lignin would be of great interest for the production of high-value products, such as carbon fibers for automobile applications (for large reductions in fuel consumption).
Clemson researchers have discovered a powerful, versatile, yet renewable hot solvent system that can be "tuned" to reduce the sodium content of lignin derived from the Kraft pulping process 100-fold down to <150 ppm in a single step. Furthermore, preliminary evidence indicates that by incorporating compressed CO2 into the solvent system, the ultrapure lignin can be fractionated into cuts of well-defined molecular weight and chemical composition. A fundamental investigation of these phenomena, in order to lay the groundwork for future commercial application, is the overarching objective of this work. The research plan is as follows: (1) Measure elevated-temperature, liquid-liquid equilibrium phase behavior for the solvent system with lignin. Phase boundaries for the lignin-rich and solvent-rich phases, solvent compositions in each phase, and the effect of temperature will be the focus of these measurements. (2) Determine the distribution of metal salts between the lignin present in the above two phases. (3) Investigate the addition of CO2 to the lignin-solvent system for fractionating ultrapure lignin by molecular weight. (4) Characterize the lignin constituents in each phase in terms of phenolic and carboxylic acid content. (5) Evaluate ultrapure lignins as replacements for petroleum-derived polymers in coatings applications. The industrial partner will use the lignin for different polymer coating applications.
