(20a) Ultrapure Lignin Via the ALPHA Process for Materials Applications: From Carbon Fibers to Coatings

Among renewable feedstocks, lignin can play an important role as a precursor for carbon fibers, owing to its significant carbon content. Unfortunately, the inexpensive, commercial-grade lignins available today (primarily Kraft lignins) are too polydisperse with a high metals/ash content, eliminating them from contention for high-value

Thies and co-workers have developed a process for simultaneously solvating, fractionating, and purifying the lignin polymer recovered from biomass by-product streams of pulp-and-paper mills and lignocellulosic biorefineries. Aqueous Lignin Purification with Hot Acids (ALPHA) involves combining solid lignin with hot acetic acid–water solvent mixtures to produce two liquid phases: a highly solvated, (lignin) polymer-rich phase and a solvent-rich phase. ALPHA can be operated in two ways: (1) by using increasingly aggressive solvent mixtures to isolate ultrapure, metals-free (i.e., <100 ppm) lignin fractions of medium/high molecular weight (MW), or (2) by “reversing” the process and using solvent mixtures of decreasing strength to isolate metals-free lignin fractions of low molecular weight (MW).

ALPHA was used to generate ultrapure, high MW fractions of lignin that were then converted into carbon fibers; the resulting tensile strengths and moduli were almost doubled than any reported to date. Furthermore, “reverse” ALPHA was used to generated ultrapure, low MW fractions of lignin for application in coatings. Also discussed in this work will be how one can tune the parameters of ALPHA (e.g., temperature, acetic acid/water ratio, solvent-to-lignin ratio) to generate lignin fractions with the properties (including phenolic content, molecular weight, and purity) required for specific applications.