(597b) Recovery of Both Low and High Molecular Weight Lignin Fractions By Extraction with Hot Aqueous Organic Solvent Systems

The current technologies being used to recover lignins from the black liquor by-product streams of both pulp-and-paper mills and lignocellulosic biorefineries all generate lignins that are “low-ash” compared to the parent black liquors, which are 50% ash and 20% Na. These so-called Kraft or alkali lignins have been proposed for low- and medium-value materials applications such as the partial substitution of phenol in phenol–formaldehyde resins and of polyols in polyurethane. However, for many higher-value applications, this level of metal content renders the lignin unfit for use. For example, if lignin is to be considered for applications such as high-strength carbon fibers and electrodeposited coatings, it must be much cleaner or “ultrapure”, with a total metals content of no more than 100-150 ppm. Furthermore, the molecular weight (MW) of the lignin should be controllable within well-defined limits.

The Aqueous Lignin Purification using Hot Acids (ALPHA) processwas invented with this requirement in mind, as ALPHA uses a biorenewable solvent system for simultaneously cleaning and fractionating Kraft lignins. Lignins of controlled MW with a total metals content of less than 100 ppm (and <20 ppm Na) can be obtained in only 1-2 extraction steps. The essence of the discovery is that when Kraft lignins are combined with hot acetic acid–water mixtures at the appropriate conditions, two equilibrium liquid phases are formed, with the metal salts being extracted into the solvent phase and a purified, solvated lignin-rich liquid phase that is remarkably free of metals being obtained as the desired product. Very low metals content in the solvated lignin phase is achieved because the metals can easily diffuse out of the liquefied lignin phase and into the solvent phase. With other extraction methods, the lignin is processed as a solid, so diffusion is far slower.

In this work, the fractionation of lignin into low and high MW fractions was investigated. High MW fractions are of interest as precursors for carbon fibers, and low MW fractions are being considered for coatings applications. In both cases, the lignins must also be “ultrapure”, with a Na content less than 50 and total metals content less than 100 ppm being present. Two-three stages were used in ALPHA to obtain the desired lignin products. For the high MW lignin, an acetic acid/water (AA/H2O) solvent ratio of 50/50 was initially used to isolate a relatively pure fraction of lignin containing the highest 50% MW cut of the lignin. That fraction was then fractionated yet again with a 70/30 AA/H2O solvent mixture, and the resulting lignin contained less than 25 ppm Na and contained the highest 10% MW cut of the lignin. That lignin cut was then converted into carbon fibers, and the best fiber properties reported to date were obtained. In an analogous manner, ALPHA was operated as three stages so as to obtain a low MW cut of lignin. This time, a solvent composition rich in AA was started with, and water was then added to the solvent mixture to weaken the solvent power of the solvent mixture, precipitating out lower and lower MW cuts of lignin. One of the lowest MW cuts obtained in the aforementioned ALPHA setup contained less than 20 ppm Na and <100 ppm total metals, and is currently undergoing product testing in a coatings application.