Purification, Fractionation, and Characterization of Lignin from Kraft Black Liquor for Use as a Renewable Biomaterial

Lignin is unique among biopolymers as it is the only abundant renewable source of aromaticity, giving it potential both as a replacement for traditionally petroleum-derived products, and as a biopolymer in its own right. Unfortunately, most commercial-grade lignins available today have a relatively high metals content and broad molecular weight distribution making them unsuitable for many applications. Thus, cost-eective separation processes must be developed if lignin is to achieve its potential as a renewable biopolymer. In this work, a technique called Aqueous Lignin Purication with Hot Acids (ALPHA) was developed to purify lignin to metal impurity levels of less than 75 ppm and fractionate the lignin based on molecular weight. ALPHA is based on the discovery in our lab that by combining lignin with acetic acid-water mixtures at elevated temperatures, two liquids are formed: a solvated, lignin-rich phase containing a puried, higher-MW lignin fraction and a solvent-rich phase containing the metals and a lower-MW fraction. Both batch and continuous versions of the ALPHA process were developed and investigated to obtain a better fundamental understanding of the pseudoternary system of acetic acid-water-lignin. Fundamental work was carried out to measure the solid-liquid to liquid-liquid phase-transition temperature for this system; a method was developed utilizing electrochemical impedance spectroscopy, a technique that could also be extended to other polymer/solvent systems. Also, liquid-liquid phase compositions of the system were measured in the regions applicable to ALPHA, with the partitioning of the lignin and metal salts between of two phases being of particular interest. An alternative method for the fractionation of the lignin was also achieved using gas-expanded liquids, namely CO2 expanded solvents composed of acetic acid and water. Both gaseous and near critical CO2 were used as an anti-solvent to precipitate lignin at various pressures, and the resulting fractions were characterized. Signicant dierences in molecular weights between the fractions were achieved; however, no evidence of signicant functionality dierences were observed. In summary, ALPHA has shown itself to be a robust technique for producing ultrapure (i.e., <75 ppm key metals) lignin fractions of well-dened molecular weight from dierent lignin sources, and the molecular properties and phase-equilibrium data that were collected are valuable resources for developing processing technologies to convert current lignin by-product streams into upgraded, value-added products.