Benign Fractionation of Lignin with CO2-Expanded Solvents of Acetic Acid + Water

Abstract

Kraft lignin was fractionated by molecular weight (MW), using CO2-expanded solutions of acetic acid/water in a 90/10 wt/wt ratio. In particular, as CO2 pressures were increased from 7 bar to 48 bar, expanding the liquid-solvent phase and reducing its dielectric strength, lignin fractions decreasing in MW from 15 000 to 1250, with polydispersity index (PDI) values decreasing from 3.7 to 1.6, were obtained. The recovered lignin fractions were similar, in terms of chemical functionality. With the use of gas-expanded liquids (GXL), only one solvent composition is required, and recovery and reuse of the biorenewable CO2 + acetic acid–water solution is facilitated through pressure release and recompression. The process was demonstrated for the recovery of seven lignin fractions to demonstrate its versatility and effectiveness, but simpler operation with recovery of just a low MW lignin fraction and a high MW lignin fraction is more consistent with anticipated applications.