(91e) Fine Fractionation of Lignin By Molecular Weight Using Supercritical Fluids

As lignin is the only major constituent of biomass with aromaticity, it has been proposed as a renewable biopolymer replacement for petroleum-derived applications such as the polyols in polyurethane foams, the phenols in phenolâ??formaldehyde resins, and the polyacrylonitrile in carbon fibers. However, increasing evidence indicates that the molecular weight of the lignin should best be â??tunedâ? for a given product application. In this study, concentrated acetic acidâ??water mixtures were used to dissolve most of a low-ash (1-2%) lignin recovered from a kraft black liquor via the SLRP process. Supercritical carbon dioxide (CO₂) was then added as an anti-solvent to the acetic acidâ??waterâ??lignin mixture in increments of increasing pressure. Both the molecular weight and metals impurities of the lignin fractions that precipitated from solution were found to be a strong function of the pressure of the supercritical CO₂ being added to the acetic acidâ??water mixtures. The precipitated lignin fractions were characterized by GPC, ¹H, ¹³C, and HSQC-NMR, and unexpected relationships between molecular weight and functional-group composition were obtained. Based on these limited results, we conclude that supercritical CO₂ can be an energy-efficient, alternative method for fractionating lignin in terms of molecular weight, while holding other molecular properties constant.