Solvent Fractionation of Technical Lignin Materials with Industrially Relevant Solvents

Lignin fractionation aims at dividing the original lignin material into more uniform fractions that have the qualities desired for further processing of the material. Solvent fractionation is a versatile method that produces a soluble and an insoluble fraction with varying compositions, including molecular weight distribution, polydispersity (PDI), and phenolic hydroxyl (OHph) content. These properties are key factors in the further downstream applicability of lignin. Kraft, organosolv, and hydrolysis lignins were fractionated using multiple solvents and their water solutions with a single-stage fractionation protocol. The produced fractions were analyzed for their molecular size distribution and OHph content. Soluble fractions were discovered with desirable qualities, including a narrow size distribution (lowest PDI 1.4) and accumulation of OHph groups (up to 3.05 mmolOH/glignin). Three fractionation solvents, i.e., pure isopropanol, pure ethanol, and 30% γ-valerolactone, were found to produce solvent-specific soluble fractions that had relatively identical characteristic Mw, Mn, and PDI values regardless of the initial lignin material. This is crucially important new data that is beneficial for robust lignin applicability at industrial scale. Other important observations were made regarding water-alcohol solutions, as the increase in initial solid content resulted in changes in the composition of the fractions produced. Further applicability of the obtained fractions is examined in this paper. Patterns as well as differences between the three lignin materials were observed in their dissolution and the resulting compositions of the soluble fractions. This wide comparable dataset of industrially relevant solvents and lignin materials provides significant insight into the possibilities of lignin downstream processing.