(329i) Polymer Products from Lignin through De-Aromatization and COOH Functionalization

Background and motivation. Valorizing lignin has the potential to significantly improve the economics of biomass conversion technologies. Lignin comprises 15-30% of lignocellulosic biomass and is currently burned for its heating value within a biorefinery. Generating greater value out of this carbon is critical for the economic viability of producing fuels and chemicals from lignocellulosic biorefineries.

We report a method to convert lignin into valuable polymer products. Compared to depolymerizing lignin, functionalizing lignin has the advantage of yielding a product directly without requiring further upgrading. We use an inexpensive aqueous oxidative process near room temperature to open the aromatic rings within the lignin structure and generate carboxylic acid groups. This polymeric polyacid material functions as a commercial agricultural dispersant, micronutrient complexation agent, or water-absorbent material. No separation processes are required and all of the lignin is converted to product.

Methods. Chemical modification and functionalization of lignin was achieved using a chelator-mediated Fenton (CMF) reaction. A range of samples were prepared from kraft lignin and also from a lightly-processed near-native lignin from a lignocellulosic biorefinery (NREL’s DMR-EH lignin). The H₂O₂ requirement for these samples, expressed as mass of H₂O₂ (pure basis) per mass of lignin, ranged from 1 to 0.25 depending upon the extent of functionalization. Upon optimizing reactant concentrations and pH, the COOH content ranged up to ~2.5 mmol/g. Further, under certain conditions the molecular weight was increased by a factor of 10. Increasing the molecular weight is important for certain applications. The samples were tested for performance as a dispersant, for micronutrient complexation, and for water purification. Further, crosslinked hydrogels were generated from the functionalized lignin using acrylamide and N, N’-methylenebisacrylamide after imidization of the acid groups on the lignin, as reported in Giant 2022, 10, 100106.

Results. A set of samples were prepared from each lignin source that differ in COOH content and molecular weight. These samples were evaluated for i) their performance in formulations to disperse the herbicide mesotrione, insoluble metal oxides, and soluble metal salts, ii) their ability to bind divalent heavy metal ions (Pb(II), Co(II), Ni(II) Cu(II), and Zn(II)), and iii) for formation of hydrogels and water-absorption performance. The results are summarized in Figure 1 and will be discussed in more detail in the presentation. This chart shows that many of the functionalized lignin samples derived from kraft lignin meet performance specifications for all applications except dispersing soluble metal salts, whereas the samples derived from biorefinery lignin only met performance specifications for hydrogels. The poor performance of the samples derived from biorefinery lignin in the other applications was due to the high content of polysaccharides.

Implications. This work demonstrates that a simple low-cost aqueous oxidation treatment can be used to tailor both chemical functionality and molecular weight of lignin, and thereby generates useful polymer products from lignins.