Comparative Analysis of Chemical and Enzymatic Pathways to Improve Suitability of Industrial Lignins for Polyurethane Applications

The utilization of all biorefinery products and by-products, including lignin, will be a game-changer to optimize the biorefinery concept and make corresponding processes economically viable. Softwood kraft lignin (SKL) is a significant bioresource that could play an essential role in achieving a sustainable bio-based economy. Due to existing aliphatic and phenolic hydroxyl groups in lignin, it could be used as biobased polyol for polyurethane (PU) application. PUs represent a large class of polymers with urethane repeating units produced by reacting polyols and diisocyanates. Major hurdles to the valorization of kraft lignins include high heterogeneity, inaccessibility of hydroxyl groups for reaction with isocyanates, and low solubility in most organic solvents. As such, technologies to enhance kraft lignin homogeneity are particularly crucial to unlocking its enormous potential and entirely replace petroleum-based polyol in PUs. To overcome the bottlenecks in valorization of SKL for applications beyond energy production, chemoenzymatic methods were established and compared to propel our knowledge of effective ways to tailor lignin structure for PU resin applications. The first method focused on fragmentation of lignin structures using ionic liquid 1-butyl-3-methylimidazolium [BMIM]Br treatment to gain access to the existing hydroxyl groups. However, the expected compositional changes were not detected on SKL (Indulin-AT) after IL treatment, as confirmed by phosphorus nuclear magnetic resonance spectroscopy (31P NMR) and gel permeation chromatography (GPC) analysis. The second method was a one-step acetone fractionation of LignoBoost SKL that resulted in acetone soluble and insoluble fractions, offering components suitable to entirely replace phenol and polyol in phenolic and PU resins, respectively, without the need for further chemical modification of the lignin structure. Additionally, the possibility of using a cellulose-derived solvent in PU formulation was successfully tested. Finally, to eliminate organic solvents and mimic the oxidation of lignin phenolic compounds in nature, an alkaline tolerant laccase was used to graft acrylate monomer onto LignoBoost SKL. Enzymatic grafting was confirmed by Fourier-transform infrared spectroscopy and 1H NMR techniques, and was shown to enhance the reactivity of lignin as a polyol substitute in PU resin applications. The reported enzymatic and aqueous process presents an opportunity for the sustainable valorization of SKL.Ph.D