One-pot route to convert technical lignin into versatile lignin esters for tailored bioplastics and sustainable materials

Abstract

The valorization of lignin resources requires a functionalization route that satisfies the “greenness” and scale-up efficiency simultaneously. In this study, an efficient one-pot scalable method was created to convert kraft lignin into esterified lignin derivatives of controlled size, structure, and T_g. Specifically, the two-step reaction performed using a “one-pot method” involved the hydroxyethyl derivatization of phenolics and carboxylic acids on lignin with ethylene carbonate, followed by the direct esterification using an organic acid. The resulting homogeneous mixtures were fractionated by downward precipitation with water to obtain five different lignin fractions resulting in low process mass intensity. This one-pot route improved upon past work that involved recovery of the lignin after the first modification step. The one-pot result can esterify around 90% aliphatic hydroxyl groups in the hydroxyethyl derivatives. Analysis with NMR, GPC using multiple detectors, and DSC provided a greater view towards the structure and properties of technical lignin. 50–60% of the obtained esterified lignin derivatives have a polymeric characteristic with resulting T_g around 80 °C. The remaining 40%–50% of the technical lignin is oligomeric in size with more end group units and a corresponding T_g near or below room temperature. In solution, these fractions had conformation ranging from a rod-like structure to dense spheres, indicating a highly compact structure for the lignin. As a result, this route provided a starting material with a near-complete defined conformation and functionality to aid in-depth understanding of the structural features and properties of lignin-based materials. Integrating ethylene carbonate and organic acid as multi-functional reagents in this route can satisfy many sustainable chemistry requirements, delivering a clear path towards uniform esterified kraft lignin potentially available for various applications at the Mt level.