A simple route to synthesize esterified lignin derivatives

Abstract

Esterified lignin shows improved compatibility in polymer blends, which can be utilized for 3D printing inks, single use packaging, and compostable plastics. However, traditional esterification reactions using anhydride and acyl chloride compounds have drawbacks, when considering requirements of sustainable chemistry. In this study, we developed a greener direct esterification (catalyst-free esterification) to obtain modified lignin of various carbon lengths. Specifically, without catalyst, organic acid was used as the solvent and reagent to modify softwood kraft lignin (SKL) and organosolv hardwood lignin (OSHL) at 90–160 °C for 24–96 h with limited side reactions in this process. This reaction had a high selectivity toward aliphatic hydroxyl groups based on ³¹P NMR analysis. Therefore, hydroxyethyl lignin was synthesized utilizing ethylene carbonate as the solvent and reagent under alkaline conditions to substitute free phenolics and carboxylic acid groups. HELignin was directly esterified with propionic acid and had a controlled degree of substitution (DS, 61%–91% for HESKL and 53%–87% for HEOSHL) based on different temperatures and reaction times. This two-step route was further applied utilizing longer chain organic acids including valeric acid, octanoic acid and oleic acid. Esterified lignin showed a lower T_g of 36.8–18.1 °C (HEOSHL) and 34.0–3.0 °C (HESKL) as a function of increased carbon chain length. Moreover, this simple, direct esterification route satisfied many green principles including the utilization of renewable and safe chemicals, high atom economy, and reactions with a low environmental (E)-factor compared to alternative esterification routes.