Uniform Chemical Functionality of Technical Lignin Using Ethylene Carbonate for Hydroxyethylation and Subsequent Greener Esterification

Full utilization of plant-based feedstocks for sustainable societies must include the valorization of lignin, an abundant aromatic component of the cell wall of plant stems, in processes that follow green chemistry principles. However, one of the major issues limiting lignin utilization is the chemical nonuniformity of the isolated polymer, along with the thermal sensitivity of the free phenolics which hinders processing at elevated temperatures. To address these issues, free phenolics and carboxylic acids of industrial lignins were hydroxyalkylated through the use of ethylene carbonate, which served as both the reagent and the solvent. This facile and safe reaction resulted in near complete conversion of phenolics and carboxylic acids into aliphatic hydroxyls resulting in uniform chemical functionality and enhanced thermal stability. However, the increased reaction temperature decreased the total hydroxyl content and increased the molecular weight of the lignin; the work identified a narrow processing window that achieved derivitazation without extensive structural modification, which plagued earlier work with carbonate modification of lignin. 13C NMR spectroscopy of hydroxyalkylated lignin showed a low degree of condensation and limited to no copolymerization of the ethylene carbonate when reacted under modest conditions. The hydroxyethylated derivative had enhanced solubility in propionic acid, which was used as a solvent and reagent in order to directly esterify the lignin. The reaction achieved over 95% substitution of the lignin hydroxyls creating ethylpropionate derivatives where excess propionic acid could be recycled under vacuum. The hydroxyalkylation followed by direct esterification provided a route toward the development of greener lignin esters by avoiding added solvents, carcinogens such as ethylene oxide, and halogens like acid chlorides for lignin-based polymeric materials synthesized utilizing green chemistry principles.