Lignin Depolymerization and Esterification with Carboxylic Acids to Produce Phenyl Esters

The possibility of phenyl ester formation from lignin as an alternative to methyl esters for the production of a diesel fuel substitute is explored for the first time. The depolymerization of technical lignin at high temperatures (200-250 degrees C) was conducted using carboxylic acids. Successful depolymerization occurred in acetic acid (AA), generating 8-10 mol equiv of ester per gram of liquid product within 30 min. The inclusion of a molecular sieve as an in situ water sorbent not only increased ester production by 75% but also facilitated lignin depolymerization. Additionally, AA played a dual role in esterifying lignin and preventing undesired condensation reactions through the stabilization of the depolymerization products. Adding tetrahydrofuran as a cosolvent with AA boosted ester production by 29 and 22% after 30 and 60 min, respectively. 1,4-dioxane as a cosolvent had no significant effect. The substitution of longer-chain carboxylic acids for AA proved to be more challenging due to solubility issues. Fourier transform infrared spectroscopy analysis of the liquid product confirmed the formation of esters, but the conversion of lignin in hexanoic acid at 250 degrees C after 1 h of reaction was only approximately 40 wt %. By adding the cosolvent, lignin solubility was enhanced, but the resulting products had higher molecular weights compared to those from AA reactions.