Catalytic oxidative conversion of C/G-type lignin coexisting in Tung nutshells to aromatic aldehydes and acids

Abstract

Catechyl lignin (C-lignin) is a unique and attractive natural biopolymer used as raw material for the production of chemicals and materials due to its homogeneous linear aromatic structure. Tung nutshells, an important component of industrial solid waste, are rich in C/G-type Lignin. Herein, for the first time, we report that the catalytic oxidation strategy is effective for the conversion of Tung nutshell lignin to value-added C/G-type aromatic monomers using the commercially available CuCl₂ catalyst. The reaction conditions, including solvent, temperature, and catalyst dosage, were comprehensively investigated. The optimal yield of 60.8 mg g⁻¹ (or 12.9 wt% Klason lignin) C/G-type aromatic aldehydes and acids was obtained using the CuCl₂ catalyst in the CH₃CN/H₂O (8/2, v/v) solvent mixture at 190 °C for 4 h. The C-type aromatic monomers were formed with an yield of 7.7 mg g⁻¹ (or 1.6 wt%) in the initial 0.2 h and gradually decreased as the reaction progressed. The less amount of C-type lignin-derived aromatic monomers can be attributed to their lower thermal stability under oxidative conditions. The structural evolution study of lignin by GPC and HSQC NMR reveals that the C/G units are effectively depolymerized and transformed into aromatic aldehydes and acids by the CuCl₂/CH₃CN/H₂O catalytic oxidative system.