Depolymerization and product evolution of kraft lignin in a novel acid deep eutectic solvents (DESs)

The high-efficiency depolymerization of kraft lignin in DESs is very attractive and has potential applications. In the present study, we investigated the factors affecting lignin depolymerization in DESs and its product generation mechanism. The results indicated that the acidity of hydrogen bond donors (HBD) was positively correlated to the depolymerization of lignin. We synthesized a new binary DESs (Choline chloride (ChCl):4-chlorobenzene sulfonic acid (4-Cl-BSA)), and investigated the depolymerization of kraft lignin under different reaction conditions. Under the optimized conditions (150 ℃, 3 h, ChCl:4-Cl-BSA=1:0.7), the depolymerization rate of lignin reached 52.46 %. The yield of lignin oil reached 41.2 % with low molecular weight (M w ) and narrow polydispersity index (M w /M n ). Depolymerization experiment of lignin model compound, 2-phenoxy-1-phenylethanol (PPE) confirmed that condensation reaction occurred simultaneously with depolymerization reaction. Moreover, the lignin residue (RL) obtained was characterized by high hydroxyl content, narrow polydispersity index and high thermal stability. This is beneficial to the comprehensive utilization of RL. In the new binary DESs of ChCl:4-Cl-BSA, kraft lignin could be efficiently depolymerized into phenolic compounds. This study provides a new path for high-value utilization of kraft lignin.