Production of Methyl p-Hydroxycinnamate by Selective Tailoring of Herbaceous Lignin Using Metal-Based Deep Eutectic Solvents (DES) as Catalyst

Catalytic conversion of lignin to versatile aromatic compounds is attracting increasing attention. However, it is highly desirable but challenging to produce a specific chemical with high yield through lignin depolymerization in a one-pot process because of the complex structure of the lignin molecule (mainly composed of H, G, and S units). In this study, a series of metal-based deep eutectic solvents (M-DESs) were prepared and used for the catalytic tailoring of lignin H units to produce value-added methyl p-hydroxycinnamate (MPC). In particular, M-DES ChCl[FeCl3]2 showed excellent catalytic performance for the selective production of MPC as the sole product with high yield and selectivity (105.8 mg g–1 and 74.1%, respectively). Extensive characterizations using 2D HSQC NMR, 13C NMR, and GPC demonstrated that MPC was obtained from the selective tailoring of p-coumaric acid (pCA) units via the cleavage of ester bonds in lignin. Furthermore, M-DES ChCl[FeCl3]2 also exhibited efficiency to other herbaceous lignins, and showed excellent recyclability. Therefore, this work provides an effective strategy for the valorization of lignin.