Metal-based ionic liquids: effective catalysts in aqueous media for the selective production of vanillin from alkali lignin at room temperature

Abstract

Catalytic depolymerization of lignin to value-added compounds is receiving increasing interest as part of the circular bioeconomy. This work describes the selective depolymerization of alkali lignin to vanillin at room temperature using metal-based ionic liquids (MBILs) as catalysts in an aqueous medium. Using a reaction mixture of 20 wt% lignin, and 5 wt% [BMIM][FeCl₄] catalyst in water, 12.5 wt% of volatile aromatic products were obtained of which 105 mg g⁻¹ of vanillin could be extracted with 85% selectively. For this first, we optimized various solvents (ionic liquids; ILs, deep eutectic solvents; DESs, conventional organic solvents, and water) as reaction media and found water to be the best for lignin depolymerization. Following this, we optimized the effect of time, temperature, and catalyst loading on lignin conversion, product yield, and selectivity. We found that high solubility of alkali lignin in the water results in alkali lignin hydrolysis and subsequent conversion by [BMIM][FeCl₄] via heterolytic cleavage of C_α–C_β bond along with β-O-4 to facilitate higher vanillin production. The purity and structure of extracted vanillin have been confirmed by GC-MS, ¹H, and ¹³C NMR analysis, whereas lignin depolymerization has been confirmed from ¹H NMR, ¹³C NMR, SEM, FT-IR, 2D-HSQC, TGA, and PXRD.