Production of aromatic monomers at one atmospheric pressure through depolymerization of lignin using combined alkaline solution and aqueous ChCl:urea

Lignin is a promising sustainable feedstock for the production of renewable aromatic chemicals. However, conventional depolymerization techniques often require high pressure and temperature. This study aimed to address this limitation by investigating the application of a combined solvent containing aqueous deep eutectic solvent and sodium hydroxide in the depolymerization of Kraft lignin at atmospheric pressure. The effects of temperature, duration, and NaOH concentration were investigated. Under the depolymerization conditions of 150 °C, 20 min, 7.5% (w/v) NaOH, an aromatic monomer yield of 23.06 mg/g was achieved. Various metal chloride catalysts, including LiCl, CuCl2, and FeCl3, were added as oxidizing agents to promote lignin depolymerization. The synergistic effects of the combined solvent with CuCl2 resulted in an aromatic monomer yield of 69.75 mg/g. Physical and chemical characterization of lignin samples was performed to provide further evidence to support the effectiveness of the depolymerization process proposed in this study. The use of NaOH-aqueous DES as a depolymerization medium could be a promising method for promoting a milder and inherently safer depolymerization system.