Microwave-assisted depolymerization of lignin with synergic alkali catalysts and a transition metal catalyst in the aqueous system

Abstract

In this study, synergic alkali catalysts (NaOH + NaAlO₂) and Ni/ZrO₂ were used for microwave-assisted lignin depolymerization. The effect of temperature, alkali dosage, Ni/ZrO₂ dosage, and solid-to-liquid mass ratio was studied to improve the catalytic degradation efficiency and the yield of phenolic compounds. The results showed that more than 87.7% of lignin was converted and more than 0.582 mmol g⁻¹ total phenolic hydroxyl concentration was obtained at 180 °C. Furthermore, the characterization results of FT-IR, UV-VIS, TG-DSC, GPC and ¹H-NMR clearly demonstrated that the synergistic system of an alkali and Ni/ZrO₂ effectively improved the degradation efficiency and inhibited the formation of char, and resulted in a series of compounds with low molecular weight (<1000). In addition, the GC-MS results showed that the selectivity of the phenolic monomer product (68.2%) significantly improved after degradation. The lignin depolymerization mechanisms involved demethoxylation, dealkylation and decarbonylation.