Enhancing the Efficacy of the Subcritical Water-Based Alkali Lignin Depolymerization by Optimizing the Reaction Conditions and Using Heterogeneous Catalysts

Abstract

The catalytic depolymerization of alkali lignin into phenolic monomers was studied using subcritical water. In this study, subcritical water was used as the greener solvent with heterogeneous catalysts. The goal of this study was to screen for the best catalyst for the depolymerization, to optimize the reaction conditions, and to increase the yield of the phenolic monomers. The depolymerization reactions were performed at 200 and 240 °C for 5, 10, and 15 min, using subcritical water as the solvent with different catalysts. The treatment of the lignin sample with Ni-Graphene catalyst in subcritical water at 240 °C for 10 min resulted in the highest total yield of phenolic monomers, which was 41.16 ± 0.27 mg/g of alkali lignin. The catalysts also resulted the highest yield for each of the phenolic monomers guaiacol (G), vanillin (G), and homovanillic acid (G) compared to other catalysts studied. The optimized method proved to be an excellent approach to depolymerize alkali lignin.