Microwave-Assisted Oxidative Degradation of Lignin Catalyzed by Hydrogen Peroxide–Alkaline Ionic Liquid System

In recent years, various green solvents have played more and more important roles in catalysis and biomass studies. In this work, three imidazolium anion-based alkaline ionic liquids (ILs, including [BMIM]Im, [Ch]Im, and [N4222]Im) were selected to catalyze the oxidative degradation of alkaline lignin by a microwave-assisted hydrogen peroxide–alkaline ionic liquid system for the first time, which aimed to promote the depolymerization and high-value conversion of lignin and increase the number of alcohol hydroxyl groups and the reactivity of lignin. The changes in the number of the alcohol hydroxyl groups of lignin before and after degradation were taken as the primary indices. As the main conditions, the influence of the microwave exposure time, microwave power, ionic liquid concentration, and hydrogen peroxide concentration on the degradation efficacy was subsequently examined for the ionic liquid that exhibited the most effective degradation performance. In addition, the extracted lignin degradation reaction solution was analyzed in combination with gas chromatography–mass spectrometry (GC–MS), and the degraded lignin solids were characterized by scanning electron microscopy (SEM), ultraviolet and visible (UV–Vis) spectroscopy, Fourier-transform infrared spectroscopy (FT–IR), and thermogravimetric and derivative thermogravimetric (TG–DTG) methods, which determined the composition of the degradation products, the degradation mechanism, and the intuitive structural changes in the lignin, thereby providing insights into the extent of lignin degradation with green solvents.