Selective Oxidation Cleavage of Cα–Cβ Bonds between Lignin Aromatic Units via Phosphomolybdic Acid-Ionic Liquid

Lignin, the most abundant renewable resource of aromatics in nature, is recognized as an alternative to fossil-based fuels and chemicals. In this study, a phosphomolybdic acid-ionic liquid (PMA-IL) [BSHMIM]H2PMo12O40 was synthesized and used as a catalyst for further oxidative depolymerization of extracted lignin to prepare valuable chemicals. Under optimized conditions, the conversion of lignin reached 61.33%, and the total yield of target products (vanillin, vanillic acid, and methyl vanillate) was 28.00 mg·g–1, where the selectivity of methyl vanillate reached 56.43%. The liquid products and residues obtained under the optimum conditions were analyzed, further confirming the advantages of PMA-IL compared with single PMA. In addition, this method also has an excellent catalytic effect for other types of lignin and presents a good substrate applicability. Finally, based on the research results of model compounds, the possible reaction pathway of the formation of vanillin, vanillic acid, and methyl vanillate was proposed.