Highly Efficient Lignin Depolymerization via Effective Inhibition of Condensation during Polyoxometalate-Mediated Oxidation

Depolymerization of lignin to low-molecular-weight products plays a vital role in upgrading of lignin into high-value chemicals. One of the problems of lignin depolymerization is the recondensation of the lignin fragments through positive α carbon (Cα+) coupled with electron-rich lignin aromatic rings during the depolymerization process. Many pretreatment methods have been proposed to efficiently inhibit the recondensation by blocking active Cα–OH groups, but barely a few oxidative methods were reported for the following lignin depolymerization. Herein, we reported an effective lignin depolymerization process in which the active Cα–OH groups were first converted to stable Cα═O by TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) and then depolymerized by oxidation of polyoxometalate (POM) under relatively mild conditions (150 °C, 10 bar O2). The POM is an intermediate oxidation agent that was completely regenerated during the lignin depolymerization process. Under 150 °C and 10 bar O2 for 6 h, 74.5 wt % lignin was converted to DCM-extractable low-molecular-weight compounds (average molecular weight of 363 Da), yielding 19.4 wt % aromatic monomers (based on total lignin mass). The novel oxidative depolymerization method reported here is more effective than common oxidative depolymerization methods reported in the literature.