Enhanced Depolymerization Performances in Converting Larch Lignin to Monophenolic Compounds by Heterogeneous Catalysts From Transition Metal-Substituted Molybdovanadophosphoric Heteropolyacid and Alkaline Ionic Liquids With Hydrogen Peroxide.

The selective preparation of vanillin (Van) and ethyl vanillate (EV) remains a major challenge in depolymerizing lignin. Larch lignin is full of coniferyl alcohol unit, which is preferred to produce Van and EV. In this study, the counterions in H5PMo10V2O40 were replaced with different metal ions (Cu2+, Fe3+, and Co2+) to develop the modified polyoxometalates, which were then combined with alkaline ionic liquid (IL) to construct a multiactive catalyst possessing acidic, basic, and oxidizing properties. The incorporation of Cu significantly enhanced the cleavage of C-C and C-O bonds in larch lignin at 160°C for 6 h in an ethanol/water/H2O2 system. Under these conditions, the total yield of monophenol reached 15.62 wt%, Van and EV were dominant with the yield of 5.93 and 9.1 wt%, respectively. The fracture of lignin model compounds showed Cu-based polyoxometalates/IL catalysts are effective in selectively preparing EV and Van. The catalyst can be recycled up to 4 times with excellent stability and recoverability. This study provides a more effective strategy for converting waste biomass into valuable platform chemicals.