MoO3-Based Photocatalysts for the Depolymerization of Lignin Under UV-Vis Light

In this explorative work, molybdenum trioxide (MoO3) and representative doped MoO3 materials, i.e., Cu-doped MoO3 (2% Cu, “Cu-MoO3”) and H-doped MoO3 (H0.31MoO3, “H-MoO3”), have been tested for the first time as photocatalysts in the UV-vis light-driven depolymerization of lignin. The catalysts have been characterized by XRD, TEM, ATR-FTIR, and UV-vis DRS. Under the adopted conditions (UV-vis irradiation, solvent 0.01 M aqueous NaOH, lignin 200 ppm, catalyst 1 g/L, rt, 5 h), photocatalytic depolymerization of wheat-straw lignin (WSL) produced increasing amounts of bio-oil on changing the catalyst from pristine MoO3 to Cu-MoO3 and H-MoO3 (23%, 28% and 30%, respectively). Also, quantification of vanillin and vanillic acid shows a similar increasing trend. These results appear in line with the estimated band gap energies, which decrease in the order: MoO3 (2.91 eV) > Cu-MoO3 (2.86 eV) > H-MoO3 (2.77 eV). H-MoO3 shows the best catalytic performance, which was then fruitfully explored in the photocatalytic depolymerization of benchmark commercial Kraft lignin (bio-oil yield 32%, vanillin and vanillic acid yields 1.28% and 0.78%, respectively). In view of the results obtained, this work is expected to provide new ideas for the design of heterogeneous photocatalytic system for lignin cleavage.