Mild photocatalysed and catalysed green oxidation of lignin:A useful pathway to low-molecular-weight derivatives

Two commercial Ca2+ and NH4 + lignin derivatives were catalytically oxidised and photooxidised both aerobically and in presence of H2O2 (Fenton system), under green conditions (water solution, 20C, 1 atm), to obtain fractions with reduced degrees of polymerisation. Analyses of the oxidised solutions were carried out using NMR, MS and UV spectrometry. The catalytic and photocatalytic systems used showed satisfactory conservation of the organic material, except for the Fenton system. Lignins showed some mineralisation when irradiated in the presence of H5[PMo10V2O40] 9 H2O (POM-1), K5[Ru(H2O)PW11O39] (POM-2), K4[SiW12O40]8H2O (POM-3) and TiO2. When thePOMs were used as thermal catalysts, POM-3 was almost inactive, while the reactivities of POM-1 and POM-2 under both thermal and photochemical conditions were comparable, revealing little effect of irradiation on the reaction mechanism. The best compromise appears to be the TiO2 photosystem, which shows low carbon consumption, good preservation of aromatic rings, and greatly reduced mineralisation. Alternatively,POM-1 can be used, and particularly under thermal conditions.