Chiang Mai Journal of Science

 The biomimetic catalysts were used in catalytic oxidation of dibenzodioxocin (DBDO), a recalcitrant lignin model compound, and examined for conversion of organosolv lignin. The catalytic efficiency of Cu([H2]salen), Co([H2]salen), Cu([H4]salen), Co([H4]salen), Cu([H4]salen)/Co([H4]salen) was compared on the basis of the oxidation extent of DBDO. The combined biomimetic catalyst Cu([H4]salen)/Co([H4]salen), never used before in lignin oxidation, was found most effective among the five catalysts in transforming the model compound. The reaction pathway for catalytic oxidation of DBDO was also proposed according to the characterization upon the structural change and degradation products of DBDO, respectively with 13C-NMR and GC/MS. During oxidation of organosolv lignin, Cu([H4]salen)/Co([H4]salen) afforded much high percentage of carbonyl compounds in total degradation products (84.6%). The molecular weight of residual lignin samples obtained with GPC showed that Cu([H4]salen)/Co([H4]salen) could carry out the most extensive depolymerization of the lignin structure. The quantitative 31P-NMR allowed the focusing on the structural modifications induced on the lignin. The content β-O-4› and DBDO units present in organosolv lignin markedly decreased with the enhanced catalysis by Cu([H4]salen)/Co([H4]salen). The data of hydroxyl groups further confirmed these findings.