Catalytic oxidation of biorefinery corncob lignin via zirconium(IV) chloride and sodium hydroxide in acetonitrile/water: A functionality study

Abstract

In order to realize the efficient utilization of biorefinery corncob lignin, the promising catalytic oxidation strategy was carried out by using ZrCl4 and NaOH as the co-catalyst and dioxygen as the oxidant in MeCN/H2O. GC/MS, GC-FID, and MALDI-TOF/MS were employed to recognize the produced monomers and oligomers, and GPC was used to monitor the molecular weight changes of lignin fragments. In addition, specific structural evolution of corncob lignin during ZrCl4/NaOH-catalyzed oxidation were revealed by quantitative 13C (Q13C) and 2D HSQC NMR techniques. Results showed that the total yields of produced oxidation monomers reached 6.8 wt%, and aromatic aldehydes were the major species, in which vanillin and 4-hydroxybenzaldehyde were the two dominant products. After ZrCl4/NaOH-catalyzed oxidation, the weight-average molecular weight of corncob lignin and its products decreased from 2000 Da to 300 Da after oxidation with 16 h. Moreover, Q13C NMR analysis showed the decrease percentage of CO aliphatic carbons (including methoxyl carbons), the increase percentage of CC aliphatic and carbonyl carbons, and the relative stable percentage of aromatic carbons with reaction prolonged. These results combined with the further confirmation from HSQC indicated the oxidative cleavage of CO aliphatic linkages and removal of methoxy groups within corncob lignin, as well as the formation of CC aliphatic bonds and carbonyl groups. The work presented a comprehensive insight into the catalytic oxidative depolymerization of biorefinery corncob lignin.