Oxovanadium complexes catalyzed oxidation of lignin and lignin dimers in acetonitrile/water under O2

Selective oxidation of lignin is increasingly investigated due to its advantage of retaining the aromatic rings to produce value-added platform chemicals. In this paper, oxovanadium complexes catalyzed oxidation of lignin and lignin dimers in acetonitrile/water system under O-2 was reported. Under optimal conditions of VO(acac)(2) catalyzed oxidation, 95 mol% of lignin dimer 2-phenoxy acetophenone was converted, producing 43 mol% of phenol and 83 mol% of benzoic acid. The catalytic ability of V(acac) 3 was higher than that of VO(acac)(2), but it caused heavier repolymerization and lower product yields. VO(acac)(2) catalyzed oxidation system also cleaved various beta-O-4 lignin dimers, achieving > 95 mol% conversion. Whereas, yields of phenols from these dimers were lower than 5 mol%. These phenolics were shown to polymerize under reaction conditions. Furthermore, this oxidation system depolymerized poplar organosolv lignin. After oxidation, aromatic and C-O aliphatic functionalities in the heavy fractionation largely disappeared, and the weight-average molecular weight decreased from 5720 to 1140 Da.