A combined experimental and theoretical study on vanadium-catalytic oxidation of lignin to produce carboxylic acids

Catalytic oxidation of lignocellulose to formic acid by vanadium-based catalysts is a promising method to obtain fine chemicals. Considerable progresses have been achieved on the conversion of polysaccharides components of lignocelluloses. However, the oxidation behavior and mechanism of lignin, which quite differs from the structures of polysaccharides, have not been studied in detail. This work presents an H5PV2Mo10O40-H2SO4 catalytic oxidation of lignin to produce carboxylic acids (CAs, mainly including formic acid) using O2. A total yield of CAs can be up to 41.9 wt%. The oxidation process includes the fast hydrolysis stage and the slow oxidation stage. A combined experimental and theoretical study is applied to further investigate the mechanism of the latter oxidation stage. The ring-opening reaction of hydrolysis products is the rate-controlling reaction in the CA formation pathway. Both phenol and o-quinone are active sites and indispensable intermediates, and the ring-opening reactions of them are faster than those of other aromatic structures. The vanadium catalyst helps the formation of o-quinone and reduces the activation energy of ring-opening reaction, thus accelerate the CA formation. The combined experimental and theoretical study could lead to an in-depth understanding of catalytic oxidation process and be helpful to make better utilization of biomass.