A Combined Experimental and Theoretical Study on Vanadium-Catalytic Oxidation of Lignin to Produce Carboxylic Acids

Abstract

For the catalytic oxidation of biomass to carboxylic acids (CAs), previous research has focused mainly on cellulose and hemicellulose conversion. Lignin, the most abundant sustainable source of aromatics on Earth, has not yet been systematically studied due to the complex molecular system and amorphous structure. This work presents an H5PV2Mo10O40-H2SO4 catalytic oxidation of lignin to produce CAs in aqueous solution using O2. A total yield of CAs can be up to 41.9 wt%. The catalytic oxidation of lignin includes two simultaneous stages, e.g. the fast hydrolysis stage and the slow oxidation stage. Then, a combined experimental and theoretical study is applied to further investigate the pentavalent vanadium-catalytic 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 the 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 the 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.

Keywords: lignin, Carboxylic acids, Catalytic oxidation, Vanadium, reaxFF