Alkali-oxygen oxidation mechanism for producing benzene carboxylic acids from lignite: Experimental and molecular modelling studies

Abstract

Alkali-oxygen oxidation of lignite is a potential route to produce high-value carboxylic acids, especially benzene carboxylic acids. However, this method leads to an unfavorably large consumption of inorganic alkali and a loss of more than 50% of the carbon in lignite as byproduct CO2. A combined theoretical and experimental study was applied to investigate the lignite alkali-oxygen oxidation mechanism at a molecular level. By analyzing lignite model simulations and the experimental results of model compounds, we obtained the formation pathways of carboxyl moieties and the alkali catalytic effect. The hydroxy radical is the major oxidant for dehydrogenation and oxygenation. The carboxyl group formation followed a chain-reaction pathway, which included side-chain/bridge-bond oxidation, aromatic carbon ring-opening oxidation and decarboxylation. Active sites for carboxyl group formation were clarified and confirmed by model compound experiments. Density functional theory calculations showed that the added alkali could promote side-chain/bridge-bond oxidation and the ring-opening reaction of polycyclic moieties by decreasing their reaction energies.