A novel method for producing benzene polycarboxylic acids by electrochemical oxidation of Zhaotong lignite in aqueous NaCl solution

The development of a coal-based synthetic route to produce benzene polycarboxylic acids (BPCAs) is of great importance for the highly efficient utilization of lignites. In this paper, aqueous NaCl electrolytic system was used to oxidize Zhaotong lignite to prepare BPCAs. The electrochemical oxidation of lignite in aqueous NaCl solution could produce more BPCAs than that in aqueous NaOH solution. The aqueous NaCl electrolytic system could in-suit produce a stable OCl⁻, which was synthesized by the combination reaction between Cl₂ and OH⁻ generated in the anode or cathode, respectively. The in-suit produced OCl⁻ would degrade the organic structures of the lignite dispersing in the electrolyte to generate BPCAs. The formation of BPCAs could be greatly affected by current density, electrolysis time and the addition amount of NaCl in the electrolytic system, which resulted from that the factors played an important role in the generation of OCl⁻. The coal related model compounds including anthracene and phenanthrene were used to investigate the electrochemical oxidation mechanism of the lignite. The results indicated that the aromatic ring structures in the lignite were attacked by O₂⁻ from the OCl⁻ to afford BPCAs.