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 Cl_(2)and OHgenerated 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 O_(2)•^(-)from the OCl^(-)to afford BPCAs.

Effect of pyrolysis on Zhaotong lignite oxidation with aqueous sodium hypochlorite

The pyrolysis treatment of lignites could remove thermal-unstable aliphatic structures but enrich aromatic structures in resulting pyrolysis residues,which would facilitate producing benzene polycarboxylic acids(BPCAs).Herein,the demineralized Zhaotong lignite(DZL)was pyrolyzed at 250-550℃ to afford the corresponding pyrolysis residues.Subsequently,DZL and its pyrolysis residues were subjected to oxidation with aqueous sodium hypochlorite.By XRD,FTIR,and element analyses,it was found that the content and polycondensation degree of aromatic structures simultaneously increased with increasing pyrolysis temperature.Furthermore,the yield and selectivity of BPCAs both increased along with raising pyrolysis temperature in resulting products from fully oxidation of DZL and its pyrolysis residues.Based on the distribution of BPCAs,peri-condensed aromatic structures were dominant in DZL and its DPRs,and obviously increased with increasing pyrolysis temperature.More interestingly,good correlation was found between the yield of BPCAs and aromaticity parameters.Meanwhile,the yield of benzenepentacrboxylic acid was well associated with polycondensation degree parameters.