Inhibition Effect and Mechanism of Thiourea on Electrophilic Chlorination of Aromatics in Combustion Flue Gas.

Thiourea can effectively inhibit the formation of highly toxic chlorinated aromatics in postcombustion zone. However, the inhibition mechanism was still not adequately understood. In this study, naphthalene was adopted as a model aromatic compound to investigate the inhibition effect and mechanism of thiourea on the formation of chlorinated aromatics via electrophilic chlorination over Cu and Fe chlorides. Thiourea addition resulted in the reductions of 77.6-99.8% and 36.4-98.1% in the yield of polychlorinated naphthalenes from naphthalene chlorination mediated by CuCl2 and FeCl3 at 150-300 °C, respectively. The inhibition efficiency of chlorination presented a decreasing tendency with increasing reaction temperature and O2 content in flue gas. X-ray diffraction analysis revealed that the thiourea-induced reduction of highly active Cu (II) and Fe (III) chlorides to less active Cu (I) and Fe (II) chlorides was a primary mechanism for inhibiting aromatic chlorination. [thiourea-Cu]+ complex generated during the reduction process could be transformed into CuS and Cu2S, and isomeric reaction product NH4SCN could react with Cu2+ to produce Cu(SCN)2 and then also form Cu sulfides, suggesting sulfidization of Cu chloride was another important inhibition mechanism. Chlorination inhibition induced by the volatile decomposition products of thiourea might only play a minor role.