Synergistic effect of mixed Cu and Fe oxides and chlorides on electrophilic chlorination of dibenzo-p-dioxin and dibenzofuran

摘要

The direct chlorination of dibenzo-p-dioxin (DD) and dibenzofuran (DF) is an important source of dioxins in combustion flue gas. The chlorination reaction mainly occurs via electrophilic substitution induced by Cu and Fe chlorides, which must cohabit on particulate matters in mixed state. To explore the mechanism for DD/DF chlorination in real combustions flue gas, 8 kinds of CuO/Fe_2O_3/CuCl_2/FeCl_3 composites impregnated onto silica powder were prepared to simulate the coexisting state of Cu and Fe species in combustion flue gas. Mixed Cu and Fe oxides and chlorides induced a significant synergistic effect on electrophilic chlorination of DD/DF. The efficiencies of DD/DF chlorination over composites containing both Cu and Fe species were 1-2 orders of magnitude higher than those over composites containing only Fe species at 250 °C. CuCl_2 species were highly active sites for electrophilic chlorination. FeCl_3 acted as an excellent promoter to accelerate DD/DF chlorination over CuCl_2 species. The elevated proportion of Cu and Fe oxides was also favora ble for electrophilic chlorination. Compared with DF, DD was more prone to be chlorinated. Chlorine substitution primarily occurred at 2,3,7 and 8 positions of DD and DF. Furthermore, the possible mechanism for synergistic effect on electrophilic chlorination of DD/DF was speculated.