Mechanistic and kinetic studies on the homogeneous gas-phase formation of PCDD/Fs from 2,4,5-trichlorophenol.

An understanding of the reaction mechanism of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) formation is crucial for any attempt to prevent PCDD/Fs formation. Among the polychlorophenols, 2,4,5-trichlorophenol (2,4,5-TCP) has the minimum number of CI atoms needed to form 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (2,3,7,8-TeCDD), which is the most toxic among all 210 PCDD/F isomers. Experiments on the formation of PCDD/Fs from the 2,4,5-TCP precursor have been hindered by the strong toxicity of 2,3,7,8-TeCDD. In this work, we carried out molecular orbital theory calculations for the homogeneous gas-phase formation of PCDD/Fs from the 2,4,5-TCP precursor. Several energetically favorable formation pathways were revealed for the first time. The rate constants of crucial elementary steps were deduced over a wide temperature range of 600-1200 K, using canonical variational transition state theory with small curvature tunneling contribution. The rate temperature formulas were fitted. This study shows that the formation of polychlorinated dibenzo-p-dioxins (PCDDs) from the 2,4,5-TCP precursor is preferred over the formation of polychlorinated dibenzofurans (PCDFs). The chlorine substitution pattern has a significant effect on the dimerization of chlorophenoxy radicals.