Dioxin Formations from the Radical/Radical Cross-Condensation of Phenoxy Radicals with 2-Chlorophenoxy Radicals and 2,4,6-Trichlorophenoxy Radicals
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Abstract
It is important to understand the role of phenol in the dioxin formations because it is present in the high amount in municipal waste incinerators (MWIs). The formation mechanism of dioxins from the cross-condensation of PhRs with 2-CPRs and 2,4,6-TCPRs was investigated by using hybrid density functional theory (DFT) and compared with the dioxin formation mechanism from the self-condensation of single chlorophenol precursors. The geometrical parameters were optimized at the MPWB1K level with the 6-31+G(d,p) basis set without symmetry constraints. Single-point energy calculations were carried out at the MPWB1K/6-311+G(3df,2p) level of theory. The rate constants were deduced by using canonical variational transition-state (CVT) theory with small curvature tunneling (SCT) contribution over the temperature range of 600-1200 K. The Arrhenius formulas were reported for the first time. Results show that phenol is responsible for the formation of dioxin congeners. This work, together with results already published from our group, provides a comprehensive investigation of the homogeneous gas-phase formation of dioxins from (chloro)phenol precursors and should help to clarify the formation mechanism of dioxins in real waste combustion and to develop more effective control strategies.