Density functional theory (DFT) calculations have been used to obtain thermochemical parameters for formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) from the oxidation of 2-chlorophenol. Formation mechanisms of PCDD through radical−radical coupling have been investigated in detail. The sequence of 2-chlorophenoxy radical coupling has been studied. The formation of chlorinated bis keto dimers which results from cross coupling of 2-chlorophenoxy at the ortho carbon bearing hydrogen (a known direct route for PCDF formation) passes through a tight transition structure whose barrier is 9.4 kcal/mol (0 K). Three routes for the formation of the most abundant PCDD/PCDF species (viz., 4,6-dichlorodibenzofuran, 4,6-DCDF, and 1-monochlorodibenzo-p-dioxin, 1-MCDD) in oxidation and pyrolysis of 2-chlorophenol are discussed. In the case of 4,6-DCDF, formation through H or HO + keto−keto ⇄ H₂ or H₂O + keto−keto· ⇄ H₂ or H₂O + enol−keto· ⇄ H₂ or H₂O + 4,6-DCDF + HO is shown to be the preferred route. The other two routes proceed via closed shell processes (keto−keto ⇄ enol−keto ⇄ enol−enol ⇄ H₂O + 4,6-DCDF) and (keto−keto ⇄ enol−keto ⇄ (H-,OH-) 4,6-DCDF ⇄ H₂O + 4,6-DCDF). Results indicate that 1-MCDD should be the favored product in 2-chlorophenol pyrolysis in agreement with experimental findings. According to our results, tautomerization (inter-ring hydrogen transfer) and intra-annular displacement of HCl would not be competitive with paths deriving from H abstraction from the phenolic oxygen and the benzene ring followed by displacement of Cl in the formation of dibenzo-p-dioxin (DD) and 1-MCDD. The results presented here will assist in construction of detailed kinetic models to account for the formation of PCDD/PCDF from chlorophenols.
Journal of Physical Chemistry A Vol. 111, Issue 13, p. 2563-2573