Pathways to Chlorinated Dibenzodioxins and Dibenzofurans from Partial Oxidation of Chlorinated Aromatics by OH Radical: Thermodynamic and Kinetic Insights

Abstract

Polychlorinated-dibenzodioxins (PCDD's) and polychloro-dibenzofurans (PCDF's) are considered to be formed in combustion environments below 600°C, where an important reactive species is OH Thermodynamic properties, ▴H_f ⁰ (298 K), S ⁰ (298 K) and Cp(T) are calculated in this study for selected poly-chlorinated biphenyls (PCB's), dibenzofurans (PCDF), dibenzodioxins (PCDD) and corresponding radical intermediates resulting from hydroxyl radical addition reactions. These thermodynamic properties are combined with thermochemical kinetic analysis and are used to examine potentially important pathways to homogeneous PCDD and PCDF formation. Analysis of the reaction pathways show several likely routes to formation of chlorinated PCDF's and PCDD's involving unimolecular HCl elimination or loss of Cl, which occur following hydroxy radical addition at Cb-Cl sites. Equilibrium constants for these reactions are calculated using the estimated thermodynamic properties, and strongly favor formation of the PCDD and PCDF products. The addition reactions of OH resulting in PCDF/PCDD formation compete with addition of OH radical at fused ring sites, which are responsible for ring cleavage and destruction of these species.