Catalytic effect of CuO and other transition metal oxides in formation of dioxins: Theoretical investigation of reaction between 2,4,5-trichlorophenol and CuO

Density functional theory (DFT) calculations have been carried out to explore the potential energy surface (PES) associated with the gas-phase reaction between 2,4,5-trichlorophenol and CuO. A gas-phase model was constructed to account, from a theoretical perspective, for the most important reaction steps reported experimentally for the interaction between chlorinated phenol and a CuO surface. This involves the facile production of the chlorophenoxy radical through hydroxyl H abstraction, formation of HO-Cu-2,4,5-trichlorophenolate complex, and reduction of Cu-(II) into Cu(I) through chlorophenoxy desorption from the chlorophenolate complex. The overall process: 2,4,5-trichlorophenol + CuO → 2,4,5-trichlorophenoxy radical + CuOH is significantly exothermic and facile (unlike the strongly endothermic process of 2,4,5-trichlorophenol → 2,4,5-trichlorophenoxy radical + H) suggesting that in the gas phase, at least, CuO would be an efficient catalyst for production of polychlorinated phenoxy radicals which are known precursors of dioxins. Hence, the present study should be an important preliminary to a detailed investigation of the efficacy of CuO surfaces toward catalysis of dioxin formation. Lastly, we estimate the reaction energies for the reaction 2,4,5-trichlorophenol + MO → 2,4,5-trichlorophenoxy radical + MOH for the first-row transition metal monoxides. This reaction only becomes exothermic for elements which have at least a half-filled d shell. Although the results of the present thermodynamic analysis match the observed catalytic effect toward dioxin formation, kinetic considerations are expected to play a major role as well.