Mechanism and kinetic analysis of PCDD/Fs formation from aliphatic hydrocarbons (C2H2, C2H4, C3H6, C4H8) precursors.

Density functional theory calculations were performed to gain insight into the mechanism and kinetic studies of homogeneous gas-phase formation of polychlorinated dibenzodioxins and polychlorinated dibenzofurans (PCDD/Fs) via aliphatic hydrocarbons (C2H2, C2H4, C3H6 and C4H8). The calculated results demonstrated that the intra-annular elimination of H is the rate-determining step throughout the reaction chain; the presence of ortho-Cl increases the abstraction barrier of arene H and decreases the reactivity of the molecule. The phenoxy radicals undergoes dimerization via carbon-carbon or carbon-oxygen coupling to form PCDD/Fs and the two coupling pathways are competitive. Our work indicates that aliphatic hydrocarbons are less reactive precursors in PCDD/F formation compared with chlorophenoxy radicals and phenoxy radicals among primary precursors of PCDD/Fs. The results presented here could be used to evaluate the contribution of aliphatic hydrocarbons acting as precursors to PCDD/Fs formation.