Oxidation of dibenzo-p-dioxin: Formation of initial products, 2-methylbenzofuran and 3-hydro-2-methylenebenzofuran

This study combines the results of experimental measurements and theoretical computations to investigate the initial steps in the oxidation of dibenzo-p-dioxin (DD). The aim is to discover new pathways leading to the formation of toxic species in combustion systems, as well as compounds that may act as precursors for polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F). The experiments were performed in a tubular reactor equipped with sampling systems for condensable and gaseous products. The reaction conditions were characterised by a residence time of 5 s and a fuel equivalence ratio of 0.12. We investigated the effect of temperature (400–800 °C) on the type and the yield of the oxidative products, trapped in a cartridge, and gases collected in tedlar bags for subsequent FTIR analyses. The analyses of VOC, performed on a high resolution gas chromatograph–triple quadrupole mass spectrometer (HRGC–QQQMS), identified 2-methylbenzofuran and 3-hydro-2-methylenebenzofuran as the initial products. This has been confirmed by injection of authentic standards and the application of collision induced dissociation (CID) that fragmented the isolated parent ions into specific product ions affording the identification of parent species. The oxidative decomposition of DD initiated at around 450 °C, with the evolution of VOC peaking at between 650 and 700 °C. In excess of 750 °C, all VOC were completely oxidised. The potential energy surface (PES), based on the density functional theory (DFT) of B3LYP, mapped the initial steps involved in the oxidation of DD, and yielded a detailed reaction scheme for the onset of oxidation of DD that results in the formation of 2-methylbenzofuran and 3-hydro-2-methylenebenzofuran. Initial oxidation of DD is most likely to commence via addition of the singlet oxygen molecule and the subsequent facile formation of the propagating OH radical.