Abstract
The influence of halogen substituents on the catalytic oxidation of 2,4,6-trihalogenated phenols (TrXPs) by iron(III)-porphyrin/KHSO5 catalytic systems was investigated. Iron(III)-5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin (FeTHP) and its supported variants were employed, where the supported catalysts were synthesized by introducing FeTHP into hydroquinone-derived humic acids via formaldehyde poly-condensation. F (TrFP), Cl (TrCP), Br (TrBP) and I (TrIP) were examined as halogen substituents for TrXPs. Although the supported catalysts significantly enhanced the degradation and dehalogenation of TrFP and TrCP, the oxidation of TrBP and TrIP was not enhanced, compared to the FeTHP catalytic system. These results indicate that the degree of oxidation of TrXPs is strongly dependent on the types of halogen substituent. The order of dehalogenation levels for halogen substituents in TrXPs was F > Cl > Br > I, consistent with their order of electronegativity. The electronegativity of a halogen substituent affects the nucleophilicity of the carbon to which it is attached. The levels of oxidation products in the reaction mixtures were analyzed by GC/MS after extraction with n-hexane. The most abundant dimer product from TrFP via 2,6-difluoroquinone is consistent with a scenario where TrXP, with a more electronegative halogen substituent, is readily oxidized, while less electronegative halogen substituents are oxidized less readily by iron(III)-porphyrin/KHSO5 catalytic systems.
Highlights
Halogenated phenols, especially chlorophenols and bromophenols, are frequently included in plastics as flame retardants and for wood preservation
TrXPs were used as organic substrates, and two HQ-humic acid (HA)-FeTHP catalysts (HQ-SHA-FeTHP and HQ-Tohro ando soil (THA)-FeTHP) were prepared from two types of HAs as described in the Experimental Section
TrIP using the FeTHP catalyst, the percentage of TrXPs was somewhat higher at higher pH, while the percent TrXP degradation using the hydroquinone-derived HA (HQ-HA)-FeTHPs reached a minimum at pH 5
Summary
Halogenated phenols, especially chlorophenols and bromophenols, are frequently included in plastics as flame retardants and for wood preservation. Iron(III)-porphyrin complexes have been regarded as biomimetic models of oxidative enzymes such as ligninases and peroxidases [4], and can catalyze the oxidative dechlorination of chlorophenols in the presence of an oxygen donor such as H2O2 and KHSO5 [5,6,7,8,9,10]. Some iron(III)-porphyrin catalysts have been examined for their ability to catalyze the oxidation of bromophenols, which are mainly utilized as brominated flame retardants. In this case, the degree of debromination was much lower than that of the dechlorination for chlorophenols [21,22]. Systems were investigated, in terms of degradation rates, the levels of dehalogenation and the oxidation products that are produced
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