Catalytic oxidation of 2,4,6-tribromophenol using iron(III) complexes with imidazole, pyrazole, triazine and pyridine ligands

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Highlights

  • Catalytic oxidation of TrBP was tested by using five non-heme Fe(III) complexes.

  • The mer-[FeCl3(terpy)] had the highest capability to TrBP degradation with KHSO5.

  • DBQ was detected as a major byproduct and further degraded to organic acids.

  • 15% of the degraded TrBP was mineralized to CO2 in mer-[FeCl3(terpy)]/KHSO5 system.

  • The mer-[FeCl3(terpy)] was activated by forming peroxide complex with oxygen donor.

Abstract

Five types of non-heme iron complexes, coordinated with imidazole, pyrazole, triazine and pyridine ligands, which had been previously synthesized, were used in the following studies. Among these complexes, the mer-[FeCl3(terpy)] complex showed the highest catalytic activity for the oxidative degradation of 2,4,6-tribromophenol (TrBP) using KHSO5 as an oxygen donor. The turnover numbers for the degradation and debromination of TrBP in the mer-[FeCl3(terpy)]/KHSO5 catalytic system were estimated to be 1890 ± 1 and 4020 ± 216, respectively. The catalytic activity was significantly inhibited at pH 4–7 in the presence of a humic acid, a major component of landfill leachates. However, the percent of TrBP degradation and debromination increased at pH 8. GC/MS analyses showed that a major oxidation product was 2,6-dibromo-p-benoquinone (DBQ) and its level decreased with increasing reaction time, suggesting that organic acids (identified by LC/TOF-MS) are formed via the ring-cleavage of DBQ. Mineralization to CO2 was observed to be 15% as a result of the oxidation for a 3 h period, where TOC values before and after the reaction were measured. Absorption spectra of mer-[FeCl3(terpy)] with m-chloroperoxybenzoic acids as an oxygen donor in acetonitrile showed that a center metal, Fe, formed a peroxide complex with the oxygen donor.

Introduction

2,4,6-Tribromophenol (TrBP) is used in the production of fungicides, wood preservatives [1] and an intermediate in the production of brominated flame retardants (BFRs) [2]. TrBP is eluted from landfills where wastes derived from electrical devices including BFRs are disposed of, and this leads to water pollution [3]. In particular, humic acids (HAs) are present in landfill leachates in the form of dissolved organic matter and interactions with HA can enhance water solubility and inhibit the oxidative degradation of organic pollutants [4], [5]. Because TrBP has been reported to have endocrine disrupting effects, the TrBP in landfill leachates must be reduced.

Previous studies suggest that Fe(III)-porphyrins, a model of heme enzymes, can be effective for the oxidative degradation of bromophenols and chlorophenols [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Among the halogenated phenols, bromophenols are relatively resistant to oxidative degradation and dehalogenation [8]. However, mineralization to CO2 has been reported in the case of a heterogeneous Fe(III)-porphyrin catalyst, which was introduced into an ionic liquid supported Fe3O4 [19]. However, in a homogeneous catalytic system, mineralization is minimal, because of catalyst self-degradation.

On the other hand, non-heme enzymes, such as methane monooxygenase, riske dioxygenase, lipoxygenase and catechol dioxygenase, involve iron as a center metal. These enzymes can be activated by an oxygen donor, and organic substrates are oxidized by active species such as Lradical dot+-Fe(IV) = O, L-Fe(IV) = O and HOradical dot that are produced in such systems [18]. Stable non-heme iron complexes with tetradentate ligands had been synthesized and applied as catalysts for the selective oxidation of olefins [19], [20], [21], [22], [23], [24]. However, there are no reports, regarding the catalytic oxidation of halogenated phenols using non-heme iron complexes. Catechol dioxygenases can oxidize catechol, a phenol derivative, via ring-cleavage [20]. In particulars, Dhanalkshmi et al [25]. reported that iron complexes, when coordinated with tridentate ligands, such as pyridine-2,6-dicarboxylic acid, 2,2′:6,2′′-terpyridine (terpy) and 2,6-bis(benzimidazol-2′-yl) pyridine, showed higher activities for the oxidation of catechol. This finding suggests that a non-heme iron complex with a tridentate ligand might be effective for the oxidation of phenol derivatives.

Five types of non-heme iron complexes with monodentate imidazole, pyrazole and tridentate nitrogen ligands (Fig. 1), which had been previously synthesized and their structures determined by X-ray crystallography [26], were prepared. In the present study, the catalytic activities of the non-heme iron complexes for the oxidative degradation of TrBP and the influence of HA on the oxidation of TrBP were investigated. In addition, to characterize the oxidation products of TrBP produced by the non-heme catalyst, the reaction mixtures and CH2Cl2 extracts derived from them were analyzed by LC/TOF-MS and GC/MS, respectively. The detection of the activated species of the non-heme complex by the oxygen donor was examined by observing the visible absorption spectrum.

Section snippets

Materials

Imidazole (im), 1-methylimidazole (meim) (Wako Pure Chemical Industries), 3-methylpyrazole (mepy), 2,4,6-tris(2-pyridil)-1,3,5-triazine (tptz), terpy and TrBP (Tokyo Chemical Industry) were used without further purification. An iron(III) chloride anhydrate was obtained from Nacalai Tesque. KHSO5 was obtained as a triple salt, 2KHSO5∙KHSO4∙K2SO4 (Merck). Other reagents were obtained from Wako Pure Chemical Industries and were used without further purification. An HA used in this study was

Catalytic activities for the synthesized complexes

Table 1 shows the percent TrBP degradation for the synthesized complexes using H2O2 or KHSO5 as the oxygen donor at pH 7 after a 30 min reaction period. In control cases, no TrBP degradation was observed in the presence of H2O2 or KHSO5 alone. For the case of complexes with monodentate ligands mer-[FeCl3(meim)3], [FeCl2(mepy)3]Cl and [FeCl2(im)4]Cl, the percent TrBP degradation was less than 10%, while the iron complex with a tridentate ligand, mer-[FeCl3(tptz)], indicated 7–8% of degradation.

Conclusion

Catalytic activities of five non-heme complexes (mer-[FeCl3(meim)3], [FeCl2(im)4]Cl, [FeCl2(mepy)]Cl, mer-[FeCl3(tptz)] and mer-[FeCl3(terpy)]), which had been previously synthesized, were evaluated, in terms of the degradation of TrBP in the presence of H2O2 or KHSO5 as oxygen donors. The mer-[FeCl3(terpy)]/KHSO5 catalytic system showed a higher activity than other catalytic systems. TON values for the degradation and debromination of TrBP by the mer-[FeCl3(terpy)]/KHSO5 catalytic system were

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number 25241017.

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