Structure activity relationship study of N-doped ligand modified Fe(III)/H2O2 for degrading organic pollutants

The performance of Fe(III)/H₂O₂ was extremely enhanced by a novel N-doped ligand dipicolinamide (Dpa) for removing various organic pollutants. This dramatic enhancement of contaminants degradation in Fe(III)-Dpa/H₂O₂ system under pH≥ 7 was ascribed to the coordinating capacity of Dpa to form the dissolved Fe(III)-Dpa/Fe(II)-Dpa, and the reductive capacity of Dpa to maintain the concentration of Fe(II), which made Dpa improve the catalytic performance of Fe(III) nearly twice as much as Fe(II). Dpa has a strong complexing ability than Cit, NTA, and EDTA to maintain the catalytic activity of Fe(III) without light. The single crystal of Fe-Dpa was obtained to reveal its structure activity relationship. Fe-Dpa was composed of four bonds of Fe-N and two bonds of Fe-Cl. The Fe-Cl bonds were labile sites, which was easily experienced ligand exchange with H₂O₂, resulting Fe-H₂O₂ bonds to initiate degradation reaction. The remaining Fe-N bonds were effectively planar, which had a large delocalized π electrons flow domain, enhancing the production of multiple reactive species, including iron(IV/V)-oxo species, HO· and O₂^-·. An empirical kinetic model of Fe(III)-Dpa/H₂O₂ system was established. In addition, the evaluation results of the toxicity of Fe-Dpa to larval zebrafish and chinese cabbage displayed that Fe-Dpa possesses low toxicity.

Application of FeIII-TAML/H2O2 system for treatment of fluoroquinolone antibiotics

Over the recent past, fluoroquinolone antibiotics (FQs) have raised extensive attention due to their potential to induce the formation of resistance genes and "superbugs", thus various advanced oxidation techniques have been developed to eliminate their release into the environment. In the present study, the prototype tetraamido macrocyclic ligand (Fe^III-TAML)/hydrogen peroxide (H₂O₂) system is employed to degrade FQs (i.e., norfloxacin and ciprofloxacin) over a wide pH range (i.e., pH 6-10), and the reaction rate increases with the increase in pH level. The effect of dosage of Fe^III-TAML and H₂O₂ on the degradation of FQs is evaluated, and the reaction rate is linearly correlated with the added amount of chemicals. Moreover, the impact of natural organic matters (NOM) on the removal of FQs is investigated, and the degradation kinetics show that both NOM type and experimental concentration exhibit negligible influence on the oxidative degradation of selected antibiotics. Based on the results of liquid chromatography-high resolution mass spectrometry and theoretical calculations, the reaction sites and pathways of FQs by Fe^III-TAML/H₂O₂ system are further predicted and elucidated.

Self-assembly of FeIII-TAML-based microstructures for rapid degradation of bisphenols

Iron(III)-tetraamidomacrocyclic ligand (Fe^III-TAML) activators have drawn great attentions due to the high reactivity to degrade organic pollutants. However, previous studies showed that the reactivity and stability of Fe^III-TAML were both strongly pH-dependent, which dramatically decrease at lower pH levels. Herein, Fe^III-TAML/DODMA (dimethyldioctadecylammonium chloride) microspheres with diameters ranging from 100 to 2000 nm were synthesized via a surfactant-assisted self-assembly technique. The newly synthesized Fe^III-TAML/DODMA composite exhibits superior reactivity compared to free Fe^III-TAML as indicated by the degradation of bisphenols (i.e., bisphenol A and its analogues) over a wide pH range (i.e., pH 4.5-10.0). Based on the adsorption results and quantitative structure-activity relationship (QSAR) models, the enhanced reactivity of Fe^III-TAML/DODMA is mainly ascribed to the hydrophobic sorption of bisphenols. Moreover, the enhanced ionization of the axial water molecule associated with Fe^III-TAML could further enhance the reactivity of synthesized microcomposites, which was confirmed by the results of infrared and Raman spectra. Furthermore, Fe^III-TAML/DODMA shows distinct acid-resistance as explained by the protection of the hydrophobic alkyl chains of DODMA. This novel method would provide a simple and effective strategy to expand the application of Fe^III-TAML in a wide pH range and render Fe^III-TAML/DODMA microstructure as a potential catalyst for treatment of bisphenol compounds.

Peroxymonosulfate activation by iron(III)-tetraamidomacrocyclic ligand for degradation of organic pollutants via high-valent iron-oxo complex

Herein, we proposed a new catalytic oxidation system, i.e., iron(III)-tetraamidomacrocyclic ligand (Fe^III-TAML) mediated activation of peroxymonosulfate (PMS), for highly efficient organic degradation using p-chlorophenol (4-CP) as a model one. PMS/Fe^III-TAML is capable of degrading 4-CP completely in 9 min at the initial 4-CP of 50 μM and pH = 7, whereas the recently explored system, H₂O₂/Fe^III-TAML, could only result in ∼22% 4-CP removal in 20 min under otherwise identical conditions. More attractively, inorganic anions (i.e., Cl^-, SO₄^2-, NO₃^-, and HCO₃^-) exhibited insignificant effect on 4-CP degradation, and the negative effect of natural organic matters (NOM) on the degradation of 4-CP in PMS/Fe^III-TAML is much weaker than the sulfate radical-based oxidation process (PMS/Co²⁺). Combined with in-situ XANES spectra, UV-visible spectra, electron paramagnetic resonance (EPR) spectra, and radical quenching experiments, high-valent iron-oxo complex (Fe^IV(O)TAML) instead of singlet oxygen (¹O₂), superoxide radical (O₂^•-), sulfate radicals (SO₄^•-) and hydroxyl radicals (HO^•) was the key active species responsible for 4-CP degradation. The formation rate (k_I) and consumption rate (k_II) of the Fe^IV(O)TAML in PMS/Fe^III-TAML were pH-dependent in the range of 6.0-11.5. As expected, increasing the Fe^III-TAML and PMS dosage resulted in a higher steady-state concentration of Fe^IV(O)TAML and enhanced the 4-CP degradation accordingly. In addition, the oxidation capacity of PMS was almost totally utilized in PMS/Fe^III-TAML for 4-CP oxidation due to the two-electron abstraction from 4-CP by one PMS. We believe this study will shed new light on effective PMS activation by Fe-ligand complexes to efficiently degrade organic contaminants via nonradical pathway.

Development of Fe(II) system based on N, N'-dipicolinamide for the oxidative removal of 4-chlorophenol

A novel catalyst system was investigated based on Fe-N, N'-dipicolinamide complex for the degradation of 4-chlorophenol (4-CP) by using hydrogen peroxide as an oxidant under mild alkaline conditions. This complex was stabilized by a ligand that assembles pyridyl and amide groups with a suitable linker. The optimization of the synthesized catalysts was evaluated in terms of the removal efficiency of 4-CP, by using Fe(II) and N, N'-1,2-phenyl-enedipyridine-2-carboxamide with a molar ratio of 1:1. The effects of reaction parameters on the oxidation of 4-CP were investigated by applying the selected catalyst with 4-CP removal rate of 99%. The results indicated that the pH and catalyst concentration could significantly affect the degradation rate of 4-CP. The mineralization level of 4-CP during the reaction was also examined, and almost 62.5% of 4-CP was absolutely mineralized into carbon dioxide and water. The preliminary analysis on the degradation mechanism indicate that the main active species are not hydroxyl radicals, and another kind of active species, called iron-oxo species, were proposed. This study explores a resultful linker between pyridyl and amide and presents a new method to expand the application of pH range of Fenton-like system.

Spectroscopic investigation and direct comparison of the reactivities of iron pyridyl oxidation catalysts

The growing interest in green chemistry has fueled attention to the development and characterization of effective iron complex oxidation catalysts. A number of iron complexes are known to catalyze the oxidation of organic substrates utilizing peroxides as the oxidant. Their development is complicated by a lack of direct comparison of the reactivities of the iron complexes. To begin to correlate reactivity with structural elements, we compare the reactivities of a series of iron pyridyl complexes toward a single dye substrate, malachite green (MG), for which colorless oxidation products are established. Complexes with tetradentate, nitrogen-based ligands with cis open coordination sites were found to be the most reactive. While some complexes reflect sensitivity to different peroxides, others are similarly reactive with either H₂O₂ or tBuOOH, which suggests some mechanistic distinctions. [Fe(S,S-PDP)(CH₃CN)₂](SbF₆)₂ and [Fe(OTf)₂(tpa)] transition under the oxidative reaction conditions to a single intermediate at a rate that exceeds dye degradation (PDP=bis(pyridin-2-ylmethyl) bipyrrolidine; tpa=tris(2-pyridylmethyl)amine). For the less reactive [Fe(OTf)₂(dpa)] (dpa=dipicolylamine), this reaction occurs on a timescale similar to that of MG oxidation. Thus, the spectroscopic method presented herein provides information about the efficiency and mechanism of iron catalyzed oxidation reactions as well as about potential oxidative catalyst decomposition and chemical changes of the catalyst before or during the oxidation reaction.

Rapid Destruction of Tetrabromobisphenol A by Iron(III)-Tetraamidomacrocyclic Ligand/Layered Double Hydroxide Composite/H2O2 System

Iron(III)-tetraamidomacrocyclic ligand (Fe(III)-TAML) activators have received widespread attentions for their abilities to activate hydrogen peroxide to oxidize many organic pollutants. In this study, Fe(III)-TAML/layered double hydroxide (LDH) composite was developed by intercalating Fe(III)-TAML into the interlayer of LDH. Electrostatic interaction and hydrogen bonding might account for the adsorption of Fe(III)-TAML on LDH. The newly synthesized Fe(III)-TAML/LDH composite showed superior reactivity as indicated by efficient decomposition of tetrabromobisphenol A (TBBPA) in the presence of hydrogen peroxide, which can be fully degraded within 20 s and the degradation rate increased up to 8 times compared to free Fe(III)-TAML. In addition, the toxicity of the system was significantly reduced after the reaction. The higher reactivity of Fe(III)-TAML/LDH system is attributed to the enhanced adsorption of TBBPA on LDH, which could increase the contact possibility between Fe(III)-TAML and TBBPA. On the basis of the analysis of reaction intermediates, β-scission at the middle carbon atom and C-Br bond cleavage in phenyl ring of TBBPA were involved in the degradation process. Furthermore, our results demonstrated that the Fe(III)-TAML/LDH composite can be reused several times, which could lower the overall cost for environmental implication and render Fe(III)-TAML/LDH as the potential environmentally friendly catalyst for future wastewater treatment under mild reaction conditions.

Spectroscopic, Electrochemical and Computational Characterisation of Ru Species Involved in Catalytic Water Oxidation: Evidence for a [Ru(V) (O)(Py2 (Me) tacn)] Intermediate

A new family of ruthenium complexes based on the N-pentadentate ligand Py2 (Me) tacn (N-methyl-N',N''-bis(2-picolyl)-1,4,7-triazacyclononane) has been synthesised and its catalytic activity has been studied in the water-oxidation (WO) reaction. We have used chemical oxidants (ceric ammonium nitrate and NaIO4 ) to generate the WO intermediates [Ru(II) (OH2 )(Py2 (Me) tacn)](2+) , [Ru(III) (OH2 )(Py2 (Me) tacn)](3+) , [Ru(III) (OH)(Py2 (Me) tacn)](2+) and [Ru(IV) (O)(Py2 (Me) tacn)](2+) , which have been characterised spectroscopically. Their relative redox and pH stability in water has been studied by using UV/Vis and NMR spectroscopies, HRMS and spectroelectrochemistry. [Ru(IV) (O)(Py2 (Me) tacn)](2+) has a long half-life (>48 h) in water. The catalytic cycle of WO has been elucidated by using kinetic, spectroscopic, (18) O-labelling and theoretical studies, and the conclusion is that the rate-determining step is a single-site water nucleophilic attack on a metal-oxo species. Moreover, [Ru(IV) (O)(Py2 (Me) tacn)](2+) is proposed to be the resting state under catalytic conditions. By monitoring Ce(IV) consumption, we found that the O2 evolution rate is redox-controlled and independent of the initial concentration of Ce(IV) . Based on these facts, we propose herein that [Ru(IV) (O)(Py2 (Me) tacn)](2+) is oxidised to [Ru(V) (O)(Py2 (Me) tacn)](2+) prior to attack by a water molecule to give [Ru(III) (OOH)(Py2 (Me) tacn)](2+) . Finally, it is shown that the difference in WO reactivity between the homologous iron and ruthenium [M(OH2 )(Py2 (Me) tacn)](2+) (M=Ru, Fe) complexes is due to the difference in the redox stability of the key M(V) (O) intermediate. These results contribute to a better understanding of the WO mechanism and the differences between iron and ruthenium complexes in WO reactions.

Rapid degradation of oxidation resistant nitrophenols by TAML activator and H2O2

TAML and H₂O₂remove toxic nitrophenol pollutants producing innocuous minerals. Mechanistic studies reveal the substrate inhibition due to the reversible binding of nitrophenolate to iron(iii) of the TAML resting state.

Activation parameters as mechanistic probes in the TAML iron(V)-oxo oxidations of hydrocarbons

The results of low-temperature investigations of the oxidations of 9,10-dihydroanthracene, cumene, ethylbenzene, [D10]ethylbenzene, cyclooctane, and cyclohexane by an iron(V)-oxo TAML complex (2; see Figure 1) are presented, including product identification and determination of the second-order rate constants k2 in the range 233-243 K and the activation parameters (ΔH(≠) and ΔS(≠)). Statistically normalized k2 values (log k2') correlate linearly with the C-H bond dissociation energies DC-H, but ΔH(≠) does not. The point for 9,10-dihydroanthracene for the ΔH(≠) vs. DC-H correlation lies markedly off a common straight line of best fit for all other hydrocarbons, suggesting it proceeds via an alternate mechanism than the rate-limiting C-H bond homolysis promoted by 2. Contribution from an electron-transfer pathway may be substantial for 9,10-dihydroanthracene. Low-temperature kinetic measurements with ethylbenzene and [D10]ethylbenzene reveal a kinetic isotope effect of 26, indicating tunneling. The tunnel effect is drastically reduced at 0 °C and above, although it is an important feature of the reactivity of TAML activators at lower temperatures. The diiron(IV) μ-oxo dimer that is often a common component of the reaction medium involving 2 also oxidizes 9,10-dihydroanthracene, although its reactivity is three orders of magnitude lower than that of 2.

Zebrafish Assays as Developmental Toxicity Indicators in The Design of TAML Oxidation Catalysts

TAML activators promise a novel water treatment approach by efficiently catalysing peroxide-based degradation of chemicals of high concern at environmental concentrations. Green design ethics demands an exploration of TAML toxicity. Exposure to high concentrations of certain activators caused adverse effects in zebrafish. At typical TAML operational concentrations, development was not perturbed.