Arani Chanda

FeIII–TAML-catalyzed green oxidative degradation of the azo dye Orange II by H2O2 and organic peroxides: products, toxicity, kinetics, and mechanisms

Oxidation of Orange II ([4-[(2-hydroxynaphtyl)azo]benzenesulfonic acid], sodium salt) by hydrogen peroxide catalyzed by iron(III) complexed to tetra amido macrocyclic ligands (FeIII–TAML activators) in aqueous solutions at pH 9–11 leads to CO2, CO, phthalic acid and smaller aliphatic carboxylic acids as major mineralization products. The products are non-toxic according to the Daphnia magna test. Several organic intermediates have been identified by HPLC and GC-MS that allowed the detailed description of Orange II degradation. The catalytic oxidation can also be performed by organic oxidants such as benzoyl peroxide, tert-butyl and cumyl hydroperoxides. Kinetic studies of the catalyzed oxidation indicated that FeIII–TAML activators react first with ROOR′ to form an oxidized catalyst (kI), which then oxidizes Orange II (kII). Neglecting the reversibility of the first step, the rate equation is rate = kIkII[FeIII][ROOR′][Dye]/(kI[ROOR′] + kII[Dye]); here FeIII and ROOR′ represent the catalyst and peroxide, respectively. The rate constant kI equals (74 ± 3) × 103, (1.4 ± 0.1) × 103, 24 ± 2, and 11 ± 1 M−1 s−1 for benzoyl peroxide, H2O2, t-BuOOH, and cumyl hydroperoxide at pH 9 and 25 °C, respectively. An average value of kII equals (3.1 ± 0.9) × 104 M−1 s−1 under the same conditions. The unraveling of the kinetic mechanism allows the comprehension of the robust reactivity, and this is discussed in detail using the representative results of DFT calculations.

(TAML)FeIV═O Complex in Aqueous Solution: Synthesis and Spectroscopic and Computational Characterization

Recently, we reported the characterization of the S = (1)/ 2 complex [Fe (V)(O)B*] (-), where B* belongs to a family of tetraamido macrocyclic ligands (TAMLs) whose iron complexes activate peroxides for environmentally useful applications. The corresponding one-electron reduced species, [Fe (IV)(O)B*] (2-) ( 2), has now been prepared in >95% yield in aqueous solution at pH > 12 by oxidation of [Fe (III)(H 2O)B*] (-) ( 1), with tert-butyl hydroperoxide. At room temperature, the monomeric species 2 is in a reversible, pH-dependent equilibrium with dimeric species [B*Fe (IV)-O-Fe (IV)B*] (2-) ( 3), with a p K a near 10. In zero field, the Mössbauer spectrum of 2 exhibits a quadrupole doublet with Delta E Q = 3.95(3) mm/s and delta = -0.19(2) mm/s, parameters consistent with a S = 1 Fe (IV) state. Studies in applied magnetic fields yielded the zero-field splitting parameter D = 24(3) cm (-1) together with the magnetic hyperfine tensor A/ g nbeta n = (-27, -27, +2) T. Fe K-edge EXAFS analysis of 2 shows a scatterer at 1.69 (2) A, a distance consistent with a Fe (IV)O bond. DFT calculations for [Fe (IV)(O)B*] (2-) reproduce the experimental data quite well. Further significant improvement was achieved by introducing hydrogen bonding of the axial oxygen with two solvent-water molecules. It is shown, using DFT, that the (57)Fe hyperfine parameters of complex 2 give evidence for strong electron donation from B* to iron.

Mechanistically Inspired Design of FeIII−TAML Peroxide-Activating Catalysts

Recent broad-ranging mechanistic studies of FeIII-TAML peroxide activators enable a strategy for designing catalysts with improved (i) hydrolytic and (ii) operational stabilities, (iii) faster activation of H2O2 and other peroxides, and (iv) a pH of highest activity closer to 7. Combining all items of insight leads to [Fe{1-NO2C6H3-3,4-(NCOCMe2NCO)2CF2}(OH2)]- (1a) which exhibits the most desirable technical performance in its class.

TAML Activator/Peroxide-Catalyzed Facile Oxidative Degradation of the Persistent Explosives Trinitrotoluene and Trinitrobenzene in Micellar Solutions

TAML activators are well-known for their ability to activate hydrogen peroxide to oxidize persistent pollutants in water. The trinitroaromatic explosives, 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitrobenzene (TNB), are often encountered together as persistent, toxic pollutants. Here we show that an aggressive TAML activator with peroxides boosts the effectiveness of the known surfactant/base promoted breakdown of TNT and transforms the surfactant induced nondestructive binding of base to TNB into an extensive multistep degradation process. Treatment of basic cationic surfactant solutions of either TNT or TNB with TAML/peroxide (hydrogen peroxide and tert-butylhydroperoxide, TBHP) gave complete pollutant removal for both in <1 h with >75% of the nitrogen and ≥20% of the carbon converted to nitrite/nitrate and formate, respectively. For TNT, the TAML advantage is to advance the process toward mineralization. Basic surfactant solutions of TNB gave the colored solutions typical of known Meisenheimer complexes which did not progress to degradation products over many hours. However with added TAML activator, the color was bleached quickly and the TNB starting compound was degraded extensively toward minerals within an hour. A slower surfactant-free TAML activator/peroxide process also degrades TNT/TNB effectively. Thus, TAML/peroxide amplification effectively advances TNT and TNB water treatment giving reason to explore the environmental applicability of the approach.

Facile destruction of formulated chlorpyrifos through green oxidation catalysis

The organophosphorus (OP) insecticide, chlorpyrifos (CP, O,O-diethyl-O-3,5,6- trichloro-2-pyridyl phosphorothioate) in an emulsifiable concentrate formulation (CP-EC) is totally degraded in water by hydrogen peroxide catalytically activated by the TAML activator (1), to a combination of small aliphatic acids and minerals. CP-EC rapidly forms an oil-in-water emulsion when added to water. The CP in this emulsion is more resistant to oxidation than pure CP in aqueous solution. A one-pot, two-step process consisting of perhydrolysis followed by 1/H2O2 oxidation achieved total degradation of CP in the emulsion. In the first step, emulsified CP was hydrolyzed by H2O2 at high pH to induce the release of 3,5,6-trichloropyridin-2-ol (TCPy). The cationic surfactants CTAB or CTAC accelerated this hydrolysis. Addition of tert-butanol or ethanol also enhanced the hydrolysis rate. Xylenes serving as the solvent in CP-EC were shown to be the cause of the impeded hydrolysis. In the second step, the CP-EC hydrolysate was treated with 1/H2O2 to readily degrade the TCPy. In water, TCPy exists in the enol and not the keto form, which was found to facilitate its rapid oxidation. This degradation procedure produced a 72.5-fold reduction in toxicity (Microtox® assays) of the treated reaction mixture, compared to the untreated CP-EC emulsion. This ambient catalytic process establishes a promising line of investigation for alleviating the decades-old problem of obsolete thiophosphate pesticides.

Oxidation of pinacyanol chloride by H2O2 catalyzed by FeIII complexed to tetraamidomacrocyclic ligand: unusual kinetics and product identification

Oxidative degradation of pinacyanol chloride (PNC) dye by H2O2, as catalyzed by the 1 FeIII-TAML activator (TAML = tetraamidomacrocyclic ligand), occurs rapidly in water, goes to completion, and exhibits noticeably complex kinetics at pH 11. The reaction achieves a carbon mineralization of 51%. The detected products are acetate, formate, oxalate, maleate, 2-nitrobenzoate, nitrite, and nitrate. The catalytic reaction is a first-order process in 1 and the reaction rate has a Michaelis dependence in hydrogen peroxide (H2O2). The reaction rate increases sharply with increasing PNC concentration, reaches a maximal value, and then declines as the PNC concentration is further increased. The initial rate (v) versus [PNC] profile has been quantified in terms of the equation: v = (c 1[PNC] + c 2[PNC]2)/(c 3 + c 4[PNC] + [PNC]2) which accounts for the maximum and further rate decline. Kinetic analysis at a more acidic pH (9 vs. 11) revealed that there is no initial rate increase and only the hyperbolic retardation by PNC is observed, in accord with the rate law v = (b 1 + b 2[PNC])/(b 3 + [PNC]). The kinetic data has been rationalized using the adopted mechanism of catalysis by FeIII-TAML activators, which involves the reaction between 1 and H2O2 to form reactive, oxidized TAML (k I, k −I) followed by its reaction with the dye (k II). The minimalistic addition to the scheme to account for the PNC case is the assumption that 1 may rapidly and reversibly associate with PNC (K), and the associated complex reacts also with H2O2 (k ID, k −ID) to form also the oxidized TAML. Spectral evidence for this association is presented. The optimization of PNC structure by density functional theory rules out coordination of PNC to 1 via the formation of a Fe–N bond. The kinetic data indicate that the rate constant k II exceeds 1 × 105 (mol L−1)−1 s−1 at pH 11 and 25°C.

Rapid degradation of oxidation resistant nitrophenols by TAML activator and H2O2

Nitrophenols (NPs) are widely prevalent recalcitrant anthropogenic pollutants. TAML activators in conjunction with peroxides have proven to be effective in the remediation of myriad organic pollutants. In the present study, we have discovered that one of the most reactive TAML activators (1) catalyses the oxidative degradation by H2O2 of mono- and dinitrophenols (including all US-EPA classified priority pollutants) under ambient conditions at pH 8 which is close to pH of environmental waters. Individual nitrophenols as well as mixtures thereof undergo fast decontamination (reaction time ≤45 min) resulting in deep oxidation producing HCO2− and minerals, CO, CO2, NO2−, and NO3− (up to 99% of N and 70% of C). The remarkable efficacy of the 1/H2O2-mediated decontamination process is matched by complete elimination of the toxicity of the nitrophenols (Microtox® assays). Detailed mechanistic studies of the catalyzed oxidation revealed a strong substrate inhibition of the catalytic activity for some nitrophenols, the strongest being observed for 4-nitrophenol. DFT calculations suggest that the inhibition is likely due to reversible binding of nitrophenolate anions to the iron(III) center of the resting state of the 1 catalyst, which compromises its reactivity toward H2O2.