Robust Iron Coordination Complexes with N-Based Neutral Ligands

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Robust Iron Coordination Complexes with N‑Based Neutral Ligands As Efficient Fenton-Like Catalysts at Neutral pH Maite Canals,†,‡ Rafael Gonzalez-Olmos,*,† Miquel Costas,*,‡ and Anna Company*,‡ †

LEQUIA, Institute of the Environment, Universitat de Girona, Campus Montilivi, E17071 Girona (Catalonia − Spain) Grup de Química Bioinorgànica i Supramolecular (QBIS), Institut de Química Computacional i Catàlisi (IQCC), Departament de Química, Universitat de Girona, Campus Montilivi, E17071 Girona (Catalonia − Spain)

‡

S Supporting Information *

ABSTRACT: The homogeneous Fenton-like oxidation of organic substrates in water with hydrogen peroxide, catalyzed by six different metal coordination complexes with N-based neutral ligands, was studied at ambient conditions and initial pH 7, employing hydrogen peroxide as the terminal oxidant. At low catalyst concentration, the catalytic oxidative depletion of toluene achieved by selected catalysts was much more efficient than that obtained by the Fenton reagent at pH 3. The influence of pH, the water matrix and the catalyst/hydrogen peroxide concentration were investigated for the oxidation of toluene employing [FeCl2(bpmcn)] (1, bpmcn = N,N′-bis(2pyridylmethyl)-N,N′-dimethyl-trans-1,2-diaminocyclohexane), the most efficient catalyst of the series. Moreover, the evolution of catalysts [FeCl2(bpmcn)] (1) and [Fe(OTf)2(Pytacn)] (3, Pytacn = 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7triazacyclononane, OTf = trifluoromethanesulfonate anion) during the course of the reaction was also studied by electrospray ionization mass spectrometry (ESI-MS). The oxidation products derived from toluene oxidation were also analyzed. A plausible mechanism of toluene degradation using [FeCl2(bpmcn)] (1) and [Fe(OTf)2(Pytacn)] (3) as catalysts was proposed, which involves the coexistence of a metal-based path, analogous to that operating in organic media where substrate oxidation is executed by an iron(V)-oxo-hydroxo species, in parallel to a Fenton-type process where hydroxyl radicals are formed.

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INTRODUCTION The presence of organic pollutants in the aquatic environment, particularly toxic and persistent, poses a serious problem for global pollution. The large number and diversity of compounds that are produced and released into the environment as a result of the industrial progress calls for the development of new methods for water treatment.1 These recalcitrant pollutants are introduced into the water cycle through scattered sources including the use of pesticides in agriculture or punctual sources such as the occasional and accidental discharges of industrial wastewater and poorly treated urban sewage.2 Although biological treatment is the most common process used to treat organic-containing wastewaters, a large number of organic pollutants can have low biodegradability and pass through the process without any transformation.3,4 Some organic compounds may also upset the biological treatment because of their toxicity to the microorganisms.5 Advanced oxidation processes (AOP) usually involve the use of O3 or H2O2 for the generation of hydroxyl radicals (OH•) to oxidize organic compounds.6,7 The Fenton reaction is one of the most used methods to generate hydroxyl radicals, which consists in a combination of hydrogen peroxide and a soluble ferrous salt.8 Fenton and related reactions have become of great interest due to their relevance to biological chemistry, chemical © 2013 American Chemical Society

synthesis, the chemistry of natural waters, and the treatment of hazardous wastes.9,10 Its popularity is based on the fact that hydrogen peroxide is inexpensive, relatively safe, and easy to handle, with respect to other oxidants such as O3, and poses no lasting environmental threat since it readily decomposes to water and oxygen. Likewise, iron is comparatively inexpensive, safe and environmentally friendly with regard to any other transition metal.9 These characteristics convert iron in one of the preferred metals in catalytic applications both from an environmental and an economic perspective. However, Fenton reactions show several drawbacks that limit their use in the treatment of polluted waters. One of the most important limitations is the optimum pH needed for the oxidation of organic compounds which is found between 2.5 and 3.5.11,12 At higher pH, ferric ions form oxides or hydroxides that precipitate, thus inhibiting the recycling of iron(III) back to iron(II), a key step in Fenton processes.9 Because of this pH requirement, the application of the Fenton treatment is restricted to acidic and small volumes of water, and it shows Received: Revised: Accepted: Published: 9918

April 12, 2013 July 19, 2013 July 29, 2013 July 29, 2013 dx.doi.org/10.1021/es401602t | Environ. Sci. Technol. 2013, 47, 9918−9927

Environmental Science & Technology

Article

Scheme 1. Structure of the Transition Metal Complexes Used As Catalysts in the Present Study

Selected iron coordination complexes are highly active and selective catalysts in the oxidation of the strong C−H bonds of alkanes in organic medium using hydrogen peroxide as oxidant,29 and they are also competent for catalyzing the CeIV-driven oxidation of water at pH 1.30 These complexes are therefore very robust and resistant to oxidant and acidic conditions. They are mononuclear FeII complexes bearing tetradentate ligands that combine pyridines and tertiary amines as the donor atoms.31−35 Pioneer works by Que and coworkers31 have established that these catalysts react with H2O2 in organic medium to form highly electrophilic iron(V)-oxohydroxo species that are competent for breaking the strong C− H bond of an alkane. In fact, direct detection of this highly elusive intermediate under catalytic conditions has recently been achieved by low temperature ESI-MS techniques.36 Given the easy access to high-valent iron-oxo species with the above-mentioned type of complexes and their remarkable stability under oxidative conditions, in this work we have studied the activity of six of these complexes as catalysts for the removal of organic molecules from water through their oxidative degradation (Scheme 1). Four of them, that is [FeCl2(bpmcn)] (bpmcn = N,N′-bis(2-pyridylmethyl)-N,N′dimethyl-trans-1,2-diaminocyclohexane) (1), [Fe(OTf)2(tpa)] (tpa = tris(2-pyridylmethyl)amine), OTf = trifluoromethanesulfonate) (2), [Fe(OTf)2(Pytacn)] (Pytacn =1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane) (3) and [Fe(OTf)2(6Me-Pytacn)] (4) have been reported as highly efficient iron-based oxidation catalysts in the oxidation of C− H bonds29−32 and water in some instances.30 In contrast, [Mn(OTf)2(Pytacn)] (5) is highly active in epoxidation reactions,37,38 and it has been tested for comparative purposes. Finally, the dinuclear FeII complex [Fe2(OTf)4(Me4dtne)(CH3CN)2] (6) (Me4dtne = 1,2-bis(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane) was selected to evaluate the effect of the catalyst nuclearity in the efficiency in Fenton-like oxidations. In the present work, toluene and methyl tert-butyl ether (MTBE) have been selected as target organic compounds susceptible to being oxidatively degraded by the abovementioned catalysts using H2O2 as oxidant. These two compounds are commonly found in groundwater polluted by gasoline or petrochemical spills.39 Moreover, 2,4,6-trichlorophenol (TCP)40 and 1,1,2,2-tetrachloroethane (TeCA)41 have

a limited application in natural waters in which the common pH is found in the range of 6−8.5 and the buffer capacity is so high that significant volumes of acid would be needed to decrease the pH.9,13 In order to increase the pH range for the application of iron in the so-called Fenton-like reactions, research on iron complexes has become of interest. In this regard, the most successful systems were reported by T. J. Collins using FeIIITAML catalysts (TAML = tetraamido macrocyclic ligand).14 FeIII-TAML catalysts are low molecular-weight iron complexes that activate hydrogen peroxide via peroxidase-like cycles generating oxidizing species that degrade organic compounds, facilitating their removal from water. These catalysts work efficiently at concentrations in the range of ppb to low ppm at room temperature and their maximum efficiency is exhibited in the pH range 10−11.14 FeIII-TAML catalysts have shown a great ability to degrade hazardous environmental pollutants including toxic polychlorophenols,15 thiophosphate pesticides,16 azo dyes,17,18 dibenzothiophenes,19 natural and synthetic estrogens,20 the active pharmaceutical agent sertraline,21 and bacterial spores.16 Despite the fact that the activity exhibited by FeIII-TAML systems is incomparable, some other metal complexes have been tested as catalysts in AOP’s in water. These include metalloporphyrins, polyoxometalates or metal complexes based on nonporphyrinic nitrogen-containing ligands relying on iron or vanadium.22−24 The coordination of iron to ligands or chelators does not only allow an increase on the range of pH that can be used with respect to Fenton processes, but it may also provide an enhancement of the catalytic activity through the involvement of highly reactive species different from hydroxyl radicals. These are the so-called high-valent iron-oxo species in which the iron center adopts high oxidation states (FeIV or FeV).25 Such species are only viable in the presence of ligand scaffolds that can stabilize the metal center in these high oxidation states. In fact, the unprecedented activity of iron-TAML systems is commonly attributed to the involvement of high-valent ironoxo compounds rather than hydroxyl radicals as the active species in oxidation processes.26−28 Moreover, coordination of the iron to ligands or chelators can have an effect on important reaction parameters such as the selectivity or the hydrogen peroxide utilization. 9919

dx.doi.org/10.1021/es401602t | Environ. Sci. Technol. 2013, 47, 9918−9927


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also been tested because they constitute common contaminants derived from pesticides or halogenated organic solvents spills, respectively.

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EXPERIMENTAL SECTION Chemicals. Toluene, methyl tert-butyl ether (MTBE), 1,1,2,2-tetrachloroethane (TeCA) and 2,4,6-trichlorophenol (TCP), which were used as target organic substrates, were obtained from Merck (Purity ≥98%). Ferrous sulfate (FeSO4· 7H2O), sodium thiosulfate and hydrogen peroxide (30% w/w) were purchased from Merck. The catalysts [FeCl2(bpmcn)] (1),42 [Fe(OTf)2(tpa)] (2),43 [Fe(OTf)2(Pytacn)] (3),34 [Fe(OTf)2(6Me-Pytacn)] (4),33 [Mn(OTf)2(Pytacn)] (5),37 and [Fe2(OTf)4(Me4dtne)(CH3CN)2] (6)44 (Scheme 1) were synthesized following previously described methods, or slight modifications thereof. Instrumentation and Analysis. See Supporting Information (SI). Fenton-Like Reactions. 2.0 mM stock solutions of toluene, MTBE, TeCA, or TCP were prepared in distilled or tap water. The initial pH of the reaction mixtures was initially adjusted at 3, 7, or 10 by addition of potassium hydroxide (∼0.1 M) or diluted nitric acid (∼0.1 M) solutions. The pH measurements were performed using a Crison pH-meter BASIC 20+. All experiments were carried out in triplicate at room temperature (T = 23 ± 2 °C) in 10−100 mL glass reactors with a Mininert valve for headspace sampling. The headspace in the reactors was lower than 10% of the total reactor volume in order to minimize volatilization losses. Reactions with Fenton reagent using FeSO4·7H2O were performed concurrently under the same conditions of pH and molar ratios (1:100:4900, 1:10:490, or 1:10:40). The molar ratios are defined as FeII/MnII: toluene: H2O2. Two blanks, one with organic compound and H2O2 and the other with organic compound and catalyst, were conducted to exclude substrate degradation by the sole presence of the oxidant or the catalyst. On average, substrate degradation in the blanks was lower than 6%. The concentration of the iron complexes used as catalysts was in the range 0.02−0.2 mM (as FeII/MnII). The appropriate amount of catalysts was added into 2 mM stock solutions of substrates at pH 7. The reaction mixtures were stirred with a magnetic stirrer for five minutes and the initial concentration of the organic compound (C0, SUBSTRATE) was determined using the appropriate chromatographic technique (see SI). Reaction started by the addition of a particular amount of H2O2 to give a concentration 2−98 mM and the reactor was immediately closed in order to prevent the volatilization of the organic substrate. In the case of toluene, measurement of the overpressure generated in the reactor during the reaction allowed to dismiss volatilization losses for the most active complexes 1 and 3 (SI Figure S1).

Figure 1. Percentage of toluene oxidation in aqueous solution for several catalysts after 48 h of reaction. The red line indicates the percentage of toluene oxidation in the classical Fenton reaction (FeSO4) at pH 3. Initial reaction conditions: pH 7, Fe:toluene:H2O2 1:100:490 (violet), 1:100:0 (blue) and [FeII/MnII] = 0.02 mM.

also depicted for comparison. Remarkably, the catalyst concentration in our experiments is much lower than that generally used in Fenton reactions or in some Fenton-like processes (2 −30 mM Fe),24,45 with the exception of TAMLbased systems (2.5 μM Fe).15 As shown in Figure 1, after 48 h of reaction the catalyst with the highest catalytic activity was 1, which achieved 95% of toluene depletion. 2, 3, and 4 removed 72%, 71%, and 46% of toluene, respectively. Interestingly, in all these cases, the percentage of toluene removal was greater than that achieved with the classical Fenton reaction at pH 3 (31%). Thus at low iron concentrations (0.02 mM), 1−3 were much more active than the Fenton reagent at pH 3. On the other hand, the percentage of toluene removal in the reactions catalyzed by 5 and 6 was lower than in classical Fenton reactions, with values of 12% and 27%, respectively. By comparing 3 and 5, it is concluded that iron appears to have a higher catalytic activity than manganese. On the other hand, 6 showed only modest activity, suggesting that the presence of a dinuclear FeII complex in the catalyst structure is detrimental to toluene oxidation. It is worth mentioning that the Fenton reaction carried out at pH 7 did not allow any toluene removal at all, which might be caused by the precipitation of insoluble iron oxyhydroxides. In conclusion, the set of complexes 1−3 is efficient to oxidatively degrade toluene at initial pH 7, which ascertains the crucial impact of the coordination of the metal to a ligand in order to afford active catalysts under neutral conditions.12,46 Parallel control experiments were also performed by exposing the substrate to the catalyst but in the absence of hydrogen peroxide. In all cases, toluene oxidation was lower than 20% being negligible for 2 and 5. Toluene depletion in the absence of H2O2 might be caused by a partial catalyst oxidation in water due to the dissolved oxygen contained in the solution.47 Moreover, control experiments performed with toluene and hydrogen peroxide without metal complex showed that the oxidation of toluene by the sole presence of H2O2 was negligible. In conclusion, efficient toluene oxidation was only achieved by the combination of the metallic catalysts and hydrogen peroxide. According to the measurements of the overpressure generated in the reactor along the reaction (see SI Figure S1) volatilization losses are minimal for 1 and 3, while for catalyst 2 they cannot be excluded. For this reason and due to their high efficiency in toluene oxidation, catalysts 1 and 3 were selected for further investigation.

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RESULTS AND DISCUSSION Fenton-Like Catalytic Activity of Complexes 1−6 for Toluene Degradation at Neutral pH. In order to determine the catalytic activity of complexes 1−6 in Fenton-like reactions at initial neutral pH, a screening test was carried out. The initial conditions used in this experiment were 0.02 mM complex, 2.0 mM toluene, and 98 mM H2O2, which corresponds to a molar ratio of 1:100:4900 (FeII/MnII: toluene: H2O2). Figure 1 shows the percentage of toluene oxidized in these reactions using the six catalysts at initial neutral pH. Results corresponding to the Fenton reaction at pH 3 and 7 using the same molar ratios are 9920



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Figure 2. Oxidation of toluene in an aqueous solution catalyzed by 1 or FeSO4·7H2O using distilled or tap water. Initial reaction conditions: Fe:toluene:H2O2 1:10:490 and [FeII] = 0.2 mM. Lines were drawn as guides for the eye.

5.85 ± 0.59 when toluene degradation is complete. Despite the fact that reactions in distilled water occur at pH 4 rather than 7, the efficiency of 1 to oxidize toluene in tap water demonstrates its ability to operate not only under neutral conditions but also in a realistic matrix. The H2O2 consumption for 1 and 3 at initial pH 7 and Fenton reaction at pH 3 were analyzed during toluene oxidation (SI Figure S4) with the ratio 1:10:490. The consumption was in all cases around 39% after 500 min. However, from the point of view of H2O2 utilization, some differences were observed. The H2O2 consumption index50% was defined as the ratio of moles of H2O2 consumed per mol of toluene degraded when 50% reduction of the initial toluene concentration was reached. If we compare the H 2 O 2 consumption index50% for Fenton reagent, 1 and 3 the values obtained were 21, 10, and 15, respectively. This means that H2O2 utilization with 1 and 3 is slightly better than with Fenton. In order to determine the optimal H2O2 concentration for the degradation of toluene with 1, experiments were carried out by modifying the amount of H2O2 added to the reaction. Fe:toluene ratio was kept constant at 1:10 ([FeII] = 0.2 mM) and the initial concentration of H2O2 was modified. As shown in SI Figure S5, the optimal H2O2 concentration for toluene removal is between 4 and 8 mM. In both cases the percentage of toluene degradation was already around 95% after 30 min. An increase in the oxidant concentration up to 20 or 98 mM caused a slight decrease in toluene depletion down to 80% after 30 min. In contrast, diminishing the amount of oxidant to 2 mM caused a much slower reaction and only 44% of substrate was degraded over the same period of time. This might be explained by considering that catalytic activity is enhanced by increasing H2O2 concentration but at certain peroxide concentration the reactive species are quenched by excess H 2 O 2 . Moreover, it cannot be disregarded that H 2 O 2 disproportionation becomes more important at higher oxidant concentrations. Based on the observed H2O2 concentration effect, all subsequent experiments were performed by fixing the ratio Fe:toluene:H2O2 at 1:10:40 ([FeII] = 0.2 mM). Stability of Complexes 1 and 3 in Fenton-Like Reactions. In order to gain insight into the nature of the

Optimization of the Reaction Conditions for Toluene Oxidation. In order to check the operational pH range of 1, experiments for toluene oxidation were carried out at initial acidic, neutral, and basic pH using the molar ratios described above. According to Figure S2, at pH 3 and 10, the reaction time required to remove 95% of the initial toluene concentration exceeded 66 h. However, at initial neutral pH the reaction time is reduced below 50 h. In all experiments, once the toluene removal was complete the pH of the reaction mixture was ∼3 (see below).48 Based on the observed pH effect, it was concluded that 1 exhibited its highest activity at initial neutral pH and for this reason the following experiments were performed at initial pH 7. The influence of molar ratios between catalyst, organic substrate and hydrogen peroxide was also studied. As expected, a 10-fold increase in catalyst concentration (from 0.02 mM to 0.2 mM) while keeping the toluene and oxidant concentration constant caused a clear decrease in the reaction time; only 90 min were necessary to degrade 95% of the initial toluene. Thus, for practical reasons, further experiments were conducted at a molar ratio Fe:toluene:H2O2 1:10:490 and [FeII] = 0.2 mM. The effect of the water matrix in the efficiency of 1 to degrade toluene was also tested. As shown in Figure 2, a series of experiments were carried out using a molar ratio 1:10:490 (Fe:toluene:H2O2 and [FeII] = 0.2 mM) in both distilled and tap water, which constitutes a more realistic matrix and it has enough buffer capacity to keep a circumneutral pH. Clearly, the most inefficient system at pH 7 corresponds to the reactions catalyzed by FeSO4, while at this pH toluene oxidation catalyzed by 1 in distilled water is the most efficient combination, being even faster than the classical Fenton reaction at pH 3. In all cases, the use of tap water gives slower reactions compared to distilled water, which might be explained by quenching of the reactive species and/or ligand exchange at the catalyst induced by anions present in tap water. Remarkably, a pH value of 4.25 is measured immediately after H2O2 addition in those reactions carried out in distilled water at initial pH 7 (SI Figure S3), presumably because of the acidic character of the ferric species formed after oxidation of the ferrous catalysts. In sharp contrast, the buffer capacity of tap water prevents a decrease in pH, so that in this matrix the pH is 9921



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Figure 3. (a) ESI-MS monitoring of the oxidation of toluene in aqueous medium catalyzed by 1 using hydrogen peroxide as the oxidant (right) and ESI-MS determination of the structure of the ligand at different reaction times (left). t = 0 min (before oxidant addition), 1 min, 5 min, and 30 min (at the end of the reaction). (b) Oxidation of toluene catalyzed by 1. Initial reaction conditions: initial pH 7, Fe:toluene:H2O2 1:10:40 and [FeII] = 0.2 mM.

respectively. As previously documented, oxidation of the iron(II) center due to the presence of dissolved O2 might explain the presence of iron(III) in water even before H2O2 addition.47 Addition of 10 equiv H2O2 (2 mM) caused an immediate decrease of the spectrum overall intensity and the appearance of a new signal at m/z 432.0 and 442.0 corresponding to [FeIII(OH)(Cl)(bpmcn)]+ and [FeIII(OH)-

iron species involved in the Fenton-like oxidation of toluene catalyzed by 1, a typical reaction in distilled water at initial neutral pH with a molar ratio 1:10:40 (Fe:toluene:H2O2 and [FeII] = 0.2 mM) was monitored by ESI-MS (Figure 3a, right). Before the H2O2 addition the spectrum was dominated by one intense peak at m/z 414.0 and a minor peak at m/z 190.0 corresponding to [FeIII(OH)2(bpmcn)]+ and [FeII(bpmcn)]2+, 9922



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(HCOO)(bpmcn)]+, respectively. The partial ligand exchange occurring at the iron center might be due to the decrease in the pH of the reaction mixture when the oxidant is added (see above). The relative intensity of the three peaks is slightly modified along the reaction but, in general terms, the peaks become less intense. At the end of reaction (t = 30 min) the signal corresponding to [FeIII(OH)(Cl)(bpmcn)]+ dominates the spectrum, albeit its intensity is 8 times lower than at the beginning of the reaction. The decrease in the overall ESI-MS spectrum intensity over time might be due to the deactivation of the catalyst by ligand degradation and/or precipitation of iron oxyhydroxides. Measurement of the dissolved iron content for 1 indicated that only 17% of the iron had precipitated after 30 min of reaction, whereas ESI-MS analysis of the structure of the bpmcn ligand showed that the ligand had been fragmented over the course of the reaction, with major ligand degradation even at very short reaction times (5 min) (Figure 3a, left) (see SI for experimental details). Given the fact that a lag phase is observed for the degradation of toluene using 1 (Figure 3b), it might be concluded that substrate consumption becomes significant only when an important degree of ligand degradation is evident by ESI-MS. This correlation indicates that in the case of 1 the active catalyst might correspond to an iron center ligated to a partially decomposed bpmcn ligand. Remarkably, at longer reaction times the concentration of the bmpcn ligand or its fragments decreases (see the decrease in the intensity of ESI-MS over time), suggesting the formation of smaller organic fragments that could not be detected in the ESI-MS analysis. The ligand decomposition observed for 1 is the most plausible explanation to understand the fact that catalytic activity is only partially recovered if a second addition of hydrogen peroxide and toluene is done at the end of the reaction (SI Figure S6). A parallel experiment was carried out using 3 as catalyst, which also proved to be efficient in the degradation of toluene (Figure 1). Similarly to catalyst 1, the predominant forms of the iron species in solution during the reaction were [FeIII(OH)2(Pytacn)]+ and [FeIII(OH)(OTf)(Pytacn)]+ with a significant decrease in their signal intensity over the course of the reaction (SI Figure S7a). But in sharp contrast to 1, the structure of the Pytacn ligand in 3 remained intact at the end of the reaction (SI Figure S7b) and the loss of iron in solution was almost negligible (2%). Given the fact that neither ligand degradation nor iron precipitation seem to be clear reasons for catalyst deactivation in the case of 3, we believe that formation of catalytically inactive ferric μ-oxo oligomers through bimolecular processes might also contribute to the observed depletion in activity. Formation of such oligomers, which are usually considered as thermodynamic sinks due to their high stability, has been often invoked as the main deactivation pathway for both mononuclear heme and nonheme iron catalysts.49−51 Catalytic Activity of Complexes 1 and 3 to Degrade MTBE, TeCA and TCP in Fenton-Like Reactions at Neutral pH. The catalytic activity of the two most active catalysts in toluene oxidation, 1 and 3, was tested for the degradation at initial neutral pH of other organic compounds commonly found as water contaminants, such as MTBE (an ether), TeCA (a linear chlorinated alkane) and TCP (aromatic chlorinated hydrocarbon). Reactions were prepared for each substrate with a molar ratio 1:10:40 (Fe:substrate:H2O2 and [FeII] = 0.2 mM) in distilled water. As shown in Figure 4, catalyst 1 afforded at least 99.6%, 75% and 44% of toluene,

Figure 4. Percentage of substrate oxidation in aqueous solution by 1 (dark blue) and 3 (light blue) after 90 min. Initial reaction conditions: pH 7, Fe:substrate:H2O2 1:10:40 and [FeII] = 0.2 mM.

TCP and MTBE degradation after 90 min of reaction, respectively. The activity of 3 proved to be more modest yielding 6% MTBE and 18% TCP degradation over the same period of time, while maintaining at least 99.6% of toluene degradation. In the case of TeCA the observed percentage of removal was negligible for both catalysts under these experimental conditions. However, it is worth highlighting that 66% of TeCA degradation was achieved by 1 over a period of 48 h by increasing the hydrogen peroxide concentration up to 98 mM (SI Figure S8). Results obtained from this substrate screening might be used to draw a comparison between the two iron catalysts tested and the well-established kinetic constants corresponding to the reaction of hydroxyl radicals (kOH) with the above-mentioned substrates (SI Table S1) (The authors would like to emphasize that the comparison with the reported kOH values must be carefully done, as these values were determined under different reaction conditions. Nevertheless, these values have been used by several research groups to compare a particular process with hydroxyl radical-based oxidation, and thus they constitute a standard literature benchmark for comparison. See refs 7, 12, 52, and 53).52−53 As expected and in accordance with the kinetic constants reported for hydroxyl radicals, TeCA has the lowest kOH, which correlates with the fact that negligible degradation is observed with 1 and 3 under the experimental conditions tested. However, discordance with respect to kOH is evident when the relative reactivity against TCP and toluene is analyzed. While hydroxyl radicals react faster with TCP, the opposite trend is observed for 1 and 3, in which the reaction with toluene is faster. Therefore, these results could point out to a degradation route primarily based on an oxidant different from hydroxyl radicals. Degradation Products. In order to identify the primary products derived from toluene degradation, experiments were carried out using a large excess of this substrate (400-fold with respect to iron. See SI for experimental details). After 1 h of reaction, the oxidized organic products were extracted in CH2Cl2 and identified and quantified by GC-MS and GC-FID. Both for 1 and 3, the primary oxidation products were mainly ortho-cresol and para-cresol in relative 70/30(±2) and 65/ 35(±2) ratios. Oxidation products derived from the oxidation of the methyl C−H bond (benzaldehyde and benzyl alcohol) accounted for 13 ± 1% and 28 ± 1% of the total degradation products for 1 and 3, respectively (SI Figure S9). These results are consistent with previously reported experiments on toluene degradation by Fenton-like systems.54 Both the o/p ratio as 9923



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Scheme 2. Proposed Mechanism for the Oxidation of Toluene with Hydrogen Peroxide in Water Using 1 and 3

catalysts tested, 1 and 3, is possible from the experimental results. As observed by ESI-MS, ligand-bound iron(III)hydroxo species is the major form of the iron in water solution after hydrogen peroxide addition. Formation of these species entails the homolytic O−O bond cleavage of H 2 O 2 concomitant with the oxidation of the iron(II) center to iron(III) and the generation of hydroxyl radicals that might potentially act as oxidants (Scheme 2). Alternatively, [FeIII(OH)x(L)]n+ species (L = bpmcn, Pytacn) can be formed through iron(II) oxidation in water by reaction with atmospheric O2, as ascertained by ESI-MS before oxidant addition (Figure 3 and SI Figure S7). This process would generate superoxide radical as byproduct. Hydroxyl or superoxide radicals liberated at the initial stages of the reaction could be the oxidizing species, however, several experimental results suggest that another oxidant might be involved. These are the faster degradation of toluene over TCP by catalysts 1 and 3 compared to Fenton reactions and the observed selectivity in the oxidation of toluene (aromatic vs side-chain). Given the well-established involvement of high-valent oxoiron species in the oxidation reactions catalyzed by these two catalysts in organic medium,29 formation of such active species in reactions in aqueous medium is also reasonable. As alkene epoxidation is uncharacteristic of OH• but it is well-known for ferryl complexes,56−58 epoxidation experiments were carried out in a well-agitated multiphase mixture containing neat cyclohexene and a water phase together with the catalyst (FeSO4, 1 or 3) and H2O2 (For experimental details see SI). This provided a constant concentration of cyclohexene in the aqueous phase (solubility 0.7 mM) and allowed the extraction of cyclohexene oxide into the organic phase as it was formed. After 18 h of reaction, cyclohexene oxide was detected (0.2 mM for 1 and 0.4 mM for 3), whereas negligible amounts of epoxide were obtained in an experiment using FeSO4 as catalyst (SI Figure S11).59 Thus, at least to some extent, high-valent iron-oxo species seem to be involved in the Fenton-like reactions catalyzed by 1 and 3. Thus, we propose that formation of this high-valent species occurs by heterolytic O−O cleavage of a preceding FeIII−OOH intermediate (Scheme 2). This highly electrophilic FeV(O)(OH) then attacks the aromatic ring of the toluene substrate to give the corresponding cresols. The resting state form of the

well as the amount of aromatic vs side-chain oxidation products were found to be the same during the course of the reaction. For comparative purposes, a parallel reaction using FeSO4 as catalyst and at pH 3 was conducted under the same conditions. Importantly, the relative amount of side chain oxidation products (benzaldehyde and benzyl alcohol) was much higher in this case and they represented up to 50 ± 1% of the total oxidized compounds, while the rest corresponded to orthocresol and para-cresol in a relative amounts similar to those obtained for 1 and 3. Interestingly, a high contribution of sidechain toluene oxidation has been previously documented for the Fenton reagent.45,55 The slight different selectivity of 1 (and 3) with respect to the Fenton reagent toward aromatic or sidechain toluene oxidation suggests that apart from hydroxyl radicals, some other oxidants might be involved in the observed reactivity with 1 and 3. Once these primary oxidation products are formed, they can be further oxidized in solution to give low-molecular-weight organic acids (SI Scheme S2),48 which contribute to the final acidic pH under conditions of excess hydrogen peroxide (see above). Identification of these low-molecular-weight organic acids was carried out by ion chromatographic analysis of the final reaction mixture (after ∼30 min of reaction) obtained in distilled water at neutral pH (molar ratio 1:10:40 Fe:toluene:H2O2 and [FeII] = 0.02 mM) using catalyst 1. These analyses show the presence of 14 mg L−1 formic acid, 10 mg L−1 oxalic acid and 12 mg L−1 acetic acid (SI Figure S10). Pyruvic and malic acids were not detected. The sum represents around 20% of the initial toluene concentration; the other 80% could correspond to longer chain organic acids and some degree of mineralization. In order to assess if the system was able to achieve toluene mineralization, the TOC (total organic content) concentration was analyzed after 200 min of reaction. However, the ratio 1:10:490 was used in this case, so that the amount of added hydrogen peroxide was large enough to achieve mineralization. The TOC removal using 1 as catalyst and at initial pH 7 was 47%. In comparison, the TOC depletion in Fenton reaction at pH 3 was 39%. Mechanistic Proposal. Although a detailed mechanistic study was beyond the scope of the present work, suggestion of the mechanistic scenario operative for the two most active 9924



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catalyst, that is FeIII−OH, is regenerated, thus closing the catalytic cycle. Precedents for this mechanistic scheme for aromatic hydroxylation can be found in a recent kinetic study by Akimova et al., albeit in organic medium.60 Nevertheless, mechanistic probes presented here are not fully conclusive and most probably, iron(V)-oxo-hydroxo pathway coexists with the oxidation performed by free hydroxyl radicals, generated either in the initial oxidation step of iron(II) to iron(III) or in parallel Fenton-like reactions initiated by reaction of the high valent oxoiron species with the excess of oxidant. These reactions result in the formation of hydroperoxyl radicals (HOO•) that may decay through the formation of hydroxyl radicals.61,62 Environmental Implications. Chemically robust iron coordination complexes with tetradentate N-based ligands are promising catalysts to efficiently oxidize and degrade recalcitrant organic compounds with hydrogen peroxide in water at neutral pH. In the case of toluene, catalyst 1 exhibited high oxidation efficiencies using tap water as a realistic water matrix with high buffer capacity in order to keep circumneutral pH, whereas the Fenton reagent was inactive at pH 7. This is noteworthy from environmental and engineering points of view because this transformation could be potentially carried out in natural waters polluted with organic compounds without the need of adding high amounts of acid to keep iron ions in solution and base for the posterior neutralization. The reasons to explain the significant enhancement of the catalytic activity using catalysts 1 and 3 are related mainly to two chemical properties. First, the coordinated organic ligands allow the iron to remain in solution even at neutral pH, thus avoiding its precipitation as iron oxyhydroxides. Second, the reactive species generated in the reaction seem to behave differently from hydroxyl radicals. This hypothesis is corroborated by observing the epoxide formation in alkene epoxidation experiments with cyclohexene, the ring versus side-chain selectivity in the oxidation of toluene and the relative reactivity against different substrates. Considering the well-established involvement of high-valent oxoiron species in the oxidation reactions catalyzed by 1 and 3 in organic medium and the observed epoxidation of cyclohexene, it is reasonable to propose that such highly electrophilic species could be also operating in reactions in water together with hydroxyl radicals. In order to compare different catalysts in Fenton and Fenton-like reactions we have defined the catalytic efficiency (CE) as the moles of substrate consumed after 15 min divided by the moles of catalyst in a wet peroxide oxidation reaction. As described in the present work, toluene oxidation by the Fenton reagent (FeSO4) at pH 3 affords a CE of 1.4, whereas this value increases up to 7 when 1 is used as catalyst for toluene degradation at initial pH 7. Thus, CE is 5 times higher for 1 than for Fenton reaction even working at initial neutral pH. The data reported by Huling et al.63 regarding the toluene oxidation in Fenton-like reactions using FeIII at pH 3 gave CE values 140 times lower than the Fenton reaction (around 0.01). Moreover, other iron complexes based on different chelators such as carboxylates, hydrozomates, polyhydroxy aromatics, porphyrins, sulfur compounds, and phosphates11 afforded CE values in the range of 0−0.1 for the degradation of 2,4dichlorphenoxyacetic acid at pH 6. In sharp contrast, the wellknown Fe-TAML reported by Collins et al. has catalytic efficiencies in the range of 13−2000 for different organic substrates such as organophosphorous pesticides, azo dyes, sertraline, benzene, estrogens, and trichlorophenol.12,15,16,18,20,21 Such high catalytic efficiencies were obtained

at pH > 8.5, whereas at neutral and acidic pH, the catalytic activity was highly decreased.21 In spite of the fact that catalysts 1 and 3 do not exhibit the high catalytic efficiencies observed for Fe-TAML systems, they present a significant advantage as they are active at circumneutral pH. Although 1 and 3 show high catalytic activity, efforts to minimize their main deactivation pathways, that is ligand degradation and formation of inactive μ-oxo ferric oligomers, are still necessary for practical applications. Moreover, from an economic point of view, the production cost of the catalyst must be reduced in order to compete with conventional Fenton reagent. A possible strategy towards applicability would be the reduction of the required amount of catalyst by supporting it on zeolites or mesoporous materials giving heterogeneous systems that could be recovered or reused. Alternatively, the use of these compounds as catalysts for the removal of micropollutants (pharmaceuticals, endocrine disruptors, or pesticides) from waste waters would enable the use of significantly lower amounts of catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

Instrumentation and analysis, overpressure measurements, effect of initial pH, and hydrogen peroxide concentration, pH and H2O2 evolution over time, oxidation of toluene in two consecutive Fenton-like reactions, ESI-MS monitoring of the oxidation of toluene by 3, analysis of oxidation products from toluene oxidation, cyclohexene epoxidation and degradation of TeCA by 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +34 972419842. E-mail: [email protected] (A.C.); [email protected] (R.G.-O.); miquel.costas@ udg.edu (M. C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.C. thanks the European Research Council for a Starting Grant Fellowship (ERC-2009-StG-239910), MINECO of Spain (CTQ2012-37420-C02-01/BQU, Consolider-Ingenio CSD2010-00065) and the Catalan DIUE of the Generalitat de Catalunya (2009SGR637, and ICREA Academia Award). A.C. thanks the European Commission for a Career Integration Grant (FP7-PEOPLE-2011-CIG-303522). Ministerio de Ciencia e Innovación (Spain) is acknowledged for financial support with the project SIRENA (CTQ2011-24114), for a Juan de la Cierva contract to R.G.-O. (JCI-2010-07104) and for a Ramón y Cajal contract to A. C. (RYC-2011-08683). Mr. Z. Codolà is acknowledged for his help with the experimental setup for the determination of the overpressure generated into the reactors.

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REFERENCES

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Robust Iron Coordination Complexes with N-Based Neutral Ligands

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