A nanoscale Fe(II) metal–organic framework with a bipyridinedicarboxylate ligand as a high performance heterogeneous Fenton catalyst

Abstract

With 2,2′-bipyridine-5,5′-dicarboxylate as the ligand, a nanoscale Fe(II) MOF material (Fe-bpydc) was synthesized. Fe-bpydc could activate H₂O₂ to degrade organic pollutants in near neutral conditions, and its activity and stability are superior to reported results. Interestingly, Fe-bpydc only consumes H₂O₂ in the presence of organics, leading to high H₂O₂ utilization efficiency.

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The Fenton reaction has attracted persistent attention due to its potential in water purification.^1–4 In the Fenton system, environmentally benign H₂O₂ is used as the oxidant, and cheap Fe²⁺ ion is used as the catalyst. The reactions between them generate a series of highly oxidative intermediates, such as ˙OH, ˙OOH and high valent iron species, which induce the degradation of organic pollutants. However, the traditional Fenton reaction can only be carried out under acidic conditions (pH < 3), and large amount of ion slurry is produced in the treatment, which hinders its practical application. To overcome these drawbacks, plenty of heterogeneous Fenton systems have been developed.^5–7 The reported heterogeneous Fenton catalysts can be largely classified into two kinds. One kind is inorganic iron minerals, such as Fe₃O₄,⁸ α-FeOOH,⁹ FeS,¹⁰ FeS₂ (ref. 11) and FeVO₄.¹² Although their performance can be enhanced to some extent by the morphology modulation, the inorganic frameworks of these catalysts determine that the coordination sphere of metal ion, which is extremely important for the redox reactivity and stability of the catalyst, cannot be modified. The other kind of heterogeneous Fenton catalysts are prepared by the immobilization of Fe species (ion or complex) to various supports.^13–15 However, the low loading of active components makes these catalysts usually exhibit an unsatisfactory activity. Apparently, the development of novel heterogeneous Fenton catalysts is still in urgent need yet.

Metal–organic frameworks (MOFs) have emerged as a new class of organic–inorganic hybrid materials in recent years and shown potential application in various fields.^16–19 Especially, MOF is an ideal platform to construct catalysts, because the limitless choice of organic linker makes the coordination environment easy to be modulated, and the compact arrangement of metal ions is favorable to obtain high catalytic efficiency. Up to now, a series of MOF based Fenton catalysts have been proposed and shown great promise in the treatment of organic pollutants.^20–24 For example, Zhao et al. reported that MIL-100(Fe) could be used to the degradation of high concentration azo-dye wastewater.²⁰ Jiang et al. observed that MIL-53(Fe) could completely decompose Rhodamine B in the presence of a certain amount of H₂O₂.²¹

The reasonable selection of organic linker is the key to enhance the performance of MOF catalyst. 2,2′-Bipyridine is a widely used ligand to the synthesis of Fenton catalysts,^14,25,26 not only because of its strong binding affinity with Fe²⁺ but also the oxidative resistance of pyridine ring to ˙OH. Some studies showed that inducing carboxyl group to the 2,2′-bipyridine ligand would further stabilize the obtained complex, due to it would bind with the Fe³⁺ formed in the catalytic cycle to avoid its hydrolyzation. Therefore, carboxylbipyridine type ligands are ideal for the construction of MOF based Fenton catalysts. On the other hand, minimizing the particle size of MOF catalyst to micro/nanoscale is also a feasible way to enhance the activity, since small-sized catalyst would expose more metal sites on the surface relative to the bulk crystal.

In this work, a nanoscale Fe(II) MOF material (Fe-bpydc) was synthesized with bipyridinedicarboxylate ligand. In the presence of H₂O₂, this MOF material could catalyze the degradation of organic pollutants in near neutral conditions, and exhibited high activity, high stability and high efficiency of H₂O₂ utilization in the reaction process. These results indicate that Fe-bpydc could be used as an ideal heterogeneous Fenton catalyst in water treatment.

Fe-bpydc was obtained as brown powder from the solvothermal reaction between Fe(ClO₄)₂ and 2,2′-bipyridine-5,5′-dicarboxylic acid (H₂bpydc) in the mixed solvent of DMF and H₂O (v/v = 1/1). SEM observation shows that the as-prepared Fe-bpydc sample presents a 2D morphology of irregular shaped plates with thickness of 100–500 nm (Fig. 1a). Sharp edges are shown on the plates, indicating the crystalline structure of Fe-bpydc. The crystalline structure of the sample was also corroborated by the measured PXRD pattern (Fig. 1b), in which well-defined diffraction peaks are shown. All the diffraction peaks in the pattern could be indexed to the reported bulk phase of {[Fe(bpydc)(H₂O)]·H₂O}_n (Fig. S1, ESI†),²⁷ indicating that Fe-bpydc has the same framework structure with it.

Fig. 1 (a) SEM image and (b) PXRD pattern of Fe-bpydc sample.

Further evidence supporting the structure of {[Fe(bpydc)(H₂O)]·H₂O}_n comes from FT-IR, elemental and thermogravimetric analysis. Fig. S2 (ESI†) gives the FT-IR spectrum of Fe-bpydc. The broad band in the range of 3650–3000 cm⁻¹ is typical for the coordinated H₂O molecules with H bonds. The carboxyl group gives intense peaks at 1611 and 1378 cm⁻¹, indicating that the H₂bpydc ligand is completely deprotonated to coordinate with metal ions. The peaks at 851, 773 and 571 cm⁻¹ could be assigned to the vibration of Fe–O bonds. Elemental analysis gave the contents of C, H and N as 41.13%, 3.11% and 7.98%, respectively, which are consistent with the theoretical values based on the formula of {[Fe(bpydc)(H₂O)]·H₂O}_n (C 42.59%, H 2.97%, N 8.42%). Two stages of weight loss were shown in the measured TGA curve of Fe-bpydc (Fig. S3, ESI†). The first stage of weight loss occurred from room temperature to 190 °C (obsd. 10.64%, calcd. 10.78%), corresponding to the loss of two H₂O molecules. The organic component was lost in the temperature range of 345–445 °C (obsd. 61.39%, calcd. 65.31%). The measured weight losses are in good accordance with the calculated values.

Phenol was used as the model substrate to evaluate the catalytic property of Fe-bpydc, and the reaction was carried out under natural pH conditions. As shown in Fig. 2, phenol did not react remarkably in the solution of H₂O₂ (curve a). In contrast, the addition of Fe-bpydc sharply accelerated the degradation process of phenol (curve b). Under the catalysis of only 0.01 g L⁻¹ of Fe-bpydc, 0.25 mM phenol could be removed completely (Conv.% > 90) in 2 h, showing the high activity of Fe-bpydc as a heterogeneous Fenton catalyst. The pH value of the media showed a slow decrease from 5.3 to 4.0 during the degradation process due to the formation of carboxyl intermediates. In the absence of H₂O₂, the decay rate of phenol was negligible (curve c), showing that H₂O₂ works as the primary oxidant in the catalytic system of Fe-bpydc. After the complete degradation of phenol, the catalyst was separated from the reaction medium. The used catalyst gave nearly identical PXRD pattern (Fig. 1b) and morphology (Fig. S4, ESI†) as those of the as-prepared sample, which indicates the stability of Fe-bpydc under experimental conditions. Additionally, when phenol and H₂O₂ were supplemented to the reacted solution (without Fe-bpydc) to the initial concentration, no further reaction was observed (curve d), implying that the Fe-bpydc catalyzed degradation is truly heterogeneous in nature and not due to the leaching of active components to the solution. This oxidation system with Fe-bpydc as catalyst and H₂O₂ as oxidant can also be applied to the degradation of other substrates, such as 4-chlorophenol and methylene blue, and high removal rates were observed in these experiments (Fig. S5 and S6, ESI†). These results show the extensive applicability of Fe-bpydc in the treatment of organic pollutants.

Fig. 2 The degradation of phenol under different conditions: (a) in H₂O₂ solution, (b) in the presence of Fe-bpydc and H₂O₂, (c) in the presence of Fe-bpydc but without H₂O₂, (d) after the degradation of phenol, the catalyst was separated and the substrate and H₂O₂ were supplemented to the original concentration. 0.01 g L⁻¹ Fe-bpydc, c⁰_phenol = 0.25 mM, = 1.5 mM, 400 mL H₂O.

To characterize the reusability of Fe-bpydc, cyclic degradation experiment was carried out. As shown in Fig. 3, Fe-bpydc can realize the complete removal of phenol in three consecutive cycles. Based on the determined formula of {[Fe(bpydc)(H₂O)]·H₂O}_n (MW = 334), the total turnover number (TON) of three cycles is 23. To the best of our knowledge, although a series of MOF based Fenton catalysts have been reported, the TON values obtained in those works were usually much less than unit.^20–24 The TON calculated here definitely proves that Fe-bpydc works as a catalyst in the degradation system and its stability is superior to most of the reported MOF catalysts, probably due to the oxidative recalcitrance and versatile coordination ability of the applied carboxylbipyridine type ligand. However, the removal rates of the second and third cycles exhibited remarkable slowdown relative to the first one, indicating the decrease of the efficiency of Fe-bpydc during the reaction. The deactivation of Fe-bpydc may be due to the blockage of its active sites by the intermediates formed in the degradation of phenol.

Fig. 3 Cyclic degradation of phenol in the presence of Fe-bpydc. 0.01 g L⁻¹ Fe-bpydc, c⁰_phenol = 0.25 mM, = 1.5 mM, 400 mL H₂O. At the end of each run, substrate and H₂O₂ were supplemented to the initial concentration.

The acidity of the reaction system is known to be an important factor that affects the efficiency of Fenton catalysts, so we tested the kinetics of the Fe-bpydc catalyzed decomposition of phenol in the medium with different initial pH values. As shown in Fig. 4, the degradation can perform in a wide pH range from 3 to 6, and gives the maximum rate at pH 4–5. Further enhancement of the media pH to 7 would stop the reaction. Compared with the traditional Fenton system which can only work under strongly acidic environment, the near neutral operation condition of Fe-bpydc is more suitable for practical water treatment.

Fig. 4 Effect of pH on the degradation of phenol in the catalytic system of Fe-bpydc, 0.01 g L⁻¹ Fe-bpydc, c⁰_phenol = 0.25 mM, = 1.5 mM.

Since H₂O₂ works as the primary oxidant in the catalytic system of Fe-bpydc, the consumption of H₂O₂ during the reaction was also monitored. It was observed that the H₂O₂ decomposition occurred synchronically with the degradation of phenol (Fig. 5a). The consumption ratio between H₂O₂ to phenol was only 3.5 : 1, showing high efficiency of H₂O₂ utilization in this system. In another experiment, Fe-bpydc was added solely to the solution of H₂O₂ (without substrate). Interestingly, no remarkable decomposition of H₂O₂ was observed in 150 min (Fig. 5b). After phenol was added artificially, the consumption of H₂O₂ happened subsequently. These results clearly show that the presence of organic substrate is necessary for the decomposition of H₂O₂ in the catalytic system of Fe-bpydc, which is distinct from the substrate independent H₂O₂ consumption in the traditional Fenton system. Because the invalid decomposition of H₂O₂ (produce O₂ and H₂O) is negligible in the Fe-bpydc catalysed system, H₂O₂ is consumed primarily by the degradation of substrate, which gives a high H₂O₂ utilization efficiency.

Fig. 5 (a) The consumption of H₂O₂ with the degradation of phenol. 0.01 g L⁻¹ Fe-bpydc, c⁰_phenol = 0.25 mM, = 1.5 mM. (b) The comparison of the consumption kinetics of H₂O₂ in the presence and absence of phenol.

The intermediates formed during the degradation of phenol were determined by HPLC method. Besides the peak of residue substrate, two additional peaks were shown in the UV chromatograms of the reacted solutions (Fig. S7, ESI†). By comparing the retention time with authentic compounds, these peaks could be assigned respectively to o- and p-benzenediols, the hydroxylated products of phenol. These two intermediates accumulated with the conversion of phenol initially, and reached their maximum concentrations in 30 min (Fig. S8, ESI†). The concentrations of benzenediols decreased with the further prolongation of reaction time due to secondary reactions, and these intermediates were consumed nearly completely in 210 min. To monitor the selectivity of these two benzenediols, an experiment with high substrate concentration (to minimize the secondary consumption of intermediates) was carried out (Fig. 6). The total formation of these two intermediates in 180 min was 0.32 mM, which accounts for 58% of the consumption of substrate (0.56 mM). This result indicates that o- and p-benzenediol are the main primary oxidation intermediates of phenol. It is expected that, in the catalytic oxidation system of Fe-bpydc, phenol undergoes a hydroxylation process initially, and then the further oxidation of the formed hydroxylated intermediates leads to the ring-opening and even mineralization of the substrate, so no other intermediates (without UV absorption) were detected in our experiments.

Fig. 6 Consumption of substrate and accumulation of hydroxylated intermediates during the degradation of phenol with Fe-bpydc as catalyst. 0.01 g L⁻¹ Fe-bpydc, c⁰_phenol = 1.25 mM, = 1.5 mM.

The formation of hydroxylated products as main intermediates illustrates that the catalytic oxidation is via a ˙OH pathway. To confirm the participation of ˙OH, ESR experiment was carried out with DMPO as spin trapper. After the addition of H₂O₂, the characteristic fourfold peaks of DMPO–˙OH adduct with the intensity ratio of 1 : 2 : 2 : 1 was recorded (Fig. 7). This finding clearly proves that ˙OH works as the dominant active species in the catalytic degradation system of Fe-bpydc.

Fig. 7 The ESR signals of the system of Fe-bpydc/phenol before and after the addition of H₂O₂ (1.5 mM). 0.01 g L⁻¹ Fe-bpydc, c⁰_phenol = 0.25 mM, 20 mM DMPO was added as spin-trapper.

Conclusions

In summary, we have synthesized an Fe(II) bipyridinedicarboxylate based MOF catalyst, which presents a 2D morphology of nanoplate. This catalyst exhibited high efficiency towards the activation of H₂O₂ to induce the degradation of organic pollutants, and maintained good stability in the reaction system. The TON of Fe-bpydc can reach 23 in three consecutive degradation cycles, much exceeding previously reported MOF based Fenton systems. The degradation reaction can be carried out in the wide pH range of 3–6. The measurement of the concentration change of H₂O₂ indicates that Fe-bpydc can only consume H₂O₂ in the presence of organic substrates, which prevents the invalid decomposition of H₂O₂ and enhances its utilization efficiency. Above results show that Fe-bpydc could be used as an ideal heterogeneous Fenton catalyst to the water purification. This work also indicates that the reasonable selection of organic linker and the minimization of particle size are viable methods to enhance the performance of MOF catalyst.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (21307062, 20671053) and Tianjin Research Program of Application Foundation and Advanced Technology (14JCQNJC08000).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, FT-IR spectrum and TGA curve of the catalyst, intermediate concentration change and typical HPLC chromatogram during phenol decay, degradation kinetics of 4-CP and MB, and the SEM image of the used catalyst. See DOI: 10.1039/c5ra22779h

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