Facile synthesis of a Fe₃O₄/MIL-101(Fe) composite with enhanced catalytic performance

Abstract

A magnetic porous material, Fe₃O₄/MIL-101(Fe), has been successfully fabricated by using simple ultrasound-assisted electrostatic self-assembly technology, and was demonstrated to be a highly active heterogeneous catalyst for the dimerization reaction of o-phenylenediamine (OPD) in the presence of H₂O₂ via synergistic peroxidase-like activity.

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Metal–organic frameworks (MOFs), well-known as a new class of porous materials synthesized by assembly with metal cations/clusters and organic linkers, have attracted significant research interest in recent years,¹ not only due to their diversiform topological structure but also due to their many attractive applications in adsorption and separation,² sensing,³ drug delivery⁴ and catalysis.⁵ Recently, Fe-based MOFs have been widely used in catalytic applications due to their catalytically active coordinatively unsaturated Fe(III) sites and the existence of Fe₃-μ₃-oxo clusters, which endows iron-containing MOFs with photocatalytic activity.⁶ Moreover, intrinsic peroxidase-like activity, a new activity of Fe-based MOFs, which shows the ability to catalytically decompose hydrogen peroxide (H₂O₂) to ˙OH radicals via a Fenton-like route has been found recently and it shows a wide range of applications in biosensing platforms.⁷ As far as we know, there is no report about utilizing the peroxidase-like activity catalytic performance of Fe-based MOFs towards organic synthesis.

Magnetic nanoparticles (MNPs) have received considerable attention because of their power in separation techniques. Among various magnetic particles, iron oxides (such as Fe₃O₄ MNPs) have drawn the most attention owing to their strong magnetic responsiveness and biocompatibility. As magnetic separation is directly performed on complex samples, the functionalized Fe₃O₄ MNPs are usually used for the preconcentration and isolation of analytes from complex matrices.⁸ For instance, MNPs have been used for separating DNA, proteins, cells from samples⁹ and catalyst recycling in organic synthesis aspect.¹⁰ In addition, a research reported that Fe₃O₄ MNPs exhibit peroxidase-like activity allowing the effective detection of H₂O₂.¹¹ Since then, Fe₃O₄ MNPs pose the widespread applications in sensing and water purification by utilizing the peroxidase-like activity.¹²

Controllable integration of MOFs and functional materials is resulting in the creation of new multifunctional composites, which exhibit new properties that are superior to those of the individual components through the collective behavior of the functional units.¹³ For the magnetization of MOFs, five different synthesis approaches, including, hydrothermal,¹⁴ encapsulation,¹⁵ in situ magnetization,¹⁶ chemical bonding¹⁷ and coprecipitation assembly¹⁸ were attempted by previous report. Wherein, the magnetized MOFs were used for catalysis, extraction and separation. Intriguing by these meaningful researches and given the tremendous emerging interest in this field, we present an ultrasound-assisted electrostatic self-assembly strategy that has allowed us to rationally design and fabricate a novel type of magnetic composites in a general approach.

In our strategy, MIL-101(Fe), a kind of Fe-based MOFs, was chose as a matrix for loading Fe₃O₄ MNPs due to the outstanding peroxidase-like activity among the reported Fe-based MOFs owing to the high BET surface, mesosize cages and superior affinity for H₂O₂.¹⁹ Typically, cysteine (Cys) functionalized Fe₃O₄ MNPs and MIL-101(Fe) suspensions were mixed together under ultrasonication at room temperature for 10 minutes and the magnetic composite would be obtained by magnetic separation. The Fe₃O₄/MIL-101(Fe) magnetic composite prepared by this method displays high saturation magnetization value, making this composite more susceptible to magnetic fields and easier to isolate from aqueous solution. Furthermore, the as-synthesized Fe₃O₄/MIL-101(Fe) exhibits much higher catalytic activity than the single component for the synthesis of 2,3-diaminophenazine (2,3-DPA) due to the synergistic peroxidase-like activity between the multiple active centers, including Fe₃O₄ MNPs and MIL-101(Fe), which improved the decomposition efficiency of H₂O₂ and produced abundant reactive oxygen species (ROS) (Scheme 1).

Scheme 1 Schematic illustration of the synergistic catalytic behavior of Fe₃O₄/MIL-101(Fe) for the oxidation of OPD.

Powder XRD patterns of the parent materials (MIL-101(Fe), Fe₃O₄) and the hybrid Fe₃O₄/MIL-101(Fe) are shown in Fig. 1A. The diffraction peaks of MIL-101(Fe) and Fe₃O₄ are observed in the PXRD patterns of the as-synthesized hybrid Fe₃O₄/MIL-101(Fe), which confirms that the MIL-101(Fe) represented the major component of the composite, and it also indicated that the hybrid Fe₃O₄/MIL-101(Fe) preserved the crystalline characters of parent MIL-101(Fe). The diffraction intensity of Fe₃O₄ in the PXRD patterns of Fe₃O₄/MIL-101(Fe) was relatively weak, probably because the low content of Fe₃O₄ in the composite. The Brunauer–Emmett–Teller (BET) surface area of the as-synthesized MIL-101(Fe) and Fe₃O₄/MIL-101(Fe) were evaluated by nitrogen adsorption–desorption isotherms (Fig. 1B). The BET surface area of the naked MIL-101 and the hybrid Fe₃O₄/MIL-101(Fe) are calculated to be 1362 m² g⁻¹ and 1248 m² g⁻¹, respectively. Obviously, Fe₃O₄/MIL-101(Fe) still maintained a high specific surface area although the BET surface area of MIL-101(Fe) is minor higher than that of Fe₃O₄/MIL-101(Fe). Besides, the pore size of Fe₃O₄/MIL-101(Fe) is similar to that of MIL-101(Fe) with a little change (Fig. S1†).

Fig. 1 The characterization of Fe₃O₄/MIL-101(Fe). (A) PXRD pattern of MIL-101(Fe), Fe₃O₄ and Fe₃O₄/MIL-101(Fe); (B) the N₂ adsorption–desorption isotherms of MIL-101(Fe) and Fe₃O₄/MIL-101(Fe); (C) the hysteresis loop of Fe₃O₄ and Fe₃O₄/MIL-101(Fe); (D) SEM images of Fe₃O₄/MIL-101(Fe).

In addition, the magnetization curve illustrates the superparamagnetic characteristic of Fe₃O₄ and Fe₃O₄/MIL-101(Fe) (Fig. 1C). The saturation magnetization values were measured to be 50 emu g⁻¹ and 23 emu g⁻¹ respectively, which is susceptible to magnetic fields and easy to isolate from aqueous solution. The photographs of dispersed MIL-101(Fe) and Fe₃O₄/MIL-101(Fe) are shown in Fig. S2A and B,† respectively. A clear solution could be obtained after separation of Fe₃O₄/MIL-101(Fe) by using the external magnet in 20 s (Fig. S2C†). In order to better understand the morphological structure of the hybrid material Fe₃O₄/MIL-101(Fe), the texture of the samples were observed by scanning electron microscope (SEM). As can be seen from Fig. 1D, the surface of MIL-101(Fe) with a particle size of 1.1 μm has been successfully decorated with Fe₃O₄ nanoparticles without any influence on the morphology of MIL-101(Fe). The transmission electron microscope (TEM) images also demonstrated that the Fe₃O₄ MNPs with particle size about 10 nm were attached on the surface of MIL-101(Fe) successfully (Fig. S3†).

As we know, Cys is a kind of ampholyte with an isoelectric point of 5.05. Therefore, the prepared Cys-functionalized Fe₃O₄ MNPs should exhibit different zeta potential under various pH of the suspension. The as-prepared MIL-101(Fe) shows a strong electropositive. Hence, in order to obtain electronegative Fe₃O₄ MNPs, the pH of the suspension has been tuned from 6.0 to 10.0 (Fig. 2A). With the increase of the pH, the electronegativity of Cys-Fe₃O₄ become stronger and the corresponding electropositivity of the as-prepared Fe₃O₄/MIL-101(Fe) get weaker. Unfortunately, when the pH of Cys-Fe₃O₄ MNPs suspension was tuned to 10.0, the as-synthesized Fe₃O₄/MIL-101(Fe) would aggregate seriously. So, the pH of Cys-Fe₃O₄ MNPs suspension was selected as 9.0 with the purpose of obtaining stable dispersion of Fe₃O₄/MIL-101(Fe). As shown in Fig. 2B, the zeta potential of Cys-Fe₃O₄MNPs is −36.87 mV at pH of 9.0, and 35.37 mV for that of original MIL-101(Fe) suspension. The corresponding Fe₃O₄/MIL-101(Fe) shows weak electropositivity (11.23 mV) without obvious aggregation.

Fig. 2 (A) Zeta potential of Fe₃O₄ and the corresponding Fe₃O₄/MIL-101(Fe) under various pH; (B) the changes of zeta potential before and after reaction.

Liquid-phase dimerization reaction of OPD to 2,3-DPA in the presence of H₂O₂ was used as a model reaction system to characterize the catalytic performance of the Fe₃O₄/MIL-101(Fe). When the Fe₃O₄/MIL-101(Fe) catalyst was introduced into the solution, the fluorescence of 2,3-DPA increased along with the time's going and showed an intensive yellow-green fluorescence under UV excitation (Fig. S4†). The product 2,3-DPA shows a strong fluorescence emission (558 nm) in methanol under 439 nm excitation. So, the reaction process was monitored by fluorescence standard curve method (Fig. S5†). The standard curve has a good linear relationship (R² = 0.998) with a linear equation y = 24.60c + 37.52. The reaction proceeds slowly over the Cys-Fe₃O₄ MNPs and MIL-101(Fe) catalysts, while the catalytic activity of the Fe₃O₄/MIL-101(Fe) catalyst is noticeably better under the same conditions (Fig. 3). It also indicated that the catalytic efficiency of H₂O₂ is very low without Fe₃O₄/MIL-101(Fe). The possible reaction mechanism was shown in Fig. S6.† The Fe₃O₄/MIL-101(Fe) shows excellent catalytic activity for the decomposition of H₂O₂ and produces abundant reactive oxygen species (ROS) which was involved in the whole oxidation of OPD.

Fig. 3 Catalytic conversion of OPD to 2,3-DPA over Cys-Fe₃O₄ MNPs, MIL-101(Fe) and Fe₃O₄/MIL-101(Fe), respectively.

Similar to peroxidase, the catalytic activity of Fe₃O₄/MIL-101(Fe) was found to be closely dependent on pH, temperature, H₂O₂ concentration, and so on. Therefore, the reaction conditions, such as the concentration of H₂O₂ and Fe₃O₄/MIL-101(Fe), pH, temperature, and reaction time, have been optimized (Fig. S7†). Under the optimal reaction conditions, the Fe₃O₄/MIL-101(Fe) displayed a higher catalytic activity than that of Cys-Fe₃O₄ MNPs and MIL-101(Fe), achieving 97.79% yield of 2,3-DPA (Fig. S8†). Compared with other existing catalyst (Table S1†), the as-synthesized Fe₃O₄/MIL-101(Fe) shows highest catalytic efficiency among the reported catalyst for the oxidation of OPD. For the product 2,3-DPA (structure displayed in Fig. S9†), the characterization data of FTIR, ¹H NMR, ¹³C NMR and ESI-TOFMS are showed in Fig. S10–S13,† respectively.

From the results shown above, one can easily draw a conclusion that the catalytic activity of Fe₃O₄/MIL-101(Fe) is higher than those of Cys-Fe₃O₄MNPs and MIL-101(Fe) for the same reaction time. This catalytic activity enhancement can be attributed to the synergistic effect of Cys-Fe₃O₄MNPs and MIL-101(Fe). Furthermore, the recyclability of the Fe₃O₄/MIL-101(Fe) catalyst was also examined in the dimerization reaction of OPD. The catalyst was isolated by magnetic separation from the reaction mixture and washed with methanol, and reused for the next run under the same conditions. The results indicated that no significant loss of activity for the dimerization reaction of OPD was observed over Fe₃O₄/MIL-101(Fe) in the five successive catalytic cycles, suggesting that the Fe₃O₄/MIL-101(Fe) possesses long-term stability (Fig. 4). The atomic absorption spectrometry (AAS) has been used for monitoring the leaching test. The total iron content which leached from the composite would have a relatively high leaching quantity in the first-time using. The highest leaching amount is 1.153 ng mL⁻¹. The leaching amount dropped dramatically with the increase of cycling times, and even no iron was detected after three cycles (Fig. S14†). However, the catalytic efficiency of the composite is still maintained at a high level. It indicates that the catalytic activity is mainly derived from the composite material rather than the leached iron ion. After five recycling use, the PXRD pattern showed that the used Fe₃O₄/MIL-101(Fe) keeps close to the originality (Fig. S15†). As can be seen from the X-ray photoelectron spectroscopy (XPS) spectrums (Fig. S16†), there were no changes between the fresh and recovered Fe₃O₄/MIL-101(Fe). It indicates that the composite remains the integrity without any valence state change before and after the reaction. Besides, the morphology of Fe₃O₄/MIL-101(Fe) has no serious changes and no great influence upon the catalytic activity after five consecutive reaction cycles (Fig. S17†).

Fig. 4 The productivity of 2,3-DPA using Fe₃O₄/MIL-101(Fe) as catalyst after five consecutive reaction.

In summary, we have developed a facile, general and effective ultrasound-assisted electrostatic self-assembly strategy for immobilizing Fe₃O₄ MNPs on the surface of MIL-101(Fe). The catalytic property of Fe₃O₄/MIL-101(Fe) was evaluated by using the dimerization reaction of OPD as a model reaction system. It is noteworthy that the as-prepared Fe₃O₄/MIL-101(Fe) composite synergistically improves the catalytic activity compared to the Cys-Fe₃O₄ and MIL-101(Fe). Furthermore, the high catalytic activity was retained after a number of reaction cycles with a slightly change. This study may bring light to new opportunities in the development of high performance heterogeneous catalysts based on the peroxidase-like activity of Fe-based MOFs or nanostructures.

Acknowledgements

The authors gratefully acknowledge thank the financial support of the National Natural Science Foundation of China (NSFC, No. 21175109).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19170c

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