Synthesis and application of core–shell magnetic metal–organic framework composites Fe₃O₄/IRMOF-3

Abstract

A new type of magnetic core–shell structured MOF composite (Fe₃O₄/IRMOF-3) was successfully prepared by a facile mixed solvothermal method. Fe₃O₄ nanoparticles and polyvinylpyrrolidone (PVP) are the magnetic core and promoter, respectively. Fe₃O₄ nanoparticles are encapsulated completely inside the MOF matrix. PVP not only functions as the stabilizer to make the Fe₃O₄ nanoparticles well dispersed but also provides the ability to adsorb IRMOF-3 precursors onto the surface of the Fe₃O₄ nanoparticles. The composite possesses high crystallinity, porosity characteristics, a rapid magnetic response and good stability. In addition, the catalytic performance of Fe₃O₄/IRMOF-3 is investigated via the Knoevenagel condensation reaction of benzaldehyde and ethyl cyanoacetate. The conversion of ethyl cyanoacetate reaches 98.3% after 4 hours. More importantly, the catalyst can be quickly separated by a magnet and reused many times.

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Introduction

Metal–Organic Frameworks (MOFs) are porous crystalline materials composed of metal ions or metal cluster nodes connected by organic ligands.^1–3 They have attracted significant research interest in recent years, not only for their extraordinarily high surface areas, tunable pore size, and adjustable internal surface properties but also for their many attractive applications in heterogeneous catalysis, gas storage, separation, sensing, molecular recognition and other fields.^4–10

Although an important research stream on MOFs focuses on the fabrication of frameworks using stimuli responsive building blocks, a new research trend investigates the combination of MOFs with a variety of functional nano- and micro-particles.^11–15 Among various functional nanoparticles, magnetic Fe₃O₄ and γ-Fe₂O₃ nanoparticles, which have received the most attention owing to their strong magnetic responsiveness and biocompatibility, can be efficiently separated and purified during the synthesis and application processes by an external magnetic field. After combination with Fe₃O₄ nanoparticles, the magnetic MOF composites also can be easily positioned/collected thanks to their strong magnetic responsiveness. To date, several pioneering synthetic studies on the MOF-based core–shell structure have been reported. For example, Fe₃O₄@[Cu₃(btc)₂] and Fe₃O₄@MIL-100(Fe) core–shell microspheres were synthesized by dispersing the MAA-modified Fe₃O₄ microspheres alternately in ethanol solutions of metal ions and organic ligands after dozens of assembly cycles, which was called a step-by-step assembly strategy.^16–19 Fe₃O₄@ZIF-8 core–shell structured composites were obtained by a solvothermal method using poly(styrenesulfonate, sodium salt) or polyvinylpyrrolidone as a promoter.^20,21 In addition, several magnetic MOF composites with similar structure, such as γ-Fe₂O₃/ZIF-8, γ-Fe₂O₃/MIL-53(Al), Fe₃O₄/PDA/HKUST-1 and Fe₃O₄/MIL-101(Cr) were synthesized by some unique and novel synthetic methods.^22–25 Although a series of magnetic core–shell MOFs have been fabricated, the different kinds of composites suffer from tedious and long synthetic steps, which make them very costly or not suitable for large-scale production. Furthermore, controlling the heterogeneous growth of different MOF crystals on the magnetic nanoparticles remains a great challenge.

IRMOF-3 is a three-dimensional (3D) cubic porous framework comprised of Zn₄O secondary building units and 2-aminobenzenedicarboxylate linkers.²⁶ It has been the focus of several studies due to its large surface area, exceptional pore volume and relatively high thermal stability. In particular, research has shown that IRMOF-3 as a heterogeneous catalyst has high catalytic activity for the Knoevenagel condensation reaction.^27–29 To the best of our knowledge, the synthesis method of magnetic core–shell Fe₃O₄/IRMOF-3 has not been reported.

In this paper, a new magnetic core–shell MOF composite denoted Fe₃O₄/IRMOF-3 was synthesized by a flexible method. The synthesis route of Fe₃O₄/IRMOF-3 is shown in Scheme 1. The composites were systematically characterized by a series of characterization methods. Meanwhile, the obtained Fe₃O₄/IRMOF-3 has been applied as a heterogeneous catalyst for the Knoevenagel condensation reaction.

Scheme 1 The synthesis route of Fe₃O₄/IRMOF-3.

Experimental

Materials

Ferric chloride hexahydrate (FeCl₃·6H₂O), anhydrous sodium acetate, trisodium citrate, ethylene glycol, ethanol, N,N-dimethylformamide (DMF), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and benzaldehyde were provided by Sinopharm Chemical Reagent Company, Ltd. Polyvinylpyrrolidone (PVP, M_w = 8000) and ethyl cyanoacetate were purchased from Aladdin. 2-Amino terephthalic acid (NH₂–H₂BDC) was purchased from Tokyo Chemical Industry. All reagents and solvents were used without further purification.

Synthesis of Fe₃O₄/IRMOF-3

Fe₃O₄ superparamagnetic nanoparticles were prepared by a solvothermal method.³⁰ 3.0 g of FeCl₃·6H₂O, 0.72 g of trisodium citrate and 4.8 g of sodium acetate were dissolved in 100 mL of ethylene glycol under vigorous stirring for 30 min. The resultant mixture was then transferred into a Teflon-lined stainless-steel autoclave for heating at 200 °C for 10 h. After that, the autoclave was carefully taken out to cool to room temperature. The as-made black products were thoroughly washed with ethanol and deionized water several times, and finally vacuum dried at 60 °C. In brief, the synthesis of core–shell Fe₃O₄/IRMOF-3 includes the following steps: first, the dissolution of precursors 22.31 mg Zn(NO₃)₂ and 5.43 mg NH₂–H₂BDC into 2 mL DMF. Next, 0.2 g PVP was dissolved into the mixed solvent containing 6 mL DMF and 4 mL ethanol, then the mixture was added into the above precursor solution, followed by addition of pre-synthesized Fe₃O₄ (0.01 g) and subsequent ultrasonic treatment for 20 min. After that, the mixture was transferred into a 50 mL round-bottom flask and heated at 100 °C for 4 h under vigorous stirring conditions. The above process was repeated several times. After separation with a magnet and washing with DMF, the obtained sample was dried under vacuum at 150 °C. The samples were designated as FI-x, where x is the number of cycles of synthesis.

Characterization

Powder XRD patterns were recorded on a Rigaku D/Max 2400 diffractometer using Cu Kα radiation (40 kV and 40 mA). Fourier transform infrared (FT-IR) spectra were recorded using a Bruker EQUINOX55 infrared spectrometer. Scanning electron microscopy (SEM) images were collected using a FEI Quanta 450 instrument. Transmission electron microscopy (TEM) images were obtained using a FEI Tecnai G2 Spirit instrument with an acceleration voltage of 120 kV. The nitrogen adsorption and desorption isotherms were measured at −196 °C using Quantachrome Autosorb-IQ apparatus. The specific surface area and pore size distribution were calculated by using the BET method and a DFT model. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q50 TGA by heating from room temperature to 900 °C under N₂ at 5.00 °C min⁻¹. The magnetic properties were measured with a Magnetic Properties Measurement System (MPMS XL-7) from Quantum Design.

Catalytic reactions

In a typical batch experiment, the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate was carried out in a round bottom flask connected to a reflux condenser. 5 mL ethanol, 8 mmol benzaldehyde, 7 mmol ethyl cyanoacetate and 0.06 g of solid catalyst were placed inside the round bottom flask with an inert (N₂) atmosphere. The reaction was performed at 60 °C under vigorous stirring conditions, and the course of the reaction was followed by Shimadzu GC-2014C (HP-1 capillary column, Flame Ionization Detector) analysis. Conversion was calculated on the basis of the initial and final amounts of ethyl cyanoacetate. For the recyclability tests, the catalyst after the reaction was recovered with a magnet and washed with ethanol several times, and then reused in the next run under the same reaction conditions.

Results and discussion

Characterization of the material

The wide-angle powder XRD patterns of Fe₃O₄, FI-1, FI-2 and FI-3 are shown in Fig. 1a. The diffraction peaks for the samples obtained can be readily indexed to crystalline Fe₃O₄ and IRMOF-3, and no peak of impurities can be detected. It can be clearly seen that FI-1 possesses two sets of peaks, but the crystallinity of IRMOF-3 is low. However, the crystallinity of IRMOF-3 in the samples increased with an increasing number of cycles of synthesis. After the third round of synthetic reaction, FI-3 possessed high strength diffraction peaks of Fe₃O₄ and IRMOF-3.

Fig. 1 XRD patterns of samples with different (a) numbers of cycles of synthesis and (b) m(PVP)/m(Fe₃O₄) ratios.

PVP not only functions as the stabilizer to make the Fe₃O₄ nanoparticles well dispersed but also provides the ability to adsorb IRMOF-3 precursors onto the surface of Fe₃O₄ nanoparticles via coordination interaction between the pyrrolidone rings (C=O) and Zn²⁺ ions as well as hydrophobic interaction between the apolar groups of PVP and the NH₂–H₂BDC organic linkers.³¹ In order to investigate the influence of PVP on the synthesis, a series of samples with different m(PVP)/m(Fe₃O₄) ratios were prepared, and their powder XRD patterns are shown in Fig. 1b. Obviously, the crystallinity of IRMOF-3 in samples increased at first and then decreased with increasing the amount of PVP. The sample synthesized with m(PVP)/m(Fe₃O₄) = 20 possesses the strongest diffraction peaks of IRMOF-3. The results demonstrate that the dosage of PVP in the synthetic system shows a strong influence on the crystallization of IRMOF-3 for the composites.

The FI-3 and NH₂–H₂BDC were characterized by FT-IR spectra to further confirm the formation of IRMOF-3. Fig. 2a shows peaks at 3500 and 3400 cm⁻¹ of NH₂–H₂BDC, which were ascribed to the asymmetric and symmetric stretching absorptions of the primary amine group. However, for FI-3, the two peaks become weak and are shifted slightly to 3450 and 3350 cm⁻¹ (Fig. 2b), which is due to the occurrence of a coordination interaction between Zn²⁺ ions and the amine group of NH₂–H₂BDC to form IRMOF-3.

Fig. 2 Fourier transform infrared (FT-IR) spectra of (a) NH₂–H₂BDC and (b) FI-3.

The morphologies of the Fe₃O₄ nanoparticles and the composites in each step were observed by SEM (Fig. 3). Fig. 3a and b clearly indicate that the Fe₃O₄ particles are spherically shaped and their diameters are about 200–500 nm. As shown in Fig. 3c and d, there are a lot of micron scale spherical particle aggregates of varying degrees in FI-1. Combined with XRD test results, these particles should be IRMOF-3 crystals. In addition, a large number of Fe₃O₄ nanoparticles are not embedded and are bare on the outside surface of IRMOF-3 crystals. After two cycles of synthetic reaction, the morphology of FI-2 is similar to that of FI-1 (Fig. 3e and f), but the number of bare Fe₃O₄ nanoparticles has decreased significantly. After three cycles of synthetic reaction, the surface of FI-3 particles is smooth and almost no Fe₃O₄ nanoparticles can be found (Fig. 3g and h). Meanwhile, TEM images (Fig. 4) of FI-3 show that Fe₃O₄ nanoparticles are embedded in IRMOF-3 crystals, clearly demonstrating the formation of a core–shell structure.

Fig. 3 SEM images of (a and b) Fe₃O₄, (c and d) FI-1, (e and f) FI-2 and (g and h) FI-3.

Fig. 4 TEM images of FI-3.

N₂ adsorption–desorption experiments were performed to determine the textural properties of FI-3. The BET surface area and pore volume of FI-3 were calculated to be 237.57 m² g⁻¹ and 0.31 cm³ g⁻¹, respectively. These values are lower than those of pure IRMOF-3 synthesized in a prior report²⁷ due to the inner nonporous Fe₃O₄ particle cores. The adsorption isotherms exhibit a hysteresis loop at higher relative pressures, which is most probably due to textural porosity formed by the stacking of particles. The pore diameter distribution curve derived from the DFT model indicates that there are two mesopores with a diameter of 2.8 and 4.1 nm besides the micropores with a diameter of 1.7 nm (Fig. 5).

Fig. 5 N₂ adsorption–desorption isotherms and the DFT pore size distribution curve (inset) of FI-3.

In order to examine the thermal stability of the Fe₃O₄/IRMOF-3 core–shell structure, TG analysis was carried out. The TG and DTG curves of activated FI-3 exhibit one main step in the temperature range from 260 to 530 °C (Fig. 6). This step is due to the decomposition of the framework, demonstrating the weight-loss behavior of the IRMOF-3 component of the composites. It can be deduced from the TG results that the weight percentage of IRMOF-3 in FI-3 reached 38%.

Fig. 6 TG and DTG curves of FI-3.

The saturation magnetization values of Fe₃O₄ and FI-3 are 78.5 and 13.5 emu g⁻¹, respectively, and no remanence was detected for the samples (Fig. 7), which confirmed these samples were superparamagnets. Although the saturation magnetization value of FI-3 is lower than that of Fe₃O₄, owing to the coating of IRMOF-3 on the surface of Fe₃O₄, it was convenient to separate the composites from an ethanol solution using a magnet (inset of Fig. 7). This phenomenon demonstrates directly that the core–shell structured composites possess strong magnetic properties, which will provide an easy and efficient way to separate Fe₃O₄/IRMOF-3 particles from a solution or slurry.

Fig. 7 The magnetic hysteresis loops of Fe₃O₄ and FI-3. The inset shows the process of separation of FI-3 by using a magnet.

Catalytic tests

IRMOF-3 has been reported to be an efficient catalyst for the Knoevenagel condensation reaction, which is an important synthesis route for many pharmaceuticals and fine chemicals.³² Hence, the core–shell structured composite, FI-3, which contained both Fe₃O₄ and IRMOF-3 (38 wt%), is used as a catalyst for the Knoevenagel condensation reaction. As shown in Fig. 8, a blank experiment, in the absence of any catalyst, only gave a maximum conversion of 12.6% after 4 h. Compared with Fe₃O₄, the IRMOF-3 presents much higher activity to catalyze the Knoevenagel condensation reaction. So the IRMOF-3 component in FI-3 will play the major role in the Knoevenagel condensation reaction. It is noteworthy that the performance of FI-3 is better than that of Fe₃O₄ or IRMOF-3. A conversion of about 98.3% is obtained after 4 h of reaction and no by-products are detected. The results indicate that Fe₃O₄ and IRMOF-3 in FI-3 may have a synergistic effect in the reaction. In order to investigate the influence of the reaction conditions, a series of contrast experiments were carried out and the results are listed in Table 1. It can be seen that the conversion of ethyl cyanoacetate reaches 100% after 4 h of reaction with raising the reaction temperature or increasing the catalyst dosage.

Fig. 8 Conversion of ethyl cyanoacetate over FI-3 at 60 °C.

Table 1 Ethyl cyanoacetate conversion efficiency of the Knoevenagel reaction over FI-3 with different reaction conditions

Catalyst dosage (g)	Reaction temperature (°C)	Conversion (%)
0.06	50	94.5
0.06	60	98.3
0.06	70	100
0.04	60	97.3
0.08	60	100

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The recyclability is also a vital character for a practical solid catalyst. In this work, we examined the recyclabilities of FI-3 in the Knoevenagel condensation reaction by reusing recovered catalysts for the next run under the same conditions. In each test, the catalyst was separated with a magnet and washed with ethanol several times, and then reused in the next run. As shown in Fig. 9, the conversion of ethyl cyanoacetate slightly decreased to 92.1% from 98.3% after three cycles of reaction, and no noticeable decrease of the conversion can be observed in the subsequent 4th and 5th cycles. These results indicate that FI-3 possesses good recyclability.

Fig. 9 Recyclability of FI-3 in the Knoevenagel condensation reaction.

Conclusions

In summary, core–shell structured magnetic metal–organic framework composite Fe₃O₄/IRMOF-3 has been successfully synthesized by a novel method. Fe₃O₄ nanoparticles are encapsulated completely inside the MOF matrix. Moreover, the obtained Fe₃O₄/IRMOF-3 composite exhibited excellent catalytic performance for the Knoevenagel condensation reaction of benzaldehyde and ethyl cyanoacetate. Additionally, the magnetic catalyst can be easily separated from the reaction solution by adding an external magnetic field and can be reused many times. Furthermore, the Fe₃O₄/IRMOF-3 core–shell magnetic composite could be a potential candidate in other fields.

Acknowledgements

The authors acknowledge the financial support from the Program for New Century Excellent Talents in University (NCET-04-0270).

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