Palladium-loaded magnetic core–shell porous carbon nanospheres derived from a metal–organic framework as a recyclable catalyst

Abstract

Separation and recycling of noble metal nanocatalysts after catalytic reactions are significant challenges to reduce catalyst cost and avoid waste generation in industrial applications. In this study, Pd-loaded magnetic porous carbon nanospheres (Fe₃O₄@MC-Pd) were prepared by annealing Fe₃O₄@MIL-100/PdCl₂, which was fabricated through a facile one-pot solvothermal method, at 450 °C in a nitrogen atmosphere. The novel Fe₃O₄@MC-Pd catalyst consists of a superparamagnetic Fe₃O₄ core and a chemically inert porous carbon layer, which can protect the Fe₃O₄ core from extreme external environments and prevent the loss of Pd NPs. The resultant composite material showed excellent catalytic performance in reducing methylene blue with sodium borohydride as a reducing agent and superparamagnetic behavior that enabled the magnetic separation and convenient recovery of the nanocatalysts from the reaction mixture. Moreover, the composite material also showed good thermal and acid stability, fast regeneration ability, and high cyclic stability (>10 cycles without loss of catalytic efficiency). The result shows the nanocatalysts could overcome the drawbacks of MOF catalysts (chemical unstability). This study indicated that the as-prepared Fe₃O₄@MC-Pd composite material shows great potential for using in a wide range of applications.

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1. Introduction

Noble metal nanoparticles (NMNPs), such as Pt, Au, Ag, and Pd, have attracted increasing attention owing to their high catalytic activities toward different types of reactions.^1–8 Thus, they are considered excellent candidates for the design of highly active catalysts. However, aggregation of metal nanoparticles (NPs), which is the main obstacle in the use of catalytic NPs in real applications, severely reduce the catalytic efficiency of the catalysts.^9,10 Moreover, recycling NPs is a complicated task because they are too small.¹¹ Thus, the stability and separation of the NMNPs are two crucial issues for their further applications.

To solve the stability problem, various techniques have been developed, such as stabilizing NPs by ligands and embedding NPs inside shells.^12,13 Among these strategies, the immobilization of NMNPs on solid supports has been regarded as the most frequently used approach to preserve their catalytic properties.^14–19 In this strategy, aggregation of noble NPs can be prevented efficiently by dispersing noble NPs finely on the support.^20,21 The emergence of new nanomaterials, such as carbon-based materials (graphene oxide, carbon nanotubes, and carbon nanofibers),^22,23 mesoporous silica,²⁴ polymers,^25–29 and metal oxides,^30,31 provides promising support in fabrication of composite catalysts. Among a variety of catalyst supports, carbonaceous materials, such as graphene, carbon nanotubes, and active carbons, exhibit remarkable advantages owing to their excellent chemical stability, mechanical strength, and large surface area.^32,33 However, NMNPs that are physically attached to carbonaceous materials leach easily during catalytic process, which causes a significant loss of catalytic activity. Only functionalized with O- and N-containing functional groups, the NMNPs on these carbonaceous materials cannot be easily leached and subsequently maintain high cyclic stability.³⁴

Metal–organic frameworks (MOFs) are crystalline materials composed of metal ions and organic ligands. The pore size and functional groups of MOFs can be adjusted given the enormous variety of metal ions and organic ligands.^35–37 These distinct characteristics make them excellent candidates for various applications, such as gas storage,^38,39 sensing,⁴⁰ chemical separation,⁴¹ drug delivery,⁴² and catalysis.⁴³ In recent years, utilization of MOFs as heterogeneous catalysts has received remarkable attention. However, application of MOFs in special environments, such as acid, alkali, and high temperature, is limited because of their low stability. Moreover, recycling MOFs is a complicated task after reaction. Utilization of MOFs as templates or precursor materials to produce porous carbonaceous materials has been demonstrated by taking advantage of the fact that MOFs have large surface area and possess a significant amount of carbon source.^44,45 Meanwhile, to solve the separation problem, magnetic nanomaterials may be adopted as catalyst supports because they exhibit remarkable advantages owing to their easy separation for reuse by applying an external magnetic field. In addition, the use of magnetic separation can reduce the capital and operational costs.

In this paper, we report a facile method to synthesize Fe₃O₄@MC-Pd composite material derived from Fe₃O₄@MIL-100(Fe)/PdCl₂ at 450 °C in nitrogen atmosphere. First, Fe₃O₄ nanospheres were prepared through a traditional hydrothermal method, and then the nanospheres were modified by carboxylic acid. Subsequently, Fe₃O₄@MIL-100(Fe) was prepared using the modified Fe₃O₄ NPs as templates through a one-pot self-assembly strategy. The resultant composite not only has excellent morphology but also shows a high Brunauer–Emmett–Teller (BET) surface area. Finally, the as-synthesized Fe₃O₄@MIL-100(Fe) was added to ethanol solution of the precursor (H₂PdCl₄) with constant vigorous stirring. The magnetic porous carbon (MPC) that contains Pd (Fe₃O₄@MC-Pd) composite was prepared by annealing Fe₃O₄@MIL-100(Fe)/PdCl₂ in nitrogen atmosphere. Both one-pot self-assembly strategy for Fe₃O₄@MIL-100(Fe) preparation and one-step synthesis of Fe₃O₄@MC-Pd from Fe₃O₄@MIL-100(Fe)/PdCl₂ have rarely been reported.

The catalytic activity of the as-prepared Fe₃O₄@MC-Pd in reduction of methylene blue (MB) with NaBH₄ was evaluated. Compared with other methods of magnetic-Pd catalyst formation, additional toxic reagent is unnecessary in the present method. The Fe₃O₄@MC-Pd exhibits excellent catalytic activity compared with most of the other reported catalysts. The as-synthesized nanocatalyst shows high efficiency in magnetic separation and recovery because of the high saturation magnetization and super-paramagnetic property. In addition, the chemical and thermal stability of Fe₃O₄@MC-Pd materials are better than those of Fe₃O₄@MIL-100(Fe). These advantages make Fe₃O₄@MC-Pd materials an excellent catalyst for practical application.

2. Experimental

2.1 Materials

Sodium citrate and FeCl₃ were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Benzene-1,3,5-tricarboxylicacid (H₃BTC) and (3-aminopropyl) triethoxysilane (APTES) were purchased from Aldrich. Succinic anhydride and sodium hydroxide were purchased from Shanghai Chemical Reagents Company. Ethylene glycol was supplied by Tianjin Fine Chemicals Co., Ltd. Palladium chloride (PdCl₂) were purchased from Sigma Aldrich. All the chemicals were of analytical grade.

2.2 Synthesis of Fe₃O₄ NPs

Fe₃O₄ NPs were prepared via a one-pot hydrothermal method.⁴⁶ In a typical experiment, 2.6 g of anhydrous FeCl₃ and 1.0 g of trisodium citrate dihydrate were first dissolved in 80 mL of ethylene glycol under vigorous stirring for 30 min. Then, 4.0 g of NaAc was added while stirring for 30 min. The mixture was then sealed in a Teflon-lined stainless-steel autoclave and heated at 200 °C for 10 h. Afterward, the autoclave was carefully removed and allowed to cool to room temperature. The products were separated and washed with ultrapure water and ethanol for several times and dried under vacuum at 50 °C.

2.3 Preparation of COOH-functionalized Fe₃O₄ microspheres

Fe₃O₄ NPs were first modified by APTES, which was achieved by adding 160 mL of ethanol, 40 mL of deionized water, 3 mL of NH₃·H₂O, and 1.2 g of APTES into a three-necked round-bottom flask, which was equipped with a mechanical stirrer. Moreover, the mixture was vigorously stirred for 24 h at 60 °C. Then, the obtained Fe₃O₄/APTES microspheres were collected through magnetic separation and washed with ethanol for several times to remove excess APTES. The resultant Fe₃O₄/APTES NPs were dried in a vacuum oven at 50 °C until constant weight was achieved. Then, 200 mg of Fe₃O₄/APTES and 200 mg of succinic anhydride were dispersed in 60 mL of DMF solution and stirred for 24 h at room temperature. The solid products were collected with an external magnetic field, rinsed with ultrapure water and ethanol for several times, and finally dried under vacuum at 50 °C for further use.

2.4 Preparation of core–shell magnetic Fe₃O₄@MIL-100(Fe) microspheres

The magnetic Fe₃O₄@MIL-100(Fe) microspheres were fabricated through a one-pot assembly strategy. The COOH-functionalized Fe₃O₄ microspheres (250 mg) were obtained and dispersed in 30 mL of ethanol solution. Then, 50 mL of DMF containing FeCl₃ (500 mg) and 1,3,5-benzenetricarboxylic acid (H₃BTC) (300 mg) was added. Finally, the mixture was transferred into a Teflon-lined stainless-steel autoclave with a capacity of 100 mL and was heated at 120 °C for 24 h. After cooling to room temperature, the solid products were collected from the solution with the use of an external magnet. The solid products were washed with water for several times and dried in a vacuum at 60 °C.

2.5 Synthesis of Fe₃O₄@MC-Pd microspheres

Fe₃O₄@MC-Pd microspheres were synthesized through a one-step strategy. The as-synthesized Fe₃O₄@MIL-100(Fe) (50 mg) was typically added to 10 mL of ethanol solution containing H₂PdCl₄ (25 μmol) with constant vigorous stirring for 6 h. The resultant Fe₃O₄@MIL-100(Fe)/PdCl₂ microspheres were then recovered with a magnet and dried in a vacuum oven at 50 °C until constant weight was achieved. Finally, the Fe₃O₄@MC-Pd microspheres were prepared by annealing Fe₃O₄@MIL-100(Fe)/PdCl₂ at 450 °C in nitrogen atmosphere for 1 h. After cooling to room temperature, the as-prepared Fe₃O₄@MC-Pd microspheres were collected.

2.6 Catalysis test

The catalytic property of Fe₃O₄@MC-Pd materials was investigated in terms of the absorbance intensity change at the maximum absorbance wavelength of the MB dye. Typically, 2.75 mL of ultrapure water and an aqueous solution (200 μL) of freshly prepared NaBH₄ (100 mM) were added to a solution (25 μL) of MB (100 mM), respectively. Subsequently, 25 μL of aqueous dispersion of the catalyst (1 mg mL⁻¹) was added to start the reaction. Then, the blue solution gradually faded as the reaction proceeded, which could be observed by the naked eye. The progress of the conversion reaction was monitored by measuring the changes in absorbance at 665 nm with a UV-vis spectrophotometer.

2.7 Stability tests

A 10 mg portion of Fe₃O₄@MC-Pd microspheres was dispersed in 20 mL of boiling water and kept for 10 h to test the stability of the catalyst. Another 10 mg portion of Fe₃O₄@MC-Pd microspheres was dispersed in 10 mL of hydrochloric acid solution at pH 2 for 24 h to test the acid stability of the sample.

The recyclability of Fe₃O₄@MC-Pd microspheres for MB reduction was also investigated for 10 cycles. After each run, the solid catalyst was recovered with a magnet, washed with water and ethanol, and dried under vacuum. To evaluate the stability, the conversions were determined via UV-vis detection of the residual MB in the mixture after reaction. Furthermore, Pd wt% was also measured with ICP-AES for every cycle.

2.8 Characterization

The morphology of the sample was examined by transmission electron microscope (TEM, Tecnai G2 F30) and scanning electron microscope (SEM, JSM-6701F, JEOL, Japan). FT-IR spectra were obtained in transmission mode on a FT-IR spectrometer (American Nicolet Corp., Model 170-SX) using the KBr pellet technique. XRD (Rigaku D/MAX-2400 X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 1.54056)) was used to investigate the crystal structure of the nanoparticles. The Pd content of the prepared nanocatalysts were obtained by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Magnetization measurements at room temperature were obtained using a vibrating sample magnetometer (LAKESHORE-7304, USA) at room temperature. The X-ray photoelectron spectroscopy (XPS) spectra were obtained with an ESCALab220i-XL electron spectrometer (VG Scientific) using 300 W Al-Kα radiation. The N₂ adsorption–desorption isotherm was measured at liquid nitrogen temperature (77 K) using a Micromeritics ASAP 2020 analyzer. The specific surface area was calculated by the BET method. The pore size distribution was obtained using the Barrett–Joyner–Halenda (BJH) method. UV-Vis detection was carried out on a TU-1810PC UV-Vis spectrophotometer (Purkinje General, China).

3. Results and discussion

3.1 Morphology and structure characterization

Fig. 1 outlines the procedure to prepare Fe₃O₄@MC-Pd microspheres. Initially, Fe₃O₄ NPs were synthesized via a robust solvothermal method. Then, the NPs were modified with carboxylic acid. Fe₃O₄@MIL-100 was prepared using the modified Fe₃O₄ NPs as templates through a one-pot self-assembly strategy. Finally, the Fe₃O₄@MC-Pd composite was prepared by annealing Fe₃O₄@MIL-100(Fe)/PdCl₂ in nitrogen atmosphere. The TEM (Fig. 2A) and SEM (Fig. 2E and J) images show that the obtained magnetite particles are nearly spherical and have an average diameter of approximately 250 nm. Subsequently, magnetic Fe₃O₄@MIL-100(Fe) microspheres were then fabricated by a one-pot assembly strategy. Fig. 2 shows representative TEM (Fig. 2B) and SEM (Fig. 2F and K) images of the microspheres. The diameter of the Fe₃O₄@MIL-100 microspheres was approximately 450 nm. In the one-pot assembly strategy, DMF is used as the solvent for H₃BDC solution and it directly determines whether MIL-100(Fe) can be produced. Ethanol is also vital for the formation of core–shell NPs. Its presence can change the coordination environment of metal ions such that MIL-100(Fe) grows preferentially around Fe₃O₄ NPs instead of self-nucleating in solution.⁴⁷ The Fe₃O₄@MC-Pd microspheres prepared by annealing Fe₃O₄@MIL-100/PdCl₂ at 450 °C under nitrogen atmosphere also showed excellent morphology, and the diameter was approximately 400 nm (Fig. 2C, G and H). The Pd nanoparticles randomly disperse in the porous carbon layer of Fe₃O₄@MC-Pd microspheres. Their size distribution was evaluated statistically through measuring the diameter of 150 Pd nanoparticles in the TEM images. It can be seen that the particle size of Pd distributes mainly between 1.0 nm and 7.0 nm with an average diameter of about 5.0 nm (Fig. 3), which are small enough to be accommodated in the mesoporous cavities of the Fe₃O₄@MC-Pd microspheres. Furthermore, utilizing N₂ reduction can effectively avoid aggregation of metal NPs. Fig. 4 shows high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images that display a detailed elemental distribution of the nanocatalysts. The figures clearly demonstrate that Fe₃O₄@MC-Pd is mainly composed of C, O, Fe, and Pd elements. Moreover, the Pd mapping image (Fig. 4E) shows that the Pd NPs are distributed well in the spaces of porous carbon layer compared with the prepared MPC nanocatalysts described in Ma et al.⁴⁸

Fig. 1 Schematic illustration of the fabrication process of Fe₃O₄@MC-Pd microspheres.

Fig. 2 TEM and SEM images of the Fe₃O₄ (A, E and J), Fe₃O₄@MIL-100 (B, F and K), Fe₃O₄@MC-Pd (C, G and H).

Fig. 3 Size distribution of Pd nanoparticles on the surface of Fe₃O₄@MC-Pd.

Fig. 4 HAADF-STEM image of (A) Fe₃O₄@MC-Pd, (B) C mapping image (C-K), (C) O mapping image (O-K), (D) Fe mapping image (Fe-L), (E) Pd mapping image (Pd-K).

Surface modification of Fe₃O₄ microspheres and further functionalized microspheres with –COOH was investigated via FTIR spectroscopy (Fig. 5). The absorption peaks for the Fe₃O₄ microspheres at 455 and 578 cm⁻¹ can be assigned to the vibrations of the Fe–O functional group, which corresponded to reported stretching vibrations of Fe–O in bulk Fe₃O₄.⁴⁹ The peaks at 3415 cm⁻¹ is caused by the stretching vibrations of N–H. Moreover, 1440 and 1562 cm⁻¹ are caused by the stretching vibrations of C=C in benzene ring, whereas the peaks at 1371 and 1619 cm⁻¹ are attributed to C=O stretching vibrations of carboxylic acid, thereby suggesting that MIL-100 shell has been successfully coated on the surface of Fe₃O₄.

Fig. 5 FT-IR spectra of Fe₃O₄ (a), Fe₃O₄/APTES (b), Fe₃O₄/COOH (c) and Fe₃O₄@MIL-100 (d).

PXRD analysis also provided information regarding the composition of the as-synthesized samples. To verify the formation of Fe₃O₄@MC-Pd microspheres, XRD patterns of Fe₃O₄, Fe₃O₄@MIL-100(Fe), and Fe₃O₄@MC-Pd samples were obtained, as shown in Fig. 6. The diffraction peaks of the Fe₃O₄@MIL-100(Fe) microspheres and Fe₃O₄@MC-Pd at 2θ = 30.0°, 35.5°, 43.2°, 53.8°, 57.2°, and 62.6° were ascribed to (220), (311), (400), (422), (511), and (440) planes, which agree well with the standard XRD data for the cubic phase Fe₃O₄ (JCPDS card, file no. 89-4319), with a face-centered cubic structure.⁵⁰ In addition to these peaks of Fe₃O₄@MIL-100(Fe), additional peaks exist at 3.4° (022), 3.9° (113), 5.3° (333), 11° (428), 14.2° (088), 18.2° (7911), 20.1° (4814), 24° (6618), and 27.7° (9321), which correspond to the crystalline MIL-100, as reported previously.⁵¹ This finding demonstrates the formation of the crystal Fe₃O₄@MIL-100(Fe). After carbonization process, some diffraction peaks, which belong to crystal MIL-100, had disappeared. However, the weak diffraction (111) reflections of typical cubic Pd NPs were detected, which indicate the formation of very small Pd NPs.⁵² These observations match well with TEM observations. The XRD results indicated that the metal precursors are successfully reduced to metallic NPs.

Fig. 6 XRD patterns of (a) Fe₃O₄, (b) the crystallographic data of MIL-100, (c) Fe₃O₄@MIL-100, (d) Fe₃O₄@MC-Pd.

XPS was employed to verify the composition of the resulting hybrid magnetic microspheres by investigating the chemical state of the surface of obtained Fe₃O₄@MC-Pd microspheres. Fig. 7A exhibits the overall surveys of the as synthesized Fe₃O₄@MC-Pd microspheres, which clearly show the signals of elemental C, O, and Fe. Fig. 7A clearly illustrates that the signal of C is much stronger than that of other elements (i.e., O, Pd, and Fe) on the surface of the nanocatalysts. This phenomenon results from carbonization process, which is in accordance with the TEM observations. In addition, Fig. 7B shows the XPS spectra of Pd 3d. The peaks at 335.8 and 341.2 eV are assigned to Pd 3d_5/2 and Pd 3d_3/2, respectively. The position of the Pd 3d peaks corresponds to Pd⁰, which is in good agreement with that described in the previous literature.⁵³ According to these spectra, all of the metal ions are reduced to NMNPs after carbonization process. The Fe 2p spectrum can be deconvoluted into two peaks centered at 725.2 and 711.1 eV, which correspond to the peaks of Fe 2p_1/2 and Fe 2p_3/2 (Fig. 7C). The position of the Fe 2p peaks corresponds to that of Fe₂O₃ or Fe₃O₄.⁵⁴ The C 1s peak at 284.6 eV (Fig. 7D) corresponds to C–C/C=C species.⁵⁵

Fig. 7 XPS spectra of Fe₃O₄@MC-Pd: survey spectrum (A), high resolution of Pd spectrum (B), high resolution of Fe spectrum (C), and high resolution of C spectrum (D).

The shell also contained iron oxide after carbonization process. Thus, to analyze the composition of iron oxide, MS is indispensable. Fig. 8 shows that the spectrum was interpreted by two sextets and one doublet. The two sextets (H_eff = 47.96 and 45.31 T and IS = 0.32 and 0.25 mm s⁻¹) belong to magnetite,⁵⁶ which accounts for 81.60%. The doublet accounting for 18.5% with IS = 0.39 mm s⁻¹ and QS = 0.81 mm s⁻¹ may be Fe₂O₃,⁵⁷ which corresponds to the analysis of XPS.

Fig. 8 Mössbauer spectrum of Fe₃O₄@MC-Pd.

The BET surface area were measured using the N₂ adsorption/desorption isotherms at −197.2 °C. Before measurements, the samples were evacuated at 100 °C for 10 h. The BET surface area, pore volume, and adsorption average pore width (Fig. S1†) of Fe₃O₄@MIL-100(Fe) are 505.09 m² g⁻¹, 0.53 cm³ g⁻¹, and 4.16 nm, respectively. The surface areas of the samples obtained via the one-pot assembly strategy are higher and their pore sizes are larger than those prepared via the step-by-step assembly strategy. Thus, the one-pot assembly strategy is beneficial for diffusion of the metallic precursors into the pores. After carbonization, the BET surface areas and pore volumes of Fe₃O₄@MC-Pd are only 51.47 m² g⁻¹ and 0.078 cm³ g⁻¹, respectively. Compared with Fe₃O₄@MIL-100(Fe), the surface areas and the pore volumes of Fe₃O₄@MC-Pd had considerably decreased. The results indicated that the structure is partly damaged in the carbonization process, and the pores of the host frameworks are occupied by dispersed Pd NPs. However, the adsorption average pore width of the nanocatalysts increased to 6.06 nm, which provides valuable sites for catalytic reaction.

The magnetic properties of the resultant core–shell microspheres were investigated at room temperature with vibrating sample magnetometry in the field range of −10 kOe to 10 kOe (Fig. 9). The magnetization saturation values of Fe₃O₄, Fe₃O₄@MIL-100(Fe), and Fe₃O₄@MC-Pd were measured as 69.5, 22.7, and 60.4 emu g⁻¹, respectively, which are much higher than the value in literature (i.e., 45.07 emu g⁻¹) with Fe₃O₄@Pd/MIL-100(Fe) as nanocatalysts.⁵⁸ The measured values indicated the strong magnetic properties of the prepared nanocatalysts. All of the curves present a magnetic hysteresis loop, which also depicts the strong magnetic response to a varying magnetic field. Furthermore, Fe₃O₄, Fe₃O₄@MIL-100(Fe), and Fe₃O₄@MC-Pd indicated ferromagnetic-type curves, which show a hysteresis loop. The values of coercivity and remanence are summarized in Table 1. Fast aggregation of the nanocatalysts can be observed from their homogeneous dispersion in the presence of an external magnetic field applied for 10 s. This observation directly demonstrates the convenient separation of the nanocatalysts through an external magnetic field, which is important in terms of their practical manipulation.

Fig. 9 Magnetic hysteresis loops of Fe₃O₄ (a), Fe₃O₄@MIL-100 (b) and Fe₃O₄@MC-Pd (c). The inset is a magnified view of the magnetization versus field curves.

Table 1 Magnetic properties of Fe₃O₄, Fe₃O₄@MIL-100(Fe) and Fe₃O₄@MC-Pd

Samples	Magnetic properties
	MS (emu g^−1)	HC (Oe)	MR (emu g^−1)
Fe3O4	69.5	27.6	4.8
Fe3O4@MIL-100(Fe)	22.7	16.2	1.1
Fe3O4@MC-Pd	60.4	65.6	7.6

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3.2. Catalytic reduction tests

Metallic Pd nanostructures have been demonstrated as excellent catalysts with high activity and selectivity.^59,60 In the current investigation, the catalytic performance of Fe₃O₄@MC-Pd microspheres for reduction of MB with NaBH₄ was investigated. The main merit for the reduction of MB with NaBH₄ is its convenient monitoring via UV-vis absorption spectroscopy, given that no side reactions exist. Fig. 10 depicts the performance of the Fe₃O₄@MC-Pd microspheres as catalysts in the reduction of MB by NaBH₄. The Fe₃O₄@MC-Pd microspheres serve as millions of nanoreactors, where the reduction actions take place via relaying electrons from the donor BH₄⁻ to the acceptor MB. In aqueous medium BH₄⁻ is adsorbed onto the surface of the catalyst. The hydrogen atom, which is formed from the hydride, after electron transfer to the noble metallic NPs, attacks MB molecules to reduce it.

Fig. 10 Schematic of Fe₃O₄@MC-Pd microspheres as catalysts for the reduction of MB.

The progress of the catalytic reduction of MB can be easily followed by the decrease in absorbance at λ_max of MB with time. Fig. 11A shows the successive UV-vis spectra of MB in the presence of Fe₃O₄@MC-Pd and NaBH₄ in aqueous solution. A control experiment was conducted with Fe₃O₄@MC-Pd as the catalysts to clarify the catalytic effect of NPs. To elucidate the reaction mechanism, the concentration of NaBH₄ can be considered constant throughout the reaction, given that it was in a considerable excess. Therefore, pseudo-first-order kinetics with regard to the catalytic reduction of MB, described as ln(C/C₀) = −kt, can be applied, where C is the concentration of MB at time t, C₀ is the initial concentration of MB, and k is the rate constant. Fig. 11B reveals the linear relationship between ln(C/C₀) and the reaction time t, which matches well with the first-order reaction kinetics. The rate constant, k, of MB catalytic reduction is calculated as 5.97 min⁻¹ at 25 °C for the case with Fe₃O₄@MC-Pd microspheres, which is much larger than that reported by the literature, that is, 0.430 min⁻¹ with Fe₃O₄@PDA-Ag as catalyst,⁶¹ 0.266 min⁻¹ with Au@polypyrrole/Fe₃O₄ hollow capsules as nanocatalyst,⁵³ and 0.05 min⁻¹ with Ag NP-embedded hybrid microgels as catalyst.⁶² Therefore, the catalytic activity of as-synthesized catalysts in the present work is higher than that of most of the other catalysts. This outstanding catalytic performance can be ascribed to the high loading amount and well distributing of Pd NPs in the developed microspheres. The inset photos in Fig. 11 show that the colors of MB + NaBH₄ are different in the absence of catalyst (A) and in the presence of Fe₃O₄ (B), Fe₃O₄@MC (C), and Fe₃O₄@MC-Pd (D). The first three solutions almost maintain the same colors. However, by adding a small amount of Fe₃O₄@MC-Pd, the color of MB solution changed in only several seconds, thereby indicating that the reduction of MB via NaBH₄ occurs very fast in the presence of Fe₃O₄@MC-Pd. The superior catalytic performance of Fe₃O₄@MC-Pd microspheres confirms that the composite microspheres can serve as excellent supports for NMNPs.

Fig. 11 (A) Time dependent absorption spectra for the catalytic reduction of MB by NaBH₄ in the presence of Fe₃O₄@MC-Pd catalyst. (B) The relationships of ln(C/C₀) versus reaction time plot in the presence of 10 mg different catalysts: (a) no catalysts, (b) Fe₃O₄, (c) Fe₃O₄@MC, (d) Fe₃O₄@MC-Pd. The inset indicate the colors of MB + NaBH₄ are different in the absence of catalyst (A) and in the presence of Fe₃O₄ (B), Fe₃O₄@MC (C) and Fe₃O₄@MC-Pd (D).

To evaluate the pH effect of the MB solution on the catalytic activity of Fe₃O₄@MC-Pd microspheres, MB solutions with three different pH values (pH 3, 7, 9) were used for the tests. Fig. 12A–C shows that there is no significant change on the catalytic activity of Fe₃O₄@MC-Pd microspheres under different pH conditions, which implies that the high catalytic activity of the microspheres could withhold in various solution conditions of different pHs. The MOFs is effective only within a low and narrow pH range due to its chemical unstability, but the Fe₃O₄@MC-Pd microspheres resulted in a broader pH range.

Fig. 12 Successive UV-vis spectra for catalytic reduction of MB aqueous solution by NaBH₄ and Fe₃O₄@MC-Pd microspheres at (A) pH 3, (B) pH 7, and (C) pH 9.

In addition, the reduction of 4-nitrophenol (4-NP) with NaBH₄ has also been used as another model reaction to check the catalytic activity of the as-prepared Fe₃O₄@MC-Pd microspheres. Fig. S2† shows the successive UV-vis spectra of 4-NP in the presence of Fe₃O₄@MC-Pd microspheres and NaBH₄ in aqueous solution. The color of the 4-NP solution vanishes gradually, indicated by the gradual decrease in absorbance value at 400 nm. Meanwhile, a new peak centered at 296 nm appears and increases with reaction time. This peak could be indexed to the characteristic absorbance peak of 4-AP, further confirming the reduction of 4-NP to 4-AP. Therefore, the Fe₃O₄@MC-Pd microspheres exhibit the potential application as an effective catalyst for the reduction of various organic dyes.

3.3. Stability tests

Stability of Fe₃O₄@MC-Pd microspheres is a critical factor for their practical applications. In this work, the catalyst maintained the same morphology as that of fresh samples after 10 h in boiling water (Fig. 13A). Meanwhile, given that Fe₃O₄ microspheres can be easily etched under acid conditions, the acid stability of Fe₃O₄@MC-Pd microspheres was also tested under a strong acid environment (pH 2) for over 24 h. The TEM images (Fig. 13B) showed that the Fe₃O₄@MC-Pd microspheres maintain the core–shell structure under strong acid condition. The catalyst also maintained the same morphology under a strong alkali environment (pH 12) for over 24 h (Fig. 13C). Furthermore, the ICP-AES results (Table 2) confirmed that the catalyst can withstand the strong acid and alkali condition. Therefore, the robust C layer can effectively protect Fe₃O₄ cores, bind the Pd NPs under strong acid and alkali conditions, and show good stability in practical applications.

Fig. 13 TEM images of (A) Fe₃O₄@MC-Pd microspheres dispersed in boiling water for 10 h. (B) Fe₃O₄@MC-Pd microspheres dispersed in acid solution with pH 2 for 24 h. (C) Fe₃O₄@MC-Pd dispersed in alkali solution with pH 12 for 24 h.

Table 2 ICP-AES results of Fe₃O₄@MC-Pd under acid and alkali conditions

Entry	Catalyst	Pd content (wt%)	k (min^−1)
1	Fe3O4@MC-Pd	5.44	5.97
2	Fe3O4@MC-Pd (pH 2, 24 h)	5.27	5.74
3	Fe3O4@MC-Pd (pH 12, 24 h)	5.41	5.90

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Recyclability of the as-prepared catalyst was also evaluated. The catalytic activity remains stable even after 10 cycles, and the conversion of each cycle remains nearly 100% (Fig. 14). In addition, no changes were found in the XPS spectra of the microspheres after 10 reduction cycles (Fig. S3†), which confirms the long-term stability of the nanocatalysts. This high stability can be attributed to the core–shell porous architecture of the as-prepared microspheres, which provides both docking sites for MB and a superparamagnetic core that allows easy manipulation by a magnet. The solid catalyst was magnetically separated from the reactant mixture after each catalytic cycle. The residual solution was collected and measured by ICP-AES to determine the loss of Fe and Pd. The results showed that no Fe element was determined in residual solution and only trace amounts of Pd element was detected in the residual solution. As shown in Fig. 15, nearly 80% of Pd content was maintained on the catalyst even after 10 cycles. These results suggested that the solid catalyst has high stability.

Fig. 14 The reusability of the as-prepared catalysts for the reduction of MB with NaBH₄.

Fig. 15 Changes of Pd content after each cycle.

4. Conclusion

In conclusion, a facile and efficient approach to prepare Fe₃O₄@MC-Pd microspheres was developed. The resultant composite material showed excellent catalytic performance in reduction of MB and superparamagnetic behavior that enabled magnetic separation and convenient recovery of the nanocatalysts from the reaction mixture. Moreover, the composite catalysts showed good thermal and acid stability, fast regeneration ability, and high cyclic stability, indicating that it could overcome the drawbacks of MOF catalysts (chemical unstability). This study may aid in the further development of effective reduction catalysts for organic reactions. Therefore, the composite catalysts show great potential for industrial application.

Acknowledgements

The authors would like to express their appreciation for research funding provided by the National Natural Science Foundation of China (No. 21374045, 21074049) and the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China (Grant No. J1103307).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17971h

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