Introduction

In the recent years, the yolk-shell (YS) structured nanomaterials have demonstrated a new type of structures containing a void between core and shell. These materials have received increasing attention owing to tunable their physiochemical properties and also high capability in the adsorption and catalytic processes1,2,3,4,5,6,7,8,9. These nanomaterials have a lot of application in areas of data storage, catalysis and environmental remediation5,6,10,11,12. Due to the aforementioned notes and also widespread applications of YSs in green chemistry, various methods such as Kirkendall, etching, ship in bottle and Ostwald have been employed for the synthesis of these materials13,14,15,16. Among these, selective etching is very interested in between researchers17,18. YSs with magnetite core and PMO shell (Mag@YS-PMOs) have attracted much research attention because of their combined properties of YSs, magnetic materials and ordered mesoporous structures12,19,20,21,22. The Mag@YS-PMOs have the advantages of magnetic nanoparticles such as easy magnetic separation, high dispersion ability in aqueous media as well as high chemical reactivity and stability. In addition, these have also the advantages of PMOs such as highly-ordered mesostructure, excellent loading of uniformly distributed organic functions in their framework and high moisture stability12,19,23. These characteristics make Mag@YS-PMOs as promising applicant for supporting metals in different chemical processes5,6,24,25.

On the other hand, aromatic amines are very important in herbicide, dye, agrochemical and pesticide industries26,27,28. In addition, they are substrates for different intermediates such as diazonium salts, isocyanate, azo and amide molecules29,30. Recently, sodium borohydride, NaBH4, has been suggested as a new fuel source for supplying hydride ions to reduce nitroarene compounds to the corresponding amines under mild conditions. However, NaBH4 exhibits limited capability in the absence of additives31,32,33,34,35. Therefore, this reagent is usually used in the presence of metallic catalysts. Another important and conventional method for preparing the aromatic amines is the reduction of corresponding nitroarenes via catalytic hydrogenation36,37. The most metallic complexes and metallic nanoparticles applied for catalytic reduction of nitroarenes are based on Rh, Ru, Pt, Pd, Au, Cu, Ir and Ni38,39,40. Some of recently developed catalytic systems in this matter are Fe3O4/Ni MNPs41, Rh–Fe3O4 heterodimer42, Cu/Fe2O4–G43, graphene-Fe3O444, Au–GO45, Fe3O4@GO46, Ag@Ni47 and NiFe2O4@Cu48. However, the homogeneous protocols for reduction of nitroarenes have different disadvantages such as separation of catalyst and product. Therefore, it is still necessary to design an effective catalytic system resolving these issues.

According to the advantages of Mag@YS-PMO-based nanocatalysts as well as the importance of reduction of nitroarenes in green chemistry, in continuation of our previous works49,50,51, herein, a YS nanomaterial with magnetite core and PMO shell-supported palladium species (Fe3O4@YS-Ph-PMO/Pd) is synthesized through sol–gel mediated hydrolysis and co-condensation of 1,4-bis(triethoxysilyl)benzene (BTEB) around Fe3O4@SiO2 cores using CTAB surfactant followed by treatment with palladium acetate (Fig. 1). Furthermore, the physiochemical properties of the synthesized Fe3O4@YS-Ph-PMO/Pd nanocatalyst were studied by using FT-IR, TGA, PXRD, SEM, TEM and VSM techniques. After characterization, the catalytic efficiency of this magnetic nanomaterial was investigated in the green reduction of nitroarenes under moderate conditions.

Figure 1
figure 1

Preparation of Fe3O4@YS-Ph-PMO/Pd.

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Experimental

General

All chemicals and reagents such as iron (II) chloride tetrahydrate, iron (III) chloride hexahydrate, ammonia, 1,4-bis(triethoxysilyl)benzene, NaBH4, cetyltrimethylammonium bromide (CTAB) and all applied nitroarenes and alcohols were purchased from Fluka, Merck and Aldrich companies. Solvents were dried and purified following standard procedures. The characterization of the materials was conducted using instruments previously reported52,53. The purity determination of the products and reaction monitoring were carried out by using TLC on silica gel polygram SILG/UV 254 plates.

Preparation of Fe3O4@YS-Ph-PMO

For the preparation of Fe3O4@YS-Ph-PMO nanomaterial, firstly, Fe3O4 and silica coated magnetite nanoparticles (Fe3O4@SiO2) were synthesized according to known procedures23. Then, 100 mg of Fe3O4@SiO2 microspheres were added into a flask containing EtOH (60 mL) and H2O (80 mL). This mixture was homogenized for 25 min under ultrasound irradiations. Next, CTAB (140 mg) and NH3 (25%, 2 mL) were added while stirring at RT for 30 min. After that, BTEB (0.3 mL) was added and it was further stirred at RT for 2 h. This mixture was heated statically at 100 °C for 24 h. The obtained Fe3O4@SiO2@CTAB@Ph-PMO product was dispersed in a basic solution (H2O:Na2CO3, 80:4). The resulting mixture was heated to 50 °C for 4 h to eliminate the SiO2 shell. The CTAB surfactant was successfully eliminated by refluxing of as-made material in an acidic EtOH solution (EtOH:HCl 100:2). The final product was dried at 70 °C for 12 h and denoted as Fe3O4@YS-Ph-PMO.

Preparation of Fe3O4@YS-Ph-PMO/Pd

For this, Fe3O4@YS-Ph-PMO (0.4 g) was added to an aqueous solution of palladium acetate (0.05 M, 14 mL). Then, this mixture was stirred at RT for 24 h. The product was magnetically collected, washed completely with H2O, dried at 70 °C for 7 h and denoted as Fe3O4@YS-Ph-PMO/Pd.

Procedure for the reduction of nitrobenzenes

For this, nitrobenzene (1 mmol), Fe3O4@YS-Ph-PMO/Pd (0.9 mol %) and H2O (10 mL) were added into a reaction vessel. Then, an aqueous solution of NaBH4 (3 mmol) was added while stirring at RT. After completion of reaction, the catalyst was magnetically separated and the amine product was obtained after extraction with EtOAc and evaporation of solvent. The IR and NMR spectra of prepared aminobenzenes are available in the supporting information.

Results and discussion

The preparation of Fe3O4@YS-Ph-PMO/Pd nanocatayst with yolk-shell structure is shown in Fig. 1. At first, the magnetic silica (Fe3O4@SiO2) was prepared by chemical modification of Fe3O4 nanoparticles with tetramethoxysilane (TMOS). Then, 1,4-bis(triethoxysilyl)benzene (BTEB) was hydrolyzed and co-condensed on the Fe3O4@SiO2 spheres in the presence of CTAB surfactant in a basic ammonia–water–ethanol solution through sol–gel process. Then, the silica layer was removed in an aqueous solution of Na2CO3 through an etching process. After that, the CTAB surfactant was removed by a Soxhlet apparatus to give a yolk-shell structured material called Fe3O4@YS-Ph-PMO. The resulting material was then treated with a sub-stoichiometric amount of Pd(OAc)2 in water to produce the Fe3O4@YS-Ph-PMO/Pd nanocatalyst.

The Fourier transform infrared (FT-IR) spectra of prepared materials are shown in Fig. 2. For all samples, the Fe–O bond is cleared at 582 cm−1. The band about 3552 cm−1 is due to the O–H bonds of material surface. For Fe3O4@SiO2, Fe3O4@SiO2@CTAB@YS-Ph-PMO and Fe3O4@YS-Ph-PMO/Pd, the peaks at 1122 and 925 cm−1 are attributed to Si–O-Si bonds. The bands at 2924 and 2844 cm−1 are related to the C–H vibrations of CTAB surfactant (Fig. 2C). Interestingly, for Fe3O4@YS-Ph-PMO/Pd, the latter peaks are eliminated (Fig. 2D), indicating the successful removal of CTAB surfactant during extraction process. For Fe3O4@SiO2@CTAB@YS-Ph-PMO and Fe3O4@YS-Ph-PMO/Pd, the peaks observed at 3100 and 1620 cm−1 are, respectively, correspond to C–H and C=C vibrations of phenyl rings. These confirm the successful formation of Ph-PMO shell on magnetite NPs. Notably, the Fe–O absorption peaks of Fe3O4 and Fe3O4@SiO2 exhibited a slight red shift compared to Fe3O4@SiO2@CTAB@Ph-PMO and Fe3O4@YS-Ph-PMO/Pd nanomaterials. This shift is in line with the Bouguer-Beer-Lambert (BBL) law, where spectral positions correspond to sample thickness and absorbing entity concentration. The presence of silica, CTAB, and PMO layers on Fe3O4 nanoparticles contributes to this red shift, indicating a modified chemical environment surrounding the nanoparticles54,55,56. Moreover, an increase in layer thickness enhances this effect57,58,59.

Figure 2
figure 2

FT-IR spectra of (A) Fe3O4, (B) Fe3O4@SiO2, (C) Fe3O4@SiO2@CTAB@Ph-PMO and (D) Fe3O4@YS-Ph-PMO/Pd.

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Figure 3 shows the wide-angle powder X-ray diffraction (PXRD) patterns of Fe3O4, Fe3O4@SiO2, Fe3O4@YS-Ph-PMO and Fe3O4@YS-Ph-PMO/Pd nanomaterials. The PXRD patterns of all the materials showed six sharp peaks at 2θ of 30.38, 35.65, 43.34, 53.9, 57.40 and 62.98°, corresponding to the Miller indices of 220, 311, 400, 422, 511, and 440, respectively, which is completely matched with that of the Fe3O4 standard sample (JCPDS file No. 19-0629)60. This finding proves that the crystalline structure of the magnetic iron oxide NPs is preserved during the modification processes. Additionally, a continuous decrease in the intensity of Fe3O4 peaks was observed, indicating the successful modification of magnetic iron oxide with the organic and inorganic species. After coating with silica, a new and broad peak at 2θ = 20–25 appeared, which is attributed to the presence of amorphous silica (Fig. 3B). This peak disappeared in both Fe3O4@YS-Ph-PMO and Fe3O4@YS-Ph-PMO/Pd (Fig. 3C and D), confirming the successful removal of the silica layer through the etching process.

Figure 3
figure 3

PXRD patterns of (A) Fe3O4, (B) Fe3O4@SiO2, (C) Fe3O4@YS-Ph-PMO and (D) Fe3O4@YS-Ph-PMO/Pd.

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The low-angle powder X-ray diffraction (PXRD) pattern of the Fe3O4@YS-Ph-PMO/Pd is shown in Fig. 4. This illustrates a broad peak at 2θ = 2.2°, which is characteristic of ordered mesoporous structures. This pattern confirm well formation of Ph-PMO shell on magnetite NPs.

Figure 4
figure 4

Low-angle PXRD pattern of Fe3O4@YS-Ph-PMO/Pd.

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Thermal stability of Fe3O4@YS-Ph-PMO/Pd was studied by using thermal gravimetric analysis (TGA) (Fig. 5). This showed a low weight loss at temperature below 100 °C, corresponding to removal of adsorbed solvents. The second weight loss observed between 100 to 450 °C is related to the elimination of the remaining CTAB surfactant. The main weight loss, cleared at 451–620 °C, is corresponded to the removal of organic (phenylene) moieties incorporated in the mesoporous shell. These data successfully prove the high thermal stability of the designed nanocatalyst.

Figure 5
figure 5

TGA of Fe3O4@YS-Ph-PMO/Pd.

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The magnetic properties of materials were investigated by vibrating sample magnetometer (VSM) (Fig. 6). The saturated magnetization values of the Fe3O4 (Fig. 6A), Fe3O4@SiO2 (Fig. 6B) and Fe3O4@YS-Ph-PMO/Pd (Fig. 6C) were found to be 65, 30 and 14 emu g−1, respectively. The decrease in magnetic properties following the modification processes is attributed to the well coating of SiO2 and PMO shells around Fe3O4 NPs. The magnetic separation capability of the Fe3O4@YS-Ph-PMO/Pd nanocomposite was also evaluated by introducing an external magnet near the reaction vessel, as depicted in Fig. 6b. As demonstrated, these nanomaterials can be efficiently collected by using an external magnet within seconds, confirming the high magnetic properties of the designed catalyst, rendering it readily recoverable.

Figure 6
figure 6

(a) VSM diagrams of Fe3O4 (A), Fe3O4@SiO2 (B) and Fe3O4@YS-Ph-PMO/Pd (C) nanomaterials and (b) magnetic separation ability of Fe3O4@YS-Ph-PMO/Pd nanocatalyst.

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The scanning electron microscopy (SEM) was performed to study the morphology of the particles at different steps of nanocatalyst preparation. According to this analysis, a uniform spherical morphology was observed for all prepared nanomaterials with an increase in size observed at each step (Fig. 7). The progressive increase in particle size after each step confirms the successful formation of the shell and modification of magnetite nanoparticles, as outlined in Fig. 1.

Figure 7
figure 7

SEM images of (A) Fe3O4, (B) Fe3O4@SiO2, (C) Fe3O4@YS-Ph-PMO and (D) Fe3O4@YS-Ph-PMO/Pd.

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The TEM image of the Fe3O4@YS-Ph-PMO/Pd nanocatalyst showed spherical particles with a black core (magnetite NPs) and a grey shell (PMO layer) for the designed nanocomposite (Fig. 8). Notably, recently, the similar TEM images have been reported for a number of yolk-shell structured magnetic nanocomposites61,62.

Figure 8
figure 8

TEM image of the Fe3O4@YS-Ph-PMO/Pd nanocatalyst.

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The Fe3O4@YS-Ph-PMO/Pd nanocatalyst was then employed in the reduction of nitroarenes in the presence of NaBH4 (Table 1). The effect of different parameters was studied in the reduction of nitrobenzene as a reaction model. As shown, in the absence of catalyst, along with 3 mmol of NaBH4, no reduction was observed (Table 1, entry 1). However, after addition of the catalyst, the reaction proceeded successfully, and the highest yield achieved using 0.9 mol% of the Fe3O4@YS-Ph-PMO/Pd (Table 1, entry 3). Next, the effect of different solvents including methanol, ethanol and water was tested. As depicted in Table 1, the use of MeOH, EtOH, aqueous methanol or ethanol, as well as solvent-free media, resulted in low to moderate yields (Table 1, entries 5–9). Pleasantly, in aqueous media at RT, the reaction was completed and excellent yield of aniline was obtained. To investigate the neat effect of Pd, the efficiency of Fe3O4@YS-Ph-PMO/Pd was compared with Fe3O4 and Fe3O4@YS-Ph-PMO nanomaterials. This demonstrated that when using Pd-free Fe3O4 and Fe3O4@Ph-PMO materials, the reaction did not progress, and only a trace conversion (< 4%) was observed (Table 1, entries 10 and 11). This confirms that the reduction process is catalyzed by supported palladium species. Based on the aforementioned results, NaBH4 (3 mmol), H2O solvent (3 mL), Fe3O4@YS-Ph-PMO/Pd catalyst (0.9 mol %) and RT were chosen as the optimum conditions (Table 1, entry 3).

Table 1 Reduction of nitrobenzene in the presence of Fe3O4@YS-Ph-PMO/Pda.
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The efficiency and the scope of the Fe3O4@YS-Ph-PMO/Pd nanocatalyst were next studied using a variety of nitrobenzenes (Table 2). As shown, all nitroarenes deliver corresponding anilines in high yields. Importantly, in all cases, it was found that the aniline derivatives were the only products of the reactions. These results demonstrate high selectivity of the designed catalytic system.

Table 2 Reduction of different nitrobenzenes using Fe3O4@YS-Ph-PMO/Pd catalysta.
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Recoverability and reusability of the Fe3O4@YS-Ph-PMO/Pd nanocatalyst in the reduction of nitrobenzene were also examined. For this, after completion of the reaction, the Fe3O4@YS-Ph-PMO/Pd catalyst was magnetically separated, washed and reused in the next run. It was found that the Fe3O4@YS-Ph-PMO/Pd catalyst can be recovered and reused for at least 11 times with no important decrease in efficiency (Fig. 9).

Figure 9
figure 9

Recoverability and reusability of the Fe3O4@YS-Ph-PMO/Pd nanocatalyst in the reduction of nitrobenzene.

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A leaching experiment was then accomplished on the model reaction to investigate the leaching behavior of the palladium species under applied conditions. For this, after a conversion of about 40%, the Fe3O4@YS-Ph-PMO/Pd catalyst was magnetically separated and the residue mixture was allowed to progress under the optimized conditions. Notably, after 140 min, no further product was observed indicating that the removal of the catalyst resulted in a complete stop of the conversion of nitrobenzene to aniline (Fig. 10). These results confirm no leaching of palladium species and also demonstrate the high stability of the designed catalyst under applied conditions.

Figure 10
figure 10

Result of the leaching test in the reduction of nitrobenzene in the presence of Fe3O4@YS-Ph-PMO/Pd nanocatalyst.

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The catalytic performance of Fe3O4@YS-Ph-PMO/Pd was also compared with former heterogeneous catalysts in the reduction of nitrobenzene (Table 3). The result showed that the efficiency of Fe3O4@YS-Ph-PMO/Pd is much better than previous catalytic systems, particularly in terms of reaction temperature and recovery times.

Table 3 Comparison of catalytic activity of Fe3O4@YS-Ph-PMO/Pd nanocatalyst with previously reported catalytic systems in the reduction of nitrobenzene.
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Conclusion

In this work, the preparation, characterization and application of a new nanocatalyst named Fe3O4@YS-Ph-PMO/Pd were developed. The FT-IR and TG analyses clearly demonstrated high stability and well incorporation/immobilization of expected organic and inorganic moieties onto/into prepared nanomaterial. The SEM and TEM images showed a spherical morphology for the designed catalyst. The VSM analysis confirmed well-magnetic properties of the catalyst. The wide-angle PXRD analysis confirmed high stability of crystalline structure of magnetite NPs during catalyst preparation steps. The Fe3O4@YS-Ph-PMO/Pd was powerfully used in the reduction of nitrobenzenes giving corresponding anilines in high yield and selectivity. The leaching and recovery experiments confirmed that the designed catalyst operate in a heterogeneous manner. Other advantages of this methodology were the use of water as green solvent, performing reactions at RT, short reaction times, clean conditions, as well as high recoverability, durability and stability of the designed catalyst. Some applications of this catalytic system in other organic processes are underway in our laboratory.