Introduction

In recent decades, mesoporous silica materials have been subjected to numerous investigations, due to their applications in many fields such as biotechnology [1], biomedicine [2], optical imaging [3], and catalysis [4]. The great number of advantages such as the tunable structural parameters (pore size, pore volume, and surface area), high surface area, and particularly sufficient silanol groups for surface modification has led to the emerging of this important class of materials as a promising catalyst [5]. Flexible modification regimes with nanocrystals of other inorganic materials are proposed to expand their practical uses in the area of heterogeneous catalytic systems [6,7,8].

Nanoscopic materials have received a major interest in particular, magnetic nanoparticles (MNPs) which have brought out some new kind of catalysts [9,10,11]. In this sense, a lot of attention has been focused on Fe3O4 magnetic nanoparticles thanks to their supermagnetism, high concavity, low Curie temperature, and nontoxicity [12,13,14]. Pure Fe3O4 nanoparticles tend to be oxidation in an air atmosphere and aggregation. Therefore, in the absence of any proper surface coating, they become stuck together, forming large clusters, resulting in limited functional groups and losing the specific properties. Therefore, a proper protection strategy has been employed to stabilize bare iron oxide nanoparticles over long periods chemically [15].

The core–shell composite nanostructures consisting of an Fe3O4 magnetic nanoparticles core and a mesoporous silica shell with great potential in catalytic applications integrate the magnetic properties of Fe3O4 MNPs with the features of mesoporous materials [16,17,18]. Subsequently, it can greatly improve the performance of the catalyst and even generate new synergetic properties [19, 20]. Its surface modification can manipulate the mesoporous silica shell with silanol groups, which can conjugate with a range of different chemical entities [21]. The unique compatibility and consistency between these nanocatalysts and green chemistry are due to the faster and more practical separation methods by the use of an external magnet without any filtration that reduces solvent consumption, so that it can be a savior for environmental and energy challenges. The other noticeable features of these porous magnetic nanocatalysts can refer to chemically stability, low toxicity, being affordable, and high potency and activity [22,23,24,25,26,27].

To the best of our knowledge, coumarin and its derivatives constitute one of the major classes of naturally occurring compounds. A wide variety of biological activities such as anti-HIV [28], anti-oxidant [29], antifungal [30], anthelmintic [31], and antibacterial properties [32] have been reported for these compounds. Furthermore, they have found applications in optical [33], food [34], and cosmetic industries [35]. Coumarin derivatives also occupy an important position in organic synthesis ascribed to their key role in the preparation of pharmacologically active heterocyclic compounds [36]. Accordingly, much effort has been focused towards the development of versatile and efficient procedures for the preparation of coumarins [37, 38]. However, some of these methods suffer from at least one of the following drawbacks: long reaction time, difficult working conditions, tedious work-up procedures, and low yields [39].

Herein, in continuation of our studies in the field of heterogeneous catalysts [40,41,42] and according to the importance of coumarins, we wish to present the preparation and characterization of novel immobilized molybdic acid onto Fe3O4@MCM-41 nanoparticles [Fe3O4@MCM-41-(CH2)3-OMoO3H] as well as studying its catalytic application in the synthesis of coumarin derivatives.

Results and discussion

The new magnetic nanocatalyst Fe3O4@MCM-41-(CH2)3-OMoO3H was prepared according to the protocol shown in Scheme 1. The core–shell structure was obtained by coating of a layer of prepared MCM-41 on the synthesized Fe3O4 core. Then, the surface of MCM-41 was functionalized by (3-chloropropyl)trimethoxysilan molecule. Finally, chloride group replaced by molybdic acid and the desired magnetic nanocatalyst was prepared. Chemical analysis of prepared Fe3O4@MCM-41-(CH2)3-OMoO3H was performed using FT-IR, energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and transmission electron microscopy (TEM).

scheme 1

Figure 1 shows XRD patterns of MCM-41 (a) and Fe3O4@MCM-41-(CH2)3-OMoO3H (b) in low angle (0.7–10). The characteristic peaks of MCM-41 are seen around 2θ of 2.4°, 4.7°, and 5.5°, attributed to the (100), (110), and (200) reflections, confirming the highly ordered and hexagonal mesoporous structure of MCM-41 [43]. In the low angle XRD pattern of Fe3O4@MCM-41-(CH2)3-OMoO3H (Fig. 2), the intensity of the main peak was decreased and its width was increased, and both peaks at 2θ = 4.7° and 5.5° disappeared. These observations indicate that Fe3O4@MCM-41-(CH2)3-OMoO3H still has a hexagonal mesoporous structure but with less order, which might be ascribed to the location of Fe3O4 inside the lattice of MCM-41. Wide angle XRD patterns of Fe3O4@MCM-41-(CH2)3-OMoO3H exhibit six strong diffraction peaks at 2θ = 29.5°, 36.2°, 43.7°, 54.4°, 57.1°, and 63.4°, as can be seen in Fig. 2, which is quite similar to that of pure Fe3O4 [44], which suggest that almost no change occurs in the structure of Fe3O4 after coating by MCM-41 and reveal the inverse cubic spinal structure of iron oxide phrase. The confirming peak showing the presence of molybdate group has appeared in the range of 2θ = 20°–30° which is covered by the broad peak of SiO2.

Fig. 1
figure 1

Low angle XRD patterns of (a) MCM-41 and (b) Fe3O4@MCM-41-(CH2)3-OMoO3H

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Fig. 2
figure 2

Wide angle XRD patterns of Fe3O4@MCM-41-(CH2)3-OMoO3H

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Energy-dispersive X-ray analysis (EDX) was used for the structural characterization of Fe3O4@MCM-41-(CH2)3-OMoO3H. As can be seen in Fig. 3, the components of this catalyst include C, O, Fe, Si, and Mo, which indicate the acceptable concordance with the expectations and also confirm the successful incorporation of molybdate groups.

Fig. 3
figure 3

Energy-dispersive X-ray analysis (EDX) of Fe3O4@MCM-41-(CH2)3-OMoO3H

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The FT-IR spectra of Fe3O4 MNPs, MCM-41, and prepared Fe3O4@MCM-41-(CH2)3-OMoO3H are shown in Fig. 4. A broad peak of the OH group was located at about 3305 cm−1. The bands near 589 cm−1 and 1085 cm−1 can be attributed to the stretching vibration of Fe–O and asymmetric stretching vibration of Si–O–Si, respectively. It can be concluded from the FT-IR pattern of Fe3O4@MCM-41-(CH2)3-OMoO3H that Fe3O4 is coated with MCM-41, because all the characteristic bands of Fe3O4 as well as MCM-41 are found in the spectrum of the catalyst. On the other hand, the band observed at 2930 cm−1 may be assigned to stretching vibration of a C–H band. Furthermore, the presence of molybdic acid is confirmed by the band at 995 cm−1, which is related to the stretching vibration of an Mo–O band.

Fig. 4
figure 4

FT-IR spectra of (a) Fe3O4 MNPs, (b) MCM-41, and (c) Fe3O4@MCM-41-(CH2)3-OMoO3H

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Figure 5 shows the representative TEM images of the Fe3O4@MCM-41-(CH2)3-OMoO3H nanocatalyst. As can be seen, most of the catalyst is nearly spherical, so that dark magnetic nanoparticle core is surrounded by a lighter MCM-41. Based on TEM images, the particle diameter is about 80–120 nm with almost uniform distribution of magnetic nano particles.

Fig. 5
figure 5

TEM images of Fe3O4@MCM-41-(CH2)3-OMoO3H

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After full characterization of the catalyst, it has been successfully applied to the synthesis of coumarin derivatives 4 by the three-component reaction of 5,7-dihydroxycoumarin derivatives 1, aryl aldehydes 2, and benzamide (3) (Scheme 2).

scheme 2

The reaction efficiency was highly dependent on the different parameters including the amount of catalyst, solvent, and reaction temperature. Therefore, to get the best experimental conditions, we investigated the model reaction of equimolar quantities of benzamide, benzaldehyde, and 5,7-dihydroxy-4-methylcoumarin. The reaction in the absence of the catalyst was inefficient, and only about 2–5% conversion was obtained even after a long time. Then, we attempted with different amounts of Fe3O4@MCM-41-(CH2)3-OMoO3H as a catalyst under various conditions. Considering the obtained results, the desired reaction took place efficiently using 0.007 g of the catalyst in ethyl acetate under reflux conditions (Table 1, entry 11). After establishing the optimal conditions, to evaluate the generality and versatility of the introduced procedure, the reaction was loaded with a wide range of aromatic aldehydes and 5,7-dihydroxycoumarins. As shown in Table 2, in all cases, the corresponding products were obtained in good-to-excellent yields.

Table 1 Optimization of the model reaction
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Table 2 Synthesis of substituted coumarins 4 using Fe3O4@MCM-41-(CH2)3-OMoO3H (Scheme 2)
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The proposed mechanism of formation of the product 4 is given in Scheme 3. First, the reaction occurs between benzamide and aryl aldehyde to afford the adduct 5. Here, it is thought that the acid nanocatalyst can activate the carbonyl group of aldehyde and accelerate the condensation reaction. It is reasonable to assume that, in the presence of nanocatalyst, the adduct 5 to be attacked by C-8 of substituted coumarin which rearranges into product 4. It should be noted that the existing steric effect at C-6 causes the hydroxyl groups at the C-5 and C-7 positions directly are attack by positions C-8, so the reaction is regioselective [45].

scheme 3

Fe3O4@MCM-41-(CH2)3-OMoO3H for the one-pot, three-component reaction of benzamide, benzaldehyde, and 5,7-dihydroxy-4-methylcoumarin was checked by separating the catalyst, washing it with MeOH and drying it at 110 °C. As outlined in Fig. 6, the catalyst exhibited no considerable decrease in activity after five cycles.

Fig. 6
figure 6

Reusability of Fe3O4@MCM-41-(CH2)3-OMoO3H in the reaction of benzamide, benzaldehyde, and 5,7-dihydroxy-4-methylcoumarin

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To test the stability of the catalyst structure, the recycled nanocatalyst was examined by XRD analysis. The diffraction patterns and relative intensities of all peaks matched well to those of the primary catalyst (Fig. 7).

Fig. 7
figure 7

(a) Wide angle and (b) low angle XRD patterns of recycled catalyst

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Conclusion

In this research, Fe3O4@MCM-41-(CH2)3-OMoO3H as a novel nanocatalyst was introduced and characterized by FT-IR, XRD, SEM, EDS, and vibrating sample magnetometer (VSM) analysis. This nanocatalyst demonstrated extremely high reactivity in the environmentally friendly synthesis of coumarin derivatives in high yield and purity. This series of coumarins were designed by the reaction of aryl aldehydes, benzamide, and 5,7-dihydroxycoumarin derivatives. The catalyst can be recovered and reused several times with no decrease in activity.

Experimental

All chemicals used in this research were purchased from Fluka and Merck chemical companies. The obtained products were identified by comparing their spectral data and physical properties with their reported data in the references. The monitoring of the reaction progress and the purity of the compounds were accomplished using TLC performed with silica gel SIL G/UV254 plates. Melting points were determined by an electrothermal KSB1N apparatus. 1H NMR spectra were recorded in DMSO-d6 on a Bruker Avance UltraShield 400 MHz instrument spectrometers and 13C NMR spectra were recorded at 100 MHz. IR spectra were obtained with a JASCO FT-IR/680 instrument spectrometer using KBr pellets. X-ray powder diffraction (XRD) patterns were recorded using a Bruker AXS (D8 Advance) X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The measurement was made in 2θ ranging from 10° to 80° at the speed of 0.05° min−1. X-ray spectroscopy (EDX) was obtained using TESCAN vega model instrument. Transmission electron microscopy (TEM) analysis was performed on a JEM 3010 electron microscope (JEOL, Japan) with an accelerating voltage of 300 kV.

Procedure for the preparation of Fe3O4 MNPs

For the synthesis of Fe3O4 MNPs, 2.5 g FeCl2·4H2O and 5 g FeCl3·6H2O (dissolved in 50 cm3 of distilled water) were added to 6 cm3 concentrated HCl (12 M), degassed with nitrogen gas, heated to 80 °C, and stirred for 10 min. Then, 10 cm3 NaOH solution (1.5 M) was added dropwise to the mixture until the brown color solution turned out to be black. The magnetite precipitate (Fe3O4 NPs) was separated by an external magnet and washed using double distilled water and ethanol until the pH < 7.5 [46].

Procedure for the synthesis of MCM-41

According to the current method in the literature [47], 4 g cetyltrimethylammonium bromide (CTAB) was dissolved in 1250 cm3 ammonium hydroxide (1.1 M). Then, 20 cm3 tetraethoxysilan (TEOS) was added to the prepared solution under vigorous stirring for 30 min. The resulting precipitate was filtered and washed with distilled water, and dried at room temperature and calcinated at 560 °C for 48 h [48].

Preparation of Fe3O4@MCM-41-(CH2)3Cl

MNPs-Fe3O4 (2.0 g) was dispersed to a mixture of ethanol and distilled water (70:10 cm3) by sonication. Then, 5 cm3 aqueous ammonia (25 wt%) and 1.72 g MCM-41 were slowly added under vigorous stirring at ambient temperature for 24 h. The prepared Fe3O4@MCM-41 was separated and washed with ethanol and distilled water and dried at 80 °C. In the next step, in a round-bottomed flask equipped with a condenser and a magnetic stirrer, 3.0 g Fe3O4@MCM-41 and 1.5 g (3-chloropropyl)triethoxysilan was suspended in 40 cm3 dry toluene at 100 °C under N2 atmosphere and stirred for 24 h. Then, the obtained Fe3O4@MCM-41-(CH2)3Cl was collected by an external magnet and washed with CH2Cl2 and dried in an oven at 50 °C [49].

Preparation of Fe3O4@MCM-41-(CH2)3-OMoO3H

In the final step, Fe3O4@MCM-41-(CH2)3-OMoO3H was prepared according to the following procedure. A flask was charged with 0.80 g Fe3O4@MCM-41-(CH2)3Cl and dispersed in 40 cm3 DMSO in an ultrasonic bath for 30 min. Then, 1.08 g dehydrated sodium molybdate salt was added to the mixture, degassed and stirred under reflux condition. After 24 h, the reaction mixture was collected and washed with DMSO and distilled water and dried. Finally, the brown precipitate was stirred in the presence of 40 cm3 HCl (0.1 N) for 5 h. The obtained nanocatalyst was separated by a normal magnet and washed with distilled water and dried overnight under vacuum at 80 °C.

General procedure for the synthesis of coumarins 4

Fe3O4@MCM-41-(CH2)3-OMoO3H (0.007 g) was added to a mixture of 5,7-dihydroxycoumarins (1 mmol), aryl aldehyde (1 mmol), and benzamide (1 mmol) in 5 cm3 ethyl acetate and the mixture was stirred under reflux for the requisite time. After completion of the reaction, the catalyst was collected easily by an external magnet and the solvent was removed. Further purification was achieved by recrystallization from ethanol.

Method for the recovery of the catalyst

Fe3O4@MCM-41-(CH2)3-OMoO3H (0.007 g) was added to a stirred solution of 5,7-dihydroxy-4-methylcoumarin (1 mmol), benzaldehyde (1 mmol), and benzamide (1 mmol) in 5 cm3 refluxing ethyl acetate. After completion of the reaction, the catalyst was separated by an external magnet and washed with methanol, dried, and reused in subsequent runs.