Introduction

Magnetic nanoparticles (NPs) have wide applications in various fields of medicine, catalysis, environment, materials science and biotechnology due to their unique magnetic properties and ability to respond to external magnetic fields. Therefore, in recent years, many researchers have focused on making different types of these NPs. The cobalt, iron, nickel elements and their chemical compounds are precursors that commonly used to prepare magnetic NPs1,2,3,4,5,6,7,8,9. The use of nickel and cobalt is limited due to their toxicity and high tendency to oxidize. Among these, magnetic iron oxide NPs, especially superparamagnetic Fe3O4 NPs, have been considered by researchers due to their non-toxicity, good biocompatibility and good magnetic properties10,11. Magnetic NPs, despite having many advantages, suffer from a series of inherent disadvantages such as high chemical activity and a high tendency to aggregate due to their high surface area. Therefore, the development of effective strategies to improve the stability of these NPs is an essential need. Coating the surface of Fe3O4 NPs is one of the effective methods to stabilize them. The species including organic polymers such as dextran, chitosan, polyethylene glycol, polyaniline; organic surfactants such as CTAB, DTAB, DPB and SOS; metals such as Au and Ag; mineral oxides such as carbon and silica; biological molecules and structures such as ligands/receptors, peptides, liposomes have been used as coating shell for Fe3O4 NPs to form core–shell structured MNPs10,12,13,14,15,16,17,18,19,20,21,22,23,24. Some of the recently developed reports in this matter are dextran-coated Fe3O4 MNPs12, Fe3O4/chitosan13, Fe3O4/CTAB17, Fe3O4/Aun.Ac-FA NCPs18, Fe3O4@Ag19, Fe3O4@C20, Fe3O4-OS-SO3H10 and DOX–Fe3O4–TSL24. Among the various protective shells, silica has attracted more attention between many researchers. The silica shell can reduce the magnetic dipole adsorption between nanoparticles, helping to diffuse magnetic NPs in aqueous and organic environments. Also due to poor chemical permeability, silica can prevent the destruction of MNPs in different chemical environments. Moreover, the abundant silanol groups on the silica surface provide suitable conditions for different types of modification10,25,26,27,28. Some of recently developed magnetic nanostructures with silica shells are Fe3O4@BOS@SB/In29, Fe3O4@SiO2@PMO30, Re–SiO2–Fe3O431, Mag@Ti-NOS32, Fe3O4@RF@void@PMO(IL)/Cu33, Fe3O4@SiO2@propyl‐ANDSA34 and Fe3O4@Au@mSiO2-dsDNA/DOX35. The MNPs with silica shell can be used as electrode36, adsorbent37, sensor38, catalyst support10,29,32,33,34,39,40, ion exchanger41 and so on. Especially, in the field of catalysis, magnetic silicas with a core–shell structure have been considered by many researchers due to their magnetic recoverability, high hydrophobicity and ability to modify their surface42,43,44,45,46. Some of recently developed nanocatalysts are Fe3O4@SiO2/Schiff-base/Cu(II)42, Fe3O4@SiO2–EDTA–Ni43, Fe3O4@SiO2-IL44, Fe3O4@SiO2/Ru-WOx45 and IL-Fe3O4@SiO246.

Transition metal catalysts have been useful in modern synthetic organic chemistry due to their diverse reactivity in enabling various molecular conversions47. The reactions performed using these catalysts can be classified into three groups based on the role of the metal: 1. catalytic reactions based on the oxidation/reduction cycle of the transition metal, 2. catalytic reactions in which the transition metal acts as a Lewis acid and 3. reactions catalyzed by coinage metals (Cu, Ag and Au)48. In recent years, silver metal has been more considered by researchers as an effective transition metal catalyst, due to the processes catalyzed by silver perform under mild conditions and silver is cheaper and environmentally friendly than many rare metals (Pd, Pt, Rh, Ru, etc.). Among the various silver species, silver carbonate (Ag2CO3) can be employed as a Lewis acid, an inorganic base and a good oxidant in different organic reactions. Also, Ag2CO3 can be coordinated with various unsaturated systems (carbonyls, imines, isocyanides, alkynes and alkenes) and create very stable intermediates in the course of various processes48,49,50,51,52,53.

On the other hand, the Knoevenagel condensation of active methylene and carbonyl compounds is among the most commonly used methods in organic chemistry for the synthesis of low-electron olefins. In recent years, many catalysts were used for Knoevenagel condensation, in which heterogeneous ones have received much attention due to the easy recovery of the catalyst and also the easy separation of the products54,55,56,57,58,59,60,61. Some of the recently reported heterogeneous catalytic systems are Fe3O4@OS-NH254, CAU-1-NH255, MgOS_40056, PMO-IL-NH257, IL–H2O–DABCO58, MgO/ZrO259, CoFe2O460 and LDH-ILs-C1261. In view of the above, especially the advantages mentioned for Ag2CO3, our motivation in this study is the design and preparation of a novel core–shell structured MS/Ag2CO3 nanocomposite as a powerful, effective, recyclable and reusable nanocatalyst for the Knoevenagel condensation.

Experimental section

Preparation of MS/Ag2CO3

For this, the Fe3O4 NPs (0.6 g)29 were dispersed in deionized water (25 mL) and EtOH (75 mL) for 0.5 h. After adding NH3 (3.5 mL, 25% wt), the mixture was stirred at RT for 20 min. Then, tetramethoxysilane (TMOS, 0.5 mL) was added and stirring was continued at RT for 16 h. After that, the resulting solid material was magnetically collected, washed with deionized water and EtOH, dried at 80 °C for 6 h and defined as MS. For preparation of MS/Ag2CO3, MS (0.6 g) was well-dispersed in deionized water (30 mL). After 0.5 h, NaHCO3 (2.5 mmol) was added and stirring was continued at RT for 2 h. Then, AgNO3 (5 mmol) was added under lightless conditions. After that, the reaction combination was stirred for 12 h in an ice bath. The resulted material was collected using a magnetic field, washed with deionized water, dried and designated as MS/Ag2CO3.

Procedure for the Knoevenagel reaction using MS/Ag2CO3

For this, MS/Ag2CO3 (0.015 g), ethyl cyanoacetate (1 mmol) and aldehyde (1 mmol) were added in a reaction flask and the resulted mixture was sonicated at 60 °C under solvent-free conditions. After completing of the process, EtOH (10 mL) was added and MS/Ag2CO3 was magnetically separated. Finally, the solvent was evaporated and pure Knoevenagel products were resulted after recrystallization in EtOH and n-hexane solvents.

IR, 1H NMR and 13C NMR data of Knoevenagel products

(E)-ethyl 2-cyano-3-(2-nitrophenyl)acrylate (Table 2, entry 2)

Pale yellow solid; yield: 95%; M.P.: 98–100 °C (ref: 102 °C62), IR (KBr, cm−1): 3097 (=C–H, stretching vibration sp2), 2989 (C–H, stretching vibration sp3), 2221 (CN, stretching vibration), 1723 (C=O, stretching vibration), 1565, 1462 (C=C, Ar stretching sp2), 1264 (C–O, stretching vibration), 1529, 1358 (NO2, stretching vibration). 1H NMR (300 MHz, DMSO): δ (ppm) 1.34 (t, 3H, J = 6.0 Hz), 4.38 (q, 2H), 7.84–7.89 (m, 1H), 7.93–8.02 (m, 2H), 8.33 (d, 1H, J = 9.0 Hz), 8.86 (s, 1H). 13C NMR (75 MHz, DMSO): δ (ppm) 14.4, 63.1, 107.8, 114.7, 125.7, 128.7, 131.0, 132.9, 135.2, 147.7, 155.6, 161.4.

(E)-ethyl 2-cyano-3-(4-nitrophenyl)acrylate (Table 2, entry 3)

Pale yellow solid; yield: 97%; M. P.: 170–171 °C (ref: 168 °C63), IR (KBr, cm−1): 3095 (=C–H, stretching vibration sp2), 2990 (C–H, stretching vibration sp3), 2226 (CN, stretching vibration), 1718 (C=O, stretching vibration), 1593, 1469 (C=C, Ar stretching sp2), 1259 (C–O, stretching vibration), 1510, 1350 (NO2, stretching vibration). 1H NMR (300 MHz, DMSO): δ (ppm) 1.31 (t, 3H, J = 6.9 Hz), 4.33 (q, 2H), 7.82 (d, 2H, J = 13.2 Hz), 8.00 (d, 2H, J = 10.80 Hz), 8.40 (s, 1H). 13C NMR (75 MHz, DMSO): δ (ppm) 14.4, 62.9, 103.7, 115.8, 127.7, 131.0, 132.9, 133.0, 154.3, 162.1.

(E)-ethyl 3-(2-chlorophenyl)-2-cyanoacrylate (Table 2, entry 4)

White solid; yield: 92%; M.P.: 52–54 °C (ref: 52–54 °C64), IR (KBr, cm−1): 3072 (=C–H, stretching vibration sp2), 2955 (C–H, stretching vibration sp3), 2229 (CN, stretching vibration), 1718 (C=O, stretching vibration), 1619, 1475 (C=C, Ar stretching sp2), 1264 (C–O, stretching vibration). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.43 (t, 3H, J = 7.2 Hz), 4.43 (q, 2H), 7.40–7.47 (m, 1H), 7.50–7.55 (m, 2H), 8.24 (d of d, 1H, J1 = 4.6 Hz, J2 = 1.6 Hz,), 8.71 (s,1H). 13C NMR (100 MHz, CDCl3): δ (ppm) 14.2, 62.9, 106.2, 114.8, 127.5, 129.8, 129.9, 130.3, 133.7, 136.4, 151.1, 161.8.

(E)-ethyl 2-cyano-3-(p-tolyl)acrylate (Table 2, entry 7)

White solid; yield: 91%; M.P.: 95–97 °C (ref: 93–94 °C62), IR (KBr, cm−1): 3025 (=C–H, stretching vibration sp2), 2961 (C-H, stretching vibration sp3), 2217 (CN, stretching vibration), 1725 (C=O, stretching vibration), 1604, 1515 (C=C, Ar stretching sp2), 1261 (C–O, stretching vibration). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.42 (t, 3H, J = 7.2 Hz), 2.46 (s, 3H), 4.42 (q, 2H), 7.33 (d, 2H, J = 8.4 Hz), 7.93 (d, 2H, J = 8.4 Hz), 8.25 (s, 1H). 13C NMR (100 MHz, CDCl3): δ (ppm) 14.2, 22.0, 62.6, 101.5, 115.8, 128.8, 130.1, 131.3, 144.7, 155.0, 162.8.

Results and discussion

The synthesis of MS/Ag2CO3 is shown in Fig. 1. Firstly, the Fe3O4 NPs were modified with a silica shell to give MS NPs. Then, the MS NPs were treated with NaHCO3 and AgNO3 to deliver the desired MS/Ag2CO3 nanocomposite.

Figure 1
figure 1

Preparation of the MS/Ag2CO3 nanocomposite.

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The FT-IR spectra of Fe3O4, MS and MS/Ag2CO3 are shown in Fig. 2. For all samples, the characteristic signals at 3397 and 583 cm−1 are, respectively, due to O–H and Fe–O bonds. Also, the band cleared at 1662 cm−1 is due to bending vibration of O–H bonds32,65. For MS and MS/Ag2CO3, the signals at 823 and 1069 cm−1 are assigned to Si–O-Si, confirming the construction of SiO2 shell around the Fe3O4 core (Fig. 2b,c)26. Importantly, for the MS/Ag2CO3 nanocomposite, the observed peaks at 705, 884, 1381 and 1448 cm−1 are attributed to the absorption bands of CO32 (Fig. 2c), indicating successful immobilization of Ag2CO3 particles on the surface of MS66.

Figure 2
figure 2

FT-IR of (a) Fe3O4, (b) MS and (c) MS/Ag2CO3.

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The wide-angle PXRD pattern of Fe3O4 and MS/Ag2CO3 nanomaterials are shown in Fig. 3. As shown, for both samples, six characteristic peaks are observed at 2θ of 30.10, 35.58, 43.29, 53.81, 57.44 and 63.24 degree, corresponding to the crystal planes of (220), (311), (400), (422), (511) and (440), respectively. These are related to the crystalline structure of magnetite NPs confirming high stability of Fe3O4 during catalyst preparation. The pattern of MS/Ag2CO3 nanocomposite also showed two sharp peaks at 2θ of 33.2 and 38.5 degree corresponding to the Ag2CO3 NPs (Fig. 3b)10,66,67. This proves successful construction of Ag2CO3 NPs on MS core.

Figure 3
figure 3

Wide angle-PXRD pattern of the (a) Fe3O4 and (b) MS/Ag2CO3 nanomaterials.

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The VSM analysis was done to investigate the magnetic property of MS/Ag2CO3 nanocomposite (Fig. 4). As shown, the saturation magnetization of 17.5 emu/g was found for this material. Also, the VSM curve showed that this material has a superparamagnetic behavior.

Figure 4
figure 4

VSM of the MS/Ag2CO3.

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The surface morphology of MS/Ag2CO3 nanocomposite was studied by using SEM analysis. This showed that the MS/Ag2CO3 nanocomposite has a uniform spherical structure (Fig. 5).

Figure 5
figure 5

SEM image of MS/Ag2CO3.

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The EDX spectrum showed that the designed MS/Ag2CO3 is composed of Fe, Si, O, Ag and C elements confirming the successful incorporation/immobilization of expected species in the material framework (Fig. 6).

Figure 6
figure 6

EDX of MS/Ag2CO3 nanocomposite.

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In the following, the catalytic activity of MS/Ag2CO3 was evaluated in the Knoevenagel reaction. To obtain the optimum conditions, the reaction of benzaldehyde with ethyl cyanoacetate was chosen as a model (Table 1). The study showed that the catalyst loading is a very important factor in the reaction progress, in which the best result was obtained using 0.015 g of catalyst (Table 1, entries 1–4). The solvent screening demonstrated that solvent-free condition is the best for the reaction (Table 1, entries 4–8). Evaluation of temperature showed that the highest activity of MS/Ag2CO3 is resulted at 60 °C (Table 1, entry 4 versus entries 9, 10). In the next, the activity of Ag2CO3-free Fe3O4 and MS nanomaterials was compared with that of MS/Ag2CO3 showing that the presence of Ag2CO3 particles as active catalytic centers are necessary for the development of the reaction (Table 1, entry 4 versus entries 11, 12). To prove the effect of both Ag and CO3 species in the reaction progress, the catalytic activity of MS/Ag2CO3 was compared with AgCl, Na2CO3 and NaNO3 salts (Table 1, entry 4 versus entries 13–15). The results showed that AgCl and Na2CO3 deliver a low to moderate yield of desired product. While, using NaNO3, no progress was observed in the reaction. Interestingly, in the presence of designed MS/Ag2CO3 the best result was obtained. These confirm that both Ag and CO3 species are necessary for the development of reaction. According to these results, it can be concluded that the designed MS/Ag2CO3 acts as a bifunctional catalyst in the reaction.

Table 1 The effect of catalyst, solvent and temperature in the Knoevenagel condensation.
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With the optimal conditions in hand that are bolded in Table 1 (entry 4), a variety of aldehydes were employed as substrate (Table 2). Generally, for aromatic aldehydes bearing both electron-withdrawing or electron-donating substituents, electronic nature or substitution pattern had little effect on this process and MS/Ag2CO3 was able to effectively catalyze the reaction to give the Knoevenagel products in high to excellent yields. Also, terephthalaldehyde, hetero-aromatic aldehydes such as thiophene-2-carbaldehyde and furan-2-carbaldehyde, 1-naphthaldehyde and hexanal also gave the corresponding Knoevenagel adducts in good to high yield at relatively short time. It is important to note that, as previously summarized by Tietze et al., all condensations of cyanoacetate with aromatic and aliphatic aldehydes give E-isomer almost exclusively68. In the present study, all synthesized Knoevenagel products were identified as E-isomer by comparing their melting points, IR, and NMR spectra with valid samples62,63,64,69,70,71,72.

Table 2 Preparation of the Knoevenagel products using MS/Ag2CO3.
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The ability to recycle and reuse of catalysts are important issues that should be considered in heterogeneous catalytic systems. In this regard, the recyclability and reusability of MS/Ag2CO3 catalyst were evaluated in the condensation of benzaldehyde with ethyl cyanoacetate as a test model. For this, in the end of reaction, the catalyst was magnetically separated and reused in the next reaction run at conditions the same as first run. As it is clear in Fig. 7, MS/Ag2CO3 can be recovered and reused at least 6 times without the significant loss in its activity and productivity.

Figure 7
figure 7

Reusability of the MS/Ag2CO3 nanocatalyst.

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For determining the nature of the MS/Ag2CO3 catalyst, a leaching test was done at optimal conditions. For this, after progress of about 50% of the process, the MS/Ag2CO3 catalyst was collected using a magnet and the reaction progress of the catalyst-free residue was monitored. Importantly, after 1 h, no progress was observed in converting the starting material to product. This proves the heterogeneous nature of the MS/Ag2CO3 catalyst and also confirms no-leaching of active Ag2CO3 particles during the applied conditions.

Although the exact reaction pathway for the Knoevenagel condensation with the MS/Ag2CO3 catalyst is not clear for us, however, based on the results presented for the Ag2CO3 catalyst in other organic reactions, a plausible mechanism for this reaction is presented in Fig. 8. Since Ag2CO3 has a dual role as a base and a one-electron oxidant48, it picks up one acidic proton from the active methylene group of ethyl cyanoacetate to give radical intermediate 1. Simultaneously, Ag2CO3 coordinates to an aldehyde to generate complex 2. Then, the intermediate 1 is coupled with complex 2 to give radical intermediate 3. In the next step, radical intermediate 3 provides β-hydroxyl compound 4 by picking up a H atom from the produced AgHCO3 during the one-electron oxidation. Finally, the desired Knoevenagel product 5 is resulted after dehydration of the β-hydroxyl compound.

Figure 8
figure 8

Proposed mechanism for the Knoevenagel condensation using the MS/Ag2CO3 catalyst.

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At the end, the performance of the MS/Ag2CO3 catalyst was compared with the previous catalysts in the Knoevenagel condensation (Table 3). As demonstrated, the study showed that MS/Ag2CO3 is a catalyst with higher efficiency, stability and durability time than other catalysts. These findings are attributed to the magnetic nature and the chemically immobilized Ag2CO3 particles. In fact, the high performance of Ag2CO3 NPs in the catalytic processes is due to its bifunctional role as both inorganic base and Lewis acid.

Table 3 Comparison of the catalytic activity of MS/Ag2CO3 with other catalysts.
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Conclusion

In summary, a novel magnetic silica-supported Ag2CO3 (MS/Ag2CO3) was successfully prepared and its catalytic performance was studied. The FT-IR and EDX techniques showed the well immobilization of Ag2CO3 particles on the MS nanomaterial. The wide-angle PXRD analysis demonstrated the high stability of Fe3O4 NPs during steps of catalyst preparation. The PXRD pattern also confirmed the well formation of Ag2CO3 NPs on the MS nanocomposite. The superparamagnetic behavior of MS/Ag2CO3 was confirmed by the VSM analysis. The SEM image also demonstrated a uniform spherical structure for this catalyst. The MS/Ag2CO3 nanocatalyst was efficiently employed in the Knoevenagel condensation under moderate conditions and delivered the desired products in high to excellent yield. Also, MS/Ag2CO3 could be recycled and reused with maintaining its activity in several times.