The procedure for preparing PVAm-coated Fe₃O₄@SiO₂ composite microspheres has four steps. The schematic procedure is illustrated in Scheme 1. The detailed procedure is as follows.

Scheme 1.

Scheme 1. Schematic of the procedure for the preparation of Fe3O4@SiO2-PVAm composite microspheres with core-shell structure.

In the first step, Fe₃O₄ nanoparticles were synthesized by chemical co-precipitation ^[27]. FeCl₃·6H₂O (12.0 g) and FeCl₂·4H₂O (4.90 g) were dissolved in 50 mL deionized water with mechanical stirring at 80 °C, followed by the addition of 80 mL ammonia (25 wt%, aqueous solution). The dispersion was stirred for 30 min upon addition of trisodium citrate (1.40 g). The resultant magnetite particles were separated with a magnet, washed several times with deionized water, and dried at 60 °C under vacuum for 24 h.

In the second step, Fe₃O₄@SiO₂ core-shell microspheres were prepared according to the Stöber method ^[28]. Fe₃O₄ (1 g), 4 mL of ammonia (25 wt%), and 20 mL of deionized water were mixed with 70 mL of ethanol. After ultrasonication for 10 min, 0.5 mL of tetraethyl orthosilicate (TEOS) was added to the mixture with continuous stirring. After reaction for more than 8 h, the product was collected by a magnet, washed several times with water and absolute ethanol. Finally, the product was dried in a vacuum oven at 60 °C for 24 h to yield Fe₃O₄@SiO₂.

In the third step, surface modification of the magnetic Fe₃O₄@SiO₂ core-shell microspheres was carried out using the method reported in the literature ^[26]. Fe₃O₄@SiO₂ (0.1 g) and acrylamide monomer (0.05 g) in THF (7 mL) were placed in a round bottom flask and sonicated for 10 min. Benzoyl peroxide (0.005 g) as initiator was then added, and the mixture was heated to 75 °C for 5 h while stirring. Finally, the resulting solid material was removed by a magnet and dried at 60 °C overnight under vacuum to obtain Fe₃O₄@SiO₂-PAM.

In the last step, in order to convert polyacryl amide to polyvinyl amine, Hofmann degradation was carried out with Ca(OCl)₂. Fe₃O₄@SiO₂-PAM (0.1 g), deionized water (10 mL), and Ca(OCl)₂ (0.08 g) were placed into a round bottom flask and sonicated for 10 min. The suspension was then refluxed for 6 h. Afterwards, it was separated using a magnet, washed with deionized water, and finally dried at 60 °C overnight under vacuum to yield polyvinyl amine-coated Fe₃O₄@SiO₂ (Fe₃O₄@SiO₂-PVAm).

The basic site content of the Fe₃O₄@SiO₂-PVAm composite was measured by back titration using NaOH. HCl (0.2 mol/L, 3 mL) was added to 0.02 g of the composite and stirred for 30 min. The catalyst was removed and washed successively with deionized water. The excess amount of HCl was titrated with NaOH (0.1 mol/L) using phenolphthalein as indicator. The amine content of catalyst was 9.24 mequiv/g.

The crystalline structure of the samples was characterized by X-ray diffraction (XRD) analysis on a Bruker D8 Advance diffractometer with Cu K_α radiation at 40 kV and 20 mA. Fourier transform infrared (FT-IR) spectra were recorded with a Perkin Elmer 65 spectrometer in the range of 400-4000 cm^-1. Transmission electron microscopy (TEM) analysis was performed on a Phillips CM10 microscope at an accelerating voltage of 200 kV. Magnetization measurements were carried out on a BHV-55 vibrating sample magnetometer (VSM).

In a typical reaction, a mixture of Fe₃O₄@SiO₂-PVAm composite (0.05 g), benzaldehyde (1 mmol), and ethanol (5 mL) was placed in a round bottom flask. The reaction vessel was sonicated for 5 min at room temperature to disperse the magnetic nanoparticles in the solution. Then, ethyl cyanoacetate (1 mmol) was added to the mixture and stirred at room temperature for 30 min. The progress of the reaction was monitored by thin layer chromatography (TLC). After completion of the reaction, the catalyst was separated with a magnet and was washed with hot ethanol. Then, the solvent was evaporated. The products were identified using ¹H NMR, ¹³C NMR, and FT-IR spectroscopy. Quantitative analyses were conducted with an Agilent 6820 gas chromatograph equipped with an FID detector.

The crystalline structure of the different synthesized materials was investigated by XRD. For the Fe₃O₄ particles (Fig. 1(1)), the diffraction peaks at 2θ = 30.1°, 35.4°, 43.1°, 53.4°, 56.9°, and 62.5° corresponded to the spinel structure of Fe₃O₄ ^[29], which can be assigned to the diffraction of the (220), (311), (400), (422), (511), and (440) planes of the crystals, respectively. For the Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-PVAm samples (Fig. 1(2) and (3)), the XRD patterns indicated that the crystalline structure of the Fe₃O₄ particles was retained after the deposition of the polymer layers. The broad peak at around 2θ ≈ 24° indicated the presence of amorphous silica in Fe₃O₄@SiO₂ (Fig. 1(2)). The intensity of this peak increased with the introducing of polyvinyl amine on the silica-coated magnetic nanoparticles (Fig. 1(3)), which can be attributed to the amorphous polymer supported on the composite ^[30]. The XRD results showed that the Fe₃O₄@SiO₂ particles have been successfully coated with polyvinyl amine.

Fig. 1

Fig. 1. XRD patterns of Fe3O4 nanoparticles (1), Fe3O4@SiO2 (2), and Fe3O4@SiO2-PVAm composite microspheres (3).

Figure 2 depicts the FT-IR spectra of the Fe₃O₄@SiO₂, PVAm, and Fe₃O₄@SiO₂-PVAm composites. An absorption peak at 596 cm⁻¹ is the characteristic absorption of a Fe−O bond, confirming the presence of Fe₃O₄ nanoparticles. This was present in the spectra of Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-PVAm. The adsorption peaks at 1090 and 803 cm⁻¹ corresponds to the asymmetric and symmetric stretching vibration of Si−O−Si bond in oxygen-silica tetrahedron, respectively, indicating that the Fe₃O₄ microspheres were successfully coated with a SiO₂ layer (Fig. 2(1) and (3)). For Fe₃O₄@SiO₂-PVAm, the new band located at 1439 cm⁻¹ can be attributed to the bending vibration absorption of the N-H bond ^[26]. In addition, the peak at 3425 cm⁻¹ was stronger than in the FT-IR spectrum of Fe₃O₄@SiO₂, showing the presence of the stretching vibration of the N-H groups in the structure of Fe₃O₄@SiO₂-PVAm ^[26]. The bands in the 2800-3000 cm⁻¹ region are attributed to the stretching of C-H bonds of the saturated alkane (Fig. 2(3)) ^[31], which were in agreement with the spectrum of PVAm. A closer examination of the spectrum of Fe₃O₄@SiO₂-PVAm showed that there was no characteristic absorption peak of the carbonyl group (C=O), which is located at around 1625 cm^-1. This indicated that the amide groups in the composite have been converted to amine groups by the Hofmann degradation used. Thus, it was concluded that a Fe₃O₄@SiO₂-PVAm composite was obtained.

Fig. 2

Fig. 2. FT-IR spectra of Fe3O4@SiO2 (1), PVAm (2), and Fe3O4@SiO2- PVAm composite microspheres (3).

The morphology of the composite microspheres was observed by TEM. Figure 3 displays the TEM micrographs of the Fe₃O₄@SiO₂ core-shell and Fe₃O₄@SiO₂-PVAm composite microspheres. In the case of Fe₃O₄@SiO₂ (Fig. 3(a)), a continuous layer of SiO₂ was clearly observed on the outer shell of the Fe₃O₄ microsphere core, which had a diameter size of 40 nm. After compositing with PVAm, a thin PVAm layer was deposited on the surface of the Fe₃O₄@SiO₂ microspheres, and the particles were fully encapsulated by the PVAm polymer layer (Fig. 2(b)) to form a core-shell structure (Fig. 3(b)), with the size of 250 to 300 nm. Furthermore, these composites still maintained their spherical shape after the coating of SiO₂ and PVAm.

Fig. 3

Fig. 3. TEM images of Fe3O4@SiO2 (a) and Fe3O4@SiO2-PVAm composite microspheres (b).

In order to investigate the influence of the PVAm polymer on the magnetic properties of Fe₃O₄@SiO₂ core-shell microspheres, the magnetic properties of Fe₃O₄@SiO₂ and Fe₃O₄@ SiO₂-PVAm composites were measured by VSM. Typical magnetization curves of the samples with the applied magnetic field at room temperature are shown in Fig. 4. The absence of hysteresis for both samples indicated that the products have paramagnetism at room temperature. The magnetic saturation value reached 61.4 emu/g for the Fe₃O₄ nanoparticles coated with SiO₂. The magnetic saturation value decreased to 45.2 emu/g for the Fe₃O₄@SiO₂-PVAm composite microspheres, which was possibly due to the non-magnetic polyvinyl amine coating. The magnetic properties and TEM analysis further proved the presence of a PVAm shell around the Fe₃O₄@SiO₂ particles.

Fig. 4

Fig. 4. Room temperature magnetization curves of Fe3O4@SiO2 (1) and Fe3O4@SiO2-PVAm composite microspheres (2).

Fe₃O₄@SiO₂-PVAm as a solid basic catalyst for C-C bond formation in Knoevenagel condensation was evaluated. A model reaction for benzaldehyde and ethyl cyanoacetate as the substrates in Knoevenagel condensation was investigated over the catalyst at room temperature to optimize the reaction conditions and to obtain the best catalytic activity. Ethanol was chosen as the best solvent for the Knoevenagel condensation according to a previous work ^[26].

In order to find the maximum amount of PVAm functionalized on the surface of Fe₃O₄@SiO₂, catalysts with different ratios of Fe₃O₄@SiO₂ (0.1 g) to PVAm (0-0.5 g) were prepared, and their catalytic activity was tested for Knoevenagel condensation (Table 1). It can be seen that as the ratio of Fe₃O₄@SiO₂-PVAm increased, the reaction yield increased. The probable reason is that when the amount of Fe₃O₄@SiO₂ increased, the polymeric material gets more ordered on the surface of the magnetic microspheres ^[26]. Therefore, the functional groups of the polymer were more exposed and more effective. From the results, the best mass ratio of Fe₃O₄@SiO₂ to acryl amide was 2.5 for these components. It should be noted that the non-functionalized magnetic nanoparticles (Fe₃O₄@SiO₂) were not active in the Knoevenagel condensation, with less than 5% conversion observed after 4 h. This indicated the need for the polymer coating on the surface of the magnetic nanoparticles to create basic sites.

Table 1 Effect of the Fe3O4@SiO2 to PVAm ratio on catalytic activity of Fe3O4@SiO2-PVAm. Fe3O4@SiO2/PVAm mass ratio Isolated yield (%) 3.5 38 3.0 55 2.5 69 2.0 48 Without PVAm 4 a Reaction conditions: benzaldehyde 1 mmol, ethyl cyanoacetate 1 mmol, catalyst 0.03 g, ethanol 5 mL, reaction time 30 min (a 4 h), room temperature, selectivity 100%.

Table 1 Effect of the Fe3O4@SiO2 to PVAm ratio on catalytic activity of Fe3O4@SiO2-PVAm.

To study the effect of catalyst amount, the Knoevenagel condensation was carried out with different amounts of catalyst (Table 2). When the amount of catalyst increased from 0.03 to 0.05 g, the product yield increased significantly from 69% to 95%, which was due to the availability of more basic sites. After that, the yield remained almost stable between 0.05 g and 0.06 g. Therefore, 0.05 g was chosen as the optimized amount of catalyst for further experiments.

Table 2 Effect of catalyst amount on the Knoevenagel condensation. Amount of catalyst (g) Isolated yield (%) 0.03 69 0.04 82 0.05 95 0.06 95 Reaction conditions: benzaldehyde 1 mmol, ethyl cyanoacetate 1 mmol, reaction time 30 min, room temperature, selectivity 100%.

Table 2 Effect of catalyst amount on the Knoevenagel condensation.

In addition, in order to investigate the importance of the support (Fe₃O₄@SiO₂), the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate over PVAm was examined under optimum reaction conditions. A yield of 38% was observed after 2 h. This showed that the existence of the support (Fe₃O₄@SiO₂) in this magnetic composite played a critical role in the efficiency of the catalyst.

The activity of Fe₃O₄@SiO₂-PVAm for Knoevenagel condensation reaction was determined using ethyl cyanoacetate, diethyl malonate, and acetoacetic ester as active methylene reagents (Table 3). It was observed that ethyl cyanoacetate (pK_a ≈ 9) and acetoacetic ester (pK_a ≈ 11) can be deprotonated by the catalyst to generate the corresponding carbanion, followed by attack on the electron acceptor center. Moreover, it was found that the catalyst was not able to pull off protons from diethyl malonate (pK_a ≈ 13) and the Knoevenagel adducts with this substrate were not obtained. This observation showed that the catalyst has surface basic sites with pK_a lower than 13. The highest benzaldehyde yield (95%) was obtained using ethyl cyanoacetate.

Table 3 Effect of active methylene reagents on the Knoevenagel condensation over Fe3O4@SiO2-PVAm. Active methylene reagent Time (min) Isolated yield (%) Ethyl cyanoacetate 30 95 Diethyl malonate 360 — Acetoacetic ester 30 42 Reaction conditions: benzaldehyde 1 mmol, catalyst 0.05 g, active methylene reagent 1 mmol, reaction time 30 min, room temperature, 100% selectivity.

Table 3 Effect of active methylene reagents on the Knoevenagel condensation over Fe3O4@SiO2-PVAm.

In general, when using a supported catalyst, a crucial issue is the possibility that some active sites are lost from the solid support to the liquid phase, and these leached species are responsible for a significant loss of the catalytic activity. Therefore, in order to prove the heterogeneous nature of the catalyst, a heterogeneity test was performed, in which the catalyst was separated from the reaction mixture at approximately 50% conversion of the starting material by a magnet. The reaction progress in the filtrate was monitored (Fig. 5). No further reaction conversion was observed even at extended time, indicating that the nature of reaction process was heterogeneous and there was no reaction in the homogeneous phase.

Fig. 5

Fig. 5. Heterogeneity test for Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate.

For use for industrial purposes, reusability of the catalyst was tested by carrying out repeated runs of the reaction with the same batch of the catalyst with the model reaction (Fig. 6). In order to regenerate the catalyst after each cycle, it was separated by a permanent magnet and washed several times with deionized water and ethanol. Then, it was dried in an oven at 60 °C and used in the Knoevenagel condensation. The results showed that this catalyst can be reused ten times without significant loss of activity or selectivity in the Knoevenagel condensation.

Fig. 6

Fig. 6. Recyclability of the catalyst. Reaction conditions: benzaldehyde 1 mmol, ethyl cyanoacetate 1 mmol, catalyst 0.05 g, reaction time 30 min, room temperature, 100% selectivity.

The Fe₃O₄@SiO₂-PVAm catalyst also gave good activity and selectivity in the Knoevenagel condensation of different kinds of aromatic and aliphatic aldehydes and ethyl cyanoacetate (Table 4). All the investigated reactions were almost completed in 10-45 min and produced the corresponding alkenes in high yield without the formation of any side product.

Table 4 Knoevenagel condensation of various aromatic and aliphatic aldehydes and ethyl cyanoacetate catalyzed by Fe3O4@SiO2-PVAm. Entry Substrate Product Time (min) Isolated yield (%) 1 30 95 2 10 95 3 45 78 4 20 86 5 20 95 6 10 98 7 10 98 8 20 45 9 45 87 a 10 40 90 a Reaction conditions: substrate 1 mmol, ethyl cyanoacetate 1 mmol, catalyst 0.05 g, ethanol 5 mL, room temperature, 100% selectivity. a GC yield.

Table 4 Knoevenagel condensation of various aromatic and aliphatic aldehydes and ethyl cyanoacetate catalyzed by Fe3O4@SiO2-PVAm.

The catalytic activity of Fe₃O₄@SiO₂-PVAm in Knoevenagel condensation was compared with that of the SBA-15/PVAm composite reported in the literature ^[26]. The results are summarized in Table 5. As can be seen, Fe₃O₄@SiO₂-PVAm gave a higher activity in Knoevenagel condensation at a shorter time and under a milder condition (room temperature) compared to SBA-15/PVAm.

In general, when porous inorganic materials are used as support, a substantial decrease in activity of the immobilized catalyst is commonly observed, especially in a liquid phase reaction, due to the problem of slow reactant diffusion to the catalyst anchored on the interior surface ^[32]. In contrast, nanoparticles, especially magnetic nanoparticles, are more efficient support materials for homogeneous catalyst immobilization ^[33]. When nonporous nanoparticles are used as the support, the size of the support materials is decreased to the nanometer scale, and all the catalytic sites are on the external surface of the particles and are accessible to the substrates. As a consequence, the activity of nanoparticle-supported catalysts is improved compared to homogeneous catalysts immobilized on conventional porous supports, where internal pore diffusion in the porous catalysts can be rate limiting. This would explain the higher activity at a shorter time and milder condition for the catalyst here (Fe₃O₄@SiO₂-PVAm) compared to SBA-15/PVAm.

Table 5 Comparison of the catalytic activity of Fe3O4@SiO2-PVAm and SBA-15/PVAm in Knoevenagel condensation. Catalyst Time (min) Temperature Yield (%) Ref. Fe3O4@SiO2-PVAm magnetic microspheres a 30 room temperature 95 c this work SBA-15/PVAm mesoporous composite b 120 78 °C (reflux) 90 d [26] a Reaction conditions: benzaldehyde 1 mmol, ethyl cyanoacetate 1 mmol, catalyst 0.05 g, ethanol 5 mL. b Reaction conditions: benzaldehyde 2 mmol, ethyl cyanoacetate 2 mmol, catalyst 0.12 g, ethanol 10 mL. c Isolated yield. d GC yield.

Table 5 Comparison of the catalytic activity of Fe3O4@SiO2-PVAm and SBA-15/PVAm in Knoevenagel condensation.