2.1 Materials

APTES and tetraethyl orthosilicate were purchased from Merck, Germany. Hydrochloric acid (36.5%), acetanilide, zinc acetate, nickel chloride, ethanol, acetone, dichloromethane, acetonitrile, and hydrogen peroxide were obtained from Research Laboratory.

2.2 Characterizations

Perusal of nanocatalyst morphology was performed by scanning electron microscopy (SEM) on a KYKY-EM3200 scanning electron microscope. The phases of the products were assayed by X-ray diffraction (XRD) on a powder diffractometer using Cu Kα radiation (λ = 1.54060 Å) in the 2θ interval. Fourier transform infrared spectroscopy (FT-IR) were recorded using a Thermo Scientific Nicolet 8700 FT-IR spectrometer. The product of oxidation was analyzed and validated using gas chromatography–mass spectrometry (GC–MS).

2.3 Amino-functionalized SiO₂-coated Fe₃O₄ nanoparticle preparation

We prepared MNPs using a coprecipitation approach. To this end, we dissolved 6.0 g of ferric sulfate and 4.2 g of ferrous chloride in 250 mL of water and stirred the mixture at 60 °C to obtain an orange solution. Next, we subjoined 15 mL of ammonium hydroxide (25%) by stirring, which turned the color of the solution to black, and continued stirring for 30 min. We separated the MNPs and washed them with deionized water and ethanol. We carried out coating of the MNPs by the sol–gel method. We sonicated the solution of activated MNPs in 0.5 g of HCl (0.1 M) combined with 200 mL of ethanol and 50 mL of water, and then added 5 mL of ammonium hydroxide (25%) and 1 mL of tetraethyl orthosilicate to the suspension. We continued stirring at a temperature of 60 °C for 6 h. We separated the resulting SiO₂-coated MNPs (SMNPs) and washed them with ethanol. Finally, we sonicated 1.0 g of SMNPs in 700 mL of ethanol and charged the flask containing dispersed SMNPs with 5 mL of APTES, refluxing for 6 h at 80 °C to produce amino-functionalized SMNPs (ASMNPs). Separation and washing of the ASMNPs with ethanol were performed to remove the unreacted silylating agent.

2.4 Synthesis of Zn(II)–acetanilide complex anchored to ASMNPs

To anchor acetanilide onto the ASMNPs, we refluxed 3.0 g of ASMNPs and 6.0 mmol of acetanilide in 375 mL of ethanol at 80 °C for 3 h. To synthesize the nanocatalyst, we stirred 2.0 g of anchored ASMNPs in a solution of 8.0 mmol of zinc acetate in acetone for 4 h. Finally, we separated the obtained Zn(II)–acetanilide@ASMNP nanocatalyst and washed it completely with water.

2.5 Synthesis of Ni(II)–acetanilide complex anchored to ASMNPs

The above-mentioned procedure was repeated to prepare the Ni(II)–acetanilide complex nanocatalyst, with only two differences: nickel chloride was used instead of zinc acetate and methanol instead of acetone (Scheme 1).

2.6 Evaluation of catalyst activity in oxidation of aldehydes

We poured 10 mmol of benzaldehyde and 200 mg of a nanocatalyst into 10 mL of acetonitrile and refluxed the mixture at 80 °C for 3 h, adding 1.5 mL hydrogen peroxide (30%) dropwise to the reaction mixture during the reflux. After 3 h, we allowed the reaction mixture to cool and aggregated the nanocatalyst using a magnet at the side of the flask. The residual solution was collected with a pipette and extracted using dichloromethane and 10% sodium bicarbonate. The organic layer was dried with sodium sulfate and the solvent removed under vacuum at 66 °C to obtain the final product. GC–MS was used to validate the structure of the product.

3.1 Characterization of Zn(II)–acetanilide@NH₂–SiO₂–Fe₃O₄ and Ni(II)–acetanilide@NH₂–SiO₂–Fe₃O₄

The SEM image in Fig. 1 shows the monotonic scattering of zinc complex upon the surface of the silica-coated magnetite nanoparticles and is further documented by comparison of the SEM image gained in this study with a previous study [13]. Measurements of the crystalline structures for MNPs and ASMNPs were performed by powder XRD (Fig. 2), which shows the peaks of a cubic inverse spinel structure. The spacings and relative intensities in the diffractogram are consistent with the Joint Committee on Powder Diffraction Standards XRD data (Card No. 19-06291) for Fe₃O₄. Previous study has validated coating of MNPs for widening of XRD peaks of SMNPs [14]. In our study, synthesis of MNPs, SMNPs, and ASMNPs is evident.

In FT-IR studies (Fig. 3), a peak at 1632 cm⁻¹ in Fig. 3d indicates anchoring of acetanilide as a ligand on NH₂–Si–Fe₃O₄. This peak is characteristic of the CN stretching vibration. The absorption peak at 1636 cm⁻¹ in Fig. 3e corresponds to binding of zinc as a metal to the ligand, which is confirmed in comparison with previous research [15].

Fig. 4 displays the magnetization curves of the MNPs [16] and the two magnetic nanocatalysts. They exhibited no hysteresis at room temperature. The saturation magnetization of three samples was 60, 30, and 18 emu/g, respectively. In comparing the vibrating sample magnetometry of magnetic nanocatalysts with Zn as the metal in this study with previous research [13], higher magnetization of our catalyst is obvious; the other nanocatalyst with Ni as the metal (which to the best of our knowledge has not been synthesized before) had equal magnetization to the Zn magnetic nanocatalyst in the previous study [13]. The decreased saturation magnetization seen in the nanocatalysts compared with the MNPs is because of the existence of nonmagnetic elements (such as SiO₂, Zn/Ni, and acetanilide) in the nanocatalysts. Despite this reduction in saturation magnetization, these particles were separated from the reaction medium with an external magnet.

3.2 Catalytic efficiency

The prepared nanocatalysts Zn(II)–acetanilide@NH₂–SiO₂–Fe₃O₄ and Ni(II)–acetanilide@NH₂–SiO₂–Fe₃O₄ were used for oxidation of benzaldehyde. Although both nanocatalysts transmuted benzaldehyde into benzoic acid with high efficiency, the zinc nanocatalyst had higher transformation (100%) than the nickel nanocatalyst. The optimal time and temperature for conversion were found to be 3 h and 80 °C, respectively.

As these nanocatalysts retained their high catalytic efficiency (100% for the nanocatalyst with zinc and 55% for the nanocatalyst with nickel) after six cycles, their reusability is confirmed. Different time periods (1.5, 3, and 6 h) were tested on the zinc nanocatalyst. The highest rate of transformation (100%) was obtained at 3 h. When the time was decreased to 1.5 h, transformation decreased to 65%. When the time was increased to 6 h, transformation decreased to 36% (Fig. 5). For these time experiments, the temperature was set at 80 °C.

The nanocatalyst with zinc was used for the oxidation of different aldehydes. Oxidation of 3-hydroxybenzaldehyde and 3-nitrobenzaldehyde was carried out under the same conditions. Acetonitrile was used as a solvent (10 mL), H₂O₂ (1.5 mL) as an oxidant, and dichloromethane was used for extraction of the organic phase. The reaction was continued for 3 h at 80 °C. For 4-methoxybenzaldehyde, the reaction was continued for 6 h. GC–MS was used for evaluation of the conversion. The highest conversion was obtained for the oxidation of 3-hydroxybenzaldehyde, with oxidations of 4-methoxybenzaldehyde and 3-nitrobenzaldehyde having lower conversions (Fig. 6).

3.3 History of nanocatalysts used for the oxidation of aldehydes

Different nanocatalysts have been used for the oxidation of aldehydes. Of note is the use of Fe₃O₄ nanoparticles activated by ethyl acetoacetate for the oxidation of different aldehydes in solvent-free conditions and comparing two oxidants: H₂O₂ and tert-butyl hydroperoxide; higher conversion was achieved with tert-butyl hydroperoxide [17]. Another study used biosilica-supported Fe₂O₃ nanoparticles as a nanocatalyst, using H₂O₂ as an oxidant and acetonitrile as the solvent [18]. Other research used an iron–oxalate capped iron–copper bimetallic oxide nanomaterial for the oxidation of aldehydes to their acids and esters [19]. Other catalysts investigated for this purpose include bio-derived CuO nanoparticles [20], oxoammonium salt, Mohr's salt [21], and gold [22].