Magnetically recoverable MFe₁₂O₁₉ nanoparticles as efficient and environmentally benign catalysts for gram-scale selective oxidation of olefins

Abstract

Catalytic oxidation is an efficient route for synthesizing oxygenated compounds such as epoxides and aldehydes. However, developing cost-effective, environmentally friendly and selective gram-scale catalysts, in full agreement with circular economy and green chemistry principles, remains a significant challenge. Herein, we report on MFe₁₂O₁₉ magnetic nanoparticles (MNPs) as a novel, magnetically recoverable and selective catalyst for the oxidation of olefins. Three different MFe₁₂O₁₉ (M = Cu, Sn and Sr) MNPs were synthesized using the coprecipitation method and characterized by XRD, FTIR, Raman, XPS, SEM-EDX, TEM, BET, zeta potential and ⁵⁷Fe Mössbauer spectroscopy. XRD analysis demonstrates that the patterns of SnFe₁₂O₁₉ and CuFe₁₂O₁₉ are totally different from those of the magnetoplumbite hexaferrite structure due to the confirmed coexistence by Rietveld refinements of SnO₂–Fe₂O₃ and CuFe₂O₄–Fe₂O₃ as composite structures, respectively. In good agreement, Raman studies exclusively confirms the coexistence of MO_x−Fe₂O₃ as a composite structure in MFe₁₂O₁₉, due to the presence of α-Fe₂O₃ and γ-Fe₂O₃ as intermediate phases during the formation process of the hexaferrite structure. Moreover, the XPS and Mössbauer results are consistent with the experimental evidence and spectroscopic characterizations. Subsequently, the catalytic activity of the as-synthesized MNPs was evaluated for the oxidation of styrene as a model olefinic substrate. Among the as-prepared MFe₁₂O₁₉ MNPs, the composite structure CuFe₂O₄–Fe₂O₃ in CuFe₁₂O₁₉ effectively enhances catalytic activity, selectivity and reusability due to the synergistic catalytic effect within a single magnetically recoverable nanostructure. Overall, MFe₁₂O₁₉ MNPs present a facile and greener approach using magnetically recyclable hexaferrites for selective catalytic oxidation reactions.

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1. Introduction

Styrene oxide and benzaldehyde are important organic intermediates widely used in the industrial manufacture of fragrances, pharmaceuticals, and organic synthesis.^1–4 Hence, the oxidation of olefins has been widely regarded as one of the most important routes to accessing epoxides and aldehydes.⁵ To date, both homogeneous and heterogeneous catalysts have been extensively reported for the oxidation of olefins.^6–15 Catalysis plays an important role in green chemistry by enabling less polluting chemical processes and providing sustainable pathways to synthesize desired products.¹⁶ Heterogeneous catalysts are widely preferred over homogeneous ones owing to their recyclability and atom economy, which contribute to sustainability by reducing time- and energy-consuming catalytic processes in alignment with the circular economy. Particular attention has been given to the development of selective heterogenous catalysts for the oxidation of styrene to benzaldehyde or styrene oxide. Magnetic nanoparticles (MNPs) are especially attractive candidates for this reaction owing to their high activity, selectivity, and facile reusability.¹⁷ Moreover, MNPs exhibit numerous advantageous properties, including a large surface-to-volume ratio, good crystallinity, excellent thermal stability and homogeneous composition as heterogeneous bulk catalysts, all of which contribute to their high catalytic performance.^17–20

M-type hexaferrite, with a chemical formula of MFe₁₂O₁₉, is a class of promising chemically and thermally stable magnetic materials for a wide range of applications. Regarding their high magneto-crystalline anisotropy with a single easy magnetization axis, they are classified as hard magnets with high coercivity.²¹ They have been widely used as permanent magnets, recording media, photocatalysts and components in microwave, telecommunication, higher-frequency, and magnetooptical devices.^22–29 Moreover, special attention has been given to magnetic materials based on barium, strontium, palladium and lead with unique properties.³⁰ Previously, Ansari et al. reported on the synthesis of CuFe₁₂O₁₉ nanostructures by using sol–gel auto-combustion method and its magnetic properties.³¹ Recently, SnFe₁₂O₁₉ nanostructures were synthesized through ultrasonic irradiation and utilized in an environmental application.³² Based on the current understanding, the XRD patterns of these reported structures MFe₁₂O₁₉ (M = Cu or Sn) are not yet confirmed, and they are totally different from those of the magnetoplumbite hexaferrite structure. Hence, there is a lack of advanced spectroscopic investigations in the reported literature to accurately assess the structural properties of such MFe₁₂O₁₉ compounds. For this reason, Cu and Sn have been selected to study the obtained MFe₁₂O₁₉ compounds and compare them with the SrFe₁₂O₁₉ M-type hexaferrite.

In the last decades, owing to their magnetic properties, M-type hexaferrites have attracted the attention of scientists in various fields of catalytic applications. Recently, the SrFe₁₂O₁₉ M-type hexaferrite was reported as high catalytically active MNPs for the improvement of the catalytic epoxide ring-opening reaction with amines, as pristine³³ or in hybrid graphene-derived nanocomposites,³⁴ as well as in the synthesis of 2-amino-4,6-diphenylnicotinonitrile³⁵ and 1,5-benzodiazepine derivatives.³⁶ However, the presumed MFe₁₂O₁₉ (M = Cu or Sn) nanoparticles have only been reported in a few studies. According to the literature, CuFe₁₂O₁₉ MNPs and the grafted ones on CNT have only been reported for photocatalytic elimination of water contamination.^37,38 To the best of our knowledge, CuFe₁₂O₁₉ and SnFe₁₂O₁₉ nanoparticles have not yet been used as heterogeneous catalysts for any application. In this regard, the growing potential of hexaferrite MNPs in catalysis motivates further study of their catalytic activity in alignment with green chemistry principles and circular economy.

In this study, we focus on the development of novel hexaferrite nanostructures as magnetically recoverable nanoparticles (MNPs) for catalytic applications, leveraging their magnetic properties, stability and high specific surface area. Various chemical methods, including sol–gel,^31,39,40 salt melting process,⁴¹ sonochemical^32,42 and coprecipitation,^33,43 have previously been employed for the synthesis of M-type hexaferrites. Among these, the chemical coprecipitation method stands out due to its simplicity and precision in controlling the grain size, making it an excellent approach for preparing magnetic oxide nanoparticles. Herein, we present the synthesis of CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉ NPs using a reproducible coprecipitation method. To the best of our knowledge, this is the first study to evaluate the catalytic performance of as-prepared MFe₁₂O₁₉ NPs in the oxidation of styrene derivatives, chosen as model substrates for olefin oxidation.

2. Experimental

2.1. Materials

All reagents and solvents were obtained from commercial suppliers (Aldrich and Acros), and used directly without further purification. As-synthesized nanoparticles (NPs) were characterized by X'Pert MPD Powder instrument from Malvern Panalytical with Cu Kα radiation (λ_Cu = 1.5406 Å). A Bruker vertex 70 DTGSFTIR spectrophotometer was used to record the stretching vibrational frequencies of NPs in the range of 400–4000 cm⁻¹. X-ray photoelectron spectra (XPS) were obtained using a Thermo Scientific K-Alpha spectrometer using an aluminum anode (Al Kα_1/4 1468.3 eV). The binding energies were calibrated by referencing the C 1s signal at 284.4 eV. Morphology and topology of the materials were characterized by using a scanning electron microscope (SEM) (FEI, Quanta FEG 450). Transmission Mössbauer spectra were collected by using a conventional constant acceleration spectrometer with a 25 mCi source ⁵⁷Co sealed in a Rh matrix at room temperature. In this configuration, the γ-ray direction is perpendicular to the powder plane. NORMOS DIST program was used to interpret the experimental spectra.⁴⁴ All isomer shift values are reported with respect to that of the α-iron foil at room temperature. Transmission electron microscope (TEM) images were recorded on a 120 kV Hitachi HT7800 TEM instrument with EXALENS module capable of working at high-resolution (HR) mode in the magnification range of 10–600k×. BET measurements were conducted using the Micromeritics Tristar II automated gas sorption system. The zeta potential was measured using a Malvern Panalytical Zetasizer, after dispersing 0.1 g of nanopowders in 3 ml of solvent (acetonitrile; acetone or deionized water). Aliquot samples from the reaction mixture were monitored by gas chromatography (GC, Shimadzu) equipped with a flame ionization detector and nitrogen as the carrier gas. The GC conditions for the BP capillary columns (25 m × 0.25 mm, SGE) were set as follows: injector temperature at 250 °C; detector temperature at 250 °C and oven temperature initially set at 70 °C for 5 min, followed by a ramp of 3 °C min⁻¹ until reaching 250 °C for 30 min. The column pressure was maintained at 20 kPa with a flow rate of 6.3 ml min⁻¹, linear velocity of 53.1 cm s⁻¹ and total flow of 138 ml min⁻¹. The products were confirmed by injecting the reaction mixture into an ISQ LT single quadrupole mass spectrometer, operating in positive EI mode, and scannig a mass range of 50 to 400 m/z.

2.2. Synthesis of the MFe₁₂O₁₉ nanoparticles

The magnetic MFe₁₂O₁₉ (M = Sr; Cu; or Sn) nanoparticles were synthesized by following a coprecipitation protocol.^33,34 In a typical synthesis protocol, stoichiometric quantities of metal chlorides (SrCl₂·6H₂O; CuCl₂; SnCl₂·2H₂O; FeCl₃) were individually dissolved in 30 ml of deionized water to form homogeneous solutions. These solution were then mixed and stirred for 30 min at 80 °C. Next, pH of the reaction mixture was adjusted to 11–12 via dropwise addition of NaOH (1.5 M) under continuous stirring for 1.5–2 h at 80 °C, ensuring the homogeneity of the mixture. The precipitated CuFe₁₂O₁₉ and SnFe₁₂O₁₉ nanopowders were separated magnetically. In contrast, SnFe₁₂O₁₉ was separated via centrifugation due to the lack of its magnetization. The prepared MFe₁₂O₁₉ nanopowders were washed several times with deionized water to remove the excess salt and dried at 80 °C overnight. The obtained nanopowders were calcined at 900 °C for 8 h to obtain the pure hexaferrites.

2.3. Catalytic studies

2.3.1. Selective synthesis of epoxide or aldehyde derivatives. In a typical reaction, the synthesis of epoxide derivatives was performed by combining olefin (1.92 mmol), tBHP 70 wt% (3 eq.), CuFe₁₂O₁₉ (1.92 × 10⁻² mmol, %mol (CuFe₁₂O₁₉) = 1%) and 2 ml of acetonitrile as solvent. The mixture was then placed in a 50 ml Rotaflow tube and stirred at 60 °C for 24 h. The progress of the reaction was monitored by GC using dodecane as the internal standard. At the end of the reaction, the catalyst was recovered by an external magnetic field, washed with deionized water and acetone, and dried in an oven at 80 °C for 6 h before reuse. For the synthesis of the aldehyde derivatives, the oxidation reaction was carried out in the presence of H₂O₂, 30 wt% (3 eq.), in 2 ml of acetone as the solvent at 80 °C for 24 h.

3. Results and discussion

3.1. MFe₁₂O₁₉ characterization

The structures of the as-synthesized MFe₁₂O₁₉ NPs (M = Cu, Sn and Sr) were first characterized by X-ray diffraction (XRD) (Fig. 1). While all the diffraction lines of CuFe₁₂O₁₉ MNPs match the reported ones in the literature,³¹ the diffraction patterns of CuFe₂O₄ (JCPDS: 77-0010) and Fe₂O₃ (JCPDS: 24-0072) reveal the coexistence of CuFe₂O₄–Fe₂O₃ as a composite structure in the as-synthesized CuFe₁₂O₁₉ MNPs. Accordingly, while the XRD pattern of SnFe₁₂O₁₉ also matched the reported literature,³² the crystalline phases of SnO₂ (JCPDS: 72-1147) and Fe₂O₃ (JCPDS: 24-0072) indicate the coexistence of SnO₂–Fe₂O₃ as possible composite structure. However, the XRD pattern of the as-prepared SrFe₁₂O₁₉ presents a very similar diffraction pattern as reported for the magnetoplumbite hexaferrite structure.³⁴ In addition, the SrFe₁₂O₁₉ crystal phase clearly matches well with the known strontium hexaferrite crystal phase (JCPDS: 33-1340). No obvious peaks of metal oxides (SrO and Fe₂O₃) are observed, clearly indicating only the presence of the pure SrFe₁₂O₁₉ hexaferrite structure.

Fig. 1 XRD patterns of CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉.

Based on the XRD results, Rietveld refinement was carried out to accurately assess the observed structural differences of both compounds CuFe₁₂O₁₉ and SnFe₁₂O₁₉ compared to the magnetoplumbite M-type hexaferrite structure of the SrFe₁₂O₁₉ MNPs. The recorded and Rietveld refined XRD patterns of the as-presumed prepared hexaferrites are displayed in Fig. 2. Clearly, in SnFe₁₂O₁₉ and CuFe₁₂O₁₉, the refined XRD patterns exhibit a combination of two distinct parent phases. The refinement confirmed that CuFe₁₂O₁₉ exhibits a spinel structure with the Fd3̄m space group and hematite (Fe₂O₃) as the main phase (73.5%). However, the SnFe₁₂O₁₉ XRD pattern presents the hematite phase (92.5%) as the major component and the minor crystalline phase of SnO₂ (7.5%). Table 1 presents the fitting parameters and crystallite sizes (D), using the most intense and single indexed reflection of all phases, as found in the as-prepared MFe₁₂O₁₉ NPs.

Fig. 2 Rietveld refinement of CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉.

Table 1 Refined parameters of the XRD data for MFe₁₂O₁₉ (M = Cu, Sn and Sr)

	CuFe12O19 = {CuFe2O4–Fe2O3}		SnFe12O19 = {SnO2–Fe2O3}		SrFe12O19
Phase	CuFe2O4	Fe2O3	SnO2	Fe2O3	SrFe12O19
Crystal structure	Cubic spinel	Hexagonal	Tetragonal	Hexagonal	Hexagonal
Group space	Fd3̄m	R3̄c	P42/mnm	R3̄c	P63/mmc
Composition (%)	23.8	76.2	7.5	92.5	100
a (Å)	8.42	5.033	4.735	5.037	5.882
b (Å)	8.42	5.033	4.735	5.037	5.882
c (Å)	8.42	13.741	3.179	13.748	23.051
V (Å^3)	598.69	301.53	71.28	302.11	690.85
D (nm)	52.2	119.9	37.4	160.1	118.1
R wp (%)	8.09		2.55		3.77
R p (%)	4.61		1.99		2.62
R exp (%)	1.77		1.93		1.61
χ ^2	4.56		1.32		2.33

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The Rietveld refinement study reveals good agreement between the experimental and calculated patterns. Moreover, based on what is currently known, we can assume that the structural properties of both CuFe₁₂O₁₉ and SnFe₁₂O₁₉ have never been reported with such experimental proof, which may explain the XRD outcomes compared to the magnetoplumbite structure due to the coexistence of SnO₂–Fe₂O₃ and CuFe₂O₄–Fe₂O₃. Consequently, we can assume that they are not isostructural with the SrFe₁₂O₁₉ M-type hexaferrite.

The FTIR spectra of all synthesized MFe₁₂O₁₉ NPs present the characteristic peaks of metal–oxygen stretching vibrations at around 460–630 cm⁻¹ (Fig. 3).³⁴ However, the peaks around 3400 and 1600 cm⁻¹ are attributed to the stretching and the bending vibrations, respectively, of the adsorbed water on the MFe₁₂O₁₉ NPs.^32,38

Fig. 3 FTIR spectra of CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉.

Fig. 4 shows the Raman spectra of the obtained MFe₁₂O₁₉ NPs. The Raman shifts observed for SrFe₁₂O₁₉ are attributed to the A_1g, E_1g, and E_2g modes. The tetrahedral 4f1 and bipyramidal 2b sites of SrFe₁₂O₁₉ exhibit A_1g vibrations of Fe–O bonds at 724 and 690 cm⁻¹, respectively. Meanwhile, the octahedral 4f2, 2a and 12k sites exhibit the same A_1g vibrations at 620 and 479 cm⁻¹, respectively. At the octahedral 12k dominant site, the peak of the A_1g vibrations was detected at 407 cm⁻¹. The band at 347 cm⁻¹ is attributed to an octahedra mode mix with A_1g and E_1g symmetries. While the E_1g vibrations of the entire spinel block are shown at 185 cm⁻¹, the E_1g vibration modes are also seen at 295 and 225 cm⁻¹. The bands at 534 and 432 cm⁻¹ were both attributed to E_2g modes.^45,46 On the other hand, the Raman spectrum of SnFe₁₂O₁₉ reveals three Raman active modes (Fig. 4). While the observed peaks at 222 and 497 cm⁻¹ are both attributed to A_1g, the remaining four peaks at 243, 284, 407, and 609 cm⁻¹ may be assigned to the E_g mode of α-Fe₂O₃ NPs.⁴⁷ However, the peaks at 499 and 701 cm⁻¹ are assigned to the E_g and A_2u active modes of SnO₂.⁴⁷ In addition, the Raman spectrum of CuFe₁₂O₁₉ reveals peaks at 226, 228, 409, 611, and 656 cm⁻¹, matching the Raman active modes of α-Fe₂O₃ NPs.⁴⁸ Moreover, the broad peak around 1303 cm⁻¹ observed in all samples, with a noticeable differentiation in intensity, could be attributed to the Raman mode of α-Fe₂O₃,⁴⁹ which is in good agreement with the Rietveld study. Furthermore, the significant intensity of the α-Fe₂O₃ peak in the SnFe₁₂O₁₉ structure may explain its lack of magnetization. In conclusion, the Raman spectra support the recorded and Rietveld refined XRD patterns, especially those of CuFe₁₂O₁₉ and SnFe₁₂O₁₉, about a possible coexistence of MO_x–Fe₂O₃ as a composite structure in the MFe₁₂O₁₉ composition. Indeed, it was reported that α-Fe₂O₃ and γ-Fe₂O₃ are intermediate phases during the formation process of the hexaferrite structure.^49,50 Moreover, their existence as intermediate phases supports the formation mechanism of MFe₁₂O₁₉. Specifically, the γ-Fe₂O₃ phase is formed at low temperatures and incorporates cations into MFe₂O₄, which further reacts with γ-Fe₂O₃ to form the MFe₁₂O₁₉ phase. However, the α-Fe₂O₃ phase appears at higher temperatures, accompanied by the formation of the hexaferrite structure.

Fig. 4 Raman spectra of CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉.

To understand the surface electronic and chemical configurations of the as-synthesized MFe₁₂O₁₉ NPs, XPS analyses were carried out (Fig. 5). The chemical environment and the oxidation state of Cu, Sr, Sn and Fe were studied through the Cu 2p, Sr 3d, Sn 3p, Sn 3d and Fe 2p regions (Fig. 5b and c). All the expected elements, Cu, Sr, Sn, Fe and O, were detected in the XPS survey spectra of MFe₁₂O₁₉ (Fig. 5a₁–a₃). The high-resolution XPS spectrum of the Cu 2p region in CuFe₁₂O₁₉ shows the characteristic peaks of the Cu 2p_3/2 and Cu 2p_1/2 core-levels at the binding energies (BEs) of 933.38 and 953.10 eV, as well as two additional peaks attributed to shakeup satellites at BEs of 940.49 and 961.78 eV (Fig. 5b₁), respectively. Deconvolution of the Cu 2p XPS spectrum indicated the presence of Cu²⁺ of CuFe₂O₄ in the as-synthesized CuFe₁₂O₁₉.⁵¹ Meanwhile, the peaks located at BEs of 486.13 and 494.60 eV are attributed to 3d_5/2 and 3d_3/2 of Sn⁴⁺ (Fig. 5b₃), respectively. In addition, the Sn 3d_5/2 and Sn 3d_3/2 core-levels are presented as a doublet with a spin–orbit split of ≈8.5 eV, which confirms the presence of SnO₂ in the SnFe₁₂O₁₉ composite structure.⁵² However, the XPS spectrum of Sr 3d in SrFe₁₂O₁₉ exhibits two peaks at BEs of 133.38 and 134.98 eV, attributed to the characteristic doublets of Sr²⁺ (Sr 3d_5/2 and Sr 3d_3/2, respectively) (Fig. 5b₃).⁵³

Fig. 5 XPS survey (a_1–3), high-resolution M (M = Cu 2p, Sr 3d, and Sn 3d) (b_1–3), Fe 2p (c_1–3) and O 1s spectra (d_1–3) of CuFe₁₂O₁₉, SnFe₁₂O₁₉, and SrFe₁₂O₁₉, respectively.

The high-resolution XPS spectra of the Fe 2p regions of the MFe₁₂O₁₉ NPs (CuFe₁₂O₁₉, SnFe₁₂O₁₉, and SrFe₁₂O₁₉) show similar two spin–orbit doublets at BEs of around 710 and 720 eV, corresponding to the Fe 2p_3/2 and Fe 2p_1/2 core-levels, as well as two additional peaks attributed to shakeup satellites at BEs of around 719 and 732 eV (Fig. 5c), respectively. Hence, the obtained 2p peaks and satellite peaks of Fe are in close agreement with the reported values for Fe³⁺ and the energy differences between the main 2p_3/2 and 2p_1/2 peaks (≈13.5 eV), confirming the electronic state of Fe³⁺.⁵⁴ Moreover, this electronic state in the as-synthesized MFe₁₂O₁₉ NPs can be assigned to the contributions from Fe³⁺ in the octahedral and tetrahedral sites.⁵⁴ However, the noticeable difference in XPS spectrum of Fe in SnFe₁₂O₁₉ is due to the appearance of Sn 3p_3/2 at the BE of 725.61 eV, which is in the same region as the shakeup satellite (at BE of 718.98 eV).⁵⁵ The deconvoluted spectra of O 1s show two peaks in the BE region of around 529–534 eV (Fig. 5d). While the peak at BE of around 530 eV is ascribed to the lattice oxygen in the metal–oxygen bond, the other one at BE of around 531.5 eV corresponds to the adsorbed O⁻ or O₂²⁻ species, which are associated with the intrinsic oxygen vacancies on the surface. However, the peak at BE of 530.04 eV for SrFe₁₂O₁₉ is shifted to BE of around 529.5 eV in both CuFe₁₂O₁₉ and SnFe₁₂O₁₉, which could be attributed to the characteristic peak of O²⁻ in the hybrid oxide framework.⁵⁶

⁵⁷Fe Mössbauer spectroscopy is a highly effective technique for examining the oxidation state, local environment and magnetic characteristics of Fe atoms in the studied MFe₁₂O₁₉ NPs. Fig. 6 shows the room temperature Mössbauer spectra of CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉ NPs. For CuFe₁₂O₁₉, the Mössbauer spectrum is well fitted using two sextets and one doublet, which confirms the presence of the CuFe₂O₄–Fe₂O₃ composite structure. While the two sextets are characteristic of the A and B sites of the Fe³⁺ ions present in the magnetically ordered structure of CuFe₂O₄ at the tetrahedral and octahedral sites, the doublet is associated with Fe³⁺ ions in the paramagnetic state of α-Fe₂O₃. However, the fitted experimental spectrum of SrFe₁₂O₁₉ reveals a superposition of five magnetically split spectral components (sextets) characteristic of five different atomic environments around the Fe nuclei in the SrFe₁₂O₁₉ M-type hexaferrite structure. Each one of these sextets is associated with a specific Fe³⁺ crystallographic site: one tetrahedral (4f1), one bipyramidal (2b) and three octahedral sites (12k, 4f2, and 2a). On the other hand, the spectrum of SnFe₁₂O₁₉ shows a doublet characteristic of the paramagnetic iron atoms, confirming the lack of magnetization of the as-synthesized SnFe₁₂O₁₉ nanoparticles due to the presence of the SnO₂–Fe₂O₃ composite structure. Table 2 lists all of the hyperfine parameters, namely the percentage of area, isomer shift (IS), hyperfine field (B_hf) and quadrupole splitting (QS). It should also be noted that the Mössbauer results are consistent with the experimental evidence and the outcomes of the spectroscopic characterizations (XRD, FTIR, Raman and XPS).

Fig. 6 ⁵⁷Fe Mössbauer spectra recorded at room temperature of CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉.

Table 2 Hyperfine parameters of the room temperature Mössbauer spectra of CuFe₁₂O₁₉, SrFe₁₂O₁₉ and SnFe₁₂O₁₉

MFe12O19	Component	Area (%)	IS (mm s^−1)	B hf (T)	QS (mm s^−1)	Site
CuFe12O19 = {CuFe2O4–Fe2O3}	Sextet-1	72.35	0.36	51.71	—	Octahedral
	Sextet-2	12.15	0.36	50.16	—	Tetrahedral
	Doublet	15.50	0.59	—	2.65	—
SrFe12O19	Sextet-1	25.12	0.28	51.95	—	4f2
	Sextet-2	7.00	0.39	50.93	—	2a
	Sextet-3	10.96	0.26	49.13	—	4f1
	Sextet-4	55.94	0.36	40.90	—	12k
	Sextet-5	0.98	0.38	34.95	—	2b
SnFe12O19 = {SnO2–Fe2O3}	Doublet	100	0.41	—	0.20	—

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To investigate the morphology and topology of the as-prepared MFe₁₂O₁₉ nanoparticles, SEM images were recorded for all MFe₁₂O₁₉ NPs (Fig. 7). CuFe₁₂O₁₉ shows a uniform nanostructure with a homogeneous shape distribution and slight agglomerations (Fig. 7a₁). However, SnFe₁₂O₁₉ reveals a slightly uniform distribution of crystallized NPs (Fig. 7b₁). On the other hand, SrFe₁₂O₁₉ shows the formation of nano-sized grains and uniform particle shape distribution (Fig. 7c₁). Moreover, all EDX elemental mapping images indicate that the main constituent elements of the synthesized MFe₁₂O₁₉ nanostructures are well-dispersed throughout the structure (Fig. 7a₂–c₂).

Fig. 7 SEM images and EDX elemental maps of (a) CuFe₁₂O₁₉, (b) SnFe₁₂O₁₉ and (c) SrFe₁₂O₁₉.

TEM analysis was performed to further investigate the shape and size of the as-synthesized MFe₁₂O₁₉ nanostructures (Fig. 8). The TEM images of CuFe₁₂O₁₉ reveal spherical nanoparticles with a size range of approximately 75–85 nm, and confirm the presence of tightly agglomerated NPs (Fig. 8a). The TEM images of SnFe₁₂O₁₉ reveal small platelets forming large spherical grains, with an average size of about 40–80 nm (Fig. 8b), while the TEM images of SrFe₁₂O₁₉ reveal interconnected spherical NPs with a size range of about 25–55 nm (Fig. 8c).

Fig. 8 TEM images of (a) CuFe₁₂O₁₉, (b) SnFe₁₂O₁₉ and (c) SrFe₁₂O₁₉.

Fig. 9 shows the nitrogen adsorption–desorption isotherms of CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉ NPs. According to the IUPAC classification, the isotherms of the MFe₁₂O₁₉ NPs may be assigned to type III, III and II for the CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉ NPs, respectively. The pore size distributions (BJH model) of all MFe₁₂O₁₉ NPs are presented in the inset spectra in Fig. 9a–c. The BJH average pore diameters of CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉ NPs are found to be 12.98, 14.95 and 4.89 nm, respectively. According to the IUPAC, these average values indicate that the as-synthesized MFe₁₂O₁₉ are classified as a mesoporous nanomaterial.^57,58 Moreover, the BET surface area of the obtained NPs was found to be 1.87, 5.25 and 24.91 m² g⁻¹, respectively.

Fig. 9 N₂ adsorption–desorption isotherm and BJH pore diameter distribution of (a) CuFe₁₂O₁₉, (b) SnFe₁₂O₁₉ and (c) SrFe₁₂O₁₉.

To evaluate the stability of the as-prepared MFe₁₂O₁₉ for potential catalytic applications, zeta potential (ZP) measurements were carried out on MFe₁₂O₁₉ NPs dispersed in a solvent (acetonitrile; acetone or deionized water). The CuFe₁₂O₁₉, SnFe₁₂O₁₉ and SrFe₁₂O₁₉ NPs present ZP values of −24.8, −16.9 and −20.6 mV, respectively. Accordingly, it was reported that NPs manifesting −25 > ZP > +25 mV usually indicate a high degree of stability.⁵⁹ Indeed, the obtained surface charge values indicate that the prepared NPs exhibit stabilities ranging from good to a high degree of stability. In addition, the obtained ZP values of the synthesized MFe₁₂O₁₉ confirm the observed fluctuation in particle agglomeration (Cu > Sr > Sn) during the microscopic study (CuFe₁₂O₁₉ presents a threshold ZP value).

3.2. Catalytic studies

The catalytic activity of the as-synthesized MFe₁₂O₁₉ NPs were evaluated in the selective oxidation reaction of aromatic olefins. Hence, styrene was chosen as a model substrate to conduct the optimization study of the catalytic oxidation. Moreover, preliminary catalytic tests of MFe₁₂O₁₉ were focused on the selective formation of styrene oxide or benzaldehyde, while tBHP or H₂O₂ were used as the oxidizing agent (Scheme 1 and Table 3).

Scheme 1 Selective catalytic oxidation of styrene.

Table 3 Catalytic oxidation^a of styrene in the presence of t-BHP^b or H₂O₂^c

Reaction	Catalyst	T (°C)	Ox. agent	Solvent	Conversion^d (%)	Yield^d (%)
						Benzaldehyde	Styrene oxide
a General reaction conditions: 1.92 mmol of styrene with 3 eq. of oxidizing agent and 2 ml of solvent for 24 h. b Condition 1: tBHP in acetonitrile at 60 °C. c Condition 2: H2O2 in acetone at 80 °C. d The conversion and yields were determined by GC using dodecane as the internal standard.
Condition 1	—	60	tBHP	Acetonitrile	8	7	—
	CuFe 12 O 19	60	tBHP	Acetonitrile	93	30	61
	SnFe12O19	60	tBHP	Acetonitrile	20	11	8
	SrFe12O19	60	tBHP	Acetonitrile	30	11	18
Condition 2	—	80	H2O2	Acetone	4	4	—
	CuFe 12 O 19	80	H 2 O 2	Acetone	100	80	—
	SnFe12O19	80	H2O2	Acetone	100	28	—
	SrFe12O19	80	H2O2	Acetone	72	40	—

---

In the absence of a catalyst, the oxidation reaction gives only a conversion of up to 8% (Table 3). However, the CuFe₁₂O₁₉ nanoparticles exhibited the highest efficiency and selectivity in the presence of both oxidizing agents (tBHP or H₂O₂). While the use of H₂O₂ in acetone provides a high selectivity towards benzaldehyde (80%) in the presence of CuFe₁₂O₁₉, switching to tBHP in acetonitrile resulted in two times higher selectivity towards styrene oxide formation (61%) compared to benzaldehyde (30%). Therefore, both reaction conditions (1 and 2) were optimized for a better catalytic selectivity throughout the study of various parameters: the catalyst amount, reaction time, temperature, amount of oxidizing agent and solvent nature (Scheme 2).

Scheme 2 Catalytic oxidation of styrene in the presence of tBHP or H₂O₂.

To investigate the active site responsible for the obtained catalytic performance, we studied the catalytic activity of both CuFe₂O₄ and Fe₂O₃ compared to the as-prepared composite structure CuFe₁₂O₁₉ (Table 4). Indeed, the XRD pattern and Rietveld refinement of CuFe₁₂O₁₉ illustrate the presence of both CuFe₂O₄ and Fe₂O₃ phases as the composite nanostructure (CuFe₁₂O₁₉ = {CuFe₂O₄–Fe₂O₃} composite). Hence, the catalytic oxidation in the presence of CuFe₁₂O₁₉ reveals high conversion and good selectivity compared to CuFe₂O₄ or Fe₂O₃ for both reaction conditions. Consequently, the composite structure of the as-prepared CuFe₁₂O₁₉ NPs effectively enhances the catalytic activity and reusability of the developed nanocatalyst, owing to the synergistic catalytic effect in a single magnetically recoverable nanostructure.

Table 4 Comparison of the catalytic activity of the as-prepared nanomaterials in catalytic styrene oxidation^a

Oxidizing agent	Catalyst	Conversion^b (%)	Yield^b (%)
			Benzaldehyde	Styrene oxide
a General reaction conditions: 1.92 mmol of styrene, 3 eq. of oxidizing agent (tBHP, acetonitrile at 60 °C or H2O2, acetone at 80 °C), catalyst (20 mg) and 2 ml of solvent for 24 h. b The conversion and yields were determined by GC using dodecane as the internal standard.
tBHP	Fe2O3	86	34	39
	CuFe2O4	99	14	37
	CuFe 12 O 19	93	30	61
H2O2	Fe2O3	18	11	Trace
	CuFe2O4	5	Trace	—
	CuFe 12 O 19	100	81	—

---

3.3. Optimization of the CuFe₁₂O₁₉ catalyzed oxidation of styrene

3.3.1. Effect of solvent. An examination of the protic and aprotic solvents on the model reaction using CuFe₁₂O₁₉ as the catalyst and tBHP or H₂O₂ as the oxidizing agent has been carried out (Table 5). On the one hand, the presence of tBHP in polar and aprotic solvents (acetonitrile, acetone and THF) promotes the selective formation of styrene oxide, compared to the protic ones. On the other hand, acetone provides the best selectivity towards benzaldehyde when using H₂O₂ as the oxidizing agent. For further optimizations, the catalytic oxidation will be carried out in the presence of tBHP in acetonitrile for the selective formation of epoxide (condition 1), while H₂O₂ in acetone will be used for the selective synthesis of aldehyde (condition 2).

Table 5 Effect of solvent on the styrene oxidation in the presence of CuFe₁₂O₁₉ NPs as the catalyst^a

Oxidizing agent	Solvent	Conversion^b (%)	Yield^b (%)
			Benzaldehyde	Styrene oxide
a General reaction conditions: 1.92 mmol of styrene, 3 eq. of oxidizing agent (tBHP at 60 °C or H2O2 at 80 °C), CuFe12O19 (20 mg) and 2 ml of solvent for 24 h. b The conversion and yields were determined by GC using dodecane as the internal standard.
tBHP	None	24	6	17
	H2O	17	16	—
	Methanol	45	26	13
	Ethanol	18	14	3
	Acetonitrile	93	30	61
	THF	65	19	43
	Acetone	63	17	39
H2O2	None	5	5	—
	H2O	4	4	—
	Methanol	82	10	—
	Ethanol	100	17	—
	Acetonitrile	22	20	—
	THF	79	45	—
	Acetone	100	81	—

---

3.3.2. Effect of the reaction temperature. Table 6 shows the effect of temperature on the selective catalytic oxidation of styrene in the presence of CuFe₁₂O₁₉ nanoparticles as the catalyst. In the presence of tBHP, the optimum conversion/selectivity was obtained at 60 °C and the elevated temperature (80 °C) resulted in a decrease of the styrene oxide selectivity due to the epoxide ring-opening. Moreover, when the reaction was conducted in the presence of H₂O₂ as the oxidizing agent, the increase of temperature to 80 °C resulted in a total styrene conversion and a high selectivity towards benzaldehyde. Therefore, further optimization studies were conducted at 60 °C and 80 °C for conditions 1 and 2, respectively.

Table 6 Effect of the reaction temperature on the styrene oxidation in the presence of CuFe₁₂O₁₉ NPs as the catalyst^a

Reaction	Temperature (°C)	Conversion^d (%)	Yield^d (%)
			Benzaldehyde	Styrene oxide
a General reaction conditions: 1.92 mmol of styrene with 3 eq. of oxidizing agent, CuFe12O19 (20 mg) and 2 ml of solvent for 24 h. b Condition 1: tBHP in acetonitrile. c Condition 2: H2O2 in acetone. d The conversion and yields were determined by GC using dodecane as the internal standard.
Condition 1^b	50	64	20	40
	60	93	30	61
	70	100	32	59
	80	100	44	11
Condition 2^c	50	23	21	—
	60	24	22	—
	70	52	50	—
	80	100	81	—

---

3.3.3. Effect of the catalyst amount. The selective catalytic oxidation of styrene was carried out in the presence of different amounts of CuFe₁₂O₁₉ as the catalyst (Table 7). When the reaction was performed with tBHP as the oxidant, the best result was obtained with 20 mg of CuFe₁₂O₁₉, giving a selective formation of styrene oxide of 61%. Similarly, in the presence of H₂O₂, the optimal CuFe₁₂O₁₉ amount was 20 mg with a total conversion and a high selectivity of benzaldehyde of up to 81%. Therefore, further optimization studies were conducted using 20 mg of catalyst.

Table 7 Effect of the catalyst amount on the styrene oxidation in the presence of CuFe₁₂O₁₉ NPs as the catalysts^a

Reaction	Catalyst amount (mg)	Conversion^d (%)	Yield^d (%)
			Benzaldehyde	Styrene oxide
a General reaction conditions: 1.92 mmol of styrene with 3 eq. of oxidizing agent, CuFe12O19 and 2 ml of solvent for 24 h. b Condition 1: tBHP in acetonitrile at 60 °C. c Condition 2: H2O2 in acetone at 80 °C. d The conversion and yields were determined by GC using dodecane as the internal standard.
Condition 1^b	10	87	33	51
	15	91	32	57
	20	93	30	61
	25	96	36	56
	30	85	33	51
Condition 2^c	10	85	68	—
	15	88	70	—
	20	100	81	—
	25	100	67	—
	30	100	60	—

---

3.3.4. Effect of oxidizing agent/styrene molar ratio. The selective catalytic oxidation of styrene was performed by varying the molar ratio of the oxidizing agent/styrene (Table 8). The increase of the tBHP/styrene molar ratio to 3 enhances both conversion and selectivity of styrene oxide. However, with a tBHP/styrene molar ratio of 4, the selectivity of styrene oxide decreases due to the formation of benzoic acid. Similarly, a H₂O₂/styrene molar ratio of 3 gave the best result.

Table 8 Effect of the oxidizing agent/styrene molar ratio on the styrene oxidation in the presence of CuFe₁₂O₁₉ NPs as catalysts^a

Reaction	Oxidizing agent/styrene	Conversion^d (%)	Yield^d (%)
			Benzaldehyde	Styrene oxide
a General reaction conditions: 1.92 mmol of styrene, oxidizing agent, CuFe12O19 (20 mg) and 2 ml of solvent for 24 h. b Condition 1: tBHP in acetonitrile at 60 °C. c Condition 2: H2O2 in acetone at 80 °C. d The conversion and yields were determined by GC using dodecane as the internal standard.
Condition 1^b	1	43	15	24
	2	66	19	42
	3	93	30	61
	4	80	30	45
Condition 2^c	1	10	9	—
	2	40	26	—
	3	100	81	—
	4	100	48	—

---

3.3.5. Effect of the reaction time. A kinetic study of the selective catalytic oxidation of styrene was carried out (Table 9). The evolution of the reaction versus time shows that styrene oxide or benzaldehyde were formed as major products in the presence of tBHP and H₂O₂, respectively. The best result was obtained within 24 h.

Table 9 Effect of the reaction time on the styrene oxidation in the presence of CuFe₁₂O₁₉ NPs as catalysts^a

Reaction	Time (h)	Conversion^d (%)	Yield^d (%)
			Benzaldehyde	Styrene oxide
a General reaction conditions: 1.92 mmol of styrene with 3 eq. of oxidizing agent, CuFe12O19 (20 mg) and 2 ml of solvent. b Condition 1: tBHP in acetonitrile at 60 °C. c Condition 2: H2O2 in acetone at 80 °C. d The conversion and yields were determined by GC using dodecane as the internal standard.
Condition 1^b	3	28	12	12
	6	48	20	21
	12	78	29	46
	15	83	30	50
	18	87	30	55
	24	93	30	61
Condition 2^c	3	9	8	—
	6	40	36	—
	12	68	61	—
	15	77	67	—
	18	83	68	—
	24	100	81	—

---

3.3.6. Effect of basicity. To improve the epoxide selectivity in the oxidation of olefins, the effect of the reaction medium basicity should be taken into consideration. Indeed, the selective catalytic oxidation of styrene-to-styrene oxide was performed using different amounts of urea to assess the effect of basicity (Fig. 10). In the presence of 20 mg of urea (17.34 mol%), a total conversion was obtained and the selectivity of styrene oxide increases to 72%. This result can be explained by the stabilization effect of urea on the epoxide function, which avoids the epoxide ring-opening via hydrolysis reaction. Indeed, Liu et al. have reported that the basicity of the solvent medium increases the styrene oxide selectivity.¹⁷

Fig. 10 Effect of urea in the presence of tBHP and CuFe₁₂O₁₉ as the catalyst.

3.4. Heterogeneity test

A hot-filtration test was performed to investigate the heterogeneity of the CuFe₁₂O₁₉ NPs catalytic system (Fig. 11). After 12 h of the catalytic oxidation of styrene, the catalyst was separated from the reaction mixture by an external magnetic field and the obtained filtrate was continually stirred under the same reaction conditions. The results indicated that no significant enhancement of the styrene conversion was observed even after completing 24 h of reaction time. This result clearly indicates that the CuFe₁₂O₁₉ nanoparticles act as a heterogeneous catalyst in the styrene oxidation.

Fig. 11 Heterogeneity test of the CuFe₁₂O₁₉ nanoparticles.

3.5. Recycling of catalyst

Since catalyst recyclability has great potential for practical applications, the reusability of magnetic CuFe₁₂O₁₉ NPs was evaluated in five consecutive cycles in the selective styrene oxidation reaction under gram-scale conditions (Fig. 12). In the presence of tBHP, the catalytic activity of CuFe₁₂O₁₉ NPs slightly decreases in terms of the styrene conversion and styrene oxide selectivity during the consecutive runs. Hence, the investigation of XRD analysis shows that the identity of the recovered CuFe₁₂O₁₉ catalyst remains like the fresh one (Fig. 13a). However, the SEM image shows agglomerated particles with a change in the surface morphology (Fig. 13b), which could explain the noticeable drop in the reaction conversion and selectivity. On the other hand, the selective catalytic reaction carried out in presence of H₂O₂ and CuFe₁₂O₁₉ exhibits a total and stable conversion within the five runs with a slight decrease in benzaldehyde selectivity over the runs. Furthermore, the XRD patterns and SEM images of the recycled CuFe₁₂O₁₉ catalyst present no obvious changes compared to the fresh one (Fig. 13a and c).

Fig. 12 CuFe₁₂O₁₉ reusability in the gram-scale selective catalytic oxidation of styrene.

Fig. 13 XRD patterns of the fresh and recycled CuFe₁₂O₁₉ (a), and SEM images of CuFe₁₂O₁₉ after five runs while using (b) tBHP or (c) H₂O₂.

3.6. Catalytic oxidation of styrene derivatives

With the optimized reaction conditions (1 and 2) in hand, the scope and limitations of the as-developed catalytic system MFe₁₂O₁₉ were investigated in the catalytic oxidation of various aromatic olefins in the presence of CuFe₁₂O₁₉ NPs as catalysts (Table 10). All studied olefins were selectively converted to the corresponding oxygenated products in excellent to good yields.

Table 10 Catalytic oxidation of styrene derivatives in presence of CuFe₁₂O₁₉ NPs as catalyst^a

Reaction	Substrate	Conversion^d (%)	Yield^d (%)
a General reaction conditions: 1.92 mmol of styrene with 3 eq. of oxidizing agent, CuFe12O19 (20 mg) and 2 ml of solvent for 24 h. b Condition 1: tBHP in acetonitrile with 20 mg of urea at 60 °C. c Condition 2: H2O2 in acetone at 80 °C. d The conversion and yields were determined by GC using dodecane as the internal standard.
Condition 1^b		100
		98
		100		—	—
		100
		80		—	—
Condition 2^c		100
		71			—
		80			—
		58			—

---

3.7. Comparison of the catalytic performance of MNPs

To shed more light on the scope of CuFe₁₂O₁₉ NPs in organic synthesis as MNPs catalysts, the catalytic performance of other MNPs-based catalysts that have been tested in styrene catalytic oxidation so far is summarized in Table 11. Compared to the previous studies, CuFe₁₂O₁₉ exhibits a high catalytic conversion with good selectivity under both conditions (1 and 2). The higher efficiency of CuFe₁₂O₁₉ MNPs can be attributed to the superior catalytic role of Cu and its synergistic effect with Fe, in comparison to Sn and Sr in the studied MFe₁₂O₁₉ NPs. This can be explained by the synergy between CuFe₂O₄ and Fe₂O₃ during the oxidation process, where both Cu and Fe are involved in the coordination with the activated oxidizing agent (tBHP or H₂O₂). This could lead to the formation of peroxo or superoxo species, such as Fe(III)OO˙ and Cu(II)OO˙, complexes that may not be formed with Sn and Sr. Additionally, the possible synergistic intersection between Cu⁺/Cu²⁺ and Fe²⁺/Fe³⁺ redox pairs at the catalyst surface could also contribute to the observed catalytic performance.

Table 11 Comparison of the catalytic activity of MNPs towards the selective oxidation of styrene

Catalyst		Ox. agent	Conv. (%)	Select. SO (%)	Select. BA (%)	Reusability (conv. % : run)	Ref.
CoFe2O4		tBHP	81	76	23	92 : 5	17
α-Fe2O3		tBHP	73	77	ND	—	60
Au/L-Fe3O4		tBHP	76.1	70.1	27.5	—	61
Ag/KOH-γ-Fe2O3		tBHP	89.6	89.7	10.3	89 : 5	62
MFe 12 O 19	CuFe 12 O 19 = {CuFe 2 O 4 –Fe 2 O 3 }	tBHP	100	72	24	45 : 5	Present work
	SnFe 12 O 19 = {SnO 2 –Fe 2 O 3 }	tBHP	65	26	5	—
	SrFe 12 O 19	tBHP	30	11	18	—
SrFe2O4		H2O2	63.7	ND	32	42 : 2	20
BaFe2O4		H2O2	45.1	9.5	88.5	31 : 3	63
CaFe2O4		H2O2	37.9	ND	91.1	—	64
NiFe2O4		H2O2	31.4	ND	55.6	—	18
MFe 12 O 19	CuFe 12 O 19 = {CuFe 2 O 4 –Fe 2 O 3 }	H 2 O 2	100	—	82	100 : 5	Present work
	SnFe 12 O 19 = {SnO 2 –Fe 2 O 3 }	H 2 O 2	100	—	28	—
	SrFe 12 O 19	H 2 O 2	72	—	40	—

---

4. Conclusion

Magnetically separable MFe₁₂O₁₉ NPs were successfully synthesized using the coprecipitation method. As-prepared MFe₁₂O₁₉ (M = Cu, Sn and Sr) NPs exhibited nanoscale particle size, magnetic behavior, mesoporous structure and good recyclability. Advanced characterization studies revealed that SnFe₁₂O₁₉ and CuFe₁₂O₁₉ are not isostructural with the SrFe₁₂O₁₉ M-type hexaferrite, due to confirmed coexistence of SnO₂–Fe₂O₃ and CuFe₂O₄–Fe₂O₃ as a composite structure, respectively. The magnetic MFe₁₂O₁₉ nanocatalysts were found to be highly efficient for the selective oxidation reaction of various styrene derivatives compared to other reported MNPs heterogeneous catalysts. Among the as-synthesized MFe₁₂O₁₉ NPs, CuFe₁₂O₁₉ was found to be the best catalyst either in the presence of tBHP or H₂O₂ as the oxidizing agent. The high catalytic activity and good selectivity of CuFe₁₂O₁₉ MNPs could be associated with the synergistic catalytic effect of CuFe₂O₄ and Fe₂O₃ in a single magnetically recoverable nanostructure, compared to SnFe₁₂O₁₉ and SrFe₁₂O₁₉. We can assume that the developed MFe₁₂O₁₉ MNPs present a facile and greener approach using magnetically recyclable nanostructures for the selective catalytic oxidation reaction of olefins.

Data availability

The device types used for recording spectra and other analytical data that support the findings of this study are included in the manuscript.

Conflicts of interest

The authors declare that they have no conflicts of interest.

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