An Fe₃O₄@P4VP@FeCl₃ core–shell heterogeneous catalyst for aerobic oxidation of alcohols and benzylic oxidation reaction

Abstract

A novel magnetic Fe₃O₄@P4VP(poly(4-vinylpyridine))@FeCl₃ core–shell structure was successfully synthesized. Its Fe₃O₄@P4VP core was initially prepared via polymerization of 4-vinyl pyridine on the surface of Fe₃O₄ microspheres. The successful introduction of the FeCl₃ moiety as a catalytic active site was achieved through coordination interaction between P4VP and FeCl₃. The obtained Fe₃O₄@P4VP@FeCl₃ catalyst was applied in the selective oxidation of alcohols using molecular oxygen as the oxidant. It was shown that a variety of alcohol substrates is tolerated under optimized reaction conditions. Additionally, benzylic oxidation of hydrocarbon compounds was also evaluated using Fe₃O₄@P4VP@FeCl₃ as the catalyst and tBuOOH as the oxidant, achieving high yields and very good selectivities. The heterogeneity of the Fe₃O₄@P4VP@FeCl₃ core–shell catalyst was tested and the initial activity was maintained after five reuses.

---

1. Introduction

The catalytic selective oxidation of alcohols to the corresponding carbonyl compounds is one of the most industrially significant reactions in fine and industrial organic chemistry.¹ The use of transition metal-based homogeneous² and heterogeneous³ catalysts is a good strategy as the use of stoichiometric oxidants such as chromate or permanganate is replaced by atom-economical clean oxidants. However, heterogeneous processes are preferable for oxidation, as they display several advantages during the purification and recovery steps of products and catalysts.⁴ Oxidizing alcohols heterogeneously using molecular oxygen is an ideal reaction process, since it produces H₂O as the sole by-product.⁵ Recently, aerobic oxidations received much attention because of their use of inexpensive oxidants and controllable selectivities. Studies about homogenous catalysts for aerobic oxidation reactions are abundant in the literature.⁶ Several successful noble metal catalytic systems, such as gold,⁷ palladium,⁸ platinum,⁹ rhodium,¹⁰ ruthenium,¹¹ etc. were reported for having achieved efficient oxidation of alcohols.

Over the years, the utilization of relatively inexpensive transitional metals like copper or iron have been considered an approach towards the replacement of such expensive precious metal catalysts. Although copper derived catalysts have been studied thoroughly over the years as an alternative to platinum-based ones in the oxidation of alcohols,¹² iron derived ones remain relatively underexplored. Iron is less expensive and more naturally abundant than copper and other transitional metals. Since the first iron-promoted oxidation of alcohols was reported in 2002,¹³ this field witnessed an increase in iron use for the promotion of oxidation reactions. Wang and coworkers reported a FeCl₃-TEMPO-NaNO₂ system,¹⁴ while Ma and colleagues have published an Fe(NO₃)₃-TEMPO-NaCl system¹⁵ for oxidation reactions. However, homogeneous iron catalytic systems make the recovery of the catalyst extremely difficult, which might pose limitations to industrial scale applications. Thus, in recent years, numerous heterogeneous supports, such as molecular sieves,¹⁶ aluminosilicates,¹⁷ graphene-oxide¹⁸ polymer¹⁹ and inorganic microspheres, have been developed and applied to the alcohol oxidation reaction.²⁰

Although heterogeneous iron catalysts were achieved in a stable and reusable manner, they commonly showed lower efficiency and metal leaching compared to their homogeneous counterparts. Among these heterogeneous catalysts, core–shell structures have been considered highly practical as they take advantage of their unique physicochemical and multi-functional properties.²¹ Magnetic core-shell structures are of particular interest as an effective separation method since they could easily and quickly respond to an external magnetic field.²² Many magnetic core–shell composites have been developed utilizing different coatings, such as silica, carbon, polymeric and porous materials on a Fe₃O₄ core.²³ Several hybrid magnetic core–shell nanocatalysts have been utilized in reductions, oxidations, epoxidations, coupling reactions and photo-catalytic reactions.²⁴ To the best of our knowledge, the combination of magnetic functionality and iron catalytic sites have not been reported in the literature yet. In this work, a core–shell Fe₃O₄@P4VP@FeCl₃ catalyst was synthesized and it was characterized by transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), FTIR spectroscopy, thermogravimetric analysis (TGA) and vibrating sample magnetometry (VSM). Numerous pyridine functional groups provided multi-dentate coordination to the iron(III) ion, which helps stabilizing the metal center. The aerobic oxidation of alcohols and benzylic carbons were both investigated using our synthesized Fe₃O₄@P4VP@FeCl₃ catalyst. High chemical stability and low metal leaching were observed for the oxidation catalysis using our core–shell catalyst.

2. Experimental section

2.1 General information

All reaction substrates and solvents were purchased from Sigma–Aldrich, Alfa Aesar or Aladdin. Ferric chloride hexahydrate (FeCl₃·6H₂O), polyvinyl pyrrolidone (PVP; M_r = 58 000), divinylbenzene (DVB, 80%), potassium persulfate (KPS, 99%) were purchased from Alfa Aesar and were used as received. Poly(acrylic acid) (PAA; M_r = 1800) and 2,2′-azobis (2-methylpropionamidine) dihydrochloride (97%) were purchased from Sigma–Aldrich. Anhydrous sodium acetate (99%) was supplied by Aladdin. 4-Vinylpyridine monomer (96%, 4VP; Aldrich) was stabilized with 100 ppm hydroquinone and used after distillation.

2.2 Characterization

The morphology and size of the as-obtained samples were characterized on a ZEISS SUPRA55 scanning electron microscopy (FESEM). High-resolution transmission electron microscopy (HRTEM) studies were carried out on a JEOL JEM 2010 transmission electron microscope with an accelerating voltage of 200 kV. The phase structure was determined by powder X-ray diffraction (p-XRD) experiments on a DSADVANCE diffractometer with Cu Kα radiation scanning from 10° to 80°. Fourier-transformed infrared spectra (FTIR) were recorded using a NICOLET 6700 infrared spectrophotometer using KBr pellet samples. The magnetization curves of samples were measured by a MPMS-XL superconducting quantum interference device (SQUID) at room temperature. The thermogravimetric analysis (TGA) was carried out on a NETZSCH STAf409 at a heating rate of 10 °C min⁻¹ under nitrogen. X-ray photoelectron spectroscopy data were obtained with a PHI Quantera SXM. Gas chromatography-mass spectrum was recorded using Agilent 7890A/5975C. Products were determined by GC-MS using internal standard technique, and nitrobenzene was used as an internal standard.

2.3 Synthesis of the Fe₃O₄@P4VP materials

The PAA-modified Fe₃O₄ nanoparticles were prepared through an improved one-step solvothermal method reported in the literature.²⁵ In a typical procedure, FeCl₃·6H₂O (1.08 g), PAA (0.108 g), and sodium acetate (9 g) were all dissolved in ethylene glycol (40 mL) to form a homogeneous solution. Then, the solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave and maintained at 200 °C for 12 h. After being cooled to room temperature, the obtained black products were washed with deionized water and ethanol several times, and then dried in a vacuum for 12 h. The core–shell Fe₃O₄@P4VP microspheres were prepared as follows: PAA-modified Fe₃O₄ (0.1 g) was dispersed with PVP (0.15 g) in 100 mL deionized water under ultrasonication. After being transferred into a 250 mL four-necked flask, an emulsion contained PVP (0.05 g), 4-VP (0.125 g), DVB (0.125 g), and deionized water (20 mL) were added. All of the solution was bubbled with nitrogen for 12 h under stirring. When the temperature was raised to 70 °C, a KPS solution (0.25 mL, 0.04 g mL⁻¹) was added into the reaction system. After 4 h of polymerization, the obtained products were washed with ethanol several times and dried under vacuum for 12 h.

2.4 Synthesis of the Fe₃O₄@P4VP@FeCl₃

5.0 g Fe₃O₄@P4VP nanoparticles were dispersed in 100 mL of FeCl₃·6H₂O ethanol solution (0.2 M). The solution was stirred for 12 h at 70 °C in an oil bath. The sample was recovered using a permanent magnet and washed with ethanol, and dried under vacuum at 80 °C. The amount of iron was analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES); the contents of iron in Fe₃O₄@P4VP@FeCl₃ was determined to be 4.36 wt%.

2.5 General procedure for the aerobic oxidation of alcohol

In general, the catalytic reaction was carried out under the following conditions: 2 mol% Fe₃O₄@P4VP@FeCl₃ core–shell catalyst (mole percent was based on iron), 0.1 mmol NaNO₂, 0.2 mmol of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and 1.0 mmol alcohol were mixed in 2.5 mL of CH₃CN. The air in the catalytic reaction system was evacuated and oxygen gas was supplied through an O₂ balloon. After each catalytic cycle, the solution was magnetically separated and the filtered liquid solution was analysed via gas chromatography-mass spectrometry using nitrobenzene as the internal standard.

2.6 General procedure for the oxidation of hydrocarbon compounds

In general, a mixture of 2 mol% Fe₃O₄@P4VP@FeCl₃ core–shell catalyst (mole percent was based on iron), 1.0 mmol of diphenylmethane, tert-butyl hydroperoxide (TBHP, 5.0–6.0 M in decane, 545.0 μL, 3.0 mmol) and 1.0 mL acetonitrile was mixed in a 25 mL single-necked flask fitted with a reflux condenser. The mixture was heated at 80 °C for 24 h under air atmosphere in an oil bath. After each catalytic cycle, the solution was magnetically separated and the filtered liquid solution was analysed via gas chromatography-mass spectrometry using nitrobenzene as the internal standard.

2.7 Leaching test

The Fe₃O₄@P4VP@FeCl₃ core–shell catalyst was magnetically separated after 4 h reaction time, the conversion of benzyl alcohol and selectivity of benzaldehyde was tested by GC/MS using nitrobenzene as the internal standard. The mixture was further stirred for an additional 8 hours. After the reaction, products were again analysed by GC/MS using nitrobenzene as the internal standard.

3. Results and discussion

The Fe₃O₄ microspheres modified with PAA were synthesized by a solvothermal method reported in literature.²⁵ These microspheres were composed of tiny Fe₃O₄ nanocrystals within 10 nm (Fig. 2a). The P4VP shell was successfully grafted on Fe₃O₄ to form core–shell composite microspheres held together by hydrogen bonds between carboxylic groups of PAA chains and pyridine. The diameters of the magnetic core and polymer shell were about 200 and 38 nm, respectively (Fig. 2b). Furthermore, the SEM images show that Fe₃O₄ and Fe₃O₄@P4VP microspheres are monodispersed with a narrow size distribution of about 200 nm and 280 nm, respectively (Fig. 2c and d).

Fig. 1 Schematic illustration of the synthesis of Fe₃O₄@P4VP@FeCl₃.

Fig. 2 TEM images of (a) Fe₃O₄, (b) Fe₃O₄@P4VP; SEM images of (c) Fe₃O₄, (d) Fe₃O₄@P4VP.

Fe₃O₄@P4VP@FeCl₃ core–shell catalysts were successfully prepared taking advantage of the multi-dentate coordination between FeCl₃ and poly(4-vinylpyridine) (Fig. 1). Several nitrogen sites on poly(4-vinylpyridine) ensures the stability of Fe³⁺ ions on the surface of the core–shell structure. The EDX elemental maps further provides the firm evidence of the synthesis of Fe₃O₄@P4VP@FeCl₃ (Fig. 3). Fe₃O₄ nanoparticles were encapsulated into polymer shells, and the iron(III) was well grafted on the surface of Fe₃O₄@P4VP. The iron elemental maps of the Fe₃O₄@P4VP@FeCl₃ also demonstrated an excellent distribution of iron content on the surface. The existence of iron content in the Fe₃O₄ core structure was also observed, as shown in Fig. 3.

Fig. 3 HRTEM images of the Fe₃O₄@P4VP@FeCl₃ and EDX elemental maps of Fe, Cl and N, respectively.

Wide-angle p-XRD spectra of Fe₃O₄ nanoparticles, Fe₃O₄@P4VP core/shell nanospheres, and Fe₃O₄@P4VP@FeCl₃ core–shell structure were collected and shown in Fig. 4. The diffraction peaks of Fe₃O₄ (Fig. 4a) agree with standard JCPDS 75-1609.39 indicating a face-centered cubic lattice. After coating with P4VP, the peaks of the core–shell Fe₃O₄@P4VP microspheres are highly similar as those for Fe₃O₄ due to the low-crystalline nature of the polymeric shell (Fig. 4b). As expected, the introduction of the FeCl₃ to the Fe₃O₄@P4VP nanostructure did not affect the crystalline structures of Fe₃O₄@P4VP significantly. The high chemical stability of Fe₃O₄ and P4VP ensures the structure integration in presence of acidic FeCl₃·6H₂O.

Fig. 4 PXRD patterns of (a) Fe₃O₄, (b) Fe₃O₄@P4VP and (c) Fe₃O₄@P4VP@FeCl₃.

The FTIR spectrum of the product is shown in Fig. 5. The relatively high intensity of the band at 592 cm⁻¹ is characteristic of the Fe–O vibrations. The characteristic absorption of the band at 1707 and 1165 cm⁻¹ corresponds to the C=O stretching of the carboxylic acid group and in-plane deformation of C–O–H stretching, which proves that –COOH functional groups are located on the surface of Fe₃O₄ nanoparticles (Fig. 5a). In the FTIR spectrum of Fe₃O₄@P4VP microspheres, the characteristic absorptions at 1603, 1562, and 1417 cm⁻¹ are attributed to the vibration of the pyridine ring. The band at 1603 cm⁻¹ corresponds to the stretching vibration absorption of the C–N bond, and the bands at around 1562 and 1417 cm⁻¹ are attributed to the stretching vibration absorption of the C=C bond, which further prove the encapsulation of P4VP on the surface of the Fe₃O₄ particles (Fig. 5b). Furthermore, a new peak was observed at 1627 cm⁻¹ after the introduction of the FeCl₃ moiety, which shows chemical interaction between the iron and the polymer (Fig. 5c).

Fig. 5 FTIR spectra of (a) Fe₃O₄, (b) Fe₃O₄@P4VP and (c) Fe₃O₄@P4VP@FeCl₃.

The thermal stability of the Fe₃O₄, Fe₃O₄@P4VP and Fe₃O₄@P4VP@FeCl₃ nanostructures was also examined by TGA in the temperature range of 50–800 °C (Fig. 6). The mass loss of Fe₃O₄ was due to the decomposition of organic PAA layer on the surface of Fe₃O₄ microspheres (Fig. 6a). Fe₃O₄@P4VP microspheres show two steps of mass loss. The first one, from room temperature to 440 °C, is related to the desorption of adsorbed water and the degradation of secondary nucleation polymer chains on the surface of microsphere. The second loss, from 440 to 800 °C, is instead correlated to the polymer shell and the PAA modification on the Fe₃O₄ (Fig. 6b). For the Fe₃O₄@P4VP@FeCl₃ catalyst, the main decomposition stage was similar to Fe₃O₄@P4VP microspheres. The results demonstrated that the Fe₃O₄@P4VP@FeCl₃ catalyst is thermally stable under the conditions of the aerobic oxidation reaction.

Fig. 6 TGA of (a) Fe₃O₄, (b) Fe₃O₄@P4VP and (c) Fe₃O₄@P4VP@FeCl₃.

The magnetic performance of the Fe₃O₄@P4VP@FeCl₃ core–shell nanostructure was investigated by SQUID at room temperature. The magnetization curves of synthesized samples are shown in Fig. 7. The magnetization curves of composite nanospheres exhibit no remanence at room temperature. Again, the results indicate their superparamagnetism, which is crucial for controllable flocculation and dispersion in solution by an external magnetic field. A saturation magnetization value of the Fe₃O₄@P4VP nanospheres of 47.2 emu g⁻¹ ensures its rapid recoverability as catalyst support under an external magnetic field. The value is lower than that of Fe₃O₄ nanoparticles (70.8 emu g⁻¹) due to the capsulation of magnetic components into the P4VP. In addition, Fe₃O₄@P4VP@FeCl₃ catalyst could be easily recovered by an external magnetic field as shown in Fig. 7; its saturation magnetization value is 39.8 emu g⁻¹.

Fig. 7 Room-temperature magnetic hysteresis loops of (a) Fe₃O₄, (b) Fe₃O₄@P4VP and (c) Fe₃O₄@P4VP@FeCl₃. Photos of the insets depict magnetic recycling of Fe₃O₄@P4VP@FeCl₃ catalyst.

The high resolution XPS spectra is obtained and the results are shown in Fig. 8, which shows the XPS spectra of the Fe₃O₄@P4VP@FeCl₃ core–shell structure. The strong peaks in the Fe₃O₄@P4VP@FeCl₃ signal are attributed to the N 1s and C 1s binding energies, respectively. The binding energies at 711 and 55 eV belong to the Fe 3p and Fe 2p_2/3, respectively. The peaks at 268 and 198 eV are due to the existence of Cl 2p and Cl 2s binding, respectively. The results strongly indicate the existence of iron(III) moieties on the surface of the Fe₃O₄@P4VP@FeCl₃ catalyst.

Fig. 8 XPS Spectra of Fe₃O₄@P4VP@FeCl₃.

The catalytic activity of the synthesized Fe₃O₄@P4VP@FeCl₃ toward the aerobic oxidation reaction was evaluated employing alcohol substrates and using TEMPO as the radical initiator. The control experiment showed that no benzaldehyde was detected in absence of catalyst (Table 1, entry 1). Also, the employment of Fe₃O₄ and Fe₃O₄@P4VP as catalysts failed to oxidize any benzyl alcohol to its corresponding aldehyde (Table 1, entries 2 and 3). Homogeneous iron catalysts were evaluated and almost quantitative conversion was observed. However, slightly lowered selectivity was observed due to overoxidation to benzoic acid (Table 1, entries 4–5). The amount of Fe₃O₄@P4VP@FeCl₃ added in this reaction was 2 mol%, based on the same iron loading; quantitative yields were achieved using KNO₂ additive (Table 1, entry 6). Several additives were tested under the same reaction conditions and KNO₂ was the most optimal one. Basic inorganic additives were tested and only low to medium yields were achieved (Table 1, entries 7–8). A basic organic additive was evaluated but a poor yield was observed (Table 1, entry 9). Further solvent screening showed that CH₃CN is the most suitable solvent for the aerobic oxidation of benzyl alcohol in the presence of Fe₃O₄@P4VP@FeCl₃ (Table 1, entries 6, 10–12). Aromatic solvents such as toluene gave lower yields (Table 1, entry 10). Moreover, oxygen containing solvents, such as THF and ethanol, provided much lower yield, presumably due to their ability to excessively coordinate the metal (Table 1, entries 11 and 12).

Table 1 Reaction condition optimization using benzyl alcohol^a

Entry	Catalyst	Solvent	Additive	Selec.^b	Yield^b
a Reaction conditions: 1.0 mmol of benzyl alcohol, 2 mol% of iron catalyst, 5.0 mL of solvent, 0.2 mmol of TEMPO and 0.2 mmol of additive; 60 °C, 12 h under 1 atm O2.b Selectivities and yields were calculated by GC-MS using nitrobenzene as the internal standard.
1	—	CH3CN	—	—	<5%
2	Fe3O4	CH3CN	KNO2	—	<5%
3	Fe3O4@P4VP	CH3CN	KNO2	—	<5%
4	FeCl3·6H2O	CH3CN	KNO2	91%	99%
5	Fe(NO3)3	CH3CN	KNO2	78%	99%
6	Fe3O4@P4VP@FeCl3	CH3CN	KNO2	99%	99%
7	Fe3O4@P4VP@FeCl3	CH3CN	NaHCO3	99%	76%
8	Fe3O4@P4VP@FeCl3	CH3CN	Na2CO3	99%	68%
9	Fe3O4@P4VP@FeCl3	CH3CN	Pyridine	99%	43%
10	Fe3O4@P4VP@FeCl3	PhCH3	KNO2	95%	71%
11	Fe3O4@P4VP@FeCl3	THF	KNO2	99%	55%
12	Fe3O4@P4VP@FeCl3	EtOH	KNO2	99%	21%

---

With the optimal reaction conditions in hand, a variety of alcohol substrates were tested using Fe₃O₄@P4VP@FeCl₃ core–shell catalyst. Benzyl alcohol was transformed to the corresponding benzaldehyde in 99% yield after 12 h (Table 2, entry 1). For the aerobic oxidation of benzyl alcohol in presence of Fe₃O₄@P4VP@FeCl₃ as the catalyst, the turnover number (TON) was calculated to be 50. p-Methyl benzyl alcohol and p-methoxy benzyl alcohol were reactive as electron-rich benzyl alcohols under the optimal reaction conditions, affording 99% and 95% yield, respectively (Table 2, entries 2 and 3). Alcohol substrates bearing electron-withdrawing functional groups, such as p-fluorobenzyl alcohol, were also tolerated. A slightly lowered yield was observed due to the reduction in electron efficiency (Table 2, entry 4). The heterocyclic alcohol pyridin-2-ylmethanol was selectively converted to its corresponding aldehyde, in the presence of the Fe derived core–shell catalyst (Table 2, entry 5). Moreover, cinnamaldehyde was obtained in good yield starting from the corresponding allylic alcohol (Table 2, entry 6). The transformation of secondary alcohols to ketones is also highly important in synthetic chemistry; 1-phenylethanol and cyclohexanol were both tested as secondary alcohols (Table 2, entries 7 and 8). Unfortunately, only 50% and 22% of acetophenone and cyclohexanone respectively were obtained.

Table 2 Aerobic oxidation of a variety of alcohols^a

a Reaction conditions: 1.0 mmol of benzyl alcohol, 2 mol% of Fe₃O₄@P4VP@FeCl₃ catalyst, 5.0 mL of CH₃CN, 0.2 mmol of TEMPO and 0.2 mmol of additive; 60 °C, 12 h under 1 atm O₂.

---

The oxidation of diphenylmethane 3a for the generation of benzophenone 4a was carried out in 2.0 mL acetonitrile at 80 °C for 24 h (Table 3). The activation of C–H bond is difficult and tert-butyl hydroperoxide (TBHP) was employed as the oxidant under extended reaction time. The catalytic system has been re-optimized to adapt the benzylic oxidation reaction of hydrocarbon derivatives. In the absence of any additive, FeCl₃·6H₂O and Fe(NO₃)₃·9H₂O failed to promote any conversion (Table 3, entries 1 and 3). The employment of basic pyridine additive strongly increased the yield of the desired benzophenone product (Table 3, entries 2 and 4). Initially, 91% conversion and 99% selectivity were obtained for Fe₃O₄@P4VP@FeCl₃ catalyst in the absence of additive (Table 1, entry 5). The high efficiency of Fe₃O₄@P4VP@FeCl₃ catalyst was due to the pyridine sites on the surface of the heterogeneous iron catalyst. The addition of a small amount of homogeneous pyridine can further boost the conversion to over 99% (Table 1, entry 6). Furthermore, the amount of pyridine can be reduced to 2 mol% without affecting the yield and selectivity of the diphenylmethane oxidation (Table 1, entry 7). In addition, other basic additives were examined. However, almost no improvement was observed when triethylamine (Et₃N) was employed (Table 3, entry 8). Hexamethylenetetramine (HMTA) failed to increase the yield as well (Table 3, entry 9). These results indicate that pyridine was the best base additive among the organic bases screened. Our Fe₃O₄@P4VP@FeCl₃ catalyst was designed so that the heterogeneous P4VP layer would provide large amounts of pyridine sites. These heterogeneous pyridine sites were highly helpful for the oxidation reaction process, while homogeneous FeCl₃ failed to convert diphenylmethane to the desired product. Several organic solvents were screened and acetonitrile demonstrated good compatibility in the catalytic system, which resulted in 99% isolated yield (Table 3, entry 2). Other solvents showed a trend similar to the results in Table 1. Therefore the optimal catalytic system was set to be 2.0 mol% Fe₃O₄@P4VP@FeCl₃ catalyst, 0.02 equivalent of pyridine additive and 3.0 equivalents of TBHP in acetonitrile solvent.

Table 3 Benzylic oxidation in presence of different additives^a

Entry	Catalyst	Additive	Selec.	Yield^b
a Reaction conditions: 1.0 mmol 3d, 2.0 mol% Fe3O4@P4VP@FeCl3 catalyst, 5 mol% additive, 3.0 mmol TBHP, 2.0 mL acetonitrile, 80 °C for 24 h.b Isolated yield.c 2 mol% pyridine.
1	FeCl3·6H2O	None	—	<5%
2	FeCl3·6H2O	Pyridine	99%	91%
3	Fe(NO3)3·9H2O	None	—	<5%
4	Fe(NO3)3·9H2O	Pyridine	99%	75%
5	Fe3O4@P4VP@FeCl3	None	99%	84%
6	Fe3O4@P4VP@FeCl3	Pyridine	99%	99%
7^c	Fe3O4@P4VP@FeCl3	Pyridine	99%	99%
8	Fe3O4@P4VP@FeCl3	Et3N	99%	85%
9	Fe3O4@P4VP@FeCl3	HMTA	99%	87%

---

The optimized reaction conditions were utilized for the oxidation of other substrates in the presence of tBuOOH (Table 4). C–H activation/oxidation was further extended to other hydrocarbon compounds. Ethylbenzene 3a was oxidized to 1-phenylethanone 4a in 91% conversion and 97% selectivity (Table 4, compound 4a). Ethylthiophene 3b containing a heterocyclic moiety was also employed as the substrate and over 96% yield and up to 99% selectivity were achieved (Table 3, compound 4b). Isochromane 3c underwent oxidation to give 4-isochromanone 4c in high yield (Table 4, compound 4c). The oxidation of diphenylmethane 3d and bis(4-fluorophenyl)methanone 3f for the generation of corresponding ketones 4d and 4f were smooth (Table 4, compounds 4d and 4f). Moreover, by oxidation of 9H-fluorene 3g, the reaction proceeded smoothly to form the corresponding ketones in high yield and selectivity (Table 4, compound 4g). In summary, the Fe₃O₄@P4VP@FeCl₃ was a versatile catalyst for benzylic methylene compounds (Table 4).

Table 4 Benzylic oxidation of hydrocarbon derivatives

---

The recyclability of the catalyst was studied under the optimized conditions and utilizing 2 mol% and 0.5 mol% of Fe₃O₄@P4VP@FeCl₃ catalyst in acetonitrile (Fig. 9). The same batch of the Fe₃O₄@P4VP@FeCl₃ catalyst was reused for five catalytic cycles. Conversion and selectivity were retained at 99% and the yield of benzaldehyde was compromised only slightly after five cycles (Fig. 9). At a lower catalyst loading of 0.5 mol%, 44% yield was achieved in the initial cycle. Only slight yield decrease (98%) was observed after 5 cycles of reaction (Fig. 9). The ability to retain conversion rates and selectivities after five cycles indicates that the core–shell heterogeneous catalyst is highly stable. TEM and p-XRD spectra showed no significant difference between fresh Fe₃O₄@P4VP@FeCl₃ and five-time reused samples (Fig. S1 and S2†).

Fig. 9 Core-shell iron catalyst recyclability test at 2 mol% and 0.5 mol% catalyst loading for aerobic oxidation of benzyl alcohol.

The hot filtration test was conducted to confirm the heterogeneous nature of the catalytic aerobic oxidation and low reaction leaching (Fig. 10). The Fe₃O₄@P4VP@FeCl₃ catalyst was isolated using an external magnet after 4 hours of reaction and the mixture was stirred for another 8 h. Conversion of benzyl alcohol stopped after the catalyst was removed at 4 h reaction time. This result indicates that the multi-dentate Fe⋯N bonds ensure the stability of the Fe₃O₄@P4VP@FeCl₃ catalyst during the liquid oxidation process. Furthermore, the reaction solution after solid catalyst removal was tested by ICP-AES, which suggested 2.3 ppm of iron content. These results demonstrated the high stability of Fe₃O₄@P4VP@FeCl₃ core–shell catalyst during the oxidation reaction condition.

Fig. 10 Hot filtration test of Fe₃O₄@P4VP@FeCl₃ in aerobic oxidation of benzyl alcohol.

Several control experiments were designed and conducted in order to reveal the reaction mechanism. First of all, the oxidation reaction was carried out without TEMPO (Scheme 1, reaction 1). In the absence of radical initiator, reaction was not able to proceed. This observation suggested radical reaction pathway. Furthermore, reaction was performed under open air condition, decent yield of the desired aldehyde product was obtained (Scheme 1, reaction 2). Oxygen was crucial in the oxidation process and the certain density of oxygen ensures the high conversion of the alcohol product.

Scheme 1 Controlled experimental reaction.

A overall mechanism of Fe(III) promoted catalytic oxidation can be described as a redox reaction involving two separated cycles in Fig. 11. KNO₂ was acting as a source of NO₂. Therefore, TEMPO is envisioned to carry out the main oxidation reaction of alcohols with the help of Fe(III) that initiates a series of electron and proton transfer steps in the right cycle where TEMPO-Fe(III) is reduced to generate TEMPOH-Fe(II). The overall oxidation process involves the oxidation of Fe(II) to Fe(III) by NO₂ and the oxidation of TEMPOH to TEMPO by Fe(III). The conversion of NO to NO₂ was performed in the presence of molecular oxygen.

Fig. 11 Reaction mechanism of Fe(III) promoted oxidation.

4. Conclusions

In summary, a novel magnetic core–shell Fe₃O₄@P4VP@FeCl₃ structure was designed, prepared and fully characterized. The iron(III) immobilized core–shell microspheres were composed by a magnetic Fe₃O₄ core and a P4VP middle layer. Taking advantage of active iron(III) catalytic site, the magnetic composite microspheres were utilized as an efficient catalyst for the selective aerobic oxidation of alcohols. A variety of alcohol substrates were tolerated under our optimized conditions at only 2 mol% catalyst loading. In addition, benzylic oxidation of hydrocarbon compounds was also carried out using the Fe₃O₄@P4VP@FeCl₃ catalyst and tBuOOH oxidant. The initial catalytic activity of the Fe₃O₄@P4VP@FeCl₃ catalyst was retained after at least five consecutive reaction cycles. A hot filtration test suggested low leaching of iron into the solution. Further applications of the various heterogeneous core–shell catalysts are currently under investigation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by Beijing Natural Science Foundation (2172037) and National Natural Science Foundation of China (No. 51503016). P. Y. thanks the State Key Laboratory of Chemical Resource Engineering and Y. L. thanks the Fundamental Research Funds for the Central Universities (Grant No. FRF-TP-16-004A3) for funding support.

Notes and references

1. Q. Cao, L. M. Dornan, L. Rogan, N. L. Hughes and M. J. Muldoon, Chem. Commun., 2014, 50, 4524–4543.
2. (a) R. A. Sheldon, Catal. Today, 2015, 247, 4; (b) C. Parmeggiani and F. Cardona, Green Chem., 2012, 14, 547; (c) Z. Shi, C. Zhang, C. Tang and N. Jiao, Chem. Soc. Rev., 2012, 41, 3381; (d) Q. Cao, L. M. Dornan, L. Rogan, N. L. Hughes and M. J. Muldoon, Chem. Commun., 2014, 50, 4524; (e) M. J. Schultz and M. S. Sigman, Tetrahedron, 2006, 62, 8227.
3. (a) Z. Guo, B. Liu, Q. Zhang, W. Deng, Y. Wang and Y. Yang, Chem. Soc. Rev., 2014, 43, 3480; (b) S. E. Devis, M. S. Lde and R. J. Davis, Green Chem., 2013, 15, 17.
4. Z. Guo, B. Liu, Q. Zhang, W. Deng, Y. Wang and Y. Yang, Chem. Soc. Rev., 2014, 43, 3480–3524.
5. M. Hajimohammadi, N. Safari, H. Mofakham and F. Deyhimi, Green Chem., 2011, 13, 991.
6. B. Chen, L. Wang and S. Gao, ACS Catal., 2015, 5, 5851–5876.
7. (a) P. Miedziak, M. Sankar, N. Dimitratos, J. A. Lopez-Sanchez, A. F. Carley, D. W. Knight, S. H. Taylor, C. J. Kiely and G. J. Hutchings, Catal. Today, 2011, 164, 315; (b) Y. Li, Y. Gao and C. Yang, Chem. Commun., 2015, 51, 7721; (c) X. Yu, Y. Huo, J. Yang, S. Chang, Y. Ma and W. Huang, Appl. Surf. Sci., 2013, 280, 450; (d) R. H. Adnan, G. G. Andersson, M. I. J. Polson, G. F. Metha and V. B. Golovko, Catal. Sci. Technol., 2015, 5, 1323; (e) T. Wang, X. Yuan, S. Li, L. Zeng and J. Gong, Nanoscale, 2015, 7, 7593; (f) C. D. Pina, E. Falletta, L. Prati and M. Rossi, Chem. Soc. Rev., 2008, 37, 2077.
8. (a) K. M. Gligorich and M. S. Sigman, Chem. Commun., 2009, 3854; (b) Y. Ye, M. D. Johnson, T. Diao, M. H. Yates and S. S. Stahl, Green Chem., 2010, 12, 1180; (c) B. Karimi, S. Abedi, J. H. Clark and V. Budarin, Angew. Chem., Int. Ed., 2006, 45, 4776; (d) B. Karimi, A. Zamani, S. Abedi and J. H. Clark, Green Chem., 2009, 11, 109; (e) D. Sahu, C. Sarmah and P. Das, Tetrahedron Lett., 2014, 55, 3422; (f) S. F. J. Hackett, R. M. Brydson, M. H. Gass, I. Harvey, A. D. Newman, K. Wilson and A. F. Lee, Angew. Chem., Int. Ed., 2007, 46, 8593; (g) H. Wu, Q. Zhang and Y. Wang, Adv. Synth. Catal., 2005, 347, 1536; (h) X. Ye, M. D. Johnson, T. Diao, M. H. Yates and S. S. Stahl, Green Chem., 2010, 12, 1180; (i) B. Karimi, D. Elhamifar, J. H. Clark and A. J. Hunt, Org. Biomol. Chem., 2011, 9, 7420; (j) H. Yang, X. Han, Z. Ma, R. Wang, J. Liu and X. Ji, Green Chem., 2010, 12, 441; (k) C. M. A. Parlett, D. W. Bruce, N. S. Hondow, A. F. Lee and K. Wilson, ACS Catal., 2011, 1, 636; (l) J. Chen, Q. Zhang, Y. Wang and H. Wan, Adv. Synth. Catal., 2008, 350, 453; (m) B. Karimi, A. Zamani and J. H. Clark, Organometallics, 2005, 24, 4695; (n) M. J. Schultz, C. C. Park and M. S. Sigman, Chem. Commun., 2002, 3034.
9. (a) Y. Hong, X. Yan, X. Liao, R. Li, X. Xu, L. Xiao and J. Fan, Chem. Commun., 2014, 50, 9679; (b) T. Lu, Z. Du, J. Liu, H. Ma and J. Xu, Green Chem., 2013, 15, 2215; (c) T. Wang, H. Shou, Y. Kou and H. Liu, Green Chem., 2009, 11, 562; (d) K. Kon, S. M. A. H. Siddiki and K.-i. Shimizu, J. Catal., 2013, 304, 63; (e) T. Wang, C.-X. Xiao, L. Yan, L. Xu, J. Luo, H. Shou, Y. Kou and H. Liu, Chem. Commun., 2007, 4375.
10. (a) A. Wusiman and C.-D. Lu, Appl. Organomet. Chem., 2015, 29, 254; (b) J. Olguin, H. Muller-Bunz and M. Albrecht, Chem. Commun., 2014, 50, 3488; (c) L. Liu, M. Yu, B. B. Wayland and X. Fu, Chem. Commun., 2010, 46, 6353.
11. (a) K. Yamaguchi and N. Mizuno, Angew. Chem., Int. Ed., 2002, 41, 4538; (b) Z. W. Yang, X. Zhao, T. J. Li, W. L. Chen, Q. X. Kang, X. Q. Xu, X. X. Liang, Y. Feng, H. H. Duan and Z. Q. Li, Catal. Commun., 2015, 65, 34; (c) D. Canseco-Gonzalez and M. Albrecht, Dalton Trans., 2013, 42, 7424.
12. (a) B. Xu, J. P. Lumb and B. A. Arndtsen, Angew. Chem., Int. Ed., 2015, 54, 4208; (b) L. Vanoye, M. Pablos, C. D. Bellefon and A. Favre-Reguillon, Adv. Synth. Catal., 2015, 357, 739; (c) B.-T. Chen, K. V. Bukhryakov, R. Sougrat and V. O. Rodionov, ACS Catal., 2015, 5, 1313; (d) G. Zhang, C. Yang, E. Liu, L. Li, J. A. Golen and A. L. Rheingold, RSC Adv., 2014, 4, 61907; (e) P. Gamez, I. W. C. E. Arends, J. Reedijk and R. A. Sheldon, Chem. Commun., 2003, 2414–2415; (f) G. Zhang, X. Han, Y. Luan, Y. Wang, X. Wen, L. Xu, C. Ding and J. Gao, RSC Adv., 2013, 3, 9255; (g) S. Samanta, S. Das, P. K. Samanta, S. Dutta and P. Biswas, RSC Adv., 2013, 3, 19455.
13. S. E. Martin and D. F. Suarez, Tetrahedron Lett., 2002, 43, 4475.
14. N. Wang, R. Liu, J. Chen and X. Liang, Chem. Commun., 2005, 5322.
15. J. Liu and S. Ma, Org. Lett., 2013, 15, 5150.
16. (a) M. A. Chari and K. Syamasundar, Synthesis, 2005, 5, 0708–0710; (b) M. R. Farsani and B. Yadollahi, J. Mol. Catal. A: Chem., 2014, 392, 8–15; (c) Y. Kuang, Y. Nabae, T. Hayakawa and M. Kakimoto, Appl. Catal., A, 2012, 423–424, 52–58; (d) P. Das, N. Aggarwal and N. R. Guha, Tetrahedron Lett., 2013, 54, 2924–2928.
17. F. Rajabi, A. Pineda, S. Naserian, A. M. Balu, R. Luque and A. A. Romero, Green Chem., 2013, 15, 1232.
18. M. Mahyari and A. Shaabani, Appl. Catal., A, 2014, 469, 524.
19. P. R. Likhar, R. Arundhathi, S. Ghosh and M. L. Kantam, J. Mol. Catal. A: Chem., 2009, 302, 142–149.
20. J. Mao, X. Hu, H. Li, Y. Sun, C. Wang and Z. Chen, Green Chem., 2008, 10, 827–831.
21. R. G. Chaudhuri and S. Paria, Chem. Rev., 2012, 112, 2373–2433.
22. M. B. Gawande, A. Goswami, T. Asefa, H. Guo, A. V. Biradar, D. L. Peng, R. Zboril and R. S. Varma, Chem. Soc. Rev., 2015, 44, 7540–7590.
23. (a) C. Hui, C. Shen, J. Tian, L. Bao, H. Ding, C. Li, Y. Tian, X. Shi and H. J. Gao, Nanoscale, 2011, 3, 701–705; (b) Y. D. Chiang, S. Dutta, C. T. Chen, Y. T. Huang, K. S. Lin, J. C. S. Wu, N. Suzuki, Y. Yamauchi and K. C. W. Wu, ChemSusChem, 2015, 8, 789–794.
24. (a) Q. Zhang, I. Lee, J. B. Joo, F. Zaera and Y. Yin, Acc. Chem. Res., 46, 1816–1824; (b) P. Hu, J. V. Morabito and C. K. Tsung, ACS Catal., 2014, 4, 4409–4419; (c) Y. Long, M. Xie, J. Niu, P. Wang and J. Ma, Appl. Surf. Sci., 2013, 277, 288–292; (d) X. Zhang, J. Wu, G. Meng, X. Guo, C. Liu and Z. Liu, Appl. Surf. Sci., 2016, 366, 486–493; (e) Z. Sun, H. Li, G. Cui, Y. Tian and S. Yan, Appl. Surf. Sci., 2016, 360, 252–262.
25. H. Deng, X. Li, Q. Peng, X. Wang, J. Chen and Y. Li, Angew. Chem., Int. Ed., 2005, 44, 2782–2785.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra09005f

---