Copper ferrite–graphene hybrid: a highly efficient magnetic catalyst for chemoselective reduction of nitroarenes

Abstract

In this paper, a superparamagnetic copper ferrite–graphene hybrid nanocomposite (CuFe₂O₄–G) was synthesized by a hydrothermal method and used as an efficient catalyst for the reduction of nitroarenes for the first time. The method has been applied to a broad range of compounds with different properties. The CuFe₂O₄–G catalyst can be readily recovered and reused for at least five consecutive cycles without significant loss of its catalytic activity.

---

Introduction

Development of new catalytic transformations with easy separation and recyclability of the catalyst is an essential task in chemical synthesis. Aromatic amines are important starting materials and intermediates for the manufacture of a great variety of chemicals such as dyes, pesticides, herbicides, and agrochemicals.^1–3 They are generally synthesized by the chemical reduction of nitroarenes. Iron-compound,⁴ mixed metal compounds,⁵ transition metals (Cu, Ni)⁶ and noble metals (Pt, Pd, Au)^7–9 are commonly employed as the catalyst for the reduction reaction. However, some of the methods employed have drawbacks, for example using expensive reagents, prolonged reaction time, using toxic and volatile solvent, unsatisfactory yield, etc. Hence, the development of an inexpensive, facile and effective method for the synthesis of aromatic amines is of high demand.

The copper ferrite, CuFe₂O₄ with a spinel structure, is one of the most important ferrites in powder form, has been widely applied in sensors, electronics and catalysts owing to its advantages of environmentally compatible, moisture insensitive, high dispersion, high reactivity and easy separation at an external magnet in recent years. Amini et al.¹⁰ investigated the low temperature CO oxidation over mesoporous CuFe₂O₄ nanopowders synthesized by a novel sol–gel method. Parella et al.¹¹ explored the catalytic application of CuFe₂O₄ nanoparticles for the Friedel–Crafts acylation. Feng et al.⁶ investigated the catalytic activity of CuFe₂O₄ nanoparticles for the reduction of 4-nitrophenol to 4-aminophenol with excess amount of NaBH₄.

Graphene(G), discovered in 2004 by Geim and co-workers,¹² has enjoyed tremendous research interest in a wide range of fields over the past decades. Graphene-related nanomaterials exhibit a great application potential in catalysis¹³ thanks to their large specific surface area, easy modification, remarkable electrical conductivity, excellent adsorptivity, ultrathin thickness, superior structural flexibility, and high chemical and thermal stability. To date, various kinds of catalysts, including TiO₂,¹⁴ SnO₂,¹⁵ ZnO,¹⁶ Pt,^17,18 Pt–Ru,¹⁹ Pt–Pd,²⁰ Pd²¹ and so on, have been supported on graphene-based templates for catalyzing different chemical transformations, energy conversion and photocatalytic reactions. Recently, superparamagnetic graphene–Fe₃O₄ nanocomposite (G–Fe₃O₄) was fabricated and used as an efficient catalyst for the reduction of nitroarenes with hydrazine hydrate as reductant, which is a toxic reagent.²² Copper ferrite–graphene hybrid was developed as an effective photocatalyst for the degradation of methylene blue under visible light irradiation, which also showed excellent electrochemical behaviors for use as the anode in lithium-ion batteries.²³ However, to our best knowledge, there has been no report yet about the use of copper ferrite–graphene (CuFe₂O₄–G) magnetic nanocomposite as catalyst for organic synthesis.

In the present work, the catalytic activity of magnetic CuFe₂O₄–G nanocomposite for the chemoselective reduction of nitroarenes was investigated using sodium borohydride as a hydrogen donor. The results demonstrate that the combination of CuFe₂O₄ with graphene results in a dramatic enhancement of the catalytic activity of CuFe₂O₄.

Results and discussions

Material characterizations

X-ray powder diffraction analysis was used to identify the crystal structure of the CuFe₂O₄–G (0.25) and CuFe₂O₄. As shown in Fig. 1, except some Cu impurity peaks at 43.0° and 50.6°, all peaks were indexed to be CuFe₂O₄ (JCPDS 77-0010), in detail, the peaks at 30.5°, 35.2°, 57.0°, 62.8° and 74.1° are attributed to (220), (311), (511), (440) and (533) crystal planes of CuFe₂O₄. While no typical diffraction peak of reduced graphene oxide was observed. It is speculated that the GO in the CuFe₂O₄–G(0.25) heteroarchitecture was fully exfoliated due to the crystal growth of CuFe₂O₄ nanoparticles between the interlayer of GO sheets, which result in the low diffraction intensity of GO. The formation of copper was identified based on the reported data (JCPDS 85-1326) at 43.0° and 50.6°. The reason of existence of metallic copper was that some of Cu²⁺ was reduced to copper metal in the formation process of CuFe₂O₄ due to the strong reducing capability of ethylene glycol.⁶

Fig. 1 XRD pattern of as-prepared CuFe₂O₄–G(0.25) and CuFe₂O₄. Symbols (△) represent peaks of Cu.

Fig. 2 shows the SEM image of CuFe₂O₄–G (0.25). It can be seen that CuFe₂O₄–G(0.25) were composed of quasi-sphere particle with particle sizes of about 100 nm.

Fig. 2 SEM image of CuFe₂O₄–G(0.25).

The FTIR spectra were recorded to further testify the hybrid material. As shown in Fig. 3, the spectrum of GO is in good agreement with previous work.²⁴ The broad, intense band at 3250 cm⁻¹ is assigned to the stretching of O–H. The peak at 1728 cm⁻¹ corresponds to the stretching of the –C=O and –COOH groups on GO sheets. The peak at 1616 cm⁻¹ (aromatic C=C) can be ascribed to the skeletal vibrations of unoxidized graphene domains. The C–O bond is associated with the band at 1047 cm⁻¹. From the FTIR spectrum of GO and CuFe₂O₄–G (0.25) samples, it is can be clearly seen that the graphene oxide exhibits an obvious characteristic absorption peak at about 1728 cm⁻¹ corresponds to the stretching of the –C=O and –COOH groups. However, it is can not be seen from the FTIR spectrum of CuFe₂O₄–G (0.25). It turned out that graphene oxide was reduced to graphene due to the strong reducing capability of ethylene glycol. From the FTIR spectrum of CuFe₂O₄ and CuFe₂O₄–G (0.25) samples, the spectra show two main absorption corresponding to the stretching vibration of the tetrahedral and octahedral sites around 586 and 400 cm⁻¹, respectively. The observed values illustrate that the frequency bands appearing at 586 and 400 cm⁻¹ are responsible for the formation of CuFe₂O₄. The absorption band at 1596 cm⁻¹ on spectrum referred to the vibration of remainder H₂O in the sample.⁶

Fig. 3 FT-IR spectra of the GO, CuFe₂O₄–G(0.25) and CuFe₂O₄.

Catalytic activity CuFe₂O₄–G

To obtain the optimal reaction conditions, the effect of type of catalyst and catalyst dosage on the reduction of p-nitrophenol were initially investigated (Table 1). As shown in Table 1, no product was obtained in the absence of the catalyst (Table 1, entry 1), indicating that the catalyst was necessary for the reaction. Only low yields of the product were obtained with either graphene, Fe₃O₄ or Fe₃O₄–graphene (with 25 wt% graphene) as catalyst (Table 1, entries 2–4). While CuFe₂O₄ could catalyze the reduction of p-nitrophenol efficiently, 99% yield was obtained when the reaction was performed at 343 K for 16 min (Table 1, entry 5). The combination of CuFe₂O₄ with appropriate amount of graphene results in a dramatic enhancement of the catalytic activity of CuFe₂O₄. The reaction time required to complete the reaction decreased with increasing the loading amount of graphene in CuFe₂O₄–G until 25 wt% (Table 1, entries 7–9). Compare with CuFe₂O₄ catalyst, the reaction time could lower down to 9 min from 16 min with CuFe₂O₄–G (0.25) as the catalyst, which can be attributed to its unique heteroarchitechture, which provides the remarkable synergistic effect between the CuFe₂O₄ and the graphene sheets. The presence of graphene could enhance the adsorption of reactant molecules onto the catalytic sites of the CuFe₂O₄–G through the π–π stacking and/or electrostatic interaction. However, the catalytic activity of CuFe₂O₄–G decreased when the content of graphene was increased to 35% (Table 1, entry 6). The large amount of graphene in CuFe₂O₄–G might decrease the number of the catalytic sites, which resulted in the decreasing of the catalytic activity of CuFe₂O₄–G (0.35). The above results also indicating that there is the most synergistic effect between CuFe₂O₄ and the graphene sheets. The reaction time could be further reduced with increasing the dosage of CuFe₂O₄–G (0.25) (Table 1, entries 11 and 12). But a longer reaction time was required to complete the reaction with decreasing the reaction temperature from 343 K to 333 K (Table 1, entry 13).

Table 1 Screening and control experiments for hydrogenation of p-nitrophenol^a

Entry	Catalyst	Catalyst (mg)	Time (min)	Yield (%)
a Reaction condition: p-nitrophenol (1 mmol), solvent (10 mL, EtOH : water = 1 : 1), NaBH4 (5 mmol), temperature (343 K).b Temperature (333 K).
1	—	20	500	—
2	G	20	500	Trace
3	Fe3O4	20	60	26
4	Fe3O4–G(0.25)	20	60	39
5	CuFe2O4	20	16	99
6	CuFe2O4–G(0.35)	20	35	99
7	CuFe2O4–G(0.25)	20	9	99
8	CuFe2O4–G(0.15)	20	12	99
9	CuFe2O4–G(0.1)	20	14	99
10	CuFe2O4–G(0.25)	10	12	99
11	CuFe2O4–G(0.25)	30	4	99
12	CuFe2O4–G(0.25)	50	3	99
13	CuFe2O4–G(0.25)	20	40^b	99

---

Fig. 4 showed the UV-vis absorption spectra of the reduction of p-nitroaniline by NaBH₄ at various reaction times in the presence of CuFe₂O₄–G (0.25). The observed peak at 385 nm for the p-nitroaniline shows a gradual decrease in intensity with time and a new peak appeared at 295 nm indicating the formation of p-phenylene diamine. The results indicated that CuFe₂O₄–G (0.25) exhibited considerably high activity for the reduction of nitroarenes with sodium borohydride as the hydrogen donor.

Fig. 4 Successive UV-vis absorption spectra of the reduction of p-nitroaniline with NaBH₄ in the presence of CuFe₂O₄–G (0.25).

To show the generality of this model reaction, the reduction of a series of nitro aromatics was studied under the optimized reaction conditions. As shown in Table 2, aromatic nitro compounds containing various electrons donating (Table 2, entries 1–6) or electron-withdrawing groups (Table 2, entries 8) were converted to the corresponding amino aromatics in good yields and all the reactions could proceed smoothly. Moreover, the reduction was also successfully carried out on bulkier molecule such as 1-nitronaphthalene with high yield (Table 2, entry 7).

Table 2 Chemoselective reduction of nitro compounds into amines by sodium borohydride catalyzed by CuFe₂O₄–G(25%)^a

Entry	Carbonyl compound	Product	NaBH4 (mmol)	Time (min)	Yield^b (%)
a Reaction condition: nitro compounds (1 mmol), solvent (10 mL, EtOH : water = 1 : 1), catalyst (20 mg), temperature (343 K).b Isolated yields.
1			5	9	99
2			5	4	99
3			5	10	96
4			5	30	95
5			5	45	92
6			5	10	99
7			5	20	91
8			6	30	93
9			5	30	92

---

The reusability and recycling of CuFe₂O₄–G(0.25) was also investigated. The catalyst was separated from the reaction mixture using an external magnet, washed with ethanol for three times, dried at 60 °C in a vacuum oven for 2 h and reused in another reaction. The catalytic activity of CuFe₂O₄–G(0.25) did not show any significant decrease even after five runs.

Experimental

Materials and instrumentation

Ferric trichloride hexahydrate (FeCl₃·6H₂O), copper sulfate pentahydrate (CuSO₄·5H₂O), sodium borohydride (NaBH₄), glycol, sodium hydroxide (NaOH), ethanol were all obtained from Chengxin Chemical Reagents Company (Baoding, China). Nitroaromatics were purchased from Aladdin Reagent Limited Company. The water used throughout the work was double-distilled on a SZ-93 automatic double-distiller from Shanghai Yarong Biochemistry Instrumental Factory (Shanghai, China).

IR spectra (cm⁻¹) were measured with a WQF-510 spectrometer. UV-vis absorption spectra were measured with a UV-vis spectrometer (UV-3600, Shimadzu). The size and morphology of the catalyst were observed by scanning electron microscopy (SEM) using a Hitachi S4800 field emission electron microscope operated at 30 kV. The XRD patterns of the samples were recorded with a Rigaku D/max 2500 X-ray diffractometer using Cu Kα radiation (40 kV, 150 mA) in the range 2θ = 10°–80°. ¹H NMR (400 MHz) spectra were obtained with a Brucker AVANCE 400 spectrometer (DMSO-d₆) using TMS as an internal standard.

Synthesis of CuFe₂O₄–graphene heteroarchitecture

Graphite oxide (GO) was prepared according to the procedure reported by us.^25,26 CuFe₂O₄–graphene heteroarchitectures with differing graphene content (10, 15, 25, 35 wt%) were synthesized by a modified hydrothermal method reported in the literature.

A typical experiment procedure for the synthesis of CuFe₂O₄–graphene heteroarchitecture with 25 wt% graphene content is as follows: 40 mg of GO was dispersed into 30 mL of ethylene glycol with sonication for 12 h. 0.1675 g (0.67 mmol) of CuSO₄·5H₂O and 0.3618 g (1.34 mmol) of FeCl₃ were added to 10 mL of ethylene glycol and sonicated for 1 h. The above two solutions were then mixed together and stirred for 30 min. After that, the mixture was adjusted to pH of 8 with 6 mol L⁻¹ NaOH aqueous solution and stirred for 30 min, yielding a stable bottlegreen homogeneous emulsion. The resulting mixture was transferred into a 70 mL Teflon-lined stainless steel autoclave and heated to 180 °C for 24 h under autogenous pressure. After the reaction mixture was cooled down to room temperature, the precipitate was filtered, washed with distilled water and ethanol, and dried in a vacuum oven at 30 °C for 12 h. The product was labeled as CuFe₂O₄–G (0.25). For comparison, the same method was used to synthesize CuFe₂O₄ without adding GO.

Reduction reactions of nitroarenes

In a 25 mL round bottom flask, nitroarenes (1.0 mmol, 1 eq.) was dissolved in a mixture of 10 mL H₂O–EtOH (1 : 1, v/v). Then, sodium borohydride (5.0 mmol, 5 eq.) and CuFe₂O₄–G (0.25) (20 mg) were added. The mixture was stirred at 70 °C for an appropriate time depending upon the nature of the substrate. Upon completion of the reaction (monitored by TLC), the mixture was cooled to room temperature and the catalyst was separated by a magnet for recycling tests. The reaction mixture was extracted with ether (3 × 10 mL). The organic phase were combined together and dried over anhydrous MgSO₄. The solvent was evaporated under vacuum. The pure products were obtained by silica-gel column chromatography using petroleum ether : ethylacetate (4 : 1) as the eluent. The products were identified by IR and ¹H NMR.

Conclusions

In conclusion, an inexpensive and magnetically recyclable catalyst, CuFe₂O₄–G (0.25), was synthesized by a hydrothermal method. CuFe₂O₄–G (0.25) was used as an efficient catalyst for the reduction of nitroarenes. The results demonstrate that the combination of CuFe₂O₄ with graphene results in a dramatic enhancement of the catalytic activity of CuFe₂O₄, which can be attributed to the remarkable synergistic effect between the CuFe₂O₄ and the graphene sheets. The CuFe₂O₄–G (0.25) catalyst can be readily isolated from the reaction mixture using an external magnet due to its superparamagnetism and it can be reused at least five consecutive cycles without significant loss of its catalytic activity. We believe that this method is important addition to known procedures for reduction of aromatic nitro compounds either on a lab as well as on larger scale.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 31171698), the Innovation Research Program of Department of Education of Hebei for Hebei Provincial Universities (LJRC009) and Natural Science Foundation of Hebei Province (B2011204051).

Notes and references

1. P. L. Gkizis, M. Stratakis and I. N. Lykakis, Catal. Commun., 2013, 36, 48.
2. M. M. Dell'Anna, P. Mastrorilli, A. Rizzuti and C. Leonelli, Appl. Catal., A, 2011, 401, 134.
3. A. Tanaka, K. Fuku, T. Nishi, K. Hashimoto and H. Kominami, J. Phys. Chem. C, 2013, 117, 16983.
4. L. Pehlivan, E. Métay, S. Laval, W. Dayoub, P. Demonchaux, G. Mignani and M. Lemaire, Tetrahedron Lett., 2010, 51, 1939.
5. S. Ghosh, Appl. Catal., A, 2004, 268, 61.
6. J. Feng, L. Su, Y. Ma, C. Ren, Q. Guo and X. Chen, Chem. Eng. J., 2013, 221, 16.
7. R. Nie, J. Wang, L. Wang, Y. Qin, P. Chen and Z. Hou, Carbon, 2012, 50, 586.
8. A. K. Shil, D. Sharma, N. R. Guha and P. Das, Tetrahedron Lett., 2012, 53, 4858.
9. J. Li, C.-y. Liu and Y. Liu, J. Mater. Chem., 2012, 22, 8426.
10. E. Amini, M. Rezaei and M. Sadeghinia, Chin. J. Catal., 2013, 34, 1762.
11. R. Parella, A. Naveen Kumar and S. A. Babu, Tetrahedron Lett., 2013, 54, 1738.
12. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666.
13. B. F. Machado and P. Serp, Catal. Sci. Technol., 2012, 2, 54.
14. Y. Liang, H. Wang, H. Sanchez Casalongue, Z. Chen and H. Dai, Nano Res., 2010, 3, 701.
15. J. Zhang, Z. Xiong and X. S. Zhao, J. Mater. Chem., 2011, 21, 3634.
16. B. Li and H. Cao, J. Mater. Chem., 2011, 21, 3346.
17. E. Yoo, T. Okata, T. Akita, M. Kohyama, J. Nakamura and I. Honma, Nano Lett., 2009, 9, 2255.
18. Y. Li, L. Tang and J. Li, Electrochem. Commun., 2009, 11, 846.
19. L. Dong, R. R. S. Gari, Z. Li, M. M. Craig and S. Hou, Carbon, 2010, 48, 781.
20. S. Guo, S. Dong and E. Wang, ACS Nano, 2009, 4, 547.
21. A. R. Siamaki, A. E. R. S. Khder, V. Abdelsayed, M. S. El-Shall and B. F. Gupton, J. Catal., 2011, 279, 1.
22. H.-Y. Zhang, C. Feng, N.-Z. Shang, S.-T. Gao, C. Wang and Z. Wang, Lett. Org. Chem., 2013, 10, 17.
23. Y. Fu, P. Xiong, H. Chen, X. Sun and X. Wang, Ind. Eng. Chem. Res., 2012, 51, 725.
24. N. Shang, C. Feng, H. Zhang, S. Gao, R. Tang, C. Wang and Z. Wang, Catal. Commun., 2013, 40, 111.
25. W. Wang, Y. Li, Q. Wu, C. Wang, X. Zang and Z. Wang, Anal. Methods, 2012, 4, 766.
26. C. Wang, C. Feng, Y. Gao, X. Ma, Q. Wu and Z. Wang, Chem. Eng. J., 2011, 173, 92.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05059b

---