Magnetic Co₃O₄/reduced graphene oxide nanocomposite as a superior heterogeneous catalyst for one-pot oxidative esterification of aldehydes to methyl esters

Abstract

A magnetically separable hybrid material consisting of Co₃O₄ nanoparticles supported on reduced graphene oxide (Co₃O₄/rGO) was synthesized through a simple co-reduction process of graphene oxide (GO) and cobalt chloride (CoCl₂) using sodium borohydride (NaBH₄). The Co₃O₄/rGO heterogeneous catalyst exhibited a high-performance for the oxidative esterification of aldehydes to the corresponding methyl esters using tert-butyl hydroperoxide (TBHP) as an oxidant. Owing to the synergistic effect of rGO support, the hybrid catalyst exhibited superior catalytic activity to the corresponding cobalt oxide catalyst. Importantly, the synthesized hybrid possesses good magnetic properties, which enable facile recovery of the catalyst by using an external magnet.

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Introduction

Graphene oxide (GO), derived from the exfoliation of graphite oxide possesses several fascinating features such as large surface area, as well as good electrical, thermal, and mechanical properties.^1,2 Owing to these outstanding properties, graphene has been considered to be a promising candidate for the synthesis of composite materials by combining it with various nanoparticles (NPs).^3,4 The immobilization of nanoparticles, such as Fe₃O₄, MnO₂, TiO₂, Co₃O₄, and Au onto graphene sheets provided hybrid materials with special features suitable for use in various applications, including catalysis,^5,6 supercapacitors,^7,8 and biotechnology.⁹ These hybrid materials are well known to show higher stability as well as improved catalytic activity than the individual components due to the strong synergistic interaction between both components. Owing to their low cost and high electrochemical stability, cobalt and cobalt oxide NPs have widely been used for various practical applications including biomedical¹⁰ and catalysis.^11–14 However, pure Co₃O₄ NPs have a tendency to aggregate; the aggregation is generally accompanied by the decrease of their surface area and their catalytic activity.¹⁵ Therefore, the synthesis of hybrid materials by combining these NPs with carbon materials such as graphene provides a better dispersion of the NPs and thus represents an effective approach to improve the catalytic activity as well as durability.

The direct synthesis of esters from the oxidation of aldehydes or alcohols is an important transformation in organic synthesis as these compounds find extensive applications in the synthesis of several natural and bioactive products.¹⁶ The conventional approach for the synthesis of esters involves the reaction of carboxylic acids or their derivatives (acyl chlorides and anhydrides) with alcohols,^17,18 which is a multistep process and often leads to the formation of large amounts of undesired byproducts. In the recent years, direct catalytic transformation of aldehydes or alcohols to esters,^19–22 without the use of the corresponding acid or acid-derivative has gained considerable interest. In this context, a number of metal free²³ and metal based catalysts²⁴ for oxidative esterification of aldehydes to the corresponding esters have been reported in the literature. Among the reported systems heterogeneous catalysts are preferred due to their facile recovery, less contamination of the product and efficient recyclability. However, most of the reported heterogeneous catalysts such as gold nanoparticles supported on Ce–Zr oxides,^24b magnetic gold nanoparticles,^24d gold–nickel nanoparticles,^24f alumina supported palladium catalyst^24g etc. are based on the expensive metals such as gold and palladium and involve the tedious multi-step synthetic procedures.

Herein, we report for the first time a highly efficient and reusable hybrid i.e. Co₃O₄/rGO nanocomposite as catalyst for direct oxidation of aldehydes to esters using tert-butyl hydroperoxide (TBHP) as oxidant (Scheme 1). In comparison to the known methods, the developed methodology has several advantages such as the use of a low cost metal, shorter reaction times and excellent product yields. Furthermore, the synergestic effect of rGO support has been observed in the hybrid Co₃O₄/rGO catalyst which exhibited superior catalytic activity than the corresponding cobalt oxide. Furthermore, good magnetization of the synthesized hybrid allowed it to be readily separated from the reaction mixture by using external magnet.

Scheme 1 Oxidative esterification of aldehydes.

Experimental

Materials

All chemicals were reagent grade or higher and were used as received unless otherwise specified. Graphite powder (<20 micron), potassium permanganate (KMnO₄), sulphuric acid (H₂SO₄), phosphoric acid (H₃PO₄), hydrogen peroxide (H₂O₂), cobalt chloride (CoCl₂), sodium borohydride (NaBH₄), potassium carbonate (K₂CO₃), hexane, ethyl acetate, methanol, ethanol and tert-butyl hydroperoxide (TBHP) were purchased from Sigma-Aldrich. Silicon wafers were purchased from Siltronics. The water used throughout the experiments was purified with a Milli-Q system from Millipore Co. (resistivity = 18 MΩ.cm).

Preparation of graphene oxide (GO)

GO nanosheets were produced from natural graphite powder by an improved Hummers and Offeman method. The detailed experimental conditions are reported in a recently published work.²⁵ A homogeneous yellow brown suspension (1 mg mL⁻¹) of GO sheets in deionized water (DI) was achieved by ultrasonication for 3 h.

Preparation of Co₃O₄/rGO catalyst

Co₃O₄/rGO nanocomposite was prepared by simultaneous chemical reduction of GO and CoCl₂ with sodium borohydride (NaBH₄). Typically, to 5 mL of GO (1 mg mL⁻¹) in DI water, sonicated for about 20 min to form a homogeneous dispersion, was added (50 mg) of CoCl₂ and the mixture was sonicated for 30 min. Then 5 mL of NaBH₄ (0.1 M) aqueous solution was added to the mixture at room temperature. A spontaneous formation of a black precipitate was observed. The precipitate was washed repeatedly with water and separated by centrifugation. The percentage of cobalt in the synthesized catalyst was found to be 26.4% (1.5 mmol g⁻¹) as determined by ICP-AES analysis.

Synthesis of Co₃O₄ nanoparticles

For comparison, we have synthesized Co₃O₄ nanoparticles by chemical reduction of CoCl₂ with NaBH₄. To an aqueous solution of CoCl₂ (50 mg) in 5 mL of DI was added 5 mL of NaBH₄ (0.1 M) aqueous solution at room temperature. The precipitate was washed repeatedly with water and separated by centrifugation.

General experimental procedure for oxidative esterification of aldehydes

Aldehyde (1 mmol), K₂CO₃ (0.1 mmol), methanol (2 mL), TBHP (1.8 mmol, 0.25 mL of a 70% aqueous solution) and Co₃O₄/rGO catalyst (25 mg, 2.5 mol% Co) were mixed together in a round bottomed flask. The reaction mixture was heated in an oil bath at 60 °C for 6 h with constant stirring. The progress of the reaction was monitored by TLC. After completion of the reaction, the catalyst was recovered by filtration and the mixture was extracted with ethyl acetate. Removal of the solvent under vacuum afforded the crude product, which was purified by column chromatography using a hexane/ethyl acetate mixture, and was analyzed by GC and GC-MS. The recovered catalyst was washed with methanol, dried at 60 °C and was reused for recycling experiments.

Sample characterization

Transmission electron microscopy (TEM) imaging was performed with a Philips CM30 microscope operating at 300 kV. It was equipped with a Gatan SS CCD camera and a Digital Micrograph software for the acquisition of bright-field and high-resolution imaging (HRTEM). The structural characterization of the Co₃O₄/rGO nanocomposite was carried out on a 9 kW Rigaku Smartlab rotated anode X-ray diffractometer using Cu Kα (1.5406 Å) wavelength, operated in Bragg-Brentano reflexion geometry. X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALAB 220 XL spectrometer from Vacuum Generators featuring a monochromatic Al Kα X-ray source (1486.6 eV) and a spherical energy analyzer operated in the CAE (constant analyzer energy) mode (CAE = 100 eV for survey spectra and CAE = 40 eV for high-resolution spectra), using the electromagnetic lens mode. The detection angle of the photoelectrons is 30°, as referenced to the sample surface. After subtraction of the Shirley-type background, the core-level spectra were decomposed into their components with mixed Gaussian–Lorentzian (30 : 70) shape lines using the CasaXPS software. Quantification calculations were performed using sensitivity factors supplied by PHI. The sample was prepared by drop casting a concentrated ethanoic solution of the nanocomposite onto a silicon wafer followed by oven drying at 50 °C for 2 h. Thermogravimetric analysis (TGA) measurements were carried out in Al₂O₃ crucibles in nitrogen atmosphere at a heating rate of 30 °C min⁻¹ using a TA Instruments Q50 thermogravimetric analyzer.

Results and discussion

Synthesis and characterization of catalyst

A schematic of the synthesis of Co₃O₄/rGO nanocomposite is shown in Scheme 2. For comparison Co₃O₄ NPs were synthesized by direct reduction of CoCl₂ with NaBH₄.

Scheme 2 Schematic illustration of the synthesis of Co₃O₄/rGO nanocomposite.

The HR-TEM images of the synthesized Co₃O₄ NPs and Co₃O₄/rGO nanocomposite are displayed in Fig. 1. In case of Co₃O₄ NPs, most of the nanoparticles of about 20 nm were found to be agglomerated in the form of clusters. Only nanoparticles located on the surface of the clusters could be observed and were in the range of 2 to 4 nm. If the nanoparticles self-agglomerate during the catalysis experiment, it can be easily understood that their efficiency is greatly reduced in comparison to that of nanoparticles dispersed on rGO sheets. On the other hand, the HR-TEM image of the Co₃O₄/rGO nanocomposite exhibits well dispersed and well-textured Co₃O₄ NPs (Fig. 1b). It can be clearly seen that small Co₃O₄ NPs are closely anchored on the graphene sheets surface, implying a strong interaction between graphene and Co₃O₄. The inter-planar distance between adjacent planes is 0.47 nm, corresponding to the interplanar spacing of (111) plane of Co₃O₄.²⁶

Fig. 1 HRTEM images of (a) Co₃O₄ NPs, (b) Co₃O₄/rGO and (c) SAED of Co₃O₄/rGO.

The XRD pattern of Co₃O₄/rGO displays diffraction peaks (2θ) at 31.27°, 36.84°, 38.56°, 44.8°, 59.35°, 65.23°, 74.11°, 77.34° and 78.4° which correspond to the (220), (311), (222), (400), (511), (440), (620), (533) and (622) crystalline planes of Co₃O₄ (ICDD file number 043-1003) (Fig. 2). The crystallite size (i.e. coherent domain size) was evaluated at 5 nm using the Scherer formula for the two single peaks located at 59.35° (511) and 65.23° (440).

Fig. 2 XRD of Co₃O₄/rGO nanocomposite.

X-ray photoelectron spectroscopy (XPS) analysis also confirms the formation of Co₃O₄/rGO nanocomposite. Fig. 3a displays the C_1s core level XPS spectrum of GO nanosheets. It can be deconvoluted into three components with binding energies at about 284.9, 287.0 and 288.2 eV assigned to C–H/C–C, C–O and C=O species, respectively. The spectrum is dominated by the peak at 287.0 eV due to C–O, in accordance with a high oxidation degree of GO. In contrast, the C_1s high resolution XPS spectrum of Co₃O₄/rGO nanocomposite showed a significant decrease in the peak intensities associated with C–H/C–C, C–O and C=O groups (Fig. 3b). The spectrum is dominated by a band with a binding energy at 283 eV due to sp² carbon, suggesting the restoration of the graphene network in the synthesized hybrid composite.

Fig. 3 XPS analysis of Co₃O₄/rGO hybrid material C_1s spectra of (a) GO and (b) Co₃O₄/rGO; (c) Co_2p of Co₃O₄/rGO.

The high resolution Co_2p XPS spectrum of Co₃O₄/rGO exhibits two peaks at 780 and 795 eV, corresponding to the Co_2p1/2 and Co_2p3/2 spin–orbit peaks of Co₃O₄. The spectrum displays also two shake-up satellite peaks of the Co_2p3/2 and Co_2p1/2 located at around 784 and 801 eV with deviations from the main peaks of 4 and 7 eV, respectively. The results suggest that cobalt exists in the form of Co₃O₄, in good agreement with the previously reported data.^27,28

The thermal properties of Co₃O₄/rGO nanocomposite were investigated using thermogravimetric analysis (TGA). Fig. 4 shows a weight loss of ∼3% below 100 °C due to the evaporation of adsorbed water molecules. Then, a weight loss of ∼9% occurred between 100 and 320 °C, which can be assigned to the removal of the labile oxygen-containing functional groups such as CO, CO₂, and H₂O within the nanocomposite. Correspondingly, the DTG curve displays a strong exothermal peak centred at 320 °C, and abrupt weight loss of 11.73%. The weight loss is usually attributed to the loss of the residual (or adsorbed) solvent and to the decomposition of residual organic functional groups on the graphene sheets. From 320 to 900 °C, the Co₃O₄/rGO nanocomposite exhibits much higher thermal stability.

Fig. 4 TG-DTA curve of Co₃O₄/rGO nanocomposite.

Fig. 5 depicts the Raman spectra of Co₃O₄ NPs and Co₃O₄/rGO hybrid. The Raman spectrum of the Co₃O₄/rGO composite shows characteristic peaks of Co₃O₄ in the region of 500–700 cm⁻¹ and characteristic peaks of the D and G bands from graphene at around 1335 and 1600 cm⁻¹, respectively. The presence of these peaks clearly indicates the existence of both rGO and Co₃O₄ in the as prepared hybrid. Furthermore, the higher intensity of D band in the composite showed a largely disordered structure of the obtained rGO. The loading of cobalt in the synthesized hybrid was found to be 26.4 wt% (1.5 mmol g⁻¹) as determined by ICP-AES analysis.

Fig. 5 Raman spectra of Co₃O₄ and Co₃O₄/rGO.

Catalytic activity

The catalytic activity of the synthesized Co₃O₄/rGO catalyst was tested for the oxidative esterification of benzaldehyde with methanol to give methyl benzoate. Typically, the model reaction was performed at 60 °C using THBP (70 wt% aq. solution) as an oxidant in the presence of catalytic amount of K₂CO₃ as base (0.1 mmol) for 6 h. The results of the optimization experiments are summarized in Table 1. In the absence of catalyst under similar conditions, no conversion of benzaldehyde was observed (Table 1, entry 1). Similarly, no reaction occurred when reduced graphene oxide (rGO) was used as a catalyst under similar reaction conditions (Table 1, entry 2). In order to compare the activity of hybrid Co₃O₄/rGO catalyst, the oxidation of benzaldehyde was performed using bare Co₃O₄ nanoparticles under otherwise identical reaction conditions. The reaction was found to be slow and gave poor yield of the product (Table 1, entry 3). Furthermore, the reaction was performed with other metal oxides such as Fe₃O₄, MnO₂, and TiO₂ and their rGO composites under described experimental conditions. The results of these experiments are summarized in Table 1 (entries 4–9). Among all the metal oxides and their rGO composites, only Fe₃O₄ and Fe₃O₄/rGO gave corresponding methyl ester in moderate yield (Table 1, entries 4 and 7). Other metal oxides such as MnO₂ and TiO₂ and their rGO composites yielded the corresponding acid selectively in moderate yields (Table 1, entries 5, 6, 8 and 9). The reaction of benzaldehyde with TBHP in the presence of Co₃O₄/rGO catalyst was very slow at room temperature; 60 °C was found to be the optimum temperature for this organic transformation. Further increase of temperature to 80 °C showed only marginal enhancement in the yield of the final product under similar reaction conditions (Table 1, entry 10). Furthermore, the same reaction was performed with other oxidants such as molecular oxygen and hydrogen peroxide (H₂O₂) in place of TBHP. The reaction was found to be very slow with molecular oxygen and afforded poor product yield (Table 1, entry 11) in longer reaction time (24 h). In the case of H₂O₂ under otherwise similar reaction conditions, moderate yield of the desired product was obtained (Table 1, entry 12). The presence of a base was of prime importance as in its absence the reaction gave only 30% conversion after 72 h under described reaction conditions (Table 1, entry 13). Finally to demonstrate the effectiveness of the base, we performed the reaction using different bases like KOH, K₃PO₄, Na₂CO₃ and K₂CO₃ (Table 1, entries 14–16). Among the various bases studied, K₂CO₃ proved to be optimum and gave the best results for this reaction (Table 1, entry 10). Furthermore, we checked the catalytic activity of the GO, physical mixture of GO and Co₃O₄ NPs and rGO/Co₃O₄ under identical experimental conditions (Table 1, entries 17–19). In case of GO, the corresponding acid was obtained as the sole product, whereas mixtures of GO and rGO with Co₃O₄ NPs yielded poor yield of the corresponding methyl ester along with the acid as by-product. These results confirmed the synergistic effect of the rGO support in enhancing the reaction rate as well as selectivity for the desired product.

Table 1 Results of the optimization experiments^a

Entry	Catalyst	Oxidant	Time	Base	Conv.^b (%)
a Reaction conditions: benzaldehyde (1 mmol), CH3OH (2 mL), base (0.1 mmol), catalyst (5 mol% Co, 20 mg) and oxidant (1.5 mmol) at 60 °C.b Determined by GC-MS.c Temperature at 80 °C.d Acid is the product.e In the absence of base K2CO3.
1	—	TBHP	12	K2CO3	0
2	rGO	TBHP	10	K2CO3	—
3	Co3O4	TBHP	6	K2CO3	56
4	Fe3O4	TBHP	6	K2CO3	62
5	MnO2	TBHP	6	K2CO3	50^d
6	TiO2	TBHP	6	K2CO3	35^d
7	Fe3O4/rGO	TBHP	6	K2CO3	70
8	MnO2/rGO	TBHP	6	K2CO3	52^d
9	TiO2/rGO	TBHP	6	K2CO3	48^d
10	Co3O4/rGO	TBHP	6	K2CO3	93, 95^c
11	Co3O4/rGO	O2	24	K2CO3	25
12	Co3O4/rGO	H2O2	10	K2CO3	85
			48	—	12^e
13	Co3O4/rGO	TBHP	72	—	30^e
14	Co3O4/rGO	TBHP	6	KOH	70
15	Co3O4/rGO	TBHP	6	Na2CO3	85
16	Co3O4/rGO	TBHP	6	K3PO4	75
17	GO	TBHP	6	K2CO3	72^d
18	GO + Co3O4	TBHP	6	K2CO3	50, 20^d
19	rGO + Co3O4	TBHP	6	K2CO3	35, 15^d

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After having demonstrated the excellent activity of Co₃O₄/rGO in the model reaction, a wide range of aldehydes containing electron donating as well as withdrawing groups were oxidized under described experimental conditions. The results of these experiments are summarized in Table 2. All the substrates were smoothly oxidized to give the desired product with good to excellent yields. In general, aromatic aldehydes substituted with electron-withdrawing groups were found to be more reactive and afforded desired products with good to excellent yields (Table 2, entries 2–8) than those containing electron donating groups (Table 2, entries 8–12). Under the optimized experimental conditions, aliphatic aldehydes gave moderate to high yield of the desired product (Table 2, entries 13 and 14).

Table 2 Co₃O₄/rGO catalyzed oxidative esterification of aldehydes with methanol^a

Entry	Reactant	Product	Conv.^b (%)	Yield^c (%)
a Reaction conditions: substrate (1.0 mmol), CH3OH (4 mL), K2CO3 (0.1 mmol), catalyst (5 mol%).b Determined by GC-MS.c Isolated yield.
1			93	91
2			85	83
3			88	85
4			89	86
5			87	84
6			91	87
7			90	88
8			78	70
9			72	75
10			79	77
11			80	72
12			80	75
13			91	88
14			90	86

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To establish the superiority of the developed catalyst, we also compared the potential of Co₃O₄/rGO catalyst with the state of the art systems using TBHP as oxidant (Table 3). As shown in Table 3, all the reported methods required comparatively longer reaction time, higher temperature and afforded moderate product yield.

Table 3 Comparison of the catalytic activity of Co₃O₄/rGO with state of the art methods

Entry	Catalyst	Time/h	Temp./°C	Yield (%)	Ref.
1	B(C6F5)3	18	80	86	23d
2	CuF2	24	120	85	29
3	KI	17	65	78	30
4	Cu(ClO4)2·6H2O, InBr3	16	80	91	31
5	Ti-superoxide	10	90	82	32
6	Co3O4/rGO	6	60	93	This work

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Next, we have checked the recycling of the catalyst by using benzaldehyde as a model substrate. After completion of the reaction, the catalyst was easily recovered from reaction mixture by using external magnet, washed with methanol and then dried. The recovered catalyst was used for six subsequent runs under similar reaction conditions. The results of recycling experiments are summarized in Fig. 6. The yield of the desired product remained in all cases almost similar. This confirmed that the developed catalyst was quite stable and could be reused efficiently without any significant loss in activity. Furthermore, the cobalt content in the recovered catalyst after six runs was found to be almost similar (26.36 wt%) as in the fresh catalyst (26.4 wt%) as determined by ICP-AES analysis.

Fig. 6 Results of catalytic recycling experiments.

Conclusion

We have synthesized an efficient, stable and low cost cobalt metal based hybrid Co₃O₄/rGO nanocomposite catalyst for the one-pot oxidative methyl esterification of aldehydes using aqueous TBHP as oxidant. We have shown the first successful oxidative esterification methodology using TBHP as oxidant in significantly shorter reaction times than those usually used.^23d,29–32 The synergistic effect of rGO support was observed and the hybrid catalyst exhibited superior catalytic activity than the corresponding cobalt oxide catalyst. Importantly, the hybrid catalyst was efficiently recycled for several runs without any loss in the catalytic activity.

Acknowledgements

We acknowledge Director, CSIR-Indian Institute of Petroleum (IIP) for his kind permission to publish these results. VP thanks Council of Scientific and Industrial Research (CSIR) for providing fellowship in the form of Junior Research Fellowship (JRF). The Centre National de la Recherche Scientifique (CNRS), Lille1 University and Nord Pas de Calais region are acknowledged for financial support.

Notes and references

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