Highly dispersed cobalt oxide nanoparticles on CMK-3 for selective oxidation of benzyl alcohol

Abstract

Nano cobalt oxide particles supported on ordered mesoporous carbon CMK-3 (CMK: carbon material from Korea) were prepared by a hydrothermal synthesis method. The catalysts were characterized by various characterization methods such as XRD, FT-IR, TEM, FESEM, N₂ adsorption and desorption, XPS, Raman and ICP-AES, which indicated that the size of the cobalt oxide nanoparticles was estimated to be ∼2 nm and that they were uniformly dispersed on CMK-3. The approach obtained smaller and narrow distributed cobalt oxide nanoparticles which was the key to complete the reaction with 82.1% conversion and 97.7% benzaldehyde selectivity within 6 h. The average TOF of Co₃O₄/CMK-3 can reach as high as 25.4 h⁻¹ in the initial two hours, which was far higher than that of the unsupported nano Co₃O₄ (less than 1.0 h⁻¹) and so far reported cobalt-based catalysts (e.g. 1.34 h⁻¹ for Co₃O₄/RGO_N). In addition, the catalytic activity of Co₃O₄/CMK-3 was not obviously deteriorated after 4 runs.

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1. Introduction

Selective oxidation of benzyl alcohol to the corresponding aldehyde is a promising and important organic reaction because its product is used in many fields, e.g. drugs, the agrochemical industry, perfumery, and dyestuffs, etc.¹ The traditional industrial method to synthesize benzaldehyde was hydrolysis of benzyl chloride² or vapor/liquid-phase oxidation of toluene.³ These methods not only need a long period and but also reach low efficiency. In addition, heavy metals⁴ or peracetic acid⁵ are frequently applied to the process of oxidation that are increasingly threatening the environment. In contrast with these traditional routes, direct oxidation of benzyl alcohol to benzyl aldehyde with oxygen is a sustainable protocol. Such transformation with molecular oxygen as the terminal oxidant and water as the sole by-product is highly desired for improving reaction atom efficiency.⁶

Up to now, much effort has been made in the heterogeneous systems based on noble metals including Pt⁷, Pd,⁸ Au,⁹ and Ru.¹⁰ Most of them have good catalytic performances. However, these catalysts not only have high cost but also need the addition of non-green alkaline promoters (NaOH¹¹ or K₂CO₃ ¹²) in the reaction system to control the selectivity. Therefore, it is urgent to design and develop new catalysts to resolve these problems. Alternatively, low-cost transition metals (Fe,¹³ Cu,¹⁴ Mn,¹⁵ etc.) have been used as catalysts for the oxidation of alcohols without promoters in recent years. For example, Geng¹⁶ et al. found that the catalyst of iron oxide supported on CMK-3 treated with HNO₃ was active in the selective oxidation of benzyl alcohol with oxygen. It exhibited 72% conversation of benzyl alcohol with nearly 100% benzaldehyde selectivity after 8 h. Meanwhile, cobalt-based catalysts have shown considerable performance in catalysis such as in alcohol oxidation reactions¹⁷ and other oxidation reduction reactions.^18,19 Zhu and co-workers²⁰ reported that Co₃O₄/AC was a catalyst for the oxidation of alcohols. Recently, Xiao’s group²¹ reported a one-step synthesis of a sandwich N-doped graphene/Co₃O₄ hybrid catalyst which possessed good conversion for alcohols and olefins. The non-noble metal provides a new pattern for heterogeneous catalysis. However, although most of the catalysts could obtain a high conversion for the selective oxidation of benzyl alcohol, the TOF of the total metal sites is still at a low level state due to the poor utility of the active sites. Therefore, the design of high efficiency heterogeneous catalysts for the oxidation of benzyl alcohol by oxygen is still a great challenge.

Carbonaceous materials exhibit high specific surface areas, large adsorption capacities, and high chemical stability.^22,23 The ordered mesoporous material CMK-3 is a very popular material in the catalysis field since it can provide large surface areas for the immobilization of metal active sites.²⁴ Nanomaterials are widely used in various fields, especially for catalysis. The catalyst preparation parameters influence the morphology eventually affecting the catalytic activity. In this study, we attempt to prepare a novel and high efficiency catalyst, nano Co₃O₄/CMK-3, by a simple hydrothermal reaction, which was tested to show excellent catalytic performance in the aerobic oxidation of benzyl alcohol.

2. Experimental

2.1 Materials

All chemicals used in the experiment were of analytical grade, and were used without further purification. Pluronic P123 (EO₂₀PO₇₀EO₂₀, Aldrich), tetraethyl orthosilicate (Si(OC₂H₅)₄, 99%), HCl, H₂SO₄, sucrose, benzyl alcohol, NH₃·H₂O (28 wt%), Co(NO₃)₂·6H₂O and all organic solvents were of AR grade.

2.2 Catalyst preparation

Ordered mesoporous silica material SBA-15 (SBA: santa barbara amorphous) was prepared using the triblock copolymer, EO₂₀–PO₇₀–EO₂₀ (P123), as the surfactant and tetraethyl orthosilicate (TEOS) as the silica source, following the synthesis procedure reported by Zhao et al.²⁵ Typically, 4.68 mL of TEOS was added to 60 mL of 2 M HCl containing 2 g of P123 at 35 °C. The mixture was stirred with magnetic stirring until P123 was completely dissolved. The mixture was placed in an oven at 35 °C for 24 h, and subsequently at 100 °C for 3 d. The product was filtered, dried at 100 °C for 12 h, and calcined at 550 °C for 6 h.

The ordered mesoporous carbon material CMK-3 was prepared with SBA-15 as a hard template and sucrose as the carbon source.²⁴ Typically, 1.0 g of SBA-15 silica was added to a solution containing 1.25 g of sucrose, 0.14 g of sulphuric acid and 5.0 g of distilled water. The mixture was kept in an oven at 80 °C for 6 h and then the temperature was increased to 160 °C, and maintained at this temperature for another 6 h. The heating procedure was repeated after the addition of the carbon precursor (0.8 g of sucrose, 0.09 g of H₂SO₄ and 5.0 g of H₂O), aiming to completely infiltrate the internal pores of SBA-15 silica. The carbon–silica composite was obtained after pyrolysis under argon flow at 900 °C for 6 h and successive washing in 10 wt% hydrofluoric acid aqueous solution to remove the silica template.

The Co₃O₄/CMK-3 catalyst was prepared through a simple one-pot hydrothermal reaction in a solution of NH₃.²¹ Before being added to the solution, CMK-3 supports were treated in a 4 M HNO₃ solution at 60 °C for 6 h (denoted as H-CMK-3). Then, H-CMK-3 (125 mg) was dispersed in ethanol (EtOH) (189 mL) and pretreated with ultrasonication for 2 h. Subsequently, 13 mg of Co(NO₃)₂·6H₂O in aqueous solution (0.2 M) was added to the above suspension, followed by the addition of 0.093 mL NH₃·H₂O (28 wt%) and 0.13 mL of water at room temperature. The reaction mixture was kept at 80 °C with stirring for 10 h. Then, it was transferred to a 500 mL autoclave with a Teflon inner layer for a hydrothermal reaction at 150 °C for 3 h. The hybrid, labelled as 1.3-Co₃O₄/CMK-3 (1.3 wt% Co, measured by ICP-AES), was then filtered, washed, and finally dried overnight. Bare Co₃O₄ was made through the same steps as 1.3-Co₃O₄/CMK-3 without adding CMK-3 in the first step. In addition, we also prepared different contents of nano cobalt oxide (2.4 wt% Co, 3.3 wt% Co, and 4.0 wt% Co, measured by ICP-AES) supported on CMK-3 to compare their catalytic performance for the oxidation of benzyl alcohol. These catalysts were marked as 2.4-Co₃O₄/CMK-3, 3.3-Co₃O₄/CMK-3, and 4.0-Co₃O₄/CMK-3, respectively.

2.3 Characterization

X-ray diffraction (XRD) was carried out on a RIGAKU D/MAX2550/PC diffractometer at 40 kV and 100 mA with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 10 to 70°. N₂ adsorption–desorption isotherms were measured with a Micromeritics ASAP 2020 system at liquid N₂ temperature. Before measurements, the sample was out-gassed at 130 °C for 6 h. The BET surface area was calculated using the Brunauer–Emmett–Teller (BET) method. Transmission electron microscopy (TEM) images were observed by an Hitachi HT7700. Field emission scanning electron microscopy (FESEM) images were observed by an Hitachi S-5500. The infrared spectra were recorded in KBr disks using a UICOLET impact 410 spectrometer in the range 400–4000 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was measured on an ESCALAB 250 X-ray electron spectrometer using Al Kα radiation. The metal content was estimated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis conducted on a PerkinElmer emission spectrometer. Raman spectra were collected from 100 to 2000 cm⁻¹ on a Renishaw 2000 Confocal Raman Microprobe (Renishaw Instruments, England) using a 532 nm argon ion laser.

2.4 Catalytic reaction evaluation

In a typical reaction, a varying amount of the substrate (benzyl alcohol), a certain amount of catalyst (50 mg) and 10 mL of solvent were added into a flask, followed by bubbling O₂ at a flow rate of 20 mL min⁻¹ under magnetic stirring (500 rpm). After the reaction, the solid catalyst was separated by being filtered off and the liquid phase was analysed by gas chromatography with a Shimadzu GC-8A (Japan) instrument equipped using an HP-5 capillary column and FID detector.

3. Results and discussion

3.1 Characterization of catalysts

As shown in Fig. 1, the low-angle XRD pattern of CMK-3 exhibits an intense diffraction peak (100) and two weak peaks (110) and (200), which demonstrates that the sample has a two-dimensional hexagonal mesoporous structure.²⁶ It is remarkable that CMK-3 still holds the hexagonal mesoporous structure even after supporting a high cobalt oxide content (4.0 wt% Co). Compared with the CMK-3 support, all catalysts show a peak of (100) at 2θ = 1.0°, and the intensity of the XRD peaks are weaker than that of CMK-3, while the (110) and (200) peaks are hardly seen, which may be due to the destruction of the pores during the process of hydrothermal synthesis or the introduction of cobalt oxide into the mesopores.

Fig. 1 Low-angle XRD patterns of (a) CMK-3, (b) 1.3-Co₃O₄/CMK-3, (c) 2.4-Co₃O₄/CMK-3, (d) 3.3-Co₃O₄/CMK-3, and (e) 4.0-Co₃O₄/CMK-3.

Fig. 2 shows the wide-angle XRD patterns of CMK-3, the catalysts with a different cobalt content and the bare nano Co₃O₄. CMK-3 (Fig. 2a) shows two broad peaks at 24.5° and 44° corresponding to (002) and (100), representing the graphite structure.²⁷ The two broad diffraction peaks are also observed in the patterns of the other different cobalt-containing catalysts (Fig. 2b–e), which suggests that the structure of the mesoporous carbon is well kept after supporting cobalt oxide. In addition, the peaks at 2θ = 37°, 44° and 63° are detected, which are attributed to (311), (400), (511) of Co₃O₄ with a face-centered cubic structure (JCPDS no. 421467).²¹ Compared with the pure nano Co₃O₄ (Fig. 2B), it fails to show the characteristics of the cobalt oxide nanoparticles with the low content (1.3–2.4 wt%), demonstrating that the cobalt oxide particles are too small to be detected by XRD or the high dispersion of the cobalt oxide nanoparticles. With the cobalt content increasing, the intensity of the peaks increases, which indicates that larger cobalt oxide particles are present as the cobalt content increases. It could be confirmed by the following TEM characterization.

Fig. 2 The wide-angle XRD patterns of (A) (a) CMK-3, (b) 1.3-Co₃O₄/CMK-3, (c) 2.4-Co₃O₄/CMK-3, (d) 3.3-Co₃O₄/CMK-3, and (e) 4.0-Co₃O₄/CMK-3, and (B) nano Co₃O₄.

FT-IR is used to detect the surface structure of the materials. As shown in Fig. 3a, the pure support CMK-3 exhibits a broad adsorption band at 3420 cm⁻¹ corresponding to the O–H stretching vibration.²⁸ Meanwhile, the bands at 1729 cm⁻¹ and 1595 cm⁻¹ are assigned to C=O stretching vibrations of non-aromatic carboxyl groups and aromatic ring stretching coupled to highly conjugated keto groups, respectively. Moreover, a broad band centered at around 1236 cm⁻¹ can be ascribed to the C–O–C vibration. Furthermore, some weak desorption bands appear at 2983–2874 cm⁻¹, which can be attributed to the stretching of C–H bonds.²⁹ The spectrum of 1.3-Co₃O₄/CMK-3 (Fig. 3b) is very similar to that of CMK-3 except that the intensity of the C=O bond peak at 1729 cm⁻¹ decreases. Moreover, 2.4-Co₃O₄/CMK-3, 3.3-Co₃O₄/CMK-3 and 4.0-Co₃O₄/CMK-3 (Fig. 3c–e) exhibit two additional bands at 667 and 580 cm⁻¹ which are assigned to the Co–O vibrations in the cobalt oxide lattice.³⁰ It is interesting that the strength of the two peaks increases with the increase of the cobalt loading.

Fig. 3 FT-IR spectra of (a) CMK-3, (b) 1.3-Co₃O₄/CMK-3, (c) 2.4-Co₃O₄/CMK-3, (d) 3.3-Co₃O₄/CMK-3, and (e) 4.0-Co₃O₄/CMK-3.

The morphology and structure of all the samples are further characterized by transmission electron microscopy (TEM). From Fig. 4a, CMK-3 exhibits ordered mesopores parallel to each other, which is in agreement with the result of low-angle XRD. When the cobalt loading is low (1.3–2.4 wt%), the cobalt oxide nanoparticles are highly dispersed in the channel of CMK-3 or on the surface of CMK-3, and the size of the cobalt oxide nanoparticles is ca. 2 nm. However, when the cobalt loading increases to 3.3 wt% or 4.0 wt% (Fig. 4d and e), the particle size of the cobalt oxide is about 5 nm, which is still less than that of the pure Co₃O₄ with an average particle size of ca. 10 nm. Field emission scanning electron microscope technology (FESEM) technology is used to further investigate the size of the cobalt oxide nanoparticles. From Fig. 4g, the size of the cobalt oxide nanoparticles is about 2 nm, which is coincident with the result of TEM. Moreover, some aggregation of the cobalt oxide nanoparticles is also found from Fig. 4g.

Fig. 4 TEM images of (a) CMK-3, (b) 1.3-Co₃O₄/CMK-3, (c) 2.4-Co₃O₄/CMK-3, (d) 3.3-Co₃O₄/CMK-3, (e) 4.0-Co₃O₄/CMK-3, and (f) nano Co₃O₄, and (g) FESEM image of 1.3-Co₃O₄/CMK-3. The circles and squares in the figures represent the single cobalt oxide nanoparticles and the aggregation of the cobalt oxide nanoparticles, respectively.

From Fig. 5, the N₂ adsorption–desorption isotherms show that all samples possess a type IV curve according to the IUPAC classification, indicating the characterization of the ordered mesoporous material. The isotherms of all the cobalt-based catalysts are similar to that of CMK-3, suggesting that the mesoporous structure is kept well. The sharp rise at a relative pressure (P/P₀) of about 0.4 demonstrates the existence of mesopores with a narrow pore size distribution.¹⁶ The structural parameters of CMK-3 and the cobalt-based samples are listed in Table 1. It is remarkable that the specific surface areas of all the Co₃O₄/CMK-3 catalysts (1480, 1235, 1190 and 1168 m² g⁻¹) are lower than that of CMK-3 (1506 m² g⁻¹) and the BET specific surface area decreases gradually with the increasing cobalt loading. In the meantime, the pore diameter also decreases, which suggested that cobalt oxide has been successfully loaded into CMK-3.

Fig. 5 N₂ adsorption–desorption isotherms of (a) CMK-3, (b) 1.3-Co₃O₄/CMK-3, (c) 2.4-Co₃O₄/CMK-3, (d) 3.3-Co₃O₄/CMK-3, and (e) 4.0-Co₃O₄/CMK-3 (inset: pore size distribution).

Table 1 The BET parameters of the samples

Sample	SBET (m^2 g^−1)	Pore size (nm)
CMK-3	1506	3.90
1.3-Co3O4/CMK-3	1480	3.72
2.4-Co3O4/CMK-3	1235	3.67
3.3-Co3O4/CMK-3	1190	3.50
4.0-Co3O4/CMK-3	1168	3.70

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XPS measurements are further characterized to demonstrate the oxidation state of cobalt in the mesoporous carbon supported catalyst. Fig. 6a shows the peak of Co 2p_3/2 located at ∼781 eV and that of Co 2p_1/2 located at ∼796 eV. In addition, the energy between the Co 2p_3/2 peak and the Co 2p_1/2 peak is approximately 15 eV.³¹ The XPS spectrum of the Co 2p peak is deconvoluted to six peaks. The main four peaks are located at 780.6 eV, 782.5 eV, 796.4 eV and 798.0 eV, which are assigned to Co^III_3/2, Co^II_3/2, Co^III_1/2 and Co^II_1/2.³² The peaks at 787 eV and 803 eV are Co²⁺ shake-up satellite peaks, which are coincident with Co₃O₄.^33,34 After deconvolution, the O 1s peak (Fig. 6b) is split into four peaks.³⁵ The first peak is located at 531.2 eV, which is due to the surface lattice oxygen in the Co₃O₄ crystal.³⁶ The other three peaks situated at 532.0 eV, 532.7 eV, and 533.7 eV belong to adsorbed oxygen, adsorbed H₂O and oxygenic groups,^31,32 respectively.

Fig. 6 XPS spectra of (a) Co 2p, and (b) O 1s in 1.3-Co₃O₄/CMK-3.

Raman spectroscopy is a frequently used technique to study the properties of carbon materials. Fig. 7A displays the Raman spectra of CMK-3 and a series of different contents of cobalt oxide supported catalysts. All the samples possess two main peaks at 1335 cm⁻¹ and 1590 cm⁻¹. The peak around 1335 cm⁻¹ (D band) is attributed to the vibrations of sp³ carbon atoms of the disordered graphite structure in CMK-3, while the peak near 1590 cm⁻¹ (G band) is assigned to the E_2g stretching mode of graphite and reflects the structural intensity of the sp² carbon atoms in CMK-3. The ratios of I_D/I_G of the samples (Fig. 7A(a–e)) are 2.0, 2.2, 2.1, 2.2, and 2.2, respectively. Compared with that of CMK-3, the value of I_D/I_G for the Co₃O₄/CMK-3 catalysts slightly increases which suggests the increase of defects after the hydrothermal reaction. As shown in Fig. 7B, the weak peaks at 189, 471, 512, 608, and 677 cm⁻¹ are detected in the catalysts of 2.4-Co₃O₄/CMK-3, 3.3-Co₃O₄/CMK-3, 4.0-Co₃O₄/CMK-3 and nano Co₃O₄, represented by the five Raman-active modes (A_1g, E_g and 3F_2g), which are coincident with Co₃O₄ (ref. 37). What is noted is that the A_1g peak position of nano Co₃O₄ is higher than that of a series of cobalt-based catalysts because of the combined effects of strain and phonon confinement, and a similar phenomenon has been reported previously in ref. 21. In addition, compared with pure nano Co₃O₄, the peak position of these supported catalysts shifts to a low wavenumber, suggesting that there may be some interaction between Co₃O₄ and CMK-3. Moreover, 1.3-Co₃O₄/CMK-3 doesn’t show the peaks since the cobalt oxide nanoparticles are very small and are highly dispersed on the CMK-3, in good agreement with the above TEM and HRSEM results.

Fig. 7 Raman spectra of (A) (a) CMK-3, (b) 1.3-Co₃O₄/CMK-3, (c) 2.4-Co₃O₄/CMK-3, (d) 3.3-Co₃O₄/CMK-3, (e) 4.0-Co₃O₄/CMK-3 and (B) (b) 1.3-Co₃O₄/CMK-3, (c) 2.4-Co₃O₄/CMK-3, (d) 3.3-Co₃O₄/CMK-3, (e) 4.0-Co₃O₄/CMK-3, and (f) nano Co₃O₄.

3.2 Catalytic oxidation of benzyl alcohol

Selective oxidation of benzyl alcohol by oxygen is carried out as a model reaction for the as-prepared catalyst. Acetonitrile, toluene and DMF are the common solvents for many oxidation reactions. Toluene is a non-polar solvent, while acetonitrile and DMF are polar solvents. From Table 2, toluene and acetonitrile are not the desired solvents, in which just 2.4% and 12.5% conversion are obtained after 8 h in the benzyl alcohol oxidation reaction. However, when DMF is used as the solvent for the reaction, the conversation of benzyl alcohol could reach as high as 83.5% after 8 h. The influence of the reaction temperature on the catalytic reaction is also investigated as listed in Table 2. The conversion decreased to 75.5% with the decrease in the temperature to 100 °C. In the meantime, the conversion slightly increases when the reaction temperature rises to 120 °C, while the selectivity to benzaldehyde obviously decreases. Considering the above information, the optimal reaction temperature should be 110 °C. Additionally, the effect of reaction time on the catalytic behavior has also been studied. The average TOF value of benzyl alcohol has reached 25.4 h⁻¹ in the initial 2 h. On prolonging the reaction time, the conversion of benzyl alcohol achieves a superior value at 6 h and the average TOF value decreases to 12.7 h⁻¹. With further prolonging of the reaction time, the selectivity to benzaldehyde would decrease.

Table 2 Catalytic performance of oxidation of benzyl alcohol^a

Catalyst	Solvent	Time (h)	Temperature (°C)	Conversion (%)	Co content^b (wt%)	TOF^c (h^−1)	Selectivity (%)
							Benzaldehyde	Benzoic acid
a Reaction conditions: 1.3-Co3O4/CMK-3 (50 mg), benzyl alcohol (1 mmol), solvent (10 mL), and flow rate of O2 (20 mL min^−1).b The metal content was tested by ICP-AES analysis.c TOF = turnover frequency, moles of substrate converted per mole metal ion per hour.
1.3-Co3O4/CMK-3	Toluene	8	110	12.5	1.3	1.4	98.5	1.5
1.3-Co3O4/CMK-3	Acetonitrile	8	110	2.5	1.3	0.3	>99	—
1.3-Co3O4/CMK-3	DMF	2	110	55.3	1.3	25.4	98.8	1.3
1.3-Co3O4/CMK-3	DMF	4	110	67.7	1.3	15.6	98.3	1.7
1.3-Co3O4/CMK-3	DMF	6	110	82.1	1.3	12.7	97.7	2.3
1.3-Co3O4/CMK-3	DMF	8	110	83.5	1.3	9.6	94.4	5.6
1.3-Co3O4/CMK-3	DMF	6	100	75.5	1.3	11.6	98.0	2.0
1.3-Co3O4/CMK-3	DMF	6	120	87.7	1.3	13.4	83.5	16.5
1.3-Co3O4/CMK-3^2nd	DMF	6	110	83.5	1.3	12.8	97.0	3.0
1.3-Co3O4/CMK-3^3rd	DMF	6	110	81.6	1.3	12.5	97.4	2.6
1.3-Co3O4/CMK-3^4th	DMF	6	110	80.2	1.3	12.3	98.0	2.0

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Under the optimized conditions, the catalytic performance of the pure CMK-3, Co₃O₄ and different cobalt content catalysts was evaluated. As seen from Table 3, within the series of catalysts prepared by the hydrothermal method, the order of catalytic activity decreases as the cobalt content increases, namely, 1.3-Co₃O₄/CMK-3 (83.5%) > 2.4-Co₃O₄/CMK-3 (80.2%) > 3.3-Co₃O₄/CMK-3 (73.3%) > 4.0-Co₃O₄/CMK-3 (67.7%). The catalytic performance of 2.4-Co₃O₄/CMK-3 is close to that of 1.3-Co₃O₄/CMK-3, while 4.0-Co₃O₄/CMK-3 is evidently less active than 1.3-Co₃O₄/CMK-3, which is possibly caused by the less active sites of the larger size of cobalt oxide. A similar phenomenon has been reported for supported (SBA-15) cobalt oxide catalysts for the oxidation of cyclohexanol.³⁸ Nano Co₃O₄ exhibits a relatively lower conversion (79.5%) and selectivity to benzaldehyde (80.3%) and a lower TOF (0.4 h⁻¹) after a 6 h reaction. It can be found that the TOF of 1.3-Co₃O₄/CMK-3 exceeds that of Co₃O₄ by more than 31.7 times. The catalytic activity of 1.3-Co₃O₄/CMK-3 for selective oxidation of benzyl alcohol is compared with the previously reported Co-containing catalysts. It is obviously seen that 1.3-Co₃O₄/CMK-3 has a higher turnover frequency than those catalysts. For example, the TOF of 1.3-Co₃O₄/CMK-3 is 42.3 times higher than that of Co₃O₄/AC and 254 times higher than that of Co₃O₄/CTF.²⁰ Co₃O₄/RGO_N²¹ was also examined for benzyl alcohol catalytic performance, while its TOF value just reaches 1.34 h⁻¹. Although the homogeneous catalyst of Co(TPP)Cl possesses a high conversion of benzyl alcohol, its selectivity to benzaldehyde is just 42%.³⁹ In the meantime, the selectivity to benzaldehyde is 20% when solid Co₃O₄ is used as the catalyst.⁴⁰

Table 3 Catalytic performance of oxidation of benzyl alcohol of Co-containing catalysts

Catalyst^a	Conversion (%)	TOF (h^−1)	Selectivity (%)		Reference
			Benzaldehyde	Benzoic acid
a Reaction conditions: a series of Co3O4/CMK-3 (50 mg) or nano Co3O4 (25 mg), benzyl alcohol (1 mmol), DMF (10 mL), temperature (110 °C) and flow rate of O2 (20 mL min^−1).
No catalyst	2.0	—	>99	—	This study
Nano Co3O4	79.5	0.4	80.3	20.7	This study
CMK-3	5.0	—	>99	—	This study
1.3-Co3O4/CMK-3	82.1	12.7	97.7	2.3	This study
2.4-Co3O4/CMK-3	80.2	6.6	96.6	3.4	This study
3.3-Co3O4/CMK-3	73.3	4.4	97.0	3.0	This study
4.0-Co3O4/CMK-3	66.7	3.3	96.4	3.6	This study
Co3O4/AC	99	0.3	87.3	—	20
Co3O4/CTF	17.8	0.05	25.1	—	20
Co3O4/RGON	93.9	1.34	>99		21
Co(TPP)Cl	99	—	42	58	39
Solid Co3O4	100	0.19	20	80	40

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The stability and reusability of the 1.3-Co₃O₄/CMK-3 catalyst is investigated. After a reaction run, the catalyst was collected by filtration, washed thoroughly with water and ethanol, dried under vacuum and reused for the next reaction. The reactions have been repeated for 4 runs (Table 2). It is found that the conversion and selectivity almost remain intact. The slight decrease in the conversion might be due to a little loss of catalyst by filtration and washing.

In order to assess the stability of 1.3-Co₃O₄/CMK-3 during the catalytic process, a leaching test was carried out, as shown in Fig. 8. The catalyst was filtered after the reaction for 2 h and the resulting clear solution was further stirred for another 4 h at 110 °C. The result shows that there is no obvious improvement in benzyl alcohol conversion after removal of the catalyst, which suggests that the catalyst is rather stable in the catalytic process and no metal leaching happens.

Fig. 8 Leaching experiment of 1.3-Co₃O₄/CMK-3.

Based on the relevant literatures about the selective oxidation of alcohol,^41,42 the surface of the carbon materials could activate oxygen and also regenerate to the original state during the course of the reactions (Scheme 1). Meanwhile, Co₃O₄ is a good material in the dehydrogenation reaction.⁴³ During the reaction process, the mesoporous CMK-3 provides defects, which originated from thermal decomposition (or are due to the HNO₃ solution), and the surface groups act as the adsorbed and activated sites for oxygen. Then benzyl alcohol is adsorbed on the highly distributed and small cobalt oxide nanoparticles and is transformed into benzaldehyde. The collaboration of CMK-3 with Co₃O₄ is the key to improve the catalytic activity.

Scheme 1 The proposed catalytic mechanism of the oxidation of benzyl alcohol over Co₃O₄/CMK-3.

4. Conclusions

In summary, we have successfully synthesized a range of cobalt oxide nanoparticles supported on CMK-3 with different cobalt contents by a simple hydrothermal method, as characterized by XRD, TEM, FESEM, FT-IR, N₂ adsorption–desorption, XPS, Raman and ICP-AES techniques. The results showed that very small Co₃O₄ nanoparticles (∼2 nm) had been formed inside the mesoporous CMK-3 or on the CMK-3 surface with a low cobalt concentration (1.3–2.4 wt%). The low cobalt content catalysts possessed a smaller cobalt oxide size and a better distribution on CMK-3 than that of the high content cobalt-containing catalysts. As a heterogeneous catalyst, 1.3-Co₃O₄/CMK-3 exhibited a more excellent catalytic activity (82.1% conversion and 97.7% selectivity to benzaldehyde) than that of other high content cobalt-containing catalysts in the aerobic oxidation of benzyl alcohol. The TOF of 1.3-Co₃O₄/CMK-3 was 31.8 times higher than that of nano Co₃O₄ (∼10 nm) in the selective oxidation of benzyl alcohol using DMF as the solvent.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21303069) and Jilin province (20150520013JH).

Notes and references

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