Highly dispersed cobalt nanoparticles supported on a mesoporous Al2O3: An efficient and recyclable catalyst for aerobic oxidation of alcohols in aqueous media

Molecular Catalysis 440 (2017) 133–139

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Research Paper

Highly dispersed cobalt nanoparticles supported on a mesoporous Al2 O3 : An efficient and recyclable catalyst for aerobic oxidation of alcohols in aqueous media Jalal Albadi a,∗ , Amir Alihosseinzadeh b , Mehdi Jalali c , Mahdi Shahrezaei d , Azam Mansournezhad e a

Department of Chemistry, Faculty of Science, Shahrekord University, Shahrekord, Iran School of Chemical Engineering, University of Tehran, Tehran, Iran c National Petrochemical Company, Petrochemical Research and Technology Company, Tehran, Iran d Department of Chemical Engineering, Sahand University of Technology, Tabriz, Iran e Department of Chemistry, Payame Noor University, Tehran, Iran b

a r t i c l e

i n f o

Article history: Received 17 November 2016 Received in revised form 24 July 2017 Accepted 25 July 2017 Keywords: Co/Al2 O3 nanocatalyst Aerobic oxidation Alcohols Carbonyl compounds

a b s t r a c t In this paper the catalytic performance of a Co/Al2 O3 nanocatalyst is investigated on aerobic oxidation of alcohols in a water media at reflux condition. The catalyst was synthesized via a co-precipitation method and characterized by XRD, BET surface area, atomic absorption spectroscopy, SEM, TEM and EDS analysis. The catalyst exhibited a high efficiency on the oxidation of various alcohols into their corresponding carbonyl compounds at relatively short reaction time. Also the results exhibit a consistant activity after several consecutive runs of reaction and regeneration that demonstrates the stability and recyclability of the Co/Al2 O3 nanocatalyst. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The selective oxidation of alcohols into aldehydes and ketones plays a key role in synthesis of organic materials, so, a lot of catalytic systems employing different oxidants have been developed for these reactions [1–3]. Aerobic oxidation of alcohols using inexpensive and non-toxic air or O2 as the exclusive terminal oxidant has continued to achieve much attention in recent years. Moreover, such catalytic systems are industrially attractive alternatives for sustainable alcohol oxidation [4,5]. Various catalytic systems including metal ions and metal nanoparticles such as nanogold, ruthenium, copper, cobalt, palladium and vanadium have been investigated as catalysts for aerobic oxidation of alcohols [6–23]. However, many of these catalytic systems are performed in aromatics or halogenated hydrocarbon solvents. Therefore, introduction of an efficient catalyst for these reactions with an acceptable reactivity in green solvents like water is indispensable.

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (J. Albadi). http://dx.doi.org/10.1016/j.mcat.2017.07.020 2468-8231/© 2017 Elsevier B.V. All rights reserved.

Metallic nanoparticles supported on different metal oxides play a significant role in nanoscience and nanotechnology. They have many applications such as inoptics, electronics, sensors and heterogeneous catalysis [24]. From the perspective of heterogeneous catalysis, the metallic supported nanoparticles can provide an optimal architecture for bifunctional catalytic systems [25]. In these catalysts, the nanoparticles on the catalyst surface contribute as an active site for the reaction, and the support act as a substrate to stabilize the whole structure and provide a high surface area for the reaction. Preparation of a catalyst with an appropriate interaction between support and nanoparticles can either tailor active sites with an enhanced electronic states for the reaction, and optimized size of nanoparticles [26]. The performance of different alumina-supported cobalt catalysts were investigated in the literature [27–32]. A particular aspect of these catalysts is their partial reduction in the catalytic reaction condition and the possibility of cobalt ions to insert in the aluminum oxides lattice to form a spinel structure that results in an exceptional reducibility and catalysit activity [33]. In this paper, in continuation of our research on the performance of Cu-, Au- and bimetal-nanocatalysts on aerobic oxidation reactions [34–38], we investigated the preparation, characterization

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2.4. General procedure

Scheme 1. Aerobic oxidation of alcohols on Co/Al2 O3 nanocatalyst.

and application of a highly dispersed cobalt nanocatalyst supported on a mesoporous Al2 O3 structure, as an efficient recyclable catalyst for the aerobic oxidation of alcohols in water under reflux condition (Scheme 1).

2. Experimental 2.1. General Chemicals were purchased from Merck chemical companies. Products were characterized by comparison of their spectroscopic data (1 HNMR, 13 CNMR and IR) and physical properties with those reported in the literature. All yields refer to isolated products.

2.2. Catalyst preparation The 10.0 wt% Co supported on Al2 O3 support were synthesized by a co-precipitation method. An aqueous solution of 0.5 M NaOH was added drop-wise into a mixture of 0.5 M Co(NO3 )2 ·6H2 O and Al(NO3 )3 ·9H2 O solution under vigorous stirring at 50 ◦ C. The resulted solution was aged at pH of 9.0 for 1 h at the same temperature, then filtered and washed with plenty of warm deionized water to remove the interfering ions. The precipitatnt was dried overnight at 100 ◦ C and calcined in air at 450 ◦ C for 4 h. Also, a batch of Al2 O3 substrate without Co was prepared by the same method for the supplementary analysis.

2.3. Catalyst characterization The specific surface area of the catalyst was analyzed by N2 adsorption-desorption using BET method. BET tests were carried out using an automated gas adsorption analyzer (Tristar 3020, Micromeritics). The samples were purged with N2 gas for 3 h at 300 ◦ C using VacPrep 061 degas system (Micrometrics). The XRD analysis was performed using an X-ray diffractometer (PANalytical X’Pert-Pro) with a Cu-K␣ monochromatized radiation source and a Ni filter in the range 2␪ = 5–100◦ , in order to study the structure and crystallinity of the catalysts. The average crystallite size of the sample was determined based on Scherrer equation. A flame atomic absorption spectrophotometer (GBC 906AA) was used to determine the cobalt content of the catalyst. Scanning electron microscopy (SEM) was performed by a JEOL JSM-6500F instrument, equipped with an EDS analytical system, in order to study the morphology of the prepared catalysts and the presence of different components of the catalyst. Transmission electron microscopy (TEM) analysis was performed using a JEOL JEM-2100 (200 kV) microscope with an EDS analytical system. The powdered samples were ultrasonically dispersed in ethanol and the obtained suspensions were deposited on to a thin carbon film supported on a copper micro-grid. For each catalyst, about 200 cobalt nanoparticles were measured in order to determine average particle size and its distribution.

A heterogonous mixture of alcohol (1 mmol), Cs2 CO3 (0.5 mmol) and Co/Al2 O3 nanocatalyst (0.05 g) in water was stirred under oxygen atmosphere in a slurry reactor connected to an O2 tube for atmosphere control at total reflux condition for an appropriate time. After reaction completion, catalyst was recovered by filtration, then washed with hot ethanol (2 × 5 mL), and dried for consecutive reaction runs. The filtrate was quenched with 2 M HCl aqueous solution, extracted with EtOAc three times and dried over anhydrous MgSO4 . Evaporation of the solvent followed by column chromatography on silica gel obtained the pure products. 2.5. Kinetic experiment A set of experiment was carried out to evaluate the kinetic order of the first and second alcohol’s oxidation. To perform this part of research, the oxidation of 2-phenylethylealcohol (0.1 mmol mL−1 ) and 2-propanol (0.1 mmol mL−1 ) were separately investigated. In this case, after the reaction was initiated, 0.1 mL of the reaction mixture (10 mL) was removed at 5-min intervals, and diluted with double-distilled water up to 1 mL. Subsequently, 0.5 ␮L of this analytical sample was injected into the gas chromatography-flame ionization detector (GC-FID). The concentration of alcohol in the analytical sample, was determined by a GC-FID method calibrated in the range of 0.02–0.1 mmol mL−1 . However, the dilution factor was considered and the remaining concentration of alcohol was obtained at each interval. 2.6. Gas chromatography analysis A gas chromatograph (Varian GC CP3800) with a split/splitless injection system and a flame ionization detector was used for separation and determination of 2-phenylethylealcohol and 2-propanol in aqua matrixes. Ultra-pure Nitrogen (99.999%, Fajr Co., Iran) was used as the carrier gas (5 mL min−1 ). The injection port was held at 120 and 230 ◦ C for 2-propanol and 2-phenylethylealcohol, respectively. Separation was carried out on a Wax column, 30 m × 0.32 mm capillary column with a 0.5 ␮m stationary film thickness. The oven temperature was programmed as follows: for 2-propanol; initial 60 ◦ C (held 1 min), from 80 to 130 ◦ C at the rate of 10 ◦ C min−1 , and for 2-phenylethylealcohol; initial 90 ◦ C (held 1 min), from 90 to 230 at the rate of 20 ◦ C min−1 , and held at this temperature. The total time for one GC run was 10 min for 2-propanol and 15 min for 2- phenylethylealcohol. The FID temperature was maintained at 300 ◦ C, hydrogen gas was generated by the hydrogen generator (OPGU–2200s, Shimadzu) for FID and was used at a flow of 30 mL min−1 . The flow of zero air (99.999, Sabalan Co, Tehran, Iran) was 300 mL min−1 and the split ratio was selected as 1:10. In this condition, the retention time was approximately 6 and 11 min for 2-propanol and 2-phenylethylalchol, respectively. 3. Results and discussion 3.1. Catalyst characterization results The XRD pattern of Co/Al2 O3 catalyst are shown in Fig. 1. The catalyst shows diffraction peaks at 2␪ = 37.60, 45.79, 66.76 and 85.02◦ , which are corresponded to (110), (111), (211) and (300) planes of ␥-Al2 O3 (code No. 01-1303). Also, the diffraction peaks at 19.04, 31.35, 36.94, 44.92, 59.51, 65.41 and 77.56◦ are attributed to (111), (220), (311), (400), (511), (440) and (533) planes of cubic Co3 O4 (code No. 01-074-1657), respectively. Moreover, the presence of a distinc peak at 94.38◦ which is corresponded to (731) plane of CoAl2 O4 (code No. 03-0896) demonstrates the incorporation of Co

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Table 1 Structural properties of Al2 O3 support and Co/Al2 O3 catalyst. Sample

Co Contenta (wt%)

Co crystallite sizeb (nm)

BET Surface area (m2 g−1 )

Avg. Pore size (nm)

Pore volume (cm3 g−1 )

Al2 O3 Co/Al2 O3

0.0 9.6

– 11.1

269 252

2.8 4.0

0.21 0.24

a b

Measured by atomic absorption method. Average CO crystallite size calculated by XRD.

Fig. 1. XRD pattern of Co/Al2 O3 nanocatalyst.

species into the Al2 O3 structure at calcination temperatures as high as 450 ◦ C. The average crystallite size of cobalt oxide in the catalyst structure, calculated by the Debay-Scherrer equation, is about 11.1 nm. The N2 adsorption/desorption profiles of Al2 O3 support and Co/Al2 O3 catalyst are shown in Fig. 2-a. The results display that both of the support and the catalyst show an adsorption/desorption isotherm profiles with a hysteresis at about p/p0 = 0.45–0.9. According to the IUPAC classification, this type of isotherms is ascribed to mesoporous structures and can be categorized as type IV. However, the Al2 O3 support adsorption profiles exhibits a H3 type hysterisis loop which is attributed to aggregates of nonuniform narrow slit-like pores of plate like and/or cubic nanoparticles, while the Co/Al2 O3 sample shows a H2 hysterisis loop. The H2 hysteric behaviour is common for mesoporous carrier and catalysts and is corresponding to aggregates and agglomerates of spheroidal par-

ticles with nonuniform size and shape pores [39,40]. The Fig. 2-b exhibits a narrow pore size distribution for both the alumina support and the Co/Al2 O3 catalyst with a maximum at about 3 and 4 nm, respectively. The structural properties of the prepared samples are presented in Table 1. The amount of cobalt loading analyzed by atomic absorption spectroscopy is about 9.6 wt.%. The prepared alumina has a high surface area of about 270 m2 g−1 , however, with the addition of Co content to the support the BET is decreased to about 252 m2 g−1 . Moreover, with increasing the cobalt content to the catalyst the average pore size of the support is decreased which is in consistent with the pore size distribution of the samples in Fig. 2-b. Fig. 3 displays the SEM micrographs and the EDS results of the catalyst. The Co/Al2 O3 micrographs (Fig. 3-b and c) show an aggregates of nanoparticles of about 21 nm in diameter. This results are in agreement with the type IV hysterisis H2 isotherms obtained by the N2 adsorption/desorption analysis. The EDS analysis shows the presence of the Co content on the surface of the catalyst by about 15.4 and 13.8 wt.% for Spectrum 1 and Spectrum 2, respectively. The higher Co content on the catalyst surface comparing to the nominal Co loading of 10 wt.% could be because of the procedure of precipitation during the synthesis of the catalyst. Considering the higher solubility product constant (Ksp) of Co(OH)2 by 3*10−16 in comparison to Al(OH)3 of about 3*10−34 , it can be concluded that with an increase of pH during the co-precipitation the cobalt clusters tend to precipitate at higher pH values and the amount of the Co content on the catalyst surface locally increases. Fig. 4 shows the TEM image, particle size distribution of Cobalt nanoparticles and the EDS analysis of the Co/Al2 O3 catalyst. It can be seen that the cobalt nanoparticles are dispersed on the Al2 O3 support with a Co particle size distribution of about 3–13 nm and an average particle size of about 6.5 nm. EDS analysis results are in good agreement with the nominal amount of Co loading on the catalyst surface.

Fig. 2. (a) N2 adsorption/desorption isotherms, and (b) pore size distributions of Al2 O3 support and Co/Al2 O3 nanocatalyst.

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Fig. 3. SEM micrographs and EDS analysis results of Co/Al2 O3 nanocatalyst. Table 2 Effect of different solvents on the aerobic oxidation of alcohols on Co/Al2 O3 nanocatalyst.a

Table 3 Effect of various bases on the aerobic oxidation of alcohols on Co/Al2 O3 nanocatalyst.a

Entry

Conditions

Time (h)

Yield (%)b

Entry

Base

Base loading (mmol)

Time (h)

Yield (%)b

1 2 3 4 5 6 7 8 9 10

H2 O/r.t. H2 O/reflux EtOH/r.t. EtOH/reflux MeOH/r.t. MeOH/reflux CH3 CN/r.t. CH3 CN/reflux DMF/r.t. DMF/reflux

18 1 18 4 18 6 18 8 18 8

– 93 – 65 – 50 – 50 – 65

1 2 3 4 5 6 7 8 9 10 11 12 13

Cs2 CO3 Cs2 CO3 Cs2 CO3 K2 CO3 K2 CO3 Na2 CO3 Na2 CO3 KOH KOH NaOH NaOH Et3 N Et3 N

0.3 0.5 0.7 0.5 0.7 0.5 0.7 0.5 0.7 0.5 0.7 0.5 0.7

1 1 1 4 3 3 3 2 2 3 3 4 4

70 93 93 60 80 50 60 75 75 69 75 30 50

a b

Reaction condition: Benzyl alcohol (1 mmol), Cs2 CO3 (0.5 mmol), O2 . Isolated pure products.

a

The HRTEM image of the catalyst is shown in Fig. 4-c. The reflections with d- spacing values of about 0.24 Å correspond to Co3 O4 (311) lattice plane. 3.2. Catalytic performance The catalytic activity of the Co/Al2 O3 nanocatalyst was studied on the oxidation of benzyl alcohol with O2 as the oxidant. The effects of various solvents on the performance of Co/Al2 O3

b

Reaction conditions: Benzyl alcohol (1 mmol), in water at reflux conditions. Isolated pure products.

nanocatalyst are summarized in Table 2. Under the same reaction conditions, the catalytic activity of Co/Al2 O3 nanocatalyst in water under reflux conditions is considerably higher than the typical polar and non-polar organic solvents such as ethanol, Toluene and the solvent-free condition.

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Fig. 4. TEM image of Co/Al2 O3 nanocatalyst: Particle size distribution of Co and EDS analysis results of the catalyst (a), dispersion and size of Co nanoparticles (b); and HRTEM image of the catalyst.

Table 4 Optimization of catalyst loading on the aerobic oxidation of alcohols on Co/Al2 O3 nanocatalyst.a Entry

Co loading (wt.%)

Catalyst loading(g)

Time (h)

Yield(%)b

TONc

TOF (s−1 )d

1 2 3 4 5 6 7 8 9 10 11

– 5 5 5 10 10 10 10 10 15 15

– 0.03 0.05 0.07 0.02 0.03 0.04 0.05 0.07 0.03 0.05

12 12 12 12 5 4 3.5 1 1 2 1

– Trace 20 60 50 60 75 93 93 75 92

– Trace 4.43 10.17 14.7 11.32 11.02 10.94 7.75 9.86 9.68

– Trace 1.02 × 10−4 2.35 × 10−4 8.16 × 10−4 7.86 × 10−4 8.74 × 10−4 3.03 × 10−3 2.15 × 10−3 1.37 × 10−3 2.70 × 10−3

a b c d

Reaction conditions: Benzyl alcohol (1 mmol), Cs2 CO3 (0.5 mmol), O2 , in water at reflux conditions. Isolated pure products. Turnover number. Turnover frequency.

Moreover, the effect of various bases such as Cs2 CO3 , Na2 CO3 , K2 CO3 , Et3 N and NaOH were investigated. The best results are obtained in the presence of Cs2 CO3 (Table 3). The perofrmance of various Co/Al2 O3 nanocatalysts containing 5, 10 and 15 wt.% Co, on the aerobic oxidation of alcohols were investigated. To compare the tests’ results, the turnover number, as the number of desired product produced per mole of Co catalyst, and turnover frequency, as the turnover number per second were calculated and presented in Table 4. As can be seen, the 10Co/Al2 O3 (0.05 g) catalyst shows the highest yield and turnover frequency compare to the other ones.

Although lower amount of catalyst decreases the yield of reaction, overloading the catalyst has no significant effect on the yield and time of the reaction (Table 4). Furthermore, the supplementary tests exhibit the advancement of the reaction up to 2% in the absence of the catalyst. For the additional experiments, a reflux condition in an aqueous solution over 0.05 g of Co/Al2 O3 were selected as the optimized reaction condition, and the procedure is applied to a range of various alcohols, as shown in Table 5 . The results represent the oxidation of benzyl alcohol into benzaldehyde with a high selectivity (Entry 1).

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J. Albadi et al. / Molecular Catalysis 440 (2017) 133–139 Table 5 Aerobic oxidation of alcohols catalyzed by Co/Al2 O3 nanocatalyst.a Entry

Alcohol

Time (h)

Yield (%)b

1 2 3 4 5 6 7 8 9 10

C6 H5 CH2 OH 3-ClC6 H4 CH2 OH 4-ClC6 H4 CH2 OH 2-BrC6 H4 CH2 OH 4-BrC6 H4 CH2 OH 3-NO2 C6 H4 CH2 OH 4-NO2 C6 H4 CH2 OH 3,4,5-(MeO)3 C6 H2 CH2 OH 4-MeOC6 H4 CH2 OH 4-Me3 C6 H4 CH2 OH

1 1 1.2 1 1 1.25 1.5 0.5 0.5 1

93 90 90 91 91 90 90 93 91 91

0.5

92

1 1 1.5 1

92 91 90 91

1.5

87

1.25

90

18

1.75

89

19

1.7

89

20

2.2

88

21

1.5

90

22

2.5

88

23

1

92

24

1.25

87

11

12 13 15 15

C6 H5 CH2 CH2 OH C6 H5 CH2 CH2 CH2 OH C6 H4 CH(Me)CH2 OH CH3 CH2 CH2 CH2 OH

16 17

Fig. 5. The kinetic of pseudo-second order for 2-phenylethylalcohol oxidation (a) and the kinetic of pseudo-first order for 2-propanol oxidation (b).

The catalyst performance displays a high activity on the oxidation of secondary and benzylic alcohols sontaining electrondonating and electron-withdrawing groups. Also, aldehydes or ketones are the only products of the reaction and no other products were detected in the reaction mixture implying the selectivity of the catalyst towards these components. The oxidation of primary aliphatic alcohols under the same reaction condition brings about the production of their corresponding aldehydes with a high yield (Table 5, entries 12–16). Moreover, the selectivity toward ketones on the oxidation of secondary alcohols is quite high, as tabulated in Table 5, entries 17–20. It is found out that, comparing to cyclic alcohols, the sterically hindered alcohols such as menthol or 2-adamantanol require longer reaction time for the same conversion rate (Table 5). The obtained products could be easily separated and purified by column chromatography with an appropriate combination of ethyl acetate and n-hexane. Finally, in order to study the recyclability and stability of the catalyst, the performance of the catalyst was investigated on oxidation of benzyl alcohol under optimized reaction conditions. After reaction completion, the catalyst was recovered by filtration, then washed with hot ethanol, and after drying was reused in the consecutive reaction runs. Table 6 indicates a sustained activity of the catalyst during five runs without no significant loss in catalytic activity. As addressed in literatures, in these reactions the pathway may be proceed non-radical intermediates as follow [23]. From XRD and TEM analysis, it can be assumed that the Cobalt oxide is presented mainly as Co3 O4 and is highly dispersed on the Al2 O3 as the support. Moreover, because of the water is the best solvent for the activity of the catalyst, therefore, it can be assumed that the support could be used for oxygen activation and regeneration, whereas Co3 O4 is a suitable for the dehydrogenation reaction. It can be proposed that the support offers the active site for oxygen adsorption and activation whereas Co3 O4 acts as the site for alcohol dehydrogenation.

CH3 CH(OH)CH3

a Reaction conditions: Benzyl alcohol (1 mmol), Cs2 CO3 (0.5 mmol), catalyst amount (0.05 g), O2 , in water at reflux conditions. b Isolated pure products.

3.3. Kinetic investigation As demonstrated in literatures, the integrated rate law of pseudo-zero order, pseudo-first order and pseudo-second order are expressed as follow [41]: Pseudo − zero order :

[A]t = −k0 t + [A]0

Pseudo − first order :

ln [A]t = −k1 t + ln [A]0

Pseudo − second order :

1 1 = k2 t + [A]t [A]0

Where the [A]t and [A]0 is referred to the concentration of the reactant at 0 s and t s and k0 , k1 and k2 are the relevant rate constants.

J. Albadi et al. / Molecular Catalysis 440 (2017) 133–139 Table 6 Recyclability study of Co/Al2 O3 nanocatalyst.a

139

References

Run

1

2

3

4

5

Time (h) Yield (%)b

1 93

1 92

1.2 92

1.5 90

2 89

a Reaction conditions: Benzyl alcohol (1 mmol), Cs2 CO3 (0.5 mmol), O2 , in water at reflux conditions. b Isolated pure products.

The initial alcohol concentration ([A]0 ) was 0.1 mmol mL−1 and the concentration at each 5-min interval ([A]t ) was determined by GC-FID. Subsequently, based on the GC-FID data, the integrated equations were plotted for each alcohol. As a sufficient result of linearity, the oxidation of first and second alcohol represents the pseudo-second order and pseudo-first order, respectively as shown in Fig. 5. However, the dependence of the reaction rate with the square of the first alcohol concentration, compared to the second alcohol, can be attributed to the spatial effects of the second alcohol. Hence, the rate constant for oxidation of 2-phenylethylealcohol and 2-propanols were estimated to be 3.5 × 10−2 mL mmol−1 s−1 and 26.5 × 10−2 s−1 , respectively. 4. Conclusion A highly dispersed cobalt nanocatalyst supported on a mesoporous Al2 O3 was synthesized and scrutinized for the aerobic oxidation of a variety of alcohols in water at reflux condition. The prepared catalyst demonstrates a high yield of reaction with a superior selectivity. Under the same reaction conditions, the catalytic activity of Co/Al2 O3 nanocatalyst in water under reflux conditions is considerably higher than the typical polar and non-polar organic solvents. Moreover, the activity of the catalyst also remained almost intact during five repeated runs. High yield of reaction, relatively short time of reaction, ease of work-up and clean procedure can represent this method as an appropriate and applicable substitute to the common methods for the aerobic oxidation of alcohols. It was observed that the proposed catalyst represents the pseudofirst order and pseudo-second order kinetic for the oxidation model reaction of the second and first alcohols, respectively. Acknowledgement We are grateful to the research council of Shahrekord University of Technology, for the support of this research.

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