Oxidation of Alkyl Aromatics Over SBA-15 Supported Cobalt Oxide

Copyright © 2013 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 13, 2528–2537, 2013

Oxidation of Alkyl Aromatics Over SBA-15 Supported Cobalt Oxide P. Visuvamithiran, B. Sundaravel, M. Palanichamy, and V. Murugesan∗ Department of Chemistry, Anna University, Chennai 600025, India SBA-15 supported Co3 O4 with different weight percentage have been prepared by wet impregnation method. The prepared catalysts were characterised by XRD, DRUV-vis, FT-IR, N2 adsorption– desorption, TPR, XPS, SEM and TEM. The TPR profile revealed good dispersion of active sites and interaction of Co3 O4 on SBA-15 support. The catalytic activity of SBA-15 supported Co3 O4 was evolved in liquid phase benzylic oxidation under mild condition using H2 O2 as the oxidant. The benzylic oxidation was ascribed by hydrogen abstraction of the C H bond. The various reaction parameters such as Co3 O4 loading, solvent, temperature and reaction time on the catalytic activity were also investigated. The 5 wt% of Co3 O4 /SBA-15 catalyst was found to exhibit high conversion (∼ 75 %) with product selectivity of ∼ 87%.

Keywords: Co3 O4 Nano Particle, SBA-15, Wet Impregnation, Benzylic Oxidation, H2 O2 .

material for the oxidation reaction.14–16 Generally bulk Delivered by Publishing Technology to: Korea Advanced Institute of Science & Technology (KAIST) metal oxides have low surface area, and this limits their IP: 143.248.131.73 13 May 2013 02:22:45 Catalytic oxidation is an important technology forOn: theMon,catalytic activity. Mesoporous SBA-15 and metal organic Copyright American Scientific Publishers conversion of hydrocarbons to industrially important oxyframeworks (MOFs) have been widely used for various genated organic compounds.1 The benzylic oxidation of applications such as supports, catalysis, biomedical applialkyl aromatics is considered to be one of the important cations, gas storage and separation.17–21 Hence mesoporous catalytic reactions for the preparation of corresponding SBA-15 has been considered as one of the suitable supcarbonyl compound of the reactant as they are used in ports for nanoparticles. In general mesoporous silica host the synthesis of fine chemicals and pharmaceuticals.2 The has high dispersion capacity of active sites and additionlarge scale preparation of benzylic oxidative products was ally exhibits favourable interaction with the particles to carried out over hazardous Cr and Mn based reagents.3 form small size.22 23 The scientific and industrial important A good variety of organometallic complexes containing of catalytic reactions are mainly depending on the particle transition metal ions such as Co, Cr, Mn, Fe and Ru were size of the catalyst.24 In order to achieve high dispersion used as catalyst for benzylic oxidation.4–8 The main disof transition metal oxide, high surface area support mateadvantages of these complexes are their toxicity, waste rial is essential. The dispersed metal oxide particles on disposal problem and recovery. Several heterogeneous catthe support made the particle small and thereby enhanced alysts viz., Cr-PILC, Cr-AlPO-5 and Cr-MCM-41 have the catalytic activity. It was observed that small particle been reported for benzylic oxidation in combination with size increased the surface area and enhanced the catalytic TBHP as an oxidant.9–11 In addition, most of the oxidisactivity.25 Co3 O4 is being used for various catalytic reacing agents such as periodic acid, TBHP, etc are hazardous tions such as dehydrogenation,26 CO oxidation,27 cycloin nature and cause lot of inconvenience during usage in hexane oxidation,28 cyclohexanol oxidation29 and ethyl the industries. Hence there is a constant strive to preacetate oxidation.30 However, only a few reports are availpare active, stable, reusable and environmentally friendly able in the literature on supported Co3 O4 for catalytic reagents for benzylic oxidation. The selective catalytic oxioxidative transformations. The main advantages of Co3 O4 dation of alkyl aromatics and the use of safe oxidant such are its high catalytic activity, multi oxidation sites and low as O2 and H2 O2 are gaining importance.12 13 cost compared to noble metals. The more abundant and less expensive transition metal The catalytic activity of SBA-15 supported Co3 O4 for oxides are generally considered as promising catalytic the oxidation of alkyl aromatics in the liquid phase under mild reaction condition is reported in this article. Further, ∗ Author to whom correspondence should be addressed. the effect of various reaction parameters such as solvent,

RESEARCH ARTICLE

1. INTRODUCTION

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J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 4

1533-4880/2013/13/2528/010

doi:10.1166/jnn.2013.7388

Visuvamithiran et al.

Oxidation of Alkyl Aromatics Over SBA-15 Supported Cobalt Oxide

Co3 O4 loading, temperature and reaction time were also carried out to optimise the reaction condition, and to increase the conversion as well as selectivity. In addition, various substrates were attempted in the oxidation to find product distribution in the benzylic oxidation.

2. EXPERIMENTAL DETAILS 2.1. Materials Tetraethylorthosilicate (TEOS) (Aldrich) and triblockcopolymer(poly(ethylene glycol)-poly(propylene glycol)block-poly(ethylene glycol) (Pluronic P123) of molecular weight 5800 (Aldrich) were used as silica source and structure directing agent respectively. Cobalt nitrate and hydrogen peroxide (Merck) were used as cobalt source and oxidant respectively. Ethyl benzene, cumene, xylene and 4-ethyltoluene purchased from Alfa aeser and diphenymethane, 4-bromo ethylbenezene, 4-methoxy ethylbenezene and 4-nitro ethylbenzene purchased from Fluka were used as reactants. All solvents (Merck) were used without further purification. 2.2. Synthesis of SBA-15 Supported Co3 O4

Prior to each adsorption measurement, the samples were degassed at 350 C under vacuum (p < 10−5 mbar) in the port of the adsorption analyzer. The specific surface area was calculated by Brunauer–Emmett–Teller (BET) method, and the pore size distribution was calculated using Barrett–Joyner–Halenda (BJH) method. Temperature programmed reduction (TPR) experiments were carried out using a Micromeritics chemisorb 2750. Typically 15 mg of calcined samples was flushed with He at 150 C for 2 h and then cooled to 40 C. The gas flow of 10%H2 /Ar was maintained and then the temperature raised from 40 to 900 C. X-ray photoelectron spectrum (XPS) was performed on a Thermo Multilab 2000 using monochrome AlK as the radiation source. The morphology of the materials was examined by scanning electron microscope (SEM) after gold coating using SEM-JEOL, JSM-5600 model. Cressington 108 auto sputter coater was used for coating. The accelerating voltage of the SEM was 20 keV. High-resolution transmission electron microscopic (HR-TEM) images were captured using Tecnai G2 20S model. The accelerating voltage was 200 keV. 2.4. General Procedure for Liquid Phase Catalytic Benzylic Oxidation

2.3. Characterization of the Catalyst

3. RESULTS AND DISCUSSION

The small angle X-ray diffraction patterns were recorded on a Bruker D8 advanced powder X-ray diffractometer using Cu K ( = 15418 Å) as the radiation source in the 2 range 0.5–6 with a step size of 0.01 and a step time of 1 s. The high angle X-ray diffraction patterns were recorded on a PANalytical X pert PRO diffractometer equipped with Cu K ( = 15418 Å) as the radiation in the 2 range of 5–80 with a step size of 0.01 and a step time of 1 s. The DRUV–vis absorption spectra were recorded between 190 and 800 nm on a UV–Vis spectrophotometer (Shimadzu model 2450) using BaSO4 as the reference. The FT-IR spectra were recorded in the range of 4000–400 cm−1 on a FT-IR spectrometer (Nicolet, Avatar 360) using KBr pellet technique. The N2 adsorption– desorption isotherms were measured at 77 K on a volumetric adsorption analyzer (Micromeritics ASAP 2020).

3.1. XRD

J. Nanosci. Nanotechnol. 13, 2528–2537, 2013

The low angle XRD patterns of SBA-15 and SBA-15 supported Co3 O4 (2, 5 and 10 wt%) are shown Figure 1. The diffraction patterns exhibited highly ordered 2D hexagonal mesopores with three reflections in the 2 range from 0.5 to 5. The SBA-15 showed sharp intense peak below 1 (2) corresponding to (100) and a small hump at 1.65 and 1.8 (2) corresponding to (110) and (200) respectively.33 The intensity of the peaks significantly decreased with increasing weight percent of Co3 O4 on SBA-15 support due to pore filling effect that resulted weak diffraction. The results exhibited retainment of high degree of mesoporous structure even after higher loading of Co3 O4 . The high angle XRD patterns of SBA-15 and SBA-15 supported Co3 O4 (2, 5, and 10 wt%) are shown in Figure 2. 2529

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SBA-15 was prepared using standard synthesis proce5 wt% Co3 O4 /SBA-15 catalyst (75 mg) was suspended in dure adopted by Choi et al.31 SBA-15 supported Co3 O4 5 ml acetonitrile followed by the addition of ethylbenzene Delivered Publishing Koreaby Advanced Institute of Science & Technology (KAIST) (2, 5 and 10 wt%)bywere preparedTechnology in alcoholic to: medium mmol) 5 equivalent IP: 143.248.131.73 On: Mon,(313 May and 2013 02:22:45 of H2 O2 . The reaction mixture 2 wt% of wet impregnation method.32 SBA-15 supported was stirred magnetically for 6 h at 70 C. The product Copyright American Scientific Publishers Co3 O4 was prepared as follows: SBA-15 (2 g) was added was filtered, washed thoroughly with water and extracted to ethanolic (50 ml) solution of Co(NO3 )2 · 6H2 O (0.15 g). with ether. The collected products were analyzed by gas The reaction mixture was stirred at ambient temperature chromatograph (Shimadzu GC-17A model) equipped with for about 24 h to obtain dry powder. The dry powder was DB-5 capillary column (30 m × 025 mm × 025 m) and calcined at 550 C for 6 h in air. The same procedure was a flame ionization detector. The products were also identiadopted for 5 and 10 wt% of Co3 O4 on SBA-15 support. fied by gas chromatograph coupled with mass spectrometer The sample was denoted as xCo3 O4 /SBA-15 (x = 2, 5 and (Perkin Elmer Clarus 500) using helium as the carrier gas 10 wt%). at a flow rate of 1 ml/min.



Intensity (a.u.)

Table I. Textural properties of SBA-15 and Co3 O4 /SBA-15(2, 5 and 10 wt%). Nominal Co3 O4 content (wt%)

(100) (110) (200)

Material

(d)

SBA-15 2Co/SBA-15 5Co/SBA-15 10Co/SBA-15

(c) (b)

Actual Surface Pore Pore Co3 O4 Particle area, volume, diameter, content size ABET Vp Dp (nm) BJH (wt%)a (nm)b (m2 /g)c (cm3 /g)

– 2 5 10

– 1.8 4.2 8.5

– 9.2 13.5 26.8

720 665 635 607

0.8631 0.8287 0.6956 0.4776

6.93 6.23 6.05 5.46

(a) 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Notes: a Determined by AAS analysis; b Determined from XRD; c BJH method from N2 adsorption isotherm.

2θ (degree) Fig. 1. Low angle XRD patterns of (a) SBA-15, (b) 2 wt% of Co3 O4 /SBA-15, (c) 5 wt% of Co3 O4 /SBA-15 and (d) 10 wt% of Co3 O4 /SBA-15.

using Scherrer equation (d = K/ cos ) for (311) plane and the values are given in Table I.

The characteristic amorphous silica diffraction showed The DRUV-vis spectra of SBA-15 and SBA-15 supported broad band in the 2 range between 15 and 30 . In addiCo3 O4 (2, 5, and 10 wt%) are shown in Figure 3. SBA-15 tion, the distinct diffraction peaks appeared at about 18.4 , supported Co3 O4 showed three broad peaks around 216, 31.2 , 36.7 , 38.4 , 44.6 , 55.6 , 59.4 , 65.3 and 77.6 cor452 and 716 nm. The broad band at 216 nm is assigned to respond to (111), (220), (311), (222), (400), (422), (511), charge transfer from oxygen to cobalt. The broad band at (440) and (622) plane respectively of crystalline Co3 O4 .34 452 nm is assigned to Co3+ ion in octahedral environment Figure 2 showed all diffractions patterns corresponding and the absorption band at 716 nm is due to Co2+ ion in to Co3 O4 with low intensity for 2 wt% of Co3 O4 /SBAtetrahedral environment.35 The resulting absorption bands 15 catalyst. However, 5 wt% of Co3 O4 /SBA-15 catalyst clearlyInstitute showedof theScience existence well ordered Co3 O4 spinal Delivered by Publishing Technology to: Korea Advanced & of Technology (KAIST) showed high intensity with allIP:characteristic reflections theMay wall2013 of SBA-15. 143.248.131.73 On: Mon,in13 02:22:45 the diffraction of crystalline spinal Co3 O4 . In addition, Copyright American Scientific Publishers peak of amorphous silica did not change. It confirmed that 3.3. FT-IR Spectroscopy Co3 O4 particles were uniformly dispersed in the host of SBA-15 wall. However, increase of Co3 O4 wt% to 10 in The FT-IR spectra of SBA-15 and SBA-15 supported Co3 O4 /SBA-15 showed high intensity diffraction patterns Co3 O4 (2, 5 and 10 wt%) are shown in Figure 4. In the which could be possibly the formation of Co3 O4 of larger FT-IR spectrum of SBA-15, strong peaks observed at 1082 particles, and the high crystalline nature led to suppress the and 970 cm−1 are assigned to the stretching vibration amorphous silica nature. The particle size was determined of Si O Si bond. The peaks at 783 and 464 cm−1 are assigned to SiO4 tetrahedral structural unit. The IR (311) (d)

Intensity (a.u.)

(111)

(220)

0.7

(400) (222)

(511) (422)

2

0.6

(440)

3+

O -Co

(622) (620)

3+

Co (Oh) 452 nm

0.5 (c)

Absorbance

RESEARCH ARTICLE

3.2. DRUV-Vis Spectroscopy

(b)

(a)

10

20

30

40

50

60

70

0.4

2+

Co (Td) 716 nm

(d)

0.3

(c)

0.2

(b)

0.1

(a)

80

2θ (degree)

200

300

400

500

600

700

800

Wavelength (nm) Fig. 2. High angle XRD patterns of (a) SBA-15, (b) 2 wt% of Co3 O4 /SBA-15, (c) 5 wt% of Co3 O4 /SBA-15 and (d) 10 wt% of Co3 O4 /SBA-15.

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Fig. 3. DRUV-vis spectra of (a) SBA-15, (b) 2 wt% of Co3 O4 /SBA-15, (c) 5 wt% of Co3 O4 /SBA-15 and (d) 10 wt% of Co3 O4 /SBA-15.


Oxidation of Alkyl Aromatics Over SBA-15 Supported Cobalt Oxide 1000

(a) (b) (c) (d)

Co-O Td Si-O-Si

4000

3800

3600

3400

3200

1200

1000

Co-Ooh

800

600

Volume of N2 adsorbed (cm3/g)

Transmittance (a.u.)


(A)

(a)

900 800

(b)

700 600

(c)

500 (d)

400 300 200 100 0

400

–1

Wave number (cm )

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Relative pressure (p/po)

Fig. 4. FT-IR spectra of (a) SBA-15, (b) 2 wt% of Co3 O4 /SBA-15, (c) 5 wt% of Co3 O4 /SBA-15 and (d) 10 wt% of Co3 O4 /SBA-15.

2.0

(B)

dVp/drp

spectrum of SBA-15 supported Co3 O4 (2, 5 and 10 wt%) showed two additional strong peaks at 652 and 560 cm−1 1.5 corresponding to M O stretching vibration in the Co3 O4 (a) spinal lattice.36 The peak at 652 cm−1 is assigned to tetra1.0 hedral coordination environment of Co2+ O stretching (b) and the peak at 560 cm−1 is due to octahedral coordination environment of Co3+ O stretching. The Si O Si (c) 0.5 stretching frequency at 1082, 970 and 784 cm−1 are slightly shifted to lower frequency due to loading of (d) Co3 O4 . Delivered by Publishing Technology to: Korea Advanced0.0Institute of Science & Technology (KAIST) IP: 143.248.131.73 On: Mon, 13 May 2013 02:22:45 Copyright American Scientific0Publishers 2 4 6 8 10 12 14 16 18 3.4. N2 Physisorption Figure 5(a) depicts N2 adsorption–desorption isotherms of SBA-15 and SBA-15 supported Co3 O4 (2, 5 and 10 wt%). All the samples exhibited type IV adsorption isotherms and showed H1 hysteresis loop in the relative pressure range of 0.6–0.8 with narrow pore size distribution. The textural properties such as surface area, pore volume and pore diameter of SBA-15 and x wt% Co3 O4 /SBA-15 are given in Table I. The surface area, pore volume and pore diameter decreased (720 to 607 m2 /g, 0.86 to 0.47 cm3 /g and 6.93 to 5.46 nm respectively) with increasing amount of Co3 O4 from 2 to 10 wt % on SBA-15 support.37 After the increase of Co3 O4 wt% loading the capillary condensation of adsorption isotherm slightly shifted to lower relative pressure in the desorption branch due to partial pore blocking by Co3 O4 particles. The shape of the isotherms and hysteresis loop were identical and retained the mesopores even after higher wt% Co3 O4 . Figure 5(b) showed narrow pore size distribution of SBA-15 and Co3 O4 on SBA-15 support. The decrease of pore size distribution is due to partial pore blocking of Co3 O4 nanoclusters inside the silica walls. 3.5. TPR Spectroscopy Figure 6 depicts the effect of interaction of Co3 O4 particles with SBA-15 support and reducibility. TPR studies J. Nanosci. Nanotechnol. 13, 2528–2537, 2013

Fig. 5. N2 adsorption–desorption isotherms and pore size distribution of (a) SBA-15, (b) 2 wt% of Co3 O4 /SBA-15, (c) 5 wt% of Co3 O4 /SBA-15 and (d) 10 wt% of Co3 O4 /SBA-15.

showed two reduction peaks, one at ∼ 410 C and another at ∼ 650 C illustrating the step wise reduction of Co3 O4 to CoO and then to metallic Co.38 The intensity of reduction area increased with increasing Co3 O4 wt%. The particles were well dispersed and strongly interacted with SBA-15 support in the case of 5 wt% Co3 O4 loading. Further, increase upto 10 wt% of Co3 O4 , the maximum of reduction area and high H2 consumption were due to agglomeration of Co3 O4 particles into larger particles. 3.6. XPS Spectrum The XPS spectrum elucidates the chemical state of Co3 O4 on SBA-15 support. The XPS spectrum showed two peaks separated by spin orbit splitting in Co 2p3/2 and Co 2p1/2 states and the corresponding binding energies were 780.7 eV and 794.8 eV respectively. It is reported that spin orbit coupling for Co3 O4 is 15 eV and 16 eV for CoO.39 The Co 2p3/2 and Co 2p1/2 peaks are in accordance to the presence of Co3 O4 (Fig. 7). Obviously the shake up peak at 788.5 eV is due to Co2+ in paramagnetic high spin state.40 2531

RESEARCH ARTICLE

20

Pore diameter (nm)

Oxidation of Alkyl Aromatics Over SBA-15 Supported Cobalt Oxide 3.0


Co3 O4 nanoparticles. The average particle size of 5 wt% of Co3 O4 /SBA-15 catalyst is 13 nm.

2.5

3.8. Catalytic Activity 2.0

Intensity

RESEARCH ARTICLE

TCD signal

Catalytic activity of SBA-15 supported Co3 O4 of different wt% was evaluated in the benzylic oxidation using H2 O2 as the oxidant (Scheme 1). The oxidation of alkyl aromatics in the benzylic C H bond of the side chain 1.0 gave exclusively the corresponding oxidative product with (c) high conversion and selectivity. In addition, small amounts 0.5 (b) of ring hydroxylated alkyl aromatics were also obtained. (a) The influence of various reaction parameters such as sol0.0 vent, Co3 O4 loading, temperature and reaction time on 100 200 300 400 500 600 700 800 900 the catalytic behaviour was also studied and optimised the Temperature (ºC) reaction conditions to obtain high conversion and selecFig. 6. TPR profile of (a) 2 wt% of Co3 O4 /SBA-15, (b) 5 wt% of tivity. The crystalline structure of Co3 O4 corresponds to Co3 O4 /SBA-15 and (c) 10 wt% of Co3 O4 /SBA-15. normal spinal structure with Co3+ and Co2+ in octahedral and tetrahedral sites respectively. Among Co3+ and This further confirmed that Co3 O4 particles are strongly Co2+ species, Co3+ is found to be more active than Co2+ interacted with mesoporous SBA-15 if the particle size is for benzylic oxidation because of its ability to reduce to small. Co2+ and thus forming redox couple (Co3+ ↔ Co2+ ).43 H2 O2 decomposed into HO and HOO radicals. The plausible mechanistic pathway for the benzylic oxidation is 3.7. SEM and TEM shown in Scheme 2. The first step involved the chemisorpThe SEM morphological images of SBA-15 and SBA-15 tion of alkyl aromatics on Co3+ sites followed by polarsupported Co3 O4 (2, 5 and 10 wt%) are shown in Figure 8. isation of benzylic C H bond. The lattice oxygen first by Publishing to: morpholKorea Advanced Institute of Science & Technology (KAIST) FigureDelivered 8(a) showed a bundle Technology of worm like with C 02:22:45 H bond to form C O bond. The secIP: 143.248.131.73 On: Mon,reacted 13 May 2013 ogy, and this is the characteristics of well ordered meso3+ ond step was the Copyright American Scientific Publishersadsorption of alkyl aromatics on Co porous SBA-15.41 In addition, different weight percent of 2+ sites which in turn reduced to Co . The lattice vacancy Co3 O4 on SBA-15 support SEM images are shown in created on Co3+ sites were compensated by neighbouring Figures 8(b)–(d). The Co3 O4 on SBA-15 support showed Co2+ species. The highly reactive oxygen of hydroperoxsimilar morphology as that of the parent siliceous SBA-15. ide species chemisorbed on Co2+ sites and converted into This confirmed that even after the impregnation of Co3 O4 lattice oxygen. Finally the desorbed oxygenated product on the host did not change the morphology of meso(major) was formed in the last step. The hydroxyl radiporous SBA-15. TEM images of SBA-15 and 5 wt% of cal reacted with benzylic compound to form hydroxylated Co3 O4 /SBA-15 catalysts are shown in Figure 9. The TEM benzylic compounds (minor). images in Figure 9(a) illustrated ordered hexagonal array of uniform pore characteristics of mesoporous SBA-15.42 3.8.1. Effect of Solvent Figure 9(b) revealed dark spot inside the mesoporous channels that could be assigned to the well dispersion of The solvents play significant role in the liquid phase oxidation especially in the product distribution. The values 18000 in Table II revealed the influence of various solvents in 17000 the benzylic oxidation under the same reaction conditions. 780.1 eV 16000 It is noticed that solvents such as dichloromethane and Co 2p3/2 15000 794.8 eV chloroform as solvent resulted low conversion compared Co 2p1/2 14000 to acetone and acetonitrile due to slow decomposition 13000 788.5 eV of hydrogen peroxide. Whereas, polar solvent like aceShake up 12000 tonitrile led to benzylic C H oxidation. The conversion 11000 of ethylbenzene was found to be 75% with 87% ace10000 tophenone selectivity. The increase in the catalytic activ9000 ity of 5 wt% of Co3 O4 /SBA-15 for benzylic oxidation 8000 was in the order: acetonitrile > acetone > chloroform > 805 800 795 790 785 780 775 dichloromethane. The ethylbenzene conversion and prodBinding energy/eV uct selectivity (acetophenone) increased with increase of solvent polarity. Fig. 7. XPS spectrum of Co 2p. 1.5

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Fig. 8.


(a)

(b)

(c)

(d)

SEM images of (a) SBA-15, (b) 2 wt% of Co3 O4 /SBA-15, (c) 5 wt% of Co3 O4 /SBA-15 and (d) 10 wt% of Co3 O4 /SBA-15.

3.8.2. Effect of Co3 O4 Loading

3.8.3. Effect of Reaction Temperature

Fig. 9.

TEM images of (a) SBA-15 and (b) 5 wt% of Co3 O4 /SBA-15.


The influence of reaction temperature on benzylic oxidation over 5 wt% of Co3 O4 /SBA-15 catalyst was investigated in the temperature range between 40 and 70 C. The percentage of ethylbenzene conversion increased from 18 to 75% with increase of reaction temperature from 40 to 70 C (Fig. 10). The increase in the percentage conversion of ethylbenzene is attributed to enhanced decomposition of H2 O2 with increase in temperature. Further increase of reaction temperature above 80 C vaporization of benzylic compound occurred and the conversion decreased. 2533

RESEARCH ARTICLE

Delivered by Publishing Technology to: Korea Advanced Instituteactivity of Science & Technology (KAIST) The catalytic of different weight percent of Co3 O4 IP: 143.248.131.73 On: Mon,on13 May 2013 02:22:45 SBA-15 support are shown in Table III. The dispersed Copyright American Scientific Publishers particles effectively interacted with support and increased the pore accessibility in 5 wt% of Co3 O4 /SBA-15 catalyst. Hence the reagents freely accessed to the highly dispersed active sites and increased the conversion of ethylbenzene as well as the selectivity of the product. The 5 wt% of Co3 O4 /SBA-15 showed 75% ethylbenzene conversion with 87% selectivity of acetophenone. Further, increase of Co3 O4 loading upto 10 wt%, the particles formed were larger size with low pore volume. So, the reactants could not freely access to the reactive sites and hence the larger particle size showed relatively low reactivity. Similar phenomenon with high loading of Co3 O4 exhibited poor catalytic activity in the methane combustion over Co3 O4 loaded SBA-15.44


Visuvamithiran et al. O

O

Co/SBA-15

OH +

+

+

ACN/H2O2

+ OH

HO

70 ºC, 6 h Major product

O

Others

Scheme 1. Benzylic oxidation of alkyl aromatics over 5 wt% of Co3 O4 /SBA-15.

O R

HO

Co3O4/SBA-15

OH O

R

Solvent

R

Acetonitrile Acetone Chloroform Dichloromethane

R OH O

R Co2+ O OH

R O Co3+

Co2+

R

Selectivity (%) Conversion Acetophenone 1-phenylethanol Others (%)b

OH R

O Co3+

Effect of solvents on benzylic oxidationa .

Table III.

OH

75.2 59.4 31.5 26.7

87.4 76.7 53.6 37.9

73 167 342 516

53 66 122 105

Notes. a Reaction condition: Ethylbenzene (3 mmol), H2 O2 (5 equi), solvent (5 mL), catalyst 5Co/SBA-15 (75 mg), for 6 h at 70 C; b Determined by gas chromatography.

OH R

100

OH

R

O Co2+ Co3+

H2O

90

Conversion Selectivity

80

Conversion (%)

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Scheme 2. Plausible mechanism for the benzylic oxidation of alkyl Delivered by Publishing Technology to: Korea Advanced Institute aromatics. 60

of Science & Technology (KAIST) IP: 143.248.131.73 On: Mon, 13 May 2013 02:22:45 Copyright American Scientific Publishers

3.9. Effect of Reaction Time The influence of reaction time on the benzylic oxidation of ethylbenzene at 70 C is depicted in Figure 11. The percentage of ethylbenzene conversion increased from 16 to 75% with increase in the reaction time from 1 to 6 h. After 7 h reaction time the conversion remains constant and selectivity decreased because acetophenone formed in the reaction adsorbed over the catalyst surface and

60

40

Selectivity (%)

O

30 20

0

0 40

50

60

70

Temperature (ºC)

Fig. 10. Effect of reaction temperature in the benzylic oxidation.

Table II. Effect of Co3 O4 loading on SBA-15 for benzylic oxidationa . O Co3O4/SBA-15 ACN/H2O2 70 ºC, 6 h

O

OH +

+

+

+

HO

OH

O

Others

Selectivity (%) Catalyst 2Co/SBA-15 5Co/SBA-15 10Co/SBA-15

Conversion (%)b

Acetophenone

1-phenylethanol

Others

36.6 75.2 43.8

77.2 87.4 73.6

164 73 147

64 53 117

Notes: a Reaction condition: Ethylbenzene (3 mmol), H2 O2 (5 equi), ACN (5 mL), catalyst (75 mg) for 6 h at 70 C; b Determined by gas chromatography.

2534



Oxidation of Alkyl Aromatics Over SBA-15 Supported Cobalt Oxide 100

100

40

40

20

20

0

0 1

2

3

4 5 Time (h)

6

7

8

Conversion/selectivity (%)

60

Selectivity (%)

Conversion (%)

80

60

0

Selectivity

90

Conversion Selectivity

80

100 Conversion

80 70 60 50 40 30 20 10 0

9

1

2

3

4

5

No. of cycles

Fig. 11.

Effect of reaction time in the benzylic oxidation. Fig. 12. Recyclability of 5 wt% of Co3 O4 /SBA-15 catalyst in the benzylic oxidation.

further oxidised to the corresponding acid. The activity also decreased due to polar oxidation of –C O group.

Table IV. Effect of substituents on Co3 O4 benzylic oxidationa .

R

O H

Co3O4/SBA-15 ACN/H2O2

R2

70 ºC,6 h

R1 Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

R2

R1

R

R1

R2

Conversion (%)e

Selectivity (%)

Major product

H H H H H H H H H –OH H H H H H

–CH3 –CH3 –CH3 –CH3 H H H H –CH3 –CH3 –CH3 –CH3 –CH3 –CH3 –C6 H5

– – – – – o-CH3 p-CH3 p-NO2 -CH3 – p-OCH3 p-NO2 p-Br p-CH3 –

752 743 23 126 687 692 689 675 894 887 634 672 691 624 826

874 827 62 275 728 737 718 698 782 964 819 716 752 793 928

Acetophenone Acetophenoneb Acetophenonec Acetophenoned Benzaldehyde 2-Methylbenzaldehyde 4-Methylbenzaldehyde 4-Nitrobenzaldehyde 2-Phenyl-2-propanol Acetophenone 4-Methoxyacetophenone 4-Nitroacetophenone 4-Bromoacetophenone 4-Methylacetophenone Benzophenone

Notes. a Substrate (3 mmol), H2 O2 (5 equi), catalyst 5Co/SBA-15 (75 mg) in 5 mL ACN for 6 h at 70 C; e Determined by gas chromatography.


b

Reused catalyst;

c

Without catalyst; d Without solvent.

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RESEARCH ARTICLE

in the benzylic C H oxidation. Thus, the results highlighted the catalytic role played by Co2+ /Co3+ in the benzylic oxidation thereby facilitating the redox cycle.45 46 3.9.1. Effect of Various Substituents From entries 6–9 and 11–14 it is observed that there is Benzylic oxidation of different substrates was carried out no relationship between the rate of oxidation and electron with 5 wt% of Co3 O4 /SBA-15 using H2 O2 in acetonitrile density in the aromatic substrates. The reaction carried medium and the results are given in Table IV. The results out with 1-phenylethanol (entry 10) as a substrate gave of fresh and reused catalyst in entries 1 and 2 did not show acetophenone as major product and minor ring hydroxany appreciable change in the conversion and selectivity. ylatedInstitute product.ofThe conversion remained (KAIST) the same. The Delivered by Publishing Technology to: Korea Advanced Science & Technology But, the entries in 3 and 4 were out with blank in 52013 and 15 indicated high conversion of diphenylIP:carried 143.248.131.73 On: Mon,entries 13 May 02:22:45 and SBA-15 respectively which resulted low conversion. methane Publishers with high selectivity compared to toluene converCopyright American Scientific sion. This is attributed to the relative reactivity of benzylic This clearly indicated that Co3 O4 catalyst is imperative



RESEARCH ARTICLE

C H bond. This confirmed that the mechanism proceeded through hydrogen abstraction.

7. S. S. Kim, K. S. Sar, and P. Tamrakar, Bull. Korean Chem. Soc. 23, 937 (2002). 8. S. I. Murahashi, N. Komiya, Y. Oda, T. Kuwabara, and T. Naota, J. Org. Chem. 65, 9186 (2000). 9. B. M. Choudhary, A. D Prasad, V. Bhuma, and V. J. Swapna, Org. 3.9.2. Reusability of the Catalyst Chem. 57, 5841 (1992). 10. R. A. Sheldon, J. D. Chen, J. Dakka, and E. Neeleman, Stud. Surf. Since the recyclability of the catalyst is important for Sci. Catal. 83, 407 (1994). industrial application, the recycled catalyst was tested for 11. T. K. Das, K. Chaudhari, E. Nandanan, A. J. Chandwadkar, their catalytic activity and the results are depicted in A. Sudalai, T. Ravindranathan, and S. Sivasanker, Tetrahedron Lett. Figure 12. The recovered catalyst was recycled by washing 38, 3631 (1997). 12. J. M. Bregeault, J. Chem. Soc., Dalton. Trans. 3289 (2003). with acetonitrile and then dried at 150 C. The recyled cat13. W. D. Wang, A. Bakac, and J. H. Espenson, Inorg. Chem. 34, 6034 alyst exhibited almost the same up to five cycles. 5 wt % (1999). of Co3 O4 /SBA-15 was reused for five times without loss 14. Q. Tang, Q. Zhang, H. Wu, and Y. Wang, J. Catal. 230, 384 of activity. (2005). 15. T. Garcia, S. Agouram, J. F. Sanchez-Royo, R. Murillo, A. M. Mastral, A. Aranda, I. Vazquez, A. Dejoz, and B. Solsona, Appl. Catal. A: Gen. 386,16 (2010). 4. CONCLUSIONS 16. Y. Xia, H. Dai, H. Jiang, and L. Zhang, Catal. Commun. 11, 1171 (2010). In this study, SBA-15 supported Co3 O4 was successfully 17. K. Ariga, A. Vinu, Y. Yamauchi, Q. Ji, and J. P. Hill, Bull. Chem. prepared, and their catalytic performance in the benzylic Soc. Jpn. 85, 1 (2012). oxidation under mild condition using H2 O2 as the oxi18. M. C. Marchi, J. J. S. Acuna, and C. A. Figueroa, J. Nanosci. Nandant yielded high conversion with good selectivity of the otechnol. 12, 6439 (2012). 19. M. B. Zakaria, N. Suzuki, K. Shimasaki, N. Miyamoto, Y. T. Huang, product. XRD, TPR and N2 sorption results revealed the and Y. Yamauchi, J. Nanosci. Nanotechnol. 12, 4502 (2012). presence of crystalline Co3 O4 in the host of SBA-15. This 20. W. Xuan, C. Zho, Y. Liu, and Y. Cui, Chem. Soc. Rev. 41, 2590 also demonstrated high degree of dispersion of Co3 O4 with (2012). interaction on the support and pore accessibility of Co3 O4 . 21. Z. Li, J. C. Barnes, A. Bosoy, J. F. Stoddart, and J. I. Zink, Chem. Soc. Rev. 41, 1677 (2012). The catalytic activity as well as the characterization results 22. J. P. de Breejen, P. B. Radstake, G. L. Bezemer, J. H. Bittere, suggested that the redox cycle Co3+ /Co2+ played an imporDelivered by Publishing Technology to: Korea Advanced Science Technology V. Institute Froseth, A.ofHolmen, and&K. P. de Jong, J.(KAIST) Am. Chem. Soc. tant role in the benzylic oxidation. high catalytic activIP:The 143.248.131.73 On: Mon, 13131, May 2013 02:22:45 7197 (2009). with Scientific ity was found to be over 5 wt% of Co Copyright American Publishers 3 O4 /SBA-15 23. Y. Ohtsuka, Y. Takahashi, M. Noguchi, T. Arai, S. Takasaki, 75% conversion of ethylbenzene and 87% of selectivity of N. Tsubouchi, and Y. Wang, Cat. Today 89, 419 (2004). 24. S. Rojluechai, S. Chavadej, J. W. Schwank, and V. Meeyoo, Cat. acetophenone in acetonitrile at 70 C for 6 h. The present Commun. 8, 57 (2007). catalyst is associated with several advantages such as mild 25. T. E. Davis, T. Garcia, B. Solsona, and S. H. Taylor, Chem. Commun. reaction condition, simple workup, stability and recycla3417 (2006). bility of the catalyst for 5 cycles. 26. S. Lee, M. D. Vece, B. Lee, S. Seifert, R. E. Winans, and S. Vajda, Phys. Chem. Chem. Phys. 14, 9336 (2012). 27. C. J. Jia, M. Schwickardi, C. Weidenthaler, W. 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Received: 21 September 2012. Accepted: 21 October 2012.


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