Advances in Materials Science and Engineering
Volume 2018 (2018), Article ID 2716435, 7 pages
https://doi.org/10.1155/2018/2716435

Research Article

Selective Oxidation of Styrene to Benzaldehyde by Co-Ag Codoped ZnO Catalyst and H₂O₂ as Oxidant

Abderrazak Aberkouks,¹ Ayoub Abdelkader Mekkaoui,1,2 Brahim Boualy,² Soufiane El Houssame,² Mustapha Ait Ali,¹ and Larbi El Firdoussi¹

¹Laboratoire de Chimie de Coordination et de Catalyse, Département de Chimie, Faculté des Sciences Semlalia, BP 2390, 40001 Marrakech, Morocco
²Laboratoire de Chimie et Modélisation Mathématique, Faculté Polydisciplinaire de Khouribga, Université Hassan 1er, BP 145, 25000 Khouribga, Morocco

Correspondence should be addressed to Larbi El Firdoussi; elfirdoussi@uca.ac.ma

Received 7 November 2017; Revised 20 February 2018; Accepted 25 March 2018; Published 24 April 2018

Academic Editor: Marco Cannas

Copyright © 2018 Abderrazak Aberkouks et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Various ratio of Co-Ag supported on ZnO have been evaluated in the selective catalytic oxidation of styrene to benzaldehyde, using H₂O₂ as an oxidant. The catalysts were prepared by a sol-gel process and were characterized using XRD, FT-IR, TG-DTG, BET, and SEM/EDX. The performance of the prepared catalyst was investigated under different parameters such as solvent, temperature, substrate/oxidant molar ratios, reaction time, and doping percent. The Zn1−x−yAgxCoyO catalysts exhibit a good activity and a high selectivity towards benzaldehyde (95%) with the formation of only 5% of acetophenone.

1. Introduction

Benzaldehyde is an important intermediate in the industrial fine chemical synthesis for fragrances, flavorings, pharmaceuticals, and in the organic synthesis [1, 2]. Conventional preparation of benzaldehyde is based on the hydrolysis of benzyl chloride, and it can be obtained as an intermediate of the oxidation of toluene to benzoic acid [3, 4]. This process was not environmentally friendly because of using organic solvents and producing traces of chlorine [5]. The development of recyclable catalysts, to simply separate from the reaction mixture, has attracted an economic and ecological interest [6]. In this context, there are two categories of catalysts: transition metal immobilized or constituted molecular sieves, such as NbCo-MCM-41 [7], VSB-5 [8], and Co-ZSM-5 [9], and application of metal oxides or complexes, such as MgxFe3−xO₄ [1] and TiO₂/SiO₂ [10]. Recently, Long et al. [11] have developed magnetic microparticles immobilizing palladium acetate as heterogeneous coordination catalysts for selective oxidation of styrene allowing mainly to prepare acetophenone. Earlier, it was shown that in the presence of activated carbon (AC) and H₂O₂ as the oxidant, the styrene was selectively converted to benzaldehyde [12].

The main objective of this work was the preparation of heterogeneous supported catalysts (Zn1−x−yAgxCoyO) by a sol-gel method and their evaluation in the oxidation of styrene to benzaldehyde (supplementary data 1).

2. Experimental Details

2.1. Materials

Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), silver nitrate (Ag(NO₃)), citric acid (HOC(COOH)(CH₂COOH)₂), hydrogen peroxide (H₂O₂ 30 wt.%), ethanol, styrene, acetonitrile, acetone, and methanol were purchased from Sigma-Aldrich chemical reagent grade and used as received.

2.2. Characterization

Thermogravimetric analysis (TGA) was recorded on a TA Instrument Q500 apparatus in flowing air at a heating rate of 10°C·min⁻¹. BET specific surface areas and average pore diameter of the prepared catalyst were measured by N₂ adsorption-desorption technique using a Micromeritics analyser P/N 05098-2.0 Rev A. The stretching vibration frequency of the catalyst was recorded by FT-IR spectroscopy in the range of 400–4000 cm⁻¹ using a Bruker vertex 70 DTGS. Spectrometer XRD measurements were performed on a XPERT-MPD Philips diffractometer using Cu-Kα radiation as the X-ray source in the 2θ range of 20°–80°. The size and morphology of the microstructures were carried out on VEGA3 TESCAN microscope equipped with an energy dispersive X-ray spectrometer (EDAX TEAM).

2.3. Preparation of the Catalyst

Pure ZnO nanopowders were prepared by a sol-gel method. Zn(CH₃COO)₂ 2H₂O (50 mL, 0.5 M in ethanol) and citric acid (50 mL, 0.5 M in ethanol) were stirred separately for 30 min. Then, the citric acid solution was added slowly into the solution of zinc acetate. This hydroalcoholic solution was heated at 80°C for 24 h, giving an opaque white gel which was dried at 120°C overnight and calcined at 600°C for 5 hours under air to offer pure ZnO. The same procedure was used to prepare nanopowders of Zn1−x−yAgxCoyO by adding the needed amount of Co(NO₃)₂·6H₂O (0.5 M) and Ag(NO₃) (0.5M) (Figure 1). Six samples were prepared: C1 (pure ZnO); C2 (1% mol Ag-doped ZnO); C3 (1% mol Co-doped ZnO); C4 (1% mol Co-1% mol Ag codoped ZnO); C5 (2.5% mol Co-2.5% mol Ag codoped ZnO), and C6 (5% mol Co-5% mol Ag codoped ZnO).

Figure 1: Flow chart for synthesis of Co-Ag/ZnO nanopowders by a sol-gel method.

2.4. Catalytic Activity

The oxidation of styrene with hydrogen peroxide as the oxidant was carried out in a 50 mL rotaflow tube with magnetic stirring immersed in bath oil. The activity of the catalyst (Zn1−x−yAgxCoyO) was studied varying the following parameters: doping percents of 0 ≤ x ≤ 0.05 and 0 ≤ y ≤ 0.05, H₂O₂/styrene molar ratios from 1.1 to 3, reaction temperatures range (40°C–120°C), and the amount of the catalyst (0.005–0.02 g) in aprotic and protic solvents. Aliquots samples from the reaction mixture were taken at regular intervals and were monitored by gas chromatography (GC) equipped with FID using Rtx-5 capillary column. Dodecane was used as an internal standard for the quantitative analysis of the reaction products. The conversion of styrene was based on the unreacted substrate.

3. Results and Discussion

3.1. Characterization of the Catalysts

Figure 2 shows XRD patterns of pure ZnO and Zn1−x−yAgxCoyO nanopowders calcined at 600°C. According to pure ZnO reference (00-036-1451), the sample corresponding to C3 is highly crystallized and all diffraction peaks are well indexed to the diffraction pattern of hexagonal ZnO with the P63mc space group. However, the ZnO doped with 1% mol of Ag (C2) revealed new peaks with weak intensities at 38.20° and 44.62°, as it was reported by Zeferino et al. [13]. The doping concentration of 2% to 10% mole of Co-Ag (C5 and C6) can lead to the formation of Co₃O₄ with a corresponding trace at 64.66° [14].

Figure 2: XRD patterns of Zn1−x−yAgxCoyO.

Table 1 summarizes the position and the width at midheight (FWHM) of the main peaks, along with the lattice parameter values calculated from the XRD patterns of the pure and codoped ZnO nanopowders. The incorporation of Co-Ag on ZnO results in the decrease of the FWHM, indicating an increase in crystallinity. A shift position of the diffraction peak (002) could be attributed to the incorporation of Ag⁺ and Co²⁺ ions in the ZnO lattice sites. As it was shown earlier, Ag⁺ ions in the ZnO lattice sites, probably substitute Zn²⁺ ions [13].

Table 1: Position, FWHM, and the lattice parameters of Zn1−x−yAgxCoyO nanopowders.

Nominal Ag-Co concentration
Position of the (101) peak
FWHM (degree)
Lattice parameters (Å)

a
c

0% mol
36.2583
0.2134
3.2498
5.2066

1% mol Ag
36.3177
0.082
3.24509
5.18829

1% mol Co
36.1927
0.325
3.25037
5.19644

1% mol Ag-1% mol Co
38.279
0.200
3.28825
5.23984

2.5% mol Ag-2.5% mol Co
36.4427
0.078
3.24454
5.19797

5% mol Ag-5% mol Co
36.3177
0.075
3.24016
5.21778

On the other hand, the peaks associated with the cobalt oxide or cobalt hydroxide phases such as CoO or Co(OH)₂ were not detected as shown in the XRD patterns (Figure 2). According to Wojnarowicz et al. [15], some precipitation of Co₃O₄ was observed in the XRD investigation for the 10% mol Co-Ag sample in synthetic air.

TG/DTG curves of the prepared materials (supplementary data 2) reveal one thermal event at 162°C attributed to the loss of crystal water from the catalyst (2.42 wt.%).

To investigate the specific areas and the porous nature of the C4 nanopowders, Brunauer–Emmett–Teller (BET) gas sorption measurements were performed. The nitrogen adsorption-desorption isotherm and the pore size distribution plots of the C4 sample are shown in supplementary data 3. The sample corresponds to type IV and V isotherm and type H3 hysteresis loop appearing which is attributed to the predominance of mesopores [16]. The determined surface area of the C4 sample was 5.90 m²/g, and the calculated BJH pores size was 5.42 nm.

FT-IR spectrum (supplementary data 4) of C4 nanopowder shows a strange band at 600 cm⁻¹ and a relatively weak band at 500 cm⁻¹ which could be assigned to the stretching modes of Zn–O [17]. The bands in the range 910–1537 cm⁻¹ may be attributed to the residual NO₃– ions [18]. A weak band at 2375 cm⁻¹ is attributed to the CO₂ present in air. The bands at 3400 cm⁻¹ and 1630 cm⁻¹ correspond to the stretching of hydroxyl groups due to the adsorption of water on the particles’ surface [19].

The scanning electron microscopy (SEM) of the undoped and the Ag-Co codoped ZnO nanopowders are shown in Figure 3. Ag and Co do not bring significant change in the morphology. The EDX results confirmed that the products consist of zinc, cobalt, and silver elements in ZnO nanopowders, which are in good agreement with XRD patterns.

(a)

(b)

(c)

Figure 3: SEM images and EDX spectra of (a) C1, (b) C4 before used, and (c) C4 after the 4th cycles.

3.2. Catalytic Experiments

The oxidation reaction of styrene is influenced by several parameters. In an attempt to find out the suitable reaction conditions providing an optimum of conversion and yield, effects of different reaction conditions have been carried out.

3.2.1. Effect of Catalyst Loading

As shown in Table 2, when the loading amount of Co and Ag was increased from 2 to 10% mol, there was an obvious increase in styrene conversion from 61 to 80% (entries 5–7), but the selectivity of benzaldehyde decreases for more than 2% mol (entries 6-7). However, with 1% mol, either with Co or Ag, a similar result was obtained (entries 3-4). In the presence of pure ZnO or in the absence of the catalyst, no significant conversion was observed.

Table 2: Effect of the catalyst’s amount and loading.

Entry
Catalyst
Amount of catalyst (g)
Styrene conversion
Yielda (% mol)

Benzaldehyde
Acetophenone

1
Without
—
1
1
0

2
C1
0.02
5
5
0

3
C2
0.02
30
30
0

4
C3
0.02
25
25
0

5
C4
0.02
61
58
3

6
C5
0.02
75
45
33

7
C6
0.02
80
48
16

8
C4
0.005
30
30
0

9
C4
0.01
50
50
0

10
C4
0.015
53
53
0

Reaction conditions: H₂O₂/styrene, 1; temperature, 80°C; solvent, acetonitrile; reaction time, 24 h; ayields determined by GC using dodecane as an internal standard.

3.2.2. Effect of Catalyst Amount

To investigate the catalyst amount effect on the oxidation reaction, different amounts of the C4 catalyst have been studied. As shown in Table 2, the optimal catalyst amount was found to be 0.02g with high selectivity of benzaldehyde (95%).

3.2.3. Effect of the Reaction Time

In order to shed more light on this point, a kinetic study was carried out with 0.02 g of C4 catalyst using GC to determine the conversion and product distribution (Figure 4). As depicted, the evolution of styrene versus time shows that benzaldehyde was formed as the major product. This compound reached a maximum after 24 hours with 61% in conversion and 95% in selectivity.

Figure 4: Effect of reaction time on the oxidation of styrene. Reaction conditions: H₂O₂/styrene, 1; temperature, 80°C; solvent, acetonitrile. Yields are determined by GC using dodecane as an internal standard.

3.2.4. Effect of Molar Ratio H₂O₂/Styrene

The catalytic oxidation of styrene using the C4 catalyst was performed by varying the molar ratio of the H₂O₂/styrene. The increase in molar ratio from 1 : 1 to 1 : 2.5 resulted in a significant increase in the conversion. The yield of benzaldehyde increased from 57% to 72% mol. However, the selectivity of benzaldehyde slightly decreases (Table 3).

Table 3: Effect of styrene, H₂O₂ molar ratio, temperature, and solvent.

Entry
H₂O₂/Styrene
Solvant
Temperature (°C)
Styrene conversion
Yielda (% mol)

Benzaldehyde
Acetophenone

1
1.1
Acetonitrile
80
61
57
3

2
1.5
Acetonitrile
80
66
62
3

3
2
Acetonitrile
80
70
67
3

4
2.5
Acetonitrile
80
81
72
3.5

5
3
Acetonitrile
80
95
73
5

6
2.5
Acetonitrile
40
10
10
0

7
2.5
Acetonitrile
70
50
50
0

8
2.5
Acetonitrile
100
94
60
4

9
2.5
Acetonitrile
120
98
50
9

10
2.5
Acetone
80
90
77
12

11
2.5
Water
80
12
12
0

12
2.5
Methanol
80
75
59
6

13
2.5
Ethanol
80
50
31
7

Reaction conditions: C4 catalyst (0.02 g); reaction time, 24 h; ayields determined by GC using dodecane as an internal standard.

3.2.5. Effect of Reaction Temperature

Table 3 depicts the influence of reaction temperature on the oxidation of styrene catalyzed by the C4 catalyst. The increase of the temperature from 40°C to 120°C results in the increased conversion from 10 to 98, indicating that the styrene conversion has a strong dependency on the reaction temperature. Hence, at high temperature, the benzaldehyde selectivity decreases in competition with the formation of acetophenone and other products. This confirms that the cleavage of C = C bond is greater at lower temperatures and epoxidation participates more favorably against C = C cleavage to high temperatures [20, 21].

3.2.6. Effect of the Solvent

An examination of the effects of a variety of protic and aprotic solvents on the model reaction using C4 as the catalyst has been carried out (Table 3).

According to Kumar [22], aprotic solvents are more favorable for the styrene oxidation than protic ones. The selectivity towards benzaldehyde with respect to percentage yield is in the following order: CH₃CN > CH₃COCH₃ > CH₃OH > C₂H₅OH > H₂O. The styrene conversion in acetone, methanol, ethanol, and water is, respectively, equal to 90%  mol, 75%  mol, 50% mol, and 12%  mol, with 77% mol, 59% mol, 31% mol, and 12%  mol yield of benzaldehyde, respectively. Among the solvents, acetonitrile appears to be the best one (entry 4).

3.2.7. Recycling of Catalyst

To investigate the reusability of C4, the catalytic activity of this catalyst was evaluated in four consecutive cycles (Figure 5). Before reusing the catalyst, it was separated from the reaction mixture by filtration, washed with water, and finally dried at 200°C for 2 hours. As shown in Figure 5, the catalyst still exhibits a good catalytic activity in the first three reuse cycles. However, a noticeable drop in styrene conversion was observed after the fourth consecutive run due to agglomerated particles.

Figure 5: Reusability of the catalyst in oxidation of styrene. Reaction conditions: C4 catalyst (0.02 g); H₂O₂/styrene,  2.5, temperature, 80°C; solvent, acetone; reaction time, 24 h. Yields are determined by GC using dodecane as an internal standard.

SEM image of C4 before and after four cycles of oxidation has been studied (Table 4). Before the catalytic test, SEM image of the Ag-Co codoped ZnO shows relatively dispersed nanoparticles (Figure 3(b)). However, SEM morphology of the catalyst after four cycles of oxidation showed agglomerated particles with the presence of smaller single particles (Figure 3(c)). Even by SEM, the physical change of the matrix is well visible due to probable decomposition during the reaction. In fact, EDX on various regions of the catalyst after 4 cycles confirmed the presence of Co and Ag elements without any significant variation. The analysis indicates a predominance of oxygen species, which comes mainly from the oxidation of Co and Ag species, leading to the deactivation of the catalyst.

Table 4: EDX analysis of C4 before using and at the 1st, 2nd, 3rd, and 4th cycles.

Elements
Wt.%
Atomic %

As-prepared catalyst
OK
14.73
41.58

AgL
1.88
0.76

CoK
0.42
0.32

ZnK
82.96
57.31

1st cycle
OK
18.05
47.58

AgL
1.97
0.76

CoK
0.45
0.32

ZnK
79.54
51.31

2nd cycle
OK
20.80
52.01

AgL
2.08
0.77

CoK
0.44
0.30

ZnK
76.68
46.92

3rd cycle
OK
25.54
58.63

AgL
2.26
0.77

CoK
0.43
0.27

ZnK
71.77
40.32

4th cycle
OK
31.36
65.43

AgL
2.49
0.77

CoK
0.36
0.20

ZnK
65.73
33.59

4. Conclusion

In this study, the C4 catalyst was prepared by a sol-gel method, and its catalytic activity was investigated in the oxidation of styrene. Among all the catalysts examined, the C4 catalyst exhibited the best activity and provided 76% yield of benzaldehyde in the presence of H₂O₂ as an oxidant after 24 hours. The optimum reaction conditions was acetonitrile, temperature reaction 80°C, H₂O₂/styrene molar ratio equal to 2.5, catalyst loading of 1% mol Ag-1% mol Co, and the catalyst amount of 0.02 g. Under the optimized conditions, the Co-Ag codoped ZnO shows a high degree of efficiency and selectivity towards the oxidation reactions.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Supplementary Materials

General scheme, SEM images, EDX, FT-IR spectra, and other analytical data are available free of charge via the Internet in the Electronic Supporting Information (ESI). Supplementary data 1: catalytic oxidation of styrene. Supplementary data 2: TG and DTG curves of C4. Supplementary data 3: N2 adsorption-desorption isotherm and BJH pore diameter distribution of C4. Supplementary data 4: FT-IR spectrum of C4. Supplementary data 5: SEM images and EDX spectra of (a) C1, (b) C2, (c) C3, (d) C4, (e) C5, (f) C6, (g) 1st cycle, (h) 2nd cycle, (i) 3th cycle, and (j) 4th cycle of the reused catalyst. (Supplementary Materials)

References

N. Ma, “Selective oxidation of styrene over nanosized spinel-type MgxFe3−xO₄ complex oxide catalysts,” Applied Catalysis A: General, vol. 251, no. 1, pp. 39–47, 2003.

J. Liu, F. Wang, Z. Gu, and X. Xu, “Vanadium phosphorus oxide catalyst modified by silver doping for mild oxidation of styrene to benzaldehyde,” Chemical Engineering Journal, vol. 151, no. 1–3, pp. 319–323, 2009.

K. S. Suslick, “Sonochemistry,” in Kirk-Othmer Encyclopedia of Chemical Technology, vol. 26, John Wiley & Sons, New York, NY, USA, 4th edition, 1998.

V. R. Choudhary and D. K. Dumbre, “Solvent-free selective oxidation of benzyl alcohol to benzaldehyde by tert-butyl hydroperoxide over U₃O₈-supported nano-gold catalysts,” Applied Catalysis A: General, vol. 375, no. 2, pp. 252–257, 2010.

X. Zhu, R. Shen, and L. Zhang, “Catalytic oxidation of styrene to benzaldehyde over a copper Schiff-base/SBA-15 catalyst,” Chinese Journal of Catalysis, vol. 35, no. 10, pp. 1716–1726, 2014.

A. Suresh Kumar, B. Thulasiram, S. Bala Laxmi, V. S. Rawat, and B. Sreedhar, “Magnetic CuFe₂O₄ nanoparticles: a retrievable catalyst for oxidative amidation of aldehydes with amine hydrochloride salts,” Tetrahedron, vol. 70, no. 36, pp. 6059–6067, 2014.

V. Parvulescu, C. Constantin, and B. L. Su, “Liquid phase oxidation of aromatic hydrocarbons using highly ordered Nb and NbCo-MCM-41 nanoreactors,” Journal of Molecular Catalysis A : Chemical, vol. 202, no. 1-2, pp. 171–178, 2003.

D. Gao and Q. Gao, “Selective oxidation of styrene to benzaldehyde over VSB-5 and isomorphously substituted cobalt VSB-5,” Catalysis Communications, vol. 8, no. 4, pp. 681–685, 2007.

L. B. Pierella, C. Saux, S. C. Caglieri, H. R. Bertorello, and P. G. Bercoff, “Catalytic activity and magnetic properties of Co-ZSM-5 zeolites prepared by different methods,” Applied Catalysis A: General, vol. 347, no. 1, pp. 55–61, 2008.

N. T. Thao and H. H. Trung, “Selective oxidation of styrene over Mg-Co-Al hydrotalcite like-catalysts using air as oxidant,” Catalysis Communications, vol. 45, pp. 153–157, 2014.

Y. Long, Z. Zhao, L. Wu et al., “Distinctive ligand effects of functionalized magnetic microparticles immobilizing palladium acetate as heterogeneous coordination catalysts for selective oxidation of styrene to acetophenone,” Molecular Catalysis, vol. 433, pp. 291–300, 2017.

N. Li, Y. Gao, X. Zhang, Z. Yu, L. Shi, and Q. Sun, “Oxidation of styrene to benzaldehyde by p-toluenesulfonic acid using hydrogen peroxide in the presence of activated carbon,” Chinese Journal of Catalysis, vol. 36, no. 5, pp. 721–727, 2015.

R. S. Zeferino, M. B. Flores, and U. Pal, “Photoluminescence and Raman scattering in Ag-doped ZnO nanoparticles,” Journal of Applied Physics, vol. 109, no. 1, p. 014308, 2011.

H. Yang and S. Nie, “Preparation and characterization of Co-doped ZnO nanomaterials,” Materials Chemistry and Physics, vol. 114, no. 1, pp. 279–282, 2009.

J. Wojnarowicz, S. Kusnieruk, T. Chudoba et al., “Paramagnetism of cobalt-doped ZnO nanoparticles obtained by microwave solvothermal synthesis,” Beilstein Journal of Nanotechnology, vol. 6, pp. 1957–1969, 2015.

Z. A. Alothman, “A review: fundamental aspects of silicate mesoporous materials,” Materials, vol. 5, no. 12, pp. 2874–2902, 2012.

C. Pholnak, C. Sirisathitkul, S. Suwanboon, and D. J. Harding, “Effects of precursor concentration and reaction time on sonochemically synthesized ZnO nanoparticles,” Materals Research, vol. 17, no. 2, pp. 405–411, 2014.

M. Viertelhaus, E. Taylor, L. Kloo, I. Gameson, and P. Anderson, “Silver nitrate in silver zeolite A: three-dimensional incommensurate guest ordering in a zeolite framework,” Dalton Transactions, vol. 19, pp. 2368–2373, 2006.

Y.-M. Hao, S.-Y. Lou, S.-M. Zhou, R.-J. Yuan, G.-Y. Zhu, and N. Li, “Structural, optical, and magnetic studies of manganese-doped zinc oxide hierarchical microspheres by self-assembly of nanoparticles,” Nanoscale Research Letters, vol. 7, no. 1, p. 100, 2012.

C. Saux and L. B. Pierella, “Studies on styrene selective oxidation to benzaldehyde catalyzed by Cr-ZSM-5: Reaction parameters effects and kinetics,” Applied Catalysis A: General, vol. 400, no. 1-2, pp. 117–121, 2011.

M. V. Cagnoli, S. G. Casuscelli, A. M. Alvarez et al., “clean limonene epoxidation using Ti-MCM-41 catalyst,” Applied Catalysis A: General, vol. 287, no. 2, pp. 227–235, 2005.

S. Kumar, “Epoxidation of styrene over a titanium silicate molecular sieve TS1 using dilute H₂O₂ as oxidizing agent,” Journal of Catalysis, vol. 156, no. 1, pp. 163–166, 1995.