Selective Oxidation of Styrene to Benzaldehyde by CoAg Codoped ZnO Catalyst and H 2 O 2 as Oxidant

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. *e catalysts were prepared by a sol-gel process and were characterized using XRD, FT-IR, TG-DTG, BET, and SEM/EDX. *e performance of the prepared catalyst was investigated under different parameters such as solvent, temperature, substrate/oxidant molar ratios, reaction time, and doping percent. *e Zn1−x−yAgxCoyO catalysts exhibit a good activity and a high selectivity towards benzaldehyde (95%) with the formation of only 5% of acetophenone.


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].is process was not environmentally friendly because of using organic solvents and producing traces of chlorine [5].e 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 Mg x Fe 3−x O 4 [1] and TiO 2 /SiO 2 [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 H 2 O 2 as the oxidant, the styrene was selectively converted to benzaldehyde [12].
e main objective of this work was the preparation of heterogeneous supported catalysts (Zn 1−x−y Ag x Co y O) by a sol-gel method and their evaluation in the oxidation of styrene to benzaldehyde (supplementary data 1)., hydrogen peroxide (H 2 O 2 30 wt.%), ethanol, styrene, acetonitrile, acetone, and methanol were purchased from Sigma-Aldrich chemical reagent grade and used as received.

Characterization.
ermogravimetric 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 N 2 adsorption-desorption technique using a Micromeritics analyser P/N 05098-2.0Rev A. e 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 di ractometer using Cu-Kα radiation as the X-ray source in the 2θ range of 20 °-80 °.
e size and morphology of the microstructures were carried out on VEGA3 TESCAN microscope equipped with an energy dispersive X-ray spectrometer (EDAX TEAM).

Preparation of the Catalyst.
Pure ZnO nanopowders were prepared by a sol-gel method.Zn(CH 3 COO) 2 2H 2 O (50 mL, 0.5 M in ethanol) and citric acid (50 mL, 0.5 M in ethanol) were stirred separately for 30 min.en, the citric acid solution was added slowly into the solution of zinc acetate. is 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 o er pure ZnO.

Catalytic Activity.
e oxidation of styrene with hydrogen peroxide as the oxidant was carried out in a 50 mL rota ow tube with magnetic stirring immersed in bath oil.
e activity of the catalyst (Zn 1−x−y Ag x Co y O) was studied varying the following parameters: doping percents of 0 ≤ x ≤ 0.05 and 0 ≤ y ≤ 0.05, H 2 O 2 /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.e conversion of styrene was based on the unreacted substrate.

Characterization of the Catalysts.
Figure 2 shows XRD patterns of pure ZnO and Zn 1−x−y Ag x Co y O nanopowders calcined at 600 °C.According to pure ZnO reference (00-036-1451), the sample corresponding to C3 is highly crystallized and all di raction peaks are well indexed to the di raction 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].e doping concentration of 2% to 10% mole of Co-Ag (C5 and C6) can lead to the formation of Co 3 O 4 with a corresponding trace at 64.66 ° [14].
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.e incorporation of Co-Ag on ZnO results in the decrease of the FWHM, 2 Advances in Materials Science and Engineering indicating an increase in crystallinity.A shift position of the di raction peak (002) could be attributed to the incorporation of Ag + and Co 2+ ions in the ZnO lattice sites.As it was shown earlier, Ag + ions in the ZnO lattice sites, probably substitute Zn 2+ ions [13].
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 Co 3 O 4 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 speci c areas and the porous nature of the C4 nanopowders, Brunauer-Emmett-Teller (BET) gas sorption measurements were performed.
e nitrogen adsorption-desorption isotherm and the pore size distribution plots of the C4 sample are shown in supplementary data 3. e sample corresponds to type IV and V isotherm and type H3 hysteresis loop appearing which is attributed to the predominance of mesopores [16].e determined surface area of the C4 sample was 5.90 m 2 /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].
e bands in the range 910-1537 cm −1 may be attributed to the residual NO 3 -ions [18].A weak band at 2375 cm −1 is attributed to the CO 2 present in air.
e 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].
e 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 signi cant change in the morphology.e EDX results con rmed that the products consist of zinc, cobalt, and silver elements in ZnO nanopowders, which are in good agreement with XRD patterns.

Catalytic Experiments.
e oxidation reaction of styrene is in uenced by several parameters.In an attempt to nd out the suitable reaction conditions providing an optimum of conversion and yield, e ects of di erent reaction conditions have been carried out.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 signi cant conversion was observed.Advances in Materials Science and Engineering 3

E ect of Catalyst Amount.
To investigate the catalyst amount e ect on the oxidation reaction, di erent 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%).

E ect 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.is compound reached a maximum after 24 hours with 61% in conversion and 95% in selectivity.

E ect of Molar Ratio H 2 O 2 /Styrene.
e catalytic oxidation of styrene using the C4 catalyst was performed by varying the molar ratio of the H 2 O 2 /styrene.e increase in molar ratio from 1 : 1 to 1 : 2.5 resulted in a signi cant increase in the conversion.
e yield of benzaldehyde increased from 57% to 72% mol.However, the selectivity of benzaldehyde slightly decreases (Table 3).Advances in Materials Science and Engineering

E ect of Reaction
Temperature.Table 3 depicts the in uence of reaction temperature on the oxidation of styrene catalyzed by the C4 catalyst.e 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.is con rms 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].Yields are determined by GC using dodecane as an internal standard.Reaction conditions: C4 catalyst (0.02 g); reaction time, 24 h; a yields determined by GC using dodecane as an internal standard.
Advances in Materials Science and Engineering 3.2.6.E ect of the Solvent.An examination of the e ects 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.e selectivity towards benzaldehyde with respect to percentage yield is in the following order: CH 3 CN > CH 3 COCH 3 > CH 3 OH > C 2 H 5 OH > H 2 O. e 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).

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 ltration, washed with water, and nally dried at 200 °C for 2 hours.As shown in Figure 5, the catalyst still exhibits a good catalytic activity in the rst three reuse cycles.However, a noticeable drop in styrene conversion was observed after the fourth consecutive run due to agglomerated particles.
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 con rmed the presence of Co and Ag elements without any signi cant variation.e 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.

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 H 2 O 2 as an oxidant after 24 hours.e optimum reaction conditions was acetonitrile, temperature reaction 80 °C, H 2 O 2 /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 e ciency and selectivity towards the oxidation reactions.

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

Figure 2 :
Figure 2: XRD patterns of Zn 1−x−y Ag x Co y O.

Table 1 :
Position, FWHM, and the lattice parameters of Zn 1−x−y Ag x Co y O nanopowders.

Table 2 :
E ect of the catalyst's amount and loading.

Table 3 :
E ect of styrene, H 2 O 2 molar ratio, temperature, and solvent.