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Short Communication

Selective oxidation of styrene over Mg–Co–Al hydrotalcite like-catalystsusing air as oxidant

Nguyen Tien Thao ⁎, Ho Huu TrungFaculty of Chemistry, Vietnam National University, Hanoi, 19 Le Thanh Tong St, Hanoi, Viet Nam

a b s t r a c ta r t i c l e i n f o

Article history:Received 16 September 2013Received in revised form 31 October 2013Accepted 5 November 2013Available online 21 November 2013

Keywords:Metal-doped hydrotalciteStyrene oxidationBenzaldehydeEpoxideMg–Co–Al

A set of synthesized Mg/Co/Al hydrotalcites was synthesized and characterized by XRD, XPS, BET, SEM, TEM, andFT-IR physical techniques. The partial substitution ofMg2+ by Co2+ in brucite layers has not significantly affectedthe layered double hydroxide structure, but plays a crucial role in the oxidation of styrene in the presence of air.The prepared Mg/Co/Al hydrotalcite-like compounds express a good activity and stability in the oxidation ofstyrene in the free-solvent condition. Both styrene conversion and desired product selectivities are stronglydependent on the cobalt substitution content. The intra-hydrotalcite lattice Co2+ ions are active sites for theepoxidation of styrene.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Oxidation of styrene is a reaction of great interest because its prod-ucts act as versatile and useful intermediates [1]. Conventionally, thisprocess has been usually carried out by homogeneous catalysts and re-sulted in a huge amount of toxically corrosive chemical wastes. Recent-ly, there has been much interest in solid catalysts and uses ofenvironmentally friendly cheap oxidants [2,3]. Several transitionmetal-containing catalysts based on Ru, Cu, Fe, Mn, V, Ti… have beenused in the liquid phase oxidation of olefinic compounds to oxygenates[3–6]. Among those, Ru- and Cu-based heterogeneous solids arerestricted only to doubly activated alkylaromatics while Fe- and Ni-containing catalysts usually give a rather low yield of oxygenatedproducts [3–7]. Therefore, the synthesis of novel easily recyclablecatalyst for the oxidation of alkylbenzenes is still a great challenginggoal of fine chemical industry.

Hydrotalcite-like compound is known as a layered double hydroxide(LDH) mineral with the general formula of [A(1 − x)

2+ Bx3+(OH)2(CO3)0.5x·nH2O]. Cations are usually located in coplanar [M(OH)6] octahedrasharing vertices and forming M(OH)2 layers with the brucite struc-ture [8]. Partial substitution of divalent cations by trivalent cationsleads to the appearance of positive layerswhich is usually compensatedby anions between layers. Thus, the complexity of chemical composi-tion in hydrotalcite-like compoundmakes it be able to act as basic solidsand oxidation–reduction catalysts [9–11]. For example, Ni-containingbasic hydrotalcites were used for the selective oxidation of benzylicC\H bonds of ethyl benzene [11]. Mn–MgAl and MoO4

−/MgAl

hydrotalcite-like catalysts present a good activity in the oxidation ofalkylbenzenes [9,12]. Cobalt-containing hydrotalcites have been usedfor the steam reforming of ethanol [13] and synthesis of benzoinmethylether [14]. In these cases, transition metal ions in layered structure arethe key for the catalytic activity. This article provides a novel applicabil-ity of Mg–Al hydrotalcites partially substituted by cobalt ions as effec-tive catalyst for the oxidation of styrene under milder conditions.

2. Experimental

2.1. Preparation and characterization of the catalysts

Mg/Co/Al hydrotalcite-like compounds were prepared by thecoprecipitation method. The detailed procedure was described in ourprevious publication [15]. In brief, 150 mL-mixed aqueous solution ofMg(NO3)2·6H2O (99%), Co(NO3)2·6H2O (98%) and Al(NO3)3·9H2O(N98%) with Co2+/(Mg2+ + Al3+) molar ratios ranging from 0 to0.44was added dropwise to 25 mL of 0.6 MNa2CO3 under vigorous stir-ring. The exact amounts of starting materials for each catalyst are givenin Supporting information (Table 1S). The solution pH was adjusted to9.50 using 1.5 M NaOH and was kept for 24 h. Then, the resulting gel-like material was aged at 65 °C for 24 h. The resultant slurry was thencooled to room temperature and separated by filtration, washed withhot distilled water several times, and then dried at 80 °C for 24 h inair. The prepared catalysts are denoted as Mg/Co/Al-1, -2, and -3(Table 1).

The elemental composition (Mg, Co, Al) of catalyst was measuredusing an ICP-MS Elan 9000 (PerkinElmer, USA) and carbon content in-strument PE 240 (USA). Powder X-ray diffraction (XRD) patterns wererecorded on a D8 Advance-Bruker instrument using CuKα radiation

Catalysis Communications 45 (2014) 153–157

⁎ Corresponding author. Tel.: +84 4 3825 3503; fax: +84 4 3824 1140.E-mail address: ntthao@vnu.edu.vn (N.T. Thao).

1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.catcom.2013.11.004

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(λ = 0.1549 nm). Fourier transform infrared (FT-IR) spectra wereobtained in 4000–400 cm−1 range on a FT/IR spectrometer(DX-PerkinElmer, USA). TEM images were collected on a JapanJEOL JEM-1010. The nitrogen physisorption was run on an AutochemII 2920 (USA). The X-ray photoelectron spectra (XPS) of catalystswere recorded with a Thermo K-Alpha.

The catalytic oxidation of styrene in the absence of solvent wascarried out in a 100 mL three-neck glass flask fitted with a reflux con-denser. For a typical run, 87.28 mmol of styrene and 0.2 g of catalystwere loaded into the flask unless some particular tests were indicated.After the reaction mixture was magnetically stirred and heated to thedesired temperature, the flow of air (5 mL/min) was bubbled throughthe vigorously stirred reaction mixture and the reaction time startsrecorded. After the reaction, the mixture was quenched to room tem-perature and then catalyst was filtered off. The filtrate was analyzedby a GC–MS (HP-6890 Plus) and a frame ionization detector (FID) isused as a detector.

3. Results

3.1. Catalyst characteristics

The prepared catalyst characteristics and chemical composition aresummarized in Table 1. Fig. 1 displays the powder X-ray diffraction pat-terns for all synthesized Mg–Co–Al hydrotalcite-like materials. Overall,all samples present a set of reflection lines matching to those character-istics of layered double hydroxide structure [8,10,13,15]. Indeed, twosharp and intense peaks at low diffraction angles of 23.2 and 34.4° areascribed to the diffraction by basal planes (006) and (102), respectively[10,15]. Furthermore, broad, less intense peaks at higher angles around38, 46, and 60° indexed to (105), (108), and (110) planes also confirmthe hydrotalcite structure [14,15]. The positions of these reflectionlines are slightly changed but the signal to noise ratio and full width at

half maximum peaks vary with increased cobalt content. The lattercould possibly be explained by only subtle differences in the octahedralionic radii of Co2+ (0.74 Å) and Mg2+ (0.72 Å) [13]. The XRD patterns(Fig. 1) reveal that the cobalt rich-samples are somewhat poorer crys-tallinity because the affinity of CO3

2− to Co2+ is less than that to Mg2+

[10,13]. No reflection lines corresponding to cobalt oxides are observed,suggesting that cobalt ions are present in LDH structure [13,14].

The major photoelectron lines of the elements in a representativeMg/Co–Al-1 are reported in Fig. 2A. Clearly, magnesium, cobalt, oxygen,carbon and aluminum have photoelectron lines at 1s (1303.93 eV), 2s(88,08); 2p (781.08 eV); 2p (531.90 eV); 2p (289.08 eV) and 2p(74.34 eV), respectively [13]. To investigate the oxidation state in thenear-surface region, the spectrum corresponding to the Co 2p corelevel is represented in Fig. 2BwhileMg 1s andAl 2p scans are elucidatedin Supporting information. XPS spectrum of Co 2p in Mg/Co/Al sampleshows two clear peaks positioned at binding energy values of 781.1(Co 2p3/2) and 797.1 eV (Co 2p1/2), along with shake-up satellites.These binding energy values and the peak separation are essentiallyascribed to Co2+ species. Furthermore, the high intensities of the satel-lites are typical characteristics for the cobalt containing layered doublehydroxide structure. Thus, it is suggested that Co2+ ions locate at octa-hedral sites in brucite-like layers [13,16].

FT-IR spectra of Mg/Co/Al hydrotalcite-like materials present themain band around 3454 cm−1 assigning to the OH stretching mode ofwater molecules and hydroxyls in the layers [10,12]. This band showsa prominent shoulder around2950 cm−1 ascribed to hydrogen bondingof OHs of layered lattice and/orwatermolecules with interlayer carbon-ate anions (see Fig. 1S in Supporting information). A sharp band at1365 cm−1 is firmly assigned to the asymmetric stretching vibrationof the CO3

2− in the hydrotalcite layers. A set of bands at 437, 663, 742,and 927 cm−1 is associated to Al\O, Co\O, Al\OH translation, anddoublet Al\OH deformation modes, respectively [13,17].

The textural properties of nominal Mg/Co/Al hydrotalcite-like com-pounds were insignificantly changed with molar ratios of Mg/Co/Al.

Table 1Physical properties of the prepared Mg/Co/Al hydrotalcite-like compounds.

Catalystbatch

Molar ratioofCo2+/(Mg2++Al3+)

Elemental analysis (wt.%) BETsurfacearea(m2/g)

Porevolume(cm3/g)

Mg Co Al C

Mg/Al-0 0 24.74 – 11.93 2.39 83.4 0.62Mg/Co/Al-1 0.10 21.34 8.16 12.30 1.98 78.9 0.60Mg/Co/Al-2 0.24 14.20 12.72 8.14 1.87 74.6 0.58Mg/Co/Al-3 0.44 9.18 17.89 8.10 1.89 74.5 0.58Mg/Co/Al-2reacted

0.24 10.13 9.71 7.24 2.11 44.2 0.49

20 25 30 35 40 45 50 55 60 65 702-theta (o)

Mg/Al -0

Mg/Co/Al -1

Mg/Co/Al -2

Mg/Co/Al -3

Mg/Co/Al -2- Reacted

Fig. 1. XRD patterns of as-synthesized hydrotalcite-like compounds and the used sample.

0

60000

120000

180000

240000

300000

360000

420000

020040060080010001200Binding Energy (eV)

Cou

nts/

s

Mg 1s

Co

O 1s

C1s

Al 2p

A

Co2p scan

1000011000120001300014000150001600017000180001900020000

770773776779782785788791794797800803806809812Binding Energy (eV)

Cou

nts

/s

781.18

797.0

B

Fig. 2. Survey scan (A) and Co 2p XPS spectrum (B) of as-synthesized Mg/Co/Al-2 hydro-talcite-like material.

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BET specific surface area of the cobalt-free-sample (Mg/Al-0) is only83.4 m2/g while that of the others is approximately 74–78 m2/g(Table 1). The nitrogen isothermal curves likely expresses a plateaufrom 0 to 0.6 and are gradually skewed in the range of 0.6–0.85,reflecting the nitrogen physisorption and condensation in micropores[14,16]. Furthermore, the condensation process at relative pressureshigher than 0.8 alongwith a sharp adsorption volume increase is firmlyresponsible for the physisorption in mesopores (Supporting informa-tion) [15].

The morphology of Mg/Co/Al-1 LDH is illustrated in Fig. 3 and someadditional micrographs are depicted in Supporting information. Fig. 3Ashows that the hydrotalcite-like compound particles are regularly hex-agonal plates [15]. The particle sizes are relatively uniform with themean crystal domain of 70–100 nm [11,13]. More details, the TEMimage ofMg/Co/Al-1 LDH shows laminar structure which is an essentialcharacteristic for hydrotalcite mineral and the stacking of the layers(Fig. 3C) [17]. The flat particles with hexagonal shapes are presentedand the grain boundaries are clearly observed. The aggregation ofuniform particles leads to the formation of voids between primarynanoparticles [13,17].

3.2. Catalytic results

The catalytic activity of Mg/Co/Al-hydrotalcite-like catalysts in theliquid oxidation has been examined at atmospheric pressure and airwas bubbled into the reaction system without any further purification.

3.2.1. Oxidation of styrene catalyzed by cobalt ions in hydrotalcitesFig. 4 presents the reaction results of three Mg/Al/Co hydrotalcite-

like materials in the oxidation of styrene. By comparison, a blank testand the cobalt-free-sample (Mg/Al-0) have been also performed

under the same reaction conditions. The former test shows a null con-version of styrene while the cobalt-free sample (Mg/Al-0) converts anegligible amount of styrene (b1%) over Mg/Al-hydrotalcite-likematerial basic sites to benzaldehyde [10,19]. Meanwhile the cobalt-low-sample (Mg/Co/Al-1) selectively oxidizes about 6% styrene to benz-aldehyde [18,19]. Furthermore, styrene conversion reaches 54% after4 h of reaction time over Mg/Co/Al-3 sample (Fig. 4) and two majorproducts are styrene oxide and benzaldehyde in addition to smallamounts of phenylacetaldehyde, benzoic acid, styrene glycol, and ben-zyl benzoate…. Therefore, it is suggested that the presence of Co2+ inLDH structure (Fig. 2) has a synergetic effect on the formation of alde-hyde and yielded a major amount of styrene oxide [11,19–22]. Indeed,the intra-hydrotalcite lattice cobalt ions are more stable and avoidedthe oxidation to higher oxidation states (e.g. Co3O4, Co2O3), in accor-dance with those observed for Co2+ exchanged in zeolites [20,21].

A B

C D

Fig. 3. SEM micrographs of as-synthesized (A) and the used (B) and TEM images of as-synthesized (C) and the used (D) Mg/Co/Al-1 hydrotalcite-like compound.

0

10

20

30

40

50

60

70

80

90

100

Mg/Co/Al = 6/1/3 Mg/Co/Al =5/2/3 Mg/Co/Al = 4/3/3Hydrotalcite catalysts

(%)

Conversion (%)

Benzaldehyde Sel.

Styrene oxide Sel.

Other Product Sel.

Fig. 4. The correlation between catalytic activity in the oxidation of styrene and cobalt con-tents in Mg/Co/Al hydrotalcite-like catalysts (other products: phenyl acetaldehyde,benzoic acid, styrene glycol, benzyl benzoate, and polymerized products).

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3.2.2. Effect of reaction temperaturesTable 2 describes the variation of activity andmain product selectiv-

ities over all hydrotalcite-like samples in the reaction temperaturerange of 60–95 °C. At higher temperatures the reaction becomesquite complicated because some side reactions like overoxidationand polymerization occurring simultaneously [3,19,22–24]. In gen-eral, styrene conversion varies dramatically with the temperaturesfrom 60 to 95 °C, but the selectivity to epoxide approaches ahighest value at 80–90 °C while that to benzaldehyde reaches amaximum level at lower temperatures of 60–70 °C. Moreover, theoverall conversion of styrene was found to decrease in order ofMg/Co/Al-3 N Mg/Co/Al-2 N Mg/Co/Al-1 in a whole range of reac-tion temperatures (Table 2). Since the reaction has a negligibleactivity over the cobalt-free sample, this order indicates a strongrelation between catalytic activity and the surface density of cobaltions [20,22]. Table 2 also presents that the selectivity towardsphenyloxirane significantly increases with the Co/(Mg + Al) molarratio order of 0.44 N 0.24 N 0.10 N 0. The possible incorporation ofCo(II) into the brucite layers of hydrotalcite-like materials providesavailable sites for the epoxidation of styrene. It is well knownthat Co(II) complexes can activate molecular oxygen to form atransition complex of (Co–O)* [2,22]. In the present work, Co(II)octahedral sites in hydrotalcite structure are responsible for theformation of the (Co3+–O2

−) species which further generate radicaloxygen species for the initiation of the oxidation reaction undermild conditions [2,20–22].

3.2.3. Effects of reaction timeThe influence of the reaction time on the reaction over Mg/Co/Al

hydrotalcite-like materials is represented in Fig. 5. The conversion ofreactant gradually increases from beginning time to 6 h and reaches aplateau after 7 h. Overall, the styrene conversion over the cobalt-richsample is always higher than the cobalt-low-catalyst. Fig. 5 also indi-cates that the selectivity to styrene oxide slightly increases with reac-tion time whereas that to benzaldehyde decreases from 70% to 46%over Mg/Co/Al-3 catalyst (Fig. 5B). [14,21,22,24]. It is noted that no sig-nificant change in structural feature and morphology during the oxida-tion reaction although the specific surface area of spent hydrotalcitecatalysts slightly decreases (74.6 to 44.2 m2/g for Mg/Co/Al-2). TheCo/(Mg + Al) molar ratio of the reacted sample is almost unchangedafter 6 h, demonstrating that the intra-LDH lattice cobalt ions are thekey for the oxidation of styrene.

The catalytic tests reported in Fig. 5 were lasting for 8 h with no sig-nificant changes in conversions. In the case of sample Mg/Co/Al-1, thecatalyst was recorded infrared bands and its IR spectrum was reportedin Fig. 1S.

4. Conclusion

Three Mg/Co/Al materials show good characteristics of layereddouble hydroxides: the presence of carbonate ions between the layers,homogeneous and laminar structure, and a medium surface area. Thecatalysts were tested for the oxidation of styrene in solvent free condi-tions using air as a friendly cheap oxidant. All synthesized Mg–Co–Alcatalysts exhibit good activity and relative stability in the selectiveoxidation of styrene to benzaldehyde and epoxide. Both reactant con-version and product selectivities are dependent on the surface areaof cobalt ions and reaction variables. Co2+ in octahedral sheets is sug-gested to be acting as active sites for the oxidation of styrene into sty-rene oxide while both Co2+ intra-lattice ions and basic sites inhydrotalcite are responsible for the formation of benzaldehyde. Theconversion of styrene reaches about 70–90% and selectivity to desiredproducts (benzaldehyde + styrene oxide) is about 92–99%.

Acknowledgment

This research is funded by the Vietnam National Foundation forScience and Technology Development (NAFOSTED) under grant num-ber 104.99-2011.50.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.catcom.2013.11.004.

Table 2Catalytic activity of Mg/Co/Al-hydrotalcite-like compounds at different reaction tempera-tures after 4 h.

Catalysts Reactiontemperature(°C)

Styreneconversion(%)

Product selectivity (%)

Benzaldehyde Styrene oxide Othersa

Mg/Co/Al-1 65 2.6 91 – 975 3.3 98 1 185 4.5 91 8 195 12.1 82 11 7

Mg/Co/Al-2 65 5.0 93 9 175 13.4 71 24 585 32.3 57 27 1695 38.1 59 36 5

Mg/Co/Al-3 65 18.7 99 - 175 46.2 64 30 685 52.6 55 38 795 93.0 45 36 19

a Other products: phenyl acetaldehyde, benzoic acid, styrene glycol, benzyl benzoate,polymerized products.

A

0

10

20

30

40

50

60

70

80

90

100

31 2 4 5 6 7 8 9

Reaction time (h)

Per

cent

(%

)

Conversion

Benzaldehyde Sel

Styren oxide Sel

Other product Sel.

B

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6 7 8 9

Reaction time (h)

Per

cent

(%

)

ConversionBenzaldehyde SelStyren oxide SelOther product Sel.

Fig. 5. Catalytic activity in the oxidation of styrene over Mg/Co/Al-1 sample (A) and Mg/Co/Al-3 catalyst (B) 85 °C (other products: phenyl acetaldehyde, benzoic acid, styrene gly-col, benzyl benzoate, and polymerized products).

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