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Short Communication
Selective oxidation of styrene over Mg–Co–Al hydrotalcite like-catalysts
using air as oxidant
Nguyen Tien Thao ⁎, Ho Huu Trung
Faculty 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 2013
Received in revised form 31 October 2013
Accepted 5 November 2013
Available online 21 November 2013
Keywords:
Metal-doped hydrotalcite
Styrene oxidation
Benzaldehyde
Epoxide
Mg–Co–Al
A set of synthesized Mg/Co/Al hydrotalcites was synthesized and characterized by XRD, XPS, BET, SEM, TEM, and
FT-IR physical techniques. The partial substitution of Mg2+
by Co2+
in brucite layers has not significantly affected
the 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 of
styrene in the free-solvent condition. Both styrene conversion and desired product selectivities are strongly
dependent on the cobalt substitution content. The intra-hydrotalcite lattice Co2+
ions are active sites for the
epoxidation 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, this
process 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 of
environmentally friendly cheap oxidants [2,3]. Several transition
metal-containing catalysts based on Ru, Cu, Fe, Mn, V, Ti… have been
used in the liquid phase oxidation of olefinic compounds to oxygenates
[3–6]. Among those, Ru- and Cu-based heterogeneous solids are
restricted only to doubly activated alkylaromatics while Fe- and Ni-
containing catalysts usually give a rather low yield of oxygenated
products [3–7]. Therefore, the synthesis of novel easily recyclable
catalyst for the oxidation of alkylbenzenes is still a great challenging
goal 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+
Bx
3+
(OH)2(CO3)0.5x·
nH2O]. Cations are usually located in coplanar [M(OH)6] octahedra
sharing vertices and forming M(OH)2 layers with the brucite struc-
ture [8]. Partial substitution of divalent cations by trivalent cations
leads to the appearance of positive layers which is usually compensated
by anions between layers. Thus, the complexity of chemical composi-
tion in hydrotalcite-like compound makes it be able to act as basic solids
and oxidation–reduction catalysts [9–11]. For example, Ni-containing
basic hydrotalcites were used for the selective oxidation of benzylic
CH bonds of ethyl benzene [11]. Mn–MgAl and MoO4
−
/MgAl
hydrotalcite-like catalysts present a good activity in the oxidation of
alkylbenzenes [9,12]. Cobalt-containing hydrotalcites have been used
for the steam reforming of ethanol [13] and synthesis of benzoin methyl
ether [14]. In these cases, transition metal ions in layered structure are
the 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 the
coprecipitation method. The detailed procedure was described in our
previous publication [15]. In brief, 150 mL-mixed aqueous solution of
Mg(NO3)2·6H2O (99%), Co(NO3)2·6H2O (98%) and Al(NO3)3·9H2O
(N98%) with Co2+
/(Mg2+
+ Al3+
) molar ratios ranging from 0 to
0.44 was added dropwise to 25 mL of 0.6 M Na2CO3 under vigorous stir-
ring. The exact amounts of starting materials for each catalyst are given
in Supporting information (Table 1S). The solution pH was adjusted to
9.50 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 then
cooled to room temperature and separated by filtration, washed with
hot distilled water several times, and then dried at 80 °C for 24 h in
air. The prepared catalysts are denoted as Mg/Co/Al-1, -2, and -3
(Table 1).
The elemental composition (Mg, Co, Al) of catalyst was measured
using an ICP-MS Elan 9000 (PerkinElmer, USA) and carbon content in-
strument PE 240 (USA). Powder X-ray diffraction (XRD) patterns were
recorded 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
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom

3. Author's personal copy
(λ = 0.1549 nm). Fourier transform infrared (FT-IR) spectra were
obtained in 4000–400 cm−1
range on a FT/IR spectrometer
(DX-PerkinElmer, USA). TEM images were collected on a Japan
JEOL JEM-1010. The nitrogen physisorption was run on an Autochem
II 2920 (USA). The X-ray photoelectron spectra (XPS) of catalysts
were recorded with a Thermo K-Alpha.
The catalytic oxidation of styrene in the absence of solvent was
carried 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 catalyst
were loaded into the flask unless some particular tests were indicated.
After the reaction mixture was magnetically stirred and heated to the
desired temperature, the flow of air (5 mL/min) was bubbled through
the vigorously stirred reaction mixture and the reaction time starts
recorded. After the reaction, the mixture was quenched to room tem-
perature and then catalyst was filtered off. The filtrate was analyzed
by a GC–MS (HP-6890 Plus) and a frame ionization detector (FID) is
used as a detector.
3. Results
3.1. Catalyst characteristics
The prepared catalyst characteristics and chemical composition are
summarized 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, two
sharp and intense peaks at low diffraction angles of 23.2 and 34.4° are
ascribed to the diffraction by basal planes (006) and (102), respectively
[10,15]. Furthermore, broad, less intense peaks at higher angles around
38, 46, and 60° indexed to (105), (108), and (110) planes also confirm
the hydrotalcite structure [14,15]. The positions of these reflection
lines are slightly changed but the signal to noise ratio and full width at
half maximum peaks vary with increased cobalt content. The latter
could possibly be explained by only subtle differences in the octahedral
ionic 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 representative
Mg/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 the
near-surface region, the spectrum corresponding to the Co 2p core
level is represented in Fig. 2B while Mg 1s and Al 2p scans are elucidated
in Supporting information. XPS spectrum of Co 2p in Mg/Co/Al sample
shows 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 essentially
ascribed to Co2+
species. Furthermore, the high intensities of the satel-
lites are typical characteristics for the cobalt containing layered double
hydroxide 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 the
main band around 3454 cm−1
assigning to the OH stretching mode of
water molecules and hydroxyls in the layers [10,12]. This band shows
a prominent shoulder around 2950 cm−1
ascribed to hydrogen bonding
of OHs of layered lattice and/or water molecules with interlayer carbon-
ate anions (see Fig. 1S in Supporting information). A sharp band at
1365 cm−1
is firmly assigned to the asymmetric stretching vibration
of the CO3
2−
in the hydrotalcite layers. A set of bands at 437, 663, 742,
and 927 cm−1
is associated to AlO, CoO, AlOH translation, and
doublet AlOH 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 1
Physical properties of the prepared Mg/Co/Al hydrotalcite-like compounds.
Catalyst
batch
Molar ratio
of
Co2+
/(Mg2+
+Al3+
)
Elemental analysis (wt.%) BET
surface
area
(m2
/g)
Pore
volume
(cm3
/g)
Mg Co Al C
Mg/Al-0 0 24.74 – 11.93 2.39 83.4 0.62
Mg/Co/Al-1 0.10 21.34 8.16 12.30 1.98 78.9 0.60
Mg/Co/Al-2 0.24 14.20 12.72 8.14 1.87 74.6 0.58
Mg/Co/Al-3 0.44 9.18 17.89 8.10 1.89 74.5 0.58
Mg/Co/Al-2
reacted
0.24 10.13 9.71 7.24 2.11 44.2 0.49
20 25 30 35 40 45 50 55 60 65 70
2-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
020040060080010001200
Binding Energy (eV)
Counts/s
Mg 1s
Co
O 1s
C1s
Al 2p
A
Co2p scan
10000
11000
12000
13000
14000
15000
16000
17000
18000
19000
20000
770773776779782785788791794797800803806809812
Binding Energy (eV)
Counts/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.
154 N.T. Thao, H.H. Trung / Catalysis Communications 45 (2014) 153–157

4. Author's personal copy
BET specific surface area of the cobalt-free-sample (Mg/Al-0) is only
83.4 m2
/g while that of the others is approximately 74–78 m2
/g
(Table 1). The nitrogen isothermal curves likely expresses a plateau
from 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 pressures
higher than 0.8 along with a sharp adsorption volume increase is firmly
responsible for the physisorption in mesopores (Supporting informa-
tion) [15].
The morphology of Mg/Co/Al-1 LDH is illustrated in Fig. 3 and some
additional micrographs are depicted in Supporting information. Fig. 3A
shows that the hydrotalcite-like compound particles are regularly hex-
agonal plates [15]. The particle sizes are relatively uniform with the
mean crystal domain of 70–100 nm [11,13]. More details, the TEM
image of Mg/Co/Al-1 LDH shows laminar structure which is an essential
characteristic for hydrotalcite mineral and the stacking of the layers
(Fig. 3C) [17]. The flat particles with hexagonal shapes are presented
and the grain boundaries are clearly observed. The aggregation of
uniform particles leads to the formation of voids between primary
nanoparticles [13,17].
3.2. Catalytic results
The catalytic activity of Mg/Co/Al-hydrotalcite-like catalysts in the
liquid oxidation has been examined at atmospheric pressure and air
was bubbled into the reaction system without any further purification.
3.2.1. Oxidation of styrene catalyzed by cobalt ions in hydrotalcites
Fig. 4 presents the reaction results of three Mg/Al/Co hydrotalcite-
like materials in the oxidation of styrene. By comparison, a blank test
and 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 a
negligible amount of styrene (b1%) over Mg/Al-hydrotalcite-like
material 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% after
4 h of reaction time over Mg/Co/Al-3 sample (Fig. 4) and two major
products are styrene oxide and benzaldehyde in addition to small
amounts of phenylacetaldehyde, benzoic acid, styrene glycol, and ben-
zyl benzoate…. Therefore, it is suggested that the presence of Co2+
in
LDH 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 avoided
the 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 = 4/3/3
Hydrotalcite 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).
155N.T. Thao, H.H. Trung / Catalysis Communications 45 (2014) 153–157

5. Author's personal copy
3.2.2. Effect of reaction temperatures
Table 2 describes the variation of activity and main product selectiv-
ities over all hydrotalcite-like samples in the reaction temperature
range of 60–95 °C. At higher temperatures the reaction becomes
quite complicated because some side reactions like overoxidation
and polymerization occurring simultaneously [3,19,22–24]. In gen-
eral, styrene conversion varies dramatically with the temperatures
from 60 to 95 °C, but the selectivity to epoxide approaches a
highest value at 80–90 °C while that to benzaldehyde reaches a
maximum level at lower temperatures of 60–70 °C. Moreover, the
overall conversion of styrene was found to decrease in order of
Mg/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 negligible
activity over the cobalt-free sample, this order indicates a strong
relation between catalytic activity and the surface density of cobalt
ions [20,22]. Table 2 also presents that the selectivity towards
phenyloxirane significantly increases with the Co/(Mg + Al) molar
ratio order of 0.44 N 0.24 N 0.10 N 0. The possible incorporation of
Co(II) into the brucite layers of hydrotalcite-like materials provides
available sites for the epoxidation of styrene. It is well known
that Co(II) complexes can activate molecular oxygen to form a
transition complex of (Co–O)* [2,22]. In the present work, Co(II)
octahedral sites in hydrotalcite structure are responsible for the
formation of the (Co3+
–O2
−
) species which further generate radical
oxygen species for the initiation of the oxidation reaction under
mild conditions [2,20–22].
3.2.3. Effects of reaction time
The influence of the reaction time on the reaction over Mg/Co/Al
hydrotalcite-like materials is represented in Fig. 5. The conversion of
reactant gradually increases from beginning time to 6 h and reaches a
plateau after 7 h. Overall, the styrene conversion over the cobalt-rich
sample 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 hydrotalcite
catalysts slightly decreases (74.6 to 44.2 m2
/g for Mg/Co/Al-2). The
Co/(Mg + Al) molar ratio of the reacted sample is almost unchanged
after 6 h, demonstrating that the intra-LDH lattice cobalt ions are the
key 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, the
catalyst was recorded infrared bands and its IR spectrum was reported
in Fig. 1S.
4. Conclusion
Three Mg/Co/Al materials show good characteristics of layered
double hydroxides: the presence of carbonate ions between the layers,
homogeneous and laminar structure, and a medium surface area. The
catalysts were tested for the oxidation of styrene in solvent free condi-
tions using air as a friendly cheap oxidant. All synthesized Mg–Co–Al
catalysts exhibit good activity and relative stability in the selective
oxidation of styrene to benzaldehyde and epoxide. Both reactant con-
version and product selectivities are dependent on the surface area
of 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 in
hydrotalcite are responsible for the formation of benzaldehyde. The
conversion of styrene reaches about 70–90% and selectivity to desired
products (benzaldehyde + styrene oxide) is about 92–99%.
Acknowledgment
This research is funded by the Vietnam National Foundation for
Science 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 2
Catalytic activity of Mg/Co/Al-hydrotalcite-like compounds at different reaction tempera-
tures after 4 h.
Catalysts Reaction
temperature
(°C)
Styrene
conversion
(%)
Product selectivity (%)
Benzaldehyde Styrene oxide Othersa
Mg/Co/Al-1 65 2.6 91 – 9
75 3.3 98 1 1
85 4.5 91 8 1
95 12.1 82 11 7
Mg/Co/Al-2 65 5.0 93 9 1
75 13.4 71 24 5
85 32.3 57 27 16
95 38.1 59 36 5
Mg/Co/Al-3 65 18.7 99 - 1
75 46.2 64 30 6
85 52.6 55 38 7
95 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)
Percent(%)
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)
Percent(%)
Conversion
Benzaldehyde Sel
Styren oxide Sel
Other 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).
156 N.T. Thao, H.H. Trung / Catalysis Communications 45 (2014) 153–157

6. Author's personal copy
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