Selective oxidation of styrene catalyzed by cerium-doped cobalt ferrite nanocrystals with greatly enhanced catalytic performance

Article abstract of DOI:10.1016/j.jcat.2016.10.003

The rare earth metal Ce-doped cobalt ferrite samples Ce_xCo_1?xFe₂O₄ (x?=?0.1, 0.3, 0.5) were prepared by the sol–gel autocombustion route. The as-prepared samples were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, ICP–atomic emission spectroscopy, and N₂ physisorption. Their catalytic performance was evaluated in oxidation of styrene using hydrogen peroxide (30%) as oxidant. Compared with pristine CoFe₂O₄, the Ce-doped samples were found to be more efficient catalysts for the oxidation of styrene to benzaldehyde, with greatly enhanced catalytic performance. Especially, when Ce_0.3Co_0.7Fe₂O₄ was used as catalyst, 90.3% styrene conversion and 91.5% selectivity for benzaldehyde were obtained at 90?°C for 9?h reaction. The catalyst can be magnetically separated easily for reuse, and no obvious loss of activity was observed when it was reused in five consecutive runs.

Full text of DOI:10.1016/j.jcat.2016.10.003

Journal of Catalysis 344 (2016) 474–481
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective oxidation of styrene catalyzed by cerium-doped cobalt ferrite
nanocrystals with greatly enhanced catalytic performance
Jinhui Tong a, , Wenyan Li , Lili Bo , Huan Wang , Yusen Hu , Zhixia Zhang , Abdulla Mahboob
⇑
a
b
b
b
b
c,
⇑
a
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China
College of Science, Gansu Agricultural University, Lanzhou 730070, People’s Republic of China
Department of Chemistry, School of Science and Technology, Nazarbayev University, 53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
x 1ꢀx 2 4
The rare earth metal Ce-doped cobalt ferrite samples Ce Co Fe O (x = 0.1, 0.3, 0.5) were prepared by
the sol–gel autocombustion route. The as-prepared samples were characterized by X-ray diffractometry,
scanning electron microscopy, transmission electron microscopy, ICP–atomic emission spectroscopy, and
Received 21 July 2016
Revised 1 October 2016
Accepted 3 October 2016
N
2
physisorption. Their catalytic performance was evaluated in oxidation of styrene using hydrogen per-
oxide (30%) as oxidant. Compared with pristine CoFe , the Ce-doped samples were found to be more
efficient catalysts for the oxidation of styrene to benzaldehyde, with greatly enhanced catalytic perfor-
mance. Especially, when Ce0.3Co0.7Fe was used as catalyst, 90.3% styrene conversion and 91.5% selec-
2 4
O
Keywords:
2 4
O
Sol–gel autocombustion
Magnetic nanocrystals
Rare-earth-doped ferrites
Styrene oxidation
tivity for benzaldehyde were obtained at 90 °C for 9 h reaction. The catalyst can be magnetically
separated easily for reuse, and no obvious loss of activity was observed when it was reused in five con-
secutive runs.
Hydrogen peroxide
Ó 2016 Elsevier Inc. All rights reserved.
1
. Introduction
Nanospinel ferrites with the general formula AB
The physical and chemical properties of the ferrites are strongly
dependent on their shapes, sizes, and constituents, which are clo-
2
O
4
, as a class of
sely related to the method of preparation [27–29]. A widely
adopted strategy for synthesizing new ferrites with various con-
stituents and properties is doping other elements in the pristine
ferrites [30–35]. As for preparation methods, a most popular one
is the sol–gel autocombustion method. This method combines
the advantages of chemical sol–gel and combustion processes
and gives rise to a thermally induced anionic redox reaction. The
energy released from the reaction between oxidant and reductant
is adequate to form a desirable phase within a very short time. The
process exhibits the advantages of inexpensive precursors and a
simple preparation process and can produce highly reactive nano-
sized powder [36,37].
Catalytic oxidation of styrene at side chains is of great interest
for both academic research and utilization in the industry, because
the products styrene oxide and benzaldehyde are important and
versatile synthetic intermediates in chemical industries [38]. Espe-
cially, benzaldehyde is a very valuable chemical that has wide-
spread applications in perfumery, pharmaceuticals, dyestuffs, and
agrochemicals [39]. Traditionally, this reaction has been carried
out in both homogeneous and heterogeneous catalyst systems
chemically and thermally stable materials, are of great fundamen-
tal and technological importance due to their structural, electronic,
magnetic, and catalytic properties [1–4]. The complex metal oxides
have been widely used in diverse areas such as magnetic recording
and separation [5], ferrofluids [6], magnetic resonance imaging
(
[
MRI) [7], biomedicine [8], gas sensors [9], high-quality ceramics
10,11], and superparamagnetic materials [12–14]. Especially, they
have been widely used as catalysts in liquid phase reactions due to
their redox and magnetic recovery properties [15]. As a matter of
fact, these catalysts can be separated from a reaction medium by
simply placing a magnetic field on the surface of a flask. Spinel fer-
rites are found to be highly active toward a number of industrial
processes, such as oxidative dehydrogenation of hydrocarbons
[
16,17], decomposition of hydrogen peroxide [18], treatment of
automobile-exhaust gases [19], oxidation of various compounds
such as chlorobenzene [20], phenol hydroxylation [21], alkylation
reactions [22–24], hydrodesulfurization of crude petroleum [20],
and catalytic combustion of methane [25,26].
[
40–42]. However, homogeneous catalysts have recently become
less attractive because of difficulty in separating products and cat-
alysts. Thus, heterogeneous catalysts have currently been paid
much attention [43]. Various kinds of catalysts have been
⇑
Corresponding authors. Fax: +86 931 7971533 (J. Tong).
E-mail address: jinhuitong@126.com (J. Tong).
http://dx.doi.org/10.1016/j.jcat.2016.10.003
021-9517/Ó 2016 Elsevier Inc. All rights reserved.
0

J. Tong et al. / Journal of Catalysis 344 (2016) 474–481
475
employed to catalyze styrene oxidation [44]. From the viewpoint of
environmental protection, molecular oxygen was used as a green
and cheap oxidant, for which MOF-74(Cu/Co) [45], bulk Au parti-
cles [46], and gold nanoparticles supported on dendrimer resin
evaporated in an oil bath at 80 °C under continuous stirring until
a brown gel formed. After the reaction, the formed gel was dried
at 120 °C until a foamy xerogel was obtained. Then the produced
xerogel was ignited at 650 °C, a self-propagating combustion pro-
cess occurred, and an olive brown product was obtained after it
combusted completely. The Ce-doped samples were prepared as
[47], sulfur-doped graphene [48], and hollow iron oxide nanoshells
49] have been reported to be active catalysts. However, as a mild
[
oxidant, molecular oxygen is usually less efficient, with a low reac-
tion rate at ambient pressure. Compared with molecular oxygen,
described above and were designated as CFO-Ce0.1, CFO-Ce0.3
,
and CFO-Ce0.5, respectively. For comparison, pristine CoFe O
2 4
H
2
O
2
is a green and more efficient alternative. Various catalysts
have been used to catalyze styrene oxidation using H as oxi-
dant, such as N -type bis(diazoimine) complexes supported on
silica [50], platinum nanoclusters supported on TiO anatase [51],
phosphomolybdic acid supported on ionic-liquid-modified MCM-
1 [52], zinc phthalocyanine supported on multiwalled carbon
nanotubes [53], Ag–WO catalyst [54], V-MCM-48 [55], Ti-
was also prepared and was designated as CFO-A. The samples were
ground finely and screened by a 300 mesh sieve before character-
ization and employment as catalysts for oxidation of styrene with
hydrogen peroxide.
2 2
O
4 4
O
2
4
2.3. Oxidation of styrene
3
containing mesoporous silica [56], and iron oxide nanoparticles
supported on mesoporous silica-type materials [57]. Based on the
mentioned advantages of ferrites, several examples have been
reported for oxidation of styrene using ferrites as catalysts, such
The selective oxidation of styrene was carried out in a 25 mL
Schlenk tube. In a typical procedure, 0.06 mmol (ca. 15.0 mg, based
on the given formula Ce Co1ꢀxFe of catalyst, 2.0 mL
17.4 mmol) of styrene, 10 mL of solvent, and 2.7 mL of hydrogen
peroxide (30%), styrene:H molar ratio of 2:3, were added suc-
x
2 4
O )
(
2 4 x 4
as nickel and zinc ferrites [26], SrFe O [28], Mg Fe3ꢀxO [29],
2 2
O
CaFe [58], Ni–Gd ferrites [59], spinel Mg–Cu ferrites [60], and
2 4
O
cessively into the flask. The flask was then immersed in an oil bath
at a desired temperature for a desired reaction time under stirring
with an optimum stirrer speed of 1200 rpm, at which the highest
conversion rate could be obtained (Fig. S1; see the Supporting
Information). It is clear from Fig. S1 that the reaction is effected
by diffusion limitation. Under the above conditions, the atmo-
sphere in the tube included mainly air, vapor of substrate, water,
supported noble metals [1,54,61]. Being a terminal olefin, styrene
is difficult to oxidize, and most of these oxidation reactions were
limited by complicated preparation procedures, high costs, harsh
conditions, or low efficiency.
2 4
In our previous work [62], CoFe O nanocrystals synthesized by
sol–gel autocombustion were proved to be a highly active and
easily recovered catalyst for the oxidation of cyclohexane by
molecular oxygen without addition of solvents or reductants. As
part of our interest in hydrocarbon oxidation catalyzed by spinel
ferrites, we are reporting here that rare earth metal Ce-doped
cobalt ferrites synthesized by a sol–gel autocombustion method
can efficiently catalyze the oxidation of styrene to produce ben-
zaldehyde, and their catalytic activities can be greatly enhanced
2 2
and solvent, and oxygen from decomposition of H O . The pressure
in the tube ranged from 1.1 to 1.2 atm. After the reaction, the tube
was cooled to room temperature. The gas-phase mixture was col-
lected and analyzed by gas chromatography (GC) equipped with
a 5A molecular sieve column and a thermal conductivity detector
(
TCD). Liquid-phase aliquots were identified by GC-MS and
quantified by GC equipped with an SE-54 capillary column and a
hydrogen flame ionization detector (FID) using toluene as internal
standard. The detected products in the liquid phase included the
main product benzaldehyde and byproducts phenylacetaldehyde,
styrene oxide, benzoic acid, phenylacetic acid, and formaldehyde.
Trace of CO was detected in the gas phase. The amount of resid-
2 4 2
when compared with those of pristine CoFe O and CeO .
2
. Experimental
2.1. Materials and equipment
ual H
2
O
2
was determined by iodometric titration [63,64] and the
All reagents were of analytical grade and were used as received.
X-ray diffraction (XRD) patterns of the samples were collected
using a PANalytical X’Pert Pro diffractometer with CuK radiation.
Scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) micrographs were obtained using a JSM-
2
H O
2
utilization efficiency was defined as follows: H utiliza-
2 2
O
tion efficiency = [(mol (benzaldehyde + phenylacetaldehyde +
a
styrene oxide) + 2 ꢂ mol (benzoic acid + phenylacetic acid))/mol
(
2
H O
2
)
consumed] ꢂ 100%.
5
600LV and a Hitachi H-600 microscope, respectively. Metal con-
tent was determined by inductively coupled plasma (ICP) on a Per-
kin–Elmer ICP/6500 atomic emission spectrometer. BET surface
area measurements were performed on a Micromeritics ASAP
3. Results and discussion
3.1. Characterization of the catalysts
2
010 instrument at liquid nitrogen temperature. The oxidation
products were determined by an HP 6890/5973 GC/MS instrument
and quantified by a Shimadzu GC-2010 gas chromatograph using
toluene as internal standard.
3.1.1. The XRD characterization
The XRD patterns of the Ce-doped samples are shown in Fig. 1.
The average particle sizes (based on the Scherrer equation) and lat-
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
2
tice parameters (based on the formula a ¼ d h þ k þ l ,
2d sin h ¼ nk) based on the most intense diffraction peaks of the
spinel phase of the samples are shown in Table 1. It can be con-
firmed from Fig. 1 that the main phase in Ce-doped samples is spi-
nel (JCPDS No. 22-1086). It can also be confirmed that the minor
2.2. Preparation of the catalysts
The rare earth metal cerium-doped cobalt ferrite samples
Ce Co1ꢀxFe O (x = 0.1, 0.3, 0.5) were prepared by a sol–gel auto-
x 2 4
combustion route under optimized conditions reported in our pre-
2
new phases of CeO (PDF No. 801792) formed and their contents
vious work [62]. In a typical procedure, stoichiometric amounts of
increased with increasing Ce content.
Co(NO
3
)
2
ꢁ6H
2
O, Ce(NO
3
)
3
ꢁ6H
2
O, Fe(NO
3
)
3
ꢁ9H
2
O, and citric acid
Table 1 shows that the lattice parameters of the Ce-doped sam-
ples increased from 8.3812 to 8.3870 Å with increase of Ce from 0
were completely dissolved in distilled water with a 1:1 molar ratio
ꢀ
1
4+
of metals to citric acid and a 0.1 mol L concentration of metals.
Concentrated ammonia (25–28%) was added slowly under con-
stant stirring to adjust the solution to neutral. The solution was
to 0.5. This confirms that Ce , with a larger radius of 0.101 nm,
2
+
3+
partly replaced Co
0.067 nm, respectively.
or Fe , with small radii of 0.082 and

4
76
J. Tong et al. / Journal of Catalysis 344 (2016) 474–481
1
). All the samples can efficiently catalyze the oxidation of styrene,
ƽ
and benzaldehyde was found to be the major product in all cases.
As expected, the Ce-doped ferrite samples have shown better per-
ƽ
Spinel
*
CeO2
2 4 2
formance than pristine CoFe O and CeO in enhancement of styr-
ƽ
ene conversion and benzaldehyde selectivity. Especially in the case
of the sample CFO-Ce0.3, 59.4% styrene conversion and 88.2% ben-
zaldehyde selectivity were obtained, which is 26.5%, 28.6%, and
ƽ
ƽ
CFO-A
ƽ
ƽ
ƽ
ƽ
ƽ
CFO-Ce
22.9%, 41.0% higher than those obtained over pristine CoFe
and CeO , respectively. The Ce-doped samples also obtained higher
utilization efficiency, except for the sample CFO-Ce0.5 and
. It is also clear that the samples have shown much higher
2 4
O
0
.5
ƽ
*
ƽ
ƽ
*
*
ƽ
ƽ
ƽ
ƽ
*
*
ƽ
ƽ
ƽ
ƽ
2
CFO-Ce
0
.3
ƽ
ƽ
ƽ
2 2
H O
ƽ
CeO
2
ƽ
activities in both styrene conversion and benzaldehyde selectivity
than the reported analogous ferrites [26,65] (entries 7, 8) and sup-
ported noble metal catalysts [39] (entry 9).
CFO-Ce
ƽ
ƽ
0
.1
ƽ
ƽ
*
ƽ
ƽ
ƽ
*
Increase in catalytic activity can be thought of as linked to the
appearance of the CeO
catalyst. The likely mechanism involves coordination of the organic
2 2
face in the XRD. CeO is a known oxidation
1
0
20
30
40
50
60
70
80
o
(
)
3
+
ligand and the H
2 2 2
O on Ce sites on CeO , as previous kinetics
Fig. 1. The XRD patterns of the Ce-doped samples.
studies support this hypothesis [65]. However, it is possible to
3
+
overload the Ce site on CeO
2 2 2
with H O . In addition to the expan-
sion of the lattice due to the introduction of the larger Ce atom in
Table 1
3
+
lieu of Fe, the overloading of the Ce by H
explanation for the optimal performance of the CFO-Ce0.3 when
compared with CFO-Ce0.5 and CeO . We are currently investigating
2 2
O could be a plausible
The XRD average particle sizes, lattice parameters, and BET surface areas of the
samples.
2
Catalyst
XRD average
particle size (nm)
Lattice
parameter (Å)
BET surface
area (m2
ꢀ1
the possibility of cooperative interactions between the Fe atoms
and the Ce atoms, as it relates to the optimal performance of the
g )
CFO-A
24
29
23
21
8.3812
8.3824
8.3847
8.3870
38.6
37.5
38.4
38.9
CFO-Ce0.3
We have performed geometry optimization of a plausible coor-
dination geometry of styrene and H at CeO face using the pseu-
.
CFO-Ce0.1
CFO-Ce0.3
CFO-Ce0.5
2
O
2
2
dopotential method implemented in the Siesta package [66]. The
coordination geometry (Fig. S2; see the Supporting Information)
shows a slight sp3 character on the terminal vinyl carbon, suggest-
The mean particle sizes of the samples, based on the Scherrer
equation, are 24, 29, 23, and 21 nm for CFO-A, CFO-Ce0.1, CFO-
Ce0.3, and CFO-Ce0.5 respectively.
Metal content of the samples determined by ICP–atomic emis-
sion spectroscopy is listed in Table 2. As can be seen, the deter-
mined metal contents are consistent with the calculated ones.
3
+
ing that coordination onto the Ce can activate the terminal vinyl
carbon for attack by H
2 2
O .
In pursuit of high benzaldehyde selectivity, the sample CFO-
Ce0.3 was chosen as catalyst to optimize the reaction conditions
and investigate the recyclable performance of the catalyst.
3
.1.2. The SEM and TEM characterization
The morphologies of the samples were also characterized by
3.3. Effect of catalyst amount
SEM and TEM. The SEM and TEM images of the sample CFO-Ce0.3
are shown in Fig. 2 as representations. The sample CFO-Ce0.3 fea-
tures nanoparticles with an irregular morphology and a broad par-
ticle size distribution ranging from 10–20 to 30–50 nm, with a
large percentage of small particles (25–30 nm). The other samples
showed similar morphologies, except for different particle sizes, as
calculated from the Scherrer equation based on the XRD patterns.
Table 4 shows the effects of the catalyst amount on the results
of styrene oxidation. As can be seen, 5.9 and 9.6% increases in the
styrene conversion and benzaldehyde selectivity, respectively,
were obtained when the catalyst amount increased from 10.0 to
5.0 mg. However, the selectivity for benzaldehyde decreased to
2.4% when the catalyst amount increased further to 20.0 mg, with
only a marginal increase of 2.6% in styrene conversion, possibly
due to increasing magnetic agglomeration of the catalyst with
increased catalyst amount, which reduced the contact of reactants
with the active sites of the catalyst. This is based on the fact that
the reaction is affected by diffusion limitation (Fig. S1 in the Sup-
1
8
2
3.1.3. N physisorption characterization
N
2
physisorption measurements were also performed for the
as-prepared samples. The BET surface areas of the samples are
summarized in Table 1. It can be seen that the samples showed
2
2 2
porting Information). The highest H O utilization efficiency was
similar BET surface areas, around 38 m /g.
obtained when 15.0 mg catalyst was used. Based on these facts,
5.0 mg was selected as the optimum catalyst amount in the fol-
lowing investigations.
1
3.2. Catalysis tests
The results of styrene oxidation over the as-prepared samples
are listed in Table 3. As shown in Table 3, in the absence of catalyst,
almost no reaction took place, as seen from GC/MS analysis (entry
3
.4. Effect of the solvents
Table 5 shows effects of different solvents on styrene conver-
Table 2
Metal content of the samples.
sion and products distribution over the sample CFO-Ce0.3. The
order of decrease of dielectric constants for the solvents is H
(78.3) > DMF (36.7) > CH CN (35.9) > acetylacetone (23.1) > tert-
2
O
Catalyst
Co content (wt.%)
Fe content (wt.%)
Ce content (wt.%)
3
butanol (12.9) > pyridine (12.5) > toluene (2.4) > 1,4-dioxane
(2.0). This is also the order of decrease of styrene conversion and
the order of increase of benzaldehyde selectivity, except for 1,4-
dioxane (entry 5) and water (entry 8). The results have shown that
CFO-A
25.62
22.29
16.25
10.91
46.64
45.08
42.25
39.76
–
CFO-Ce0.1
CFO-Ce0.3
CFO-Ce0.5
5.66
15.92
24.96

J. Tong et al. / Journal of Catalysis 344 (2016) 474–481
477
Fig. 2. The SEM and TEM images of the sample CFO-Ce0.3
.
Table 3
a
Oxidation of styrene over the as-prepared samples.
Entry
Catalyst
Conversion (mol%)
H
–
39.2
58.8
65.8
68.4
57.7
–
2
O
2
utilization efficiency (%)
Selectivity (mol%)b
BZAc
PHAc
SOc
Othersd
1
2
–
–
–
–
–
–
e
CeO
2
36.5
32.9
47.6
59.4
41.0
30.7
51.8
41
47.2
59.6
75.5
88.2
74.0
52.1
46.4
61.0
25.3
19.2
8.5
4.5
11.2
–
17.2
15.0
10.2
3.3
11.2
–
10.3
6.2
5.8
4.0
3.6
42.0
46.7
–
3
4
5
6
CFO-A
CFO-Ce0.1
CFO-Ce0.3
CFO-Ce0.5
Ni0.5Zn0.5Fe
f
7
2
4
O
4
g
8
Mg0.4Fe2.6
O
–
–
2.31
–
4.63
39.0
h
9
Au/TiO
2
a
b
c
Reaction conditions: styrene: 2.0 mL (17.4 mmol), catalyst: 0.06 mmol, styrene/H
Selectivity = mol of the given product/mole of the converted styrene.
BZA = benzaldehyde, PHA = phenylacetaldehyde, SO = styrene oxide.
The main products are benzoic acid and phenylacetic acid.
CeO was purchased from Sigma-Aldrich; average particle < 20 nm.
2
From Ref. [26]. Reaction conditions: styrene: 10 mmol, catalyst: 0.42 mmol, styrene/H
From Ref. [29]. Reaction conditions: styrene: 10 mmol, catalyst: 0.46 mmol, styrene/H
2 2
O : 2:3, 1,4-dioxane: 10 mL, 70 °C, 9 h.
d
e
f
2
O
O
2
: 1:1, acetone: 10 mL, 60 °C, 12 h.
:1:2, acetone: 10 mL, 50 °C, 24 h.
g
h
2
2
From Ref. [39]. Reaction conditions: styrene: 2.6 mmol, catalyst, 0.006 mmol (based on Au), styrene/TBHP: 1:1, toluene: 10 mL, 70 °C, 15 h.
Table 4
a
Effect of catalyst amount on styrene oxidation.
Amount of catalyst (mg)
Conversion (mol%)
H
2
O
2
utilization efficiency (%)
Selectivity (mol%)b
BZAc
PHAc
SOc
Othersd
1
1
2
0.0
5.0
0.0
53.5
59.4
62.0
67.1
68.4
63.4
78.6
88.2
82.4
11.6
4.5
8.2
7.6
3.3
6.5
2.2
4.0
2.9
a
b
c
2 2
Reaction conditions: styrene: 2.0 mL (17.4 mmol), styrene/H O : 2:3, 1,4-dioxane: 10 mL, 70 °C, 9 h.
Selectivity = moles of the given product/mole of the converted styrene.
BZA = benzaldehyde, PHA = phenylacetaldehyde, SO = styrene oxide.
The main products are benzoic acid and phenylacetic acid.
d
the solvent with a high dielectric constant favors styrene conver-
sion, while that with low dielectric constant favors formation of
benzaldehyde. The obtained styrene conversion in water, with
the highest dielectric constant, was much lower than expected,
possibly due to insolubility of styrene in water. The contrary case
is that much higher styrene conversion was obtained in 1, 4-
dioxane, with the lowest dielectric constant. Much more effort is
needed to find convincing explanations for this in future work.
From consideration of both styrene conversion and benzaldehyde
selectivity, 1,4-dioxane was selected as the proper solvent for the
following investigations, which obtained the second highest styr-
ene conversion of 59.4% and the highest benzaldehyde selectivity
3.5. Effect of reaction temperature
The influences of reaction temperature on styrene oxidation
were evaluated in the range of 50 to 90 °C for 9 h reaction and
the results are listed in Table 6. It can be seen that high tempera-
ture favors both styrene conversion and benzaldehyde production.
As a result, when temperature was raised from 50 to 90 °C, styrene
conversion increased from 21.9 to 90.3% and the selectivity for
2 2
benzaldehyde increased from 67.3 to 91.5%. The highest H O uti-
lization efficiency of 75.1% was also obtained at 90 °C. The selectiv-
ities for phenylacetaldehyde and styrene oxide showed a tendency
to decrease with increased temperature, while total selectivity for
other byproducts increased a little. This indicates that more deeply
oxidized products formed at higher temperature.
of 88.2%, as well as the highest H
8.4% (entry 5).
2 2
O utilization efficiency of
6

4
78
J. Tong et al. / Journal of Catalysis 344 (2016) 474–481
Table 5
a
Effect of solvent on styrene oxidation.
Entry
Solvent
Conversion (mol%)
H
2
O
2
utilization efficiency (%)
Selectivity (mol%)b
BZAc
PHAc
SOc
Othersd
1
2
3
4
5
6
7
8
DMF
Pyridine
64.2
22.7
43.2
7.3
59.4
38.1
22.9
44.6
68.2
43.1
59.0
19.6
68.4
55.9
43.3
64.0
52.8
78.4
66.7
83.8
88.2
75.8
80.4
41.3
9.4
7.6
11.2
4.1
4.5
8.5
6.2
32.6
12.3
8.2
7.6
5.0
3.3
8.1
7.8
22.3
25.5
5.8
14.5
7.1
4.0
7.6
CH
3
CN
Toluene
1,4-dioxane
Acetylacetone
tert-Butanol
5.6
3.8
H
2
O
a
b
c
Reaction conditions: styrene: 2.0 mL (17.4 mmol), CFO-Ce0.3: 15.0 mg (0.06 mmol), styrene/H
Selectivity = moles of the given product/mole of the converted styrene.
BZA = benzaldehyde, PHA = phenylacetaldehyde, SO = styrene oxide.
The main products are benzoic acid and phenylacetic acid.
2
O
2
: 2:3, solvent: 10 mL, 70 °C, 9 h.
d
Table 6
Effect of reaction temperature on styrene oxidation.a
Reaction temperature (°C)
Conversion (mol%)
H
2
O
2
utilization efficiency (%)
Selectivity (mol%)b
BZAc
PHAc
SOc
Othersd
5
6
7
8
9
0
0
0
0
0
21.9
39.6
59.4
72.4
90.3
42.2
56.9
68.4
70.7
75.1
67.3
73.7
88.2
89.3
91.5
13.0
11.1
4.5
2.9
1.2
17.9
12.7
3.3
1.9
–
1.8
2.5
4.0
5.9
7.3
a
b
c
2 2
Reaction conditions: styrene: 2.0 mL (17.4 mmol), CFO-Ce0.3: 15.0 mg (0.06 mmol), styrene/H O : 2:3, 1,4-dioxane: 10 mL, 9 h.
Selectivity = moles of the given product/mol of the converted styrene.
BZA = benzaldehyde, PHA = phenylacetaldehyde, SO = styrene oxide.
The main products are benzoic acid and phenylacetic acid.
d
3
.6. Effect of styrene/H
O
2 2
molar ratio
3.7. Effect of reaction time
The effects of different styrene/H
gated and the results are shown in Table 7. It can be seen clearly
that, with the molar ratio of styrene/H increased, styrene con-
version also increased. As a result, when the molar ratio of styr-
ene/H increased from 2:1 to 2:5, styrene conversion increased
2
O
2
molar ratios were investi-
The influence of reaction time on styrene oxidation was inves-
tigated at 90 °C in the range 1–10 h with styrene:H molar ratio
2 2
O
2
O
2
2:3. As shown in Table 8, when the reaction time was prolonged
from 1 to 10 h, styrene conversion increased from 25.0 to 92.2%;
the selectivity for benzaldehyde increased first, but then dropped,
and the highest value of 91.5% was obtained for 9 h reaction; the
selectivity for phenylacetaldehyde and styrene oxide dropped from
17.7 to 2.5% and from 22.8% to zero, respectively, while the selec-
tivity for total byproducts increased from 0.8 to 13.7% due to more
deeply oxidized products being formed. According to the proposed
pathway for styrene oxidation (Scheme 1) [67,68], the high yield of
benzaldehyde is possibly due to further oxidation of styrene oxide
formed in the first step. To confirm this, the stability of styrene
oxide and phenylacetaldehyde under the reaction conditions was
tested by adding equal molar quantities of styrene oxide (entry
8) or phenylacetaldehyde (entry 9) to the converted styrene and
2 2
O
from 38.4 to 96.4%. As for the products distribution, the benzalde-
hyde selectivity increased first from 61.0 to 91.5% when the molar
ratio of styrene/H
to 70.5% when the molar ratio of styrene/H
:5; the selectivity of phenylacetaldehyde and styrene oxide
showed a tendency to decrease with the increased molar ratio of
styrene/H , while total selectivity for other byproducts increased
greatly. This indicates that more deeply oxidized products formed
when more oxidant was used. Furthermore, the H utilization
efficiency decreased with the increased styrene/H molar ratio.
2
O
2
increased from 2:1 to 2:3, but then dropped
2 2
O
further increased to
2
2 2
O
2 2
O
O
2 2
Considering conversion of styrene and selectivity for benzalde-
hyde, 2:3 was selected as the optimum molar ratio of styrene to
equal molar quantities of H
2 2 2 2
O to the residual H O based on the
2 2
H O .
first hour’s reaction results (entry 1) under the same conditions
Table 7
Effect of H
amount on styrene oxidation.a
2
O
2
Styrene/H
2
O
2
(mol/mol)
Conversion (mol%)
H
2
O
2
utilization efficiency (%)
Selectivity (mol%)b
BZAc
PHAc
SOc
Othersd
2
1
2
1
2
:1
:1
:3
:2
:5
38.4
76.3
90.3
94.6
96.4
83.7
79.2
75.1
65.4
56.2
61.0
78.4
91.5
82.4
70.5
13.9
6.7
3.2
2.4
1.0
22.1
10.7
–
2.0
1.2
3.0
4.2
5.3
13.2
27.3
a
b
c
Reaction conditions: styrene: 2.0 mL (17.4 mmol), CFO-Ce0.3: 15.0 mg (0.06 mmol), 1,4-dioxane: 10.0 mL, 90 °C, 9 h.
Selectivity = moles of the given product/mole of the converted styrene.
BZA = benzaldehyde, PHA = phenylacetaldehyde, SO = styrene oxide.
d
The main products are benzoic acid and phenylacetic acid.

J. Tong et al. / Journal of Catalysis 344 (2016) 474–481
479
Table 8
a
Effect of reaction time on styrene oxidation.
Entry
Reaction time (h)
Conversion (mol%)
H
2
O
2
utilization efficiency (%)
Selectivity (mol%)b
BZAc
PHAc
SOc
Othersd
1
2
3
4
5
6
7
1
3
5
7
8
9
10
1
1
25.0
65.7
79.9
83.2
85.6
90.3
92.2
74.8
3.7
45.5
68.7
72.7
73.5
74.0
75.1
71.5
–
58.7
62.8
69.1
74.3
78.9
91.5
83.8
55.3
–
17.7
15.1
11.8
9.5
7.4
3.2
2.5
19.3
91.3
22.8
20.4
15.3
11.9
8.7
–
–
24.2
–
0.8
1.7
3.8
4.3
5.0
5.3
13.7
e
f
f
f
f
f
8
1.2
g
h
9
–
3.7
a
b
c
Reaction conditions: styrene: 2.0 mL (17.4 mmol), CFO-Ce0.3: 15.0 mg (0.06 mmol), styrene/H
Selectivity = moles of the given product/mole of the converted styrene.
BZA = benzaldehyde, PHA = phenylacetaldehyde, SO = styrene oxide.
The main products are benzoic acid and phenylacetic acid.
Only styrene oxide was added.
Equal to moles of the given substance/mole of the initially added substrate.
Only phenylacetaldehyde was added.
Phenylacetic acid.
2 2
O : 2:3, 1,4-dioxane: 10 mL, 90 °C.
d
e
f
g
h
for 1 h reaction. It can be seen that under the reaction conditions,
benzaldehyde transformed from styrene oxide (entry 8) was only
1
00
Conversion
Selectivity
3
3
.4% less than that from direct oxidation of styrene (entry 1), while
.7% of phenylacetaldehyde was deeply oxidized to phenylacetic
8
0
acid (entry 9). This confirms that most of the benzaldehyde is
derived from styrene oxide. The formation of benzaldehyde may
also be facilitated by direct oxidative cleavage of the styrene side
chain double bonds [23]. In pursuit of high selectivity for the main
product, benzaldehyde, 9 h was selected as the optimum reaction
time.
60
4
0
0
0
3.8. Reuse of the catalyst
2
After the reaction, the catalyst can be seen being adsorbed on
the magnet. The catalyst together with the magnet can be easily
separated by simple decantation after a magnetic field is applied
to the surface of the tube, and then subjected to a second run under
the same conditions. The results are shown in Fig. 3. The selectivity
for benzaldehyde changed slightly after five runs, but the conver-
sion of styrene decreased slightly from 90.3 to 86.8%. The decrease
in styrene conversion could be attributed mainly to unavoidable
loss of the catalyst during the process of collection. The average
1
2
3
4
5
Reaction run
Fig. 3. Reuse of the catalyst CFO-Ce0.3
17.4 mmol), CFO-Ce0.3 15.0 mg (0.06 mmol), styrene/H
0 mL, 90 °C, 9 h.
.
Reaction conditions: styrene: 2.0 mL
(
1
:
2 2
O :
2:3, 1,4-dioxane:
values of styrene conversion and benzaldehyde selectivity were
8.5 and 91.1%, respectively. The results confirm that the sample
8
CFO-Ce0.3 has good stability and recyclable applicability for the
oxidation of styrene under our investigation conditions.
O
3.9. Extension of substrates
Table 9 shows the results of the investigation of catalysis of oxi-
dation of other olefins by the sample CFO-Ce0.3 under our opti-
mized conditions. As can be seen, it can effectively catalyze the
oxidation of many other cyclic and linear olefins, and above 30%
conversion can be obtained. Especially in the case of cyclopentene
O
H O
2
2
Others
(entry 1) and
a-methylstyrene (entry 3), above 60% conversion and
Step 1
76% selectivity for the main product were obtained.
S
t
e p
4
. Conclusions
2
Nanosized spinel-type Ce-doped cobalt ferrite complex oxides
O
were prepared by a simple and effective route of sol–gel autocom-
bustion using cheap precursors. The complex ferrite catalysts are
more active and easily reusable catalysts for styrene oxidation,
and benzaldehyde is the main product. The Ce-doped samples
Scheme 1. Proposed pathway for styrene oxidation.

4
80
J. Tong et al. / Journal of Catalysis 344 (2016) 474–481
Table 9
Extension of substrate.a
Entry
1
Substrate
Conversion (%)b
Selectivity (%)b
62.9
O
O
O
O
Others
(
(
76.7)
(11.6)
OH
(4.6)
O
(7.1)
Others
2
3
49.3
67.9
O
2.7)
(31.5)
(54.3)
(11.5)
Others
O
HO
OH
O
(
(
76.8)
(6.8)
4
(6.1)
(10.3)
Others
4
5
39.2
38.0
O
O
O
4
5
4
3
22.7)
(42.8)
(23.8)
(10.7)
Others
O
O
O
5
4
4
(
(
32.0)
(24.2)
(23.9)
(19.9)
Others
6
34.1
O
9
O
O
9
8
9
19.2)
(39.3)
(35.6)
(5.9)
a
Reaction conditions: catalyst 15.0 mg (0.06 mmol); substrate 2.0 mL; substrate/H
Selectivity = moles of the given product/mole of the converted substrate. Entries 1–3 were quantified by GC using toluene as the internal standard, entries 4–6 using
2
O
2
ratio 2:3 (mol/mol); 1,4-dioxane: 10 mL; 90 °C, 9 h.
b
decane as the internal standard.
show better performance than pristine CoFe
2
O
4
and CeO
2
in
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Acknowledgments
The authors are grateful to the National Natural Science Foun-
dation of China (51302222, 21363021, 21202133), the Natural
Science Foundation of Gansu Province (1308RJYA017), and the Pro-
gram for Changjiang Scholars and Innovative Research Team in
University (IRT15R56) for financial support. Siesta DFT calculations
in this work were made possible by the facilities of the Shared
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