Synthesis of acetophenone from aerobic catalytic oxidation of ethylbenzene over Ti-Zr-Co alloy catalyst: Influence of annealing conditions

Article abstract of DOI:10.1016/j.apcata.2015.12.008

The aerobic catalytic oxidation of ethylbenzene to produce acetophenone over an effective and robust heterogeneous Ti-Zr-Co has been studied. Under the optimum conditions, a high selectivity of 69.2% to acetophenone was obtained at a 61.9% conversion of ethylbenzene over Ti-Zr-Co catalyst, which is much better than the most results reported with heterogeneous catalysts. The catalytic performance and the active species of Ti-Zr-Co catalyst were discussed based on the changes of conversion and selectivity with varying the annealing temperature and atmosphere. It is confirmed that the surface CoO, Co₃O₄ species are the active sites, and the bulk structure of the alloy, CoTi₂ phase also present a large effect on the catalytic activity in present oxidation. As a cheap, robust and separable catalyst as well as a green oxidant is used, the present work provides a facile and green catalytic process for the oxidation of hydrocarbons.

Full text of DOI:10.1016/j.apcata.2015.12.008

Applied Catalysis A: General 512 (2016) 9–14
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis of acetophenone from aerobic catalytic oxidation of
ethylbenzene over Ti–Zr–Co alloy catalyst: Influence of annealing
conditions
Tong Liua,b,c, Haiyang Chenga,b,∗, Lianshan Sund, Fei Liangd, Chao Zhanga,b
,
Zhong Yinga,b,c, Weiwei Lina,b, Fengyu Zhaoa,b,∗
a State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
b Laboratory of Green Chemistry and Process, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
c University of Chinese Academy of Sciences, Beijing 100049, PR China
d State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR
China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 October 2015
Received in revised form 1 December 2015
Accepted 7 December 2015
Available online 8 December 2015
The aerobic catalytic oxidation of ethylbenzene to produce acetophenone over an effective and robust
heterogeneous Ti–Zr–Co has been studied. Under the optimum conditions, a high selectivity of 69.2% to
acetophenone was obtained at a 61.9% conversion of ethylbenzene over Ti–Zr–Co catalyst, which is much
better than the most results reported with heterogeneous catalysts. The catalytic performance and the
active species of Ti–Zr–Co catalyst were discussed based on the changes of conversion and selectivity with
varying the annealing temperature and atmosphere. It is confirmed that the surface CoO, Co3O4 species
are the active sites, and the bulk structure of the alloy, CoTi2 phase also present a large effect on the
catalytic activity in present oxidation. As a cheap, robust and separable catalyst as well as a green oxidant
is used, the present work provides a facile and green catalytic process for the oxidation of hydrocarbons.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Aerobic oxidation
Ethylbenzene
Ti–Zr–Co alloy
Anneal
1. Introduction
ronmental benign compared to the chemical oxidants mentioned
above. Therefore, the aerobic catalytic oxidation is an important
The selective oxidation of hydrocarbon is of great importance in
petrochemical industry for that it can transform petroleum feed-
stocks such as alkanes, olefins, and aromatics into a plenty of
valuable organic chemicals, which are widely used in the prepa-
ration of polymer materials and fine chemicals etc. [1]. Generally,
the oxidants such as tert-butylhydroperoxide, hydrogen peroxide,
nitric acid, halides or ozone, etc. are used for the catalytic oxida-
tion of hydrocarbons [2–6]. However, from the points of industrial
application, these oxidants are expensive and un-environmentally
benign such as tert-butylhydroperoxide, nitric acid and halides
will leave large volume of organic waste, and hydrogen peroxide
decomposes easily into water which will reduce the activity of the
catalyst [7]. In term of eco-friendly sustainability, molecular oxygen
is an ideal terminal oxidant, as it is much cheaper and more envi-
and green chemical process for the chemical industrial fabrication
and it has attracted more attention in these years. Some effective
catalysts have been developed for aerobic catalytic oxidation. For
example, MnCP@SiO2 catalyst was studied for the aerobic selective
oxidation of ethylbenzene, in which the conversion of ethylbenzene
was 22.9% and the selectivity of acetophenone reached to 74.9%
at 100 ◦C [8]. Jin et al. reported a catalyst of hierarchically porous
aluminophosphate-based zeolite with AEL and AFI structures, with
it a 77.9% selectivity to acetophenone was obtained at a 14.8%
conversion of ethylbenzene at 140 ◦C, 3 MPa O2 in the absence of
solvent [9]. Most recently, the non-metal catalyst is of high popular
in the hydrocarbon oxidation, Luo et al. reported that carbon nano-
tubes were effective for the selective oxidation of ethylbenzene,
the selectivity of acetophenone reached to 60.9% at a conversion of
38.2% at 155 ◦C, 1.5 MPa in CH3CN [10]. Gao et al. reported that N-
doped materials were active for the oxidation of hydrocarbons and
alcohols [11]. In addition, they developed an effective palladium
catalyst of Pd@N-doped carbon for the aerobic selective oxida-
tion of ethylbenzene, in which the conversion of ethylbenzene was
14.2% and the selectivity of acetophenone reached up to 94% under
∗
Corresponding authors at: State Key Laboratory of Electroanalytical Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun
130022, PR China. Fax: +86 431 85262410.
E-mail addresses: hycyl@ciac.ac.cn (H. Cheng), zhaofy@ciac.ac.cn (F. Zhao).
http://dx.doi.org/10.1016/j.apcata.2015.12.008
0926-860X/© 2015 Elsevier B.V. All rights reserved.

10
T. Liu et al. / Applied Catalysis A: General 512 (2016) 9–14
atmosphere at 120 ◦C [12]. Chen et al. prepared Co
alyst by a thermal annealing carbonization strategy, it exhibited
a 74.8% selectivity to acetophenone at a 33.1% conversion for the
ethylbenzene oxidation at 120 ◦C, 0.8 MPa O2 [13].
N
C/CeO2 cat-
water-cooled cuprum hearth in a high-purity argon atmosphere at
250 A. To make the chemical compositions homogenous, the alloy
ingot was turned over and remelted at least three times. After that,
the surface of the cast ingot was burnished in order to eliminate
the oxide layer. Then the alloy ingot was crushed to powders by
repeated manual beating with a steel pestle and mortar, and sepa-
rated by 140 meshes screen, the powders with size smaller than 140
meshes (109 ␮m) were used for the reaction experiment. In some
cases, the Ti–Zr–Co catalyst, before use, was annealed at 450 ◦C or
750 ◦C for 2 h under N2 (denoted as Ti–Zr–Co-450-N2 and Ti–Zr–Co-
750-N2) or at 450 ◦C for 2 h under air (denoted as Ti–Zr–Co-450-air)
with a heating rate of 5 ◦C min−1 in a tube furnace to obtain different
treated Ti–Zr–Co catalysts. A CoTi2 alloy was prepared by a method
similar to the Ti–Zr–Co alloy catalyst as described above, except for
without Zr involved. Co3O4 was purchased from Sinopharm Chem-
ical Reagent Co., Ltd. (Beijing, China), is of analytical reagent. 7 wt.%
Co3O4/TiO2 was prepared according to literature [21].
The phase composition and microstructure of the alloys were
examined by X-ray diffraction (XRD) on a Bruker-AXS D8 ADVANCE
with K␣. The leaching of Ti, Zr or Co in the filtrate was not detected
by ICP-OES measurement (iCAP6300, Thermo USA). XPS measure-
ment was performed by using a VG Microtech 3000 Multilab, and
the spectra of Co 2p, Zr 3d, Ti 2p, O 1s and C 1s were recorded,
which were corrected to the C 1s peak at 284.6 eV. Scanning elec-
tron microscopy (SEM) image was performed on a Hitachi S-4800
field emission scanning electron microscope at an accelerating volt-
age of 10 kV, and the size of particles was in a range of 50–100 ␮m
(Fig. S1).
The selective oxidation of ethylbenzene to acetophenone is an
important reaction for that acetophenone is a very useful interme-
diate of the fine chemicals such as pharmaceuticals and perfume
[14,15]. Usually, acetophenone is commercially produced from cat-
alytic oxidation of ethylbenzene with molecular oxygen in the
presence of homogeneous cobalt based catalyst and additives
such as manganese and bromide species in an acetic acid solvent
[16–18]. Herein, we will report an effective heterogeneous cata-
lyst, Ti–Zr–Co metallic alloy, for production of acetophenone from
oxidation of ethylbenzene with molecular oxygen. In our previous
work, the Ti–Zr–Co alloy was found to be active and selective for
the oxidation of cyclohexane, in which cyclohexanol and cyclohex-
anone were produced with a high selectivity of 90% at a conversion
around 7% [19,20]. The Ti–Zr–Co catalyst is easy to handle and
transport, simple and cheap in production, sturdy to wearing in
the utilization, with comparing to those reported catalysts such
as metal-organic complex, metal nanoparticles and nanocarbon
materials. Therefore, it stimulates us to extend further study of
the Ti–Zr–Co alloy catalyst in the aerobic catalytic oxidations. The
catalyst showed a higher catalytic performance for the aerobic oxi-
dation of ethylbenzene, in which acetophenone was produced as
the main product with a selectivity of 69.2% at a 61.9% conver-
sion, which are much better than the results reported in literature.
The annealing temperature and atmosphere presented significant
effects on the oxidation of ethylbenzene, after annealing at 450 ◦C
in N2 flow, the activity of Ti–Zr–Co increased significantly with a
stable selectivity to acetophenone. The active species was discussed
in detail by experimental data and the characterizations of the cat-
alysts annealed under the different conditions. This study not only
presents an effective and robust catalyst for the selective oxida-
tion of ethylbenzene to acetophenone, but also gives a new insight
into the effect of surface phase composition and bulk structure of
Ti–Zr–Co alloy on the catalytic performance of aerobic oxidation.
2.2. Ethylbenzene oxidation
Ethylbenzene and acetonitrile purchased from Beijing chemical
plant are of analytical grade, and used without further purification,
gas of O2 (99.99%) (Changchun Xinxing Gas Company) is used as
delivered. Typically, ethylbenzene (5 mL), acetonitrile (5 mL) and
the Ti–Zr–Co catalyst (20 mg) were charged into a stainless steel
autoclave with a Teflon inner liner (50 mL) at room temperature,
then the reactor was sealed and preheated in an oil bath, after
the reactor was heated up to the desired temperature, O2 (2 MPa)
was introduced and the reaction was started with a continuously
stirring at 1200 rpm, and the reaction time was recorded. When
the reaction finished, the reactor was cooled to room temperature
and then depressurized carefully. The solid catalyst was separated
2. Experimental
2.1. Ti–Zr–Co alloy preparation and characterization
Ti–Zr–Co catalyst was prepared by arc-melting of Ti (99 wt.%), Zr
(97 wt.%) and Co (99 wt.%) metals with a mole ratio of 60:10:30 on a
Fig. 1. The effect of temperature on the oxidation of ethylbenzene. Reaction conditions: 5 mL ethylbenzene, 5 mL CH3CN, 20 mg Ti–Zr–Co alloy catalyst, 2 MPa O2, 3 h.

T. Liu et al. / Applied Catalysis A: General 512 (2016) 9–14
11
Table 1
The results for the ethylbenzene oxidation over Ti–Zr–Co catalyst in acetonitrile.
Entry
CH3CN (mL)
Conv. (%)
Sel. (%)
ACP
PEA
Othersa
1
2
3
4
5
6
–
3
5
7
9
15
13.8
33.9
36.2
35.4
33.7
26.8
73.4
57.9
65.1
60.9
65.0
62.0
16.6
12.2
13.4
14.0
12.9
12.9
10.0
29.9
21.5
25.1
22.1
25.1
Reaction conditions: 5 mL ethylbenzene, 20 mg Ti–Zr–Co alloy catalyst, 2 MPa O2,
160 ◦C, 3 h.
a
Others are the main byproducts of benzaldehyde and benzoic acid.
Table 2
Effects of annealing temperature and atmosphere.
Entry
Catalyst
Conv. (%)
Sel. (%)
ACP
PEA
Others
1
2
3
4
Ti–Zr–Co
36.3
48.7
60.3
25.7
65
13.4
11.2
9.76
9.31
21.6
37.7
30.14
36.19
Ti–Zr–Co-450-air
Ti–Zr–Co-450-N2
Ti–Zr–Co-750-N2
51.1
60.1
54.5
Reaction conditions: 5 mL ethylbenzene, 5 mL CH3CN, 20 mg catalyst, 160 ◦C, 2 MPa
O2, 3 h.
out by filtration, and then the reaction solution was diluted with
ethanol to 50 mL, the composition of reaction products was con-
firmed by gas chromatography/mass spectrometry (GC/MS, Agilent
5890) and analyzed with toluene as an internal standard by gas
chromatograph (Shimadzu GC-2010, FID, capillary column, RTX-50,
30 m × 0.25 mm × 0.25 ␮m).
Fig. 2. Changes of conversion and selectivity with reaction time in the oxidation of
ethylbenzene at 160 ◦C (a) and 170 ◦C (b). Reaction conditions: 5 mL ethylbenzene,
5 mL CH3CN, 20 mg Ti–Zr–Co alloy catalyst, 2 MPa O2.
3. Results and discussion
3.1. Effects of solvent, reaction temperature and time
further increase, the concentration of ethylbenzene decreases and
the dilution effects will appear, resulting in a decrease of conver-
sion. In the presence of acetonitrile, the selectivity of the main
product of acetophenone is around 60%, which is a little lower than
the result (73.4%) obtained in the absence of solvent. It is ascribed
to that the presence of acetonitrile could accelerate the reactions
of ehylbenzene to acetophenone and 1-phenethanol, and acceler-
ate their further oxidation to benzaldehyde and benzoic acid also.
Temperature has a remarkable influence on the oxidation reac-
tion as seen in Fig. 1. Generally, high temperature is favor for the
proceeding of the oxidation of hydrocarbon, as expected, the con-
version increases with reaction temperature, and it increases from
23.7% to 70.9% with enhancing temperature from 150 ◦C to 180 ◦C.
While, the selectivity of acetophenone increases initially and then
decreases with further raising temperature, and it presents an
optimum value of 73.7% at 170 ◦C, at which the total selectivity
of both acetophenone and 1-phenethanol reached to 82.6%. With
further enhancing temperature, the selectivity of acetophenone
decreases to 60.2%, as the deep oxidation of both acetophenone and
1-phenethanol to benzaldehyde and benzoic acid occurs. ) and XPS
(Fig. 4 In addition, the changes of the conversion and selectivity
with the reaction proceeding were checked at 160 ◦C and 170 ◦C,
respectively. As shown in Fig. 2, the conversion increases largely
with extending reaction time up to 6 h and then it keeps at a con-
stant level with further extending to 12 h at 160 ◦C, it suggested that
the reaction has reached chain-termination after proceeding for 6 h
(Fig. 2a). When the reaction was performed at 170 ◦C (Fig. 2b), the
reaction reaches the chain-termination within 4.5 h, as the conver-
sion increases at the first 4.5 h, and then increases very less at the
stage of 4.5–6 h. In addition, the selectivity of acetophenone reaches
The effects of solvent, reaction temperature, and reaction time
on the ethylbenzene oxidation over the fresh Ti–Zr–Co alloy were
discussed. Acetophenone (ACP) was produced as the main prod-
uct with accompanying of 1-phenethanol (PEA) and benzaldehyde
(BA), and benzoic acid (BC) was also detected in the products.
Firstly, the efficiency of several organic solvents such as benzene,
ethanol, acetonitrile and acetone was examined for the oxidation
of ethylbenzene (Table S1). Compared to the case of reaction in
the absence of solvent, the conversion increased, however, the
selectivity to acetophenone and 1-phenethanol decreased with a
different range in the presence of organic solvents as the formation
of undesirable over-oxidized products like acetic acid, esters etc.
Among these solvents, the acetonitrile presented to be more effec-
tive for the present ethylbenzene oxidation, the conversion was
improved more than two times and the selectivity reduced slightly
with comparing to the case in the absence of solvent, resulting in
the yield of acetophenone and 1-phenethanol increased to 28.4%
from 12.4%. Therefore, acetonitrile was selected as solvent in the
following studies. Table 1 shows the effect of the amount of ace-
tonitrile on the conversion and the product selectivity. It is clear
that the conversion increased largely in the presence of acetonitrile,
and changed slightly with the amount of acetonitrile at the range of
3–9 mL. But the conversion decreased with further increasing of the
amount of acetonitrile to 15 mL, it decreased to 26.8%. These results
are in agreement to the literature [22,23]. These will be ascribed to
the solubility of O2 and the dilution effect of acetonitrile [24,25].
That acetonitrile has a better ability to dissolve O2, thus with its
presence, the content of O2 will increase in the reaction solu-
tion, and so the reaction conversion increases. However, with its

12
T. Liu et al. / Applied Catalysis A: General 512 (2016) 9–14
Table 3
The bulk phase and the surface composition based on XRD and XPS measurements.
Entry
Catalyst
Bulk phasea
Surface compositionb
Co species molar ratio (%)b
Co
CoO
Co3O4
1
2
3
4
5
CoTi2
Ti–Zr–Co
Ti–Zr–Co-450-air
Ti–Zr–Co-450-N2
Ti–Zr–Co-750-N2
CoTi2
CoTi2
CoTi2
CoTi2
Co, CoO, Co3O4, TiO2, ZrO2
Co, CoO, Co3O4, TiO2, ZrO2
Co3O4, TiO2, ZrO2
Co, CoO, Co3O4, TiO2, ZrO2
Co3O4, TiO2, ZrO2
31.5
20.1
0
25.7
0
37.8
32.8
0
51.4
0
30.7
47.1
100
22.9
100
Co3O4, TiO2, CoTiO3
a
The bulk phases of the alloy catalysts were obtained based on the XRD patterns.
The surface compositions of the alloy catalysts were obtained based on the XPS spectra.
b
Fig. 3. XRD patterns (left) and a partial enlarged drawing (right) of (a) the CoTi2 alloy and (b) the fresh Ti–Zr–Co alloy catalyst, (c) Ti–Zr–Co-450-air, (d) Ti–Zr–Co-450-N2,
(e) Ti–Zr–Co-750-N2, and the JCPDS 07-0141 file identifying CoTi2. (ꢀ: laves phase; ᮀ: Co3O4; ♦: CoTiO3; ꢀ: TiO2).
73.7% after reaction performed for 3 h, and then decreases with
extending reaction time, especially after the conversion of ethyl-
benzene reaches chain-termination, the selectivity decreases from
69.2% at 4.5 h to 61.3% at 6 h. By contrast, the selectivity of acetophe-
none decreases much slowly at 160 ◦C, from 65.1% at 6 h to 59.7%
at 12 h. It is clearly that the oxidation proceeds much faster at the
higher temperature, including the deep oxidation of acetophenone.
The best results for the ethylbenzene oxidation over the Ti–Zr–Co
catalyst were obtained at the conditions of 170 ◦C, 2 MPa, 4.5 h, at
which the conversion of ethylbenzene is 61.9% and the selectivity
of acetophenone reaches up to 69.2%, these are comparable to the
best results reported in the latest literature (Table S2).
3.2. Effects of annealing temperature and atmosphere
The annealing temperature and atmosphere present large
effects on the conversion and product selectivity as shown in
Table 2. As Ti–Zr–Co-450-air calcined at 450 ◦C in air, the con-
version (48.7%) increased largely with comparing to the fresh one
(36.3%) (Entries 1,2). For Ti–Zr–Co-450-N2 annealed at 450 ◦C in
N2, the conversion increased further to 60.3% (Entry 3). However,
as Ti–Zr–Co-750-N2 was annealed at 750 ◦C in N2 flow, the con-
version decreased sharply to 25.7% (Entry 4). It is clear that the
annealing temperature and atmosphere have a significant effect
on the conversion. Thus, the structure and surface composition of
Fig. 4. The XPS spectra of Co 2p3/2 of (a) CoTi2 alloy and (b) fresh Ti–Zr–Co alloy
catalyst, (c) Ti–Zr–Co-450-air, (d) Ti–Zr–Co-450-N2, (e) Ti–Zr–Co-750-N2.
these catalysts may affect the catalytic activity markedly, which
were well characterized and discussed.
3.3. The relation between structure and catalytic performance
Ti–Zr–Co alloy catalyst give the typical diffraction peaks of the
intermetallic compound of CoTi2 phase (Fig. 3a,b). The diffraction
peaks of Ti–Zr–Co shift slightly to the small angles with comparing
to the prepared CoTi2 alloy, which is ascribed to that the addi-
tive element Zr has a much larger atomic radius than Ti and Co.
In addition, the addition of Zr also caused the diffraction peaks to
broaden for that the insertion of Zr atom can separate the grain
To study the structure or surface composition of the fresh and
annealing Ti–Zr–Co catalysts and their relationship to the catalytic
performance, the crystal structure and surface composition or elec-
tronic states of these samples were characterized with XRD (Fig. 3).
It is obvious that both the prepared CoTi2 alloy and the fresh

T. Liu et al. / Applied Catalysis A: General 512 (2016) 9–14
13
Fig. 5. Possible reaction processes for aerobic oxidation of ethylbenzene over the surface of Ti–Zr–Co catalyst.
CoTi2, resulting in a highly dispersion of the smaller CoTi2 crys-
tal grain in Ti–Zr–Co (Fig. 3a,b). At a annealing temperature of
450 ◦C in air (Ti–Zr–Co-450-air) and N2 (Ti–Zr–Co-450-N2), the
main bulk is composed of CoTi2 phase (Fig. 3c,d), and a laves phase
presents in the samples of Ti–Zr–Co-450-air, Ti–Zr–Co-450-N2 and
the prepared CoTi2 alloy (Fig. 3a,c,d). However, at the high anneal-
ing temperature of 750 ◦C in N2 flow (Ti–Zr–Co-750-N2), some new
crystal phases such as Co3O4, TiO2 and CoTiO3 were formed, while
the CoTi2 and the laves phase disappeared (Fig. 3e).
As the heterogeneous catalytic reaction is occurring on the sur-
face of catalyst, thus the effective and active species should locate
on the surface of solid catalyst. Thus, we further characterized the
surface composition for the catalysts. The Co 2p XPS spectra are
shown in Fig. 4. The Co, Co2+ and Co3+ species are found in the
samples containing Co elements, CoO is confirmed by the intense
shake-up satellites around 6 eV above the primary spin-orbit Bes
[26–28], and Co3O4 is confirmed by Co3+/Co2+ (2/1) [29]. There-
fore, the Co metal, CoO and Co3O4 species are co-existence on the
surface of the CoTi2 alloy, the fresh Ti–Zr–Co and Ti–Zr–Co-450-N2
(Fig. 4a,b,d; Table 3, entries 1,2,4). While, the sole Co3O4 species
is found on the surface of Ti–Zr–Co-450-air and Ti–Zr–Co-750-N2
(Fig. 4c,e; Table 3, entries 3,5), because the Co0 and CoO species
were transformed to the Co3O4 species when calcinated in air or
at the high temperature (750 ◦C) in N2. Moreover, Ti and Zr species
are mainly existence with TiO2 and ZrO2 on the surface of catalysts.
For Ti species, the peaks of Ti 2p move slightly to the higher bind-
ing energy for all the samples annealed in air and N2 flow (seen
in Fig. S2), indicating their surfaces are of little electron deficient
compared with the fresh one.
and the same Co species such as Co, CoO and Co3O4 on their surface
(Table 3, entries 1,2). Compared with the CoTi2 alloy, the addition
of Zr with high oxygen affinity will produce much more surface
defect for that the large atom radius Zr will make Co oxidize easily
to Co3O4 (Table 3, entries 1,2), indicating that the Co3O4 species
may be one of the main catalytic active species. Thus, Ti–Zr–Co
presented higher activity compared to the CoTi2 alloy. Based on
the analysis and comparison of the activities and phase composi-
tions among the samples (Table 3, entries 2–5), the CoO species
should also play an important role in the present oxidation. More-
over, the bulk CoTi2 phase may own a promoting function in present
oxidation as comparison Ti–Zr–Co-450-air with Ti–Zr–Co-750-N2,
although they are the same in Co species composition, but different
in the bulk phases, and Ti–Zr–Co-450-air with CoTi2 phase presents
higher activity than Ti–Zr–Co-750-N2 without CoTi2 phase (Table 3,
entries 3,5). The function of the bulk CoTi2 phase is also demon-
strated by following results. Co3O4 is usually an effective oxidation
catalyst, but both the pure Co3O4 and Co3O4/TiO2 presents lower
activity than Ti–Zr–Co-450-air (Table 4, entries 3,4 vs. Table 2, entry
2), as the CoTi2 phase exists in Ti–Zr–Co-450-air but not in Co3O4
and Co3O4/TiO2. According to the XPS analysis, the electron may
transfer from the surface TiO2 species to the bulk CoTi2 phase, and
which benefits for the present oxidation (Fig. S2). Therefore, both
CoO and Co3O4 are the major active species, and the bulk CoTi2
phase may affect the activity through the electron transfer on the
surface of Ti–Zr–Co catalysts. Ti–Zr–Co-450-N2 shows the highest
catalytic activity due to its most abundant surface CoO species and
the presence of the bulk CoTi2 phase.
The activity of the CoTi2 alloy is as low as the blank reaction
(Table 4, entries 1,2). However, the activity of the fresh Ti–Zr–Co
alloy is much higher than the CoTi2 alloy (Table 2, entry 1 vs. Table 4,
entry 2), although they have the same bulk structure as CoTi2 phase
3.4. Possible reaction processes
Generally, the aerobic oxidation of hydrocarbons can sponta-
neously occur by a radical reaction in some cases without catalyst

14
T. Liu et al. / Applied Catalysis A: General 512 (2016) 9–14
Table 4
Acknowledgements
Comparison of the catalytic performances of different catalysts in the ethylbenzene
oxidation.
The authors gratefully acknowledge the financial support from
the One Hundred Talent Program of CAS and Youth Innovation
Promotion Association CAS. And we also thank Prof. Limin Wang
of the State Key Laboratory of Rare Earth Resources Utilization,
Changchun Institute of Applied Chemistry, CAS, for his helpful dis-
cussion.
Entry
Catalyst
Conv. (%)
Sel. (%)
ACP
PEA
Others
1
2
3
4
–
19.4
19.5
43.5
27.7
53.2
71.9
63
14.2
12.3
12.5
15.9
32.6
15.8
24.5
22.7
CoTi2
Co3O4
Co3O4/TiO2
61.4
Reaction conditions: 5 mL ethylbenzene, 5 mL CH3CN, 20 mg catalyst, 160 ◦C, 2 MPa
O2, 3 h.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2015.12.
008.
[30,31]. But the reaction is very slow therefore the researchers
make their best effort to develop effective catalysts to accelerate
the reaction. As reported in literature, the oxidation of ethylben-
zene could occur automatically somewhat at certain conditions in
the absence of catalyst [32]. Herein, it was also found that the oxi-
dation of ethylbenzene can occur without addition of catalyst, but
it is much lower (Table 3 entry 1). Ti–Zr–Co was effective for the
present ethylbenzene oxidation, with it the reaction was promoted
largely.
References
[1] A.K. Suresh, M.M. Sharma, T. Sridhar, Ind. Eng. Chem. Res. 39 (2000)
3958–3997.
[2] B. Sarkar, P. Prajapati, R. Tiwari, R. Tiwari, S. Ghosh, S.S. Acharyya, C. Pendem,
R.K. Singha, L.N.S. Konathala, J. Kumar, T. Sasaki, R. Bal, Green Chem. 14 (2012)
2600–2606.
[3] L. Chen, B.D. Li, D.B. Liu, Catal. Lett. 144 (2014) 1053–1061.
[4] Y. Wang, J.S. Zhang, X.C. Wang, M. Antonietti, H.R. Li, Angew. Chem. Int. Ed. 49
(2010) 3356–3359.
[5] K.N. Bhagya, V. Gayathri, J. Porous Mater. 21 (2014) 197–206.
[6] Y.B. He, Z.B. Rui, H.B. Ji, Catal. Commun. 14 (2011) 77–81.
[7] B. Sundaravel, C.M. Babu, B. Palanisamy, M. Palanichamy, K. Shanthi, V.
Murugesan, J. Nanosci. Nanotechnol. 13 (2013) 2507–2516.
[8] D.H. Shen, L.T. Ji, Z.G. Liu, W.B. Sheng, C.C. Guo, J. Mol. Catal. A: Chem. 379
(2013) 15–20.
[9] Y.Y. Jin, X. Chen, Q. Sun, N. Sheng, Y. Liu, C.Q. Bian, F. Chen, X.J. Meng, F.S. Xiao,
Chem. Eur. J. 20 (2014) 17616–17623.
[10] J. Luo, F. Peng, H. Yu, H.J. Wang, W.X. Zheng, ChemCatChem 5 (2013)
1578–1586.
[11] Y.J. Gao, G. Hu, J. Zhong, Z.J. Shi, Y.S. Zhu, D.S. Su, J.G. Wang, X.H. Bao, D. Ma,
Angew. Chem. Int. Ed. 52 (2013) 2109–2113.
[12] P.F. Zhang, Y.T. Gong, H.R. Li, Z.R. Chen, Y. Wang, Nat. Commun. 4 (2013) 1593.
[13] Y. Chen, S.F. Zhao, Z.G. Liu, Phys. Chem. Chem. Phys. 17 (2015) 14012–14020.
[14] S. Devika, M. Palanichamy, V. Murugesan, Appl. Catal. A: Gen. 407 (2011)
76–84.
Herein, a radical reaction mechanism for the ethylbenzene oxi-
dation over the Ti–Zr–Co catalyst was proposed (Fig. 5), that oxygen
firstly is adsorbed onto the surface of the solid catalyst and acti-
vated, and then the activated oxygen reacts with the activated
ethylbenzene to produce 1-phenylethyl hydroperoxide [32]. In
the absence of an initiator system, the homolytic dissociation of
hydroperoxide molecules is considered to be the dominant chain-
initiation step, which dissociates to alkoxy and hydroxy radicals
[30]. These radicals react rapidly with substrate to form alkyl rad-
icals for lighting off the propagation reactions. Molecular oxygen
is added to alkyl radicals before forming peroxyl radicals, which
afterwards abstracted H-atoms from the 1-phenylethyl hydroper-
oxide to produce acetophenone and hydroxy radicals. The radicals
will react with substrate and oxygen for recycle to produce peroxyl
radicals continuously. Phenylethyl hydroperoxide reacts with the
alkyl radicals to generate alkoxy radical and 1-phenethanol also. 1-
Phenethanol could react with peroxyl radicals and oxygen to form
1-phenethanol peroxyl radicals, and then reversibly produce ace-
tophenone [32]. It can explain that acetophenone is produced as the
main product other than the same proportion of alcohol and ketone
[33,34]. These alkoxy radicals can abstract H-atoms from the ethyl-
benzene substrate in chain-propagation reactions or decompose
via ␤-scission to benzaldehyde. Benzaldehyde will be promptly
further oxidized to benzoic acid in the presence of oxygen [10].
[15] G.Y. Yang, Y.F. Ma, J. Xu, J. Am. Chem. Soc. 126 (2004) 10542–10543.
[16] J.Y. Qi, H.X. Ma, X.J. Li, Z.Y. Zhou, M.C.K. Choi, A.S.C. Chan, Q.Y. Yang, Chem.
Commun. (2003) 1294–1295.
[17] C. Ricca, F. Labat, N. Russo, C. Adamo, E. Sicilia, J. Phys. Chem. C 118 (2014)
12275–12284.
[18] G.X. Li, Y.Z. Li, R.N. Mu, Y.D. Xu, P. Dong, React. Kinet. Mech. Catal. 109 (2013)
199–212.
[19] J.M. Hao, B.Z. Liu, H.Y. Cheng, Q. Wang, J.Y. Wang, S.X. Cai, F.Y. Zhao, Chem.
Commun. (2009) 3460–3462.
[20] J.M. Hao, J.Y. Wang, Q. Wang, Y.C. Yu, S.X. Cai, F.Y. Zhao, Appl. Catal. A: Gen.
368 (2009) 29–34.
[21] Q. Yang, H. Choi, D.D. Dionysiou, Appl. Catal. B: Environ. 74 (2007) 170–178.
[22] M.R. Maurya, M. Kumar, U. Kumar, J. Mol. Catal. A: Chem. 273 (2007) 133–143.
[23] K.N. Bhagya, V. Gayathri, J. Porous Mater. 19 (2012) 1037–1045.
[24] E.L. Pires, J.C. Magalhaes, U. Schuchardt, Appl. Catal. A: Gen. 203 (2000)
231–237.
4. Conclusions
[25] R.J. Correa, G.C. Salomao, M.H.N. Olsen, L. Cardozo, V. Drago, C. Fernandes,
O.A.C. Antunes, Appl. Catal. A: Gen. 336 (2008) 35–39.
[26] D. Barreca, C. Massignan, S. Daolio, M. Fabrizio, C. Piccirillo, L. Armelao, E.
Tondello, Chem. Mater. 13 (2001) 588–593.
[27] D.H. Kim, S.Y. Lee, J.E. Jin, G.T. Kim, D.J. Lee, Phys. Chem. Chem. Phys. 16 (2014)
6980–6985.
[28] F. Yang, J.Y. Yao, F.L. Liu, H.C. He, M. Zhou, P. Xiao, Y.H. Zhang, J. Mater. Chem.
A 1 (2013) 594–601.
[29] S. Todorova, H. Kolev, J.P. Holgado, G. Kadinov, C. Bonev, R. Pereniguez, A.
Caballero, Appl. Catal. B: Environ. 94 (2010) 46–54.
[30] I. Hermans, P.A. Jacobs, J. Peeters, J. Mol. Catal. A: Chem. 251 (2006) 221–228.
[31] U. Neuenschwander, I. Hermans, J. Catal. 287 (2012) 1–4.
[32] I. Hermans, J. Peeters, P.A. Jacobs, J. Org. Chem. 72 (2007) 3057–3064.
[33] I. Hermans, T.L. Nguyen, P.A. Jacobs, J. Peeters, ChemPhysChem 6 (2005)
637–645.
In summary, a Ti–Zr–Co catalyst was studied at the first time for
the oxidation of ethylbenzene. High selectivity (69.2%) to acetophe-
none was obtained at a high conversion (61.9%) of ethylbenzene.
The annealing temperature and atmosphere of the Ti–Zr–Co alloy
catalyst presented significant effects on the oxidation of ethylben-
zene, due to the change between the surface species and the bulk
phase. CoO and CO3O4 on the surface are the main active species
and the bulk CoTi2 phase also plays a very important role in acceler-
ating the reaction rate for the present ethylbenzene oxidation. The
addition of Zr could separate the CoTi2 grains and thus increased
its dispersion and make Co be oxidized easily to CoO and/or Co3O4.
The present catalytic system presents a promising heterogeneous
catalyst for the aerobic oxidation of hydrocarbons, its extensive
applications in the aerobic oxidation of other hydrocarbons is worth
expected.
[34] I. Hermans, P.A. Jacobs, J. Peeters, Chem. Eur. J. 12 (2006) 4229–4240.