DSpace at VNU: Synthesis and study on catalytic activity of spinel metallic oxides in styrene preparation from ethylbenzene

e-Journal of Surface Science and Nanotechnology

23 June 2012

Conference - IWAMN2009 -

e-J. Surf. Sci. Nanotech. Vol. 10 (2012) 263-267

Synthesis and Study on Catalytic Activity of Spinel Metallic Oxides in Styrene
Preparation from Ethylbenzene∗
Le Thanh Son,† Hoa Huu Thu, Nguyen Thanh Binh, Tran Thi Nhu Mai, and Nguyen Hong Vinh
Department of Petroleum Chemistry, Faculty of Chemistry,
Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
(Received 6 December 2009; Accepted 21 December 2011; Published 23 June 2012)
A series of spinel oxides AB2−x B’x O4 /γ-Al2 O3 (A: Ni, Cu; B: Cr; B’: Fe and x = 0, 0.5, 1, 1.5, 2) were synthesized
by two methods: solid-state reaction and coprecipitation. The oxides obtained were characterized by XRD, SEM
and BET to determinate their textural and structural properties. Their catalytic activity was evaluated by reaction
of oxidative dehydrogenation of ethylbenzene to styrene. The XRD showed the spinel phase formed for all oxides
synthesized by two methods. However, the coprecipitation method seems to be more favorable for formation
of spinel phase. All samples showed a high catalytic activity and selectivity for oxidative dehydrogenation of
ethylbenzene to styrene, especially, in the case of NiCr2−x Fex O4 obtained by coprecipitation method.
[DOI: 10.1380/ejssnt.2012.263]
Keywords: Nano spinel; Ethylbenzene; Dehydrogenation

I.

INTRODUCTION

Recent years, the styrene quantity consumed is increasing as start materials to synthesize the polymers and
copolymers. The worldwide capacity for production of
stryrene is approximately 15.106 t/year [9]. Stryrene is

produced by two processes: (i) dehydrogenation of ethylbenzene and (ii) as a by-product in the epoxidation of
propene with ethylbenzene hydroperoxide and Molybdenum complex-based catalysts [1]. The ethylbenzene dehydrogenation is similar to the hydrogenation of alkanes. The actual ethylbenzene dehydrogenation process
is highly endothermic, reversible and needing reactant recycle, high steam-to-ethylbenzene ratios. So, it needs the
presence of catalysts. The traditional catalysts for ethylbenzene dehydrogenation are iron oxides promoted by alkali metal ions [3, 4, 6]. However, it is observed a slight
irreversible deactivation of the catalysts with usage because of migration of potassium from the styrene to the
bulk [4, 7]. That is why, catalyst research for ethylbenzene dehydrogenation has been of interest to many chemical manufactures, at the same time, many techniques
have been proposed to find out a best solution producing
styrene. These techniques are the following alternative
ones:
• Ethylbenzene dehydrogenation followed by oxidation of hydrogen in order to furnish the heat of reaction to the former and shift the reaction equilibrium
toward the right, styrene formation.
• Oxidative dehydrogenation in order to realize an
exothermic reaction and shift the reaction equilibrium toward the product formation and to carry out
the reaction at lower temperature.

∗ This paper was presented at the International Workshop on Advanced Materials and Nanotechnology 2009 (IWAMN2009), Hanoi
University of Science, VNU, Hanoi, Vietnam, 24-25 November, 2009.
† Corresponding author:

• Membrane catalysis in order to shift the equilibrium and to carry out the reaction at lower temperature [1].
Table I summarizes the catalytic performances obtained
with the different techniques, inside the ethylbenzene dehydrogenation is the only process widely used at a commercial level [8].
In recent years, the spinel metallic oxide having move
activity for dehydrogenation and oxidative dehydrogenation of ethylbenzene to styrene is reported [5? ]. Spinel
oxides having cation distribution in the planes of (110)
and (111) showed high catalytic activity for dehydrogenation of hydrocarbon, isopropanol, cyclohexanol [1]. In
the present investigation, we have prepared several series
of spinel oxides AB2−x B’x O4 (A=Ni2+ , Cu2+ , B=Cr3+ ,
B’=Fe3+ ), determined textural and structural characteristics and evaluated their catalyst ability for oxidative dehydrogenation of ethylbenzene to styrene.

II.

EXPERIMENTAL

A.

Spinel preparation

There have been a lot of methods to prepare spinel
materials. Here, we have used two methods of preparing
the spinels AB2−x B’x O4 (x = 0, 0.5, 1.0, 1.5, 2.0), which
are described in the following. Details of samples used in
the present study are summarized in Table II.

1.

Solid-state reaction method

In this method, iron (III) oxide, chromium (III)
nickel oxide all in PA, were used as sources of metallic irons in spinel ternary structure NiCr2−x Fex O4 (x =
0, 0.5, 1.0, 1.5, 2.0). Spinel NiCr2−x Fex O4 was prepared
as following: first, the quantities calculated in advance of
the oxides above were mixed carefully in a porcelain mortar for 30 minutes. Then, the powder mixture granulated
at pressure of 2.000 N/cm2 . Transfer the granules in a

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Le, et al.

Volume 10 (2012)

TABLE I: Comparison of the catalytic performances for different technique of styrene synthesis from ethylbenzene.
Techniques
Dehydrogenation
Dehydrogenation/H2 Oxidative
Oxidative Dehydrogenation
Oxidative Dehydrogenation
Membrane technique

Selectivity in styrene (%)
90
90
90
90
94

Reaction temperature (◦ C)
600-650
605
350
480
625

Catalyst
Fe/K/Cr/O
SMART technology
Carbon molsieve

V/Mg/O
Fe/K/Li/Cr/O

States
Industrial
Commercial
Research
Research
Research

scheme (fig.1)

TABLE II: Spinel samples obtained by different methods.
Sample
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

Method of
preparation
NiCr2 O4
NiCr1.5 Fe0.5 O4
Solid-state reaction
NiCr0.5 Fe1.5 O4
NiCr2 O4
NiCr2 O4
NiCr1.5 Fe0.5 O4
Coprecipitation
NiCr0.5 Fe1.5 O4
NiCr2 O4
CuCr2 O4
CuCr1.5 Fe0.5 O4
Coprecipitation
CuCr0.5 Fe1.5 O4
CuCr2 O4

Spinels
Spinel formula
Sign
NC 1 (I)
NCF 2 (I)
NiCrFeO4
NCF 3 (I)
NCF 4 (I)
NF 5(II)
NC 1 (I)
NCF 2 (II)

NiCrFeO4
NCF 3 (II)
NCF 4 (II)
NF 5(II)
CC 1 (I)
CCF 2 (II)
CuCrFeO4
CCF 3 (II)
CCF 4 (II)
CF 5(II)

cup and place this cup in a furnace, heat the electrical
furnace at 1300◦ C for 4 hours. We cooled the solid obtained in desiccators and cracked them into small grains
of 0.1-1.0 mm in diameter as catalyst grains.
The reaction of spinel formation at 1300◦ C is generally
represented after the following equation:
2NiO + xFe2 O3 + (2 − x)Cr2 O3 → 2NiCr2−x FeO4 (1)
In the case of x = 0, the reaction is as follows:
NiO + Cr2 O3 → NiCr2 O4

(2)

As a comparison, we also prepared two series of spinels:
NiCr2−x Fex O4 and CuCr2−x Fex O4 by coprecipitation
method, using the sources of respective metal nitrates.
2.

Figure 1: Scheme of formation of ternary spinels NiCr2-xFexO4 at high temperature

FIG. 1: Scheme of formation of ternary spinels NiCr2−x Fex O4

at high temperature.

4
the precipitate was dried at 120◦ C for 6 hours in order
to eliminate the adsorbed water and form links of metaloxygen-metal existing in the solid mass obtained. Finally,
the solid was calcined at 750◦ C for 4 hours. By these ways,
we have obtained the following spinel.

B.

X-ray diffraction (XRD) patterns were recorded for all
samples of spinel obtained on a SIEMENS D5000 diffractometer single X-ray with wavelength of 1.5406 ˚
A. Scanning electron microscope, SEM image were performed several samples representative. Infrared (IR) spectra for
all samples were measured on a Fourier transform IR
spectrometer (Nicolet 760 Magara, Japan). Specific surface of samples was determined by nitrogen adsorptiondesorption at −196◦ C on Autosorb01 equipment.

Coprecipitation method
C.

This is a simple method and very favorable in making the ternary spinels.
Here, we have used the
source of metallic irons under from of their nitrates:
Ni(NO3 )3 ·6H2 O, Cu(NO3 )2 ·6H2 O. Cr(NO3 )3 ·9H2 O and
Fe(NO3 )3 ·9H2 O, all in PA (Aldrich).
The spinel
NiCr2−x Fex O4 and CuCr2−x FexO4 were prepared as follows: first, the quantities of metallic salts after the general formula of spinel and weighed were dissolved in the
10% salts solution. The obtained solution was mixed and
heated at 80◦ C. Then, 5% NH4 OH solution was added in
the last solution until pH=7. This one was maintained
at 80◦ C for 5 hours in order to precipitate completely the

desired solid. The precipitate was filtered and washed
with distillated water until absence of NO−
3 ions. Then,
264

Characterization

Reaction system and analysis of liquid products
obtained

The reaction of oxidative dehydrogenation was carried
out in the vapor phase in a fixed bed flow type reactor
consisting of a quarts tube in which the catalyst bed was
placed in the middle of the tube. The reactor was heated
by electricity and controlled by digital temperature controller. The temperature was measured by thermocouple
placed in the center of the catalyst bed. The reactants
were fed into the catalyst bed by a syringe infusion pump
following the ethylbenzene flow rate desired. The liquid products collected for the first 30 min were discarded
and analyzed on Gas Chromatography-Mass spectroscopy
(GC-MS HP 6890).

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Volume 10 (2012)

(a)of splitting hydride on metallic sites Fe or Cr (Me ):
Process

Break
of C-C bond:
(b)

FIG. 2: XRD patterns of spinels NiCr2−x Fex O4 (x = 0, 0.5,
1.0, 1.5, 2.0) obtained by solid-state reaction method.

-

(c)
(a)

(b)




FIG. 4: Schemes of the monomolecular reaction after the
mechanism of Langmuir-Hinshelwood to form the reaction
Figure3: SEM images of spinel samples: (a) sample NiCr O obtained by solid-stateproducts. (a) Process of splitting hydride on metallic sites
Fe3+ or Cr3+ (Me3+ ); (b) Breaking of C–C bond; (c) OxidaFIG. 3: SEM images of spinel samples: (a) sample NiCr2 O4
tive dehydrogenation of intermediate.
obtained by solid-state reaction method; (b) sample NiCr2 O4
obtained by coprecipitation method.

III.

RESULTS AND DISCUSSIONS

The solid solution of metallic oxides mixture having
spinel structure or the ceramic materials are often prepared to suit their applications. Generally, the spinel solid
solutions are formed at different temperatures according

to chemical precursors used for preparing spinels desired.
In the solid-state reaction method, the precursors are all
metallic oxides, the reaction temperature is used being
1300◦ C.
2NiO + xFe2 O3 + (2 − x)Cr2 O3 → 2NiCr2−x Fex O4
(x = 0, 0.5, 1.0, 1.5, 2.0)
(3)

In these reactions, NiO existing at solid state with
body-centered cubic structure coordination number of
Ni2+ , O2− ions being 6; Cr2 O3 and Fe2 O3 having hexadiTABLE III: Characteristic absorption bands in IR region of
rection structure, while the spinels NiCr2−x Fex O4 represpinel samples.
sented face-centered cubic structure. So, the formation
Absorption bands in the IR of samples (cm−1 )
of spinels is easy because of their structure being apSpinels
Vibration of Vibration of Presence Presence proachable although the reaction temperature 1300◦ C was
tetrahedral
octahedral of nitrate of water far from their fusion temperature. In the reaction prometal-oxygen metal-oxygen
NO−
∼1670 [3]
3
cess, the ions consisting of anion O2− and cation Ni2+ ,
bond ∼620 [3] bond ∼530 [3] ∼1390 [3]

Cr3+ and Fe3+ at different phase interface of the oxNC 1 (I)
600
447
—
—
ides NiO, Cr2 O3 , Fe2 O3 diffuse one an other resulting
NCF 2 (I)
638
495
—
—
spinel structure. This can be imagined after the scheme
NCF 3 (I)
627
466
—
—
shown in Fig. 1. Thus, all XRD patterns of five samNCF 4 (I)
633
433
—
—
ples NC 1(I), NCF2(I), NCF3(I), NCF4(I) and NF5(I)
NF 5(II)
627
423
—
—
(Fig. 2), demonstrated that the spinels NiCr2−x Fex O4
NC 1 (I)

610
517
—
1634
NCF 2 (I)
604
507
1378
1664
(x = 0, 0.5, 1.0, 1.5, 2.0) were formed.
NCF 3 (I)
664
554
1344
1638
This solid-state reaction process can be analogous to
NCF 4 (I)
621
514
1439
—
crystallization one of spinels through reorganization of
NF 5(II)
—
—
—
—
metallic cation Cr3+ and Fe3+ in the octagonal sites and
CC 1 (I)
630

528
—
1644
Ni2+ in the tetragonal sites of face-centered cubic strucCCF 2 (II)
620
456
1342
1634
ture. This favor formation of big crystals. SEM image of
CCF 3 (II)
625
529
1351
1656
sample NC1(I) illustrated our explication (see Fig. 3).
CCF 4 (II)
625
449
1341
—
The size of NC1(I) crystal is bigger than sample
CF 5(II)
595
443
1338
—
NCF3(II). The IR results were represented in Table III
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Volume 10 (2012)

TABLE IV: X-ray d-spacing for the series of samples: NiCr2−x Fex O4 and CuCr2−x Fex O4 obtained by coprecipitation method
compared with NiCr2−x Fex O4 obtained by solid-state reaction method and reference [3].
Plane
(hkl)
111
220
311
222
400
422
Plane
(hkl)
111
220
311
222
400
422
Plane
(hkl)
111
220
311
222
400

422

dref
(˚
A)
4.81
2.93
2.40
2.40
2.08
1.70
dref
(˚
A)
4.81
2.93
2.40
2.40
2.08
1.70
dref
(˚
A)
4.81
2.93
2.40
2.40
2.08
1.70

NC 1(II)
4.81
2.95
2.50
2.40
2.08
1.70

CC 1 (II)
4.81
2.95
2.51
2.41
2.08
1.70

NC 1(I)
4.80
2.95
2.51
2.41
2.08
1.70

d-spacing for NiCr2−x Fex O4 obtained by coprecipitation
NCF 2 (II)
NCF 3 (II)
NCF 4 (II)
4.80
4.80

4.79
2.95
2.95
2.95
2.51
2.51
2.51
2.41
2.41
2.40
2.08
2.08
2.08
1.69
1.70
1.69
d-spacing for CuCr2−x Fex O4 obtained by coprecipitation
CCF 2 (II)
CCF 3 (II)
CCF 4 (II)
4.79
4.80
4.79
2.93
2.94
2.94
2.51
2.51
2.51
2.41

2.31
2.42
2.06
2.07
2.09
—
—
—

d-spacing for NiCr2−x Fex O4 obtained by solid-state reaction
NCF 2 (I)
NCF 3 (I)
NCF 4 (I)
4.79
4.79
4.81
2.90
2.93
2.93
2.48
2.50
2.50
—
—
2.40
2.07
2.07
2.07
1.69
1.69

1.69

affirming spinel structure of our products. The solid samples obtained by coprecipitation method were affirmed to
be expected ternary spinels by XRD results represented
in Table IV.
As all what we have represented above, the coprecipitation method permit to prepare ternary spinels of
NiCr2−x Fex O4 type (A = Ni2+ , Cu2+ ; Br = Cr3+ and
Bf=Fe3+ ) at lower temperature, 750◦ C than the reaction temperature of spinels preparation NiCr2−x Fex O4 by
solid-state reaction, 1300◦ C and the size of spinel grain
is smaller with the specific surface around 20 m2 /g (see
Fig. 3). That’s catalysts composition, catalyst preparation method, conditions of catalyst preparation as temperature, reaction medium influence their catalytic capacity in oxidative dehydrogenation of ethylbenzene to
styrene. Table V represents the results of catalytic activity evaluations of spinels in oxidative dehydrogenation of
ethylbenzene to styrene.
The results presented in the Table V showed catalytic performance of these spinels in oxidative dehydrogenation of ethylbenzene to styrene. These data also
showed that when the reaction temperature was increasing, the ethylbenzene conversion increasing, the selectivity in styrene decreasing. In the reaction temperature range from 350◦ C to 450◦ C, the catalyst NFC3(I),
NFC3(II) and CCF3(II) or the NiCrFeO4 3(I), NiCrFeO4
3(II) and CuCrFeO4 3(II) (x = 1) represent the highest catalytic activity and selectivity in styrene. In oxidative dehydrogenation of ethylbenzene to styrene on
spinel catalyst CuCr2−x Fex O4 , the role of water was very
important. The presence of water has eliminated secondary reactions as deakylation ethybnezene molecular.
While these secondary reactions took place styrene at the
266

CCF 5 (II)
4.79
2.95
2.49
4.42
2.09
—

same time with the reaction, oxidative dehydrogenation
of ethylbenzene.
Basing on several publications in recent years and the
results represented in Table V, the main reaction and the
secondary reactions in the oxidative dehydrogenation of
ethylbenzene can be explained after the following steps:
First, oxygen that comes from the air was adsorbed on
the hole vacant catalyst surface to form oxygen adsorbed
(O−
ad ):
3+
1/2O2 + Me2+ → (O−
ad ) + Me

(4)

And then, the monomolecular reaction went on after
mechanism of Langmuir-Hinshelwood to form the reaction
products (see Fig. 3): (a) Process of splitting hydride on
metallic sites Fe3+ or Cr3+ (Me3+ ); (b) Breaking of C–C
bond; (c) Oxidative dehydrogenation of intermediate; (d)
Process repeated:
2OH− → H2 O + O2−
net
Me2+ + 1/2O2 → Me3+ + O−
ad ,

(5)

where O2−

net is oxygen of network crystalline of spinel and
O−
oxygen
adsorbed. Here, Me3+ can be either Cr3+ or
ad
3+
Fe . The both these cations are Lewis acid cites and
represent catalytic possibility of hydride elimination analogue.
IV.

CONCLUSION

1) Three series of catalyst spinel samples were synthesized by solid-state reaction and coprecipitation
method. The coprecipitation method has revealed

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e-Journal of Surface Science and Nanotechnology

Volume 10 (2012)

TABLE V: Composition of liquid product obtained in oxidative dehydrogenation of ethylbenzene to styrene, at different temperatures, air flow of 1.0 l/min, special velocity 0.6 h−1 .
Reaction
temperature (◦ C)
Catalyst

NC 1 (I)
NCF 2 (I)
NCF 3 (I)
NCF 4 (I)

NF 5(I)
NC 1 (II)
NCF 2 (II)
NCF 3 (II)
NCF 4 (II)
NCF 5(II)
CC 1 (I)*
CCF 2 (II)*
CCF 3 (II)*
CCF 4 (II)*
CF 5(II)*

350

400

450

Overall
Selectivity Toluene
Overall
Selectivity Toluene
Overall
Selectivity Toluene
conversion in styrene + benzene conversion in styrene + benzene conversion in styrene + benzene
of ethyl(%)
yield (%)
of ethyl(%)
yield (%)
of ethyl(%)

yield (%)
benzene (%)
benzene (%)
benzene (%)
12.31
45.31
54.69
17.71
39.52
60.48
33.79
36.80
63.20
14.15
66.29
32.71
21.32
61.30
38.70
38.14
53.20
45.19
19.20
92.90
6.18
34.47
92.40
6.54
61.43
91.21

8.70
18.71
78.07
21.41
31.38
70.07
28.16
47.65
64.30
35.40
16.25
78.29
21.45
28.69
69.45
30.06
46.44
59.12
35.22
21.72
56.06
40.15
22.52
57.63
41.23
32.37
40.76
55.58
26.45
72.60

20.33
34.15
65.15
30.75
40.26
64.47
33.51
21.17
92.14
6.74
43.43
85.60
13.21
58.82
82.60
16.79
16.67
87.58
11.56
38.43
71.13
24.43
43.52
69.63
30.04
15.71
79.83
18.81
28.36
72.52

24.63
40.08
60.13
37.69
35.27
93.69
—
47.03
70.72
—
98.21
47.76
—
30.42
92.78
—
51.62
89.43
—
70.14
57.15
—
40.01
78.27
—
70.23
90.17
—
70.23
69.20

—
39.56
81.17
—
65.42
72.06
—
78.95
70.45
—
29.94
95.19
—
38.18
96.72
—
51.12
78.74
—

*reaction conditions on these catalysts are identical to reaction above, but in addition with water presence.

to be more favorable with formation of spinels at
lower temperature.
2) It was used physical method to verify structural
characteristics of the spinel products obtained. The
data obtained have affirmed the structure of spinels
synthesized.
3) Generally, the spinel materials showed a high catalytic activity and selectivity in styrene in the oxidative dehydrogenation of ethylbenzene to styrene.
4) The oxidative dehydrogenation of ethylbenzene on

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spinels NiCr2−x Fex O4 (I) and (II) was complicated
beside the main product, styrene there was secondary reaction influencing quality of styrene obtained.

Acknowledgments

the authors gratefully acknowledge financial support
from the National Foundation for Science and Technology Development of Vietnam (NAFOSTED).

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