Dehydrogenation of ethylbenzene in presence of carbon dioxide over supported Fe2O3-Cr2O3 mixed oxides catalysts

Article abstract of DOI:10.14233/ajchem.2014.15601

Dehydrogenation of ethylbenzene to styrene in the presence of carbon dioxide was investigated over a series of binary iron-chromium oxides (Fe ₂O₃-Cr₂O₃) supported on alumina, silica and titania. The catalysts w

Full text of DOI:10.14233/ajchem.2014.15601

Asian Journal of Chemistry; Vol. 26, No. 2 (2014), 504-508
http://dx.doi.org/10.14233/ajchem.2014.15601
Dehydrogenation of Ethylbenzene in Presence of Carbon Dioxide
Over Supported Fe
2
O
3
-Cr O Mixed Oxides Catalysts
2
3
*
AHMED AOUISSI , HOCEINE BAYAHIA, ZEID A. AL-OTHMAN and M. RAFIQ H. SIDDIQUI
Department of Chemistry, College of Science, King Saud University, Riyadh, Kingdom of Saudi Arabia
*
Corresponding author: E-mail: aouissed@yahoo.fr
Received: 8 April 2013; Accepted: 21 May 2013;
Published online: 15 January 2014;
AJC-14576
Dehydrogenation of ethylbenzene to styrene in the presence of carbon dioxide was investigated over a series of binary iron-chromium
oxides (Fe -Cr ) supported on alumina, silica and titania. The catalysts were characterized by means of XRD, IR and SEM. XRD and
2
O
3
2
O
3
FT-IR characterization indicated that iron and chromium ions are dispersed into the support matrix. Compared to Fe-Cr-Si-30 and Fe-Cr-
Ti-30 catalysts, Fe-Cr-Al-30 was found to be the most active (conversion = 57.2 %) and the most selective in styrene (94.1 %). The
activity value was observed in the following order Fe-Cr-Al > Fe-Cr-Ti.> Fe-Cr-Si. SEM images indicated that there are no significant
carbonaceous deposits on the spent Fe-Cr-Al-30 catalyst, whereas modifications of its morphological aspect were observed. The variation
of Fe-Cr content in the Fe-Cr-Al catalysts showed that the activity depends strongly on the content. The optimal content determined was
50 wt- %.
Keywords: Mixed oxides, Ethylbenzene dehydrogenation, Styrene, Carbon dioxide, Support.
this commercial iron based catalyst does not work effectively
for the ethylbenzene in the presence of CO . Consequently
various supports such as Al , MgO, WO , SiO , ZrO and
activated carbon have been tested on the dehydrogenation of
INTRODUCTION
2
Styrene is one of the most important monomer for
2
O
3
3
2
2
synthetic polymers. More than 90 % of its world production
is obtained by the dehydrogenation of ethylbenzene (EB).
Currently, the dehydrogenation of EB is carried out at 600-
20-30
EB in the presence of CO
oxides supported on activated carbon and Fe oxide supported
on ZrO exhibited high initial catalytic activity, but suffered
from severe deactivation
was activated by the acidic sites over γ-Al
catalysts while CO was activated by basic sites; CO
2
. It has been found that Fe or Cr
7
00 ºC, just below the temperature where thermal cracking
2
1
,2
becomes significant on the promoted iron oxide catalysts .
Due to the highly endothermic and volume increasing character
of the reaction, a large amount of superheated steam is required
to supply heat, lower the partial pressure of the reactant, thus
shifting the chemical equilibrium to higher styrene formation .
The large amount of wasted energy in the form of excess steam ,
the thermodynamic equilibrium limitations and the deactivation
of catalyst are among the several problems which must be
solved . In order to circumvent some of these cited difficulties,
22,23,28,31
32
. Saito et al. found that EB
2
O
3
supported
conversion
2
2
was then correlated with the amounts of strong basic sites.
The acid-basic and redox properties of the catalyst, at an
optimum composition, will influence the conversion and
3-5
6
24,29
selectivity to styrene . Strong basic sites lead to the toluene
5
33
formation, whereas acidic ones lead to the benzene formation .
7
In this work, the performance of the binary oxides Fe-Cr cata-
the process of dehydrogenation of EB using CO instead of
2
lyst system for the dehydrogenation of EB with CO has been
2
8-12
steam has received much attention . This process, if developed
is believed to be an energy-saving and environmentally friendly
process. In fact, the oxidative dehydrogenation of EB with
investigated. The effect of the support and Fe-Cr loading on
the support was explored.
13
10,14-17
EXPERIMENTAL
CO
2
as a soft oxidant possesses several advantages
than
O
2
(strong oxidant) which leads the production of undesired
Catalysts preparation: The supported catalysts were
prepared by the wet impregnation method. Typically, the
support (alumina, silica and titania) was impregnated with
18
by-products (oxygenates) along with styrene . It is well known
that in the present commercial iron catalyst, hematite (α-Fe
is preferred for styrene production. Small amounts of other
2
O )
3
aqueous mixture solution of 1 M Cr(NO
3
)
3
·9H O and 1 M
2
19
metal oxides like Cr
O
2 3
are added as promoters . However,
Fe(NO ·6H O. The volumes of the mixture solution were
3
)
3
2

Vol. 26, No. 2 (2014)
Dehydrogenation of Ethylbenzene in Presence of Carbon Dioxide 505
selected to obtain Fe/Cr = 10/90 (molar ratio) in the final binary
2 3 2 3 2 3 2 3 2 3
oxides. Three catalysts, α-Fe O -Cr O -Al O , α-Fe O -Cr O -
SiO and α-Fe -Cr -TiO
2
2
O
3
2
O
3
2
, referred to as Fe-Cr-Al-30,
Fe-Cr-Si-30 and Fe-Cr-Ti-30 constituted of 30 % by weight
of the binary oxides Fe-Cr in the catalyst were prepared by
stirring the support and impregnation solutions maintained at
(c)
5
1
0 ºC until dryness evaporation and then dried overnight at
20 ºC. The precursor salt was calcined at 750 ºC in stainless
steel reactor with air flow of 6 L/h. The sample was heated
first to 120 ºC for 0.5 h then the temperature was increased to
(b)
750 ºC for 3.5 h.
Catalysts characterization: Powder X-ray diffraction
patterns were obtained on a Siemens D5000 diffractometer
with CuK (λ = 1.5418 Å) radiation. Fourier transform-infra-
α
red spectra of the samples were recorded with Shimadzu FTIR
Spetrometer 8400S at ambient conditions via grinding the
sample with KBr and then pressed. Scanning electron micro-
scopy and elemental analysis (Energy Dispersive X-Ray Analysis:
EDAX) were carried out using Jeol SEM model JSM 6360A
(a)
(
Japan).
Catalyst testing: The dehydrogenation of EB in the
presence of CO was carried out in flow fixed bed reactor
1
0
20
30
40
(°)
50
60
70
2
2θ
loaded with a sample of catalyst under atmospheric pressure.
Prior to dehydrogenation of EB, a sample of a mixed oxides
Fig. 1. XRD patterns of the supported Fe-Cr binary oxides calcined at 750
ºC: (a) Fe-Cr-Al-30; (b) Fe-Cr-Si-30; (c) Fe-Cr-Ti-30
-1
catalyst was pretreated at 700 ºC for 1 h with 50 mL min CO
gas. Then, the reactant mixture CO and EB at a flow rate of
0 mL min was passed through the catalyst at 660 ºC. The
molar ratio of CO to EB was fixed at 11.7 and the W/F was
controlled at 13.9 gcat h/mol. The flow of CO gas was cont-
2
2
26,27
coexistence of various phases besides the main phase . The
-1
5
XRD results indicated that the mixed oxides have slightly diffe-
rent values than the unsupported binary oxides depending on
the support. Table-1 presents the d-spacings that correspond
to the most significant peaks in the patterns of the unsupported
and the supported binary oxides. The deviation from the
corresponding unsupported binary oxides, α-Fe
respectively may suggest that some iron and chromium ions
are dispersed into the support matrix.
2
2
rolled by mass flow controller. Ethylbenzene was fed to the
reaction system with syringe pump. The reaction products were
analyzed with a gas phase chromatograph (Agilent 6890N)
equipped with a flame ionization detector, a thermal conduc-
tivity detector and a capillary column (HP-PLOT Q length
2
O
3
and Cr
2
O ,
3
3
0 m ID 0.53 mm).
RESULTS AND DISCUSSION
X-Ray powder diffraction: The XRD patterns of samples
FTIR analysis: The mixed oxides were characterized by
FT-IR spectroscopy. The IR spectra of the samples in the range
from 4000-400 cm are shown in Fig. 2. The IR band at 3464
-
1
-1
cm corresponds to the stretching vibrations of adsorbed H O
2
with different compositions calcined at 750 ºC are shown in
Fig. 1. Phase identification was performed by comparing the
experimental XRD pattern results (Fig. 1) with the reported
XRD data in ICDD (International Centre for Diffraction Data)
and JCPDS (Joint Committee on Powder Diffraction Standards).
The mixed oxides, Cr-Fe-10, showed the typical XRD pattern
molecules. The IR band at 1626 band is normally attributed to
O-H bending vibrations . Compared with the unsupported
34
α-Fe O /Cr O mixed oxides, the supported ones showed
2
3
2
3
almost the same features in their FT-IR spectra that can be
attributed to changes in the metal-oxygen bonding. The peaks
are characteristic of IR-active fundamental and combination
35
of Cr O
2 3
andα-Fe O
2 3
with hematite structure (JCPDS files No.
lattice modes of crystalline Cr and Fe oxides . The bands in
-1
3
3-664 and 38-1479, respectively). The peaks are relatively
800-400 cm region could be assigned to Fe-O or Cr-O lattice
3
6-41
-1
sharp and strong indicating crystalline product with relatively
large crystallites. Nevertheless, other phases could be present
as traces. In fact, it has been reported for several oxides, the
vibration . The band at 947 cm is assigned to Cr-O-Cr
-
1
vibrations. The bands at 897 cm is assigned to Cr-O-Cr
vibrations and Fe-O-Fe . The deviation from the unsupported
22,35
TABLE-1.
d-SPACINGS CORRESPONDING TO THE MOST SIGNIFICANT PEAKS IN THE XRD PATTERNS
OF THE SUPPORTED BINARY Fe-Cr OXIDES (THE 100 % INTENSITY IS IN BOLD)
Composition
d-Spacings
α-Fe O
–
–
–
4.20
–
–
–
3.62
3.59
–
–
–
–
3.31
2.67
2.63
2.66
2.65
2.66
2.49
2.45
2.48
2.47
2.48
–
–
–
–
2.18
–
–
1.81
1.87
–
1.68
1.66
1.67
1.67
1.69
1.48
1.42
–
–
–
2
3
Cr O
2
3
Fe-Cr-Al-30
Fe-Cr-Si-30
Fe-Cr-Ti-30
3.24

506 Aouissi et al.
Asian J. Chem.
100
Fe-Cr-Al
Fe-Cr-Si
Fe-Cr-Ti
(
c)
90
80
70
60
50
40
30
20
10
0
(b)
(a)
1
2
3
4
5
6
7
3600
2800
2000
1600
1200
1
800
400
–
Time on stream (h)
Wavenumbers (cm
)
Fig. 3. Effect of the support on the EB conversion over supported Fe-Cr
binary oxides catalysts as a function of reaction time. Reaction
Fig. 2. FTIR spectra of supported Fe-Cr binary oxides calcined at 750 ºC:
a) Fe-Cr-Al-30; (b) Fe-Cr-Si-30; (c) Fe-Cr-Ti-30
(
conditions: Temperature = 660 ºC, CO
13.9 g h/mol
2
/EB = 11.7 (molar ratio) W/F
=
mixed oxides, α-Fe
2
O
3
/Cr
2
3
O may suggest that some iron and
chromium ions are dispersed into the support matrix.
Change in the activity of catalyst with time on stream:
The dehydrogenation of EB was performed in the presence
7
6
5
4
3
2
1
0
Fe-Cr-Al
Fe-Cr-Si
Fe-Cr-Ti
of CO as a mild oxidant over the supported Fe-Cr oxide
2
catalysts. The overall EB conversion and the selectivity of
the products for the different catalysts were studied. Fig. 3
showed that the time-on-stream experiments revealed that the
EB conversion over Fe-Cr -Al decreased rapidly from 76.8 to
58.5 % after 7 h, whereas no evident decrease in the conversion
is observed over Fe-Cr -Si and Fe-Cr -Ti catalysts after 4 h.
The acidity of catalysts had significant impact on the catalytic
performance. Strong acidity could promote the cracking of
EB and make the styrene selectivity decrease. The deactivation
of Fe-Cr-Al might not be due only to the coke formation, but
also to the sintering of the catalyst. In fact, the evolution of
the yield with time on stream for all these catalysts showed
that they form the cracking products with almost the same
yield (Fig. 4). The yield obtained over Fe-Cr-Al, Fe-Cr-Si and
Fe-Cr-Ti are 3.7, 2.9 and 3.6 %, respectively. This result indi-
cated that all the catalyst underwent the same effect of carbon
deposition. Thus, the deactivation of Fe-Cr-Al might be due
also to the sintering of the catalyst. This result was evidenced
by SEM measurements. The morphological aspects of fresh
and spent catalysts were evidenced by SEM images.As shown
in Fig. 5a, fresh Fe-Cr -Al sample is characterized by irregular
prismatic crystals. By contrast, spent Fe-Cr-Al catalyst is
composed of large polygonal aggregates, particles (Fig. 5b).
No significant carbon deposits are identified; only the solid
1
2
3
4
5
6
7
Time on stream (h)
Fig. 4. Effect of the support on the styrene yield over supported Fe-Cr
binary oxides catalysts as a function of reaction time. Reaction
conditions: Temperature = 660 ºC, CO
2
/EB = 11.7 (molar ratio) W/F
= 13.9 g h/mol
(
a)
Fig. 5. SEM images of fresh and spent Fe-Cr-Al-30 catalysts. Reaction
conditions: Temperature = 660 ºC, CO /EB = 11.7 (molar ratio) W/F
= 13.9 g h/mol
(b)
structure was modified. Thus, CO
removal. In fact, Mamedov and Corberan have reported that
CO has two main roles: first is to remove the hydrogen
2
contributed in the carbon
42
2
2
produced as water by the reverse water gas shift reaction; there-
fore, the equilibrium limitation of dehydrogenation shifts to
the products side and the second is to remove the deposited
coke.
The evolution of the styrene selectivity with time on stream
is shown in Fig. 6. It can be seen that styrene is the major
product in this reaction whereas benzene and toluene are minor
by-products. After 7 h on stream, the Fe-Cr/Si and Fe-Cr/Ti
catalysts exhibited almost identical styrene selectivity 90.0 and
90.9 %, respectively. The stability of styrene selectivity with
time on stream while the activity decreased indicated that
the catalyst deactivation was mostly due to sintering but not
to carbon deposition (carbon deposition alters the styrene
formation). Thus, besides its role as a soft oxidant, CO
2
played
also the role of coke removal.As shown in Table-2, the Fe-Cr/Al

Vol. 26, No. 2 (2014)
Dehydrogenation of Ethylbenzene in Presence of Carbon Dioxide 507
1
05
Fe-Cr-Al-10
Fe-Cr-Al-70
Fe-Cr-Al-30
Fe-Cr-Al-90
Fe-Cr-Al-50)
Fe-Cr-Al
Fe-Cr-Si
Fe-Cr-Ti
100
90
1
00
80
70
60
50
40
30
20
10
0
9
9
8
8
7
5
0
5
0
5
0
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
Ti
Fig. 7. Effect of Fe-Cr loading on the conversion of dehydrogenation of
EB with CO over alumina supported catalysts. Reaction conditions:
Temperature = 660 ºC, CO /EB = 11.7 (molar ratio) and W/F =
3.9 g h/mol
me on stream (h)
Time on stream (h)
Time on stream (h)
Time on stream (h)
Fig. 6. Effect of the support on the styrene selectivity over supported
Fe-Cr binary oxides catalysts as a function of reaction time
2
2
1
TABLE-2
CONVERSION AND SELECTIVITIES AT THE STEADY STATE
OBTAINED OVER SUPPORTED Cr-Fe BINARY OXIDES
CATALYSTS FOR THE EB DEHYDROGENATION WITH CO2
(70 and 90 %). It is worth noting that the most stable catalysts
were the less selective in styrene production (Fig. 8), i.e., the
most selective in cracking products. This result suggested that
Conversion
%)
Selectivity (%)
Toluene
Catalyst
(
Styrene
Benzene
the main deactivation was not due to the coke formation as it
Cr-Fe-Al
Cr-Fe-Si
Cr-Fe-Ti
57.18
29.70
33.55
94.04
91.98
89.35
1.94
2.15
3.48
4.01
5.87
7.17
22,43,44
was the case for many catalyst systems
.
1
00
Reaction conditions: Temperature = 660 ºC, CO /EB = 11.7 (molar
2
ratio) and W/F = 13.9 g h/mol.
90
catalyst is the most active solid (conversion = 57.2 %) and the
most selective in styrene (94.1 %). Its high activity compared
to its counterpart silica and titania supported Fe-Cr oxides is
due to the combined effects of acid-base and redox properties.
The activity values suggest the following order of the
favourable acid-base /redox and catalytic properties under
these reaction conditions: Fe-Cr-Al > Fe-Cr-Ti > Fe-Cr-Si.
Effects of binary oxides loading on the catalytic activity
of Fe-Cr-Al: It is well known that the conversion and selectivity
to styrene is influenced by the acid-basic and redox properties
of the catalyst.An optimum of configuration in acid-basic and
redox properties needs to be tailored base on the binding
energies in EB molecule. If the surface basic sites are strong
enough to abstract b-hydrogen from EB, the break of the lateral
C-C bond is promoted and, therefore, the selectivity to toluene
will increase. If the catalyst surface acidity is larger, α-hydrogen
can be abstracted from EB and the break of the phenyl-C bond
become more probably, therefore, a higher selectivity to benzene
80
70
60
50
Fe-Cr-Al-10
Fe-Cr-Al-30
Fe-Cr-Al-50
Fe-Cr-Al-70
Fe-Cr-Al-90
40
3
0
0
2
10
0
1
2
3
4
5
6
7
8
9
Tim
T
im
e
e
o
o
n
n
s
s
t
t
r
r
e
e
a
a
m
m (
(
h
h
)
)
Fig. 8. Effect of Fe-Cr loading on the styrene selectivity for dehydroge-
nation of EB with CO over alumina supported catalysts. Reaction
2
conditions: temperature = 660 ºC, CO /EB = 11.7 (molar ratio) and
W/F = 13.9 gh/mol
2
The activity and the selectivity at the steady state for
dehydrogenation of EB in the presence of CO are presented
33
will be obtained . Since EB dehydrogenation occurs at the
redox sites on the solid surface and CO behaves as an oxidant
2
in the Fig. 9. It can be seen that the catalytic behavour of
Fe-Cr/Al depends strongly on the loading. Ethylbenzene
conversion initially increased from 31.93-95.38 % when
Fe-Cr content increased from 10 to 70 %. Up to 70 % the EB
conversion remained unchanged.As for the selectivities, Fig. 9
showed that the selectivity of styrene and the cracking
products (benzene and toluene) remained unchanged when
the loading varied from 10-50 %. A further increase in the
binary oxides content decreased the styrene selectivity in
favour of that of cracking. Thus, the optimal loading of the
catalyst was determined to be 50 %.
2
for Fe and Cr species, it is useful to determine the optimal
composition of the catalyst for dehydrogenation of EB with
CO . For that purpose a series of Fe-Cr-Al catalysts with diffe-
2
rent Fe-Cr content (10, 30, 50, 70 and 90 wt %) marked as
Fe-Cr-Al-x (x denotes the weight percent of Fe-Cr in Fe-Cr-
Al) was prepared and tested. Fig. 7 represented the variation
of the overall conversion of EB dehydrogenation as a function
of time on stream over the series of catalysts. It can be seen
that the EB conversion declined gradually with time on stream
for low loadings while it remained stable for higher loadings

508 Aouissi et al.
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