Ethylbenzene dehydrogenation over Mg3Fe0.5-xCo xAl0.5 catalysts derived from hydrotalcites: Comparison with Mg3Fe0.5-yNiyAl0.5 catalysts

Article abstract of DOI:10.1016/j.apcata.2011.02.003

A series of Mg₃Fe_0.5-xCo_xAl_0.5 (x = 0-0.5) catalysts were prepared from hydrotalcite precursors and their activities in the dehydrogenation of ethylbenzene were compared with those of a series of Mg₃F

Full text of DOI:10.1016/j.apcata.2011.02.003

Applied Catalysis A: General 396 (2011) 107–115
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Ethylbenzene dehydrogenation over Mg Fe
Co Al catalysts derived from
0.5
3
0.5−x
x
hydrotalcites: Comparison with Mg Fe
Ni Al catalysts
3
0.5−y
y
0.5
a
a
a
a
b
Luqman A. Atanda , Rabindran J. Balasamy , Alam Khurshid , Ali A.S. Al-Ali , Kunimasa Sagata ,
b
b
c
d
a,∗
Makiko Asamoto , Hidenori Yahiro , Kiyoshi Nomura , Tsuneji Sano , Katsuomi Takehira
Sulaiman S. Al-Khattaf
KAUST Center in Development, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
,
a,∗∗
a
b
c
School of Engineering, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
Department of Applied Chemistry, Graduate School of Engineering, Kagamiyama, 1-4-1, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
d
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Mg Fe
Co Al
0.5
(x = 0–0.5) catalysts were prepared from hydrotalcite precursors and
3
0.5−x
x
Received 26 October 2010
Received in revised form 29 January 2011
Accepted 2 February 2011
Available online 25 February 2011
their activities in the dehydrogenation of ethylbenzene were compared with those of a series of
Mg3Fe0.5−yNiyAl0.5 (y = 0–0.5) catalysts also derived from hydrotalcite. The hydrotalcites prepared by
co-precipitation were calcined at 550 C to the mixed oxides with a high surface area of 150–240 m g
◦
2
−1
;
cat
they were composed of Mg(Fe,Me,Al)O periclase and Mg(Me)(Fe,Al)2O4 spinel (Me = Co or Ni). Bimetallic
3
+
2+
3+
2+
Fe –Co system showed a synergy, i.e., an increase in the activity, whereas Fe –Ni bimetallic system
showed no synergy. The high styrene yield was obtained on Mg3Fe0.1Co0.4Al0.5; however, a large substi-
Keywords:
Ethylbenzene dehydrogenation
Styrene
3
+
2+
tution of Fe with Co caused a decrease in styrene selectivity along with coking on the catalysts, due to
an isolation of CoOx on the catalyst surface. The highest yield as well as the highest selectivity for styrene
production was obtained at x = 0.25 at time on stream of 30 min. The coprecipitation at pH = 10.0 and the
composition of Mg3Fe0.25Co0.25Al0.5 were the best for preparing the active catalyst. This is partly due to
Mg3Fe0.25Co0.25Al0.5 catalyst
3+
2+
Fe –Co active species
Hydrotalcite
3
+
the formation of a good hydrotalcite structure. On this catalyst, the active Fe species was reduced at
a low temperature by the Fe –Co bimetal formation, leading to a high activity. Simultaneously, the
3
+
2+
3
+
3+
amount of reducible Fe was the smallest, resulting in a high stability of the active Fe species. It is likely
that the dehydrogenation was catalyzed by the reduction–oxidation between Fe and Fe2+ and that Co
3
+
2+
3
+
2+
assisted the reduction–oxidation by forming Fe –Co (1/1) bimetallic active species.
2011 Elsevier B.V. All rights reserved.
©
1
. Introduction
presses the coke formation on the catalyst [8]. However, one should
notice that steam is used in a large excess molar amount with
respect to ethylbenzene (6-13:1), leading to a huge amount of
energy consumption (1.5 × 10 kcal/styrene ton) [9].
Styrene, an important basic chemical as a raw material for
6
polymers, is produced commercially by the dehydrogenation of
ethylbenzene using an Fe–K oxide catalyst in the presence of a
large excess amount of superheated steam at 600–700 C [1]. KFeO2
was identified as the active phase for the dehydrogenation [2–4];
potassium enhances the activity of iron oxide [5] and reduces the
formation of coke deposits that deactivate the catalysts [5,6]. Steam
shifts the chemical equilibrium to higher styrene conversions [7],
assists the surface formation of active KFeO2 species [2] and sup-
Moreover, the Fe–K oxide catalysts have low surface area and
are deactivated with time by residual organic chloride impurities
[1]. The active KFeO2 tends to be reduced to lower oxides and
even to elemental iron, which would catalyze coke formation and
dealkylation [10]. The most serious deactivation is caused by the
migration and loss of potassium promoter [11]. The search for new
catalyst systems which have high surface areas and can stabilize
the active state of iron, in the absence of potassium and steam,
is important. MgO has especially good characteristics as an addi-
tive to a K-promoted iron oxide catalyst among a series of alkaline
earth oxides [12]. Mg2+ ions possess a small ionic radii, leading to a
high electrostatic potential due to the stable valence state, and may
◦
∗ Corresponding author. Present address: Hiroshima University, Kagamiyama, 1-
4-1, Higashi-Hiroshima, 739-8527, Japan.
∗
2+
effectively suppress Fe sintering caused by the reduction of Fe to
∗ Corresponding author. Tel.: +966 3 860 1429; fax: +966 3 860 4234.
E-mail addresses: takehira@hiroshima-u.ac.jp (K. Takehira),
0
Fe . Aluminum was proved to be an excellent promoter, preventing
skhattaf@kfupm.edu.sa (S.S. Al-Khattaf).
sintering in iron-oxide catalysts [13,14].
0
926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.02.003

1
08
L.A. Atanda et al. / Applied Catalysis A: General 396 (2011) 107–115
There has been for the past decades an increased interest in
XRD was recorded on a Mac Science MX18XHF-SRA pow-
der diffractometer with monochromatized CuK˛ radiation
(ꢀ = 0.154 nm) at 40 kV and 30 mA. The diffraction pattern was
identified through comparison with those included in the JCPDS
(Joint Committee of Powder Diffraction Standards) database.
Mössbauer spectra of pelletized powder samples were recorded
at room temperature, using a constant acceleration mode (Topo-
logic System Co.) of a radiation source with about 40 MBq 57 Co(Cr)
and a YAP scintillation counter. Doppler velocity was calibrated
with reference to ␣-Fe.
using hydrotalcite-like compounds as precursors for mixed oxide
catalysts in various reactions [15]. The catalysts obtained by ther-
mal decomposition of hydrotalcite precursors have high thermal
stability and good metal dispersion; these properties are attributed
to the particular structure of the hydrotalcite through a homoge-
neous distribution of the metallic cations in the brucite-type sheets
[
[
15]. FeOx/Mg(Al)O [16], FeOx/Mg(Zn,Al)O [17], and FeOx/Zn(Al)O
18] catalysts derived from hydrotalcites were tested in the dehy-
drogenation of ethylbenzene in a CO2 atmosphere. The activity
losses over the Fe/Mg(Al)O catalysts with time-on-stream were
completely restored by oxygen pulses [16]. The authors reported
that behaviors of FeOx/Mg(Al)O catalysts derived from hydrotalcite
are different from those reported in the preceding papers [16–18];
neither the addition of O2 nor the oxidation pre-treatment of the
catalyst showed favorable effect on the activity [19]. The authors
recently studied the activity of various FeOx–MeOy/Mg(Al)O cata-
lysts derived from hydrotalcites, among which FeOx–CoOy/Mg(Al)O
TG–DTA of the catalyst was performed under an inert atmo-
−
1
sphere of N2 (20 ml min ) with a TA Instrument SDT Q 600 using
◦
−1
.
50 mg of sample at a rate of 10 C min
◦
N adsorption–desorption isotherms at −196 C were measured
2
using a conventional volumetric apparatus (Bel Japan, BELSORP
Mini). Before adsorption measurements, samples (ca. 0.1 g) were
heated at 400 C for 10 h under N2 flow. Surface areas were calcu-
◦
lated by the Brunauer–Emmett–Teller (BET) method.
exhibited high activity by the formation of Fe3 –Co active species
+
2+
H -TPR of the calcined catalysts was carried out by flow-
2
3
−1
[
20].
In this paper, we compare the activity of Mg Fe
ing 5 vol.% H /N (30 cm min ) in the temperature range of
100–900 C. The sample temperature increased at a rate of
10 C min and the amount of H2 consumed was monitored by a
thermal conductivity detector of a gas chromatograph (Shimadzu,
GC-8AIT) [21].
2
2
◦
−1
CoxAl0.5
0.5−x
3
◦
(
x = 0–0.5) and Mg Fe
NiyAl0.5 (y = 0–0.5) in the dehydrogena-
3
0.5−y
tion of ethylbenzene in a He atmosphere. Further we report on
the effects of metal composition, pH value in the coprecipitation,
and metal loading on the structure and the activity of the former
catalysts.
XPS measurements were performed on a Perkin Elmer 1600E
spectrometer using Mg K␣ radiation as excitation source. In charge-
up correction, the calibration of binding energy (BE) of the spectra
was referenced to the C 1 s electron bond energy corresponding to
graphitic carbon at 284.5 eV. In addition, relative atomic sensitiv-
ity factors (ASF) were used to determine practically more accurate
chemical compositions on the surface.
2
. Experimental
2
.1. FeOx/Mg(Al)O-based catalysts’ preparation
Two series of catalyst, i.e., Mg Fe
CoxAl0.5 (x = 0, 0.1, 0.2, 0.25,
3
0.5−x
0
.3, 0.4 and 0.5) and Mg Fe
NiyAl0.5 (y = 0, 0.1, 0.2, 0.25, 0.3, 0.4
2.3. Catalyst test
3
0.5−y
and 0.5), were prepared by coprecipitation [20]; both hydrotalcite
precursors prepared by coprecipitation of metal nitrates were cal-
Dehydrogenation of ethylbenzene was conducted using a con-
tinuous gas-flow reactor with a fixed bed catalyst (Autoclave
Engineers Ltd., Model 401 C 0286) at atmospheric pressure. In
the dehydrogenation reactions, typically 0.15 g of catalyst, which
had been pelletized into particles 0.3–0.8 mm in diameter, was
loaded into the reactor. The catalyst was pre-treated in a He gas
◦
cined at 550 C. Mg Fe0.25Co0.25Al2 and Mg Fe0.25Co0.25Al0.25 were
2
4
also prepared in the same way to test the effects of Mg/Al molar
2+
3+
ratio. An aqueous solution containing the nitrates of Mg , Fe
,
2+
2+
2+
3+
Me (Co or Ni ) and Al (ca. 0.05 total mol/200 ml) was added
slowly with vigorous stirring into an aqueous solution of sodium
carbonate (0.04 mol/400 ml). The pH of the solution was adjusted at
−
1
◦
flow (100 ml min ) at 550 C for 1 h. The reaction was started by
introducing a gas mixture of ethylbenzene and He into the reac-
1
0.0 by dropping a 1 M aqueous solution of sodium hydroxide, lead-
−
1
−1
ing to a precipitation of heavy slurry. After the solution was aged
tor. Ethylbenzene (0.08 ml min ; ca. 0.7 mmol min ) was fed by
◦
−1
at 60 C for 24 h, the precipitates were filtrated, washed with de-
a micro-feeder under a He flow (100 ml min ). Helium was used as
◦
ionized water (1000 ml), dried in air at 100 C for 4 h, and calcined
a carrier gas instead of N2, because N2 can be activated to form NH3
in the presence of H2 over Fe catalysts. The reaction was carried out
◦
at 550 C for 12 h in a muffle furnace in a static air atmosphere. To
◦
test the effect of pH, we carried out the coprecipitation at pH = 9.5
and 10.5. The concentration of Na+ in the catalysts after the calci-
nation was confirmed to be below 10 ppm by atomic absorption.
As a control, FeOx/MgO (Mg Fe ) and FeOx/Al O (Fe Al ) catalysts
for 3 h of time-on-stream at 550 C.
Thereactionproducts (styrene, toluene, and benzene) and ethyl-
benzene were analysed using on-line gas chromatograph equipped
with FID using a HP-INNOWAX column. None of other hydrocar-
bons was detected. Analysis of hydrogen was performed with a TCD
gas chromatograph using a packed Molecular Sieve-5A column. All
the lines and valves between the cold trap and the reactor were
3
1
2
3
1
1
were prepared by co-precipitation of the nitrates in the same way.
®
Also as a control, a commercial Fe–K oxide catalyst, Styromax -4,
supplied from Süd-Chemie Catalysts Japan, Inc. was used.
◦
heated to 150 C to prevent any condensation of ethylbenzene or
of the dehydrogenation products.
2.2. Characterizations of catalysts
The catalyst precursors and the catalysts were character-
3. Results and discussion
ized by atomic absorption (AA), powder X-ray diffraction
XRD), transmission Mössbauer spectra (Mössbauer),
thermogravimetric–differential thermal analysis (TG–DTA),
nitrogen adsorption–desorption (N2 absorption–desorption),
(
3.1. Surface area, metal composition and crystal structure of the
catalysts
temperature-programmed reduction (H -TPR) and X-ray
photoelectron spectroscopy (XPS) methods.
Specific surface area of Mg Fe
CoxAl0.5 (x = 0–0.5) and
2
3
0.5−x
Mg Fe
NiyAl0.5 (y = 0–0.5) catalysts are shown in Table 1
3
0.5−y
AA measurements were carried out with a Perkin Elmer AAna-
together with those of Mg Fe0.25Co0.25Al0.5(9.5), Mg Fe
3
3
0.25
lyst 100 using a mixed gas of acetylene–N O–air.
Co0.25Al0.5(10.5),
Mg Fe0.25Co0.25Al ,
Mg Fe0.25Co0.25Al0.25,
2
2
2
4

L.A. Atanda et al. / Applied Catalysis A: General 396 (2011) 107–115
109
Table 1
a
Specific surface area and activity of Mg–Fe–Co–Al and Mg–Fe–Ni–Al catalysts .
g)
f)
Catalyst
Specific surface
Yield of
styrene /%
area/m2
g
−1
b
cat
Mg3Fe1
Fe1Al1
122
123
0
0
e)
d)
Mg3Fe0.5Al0.5
197
190
193
187
169
158
171
21.8
21.8
25.1
26.2
22.6
21.8
18.5
Mg3Fe0.4Co0.1Al0.5
Mg3Fe0.3Co0.2Al0.5
Mg3Fe0.25Co0.25Al0.5
Mg3Fe0.2Co0.3Al0.5
Mg3Fe0.1Co0.4Al0.5
Mg3Co0.5Al0.5
c)
b)
1
00 cps
Mg3Fe0.25Co0.25Al0.5(9.5)c
Mg3Fe0.25Co0.25Al0.5(10.5)
Mg2Fe0.25Co0.25Al2
180
172
182
178
21.5
19.4
22.9
20.9
a)
d
0
10
20
30
40
2
50
60
70
80
90
100
Mg4Fe0.25Co0.25Al0.25
theta / degree
Mg3Fe0.4Ni0.1Al0.5
Mg3Fe0.3Ni0.2Al0.5
Mg3Fe0.25Ni0.25Al0.5
Mg3Fe0.2Ni0.3Al0.5
Mg3Fe0.1Ni0.4Al0.5
Mg3Ni0.5Al0.5
220
192
238
182
187
181
21.1
20.0
22.4
19.4
19.6
17.5
Fig. 1. XRD patterns of Mg3Fe0.5−xCoxAl0.5 after drying. (a) x = 0; (b) x = 0.1; (c) x = 0.2;
d) x = 0.25; (e) x = 0.3; (f) x = 0.4; (g) x = 0.5. ᭹, hydrotalcites; ꢀ, cobalt hydroxylcar-
(
bonates.
a
c
form hydrotlcite-like compounds (HTlc), whereas no example of
Mg –Co or Co –Al system is known as HTlc in minerals
[15].
The catalysts were prepared by co-precipitation of metal nitrates at pH = 10.0 (
2+
3+
2+
3+
d
◦
pH = 9.5 and pH = 10.5) and calcined at 550 C for 12 h.
b
at 30 min of time-on-stream.
Mg Fe
CoxAl0.5 (x = 0–0.5) after calcination (Fig. 2) exhibited
3
0.5−x
reflections of periclase and spinel. Although cobalt hydroxylcar-
bonates were detected on Mg Fe0.1Co0.4Al0.5 and Mg Co0.5Al0.5
Mg Fe and Fe Al as comparison. The number in parenthe-
3
1
1
1
3
3
ses after the catalyst symbol shows the pH value used in the
coprecipitation. Metal compositions of the catalysts were con-
firmed to coincide with the amount of metal nitrates used
in the coprecipitation, previously reported [20]. Mg–Al [15],
Ni–Al [15] and Mg–Fe [22] are well known to form hydrotalcite
structure, resulting in a large surface areas after the calcina-
after drying (Fig. 1f and g), no reflection of any Co species was
observed for either of the two samples after calcination (Fig. 2f and
g). This indicates that Co species derived from the cobalt hydrox-
ylcarbonates were amorphous or of finely dispersed particles. All
Mg Fe
NiyAl0.5 (y = 0–0.5) after calcination showed periclase
3
0.5−y
and spinel reflections with the higher S/N ratios than those of
Mg Fe Co Al (x = 0–0.5) after calcination. This may be again
tion. However, Mg Fe1 exhibited a similar surface area to that
3
3
0.5−x
x
0.5
of Fe Al which does not form a hydrotalcite structure. All
1
1
due to the well grown hydrotalcite structure of the precursors, as
mentioned above.
Mg Fe
CoxAl0.5 (x = 0–0.5) and Mg Fe
NiyAl0.5 (y = 0–0.5)
3
0.5−x
3
◦
0.5−y
catalysts after the calcination at 550 C exhibited large surface
areas between 150 and 240 cm g . This is due to the forma-
When the Mg/Al ratio was decreased, the reflections of cobalt
hydroxylcarbonate again appeared, together with those of hydro-
talcite for Mg Fe0.25Co0.25Al after drying (data are not shown). The
2
−1
cat
tion of layered hydrotalcite structure, as the precursors during
the co-precipitation [15], which is confirmed by the XRD mea-
surements of the samples after drying (vide infra). The surface
area slightly increased by Fe–Ni bimetal formation, whereas
no significant increase was observed by Fe–Co bimetal forma-
tion; the surface area decreased with increasing Co content. The
pH value in the coprecipitation also influenced the surface area;
2
2
ratio of Mg/Al = 1/1 is not favorable for the formation of hydrotal-
2+
cite, and therefore Co ions could not be sufficiently incorporated
in the hydrotalcite structure. Mg Fe0.25Co0.25Al2 after calcination
2
showed again periclase and spinel reflections alone, suggesting that
Mg Fe0.25Co0.25Al0.5 prepared at pH = 9.5 and 10.5 afforded smaller
3
surface area than that at pH = 10.0.
50 cps
XRD patterns of Mg Fe
CoxAl0.5 (x = 0–0.5) samples after
3
0.5−x
g)
drying (Fig. 1) showed typical hydrotalcites reflections, indi-
2
+
cating that Co
ions were incorporated into the hydrotalcite
f)
e)
d)
structure except Mg Fe0.1Co0.4Al0.5 and Mg Co0.5Al0.5. The lat-
ter two samples of large Co contents showed the reflections of
cobalt hydroxylcarbonates (Fig. 1f and g). These reflections can
be assigned to mixtures of hexagonal cobalt hydroxylcarbon-
ate with the lattice parameters of a = 10.335(5) A˚ , c = 3.124(1) A˚
3
3
c)
b)
[
23], cobalt hydroxylcarbonates of dandelion-like form (JCPDS
4
8-0083, Co(OH)x(CO3)0.5·0.11H O) and those of rose-like from
2
(
JCPDS 29-1416, Co (OH) CO ) [24]. Although the structure of
2 2 3
cobalt hydroxylcarbonate cannot be precisely determined, it is
concluded that the use of an excess amount of Co caused an isola-
a)
tion of Co species from the hydrotalcite in both Mg Fe0.1Co0.4Al
3
0.5
0
10
20
30
40
2
50
60
70
80
90
100
and Mg Co0.5Al0.5. Contrarily Mg Fe NiyAl0.5 (y = 0–0.5) after
3
3
0.5−y
theta / degree
drying showed typical reflections of hydrotalcite over all metal
compositions (data are not shown). This coincided with the fact
◦
Fig. 2. XRD patterns of Mg3Fe0.5−xCoxAl0.5 after calcination at 550 C. (a) x = 0; (b)
x = 0.1; (c) x = 0.2; (d) x = 0.25; (e) x = 0.3; (f) x = 0.4; (g) x = 0.5. ꢀ, periclase; ꢁ, spinel.
that bimetallic systems of Mg2 –Fe , Mg –Al and Ni –Al
+
3+
2+
3+
2+
3+

1
10
L.A. Atanda et al. / Applied Catalysis A: General 396 (2011) 107–115
g)
g)
20 wt%
f)
e)
d)
c)
b)
a)
f)
e)
d)
c)
b)
a)
End.
-1
2
μV mg
0
100 200 300 400 500 600 700 800 900
Temperature / °C
0
100 200 300 400 500 600 700 800 900
Temperature / °C
Fig. 3. TG–DTA of Mg3.0Fe0.5−xCoxAl0.5 after drying. (a) Mg3Fe0.5Al0.5; (b) Mg3Fe0.4Co0.1Al0.5; (c) Mg3Fe0.3Co0.2Al0.5; (d) Mg3Fe0.25Co0.25Al0.5; (e) Mg3Fe0.2Co0.3Al0.5; (f)
Mg3Fe0.1Co0.4Al0.5; (g) Mg3Co0.5Al0.5.
◦
Co species exist as amorphous or of small-sized particles on the
catalyst (vide infra).
shifted to 310 C and a new endothermic peak appeared around
◦
430 C. It was reported that Co–Al hydrotalcites decomposes at the
lower temperature than Ni–Al [25] or Mg–Al [15] hydrotalcites.
Sometimes the second peak, due to the loss of hydroxyl groups
from the brucite layer and of the carbonate anions, appeared just
after the first peak due to the loss of interlayer water. The peak
3
.2. Thermal behaviors during the preparation of the catalysts
TG–DTA results of Mg Fe CoxAl0.5 (x = 0–0.5) after drying are
3
0.5−x
◦
3+
around 310 C may be attributed to such peak shift due to the for-
shown in Fig. 3. Fe species likely exists as an Fe valence state
in Mg Fe0.5Al0.5 based on Mössbauer measurements (vide infra).
Mg –Al is well known to form a typical hydrotalcite [15], and
Mg –Fe also forms hydrotalcite [22]. Therefore Mg Fe0.5Al0.5
after drying is composed of (Mg )3–(Fe ,Al )1 hydrotalcites
structure, as seen in Fig. 1a. Indeed, TG–DTA curves of Mg Fe0.5Al0.5
after drying (Fig. 3a) exhibited typical thermal changes in two
steps, i.e., the first weight decrease around 160 C and the second
around 370 C. The first one, endothermic, corresponds to the loss
of interlayer water, without collapse of the structure, and the sec-
ond one, endothermic, is due to the loss of hydroxyl groups from the
brucite-like layer as well as of the carbonate anions [15]. When x
in Mg Fe
1
mation of Co2+–Al hydrotalcites. Another possible assignment of
3+
3
◦
the endothermic peak around 310 C would be the formation of
2+
3+
◦
cobalt hydroxylcarbonate which decomposes around 300 C [24].
2+
3+
3
2+
2+
3+
3+
The valence state of Co is not limited to Co but exists partly as
3
+
2+
3+
Co [26]; Xu and Zeng [27] observed the oxidation of Co to Co
3
2+
2+
3+
during the preparation of Mg Co Co hydrotalcites. In such case,
3+
◦
Co species of higher valence state strongly hold carbonate anions,
resulting in an increase in the temperature of the loss of carbonate
◦
◦
anions; the second endothermic peak observed around 430 C with
a weight loss may be assigned to the loss of such carbonate species
(
Fig. 3e–g).
TG–DTA results of Mg Fe NiyAl0.5 (y = 0–0.5) after drying
0.5−y
CoxAl0.5 was increased, the endothermic peak around
60 C was weakened and simultaneously its weight decrease was
3
3
0.5−x
◦
are shown in Fig. 4; the changes in the TG–DTA curves are
◦
more simpler than those of Mg Fe
0.5−x 0.5
x
Co Al (x = 0–0.5). The first
reduced. Moreover, the other endothermic peak around 370 C
3
e)
e)
d)
20 wt%
d)
c)
b)
c)
b)
a)
a)
End.
-1
2
μV mg
0
100 200 300 400 500 600 700 800 900
Temperature / °C
0
100 200 300 400 500 600 700 800 900
Temperature / °C
Fig. 4. TG–DTA of Mg3.0Fe0.5−xNixAl0.5 after drying. (a) Mg3Fe0.5Al0.5; (b) Mg3Fe0.3Ni0.2Al0.5; (c) Mg3Fe0.25Ni0.25Al0.5; (d) Mg3Fe0.2Ni0.3Al0.5; (e) Mg3Ni0.5Al0.5.

L.A. Atanda et al. / Applied Catalysis A: General 396 (2011) 107–115
111
3
5
100
10
100
95
30
95
25
20
90
15
5
10
85
5
0
80
0
50
100 150
200
0
50
100
150
200
Time on stream / min
Time on stream / min
Fig. 5. Ethylbenzene dehydrogenation over Mg3Fe0.5−xCoxAl0.5 catalysts. Reaction
0
90
◦
®
temperature, 550 C; catalyst, 0.15 g (0.30 g for the commercial catalyst, Styromax
4
-
); ethylbenzene, 0.08 ml min 1(ca. 0.7 mmol min ); He, 100 ml min . Full line,
−
−1
−1
0
0.1
0.2
0.3
0.4
0.5
ethylbenzene conversion; dotted line, styrene selectivity. ᭹, x = 0; ꢀ, x = 0.1; ꢁ,
®
x and y
x = 0.2; ꢀ, x = 0.25; ꢂ, x = 0.3; +, x = 0.4; ×, x = 0.5; , Styromax -4.
in Mg3Fe0.5-xCoxAl0.5 and Mg3Fe0.5-yNiyAl0.5, respectively
◦
endothermic peak around 160 C was weakened and a new peak
Fig. 6. The activities of Mg3Fe0.5−xCoxAl0.5 and Mg3Fe0.5−xNixAl0.5 catalysts for
◦
◦
appeared around 60 C with increasing y. The new peak can be
ethylbenzene dehydrogenation. Reaction temperature, 550 C; catalyst, 0.15 g;
assigned to the loss of interlayer water in Mg2 Ni –Al hydro-
+
2+
3+
−1
−1 −1
(ca. 0.7 mmol min ); He, 100 ml min ; at 30 min
ethylbenzene, 0.08 ml min
2+
3+
of time-on-stream. Full line, rate of styrene production over Mg3Fe0.5−xCoxAl0.5
talcites as reported by Kloprogge and Frost [28]; both Mg –Al
and Ni –Al hydrotalcites exhibited the peak of dehydration
from interlayer water below 100 C. The second endothermic peak
around 370 C, ascribed to the loss of hydroxyl groups from the
(
᭹) and Mg3Fe0.5−xNixAl0.5 (ꢀ) catalysts; dotted line, styrene selectivity over
2
+
3+
Mg3Fe0.5−xCoxAl0.5 (ꢀ) and Mg3Fe0.5−xNixAl0.5 (ꢂ) catalysts.
◦
◦
brucite-like layer as well as of the carbonate anions, shifted to lower
temperature of 350 C. This may be due to the metal composition
ited a rather stable conversion (Fig. 5). However, the styrene
selectivity decreased over these Co-rich catalysts, whereas the
benzene selectivity increased, compared with the Fe-rich cata-
lysts. Moreover, the Co-rich catalysts frequently provoked the
reactor plugging due to the coking on the catalysts. Actually,
◦
2
+
3+ 3+
2+
2+
3+
change from Mg –Fe Al to Mg Ni –Al hydrotalcites; the
decrease in the positive charge may enhance the elimination of
−
−
OH as well as CO3 as anions.
.3. Activity of the catalysts
The activities of Mg Fe
on Mg Fe0.1Co0.4Al0.5, the reactor was plugged just after the
3
3
last sampling at 180 min of time-on-stream. XRD analyses of
Mg Fe0.1Co0.4Al0.5 after the reaction clearly showed the reflections
3
◦
CoxAl0.5 (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4
0.5−x
of graphite at 2ꢁ = 25 [29,30], indicating that the reactor plugging
3
and 0.5) catalysts were tested for 3 h of time-on-stream together
with that of a commercial catalyst. Time courses of the ethyl-
benzene dehydrogenation are shown in Fig. 5. After the start of
the reaction, both ethylbenzene conversion and styrene selectivity
were not stable at 10 min but stabilized at 30 min of time-on-
stream. Styrene yields at 30 min of time-on-stream are shown in
Table 1. Both Mg Fe1 and Fe Al1 as comparison showed a plug-
was due to the severe coking on the catalyst. No graphite reflec-
tion was observed in the XRD of Mg Fe0.5Al0.5 after the reaction.
3
The Mg Fe0.1Co0.4Al0.5 catalyst after the reaction showed also the
3
reflections of Co O4 [31,32] and CoAl O4 spinel [25]. This indi-
3
2
cates that such isolated Co species accelerated the cracking of
ethylbenzene to benzene, leading to the coke formation on the cat-
alyst. Moreover, the reactor plugging was substantially affected by
3
1
ging of the reactor just after starting the reaction due to coking,
resulting in no production of styrene. We previously reported that
the preparation procedure of Mg Fe0.1Co0.4Al0.5 catalyst; a quick
3
coprecipitation enhanced the isolation of cobalt hydroxylcarbon-
ate, resulting in a quick plugging of the reactor. In the present work,
coprecipitation was carried out slowly by using a highly diluted
aqueous solution in order to avoid such phase separation.
Mg Fe0.5Al0.5 derived from hydrotalcites showed high activity in
3
the dehydrogenation of ethylbenzene in N2 atmosphere [19]. Such
high activity of Mg Fe0.5Al0.5 was again confirmed in the present
3
work (Fig. 5), although the ethylbenzene conversion was lower
and the deactivation was more significant than reported previously
The styrene yields over Mg Fe
NiyAl0.5 (y = 0–0.5) catalysts
3
0.5−y
are shown in Table 1. Although the ethylbenzene conversion at the
beginning slightly decreased with increasing y, no substantial dif-
ference was observed between the catalysts (data are not shown).
The activity was slightly stabilized by increasing y. The rates of
styrene production calculated at 30 min of time-on-stream are plot-
[
19]. This may be due to the difference in the reaction conditions:
flow rates of ethylbenzene and carrier gas were increased from
− −1
.025 mmol min and 10 ml min (N ) in the previous report to
2
1
0
0
−1
−1
.7 mmol min and 100 ml min (He), respectively, in the present
®
reactions. The commercial catalyst, Styromax -4, showed a low
activity as well as a significant deactivation even with the use of
ted against y in Mg Fe
NiyAl0.5 together with those against x in
3
0.5−y
Mg Fe
CoxAl0.5 (Fig. 6). The selectivity to styrene was higher
3
0.5−x
2
times larger amount. This may be due to the absence of steam,
on the Fe–Ni system than on the Fe–Co system, whereas the rate
of styrene production was higher on the Fe–Co system than on the
because a large amount of steam was used in the commercial pro-
cess. Moreover, steam assists the formation of active KFeO species
on the commercial catalyst [2].
Fe–Ni system. The rate of styrene production on Mg Fe
CoxAl0.5
2
3
0.5−x
showed the maximum value around x = 0.4, indicating an appear-
ance of synergy between Fe3+ and Co species. Contrarily, the
2+
Among Mg Fe
CoxAl0.5 catalysts, the highest styrene yield
3
0.5−x
was obtained with x = 0.25 at 30 min of time-on-stream (Table 1).
Mg Fe CoxAl0.5 with small x values showed a decrease in the
activity of Mg Fe
and the selectivity to styrene was almost constant over the entire
range of the Fe/Ni composition. Although the maximum rate of
NiyAl0.5 did not show any maximum value
3
0.5−y
3
0.5−x
conversion during the reaction, whereas those with x ≥ 0.4 exhib-

1
12
L.A. Atanda et al. / Applied Catalysis A: General 396 (2011) 107–115
Table 2
Mössbauer parameters of the catalysts.
Sample
Component
Site
ISa
QSb
LWc
Rel. contr. (%)d
Phases by XRD
Fe3 (1)
+
0.40
0.19
0.42
0.22
0.38
0.15
0.77
0.84
0.77
0.80
0.78
0.89
0.49
0.49
0.48
0.48
0.52
0.52
61.5
38.5
55.0
45.0
74.4
25.6
Mg(Fe,Al)2O4
Mg(Fe,Al)O
(Mg,Co)(Fe,Al)2O4
Mg(Co,Fe,Al)O
(Mg,Ni)(Fe,Al)2O4
Mg(Ni,Fe,Al)O
Mg3Fe0.5Al0.5
Doublet
Doublet
Doublet
Doublet
Doublet
Doublet
3
+
Fe (2)
3
+
Mg3Fe0.25Co0.25Al0.5
Mg3Fe0.25Ni0.25Al0.5
Fe (1)
3
+
Fe (2)
3+
Fe (1)
3+
Fe (2)
a
−1
Isomer shift (mm s ).
b
c
−1
Quadrupole splitting (mm s ).
−1
Line width (mm s ).
d
Relative contribution.
◦
styrene formation appeared on Mg Fe0.1Co0.4Al0.5, the selectivity
to reduction of Co O4 and CoO [34]. The peaks at 150–350 C are
3
3
to styrene decreased at x ≥ 0.4 due to the formation of benzene
as byproduct, indicating that cracking of ethylbenzene proceeded.
Judging from both rate of production and selectivity of styrene, the
assigned to the reduction of small crystallites of segregated Co O
3
4
or CoO and surface reduction of Co3+/Co cations dissolved in
2+
◦
spinel or periclase phase [35]. The peak around 850 C can be
3+
best catalyst is composed of almost equimolar amounts of Fe
assigned to CoAl O4 reduction [35]. Mg Fe0.25Co0.25Al0.5 (Fig. 7c)
2
2
and Co2+, e.g., the active species is composed of Fe –Co (1/1)
3+
2+
and Mg Fe0.25Co0.25Al0.5 (Fig. 7d) showed a broad reduction peak
4
◦
◦
bimetallic species.
or a shoulder around 300 C. All these peaks below 300 C can be
attributed to the reduction of Co3+ to Co as mentioned above.
2+
In the case of Mg Ni0.5Al0.5, a strong reduction peak alone was
observed above 700 C; this is attributed to the reduction of Ni in
3
3.4. Mössbauer analyses of the catalyst
◦
2+
Mg(Ni,Al)O periclase [35,36] (Fig. 7i). Mg Fe0.25Ni0.25Al0.5 showed
3
Mössbauer spectra of Mg Fe0.5Al0.5, Mg Fe0.25Co0.25Al0.5 and
3
3
◦
◦
a broad peak around 350 C together with the strong peak above
Mg Fe0.25Ni0.25Al0.5 after calcination at 550 C displayed a dou-
3
◦
−
1
700 C (Fig. 7h) and the latter can be assigned to the reduction of
blet with an isomer shift (IS) of around 0.3–0.4 mm s and with
2+
−
1
Ni in Mg(Ni,Al)O.
All broad peaks observed around 400 C for iron
a quadrupole splitting (QS) of around 0.6–0.8 mm s ; such values
◦
are typical of super paramagnetic Fe3+ species. All spectra of the
containing catalysts, i.e., Mg Fe0.25Co0.25Al0.5 (Fig. 7a),
3
samples can be fitted with two doublets, whose calculated param-
eters are shown in Table 2. One of the two doublets is attributed to
Mg Fe0.25Co0.25Al0.5 (Fig. 7c), Mg Fe0.25Co0.25Al0.5 (Fig. 7d),
2
4
3+
Mg3Fe0.25Co0.25Al0.5(9.5) (Fig. 7f), Mg3Fe0.25Co0.25Al0.5(10.5)
(Fig. 7g) and Mg3Fe0.25Ni0.25Al0.5 (Fig. 7h) are attributed to the
the Fe in spinel, whereas the other doublet is attribute to the
3+
Fe in peliclase, as seen in the XRD (Fig. 2). The XRD patterns
showed the presence of periclase as a main phase, while spinel
was observed as a minor phase or as an amorphous phase. The
reduction of Fe3+ to Fe in Fe–Co and Fe–Ni bimetallic species [20].
2+
◦
After the TPR measurements up to 950 C, Mg Fe0.25Co0.25Al0.5 and
3
3
+
Mg3Fe0.25Ni0.25Al0.5 showed the reflection lines of Fe–Co and Fe–Ni
alloys, respectively. Mg3Fe0.25Co0.25Al0.5 showed the reflections of
concentration of Fe is higher in spinel than in periclase, since
3+
Fe possesses a regular site in the former crystal structure but
◦
◦
◦
bcc-Fe metal at 2ꢁ = 43.0 (1 1 0), 65.3 (2 0 0) and 82.9 (2 1 1) and
not in the latter. No clear correlation was thus observed between
the abundance of Fe3+, shown as the value of relative contribution
no reflections of Co metal, indicating that Co dissolved into Fe to
form bcc-like Fe–Co alloy [37]. Also Mg Fe0.25Ni0.25Al0.5 showed
3
(
Table 2), and the analytical results obtained by XRD. Mg Fe0.5Al0.5
3
◦
◦
3
+
3+
the reflections of Fe–Ni alloy at 2ꢁ = 43.5 (1 1 1) and 50.7 (2 0 0)
showed two doublets, assigned to Fe (1) in Mg(Fe ,Al) O spinel
2
4
[38]. This indicates that both Co and Ni2+ are well dispersed
together with Fe3+ in/on the catalysts, resulting in an appearance
of strong interaction between Fe and Co as well as Fe and Ni.
2+
3
+
3+
and to Fe (2) in Mg(Fe ,Al)O periclase. The relative contribu-
3+
tion of spinel to periclase decreased by replacing a half of Fe
with Co2+ (Mg Fe0.25Co0.25Al0.5), whereas it increased by replac-
3
3
+
2+
ing a half of Fe with Ni (Mg Fe0.25Ni
may conclude that iron species exists as Fe state in Mg Fe0.5Al0.5
Al0.5) (Table 2). One
3
0.25
3
+
,
3
3.6. Reducibility of active iron species
Mg Fe0.25Co0.25Al0.5 and Fe Fe0.25Ni0.25Al0.5
.
3
3
The H2 consumption of the Fe3+ reduction peak around 400 C
◦
3
.5. H -TPR measurements of the catalyst
in the H -TPR (Fig. 7) was calculated per molar amount of Fe
2
2
−
1
in the catalysts, i.e., [H2 cons.]max (mol mol ), and is shown
Fe
H -TPR data of the catalysts are shown in Fig. 7.
in Table 3 together with the peak temperature. Each peak was
not well separated from the others and therefore the peak
height was tentatively used for the calculation. In all cases,
the Fe3+ reduction peak was shifted to lower temperature by
2
Mg Fe0.25Co0.25Al0.5 showed
00 C together with a strong reduction peak around 850 C
a
weak reduction peak around
3
◦
◦
4
(
Fig. 7a). In the previous paper [20], we assigned the former
◦
3+
2+
◦ ◦
bimetal formation compared to 418 C for Mg Fe and 417 C
3 1
peak around 400 C to the reduction of Fe in spinel to Fe in
◦
periclase and the latter peak around 850 C to the reduction of
Fe in periclase to Fe . When the Fe/Co loading was decreased
to 0.1/0.1, the peak around 400 C almost disappeared (Fig. 7b).
for Mg Fe0.5Al0.5 [20]. The lowest value of [H2 cons.]max was
3
2+
0
observed for Mg Fe0.25Co0.25Al0.5, followed by Mg Fe0.25Ni0.25Al0.5
3
3
◦
and the others, whereas the peak temperature was the lowest for
Mg Fe0.25Ni0.25Al0.5, followed by Mg Fe0.25Co0.25Al0.5 and the oth-
Mg Co0.5Al0.5 showed the broad reduction peaks between 150 and
3
3
3
◦
◦
3+
3
50 C and an intensive reduction peak around 850 C (Fig. 7e). This
ers. It is concluded that the Fe species was the most reducible
on Mg Fe Ni0.25Al0.5, followed by Mg Fe0.25Co0.25Al0.5, and the
reducible Fe amount was the smallest on Mg Fe0.25Co0.25Al0.5,
reduction profile is very similar to that reported by Khassin et al.
3
0.25
3
3+
[
33]; Co/Mg/Al mixed oxides derived from hydrotalcites showed
3
◦
broad reduction peaks between 230 and 350 C and an intensive
followed by Mg Fe0.25Ni0.25Al0.5. The pH in the coprecipitation
3
◦
◦
peak above 600 C after calcination at 450–500 C. The peaks
of Mg Fe0.25Co0.25Al0.5 and the Mg/Al ratio in the catalyst also
3
at low temperature are similar to those observed for Co/Al O
affected the peak temperature and the peak intensity. Both
Mg Fe0.25Co0.25Al0.51(9.5) and Mg Fe0.25Co0.25Al0.5(10.5) afforded
2
3
prepared by impregnation with Co2 nitrate and are attributed
+
3
3

L.A. Atanda et al. / Applied Catalysis A: General 396 (2011) 107–115
113
-1
-1
50 μmol gcat
50 μmol gcat
i)
e)
d)
c)
h)
g)
b)
a)
f)
1
00 200 300 400 500 600 700 800 900
0
100 200 300 400 500 600 700 800 900
Temperature / °C
Temperature / °C
Fig. 7. H2-TPR of Mg–Fe–Co–Al and Mg–Fe–Ni–Al catalysts. (a) Mg3Fe0.25Co0.25Al0.5; (b) Mg3Fe0.1Co0.1Al0.5; (c) Mg2Fe0.25Co0.25Al2; (d) Mg4Fe0.25Co0.25Al0.25; (e) Mg3Co0.5Al0.5;
f) Mg3Fe0.25Co0.25Al0.5(9.5); (g) Mg3Fe0.25Co0.25Al0.5(10.5); (h) Mg3Fe0.25Ni0.25Al0.5; (i) Mg3Ni0.5Al0.5.
(
Table 3
3+
◦
Fe reduction around 400 C in H2-TPR.
◦
[H2 cons.]max/ꢂmol g-1
Fe2O3/ꢂmol g-1
−1
Catalyst
Peak temperature/ C
[H2 cons.]max/mol molFe
cat
cat
Mg3Fe1
Mg3Fe0.5Al0.5
418
417
391
353
400
396
410
409
200
2491
1342
668
711
560
582
668
668
0.0803
0.0583
0.0220
0.0315
0.0680
0.0715
0.0451
0.0371
78.3
14.7
22.4
38.1
41.6
30.1
24.8
Mg3Fe0.25Co0.25Al0.5
Mg3Fe0.25Ni0.25Al0.5
Mg2Fe0.25Co0.25Al2
Mg4Fe0.25Co0.25Al0.25
Mg3Fe0.25Co0.25Al0.5(9.5)
Mg3Fe0.25Co0.25Al0.5(10.5)
the higher peak temperature as well as the larger [H cons.]max for
SiO2 [42]. The Mg peaks appeared at 48.9–49.1 eV and are assigned
to 2p3/2 of Mg well fitted with a Gaussian curve as reported for
2
3+
2+
Fe reduction than Mg Fe0.25Co0.25Al0.5(10.0). Both Mg/Al ratios
3
of 2/2 (Mg Fe0.25Co0.25Al ) and 4/0.25 (Mg Fe0.25Co0.25Al0.25) also
Pt(Sn)/Mg(Al)O catalysts [43]. Al 2p peaks overlapped on the Pt 4f
doublet arising from the Pt gauge used for the analyses and are
therefore not shown.
2
2
4
showed the higher peak temperature and the larger [H cons.]max
2
3
+
for Fe reduction than Mg Fe0.25Co0.25Al0.5. Indeed the activity
3
of the catalyst significantly varied by changing the pH in copre-
cipitation or the Mg/Al ratio in the catalyst; both pH = 9.5 and
Olsbye et al. [43] reported that the depth profiling of
Pt(Sn)/Mg(Al)O catalysts by XPS showed that the Mg/Al ratio in
1
0.5 afforded the lower activity compared with pH = 10.0 and both
the Mg(Al)O crystallites does not vary significantly in the analysis
depth range of 10–21 A˚ . This result indicates that both Mg and Al
Mg Fe0.25Co0.25Al0.25 and Mg Fe0.25Co0.25Al2 showed the lower
4
2
activity than Mg Fe0.25Co0.25Al0.5 (Table 1). One may conclude that
the pH in the coprecipitation and the Mg/Al ratio in the catalyst
are homogeneously distributed in the Mg(Al)O crystallites derived
from hydrotalcites. In the present work, the surface molar ratios
of Mg/Fe, Mg/Co and Mg/Ni calculated from the XPS results exhib-
ited smaller values than the bulk molar ratios obtained by the AA
analyses (Table 4). Although the data of AA were not shown for
3
also played important roles in the catalytic activity through the
reduction behaviors of Fe3
+
.
Mg Co0.5Al
and Mg Ni0.5Al0.5, it was confirmed that the metal
3
.7. Comparison between Mg Fe0.25Co0.25Al0.5 and
3
0.5
3
3
compositions by AA analyses coincided with those expected by
the amount of raw materials [20]. This indicates that Fe, Co and
Ni species are localized in the surface layer of the catalyst particles;
such surface enrichment was most enhanced for Ni, followed by Co
Mg Fe0.25Ni0.25Al0.5
3
The XPS analytical results of several catalysts are shown in
Table 4. In the spectra of Fe 2p region for all samples, peaks were
and Fe. The experimental values scattered for Mg Co0.5Al0.5, sug-
observed at 710.5–710.6 and 723.7–724.1 eV and are assigned to
3
the 2p3/2 and 2p1/2 spin orbit components, respectively, of Fe3+
gesting that Co dispersion on the catalyst was not homogeneous
due to the phase separation (vide supra). However, no clear phase
species [39]. The peaks of Co and Ni were observed as follows: Co
separation of Co was observed for the active Mg Fe0.25Co0.25Al
at 779.8–779.9 and 785.6–785.7 eV assigned to 2p
and 2p1/2 of
3
0.5
3/2
2+
catalyst. When the surface concentrations of Fe, Co and Ni were
compared on Mg Fe0.25Co0.25Al
Co [40] and Ni at 853.8–853.9 and 872.5–872.7 eV assigned to
2
+
and Mg Fe0.25Ni0.25Al0.5, the
0.5 3
2
p3/2 and 2p1/2 of Ni , although the 2p binding energy was con-
3
3/2
active Fe3+ was enriched more on the former than on the lat-
2
+
siderably lower than 856.2 eV of Ni and close to 852.5 eV reported
2+
0
ter, and the Ni enrichment on the latter was higher than the
Co enrichment on the former. It is concluded that, on the active
for 2p3/2 of Ni on Rh–Ni/CeO –Al O catalyst [41]. This may be
2
2
3
2+
due to the substantial reduction of Ni to Ni by Ar+ bombard-
2+
0
Mg Fe0.25Co0.25Al0.5, nearly equal amounts of Fe and Co appeared
ment during XPS measurements as observed for NiO dispersed on
3

1
14
L.A. Atanda et al. / Applied Catalysis A: General 396 (2011) 107–115
Table 4
XPS analytical data of the catalysts .
a
Catalyst
Molar ratiob
Binding energyc /eV
Surface molar ratiod
Mg/Fe
Mg/Mec
Mg 2p3/2
Fe 2p3/2
Me 2p3/2
Mg/Fe
Mg/Mec
Mg3Fe0.5Al0.5
5.9
11.5
10.2
–
49.1
48.9
49.0
49.0
48.9
710.5
710.6
710.6
–
–
3.7
6.5
8.0
–
–
Mg3Fe0.25Co0.25Al0.5
Mg3Fe0.25Ni0.25Al0.5
Mg3Co0.5Al0.5
11.5
11.3
–
779.9
853.8
779.8
853.9
5.0
2.4
2.5
2.4
e
Mg3Ni0.5Al0.5
–
–
–
–
a
◦
All catalysts were prepared by co-precipitation of metal nitrates at pH = 10.0 and calcined at 550 C for 12 h.
By AA analyses.
Me = Co or Ni.
By XPS analyses.
b
c
d
e
The data scattered between 2.1 and 2.9.
and formed Fe3+–Co2+ (5.0:6.5) bimetallic system on the surface.
sors and tested in the dehydrogenation of ethylbenzene. After
the calcination at 550 C, the catalysts were composed of spinel
3+
◦
Contrarily, on Mg Fe0.25Ni0.25Al0.5, Fe was more deeply incorpo-
3
2
+
rated, whereas Ni was more located on the surface (surface ratio
and periclase-type compounds with
a
high surface area of
3+
2+
2
−1
3+
2+
Fe :Ni = 2.4:8.0).
150–240 m g . Fe –Co system showed a synergy in the cat-
cat
3+
2+
alytic activity, whereas the Fe –Ni system showed no synergy.
On Mg Fe CoxAl0.5, the rate of styrene formation was the
3
.8. Some characteristics of active Mg Fe0.25Co0.25Al0.5 catalyst
3
0.5−x
3
highest at x = 0.4, but CoOx was isolated at x ≥ 0.4, resulting in
a decrease in styrene selectivity as well as some coking on the
catalysts. The best catalyst was obtained around x = 0.25. The
coprecipitation of hydrotalcites at pH = 10.0 and the Mg/Al ratio
is 3/0.5 caused formation of well grown hydrotalcite structure,
One of the authors reported that the good catalytic performance
of Mg Fe0.5Al0.5 can be attributed to the formation of partially
3
3
+
2+
reduced iron oxides (Fe /Fe ) and the dehydrogenation proceeds
3
+
via reduction–oxidation of Fe species on the surface of catalyst
19]. Krishnasamy et al. [44] reported that dehydrogenation of
ethylbenzene over Zn–Fe–Cr ternary spinel system was catalyzed
3+
leading to the highest activity. The surface enrichment of Fe as
[
well as the surface composition of Fe3+/Co = 1/1 was confirmed
2+
by the presence of stable Fe3+/Fe reduction–oxidation couple and
2+
on the active Mg Fe0.25Co0.25Al0.5 catalyst. The dehydrogenation
3
3
+
2+
2+
0
was catalyzed by the reduction–oxidation of Fe /Fe . The H -TPR
the deactivation of catalyst is due to the reduction of Fe to Fe .
It is suggested by the H -TPR measurements (Fig. 7 and Table 3)
2
3+
2+
observation showed that Fe was reduced, assisted by Co at low
temperature and the amount of reducible Fe3+ was small on the
2
3
+
that, on the active catalysts, Fe species must be reducible at the
lower temperature for accelerating the reduction–oxidation cycle
active Mg Fe0.25Co
Al
catalyst. This leads to the formation
3
0.25 0.5
of active and stable Fe –Co2+ bimetallic species on the catalyst
3+
and simultaneously the reducible Fe3+ amount must be small for
stabilizing the active sites on the catalysts. Indeed, the activity
surface.
Mg Fe0.1Co0.1Al0.5 of small amount of Fe3+–Co couple was sta-
2+
3
bilized; ethylbenzene conversion was almost constant during the
time-on-stream (data are not shown), although the reduction peak
around 400 C was negligibly small (Fig. 7b) and the activity was
Acknowledgements
◦
This publication was based on work supported by Award No.
K-C1-019-12 made by King Abdullah University of Science and
Technology (KAUST). The support of King Fahd University of
Petroleum and Minerals (KFUPM) is also highly appreciated. The
authors also acknowledge Japan Cooperation Center, Petroleum
lower than that of Mg Fe0.25Co0.25Al0.5. Contrarily, no increase in
3
3+
2+
the activity was observed even when Fe /Co loading increased
from Mg Fe0.25Co0.25Al0.5 to Mg Fe0.5Co0.5Al0.5
.
3
3
2
+
We conclude that Co addition was effective for enhancing
3
+
the activity of Fe species on Mg–Fe–Al catalysts for ethylben-
zene dehydrogenation. Fe3+–Co bimetallic species can work as
the active sites on the Mg(Al)O supports. The surface enriched
(JCCP) for giving opportunity of this collaborative research.
2+
3+
Fe species suggests that spinel phase was located in the surface
layer of the catalyst particles. Although Mg3.0Co0.5Al0.5 catalyst was
active, the styrene selectivity decreased by the dealkylation and
the reactor plugging occurred due to the coking on the catalysts, as
mentioned previously. The highest activity was observed around
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