Dehydrogenation of ethylbenzene with carbon dioxide as oxidant over Mg-modified alumina-supported V-Sb oxide catalysts

Article abstract of DOI:10.1246/cl.2006.28

Modification of alumina support with an appropriate amount of magnesia (Mg/Al = 0.1) leads to the stable activity of the supported vanadium-antimony oxide catalyst for the dehydrogenation of ethylbenzene into styrene in the presence of carbon dioxide as o

Full text of DOI:10.1246/cl.2006.28

28
Chemistry Letters Vol.35, No.1 (2006)
Dehydrogenation of Ethylbenzene with Carbon Dioxide
as Oxidant over Mg-modified Alumina-supported V–Sb Oxide Catalysts
Do-Young Hong,1;2 Jong-San Chang,ꢀ1 Vladislav P. Vislovskiy,1 Sang-Eon Park,ꢀ3 Yeung-Ho Park,2 and Jin S. Yoo1
1Catalysis Center for Molecular Engineering, Korea Research Institute of Chemical Technology (KRICT),
P. O. Box 107, Yuseong, Daejeon 305-600, Korea
2Department of Chemical Engineering, Hanyang University, Seoul 133-791, Korea
3Department of Chemistry, Inha University, Incheon 402-751, Korea
(Received August 1, 2005; CL-050998)
Modification of alumina support with an appropriate amount
2400), X-ray diffraction (XRD) (Rigaku D/MAX-3B diffrac-
tometer) and the NH3-temperature-programmed-desorption
method (NH3-TPD) (Micrometrics, TPD/TPR 027 Option).
The catalytic tests were performed with 1 g of catalyst at
595 ꢁC under atmospheric pressure in an isothermal fixed bed
flow reactor of the continuous micro-activity test unit. The ethyl-
benzene feed rate was 8.2 mmol/h, molar ratio CO2/EB ¼ 5, to-
tal flow rate of the gas mixture of CO2 and N2 was 45 mL/min.
The components of reaction mixtures were analyzed by on-line
gas chromatograph.
of magnesia (Mg/Al ¼ 0:1) leads to the stable activity of the
supported vanadium–antimony oxide catalyst for the dehydro-
genation of ethylbenzene into styrene in the presence of carbon
dioxide as oxidant. Correlation between catalytic performance
and surface acidity has been found.
Catalytic dehydrogenation of ethylbenzene in the presence
of carbon dioxide (CO2-EBDH) has recently been attempted to
explore a new technology for producing styrene selectively be-
cause it could provide us with an energy-saving and environ-
mentally friendly process to utilize CO2, a greenhouse gas, as
a mild oxidant.1–3 Several families of the catalysts, mainly based
on supported or mixed oxides of Fe, V, or Cr were found to be
active and selective in the CO2-EBDH.2–14 However, most of
the catalysts have usually shown strong deactivation for this re-
action during even a few hours on stream. As a typical example,
over Mg–Al–V mixed oxides, the initial styrene yield, 64% after
1 h on stream, decreased to 40% after 7 h.7 The similar deactiva-
tion rate was observed also for VOx/MgO catalyst.3 We have
proposed a very active and selective catalyst, V0:43Sb0:57Ox/
Al2O3 (VSb/Al), but its deactivation was also significant mainly
owing to coke formation although the deactivation rate was less
than other reported catalysts.12,13
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Since the coke precursors are easily formed through the
oligomerization of olefins, facilitation of olefins desorption from
a catalyst surface could minimize the carbon accumulation.
Taking into consideration a basic character of electron-rich
olefins including styrene, lower surface acidity of the catalyst
appears to induce easier desorption of olefins from the catalyst
surface. VOx catalysts loaded on MgAl oxide support exhibited
a decreased adsorption and rapid desorption of propylene as
compared with those on alumina.14,15 Based on these considera-
tions, an attempt has been made to stabilize the activity of VSb/
Al catalyst in the CO2-EBDH by the modification of the alumina
support with a basic MgO component.
Time-on-stream / h
Figure 1. Catalytic dehydrogenation of ethylbenzene with car-
bon dioxide as oxidant over supported V–Sb oxide catalysts at
595 ꢁC: VSb/Al ( ) and VSb/MgnAl with n ¼ 0:1( ),
n ¼ 0:3( ), or n ¼ 0:5( ).
VSb/Al and VSb/MgnAl catalysts are highly selective with
respect to styrene (>95%). The VSb/Al and VSb/Mg0:1Al sam-
ple showed similar initial styrene yield, YST ꢂ 75 mol % after
1 h on-stream. However, considering the difference in their
SBET values (87 and 67 m2/g, respectively), the VSb/Mg0:1Al
catalyst can be regarded as a little more active one due to its
higher specific yield based on the SBET value (YST/SBET),
1.1%/m2 against 0.9%/m2 for the VSb/Al. The VSb/Mg0:3Al
and VSb/Mg0:5Al catalysts displayed substantially lower sty-
rene yield (ꢂ45 and 30%, respectively) due to their reduced spe-
MgO-modified alumina (MgnAl) supports were obtained by
impregnation of an activated alumina with aqueous solutions
.
containing a certain amount of Mg(NO3)2 6H2O (MgnAl; n is
an atomic ratio of Mg/Al ¼ 0:1, 0.3, or 0.5) followed by calci-
nation at 670 ꢁC. Alumina (Al)- or MgnAl-supported vanadium–
antimony oxide catalysts were prepared by impregnation with
the solutions of NH4VO3 and SbCl3 followed by calcination at
650 ꢁC. The loading of the supported V0:43Sb0:57Ox component
(20 wt %) was the same for all samples. The catalyst supports
and supported catalysts were characterized by BET specific sur-
face area (SBET) measurements (Micrometrics model ASAP
cific surface areas (48 and 38 m2/g, respectively) but their YST
/
S
BET values (0.9–0.8%/m2) are very close to that for the VSb/Al
catalyst. Thus, the specific surface areas of the catalysts decrease
with an increase of MgO-content, affecting only slightly their
specific activities. The activities of the supported V–Sb oxide
catalysts in the CO2-EBDH are mainly determined by the sup-
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.1 (2006)
29
ported VOx species whereas the presence of SbOx positively af-
fects the catalyst stability through the facilitation of the redox
cycle.12,13 Moreover, it is noted that the Mg-containing catalysts
studied exhibit much stable catalytic behavior compared to the
VSb/Al (Figure 1). When we checked the content of carbon
formation on each catalyst after 12 h on stream, we observed that
Mg-modified catalysts reveal less carbon formation (4.4–
6.0 wt %) as compared with VSb/Al (13.1 wt %), indicating
that modification of alumina support with magnesia leads to
the stable activity, at least partly, due to strong resistance against
carbon formation.
As illustrated in NH3-TPD profiles (Figure 2), the VSb/
MgnAl catalysts contain smaller amounts of surface acid
sites compared with the un-modified VSb/Al catalyst. Thus,
the higher Mg-content leads to the lower acidity. Therefore,
the better stability of the MgO-containing catalysts is ascribed
to their reduced surface acidities.
VSb/Mg0.5Al
Mg0.5Al
VSb/Mg0.3Al
Mg0.3Al
VSb/Mg0.1Al
Mg0.1Al
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40
50
60
70
80
2θ / degree
Figure 3. XRD patterns of MgnAl supports and VSb/MgnAl
catalysts: ( ) MgAl6O10; ( ) MgAl2O4; ( ) MgO; ( )
25
V
1:1Sb0:9O4; ( ) V0:95Sb0:95O4.
(a)
In summary, this work demonstrates that a favorable balance
20
between the decreased acidity of the V–Sb oxide catalyst sup-
ported on MgO-modified alumina and an amount of V–Sb oxide
species is needed for the stable and active catalytic performance
in the CO2-EBDH. As a result, modification of alumina support
with an appropriate amount of magnesia (Mg/Al ¼ 0:1) leads to
the stable activity of the VSb/Mg0:1Al catalyst for the reaction.
(b)
15
(c)
10
(d)
5
The financial support of the Ministry of Science and
Technology of Korea (MOST) is gratefully acknowledged.
VPV thanks the MOST for the visiting scientist fellowship.
We also thank Dr. S. H. Jhung for helpful discussions.
0
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o
Temperature / C
References
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Figure 2. NH3-TPD profiles of (a) VSb/Al and VSb/MgnAl
catalysts: (b) n ¼ 0:1, (c) n ¼ 0:3, and (d) n ¼ 0:5.
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Published on the web (Advance View) December 3, 2005; DOI 10.1246/cl.2006.28