Highly ordered mesoporous carbon as catalyst for oxidative dehydrogenation of ethylbenzene to styrene

Article abstract of DOI:10.1002/asia.200800424

We demonstrate that mesoporous carbon without deposition of metal particles is a highly active catalyst. It exhibits both high activity and selectivity in oxidative dehydrogenation of ethylbenzene to styrene, as well as long catalytic stability when compa

Full text of DOI:10.1002/asia.200800424

FULL PAPERS
DOI: 10.1002/asia.200800424
Highly Ordered Mesoporous Carbon as Catalyst for Oxidative
Dehydrogenation of Ethylbenzene to Styrene
[
a]
[a]
[a]
[a]
[a]
Dang Sheng Su,* Juan Jose Delgado, Xi Liu, Di Wang, Robert Schlçgl,
[
b]
[b]
[b]
[b]
Lifeng Wang, Zhe Zhang, Zhichao Shan, and Feng-Shou Xiao*
Abstract: We demonstrate that meso-
porous carbon without deposition of
metal particles is a highly active cata-
lyst. It exhibits both high activity and
selectivity in oxidative dehydrogena-
tion of ethylbenzene to styrene, as well
as long catalytic stability when com-
pared with activated carbon. Both the
as-prepared mesoporous carbon and
the active coke formed during the ini-
tial stage of the reaction play an impor-
tant role in the catalytic performance.
XPS and IR techniques reveal that the
surface oxygen functional groups
formed during the reaction are the
active sites for the reaction. The or-
dered mesoporous structure is benefi-
cial for mass transport in catalytic reac-
tion exhibiting long term stability in
contrast to activated carbon.
Keywords: dehydrogenation
·
ethylbeneze · heterogeneous catal-
ysis · mesoporous materials · sty-
rene
Introduction
a superior performance for O reduction in a fuel-cell
2
[3]
setup. Recently, an ordered mesoporous carbon has been
used for the assembly of a magnetically separable hydroge-
Synthesis of ordered mesoporous carbon materials with vari-
[1]
[12]
ous structures has achieved great progress. Such carbon
materials are usually synthesized by carbonization of su-
crose, furfuryl alcohol, resin, or other suitable carbon sour-
ces inside silica or aluminosilicate mesopores. The high sur-
face areas and controllable pore sizes of the ordered meso-
porous carbons with large pore volumes make it a promising
candidate for many applications, such as for the storage of
hydrogen and methane, as super capacitor, and for molecu-
nation catalyst. Magnetic nanoparticles are deposited on
the external surface of mesoporous carbon protected by a
thin carbon shell. Such magnetic carbon supports loaded
with palladium as hydrogenation catalyst could be complete-
ly removed by a magnet from the reaction solution of the
hydrogenation of octene. While in all the reported works
(including those in electrocatalysis), ordered mesoporous
carbons are only used as supporting materials, no effort has
been made up to now to use mesoporous carbon itself as a
catalyst.
[2–11]
lar separation.
In catalysis, ordered mesoporous carbons
were used to support a high loading of platinum in a highly
dispersed state. Even for a Pt loading of 50 wt%, a mean
size of 2.5 nm of Pt particles could be maintained, leading to
In fact, mesoporous carbon with ordered and adjustable
porosity, large pore size and pore volume, and high surface
[13]
area can be a candidate as a high performance catalyst in
the gas-phase reaction. Carbon materials, especially nano-
structure carbon, have been reported to be highly active cat-
alysts for oxidative dehydrogenation (ODH) of ethylben-
[
a] Dr. D. S. Su, Dr. J. J. Delgado, Dr. X. Liu, Dr. D. Wang,
Prof. Dr. R. Schlçgl
Fritz Haber Institute of the Max Planck Society
Faradayweg 4–6
[14,15]
zene to styrene.
Herein, we report on the catalytic per-
formance of highly ordered mesoporous carbon in the ODH
of ethylbenzene to styrene synthesis. Searching for new cata-
lysts for use in the synthesis of the styrene monomer has
drawn a lot of attention as the monomers are needed in sev-
eral polymer syntheses. The current industrial processes are
based on metal-oxide catalysts working at high temperatures
in excess of steam. Our results reveal that ordered mesopo-
rous carbons are active and stable for the synthesis of sty-
rene under more favorable reaction conditions (low temper-
1
4195 Berlin (Germany)
Fax : (+49)30-8413-4401
E-mail: dangsheng@fhi-berlin.mpg.de
[
b] Dr. L. Wang, Z. Zhang, Z. Shan, Prof. F.-S. Xiao
State Key Laboratory of Inorganic Synthesis
and Preparative Chemistry & College of Chemistry
Jilin University
Changchun 130012 (China)
Fax : (+86)431-85168624
E-mail: fsxiao@mail.jlu.edu.cn
1108
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 1108 – 1113

ature, without steam, energy saving) than that of the indus-
trial catalyst.
lel with and perpendicular to the well-ordered arrays of
channels confirm the well-ordered hexagonal mesostructure.
The catalytic performance of ODH of ethylbenzene to
styrene over CMK-3 carbon at different temperature and
flow rate are summarized in Table 1. The conversion of EB
increases from 36% to 69% when the temperature is
Results and Discussions
Sucrose was used as a carbon
source and SBA-15 as the tem-
plate for the synthesis of
Table 1. Comparison of styrene yields over CMK-3 carbon at different operation conditions.
Operation conditions
Conversion* [%]
Selectivity* [%]
Yield* [%]
[1b]
CMK-3 mesoporous carbon.
Scanning electron microscopy
SEM) of the mesoporous
ꢀ
1
3
3
508C, 10 mLmin , O/EB=5:1
36.1
51.2
69.0
74.1
77.7
77.8
76.0
62.0
28.0
39.8
52.4
45.9
ꢀ1
758C, 10 mLmin , O/EB=5:1
ꢀ
1
(
4008C, 10 mLmin , O/EB=5:1
ꢀ
1
carbon after the removal of
3508C, 5 mLmin , O/EB=5:1
the silica template reveals the
[*]Values obtained after 5 hour at the steady state.
highly ordered mesostructure
replicated from SBA-15 (Fig-
ure 1A). Figure 1B shows the image obtained from trans-
mission electron microscopy (TEM) of the mesoporous car-
bons. The images taken with the electron beams both paral-
changed from 3508C to 4008C, while the selectivity of EB to
ST remains nearly constant (about 76%). The only by-prod-
ucts observed during the whole catalytic test were CO and
2
H O, formed from the total oxidation of ethylbenzene. A
2
ꢀ
1
ꢀ1
change of flow rate from 10 mLmin down to 5 mLmin
at 3508C leads to an increased conversion of ethylbenzene
(
(
from 36% to 74%), but a decreased selectivity to styrene
from 77% to 62%). It is remarkable that an increase of the
conversion rate by increasing the temperature did not
change the selectivity of the catalyst, while the selectivity
decreases by decreasing the flow rate. It seems that the high
surface area of the CMK-3 would lead to the re-adsorption
of the styrene even for short retention times, rendering a
further reaction with oxygen and therefore, a total oxida-
tion. For comparison, the yield of styrene at steady state
over various tested nanocarbons is summarized in Table 2.
CMK-3 carbon exhibits the highest selectivity under the
given conditions. This material exhibits a styrene yield that
is 2–4 times higher than the commercial carbon nanotubes.
Figure 2 displays the long term stability test of the ODH
of ethylbenzene to styrene as a function of time on flow
rate. After a short induction time at 4008C and at a flow
ꢀ
1
rate of 10 mLmin , a selectivity of 76% EB to ST is ach-
ieved at a conversion of 69%. The high activity at the begin-
ning of the reaction is accompanied by a slightly lower selec-
tivity, but both level-off after 3 h. The C-balance of the inlet
and outlet stream indicates that coking took place during
the initial deactivation period at the beginning of the reac-
tion, leading to a slightly lower concentration of carbon in
the stream. After this period, the highly ordered CMK-3
carbon is structurally stable during the catalytic test.
The SEM image in Figure 3A reveals coke formation on
the surface of the mesoporous carbon, and the TEM image
in Figure 3B reveals that the mesopores are, to a certain
degree, blocked by the coke formed at the beginning of the
reaction. It is obvious that the mesoporous carbon is initially
highly active for the ODH of EB, but coke formed at the
beginning of the reaction may also be an active phase as has
[22b]
been reported by Cadus et al. for Al O .
The combina-
2
3
Figure 1. A) SEM image of CMK-3 carbons, B) TEM micrographs of
CMK-3 carbon taken with the electron beam parallel the channels. The
inset shows the channel structure.
tion of both the initial CMK-3 carbon and the active coke
gives the high reaction rate shown in Table 1 and the long
Chem. Asian J. 2009, 4, 1108 – 1113
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1109

D. S. Su, F.-S. Xiao et al.
FULL PAPERS
Table 2. Comparison of styrene yields over various catalysts.
Figure 6A shows the C 1s X-
ray photoelectron spectrum
(XPS) of CMK-3 before and
after reaction. The carbon
Sample
Operation conditions
Conversion [%]
Selectivity [%]
Yield [%]
ꢀ
1
CMK-3
Applied Science
FC: CNF-PL
FC: CNF-SC
4008C, 10 mLmin , O/EB=5:1
69.0
20.1
26.2
45.6
76.0
70.0
64.9
50.4
52.4
14.1
17.0
23.0
[
a]
ꢀ1
4008C, 10 mLmin , O/EB=5:1
[
b]
ꢀ1
4008C, 10 mLmin , O/EB=5:1
spectra can be fitted to five
[
c]
ꢀ1
4008C, 10 mLmin , O/EB=5:1
[19,20]
peaks:
one peak for
[
a] Multiwalled carbon nanotubes (provided by Applied Science Ltd.). [b] Carbon nanofibers with platelet
carbon atoms with bonds to
carbon and/or hydrogen atoms
structure (provided by Future Carbon GmbH). [c] Carbon nanofibers with screw structure (provided by
Future Carbon GmbH).
Figure 2. Conversion (^) and selectivity (&) of oxidative dehydrogena-
tion of ethyl benzene to styrene over highly ordered CMK-3 carbon at
o
4
00 C
term high stability shown in Figure 2. The electron-energy
loss spectra (EELS) of the carbon K edge of the CMK-3
carbon before and after reaction are shown in Figure 4. The
spectrum of the as-prepared sample exhibits a maximum at
286 eV, typical for the transition of carbon 1s electrons to
the unoccupied p* states. A broad feature from 292 eV to
about 313 eV is attributed to 1s–s* transitions. The as-pre-
pared sample shows a sharper peak at the onset of the s*
band than that of the sample after reaction. The prominent
p* and s* features in the as-prepared sample indicate a
more graphitic feature of the prepared CMK-3 carbon. The
EELS spectrum of the sample after reaction is quite similar
to that of disordered carbon, revealing that the coke formed
during the reactions differs in structure from the CMK-3
carbon.
The change in the pore structure of the mesoporous
carbon is also confirmed by the nitrogen-isotherm plot
shown in Figure 5. Nitrogen isotherms of CMK-3 before and
after reaction exhibit typical IV curves, suggesting that both
samples have mesopores with cylindrical shape. However,
the sample after reaction shows a less obvious step at low
partial pressures (before 0.4). After reaction, the BET sur-
Figure 3. SEM (A) and TEM (B) micrographs of the CMK-3 carbon
after 100 h catalytic test.
(C , BE=284.5 eV); three peaks for carbon atoms with one,
1
two, and three bonds to oxygen atoms (C , C , C ; CꢀOH,
2
3
4
2
ꢀ1
face area of the CMK-3 carbon is reduced from 983 m g
C=O, COOH, BE=285.0, 286.5, and 288.3 eV, respectively)
and a p!p* peak (C , BE=290). The C peak had an asym-
2
ꢀ1
for the fresh catalyst to 211 m g , which is obviously a
result of the formation of coke during the reaction. Howev-
er, the high stability shown in Figure 2 indicates that the loss
of the surface area at the very beginning of the reaction
does not have a strong influence to the catalytic perfor-
mance of the CMK-3 carbon.
5
1
metrical shape, whereas the other peaks were symmetrical.
C1s spectra before and after reaction were dominated by
the intense asymmetrical C and a smaller p!p* (C ) peak,
1
5
indicating a polyaromatic surface such as carbon black and
[19,21]
carbon fiber.
The ODH reaction had a pronounced
1110
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Chem. Asian J. 2009, 4, 1108 – 1113

Mesoporous Carbon as Catalyst for Oxidative Dehydrogenation
Figure 4. EELS spectra of carbon of the tested catalyst before (A) and
after (B) reaction.
Figure 6. A) C1s and B) O1s XPS spectra of CMK-3 carbon before and
after 100 h catalytic test.
Figure 6B shows the O 1s XPS of mesoporous carbon
before and after reaction. The oxygen signal increases dra-
matically after reaction. The oxygen content on the surface
of the fresh samples was around 1.5% and it increased to
1
7.8% after the catalytic test. The spectra were fitted using
[21]
the model proposed by Xie and Sherwood to three peaks
corresponding to chinoidic carbonyl groups (O , BE=531.2–
1
5
31.6 eV), CꢀOH and/or CꢀOꢀC groups (O , BE=533–
2
5
34 eV), and chemisorbed oxygen and adsorbed water (O ,
3
[21]
BE=535.4–536 eV).
These results are in good concord-
ance with the C 1s spectra. The presence of strongly basic
sites has been reported as active sites during the ODH reac-
[15,22]
tion.
The obtained results correspond to the fact that
carbonyl-quinone groups are the active sites in the ODH re-
[22]
action.
The IR spectra of the CMK-3 before and after reaction
ꢀ1
are displayed in Figure 7. The 1713 cm band is associated
with the C=O vibrations of carboxyl, lactone, or ketone
groups. The peaks observed in the range from 1559 to
ꢀ
1
1
509 cm can be assigned to carbon skeleton vibration of
ꢀ
1
the aromatic ring. The strong peaks at 1222 cm were at-
tributed to the combination of CꢀO stretching vibration.
ꢀ
1
The peaks at 1041 cm may be associated with CꢀO or Cꢀ
Figure 5. N
tion.
2
-isotherms of CMK-3 carbon before (A) and after (B) reac-
effect on the carbon spectra. After the reaction, the contri-
bution of the C to the total C 1s spectrum decreases from
1
4
3 to 33%. The C peak becomes also broader, which can
1
be related with the disordered structure of the coke. In fact,
it was previously reported that styrene exhibit a peak at
[23]
284.9 eV.
It can be seen that the area of the C (C=O)
3
peak increased after reaction, indicating an increasing car-
bonyl and quinone character of the surface. It is noticeable
that the contribution of the C component in the C 1s spec-
trum increases after reaction.
4
Figure 7. Attenuated total reflectance IR spectra of CMK-3 carbon
before and after 100 h catalytic test.
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1111

D. S. Su, F.-S. Xiao et al.
FULL PAPERS
OꢀC vibrations of ester, ether, phenol, or carboxyl groups.
Conclusion
Clearly, after 100 h of the catalytic test, the sample exhibits
ꢀ
1
an obvious peak at 1713 cm , which is a typical signal of
surface C=O groups. This result confirms that surface C=O
groups are catalytically active for ODH. These groups
would be formed by the activation of molecular oxygen on
the basal planes of the graphite layers or on the defected
carbon surface. The possible presence of acetophenone-type
In conclusion, we demonstrate that the CMK-3 carbon ex-
hibits both high activity and selectivity in oxidative dehydro-
genation of ethylbenzene to styrene, as well as good stability
when compared with other conventional catalysts. Both the
initial material and the active coke formed during the initial
stage of the reaction play an important role in the catalytic
performance. XPS and IR techniques reveal that the surface
oxygen functional groups formed during the reaction are the
active sites for the reaction. The ordered mesopores of the
CMK-3 carbon should be beneficial for mass transport in
catalytic reaction exhibiting long time stability in contrast to
activated carbon.
ꢀ
1
carbon in coke may also cause the vibration at 1713 cm .
Nanocarbons with graphitic structure, such as carbon
[
14]
nanofilaments, onion-like carbons,
and carbon nano-
[15]
tubes, are reported to be highly active for the ODH of
ethylbenzene to styrene. The common characteristic of the
mentioned nanocarbons are the long-range ordering of the
2
graphitic sp structure. The activity of nanocarbon in ODH
of ethylbenzene to styrene is assigned to the surface defects
2
of the nanocarbons and the sp character of electrons in the
Experimental Section
basal planes. The defects allow the anchoring of carbonyl
groups dehydrogenating ethylbenzene to styrene and there-
fore the formation of surface OH-groups. The gas-phase
oxygen dissociated on the basal planes of the graphite layers
by delocalized p-electrons, diffuse to the hydroxyl groups to
Catalyst Synthesis
The CMK-3 ordered mesoporous carbon was synthesized using mesopo-
[3]
rous silica of SBA-15 as a hard template. Typically, mesoporous silica
was impregnated with sucrose solution in the presence of sulfuric acid
and dried at 323 K and subsequently at 433 K. Then the impregnation/
drying step was repeated once. The obtained sample was carbonized
under nitrogen atmosphere at 1173 K for 4 h. Finally, the obtained silica/
carbon composite was stirred in a hydrofluoric acid solution (35%) for
[15]
oxidize the OH groups with water molecules released.
CMK-3 carbon can be efficiently used for ODH of ethyl
benzene to styrene. Besides the outstanding stability, it gives
a high styrene yield among the tested nanocarbons under
the same reaction condition. The XPS and IR spectra of the
CMK-3 before and after reactions confirm that the reaction
mechanism is the same as that when nanocarbons are used.
The presence of oxygenated species is essential for a good
catalytic activity. However, CMK-3 acts in the reaction quite
differently from nanocarbons. While no coke formation was
observed when carbon nanotubes were used for this reac-
tion, CMK-3 plays a “support” role for active coke that is
formed during the ODH of ethyl benzene, as it is revealed
by the catalytic performance in Figure 2. This is also indicat-
ed by the BET surface area and nitrogen isotherms of the
CMK-3 and confirmed by the electron micrograph shown in
Figure 5. Once this active coke is formed on the CMK-3, the
material is stable. The microstructure of the CMK-3 carbon
is similar to activated carbon showing a disordered long-
range ordering of carbon atoms. Both carbon materials are
characteristic for the high-surface area. Activated carbon is
also active in the ODH of ethylbenzene to styrene, but de-
1
2 h, then filtrated and washed with distilled water, and dried at 393 K
overnight.
Characterization
The XRD pattern was obtained by a Siemens D5005 diffractometer using
CuKa radiation. The nitrogen adsorption at 77 K was measured using a
Micromeritics ASAP 2010m system. The samples were degassed for 10 h
at 3008C before the measurements. Scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) experiments were per-
formed on Hitachi S-4000 and Philips CM 200 electron microscopes, re-
spectively. An electron energy-loss spectrometer (EELS) equipped on
the Philips CM200 is used for the measurement of energy-loss spectra of
carbon inner-shell electrons. The XPS experiments were carried out in a
modified LHS/SPECS EA200 MCD system equipped with facilities for
XPS (MgKa 1253.6 eV, 168 W power) measurements. XPS spectra areas
have been corrected considering that C 1s peaks have the same area in
all the samples. The binding energy determined from the C 1s peak was
referenced at 284.4 eV. Attenuated total reflectance infrared spectra (IR)
were obtained on a Perkin–Elmer System 2000 FTIR spectrometer by
using a KBr wafer. Surface analysis was performed on a VG RSCA
LABMK II spectrometer equipped with AlKa radiation. Textural proper-
ties (specific surface area, pore size, and so forth) were determined by ni-
trogen physisorption at ꢀ1968C.
[16,17]
activates rapidly until nearly no activity is exhibited.
The CMK-3 used in the present work exhibits an unusual
stability that can be related with its unique structure: the
larger, but well-ordered porosity of mesoporous carbon is
advantageous for mass transport and good thermal stability.
The specific surface area of the used mesoporous carbon
Catalytic Test
Oxidative dehydrogenation (ODH) of ethylbenzene to styrene reaction
was carried out in a quartz tube reactor (4 mm i.d.ꢁ320 mm) at tempera-
tures between 350 and 4008C, holding 0.06 g of catalyst particles between
two quartz wool plugs in the isothermal zone. Ethylbenzene was evapo-
rated at 38.58C in a flowing mixture of helium and oxygen, with an ethyl-
benzene concentration of 2.6 vol.% and an ethylbenzene/oxygen ratio of
5. The total flow was 5 mLmin or 10 mLmin . The inlet and outlet gas
analysis was performed on an online gas chromatograph equipped with
two columns for simultaneous analysis of aromatics and permanent
gases: a 5% SP-1200/1.75% bentone 34 packed column for the hydrocar-
bons and a carboxen 1010 PLOT column for the permanent gases, cou-
pled to FID and TCD detectors, respectively.
2
ꢀ1
after the reaction is still as high as 211 m g , while the spe-
cific surface area of activated carbon typically decreases
2
ꢀ1
2
ꢀ1
ꢀ1
ꢀ1
from above 1000 m g to below 80 m g after 24 hours of
reaction.
[22a]
1112
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Chem. Asian J. 2009, 4, 1108 – 1113

Mesoporous Carbon as Catalyst for Oxidative Dehydrogenation
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Published online: May 21, 2009
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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