Ethylbenzene dehydrogenation to styrene in the presence of carbon dioxide over chromia-based catalysts

Article abstract of DOI:10.1039/b308450g

The dehydrogenation of ethylbenzene to styrene in the presence of carbon dioxide over chromia-based catalysts prepared in different ways was investigated. The 25% Cr₂O₃/Al₂O₃ supported catalyst and the 20% Cr

Full text of DOI:10.1039/b308450g

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P A P E R
Ethylbenzene dehydrogenation to styrene in the presence of carbon
dioxide over chromia-based catalysts
Xingnan Ye,a Weiming Hua,a Yinghong Yue,a Weilin Dai,a Changxi Miao,b Zaiku Xieb and
Zi Gao*a
a
Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry,
Fudan University, Shanghai 200433, P. R. China. E-mail: zigao@fudan.edu.cn;
Fax: þ86-21-65641740; Tel: þ86-21-65642792
b
Shanghai Research Institute of Petrochemical Technology, Shanghai 201208,
P. R. China
Received (in Montpellier, France) 24th July 2003, Accepted 13th November 2003
First published as an Advance Article on the web 9th February 2004
The dehydrogenation of ethylbenzene to styrene in the presence of carbon dioxide over chromia-based catalysts
prepared in different ways was investigated. The 25% Cr2O3/Al2O3 supported catalyst and the 20% Cr2O3–SiO2
mixed oxide catalyst display the highest activities for the reaction. XRD characterization of the catalysts reveals
that the activity depends on the amount of dispersed chromia species in the catalysts. A combination of XPS
and TPR studies shows that both Cr6þ and Cr3þ species are present in the precalcined catalysts and the Cr6þ
species are probably the precursors of the active sites of the catalysts with higher activity. Under the same
reaction conditions, the supported chromia and chromia mixed oxide catalysts give better catalytic performance
than Fe2O3/Al2O3 and V/MgO catalysts.
Introduction
Experimental
Styrene is one of the most important monomers in the petro-
chemical industry; it is commercially produced mainly by the
catalytic dehydrogenation of ethylbenzene (EB) over potas-
sium-promoted iron oxide in the presence of an excess of
overheated steam as a diluent and heat carrier.1,2 This process
is equilibrium limited and has a high energy consumption.
Hence, an alternative way is still being pursued. The oxidative
dehydrogenation of EB in the presence of oxygen has
attracted much attention, because this reaction is free from
thermodynamic constraints and can be operated at lower
temperatures.3–5 Nevertheless, a considerable decrease in styr-
ene selectivity owing to deep oxidation of hydrocarbons to
carbon oxide makes it uneconomical in view of industrial
application.
EB dehydrogenation using carbon dioxide as a mild oxidant
was first reported by Sato and co-workers as early as in the
late 1980s.6 Besides the necessity to solve the problem of
CO2 emission, the use of CO2 in the reaction has aroused
widespread interest later on because the energy required for
this new process is estimated to be lower than that of the pre-
sent commercial process and the equilibrium yield of styrene
in an CO2 atmosphere is higher than that in steam.7 The
catalyst systems giving good performance for this reaction
comprise iron-oxide-based,7–11 vanadium-oxide-based,12–15
zirconia-based16,17 catalysts and activated-carbon-supported
chromium and cerium oxides18 as well as calcined hydro-
talcite-like compounds.19,20 The positive roles of CO2 in the
reaction have been suggested to be the following: (1) CO2
removes the deposited coke by the Boudouard reaction and
(2) the generated hydrogen reacts with CO2 via the reverse
water gas shift reaction, shifting the equilibrium to the
product side.21
Catalyst preparation
The g-Al2O3 and SiO2 supports used in this work have BET
surface areas of 214 and 234 m2 gꢀ1, respectively. An aqueous
solution of chromic nitrate was applied to the g-Al2O3 and
SiO2 supports until incipient wetness, followed by drying at
383 K overnight. The supported catalysts obtained are denoted
as x%Cr/Al and x%Cr/Si, where x% represents the weight
percentage of Cr2O3 in the catalysts. The mixed oxide catalysts
were prepared by co-precipitation and sol-gel methods. To pre-
pare Cr–Al mixed oxide catalysts, an aqueous solution of
ammonia was added dropwise under vigorous stirring to a
mixed solution of Al(NO3)3 and Cr(NO3)3 until the final
pH ¼ 8.5 was attained. The precipitate was then filtered,
washed with distilled water and dried at 383 K overnight.
The Cr–Si mixed oxide catalysts were prepared by hydrolyzing
TEOS (tetraethyl orthosilicate) in an aqueous ethanol solution
containing a given amount of Cr(NO3)3 at 323 K and then
adding a stoichiometric amount of NH3ꢁH2O to the clear sol.
The gel obtained was washed with distilled water and dried
at 383 K overnight. The mixed oxide catalysts are labelled
x%Cr–Al and x%Cr–Si, where again x% represents the weight
percent of Cr2O3 in the catalysts. All the catalysts were
calcined in air at 823 K for 5 h, unless otherwise noted.
For comparison, 10%Fe2O3/Al2O3 and 20%Fe2O3/Al2O3
catalysts were also prepared in the same way as the Cr/Al
catalysts.
Characterization of catalysts
The BET surface areas of the supports and catalysts were mea-
sured by N2 adsorption at 77 K on a Micromeritics ASAP
2000 instrument. X-Ray powder diffraction (XRD) patterns
were recorded on a Rigaku D/MAX-IIA diffractometer with
Ni-filtered CuKa radiation, operating at 30 kV and 20 mA.
XRD quantitative phase analysis was carried out using pure
In this work, ethylbenzene dehydrogenation on supported
chromia and chromia mixed oxide catalysts in the presence
of CO2 was studied. The main factors affecting the activity
of the catalysts are discussed.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 7 3 – 3 7 8
373

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silicon as an internal standard. The ratio of the weight of the
sample to that of silicon was kept constant at 9. The peak
intensity ratio ICr O /ISi was measured and it was assumed to
maximum conversion is 99.0%. Over Cr/Si catalysts, the con-
version of EB initially increases with Cr2O3 loading up to 5
wt. %, then levels off at a conversion of 22.7% and a selectiv-
ity of 99.5%.
2
3
be proportional to the content of crystalline Cr2O3 . Tempera-
ture programmed reduction (TPR) experiments were carried
out using a Micromeritics ASAP 2900 instrument. Samples
of 200 mg of catalyst were pretreated in N2 at 573 K for 3 h.
A reduction run was then performed from 323 to 873 K at a
The mixed oxide catalysts behave differently from the sup-
ported ones. The activities of the Cr–Al and Cr–Si mixed oxide
catalysts after dehydrogenation for 6 h are shown in Fig. 2.
Contrary to the supported catalysts, the activities of the Cr–
Al catalysts are lower than those of the Cr–Si catalysts, but
the selectivities to styrene on the Cr–Al catalysts are slightly
higher than those on the Cr–Si catalysts. For the Cr–Al cata-
lysts, the EB conversion increases with the Cr2O3 loading initi-
ally and then decreases. A maximum conversion of 45.8% and
a selectivity of 99.5% are achieved at a Cr2O3 loading of 25 wt.
%, showing that the Cr–Al mixed oxide catalysts are less active
than the Cr/Al supported catalysts. The relation of the conver-
sion of EB on Cr–Si catalysts to the Cr2O3 loading shows a dif-
ferent pattern. The activity rises more steeply at low Cr2O3
loading. Then, a maximum conversion of 56.0% and a selectiv-
ity of 98.8% are achieved at a Cr2O3 loading of 20 wt. %, that
is, the Cr–Si mixed oxide catalysts are much more active than
the Cr/Si supported catalysts.
Iron oxide based catalysts were reported to be effective for
EB dehydrogenation in the presence of CO2 .7–11 For compar-
ison, the activities of 10% Fe2O3/Al2O3 and 20% Fe2O3/Al2O3
supported catalysts were tested under the same reaction condi-
tions. The conversions of EB after 6 h on stream on 10%
Fe2O3/Al2O3 and 20% Fe2O3/Al2O3 catalysts are 24.2% and
32.8%, respectively, and the selectivity to styrene on the two
catalysts is 99.7%. This shows that the chromia catalysts are
more active than the iron oxide catalysts for EB dehydrogena-
tion in the presence of CO2 .
heating rate of 10 K minꢀ1 under a gas flow (40 ml minꢀ1
)
of hydrogen (10 vol. %) and argon (90 vol. %). X-Ray photo-
electron spectroscopy (XPS) studies were performed on a Per-
kin–Elmer PHI-5000C ESCA system using MgKa radiation at
14 kV and 250 W. All the binding energies are referenced to the
C1s peak at 284.6 eV. The data were treated with an XPS peak
fitting program (XPSPEAK Version 4.1 supplied by CUHK).
Activity measurement
The oxidative dehydrogenation of EB in the presence of CO2
was carried out in a flow-type fixed-bed microreactor under
atmospheric pressure. To supply the reactant, a gas mixture
of N2 and CO2 (19:1 molar ratio, unless otherwise stated) at
a flow rate of 60 ml minꢀ1 was passed through a glass evapora-
tor filled with liquid EB maintained at 273 K. The catalyst load
was 100 mg and the reaction temperature was 773 K. The
molar ratio of CO2 to EB was kept constant at 19, unless
otherwise stated. Prior to the reaction, the catalyst was pre-
treated at 773 K in N2 for 2 h. The hydrocarbon products were
analyzed with a gas chromatograph equipped with a flame
ionization detector (FID).
Results
EB dehydrogenation in the presence of CO2
Effect of reaction conditions
The main product of EB dehydrogenation in the presence of
CO2 is styrene and the minor hydrocarbon by-products are
benzene and toluene. As the reaction goes on, a slow decline
in activity occurs and meanwhile the selectivity to styrene
increases gradually. For all the catalysts in this work, the selec-
tivity is higher than 98.5%.
The activities after 6 h on stream over the supported Cr/Al
and Cr/Si catalysts in the presence of CO2 as a function of
chromia loading are illustrated in Fig. 1. In general, the
activities of the Cr/Al catalysts are higher than those of
the Cr/Si catalysts. As the Cr2O3 loading is increased, the
conversion of EB on Cr/Al catalysts first increases and then
decreases. A maximum of 59.0% conversion appears at a
Cr2O3 loading of 25 wt. %. The selectivity to styrene at the
The dependence of the activity and selectivity on the reaction
temperature for the supported and mixed oxide catalysts is
given in Table 1. The dehydrogenation activity of all the cata-
lysts increases with reaction temperature, but the selectivity to
styrene is slightly decreased. It is noteworthy that the activity
rises differently with temperature on the different catalysts,
implying that the apparent activation energies of the dehydro-
genation reaction for these catalysts are different.
The variations of the activity and selectivity of 20%Cr/Al
and 20%Cr–Si catalysts with CO2 partial pressure are given
in Table 2. The partial pressure was adjusted by changing
the molar ratio of CO2 and N2 , while the EB partial pressure,
contact time factor W/F and other reaction conditions were
kept unchanged. Upon introducing a small amount of CO2
Fig. 1 Effect of Cr2O3 content on the conversion of EB and selectiv-
ity to styrene at 773 K after 6 h on stream for the supported catalysts.
Open symbols: selectivity; filled symbols: conversion. Cr/Al catalysts:
/, N; Cr/Si catalysts: S, X.
Fig. 2 Effect of Cr2O3 content on the conversion of EB and selectiv-
ity to styrene at 773 K after 6 h on stream for the mixed oxide cata-
lysts. Open symbols: selectivity; filled symbols: conversion. Cr–Al
catalysts: /, N; Cr/Si catalysts: S, X.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
374
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Table 1 Effect of reaction temperature on the catalytic properties of
chromia catalystsa
% Selectivity
Catalyst
T/K % Conv. % Yield Styrene Benzene Toluene
20%Cr/Al 723
773
26.8
57.5
76.9
9.9
26.7
57.0
75.8
9.9
99.5
99.0
98.6
0.5
0.8
0.8
–
–
0.2
0.6
–
823
20%Cr/Si 723
100
773
823
22.1
30.3
19.0
42.5
73.4
31.8
56.0
61.3
22.0
30.2
18.9
42.2
72.6
31.6
55.3
60.6
99.6
99.6
99.4
99.3
98.8
99.3
98.8
98.8
0.4
0.4
0.6
0.6
0.8
0.7
0.7
0.5
–
–
20%Cr–Al 723
773
–
0.1
0.4
–
823
20%Cr–Si 723
773
Fig. 3 Effect of W/F on the conversion of EB and selectivity to
styrene at 773 K after 6 h on stream. Open symbols: selectivity; filled
symbols: conversion. 20%Cr/Al: L, K; 20%Cr–Si: /, N.
0.5
0.7
823
a
Reaction conditions: 100 mg catalyst; N2:CO2:EB ¼ 361:19:1;
time-on-stream 6 h.
diffraction peaks of crystalline Cr2O3 were observed on the
Cr/Si catalysts with a Cr2O3 loading as low as 3 wt. %.
Increasing the Cr2O3 loading on the SiO2 support results in
an increase of the diffraction intensity. The peak intensities
of the reflections (104) for Cr2O3 in Cr/Al catalysts and
(116) for Cr2O3 in Cr/Si catalysts were measured and com-
pared with those of the silicon standard. The quantitative
phase analysis data are depicted in Fig. 6, showing that the dis-
persion thresholds of Cr2O3 on g-Al2O3 and SiO2 are 21.3 and
1.2 wt. %, respectively. According to the crystallographic data
of Cr2O3 , one monolayer of Cr2O3 is equivalent to ca. 10 atoms
of Cr per 1 nm2 of the support.22 Therefore, the formation of a
monolayer of Cr2O3 on the surface of the g-Al2O3 and SiO2
supports corresponds to Cr2O3 loadings of ca. 21 and 23 wt.
%, respectively. The fact that the dispersion threshold of
Cr2O3 on g-Al2O3 detected by the XRD method is close to
the estimated monolayer surface coverage implies that chromia
is probably dispersed as a monolayer on the g-Al2O3 support
when its loading is lower than the threshold, due to the strong
interactions between chromia and the Al2O3 support. On
the other hand, the dispersion threshold of Cr2O3 on SiO2 is
much smaller than the theoretical monolayer surface coverage,
indicating that chromia tends to aggregate and form micro-
crystals rather than to disperse on the surface of the SiO2 sup-
port. The above results are consistent with those found by
Wang et al.23 on similar supported chromia catalysts.
into the reaction, the EB conversion on both catalysts
increased. The EB conversion on 20%Cr/Al catalyst remains
constant at a level of 58–59% as the partial pressure of CO2
was raised above 50 kPa, but the conversion on 20%Cr–Si
catalyst drops off to 45% as the partial pressure of CO2 is
raised above 5 kPa.
The effect of the contact time factor (W/F) on the dehydro-
genation of EB is illustrated in Fig. 3. The contact time factor
was adjusted by changing the catalyst load while the feed flow
was kept constant (N2:CO2:EB ¼ 361:19:1, 60 ml minꢀ1). The
EB conversion on 20%Cr/Al and 20%Cr–Si catalysts increases
with an increase in W/F, but the styrene selectivity decreases
gradually. In the literature,14 the EB conversion and selectivity
to styrene of the V/MgO catalyst in the presence of CO2
(CO2/EB ¼ 45) at 773 K and W/F ¼ 70 (g cat) h molꢀ1 after
1 h on stream were reported to be 17.2% and 92.2%, respec-
tively. The results in Fig. 3 show that 20%Cr/Al and
20%Cr–Si catalysts are probably more active and selective
than the supported vanadium oxide catalyst under the same
reaction conditions.
Dispersion of Cr2O3 on the catalysts
XRD patterns of some representative Al2O3 and SiO2 sup-
ported chromia catalysts calcined at 823 K are shown in Figs.
4 and 5. For the Cr/Al series of catalysts, no distinct diffrac-
tion peaks corresponding to Cr2O3 crystals appear until a 25
wt. % Cr2O3 loading is reached, demonstrating that chromia
is readily dispersed on the Al2O3 support. In contrast,
The XRD patterns of the Cr–Al and Cr–Si mixed oxide
catalysts calcined at 823 K are shown in Figs. 7 and 8. The
Cr–Al and Cr–Si mixed oxide catalysts do not display distinct
diffraction peaks due to crystalline Cr2O3 until the content of
Cr2O3 reaches 30 and 20 wt. %, respectively, indicating that
Table 2 Effect of CO2 partial pressure on the catalytic properties of
chromia catalystsa
% Selectivity
P(CO2)/
kPa
Catalyst
% Conv. % Yield Styrene Benzene Toluene
20%Cr/Al
0
5
48.9
57.5
59.0
58.3
51.7
56.0
44.9
45
48.3
57.0
58.3
57.7
51.3
55.3
44.3
44.4
98.9
99.0
98.9
98.9
99.1
98.8
98.8
98.5
1.0
0.8
0.9
0.9
0.6
0.7
0.7
1.0
0.1
0.2
0.2
0.2
0.3
0.5
0.5
0.5
51
99
0
20%Cr–Si
5
51
99
a
Reaction conditions: 100 mg catalyst; EB feed rate 0.42 mmol hꢀ1
773 K; time-on-stream 6 h.
;
Fig. 4 XRD patterns of a series of Cr/Al catalysts calcined at 823 K;
(S) crystalline Cr2O3 .
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 7 3 – 3 7 8
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Fig. 7 XRD patterns of a series of Cr–Al catalysts calcined at 823 K;
(S) crystalline Cr2O3 .
Fig. 5 XRD patterns of a series of Cr/Si catalysts calcined at 823 K;
(S) crystalline Cr2O3 .
Partial reduction of the Cr6þ species is observed in the 20%Cr/
Al catalyst after reaction for 6 h.
aggregation of Cr2O3 crystallites in the mixed oxide catalysts
begins at Cr2O3 contents higher than those of the supported
catalysts. The discrepancy is brought about by the different
distribution patterns of chromia in the catalysts. For the mixed
oxide catalysts, chromia is certainly distributed not only on the
surface but also in the bulk of the catalysts.
The BET surface areas of the supported and mixed oxide
catalysts calcined at 823 K are listed in Table 3. For the
Cr/Al and Cr/Si supported catalysts, the surface area
decreases gradually with an increase in Cr2O3 loading, which
is common for these supported catalysts.24 The Cr–Al and
Cr–Si mixed oxide catalysts have larger surface areas than
the analogous supported catalysts and the surface areas for
these catalysts first rise and then fall with chromia content.
The TPR profiles of the catalysts are depicted in Fig. 9. For
bulk Cr2O3 , only one weak reduction peak appears in the pro-
file at 538 K, corresponding to the reduction of Cr6þ in the
oxide.26 There is also one reduction peak on the TPR profiles
of 5%Cr/Al, 5%Cr–Al and 5%Cr–Si catalysts, but the peak
temperature is shifted towards higher temperatures, suggesting
that the oxygen is more strongly bound in the dispersed chro-
mia on the catalysts.27 Both the low temperature (peak I) and
high temperature (peak II) peaks are observed in the profile of
the 5%Cr/Si catalyst, showing that large crystals of chromia
exist on the catalyst as well as the dispersed oxide. This is con-
sistent with the result of the XRD measurement. For all the
supported and mixed oxide catalysts containing 20 wt. %
Cr2O3 , a small peak or a shoulder peak around 545 K is
observed along with the high temperature reduction peak,
implying that some undispersed crystalline chromia is present
in these catalysts. Since the Cr loadings of our catalysts are
high, it can be assumed that the Cr6þ in the catalysts is mostly
XPS and TPR studies
The XPS spectra of some of the studied catalysts were
recorded. The spectra were deconvoluted into two bands at
about 576 and 579 eV assigned to Cr3þ and Cr6þ ions, respec-
tively.25 The fraction of Cr6þ ions in the total amount of chro-
mium calculated from the relative intensities of the two bands
is listed in Table 4, showing that a considerable amount of
Cr6þ species is present on the surface of the catalysts after cal-
cination at 823 K in air. The atomic ratios between Cr and
Al(Si) calculated from the Cr(2p) and Al(2p) or Si(2p) bands
are also given in Table 4. The Cr/Al(Si) ratio increases with
the Cr loading of the supported catalysts and, as expected, it
is smaller for mixed oxide catalysts with the same Cr content.
reduced to Cr3þ 28,29
The TPR data as well as the amount of
.
Cr6þ in the catalysts calculated from the total H2 consumption
are listed in Table 5. The concentration of Cr6þ varies with
the type of catalyst and the total amount of chromia in the
catalyst.
Discussion
The variation of EB dehydrogenation activity with Cr loading
in the presence of CO2 in Fig. 1 reveals that the dispersed
chromia phase on the supports is crucial for the reaction.
Fig. 6 Dispersion thresholds of Cr2O3 on g-Al2O3 and SiO2
supports; (/) g-Al2O3 , (S) SiO2 .
Fig. 8 XRD patterns of a series of Cr–Si catalysts calcined at 823 K;
(S) crystalline Cr2O3 .
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
376
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Table 3 BET surface areas of the catalysts calcined at 823 K
Surface area/m2 gꢀ1
Cr2O3 content/wt. %
Cr/Al
Cr/Si
Cr–Al
Cr–Si
0
5
214
203
198
173
144
124
234
170
164
148
124
107
–
–
298
310
247
200
151
421
447
446
–
11
20
30
40
349
The catalytic activity of the Cr/Al supported catalysts exhibits
a maximum at a Cr2O3 loading of 25 wt. %, corresponding
approximately to monolayer surface coverage of Cr2O3 on
the g-Al2O3 support, whereas the activity of the Cr/Si catalysts
levels off at a much lower value with a Cr2O3 loading far below
monolayer surface coverage. The difference in activity between
the two types of supported catalysts arises from the low disper-
sion of chromia on the surface of the SiO2 support. Chromia is
hardly spread over the support surface after calcination. This
particular behavior of SiO2 limits its use as a support for EB
dehydrogenation catalyst.
The variation of catalytic activity with Cr loading on mixed
oxide catalysts in Fig. 2 is significantly different from that on
supported catalysts. For both Cr–Al and Cr–Si mixed oxide
catalysts, the activity initially increases with Cr2O3 loading
and then decreases. The maximum activity of Cr–Al and
Cr–Si catalysts appears at Cr2O3 loadings of 25 and 20 wt.
%, respectively. XRD patterns of the Cr–Al and Cr–Si cata-
lysts in Figs. 7 and 8 show that the aggregation of large
Cr2O3 crystallites in Cr–Al and Cr–Si catalysts begins at
Cr2O3 loadings of 30 and 20 wt. %, respectively. These results
further confirm that the dispersity of chromia in the mixed
oxide catalysts is also related to the dehydrogenation reactiv-
ity. The Cr–Si mixed oxide catalyst gives better performance
for the reaction than the Cr/Si supported catalyst with the
same Cr content, since chromia is more homogeneously dis-
tributed in the mixed oxide catalyst. The Cr–Si mixed oxide
catalyst gives a better performance for the reaction than the
Cr/Si supported catalyst with the same Cr content. In particu-
lar, the Cr–Si mixed oxide catalysts with low Cr content dis-
play exceedingly high activity in comparison to all the other
catalysts. This is probably due to the more homogeneous dis-
tribution of chromia species in the catalysts, resulting from the
sol-gel preparation method.
Fig. 9 TPR profiles of some samples: (a) bulk Cr2O3 ; (b) 5%Cr/Al;
(c) 5%Cr–Al; (d) 5%Cr/Si; (e) 5%Cr–Si; (f) 20%Cr/Al; (g) 20%Cr–Al;
(h) 20%Cr/Si; (i) 20%Cr–Si.
The EB conversion of our catalysts is plotted against the
amount of Cr6þ determined by the TPR method in Fig. 10.
The correlation is not as linear as that shown in the literature31
due to the diversity of the catalysts studied in this work, but
the positive effect of the Cr6þ species on the reaction is evident,
suggesting that the Cr6þ species are probably the precursors of
the active sites, whch have a higher activity for the EB conver-
sion as well.30,31 Besides the Cr6þ content, there are probably
other minor factors, such as the difference in specific surface
area among the catalysts (see Table 3), the difference in distri-
bution of the Cr species between the supported and the mixed
oxide catalysts and the difference in interactions between Cr–
Al and Cr–Si in the catalysts, that may also affect the activity
of the catalysts.
The XPS and TPR measurements have shown the coexis-
tence of Cr3þ and Cr6þ species in the catalysts before the reac-
tion. In previous literature,30,31 the n-butane dehydrogenation
activity of Cr2O3/Al2O3 catalysts increased parallel to the
initial amount of Cr6þ in the catalysts. The high-oxidation-
state Cr species were suggested to be the precursors of the
active sites of the catalysts.
It is noteworthy that the reducibility of the Cr6þ species in
the Cr/Si and Cr–Si catalysts represented by the reduction
peak temperature (see Table 5) is lower than that in Cr/Al
and Cr–Al catalysts, indicating that the oxygen is more
strongly bound in the former catalysts. However, this differ-
ence in reducibility does not exert much influence on the EB
dehydrogenation activity of the catalysts. This is probably
because the dehydrogenation reaction is carried out at tem-
peratures much high than the reduction temperature of the
chromia species.
Table 4 XPS data for the catalysts
B.E. Cr(2p3/2)/eV
Catalyst
Cr3þ
Cr6þ
Cr6þ/Cr
Cr/Al(Si)
In the presence of an appropriate amount of CO2 , the con-
version of EB on the catalysts is increased, whereas the selec-
tivity to styrene is almost unchanged. The enhancement in
activity can be attributed to the oxidative dehydrogenation
of ethylbenzene by the oxygen species originating from
CO2 .9,16 An excess of CO2 may somewhat reduce the activity
of the catalysts. The reason for this behavior may be related to
the decreased adsorption of EB in the presence of large
amounts of CO2 .
5%Cr/Al
5%Cr/Si
576.8
576.4
576.7
576.7
576.8
576.1
579.1
578.8
579.3
579.3
579.3
578.6
0.42
0.28
0.36
0.18
0.31
0.48
0.13
0.10
0.35
0.31
0.26
0.15
20%Cr/Al
20%Cr/Ala
20%Cr–Al
20%Cr–Si
a
After reaction at 773 K for 6 h.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 7 3 – 3 7 8
377

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Table 5 TPR data for the catalysts
Peak I
Peak II
Catalyst
T/K
H2/mmol (g cat)ꢀ1
T/K
H2/mmol (g cat)ꢀ1
Total H2/mmol (g cat)ꢀ1
Cr6þ/mmol (g cat)ꢀ1
5%Cr/Al
5%Cr/Si
5%Cr–Al
5%Cr–Si
20%Cr/Al
20%Cr/Si
20%Cr–Al
–
–
0.050
626
696
0.564
0.185
0.489
0.549
0.780
0.248
0.743
0.564
0.235
0.489
0.549
0.780
0.322
0.743
0.376
0.157
0.326
0.366
0.520
0.215
0.495
548
–
–
650
687
607sha
666
612sh
–
–
–
–
547
–
0.074
–
a
sh: with shoulder peak at lower temperature.
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The work was supported by the Chinese Major State Basic
Research Development Program (Grant 2000077507) and
Shanghai Research Institute of Petrochemical Technology.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
378
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 7 3 – 3 7 8