Dehydrogenation of Isobutane to Isobutene with Carbon Dioxide over SBA-15-Supported Chromia-Ceria Catalysts

Article abstract of DOI:10.1002/cjoc.201700120

A series of SBA-15-supported chromia-ceria catalysts with 3% Cr and 1%–5% Ce (3Cr-Ce/SBA) were prepared using an incipient wetness impregnation method. The catalysts were characterized by XRD, N₂ adsorption, SEM, TEM-EDX, Raman spectroscopy, UV–vis spectroscopy, XPS and H₂-TPR, and their catalytic performance for isobutane dehydrogenation with CO₂ was tested. The addition of ceria to SBA-15-supported chromia improves the dispersion of chromium species. 3Cr-Ce/SBA catalysts are more active than SBA-15-supported chromia (3Cr/SBA), which is due to a higher concentration of Cr⁶⁺ species present on the former catalysts. The 3Cr-3Ce/SBA catalyst shows the highest activity, which gives 35.4% isobutane conversion and 89.6% isobutene selectivity at 570?°C after 10?min of the reaction.

Full text of DOI:10.1002/cjoc.201700120

FULL PAPER
DOI: 10.1002/cjoc.201700120
Dehydrogenation of Isobutane to Isobutene with Carbon
Dioxide over SBA-15-Supported Chromia-Ceria Catalysts
Chunling Wei,a,b Fangqi Xue,a Changxi Miao,*,b Yinghong Yue, a Weimin Yang, b
Weiming Hua,*,a and Zi Gaoa
a Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry,
Fudan University, Shanghai 200433, China
b Shanghai Research Institute of Petrochemical Technology SINOPEC, Shanghai 201208, China
A series of SBA-15-supported chromia-ceria catalysts with 3% Cr and 1%－5% Ce (3Cr-Ce/SBA) were pre-
pared using an incipient wetness impregnation method. The catalysts were characterized by XRD, N2 adsorption,
SEM, TEM-EDX, Raman spectroscopy, UV-vis spectroscopy, XPS and H2-TPR, and their catalytic performance for
isobutane dehydrogenation with CO2 was tested. The addition of ceria to SBA-15-supported chromia improves the
dispersion of chromium species. 3Cr-Ce/SBA catalysts are more active than SBA-15-supported chromia (3Cr/SBA),
＋
which is due to a higher concentration of Cr6 species present on the former catalysts. The 3Cr-3Ce/SBA catalyst
shows the highest activity, which gives 35.4% isobutane conversion and 89.6% isobutene selectivity at 570 ℃ af-
ter 10 min of the reaction.
Keywords isobutane dehydrogenation, carbon dioxide, chromia-ceria, SBA-15, supported catalyst
Introduction
respective alkenes has received much attention. Deep
oxidation of the desirable dehydrogenation product to
CO and CO2 can be avoided through employing CO2
instead of O2.[6] In comparison with the direct dehydro-
genation of small alkanes, CO2 can act as a weak oxi-
dant to improve the alkenes yields[7-9] and/or enhance
the dehydrogenation through reaction coupling between
a simple dehydrogenation of light alkanes and the re-
verse water–gas shift reaction.[10-13] On the other hand,
this new dehydrogenation process opens a new attrac-
tive pathway for the utilization of CO2 which is one of
the major greenhouse gases, because CO2 is reduced to
CO which is more valuable in chemical industry. Hence,
the CO2-assisted dehydrogenation of light alkanes is
believed to be an attractive alternative to the direct de-
hydrogenation.
Compared to the extensive studies on the CO2-as-
sisted dehydrogenation of ethane and propane,[6,14-16]
there is less work reported on the dehydrogenation of
isobutane to isobutene in the presence of CO2. The cat-
alysts reported for this reaction mainly comprise of iron
oxide,[17] vanadium–magnesium oxide,[18] V2O5,[19,20]
LaBaSm oxide[21] and NiO.[13] Nakagawa et al. reported
that oxidized diamond-supported chromia is more active
than supported vanadia for propane dehydrogenation in
the presence of CO2, but the former catalyst is less ac-
tive than the latter one for isobutane dehydrogenation in
Because C2－C4 small alkenes are important build-
ing blocks in chemical industry, attention has been paid
remarkably to the catalytic dehydrogenation of small
alkanes over the last few decades.[1,2] Nowadays, the
industrial uses of isobutene comprise production of pol-
yisobutene, methyl tert-butyl ether (MTBE), butyl rub-
ber, methyl acrylates and so on. Naphtha steam cracking
and fluidized catalytic cracking, which are the main two
methods to obtain isobutene, can not meet the require-
ments for the expanding isobutene market. Hence, cata-
lytic dehydrogenation of isobutane to isobutene has re-
ceived considerable attention.[3]
There are some disadvantages for direct isobutane
dehydrogenation, such as equilibrium limitations re-
garding isobutane conversion, large energy consumption
due to endothermic reaction and high temperature oper-
ation, and fast catalyst deactivation due to coke for-
mation. Alternatively, the oxidative dehydrogenation of
isobutane with O2 can overcome the above drawbacks.[1]
Nevertheless, this process suffers from a significant loss
of selectivity toward isobutene owing to the deep oxida-
tion of isobutene.[4,5] Moreover, it is difficult to deal
with the oxygen-containing mixture with potential ex-
plosion and remove heat.
Recently, the utilization of CO2 as a mild oxidant for
the selective dehydrogenation of light alkanes to their
*
E-mail: miaocx.sshy@sinopec.com, wmhua@fudan.edu.cn
Received February 14, 2017; accepted May 10, 2017; published online XXXX, 2017.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201700120 or from the author.
Chin. J. Chem. 2017, XX, 1—8
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Wei et al.
FULL PAPER
the presence of CO2.[19] Thus, it is interesting and a
challenge to improve the catalytic activity of supported
chromia for the dehydrogenation of isobutane with CO2.
Silica mesoporous molecular sieves such as MCM-41
and SBA-15 are widely employed as catalyst supports
due to their very high surface areas and pore volumes.
In this work, a detailed investigation of isobutane dehy-
drogenation with CO2 over SBA-15-supported chromia-
ceria catalysts was studied. To the best of our knowl-
edge, there has been no work reported on it. These cata-
lysts were found to be more active than SBA-15-sup-
ported chromia. The catalysts were characterized in de-
tail, and the reasons for the superior activities of SBA-
15-supported chromia-ceria were elucidated.
and under ambient conditions. UV-vis spectra were rec-
orded at ambient temperature on a Perkin-Elmer Lamb-
da 650S spectrometer equipped with a diffuse reflec-
tance accessory. The spectra were measured in reflec-
tance mode and converted with the Kubelka-Munk
function F(R). X-ray photoelectron spectroscopy (XPS)
measurements were carried out on a Perkin-Elmer PHI
5000C spectrometer with Mg Kα radiation as the excita-
tion source. All binding energy values were referenced
to the C 1s peak at 284.6 eV.
Temperature-programmed reduction (TPR) profiles
were obtained on a Micromeritics AutoChem II appa-
ratus loaded with 0.1 g of catalyst. The samples were
pretreated at 500 ℃ for 1 h in Ar flow, and then the
samples were cooled to 100 ℃ in Ar flow. The samples
were subsequently contacted with a 10% H2/Ar mixture
flowing at 30 mL•min−1, then heated to 650 ℃ with a
ramping rate of 10 ℃•min−1. H2 consumption was
monitored using a TCD.
Experimental
Catalyst preparation
SBA-15 (BET surface area＝555 m2•g−1, total pore
volume＝0.875 cm3•g−1) was purchased from Nanjing
Pioneer Nanomaterials Science and Technology Co.,
Ltd. in Nanjing, China. The SBA-15-supported chro-
mia-ceria catalysts were prepared by impregnating a
mixed aqueous solution of Cr(NO3)3•9H2O and
Ce(NO3)3•6H2O on SBA-15 using the incipient wetness
impregnation method. The impregnated samples were
dried at 100 ℃ and calcined at 600 ℃ for 4 h in static
air. The resulting catalysts were labelled as mCr-nCe/
SBA, where m and n represent the weight percentage of
Cr and Ce in the catalysts, respectively. The weight
content of Cr is 3%, and the weight contents of Ce are
1%, 3% and 5%, respectively. For comparison, a
3Cr/SBA catalyst with a Cr weight content of 3% was
prepared in the same way using Cr(NO3)3•9H2O as the
precursor.
Activity measurement
The oxidative dehydrogenation of isobutane with
CO2 was carried out in a homemade flow-type fixed-bed
microreactor at 570 ℃ under atmospheric pressure. 0.5
g of catalyst (40－60 mesh) was first pretreated at
570 ℃ in N2 flow for 1 h. Then, a gas mixture of iso-
butane and CO2 (1∶5 molar ratio) flowed through the
catalyst at a flow rate of 19.3 mL•min−1. The hydrocar-
bon products were analyzed using an on-line gas chro-
matograph (GC) equipped with a FID and an HP-AL/S
capillary column (50 m×0.32 mm×8.0 µm). CO and
CO2 were analyzed on-line by another GC equipped
with a TCD and a 6 m long stainless steel column
packed with Porapak Q. The conversion, selectivity and
yield were calculated as follows:
i-C4H10 in－i-C4H10
i-C4H10 conversion＝
out × 100%
Catalyst characterization
i-C4H10
in
X-ray diffraction (XRD) patterns were collected on
an MSAL XD2 X-ray diffractometer using Cu Kα radia-
tion at 40 kV and 30 mA. The BET specific surface are-
as and pore volumes of the catalysts were determined by
the adsorption-desorption of nitrogen at liquid N2 tem-
perature with a Micromeritics Tristar 3000 instrument.
The pore sizes of the catalysts were obtained from the
peak positions of the distribution curves determined
from the adsorption branches of the isotherms. Field-
emission SEM images were recorded on Nova Nano-
SEM 450. TEM images and 2-D atomic mapping were
recorded on an FEI Tecnai G2 F20 S-TWIN instrument.
Thermogravimetric (TG) analysis was performed in air
flow on a Perkin–Elmer 7 Series Thermal Analyzer ap-
paratus to determine the amount of coke deposited on
the catalyst after reaction.
i-C4H8
out
i-C4H8 selectivity＝
× 100%
i-C4H10 in－i-C4H10
out
i-C4H8
i-C4H8 yield＝
out × 100%
i-C4H10
in
Results and Discussion
Structure and physicochemical properties
The low-angle XRD patterns of SBA-15-supported
chromia-ceria samples with 3% Cr and different Ce
content (3Cr-Ce/SBA) are displayed in Figure 1. An
intense diffraction peak and two other weak peaks,
which correspond to (100), (110) and (200) reflections
of the hexagonal regularity of SBA-15, were observed
for all samples. This result indicates that the hexagonal
array of mesopores in SBA-15 is retained after the in-
troduction of chromia-ceria. Figure 2 shows the wide-
angle XRD patterns of SBA-15-supported chromia-ceria
Laser Raman spectra were recorded on a HORIBA
Jobin Yvon XploRA spectrometer. The wavelength of
exciting light was selected as 532 nm. The laser power
was 25 mW, and the spectral resolution was 1－2 cm1.
The Raman spectra were obtained at room temperature
2
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Dehydrogenation of Isobutane to Isobutene with Carbon Dioxide
samples. Distinct diffraction peaks corresponding to
Cr2O3 crystallites can be observed for all samples. The
3Cr-Ce/SBA samples display weaker diffraction peaks
assigned to crystalline Cr2O3 than 3Cr/SBA, suggesting
that the dispersion of chromia is better in the former
samples than in the latte one. When the content of Ce is
3% or higher, diffraction peaks of crystalline CeO2 were
also observed.
preserved after loading of chromia-ceria onto SBA-15.
The N2 adsorption result is consistent with that of XRD.
Table 1 Textural properties of the samples
Sample
S
BET/(m2•g−1)
Vta/(cm3•g−1)
Db/nm
6.46
6.51
6.45
6.47
3Cr/SBA
476
0.745
3Cr-1Ce/SBA
3Cr-3Ce/SBA
3Cr-5Ce/SBA
472
0.737
462
0.753
443
0.706
a Total pore volume. b Pore diameter at the maximum of the pore
size distribution.
Figure S2 presents the SEM images of 3Cr/SBA and
3Cr-3Ce/SBA samples. Both samples exhibit wheat-like
macro-structures aggregated with rode-like domains of
about 1 m length. Figure S3 shows the TEM images of
3Cr/SBA and 3Cr-3Ce/SBA samples. Both samples
show well-ordered hexagonal arrays of mesopores. The
TEM result indicates that the hexagonal pore structure
of the parent SBA-15 is retained after loading of chro-
mia and chromia-ceria, which is in accordance with the
results obtained from XRD and N2 adsorption meas-
urements. An analytical HR-TEM with large angel de-
tection was used for the 2-D atomic mapping. As shown
in Figure 3, the distribution of chromium is more ho-
mogeneous in 3Cr-3Ce/SBA than in 3Cr/SBA. The 2-D
mapping result is in line with that of XRD.
Figure 1 XRD patterns of 3Cr-Ce/SBA samples with different
Ce content in the small-angle region. (a) SBA, (b) 3Cr/SBA, (c)
3Cr-1Ce/SBA, (d) 3Cr-3Ce/SBA, (e) 3Cr-5Ce/SBA.
The dark-field TEM images of 3Cr/SBA and
3Cr-3Ce/SBA samples are shown in Figure S4. No
chromia and ceria can be seen in the mesopores. We
think that chromia and ceria are on the out surface of the
mesopores in SBA-15.
Laser Raman is a useful technique to characterize
chromium species dispersed on the SBA-15 support.
Figure 4 shows the Raman spectra of SBA-15-supported
chromia-ceria samples. An intense and sharp band at
547 cm1, which is commonly assigned to crystalline
Figure 2 XRD patterns of 3Cr-Ce/SBA samples with different
Ce content in the wide-angle region. (a) 3Cr/SBA, (b)
3Cr-1Ce/SBA, (c) 3Cr-3Ce/SBA, (d) 3Cr-5Ce/SBA.
Cr2O3,[22-25] was observed for all samples. The band at
＋
985 cm1 can be attributed to Cr6 species in the form
of monochromate, while the band at 1012 cm1 can be
＋
ascribed to Cr6 species in the form of polychro-
Figure S1 illustrates the N2 adsorption–desorption
isotherms of SBA-15-supported chromia-ceria samples,
and their textural properties are listed in Table 1. Ac-
cording to the IUPAC classification, all samples exhibit
IV type adsorption isotherms and a clear H1 type hyste-
resis loop in the relative pressure range between 0.6 and
0.8, suggesting that these samples have regular meso-
porous channels. The BET surface areas of these sam-
ples are 443－476 m2•g−1, and the total pore volumes
are 0.706－0.753 cm3•g−1, which are lower than those
of the parent SBA-15 (555 m2•g−1 and 0.875 cm3•g−1,
respectively). This can be ascribed to the blocking of
some mesopores in SBA-15 after the impregnation of
chromia-ceria. Nevertheless, SBA-15-supported chro-
mia-ceria samples still possess high surface areas and
pore volumes, implying that the SBA-15 framework is
mate.[24,26] Compared to the SBA-15-supported chromia
sample (3Cr/SBA), SBA-15-supported chromia-ceria
samples (3Cr-Ce/SBA) display weaker Raman band at
547 cm1 and higher intensity ratio of Raman band at
985 to 1012 cm1. The presented Raman data reveal that
the addition of ceria to the 3Cr/SBA sample improves
the dispersion of chromium species on the SBA-15
support. The Raman result is in agreement with the re-
sults measured by XRD and 2-D mapping. The broad
and weak band at 457 cm1, which is assigned to CeO2
with cubic fluorite structure,[27] appears only in both
3Cr-3Ce/SBA and 3Cr-5Ce/SBA samples. The band at
607 cm1 observed in all samples can be attributed to
the vibration of tri-siloxane ring of siliceous
SBA-15.[23,28]
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Wei et al.
FULL PAPER
cies is higher over SBA-15-supported chromia-ceria
samples (3Cr-Ce/SBA) than over the SBA-15-supported
chromia sample (3Cr/SBA).
Figure 5 Diffuse reflectance UV-vis spectra of 3Cr-Ce/SBA
samples with different Ce content. (a) 3Cr/SBA, (b) 3Cr-1Ce/
SBA, (c) 3Cr-3Ce/SBA, (d) 3Cr-5Ce/SBA.
Figure S5 illustrates the XPS spectra of Cr 2p on the
fresh and spent 3Cr-3Ce/SBA catalyst. There are two
sets of Cr peaks at 570－583 eV and 583－593 eV, due
to the splitting of the 3p orbit of Cr into Cr 3p3/2 and Cr
3p1/2. The following discussion will be focused on the
Cr 3p3/2 peak due to its higher intensity. The XPS spec-
tra were deconvoluted into two bands at ca. 576 and 580
＋
＋
eV which are attributed to Cr3 and Cr6 ions, respec-
tively.[32-36] The XPS data obtained by applying a
peak-fitting program are listed in Table 2. After the de-
hydrogenation of isobutane with CO2 at 570 ℃ for 250
Figure 3 2-D mapping images of (a) 3Cr/SBA and (b)
3Cr-3Ce/SBA.
＋
＋
min, the ratio of Cr6 to Cr3 declines significantly
from 0.74 to 0.25, indicative of the reduction of most
＋
＋
Cr6 species to Cr3 species in the reaction. This value
is higher than that (0.14) after the dehydrogenation in
the absence of CO2 (i.e., using N2 instead of CO2), con-
firming that a higher amount of chromium species were
＋
retained in a high oxidation state (Cr6 ) under a CO2
atmosphere than under a N2 atomosphere during the
dehydrogenation reaction. A subsequent treatment of the
spent 3Cr-3Ce/SBA catalyst with CO2 at 570 ℃ for 30
Table 2 XPS results of the fresh and spent 3Cr-3Ce/SBA cata-
lyst after isobutane dehydrogenation at 570 ℃ in different at-
mospheres
Figure 4 Raman spectra of 3Cr-Ce/SBA samples with different
Ce content. (a) 3Cr/SBA, (b) 3Cr-1Ce/SBA, (c) 3Cr-3Ce/SBA, (d)
3Cr-5Ce/SBA.
Binding energy of Cr 2p3/2/eV
＋
Cr6 /Cr3
＋
Catalyst
＋
＋
Cr6
Cr3
The oxidation state and coordination of chromium in
SBA-15-supported chromia-ceria samples were investi-
gated by UV-vis diffuse reflectance measurements, and
Fresh
Spenta
Spentb
Spentc
579.9
579.9
579.9
579.7
575.5
576.0
576.0
576.0
0.74
0.25
0.14
0.59
the results are shown in Figure 5. Two intense bands at
＋
273 and 358 nm are usually assigned to O2  Cr6
charge transfer transition of chromate species with a
tetrahedral symmetry.[24,29-31] The bands at 453 and 603
a Isobutane dehydrogenation in the presence of CO2 for 250 min.
b Isobutane dehydrogenation in the absence of CO2 (i.e., using N2
instead of CO2) for 250 min. c Isobutane dehydrogenation in the
absence of CO2 for 250 min, followed by treatment with CO2 at
570 ℃ for 30 min.
nm are attributed to the A2g  T1g and A2g  T2g transi-
＋
tions of Cr3 in crystalline Cr2O3 or CrOx clusters with
a octahedral symmetry, respectively.[24,29-31] It can be
＋
concluded that the surface concentration of Cr6 spe-
4
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Dehydrogenation of Isobutane to Isobutene with Carbon Dioxide
＋
＋
min brings about an increase of the Cr6 to Cr3 ratio
from 0.14 to 0.59, but this value is still lower than that
for the fresh catalyst (0.74). The XPS result reveals that
CO2 can act as a mild oxidant to oxidize the reduced
and the minor hydrocarbon products are methane,
ethane, ethylene, propane, propylene and butenes (ex-
cept isobutene). Figure S7 shows the activities of
SBA-15-supported chromia catalysts with different Cr
content as a function of reaction time. It is seen that the
supported chromia catalyst with a Cr content of 3% is
the most active. Figure 7 displays the catalytic perfor-
mance of SBA-15-supported chromia-ceria catalysts
with 3% Cr and different Ce content (3Cr-Ce/SBA) as a
function of reaction time. The SBA-15-supported chro-
mia catalyst with 3% Cr (3Cr/SBA) gives a 26.9% con-
version of isobutane after 10 min of the reaction and a
16.7% conversion of isobutane after 250 min of the re-
action. The 3Cr-Ce/SBA catalysts are obviously more
active than 3Cr/SBA. The highest activity was achieved
on the 3Cr-3Ce/SBA catalyst with 3% Cr and 3% Ce,
but the conversion of isobutane for 3Cr-Ce/SBA cata-
lysts does not differ significantly. The 3Cr-3Ce/SBA
catalyst gives a 35.4% conversion of isobutane after 10
min of the reaction and a 22.4% conversion of isobutane
after 250 min of the reaction. The isobutane conversion
is ca. 2% for 1Ce/SBA, 3Ce/SBA and 5Ce/SBA, which
is similar to the value of the blank test (without catalyst)
under the same reaction conditions. As shown in Figure
7b, the selectivity to isobutene is slightly higher for
3Cr-Ce/SBA catalysts than 3Cr/SBA. Therefore, as far
as the isobutene yield is concerned, the 3Cr-Ce/SBA
catalysts present higher isobutene yield than 3Cr/SBA
(Table 3). The 3Cr-3Ce/SBA catalyst gives the highest
isobutene yield of 31.7% after 10 min of the reaction. It
can be also found in Table 3 that the 3Cr-Ce/SBA
＋
＋
Cr3 species to Cr6 species. Nevertheless, the com-
plete recovery of the catalyst to its original state cannot
be achieved. The XPS result demonstrates that the de-
hydrogenation of isobutane in the presence of CO2 car-
ried out mainly via a redox mechanism in which CO2
reoxidized the catalyst which was reduced by isobutane,
as shown in Figure S6.
The redox properties of SBA-15-supported chro-
mia-ceria samples were investigated by H2-TPR, and the
resulting profiles obtained between 100 and 650 ℃ are
displayed in Figure 6. The 3Cr/SBA sample displays
only one reduction peak at 395 ℃, which corresponds
＋
＋
to the reduction of Cr6 to Cr3 .[34,35,37-39] In addition to
the main reduction peak at ca. 400 ℃, the weak peak at
ca. 270 ℃ assigned to the reduction of surface cerium
oxide and probably the active chromium species tightly
interacted with ceria was observed for the 3Cr-Ce/SBA
＋
＋
samples.[7] The peak temperature of the Cr6 to Cr3
reduction is equivalent for 3Cr/SBA, 3Cr-1Ce/SBA and
3Cr-3Ce/SBA samples (ca. 395 ℃), but lower than that
for the 3Cr-5Ce/SBA sample (414 ℃). This result sug-
gests that the oxygen is more strongly bound in chromium
＋
oxide on the latter sample. The amount of Cr6 in the
fresh catalysts determined by the TPR method is given
in Table 3. It can be found that the incorporation of ceria
to the 3Cr/SBA sample enhances the concentration of
＋
Cr6 species in the catalysts, which could be related to
the better dispersion of chromium species on SBA-15
after the doping of ceria, as revealed by the XRD, 2-D
mapping and Raman results. This result is consistent
with the data obtained from the UV-vis diffuse reflec-
tance measurement.
Figure 6 H2-TPR profiles of 3Cr-Ce/SBA samples with
different Ce content. (a) 3Cr/SBA, (b) 3Cr-1Ce/SBA, (c)
3Cr-3Ce/SBA, (d) 3Cr-5Ce/SBA.
Catalytic performance
Figure 7 Conversion of isobutane (a) and selectivity to
isobutene (b) as a function of reaction time for 3Cr-Ce/SBA
catalysts with different Ce content at 570 ℃.
The dehydrogenation of isobutane to isobutene in the
presence of CO2 was investigated at 570 ℃. The major
hydrocarbon product formed in the reaction is isobutene,
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Wei et al.
FULL PAPER
catalysts exhibit higher activity for CO2 conversion to
CO than 3Cr/SBA.
The reducibility of a catalyst plays a key role in the
dehydrogenation reaction that involves a redox mecha-
＋
nism. It is generally suggested that Cr6 species are
responsible for the high catalytic activities of supported
chromium oxide catalysts in the oxidative dehydrogena-
tion of small alkanes with CO2.[7,30,40] Therefore, the
evidently higher conversion of isobutane on the 3Cr-Ce/
SBA catalysts than 3Cr/SBA is due to the fact that the
＋
former catalysts possess higher content of Cr6 species,
as revealed by TPR and UV-vis diffuse reflectance re-
sults. On the other hand, the 3Cr-5Ce/SBA catalyst is a
Figure 8 Yield of isobutene in the presence and absence of CO2
at 570 ℃ as a function of reaction time for the 3Cr-3Ce/SBA
catalyst.
bit less active than 3Cr-3Ce/SBA, although the former
＋
catalyst possesses more Cr6 species. This can be at-
tributed to the lower reducibility of the former catalyst
than the latter one, as suggested from the peak tempera-
As shown in Figure 7a, isobutane conversion de-
creases relatively fast in the initial 50 min, followed by
a slow diminishment with reaction time, which suggests
a deactivation of the catalyst. The colour of the catalysts
became black after the dehydrogenation of isobutane
with CO2, indicative of the coke formation. The amount
of coke (determined by TG) deposited on the catalysts
after 250 min of the reaction in the presence of CO2 is
listed in Table 3. It can be seen that the catalyst with
higher activity has higher amount of coke. The amount
of coke deposited on the 3Cr-3Ce/SBA catalyst after the
dehydrogenation of isobutane in the presence of N2 is
2.5%, less than that in the presence of CO2. This result
indicates that CO2 can not suppress the coke deposition.
Chen et al. observed similar phenomena for ethylben-
zene dehydrogenation in the presence of CO2 over alu-
mina-supported vanadia catalysts.[43] The Raman analy-
sis on the spent 3Cr-3Ce/SBA catalyst reveals the nature
of carbonaceous deposits. As presented in Figure S8, the
G-band and D-band were observed for the spent catalyst
at 1598 and 1347 cm1, respectively, which suggests the
graphitic and amorphous carbon deposition on the cata-
lyst.[44,45]
＋
＋
ture of the Cr6 to Cr3 reduction (414 vs. 395 ℃).
Cerium oxide is a solid base.[41] Thus, the desorption of
dehydrogenation product isobutene is more favorable
from SBA-15-supported chromia-ceria catalysts than
SBA-15-supported chromia, which leads to the higher
isobutene selectivity achieved on the former catalysts.
Figure 8 compares the isobutene yield on the
3Cr-3Ce/SBA catalyst at 570 ℃ as a function of reac-
tion time in the presence of CO2 and in the absence of
CO2 (i.e., using N2 instead of CO2). The isobutene yield
is obviously greater in the former case (31.7% after 10
min of the reaction) than in the latter one (23.7% after
10 min of the reaction). This result indicates that CO2
has a promoting effect on the dehydrogenation of iso-
butane, which can be ascribed to the higher concentra-
＋
tion of Cr6 species kept under a CO2 atmosphere than
under a N2 atomosphere as a consequence of weak oxi-
dation ability of CO2, as demonstrated by the XPS re-
sult.
The catalytic performance of the best catalyst
(3Cr-3Ce/SBA) in this work is compared with the per-
formance of other catalysts reported in the literature.
Table S1 lists the conversion, selectivity and space-time
yield of isobutene, together with the reaction conditions.
In fact, it is not appropriate to compare the catalytic ac-
tivity because of the different reaction conditions. As far
as the space-time yield of isobutene is concerned, the
3Cr-3Ce/SBA catalyst is less active than supported
V-based and Fe-based catalysts, but more active than
supported Cr-based catalyst.[17-20,42]
On the other hand, the XPS data confirm that most
＋
＋
Cr6 species present on the catalyst are reduced to Cr3
species during the dehydrogenation of isobutane with
CO2. In the oxidative dehydrogenation of hydrocarbons,
＋
＋
both oxidized and reduced forms (Cr6 /Cr3 ) are need-
＋
＋
ed.[46] Thus, the reduction of Cr6 to Cr3 will lead to a
decrease in catalytic activity. To further demonstrate that
＋
＋
the reduction of Cr6 to Cr3 is another cause for the
Table 3 Reaction data of SBA-15-supported chromia-ceria catalystsa
＋
Cr6 content/
(mmol•g−1)
Conversion/%
i-C4H10 CO2
Selectivity/%
i-C4H8 Coke/
Catalyst
b
yield/%
%
i-C4H8
CH4
C2H4 C2H6 C3H6
C3H8 C4H8
3Cr/SBA
0.044
26.9(16.7) 4.9(2.6) 89.7(89.9) 2.7(2.6) 0.2(0) 0(0) 6.0(6.6) 0.7(0.9) 0.7(0) 24.1(15.0) 2.5
33.0(20.6) 7.6(3.4) 90.1(91.5) 3.1(2.4) 0.2(0) 0(0) 5.3(5.5) 0.6(0.6) 0.7(0) 29.7(18.8) 3.2
35.4(22.4) 7.1(3.7) 89.6(91.6) 3.0(2.4) 0.3(0) 0.1(0) 5.7(5.4) 0.6(0.6) 0.7(0) 31.7(20.5) 3.2
34.6(21.2) 7.5(3.8) 89.8(91.6) 3.4(2.4) 0.3(0) 0.2(0) 5.1(5.4) 0.6(0.6) 0.6(0) 31.1(19.4) 3.2
3Cr-1Ce/SBA
3Cr-3Ce/SBA
3Cr-5Ce/SBA
0.054
0.060
0.080
a The values outside and inside the bracket are the data obtained at 10 and 250 min, respectively. b Butenes except isobutene.
6
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Chin. J. Chem. 2017, XX, 1—8

Dehydrogenation of Isobutane to Isobutene with Carbon Dioxide
Figure 9 Variation of conversion and selectivity as a function of reaction time in consecutive cycles at 570 ℃. () conversion, ()
selectivity.
alyst with 3% Cr and 3% Ce, which affords 35.4% iso-
butane conversion and 89.6% isobutene selectivity at
570 ℃ after 10 min of the reaction. Both coke for-
catalyst deactivation, the activity of the 3Cr-3Ce/SBA
catalyst was investigated after the pretreatment with
hydrogen (10 vol.% H2/Ar, 400 ℃ and 1 h). As shown
in Figure S9, the H2-pretreated 3Cr-3Ce/ SBA catalyst
exhibits an obvious decrease in isobutane conversion
than the fresh one at the initial stage of the reaction but
a relatively small decline in isobutane conversion from
110 to 250 min. This result suggests that the reduction
＋
＋
mation and reduction of Cr6 to Cr3 are responsible for
the catalyst deactivation.
Acknowledgement
＋
＋
W. Hua thanks the Shanghai Research Institute of
of Cr6 to Cr3 contributes more to the catalyst deac-
tivation than to the coke formation during the dehydro-
genation reaction.
Petrochemical
Technology
SINOPEC
(No.
14ZC06070009) and the Science and Technology
Commission of Shanghai Municipality (13DZ2275200)
for financial support. Y. Yue thanks the National Natural
Science Foundation of China (No. 21273043) for finan-
cial support.
Attempt was also carried out to regenerate the best
performance catalyst of 3Cr-3Ce/SBA after isobutane
dehydrogenation with CO2 at 570 ℃ for 250 min, and
the result is depicted in Figure 9. The conversion of
isobutane diminishes from 35.4% to 22.4% after the
dehydrogenation reaction for 250 min. The spent cata-
lyst was regenerated by re-calcination in flowing air at
550 ℃ for 2 h, followed by subsequent purge with N2
for another 1 h. The original activity of 3Cr-3Ce/SBA
can be almost completely restored, with no noticeable
deactivation being detected even after the third regener-
ation. This can be ascribed to the removal of deposited
carbon on the catalyst and reoxidation of the reduced
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