Oxidative dehydrogenation of: N -butane to butenes on Mo-doped VMgO catalysts

Article abstract of DOI:10.1039/c7ra04936f

VMgO catalysts with different molybdenum doping amounts were prepared by an impregnation method. The structure, specific surface area, and basic and redox properties of these catalysts were determined by XRD, BET, CO₂-TPD, and H₂-TPR. The XRD results revealed that all catalysts contained an orthovanadate phase (Mg₃(VO₄)₂), while no metavanadate and pyrovanadate phases were detected. BET surface area analysis showed that the Mo-doped catalysts possessed lower surface areas than the undoped one. The reducing and basicity properties of the catalysts were characterized by H₂-TPR and CO₂-TPD measurements, which demonstrated that Mo-doping improved the redox temperature and reduced the number of basic sites. The performances of these catalysts were investigated at different C₄H₁₀/O₂ molar ratios, temperatures, and contact times. The Mo-doping not only improved the selectivity of butenes but also inhibited the deep oxidation reactions, although cracking reactions occurred with high levels of Mo doping. When the Mo/V atomic ratio was 3:100, n-butane conversion of 34.5% and total butene selectivity of 79.3% were achieved at 630 °C. To the best of our knowledge, the oxidative dehydrogenation performance of the synthesized Mo-doped VMgO catalysts described in this work represents a remarkable improvement compared to previous reports.

Full text of DOI:10.1039/c7ra04936f

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Oxidative dehydrogenation of n-butane to butenes
on Mo-doped VMgO catalysts
Cite this: RSC Adv., 2017, 7, 34131
ab
a
b
and Xuefeng Zhu *b
Xue Liu,
Linhai Duan,* Weishen Yang
VMgO catalysts with different molybdenum doping amounts were prepared by an impregnation method.
The structure, specific surface area, and basic and redox properties of these catalysts were determined
by XRD, BET, CO
orthovanadate phase (Mg
surface area analysis showed that the Mo-doped catalysts possessed lower surface areas than the
undoped one. The reducing and basicity properties of the catalysts were characterized by H -TPR and
CO -TPD measurements, which demonstrated that Mo-doping improved the redox temperature and
reduced the number of basic sites. The performances of these catalysts were investigated at different
10/O molar ratios, temperatures, and contact times. The Mo-doping not only improved the
2 2
-TPD, and H -TPR. The XRD results revealed that all catalysts contained an
3
(VO ), while no metavanadate and pyrovanadate phases were detected. BET
4 2
)
2
2
C
4
H
2
selectivity of butenes but also inhibited the deep oxidation reactions, although cracking reactions
Received 2nd May 2017
Accepted 20th June 2017
occurred with high levels of Mo doping. When the Mo/V atomic ratio was 3 : 100, n-butane conversion
ꢀ
of 34.5% and total butene selectivity of 79.3% were achieved at 630 C. To the best of our knowledge,
DOI: 10.1039/c7ra04936f
the oxidative dehydrogenation performance of the synthesized Mo-doped VMgO catalysts described in
this work represents a remarkable improvement compared to previous reports.
rsc.li/rsc-advances
and C
4
selective oxidation reactions occurring in the gas phase
1
Introduction
over solid catalysts, and suggested that there is a clear limita-
Butenes, including 1-butene, 1,3-butadiene, 2-butene, and iso- tion in terms of selectivity and conversion, beyond which
butene, are the third most important petrochemical interme- experimental progresses are difficult to achieve. In their study,
1
diate products among the olens. They are widely used in the selectivity of n-butane to butenes could only reach 60%,
14
various chemical processes for the production of synthetic when the conversion was 30%.
resins, plastics, rubber, automotive fuel components, and other
The effects of the supports, e.g. TiO
, CeO , ZrO , Al O , and
2 2 2 3
2
2
–6
8,15–25
valuable products. In view of the increasing demand for MgO, have been widely investigated for V-based catalysts.
butenes, the oxidative dehydrogenation (ODH) of n-butane is To the best of our knowledge, MgO is the best among these
regarded as a low-cost and promising process for the produc- supports, and VMgO catalysts have been extensively studied as
7
8,15,20–24
tion of butenes.
The types of side reactions that would happen during the been reported that Mg
highly efficient catalysts for the ODH of n-butane.
It has
3
(VO is the active phase for the ODH of
4 2
)
process of n-butane ODH, such as deep oxidation reactions, n-butane, while other vanadate phases (such as metavanadate
cracking reactions and isomerization reactions, are shown in and pyrovanadate) are unfavorable for the production of
Scheme 1. The ODH of n-butane is very complex and difficult to butenes. However, most studies disclosed that the as-prepared
control. Therefore, a catalyst with high performance is required catalysts contained minor quantities of other vanadate
8,9
for the ODH of n-butane. In recent studies, much effort has
been devoted to the development of more efficient catalysts in
10–13
order to achieve both high activity and selectivity.
The crit-
ical issue related to the ODH of butane is that deep oxidation
reactions can occur, leading to the low selectivity of butenes.
Costine et al. conducted an extensive data mining study on C
3
a
College of Chemistry, Chemical Engineering and Environmental Engineering,
Liaoning Shihua University, Fushun, 113001, P. R. China. E-mail: lhduan@126.
com; Tel: +86-024-5686-0757
b
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian, 116023, P. R. China. E-mail: zhuxf@dicp.ac.cn; Tel:
+
86-411-8437-9182
Scheme 1 Reactions of the ODH of n-butane.
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22,26
phases.
Therefore, the preparation method is critical for 2.2 Catalyst characterization
obtaining a good VMgO catalyst.
The surface areas of the catalysts were measured using a BET
apparatus (OMNISORP 100CX). The crystalline phases of the
catalysts were investigated using a XRD diffractometer (Rigaku,
D/MAX-RB) with Cu Ka radiation operated at 40 kV and 100 mA.
The infrared spectra were recorded on a Nicolet 6700 Fourier-
transform infrared spectroscopy-attenuated total reectance
The catalytic performance of the VMgO catalysts can be
slightly improved by adding transition metals, such as Ni, Mo,
or Zr. Liu et al. examined the inuence of the addition of Ni to
the VMgO catalysts in the ODH of n-butane. The authors found
that a moderate doping amount of Ni (n(Ni)/n(V) ¼ 0.3) could
3 4 2
promote the formation of the Mg (VO ) phase and increase the
(
FTIR-ATR, Thermo Scientic Co.) spectrometer. H tempera-
2
26
selectivity of butenes. Dejoz et al. prepared Mo-doped VMgO
catalysts with a Mo : V atomic ratio in the range 0–1.4 to
determine the inuence of the addition of Mo on the ODH of n-
butane. The results demonstrated that the Mo–VMgO-based
catalysts showed an enhanced selectivity of the ODH prod-
ucts, and especially to butadiene. However, the selectivity of
cracking products increased with increasing Mo-doping
ture programmed reduction (H -TPR) was performed to eluci-
date the reducibility of the catalysts using a ChemStar
2
chemisorption analyzer. CO
2
temperature programmed
desorption (CO -TPD) was performed to determine the amount
2
of surface basic sites with a ChemStar chemisorption analyzer.
15
2.3 Catalyst performance test
amount. Lee et al. studied Mg(VO
with different Mg : Zr ratios. The MgO–ZrO
prepared by a gel-oxalate coprecipitation method, while the
Mg (VO ) /MgO–ZrO catalysts were obtained by a wet impreg-
4
)
2
2
/MgO–ZrO catalysts
2
supports were
The ODH of n-butane was performed in a xed bed quartz
reactor (10 mm i.d.) under atmospheric pressure. For each
catalytic test, 0.50 g of catalyst (40–60 mesh) was used. A ther-
mocouple tube with an o.d. of 5 mm was placed at the center of
the quartz reactor. The catalytic reaction conditions were as
follows unless otherwise stated: the total space velocity (GHSV)
3
4 2
2
nation method. The results of this study indicated that
Mg (VO /MgO–ZrO
3
4
)
2
2
(Mg : Zr ratio ¼ 4 : 1) exhibited the best
catalytic performance for the ODH of n-butane, providing a total
ꢂ1
butenes (including 1-butene, 2-butene, and 1,3-butadiene) yield
was kept at 58 800 h ; the concentration of n-butane at the feed
27
of ꢁ23%. Rischard et al. prepared Mo-doped VMgO catalysts,
inlet was 2 vol%; the molar ratio of n-butane to oxygen was 1 : 1
or 1.5 : 1; nitrogen was used as balance gas. The contact time
was adjusted by changing the total ow rate, while maintaining
constant the composition of the feed gas mixture. The products
of the reaction were analyzed by an online gas chromatograph
equipped with a thermal conductivity detector (TCD). The
conversion of n-butane, selectivity to total butenes (including 1-
and performed the ODH reaction at different O /n-butane molar
2
ratios in a two-zone uidized bed reactor. When the reaction
ꢀ
was conducted at 580 C and the O /n-butane molar ratio was
2
1
.9, the selectivity of 1,3-C H reached 50.7% with an n-butane
4 6
conversion of 64.5%. Particularly, the authors reported that the
28
catalysts could be regenerated in a single vessel.
In this work, a series of 10 wt% V /MgO catalysts devoid of
2
O
5
C
H
, 2-t-C
butenes (including 1-C
H
, 2-c-C
H
, and 1,3-C
4 6
H
), selectivity to mono-
4
8
4
8
4
8
metavanadate and pyrovanadate phases were prepared via an
impregnation method. Oxalic acid was used to dissolve
ammonium metavanadate and ammonium molybdate in order
to allow a good dispersion of the V and Mo species on the
catalyst surface. The inuence of the Mo : V atomic ratio on the
physical and chemical properties was determined by various
analytical techniques and the catalytic performance was inves-
tigated in a xed bed reactor. We found that the addition of
a small amount of Mo can signicantly change the redox and
basic properties of the corresponding catalyst, improving the
total butenes selectivity.
4
H
8
, 2-t-C , and 2-c-C H
4
H
8
4 8
) and selec-
tivities to other products were calculated based on the following
equations, respectively:
X
M
i
ꢃ n
i
X
n-C H ð%Þ ¼ X
ꢃ 100
(1)
(2)
4
10
M
i
ꢃ n
i
þ 4 ꢃ mn-C4H10
M
i
ꢃ n
i
S ð%Þ ¼ X
ꢃ 100
i
M
i
ꢃ n
i i
where M are the moles of products; n are the number of carbon
atoms of the products, and mn-C H are the mole of the
4
10
unreacted n-butane, respectively. The carbon balance in this
study was in the range of 100 ꢄ 2%. Relative errors of the
conversion and selectivities are less than 1%.
2
Experimental
2.1 Catalyst preparation
All catalysts were prepared by an impregnation method. The
different Mo : V atomic ratios used were 0, 3 : 100, and 5 : 100.
The MgO support was pretreated in a muffle furnace at 900 C
for 3 h. Appropriate amounts of oxalic acid, ammonium meta-
3
Results and discussion
ꢀ
3.1 Catalyst characterization
vanadate, and ammonium heptamolybdate were dissolved in The BET surface areas of all the samples before and aer the
deionized water. The mixed solution was added into a beaker reaction are listed in Table 1. A slight decrease in the surface
containing the MgO powder and the stirring was continued for area can be noticed as the doping amount of molybdenum
2
1
h at room temperature. The resultant slurry was dried at increases. The specic surface area of all the catalysts decreased
20 C overnight, and then calcined at 700 C for 10 h. The to about 20 m g aer the reaction. This decrease may be
ꢀ
ꢀ
2
ꢂ1
resultant powders were pressed into pellets, and then crushed ascribed to the sintering of the catalysts under the reaction
29
and sieved to 40–60 mesh.
conditions.
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3
ꢂ
32
Table 1 BET specific area of the catalysts with different Mo contents which correspond to the stretching vibration of VO
.
As
4
3
ꢂ
shown in Fig. 2, the stretching vibration of VO4 becomes
stronger with the increase in Mo content. The FTIR results are
consistent with those of XRD, further conrming the absence of
2
ꢂ1
S
BET/(m g
)
V
2
O
5
Mo/V
Before
Aer
Samples
VMgO
content (wt%)
atomic ratio
reaction
reaction metavanadate and pyrovanadate phases except for the
Mg
The number of basic sites and the basic strength that
signicantly affect the CO desorption were measured by means
of CO -TPD. The results are shown in Fig. 3. For comparison,
3 4 2
(VO ) phase.
10
10
10
0
44.2
33.8
26.6
22.0
21.0
17.1
3
5
Mo–VMgO
Mo–VMgO
3 : 100
5 : 100
2
2
the CO -TPD curve of the MgO support was also provided, as
2
shown in Fig. 3(a). The VMgO catalyst without Mo doping
Fig. 1 shows the XRD patterns of the as-prepared catalysts. exhibited a similar desorption peak area to the peak of the MgO
The two crystalline phases can be assigned to MgO (JCPDS no. support. This suggests that the VMgO catalyst has a similar
15,30
4
5-0946) and Mg (VO ) (JCPDS no. 19-0778) respectively.
No number of basic sites to that of the MgO support. The increase
3
4 2
metavanadate and pyrovanadate phases were detected, and no in the Mo-doping amount strongly reduced the number of basic
Mo-based crystal phase was found due to the extremely low Mo sites, but did not obviously change the basic strength since the
31
ꢀ
content and its high dispersion in the catalysts.
CO desorption peak at about 300 C did not shi to high or low
2
Fig. 2 shows the FTIR spectra of the as-prepared catalysts. All temperatures aer the Mo-doping. With the increase in the Mo-
ꢂ1
samples exhibited strong absorption bands at 800–900 cm
,
doping amount, the CO desorption peak area decreased
2
signicantly according to the order 5Mo–VMgO < 3Mo–VMgO <
VMgO.
The catalytic behavior of VMgO and the Mo-doped VMgO
catalysts during the ODH of n-butane strongly depends on the
redox properties of the V-species. Therefore, the redox proper-
2
ties of the catalysts were characterized by H -TPR, and the
results are shown in Fig. 4. It can be seen that the content of
reducible lattice oxygen slightly increases with the increase in
Mo-doping amount, while the reduction peak shis to higher
temperatures. This indicates that the V–O bonding energy
increases with the addition of Mo.
It is generally believed that the ODH of alkanes requires
28,33–35
moderately active lattice oxygen,
since more highly active
lattice oxygen can easily lead to the deep oxidation of alkanes,
thus resulting in a low selectivity, while much less active lattice
oxygen is responsible for low conversion. The H consumption
2
peak appearing at high temperatures suggests that the lattice
Fig. 1 XRD patterns 10 wt% V
doping amounts. (a) VMgO, (b) 3Mo–VMgO, (c) 5Mo–VMgO. (*) MgO,
JCPDS no. 45-0946; (A) Mg (VO , JCPDS no. 19-0778.
2
5
O /MgO catalysts with different Mo oxygen of the catalyst is less active upon Mo-doping, as shown
3
4 2
)
Fig. 3 CO
2 2 5
-TPD profiles of the 10 wt% V O /MgO catalysts with
Fig. 2 FTIR spectra of the 10 wt% V O /MgO catalysts with different different Mo-doping amounts. (a) MgO, (b) VMgO, (c) 3Mo–VMgO, (d)
2 5
Mo doping amounts. (a) VMgO, (b) 3Mo–VMgO, (c) 5Mo–VMgO.
5Mo–VMgO.
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carbon-containing products. Some small amounts of other
carbon-containing products like C –C olens were also detec-
2
3
ted as by-products of the ODH of n-butane.
As expected, with the rising of the temperature the conver-
sion increased, while the selectivity decreased. Furthermore,
the selectivity of monobutenes gradually decreased, whereas
that of butadiene increased upon increasing temperatures,
suggesting that the monobutenes were gradually converted into
butadiene during the ODH of n-butane. This phenomenon has
9,36–38
also been reported by other researchers.
The selectivity to
total butenes gradually decreased with the rising of tempera-
ture. The selectivities to other products, such as C –C ,
2
3
increased with the addition of Mo, indicating that more Mo-
doping leads to cracking reactions. In the temperature range
ꢀ
of 540–630 C, the Mo-free sample, VMgO, acted as the most
Fig. 4
2 2 5
H -TPR profiles of the 10 wt% V O /MgO catalysts with
active catalyst. This result was consistent with the H
results. At 630 C, the n-butane conversion over the VMgO
2
-TPR
different Mo-doping amounts. (a) VMgO, (b) 3Mo–VMgO, (c) 5Mo–
VMgO.
ꢀ
catalyst reached 42.8% at a C H /O molar ratio of 1 : 1. This
4
10
2
value was higher than those obtained using 3Mo–VMgO (37.0%)
and 5Mo–VMgO (30.1%) under the same conditions. Thus, the
addition of Mo can decrease the activity of the catalyst,
accompanied by an improvement of the selectivity to total
in Fig. 4. As the Mo/V ratio increased from 0 to 5 : 100, the H
2
ꢀ
ꢀ
consumption peak gradually shied from 580 C to 630 C.
Therefore, a lower selectivity to CO is expected to be achieved
on the 5Mo–VMgO catalyst due to the lower oxygen lattice
reactivity of the Mg (VO phase, as measured by H -TPR.
x
butenes and inhibition of the deep oxidation reactions. The
ꢀ
total butenes selectivity over the VMgO catalyst at 630
C
3
4
)
2
2
reached 71.6%, while that on the 3Mo–VMgO and 5Mo–VMgO
catalysts was 75.4 and 79.2%, respectively. For the selectivity to
butadiene, the value obtained using the VMgO catalyst was
3.2 Catalytic performance
ꢀ
similar to that of 3Mo–VMgO and close to 45% at 630 C, while
3
.2.1 Effects of reaction temperature and C H /O molar
4
10
2
that of 5Mo–VMgO was 41.6%. All these catalysts exhibited
ratio. Fig. 5 shows a comparison of the catalytic performance of
the VMgO, 3Mo–VMgO and 5Mo–VMgO catalysts in the ODH of
n-butane to butenes. The catalytic performance of the Mo–
better performances than those reported in the
8,15,26,30,35
literature.
As discussed above, while the conversion of n-butane
decreased, the selectivity to total butenes increased with the
increase of the Mo-doping amount. This change can be
VMgO catalysts with different Mo/V atomic ratios was studied in
ꢀ
the temperature range of 540–660 C and at a C
4
H10/O
2
molar
ratio of 1 : 1. Monobutenes, butadiene, and CO
x
were the main
ꢂ1
Fig. 5 Catalytic performance of the three catalysts at GHSV of 58 800 h and a C
4
H10/O
2
molar ratio of 1 : 1. ( ) VMgO, ( ) 3Mo–VMgO, ( )
5
Mo–VMgO.
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attributed to the fact that the addition of Mo might deactivate shown in Fig. 5 and 6, when the Mo/V ratio was 3 : 100, no effect
part of the high active sites on the catalyst surface and suppress on the cracking reactions was observed, while a signicant
the deep oxidation reactions to produce CO . Therefore, a small increase in the selectivity to total butenes was noticed. However,
x
amount of Mo oxide can enhance the ODH performance of as the Mo/V ratio slightly increased to 5 : 100, the selectivity to
3
1
butane, in agreement with other literature reports.
cracking reactions was more than doubled. The results of the
-TPD experiments indicate that the Mo-doping reduced the
For selective oxidation reactions, the selectivity is usually CO
2
more important than the conversion; in the presence of number of basic sites on the catalysts, thus higher levels of Mo-
a certain catalyst, a higher conversion oen means lower doping might promote the cracking reactions. Dejoz et al. found
selectivity, higher consumption of raw materials, more heat that the Lewis acid sites increased with the addition of Mo to
release, and higher risk of thermal runaway, thus the conver- the VMgO catalyst, and reported a similar nding in their study,
sion is usually regulated at low levels. The ODH of butane is i.e., that the selectivity of the cracking reactions increased with
15
strongly exothermic, and during our study it was found difficult Mo-doping increases. Therefore, the optimal Mo/V ratio for
to control the catalyst bed temperature at a C ratio lower the Mo–VMgO catalysts is 3 : 100 by considering the n-butane
than 1. Thus, the three catalysts were tested at a higher C
ratio. Fig. 6 shows the performance of the three catalysts at
4 2
H10/O
4
H
10
/
conversion and total butenes selectivity.
O
2
3.2.2 Effect of contact time. A series of tests were per-
a C
at a C
4
H
10/O
2
molar ratio of 1.5 : 1. In comparison to the reaction formed to investigate the effect of the contact time on the
ꢀ
4
H10/O
2
molar ratio of 1 : 1, an increase of total butenes performance of the 3Mo–VMgO catalyst at 630 C. As shown in
selectivity was accompanied by a slight decrease of butane Fig. 7, the n-butane conversion increased from 34.1 to 36.3%,
ꢀ
conversion. For example, for the 3Mo–VMgO catalyst at 630 C while the selectivity to total butenes decreased from 77.7 to
and C H /O molar ratio of 1 : 1, the total butenes selectivity 75.5% with the contact time increasing from 42.9 to 150 ms.
4
10
2
and the butane conversion were 75.4 and 37.0%, respectively, Usually, under the condition that the amount of oxygen is
while those at a 1.5 : 1 C
4.5%, respectively.
4
H10/O
2
molar ratio were 79.3 and enough for the oxidation reactions to occur, the conversion
decreases by shortening the contact time, and it is accompanied
3
The catalytic activity of vanadium-based catalysts is closely by an increase of oxidation product selectivity. The results are
related to the activity of the lattice oxygen, and it is based on the consistent with this conclusion. Though longer contact times
5
+
4+ 15,39
redox cycle V 4 V .
For the reaction, the lattice oxygen is are favourable for the conversion of n-butane which lead to
consumed by oxidation reactions, while being supplied by lower selectivity of total butenes. While decrease the contact
oxygen gas. The results of the H -TPR analysis indicate that the time between n-butane and catalyst, the generated butenes
2
addition of Mo to the VMgO catalyst can increase the reduction would timely escape from the surface of the catalyst which
5
+
4+
temperature of V to V , therefore the catalytic activity is would give rise to the higher butenes selectivity. In addition, the
reduced but accompanied by an improvement of the selectivity conversion and selectivity not change obviously with contact
to total butenes. A small amount of Mo-doping can signicantly time because the conversion of butane is limited by the insuf-
improve the selectivity to total butenes, however further addi- cient oxygen, i.e. no oxygen remaining in the products.
tions of Mo can lead to an increase of cracking reactions. As
ꢂ1
Fig. 6 Catalytic performance of the three catalysts at GHSV of 58 800 h and a C
4
H
10/O
2
molar ratio of 1.5 : 1. ( ) VMgO, ( ) 3Mo–VMgO, ( )
RSC Adv., 2017, 7, 34131–34137 | 34135
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Mo–VMgO.
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surface area tests revealed that the Mo-doped catalysts
possessed a lower surface area as comparing to the undoped
sample. H -TPR and CO -TPD results indicated that the Mo-
2
2
doping improved the redox temperature, while decreasing the
number of basic sites. On the basis of the n-butane ODH cata-
lytic tests, it was found that the addition of a small amount of
Mo effectively improved the selectivity of butenes and inhibited
the deep oxidation reactions. However, further addition of Mo
decreased the catalyst activity, leading to an increase in the
selectivity of cracking reactions. Therefore, the optimal Mo/V
ratio for this series of catalysts was 3 : 100 by considering the
n-butane conversion and total butenes selectivity. Changing the
contact time produced only a slight increase in the selectivities
to monoolens and total butenes. The 3Mo–VMgO catalyst
maintained a stable performance during the initial 50 h, but it
then underwent a gradual deactivation due to sintering, which
induced a reduction of the active sites.
Fig. 7 Effect of the contact time on the catalytic performance.
ꢀ
Reaction conditions: 630 C, C
Butane conversion, ( ) total butenes selectivity, ( ) butadiene selec-
4
H
10/O
2
molar ratio ¼ 1.5 : 1. ( ) n-
x
tivity, ( ) monoolefin selectivity, ( ) CO selectivity, ( ) other products
selectivity.
Acknowledgements
The authors gratefully acknowledge nancial support from the
Fundamental Research Program for Clean Energy and National
Natural Science Foundation of China (21476101), Youth Inno-
vation Promotion Association of the Chinese Academy of
Sciences, the Strategic Priority Research Program of the Chinese
Academy of Sciences (Grant No. XDB17020400) and Dalian
Institute of Chemical Physics (DICP DMTO201503).
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the 3Mo–VMgO catalyst was tested at the C
of 1.5 : 1 and 630 C, and the results are shown in Fig. 8. As it
4
H
10/O
2
molar ratio
7
8
9
G. Raju, B. M. Reddy, B. Abhishek, Y.-H. Mo and S.-E. Park,
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ꢀ
can be seen, the conversion of n-butane was maintained around
34%, the total butenes selectivity remained close to 80%, and
the butadiene selectivity was around 45% within the rst 50 h.
Aer 50 h, the n-butane conversion and butadiene selectivity
decreased slightly, however the total butenes selectivity gradu-
ally increased. This change in conversion and selectivity was
related to the catalyst sintering, which resulted in the reduction
of the specic surface area (as shown in Table 1) and number of
active sites.
1
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