Catalytic dehydrogenation of isobutane over Co-based catalysts

Article abstract of DOI:10.1039/c4ra08849b

In this work, to optimize the catalytic performance of Co-based catalysts for isobutane dehydrogenation, the effects of support, calcination temperature and some promoters were investigated systematically. Results of activity tests and catalyst characterization jointly indicated that both the support and calcination temperature influenced Co dispersion, catalyst acid properties and the interaction between support and Co species significantly. Consequently, the adsorption-desorption behaviors for isobutane and isobutene as well as the dehydrogenation performance were further affected. Sulfided Co/SiO₂, with no acidity, exhibited the best performance. Although high calcination temperature was beneficial for achieving a high selectivity to isobutene over Al₂O₃ and MgAl₂O₄ supported Co-based catalysts, the inevitable formation of Co₂SiO₄ over Co/SiO₂ at high temperature led to reduced active sites and dehydrogenation activity. In addition, by comparing the different effects of the separate introduction of S, Sn, Cu and Cl, it was concluded that an efficient promoter should present the following characteristics: inhibition of the formation of metal ensembles, and strong binding affinity to two hydrogen atoms of one additive atom.

Full text of DOI:10.1039/c4ra08849b

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Catalytic dehydrogenation of isobutane over Co-
based catalysts†
Cite this: RSC Adv., 2014, 4, 57071
a
ab
a
a
a
Guowei Wang, Xiaolin Zhu, Jiaoyu Zhang, Yanan Sun, Chunyi Li*
a
and Honghong Shan
In this work, to optimize the catalytic performance of Co-based catalysts for isobutane dehydrogenation,
the effects of support, calcination temperature and some promoters were investigated systematically.
Results of activity tests and catalyst characterization jointly indicated that both the support and
calcination temperature influenced Co dispersion, catalyst acid properties and the interaction between
support and Co species significantly. Consequently, the adsorption–desorption behaviors for isobutane
and isobutene as well as the dehydrogenation performance were further affected. Sulfided Co/SiO
no acidity, exhibited the best performance. Although high calcination temperature was beneficial for
achieving a high selectivity to isobutene over Al and MgAl supported Co-based catalysts, the
inevitable formation of Co SiO over Co/SiO at high temperature led to reduced active sites and
2
, with
O
2 3
2 4
O
2
4
2
Received 18th August 2014
Accepted 27th October 2014
dehydrogenation activity. In addition, by comparing the different effects of the separate introduction of
S, Sn, Cu and Cl, it was concluded that an efficient promoter should present the following
characteristics: inhibition of the formation of metal ensembles, and strong binding affinity to two
hydrogen atoms of one additive atom.
DOI: 10.1039/c4ra08849b
www.rsc.org/advances
selectivity to isobutene of around 87 wt%. In general, the
1
. Introduction
promoting effects of introduced sulfur can be understood in
3
Lately, dehydrogenation of light alkanes has received great two aspects: one is a geometric effect to dilute aggregated
attention as a route to produce the corresponding alkenes, metallic species and inhibit C–C bond rupture; another is an
which constitute an important and versatile class of chemical electronic effect to facilitate olen desorption and further
intermediate. However, due to the high cost of platinum and increase product selectivity. Meanwhile, sulfur loss is specu-
1
adverse environment effects of chromia, the application of lated to be the origin of catalyst deactivation. To replenish the
industrialized Pt and Cr
2
O
3
-based catalysts has been limited to loss of sulfur, continuous suldation-reaction cycles have been
some extent.
carried out and the catalyst performance can be maintained at a
Although high catalytic activity can be obtained over metal high level. Moreover, it should also be noted that, the catalytic
such as Fe, Co and Ni) based catalysts for the conversion of performance can be stable for a long period when diluted H S is
isobutane, the reaction is suffered from an extremely poor fed together with isobutane.
(
2
10
selectivity because of aggravated hydrogenolysis reaction, of
As mentioned above, the active sites of metal sulde catalysts
which the main product is methane rather than isobutene. To and how they interact with isobutane molecules were specu-
address this issue, sulfur was introduced into metal based lated in our previous study. However, it is still not enough for
2,3
4
–6
catalysts in spite that it is usually regarded as an impurity.
Fortunately, we have found that a series of metal sulde cata- catalyst support, signicantly inuencing the dispersion and
the preparation of an ideal dehydrogenation catalyst. Firstly, the
7–9
11,12
lysts, either prepared by impregnation with sulfate or sul- electronic properties of the active components
as well as the
3,10
13–15
dation with H S,
exhibit excellent performance in alkane acid–base properties of the catalysts,
is an important factor
2
dehydrogenation. The conversion of isobutane over a series of to be considered. Meanwhile, some literature reported that
sulde catalysts of Fe, Co, and Ni reached up to 70 wt%, with a high-temperature calcination could modify the catalyst struc-
ture and increase the activity and stability of Pd@CeO /Si–Al O
2 3
2
16
catalyst obviously. Therefore, the effects of both support and
calcination condition, directly related to catalyst structure
a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao
16–18
2
66580, China. E-mail: chyli_upc@126.com; chyli@upc.edu.cn; Fax: +86 532
6981718; Tel: +86 532 86981862
1
9,20
and properties
as well as the interaction between active
8
b
21,22
Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, component and support,
should be further investigated.
Given the difference in acid–base properties and the inter-
action with active component, Al , MgAl and SiO were
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra08849b
2
O
3
2
O
4
2
1
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chosen as the supports of Co-based catalysts separately in this prepared samples are denoted as 13Co/support-x, where “x”
work. Moreover, to examine the effect of calcination tempera- indicated the calcination temperature. Moreover, sulded
ture, the catalysts with different supports were calcined at both catalysts were obtained by pretreatment of the samples with
ꢁ
1
ꢀ
H S/H (30 mL min ) for 3 h at 560 C. Co/SiO catalysts with
2 2 2
low and high temperatures.
In addition to the support and calcination temperature, the the introduction of Sn or Cu were prepared by subsequent
promoter plays a more important role in this catalyst system. As impregnation of above as-prepared Co/SiO catalysts with
2
reported in our previous study, metal ensembles catalyze ethanol solution of stannous chloride or aqueous solution of
hydrogenolysis reaction to generate methane. Since the hydro- copper nitrate, respectively. Cl-modied catalyst was prepared
23–27
genolysis reaction is structure sensitive,
2
breaking these by impregnation of SiO with certain amount of cobalt chloride
metal ensembles and isolating small metal particles seem to be solution. Aer the introduction of the promoters, the catalysts
ꢀ
the key in restricting this reaction. To achieve this goal, there were calcined again at 560 C for 2 h.
are two available methods: reducing metal loading or intro-
ducing barriers into the ensembles. As for the former case,
although the selectivity to isobutene increased obviously with
2.2 Catalyst characterization
decreasing metal loading, the catalyst activity decreased Nitrogen adsorption–desorption measurements were per-
7
sharply, indicating the lack of active sites. In contrast, the later formed on a Quadrasorb SI instrument and a porosimetry
approach seems to be more effective: sulfur introduction analyzer at liquid nitrogen temperature. The specic surface
inhibits the formation of metal ensembles and improves the areas and pore volumes of the catalyst samples were determined
dehydrogenation
performance
signicantly.
Similarly, by the BET and BJH methods. X-ray diffraction (XRD) analysis of
28
Apestegu ´ı a and Barbier also pointed out that presuldation the samples was carried out on an X'Pert PRO MPD diffrac-
treatment of Pt-based catalyst could inhibit hydrogenolysis tometer system using Cu Ka radiation at 40 kV and 40 mA,
reaction effectively.
ꢀ
ꢀ
ꢀ
ꢁ1
running from 5 to 75 with scanning speed at 10 min . To
Some open literatures reported that the addition of Sn into determine the content of chlorine on 13Co/SiO –Cl catalyst
2
Pt-based catalyst generated certain geometric and electronic before and aer reaction, elemental analysis was carried out by
effects, which effectively reduced platinum particles and sup- an Axios X-ray ourescence spectrometer (XRF).
29–32
pressed coke formation and hydrogenolysis reaction.
Simi-
Metal dispersion and particle size were determined by H2
larly, copper addition into nickel-based catalyst also played pulse chemisorption using a FINESORB-3010 chemisorption
33,34
such a role through the formation of Cu–Ni alloys.
In addi- analyzer, with the gas released from the reactor being analyzed
tion, resulting from blockage/electronic effects of the adatom, by an online GAS-100Q quadrupole mass spectrometer. About
chlorine modication inuenced the adsorptive and catalytic 0.3 g of fresh catalyst was reduced in situ under a ow of 10 vol%
35–37
ꢁ1
properties of the catalyst remarkably.
explore the promoting mechanism of the additives in this work, 100 C under the purge of Ar. Aerwards, H
Sn, Cu, and Cl have been introduced into Co/SiO catalyst to the catalyst bed by the application of a six-way valve until the
compare their effects with that of S on the dehydrogenation signal intensity of H did not increase with the pulse number
Therefore, to further H /N (30 mL min ) at 773 K for 1 h, and then cooled down to
2 2
ꢀ
2
2
/N was pulsed into
2
2
performance.
any more. Ultimately, according to the change of area for each
pulse, the amount of chemisorbed H was determined, and thus
2
the metal dispersion and particle size can be further calculated.
To investigate the desorption behaviors of the catalysts for
alkenes, isobutene desorption test was performed on the same
2
. Experimental
2.1 Catalyst preparation
Alumina was prepared by the sol–gel method. Firstly, pseudo- chemisorption analyzer, also equipped with an online MS. For
boehmite powder was mixed with distilled water to obtain a each test, about 0.3 g of sulded catalyst was loaded into a
suspended solution. Aer that, HCl solution (36–38 wt%) was quartz tube reactor, preheated in a ow of nitrogen (60 mL
ꢀ
ꢁ1
ꢀ
ꢀ
added dropwise under vigorous stirring at 70 C, until a gel-like min ) at 450 C for 0.5 h, and then cooled down to 200 C.
mixture was formed. To prepare magnesia-alumina spinel, an Subsequently, with the application of a four-way valve, nitrogen
appropriate amount of magnesium nitrate solution was added ow was instantaneously switched to a mixture of Ar and iso-
ꢁ1
into the above gel-like mixture. Then, the mixture was stirred for butene at separate ow rate of 30 mL min . Aer adsorption
h, dried at 140 C for 12 h, and calcined at 700 C for another 2 saturation, the valve was switched back to nitrogen. At that
ꢀ
ꢀ
2
h in air. Finally, both gel-like mixtures were crushed and sieved time, the signal for the inert gas decreased immediately since it
to 80–180 mm for later use. In addition, mesoporous silica (80– is hardy adsorbed on catalyst surface. However, the decrease of
1
80 mm) was obtained from China Qingdao Haiyang Chemical signal for isobutene was delayed due to its adsorption on the
2
ꢁ1
Company, with a specic surface area of 378.3 m g and an catalyst. Precisely the difference between the signals of inert gas
average pore diameter of 9.6 nm. and isobutene gives useful desorption information, for
Supported catalyst samples were prepared by wetness instance, a larger delay extent is indicative of a more difficult
impregnation of the support with certain amount of cobalt isobutene desorption process. Furthermore, such test was also
ꢀ
ꢀ
nitrate solution (Co O content of the catalyst was 13 wt% in performed at 300 C and 400 C.
3
4
this work). Subsequently, the samples were dried overnight at
To compare the adsorption amounts of isobutane or iso-
ꢀ
140 C and calcined for 2 h at different temperatures. The as- butene over different catalysts, pulse chemisorption tests were
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also carried out using the same equipment with H
2
chemi-
Furthermore, isobutane dehydrogenation in a pulse mode
sorption. Aer pretreatment of 0.3 g of sulde catalysts in a ow xed-bed reactor over Cu-modied catalysts was also con-
of nitrogen, 0.803 mL of isobutane or isobutene was pulsed into ducted. For each test, about 0.1 g of catalyst was loaded and the
ꢁ
1
ꢀ
ꢀ
the catalyst bed with a ow of nitrogen (30 mL min ) at 300 C. reaction temperature was maintained at 560 C. Prior to the
Meanwhile, an online quadrupole mass spectrometer was reaction, the catalyst was pretreated by a ow of H /N (30 mL
2
2
ꢁ
1
applied to detect the signals of released hydrocarbons, and the min ) for 2 h. Then, pulse feeding was performed using a six-
adsorption amount could be further determined by calculating way valve, and 0.803 mL of isobutane was driven into the reactor
ꢁ
1
the signal area of each pulse.
by nitrogen ow (with a ow rate of 30 mL min ) in one pulse.
The acidic properties of the catalyst samples were deter- Meanwhile, the effluent products were characterized using an
mined by temperature-programmed desorption (TPD) of NH3. online MS, and their mass peaks were registered as follows: H2,
For each test, 0.1 g of sample was preheated in situ under a 2; CH , 16; i-C H , 56 and i-C H , 58.
4
4
8
4
10
ꢀ
ꢀ
helium ow at 600 C for 0.5 h, and then cooled down to 100 C.
Adsorption was proceeded for 0.5 h under NH ow. Subse-
quently, the sample was purged under a dry helium ow to
3
3
. Results and discussion
3
remove physically adsorbed NH . Ultimately, desorption was
initiated at a heating rate of 10 C min until 600 C was
ꢀ
ꢁ1
ꢀ
3.1 Structural characterization and Co dispersion
reached. The amount of desorbed NH
area under the TPD curve.
3
was calculated from the The XRD patterns of Co-based catalysts supported on the three
different supports are illustrated in Fig. 1a. For low-temperature
calcined catalysts, diffraction peaks characteristic of both the
support and Co O were observed. However, with the increase of
3
4
2
.3 Catalytic activity test
calcination temperature, some newly emerged diffraction peaks
Catalytic activity tests were performed in a xed bed micro- assigned to CoAl O and Co SiO were detected over Co/Al O
2
4
2
4
2 3
ꢀ
reactor at atmospheric pressure and 560 C. Prior to reaction, and Co/SiO
S-modied catalysts were obtained by in situ suldation with of peaks corresponding to Co
2
catalysts, respectively. Meanwhile, the intensities
decreased to some extent.
2 4
(30 mL min ) for 3 h at 560 C, while Sn, Cu, and Cl- However, no obvious phase change took place over Co/MgAl O
3 4
O
ꢁ
1
ꢀ
H
2
S/H
2
ꢁ
1
promoted catalysts were reduced by H
at 560 C. During the test, 4 g of catalyst was loaded into the
2
(30 mL min ) for 2 h catalyst with increasing calcination temperature.
Moreover, phase analysis was further carried out over sul-
ꢀ
reactor, then a ow of isobutane diluted by nitrogen with a xed ded Co-based catalysts with different supports calcined at both
molar ratio of 1 : 6 was fed into the reactor, at a total ow rate of low and high temperatures, and the XRD patterns are shown in
ꢁ
1
1
4 mL min . The inner diameter of the reactor is 14 mm, with a Fig. 1b. CoS was present on the surface of catalysts calcined at
ꢀ
length of 160 mm, and the catalyst bed (50 mm long) was loaded 560 C aer suldation, indicating that suldation treatment
between two layers of silica sand. To compare the dehydroge- transformed Co
nation performance of different catalysts, the measurements calcination temperature increased to 800 C, stable CoAl
3
O
4
into CoS successfully. However, when the
ꢀ
2
O
4
and
were taken at the initial stage of the reaction. Both the feed and Co
products were analyzed by a Bruker 450 Gas Chromatograph surface of Co/Al
GC) equipped with an FID detector and two TCD detectors. addition, compared with low temperature calcined samples, the
2
SiO
4
, which were difficult to be sulded, formed on the
2
O
3
and Co/SiO catalysts, respectively. In
2
(
Fig. 1 XRD patterns of different Co-based catalysts before (a) and after (b) sulfidation: (1) 13Co/Al
MgAl -560; (4) 13Co/MgAl -800; (5) 13Co/SiO -560; (6) 13Co/SiO -800.
2 3 2 3
O -560; (2) 13Co/Al O -800; (3) 13Co/
2
O
4
2
O
4
2
2
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Table 1 Physicochemical properties and activity test results of supported Co-based catalysts
Cosurf
density, atomCo/nm
.
a
2
ꢁ1
b
p
3
ꢁ1
c
p
2
d
e
f
ꢁ1
Sample
S
BET , m g
V
, cm g
D
, nm
Co dispersion , %
D
Co , nm
TOF , h
1
1
1
1
1
1
3Co/Al
3Co/Al
3Co/MgAl
3Co/MgAl
3Co/SiO
3Co/SiO
2
2
O
O
3
3
-560
-800
146
130
84
73
225
168
0.25
0.25
0.19
0.19
0.86
0.73
5.7
6.5
7.9
7.9
8.7
9.6
6.7
7.5
11.6
13.4
4.3
43.6
31.2
28.5
23.4
20.3
11.5
2.3
3.1
3.4
4.2
4.3
8.5
0.4
0.7
1.2
1.9
2.5
3.9
2
O
O
4
-560
-800
2
4
2
-560
-800
2
5.8
a
b
c
d
2
Specic surface area. Pore volume. Pore size. Co dispersion was determined by H chemisorption, referred to the molar ratio of surface Co to
f
e
bulk Co, with the latter being calculated from the cobalt nitrate impregnation. Particle size of Co was determined by H
2
chemisorption. Turn over
frequency was calculated as the amount of formed isobutene per hour divided by the amount of active metal atom (determined by H
2
ꢁ1
ꢀ
chemisorption) supported on catalyst aer presuldation treatment with H S/H (30 mL min ) for 3 h at 560 C.
2 2
intensity of diffraction peaks corresponding to CoS decreased to isobutane conversion and isobutene selectivity decreased
some extent. As for MgAl
phase change was observed at high calcination temperature.
2
O
4
-supported catalyst, no obvious slightly with increasing calcination temperature. As a result,
ꢀ
isobutene yield reduced from 61.8 wt% at 560 C to 52.6 wt% at
ꢀ
Table 1 lists the BET surface area, pore properties and Co 800 C. At the same time, both the selectivity to methane and n-
dispersion determined by H
lysts calcined at different temperatures. With the increase of 13Co/SiO
2
chemisorption of Co-based cata- butane were kept at a very low level, revealing that the sulded
catalyst was almost inactive for cracking and isom-
2
calcination temperature, the surface area decreased noticeably erization reactions. In summary, SiO2 seems to be the most
for all the samples, which was indicative of the collapse of small appropriate support for Co-based catalysts in dehydrogenation.
pore walls. And this result was also consistent with the
increasing trend of pore size. As for the catalysts with different the stability of sulded 13Co/SiO
Continuous activity test was further carried out to evaluate
ꢀ
2
catalyst calcined at 560 C,
supports, the surface area decreased in the following order: with the results shown in Fig. S1 in ESI.† Isobutane conversion
3Co/SiO > 13Co/Al > 13Co/MgAl . While, for the decreased signicantly with time on stream, especially at the
1
2
2
O
3
2 4
O
sequence of surface density of Co atom, a reverse trend to that initial stage, showing an obvious catalyst deactivation
of surface area was obtained. Furthermore, Co dispersion was phenomenon. Fortunately, the decreased catalyst activity can be
determined by H chemisorption, and it decreased with calci- recovered by another suldation treatment. Within ve
2
nation temperature. Meanwhile, Co dispersion increased as continuous suldation-reaction cycles (see Fig. S2†), the initial
follows: 13Co/SiO2 < 13Co/MgAl O < 13Co/Al O , and the isobutane conversion was maintained at a relatively steady level.
2
4
2 3
calculated particle size of Co presented an opposite trend. To achieve continuous operation, a circulating uidized bed
Above results indicated that the dispersion of Co species had no unit equipped with a reactor for dehydrogenation, a regenerator
direct relationship with the catalyst surface area, and the for coke burning and a suldation section, is suggested to be
interaction between Co species and the support might play a employed in future industrial application.
more signicant role.
3.2.1 Adsorption–desorption
behaviors.
Adsorption,
surface reaction and desorption are three essential steps in
heterogeneous reaction. Among these processes, adsorption
and desorption behaviors signicantly inuence the conversion
3.2 Effects of support and calcination temperature
The inuence of calcination temperature on dehydrogenation of the reactants, the prompt departure of the desired product
performance of sulded Co-based catalysts was rstly investi- from the catalyst, and further the catalytic performance. To
gated, and the results are illustrated in Fig. 2. With regard to obtain an ideal performance, the interaction between the
38
2 3
Al O
-supported catalyst, isobutane conversion decreased reactants and catalyst surface should be properly adjusted. If
ꢀ
ꢀ
obviously from 76.2 wt% at 560 C to 57.1 wt% at 800 C. the interaction is too strong, low steady state coverage of free
Meanwhile, the selectivity to methane and n-butane also sites will be formed, resulting in a low reaction rate. For the
exhibited a gradually decreasing trend, indicating that both opposite case, the reactant coverage on the surface will be low,
cracking and isomerization reactions were weakened with also leading to a low reaction rate. Therefore, the optimum state
increasing calcination temperature. Even though, the selectivity is an ideal coverage of adsorbed species and free sites on cata-
to isobutene only increased to 46.6 wt%, a relatively low level. As lyst surface, facilitating both the adsorption of reactants and the
for 13Co/MgAl
2
O
4
catalyst, the selectivity to isobutene was desorption of products.
Table 2 compares the adsorption amount of isobutane and
much higher, and it increased from 60.4 wt% to 84.0 wt% with
the increase of calcination temperature. Meanwhile, isobutane isobutene (determined by pulse experiment) over different sul-
conversion declined slightly at the initial stage, and then ded Co-based catalysts. The adsorption amount of isobutane
approached constant aerwards. When taking SiO2 as the decreased with increasing calcination temperature, which was
support, the selectivity to isobutene was maintained above 80 consistent with the decrease of isobutane conversion.
wt% at all tested calcination temperatures. However, both Combined with the results of activity test, it can be inferred that
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2 3 2 4 2
Fig. 2 Dehydrogenation performance of sulfided Co-based catalysts: (a and b) 13Co/Al O ; (c and d) 13Co/MgAl O ; (e and f) 13Co/SiO .
the adsorption of isobutane is not the bottleneck for isobutane procedure are illustrated in Fig. 3. In this gure, a larger signal
dehydrogenation. As for isobutene, the adsorption amount delay extent between Ar and isobutene indicates a harder
decreased remarkably with increasing calcination temperature desorption behavior of the catalysts for isobutene and a higher
except for 13Co/SiO
ported catalyst, the adsorption ability of 13Co/Al
MgAl catalysts for isobutene was much stronger.
The transient response signals of Ar and isobutene over which well explained the worst selectivity to isobutene over
2
catalyst. And compared with SiO
and 13Co/ different supports, the signal delay extent decreased in the
2
following order: 13Co/Al > 13Co/MgAl > 13Co/SiO ,
2
-sup- possibility of secondary reactions. As for sulded catalysts with
2 3
O
2
O
4
2
O
3
2
O
4
different sulded Co-based catalysts during the desorption Al
2
O
3
-supported catalysts. Moreover, with the increase of
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Table 2 Comparison of adsorption amounts of isobutane and iso- catalysts calcined at different temperatures, almost with no
butene over sulfided Co-based catalysts calcined at different
temperatures
acidity, exhibited a superior dehydrogenation activity and
selectivity. This result indicates that the introduced sulfur also
has the ability to capture the rst hydrogen from isobutane
Adsorption amount,
mL g cat
ꢁ1
molecule, and further initiates the dehydrogenation reaction.
Therefore, it can be concluded that the acid sites are not
necessary for this catalytic system.
ꢀ
Sample
Calcination temperature, C Isobutane Isobutene
According to above discussions, the support not only inu-
ences the interaction with active component but also its
dispersion. As listed in Table 1, although the dispersion of Co
species on SiO2 was the worst, the sulded Co/SiO2 catalyst
exhibited the highest TOF for isobutene formation. In contrast,
alumina-supported catalysts with the best Co dispersion gave
the lowest TOF value. This result suggests that Co dispersion is
not the most important factor affecting the dehydrogenation
performance in this catalytic system. In addition, it should also
be noticed that the three evaluated supports with different acid
properties, exhibited different adsorption–desorption behaviors
for isobutane and isobutene and dehydrogenation performance
as discussed above. From these aspects, silica, with no acidity,
showed the highest prospect for application.
1
1
1
3Co/Al
3Co/MgAl
3Co/SiO
2
O
3
560
00
560
00
560
00
0.68
0.49
0.56
0.53
0.65
0.55
2.47
1.42
1.21
0.31
0.25
0.28
8
2 4
O
8
2
8
calcination temperature, the signal delay extent decreased
obviously over Al and MgAl -supported catalysts, which
was consistent with the increase of selectivity to isobutene.
Considering the above discussions, the desorption behaviors of
the products are crucial to the selectivity.
2
O
3
2 4
O
3
.2.2 Acidic and structural properties. Given the formation
of a large amount of methane, propene and n-butane over sul-
ded Co-based catalysts, and the fact that the presence of acid
sites is prone to affect the adsorption–desorption behaviors of
During catalyst preparation, the calcination temperature
4
7–49
50,51
also inuence the surface area,
composition
properties
and phase

52–55
of the catalyst remarkably. For all tested
39
samples, the surface area decreased obviously with increasing
calcination temperature, especially for silica-supported cata-
the catalysts and facilitate cracking and isomerization reac-
tions,
determined by NH
Fig. 4. As for sulded Co-based catalysts with different supports,
40–42
the acid properties of the catalysts were further
-TPD, with the proles demonstrated in
lysts. Meanwhile, the acid amount also decreased for Al O and
2 3
3
MgAl O -supported catalysts, which was benecial for obtain-
2 4
ing a high selectivity to isobutene. In addition, the phase
composition on the catalyst changed with calcination temper-
ature. At high temperature, the interaction between Co species
and the support became strong, and some solid state reactions
both the peak temperature and the amount of NH desorbed
3
increased in the following order: 13Co/SiO < 13Co/MgAl O <
2
2 4
1
3Co/Al O , which was the same with the sequence of adsorp-
2 3
tion ability for isobutene. Such results can be explained by the
fact that isobutene molecule containing a p-bond is more
alkalinic than isobutane, and more likely to interact with the
took place. For instance, CoAl
2 4 2 4
O and Co SiO formed on the
surface of Co/Al O and Co/SiO2 catalysts, respectively. The
2
3
43
generation of CoAl O decreased the catalyst acidity and
2 4
electron-decient Lewis acid sites on catalyst surface. There-
fore, the increase of acid strength and amount promoted the
adsorption of isobutene, and enhanced the secondary reactions
to side products.
inhibited secondary reactions effectively. In contrast, the
formation of Co SiO (difficult to be sulded) probably led to
2
4
the decrease of active sites, and thus worsened the dehydroge-
nation performance.
With regard to sulded Al O -supported catalysts, the acidity
2
3
decreased with calcination temperature, which can possibly be
attributed to the consumption of Al for the formation of
CoAl on the catalyst (as evidenced by the XRD patterns in
Fig. 1). And the acid amount of MgAl -supported catalysts
also exhibited the same variation trend with increasing calci-
nation temperature.
2 3
O
3
.3 Mechanism of different promoters
2 4
O
3.3.1 Sn and Cu modication. In addition to the support
2 4
O
and calcination temperature, the effects of promoters should
also be investigated. Based on our previous study, sulfur seems
3
to act as a barrier to break the metal ensemble which is active
for hydrogenolysis reaction, and consequently the dehydroge-
nation performance is improved dramatically. Accordingly, to
examine if the geometric or alloy effect can inhibit hydro-
genolysis reaction by isolating metal particles, different
Based on above discussions, it can be concluded that the
decrease of acid amount results in an increasing selectivity to
41,44
isobutene, which is consistent with other reports.
That is,
the presence of acid sites seems to have no promoting effect on
the dehydrogenation performance. However, according to
43
contents of Sn and Cu have been introduced into 13Co/SiO2
Abello et al., acid sites on the catalyst facilitate the abstraction
of the rst hydrogen atom from C–H bond of alkane molecule,
which is generally regarded as the rate-determining step. Some
catalyst in this work, and the activity test results are illustrated
in Fig. 5.
45,46
To reduce the metal oxide on catalyst surface, the catalysts
other researchers
priate amount of acid sites facilitates the activation of alkanes.
However, in this present study, the sulded SiO -supported
also believe that the presence of appro-
were prereduced by H for 2 h prior to reaction. During the
2
activity test, the following three reactions (eqn (1)–(3)) probably
take place over Co-based catalysts. It can be observed in Fig. 5
2
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Fig. 3 Transient response signals of Ar (dotted lines) and isobutene (solid lines) over different sulfided Co-based catalysts during the desorption
procedure: (a) 13Co/Al -560; (b) 13Co/Al -800; (c) 13Co/MgAl -560; (d) 13Co/MgAl -800; (e) 13Co/SiO -560; (f) 13Co/SiO -800.
2
O
3
2
O
3
O
2 4
2
O
4
2
2
that, with hydrogen being supplied by deep dehydrogenation
Aer introduction of Sn and Cu, the selectivity to isobutene
56
(
eqn (1)) of isobutane, the reduced 13Co/SiO catalyst without increased sharply with increasing Sn and Cu contents, espe-
2
any promoter exhibited extremely high activity for hydro- cially for Sn-modied catalysts. When 1 wt% Sn was introduced,
genolysis reaction and the selectivity to methane was up to 89.9 the selectivity to isobutene increased from 0 to 54.6 wt%, and
wt%.
the selectivity to methane decreased to 5.2 wt%. Such results
indicated that the introduction of Sn and Cu could suppress
hydrogenlysis reaction effectively. However, isobutane conver-
sion decreased obviously at the same time, making the modi-
Deep dehydrogenation: i-C
4
H
10 / 4C + 5H
2
(1)
(2)
(3)
Hydrogenolysis: i-C H + 3H / 4CH
4
10
2
4

cation less attractive.
Dehydrogenation: i-C
4
H10 / i-C
H
4 8
+ H
2
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introduced, the signals of isobutane (m/z ¼ 58) and isobutene
(m/z ¼ 56) can hardly be detected except for the rst pulse (as
illustrated by the MS spectra in Fig. 6a). However, the signal of
methane (m/z ¼ 16) was obvious, demonstrating that 13Co/SiO2
catalyst exhibited extremely high hydrogenolysis activity,
consistent with the results obtained by GC. Aer the introduc-
tion of Cu, the signal intensity of methane decreased gradually
with increasing Cu content, meanwhile, that of isobutane
increased obviously. In contrast, the signal intensity of iso-
butene increased for the initial introduction of Cu, and
decreased subsequently. This observation can be attributed to
the combined effects of decreasing isobutane conversion and
increasing selectivity to isobutene.
As shown in Table 3, for 13Co/SiO
and Cu, Co dispersion was much larger than that over the
original 13Co/SiO catalyst (see Table 1), at the same time, the
2
catalysts modied by Sn
2
size of Co particles decreased obviously, indicating that the
introduction of Sn or Cu destroyed the aggregated metallic
species. In general, the performance of Sn or Cu promoted
catalysts was similar to that of catalyst with low metal loading
3
Fig. 4 NH -TPD profiles of different sulfided 13Co/support catalysts.
To study the dehydrogenation performance of Sn and Cu
promoted catalysts intuitively, pulse-mode isobutane dehydro-
genation was carried out with the equipment of an online MS.
Taking Cu-modied catalyst for example, when no Cu was
(i.e., high selectivity to isobutene but low activity). This fact
revealed that the introduction of Sn or Cu destroyed metal
ensembles through geometric or alloy effect, and suppressed
hydrogenolysis
reaction
effectively.
However,
the
2
Fig. 5 Catalytic performance of Sn or Cu promoted 13Co/SiO catalyst for isobutane dehydrogenation.
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2 2 2
Fig. 6 Mass spectra of pulse-mode isobutane dehydrogenation over Cu-modified Co/SiO catalysts: (a) 13Co/SiO ; (b) 1Cu–13Co/SiO ; (c)
2 2 2 2
2Cu–13Co/SiO ; (d) 3Cu–13Co/SiO ; (e) 4Cu–13Co/SiO ; (f) 5Cu–13Co/SiO .
dehydrogenation activity was not improved, and isobutene yield molecule adsorbs on the surface of the catalyst dissociatively,
only reached to 27.5 wt% and 14.1 wt% over Sn and Cu forming an intermediate with cobalt bonded to the tertiary butyl
promoted catalysts, respectively, much lower than S-modied and sulfur bonded to the le hydrogen. Subsequently, the bond
catalyst. And the reason might lie in the difference of binding between b-hydrogen and sulfur weakens the C–H bond in
affinity between S and Sn or Cu to hydrogen atom in isobutane isobutane, leading to the formation of a molecule of hydrogen.
molecule.
Based on our previous studies,
The le intermediate with a p-bonded alkene releases an iso-
involving combined butene molecule, meanwhile the active center is recovered.
3,7,10
characterizations of H -TPR, SEM, XPS and purposive activity During this procedure, the high binding affinity of sulfur to two
2
tests, the catalytic cycle of cobalt sulde catalyst for isobutane hydrogen atoms facilitates the formation of hydrogen, and the
dehydrogenation can be speculated as Scheme 1. An isobutane reaction equilibrium is shied towards the right side. As a
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Table 3 Physicochemical properties of 13Co/SiO
by different elements at calcination temperature of 560 C
2
catalysts modified
3
.3.2 Cl modication. Given a higher electronegativity than
ꢀ
sulfur, chlorine is considered to have a stronger ability to
combine with hydrogen, therefore, it has also been introduced
into 13Co/SiO catalyst for the purpose of improving the dehy-
2
a
b
c
S
BET
,
Cosurf. density,
atomCo/nm
Co dispersion ,
%
Co
D ,
2
ꢁ1
2
Sample
m g
nm
drogenation performance. It should be noted here that this
modication was also effective in decreasing the size of Co
particle size (see Table 3) and breaking the large metal ensem-
bles into smaller ones.
Before the activity test, the catalyst sample was prereduced
by hydrogen for 2 h, and then the reaction lasted for 2 h. To
d
1
1
1
3Co/SiO
3Co/SiO
3Co/SiO
2
2
2
-Sn
221
218
230
4.4
4.5
4.2
34.1
31.9
27.1
2.5
2.7
3.2
d
-Cu
e
-Cl
a
b
Specic surface area.
chemisorption, referred to the molar ratio of surface Co to bulk Co,
with the latter being calculated from the cobalt nitrate impregnation.
Particle size of Co was determined by H
of Cu and Sn in the catalyst were 5 wt%. Content of Cl in the
catalyst was 3.7 wt%, which was determined by XRF.
Co dispersion was determined by H
2
c
d
chemisorption. Contents determine the content of chlorine on 13Co/SiO –Cl catalyst
2
2
e
before and aer reaction, elemental analysis was further carried
out by XRF. Cl content of the fresh catalyst was 3.72 wt%,
indicating that part of Cl was removed from the catalyst during
the calcination process. Aer 2 h reaction, Cl content on the
catalyst was still up to 3.58 wt%, remaining almost stable. It can
2
be seen in Fig. 7 that, compared with 13Co/SiO catalyst, the
selectivity to isobutene increased remarkably aer chlorine
introduction, meanwhile, the selectivity to methane decreased
sharply, indicating that the hydrogenolysis reaction was effec-
tively inhibited. The decreasing catalyst activity with time on
stream was probably resulted from the coke formation. Mean-
while, under the coverage and isolation effects of coke, the
selectivity to isobutene increased consequently. However,
isobutane conversion was constantly lower than 10 wt% for the
continuous 2 h reaction. This result indicates that, although
chlorine has the ability to bond with hydrogen, the introduction
of chlorine could not improve the dehydrogenation activity as
well. Primarily, it is worth mentioning that one chlorine atom
can only bind to one hydrogen atom and can not capture b-
hydrogen atom at the same time. Moreover, the distance
between two chlorine atoms on the catalyst surface is probably
larger than that between the rst hydrogen and b-hydrogen of
one isobutane molecule. And even if two chlorine atoms capture
the two hydrogen atoms of one isobutane simultaneously, the
formation of hydrogen would still be restricted due to the
untouchable distance. Therefore, the dehydrogenation activity
is limited aer chlorine modication.
2
Scheme 1 Proposed catalytic cycle of sulfided Co/SiO catalyst for
isobutane dehydrogenation.
result, the dehydrogenation activity is improved dramatically
aer sulfur modication. However, as for Sn and Cu, the
binding ability to hydrogen is not strong enough, therefore, the
effect of Sn and Cu introduction only involves breaking metal
ensembles and inhibiting hydrogenolysis reaction.
2
Fig. 7 Catalytic performance of Cl promoted 13Co/SiO catalyst for isobutane dehydrogenation.
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Scheme 2 Schematic of promoting mechanism of different methods.
Based on the above discussions, the promoting mechanisms
of different additives are summarized and depicted in Scheme
Acknowledgements
2. In general, an efficient promoter should at least satisfy the This work was nancially supported by the National Natural
following requirements: the rst is to effectively suppress the Science Foundation of China (no. U1362201) and the National
formation of metal ensembles, which are active for hydro- 973 Program of China (no. 2012CB215006).
genolysis reaction; the second is a strong bonding affinity to
hydrogen atoms; lastly, to shi the reaction equilibrium
towards right side, the promoter should have the ability to
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