Promoting effect of sulfur addition on the catalytic performance of Ni/MgAl2O4 catalysts for isobutane dehydrogenation

Article abstract of DOI:10.1021/cs400705p

Ni/MgAl₂O₄ catalysts with high NiO loadings were highly active for isobutane cracking, which led to abundant formation of methane, hydrogen and coke. The results of activity testing and XRD characterization jointly revealed that large ensembles of metallic nickel species formed during reaction notably catalyzed cracking instead of dehydrogenation. However, after introduction of sulfur into Ni/MgAl ₂O₄ catalyst through impregnation with ammonium sulfate, undesired cracking reactions were effectively inhibited, and the selectivity to isobutene increased remarkably. Totally, up to ～42 wt % isobutene could be obtained at 560 C in a single pass after the modification. From the characterization results, it was also concluded that, after sulfur introduction, NiO particles became much smaller and better dispersed on the catalyst surface. NiS species, formed during the induction period of the reaction, not only facilitated isobutene desorption from the catalyst, but also constituted the active sites for isobutane dehydrogenation. In addition, due to the appearance of NiS species, Ni/MgAl₂O₄ catalyst after H ₂S/H₂ sulfuration exhibited a high initial activity without experiencing an induction period, further confirming the crucial role that introduced sulfur played.

Full text of DOI:10.1021/cs400705p

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Article
Promoting Effect of Sulfur Addition on the Catalytic Performance
of Ni/MgAl2O4 Catalysts for Isobutane Dehydrogenation
Guowei Wang, Zhe Meng, Jianwei Liu, Chunyi Li, and Honghong Shan
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 11 Nov 2013
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Promoting Effect of Sulfur Addition on the Catalytic Performance of Ni/MgAl
Catalysts for Isobutane Dehydrogenation
2 4
O
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Guowei Wang, Zhe Meng, Jianwei Liu, Chunyi Li*, Honghong Shan
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, PR China
*
Corresponding author’s eꢀmail: chyli@upc.edu.cn, chyli_upc@126.com (C. L.).
ABSTRACT
2 4
Ni/MgAl O catalysts with high NiO loadings were highly active for isobutane cracking, which
led to abundant formation of methane, hydrogen and coke. The results of activity testing and XRD
characterization jointly revealed that large ensembles of metallic nickel species formed during
reaction notably catalyzed cracking instead of dehydrogenation. However, after introduction of
sulfur into Ni/MgAl O catalyst through impregnation with ammonium sulfate, undesired cracking
2
4
reactions were effectively inhibited, and the selectivity to isobutene increased remarkably. Totally,
o
up to ~42 wt% isobutene could be obtained at 560 C in a single pass after the modification. From
the characterization results, it was also concluded that, after sulfur introduction, NiO particles
became much smaller and better dispersed on the catalyst surface. NiS species, formed during the
induction period of the reaction, not only facilitated isobutene desorption from the catalyst, but also
constituted the active sites for isobutane dehydrogenation. In addition, due to the appearance of NiS
2 4 2 2
species, Ni/MgAl O catalyst after H S/H sulfuration exhibited a high initial activity without
experiencing an induction period, further confirming the crucial role that introduced sulfur played.
2 4
KEYWORDS: isobutane, dehydrogenation, Ni/MgAl O catalysts, sulfur addition, NiS
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. INTRODUCTION
Driven largely by the increasing demand for isobutene, which has been widely applied in the
production of polyisobutene (PIB), methyl tertiary butyl ether (MTBE) and methyl methacrylate
MMA), the technologies for isobutane dehydrogenation have attracted much attention in recent
years.
Two kinds of catalysts commercially applied for catalytic dehydrogenation of isobutane are
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chromiaꢀbased and platinumꢀbased catalysts . Although relatively satisfactory performance has
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been achieved, both of them possess their own disadvantages. Cr species are harmful to humans
and the environment during both preparation and application processes, while for Ptꢀbased catalysts,
high investment and operating costs are inevitable. Up to now, the development of a novel catalyst
system with relatively inexpensive and environmentally friendly characteristics is still one of the
urgent issues.
Nickel oxide, known to be active in many important reactions for decades, such as waterꢀgas
7
ꢀ9
10ꢀ11
12ꢀ13
shift , and steam reforming of methane
and ethanol
have investigated the reaction mechanism of gas phase transition metal ions with
light alkanes to determine the factors governing CꢀC and CꢀH bond cleavage ratios. Georgiadis et
, may be an attractive alternative. Some
14ꢀ20
researchers
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al. and Jacobson et al. reported that nickel ions were prone to insert into the weakest CꢀC bond
rather than the CꢀH bond; consequently, cracking reactions occurred. Although these homogeneous
catalytic reactions investigated in the gas phase have little practical value, the theories developed
provide profound guidance to deepen the understanding of the nature of dehydrogenation.
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In 1994, Resasco and his coworkers found that nickelꢀbased catalysts exhibited high catalytic
activity but low selectivity towards olefins in isobutane dehydrogenation, and as a result of the rapid
cracking reactions, abundant methane and coke were generated. Since the breakage of CꢀC bond
occurs preferentially under the catalysis of nickel species, greater efforts should be made to
selectively activate alkanes to achieve higher selectivities towards olefins in alkane
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dehydrogenation . To activate the alkanes selectively, two important aspects should not be
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overlooked: (1) to inhibit CꢀC bond rupture to reduce the possibility of cracking reactions, (2) to
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facilitate desorption of the olefin produced
to avoid undesired secondary reactions. To achieve
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the above objectives, the researchers introduced a certain amount of dimethyl sulfoxide into
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Ni/Al O catalyst and found that the selectivity to isobutene was improved effectively, but both the
activity and stability still needed to be improved. Moreover, the liquids containing sulfur are
generally hazardous and malodorous. Therefore, in this study, relatively safe sulfate species are
introduced to modify the nickelꢀbased catalysts instead.
2 4
In this study, Ni/MgAl O catalysts with different NiO loadings were prepared and evaluated
for catalytic dehydrogenation of isobutane. Moreover, sulfur modified Ni/MgAl O catalysts were
2
4
prepared by impregnation of ammonium sulfate and catalytic performance was also evaluated.
Experimental results demonstrated that improved dehydrogenation performance was achieved for the
sulfur modified catalysts. Subsequently, the effects of sulfur addition on surface structure, redox
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properties and adsorptionꢀdesorption behaviors of Ni/MgAl O catalyst were investigated, and the
2 4
active species for dehydrogenation were also speculated. Furthermore, sulfuration of Ni/MgAl O
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catalyst with H S/H was carried out to gain deeper insight into the nature of the active phase.
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. EXPERIMENTAL
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2.1. Catalyst Preparation.
MgAl O was prepared by the solꢀgel method. Pseudoꢀboehmite powder was mixed with
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distilled water in a vessel to obtain a suspended solution, and then HCl solution was added dropwise
under vigorous stirring at 70 °C. After that, an appropriate amount of magnesium nitrate solution
was added. The gel was stirred for 4 h, dried at 140 °C overnight, and calcined at 700 °C in air for 4
h. Finally, the support was crushed and sieved to 80ꢀ180 ꢁm for later use.
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Nickel oxide catalysts supported on MgAl O were prepared by incipient wetness impregnation
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with appropriate amounts of nickel nitrate. Sulfur modified Ni/MgAl O catalysts were obtained by
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sequential impregnation of MgAl O with aqueous ammonium sulfate and nickel nitrate solutions.
o
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After impregnation, the catalysts were dried at 140 C overnight and calcined at 700 C in air for 2 h.
2 2
The asꢀprepared catalysts are denoted as xNi/MgAl for the unmodified catalysts and xNi/MgAl ꢀS
for the catalysts with sulfur addition, respectively, where x represents the content of nickel oxide by
2
ꢀ
weight, and the content of SO
4
was 5 wt% for the sulfur modified catalysts.
2
.2. Catalyst Characterization.
Specific surface areas and pore structure properties of the catalysts were determined by
adsorptionꢀdesorption measurements of nitrogen at liquid nitrogen temperature using a Quadrasorb
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SI instrument. Prior to the measurement, all samples were evacuated at 300 C for 4 h at a pressure
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of 1.0 × 10 kPa to ensure complete removal of adsorbed moisture. Xꢀray diffraction (XRD) of the
catalysts was carried out on an X’Pert PRO MPD diffractometer system using Cu Kα radiation at 40
kV and 40 mA, running from 5° to 75° with a speed of 10 °/min. And SEM images were obtained
using HITACHI Sꢀ4800 scanning electronic microscope.
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The redox behavior of catalyst samples was determined by temperatureꢀprogrammed reduction
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of hydrogen (H ꢀTPR). During the test, about 0.1 g of sample was loaded into the apparatus,
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preheated in a flow of helium at 200 C for 0.5 h, and then cooled down to 80 C. Subsequently, the
sample was brought into contact with a mixture of 10 vol. % H /N (30 mL/min) and heated at a rate
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of 10 °C/min to the final temperature. The effluent gas passed through a thermal conductivity
detector and was analyzed with a GASꢀ100Q mass spectrometer.
To investigate the adsorption and desorption behaviors of the catalysts for alkanes and alkenes,
isobutane and isobutene adsorptionꢀdesorption tests were performed with a FINESORBꢀ3010
chemisorption analyzer, and the gas released from the reactor was analyzed by an online GASꢀ100Q
quadrupole mass spectrometer. A schematic of the apparatus is shown in Figure S1 in the Supporting
Information Section. For each test, about 0.2 g of catalyst was loaded into a quartz tube reactor,
o
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preheated in a flow of nitrogen (60 mL/min) at 450 C for 0.5 h, and then cooled to 50 C.
Subsequently, the nitrogen flow was instantaneously switched to a mixture of Ar (30 mL/min) and
isobutane or isobutene (30 mL/min). Compared with the insert gas, detection of hydrocarbon was
delayed to some extent due to adsorption on the catalyst surface, and a large difference between the
signals of inert gas and hydrocarbon implied a stronger adsorption ability of the catalysts for
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isobutane or isobutene. For the tests of hydrocarbon desorption, the flow was switched back to
nitrogen after adsorption saturation.
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.3. Catalytic Activity Test.
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atmospheric pressure with 4 g catalysts (80ꢀ180 ꢀm) loaded into the reactor. Prior to reaction,
catalysts were degassed at reaction temperature in a nitrogen flow to remove adsorbed oxygen and
water from the catalyst surface. During the test, isobutane was fed to the reactor at a fixed flow rate
of 2 mL/min with a nitrogen flow of 12 mL/min. The composition of effluent gaseous products was
analyzed by a Bruker 450 Gas Chromatograph equipped with a FID detector to determine the
composition of hydrocarbons and two TCD detectors to analyze the content of hydrogen, carbon
monoxide and carbon dioxide. Through analyzing the quantity of CO generated during in situ coke
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combustion with the chromatogram, the amount of coke deposited on the catalysts after reaction was
determined.
Isobutane conversion and product selectivity are calculated as follows:
Isobutane, in − Isobutane, out
Isobutane conversion =
× 100 wt%
× 100 wt%
(1)
(2)
Isobutane, in
i, out
Selectivity to product i =
Isobutane, in − Isobutane, out
3
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. RESULTS AND DISCUSSION
2 4
.1. Dehydrogenation Performance of Ni/MgAl O Catalysts.
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Isobutane dehydrogenation over Ni/MgAl O catalysts with different NiO loadings was
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evaluated in the fixed bed microꢀreactor at 560 C, and the results are displayed in Figure 1. It is
apparent that the dehydrogenation performance is strongly affected by NiO loading. As for
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1Ni/MgAl catalyst, a very low and steady activity lasting for 2 h was observed, while the selectivity
to isobutene was maintained at a high level. With increasing NiO loading, the catalytic performance
changed significantly. As the reaction proceeded, the isobutane conversion firstly increased and then
2 2
decreased dramatically for both 5Ni/MgAl and 13Ni/MgAl catalysts; meanwhile, the selectivity to
isobutene exhibited the opposite trend with time on stream (TOS). It is also worth noting that, with
decreasing selectivity towards isobutene, abundant methane was generated, indicating the
transformation from dehydrogenation to undesirable cracking reactions. In general, due to the low
isobutane conversion at low NiO loadings and inferior selectivity to isobutene at high loadings,
isobutene yield was unsatisfactory for the investigated catalysts.
Possibly resulting from structural changes in surface nickel species during reaction, the desired
dehydrogenation reaction was transformed to cracking reactions, most probably hydrogenolysis
reactions with the presence of hydrogen produced in the dehydrogenation reaction, and consequently,
abundant methane was generated at reaction time 0.5 h. After 2 h reaction, the coke contents of
2 4
Ni/MgAl O catalysts with NiO loadings of 1, 5 and 13 wt% were 0.09, 1.93 and 3.55 wt%,
respectively. Therefore, the decreasing isobutane conversion with TOS could mainly be attributed to
the coverage of active sites by coke deposition. And consistent with the variation trend of coke yield,
the deactivation rate also increased with NiO loading.
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Figure 1. Dehydrogenation performance of Ni/MgAl O catalysts with different NiO loadings.
To further investigate the possible transformation of nickel phase during the reaction, XRD
characterization of both the fresh 13Ni/MgAl catalyst and the catalyst reacted for 0.5 h, the latter of
2
which exhibited highest cracking activity and yielded most methane, was carried out. In the XRD
2 4 2 4
patterns displayed in Figure 2, NiO, MgAl O and NiAl O (the latter two presented
indistinguishable characteristic peaks) were all detected on the fresh catalyst. With regard to
1
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3Ni/MgAl catalyst reacted for 0.5 h, the intensity of diffraction peaks for NiO decreased, and
additional peaks characteristic of metallic nickel were observed. It has been reported that metallic
26
nickel is extremely active for CꢀC bond scission ; moreover, it is also believed that the
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hydrogenolysis of alkanes occurs rapidly under the catalysis of Niꢀbased catalysts , which leads to
abundant formation of methane, coke and hydrogen. Therefore, it could be concluded that the
metallic nickel species generated during the reaction facilitated the undesired cracking reactions, and
at the same time, the dehydrogenation reaction was weakened. One possible explanation for the low
cracking activity of the catalysts with low NiO loading is that the hydrogenolysis reaction is
structure sensitive and a large ensemble of nickel atoms are required to constitute the active sites and
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catalyze the reaction . Furthermore, to detect the nickel ensembles directly, SEM images of both
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Ni/MgAl and 13Ni/MgAl catalysts after 0.5 h of reaction have been taken, as shown in Figure S2
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in the Supporting Information. As for 1Ni/MgAl catalyst, the nickel species on the support were
2
highly dispersed and hardly aggregated together. In contrast, the surface of 13Ni/MgAl catalyst was
covered by plentiful nickel species, and nickel ensembles with average diameters of about 1.5 ꢀm
were also observed, which were likely responsible for hydrogenolysis. Moreover, the reduction
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process becomes much more difficult for smaller NiO particles , which also restricted the undesired
reactions.
In general, Ni/MgAl O catalysts with low NiO loadings exhibited poor activity, while
2
4
undesired cracking reactions, most probably hydrogenolysis reactions catalyzed by the aggregated
metallic nickel formed during the reaction, occurred at high loadings. Consequently, further work is
still required to improve the catalytic performance.
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•
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MgAl O4
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NiAl O4
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3Ni/MgAl reacted for 0.5 h
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Ni S6
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fresh 13Ni/MgAl -S
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3Ni/MgAl -S reacted for 5 h
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∇
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3Ni/MgAl sulfurated by H S/H
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13Ni/MgAl sulfurated by H S/H reacted for 3 h
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o
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Theta ( )
Figure 2. XRD patterns of different 13Ni/MgAl
2
catalysts.
2 4
3.2. Effect of Sulfur Introduction into Ni/MgAl O Catalysts.
3
.2.1. Dehydrogenation Performance and Reaction Kinetics.
To improve the dehydrogenation performance of the catalysts, many attempts have been carried
out. Among them, sulfur modification by impregnating the catalyst with ammonium sulfate
improved the catalyst performance impressively, which was in accordance with the excellent
28ꢀ30
activation ability that the sulfide catalysts behaved in the hydrogenation reaction
, i.e., reverse
2
reaction to dehydrogenation. Activity test results of the 13Ni/MgAl ꢀS catalyst are displayed in
2
Figure 3, and detailed comparison with the properties and performance of 13Ni/MgAl catalyst
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without sulfur addition is shown in Table 1.
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0
Isobutane conversion
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60
Selectivity to isobutene
Selectivity to methane
Yield of isobutene
6
40
20
0
0
2
4
6
8
10
12
Time on stream (h)
Figure 3. Dehydrogenation performance of 13Ni/MgAl ꢀS catalyst.
2
Compared with the catalyst with no sulfur addition, the initial isobutane conversion over
2
13Ni/MgAl ꢀS catalyst exhibited little difference, while the initial selectivity to isobutene was much
lower due to the formation of a large amount of CO (not shown). Moreover, both the isobutane
x
conversion and selectivity towards isobutene increased gradually with TOS in the first 5 h of the
reaction, suggesting the existence of an induction period for 13Ni/MgAl
2
ꢀS catalyst. Similar
3
1
32ꢀ34
induction periods have also been reported for Moꢀbased catalysts and Inꢀbased catalysts
, which
were considered to be the result of the variation of valence state or structure of surface species.
Moreover, the selectivity to methane was constantly kept at a very low level during the continuous
1
2 h reaction, which indicated that the cracking reactions were effectively inhibited by the sulfur
modification. At the steady stage of the reaction for sulfur modified catalyst, lasting from 6 h to 10 h,
isobutene selectivity up to 90 wt% and isobutene yield higher than 40 wt% were both obtained. And
after that, slightly decreasing isobutane conversion due to catalyst deactivation was observed.
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Table 1. Properties and Reaction Results of 13Ni/MgAl Catalysts Before and After Sulfur
2
Addition
e
f
e
g
item
13Ni/MgAl
2
13Ni/MgAl
2
13Ni/MgAl
2
ꢀS 13Ni/MgAl
2
ꢀS
a
2
SBET , m /g
80
84
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
b
D
p
, nm
7.9
6.5
c
ꢀ1 ꢀ1
R , mmol h g
ꢀ
54.6
0
0.6
2.6
0.2
0.2
0.3
1.5
0.1
1.4
methane
ethane
56.2
0
9.3
4.0
3.9
3.0
10.5
2.5
3.2
52.6
5.4
0
ethene
0
0
propane
propene
nꢀButane
0
0
0
0
0
0.1
1.8
8.4
3.3
0.6
2.6
26.8
0.2
48.9
d
selectivity , wt% nꢀButene
0
iꢀButene
0
86.5
3.9
0
H2
3.1
0.2
2.2
39.9
0
CO
2
CO
2.7
0
1.7
1.4
2.5
90.7
coke
other product
0.6
48.4
d
conversion , wt%
91.0
a
Specific surface area.
Pore size.
b
c
Formation rate of isobutene per mass of catalyst.
d
2
Data were collected after 0.5 h reaction for 13Ni/MgAl catalyst and after 5 h reaction for
1
3Ni/MgAl ꢀS catalyst, when maximum isobutane conversion was achieved. And product selectivity
2
data have been normalized.
e
2 2
Catalytic performances of 13Ni/MgAl and 13Ni/MgAl ꢀS catalysts obtained at the same reaction
conditions as described in the Experimental Section.
f
Catalytic performance of 13Ni/MgAl
2
catalyst obtained at similar conversion level with
1
3Ni/MgAl ꢀS catalyst under different reaction conditions.
2
g
Catalytic performance of 13Ni/MgAl ꢀS catalyst obtained at similar conversion level with
2
1
3Ni/MgAl
2
catalyst under different reaction conditions.
For 13Ni/MgAl
2
catalysts before and after sulfur introduction, the reaction conditions were
adjusted to obtain the product distributions at similar conversion levels. As listed in Table 1, at
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similar conversion levels, the isobutene selectivity increased significantly after sulfur introduction to
the catalyst; simultaneously, the selectivity to methane decreased remarkably. It should be mentioned
here that the reaction temperature was elevated significantly to obtain the high isobutane conversion
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2
over 13Ni/MgAl ꢀS catalyst; therefore, the undesired thermal cracking reactions inevitably occurred,
leading to decreased selectivity to isobutene.
2
To investigate possible phase transformation of the catalysts with TOS, 13Ni/MgAl ꢀS catalysts
before and after reaction were both characterized. XRD patterns (see Figure 2) of the fresh
2ꢀ
1
3Ni/MgAl ꢀS catalyst were similar with the catalyst without sulfur addition due to the low SO4
2
2
content of 5 wt%. As for the 13Ni/MgAl ꢀS catalyst after 5 h reaction, no diffraction peak
corresponding to metallic nickel was detected, suggesting that the nickel oxide on the catalyst
surface was possibly not yet reduced during the continuous 5 h reaction, or the metallic nickel
species formed were likely to be well dispersed on the catalyst surface. Consequently, the rapid
cracking reactions activated by large ensembles of metallic nickel were inhibited remarkably.
Moreover, after the introduction of sulfur, the coke content of the used catalyst decreased
dramatically, from 3.55 wt% to less than 0.01 wt%, which was consistent with the reported results of
21
Resasco et al. that the pretreatment of nickelꢀbased catalysts with dimethyl sulfoxide significantly
inhibited the coke formation in isobutane dehydrogenation. As such, a low coke formation rate over
the catalyst with sulfur addition guaranteed a relatively stable catalytic activity after the induction
period.
2
To further study the reaction kinetics, isobutane dehydrogenation over 13Ni/MgAl ꢀS catalyst
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o
o
was carried out in a fixed bed reactor at different temperatures ranging from 540 C to 600 C with
various isobutane flow rates under atmospheric pressure, and all experimental data were collected at
3
5ꢀ36
0
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0
the steady state rather than the induction period of the reaction. According to the open literature
,
a power law model neglecting adsorption of isobutane and isobutene was adopted for the
dehydrogenation kinetics as follows, where r represents reaction rate, k and K stand for reaction
1
eq
rate constant of isobutane dehydrogenation and equilibrium constant, respectively.

P
P 
C H
H
4
8
2
-
r
= k  P
-
C H
4 10


(3)
C H
1
4
10

Keq


Accordingly, the value of the frequency factor for isobutane dehydrogenation in this work was
6
estimated to be 9.2 × 10 , and an activation energy of 84 kJ/mol was also obtained, which was much
3
7
2 3 2 3
lower than that of 133ꢀ142 kJ/mol for isobutane dehydrogenation over Cr O /Al O catalyst and 95
38
kJ/mol over Pt/Sn/SiO catalyst .
2
3
.2.2 Structure Characteristics of the Catalysts.
To investigate the interaction of introduced sulfur with the support and nickel species after
2
ꢀ
calcination, 13Ni/MgAl ꢀS catalyst with SO4 content of 15 wt% was prepared and characterized by
2
XRD, with the patterns illustrated in Figure S3 in the Supporting Information. In addition to peaks
2 4 2 4 4
characteristic of MgAl O , NiAl O and NiO, additional peaks characteristic of MgSO were also
detected, indicating that the sulfate introduced was interacting preferentially with MgO in the
support.
SEM images shown in Figure 4 illustrate the differences in the morphology of surface species
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on 13Ni/MgAl catalysts before and after sulfur introduction. For the catalyst without sulfur addition
2
(
see Figure 4aꢀb), the catalyst surface was densely covered by a large amount of uniformly
distributed cubic crystals (100ꢀ200 nm in length). Given the morphology of 1Ni/MgAl catalyst (see
Figure S2a in the Supporting Information) in which the support MgAl was basically exposed,
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2
2
O
4
these semiꢀcubic particles were speculated to be NiO. Meanwhile, a certain amount of aggregated
NiO particles were also observed. In contrast, after the introduction of sulfur, relatively long cubic
NiO crystals were transformed to small crystals (20ꢀ80 nm in length) (see Figure 4cꢀd).
2 2
Figure 4. SEM images of 13Ni/MgAl catalysts before and after sulfur addition: aꢀb. 13Ni/MgAl ;
cꢀd. 13Ni/MgAl ꢀS.
2
This observation is consistent with the increase in specific surface area and the decrease in pore
size (as listed in Table 1). The large ensembles of VIII metal were reported to be active for cracking
39ꢀ40
41
reactions
. It has been reported that , with the combined effect of rhenium and sulfur, the surface
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platinum on the catalyst was divided into tiny particles, and then a highly selective catalyst with mild
dehydrogenation activity and good stability was obtained. In this work, the introduction of sulfur
also seemed to prevent the nickel species from aggregating, and thus inhibiting the formation of
agglomerated metallic nickel species and undesired cracking reactions, consistent with the results of
XRD characterization and activity tests.
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3
.2.3. Redox Behaviors of the Catalysts.
As evidenced by the experimental results, metallic nickel and nickel oxide species on the
catalyst surface exhibited different catalytic behaviors; therefore, the dehydrogenation performance
2
of the catalysts is directly related to their reduction properties. Accordingly, H ꢀTPR was carried out
2 4
to investigate the effect of sulfur addition on the redox behaviors of Ni/MgAl O catalysts, and the
H ꢀTPR profiles are shown in Figure 5.
2
2
As for the fresh 13Ni/MgAl catalyst (as displayed in Figure 5a), the appearance of a weak
o
o
reduction peak extending from 350 C to 500 C was speculated to be caused by the reduction of
nickel oxide species weakly interacting with the support to metallic nickel species. In addition, a
o
major peak around 760 C was also observed, which was reported to result from the reduction of
42ꢀ44
other nickel species closely interacting with the support - for instance, in the form of NiAl
2
O
4
.
2 2
For the H ꢀTPR profile of 13Ni/MgAl ꢀS catalyst, three reduction peaks appeared (as displayed
2
in Figure 5b). Compared with 13Ni/MgAl catalyst, the intensity of the low temperature peak
increased to some extent, indicating that, apart from the reduction of dispersed NiO species, partial
reduction of introduced sulfate species might also occur. Moreover, a newly emerged large reduction
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peak around 600 C was observed. To determine the origin of this peak, an online MS spectrometer
has been applied to detect the released gases during the TPR process. As illustrated in Figure 5c, a
o
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peak around 600 C in the MS spectra corresponding to the formation of H
S was detected,
2
indicating that the newly emerged peak was attributed to the reduction of sulfate species introduced.
And it was also worth noting that the sulfate species could even be reduced at a reaction temperature
o
of 560 C, which made possible the formation of metal sulfides. Moreover, as for the high
temperature reduction peak in Figure 5a and Figure 5b, no obvious variation was observed.
a. H -TPR profile of 13Ni/MgAl
2
2
α. Weakly interacted surface NiO
β. Surface sulfate
γ
γ. NiAl O spinel
2
4
α
b. H -TPR profile of 13Ni/MgAl -S
β
2
2
γ
α
c. Real time MS profile of 13Ni/MgAl -S
2
m/z = 34
300
400
500
600
700
800
o
Temperature ( C)
2 2
Figure 5. H ꢀTPR and real time MS profiles of different 13Ni/MgAl catalysts.
3
.2.4. Adsorption and Desorption Behaviors for Isobutane and Isobutene.
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Adsorption and desorption are important steps in heterogeneous catalytic reactions, and
generate significant impacts on the catalytic behaviors of the catalysts. Strong adsorption of the
reactants increases the possibility of the reaction and thus facilitates the conversion of the reactants.
However, if the reactant molecules are strongly adsorbed but hardly react, the active sites will be
poisoned. Moreover, fast desorption of the products reduces the chances of undesired secondary
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5ꢀ47
reactions and further increases the selectivity towards target products
.
To investigate the adsorption and desorption behaviors of the catalysts for isobutane and
isobutene and their impacts on dehydrogenation performance, the transient response experiment as
depicted in the Experimental Section was carried out on the apparatus displayed in Figure S1 in the
2
Supporting Information. For both 13Ni/MgAl catalyst reacted for 0.5 h (when maximum cracking
2
activity was achieved) and 13Ni/MgAl ꢀS catalyst reacted for 5 h (when maximum dehydrogenation
activity was obtained), the transient response signals of Ar and isobutene during the adsorption
processes at different temperatures are shown in Figure 6. It is apparent that the signal delay extent
2 2
between isobutene and Ar of 13Ni/MgAl catalyst was much larger than that of 13Ni/MgAl ꢀS
catalyst. As described previously, a larger signal delay extent between hydrocarbon and inert gas in
the adsorption process implies a stronger adsorption ability of the catalysts, while, that in the
desorption process indicates more difficult desorption of the hydrocarbon instead. Therefore,
compared with 13Ni/MgAl ꢀS catalyst, the adsorption ability of 13Ni/MgAl catalyst for isobutene is
2
2
much stronger.
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a. 13Ni/MgAl2
1.0
0.8
0.6
0.4
0.2
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Dotted line
Solid line
Isobutene
Ar
0.0
0
40
80
120
160
200
TOS (S)
b. 13Ni/MgAl -S
2
1
.0
.8
0
o
0.6
50 C
o
1
1
2
2
00 C
o
50 C
0.4
0.2
0.0
o
00 C
o
50 C
o
300 C
o
350 C
o
4
00 C
o
450 C
0
40
80
120
160
200
TOS (S)
Figure 6. Transient response signals of Ar and isobutene over 13Ni/MgAl
2
catalyst reacted for 0.5 h
and 13Ni/MgAl ꢀS catalyst reacted for 5 h during the adsorption procedure.
2
To clearly compare the distinction between the transient response signals, the area between the
curves of Ar and isobutene was computed through integration of the difference, and the calculated
adsorption areas are plotted as a function of temperature in Figure 7a. Similarly, the results of the
desorption data for isobutene are also obtained and displayed in Figure 7b. Difficult desorption of
alkenes from the nickel sites probably facilitates undesired secondary reactions and coke formation,
and consequently, the active sites are covered by deposited coke and further deactivate. In this work,
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2
indicating much harder adsorption and easier desorption of the generated isobutene, and
consequently, undesired secondary reactions were effectively inhibited and isobutene selectivity
increased remarkably.
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a. Isobutene adsorption
b. Isobutene desorption
13Ni/MgAl2
1
3Ni/MgAl -S
2
0
0
9
0
100
200
300
o
400
500
0
100
200
300
o
400
500
Temperature ( C)
Temperature ( C)
1
5
0
5
0
c. Isobutane adsorption
d. Isobutane desorption
1
6
3
0
0
100
200
300
400
500
0
100
200
300
o
400
500
o
Temperature ( C)
Temperature ( C)
2
Figure 7. Integral area between Ar and hydrocarbon over 13Ni/MgAl catalyst reacted for 0.5 h and
1
3Ni/MgAl ꢀS catalyst reacted for 5 h during both adsorption and desorption procedures.
2
Furthermore, both larger integrated adsorption (Figure 7c) and desorption (Figure 7d) areas of
2
isobutane were detected for the Ni/MgAl catalyst, especially at higher temperatures. This result
suggests that the catalyst before sulfur introduction possessed stronger adsorption ability of
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isobutane, but lower desorption ability, thus facilitating the conversion of isobutane.
2 4
3.3. Active Phase of Ni/MgAl O Catalysts with Sulfur Addition.
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Based on the above discussions, the introduction of sulfur prevented the nickel oxide species
from being aggregated, facilitated the desorption of isobutene generated, and thus reduced the
possibility of occurrence of cracking reactions and undesired secondary reactions. However, the
2
initial dehydrogenation activity of 13Ni/MgAl ꢀS catalyst was relatively low, and an induction
period was required to achieve the maximum activity. Observation of such an induction period
suggested that the improved performance was probably related to the phase transformation of nickel
species during the induction period, and the formation of a new active phase might be the real
reason.
To investigate the variation of valence states of sulfur species during the induction period, XPS
2
measurements were performed over both the fresh 13Ni/MgAl ꢀS catalyst and the catalyst reacted
for 5 h, when maximum catalytic activity was obtained. The S 2p XPS spectra are illustrated in
Figure 8. For the fresh 13Ni/MgAl ꢀS catalyst, only one oxidation state of sulfur with binding energy
2
4
8ꢀ49
of S 2p between 168.5 and 169.5 eV, characteristic of sulfate species
, was detected. After the
induction period, additional features with binding energy of S 2p around 161.7 eV, corresponding to
2
ꢀ
50ꢀ51
the formation of S species
, appeared. Therefore, it could be speculated that the nickel sulfide
species formed during the reaction were probably the active phase for dehydrogenation, which
contributed to the improved performance.
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S 2p
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2-
S
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-
SO4
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+
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+
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175
170
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B.E.(eV)
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175
170
165
B.E.(eV)
160
Figure 8. XPS spectra of 13Ni/MgAl
2 2
ꢀS catalysts before and after reaction: a. fresh 13Ni/MgAl ꢀS
catalyst; b. 13Ni/MgAl ꢀS catalyst after 5 h reaction.
2
In order to verify the above speculation that NiS species constitute the active phase of
2
ammonium sulfate modified catalysts for isobutane dehydrogenation, 13Ni/MgAl catalyst was
2 2
sulfurated by 10 vol.% H S/H and further evaluated, which was similar with the sulfuration process
5
2ꢀ53
in the hydrogenation reaction
. Figure S4 in the Supporting Information illustrates the
dehydrogenation performance of 13Ni/MgAl
2 2 2
catalyst sulfurated by H S/H at a reaction
o
temperature of 560 C for 1 h, 2 h, 3 h and 5 h, respectively. For all the cases, the induction period
disappeared, and the relatively high initial isobutane conversion indicates that the active phase NiS
was probably formed during the sulfuration process. The initial isobutane conversion increased
gradually with sulfuration time until 3 h, and after that the conversion decreased to some extent. Due
to slight changes in isobutene selectivity arising from the different sulfuration processes used,
isobutene yield exhibited a similar variation trend with that of isobutane conversion. To study the
o
o
o
o
effect of sulfuration temperature (350 C, 450 C, 560 C and 600 C) on catalyst dehydrogenation
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performance, the sulfuration time was fixed at 3 h. As illustrated in Figure S5 in the Supporting
o
Information, for the catalyst sulfurated at 350 C, an obvious induction period could still be observed.
o
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With the increase of sulfuration temperature above 450 C, the induction period disappeared, and the
initial isobutane conversion first increased, and then decreased significantly. In consideration of
o
isobutene yield, the optimum sulfuration temperature was about 560 C, at which the highest initial
isobutane conversion and isobutene yield were both obtained. In conclusion, the catalyst sulfurated
o
at 560 C for 3 h exhibited the best dehydrogenation performance (as shown in Figure 9), for which
the initial isobutane conversion up to ~64 wt% and relatively high selectivity to isobutene were
obtained.
100
80
60
40
20
0
Isobutane conversion
Selectivity to isobutene
Yield of isobutene
0
3
6
9
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30
TOS (h)
2
Figure 9. Catalytic performance of 13Ni/MgAl catalyst (without sulfur addition) sulfurated by
H
2
S/H for 3 h.
2
2
The sulfurated 13Ni/MgAl catalyst under the optimum sulfuration conditions was also
characterized by XRD and the patterns are illustrated in Figure 2. In addition to diffraction peaks of
o
o
o
2
fresh 13Ni/MgAl catalyst, additional characteristic peaks of NiS at 30.1 , 34.7 and 53.5 were also
2
detected, in accordance with the results of XPS characterization obtained for 13Ni/MgAl ꢀS catalyst.
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Therefore, the conclusion can be drawn that, for the Ni/MgAl O catalyst with sulfur addition, NiS
2
4
species, formed during both the reaction and sulfuration processes, are the real active phase for
isobutane dehydrogenation.
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Furthermore, possibly due to the consumption of the active phase, the catalyst activity
decreased gradually with TOS. For the sulfurated catalyst reacted for 3 h, characterization peaks of
7 6
Ni S were observed in the XRD patterns (see Figure 2), suggesting that sulfur loss was the real
reason for catalyst deactivation during the reaction. An online MS was applied to detect the gas
released during isobutane dehydrogenation over the sulfurated catalyst. In the MS spectra (as
2
illustrated in Figure S6 in the Supporting Information), H S (m/z = 34) was detected and its intensity
decreased gradually with time on stream, suggesting that the sulfur was lost mainly in the form of
H
2
S.
2 2
In order to recover the activity, the catalyst was sulfurated by H S/H for another 3 h after every
3
h reaction, and totally five such sulfurationꢀreaction cycles were carried out. As displayed in
Figure 9, for each cycle, the catalyst activity could be completely restored by the sulfuration
treatment, further confirming that the existence of sulfur is essential for achieving excellent
dehydrogenation performance.
4
. CONCLUSIONS
2 4
Ni/MgAl O catalysts with different NiO loading amounts exhibited different performances in
isobutane dehydrogenation. Catalysts with low NiO content possessed low catalytic activity.
Although the activity was improved at higher NiO loadings, due to the formation of aggregated
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metallic nickel species during the initial reduction period, which was verified by XRD
characterization, undesirable cracking reactions were intensified and isobutene selectivity was
worsened.
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To improve the catalytic performance, sulfur was introduced into the Ni/MgAl O catalysts.
Characterization results obtained in SEM and adsorptionꢀdesorption tests for isobutane and
isobutene indicated that NiO particles were well dispersed and isobutene desorption was facilitated
after sulfur introduction. Consequently, cracking reactions were effectively inhibited, and the
selectivity to isobutene increased remarkably. Furthermore, an XPS study together with an activity
2 4
test jointly revealed that, for Ni/MgAl O catalyst with sulfur addition, NiS was the real active phase
for isobutane dehydrogenation.
2 2 2 4
Through sulfuration with H S/H , a high initial activity was achieved for Ni/MgAl O catalyst.
Subsequently, with the consumption of sulfur species, the isobutane conversion decreased gradually
2 2
with TOS. After sulfuration treatment with H S/H again, the catalyst activity could be completely
recovered, fully demonstrating the crucial role that the introduced sulfur played.
ASSOCIATED CONTENT
Supporting Information
Further details are given in Figures S1 to S6. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
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Corresponding Author
Eꢀmail: chyli@upc.edu.cn, chyli_upc@126.com (C. L.).
*
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was financially supported by the National Natural Science Foundation of China (No.
U1362201), the National 973 Program of China (No. 2012CB215006) and the Fundamental
Research Funds for the Central Universities (No. 12CX06040A).
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