Catalytic performance of a novel Cr/ZnAlLaO catalyst for oxidative dehydrogenation of isobutane

Article abstract of DOI:10.1039/c4cy00990h

The oxidative dehydrogenation of isobutane using a chrome-based catalyst supported on a novel ZnAlLaO mixed oxide was investigated. The characterization and performance of Cr/ZnAlLaO catalysts were compared with those of several traditional catalysts. The Cr/ZnAlLaO catalyst showed the best selectivity and conversion rate with the highest stability. The performance of the Cr/ZnAlLaO catalyst strongly depends on the Cr content and support properties. The activity peak of isobutane oxidative dehydrogenation occurs at 18 wt% of Cr₂O₃ loading. The selectivity and conversion of isobutene reach their maximum at 84.07% and 52.44%, respectively. Correlation was observed between coke formation and catalyst deactivation. Moderate alkalinity on the ZnAlLaO support surface is helpful to suppress coke formation as well as to improve the catalytic activity and stability. Appropriate water vapour addition to the feed can improve catalytic stability, while both the selectivity of isobutene and the conversion of isobutane decreased.

Full text of DOI:10.1039/c4cy00990h

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Catalytic performance of a novel Cr/ZnAlLaO
catalyst for oxidative dehydrogenation of
isobutane
Cite this: DOI: 10.1039/c4cy00990h
a
b
c
Li He,* Li Fu and Yingzhan Tang
The oxidative dehydrogenation of isobutane using a chrome-based catalyst supported on a novel ZnAlLaO
mixed oxide was investigated. The characterization and performance of Cr/ZnAlLaO catalysts were
compared with those of several traditional catalysts. The Cr/ZnAlLaO catalyst showed the best selectivity
and conversion rate with the highest stability. The performance of the Cr/ZnAlLaO catalyst strongly depends
on the Cr content and support properties. The activity peak of isobutane oxidative dehydrogenation occurs
at 18 wt% of Cr O loading. The selectivity and conversion of isobutene reach their maximum at 84.07%
Received 1st August 2014,
Accepted 23rd September 2014
2
3
and 52.44%, respectively. Correlation was observed between coke formation and catalyst deactivation.
Moderate alkalinity on the ZnAlLaO support surface is helpful to suppress coke formation as well as to
improve the catalytic activity and stability. Appropriate water vapour addition to the feed can improve
catalytic stability, while both the selectivity of isobutene and the conversion of isobutane decreased.
DOI: 10.1039/c4cy00990h
www.rsc.org/catalysis
4
TiO
2
(8.2%) > Cr
2
O
3
/MgO (7.1%). Sloczynski et al. studied
Introduction
the chrome activities on different supports with the following
In recent years, the oxidative dehydrogenation (ODH) of
isobutane has received considerable research attention as a
route to obtain isobutene (an additive of petrol used for
sequence: Al O ≈ ZrO > MgO > TiO > SiO . Recently, stud-
2
3
2
2
2
ies demonstrated that the chromium oxide-based catalysts
used for the ODH of isobutane on different supports could be
1
5,6
replacing lead-containing compounds). Due to the develop-
activated at relatively low temperatures, 200–300 °C.
ment of the petroleum and chemical industries, many green
technologies were developed for using alkanes. Isobutene is
an intermediate mainly from catalytic cracking of petroleum
by-products. Chromium supported on alumina was widely
employed in this reaction as a catalyst. However, it also
exhibits several undesired features such as catalysing side
reactions like coking and cracking, finally leading to catalyst
According to the different systems, the maximum selectivity
of isobutene was between 50% and 80% and the conversion
of isobutene was about 10–20%. The Al O catalysts exhibit
the best performance among these catalysts. However, the
action stability and the activity of the catalysts are still not
satisfactory. Thus, it is necessary to further improve the cata-
2
3
2 3
lytic performance of Al O catalysts in the ODH of isobutane.
2
deactivation. Most of these disadvantages could be avoided
Recently, numerous studies have been carried out to mea-
sure the modification of the stability and activity of Al O cat-
by improving the properties of the catalyst. Hence, the most
important challenge is to develop a more effective catalytic
system which could produce a high yield of the required iso-
butene products. Supported chromium oxide-based catalysts
have been studied for many decades as an active component
in many reactions, one of which is the ODH of isobutane to
2
3
alysts by adding some alkali metals as active components,
such as Pt, Cr, Sn and Ni. Systems with relatively low Cr sur-
face concentration and chromium oxide-based catalysts with
higher concentrations of Cr applied in industrial plants, gen-
erally supported on alumina with alkali oxides as promoters,
3
7
isobutene. Jibril et al. studied the chromium oxide-based cat-
were compared. After addition of the Cr active component
alytic activity on different support surfaces. The result showed
that the order of isobutane conversion rate under 250 °C is as
follows: Cr O /Al O (11.0%) > Cr O /SiO (8.4%) ≈ Cr O /
2 X 2 3
and the additive K O, K–CrO /Al O , the catalyst exhibited
8
the best performance. McVicker et al. reported the relation-
ship between the acidity and the basicity on the support
surface as well as the relationship between catalytic reaction
performance and resistance to carbon deposition. Moreover,
a further attraction of the ODH of isobutane is the modifica-
2
3
2
3
2
3
2
2
3
a
Beijing Petrochemical Engineering Corp. LTD, Chaoyang District Tianjuyuan
Building No. 7, Beijing, China. E-mail: heli_0619@163.com
b
Department of Chemistry and Biotechnology, Faculty of Science, Engineering and
Technology, Swinburne University of Technology, Hawthorn VIC 3122, Australia
9
tion of supports with elements such as Zn and La.
c
In the present study, various chrome-based catalyst sup-
ports and a Cr/ZnAlLaO catalyst with different Cr contents
Shenzhen institute, Hong Kong City of University of Research Centre, Nanshan
District Yuexing 2 Way No. 8, China
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were prepared for the dehydrogenation of isobutane. The
objective was to study the catalytic and chemical–physical
properties in the ODH of isobutane. Cr/ZnAlLaO catalysts
with different amounts of Cr loading were investigated by
calculated by keeping the same ratio of i-C H /O and SV.
4
10
2
The activity measurements were carried out at 560–600 °C
under atmospheric pressure. The reaction temperature was
measured with three thermocouples, one at the top, one at
the middle, and another at the bottom, placed near the cata-
lyst bed, and was maintained constant at 580 °C for 10 h
under normal conditions. Fig. 1 displays the flow diagram of
the ODH.
Reactants and products were analysed online using gas
chromatography, and products were analysed online using a
gas chromatograph–mass spectrometer (SP2100) equipped
with three sample loops and a 13-port sampling valve. One
sample loop was injected into an alumina PLOT column, and
then eluting products were detected with a flame ionization
detector (FID). The second sample loop was injected into a
second alumina PLOT column (Φ = 1/8 in × 3 m) and
analysed by mass spectrometry. The contents of the third
sample loop were injected into a Hayesep N column (Φ =
several techniques, including XRD, CO -TPD and XPS. The
2
reaction conditions such as reaction temperature, alkoxy oxy-
gen ratio and the influence of water vapour were optimized
in detail. The catalytic performance was carried out at a rela-
tively lower temperature than that applied industrially. All
the results obtained are compared with literature data in an
effort to provide important information and form a better
understanding of the Cr/ZnAlLaO catalyst on this catalytic
system of ODH.
Experimental
Catalyst preparation
Catalysts were prepared according to the procedure described
1
0
as follows. Pseudo-boehmite (AlOOH) powder was originally
calcined at 600 °C for 3 h in order to obtain γ-Al . Appro-
priate amounts of γ-Al O with ZnIJNO ) ·6H O (Al/Zn =
1
/8 in × 2 m) used to adsorb CO , H O and heavy hydrocar-
2 2
2
O
3
bons connected in series with a Molecular Sieve 5A packed
column. The products eluting from this column were detected
with a thermal conductivity detector (TCD). The analytical
conditions were as follows: carrier gas, nitrogen; column
temperature, 50 °C; temperature of the TCD detector, 100 °C;
sample amount, 5 μL. The catalyst behaviour was evaluated
according to the formulas given below.
2
3
3 2
2
8
3 3 2
.5–10.5, molar ratio) and LaIJNO ) ·6H O (La/Al = 0.025,
molar ratio) were mixed evenly, and then an appropriate
amount of diluted nitric acid aqueous solution (AR, 99%)
(2 wt.%) as an adhesive solvent was added dropwise until the
powder mixture bonded together evenly to form a dough.
Then, it was extruded via a banded extruder (using an extru-
sion moulding method) to form original supports. After
drying in a stoving chest at 50 °C for 12 hours, the supports
were cut into cylindrical particles with lengths of about 2 mm
to 3 mm. Finally, the cylindrical supports were calcined in a
muffle furnace at 650 °C for 5 hours.
Conversation of isobutane:
Fisobutane, in  Fisobutane, out
Xisobutane

Fisobutane, in
Selectivity of isobutene:
Sisobutane
Subsequently, Cr was introduced into calcined ZnAlLaO by
immersing the ZnAlLaO support in different concentrations
Fisobutane

of CrIJNO
3
)
3
·9H
2
O aqueous solution. Typically, the support
F
isobutane, in  Fisobutane, out
was first immersed in the prepared CrIJNO ) ·9H O solution
3
3
2
for about 15 min in a stirred vessel with the temperature
maintained at 60 °C. After drying in a stoving chest at 105 °C
for 12 h, the dried catalyst was calcined at 500 °C for 4 h with
where F is the molar flow rate.
X-ray diffraction (XRD) of the catalysts was performed
using a D8 Advance (Bruker, Germany) instrument with Cu
Kα radiation (40 kV and 30 mA). XRD data were collected in
the 2θ range of 10° to 90° using a scan speed of 12° min
and a step size of 0.02°.
−
1
steam for dechlorination. The heating rate was 2 °C min
.
−
1
Then the catalyst was heated to 700 °C and maintained at
this temperature for a further 5 h.
Catalyst testing
The catalytic activity measurements were carried out in a
continuous-flow reaction system consisting of stainless steel
single-pipe fixed-bed reactors (Φ = 9 × 2 mm, 500 mm in
length) with a gas chromatograph for online product analy-
sis. 2 g of catalysts were used each time. The reactant gases
were fed through the mass flow controllers (MFC), which
were used to deliver a defined flow of each gas. The ratio of
i-C H /O was 3.5 (molar ratio) at a space velocity (SV) of
4
10
2
−
1
−1
2
00 mL g
h . The feed composition was obtained by
−
1
−1
mixing 6.67 mL min of isobutane, 1.67 mL min of oxygen
and 0.05 mL min of water vapour under normal conditions.
For the tests without water vapour, the feed composition was
−
1
Fig. 1 Flow diagram of the dehydrogenation of isobutene.
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Temperature-programmed desorption (TPD) characteriza-
tion of the catalysts was performed with a quadrupole mass
spectrometer (AMETEK HQ 200 M, USA) in a quartz fixed-bed
reactor with an inner diameter of 6 mm. Catalysts (0.4 g) were
placed in the TPD reactor and reduced under 10% (v/v) H /Ar
2
flow at 600 °C for 0.5 h. Then the temperature was dropped to
4
00 °C for 10 min and the reactor was purged with Ar gas
−
1
(30 mL min ) to remove the H and steam. After naturally
2
cooling down to room temperature, the temperature of the reac-
tor was adjusted by a cooling machine. CO or isobutene was
2
adsorbed on the catalyst at −5 to −8 °C for 1 h. After the reactor
−
1
was purged under Ar flow (30 mL min ) for 1 h, the adsorbate
−1
was desorbed by heating at 15 °C min under Ar flow.
X-ray photoelectron spectroscopy (XPS) measurements
were performed with an Axis Ultra DLD (a delay-line detector)
spectrometer (ThermoScientific, USA) using a focused mono-
chromatized AlKα radiation (1486.6 eV) in the CAE mode.
The material was introduced into the treatment chamber
using double-sided tape. The sample was maintained under
Fig. 3 XRD patterns of different types of supports.
observed that the active component not only existed on the
external surface of the catalyst but also penetrated into the
interior. Hence, the utilization rate of the Cr active component
on the catalyst's external surface should be lower than those
of the MgAlLaO and ZnAlLaO catalysts. The reaction perfor-
mance of various catalysts also proved this speculation
(Fig. 4). As shown in the XRD pattern of the MgAlLaO catalyst,
the magnesium aluminate spinel structure has been formed
on the catalyst surface. The peaks at 37.904°, 46.102° and
−
9
high vacuum at 5.0 × 10 mbar. It was reported that the
high-resolution XPS region scan spectra were obtained within
a short period (ca. 12 min) at 20 eV pass energy to minimize
the photo-reduction of the surface chromium species that
1
1–13
may be induced by X-rays.
The composition of different
chemical states of surface chromium in the samples was
obtained from XPS spectra using Vision 2.0 software.
6
7.275° can be indexed as IJMg0.4Al0.6)Al1.8
of the ZnAlLaO catalyst also shows that the diffraction peaks
are assigned to the zinc–aluminum spinel phase IJZn0.3Al0.7
, which are slightly stronger than the diffraction peaks
4
O . The XRD pattern
Results and discussion
)
XRD and CO -TPD characterization of different
2
Al1.7O
4
support catalysts
of MgAlLaO. It is probably due to the stronger interaction of
the active component Cr with the ZnAlLaO support than that
with the MgAlLaO support, which further enhances the dif-
fraction peak intensity of Al O . Furthermore, no obvious dif-
Fig. 3 shows the XRD patterns of the same Cr active compo-
nent on different types of supports. The diffraction peaks of
different supports indicate that the Cr in the samples is a par-
tially amorphous phase or a small crystalline particle due to
2
3
2 3
fraction peaks of Cr O were observed for both Cr/MgAlLaO
the difficulties in interaction between Cr with TiO
2
and
and Cr/ZnAlLaO catalysts. This implies that the active compo-
nent dispersed evenly on the support surface. However, in Fig. 2,
it can be seen clearly that the penetrated thickness of the
active component in the ZnAlLaO catalyst is lower than that in
the MgAlLaO catalyst. The active component of the ZnAlLaO
1
4
SiO2. As shown in the XRD patterns of both Cr/TiO2 and
Cr/SiO , it can be deduced that the bulk Cr O gathered on
2
2 3
the support surface and the active component was dispersed
unevenly. In Fig. 2, it can be seen from the pictures that the
active component Cr over the CaAlLaO support is caked on
the external surface, indicating uneven dispersity. The Cr on
the CaAlLaO catalyst showed agglomeration; the active
component cannot be fully utilized on the surface. For the
CrAlLaO catalyst, due to the self-containing property of the Cr
element, the CrAlLaO interacted with the active component
and exhibited strong syn-Cr O diffraction peaks. It can be
2
3
Fig. 2 Pictures of different supports used for loading Cr. (a) Cr/CaAlLaO,
b) Cr/CrAlLaO, (c) Cr/MgAlLaO, and (d) Cr/ZnAlLaO. Cr loading: 18 wt%.
Fig. 4 CO
2 2 2
-TPD profile of different supports: (1) TiO , (2) SiO , (3)
(
Cr/ZnAlLaO, (4) Cr/MgAlLaO, (5) Cr/CrAlLaO, and (6) Cr/CaAlLaO.
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catalyst is mainly distributed on the external surface, sug-
gesting that the Cr utilization rate of the ZnAlLaO catalyst on
the outside layer is higher than that of the MgAlLaO catalyst.
catalytic activity along with the reaction time. The initial
selectivities of isobutene catalyzed by Cr (18%)/MgAlLaO and
Cr (18%)/ZnAlLaO are 78.87% and 84.07%, respectively. In
our experiments, the Cr (18%)/ZnAlLaO catalyst exhibits the
best catalytic performance and the highest selectivity conver-
sion compared with the other five catalysts. This result is
superior to that of previously reported 10% Cr/Al O catalyst
15,16
In order to further study the influence of different
supports on catalytic properties, CO -TPD experiments were
2
carried out. Fig. 4 shows the CO
2
-TPD profiles of the different
samples. It can be observed that for SiO and TiO catalysts,
2
2
2
3
there are no obvious desorption peaks observed, indicating
almost no alkalinity on the support surfaces of SiO and
TiO . The CO -TPD profile of the CaAlLaO support shows
(63.1%) selectivity and 10.7% conversion when using
CrIJNO ·9H O as a precursor, and 100% selectivity and 0.1%
conversion when using CrKIJSO ) ·12H O as a precursor. How-
2
3
)
3
2
2
2
4 2
2
the highest intensity with a desorption temperature of
2.2 °C. However, the CO desorption peak areas of CrAlLaO,
ever, the Cr (18%)/ZnAlLaO catalyst also showed relatively
unsatisfactory stability after 6 h of reaction.
6
2
MgAlLaO and ZnAlLaO supports are relatively smaller than
that of the CaAlLaO support, which indicates that these three
supports have weaker alkalinity than that on the CaAlLaO
support surface. Moreover, from the ZnAlLaO support
In order to obtain better catalytic performance, we further
studied the influence of the Cr content and water vapour.
2
XRD and CO -TPD characterization of different loadings on
the ZnAlLaO catalyst
emerged two CO desorption peaks. The first peak located at
2
1
6.6 °C is assigned to the weak physical desorption peak.
Another peak is the weak alkaline desorption centre that
appeared at a temperature of 46.3 °C.
Several studies have proved that the addition of Cr to the
ZnAlLaO catalyst could enhance the catalytic performance
and the reaction stability. The loading amount of Cr in this
case acts as a promoter in ODH. As the loading amount of
2
4
Generally, appropriate alkaline sites can significantly
improve the catalytic ability of carbon elimination as well as
1
7,18
enhance the catalytic stability.
The acidic sites are carbon
ion activation centres, and these easily lead to catalyst carbon
deposition. This is an important factor that causes the
decrease in catalytic activity and stability. Thus, the appropri-
ate alkalinity on the catalyst surface can improve the selectivity
of isobutene and the resistance to carbon deposition. Weak
acid sites adsorb isobutene easily and then form tertiary
carbonium ions. Then the intermediates leave the catalyst sur-
1
9,20
face to produce isobutene products.
If the isobutane is
adsorbed on the acid sites, its intermediates cannot undergo
desorption of the catalyst surface in a timely manner or imme-
diately undergo adsorption again after desorption. This pro-
cess could lead to deep oxidation of isobutane and then
induce carbon deposition. The carbon deposition can cause
catalyst deactivation and can decrease the selectivity of iso-
2
1
butene. However, the alkaline centre can activate the oxygen
element effectively on the ZnAlLaO support surface, promot-
ing ODH reaction. We believe that the selectivity is associated
with the lattice oxygen of the catalyst surface. The number of
lattice oxygen atoms can improve the conversion rate of the
2
2,23
catalyst and reduce its selectivity.
In addition, when the
deep oxidation occurred on the catalyst surface and induced
carbon deposition, the adjacent weak alkalinity site can acti-
vate the oxygen of the ZnAlLaO support and carbon deposition
can be eliminated, consequently improving the stability and
activity of the Cr/ZnAlLaO catalysts. It has also been proven, as
shown in Fig. 4, that the selectivity and conversion rate of the
ZnAlLaO catalyst are much higher than that of other catalysts.
Therefore, we believe that moderate alkalinity and acidity are
important factors for catalytic activity and stability.
Fig. 5 (a) Selectivity and (b) conversion as functions of time for the
different support catalysts: (1) Cr (18%)/TiO
(
(
2
, (2) Cr (18%)/SiO
18%)/CaAlLaO, (4) Cr (18%)/CrAlLaO, (5) Cr (18%)/MgAlLaO, and (6) Cr
18%)/ZnAlLaO. Reaction conditions: temperature = 580 °C, 1 atm, 2 g
2
, (3) Cr
Catalytic performance of different support catalysts
The catalytic performances of the different support catalysts
are presented in Fig. 5. All catalysts showed decreasing
−1
−1
of catalysts, i-C
4
H10/O
2
= 3.5 (molar ratio), SV = 200 mL g
h .
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the active component Cr increases, the loading thickness on
the external surface is also increased, which can be easily
observed on the digital pictures (Fig. 6). In Fig. 5, the sam-
ples with Cr loadings of 8.56% and 18.12% did not obviously
appear in the diffraction peaks associated with the Cr O
2
3
2 3
crystalline structure. The results suggest that Cr O is highly
dispersed in the loading range of 8% to 18%. The XRD pat-
terns of the 8.56% and 18.12% Cr loadings show the appear-
ance of the IJZn0.3Al0.7)Al1.7
4
O aluminium-rich zinc spinel
lattice structure. It has been reported that Cr can diffuse into
2
5
the oxygen vacancies in the ZnAl O spinel lattice. It can be
2
4
observed that the IJZn0.3Al0.7)Al1.7
32.03%)/ZnAlLaO catalyst is damaged by the loading amount
of Cr up to 32% on the catalyst surface. The diffraction peaks
of Cr are significantly strengthened as well as the appear-
ance of the Cr O crystalline phase on the catalyst surface.
4
O spinel structure of the Cr
(
2 3
O
Fig. 7 XRD patterns of the catalysts with different Cr additions.
2
3
Fig. 8 shows the CO -TPD profiles of the different Cr load-
2
ing samples. Generally, various catalysts appear in the CO
weak alkalinity peak range of 48 °C to 62 °C; low-temperature
2
2
6
desorption peaks are assigned to CO
2
on Cr
2
O
3
.
It can be
observed that the ZnAlLaO catalyst without active component
Cr loading showed the strongest alkalinity compared with
other catalysts. However, the ZnAlLaO catalyst without the
active component has the lowest active performance. Conse-
quently, higher alkalinity has disadvantages for the ODH
reaction. As the active component Cr loading increases, the
2
area of the CO desorption peak decreases. The addition of
Cr can lead to the formation of weak alkaline sites. In
addition, the weak alkaline sites can effectively activate the
2
6
oxygen element in the ZnAlLaO support, inhibiting the pro-
duction of coke. However the overloading of Cr results in the
blocking effect of a worse Cr particle distribution on the cata-
lyst surface, which has been proven from the XRD pattern
in Fig. 7.
Fig. 8 CO
Cr (0%)/ZnAlLaO, (2) Cr (8.56%)/ZnAlLaO, (3) Cr (18.12%)/ZnAlLaO, and
4) Cr (32.03%)/ZnAlLaO.
2
-TPD profile of the catalysts with different Cr loadings. (1)
(
results obtained at a better performance level confirm the
increase in dehydrogenation activity partly due to a better
particle distribution prompted by suitable loading of Cr on
the ZnAlLaO catalyst. However, the continuous addition of Cr
leads to the decrease in catalytic activity due to the blocking
effect of a worse Cr particle distribution on the catalyst sur-
Catalytic performance of different Cr loadings on the
ZnAlLaO catalyst
Fig. 9 shows the effect of the Cr content on the catalytic per-
formance of the isobutane ODH over 10 h. The Cr (18.12%)/
ZnAlLaO catalyst achieves the maximum isobutene selectivity
2
7
(
84.07% after reaction for 6 h). It has been studied that Cr
face. The reactant gas can easily interact with the active
sites due to overloading of Cr species and thus enhances
uneven active component dispersion on the catalyst surface.
This behaviour causes the occurrence of side reactions dur-
ing this process and then leads to the decrease in the selec-
tivity eventually. Hence, the active component disperses
evenly and appropriate Cr loading (about 18%) on the cata-
lyst surface is the reason for the enhancement of the catalytic
activity and stability.
has become a key ingredient in oxidative dehydrogenation
catalysts because of its high activity for Cr–O–Al bond forma-
2
5
tion. This is the reason for an intrinsic high selectivity of
unprompted Cr toward isobutene ODH. On the other hand, it
has been proven from the XRD pattern (Fig. 7) that these
2
8,29
According to the literature,
X
the formation of CO and
CH species from readsorption of isobutane and isobutene is
X
one of the significant factors for coke deposition on the cata-
lyst surface; in this case, the carbon deposition on the cata-
lyst surface may act as a reason to decrease the catalytic
Fig. 6 Pictures of different supports used for loading Cr. (1) Cr
3
0,31
(
8.56%)/ZaAlLaO, (2) Cr (18.12%)/ZnAlLaO, and (3) Cr (32.03%)/ZnAlLaO.
activity by deactivating the active sites.
Therefore,
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Fig. 10 100 h test of the Cr (18%)/ZnAlLaO catalyst with steam as a
diluent. Reaction conditions: temperature = 580 °C, 1 atm, 2 g of
−1
−1
catalysts, i-C
4
H10/O
2
= 3.5 (molar ratio), SV = 200 mL g
h .
the reaction gas and promote the ODH reaction equilibrium
towards the right. On the other hand, water vapour is respon-
sible for reducing the acidity of the support surface and inter-
acts with some carbon deposits to induce carbon deposit
gasification. Thus, the CO2 selectivity increased and side
reactions occurred, such as steam reforming. It can also delay
the coverage rate of carbon formation on the catalyst sur-
3
5
face. Due to adsorption of water molecules on the Cr O
2
3
surface, the adsorption and desorption capacity of oxygen
species are decreased on the support surface. The interaction
between the active component and the support can be weak-
ened gradually. Therefore, the addition of an appropriate
amount of water vapour to the feed can continuously enhance
the catalytic stability, while both the isobutane conversion
and the isobutene selectivity decreased at the same time.
Fig. 9 (a) Selectivity and (b) conversion as functions of time for the
different loading catalysts: (1) Cr (0%)/ZnAlLaO, (2) Cr (8.56%)/ZnAlLaO,
3
6
(
3) Cr (18.12%)/ZnAlLaO, and (4) Cr (32.03%)/ZnAlLaO. Reaction
conditions: Al/Zn = 8.5–10.5, temperature = 580 °C, 1 atm, 2 g of
−
1
−1
catalysts, i-C
4
H10/O
2
= 3.5 (molar ratio), SV = 200 mL g
h .
XPS analysis
minimizing the readsorption and the dissociation of the C–C
bond will effectively eliminate carbon deposition on the cata-
Fig. 12(a) shows the XPS spectra in the Cr 2p region for vari-
ous catalysts. The quantitative data of binding energies are
illustrated in Table 1. Curve fitting of the experimental data
showed the presence of two distinct chromium species. The
oxidation states of the species have been totally assigned to
CrIJIII) and CrIJVI) at the binding peaks of 577 and 579 eV,
respectively. It has been reported in ref. 37 that the oxidation
states of chromium were determined by the binding energy
of the Cr 2p(3/2) peak and the value of spin–orbit splitting
Cr 2pIJ1/2)–Cr 2p(3/2). The binding energy decreases in the
range of 576.4 eV to 576.9 eV for CrIJIII). Due to the intrinsic
3
2
lyst surface. Subsequently, we did some experiments on the
addition of water vapour and its influence as part of the feed
gas for ODH.
The effects of water vapour
Water vapour is used as a diluent and oxidant in many dehy-
drogenation reactions in order to eliminate carbon deposi-
3
3,34
tion.
However, few studies have been reported on the use
of water vapour as a feed gas for ODH reaction. Fig. 10 shows
the 100 h reaction of the Cr/ZnAlLaO catalyst with steam as a
diluent; it can be observed that the conversion of isobutene
decreased significantly (32.74%) after 6 h, and CO selectivity
Table 1 Curve-fitting parameters for Cr 2p region XPS of the spent and
fresh catalysts
2
(
10.92%) obviously increased after addition of water vapour.
Without water vapour, small amounts of dehydrogenation
products, such as unsaturated olefin, would be polymerized
on a strong acid centre of the catalyst surface. Then, deep
dehydrogenation would occur and eventually result in the for-
mation of carbon deposit precursors. The water vapour could
act as a diluent to reduce the partial pressure of oxygen in
Cr 2p3/2 BE (eV)
Cr 2p1/2 BE (eV)
Catalysts
With H O-100 h
CrIJVI)
CrIJIII)
CrIJVI)
CrIJIII)
2
—
575.96
576.71
577.13
576.87
586.88
—
588.17
586.97
585.90
586.69
586.77
586.69
Without H O-8 h
Fresh catalyst
With H O-30 h
2
577.02
577.56
578.47
2
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complexity of CrIJIII), it is quite hard to ascertain the contribu-
tion of the intermediate chromium state in the Cr 2p3/2
region. Studies have shown that both CrIJVI) and CrIJIII) are the
catalytic activity centres of ODH; however, CrIJVI) is usually
is generally shifted toward the lower binding energy with
increasing reaction time. In contrast, the C1s spectrum also
displays the asymmetric peak shape on the used catalysts
(without H O-8 h and with H O-100 h) at binding energies of
284.28 eV and 283.88 eV. It is obvious that the catalyst with
H O-100 h achieves the maximum peak intensity (with a bind-
2
2
2
2
5
unstable and easily reduced. It can be observed that the
fresh catalyst and the catalyst with H O-30 h mainly appear
at the binding energies of CrIJIII) at 577.08 eV and 577.28 eV
2
ing energy of 283.88 eV), and the coke deposits are possibly
3
7,38
2
(
2p3/2).
At the same time, the catalysts without H O-8 h
composed of the sp hybridized carbon; this type of coke
2
and with H
2
O-100 h displayed Cr (close to a zero-valent state)
probably consists of aromatic rings according to the previous
4
0–42
at the binding energies of 576.88 eV and 576.28 eV. The rela-
tive amount of CrIJIII) increased in the used sample compared
with a fresh catalyst, indicating the reduction of CrIJVI) to
CrIJIII), and the result is in agreement with the conclusions
studies.
Furthermore, the produced coke deposits of used
catalysts without H O-8 h and with H O-100 h are assigned to
2
2
4
3
the pregraphite-like carbon, which was present in the study.
It has a lower binding energy than the graphitic carbon,
1
5,39
42
reported in the recent literature.
The binding energy of
suggesting a high electron density related to its structure.
CrIJIII) decreased with increasing reaction time; meanwhile, a
close to zero-valent state of Cr generally formed, leading to
the decrease in the catalytic activity on the catalyst surface,
and eventually the catalytic performance dropped. Further-
more, the decrease in interaction between adsorbed oxygen
and the active component is probably due to the adsorption
of water molecules on the active component surface. There-
fore, the addition of water vapour is beneficial to inhibit car-
bon deposition and deep oxidation on the catalyst surface.
Because coke formation is the main factor of the deactivation
process of ODH, we characterized the basic coke information
of the catalyst before and after catalytic reaction. Fig. 12
shows the XPS of C1s of the fresh catalyst and spent 18%
Cr/ZnAlLaO catalysts. One main peak and some broad peaks
are observed in each case. The area percentages and quantita-
tive data of binding energies are summarized in Table 2. The
C1s region spectra display the features corresponding to the
various chemical nature of carbon: pregraphite-like, aromatic,
aliphatic, and partially oxidized, and the binding energies
2
Moreover, compared to the catalyst without H O-8 h, the coke
deposit of the pregraphite-like carbon has not been deter-
mined in the catalyst with H O-30 h, which suggests that the
2
presence of water vapour can decrease the amount of coke on
the catalyst surface and improve the stability of ODH reaction.
It has also been proven by the consequence of catalytic per-
formance shown in Fig. 11 and 12. The C1s spectrum of the
catalyst with H O-30 h is similar to that of the fresh catalyst.
2
However, the main peak at 283.88 eV in the catalyst with
9
were identified according to the previous reports. As for the
fresh catalyst, two main peaks at binding energies of 284.88
and 288.76 eV are detected, which are assigned to the adventi-
tious hydrocarbon (C–C/C–H) and oxidized carbon species
(
O–CO), respectively. These are probably from residues after
Fig. 11 100 h test comparison of the Cr (18%)/ZnAlLaO catalyst with
and without water vapour. Reaction conditions: temperature = 580 °C,
1 atm, 2 g of catalysts, i-C H10/O = 3.5 (molar ratio), SV = 200 mL g h .
4 2
the calcination of the synthesized catalyst or the interaction
of the feed gas on the catalyst surface. The value of C–C/C–H
−
1
−1
Table 2 Curve-fitting parameters for C1s XPS of the spent and fresh catalysts
C1s BE (eV)
Catalysts
Pregraphite-like
C–C/C–H
C
H
x y
–CO
O–CO
2
With H O-100 h
BE (eV)
Area (%)
283.88
84.05
—
—
—
—
287.38
0.33
—
—
Without H
BE (eV)
Area (%)
Fresh
2
O-8 h
284.28
19.80
—
—
285.88
0.02
—
—
—
—
BE (eV)
Area (%)
—
—
284.88
77.67
—
—
—
—
288.76
5.07
2
With H O-30 h
BE (eV)
Area (%)
—
—
284.68
68.18
—
—
—
—
288.88
3.25
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Optimization of reaction temperature
Fig. 13 shows the catalytic activities of C
4 4 8
H10 and C H under
various temperatures. It can be seen that the catalytic activity
and isobutane conversion are relatively low at temperatures
between 560 °C and 570 °C. As the reaction temperature
increases, the conversion rate significantly increases. The iso-
butene selectivity reaches the maximum and stabilizes at
about 85% between 570 °C and 585 °C. As the reaction time
is prolonged, both the conversion and the selectivity gener-
ally decreased. When the temperature becomes too high
(
600 °C), not only the cracking reaction rate and coke deposi-
tion increase but also the catalyst's specific surface area and
4
5,46
the active catalysts decrease.
As shown in Fig. 14, as the
temperature increases, cracking reaction generally occurs,
which results in the increase in selectivity of CH and C H
4
3
6
2
and the decrease in selectivity of CO . Hence, the optimal
reaction temperature is in the range of 575 °C to 580 °C. The
optimal temperature of the fixed bed is in the range of
5
80 °C to 590 °C. The conversion of isobutane (52.44%) and
selectivity of isobutene (84.07%) achieve the optimum at 577 °C.
Optimization of alkoxy oxygen ratio
Table 3 shows the Cr/ZnAlLaO catalytic performances of dif-
ferent alkane oxygen ratios. It can be observed that the iso-
butene selectivity achieves the maximum when alkoxy ratio is
3
.5. When alkoxy ratio is relatively low (ranging from 1 to 3),
Fig. 12 (a) XPS spectra of chrome over various catalyst surfaces, (b) XPS
spectra of carbon over various catalyst surfaces. (1) Cr (18%)/ZnAlLaO
the intermediates and products are easily converted to by-
products such as C H , CH and CO in the oxygen-enriched
3
6
4
2
catalyst after 100 h test with water vapour (with H
2
O-100 h), (2) Cr
environment. In addition, the oxygen-enriched environment
can bring competitive adsorption of isobutane to a disadvan-
tageous situation. It leads to the decrease in selectivity of iso-
butene and the raw material waste. As the alkoxy ratio
increases to 3.5, the catalytic activity reaches maximum.
When the isobutane content is increased, it can be seen that
(
18%)/ZnAlLaO catalyst after 8 h test without water vapour (without
H
2
O-8 h), (3) Cr (18%)/ZnAlLaO fresh catalyst (fresh catalyst), and (4) Cr
O-30 h).
(
18%)/ZnAlLaO catalyst after 30 h test with water vapour (with H
2
H
2
O-100 h has a lower binding energy than that of the C1s
spectra in the catalyst without H O-30 h. This indicates that
2
2
the coke structure formed over the catalyst with H O-100 h
has a higher electron density than that over the catalyst
without H O-30 h.
2
Apparently, the coke deposit was an important factor for
the activity and stability of catalysts. The possible regenera-
3
3,44
tion routes of the catalysts were reported in the literature
in which several sequential regeneration cycles of various
catalysts can be carried out through the method of burning
3
8
the formed coke under some controlled conditions. For
instance, the temperature of the reactor is cooled to about
2
50 °C, simultaneously using an inert steam such as Ar as
the only feeding gas. After reaching this temperature, the
inert Ar steam was replaced by an air flow, and then the tem-
perature generally increased to about 550 °C. During this
process, the coke formed on the surface of the catalyst was
burned through interaction with oxygen gas. Approximately
1
h later, the catalysts were cooled to room temperature and
Fig. 13 The C
temperatures. Reaction conditions: 1 atm, 2 g of catalysts, i-C
3.5 (molar ratio), SV = 200 mL g
4
H
10 conversion and C
4
H
8
selectivity at different
10/O
maintained that way overnight. By this measure, the samples
were ready to be used in a new cycle.
4
H
2
=
−1
−1
h , without water vapour.
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the active component Cr O and the moderate alkalinity over
2
3
the ZnAlLaO catalyst surface are beneficial for the activity
and stability of the catalyst. The addition of an appropriate
amount of water vapour as part of the feed gas can effectively
inhibit carbon deposition on the catalyst surface and
enhance the catalytic stability, whereas both the selectivity
and the conversion are decreased. In this case, the optimum
reaction temperature is in the range of 575 °C to 580 °C. The
alkoxy ratio has been chosen in the range of 3 to 4. In the
present study, the catalytic activity of the Cr/ZnAlLaO catalyst
has obtained a significant improvement in order to compete
with commercial catalysts.
Acknowledgements
This work was supported by the key laboratory research of
natural gas and light hydrocarbon catalytic conversion of
China University of Petroleum (Beijing).
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