OXIDATIVE DEHYDROGENATION OF ETHANE IN THE
PRESENCE OF NANOCATALYST BASED ON NiO/γ-Al2O3
Dafina Momchilova ∗1, Desislava Ahchieva ∗∗, Lothar Moerl∗∗,
Christo Karagiozov ∗, Adriana Slavova ∗
∗University “Assen Zlatarov”,
Faculty of Technical Sciences
Departement of “Chemical Engineering”
8010 Bourgas, Bulgaria
e-mail: [email protected]; [email protected]∗
∗∗Otto – von Guericke – University, Magdeburg
Fakultat fur Verfahrens und Systemtechnik
Institut fur Apparate und Umwelttchnik
39016 Magdeburg, Germany
1. INTRODUCTION
The chemical transformation of alkanes to useful intermediates has
recently been subjected to intensive research in the field of catalysis. The
catalytic oxidative dehydrogenation of alkanes is known to represent a modern
process for the production of alkenes, principally, because of the significant
reduction of energy expenses, as compared to the conventional cracking
processes [1].
The main challenge, associated with this process has been to increase the
reactivity of alkanes, and to prevent the re-oxidation of both the starting
materials and products to carbon oxides [2].
Numbers of catalytic systems for the oxidative dehydrogenation of ethane
have been reported in the literature. According to their activity, these systems
can be classified as follows:
(i)
Catalysts, acting at temperatures higher than 873 K;
(ii) Catalysts acting at temperatures lower than 873 K
The first group is characterized by the use of basic oxides of alkaline,
alkaline earth and/or rare-earth metals. The second group includes metals,
belonging to V and/or VI group of the periodical system [3].
Many of the efficient catalysts from the group (ii) are known to contain
several blends of inorganic compounds (mainly, metal oxides). For example, the
catalyst based on mixed oxides of Mo, V and Nb is a typical representative of
this group. The addition of Nb to the blend of Mo- and V-oxides has increased
the selectivity of dehydrogenation with respect to ethylene to, approximately, 20
%. It was suggested that Nb eliminated the possibility of full oxidation to MoO3
1
To whom the correspondence should be addressed
and V2O5 and, also, caused the formation of new type of mechanical blends,
containing “selective” sites on the catalyst surface.
The process of oxidative dehydrogenation of alkanes was conducted in the
presence of α-Sb2O4, mixed with Mo- and V-oxides as well as with blend,
consisting of Ni- and V-oxides on one hand, and V-and Sb-oxides, on another.
In both cases the ratio between components was 9:1. The composition of these
blends was expressed by the mass ratio parameter Rm [3],
Rm =
mA
mA + mB
(1)
Where:
mi was the mass of the corresponding component (i) in the blend, i = A, B;
A was the assumed acceptor (Mo – V – O or Ni – V – O); and
B represented α-Sb2O4.
Another catalyst, employed in the process of oxidative dehydrogenation
of ethane was CeO2 (pure oxide or CeO2, subjected to modification with CaO
within the temperature range of 953 – 993 K). Carbon dioxide was used as an
oxidant in this process. The redox couple Ce4+/Ce3+ could be used for activation
of CO2 and the formation of an “active oxygen” [4, 5]. The authors synthesized
this catalyst by using two principal methods:
(i)
Freeze drying for the preparation of CeO2 with nanostructure;
(ii) The so-called “ceramic” method.
Still another new catalyst for the oxidative dehydrogenation of ethane,
which was effective at temperatures of 573 – 723 K was Al2O3 combined with
NiO and NiO-K, with Ni/K ratio of 10:2, and, also, NiO-W in 10:2 ratio
deposited on the Al2O3 surface [6]. The amount of NiO deposited on Al2O3 was
found to be 0.24 g/g catalyst. These studies led to the discovery of two types of
oxygen species with different activities. The more active oxygen converted both
the ethane and ethylene to carbon dioxide, whereas the less active one
transformed ethane to ethylene without further conversion to CO2. Heating the
catalyst from 623 to 723 K led to the removal of the more active oxygen species
from the catalyst surface. Therefore, the mixed catalysts changed the amounts of
the separated oxygen species and, accordingly, the selectivity towards ethylene.
The amount of the removed (i.e., desorbed) oxygen species from the surface of
the fresh catalyst was found to be proportional to the selectivity with respect to
CO2 [6].
2. OXIDATIVE DEHYDROGENATION OF ETHANE
2.1.
AIMS AND TASKS
The aim of the present work was to study the efficiency of a nanostructured catalyst, consisting of NiO deposited on Al2O3 for the process of
oxidative dehydrogenation of ethane in order to obtain ethylene as a target
product.
SYNTHESIS OF NIO/γ-AL2O3 AS A CATALYST
2.2.
The catalyst support was γ-Al2O3 (Sasol) with an average particle size of
1.8 mm, average pore size and pore volume of 10.33 nm and 0.5071 cc/g
respectively, and specific surface of 196.4 m2/g. Following heating at 423 K, the
sample with initial water content of 12 mass % was completely dried.
Nano-particles of NiCO3 were prepared, according to the method
described previously [7]. The deposition of NiCO3 onto the surface of the γAl2O3 support was conducted by impregnation, accompanied by continuous
agitation of the suspension. The catalyst prepared in this manner was subjected
to calcination at 473 K, which resulted in the formation of NiO. The nickel
content was 0.41 g/kg as determined by the atom absorption analysis.
2.3.
EQUIPMENT
The process of oxidative dehydrogenation of ethane was conducted in a
metal fluidized-bed reactor with cross section of 10-2 m2 at temperatures of 823
and 873 K. Details, concerning this reactor and its operation have been
described previously [8]. The mass of the NiO/γ-Al2O3 catalyst layer was 150 g.
The principal scheme of the installation is shown on Fig. 2. The fluidizing gas
contained ethane and nitrogen. The oxygen was fed under the gas-distributing
grating by means of ceramic membranes. Ethane and oxygen were supplied in
different ratios of C2H6/O2 = 0.2; 0.5; 1.1; 1.5; 2.0. The stoichiometric ratio of
the reactants was 2. The composition of the gas mixture was determined by
online gas chromatography. The temperature was observed by thermocouple
inserted into the catalyst layer.
6
GC
7
9
1
T1,P1
Fig. 2. Scheme of a fluidized-bed catalytic
reactor
4
T2,P2
T3,P3
3
2
O2
T4,P4
T5,P5
5
C2H6 + азот (N2)
8
1 – reactor with steady layer; 2 – gas-distributing grating; 3
– membranes for oxygen supply; 4 – NiO/γ-Al2O3 catalyst; 5 –
ethane and air supply; 6 – gas chromatograph; 7 – cyclone; 8 – gascollecting vessel; 9 – indicator for temperature and pressure.
3. RESULTS AND DISCUSSION
The process of oxidative dehydrogenation of ethane in the presence of
NiO/γ-Al2O3 catalyst was conducted under the reaction conditions, indicated in
Table 1:
Table 1
Experimental Parameters of the Oxidative Dehydrogenation of Ethane at
Temperatures of 823 and 873 K
№
1
1
2
3
4
5
6
7
8
9
T [°C]
2
815
816
817
813
813
811
811
811
811
p[Pa]
3
126.25
122.5
96.25
107.5
103.75
100
97.5
101.25
110
c_C2H6[vol%]
4
1.22
1.37
1.37
1.23
1.23
1.00
1.00
1.00
1.00
C2H6/O2
5
0.24
0.27
0.27
0.53
0.53
1.14
1.14
1.78
1.78
№
1
10
11
12
13
14
15
16
17
18
19
T [°C]
2
851
850
850
849
850
849
853
853
857
857
p[Pa]
3
120,00
98.75
101.25
106.25
126.25
96.25
71.25
103.75
120,00
110,00
c_C2H6[vol%]
4
1.00
1.00
1.00
1.00
1.00
1.00
1.23
1.23
1.37
1.37
C2H6/O2
5
2.29
2.29
1.78
1.78
1.14
1.14
0.53
0.53
0.27
0.27
The selectivity of the process towards C2H4, CO and CO2 was calculated
from the experimental results obtained. The conversion of C2H6 and the yields of
C2H4, CO and CO2 products were also evaluated, according to the following
equations:
ethane conversion (X)
X=
-
C C 2 H 4 + 0.5C CO + 0.5C CO 2
C C 2 H 4 + C C2 H 6 + 0.5C CO + 0.5C CO 2
* 100
(%)
(7)
selectivity (S)
S=
Ci
* 100
∑С
(%)
(8)
where Ci is the target product concentration;
∑ Ci was the total concentration of products;
n
-
yield (A)
А=
X .S
100
(%)
(9)
The data on the selectivity towards ethylene, carbon monoxide and carbon
dioxide as well as ethylene yield and ethane conversion are shown in Table 2.
Table 2
Experimental Results Obtained from the Oxidative Dehydrogenation of
Ethane
Номер на
опита
Температура
в слоя
Отношение
на
реагентите
Концентрация
на етена в слоя
Концентрация
на етана в слоя
Конверсия
на етана
спрямо етен
спрямо СО
спрямо СО2
Добив на
етен
№
1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
T[K]
2
815
816
817
813
813
811
811
811
811
851
850
850
849
850
849
853
853
857
857
C2H6/O2
3
0.24
0.27
0.27
0.53
0.53
1.00
1.14
1.57
1.78
2.29
2.29
1.78
1.78
1.14
1.14
0.53
0.53
0.27
0.27
C C2H4_p
4
0.01
0.02
0.02
0.02
0.02
0.01
0.01
0.01
0.01
0.03
0.03
0.03
0.03
0.03
0.03
0.04
0.04
0.04
0.04
c_C2H6_p
5
0.86
0.96
0.95
1.01
1.00
0.90
0.89
0.91
0.90
0.91
0.89
0.88
0.88
0.86
0.85
0.94
0.94
0.87
0.85
X
6
7.68
7.96
8.57
5.02
5.16
2.68
2.66
2.30
2.49
5.06
5.41
5.66
5.78
6.60
6.75
11.13
10.94
16.59
16.76
S_C2H4
7
19.38
19.08
18.74
28.85
28.20
50.09
51.37
62.24
58.33
69.67
66.19
62.92
62.27
53.93
52.65
34.17
35.33
24.42
24.65
S_CO
8
61.42
58.86
62.85
58.24
58.33
44.54
40.24
32.72
35.25
25.30
26.59
29.46
28.86
38.72
35.37
50.83
49.32
57.06
56.37
S_CO2
9
20.51
20.04
21.12
16.79
17.26
9.69
11.32
7.23
9.09
6.43
8.89
8.48
9.75
11.67
11.61
16.41
15.85
19.92
19.85
A_C2H4
10
1.49
1.52
1.61
1.45
1.46
1.34
1.37
1.43
1.45
3.52
3.58
3.56
3.60
3.56
3.55
3.80
3.87
4.05
4.13
Селективност
The results in Table 2 were also represented as graphs. For temperature of
823 K, the dependences of ethane conversion, selectivity and yield with respect
to ethylene on the C2H6/O2 ratio were respectively given (Figs. 3 – 5).
1.55
10
X C 2 H6
X%
А%
A
1.5
8
1.45
6
1.4
4
1.35
2
1.3
0
0
0.5
1
1.5
C 2 H 6 :O 2
0
2
Fig. 3. Dependence of the conversion of С2Н6
from the ratio С2Н6/О2
S%S
0.5
50
40
30
20
10
0
0.5
2
Fig. 4. Dependence of the yield of С2Н4
from the ratio С2Н6/О2
70
60
0
1
1.5
C2H6:O2
1
1.5
C 2 H 6 :O 2
2
Fig.5. Dependence of the selectivity of
С2Н4 from the ratio С2Н6/О2
As seen from these graphs, the higher C2H6/O2 ratio led to lower
conversion and higher selectivity; minimum yield was obtained under certain
conditions. C2H6/O2 ratio within the 0.2 – 0.5 ranges produced higher yield of
ethylene. Low yield was obtained with C2H6/O2 ratio of unity. However, at
stoichiometric ratio between the reactants (C2H6/O2 ratio = 2), trend towards an
increase of ethylene yield was observed.
Comparison of the yield of the target product (ethylene) with that of the
side products (CO and CO2) was also made (Fig. 6).
5
A%
4
3
2
1
0
0
0.5
1
1.5
C 2 H 6 :O 2
2
Fig.6. Dependence of the yield of ethen COand
CO2 from the ratio С2Н6/О2
- yield of CO2
- yield of CO
- yiel d of C2H4
The increase of C2H6/O2 ratio resulted in decreased yields of CO and CO2
but ethylene yield remained almost unchanged. Ethylene yield was also
compared with that of CO and CO2 at higher temperature (873 K, Fig. 7).
A % 12
A
10
8
6
4
2
0
0
0,5
1
1,5
2
2,5
C2H6:O2
Fig.7. Dependence of the yield of ethen COand
CO2 from the ratio С2Н6/О2 for catalyst NiO/γ Al2O3
- yield of CO2
- yield of CO
- yiel d of C2H4
The yields of carbon oxides were found to decrease as C2H6/O2 ratio
became higher. At the same time, ethylene yield did not essentially change (Fig.
11). Stoichiometric C2H6/O2 ratio gave ethylene yield, that was certainly higher
than that of CO and CO2.
The experimental data obtained at 873 K in the presence of NiO/γ-Al2O3
catalyst were compared to the data for the same catalytic process, conducted
earlier in the presence of VOx/ γ-Al2O3 [8].
X%
X
18
16
14
12
10
8
6
4
2
0
X%
0
1
2
3
C2H6:O2
C2H6/O2
C2H6/O2
Fig.8 Dependence of the conversion of С2Н6 from
the ratio С2Н6/О2 for catalyst NiO/γ - Al2O3
Fig.9. Dependence of the conversion of С2Н6 from
the ratio С2Н6/О2 for catalyst VOx/γ - Al2O3
The increase of the C2H6/O2 ratio in the presence of both catalysts resulted
in reduced conversion of ethane. However, higher ethane conversion was
observed in the presence of VOx/γ-Al2O3 catalyst within the whole range of the
C2H6/O2 ratio.
A%
4,2
A
A%
4,1
4
3,9
3,8
3,7
3,6
3,5
3,4
0
0,5
1
1,5
C2H6:O2
2
2,5
C2H6/O2
Fig.10. Dependence of the yield of С2Н4 from the
ratio С2Н6/О2 for catalyst NiO/γ - Al2O3
Fig.11. Dependence of the yield of С2Н4 from the
ratio С2Н6/О2 for catalyst VOx/γ - Al2O3
The comparison of both catalysts with respect to ethylene yield (Figs. 10
and 11) showed that higher yield was obtained in the presence of VOx/ γ-Al2O3
catalyst. The use of NiO/γ-Al2O3 produced higher ethylene yield, again, with
C2H6/O2 ratio that was lower than the stoichiometric one (C2H6/O2 = 2).
S%
80
S
S%
70
60
50
40
30
20
10
0
0
0,5
1
1,5
2
2,5
C H :O2
C22 H6 6/O
2
Fig.12. Dependence of the selectivity of С2Н4
from the ratio С2Н6/О2 for catalyst NiO/γ - Al2O3
C2H6/O2
Fig.13. Dependence of the selectivity of С2Н4
from the ratio С2Н6/О2 for catalyst VOx/γ - Al2O3
The selectivity towards ethylene (Figs 12, 13) increased with the higher
C2H6/O2 ratios for both catalysts. Stoichiometric ratio of reactants produced high
selectivities of almost the same values for both catalysts.
The higher temperature studied (873 K), Figs 4, 10) was associated with
higher yields of ethylene. Therefore the process of oxidative dehydrogenation of
ethane took place more efficiently at higher temperature and in the presence of
the more selective catalyst.
4. CONCLUSIONS
1.
Heterogeneous catalyst, consisting of nano-particles of NiO
deposited on γ-Al2O3 support has been synthesized.
2.
Experiments on the process of oxidative dehydrogenation of ethane
in the presence of NiO/γ-Al2O3 catalyst have been conducted under the
following conditions: temperatures 550 and 600oC and ethane/oxygen ratio
within the 0.2 – 2.0 interval.
3.
The change in the ethane/oxygen ratio at temperature 823 K results
in lower conversion and higher selectivity towards ethylene; under certain
conditions, the yield of the latter is minimal. Higher ethylene yield is obtained
for ethane/oxygen ratio within the 0.2 – 0.5 range. At the stoichiometric ratio
between these reactants, trend towards increase of the ethylene yield can be
observed.
4.
Ethane conversion decreases and selectivity towards ethylene
becomes higher with simultaneous reduction in the yield at 873 K as the ratio
between the reactants changes. Again, stoichiometric ethane/oxygen ratio of 2
gives higher yield of ethylene.
5.
The comparison of temperature effect on the ethylene yield
indicates that at higher temperature the yield increases. This observation
confirms that the oxidative dehydrogenation of ethane occurs more efficiently at
higher temperature and in the presence of catalyst, exhibiting higher selectivity
towards ethylene as a target product.
6.
The experimental data obtained at temperature of 873 K and in the
presence of NiO/γ-Al2O3 catalyst have been compared to those obtained with
VOx/γ-Al2O3. The increase in the ethane/oxygen ratio results in the decrease of
ethane conversion and the selectivity with respect to ethylene becomes higher in
the presence of both catalysts. High selectivities of almost the same values for
both catalysts are observed for stoichiometric ratio between these reactants.
Higher yield of ethylene, however, is produced in the presence of VOx/γ-Al2O3
as catalyst. This is most likely to be caused by the low concentration of Ni nanoparticles in the catalyst blend.
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