Mendeleev Commun., 2020, 30, 359–361
Table 1 Oxidative dehydrogenation of ethane on a FeCrOx/C catalyst.
Conversion
(mol%)
Molar content
Selectivity
(%)
Carbon
balance
(%)
of products (%)a
ꢆ0
ꢅ0
ꢄ0
ꢃ0
ꢂ0
ꢁ0
20
ꢀ0
0
T/°C
C2H6 CO2 CO CH4 C2H4 H2 C2H4 CO
650
700
750
12
18
43
19
30
40
52
45
40
7
8
18
22
16
23
25
34
80
82
52
20
18
48
96
95
92
10
a Excluding water; v(C2H6) = v(CO2) = 21 ml min–1.
‘Sibunit’ is not neutral to hydrogen at temperatures above 400 °C
and undergoes hydrogenation that corresponds to a broad peak in
the temperature range of 370–850 °C with a maximum at
676 °C. Two peaks are observed on the TPR profile of the
FeCrOx/C catalyst. The gradual decrease in magnetization in the
temperature range of 20–420 °C indicates the transition of
magnetite Fe3O4 (ferromagnetic) to wustite FeO. The series of
shoulders at the first peak in the TPR curve is probably due to the
presence of yet another phase (Fe2O3) in the catalyst that has no
magnetic properties but is also reduced in the temperature range of
200–500 °C. The subsequent increase in magnetization reflects the
formation of metallic iron. The second peak with a maximum at
619 °C apparently corresponds to the hydrogenation of the carbon
support with a shift towards lower temperatures compared to the
pure support (T = 676 °C). This is due to the catalytic effect of
Fe-containing active sites on the hydrogenation of ‘Sibunit’.
Therefore, based on the difference in the phase composition
of the catalysts studied and the reducing properties of the carbon
support, one should expect to observe changes in the ODE
process on the FeCrOx/C catalyst in comparison with the other
catalytic systems. This is confirmed by the results of catalytic
experiments (Table 1).
Along with reaction (1) of direct dehydrogenation of ethane
to ethylene, ODE reaction (2) also occurs due to CO2 activation
by the catalyst, where CO2 acts as the oxidizing agent. The
hydrogen/ethylene ratio obtained exceeds 1, indicating that a
fraction of ethane undergoes deep dehydrogenation by reaction
(3) to give amorphous deposits of carbon. In turn, carbon reacts
with water to produce synthesis gas (4) and carbon dioxide by
the Boudoir–Bell reaction (5). Wherein, the loss in the catalyst
weight after the reaction (12 h) did not exceed 5%.
Figure 2 Comparison of the selectivity of catalysts for ethylene in ODE
with carbon dioxide (T = 650 °C).
hydrogenation of CO2/H2 = 1:1 on a FeCrOx/C catalyst under
atmospheric pressure (Table S1). At 500 °C, CO2 conversion is
30% with CO selectivity equal to 94% and CH4 selectivity of 6%.
At 600 °C, CO2 conversion reaches 54% with CH4 selectivity of
57% and CO selectivity of 43%. Thus, a high hydrogen content in
the gas mixture favours the Sabatier reaction (see Table 1).
Figure 2 presents data on the selectivity of ethane conversion
to ethylene on Fe–Cr catalysts supported on various supports.
Despite the higher initial activity of Fe–Cr catalytic systems
on oxide supports (see Figure S1), all these samples were
deactivated irreversibly in several hours and the selectivity for
ethylene did not exceed 40%, while on the FeCrOx/C catalyst it
was possible to reach 80% selectivity for the olefin with a
possibility of in situ regeneration in a stream of CO2. The data
on the catalyst reactivation are presented in Figure S3.
First, the C2H6/CO2 = 1:1 mixture was passed through the
activated catalyst for 80 min at 650 °C, then pure CO2 was
supplied for 10 min. After that, the cycle was repeated. In the
very beginning, ethane conversion had a maximum and
amounted to 20% but then decreased abruptly to 12%. This
type of behavior of the catalytic system may be due to the
reduction of FeII,III to Fe0 and fast carbonization of the active
centers. When the supply of ethane is stopped, CO2 would react
with carbon to give a stoichiometric amount of CO by the
Boudoir–Bell reaction [reaction (5)]. The catalyst surface is
reactivated and, upon subsequent supply of ethane, the
conversion again reaches 20%.
In summary, the ODE process with carbon dioxide carried out
on a FeCrOx/C catalyst allows one not only to achieve up to 80%
selectivity for ethylene but also reactivate the catalyst in situ in a
CO2 stream at the reaction temperature. After reactivation, the
catalyst shows stable selectivity and activity values for more
than one hour.
C2H6 ® C2H4 + H2
(1)
(2)
(3)
(4)
(5)
C2H6 + CO2 ® C2H4 + H2O + CO
C2H6 ® 2C + 3H2
C + H2O ® H2 + CO
C + CO2 ® 2CO
The authors are deeply grateful to Dr. P. A. Chernavsky
(M. V. Lomonosov Moscow State University) for his help in the
magnetometric studies.
Calculation of the equilibrium concentrations of hydrogen for
the mixture CO2/C2H6 = 1:1 at 750 °C (in the software package
HSC 4.0) confirms the experimental fact that the amount of
hydrogen evolved exceeds the amount of ethylene formed more
than twofold. The high content of carbon monoxide in the
products is explained by an increase in its concentration by
reactions (2), (4), (5), (6), (7), (8).
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2020.05.033.
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Bogdan, Victor I.
