Oxidative coupling of methane in the redox cyclic mode over the catalysts on the basis of CeO2 and La2O3

Article abstract of DOI:10.1016/j.mencom.2010.01.011

The 1% CeO₂, 9% La₂O₃/SiO₂ and 2% CeO₂, 8% La₂O₃/SiO₂ catalysts show reliable efficiency in the OCM reaction, as well as stable work in the redox cyclic mode. Selectivity to C₂ products remarkably increases if preliminary reduction of the catalyst by a small amount of hydrogen is used.

Full text of DOI:10.1016/j.mencom.2010.01.011

Mendeleev
Communications
Mendeleev Commun., 2010, 20, 28–30
Oxidative coupling of methane in the redox cyclic mode
over the catalysts on the basis of CeO2 and La2O3
Alexander A. Greish,*a Lev M. Glukhov,a Elena D. Finashina,a
Leonid M. Kustov,a Jae-Suk Sung,b Ko-Yeon Choob and Tae-Hwan Kimb
a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 499 135 5328; e-mail: greish@ioc.ac.ru
b Korea Institute of Energy Research, 71-2 Jangdong, Yusunggu, Daejeon 305-343, Republic of Korea.
E-mail: jssung@kier.re.kr
DOI: 10.1016/j.mencom.2010.01.011
The 1% CeO2, 9% La2O3/SiO2 and 2% CeO2, 8% La2O3/SiO2 catalysts show reliable efficiency in the OCM reaction, as well as
stable work in the redox cyclic mode. Selectivity to C2 products remarkably increases if preliminary reduction of the catalyst by a
small amount of hydrogen is used.
Oxidative coupling of methane (OCM) is one of the challenges
in natural gas processing. A search for efficient oxidation
catalysts for the OCM process remains the topical problem in
the oxidation catalysis. Now, many attempts to increase the
efficiency of this process are made. One of the ways to enhance
the OCM selectivity consists in alternating feeding of methane
and oxygen to the catalyst. Earlier, this approach has been
applied both for OCM1–5 and other processes.6–9
If OCM occurs over an oxide catalyst, the lattice of the
catalyst can supply its oxygen for oxidative conversions of
methane accompanying OCM. Then, formation of the reaction
products, such as C2 hydrocarbons and CO2, proceeds from the
interaction of methane molecules with lattice oxygen. As a
result, reduction of the oxygen content in the catalyst composi-
tion takes place. The lost of lattice oxygen in the catalyst is
recovered by the following re-oxidation procedure. Thus, using
consecutive reduction and oxidation stages in OCM allows one
to carry out this process in the so-called redox cyclic mode. The
main feature of this mode is that oxidation recovery of the
catalyst and oxidative methane conversions are separated in
time and are performed in the alternating flows.
The absence of molecular oxygen in the reaction zone is
favourable for lowering the yield of carbon dioxide that is a
product of complete oxidation of both methane and C2 products
produced in OCM. As a result, the OCM selectivity to C2
products can be enhanced.
Earlier, it was found that the Mn–Na2WO4/SiO2 catalyst is
rather suitable for OCM performed in the redox cyclic mode.1,2
The main reason is that the Mn–Na2WO4/SiO2 catalyst includes
two transient metals that are capable to take part in reduction–
oxidation reactions due to changing their valency.
to study the opportunity of using similar catalysts for carrying
out the OCM process in the redox cyclic mode. For our study, a
series of catalysts on the basis of La2O3 and CeO2 supported on
silica have been prepared.
Investigations were focused on the study of the factors that
effect the contributions of oxidative reactions resulting, on one
hand, in the formation of C2 hydrocarbons and, on the other hand,
in the formation of CO2. Also there was the intention to find
the proper reaction conditions that would be most suitable for
increasing the OCM selectivity.†
To perform the redox cyclic testing of the catalysts in the OCM
process, air and methane gases are injected into the reactor as
separate pulses in the inert gas flow (Figure 1). In this case,
methane passes over the previously oxidized catalyst bed in the
†
The experimental setup for the study of the OCM reaction in a redox
cyclic mode made it possible to inject alternating pulses of air and
methane into the catalytic reactor, followed by GC analysis of the reac-
tion products.
The carrier gas was He. Air and methane pulses were injected by
syringes. Methane pulses were introduced to the oxidized catalyst. After
the reaction products were removed from the reactor, the catalyst was re-
oxidized by the air pulse.
The amount of oxygen required for recovery of the catalyst after OCM
can be introduced by feeding the inert gas (He) containing 0.04–0.05%
of oxygen to the catalyst bed for 10–15 min.
The catalytic reactor was a quartz tube with 5.6 mm or 2.5 mm i.d. and
45 cm length. To minimize the free space in the reactor, quartz chips or
insets were used. This allowed us to reduce an opportunity of gas-phase
reactions in the space out of the catalyst bed.
At the outlet of the reactor, the reaction products were collected in a
U-shape trap at the temperature of liquid nitrogen (–196 °C), and after fast
heating to 150–200 °C they were analyzed by gas chromatography. GC
analysis was performed on two chromatographic columns packed with
Polychrom and Zeolite CaA. The temperature range of the process was
700–850 °C.
The following catalyst samples were prepared and tested in this work:
10% La2O3/SiO2; 1% CeO2, 9% La2O3/SiO2; 2% CeO2, 8% La2O3/SiO2;
10% CeO2, 90% La2O3 (bulk).
All supported catalysts were prepared by impregnation of grinded silica
with a particle size 0.25–0.5 mm with appropriate metal nitrates solu-
tions followed by drying at 120 °C and calcination in dry air at 800 °C.
The bulk CeO2–La2O3 catalyst was prepared by co-precipitation of
oxalates of La and Ce using dropwise addition of an oxalic acid solution
to a solution of La(NO3)3 and Ce(NO3)3. Then the sample was dried at
120 °C and calcined at 800 °C.
The goal of this work was searching the catalysts for the OCM
process carried out in the redox cyclic mode. This mode can
be realized if the catalyst consists of a metal oxide that provides
its lattice oxygen for oxidative activation of methane mole-
cules followed by cleavage of C–H bonds. In this case, the
OCM efficiency must depend on the dynamics of oxygen supply
from the oxide lattice for oxidation conversions on the catalyst
surface.
Recently,10–14 the catalytic properties of rare earth element
oxides in the OCM reaction were studied. It was shown that
the catalysts on the basis of La2O3 and CeO2 oxides and their
compositions exhibited high activity in this reaction. We decided
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© 2010 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2010, 20, 28–30
100
Air
He CH4 He
OCM
Air
He CH4
5
80
60
40
20
Oxidation of
the catalyst
Oxidation of
the catalyst
OCM
4
2
Figure 1 The gas supply in the OCM experiments.
absence of molecular oxygen in the gas phase. After reaction
products were removed from the reactor, the catalyst was re-
oxidized by introducing an air pulse. The amount of air needed
for entire re-oxidation (recovery) of the catalyst depends on the
amount of oxygen lost by the catalyst during oxidative conver-
sions of methane.
1
3
0
2
4
6
8
10
Yield of C2 products (%)
Figure 2 Correlation between OCM selectivity and the yield of C2
products for the catalysts tested in the redox cyclic mode at 800 °C. Loading
of each catalyst, 300 mg; methane pulse is changed from 0.1 to 6 cm3;
gas flow rate, 40 cm3 min–1. (1) 10% CeO2/SiO2; (2) 10% La2O3/SiO2;
(3) 10% CeO2–90% La2O3 (bulk); (4) 1% CeO2, 9% La2O3/SiO2 and
(5) 2% CeO2, 8% La2O3/SiO2.
In the absence of molecular oxygen in the reaction zone, the
lattice of the oxide catalyst delivers oxygen atoms to activate
methane molecules for the OCM reaction. In this case, methyl
radicals are produced from methane molecules due to their
interaction with mobile oxygen atoms of the catalyst lattice.
Certainly, at high temperatures of the process a part of lattice
oxygen can be removed from the catalyst and takes part in
oxidation reactions and thus in the following formation of all
reaction products. The lost of lattice oxygen spent in oxidation
reactions is recovered by re-oxidation of the catalyst. The pro-
cesses of oxygen evolution from metal oxide and next recovery
of anion vacancies appeared after oxidation reactions can be
presented in a general way by the following equations:
cedure the catalyst recovers entirely its activity. The data allow
us to conclude that the CeO2/SiO2 catalyst is not suitable for
OCM carried out in a redox cyclic mode.
It was decided to lower remarkably the content of CeO2 in
the catalyst composition and replace it with such an oxide as
La2O3. The latter is known to possess less pronounced oxida-
tion properties than CeO2. The maximum positive valency of
Ce is 4+, while that of La is only 3+. The total molar amount
of the rare earth element oxide in the catalyst composition was
the same, i.e., 10%. Such samples as 1% CeO2, 9% La2O3/SiO2
and 2% CeO2, 8% La2O3/SiO2 were prepared and tested also in
the redox cyclic mode of OCM. The data show that these
catalysts are also able to supply their lattice oxygen for oxidative
conversions of methane (Figure 2, plots 4, 5). Moreover, the 1%
CeO2, 9% La2O3/SiO2 and 2% CeO2, 8% La2O3/SiO2 catalysts
exhibit proper oxidative properties and high stability. With the
use of these catalysts it is possible to approach the OCM
selectivity higher than 85–90%, but in the case when the yield
of C2 products does not exceed 2–3% (Figure 2, plots 4, 5).
The amount of lattice oxygen of these catalysts taking part in
oxidation reactions increases with reaction temperature, likely
due to increasing oxygen mobility in the oxide lattice at high
temperatures. According to experimental data, when injecting
of 3 ml methane pulses, the increase in temperature from 700
to 800 °C results in about double increase in the amount of
converted methane, from 0.8 to 1.7 μmol. Meantime, in the
800–850 °C temperature range, the quantity of mobile oxygen
changes insignificantly.
Me2O3 ® Me2O3 – x + x[O]surf.
;
Me2O3 – x + x[O]gas ® Me2O3.
Participation of lattice oxygen in the oxidation reactions accom-
panying OCM on the catalyst surface can be presented as follows:
Coupling of methane
2CH4 + [O] ® C2H6 + H2O;
Partial oxidation of ethane
C2H6 + [O] ® C2H4 + H2O.
Noteworthy the amount of oxygen required for conversion of
methane to CO2 (complete oxidation) exceeds several times
the amount consumed in the reactions of formation of such
products as ethylene or ethane. For example, the reaction of
complete oxidation of methane demands four times more oxygen
than oxidative activation of methane molecules followed by the
formation of ethane.
Taking this into account, it is very important to know the
factors that can change the ratios of presented oxidation reac-
tions in favour of C2 hydrocarbons formation. It is most likely
that the formation of CO2 is related with the participation of
mobile oxygen which has a lower binding energy in the oxide
lattice, i.e. with nonselective oxygen.
Note that the total amount of formed CO2 practically does
not depend on the volume of the methane pulse. It allows us to
suppose that in the catalyst there is a fixed quantity of non-
selective oxygen that takes part in complete oxidation.
It is well known that CeO2 oxide can be a supplier of lattice
oxygen for different oxidation reactions, e.g., oxidation of light
hydrocarbons. Its lattice oxygen is rather mobile. There was
intension to check the properties of CeO2 as an oxygen supplier
for the OCM reaction carried out in the redox cyclic mode.
Therefore, the 10% CeO2/SiO2 catalyst was chosen as the
initial sample for investigation of main features of OCM carried
out in a redox cyclic mode. The use of the CeO2/SiO2 catalyst
in OCM was expected to result in relatively high yields of C2
hydrocarbons.
The very first experiments with the CeO2/SiO2 catalyst showed
that this catalyst possesses a certain amount of mobile lattice
oxygen that takes part in both methane conversion to C2 and
CO2 (Figure 2, plot 1). But this catalyst exhibits a very low
selectivity in OCM because the complete oxidation of methane
to CO2 prevails, although after the following re-oxidation pro-
In most of the runs, the ethylene portion in the C2 products
does not exceed 45–50%. This indicates that a relatively small
amount of lattice oxygen participates in partial oxidation of ethane.
The data obtained lead to a conclusion that in the catalyst
there are two types of lattice oxygen delivered for such oxida-
tion reactions as OCM and complete oxidation. These types of
oxygen can be considered as selective and nonselective oxygen.
Selective oxygen takes part in OCM only, while nonselective
oxygen participates in complete oxidation and partial oxida-
tion of ethane to ethylene. These types of oxygen differ by the
binding energy in the oxide lattice. Selective oxygen is charac-
terized by a larger binding energy than nonselective oxygen.
Dilution of injected methane with helium results in a decrease
in the OCM selectivity and increase in the yield of CO2. The
1% CeO2, 9% La2O3/SiO2 catalyst starts to work stable after a
few pulses of methane, i.e., its surface must be somewhat reduced.
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Mendeleev Commun., 2010, 20, 28–30
In comparison with the supported catalyst, the bulk sample
100
80
60
40
20
2
10% CeO2–90% La2O3 was prepared and tested (Figure 2,
plot 3). It was found that the presence of CeO2 in the bulk
10% CeO2–90% La2O3 catalyst results in a sharp drop of the
selectivity to C2 products, it was about 15–25% at 1% yield of
C2 products. There is a large slope in the dependence of the
OCM selectivity on the yield of C2 products. When the amount
of injected methane is reduced, there is a sharp drop of the
OCM selectivity. It is obvious that lattice oxygen in the CeO2
oxide is rather mobile and relatively easy leaves the crystal lattice
for participation in oxidation reactions. However, one should
take into account that the bulk density of the bulk 10% CeO2–
90% La2O3 catalyst exceeds essentially that of the supported
1% CeO2, 9% La2O3/SiO2 catalyst. It means that at equal loadings
of the bulk and supported catalysts, the accessible surface area
of the first catalyst is remarkably lower than that of the second one.
In any case, one can consider that the bulk catalyst is inferior to
the supported sample in its activity in OCM.
1
0
2
4
6
8
10
Yield of C2 products (%)
Figure 3 Correlation between the OCM selectivity and the yield of C2
products for the 2% CeO2, 8% La2O3/SiO2 catalyst at 800 °C in the reactor
of 2.5 mm i.d. Loading of the catalyst, 300 mg; methane pulse is changed
from 0.1 to 6 cm3; gas flow rate, 40 cm3 min–1. (1) Without injecting of
hydrogen; (2) with preliminary injecting of hydrogen.
Extrapolation of the curve 2 in Figure 3 to the larger values
of the C2 yield allows one to consider that under our conditions
the 2% CeO2, 8% La2O3/SiO2 catalyst is able to give the yield
of C2 hydrocarbons at the level of 30% when the OCM selec-
tivity is close to 65–70%.
Thus, the data show that the catalysts consisted of the La2O3
and CeO2 oxides provide the opportunity to carry out the OCM
process in the redox cyclic mode. A general feature of this
process is the supply of lattice oxygen of the catalyst for oxida-
tion reactions proceeding on the catalyst surface.
The best results are found on 1% CeO2, 9% La2O3/SiO2 and
2% CeO2, 8% La2O3/SiO2 catalysts, in particular when the
process is carried out in the narrow catalytic reactor and with
preliminary injection of a small amount of hydrogen. Prelimi-
nary reduction of the catalyst by hydrogen decreases the surface
concentration of non-selective oxygen. This results in an increase
in the selectivity to C2 products.
It was interesting to check the catalytic activity of La2O3
supported on SiO2, i.e. La2O3/SiO2.
Some results obtained in the runs with the 10% La2O3/SiO2
catalyst are presented in Figure 2 (plot 2). It can be noticed that
the 10% La2O3/SiO2 catalyst allows one to reach a 80–90%
selectivity to C2 products but when the methane conversion to
C2 hydrocarbons is lower than 1%. Injection of small pulses of
methane at 800 °C gives the yield of C2 hydrocarbons no more
than 2%. Increasing the reaction temperature leads to a sharp
drop of the catalyst productivity in oxidation reactions.
Thus, introduction of a small additive of CeO2 to La2O3/SiO2
system allows one to get a catalyst that exhibits a higher selec-
tivity in OCM than initial La2O3/SiO2. A plot of the selectivity
vs. the C2 yield becomes more flat, i.e., with a less pronounced
slope.
According to our results, the catalysts with a mixture of
La2O3 and CeO2 oxides taken in ratios 9:1 or 8:2 make it possible
to reach a relatively high selectivity to C2 products in OCM
performed in the redox cyclic mode. These values of the OCM
selectivity are obtained when we use small pulses of methane.
In this case, a large part of methane undergoes complete oxida-
tion and is converted into CO2. C2 products formed previously
in OCM can burn also giving an additional amount of CO2 but
the contribution of the former process in the total formation of
CO2 is more remarkable. One reason of the foreground burning
of methane under these conditions may be dilution of the methane
pulse with inert gas. Really, OCM is known to be a reaction of a
second order. Therefore, a decrease in the partial pressure of
methane in the reaction zone results in a decrease of the
possibility of OCM and corresponding lowering of the C2 yield.
In order to decrease the dilution of methane pulse with He, it
was decided to use the reactor with a diameter of 2.5 mm.
Our expectations were met. The dependence of the OCM selec-
tivity on the yield of C2 products became more flat (Figure 3).
The catalyst shows a rather good stability.
We thank Korea Institute of Energy Research for financial
support of this work.
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It is a very important result. This confirms once more that in
the catalyst sample there are two types of mobile lattice oxygen,
i.e. weakly and strongly bound oxygen atoms. Injection of a
small amount of hydrogen directly before the methane pulse
reduces the content of weakly bound oxygen that leads to an
increase in the OCM selectivity.
Received: 7th September 2009; Com. 09/3389
– 30 –