Oxidative coupling of methane in the redox cyclic mode over the Ag-La2O3/SiO2 catalytic system

Article abstract of DOI:10.1016/j.mencom.2010.03.009

Synergism of Ag and La₂O₃ in the Ag-La₂O₃/SiO₂ catalytic system provides substantial efficiency in oxidative coupling of methane carried out in the redox cyclic mode. Selectivity to C₂ products can be raised by preliminary injection of small amount of hydrogen to the catalyst.

Full text of DOI:10.1016/j.mencom.2010.03.009

Mendeleev
Communications
Mendeleev Commun., 2010, 20, 92–94
Oxidative coupling of methane in the redox cyclic
mode over the Ag–La2O3/SiO2 catalytic system
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, Yusunggu, Daejeon 305-343, Republic of Korea. E-mail: thkim@kier.re.kr
DOI: 10.1016/j.mencom.2010.03.009
Synergism of Ag and La2O3 in the Ag–La2O3/SiO2 catalytic system provides substantial efficiency in oxidative coupling of
methane carried out in the redox cyclic mode. Selectivity to C2 products can be raised by preliminary injection of small amount
of hydrogen to the catalyst.
Oxidative coupling of methane
1. 2CH4 + [O] ® C2H6 + H2O
2. C2H6 + [O] ® C2H4 + H2O
A search for efficient catalysts for oxidative coupling of methane
(OCM) is the topical problem in the oxidation catalysis. Numerous
catalytic systems on the basis of the metal oxides often possess
low selectivity to the target products of OCM, in particular C2
hydrocarbons, mostly due to complete oxidation of methane as
well as C2 hydrocarbons into carbon oxides.
Application of the redox cyclic mode to OCM can be a solu-
tion to enhance OCM selectivity.1–9 In this case the catalyst
should contain mobile lattice oxygen that is able to leave its
location in the oxide lattice and then take part in oxidation
reactions on the catalyst surface.
Complete oxidation
3. CH4 + 4[O] ® CO2 + 2H2O
4. C2H6 + 7[O] ® 2CO2 + 3H2O
5. C2H4 + 6[O] ® 2CO2 + 2H2O
In the redox cyclic mode oxidation and reduction stages of
the process are performed in the reverse order. Figure 1 shows a
scheme of the gases feed used in the experiments.
In redox mode OCM partial oxidation of methane followed
by formation of C2 hydrocarbons as well as complete oxidation
are caused by interaction of methane molecules or intermediates
with oxygen species of the catalyst lattice. The lost of lattice
oxygen spent in the oxidation reactions is recovered by following
feed of air (or oxygen) over the catalyst. Thus, consecutive
pulses of air and methane to the catalyst give opportunity to
carry out OCM in the redox cyclic mode, i.e., when oxidation
and reduction on the catalyst are separated in time and proceed
independently.
During passing air through the catalyst bed the oxide lattice
of the catalyst fills its anion vacancies with oxygen. In the next
†
Oxidative coupling of methane was carried out in the quartz reactors
with a fixed catalyst bed mounted inside a cylindrical furnace. One reactor
was of 45 cm length, 5.6 mm i.d., another reactor was of 45 cm length,
3 mm i.d. Reaction temperatures were measured by a thermocouple that
was inserted in a jacket inside the reactor. The catalyst sample was placed
in the central part of the reactor. In order to minimize gas-phase reactions
outside the catalyst bed free space of the reactor was filled with quartz
chips or quartz insets.
Noteworthy, if a redox mode is used all oxidation reactions
occur in the absence of molecular oxygen in the gas phase, the
yield of CO2 becomes substantially lower. Under conven-
tional conditions of OCM carbon oxides are produced due to
participation of molecular oxygen. The literature data10–14 allow
one to suppose, that the catalysts on the basis of rare earths, in
particular, lanthanide oxides, are suitable for redox mode OCM.
Addition of metal component (e.g., silver) to the catalyst com-
position could also increase efficiency of OCM.15–17 Metallic
silver is known to have a good capacity to dissolve oxygen at
high temperatures. So, if OCM is carried out in the redox cyclic
mode silver can serve as a buffer for mobile oxygen left pre-
viously the oxide lattice. Additionally, silver crystallites could
store and keep a part of oxygen that is input into the reactor
before the pulse of methane.
In the present work OCM in the redox cyclic mode using
catalytic system on the basis of Ag and La2O3 supported on
SiO2 was studied.† Factors that influence the participation of
lattice oxygen in the OCM reaction and selectivity to C2
products were investigated thoroughly.
Major catalytic reactions that accompany OCM on the
catalyst surface and proceed with participation of lattice oxygen
can be presented as follows:
The temperature was ranged in the 700–800 °C interval.
A setup was destined for injection of alternative pulses of air and
methane into the reactor. Pulses of methane were injected into the carrier
gas flow with the syringes. The carrier gas was He.
Oxidation recovery of the catalyst was carried out by two methods:
(i) injecting of air pulse into the reactor; (ii) passing of inert gas flow
containing 0.04–0.05% oxygen over the catalyst bed. In the last case
oxidation recovery of the catalyst with loading of 300–500 mg took
10–12 min at 40 cm3 min–1 gas flow rate.
Injections of small quantity of hydrogen (0.02 ml) used in some runs
were done 30–60 s before the methane pulse.
At the outlet of the reactor reaction products were caught in the U-shape
trap at temperature of liquid nitrogen (–196 °C) and after rapid heating to
150–200 °C they were analyzed by GC on two chromatographic columns
packed with Polychrom and Zeolite CaA.
The catalyst samples prepared and tested in the work were as follows:
10% La2O3/SiO2; 3%Ag–10% La2O3/SiO2; 6% Ag–10% La2O3/SiO2;
15% Ag–10% La2O3/SiO2; 6% Ag/La2O3.
All samples were prepared by the incipient wetness impregnation method.
Grinded silica (KSK, Russia) with 0.25–0.50 mm or 0.09–0.2 mm particles
was used as a support. Silica was preliminary washed by a solution of
hydrochloric acid and dried. Prepared silica was impregnated with corre-
sponding water solutions of silver nitrate or lanthanum nitrate, or their
mixtures. Then the samples were dried at 120 °C for 2 h and calcined at
800 °C for 4 h.
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© 2010 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2010, 20, 92–94
100
80
60
40
20
0
Air
He CH4 He
OCM
Air
He CH4
OCM
1
Oxidation of
the catalyst
Oxidation of
the catalyst
2
3
GC analysis
Trapping
of products
GC analysis
Trapping
of products
4
Figure 1 The gas supply scheme in the OCM experiments.
step, the catalyst is blown up by the inert carrier gas (e.g., He)
for removing molecular oxygen from the reaction zone as well
as oxygen species weakly adsorbed on the catalyst surface. The
third step of the procedure is input of the methane pulse into
the carrier gas flow, and this is the OCM reaction itself. Then a
sequence of the gases feed is repeated.
0
2
4
6
8
10
Yield of C2 products (%)
Figure 2 Correlation between selectivity and the yield of C2 products for
OCM carried out in a redox cyclic mode in the runs with (1) 3% Ag–
10% La2O3/SiO2, (2) 6% Ag–10% La2O3/SiO2, (3) 6% Ag/SiO2 and (4)
10% La2O3/SiO2. Methane impulse is varied from 0.1 to 6 cm3; loading of
each catalyst, 300 mg; temperature, 750 °C; gas flow rate, 40 cm3 min–1
.
In the redox mode the reaction products are formed due to the
interaction of mobile lattice oxygen with methane molecules or
intermediate species produced on the catalyst surface. It is very
important that under these conditions either OCM or complete
oxidation occurs without participation of molecular oxygen.
Note that complete oxidation of methane requires oxygen
several times higher than oxidative activation of methane fol-
lowed by formation of methyl radicals (see equations above).
It is reasonable to consider that complete oxidation proceeds
with participation of unselective oxygen. The latter is charac-
terized by lower binding energy in the oxide lattice and thus is
rather available for methane molecules or intermediate species.
There is every reason to suppose that unselective oxygen is either
molecular oxygen adsorbed on the catalyst surface or oxygen
atoms located on the edges of the oxide lattice.
Oxidative generation of methyl radicals from methane mole-
cules followed by their coupling into C2 hydrocarbons probably
has to occur in the active sites characterized by low concentra-
tion of mobile oxygen, and should exhibit higher binding energy
in the lattice, e.g., on its planes. Additionally, formation of C2
hydrocarbons can take place in the locations where the OCM
reaction rate is limited by slow diffusion of lattice oxygen (or
oxygen dissolved in metallic silver) to outer surface of the catalyst.
This can let methyl radicals to stay on the active surface for a
relatively long time without next interaction with oxygen species.
In turn, it results in increase in opportunity for dimerization
(coupling) of methyl radicals into C2 product.
It is reasonable to suppose that decrease in the concentra-
tion of weakly bound oxygen (i.e., unselective oxygen) in the
catalyst has to result in increase in selectivity to C2 hydro-
carbons. Lowering of percentage of unselective oxygen can be
achieved by passing small amount of hydrogen over the catalyst.
In order to study the influence of silver constituent on cata-
lytic properties of the Ag–La2O3/SiO2 system, such samples as
10% La2O3/SiO2, 3% Ag–10% La2O3/SiO2, 6% Ag–10% La2O3/
SiO2 and 6% Ag/SiO2 were prepared and tested in OCM. The
runs with these catalysts were carried out under the identical
conditions. Figure 2 presents results of these experiments. To
compare the samples in their OCM efficiency, the correlation
between C2 selectivity and the yield of the C2 products was used.
According to the results such catalysts as 10% La2O3/SiO2,
3% Ag–10% La2O3/SiO2 and 6% Ag–10% La2O3/SiO2 allow one
to reach OCM selectivity close to 80–90%. But it takes place just
when methane conversion is relatively low: less than 0.5% for
10% La2O3/SiO2, and less than 4% for 3% Ag–10% La2O3/SiO2
and 6% Ag–10% La2O3/SiO2.
It is important to emphasize that combined action of Ag and
La2O3 supported on silica enhances both OCM activity and
selectivity to the C2 products. It means that there is a considerable
synergetic effect in this case. The curves for 3% Ag–10% La2O3/
SiO2 and 6% Ag–10% La2O3/SiO2 samples are located above
the corresponding curves for 10% La2O3/SiO2 and 6% Ag/SiO2.
Joint action of Ag and La2O3 makes it possible to get the yield
of C2 products of about 9% when OCM selectivity is close to
60%. These values of the C2 yield as well as OCM selectivity
are not available on either 10% La2O3/SiO2 or 6% Ag/SiO2
catalysts. Increase in silver percentage from 3% to 6% in the
Ag–La2O3/SiO2 catalyst system does not practically have effect
on the OCM efficiency. It was noticed that total amount of
lattice oxygen taking part in oxidation reactions grows if a
methane pulse injected into the reactor is increased.
It was found that addition of a small amount of air to the
methane pulse resulted in a sharply drop of OCM selectivity. In
particular, if 1.6 cm3 mixture consisted of the equal quantities
of methane and air was input over the 6% Ag–10% La2O3/SiO2
catalyst OCM selectivity was about 40%, while in the runs with
0.8 cm3 methane pulses without air additive OCM selectivity could
reach 86%. These results prove that the presence of molecular
oxygen in the reaction zone promotes complete oxidation of light
hydrocarbons to an essential extent. Thus, molecular oxygen can
be considered to be responsible for decrease in OCM selectivity.
The 6% Ag–10% La2O3/SiO2 catalyst was characterized by
XRD. XRD pattern of the sample in the 2q range from 20 to 85°
has five diffraction maxima. The most intense reflections were
observed at 2q = 38.14, 44.32, 64.44 and 77.42°. These peaks
correspond to the reflections from the different planes Ag crys-
tallites. It means that Ag is in the reduced metallic state. Any
peaks associated with the crystalline phase La2O3 were not found.
It allows one to consider that in our sample the La2O3 oxide
supported on silica is amorphous and does not form definite
crystallite phase which could give corresponding XRD peaks.
Temperature in the interval of 700–800 °C was found to be
optimal for OCM proceeding on the catalysts studied. The results
showed that the amount of lattice oxygen participating in catalytic
reactions lowers with the temperature growth.
Decrease in the gas flow rate does not increase the yield
of C2 hydrocarbons probably because of dilution of injected
methane with helium in the reactor volume.
It was found that during starting pulses of methane to the
3% Ag–10% La2O3/SiO2 catalyst the latter enhances its activity
in OCM, and the yield of C2 hydrocarbons becomes several
times higher (Figure 3).
It is obvious that training of the catalyst sample occurs in the
starting pulses of methane. It can be related with reduction of
Ag crystallites followed by formation of the active sites that are
Noteworthy, catalytic properties of 3% Ag–10% La2O3/SiO2
and 6% Ag–10% La2O3/SiO2 are very close to each other. Catalytic
activity in OCM as well as selectivity to C2 products of these
catalysts exceed essentially those of two other samples, in which
either La2O3 or Ag alone is supported on silica.
– 93 –

Mendeleev Commun., 2010, 20, 92–94
between silver and La2O3. In this case silver is a carrier of
3
2
1
0
mobile oxygen from La2O3 to methane. Meantime, the data
evidence that complete oxidation proceeds with participation of
nonselective oxygen.
In summary, the Ag–La2O3/SiO2 system is favourable for
carrying out OCM in the redox cyclic mode. This system pos-
sesses lattice oxygen capable of leaving the crystal lattice and
participating in oxidation reactions accompanying OCM. The
lost of mobile oxygen spent in oxidation reactions is recovered
by next feed of air to the catalyst.
1
2
0
2
4
6
8
10
12
Number of methane impulses
Combination of Ag and La2O3 in the catalyst results in
essential synergetic effect and the OCM efficiency. The active
sites being responsible for OCM are generated during silver
reduction with methane or hydrogen. At the same time proper
catalyst reduction increases OCM selectivity. In our opinion, a
silver phase is a buffer transferring mobile oxygen from lanthana
to methane molecules. In order to achieve high OCM selectivity,
it is required to keep reduced state of the catalyst. Rather high
efficiency in OCM performed in the redox mode is exhibited
by the 3% Ag–10% La2O3/SiO2 catalyst. It allows one to reach
the 30% yield of C2 products with OCM selectivity being close
to 65–70%.
Figure 3 Variation of the yield of C2 products and CO2 in the starting
pulses of methane during OCM over the 3% Ag–10% La2O3/SiO2 catalyst.
Loading of the catalyst, 300 mg; temperature, 750 °C; methane pulse,
0.8 cm3; gas flow rate, 40 cm3 min–1
.
responsible for OCM proceeding. On the contrary, the yield of
CO2 firstly lowers and then remains at the about constant level,
i.e., the rate of complete oxidation does not change.
There was every reason to consider that partial reduction of
the Ag–La2O3/SiO2 catalyst before methane pulse has to remove
a part of unselective oxygen from the catalyst and thus to cause
enhance in OCM selectivity. In order to check this supposition a
series of the runs with preliminary injections of small amounts
of hydrogen (0.02 cm3) were carried out (Figure 4). In these
runs the 3 mm i.d. reactor was used.
This work was supported by Korea Institute of Energy
Research.
It is clear that preliminary reduction of the catalyst with
hydrogen increases reaction selectivity to the C2 products. How-
ever, if the quantity of injected hydrogen is relatively large there
is a drop of the catalytic activity in both OCM and complete
oxidation, whereas OCM selectivity remains unchanged. It is
obvious that large amount of hydrogen is able to remove simul-
taneously both unselective and selective oxygen species. There
is likely an optimal amount of hydrogen injected into the catalyst
that is capable of enhancing OCM selectivity.
In general, the catalyst developed can provide 30% yield of
C2 hydrocarbons with 60% OCM selectivity. However, when
the yields of C2 hydrocarbons are lower than 5%, the selectivity
can reach 80% and above.
References
1
2
3
V. Salehoun, A. Khodadadi, Y. Mortazavy and A. Talebizadeh, Chem.
Eng. Sci., 2008, 63, 4910.
N. Ya. Usachev, E. P. Belanova and G. A. Shirokova, Neftekhim., 1998,
38, 438 [Petrol. Chem. (Engl. Transl.), 1998, 38, 435].
A. M. Gaffney, C. A. Jones, J. J. Leonard and J. A. Sofranko, J. Catal.,
1988, 114, 422.
4
5
6
J. A. Sofranko, J. J. Leonard and C. A. Jones, J. Catal., 1987, 103, 302.
K. Otsuka and A.-A. Said, Inorg. Chim. Acta, 1987, 132, 123.
X. Dai, C.-C. Yu, R.-J. Li and Z. Hao, Stud. Surf. Sci. Catal., 2007,
167, 391.
7
8
9
R.-J. Li, C.-C. Yu, W.-J. Ji and S.-K. Shen, Stud. Surf. Sci. Catal.,
2004, 147, 199.
Y. Wei, H. Wang, F. He, X. Ao and C. Zhang, J. Nat. Gas Chem., 2007,
16 (1), 6.
K. Otsuka, Y. Wang, E. Sunada and I. Yamanaka, J. Catal., 1998, 175,
152.
The experiments confirm that formation of the C2 products
(i.e., coupling of methyl radicals) occurs on the active sites that
are located either on the silver crystallites or on the boundary
10 A. G. Dedov, A. S. Loktev, I. I. Moiseev, A. Aboukais, J.-F. Lamonier
and I. N. Filimonov, Appl. Catal. A: Gen., 2003, 245, 209.
11 J. Wang, L. Chou, B. Zhang, H. Song, J. Yang, J. Zhao and S. Li, Catal.
Commun., 2006, 7, 59.
100
2
80
12 S. Kus, M. Otremba and M. Taniewski, Fuel, 2003, 82, 1331.
13 E. M. Kennedy and N. W. Cant, Appl. Catal., 1992, 87, 171.
14 E. M. Kennedy and N. W. Cant, Appl. Catal., 1991, 75, 321.
15 A. J. Nagy, G. Mestl, D. Herein, G. Weinberg, E. Kitzelmann and
R. Schlogl, J. Catal., 1999, 182, 417.
60
1
40
20
0
16 A. J. Nagy, G. Mestl and R. Schlogl, J. Catal., 1999, 188, 58.
17 A. J. Nagy and G. Mestl, Appl. Catal. A: Gen., 1999, 188, 337.
0
5
10
15
20
25
Yield of C2 products (%)
Figure 4 Correlation between selectivity and the yield of C2 products for
OCM in the redox cyclic mode over the 3% Ag–10% La2O3/SiO2 catalyst:
(1) without preliminary injection of H2, (2) with preliminary injection of
0.02 cm3 H2. Methane pulse is varied from 0.1 to 3 cm3; loading of the
catalyst, 300 mg; fraction of the catalyst particles, 0.09–0.20 mm; tempera-
ture, 750 °C; gas flow rate, 40 cm3 min–1
.
Received: 20th October 2009; Com. 09/3407
– 94 –