Unusual ceria promoting effect on oxidative methane coupling catalysts

Full text of DOI:10.1023/A:1012388309222

Doklady Chemistry, Vol. 380, Nos. 4–6, 2001, pp. 301–304. Translated from Doklady Akademii Nauk, Vol. 380, No. 6, 2001, pp. 791–794.
Original Russian Text Copyright © 2001 by Dedov, Loktev, Men’shchikov, Kartasheva, Parkhomenko, Moiseev.
CHEMICAL
TECHNOLOGY
Unusual Ceria Promoting Effect
on Oxidative Methane Coupling Catalysts
A. G. Dedov*, A. S. Loktev*, V. A. Men’shchikov**, M. N. Kartasheva*,
K. V. Parkhomenko*, and Academician I. I. Moiseev***
Received August 1, 2001
Catalytic oxidative methane coupling is a promising and the yield of deep oxidation products. Contrary to
direct method of converting methane to ethylene [1]. Of these expectations, our experiments showed that addi-
the numerous oxidative methane coupling catalysts, the tion of ceria to catalysts to a concentration of 10 wt %
most efficient are rare earths [2–4], such as samaria, lan- of the total weight of rare earths caused a synergistic
thana, and neodymia, whereas ceria and praseodymia are effect, thereby increasing the efficiency of OCM cata-
inefficient in oxidative coupling of methane (OCM) and lysts.
catalyze predominantly deep methane oxidation to form
The effect produced by ceria addition can be dem-
onstrated by comparing the efficiencies of two catalyst
samples, one of which comprised 90 wt % lanthana and
10 wt % ceria, and the other consisted of 90 wt % lan-
thana and 10 wt % neodymia. Neodymia is as active in
OCM as lanthana. Figure 1 presents the results of OCM
over these catalysts at various temperatures. One can
see that the catalyst containing ceria instead of
neodymia ensured higher methane conversion (curve 1)
and a higher yield of ë2+ products (curve 3) than the
lanthana–neodymia catalyst (curves 2 and 4, respec-
tively). This ratio between the catalyst characteristics
persisted throughout the temperature range studied.
Thus, ceria unexpectedly proved to be a more efficient
promoter for lanthana than neodymia. The catalytic
activities of the mixtures of rare earths in OCM turned
carbon oxides.
In this study, we, for the first time, revealed the pro-
moting effect of ceria on lanthana, which manifested
itself in an increase not only in methane conversion but
also in the selectivity of formation of ë2+ products
(ethane, ethylene, propane, propylene) during OCM.
OCM was explored in a quartz reactor, 11 mm inner
diameter, with a pocket 8 mm in diameter for a thermo-
couple (which left an annular gap 1.5 mm in width) at
atmospheric pressure using methane–air mixtures. The
catalyst weight was 0.2 g, the particle size was 0.5–
1 mm, and the catalyst bed height was 2–3 mm. As cata-
lysts, we applied mixtures of rare earths, which were
obtained by calcining oxalates coprecipitated from
solutions of nitrates. In addition, we used oxides depos-
ited onto molten magnesia (periclase); for producing
such catalysts, periclase particles were wetted with a
solution of rare-earth metal nitrates of the necessary
composition and were then dried and calcined. The
experimental temperature was varied from 700 to
900°ë, and the gas flow rate was changed from 40 to
60 L per g catalyst per h. The conversion and the selec-
tivity were calculated from gas-chromatographic data
(with a relative error of 12%).
%
1
2
3
4
34
30
26
22
18
14
10
1
3
The available data [3, 4] on the high activity of ceria
in deep methane oxidation led us to expect the addition
of some amount of ceria to lanthana and other rare
earths to lead to an increase in both methane conversion
2
4
700 720 740 760 780 800 820 840 860
T, °C
* Gubkin Russian State University of Oil and Gas,
Leninskii pr. 65, Moscow, GSP-1 119991 Russia
** OAO VNIIOS, ul. Radio 12, Moscow, 107005 Russia
*** Kurnakov Institute of General and Inorganic Chemistry,
Russian Academy of Sciences, Leninskii pr. 31,
Moscow, 119991 Russia
Fig. 1. (1, 2) Methane conversion and (3, 4) yield of ë
2+
hydrocarbons in OCM over (1, 3) lanthana–ceria and (2, 4)
lanthana–neodymia catalysts vs. temperature at gas mixture
flow rates of 44–49 L per g catalyst per h and ëç -to-é
4
2
ratios of 4.9–5.4, as in Fig. 2.
0012-5008/01/0010-0301$25.00 © 2001 åÄIä “Nauka /Interperiodica”

302
DEDOV et al.
g per g catalyst per h
Note that the promoting effect of the addition of
ceria to lanthana in OCM was observed not only for cat-
alysts that were mixtures of rare earths but also for cat-
alysts obtained by depositing active components onto
an inert support. As such a support, we chose molten
magnesia (periclase), which has a high mechanical and
thermal durability.
1
2
1
2
3.5
3.0
2.5
2.0
1.5
1.0
0.5
The table lists the results of OCM at various tem-
peratures over two catalysts of this type: one was a
mixture of 0.9 wt % lanthana and 0.1 wt % ceria
deposited onto molten magnesia; the other, 1 wt %
lanthana deposited onto the same support. The table
shows that the total yield of OCM products over the
0
760 780 800 820 840 860
700 720 740
T, °C La2O3/MgO catalyst was lower than that over the
(La2O3 + ëÂé2)/MgO catalyst throughout the temper-
ature range studied. Consequently, both unsupported
and supported catalysts supplemented with ceria
ensured a higher yield of the desired products.
Fig. 2. Output of ë hydrocarbons in OCM over (1) lan-
2+
thana–ceria and (2) lanthana–neodymia catalysts at gas
mixture flow rates of 44–49 L per g catalyst per h, ëç -to-
4
é ratios of 4.9–5.4, and various temperatures.
2
The (La2O3 + ëÂé2)/MgO catalyst, which had a
high mechanical strength and contained a small
out to be unequal to the sums of the activities of their
constituents. The resultant effect is likely to have a
more complex character and be determined by the
interactions of the catalyst components both with the
reactants and with each other.
amount of rare earths, was tested on a larger scale.
Tests were performed on a continuous-flow setup
consisting of a reactor and a cooling separator. The
reactor was a vertical quartz tube with an inner diam-
eter of 48 mm. A thermocouple pocket 6 mm in diam-
eter was located along the longitudinal axis of the
reactor. A catalyst bed 0.1 L in volume with a particle
size of 1–2 mm was placed in the central part of the
reactor and rested on a quartz glass cushion. The
reactor was equipped with two electric heaters,
whose power was regulated by two autotransformers.
The lanthana–ceria catalyst had a higher selectivity
and led to a higher methane conversion than the lan-
thana–neodymia catalyst; therefore, the lanthana–ceria
catalyst ensured a higher output of ë2+ products, which
reached quite high values (Fig. 2).
Effect of temperature on the yield of the C2+ products of OCM over rare earths deposited onto magnesia at gas mixture flow
rates of 53–57 L per g catalyst per h and at CH4-to-O2 molar ratios of 4.0–5.1
Formation selectivity, %
C2H6
Rate of
CH4
conversion, %
conversion of
CH4 into C2+,
mmol/(min g)
No.
T, °C
C2+ yield, %
C2H4
COx
(La2O3 + CeO2)/MgO
1
2
3
4
795
815
825
850
14.1
19.8
13.4
22.3
38.9
36.1
47.2
48.0
23.3
19.2
25.3
16.9
32.4
25.4
19.1
27.6
9.5
14.8
10.8
16.1
1.957
2.803
2.111
3.156
La2O3/MgO
5
6
7
8
795
815
825
850
11.2
16.3
17.5
21.3
21.0
45.1
39.3
43.7
24.1
26.0
13.9
14.8
55.0
28.9
46.8
41.5
5.0
11.6
9.3
0.948
2.136
1.679
2.220
12.4
DOKLADY CHEMISTRY Vol. 380 Nos. 4–6 2001

UNUSUAL CERIA PROMOTING EFFECT
303
The upper heater was mounted over the catalyst bed catalysts. In lanthana, which has a hexagonal lattice, the
and served to heat the initial gas mixture fed from the concentration of oxygen vacancies is much lower. In
top. The lower heater encompassed the catalyst bed the La2O3–CeO2 system [6], at a lanthana concentration
and was used mainly for heating the catalyst when
starting the reactor.
to 66 mol %, a solid solution having a fluorite-type
structure forms and at a lanthana concentration above
69 mol %, a solid solution having a body-centered
cubic structure and pure lanthana coexist; at high tem-
peratures, the latter has a hexagonal structure. Hence,
under the conditions of performing OCM, the forma-
tion of a lanthana–ceria solid solution having a cubic
structure can give rise to additional active sites for
OCM. In the lanthana–neodymia system, solid solu-
tions having only a hexagonal structure were detected
[6], which should not favor an increase in activity in
OCM.
The initial gas mixture used was natural gas from a
domestic gas supply network, which contained
98.7 vol % of methane (and also admixtures of N2,
C2H6, and CO2) and was fed into the apparatus
through a compressor. The oxidant used was industrial
oxygen. The gas mixture began to be introduced once
the catalyst bed temperature reached 600–650°ë.
After the temperature started intensely increasing,
which indicated that the reaction had begun, the lower
electric heater was switched off. The time of attaining
a stable thermal mode from the beginning of the reac-
tion and also during passing to new values of the pro-
cess parameters was ~1 h. Thereafter, the movable
thermocouple found and fixed the maximum catalyst
bed temperature and gas samples for chromatographic
analysis were taken.
Another important factor affecting the efficiency of
oxide systems with oxygen vacancies in OCM is the
presence of transition metal impurity ions in their crys-
tal lattices. Through oxidative transformations, these
ions [5] can present the lacking electrons required for
dioxygen activation, during which the catalyst donates
and an oxygen molecule accepts at least one electron.
For example, variable-valence cerium ions can favor
the formation of oxygen active sites O(–sur) for OCM,
which are stabilized near cation vacancies of lanthana,
through the reaction
The catalyst operation modes in the reactor can be
divided into two groups. The first group includes adia-
batic experiments, in which a steady-state process takes
place without heating, since the heat of reaction is suf-
ficient for maintaining a necessary temperature. The
ëç4-to-é2 ratio in these experiments did not exceed 5.
It was found that the optimum maximum temperatures
were within the range 860–950°C. The temperature that
was optimum at each ëç4-to-é2 ratio depended on the
flow rate of the initial mixture. With decreasing this
ratio, i.e., with increasing the oxygen concentration, the
gas flow rate decreases. The yield of ë2 hydrocarbons
was maximum (14.5%) at a ë2ç4-to-é2 ratio of 3.0, a
maximum catalyst bed temperature of 890°C, a meth-
ane conversion of 28.7%, and a selectivity with respect
to ë2 hydrocarbons of 50.4%. The ë2ç4-to-ë2ç6 ratio
was 2. These data were obtained at a gas mixture flow
rate of ~170 nL/h. The catalyst worked for a total of
about 100 h without intermediate regeneration and
without any signs of loss of activity. Thus, the studied
lanthana–ceria catalysts can be a promising object of
investigation aimed at industrial production of ethylene
from methane.
2–
2(ads)
O2(gas) + 2Ce3+
O
+ 2Ce4+
2O(–sur) + 2Ce4+.
The additional pathway of dioxygen activation can lead
to an increase in the activity of the catalyst in OCM.
The rise in the yield of OCM products by a seem-
ingly unimpressive value of about 5% is actually a con-
siderable step toward creating OCM catalysts, since the
desired goal is a new method of production of ethylene,
which is a basic petrochemical product, whose annual
output is tens of millions of tons. Moreover, the
revealed beneficial effect of ceria enables one to use
one of the commercially available concentrates of rare
earths as an OCM catalyst without additional purifica-
tion [7–9].
ACKNOWLEDGMENTS
The found effect of ceria addition on the activity of
rare earths in OCM needs further exploration, first of
all, to reveal the nature of active sites in these catalysts.
For now, note that the character of active sites for OCM
can, in this case, be governed by two factors. The first
of them is the fact that the activation of a C–H bond in
This work was supported by the Russian Foundation
for Basic Research (project no. 01–03–32508a), the
Federal Program of State Support of the Integration
between Higher Education and Fundamental Science
(project no. A 0072), and OAO Gazprom (contract
methane and its subsequent transformations into the no. 545–01).
products of dimerization or deep oxidation are deter-
mined by the presence of oxygen vacancies in a catalyst
[5]. The concentration of oxygen vacancies is particu-
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