Deep oxidation of methane on granulated and monolith copper-manganese oxide catalysts

Article abstract of DOI:10.1023/A:1013078316977

Deep oxidation of methane on the granulated Cu - Mn-mixed oxide catalyst and metallic monolith catalysts coated with the same oxide was studied. The experimental kinetic curves for both monolith and granulated catalysts are satisfactorily described by the first-order rate law. The values of activation energies, reaction rate constants, and feed flow rates for the specified conversion almost coincide for both types of the catalysts. The data obtained confirm the possibility of a quantitative comparison of the activities of the granulated and monolith catalysts. The activity of the monoliths is proportional to the concentration of the active component.

Full text of DOI:10.1023/A:1013078316977

Russian Chemical Bulletin, International Edition, Vol. 50, No. 9, pp. 1589—1592, September, 2001
1589
Deep oxidation of methane on granulated and monolith
copper—manganese oxide catalysts
«
K. I. Slovetskaya, A. A. Greish, M. P. Vorob´eva, and L. M. Kustov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
4
7 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5328. E-mail: greish@ioc.ac.ru
Deep oxidation of methane on the granulated Cu—Mn-mixed oxide catalyst and metallic
monolith catalysts coated with the same oxide was studied. The experimental kinetic curves
for both monolith and granulated catalysts are satisfactorily described by the first-order rate
law. The values of activation energies, reaction rate constants, and feed flow rates for the
specified conversion almost coincide for both types of the catalysts. The data obtained
confirm the possibility of a quantitative comparison of the activities of the granulated and
monolith catalysts. The activity of the monoliths is proportional to the concentration of the
active component.
Key words: methane, oxidation, kinetics, Cu—Mn-mixed oxide catalyst, monolith catalyst.
1
Monolith catalysts have a lower gas-dynamic resis-
tance than granulated catalysts. In addition, in mono-
lith catalysts cross sections of heat and mass flows are
uniform and that decreases the probability of the forma-
tion of "hot" zones and improves the time-on stream
behavior of the catalyst and process selectivity. Finally,
when the active component is supported on the mono-
lith surface as the secondary coating, its fraction in the
reaction space unit is 0.01—0.25, which is much lower
than 0.5—1 characteristic of granulated catalysts.
Thus, the volume of monolith catalysts exceeds that
of granulated catalysts by several times at a lower con-
tent of the active component. All this impedes strongly
to compare activities of granulated and monolith cata-
lysts under identical conditions.
stants showed that the rate constants of ÑÍ4 oxi-
dation per unit volume of the active component
(La—Mn—O—perovskite) were identical for the mono-
lith and granules when the process was controlled by
kinetics. The method for comparison of activities of
granulated and monolith catalysts is applicable for ce-
ramic monoliths only, which can be subjected to crash-
ing with fractionation. For metallic monoliths, the prepa-
ration of similar fractions by crashing or grinding of the
starting catalyst is completely impossible and, hence,
another method for comparison of activities is necessary.
The purpose of this work is under identical condi-
tions and compare the activities of the granulated oxide
catalyst based on Mn and Cu oxides (Carulite-200) and
monolith metallic catalyst with supported Carulite as an
active component.
In several works, activities of monolith catalysts are
compared comparing the reaction temperature Ò at
α
which the conversion has a set value α or the tempera-
Experimental
ture dependence of the conversion (α ) at a set catalyst
Ò
2
—5
volume and a set feed gas space velocity.
These
The starting granulated catalyst Carulite-200 with a specific
surface area of 273 m2 g–1 contained 52.7 wt.% manganese
oxide and 11.40 wt.% copper oxide. To obtain monolith cata-
lysts, the foil of the Fe—Cr—Al alloy 80 µm thick was coated
with a thin layer of Carulite. The oxide coating was supported
by the electrophoretic deposition9 of a suspension containing
finely ground Carulite (8.2 g L–1, particle size 5—10 µm) in a
solution of aluminum hydroxide sol in ethanol (the content of
AlO(OH) was 8.2 g L–1). After deposition the samples obtained
were dried in air for 36 h at 20 °C and then calcined in an air
flow for 2 h at 500 °C. The smooth and corrugated foil layers
coated with Carulite were put together and rolled to form a
monolith with a height of 2.6 cm and a cross section of
.10 cm . The monoliths obtained had 40 triangular channels
per 1 cm of the cross section. The content of Carulite in each
monolith was 0.06—0.07 g.
An air mixture containing 2.52 vol.% ÑÍ4 was used for
methane oxidation. Experiments were carried out at an atmo-
methods do not allow the quantitative comparison of
6
activities. In addition, the reaction volumes of mono-
lith and granulated catalysts differ so strongly that their
activities cannot be compared when experiments are
carried out under identical conditions.
7
The authors compared the adsorption properties of
CuZSM-5 zeolite and the related monolith catalyst
using the TPD. To compare the catalytic properties of
these catalysts, the activation energy was used that was
determined in the kinetic and internal-diffusion re-
gimes. Catalytic activity was measured in a flow-type
reactor with a vibrofluidized bed catalyst on different
fractions of grains obtained by grinding of the starting
monolith catalyst. Comparison of activities of the grains
and monolith fragments using reaction rate con-
2
4
2
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1512—1515, September, 2001.
066-5285/01/5009-1589 $25.00 ©ꢀ2001 Plenum Publishing Corporation
1

1
590
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001
Slovetskaya et al.
spheric pressure in a flow-type reactor at 360—500 °C. In
experiments with the granulated catalyst, a quartz reactor with
a cross section of 0.82 cm2 was used. The height of the catalyst
bed in two experiments was 4 and 6 mm, and the weighed
samples were 0.40 and 0.60 g, respectively. The reaction on
α(%)
6
0
monolith catalysts was performed in a quartz reactor with a
2
40
20
cross section of 4.15 cm . The deep oxidation of ÑÍ on
4
granulated Carulite was studied in the 360—400 °Ñ temperature
interval. The methane conversion was varied in experiments
from 20 to 99 vol.%. The oxidation of ÑÍ4 on the metal-
lic monoliths with the supported catalyst was studied at
1
2
3
4
00—500 °Ñ, and the methane conversion was varied from 10
to 99%. The reaction products were analyzed by GLC on a
column packed with Polysorb-1 (thermal-conductivity detec-
tor, helium served as a carrier gas, and the temperature was
0
0
.1
0.2
0.3
0.4
τ/s
Fig. 1. Conversion of ÑÍ4 into ÑÎ2 by the oxidation with air
on the granulated catalyst at 400 °C and different weighed
samples: 1, 0.4 g, bed height 4 mm; 2, 0.6 g, bed height 6 mm;
and 3, 0.6 g, after preliminary heating to 450 °C.
2
5 °Ñ). Chromatograms were detected and processed in the on-
line mode on PC. Only ÑÎ2 and Í Î were found as reaction
2
products, indicating a high selectivity of the process. The
methane conversion measured in experiments was determined
from the amount of ÑÎ formed. Activities of the catalysts were
2
Table 1. Calculated rate constants of ÑÍ4 oxidation to ÑÎ2
compared by monitoring the feed gas flow rate (v ) correspond-
0
ing to equal conversions of methane to ÑÎ . The flow rate of
2
the initial mixture at a specified conversion is proportional to
k/s–1
Catalyst
T/°C
h/mm
(monolith)*
k
6
,10
the reaction rate constant
and, unlike T and α , can be
/cm3 (s g–1
α
T
)
related to the catalyst weight. Since the volume of monoliths is
determined by their construction and is not directly related to
the volume of the active component, it is difficult to determine
the real volume of the active component, therefore, instead of
the contact time (s)
Granules
400
400
400
400
4
6
6
(2)
(1)
(2)
2.80
2.53
1.15
—
—
—
2.13
1.93
0.88
1.88
16.66
16.77
Granules**
Monolith
5
00
τ´ = V /v ,
c
0
500
where Vc is the catalyst volume, for the monolith catalysts we
used the proportional value of the effective contact time
(
* Bed height.
*
* Granules after calcination at 450 °C.
s g cm–3
)
τ´ = m/v0
sponding to the same conversion are proportional to the
reaction rate constants
where m is the weight of the active component (Carulite). The
obtained values of activities were independent of the sizes of
reactors in which the reaction was performed.
(LT1/L ) = kT1/kT2,
T2 α
and the following equation is valid:
Results and Discussion
2
E = RT (∂lnL/∂T) .
α
To study the influence of mass exchange on the
reaction kinetics, we studied the catalyst activity by
changing the height of the catalyst bed and the gas flow
rate at a constant space velocity. A good coincidence of
–
1
Based on the Arrhenius plots of V vs. Ò
(Fig. 2), we
α
determined the activation energy of methane oxidation.
It was 25.8±1.9 kcal mol–1 for the granulated catalyst.
the plots of the conversion of ÑÍ vs. contact time τ
4
lnL0
(
Fig. 1) was obtained for two different catalyst beds,
suggesting the absence of diffusion limitations.11 The
kinetic data obtained are well described by the first-
order rate law. The reaction rate constants were deter-
mined from the plot
1
0.5
9
8
.5
.5
ln(1 – x) = –kτ,
1
2
where x is the conversion, k is the reaction rate con-
stant, and τ is the contact time. The calculated reaction
rate constants are presented in Table 1.
3
1.48
1.53
T –1•10 /K
3
The feed gas flow rates corresponding to conversions
of 20, 30, and 40% were determined from the kinetic
data on methane conversion obtained at 365—409 °C.
In the flow-type reactor, the feed gas flow rates corre-
Fig. 2. Temperature dependence of the activity of the granu-
lated catalyst in ÑÍ4 oxidation to ÑÎ2 at conversion (%):
20 (1), 30 (2), and 40 (3).

Deep oxidation of methane
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001 1591
L•10–3/cm3 h–1
It was found that heating of the catalyst in the reaction
mixture for 1 h at 500 °C decreases its activity
9
(
see Fig. 1), and the activation energy increases to
–
1
3
0.1±2.2 kcal mol .
6
As in the case of the granulated catalysts, the cata-
1
lyst activity of monoliths was studied at different flow
rates and the same space velocity. With this purpose, the
reaction was first performed on one monolith and then
on two similar monoliths placed one above another in
the reactor. The plots of the methane conversion into
2
3
0
0
.2
0.4
Weight of the catalyst/g
ÑÎ vs. effective contact time obtained in these two
Fig. 5. Plot of the activity of the granulated (1) and monolith
catalysts (2) in ÑÍ4 oxidation to ÑÎ2 vs. amount of the
component; temperature 400 °C, conversion 40%.
2
experiments virtually coincide (Fig. 3). This indicates
the absence of external diffusional limitations. The acti-
vation energy calculated from the feed gas flow rates
corresponding to conversions of 20, 30, and 40% is
law. The calculated constants are presented in Table 1,
and the curves are shown in Fig. 3. The obtained rate
2
5.9±1.1 kcal mol–1 (Fig. 4) and coincides with the
activation energy for the starting granulated catalyst.
This indicates that the same mechanism is operative in
the reaction on the granulated and monolith catalysts.
The data obtained for both the monolith and granulated
catalysts are well approximated by the first-order rate
constants for ÑÍ oxidation at 400 °C related to the
4
unit weight of the active component are virtually identi-
cal for the monolith and granulated catalysts.
Since the kinetics of methane oxidation on the
monolith and granulated catalysts is described by the
same rate law, their activities can be compared using the
feed gas flow rate corresponding to the same conversion
level. Comparison of the activities of the granulated and
monolith catalysts at 400 °C by the feed gas space
velocity corresponding to 40% conversion shows that
the activity increases linearly with the amount of the
active component (Fig. 5). This linear dependence indi-
cates the absence of limitations due to bulk and internal
diffusion.1
α(%)
a
1
00
8
6
4
2
0
0
0
0
b
0,12
Thus, it follows from the data obtained that ÑÍ4
oxidation at 400 °C on granulated Carulite and on the
metallic monolith with the Carulite coating is controlled
by kinetics. The reaction on both catalysts is character-
ized by the same activation energy and the rate con-
stant, and the activities measured by the feed gas flow
rate corresponding to the same conversion and related
to the catalyst unit weight virtually coincide (Table 2).
The deposition of the initial copper—manganese oxide
catalyst by the electrophoretic method on a metallic
1
2
3
4
0
0
.1
0.2
0.3
0.4 τ´/s g cm–3
Fig. 3. Oxidation of ÑÍ4 to ÑÎ2 with air on the monolith
catalyst at different temperatures: à, 500 °C and b, 400 °C:
3
1
, one monolith (height 26 mm, volume 10.8 cm , content of
support using Al O as a binder has no effect on the
2 3
Carulite 0.067 g); 2 and 3, 2 monoliths (height 52 mm, volume
catalytic activity of Carulite. At the same time, the
distribution of the active component as a thin layer over
the metallic monolith surface favors an increase in the
3
2
0.7 cm , content of Carulite 0.127 g); 4, granulated catalyst.
lnL0
Table 2. Comparison of activities in ÑÍ4 oxidation to ÑÎ2 at
1
1
1
0
9
8
4
00 °C and conversion 40%
Ma
b
c
L
Catalyst
L
α
/
g
/h–1
cm3 h–1
cm (h g)
3
–1
1
2
3
Granulated
Monolith
0.6
0.4
0.067
10000
6670
1100
2100
16670
16670
16420
16540
20400
20400
100
0
.127
100
3
1
.35
1.45
T –1•10 /K
a
Fig. 4. Temperature dependence of the activity of the monolith
catalyst in ÑÍ4 oxidation to ÑÎ2 at conversion (%): 20 (1),
Weight of active component.
Feed gas flow rate.
Space velocity.
b
c
3
0 (2), and 40 (3).

1
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Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001
Slovetskaya et al.
thermal stability of the catalytic system. The perfor-
mance of the monolith catalyst after thermal treatment
in air at 500 °C was stable under the reaction conditions
for 15 h at 500 °C (see Figs. 3 and 4).
The comparison of the activities of the granulated
Cu—Mn-mixed oxide and metallic monolith catalysts
shows that the feed gas flow rate corresponding to the
achievement of a specified conversion and related to the
unit weight of the active component can be used as a
measure of activity of the monolith catalyst. The use of
this value allows comparison of the activities of granu-
lated and monolith catalysts to be conducted in a
quantitative way.
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