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Reaction Performances of Methane Oxidative Coupling Along Catalyst Bed with Sm-CaO Catalyst
Guczi, L et al. (Editors), New Frontiers in Cafalysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights resewed
REACTION PERFORMANCES OF METHANE OXIDATIVE COUPLING ALONG CATALYST BED WITH Sm-CaO CATALYST
C.Tang, L. Lin,Z. Xu and J. Zang Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O.Box 110. Dalian 116023, China
Abstract Mass spectrometer has been used to measure in situ the reactant and products (CH4, 02, CO, COZ, C2H4, C2H6) distributions of methane oxidative coupling over 2%Sm/CaO. The results indicate that the rate of CH4 conversion does not vary with the continual consumption at 1023K. At 873K, the poisoning effect of COz in the catalyst bed must be considered. 1. INTRODUCTION
Methane oxidative coupling(M0C) is a complex reaction comprising of homogeneousheterogeneous processes[ 11. With the conventional reaction system, it is difficult to estimate the relative contribution of heterogeneous vs homogeneous reactions during CH4 oxidative coupling to C2 hydrocarbons. In this work, the product distribution along 2%Sm/CaO catalyst bed has been measured in situ and the characterization of MOC reaction is illustrated. 2. EXPERIMENTAL
The 2%Sm/CaO catalyst was prepared by impregnating calcium oxide with an aqueous solution of samarium nitrate followed by drying and calcination. The product distribution was measured by locating a molecular leak which was at the end of a lmm-I.D. quartz tube at different height of the catalyst bed during the reaction. Part of the effluence of the reacting gas was directly led through the leak into a quadrupole MS analyzer. The reaction was carried out at ambient pressure, while the pressure at the molecular leak was below 1 ~ 1 0Torr. . ~ Moreover, the length of the leak was less than lmm to avoid further reaction of inlet material. The amount of catalyst was about 250mg(30mm bed height) and the flow of reaction gas (46%CH4+ 16%02+38%He)was 46ml/min(STP). 3. RESULTS AND DISCUSSION
The product distribution of the temperature programmed reaction of MOC over a 2%Sm/CaO catalyst shows that the conversion of CH4 to COX is initiated at about 823K,
2222
the coupling reaction starts at 923K and the yield of Cz reaches a maximum at about 1073K. When the temperature exceeds 1073K, the yield of Cz drops. Fig.lA shows a set of data of product distribution along 2%Sm/CaO catalyst bed at 1073K. From the slopes of the CH4 curve of Fig.lA, it can be calculated that the rate of CH4 conversion in the catalyst region is 40-times larger than that in the preheating zone of packed A1203. This result clearly reveals that the MOC reaction mainly occurs in the catalyst bed and the contribution of the homogeneous reaction of CH4 and 02 can be neglected in our experiment.
'+.+l+ Bp
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O
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0.0 1-
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A
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1-
1,
d,A
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0
A
A
-
-
0
-
10
15
20
25
BED H E I G H T , m m
(A) 1073K
I
5
I
I0
1
15
I
20
BED H E I G H T , m m
(B) 873K
A1203 Fig. 1 Reactants and products distribution of MOC reaction along catalyst bed. @%@2%Sm/CaOmempty,CH4(+),a( O),CO( n),CO2( N),CzH6( A),CzH4( A).
Fig.lA also shows that the concentrations of reactants and products along the catalyst bed vary significantly only within a certain height of the catalyst bed where 02 is not
2223 completely eliminated. Behind this region, all the curves for concentration vs bed height reach an inflexion point, then only a slightly change can be. observed. In other experiment(see Fig. lB), MOC reaction and CzHa dehydrogenation can not proceed behind the catalyst bed where remaining 0 2 is still present. These results indicate that the transformation of CH4 to C2 and COX and dehydrogenation of CzHa to C2H4 proceed remarkably only when both the catalyst and 02 are co-existing. All the results suggest that C2 formation and other main reactions occur mainly over the catalyst surface. In Fig.lA the concentration of 02 decreases linearly along the catalyst bed until it reaches zero. Apparently, the rate of CH4 conversion does not vary with the continual consumption of 02, even near zero concentration of oxygen. From this phenomenon we can conclude that MOC reaction is a zero order reaction with oxygen and the forming of oxygen active sites is not the rate determining step of the MOC reaction at 1073K for the 2%Sm/CaO. The same relationship can also be observed on a Sm203 catalyst even at a lower temperature of 873K. Fig.lB shows the product distribution of MOC reaction over the 2%Sm/CaO catalyst at 873K. In Fig. 1B oxygen is not consumed linearly at the catalyst bed, and at the end of the catalyst bed the rates of CH4 and 02 conversion approach zero, though the catalyst and large amount of CH4 and 02 are still existing. For studying the reason of these fact, after the reaction at 873K, the temperature of the catalyst bed was increased to 1273K. A large amount of C02 was desorbed from the catalyst bed during temperature raising and C02 was wholly desorbed at 1073K. Therefore the inhibition scheme of active sites by C02 can be proposed as follows: ki
02 ---+2[0-] k2
CH4
+ [O-]
c02
+ [o-]
lo
__+
CH3.
+ [OH-]
[CO3-]
According to the work of Lunsford et al[2], the methane activation is due to the [0-] active sites detected from the Li/MgO catalyst in the MOC reaction. In the Sm/CaO catalyst, the active sites may be [O-] too. In our experiments we find that the rate of CH4 conversion is independent from the gas phase concentration of 02 at 1073K so that it may be true also at 873K. If we assume that this kind of active sites adsorbs C02 at lower temperature, which causes the deactivation of the catalyst, and the amount of the [O-] sites covered by C02 is proportional to the partial pressure of COz, elsewhere, the rate of CH4 conversion is first order with methane in nature, the relation between reaction rate (dx/dh)/Pc~4along the bed and C02 partial pressure Pc02 can be expressed as: (dx/dh)/Pc~4= k2(h- lOPc02) where P C His~ the partial pressures of CH4 and C02 respectively. kz denotes the rate
2224
constant of CH4 conversion and b is a constant correlated to COz adsorption. 80 is a constant correlated to zero order of 02 consumption.
0.018 0
0.016
0.014
-
n
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0.012
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rd
7
0.010
E E 0.008
v
‘d
X
0.006 ?c
x
-Q
0.004 0.002
0.000
0.0
0.2
0.4 0.6 pco2 ( K
W
0.8
1.0
1.2
Fig.2 The relation between (dx/dh)/Pcw and the partial pressure Pc02. The plot of (dx/dh)/Pc~4vs Pc02 gives a linear relation as shown in Fig.2 which indicates that the above speculation is right. This result implies that when the catalysts with basic supports or promoters for MOC reaction are studied at lower temperature, the poisoning effect of CO2 in the catalyst bed must be considered. 4. REFERENCES 1 T. Ito, J.Wang,C. Lin,and J.H. Lunsford, J. Am. Chem. Soc.,107(1985)5062. 2 C-H. Lin, J-X. Wang, and J.H. Lunsford, J. Am. Chem. Soc.,109(1987)4808.
REACTION PERFORMANCES OF METHANE OXIDATIVE COUPLING ALONG CATALYST BED WITH Sm-CaO CATALYST
C.Tang, L. Lin,Z. Xu and J. Zang Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O.Box 110. Dalian 116023, China
Abstract Mass spectrometer has been used to measure in situ the reactant and products (CH4, 02, CO, COZ, C2H4, C2H6) distributions of methane oxidative coupling over 2%Sm/CaO. The results indicate that the rate of CH4 conversion does not vary with the continual consumption at 1023K. At 873K, the poisoning effect of COz in the catalyst bed must be considered. 1. INTRODUCTION
Methane oxidative coupling(M0C) is a complex reaction comprising of homogeneousheterogeneous processes[ 11. With the conventional reaction system, it is difficult to estimate the relative contribution of heterogeneous vs homogeneous reactions during CH4 oxidative coupling to C2 hydrocarbons. In this work, the product distribution along 2%Sm/CaO catalyst bed has been measured in situ and the characterization of MOC reaction is illustrated. 2. EXPERIMENTAL
The 2%Sm/CaO catalyst was prepared by impregnating calcium oxide with an aqueous solution of samarium nitrate followed by drying and calcination. The product distribution was measured by locating a molecular leak which was at the end of a lmm-I.D. quartz tube at different height of the catalyst bed during the reaction. Part of the effluence of the reacting gas was directly led through the leak into a quadrupole MS analyzer. The reaction was carried out at ambient pressure, while the pressure at the molecular leak was below 1 ~ 1 0Torr. . ~ Moreover, the length of the leak was less than lmm to avoid further reaction of inlet material. The amount of catalyst was about 250mg(30mm bed height) and the flow of reaction gas (46%CH4+ 16%02+38%He)was 46ml/min(STP). 3. RESULTS AND DISCUSSION
The product distribution of the temperature programmed reaction of MOC over a 2%Sm/CaO catalyst shows that the conversion of CH4 to COX is initiated at about 823K,
2222
the coupling reaction starts at 923K and the yield of Cz reaches a maximum at about 1073K. When the temperature exceeds 1073K, the yield of Cz drops. Fig.lA shows a set of data of product distribution along 2%Sm/CaO catalyst bed at 1073K. From the slopes of the CH4 curve of Fig.lA, it can be calculated that the rate of CH4 conversion in the catalyst region is 40-times larger than that in the preheating zone of packed A1203. This result clearly reveals that the MOC reaction mainly occurs in the catalyst bed and the contribution of the homogeneous reaction of CH4 and 02 can be neglected in our experiment.
'+.+l+ Bp
10-
z" 0
0
4
i -
M
F:
2b
-
w u
u
=='t
--
4-
2
z 0
5
1.0 -
D
O
0
0.5 -
0.0 1-
' 0
A
0.05
1-
1,
d,A
t"
0
A
A
-
-
0
-
10
15
20
25
BED H E I G H T , m m
(A) 1073K
I
5
I
I0
1
15
I
20
BED H E I G H T , m m
(B) 873K
A1203 Fig. 1 Reactants and products distribution of MOC reaction along catalyst bed. @%@2%Sm/CaOmempty,CH4(+),a( O),CO( n),CO2( N),CzH6( A),CzH4( A).
Fig.lA also shows that the concentrations of reactants and products along the catalyst bed vary significantly only within a certain height of the catalyst bed where 02 is not
2223 completely eliminated. Behind this region, all the curves for concentration vs bed height reach an inflexion point, then only a slightly change can be. observed. In other experiment(see Fig. lB), MOC reaction and CzHa dehydrogenation can not proceed behind the catalyst bed where remaining 0 2 is still present. These results indicate that the transformation of CH4 to C2 and COX and dehydrogenation of CzHa to C2H4 proceed remarkably only when both the catalyst and 02 are co-existing. All the results suggest that C2 formation and other main reactions occur mainly over the catalyst surface. In Fig.lA the concentration of 02 decreases linearly along the catalyst bed until it reaches zero. Apparently, the rate of CH4 conversion does not vary with the continual consumption of 02, even near zero concentration of oxygen. From this phenomenon we can conclude that MOC reaction is a zero order reaction with oxygen and the forming of oxygen active sites is not the rate determining step of the MOC reaction at 1073K for the 2%Sm/CaO. The same relationship can also be observed on a Sm203 catalyst even at a lower temperature of 873K. Fig.lB shows the product distribution of MOC reaction over the 2%Sm/CaO catalyst at 873K. In Fig. 1B oxygen is not consumed linearly at the catalyst bed, and at the end of the catalyst bed the rates of CH4 and 02 conversion approach zero, though the catalyst and large amount of CH4 and 02 are still existing. For studying the reason of these fact, after the reaction at 873K, the temperature of the catalyst bed was increased to 1273K. A large amount of C02 was desorbed from the catalyst bed during temperature raising and C02 was wholly desorbed at 1073K. Therefore the inhibition scheme of active sites by C02 can be proposed as follows: ki
02 ---+2[0-] k2
CH4
+ [O-]
c02
+ [o-]
lo
__+
CH3.
+ [OH-]
[CO3-]
According to the work of Lunsford et al[2], the methane activation is due to the [0-] active sites detected from the Li/MgO catalyst in the MOC reaction. In the Sm/CaO catalyst, the active sites may be [O-] too. In our experiments we find that the rate of CH4 conversion is independent from the gas phase concentration of 02 at 1073K so that it may be true also at 873K. If we assume that this kind of active sites adsorbs C02 at lower temperature, which causes the deactivation of the catalyst, and the amount of the [O-] sites covered by C02 is proportional to the partial pressure of COz, elsewhere, the rate of CH4 conversion is first order with methane in nature, the relation between reaction rate (dx/dh)/Pc~4along the bed and C02 partial pressure Pc02 can be expressed as: (dx/dh)/Pc~4= k2(h- lOPc02) where P C His~ the partial pressures of CH4 and C02 respectively. kz denotes the rate
2224
constant of CH4 conversion and b is a constant correlated to COz adsorption. 80 is a constant correlated to zero order of 02 consumption.
0.018 0
0.016
0.014
-
n
‘E
0.012
J
rd
7
0.010
E E 0.008
v
‘d
X
0.006 ?c
x
-Q
0.004 0.002
0.000
0.0
0.2
0.4 0.6 pco2 ( K
W
0.8
1.0
1.2
Fig.2 The relation between (dx/dh)/Pcw and the partial pressure Pc02. The plot of (dx/dh)/Pc~4vs Pc02 gives a linear relation as shown in Fig.2 which indicates that the above speculation is right. This result implies that when the catalysts with basic supports or promoters for MOC reaction are studied at lower temperature, the poisoning effect of CO2 in the catalyst bed must be considered. 4. REFERENCES 1 T. Ito, J.Wang,C. Lin,and J.H. Lunsford, J. Am. Chem. Soc.,107(1985)5062. 2 C-H. Lin, J-X. Wang, and J.H. Lunsford, J. Am. Chem. Soc.,109(1987)4808.