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Utilization of oxide ion transfer for oxidative coupling of methane with membrane reactors
T. Inui et al. (Editors), New Aspects of Spillover Effect in Catalysis 0 1993 Elsevier Science Publishers B.V. All rights reserved.
371
Utilization of oxide ion transfer for oxidative coupling of methane with membrane reactors T. Nozaki, K. Omata, and K. Fujimoto Department of Synthetic Chemistry, The University of Tokyo Bunkyo-ku Hongo 7-3-1, Tokyo, 113 Japan Abstract Oxide ion transportation between metal oxide was effectively utilized for the selective oxidation of methane. Oxide ion of lead oxide supported on the membrane of oxide ion conducting materials (Y,O,-ZrO,, Ca(Co,~,Fe,,,)O,) exhibited high activity for the oxidative coupling of methane with high selectivity (>90%), although oxide ion conductor itself exhibited poor activity for C, hydrocarbon synthesis. The amount of oxide ion consumption from PbO/(Y,O,-ZrO,) catalyst was measured in the methane stream to clarify that the supported PbO played the role as the site of methane oxidation by oxide ion supplied from Y,O,-ZrO,.
1. INTRODUCTION Methane conversion technologies have been studied by many investigators with the aim of developing a novel use of natural gas. The authors found that oxide ion of PbO on MgO carrier converted methane to C, hydrocarbons selectively[l], and successfully applied a non-porous membrane reactor made of PbO for the selective oxidative coupling of methane[2-51. However, it is very difficult to make a pinhole-free PbO film on the porous support materials. Therefore, it is favorable to employ a non-porous support material of oxide ion conductor. Although stabilized zirconia is well-known 0’- conductor, zirconia itself has low activity for methane oxidation[6-81. A perovskite-type material such as Ca(CoO,,FeO,,)O, is also an oxide ion conductor, which is also necessary to be modified because it converts methane into CO or carbon. In this study, the present author tried to establish the oxide ion transfer system for the selective oxidation of methane where oxide ion, which is transported to PbO through the non-porous support materials of the oxide ion conductor, acted as the oxidant. 2. EXPERIMENTAL Lead oxide was coated on the supporting materials by painting aqueous solution (lead nitrate) and calcining at 800°C. The supporting materials of stabilized zirconia were supplied by Nippon Kagaku Togyo. A non-porous tube of Ca(Coo,,Feo,,)O, was prepared by a cold isostatic pressurizing and calcination at 1200°C. Composition and thickness of the membrane are shown in Table 1.
378
The membrane reaction system was presented in figure 5[2-51. Methane was supplied into annular space between the outer tube and inner tube on which PbO film was coated. Oxygen consumption from PbO/(Y,O,-ZrO,) catalyst was measured in the methane stream from the amount of COX and H,O formed. All the products were measured by gas chromatography. Table 1 Composition of the membrane reactors Membrane "hickness(p m) Support Materials KO,S-PbO KOo,s-PbO KOo,s-PbO KO0,,-PbO
3 3 3 3
Abbreviations
csz
(CaO)o.,1 ( ~ ~ 0 ~ ) 0 , * 9 (MgO)o.,s(Z~O,)o.,,
MSZ YSZ CCF
~y01S~0.08~~r0~~0.92
Ca(Coo,,Feo.2)0,
3.FtESULTS AND DISCUSSION 3.1.Methane conversion by membrane reactors Figure 1 shows the activity change of C, formation on CSZ based membrane reactor as a function of process time. C, formation was not observed on CSZ without PbO. It was about two hours until the activity achieved to the steady state level. The steady ' ' transfer rate. state should be realized when the surface reaction rate is equal to the 0 Most of product was ethane while little ethylene and COX were formed. The Arrhenius plots of C, formation rate over PbO/zirconia membranes are shown in figure 2. The effect of supporting materials and that of outer circuit on the activity were examined. The membranes supported by CSZ and MSZ showed lower activity than supported by YSZ. The activation energies on PbO/CSZ and PbOMSZ were 28
-
loo
N
E
f
8E
I
Reaction nies -- Elsmn mndueuvq *
Process Time [ min]
Figure 1. Activity change with process time
9.6 T - ' [ lo-" K - ' ]
10.0
Figure 2. Temperature dependence of the activity of PbO/zirconia membrane
379
kcal/mol while 44 kcal/mol on PbO/YSZ membrane. The transportation of oxide ion is controlled by the counter current of electron, whose activation energy is also 28 kcal/mol as is indicated by broken lines in figure 2. This indicates that the reaction rates of C, formation over PbO/CSZ and PbO/MSZ membranes were controlled by their electron conductivity. On the other hand, electron conductivity of YSZ is quite higher than CSZ and MSZ. However, the difference in the activity is smaller than that in the electron conductivity. Oxide ion conductivity can be enhanced by outer circuit, in which electron is conducted with lower resistance. Therefore, the outer circuit was attached to PbO/MSZ membrane (PbO/MSZ/O.C.). It is clear from the data in figure 2 that the C, formation activity of PbO/MSZ was enhanced by several times by equipping with the outer circuit. However, the C, selectivity was lowered (100%+70%). The activation energy of C, formation on PbO/YSZ membrane (44kcal/mol) and that on PbO/MSZ/O.C. (36 kcal/mol) indicates that the total reaction rate was limited by the surface reaction of CH,-PbO, in spite of the sufficient supply of oxide ion. Therefore, it is unnecessary to accelerate transportation of oxide ion by outer circuit and electrical power, but it is necessary to promote the rate of surface reaction of CH, with 02-. Figure 3 shows the methane conversion activities of the Ca(Co,,,Fe,,,)O, ("CCF") membrane. In this case, no C, formation was observed on bare membrane, while C, formation with 50% selectivity was observed when it was modified by PbO-K,O. The C, formation activity of this membrane was quite higher than any other membrane used, which should be caused by the high conductivity of oxide ion and the effective modification of PbO-K,O. We found CH, conversion activity of Ca(CoFe)O, membrane was quite high, and that it is possible to enhance the selectivity for C, hydrocarbons by the modification with some component coated on its surface.
3.2.Amount of oxide ion participating in the membrane reaction Methane oxidation was conducted in the alumina tube reactor with packed bed catalysts (YSZ, PbO/YSZ and PbO/MgO). Amount of consumed oxygen was calculated from the amounts of COXand H,O in the effluent methane stream. Figure 4 shows the integrated amount of consumed oxygen with process time. In the case of PbO/MgO, the amount
1 .o
-1
PbOIK20NSZ PbOK201CCF
0
Yo CCF (720"C) 0
loo0 Moo 3wo 4 w o 5Mx)
63ooo
Methane conversion rate [mmolM~?]
Figure 3. Methane conversion rate on modified membrane at 750°C.
B
5 B v 0
100
200
Time [ min 1
300
Figure 4. Oxide ion consumption from OSmmol PbO in CH, stream at 750°C.
380
of PbO oxygen was consumed in a few minutes and oxygen consumption was scarcely observed in the following period. The total amount of oxygen consumed by methane conversion was about 150% of the amount of PbO oxygen. It is possible that oxygen in carbon dioxide adsorbed on the surface of MgO, which could not release at calcining temperature 800"C, or in MgO might convert methane. However, the total amount was not significantly different from the amount of PbO. On the other hand, the integrated amount of consumed oxygen from PbO/YSZ was over 200% of PbO oxygen in two hours, and it was still increasing rapidly at after three hours. The performance was apparently different from the case of PbO/MgO. This result indicates that the oxide ion in YSZ was consumed as well as that in PbO through the membrane. Oxygen from YSZ was also consumed slowly even in the absence of PbO. However, the consumption rate of oxide ion in YSZ was 100 pmol/min * mol,,d oxy en, which was one fifth of PbOiYSZ (500 pmol/min moltotdox ,J. This means P b d played the role as the site of oxide ion reaction supplied from ?SZ as shown in figure 5.
-
Figure 5. Oxide ion transfer in the membrane reaction system 4. CONCLUSION Oxide ion transportation was successfully utilized for the selective oxidative coupling of methane with high activity and selectivity. Oxide ion was supplied from the supporting materials to PbO film membrane, which was experimentally clarified by measuring the amount of consumed oxygen from PbO/YSZ catalyst in a methane stream. 5. REFERENCES 1 K. Asami, T. Shikada, K. Fujimoto and H. Tominaga, Ind. Eng. Chem. Res., 26 (1987)2348. 2 K. Omata, S . Hashimoto, H. Tominaga, and K. Fujimoto. Appl. Catal., 52(1989)Ll. 3 K. Fujimoto, K. Asami, K. Omata and S. Hashimoto, Stud. Surf. Sci. Catal., 61(1991) 625. 4 T. Nozaki and K. Fujimoto, J. Chem. Soc., Chem. Commun.,(1992)1248. 5 T. Nozaki, 0. Yamazaki, K. Omata and K. Fujimoto, Chem. Eng. Sci., 47(1992)2945. 6 K. Otsuka, S. Yokoyama, and A. Morikawa, Chem. Lett., (1989319. 7 H. Nagamoto, K. Hayashi, and H. Inoue, J. Catal., 126(1990)671. 8 D. Eng and M. Stoukides, J. Catal., 130(1991)306.
371
Utilization of oxide ion transfer for oxidative coupling of methane with membrane reactors T. Nozaki, K. Omata, and K. Fujimoto Department of Synthetic Chemistry, The University of Tokyo Bunkyo-ku Hongo 7-3-1, Tokyo, 113 Japan Abstract Oxide ion transportation between metal oxide was effectively utilized for the selective oxidation of methane. Oxide ion of lead oxide supported on the membrane of oxide ion conducting materials (Y,O,-ZrO,, Ca(Co,~,Fe,,,)O,) exhibited high activity for the oxidative coupling of methane with high selectivity (>90%), although oxide ion conductor itself exhibited poor activity for C, hydrocarbon synthesis. The amount of oxide ion consumption from PbO/(Y,O,-ZrO,) catalyst was measured in the methane stream to clarify that the supported PbO played the role as the site of methane oxidation by oxide ion supplied from Y,O,-ZrO,.
1. INTRODUCTION Methane conversion technologies have been studied by many investigators with the aim of developing a novel use of natural gas. The authors found that oxide ion of PbO on MgO carrier converted methane to C, hydrocarbons selectively[l], and successfully applied a non-porous membrane reactor made of PbO for the selective oxidative coupling of methane[2-51. However, it is very difficult to make a pinhole-free PbO film on the porous support materials. Therefore, it is favorable to employ a non-porous support material of oxide ion conductor. Although stabilized zirconia is well-known 0’- conductor, zirconia itself has low activity for methane oxidation[6-81. A perovskite-type material such as Ca(CoO,,FeO,,)O, is also an oxide ion conductor, which is also necessary to be modified because it converts methane into CO or carbon. In this study, the present author tried to establish the oxide ion transfer system for the selective oxidation of methane where oxide ion, which is transported to PbO through the non-porous support materials of the oxide ion conductor, acted as the oxidant. 2. EXPERIMENTAL Lead oxide was coated on the supporting materials by painting aqueous solution (lead nitrate) and calcining at 800°C. The supporting materials of stabilized zirconia were supplied by Nippon Kagaku Togyo. A non-porous tube of Ca(Coo,,Feo,,)O, was prepared by a cold isostatic pressurizing and calcination at 1200°C. Composition and thickness of the membrane are shown in Table 1.
378
The membrane reaction system was presented in figure 5[2-51. Methane was supplied into annular space between the outer tube and inner tube on which PbO film was coated. Oxygen consumption from PbO/(Y,O,-ZrO,) catalyst was measured in the methane stream from the amount of COX and H,O formed. All the products were measured by gas chromatography. Table 1 Composition of the membrane reactors Membrane "hickness(p m) Support Materials KO,S-PbO KOo,s-PbO KOo,s-PbO KO0,,-PbO
3 3 3 3
Abbreviations
csz
(CaO)o.,1 ( ~ ~ 0 ~ ) 0 , * 9 (MgO)o.,s(Z~O,)o.,,
MSZ YSZ CCF
~y01S~0.08~~r0~~0.92
Ca(Coo,,Feo.2)0,
3.FtESULTS AND DISCUSSION 3.1.Methane conversion by membrane reactors Figure 1 shows the activity change of C, formation on CSZ based membrane reactor as a function of process time. C, formation was not observed on CSZ without PbO. It was about two hours until the activity achieved to the steady state level. The steady ' ' transfer rate. state should be realized when the surface reaction rate is equal to the 0 Most of product was ethane while little ethylene and COX were formed. The Arrhenius plots of C, formation rate over PbO/zirconia membranes are shown in figure 2. The effect of supporting materials and that of outer circuit on the activity were examined. The membranes supported by CSZ and MSZ showed lower activity than supported by YSZ. The activation energies on PbO/CSZ and PbOMSZ were 28
-
loo
N
E
f
8E
I
Reaction nies -- Elsmn mndueuvq *
Process Time [ min]
Figure 1. Activity change with process time
9.6 T - ' [ lo-" K - ' ]
10.0
Figure 2. Temperature dependence of the activity of PbO/zirconia membrane
379
kcal/mol while 44 kcal/mol on PbO/YSZ membrane. The transportation of oxide ion is controlled by the counter current of electron, whose activation energy is also 28 kcal/mol as is indicated by broken lines in figure 2. This indicates that the reaction rates of C, formation over PbO/CSZ and PbO/MSZ membranes were controlled by their electron conductivity. On the other hand, electron conductivity of YSZ is quite higher than CSZ and MSZ. However, the difference in the activity is smaller than that in the electron conductivity. Oxide ion conductivity can be enhanced by outer circuit, in which electron is conducted with lower resistance. Therefore, the outer circuit was attached to PbO/MSZ membrane (PbO/MSZ/O.C.). It is clear from the data in figure 2 that the C, formation activity of PbO/MSZ was enhanced by several times by equipping with the outer circuit. However, the C, selectivity was lowered (100%+70%). The activation energy of C, formation on PbO/YSZ membrane (44kcal/mol) and that on PbO/MSZ/O.C. (36 kcal/mol) indicates that the total reaction rate was limited by the surface reaction of CH,-PbO, in spite of the sufficient supply of oxide ion. Therefore, it is unnecessary to accelerate transportation of oxide ion by outer circuit and electrical power, but it is necessary to promote the rate of surface reaction of CH, with 02-. Figure 3 shows the methane conversion activities of the Ca(Co,,,Fe,,,)O, ("CCF") membrane. In this case, no C, formation was observed on bare membrane, while C, formation with 50% selectivity was observed when it was modified by PbO-K,O. The C, formation activity of this membrane was quite higher than any other membrane used, which should be caused by the high conductivity of oxide ion and the effective modification of PbO-K,O. We found CH, conversion activity of Ca(CoFe)O, membrane was quite high, and that it is possible to enhance the selectivity for C, hydrocarbons by the modification with some component coated on its surface.
3.2.Amount of oxide ion participating in the membrane reaction Methane oxidation was conducted in the alumina tube reactor with packed bed catalysts (YSZ, PbO/YSZ and PbO/MgO). Amount of consumed oxygen was calculated from the amounts of COXand H,O in the effluent methane stream. Figure 4 shows the integrated amount of consumed oxygen with process time. In the case of PbO/MgO, the amount
1 .o
-1
PbOIK20NSZ PbOK201CCF
0
Yo CCF (720"C) 0
loo0 Moo 3wo 4 w o 5Mx)
63ooo
Methane conversion rate [mmolM~?]
Figure 3. Methane conversion rate on modified membrane at 750°C.
B
5 B v 0
100
200
Time [ min 1
300
Figure 4. Oxide ion consumption from OSmmol PbO in CH, stream at 750°C.
380
of PbO oxygen was consumed in a few minutes and oxygen consumption was scarcely observed in the following period. The total amount of oxygen consumed by methane conversion was about 150% of the amount of PbO oxygen. It is possible that oxygen in carbon dioxide adsorbed on the surface of MgO, which could not release at calcining temperature 800"C, or in MgO might convert methane. However, the total amount was not significantly different from the amount of PbO. On the other hand, the integrated amount of consumed oxygen from PbO/YSZ was over 200% of PbO oxygen in two hours, and it was still increasing rapidly at after three hours. The performance was apparently different from the case of PbO/MgO. This result indicates that the oxide ion in YSZ was consumed as well as that in PbO through the membrane. Oxygen from YSZ was also consumed slowly even in the absence of PbO. However, the consumption rate of oxide ion in YSZ was 100 pmol/min * mol,,d oxy en, which was one fifth of PbOiYSZ (500 pmol/min moltotdox ,J. This means P b d played the role as the site of oxide ion reaction supplied from ?SZ as shown in figure 5.
-
Figure 5. Oxide ion transfer in the membrane reaction system 4. CONCLUSION Oxide ion transportation was successfully utilized for the selective oxidative coupling of methane with high activity and selectivity. Oxide ion was supplied from the supporting materials to PbO film membrane, which was experimentally clarified by measuring the amount of consumed oxygen from PbO/YSZ catalyst in a methane stream. 5. REFERENCES 1 K. Asami, T. Shikada, K. Fujimoto and H. Tominaga, Ind. Eng. Chem. Res., 26 (1987)2348. 2 K. Omata, S . Hashimoto, H. Tominaga, and K. Fujimoto. Appl. Catal., 52(1989)Ll. 3 K. Fujimoto, K. Asami, K. Omata and S. Hashimoto, Stud. Surf. Sci. Catal., 61(1991) 625. 4 T. Nozaki and K. Fujimoto, J. Chem. Soc., Chem. Commun.,(1992)1248. 5 T. Nozaki, 0. Yamazaki, K. Omata and K. Fujimoto, Chem. Eng. Sci., 47(1992)2945. 6 K. Otsuka, S. Yokoyama, and A. Morikawa, Chem. Lett., (1989319. 7 H. Nagamoto, K. Hayashi, and H. Inoue, J. Catal., 126(1990)671. 8 D. Eng and M. Stoukides, J. Catal., 130(1991)306.