3576
Ind. Eng. Chem. Res. 1997, 36, 3576-3582
Oxidative Coupling of Methane in a Solid Oxide Membrane Reactor Xiu-Mei Guo,* Kus Hidajat, and Chi-Bun Ching Department of Chemical Engineering, National University of Singapore, Singapore 119260, Singapore
Hong-Fang Chen Department of Chemical Engineering, Tianjin University, Tianjin, China 300072
The oxidative coupling of methane (OCM) has been studied using the 1 wt % Sr/La2O3-Bi2O3Ag-YSZ solid oxide membrane reactor. The effects of oxygen flux, temperature and feed concentration on the performance of oxidative coupling of methane reaction have been investigated. Two mathematical models based on well-mixed flow (CSTM) or plug flow (PFM) in the reactor have been developed to describe the behavior of the solid oxide membrane reactors. The simulation results show good agreement with experimental data. However, the simulation results of the CSTM fit experimental data much better than that of PFM over a broad range of experimental conditions. The simulation results also show it is very important to match reaction conditions and reactor parameters. Introduction The oxidative coupling of methane (OCM) to ethylene and ethane is one of important routes of natural gas utilization. A lot of research work on the OCM has been made since the pioneering work of Keller and Bhasin (1982). However, most of the research has focused on converting methane and molecular oxygen over a catalyst (Amenomiya et al., 1990; Fox, 1993). Little work has been done on the configuration of the reactors, modes of gas, and solid contact. Solid oxide membrane reactors have the advantages of enhanced catalytic activity and selectivity, better process integration, reduced feedstock, ease of reaction rate control and lower risk of explosion (Stoukides, 1988). Recently, increasing attention is on the testing of the OCM reaction in the solid oxide membrane reactors. Otsuka et al. (1985) first studied the OCM reaction in this type of membrane reactors with Ag or Ag/Bi2O3 applied on yttria-stabilized zirconia (YSZ) in 1985. The results showed that oxygen supplied by YSZ was more active than gas-phase supplied oxygen. Two years later, they studied the same system with LiCl/NiO (Otsuka and Suga, 1988). The C2 selectivity could reach 92%. They concluded that O2- pumping created active oxygen species for C2 hydrocarbons on the LiCl/NiO. Work in this area has been reviewed by Eng and Stoukides (1991). More recently, Vayenas et al. (1992) have reported the non-faradaic electrochemical modification of the catalytic activity (NEMCA) effect, which can enhance the rates dramatically. Although a lot of catalyst compositions have been studied in a conventional reactor, only a few catalysts are capable of achieving C2 selectivity g 80% at CH4 conversion g15% (Lunsford, 1994). The 1 wt % Sr/ La2O3 catalyst is one of the few catalysts that are known to achieve the above criteria (Mimoun et al., 1989). In this paper, the OCM reaction is studied in the solid oxide membrane reactor using a combination of 1 wt % Sr/La2O3 and Bi2O3 as the catalyst and YSZ as the oxygen ion conductor. In addition, few simulation and theoretical analyses on the OCM reaction in the membrane reactor have been reported (Wang and Lin, 1995). * Author to whom correspondence is addressed. Telephone: (65) 7722196. Fax: (65) 7791936. Email:
[email protected]. S0888-5885(96)00700-2 CCC: $14.00
However, the work is important to gain a better insight into the membrane reactors and provide a better experimental design for the OCM reaction in the membrane reactors. In this paper, the simulation studies on the OCM reaction in the solid oxide membrane reactor are also reported. Experimental Section Experimental Apparatus. The experimental apparatus used is shown in Figure 1. It consists of three parts: the reactant gas delivery system, the membrane reactor system, and the analytical and measuring system. Reactant gases, methane (diluted in nitrogen) and air, are flowing through a set of mass-flow controllers and enter the shellside and the tubeside of the reactor, respectively. The reactor is of a tube-and-shell type configuration, as shown in Figure 2. It consists of a yttria-stabilized zirconia (YSZ) tube (17 mm i.d., 20 mm o.d.) inside a quartz tube (22 mm i.d., 25 mm o.d.). The reactor is operated under atmospheric pressure. The reactor temperature is controlled within 1-2 Κ by a YCC-161 temperature controller and monitored by a thermocouple near the catalyst. The reactor is driven galvanostatically by a E3631A triple output dc power supply. The reactor working voltage and current are measured by a A502 galvanometer and a PZ-92 dc digital voltmeter, respectively. Analysis of reactants and products is performed using a gas chromatograph with a thermal conductivity detector. Two columns are required. Porapak Q and molecular sieve 5A are used as the column packings. To start an experimental run, the reactor is first heated slowly (2-4 Κ/min) to reaction temperature (923-1073 K) under N2 fed to the shellside. When the temperature reaches the reaction temperature, N2 is replaced by CH4 to start the reaction. For the experiments reported here, the total flow rate in the shellside is varied in the range of 6.82 × 10-6-2.73 × 10-4 mol/ s. In the course of the experiments, the feed flow rate and composition, reactor temperature, and current (00.7 A) are adjusted. The cell voltage is monitored, less than 2.0 V, until it stabilized at its steady-state value. Generally, it takes 0.5-1.0 h to reach the steady state. Then the voltage, current, and chemical composition of the feed and product streams are measured or analyzed. © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3577
Figure 1. Schematic diagram of the apparatus: 1, mass flow controllers; 2, mixer; 3, membrane reactor; 4, tubular furnace; 5, temperature controller; 6, gas chromatograph.
Figure 3. Coating apparatus: 1, rotating axle; 2, coating cell; 3, Ag-YSZ; 4, coating mixture; 5, plug.
Figure 2. Solid oxide membrane reactor: 1, quartz tube; 2, YSZ tube; 3, Ag wire; 4, galvanometer; 5, digital voltmeter; 6, power supply.
The main detectable products are C2H6, C2H4, CO2, H2, and H2O. The carbon atom balance does not exceed (2%, which indicates that no consistent loss of material, no significant formation of other oxygenated species, and coking of the catalyst. CH4 conversion and C2 selectivity are calculated by:
XCH4 ) (F0CH4 - FCH4)/F0CH4
(1)
Sc2 ) 2(FC2H4 + FC2H6)/(F0CH4 - FCH4)
(2)
Electrode-Catalyst Preparation. The solid oxide membrane consists of three layers: YSZ layer, electrode layer, and catalyst layer. The YSZ layer (YSZ tube) is composed of 8 mol % yttria in zirconia, which is available commercially (purchased from Luoyang Institute of Refractories Research, China). The porous electrode layer is made of Ag, prepared according to the following procedure. First, the YSZ tube is cleaned by rinsing in distilled water in an ultrasonic bath, followed by drying and calcining for 4 h at 1223 K. After the YSZ tube is cooled down naturally, it is slightly heated and coated using AgNO3 in nitrocellulose/butyl acetate, followed by drying and calcining. The procedure is repeated several times in order to achieve an electrode resistance of less than 0.30 Ω. The catalyst layer is composed of 1 wt % Sr/La2O3-Bi2O3, prepared from Sr(NO3)2, La2O3, and Bi(NO3)3‚5H2O. At first, Bi2O3 is obtained by calcining Bi(NO3)3‚5H2O, and 1 wt % Sr/ La2O3 is prepared by impregnation and ground to powder. Then, the Bi2O3 powder is added to nitrocel-
Figure 4. Schematic illustration of CSTM and PFM: (a) CSTM; (b) PFM.
lulose/butyl acetate and applied to the external surface of the Ag-YSZ tube using the method shown in Figure 3, followed by drying and calcining in air. Finally, the powder product of 1 wt % Sr/La2O3 is added to nitrocellulose/butyl acetate and applied to the surface of the Bi2O3-Ag-YSZ tube using the same method, followed by drying and calcining in air. The surface and the section of the 1 wt % Sr/La2O3-Bi2O3-Ag-YSZ membrane have been studied by a scanning electron microscope (SEM). The micrographs have been published elsewhere (Guo and Chen, 1995a). Modeling. Two ideal theoretical models used to describe the behavior of the solid oxide membrane reactors are the continuously stirred tank model (CSTM) and the plug flow model (PFM). The two models are based on the following assumptions: constant pressure, isothermal, steady-state operation, negligible masstransfer resistance, and ideal gas behavior. The rates for the reactions are assumed to be of the form: m
Rj )
γijri ∑ i)1
(j ) 1, 2, ..., n)
(3)
The kinetic rate expressions have been reported elsewhere (Guo and Chen, 1995b), which are as appended. Based on these assumptions, the CSTM equations for the well-mixed flow in both the shellside and the tubeside of the membrane reactor (as shown in Figure 4) are developed by taking the material balance for each
3578 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
component in the shellside and the tubeside:
In the shellside: F0j - Fj + Nj - SrRj ) 0 (j ) 1, 2, ..., n)
(4)
In the tubeside: G0j - Gj - Nj ) 0 (j ) 1, 2, ..., n)
(5)
The PFM equations for the plug flow in both the shellside and the tubeside of the membrane reactor (as shown in Figure 4) are as follows:
In the shellside: dFj Nj SrRj + ) 0 (j ) 1, 2, ..., n) dz L L
(6)
In the tubeside: dGj Nj ) 0 (j ) 1, 2, ..., n) dz L
(7)
The boundary conditions are
at z ) 0, in the shellside
Fj ) F0j (j ) 1, 2, ..., n) (8)
at z ) 0, in the tubeside
Gj ) G0j (j ) 1,2, ..., n) (9)
Figure 5. C2 selectivity and CH4 conversion as a function of oxygen flux (T ) 1003 K; in shellside, Ft ) 6.82 × 10-5 mol/s, yCH4 ) 0.20; in tubeside, Gt ) 1.70 × 10-4 mol/s, yO2 ) 0.21).
Due to the fact that YSZ is a good O2- conductor, it may be considered that other components except oxygen cannot permeate through the membrane. According to Faraday’s law, the oxygen permeation flux can be written as
NO2 ) I/4f, Nj ) 0 (j * O2, j ) 1, 2, ..., n)
(10)
The set of model equations (4) and (5) are nonlinear. The modified Newton-Raphson method is employed to solve them numerically. The Jacobian matrix is divided into two parts: an analytical expression part and an approximate part. The later part is computed by numerical differentiation. The set of model equations (6) and (7) are ordinary differential equations, which can be integrated by the implicit Adams-Moulton’s method or Gear’s backward differentiation formulas (BDF) method. The exact numerical details can be found elsewhere (Gear, 1971). Results and Discussion Both the experimental and simulation results with the membrane reactor are shown in Figures 5-10. (The scattered points stand for experimental data. The lines stand for the model results). Figure 5 shows the effect of oxygen flux on C2 selectivity and CH4 conversion. It can be seen that the CH4 conversion increases and C2 selectivity decreases with a rise in oxygen flux. This indicates that the C2 hydrocarbons are much more active than methane and form the complete oxidation products much easier. The results are consistent with the results experimented with the conventional cofeed reactor (Amenomiya et al., 1990). Figure 6 shows the effect of temperature on C2 selectivity and CH4 conversion at constant feed concen-
Figure 6. C2 selectivity and CH4 conversion as a function of temperature (in shellside, Ft ) 6.82 × 10-5 mol/s, I ) 0.25 A, yCH4 ) 0.20; in tubeside, Gt ) 1.70 × 10-4 mol/s, yO2 ) 0.21).
tration and flow rate. It can be seen that CH4 conversion shows an increase with increasing temperature. C2 selectivity increases initially with a rise in temperature and then appears to reach a plateau, and for T > 993 K, it starts decreasing. C2 selectivity shows a maximum at around 993 K, which is the optimal temperature for the synthesis of C2 hydrocarbons. This indicates that the complete oxidation reaction occurs much easier under high temperatures. The effect of temperature on the ethylene-to-ethane ratio is shown
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3579
Figure 7. Effect of temperature on C2H4/C2H6 (conditions as in Figure 6).
Figure 9. Profiles of C2 selectivity and CH4 conversion in the shellside of the reactor (T ) 1003 K; in shellside, Ft ) 6.82 × 10-5 mol/s, I ) 0.25 A, yCH4 ) 0.20; in tubeside, Gt ) 1.70 × 10-4 mol/s, yO2 ) 0.21).
Figure 10. Profiles of C2 selectivity and CH4 conversion in the shellside of the reactor (T ) 1003 K; in shellside, Ft ) 6.82 × 10-5 mol/s, I ) 0.25 A, yCH4 ) 0.20; in tubeside, Gt ) 1.70 × 10-4 mol/s, yO2 ) 0.21).
Figure 8. C2 selectivity and CH4 conversion as a function of feed methane mole fraction (T ) 1003 K; in shellside, Ft ) 6.82 × 10-5 mol/s, I ) 0.25 A; in tubeside, Gt ) 1.70 × 10-4 mol/s, yO2 ) 0.21).
in Figure 7. It is observed that the ratio increases with increasing temperature. The result is consistent with that in a conventional reactor. This indicates that ethylene is formed from ethane as a secondary product. The reaction of ethane oxidative dehydrogenation is enhanced with a rise in temperature. Figure 8 shows C2 selectivity and CH4 conversion as a function of feed methane concentration. N2 is used as a diluent. It can be seen that C2 selectivity increases and CH4 conversion decreases with a rise in feed CH4 concentration. The amount of oxygen transported limits
the reaction. The feed methane concentration should be set as high as possible if only high C2 selectivity is desirable. The above observations may be explained by analyzing the mechanism. Although the detailed mechanism of the OCM reaction is unknown at present, it is generally accepted that methane is transformed to methyl radicals which combine to generate C2H6. Based on the literature and our previous studies, the reaction mechanism in the membrane reactor is proposed elsewhere (Guo and Chen, 1995b), which has been attached in the Appendix. The model does not attempt to describe all of the reaction steps and intermediates involved in the OCM reaction due to the complex mechanism. In the mechanism, methane reacts with the active oxygen species assured as O- to produce CH3• radicals on the surface of the membrane, and then two
3580 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Table 1. Kinetic Parameters Arrhenius constants parameter
A
E*
k1 k3 k4 k-4 k5 k6 k7 K4 K8
1.29 1.03 × 104 2.28 × 102 1.26 × 102 1.05 1.54 × 10 1.65 × 10 9.87 × 10-1 1.20
137.10 × 103 145.37 × 103 111.86 × 103 100.68 × 103 166.11 × 103 78.63 × 103 71.57 × 103 35.87 × 103 20.99 × 103
CH3• radicals couple to form C2H6. C2H6 is oxidized and dehydrogenated further to produce C2H4. C2H6 and C2H4 are oxidized further by another form of oxygen species O to form CO2. The active oxygen species Oattend three reactions in which CH3• radicals, C2H4, and O are produced. With increasing in the oxygen flux, the oxygen species O becomes an abundant species. Thus, more C2H6 and C2H4 are oxidized completely, leading to decreasing C2 selectivity. As shown in Table 1, the activation energy of C2H6 and C2H4 formation is higher than that of CO2 formation. Therefore, C2 selectivity increases with increasing temperature initially. However, after the temperature reaches a maximum, C2 selectivity is controlled mainly by the gas phase reactions, resulting in decreasing C2 selectivity. The activation energy of C2H4 formation is higher than that of C2H6 formation. Thus, the ethylene-to-ethane ratio increases with a rise in temperature. When methane concentration in the feed stream is increased, more CH3• radicals are generated, leading to a rise in C2 selectivity, but the oxygen flux controls the reaction. Comparisons of the simulation results of the two membrane reactors with the experimental data show that the simulation results of CSTM agree better with the experimental data. This is due to the small lengthto-radius of the experimental reactor which results in the behavior of the membrane reactor used for the experiments resembling the CSTR. However, the tendency of the effects of reaction conditions on the reaction is the same between the two models. In addition, there are distinct differences between the simulation results of the two models. Figures 5-8 show that both C2 selectivity and CH4 conversion for PFM are higher than those for CSTM, but the ethylene-to-ethane ratio for PFM is lower than that for CSTM due to the effect of the different degrees of backmixing in the membrane reactor. The OCM reaction is a homogeneous-heterogeneous reaction. Part of the oxygen transported through the membrane can be changed into the oxygen species, which can react with C2 hydrocarbons further. As a result, a lower C2 selectivity occurs. This indicates that C2 hydrocarbons are relatively unstable and easily involve sequential oxidation reactions. Therefore, the degree of backmixing in the membrane reactor should be minimized to achieve a higher yield. On the other hand, backmixing is favorable to producing ethylene. This indicates further that ethylene is formed from ethane. Figure 9 shows the calculated profiles of C2 selectivity and CH4 conversion in the shellside along the longitudinal direction of the membrane reactor with PFM. Values of the reactor parameters used in the calculations are the same as that of the experimental membrane reactor. This shows that C2 selectivity and CH4 conversion increase with increasing the reactor length. C2 selectivity and CH4 conversion may be
improved further if the length of the reactor is extended. Figure 10 shows the calculated profiles of C2 selectivity and CH4 conversion in the shellside when the reactor length is 1.0 m. Values of other parameters in Figure 10 are the same as those in Figure 9. This shows that C2 selectivity increases with increasing the reactor length, reaches 100% at about Z ) 0.5 m, and becomes constant when Z > 0.5 m. CH4 conversion increases slightly with increasing Z. This is because the amount of oxygen transported is dependent on the external current. There is no point in increasing the reactor length further when it reaches a certain value under constant reaction conditions because the amount of oxygen transported controls the performance of the reaction. This information is useful for future membrane reactor design. It indicates that a match of reaction conditions and reactor parameters is very important to achieve a high C2 selectivity and CH4 conversion. The results are different from those obtained in dense oxide membrane reactors. The simulation study of OCM in dense oxide membrane reactors by Wang and Lin (1995) showed that C2 selectivity decreases and CH4 conversion increases with increasing the reactor length. The different results are caused by the different permeation mechanisms of oxygen between the two types of membranes, the different reaction conditions and reactor parameters. Conclusions 1 wt % Sr/La2O3-Bi2O3-Ag-YSZ solid oxide membrane reactor has been used to carry out the OCM reaction. CH4 conversion increases and C2 selectivity decreases sharply with increasing oxygen flux. There is an optimal temperature for C2 selectivity. If only C2 selectivity is concerned, the methane feed concentration should be as high as possible. Two mathematical models based on well-mixed flow and plug flow models have been developed to describe the solid oxide membrane reactors. A comparison between the simulation results and experimental data shows that the agreement can be considered as satisfactory. Higher C2 selectivity and CH4 conversion can be reached under plug flow conditions compared with well mixed flow conditions in the solid oxide membrane reactor. Reaction conditions and reactor parameters must match to achieve a high C2 yield. Nomenclature A ) preexponential factor E* ) activation energy, J‚mol-1 E ) active site e ) electron F ) mole flow rate in the shellside, mol‚s-1 f ) Faraday constant G ) mole flow rate in the tubeside, mol‚s-1 I ) current, A J ) oxygen flux, mol‚s-1 K ) equilibrium constant k ) reaction rate constant, mol‚m2‚s-1 L ) reactor length, m m ) number of reactions N ) permeation flux, mol‚s-1 n ) number of components Pi ) i component partial pressure, Pa R ) total reaction rate, mol‚m2‚s-1 r ) reaction rate, mol‚m2‚s-1
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3581 Sc2 ) C2 selectivity, % Sr ) effective reaction area, m2 T ) temperature, K XCH4 ) CH4 conversion, % y ) mole fraction, % Z ) longitudinal distance, m
rC2H4 )
k3yC2H6JO2
k7K4yC2H4JO2
k1yCH4 + k3yC2H6
-
k1yCH4 + k3yC2H6 + K4JO2
+
k5yC2H6 (A12) rCO2 )
Greek Symbol γ ) stoichiometric coefficient Superscript
(k6yC2H6 + k7yC2H4)K4JO2 k1yCH4 + k3yC2H6 + K4JO2
(A13)
(2) Existence of Gas-Phase Oxygen. In the case of the existence of gas-phase oxygen, there is an equilibrium in step (A9):
0 ) input Subscripts g ) gas phase t ) total i ) reaction i j ) component j
k8/k-8 ) K8
(A14)
The formation rate of C2H6, C2H4, and CO2 are as follows:
Appendix The kinetic equations for OCM in the membrane reactor are derived based on the following mechanism (Guo and Chen, 1995b): JO 2
O2–
k1
CH4(g) + O–E1
k2
2CH•3(g) C2H6(g) + O–E1
k3
O– + e
(A1)
CH•3(g) + HO–E1
(A2)
C2H6(g)
(A3)
HO–E1 + C2H•5
(A4)
C2H4(g) O–E1
k4
+ E2
C2H6(g)
k5
C2H6(g) + 7OE2 C2H4(g) + OE2 2OE2
(A5)
C2H4(g) + H2(g)
(A6)
k6 k7 k8 k–8
2CO2 + 3H2O 2CO2 + 2H2O O2 + 2E2
(A9)
(A10)
(k1yCH4 - k3yC2H6)JO2 k1yCH4 + k3yC2H6
-
k6K4yC2H6JO2 k1yCH4 + k3yC2H6 + K4JO2
k6yC2H6xyO2
(xK8 + xyO2)
-
- k5yC2H6 (A15)
k3yC2H6[JO2(xK8 + xyO2) + k-4xyO2] (xK8 + xyO2)(k1yCH4 + k3yC2H6) + k4xK8 k7yC2H6xyO2
(xK8 + xyO2)
-
+ k5yC2H6 (A16)
(A8)
The formation rates of C2H6, C2H4, and CO2 are as follows:
r C 2H 6 )
(xK8 + xyO2)(k1yCH4 + k3yC2H6) + k4xK8
(A7)
in which E1 and E2 are assumed to be two active sites on the membrane surface. Assuming that the rate-determining step in each reaction is first order in the reactants, the coverage of the species absorbed on E1 is low, there is a quasi steady state for the intermediate species, etc., the following expressions can be obtained. Two situations with the absence and existence of gas-phase oxygen can be considered due to the complex mechanism. (1) Absence of Gas-Phase Oxygen. In the case of the absence of gas-phase oxygen, there is no step (A9). There is an equilibrium in step (A5):
k4/k-4 ) K4
(k1yCH4 - k3yC2H6)[JO2(xK8 + xyO2) + k-4xyO2]
r C 2H 4 )
OE2 + E1 + e
k–4
r C 2H 4 )
- k5yC2H6 (A11)
rCO2 )
(k6yC2H6 + k7yC2H4)xyO2 (xK8 + xyO2)
(A17)
The kinetic parameters estimated are shown in Table 1. Literature Cited Amenomiya, Y.; Birss, V. I.; Golegzinowski, M.; Galuszka, J.; Sanger, A. R. Conversion of Methane by Oxidative Coupling. Catal. Rev.sSci. Eng. 1990, 32 (3), 163. Eng, D.; Stoukides, M. Catalytic and Electrocatalytic Methane Oxidative with Solid Oxide Membranes. Catal. Rev.sSci. Eng. 1991, 33 (3 & 4), 375. Fox, J. M. the Different Catalytic Routes for Methane Valorization: An Assessment of Processes for Liquid Fuels. Catal. Rev.sSci. Eng. 1993, 35 (2), 169. Gear, C. W. In Numerical Initial-Value Problems in Ordinary Differential Equations; Prentice-Hall: Englewood Cliffs, NJ, 1971. Guo, X. M.; Chen, H. F. Oxidative Coupling of Methane in Membrane Reactors with Oxygen Pumping Type. J. Mol. Catal. (China) 1995a, 9 (5), 393. Guo, X. M.; Chen, H. F. Oxidative Coupling of Methane in Membrane Reactors with Oxygen Pumping Type (II) Reaction Kinetic Study. J. Chem. Ind. Eng. (China) 1995b, 46 (3), 310. Keller, G. E.; Bhasin, M. M. Synthesis of Ethylene via Oxidative Coupling of Methane. J. Catal. 1982, 73, 9.
3582 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Lunsford, J. H. Recent Advances in the Oxidative Coupling of Methane. In Natural Gas Conversion II; Curry-Hyde, H. E., Howe, R. F., Eds.; Elsevier Science B.V.: Amsterdam, The Netherlands, 1994. Mimoun, H.; Robine, A.; Bonnaudet, S.; Cameron, C. J. Oxidative Coupling of Methane followed by Ethane Pyrolysis. Chem. Lett. 1989, 2185. Otsuka, K.; Suga, K. Electrochemical Enhancement of Oxidative Coupling of Methane over LiCl-doped NiO Using Stabilized Zirconia Electrolyte. Catal. Lett. 1988, 1, 423. Otsuka, K.; Yokoyama, S.; Morikawa, A. Catalytic Activity- and Selectivity-Control for Oxidative Coupling of Methane by Oxygen-Pumping through Yttria-stabilized Zirconia. Chem. Lett. 1985, 319. Stoukides, M. Applications of Solid Electrolytes in Heterogenous Catalysis. Ind. Eng. Chem. Res. 1988, 27, 1745.
Vayenas, C. G.; Bebelis, S.; Yentekakis, I. V.; Lintz, H.-G. NonFaradaic Electrochemical Modification of Catalytic Activity: A Status Report. Catal. Today 1992, 11 (3), 303. Wang, W.; Lin, Y. S. Analysis of Oxidative Coupling of Methane in Dense oxide Membrane Reactors. J. Membr. Sci. 1995, 103, 219.
Received for review November 4, 1996 Revised manuscript received February 4, 1997 Accepted February 6, 1997X IE9607006
X Abstract published in Advance ACS Abstracts, March 15, 1997.
