Ind. Eng. Chem. Res. 2009, 48, 641–646
641
Activation of Methane over Perovskite Catalysts Li Jun,*,† Zhao Ling,‡ and Lu Guanzhong† Department of Chemical Engineering, Shanghai Institute of Technology, Shanghai 200235, China, and UNILAB Research Center of Chemical Reaction Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China
Perovskite-type catalysts group one Ln0.6Sr0.4FexCo1-xO3 (Ln ) Nd, Pr, Gd, Sm, La, 0 < x < 1), (La0.8Sr0.2)0.9MnO3 and La0.8Sr0.2CrO3, and group two sodium-doped ACoO3 (Na:A ) 1:4) and A0.8Na0.2MnO3 (A ) La, Gd, Sm) were synthesized, and their properties for activation of methane were evaluated in a quartz reactor first. Catalysts group one presented much higher activity than catalysts group two at temperatures below 740 °C, and the main product was, however, carbon dioxide. Group two showed much higher selectivity to C2 hydrocarbons than group one. Electrochemical measurements were conducted in a solid oxide membrane reactor with sodium-doped LaCoO3 as the catalyst electrode. It was found that electrochemical supply of O2to the catalyst film can significantly change the rate of methane consumption and C2 hydrocarbon selectivity. The total selectivity to C2 hydrocarbon exceeded 80% in the case of the electrochemical supply of oxygen. On the basis of the experimental results, a kinetic model was suggested to describe the reaction results. Introduction Methane is the main component of natural gas. The world reserve of natural gas is abundant, and methane may well become a primary energy source and a raw material of chemicals in the 21st century.1 The quantity of methane used as raw materials of the chemical industry accounts for only 5-7% of the total consumption. In the long run, methane may well become the main energy source and the primary raw material for many chemical products in the 21st century with the rapid depletion of crude oil. Research and development of methaneutilizing techniques is a hot subject in the world. Basically, utilization of methane can be divided into direct and indirect methods. Indirect utilization of methane is a more ripe method compared to the direct one, according to which methane is converted to syngas first, and the resultant syngas can be converted to methanol or other products; however, there are many disadvantages, for instance, it is a complicated process, high amounts of energy are consumed, there is a high production cost, and so forth. Direct conversion of methane to valuable products seems to be attractive for overcoming the economic problem, especially partial oxidation or oxidative coupling of methane (OCM). It is promising for methane being partially oxidized to methanol and formaldehyde through one step,2-4 but it is also challenging because methanol and formaldehyde are much more active than methane. Up to now, great efforts have been devoted to increase the conversion of methane, and selectivity to the goal products, methods of heterogeneous oxidation,5,6 and homogeneous oxidation7,8 have been developed, and significant progress has been made.6,8 The oxidative coupling of methane leading to C2 hydrocarbons ethane and ethylene has received much attention. Numberous papers have been published each year, but determining how to convert methane to C2 hydrocarbons with high selectivity is difficult because complete oxidation of CH4, C2H6, and C2H4 to CO2 and H2O must be suppressed.9 In past decades, significant work has been reported toward the development of efficient catalysts for OCM. These catalysts may be divided into three * To whom correspondence should be addressed. E-mail: lij82@ tom.com. † Shanghai Institute of Technology. ‡ East China University of Science and Technology.
major groups: alkali and alkaline earth metal compounds, rare earth metals, and other transition-metal compounds. Work on catalysts for OCM has been reviewed,10 and many details can be found therein. The activity and selectivity of catalysts for this reaction are very dependent on the experimental conditions, such as, temperature, space velocity, CH4/O2 ratio, and so forth. Many methods have been suggested to achieve high yields and selectivity. These include the work of Keller and Bhasin,11 who obtained most of their results by a sequential feed of oxygen and methane so that the reactant oxygen is either adsorbed on the catalyst surface or incorporated into the lattice of an oxide catalyst; the work of Tonkovich and coworks12 on a moving bed reactor; and that of Vayenas and co-workers13 on a gas recycle solid oxide electrolyte reactor. High C2 yields and selectivity were obtained by the above methods, but the conversion of methane per pass is expected to be enhanced further. Owing to high reaction activity of ethane and ethylene, development of a new reactor is extremely important. Membrane reactors have attracted increasing attention in recent years, and substantial progress has been made.14,15 Studies in this aspect can be classified into two groups: one is a dense membrane reactor, and the other is a porous ceramic membrane reactor. In the previous studies, a porous ceramic membrane served as an oxygen distributor, and catalysts were coated on the surface of the membrane. Results of OCM in the porous membrane reactor depend on the catalysts and the distribution of oxygen along the axial direction.16,17 In the dense membrane reactor for OCM, methane is fed to one side of the membrane and reacts with oxygen permeating from the other side of the membrane. Dense membrane materials possess O2--conducting properties, and these materials are solid solutions of oxides of divalent or trivalent cations in oxides of tetravalent metals. Out of these O2- conductors, yttria-stabilized zirconia (YSZ) showed good chemical and mechanical stability and was frequently used. The flux of O2- can be controlled by an external electrical circuit; oxygen permeating through the membrane may not get into the gas phase before being adsorbed onto the surface of the coated catalytic particles, and further oxidation of methyl radicals or C2 products in the gas phase may be reduced. Oxidative coupling of methane in the dense
10.1021/ie8008007 CCC: $40.75 2009 American Chemical Society Published on Web 12/04/2008
642 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009
membrane reactor has been reviewed,18-22 and many details about this work can be found therein. Some perovskite-type catalysts were reported to be effective for OCM,20,22,15,23 such as, Sm0.5Ce0.5CuO3, Gd0.9Na0.1MnO3,24 La1-ySryCo1-xFexO3,15,25 Ba0.5Sr0.5Co0.8Fe0.2O3-δ,23 and so forth. These catalysts were also considered for their ionic and electronic conductivity, chemical and mechanical stability, and compatibility with YSZ. They can be deposited on the surface of the dense ceramic membrane, and so perovskite-type oxides deserved further investigation. To find suitable perovskite-type catalysts for OCM, the following catalysts were synthesized: group one Ln0.6Sr0.4FexCo1-xO3 (Ln ) Nd, Pr, Gd, Sm, La, 0 < x < 1), (La0.8Sr0.2)0.9MnO3 and La0.8Sr0.2CrO3; group two sodium-doped ACoO3 and A0.8Na0.2MnO3 (A ) La, Gd, Sm). First, the newly synthesized catalysts were tested in a fixed-bed reactor made of quartz tube for activation of methane. On the basis of the experimental results, one of the above catalysts was selected and coated on the outside surface of the YSZ tube. Effects of electrochemical oxygen supply on the rate of CH4 consumption and selectivity to C2 hydrocarbons were investigated.
Figure 1. Schematical diagram of electrochemical reactor system: 1, fine metering valve; 2, rotameter; 3, gas mixing bottles; 4, YSZ tube; 5, quartz tube; 6, oven; 7, gavanostat-potentiostat; 8, temperature controller; 9, gas chromatograph.
Experiment Section The perovskite-type oxide, Ln0.6Sr0.4FexCo1-xO3 (Ln ) Nd, Pr, Gd, Sm, La, 0 < x < 1), (La0.8Sr0.2)0.9MnO3, La0.8Sr0.2CrO3, Na-ACoO3 (Na:A ) 1:4), and A0.8Na0.2MnO3 (A ) La, Sm, Gd), were synthesized by the solid-reaction method.26 The starting materials are: Nd2O3 (99.9%), Pr6O11 (99.9%), Gd2O3 (99.9%), Sm2O3 (99.9%), SrCO3 (99.0%), Fe2O3 (99.9%), Co3O4 (99.9%), La2O3 (99.9%), Mn2O3 (99.9%), Cr(NO3)3, and Na2CO3 (99.0%). These materials are obtained from Shanghai Chemical Reagent Corp. The starting powders were well mixed in proper ratios in absolute alcohol using an agate mortar and pestle. The resulting mixture was exposed to air for about 30 min, then poured into a module, and was pressed into disk. The disk was calcined at 1200°C for 24 h or more in air. X-ray diffraction (XRD) patterns of the synthesized sample were obtained on a RigakuRC (12 kW) diffractometer with monochromated Cu KR radiation. The temperature-programmed studies were carried out by passing ultrahigh-purity O2 over the synthesized catalysts while the temperature was from room temperature to 850 °C with a heating rate of 5 °C/min. The catalyst was then cooled down to room temperature under oxygen atmosphere. The excess oxygen was flushed out by high-purity helium. Temperature-programmed desorption of oxygen was carried out by raising the temperature linearly with a heating rate of 60 °C/min and a flow rate of 30 mL/min of high-purity He. Products were analyzed by an online gas chromatograph. The fixed-bed reactor was made of quartz tube (15 mm inside diameter and 30 cm length). Methane (99.9%), oxygen (99.9%), and nitrogen (99.99%) were used as the feed gases. O2 and CH4 were diluted by N2, and premixed before entering the fixedbed reactor. The mixture of O2, CH4 and N2 was carried to the reactor. The total volumetric flow rate of the feed mixture was 110 mL (NTP)/min. The catalyst to be tested was placed in the quartz tube. The composition of the feed and product gases were analyzed by gas chromatography (Wenling Scientific Instrument Corp.) equipped with FID and TCD detectors, using nitrogen gas as a carrier. A known gas sample was used to quantify the feed gases and products. All gas samples were taken 20 min or a longer time apart to ensure complete equilibration after changing a given parameter.
Figure 2. Configuration of electrodes system.
Figure 3. X-ray power diffraction patterns of the samples: (a) LaCoO3, (b) Na-GaCoO3.
The solid oxide membrane reactor or electrochemical reactor was constructed with a closed-one-end YSZ tube (12 mm outside diameter, 10 mm inside diameter), obtained from Shanghai Silicate Institute, and quartz tube (22 mm outside diameter, 20 mm inside diameter), and the volume available was about 20 cm3. Catalyst Na-LaCoO3 was selected and coated on the outside surface of the YSZ tube for oxidative coupling of methane in the electrochemical reactor. Ag paste was deposited on the inside surface of YSZ tube and was used as a counter electrode and a reference electrode. The area available was about 7 cm2. The general electrochemical reactor system and configuration of electrodes system are shown schematically in Figures 1 and 2. The reactor was controlled galvanostatically by a potentiostat/ galvanostat. The current and voltage between electrodes were measured by a digital multimeter.
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 643 Table 1. Cell Parameters of the Synthesized Catalysts catalysts
crystal structure a (Å) b (Å)
Nd0.6Sr0.4Fe0.2Co0.8O3 Gd0.6Sr0.4Fe0.2Co0.8O3 Pr0.6Sr0.4Fe0.2Co0.8O3 Sm0.6Sr0.4Fe0.2Co0.8O3 La0.6Sr0.4Fe0.2Co0.8O3 (La0.8Sr0.2)0.9MnO3 La0.8Sr0.2CrO3 La0.8Na0.2MnO3 Gd0.8Na0.2MnO3 Sm0.8Na0.2MnO3
orthorhombic orthorhombic orthorhombic orthorhombic orthorhombic orthorhombic orthorhombic rhombohedral rhombohedral rhombohedral
5.376 5.379 5.377 5.379 5.435 5.521 5.475 5.515 5.508 5.509
5.427 5.392 5.417 5.387 5.445 5.551 5.503
c (Å)
volume (Å3)
7.622 7.608 7.663 7.612 7.665 7.825 7.674 13.39 13.34 13.37
222.4 220.7 223.2 220.6 226.8 239.8 231.2 351.6 349.3 350.1
Oxygen flowing into the YSZ tube was reduced to O2- on the catalyst Ag membrane deposited on the inside of the YSZ tube. These ions (O2-) were pumped electrochemically through the electrolyte to the other side. Methane diluted by nitrogen went into the room between the YSZ tube and the quartz tube and was oxidized over a catalyst electrode of Na-LaCoO3. Methane conversion is defined as follows:28 X)
(GinYin,CH4 - GoutYout,CH4) GinYin,CH4
(1)
where G stands for volumetric flow rate and Yin,CH4 and Yout,CH4 are molar fractions in the feed gas and the effluent gas, respectively. The selectivity to hydrocarbon j is calculated by: 28
Sj )
2GoutYj,out GinYin,CH4 - GoutYout,CH4
(2)
where j stands for C2H4 or C2H6. The total selectivity to C2 hydrocarbons is expressed by SC2, and is calculated by:28 SC2 ) SC2H4 + SC2H6
(3)
The instant formation rates of product j are calculated by:28 rj ) GoutYj,out
(4)
Results XRD Characterizations and TPD Measurement. The solid solutions and perovskite phase structure of Ln0.6Sr0.4FexCo1-xO3 (Ln ) Nd, Pr, Gd, Sm, La, 0 < x < 1), (La0.8Sr0.2)0.9MnO3, La0.8Sr0.2CrO3, ACoO3 and A0.8Na0.2MnO3 (A )La, Sm, Gd) were confirmed by the XRD patterns. Na-ACoO3 (Na:A ) 1:4, A ) La, Sm, Gd) samples consists of a sodium oxide layer coated on the ACoO3 (A ) La, Sm, Gd) particles. The desired single perovskite phase of A0.8Na0.2CoO3 (A ) La, Sm, Gd) were not confirmed as shown in Figure 3. The XRD results indicate that the prepared powders Ln0.6Sr0.4FexCo1-xO3 (Ln ) Nd, Pr, Gd, Sm, La, 0 < x < 1) and La0.8Sr0.2CrO3 are perovskite type oxides with the crystal structure of orthorhombic, whereas A0.8Na0.2MnO3 (A ) La, Sm, Gd) and (La0.8Sr0.2)0.9MnO3 are perovskite-type oxides with the crystal structure of rhombohedral. The crystal cell parameters of the synthesized catalysts are presented in Table 1. Temperature-programmed desorption (TPD) of oxygen was used in the present study to investigate the metal-oxygen bonds of the synthesized catalytic materials and relate the binding energy with the catalytic performance for OCM. The TPD curves of the synthesized catalysts are shown in Figure 4. All samples are characterized by a major peak, although a very weak signal also appears at lower temperature for every sample tested.
Figure 4. TPD spectra of oxygen (m/e ) 32) of the samples: (a) La0.6Sr0.4Fe0.2Co0.8O3, (b) Na-GaCoO3, (c) La0.8Sr0.2CrO3, (d) La0.8Na0.2MnO3, and (e) Ga0.6Sr0.4Fe0.2Co0.8O3.
The top temperature (Tmax) corresponding to the major peak reflects the energy of the metal-oxygen bond, the higher Tmax means the stranger binding energy. The values for Tmax ranges from 732 for La0.8Sr0.2CrO3 to 868 for Na-GaCoO3, introduction of alkali-metal ion increases Tmax prominently, and the selectivities to C2 are consistent with Tmax in order for the present study. The binding energy were correlated with the selectivity to C2 hydrocarbons by Elliott,27 and the present observation agrees with the earlier claim. Catalytic Properties of the Catalysts in a Fixed-Bed Reactor. Experimental variables and results are presented in Table 2. Conversion is reported as the mole percent of the methane converted to products. The main products included carbon dioxide, ethane, ethene, and water. For convenience, the catalysts No. 1 to No. 7 are referred to as group one, and the rest catalysts are referred to as group two. Under the experimental conditions, group one presents much higher activity than group two at temperatures below 740 °C. The selectivity to C2 hydrocarbon is, however, very poor. Moreover, conversion of methane changes smaller for group one than for group two with an increase of temperature. Group one is not suitable to methane oxidative coupling; however, they may be good catalysts materials for fuel cells with methane as fuel, where the main goal is to get as much electrical power as possible. The catalytic behaviors of group two are different from those of group one. The catalytic activities of group two are much lower than those of group one at lower temperatures. Conversion of methane increases obviously with an increase of temperature, and selectivity to the goal products ethane and ethene are much higher. So, group two may be the proper candidate for OCM. Group one shows higher activity for complete oxidation of methane than the remaining six catalysts in the table at the lower temperatures. These may result from introduction of strontium and iron in the perovskite-type catalysts. The good selectivity of the last six catalysts for OCM may stem from the introduction of sodium in the catalysts. In the selective oxidation of methane to higher hydrocarbons, the important factor may be the binding energy of oxygen to a surface site of the catalyst. Higher metal-oxygen binding energy is favorable in the selective oxidation of methane to higher hydrocarbons.7 So, the introduction of sodium in the catalysts may increase the binding energy of oxygen to a surface site of the catalyst. Oxidative Coupling of Methane in the YSZ Electrochemical Reactor. Na-LaCoO3 (Na:La ) 1:4) were selected and deposited on the outside surface of the YSZ tube. Experimental measurements were conducted as a function of the following variables: (1) residence time of methane-contained gas intro-
644 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 Table 2. Performance of the Catalysts Presented in a Fixed-Bed Reactora 700 °C
No.
720 °C
740 °C
760 °C
780° C
XCH4
SC2H4
SC2H6
XCH4
SC2H4
SC2H6
XCH4
SC2H4
SC2H6
XCH4
SC2H4
SC2H6
XCH4
SC2H4
SC2H6
1 2 3 4 5 6 7 8 9 10 11 12 13
12.9 11.1 12.3 11.5 11.8 11.8 11.2 9.4 4.7 6.0 3.4 5.7 3.3
0.37 0.40 0.75 0.78 0.36 0.00 0.00 3.3 6.8 20.5 11.2 17.6 4.08
3.21 3.32 4.80 4.39 2.87 1.15 0.31 14.8 15.2 25.9 35.0 29.7 19.5
13.1 11.9 12.4 12.3 12.1 11.9 11.6 10.3 6.5 9.5 5.1 6.8 4. 6
0.68 0.81 0.95 1.00 0.68 0.12 0.00 6.8 8.9 25.4 17.3 21.7 8.09
4.12 4.60 5.41 5.09 3.98 1.70 0.48 18.7 17.8 26.6 35.6 31.3 24.4
14.3 13.4 13.0 12.4 12.6 12.2 12.4 11.5 9.3 11.2 7.0 9.1 7.8
1.04 1.34 1.35 1.81 1.13 0.31 0.00 9.1 12.2 31.8 25.0 29.4 14.8
4.88 5.41 5.96 6.99 4.76 2.35 0.73 20.4 19.2 22.7 33.8 29.0 25.8
14.9 14.6 13.5 13.0 13.6 13.0 12.6 14.2 10.7 15.5 9.1 13.7 9.5
1.47 2.03 2.08 3.72 1.72 0.75 0.19 15.8 14.0 34.8 34.6 34.3 22.0
5.72 6.64 6.90 8.01 5.59 3.09 1.22 26.9 18.5 21.4 30.0 27.0 23.6
15.3 15.5 14.3 14.5 13.8 13.9 13.3 15.1 11.3 17.2 11.9 17.0 13.2
2.16 2.92 3.12 5.98 2.73 1.27 0.40 21.7 18.1 39.1 41.1 38.3 27.6
6.91 7.83 8.15 11.15 6.27 4.09 1.69 25.4 17.8 18.4 27.3 25.7 27.0
a
NOTE: (1) Numbers in the first column: 1, Nd0.6Sr0.4Fe0.2Co0.8O3; 2, Gd0.6Sr0.4Fe0.2Co0.8O3; 3, Pr0.6Sr0.4Fe0.2Co0.8O3; 4, Sm0.6Sr0.4Fe0.2Co0.8O3; 5, La0.6Sr0.4Fe0.2Co0.8O3; 6, (La0.8Sr0.2)0.9MnO3; 7, La0.8Sr0.2CrO3; 8, La0.8Na0.2MnO3; 9, Gd0.8Na0.2MnO3; 10, Sm0.8Na0.2MnO3; 11, Na-LaCoO3 (Na:La ) 1:4); 12, Na-GdCoO3 (Na:Gd ) 1:4); 13, Na-SmCoO3 (Na:Sm ) 1:4). (2) XCH4-conversion of methane (mol%); SC2H4-selectivity to C2H4 (%), SC2H6-selectivity to C2H6 (%). (3) Operation conditions: total pressure 1 atm, total flow rate 110 mL/min(NTP), N2:CH4:O2 ) 75:25:10, catalyst weight 1 g.
Figure 5. Selectivity as a function of residence time.
duced into the reaction room, (2) operation temperature, (3) concentration of methane introduced into the reaction room, (4) the flux of oxygen pumped to the perovskite-type catalyst. First, the effect of residence time on methane oxidative coupling was examined. A long residence time means that the reactant has more chances to make contact with catalyst electrode Na-LaCoO3, and more reactants may be conversed to products, whereas the goal products C2 hydrocarbons may have much time to be oxidized further. The further oxidation of C2 hydrocarbons may result in lower selectivity to the goal products. Experimental results shown in Figure 5 suggest that there is much methane and ethane being converted to carbon dioxide and ethene when the residence time becomes longer. So, selectivity to ethane decreases, and selectivity to ethene and carbon dioxide increases as is shown in the Figure 5. Second, the effects of temperature on OCM were examined. Temperature is an important parameter that may affect the reaction rate and the profile of products. It is difficult to activate methane at low temperature, and the ionic O2- conductivity of the YSZ is poor when the temperature is low, especially lower than 700 °C, and therefore our experiment was carried out above 700 °C. The results shown in Figure 6 indicate that the formation rates of products increase obviously with the increase in operation temperature. Selectivity to C2 hydrocarbons also becomes better when the temperature gets high. Thus, high operation temperature is advantageous to the oxidative coupling of methane in the electrochemical reactor. Third, the effects of the partial pressure of methane in the inlet gas on the formation rates of products and selectivity to C2 hydrocarbons were examined. Also, the effects of the
Figure 6. Formation rate and C2 selectivity as a function of temperature.
Figure 7. Formation rates of products with respect to methane partial pressure.
flux of electrochemical oxygen were examined. High partial pressure of methane means there is much methane to react with electrochemical oxygen. Hence, the chance that C2 hydrocarbons are oxidized deeply may becomes less. The results shown in Figure 7 indicate that the formation rates of ethane and ethene increase more quickly than that of carbon dioxide with the increase of methane partial pressure. So, selectivity to C2 hydrocarbons grows, as is presented in Figure 8. Oxygen was pumped electrochemically to the outside surface of the YSZ tube (coated with perovskite-type catalyst) and then reacted with methane adsorbed on the catalyst. Results shown in Figure 9 indicate that the formation rate of products increases with an increase of oxygen flux. The
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 645
suggested to describe the reaction kinetics. There are three major assumptions: first, the charge-transfer reaction on the perovskitetype electrode is that oxygen ions are rapidly oxidized to neutral atoms; second, two forms of oxygen coexist on the perovskitetype catalyst, one is chemisorbed oxygen, the other is lattice oxygen; third, the interchange between two forms of oxygen is rapid. The perovskite-type catalyst surface is therefore modeled to have two types of sites, S1 and S2. In the absence of gasphase oxygen, oxygen atoms are supplied to S1 sites at a rate equal to the oxygen ion flux to the electrode Jo ) I/2F. The charge-transfer reaction on the perovskite-type catalyst electrode is: Figure 8. Selectivity as a function of methane partial pressure.
Jo
O-2 98 OS1 + 2e-
(5)
Reactions on the perovskite-type catalyst are as follows: At site S1 k1
CH4(g) + OS1 98 CH3(g) + HOS1
(6)
k1′
2CH3(g) 98 C2H6(g)
(7)
k2
C2H6(g) + OS1 98 HOS1 + C2H4(g) + H(g) k3
C2H6(g) 98 C2H4(g) + H2(g)
Figure 9. Formation rates of the products as a function of the flux of electrochemical oxygen.
(8)
(9)
k4
S1 + S2 y\z OS2 + S1
(10)
k-4
At Site S2 k5
CH4(g) + 4OS2 98 CO2(g) + 2H2O(g)
(11)
k6
C2H6(g) + 7OS2 98 2CO2(g) + 3H2O(g)
(12)
k7
C2H4(g) + 6OS2 98 2CO2(g) + 2H2O(g)
Figure 10. Selectivity with respect to the flux of electrochemical oxygen
selectivity to C2 hydrocarbons decreases, however, with the increase of electrochemical oxygen as shown in Figure 10. The above experiment results mean the perovskite-type catalysts with perfect performance in a fixed-bed reactor can serve as the catalyst electrode for OCM in an electrochemical reactor. Moreover, the perovskite-type catalysts present good properties with an electrochemical oxygen supply, and selectivity to C2 hydrocarbons can be up to approximately 80%.
Owing to high temperature, the coverage ratio of the catalyst by the adsorbates is very low. Radical CH3 · is thought to remain constant in the absence of gas-phase oxygen. On the basis of the simple mechanism, the following expressions can be deduced. rCO2 ) rC2H6 )
k5yCH4K4JO + k6yC2H6K4JO + k7yC2H4K4JO k1yCH4 + k2yC2H6 + K4JO
k1yCH4JO - k2yC2H6JO k1yCH4 + k2yC2H6
-
k6yC2H6K4JO k1yCH4 + k2yC2H6 + K4JO
(14)
- k3yC2H6 (15)
Discussion The above experimental results suggest that methane is oxidized to carbon dioxide and ethane by two parallel reactions. Ethane is oxidized to ethene and carbon dioxide. A fraction of ethene is oxidized deeply to carbon dioxide. On the basis of the results and related literatures,1,7,8 the following model is
(13)
rC2H4 )
k2yC2H6JO k1yCH4 + k2yC2H6
-
k7yC2H4K4JO k1yCH4 + k2yC2H6 + K4JO
- k3yC2H6 (16)
Where
646 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009
K4 )
k4 k-4
(17)
Rate constants are calculated using the POWELL nonlinear, multivariate least-squares algorithm29 at 760 °C. k1 ) 0.32 mol ⁄ min k2 ) 68.64 mol ⁄ min k3 ) 0.0063 mol ⁄ min K4 ) 0.078 k5 ) 1.12 mol ⁄ min k6 ) 0.013 mol ⁄ min k7 ) 190.3 mol ⁄ min Formation rates of products calculated with eqs 14, 15, and 16 are shown by the solid lines in Figures 5 and 7. The agreement between the calculated and the measured rates is quantitative in most cases. This agreement does not constitute proof of the proposed mechanism, but the model is capable of correctly describing the complex behavior of this catalytic system. Conclusion Perovskte-type catalysts Ln0.6Sr0.4FexCo1-xO3 (Ln ) Nd, Pr, Gd, Sm, La, 0 < x < 1), (La0.8Sr0.2)0.9MnO3, and La0.8Sr0.2CrO3 can promote activation of methane. The main products is, however, carbon dioxide. They may be perfect candidates for complete oxidation of methane. Na-ACoO3 (Na:A ) 1:4) and A0.8Na0.2MnO3 (A ) La, Sm, Gd) are effective catalysts for OCM. These catalysts need to be studied further. The oxidative coupling of methane in an electrochemical reactor is affected by many variables, including residence time, content or partial pressure of methane in the inlet gas, temperature, and the flux of the electrochemical oxygen. Effects of the later two variables are stronger than the former two. High temperature and small flux of the electrochemical pumped oxygen are favorable to the subject products. The perovskitetype catalyst sodium-doped LaCoO3 deposited on the surface of YSZ is promising for the oxidative coupling of methane, which gives a selectivity of more than 80% to C2 hydrocarbons in an electrochemical reactor. The kinetics model suggested can describe the reaction results. From a practical viewpoint, it is necessary to develop a catalyst that possesses good conductivity of electrons and ions and that can be deposited on the surface of a YSZ tube. Also, the catalyst should have the ability to activate methane to C2 hydrocarbons. Acknowledgment The authors acknowledge financial support from the Education Committee of Shanghai (06-OZ-003) and the Committee of Science and Technology of Shanghai (06-JC-14095), China.
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ReceiVed for reView May 19, 2008 ReVised manuscript receiVed October 19, 2008 Accepted October 23, 2008 IE8008007
