Partial Oxidation of Methane and Oxygen Permeation in SrCoFeOx

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Partial Oxidation of Methane and Oxygen Permeation in SrCoFeOx Membrane Reactor with Different Catalysts Jay Kniep and Y.S. Lin* Chemical Engineering, School for Engineering of Matter, Energy and Transport, Arizona State University, Tempe, Arizona 85287-6106, United States ABSTRACT: Partial oxidation of methane (CH4) and oxygen permeation in a dense SrCoFeOx disk membrane reactor were studied with the reducing side of the membrane packed with different catalysts (catalyst support γ-Al2O3, La0.6Sr0.4Co0.8Fe0.2O3
δ, and 10 wt % Ni/γ-Al2O3) of increasing reaction activities for CH4 oxidation. The influence of temperature, flow rates, and inlet CH4 concentration (diluted with helium) on the performance of the different membrane reactors was investigated. The oxygen permeation flux and CH4 conversion increased in the following order: γ-Al2O3 < La0.6Sr0.4Co0.8Fe0.2O3
δ < 10% Ni/γ-Al2O3. The membrane reactor with the reforming catalyst of 10 wt % Ni/γ-Al2O3 had the highest CH4 conversion (∼90%), CO selectivity (97%), and oxygen permeation flux (2.40 mL/(cm2 min)) at 900 °C. The improvement of the oxygen permeation through the membranes with different catalysts emphasizes the effect of the CH4 oxidation reaction rate on the reducing side of the membrane on the oxygen permeation flux through the mixed-conducting ceramic membranes. Under identical conditions, the oxygen permeation flux through mixed-conducting ceramic membrane with a reducing gas is a strong function of the catalytic activity for the oxidation of the reducing gas.

’ INTRODUCTION Mixed-conducting oxygen ionic and electronic ceramic-based dense membranes continue to attract attention from both academia and industry.1 The advantage of these materials is the ability to selectively transport oxygen ions from one side of the membrane to the other without the use of external electrodes. Industrial applications of these types of materials include oxygen separation and purification, sensor, fuel cells, and use in catalyzed membrane reactors with chemical reactions.2 One process that has been extensively studied over the years is the conversion of methane (CH4) to syngas. Syngas (H2 þ CO) is an important feedstock that can later be converted to several products (including paraffins, olefins, or methanol) or used in the Fischer
Tropsch process.3
5 The current commercial process to produce syngas is steam reforming of methane (SRM) in a conventional packed-bed reactor: CH4 þ H2 O T CO þ 3H2

0 ΔH298 ¼ 206 kJ=mol ð1Þ

Steam reforming is a highly endothermic reaction, which requires a substantial energy input and capital investment.6
8 In addition, the H2/CO ratio in the steam reforming reaction is 3, which is unsuitable for methanol synthesis or use in the Fischer
Tropsch process.8,9 The partial oxidation of methane (POM) is another method that can be used to convert CH4 to syngas. In the direct partial oxidation of methane (DPO) reaction, oxygen reacts with the CH4 to form hydrogen and carbon monoxide: 1 CH4 þ O2 T CO þ 2H2 2

0 ΔH298 ¼
36 kJ=mol ð2Þ

This reaction is slightly exothermic and the H2/CO ratio of 2 is within the range for both methanol synthesis and the r 2011 American Chemical Society

Fischer
Tropsch reaction.10 However, the desired products are more reactive than CH4 and can be further oxidized to CO2 and H2O, so this mechanism has only been reported for reactors with extremely small residence times.6,7,10,11 Most likely, the actual mechanism for the partial oxidation of CH4 to syngas occurs via the combustion and reforming reactions (CRR) mechanism.7,10 In this mechanism, oxygen fed into the reactor completely combusts a portion of the CH4 via the reaction CH4 þ 2O2 T CO2 þ 2H2 O 0 ΔH298 ¼
802 kJ=mol

ð3Þ

The remaining CH4 is then reformed either by steam (reaction 1) or by CO2. CH4 þ CO2 T 2CO þ 2H2

0 ΔH298 ¼ 247 kJ=mol ð4Þ

Although POM has received significant attention recently as a viable alternative method to produce syngas from CH4, there are still some barriers preventing commercial-sized POM plants. The reactor must be carefully designed, because the high exothermic value of reaction 3 could cause local hot spot problems and possible reactor runaway.3 The main drawback of POM is that pure oxygen is required, because the downstream processing of syngas cannot tolerate nitrogen.3,6,7,9 Therefore, the cost of a POM plant is significantly higher, because of the need for a cryogenic oxygen separation plant. Membrane reactors can overcome many of the shortcomings of conventional packed-bed reactors for POM reactions. The Received: January 19, 2011 Accepted: May 14, 2011 Revised: April 20, 2011 Published: May 14, 2011 7941

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Industrial & Engineering Chemistry Research reaction stoichiometry, as well as the interaction of the reactants, can be controlled, which can eliminate the hot-spot problem associated with POM.3,9 An oxygen-selective membrane reactor can also eliminate the need for a cryogenic oxygen separation plant for POM.3,7,9 While mixed-conducting oxygen-permeable membranes can be used for this application, the industrial material requirements are quite stringent. The membrane must have an economically feasible oxygen flux, estimated to be 5
10 mL (STP)/(cm2 min).10 Chemical stability is also very important, because one side of the membrane will be exposed to a gas mixture containing CH4, hydrogen, CO, CO2, and steam at high temperatures. The membrane will be exposed to an extreme oxygen partial pressure gradient, so the material must also be mechanically strong. Various materials have been investigated as possible candidates for use in POM membrane reactors. SrCo0.8Fe0.2O3
δ and La0.2Sr0.8Co0.8Fe0.2O3
δ tubular membranes fractured under POM conditions at 850 °C, because of lattice expansion on the reducing side that led to mechanical strain.6,11 POM membrane reactors with Ba0.5Sr0.5Co0.8Fe0.2O3
δ disk and tubular membranes have been shown to be mechanically stable with high CH4 conversion, CO selectivity, and a steady-state oxygen flux of >10 mL (STP)/(cm2 min) for over 500 h.5,7,9,12,13 Other perovskite materials that have shown an appreciable oxygen flux and stability in a POM membrane reactor include BaCo0.4Fe0.4Zr0.2O3
δ,3 BaCo0.7Fe0.2Ta0.1O3
δ,8 Sm0.4Ba0.6Fe0.8Co0.2O3
δ,10 and BaCe0.15Fe0.85O3
δ.14 Balachandran et al.6,11 have also reported POM results using the multiphase SrFeCo0.5Ox membrane reactors for over 1000 h. They reported that the SrFeCo0.5Ox membrane is mechanically and chemically much more stable than the classical La
Sr
Co
Fe oxide membranes under a large oxygen partial pressure gradient. When a reducing gas is present downstream, the oxygen partial pressure is determined by the thermodynamic equilibrium between oxygen and the reducing gas. However, the actual downstream oxygen partial pressure is determined by many factors, including temperature, sweep gas flow rate, configuration of the permeation cell, and, perhaps most importantly, the reaction kinetics between oxygen and the reducing gas. Akin and Lin15 studied the effect of oxygen reaction kinetics on the oxygen permeation through Bi1.5Y0.3Sm0.2O3
δ fluorite membranes using either methane or ethane oxidation on the sweep side. The thermodynamic equilibrium oxygen partial pressure for either reaction is very low, but the oxygen permeation through the membranes with ethane on the sweep side was an order of magnitude higher than methane, because of faster oxidation kinetics. Zhang et al.16 has reported an order of magnitude difference in the oxygen permeation through a (La,Ca)(Co, Fe)O3
δ perovskite membrane when CH4, hydrogen, or CO is the reducing gas present. The detailed effects of the downstream reaction conditions on oxygen permeation through mixed-conducting membranes with a reducing gas that has a finite reaction kinetic rate have been systematically examined by Rui et al.17 However, no experimental data have been reported comparing oxygen permeation through the mixed-conducting ceramic membranes with the downstream, involving the same reaction but with different catalytic activities. Recently, we reported a mixed-conducting ceramic membrane of a new composition, SrCoFeOx, as a potential material for membrane reactor applications.18 SrCoFeOx membrane exhibits much higher oxygen permeability than SrFeCo0.5Ox reported by Balachandra et al., although both are

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similar in composition.18 The objective of this work was to study the effects of the catalytic activities in the downstream of the SrCoFeOx membrane on oxygen permeation and methane conversion. Oxygen permeation and POM reaction experiments were conducted in a SrCoFeOx membrane reactor with γ-Al2O3, La0.6Sr0.4Co0.8Fe0.2O3
δ, or 10 wt % Ni/γ-Al2O3 packed in the downstream (methane) chamber under the same experimental conditions.

’ EXPERIMENTAL SECTION SrCoFeOx powder and membranes were prepared using the solid-state reaction method. The method consisted of mixing stoichiometric amounts of SrCO3 (99.9%, Sigma
Aldrich), Fe2O3 (99.945%, Alfa-Aesar), and Co(NO3)2 3 6H2O (99.0%, Sigma
Aldrich) with isopropanol (99.8%, Pharmo) and ball milling the resulting slurry with zirconia (ZrO2) balls for 24 h. The resulting solution was transferred to a beaker and allowed to dry. The dried red sample was then ground with a mortar and pestle. The powder was then calcined at 850 °C for 8 h, followed by further grinding. Samples of the resulting powder were placed into a die with a diameter of 2.30 cm and pressed with a hydraulic press (Carver, Model 3853) to 180 MPa. The resulting green disks were sintered in air at 1150 °C for 24 h in a furnace (Thermolyne, 46100) with a ramp rate of 2 °C/min. After sintering, all SrCoFeOx membranes were 1.0 mm in thickness and 20 mm in diameter. The gas tightness of each membrane was verified using a room-temperature unsteady-state permeation system with helium. X-ray diffraction (XRD) (Bruker, Cu KR1 radiation) was used to characterize the crystal structure of each composition. For membrane samples, XRD measurements were performed directly on the surface of the membrane disk placed on the XRD sample stage. Characterization of the powders and membranes were evaluated in the 2θ range of 20°
70°. As a support, γ-Al2O3 was purchased from Alfa Aesar and also was used a blank catalyst support of low activity. To synthesize the 10 wt % Ni/γ-Al2O3 catalyst (referred to as the reforming catalyst), appropriate amounts of Ni(NO3)2 3 6H2O (Alfa Aesar, 98% purity) and γ-Al2O3 were weighed out. The γ-Al2O3 support was ground with a mortar and pestle and sieved with a #80 sieve so the support particles were e180 μm in size. The Ni(NO3)2 3 6H2O and γ-Al2O3 were mixed with deionized water at room temperature. The solution was then heated to 80 °C to allow the excess water to evaporate off. The resulting slurry was then dried overnight at 120 °C. The dried sample was grinded with a mortar and pestle and then calcined for 4 h at 700 °C (ramp rate = 5 °C/min). Once the catalyst has been synthesized, samples were reduced in a 10% H2/He gas mixture for 6 h at 600 °C before use in a steam reforming of methane membrane reactor. Calcined and reduced catalyst samples were characterized with XRD in the 2θ range of 30°
80° with a step size of 0.02°/s. The BET surface area of the 10 wt % Ni/γ-Al2O3 catalyst was determined to be 154 m2/g. La0.6Sr0.4Co0.8Fe0.2O3
δ powder (referred to as the LSCF catalyst) was prepared by the liquid citrate method. Appropriate amounts of La(NO3)3 3 6H2O (Alfa Aesar, 99.9%), Sr(NO3)2 (Alfa Aesar, 99.0%), Co(NO3)3 3 6H2O (Sigma
Aldrich, 98%), Fe(NO3)3 3 9H2O (Sigma
Aldrich, 98%) were dissolved with 50% excess of the stoichiometric amount of citric acid in a beaker with deionized water. The liquid was heated to 95
100 °C and remained at that temperature under reflux and stirring for 4 h for the polymerization reaction to occur. The lid was then removed, 7942

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Figure 1. Experimental high-temperature membrane reactor setup.

and excess water was evaporated off at 100 °C, changing the solution to a viscous gel. The gel was dried for 24 h at 120 °C, and the resulting brittle, porous material was heated to 400 °C for self-ignition, to burn out the organics that were present. The material was then ground with a mortar and pestle for 10 min, followed by calcination at 600 °C for 5 h (ramp rate = 2 °C/min) to burn off the organic residues. The resulting powder was reground with a mortar and pestle and then sintered for 8 h at 1000 °C (ramp rate = 2 °C/min). The sintered powder was ground once again using a mortar and pestle. The ground La0.6Sr0.4Co0.8Fe0.2O3
δ particles were ∼10 μm in size, with a BET surface area of 2.85 m2/g. The sintered powder was characterized with XRD in the 2θ range of 20°
80° with a step size of 0.02°/s. The O2 flux through dense SrCoFeOx membranes was measured in a catalyzed membrane reactor in the temperature range of 825
900 °C, using a high-temperature gas permeation system. A schematic of the system is shown in Figure 1. The gas permeation module (Probostat, Norwegian Eletro Ceramics AS) utilizes the spring force and metal seals to ensure sealing for the O2 flux measurements. The feed gas mixture contained a mixture of CH4 and helium, while the sweep gas was a 20% O2/Ar gas mixture. Using helium and argon in two gas steams allows the detection of leaks through the seal (if argon was found in the helium stream) or through the reactor system (if N2 was detected in the system). For O2 permeation measurements, a gas-tight SrCoFeOx membrane and silver seal (Alfa Aesar, 99.9%) were mounted on the inner alumina tube and held in place by spring pressure applied by the alumina spacer on top of the membrane. Once the membrane and seal were mounted, a catalyst was placed on top of the membrane. The catalyst bed height was 4 mm for all experiments, and the amount of the catalyst placed above the membrane was, respectively, 0.25 g for catalyst support γ-Al2O3, 1.20 g for La0.6Sr0.4Co0.8Fe0.2O3-δ and 0.30 g for 10 wt % Ni/γ-Al2O3. The sealing procedure consisted initially of heating the setup from ambient conditions to ∼950 °C (ramp rate = 1 °C/min) to soften the silver ring. Next, helium and argon were introduced on the feed and sweep side, respectively. The flow rate of the inert gases on either side of the membrane was 30 mL/min, which was regulated by mass flow controllers (MKS, Model 1179) and a four-channel readout (MKS, Type 247). The amount of helium in the argon stream (and, therefore, the leakage rate through the seal) was determined by running gas samples through a gas chromatography (GC) system (Agilent, Model 6890N) with a packed column (Alltech, Model 2836PC)

Figure 2. Effect of temperature on the membrane reactor packed with catalyst support γ-Al2O3 (reactor conditions: CH4 inlet concentration = 50%, inlet CH4/He flow rate = 20 mL/min, O2/Ar sweep rate = 100 mL/min).

and a TCD detector. After the helium content in the sweep stream was minimized, the system was ramped down (1 °C/min) to experimental conditions. For O2 permeation flux measurements, both exiting gas streams were analyzed using a gas chromatography (GC) system. The system was allowed to equilibrate for 5 h after a gas mixture change or 1 h after a temperature change for all experiments. The error in determining the O2 permeation flux using this procedure is approximately (10%. The standard temperature and pressure used to determine the oxygen permeation are 0 °C and 1 atm.

’ RESULTS AND DISCUSSION Reaction and Oxygen Permeation with Different Catalysts. Oxygen permeation through the SrCoFeOx membrane was

first studied with the downstream (CH4) side of the SrCoFeOx membrane reactor packed with the catalyst support γ-Al2O3 as a blank catalyst. Figure 2 shows the effect of temperature on the catalytic performance of the blank membrane reactor. The oxygen flux and CH4 conversion increase with increasing temperature, while the CO selectivity is fairly independent of temperature. Overall, the CH4 conversion was fairly low, with the highest conversion being ∼20% at 900 °C. The low CH4 conversion is consistent with other membrane reactors tested without a solid catalyst.6,10,12
14 The main carbon-containing products were CO and CO2, indicating that complete oxidation and reforming of CH4 were the main reactions taking place. Ethylene (C2H4), which is formed by the oxidative coupling of CH4, was the only C2 species detected; however, the C2H4 selectivity was significantly less than 1%. While the CH4 conversion rates were quite low for the blank membrane reactor, the oxygen flux through the SrCoFeOx membrane with CH4 as the sweep gas is still significantly higher than that under an inert gas and with a 20% O2/Ar oxygen partial pressure gradient.18 With an inert gas in the sweep side, any permeation oxygen increases the oxygen partial pressure and reduces the driving force for oxygen permeation through the membrane. On the other hand, even with a low CH4 conversion rate, all of the oxygen permeating through the membrane reacted 7943

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Figure 3. Effect of temperature on the membrane reactor packed with LSCF catalyst (reactor conditions are the same as Figure 2).

Figure 4. Influence of the inlet CH4 concentration on the membrane reactor with a LSCF catalyst (reactor conditions: reactor temperature = 900 °C, inlet CH4/He flow rate = 20 mL/min, O2/Ar sweep rate = 100 mL/min).

and, therefore, a low oxygen partial pressure on the methane side of the membrane was maintained. POM and oxygen permeation for the membrane reactor of improved CH4 oxidation reactivity in the downstream were studied with the LSCF catalyst. Perovskites of various compositions have been found to be good CH4 combustion catalysts under certain conditions.19
22 Figure 3 shows the effect of temperature on the performance of the SrCoFeOx membrane reactor with the LSCF catalyst. The species in the exit stream of the CH4/He gas mixture included unreacted CH4, hydrogen, CO, CO2, and water. No C2 products were detected. The oxygen flux through the membrane and the CH4 conversion increase while the CO selectivity slightly decreases with increasing temperature. The increase in the oxygen flux is due to several factors, including more-favorable transport properties and surface kinetics with the increase of temperature. In addition, the combustion and reforming reactions introduce highly reducing gases (H2 and CO) and a lower oxygen partial pressure on the reactor side, which increase the driving force for oxygen permeation. Since all of the oxygen that permeated through the membrane reacts with CH4, an increasing oxygen flux also increases the CH4 conversion. However, with an increasing oxygen flux and a constant CH4 flow rate with increasing temperature, the CH4:O2 ratio varies with temperature and can affect the methane conversion and product selectivity. At higher temperatures, a lower CH4:O2 ratio leads to the formation of CO2, so the CO selectivity slightly decreases.10,14 Perovskite catalysts, such as La0.6Sr0.4Co0.8Fe0.2O3
δ powder, have been shown to be highly selective to the complete combustion of CH4, so one would expect CO2 and H2O (the products of reaction 3) to be the main components in the CH4 exit stream, instead of a mixture of hydrogen, CH4, CO, and CO2. Tan et al.23 studied CH4 combustion using La0.6Sr0.4Co0.2Fe0.8O3
δ hollow fiber membranes with La0.6Sr0.4Co0.2Fe0.8O3
δ granules as the combustion catalyst. In the temperature range of 790
950 °C, CO2 was the main product, with ethylene, ethane, and CO produced at low concentrations. However, unreacted oxygen was detected in the product stream and increased with increasing temperature, indicating that the oxygen permeation rate was higher than the oxygen consumed by reactions.23 In the case of the membrane reactor in Figure 3, the oxygen permeating

Figure 5. Influence of the CH4/He gas mixture inlet flow rate on the membrane reactor with LSCF catalyst (reactor conditions: reactor temperature = 900 °C, inlet CH4 concentration = 50%, O2/Ar sweep rate = 100 mL/min).

through the SrCoFeOx membrane is less than what is needed to completely combust the CH4 in the system. For example, at 900 °C, the oxygen flux through the membrane is ∼1.75 mL/(cm2 min). Taking the surface area of the membrane into account, under steady-state conditions, ∼3.0 mL/min of oxygen is permeating through the membrane while CH4 is entering the system at a rate of 10 mL/min. The kinetics of CH4 combustion (reaction 3) is extremely fast; therefore, the CO2 and steam formed can then react with the residual CH4 that is still in the system (via reaction 1 or reaction 4) to produce hydrogen and CO. Figure 4 shows the effect that the inlet CH4 concentration has on the oxygen flux, CH4 conversion, and CO selectivity of the system at 900 °C. As shown in Figure 4, at lower inlet CH4 concentrations, the CO selectivity is very low, because there is little residual CH4 available to be reformed by either steam or CO2, which would produce CO. Figure 5 shows the influence 7944

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Figure 6. Effect of temperature on the membrane reactor with 10 wt % Ni/γ-Al2O3 catalyst (reactor conditions are the same as those given for Figure 2).

Figure 7. Influence of the CH4/He gas mixture inlet flow rate on the on the membrane reactor with 10 wt % Ni/γ-Al2O3 catalyst (reactor conditions: reactor temperature = 900 °C, inlet CH4 concentration = 50%, O2/Ar sweep rate = 100 mL/min).

that the inlet CH4/helium gas mixture flow rate has on the SrCoFeOx membrane reactor with a La0.6Sr0.4Co0.8Fe0.2O3
δ combustion catalyst. As expected, reducing the CH4 mixture flow rate increases the CH4 conversion, because of the longer residence time of CH4. The initial increase in the oxygen permeation rate can be attributed to a reduction in the oxygen partial pressure on the reactor side of the membrane and, therefore, a larger driving force for oxygen permeation. However, the oxygen permeation rates are almost unchanged at higher CH4 mixture flow rates. This is due to the low CH4 conversions yielding roughly the same driving force for oxygen permeation.23 The data in Figures 4 and 5 (at 20 mL/ min) show that CO selectivity and oxygen permeation flux increases with increasing inlet CH4 concentration. However, CH4 conversion decreases with increasing inlet CH4 concentration in the range of 5%
20%, and, after possibly reaching a minimum, increases to ∼63% as the inlet CH4 concentration reaches 50%. Since the membrane reactor is a complex system where a variety of interdependent variables influence the reactor performance, it is possible that the inlet CH4 concentration has the observed effects on CH4 conversion, CO selectivity, and oxygen permeation flux. In order to investigate how different catalysts affect the oxygen permeation properties of the SrCoFeOx membrane reactor, 10 wt % Ni/γ-Al2O3 was used as a reforming catalyst. Figure 6 shows the effect of temperature on the performance of the SrCoFeOx membrane reactor with a reforming catalyst. The species in the exit stream included unreacted CH4, hydrogen, CO, CO2, and H2O. Neither ethane nor ethylene was not detected under the experimental conditions. Similar to the membrane reactor with a combustion catalyst (see Figure 3), the oxygen flux and CH4 conversion increase while the CO selectivity decreases slightly with increasing temperature for the membrane reactor with a reforming catalyst. As stated earlier, the increase in oxygen permeation flux is due to the increase in the oxygen diffusion rate through the SrCoFeOx membrane and improved surface kinetics with increasing temperature. The influence of the CH4/He gas mixture flow rate on the CH4 conversion, CO selectivity, and oxygen permeation of a SrCoFeOx membrane reactor with a reforming catalyst is shown in Figure 7. With an increasing CH4 mixture flow rate, the CH4 conversion decreases, the CO selectivity increases (and

Figure 8. Oxygen permeation flux of SrCoFeOx membrane reactors with different catalysts (reactor conditions are the same as those given for Figure 2).

approaches 100%), and the oxygen permeation flux initially increases and then levels off at higher CH4 mixture flow rates. At higher CH4 mixture flow rates, CH4 is fed to the membrane reactor in excess and the decreasing CH4 conversion rates do not significantly affect the driving force for oxygen permeation. Also, when dealing with the oxygen permeation with a chemical reaction of finite rate, the flow rate of the reactant can be very important. With a higher flow rate, the residence time of the reactant decreases, leading to a lower reactant conversion.15 However, at an inlet CH4 mixture flow rate of 10 mL/min, the high CH4 conversion affects the oxygen partial pressure on the CH4 side enough to significantly reduce the oxygen flux through the membrane. With a very low flow rate, the residence time of the reactant is increased to the point that the reactant is likely to be converted and the CH4 conversion rate approaches equilibrium values.15,17 Comparison of Membrane Reactors. In order to compare the performance of each type of membrane reactor, the oxygen 7945

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Table 1. Results of SrCoFeOx Membrane Reactors at 900 °C catalyst oxygen flux,

La0.6Sr0.4-

10% Ni/

blank (γ-Al2O3)

Co0.8Fe0.2O3
δ

γ-Al2O3

0.71

1.75

2.40

mL/(cm2 min) CO selectivity

57.63

83.7

97.2

CH4 conversion

18.8%

63.7%

88.7%

fluxes through the SrCoFeOx membrane reactors under the same experimental conditions are shown in Figure 8. The results of each type of membrane reactor at 900 °C are summarized in Table 1. As discussed by Akin and Lin,15 the oxygen permeation for systems with a finite rate chemical reaction depends on the reactant reactivity with oxygen on the reaction side. The thermodynamic equilibrium of the reaction between CH4 and oxygen at high temperatures is complete conversion to CO2. However, each membrane reactor has significantly different oxygen permeation flux, CO selectivity, and CH4 conversion values. Gu et al.24 has also reported higher oxygen permeation flux values for a membrane reactor with a nickel-based catalyst, compared to a membrane reactor without a catalyst. In either case, a catalyst enhances the reactivity of CH4 with oxygen that had permeated to the reaction side.15 For the three cases listed in Table 1, under the same reaction conditions, the oxygen permeation flux increases as the activity for CH4 oxidation increases. As the CH4 conversion increases, the amount of reducing species (H2, CO) on the reaction side increase, reducing the oxygen partial pressure and increasing the driving force for oxygen permeation. For the case of the SrCoFeOx membrane reactor with the blank γ-Al2O3, the SrCoFeOx membrane has weak catalytic properties and has very little surface area for catalysis to take place, because the membrane is dense. The result is minimal improvement of the reactivity of CH4. The low CH4 conversion does lead to the presence of highly reducing species H2 or CO and oxygen flux values that are higher than the case with no CH4 conversion (the case where methane performs like an inert gas) at all. When La0.6Sr0.4Co0.8Fe0.2O3
δ was used as a combustion catalyst, the reactivity of CH4 with oxygen was significantly enhanced. The oxygen flux of the membrane reactor more than doubled when La0.6Sr0.4Co0.8Fe0.2O3
δ was used as a catalyst instead of a blank. The CO selectivity and CH4 conversion also increased, so the enhanced reactivity of the system moved closer to complete conversion and thermodynamic equilibrium. However, typical perovskite granules that are used as a combustion catalyst usually have a surface area of