Improving oxygen permeation of MIEC membrane reactor by enhancing the electronic conductivity under intermediate-low oxygen partial pressures

Journal of Membrane Science 520 (2016) 607–615

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Improving oxygen permeation of MIEC membrane reactor by enhancing the electronic conductivity under intermediate-low oxygen partial pressures Lili Cai a,b, Wenping Li a,b, Zhongwei Cao a, Xuefeng Zhu a,n, Weishen Yang a,n a b

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China University of Chinese Academy of Sciences, Beijing 100039, China

art ic l e i nf o

a b s t r a c t

Article history: Received 11 June 2016 Received in revised form 3 August 2016 Accepted 9 August 2016

Oxygen chemical potential decreases continuously across a mixed ionic-electronic conducting membrane as one side is fed with air and the other side is fed with methane for syngas production. At a certain position in the membrane bulk, the oxygen chemical potential is intermediate-low, i.e. corresponding to an oxygen partial pressure of 10
10–10
15 atm. Insufficient electronic conductivity at an intermediatelow oxygen chemical potential may limit the oxygen transport across the membrane bulk under the syngas production condition. In this work, a new ceria based dual-phase membrane 75 wt% Ce0.85Sm0.15O1.925 – 25 wt% Sm0.6Sr0.4Cr0.3Fe0.7O3
δ (SDC–SSCF) was prepared as a membrane reactor for the syngas production. The conductivities of SDC–SSCF, 75 wt% Ce0.85Sm0.15O1.925 – 25 wt% Sm0.6Sr0.4Al0.3Fe0.7O3
δ (SDC–SSAF, as a control material) and related single-phase materials were investigated under various oxygen partial pressures. SDC–SSAF has enough high electronic conductivity under high and low oxygen partial pressures but limited electronic conductivity under the intermediatelow oxygen partial pressure, thus the low electronic conductivity limits the ambipolar diffusion in the membrane bulk. However, the electronic conductivity of SDC–SSCF is high enough in the whole range of oxygen partial pressure compared with the ionic conductivity. As a result, the oxygen permeation flux through a 0.5-mm-thick SDC–SSCF membrane is high up to 7.6 mL cm
2 min
1 at 950 °C for the syngas production, which is 1.8 times that of the SDC–SSAF membrane under the same condition. In addition, the SDC–SSCF membrane reactor was steadily operated for 220 h, and reached 4 95% methane conversion and 498% CO selectivity. The scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses reveal the good stability of SDC–SSCF as a membrane reactor. & 2016 Elsevier B.V. All rights reserved.

Keywords: Dual-phase membrane Conductivity Oxygen permeation Syngas Oxygen partial pressure

1. Introduction Mixed oxygen ionic-electronic conducing (MIEC) oxides have drawn considerable attention due to their versatile applications as catalysts [1,2], electrodes of solid oxide fuel cells (SOFCs) [3–5], oxygen-permeable membranes for pure oxygen production [6–11], or for the combination with oxy-fuel process for CO2 capture [12– 16], and membrane reactors for the partial oxidation of light hydrocarbons [17–20] or water splitting for hydrogen production [21–23], or co-producing two types of syngas for ammonia and liquid-fuel synthesis [24]. The catalytic membrane reactor for the partial oxidation of methane (POM) to syngas (H2 þ CO), the important feedstock for methanol production and Fischer–Tropsch (F–T) reaction, is energy- and cost-efficient in comparison with conventional technologies [25,26]. n

Corresponding authors.

http://dx.doi.org/10.1016/j.memsci.2016.08.012 0376-7388/& 2016 Elsevier B.V. All rights reserved.

Many oxygen-permeable materials have been extensively investigated in recent years as membrane reactors for the POM reaction. Amongst them, dual-phase membrane materials are indicated as promising candidates with adjustable compositions according to practical requirements, owing to their good stability under strong reducing conditions [27–34]. The main challenges for dual-phase membranes lie in the chemical compatibility between the electronic conducting phase and ionic conducting phase, as well as the performance of oxygen permeation. Fluorite oxides such as zirconia- and ceria-based solid electrolytes are usually used as the oxygen ionic conducting phase for their high ionic conductivities. In the traditional dual-phase membranes, pure electronic conductors, such as La1
xSrxMnO3 [27] and La1
xSrxCrO3 [29], with extremely low ionic conductivity, block the transport of oxygen ions between the fluorite phase grains and thus leads to the low oxygen permeability of the dual-phase materials. Therefore, our group has developed some new dual-phase membranes that are made of ionic conducting oxides and MIEC

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oxides. Their oxygen fluxes are one order of magnitude higher than those of the traditional dual-phase membranes, and these membranes show high stability as membrane reactors for POM reaction [30,32,33]. For example, the dual-phase membranes based on strontium ferrite perovskites, 75 wt% Ce0.85Sm0.15O1.925 – 25 wt% Sm0.6Sr0.4FeO3
δ (SDC–SSF) [32] and 75 wt% Ce0.85Sm0.15O1.925 – 25 wt% Sm0.6Sr0.4Al0.3Fe0.7O3
δ (SDC–SSAF) [33] exhibit excellent stabilities under the syngas production condition and good oxygen permeation fluxes up to 4.2 and 4.3 mL cm
2 min
1 at 950 °C, respectively. We found that Al doping indeed improves the stability under the syngas production conditions, but does not improve the oxygen permeation flux for POM reaction. The oxygen permeation fluxes through MIEC membranes are mainly influenced by the conductivity (including electronic and ionic conductivities) and the oxygen partial pressure gradient when bulk diffusion is the rate-determining step. The dependences of the total conductivities on oxygen partial pressure for La1
xSrxFeO3
δ [35–37] and La1
xSrxM1
yFeyO3
δ (M ¼Al, Ti, Cr etc.) [35,38–42] were analyzed in detail in literatures. The total conductivities of these perovskite materials are predominantly p-type electronic conduction in a high oxygen partial pressure regime (4 10
10 atm), and n-type electronic in a very low oxygen partial pressure regime ( o10
15 atm). In the intermediate-low oxygen partial pressure range, usually 10
10–10
15 atm, at elevated temperatures, a minimum value appears. At the point, oxygen ionic conduction dominates and the contributions of p-type and n-type electronic transport are low and comparable. In a POM membrane reactor, the oxygen partial pressure gradient across the membrane is very large. The membrane must withstand oxidizing atmosphere (air) on one side and highly reducing atmosphere (CH4, CO and H2) with oxygen partial pressure down to 10
21 atm on the other side at high temperatures. Fig. 1 shows the decrease of oxygen chemical potential across an MIEC membrane. According to the dependence of electronic conductivities on oxygen partial pressure for La1
xSrxFeO3
δ and La1
xSrxM1
yFeyO3
δ, the electronic conductivity of an MIEC membrane should decrease with oxygen chemical potential in zone I, reach a minimum value in zone II and then gradually increase in zone III. In zone II, the electronic conductivity is even lower than its ionic conductivity. Therefore, although an MIEC membrane has high electronic conductivities under both oxidizing and strong reducing atmospheres, its oxygen permeation may be still limited by the low electronic conductivity in zone II. For the ceria-based dual-phase membranes, the ceria is the main phase.

Fig. 2. XRD patterns of single phase oxides and dual-phase membrane. The standard JCPDS files for perovskite (SmFeO3, PDF#39-1490) and fluorite (CeO2, PDF#431002) oxides are also listed in this figure.

Ionic conductivity of ceria is marginally affected by oxygen partial pressure [43]; thus, the low electronic conductivity in zone II may significantly affect the oxygen permeation flux because ceria is still not reduced to produce remarkable n-type electronic conduction in the intermediate-low oxygen partial pressure range. It is possible to improve the electronic conductivity in the zone II by changing the chemical composition of the MIEC phase in ceriabased dual-phase membranes. Cr-based perovskite oxides exhibit high electronic conductivity in the oxygen partial pressure range of 0.21–10
21 atm [44]. Therefore, a combination of transition metal Cr and Fe on the B-site of perovskites may produce considerable ionic conductivity and high electronic conductivity at intermediate-low oxygen partial pressures. Herein, we design a ceria based new dual-phase membrane of 75 wt% Ce0.85Sm0.15O1.925 – 25 wt% Sm0.6Sr0.4Cr0.3Fe0.7O3
δ (SDC– SSCF). Its conductivity is higher than SDC–SSAF under various oxygen partial pressures, especially in the oxygen partial pressure range of 10
2–10
15 atm. Its oxygen permeation flux reaches to 7.6 mL cm
2 min
1 in the POM reactor at 950 °C, and the flux is about 1.8 times that of SDC–SSAF under the same condition. This result indicates that the improvement of electronic conductivity in the intermediate-low oxygen partial pressure range is effective to improve the oxygen permeation flux of dual-phase membranes for POM reaction.

2. Experimental 2.1. Preparation of membranes and catalysts

Fig. 1. Schematic illustration of oxygen chemical potential drop and conductivity change across an MIEC membrane in the POM reactor, where L represents the thickness of membrane, and se is the electronic conductivity including p-type and n-type.

The SDC–SSCF powder was prepared by a simple one-pot solid state reaction method (SSR). The required amounts of CeO2, Sm2O3, Fe2O3, SrCO3 and Cr2O3 were mixed and ball-milled in ethanol for 5 h and then calcined at 1100 and 1200 °C for 10 h, respectively. The particle size of the resultant powder is 100– 600 nm. The MIEC and fluorite phases cannot be distinguished as they are ball-milled together. The resultant powder was pressed into green disks under a pressure of  200 MPa and then sintered at 1450 °C for 5 h in stagnant air with heating and cooling rates of 2 °C min
1. All membranes used for oxygen permeation tests and membrane reactors were polished to 0.5 mm in thickness by 500mesh waterproof sandpaper. One or both sides of the membrane were coated with a Sm0.5Sr0.5CoO3
δ (SSC, prepared via a sol–gel

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Fig. 3. SEM images of the (a) surface of the dual-phase membrane sintered at 1450 °C for 5 h, (b) surface and (c) cross-section of the fresh membrane coated with SSC porous layer.

Fig. 4. Dependence of the total conductivities on temperature for different samples in (a) air and (b) wet H2 atmospheres.

method) porous layer to improve the surface exchange rate of oxygen [45,46]. The SSC slurry was prepared by mixing terpineol and SSC powder with a weight ratio of 1:1. Porous layers were calcined in situ at the sealing temperature for several hours. The thicknesses of the porous layers were usually 5–10 mm. For comparison, SDC, SSCF, SSAF and SDC–SSAF powders were prepared

Fig. 5. Dependence of the total conductivities on oxygen partial pressure for different samples at 950 °C.

separately by the SSR method. And these powders were pressed into bar-shaped samples under a pressure of  200 MPa and then sintered at 1450–1480 °C for 5 h in stagnant air with heating and cooling rates of 2 °C min
1. The LiLaNiO/γ–Al2O3 catalyst used for syngas production was

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Fig. 6. Dependences of oxygen permeation performance on various conditions. (a) Temperature, (b) oxygen partial pressure, (c) flow rate of the sweep gas and (d) time. He flow rate in (d) is  35 mL min
1; air flow rate is 100 mL min
1; SDC–SSCF membrane thickness is 0.5 mm. The oxygen partial pressure of the permeation side is fixed at 0.005 atm in the temperature-dependent experiment.

prepared via the impregnation method, as described in detail elsewhere [47]. Briefly, the γ–Al2O3 supports were impregnated in aqueous solution of appropriate amounts of LiNO3, La(NO3)3 and Ni(NO3)2 for 24 h, followed by drying at 120 °C and calcination in air at 800 °C for 4 h. Then the catalyst was pressed and crushed into 40–60 mesh particles. 2.2. Membrane reactor configurations and operation A silver ring was used to seal the membrane onto an alumina tube at the melting point of silver (961 °C) for oxygen permeation tests, while a gold ring was used to seal the membrane at 1000 °C for POM reaction. The operations for permeation and POM experiments are described in our previous works [48]. The effective membrane areas in the experiments were around 0.9 cm2. Dried air and high-purity helium were used as the feed and sweep gases on the two sides of the membrane, respectively. In addition, highpurity methane without dilution was used as the reactant. 300 mg LiLaNiO/γ–Al2O3 catalyst was directly packed onto the top of membrane surface for the POM reaction. All of the effluents were analyzed on a gas chromatograph (GC, Agilent 6890) equipped with a Porapak Q column and a 13X column. The oxygen permeation fluxes through the SDC–SSCF membranes were calculated based on the following equation,

JO

2

⎛ VO = ⎜⎜ CO 2 − CN2 × 2 × VN2 ⎝

MN2 ⎞ ⎟ × F /S MO 2 ⎟⎠

are the molecular masses of oxygen and nitrogen molecules, respectively, F is the flow rate of the effluent, and S is the effective membrane area. The oxygen leakages due to imperfect sealing were no more than 1% in both the oxygen permeation and POM experiments. The equations for the calculation of oxygen permeation flux, selectivity and conversion for POM reaction are as follows:

JO =

FCO + 2FCO 2 + F H2O + 2FO 2(unreacted) (2)

2S

2

(

)

F H2O = 2 FCH4(in) − FCH4(out ) − F H2

(3)

F H2 = Ftotal − FCO − FCO 2 − FCH4(out )

(4)

XCH4 = 1 −

SCO =

FCH4(out ) FCH4(out ) + FCO + FCO 2

FCO FCO + FCO 2

(5)

(6)

(1)

where CO2 and CN2 are the concentrations of oxygen and nitrogen in the effluents, respectively, VO2 and VN2 are the volume percentages of oxygen and nitrogen in feed gas, respectively, MO2 and MN2

where Fi is the flow rate of species i, Ftotal is the total flow rate of the effluent outflowing from CH4 side, XCH4 and SCO are conversion of CH4 and selectivity of CO, respectively.

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pressures were acquired by adjusting various atmospheres, namely air, 5% air, N2, 1% wet H2, 5% wet H2, 10% wet H2 and wet pure H2 atmospheres, respectively. The voltage or resistance value was not recorded until a steady state was reached under a certain condition. 2.4. Characterizations X-ray diffraction (XRD, Rigaku D/Max-2500, Cu Kα radiation) was used to determine phase structures of the as-prepared powders and disks in the 2θ range of 20–80° with a step width of 0.02°. The morphologies of the membranes and local-area composition analysis were determined on a scanning electron microscope and an energy dispersive X-ray spectroscopy (SEM/EDX, Quanta 200 FEG, FEI Company) operated at 20 kV.

3. Results and discussion 3.1. Structures and morphologies Fig. 2 shows the XRD patterns of SDC, SSCF and dual-phase membrane SDC–SSCF. Compared with patterns of the single-phase oxides, the dual-phase composite oxides contain a fluorite phase (SDC) and a perovskite phase (SSCF), and no other impurities were detected. The surface of the membrane is dense without any cracks. The grain size of perovskite phase is around 200 nm, while that of fluorite phase is in the range of 1–2 mm (Fig. 3a). The surface and cross section of the fresh membrane coated with SSC porous layer are also shown in Fig. 3b and c. The thickness of the porous layer is about 10 mm. Fig. 7. Effect of CH4 flow rate on (a) CH4 conversion, CO selectivity and (b) O2 permeation flux at different temperatures.

Fig. 8. Dependence of oxygen permeation flux on temperature under the POM conditions, where CH4 conversion and CO selectivity are both higher than 95%.

3.2. Conductivities of membrane materials The total conductivities of the SDC–SSCF, SDC–SSAF, SDC, SSCF and SSAF were measured under various atmospheres. As shown in Fig. 4a, among the three single phase oxides, SSCF has the highest total conductivity in air and keeps almost constant at 35 S cm
1 in the temperature range of 750–950 °C. Compared with SSAF, doping of Cr element is better than Al element in term of total conductivity. Unlike the valence changeable Cr3 þ /4 þ , the Al3 þ cations in the sub-lattice partially block the transfer of holes between Fe3 þ and Fe4 þ . The total conductivities of the two single-phase perovskite oxides are almost equal to their p-type electronic conductivity, respectively, because the ionic conductivity of the dual-phase membranes is close to the ionic conductivity of the main phase, SDC. The decrease of total conductivity with the in∙ crease of temperature for SSAF is induced by the decrease of FeFe concentration in the perovskite lattice. It is the result of the evolution of lattice oxygen to oxygen molecules and the reduction of Fe4 þ to Fe3 þ , as indicated by Eq. (7). For SSCF, the reduction of Fe4 þ to Fe3 þ likes that happens in SSAF, but the electron transfer × ∙ to CrFe is thermal activated, thus the electronic confrom CrFe ductivity keeps constant with the increase of the temperature from 750 to 950 °C.

2.3. Conductivities of membrane materials ∙ × OO×+2FeFe ↔ VO∙∙+2FeFe +

Conductivities of the samples were measured by four- or twoprobe method on the sintered bars under different conditions. Princeton Applied Research 263 A was used to supply a constant current, and a multimeter (Keithley 2000) was used to measure voltages for the four-probe method or resistances for the twoprobe method. The total conductivities in air or wet H2 atmospheres were recorded in a temperature range of 950–750 °C temperature range, with a cooling rate of 1 °C min
1. Data were collected with an interval of 1 °C. The different oxygen partial

1 O2 2

(7)

For the two dual-phase membranes, they have similar total conductivities and are all less temperature-dependent in air in the temperature range of 750–950 °C. The order of total conductivities of the five oxides in air is SSCF 4SSAF4SDC–SSCF ESDC– SSAF 4SDC (Fig. 4a), and the conductivity values are 35, 3.9, 0.71, 0.49 and 0.16 S cm
1 at 950 °C, respectively. However, in the same temperature range the conductivity order in wet pure H2 atmosphere becomes SDC 4SDC–SSCF 4SDC–SSAF4 SSCF 4SSAF

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Table 1 Comparison of dual-phase membranes for oxygen permeation under similar POM conditions. Membrane materials

Membrane configurations

Thickness (mm)

JO (mL cm
2 min
1) Catalysts

XCH4 & SCO

Ref.

60 vol% YSZ – 40 vol% La0.8Sr0.2MnO3
δ

Hollow fiber

0.075

 3a

Disk

0.5

2–5

75 wt% Ce0.85Sm0.15O1.925 – 25 wt% Sm0.6Sr0.4FeO3
δ 75 wt% Ce0.85Sm0.15O1.925 – 25 wt% Sm0.6Sr0.4Al0.3Fe0.7O3–δ Zr0.86Y0.14O1.92 –La0.8Sr0.2Cr0.5Fe0.5O3
δ

Disk

0.6

4.2

LiLaNiO/γ–Al2O3

Disk

0.5

4.3

LiLaNiO/γ–Al2O3

Hollow fiber

0.08b

7.9

33 wt% Ru/LSCF

60 wt% Ce0.9Pr0.1O2
δ – 40 wt% Pr0.6Sr0.4FeO3
δ

Disk

0.6

4.4

Ni based catalysts

75 wt% Ce0.85Sm0.15O1.925
25 wt% Sm0.6Sr0.4Cr0.3Fe0.7O3
δ

Disk

0.5

7.6

LiLaNiO/γ–Al2O3

 36%,  92% 498%, 4 98% 498%, 4 98% 498%, 4 98%  91%,  90% 499%, 4 97% 495%, 4 98%

[49]

60 wt% Ce0.8Gd0.2O1.9 – 40 wt% Gd0.2Sr0.8FeO3
δ

30 vol% NiO/ 70 vol% YSZ LiLaNiO/γ–Al2O3

2

[30] [32] [33] [50,51] [52] This work

All the membrane reactors were operated at 950 °C. a

The reactant was diluted CH4 (10% CH4/Ar) in this literature. The thickness of dense layer is not given directly in the literatures, but is observable in the SEM images. The wall thickness of the hollow fiber is 0.34 mm, and the thickness of porous layer is about 0.26 mm. b

phase perovskite oxides, as shown in Fig. 5b. However, the reduction of Cr4 þ to Cr3 þ (Eq. 10) only happens at a lower oxygen partial pressure than the reduction of Fe4 þ to Fe3 þ . Therefore, SSCF still keep a high electronic conductivity at low oxygen partial pressures. ∙ × OO×+2CrFe ↔ VO∙∙+2CrFe +

Fig. 9. Time-dependent operation of the POM reaction in the SDC–SSCF membrane reactor at 950 °C. L¼ 0.5 mm; air flow rate: 200 mL min
1; CH4 flow rate: 13.1 mL min
1.

(Fig. 4b), and the conductivity values are 2.1, 1.32, 0.91, 0.22 and 0.002 S cm
1 at 950 °C, respectively. For the single-phase perovskite oxides, their conductivities decrease by more than two orders due to the significant reduction of the p-type conduction according to the Eq. (7). While the remarkable increase of the total conductivity of SDC is due to the reduction of Ce4 þ to Ce3 þ , which produces n-type conduction according to the following equations,

OO× ↔ VO∙∙ +

1 O2+2e′ 2

× ′ CeCe + e′ ↔ CeCe

(8) (9)

As indicated by the conductivity values of the two dual-phase membranes, the electronic conductivities are sufficient higher than their ionic conductivity assuming the ionic conductivity of dual-phase membrane is close to the main phase (i.e. SDC) whenever in oxidative or reducing atmosphere. Therefore, one may infer that the electronic conductivities are sufficient higher for oxygen permeation under a gradient of air/syngas. However, the conductivities of the two dual-phase membranes decrease gradually with the decrease of oxygen partial pressure from 0.21 to about 4  10
15 atm, as shown in Fig. 5a. The decrease is attributed to the reduction of p-type electronic conduction of the single-

1 O2 2

(10)

The conductivity of SDC is independent of the oxygen partial pressure in the oxygen partial pressure range from 0.21 down to 3  10
6 atm, because minor oxygen nonstoichiometry variation happens in oxidizing atmospheres [36,43]. As the oxygen partial pressure is lower than 10
15 atm, SDC starts to be reduced and produce n-type electronic conduction. Therefore, the conductivities of the two dual-phase membranes increase at the same conditions. The conductivity of SDC–SSAF is 0.28 S cm
1, nearly equal to the one of SDC, at the oxygen partial pressure of about 10
15 atm. This result indicates that the contribution of SSAF to the total conductivity is negligible, and that can be also inferred through the low total conductivity ( o0.1 S cm
1) of single-phase SSAF at the oxygen partial pressure of about 10
15 atm (Fig. 5b). If the ionic conductivity of the SDC main phase does not change remarkably as the oxygen partial pressure decreasing from 0.21 to 10
15 atm, the electronic conductivity of SDC–SSAF is even lower than its ionic conductivity. Thus, electron transfer would limit the ambipolar transport of oxygen ions and electrons in the zone II of the membrane bulk. However, in the case of SDC–SSCF, the total conductivity is higher than 0.5 S cm
1 as the oxygen partial pressure decreasing from 0.21 to 10
15 atm. Obviously, the high total conductivity profits from the high total conductivity of SSCF in the aforementioned oxygen partial pressure range, as shown in Fig. 5b. Therefore, it can be expected that the oxygen permeation flux of SDC–SSCF membrane is higher than that of SDC–SSAF membrane for POM reaction. 3.3. Oxygen permeation through SDC–SSCF membranes Fig. 6a shows the oxygen permeation fluxes of dual-phase membrane SDC–SSCF at different temperatures. The oxygen permeation flux increases from 0.22 to 0.53 mL cm
2 min
1 with increasing temperature from 800 to 950 °C under an oxygen partial pressure gradient of 0.21 atm/0.005 atm through a 0.5-mmthick SDC–SSCF membrane. The increase of the oxygen permeation flux is attributed to the increase of an enhanced surface exchange

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Fig. 10. SEM images of the spent membranes after 220 h operation under the POM condition. (a and b) air side coated with SSC porous layer; (c and d) CH4 side of the spent membrane; surface: (a and c); cross-section: (b and d).

Table 2 Local–area EDX analysis of the fresh and spent membranes. Membranes

Fresh Spent

Position

Cross section Methane side Air side

Elements

Surface 10 mm 10 mm

Ce

Sm

Sr

Cr

Fe

54.6 32.0 51.1 54.1

16.7 11.9 19.5 17.5

10.6 10.2 8.9 10.5

2.8 2.1 4.9 3.8

15.2 6.8 15.7 14.2

rate and the oxygen diffusion rate through the oxygen membrane with increasing temperatures. The oxygen permeation flux of SDC– SSCF is lower than that of SDC–SSAF under the same condition [45,46]. The activation energy for oxygen permeation is 64 70.8 kJ mol
1, which is similar to that of SDC–SSAF (68.6 72.3 kJ mol
1) [45]. The oxygen permeation flux increases linearly with the logarithm of oxygen partial pressure gradients (Fig. 6b), which indicates that the oxygen permeation is controlled by bulk diffusion. It reaches to 0.79 mL cm
2 min
1 at 950 °C under the oxygen partial pressure gradient of 1.0 atm/0.005 atm. Fig. 6c shows the dependence of oxygen permeation fluxes over the flow rates of sweep gas. With the increase of the flow rates of helium, oxygen permeation flux enhances gradually, from 0.31 to

Al

La

Ni

32.8

2.0

0.5

0.51 mL cm
2 min
1 at 950 °C, which ascribes to the increase of the oxygen partial pressure gradient across the membrane. The permeation stability of the SDC–SSCF membrane was tested for 110 h under air/He gradient at 950 °C, and the oxygen permeation flux stabilizes at 0.45 mL cm
2 min
1, as shown in Fig. 6d. 3.4. Dual-phase membrane reactor for POM reaction The POM experiment was studied in a planar membrane reactor packed with unreduced LiLaNiO/γ–Al2O3 catalysts. There is no ignition step in this membrane reactor, and the same phenomena were observed for SDC–SSF [30] and SDC–SSAF [33] membrane reactors. Both the CH4 conversion and CO selectivity

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reach to 95% rapidly. After the reaction reached to the steady states of different conditions, the effects of CH4 flow rate and temperature were investigated. Fig. 7 shows the effect of the flow rate of CH4 on the performance of the membrane reactor at different temperatures. The flow rate of air was kept at 200 mL min
1. As shown in Fig. 7(a), the CH4 conversion decreases with the increase of CH4 flow rate at each temperature, while the CO selectivity increases with the increase of CH4 flow rate. There is an optimal CH4 flow rate at each temperature in terms of CH4 conversion and CO selectivity. The oxygen permeation flux increases from 5.2 to 7.9 mL cm
2 min
1 by increasing the flow rate from 8 to 15 mL min
1 at 950 °C, as shown in Fig. 7(b). Accordingly, the selectivity of CO increases from 83% to 99%, while the conversion of CH4 decreases from 98% to 94%, as shown in Fig. 7(a). The permeated oxygen is consumed instantly when increasing the flow rate of CH4 at the sweep side of SDC–SSCF membrane; meantime, the driving force across the membrane is increased and thus enhancing the oxygen permeation flux. In addition, the CH4 conversion and CO selectivity are influenced by the ratio of CH4/O2 in the membrane reactor. The increase of CH4 flow rate leads to the increase of the ratio of CH4/O2; as the ratio is larger than 2, oxygen is lack for POM reaction. Temperature has a significant effect on the membrane reaction because the oxygen permeation flux of the membrane is strongly dependent on temperature. As shown in Fig. 8, the oxygen permeation flux increases from 4.0 to 7.6 mL cm
2 min
1 as the temperature increasing from 800 to 950 °C if CH4 conversion and CO selectivity are both above 95%. The H2/CO ratio keeps at around 2.0 during the temperature change. The oxygen permeation flux of 7.6 mL cm
2 min
1 is 1.8 times that of the SDC–SSAF under the same condition [33]. As indicated by the conductivity experiments, the enhancement of oxygen permeation flux benefits from the improved electronic conductivity of SDC–SSCF in the oxygen partial pressure range of 10
2–10
15 atm. The performance with the non-noble metallic catalyst (LiLaNiO/γ–Al2O3) of our SDC–SSCF membrane is better than most of the other dual-phase membranes, as listed in Table 1. The oxygen permeation flux is comparable with the hollow fiber Zr0.86Y0.14O1.92 – La0.8Sr0.2Cr0.5Fe0.5O3
δ dual-phase membrane with a thinner dense layer and a 33 wt% Ru/La0.8Sr0.2Cr0.5Fe0.5O3
δ catalyst [50]. Besides the high oxygen permeation flux, good structural stability of the membrane is another essential factor for a POM membrane reactor. The membrane reactor should withstand the rigorous reaction conditions, and maintain the high performance during long period of operation as well. A durability test (Fig. 9) at 950 °C was operated for the SDC–SSCF membrane reactor, and then it was voluntarily terminated after 220 h on-stream. The flow rates of CH4 and air were kept at 13.1 mL min
1 and 200 mL min
1, respectively. It can be seen that there is no degradation during the 220 h POM operation. The oxygen permeation flux, CH4 conversion, CO selectivity, and H2/CO ratio kept at 7.2 mL cm
2 min
1, 95%, 98%, and 2.0 respectively. The oxygen permeation flux is about 15 times higher than that of He as the sweep gas, owing to the enhanced oxygen partial pressure gradient. 3.5. Characterizations of the SDC–SSCF membranes after POM operation After 220 h operation for the POM reaction, the spent membrane was characterized with SEM and EDX. Fig. 10 shows the SEM images of the spent membranes with one side coated with the SSC porous layer. The grains of SSC (Fig. 10a) grew bigger to some degree after POM. There was no difference from the cross-section view of the air side between the spent and fresh membranes. There were some residual LiLaNiO/γ–Al2O3 catalysts on the surface

of CH4 side of the spent membrane (Fig. 10c). The cross section of the spent membrane remained intact after being exposed to syngas and air for 220 h, as shown in Fig. 10b and d. The EDX analysis of the local-area elemental composition of the as-prepared and spent membranes is listed in Table 2. There is no big difference in elemental content between the as-prepared and spent membranes at 10-μm-depth from the membrane surfaces considering the accuracy of EDX analysis. The SEM and EDX results confirm the good stability of SDC–SSCF as a membrane reactor for syngas production.

4. Conclusions A new designed dense dual-phase membrane SDC–SSCF was synthesized by a simple one-pot SSR method. The membrane comprised only fluorite and perovskite phases. At the intermediate-low oxygen partial pressures, the electronic conductivity of SDC–SSCF is still high enough for the oxygen permeation, but that of SDC–SSAF limits the oxygen permeation. Thus, the oxygen permeation flux through a 0.5-mm-thick SDC–SSCF membrane reaches to 7.6 mL cm
2 min
1 for the syngas production, which is 1.8 times that of the SDC–SSAF membrane under the same condition. In addition, over a period of 220 h, 495% methane conversion and 498% CO selectivity were remained at 950 °C. The SEM and EDX results confirm the good stability of SDC–SSCF as a membrane reactor for syngas production. And the performance of the SDC–SSCF membrane can be further improved by reducing its thickness, such as fabricating into asymmetric membranes via tape-casting or other techniques.

Acknowledgments All the authors thank the financial support from National Natural Science Foundation of China (21476225, U1508203 and 91545202), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDB17020400), Youth Innovation Promotion Association of the Chinese Academy of Sciences and DICP (DICP DMTO201503).

References [1] M.P. Lobera, J.M. Serra, S.P. Foghmoes, M. Søgaard, A. Kaiser, On the use of supported ceria membranes for oxyfuel process/syngas production, J. Membr. Sci. 385 (2011) 154–161. [2] A. Aguadero, H. Falcon, J.M. Campos-Martin, S.M. Al-Zahrani, J.L. Fierro, J.A. Alonso, An oxygen-deficient perovskite as selective catalyst in the oxidation of alkyl benzenes, Angew. Chem. Int. Ed. 50 (2011) 6557–6561. [3] Z.P. Shao, S.M. Haile, A high-performance cathode for the next generation of solid-oxide fuel cells, Nature 431 (2004) 170–173. [4] S. Cho, D.E. Fowler, E.C. Miller, J.S. Cronin, K.R. Poeppelmeier, S.A. Barnett, Fesubstituted SrTiO3
δ–Ce0.9Gd0.1O2 composite anodes for solid oxide fuel cells, Energy Environ. Sci. 6 (2013) 1850–1857. [5] J.M. Serra, V.B. Vert, O. Buchler, W.A. Meulenberg, H.P. Buchkremer, IT-SOFC supported on mixed oxygen ionic–electronic conducting composites, Chem. Mater. 20 (2008) 3867–3875. [6] X.F. Zhu, M.R. Li, H.Y. Liu, T.Y. Zhang, Y. Cong, W.S. Yang, Design and experimental investigation of oxide ceramic dual-phase membranes, J. Membr. Sci. 394–395 (2012) 120–130. [7] P.-M. Geffroy, J. Fouletier, N. Richet, T. Chartier, Rational selection of MIEC materials in energy production processes, Chem. Eng. Sci. 87 (2013) 408–433. [8] J. Sunarso, S. Baumann, J.M. Serra, W.A. Meulenberg, S. Liu, Y.S. Lin, J.C. Diniz da Costa, Mixed ionic–electronic conducting (MIEC) ceramic-based membranes for oxygen separation, J. Membr. Sci. 320 (2008) 13–41. [9] Z.P. Shao, W.S. Yang, Y. Cong, H. Dong, J.H. Tong, G.X. Xiong, Investigation of the permeation behavior and stability of a Ba0.5Sr0.5Co0.8Fe0.2O3
δ oxygen membrane, J. Membr. Sci. 172 (2000) 177–188. [10] X.Y. Tan, N. Liu, B. Meng, S.M. Liu, Morphology control of the perovskite hollow fibre membranes for oxygen separation using different bore fluids, J. Membr. Sci. 378 (2011) 308–318.

L. Cai et al. / Journal of Membrane Science 520 (2016) 607–615

[11] P.Y. Zeng, R. Ran, Z.H. Chen, H.X. Gu, Z.P. Shao, S.M. Liu, Novel mixed conducting SrSc0.05Co0.95O3-δ ceramic membrane for oxygen separation, AIChE J. 53 (2007) 3116–3124. [12] Q. Zeng, Y.B. Zu, C.G. Fan, C.S. Chen, CO2-tolerant oxygen separation membranes targeting CO2 capture application, J. Membr. Sci. 335 (2009) 140–144. [13] S. Smart, C.X.C. Lin, L. Ding, K. Thambimuthu, J.C. Diniz da Costa, Ceramic membranes for gas processing in coal gasification, Energy Environ. Sci. 3 (2010) 268–278. [14] X.F. Zhu, H.Y. Liu, Y. Cong, W.S. Yang, Novel dual-phase membranes for CO2 capture via an oxyfuel route, Chem. Commun. 48 (2012) 251–253. [15] H.X. Luo, T. Klande, Z.W. Cao, F.Y. Liang, H.H. Wang, J. Caro, A CO2-stable reduction-tolerant Nd-containing dual phase membrane for oxyfuel CO2 capture, J. Mater. Chem. A 2 (2014) 7780–7787. [16] H.B. Li, X.F. Zhu, Y. Liu, W.P. Wang, W.S. Yang, Comparative investigation of dual-phase membranes containing cobalt and iron-based mixed conducting perovskite for oxygen permeation, J. Membr. Sci. 462 (2014) 170–177. [17] W.S. Yang, H.H. Wang, X.F. Zhu, L.W. Lin, Development and application of oxygen permeable membrane in selective oxidation of light alkanes, Top. Catal. 35 (2005) 155–167. [18] C.M. Chen, D.L. Benner, M.F. Carolan, E.P. Foster, W.L. Schinski, D.M. Taylor, ITM syngas ceramic membrane technology for synthesis gas production, Stud. Surf. Sci. Catal. 147 (2004) 55–60. [19] X.Y. Tan, Z.B. Zhao, Z. Gu, S.M. Liu, Catalytic perovskite hollow fibre membrane reactors for methane oxidative coupling, J. Membr. Sci. 302 (2007) 109–114. [20] X.L. Dong, W.Q. Jin, N.P. Xu, K. Li, Dense ceramic catalytic membranes and membrane reactors for energy and environmental applications, Chem. Commun. 47 (2011) 10886–10902. [21] W.P. Li, X.F. Zhu, Z.W. Cao, W.P. Wang, W.S. Yang, Mixed ionic–electronic conducting (MIEC) membranes for hydrogen production from water splitting, Int. J. Hydrog. Energy 40 (2015) 3452–3461. [22] R.V. Franca, A. Thursfield, I.S. Metcalfe, La0.6Sr0.4Co0.2Fe0.8O3
δ microtubular membranes for hydrogen production from water splitting, J. Membr. Sci. 389 (2012) 173–181. [23] H.Q. Jiang, H.H. Wang, S. Werth, T. Schiestel, J. Caro, Simultaneous production of hydrogen and synthesis gas by combining water splitting with partial oxidation of methane in a hollow-fiber membrane reactor, Angew. Chem. Int. Ed. 47 (2008) 9341–9344. [24] W.P. Li, X.F. Zhu, S.G. Chen, W.S. Yang, Integration of nine steps into one membrane reactor to produce synthesis gases for ammonia and liquid fuel, Angew. Chem. Int. Ed. 55 (2016) 8566–8570. [25] P.N. Dyer, R.E. Richards, S.L. Russek, D.M. Taylor, Ion transport membrane technology for oxygen separation and syngas production, Solid State Ion. 134 (2000) 21–33. [26] D.J. Wilhelm, D.R. Simbeck, A.D. Karp, R.L. Dickenson, Syngas production for gas-to-liquids applications: technologies, issues and outlook, Fuel Process. Technol. 71 (2001) 139–148. [27] V.V. Kharton, A.V. Kovalevsky, A.P. Viskup, A.A. Yaremchenko, E.N. Naumovich, F.M.B. Marques, Oxygen permeability of Ce0.8Gd0.2O2
δ – La0.7Sr0.3MnO3
δ composite membranes, J. Electrochem. Soc. 147 (2000) 2814–2821. [28] V.V. Kharton, A.V. Kovalevsky, A.P. Viskup, A.L. Shaula, F.M. Figueiredo, E. N. Naumovich, F.M.B. Marques, Oxygen transport in Ce0.8Gd0.2O2
δ-based composite membranes, Solid State Ion. 160 (2003) 247–258. [29] J.X. Yi, Y.B. Zuo, W. Liu, L. Winnubst, C.S. Chen, Oxygen permeation through a Ce0.8Sm0.2O2
δ–La0.8Sr0.2CrO3
δ dual-phase composite membrane, J. Membr. Sci. 280 (2006) 849–855. [30] X.F. Zhu, W.S. Yang, Composite membrane based on ionic conductor and mixed conductor for oxygen permeation, AIChE J. 54 (2008) 665–672. [31] X.F. Zhu, H.H. Wang, W.S. Yang, Relationship between homogeneity and oxygen permeability of composite membranes, J. Membr. Sci. 309 (2008) 120–127. [32] X.F. Zhu, Q.M. Li, Y.F. He, Y. Cong, W.S. Yang, Oxygen permeation and partial oxidation of methane in dual-phase membrane reactors, J. Membr. Sci. 360 (2010) 454–460.

615

[33] X.F. Zhu, Q.M. Li, Y. Cong, W.S. Yang, Syngas generation in a membrane reactor with a highly stable ceramic composite membrane, Catal. Commun. 10 (2008) 309–312. [34] Q.M. Li, X.F. Zhu, W.S. Yang, Single-step fabrication of asymmetric dual-phase composite membranes for oxygen separation, J. Membr. Sci. 325 (2008) 11–15. [35] E.V. Tsipis, M.V. Patrakeev, V.V. Kharton, A.A. Yaremchenko, G.C. Mather, A.L. Shaula, I.A. Leonidov, V.L. Kozhevnikov, J.R. Frade, Transport properties and thermal expansion of Ti-substituted La1
xSrxFeO3
δ (x¼ 0.5–0.7), Solid State Sci. 7 (2005) 355–365. [36] M.V. Patrakeev, J.A. Bahteeva, E.B. Mitberg, I.A. Leonidov, V.L. Kozhevnikov, K.R. Poeppelmeier, Electron/hole and ion transport in La1
xSrxFeO3
δ, J. Solid State Chem. 172 (2003) 219–231. [37] M.V. Patrakeeva, I.A. Leonidov, V.L. Kozhevnikov, V.V. Kharton, Ion–electron transport in strontium ferrites: relationships with structural features and stability, Solid State Sci. 6 (2004) 907–913. [38] V.V. Kharton, A.A. Yaremchenko, A.L. Shaula, A.P. Viskup, F.M.B. Marques, J.R. Frade, E.N. Naumovich, J.R. Casanova, I.P. Marozau, Oxygen permeability and thermal expansion of ferrite-based mixed conducting ceramics, Defect Diffus. Forum 226–228 (2004) 141–159. [39] V.L. Kozhevnikov, I.A. Leonidov, J.A. Bahteeva, M.V. Patrakeev, E.B. Mitberg, K.R. Poeppelmeier, Disordering and mixed conductivity in the solid solution LaSr2Fe3
yCryO8 þ δ, Chem. Mater. 16 (2004) 5014–5020. [40] D.E. Fowler, J.M. Haag, C. Boland, D.M. Bierschenk, S.A. Barnett, K. R. Poeppelmeier, Stable, low polarization resistance solid oxide fuel cell anodes: La1
xSrxCr1
xFexO3
δ (x¼ 0.2–0.67), Chem. Mater. 26 (2014) 3113–3120. [41] J.M. Haag, S.A. Barnett, J.W. Richardson Jr., K.R. Poeppelmeier, Structural and chemical evolution of the SOFC anode La0.30Sr0.70Fe0.70Cr0.30O3
δ upon reduction and oxidation: an in situ neutron diffraction study, Chem. Mater. 22 (2010) 3283–3289. [42] J. Yoo, S. Kim, H. Choi, Y. Rhim, J. Lim, S. Lee, A.J. Jacobson, Measurement of electrical conductivity of La0.2Sr0.8Cr0.2Fe0.8O3
δ using gas-tight electrochemical cells, J. Electroceram. 26 (2011) 56–62. [43] M. Mogensen, N.M. Sammes, G.A. Tompsett, Physical, chemical and electrochemical properties of pure and doped ceria, Solid State Ion. 129 (2000) 63–94. [44] S. Gupta, M.K. Mahapatra, P. Singh, Lanthanum chromite based perovskites for oxygen transport membrane, Mater. Sci. Eng. R: Rep. 90 (2015) 1–36. [45] Z.W. Cao, X.F. Zhu, W.P. Li, B. Xu, L.N. Yang, W.S. Yang, Asymmetric dual-phase membranes prepared via tape-casting and co-lamination for oxygen permeation, Mater. Lett. 147 (2015) 88–91. [46] X.F. Zhu, Y. Liu, Y. Cong, W.S. Yang, Ce0.85Sm0.15O1.925–Sm0.6Sr0.4Al0.3Fe0.7O3 dual-phase membranes: one-pot synthesis and stability in a CO2 atmosphere, Solid State Ion. 253 (2013) 57–63. [47] S.L. Liu, G.X. Xiong, S.S. Sheng, W.S. Yang, Partial oxidation of methane and ethane to synthesis gas over a LiLaNiO/γ–Al2O3 catalyst, Appl. Catal. A: Gen. 198 (2000) 261–266. [48] X.F. Zhu, H.H. Wang, Y. Cong, W.S. Yang, Partial oxidation of methane to syngas in BaCe0.15Fe0.85O3
δ membrane reactors, Catal. Lett. 111 (2006) 179–185. [49] Z.T. Wu, B. Wang, K. Li, A novel dual-layer ceramic hollow fibre membrane reactor for methane conversion, J. Membr. Sci. 352 (2010) 63–70. [50] J.J. Liu, S.Q. Zhang, W.D. Wang, J.F. Gao, W. Liu, C.S. Chen, Partial oxidation of methane in a Zr0.84Y0.16O1.92–La0.8Sr0.2Cr0.5Fe0.5O3
δ hollow fiber membrane reactor targeting solid oxide fuel cell applications, J. Power Sources 217 (2012) 287–290. [51] J.J. Liu, T. Liu, W.D. Wang, J.F. Gao, C.S. Chen, Zr0.84Y0.16O1.92– La0.8Sr0.2Cr0.5Fe0.5O3
δ dual-phase composite hollow fiber membrane targeting chemical reactor applications, J. Membr. Sci. 389 (2012) 435–440. [52] H.X. Luo, H.Q. Jiang, T. Klande, Z.W. Cao, F.Y. Liang, H.H. Wang, J. Caro, Novel cobalt-free, noble metal-free oxygen-permeable 40Pr0.6Sr0.4FeO3
δ– 60Ce0.9Pr0.1O2
δ dual-phase membrane, Chem. Mater. 24 (2012) 2148–2154.