155
Topics in Catalysis Vol. 35, Nos. 1–2, June 2005 ( 2005)
DOI: 10.1007/s11244-005-3820-6
Development and application of oxygen permeable membrane
in selective oxidation of light alkanes
Weishen Yang*, Haihui Wang, Xuefeng Zhu, and Liwu Lin
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
In this paper, oxygen permeable membrane used in membrane reactor for selective oxidation of alkanes will be discussed in
detail. The recent developments for the membrane materials will be presented, and the strategy for the selection of the membrane
materials will be outlined. The main applications of oxygen permeable membrane in selective oxidation of light alkanes will be
summarized, which includes partial oxidation of methane (POM) to syngas and partial oxidation of heptane (POH) to produce H2,
oxidative coupling of methane (OCM) to C2, oxidative dehydrogenation of ethane (ODE) to ethylene and oxidative dehydrogenation
of propane (ODP) to propylene. Achievements for the membrane material developments and selective oxidation of light alkanes in
membrane reactor in our group are highlighted.
KEY WORDS: Oxygen permeable membrane; selective oxidation; hydrocarbon conversion; membrane reactor.
1. Introduction
Dense oxygen permeable membrane made from material that can conduct oxygen ions at elevated temperatures has been attracted a great attention in academy and
industry [1]. For oxygen separation, each side of the
membrane is exposed to different oxygen partial pressure
environments, so the chemical potential gradient created
cross the membrane makes the oxygen ions transported
from the high oxygen partial pressure side (cathode) to
the low oxygen partial pressure side (anode). At the same
time, electrons transport through the membrane from
anode to cathode. An external circuit or a second phase
(which can transport electrons, such as metals or electronic conducting oxides) is needed if the membrane is
made from a pure oxygen ion conductor. The external
circuit is not necessary if the membrane is made from a
mixed oxygen ionic and electronic conducting oxide. The
membrane is so-called mixed ionic and electronic conducting (MIEC) membrane. Figure 1 schematically
shows the mechanism of MIEC membrane for separation
of oxygen. It is generally accepted that the MIEC
membrane have great potential to meet the needs of
many segments of the oxygen market ranging from smallscale oxygen pumps for medical purpose to large-scale
applications in combustion process if they can be
developed with sufficient durability and reliability. Furthermore, the MIEC membrane can also be used as
membrane reactor to enhance products selectivity for
selective oxidation of hydrocarbons by controlling the
distribution of oxygen supply along the membrane
reactor and/or providing different types of active oxygen
species (e.g. O2), O), O2)) at the membrane surface where
the catalyst and reactants are located [2].
* To whom correspondence should be addressed.
E-mail: [email protected]; URL: http://www.yanggroup.dicp.ac.cn
For practical application, the membrane materials
used in the membrane reactor must meet a number of
requirements, as shown in figure 2. (1) the materials
must be stable for long-term operation under strongly
reducing atmosphere, such as the mixture of carbon
monoxide and hydrogen, at elevated temperatures
(>700 C); (2) the materials must have considerable
high oxygen permeability under the operation conditions; (3) the materials must have enough mechanical
strength for constructing the membrane reactor; (4) the
oxygen permeability of the membrane materials should
avoid a decline with time; (5) the materials should be
cheap enough for large-scale industrial applications.
2. Development of MIEC membrane materials
2.1. Perovskite-type membrane materials
2.1.1. Materials based on LaCoO3
Many perovskite oxides, such as La1)xSrxCo1)yFeyO3)d, have a comparable oxygen ionic conductivity
with but much higher electronic conductivity than
yttrium-stabilized zirconia (YSZ) when B-site is doped
with multi-valent transition metals. So these mixed
conductors are suitable for making oxygen permeable
membrane without external circuit. In recent decade,
much research effort is focused on doped perovskite
oxides A1)xA¢xB1)yB¢yO3)d (A ¼ lanthanide and Y;
A¢ ¼ Ca, Sr, Ba; B or B¢ ¼ Mg, Al, Ti, Cr, Mn, Fe, Co,
Ni, Cu, Ga, Zr; 0 £ x £ 1; 0 £ y £ 1). The oxygen
permeation fluxes of perovskite materials reported in
the literatures are listed in table 1. Teraoka et al. [3–6]
were the first to report high oxygen flux through
La1)xSrxCo1)yFeyO3)d perovskite membranes. They
substituted Sr with Na, Ca, Ba, and substituted La with
Pr, Nd, Sm, Gd, respectively, and found that the oxygen
permeability increased as Na < Sr < Ca < Ba to the
1022-5528/05/0600–0155/0 2005 Springer Science+Business Media, Inc.
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W. Yang et al./Development and application of oxygen permeable membrane
Figure 1. Mechanism of mixed oxygen ionic and electronic conducting
ceramic membrane for oxygen separation.
Figure 2. Criteria for oxygen permeable membrane.
former, and as La < Pr < Nd < Sm < Gd to the
latter. They also investigated Co substituted by different
light transition metals. As a result, the oxygen permeability was improved by substitution of Cu and Ni due to
the increase of oxygen ion vacancie while deteriorated by
Fe, Cr, and Mn. Based on the above results, they
designed Gd0.2Ba0.8Co0.7Fe0.1Cu0.2O3)d as an optimum
material. Oxygen flux of the membrane was reported to
be 5.5 ml/cm2 min at 870 C. Unfortunately this result
has been hardly repeated by other researchers.
Since Teraoka’s pioneer work, a wide range of such
MIEC membrane materials was developed by now
[7–34]. Bouwmeester [1] reviewed the oxygen permeability of these membrane materials, and pointed out
that the oxygen permeability of a given material
reported by different authors has great difference.
For example, the oxygen permeation flux of SrCo0.8Fe0.2O3)d reported by Teraoka et al. [3] was ten times
higher than that of the permeation flux measured by
Kruidhof et al. [7] over the same temperature range.
Many researchers also found that the oxygen permeation fluxes of La1)xSrx,Co1)yFeyO3)d membrane
reported by Teraoka and coworkers were obviously
higher than others [8–17], but they verified that
La(Ba,Sr,Ca)CoFeO3 was a series of materials with
high oxygen permeability.
With in-depth investigation, many researchers realized that except the oxygen permeability of the membrane materials, the stability of the materials under
reducing environments at elevated temperatures is also
important. Due to the high oxygen permeability of
SrCo0.8Fe0.2O3)d membrane, its phase structure and
stability in reducing environments were extensively
investigated [7,9,35,36]. For SrCo0.8Fe0.2O3)d membrane material, the perovskite phase is thermodynamically stable only at higher oxygen partial pressure
(>0.1 atm) and high temperatures (>790 C) [17]. The
oxygen vacancies disordered perovskite phase was
transformed to an oxygen vacancies ordered brownmillerite phase when the temperature was lower than
790 C [7]. Ordering of oxygen vacancies can lead to
significant reduction of ionic conductivity. However, the
proper substitution of metal ion in SrCo0.8Fe0.2O3)d may
improve the phase stability of the material. For example,
partial substitute of Sr by La [37] or Co by Ti [38], Zr
[18,19,39,40] can significantly improve the stability of
the material, but at the cost of oxygen permeability.
Prado et al. [37] studied the effect of La3+ doping on the
perovskite-to-brownmillerite
transformation
in
LaxSr1)xCo0.8Fe0.2O3)d, membrane. They found that
only the La3+ dopant was at 0.4 that perovskite
structure could be kept in pure N2 (PO2 < 10)5 atm)
Table 1
Oxygen permeation fluxes of some MIEC membrane materials
Materials
La0.6Sr0.4Co0.8Fe0.2O3)d
La0.6Sr0.4Co0.2Fe0.8O3)d
La0.2Ba0.8Co0.2Fe0.8O3)d
La0.8Sr0.2Ga0.6Fe0.4O3)d
La0.3Sr0.14Ga0.6Fe0.1.4O5+d
SrCo0.8Fe0.2O3)d
SrCo0.4Fe0.6O3)d-ZrO2
Ba0.5Sr0.5Co0.8Fe0.2O3)d
BaZr0.2Co0.4Fe0.4 O3)d
BaCe0.15Fe0.85O3)d
O2 flux (ml cm)2min)1)
Temperature (C)
Thickness (mm)
References
0.62
0.3
0.4
0.85
0.75
3.1
0.4
1.3
0.7
0.42
850
900
900
900
900
850
900
900
900
900
2.0
2.3–3.1
2.3–3.1
0.5
1.0
1.0
2.0
1.5
1.0
1.0
3
8
8
21
52
3
39
20
19
34
W. Yang et al./Development and application of oxygen permeable membrane
at high temperatures. In our laboratory, we used Ba to
partially substitute Sr, such as Ba0.5Sr0.5Co0.8Fe0.2O3)d,
to improve the stability of the material [20]. The results
showed that the. perovskite structure of the material
could be kept in lower oxygen partial pressure
(PO2 < 10)5 atm) and with even higher oxygen permeability.
The
O2-TPD.(under
pure
He,
)5
PO2 < 10 atm) results showed that the oxygen
desorption peak of Ba0.5Sr0.5Co0.8Fe0.2O3)d (BSCF)
was much smaller than that of the SrCo0.8Fe0.2O3)d
(SCF), which implied that the oxidation of Co3+ and
Fe3+ to higher valence state Co4+ and Fe4+ were
effectively suppressed by the introduction of barium. The
samples after O2-TPD were subjected to X-ray diffraction
analysis. The results are shown in figure 3. BSCF
sustained its perovskite structure while SCF changed
from perovskite to brownmillerite-type structure [20].
500 h’s syngas production experiment further confirmed
the substitution of Sr by Ba could effectively enhance the
stability of the perovskite phase [41–43].
Doping less reducible ions, such as Ga3+ [21], Ti4+
[44], Zr4+ [18,19,39,40], is regarded as another effective
method to improve the stability of the membrane
materials under reducing environments at high temperatures. BaZr0.2Co0.4Fe0.4O3)d developed in our group
[19,45] is an example, which can withstand syngas
production experiment at 850 C for more than 2200 h
[45]. However, materials with B-site doped by Zr4+ and
Ti4+ usually have lower oxygen permeability compared
to the counterparts without tetravalent cations doping.
Kharton et al. [22] attributed this effect to a higher
coulombic interaction between B-cations and oxygen
anions, which agreed with the observation that ionic
conductivity of A2+B4+O3 type perovskite was usually
lower than that of A3+B3+O3 type perovskite [46]. But
just the strong bond between Zr4+, Ti4+ and oxygen
makes the materials had higher mechanical strength and
stability under reducing environments.
Figure 3. X-ray diffraction patterns of (a) SrCo0.8Fe0.2O3)d, (b)
Ba0.5Sr0.5Co0.8Fe0.2O3)d after O2-TPD.
157
2.1.2. Materials based on LaGaO3
Perovskite-like solid electrolytes of La(Sr)Ga(Mg)O3
were reported to have a high oxygen ionic conductivity
(r0 ¼ 0.08 S/cm at 800 C) [47]. Light transition metal,
such as Fe, Co and Ni, doped in B-site of La(Sr)Ga(Mg)O3 can improve the electronic conductivity, thus
makes it become an interesting materials for membrane
applications [21,48,49]. Ishihara et al. [21] investigated
double-doped LaGaO3 by Sr for La and transition metal
(e.g. Fe, Co, Ni) for Ga, and found that Fe-doped
La0.8Sr0.2Ga1)xFexO3)d was stable under reducing
atmosphere, though the oxygen permeability was lower
than that of Co-doped and Ni-doped counterparts, i.e.,
La0.8Sr0.2Ga1)xMx03)d (M ¼ Co, Ni). It was found that
the oxygen permeability increased with increasing of Fe
content at all temperatures, and reached at a maximum
at x ¼ 0.4, i.e., La0.8Sr0.2Ga0.6Fe0.4O3)d. To improve the
stability of the materials under water vapor and CO2 at
high temperatures, Yaremchenko et al. [48] suggested
that doping B-site of LaGaO3 with bivalent cations
(such as Mg, Ni and Cu) to improve ionic conductivity
should be preferable as compared to incorporating
alkaline earth (such as Ca, Sr and Ba) into the A-site.
They reported that incorporating Mg into B-site
resulted in an increase both electronic and oxygen ionic
conductivity due to an increase in average oxidation
state of transition metal ions and vacancy concentration, respectively. Ishihara et al. [50] and Trofimenko
et al. [51] reported that doping slight amount of Co at
Ga site of La(Sr)Ga(Mg)O3 could result in the increase
of the ionic conductivity with a wide ionic domain.
Kharton et al. [49] reported that the ionic conductivities
of the materials doped with Fe, Co and Ni were lower,
but slight amount doping, than that of parent compounds La1)xSrxGa1)yMgyO3)d due to partial oxygenvacancy ordering and higher average cation-anion bond
energy in transition metal-containing gallates [48].
Mackay et al. [52] reported a series of membrane
materials of La1)xSrxGa1)yFeyO3)d (LSGF) with substitution of Fe with Co and Mg, i.e., La0.3Sr1.7Ga0.6Fe1.1Co0.3O5+d, La0.4Sr1.6Ga0.6Fe1.2Co0.2O5+d and
La0.3Sr1.7Ga0.6Fe1.1Mg0.2O5+d. The oxygen permeation
fluxes of these materials were 1.2, 1.3 and 0.94 ml/
cm2 min for 1 mm-thickness membrane at 900 C with
oxidation and reduction catalyst on the both surfaces,
respectively, which were higher than that of La0.3Sr1.7Ga0.6Fe1.4O5+d, (0.75 ml/cm2 min) at the same experimental conditions. The improvement of permeability are
perhaps due to the bonds of Co–O and Mg–O is weaker
than Fe–O and the lower oxidation states of Co and Mg
create more oxygen vacancies.
2.1.3. Materials based on BaCeO3
Recently, our group developed a series of new cobaltfree perovskite membrane materials BaCexFe1)xO3)d
(BCF) [34], based on the point that perovskite BaCeO3
with excellent resistance to reducing atmosphere is
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W. Yang et al./Development and application of oxygen permeable membrane
employed as hydrogen permeable membrane. In general,
MIEC membrane materials doped with iron instead of
cobalt in B site can improve the stability of materials.
Barium is chosen as A site ion to decrease the average
metal–oxygen bond energy (ABE) within the lattice and
increase the lattice free volume (Fv). Certainly, only the
large ionic radii of Ba2+ can insure the Goldschimdt
tolerance factor close to 1. As shown in figure 4, the
oxygen fluxes of the series of membranes increase
monotonously with the increase of the temperatures. It
is obvious that the oxygen fluxes increase remarkably
with the increasing of the iron-doping amount. The oxygen
permeation’ flux of BaCe0.15Fe0.85O3)d (BCF1585), exhibiting the highest oxygen permeaction flux among this series,
is several times higher than that of La1)xSrxGa1)yFeyO3)d
(LSGF) and also can compare to that of La0.6Sr0.4Co0.2Fe0.8O3)d at the same temperature [53].
For syngas generation, oxygen permeable membrane
is operated under severe reducing environment at
elevated temperatures. Few MIEC membrane materials
can withstand such reducing environment, and the
surface exposed to reducing atmosphere is often partially decomposed up to a certain degree. The structural
failure will affect the long-term performance of the
membrane reactors. So it is necessary to test the
perovskite structural stability for the materials we
developed. However, it is a time-consuming process to
make sure the stability of the membrane materials in real
syngas generation environment, because once the membrane is partially reduced by syngas, it will be quickly reoxidized by permeated oxygen from air side of
membrane. It is a dynamically controlled process. As
results, many oxygen permeable materials can sustain
syngas generation experiment for hundreds of hours
without failure even for Co containing materials (such
as BSCF can sustain more than 500 h under syngas
generation experiment [41–43]). For materials without
Figure 4. Comparison of oxygen permeation fluxes of BaCexFe1)xO3)d
series of membranes as a function of temperature. (a), x ¼ 0.15
(d ¼ 1.0 mm); (b), x ¼ 0.2 (d ¼ 1.0 mm); (c), x ¼ 0.2 (d ¼ 1.5 mm);
(d), x ¼ 0.4 (d ¼ 1.4 mm); (e), x ¼ 0.6 (d ¼ 0.82 mm).
Co, such as LSGF, it can be operated in syngas
generation conditions for more than 1000 h [54]. Therefore, to quickly evaluate the stability of the membrane
materials in the reducing environment, we just subject
the materials in 5%H2+Ar atmosphere at different
temperatures for 1 h, then check their structure changes.
Since there are few reports concerning on direct reducing the oxygen permeable materials, two materials were
selected for comparison. For BSCF material, the perovskite structure is completely destroyed at 800 C (see
figure 5a) while LSGF material still maintains their
Figure 5. XRD patterns of fresh membrane materials and treated for
1h in 5% H2+Ar mixture at different temperatures (a) Ba0.5Sr0.5Co0.8Fe0.2O3)d; (b) La0.5Sr0.5Ga0.3Fe0.7O3)d; (c) BaCe0.15Fe0.85O3)d.
W. Yang et al./Development and application of oxygen permeable membrane
perovskite structure (see figure 5b). However, when the
temperature increases to 900 C, the perovskite structure of LSGF material is partly degraded. Ming and
Ritchie [55,56] reported that the decomposition of
La0.5Sr0.5Ga0.2Fe0.8O3)d begins at 860 C in an oxygen
partial pressure of about 10)17 atm of a syngas environment. For our material (BCF 1585), their perovskite
structure is even maintained at 950 C (See figure 5c).
Based on the above experiments, we can conclude that
the materials (BaCe0.15Fe0.85O3)d) we developed have
good structural stability under reducing environment.
Another important feature of BCF membrane materials
is its much lower cost as compared with LSGF
membrane materials.
2.2. Perovskite-related intergrowth structure materials
2.2.1. Sr4Fe6)xCoxO13 system
An alternative method to overcome the difficulties for
the practical application of the perovskite membrane
materials, such as chemical instability and low mechanical integrity, is to develop perovskite-related intergrowth oxides. Balachandran et al [57–59] firstly
reported a MIEC material based on cobalt-doped
Sr4Fe6O13, i.e. SrFeCo0.5Ox, which exhibits unusually
high oxygen ionic conductivity (r0 ~ 7 S cm)1 at 800 C
in air) and oxygen permeability (2.5 ml/cm2 min for a
2.9 mm thick membrane at 900 C) [27].
2.2.2. Ruddlesden-Popper series materials
Manthiram et al. [60–63] reported another kind of
materials with perovskite related intergrowth structure,
i.e.
Ruddlesden–Popper
(R–P)
series
An + 1BnO3n + 1(A ¼ lanthanide or alkaline earth and
n ¼ 1, 2, 3...). The crystal structure of R-P phase is
similar to perovskite, in which a number of perovskite
blocks (n is numbers of perovskite blocks) having
corner-shared BO6 octahedra alter with AO rock-salt
layers along the c-axis. K2NiF4-type oxides and perovskite-type oxides are two extremes for this series of
materials (n ¼ 1 for K2NiF4-type, and n ¼ ¥ for perovskite). The oxygen permeabilities of this series materials
are lower than that of perovskite-type counterparts [60–
63]. Additionally, R–P series materials with larger
oxygen vacancies along c-axis, such as Sr3FeCoO7)d,
are extremely sensitive to atmospheric moisture [64].
2.2.3. K2NiF4-type materials
K2NiF4-type of oxides La2)xA¢xNi1)yB¢yO4+d, similar to the perovskite oxides, exhibits mixed oxygen ionic
and electronic conductivities. However, the oxygen ions
transport mechanism is somehow different from that for
the perovskite oxides. For perovskite oxides, oxygen
ions transport through membrane by oxygen vacancies
while transport of oxygen ions in K2NiF4-type of oxides
is realized by both interstitial oxygen ions migration in
the rock-salt-type layers of the structure and oxygen
159
ions diffusion through oxygen vacancies in the perovskite layers [65–68]. This type of oxides have many
advantages, such as fast oxygen ions diffusion [67], high
stability [68], considerable high electronic conductivity
and low TEC [68], so it is supposed to be a good
alternative material for oxygen separation membrane.
Kharton et al. [69–71] extensively investigated the oxygen permeability, conductivity, stability and catalytic
activity of La2)x,Srx,Ni1)y (Fe, Co, Cu)yO4+d for
methane oxidation. Based on the oxygen permeation
results, they found that the interstitial oxygen ions
migration in the rock-salt-type layers of the structure
was more important than oxygen ions diffusion through
oxygen vacancies to the ionic conductivity of the
K2NiF4-type membrane [66]. At membrane operation,
the oxygen partial pressure at permeation side is usually
low. With decrease of oxygen partial pressure at
permeation side, the contribution of interstitial oxygen
ions to ionic conductivity decreased even though the
contribution of oxygen ions diffusion through oxygen
vacancies increased. As net result, the oxygen permeability of La2NiO4+d-based membranes is lower than
that of La1)xSrxCo1)yFeyO3)d system [69–71]. Therefore, this type of materials is actually not very suitable to
construct membrane reactors for hydrocarbon
conversions [70,71].
2.3. Dual phase composite materials
It is difficult to meet all the requirements, such as
high permeability, stability, mechanical strength etc.,
in one single-phase membrane material. In general,
the improvement in some aspects is simultaneously
degraded on the others. For example, the increase of
oxygen permeability of membrane material is usually
at the cost of the stability in reducing atmosphere for
single-phase material. So, dual-phase composite materials, which are made from oxygen ionic conducting
phase and electronic conducting phase, were suggested as MIEC membrane materials to avoid the
dilemma.
2.3.1. Oxygen ionic conductor and noble metal composite
membrane
Stabilized bismuth oxide and zirconia are the extensively investigated as oxygen ionic conductors. These
oxygen ionic conductors mixed with noble metals forms
a dual phase membrane. The dual-phase composite
membrane material was proposed and examined by
Mazanec et al. [72], and was further investigated in a
systematic and in-depth manner by Chen and Burggraaf
[73,74]. For this type of composite membrane, the noble
metal (Pd, Pt, Au, Ag etc.) phase should excess 30 vol.%
in order to form a continuous electronic conducting
phase [73–76]. The high material cost and low oxygen
permeability hinder this type of composite membrane
for practical applications.
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W. Yang et al./Development and application of oxygen permeable membrane
2.3.2. Composite oxide membrane
Kharton et al. [77,78], Wang et al. [79] and Sirman et
al [80] proposed a kind of composite oxide membrane to
decrease the cost of dual phase membrane materials. The
composite oxide membrane is made from an oxygen
ionic conductor (serving as an oxygen ionic conducting
phase) and a perovskite oxide (serving as an electronic
conducting phase) which has high electronic conductivity. Kharton et al. [78] prepared a composite membrane
by mixing Ce0.8Gd0.2O2)d (CGO) and La0.7Sr0.3MnO3)d
with similar volume fraction. The results showed that the
oxygen permeation flux continually declined with time
during 800 h’s long-term operation. They attributed the
degradation to the formation of layers with low ionic
conductivity at the Ce0.8Gd0.2O2)d grain boundaries by
inter-diffusion of the two phases. Similar inter-diffusion
of between La0.8Sr0.2Fe0.8Co0.2O3-(La0.9Sr0.1)0.98Ga0.8
Mg0.2O3 was also observed and resulted in the decrease
of the ionic conductivity [81]. Sirman et al. [80] reported
a series of composite membranes based on Ce0.8Gd0.2O1.9
and La0.8Sr0.2Fe0.8Co0.203)d (LSCF) with different volume
ratios of the two oxides. For example, two oxides were
mixed at a 50%/50% volume ratio, and sintered in the
temperature range of 1200–1400 C to get a heterogeneous
two-phase material with a considerable low mean TEC,
about 11.9 ppm/K. The oxygen permeation flux reached at
6.1 ml/cm2 min for a 1 mm disc at 1000 C with 90%CO/
l0%CO2 as sweeping gas. However, the long-term stability
of oxygen permeation is not included.
We also synthesized a dual-phase Lao0.15Sr0.85Ga0.3Fe0.7O3)d (LSGF)-BSCF composite membrane with a
structure defined by the closed packing of LSGF grains
with a three-dimensional thin BSCF film running
between the boundaries of the connected LSGF grains
[79]. Figure 6 shows the SEM picture of the LSGFBSCF membrane. The film phase is percolated at a
volume percent as low as 7.2%. Since the majority phase
is the chemically more stable LSGF with the structure of
perovskite and the second phase is made of a mechanically and chemically compatible BSCF, rather than a
metal. In this way, the oxygen permeation of the dualphase LSGF-BSCF membrane is much higher than that
of LSGF membrane. The oxygen permeation flux of
LSGF-BSCF membrane (~0.45 ml/cm2 min) is nine
times higher than that of the LSGF membrane
(~0.05 ml/cm2 min) at 915 C. H2 reduction experiment
shows that the LSGF-BSCF membrane has a good
stability in the H2-containing atmosphere. This membrane may be more suitable for constructing membrane
reactor for syngas generation.
3. Selective oxidation of light alkanes in MIEC
membrane reactor
In this section, an attempt has been made to highlight
the MIEC membrane used for the selective oxidation of
alkanes in our laboratory. The following reactions in the
oxygen permeable membrane reactors will be summarized: partial oxidation of methane to syngas (POM)
and partial oxidation of heptane to produce hydrogen
(POH), oxidative couple of methane to C2 (OCM),
oxidative dehydrogenation of ethane to ethylene (ODE)
and oxidative dehydrogenation of propane to propylene
(ODP).
3.1. POM and POH reactions in oxygen permeable
membrane reactor
During the past decades, a lot of extensive efforts
have focused on using the oxygen permeable membrane
to improve the process efficiency of catalytic conversion
of alkanes. Of particular interest is the partial oxidation
of methane to syngas (CO + H2). Synthesis gas is an
important intermediate for the gas-to-liquid process
(natural gas to liquid fuels) via existing processes, such
as F–T synthesis and methanol synthesis. Up to now,
steam reforming is the dominant process for producing
syngas. However, to this process, there are some
drawbacks, for examples, this reaction is highly energy
Figure 6. SEM picture of La0.15Sr0.85Ga0.3Fe0.7O
3d Ba0.5Sr0.5Co0.8Fe0.2O3)d membrane.
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W. Yang et al./Development and application of oxygen permeable membrane
intensive and also suffers from limitations, such as poor
selectivity for CO and high H2/CO production ratio,
unsuitable for the F-T synthesis. So, the catalytic POM
to syngas has attracted more and more attention in
recent years [82, 83]. Although the partial oxidation of
methane with air as the oxygen source is a potential
alternative to the steam-reforming process, however,
downstream process requirements could not tolerate
nitrogen. Therefore, pure oxygen is required. The
investment cost in a cryogenic oxygen plant may
constitute up to 45% of the total investment cost [84].
There is thus a great incentive to look for other ways of
supplying oxygen. An alternative route that seems very
promising is to use the MIEC membrane for the oxygen
separation, and to combine the catalytic partial oxidation reaction and oxygen separation into a catalytic
membrane reactor, as shown in figure 7.
Successful development of the MIEC membrane
reactor technology can reduce the cost of oxygen
production. The ceramic membrane further aids in
safety management, since it avoids the premixing of
oxygen and natural gas and reduces the formation of
hot spots as encountered in a co-feed reactor. The
Figure 7. Principle of a ceramic membrane reactor for partial oxidation of methane to syngas.
reaction rate is controlled by the oxygen permeation rate
of the ceramic membrane. So this makes the operation
process controllable and overcome the flammability
limits. Due to the potential advantages and economic
benefits offered by this technology, world-wide alliances
co-ordinated by Air Products and Praxair, respectively,
have launched some big programs to solve the scale-up
problems. The key to success of POM based-sygngas
production in membrane reactor is to develop the
ceramic membrane, which possesses high oxygen permeable flux and long-term operational stability in the
syngas environment. Table 2 summarized the performance of the membranes during the POM process
reported in the literatures.
Tsai et al. [85] studied the direct conversion of
methane to syngas in a disk-type membrane reactor
based on La0.2Ba0.8Co0.2Fe0.8O3)d at 1123 K. They
found that packing a 5%Ni/Al2O3 catalyst directly on
the membrane surface result in a fivefold increase in
oxygen permeation flux and fourfold increase in CH4
conversion, compared with the blank run. The observation demonstrates that intimate contact of the catalyst
and membrane surface is critical to deplete oxygen at the
immediate membrane surface in order to establish a high
oxygen potential gradient for oxygen transport. The
authors successfully used this membrane in syngas
generation experiments for 850 h at 850 C, albeit that
XRD, EDS and SEM analyses after the experiments
revealed morphological and compositional changes of
the membrane surfaces. Balachandran et al. [57–59]
investigated POM to syngas by using tubular La0.2Sr0.8Co0.2Fe0.8O3)d (LSCF) and SrCo0.8Fe0.2O3)d (SCF)
membrane reactors. They found that the membranes
broke into several pieces in a few minutes after the
introduction of methane to the membrane reactor at
850 C. Pei et al. [35] studied the failure mechanism of
ceramic membrane reactors on POM to syngas. They
observed two types of fractures occurring on the SCF
membrane reactor. The first type occurred shortly after
the reaction started and the second type often occurred
days after the reaction. They also found that the first
Table 2
Membrane performance reported for POM
Material
La0.2Ba0.8Fe0.8Co0.2O3)d
SrFeCo0.5Ox
Ba0.5Sr0.5Fe0.2Co0.8O3)d
BaZr0.2Fe0.4Co0.4O3)d
La0.5Sr0.5Ga0.2Fe0.8O3)d
La0.3Sr1.7Ga0.6Fe1.4O5+d
La0.7Sr0.3Ga0.6Fe0.4O3)d
SrFe0.6Co0.4O3)d-YSZ
La0.2Sr0.8Fe0.8Co0.1Cr0.1O3)d
La0.2Sr0.8Fe0.7Co0.1Cr0.2O3)d
AxA¢1)xByB¢1)xO5+d
O2 flux (ml/cm2 min)
Thickness (mm)
Temperature (C)
4
2–4
11.5
5.6
0.34
1–1.3
8–12
4.5
5.68
6
10–12
0.55
0.25–1.2
1.5
1.3
0.15
–
0.3–0.5
1.8
–
1.0–1.1
–
850
900
850
850
850
900
1000
850
950
1000
900
Operation ·(h) References
850
1000
500
2200
–
>1000
–
220
340
–
>8700
85
57–59
41,42
45
56
54
124
40
86
125
95
162
W. Yang et al./Development and application of oxygen permeable membrane
fracture was the consequence of oxygen gradient across
the membrane from reaction side to the air side, which
causes a little mismatch inside the membrane, leading to
fracture; the second type of fracture was the result of a
chemical decomposition in the reductive atmosphere.
Bouwmeester reported membranes made of La0.2Sr0.8Co0.1Cr0.1Fe0.8O3)d cracked for POM after 350 h of
operation at 900 C [86].
Our group investigated POM in a BSCF membrane
reactor by packing LiLaNiO/c-Al2O3 with 10% Ni
loading as the catalyst [41,42]. At the initial stage, oxygen
permeation flux, methane conversion and CO selectivity
were closely related with the state of the catalyst. About
21 h was needed for the oxygen permeation flux reaching
steady state. 98.5% CH4 conversion, 93.0% CO selectivity and 10.45 ml/cm2 min oxygen permeation flux were
achieved under steady state at 850 C. No fracture of the
membrane reactor was observed during 500 h operation.
H2-TPR investigation demonstrated that the BSCF
material was unstable under reducing atmosphere, but
the material was found to have excellent phase reversibility [41]. A tubular membrane reactor made of BSCF
was successfully operated for the POM reaction in pure
methane stream at 875 C for more than 500 h without
failure, as shown in figure 8. Beside the limited kinetics at
the reaction-side of the membrane, the mechanism of
POM in the membrane reactor is another factor for the
long-term operation for BSCF membrane.
In the tubular membrane reactor [43], the combustion
of methane, CH4 + 2O2 ¼ CO2 + 2H2O, with all the
permeated oxygen first takes place on NiAl2O4 and Ni/
Al2O3 catalysts in the reaction zone close to the
inner surface of the membrane tube, then the reforming
reactions of the residual methane by H2O and CO2, i.e.,
CH4 + CO2 ¼ 2CO + 2H2, CH4 + H2O ¼ CO + 3H2,
occur on Ni0/Al2O3 in the middle zone of the membrane
tube. The different nickel species are responsible for
different reactions, so different reactions take place in
different regions, as shown in figure 9. The combustion
Figure 8. Long-term POM reaction in the tubular membrane reactor
at 875 C. The feed (pure methane) flow rate in the tube side is
45.28 ml/min; the flow rate of air in the shell side is 300 ml/min.
Figure 9. Schematic representation of LiLaNiO/c-Al2O3 catalyst bed
layers and different reactions in the different regions in the tubular
membrane reactor.
reaction (CH4 + 2O2 ¼ CO2 + 2H2O) took place in
the blue and gray layers while the reforming reactions
(CH4 + CO2 ¼ 2CO + 2H2, CH4 + H2O ¼ CO +
3H2) took place in the black layer. Therefore, gases
directly contacted with the membrane tube wall are CO2
and H2O rather than H2 and CO. CO2 and H2O are not
reductive gases, so the membrane reactor based on
BSCF can be operated steadily for long time during the
POM reaction. Regarding the catalytic conversion of
methane to syngas in a tubular membrane reactor, Chen
et al. [87] proposed and verified that the reaction via a
combustion-reforming mechanism can improve the
stability of BSCF membrane materials.
Balachandran et al. [57–59] reported that a tubular
membrane reactor made from SrCo0.5FeOx (SrCo2Fe4O13+d) could be operated more than 1000 h without
fracture in the process for syngas production. A decline
in the oxygen flux (from 4 to 2 ml/cm2 min) was
observed. However, other researchers [53, 88–90] could
not get such high oxygen permeability for the material.
After systematical investigation of the Sr4Fe6)xCoxOl3
[91–94], it was found that the material has an intergrowth structure similar to Sr4Fe6O13 for x < 1.5,
mixed phases of intergrowth phase, perovskite phase
(SrFe1)yCoyO3)d) and Co3)xFexO4 phase for
1.7 < x < 2.1, and perovskite phase with Co3)xFexO4
for x ¼ 2.4. Although at 900 C the decomposition of
the majority phase into perovskite phase and rock at low
oxygen partial pressure is reversible when surrounding
atmosphere is again changed to air, membrane material
based on SrFeCo0.5Ox may not possess long-term
stability under syngas production conditions. Other
authors [89, 90] argued that different synthesis method
and synthesis/annealing temperature strongly affected
the phase composition, conductivity and oxygen permeability of the materials. Based on their experimental
results, they thought the oxygen permeation flux for
SrFeCo0.5Ox is mainly attributed to the perovskite phase
since the oxygen permeability of perovskite phase is
remarkably higher than that of intergrowth phase.
Recently, Deng et al. [90] reported that the perovskite
phase dominated SrFeCo0.5Ox, could be prepared by a
glycine-nitrate combustion method, but the permeability
was far lower than the initially reported data [27,93].
Generally, SrFeCo0.5Ox prepared by citric acid complexing method or other wet chemical methods tend to
form the intergrowth dominated phase, on the other
W. Yang et al./Development and application of oxygen permeable membrane
hand, material synthesized by solid state reaction
method likely has less intergrowth phase.
It is apparent that the cobalt-based perovskite oxide
membranes have high oxygen permeation flux, but low
chemical stability. To improve the stability, following
strategies are applied. (1) reducing the relative amount
of cobalt in the perovskite phase; (2) co-doping the
material with less reducible ions, e.g. Zr4+ [15,18,19, 39–
43], Ga3+ [21,48,49,52]; (3) adding a toughening material such as zirconia or alumina; (4) developing cobaltfree oxide membranes with sufficient oxygen permeation
flux. We developed a membrane, BaCo0.4Fe0.4Zr0.2O3)d,
with low and co-doped with less reducible ion Zr4+ [19].
This membrane exhibits better stability in the syngas
production. The membrane reactor made of BaCo0.4Fe0.4Zr0.2O3)d in syngas production experiments at
850 C can be operated steadily for more than 2200 h,
as shown in figure 10. XRD, XPS and EDS analysis
revealed that a slight chemical decomposition occurred
at both membrane surfaces [45].
Eltron Research team developed a material with
brownmillerite structure in general composition of
A2B2O5. A membrane reactor based on the brownmillerite structure materials with certain composition could
be continuously operated for over one year under syngas
atmosphere at 900 C. The syngas production rate is
60 ml/cm2 min, and equivalent oxygen permeation flux
is 10–12 ml/cm2 min [95].
Recently we attempted to explore the production of
hydrogen in the oxygen permeable reactor by steam and
partial oxidation of heptane, as shown in figure 11 and
figure 12. 100% n-heptane conversion and 93% hydrogen selectivity can be obtained in the steam reforming
and partial oxidation of heptane in the MIEC membrane reactor made of BSCF. The membrane reactor
can be operated steadily for more than 100 h for the
reaction. The applications of MIEC membrane in this
field are still in progress in our group.
Figure 10. Long-term stability in CH4 conversion, CO selectivity,
H2/CO ratio and oxygen flux observed in POM at 850 C using
BaZr0.2Co0.4Fe0.4O3)d membrane reactor and catalyst LiLaNiO/c-Al2O3.
163
Figure 11. Hepane conversion, H2O conversion and oxygen permeation flux versus reaction time.
3.2. OCM reaction in oxygen permeable membrane
reactor
Oxidative coupling of methane (OCM) is a promising
process for direct conversion of natural gas into more
useful products such as C2 hydrocarbons. The major
challenge for the commercialization of OCM process is
that C2 yield is still not high enough. High C2 yield can
be realized by either increasing methane conversion, C2
selectivity or both of them. However, higher methane
conversion often leads to lower C2 selectivity. In
conventional packed-bed reactor, the C2 yields are less
than 25% because of undesired gas-phase combustion
reaction. Since 1990s, many studies on OCM have been
shifted to develop new reactor configuration, which can
better fit OCM reaction with the found catalysts.
Guo et al. [96] investigated the effects of oxygen flux,
temperature and feed concentration on the performance of
OCM in Sr/La2O3–Bi2O3–Ag–YSZ solid oxide membrane
reactor. 5% C2 yields were achieved at C2 selectivity of
80%. A ceramic membrane reactor reported by Hazbun
was comprised of an impervious outer layer of mixed
conducting zirconia(10% yttria, 89% ZrO2, and 1%TiO2)
and a porous inner layer of magnesia stabilized zirconia
(87% ZrO2 and 13% MgO) with Li/MgO as catalyst inside.
Figure 12. H2, CO, CO2 and CH4 selectivity versus reaction time.
164
W. Yang et al./Development and application of oxygen permeable membrane
In this membrane reactor, the C2 yield was 20–25% with the
C2 selectivity of 50–60%. Fujimoto and his co-worker [97–
102] studied OCM by using a dense membrane reactor.
Although the C2 selectivity was very high (>90%), but the
methane conversion was very low (about 0.5%). Xu and
Thomson [103] investigated OCM in La–Ba–Co–Fe–O
membrane reactor. Higher C2 selectivity (up to 50%) was
achieved in the membrane reactor than that of the packedbed reactor, although they did not give the methane
conversion. ten Elshof et al. [104] used perovskite-type La–
Sr–Co–Fe–O membrane as reactor for OCM, they achieved
higher C2 selectivity (up to 70%), but the conversion was
still low (1–3%). Lu et al. [105] conducted OCM experiments in a porous c-Al2O3 membrane reactor with Mn–W–
Na/SiO2 catalyst. C2 yields up to 27.5% were obtained in
the membrane reactor. They demonstrated that it was
beneficial to distribute the feed of oxygen along the reactor
length for the OCM reaction. Lu et al. [106,107] investigated the performance of BaCe0.8Cd0.2O3 membrane reactor for OCM, and 16.5% C2 yields was achieved.
Lin et al. [108–112] studied the catalytic performance
of fluorite-structured yttria-doped bismuth ceramic
membrane material for OCM reaction. The reported
C2 selectivity and yield were in the range of 20–90% and
16–4%, respectively, in the membrane reactor of
25 mol% yttria-doped bismuth oxide (BY25). When
samarium was doped in the BY25 lattice, the oxygen
permeability and catalytic performance were improved.
In BYS membrane reactor, the highest C2 yield achieved
is 17% with a C2 selectivity of about 80%.
In a number of studies, the C2 selectivity of OCM in
membrane reactor is found significantly higher than that
obtained in conventional co-feed reactors [41,103,106–
109,113]. However, the reported yields are still low,
generally below 20%. Lin et al. [114,115] investigated
OCM in a tubular dense membrane reactor made of
catalytically active fluorite-structured Bi1.5Y0.3Sm0.2O3)d. As high as 35% C2 (C2H4 + C2H6) yield
was achieved with C2 selectivity of 54% at 900 C. In
order to improve the OCM catalytic performance of the
membrane surface, we deposited La–Sr/CaO catalyst on
the BSCF membrane surface. 17% C2 yield was
obtained in the membrane reactor, which is similar to
that in the co-feed reactor with La–Sr/CaO catalyst
under the same reaction conditions. However, the ratio
of C2H4/C2H6 in membrane reactor was much higher
than that in the co-feed reactor, as shown in figure 13,
which meant that much more desired ethylene (not
ethane) was produced in the membrane reactor [116].
All the researches showed that the MIEC membrane
reactor could improve the C2 selectivity of OCM.
However, there is a long way to go for the MIEC
membrane used in the commercial OCM process. The
key challenge is to develop the membrane material,
which possesses both excellent OCM catalytic performance and sufficient oxygen permeation at the reaction
temperature.
Figure 13. Comparison of the ratio of C2H4/C2H6 between the
membrane reactors packed with La-Sr/CaO catalyst and the fixedbed reactor.
3.3. ODE and ODP in oxygen permeable membrane
reactor
Selective oxidation of alkanes to corresponding
olefins and oxygenates is an important catalytic process.
One of challenges for the process is how to achieve a
high selectivity for the aimed products, i.e., olefins and
oxygenates, because they are usually more reactive than
the feed materials, i.e., alkanes and easier to be deeply
oxidized to COx by O2. One way to overcome these
difficulties is to use lattice oxide (O2)) as oxidant, such
as in a periodic shift reactor, which has been used in the
ammoxidation of propylene to acrylonitrile [117] and of
metaxylene to isophthalonitrile [118]. In this way, a high
selectivity of the target product was achieved due to the
utilization of lattice oxide (O2)). However, these processes can only be operated periodically.
We exploited the possibility to continuously supply lattice
oxygen (O2)) to control the oxidative dehydrogenation of
alkanes to alkenes by using an MIEC membrane reactor
[119–121]. The primary idea is shown in the figure 14. As
shown in figure 14, the molecular oxygen in air get electrons
on one surface of the membrane to form O2), then O2)
transport through the bulk of the membrane to another
surface and react with ethane before O2) recombined to
molecular oxygen. The fact that no gaseous O2 is detected in
the reaction side with the presence of ethane indicates that the
Figure 14. The mechanism of oxygen-permeable membrane for selective oxidation of hydrocarbons.
W. Yang et al./Development and application of oxygen permeable membrane
rate of the reaction between the lattice oxide and ethane is
much faster than the rate of lattice oxide recombining into
O2. Therefore, the mechanism of reaction on the reaction
side is CnH2n+2+O2) ¼ CnH2n+H2O+2e. As a result, O2)
converted from molecular oxygen by MIEC membrane will
be continuously supplied to the reaction system, and the
selectivity of the oxidation reaction can be controlled at a
very high level.
The oxidative dehydrogenation of ethane to ethylene
(ODE) was acted as the probe reaction to confirm this
idea. First, we investigated ODE in a dense disk-type
membrane reactor made of mixed conductor BSCF at
temperatures higher than 800 C [119,121]. Per pass
ethylene yield of 67% with ethylene selectivity of 80%
was achieved, while 53.7% ethylene selectivity was
obtained using a conventional fixed-bed reactor under
the same reaction conditions with the same catalyst at
800 C. A 100 h continuous operation of this process
was carried out and the result indicates the feasibility for
practical applications. Lin et al. [122] also studied ODE
in a dense tubular ceramic membrane reactor made of
fluorite structured Bi1.5Y0.3Sm0.2O3 (BYS) at temperatures 825–875 C. A dead-end tube and shell configuration was used where ethane was fed inside the tube and
air was fed into the shell side of the membrane reactor.
At 875 C, per pass ethylene yield of 56% with ethylene
selectivity of 80% was obtained in the membrane
reactor. However, the thermal dehydrogenation of
ethane should be considered when the reaction temperature is higher than 800 C. In order to confirm the
reaction mechanism, the thermal dehydrogenation of
ethane should be avoided. So, ODE reaction in the
tubular MIEC membrane reactor made of BSCF was
investigated at 650 C in our laboratory. It emphasized
that a blank experiment using a quartz tube instead of
the BSCF tubular membrane showed that the ethane
conversion is ~0.5% at 650 C, which was neglect to our
result. For comparison, the co-feed operation mode and
periodic shift operation mode with lattice oxygen were
also investigated. The results are shown in table 3. More
than 90% selectivity for ethylene at ethane conversion of
Table 3
Typical results of oxidative dehydrogenation of ethane to ethylene at
650C in different operation modes
Operation mode
Membrane mode
(air to air side)
Periodic shift mode
Co-feed mode
C2H6
Conversion
%
Product Selectivity, %
C2H4
CH4
CO
CO2
18.0
90.6
3.50
2.20
3.73
3.40
15.8
91.0
57.6
4.53
1.55
3.32
0.62
1.14
40.2
The feed of 6.1 ml/min ethane and 53.9 ml/min helium was fed to the
reaction side. In the co-feed mode, 0.91 ml/min O2 was also fed to
reaction side.
165
18% was obtained in MIEC membrane reactor. Only
57.6% selectivity of ethylene was obtained in the co-feed
mode, indicating that molecular oxygen is responsible
for the deep oxidation of ethane to COx. However, the
ethylene selectivity in the periodic shift operation mode
was 91.0%, which is very similar to that in the MIEC
membrane reactor, as shown in table 3. The similarity in
the selectivity between the two modes strongly suggests
that our mechanism of reaction in the MIEC membrane
reactor is correct.
We also investigated ODP to propylene in a dense
tubular membrane reactor made of BSCF at 700 C and
750 C [123]. The propylene selectivity in the membrane
reactor (44.2%) is much higher than that in the fixedbed reactor (15%) at the similar propane conversion
(23–27%). Higher propylene selectivity in the membrane
reactor was attributed to the lattice oxygen (O2))
supplied through the membrane.
Our experiments clearly demonstrated the concept
that the MIEC membrane can provide lattice oxygen for
oxidative dehydrogenation of alkanes. The relatively
low oxygen flux through current MIEC membrane at
intermediate temperatures largely limits its applications
in other selective oxidation reactions of alkanes.
Another challenge for oxygen permeable membrane is
to develop a new membrane material with sufficient
oxygen permeation flux at low temperatures (<500 C).
4. Conclusions
The paper summarized the development and design
of the oxygen permeable membrane material. Both
permeability and stability are concerned for design of
the MIEC membrane materials. As for its application in
the selective oxidation of alkanes, the oxygen permeable
membrane can provide gas-phase oxygen for POM and
POH reaction at more economics and environmental
way, and can also provide the lattice oxygen for
converting alkanes to alkene or oxygenates, such as
OCM to ethylene, ODE to ethylene, ODP to propylene,
with high selectivity. At present, the membranes are
usually operated at high temperature (>600 C). However, most important selective oxidation of alkanes are
carried out at low temperatures (<500 C). So, it is
necessary to develop a MIEC membrane with sufficient
oxygen permeation at low temperatures (<500 C) to
meet more need for the selective oxidation of alkanes.
Acknowledgments
The authors gratefully acknowledge financial supports from the National Nature Science Foundation of
China (Grant No. 50332040) and the Ministry of
Science and Technology, China (Grant No.
G1999022401). The authors would also like to thank
the colleagues in our research group for their work
summarized in this paper.
166
W. Yang et al./Development and application of oxygen permeable membrane
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