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Facilitated transport of small molecules and ions
for energy-efficient membranes†
Yifan Li,ab Shaofei Wang,ab Guangwei He,ab Hong Wu,ab Fusheng Panab and
Zhongyi Jiang*ab
In nature, the biological membrane can selectively transport essential small molecules/ions through
facilitated diffusion via carrier proteins. Intrigued by this phenomenon and principle, membrane
researchers have successfully employed synthetic carriers and carrier-mediated reversible reactions to
enhance the separation performance of synthetic membranes. However, the existing facilitated transport
membranes as well as the relevant facilitated transport theories have scarcely been comprehensively
reviewed in the literature. This tutorial review primarily covers the two aspects of facilitated transport
theories: carrier-mediated transport mechanisms and facilitated transport chemistries, including the
Received 25th June 2014
design and fabrication of facilitated transport membranes. The applications of facilitated transport
DOI: 10.1039/c4cs00215f
membranes in energy-intensive membrane processes (gas separation, pervaporation, and proton
exchange membrane fuel cells) have also been discussed. Hopefully, this review will provide guidelines
www.rsc.org/csr
for the future research and development of facilitated transport membranes with high energy efficiency.
Key learning points
(1)
(2)
(3)
(4)
(5)
The panorama of facilitated transport and its important implications.
Chemistries and reactions involved in facilitated transport.
Approaches to exploring advanced functional materials to facilitate the transport of molecules and ions.
Application paradigms of facilitated transport in membrane processes.
Design of energy-efficient, high-performance membranes with a facilitated transport feature through biomimetic and bioinspired strategies.
1. Introduction
A high-performance membrane which allows fast and selective
transmembrane permeation of small molecules/ions is essential
for triggering revolutionary changes in many significant
chemical processes.1 As the pore size of the membrane falls
below 1 nm, the relevant membrane processes can easily inherit
the benefits from equilibrium-governed separation and rategoverned separation, thus acquiring high energy efficiency.
The permeation of small molecules through the membrane is
usually described by the well-known ‘‘solution–diffusion’’
mechanism, where solubility and diffusivity are governed by
a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of
Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.
E-mail: [email protected]
b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, China
† Electronic supplementary information (ESI) available: Additional information
and references for the facilitated transport membranes discussed. See DOI:
10.1039/c4cs00215f
This journal is © The Royal Society of Chemistry 2015
thermodynamic and kinetic/transport factors, respectively.
Therefore, the ideal membranes should render an appropriate
chemical microenvironment to ensure high solubility, and also
possess a well-tailored microstructure to ensure high diffusivity.
In this regard, molecular sieve membranes seem to be the
preferred choice for molecular transport, which have great
potential for simultaneous enhancement of permeability and
selectivity. However, molecular sieve membranes may not be the
best choice for ion transport, which strongly relies on electrochemical interactions. Also, the difficulties in fabricating molecular sieves into defect-free thin membranes impede the broad
applications of the molecular sieving membrane.2 Consequently,
interest has been growing in the selective transport mechanisms
which allow efficient enrichment of the desired permeant based
on broader material chemistries and more specific interactions.
If we take a look at nature, we can easily find an ideal modelbiological membrane, which can selectively transport essential
small molecules/ions through facilitated diffusion via carrier
protein. Early evidence of carrier-mediated facilitated diffusion
was traced back to half a century ago.3 As one important type of
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Tutorial Review
structure proteins of the cell membrane, a carrier protein can
specifically and reversibly bind small molecules (e.g. sugar,
amino acid, and nucleotide) or ions (e.g. Na+, K+, Mg2+, Ca2+,
Cl, HCO3), and transport them to the other side of the cell
membrane via conformational variation. The specificity of
facilitated diffusion arises from the ingenious integration of
multiple types of interactions between carrier protein and the
target permeant, e.g. electrostatic interactions, hydrogen bond
interactions, hydrophobic interactions, cation–p interactions,
etc. Herein the biological term ‘‘facilitated diffusion’’ highlights
the contribution of molecular recognition based on specific
interactions to the overall transmembrane permeability.
According to the function of carrier protein, ‘‘facilitated transport’’
is also often used as a substitute of facilitated diffusion, and carrier
protein is also called ‘‘transport protein’’ or a ‘‘transporter’’.4
Intrigued by the carrier-involved model developed for
the cell membrane, we can design a ‘‘carrier’’ for synthetic
membranes by mimicking the function of carrier protein, so as
to enable facilitated transport of small molecules or ions
through membranes. Considering the complexity of molecular
recognition and conformational variation for carrier protein,
the simplified carrier for ‘‘in vitro’’ facilitated transport is
expected to reversibly react with the target species/permeants
followed by the formation of a transient complex. The transport
of the target permeant is thus enabled by the motion of the
complex (mobile carrier) or the hopping of the target permeant
from one carrier to another (fixed carrier). Compared to the
solubility-selective and diffusivity-selective transport, reactivityselective transport mediated by carriers appears to be a much
more specific transport manner. From a thermodynamics viewpoint, the introduction of reversible reactions is favorable for
acquiring high energy efficiency. In this way, an excellent
paradigm for overcoming the ‘‘tradeoff’’ effect between permeability and selectivity can be portrayed by borrowing reactivity
selectivity from carriers.5 For example, facilitated transport
membranes have been successfully explored for diverse membrane
systems: (1) CO2 removal from diverse sources (e.g. natural gas,
flue gas, and shift gas, where CO2/CH4, CO2/N2, CO2/H2 separation
are the common tasks, respectively), typically using amino group
containing membranes;5 (2) oxygen enrichment from air (O2/N2
separation), typically using cobalt porphyrin containing
membranes;6 (3) olefin/paraffin separation (ethylene/ethane,
propylene/propane separation), typically using Ag+-containing
membranes;7 (4) gasoline desulphurization (thiophene/octane
separation), typically using transition metal ion-containing
membranes;8 (5) heavy metal ion recycling, typically using
extractive membranes.9 Although the term ‘‘facilitated transport
membrane’’ has rarely been employed to describe the proton
exchange membrane in the literature, facilitated transport of
proton with acidic groups as donors and basic groups as acceptors
has already been widely acknowledged because the rapid transfer
of proton relies heavily on a fundamental proton transfer
reaction.10 Furthermore, the well-known vehicle mechanism
and Grotthus mechanism proposed for the proton exchange
membrane is akin to the facilitated transport mechanism
mentioned above.11 In this sense, integrating the proton exchange
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membrane into the topic of the ‘‘facilitated transport membrane’’
may be helpful to understand the common features of molecule
transport and ion transport, and acquire the whole picture of
facilitated transport membranes.
Facilitated transport theories play pivotal roles in the rational
design and tunable fabrication of facilitated transport membranes.
On one hand, carrier-mediated transport mechanisms should be
included, which are closely related to the carrier mobility and the
physicochemical properties of the membrane matrix. On the other
hand, facilitated transport theories should encompass the various
types of carriers and the corresponding reversible carrier–permeant
reactions, which can be merged in ‘‘facilitated transport
chemistries’’. In summary, facilitated transport theories are
comprised of carrier-mediated transport mechanisms and
facilitated transport chemistries, which reflect the physical
and chemical aspects of facilitated transport, respectively.
This tutorial review focuses on the design and fabrication of
high-performance dense membranes for energy-intensive processes
with deep insights into facilitated transport theories, encompassing
carrier-mediated transport mechanisms and facilitated transport
chemistries. The potential applications of facilitated transport
membranes are also summarized. Due to the limitation of the
length of this article as well as the scope of energy-intensive
processes, the membrane processes are confined to gas separation,
pervaporation and proton exchange membrane fuel cells. More
comprehensive review articles concerning other membrane
processes can be found elsewhere.4,9
2. Facilitated transport membranes
According to the mobility of carriers, facilitated transport
membranes can be classified into three types: (1) a mobile
carrier membrane, through which the carrier can diffuse freely;
(2) a semi-mobile carrier membrane, in which the carrier can
migrate elsewhere at a cost of high diffusional activation
energy; (3) a fixed-site carrier membrane, in which the carrier
can only vibrate within a confined nanospace, rather than migrate
elsewhere. Correspondingly, these three types of facilitated transport
membranes exhibit distinct differences in phase states, materials
types, and especially, transport mechanisms. Mobile carrier
membranes are usually liquid membranes, of which the major
ingredient is often carrier solution or carrier-bearing liquid
compounds, e.g. ionic liquid. The transport of the target
permeant through mobile carrier membranes obeys the vehicle
mechanism, in which the carrier serves as a ‘‘ferryboat’’ plying
between the two sides of membrane (Fig. 1a). Both the semimobile carrier and fixed-site carrier membranes are solid-state
membranes, of which polymer materials are often utilized as
the hosting matrix. For fixed-site carrier membranes, the target
molecule/ion has to pass through the membrane via carrier-tocarrier hopping (Fig. 1c). In a general sense, this mechanism is
called the ‘‘hopping mechanism’’, as proposed by Cussler and
co-workers.12 Semi-mobile carrier membranes are usually
composed of a plasticized polymer matrix and physically
restricted carriers whose mobility lies in between a mobile
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Fig. 1 Schematic illustration of the carrier-mediated transport mechanisms for (a) a mobile carrier membrane; (b) a semi-mobile carrier membrane; and
(c) a fixed-site carrier membrane.
carrier and a fixed-site carrier. As a consequence, both the
vehicle mechanism and the hopping mechanism dominate
the facilitated transport process (Fig. 1b). Such a definition of
a semi-mobile carrier membrane allows a novel survey of
facilitated transport membranes with different carrier types
and two basic transport mechanisms: vehicle mechanism and
hopping mechanism.
2.1
Facilitated transport membranes with mobile carrier
Mobile carrier membrane is the simplest type of facilitated
transport membrane. With the active carrier distributed within
the liquid matrix, the mobile carrier membrane is akin to
the cell membrane in view of both structures and facilitated
transport properties. The liquid matrix as a continuous phase
manipulates higher permeability than the solid polymer matrix,
and the vehicle-like carriers further facilitate the transport of
the target permeant. Presently, mobile carrier membranes have
been utilized to facilitate the transport of CO2, O2, olefins, metal
ions, and biomolecules. Generally, the total flux of the target
permeant, JA, can be illustrated as the sum of Fickian diffusion
and carrier-mediated diffusion, as shown in eqn (1):
JA ¼
DA
DAC
DCA þ
DCAC
l
l
(1)
where D is the Fickian diffusion coefficient, l is the membrane
thickness, DC is the transmembrane concentration difference,
the subscript ‘‘A’’ and ‘‘AC’’ refer to the uncomplexed target
permeant and the complexed target permeant, respectively. In
most cases, due to the enrichment of the permeant–carrier
complex at the feed side, DCAC is much higher than DCA.
Because of the low diffusion resistance of the liquid matrix,
DAC, is at least as high as DA, and therefore the contribution from
carrier-mediated is expected to be significantly greater than that
from conventional Fickian diffusion. Although this equation
assumes one-step interfacial chemical reaction and bulk diffusion,
it remains useful to further understand the intricate facilitated
transport processes due to its straightforward physical meaning
and simple mathematical expression. Nowadays it has been
possible to quantitatively study mobile carrier membranes by
combining the reaction thermodynamics and kinetics with
molecular diffusion theory. In detail, facilitated transport factor,
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F, is usually employed to describe the ratio of the total flux of
Fickian diffusion and carrier-mediated diffusion to Fickian
diffusion flux. For the case of one step reaction between carrier
and the target permeant, F can be calculated by eqn (2)–(6) in the
absence of external mass transfer resistance:13
aK
1
þK
F¼
aK
tanh l
1þ
1þK
l
1þ
kf
CA0
kr
(3)
DAC CAC0
DA CA0
(4)
K¼
a¼
(2)
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 1 þ ða þ 1ÞK
l¼
2
eð1 þ KÞ
e¼
DAC
kr l 2
(5)
(6)
where K is the dimensionless reaction equilibrium constant; kf
and kr are the forward and reverse rate coefficients, respectively;
CAC0 and CA0 are the concentration of the carrier–permeant
complex and the uncomplexed permeant at the feed side,
respectively; a represents the mobility ratio of the carrier–permeant
complex to the uncomplexed permeant; e refers to the inverse
Damköhler number, which represents the ratio of characteristic
reverse reaction time to diffusion time. According to these
equations, F is mainly determined by the value of (tanh l)/l.
A maximum F is reached when l - N and thus (tanh l)/l - 0,
while F decreases to 1 when l - 0 and thus (tanh l)/l - 1.
According to eqn (5), l monotonically increases with K and a,
and therefore high K and a values are desired. Since kr and DAC
also affect the value of e, high kf and CAC0 as well as moderate kr
and DAC are favored, which highlights the importance of both
reaction kinetics and thermodynamics.
A mobile carrier membrane is usually fabricated into supported
liquid membrane (SLM) to maintain sufficient mechanical stability.
However, the development of practical SLMs requires the prudent
consideration of the tricky liquid/carrier loss issue, because volatile
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or low-viscosity liquid is subject to being expelled out under
elevated temperature or cross-membrane pressure difference. A
supported ionic liquid membrane (SILM) might be a promising
alternative to suppressing the liquid loss, owing to its negligible
vapour pressure and high viscosity. Besides, the high flexibility of
ionic liquid chemistry allows elaborate design and synthesis of
appropriate SILMs. The carrier can be either a solute or a portion
of the ionic liquid, and both charged and uncharged carriers are
available. Although carrier mobility will be somewhat restricted by
the high viscosity of ionic liquid, the highly ionic matrix has already
allowed hopping of the ionic-state complex, especially when facilitating the transport of CO2 or olefins.7,14 In this sense, SILM has been
rendered some characteristics of semi-mobile carrier membranes.
2.2 Facilitated transport membranes with a semi-mobile
carrier
The term ‘‘semi-mobile carrier membrane’’ is new but essential
because a large quantity of facilitated transport membranes
cannot be simply classified into a mobile carrier membrane or
a fixed-site carrier membrane. For example, water is an important carrier for CO2 and proton in a hydrated polymer matrix.
If the polymer matrix is fully swollen by water, this type of
membrane is no more a fixed-site membrane in the strict sense
because the water-mediated vehicle mechanism may also play a
pivotal role in facilitated transport. In a broader sense, when
small molecules with active carriers are doped into polymer
membranes, the carriers might migrate if the polymer matrix is
plasticized or swollen by a solvent. This type of membrane was
initially called a ‘‘polymer inclusion membrane’’ or a ‘‘polymeric
plasticized membrane’’ for selectively extracting heavy metal
ions or small biomolecules from aqueous sources.9 Actually,
such semi-mobile carrier membranes have been also reported
for CO2 separation.15
Obviously, if there is a continuous liquid mesophase or
macrophase spanning the entire membrane thickness, a
semi-mobile carrier membrane will become analogous to the
mobile carrier membrane due to the reduced contribution from
the hopping mechanism. Therefore, it can be deduced that the
liquid or aqueous phase which usually exists within the semimobile carrier membrane is required to simultaneously plasticize
the polymer matrix and accommodate the carriers. With the
integration of the liquid phase and the solid polymer matrix,
the semi-mobile carrier membrane is expected to combine the
merits of both a mobile carrier membrane and a fixed-site carrier
membrane by compromising permeability and stability. Unfortunately, few efforts have been devoted to elucidating the relevant
‘‘mixed-matrix’’ facilitated transport mechanism. The mixed
Grotthuss and vehicle transport mechanism in proton conducting
polymers revealed by ab initio molecular dynamics simulations
may aid in further elucidating the transport mechanism of semimobile carrier membranes at the molecular level.
2.3
Facilitated transport membranes with a fixed-site carrier
A fixed-site carrier membrane was originally developed due to
the instability of the liquid mobile carrier membrane. Traditional fixed-site carrier membranes consist of merely polymeric
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materials, and the carriers must be covalently bound to polymer
chains. In recent years, the rapid development of the mixed matrix
membrane or polymer composite membrane has created new
opportunities to design diverse fixed-site carrier membranes.16
Alternatively, a carrier can be immobilized by either the polymer
matrix or the filler phase of the mixed matrix membrane. In this
sense, the carrier does not necessarily come from the side-chain of
the polymer, which significantly broadens the range of screening
suitable matrix polymers.
Actually, in the past five years the scope of fixed-site carrier
membranes has been dramatically enlarged in the field of CO2
separation. Since carrier mobility depends on not only the
connection mode of the carrier but also the plasticization or
swelling state of the polymer matrix, a membrane containing
both covalently bound carrier and physically doped carrier may be
also classified into the fixed-site carrier membrane. Nevertheless,
the majority of carriers are not permitted to migrate in an
ordinary working state. With the definition of the semi-mobile
carrier membrane, it is explicit that a fixed-site carrier membrane
containing physically doped carrier may switch to the semimobile carrier membrane or even mobile carrier membrane with
the increase of the swelling degree of the polymer matrix.
For fixed-site carrier membranes, hopping mechanism is
the dominant facilitated transport mechanism. Despite this,
the present hopping mechanism assuming a carrier-to-carrier
hopping might be too simple. For example, facilitated transport
was thought to occur only when the distance between two
carriers was shorter than the diffusional jump distance, as
obtained from the diffusional activation energy. However, even
at extremely low carrier concentration, facilitated transport of
O2 was also observed,17 which might be attributed to the
carrier-mediated increase in the chemical potential of O2.
Considering the weak polarity of O2, its concentration around
a carrier should be much higher than that in the matrix, and
therefore the ‘‘hopping’’ model needs modification at low
carrier concentrations. Another important issue is the effect of
diffusion resistance on carrier-to-carrier hopping. Theoretically, a
carrier cannot show high activity unless it is accessible for the
target permeant. A high free volume matrix caused by swelling or
the intrinsic porosity favors hopping, while a dense matrix may
hinder hopping, even resulting in ‘‘starvation’’ of the carrier.
Based on the insufficient exploitation of carrier activity caused
by the diffusion resistance of the membrane, Li et al.5 proposed
the concept of ‘‘undesired membrane structure’’, which may be
solved by isolating the polar carrier groups from each other by
rigid aromatic groups and by packing the polymer chains in
disordered manners. As a consequence, a deep insight into
hopping mechanism for fixed-site carrier membranes is essential
to comprehensively consider carrier properties and the effect of its
surroundings.
3. Facilitated transport chemistries
An in-depth understanding of facilitated transport theories
based on facilitated transport chemistries is essential to the
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Tutorial Review
The four typical types of reversible reactions for facilitated transport membranes together with their corresponding carriers and target permeants.
rational design of facilitated transport membranes. The carriers
are traditionally classified simply based on several specific target
permeants, e.g. O2, CO2, olefin, and sugar, etc. Actually, facilitated
transport theories are applicable to a broader range of molecules
and ions, therefore it is necessary to re-arrange the knowledge
points about facilitated transport chemistries based on the
limited types of reversible reactions. Herein, four basic types of
reversible reactions are included, namely proton transfer reaction,
nucleophilic addition reaction, p-complexation reaction and electrochemical reaction. Carriers and the corresponding target permeants
are therefore closely connected with reversible reactions as the
ligament (Fig. 2).
3.1 Facilitated transport chemistries in terms of proton
transfer reaction
Proton transfer reaction is one type of acid–base reaction that
can be described by Brønsted–Lowry acid–base theory. Proton
transfer reaction can facilitate proton transport itself, as well as
the transport of some small molecules with Brønsted acidity
or basicity, e.g. H2S, NH3, etc. In other cases, for example,
facilitated CO2 transport, although proton transfer reaction is
not the first-step reaction between the carrier and the target
permeant, it remains important to complete the cycle of multistep reactions and release uncomplexed carriers.
Facilitated proton transport is primarily mediated by acid–
base pairs. Strictly, proton transfer between a conjugate acid–
base pair (generally expressed as HA/A or BH+/B) does not
belong to the proton transfer reaction because of no production
of new intermediates. Only if a second Brønsted base (B 0 ) serves
as the proton acceptor from HA or a second Brønsted acid (HA 0 )
serves as the proton donor towards B, proton transfer reaction
will occur following one formula of eqn (7) and (8):
HA + B 0 " B 0 H+ + A
(7)
B + HA 0 " BH+ + A 0 (8)
where HA 0 and B 0 are defined as proton carriers. Accordingly,
an acid group (e.g. –SO3H, PO3H2) requires a Brønsted base as a
carrier, and a Brønsted acid is often required to match a basic
group (e.g. amino group, imidazole).
Considering that most proton exchange membrane materials
bear SO3H or PO3H2 groups, Brønsted base as a carrier has
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attracted more attention. As mentioned above, water is an
important proton carrier which is capable of facilitating both
vehicle and Grotthus mechanisms. Actually, water as a Brønsted
base provides a conjugate acid–base pair as H3O+/H2O, triggering
a proton transfer reaction as eqn (9):
HA + H2O " H3O+ + A
(9)
Consequently, water often functions as a proton transfer
bridge between proton donating groups and proton accepting
groups when the distance between them exceeds the mean free
path of proton hopping. Furthermore, water plays critical roles
in dissociation of acid groups and construction of a continuous
hydrogen-bonding network for successive proton hopping.
Water is an essential proton carrier in the overwhelming
majority of cases, while other Brønsted bases may be also available
as a proton carrier based on water-independent proton transfer.
Kreuer and co-workers stated that carrier-mediated proton
conductivity was simultaneously determined by proton donating
and proton accepting capacities.18 A relatively stronger Brønsted
base than water can form an acid–base pair with an acid group
through proton transfer reaction. Taking the typical –NH2 group as
an example, it can be closely linked to a proton donating group
(typically –SO3H or –PO3H2), and the protonation/deprotonation of
each group would be facilitated by the electrostatic attraction of the
other groups, thus greatly reducing the energy barrier for proton
hopping.19,20 Azole groups (e.g. imidazole, triazole, tetrazole) as
another important family of Brønsted bases were also reported
to form acid–base pairs with –SO3H or –PO3H2 groups, which
drastically enhanced proton conduction.21–23
There are still some proton exchange membrane materials
based on basic groups, e.g. chitosan, polybenzimidazole. In
particular, those containing azole groups are expected to be
operated at anhydrous state, which is required for hightemperature, low-humidity fuel cell membrane.24 Without the
acid groups, azole rings have to flit to enable long-range proton
hopping between adjacent azole rings, and therefore the
construction of acid–base pairs would dramatically facilitate such
a rate-limiting step of long-range proton hopping. Since phosphoric
acids are also anhydrous proton conductors, the integration of
azole-based polymer matrix and phosphoric acid is expected to
facilitate proton transport in the absence of water.24,25
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3.2 Facilitated transport chemistries in terms of nucleophilic
addition reaction
Nucleophilic addition reaction often occurs at the C atom of an
asymmetric double bond (e.g. CQO, SQO). The displacement
of the electron cloud towards the O atom occurs, and therefore
the positively charged C atom is prone to be attacked by
nucleophilic agents, e.g. water, OH, amino groups, etc. In this
manner, nucleophilic addition reaction allows facilitated transport
of small molecules CQO bond, e.g. CO2, SO2, COS, etc. Since the
common nucleophilic agents belong to Brønsted base, herein the
major carrier for nucleophilic addition reaction is concluded as a
Brønsted base in Fig. 2. On the other hand, facilitated transport of
CO2 has become one of the most important and representative
topics about facilitated transport membranes. Various types of
Brønsted bases, such as OH, CO32, F, PO43, –COO, amino
groups, and even water, can be designed as CO2 carriers, which
bring about difficulty in understanding the common facilitated
transport mechanism. From the viewpoint of nucleophilic
addition reaction, it is possible to categorize different types
of CO2 carriers and systematically compare their facilitating
effects.
Based on the theory of nucleophilic addition reaction, the
reaction rate is determined by the nucleophilicity of the nucleophilic agent, which is further determined by both the basicity and
polarizability of the nucleophilic atom. For the most Brønsted
bases utilized as a CO2 carrier, the nucleophilic atoms (N, O, F
atoms) are localized in the same row of Periodic Table of the
Elements, and hence the basicity of carrier is the major factor that
determines nucleophilicity. Consequently, a strong Brønsted base
is preferred to acquire high carrier reactivity.
The conjugative base of a weak acid is usually a strong base.
These anion-type carriers are better nucleophilic agents than
non-ionic carriers. In theory, OH should have been the best
choice because of its strong basicity and easy availability. The
direct addition of OH onto CO2 results in HCO3:
CO2 + OH " HCO3
(10)
with a polymer electrolyte as a membrane matrix, HCO3 can
diffuse through the membrane much faster than gaseous
permeants mainly due to the much higher ion exchange capacity
than gas solubility. Water is often indispensible to increase the
concentration of dissociated OH. Hagg and co-workers found
that a humidified polyviny amine (PVAm) membrane showed
remarkable enhancement of CO2 transport properties with the
addition of KOH, revealing higher reactivity of OH than –NH2.26
However, if the counter ion is a small metal ion (e.g. Na+, K+, Ca2+),
OH cannot be firmly immobilized within the membrane matrix,
and the concerns on carrier stability and membrane durability
arise at high carrier content. Xiong et al.27 designed a novel polymer
with quaternary phosphonium hydroxide moieties (Fig. 3a). The
electrostatic attraction from the quaternary phosphonium group
stabilized OH, and the improved charge delocalization by the
quaternary phosphonium group ensured complete OH dissociation in the hydrated state, leading to strong basicity (Fig. 3b).
This work may evoke the interest in designing novel types of
108 | Chem. Soc. Rev., 2015, 44, 103--118
Fig. 3 (a) Chemical structure of tris(2,4,6-trimethoxyphenyl)polysulfonemethylene quaternary phosphonium hydroxide; and (b) the facilitatedtransport pathway. Reprinted with permission from ref. 27. Copyright 2014
Wiley-VCH.
polymer-bound anionic carrier, which have been already intensively studied in the field of alkaline anion exchange membranes.
Another representative anion-type carrier is carboxylate
group (–COO). Despite weaker basicity than OH, –COO as
an organic carrier can be covalently bound to polymer chains,
resulting in a more stable fixed-site carrier membrane. When
utilizing –COO as a carrier, H2O not only assists in the
dissociation of the carboxylate salt but also acts as a direct
carrier that triggers another nucleophilic addition reaction:
CO2 + H2O " H2CO3
(11)
The carboxylate group as an indirect carrier can further react
with H2CO3 and shift the equilibrium of eqn (11) to the right:
–COO + H2CO3 " –COOH + HCO3
(12)
Actually, many anion carriers facilitate CO2 transport following
the similar reaction mechanism as eqn (11) and (12). Few of them
have been explored because they exhibit weaker basicity than
OH and cannot be covalently immobilized to polymer chains
like –COO.
Amino groups are the typical non-ionic CO2 carriers which
can be covalently connected to polymer chains. Despite the
non-ionic characteristics, amino groups usually display stronger
basicity than –COO, as supported by the higher pKa value
of NH4+ (9.26) than CH3COOH (4.76). The reaction between
CO2 and primary or secondary amino groups was reported as
the following formulae:
CO2 + RR 0 NH " RR 0 NH+COO
(13)
RR 0 NH+COO + RR 0 NH " RR 0 NCOO + RR 0 NH2+
(14)
RR 0 NH+COO + H2O " RR 0 NCOO + H3O+
(15)
RR 0 NCOO + H2O " RR 0 NH + HCO3
(16)
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where R 0 is an H atom or other organic group. Eqn (13) is a
nucleophilic addition reaction, which is the rate-limiting step
whether water is present or not. Eqn (14)–(16) are rapid proton
transfer reactions, as illustrated at the beginning of Section 3.1.
Due to the production of zwitterion as an intermediate, this
mechanism is defined as the zwitterion mechanism. In this
mechanism, water is not only a reactant for eqn (15) and (16),
but also beneficial to facilitating charge separation and accelerating eqn (13). Due to the electron donating effect from the R 0
group, secondary amino group exhibits stronger basicity and
higher reactivity than the primary amino group.
For tertiary amino group, the lack of active H atom does not
permit the zwitterion mechanism. Instead, H2O acts as a carrier
to firstly react with CO2 like eqn (11), and the tertiary amino
group as another carrier can further react with H2CO3 and shift
the equilibrium of eqn (11) to the right:
RR 0 R00 N + H2CO3 " RR 0 R00 NH+ + HCO3
(18)
Intrigued by the development of amine-based chemical
adsorption agents, Ho’s group developed sterically hindered
amine to increase CO2 loading capacity and accelerate the
amine–CO2 reactions.28 A sterically hindered amine is defined
as either a primary amine in which the N atom is attached to a
tertiary C atom or a secondary amine in which the N atom is
attached to at least one secondary or tertiary C atom. The alkyl
group inhibits the formation of a carbamate ion as shown in
eqn (14), and thus facilitates the formation of HCO3 as shown
in eqn (15)–(17). Zhao and Ho firstly reported the effect of
amine steric hindrance in a solid phase and demonstrated
encouraging facilitation effects.28 They synthesized a series of
sterically hindered polyamines by grafting alkyl groups to the
primary amine group of polyallylamine (PAAm). This work also
implies the possibility of further understanding facilitated
transport by distinguishing the contributions from the steric
hindrance effect, electron donating effect and carrier content.
As mentioned above, water is an important non-ionic CO2
carrier, while it is a weak Brønsted base. Without another
stronger base as a carrier, water can also efficiently convert
CO2 into HCO3 by stabilizing the deprotonated intermediate,
OH, and transporting the synchronously generated H+ elsewhere. In biological organisms, carbonic anhydrase can catalyze
the hydration of CO2, and the active center is comprised of a zinc
ion coordinated by three imidazole groups of histidine residues.
Considering the following equilibrium:
(His)3Zn(OH2) " (His)3Zn(OH) + H+
(19)
where His refers to the histidine residues. The zinc ion center
can stabilize the coordinated OH, while both the histidine
residues and the body buffer solution can rapidly transport the
proton away. As such, the pKa value of water increases up to 7,
and the following proton transfer reaction is thought to occur:
This journal is © The Royal Society of Chemistry 2015
(His)3Zn(OH) + CO2 + H2O " (His)3Zn(OH2) + HCO3
(20)
(17)
where R, R 0 , R00 may be different or the same organic groups.
As such, the overall reaction formula is:
RR 0 R00 N + CO2 + H2O " RR 0 R00 NH+ + HCO3
Fig. 4 Chemical structure of a poly(N-vinylimidazole)–zinc complex.
Reprinted with permission from ref. 29. Copyright 2012 the Royal of Social
Chemistry.
by mimicking the zinc active site of carbonic anhydrase, Yao
et al.29 reported a novel material, a poly(N-vinylimidazole)–zinc
complex (as shown in Fig. 4), which can efficiently catalyze the
hydration of CO2. We infer that the abundant imidazole groups
greatly contributed to the synchronous proton conduction, and
hence the hydration rate of CO2 turned out drastically accelerated.
Also, Li et al. found the evidence of a weak facilitated transport
effect in salt-doped poly(ether-amide) membranes, which heavily
relied on the hydration energy of the metal ions.30 Both the ether
group and amide group complexing with metal ions could act as
proton acceptors to facilitate charge separation.
3.3 Facilitated transport chemistries in terms of pcomplexation
Facilitated transport based on p-complexation is mainly limited
to the transport of unsaturated small molecules, e.g. olefins,
aromatic compounds, thiophenes, O2 and CO2. Majority of the
carriers for p-complexation are transition metal ions, while the
positively charged transition metal atoms have been also
explored as an olefin carrier. The fundamental interactions of
p-complexation consist of cation–p and p–d interactions. From
the perspective of acid–base reaction, p-complexation can be
perceived as another type of acid–base reaction on the basis of
Lewis acid–base theory and hard–soft–acid–base (HSAB) theory.
According to the polarizability of the electron-donating atom
of an unsaturated bond, olefins, aromatic compounds, thiophenes are categorized into soft base (C or S atom provides
electron), while O2 and CO2 belong to hard base (O atom
provides electron).
Olefins are the most representative target permeants as soft
bases. Theoretically, any metal ion that belongs to soft acids
can be selected as an olefin carrier, e.g. Ag+, Pd2+, Hg2+, Cd2+,
Cu+, Au+, etc. Ag+ as an olefin carrier has been intensively
investigated because of its high activity, low toxicity and moderate
chemical stability. The high activity of Ag+ can be explained as
follows: Ag+ is a highly polarizable and deformable metal ion due
to the unoccupied 5s orbitals and the activatable 4d-orbital
electrons. The 5s orbitals of Ag+ can accept the p electrons
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Chem Soc Rev
donated from the occupied 2p orbitals of olefin molecules to
form s-bonds, and back-donation of electrons from the occupied
4d orbitals of Ag+ into the empty p*–2p antibonding orbitals
of olefin molecules results in p-bonds. The synergic effect of
s-bonds and p-bonds strengthens the complexation bond,
enabling the high specificity of Ag+–olefin complexation. One
Ag+ can reversibly bind one or two olefin molecules, while the
latter case corresponds to an intermediate state during the
exchange of the complexed olefin molecule by a new one. That
is, Ag+ usually forms a 1 : 1 complex with olefins:
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Ag+ + olefin " [Ag(olefin)]+
(21)
Among the Ag+-containing compounds, soluble silver salts
are usually utilized to provide dissociated Ag+. These silver salts
are usually added into a polar polymer matrix to form solidstated polymer electrolyte membranes, where the activity of Ag+
is mainly determined by the counter ion and the available
coordinating sites. The large anions such as BF4, CF3SO3,
ClO4, and SbF6 are preferred choices because of their low
lattice energy and relatively weak Ag+–anion interactions.
Among the available silver salts, AgBF4 and AgCF3SO3 show
relatively high activity but insufficient stability. On the other
hand, Ag+ coordinates with the polar ligand of the polymer, and
a threshold concentration of Ag+ was observed because the
coordination number by the ligands of the polymer should be
less than 3 to ensure the reaction activity with olefins.17
Silver salts can be also added into ionic liquid to prepare
SILMs, where Ag+ is immobilized within membrane via electrostatic attraction and its activity is no longer affected by the
competitive coordinating effect. It becomes possible to investigate
the effect of a large number of counter ions based on the platform
of SILMs. What’s more, new silver salts from room temperature
ionic liquid have also been reported, in which Ag+ was the only
cation. Such ionic liquids could maximize the concentration of
Ag+ within SILMs.7
A positively charged metal atom has become another critical
type of olefin carrier in recent years.31–33 By replacing Ag+ with
silver nanoparticles (AgNP) of which the surface was positively
charged by an organic electron-acceptor molecule, the Ag atoms
at the surface of AgNP were found capable of facilitating olefin
transport as efficient carriers. As shown in Fig. 5, a strong
electron acceptor, 7,7,8,8-tetracyanoquinodimethane (TCNQ),
was employed to tune the surface positive charge of a silver
nanoparticle. When TCNQ was in contact with AgNP, the
interface dipole was induced, so that high positive charge
Fig. 5 A schematic illustration of the surface positive charge on Ag
nanoparticles induced by an electron acceptor. Reprinted with permission
from ref. 33. Copyright 2011 Wiley-VCH.
110 | Chem. Soc. Rev., 2015, 44, 103--118
Fig. 6 The complex between ferrous porphyrin and O2 with the aid of
histidyl residue as a fifth ligand in haemoglobin. Reprinted with permission
from ref. 17. Copyright 2006 Wiley-VCH.
appeared on the AgNP surface. Such charged Ag atoms performed
even better than Ag+ as olefin carriers within the membrane.33
More importantly, this judicious exploration provided great
opportunities to well address the concerns on carrier stability.
Other metal nanoparticles made of Au or Cu with charged surface
atoms were also available in facilitating olefin transport.
The carriers for O2 are quite different from those for soft
bases. Considering the high electronegativity of the O atom and
the unshared pair electrons in O2, O2 is a hard base with good
affinity towards softer acids such as Co2+, Fe2+ and Mn2+. On
the other hand, due to the electron withdrawing effect of O2,
the center metal ions need higher electron density to lower the
energy of metal–O2 complexes. Consequently, other Lewis bases
such as electron-donating ligands are necessary to activate the
O2 carrier. In biological organisms, haemoglobin smartly
employs ferrous porphyrin (FeIIP) with an axial-direction histidyl
residue as a fifth ligand to reversibly bind O2 (Fig. 6).17 Planner
porphyrin ensures the stability of a metal complex, and the fifth
ligand plays key roles in donating external electrons and inhibiting
the irreversible formation of peroxido-bridged dimers. Intrigued
by the structure of FeIIP, many researchers have attempted to
design artificial carrier systems using different metal complexes.
Considering the in vitro instability of FeII complexes in the
presence of O2, the major efforts were devoted to developing
CoII complexes. Among the various types of Co complexes, cobalt
porphyrin (CoP) derivatives and cobalt–Shiff base complexes
are perceived as highly active and stable carrier systems for O2selective transmembrane permeation. However, in most relevant
works the complexes were directly doped into liquid membranes
or polymeric membranes. Examples about the covalent connection
of such complexes to polymer chains have rarely been reported.
Metal ions or charged metal atoms as CO2 carriers were not
reported until several years ago. It is notable that CO2 is a Lewis
base rather than a Lewis acid owing to the lack of unoccupied
orbitals. Analogous to O2, CO2 is also a hard base, while it is
softer than O2 due to the delocalized p bond. Based on HSAB
theory, hard acid and junction acid should be the better choices
than Ag+, yet recently reported results imply that things are not
so simple. Chung and co-workers embedded Zn2+ complexes
into a glassy polymer membrane, and firstly verified the facilitation effect.34 Both O2/N2 and CO2/CH4 selectivities showed
remarkable increment compared to the control membrane,
indicative of the similar facilitated transport mechanisms.
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Fig. 7 Dissociated and surface charged Cu nanoparticles by ionic liquid
and complexation with CO2 molecules. Reprinted with permission from
ref. 36. Copyright 2012 the Royal of Social Chemistry.
K+ was also reported as a CO2 carrier in the polyvinyl pyrrolidone
(PVP) membrane, and it was the first time that a salt-doped
solid-state membrane exhibited much higher CO2 permeance
than the control membrane in the dry state.35 This result is
surprising because the metal ions usually lead to higher polymer
rigidity and lower free volume. The authors attributed the
increase of CO2 permeance to the favorable interactions between
K+ and CO2, whereas other possibilities such as non-selective
cracks could not be excluded. In another impressive example,
positively polarized Cu nanoparticles dispersed in ionic liquid
simultaneously enhanced CO2/gas selectivity and CO2 permeance.
The ionic liquid not only dissociated micro-sized Cu flakes into
nanoparticles, but also induced surface positive charges like the
case of TCNQ (Fig. 7). However, the facilitation effect was weaker
than the case of olefin.36 We deduce that the uneven charge
distribution and the linear molecule shape of CO2 would inevitably
reduce the overlapping of electron clouds between Cu and CO2,
resulting in weaker binding strength.
3.4 Facilitated transport chemistries in terms of
electrochemical reaction
All the aforementioned reactions occur within the entire
membrane. If a membrane is both ion conductive and electron
conductive, it is feasible to trigger facilitated transport by
electrochemical reactions, which typically occur at the two sides
of membranes. This concept can be realized for O2 transport
with a ceramic-based mixed conductor membrane as both an
ion conductor and an electron conductor at high operation
temperature.37 As shown in Fig. 8a, O2 molecules are firstly ionized
by electrons at the feed side, and then are transported through
the membrane in the form of O2, followed by deionization at the
permeate side. Synchronously, the released electrons are transported back to the feed side. A high temperature above 600 1C is
usually required to activate these ion conducting processes. If the
membrane material permits, the operation temperature ought to
be as high as possible to reduce the ion conducting resistance.
This journal is © The Royal Society of Chemistry 2015
Fig. 8 O2 enrichment and CO2 capture with a mixed conductor membrane:
(a) air separation with a mixed oxide-ion and electron conductor (MOCC)
membrane; (b) pre-combustion CO2 capture with a mixed carbonate-ion
and oxide-ion conductor (MOCC) membrane; (c) post-combustion CO2
capture with a mixed carbonate-ion and electron conductor (MECC)
membrane.
Sometimes the surface reaction may be the rate-controlling step,
and a thin catalytic surface layer (usually consists of perovskite,
with a general formula of ABO3) is required to accelerate the
conversion of O2 to O2. From the viewpoint of facilitated
transport, electron can be regarded as the carrier, and the
mixed conductor membranes for selective O2 transport can be
regarded as a semi-mobile carrier membrane.
Applying similar principles, two types of mixed conductors
can be utilized to design CO2 separation membrane: mixed
carbonate-ion and oxide-ion conductor (MOCC, Fig. 8b) and
mixed carbonate-ion and electron conductor (MECC, Fig. 8c),
which are suitable for pre-combustion and post-combustion
CO2 capture, respectively.38 O2 is the carrier for the former
case, while the latter case is actually an integration of an O2
selective membrane and a CO2 selective membrane. Unlike the
case of a crystalline and solid oxide-ion conductor, molten
carbonate is often filled into the membrane pores to conduct
carbonate ion. Consequently, the membrane framework should
be highly porous to ensure continuous distribution of a
carbonate-ion conductor. According to the definition of three
types of facilitated transport membranes, mixed conductor
membranes for selective CO2 transport can be treated as
mobile-carrier membranes.
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Fig. 9 Stepwise oxidative dehydrogenation of propane with a sequence
of dehydrogenation and hydrogen combustion with the aid of a mixed
conductor membrane. Reprinted with permission from ref. 37. Copyright
2013 Elsevier.
A facilitated transport membrane involving an electrochemical
reaction is also applicable for separating other gas mixtures with
the assistance of membrane catalysis. Due to the possibility of
integrating O2 permeation and catalytic oxidation reaction, an
O2-selective mixed conductor membrane is expected to separate
the two components of redox conjugate pairs, e.g. CO/CO2, C2H4/
C2H6, C3H6/C3H8, etc. As shown in Fig. 9, the periodically placed
dehydrogenation membrane module and selective H2 oxidation
module can break the thermodynamic limitation of oxidative
dehydrogenation process, and thus propane is likely to be continuously converted to propylene, resulting in good separation
and highly efficient propylene production.
Chem Soc Rev
Among the three types of reactions, nucleophilic addition
reaction represents the main stream for facilitating CO2 transport.
In recent years, great progresses have been achieved for facilitated
CO2 transport based on nucleophilic addition reaction. The
typical examples about such type of facilitated CO2 transport
membranes are summarized in Table S1 (ESI†). Firstly, thanks
to the fascinating carrier-involved chemistry, the separation
performance of CO2 separation membrane in terms of permeability
and selectivity has been elevated to an unprecedentedly high level.
Besides the aforementioned biomimetic poly(N-vinylimidazole)–
zinc complex membranes, Wang and coworkers39 designed and
fabricated mixed matrix membranes filled with nanosized hydrotalcite (HT), an anionic pillared layered material with basicity.
Owing to the mobile ionic carriers within the interlayer gap of
HT with the aid of water condensation, high-speed CO2 transport
channels were successfully constructed, as shown in Fig. 10. With a
polyethyleinimine-based copolymer as a polymer matrix, the mixed
matrix membrane exhibited a high CO2 permeance up to 5693 GPU
(1 GPU = 106 cm3 (STP) cm2 s1 cmHg1) and a CO2/N2
selectivity of 268 at 0.11 MPa. Although carriers tended to be
saturated at elevated pressures, a CO2 permeance above 500 GPU
and a CO2/N2 selectivity above 40 was maintained at 1.0 MPa. Next,
the importance of increasing effective carrier content has been
highlighted. Qiao et al.40 found that doping piperazine (PIP) into
4. Applications of facilitated transport
membranes in energy-intensive
processes
This section describes the applications of facilitated transport
membranes in energy-intensive processes, especially gas separation, pervaporation and fuel cells. The engineering-relevant
issues such as scale-up difficulty, investment, membrane stability
are also included.
4.1
Gas separation
Facilitated transport membranes are mostly designed for gas
separation due to the weak interactions between gas molecules
and membrane materials. Reversible reactions prove very useful
for enriching the target permeants without irreversible binding.
Three typical examples are CO2 capture, air separation and olefin/
paraffin separation, respectively.
Nowadays, CO2 capture has become globally concerned due
to the anthropogenically forced carbon emission and climate
change. Fortunately, CO2 transport can be facilitated due to
the abundant facilitated transport chemistries as described in
Section 3. That is, facilitated CO2 transport is possible based on
different reversible reactions, namely nucleophilic addition
reaction, p-complexation and electrochemical reaction, providing
researchers with more opportunities.
112 | Chem. Soc. Rev., 2015, 44, 103--118
Fig. 10 (a) The anionic pillared layered structure of hydrotalcite; (b) the
mixed matrix structure of a polyethyleinimine-based copolymer and
hydrotalcite with 3-aminopropyltriethoxysilane as a interfacial linker.
Reprinted with permission from ref. 39. Copyright 2014 the Royal of Social
Chemistry.
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PVAm could yield a physically crosslinked structure without
increasing crystallinity. The multiple hydrogen bonds between
PVAm and PIP allowed very high PIP loading (as high as 230 wt%
of PVAm) without loss of mechanical stability. As such, a substantial
increase of effective carrier content was acquired, resulting in an
extremely high CO2 permeance of 6500 GPU and a CO2/N2 selectivity
of 277 at 0.11 MPa. To the best of our knowledge, this study
reported the highest CO2 permeance at low pressures so far. Last
but not the least, more carriers have been designed considering
the carrier stability under different conditions. For example,
carboxylate-type carriers have proved effective in acquiring the
desired carrier stability in an oxidative atmosphere. Aromatic
carboxylate groups as carriers could also avert the severe
compact packing of matrix polymer chains, yielding high
CO2 permeance41.
The aforementioned facilitated CO2 transport membranes rely
heavily on water, which allows appropriate degree of swelling of
the membrane matrix and thus high CO2 permeability. Kasahara
et al. developed amino acid ionic liquid-based facilitated transport
membranes to promote CO2 transport without the aid of water.
Since the liquid membrane matrix rendered low diffusion
resistance, the membranes achieved high CO2 permeability
up to 8300 Barrer and high CO2/N2 selectivity up to 146 at
100 1C under dry conditions.42 The effect of carrier saturation
at elevated pressures proved the occurrence of facilitated transport. The authors also designed polymeric ion-gel membranes
to enhance membrane stability while maintaining high CO2
permeability.43 More information about the development of amino
acid ionic liquid-based facilitated transport membranes can be
found in Table S2 (ESI†).
Another notable breakthrough was reported by Izak and
coworkers.44 They proposed the concept of separating CO2,
H2S and impurities from biogas by a ‘‘condensing-liquid
membrane’’, based on the different solubility of components
in a very thin continuously refreshed water layer supported by a
hydrophobic porous substrate. Actually the thin water layer also
acted as facilitated transport carriers for CO2, while the lack of
proton acceptors restricts the dissociation of H2CO3. By further
modifying the surface of substrate with basic groups or long
chains, better separation performance can be expected.
Facilitated CO2 transport based on p-complexation is a
burgeoning research field. Since metal ions can also complex
with the polymer matrix and lead to chain rigidification, it
remains difficult to evaluate the contribution of p-complexation
to CO2/CH4 selectivity.34 Also, considering the role of water as a
nucleophilic agent, it is also challenging to distinguish the
facilitation effect due to nucleophilic addition reaction from
that due to p-complexation. Consequently, the existing studies
on metal ion-mediated facilitated CO2 transport were conducted
under anhydrous conditions, and the corresponding CO2
permeances were much lower than the values mentioned
above.34–36 We speculate that water is an indispensible ingredient
for high CO2 permeance, because water-induced swelling on
polymeric frameworks may aid in increasing effective carrier
content and taking the full advantages of carriers. Accordingly,
it is recommended that the future work on polymer-based
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Tutorial Review
facilitated transport membranes for CO2 separation should
include the comparison of dry-state and humidified-state gas
separation performances at different carrier loadings, which
may be very helpful to more deeply understand the differences
in membrane structures and transport mechanisms.
Facilitated CO2 transport based on electrochemical reaction
requires a mixed conductor membrane, which is usually an
inorganic ceramic membrane. The electrochemical reactions
described in Section 3 are quite simple, while the overall CO2
permeance through a mixed conductor membrane is determined
by not only the intrinsic carbonate and oxygen ionic conductivities
of the carbonate and ceramic phases, but also the pore and solid
microstructure of the ceramic support. In particular, the latter is
often the controlling factor due to the difficulty in controlling the
connectivity and tortuosity of the ion conducting channels. Zhang
et al.38 reported a high-flux mixed carbonate-ion and oxide-ion
conductor (MOCC) membrane consisting of highly and efficiently
interconnected three-dimensional ionic channels. A combined
‘‘co-precipitation’’ and ‘‘sacrificial template’’ method was used
to synthesize a solid oxide porous matrix with highly interconnected solid and uniformly distributed pores. The molten
carbonate phase then fills into these pores to form a dense MOCC
membrane. Such type of membrane shows a CO2 flux density two
orders of magnitude higher than the existing ceramic–carbonate
systems fabricated by other techniques. Therefore, this type of
membrane is a promising alternative to implement hightemperature CO2 capture from flue gases or shift gases.
Olefin/paraffin separation is another typical process of
which the energy efficiency is expected to be enhanced by a
facilitated transport membrane. In the petrochemical industry,
separation olefin from paraffin is typically one of the most
expensive and most energy-consuming separation processes
because of the high cooling capacity and large number of
theoretical plates required for cryogenic distillation. Membrane
separation of olefin/paraffin is advantageous over cryogenic
distillation in energy and investment cost, while the conventional membranes based on solution–diffusion mechanism are
insufficient to produce high-purity olefin. Facilitated transport
membranes involving p-complexation prove to be promising
candidates because of the expected high separation specificity.
According to the manners of introducing metal ion carriers into
a membrane, facilitated transport membranes for olefin/paraffin
separation can be divided into four types: SILM, ion exchange
membrane, polymer electrolyte membrane, and nanocomposite
membrane. All the three basic types of facilitated transport
membranes mentioned in Section 2 are encompassed.
SILMs are often compared with ion exchange membranes
because of the strong binding of Ag+ by electrostatic force.
However, the high separation performance of ion exchange
membrane relies on the dissociation of Ag+ and swelling of the
polymer matrix, which requires introduction of water. Herein
the introduction of water is actually not a good choice like the
case for CO2 capture because of the strict limitation of water
content for olefin polymerization. The ionic and liquid like
SILMs exclude the necessity of water, and hence SILMs have
witnessed a rapid progress in these years. As one example,
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Pitsch et al.7 reported an ionic liquid nanocomposite membrane
comprising a multi-layer support structure hosting the ionic salt
[Ag]+[Tf2N]. The ionic salt renders liquid like upon complexation
with propylene, resulting in facilitated transport of propylene over
propane at benchmark-setting selectivity and permeance levels.
The membrane also showed good resistance to C2H2 poisoning.
As for the polymer electrolyte membrane, Ag+ is immobilized
by relatively weak coordinating interactions, and hence water is
also unnecessary to promote Ag+ dissociation. Large amount of
Ag+ is permitted to be doped into the membrane due to the
multiple coordinating interaction sites. At high Ag+ loading,
the strong plasticizing ability of olefin molecules allows high
fractional free volume to reduce the effect of diffusion resistance,
which as well as the enrichment of hopping site for olefin leads to
remarkable increment of olefin permeance, as mentioned as a
threshold concentration effect of carrier loading in Section 3.3.
Therefore, achieving high olefin/paraffin separation performance
of polymer electrolyte membrane is already not a challenge in
laboratory. Nevertheless, maintaining the stability of Ag+ in the
real multi-component gas feed remains a great challenge. One
possible solution is to add strong oxidant into the membrane as a
stabilizing agent, so as to prevent Ag+ from being reduced.
However, a cyclic process is required after the stabilizing agent
is exhausted. Merkel et al.45 presented a baseline study describing
the scope of addressing carrier instability. They developed an
in situ regeneration method by using peroxide/acid liquid or vapor
phase treatment to oxidize the reduced Ag+ carriers within the
polymer electrolyte membrane. However, the poisoned Ag+ by H2S
cannot be regenerated through this method because the oxidation
process may produce Ag2SO4, which is also not effective in
facilitating olefin transport. Doping the membrane with nitrate
salts has also proven effective in suppressing the reduction of
silver ion. For example, Kang et al. found that doping Al(NO3)3
into a poly(2-ethyl-2-oxazoline)–AgBF4 complex could prolong the
lifetime of Ag+ for more than 14 days, which was attributed to the
ionic aggregation between the Ag+ of AgBF4 and NO3 of Al(NO3)3.
The ionic aggregation phenomena could be further interpreted
by the favorable interactions between BF4 and Al3+ and the
weakened interactions between Al3+ and NO3.46 More examples
can be found in Table S3 of ESI.†
Nanocomposite membranes containing positively charged
metal nanoparticles have been mentioned in Section 3. Like the
case of Ag+, AgNP was identified as one of the best choices. As a
typical example, an AgNP-containing nanocomposite membrane
was successfully fabricated by a reflux method using the casting
solution of a PVP–AgBF4 polymer electrolyte membrane. In this
way, the average size of AgNPs was controlled to 20 nm with a
standard deviation of 3 nm, and a homogeneous dispersion of
AgNPs within the membrane was observed. By selecting an
appropriate electron acceptor, positive charge was induced
onto the AgNP surface, and the resultant facilitated transport
membrane exhibited a high mixed-gas propylene/propane selectivity
up to 50, with a moderate propylene permeance of 3.5 GPU.33
Although such performance could not rival the best results for
polymer electrolyte membranes, the as-prepared PVP–AgNP nanocomposite membrane is expected to show much better stability
114 | Chem. Soc. Rev., 2015, 44, 103--118
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against reductive atmosphere than most polymer electrolyte
membranes.
Air separation is also a tough task for conventional membranes
because of the similar dynamic diameters and low boiling points
of O2 and N2. In chemical industries, membrane air separation is
already regarded as a cost-effective process to produce moderately
pure streams containing 495% N2 or 60–80% O2. The production
of O2 or N2 with higher purity requires higher separation
efficiency enabled by facilitated transport membranes. Actually,
the earliest understanding on facilitated transport phenomena
and principles arose from the naturally-occurring O2 carriers.
Despite a long history of synthetic O2 carrier development, the
application prospect of facilitated transport in air separation is
not as promising as that in CO2 capture and olefin/paraffin
separation. The main reasons lie in the lack of stability of the
O2–carrier coordinating bonds and the insufficient physical
adsorption capacity of O2 in dense polymer membranes. Also,
the essential multidentate ligands for the center metal ion of
the O2 carrier may sterically hinder the motion of the unique
active site, lowering the reaction rate. One typical representation
is the carrier saturation problem. Most O2 carriers are only
effective at very low pressure and become quickly saturated
below 1 bar. As such, both O2 permeability and O2/N2 selectivity
decrease to rather low values with limited advantages over
common polymer membranes. There are two strategies to solve
this problem: (1) enhancing the intrinsic carrier activity, which is
extremely challenging since a huge number of Co complexes
have already been attempted; (2) increasing the carrier content,
which is relatively more feasible. Given the most active carrier
systems such as CoP derivatives, the main challenge is to
maintain adequate mechanical strength at high carrier loading,
because the planar and rigid ligands often aggregate in the
membrane and result in brittleness. Chikushi et al. synthesized
porphyrin network polymers via a Michael addition-type click
reaction with acetoacetate-substituted CoP (CoPac) as the
Michael addition donor and tri/tetra-acrylate as the acceptor
(Fig. 11). The as-synthesized polymer allowed high CoPac content up to 70 wt%, rendering the membrane with both high O2
permeability (10–100 Barrer) and O2/N2 selectivity (above 30).47
A mixed conductor membrane comprising an electron
conductor and an O2 conductor has shown great potential in
air separation as a special type of facilitated transport membrane.
On account of the transport of O2 through oxygen vacancies, a
mixed conductor membrane is theoretically expected to produce
Fig. 11 The chemical structure of the porphyrin network polymers
synthesized by Chikushi et al. Reprinted with permission from ref. 47.
Copyright 2014 Wiley-VCH.
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100% pure O2. Sunarso et al.48 reported a high performance
BaBiScCo hollow fiber membrane, which combined the merits
of high O2 flux from barium–cobalt in tandem with the stability
and electrical conductivity enhancement by scandium oxide. The
resultant membrane delivered high flux up to 11.4 ml cm2 min1
at 950 1C, which has exceeded the target value of 10 ml cm2
min1 sought by the research community. Whereas, from the
viewpoint of engineering, the high temperature required for
producing high concentration of oxygen vacancies is not beneficial
for acquiring high energy efficiency. A combined strategy using
the mixed conductor membrane as a membrane reactor for hightemperature oxidation/reduction reactions might be appropriate
to make better use of the high-grade energy. Also, the long term
stability of the mixed conductor membrane and the related
membrane configuration should be carefully considered to evaluate
the future of the mixed conductor membrane in air separation.
4.2
Pervaporation
Facilitated transport membranes are also effective in pervaporation,
especially for aromatic compound separation. Most aromatic
compounds exhibit weak polarity and therefore low solubility
in conventional membranes. Facilitated transport based on
p-complexation is expected to significantly increase the concentration of the target permeant within the membrane. One of
the most representative examples is gasoline desulfurization,
which is an essential tache in clean energy production. The
combustion of gasoline with high sulphur content would
enable excessive emission of SO2, which is one of the main
air pollutants. The removal of thiophenes from gasoline is the
most challenging work for gasoline desulfurization because of
the similar physical properties between thiophene and the
hydrocarbon components of gasoline, heptane and octane.
Hydrodesulfurization (HDS) processes constitute the most
effective method presently, while it requires large quantity of H2.
Although it is often impossible to deeply remove thiophenes from
gasoline solely by membrane technologies, rough removal of the
majority of thiophene by membrane technologies is also attractive
because the enrichment of sulphur in the permeate means
remarkable reduction of the treatment quantity of gasoline for
the HDS process.
The facilitated transport chemistries for thiophene are
analogous to the case of olefins, while thiophene allows a wider
range of metal ions as carriers, e.g. Ni2+, Cu2+, and Ce4+.8 The
major difference lies in the low content of thiophene in gasoline,
indicating that thiophene cannot be overwhelmingly enriched in
a membrane. In order to obtain sufficiently high flux when
abundant heptane or octane exists within the membrane, hydrophobic polysiloxane is often employed as the matrix polymer,
which can be sufficiently swollen by heptane and octane. Due to
the lack of carrier loading capacity of polysiloxane, the carriers
are often loaded by micro-/nano-sized particles, which are further
embedded in polysiloxane as a filler. Liu et al.49 found that a thin
dopamine layer on TiO2 microspheres allowed high Ag+ loading
(37.6 wt% of Ag+/TiO2 microspheres) via electron donor–acceptor
coordination bonds (Fig. 12). The Ag+-loaded fillers embedded
into the rubbery polysiloxane membrane brought about additional
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Tutorial Review
Fig. 12 The synthetic route of a Ag+/TiO2 microsphere for membrane
desulfurization. Reprinted with permission from ref. 49. Copyright 2011
Elsevier.
free volume at the polymer–filler interface, which further lowered
the negative effect of diffusion resistance on facilitated transport.
The optimum membrane presented a permeation flux of
4.14 kg m2 h1 (1.97 times as much as polysiloxane control
membrane) and an enrichment factor of 8.56 (1.95 times as
much as polysiloxane control membrane).
4.3
Proton exchange membrane fuel cell (PEMFC)
Proton exchange membrane fuel cell (PEMFC) is another representative application of the facilitated transport membrane.
The key component of PEMFC is proton exchange membrane
(PEM), which functions as an electrolyte for transporting
protons from the anode to the cathode as well as blocking
the passage of electrons and fuel between the electrodes.
Designing and optimizing the proton carriers is important for
high performance PEMs. The primary proton carriers include
water, which facilitates both vehicle and hopping mechanisms,
and other Brønsted acids–bases, which facilitate the hopping
mechanism. As for water, the major objective is to achieve high
water retention under low humidity and elevated temperature,
affording significantly enhanced system efficiency. For the
latter case, the incorporated proton carrier should balance
the proton accepting/donating properties of the existing proton
conducting group. One attractive solution is to design acid–base
pairs via proton transfer reaction.
Microcapsules as fillers in PEM have been reported recently
as promising alternatives to increasing the water retention
capacity of membrane at low humidity. Jiang and co-workers
utilized polymeric microcapsules as water reservoirs due to the
possibility of integrating elastic storage and capillary storage
mechanisms.50 The highly hydrophilic and crosslinked capsule
shell allowed steady retention of water in the bound-water state,
yielding dramatically enhanced water retention properties and
interconnected proton transfer pathways. However, it remains a
tough work to completely impede water release at low humidity.
Constructing acid–base pairs has rapidly emerged at the forefront of PEM research. Given an acidic sulfonated poly(ether ether
ketone) (SPEEK) matrix, basic amino groups were anchored
to halloysite nanotube (HNT) via dopamine chemistry, so as to
form acid–base pairs with the sulfonate group of SPEEK (Fig. 13).
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Fig. 14 A combined strategy involving water retaining and acid–base pair
designing by a composite SPEEK–imidazole capsule membrane. Reprinted
with permission from ref. 23. Copyright 2012 Wiley-VCH.
Fig. 13 Preparation of dopamine-modified halloysite nanotubes and
the interfacial chemical structures between SPEEK bulk and nanotube.
Reprinted with permission from ref. 20. Copyright 2013 Wiley-VCH.
The incorporation of dopamine-modified HNTs (DHNTs) reduced
the channel size, water uptake, and area swelling of the SPEEK
membranes, while the acid–base pairs created continuous
pathways for a 30% increase of proton conductivity and a
52% increase of power density of a single cell. By comparison,
only slight increase of proton conductivity was observed when
unmodified HNT was embedded into SPEEK, because of the
lower proton accepting properties of hydroxyl groups and the
reduced possibility of proton transfer reaction.20 On the other
hand, a basic matrix requires acidic carriers to form acid–base
pairs. Song et al. developed a tetrazole-based polymer as the
matrix of PEM, and the –PO3H2 group was connected onto the
polymer chains to construct acid–base pairs. Considering that
both tetrazoles and phosphoric acids are capable of transferring
proton under anhydrous conditions, the acid–base pair afforded
enhanced proton conductivity and peak power density at 120 1C.25
It is also possible to achieve two goals of retaining water and
designing acid–base pairs by incorporating multi-functional microcapsules. Wang et al.23 designed and fabricated imidazole microcapsules (IMCs)-embedded SPEEK composite membranes. The
hollow structure and hydrophilicity of the IMCs endowed the
composite membranes with significantly improved water retention
properties under low humidity, thus facilitating proton transport
within the well-connected water channels (Fig. 14). The concentrated imidazole groups in IMC shells facilitated proton hopping
via proton exchange. Moreover, the formation of acid–base pairs
(bioinspired proton transport highway) between the sulfonic acid
group on SPEEK and the imidazole group on IMCs, mimicking the
aspartic acid–Schiff base complex in bacteriorhodopsin, allowed
ultrafast proton transport with low energy barrier.
5. Concluding remarks
Facilitated transport phenomena and theories have opened up
novel avenues for fast and selective transport of specific small
116 | Chem. Soc. Rev., 2015, 44, 103--118
molecules or ions. With deep insights into the carrier-mediated
transport mechanisms and facilitated transport chemistries,
great progresses in designing and fabricating facilitated
transport membranes have been achieved in the past decade.
Given the classification of the available carriers and the relevant
reversible reactions for facilitated transport in this tutorial review,
facilitated transport membranes can be fabricated using a variety
of materials, e.g. polymers, polymer composites or inorganic
materials, and have shown attractive performance in a broad
range of energy-intensive processes, especially CO2 capture
and olefin/paraffin separation. Moreover, some polymer-based
membrane materials (e.g. polyvinylamine) with excellent
solution processing characteristics have been considered for
kilogram-scale synthesis and fabrication into large-area asymmetric composite membranes. The stability problems for CO2
carriers and olefin carriers have also been partially solved.
Despite the achievements mentioned above, there are still
many emerging challenges and opportunities for both the
scientific community and engineers, and some of them are
highlighted as follows. (1) The facilitated transport chemistries
need further diversification for higher carrier reactivity. In particular, metal ion-involved facilitated transport widely exists in
biological organisms. Many proteins with catalytic functions
allow high specificity reverse binding towards a guest molecule/
ion, which heavily relies on a synergy of multi-site interactions,
including reversible chemical bonds and physical interactions.
That is, the active center atom of a carrier cannot normally take
effects without the aid of the ambient microenvironments.
Moreover, like the cases of carbonic anhydrase and haemoglobin,
appropriate amino acid residues are required to tune the HSAB
acidity/basicity of the centre metal ions to the desired level. In this
way, more attentions should be paid to the unique and common
features of different acid–base reactions where the acids and
bases are defined by Brønsted theory, Lewis theory, and HSAB
theory, respectively. (2) The complex transport mechanisms need
deeper elucidation. Firstly, the relationship between the facilitated
transport mechanism and conventional solution–diffusion
mechanism remains elusive. Obviously, the facilitated transport mechanism is not an independent mechanism that can be
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separately investigated experimentally. The involvement of a
carrier would inevitably interfere with the physical structure of
the membrane matrix and therefore the transport properties
follow the solution–diffusion mechanism. Secondly, taking ion
transport into considerations, whether and to what extent the
diffusivity resistance of the membrane matrix affects hopping
mechanism remains unknown, since the carrier-to-carrier
hopping does not belong to Fickian diffusion. The existing
facilitated transport membranes employing hopping mechanism
are mostly swollen by water or plasticized by the target permeant,
and therefore this effect was often ignored. The mixed conductor
membrane using oxygen vacancies or molten carbonates may
help to understand the related mechanism. Since O2 and CO2
cannot plasticize the polymer matrix at relatively low pressure
without the aid of other plasticization agents, whether the facilitation effect of p-complexation on their permeabilities can be
enhanced by adding a plasticization agent would be an interesting
topic. (3) The membrane fabrication methods need more alternatives.
In view of the manners of introducing carriers, the concentration
and distribution of carriers should be better controlled to enhance
the facilitation transport effect. A soluble carrier ingredient is
permitted to achieve high carrier concentration within the
membrane, whereas an insoluble carrier ingredient suffers from
the insufficient carrier concentration. Also, the carriers are
expected to distribute at the accessible regions, rather than selfrigidified regions. That is, the interactions of carriers may cause
chain rigidification or carrier aggregation, which would drastically
lower the effective carrier concentration. In view of material
selection, organic polymers are selected most of the time, while
mixed matrix polymer composites composed of polymer and
functional filler seem better alternatives. Recently, nanocomposite
membranes with facilitated transport carriers have become a
promising direction to further enhance gas permeance, especially
for CO2 capture and olefin/parafin separation. With the aid of an
adsorptive filler, the carrier saturation problem is also expected to
be partially addressed, which is important to maintain the high
performance in the cases where elevated feed pressure is required,
e.g. natural gas sweetening, post-combustion CO2 capture and air
separation. On one hand, the carriers can be loaded onto the
fillers, which expands the selection range of available matrix
polymers. On the other hand, the filler phase is believed to
interrupt the crystalline or rigidified region caused by strong
cohesive interactions, producing higher fractional free volume
and effective carrier concentration. Inorganic mixed conductors
now represent a relatively independent branch, while it illumines
the possibility of converting molecular transport into ion conduction. (4) The membrane stability problems need smarter solutions.
Nowadays, the efforts devoted to enhance carrier stability are
mainly for addressing the stability of Ag+ against light, reductive
and sulphur-containing impurities. The development of antioxidation carriers for CO2 capture has also been reported. It
should be mentioned that the instability facilitated transport
membranes should be also considered in the following two cases:
(1) water is necessary as a carrier or plasticization agent, and in
this case the water retention and management in the membrane
process will be very important to maintain membrane stability;
This journal is © The Royal Society of Chemistry 2015
Tutorial Review
(2) mobile carriers are dissolved in liquid membranes, and therefore ionic liquid-based facilitated transport may be more appropriate for membrane contactors.
Altogether, incorporating facilitated transport into membranes
enriches the multiple selectivity mechanism, and significantly
intensifies the mass transfer and application efficiency of
membrane processes. Although there is still a long road ahead,
we believe that the literature published to date donate excellent
examples for future exploitation of facilitated transport membranes.
With the rapid advancement in chemistry and materials science,
facilitated transport membranes with superior transport properties
and operation stability are expected to be developed in the near
future. In particular, we should bear in mind that the inspiration
models of facilitated transport arise from nature, and therefore
the advances in protein chemistry (especially for carrier proteins,
channel proteins, and enzymes) and synthetic biology might be of
great values to pursue new breakthroughs in designing the next
generation of facilitated transport membranes.
Acknowledgements
The authors are grateful for the financial support from National
Science Fund for Distinguished Young Scholars (21125627), the
National High Technology Research and Development Program
of China (2012AA03A611), and the Programme of Introducing
Talents of Discipline to Universities (No. B06006).
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