Recent advances in polymeric membranes for CO2 capture

Yang Han,W.S.Winston Ho

William G.Lowrie Department of Chemical and Biomolecular Engineering and Department of Materials Science and Engineering,The Ohio State University,151 West Woodruff Avenue,Columbus,OH 43210-1350,USA

Keywords:Carbon capture Polymeric membrane Gas separation

A B S T R A C T Membrane and membrane process have been considered as one of the most promising technologies for mitigating CO2 emissions from the use of fossil fuels.In this paper,recent advances in polymeric membranes for CO2 capture are reviewed in terms of material design and membrane formation.The selected polymeric materials are grouped based on their gas transport mechanisms,i.e.,solution-diffusion and facilitated transport.The discussion of solution-diffusion membranes encompasses the recent efforts to shift the upper bound barrier,including the enhanced CO2 solubility in several rubbery polymers and novel methods to construct shape-persisting macromolecules with unprecedented sieving ability.The carrier-bearing facilitated transport membranes are categorized based on the specific CO2-carrier chemistry.Finally,opportunities and challenges in practical applications are also discussed,including post-combustion carbon capture(CO2/N2),hydrogen purification(CO2/H2),and natural gas sweetening(CO2/CH4).

1.Introduction

Significant advances in polymeric membrane materials for gas separations have been witnessed in the past decade.Nearly hundreds of polymer materials have been engineered,and their unprecedented transport properties could enable an energy-efficient route for largescale gas separations.These concerted research efforts are largely catalyzed by the urgent need to reduce CO2emissions from the usage of fossil fuels.CO2capture from fuel gases(e.g.,natural gas and syngas)and combustion flue gases(e.g.,coal-fired power plant flue gas)is generally believed to be the effective technological solution to mitigate the environmental and economic impacts of CO2-induced climate change[1].Compared to other CO2separation processes,membrane is advantageous for its system compactness,energy efficiency,operational simplicity,and ability to overcome thermodynamic solubility limitations[2].These features stem from the fact that membrane is a thin interphase acting as a selective barrier separating two phases[3].This thinness,typically in the range of 100 nm to a few micrometers,provides an almost natural platform to implement sophisticatedly engineered macromolecular structures.

In this review,opportunities and challenges in membrane-based carbon capture are discussed,including CO2/N2separation in postcombustion carbon capture,CO2/H2separation in syngas processing,and CO2/CH4separation in natural gas sweetening.These separation scenarios differ in the CO2partial pressure.The post-combustion flue gases are typically discharged at atmospheric pressure with 11 vol%–15 vol%CO2for coal-fired power plants and 4 vol%–8 vol%for natural gas-fired power plants[4].The low CO2partial pressure,along with the enormous volumetric flow rate,requires considerable CO2permeance and CO2/N2selectivity to achieve bulk CO2removal with high product CO2purity[5,6].In the syngas processing,the reforming of hydrocarbon produces a H2-rich syngas with around 30 vol%CO2[7].The CO2/H2separation is typically performed after the water-gas shift reaction,where a pressure as high as 5 MPa yields a large transmembrane driving force,but a sufficiently high CO2/H2or H2/CO2selectivity is required to minimize the H2loss.Natural gas dehydration and sweetening are typically carried out at 3–6 MPa with 5%–70%CO2based on the geographical location,although the feed natural gas pressure can be as high as 35 MPa[8].As a pressure driven process,membranes are well suited for these high pressure applications.However,the mechanical properties and the resistance to plasticization are of paramount importance for polymers in these separation scenarios[9].

Over the past decade,better polymeric materials with improved CO2permeability and selectivity have been extensively searched.However,a fundamental barrier is the permeability/selectivity trade-off,i.e.,more permeable polymers tend to possess less selectivity and vice versa[10].This trade-off is deeply rooted in the solution-diffusion mechanism that most polymeric materials rely on[11].Conventionally,this limiting behavior is represented by an empirical upper bound,which was firstly proposed by Robeson in 1991[12]then updated in 2008[13].The recent efforts to shift the upper bound are discussed in this review,including the enhanced CO2solubility in several rubbery polymers and the novel methods to construct shape-persisting macromolecules with unprecedented sieving ability.One approach to overcome the upper bound is by incorporating reactive sites to facilitate the CO2transport.The facilitated transport membranes are also reviewed based on the specific CO2-carrier chemistry.

One key requirement for a successful industrial gas separation membrane is the processability.Most CO2separation applications require the reliable production of a thin selective layer of 0.1–10 μm at a scale of 1000–1000000 m2membrane area.At such a scale,polymers are the preferable membrane materials and nearly all commercial membranes are made from polymers with good scalability and low cost.The membrane formation ability of the emerging polymers is discussed in this review.Perspectives and future research directions are provided to enable the use of polymeric membranes in commercial-scale gas separation processes.

It should be noted that another strategy to utilize the processability of polymers is to add inorganic materials to form nano-composites known as mixed-matrix membranes(MMMs).Due to the specific pore size and geometry of the inorganic materials,well-constructed MMMs are capable of overcoming the upper bound while simultaneously possessing good processability.While not the emphasis of this review,considerable progresses have been achieved by MMMs with new sieving materials such as metal–organic frameworks(MOFs).Comprehensive reviews on MMMs can be found elsewhere[14,15].

2.Gas Transport in Polymeric Membranes

Membrane separation relies on the membrane material's ability to control the permeation of different species.Driven by the partial pressure differential,the penetrant firstly dissolves in the membrane,diffuses down the chemical potential gradient,and desorbs to the downstream side.This well-accepted explanation of gas permeation is referred as the solution-diffusion transport mechanism,in which a separation is achieved by the difference in the amount of the penetrant that dissolves in the polymer and/or the diffusion rate of the penetrant through the polymer.Because of its unique chemical structure,CO2can reversibly react with a Brønsted base,e.g.,KOH,or a Lewis base,e.g.,amine.These reversible reactions can be implemented in the membrane as a reactive diffusion pathway for CO2permeation.Therefore,a thin polymer phase with reactive sites is typically termed as a facilitated transport membrane.Due to the different physical and chemical aspects,these two gas transport mechanisms are discussed in this section as a general understanding of the vast polymeric membrane materials in the literature.

2.1.Solution-diffusion transport mechanism

In the absence of reactive moieties,the light gas molecules firstly dissolve in the dense polymer phase,diffuse across the membrane,then desorb into the low pressure downstream.The kinetic factor of this process is characterized as the permeability coefficient(Pi),which can be written as the product of the solubility coefficient(Si)and the diffusion coefficient(Di)[16]

The most commonly used unit for Piis Barrer,where 1 Barrer=1×10-10cm3(STP)·cm·cm-2·s-1·(cm Hg)-1.A more directly measurable property of a membrane is its permeance,which represents the flux of a penetrant per unit permeation driving force.Traditionally,the permeance is expressed in Gas Permeation Unit(GPU)(1 GPU=1×10-6cm3(STP)·cm-2·s-1·(cm Hg)-1).Therefore,the permeability can be calculated as the permeance divided by the thickness of a dense and uniform membrane.In the case of asymmetric or thin-film composite membranes,the permeability is barely reported due to the difficulty of defining an effective membrane thickness.In other words,the permeability is an intrinsic property of the membrane(selective layer)material,while the permeance is a practical measure of the separation performance of a membrane in a given configuration.

The separation capability of the membrane is characterized by the ideal selectivity αij,which is the ratio of two gas permeabilities.By Eq.(1),it can be further expressed by a solubility selectivity(Si/Sj)and a diffusivity selectivity(Di/Dj)

Therefore,the separation can be achieved by the difference in solubility and/or diffusivity.The solubility of light gases in polymer is largely determined by its condensability and its affinity to the membrane material.The penetrant condensability increases with increasing critical temperature[17].The diffusion of a gas molecule in polymer arises from the free volume characteristics of the polymer.The chain-tochain spacing and the chain segment random motion allow molecules with small kinetic diameter to diffuse through[13].In typical polymers,the penetrant diffusivity increases with decreasing kinetic diameter,as well as increasing free volume.

Table 1 summarizes the physical properties of CO2,N2,H2,and CH4.CO2has a higher solubility than the other gases due to its higher critical temperature.Consequently,the solubility selectivity favors CO2.CO2also has a smaller kinetic diameter than N2and CH4,resulting in a favorable diffusivity selectivity.These two factors render a CO2-selective feature for most CO2/N2and CO2/CH4separation membranes.For CO2/H2,however,the diffusivity selectivity favors H2,a much smaller molecule.Therefore,the final CO2/H2selectivity depends on the polymer design.CO2-selective membranes,e.g.,Polaris®from MTR,typically contain CO2-philic moieties to enhance the solubility selectivity, while H2-selective membranes,i.e.,polybenzimidazole(PBI)-based membranes,utilize glassy polymers with size-sieving capability[18].

Table 1 Physical properties of CO2,N2,H2,and CH4[19]

2.2.Facilitated transport mechanism

In facilitated transport membranes,mass transfer can be enhanced by the reversible reaction between the penetrant and reactive carriers[20].Similar to the solution–diffusion transport mechanism,the penetrant first dissolves in the polymer matrix on the upstream side,where it reacts with a carrier to form a reaction product.Given certain mobility,this reaction product transports down its concentration gradient.At the downstream side,the low partial pressure of the penetrant drives the reverse reaction of the reaction product,by which the penetrant is released and the carrier is regenerated.Adding onto the normal Fickian diffusion,the permeation rate of the reactive penetrant is greatly enhanced.On the other hand,penetrants that are inert to the carrier are largely rejected due to the lack of the reactive diffusion pathway.In CO2separation processes,CO2is usually chosen to be selectively removed because the acidic nature of CO2enables it capable of fast reaction kinetics with basic carriers[21].

Fig.1.CO2 reaction with primary and secondary amines via the zwitterion mechanism.

Even though quite many inorganic and organic bases have been used as carriers for CO2facilitated transport,most successes are achieved by exploiting the rich amine-CO2chemistry.Therefore,the reaction mechanisms between CO2and amines are briefly discussed in this section.The reactivity of CO2derives from the high electron deficiency of the carbon bonded to the two highly electronegative oxygens.For primary and secondary amines with a lone electron pair on the nitrogen atom,the amine functions as a nucleophile,i.e.,a Lewis base,which attacks the electrophile carbonyl group on CO2to form a zwitterion.The zwitterion is rapidly deprotonated by another amine to form a more stable carbamate ion,which leads to 2 mol of amine for 1 mol of CO2(Reaction(1)in Fig.1).At a lower pH,water can hydrate the carbamate species to produce bicarbonate and release a free amine(Reaction(2)in Fig.1)[22].The hydration reaction is considerably slow due to the stable structure of the carbamate.Unlike primary and secondary amines,a tertiary amine can only act as a Brønsted base,which neutralizes the carbonic acid formed by CO2in the presence of water(Fig.2).This reaction involves 1 mol of amine for 1 mol of CO2.This reaction,however,is known to be slow due to the slow formation of the carbonic acid[23].

Fig.2.CO2 reaction with tertiary amine via acid–base reaction.

Fig.3 depicts the transport of CO2vs.other inert light gases through an amine-containing facilitated transport membrane[24].In this case,the amino groups are bound to the polymer backbone,and the CO2molecules hop among neighboring amino groups[24].This hopping mechanism is an intramolecular diffusion in nature,which is also driven by the chemical potential gradient arising from the concentration gradient of carbamate ion[25].The transport of non-reactive gases is governed by the solution–diffusion mechanism through a dense polymer matrix.Therefore,a high CO2/gas(N2,H2or CH4)selectivity can be achieved.

Fig.3.Schematic diagram of gas transports through an amine-containing facilitated transport membrane.

3.Solution-Diffusion Polymeric Membranes

In this section,the recent advances are discussed for polymers that are capable of selective CO2separation based on the solution-diffusion mechanism.Five classes of polymers are covered:poly(ethylene oxide)(PEO)-based polymers, perfluoropolymers, polymers of intrinsic microporosity (PIMs), thermally rearranged (TR) polymers, and iptycene-containing polymers.These classes are ordered based on when they were discovered or applied in gas separation.This sequence happens to coincide with the order of polymer chain stiffness,from the rubbery PEO to the glassy TR polymers.Strictly speaking,the iptycenecontaining polymers are not a stand-alone category.Rather,they are derived/modified polymers via the incorporation of iptycenecontaining moieties.The iptycene-containing polymers are discussed here individually,due to the uprising research efforts in this area.

3.1.PEO-based membranes

Even though CO2molecule is non-polar as a whole,the uneven distribution of charges inside grants the molecule a quadrupole moment[26].The polar ether linkage(--C--C--O--)in poly(ethylene oxide)(PEO)is known to possess a high affinity to CO2via the quadrupole–quadrupole interaction[27].Therefore,PEO-based polymers show a considerable CO2solubility,and the CO2selectivity mainly stems from the solubility selectivity.One drawback,however,is that the polar ether groups tend to form strong hydrogen bonding,which induces compact chain packing.A high degree of crystallinity has been reported in pure PEO or PEO-based materials[28].The crystalline region limits the diffusion of CO2and eventually hampers the membrane permeability.The high crystallinity also leads to a weak mechanical strength,thus a poor film forming ability of PEO.To overcome these limitations,various approaches have been devised,including(1)block copolymerization with other hard segments,(2)blending with low MW poly(ethylene glycol)(PEG)and PEG-derivatives,and(3)crosslinking to form highly branched PEO polymer network.These concerted efforts not only produced considerably permeable PEO-based membrane materials,but also led to a deep understanding of the nanostructures of the PEO-based polymers.Table 2 summarizes the CO2permeabilities and CO2/gas selectivities of the PEO-based polymers synthesized based on different strategies.The CO2permeances of a few membranes in thin-film composite configuration are also listed.

The most well-established approach to refrain the high crystallinity of PEO is to block copolymerize PEO with hard segments.The gas permeation properties of these copolymers are controlled by tuning the lengths of the soft and hard segments,as well as adjusting the fraction of the PEO phase in the copolymer.The first few groundbreaking copolymers were under the commercial names,Pebax®and Polyactive™,in which various polyamides(PA)and poly(butylene terephthalate)(PBT)served as thehard segments,respectively[19,29].These PEO block copolymers typically showed a CO2permeability of 100–200 Barrers with a CO2/N2selectivity around 50 at 25°C.Recently,several novel hard segments were incorporated with PEO to synthesize ultra-permeable PEO-based copolymers,e.g.,monodisperse tetra-amide(T6T6T)[31]and pentiptycenebased polyimide(pent-PI)[32].The self-assembly feature of the PEObased block copolymer was also intensively studied.Yave et al.observed a polymer chain rearrangement when an ultrathin selective layer was coated onto a hydrophobic PDMS surface,leading to a high CO2permeance of 1815 GPU with a CO2/N2selectivity of 50 at 30°C[34].A PEO–polystyrene(PS)block copolymer was synthesized by Xue et al.[35].The copolymer self-assembled to form cylindrical PEO domains,which rendered an impressively high CO2permeance of 20400 GPU with a CO2/N2selectivity of 27.7 at 70°C[35].

Table 2 Transport properties of selected PEO-based polymers

Blending is another strategy to further increase the ether content in the polymer matrix.Short chain PEG,with a MW in the range of 100–2000,was fixed in PEO-based copolymer through polymer chain entanglement,which not only enhanced the CO2solubility,but also interrupted the compact packing of the PEO segment in the copolymer.The end groups on PEG also provided another control to mitigate the strong hydrogen bonding among the ether groups.For this reason,PEG moieties with terminal groups,such as methyl ether,ally ether,divinyl ether,and butyl ether,were studied[37].Ultrathin composite membranes were also synthesized by this approach.Chen et al.reported a Pebax®-based membrane with poly(ethylene glycol)dimethyl ether(PEGDME-500)coated on deposited zeolite-Y nanoparticles[39].The membrane demonstrated a CO2permeance of 940 GPU with a surprisingly high CO2/N2selectivity of 30 at an elevated temperature of 57°C.

The crosslinking method refers to the bottom-up synthesis of highly branched PEO or PEO-based copolymers through the polymerization of ethylene oxide monomer or oligomer.The initial work in this area was based on various methacrylate monomers,where the crosslinking reduced the crystallinity as well as improved the film forming ability[41].Another interesting building block for the construction of PEO network is polyether terminated with amino groups.Jeffamine®,a commercially available polyether diamine,can react with multifunctional acyl chloride[48]or epoxide[47]through polycondensation reactions.A thin-film-composite membrane was synthesized by Li et al.via interfacial polymerization,which showed a CO2permeance of 1310 GPU with a CO2/N2selectivity of 33 at 22°C[49].Recently,Kline et al.demonstrated that the crosslinking density and inhomogeneity could be tuned through the platform of polyether diamine and poly(ethylene glycol)diglycidyl ether[47].Unimodal,bimodal and clustered PEO networks were synthesized(Fig.4),and the inhomogeneous crosslinking improved the CO2permeability.

A special remark is needed for the membrane formation ability of PEO-based polymers.The rubbery nature of PEO bestows it a considerable solubility in most common organic solvents.These polymers can be readily coated on nanoporous substrate to form a sub-micron,defect-free selective layer.Significant advances have been achieved by Polaris®membranes from MTR to push the large-scale application in post-and pre-combustion carbon capture[50,51].As one of the very few polymer materials that a high CO2permeance has been successfully demonstrated in thin-film-composite membranes under relevant testing conditions,PEO-based polymers have potential for commercialization.In addition,the several PEO-based materials that have shown viable selectivities at an operating temperature higher than 35°C are of special interest[35,39].The elevated temperature mitigates the intensive cooling associated with the membrane processes.

3.2.Perfluoropolymers

Fig.4.Schematics of unimodal,bimodal,and clustered crosslinked PEO networks.(Adapted from Ref.[47]with permission from Elsevier.)

Perfluoropolymers are a class of glassy hydrocarbon polymers with most or all of its hydrogen atoms replaced by those of fluorine.Bestowed by the strong C--C and C--F covalent bonds,perfluoropolymers are resistant to most chemicals,which makes them ideal for applications that are subjected to hostile conditions[52].The development of perfluoropolymers in gas separation,however,was largely hampered by their semi-crystalline nature and poor solvent processability.No gas permeation data were obtained until the introduction of several amorphous perfluoropolymers under the trade name Teflon™AF,Hyflon™AD,and Cytop™in the mid-1980s.The chemical structures of these commercially available perfluoropolymers are shown in Fig.5.They are either copolymers of tetrafluoroethylene and perfluorodioxole or cyclic homopolymers.These amorphous glasses are known for their high gas permeability due to the pre-existing microchannels[53].Actually,the permeation data for these polymers defined the high permeability end of the 2008 Robeson upper bound for a number of important gas pairs[13].The permselective properties of these commercial perfluoropolymers are listed in Table 3.Ranging from the most permeable Teflon™AF2400 to the least permeable Cytop™,these glassy polymers generally show a H2selectivity over CO2and a CO2selectivity over CH4.Stemmed from the overall low sorption for hydrocarbons,the low CH4permeability is of special interest since membranes can be tailored for separating He,H2,CO2,and N2from CH4in natural gas sweetening[54].More importantly,the development of special fluorocarbon solvents,e.g.,Vertrel®XF and Novec™7200,has granted these polymers moderate solubilities,and thin-film-composite membranes have been synthesized.For instance,a Teflon™AF2400 composite membrane was reported by Fang et al.with a CO2permeance of 13000 GPU and a CO2/CH4selectivity of 5.7[55].The solvent processability allows the perfluoropolymers to be fabricated into a variety of composite membrane configurations.However,the cost and the toxicity of the specialty fluorocarbon solvents must be taken into consideration.

Noticeably, these highly permeable commercially available perfluoropolymers typically suffer from a moderate to low selectivity.Therefore,research efforts have been concerted to develop new fluorinated monomers for tunable polymer properties.Currently,most of the promising candidates fall into the category of perfluorodioxolane monomers,e.g.,perfluoro-(2-methylene-4-methyl-1,3-dioxolane)(PFMMD)and perfluoro-(2-methylene-1,3-dioxolane)(PFMD).Their homopolymers were synthesized,and improved selectivities were reported[59].Interesting gas permeation results were also obtained for the copolymer of PFMMD and PFMD,where a high CO2permeance of 403 GPU with a considerable CO2/CH4selectivity of 55 was reported at 22°C[60].Along with its potential chemical resistance,poly(PFMMD-co-PFMD)can be a well suited membrane material for removing CO2from natural gas.Further studies are needed regarding the high pressure plasticization,physical aging,and separation performance in the presence of hydrocarbon vapors.These issues have been somewhat investigated for the commercially available perfluoropolymers[61–64].These studies suggested relatively slow physical aging of the perfluoropolymers.

3.3.Polymers of intrinsic microporosity(PIMs)

Fig.5.Structures of perfluoropolymers.The detailed grades of TeflonTMAF and HyflonTMAD depend on the ratio of tetrafluoroethylene to the copolymerized dioxole unit.

Table 3 Transport properties of selected perfluoropolymers

PIMs,firstly reported by Budd and McKeown[65],refer to a class of glassy polymers with rigid and contorted macromolecular backbone structure.Originated from the sites of contortion or spiro centers,the restricted chain rotation of the component macromolecules induces poor molecular packing,resulting in interconnected pores of less than 2 nm[66].The intrinsic microporosity of this class of polymers results in a＞20% fractional free volume (FFV), thereby an ultrahigh gas permeability regardless of the processing conditions[67].Unlike most other porous organic polymers,PIMs are solution-processable[65].These features quickly catalyzed intensive research efforts to synthesize different PIMs to improve the gas permeability and selectivity.In this section, the newly developed PIMs are reviewed, including spirobiindane(SBI)-based PIMs,Tröger's base(TB)-based PIMs,polyimide(PI)-based PIMs,and some other variations.The efforts to address the fast physical aging by crosslinking are also briefly discussed.Transport properties of some representative examples are listed in Table 4.

The kinks in PIMs polymer backbone were firstly realized by introducing the SBI moiety with large pendant groups.SBI refers to a molecule with two indanes connected by a spiro carbon center.The SBI moiety is typically polymerized with a halogen-containing aromatic monomer,which leads to the first few PIMs that have been made into gas separation membranes,e.g.,PIM-1 and PIM-7[68,69].The gas permeation in PIMs obeys the solution-diffusion mechanism.The higher condensability and smaller size of CO2yield CO2/N2and CO2/CH4selectivities in the range of 10–20.The CO2/H2selectivity,however,is at most 1–2 since the size sieving feature favors the diffusion of H2.The SBI center could be replaced with a spirofluorene(SBF)unit,and the resultant PIM-SBF exhibited less chain flexibility,yielding a very high CO2permeability of 13900 Barrers with a CO2/CH4selectivity of 12.6[70].Recently,bulky tetramethyltetrahydronaphthalene(TMN)units were fused to the SBI units in PIM-1.The further reduced conformational flexibility led to a CO2permeability of 17500 Barrers with a CO2/CH4selectivity of 8.3[71].

Unlike the dibenzodioxane-forming reaction utilized by the SBIbased PIMs,chemistry based on Tröger's base(TB)has been undertaken to incorporate this rigid bicyclic unit into PIMs.In this reaction scheme,diamino aromatic polymers with bicyclic rings,e.g.,ethanoanthracene(EA)and triptycene(Trip),are fused with or without SBI centers to form a highly contorted network.In PIM-EA-TB,a TB-based PIM without the spiro centers,the chain rigidity was realized by the combined kinks from the fused rings[72].In addition,the TB group provides additional Langmuir affinity toward CO2,which is beneficial for improving the CO2selectivity[86].Aiming for an even higher free volume,non-planar iptycene units have been used to replace EA to further disrupt the chain packing.A triptycene-based PIM with TB units was synthesized by Carta et al.,demonstrating a very high CO2permeability of 9709 Barrers with a CO2/CH4selectivity of 10.7[73].

The formation of imide via the polycondensation of anhydrides and diamines has also been employed in the synthesis of PIMs,termed as PI-based PIMs.The contortion sites can be within either the dianhydride[74]or the diamine[75].It should be noted that the imide linkage does not contain any fused ring structure to refrain the chain rotation.For this reason,aromatic diamine monomers are typically used in the PI-based PIM synthesis to inhibit the conformational rearrangement[87].The relatively flexible imide linkage also improves the polymer solubility in organic solvents,by which the established membrane formation methods may apply to this type of PIMs.

Aside from exploiting different chemistry for PIM synthesis,it is worth mentioning that several improved PIMs have been reported byincorporating bulky side groups containing aromatic rings.For instance,the hexaphenylbenzene(HPB)unit is proven to interrupt the chain packing in PIMs,and the enhanced rigidity mitigates the physical aging[77].A record-breaking CO2permeability of 333,000 Barrers with a CO2/CH4selectivity of 9.7 was reported by Carta et al.by combining bulky TMN and Trip into PIM[71].These PIMs have shown CO2/N2separation performances surpassing the 2008 Robeson upper bound.

Table 4 Transport properties of selected PIMs

Like other glassy polymers with high free volume,PIMs are plagued by physical aging,where the relaxation of nonequilibrium chain conformations results in the loss of permeability over time[88].One heavily studied approach is crosslinking to create a more robust polymer network.Thermal,UV,and chemical crosslinking methods have been reported for PIM-1.Song et al.reported a thermal oxidative crosslinking of PIM-1 in the presence of trace amount of oxygen.Stabilized permeation properties was observed over a period of one year[78].A UV-induced crosslinking was also reported by the same research group to enhance the gas selectivities[79].Several crosslinkers have also been used,such as polyethylene glycol biazide[89]and pyrene[80].McDonald et al.reported a PIM-1 membrane crosslinked by 1-aminopyrene,where the pore size decreased over time and the CO2sorption was enhanced[80].

Fig.6.Schematic of PIM-1/POSS–PEG.

Despite the intensive research of PIMs on the aspect of material synthesis,studies of membrane formation are still in the early stage.On the one hand,the CO2/gas(N2and CH4)selectivities of most PIMs are lower than those offered by PEO-based membranes.Whether these highly permeable but only mediocrely selective materials suit the actual CO2/N2or CO2/CH4separation is debatable.On the other hand,the complicated synthesis route of PIMs adds additional cost onto membrane fabrication.A high membrane cost hampers applications in energy-related industries where the luxury to produce highvalue chemicals does not exist.Driven by these reasons,other polymeric materials have been blended with PIMs to increase the gas selectivity as well as reduce the fabrication cost.Chung's group has dispersed PIM-1 or carboxylate PIM-1 in Ultem[82]and Matrimid®[81]as highly permeable nanofillers.Contrarily,PIM-1 has been used as the continuous phase by Yang et al.to bear nano-sized poly(ethylene glycol)functionalized polyhedral oligomeric silsesquioxane(POSS–PEG)[83].Both approaches led to an increased CO2/N2or CO2/CH4selectivity,which was however compromised by a proportionally reduced CO2permeability.Interestingly,a high CO2/CH4selectivity of 30 was reported for PIM-1/POSS–PEG with 10%of filler loading,which was significantly higher than the other PIM-based materials.The schematic of this membrane is shown in Fig.6.Blending can also retard the physical aging of PIMs[88].Hypercrosslinked polystyrene(HCP)was dispersed in PIM-1 as a filler phase[84].The addition of HCP enhanced the CO2permeability to 19086 Barrers as well as slowed down the physical aging.

In terms of membrane formation,reports of PIM membranes in common thin-film-composite or hollow-fiber(HF)configuration are very limited.Recently,Jue et al.reported a defect-free HF asymmetric PIM-1 membrane prepared by phase inversion[85].A skin-layer thickness of 3–6 μm was produced,and a CO2permeance of 360 GPU with a CO2/CH4selectivity of 22.5 was obtained(Fig.7).Structural designs that are amenable to large-scale applications are needed to capture PIM's potential as a high performance membrane material.

Fig.7.SEM images of a defect-free PIM-1 hollow fiber(a)cross section,(b)asymmetric substructure,(c)internal boundary,and(d)skin layer.(Adapted from Ref.[85]with permission from Elsevier.)

3.4.Thermally rearranged(TR)polymers

In comparison with the three-dimensional contorted chains of PIMs,more rigid and planer macromolecules can be formed by the thermally rearrangement of ortho-functionalized polyimides(PIs)or polyamide(PAs).First reported by Park et al.,this class of macromolecules is referred as thermally rearranged(TR)polymers[90].The microporous nature arises from the high torsional energy barrier against rotation between the phenylene-heterocyclic rings.Therefore,the TR polymers are typically featured with unprecedented polymer chain rigidity and a narrow cavity size distribution.As shown in Fig.8,the major precursors for the TR conversion are ortho-functionalized PIs or PAs.Upon a high temperature above 350°C,an intramolecular cyclization is initiated,and a rigid polymer,e.g.,polybenzoxazole(PBO),polybenzimidazole(PBI)or polypyrrolone(PPL),is formed if the ortho-functional group is hydroxyl or amino[91].The TR polymers derived from the polycondensation of dianhydride and ortho-functional diamine are termed as TR-α,while the ones from the polycondensation of hydroxy-diamine and diacid chloride are referred as TR-β.

Table 5 summarizes the transport properties of several selected TR polymers.The flexible hydroxyl group on the PA-precursor as well as the less bulky diacid chloride moiety renders TR-β a ca.100°C lower TR conversion temperature than TR-α.More importantly,the smaller cavity size also causes a lower permeability,as shown in Table 5 for the TR-β-PBO[90,92].Regardless,the CO2permeability can be tuned spanning 3 orders of magnitude.Specially,the CO2/CH4selectivity can be as high as 40–50 with decent CO2permeability,projecting a promising application of TR polymers in natural gas sweetening.A recent research focus is to use precursors with an even higher free volume.One example is shown in Fig.9 for the bulky 2,2-bis(3-amino-4-hydroxyphenyl)adamantane(ADHAB)monomer synthesized by Aguilar-Lugo et al.[93].The bulky adamantane group was proven to be thermally stable at the TR conversion temperature and led to CO2permeability and CO2/CH4selectivity exceeding the 2008 Robeson upper bound.

Even though the high polymer chain rigidity grants TR polymers less tendency for physical aging,plasticization is still a pressing issue,especially for high pressure gas separation applications,such as natural gas sweetening.The widely investigated approach is the intermolecular crosslinking during thermal conversion.The primary crosslinking method is to incorporate 3,5-diaminobenzoic acid in the ortho-hydroxy PI precursors,followed by a mono-esterification reaction with diol[97].The rigid linkages created by the crosslinking reaction can also increase the cavity size and free volume,which benefit the CO2permeability and selectivity[98].

Fig.8.Thermal rearrangement of(1)TR-α,where the precursor is an ortho-functional polyimide(PI)and(2)TR-β,where the precursor is an ortho-functional polyamide(PA).

Table 5 Transport properties of selected TR polymers

Fig.9.Structure of 6FDA+bisAPAF with bulky ADHAB[93].

Copolymerization of TR polymers with other functional segments that are not capable of the cyclic rearrangement is another competitive approach to tune the chain rigidity.Scholes et al.copolymerized a series of TR-α-PBI with 2,3,5,6-tetramethyl-1,4-phenylenediamine(4MPD)or 9,9′-bis(4-aminophenyl)fluorene(FDA),which were monomers that could not undergo the thermal rearrangement[100].Different TR polymers can also be copolymerized as an attempt to combine the different transport features.Choi et al.synthesized a TR-PBO-co-PPL copolymer,which could be tuned to be highly CO2permeable by increasing the PBO content,or to be highly CO2selective vs.CH4by increasing the PPL content[101].

The cavity size and free volume of TR polymers can also be modified by grafting thermally labile moieties or introducing the spirobiindane center into the polymer backbones.Chung's group substituted the carboxylic acid group on the PI precursors with β-cyclodextrin(β-CD)or β-CD-ferrocene. The cyclodextrin was thermally labile and decomposed at 300–450°C,which left micropores in the PI during the TR conversion[102,103].This process is depicted in Fig.10.Improved plasticization resistance and good mechanical strength were observed for this modified TR-α-PBO.Polymers with both TR and PIM segments were synthesized by Li et al.[104].The main merit,however,was the improved mechanical properties compared to the pristine PIMs.

Due to the high temperature required by the TR conversion,the TR polymer membranes are typically synthesized by non-solvent induced phase inversion.Several asymmetric TR-β-PBO hollow-fiber membranes have been reported in the literature with a skin-layer thickness as thin as 200 nm[105,106].A CO2permeance as high as 2500 GPU was reported with a CO2/CH4selectivity of 22.Research efforts to reduce the thermal treatment temperature have also been reported.Guo et al.employed a 6FDA-2,2′-bis-[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride precursor,which reduced the TR temperature by ca.100°C.A lower TR temperature improves the processability during the membrane formation[95].Overall,as the glassy polymers with the highest chain rigidity,TR polymers are promising for high pressure,high temperature CO2separation applications.However,the high TR temperature and the associated membrane brittleness limit their scalability.Further research efforts on membrane formation are required for practical gas separations.

3.5.Iptycene-containing polymers

Two recently emerging building blocks for shape persisting macromolecular design are triptycene(Trip)and pentiptycene(Pent).These two molecules belong to the iptycene family,which is a class of three-dimensional molecules with arene units fused to the[2,2,2]bicyclooctatriene bridgehead system[107].What makes iptycenes unique is that the clefts of the benzene“blades”form an internal free volume,which is comparable to the kinetic diameters of the light gases(Table 1).Due to its shape persisting nature,the internal free volume is not susceptible to collapse,thereby a less tendency for physical aging[108].When incorporated into other polymer systems,the bulky iptycenes also disrupt the polymer chain packing and increase the overall free volume.A few iptycene-containing PEO and PIMs have been discussed in the previous sections[32,71].In this section,polymers with iptycene units are reviewed in a more systematic manner.Table 6 summarizes the representative examples.

As suggested by Weidman et al.,these polymers can be grouped into three categories based on the backbone architecture:nonladder,semiladder and ladder polymers [117]. The iptycene-containing nonladder polymers are mainly polyimides with Trip or Pent built in the diamine and/or dianhydride monomers.One example is given by Cho et al.,where 6FDA was polymerized with 2,6-diaminotriptycene(DATRI)through a polycondensation reaction[109].The fractional free volume increased to 0.226 from the 0.16–0.18 of 6FDA-based polyimides,and a CO2permeability of 189 Barrers with a CO2/CH4selectivity of 30.5 was reported.Other iptycene-based diamines were also used,including DAT2[88]and pentiptycene diamine(PPDA)[110].The 6FDA can also be replaced with an iptycene-containing dihydride,e.g.,TPDAn,to synthesize polyimide fully based on the iptycene structures[111].For this membrane,an interesting CO2/CH4selectivity of 52 was reported.The highest CO2/CH4selectivity,however,was registered by an iptycene-containing TR-PI,which demonstrated a selectivity as high as 67 with a CO2permeability of 270 Barrers.Due to the relatively“soft”imide linkage,these nonladder polymers feature a better CO2selectivity than CO2permeability,although the permeability is at least one-order-of-magnitude higher than the traditional polyimides.

Most of the semiladder iptycene-contained polymers are synthesized by incorporating iptycene units into certain PIM-polyimide copolymers.The fused rings of PIM partially replace the single-bond connections in the polymer backbone,hence these polymers are termed as semiladder.A series of Trip-based PIM-polyimides were synthesized at KAUST,e.g.,KAUST-PI-1 and KAUST-PI-2,which showed high CO2permeabilities above 2000 Barrers with CO2vs.N2or CH4selectivities ca.20.The polymer network can be further tightened up by the introduction of hydroxyl groups,which provides additional chain-to-chain interaction via hydrogen bonding.This modification method led to the synthesis of more selective PIM-polyimides,which showed a CO2/CH4selectivity as high as 96[114].One semiladder polymer without the PIM structure was reported by Mao and Zhang,where the iptycene structure was built in the multi-amine monomer while the dianhydride monomer contained fused rings to stiffen the final polyimide backbone[115].

Lastly,ladder polymers were synthesized based on Tröger's base,a V-shape diamine described in Section 3.3[116].The introduction of iptycene structure increased the CO2/CH4selectivity;the π–π interaction between the benzene rings on iptycene moderately reduced the free volume.

4.Facilitated Transport Membranes

In this section,the recent advances are discussed for polymers that are capable of selective CO2separation based on the facilitated transport mechanism.The most extensively studied amine carriers are discussed in Section 4.1.These amine-containing polymeric materials are further categorized based on whether amino groups are covalently bonded to the polymer backbone or freely dispersed in the polymer matrix.Other carriers,including various Brønsted bases,ionic liquids,and mimic enzymes,are discussed in Section 4.2.

4.1.Amine-containing membranes

Amines are the most exploited carriers for reactive CO2transport for three reasons.Firstly,with their importance in organic synthesis,amines encompass a rich structural variety[118].The diverse synthesis routes provide certain easiness to impregnate amino groups into polymeric membranes.Secondly,the amine–CO2reactions are easily tunable to feature either fast reaction kinetics or high CO2loading capacity as briefed in Section 2.2.The balance of these two characteristics determines the overall facilitation effect.Lastly,the abundant experience in aqueous amine absorption for acid gas removal is transferable to the membrane synthesis.Polymeric facilitated transport membranes bear amines in two forms.The amino groups can be covalently bonded to the polymer backbone,which serve as the fixed-site carriers.The CO2molecules hop among neighboring amino groups,and the statistically more preferable hopping down the chemical potential gradient yields the facilitated CO2transport[24,25].Alternatively,the amino groups can be incorporated in the polymer matrix via blending small molecule amines.These amines not only densify the amino group content in the membrane,but also can serve as mobile carriers for CO2transport,provided that the amine molecule size and the free volume of the polymer allow for certain carrier mobility.Table 7 categorizes the transport properties of selected amine-containing facilitated transport membranes based on the carrier types.

As for the fixed-site carriers,the transport properties of several polymers bearing amino groups have been studied,including polyvinylamine(PVAm)[119],poly(allylamine)(PAA)[24],poly(ethyleneimine)(PEI)[122],and chitosan[124],with a reducing density of primary amine groups.Limited by their low MW or high crystallinity,PAA,PEI,and chitosan are typically blended with other polymers.In this case,pristine or crosslinked poly(vinyl alcohol)(PVA)is the most common choice as the polymer matrix[24,122].PVA not only enables the polymer mixture a suitable film forming ability.The hydrophilic nature of PVA also enhances the water retention ability,which eventually benefits the amine–CO2reaction[142,143].Alternatively,PEO-based block copolymers have also been reported as a suitable polymer matrix.A blend of chitosan and Pebax®MH 1657 was reported by Liu et al.,which demonstrated a high CO2permeability of 2884 Barrers with a CO2/CH4selectivity of 23.2 at 85°C[124].Unlike these polymers,PVAm has the highest amino group density.More importantly,the high MW PVAm can be easily synthesized via the polymerization of N-vinylformamide followed by an acid or base hydrolysis[144].Consequently,PVAm is typically used alone as the polymer matrix.A CO2permeability of 103 Barrers as well as an unprecedented CO2/CH4selectivity of 1143 was reported by Kim et al.[119].

These amine-containing polymers also provide a platform to carry out sophisticated amine chemistry.Zhao and Ho explored the steric hindrance effect in a solid state polymer system[24].Bulky alkyl substitutes were incorporated on PAA via a series of SN2 reactions.A membrane containing the moderately hindered poly-N-isopropylallylamine(PAA-C3H7)increased the CO2permeability to 297 Barrers from the 55 Barrers of the unmodified PAA membrane.A high CO2/H2selectivity of 40 as well as a CO2/N2selectivity of 341 was also achieved due to the enhanced CO2transport(Fig.11).When the amine is sterically hindered,the carbamate ion formed by the nucleophile reaction with CO2is unstable due to the bulky alkyl group attached(Reaction(1)in Fig.1).In this case,the carbamate ion can be easily hydrolyzed in the presence of water to form bicarbonate and regenerate a free amine(Reaction(2)in Fig.1).Overall,1 mol of CO2reacts with 1 mol of hindered amine;therefore,the CO2loading capacity is doubled compared to that of the unhindered amine.However,if the steric hindrance is too severe,the bulky alkyl group shields the nitrogen on the amino group from CO2,thereby the reaction rate reduces.This tradeoff wasalso demonstrated by Zhao et al.for poly-N-tert-butylallylamine(PAA-C(CH3)3),which exhibited a reduced CO2permeability of 119 Barrers.A similar modification was conducted by Tong and Ho on PVAm,where poly(N-methyl-N-vinylamine)(PVAm-CH3)showed a CO2permeability of 264 Barrers at 57°C[121].The permeation properties of this moderately hindered PVAm were affected by the temperature significantly.By increasing the temperature to 102°C,the CO2permeability increased to 6804 Barrers with an impressive CO2/H2selectivity of 162.

Fig.11.CO2/H2 selectivity versus CO2 permeability for membranes containing 70 wt%polyamines and 30 wt%crosslinked PVA at 110°C and a feed pressure of 0.2 MPa.

The abovementioned polymers also possess excellent processability.Membrane formation is much less challenging than most of the glassy polymers reviewed in Section 3.In most cases,water is a good solvent for the amine-containing polymers,and an ultrathin selective layer,e.g.,100 nm,can be coated on proper substrate[127].A thin selective layer can also be synthesized by a bottom-up approach.Wang's group has reported an interfacial polymerization method to prepare highly crosslinked amine-containing polymers by reacting multi-amines with trimesoyl chloride(TMC)[128–130].A high CO2permeance of 1613 GPU with a CO2/N2selectivity of 138 was registered[128].

Blending with small molecule amines is another facile approach to further enhance the facilitation effect of amine-containing membranes.Three categories of small molecule amines are widely adopted:amino alcohols [136], multi-amines [131], and amino acid salts [138].Monoethanolamine(MEA)was incorporated in PVAm by Qiao et al.to achieve a CO2permeance of 2661 GPU with a CO2/H2selectivity of 63.Another series of blended membranes were reported by Ho's group by adding the potassium salt of 2-aminoisobutyric acid(AIBA-K)in sulfonated polybenzimidazole(SPBI)[131],PAA-C3H7[133],and PVAm[134],which demonstrated a CO2/H2selectivity as high as 300 at 110°C.These highly CO2-selective membranes are interesting materials for H2purification.For CO2/N2separation,piperazine(PZ)[137]and PZ-derivatives[139]have been proven as effective additives to PVAm membranes.The transport properties of these membranes largely depended on the detailed fabrication and testing conditions.However,a CO2permeance ＞1000 GPU and a CO2/N2selectivity ＞140 were generally reported.This has attracted increasing attention of applying these membranes in post-combustion carbon capture.

For the superior processability and CO2/N2selectivity,aminecontaining membranes are among the few membrane materials that are in the stage of pilot-scale study for post-combustion carbon capture.The key concern for practical application,however,is the long-term stability in the presence of contaminants,especially SO2.The more acidic SO2competes with CO2to react with amines,which might lead to membrane performance degradation.This issue has been intensively investigated.In general,a reduced CO2permeance was observed in the presence of ppm level SO2but not much for CO2/N2selectivity;at moderate temperatures,e.g.,57–67°C,the separation performance was stable[145,146].The PVAm-based membranes have been fabricated into spiral-wound(SW)and plate-and-frame(PF)membrane modules for actual flue gas testing.Researchers at The Ohio State University(OSU)have tested their SW module at the National Carbon Capture Center(NCCC),Wilsonville,Alabama,U.S.A.,with real flue gas,and a high CO2permeance of 800 GPU with a CO2/N2selectivity of 200 has been reported[140].The PF module developed by Hägg's group has been tested in the Nanoglowa EU project continuously for 6.5 months[141].No performance degradation was observed under high level of SOxand NOx.

4.2.Other carriers for facilitated transport membranes

This section discusses the CO2carriers beyond amines.As listed in Table 8,these carriers are grouped into three categories:(1)base,(2)ionic liquid(IL)gel or poly(ionic liquid)(PIL),and(3)mimic enzyme.

Aside from amines,several other bases have been explored for their facilitation effect on CO2transport.These carriers either serve as a Brønsted base to react with CO2,or catalyze the hydration reaction of CO2to form carbonate and bicarbonate.In terms of reaction kinetics,these basic carriers are incomparable to amines.However,they can be applied to several niche separation scenarios where the harsh conditions exclude the use of amines.The hydroxide OH-group has been reported as an efficient CO2carrier at high temperature by Vakharia et al.[147].Different forms of quaternary ammonium ion were chosen as the counterion due to their relatively good thermal and oxidative stabilities,which were introduced in the membrane by poly(diallyldimethylammonium hydroxide)(PDADMQ-OH)and tetramethylammonium hydroxide(TMAOH).The PDADMQ-OH was also replaced with poly(diallyldimethylammonium fluoride)(PDADMQ-F)in the same study,where the fluoride ion catalyzed the CO2hydration reaction.Overall,a CO2permeance of ＞50 GPU and a high CO2/H2selectivity of ＞100 were reported at 120°C in the presence of CO and O2.A similar quaternary ammonium-based polyelectrolyte was used by Li et al.to copolymerize with PVAm[151].However,CO32-was chosen as the anion to react with CO2as well as to reduce the copolymer crystallinity.Facilitation effect on CO2was also reported for carboxylate ion[152,153].These much weaker Brønsted bases were investigated mainly for their oxidative stability in the presence of O2.Lastly,organic super bases,e.g.,amidine[149]and guanidine[150],have also been employed as CO2carriers.A facile method to graft guanidine to polyaniline(PANI)membrane was demonstrated by Blinova and Svec[150].Provided sufficient hydration,the guanidine-modified PANI showed a high CO2permeability of 2460 Barrers with a CO2/CH4selectivity of 540,which is interesting for natural gas sweetening.

Room temperature ILs bearing CO2-reactive functional groups are another class of emerging materials for CO2separation.A combination of the high CO2solubility and the facilitated transport effect provides ILs not only high CO2selectivity,but also high CO2permeability in the membrane form[163].The various IL-based supported liquid membranes are out of the scope of this review.Here,only free-standing IL membranes are discussed.Two methods have been developed to anchor ILs in a free-standing membrane:(1)polymerize IL-containing monomers to form PIL and(2)embed IL into a compatible polymer matrix to form IL gel.Noble's group has used several bis(epoxide)-functionalized IL monomers to crosslink with tris(2-aminoethyl)amine(TAEA)[154,155,164].The tertiary amine linkages formed by the crosslinking facilitates the CO2transport,yielding a CO2permeability as high as 900 Barrers with a CO2/N2selectivity of 138 at room temperature.The carrier site can also be incorporated into the IL units,e.g.,tetrabutylphosphonium glycine([P4444][Gly]),serine([P4444][SER]),lysine([P4444][Lys]),and proline([P4444][Pro])[157].In this case,the IL carriers are typically embedded in a gel network to form an“elastomer-like”film at room temperature.One example is given by Moghadam et al.,where[P222(101)][Inda]was synthesized by neutralizing a phosphonium hydroxide by indazole[159].This IL was embedded in a polymer network formed by poly(methacryloylamino propyl trimethylammonium chloride)and poly(dimethylacrylamide)(PMAPTAC/PDMAAm).In the final ion-gel membrane,[P222(101)][Inda]served as the carrier,and a CO2permeability of 7569 Barrers was reported at 100°C.

As mentioned earlier,the fluoride ion is capable of catalyzing the hydration of CO2.A similar functionality is achieved by carbonic anhydrase(CA),a metalloenzyme in biological processes.The activity of CA derives from a Zn2+coordinate to three histidine groups,known as α-CA,or to two cysteine groups and a histidine group,known as β-CA[165].These active sites have been artificially constructed by complexing Zn2+with imidazole groups in poly(N-vinyl imidazole)(Fig.12(a))or impregnate a Zn atom into cyclen(Fig.12(b))[160–162].Depending on the hosting polymer,these mimic enzymes facilitated the CO2transport,and a high CO2/N2selectivity above 80 was reported.It is worth noting that the activities of the mimic enzymes are about an order of magnitude lower than those of the native enzymes.Further studies are needed in this area,e.g.,changing Zn2+to Cd2+to mimic γ-CA,an enzyme with an even lower activation energy for the CO2hydration reaction[166].

5.Conclusions and Outlook

The recent advances in polymeric membranes for CO2capture are reviewed in terms of the material design and membrane formation.Opportunities and challenges in practical applications are also discussed,including post-combustion carbon capture(CO2/N2),hydrogen purification(CO2/H2),and natural gas sweetening(CO2/CH4).To compare the various polymeric membrane materials,selected data from Tables 2–8 are plotted in Fig.13,along with the 2008 Robeson upper bound.A Robeson upper bound does not exist when plotting CO2/H2selectivity against CO2permeability.Inverting the H2/CO2Robeson upper bound gives the CO2/H2lower bound.Therefore,the theoretical upper bound for rubbery polymers predicted by Freeman is used[11,167].The final remarks and outlook are summarized below for these three gas pairs.

Table 8 Transport properties of selected facilitated transport membranes

Fig.12.Two types of mimic enzymes:(a)Zn2+–imidazole complex for α-CA;(b)Zn-cyclen for β-CA.

(1)The shape persisting glassy polymers define the highly permeable end of the CO2/N2upper bound.However,the low CO2partial pressure in the coal-derived flue gases requires a significant CO2/N2selectivity to achieve ＞95%CO2purity at 90%CO2capture.The abundant techno-economic analyses generally agree that a minimal selectivity of 50 is needed for membrane to compete with aqueous amine absorption[50,168,169].In this case,the PEO-based membranes and amine-based facilitated transport membranes provide sufficient permselectivity to allow for practical membrane process design.Another advantage of these two classes of polymers is their processability.A thin selective layer ca.100 nm has been demonstrated in flat-sheet and hollowfiber composite membranes,and suitable membrane modules in commercial configurations have also been fabricated.Their technical potential is also reflected in the pilot-scale field tests conducted in the U.S.and Europe.An on-going research focus is to further boost the CO2permeance exceeding 3000 GPU.The reproducible and cost-effective fabrication of ultrathin selective layer is crucial.In addition,an ultrathin and ultra-permeable selective layer calls for innovations in substrate synthesis[170].The nanoscale pore structure of the substrate affects the composite membrane separation performance.The Knudsen diffusion in the substrate and the pore restriction at the selective layersubstrate interface can account for a significant portion of the transport resistance[171,172].Novel isoporous substrate selfassembled by block copolymers might lead the research efforts in membrane formation.The application of facilitated transport membranes is not limited to decarbonizing the coal-derived flue gases.Their unprecedented CO2/N2selectivity is a necessity for carbon capture from gas sources with even lower CO2concentration,e.g.,natural gas-derived flue gas,coal mine gas,and residual flue gas after the primary carbon capture.

Fig.13.Transport properties of selected polymeric materials vs.2008 Robeson upper bound[13]or the theoretical upper bound for rubbery polymers in CO2/H2 separation[167].

(2)Once again,PEO-based membranes and facilitated transport membranes are the only feasible materials for selective CO2removal from hydrogen.The reviewed polymers with rigid chains are more suitable for H2-selective membrane synthesis,although they face the competition against several other glassy polymers featuring high H2/CO2selectivity,e.g.,polybenzimidazole and crosslinked polyimide[173].The required CO2/H2selectivity for pre-combustion carbon capture is not well-defined.However,the high CO2/H2selectivity demonstrated by the aminecontaining facilitated transport membranes is expected to be beneficial for enhancing the H2recovery upon a high degree of CO2removal,thus an overall higher energy efficiency.More importantly,these facilitated transport membranes have been reported to be stable at elevated temperatures ＞100°C in the presence of H2S[133].These characteristics eliminate the intensive cooling and pre-treatment in pre-combustion carbon capture,which are usually required for solution-diffusion membranes.One disadvantage of facilitated transport membrane is the deteriorated CO2permeability and CO2/H2selectivity at high CO2partial pressure due to the carrier saturation phenomenon[136].For a given CO2partial pressure,the carrier saturation can be mitigated by increasing the carrier content.However,the hosting polymer should be able to form a mechanically robust yet highly permeable network to accommodate the carrier molecules.The design principle of polymer network for IL-gel membranes might shed light on polymer synthesis to host other type of CO2-carriers.In addition,other CO2-philic moieties can be blended with the CO2-carriers to enhance the CO2solubility at high CO2partial pressure.For facilitated transport membranes,the membrane stability at high temperature and pressure needs to be demonstrated.Opportunities in several niche applications are also available,e.g.,H2purification and recycling from fuel cell discharge gas[174].Novel membrane design,however,is needed to accommodate for the different operating conditions and contaminants.

(3)TR polymers,perfluoropolymers,PIMs,and facilitated transport membranes cover a wide feasible range for CO2/CH4separation.TR polymers and perfluoropolymers are of special interest since their transport properties are well above the 2008 Robeson upper bound.The chemical resistance of perfluoropolymers is well suited for natural gas sweetening.One concern,however,is the membrane plasticization in the presence of water,which affects both TR polymers and perfluoropolymers.Further studies in polymer properties and innovative engineering are needed to realize the separation performances in practical applications.In addition,several modified polyimides have been reported recently with notable tolerance to CO2and H2O plasticization,where the polymer chain mobility was restricted by introducing bulky Trip linkages in the polymer backbone[109]or grafting bulky--CF3as pendent groups[175].Amine-and guanidinecontaining facilitated transport membranes can take advantage of the high water content in some crude natural gases.The likely high H2O permeance of these membranes can also dehydrate the natural gas upon CO2removal.Carrier saturation at high feed pressure is the main challenge for facilitated transport membranes in CO2/CH4separation.

Nomenclature

Didiffusion coefficient of gas component CO2

Djdiffusion coefficient of another gas component(N2,H2or CH4)

i gas component CO2

j another gas component(N2,H2or CH4)

Pipermeability of species i

Sisolubility coefficient of gas component CO2

Sjsolubility coefficient of another gas component(N2,H2or

CH4)

αijideal selectivity of species i over species j

Acknowledgments

We would like to thank José D.Figueroa of National Energy Technology Laboratory for providing helpful discussion and input.We would like to gratefully acknowledge the U.S.Department of Energy/National Energy Technology Laboratory(DE-FE0026919)and the Ohio Development Services Agency(OOE-CDO-D-13-05 and OER-CDO-D-15-09)for their financial support of this work.This work was partly supported by the U.S.Department of Energy under Award Number DE-FE0026919 with substantial involvement of the National Energy Technology Laboratory,Pittsburgh,PA,USA.