Recent advances on mixed matrix membranes for CO2 separation☆

Ming Wang,Zhi Wang*,Song Zhao,Jixiao Wang,Shichang Wang

Chemical Engineering Research Center,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300350,China Tianjin Key Laboratory of Membrane Science and Desalination Technology,Tianjin 300350,China

State Key Laboratory of Chemical Engineering,Tianjin 300350,China

Collaborative Innovation Center of Chemical Science and Engineering,Tianjin University,Tianjin 300350,China

1.Introduction

The energy-efficient and environmentally friendly CO2separation technology is increasingly necessary and has huge market in industrial application including CO2capture,CO2removal from flue gas,natural gas treatment and hydrogen purification[1-3].Membrane-based gas separation is considered as the candidate technology.However,polymer membranes are shown to suffer a permeability-selectivity trade-off limitation[4].Recently,mixed matrix membranes(MMMs)are developed to overcome the limitation[5-9].In general,MMMs are fabricated by using two or more different materials of distinct properties.One material(usually a polymer)forms a continuous phase,also known as matrix.Another material forms a dispersed phase,inorganic or organic,which is the so-called filler.The matrix and filler are immiscible and possess different transport properties.There are a larger number of scientific literatures on MMMs for CO2separation.

Permeability and selectivity are two important parameters to evaluate membrane performance.For the membranes without a support membrane or a support layer,permeability is officially called as permeability coefficient(P).It can be expressed as follows:

whereQiis the permeation rate of gasspeciesiat standard temperature and pressure(STP),lis thickness,ΔPiis the transmembrane partial pressure difference of gas species i,andAis the effective membrane area.The gas permeability coefficient is customarily expressed in the unit of mol·m·m-2·s-1·Pa-1.However,the permeance(R)is usually applied to assess the permeability for composite membranes and asymmetric membranes.It can be expressed as follows:

The gas permeance is customarily expressed in the unit of mol·m-2·s-1·Pa-1.

The selectivity reflects the capability of a membrane to separate one gas from gas mixture.The ideal selectivity(α?i/j) is given by the ratio of the two pure gas permeabilities shown in Eq.(3).

wherePi,Ri,PiandRjare the permeability coefficient and permeance of gas species i and j in the membrane,respectively.

For permeation of actuali-jmixtures,the mixed gas selectivity,also called as separation factor(αi′/j),is calculated by Eq.(4).

whereyiandyjare the molarfraction of gas speciesiandjin the permeate side,whilexiandxjare the molar fraction of gas speciesiandjin the feed side.

2.Material Selection for MMM

As typical MMMs,the polymer acts as a continuous phase and the filler acts as a dispersed phase.To develop high performance MMMs,correct selection of polymer and filler is very important.

2.1.Selection of polymer

In general,gas transport through polymer membranes follows the solution-diffusion mechanism.In such membranes,gas molecules first dissolve in the membranes at the interface of the feed side and the membrane,and then diffuse across the membrane to the permeate side[10].Permeability is the product of gas solubility and diffusivity.As a result,polymer with specific structure or high affinity for CO2molecules can provide high CO2permselectivity.However,there are different cases inside facilitated transport polymer membranes.The facilitation of CO2transport is accomplished by the “carrier”inside a facilitated transport membrane,which can reversibly react with CO2[10].It is well-known as facilitated transport mechanism or reactivity selective mechanism.At the feed-side interface of the membrane,CO2reacts with the carrier and forms a CO2-carrier reaction product,which diffuses along its concentration gradient to the permeate side of the membrane.Due to a lower CO2partial pressure on the permeate side,CO2is released from the CO2-carrier reaction product to the permeate side,while regenerating the carrier that can then react with another CO2molecule on the feed side[10].Hence,a major part of CO2is transported by the carriers inside the membranes in addition to the physical solution-diffusion as other non-reactive gases such as N2,CH4and H2.As a result,both high CO2permeability and selectivity can be obtained for facilitated transport polymer membrane materials.

To achieve CO2separation,as a continuous phase in MMMs,the polymer should have not only high CO2permeability but also high selectivity.For CO2/N2or CO2/CH4separation,because the kinetic diameter of CO2molecule is less than that of N2or CH4molecule(Table 1),diffusivity selectivity of the polymer is higher than one.For CO2/H2separation,because the kinetic diameter of CO2molecule is larger than that of H2molecule,diffusivity selectivity of the polymer is less than one.To meet high selectivity for CO2separation,the polymer should have high solubility selectivity or reactivity selectivity.Hence,the polymer should contain functional groups which only have high affinity for CO2molecules or react with CO2molecules.Furthermore,the polymer should have high mechanical strength and good thermal stability.Due to the different sources of CO2,the content,pressure and temperatures vary widely.The polymer should be selected based on practical application conditions.High performance glassy polymers with robust mechanical strength may be applicable for high pressure conditions such as natural gas purification.Facilitated transport membrane material may be suitable for low pressure flue gas purification,biogas treatment,and hot(＞100 °C)syngas separation[11].Besides,because various kinds of impurity gases such as H2O,O2,SOx,NOxand H2S are present as minor components in industrial CO2gas feeds,the polymer should have good chemical stability.Finally,the polymer should have good processability.To be employed in large scale applications,the polymer should be capable of being formed into thin membranes with separation layers to achieve high CO2permeance.

The common polymers include polyethersulfone(PES)[12-15],polycarbonate(PC)[16],poly(vinyl acetate)(PVAc)[17-19],sulfonated poly(ether ether ketone)(SPEEK)[20,21],poly(2,6-dimethyl-1,4-phenylene oxide)(PPO)[22],cellulose acetate(CA)[23,24],polyimide(PI,such as Matrimid?[25-29]),polyetherimide(such as Ultem[30]),PIM-1[31-35],poly(ether-block-amide)(Pebax,such as Pebax 1657[36-39],Pebax 2533[40]and Pebax 1074[41])and poly(vinylamine)(PVAm)[42,43].The above polymer materials possess different characteristics.PES,PC,SPEEK,PPO,CA,PI,and Ultem are glassy polymers,and have good chemical and thermal stability,permselectivity and processability.PIM-1 is a new kind of glassy polymer.Due to its special ladder-type structure with contorted sites that prevent polymer chains from rotating and packing efficiently,PIM-1 has high free volumes,which results in superior gas separation performance.Pebax as a copolymer has a soft(rubbery)polymer segment such as polyethylene oxide(PEO)and a hard(glassy)polymer segment such as polyamide(PA).On the one hand,PEO soft segment provides enough adhesion between polymer and filler.On the other hand,PEO soft segment has high affinity for CO2molecules.PA hard segment provides the mechanical strength.As the facilitated transport membrane material,PVAm containing amine groups exhibits both high permeability and high selectivity through the reversible reactions between reactive carriers—amine groups and CO2molecules.

To sum up,Matrimid?is the best polymer for CO2/CH4separation under high pressure,and PVAm is the best polymer for CO2/N2,CO2/CH4and CO2/H2separation under low pressure.

Table 1Distinctions of gases in size,condensability and reactivity

2.2.Selection of filler

To achieve CO2separation,as a dispersed phase in MMMs,the filler should have high selectivity.The selected filler must exactly correspond to the shape,size and other property difference of the targeted gas molecules,which facilitates CO2transport.Furthermore,the selected filler should have good compatibility with polymermatrix.Suitable combination of filler and polymer is a very important factor for improving CO2selectivity.Besides,the particle size of the filler should be small.To be employed in industrial applications,separation layer thickness of MMMs is only several micrometers,so the particle size of the filler should be as small as possible.

As a dispersed phase in MMMs,inorganic nanomaterials and organic nanomaterials can both be used as fillers.Inorganic nanofillers used in MMMs can be divided into two classes:solid or impermeable( filled)nanofillers and porous or permeable nanofillers.The impermeable nanofillers include silica and TiO2.The permeable nanofillers include zeolite,carbon molecular sieve,carbon nanotube,montmorillonite,metal-organic framework,graphene oxide and so on.The critical reviews on nanofillers were made[44-48].

In this section,we critically review the recent progress made in nanofillers.The typical size of the fillers ranges from dozens of nanometers to hundreds of nanometers,and the biggest size of fillers is 20 μm.The typical loading of porous inorganic fillers,laminar inorganic fillers and organic fillers ranges from 10 wt%to 30 wt%,from 1.5 wt%to 6 wt%,and from 15 wt%to 30 wt%,respectively.For most of the porous fillers used in MMMs for CO2separation,the pore size ranges from 0.34 nm to 2.6 nm.

For porous filler,the pore size determines CO2transport and CO2separation mechanism.Generally,CO2transport through porous filler follows molecular sieving mechanism and surface diffusion mechanism.When the pore size of the porous filler is roughly the same as kinetic diameter of the permeating gas molecule,gas transport through porous filler follows molecular sieving mechanism[49].If the pore size of the porous filler is between the diameters of the CO2and other gas molecules,only the smaller gas molecule can permeate through the porous filler leading to a more efficient separation.When the pore size of the porous filler increases,if the gas molecule exhibits a strong affinity for the filler surface and adsorption along the pore walls,gas transport through porous filler follows surface diffusion mechanism[49].Efficient CO2separation can take place by this mechanism due to differences in the amount of adsorption of the CO2and other gas molecules.When the pore size of the porous filler is big enough,molecular sieving and surface diffusion mechanisms often coexist.For the laminar inorganic fillers,interlamellar spacing determines CO2transport and CO2separation mechanism.In general,CO2transport through galleries between the neighboring nanosheets follows molecular sieving mechanism.

Effect of the fillers on CO2separation performance of MMMs is summarized in Table 2.The fillers not only disturb polymer chain packing and increase free volume,but also facilitate CO2transport by itself,which results in improvement of membrane performance.The fillers are reviewed in the section in detail.

2.2.1.Carbon-silica nanocomposite materials

Carbon-silica nanocomposite materials(CSMs)have a tunable porosity and surface chemistry which is controlled by the carbon deposition,the pyrolysis conditions and post-synthetic treatments.The carbon fraction of such nanocomposite fillers increases the affinity for CO2.

Anjumet al.[50]developed MMMs by adding porous CSM fillers to Matrimid?matrix.Owing to the addition of a carbon phase,providing an increased affinity for the CO2molecules next to the creation of extra porosity and free volume,the overall separation efficiency of MMMs increased.

2.2.2.Graphene oxide

The effective gas separation for graphene oxide(GO)is based on the formation of the molecular sieving galleries between the neighboring nanosheets or possible defects on the nanosheets.

Donget al.blended the partially porousreduced graphene oxide(PRG)nanosheets into Pebax 1657 polymer to prepare MMMs[39].For PRG,the narrow gas flow galleries(average width of 0.34 nm)between the neighboring nanosheets ensured effective molecular sieving of CO2against other larger gas molecules,while the mesoscopic pores on the laminate provided rapid gas transport pathways.Hence,the MMMs had substantially improved CO2permeability as well as CO2/N2selectivity.

2.2.3.Attapulgite

Attapulgite(ATP)is one kind of natural clay with low cost and high availability.In view of its narrow size of the tunnel-like rectangular microspores(0.37 nm×0.60 nm),ATP is anticipated to distinguish CO2(0.33 nm)from N2(0.364 nm).

Xianget al.blended Pebax 1657 and ATP nanorods to fabricate MMMs[51].Both the CO2permeability and CO2/N2selectivity of the MMMs increased at low ATP loadings(＜6.3 wt%).Compared with the pristine Pebax membrane,CO2permeability and CO2/N2selectivity of the MMMs with 1.7 wt%ATP increased by 37.5%and 30%,respectively.

2.2.4.Metal-organic frameworks

Metal-organic frameworks(MOFs)are a large emerging class of hybrid materials with porous crystalline structures,and combine the connectivity of metal centers with the bridging ability of organic ligands.Careful choice of metal and linker allows MOFs to be designed and synthesized with the desired functionalities,pore sizes and pore shapes.

Cu-BTC is made of copper clusters linked to each other via trimesic acid.Cu-BTC consists of main channels of a square cross-section of～0.9 nm diameter and tetrahedral side pockets of～0.5 nm,which are connected to the main channels by triangular windows of～0.35 nm diameter.Geet al.[22]developed MMMs by incorporating sizereduced Cu-BTC in poly(2,6-dimethyl-1,4-phenylene oxide)(PPO)matrix,and demonstrated that the incorporation of the Cu-BTC led to the improvement of both gas permeability and selectivity.sod-ZMOF has micropores of approximate 0.96 nm.For CO2/light gas mixture,molecular sieving does not happen throughsod-ZMOF due to the large micropores.However,sod-ZMOF has high affinity for CO2.K?l??et al.[52]fabricated Matrimid-sod-ZMOF MMMs,and found that with increasingsod-ZMOF,both CO2permeability and selectivity of the MMMs increased.

MIL-53 has open pores of diameter 0.85 nm at room temperature.MIL-53 was added to Matrimid?and poly(4-methyl-1-pentyne)(PMP)for CO2/CH4and CO2/H2separation,respectively[27,53].MIL-53 with polar functional groups is selected as filler.Rodenaset al.[54]fabricated MMMs by incorporating NH2-functionalized MIL-53(Al)in PI.

MIL-101(Cr)exhibits two types of cages:small cages,which possess a free diameter of2.9 nm and pentagonal windows of1.2 nm,and larger cages with a free diameter of 3.4 nm and both pentagonal and hexagonal windows,the latter with a 1.45 nm by 1.6 nm free aperture.Naseriet al.[55]prepared the Matrimid-MIL-101 MMMs.Compared with the neat Matrimid?membrane,CO2/CH4and CO2/N2ideal selectivities of the MMMs increased.MIL-101 with polarfunctional groups is selected as filler.Xinet al.[21]modified MIL-101(Cr)by concentrated sulfuric acid and trifluoromethanesulfonic anhydride to prepare sulfonated MIL-101(Cr)[S-MIL-101(Cr)],and then incorporated the S-MIL-101(Cr)into SPEEK to prepare MMMs.The addition of the S-MIL-101(Cr)increased the CO2/CH4and CO2/N2selectivity of the MMMs due to the increased CO2solubility.Seoaneet al.[56]developed MMMs by dispersing amino functionalized MOFs(NH2-MIL-53(Al)or NH2-MIL-101(Al))in sulfur-containing copolyimide.Rodenaset al.[57]prepared MMMs by dispersing NH2-MIL-53(Al)and NH2-MIL-101(Al)in polysulfone(PSf)and PI,and found that the incorporation of the MOF fillers had a positive effect on the separation performance.

MIL-125 has a quasi-cubic tetragonal structure in which the octahedral(1.07 nm)and tetrahedral(0.47 nm)cages are accessible through a window of about 0.5-0.7 nm.Guoet al.[58]fabricated MMMs by incorporating NH2-MIL-125 into PSf matrix,and demonstrated that the incorporation of NH2-MIL-125(Ti)particles could significantly improve the CO2permeability,and slightly enhance CO2/CH4separation factor.Anjumet al.[29]added MIL-125(Ti)and the amine-functionalized counterpart(NH2-MIL-125)as fillers to Matrimid?polyimide.The synthesized MMMs had the good adhesion between the fillers and the polymer matrix,and the NH2-functionalized filler was preferred as it led to higher selectivities and permeabilities.

UiO-66 can exhibit strong affinity for CO2molecules owing to the--OH groups coordinated to Zr cluster,and triangular windows possess the size of 0.6 nm.Shenet al.[59]embedded CO2-philic zirconium metal organic framework UiO-66 and UiO-66-NH2nanocrystals into Pebax membranes.The hydrogen bonding frameworks between UiO-66-NH2and Pebax were enhanced.The as-prepared Pebax-UiO-66-NH2MMM with MOF loading of 10 wt%displayed a CO2permeability of 4.36 × 10-14mol·m·m-2·s-1·Pa-1and CO2/N2selectivity of 72.

MOFs with hydroxyl groups are selected as fillers.Mg-MOF-74 exhibits exceptionally high CO2selective adsorption over CH4and 1-Dhexagonal channels of 1.1 nm diameter.Tien-Binhet al.[34]added a novel filler having hydroxyl functional groups on the surface(Mg-MOF-74)to PIM-1.Under optimized conditions,chemical cross linking between the hydroxylgroups and the fluoride chain-ends of PIM-1 was facilitated to completely remove interfacial defects.Compared with the neat PIM-1 membrane,CO2permeability of the MMMs with 20 wt%MOF-74 increased by 3.2 times to 7.12 × 10-12mol·m·m-2·s-1·Pa-1,meanwhile CO2/CH4selectivity was improved to 19.1.

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2.2.5.Zeolite imidazolate frameworks

Zeolite imidazolate frameworks(ZIFs)are built of tetrahedral metal ions(e.g.,Zn,Co)bridged by imidazolates.ZIFs have permanent porosity,and relatively high thermal and chemical stability,which makes them attractive candidates for nanofiller used in MMMs.

ZIF-71 has its rhombic structure with an aperture size of0.42 nm and a pore cavity of 1.65 nm.Haoet al.[60]developed MMMs consisting of a zeolite imidazolate framework-71(ZIF-71,Zn(cbIm)2)and PIM-1 with and without UV irradiation,and found that the addition of ZIF-71 considerably enhanced the gas permeability without compromising the gas pair selectivities of CO2/N2and CO2/CH4,and the UV treated MMM with 20 wt%ZIF-71 had a CO2permeability of around 6.38 × 10-13mol·m·m-2·s-1·Pa-1and a CO2/CH4selectivity of 32.2 under mixed gas tests.

ZIF-8 has large(1.16 nm)pore cavities that are accessible through small(0.34 nm)pore apertures,which complement the kinetic diameter of CO2(0.33 nm),allowing for CO2separationviaa sieving mechanism.Askari and Chung[61]fabricated MMMs by directly mixing ZIF-8 suspension into three 4,4-(hexa fluoroisopropylidene)diphthalic anhydride(6FDA)-based polyimide solutions,and the MMM made of 6FDA-Durene/DABA(9/1)and 20 wt%ZIF-8 displayed an impressive CO2permeability of 2.44 × 10-13mol·m·m-2·s-1·Pa-1and a CO2/CH4selectivity of 19.61 in mixed gas tests.Bushellet al.developed MMMs consisting of PIM-1 and ZIF-8,and found that an increase in ZIF-8 loading led to increases in the permeability as well as in the separation factors,and data points on several Robeson diagrams were located above the 2008 upper bound.Na fisi and H?gg[40]developed MMMs by using ZIF-8 as inorganic filler in Pebax 2533 polymer matrix.As the inorganic filler content increased,the permeability of all examined gases increased.Nafisi and H?gg[62]also added ZIF-8 to 6FDA-Durene PI.Chiet al.[63]prepared MMMs consisting of polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene(SEBS)block copolymers and size-controlled ZIF-8 nanoparticles to investigate the effect of filler particle size on MMM gas separation performance,and found that ZIF-8(M)(240 nm)was the most effective in improving gas permeability and selectivity.

Hwanget al.[64]fabricated MMMs by dispersing hollow zeolite imidazole frameworks(H_ZIF-8) filler in poly(vinyl chloride)-gpoly(oxyethy-lene methacrylate)(PVC-g-POEM)graft copolymer matrix.Compared with pure PVC-g-POEM membranes,the MMMs exhibited an 8.9-fold increase in CO2permeability with only a small decrease in CO2/CH4selectivity.

ZIF-11 has cages with diameter of 1.46 nm which are connectedviaapertures with diameter of 0.3 nm.Ehsani and Pakizeh[65]incorporated ZIF-11 into Pebax?2533 polymer in the range of 10wt%-70 wt%to fabricate MMMs.Excellent adhesion existed between ZIF-11 and polymer matrix,especially at 30 wt%loading.However,at 50 and 70 wt%ZIF-11 loadings,a few voids were observed throughout the membranes.

2.2.6.Two-componentfillers

Except for single kinds of inorganic nanoparticle as filler,two kinds of inorganic nanoparticles can be added together as two component fillers.Liet al.[66]prepared MMMs by incorporating carbon nanotubes(CNTs)and GO into a Matrimid?matrix.The extraordinary smooth walls of CNTs acted as a highway to render high permeability,whereas the GO nanosheets acted as a selective barrier to render high selectivity through the hydroxyl and carboxyl groups on GO surface in MMMs.The MMM with 5 wt%CNTs and 5 wt%GO displayed the optimum performance with a CO2permeability of 1.28 × 10-14mol·m·m-2·s-1·Pa-1,a CO2/CH4selectivity of 84.60 and a CO2/N2selectivity of 81.00.Furthermore,the composites consisting of two kinds of inorganic nanoparticles can be also used as two-component fillers.Donget al.[67]fabricated MMMs by incorporating ZIF-8@GO composites into Pebax matrix.On the one hand,the high-aspect ratio GO nanosheets in polymer matrix increased the length of the tortuous path of gas diffusion,which enhanced the diffusivity selectivity.On the other hand,the inherent high permeability of ZIF-8 with ultra-microporosity could enhance the gas permeability and solubility selectivity of MMMs.The MMM containing 6 wt%ZIF-8@GO exhibited the optimum performance with a CO2permeability of 8.34 × 10-14mol·m·m-2·s-1·Pa-1and a CO2/N2selectivity of 47.6.

Compared with inorganic nanofillers,organic nanofillers are emerging fillers.Organic nanofillers have advantages such as improved adhesion to polymermatrix.This superiority may be attributed to the organic feature of the filler.In addition,some organic nanofillers can provide unique properties.For instance,nanohydrogels can absorb and retain extremely high water content,and the incorporation of nanohydrogels in MMMs can increase water uptake of MMMs,which is beneficial to facilitating CO2transport.

Mixed matrix material with organic nanofillers and polymerblends are both important membrane materials for CO2separation.Polymer blends can be categorized as miscible and phase-separated blends(immiscible and partially miscible blends)[73].In miscible blends,both the polymers are dissolved in each other at molecular levels representing a homogeneous single-phase behavior.However,in phase-separated blends both the polymersare not dissolved in each other and are separated by an interface between the two phases[74].Hence,mixed matrix material with organic nanofillers should be phase-separated blends.Its performance is strongly dependent on interface morphology,specific volume fraction,and size and shape of the dispersed and continuous phase[74].In many cases,mixed matrix material with organic nanofillers is beneficial to CO2transport compared with miscible blends.It is mainly attributed to suitable interface morphology between two phases and the specific structure of dispersed phase such as pore size and functional groups.

Organic nanofillers used in the MMMs include polyaniline[68,69],poly(N-isopropylacrylamide)nanohydrogels[70],carboxylic acid nanogels[71],poly(ethylene glycol)-containing polymeric submicrospheres[72],and hypercrosslinked polystyrene[33].The organic nanofillers are reviewed in the following sections.

2.2.7.Polyaniline

The incorporation of polyaniline(PANI)disturbs chain packing,and increases fractional free volume.Moreover,PANIwith amine groups can facilitate CO2transport through reversible reaction between amine groups and CO2molecules,which results in the improvement of membrane performance[68,69].

2.2.8.Nanohydrogels

The addition of nanohydrogels increases the fractional free volume,water uptake and water retention capacity of the MMMs,which is beneficial to improving CO2separation performance of polymermembranes.

Liet al.[70]incorporated poly(N-isopropylacrylamide)nanohydrogels(NHs)into Matrimid?to prepare MMMs.The NHs homogeneously embedded in the Matrimid?matrix acted as water reservoirs to not only provide more water for dissolving CO2,but also construct interconnected CO2transport passageways.The as-prepared Matrimid-NHs-20 membrane exhibited CO2/CH4and CO2/N2selectivities of 61 and 52 with a CO2permeability of 9.31 × 10-14mol·m·m-2·s-1·Pa-1.Liet al.[71]added carboxylic acid nanogels(CANs)into Pebax 1657 to fabricate MMMs.The incorporation of CANs simultaneously tailored favorable water environment and increased CO2transport sites within the membranes.The Pebax-CANs-30 membrane displayed CO2/CH4and CO2/N2selectivities of 33 and 85 with a CO2permeability of 6.79 × 10-13mol·m·m-2·s-1·Pa-1.

2.2.9.Poly(ethylene glycol)-containing polymeric submicrospheres

Wanget al.[72]incorporated poly(ethylene glycol)(PEG)-containing polymeric submicrospheres(PEGSS)into PI to prepare MMMs.The favorable affinity between PEGSS and CO2greatly increased CO2solubility,which led to an increase in CO2permeability.Compared with those of pristine PI membrane,CO2permeability and CO2/N2selectivity of the PI-PEGSS(20)membrane with 20 wt%PEGSS increased by 35%and 104%,respectively.

2.2.10.Hypercrosslinked polystyrene

Mitraet al.[33]added hypercrosslinked polystyrene(HCP)into PIM-1 to fabricate MMMs.Because the nanosized HCP possessed rigid nanoporous structure,the addition of the HCP not only led to higher permeability but also to a significant arrest in polymer aging and permeability loss.

To sum up,in view of selectivity,compatibility and particle size,PANI is the best filler for CO2/N2separation,NHs is the best filler for CO2/CH4separation,and MIL-53 is the best filler for CO2/H2separation.

3.Interface Morphologies

Interface morphology is a determinant factor for the overall transport property.Fig.1 displays a schematic diagram of various nanoscale structures at the polymer/ filler interface[5].Case 1 shows an ideal morphology.Case 2 represents the detachment of polymer chains from the filler surface,causing the interface voids.Case 3 displays that the polymer chains in direct contact with the filler surface can be rigidified compared to the bulk polymer chains.Case 4 indicates a situation in which the surface pores of the filler have been partially sealed by the rigidified polymer chains.

When there is a poor compatibility between polymer matrix and filler,Case 2 appears.Due to the less resistance in interface voids,gas transports through interface voids instead of polymer matrix or filler,which improves gas permeability.Meanwhile,change on gas selectivity is dependent on the size of interface voids.Generally,gas selectivity decreases.In other words,as gas molecules take this non-selective and less resistant by-pass instead passing through pores in the filler,membrane performance deteriorates more or less.When there is a very good compatibility between polymer matrix and filler,Case 3 and Case 4 occur.If Case 3 happens,the movement of polymer chain at polymer/ filler interface is restrained,which reduces gas adsorption,and then reduces gas permeation.Hence,gas permeability decreases,but an increase in gas selectivity is not obvious.If Case 4 happens,because pores of the filler are partially sealed,gas permeability declines.

In order to discuss the structure-property relationship of the MMMs conveniently,interface morphologies can be roughly divided into two classes:there are interface voids and there are no interface voids.Based on this,Wanget al.[75]systematically analyzed the relationship between polymer- filler interfaces and gas transport properties of the MMMs with different nanofillers.As shown in Fig.2,considering interface morphology,types of nanofillers and distribution of nanofillers in membrane,ten kinds of possible separation layer structures of MMMs were proposed,and their corresponding gas transport pathways were discussed.Moreover,for each case,the change on gas permeance of the MMMs with increasing feed pressure was analyzed.If the size of interface voids was larger than the mean free path of the gas molecules,and the interconnectivity of interface voids formed interface void channels across the separation layer of the MMM,viscous flow could occur,which led to an increase in gas permeance and a decrease in gas selectivity with increasing feed pressure.Conversely,when viscous flow could not occur for N2,O2or CH4transport through nanofillers,an increase in gas permeance of the MMMs with increasing feed pressure demonstrated that there were void channels between polymer phase and nanofiller phase and the size of interface voids was larger than the mean free path of the gas molecules.

4.Methods to Improve Compatibility

Generally,there is poor compatibility between glassy polymer and filler,which results in a decrease in gas selectivity.To avoid interface void and improve membrane performance,researchers adopt the following eight methods to improve interface compatibility.

4.1.Silane coupling

Organofunctional silanes as the most prominent members of coupling agents can be hydrolyzed to form silanol groups.These silanol groups then react with the hydroxyl groups on the surface of inorganic filler during condensation reaction to form stable siloxane bonds.By introducing the silylated inorganic fillers into a polymeric matrix,the dual reactivity of silicone in organosilane serves as bridges between fillers and matrix.

Fig.1.The schematic diagram of various nanoscale morphology of the mixed matrix structure[5].

Sanaeepuret al.[76]modified micro-sized nanoporous sodium zeolite-Y(NaY)by 3-aminopropyl(diethoxy)methylsilane(APDEMS),and incorporated the silylated particles into a homogeneous CA membrane to achieve better polymer-zeolite adhesion in MMMs.Amooghinet al.[77]also modified NaY by APDEMS,and embedded the modified particles into the Matrimid?5218 matrix to prepare MMMs with defectfree polymer/ fillerinterface.Fig.3 shows the grafting reaction between the APDEMS and zeolite surface,and also the proposed possible reaction between silane modified zeolite and Matrimid.As shown in Fig.3,APDEMS reacts with the hydroxyl groups on zeolite surface,the amino group of APDEMS reacts with the imide group of Matrimid and consequently forms the covalent bonding between zeolite and Matrimid,which results in the good interface compatibility between the two phases.Compared with pure Matrimid?membrane,the CO2permeability and CO2/CH4selectivity of the MMMs with 15 wt%loading increased by 16%to 3.25 × 10-15mol·m·m-2·s-1·Pa-1and by 57%to 57.1,respectively.Laghaeiet al.[78]modified the surface of MCM-41 by 3-aminopropyltrimethoxysilane(APTMS)and trimethylchlorosilane(TMCS).The APTMS-modified MCM-41 with polar N-H groups and long side chains had a good compatibility with PES matrix,which facilitated the preparation of defect free membranes with gas separation performance by 250%and 40%increment in CO2permeability and CO2/CH4selectivity,respectively.However,the TMCS did not interact with the PES as strong as APTMS.Donget al.[79]developed novel MMMs by establishing montmorillonite(MMT)functionalized with PEG and aminosilane coupling agents in a Pebax membrane.There were no evident interfacial voids in all MMMs,and the prepared MMM with 40 wt%of MMT-HD702-PEG5000 displayed a CO2permeability of 1.50 × 10-13mol·m·m-2·s-1·Pa-1and a CO2/N2selectivity of 70.73.

Fig.2.Schematic diagram of separation layer structure of the MMMs and its corresponding gas transport pathway[75].

Fig.3.The grafting reaction between APDEMS and zeolite surface,and also the reaction between Matrimid and the surface modified zeolite[77].

4.2.Grignard treatment

The Grignard treatment method can modify the filler surface,reduce the solvent- filler interaction,and recuperate the interfacial adhesion between filler and polymer.The Grignard treatment involves growing Mg(OH)2whiskers on the filler surface,and consists of two steps:(i)a crystal seeding step and(ii)the crystal growth step.The Grignard treatment method creates roughened surface morphologies composed of whisker-and platelet-shaped nanocrystals,and the highly roughened filler surfaces are thought to promote adhesion at the polymer particle interfaceviathermodynamically-induced adsorption and physical entanglement of polymer chains in the whisker structures by minimizing the entropy penalty[46].

Zornozaet al.[80]prepared MMMs by adding ordered mesoporous silica MCM-41 spheres(MSSs),Grignard surface functionalized MSSs(Mg-MSSs)and hollow zeolite spheres into 6FDA-DAM polymer matrix.Because the polymerchain tends to be adsorbed onto the heterogeneous surface compared with the flat surface,once embedded in the 6FDA-DAM polymer matrix,Mg-MSSs external whisker-like structure promoted interfacial filler-polymer contact,which resulted in excellent adhesion between filler and polymer as shown in Fig.4.The 6FDADAM-Mg-MSS MMM had the best performance with CO2/N2and CO2/CH4separation selectivities of24.4(4.07×10-13mol·m·m-2·s-1·Pa-1of CO2),and 31.5(4.17 × 10-13mol·m·m-2·s-1·Pa-1of CO2),respectively.

4.3.Incorporation of additive

Expect polymer and inorganic filler,the addition of the third component as additive can improve compatibility between filler and polymer matrix.Such additives include PEG,ionic liquid(IL),and polyethylenimine(PEI).

PEG as a low molecular weight CO2selective agent can eliminate interfacial voids between the polymer chains and zeolite surface.Loloeiet al.[81]investigated the effect of low molecular weight PEG 200 on the gas separation properties of Matrimid?5218-ZSM-5 MMM,and demonstrated that there is a good compatibility between ZSM-5 and polymers.

Free ILs act like lubricants between the fillers and the polymer matrices,leading to good compatibility in the three-component MMMs and high CO2permeability of the MMMs.Hudionoet al.[82,83]demonstrated that ILs could behave like a wetting agent in polymer-zeolite MMMs and improve the compatibility between polymer and zeolite.Haoet al.[84]fabricated poly(ionic liquid)-IL-ZIF-8 MMMs and demonstrated that the addition of ZIF-8 considerably improved gas permeability without compromising CO2/N2and CO2/CH4selectivities,indicating the absence of defects in the MMMs.Casado-Coterilloet al.[85]incorporated nanometer-sized ZIF-8 or HKUST-1 particles into the mixture of[emim][Ac]IL and chitosan(Cs)to fabricate MMMs and the Cs-IL-ZIF-8 MMMs had a maximum CO2permeability of over 1.68 × 10-12mol·m·m-2·s-1·Pa-1.

Confined ILs can not only improve compatibility between the fillers and the polymer matrices,but also improve the CO2selectivity of the resulting MMMs further.Liet al.[86]incorporated a room temperature ionic liquid(IL)[bmim][Tf2N]into ZIF-8 cages,and then added the IL-incorporated ZIF-8(IL@ZIF-8)into Pebax to prepare the Pebax-IL@ZIF-8 MMMs with toughened MOF-polymer interface.A mechanism of the formation of filler-polymer interfaces was proposed in Fig.5.At the beginning of the membrane preparation,the PA blocks would assemble preferentially around the hydrophobic sites ofIL@ZIF-8 particles owing to their hydrophobic-hydrophobic interaction.As the membrane had become nearly solidified,the solvent diffused out of the ZIF-8 framework and dragged the IL to the interfaces between fillers and polymers.Because the bulkier IL clusters have good compatibility with the polymer,the bulkier IL clusters embedded in the apertures of ZIF-8 could act as cross-linking agents between the two phases.Due to a stiffer interphase between the filler and the polymer and a reduction in the effective aperture size of ZIF-8,the Pebax-IL@ZIF-8 membranes displayed improved molecular sieving properties.Compared with pure Pebax membrane,CO2permeability,CO2/N2selectivity and CO2/CH4selectivity of the MMM with 15 wt%IL@ZIF-8 increased by 45%,74%and 92%,respectively.

MIL-101(Cr)with a particle size of～550 nm was chemically decorated with PEI rich in amine groupsviaa facile vacuum-assisted method,and the obtained PEI@MIL-101(Cr)was then incorporated into SPEEK to fabricate SPEEK-PEI@MIL-101(Cr)membranes[87].Owing to the electrostatic interaction and hydrogen bond between sulfonic acid group and PEI,the PEI both in the pore channels and on the surface of MIL-101(Cr)improved the filler-polymer interface compatibility,and simultaneously rendered abundant amine carriers to facilitate the transport of CO2through reversible reaction.The SPEEK-PEI@MIL-101(Cr)membrane with 40 wt%PEI@MIL-101(Cr)displayed the highest ideal selectivities for CO2/CH4and CO2/N2which were 71.8 and 80.0,respectively,with a CO2permeability of 8.34 × 10-13mol·m·m-2·s-1·Pa-1at 0.1 MPa and 25°C,which surpassed Robeson's upper bound revised in 2008.

Fig.4.SEM images based of:(a-c)MSSs[(a)individual particle,(b)8 wt%MSS/6FDA:DAMMMM,and(c)TEM image of an embedded particle],and(d-f)Mg-MSSs[(d)individual particle,(e)8 wt%Mg-MSS/6FDA-DAM MMM,and(f)inset of(e)][80].

Fig.5.Hypothetical mechanism of the toughening of filler-polymer interface:(a)the preferential adsorption of PA block on IL@ZIF-8 and(b)the aggregation of[bmim][Tf2N]at the filler polymer interface[86].

4.4.Grafting

Inorganic nanofiller is modified by grafting with low molecular weight polymer containing EO and amine groups.Hydrogen bonding can be easier to form between polymer and modified inorganic nanofiller,which can enhance compatibility.

Liet al.[88]prepared MMMs by incorporating polyethyleneglycol and polyethylenimine-functionalized graphene oxide nano-sheets(PEG-PEI-GO)into a commercial low-cost Pebax matrix.As shown in Fig.6,GO was modified by grafting with PEI and PEG to fabricate PEG-PEI-GO nanosheets.Hydrogen bonding could form not only between the amino group of PEI and the ether oxygen group(or amide group)of Pebax,but also between the ether oxygen group of PEG and amide group of Pebax.Hence,PEG and PEI on the surface of GO improved the interface compatibility between the Pebax matrix and GO nanosheets.The MMM with 10 wt%PEG-PEI-GO showed optimal gas separation performance with a CO2permeability of 4.46× 10-13mol·m·m-2·s-1·Pa-1,a CO2/CH4selectivity of 45,and a CO2/N2selectivity of 120,which surpassed Robeson's upper bound revised in 2008.

Fig.6.Illustration of the preparation of PEG-PEI-GO[88].

4.5.In situ polymerization

The added fillers participate in the polym ersynthesis,which is called asin situpolymerization.The chemical bonding can form between polymer and filler byin situpolymerization,which is beneficial to improving compatibility.

[Cd2L(H2O)]2·5H2O(Cd-6F)synthesized using 6FDA as an organic ligand was introduced into the 6FDA-ODA polyimide matrix to achieve novel MOF MMMs[89].As shown in Fig.7,a specific interaction between the uncoordinated--COO---on the surface of Cd-6F and the--NH2groups of the ODA monomer at the terminal of poly(6FDA-ODA)chains was introduced during thein situpolymerization.Compared with the pure 6FDA-ODA polyimide membrane,the as-prepared MMM displayed both higher permeability and selectivity due to the good polymer-MOFs compatibility resulted from the targeted interfacial interaction.The MMM exhibited a CO2permeability of 1.27 × 10-14mol·m·m-2·s-1·Pa-1with a CO2/N2selectivity of 35.1,and CO2/CH4selectivity of 44.8.

4.6.Polydopamine coating

Polydopamine(PD)is a good adhesion agent.PD can conveniently depositand furtheradhere on virtually all types of inorganic and organic supports with controllable film thickness and durable stabilityviathe oxidative self-polymerization of dopamine(DOP)in a mildly alkaline environment.

The nanosized ZIF-8 was coated by an ultrathin PD layer,and then incorporated into intrinsically microporous polyimide named TBDA2-6FDA-PI with Tr?ger's Base to prepare ZIF-8@PD-PI membrane[90].The formation of hydrogen bond interaction between the abundant secondary or primary amine groups on PD molecules and the tertiary amine in TB-based PI polymers is beneficial to the improvement of compatibility between the two polymers as schematically shown in Fig.8.For CO2/N2(50/50,v:v)mixed gas,ZIF-8@PD-PI(20%)membrane exhibited a CO2permeability of 2.33 × 10-13mol·m·m-2·s-1·Pa-1and a CO2/N2selectivity of 21 at 0.1 MPa.For CO2/CH4(50/50,v:v)mixed gas,ZIF-8@PD-PI(20%)membrane exhibited a CO2permeability of 2.11 × 10-13mol·m·m-2·s-1·Pa-1and a CO2/CH4selectivity of 27 at 0.1 MPa.

Fig.7.Diagram of designed interaction between Cd-6F and 6FDA-ODA in MMM[89].

4.7.Particle fusion approach

Particle fusion method is very versatile,and can enhance compatibility between filler and polymer matrix.

Shahidet al.[91]prepared MOF based MMM with better compatibilityviaa particle fusion approach.Matrimid?polymer particles were first prepared by precipitating a Matrimid?polymer solution in water.The surface of these particles was then modified by the introduction of imidazole groups as shown in Fig.9(a).ZIF-8 nanoparticles were then grownin-situto this modified polymer particle suspension by addition of the precursor for ZIF-8 synthesis.The resulted suspension was cast to dryness and annealed in a solvent-vapor environment to induce particle fusion,forming a dense MMM as shown in Fig.9(b).The pendent imidazole units lead to a better compatibility between the polymer phase and the ZIF-8 nanoparticles.The excellent ZIF-8-polymer interfacial adhesion resulted in a significant improvement in both CO2permeability and CO2/CH4selectivity.Compared with unfilled Matrimid?,the CO2permeability of the MMMs increased by 200%and the CO2/CH4selectivity increased by 65%.

Fig.8.Schematic illustration for interface design of ZIF-8@PD-PI membrane[90].

Fig.9.Modification process of Matrimid?polymer particles(a)and SEM image of MMM containing 30 wt%ZIF-8 prepared by particle fusion(b)[91].

4.8.Polymer functionalization

Functionalized polymer is used to improve compatibility,which is called as polymer functionalization.Functionalized polymercan interact with functional groups containing filler,which lead to the enhanced compatibility.

Tien-Binhet al.[92]synthesized hydroxyl-functionalized homo-and co-polyimides 6FDA-(DAM)x-(HAB)y(withx:ymolar ratio of 1:0;2:1;1:1;1:2)and two MOFs[MIL-53(Al)and NH2-MIL-53(Al)]to prepare MMMs.A strong interaction existed between the hydroxyl groups in the copolyimides and the amine groups in NH2-MIL-53(Al),which enhanced polymer- filler compatibility.The MMM prepared with 6FDADAM-HAB(1:1)copolyimide and 10 wt%NH2-MIL-53(Al)displayed a permeability/selectivity behavior approaching the 2008 Robeson's upper bound.

In brief,during modification process by using these methods except Grignard treatment,hydrogen bonding or chemical bonding forms,which leads to the fact that the interactions between polymer matrix and inorganic nanofiller improve interface compatibility.

5.Mixed Matrix Composite Membrane

So far,most mixed matrix membranes for CO2separation are freestanding membranes without porous membrane as a support layer.The thicknesses of these membranes range from several dozen to several hundred micrometers.However,industrial demands on the membranes of higher productivity motivate researchers to fabricate integrally skinned asymmetric membranes or composite membranes.Hence,mixed matrix asymmetric membranes and mixed matrix composite membranes are developed.The mixed matrix composite membranes are reviewed in the section.A mixed matrix composite membrane consists of a thin separation layer and a support layer,and the thin separation layer known as mixed matrix layer is deposited on the asymmetric support layer.The separation layer is ultrathin,and has only several micrometers at most.When there is a poor compatibility between polymer and filler,interface voids are easier to be formed in the separation layer.For the mixed matrix composite membrane,how to avoid voids is a challenging subject.Recently,mixed matrix composite membranes have attracted the great attention.Most researches have focused on the forms of hollow fiber and flat mixed matrix composite membranes.

5.1.Hollow fiber mixed matrix composite membrane

Ekiner and Kulkarni[93]reported a patent on the hollow fiber mixed matrix composite membranes in 2003.Later,Chung's group does a lot of researches on the hollow fiber mixed matrix composite membranes[94-98].The outer diameter of these membranes ranges from～650 to～1000 μm,the inner diameter ranges from ～350 to ～600 μm,and the thickness of the dense selective layer ranges from 0.15 to 12 μm.

The hollow fiber mixed matrix composite membranes[94-97]are spun by the co-extrusion technique using a dual-layer spinneret as depicted in relative literatures[99].The flow rates of the bore fluid and both dope solutions are controlled by three pumps.The as-spun hollow fibers are rinsed in the clean water bath for several days to remove the remaining solvent and then carried out solvent exchange without further drying.Finally,these fibers are dried in the air at ambient temperature for use.By lowering the outer layer flow rate(while keeping other spinning conditions constant),the thickness of the outer layer can be reduced[95].To avoid voids,they attempt several post treatment methods such as heat treatment and two-step coating,andp-xylenediamine/methanol soaking.Liet al.[96]employed heat treatment and two-step coating processes to bring out the separation properties of zeolite beta imbedded in the polymer matrix for defect free PES-zeolite beta/P84 hollow fiber membrane.Compared with that of neat PES dense films,the CO2/CH4selectivity of the hollow fiber membranes increased by around 10%-20%.Jianget al.[97]adopted a novelp-xylenediamine/methanol soaking method to efficiently remove the PSf-zeolite interface defects of the PSf-zeolite beta/Matrimid?hollow fiber membranes,and found that CO2/CH4ideal selectivities of the PSf-zeolite beta/Matrimid?hollow fiber membranes roughly increased by 50%in comparison with that of the neat PSf/Matrimid?hollow fiber membrane.Hydrogen bonding was proposed as the possible mechanism for the tighter attachment between the PSf matrix and filler(Fig.10).In addition,Chenet al.[98]developed novel hollow fiber membranes by surface coating ultrathin layers of a PEG containing hybrid material onto the asymmetric PES hollow fiber substrate.The fabricated membranes exhibited an impressive CO2/N2selectivity of 50 with the CO2permeance of 1.01× 10-8mol·m-2·s-1·Pa-1at 25 °C and 0.2 MPa.

Fig.10.Mechanism of p-xylenediamine priming and possible structure[97].

5.2.Flat mixed matrix composite membrane

Kulprathipanja and Charoenphol[100]published a patent on mixed matrix membrane for separation of gases in 2004.In the patent,a mixed matrix composite membrane comprised PEG,silicone rubber and activated carbon on a porous support.The membrane preferably also comprised a carbonate such as potassium carbonate.Later,Kulprathipanjaet al.[101]published another patent.In that patent,a mixed matrix composite membrane comprised a nitrogen containing compound such as a mine,silicone rubber and activated carbon on a porous support.The membrane might also comprise a plasticizer such as PEG.Thereafter,a lot of academic papers on flat mixed matrix composite membranes are published.

5.2.1.Poly(dimethylsiloxane)based mixed matrix composite membrane

Some researchers select rubbery poly(dimethylsiloxane)(PDMS)as polymer matrix.de Clippelet al.[102].developed a defect-free mixed matrix composite membrane by filling a PDMS top layer with porous carbon-silica microspheres on a polyimide(PI)/polypropylene(PP)membrane,and the derived MMM displayed a CO2permeance of 2.81× 10-8mol·m-2·s-1·Pa-1,and a CO2/H2selectivity of 4.7 at 1.0 MPa and 25°C.Wanget al.[103]also incorporated mesoporous KIT-6 modified by phenyltriethoxysilane(PTES)into PVDF supported PDMS to fabricate PDMS-p-KIT-6/PVDF membranes,but the CO2/N2ideal selectivity of the membrane was too low.

5.2.2.PIM-1 based mixed matrix composite membrane

Some researchers choose glassy PIM-1 as polymer matrix.Khanet al.[104]coated the mixture of PIM-1 and multiwalled carbon nanotubes(MWCNTs)functionalized with PEG on microporous polyacrylonitrile(PAN)membrane to fabricate the PIM-1-f-MWCNT/PAN membrane.The derived MMM with f-MWCNT loading of 2 wt%reached a high CO2permeance of 3.68 × 10-6mol·m-2·s-1·Pa-1,and a CO2/N2ideal selectivity of 33.5 at 0.2 MPa and 27°C.

5.2.3.EO containing polymer based mixed matrix composite membrane

Because Pebax 1657 is a commercial candidate polymer material containing EO groups for CO2separation,it is also used as polymer matrix.Solid nanoparticles,two-dimensional materials,and three dimensional porous material are selected as nanofillers,respectively.When solid TiO2was chosen as nanofillers,defect-free Pebax 1657-TiO2/PVC membranes with TiO2loading of 3 wt%showed the best performance[105].Shenet al.[106]used the mixture of Pebax 1657 and MoS2nanosheets as the selective layer,PDMS as the gutter layer,and PSf as a support substrate to prepare the mixed matrix composite membrane.The prepared membrane with 0.15 wt%MoS2nanosheets exhibited the best performance with a CO2permeance of 5.80 × 10-9mol·m-2·s-1·Pa-1,and CO2/N2ideal selectivity of 93 at 0.2 MPa and 30°C.Zarshenaset al.[107]fabricated mixed matrix membranes by incorporating nano-zeolite NaX into Pebax 1657 as a separation layer on the PES membrane as a support layer.The membrane containing 2 wt%zeolite NaX showed a CO2permeance of 7.87 × 10-10mol·m-2·s-1·Pa-1and a CO2/N2ideal selectivity of 121.5 at 0.7 MPa and 25°C.Liet al.[108]deposited the mixture of Pebax 1657 and ZIF-7 on a porous PAN support to prepare Pebax 1657-ZIF-7/PAN membranes with polytrimethylsilylpropyne(PTMSP)gutter layer.The Pebax 1657-ZIF-7/PAN membrane with 22 wt%ZIF-7 displayed a CO2permeance of 4.59 × 10-8mol·m-2·s-1·Pa-1,and CO2/N2ideal selectivity and CO2/CH4ideal selectivity of 97 and 30,respectively at 0.375 MPa and 25°C.Jomekianet al.[109]coated Pebax 1657-ZIF-8 on PES layer to fabricate Pebax 1657-ZIF-8/PES membrane.The as-prepared membrane showed a CO2permeance of 1.21 × 10-7mol·m-2·s-1·Pa-1,and a CO2/N2ideal selectivity of 16.1 at 0.8 MPa.

In addition,PEG is also an important polymer material containing EO groups.Iron dopamine nanoparticles(FeDA NPs)were incorporated into a nanoscale thick PEG matrix on a highly permeable PDMS prelayer spin-coated onto a porous PAN substrate to form mixed matrix composite membranes[110].The as-prepared membrane displayed excellent gas separation performance with a CO2permeance of～4.02 × 10-7mol·m-2·s-1·Pa-1and an enhanced CO2/N2ideal selectivity of over 35 at 0.1 MPa and 35°C.

5.2.4.Facilitated transport based mixed matrix composite membrane

Facilitated transport based mixed matrix composite membranes have also been investigated.They are roughly reviewed in the lasted two reviews[10,111].In this section,they are discussed in detailaccording to types of polymer matrix.

Our group carried out lots of researches on flat mixed matrix composite membranes,and examined structure-property relationship.Generally,our group chose amine containing polymer as polymer matrix to prepare different flat mixed matrix composite membranes by the addition of different nanofillers.Firstly,our group selected the conventional nanomaterials as nanofillers.Yuet al.[112]fabricated the mixed matrix composite membranes by incorporating CO2-selective adsorptive LUDOX?silica nanoparticlesin situinto the tertiary amine containing polyamide membrane formed by interfacial polymerization.The membrane displayed a CO2permeance of 1.99× 10-8mol·m-2·s-1·Pa-1and a CO2/N2selectivity of 85.4 at 0.1 MPa.Wanget al.[75]found that surface modification of pristine inorganic nanofillers(MWCNT,SiO2and ZSM-5)endowed with amine groups could eliminate or reduce interface voids and improve the interface compatibility between PVAm polymer chains and modified inorganic nanofillers.

Based on this,silane coupling agents are used to couple polymer with layer nanomaterials in order to eliminate or remove interface voids.At the same time,layer nanomaterials are beneficial to CO2facilitated transport.Liaoet al.[113]synthesized the polyethyleniminebased copolymer PEIE with abundant amine groups and moderate hydroxyl groups,chose PEIE as a polymer matrix,chose nanosized hydrotalcite(HT)as a filler,used 3-aminopropyltriethoxysilane(APTES)as a molecular bridge to couple the PEIE and HT,and then coated the PEIE-HT complex on PSf membrane to fabricate the PEIE-HT/PSf membrane.In view of the mobile carriers within the interlayer gap of HT,high-speed CO2transport channels were successfully constructed,and the PEIE-HT/PSf membrane exhibited a high CO2permeance up to 1.91 × 10-6mol·m-2·s-1·Pa-1and a CO2/N2selectivity of 268 at 0.11 MPa.To verify high-speed CO2transport channels of HT further,Liaoet al.[43]chose PVAm and HT as a polymer matrix and filler,respectively,used 3-Glycidyloxypropyltrimethoxysilane(GLYMO)as a molecular bridge to couple the PVAm and HT,and then prepared the PVAm-HT/PSf membrane.The PVAm-HT/PSf membrane exhibited a high CO2permeance of 1.07 × 10-6mol·m-2·s-1·Pa-1and a CO2/N2selectivity of 296 at 0.11 MPa.To evaluate the effect of arrangement of layer nanomaterials on gas permselectivity,Qiaoet al.[114]immobilized montmorillonite layers bonded and aligned with the chain stretching orientation of polyvinylamineacid onto a porous PSf substrate to fabricate aligned montmorillonite/polysulfone(AMT/PSf)membranes.Owing to aligned interlayer gaps as high-speed CO2transport channels,the AMT/PSf membrane achieved a high CO2permeance of about 2.68 × 10-7mol·m-2·s-1·Pa-1and a high mixed-gas selectivity for CO2.

Apart from the common nanomaterials and layered nanomaterials,MOF and covalent organic framework(COF)are also used as nanofillers,respectively.On the one hand,the addition of MOF and COF disturbs polymer chain packing,and increases free volume,which results in the improvement of CO2permeance.On the other hand,CO2molecules can transport through the pore ofMOF and COF.Zhaoet al.[42]incorporated ZIF-8 into a PVAm solution,and coated the PVAm-ZIF-8 mixture on a PSf support membrane to prepare the PVAm-ZIF-8/PSf membrane.Compared with the PVAm/PSf membrane,the CO2permeance and CO2/N2selectivity of the PVAm-ZIF-8/PSf membrane with 13.1 wt%ZIF-8 increased by about 325%and 65%at 0.15 MPa and 79%and 140%at 2.0 MPa,respectively.However,owing to the nonselective voids between aggregated nanoparticles,CO2/N2selectivity of the PVAm-ZIF-8/PSf membrane with 23.1 wt%ZIF-8 decreased to a lower value than that of the PVAm/PSf membrane at a feed pressure of over 1.0 MPa.Caoet al.[115]incorporated a highly compatible covalent organic framework COF-LZU1 into PVAm to fabricate the PVAm-COF/PSf membranes.The PVAm-COF/PSf membrane with 10 wt%COF-LZU1 exhibited a CO2permeance of 1.33 × 10-7mol·m-2·s-1·Pa-1and a CO2/H2selectivity of 15 at 0.15 MPa.

In addition,organic nanoparticles are also chosen as nanofillers.Zhaoet al.[68]coated blend of polyaniline(PANI)nanoparticles and PVAm on PSf membranes to prepare the PVAm-PANI/PSf membranes.At CO2partial pressure of 0.02 MPa,the PVAm-PANI/PSf membrane with 17 wt.%PANI nanosheets showed a CO2permeance of 4.02 × 10-7mol·m-2·s-1·Pa-1and a CO2/N2selectivity of 120.To prevent the agglomeration of PANI nanomaterials and improve the interface compatibility between PVAm and PANI nanofillers,PANI nanorods modified by poly(vinylpyrrolidone)(PVP)adsorption layer were incorporated into PVAm matrix to fabricate the PVAm-PANI/PSf membranes[69].The as-prepared PVAm-PANI/PSf membrane displayed a high CO2permeance of 1.03 × 10-6mol·m-2·s-1·Pa-1and CO2/N2selectivity of 240 at 0.11 MPa.

Besides our group,other researchers also use PVAm as polymer matrix.Deng and H?gg[116]incorporated 1 wt%CNTs into the polymer matrix of PVAm and polyvinyl alcohol(PVA),and prepared the PVAm-PVA-CNT/PSf membrane on PSf membrane by dip-coating method for high pressure gas transport measurements.At 1.0 and 1.5 MPa,the CO2permeance of the PVAm-PVA-CNT/PSf membrane was more than doubled in comparison with counterpart PVAm-PVA/PSf membrane.Shenet al.[117]added GO grafted with hyperbranched PEI(HPEI-GO)into the polymer matrix of PVAm and Cs on a porous PSf support to fabricate PVAm-Cs-HPEI-GO/PSf membrane.For CO2/N2(10:90/v:v)mixed gas,the as-prepared membrane with 2.0 wt%HPEI-GO displayed a CO2permeance of1.21 × 10-8mol·m-2·s-1·Pa-1and a CO2/N2ideal selectivity of 90.The as-prepared membrane with 3.0 wt%HPEI-GO displayed a CO2permeance of 1.05 × 10-8mol·m-2·s-1·Pa-1and a CO2/N2ideal selectivity of 107 at 0.1 MPa and 25°C.

Ho's group incorporates different inorganic nanofillers into PVA matrix containing amine carriers to prepare facilitated transport mixed matrix composite membranes.Xing and Ho[118]incorporated fumed silica(FS),and the resulting membrane with 22.3 wt%FS loading displayed the best performance with a CO2permeance of 1.37 × 10-8mol·m-2·s-1·Pa-1and a CO2/H2selectivity of 87 at 1.52 MPa and 107°C.Zhaoet al.[119]incorporated MWCNTs,and the resulting membrane with 2 wt%MWCNTs showed a CO2permeance of 1.12 × 10-8mol·m-2·s-1·Pa-1and a CO2/H2selectivity of 43 at 1.52 MPa and 107°C.The membrane performance was maintained without significant change for 444 h.To improve affinity with the hydrophilic membrane matrix,Ansaloniet al.[120]incorporated amino-functionalized multi-walled carbon nanotubes(AF-MWNTs).The resulting membrane with 2.3 wt%AF-MWNTs displayed a CO2permeance of 1.11 × 10-8mol·m-2·s-1·Pa-1,a CO2/N2selectivity of 360,a CO2/CH4selectivity of 277,and a CO2/H2selectivity of 56 at 1.5 MPa and 107°C.Moreover,the resulting membrane demonstrated a good stability.

For flat mixed matrix composite membranes,in addition to material design and selection,technology of membrane formation is very critical for obtaining good separation performance.The flat mixed matrix composite membrane can be mainly fabricated by solution casting method and interfacial polymerization.The solution casting method is simple and cheap.Hence,it is universally used.The mixture solution is cast on substrate(porous support membrane)with a pre-set wet coating thickness by a casting knife,then is dried under certain conditions.After the solvent evaporation,the flat mixed matrix composite membrane is fabricated successfully.To solve the problem of void creation between polymer and filler in conventional solution-casting methods for the formation of MMMs,casting solutions were prepared as follows[107].A filler suspension was prepared by adding a specific amount of fillers to solvent and sonicating.Then,a quarter of the polymer solution was added to the filler suspension,stirred and sonicated.This procedure was continued until all of the polymer solution was added to the filler suspension and a homogenous casting solution was obtained.

Compared with solution casting method,interfacial polymerization is rather cockamamie and wastes a great amount of organic solvent.The flat mixed matrix composite membrane was fabricated by interfacial polymerization as follows[112].The flat sheet support membrane was initially immersed into the aqueous phase for a certain time and then the excess solution was drained from the membrane surface.Subsequently,the impregnated membrane was placed into the organic phase for a certain time at a certain temperature.After that,the resulted composite membrane was rinsed with pure organic solvent and then heat-treated.Furthermore,the composite membrane was washed with deionized water to eliminate excess monomers and byproducts.Finally,the resulted membrane was kept under certain conditions.To solve the problem of void creation between polymer and filler, fillers should be added to the aqueous phase or organic phase in which the fillers can be dispersed uniformly[112].

Generally speaking,compared with hollow fiber mixed matrix composite membrane, flat mixed matrix composite membrane possesses higher CO2permeance and higher CO2/gas selectivity.

6.Future Direction

MMMs are a promising new generation of membranes for CO2separation.Compared with polymer membranes,CO2permeability and CO2/gas selectivity of the MMMs both increase by the incorporation of suitable nanofillers.Therefore,in the future,MMMs are still the key research field to improve performance of polymer membranes.Some aspects on MMMs need to be explored further as follows:

(1)Developing new polymers with high permeability and selectivity.

(2)Synthesizing new nanofillers with suitable pore structure and particle size[9,121],especially organic nanofillers that have interaction with CO2molecules.

(3)Investigating the relationship between interface morphology and gas transport property systematically and qualitatively.

(4)Exploring new methods to improve compatibility between polymer and filler,and enhance gas permselectivity of the MMMs simultaneously.

(5)Developing high performance mixed matrix composite membranes for industrial application.

(6)Large pilot scale testing of membranes based on MMM approach[9].

(7)Employing high performance mixed matrix composite membranes to prepare MMM modules for industrial application.

(8)Establishing the membrane separation plant with MMM modules,which makes sure that membrane separation plant has advantages such as low cost and less energy consumption.

7.Conclusions

To improve CO2separation performance of polymer membranes,a large number of MMMs have been developed.Generally,MMMs contain two or more different components.Polymermatrix forms a continuous phase,and inorganic or organic fillers act as a dispersed phase.To prepare high performance MMMs for CO2separation,correct selection of polymer matrix and filler is essential.The polymer should possess high CO2permeability and high CO2/gas selectivity.Moreover,the polymer should have high mechanical strength,and good thermal stability,chemical stability and processability.Matrimid?is the best polymer for CO2/CH4separation under high pressure,and PVAm is the best polymer for CO2/N2,CO2/CH4and CO2/H2separation under low pressure.The filler should have high selectivity,good compatibility with polymer matrix,and small particle size.In the high performance MMMs,the fillers not only disturb polymer chain packing and increase free volume,but also facilitate CO2transport by itself.Compared with inorganic fillers,organic fillers are emerging fillers.PANIis the best filler for CO2/N2separation,NHs is the best filler for CO2/CH4separation,and MIL-53 is the best filler for CO2/H2separation.

Owing to the differences between polymer matrix and filler,there are different interface morphologies between polymer matrix and filler.When there is poor compatibility between polymer matrix and filler,interface voids form.When the size of interface voids was larger than the mean free path of the gas molecules,and the interconnectivity of interface voids formed interface void channels across the MMMs,gas permeability increased and gas selectivity decreased with increasing feed pressure.

To avoid interface voids and improve membrane performance,researchers adopt eight methods including silane coupling,Grignard treatment,incorporation of additive,grafting,in situpolymerization,PD coating,particle fusion approach and polymer functionalization to enhance interface compatibility.In principle,during modification process by using these methods except Grignard treatment,hydrogen bonding or chemical bonding forms,which leads to the fact that the interactions between polymer matrix and inorganic nano filler improve interface compatibility.

To achieve higher productivity for industrial application,mixed matrix composite membranes are developed.Compared with hollow fiber mixed matrix composite membrane, flat mixed matrix composite membrane possesses higher CO2permeance and higher CO2/gas selectivity.

In the future,further research on MMMs should be focused on the following aspects.On the one hand,researchers should develop new polymer and new filler to prepare high performance lab-scale MMMs,and investigate the effect of interface morphology on gas transport property.On the other hand,researchers should fabricate large-scale mixed matrix composite membranes with high permselectivity,and then assemble membrane modules to build membrane separation plant.

Nomenclature

Amembrane area,m2

Lccharacteristic length of the core region,m

lmembrane thickness,m

Ppermeability coefficient,mol·m·m-2·s-1·Pa-1

ΔPtransmembrane partial pressure,Pa

Qpermeation rate,m3(STP)·s-1

Rpermeance,mol·m-2·s-1·Pa-1

xthe molar fraction of gas species in the feed side

ythe molar fraction of gas species in the permeate side

αi?/jideal gas selectivity

αi//jmixed gas selectivity or separation factor

Subscripts

igas species

jgas species

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