Construction of molecule-selective mixed matrix membranes with confined mass transfer structure☆

Weidong Li,Fusheng Pan ,3,Yimeng Song ,Meidi Wang ,Hongjian Wang ,Shalik Walker,Hong Wu ,3,Zhongyi Jiang ,3,*

1 Key Laboratory for Green Chemical Technology of Ministry of Education,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

2 Collaborative Innovation Center of Chemical Science and Engineering,Tianjin 300072,China

3 Tianjin Key Laboratory of Membrane Science and Desalination Technology,Tianjin University,Tianjin 300072,China

1.Introduction

Membrane technology is an energy-efficient,low-cost and molecular-level separation technology,which has emerged as a significant and universal technology to solve the big challenges,such as energy and environmental problems[1].The membranes can be classified into porous membranes and dense(non-porous)membranes.Among which,the dense membranes are widely used in many separation processes(e.g.,pervaporation,gas separation and reverse osmosis).The dense membranes include polymeric membranes,inorganic membranes and mixed matrix membranes(MMMs).Polymeric membranes have superior process ability for large-scale industrial implementation.However,easy swelling,low selectivity,poor mechanical property and the trade-off between permeability and selectivity,are among their main disadvantages.The inorganic membranes based on zeolites,silicas,ceramics and so on have some advantages of high thermal stability,superior chemical stability and excellent separation performance,but the high cost and low processability of inorganic membranes hinder their applications[2].To overcome the limitation of both polymeric and inorganic membranes,the MMMs are fabricatedviaintroducing the fillers into the polymer matrix.The MMMs,which combine low cost and easy processability of the polymers with the excellent physical and chemical characteristics of the fillers[3],have been extensively used owing to their 4M characteristics(multi phases,multi scale,multi functionalities and multi interactions)[4].

The mass transfer mechanism in the dense membranes has been investigated during the last few decades[5-8].The mass transfer of penetrant molecules in the dense membrane occurs in the pores(or voids)whose effective size is close to the mean free path of permeate molecules(usually＜2 nm)[9-12].The mass transfer process of molecules in this confined space is quite different with that in the porous membranes.The interactions between the pore wall and the penetrant molecules would be equivalent or even stronger than the interactions among the penetrant molecules,which would show a large influence on the transport of the penetrant molecules[13,14].The mass transfer rate of penetrant molecules could be highly enhanced,even larger than those in their body phase.This extraordinary mass transfer phenomenon of molecules in the confined structure is named as “the confined mass transfer effect.”The membranes with proper confined structure would show much higher permeability and selectivity than predicted by classic theory of mass transfer.Some parameters(e.g.,free volume properties(size of the free volume voids and the corresponding intensities),glass transition temperature(Tg),crystallinity)can be utilized to evaluate the confined structure of membranes[15-17].The subtle change of the confined structure can bring huge fluctuations of the separation performance[18].For instance,free volume properties of PDMS membranes were tuned through incorporating dopamine or dopamine/Cu nanoaggregates into PDMS matrix to achieve two-fold higher permeation flux[19].The rich diversity of the fillers for MMMs will provide plenty of approaches to optimize the confined structures and probe the confined mass transfer effect in MMMs.

Recently,many kinds of fillers have been used to optimize the confined mass transfer structures of MMMs to enhance the separation performance.1D fillers such as carbon nanotubes(CNTs)[20,21],single-walled aluminosilicate nanotubes(SWANTs)[22],nanorods[23]and titanate nanotubes[24]have been employed in MMMs.2D materials with the special properties of anisotropic nature and high aspect ratio,were used for the fabrication of the MMMs with specific 2D channels,including graphene oxide(GO)[25],graphitic carbon nitride(g-C3N4)[26],and amphiphilic carbonaceous material(ACM)[27].Some novel 3D materials such as metal organic frameworks(MOFs)and covalent organic frameworks(COFs)with favorable affinities toward polymer have drawn tremendous attention to fabricating the MMMs instead of traditional 3D materials(e.g.,zeolites[28],silica[29],activated carbons[30]).In this review,the construction of MMMs containing 1D,2D and 3D fillers and their applications in liquid-phase separations(pervaporation)are described,and efforts toward comparing the MMMs and stating the research prospective of the MMMs are given.

2.Construction of MMMs with 1D Fillers

1D materials with high aspect ratio and excellent mechanical properties have widely been used as fillers to fabricate MMMs.Some 1D fillers with unique structures and specific properties can provide passageways for molecules through their hollow cores.Therefore,the incorporation of 1D fillers into polymer matrix changes the confined structure of the membrane,potentially enhancing the performance of membranes.In the 1D material family,CNTs have gained extensive attentions in preparing the MMMs with high performance,which will be emphasized in the following paragraph.Other representative 1D materials(e.g.,single-walled aluminosilicate nanotubes(SWANTs),at tapulgite(AT)nanorods)used as fillers to prepare the MMMs are also introduced in this section.

2.1.CNTs and CNT-filled MMMs

2.1.1.CNTs

CNTs,constructed from sp2carbon units,have unseamed and tubular graphitic structures and are novel carbonaceous materials,which have raised extensive attentions in the recent decades since Iijima found that CNTs are elongated fullerenes in 1991[31].CNTs are divided into two types:single-walled carbon nanotube(SWCNT)and multi-walled carbon nanotube(MWCNT)as shown in Fig.1.In the case of the MWCNTs constructed from numerous concentric and closed carbon-hexagon tubules,the diameters usually vary from 2 to 30 nm,and the space between the two adjacent sheets is 0.34 nm,approximately[31].The number of carbon-hexagon sheets for MWCNTs changes from 2 to 50.In the case of the SWCNTs composed of only one layer of the carbon-hexagon sheet.The diameters of SWCNTs generally range from 0.7 to 2.0 nm[32].Due to the strong interactions between SWCNTs,they are prone to curve and tangle together,forming bundling and entangling structures[33].

CNTs possess not only unique structures but also special properties.Owing to the ideal alignment,helicity and flexibility of lattices along with the unseamed and tubular graphitic structure,the CNTs possess distinguished electronic properties,special mechanical properties(e.g.,high stiffness,great axial strength),inertia,excellent conductivity,chemical specificity[34],thermal and optical properties[35].Besides,CNTs possess some distinct characteristics such as the high surface area as well as the smooth and straight channels in their cores.

2.1.2.CNT-filled MMMs

CNTs possess the smooth,straight and 1D channels,which provide low-resistant routes for appropriate molecules[11].Hence,CNTs can be utilized as fillers to manipulate the confined structures of polymeric membranes,improving the performance of polymer membranes.Since the application of CNTs in MMMs was investigated by Hindset al.in 2004[36],the CNT- filled MMMs have received great attentions on the separation processes,including pervaporation.Owing to superior properties of the CNTs,the CNT- filled MMMs exhibited superior pervaporation performance than the pure polymer membranes.Choiet al.fabricated the MMMs by adding pristine CNTs into PVA matrix for dehydration of 90 wt%alcohol aqueous solutions at 303 K[37].They found that the incorporation of the pristine CNTs increased theTgof membranes but decreased the crystallinity of membranes.The permeation flux of the MMMs increased with the increasing content of CNTs,while the separation factor remained a certain range.However,with the CNT content up to 3 wt%,the separation factor decreased sharply.The sharp decrease of separation factor was arisen from the aggregation and poor distribution of CNTs in membranes at high CNT concentration[38].The apparent defects between CNT aggregates and PVA resulted in the remarkable decline of the separation factor[39].Xue's group incorporated various amounts of pristine CNTs into polydimethylsiloxane(PDMS)matrix to determine their effects on ethanol recovery from model aqueous solutions[40].Due to the good affinity between pristine CNTs and PDMS,the pristine CNTs were well dispersed in PDMS matrix and led to an improvement of ethanol recovery.

Although the great potential performance of CNT- filled MMMs has been proved,challenges still exist to fabricate the well-controlled MMMs.With strong cohesive force,the CNTs can form bundling and entangling structures,tending to agglomerate during preparing MMMs.It is significant to develop effective strategies to make the CNT particles well-dispersed in the polymer matrix.The improvement to construct the MMMs in the literature mainly focused on the dispersion of CNTs in MMMs.The modifications of CNTs through physical or chemical methods can effectively enhance the dispersion of CNTs.

2.1.2.1.Physically modified CNT-filled MMMs.The physical modification methods mainly include adsorbing or winding organic molecules(e.g.,super molecules,polymers,surfactants)onto the surface of CNTs.It has been abundantly proved that the physically modified CNTs can effectively negate the van der Waals forces between the CNTs and disperse well in the polymer matrix.The merit of the physical modification method is that the structure and the pristine properties of CNTs can be preserved under moderate reaction conditions[41-43].

Fig.1.Sketch of the structure of SWCNT and MWCNTs[21].

Supermolecules can be used to improve the dispersion of CNTs effectively.Penget al.[20]used β-cyclodextrin(CD)to disperse CNTs and then embedded the physically modified CNTs into the PVA matrix.The CNTs amended by β-CD distributed better than the pristine CNTs as shown in Fig.2,which indicated that the surface of CNTs could absorb the β-CD through van der Waals forces.Then,the MMMs with the functionalized CNTs were used in pervaporation for the separation of benzene/cyclohexane(50 wt%/50 wt%)mixtures at333 K.The incorporation of the physically modified CNTs enhanced the benzene permeation flux to 61 g·m-2·h-1along with the separation factor to 41.2,which broke the trade-off effect between the permeability and the selectivity.

Apart from the supermolecules,polymers were also used to functionalize CNTs to improve the dispersion of CNTs in the polymer matrix,fabricating high-performance MMMs.Penget al.[44]added chitosan(CS)-wrapped CNTs into PVA matrix to fabricate the MMMs for pervaporation separation of benzene/cyclohexane(50 wt%/50 wt%)mixtures at 333 K.The CS successfully wrapped the CNTs as shown in Fig.3,and the CS-wrapped CNTs dispersed homogeneously in PVA matrix due to the reduction of the interactions among CNTs as well as the enhancement of the interactions between CNTs and PVA.The incorporation of CNTs increased free volume and created appropriate free volume voids with the size of 0.269 nm between benzene molecular diameter(0.263 nm)and cyclohexane molecular diameter(0.303 nm),improving the confined structure of membranes.Therefore,the CS-wrapped CNTs filled MMMs exhibited higher permeation flux(65.9 g·m-2·h-1)and separation factor(53.4)than that of pure PVA membranes(20.3 g·m-2·h-1and 9.6,respectively).Since then,fabricating MMMs with CNTs functionalized by polymers gradually became a hot topic.Sajjanet al.[45]incorporated CS-wrapped CNTs into sodium alginate(SA)matrix to fabricate the MMMs.Owing to the suitable changes of the confined structure of membranes such as the increased free volume,the decreased crystallinity and extra nano-channels provided by CNTs,the MMMs with uniform CS-wrapped CNTs showed superior pervaporation performance for separating water and isopropanol mixtures.Similarly,other researchers[41,46,47]prepared MMMsby incorporating polymer-wrapped CNTs for pervaporation.

2.1.2.2.Chemically modified CNT-filled MMMs.Chemical modification methods are modifying the CNTs by attaching functional groups(e.g.,hydroxyl groups,carboxyl groups)or modifiers on the surface of CNTs[48]to reduce the van der Waals forces between the CNTs and enhance the dispersion in the polymermatrix.Compared to the physical modification method,there are some advantages for modifying CNTs through the chemical method:there are a wide variety of reagents to functionalize the CNTs[49].Then,the surface chemical properties(such as hydrophilicity or hydrophobicity)can be modified through introducing different kinds of functional groups,such as hydroxyl groups,carboxylic groups[39].Besides,the functionalized CNTs can be further modified by grafting of functionalities through those functional groups[50].

Fig.2.The TEM morphologies of(a)pristine CNTs and(b)CNTs dispersed by β-CD[20].

Fig.3.(a)The model and(b)the TEM morphology of CS-wrapped CNT[44].

Using oxidants and/or reductants to treat the surfaces of CNTs is a conventional chemical method for the functionalization of CNTs,which can provide hydroxylgroups or carboxyl groups onto the termini of tubes or at the defect sites.Shiraziet al.[51]functionalized the CNTs synthesized by CVD method using nitric acid to graft carboxyl groups onto the CNTs.The modified CNTs were homogeneously dispersed in PVA matrix,which constructed the fine confined structure of MMMs for pervaporation.Compared with the pure polymer membranes,the MMMs containing 2 wt%CNTs exhibited better swelling resistance and higher separation factor(1794)for isopropanol/water mixtures(90 wt%/10 wt%)at 303 K,while they showed lower permeation flux(79 g·m-2·h-1)of the free volume due to reduction of free volume.Sieffertet al.[52]synthesized hydroxyl group containing CNTs through a two-step reaction.The hydroxyl groups on the CNTs not only favored the dispersion of CNTs inside MMMs but also covalently linked with the carboxyl groups on the copolyimide by esterification.Compared with the non-covalently bound MMMs,the covalently linked MMMs increased the flux by 20%.

Other groups or modifiers were further linked with the hydroxyl groups or carboxyl groups on CNTs to enhance the dispersion of CNTs and improve the affinity between CNTs and polymers.The aminated MWCNTs(MWCNT-NH2)were synthesized through a two-step reaction by Wanget al.[53]as shown in Fig.4(a).The polar functional groups were successfully functionalized onto MWCNTs and enhanced the surface polarity of MWCNTs.The MWCNT-NH2were introduced into poly(methylmethacrylate)(PMMA)matrix to fabricate the MMMs.Compared with the MWCNT-COOH,the MWCNT-NH2exhibited superiordispersion in PMMAmatrix due to the amino groups.The MWCNT-NH2filled MMMs also showed higher separation performance for benzene/cyclohexane mixtures.Shenet al.[54]treated the MWCNTs with a mixed acid,then modified with isonicotinic acid and finally grafted Ag+onto the pyridine ring on the MWCNTs to synthesize MWCNT-Ag+.Then the MWCNT-Ag+fillers were incorporated into the CS matrix to separate benzene/cyclohexane(50 wt%/50 wt%)mixtures at 293 K as shown in Fig.4(b).The MWCNT-Ag+uniformly dispersed in CS matrix without obvious aggregation.Due to the increment of free volume and the facilitated transport of benzene molecules by silver ions,the permeation flux and separation factor were enhanced to 357.96 g·m-2·h-1and 7.89,respectively.

As mentioned above,the chemical modification can improve the dispersion of CNTs.Nevertheless,the functional groups on the surface of CNTs may be inadequate to provide a good dispersion in a polymer matrix,which leads to the agglomeration and interfacial defects.A secondary treatment introducing inorganic nanoparticles onto CNTs turns out to be a processable method to enhance the dispersion of CNTs[55].Gaoet al.[48]treated the MWCNTs with mixed acid and then took a secondary treatment by incorporating hydrophilic Fe3O4nanoparticles onto CNTs to prepare Fe3O4@CNTs as shown in Fig.5.The Fe3O4@CNTs were filled into SA matrix to prepare the MMMs for ethanol dehydration,exhibiting the well-dispersion in SA matrix and the good compatibility between fillers and SA.The Fe3O4@CNTs in SA matrix would hinder the membrane crystallinity,enlarged the free volume cavities,offered fast-moving micro-channels for permeate molecules.The MMMs showed a high permeation flux of 2211 g·m-2·h-1and a separation factor of 1870 for dehydrating 10 wt%water-ethanol feed at 349 K.Similarly,Wanget al.[39]also prepared MMMs by the incorporation of the inorganic nanoparticles decorated CNTs for pervaporation.

Fig.4.Schematic of preparation process of(a)MWCNT-NH2[53]and(b)MWCNTs-Ag+[54].

Fig.5.Schematic of preparation process of Fe3O4@CNT[48].

2.2.Other 1D filler-filled MMMs

2.2.1.Attapulgite(AT)nanorod-filled MMMs

AT nanorods,a crystalline hydrated magnesium aluminum silicate,have a 1D nanostructure[23].AT nanorods have a three-layer sheet structure with alternating arrangement,which possesses open channels with free cross-section about 0.37 nm×0.60 nm paralleled to the long axis[56].The internal or intercrystalline channels of AT nanorods bear with abundant activated hydroxyl groups that can bond with water molecules through hydrogen bonds[57].In addition,AT nanorods also possess special properties such as large surface area,high adsorption properties,and high chemical stability.These properties of AT nanorods make thempromising fillers to prepare the AT nanorod- filled MMMs for pervaporation.Xinget al.[23]utilized hydrophilic AT nanorods with selective channels into SA to prepare MMMs for ethanol dehydration.The permeation flux of AT-SA/polyacrylonitrile(PAN)membranes increased by 1.4 times to that of the pristine SA/PAN membranes.

2.2.2.SWANT-filled MMMs

The hydrophobicity nature of CNTs will cause the poor dispersion of CNTs in the membrane hindering the improvement of separation performance.Metal oxide nanotubes with polar surfaces may have the potential for better dispersion[58].Single-walled aluminosilicate nanotube(SWANT),constructed from a rolled and seamless gibbsite sheet together with single silanol groups onto the internal surface as shown in Fig.6(a),have a 1D periodic wide and narrow channel structure in the internal core,which is different from the one-dimensional smooth and uniform channel of CNT.Fig.6(a)shows that the outer and inner diameters of SWNT are 2.3 and 1.0 nm,respectively[59].SWANT possesses a high hydrophilic internal surface owing to high inner surface silanol density(Fig.6(b))[60],which is conducive to water transport[61].Then,the controllable functionalization of the inner and/or outer internal surface of SWANTs can develop their properties and applications to a great extent[60,62,63].Kanget al.[22]dispersed SWANTs with high content in the PVA matrix for dehydration of water/ethanol mixtures.SWANTs were dispersed in MMMs to a high loading(up to 40 wt%),achieving a near-ideal dispersion of SAWNTs.Due to the excellent dispersion,unique structures and special properties of SWANTs,the SWANT- filled MMMs exhibited high pervaporation performance.

3.Construction of MMMs with 2D Fillers

2D materials of atomic thickness are employed as nano-building blocks to synthesize high-performance membranes for liquid separation.Due to the anisotropy,high aspect ratio,and horizontal packing orientation provided by 2D materials,MMMs with such fillers feature unique nanopores and/or nanochannels,hence exhibit significant permeability.Intrinsic porous architectures,accurate perforation,as well as the controllable assembly of 2D fillers contribute to forming nano-or sub-nanometer pathways,which lead to confined-masstransfer performances.In the large 2D materials family,graphene possesses remarkable properties to construct nanoscale confined structure,which will be emphasized in the following paragraph.Other representative 2D materials[graphitic carbon nitride(g-C3N4),molybdenum disulfide(MoS2)]used as fillers to prepare the MMMs for pervaporation are also recommended in this section.

3.1.GO and GO-filled MMMs

3.1.1.GO

As one of the most important derivatives of graphene,GO nanosheet is a laminated substance consisting of hydrophilic oxygenated graphene layers(see Fig.7),with a high surface area around 2600 m2·g-1[64].Reactive oxygen functional groups,such as hydroxyl,epoxy,carboxyl,and carbonyl groups,are “decorated”on the basal planes and edges,which render GOa good candidate for loads of applicationsviachemical or physical interactions.GO can be easily exfoliated from flake graphite through chemical oxidization.

Fig.7.A proposed schematic of GO[65].

Fig.6.(a)Cross-section structure of single-walled aluminosilicate nanotube and(b)a proposed schematic of the hydrated SWNT with hydrophilic inner wall[60].

GO nanosheets bear with unprecedented ultrafast water-transport channels,which are generated by the packing of such multilayers.Edge-to-edge slits and interlayer galleries within the GO laminates offered by the robust and scalable approach provide molecular passages.The oxygen-enrichment functional groups guaranteed the dispersion of GO well in water,as well as providing active sites for enhancing particular bonding with transport components,i.e.hydrogen bonding.The different roughness of the GO surface could affect the membrane surface characteristics from hydrophilic to hydrophobic[66].In order to achieve a better separation performance,the GO microstructure should be dense and well-aligned.Pressurized filtration method is employed to assemble such ordered free-standing GO thin film.The topology of the GO film reveals a well-stacked structure with a layer-by-layer pattern.The interlayer spacing is controlled by both packing density of GO layers and components of the feed solution.For instance,the water molecules linked with the oxygen-containing functional groups on GO nanosheetsviaintermolecular hydrogen bonds as shown in Fig.8.The connections tune the inter laminar spacing beneath the GO layers,thus form suitable pathways for targeted molecules.The research proposed by Chunget al.[67]suggested that the interactions among GO sheets and the feed solution enable the membranes to dehydrate polar solvents.Therefore,GO can serve as a decisive role as dispersed phase within MMMs for high-performance pervaporation.

Fig.8.Structural diagram of GO layers(gray)linking with water molecules by hydrogen bonds.

3.1.2.GO-filled MMMs

Researchers have focused on fabricating GO- filled membranes with good separation performance,although it remains a challenge to maximum the separation properties of GO by taking advantages of these novel pathways.In order to form the above-mentioned functional pathways,GO layers are piled up by blending with polymers.The physical blending method is employed in a broad range for preparing highly uniform MMMs with controlled morphology.In this approach,GO layers are dispersed in the dope solution.The dope solution will form MMMs after a series of post-treatments,including evaporation and annealing.The loadings of GO within the membranes are controlled by directly altering the concentrations of the dope solution.Due to π-π electrostatic interactions,GO could spontaneously form“brick-and mortar”structure(parallel to the membrane surface)in the polymer matrix.The as-prepared maze architecture interferes with the molecular diffusion,promotes the compatibility between GO particles and polymers,and optimizes the selectivity of the MMMs.Recently,some researchers found that different types of GO(e.g.,pristine GO,reduced GO,functionalized GO)affect the crystallization and arrangement of polymer chains to different degrees,resulting in lowering the movements of polymer segments,thus forming more selective pores within the thin film substrate,together with moderately changing the free volume of membrane.

3.1.2.1.Pristine GO-filled MMMs.Pristine GO- filled MMMs are often coated with hydrophilic polymers in order to increase the flexibility of the composites because the pure GO laminates are often brittle without further treatments.Zhaoet al.[68]fabricated an ultrathin pristine GO-filled MMMs(thickness less than 115 nm)by layer-by-layer(LbL)self-assembly method.Gelatin(GE)and GO were deposited alternately on hydrolyzed polyacrylonitrile(H-PAN)ultra filtration membranesviamultiple interactions,including hydrogen bonds,electrostatic attractions,and hydrophobic repulsions,as shown in Fig.9.Hydrogen bonds were generated between polar functional groups on GE and GO;Electrostatic attractions were generated between protonated amino groups on GE and ionized carboxyl groups on GO;Hydrophobic repulsions were generated between amino acid side chains on GE and carbon backbones of GO.The micrometer-scale size of GO nanosheets is favorable for the coverage of the relatively “macro-pores”on H-PAN membrane surface,because of the GO layers along the horizontal direction,and the uniform packing of GO within the MMMs.The multilayer MMMs with the bilayer number 10.5 realized a synchronous increase in separation factor(1316,with a water content 98.7 wt%in the collected permeate)and permeation flux(2275 g·m-2·h-1)for pervaporative dehydration of 20 wt%ethanol aqueous solution at 350 K.Xu's group[69]integrated a hydrophilic CS layer(thickness less than 10 nm)over the pristine GO laminates as a composite membrane to dehydrate butanol under the conditions of343 K and butanol content in feed of90 wt%,which was a further modification of the previous work[70].The overall thickness of the ultra thin membrane is 52 nm.The proposed coupled effect of facilitated water capture from the hydrophilic layer and molecular pathways from the pristine GO laminates achieved the goal of selective and fast water transport through the designed structure,which increased the separation factor to 2580 without sacrificing permeability.

3.1.2.2.Reduced GO-filled MMMs.Caoet al.[71]prepared asymmetric hybrid membranes for pervaporative ethanol dehydration by firstly dissolving a certain amount of SA in various amounts of reduced graphene oxide(rGO)aqueous dispersions under continuous stirring.The uniform mixtures were then spin-coated onto PAN ultra filtration membranes,with the following step of solvent evaporation to generate membranes.Cavities around SA-GO interfaces,defects on pGO/rGO nanosheets and gaps between GO layers contribute to the constitution of interconnected water-selective transportation channels,with average free volume radii around 0.27 nm characterized by positron annihilation lifetime spectroscopy(PALS).Water molecule with a kinetic radius of 0.15 nm could transport through the slits under confined-mass-transfer effect while ethanol molecule with a kinetic radius of 0.22 nm encountered higher transfer resistance(see Fig.10).Besides,non-oxide regions on rGO nanosheets further facilitate the water diffusion to the hybrid surfaces.Thus,composite rGO/SA/PAN membranes with a 1.6 wt%rGO loading exhibited an optimum ethanol dehydration performance with a permeation flux of 1699 g·m-2·h-1and a separation factor of 1566 under 90 wt.%ethanol aqueous solution and 350 K.

3.1.2.3.Functionalized GO-filled MMMs.The pristine GO flakes are easy to agglomerate during MMMs fabrication process.In order to increase the compatibility between GO and the polymer matrix,it is wide accepted to modify pristine GO layers with particular chemical groups.Zhaoet al.[72]modified pristine GO nanosheets with sulfobetaine methacrylate(SBMA),hence synthesizing zwitterionic graphene oxide(PSBMA@GO)nanosheets.Subsequently,the PSBMA@GO layers were incorporated into SA matrix with various loadings for alcohol dehydration.The zwitterionic surface of the functionalized fillers inside the MMMs ensured efficient water passages as shown in Fig.11.The incorporated PSBMA@GO altered the free volume properties of membranes,which provided the smaller free volume voids with the radius smaller than 0.203 nm that lies between water molecular(0.13 nm)and ethanol molecular(0.22 nm),conducive to the mass transport of water molecules.Hence,the reported MMMs led to an excellent pervaporation performance with a separation factor of 1370 and a permeation flux of 2140 g·m-2·h-1for 90 wt.%ethanol aqueous solution at 350 K.

In addition,other approaches were investigated on functionalization of GO nanosheets.Mahmoudiet al.[73]attached silver nanoparticles to pristine GO nanosheets.The decoration of silver nanoparticles strengthened the interactions between functionalized layers and polysulfone(PSf).The unique property of silver ions also blessed the MMMs with excellent antibacterial properties.Wanget al.[25]modified pristine GO nanosheets with poly(ethyleneimine)(PEI)by a simple sonication approach.The PEI-modified GOlaminates had a nanoscale stable dispersion in polyelectrolyte complexes(PECs).The MMMs showed superior substituent for dehydration of organic mixtures.

Fig.9.Schematic representation of the interfacial interactions in composite multilayer membrane[68].

Fig.10.Mechanism of selective water permeation through the composite membrane with well-aligned GO nanosheets[71].

3.2.Other 2D filler-filled MMMs

Other 2D materials have been widely utilized as fillers in MMMs because of the low aspect ratio of the exfoliated layers,i.e.graphitic carbon nitride(g-C3N4)and molybdenum disul fide(MoS2).

Fig.11.Schematic illustration for water permeation through the PSBMA@GOs/SAMMMs.The plentiful cationic and anionic groups on the surface of PSBMA@GOs provide abundant active sites with water molecules,forming hydration layers.Then,the hydration layers provide fast transport pathway for water molecules[72].

3.2.1.g-C3N4-filled MMMs

g-C3N4is an emerging 2D material and has attracted great attention due to unique properties,outstanding stability and facile synthesis.The bulk carbon nitride materials can be easily exfoliated into 2D g-C3N4nanosheets(CNs)viavarious practical approaches because the aromatics-triazine rings(C3N3)contribute to forming π-conjugated planar layers.The 2D nanosheet possesses regularly distributed triangular pores over the entire laminate.The average geometric diameter of the triangular nanopores is calculated to be 0.311 nm as shown in Fig.12,recommending that water molecules with a kinetic diameter of 0.26 nm will pass freely through the nanopores.Prepared by the thermal oxidation “etching”and ultrasonication procedures,the CNs are usually rich of defects with sizes between 0.31 nm and 0.34 nm due to the network terminations of uncondensed NH and NH2groups[74].

Caoet al.[26]reported preparing water-selective hybrid membranes by physically blending CNs into SA matrix.The membranes displayed enhanced mechanical,thermal,anti-swelling characters,as well as ethanol dehydration performance(permeation flux of 2469 g·m-2·h-1and separation factor of 1653)under 90 wt%ethanol aqueous solution and 350 K when the CNs loading reached 3 wt%.CNs played the following three roles in facilitating the membrane properties:(1)The 2D nanosheets aligned horizontally within the hybrid matrix,forming ordered water transport channels;(2)the tremendous nanopores all over the CNs sheets offered molecular sieving effect for water molecules while blocking the transport of ethanol molecules through the structures;(3)hydrogen bonds are generated between residual NH or NH2functional groups from CNs and OH or COO-groups from SA chains,thus decreased the crystallinity of the membranes.

3.2.2.MoS2-filled MMMs

MoS2,classified as a metal dichalcogenide,can be exfoliated to inorganic single-or few-layer nanosheets.Such 2D material exhibits extraordinary chemical,physicaland electronic properties[77].Choudhariet al.[78]designed composite membranes for short-chain volatile fatty acids(VFAs)separation,including butyric acid.The designed MMMs was achieved by physically dispersing layered MoS2nanosheets in polyether block amides(PEBA)matrix.Similar to GO layers,pathways for permeants were formed through the packing of 2D layers.The separation performance was enhanced slightly compared to pure PEBA substrate,suggesting that it still needs improvements compared to other MMMs with developed 2D fillers.However,other research[79]showed that a laminar MoS2membrane(with a total membrane thickness around 1.8 μm and channel size around 3 nm)exhibited superior wastewater treatment ability.Therefore,MMMs with excellent performance by incorporating MoS2nanosheets are expected.

Fig.12.(a)Structural model of CNs with triangular nanopores in the red circle and defects in the green rectangle.(b)Structure magnification of the triangular nanopores.Gray spheres represent carbon atoms,and blue spheres represent nitrogen atoms.The bond length of C--N is 0.133 nm and the bond angle of N--C--N is 120°[75].The covalent diameter of nitrogen atom denoted by a blue circle is 0.15 nm[76].The geometric pore diameter can be derived as 0.311 nm.

In addition,2DMOFs and COFs as the emerging architecture of MOFs and COFs have been favored by researchers,are used as the fillers to fabricate the MMMs.MOFs are porous coordination materials,containing metallic nodes(i.e.metal atoms and metal clusters)and bridging organic compounds(also known as “organic linkers”).COFs are pure organic materials,which are constructed from strong covalent bonds.MOFs/COFs with 2D layered crystalline structures[80,81]have been proved to be excellent fillers in MMMs.Apart from the intrinsic molecular sieving property in MOFs/COFs materials,the packing of well-aligned MOFs/COFs nanosheets could eliminate unselective permeation channels in comparison with 3D MOFs/COFs particles.Thus,such 2D architecture could achieve a simultaneous enhancement in the permeability and selectivity of pervaporative membranes.For example,Pan and coworkers[82]synthesized 2D ZIF-L nanosheets,a subfamily ofMOFs,and embedded ZIL-L nanosheets within SAsubstrate to prepare the MMMs for pervaporation ethanol dehydration.The MMMs exhibited superior separation performance,which derived from the ordered alignment,the regular apertures,and the desirable molecular sieving effect of ZIF-L nanosheets.Although 2D COFs have not been used to prepare MMMs for pervaporation,they have been applied for gas separation.Kanget al.[83]used exfoliated 2D COFs as fillers to fabricate MMMs for CO2separation,exhibiting excellent H2/CO2permselectivity.

4.Construction of MMMs with 3D Fillers

Traditional 3D nanomaterials(e.g.,zeolites,silica,activated carbons,metal oxides)have been used in the preparation of MMMs,but their applications are limited due to the rigidness and poor tunability.Nowadays,some novel 3D nanomaterials with excellent affinity and flexibility,such as MOFs,COFs,polyhedral oligosilsesquioxane(POSS),are used as fillers to prepare the MMMs.Those novel 3D fillers containing organic components have excellent affinity with the polymers,which is beneficial to prepare the homogeneous MMMs without defects.Moreover,the tunable pores of fillers give a transport passageway for permeation molecules.Hence,the confined structure of membrane could be improved by the incorporation of the novel 3D fillers into polymer matrix.In the large 3D material family,MOFs have gained extensive attention in preparing the MMMs with high performance,which will be emphasized in the following paragraph.

4.1.MOFs and MOF-filled MMMs

4.1.1.MOFs

MOFs,a kind of novel 3D nanoporous materials,are composed of inorganic metalcenters and organic ligand bridges,which interconnected to form a class of the crystals with periodic network structures through self-assembly.MOFs were first systematically investigated by Yaghiet al.in 1995[84].Then far beyond 15000 crystal structures[85]were synthesized in the past decades.Since the MOF structures can be regulated and controlled through changing various inorganic connectors and organic ligands,the structured microporous MOFs possess a variety of pore morphologies and aperture sizes along with enterable cages and channels.MOFs have high porosity,large specific surface area,excellent adsorption performance and screening performance.Besides,MOFs are provided with other special properties such as the tunability of hydrophobicity or hydrophilicity,the availability of external surface functionalization[86].Fig.13 shows some structures of typical MOFs used for the preparation of the MOF- filled MMMs for the separation process.

4.1.2.MOF-filled MMMs

Recently,some researchers have paid attention to preparing the MMMs for pervaporation by incorporating various MOFs.[Cu2(bza)4(pyz)n]was the first MOF applied as fillers in MMMs for pervaporation[88].Then many kinds of MOFs were chemically bonded or physically doped into the polymer matrix to prepare MMMs for pervaporation membranes.For chemical bonded membranes,MOFs are linked to polymer chains by chemical bonds.For physical doped membranes,interactions between MOFs and polymer are noncovalent interactions,such as dispersion,polarity,and hydrogenbonds[87].The incorporated MOFs can change the arrangement of polymer chains,adjust the chain spacing,and optimize the free volume fraction of the membranes.Meanwhile,the organic linkers of MOFs have good compatibility with polymers which can reduce the microgaps between MOFs and polymers.Additionally,the versatile architectures of MOFs provide highly ordered confined spaces for molecules,which enhance the mass transfer rate of small molecules and hinder the big molecules.In order to obtain the MOF- filled MMMs with high performance,appropriate MOFs fillers should be synthesized with the specific pore topologies and the functional groups connected with the organic linker[89].MMMs with zeolitic imidazolate frameworks(ZIFs),Materials Institute Lavoisier(MIL)and CuBTC as fillers will be described in detail.

4.1.2.1.ZIF-filled MMMs.ZIFs,a subfamily of MOFs,are 3D porous crystals composed of tetrahedral Zn2+or Co2+ions and various imidazolate linkers,which have been well studied by Yaghiand co-workers[90-92].ZIFs show excellent properties of various pore sizes,high porosity,as well as thermal and chemical stability[93-95].Due to the flexible structures and good compatibility with polymers,ZIFs have been widely used as fillers to construct the confined structure of MMMs for pervaporation.

Fig.13.Structure of some typical MOFs[87].

ZIF-8 with unique sodalite topology is constructed from Zn2+and 2-methylimidazole ligands.The hydrophobicity of ZIF-8 results in its higher affinity with organic molecules than water,which suggests that ZIF-8 nanomaterials can be applied as fillers in pervaporation MMMs for organic compounds recovery from aqueous solutions.Liuet al.[93]incorporated the ZIF-8 nanoparticles into polymethylphenylsiloxane(PMPS)matrix to fabricate the organophilic pervaporation membranes by dip-coating method.Due to the affinity with polymer chains,ZIF-8 fillers were homogeneously dispersed in PMPS matrix without defects.The as-synthesized MMMs were used for isobutanol recovery from 1.0 wt%aqueous solutions at 353 K,exhibiting the unprecedented pervaporation performance(a separation factor of 40.1 and a permeability of6454 GPUas shown in Fig.14).The well-dispersion and hydrophobicity of the ZIF-8 result in the confined structure for isobutanol preferentially transporting through the membranes.Since then,the ZIF-8 filled MMMs received tremendous attention on the pervaporation recovery of alcohol(e.g.,n-butanol[96-98],furfural[99],phenol[100]).However,the small windows(0.34 nm)endow ZIF-8 with high water diffusion selectivity toward alcohol because the kinetic diameters of alcohol are usually larger than 0.36 nm[87].Some researchers incorporated the ZIF-8 particles into the hydrophilic polymermatrix to fabricate the MMMs for dehydration of alcohol,exhibiting high pervaporation performance[101-103].

Fig.14.Comparison of pervaporation isobutanol recovery performance in literature[93].

ZIF-71,which possesses a 3D porous topology with small windows(0.48 nm)and large cavities(1.68 nm),is a hydrophobic material which is more hydrophobic than ZIF-8[104].Therefore,ZIF-71 nanoparticles can be used as fillers to fabricate organophilic pervaporation membranes to enhance the performance of alcohol recovery.Liuet al.[105]synthesized ZIF-71 nanoparticles at room temperature and incorporated ZIF-71 nanoparticles into polyether-block-amide(PEBA)matrix forn-butanol recovery from model ABE solution.With no drying procedure and a “primed”coating,the ZIF-71 nanoparticles with 25 wt%loading dispersed well in PEBA matrix without agglomeration and interfacialvoids.Simultaneously,the ZIF-71 fillers in MMMs also increased the accessible free volume.The MMMs with 20 wt%ZIF-71 exhibited high permeation flux of 520.2 g·m-2·h-1and separation factor of 18.8 for model ABE aqueous solution(0.6 wt%acetone,1.2 wt%butanol,0.2 wt%of ethanol)at 310 K.The ZIF-71 filled polydimethysiloxane(PDMS)membranes were fabricated by Wee's groups[104]for recovery of bio-alcohols(methanol,ethanol,isopropanol andsec-butanol).Fluxes of methanol and ethanol increased linearly with the increment of the ZIF-71 content,since ZIF-71 served as a gate for the free transport of methanol and ethanol molecules.Subsequently,Wee's groups[94]improved preparation method of ZIF-71 nanoparticles and synthesized submicrometer-size ZIF-71 crystals through an attractive mixed-solvent approach.The thin and smooth MMMs with the high submicrometer-size ZIF-71[Fig.15(b)]loading up to 40 wt%exhibited the excellent dispersion of submicrometer-size ZIF-71 in PDMS matrix and higher pervaporation performance compared with the micrometer-size ZIF-71[Fig.15(a)] filled MMMs.Besides,the swelling resistance,thermostability and hydrophobicity were simultaneously improved with the addition of ZIF-71 nanoparticles.Other ZIFs(e.g.,ZIF-90[106],ZIF-7[107-109])nanoparticles also have been used as fillers to prepare the ZIFs filled MMMs for enhancing the pervaporation performance.

Apart from serving as fillers directly incorporated into the polymer matrix,the ZIFs particles were loaded on the mesoporous core to synthesize hierarchical micro/mesoporous composites.The hierarchical micro/mesoporous composites possess core-shell structures combining a microporous ZIFs shell with a mesoporous core(e.g.,mesoporous silica spheres(MSS),siliceous mesocellular foams(MCF)).Generally,the microporous ZIF shell provides the appropriate pores for target molecules through the shell and possesses a high adsorption capacity,while the mesoporous core offers huge channels for permeation molecules to facilitate fast diffusion.Combining advantages of both microporous materials and mesoporous materials,the hierarchical micro/mesoporous composites can be incorporated into the polymer matrix to construct the confined structures of MMMs,improving pervaporation performance.Sueet al.[110]loaded ZIF-8 microparticles and ZIF-8 nanoparticles onto the surface of MCF to synthesize hierarchical microZIF-8@MCF and nanoZIF8@MCFviaa promising and simple strategy as shown in Fig.16.Compared with results from the literature,nanoZIF-8@MCF- filled PVA MMMs showed higher pervaporation performance on both permeation flux of 240 g·m-2·h-1and separation factor of 2000 for dehydration of 90 wt%ethanol solution at 298 K.Naiket al.[111]successfully synthesized hierarchical micro/mesoporous composites through ZIF-71 and ZIF-8 nanocrystals formed on the surface ofMSSby the seeding and regrowth approach.The experimental result showed that the MSS-ZIF doped MMMs exhibited higher pervaporation performance.

4.1.2.2.MIL-filled MMMs.MILs,constructing from metal ions(e.g.,Cr,Al,Fe,Ti)and organic ligand,have a 3D structure with the large pore size,great surface area,high porosity as well as distinguished thermal,air,water and organic solvents stability[89,112-114].Therefore,the MIL series of MOFs are promising materials as fillers for MMMs to enhance membrane pervaporation performance.

Yuet al.[114]synthesized the submicron-size MIL-101(Cr)nanoparticles with the large pore size(14 nm)and surface area(5136 m2·g-1),and incorporated the nanoparticles into PDMS matrix for pervaporation desulfurization.The addition of the MIL-101 into PDMS matrix enhanced the membrane separation performance for desulfurization of model gasoline made up of 500 ppm thiophene andn-octane at303 K:permeation flux to 5200 g·m-2·h-1and enrichment factor to 5.6 as shown in Fig.17.The excellent separation performance can be attribute to two reasons:(1)The interface between PDMS and MIL-101(Cr)can provide the extra free volume cavities,whose radius ranged from 0.38-0.39 nm and are larger than bothn-octane(0.315 nm)and thiophene(0.265 nm),endowing permeation molecules with the confined mass transfer space in MMMs.(2)The abundant channels of MIL-101(Cr)can also serve as confined mass transfer paths to accelerate the movement of permeation molecules.

Fig.17.Comparison of desulfurization performance in literature[114].

MIL-53,constructing from metal ions(e.g.,Al,Fe,Cr),1,4-benzenedicarboxylic acid and hydroxyl groups,possesses a 3Dstructure with 1D lozenge-shaped pores(0.85 nm)[86].Due to parallel aligned aromatic rings and hydroxyl groups,the MIL-53 particles possess excellent chemical and solvent stability as well as strong affinity with alcohol and polymer,and provide the confined mass transfer channels for permeation molecules.Zhanget al.[115]incorporated MIL-53 into PDMS through a sonication-enhanced dip-coating approach.The hydrogen bond interactions between MIL-53 and PDMS eliminated the interface voids.The 1Dlozenge-shaped,hydrophobic pores ofMIL-53 gave a confined mass transfer channel for ethanol molecules as shown in Fig.18.The MMMs showed the amazing permeate flux of 5467 g·m-2·h-1for ethanol recovery from 5 wt%ethanol aqueous at 343 K,while the separation factor stayed 11.1.MIL-53-NH2with the same topology of MIL-53 were successfully synthesized[116-118].Due to the free standing amino groups,MIL-53-NH2can be functionalized to synthesize the MIL-53-NH2for preparing pervaporation MMMs.Wuet al.[119]used organic agents(e.g.,formic acid,valeric anhydride,heptanoic anhydride)to modify MIL-53(Al)-NH2particles,obtaining the functionalized MIL-53-NH2such as MIL-53-NHCOH,MIL-53-NHCOC4H9and MIL-53-NHCOC6H11.The functionalized MIL-53-NH2were filled into PVAmatrix to obtain the MMMs for dehydration of 92.5 wt%ethanol aqueous solution at313 K.Both MIL-53-NHCOHand MIL-53-NHCOC4H9filled MMMs exhibited excellent water permeability and selectivity.The strong interactions between hydrophilic MIL-53-NHCOH(MIL-53-NHCOC4H9)and hydrophilic polymer PVA avoid the formation of interfacial defects and control the confined mass transfer structures.

4.1.2.3.CuBTC-filled MMMs.CuBTC(Cu3(BTC)2,HKUST-1)is a typical MOF with the active site of Lewis acid.The CuBTC contains Cu2+units combined with four carboxylate groups,forming 3D porous zeolite-like structure with large square-shaped pores[120-122].The open metal sites of Cu2+of CuBTC provide empty 3dorbital,which can coordinate with the π orbital of aromatic molecules(e.g.,toluene,thiophene)to formd-π conjugation interactions between Cu2+and aromatic molecules,facilitating mass transfer of aromatic molecules[123,124].Moreover,the 3D network pores of CuBTC give mass transfer channels for molecules.

Zhanget al.[123]synthesized the CuBTC nanoparticles through a solvothermal approach and embedded the CuBTC nanoparticles into PVA matrix by a pressure-driven assembly approach.The favorable compatibility between CuBTC and PVA along with the appropriate preparation methods resulted in a good distribution of CuBTC nanoparticles and comparatively ideal interface without obvious defects.The pervaporation performance of the CuBTC filled PVA MMMs was evaluated using toluene/n-heptane(50 wt%/50 wt%)mixture at 313 K.The CuBTC filled MMMs exhibit more excellent performance such as the permeation flux of 133 g·m-2·h-1and the separation factor of 17.9.The d-π conjugation interactions Cu2+and toluene molecules facilitate mass transfer of toluene molecules,and the pores of CuBTC provide mass transfer channels,enhancing fast diffusion for molecules as shown in Fig.19.The sizes of particles have a tremendous influence on the interface between the particles and polymers.Therefore,Yuet al.[124]synthesized unblocked submicron-size CuBTC particlesviaa green and scalable acid-base reaction approach.The submicron-size CuBTC nanoparticles were well dispersed within PDMS matrix.Due to regular mass transfer channels of CuBTC,d-π conjugation interactions between Cu2+and thiophene molecules,and extra free volume voids provide by the weak interactions between PDMS chains,the CuBTC filled MMMs showed high pervaporation performance for separation ofn-octane and thiophene mixture.

Fig.18.Schematic of the mass transfer process of ethanol and water molecules through MIL-53 filled PDMS MMMs[115].

4.2.Other3D filler-filled MMMs

4.2.1.COF-filled MMMs

COFs,a novel class ofporous organic materials,are constructed from organic building blocks through strong covalent bonds among light elements like C,H,O,N,Si and B[125].Yaghi's group successfully developed 2D COFs in 2005,which lay the landmark foundation for later research on COFs.Subsequently,they firstly synthesized 3D COFs in 2007[126].3D COFs composed of covalent bonds containing sp3carbon or silane atoms[127]and possess special properties of tunable pore topologies,permanent porosity and large surface area,which determine the adsorption behavior and facilitate transport properties for the appropriate molecules.In addition,3D COFs also possess other properties of low density,high thermal stability and facilely tailored functionality[81].

Yanget al.[128]synthesized COF SNW-1 nanoparticles that with 3D porous structure[129],and incorporated them into SA matrix to prepared MMMs for dehydration of 90 wt%ethanol aqueous solution.As shown in Fig.20,the heterogeneous distribution of COFs nanoparticles lead to filler-enriched layer and polymer-rich layer[130,131].The hydrophilicity of the filler-enriched layers enhance solubility selectivity.Meanwhile,the polymer-rich layers can increase the free volume fraction of the membranes,improving the diffusion selectivity.The heterogeneous structure benefits the confined mass transfer of water molecular.The pervaporation experiment showed high permeation flux of 2397 g·m-2·h-1and a separation factor of 1293 under the conditions of 10 wt.%aqueous solutions and 350 K.

Fig.20.Schematic diagrams for confined mass transfer in COF filled MMMs[128].

4.2.2.POSS-filled MMMs

Polyhedral oligosilsesquioxane(POSS)is a class of silsesquioxane molecules with 3D cage-like nanostructure as shown in Fig.21.Due to its special composition and structure,3D POSS endowed excellent physical properties of high thermal stability and low density[132].Besides,the chemical structure of POSS is easily altered through modifying substituent groups at its apices with a variety of functional groups.Hence,POSS has the wonderful affinity with diverse polymers and/or molecules.The incorporation of POSS into polymer matrix can improve thermal stability,oxidation resistance,physical and mechanical properties,together with separation performance of the MMMs[133,134].

Fig.21.A typical molecular structure of POSS.The substituent groups at its apices(R,represent the functional groups such as hydrogen,alkyl,alkylene,aryl,arylene and so on)[135].

Leet al.[136]embedded two types of POSS(AL0136 and SO1440)into PEBA matrix to prepare the POSS- filled MMMs for pervaporation ethanol recovery.POSS was well dispersed in PEBA matrix at a molecular-level,since POSS had a good compatibility with PEBA.The POSS- filled MMMs exhibited high pervaporation performance.The AL0136 filled MMMs showed higher performance(permeation flux of 183.5 g·m-2·h-1and separation factor of 4.6)than the SO1440 filled MMMs under the conditions of room temperature and 5 wt%aqueous ethanol solutions,which was attributed to the siloxyl groups of AL0136 facilitating ethanol molecules transport through the membranes.Liuet al.[137]incorporated molecular-level POSS into PDMS matrix to control arrangement of polymer chains,manipulate favorable interfacial morphology and adjust the free volumes of membranes.POSS increased the large free volume voids but decreased the small free volumes voids,which favored the transport of large-sized molecules through the membranes.The POSS- filled MMMs also exhibited the improvement of both permeability and selectivity.

5.Comparison of the MMMs

MMMs,which were patented by Kulprathipanja and co-workers[138],are promising membranes for pervaporation.In recent years,some novel materials,including 1D,2D and 3D materials,have been introduced into the polymers to effectively tailor the confined mass transfer structure and further enhance pervaporation performance.Tables 1,2 and 3 summarize the MMMs with 1D,2D and 3D materials and their performance of pervaporation,respectively.

By comparing Tables 1-3,it can be found that CNTs,GO laminates,and MOFs have been widely studied in pervaporation membrane process.In general,both the CNTs filled and the GO filled MMMs are applied to pervaporation dehydration of alcohols(e.g.,ethanol,isopropanol).The CNTs filled MMMs are also fabricated for separating benzene/cyclohexane mixtures[20,39,53,139]and other organic mixtures(e.g.,methanol/MTBE[140]).The GO filled MMMs are also studied for organic/organic separation(e.g.,thiophene/n-octane[141],toluene/n-heptane[142]).The MOFs filled MMMs are mainly applied to the recovery of alcohols(e.g.,ethanol[94,104,115,143,144],n-butanol[97,98,109,143])from aqueous solution and organic/organic separation(e.g.,toluene/n-heptane[114,124],toluene/n-heptane[123])since the MOFs usually exhibit hydrophobic nature.The MOFs filled MMMs are also prepared for pervaporation dehydration of alcohols(e.g.,ethanol[2,107,110,119,145],isopropanol[101,103,106]).Although the nanomaterials are applied to the same field in pervaporation,there are differences in pervaporation performances.For example,the CNTs,GO laminates,or MOFs are utilized for ethanol dehydration.Most of the GOs filled MMMs displayed superior performance compared to the CNTs and MOFs filled MMMs.However,the performance of the MOFs filled MMMs for recovery of alcohols from aqueous solution is much better than that of the CNTs and GOs filled MMMs.It is worth mentioning that most of MOFs are not stable in wet environment which is exactly the working condition of pervaporation.Therefore,long-term stability of MOFs in MMMs still needs intensive investigations.

Table 1Pervaporation performance of different MMMs with 1D fillers

Table 2Pervaporation performance of different MMMs with 2D fillers

6.Conclusions and Outlook

Different dimensions of the nanomaterials with confined mass transfer structure have been incorporated into the polymer matrix to fabricate the MMMs.The methods of preparing MMMs have already achieved a foothold due to the usage of appropriate fillers and the ease of design in the scientific and technical area.Vast amounts of studies show that the pervaporation performance of MMMs is better than that of the polymeric membranes.Obviously,the MMMs have received great attentions and are promising for pervaporation.However,it is still difficult to develop better MMMs for practical applications:(1)Problems in membrane fabrication.For MMMs with 1D fillers,1D materials with appropriate size are difficult to synthesize and the alignment of 1D materials is difficult to achieve.For MMMs with 2D fillers,it is hard to control the size of molecule transport channels generated between the stackable layers of 2D materials and the structural defects on the surface.For MMMs with 3D MOFs fillers,the stability of MOFs should be ensured during the preparation and the applications.(2)Uniform dispersion of the fillers in the MMMs.The aggregation of the fillers affects the distribution of the fillers in MMMs,which has a serious influence on the pervaporation performance.(3)Fabricating the MMMs with the ideal interfacial morphology.The non-ideal interface deteriorated the permeability and/or selectivity of the membranes.With ongoing effort in synthesis and analysis of MMMs containing fillers with confined mass transfer structure,MMMs with high performance are expected to be developed from laboratories to industrial applications.

Table 3Pervaporation performance of different MMMs with novel 3D fillers

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