Manipulation of confined structure in alcohol-permselective pervaporation membranes☆

Jing Zhao,Wanqin Jin*

State Key Laboratory of Materials-Oriented Chemical Engineering,College of Chemical Engineering,Jiangsu National Synergetic Innovation Center for Advanced Materials,Nanjing Tech University,Nanjing 210009,China

1.Introduction

In recent years,with the increasing concerns of environmental deterioration and energy shortage,alcohol fuel(mainly including ethanol and butanol)produced from biomass has gained greatattention as an environmental friendly and renewable alternative for fossilenergy[1-4].However,the production of bioalcohol by fermentation is severely impeded by the inhibition effect of products on the activity of yeast,which results in the low bioconversion efficiency and the low purity of bioalcohol in fermentation broth.Therefore,it is necessary to integrate separation technologies with fermentation process to achieve thein situand continuous removal of alcohol from fermentation broth[1-3].Compared with the conventional separation technologies such as distillation and extraction,pervaporation(PV),as a representative membrane technology for liquid separation,possesses remarkable advantages of low energy consumption,cost effectiveness,innocuousness to microorganisms,and easy coupling[5].

The flow diagram of fermentation-pervaporation coupling process is shown in Fig.1(a).First,the fermentation broth with low-concentration alcoholenters the PVmodule,in which alcohol-permselective membrane is installed.Due to the preferentially permeation of alcohol molecules through membrane,high-concentration alcohol solution is collected as permeate and transferred to the subsequent processing,while the retentate is circulated to the fermenter.For the fermentation pervaporation coupling technology,the separation performance of the corresponding membrane,i.e.,the alcohol-permselective pervaporation membrane is the key element determining the efficiency and competitiveness of this coupling technology.The molecular transport across alcohol-permselective membrane consists of three steps(Fig.1(b)):(i)sorption of permeating molecules from the feed liquid onto the upstream surface of membrane;(ii)diffusion of permeating molecules through the membrane under chemical potential difference;(iii)desorption of permeating molecules to vapor phase on the downstream of the membrane.However,the development of alcohol-permselective pervaporation membrane is relatively slow both in fundamental researches and practical applications.This phenomenon can be mainly ascribed to the following reasons.First,the mass transfer mechanism in pervaporation membrane is still not fully understood.Unlike membrane processes based on macroporous media and classical mass transfer theories such as ultra filtration and micro filtration,pervaporation process mainly involves mass transfer in narrow spaces with sizes comparable to the free distances of molecules,i.e.,mass transfer in confined structure.The present solution-diffusion mechanism is still not enough to clearly describe the mass transfer process in confined structure of pervaporation membrane.Moreover,compared with the gas separation process,which also obeys the solution-diffusion mechanism,the liquid separation process possesses much more complicated molecular interactions between permeating molecules as well as with membrane materials.Therefore,it is hard to study the mass transfer model in pervaporation membrane.Second,the intrinsic feature of larger molecules permeating preferentially and smaller molecules being intercepted increases the difficulty in achieving high separation performance.According to the solution-diffusion mechanism,the selectivity of membrane can be ascribed to two parts:solution selectivity and diffusion selectivity.For alcohol-permselective membrane,preferentially adsorption of alcohol can be achievedviaemploying alcohol-philic/hydrophobic membrane materials,while the preferentially diffusion of alcohol is hard to be obtained due to the larger size of alcohol molecules.Therefore,the selectivity of alcohol-permselective membrane is generally much smaller than water-permselective membrane.

An ideal membrane for industrial applications should possess high permeation flux(representing treatment capacity),high separation factor(representing separation efficiency),and high stability(representing lifetime and cost).Moreover,the production cost and the possibility to achieve large-scale production are also important factors.In the past decades,many efforts have been made to design advanced membrane structures and membrane materials with the aim of acquiring high efficiency alcohol recovery from aqueous solution.The main methods lie in the manipulation of confined mass transfer channels.This review is intended to present the prevalent membrane materials utilized for alcohol-permselective pervaporation,provide deep insight into the structure-performance relationships in membrane and highlight the representative and important developments in the past five years with the optimization of confined structure in membrane as the emphasis.In particular,the membrane materials are mainly classified into polymeric materials(rubbery polymer,glassy polymer,and block copolymer),inorganic materials and hybrid materials,and the diverse structure tuning methods are described with the classifications of physical structure and chemical structure.Finally,a tentative perspective on the possible future directions of alcohol-permselective pervaporation membrane is brie fly presented.

Fig.1.(a)The flow diagram of fermentation-pervaporation coupling process;(b)The schematic principle of molecular transport in alcohol-permselective membrane.

2.Membrane Materials for Alcohol-Permselective Pervaporation

The membrane materials for alcohol-permselective pervaporation can be classified into three categories:polymeric materials,inorganic materials and hybrid materials.Among them,organophilic polymers are the most widely utilized materials,which include three types:1)rubbery polymer with soft chains,2)glassy polymer with hard chains,and 3)block copolymer with both soft and hard segments.It should be mentioned that although the extensively studied mixed matrix membrane(MMM)with polymermatrix and inorganic/hybrid filler has become a promising membrane con figuration,the bulk of the membrane is still polymeric materials.Therefore,it will be considered as a strategy for structure manipulation of polymeric membrane and will not be introduced as an individual membrane material in this part.

2.1.Polymeric materials

2.1.1.Rubbery polymers

Rubbery polymers mainly refer to silicon rubbers are the most commonly utilized membrane materials for alcohol-permselective pervaporation.On the one hand,the Si-O-Sion backbone endows silicon rubbers with high chain mobility and high free volume fraction,thus favoring the high permeability.On the other hand,the high hydrophobicity is suitable for the preferential adsorption of alcohol from aqueous solution.The typical silicon rubbers include poly(dimethyl siloxane)(PDMS)[4,6-8],poly(octylmethyl siloxane)(POMS)[9]and poly(vinyltriethoxysilane)(PVTES)[10].Among them,PDMS is regarded as the benchmark membrane material due to its excellent comprehensive performance,which has been commercialized for organophilic pervaporation,such as PERVAP 4060(Sulzer Chemtech,Switzerland)and Pervatech PDMS(Pervatech,The Netherlands)[11].To date,various attempts have been performed to improve the separation performance of PDMS membrane including incorporating solid/porous fillers[8,12,13],surface modification[14],cross-linking[15],employing advanced membrane-fabrication method[4],optimizing support structure[16]and so on.It is well known that permeation flux is reversely proportional to membrane thickness.In order to acquire high permeation flux so that to improve the treatment capacity of membrane,it is necessary to decrease the thickness of PDMS layer with the premise of integrity.The thinnest PDMS membrane reported in literatures is about 800 nm fabricatedviaspray-coating method[4].Zhang and Renet al.proposed anin-situpolymerization method to fabricate thin PVTES membranes with thickness about 300 nm,which is much thinner than most of the PDMS membranes[10].Meanwhile,thein-situpolymerization process endows the strong adhesion between active layer and support layer.The as-prepared PVTES membrane exhibits a high permeation flux over 10 kg·m-2·h-1,which is about one order of magnitude higher than that of PDMS membrane.

2.1.2.Glassy polymers

Glassy polymers simultaneously possessing rigid molecular chains and high free volume fraction are attractive membrane materials to acquire high permeation flux.Poly[1-(trimethylsilyl)-1-propyne](PTMSP)is one of the most permeable polymer materials as reported[11,17,18].The rigid backbone with alternate single and double bonds and the side chain of trimethylsilane generate loose molecular conformation,large chain spacing and high free volume fraction,thus achieving high permeability for both gas and liquid molecules.Meanwhile,the hydrophobic surface is suitable for the preferential adsorption of alcohol from aqueous solution.Padukoneet al.first employed PTMSP as the membrane material for ethanol recovery from fermentation broth.The flux of PTMSP membrane is about three-fold higher and the separation factor is about two-fold higher than that of the commonly utilized PDMS membrane under similar conditions[17].In order to further improve the separation performance of PTMSP membrane,many researches have been performedviaincorporating silica nanospheres into PTMSP matrix[11,19].The membrane with optimized silica content and thickness exhibits outstanding performance with the flux of 9.5 kg·m-2·h-1,ethanol/water separation factor of 18.3 and butanol/water separation factor of 104,much superior over commercial membranes[11].However,the physical aging and the chain relaxation of PTMSP membrane with operating time inevitably leads to the decreased free volume and deteriorated separation performance,which impedes the practical application of PTMSP membrane.

Polymers of intrinsic microporosity(PIMs)are emerging membrane materials with rigid molecular chains and high free volume fraction,which combine the processability of polymeric membrane and the transport property of inorganic membrane,exhibiting promising potentials in molecular separation[20-22].The special ladder-like structure with contorted sites prevents polymer chains from rotating and packing,thus obtaining unusually high free volume and large internal surface.However,the applications of PIMs in membrane field mainly focus on gas separation.Their effects on pervaporation process have rarely been reported.Izáket al.first demonstrated the feasibility of utilizing PIM as membrane material to perform alcohol-permselective pervaporation[20].The PIM-1 membrane shows significantly decreased permeability and increased selectivity over time upon storage due to the slow rearrangement of polymer chains,and the resultant densified membrane structure.

2.1.3.Block copolymers

Apart from the above mentioned rubbery polymers and glassy polymers,various block copolymers with both rigid and flexible segments also have been employed as organophilic pervaporation membrane materials.By adjusting the relative ratio and molecular weight of different segments,membranes with diverse structural and chemical properties can be obtained.PEBA is a group of copolymers comprising rigid polyamide segments and flexible polyether segments.PEBA 2533 with 80 wt%organophilic polyether segments and 20 wt%polyamide segments is the commonly utilized one for alcohol recovery due to the high hydrophobicity[23-27].Comparatively,PEBA membrane possesses lower selectivity compared with PDMS membrane.Besides the commonly utilized PEBA,block copolymers for alcohol-permselective pervaporation also include poly(styrene-b-dimethylsiloxane-bstyrene)(SDS)triblock copolymer[28],poly(butyl acrylate-co-styrene)[29],hydroxyterminated polybutadiene-based polyurethaneurea(HTPBPU)[30],poly(vinylidene fluoride-co-hexafluoro-propylene)(VDF-HFP)[31]and so on.

2.2.Inorganic materials

Zeolite is a kind of conventional inorganic microporous nanomaterials.The much higher mechanical strength,thermal stability,chemical stability and more rigid and ordered three-dimensional porous structure than polymer render it a promising membrane material for molecular separation[32,33].Among the abundant zeolite types,MFI-type zeolites including ZSM-5 zeolite with high ratio of Si/Al and pure silica MFI-type zeolite(also called silicalite-1)are the most commonly used material for alcohol-permselective pervaporation membrane due to their medium pore size(～0.55 nm)and high hydrophobicity[34-36].The influences of synthesis method[37,38],fabrication condition[39-41]and support structure[39,40]on MFI-type zeolite membrane structure and alcohol/water separation performance have been extensively investigated.Wanget al.[36]prepared hollow fiber-supported silicalite-1 membraneviathe combination of secondary growth method and wetting-assisted rub-coating seeding method.With the optimization of fabrication conditions such as seed size,seed morphology and structure-directing agent(SDA)content,the thin membrane(～5 μm)with ultrahigh pervaporation performance(permeation flux of 9.8 kg·m-2·h-1and separation factor of 58)can be obtained.ZSM-5 is a kind of MFI-type aluminosilicate zeolite,which can be tuned from water-permselective to alcohol-permselective with the increase of Si/Al ratio due to the enhanced membrane hydrophobicity.Weydet al.[42]fabricated hydrophobic ZSM-5 zeolite membrane(with Si/Al ratio up to 300)on the inner surface of ceramic support.Porous titania support with three intermediate layers was chosen to prevent Al incorporation during the membrane fabrication process.With the feed containing 5 wt%ethanol,the ethanol concentration in permeate can be enriched up to 87 wt%(with permeation flux about 0.87 kg·m-2·h-1and separation factor about 100).Comparatively,silicalite-1 membrane draws more attention and exhibits higher separation performance for alcohol recovery than ZSM-5 membrane due to its pure-silica composition and the resultant higher hydrophobicity.

Although new synthesis methods have been explored and optimized to acquire thin zeolite membrane so that to increase permeation flux,the thicknesses of most zeolite membranes are still in the micrometer scale.How to fabricate ultrathin zeolite membrane with the premise of low defect density is one of the most top research topics for zeolite membrane.Tsapatsis's group successfully synthesized intact exfoliated MFI nanosheets from multilamellar silicalite-1 precursorsviamelt blending with polystyrene and the following sonication in toluene[32].After simple filtration of the MFI nanosheet suspension in porous support and the calcination for polymer removal,ultrathin MFI membrane comprising of well-packed nanosheets with a thickness of～200 nm is obtained.On the basis of the exfoliated zeolite nanosheets,Tsapatsis's group further proposed a gel-less secondary growth method to fabricate oriented MFI zeolite membrane with thickness varying in the range of 100-250 nm[33].First,a nanosheet seed layer(～50 nm)was prepared by simply filtering the suspension of MFI nanosheets through the silica supports.Afterwards,the gel-less secondary growth of MFI zeolite was carried out through impregnating the nanosheet membrane into the ultra-diluted MFI structure-directing agent solution followed by heattreatment.The silica layer was also consumed as silicon source during the secondary growth process.The b-out-of-plane orientation of MFI nanosheets as seed layer led to the oriented 0.55 nm pore channels of as-prepared MFI normal to the surface of the membrane,thus favoring molecular permeation.

2.3.Hybrid materials

With the features of large surface area,chemically functionalized cavities,and flexible skeleton,the hybrid metal-organic frameworks(MOFs)with metalions/clusters and organic linkers have also been considered as promising candidates for molecular separation membranes[43-45].Besides the hydrophobicity and pore size,the integrity and thickness of MOF layer,the stability of MOF crystals in aqueous solution,and the binding strength between MOF layer and support are also critical factors to achieve high-efficiency separation.Linet al.[46] first used MOF membrane in pervaporation separation of alcohol-water mixtures with ZIF-71 as the selected material.ZIF-71 is a kind ofhydrophobic and stable MOFs with small windows(0.48 nm)and large cages(1.68 nm).The ethanol-water adsorption selectivity is estimated to be 11.1.Although the separation performance of ZIF-71 membrane preparedviareactive seeding method is still not satisfactory,it demonstrates the possibility of MOF membrane in organics-water separation.Our group employed modified contra-diffusion method to fabricate thin and integrate ZIF-71 membranes on ceramic hollow fiber support(Fig.2)[47].During the membrane fabrication process,the solution of Zn2+and the solution of imidazole were placed on different sides of the hollow fiber support,and reacted on the interface of the ceramic hollow fiber support under diffusion.The self-inhibition effect of the ZIF-71 formation process similar with interfacial polymerization renders the formation of thin ZIF-71 with thickness about 2.5 μm and high integrity.Meanwhile,the diffusion of nutrients leads to the slight penetration of ZIF-71 into ceramic support,thus facilitating the favorable bonding between ZIF-71 layer and ceramic support.

Fig.2.Schematic of preparation of ZIF-71 hollow fiber membrane by contra-diffusion[47].Copyright 2015.Reproduced with permission from the American Chemical Society.

3.Tuning of Confined Structure in Membrane

For each category of membrane,the physical structure and chemical structure of confined channels are two critical elements for the mass transfer process in membrane.The physical structure comprises the size,density and connectivity of mass transfer channels.The channel size determines the diffusion resistance of permeating molecules and the impact extent of channel wall on the diffusion process,while the channel density decides the amount of mass transfer channels in membrane,the channel connectivity influences the diffusion resistance of permeating molecules.The chemical structure dictates the affinity of channels for permeating molecules,which exerts great impacts on the driving force of adsorption process and decides the level of difficulty for the molecules entering into channels.According to the above analysis,the tuning of confined structure in membrane can be performed from two aspects:physical structure and chemical structure.

3.1.The physical structure of mass transfer channels

The studies of alcohol-permselective pervaporation membrane mainly focus on polymeric materials,in which the molecular-level free volume cavities act as mass transfer channels.Free volume cavities consist of the smaller spaces between polymer chains constituting the polymer aggregates(network pores)and the larger spaces surrounding the polymer aggregates(aggregate pores)[48].The tuning of physical structure lies in two aspects:optimizing the size and amount of free volume cavities,and incorporating new mass transfer channels.

3.1.1.Optimization of free volume properties

Although crosslinking is effective in manipulating the free volume properties of polymeric membranes,it is difficult to achieve simultaneously enhanced permeability and selectivity due to the restriction of“trade-off”effect.In recent years,MMMs with polymer matrix and inorganic/hybrid filler have been considered as an efficient strategy to address this issue.The incorporation of fillers can interfere the aligned packing of polymer chains during membrane formation process,and then influence the free volume properties owing to the combined impacts of fillers'steric effect and interfacial interaction between polymer matrix and fillers.From the aspect of steric effect,the fillers'dimension is a critical factor.For instance,2D nanosheets may act as nucleating agent for the formation of highly ordered and crystalline regions in polymer,thus partially offsetting the interference for polymer chains[49].However,the systematical and in-depth research about the influence of filler dimension on free volume properties has not yet been reported.From the aspect of interfacial interaction,the relative strength of interfacial interaction compared with interfacialstress induced by the surface property difference between polymer matrix and filler determines the interfacial morphology of MMMs:weaker interfacial interaction results in interfacial voids(increased free volume cavities with lower selectivity),while too strong interfacial interaction may give rise to restricted mobility of polymer chains,i.e.,chain rigidification(decreased free volume cavities)[50].On the other hand,weak interfacial interactions generally lead to severe agglomeration of filler,in which cases the large inter- filler channels with no molecular selectivity dominate the mass transfer process in hybrid region,and deteriorate membrane performance.

As stated above,the separation performance of MMMs depends on the interfacial interactions between polymer matrix and filler to a great extent,which can be manipulatedviaversatile approaches such as physical coating,chemical grafting or replacing with organic component-containing fillers.Polyhedral oligomeric silsesquioxane(POSS)is a hybrid nanomaterial with an inner inorganic Si-O backbone(SiO1.5)nand eight external organic arms.The versatile organic arms can impart interaction sites with polymer matrix[51,52].Our group employed methyl-POSS as filler to manipulate the free volume properties of PDMS matrix[51].The enhanced intra-molecular interactions of PDMS and the promoted PDMS chain mobility(as demonstrated by the results of molecular simulation)arising from the PDMS-POSS interactions lead to the optimized free volume properties of MMMs with free volume cavities smallerthan the diameter of butanol molecules reducing and larger free volume cavities increasing(Fig.3).As a result,the selectivity and the permeability of PDMS/POSS MMMs for butanol/water separation were enhanced by 2.2-and 3.8-times,respectively,compared with PDMS membrane.

In order to achieve higher permeation flux,composite membranes with thinner separation layer have attracted great attention due to the shortened diffusion pathway.For MMMs,the fillers in thinner membrane should be with smaller size so that to acquire defect-free membrane[53].It is conceivable that the smaller-size fillers possess higher contacting area with polymer matrix,thus exerting more significant influence on the free volume properties.As a result,the effects of interfacial morphology on membrane separation performance become more remarkable,which puts forward more stringent requirements on interfacial interactions.

Fig.3.Schematic of tuning membrane free volumes and its effect on the separation process[51](1?=0.1 nm).

3.1.2.Incorporation of new mass transfer channels

Compared with the dynamic and instantaneous free volume cavities in dense membrane,the pore structure in inorganic porous materials is static and permanent,which confers lower diffusion resistance upon permeating molecules.Therefore,embedding inorganic porous materials into polymer matrix to incorporate efficient mass transfer channels has been widely adopted to intensify the alcohol-permselective pervaporation process.It is noteworthy that the aforementioned interfacial interaction between polymer matrix and filler is also a critical factor for porous filler-based MMMs.On the basis of employing porous fillers,the interfacial interaction can be further improved to simultaneously optimize free volume properties.This review will introduce the relevant research progress focusing on the most widely adopted porous fillers including zeolite,nanotube and MOFs.

3.1.2.1.Zeolite.Similar with the type of zeolite membrane,ZSM-5 zeolite with high ratio of Si/Al and silicalite-1 zeolite are also the most commonly utilized fillers in alcohol-permselective pervaporation membranes.Vankelecom and coworkers embedded solid and hollow silicalite-1(SS and HS)crystals into PDMS matrix respectively[54].The rich pore structure in solid silicalite-1 endows ethanol molecules with much lower diffusion resistance than PDMS matrix,leading to the approximate 4.5-fold increase of ethanol flux.Moreover,the hollow parts of HS further reduce the diffusion resistance of ethanol across the inorganic region(or in other words,reduce the effective membrane thickness),thus causing a more significant 7-fold increase of ethanol flux.

Despite of the extensive investigation of zeolite-based MMMs,the zeolite dispersion and interfacial morphology are still critical issues need to be addressed due to the existence of hydrophilic silanol groups on zeolite surface,which may result in interfacial or inter- filler voids and deteriorate separation performance.Moreover,according to the Maxwell model of MMMs,the volume fraction of inorganic porous filler exerts great influence on the membrane permeability[50],which brings forward higher requirements for the dispersion of filler and the interfacial morphology.Therefore,surface modification of zeolite is imperative in fabricating high-performance MMMs[12,55-57].Due to the existence of abundant hydroxyl groups on zeolite surface,the silylation reaction with silane coupling agents is generally employed for surface modification.Our group proposed a grafting/coating method to tailor the interfacial morphology between PDMS matrix and ZSM-5 zeolite[57].ZSM-5 was first grafted withn-octyltriethoxysilane(OTES)to perform hydrophobic modification,and then adsorbed a thin PDMS layerviachain entanglements of PDMS and OTES.Therefore,the modified ZSM-5 with “PDMS surface”possesses favorable interfacial compatibility with PDMS matrix.As a result,the MMMs achieve a maximal ZSM-5 loading of 40 wt%with uniform dispersion.The corresponding membrane exhibited a 75%higher ethanol/water separation factor compared with pristine ZSM-5-embedded membrane.Up to now,there have been diverse silane-coupling agents utilized for surface modification of zeolite in alcohol-permselective pervaporation membranes.The molecular structures of the mainly utilized silane-coupling agents are summarized and listed in Fig.4.

In most MMMs,the porous fillers are isolated and surrounded by polymermatrix,thus hampering the fullexploitation of the fillers' internal channels.It is conceivable that constructing continuous inorganic channels across the membraneviaincreasing filler loading is an effective strategy to solve this problem.However,the fabrication of high filler-loading MMMs is subject to the constraints of filler agglomeration and interfacial voids.In order to circumvent these issues,Yanget al.proposed a “packing- filling”method to fabricate PDMS/silicalite-1 MMMs replacing the conventional“blending-casting”method[58].The packing silicalite-1 crystals constituted the membrane matrix,while PDMS sealed the interspaces among silicalite-1 crystals and formed an extremely thin layer on membrane surface(Fig.5).Due to the continuous low diffusion resistance channels constructed by silicalite-1 play an dominantrole in molecular permeation,the as-prepared ultrathin membranes(about300 nm)with zeolite loading up to 74 vol%exhibited an especially high flux(5.0-11.2 kg·m-2·h-1)and good separation factor(25.0-41.6)fori-butanol/water pervaporation separation.

3.1.2.2.Nanotube.Nanotube is a special kind of filler possessing high aspect ratio,high specific area and 1D long-range ordered channels[7,23,59].Among various nanotubes,carbon nanotube(CNT)is a typical representative with hydrophobic and smooth surface,diverse diameters,and excellent mechanical strength.Yenet al.first incorporated CNTs into polymer matrix(PEBA)utilized for alcohol-permselective pervaporation membranes[23].The PEBA/CNTs MMM with 10 wt%CNTs loading brought about a 59%increased butanol removing rate compared with PEBA membrane.When utilized in a fermentation pervaporation coupling process,the corresponding membrane achieved a 20%increase both in butanol productivity and yield,indicating its great potential in Acetone-Butanol-Ethanol(ABE)fermentation industry.Afterwards,CNTs were also incorporated into PDMS matrix[7].At the optimal loading of 10 wt%,the butanol flux and separation factor of PDMS/CNTs MMM were significantly improved by 204.9%and 125.0%at 37°C compared with those of PDMS membrane.It was demonstrated that the influence of CNTs on membrane permeability was more significant at lower temperature,owing to the accelerated thermal motion of PDMS polymer chains at higher temperature,which declined the diffusion resistance of permeating molecules across PDMS matrix.

Fig.4.The molecular structures of silane coupling agents utilized for surface modification of zeolite in alcohol-permselective pervaporation membranes.

Fig.5.Schematic illustration of the fabrication procedure of the silicalite-PDMS nanocomposite membrane by a “Packing- filling”method[58].Copyright 2011.Reproduced with permission from Elsevier Ltd.

Compared with other inorganic porous fillers,the internal channels of CNTs are straighter and smoother,which are beneficial to the lower diffusion resistance.However,CNTs generally exhibit random orientation in polymer matrix,thus prolonging the mass transfer pathways and then exerting reverse impacts on the molecule permeation process.

3.1.2.3.MOFs.MOFs are novel hybrid porous nano-materials consisting of metal ions or clusters connected by organic linkers.Compared with conventional inorganic porous nano-materials,the existence of organic linkers on MOFs is beneficialto improving interfacial compatibility with polymer matrix,and imparts functionalization accessibility to manipulate interfacial interactions.Additionally,MOFs possess rich pore structure and high specific area(up to 6500 m2·g-1).The above features predestine MOFs as attractive fillers for high-performance MMMs[60-63].Li and coworkers first incorporated MOFs into alcoholpermselective pervaporation membranes[64].Zeolitic imidazolate frameworks(ZIF-8)with superhydrophobicity,extremely low water adsorption,and room temperature synthesis features are employed as filler and embedded into polymethylphenylsiloxane(PMPS)matrix.Although the aperture size of ZIF-8(0.34 nm)is smaller than the kinetic diameter ofi-butanol(0.50 nm),the flexible framework structure confers exceptionalhigh adsorption capacity fori-butanol.After incorporating ZIF-8 into PMPS,thei-butanol/water separation factor and thei-butanol permeability increase by 100%and 320%,respectively,compared with PMPS pristine membrane.The control experiments with ZIF-7(possessing similar superhydrophobicity,narrower aperture size,and much more rigid framework compared with ZIF-8)as filler con firms the crucial contribution of the channels inside ZIF-8 to the selective permeation ofi-butanol.

In the aforementioned study,the membranes were fabricatedviasimple solution-blending dip-coating method with the separation layer of 2.5 μm.The optimal ZIF-8 loading was determined as 10 wt%due to the poor dispersion at higher loading.In order to acquire ultrathin nanohybrid separation layer with high ZIF-8 loading and uniform ZIF-8 dispersion,simultaneous spray self-assembly technique was explored to fabricate ZIF-8/PDMS MMMs(Fig.6)[4].During the membranefabrication process,the ZIF-8/PDMS suspension and the solution containing cross-linking agent tetraethyl orthosilicate(TEOS)and catalyst dibutyltin dilaurate(DBTDL)were poured separately into two pressure barrels and simultaneously sprayed onto a polysulfone(PS)substrate.Due to the mechanical atomization of spraying process and the selfstirring of solution in barrel,ZIF-8 uniformly disperses in PDMS membrane even at the high loading of 40 wt%.The hybrid separation layer possesses a thickness of 0.8 μm,much thinner than the reported values in literatures.As a result,the ZIF-8/PDMS membrane exhibits a high separation performance forn-butanol/water separation with the total flux of 4.846 kg·m-2·h-1,and separation factor of 81.6.

A part from ZIF-8,otherhydrophobic MOFs with different pore structures and chemical compositions such as ZIF-71[24,65],materials institute Lavoisier-53(MIL-53)[6],and Zn(BDC)(TED)0.5(BDC=benzene dicarboxylate,TED=triethylenediamine)[25]have also been investigated as fillers in PDMS or PEBA matrix.

There is an interesting phenomenon that MOF membranes generally exhibit unsatisfactory alcohol/water selectivity while the MMMs with the corresponding MOF crystals can acquire high separation performance.For MOF membrane,it is difficult to precisely and efficiently capture alcohol molecules from the mixture solutions with high water content(over 90 wt%),thus leading to the poor selectivity.However,during the separation process with MMMs,the alcohol/water mixture can be pre-separatedviatheir different affinities with polymeric surface.Therefore,the mixture entering into MOF channels possesses much lower water content.Afterwards,the hydrophobic channels inside MOFs facilitate the diffusion of alcohol molecules,and eventually improve the alcohol permeability and alcohol/water selectivity.In conclusion,it is the synergistic effect between polymer and MOFs that dominantly contributes to the high separation performance of MOF-based MMMs.

3.2.The chemical structure of mass transfer channels

Fig.6.Formation of the ZIF-8/PDMS mixed matrix membrane by the simultaneous spray self-assembly technique[4].Copyright 2014.Reproduced with permission from John Wiley&Sons Ltd.

Tuning the chemical structure of mass transfer channels to improve membrane performance means modifying membrane with functional groups possessing selective affinity for alcohol molecules over water molecules.Due to the intrinsic feature of larger molecules preferentially permeating and the consequent inferiority in selective diffusion,the selective adsorption process plays a dominantrole in alcohol/water separation.First,the highly alcohol-philic groups are in favor of lowering the chemical potential of alcohol molecules at equilibrium state,thus increasing the driving force of adsorption process.Second,the employed alcohol-philic groups are of water-repellency,thus intensifying the selective capture of alcohol molecules from alcohol/water mixture.Last but not least,the strong interactions between membrane materials and alcohol molecules are beneficial to weakening the coupling effect between alcohol and water molecules,thus inhibiting the diffusion of water along with alcohol molecules.Up to now,the most extensively adopted alcohol-philic groups for alcohol-permselective pervaporation membranes are hydrophobic alkyl chains[12,15]or low-surface-energy CF3groups[14,66-69].According to the location of alcohol-philic groups in membrane and the modification methods,the tuning of chemical structure can be further divided into surface modification and bulk modification.

3.2.1.Surface modification

Covalent grafting is the most commonly utilized method for surface modification.Huanget al.constructed a superhydrophobic membrane surface(with a water contactangle of about172°)under the inspiration of lotus leaf surfaceviathe synergism of Ag nanoparticle and covalently bonded perfluorodecanethiol molecule(F-Ag-PD@Al2O3)[66].The excellently preferential adsorption ofi-butanol and the inverse exclusion of water were acquired and then gave rise to the exceptionally highi-butanol/water separation factor up to 151,significantly exceeding the previously reported hydrophobic membranes.Zhang and Jiet al.modified PDMS surfaceviaUV/ozone treatment and the subsequent(trideca fluoroctyl)triethoxysilane deposition to incorporate CF3groups(Fig.7).Due to the increased hydrophobicity of membrane surface,the ethanol/water separation factor increased from 8.5 to 13.1[14].

3.2.2.Bulk modification

Comparatively,bulk modification possesses more versatile approaches such as co-/graft-polymerization[70],incorporation of polymer additives[67,71],and surface functionalization of fillers[12,55,69].For instance,Singhet al.in-situmodified PDMS membrane with alkyl chains and per fluoroalkyl chainsviautilizingn-octadecyltrichlorosilane(OTS)and trichloro(1H,1H,2H,2H-per flourooctyl)silane(FTS)as crosslinkers,respectively[15].Araki and coworkers employed silane coupling agents with different functional groups as silica precursors to fabricate hydrophobic silica membrane,including phenyltrimethoxysilane(PhTMS),ethyltrimethoxysilane(ETMS),n-propyltrimethoxysilane(PrTMS),isobutyltrimethoxysilane(BTMS),and hexyltrimethoxysilane(HTMS)[72].Wanet al.prepared silicalite-1 particles with different surface compositionsviahydrothermal synthesis in fluoride(F)or alkaline(OH)media and investigated the influence of filler's hydrophobicity on the perm-selectivity of ethanol molecules[69].With the same loading of silicalite(40 wt%),the silicalite-1(F)-embedded membrane acquired a higher ethanol/water separation factor of 23.8,which was about 50%higher than that of silicalite-1(OH)-embedded membrane.Furthermore,from the almost constant water flux with silicalite-1(F)loading and the increased water flux with silicalite-1(OH)loading,it can be concluded that the higher hydrophobicity of silicalite-1(F)imparted a preferential pathway for ethanol molecules,and forced water to mainly diffuse through PDMS matrix with tortuous free volume cavities.Yanget al.improved the hydrophobicity ofZSM-5 zeolite membraneviapartialsubstitution of silicon in zeolite framework by boron(B)element,thus obtaining the significantly enhanced ethanol/water separation factor from 27 to 55[34].

4.Comparison of Membrane Performance

Fig.7.Schematic diagram describing the preparation of the self-assembled monolayer-modified PDMS/PSf membrane[14].Copyright 2013.Reproduced with permission from the American Chemical Society.

The pervaporation performance of various membranes with different materials and structures for alcohol recovery(mainly ethanol or butanol)is summarized in Table 1.For polymeric materials,PDMS is still the hottest research object due to its excellent comprehensive performance,facile and cost-effective fabrication,as well as the mature large-scale production technology.Various advanced methods or materials have been adopted to further improve the separation performance of PDMS membrane.PTMSP exhibits extremely high separation performance,while the physical aging phenomenon is the fatal problem restricting its practical application.PIM-1 is an emerging membrane material with preliminary attempts for alcoholpermselective pervaporation.The separation performance is still unsatisfactory and needs further optimization.PEBA membranes possess lower separation factor than PDMS-based membranes especially in separation factor.Silicalite-1 membrane exhibits attractive performance both in permeation flux and separation factor.Moreover,as a zeolite membrane,silicalite-1 membrane also possesses high thermal stability and mechanical strength.The restriction for practical application lies in the high fabrication cost.Comparatively,the separation performance of MOF membrane with ZIF-71 as a representative example is inadequate,especially in separation factor.Considering the abundant species of MOFs and their facile manipulation,the pore structure and the chemical property of MOFs can be further optimized to match the separation of alcohol/water mixtures.

From the aspect of tuning physical structure,incorporating porous nanofillers into polymer matrix is deemed to be a superb strategy with the capability of simultaneously optimizing the intrinsic free volume cavities and incorporating new mass transfer channels,which are influenced by the chemical composition and pore structure of nanofillers,respectively.The rapid development of materials chemistry and the diverse burgeoning nanomaterials impartstrong supportforthe design and optimization of nanofillers.From the aspect of tuning chemical structure,grafting alcohol-philic and water-repellency groups in membrane especially on membrane surface is desirable to intensify the selective capture of alcohol molecules from alcohol/water mixture and then acquire high selectivity.In order to acquire higher permeation flux,composite membrane has become a common membrane con figuration both for polymeric,inorganic and hybrid membranes.Furthermore,it is a research trend to design thin or even ultrathin separation layer with the premise of adequate structural stability and integrity.

Table 1PV performance of alcohol-permselective membranes in literatures

5.Conclusions and Perspectives

Due to the increasingly urgent demands for bioalcohol as clean energy,alcohol-permselectivity pervaporation with feasibility of coupling with fermentation process to improve the production efficiency of bioalcohol have aroused great research interest.In this review,we present a brief overview about the recent developments of alcoholpermselectivity pervaporation membranes with the emphasis on the optimization of confined structure(physical structure and chemical structure)in membrane and the corresponding structure-performance relationships.

Despite great progresses having been achieved,there are still several effective strategies could be employed for the further development of alcohol-permselective pervaporation membrane.1)For inorganic membranes,the synergistic effect between ultrathin polymer layer and inorganic materials can be utilized to improve the relatively low selectivity,in which case the interfacial adhesion between polymer and inorganic moieties should be a major consideration.2)For MMMs,incorporating higher-loading porous nanofillers to form more continuous channels inside nanofillers is beneficial to acquiring higher performance with the prerequisite of favorable interfacial morphology.3)Exploring advanced materials and methods to fabricate ultrathin and defect-free separation layer is effective to acquire high permeation flux.4)The physical and chemical structures of mass transfer channels should be synergistically tuned when designing high-efficiency membranes.5)From the aspect of industrial application,due to the complicated composition of the fermentation broth,the stability and anti-fouling property of membrane are critical factors determining the practical membrane performance.Meanwhile,the large-scale and cost-effective production of membrane materials should also be taken into account.

With the exploitation of advanced membrane materials and membrane structures,alcohol-permselective pervaporation is expected to be a more competitive separation technology to achieve high-efficiency production of bioalcohol.

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