Chinese Journal of Chemical Engineering 25 (2017) 1616–1626

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Chinese Journal of Chemical Engineering

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Review

Manipulation of confined structure in alcohol-permselectivepervaporation membranes☆

Jing Zhao, Wanqin Jin ⁎State Key Laboratory ofMaterials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu National Synergetic Innovation Center for AdvancedMaterials, Nanjing Tech University,Nanjing 210009, China

☆ Supported by the National Natural Science Foun21606123), the Jiangsu Province Natural Science(BK20160980), the Innovative Research Team ProgramChina (IRT13070), Top-notch Academic Programs ProjeInstitutions (TAPP) and A Project Funded by theDevelopment of Jiangsu Higher Education Institutions (PA⁎ Corresponding author.

E-mail address: wqjin@njtech.edu.cn (W.Q. Jin).

http://dx.doi.org/10.1016/j.cjche.2017.05.0041004-9541/© 2017 The Chemical Industry and Engineerin

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 December 2016Received in revised form 12 May 2017Accepted 14 May 2017Available online 1 July 2017

Alcohol-permselectivity pervaporation has been arousing increasingly more attention in bioalcohol productiondue to the advantages of environmental friendliness, low energy consumption and easy coupling with fermenta-tion process. With the intrinsic feature of larger molecules preferentially permeating and the consequent inferi-ority in selective diffusion, the development of alcohol-permselectivemembrane is relatively retarded comparedwith water-permselective membrane. This review presents the prevalent membrane materials utilized foralcohol-permselective pervaporation and emphatically expatiates the representative and important develop-ments in the past five years from the aspect of tuning confined structure inmembranes. In particular, the diversestructure tuning methods are describedwith the classifications of physical structure and chemical structure. Thecorresponding structure-performance relationships in alcohol-permselective pervaporationmembranes are alsoanalyzed to identify the objective of structure optimization. Furthermore, the tentative perspective on thepossible future directions of alcohol-permselective pervaporation membrane is briefly presented.© 2017 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.

Keywords:Confined structurePervaporationMembraneAlcohol-permselective

1. Introduction

In recent years, with the increasing concerns of environmentaldeterioration and energy shortage, alcohol fuel (mainly includingethanol and butanol) produced frombiomass has gained great attentionas an environmental friendly and renewable alternative for fossil energy[1–4]. However, the production of bioalcohol by fermentation is severe-ly impeded by the inhibition effect of products on the activity of yeast,which results in the low bioconversion efficiency and the low purity ofbioalcohol in fermentation broth. Therefore, it is necessary to integrateseparation technologies with fermentation process to achieve thein situ and continuous removal of alcohol from fermentation broth[1–3]. Compared with the conventional separation technologies suchas distillation and extraction, pervaporation (PV), as a representativemembrane technology for liquid separation, possesses remarkableadvantages of low energy consumption, cost effectiveness, innocuousnessto microorganisms, and easy coupling [5].

The flowdiagram of fermentation-pervaporation coupling process isshown in Fig. 1(a). First, the fermentation brothwith low-concentration

dation of China (21490585,Foundation for the Youth

by the Ministry of Education ofct of Jiangsu Higher EducationPriority Academic ProgramPD).

g Society of China, and Chemical Ind

alcohol enters the PVmodule, inwhich alcohol-permselectivemembraneis installed. Due to the preferentially permeation of alcohol moleculesthrough membrane, high-concentration alcohol solution is collectedas permeate and transferred to the subsequent processing, while theretentate is circulated to the fermenter. For the fermentation-pervaporation coupling technology, the separation performance of thecorresponding membrane, i.e., the alcohol-permselective pervaporationmembrane is the key element determining the efficiency and competi-tiveness of this coupling technology. The molecular transport acrossalcohol-permselective membrane consists of three steps (Fig. 1(b)):(i) sorption of permeating molecules from the feed liquid onto the up-stream surface of membrane; (ii) diffusion of permeating moleculesthrough themembrane under chemical potential difference; (iii) desorp-tion of permeating molecules to vapor phase on the downstream ofthe membrane. However, the development of alcohol-permselectivepervaporation membrane is relatively slow both in fundamentalresearches and practical applications. This phenomenon can be mainlyascribed to the following reasons. First, the mass transfer mechanism inpervaporation membrane is still not fully understood. Unlike membraneprocesses based on macroporous media and classical mass transfer theo-ries such as ultrafiltration and microfiltration, pervaporation processmainly involves mass transfer in narrow spaces with sizes comparableto the free distances of molecules, i.e., mass transfer in confined structure.The present solution-diffusion mechanism is still not enough to clearlydescribe themass transfer process in confined structure of pervaporationmembrane. Moreover, compared with the gas separation process, whichalso obeys the solution-diffusion mechanism, the liquid separation

ustry Press. All rights reserved.

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

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process possesses much more complicated molecular interactionsbetween permeating molecules as well as with membrane materials.Therefore, it is hard to study the mass transfer model in pervaporationmembrane. Second, the intrinsic feature of larger molecules permeatingpreferentially and smaller molecules being intercepted increases thedifficulty in achieving high separation performance. According to thesolution-diffusion mechanism, the selectivity of membrane can beascribed to two parts: solution selectivity and diffusion selectivity. Foralcohol-permselective membrane, preferentially adsorption of alcoholcan be achieved via employing alcohol-philic/hydrophobic membranematerials, while the preferentially diffusion of alcohol is hard to beobtained due to the larger size of alcohol molecules. Therefore, theselectivity of alcohol-permselective membrane is generally muchsmaller than water-permselective membrane.

An ideal membrane for industrial applications should possess highpermeation flux (representing treatment capacity), high separation fac-tor (representing separation efficiency), and high stability (representinglifetime and cost). Moreover, the production cost and the possibility toachieve large-scale production are also important factors. In the pastdecades, many efforts have been made to design advanced membranestructures and membrane materials with the aim of acquiring high-efficiency alcohol recovery from aqueous solution. The main methodslie in the manipulation of confined mass transfer channels. This reviewis intended to present the prevalent membrane materials utilized foralcohol-permselective pervaporation, provide deep insight into thestructure-performance relationships in membrane and highlight therepresentative and important developments in the past five years withthe optimization of confined structure in membrane as the emphasis.In particular, the membrane materials are mainly classified intopolymeric materials (rubbery polymer, glassy polymer, and blockcopolymer), inorganic materials and hybrid materials, and the diversestructure tuning methods are described with the classifications of phys-ical structure and chemical structure. Finally, a tentative perspective onthe possible future directions of alcohol-permselective pervaporationmembrane is briefly presented.

2. Membrane Materials for Alcohol-Permselective Pervaporation

The membrane materials for alcohol-permselective pervaporationcan be classified into three categories: polymeric materials, inorganicmaterials and hybrid materials. Among them, organophilic polymersare the most widely utilized materials, which include three types:1) rubbery polymer with soft chains, 2) glassy polymer with hardchains, and 3) block copolymer with both soft and hard segments. Itshould be mentioned that although the extensively studied mixed ma-trixmembrane (MMM)with polymermatrix and inorganic/hybridfillerhas become a promising membrane configuration, the bulk of themembrane is still polymeric materials. Therefore, it will be consideredas a strategy for structure manipulation of polymeric membrane andwill not be introduced as an individual membrane material in this part.

2.1. Polymeric materials

2.1.1. Rubbery polymersRubbery polymers mainly refer to silicon rubbers are the most

commonly utilized membrane materials for alcohol-permselectivepervaporation. On theonehand, the Si-O-Si on backbone endows siliconrubbers with high chain mobility and high free volume fraction,thus favoring the high permeability. On the other hand, the high hydro-phobicity is suitable for the preferential adsorption of alcohol fromaqueous solution. The typical silicon rubbers include poly(dimethylsiloxane) (PDMS) [4,6–8], poly(octylmethyl siloxane) (POMS) [9] andpoly(vinyltriethoxysilane) (PVTES) [10]. Among them, PDMS isregarded as the benchmark membrane material due to its excellentcomprehensive performance, which has been commercialized fororganophilic 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 separa-tion performance of PDMS membrane including incorporating solid/porous fillers [8,12,13], surface modification [14], cross-linking [15],employing advanced membrane-fabrication method [4], optimizingsupport structure [16] and so on. It is well known that permeation fluxis reversely proportional to membrane thickness. In order to acquirehigh permeation flux so that to improve the treatment capacity ofmembrane, it is necessary to decrease the thickness of PDMS layerwith the premise of integrity. The thinnest PDMS membrane reportedin literatures is about 800 nm fabricated via spray-coating method [4].Zhang and Ren et al. proposed an in-situ polymerization method tofabricate thin PVTES membranes with thickness about 300 nm, whichis much thinner than most of the PDMS membranes [10]. Meanwhile,the in-situ polymerization process endows the strong adhesion betweenactive layer and support layer. The as-prepared PVTES membraneexhibits a high permeation flux over 10 kg·m−2·h−1, which is aboutone order of magnitude higher than that of PDMS membrane.

2.1.2. Glassy polymersGlassy polymers simultaneously possessing rigid molecular chains

and high free volume fraction are attractive membrane materials toacquire 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 bondsand the side chain of trimethylsilane generate loose molecular confor-mation, large chain spacing and high free volume fraction, thus achiev-ing high permeability for both gas and liquid molecules. Meanwhile, thehydrophobic surface is suitable for the preferential adsorption of alcoholfrom aqueous solution. Padukone et al. first employed PTMSP as themembrane material for ethanol recovery from fermentation broth. Theflux of PTMSP membrane is about three-fold higher and the separationfactor is about two-fold higher than that of the commonly utilizedPDMS membrane under similar conditions [17]. In order to further

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improve the separation performance of PTMSP membrane, manyresearches have been performed via incorporating silica nanospheresinto PTMSP matrix [11,19]. The membrane with optimized silica contentand thickness exhibits outstanding performance with the flux of9.5 kg·m−2·h−1, ethanol/water separation factor of 18.3 and butanol/water separation factor of 104, much superior over commercial mem-branes [11]. However, the physical aging and the chain relaxation ofPTMSP membrane with operating time inevitably leads to the decreasedfree volume and deteriorated separation performance, which impedesthe practical application of PTMSP membrane.

Polymers of intrinsic microporosity (PIMs) are emergingmembranematerials with rigid molecular chains and high free volume fraction,which combine the processability of polymeric membrane and thetransport property of inorganicmembrane, exhibiting promising poten-tials in molecular separation [20–22]. The special ladder-like structurewith contorted sites prevents polymer chains from rotating and pack-ing, thus obtaining unusually high free volume and large internalsurface. However, the applications of PIMs in membrane field mainlyfocus on gas separation. Their effects on pervaporation process haverarely been reported. Izák et al. first demonstrated the feasibility of uti-lizing PIM as membrane material to perform alcohol-permselectivepervaporation [20]. The PIM-1membrane shows significantly decreasedpermeability and increased selectivity over time upon storage due tothe slow rearrangement of polymer chains, and the resultant densifiedmembrane structure.

2.1.3. Block copolymersApart from the above mentioned rubbery polymers and glassy poly-

mers, various block copolymers with both rigid and flexible segmentsalso have been employed as organophilic pervaporation membranematerials. By adjusting the relative ratio and molecular weight ofdifferent segments, membranes with diverse structural and chemicalproperties can be obtained. PEBA is a group of copolymers comprisingrigid polyamide segments and flexible polyether segments. PEBA 2533with 80 wt% organophilic polyether segments and 20 wt% polyamidesegments is the commonly utilized one for alcohol recovery due to thehigh hydrophobicity [23–27]. Comparatively, PEBA membrane pos-sesses lower selectivity compared with PDMS membrane. Besides thecommonly utilized PEBA, block copolymers for alcohol-permselectivepervaporation also include poly(styrene-b-dimethylsiloxane-b-styrene) (SDS) triblock copolymer [28], poly(butyl acrylate-co-styrene)[29], hydroxyterminated polybutadiene-based polyurethaneurea (HTPB-PU) [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 nano-materials. The much higher mechanical strength, thermal stability,chemical stability and more rigid and ordered three-dimensional po-rous structure than polymer render it a promising membrane materialfor molecular separation [32,33]. Among the abundant zeolite types,MFI-type zeolites including ZSM-5 zeolite with high ratio of Si/Al andpure silica MFI-type zeolite (also called silicalite-1) are the mostcommonly used material for alcohol-permselective pervaporationmembrane due to their medium pore size (~0.55 nm) and high hydro-phobicity [34–36]. The influences of synthesis method [37,38], fabrica-tion condition [39–41] and support structure [39,40] on MFI-typezeolite membrane structure and alcohol/water separation performancehave been extensively investigated. Wang et al. [36] prepared hollowfiber-supported silicalite-1membrane via the combination of secondarygrowth method and wetting-assisted rub-coating seeding method.With the optimization of fabrication conditions such as seed size, seedmorphology and structure-directing agent (SDA) content, the thinmembrane (~5 μm) with ultrahigh pervaporation performance(permeation flux of 9.8 kg·m−2·h−1 and separation factor of 58) can

be obtained. ZSM-5 is a kind of MFI-type aluminosilicate zeolite, whichcan be tuned from water-permselective to alcohol-permselective withthe increase of Si/Al ratio due to the enhanced membrane hydrophobic-ity. Weyd et 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 toprevent Al incorporation during the membrane fabrication process.With the feed containing 5 wt% ethanol, the ethanol concentration inpermeate can be enriched up to 87 wt% (with permeation flux about0.87 kg·m−2·h−1 and separation factor about 100). Comparatively,silicalite-1 membrane draws more attention and exhibits higher sepa-ration performance for alcohol recovery than ZSM-5 membrane dueto its pure-silica composition and the resultant higher hydrophobicity.

Although new synthesismethods have been explored and optimizedto acquire thin zeolite membrane so that to increase permeation flux,the thicknesses of most zeolite membranes are still in the micrometerscale. How to fabricate ultrathin zeolite membrane with the premiseof low defect density is one of the most top research topics for zeolitemembrane. Tsapatsis's group successfully synthesized intact exfoliatedMFI nanosheets from multilamellar silicalite-1 precursors via meltblending with polystyrene and the following sonication in toluene[32]. After simple filtration of the MFI nanosheet suspension in poroussupport and the calcination for polymer removal, ultrathin MFImembrane 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 growthmethodto fabricate oriented MFI zeolite membrane with thickness varying inthe range of 100–250 nm [33]. First, a nanosheet seed layer (~50 nm)was prepared by simply filtering the suspension of MFI nanosheetsthrough the silica supports. Afterwards, the gel-less secondary growthof MFI zeolite was carried out through impregnating the nanosheetmembrane into the ultra-dilutedMFI structure-directing agent solutionfollowed by heat treatment. The silica layerwas also consumed as siliconsource during the secondary growth process. The b-out-of-plane orienta-tion of MFI nanosheets as seed layer led to the oriented 0.55 nm porechannels 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 functionalizedcavities, and flexible skeleton, the hybrid metal–organic frameworks(MOFs)withmetal ions/clusters and organic linkers have also been con-sidered as promising candidates for molecular separation membranes[43–45]. Besides the hydrophobicity and pore size, the integrity andthickness ofMOF layer, the stability ofMOF crystals in aqueous solution,and the binding strength between MOF layer and support are also crit-ical factors to achieve high-efficiency separation. Lin et al. [46] first usedMOFmembrane in pervaporation separation of alcohol-water mixtureswith ZIF-71 as the selectedmaterial. ZIF-71 is a kind of hydrophobic andstable 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 preparedvia reactive seeding method is still not satisfactory, it demonstratesthe possibility of MOF membrane in organics–water separation. Ourgroup employed modified contra-diffusion method to fabricate thinand integrate ZIF-71 membranes on ceramic hollow fiber support(Fig. 2) [47]. During the membrane fabrication process, the solution ofZn2+ and the solution of imidazole were placed on different sides ofthe hollow fiber support, and reacted on the interface of the ceramichollow fiber support under diffusion. The self-inhibition effect of theZIF-71 formation process similar with interfacial polymerizationrenders the formation of thin ZIF-71 with thickness about 2.5 μm andhigh integrity. Meanwhile, the diffusion of nutrients leads to the slightpenetration of ZIF-71 into ceramic support, thus facilitating the favor-able 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.

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3. Tuning of Confined Structure in Membrane

For each category of membrane, the physical structure and chemicalstructure of confined channels are two critical elements for the masstransfer process in membrane. The physical structure comprises thesize, density and connectivity of mass transfer channels. The channelsize determines the diffusion resistance of permeating molecules andthe impact extent of channel wall on the diffusion process, while thechannel density decides the amount of mass transfer channels in mem-brane, the channel connectivity influences the diffusion resistance ofpermeating molecules. The chemical structure dictates the affinity ofchannels for permeating molecules, which exerts great impacts on thedriving force of adsorption process and decides the level of difficultyfor the molecules entering into channels. According to the above analy-sis, the tuning of confined structure in membrane can be performedfrom two aspects: physical structure and chemical structure.

3.1. The physical structure of mass transfer channels

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

3.1.1. Optimization of free volume propertiesAlthough crosslinking is effective in manipulating the free volume

properties of polymeric membranes, it is difficult to achieve simulta-neously enhanced permeability and selectivity due to the restrictionof “trade-off” effect. In recent years, MMMs with polymer matrix andinorganic/hybrid filler have been considered as an efficient strategy toaddress this issue. The incorporation of fillers can interfere the alignedpacking of polymer chains during membrane formation process, andthen influence the free volume properties owing to the combined im-pacts of fillers' steric effect and interfacial interaction between polymermatrix and fillers. From the aspect of steric effect, the fillers' dimensionis a critical factor. For instance, 2D nanosheets may act as nucleating

agent for the formation of highly ordered and crystalline regions inpolymer, thus partially offsetting the interference for polymer chains[49]. However, the systematical and in-depth research about the influ-ence of filler dimension on free volume properties has not yet beenreported. From the aspect of interfacial interaction, the relative strengthof interfacial interaction comparedwith interfacial stress induced by thesurface property difference between polymer matrix and filler deter-mines the interfacial morphology of MMMs: weaker interfacial interac-tion results in interfacial voids (increased free volume cavities withlower selectivity), while too strong interfacial interaction may giverise to restricted mobility of polymer chains, i.e., chain rigidification(decreased free volume cavities) [50]. On the other hand, weak interfacialinteractions generally lead to severe agglomeration of filler, in whichcases the large inter-filler channels with no molecular selectivitydominate the mass transfer process in hybrid region, and deterioratemembrane performance.

As stated above, the separation performance of MMMs dependson the interfacial interactions between polymer matrix and filler toa great extent, which can be manipulated via versatile approachessuch as physical coating, chemical grafting or replacing with organiccomponent-containing fillers. Polyhedral oligomeric silsesquioxane(POSS) is a hybrid nanomaterial with an inner inorganic Si-O backbone(SiO1.5)n and eight external organic arms. The versatile organic armscan impart interaction sites with polymer matrix [51,52]. Our groupemployed methyl–POSS as filler to manipulate the free volume proper-ties of PDMS matrix [51]. The enhanced intra-molecular interactions ofPDMS and the promoted PDMS chain mobility (as demonstrated by theresults of molecular simulation) arising from the PDMS–POSS interac-tions lead to the optimized free volume properties of MMMs with freevolume cavities smaller than thediameter of butanolmolecules reducingand larger free volume cavities increasing (Fig. 3). As a result, theselectivity 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 membraneswith thinner separation layer have attracted great attention due to theshortened diffusion pathway. ForMMMs, the fillers in thinnermembraneshould bewith smaller size so that to acquire defect-freemembrane [53].It is conceivable that the smaller-sizefillers possess higher contacting areawith polymermatrix, thus exertingmore significant influence on the free

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

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volume properties. As a result, the effects of interfacial morphology onmembrane separation performance become more remarkable, whichputs forward more stringent requirements on interfacial interactions.

3.1.2. Incorporation of new mass transfer channelsComparedwith the dynamic and instantaneous free volume cavities

in dense membrane, the pore structure in inorganic porous materialsis static and permanent, which confers lower diffusion resistanceupon permeating molecules. Therefore, embedding inorganic porousmaterials into polymermatrix to incorporate efficientmass transfer chan-nels has been widely adopted to intensify the alcohol-permselectivepervaporation process. It is noteworthy that the aforementioned interfa-cial interaction between polymer matrix and filler is also a critical factorfor porous filler-based MMMs. On the basis of employing porous fillers,the interfacial interaction can be further improved to simultaneouslyoptimize free volume properties. This review will introduce the relevantresearch progress focusing on the most widely adopted porous fillersincluding zeolite, nanotube and MOFs.

3.1.2.1. Zeolite. Similarwith the type of zeolite membrane, ZSM-5 zeolitewith high ratio of Si/Al and silicalite-1 zeolite are also the mostcommonly utilized fillers in alcohol-permselective pervaporationmembranes. Vankelecom and coworkers embedded solid and hollowsilicalite-1 (SS and HS) crystals into PDMS matrix respectively [54].The rich pore structure in solid silicalite-1 endows ethanol moleculeswith much lower diffusion resistance than PDMS matrix, leading tothe approximate 4.5-fold increase of ethanol flux. Moreover, the hollowparts of HS further reduce the diffusion resistance of ethanol across theinorganic region (or in other words, reduce the effective membranethickness), thus causing a more significant 7-fold increase of ethanolflux.

Despite of the extensive investigation of zeolite-based MMMs, thezeolite dispersion and interfacial morphology are still critical issuesneed to be addressed due to the existence of hydrophilic silanol groupson zeolite surface, which may result in interfacial or inter-filler voidsand deteriorate separation performance. Moreover, according to the

Maxwell model of MMMs, the volume fraction of inorganic porous fillerexerts great influence on themembrane permeability [50],which bringsforward higher requirements for the dispersion of filler and the interfa-cialmorphology. Therefore, surfacemodification of zeolite is imperativein fabricating high-performance MMMs [12,55–57]. Due to the exis-tence of abundant hydroxyl groups on zeolite surface, the silylation re-action with silane coupling agents is generally employed for surfacemodification. Our group proposed a grafting/coating method to tailorthe interfacial morphology between PDMS matrix and ZSM-5 zeolite[57]. ZSM-5 was first grafted with n-octyltriethoxysilane (OTES) to per-form hydrophobic modification, and then adsorbed a thin PDMS layervia chain entanglements of PDMS and OTES. Therefore, the modifiedZSM-5with “PDMS surface” possesses favorable interfacial compatibili-ty with PDMS matrix. As a result, the MMMs achieve a maximal ZSM-5loading of 40 wt% with uniform dispersion. The corresponding mem-brane exhibited a 75% higher ethanol/water separation factor comparedwith pristine ZSM-5-embeddedmembrane. Up to now, there have beendiverse silane-coupling agents utilized for surface modification of zeo-lite in alcohol-permselective pervaporation membranes. The molecularstructures of themainly utilized silane-coupling agents are summarizedand listed in Fig. 4.

In most MMMs, the porous fillers are isolated and surrounded bypolymermatrix, thus hampering the full exploitation of thefillers' inter-nal channels. It is conceivable that constructing continuous inorganicchannels across the membrane via increasing filler loading is an effec-tive strategy to solve this problem. However, the fabrication of highfiller-loadingMMMs is subject to the constraints of filler agglomerationand interfacial voids. In order to circumvent these issues, Yang et al.proposed a “packing–filling” method to fabricate PDMS/silicalite-1MMMs replacing the conventional “blending–casting” method [58]. Thepacking silicalite-1 crystals constituted the membrane matrix, whilePDMS sealed the interspaces among silicalite-1 crystals and formed an ex-tremely thin layer on membrane surface (Fig. 5). Due to the continuouslow diffusion resistance channels constructed by silicalite-1 play an dom-inant role inmolecular permeation, the as-preparedultrathinmembranes(about 300 nm)with zeolite loading up to 74 vol% exhibited an especially

R= C2H3, C2H5, C8H17, C18H37

R= C8H17, C10H21, C12H25, C16H33

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

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high flux (5.0–11.2 kg·m−2·h−1) and good separation factor (25.0–41.6)for i-butanol/water pervaporation separation.

3.1.2.2. Nanotube. Nanotube is a special kind of filler possessing highaspect ratio, high specific area and 1D long-range ordered channels[7,23,59]. Among various nanotubes, carbon nanotube (CNT) is a typicalrepresentative with hydrophobic and smooth surface, diverse diame-ters, and excellent mechanical strength. Yen et al. first incorporatedCNTs into polymer matrix (PEBA) utilized for alcohol-permselective

Fig. 5. Schematic illustration of the fabrication procedure of the silicalite-PDMS nanocomppermission from Elsevier Ltd.

pervaporation membranes [23]. The PEBA/CNTs MMM with 10 wt%CNTs loading brought about a 59% increased butanol removing ratecompared with PEBA membrane. When utilized in a fermentation-pervaporation coupling process, the correspondingmembrane achieveda 20% increase both in butanol productivity and yield, indicating itsgreat potential in Acetone–Butanol–Ethanol (ABE) fermentation indus-try. Afterwards, CNTs were also incorporated into PDMS matrix [7]. Atthe optimal loading of 10 wt%, the butanol flux and separation factorof PDMS/CNTs MMM were significantly improved by 204.9% and

osite membrane by a “Packing–filling” method [58]. Copyright 2011. Reproduced with

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125.0% at 37 °C compared with those of PDMS membrane. It was dem-onstrated that the influence of CNTs on membrane permeability wasmore significant at lower temperature, owing to the accelerated ther-mal motion of PDMS polymer chains at higher temperature, which de-clined the diffusion resistance of permeating molecules across PDMSmatrix.

Compared with other inorganic porous fillers, the internal channelsof CNTs are straighter and smoother, which are beneficial to the lowerdiffusion resistance. However, CNTs generally exhibit random orienta-tion in polymer matrix, thus prolonging the mass transfer pathwaysand then exerting reverse impacts on themolecule permeation process.

3.1.2.3. MOFs. MOFs are novel hybrid porous nano-materials consistingof metal ions or clusters connected by organic linkers. Compared withconventional inorganic porous nano-materials, the existence of organiclinkers onMOFs is beneficial to improving interfacial compatibility withpolymer matrix, and imparts functionalization accessibility to manipu-late interfacial interactions. Additionally, MOFs possess rich pore struc-ture and high specific area (up to 6500 m2·g−1). The above featurespredestine MOFs as attractive fillers for high-performance MMMs[60–63]. Li and coworkers first incorporated MOFs into alcohol-permselective pervaporation membranes [64]. Zeolitic imidazolateframeworks (ZIF-8) with superhydrophobicity, extremely low wateradsorption, and room temperature synthesis features are employed asfiller and embedded into polymethylphenylsiloxane (PMPS) matrix. Al-though the aperture size of ZIF-8 (0.34 nm) is smaller than the kineticdiameter of i-butanol (0.50 nm), the flexible framework structure con-fers exceptional high adsorption capacity for i-butanol. After incorporat-ing ZIF-8 into PMPS, the i-butanol/water separation factor and thei-butanol permeability increase by 100% and 320%, respectively, com-pared with PMPS pristine membrane. The control experiments withZIF-7 (possessing similar superhydrophobicity, narrower aperture size,and much more rigid framework compared with ZIF-8) as fillerconfirms the crucial contribution of the channels inside ZIF-8 to theselective permeation of i-butanol.

In the aforementioned study, the membranes were fabricated viasimple solution-blending dip-coating method with the separation layerof 2.5 μm. The optimal ZIF-8 loading was determined as 10 wt% due tothe poor dispersion at higher loading. In order to acquire ultrathinnanohybrid separation layer with high ZIF-8 loading and uniform ZIF-8dispersion, simultaneous spray self-assembly technique was exploredto fabricate ZIF-8/PDMS MMMs (Fig. 6) [4]. During the membrane-

Fig. 6. Formation of the ZIF-8/PDMS mixed matrix membrane by the simultaneous spray self-Sons Ltd.

fabrication process, the ZIF-8/PDMS suspension and the solution con-taining cross-linking agent tetraethyl orthosilicate (TEOS) and catalystdibutyltin dilaurate (DBTDL) were poured separately into two pressurebarrels and simultaneously sprayed onto a polysulfone (PS) substrate.Due to the mechanical atomization of spraying process and the self-stirring of solution in barrel, ZIF-8 uniformly disperses in PDMS mem-brane even at the high loading of 40 wt%. The hybrid separation layerpossesses a thickness of 0.8 μm, much thinner than the reported valuesin literatures. As a result, the ZIF-8/PDMS membrane exhibits a highseparation performance for n-butanol/water separation with the totalflux of 4.846 kg·m−2·h−1, and separation factor of 81.6.

Apart fromZIF-8, other hydrophobicMOFswith different pore struc-tures and chemical compositions such as ZIF-71 [24,65], materials insti-tute Lavoisier-53 (MIL-53) [6], and Zn(BDC)(TED)0.5 (BDC = benzenedicarboxylate, TED = triethylenediamine) [25] have also been investi-gated as fillers in PDMS or PEBA matrix.

There is an interesting phenomenon thatMOFmembranes generallyexhibit unsatisfactory alcohol/water selectivity while the MMMs withthe corresponding MOF crystals can acquire high separation perfor-mance. ForMOFmembrane, it is difficult to precisely and efficiently cap-ture alcohol molecules from the mixture solutions with high watercontent (over 90 wt%), thus leading to the poor selectivity. However,during the separation process with MMMs, the alcohol/water mixturecan bepre-separated via their different affinitieswithpolymeric surface.Therefore, the mixture entering into MOF channels possesses muchlower water content. Afterwards, the hydrophobic channels insideMOFs facilitate the diffusion of alcohol molecules, and eventuallyimprove the alcohol permeability and alcohol/water selectivity. Inconclusion, it is the synergistic effect between polymer and MOFs thatdominantly contributes to the high separation performance of MOF-based MMMs.

3.2. The chemical structure of mass transfer channels

Tuning the chemical structure of mass transfer channels to improvemembrane performance means modifying membrane with functionalgroups possessing selective affinity for alcohol molecules over watermolecules. Due to the intrinsic feature of larger molecules preferentiallypermeating and the consequent inferiority in selective diffusion, theselective adsorption process plays a dominant role in alcohol/water sep-aration. First, the highly alcohol-philic groups are in favor of loweringthe chemical potential of alcohol molecules at equilibrium state,

assembly technique [4]. Copyright 2014. Reproduced with permission from JohnWiley &

1623J. Zhao, W.Q. Jin / Chinese Journal of Chemical Engineering 25 (2017) 1616–1626

thus increasing the driving force of adsorption process. Second, theemployed alcohol-philic groups are of water-repellency, thus intensify-ing the selective capture of alcohol molecules from alcohol/water mix-ture. Last but not least, the strong interactions between membranematerials and alcohol molecules are beneficial to weakening the cou-pling effect between alcohol and water molecules, thus inhibiting thediffusion of water along with alcohol molecules. Up to now, the mostextensively adopted alcohol-philic groups for alcohol-permselectivepervaporation membranes are hydrophobic alkyl chains [12,15] orlow-surface-energy CF3 groups [14,66–69]. According to the locationof alcohol-philic groups in membrane and the modification methods,the tuning of chemical structure can be further divided into surfacemodification and bulk modification.

3.2.1. Surface modificationCovalent grafting is the most commonly utilized method for surface

modification. Huang et al. constructed a superhydrophobic membranesurface (with awater contact angle of about 172°) under the inspirationof lotus leaf surface via the synergism of Ag nanoparticle and covalentlybonded perfluorodecanethiol molecule (F-Ag-PD@Al2O3) [66]. The ex-cellently preferential adsorption of i-butanol and the inverse exclusionof water were acquired and then gave rise to the exceptionally highi-butanol/water separation factor up to 151, significantly exceedingthe previously reported hydrophobic membranes. Zhang and Ji et al.modified PDMS surface via UV/ozone treatment and the subsequent(tridecafluoroctyl)triethoxysilane deposition to incorporate CF3 groups(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 modificationComparatively, bulk modification possesses more versatile ap-

proaches such as co-/graft-polymerization [70], incorporation of poly-mer additives [67,71], and surface functionalization of fillers [12,55,69].For instance, Singh et al. in-situ modified PDMS membrane with alkylchains and perfluoroalkyl chains via utilizing n-octadecyltrichlorosilane

Fig. 7. Schematic diagram describing the preparation of the self-assembled monolayer-modAmerican Chemical Society.

(OTS) and trichloro(1H,1H,2H,2H-perflourooctyl)silane (FTS) as cross-linkers, respectively [15]. Araki and coworkers employed silane couplingagents with different functional groups as silica precursors to fabricatehydrophobic silica membrane, including phenyltrimethoxysilane(PhTMS), ethyltrimethoxysilane (ETMS), n-propyltrimethoxysilane(PrTMS), isobutyltrimethoxysilane (BTMS), and hexyltrimethoxysilane(HTMS) [72]. Wan et al. prepared silicalite-1 particles with different sur-face compositions via hydrothermal synthesis in fluoride (F) or alkaline(OH) media and investigated the influence of filler's hydrophobicity onthe perm-selectivity of ethanol molecules [69]. With the same loadingof silicalite (40 wt%), the silicalite-1(F)-embedded membrane acquireda higher ethanol/water separation factor of 23.8, which was about 50%higher than that of silicalite-1(OH)-embeddedmembrane. Furthermore,from the almost constant water flux with silicalite-1(F) loading and theincreased water flux with silicalite-1(OH) loading, it can be concludedthat the higher hydrophobicity of silicalite-1(F) imparted a preferentialpathway for ethanol molecules, and forced water to mainly diffusethrough PDMSmatrix with tortuous free volume cavities. Yang et al. im-proved the hydrophobicity of ZSM-5 zeolitemembrane viapartial substi-tution of silicon in zeolite framework by boron (B) element, thusobtaining the significantly enhanced ethanol/water separation factorfrom 27 to 55 [34].

4. Comparison of Membrane Performance

The pervaporation performance of various membranes with differ-ent materials and structures for alcohol recovery (mainly ethanol orbutanol) is summarized in Table 1. For polymeric materials, PDMS isstill the hottest research object due to its excellent comprehensive per-formance, facile and cost-effective fabrication, as well as the maturelarge-scale production technology. Various advanced methods ormaterials have been adopted to further improve the separationperformance of PDMS membrane. PTMSP exhibits extremely highseparation performance, while the physical aging phenomenon isthe fatal problem restricting its practical application. PIM-1 is an

ified PDMS/PSf membrane [14]. Copyright 2013. Reproduced with permission from the

Table 1PV performance of alcohol-permselective membranes in literatures

Membranes Feed alcohol content/wt% Temperature/°C Permeation flux/kg·m−2·h−1 Separation factor Ref.

MCM-41@ZIF-8/PDMS 5.0 (ethanol) 70 2.201 10.4 [8]ZIF-8/PDMS 1.0 (butanol) 80 4.846 81.6 [4]MIL-53/PDMS 5.0 (ethanol) 70 5.467 11.1 [6]ZIF-71/PDMS 5.0 (ethanol) 50 0.5 9.9 [65]ZIF-71/PDMS 5.0 (butanol) 50 0.66 30.2 [65]Silicalite-1/PDMS 5.0 (ethanol) 60 0.4 14.7 [55]ZSM-5/PDMS 5.0 (ethanol) 40 0.408 14 [57]Silicalite-1/PDMS 1.0 (butanol) 80 7.1 32 [58]Silicalite-1/PDMS 5.0 (ethanol) 50 0.176 34.3 [12]PVTES 5.0 (ethanol) 45 7.74 6.0 [10]Silica/PTMSP 5.0 (ethanol) 50 9.5 18.3 [11]Silica/PTMSP 5.0 (butanol) 50 9.5 104 [11]PDMSM/PTMSP 2.0 (butanol) 25 0.12 128 [18]PIM-1 5.0 (butanol) 65 9 18.5 [73]ZIF-71/PEBA 1.0 (butanol) 40 0.85 20 [24]Zn(BDC)(TED)0.5/PEBA 1.0 (butanol) 40 0.7 16.5 [25]MCM-41/PEBA 2.5 (butanol) 35 0.5 25 [26]p(VDF-HFP) 5.1 (ethanol) 40 2.4 5.88 [31]Clay/poly(butyl acrylate-co-styrene) 5.0 (ethanol) 30 0.34 26 [29]F-Ag-PD@Al2O3 5.0 (butanol) 50 5.0 151 [66]PDMS-CF3 5.0 (ethanol) 60 0.413 13.1 [14]B-ZSM-5 5.0 (ethanol) 60 2.6 55 [34]Fe-ZSM-5 5.0 (ethanol) 50 0.71 43.3 [74]Silicalite-1 5.0 (ethanol) 60 9.8 58 [36]Silicalite-1 5.0 (ethanol) 75 5.4 54 [40]Silicalite-1 5.0 (ethanol) 60 1.0 85 [35]Silicalite-1 5.0 (ethanol) 60 2.1 85 [75]ZIF-71 5.0 (ethanol) 25 0.322 6.07 [46]ZIF-71 5.0 (ethanol) 25 2.601 6.88 [47]

1624 J. Zhao, W.Q. Jin / Chinese Journal of Chemical Engineering 25 (2017) 1616–1626

emerging membrane material with preliminary attempts for alcohol-permselective pervaporation. The separation performance is stillunsatisfactory and needs further optimization. PEBA membranes pos-sess lower separation factor than PDMS-based membranes especiallyin separation factor. Silicalite-1 membrane exhibits attractive perfor-mance both in permeation flux and separation factor. Moreover, as azeolite membrane, silicalite-1 membrane also possesses high thermalstability and mechanical strength. The restriction for practical applica-tion lies in the high fabrication cost. Comparatively, the separation per-formance of MOFmembranewith ZIF-71 as a representative example isinadequate, especially in separation factor. Considering the abundantspecies of MOFs and their facile manipulation, the pore structure andthe chemical property of MOFs can be further optimized to match theseparation of alcohol/water mixtures.

From the aspect of tuning physical structure, incorporating porousnanofillers into polymer matrix is deemed to be a superb strategywith the capability of simultaneously optimizing the intrinsic freevolume cavities and incorporating new mass transfer channels, whichare influenced by the chemical composition and pore structure ofnanofillers, respectively. The rapid development of materials chemistryand the diverse burgeoningnanomaterials impart strong support for thedesign and optimization of nanofillers. From the aspect of tuning chem-ical structure, grafting alcohol-philic and water-repellency groups inmembrane especially on membrane surface is desirable to intensifythe selective capture of alcohol molecules from alcohol/water mixtureand then acquire high selectivity. In order to acquire higher permeationflux, composite membrane has become a common membrane configu-ration both for polymeric, inorganic and hybrid membranes. Further-more, it is a research trend to design thin or even ultrathin separationlayer with the premise of adequate structural stability and integrity.

5. Conclusions and Perspectives

Due to the increasingly urgent demands for bioalcohol as clean ener-gy, alcohol-permselectivity pervaporation with feasibility of couplingwith fermentation process to improve the production efficiency ofbioalcohol have aroused great research interest. In this review, we

present a brief overview about the recent developments of alcohol-permselectivity pervaporation membranes with the emphasis on theoptimization of confined structure (physical structure and chemicalstructure) in membrane and the corresponding structure-performancerelationships.

Despite great progresses having been achieved, there are still severaleffective strategies could be employed for the further development ofalcohol-permselective pervaporation membrane. 1) For inorganicmembranes, the synergistic effect between ultrathin polymer layerand inorganic materials can be utilized to improve the relatively lowselectivity, in which case the interfacial adhesion between polymerand inorganic moieties should be a major consideration. 2) ForMMMs, incorporating higher-loading porous nanofillers to form morecontinuous channels inside nanofillers is beneficial to acquiring higherperformance with the prerequisite of favorable interfacial morphology.3) Exploring advancedmaterials andmethods to fabricate ultrathin anddefect-free separation layer is effective to acquire high permeation flux.4) The physical and chemical structures of mass transfer channelsshould be synergistically tuned when designing high-efficiencymembranes. 5) From the aspect of industrial application, due to thecomplicated composition of the fermentation broth, the stability andanti-fouling property of membrane are critical factors determining thepractical membrane performance. Meanwhile, the large-scale andcost-effective production of membrane materials should also be takeninto account.

With the exploitation of advanced membrane materials and mem-brane structures, alcohol-permselective pervaporation is expected to beamore competitive separation technology to achieve high-efficiency pro-duction of bioalcohol.

References

[1] G. Liu, W. Wei, W. Jin, Pervaporation membranes for biobutanol production, ACSSustain. Chem. Eng. 2 (4) (2014) 546–560.

[2] C. Fu, D. Cai, S. Hu, Q. Miao, Y. Wang, P. Qin, Z. Wang, T. Tan, Ethanol fermentationintegrated with PDMS composite membrane: An effective process, Bioresour.Technol. 200 (2016) 648–657.

1625J. Zhao, W.Q. Jin / Chinese Journal of Chemical Engineering 25 (2017) 1616–1626

[3] S. Yi, B. Qi, Y. Su, Y. Wan, Effects of fermentation by-products and inhibitors onpervaporative recovery of biofuels from fermentation broths with novel silanemodified silicalite-1/PDMS/PAN thin film composite membrane, Chem. Eng. J. 279(2015) 547–554.

[4] H. Fan, Q. Shi, H. Yan, S. Ji, J. Dong, G. Zhang, Simultaneous spray self-assembly ofhighly loaded ZIF-8-PDMS nanohybrid membranes exhibiting exceptionally highbiobutanol-permselective pervaporation, Angew. Chem. Int. Ed. 53 (22) (2014)5578–5582.

[5] Y.K. Ong, G.M. Shi, N.L. Le, Y.P. Tang, J. Zuo, S.P. Nunes, T.-S. Chung, Recentmembrane development for pervaporation processes, Prog. Polym. Sci. 57 (2016)1–31.

[6] G. Zhang, J. Li, N. Wang, H. Fan, R. Zhang, G. Zhang, S. Ji, Enhanced flux of polydi-methylsiloxane membrane for ethanol permselective pervaporation via incorporationof MIL-53 particles, J. Membr. Sci. 492 (2015) 322–330.

[7] C. Xue, G.-Q. Du, L.-J. Chen, J.-G. Ren, J.-X. Sun, F.-W. Bai, S.-T. Yang, A carbon nano-tube filled polydimethylsiloxane hybrid membrane for enhanced butanol recovery,Sci. Rep. 4 (2014).

[8] N. Wang, G. Shi, J. Gao, J. Li, L. Wang, H. Guo, G. Zhang, S. Ji, MCM-41@ZIF-8/PDMShybrid membranes with micro- and nanoscaled hierarchical structure for alcoholpermselective pervaporation, Sep. Purif. Technol. 153 (2015) 146–155.

[9] A. Rom, A. Friedl, Investigation of pervaporation performance of POMS membraneduring separation of butanol from water and the effect of added acetone andethanol, Sep. Purif. Technol. 170 (2016) 40–48.

[10] W. Zhang, C. Xia, L. Li, Z. Ren, J. Liu, X. Yang, Preparation and application of a novelethanol permselective poly(vinyltriethoxysilane) membrane, RSC Adv. 4 (28)(2014) 14592.

[11] S. Claes, P. Vandezande, S.Mullens, K. De Sitter, R. Peeters,M.K. Van Bael, Preparationand benchmarking of thin film supported PTMSP-silica pervaporation membranes,J. Membr. Sci. 389 (2012) 265–271.

[12] X. Zhuang, X. Chen, Y. Su, J. Luo, S. Feng, H. Zhou, Y. Wan, Surface modification ofsilicalite-1 with alkoxysilanes to improve the performance of PDMS/silicalite-1pervaporation membranes: Preparation, characterization and modeling, J. Membr.Sci. 499 (2016) 386–395.

[13] Y. Lan, N. Yan, W. Wang, Application of PDMS pervaporation membranes filledwith tree bark biochar for ethanol/water separation, RSC Adv. 6 (53) (2016)47637–47645.

[14] J. Li, S. Ji, G. Zhang, H. Guo, Surface-modification of poly(dimethylsiloxane) mem-brane with self-assembled monolayers for alcohol permselective pervaporation,Langmuir 29 (25) (2013) 8093–8102.

[15] G.L. Jadav, V.K. Aswal, P.S. Singh, In-situ preparation of polydimethylsiloxane mem-brane with long hydrophobic alkyl chain for application in separation of dissolvedvolatile organics from wastewater, J. Membr. Sci. 492 (2015) 95–106.

[16] W. Wei, S. Xia, G. Liu, X. Gu, W. Jin, N. Xu, Interfacial adhesion between polymerseparation layer and ceramic support for composite membrane, AIChE J. 56 (6)(2010) 1584–1592.

[17] S.L. Schmidt, M.D. Myers, S.S. Kelley, J.D. McMillan, N. Padukone, Evaluation ofPTMSP membranes in achieving enhanced ethanol removal from fermentationsby pervaporation, Appl. Biochem. Biotechnol. 63-5 (1997) 469–482.

[18] I.L. Borisov, A.O. Malakhov, V.S. Khotimsky, E.G. Litvinova, E.S. Finkelshtein, N.V.Ushakov, V.V. Volkov, Novel PTMSP-based membranes containing elastomericfillers: Enhanced 1-butanol/water pervaporation selectivity and permeability,J. Membr. Sci. 466 (2014) 322–330.

[19] S. Claes, P. Vandezande, S. Mullens, R. Leysen, K. De Sitter, A. Andersson, F.H.J.Maurer, H. Van den Rul, R. Peeters, M.K. Van Bael, High flux composite PTMSP-silica nanohybrid membranes for the pervaporation of ethanol/water mixtures,J. Membr. Sci. 351 (1–2) (2010) 160–167.

[20] M. Žák, M. Klepic, L.Č. Štastná, Z. Sedláková, H. Vychodilová, Š. Hovorka, K. Friess, A.Randová, L. Brožová, J.C. Jansen, M.R. Khdhayyer, P.M. Budd, P. Izák, Selective remov-al of butanol from aqueous solution by pervaporation with a PIM-1 membrane andmembrane aging, Sep. Purif. Technol. 151 (2015) 108–114.

[21] O. Salinas, X.H. Ma, E. Litwiller, I. Pinnau, Ethylene/ethane permeation, diffusion andgas sorption properties of carbon molecular sieve membranes derived from theprototype ladder polymer of intrinsic microporosity (PIM-1), J. Membr. Sci. 504(2016) 133–140.

[22] H.Y. Zhao, Q. Xie, X.L. Ding, J.M. Chen, M.M. Hua, X.Y. Tan, Y.Z. Zhang, High perfor-mance post-modified polymers of intrinsic microporosity (PIM-1) membranesbased on multivalent metal ions for gas separation, J. Membr. Sci. 514 (2016)305–312.

[23] H.-W. Yen, Z.-H. Chen, I.K. Yang, Use of the composite membrane of poly(ether-block-amide) and carbon nanotubes (CNTs) in a pervaporation system incorporatedwith fermentation for butanol production by Clostridium acetobutylicum, Bioresour.Technol. 109 (2012) 105–109.

[24] S. Liu, G. Liu, X. Zhao, W. Jin, Hydrophobic-ZIF-71 filled PEBA mixed matrixmembranes for recovery of biobutanol via pervaporation, J. Membr. Sci. 446 (2013)181–188.

[25] S. Liu, G. Liu, J. Shen,W. Jin, Fabrication ofMOFs/PEBAmixedmatrixmembranes andtheir application in bio-butanol production, Sep. Purif. Technol. 133 (2014) 40–47.

[26] H.F. Tan, Y.H. Wu, Y. Zhou, Z.N. Liu, T.M. Li, Pervaporative recovery of n-butanol fromaqueous solutions MCM-41 filled PEBA mixed matrix membrane, J. Membr. Sci. 453(2014) 302–311.

[27] Y.K. Li, J. Shen, K.C. Guan, G.P. Liu, H.L. Zhou, W.Q. Jin, PEBA/ceramic hollow fibercomposite membrane for high-efficiency recovery of bio-butanol via pervaporation,J. Membr. Sci. 510 (2016) 338–347.

[28] A.E. Ozcam, N. Petzetakis, S. Silverman, A.K. Jha, N.P. Balsara, Relationship betweensegregation strength and permeability of ethanol/water mixtures through blockcopolymer membranes, Macromolecules 46 (24) (2013) 9652–9658.

[29] H.S. Samanta, S.K. Ray, Separation of ethanol from water by pervaporation usingmixed matrix copolymer membranes, Sep. Purif. Technol. 146 (2015) 176–186.

[30] C.C. Tong, Y.X. Bai, J.P. Wu, L. Zhang, L.R. Yang, J.W. Qian, Pervaporation recovery ofacetone-butanol from aqueous solution and fermentation broth using HTPB-basedpolyurethaneurea membranes, Sep. Sci. Technol. 45 (6) (2010) 751–761.

[31] J. Chen, H. Huang, L. Zhang, H. Zhang, A novel high-flux asymmetric p(VDF–HFP)membrane with a dense skin for ethanol pervaporation, RSC Adv. 4 (46) (2014)24126.

[32] K.V. Agrawal, B. Topuz, T.C.T. Pham, T.H. Nguyen, N. Sauer, N. Rangnekar, H. Zhang,K. Narasimharao, S.N. Basahel, L.F. Francis, C.W.Macosko, S. Al-Thabaiti, M. Tsapatsis,K.B. Yoon, Oriented MFI membranes by gel-less secondary growth of sub-100 nmMFI-nanosheet seed layers, Adv. Mater. 27 (21) (2015) 3243–3249.

[33] K. Varoon, X. Zhang, B. Elyassi, D.D. Brewer, M. Gettel, S. Kumar, J.A. Lee, S.Maheshwari, A. Mittal, C.-Y. Sung, M. Cococcioni, L.F. Francis, A.V. McCormick, K.A.Mkhoyan, M. Tsapatsis, Dispersible exfoliated zeolite nanosheets and theirapplication as a selective membrane, Science 334 (6052) (2011) 72–75.

[34] L. Chai, J. Yang, J. Lu, D. Yin, Y. Zhang, J. Wang, Ethanol perm-selective B-ZSM-5zeolite membranes from dilute solutions, AIChE J. 62 (7) (2016) 2447–2458.

[35] Y. Peng, H. Lu, Z. Wang, Y. Yan, Microstructural optimization of MFI-type zeolite mem-branes for ethanol–water separation, J. Mater. Chem. A 2 (38) (2014) 16093–16100.

[36] S. Xia, Y. Peng, Z. Wang, Microstructure manipulation of MFI-type zeolitemembranes on hollow fibers for ethanol–water separation, J. Membr. Sci. 498(2016) 324–335.

[37] J.A. Stoeger, J. Choi, M. Tsapatsis, Rapid thermal processing and separationperformance of columnar MFI membranes on porous stainless steel tubes, EnergyEnviron. Sci. 4 (9) (2011) 3479–3486.

[38] X. Zou, P. Bazin, F. Zhang, G. Zhu, V. Valtchev, S. Mintova, Ethanol recovery fromwater using silicalite-1 membrane: An operando infrared spectroscopic study,Chem. Plus. Chem. 77 (6) (2012) 437–444.

[39] X. Shu, X. Wang, Q. Kong, X. Gu, N. Xu, High-flux MFI zeolite membrane supportedon YSZ hollow fiber for separation of ethanol/water, Ind. Eng. Chem. Res. 51 (37)(2012) 12073–12080.

[40] L. Shan, J. Shao, Z. Wang, Y. Yan, Preparation of zeolite MFI membranes on aluminahollow fibers with high flux for pervaporation, J. Membr. Sci. 378 (1–2) (2011)319–329.

[41] X.-L. Zhang, M.-H. Zhu, R.-F. Zhou, X.-S. Chen, H. Kita, Synthesis of a silicalite zeolitemembrane in ultradilute solution and its highly selective separation of organic/water mixtures, Ind. Eng. Chem. Res. 51 (35) (2012) 11499–11508.

[42] M. Weyd, H. Richter, P. Puhlfurß, I. Voigt, C. Hamel, A.S. Morgenstern, Transportof binary water–ethanol mixtures through a multilayer hydrophobic zeolitemembrane, J. Membr. Sci. 307 (2008) 239–248.

[43] A. Huang, Q. Liu, N. Wang, J. Caro, Highly hydrogen permselective ZIF-8 membranessupported on polydopamine functionalized macroporous stainless-steel-nets,J. Mater. Chem. A 2 (22) (2014) 8246–8251.

[44] J.W. Hou, P.D. Sutrisna, Y.T. Zhang, V. Chen, Formation of ultrathin, continuousmetal-organic framework membranes on flexible polymer substrates, Angew.Chem. Int. Ed. 55 (12) (2016) 3947–3951.

[45] J. Fu, S. Das, G. Xing, T. Ben, V. Valtchev, S. Qiu, Fabrication of COF-MOF compositemembranes and their highly selective separation of H2/CO2, J. Am. Chem. Soc. 138(24) (2016) 7673–7680.

[46] X.L. Dong, Y.S. Lin, Synthesis of an organophilic ZIF-71 membrane for pervaporationsolvent separation, Chem. Commun. 49 (2013) 1196–1198.

[47] K. Huang, Q.Q. Li, G.P. Liu, J. Shen, K.C. Guan, W.Q. Jin, A ZIF-71 hollow fiber mem-brane fabricated by contra-diffusion, ACS Appl. Mater. Interfaces 7 (30) (2015)16157–16160.

[48] S.H. Kim, S.Y. Kwak, T. Suzuki, Positron annihilation spectroscopic evidence todemonstrate the flux-enhancement mechanism in morphology-controlled thin-film-composite (TFC) membrane, Environ. Sci. Technol. 39 (6) (2005) 1764–1770.

[49] C. Gao, M. Zhang, Z. Jiang, J. Liao, X. Xie, T. Huang, J. Zhao, J. Bai, F. Pan, Preparation ofa highly water-selective membrane for dehydration of acetone by incorporatingpotassium montmorillonite to construct ionized water channel, Chem. Eng. Sci. 135(2015) 461–471.

[50] T.T. Moore, W.J. Koros, Non-ideal effects in organic-inorganic materials for gasseparation membranes, J. Mol. Struct. 739 (1–3) (2005) 87–98.

[51] G. Liu, W.-S. Hung, J. Shen, Q. Li, Y.-H. Huang, W. Jin, K.-R. Lee, J.-Y. Lai, Mixed matrixmembranes with molecular-interaction-driven tunable free volumes for efficientbio-fuel recovery, J. Mater. Chem. A 3 (8) (2015) 4510–4521.

[52] M.J. Raaijmakers, M.A. Hempenius, P.M. Schon, G.J. Vancso, A. Nijmeijer, M.Wessling, N.E. Benes, Sieving of hot gases by hyper-cross-linked nanoscale-hybridmembranes, J. Am. Chem. Soc. 136 (1) (2014) 330–335.

[53] L.H. Wee, Y. Li, K. Zhang, P. Davit, S. Bordiga, J. Jiang, I.F.J. Vankelecom, J.A. Martens,Submicrometer-sized ZIF-71 filled organophilic membranes for improvedbioethanol recovery: Mechanistic insights by monte carlo simulation and FTIRspectroscopy, Adv. Funct. Mater. 25 (4) (2015) 516–525.

[54] P.V. Naik, S. Kerkhofs, J.A. Martens, I.F.J. Vankelecom, PDMS mixed matrixmembranes containing hollow silicalite sphere for ethanol/water separation bypervaporation, J. Membr. Sci. 502 (2016) 48–56.

[55] N. Wang, J. Liu, J. Li, J. Gao, S. Ji, J.-R. Li, Tuning properties of silicalite-1 for enhancedethanol/water pervaporation separation in its PDMS hybridmembrane,MicroporousMesoporous Mater. 201 (2015) 35–42.

[56] X. Han, X. Zhang, X. Ma, J. Li, Modified ZSM-5/polydimethylsiloxane mixed matrixmembranes for ethanol/water separation via pervaporation, Polym. Compos. 37 (4)(2016) 1282–1291.

[57] G. Liu, F. Xiangli, W. Wei, S. Liu, W. Jin, Improved performance of PDMS/ceramiccomposite pervaporation membranes by ZSM-5 homogeneously dispersed inPDMS via a surface graft/coating approach, Chem. Eng. J. 174 (2–3) (2011) 495–503.

1626 J. Zhao, W.Q. Jin / Chinese Journal of Chemical Engineering 25 (2017) 1616–1626

[58] X. Liu, Y. Li, Y. Liu, G. Zhu, J. Liu,W. Yang, Capillary supported ultrathin homogeneoussilicalite-poly(dimethylsiloxane) nanocompositemembrane for bio-butanol recovery,J. Membr. Sci. 369 (1–2) (2011) 228–232.

[59] Y. Huang, P. Zhang, J. Fu, Y. Zhou, X. Huang, X. Tang, Pervaporation of ethanol aque-ous solution by polydimethylsiloxane/polyphosphazene nanotube nanocompositemembranes, J. Membr. Sci. 339 (1–2) (2009) 85–92.

[60] S. Qiu, M. Xue, G. Zhu, Metal-organic framework membranes: From synthesis toseparation application, Chem. Soc. Rev. 43 (16) (2014) 6116–6140.

[61] N.C. Su, D.T. Sun, C.M. Beavers, D.K. Britt, W.L. Queen, J.J. Urban, Enhanced perme-ation arising from dual transport pathways in hybrid polymer-MOF membranes,Energy Environ. Sci. 9 (3) (2016) 922–931.

[62] J.E. Bachman, Z.P. Smith, T. Li, T. Xu, J.R. Long, Enhanced ethylene separation andplasticization resistance in polymermembranes incorporating metal-organic frame-work nanocrystals, Nat. Mater. 15 (8) (2016) 845–849.

[63] T. Rodenas, I. Luz, G. Prieto, B. Seoane, H. Miro, A. Corma, F. Kapteijn, F.X. Llabres iXamena, J. Gascon, Metal-organic framework nanosheets in polymer compositematerials for gas separation, Nat. Mater. 14 (1) (2015) 48–55.

[64] X.-L. Liu, Y.-S. Li, G.-Q. Zhu, Y.-J. Ban, L.-Y. Xu, W.-S. Yang, An organophilicpervaporation membrane derived from metal-organic framework nanoparticles forefficient recovery of bio-alcohols, Angew. Chem. Int. Ed. 50 (45) (2011) 10636–10639.

[65] Y. Li, L.H. Wee, J.A. Martens, I.F.J. Vankelecom, ZIF-71 as a potential filler to preparepervaporation membranes for bio-alcohol recovery, J. Mater. Chem. A 2 (26) (2014)10034–10040.

[66] Q. Liu, B. Huang, A. Huang, Polydopamine-based superhydrophobic membranes forbiofuel recovery, J. Mater. Chem. A 1 (38) (2013) 11970.

[67] H. Ni, H. Zhang, X. Wang, X. Wang, Pervaporation performance of polystyrene mem-brane surfacewith perfluoroalkyl groups, J. Appl. Polym. Sci. 106 (6) (2007) 3975–3982.

[68] W. Kujawski, S. Krajewska, M. Kujawski, L. Gazagnes, A. Larbot, M. Persin,Pervaporation properties of fluoroalkylsilane (FAS) grafted ceramic membranes,Desalination 205 (1–3) (2007) 75–86.

[69] X. Zhuang, X. Chen, Y. Su, J. Luo, W. Cao, Y. Wan, Improved performance of PDMS/silicalite-1 pervaporation membranes via designing new silicalite-1 particles,J. Membr. Sci. 493 (2015) 37–45.

[70] M. Krea, D. Roizard, N. Moulai-Mustefa, D. Sacco, Synthesis of polysiloxane-imidemembranes-application to the extraction of organics fromwatermixtures,Desalination163 (1–3) (2004) 203–206.

[71] T. Uragami, T. Doi, T.Miyata, Control of permselectivity with surfacemodifications ofpoly 1-(trimethylsilyl)-1-propyne membranes, Int. J. Adhes. Adhes. 19 (5) (1999)405–409.

[72] S. Araki, D. Gondo, S. Imasaka, H. Yamamoto, Permeation properties of organic com-pounds from aqueous solutions through hydrophobic silica membranes with differ-ent functional groups by pervaporation, J. Membr. Sci. 514 (2016) 458–466.

[73] L. Gao, M. Alberto, P. Gorgojo, G. Szekely, P.M. Budd, High-flux PIM-1/PVDF thin filmcomposite membranes for 1-butanol/water pervaporation, J. Membr. Sci. 529 (2017)207–214.

[74] S.F. Nai, X.F. Liu, W. Liu, B.Q. Zhang, Ethanol recovery from its dilute aqueous solu-tion using Fe-ZSM-5 membranes: Effect of defect size and surface hydrophobicity,Microporous Mesoporous Mater. 215 (2015) 46–50.

[75] B. Elyassi, M.Y. Jeon, M. Tsapatsis, K. Narasimharao, S.N. Basahel, S. Al-Thabaiti,Ethanol/water mixture pervaporation performance of b-oriented silicalite-1membranes made by gel-free secondary growth, AIChE J. 62 (2016) 556–563.