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Journal of Membrane Science

journal homepage: www.elsevier.com/locate/memsci

Pervaporation performance comparison of hybrid membranes filled withtwo-dimensional ZIF-L nanosheets and zero-dimensional ZIF-8nanoparticles

Guanhua Liua,b, Zhongyi Jianga,b, Keteng Caoa,b, Sankar Nairc, Xuanxuan Chenga,b, Jing Zhaoa,b,Hassan Gomaad, Hong Wua,b, Fusheng Pana,b,⁎

a Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin300072, Chinab Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, Chinac School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USAd Chemical and Biochemical Engineering Department, Western University, London, ON, Canada N6A 5B9

A R T I C L E I N F O

Keywords:Two-dimensional ZIF-LZero-dimensional ZIF-8Hybrid membraneSieving effectPervaporation dehydration

A B S T R A C T

Two kinds of zeolitic imidazolate frameworks (two-dimensional ZIF-L nanosheets and zero-dimensional ZIF-8nanoparticles) with the same building blocks were synthesized. Both the ZIF-L and ZIF-8 materials wereincorporated into sodium alginate (SA) matrix to fabricate hybrid membranes for pervaporation dehydration ofethanol. At the filler content of 4 wt%, the ZIF-L-filled membrane displayed permeation flux of 1218 g/(m2 h)and separation factor of 1840, while the ZIF-8-filled membrane displayed permeation flux of 879 g/(m2 h) andseparation factor of 678. The superior separation performance of the ZIF-L-filled membrane is due to thefollowing two reasons: the ordered alignment and the regular apertures of ZIF-L rendered ordered waterchannels for rapid transport of water molecules; the suitable apertures of ZIF-L rendered the desirablemolecular sieving effect. Furthermore, the hybrid membranes exhibited both good swelling resistance as well asthermal and mechanical stability. This is a step forward in realizing superior performance of hybrid membranesincorporating two-dimensional porous fillers for separation processes.

1. Introduction

Membrane technology has been applied in diverse processesincluding gas separation [1], pervaporation [2], ion-exchange mem-branes [3], wastewater treatment [4], seawater desalination [5], andmembrane reactor [6]. Many of these applications have shownpotential for substantial improvement in economic efficiency andenvironmental sustainability. For instance, pervaporation is believedto be a promising technology for dehydration of alcohols, renewablebiofuel with potential to address the concerns derived from depletionand high cost of fossil and petroleum based resources [2,7]. In thistechnology, polymeric membranes are most widely used till now owingto their low cost, ease of production as well as scale-up [8].Unfortunately, the performance of polymeric membranes is limitedby the well-known tradeoff effect between permeability and selectivitywhere an increase in the former results in decreasing the latter, andvice versa [9,10].

To solve this problem, much research has focused on incorporat-ing porous and nonporous fillers into the polymer matrix tomanipulate the synergistic effect between the polymer and fillerphases [1,8,11]. Particularly, porous fillers including zeolite [12,13],metal-organic frameworks (MOFs) [14–16], carbon molecular sieves(CMSs) [17], and carbon nanotubes (CNTs) [18] have been used.Such fillers could render lower diffusion resistance since they arepermeable and can also promote molecular sieving considering thosewith suitable pore sizes [8]. Among these fillers, MOFs with regularpore structure and organic-inorganic coordination networks havegained particular interest since they hold great promise for thedesign of novel hybrid membranes [19]. Zeolitic imidazolate frame-works (ZIFs) are a family of MOFs which exhibit zeolite-likestructural topologies with exceptional thermal and chemical stability[20–22]. ZIF-7 [23], ZIF-8 [15,24–26], ZIF-71 [27] and ZIF-90[28,29] have been used to fabricate hybrid membranes for gasseparation, pervaporation and nanofiltration.

http://dx.doi.org/10.1016/j.memsci.2016.09.064Received 20 June 2016; Received in revised form 29 September 2016; Accepted 30 September 2016

⁎ Corresponding author at: Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin300072, China.

E-mail address: fspan@tju.edu.cn (F. Pan).

Journal of Membrane Science 523 (2017) 185–196

0376-7388/ © 2016 Elsevier B.V. All rights reserved.Available online 02 October 2016

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ZIF-8 is one of the most widely investigated ZIFs and has extra-ordinary advantages in ethanol dehydration. First, ZIF-8 possesses asodalite (SOD) zeolite-type structure with a small pore aperture ofnominal diameter of ~3.4 Å and an effective aperture size for molecularsieving in the range of 4.0–4.2 Å due to its flexible framework, whichlies between the kinetic diameters of water (2.68 Å) and ethanol(4.5 Å), and is thus expected to show a sieving effect for water/ethanolmixtures [22,30–32]. Second, ZIF-8 could form water channels con-tributed by a sub-nanometer scale hydrophobic interconnected struc-ture akin to the aquaporin that is favorable for fast water transport[33–35]. Furthermore, ZIF-8 has additional advantages in terms of lowcost, easily available raw materials, simple preparation method, andtunable size and morphology [36]. Last but not least, ZIF-8 shows highwater stability, which is important for water-related applications [22].Till now, ZIF-8 nanoparticles have been incorporated into polymermatrix (polybenzimidazole (PBI) [37], polyimide [38] and (poly(vinylalcohol) (PVA) [39,40]) to fabricate hybrid membranes for dehydrationof alcohols. Unfortunately, the selectivity of these membranes showedeither a pronounced decrease or a slight increase compared to thepristine membranes. This was attributed to the isotropic nature andrandom dispersion of those ZIF-8 nanoparticles, which limited theirsieving effect as well as high separation performance. To solve theproblem, ZIF-8 nanoparticles were modified by ethanediamine toincrease the membrane selectivity [41]. In comparison, two-dimen-sional fillers with high aspect ratio and anisotropic nature couldspontaneously form “brick-and-mortar” architecture in the polymermatrix, thus effectively interfering with the molecular diffusion [42–44], and maximizing the effective diffusivity and selectivity of thehybrid membranes [45]. So far, layered oxide materials [46], layeredMFI [47] and porous graphene [44] have been used as porous fillers tomanipulate the molecular transport. MOFs, with highly ordered pores,are promising to be synthesized as two-dimensional fillers [48]. Thepioneering work by Cussler group predicts the permeability of onecomponent in hybrid membranes as a function of the volume fractionand aspect ratio of two-dimensional fillers. The permeability ofmembranes decreases after incorporating two-dimensional fillers withlow permeability [49]. While for selective permeable two-dimensionalfillers, the permeation flux and selectivity of hybrid membranes couldincrease, offering the potential for selective separations [50–53].Microporous titanosilicate JDF-L1 sheet was applied for gas separationwith preferential horizontal orientation. The permeability decreasedand the selectivity increased for H2/CH4 separation. And thepermeability was calculated using the models proposed by Cussleret al. [54]. Rodenas et al. fabricated a variety of two-dimensionalMOF-based hybrid membranes with high occupation of fillersparallel with the membrane surface to achieve outstanding gasseparation performance [14]. Accordingly, it can be expected thathybrid membranes incorporated with two-dimensional MOFs na-nosheets would show high dehydration performance. ZIF-L is a two-dimensional layered MOF with the same building blocks as ZIF-8.The ZIF-L layer is part of the SOD topology as ZIF-8, which stackedonto each other along the c axis via the H-bonds between 2-methylimidazole (MeIm) molecules [48,55]. The close similaritybetween ZIF-L and ZIF-8 provides a unique opportunity to investi-gate the dimensional effects of fillers on the separation performanceof the hybrid membranes.

In this study, we prepared two-dimensional ZIF-L nanosheets andzero-dimensional ZIF-8 nanoparticles by controlling synthesis condi-tions. ZIF-L and ZIF-8 were then incorporated into sodium alginate(SA) to prepare hybrid membranes for pervaporation dehydration ofethanol aqueous solutions. The physical and chemical structure of thetwo types of ZIFs and their corresponding hybrid membranes werecharacterized. The effects of ZIFs dimension and content on thepervaporation performance were evaluated, along with the effects ofoperating temperature and feed composition on the pervaporationperformance.

2. Experimental

2.1. Materials

Sodium alginate was obtained from Qingdao Bright Moon seaweedGroup Co. Ltd. (Shandong, China). Polyacrylonitrile (PAN) ultrafiltra-tion membranes with a molecular weight cut-off of 100,000 weregained from Shandong MegaVision Membrane Technology &Engineering Co. Ltd. (Shandong, China). 2-methylimidazole wasreceived from J&K Scientific Ltd. (Beijing, China). Zinc nitratehexahydrate (Zn(NO3)2·6H2O) was supplied by Shanghai JingchunScientifical Co., Ltd. (Shandong, China). Methanol and acetone weresupplied by Tianjin Kewei Ltd. (Tianjin, China). Cellulose acetate (CA)microfiltration membrane (0.2 μm pore size) was bought fromShanghai Mili Membrane Separation Technology Ltd. (Shanghai,China). Calcium chloride dihydrate (CaCl2·2H2O) was purchased fromTianjin Guangfu Fine Chemical Research Institute (Tianjin, China).Absolute ethanol (≥99.7 wt%) was supplied by Tianjin Guangfu Scienceand Technology Development Co. Ltd. (Tianjin, China). All thereagents were of analytical grade and not further purified. Deionizedwater was used in all the experiments.

2.2. Preparation of ZIF-L nanosheets

ZIF-L was synthesized as reported [48,55]. An aqueous solution ofZn(NO3)2 (0.05 M) was mixed with equal volume of aqueous solutionof 2-methylimidazole (0.4 M) with magnetic stirring at 30 °C for 4 h toform a milky white suspension. The suspension was subsequentlyfiltered by CA microfiltration membrane. The resultant filter cake wasdispersed and rinsed with water followed by filtration again. The rinseand filtration processes were repeated three times to obtain white filtercake. The same processes were repeated three times using acetoneinstead of water. At last the filter cake was vacuum dried at 30 °C andground to acquire ZIF-L nanosheets.

2.3. Preparation of ZIF-8 nanoparticles

ZIF-8 was synthesized by taking a rapid room-temperature route[36]. A methanol solution of Zn(NO3)2 (0.1 M) was mixed with equalvolume of methanol solution of 2-methylimidazole (0.8 M) usingmagnetic stirring at 30 °C for 1 h to form milky white suspension.The remainder of the procedure was the same as that of ZIF-L.

2.4. Preparation of SA-ZIFs hybrid membranes

First, 2 wt% SA solution was prepared by dissolving SA in waterunder stirring at 30 °C for 1 h. A certain amount of ZIFs (ZIF-L or ZIF-8) was subsequently added to the SA solution with further stirring for4 h. The mixture was filtered using filter cloth with diameter of 75 μmand kept still for 2 h at room temperature to remove air bubbles. PANultrafiltration membrane was soaked in water for 1 day and driedbefore use. Afterwards, the mixture (15 g) was spin-coated onto a PANmembrane using a spin coater (WS-400BZ-6NPP/LITE, Mycro Tech.)at 500 r/m for 20 s and then 800 r/m for 40 s, at room temperatureand atmospheric pressure. After dried at room temperature for 24 h,the hybrid membranes were soaked in 0.5 M CaCl2 solution for 10 minand rinsed three times with water then dried at room temperature for48 h. The resultant membranes were denoted as SA-ZIF-L-X/PAN orSA-ZIF-8-X/PAN, where X was the mass fraction of ZIFs to SA (wt%),ranging from 2 to 10. To allow for comparison, control SA puremembrane was also fabricated and denoted as SA pure/PAN.Furthermore, homogenous membranes were prepared by castingcorresponding solutions on glass plates for characterization andswelling study. The resultant membranes were denoted as SA-ZIF-L-X or SA-ZIF-8-X, while SA control pure membrane was denoted as SApure.

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2.5. Characterizations

The morphology of ZIFs and cross-sectional morphology of hybridmembranes were observed by field emission scanning electron micro-scopy (FESEM, Hitachi S-4800). The Zn distribution was measured byenergy-dispersive X-ray spectroscopy (EDX) coupled with FESEM. Thechemical structure of ZIFs and membranes were characterized byFourier transform infrared spectra (FTIR, Nicolet 6700). The structureof ZIFs and membranes was investigated using wide-angle XRDpatterns collected with a D/MAX-2500 X-ray diffractometer (Cu Kα,40 kV, 200 mA, λ=0.15406 nm) with 2θ range from 3 to 75° at 4°/min.Thermo gravimetric analyses (TGA) of the membranes were conductedby a thermogravimetric analyzer (NETZSCH-TG209 F3) with thetemperature range from 40 to 700 °C and heating rate of 10 °C/minin nitrogen atmosphere. Mechanical stability of the membranes wasinvestigated via an electronic stretching machine (Zhongke WDW-02)at a strain rate of 2 mm/min. The glass-transition temperature (Tg) ofmembranes was measured by a Pyris Diamond differential scanningcalorimetry (DSC, Netzsch 204 F1) with N2 flow of 50 ml/min. Themembranes were heated from room temperature to 150 °C for 0.5 h,then cooling to 0 °C, and then heated from 0 to 200 °C at a rate of10 °C/min. The static contact angles of membranes were measuredusing JCL2000D Contact Angle Meter. Every sample was testedchoosing at least 8 different places and the errors were calculatedaccordingly.

2.6. Swelling and separation experiments

The homogeneous membranes were dried in a vacuum oven andsubsequently weighted (WD, g). After being immersed in 90 wt%ethanol aqueous solution at 76 °C for 48 h, the fully swollen mem-branes were wiped with tissue paper and weighted again (WS, g). Thesolvent uptake (SU, %), which was used to represent the membraneswelling resistance, could be calculated by

SU W WW

(%) = − × 100S D

D (1)

The pervaporation dehydration experiments were conducted usingthe equipment described in our previous work with an effectivemembrane area of 2.56×10−3 m2 [56]. The feed solution (90 wt%ethanol aqueous solution) was circulated across the surface of themembrane at a flow rate of 60 L/h. The pressure of the permeate sidewas maintained below 0.3 kPa. The permeate was collected using aliquid nitrogen cold trap at certain intervals of time and analyzed by an

HP4890 gas chromatograph. The data was collected from threeseparately prepared membranes, each of which was tested for threetimes after the pervaporation process was steady. The permeation flux(J, g/(m2 h)) and separation factor (α) are defined as

J QA t

=× (2)

α P PF F

= //

W E

W E (3)

where Q (g) is the mass of permeate in operating time t (h) through aneffective membrane area A (m2). P and F are mass fractions of water(W) or ethanol (E) in the permeate and feed, respectively. Thepermeance ((P/l)i, GPU, 1 GPU=7.501×10−12 m3(STP)/m2s Pa) ofthe corresponding component and the selectivity (β) are calculated as

P l JP P

Jγ χ P P

( / ) =−

=−i

i

io il

i

io io iosat

il (4)

β P lP l

= ( / )( / )

W

E (5)

where l (m) is the membrane thickness, Ji (g/(m2 h)) is the permeation

flux of component i, pio and pil (kPa) the partial pressure in the feed andpermeate side, γio and χio the activity coefficient and mole fraction infeed, and pio

sat (Pa) is the saturated vapor pressure of the purecomponent. The permeation flux should be transformed into volumesunder standard temperature and pressure (STP).

3. Results and discussion

3.1. Characterization of ZIFs

The morphology of ZIF-8 and ZIF-L was observed by SEM as shownin Fig. 1. The ZIF-8 showed an approximate isotropic hexagonenvelope with uniform diameter of around 130.5 ± 11.5 nm, whichwas the projection of rhombic dodecahedron (a (110) crystal morphol-ogy) [36]. The morphology of ZIF-L was flat sheet-like schistose withlateral dimensions of 4.9 ± 0.7 μm length and 1.8 ± 0.2 μm width asshown in Fig. 1b. The thickness of ZIF-L was about 125.1 ± 4.5 nm asobtained from the magnified SEM view. Thus the thickness of ZIF-Lsheets was comparable to the diameter of ZIF-8.

The crystal structure of ZIFs was demonstrated by the XRD patternas shown in Fig. 2a. The diffraction pattern of the prepared ZIF-8demonstrated the typical SOD zeolite-type structure with hexatomicopen ring channels of 3.4 Å in diameter, which was consistent with that

Fig. 1. SEM images of (a) ZIF-8 and (b) ZIF-L.

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of the standard ZIF-8 sample reported in literatures, thus revealing thesuccessful synthesis of ZIF-8 [22,36]. The position of characteristicpeaks of XRD pattern of ZIF-L matched well with the literature,revealed the semi-SOD structure [48]. The intensity of the peaksshowed obvious changes compared to ZIF-8, especially the remarkableincrease of the diffraction peaks at 2θ of 15–20°, which indicated theincrease of specific crystal face derived from the two-dimensionalnetwork of ZIF-L crystal [57]. ZIF-L possessed similar crystal structureto ZIF-8 and the same apertures of 3.4 Å in diameter perpendicular tothe 2D crystal layer, so the sieving effect of these ZIFs would becomparable if ZIF-L layers dispersed parallel to the membrane surface[55].

Fig. 2b shows the FTIR spectra of ZIFs. For ZIF-8, the characteristicpeaks at 1585, 1147, around 750 (double bonds) and 423 cm−1

corresponded to the stretching vibration of C=N, bending vibrationof CH, bending vibration of imidazole ring and vibration peak of Zn-N,respectively, which agreed with the reported literature [22,25]. Thecharacteristic peaks of ZIF-L showed the same position and intensity asthose of ZIF-8 due to the same building blocks and matched well withthe literature, which indicated the same chemical bonding [57].

3.2. Characterization of the hybrid membranes

The microstructure of the membranes was observed by SEM.Fig. 3a-c shows the cross-sectional morphology of the compositemembranes. The separation layers of SA, SA-ZIF-8 and SA-ZIF-L weretightly coated on the PAN support layers with uniform thickness of1.6–1.7 μm and no obvious interfacial defects were found. Thedispersity of ZIFs in hybrid membrane matrix was presented in thecross-sectional morphology of the homogeneous membranes. Fig. 3dshowed that the cross-sectional morphology of SA pure membrane wasrelatively smooth. Fig. 3e showed ZIF-8 was uniformly dispersed in SA-ZIF-8-4 membrane, without apparent agglomeration seen in partialenlarged view. Fig. 3f showed that ZIF-L was uniformly dispersed inSA-ZIF-L-4 membrane. EDX Zn mappings of membrane cross-section(Fig. 3j, k) further showed that ZIFs were well dispersed in SA-ZIF-8-4and SA-ZIF-L-4 membranes. At excessive content ratio, the fillerparticles tended to aggregate in the membrane matrix. Fig. 3g showedapparent particle agglomeration in SA-ZIF-8-10 membrane, mean-while, Fig. 3h showed that interfacial voids between agglomeration andpolymer matrix appeared due to the stacking and agglomeration ofnanosheets. As shown in Fig. 3i, agglomeration of nanosheets alsoobviously showed up on the surface of SA-ZIF-L-10/PAN membrane.

The chemical structure of SA pure/PAN and SA-ZIFs/PAN compo-site membranes were demonstrated by FTIR. As shown in Fig. 4, theabsorption peaks of SA pure/PAN membrane at 3357, 1603 and

1430 cm−1 represented stretching vibration of hydroxyl, and symmetricand anti-symmetric stretching vibration of carboxylate radical, respec-tively [58]. In addition, the spectra of SA-ZIFs/PAN membranes didnot change in position and intensity compared with SA pure/PANmembrane, which indicated that no strong interactions existed betweenZIFs and SA matrix.

The structure of SA-ZIFs hybrid membranes was investigated byXRD. As shown in Fig. 5a,b, the two broad peaks of SA pure membranearound 2θ=14.5° and 2θ=40.5° correspond to the amorphous region,while the sharper peak at 2θ=22.6° corresponded to the crystallineregion. With the increase of ZIFs content, the peak intensities of ZIF-8and ZIF-L were gradually enhanced in the diffraction pattern of SA-ZIF-8 and SA-ZIF-L hybrid membranes. Fig. 5a showed that theintensity of the crystal peaks of SA-ZIF-8 hybrid membranes at2θ=22.6° was lower than that of SA pure membrane, which revealedthat the incorporation of ZIF-8 interfered with the chain packing of SAand extended the inter-chain spacing, thus decreasing the crystallinity.For the SA-ZIF-L membranes, Fig. 5b showed that there was no initialobvious change in the membranes crystallinity, may be due to theinterference counteracting the nucleation effect of ZIF-L on the chainpacking of SA. When the content was higher than 8 wt%, the nucleationeffect was weakened by interfacial voids in the hybrid membranes,resulting in decreased crystallinity.

To elucidate the preferential orientation of ZIF-L in the hybridmembranes, XRD patterns of ZIF-L and SA-ZIF-L-4 membrane werecompared in Fig. 5c. The crystallization peak intensity of ZIF-L in SA-ZIF-L-4 membrane was weakened because of its low concentration inSA matrix. However, except for the crystallization peak at (004), whichwas in various ZIF-L orientation, only the crystallization peak at (h00),which corresponded to the [200], [400] and [800] planes of ZIF-L,existed obviously in SA-ZIF-L-4 membrane. This phenomenon demon-strated the parallel orientation of ZIF-L to the membrane surfacetogether with the cross-sectional morphology of the membrane [54,55].As shown in Fig. 5b, all of the crystallization peaks of ZIF-L in SA-ZIF-L-2 membrane were almost disappeared, which indicated the unde-sired orientation of ZIF-L. For SA-ZIF-L-X (X≥4) membranes, thecrystallization peak intensity at (h00) of ZIF-L was enhanced, whileother crystallization peaks except for (004) were weakened, whichindicated the desired orientation of ZIF-L in theses membranes.

The static water contact angle was measured to evaluate the surfacehydrophilicity of hybrid membranes. As shown in Fig. 6, the highercontent of ZIF-L and ZIF-8 increased the membrane surface contactangle, indicating a decrease of hydrophilicity due to the hydrophobicnature of ZIFs. With filler content increasing to 8 wt%, the watercontact angle of SA-ZIF-8/PAN membranes was increased from 35.4°to 55.9°, while that was 65.9° for SA-ZIF-L-8/PAN membrane. The

Fig. 2. (a) XRD patterns and (b) FTIR spectra of ZIF-8 and ZIF-L.

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Fig. 3. SEM images of cross-section of (a) SA pure/PAN, (b) SA-ZIF-8-4/PAN, (c) SA-ZIF-L-4/PAN, (d) SA pure, (e) SA-ZIF-8-4, (f) SA-ZIF-L-4, (g) SA-ZIF-8–10 and (h) SA-ZIF-L-10membranes; SEM image of surface of (i) SA-ZIF-L-10/PAN membrane; EDX Zn mapping of cross-section of (j) SA-ZIF-8-4 and (k) SA-ZIF-L-4 membranes.

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comparison between Fig. 6a and Fig. 6b revealed that hydrophilicity ofSA-ZIF-L/PAN membranes decreased faster. The reason was that two-dimensional nanosheets with higher aspect ratio allowed larger filler/matrix interface and stronger effect on membrane hydrophilicity. Atfiller content of 10 wt%, the contact angle of SA-ZIF-8/PAN membranewas similar to SA-ZIF-L/PAN membrane, most likely due to the seriousagglomeration of nanoparticles.

Experiments were also conducted to assess the influence of ZIFs onthe membrane resistance to water swelling that occurs in pervaporationprocesses causing polymer plasticization and influencing the separa-tion performance [59]. As shown in Fig. 7, with the increase of ZIFscontent, the solvent uptake of the hybrid membranes decreasedslightly, thus revealing higher swelling resistance. The main reasonwas that the incorporation of ZIFs decreased the hydrophilicity of themembranes, thus reducing water adsorption. The solvent uptake of SA-ZIF-L hybrid membranes was lower than that of SA-ZIF-8 at the samecontent. The slight increase in the solvent uptake of SA-ZIF-L hybridmembranes at filler content of 10 wt% may be attributed to an increasein water adsorption capacity due to interfacial voids caused byagglomeration of ZIF-L.

The thermal stability of ZIFs, SA pure and SA-ZIFs hybridmembranes was studied by TGA under N2 atmosphere (Fig. 8). Theentire membrane thermal degradation process was divided into threestages. The first stage (below 210 °C) corresponded to moisture lossmainly due to the evaporation of water adsorbed by membranes. Thesecond stage (210–400 °C) corresponded to initial degradation pro-cess, which was mainly caused by the pyrolysis of thermally labilefunctional groups of SA chain such as hydroxyl and carboxyl. The laststage (400–700 °C) corresponded to the pyrolysis of SA backbonechain. The thermal degradation of SA-ZIF-8 and SA-ZIF-L hybridmembranes was lower than that of SA pure membrane in the first stage,which revealed less water adsorption of hybrid membranes. Thethermal degradation process for all the membranes started at~210 °C, and that of ZIFs were higher than 261 °C, which were higherthan the operating temperature of pervaporation, thus the thermalstability of ZIFs and the membranes could meet the operationrequirement.

The polymer chain mobility was assessed by glass-transitiontemperature (Tg) using DSC. As shown in Fig. 9, Tgs of SA-ZIF-8 andSA-ZIF-L membranes were lower than that of SA pure membrane,because ZIFs interfered with the polymer chain packing thus enhancingits mobility. With increasing ZIFs content, Tg of the hybrid membranesfirst decreased and then increased. The reason was that the well-dispersed ZIFs effectively interfered with the polymer chain packing atlow content. This, however, was weaker at higher ZIFs content due toagglomeration.

The mechanical properties of the membranes were characterizedusing an electronic stretching machine. As shown in Table 1, the tensilestrength and elastic modulus of SA-ZIFs hybrid membranes increasedwith the filler content up to 6 wt% resulting in better mechanicalstability. Further increasing the filler loading fraction resulted insomewhat decreased mechanical strength of the membranes. A similartrend of mechanical properties was also observed in SA-ZIF-L hybridmembranes. The tensile strength of SA-ZIF-L-4 increased by 15% from106 to 122 MPa, and its elastic modulus increased by 27% from 4.86 to6.16 GPa. The well-dispersed ZIFs in polymer matrix could modestlyenhance the mechanical strength of the membranes at low loadings,while the decreasing trend in these properties with further increase inZIFs content was attributed to the agglomeration of ZIFs (Fig. 3g, h).

3.3. Pervaporation performance

3.3.1. Effect of ZIFs contentThe pervaporation performance of SA pure/PAN and SA-ZIF-L/

PAN membranes was evaluated using 90 wt% ethanol aqueous solutionas feed at 76 °C. As shown in Fig. 10a, with the content of ZIF-Lincreasing to 4 wt%, the permeation flux and separation factor of themembranes synchronously and significantly increased, which demon-strated obvious anti-tradeoff effect. The increase of separation factorwas attributed to the permeation selectivity for water that was offeredby the ZIF-L. Since the surface hydrophilicity of the hybrid membraneswas decreased with the incorporation of ZIF-L, and Table S1 furtherdemonstrated the decrease of sorption selectivity after the incorpora-tion of ZIFs. So it can be concluded that the sorption selectivity was notthe crucial factor for the significant increase of separation factor. Theinherent reason was as follows. ZIF-L possessed inherently hydropho-bic framework and the hydrophobicity of the inner wall of ZIF-Lprohibited the capillary condensation in the inner pore, so ZIF-Lshowed better alcohol adsorption than water [60,61]. As a result, thediffusion selectivity played a key role according to the solution-diffusion mechanism [62]. The proper aperture size of ZIF-L couldgenerate sieving effect which lies between the kinetic diameters ofwater and ethanol [22,30–32]. Although ethanol molecules may diffusethrough attributed to its flexible framework [61] (similar as ZIF-8), thediffusivity of ethanol was ~ one magnitude lower than water [63,64].Water molecules would penetrate through the nanoporous channels ofZIF-L driven by chemical potential gradient, because of its two-dimensional anisotropic structure and the well-aligned “brick-and-mortar” architecture [2,43]. Hence, the transport paths of watermolecules were shorter with low friction, resulting in lower resistanceand then higher permeation flux. Generally speaking, sheet type fillerswith low permeability tended to reduce membrane permeability, but

Fig. 4. FTIR spectra of (a) SA pure/PAN, SA-ZIF-8-X/PAN and (b) SA-ZIF-L-X/PAN membranes.

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transport rate of water molecules in the water channels was fast andeven comparable to the exceptionally high rate in CNT, so thepermeation flux of the hybrid membrane increased unusually [33–35]. While most ethanol molecules had to wander around in theinterlayers of nanosheets and then diffused to the next interlayer due toits large kinetic diameter. The longer diffusion paths and the higherdiffusion resistance for ethanol molecules endowed the hybrid mem-branes with higher selectivity.

When the content of ZIF-L was higher than 4 wt%, the hydro-philicity of the membrane surface decreased rapidly to the intrinsicwetting threshold (IWT) of water [65], so the change of sorption

process was inferred to play a dominant role in the solution-diffusionmechanism for pervaporation. Accordingly, the concentration of watermolecules adsorbed on the membrane surface decreased, therebyreducing the permeation flux. And the decrease range of solubilityselectivity surpassed the increase range of diffusion selectivity offeredby ZIF-L, so the separation factor of the membranes decreased. Whenthe content of ZIF-L increased further to 10 wt%, the permeation fluxincreased, which could be ascribed to leaky interfacial voids caused byserious agglomeration of ZIF-L, and the higher solvent uptake [66]. Tosum up, the hybrid membranes with 4 wt% of ZIF-L showed optimumperformance with permeation flux of 1218 g/(m2 h) (37% higher thanthat of SA pure/PAN membrane) and separation factor of 1840 (2.45times higher than that of SA pure/PAN membrane).

The pervaporation performances of SA-ZIF-8/PAN membranesunder the same conditions as those used for SA-ZIF-L/PAN mem-branes are shown in Fig. 10b. The separation factor of SA-ZIF-8/PANmembranes slightly increased with increasing the content of ZIF-8 to4 wt% and then decreased. The slight increase of separation factor maybe attributed to the fact that ZIF-8 was dispersed well and offeredsieving effect similar to ZIF-L, but the enhancement was limited.Although the water transport is fast in the apertures of ZIF-8 becauseof the low resistance between hydrophobic sidewall and water mole-cules, the hydrophobic entrance of apertures in ZIF-8 is rate-limitingwater barrier [67]. Meanwhile, the weak interaction between ZIF-8 andSA rendered transport paths at the interface with low diffusionresistance. Thus, lots of water molecules would diffuse around ZIF-8[1,40] rather than penetrate through them without the well-aligned“brick-and-mortar” architecture, which disabled the sieving effect ofnanoporous channels. Consequently, the enhancement of the selectivityof hybrid membranes was not obvious. Penetrant molecules had totransport around ZIF-8 in twisted lines, resulting in low permeationflux. With further increasing ZIF-8 content to above 4 wt%, theseparation factor was decreased due to the non-selective interfacialvoids caused by the agglomeration, which increased the permeationflux [40]. When ZIF-8 content increased to 10 wt%, the contact angleincreased 15.6° and exceeded the IWT of water, which caused the rapiddecrease of membrane hydrophilicity [65]. Therefore, the permeationflux decreased. At ZIF-8 content of 4 wt%, the hybrid membraneshowed permeation flux of 879 g/(m2 h) and separation factor of678, which was much lower compared to the hybrid membranesincorporated with ZIF-L nanosheets. Therefore the two-dimensionalfillers had the superiority over zero-dimensional fillers in strengtheningthe effect of pores system for membrane separation.

3.3.2. Effect of operating temperatureOperating conditions also play an important role in affecting the

separation performance of membranes. Accordingly, it is important toinvestigate the influence on the membrane separation performance.The SA-ZIF-L-4/PAN membrane was selected as representative of thehybrid membranes to investigate the influence of feed temperature onthe pervaporation performance with 90 wt% ethanol aqueous solution.Fig. 11 showed that both the total flux and separation factor of thehybrid membrane increased continuously with the feed temperature.Furthermore, both water flux and ethanol flux increased almost linearlywith the feed temperature. The reasons for the enhancement ofpermeation flux were as follows [59,68]: (i) The downstream sidewas maintained almost under vacuum, leading to negligible changes inpartial pressure of water and ethanol, while the partial pressure on theupstream side increased with temperature resulting in continuousincrease in partial pressure difference. (ii) The polymer chain mobilitywas enhanced by the increase of temperature, resulting in higher freevolume. (iii) The molecules diffusion rate increased with temperatureby the increase of kinetic energy.

The relationship between permeation flux and operating tempera-ture is given by the following Arrhenius equation:

Fig. 5. XRD patterns of (a) SA pure, SA-ZIF-8-X, (b) SA-ZIF-L-X membranes and (c)ZIF-L, SA-ZIF-L-4 membrane.

G. Liu et al. Journal of Membrane Science 523 (2017) 185–196

191

⎛⎝⎜

⎞⎠⎟J A

ERT

= exp −i ipi

0(6)

where Ji, A i0 , Epi, R and T are the permeation flux (g/m2 h), pre-exponential factor, apparent activation energy (kJ/mol), gas constant(8.314 J/(mol K)) and feed temperature (K), respectively. According tothe best fitted lines shown in Fig. 11c, the calculated apparentactivation energy of water and ethanol were 46 kJ/mol and 24 kJ/

mol, respectively. The higher the apparent activation energy, the higherthe temperature sensitivity of permeation flux was, and the biggerincreasing range of permeation flux [69]. As a result, the enrichmentfactor increased with the temperature.

The permeance and selectivity of the membrane were calculated tofurther identify the impact of temperature. As shown in Fig. 11d, withthe increase of temperature, the water permeance increased while theethanol permeance decreased, thus increasing the selectivity.According to the solution-diffusion mechanism, on one hand, thediffusion rate of water and ethanol molecules were both elevated bythe increase of temperature. On the other hand, the effect of highertemperature on ethanol adsorption was more negative than water dueto the stronger interaction between the membrane and water molecules[11]. Consequently, the enhancement of diffusion for water surpassedthe decrease of dissolution thus increasing the water permeance, whilethe effect on ethanol was just the opposite resulting in increasing themembrane selectivity with increasing the temperature.

3.3.3. Effect of feed compositionThe effect of feed composition on pervaporation performance was

studied at 76 oC using the SA-ZIF-L-4/PAN membrane. As shown inFig. 12a, b, with the increase of water concentration in feed, the totalflux and water flux increased continuously, while the ethanol flux firstdecreased and then increased. Both the partial pressure and plasticiza-tion influenced the transport of water and ethanol molecules. On onehand, the driving force of water transport increased due to the higherwater partial pressure caused by higher water content in feed, whichwas on the contrary for ethanol. On the other hand, high waterconcentration led to excessive membrane swelling, and then serious

Fig. 6. Contact angle of (a) SA-ZIF-8-X/PAN, and (b) SA-ZIF-L-X/PAN membranes.

0 2 4 6 8 100

1

2

3

4

5

6

SU (%

)

Mass ratio of ZIFs to SA (wt%)

SA-ZIF-8 SA-ZIF-L

Fig. 7. Solvent uptake of SA pure, SA-ZIF-8-X, and SA-ZIF-L-X membranes.

100 200 300 400 500 600 70030

40

50

60

70

80

90

100

Temperature (oC)

Wei

ght (

%)

ZIF-8 SA-ZIF-8-10 SA-ZIF-8-8 SA-ZIF-8-6 SA-ZIF-8-4 SA-ZIF-8-2 SA pure

a

100 200 300 400 500 600 70030

40

50

60

70

80

90

100

ZIF-L SA-ZIF-L-10 SA-ZIF-L-8 SA-ZIF-L-6 SA-ZIF-L-4 SA-ZIF-L-2 SA pure

Wei

ght (

%)

Temperature (oC)

b

Fig. 8. TGA curves of (a) ZIF-8, SA pure, SA-ZIF-8-X membranes and (b) ZIF-L, SA-ZIF-L-X membranes.

G. Liu et al. Journal of Membrane Science 523 (2017) 185–196

192

polymer plasticization effect [59], thus increasing polymer chainspacing, which was favorable for the permeance of both water andethanol. As a result, water flux increased continuously, while ethanolflux first decreased and then increased. It was also the reason for theincrease of the enrichment factor at low water concentration. At highwater concentration, ethanol flux increased more significantly becausethe transport of ethanol molecules were more sensitive to the increaseof polymer chain spacing due to its larger kinetic diameter [70].Therefore, the separation factor decreased with further increasingwater concentration. As shown in Fig. 12c, both water and ethanolpermeance increased continuously with water concentration in feed,while the selectivity first increased and then decreased, which furtherconfirmed the membrane swelling. When water concentration was30 wt% in feed, the permeation flux was up to 2453 g/(m2 h). Althoughthe separation factor decreased, water content in permeate was stillhigher than 99.6 wt%.

4. Conclusion

In this study, two-dimensional ZIF-L nanosheets and zero-dimen-sional ZIF-8 nanoparticles were synthesized by tuning the reactionconditions and then incorporated into SA matrix to fabricate hybridmembranes for pervaporation dehydration. The effect of filler dimen-sion on separation performance was explored. The SA-ZIF-L/PANhybrid membrane showed the “brick-and-mortar” architecture, whichrendered the ordered water channels and the sieving effect for ethanoldehydration compared to the SA-ZIF-8/PAN hybrid membrane.Consequently, at filler content of 4 wt% the SA-ZIF-L/PAN hybridmembrane displayed separation performance with permeation flux of1218 g/(m2 h) and separation factor of 1840, while the SA-ZIF-8/PANhybrid membrane displayed permeation flux of 879 g/(m2 h) andseparation factor of 678. And the separation performance of the hybridmembranes was superior to SA pure/PAN membrane. In addition, theswelling resistance together with thermal and mechanical stability ofthe hybrid membranes was enhanced. This study demonstrated thesuperiority of the two-dimensional nanosheets over zero-dimensionalnanoparticles as porous fillers, which could broaden the application oftwo-dimensional materials in membrane separation processes.

Acknowledgements

The authors gratefully acknowledge the financial support from theNational Science Fund for Distinguished Young Scholars (No.21125627), the National Natural Science Foundation of China (No.21490583 and 21306131), the Program of Introducing Talents ofDiscipline to Universities (No.: B06006) and the State Key Laboratoryof Chemical Engineering (No. SKL-ChE-14B03).

Fig. 9. DSC curves of (a) SA pure, SA-ZIF-8-X and (b) SA-ZIF-L-X membranes.

Table 1Mechanical property of the membranes.

ZIFs content (wt%) Tensile strength（MPa） Elastic modulus (GPa)

SA-ZIF-8-X SA-ZIF-L-X SA-ZIF-8-X SA-ZIF-L-X

0 106 4.862 112 116 5.72 5.794 121 122 6.06 6.166 126 110 6.53 5.678 119 103 6.49 5.4110 110 100 5.92 4.52

0 2 4 6 8 10600

900

1200

1500

1800

Separation factor

Mass ratio of ZIF-L to SA (wt%)

Perm

eatio

n flu

x (g

/m2 h)

a

0

300

600

900

1200

1500

1800

0 2 4 6 8 100

300

600

900

1200

1500

Separation factor

Mass ratio of ZIF-8 to SA (wt%)

Perm

eatio

n flu

x ( g

/m2 h)

300

600

900

1200

1500b

Fig. 10. Pervaporation performance of (a) SA pure/PAN, SA-ZIF-L-X/PAN and (b) SA-ZIF-8-X/PAN membranes.

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193

40 50 60 70 800

300

600

900

1200

1500

1800

Feed temperature (oC)

Separation Factor

aT

otal

Flu

x (g

/m2 h)

0

400

800

1200

1600

2000

40 50 60 70 800

300

600

900

1200

1500b

Feed temperature (oC)

Wat

er fl

ux (g

/m2 h)

0

1

2

3

4

5

6

Ethanol flux (g/m

2h)

40 50 60 70 800

2

4

2400

2700

3000

Perm

eanc

e (G

PU)

Feed temperature (oC)

(P/l) water

(P/l) ethanol

Selectivity

0

400

800

1200

1600

2000

2400d

Fig. 11. Effect of feed temperature on separation performance of SA-ZIF-L-4/PAN membrane: (a) total flux and separation factor; (b) water flux and ethanol flux; (c) Arrhenius plots ofwater and ethanol; (d) water permeance, ethanol permeance and selectivity.

5 10 15 20 25 300

500

1000

1500

2000

2500

3000a

Separation FactorTot

al F

lux

(g/m

2 h)

Water concentration in feed (wt.%)

0

400

800

1200

1600

2000

5 10 15 20 25 300

2

1000

2000

3000

4000c

Water concentration in feed (wt.%)

Perm

eanc

e (G

PU)

(P/l) water

(P/l) ethanol

Selectivity

800

1200

1600

2000

2400

2800

Fig. 12. Effect of feed composition on separation performance of SA-ZIF-L-4/PAN membrane: (a) total flux and separation factor; (b) water flux and ethanol flux; (c) water permeance,ethanol permeance and selectivity.

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Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.memsci.2016.09.064.

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