Separation and Purification Technology 178 (2017) 105–112

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Separation and Purification Technology

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Improved ethanol recovery through mixed-matrix membranewith hydrophobic MAF-6 as filler

http://dx.doi.org/10.1016/j.seppur.2017.01.0241383-5866/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: wqjin@njtech.edu.cn (W. Jin).

Qianqian Li, Long Cheng, Jie Shen, Jiayan Shi, Guining Chen, Jing Zhao, Jingui Duan, Gongping Liu,Wanqin Jin ⇑State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University (former Nanjing University of Technology),5 Xinmofan Road, Nanjing 210009, PR China

a r t i c l e i n f o

Article history:Received 14 September 2016Received in revised form 10 January 2017Accepted 10 January 2017Available online 12 January 2017

Keywords:Metal organic frameworkHydrophobicMixed matrix membranePervaporationEthanol recovery

a b s t r a c t

Increasing the hydrophobicity of membrane was believed as an effective approach to enhance the perfor-mance of organophilic pervaporation (PV) membrane. In this work, the superhydrophobic alkyl groupmodificated metal organic framework (MOF) of RHO-[Zn(eim)2] (MAF-6) was incorporated into poly-dimethylsiloxane (PDMS) polymer to fabricate mixed matrix membranes (MMMs). The morphologies,physical and chemical properties of as-prepared MAF-6 nanocrystals and membranes were characterizedwith XRD, FESEM, EDX, QCM, contact angle and nano-scratch tests. The pervaporation experiments ofethanol/water mixture show the simultaneously enhanced flux and separation factor of MMMs comparedwith PDMS pristine membrane (with the optimal flux of 1200 g/m2 h1, and separation factor of 14.9),which stems from the hydrophilicity and high porosity of MAF-6, successfully overcoming the trade-off effect.

� 2017 Elsevier B.V. All rights reserved.

1. Introduction

Energy conservation and emissions reduction are becoming theworld development trend. In recent years, effectively recycling useof biofuels is promising to solve the global issue of fossil-fuel short-age [1]. As one of the most valuable liquid biofuel, ethanol ismainly produced by biomass fermentation, with the concentrationgenerally ranging from 1 to 10 wt%. However, the concentration ofethanol must be maintained below 8% during the fermentationprocess otherwise the activities of the yeast cells would besuppressed, which leads to the termination of the fermentationsprocess. So as to improve the bioethanol yield, ethanol has to becontinuously removed from the fermentation broth [2]. The tradi-tional purification system usually includes distillation, gasstripping and adsorption, which are energy intensive [3,4]. Onthe contrary, pervaporation (PV) is considered to be the one ofthe most promising separation technologies for efficient recoveryof ethanol with low energy cost, easy operability and high perms-electivity [5].

Polydimethylsiloxane (PDMS) is the industrial benchmarkmembrane material for ethanol permselective pervaporation, sinceits high chemical stability, high-permeability, and desirable

membrane-forming property [6–11]. However, further develop-ment of PDMS membrane is impeded by the relatively low separa-tion factor and the ‘‘trade-off” effect between flux and separationfactor [12,13]. The reported separation factors of the pure PDMSmembranes for ethanol/water mixtures are about 4–8, which can-not meet with the request of production efficiency and cost savingsfor practical application [10]. Meanwhile, the common modifica-tions of PDMS matrix for increasing separation factor generallylead to the decrease of flux. The fundamental reason lies in thestrong mobility of PDMS molecular chains and the mutual restric-tions between chain spacing and chain rigidity. The incorporationof rationally designed porous filler into PDMS matrix has beenidentified to be an effective strategy to address this issue[14–20]. Among diverse porous materials, the emerging materialsof metal organic frameworks (MOFs) formed through metal ionsbridged by organic ligands, exhibit high potential for gas and liquidseparation [21,22]. Further, their easy tuned structures and surfaceproperties by different pre- and post-synthetic approaches renderthem as one of the most promising fillers in mixed matrix mem-branes (MMMs) [23–32]. On the one hand, compared withtraditional inorganic fillers, MOFs show better compatibility withpolymer matrix because of their various chemical functionalities,thus making contributions to largely reducing non-selective voidsgenerated at the polymer-filler interface. On the other hand, manyMOFs [33–37], especially ZIF-8 [36,37] incorporated into PDMS for

106 Q. Li et al. / Separation and Purification Technology 178 (2017) 105–112

ethanol/water pervaporation have proved that doping hydrophobicinorganic porous particles in polymer could improve the pervapo-ration performance for separating organics. However, MOFs likeZIF-8 are with a hydrophobic pore surface but hydrophilic crystalsurface and small aperture size (ZIF-8 �3.4 Å), which is unfavor-able for the transport of organic molecules [38]. Therefore, a newtype of MOFs with both hydrophobic pore and crystal surfaceand large aperture size are needed and believed to be effectivelyimproving PV performance of separating organics from water[15,38].

RHO-[Zn(eim)2] (MAF-6) is an exceptional hydrophobic MOFwith high thermal/chemical stabilities [38]. MAF-6 exhibits largepore size (�18.4 Å) and large aperture size (�7.6 Å) with highhydrophobicity on both the internal pore and external crystal sur-faces. It can barely adsorb water or be wetted by water but readilyadsorb large amounts of organic molecules including ethanol. Inaddition, MAF-6 nanoparticles can be synthesized at roomtemperature using a rapid solution mixing reaction with highcrystallinity, purity, large quantity and high thermal/chemicalstabilities. Therefore, the preparation of MAF-6 particles is of lowenergy consumption and high efficiency. Also, the synthesis is veryconvenient compared to most MOFs synthesized at high tempera-ture and pressure.

In this work, MAF-6/PDMS MMMs were fabricated by incorpo-rating hydrophobic MAF-6 into PDMS matrix and utilized for etha-nol/water pervaporation separation. The effects of incorporatingMAF-6 on the morphologies, physical/chemical properties, andPV performance of PDMS membrane were investigated. Long-term PV experiment was also performed to study membranestability.

2. Experimental

2.1. Materials

PDMS (a,x-dihydroxypolydimethylsiloxane, Mw = 60,000) waspurchased from Shanghai Resin Factory Co., Ltd, China. Methanoland ethanol were purchased from Wuxi City Yasheng ChemicalCo., Ltd. Ammonia (25%) and n-heptane were obtained as analyticalreagents from Shanghai Lingfeng Chemical Reagent Co., Ltd, China.2-ethylimidazole was supplied by J&K chemical. Zinc hydroxidewas supplied by Xiya reagent. Deionized water was usedin the all experiments. Poly(vinylidene fluoride) (PVDF) supports(average pore size: 450 nm) were obtained from Solvay, America.All of the materials were used without further purification.

2.2. Synthesis of MAF-6 particles

MAF-6 was synthesized according to previous report [38].2-ethylimidazole (1.9 g, 20 mmol) was dissolved into 150 mLmethanol, and then 10 mL of cyclohexane was added. A concen-trated aqueous ammonia solution (25%, 200 mL) of Zn(OH)2 (1 g,10 mmol) was added into the afore prepared solution. The mixedsolution was stirred at room temperature for 1 h, and then cen-trifuged. White MAF-6 particles were obtained after methanolwashing and dried at 353 K in vacuum overnight.

2.3. Membrane preparation

The MAF-6/n-heptane suspension was stirred and sonicatedalternatively for 1/2 h for three times each. The MAF-6 crystalswere first ‘‘primed” [37] by adding a small amount of PDMS(wPDMS: wn-heptane = 0.5:20) into the suspension, then it was furtherstirred for 2 h. Subsequently, the suspension containing surface-modified MAF-6 particles were mixed with the cross-linked PDMS

solution which consisted of PDMS, n-heptane, TEOS and dibutyltindilaurate (wPDMS: wn-heptane: wTEOS: wdibutyltin dilaurate =0.5:10:0.1:0.005) [39] and were kept on stirring for another3–4 h. Membranes were cast on PVDF substrate by a flat mem-brane casting equipment. The prepared membranes were evapo-rated at room temperature for 24 h, and then dried in oven at70 �C for 12 h. Similarly using the above-mentioned cross linkedPDMS solution without mixing with the MOF, the unfilled PDMS/PVDF composite membrane was prepared in the same way asdescribed above. The MOF loading was defined with the followingequation:

wMOF ¼ mMOF

mPDMS� 100% ð1Þ

where WMOF represents the MOF loading, mMOF is the mass of theMOF in casting solution, mPDMS is the mass of PDMS in castingsolution.

The MAF-6 loadings of these membranes were varied as 0, 5, 10,15, 20 and 25 wt%, respectively.

2.4. Characterization

The crystal phases of samples were determined by powderX-ray diffraction (PXRD) (Rigaku, Miniflex 600, Japan). Diffractionpatterns were collected at room temperature in the angle rangeof 5� 6 2h 6 50� with a step width of 0.05� and a scan rate of0.2 s step�1. The contact angle measurement system (DSA100,Kruss) was used to measure the water contact angle of PDMS/PVDFcomposite membrane. Thermogravimetric analyzer (TGA, STA 209F1, NETZSCH, Germany) was used to determine the thermal stabil-ity of the MAF-6 powder in the temperature range of 25–800 �Cwith a heating rate of 10 �C min�1 in a nitrogen atmosphere. Theadsorption instrument (3 FLEX, micromeritics, USA) was used tomeasure ethanol and water adsorption capacity with MAF-6 parti-cles. The morphology of composite membranes was characterizedby field emission scanning electron microscopy (Hitachi Limited,S-4800, Japan). The working parameters of the SEM are high volt-age 5–15 kV, work distance 8–10 mm, and spot 2.0–3.0. Interfacialadhesion property was measured by Nano-scratch technique(NanoTest TM, MicroMaterials, UK) in this work.

2.5. Pervaporation measurement

The pervaporation experiments were conducted with a home-made apparatus [40]. The flat membranes were stationed in stain-less steel-made PV cell. The feed solution was maintained at presetconcentration and temperature, circulated between the feed tankand the membrane cell with a flow rate of 12 L/h by a circulationpump. The permeate vapor was collected in liquid nitrogen trap.The pressure at permeate side was below 300 Pa during collectionsby a vacuum pump. The concentration of feed and result weredetermined by gas chromatography (GC-2014, Shimadzu, Japan).The PV performance of a membrane is usually expressed on thebasis of the total flux J and separation factor b. To eliminate theeffect of the membrane thickness on flux, thickness-normalizedflux J0 (normalized to 10 lm) was also used.

The total flux (J, g/m2 h), thickness-normalized flux (J0, g/m2 h)and separation factor (b) at steady state was calculated by thefollowing equations:

J ¼ WAt

ð2Þ

J0 ¼ WAt

� l10

ð3Þ

Q. Li et al. / Separation and Purification Technology 178 (2017) 105–112 107

bi ¼Yi=ð1� YiÞXi=ð1� XiÞ ð4Þ

where W (g) represents the mass of permeate over a period of t (s);A (m2) is the effective area of flat membrane; l (lm) is the mem-brane thickness; Xi and Yi are the mass fractions of component iin feed and permeate side, respectively.

3. Results and discussion

3.1. Characterization of MAF-6 nanoparticle

The morphologies of as-synthesized MAF-6 particles werecharacterized by FESEM. As shown in Fig. 1a, the particle sizeof MAF-6 crystals is around 150 nm. As shown in Fig. 1b,the locations of the powder X-ray diffractions (PXRD) of MAF-6powders are consistent with the simulated data, indicating goodphase purity [41–43]. TGA result of MAF-6 nanoparticles isshown in Fig. 1c. When the temperature rises to 360 �C, the curvehas a sudden steep decline, indicating that the framework col-lapsed. Therefore the MAF-6 particles have good thermal stabilityat long range of temperature. In addition, 3 FLEX micromeriticswas employed to obtain the equilibrium adsorption and desorp-tion isotherms for ethanol and water vapor with MAF-6 particlesat 40 �C [16,44]. As shown in Fig. 1d, the adsorption capacity forethanol is much larger than that for water, confirming thehydrophobic nature of the pore surface of MAF-6, which showshigher affinity for more hydrophobic adsorbents. Thus, the resultsclearly identify that ethanol is more preferred to be absorbed byMAF-6 nanoparticles, reflecting the potential of ethanol/waterseparation.

Fig. 1. (a) FESEM image of MAF-6 crystal; (b) PXRD patterns of simulated MAF-6 powd(solid) and desorption (hollow) isotherms for ethanol and water with MAF-6 at 40 �C.

3.2. Characterization of MAF-6/PDMS MMMs

3.2.1. Structural analysisWe used X-ray diffractometer to characterize the physical

structure changes of PDMS membranes after loading. The XRD pat-terns of MAF-6/PDMS MMMs are shown in Fig. 2. The XRD patternsof MAF-6 becomes obvious with the increasing of MAF-6 loading.That means the polymer matrix does not alter the structure ofMAF-6.

3.2.2. Surface contact angle testThe hydrophilic or hydrophobic property of the membrane

materials play a key role in the pervaporation performance ofmembranes. To further demonstrate surface functions, we mea-sured the contact angle of the as-prepared membranes. Fig. 3shows the placement of a water droplet on the MMMs surface. Itcan be seen that the water droplet does not spread out. The biggestcontact angle (119.8�) of MMMs occurs with MAF-6 loading of15 wt%. With the increasing of the loading of MAF-6 nanoparticles,the membranes contact angle become larger. At loading of 20 wt%,there is a little decrease in contact angle due to excess MAF-6crystals may reunite and lead to defects at the interface betweenMAF-6 and PDMS matrix.

3.2.3. MorphologyFESEM was used to characterize the morphology of MMMs.

Fig. 4 shows morphologies of pure PDMS membrane and MAF-6/PDMS MMMs with different loadings. It can be seen that thesurface of pure PDMS membrane is very smooth and defect-free.After embedding the MAF-6 nanoparticles with the loading variedfrom 5 wt% to 25 wt%, the membrane surface are without visible

er and as-prepared MAF-6 particles; (c) TG curve of MAF-6 crystal; (d) adsorption

Fig. 2. (a) XRD spectra of pure PDMS and (b–f) MAF-6/PDMS MMMs with loadingsof 5, 10, 15, 20 and 25 wt%.

Fig. 3. Contact angle of water on MMMs with different MAF-6 loadings.

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interfacial voids, owing to the good compatibility between MAF-6and PDMS matrix. The cross-section SEM image of pristine PDMS/PVDF composite membrane is shown in Fig. 4g, For MAF-6/PDMSMMMs with loading of 15 wt%, as shown in Fig. 4h, the membranelayer is well bonded with the substrate, with a thickness of about5 lm. The EDX mapping in Fig. 4i shows uniform distribution of Znelement in the cross-section of MAF-6/PDMS MMMs. To bettershow the particle distribution in larger area of the membrane,EDX mapping of the membrane surface with lower magnificationis shown in Fig. 5. The dispersed Si element and C element areall homogeneous. Zn element (red) uniformly disperses inmembrane, which indicates the homogeneous distribution of theMAF-6 particles. Generally, the interfacial morphology between fil-ler and polymer of mixed matrix membranes is an important factoraffecting separation performance. To the best of our knowledge,there are two commonly used characterization methods in litera-tures. (1) Direct observation via SEM. Filler agglomeration [45]and severe interfacial defects [46,47] can be directly observed inthe SEM pictures. (2) Indirect analysis of separation performance.Filler agglomeration and interfacial defects will cause inter-filleror interfacial nonselective transport pathways and then deteriorateseparation performance. In this work, the SEM picture of 15 wt%loading membrane with a higher magnification is embedded in

Fig. 4d. It is still not clear to characterize the interface betweenfiller and polymer in mixed matrix membrane. And with excessMAF-6 crystals (more than 20 wt%), agglomeration can beobserved in the SEM image of membrane surface. The nonselectivevoids can be indicated by the significantly decreased separationfactor in the following pervaporation tests.

3.2.4. Interfacial adhesion properties of MAF/PDMS membraneFurthermore, we used Nano-scratch technique to analyze the

interfacial adhesion properties of MAF/PDMS membrane at loadingof 15 wt%. The Nano-Test system, which monitors and records thescratch distance relationships with the dynamic load, penetrationdepth, friction force and corresponding friction coefficient, canmake finely controlled low load to the indenter and precise move-ment of indenter relative to the membrane surface [48–52]. Duringthe test, the applied load was constant at 0.05 mN between 0 and20 lm and then was ramped to the maximum load 100 mN at theend of the scan (400 lm) with the ramping rate about 2.5 mN/s.Duplicate tests were conducted three times for each membrane.The adhesion strength of separation layer to the substrate can beevaluated by the critical load at failure, which is obtained by notingthe obvious discontinuity in the scratch depth-displacement curve(Fig. 6a). This critical load at the failure is a measure of the adhe-sive strength of the composite membrane (Fig. 6b). In scratchdepth-displacement curve, the figure of it is around 5000 nmwhich matches the thickness (5 lm). We found the critical loadof the MAF-6/PDMS composite membrane is about 16 mN, whichwould assure the MAF-6/PDMS MMM mechanically stable duringthe module installation and long-term continuous operating forseparation [18,53].

3.3. PV performance of MAF-6/PDMS MMMs

To explore the optimal fabrication and operation conditions, theeffects of MAF-6 loading, feed concentration and temperature onthe separation performance of MAF-6/PDMS MMMs were studied.On this basis, the long-term PV experiment was performed toinvestigate the membrane stability. The optimized PV performancewas then compared with literatures.

3.3.1. Effect of loadingPV performances of MAF-6/PDMS MMMs with different MAF-6

loading were systemically studied for ethanol recovery from 5 wt%aqueous solutions at 40 �C (Fig. 7). We use thickness normalizedflux to rid of the influence of membrane thickness. Fig. 7 showsthat the thickness-normalized flux increase with the loading ofMAF-6. This is because MAF-6 is a kind of porous materials, whichcan increase the molecular transfer channels in membranes.However, the separation factor first grows to �13 with the increaseof loading and then drops with the further increase of the loading,which is similar to the results of contact angle. This is because thehydrophobic MAF-6 enhances adsorption and transport of ethanolin the membrane. However, when the MAF-6 loading is higherthan 15 wt%, excess MAF-6 crystals may reunite and lead tomicrodefects at the interface between MAF-6 and PDMS matrix[54]. The MAF-6 agglomeration cannot be well bonded with PDMS,resulting in the increase of flux and decrease of separation factor.At 15 wt% loading of MAF-6, the separation factor is one timelarger than pristine PDMS membrane. At the same time, the fluxalso increased, overcoming the trade-off phenomenon of PDMSmembrane.

Therefore, the MMM with MAF-6 loading of 15 wt% possessingthe optimal performance was selected for the following operatingparameters and long-term stability studies.

Fig. 4. (a–f) FESEM images of MAF-6/PDMS MMMs with different MAF-6 loadings; (g, h) cross-section of pure PDMS and 15 wt% MAF-6/PDMS MMM; and (i) EDX mapping ofMAF-6/PDMS MMMs with MAF-6 loading of 15 wt% (Zn signal: red). (For interpretation of the references to colour in this figure legend, the reader is referred to the webversion of this article.)

Fig. 5. (a–d) SEM-EDX mappings of MAF-6/PDMS MMMs surface with 15 wt% (Zn signal: red, Si signal: white, C signal: yellow). (For interpretation of the references to colourin this figure legend, the reader is referred to the web version of this article.)

Q. Li et al. / Separation and Purification Technology 178 (2017) 105–112 109

Fig. 8. Effect of feed concentrations on the separation performace of the MAF-6/PDMS membrane (with 15 wt% MAF-6 loading) at temperature of 40 �C.

Fig. 9. Effect of temperature on the separation performance of the MAF-6/PDMSmembrane (with 15 wt% MAF-6 loading) at 5 wt% feed ethanol concentration.

Fig. 6. (a) Scratch load for the MAF-6/PDMS membrane; (b) corresponding frictionfor MAF-6/PDMS membrane.

Fig. 7. Effect of MAF-6 loading on the separation performance of the MAF-6/PDMSmembranes. Temperature: 40 �C, feed concentration: 5 wt% ethanol.

110 Q. Li et al. / Separation and Purification Technology 178 (2017) 105–112

3.3.2. Effect of feed concentrationAt first, with the increase of the feed ethanol concentration,

ethanol has a higher chance to interact with the membrane sur-face, thus enabling it to permeate through the membrane moreeasily (see Fig. 8). In addition, an increase in the feed ethanol con-centration normally results in an increasing activity and partial

pressure, which will enhance the driving force for the permeationof ethanol. When ethanol concentration continues increasing,swelling of the membrane intensifies dramatically, making watermore easily permeate through the membrane, so the flux increaseswhile separation factor decreases. What’s more, the dynamicradius of ethanol is larger than that of water, thus leading to thefaster diffusion of water than ethanol and offsetting the adsorptionselectivity of ethanol over water. As a result, the separation factordecreases gradually.

3.3.3. Effect of temperatureIt can be seen that increasing the temperature leads to the

increase of fluxes through the membrane (as shown in Fig. 9). Thisis mainly because a higher temperature promotes the motion ofpolymer molecules and weakens the interaction between the per-meate molecules and polymer molecules, thus reducing the trans-fer resistance of permeate molecules through the membrane andfacilitating their diffusion through the membrane. Temperatureinfluence on separation factors is decided by the combined effectof PDMS polymer molecule motion and adsorption/desorptionphenomena of MAF-6 particles. When increasing temperature from35 �C to 40 �C, the adsorption/desorption rate of water/ethanol forMAF-6 increases. For hydrophobic MAF-6, the increase of adsorp-tion/desorption rate of ethanol is larger than that of water. Thepromoting effect of MAF-6 adsorption/desorption overwhelmedthat of PDMS molecule motion, resulting the increase of separation

Fig. 10. The long-time pervaporation performance of the MAF-6/PDMS MMMs.

Table 1Pervaporation performances of different PDMS membranes in ethanol-water mixture.

Membranetype

T(�C)

Thickness(lm)

Total flux(g/m2 h)

Separationfactor

Reference

PDMS 40 8 1300 8.5 [55]PDMS 60 1 3275 7.5 [56]ZSM-5/PDMS 40 10 408 14.0 [14]SiO2/PDMS 40 15 200 19.0 [57]Silicalite-1 40 100 66.3 19.9 [58]MCM-41@ZIF-8/PDMS 40 3 702 7.5 [54]MIL-53/PDMS 70 3 5467 11.1 [19]ZIF-71/PDMS 40 15 648 9.9 [35]MAF-6/PDMS 40 5 1200 14.9 This work

Q. Li et al. / Separation and Purification Technology 178 (2017) 105–112 111

factor. When further increasing the temperature, the separationfactor declines due to the weakened interaction between PDMSmolecular chains and penetrations.

3.3.4. Long-term operation stabilityThe long-term operation stability is a critical factor for the

industrial application of all membranes. Long term stability ofMAF-6/PDMS MMMs were examined with 5 wt% feed solutionfor 120 h at 40 �C. As shown in Fig. 10, the performance ofMAF-6/PDMS MMM is highly stable with separation factor morethan 10 during 120 h continuous pervaporation experiment.

3.3.5. Comparison of PV performance with literaturesTable 1 listed PV performances of ethanol recovery by a series of

MMMswith different fillers. It is difficult to compare the PV perfor-mance of these membranes in Table 1 due to the different mem-brane parameters (such as different materials loading andsupport) and operating conditions. The decrease of membranethickness of the composite membrane resulted in a higher perme-ate flux, owing to the decrease of the transport resistance. Consid-ering the total flux and separation factor for practical application,the MAF-6 filled PDMS/PVDF composite membrane prepared inthis work exhibited a better performance and could be a promisingcandidate for pervaporation recovery of ethanol from ethanol-water mixtures.

4. Conclusions

In summary, the exceptional hydrophobic MAF-6 nanoparticleswere synthesized through a facile and efficient process, and thenincorporated into PDMS matrix to fabricate MMMs for ethanol/

water pervaporation separation. The MMM with the highesthydrophobicity (MAF-6 loading of 15 wt%) exhibits the highestseparation performance with the total flux of 1200 g/m2 h(1.5 times of PDMS pristine membrane) and separation factor of14.9 (2.3 times of PDMS pristine membrane), overcoming thetrade-off phenomenon of PDMS membrane. Such extraordinaryseparation performance arises from the superior adsorption abilityof MAF-6 for ethanol over water. In addition, the membraneexhibited stable PV performance after the long-term test up to120 h. The results demonstrate that the MAF-6/PDMS MMMs arepotential for ethanol recovery from aqueous solution separationvia pervaporation.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant Nos. 21406107, 21490585,21301148, 21606123), the Innovative Research Team Program bythe Ministry of Education of China (Grant No. IRT13070), the Nat-ural Science Foundation of Jiangsu Province (Nos. BK20140930,BK20160980), Topnotch Academic Programs Project of JiangsuHigher Education Institutions (TAPP), and the Innovation Projectof Graduate Research by Jiangsu Province (Grant No. KYLX_0764).

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