zeolitic imidazolate frameworks (ZIF-8) mixed matrix membranes for pervaporation dehydration of isopropanol

Journal of Membrane Science 469 (2014) 1–10

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Poly(vinyl alcohol)/zeolitic imidazolate frameworks (ZIF-8) mixed matrix membranes for pervaporation dehydration of isopropanol Mohammad Amirilargani a,n, Behrouz Sadatnia a,b a b

Petrochemical Research and Technology Company, National Petrochemical Company, P.O. Box 14358-84711, Tehran, Iran Department of Biomaterials, Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran

art ic l e i nf o

a b s t r a c t

Article history: Received 20 March 2014 Received in revised form 15 June 2014 Accepted 16 June 2014 Available online 23 June 2014

In this study, nanostructured zeolitic imidazolate frameworks (ZIF-8) were successfully synthesized and incorporated into the PVA solution to prepare PVA/ZIF-8 mixed matrix membranes (MMMs). The prepared ZIF-8 nanoparticles and MMMs were characterized by X-ray diffraction (XRD), dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis (TGA) and field emission scanning electron microscopy (FESEM). The prepared MMMs were used to separate mixtures of isopropanol (IPA)/ water at 30 1C in the pervaporation (PV) process. The different ZIF-8 loadings in PVA polymer, such as 1, 2.5, 5, 7.5 and 10 wt%, have been tried and ZIF-8 with 5 wt% loading shows the best PV performance. As a result, the 5 wt% ZIF-8 in PVA membrane increases permeation five times without much decrease in separation factor. Separation factor decreases significantly at higher loadings of ZIF-8 due to the agglomeration of nanoparticles in the PVA matrix. & 2014 Elsevier B.V. All rights reserved.

Keywords: Mixed matrix membranes Poly(vinyl alcohol) ZIF-8 Pervaporation dehydration Isopropanol

1. Introduction Pervaporation (PV) is a membrane separation technology with high selectivity, efficiency and energy saving benefits, especially in separating close-boiling and azeotropic mixtures [1,2]. PV membranes have been developed for various applications such as dehydration of organic solvents, purifying aqueous streams by removal of dilute organic compounds and separation of organicorganic mixtures [3]. Poly(vinyl alcohol) (PVA) is a very effective material for preparation of membranes used in PV dehydration due to its outstanding membrane forming ability, easy processing and abundant availability [4–6]. Recent research efforts have been focused on developing new types of membranes with selective inorganic fillers such as mixed matrix membrane (MMM), which typically incorporates zeolites into polymer matrix [7–10]. Metal organic frameworks (MOFs) composed of inorganic units of metal or metal clusters connected by organic linkages to create one-, two- and three-dimensional microporous structures are considered outstanding candidates as filler materials for MMMs [11–14]. Zeolitic imidazolate frameworks (ZIFs), a sub-family of MOFs, have been well-developed by Yaghi and his coworkers [15]. ZIFs are microporous crystalline materials appeared to be attractive fillers in preparation of MMMs due to their zeolite-like permanent

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E-mail address: [email protected] (M. Amirilargani). http://dx.doi.org/10.1016/j.memsci.2014.06.034 0376-7388/& 2014 Elsevier B.V. All rights reserved.

porosity, uniform pore size, and exceptional thermal and chemical stability [16,17]. ZIF-8 has a sodalite topology and consists of Zn (II) metal centers coordinated by four 2-methylimidazolate linkers [18]. Ordoňez et al. [19] studied Matrimid/ZIF-8 MMMs for gas separation and found that the separation performance was significantly improved with an increase in ZIF-8 loading. Many other ZIFs have been incorporated into different polymers such as chitosan [13], polybenzimidazole (PBI) [20], polyether-blockamide (PEBA) [14], polysulfone [21,22] and Matrimid [23] for fabrication of MMMs. Almost all of them have reported that permeability improved with some enhancements in selectivity for gas separation. More recently, Liu et al. studied ZIF-8/polymethyl-phenylsiloxane MMMs for recovery of bio-alcohols. They reported that adsorption selectivity enhanced by addition of ZIF-8 into the polymer matrix which resulted in significant improvement of separation performance [24]. Shi et al. [12] investigated the effect of ZIF-8 loading on the PV performance of PBI MMMs for dehydration of alcohols. Their results showed that permeability increases significantly without much decreasing in selectivity. In 2013, Kang et al. [13] synthesized chitosan/ZIF-7 MMMs for dehydration of ethanol. They showed that separation factor and the flux of the ZIF-7/CS membranes clearly exceed the upper limit of the previously reported CS based membranes. ZIF-8, with a small aperture size of 3.4 Å and a relatively large cavity size of 11.6 Å [12], possesses the right size for dehydration of alcohols, and might also increase diffusivity of water molecules. However, the high hydrophobic property of ZIF-8 may favor the sorption of

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alcohols. The effects of adding ZIF-8 into the polymeric matrix with high hydrophilic properties such as PVA for dehydration of alcohols remain unclear. In the most polymeric based MMMs used for PV dehydration, there is a trade-off between permeability and selectivity [15]. In the other words, membranes that are more permeable are often less selective and vice versa. Flux reduction in MMMs may be due to the issues of chain rigidification or partial pore blocking [12] which both of them may be controlled by using the fillers with hydrophobic properties. The main objective of this work is to investigate the effects of using ZIF-8 nanoparticles with a right pore size and hydrophobic properties for preparation of PVA MMMs used in IPA dehydration.

2. Experimental 2.1. Materials PVA (MW: 130,000) with a degree of hydrolysis of more than 98%, glutaraldehyde (GA, 25 wt% in H2O), hydrochloric acid, Zinc nitrate hexahydrate (Zn(NO3)2  6H2O) (reagent grade, 4 98%) and 2-methylimidazole (Hmim) (98%) were all purchased from SigmaAldrich.

2.2. Synthesis of ZIF-8 nanoparticles The ZIF-8 particles were synthesized using the procedure described by Cravillon et al. [25]. In summary, a solution of 3 g (10 mmol) of Zn(NO3)2  6H2O in 100 mL of methanol and another solution of 6.6 g (80 mmol) of 2-methylimidazole in 100 mL of methanol were prepared and then mixed by vigorously stirring at room temperature. The resultant solution slowly turned turbid and after 1 h the nanocrystals were separated from the milky dispersion by centrifugation and washing with methanol twice and with chloroform once. ZIF-8 nanocrystals were dried under vacuum at 120 1C for 24 h [19,26,27] and stored dry for further analysis. The yield of ZIF-8 was 40% based on zinc.

2.3. Membrane preparation The PVA/ZIF-8 MMMs were prepared via solution casting and solvent evaporation technique. PVA powder (3 g) was dissolved in 100 ml of deionized water at 90 1C by stirring and the solution was filtered to remove insoluble impurities. Afterwards, the ZIF-8 nanoparticles were added into the previously prepared PVA solution. The solution was then stirred for 12 h vigorously and then exposed to ultrasonication for 30 min. in situ cross-linking was performed by adding 0.1 ml of GA and 0.1 ml of HCL to the mixed solution and stirring for about 15 min. The solution was cast on the onto a clean glass plate. The polymer casting solution was heated in an oven at 40 1C overnight and then MMMs peeled from the glass plate. The mass ratio of ZIF-8 to PVA was varied at 0, 1, 2.5, 5, 7.5 and 10%.

2.4. Characterization 2.4.1. X-ray diffraction (XRD) X-ray diffraction patterns of the prepared membranes were recorded using a SIE-MENS, D5000 (GERMANY). The X-rays of 1.54 Å wavelengths were generated by a Cu Kα radiation source. The angle of diffraction (2θ) was varied from 51 to 401 to identify any changes in crystal morphology and intermolecular distances between inter-segmental chains.

2.4.2. Dynamic mechanical thermal analysis (DMTA) Dynamic mechanical properties of PVA MMMs were performed using DMTA-Triton (Tritec 2000 DMA, England) instrument in a tensile mode at a frequency of 1.0 Hz and at the heating rate of 5 1C/min between 25 and 120 1C. 2.4.3. Thermogravimetric analysis (TGA) Thermal behavior of the prepared samples was examined by Thermogravimetric Analyzer (TGA) (Perkin Elmer, Pyris Diamond S(II), USA) from 25 1C to 800 1C. A heating rate of 10 1C/min was used under nitrogen atmosphere and at a flow rate of 20 mL/min. 2.4.4. Field emission scanning electron microscope (FESEM) Field emission scanning electron microscope (FESEM) (MIRA 3 LM, Czech Republic) was used for analysis of the PVA/ZIF-8 MMMs.

2.4.5. Swelling study Swelling experiments on the membranes were performed gravimetrically at 30 1C in 10 wt% water containing feed mixtures. The membranes were dried completely at 70 1C for 6 h and weighed by digital microbalance (SARTORIUS AG GERMANY) sensitive to 70.01 mg. Dry membranes were equilibrated by soaking in the water–IPA mixtures in a sealed vessel at 30 1C for 48 h. The swollen membranes were weighed immediately after careful blotting of surface on a digital microbalance. Each run was performed at least three times, and the results were averaged. The percentage degree of swelling (DS %) was calculated as   Ws Wd Degree of swelling ðDS %Þ ¼  100 ð1Þ Wd where Ws and Wd are the weights of the swollen and the dried membranes, respectively. 2.5. PV experiment The prepared membranes were evaluated in a PV separation system. Mixtures of IPA–water were prepared and hold in a feed tank with a volume capacity of 1 L. Effective surface area of the membranes in the PV cell was 15.8 cm2 (.00158 m2). The system was stabilized for 1 h before the collection of samples. Thereafter, permeate samples were collected by a cold trap immersed in liquid nitrogen. The permeate pressure was maintained at 5 mm Hg using a vacuum pump (Platinum JB, USA). Concentrations of the feed and permeate were measured using Abbe refractometer (NAR-3T ATAGO, Japan) by comparing the observed refractive index with a standard graph based on the feed mixture composition (calibration curve). Permeation properties of the membranes were characterized by permeation flux (J), separation factor (α) and pervaporation separation index (PSI) using the following equations, respectively: J¼

Q At

ð2Þ

Y i =Y j X i =X j

ð3Þ

PSI ¼ Jðα 1Þ

ð4Þ

αij ¼

where Q is the mass of permeate (g), A is the effective membrane area (m2), and t is the permeation time (h). X and Y represent the weight fractions of the components i and j in the feed and the permeate, respectively.

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3. Results and discussion 3.1. ZIF-8 and membranes characterization 3.1.1. XRD Fig. 1 shows the XRD patterns of the synthesized ZIF-8 nanoparticles and different MMMs. A sharp diffraction peak is observed for the neat PVA and the MMMs at 2θ of 19.41, which corresponds to (101) planes [28]. The main peaks of the ZIF-8 nanoparticles match well with the published XRD pattern of ZIF-8 sample [29]. As observed, there is no obvious change for the XRD pattern of the membrane prepared with 2.5 wt% of ZIF-8. The XRD pattern slightly shifts towards that of ZIF-8 structure by increasing the ZIF-8 loading. As the ZIF-8 loading increases to 5 and 7.5 wt%, different diffraction peaks were observed at 2θ of 11.21, 14.51, 17.71, 24.01, 29.11 and 30.31. Incorporated ZIF-8 in PVA matrix act as nucleation sites and this phenomenon enhances crystallinity of PVA membranes [13]. On the other hand, when ZIF-8 loading is sufficiently high, agglomeration of the ZIF-8 particles tends to occur and this increases the membrane free volume and consequently decreases crystallinity of the membrane. These two opposing effects determine whether the membrane crystallinity increases or decreases as ZIF-8 content increases.

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3.1.2. DMTA analysis DMTA results indicate information about microstructures as well as thermal and viscoelastic properties of the polymeric systems over a wide range of temperature interval. Figs. 2 and 3 show the loss modulus and tan δ of the PVA MMMs with different

Fig. 2. Loss modulus of the PVA MMMs prepared with different loadings of ZIF-8.

Fig. 3. tan δ Curves of the PVA MMMs prepared with different loadings of ZIF-8.

Fig. 1. XRD patterns of the different PVA MMMs.

Fig. 4. Tg values of the PVA MMMs prepared with different loadings of ZIF-8 obtained from DMTA.

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loadings of ZIF-8. Loss modulus of all the prepared membranes shows a sharp decrease in glass transition region (Fig. 2) [30]. tan δ Curves of the different MMMs are shown in Fig. 3. The position of the peaks represents the glass transition temperature (Tg) of the prepared membranes which are changed from 65.5 to 90.2 1C for different membranes. Tg values as obtained from loss modulus and tan δ are presented in Fig. 4. As the ZIF-8 loading increases to 2.5 and 5 wt%, Tg decreases and then slightly increases by addition 7.5 wt% of ZIF-8 into the PVA matrix. For example, Tg value obtained from loss modulus is 65 1C for the neat PVA membrane. Tg value decreases to 59 and 53.7 1C and then slightly increases to 54.4 1C when the ZIF-8 loading increases to 2.5, 5 and 7.5 wt%. The decrease of Tg is may be due to the disruption of inherent organization of the polymer chains and enhancement of accessible free volume in the polymer matrix. Nevertheless, when the ZIF-8 loading reaches 7.5 wt%, the Tg increases slightly. This can be attributed to partial chain rigidification of the polymer matrix with high weight loading [14]. 3.1.3. TGA Thermal degradation and stability of the PVA MMMs membranes prepared with ZIF-8 nanoparticles were investigated using

Fig. 5. Weight loss curves of the ZIF-8 particles ad PVA MMMs prepared with different loadings of ZIF-8.

a TGA analysis. The weight loss of the pure ZIF-8 particles, neat PVA membrane and MMMs prepared with 1, 5 and 10 wt% of ZIF-8 are presented in Fig. 5. As observed in the weight loss curve of assynthesized ZIF-8, a long plateau was shown in the temperature range of 300–550 1C, indicating high thermal stability of the synthesized ZIF-8. In addition, it was also observed from Fig. 5 that ZIF-8 particles exhibited a gradual weight-loss step of 14.7% up to 300 1C, corresponding to the removal of residual solvents or reactant molecules trapped in the nanocrystals during synthesis and post-treatment [31,32]. These residual solvents may be responsible for the different peaks observed in the XRD pattern of PVA MMMs (Fig. 1). As observed, the neat PVA membrane has two degradation steps which the second step is between 305 and 460 1C corresponding to the complete decomposition phase of PVA [33]. As observed in Fig. 5, second step of degradation of the neat PVA membrane divided into two different steps of degradation for the MMMs. For example, the membrane prepared with 5 wt% of ZIF-8 has three steps of degradation. The second step is between 245 and 305 1C and the third step is between 404 and 472 1C. The weight loss temperatures of the MMMs prepared at higher loadings of ZIF-8 decreased in the region of complete decomposition phase of PVA (305–390 1C). This may be a result of the interfacial voids between the ZIF-8 and PVA matrix. The remained masses at higher temperatures are almost entirely due to the remained ZIF-8 and they are consistent with initial loading contents of ZIF-8. For example, the neat PVA membrane has a weight loss around 97.8% at 800 1C, while this value decreases to 95.5%, 92.6% and 83.5% for the membranes prepared with 1, 5 and 10 wt% of ZIF-8, respectively.

3.1.4. FESEM The FESEM image of the synthesized ZIF-8 nanoparticles is shown in the Fig. 6. As observed, the crystals do not display a distinct morphology and the majority of ZIF-8 particles have diameters smaller than 60 nm. Cross sectional FESEM images of the prepared PVA MMMs with different loadings of ZIF-8 are illustrated in Figs. 7–9. Obviously, the neat PVA membrane is free of ZIF-8 particles and by increasing ZIF-8 loading the number of light dots which presents the ZIF-8 tips increases (Fig. 7). As illustrated in Fig. 8e and f, the membrane morphology changes by increasing the ZIF-8 loading to 7.5 and 10 wt%. This may be attributed to the agglomeration of ZIF-8 particles in PVA matrix. Fig. 9 confirms the uniform dispersion of ZIF-8 nanoparticles in

Fig. 6. FESEM image of (a) ZIF-8 nanoparticles and (b) ZIF-8 nanoparticles with higher magnification.

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Fig. 7. Cross sectional FESEM images of PVA MMMs with different loadings of ZIF-8 (magnification of 2500  ): (a) neat PVA, (b) 1 wt%, (c) 2.5 wt%, (d) 5 wt%, (e) 7.5 wt% and (f) 10 wt%.

PVA MMMs prepared in the lower content of ZIF-8 loadings by mapping the Zn element with the assistance of the energydispersive X-ray spectroscopy (EDX). Since zinc element exists only in ZIF-8 nanoparticles, the uniform distribution of the Zn element noticeably confirms the uniformly dispersion of ZIF-8 in the PVA matrix. As observed in the Fig. 9d and e, ZIF-8 particles were not well dispersed in the PVA matrix when the loading increases to 7.5 and 10 wt%.

3.1.5. Effect of ZIF-8 loading on membrane swelling In PV, membrane swelling plays a key role and controls transport of permeating molecules under the influence of chemical potential gradient. The swelling behavior of the prepared MMMs membranes is illustrated in Fig. 10. As observed, the neat PVA membrane shows higher DS compared with the MMMs prepared with different loadings of ZIF-8. Incorporation of ZIF-8 into the PVA matrix may has two opposite effects on DS. Addition of ZIF-8 in

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Fig. 8. Cross sectional FESEM images of PVA MMMs with different loadings of ZIF-8 (magnification of 15,000  ): (a) neat PVA, (b) 1 wt%, (c) 2.5 wt%, (d) 5 wt%, (e) 7.5 wt% and (f) 10 wt%.

lower content may disrupt the inherent organization of the polymer chains and the membrane free volume will increase. By increasing the ZIF-8 loading partial rigidification may be occurred. On the other hand, addition of ZIF-8 with hydrophobic properties into the PVA matrix with more hydrophilic properties leads to reduction of DS. However, DS increases as the loading of ZIF-8 increases from 7.5 to 10 wt%. Aggregation of the ZIF-8 decreases the compactness of PVA membrane and penetrants can permeate through the membrane more readily.

3.2. PV performance 3.2.1. Effect of ZIF-8 loading on PV performance The effect of ZIF-8 loading on the PV performance of the PVVA MMMs is shown in Fig. 11. Operating conditions were fixed at 10 wt% water in feed mixture and feed temperature of 30 1C. Three different samples of each composition were tested. As observed, permeation flux increases significantly by addition 1 and 2.5 wt% of ZIF-8 into the PVA matrix while separation factor decreases

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Fig. 9. EDX mapping for Zn from the cross-section of PVA/ZIF-8 MMMs with different loadings of ZIF-8: (a) 1 wt%, (b) 2.5 wt%, (c) 5 wt%, (d) 7.5 wt% and (e) 10 wt%.

drastically. Increasing in permeation flux by addition of ZIF-8 should mainly be a result of the interfacial gaps between ZIF-8 nanoparticles and PVA matrix. Also, due to the superhydrophobicity of ZIF-8 nanoparticles, sorption of IPA molecules increases by addition of ZIF-8 into the PVA matrix and this leads to significantly increasing in the permeation flux. Further increment

in ZIF-8 loading results in significant enhancement in permeation flux while separation factor decreases dramatically. This obvious enhancement in partial flux may be attributed to the interfacial gaps and nanoparticles agglomeration [12]. However, as shown in the FESEM images, the agglomeration of ZIF-8 was occurred at the higher contents of ZIF-8 loading. The agglomerated ZIF-8 particles

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Fig. 10. Effect of ZIF-8 loading on DS of PVA MMMs.

Fig. 13. Graphical comparison of PV performance of the prepared PVA MMMs with the other MMMs for IPA dehydration as presented in Table 1.

reduction of PSI values for the MMMs prepared at the higher content of ZIF-8 can be due to the sharp reduction of separation factor. As observed, the MMMs prepared with 5 wt% of ZIF-8 exhibits the highest PSI value compared with the other membranes.

Fig. 11. Effect of ZIF-8 loading on PV performance of different PVA MMMs (10 wt% water in feed at 30 1C).

3.2.2. Comparison of the MMMs with the literature The obtained results in the present work were compared with the literatures. Table 1 shows performance of various MMMs compared to the performance of the neat membranes in dehydrating of IPA [12,34–37], while Fig. 13 shows the comparison graphically. As observed, improvement in separation performance, particularly the permeation flux, is impressive in this study compared with the other works. Based on the literature studies, MMMs for PV dehydration of IPA may increase flux in the range from 65% to 692% while the separation factor changes in the range of  66% to 9512% compared to neat membranes as observed in Table 1. As shown the PVA MMM prepared with 5 wt% of ZIF-8 shows significantly increases in permeation flux about 543% while separation factor slightly increases about  19%. Even though this membrane does not show agglomeration of the ZIF-8 according to the FESEM images (Figs. 7 and 8d) and EDX mapping (Fig. 9c), interfacial voids between the ZIF-8 nanoparticles and PVA matrix may play the main role significantly in increasing the permeation flux with slightly decreasing in separation factor. However, agglomeration of ZIF-8 nanoparticles in the membranes prepared with higher loadings of ZIF-8 results in dramatically decreasing in separation factor compared to the neat membrane.

4. Conclusion

Fig. 12. Effects of ZIF-8 loading on PSI of different PVA MMMs.

increase the membrane free volume and consequently penetrants can permeate through the membranes more readily. In order to investigate the overall performance of membranes, PSI for all the membranes at 30 1C using a mixture of 10 wt% water in feed mixture was calculated as presented in Fig. 12. As observed, PSI values increase with increasing the loading of ZIF-8 up to 5 wt% and then decrease by further increasing to 7.5 and 10 wt%. The

In this study, PVA/ZIF-8 MMMs prepared for PV dehydration of IPA. As-synthesized ZIF-8 nanoparticles, with sizes smaller than 60 nm, were dispersed in the matrix of PVA directly with a loading ranging from 1 to 10 wt%. FESEM observation confirmed that the agglomeration of ZIF-8 was observed only in the higher contents of ZIF-8. Tg of the prepared MMMs decreased compared to the neat PVA membrane by addition of ZIF-8 into the PVA matrix and then increases slightly by increasing the loading of ZIF-8. Weight loss temperatures of the PVA MMMs prepared at higher loadings of ZIF-8 decreased as a result of interfacial voids between ZIF-8 and PVA matrix. Increasing the ZIF-8 loading up to 5 wt%, increases permeation flux significantly without much decrease in separation factor. Separation factor decreases dramatically by increasing ZIF-8

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Table 1 Comparative study of PV performance of the prepared PVA MMMs with the MMMs reported in the literatures for dehydration of IPA. Polymer

Filler type

PVA/PVP PVA P84 co-polyimide P84 co-polyimide Matrimids PBI PVA PVA

Phosphomolybdic acid Silicalite-1 Zeolite 5A Zeolite 13X Magnesium oxide (MgO) ZIF-8 ZIF-8 ZIF-8

Feed composition Temperature (1C) Loading (wt%/wt%) (wt%)

IPA/water IPA/water IPA/water IPA/water IPA/water IPA/water IPA/water IPA/water

(90/10) 30 (90/10) 30 (90/10) 60 (90/10) 60 (90/10) 100 (85/15) 60 (90/10) 30 (90/10) 30

loading to 7.5 and 10 wt% due to the agglomeration of nanoparticles in the PVA matrix. However, the PSI values of the PVA/ZIF-8 MMMs membranes were still all higher than that of the neat PVA membrane, which indicates better PV efficiency for the separation of water and IPA after the incorporation of ZIF-8 nanoparticles into the PVA membranes.

Acknowledgment The authors gratefully acknowledge the financial support of this research by Petrochemical Research and Technology Co. of Iran (Grant no. 0870289002).

Nomenclature List of symbols DS Ws Wd J Q A t Yi Yj Xi Xj PSI

degree of swelling [%] weights of swollen membrane [g] weights of dry membrane [g] permeation flux [g/m2h] mass of permeate [g] effective membrane area [m2] time [h] mass percent of water in permeate [%] mass percent of IPA in permeate [%] mass percent of water in feed [%] mass percent of IPA in feed [%] pervaporation separation index

Greek letters α

separation factor

References [1] S. Zereshki, A. Figoli, S.S. Madaeni, S. Simone, M. Esmailinezhad, E. Drioli, Effect of polymer composition in PEEKWC/PVP blends on pervaporation separation of ethanol/cyclohexane mixture, Sep. Purif. Technol. 75 (2010) 257–265. [2] M. Mulder, Basic Principles of Membrane Technology, Second ed., Kluwer Academic Publishers, Netherlands, 1998. [3] B. Smitha, D. Suhanya, S. Sridhar, M. Ramakrishna, Separation of organicorganic mixtures by pervaporation: a review, J. Membr. Sci. 241 (2004) 1–21. [4] R.Y.M. Huang, Pervaporation Membrane Separation Processes, Elsevier Science Publishers, Amsterdam, Netherlands, 1991. [5] P. Srinivasa Rao, B. Smitha, S. Sridhar, A. Krishnaiah, Preparation and performance of poly(vinyl alcohol)/polyethyleneimine blend membranes for the dehydration of 1,4-dioxane by pervaporation: comparison with glutaraldehyde cross-linked membranes, Sep. Purif. Technol. 48 (2006) 244–254.

10 10 20 40 15 34 5 7.5

Neat membrane performance

MMM performance

J (g/m2h)

α

J (g/m2h) α

105 95 30 30 6000 13 135 135

312 77 3000 3000 900 5000 163 163

36 69 40 110 4500 103 868 952

29,991 2241 4200 2700 1800 1686 132 91

Increase Increase in J (%) in α (%)

Reference

 65  27 33 267  25 692 543 605

[34] [35] [36] [36] [37] [12] This study This study

9512 2810 40  10 100  66  19  44

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