Separation and Purification Technology

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1 Accepted Manuscript Mixed-matrix hollow fiber composite membranes comprising of PEBA and MOF for pervaporation separation of ethanol/water mixtures Quan Liu, Yukai Li, Qianqian Li, Guozhen Liu, Gongping Liu, Wanqin Jin PII: S (17) DOI: Reference: SEPPUR To appear in: Separation and Purification Technology Received Date: 11 December 2017 Revised Date: 16 January 2018 Accepted Date: 21 January 2018 Please cite this article as: Q. Liu, Y. Li, Q. Li, G. Liu, G. Liu, W. Jin, Mixed-matrix hollow fiber composite membranes comprising of PEBA and MOF for pervaporation separation of ethanol/water mixtures, Separation and Purification Technology (2018), doi: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

2 Mixed-matrix hollow fiber composite membranes comprising of PEBA and MOF for pervaporation separation of ethanol/water mixtures Quan Liu, Yukai Li, Qianqian Li, Guozhen Liu, Gongping Liu * and Wanqin Jin * State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Department of Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing , PR China. Corresponding authors: wqjin@njtech.edu.cn (Prof. W. Jin); gpliu@njtech.edu.cn (Dr. G. Liu) Abstract Pervaporation membrane technology plays an important role in production of renewable bio-fuels. To improve the separation performance of polymeric membranes, an effective approach is development of mixed-matrix membranes containing high-performing fillers such as metal-organic frameworks. Towards practical application, hollow fiber could be an ideal substrate for mixed-matrix composite membrane, while current fabrications of this kind of composite membrane are relative complicated and challenging. In this work, a novel ceramic hollow fiber supported mixed-matrix composite membrane made of RHO-[Zn(eim) 2 ] (MAF-6, Heim = 2-ethylimidazole) nanoparticles and poly (ether-block-amide) (PEBA) was fabricated via a facile dip-coating approach. SEM, XRD, TGA, contact angle and swelling measurements, nano-scratch technique were employed to study the morphology, crystal structure, thermal stability, surface property and interfacial adhesion of the resulting MAF-6/PEBA mixed-matrix hollow fiber composite membranes, respectively. These membranes were applied for recovering ethanol from aqueous solution via pervaporation. The effects of MAF-6 loading, as well as operating conditions (e.g., temperature, feed concentration and long-term stability) on the PV performance were systematically investigated. The PV performance of PEBA membrane, both flux and separation factor, were remarkably enhanced by uniformly incorporating MAF-6 nanoparticles. Total flux of 4446 g/m 2 h and separation factor of 5.6 (feed: 5 wt% ethanol/water, 60 C) was achieved for the optimized MAF-6/PEBA mixed-matrix hollow fiber composite membrane, which shows great advantages over the reported PEBA-based membranes for ethanol/water separation. Keywords: Mixed-matrix membranes, MAF-6, PEBA, ceramic hollow fiber, pervaporation, ethanol recovery 1

3 1. Introduction Energy crisis is one of the major global issues due to the excessive consumption of conventional nonrenewable resources, such as fossil fuels. As one of the alternative energy resources, the renewable biofuel has attracted an increasing attention in recent years. Biofuels (e.g., bioethanol, biobutanol) are typically produced from biomass by fermentation which however is generally inhibited by the produced solvents, causing significant decrease in yield and productivity [1]. To improve the productivity of fermentation system, many in-situ product recovery technologies such as gas stripping, adsorption, liquid-liquid extraction and pervaporation have been developed to relief the inhibition effect by continuously removing the yielded solvents from the fermentation broth. Among them, pervaporation (PV) as a membrane-based technology is now regarded as an attractive method because of its low energy cost, high efficiency and no harmful effects on the microorganisms [2-4]. As the key component of PV technology, organophilic (hydrophobic) membranes of various materials were widely studied, including polydimethylsiloxane (PDMS) [5-8], poly (1-trimethylsilyl-1-propyne) (PTMSP) [9, 10], poly (ether-block-amide) (PEBA) [11-13] and PVDF [14]. Compared with other polymers, PEBA consisting of hard polyamide (PA) segments and soft polyether (PE) segments, can be easily processed into membranes without crosslinking (sometimes is complicated and difficult to control) with comparable separation performance for organic solvents [11, 12]. The practical application of polymeric membranes is generally limited by the trade-off between permeability (or flux) and selectivity. Although inorganic membranes such as hydrophobic MFI zeolite membranes exhibited high flux and selectivity for ethanol/water separation, their scalable and cost-effective fabrication remain challenges. Mixed-matrix membranes composed of fillers dispersed in polymeric matrix have attracted much attention in recent years[15], since they could combine the advantages of high-performance from the filler and easy-processing from the polymer[16]. The PV performance of PEBA membranes for recovering ethanol from aqueous solution were enhanced by introducing ethanol-adsorptive filler in the matrix. Gu et al. incorporated silicalite-1 zeolite particles into PEBA membrane to obtain total flux of 830 g/m 2 h and separation factor of 3.6 [11]. Le at al. fabricated polyhedral oligomeric silsesquioxane (POSS)/PEBA mixed-matrix membranes filled with 2 wt% filler exhibiting an enhanced PV performance with separation factor of 4.6 and total flux of 180 g/m 2 h [13]. To reach a better economic efficiency, it s necessary to further advance the separation performance of PEBA membranes [17]. Recent works showed that metal-organic framework (MOF) is an excellent filler to improve the separation performance of polymeric membranes via mixed-matrix approach [18-25], because of its good compatibility with polymer matrix, versatile pore structure and functionality, as well as high performance. Our previous works found that introducing hydrophobic metal-organic frameworks (MOFs) could highly improve the flux meanwhile separation factor of PEBA membrane for butanol/water separation [26, 27]. It is attributed to the additional hydrophilicity and 2

4 porous structure resulting from the MOFs. As a large-pore metal-organic zeolite, RHO-[Zn(eim) 2 ] (MAF-6, Heim = 2-ethylimidazole) has been reported to be exceptional hydrophobic. He et al. showed that it can readily adsorb large amounts of organic molecules such as ethanol but barely adsorb water even wetted by water [28]. The hydrophobic MAF-6 has been used to enhance the PV performance of PDMS membrane [29]. Thus, the PEBA mixed-matrix membrane prepared by incorporating MAF-6 can be expected to improve the ethanol recovery performance of PEBA membranes. Furthermore, the preparation of MAF-6 particles is carried out at room temperature which is a timesaving way with lower cost compared to that of most MOFs by hydrothermal synthesis at high temperature and autogenous pressure. On the other hand, for practical implementation, composite or asymmetric membrane is often required to achieve sufficient flux and mechanical strength [30]. Besides of the separation layer, the support layer of composite membrane would affect the membrane integrity, transport and interfacial properties. We [31-33] and other groups [34, 35] have shown that ceramic support with excellent thermal, mechanical and chemical stability, as well as low transport resistance have shown great potential in enhancing interfacial stability and separation performance of the composite membrane. Compared with flat sheet or tube, hollow fiber with higher packing density is regarded as an important industrial-preferred membrane configuration for vapor and gas separation [30, 36-38]. In particular, it s ideal to process inorganic materials (e.g., ceramic) into hollow fiber via a facile phase-inversion and sintering method. In this work, therefore, hydrophobic MAF-6 nanoparticles were synthesized and introduced into PEBA to prepare mixed-matrix separation layer which was dip-coated on a ceramic hollow fiber substrate to form a mixed-matrix composite membrane (as shown in Schematic 1). The mixed-matrix hollow fiber composite membranes were often fabricated by a typical spinning technique that would require carefully optimizing the dope composition and controlling the spinning conditions [39]. To our best knowledge, it s rare to realize mixed-matrix hollow fiber composite membrane via the facile dip-coating approach until now. The prepared novel MAF-6/PEBA mixed-matrix hollow fiber composite membranes (MMHFCMs) were applied for recovering ethanol from aqueous solution by PV process. The morphology, crystal structure, thermal stability, surface hydrophobicity, swelling behavior and interfacial adhesion of the MAF-6/PEBA MMHFCMs were well characterized. The effect of MAF-6 loading, as well as operating conditions such as feed temperature, concentration and long-term operation on the PV performance were also investigated in detail. 3

5 Schematic 1. Structure design of ceramic-supported MAF-6/PEBA mixed-matrix hollow fiber composite membrane (MMHFCMs) and its application for ethanol recovery from aqueous solution. 2. Experimental 2.1. Synthesis of MAF-6 particles MAF-6 particles were synthesized at room temperature using a rapid solution mixing method [28]. A solution of Zn(OH) 2 (2 mmol) in 40 ml of aqueous ammonia solution was added to a solution of 2-ethylimidazole (4 mmol, 0.38 g) in 30 ml of methanol which premixed with cyclohexane. After stirring at room temperature for 0.5 h, the crystals were separated by centrifugation and washed with fresh methanol three times to remove the unreacted components. The white microcrystalline powders were obtained at 120 for 12 h Preparation of membranes The MAF-6 particles were dispersed in butanol solution under vigorous agitation for 2 h. A small amount of PEBA (Pebax 2533 containing 80 wt% of PE and 20 wt% of PA, was purchased from Arkema) (w PEBA : w butanol = 1.6:92) was added into the suspension to first prime the MAF-6 crystals and stirred for another 4 h at 80. Subsequently, the remaining bulk polymer (w PEBA : w butanol = 6.4:92) was added and kept on stirring for another 4 h. The ceramic hollow fibers were prepared and pretreated according our previous work [40]. The average pore size and porosity of the ceramic hollow fiber are 1100 nm and 56.5%, respectively. The mixed matrix membranes were prepared by dip-coating the coating solution on the pretreated ceramic hollow fiber supports. After drying in the atmosphere for 24 h, the composite membranes were heat treated at 70 4

6 for 24 h to ensure the removal of residual solvent. Then the MAF-6 /PEBA MMHFCMs were successfully fabricated. The unfilled PEBA ceramic hollow fiber composite membranes also prepared as described above. The PEBA and MAF-6/PEBA dense films were also prepared for characterization. The as-prepared membranes were stored in a constant temperature and humidity environment before use. The loading of MAF-6 in the membrane was defined as follow: where W MAF-6 is the loading of MAF-6, and are the weights of MAF-6 and PEBA, respectively. The MAF-6 loading of these membranes in this study was varied as 0, 2.5, 5, 7.5, 10 wt% Characterization X-ray diffractometer (XRD, Bruker, D8 Advance) was used to investigate the crystal structures of MAF-6, pure PEBA membrane and MAF-6/PEBA MMHFCMs in the range of 5 40 with an increment of 0.02 at room temperature. The morphologies of MAF-6 and MAF-6/PEBA hollow fiber composite membranes prepared with different MAF-6 loading were examined by field emission scanning electron microscope (FE-SEM, Hitachi-4800, Japan). The average contact angles of these composite membranes were measured by measuring the same sample at three different sites, using contact angle measurement system (DSA100, Kruss). The mechanical properties of the composite membrane were measured by nanoindentation system (NanoTestTM Vantage). The swelling degree of the dense films in different feed concentration was obtained by weighting its own mass and increased mass after swelling in the feed solution. The swelling degree of membranes was measured as below: the dried PEBA and 7.5 wt% MAF-6 filled PEBA dense films were weighted and immersed in the ethanol/water mixtures, respectively. After reaching equilibrium, the films were taken out of the solution and weighted after wiping superfluous liquid with a filter paper. The swelling degree of the membrane can be calculated by following equation: swelling degree = solvent uptake (g)/ weight of dry membrane (g) 100% Measurement of pervaporation A homemade apparatus as our pervious works introduced was employed to perform the PV experiment[41]. The MAF-6/PEBA MMHFCMs was directly put into the feed solution which stored in a feed tank and vacuumed from lumen side. The feed solution was maintained at a preset temperature by immersing the feed tank in a temperature-controlled water bath. The pressure of the permeate side was maintained below 300 Pa by using a vacuum pump. The permeate vapor was collected in cold trap which immersed in liquid nitrogen and at least three samples were collected to ensure the reliability of experimental results. The concentration of ethanol in feed and permeate side was analyzed by a gas chromatography (GC-2014, SHIMADZU, Japan), the weight of permeation samples was determined by electronic analytical balance. The PV performance of a membrane is usually evaluated in terms of the flux (J), 5 (1)

7 separation factor (β) and PV separation index (PSI); (2) where W is the mass of permeate over a permeation time interval of t;a is the effective area of the membrane; X i and Y i are the mass fractions of component i in feed and permeate side, respectively. (3) (4) 3. Results and discussion 3.1. Membrane characterizations The morphology of MAF-6 particles was investigated by SEM and shown in Figure 1a. The synthesized MAF-6 particles have an average size of ~100 nm. These nano-sized MAF-6 are favorable for reducing the minimum thickness required for a defect-free mixed-matrix membrane, thereby fabricating mixed-matrix composite membrane with a thin separation layer [42]. Here, MAF-6/PEBA MMHFCMs were prepared by dispersing MAF-6 particles in PEBA solution and dip-coating the solution on the outer surface of the ceramic hollow fiber. A typical cross-sectional SEM image of the prepared PEBA/ceramic hollow fiber composite membrane is shown in Figure 1b. Finger-like and sponge-like pore structures are observed in the ceramic hollow fiber substrate, which are used to reduce the transport resistance and to provide sufficient mechanical strength, respectively [40]. The thin polymeric coating on the outer surface of the ceramic hollow fiber was seen under higher magnification, as displayed in Figure 1c-d. The polymeric layer, either pure PEBA (Figure 1c) or PEBA filled with MAF-6 (Figure 1d), is firmly deposited on the porous ceramic substrate with a thickness of ~5 µm. There is a transition layer formed by penetration of polymer solution into the ceramic pores during the dip-coating process, providing a good adhesion between the separation layer and support layer [33]. The interfacial adhesion of the composite membrane will be discussed later. 6

8 Fig. 1. SEM images of (a) as-synthesized MAF-6 nanoparticles, cross-section of (b) typical PEBA/ceramic hollow fiber composite membrane and (c) pure PEBA/ceramic hollow fiber composite membrane and (d) MAF-6/PEBA MMHFCM with MAF-6 loading of 7.5 wt%. The arrows in (c-d) indicate the separation layer on top of the ceramic hollow fiber support. Figure 2 shows the SEM surface of the ceramic hollow fiber supported pure PEBA and PEBA mixed-matrix composite membranes with various MAF-6 loading. It is found that the surface of pure PEBA membrane is smooth while that of PEBA mixed-matrix membrane tends to be rougher as the MAF-6 loading increased from 2.5 to 10 wt%. All surface of these membranes looks dense and defect-free. The MAF-6 particles were distributed uniformly in PEBA matrix without agglomeration as its loading is no more than 7.5 wt%. The MAF-6 particles begin to aggregate as the loading increased to 10 wt%. Previous studies have demonstrated that a uniform filler dispersion is crucial for taking advantage of the high-performing filler, while a filler agglomeration in the matrix often declines the membrane selectivity due to the possible non-selective defects within the aggregated domains [43]. 7

9 Fig. 2. Surface SEM images of MAF-6/PEBA MMCHMs with various MAF-6 loading: (a) 0 wt%, (b) 2.5 wt%, (c) 5 wt%, (d) 7.5 wt%, (e) 10 wt%. The crystalline structures of MAF-6 crystals, pure PEBA and MAF-6/PEBA mixed-matrix dense films were characterized by XRD. As shown in Figure 3a, the powder X-ray diffractions (PXRD) of MAF-6 nanoparticles are consistent with its standard patterns [28], indicating a good phase purity. In Figure 3f, the XRD pattern of pure PEBA membrane shows a broad peak without sharp diffraction park because of its amorphous structure [44]. The XRD patterns of MAF-6/PEBA mixed-matrix samples are the superposition of MAF-6 and PEBA samples, in which the MAF-6 crystal peaks match with the pure MAF-6 particles, and become more and more significantly with the increase of loading. It suggests that the MAF-6 crystalline structure remained unchanged after being physically incorporated into PEBA matrix [26, 27], which provide a prerequisite for utilizing the unique pore structure of MAF-6 for molecular transport. Fig.3. XRD spectra of (a) MAF-6; (b)-(e) MAF-6/PEBA mixed-matrix membranes with various loading: (b) 2.5 wt%, (c) 5 wt%, (d) 7.5 wt%, (e) 10 wt% and (f) pure PEBA membrane. 8

10 The thermal decomposition behaviors of the MAF-6 particles, pure PEBA membrane and MAF-6/PEBA mixed-matrix membrane with 7.5% MAF-6 loading were measured by TGA. As shown in Figure 4, the MAF-6 powder shows a slight mass loss from 200 and sharp decrease happens from 500. The decomposition temperature of MAF-6/PEBA MMMs is lower than that of pure PEBA membrane which due to the lower decomposition temperature of MAF-6. As the temperature was over 400 (the decomposition temperature of pure PEBA membrane [20]), the mass loss rate of MAF-6/PEBA mixed-matrix membrane is almost the same as that of pure PEBA membrane because PEBA matrix holds the major weight of the MAF-6/PEBA mixed-matrix membrane. The decomposition temperature of MAF-6/PEBA MMMs is much higher than the operating temperature for ethanol recovery (< 100 ), which indicates that the as-prepared membranes should be thermally stable during the PV process. Fig. 4. TGA cures MAF-6 particles, pure PEBA membrane and MAF-6/PEBA mixed-matrix membrane with 7.5 wt% MAF-6 loading. The PV performance of a membrane for recovering ethanol from water is strongly related to the hydrophobicity and organophilicity of its separation layer [43]. Generally, contact angles were used to characterize the membrane surface property. The hydrophobicity and organophilicity of the MAF-6/PEBA mixed-matrix membranes prepared with various MAF-6 loadings were evaluated by measuring the surface water and ethanol contact angles, respectively. As shown in Figure 5, the water contact angle remarkably increased from 68.6 to 105 with adding MAF-6 up to 10 wt%, which suggests the hydrophobicity of PEBA membrane was significantly improved by introduction of MAF-6 particles. It is attributed to the hydrophobic nature of MAF-6 [28] and the increased surface roughness caused by the introduced MAF-6 particles (Figure 2b-e)[45]. Meanwhile, the MAF-6 filled PEBA membranes with lower ethanol contact angles exhibited more organophilic than the unfilled PEBA membranes, indicating that the affinity of PEBA membrane towards ethanol was effectively enhanced by incorporation of ethanol-adsorptive MAF-6 particles, as well. 9

11 Fig. 5. Ethanol and water contact angles of MAF-6/PEBA mixed-matrix membranes with various MAF-6 loading. The influence of MAF-6 incorporation on the sorption property of PEBA membrane was further studied by measuring the membrane swelling behavior in ethanol/water solution. The ethanol concentration ranges from 2 to 20 wt% that is often found in the ethanol recovery process. As shown in the Figure 6, the swelling degree of the PEBA membrane and MAF-6/PEBA mixed-matrix membrane both increase with ethanol concentration in the solution, confirming the strong affinity of the PEBA-based membranes toward ethanol. Moreover, the swelling degree of MAF-6/PEBA mixed-matrix membrane is higher than that of pure PEBA membrane under each ethanol concentration, which is attributed to the additional sorption capacity of ethanol resulting from the MAF-6 particles. The results from contact angles and swelling measurement indicate that the preferential adsorption of ethanol over water in the PEBA membrane was improved via incorporating the hydrophobic and organophilic MAF-6 particles, which is expected to afford enhanced sorption capacity and ethanol/water selectivity during the PV process according to the solution-diffusion model [46]. Fig. 6. The effect of ethanol concentration on swelling degree of PEBA membrane and MAF-6/PEBA mixed-matrix membrane with 7.5 wt% MAF-6 loading. 10

12 Nano-scratch test was used for measuring the interfacial adhesion force, which is an important parameter to evaluate the mechanical stability of composite membranes [47]. The test was executed under the program as shown in Figure 7a and obtained a 400 μm scratch morphology as shown in Figure 7d. It can be seen in Figure 7b that the MAF-6/PEBA MMHFCM begins to fail with a scratch depth change at 75 μm corresponding to a 5.5μm scratch depth and the SEM of scratch morphology at 75 μm exhibits the substrate exposure, considering the thickness of active layer on the support ~5 μm (Figure 1d), the load of 18.9 mn versus displacement at 75 μm was the critical load of the membrane. On the other words, the interfacial force of the MAF-6/PEBA MMHFCM is 18.9 mn which suggests the ceramic hollow fiber supported mixed-matrix composite membrane possesses a good mechanical stability. Fig. 7. Nano-scratch test of 7.5 wt% MAF-6/PEBA MMHFCM: (a) scratch load-displacement curve; (b) scratch profile; (c) and (d) SEM images of scratch morphology Membrane PV performance Effect of MAF-6 loading Figure 8 shows the effects of MAF-6 loading on PV performance of MAF-6/PEBA MMHFCMs for ethanol recovery from 5 wt% ethanol aqueous solution at 40. The permeation fluxes of all the MMMs composite membranes are higher than the unfilled PEBA membrane and increase with the MAF-6 loading. It is attributed to the strong interaction of MAF-6 and ethanol, which was proved by the contact angle and swelling measurements. Moreover, the molecular kinetic diameter of ethanol (4.3 Å) is 11

13 less than the pore size (18.4 Å) or aperture size (7.6 Å) of MAF-6 [28]. Therefore, the diffusion of ethanol in the mixed-matrix membrane is also increased with MAF-6 loading. Both factors lead to the increase of permeation flux. The separation factor of the composite membranes increased with the MAF-6 loading firstly. As a type of highly hydrophobic porous material, MAF-6 showed a very weak interaction with water molecules while much higher ethanol solubility. Thus, the sorption selectivity of ethanol to water can be improved by incorporating MAF-6 particles into PEBA matrix. Furthermore, MAF-6 can create a preferential pathway for ethanol permeation due to the preferential sorption of alcohol, forcing water molecules to move around MAF-6 and primarily permeate through polymer phase. The diffusion resistance of water is higher than that of ethanol, which also increases the separation factor. While the separation factor of the membrane with MAF-6 loading of 10 wt% is lower than that of the membrane with MAF-6 loading of 7.5 wt%, it is because of the agglomeration of MAF-6 particles as shown in SEM image (Figure 2e). Considering the combination of permeate flux and separation factor, 7.5 wt% was chosen as the optimum MAF-6 loading in the following study. It is worth noting that the flux and separation factor of MMMs increased simultaneously compared with pure PEBA membrane, breaking the trade-off limitation in conventional polymeric membranes. Fig. 8. Effect of MAF-6 loading on PV performance of MAF-6/PEBA MMHFCMs for ethanol recovery from 5 wt % aqueous ethanol solution at Effect of feed temperature The effect of feed temperature on the PV performance the 7.5 wt% MAF-6/PEBA MMHFCM was investigated in the 5 wt% ethanol/water mixtures, as shown in Figure 9. As the feed temperature increased from 25 to 60, the total permeation flux of MMMs increased from 988 to 4446 g/m 2 h. In general, the temperature dependence of the total or 12

14 partial flux (ethanol or water) can be analyzed by the Arrhenius equation [48]: where J i is the partial flux of component i; J 0 is the pre-exponential factor; E a is the apparent activation energy for permeation; R is the universal gas constant and T is the operating temperature in Kelvin. (5) Fig. 9. The effect of feed temperature on PV performance of MAF-6/PEBA MMHFCM with MAF-6 loading of 7.5 wt%. As an important parameter to indicate the effect of temperature on flux, the apparent activation energy can be calculated from logarithmic plot of permeation flux versus reciprocal of absolute temperature. As shown in Figure 10, the average activity energies of ethanol and water permeation through MAF-6/PEBA MMHFCM are 44.1 and 33.7 kj/mol, respectively. The positive values of activation energies reflect that the permeation flux increasing with temperature. Higher vapor pressure difference across the membrane can be obtained at higher temperature, which enhanced the driving forces of permeant transport. Moreover, the growth in temperature increased the thermal motions of PEBA chains and free volume of the segments. As a result, the diffusion coefficients of both ethanol and water molecules were improved leading to an increase in total flux. Usually, lower selectivity or separation factor is obtained at higher feed temperature due to the excessive swelling of the polymer layer. While it is interesting to find that the separation factor of the MAF-6/PEBA MMHFCM was enhanced with increasing the feed temperature (Figure 9). This is presumably to confined PEBA swelling provided by the rigid ceramic substrate that is not swelled under high temperature and thus restricts excessive swelling of the PEBA penetrated in the ceramic pores[49]. Also, the incorporated MAF-6 particles could prohibit the swelling of PEBA matrix[50]. The temperature-dependent study suggests that a higher productivity could be efficiently 13

15 achieved by simply increasing the feed temperature, which is a unique feature of the MAF-6/PEBA MMHFCMs for practical application. Fig. 10. Logarithmic plots of fluxes versus reciprocal of absolute temperature for MAF-6/PEBA MMHFCM with MAF-6 loading of 7.5 wt% Effect of feed concentration The effect of ethanol concentration in the feed on the PV performance of the 7.5 wt% MAF-6/PEBA MMHFCM was investigated at 40 (Figure 11). As the ethanol concentration increased, the total flux increased from 988 to 3720 g/m 2 h, with almost linear relationship. The possible reason can be that the concentration gradient between liquid and vapor phase was enlarged as increasing feed concentration, leading to greater driving force in separation process. At the same time, the swelling degree of the separation layer was enlarged at higher ethanol concentration (Figure 6), resulting in a larger free volume within the membrane. Therefore, the permeation flux increased significantly with feed concentration. Since the molecule size of water is smaller than that of ethanol, in the membrane with enlarged free volumes the flux enhancement for water is larger than that for ethanol. Thus, the separation factor of the membrane decreased at higher ethanol concentration. 14

16 Fig. 11. The effect of feed concentration on PV performance of MAF-6/PEBA MMHFCM with MAF-6 loading of 7.5 wt% Long-term stability and performance comparison Long-term stability of the 7.5 wt% MAF-6/PEBA MMHFCM in ethanol-water feed system was examined at 40, as shown in Figure 12. The total flux and separation factor of the membrane kept stable during the continuous operation of more than 200 h. In general, the operation pressure of most PV process is high-vacuum which might distort or even destroy the structure of soft PEBA membranes. Compared with organic substrate, the ceramic hollow fiber with a rigid structure can provide excellent mechanical stability for the composite membrane under negative pressure. Moreover, the appropriate penetration of coating solution into the ceramic hollow fiber pores formed a favorable interface between support and polymer layer which benefits good interfacial adhesion properties of the mixed-matrix composite membrane [33]. As a result, the MAF-6/PEBA MMHFCM shows a desirable long-term operation stability. 15

17 Fig. 12. Long-term PV operation test of MAF-6/PEBA MMHFCM with MAF-6 loading of 7.5 wt%. Compared with the reported PEBA-based membranes for ethanol/water separation, our MAF-6/PEBA MMHFCMs exhibit excellent PV performance, as shown in Table 1. With the same or even higher separation factor, our membrane has much higher flux, which is owing to the excellent hydrophobicity and pore structures of the MAF-6 fillers used in this work. Also, the low transport resistance and high packing density, resulting from the ceramic hollow fiber substrate, would allow a high productivity for practical implementation of this new MAF-6/PEBA mixed-matrix membrane. Table 1. PV performance comparison of PEBA-based membranes for ethanol recovery from aqueous solution. Membrane Feed Feed composition Total flux Separation PSI temperature ( ) (wt% ethanol) (g/m 2 h) factor (g/m 2 h) Reference PEBA membrane Silicalite-filled PEBA /PAN membrane POSS/AL0136-filled PEBA membrane MAF-6/PEBA MMHFCM This work MAF-6/PEBA MMHFCM This work 4. Conclusion In this study, a new type of MAF-6/PEBA mixed-matrix composite membranes has 16

18 been successfully fabricated onto a ceramic hollow fiber substrate via a facile dip-coating method. The surface hydrophobicity/organophilicity and ethanol preferential sorption of PEBA membrane were highly enhanced by introducing the hydrophobic and ethanol-philic MAF-6 nanoparticles. The total flux and separation factor were simultaneously improved by uniformly dispersing the MAF-6 in PEBA matrix. The optimized MAF-6/PEBA mixed-matrix hollow fiber composite membrane show an excellent and stable PV performance for recovering ethanol from aqueous solution: total flux of 4446 g/m 2 h and ethanol/water separation factor of 5.6 for 5 wt% ethanol/water in the feed at 60. The new-developed hollow fiber supported MAF-6/PEBA mixed-matrix membrane could be a promising candidate for biofuels production. Acknowledgements This work was financially supported by the National Key Basic Research Program (2017YFB ), National Natural Science Foundation of China (Grant Nos , , , , ), the Innovative Research Team Program by the Ministry of Education of China (Grant No. IRT17R54) and the Topnotch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP). Reference [1] Y. Lin, S. Tanaka, Ethanol fermentation from biomass resources: current state and prospects, Applied Microbiology and Biotechnology 69 (2006) [2] A. Jonquieres, R. Clement, P. Lochon, J. Neel, M. Dresch, B. Chretien, Industrial state-of-the-art of pervaporation and vapour permeation in the western countries, Journal of Membrane Science 206 (2002) [3] G. Liu, W. Wei, W. Jin, Pervaporation Membranes for Biobutanol Production, ACS Sustainable Chemistry & Engineering 2 (2014) [4] Y.K. Ong, G.M. Shi, N.L. Le, Y.P. Tang, J. Zuo, S.P. Nunes, T.-S. Chung, Recent membrane development for pervaporation processes, Progress in Polymer Science 57 (2016) [5] X. Zhan, J. Lu, T. Tan, J. Li, Mixed matrix membranes with HF acid etched ZSM-5 for ethanol/water separation: Preparation and pervaporation performance, Applied Surface Science 259 (2012) [6] X. Tang, R. Wang, Z. Xiao, E. Shi, J. Yang, Preparation and pervaporation performances of fumed-silica-filled polydimethylsiloxane-polyamide (PA) composite membranes, Journal of Applied Polymer Science 105 (2007) [7] T. Ikegami, H. Yanagishita, D. Kitamoto, H. Negishi, K. Haraya, T. Sano, 17

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23 Highlights Hollow fiber supported mixed-matrix composite membranes were developed MAF-5/PEBA mixed-matrix membranes were fabricated for the first time As-prepared membranes exhibited high performance for ethanol recovery 22

24 Graphic Abstract 23