PES–PI blend composite hollow fibers

PES–PI blend composite hollow fibers

Author's Accepted Manuscript High performance zeolite NaA membranes synthesized on the inner surface of zeolite/ PES-PI blend composite hollow fibers...

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Author's Accepted Manuscript

High performance zeolite NaA membranes synthesized on the inner surface of zeolite/ PES-PI blend composite hollow fibers Zhiying Zhan, Jia Shao, Yong Peng, Zhengbao Wang, Yushan Yan

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PII: DOI: Reference:

S0376-7388(14)00643-7 http://dx.doi.org/10.1016/j.memsci.2014.08.027 MEMSCI13139

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Journal of Membrane Science

Received date: 24 June 2014 Revised date: 8 August 2014 Accepted date: 13 August 2014 Cite this article as: Zhiying Zhan, Jia Shao, Yong Peng, Zhengbao Wang, Yushan Yan, High performance zeolite NaA membranes synthesized on the inner surface of zeolite/PES-PI blend composite hollow fibers, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2014.08.027 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 galley proof before it is published in its final citable 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.

High performance zeolite NaA membranes synthesized on the inner surface of zeolite/PES-PI blend composite hollow fibers

Zhiying Zhan a, Jia Shao a, Yong Penga, Zhengbao Wang a,*, Yushan Yan a,b

a

Department of Chemical and Biological Engineering, and MOE Engineering

Research Center of Membrane and Water Treatment Technology, Zhejiang University, Hangzhou 310027, PR China b

Department of Chemical and Biomolecular Engineering, University of Delaware,

Newark, DE 19716, USA Corresponding author * Fax/Tel.: +86-571-8795-2391; E-mail: [email protected]

Abstract

Zeolite/polymer blend composite hollow fibers are prepared as a new type of zeolite membrane supports by the dry-wet phase inversion method. The spinning slurry is formed by mixing zeolite NaA crystals with miscible polyethersulfonepolyimide polymer blend. Zeolite NaA membranes are hydrothermally synthesized in the lumen side of the hollow fibers under dynamic conditions. Their properties are characterized by scanning electron microscopy, mechanical test, contact angle test and pervaporation. The effects of the N2 extrusion pressure and polyimide content on 1

properties of the supports are investigated. Both of them limit the exchange rate between solvent and non-solvent during spinning process resulting in the supports with high mechanical properties. Zeolite membranes are thin and well intergrown with no macroscopic defects. The water flux is 10.6±0.4 kg m-2 h-1 with a separation factor of more than 10,000 in dehydration of a 90 wt% ethanol aqueous solution at 75 o

C. The effect of hydrophobicity of the composite supports is discussed in the context

of advantages of the supports. Keywords: zeolite membrane, polyethersulfone-polyimide blend, hollow fiber, pervaporation

1. Introduction Zeolite membranes have been shown to be effective in dehydration of organic solvents by pervaporation [1,2]. Limited commercial success was achieved after much effort was dedicated to the optimization and scale-up of membrane synthesis [3-7]. For example, the first industrial zeolite NaA membrane separation plant was developed by Mitsui Engineering and Shipbuilding Co. Ltd. in Japan in 1998 [4]. Since then, many works have been carried out to improve the separation performance of zeolite NaA membranes [8,9], increase the reproducibility of the membrane synthesis, and decrease the cost of the membranes [2,10]. Since the membrane cost is dominated by support cost rather than the cost of zeolite layer [3], an appropriate choice of the support is important. Up to now, porous ceramic (α-Al2O3 or mullite) tubes or discs have been the most commonly used 2

supports for zeolite NaA membrane synthesis [10-13]. Great progress has been made recently in the development of hollow fiber supports and they now show high potential to replace the tubes [1,8,14]; compared with ceramic tubes, hollow fibers (1) offer higher pack density and thus higher membrane surface per module volume; (2) use less materials per membrane area and thus cost less; and (3) present lower support resistance and thus higher fluxes. Among hollow fibers, zeolite/polymer composite hollow fiber (CHF) supports are more economical than ceramic hollow fiber supports because high temperature sintering (1400-1600 oC) is eliminated from the preparation process of the CHF supports. In addition, the CHF supports have uniform distribution of zeolite NaA crystals on their surface so that the seeding step can be omitted. Our group has reported that high-performance zeolite NaA membranes can be obtained on the outer surface of CHF supports, which have 70-85 wt% zeolite crystals in the polymer matrix [2,8]. CHF supports can be considered as a special type of polymer-zeolite mixed matrix membranes but their zeolite content is much higher than that in the usual mixed matrix membrane (<40 wt%). Polymer-zeolite mixed matrix membranes have been widely used for gas separations [15,16] and many polymers, such as polyethersulfone (PES) [17], polyimide (PI) [18], polyvinylidenefluorides (PVDF) [19,20], polydimethylsiloxane (PDMS) [21,22], and polysulfone (PSF) [23,24], have been chosen to fabricate mixed matrix hollow fiber membranes. Zeolite NaA membranes are most often synthesized on the outer surface of the tubes or hollow fibers, although the inner surface of the supports is more preferable 3

because the zeolite layer is better protected from mechanical impact damages and less susceptible to cracking due to thermal expansion mismatch between the zeolite layer and the support. Zeolite NaA membranes on the inner surface of the Al2O3 and TiO2 tubes were studied by Titus et al. [25-28] and Santamaria et al. [29], but the separation performances of these membranes were far from being satisfactory. While high separation performance could be achieved when the zeolite NaA membrane was synthesized on the inner surface of ceramic hollow fibers [30], it turned out that zeolite NaA membrane prepared on PES CHF supports was unable to withstand the pervaporation tests due to their weak mechanical strength. Therefore, it is of vital importance to prepare a new-type zeolite/polymer CHF support with high enough mechanical strength for pervaporation characterization of zeolite NaA membranes synthesized on the inner surface. Polymer blends combine synergistically the advantages of component polymers leading to membranes with improved performance [24,31-34]. Polyethersulfone (PES) has high chemical resistance and thermal stability, and polyimide (PI) exhibits high temperature resistance, toughness and other good properties. The thermal and rheological properties of PES-PI blends were studied [31], and it is found that blending PES with PI increased the complex viscosity and the elastic modulus of the polymer blends, which were due to the formation of a network structure because of phase separation during spinning process and the crosslink between PES with PI. It is certain that polyimide in the PSF-PI blends due to PSF and PI interactions and mixing at the molecular level offered additional thermal stability and chemical resistance [32]. 4

Effects of the spinning conditions on PES-PI hollow fibers structure and permeation were investigated and chain entanglement of the polymer blend was suggested by Koops et al. [33]. Many researches of the mixed matrix membranes about PES-PI blends were also reported [35-40]. In this paper, zeolite/polymer blend (PES-PI) composite hollow fibers (BCHFs) are prepared by dry-wet phase inversion method, and zeolite NaA membranes are synthesized on their inner surface under dynamic condition. The effects of N2 extrusion pressure and the polyimide content are investigated on the properties of the BCHF supports and the pervaporation performance of zeolite NaA membranes in dehydration of ethanol-water mixture. The BCHF supports are expected to have better mechanical properties through blending PES with PI. And this is the first time that zeolite/polymer blend composite hollow fibers are used as supports to synthesize zeolite NaA membranes on the inner surface of the hollow fibers. This will broaden the family of zeolite membrane supports and have significant implications in the study of zeolite membranes.

2. Experimental 2.1 Materials NaA zeolite (Luoyang Jianlong Chemical Industrial Co. Ltd.), Gafone-3000p Polyethersulfone (PES, Solvay Advanced Polymers), thermoplastic polyimide powder SS100P (PI, Hangzhou Surmount Science & Technology Co. Ltd.), N-methyl-2pyrrolidone (NMP, Lingfeng Chemical Reagent Co. Ltd.), sodium metasilicate 5

nonahydrate (Si source, Wako Pure Chemical Industries Ltd.), sodium aluminate (Al source, Wako), sodium hydroxide (>96%, Sinopharm Chemical Reagent Co. Ltd.) and ethanol (>99.7%, Sinopharm) were purchased as chemical reagents and used as received, while deionized water was self-produced in the laboratory.

2.2 Fabrication of zeolite/polymer blend composite hollow fibers The detailed spinning procedure has been reported previously [8,9]. Briefly here a certain amount of PES and PI was first dissolved in NMP followed by stirring for at least 6 h in a three neck flask to form homogeneous solution. Then dry zeolite NaA powders (4 µm, weight ratio of NaA/polymer: 85/15) were slowly added into the solution under agitation to form slurry, and the slurry was stirred for more than 18 h to disperse zeolite NaA crystals. Before spun, air bubbles in the slurry were removed by evacuation and then transferred to the spinning slurry tank. There still needed 30 minutes under vacuum to completely remove air bubbles produced during the transfer. Here we applied dry-wet phase inversion method [41] to spin the zeolite/polymer BCHFs, and the equipment was shown previously [8]. After the spinning slurry came out of the spinneret under N2 extrusion pressure in the range of 0.05 MPa to 0.15 MPa, the nascent fibers passed a 2-cm air gap before entering into the coagulant bath (tap water), and the bore fluid (deionized water) was simultaneously pumped at a volumetric flow rate of 1200 ml h-1 to exchange with solvent (NMP) in the spinning slurry. The nascent fibers were kept in the coagulation bath for more than 24 h to remove residual NMP and thoroughly set the geometry, and then these fibers were 6

dried at 100 oC for more than 4 h before use. We prepared zeolite/polymer BCHFs with different contents of polyimide (BCHF-n%PI, n=0, 10 and 20, where n% is the weight percentage of PI in PES-PI), and different N2 extrusion pressures (BCHF-n%PI-Nm, m=0.05, 0.07 & 0.09 MPa). For example, BCHF-10%PI-N0.05 means the PI content of 10 % and the N2 extrusion pressure of 0.05 MPa.

2.3 Synthesis of inner-side zeolite NaA membranes The synthetic mixture was prepared by mixing an aluminate solution with a silicate solution. The aluminate solution was made by dissolving 1.55 g of NaAlO2 in 28 g of deionized water under stirring, and the silicate solution was made by dissolving 3.98 g of Na2SiO3.9H2O and 2.0 g of NaOH in 45.33 g of deionized water. After adding the aluminate solution into the silicate solution under vigorous stirring, the mixture was aged for 3 h at room temperature and the final solution has a molar composition of Na2O:SiO2:Al2O3:H2O=7.5:2:1:600. After that, the synthetic mixture was transferred into an autoclave where the supports stood vertically in a holder. All supports were wrapped with Teflon tape on their outer surfaces before put into the autoclave to avoid any zeolitic deposition on the outer surfaces. Hydrothermal synthesis was carried out at 100 oC for 5 h in a rotation oven and the autoclaves were rotated at 30 rpm. After crystallization, the membranes were washed several times by deionized water and then dried at 60 oC for 4 h.

7

2.4 Characterization of the BCHFs and zeolite membranes The mechanical properties of the BCHFs were measured by three-point bending test and compressive test using SJ-018 test bed, SPJ-B hand-rack-stand and SP-50 tension-meter (Wenzhou SUNDOO Instrument Co. Ltd., China). The bending strength was calculated by the equation in [9] as follows: σF=(8F1LD)/[π(D4-d4)]

(1)

Where σF is the bending strength (MPa), F1 is the measured force (N), L, D and d are the length (m), outside diameter (m) and inside diameter (m), respectively. While compressive strength was calculated according to its definition [P=F2/S where P is the compressive strength (MPa), F2 is the compressed force (N) and S is the compressed area (m2)]. The surface hydrophobicity of the BCHFs was determined by contact angle measurement (Data Physics, OCA 20). The structure of zeolite/polymer blend composite hollow fibers and the morphology of synthesized membranes were characterized by scanning electron microscopy (SEM, HITACHI SU-70). The zeolite membranes were also characterized by pervaporation of 90 wt% ethanol-water mixture at 75 oC. The setup was the same as previously reported [30]. The zeolite membrane synthesized on the BCHF support was pasted into a module and then connected to the vacuum system. The feed solution was circulated through the lumen of the support with a peristaltic pump at a flow rate of 100 ml min-1. A cold trap in liquid N2 was used to collect the permeate after the ethanol solution was flowed in the module for 10 min. The time for collecting the permeate was 15 min. The 8

compositions of the feed and the permeate were analyzed by gas chromatography (GC-1690, Kexiao Co. in Hangzhou). The two most important variables were flux (J) and separation factor (α), which were defined as J=W/(A

×t)

α=(YH2O/YEtOH)/(XH2O/XEtOH)

(2) (3)

Where W is the total weight of the permeation (kg), A is the separation area of the membrane (m2), t is the collection time (h), YH2O/YEtOH is the weight ratio of water to ethanol in the permeation and XH2O/XEtOH is the weight ratio of water to ethanol in the feed.

3. Results and discussion 3.1 Properties of the zeolite/polymer blend composite hollow fibers Effects of polyimide content and N2 extrusion pressure on properties of the NaA zeolite/PES-PI polymer blend composite hollow fibers (BCHFs) were investigated. As shown in Fig. 1, the cross-sectional morphologies of the BCHFs with different PI contents and N2 extrusion pressures were characterized by SEM. In order to describe the morphologies distinctly, the cross section of a BCHF support was divided into three parts as shown in Fig. 2, where S1, S2 and S3 referred to poor finger-like region, spongy region and rich finger-like region, respectively. From Fig. 1, it is observed that the proportion of the three regions in the BCHFs (BCHF-0%PI, BCHF-10%PI and BCHF-20%PI) altered with the contraction of S1 and S3 and enlargement of S2 as the N2 extrusion pressure was increased. This is because that the nascent fiber spent less 9

time in the air gap region before entering into coagulant bath, resulting in faster solidification of the nascent fiber. This limits the solvent/non-solvent exchange rate on the lumen side, resulting in the decrease of S3; that is, S3 contracted and part S2 enlarged. The BCHF-0%PI showed rich finger-like region (S3) and spongy region (S2), but no poor finger-like region (S1) in the cross section (Fig. 1a1-a3), and there was still much S3 in BCHF-0%PI when the N2 extrusion pressure increased to 0.09 MPa.

Fig. 1. SEM images of the cross section of zeolite/polymer blend composite hollow fibers fabricated with different PI contents [(a) 0 %, (b) 10 % and (c) 20 %] and N2 extrusion pressures [(a1-c1) 0.05 MPa, (a2-c2) 0.07 MPa and (a3-c3) 0.09 MPa].

10

Fig. 2. Structure analysis of the cross section of the BCHFs. S1 the poor finger-like region, S2 the spongy region, and S3 the rich finger-like region.

It is also found from Fig. 1 that, when the N2 extrusion pressure was fixed, S2 enlarged and S3 contracted with the increase of PI content. At 20 % of PI content, S3 decreased significantly, resulting in much spongy region. It is generally known that the viscosity of polymer blends is higher than that of pure polymer. In other words, the viscosity of spinning slurry will increase when polyethersulfone crosslinks with polyimide. Therefore, the exchange rate between solvent and non-solvent in the lumen side is slowed down, resulting in the slow growth of finger-like pores in S3. Finally, more S2 is formed. The effect of PI content is very similar to that of N2 extrusion pressure discussed above.

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Fig. 3. Photo images of (a) BCHF-0%PI bended to a small angle, (b) BCHF-10%PI bended, and (c) broken BCHF-10%PI after bending.

As shown in Fig. 3, the BCHF-0%PI can be bended to a small angle without being broken, which means that the BCHFs without polyimide have high elasticity. However, the BCHF-10%PI will break when bended a small angle. The mechanical properties of the zeolite/polymer BCHFs with different PI contents and N2 extrusion pressures are presented in Tables 1 and 2. The bending strength was calculated according to Equation 1. As indicated in Table 1, the bending strength of the BCHFs increases with the increase of N2 extrusion pressure. For example, the bending strength of BCHF-0%PI increases from 5.59 to 8.93 MPa when the N2 extrusion pressure increases from 0.05 to 0.09 MPa, and the bending strength of BCHF-10%PI 12

correspondingly increases from 6.49 to 10.11 MPa. The bending strength also increases with the PI content. For example, the bending strength of the BCHFs increases from 8.93 to 9.50 and 12.59 MPa, respectively, when the PI content increases from 0 to 10 % and 20 % at 0.09 MPa of N2 extrusion pressure. As described above, the spongy region (S2) enlarges and finger-like pore region (S1 and S3) contracts with increasing PI content and N2 extrusion pressure. This is the reason that the bending strength increases with the PI content and N2 extrusion pressure. Table 1. Effects of N2 extrusion pressure and PI content on three-point bending strength (MPa) of zeolite/polymer BCHFs BCHF-0%PI

BCHF-10%PI

BCHF-20%PI

BS

AS

BS

AS

BS

AS

0.05

5.59

5.25

6.49

5.83

8.31

6.90

0.07

7.92

7.18

8.93

7.68

10.35

7.94

0.09

8.93

7.93

10.11

9.50

12.59

9.82

N2 Pressure (MPa)

Notice: BS before synthesis and AS after synthesis of membrane. Table 2. Effects of N2 extrusion pressure and PI content on compressive strength (MPa) of zeolite/polymer BCHFs BCHF-0%PI

BCHF-10%PI

BCHF-20%PI

BS

AS

BS

AS

BS

AS

0.05

0.48

0.61

0.55

0.74

1.06

1.49

0.07

0.90

1.26

1.21

1.88

1.57

2.30

0.09

1.29

1.60

1.67

2.26

2.02

3.03

N2 Pressure (MPa)

13

Notice: BS before synthesis and AS after synthesis of membrane.

Moreover, as shown in Table 2, the compressive strength of the BCHFs also increases with the N2 extrusion pressure and PI content. For example, the compressive strength of BCHF-10%PI increases from 0.55 to 1.67 MPa with the increase of N2 pressure from 0.05 to 0.09 MPa; at a N2 pressure of 0.09 MPa, it increased from 1.29 to 2.02 MPa with the increase of PI content from 0 to 20 %. These results may also be explained by the enlargement of spongy region (S2) and contraction of the finger-like pore regions (S1 and S3). This further confirms the positive effects of increasing N2 extrusion pressure and addition of polyimide on the mechanical strength of the BCHFs. Based on the performance of the BCHFs in the zeolite membrane synthesis and subsequent separation processes, we propose that the minimum three-point bending strength and compressive strength of the BCHFs for pervaporation membrane use are 8.0 MPa and 1.2 MPa, respectively. 3.2 Membrane synthesis and its characterization As shown in Fig. 4, all BCHFs have similar numbers of zeolite NaA crystals on the inner surface, and the distribution of zeolite crystals on the surface is not affected by the N2 extrusion pressure and PI content. The inner-side zeolite NaA membranes were synthesized under dynamic condition in a rotating oven (30 rpm). The morphologies of the synthesized inner-side zeolite NaA membranes are displayed in Fig. 5. The zeolite films of Fig.5a-d are well intergrown and no macroscopic defects, while those in Fig.5e and 5f show some intercrystalline pores. The thickness of these 14

zeolite membranes is 4-5 µm as indicated in Fig. 6.

Fig. 4. SEM images of the inner surface of different zeolite/polymer blend composite hollow fibers with different PI contents and N2 extrusion pressures: (a) BCHF-0%PI-N0.07, (b) BCHF-0%PI-N0.09, (c) BCHF-10%PI-N0.07, (d) BCHF-10%PI-N0.09, (e) BCHF-20% PI-N0.07 and (f) BCHF-20%PI-N0.09.

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Fig. 5. SEM images of the inner-side zeolite NaA membranes synthesized on different BCHF supports: (a) BCHF-0%PI-N0.07, (b) BCHF-0%PI-N0.09, (c) BCHF-10%PI-N0.07, (d) BCHF-10%PI-N0.09, (e) BCHF-20%PI-N0.07 and (f) BCHF-20%PI-N0.09.

16

Fig. 6. SEM images of cross section of the inner-side zeolite membranes [(a) BCHF-0%PI-N0.09, (b) BCHF-10%PI-N0.09, (c) BCHF-20%PI-N0.09] and (d) zeolite crystals grown up in rich finger-like region (S3).

The mechanical properties after membranes synthesis were also measured (Tables 1 and 2). The bending strength and compressive strength increase with the increase of either the N2 pressure or PI content after membrane synthesis. However, the bending strength after membrane synthesis (AS) tended to be lower than that before membrane synthesis (BS). For instance, the bending strength of BCHF-0%PI decreases from 8.93 MPa (BS) to 7.93 MPa (AS) at 0.09 MPa of N2 extrusion pressure. It is found that the decrement of bending strength from BS to AS increases with the PI content. For example, the bending strength of BCHF-10%PI declines by 0.61 MPa from 10.11 17

MPa (BS) to 9.50 MPa (AS) and that of BCHF-20%PI declines by 2.77 MPa from 12.59 MPa (BS) to 9.82 MPa (AS). The decreasing bending strength was consistent with our previous work [8] in which the bending strength of the composite hollow fiber after membrane synthesis decreased due to the brittle zeolite layer synthesized on the outer surface of the hollow fiber. It is observed that the zeolite NaA crystals in S3 of the BCHFs grew larger after membrane synthesis (Fig. 6d, the arrow direction) due to the penetration of the synthesis solution into S3 during the membrane synthesis. And it is suggested that the partially crystallized zeolite NaA crystals in S3 probably embrittled the supports, contributing to the decrease of the bending strength. Besides, the bending strengths of BCHF-10%PI and BCHF-20%PI after membrane synthesis are close to each other at 0.07 or 0.09 MPa of N2 extrusion pressure in Table 1, suggesting that there is no significant improvement in the bending strength of BCHFs (BS) by increasing the PI content. On the other hand, as indicated in Table 2, the compressive strength increases after membrane synthesis. For example, at 0.09 MPa of N2 extrusion pressure, the compressive strength of BCHF-0%PI increases from 1.29 MPa (BS) to 1.60 MPa (AS), and that of BCHF-10%PI increases from 1.67 MPa (BS) to 2.26 MPa (AS). It is possible that the intergrown zeolite NaA layer in the lumen side acts as an annular upholder to increase the compressive strength and the seeded growth of the crystals in S3 also helps to stiffen the BCHF.

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Table 3. Pervaporation properties of the inner-side zeolite NaA membranes on the zeolite/polymer BCHFs Membrane

Support

J (kg m-2 h-1)

α (-)

M1/M2

BCHF-0%PI-N0.07/0.09

-

-

M3

BCHF-10%PI-N0.07

10.28

>10,000

M4

BCHF-10%PI-N0.09

10.92

>10,000

M5

BCHF-20%PI-N0.07

10.10

2,615

M6

BCHF-20%PI-N0.09

7.90

2,644

The separation performance was measured by pervaporation of the 90/10 wt% ethanol/water mixture and the results are listed in Table 3. The pervaporation performance of membranes M1 and M2 could not be tested due to the weak mechanical properties of the BCHF-0%PI supports. The fluxes of membranes M3 and M4 synthesized on the BCHF-10%PI are 10.28 and 10.92 kg m-2 h-1, respectively, and the separation factors are greater than 10,000. When using BCHF-20%PI as the support, the flux of the M6 is 7.90 kg m-2 h-1, which is lower than 10.10 kg m-2 h-1 of the M5. The pervaporation flux of zeolite membranes increases with the porosity of the supports [1], and therefore, the lower flux of the M6 is likely due to the lower porosity of the BCHF-20%PI-N0.09 support (~35% versus >40% of other supports). The separation factors of the M5 and M6 in Table 3 are 2,615 and 2,644, respectively, which are lower than that of the M3 and M4. To understand the relatively low selectivity of M5 and M6, the contact angles of the BCHFs were measured and then shown in Fig.7. It took less than 25 seconds for water to totally 19

enter into the BCHF-10%PI, while at least 75 seconds into the BCHF-20%PI. The difference suggests that incorporation of more PI decreases the hydrophilicity of the BCHF support. Consequently, it is more difficult for the synthetic mixture to reach the inner surface and part S3 of the BCHF-20%PI than BCHF-10%PI in the dynamic synthesis. Thus, it is harder to obtain a perfect zeolite membrane layer on the inner surface of the BCHF-20%PI. This is also in agreement with the membrane morphologies indicated in Fig. 5e and 5f. Compared with the results in literature as shown in Table 4, it is clear that inner-side zeolite membranes in this work have the highest pervaporation flux with the separation factor of more than 10,000. This is due to the sufficient porosity of the BCHF-10%PI.

Fig. 7. Contact angles of the BCHF supports fabricated at different N2 extrusion pressures. 20

Table 4. Comparison of the pervaporation properties of inner-side zeolite NaA membranes Support

Ca (wt %)

Tb (oC)

J (kg m-2 h-1)

αc (-)

Ref.

α-Al2O3 tube

92.0

50

0.5

600

[25]

α-Al2O3 tube

90.0

50

0.43

16,222

[27]

TiO2

92.0

50

0.8-1.0

8,500

[28]

α-Al2O3 tube

91.8

93

2.5

130

[29]

α-Al2O3 hollow fiber

90.0

75

6.9

>10,000

[30]

BCHF-10%PI-N0.07

90.0

75

10.28

>10,000

This Study

BCHF-10%PI-N0.09

90.0

75

10.92

>10,000

This Study

a

tube

Concentration of ethanol-water mixture, b PV temperature, c separation factor.

4. Conclusions Zeolite/polymer blend (PES-PI) composite hollow fibers (BCHFs) were prepared as a new type of zeolite membrane supports by the dry-wet phase inversion method. Effects of the N2 extrusion pressure and PI content were investigated. The increase of N2 extrusion pressure and polyimide content decreased the exchange rate between solvent and non-solvent in the lumen side, resulting in contraction of finger-like pore region (S1 and S3) and enlargement of spongy region (S2). The increase of the spongy region will lead to higher mechanical strength. It is concluded that 0.07-0.09 MPa of N2 extrusion pressure range and 10 wt% of polyimide content are optimum conditions for the mechanical strength of the BCHF supports and the pervaporation performance of the inner-side zeolite membranes. The inner-side zeolite NaA 21

membranes can be synthesized by a simple hydrothermal synthesis in a rotating oven. And the zeolite NaA membranes in this work show the highest flux of >10.0 kg m-2 h-1 with the separation factor of more than 10,000 in dehydration of 90 wt% ethanol aqueous solution by pervaporation at 75 oC. This new support can reduce the membrane cost and will open a new way to prepare supports for zeolite membranes.

Acknowledgement The authors would like to thank the National Natural Science Foundation of China (21236006), the National Basic Research Program (2013CB228104), Science and Technology Department of Zhejiang Province (2009R50020), and open research fund of Top Key Discipline of Chemistry in Zhejiang Provincial Colleges and Key Laboratory of the Ministry of Education for Advanced Catalysis Materials (Zhejiang Normal University) for financial supports.

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Nomenclature F1 F2 L D d W A t J C T P S YH2O/YEtOH XH2O/XEtOH

bending force (N) compressed force (N) length of support (m) outside diameter of support (m) inside diameter of support (m) permeation water mass (kg) permeation area of membrane (m2) permeation time (h) permeation flux for water (kg m-2 h-1) concentration of ethanol-water mixture (wt %) pervaporation temperature (oC) compressive strength (MPa) compressed area (m2) weight ratio of water to ethanol in the permeation (%) weight ratio of water to ethanol in the feed (%)

Greek symbols σF bending strength (MPa) α pervaporation separation factor

Captions Fig. 1. SEM images of the cross section of zeolite/polymer blend composite hollow fibers fabricated with different PI contents [(a) 0 %, (b) 10 % and (c) 20 %] and N2 extrusion pressures [(a1-c1) 0.05 MPa, (a2-c2) 0.07 MPa and (a3-c3) 0.09 MPa]. Fig. 2. Structure analysis of the cross section of the BCHFs. S1 the poor finger-like region, S2 the spongy region, and S3 the rich finger-like region. Fig. 3. Photo images of (a) BCHF-0%PI bended to a small angle, (b) BCHF-10%PI bended, and (c) broken BCHF-10%PI after bending.

Fig. 4. SEM images of the inner surface of different zeolite/polymer blend composite hollow fibers with different PI contents and N2 extrusion pressures: (a) BCHF-0%PI-N0.07, (b) BCHF-0%PI-N0.09, (c) BCHF-10%PI-N0.07, (d) BCHF-10%PI-N0.09, (e) BCHF-20% 25

PI-N0.07 and (f) BCHF-20%PI-N0.09.

Fig. 5. SEM images of the inner-side zeolite NaA membranes synthesized on different BCHF supports: (a) BCHF-0%PI-N0.07, (b) BCHF-0%PI-N0.09, (c) BCHF-10%PI-N0.07, (d) BCHF-10%PI-N0.09, (e) BCHF-20%PI-N0.07 and (f) BCHF-20%PI-N0.09.

Fig. 6. SEM images of cross section of the inner-side zeolite membranes: [(a) BCHF-0%PI-N0.09, (b) BCHF-10%PI-N0.09, (c) BCHF-20%PI-N0.09] and (d) zeolite crystals grown up in rich finger-like region (S3).

Fig. 7. Contact angles of the BCHF supports fabricated at different N2 extrusion pressures. Highlights

· A new type of zeolite membrane supports with zeolite crystals in PES- PI blend; · Addition of PI can improve the mechanical strength of the support; · Zeolite NaA membranes can be synthesized on the inner surface of supports; · Inner-side membranes have high flux (>10 kg m-2h-1) and good selectivity (>10,000).

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