The influence of nanoseeds on the pervaporation performance of MFI-type zeolite membranes on hollow fibers

The influence of nanoseeds on the pervaporation performance of MFI-type zeolite membranes on hollow fibers

Accepted Manuscript The influence of nanoseeds on the pervaporation performance of MFI-type zeolite membranes on hollow fibers Shuixin Xia, Yong Peng,...

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Accepted Manuscript The influence of nanoseeds on the pervaporation performance of MFI-type zeolite membranes on hollow fibers Shuixin Xia, Yong Peng, Huibin Lu, Zhengbao Wang PII:

S1387-1811(15)00555-7

DOI:

10.1016/j.micromeso.2015.10.010

Reference:

MICMAT 7350

To appear in:

Microporous and Mesoporous Materials

Received Date: 1 July 2015 Revised Date:

2 October 2015

Accepted Date: 8 October 2015

Please cite this article as: S. Xia, Y. Peng, H. Lu, Z. Wang, The influence of nanoseeds on the pervaporation performance of MFI-type zeolite membranes on hollow fibers, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.10.010. 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.

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ACCEPTED MANUSCRIPT

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The influence of nanoseeds on the pervaporation performance of MFI-type zeolite membranes on hollow fibers Shuixin Xia, Yong Peng, Huibin Lu, Zhengbao Wang* College of Chemical and Biological Engineering, and MOE Engineering Research

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

* Corresponding Author, E-mail: [email protected]; Fax/Tel.: +86-571-8795-2391

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Abstract

Thin MFI-type zeolite membranes are prepared on α-Al2O3 hollow fibers by

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secondary growth. Nanoseeds with different gel amounts have been prepared and the influences of silicalite-1 nanoseeds on membrane formation and separation performance of the membrane are investigated. Both the as-synthesized zeolite seeds and membranes are characterized by X-ray diffraction (XRD), dynamic light scattering (DLS) and scanning electron microscopy (SEM) with energy dispersive

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X-ray (EDX). The separation properties of as-synthesized membranes are evaluated by pervaporation of 5 wt% ethanol aqueous solution at 60 oC. It is found that the gel amount in the nanoseeds is critical in obtaining continuous membranes with high

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separation performance. Membranes prepared via nanoseeds from ball-milled crystals show higher hydrophobicity and denser microstructure, and thus exhibit higher

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pervaporation performance than those prepared from as-synthesized nanoseeds. Keywords: MFI zeolite membranes; nanoseeds; dipcoating-wiping; pervaporation; ball-milling

1. Introduction In the past two decades, membrane-based pervaporation technology for ethanol purification has received great attention. Compared with traditional distillation, membrane pervaporation is much more energy-efficient owing to its low energy consumption and economical competition [1, 2]. Moreover, integrating the membrane 1

ACCEPTED MANUSCRIPT pervaporation with fermentation to achieve the in situ continuous ethanol extraction will greatly improve the fermentation efficiency [3]. In particular, MFI-type zeolite membranes, especially pure-silica MFI zeolite membranes (also called silicalite-1 membranes) have been investigated most intensively due to its uniform pore size

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(~0.55 nm) and desirable hydrophobicity suitable for separating organic compounds from the dilute organic solution (e.g., ethanol/water mixture) by pervaporation.

To achieve a widespread industrial application, zeolite membranes should possess both high separation factor and high permeation flux. Up to now, various efforts have

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been made to increase the permeation flux. As known, the permeation flux is in correlation with both the porous support and the membrane layer. Particularly,

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reducing the wall thickness of the support is one effective way to increase the permeation flux. Wang et al. [4] have successfully prepared α-Al2O3 hollow fibers supported zeolite LTA membranes, achieving a high flux due to the thin wall of hollow fiber. Alumina hollow fibers supported MFI membranes with a flux of 5.4 kg/(m2 h) (75 oC, 5 wt% ethanol/H2O) have been synthesized by Wang et al. [5]

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However, relatively high separation factor could only be achieved with a membrane layer as high as 12 µm. The flux was probably lowered with a thick membrane layer. The reason why a so thick membrane layer is needed to achieve a moderate separation

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factor on α-Al2O3 hollow fiber supports was not revealed in that paper. On the other hand, Gu et al. [6] have synthesized thin silicalite-1 membranes with a thickness of only 3 µm on hollow fiber substrates fabricated with yttria stabilized zirconia (YSZ)

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without aluminum, achieving a high flux of 7.4 kg/(m2 h) in pervaporation separation of 5 wt% ethanol/H2O at 60 oC. However, the separation factor of the membrane was only 47. On the contrary, macroporous alumina tubes supported MFI membranes with a thickness of ~3.5 µm could achieve a high separation factor of 62 [7]. Alumina hollow fibers are ideal supports for their high surface area-to-volume ratio (>1000 m2/m3) and relatively low price as compared with YSZ hollow fibers. Searching for new methods to prepare thin membranes with high separation factors as well as high fluxes on α-Al2O3 hollow fibers is significantly important to meet the requirements of industrial application. 2

ACCEPTED MANUSCRIPT For the preparation of MFI zeolite membrane, mainly two methods are applied, in-situ crystallization [8, 9] and secondary growth [5, 10-20]. The secondary growth with seeding induction usually requires lower temperature and shorter time owing to the separation of the crystal nucleation and growth step, and thus the microstructure of

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the membrane could be controlled easily [12]. Therefore, the secondary growth method with induction of a seed layer has been considered to be one of the most effective methods to obtain high quality membranes. Seeding plays an important role in achieving a high quality membrane. Until now, various seeding methods have been

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reported to obtain a seed layer, such as dip-coating [5, 13-15], rub-coating [16, 17], vacuum seeding [18, 19], filtration seeding [20], etc. Nano crystal (ca. 100 nm) seeds

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are commonly preferred on the hollow fiber support owing to the relatively small pore size on the surface. Up to now, the seed size and seed concentration effects on the MFI zeolite membrane performances have been investigated systematically [5, 6, 21]. For zeolite NaA membranes, our group reported that a seed paste composed of 90 wt% zeolite NaA crystals and 10 wt% gel or 100 wt% pretreated gel on the support

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surface could induce the formation of zeolite NaA membranes with high separation performance by secondary growth [22, 23]. This indicates that the synthesis gel in the seed layer has no negative effect on the formation of dense zeolite NaA membranes.

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Gel is usually unavoidable in the preparation of nano crystal MFI seeds. Then an obvious question is if amorphous gel in MFI zeolite nanoseeds has effect on the formation of dense MFI zeolite membranes. However, there is no literature report

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about this effect. In this study, a series of nanoseeds with different amounts of gel were prepared, and the influence of gel in the seeds on the properties of MFI zeolite membranes on alumina hollow fiber supports was investigated in detail. To the best of our knowledge, this is the first time to report the gel influence on the separation performance of the as-prepared MFI zeolite membranes. We also demonstrate that MFI zeolite membranes with relatively high separation factors on hollow fibers can be prepared using nanoseeds from ball-milled crystals.

2. Experimental 3

ACCEPTED MANUSCRIPT 2.1 Preparation of hollow fiber supports Alumina hollow fibers were prepared according to Refs. [24, 25]. Hollow fiber precursor was obtained by spinning polymer slurry containing suspended aluminum oxide powders, and then the precursor was calcined at elevated temperature (e.g.,

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1500 oC). In this process, polyethersulfone (PES), N-methyl-2-pyrrolidone (NMP), and polyvinyl pyrrolidone (PVP) were used as polymer binder, solvent and additive, respectively. The porosity of the support was measured by calculating the amount of adsorbed H2O within a certain time (e.g., 10 s). α-Al2O3 hollow fibers (outer diameter:

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1.2 mm, inner diameter: 1.0 mm, average pore diameter: 200 nm) are cut into 50 mm

2.2 Preparation of nanoseeds

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in length as supports.

The silicalite-1 nanoseeds of around 70 nm were synthesized by a traditional hydrothermal synthesis according to a previously reported method [26]. The molar composition of the synthesis solution was 1TPAOH:2.8TEOS:40H2O, using tetraethyl (TEOS,

98

wt%,

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Organics)

as

silicon

source

and

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orthosilicate

tetrapropylammonium hydroxide (TPAOH, 40 wt%, Sachem) as a structure-directing agent (SDA). A clear solution was obtained by mixing TEOS and TPAOH in water,

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and stirring at ambient temperature for 24 h. The hydrothermal synthesis was carried out in a Teflon-lined steel autoclave. The autoclave was placed in an oven and kept rotating at 30 rpm for 3 days at 80 oC. The seed suspension was purified with H2O for

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different washing times by repeated washing, sonication, centrifugation (12, 000 rpm) and decantation. The obtained seeds were denoted as seeds S1-S5 (see Table 1). On the other hand, nanoseeds without gel (S6) were obtained by mechanically

ball-milling homemade micro-sized silicalite-1 crystals. The micro-sized silicalite-1 crystals were synthesized following a previously reported method [27]. The molar composition of synthesis solution is 0.32TPAOH:1TEOS:165H2O. The hydrothermal synthesis is carried out in a Teflon-lined stainless steel autoclave placed in an oven and kept rotating at 20 rpm, 175 oC for 2 h. The obtained crystals are coffin-shaped and their size is ~1 µm in c-axis. Their XRD and SEM data are the same as in the 4

ACCEPTED MANUSCRIPT literature [7]. The micro-sized silicalite-1 crystals were ball-milled for 20 minutes, and then the ball-milled crystals were dispersed in water and the suspension was statically placed for 3 h. The big crystals settled down in the bottom layer were removed and the rest small crystals were collected and used as seeds (S6) for

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membrane preparation. The seed suspension was obtained by dispersing silicalite-1 seeds in deionized

Ref. [5]

2.3 Preparation of MFI zeolite membranes

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water under sonication and the concentration was controlled as 5 wt% according to

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Prior to membrane synthesis, hollow fiber supports were seeded with the as-prepared nanoseeds by dipcoating-wiping or dip-coating. The support with one end sealed was dipped into the seed suspension and kept for 10 s to ensure the seeds adsorb onto the support outer surface, and then the seeded support was dried at 60 oC. The synthesis solution for secondary growth was prepared by mixing the deionized

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H2O, TPAOH and TEOS with the molar ratio of 0.17TPAOH:1TEOS:165H2O reported by Wang et al. [5] The solution was aged at ambient temperature for 4 h under stirring. The synthesis solution was poured into a Teflon-lined stainless steel autoclave and the seeded support was immersed into the solution vertically. The

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secondary growth was carried out in a convection oven at 175 oC for 6 h. After synthesis, the obtained membrane was rinsed with deionized H2O and dried at 60 oC.

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The as-synthesized MFI zeolite membrane on the hollow fiber support was glued

onto a non-porous stainless steel tube with Torr Seal (Varian Vacuum Technologies), and then connected to the vacuum system. The pre-calcination vacuum pressure (PCVP) of the as-synthesized membrane was tested to evaluate the quality of the synthesized membrane [7]. After that the membrane was calcined at 500 oC for 12 h with rising and cooling rates of 1 oC/min to remove the SDA (TPA+) occluded in the zeolite pores during membrane synthesis.

2.4 Characterizations 5

ACCEPTED MANUSCRIPT The microstructure and morphology of seeds and membranes were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4800) and scanning electron microscope TM-1000 (Hitachi). The elemental composition of the membrane layer was measured by energy dispersive X-ray (EDX, Horiba EX350). The

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crystalline microstructure and particle size distribution of the as-prepared crystal seeds were determined by X-ray diffraction (XRD, Rigaku, D/max-rA) using CuKα radiation and dynamic light scattering (DLS, Malvern, ZEN 3600), respectively. The surface hydrophobicity of the membrane was determined by contact angle

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measurement (Data Physics, OCA 20). Pervaporation performances of zeolite membranes were evaluated using the homemade setup [4]. Zeolite membranes on the

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hollow fiber supports were pasted into a non-porous stainless steel tube by Torr Seal (Varian Vacuum Technologies) and connected to the pervaporation setup. The ethanol aqueous solution (5 wt%) in a flask was stirred and heated kept at 60 oC. The permeate was collected in a trap for 15–30 min. The total flux (J) and separation factor (α) are defined as follows:

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J = W/(A×t)

α(ethanol/water) = (Yethanol/ Ywater)/(Xethanol/ Xwater)

(1) (2)

where W is the total weight of the permeate (kg), A is the separation area of zeolite

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membrane (m2), t is the collecting time (h), X and Y are the weight fractions of species in the feed and permeate, respectively. N2 and SF6 single gas permeations through MFI zeolite membranes were carried

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out using the dead-end method on a steady state gas permeation system reported by Peng et al. [28]. The gas permeance (P) was calculated by the following equation: P = V/(A× ∆P× t)

where V is the volume of permeating gas (mol), A is the separation area of the zeolite membrane (m2), ∆P is the transmembrane partial pressure difference (Pa), and t is the permeating time (s). The N2/SF6 ideal selectivity was denoted as the ratio of its permeance values.

3. Results and discussion 6

ACCEPTED MANUSCRIPT 3.1. Characterization of nanoseeds The XRD patterns of the as-synthesized nanoseeds washed with water for different times (50 ml H2O was used for each time) are shown in Fig. 1. All the diffraction peaks were of MFI-type zeolite characteristic peaks with no evidence of other phases.

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It could be detected that the diffraction peaks for the unwashed seeds were weak and even some peaks could not be discriminated, which could be ascribed to the co-existence of large amount of amorphous gel (48 wt% in Table 1) in the seeds, which is close to the value (~40 % gel) reported in the literature [29]. And with

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increasing the washing times, the intensity of the characteristic peaks enhanced successively, indicating the ratio of silicalite-1 nano crystals in powders increased.

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Further increasing the washing times from 8 to 16, little change in the intensity of characteristic peaks could be observed. Therefore, we assumed that there was no gel in the solid product after 16 times washing and this was used to calculate the gel content in the seed crystals for different washing times (Table 1). In order to calculate the seed size, DLS test was also conducted, and the average size of the as-synthesized

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seeds was around 70 nm according to the DLS data (see Fig. 2a).

3.2. Influence of seeding method

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Secondary growth with induction of a seed layer was considered to be one of the most effective methods to obtain high quality membranes with good separation performance. The seeding on the supports has been recognized to be one crucial

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procedure in obtaining high quality zeolite membranes [4, 30, 31]. Up to now, many techniques have been reported to prepare a seed layer on the support, e.g., dipcoating [5, 13-15], rubbing [16, 17], vacuum seeding [18, 19], filtration seeding [20], etc. As for the synthesized nanoseeds with average size of ca. 70 nm, they could not be dried because they are prone to aggregate. So we could only get the seeds suspension with water or alcohols as the dispersant. In this study, we kept the nanoseeds in water. And the seed concentration was kept as 5 wt% according to optimum concentration reported previously by Wang et al [5]. First, we chose seed S5, which was completely washed, to investigate the effect of 7

ACCEPTED MANUSCRIPT the seeding method. The supports of membranes M0 and M5 were seeded by dip-coating and dipcoating-wiping, respectively, and then the membranes M0 and M5 were prepared by secondary growth. As show in Table 2, only a separation factor of 10 was obtained with a 5 wt% ethanol aqueous solution feed. The PCVP value of M0

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(120 Pa) is much higher than the blank vaccum pressure of the system (40 Pa), indicating that there might be some defects existing on the membrane surface. To further confirm our assumption, SEM characterization was conducted and the results are shown in Fig. 3. On the membrane surface, many defect pores could be detected.

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And the defect pores could be attributed to the discontinuous seed layer prepared by the dip-coating method. In addition, large cracks could also be detected on the

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membrane surface (Fig. 3c, d). The cracks occurred during the calcination and could be ascribed to the thicker membrane of some parts than the rest. A thick seed layer generally led to a thick membrane, and thus resulted in cracks during the calcination [5]. Lovallo et al. [32] also reported that too thick seed layer could result in membrane crack during the calcination to get rid of SDA. After all, the cracks could be ascribed

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to the non-uniform distribution of the seeds on the support. All the facts above indicated that the quality of the seed layer prepared only by dip-coating was poor. On the other hand, our group reported that wiping was an essential procedure to obtain a

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good quality membrane layer [4]. Therefore, wiping was also conducted followed by dip-coating to seed the support of membrane M5. The membrane M5 prepared by the dipcoating-wiping seeding method had denser structure according to their

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corresponding SEM images shown below and showed higher separation factor as high as 35 (Table 2). So we concluded that wiping was a crucial procedure in preparation of a high quality membrane layer, and in the following tests, all membranes were prepared by the dipcoating-wiping seeding method.

3.3. The influence of gel in nanoseeds As reported, the pervaporation performance of zeolite membranes was greatly influenced by the state of seeds. Zhang et al. [21] reported that poorly intergrown membranes formed using large seeds (500 nm) whereas dense and continuous 8

ACCEPTED MANUSCRIPT membranes could be obtained using zeolite seeds of 100 nm. Wang et al. [5] found that high separation performance membranes were obtained when the seed concentration was 5 wt% and membranes were easily leaking with higher seed concentration. A high separation factor of 60 could easily be achieved on alumina

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tubes supported MFI zeolite membranes with a membrane thickness of ca. 3.5 µm [7] whereas a separation factor of ~50 could be obtained only with a thick membrane layer of ~12 µm on the alumina hollow fiber supports [5]. Here the gel influence on the pervaporation performance was systematically investigated for the first time.

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The SEM images of supports coated with different seeds are shown in Fig. 4. The bare surface of hollow fiber support was lightly rough and uneven just as reported

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elsewhere [4]. When seeded with unwashed seeds, the support was only partially coated (Fig. 4a), indicating that continuous seed layers could not be obtained with the unwashed nanoseeds. On the other hand, when washed nanoseeds were used, an even and continuous seed layer could be observed on the surface of support (Fig. 4b-f). As shown in Fig. 5, after the secondary growth at 175 oC for 6 h, a MFI zeolite

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membrane layer was detected on the outer surface of the support. When seeded with unwashed seeds, the obtained MFI zeolite membrane M1 by the secondary growth was found to be coated with lots of gel and the uneven support surface still could be

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vaguely detected. The grain crystal was small and lots of pinholes and defects could be detected on the membrane surface. It is quite difficult to determine the dividing line between the support and the zeolite membrane layer from the cross-sectional

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image owing to the coverage of a thick gel layer (Fig. 5b). The ambiguous morphology of the support indicates that the membrane layer was actually very thin although its actual thickness was quite difficult to calculate, which was further evidenced by the following XRD analysis. The thin and incompact membrane structure of M1 indicated that too much gel in the seeds hindered the growth of crystal grain into continuous membranes. It is worth to be noted that the as-prepared membrane has been washed thoroughly after the secondary growth. So we deduced that the gel should result from the unpurified seeds. When the seeds were washed twice with 50 ml H2O each time, c-oriented membranes of ca. 6 µm (M2) with large 9

ACCEPTED MANUSCRIPT crystal grains could be detected after the secondary growth at 175 oC for 6 h, although the membrane surface was quite uneven and many holes could be detected (Fig. 5c). An obvious dividing line between the membrane and the support could clearly be detected from its corresponding cross-sectional image (Fig. 5d) while no gel layer was

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detected. These facts may further indicate that the thick gel layer covered upon the cross-sectional part of membrane M1 originated from the seeds. Further purifying the seeds, the obtained corresponding membrane (M3-M5) became more and more compact. No obvious differences in the membrane thickness were observed.

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The XRD patterns of MFI zeolite membranes prepared with different nanoseeds are shown in Fig. 6. Among all the peaks, only peaks at 25.5, 35.2, 37.8, 43.3o were

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designated to the characteristic diffraction peaks of the alumina support. It could clearly be detected that the characteristic peaks of membrane prepared with unpurified nanoseeds was quite weak and almost only the diffraction peaks of the alumina support could be detected (Fig. 6a), which indicates that the obtained membrane layer was quite thin. However, when purified seeds were used, obvious and strong

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characteristic peaks designated to MFI zeolite membrane could be observed. The XRD results indicated that the obtained membranes were (h0h)- and oblique c-orientation. All the results are quite in accordance with their corresponding SEM

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analysis (see Fig. 5).

The pervaporation performance tests of the obtained MFI zeolite membranes were carried out and the results are shown in Table 2. All the membranes were

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prepared at 175 oC for 6 h. When seeded with unwashed seeds (S1), the as-prepared membrane (M1) exhibited a low separation performance with a separation factor of only ~5. The synthesized membrane of M1 somewhat was leaking as evidenced by both the high PCVP value of 107 Pa and the SEM image (Fig. 5a). So the low separation performance of M1 was in close correlation with its corresponding incompact structure. However, when seeds were washed twice with water, an obvious improvement in the separation performance of as-synthesized membrane was detected, and the separation factor of the obtained membranes improved successively with increasing the washing times of seeds. A relative high separation 10

ACCEPTED MANUSCRIPT factor of 33 was achieved on the membrane prepared with seed S4. However, little improvement could be observed in the separation factor further as the number of the washing times was increased from 8 to 16.

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3.4. Membranes prepared with ball-milled nanoseeds As known, the pervaporation performance of MFI zeolite membrane is affected by many factors. The influence of hydrophobicity of the membrane surface could not be ignored. Water contact angle measurement is one of the straightforward methods to

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evaluate the hydrophobicity of the membrane surface. Fig. 7 showed the contact angles of the MFI zeolite membranes prepared with the as-synthesized nanoseeds. It

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could be observed that the hydrophobicity of the as-prepared membranes improved with increasing the washing times of seeds. In other words, too much gel in the seed layer would lower the hydrophobicity of the as-prepared membrane, which is not beneficial for the separation performance.

As well known, the gel in the nanoseeds was quite difficult to be cleaned up

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although washing, sonication and centrifugation were conducted repeatedly. Particularly for the nanoseeds with seed size below 100 nm, the washing of the seeds needed to be performed by centrifugation at an ultra-high speed (as high as 15000

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rpm). Under this condition, getting rid of the gel completely was quite difficult. And the residual gel in the seed would affect the membrane performance to some extent. In addition, small nanoseeds (less than 100 nm) were quite unstable, and they are prone

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to get gelation when kept for several weeks. Therefore, the preservation of nanoseeds for long time is particularly difficult. In addition, the preparation of nanoseeds (less than 100 nm) itself was quite time- and energy-consuming. On the other hand, pure micro-sized crystals can easily be obtained because it is easy to get rid of the gel in the synthesis solution by repeated washing. More importantly, they are quite stable and could be dried. The only problem is that the micro-sized crystals are too big to be coated on the hollow fibers with relatively small pores. Therefore, nanoseeds without gel (S6) were obtained by mechanically ball-milling the homemade micro-sized crystals, followed by settlement separate in water to get rid of the big ones. 11

ACCEPTED MANUSCRIPT Ball-milled zeolite crystals have been reported as seeds for zeolite membrane preparation [33-35]. Ball-milled NaA zeolite seeds could induce well-intergrown NaA zeolite membranes with high separation performance [33,34]. Very recently, Gu et al. reported that pure phase CHA zeolite membranes could be obtained by induction of

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ball-milled seeds in a broad SiO2/Al2O3 ratio and at a low K+ concentration [35]. To the best of our knowledge, there are no reports about fabrication of MFI-type zeolite membranes using ball-milled crystals as seeds.

The average seed size of ball-milled nano seeds (S6) detected by DLS was around

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200 nm while that of the directly synthesized seeds (S5) was about 70 nm (Fig. 2). And the XRD pattern of S6 showed stronger diffraction peaks compared with that of

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S5 (Fig. 8A), which also indicates that the particle size of seed S6 was larger than that of seed S5. The seed layer of seed S6 is shown in Fig. 4f. The support was covered by nanoseeds with some big crystals. The SEM images of MFI zeolite membranes prepared with seed S6 using the same condition as membrane M5, are shown in Fig. 9. The surface of membrane M6 prepared with seed S6 was much coarse and showed a

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continuously intergrown polycrystalline structure with coffin-like crystals and (h0h)-out-of-plane orientation evidenced by the XRD characterization (Fig. 8B), which is pretty different from that of membrane M5 prepared with seed S5. And the

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voids between crystal grains of membrane M6 were large. So it seems that membrane M5 was much denser than M6 from the surface view (Fig. 5i vs Fig. 9a). From the cross-sectional view, membrane M6 was about 6 µm thick, which is close to that of

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membrane M5. The pervaporation performance of membrane M6 was tested for a 5 wt% ethanol/water feed at 60 oC. As shown in Table 2, the membrane M6 showed much higher separation performance than membrane M5, achieving a high separation factor of 51 and a high flux of 7.6 kg/(m2 h). The higher separation performance of M6 could be ascribed to the clean seeds containing no gel in some degree according to the above drown conclusions that the less gel content of the seeds results in the higher the separation performance of the membrane. To further seek for the reason of the higher pervaporation performance of membrane M6, the water contact angle of membrane M6 was also tested and the result is shown in Fig. 7. The contact angle of 12

ACCEPTED MANUSCRIPT membrane M6 was much higher than that of M5, indicating membrane M6 had higher hydrophobicity. The higher hydrophobicity of M6 may be ascribed to the no gel containing seeds used. The dense degree of the membrane layer is also considered as an important factor

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affecting the membrane performance. From the top view of SEM image, it seems that membrane M5 was much denser than membrane M6. In order to further characterize the dense degree of the membrane structure, the measurement of N2/SF6 ideal selectivity (defined as the ratio of single-gas permeance) of membranes M5 and M6

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was also conducted, which is a most commonly employed method to characterize the quality of the membrane layer [36]. The kinetic diameter of N2 (0.364 nm) is smaller

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than SF6 (0.55 nm) which is close to the pore size of MFI-type zeolite crystals (~0.55 nm). It is reported that MFI-type zeolite membranes with more structure defects would exhibit higher SF6 permeance, resulting in the lower N2/SF6 ideal selectivity [11]. Sebastain et al. [13] found that there was almost a linear relationship between the ethanol-water pervaporation separation factor and the N2/SF6 ideal selectivity of

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MFI-type zeolite membranes. In other words, the larger the N2/SF6 ideal selectivity, the higher the pervaporation separation factor. And this linear relationship was much more evident when the separation factor is higher than ~35. The single component N2

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and SF6 permeances of membranes M5 and M6 were calculated and the results are listed in Table 3. For both membranes, the permeance of SF6 was reasonably lower than that of N2. The ideal selectivity of N2/SF6 of M6 reached 532, which is much

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higher than that of M5, indicating that the dense degree of M6 is much higher. It is generally accepted that a zeolite membrane with large grain size will have less non-zeolitic transport pathways in the form of grain boundary [37]. The larger grain size of M6 meant less grain boundary and thus showed denser microstructure. Besides, it is reported that non-zeolitic micropores (grain boundary) are hydrophilic and will reduce the ethanol/H2O selectivity of a hydrophobic zeolite membrane owing to the silanol groups along the external surface of the zeolite crystal and defects, which further explained the higher hydrophobicity of M6 [38, 39]. In addition, the Si/Al ratio of the zeolite membrane layer was measured by the EDX technique. As for the 13

ACCEPTED MANUSCRIPT synthesis solution, no aluminum was contained. The aluminum element only came from the alumina support because the Al leaching occurred under the alkaline condition of the synthesis solution. To avoid the interference of aluminum element in the alumina support on the EDX test, the measurement points were chosen on the

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membrane cross-sectional places where were close to the membrane surface as much as possible. The average Si/Al ratio values are shown in Table 3. It could be detected that the value difference in the average Si/Al ratios of both membranes was quite small. The average Si/Al value of M6 was only a little higher than that of M5. This

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indicates that the higher hydrophobicity of M6 could not be attributed to the less Al leaching during the secondary growth. According to all the facts above, we deduced

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that higher hydrophobicity of M6 could be owed to the clean seeds with no gel and the less grain boundary existed in the membrane layer. In summary, the much denser structure and higher hydrophobicity of M6 contributed to its higher pervaporation separation performance. At the same time, the higher flux of M6 could be owed to no gel trapped in the membrane layer (i.e., high hydrophobicity and low resistance) and

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the support with the clean seeds. Besides, the thickness of the membrane layer of M6 was thinner compared with that reported previously [5], which may also contribute to

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the improvement of the flux.

3.5. Comparison with literature data Table 4 shows the pervaporation performance of membranes prepared in this work

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compared with those reported in literatures. Sano et al. [40] first reported that the silicalite-1 membranes prepared on the stainless steel disc by in situ crystallization with a thick membrane layer of 400-500 µm, showed a separation factor of 58 for a 4 wt% ethanol/water mixture at 60 oC. However, the total flux was lower than 0.8 kg/(m2 h) due to the thick membrane layer. On the other hand, the secondary growth with a seed layer is an effect way to obtain thin zeolite membranes for the accelerated growth rate resulted from the seeds. A high separation factor of 89 could be achieved on alumina tubes by rubbing water slurry of MFI seeds [16]. Our group [7] synthesized macroporous alumina tubes supported MFI zeolite membranes by a novel 14

ACCEPTED MANUSCRIPT alcohol wetting agent assisted rub-seeding method, showing a separation factor of 62. These two membranes on alumina tubes had thinner membrane layers than that by in situ crystallization and showed the flux of ~1.8 kg/(m2 h). This indicates that the high flux could be obtained by minimizing the membrane thickness. However, the

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membrane flux is not inversely proportional to the membrane thickness. Our group [5] fabricated MFI zeolite membranes on α-Al2O3 hollow fibers, showing a flux of 5.4 kg/(m2 h) at 75 oC. Recently, Gu et al. [6] employed YSZ hollow fibers as the supports to eliminate the contamination of Al element from the support, and thin membrane

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layers (3 µm) was obtained with a high flux of 7.4 kg/(m2 h), however, the separation factor was lower than 50. This is due to the low diffusion resistance of hollow fibers

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with high porosity and thin wall. It seems that MFI membranes with higher fluxes had lower separation factors. However, MFI membranes on ceramic α-Al2O3 capillary supports had a low flux of 1.5 kg/(m2 h) as well as a low separation factor of 54 [13]. From Table 4, it is found that the MFI membranes had lower separation factors when silicalite-1 nanocrystals were used as seeds. This is consistent with our above result

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(e.g., membrane M5). Namely, the seeds have great influence on the pervaporation performances of the corresponding membranes. According to our above discussion, two possible reasons are proposed: (1) Some gel exists in nanocrystals, resulting in a non-perfect membrane layer; (2) More intercrystal pores exist when nanocrystals are

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used as seeds. In the view of large-scale industrial application, α-Al2O3 hollow fibers are more promising considering both their high porosity and low price. In this work,

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we have successfully synthesized α-Al2O3 hollow fibers supported MFI zeolite membranes with a flux of 7.6 kg/(m2 h) and separation factor of 51 using ball-milled MFI crystals as seeds. This is the highest value of flux at the present time.

4. Conclusions Thin MFI membranes with high fluxes have been successfully fabricated on alumina hollow fiber supports by secondary growth. In order to obtain a high quality membrane, wiping is essential in addition to dip-coating in preparing the seed layer. Gel in nanoseeds has great influence on the performance of obtained membranes. The 15

ACCEPTED MANUSCRIPT gel in the seeds could lower the hydrophobicity of the corresponding synthesized membrane, and the existence of too much gel even could prohibit the growth of crystal grains. Membranes with high separation factors could be obtained by using well-washed nanoseeds. The separation factor and flux of MFI membranes can be

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further improved by using ball-milled crystals without gel inside as seeds. That is, a high flux of 7.6 kg/(m2 h) and a separation factor of 51 were achieved on the membrane prepared from ball-milled nanoseeds. The high pervaporation performance could be attributed to the dense microstructure and high hydrophobicity of the

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membrane layer.

Acknowledgments

The financial supports provided by the National Natural Science Foundation of China (21236006), China Postdoctoral Science Foundation (2015M571874) and the National High Technology Research and Development Program of China

References

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(2015AA03A602) are gratefully acknowledged.

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ACCEPTED MANUSCRIPT Table 1. Preparation procedures of different nanocrystal seeds Synthesis Synthesis solution Washing timesa Gel contentb (TPAOH:TEOS: temperature (wt%) H2O) and time S1 1:2.8:40 80 oC, 3 days 0 48 o S2 1:2.8:40 80 C, 3 days 2 37 o S3 1:2.8:40 80 C, 3 days 4 20 o 8 ~0 S4 1:2.8:40 80 C, 3 days o S5 1:2.8:40 80 C, 3 days 16 0 o c 0.32:1:165 175 C, 2 h 16 S6 a 50 ml H2O was used each washing for nanocrystals from 54 g synthesis solution; b Assuming no gel in the solid product after 16 times washing, the gel content (xi, wt%) was calculated according to the amount of solid product (ni, g) after i times washing and the amount of solid product (m, g) after 16 times washing: xi=(ni-m)/ni×100; c Nano seeds were obtained by mechanically ball-milling the as-synthesized silicalite-1 crystals (coffin-shaped, 1 µm) for 20 minutes, followed by removal of the big crystals settled down at the bottom in water.

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Seed no.

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Table 2. Pervaporation performances of MFI zeolite membranesa

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Membrane Seeds Porosity PCVP Separation Fluxc (kg/(m2 h)) no. (%) (Pa) factor, α S5 51±5 120 10±2 10.5±0.2 M0b M1 S1 43±5 107 5±2 4.6±0.2 M2 S2 50±5 61 18±2 2.9±0.2 M3 S3 47±5 58 25±2 3.6±0.2 M4 S4 46±5 53 33±2 4.6±0.2 M5 S5 48±5 51 35±2 4.7±0.2 M6 S6 48±5 60 51±2 7.6±0.2 a The synthesis solution composition TPAOH:TEOS:H2O = 0.17:1:165, 175 oC, 6 h; b was seeded by dip-coating, others were seeded by dipcoating-wiping; c Pervaporation condition: 5 wt% ethanol aqueous solution at 60 oC.

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Table 3. Permeation properties of M5 and M6 membranes SF6 permeance (mol m-2S-1Pa-1)

N2/SF6 Average ideal selectivity Si/Al ratio

4.36×10-8 7.72×10-9

153 532

63.5 65.4

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M5 M6

N2 permeance (mol m-2S-1Pa-1) 6.66×10-6 4.11×10-6

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Table 4. Pervaporation performances of MFI membranes for ethanol/water mixture

Stainless steel disc α-Al2O3 tube α-Al2O3 tube α-Al2O3 capillary α-Al2O3 HFsb YSZ HFs α-Al2O3 HFs

400-500 ~20 3.5 2.5 12 3 6

PV conditions

Temp (oC) 60 60 60 65 75 60 60

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Separation factor; b hollow fibers.

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a

no ~3 1 ~0.05 0.07 0.1 0.2

Mem. Thickness (µm)

αa

Flux (kg/(m2 h))

Ref.

58 89 62 54 54 47 51

0.76 1.81 1.82 1.5 5.4 7.4 7.6

[40] [16] [7] [13] [5] [6] This work

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Support

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Feed (wt%) 4 5 5 5 5 5 5

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Figure Captions Fig. 1 XRD patterns of silicalite-1 nano seeds washed for different times. (a) S1, (b)

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S2, (c) S3, (d) S4, (e) S5.

Fig. 2 Particle size distributions of silicalite-1 nanoseeds. (a) S5, (b) S6.

Fig. 3 SEM images of (a-d) different places of MFI zeolite membrane M0 on the

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support seeded by dip-coating method.

(b) S2, (c) S3, (d) S4, (e) S5, (f) S6.

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Fig. 4 SEM images of hollow fiber supports seeded with different nanoseeds. (a) S1,

Fig. 5 SEM images of MFI zeolite membranes prepared with different nanoseeds. (a, b) S1, (c, d) S2, (e, f) S3, (g, h) S4, (i, j) S5; (a, c, e, g, i) top view, (b, d, f, h, j)

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cross-sectional view.

Fig. 6 XRD patterns of MFI zeolite membranes prepared with different nanoseeds.

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(a) S1, (b) S2, (c) S3, (d) S5; Peaks with index ( ) are the characteristic peaks of alumina substrate.

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Fig. 7 Contact angles of MFI zeolite membranes prepared with different nanoseeds.

Fig. 8 XRD patterns of (A) silicalite-1 seeds and (B) MFI zeolite membranes prepared with different nanoseeds. (a) S5, (b) S6; ( ) are the characteristic peaks of alumina substrate.

Fig. 9 SEM images of MFI zeolite membranes prepared with seed S6. (a) top view, (b) cross-sectional view. 20

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► Effects of gel in nanoseeds on formation of MFI zeolite membranes. ► Effects of gel in nanoseeds on pervaporation of MFI zeolite membranes. ► Effects of ball-milled nanoseeds on pervaporation of MFI zeolite membranes. ► High performance MFI membranes were obtained using ball-milled nanoseeds.