Pervaporation separation of ethanol–water mixtures through B-ZSM-11 zeolite membranes on macroporous supports

Pervaporation separation of ethanol–water mixtures through B-ZSM-11 zeolite membranes on macroporous supports

Author's Accepted Manuscript Pervaporation separation of ethanol-water mixtures through B-ZSM-11 zeolite membranes on macroporous supports Lijun Chai...

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Author's Accepted Manuscript

Pervaporation separation of ethanol-water mixtures through B-ZSM-11 zeolite membranes on macroporous supports Lijun Chai, Huazheng Li, Xiaoqin Zheng, Jinqu Wang, Jianhua Yang, Jinming Lu, Dehong Yin, Yan Zhang

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S0376-7388(15)00093-9 http://dx.doi.org/10.1016/j.memsci.2015.01.054 MEMSCI13458

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Journal of Membrane Science

Received date: 14 October 2014 Revised date: 12 January 2015 Accepted date: 14 January 2015 Cite this article as: Lijun Chai, Huazheng Li, Xiaoqin Zheng, Jinqu Wang, Jianhua Yang, Jinming Lu, Dehong Yin, Yan Zhang, Pervaporation separation of ethanol-water mixtures through B-ZSM-11 zeolite membranes on macroporous supports, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.01.054 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.

Pervaporation separation of ethanol-water mixtures through B-ZSM-11 zeolite membranes on macroporous supports Lijun Chai 1, Huazheng Li 2, Xiaoqin Zheng 1, Jinqu Wang 1, 2, Jianhua Yang 1,*, Jinming Lu 1, Dehong Yin 1, Yan Zhang 1

1 State Key Laboratory of Fine Chemicals, Institute of Adsorption and Inorganic Membrane, Dalian University of Technology, Dalian, Liaoning 116024, China 2 State Key Laboratory of Fine Chemicals, School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin, Liaoning 124221, China * Corresponding author. Tel./fax: +86 411 84986147. E-mail address: [email protected] (J. Yang).

Abstract Boron-substituted silicalite-2 (B-ZSM-11) membranes were prepared on seeded cheap macroporous α-Al2O3 tubes to separate ethanol from water by pervaporation. The incorporation of boron atom into MEL framework was confirmed by XRD and FTIR. The effects of B/Si ratio in the synthesis solution together with PV operation temperature and feed concentration on separation properties of the membranes were investigated. The B-ZSM-11 membrane prepared with B/Si ratio of 0.06 exhibited the highest separation factor of 35.0 with a total flux of 1.51 kg·m-2·h-1 for 5 wt% ethanol/water mixtures at 60 ºC. The total flux increased with increasing PV temperature from 30 to 75 ºC and the

separation factor showed the highest value at 60 ºC. The separation factor decreased with feed concentration from 1 to 7 wt% and the total flux also showed the highest value for a 5 wt% mixture. It is demonstrated that the incorporation of boron atom into MEL structure improved separation performance of B-ZSM-11 membrane for ethanol/water solutions. On the other hand, the better hydrophobicity of B-ZSM-11 membranes, evidenced

by contact

angle

measurements,

is

responsible

for

pervaporation performance for ethanol/water mixtures. Keywords:

Boron-substituted;

B-ZSM-11

zeolite

membrane;

Pervaporation; Ethanol/water mixture 1. Introduction Zeolite membranes have been widely used in many applications such as liquid separation [1], gas separation [2] and membrane reactors [3] because of their advantages of superior thermal, chemical and mechanical stabilities, uniform pore size as well as preferentially selective adsorption property over polymer membranes. So far, many types of zeolite membranes, such as LTA [4-5], T [6], MFI [1, 7-11], MOR [12], FAU [13] membranes, have been reported to separate liquid mixtures by pervaporation. Among them, MFI zeolite belongs to pentasil family and consists of straight channels (5.3 Å×5.6 Å) intersected by sinusoidal channels (5.5 Å×5.1 Å). As another member of the pentasil family, MEL zeolite contains intersecting straight channels with pore sizes of 5.4

Å×5.3 Å as shown in Fig. 1 [14]. The unique channel structure and hydrophobic nature make both MFI and MEL zeolites able to preferentially permeate organics from water. As reported previously [15-17], the diffusivity in the straight channels for MFI zeolite is higher than in the sinusoidal channels, the diffusivity in the MEL zeolite is therefore potentially higher than in the MFI zeolite because the former contains two straight channels. Accordingly, the MEL membranes potentially show higher separation efficiency in ethanol recovery than the MFI membranes. For instance, MEL membranes prepared by Kosinov et al. [17] showed a higher flux of 3.6 kg·m-2·h-1 with similar selectivity for ethanol pervaporation than a MFI membrane with similar quality and thickness. The isomorphous substitution of heteroatoms like B into zeolite framework has been regarded as an effective method for modifying pore structure and surface property of MFI zeolite due to changes in the T-O-T angles and T-O length (T=Si or B), resulting in tunable separation performance of MFI membranes [18-28]. At present, B-substituted MFI membranes have been developed to improve separation performance for gas mixtures [18-22], liquid mixtures [19, 23-26] and xylene isomers [27-28]. However, few B-substituted MEL (B-ZSM-11) membranes have been reported. Tuan et al. firstly prepared B-ZSM-11 membranes by in situ crystallization and found that B-ZSM-11 membranes had better

pervaporation performance for separation of propanol/water mixtures than B-ZSM-5 membranes prepared by similar procedures [29-30]. Compared with in situ hydrothermal growth, secondary hydrothermal growth method exhibits more advantages in manipulating the membrane microstructure in particular tuning the thickness, grain boundary and orientation because it can decouple the crystal nucleation and growth process [8]. The secondary growth is considered as one of the most effective methods for obtainment of high performance zeolite membranes. The key of secondary growth method is formation of a uniform and dense seed layer on support surface by a proper seeding technique. In our previous works, we developed a varying-temperature hot-dip coating (VTHDC) seeding method for the preparation of zeolite NaA [4], MFI [31] and T [32] membranes onto low-cost macroporous supports. This seeding method can overcome the difficulty in forming a large defect-free and thin seed layer on the cheap coarse macroporous support, leading to a high performance zeolite membrane. On the other hand, supports accounted for about 50-70% of the total cost of the zeolite membranes [1, 4]. Consequently, it is desirable that cheap macroporous supports were utilized to synthesize zeolite membranes due to their low cost and permeation resistance. In this paper, the B-ZSM-11 membranes with various B/Si ratios were successfully prepared on the low-cost coarse macroporous α-Al2O3 tubes by the

secondary growth method. The effects of B/Si ratio in the synthesis solution (from now on denoted B/Si ratio), temperature and feed concentration on pervaporation performance of the as-synthesized B-ZSM-11 membranes for ethanol/water mixtures were examined. The B role in tuning the separation performance of B-ZSM-11 membranes was also investigated. --------Fig.1-------2. Experimental 2.1. Materials The cheap coarse macroporous α-Al2O3 tubes (OD 13 mm, ID 8 mm and length 8 cm, mean pore size 2-3 µm and porosity about 36%) supplied by Foshan Ceramics Research Institute were used as supports in this study. The deionized water was home-made. Other materials were obtained commercially as reagent grade chemicals and used as received without purification, including tetraethyl orthosilicate (TEOS, 98 wt% liquid, Tianjin Kermel Chemical Reagent Co., Ltd, China), tetra-butyl ammonium hydroxide (TBAOH, 40 wt% solution, Shanghai Cainorise Chemicals Co., Ltd, China) and boric acid (H3BO3, 99.5 wt% solid, Sinopharm Chemical Reagent Co., Ltd, China). 2.2. Preparation of B-ZSM-11 seeds and seed layers The zeolite B-ZSM-11 seeds with different crystal sizes were prepared from a synthesis solution with a molar composition of 1TEOS:

0.35TBAOH:0.06H3BO3:120H2O by changing the crystallization time. The synthesis solution was obtained by dissolving H3BO3 in water, followed by adding TBAOH into the above solution under stirring. Then a certain amount of TEOS was added dropwise under vigorous stirring at room temperature until the clear solution was formed. Subsequently, the resultant synthesis solution was transferred into a Teflon-lined autoclave, which was heated to 150 ºC for 48 h and 72 h to obtain zeolite seeds with size of 600 nm and 1 µm, respectively. After synthesis, the obtained crystals were washed, dried and calcined at 550 ºC for 6 h to remove templates in the zeolite pores. For comparison, silicalite-2 zeolite crystals (B/Si=0) were obtained without the addition of boric acid into synthesis solution at 150 ºC for 48 h. Additionally, silicalite-2 zeolite seeds with similar size to the synthesized B-ZSM-11 seeds (about 600 nm and 1 µm) were also prepared by controlling H2O/SiO2 and TBAOH/SiO2 molar raitos at 120 ºC for 48 h. Their molar compositions were 0.35TABOH:1TEOS:20H2O

and

0.15TABOH:1TEOS:120H2O,

respectively. The seed layer was prepared by a VTHDC seeding method. The seeding procedure was performed as follows: (1) The preheated (175 ºC) support plugged with caps in two ends was dipped into uniformly dispersed large seed suspension (2 wt%) for 20 s, and followed by withdrawing vertically the support. (2) After drying at 100 ºC for

overnight, the support surface was rubbed carefully with degreasing cotton to remove excessive seeds loosely attached the support surface. (3) The support with two ends plugged with Teflon caps was directly dipped into well-dispersed small seed suspension (0.25 wt%) for 20 s, and then withdrew vertically the support. After drying at 100 ºC for overnight, the seeded support was calcined at 550 ºC for 6 h to consolidate seed layer and remove templates trapped in the zeolite crystals with heating and cooling rates of 1 ºC /min. 2.3. Preparation of zeolite membranes The synthesis solutions for membrane formation were prepared according to preparation of synthesis solution for B-ZSM-11 seeds in Section 2.2. The molar composition of resultant synthesis solutions was 1TEOS:0.35TBAOH:0-0.1H3BO3:120H2O.

Afterwards,

the

seeded

supports were hydrothermally crystallized in the above solutions at 150 ºC for 48 h. The synthesis step was repeated until the as-synthesized membranes were impermeable to N2 at room temperature with the pressure drop of 0.1 MPa. After synthesis, the obtained membranes were washed with deionized water, dried and calcined at 480 ºC for 8 h to remove templates with rising and cooling rates of 0.9 and 1.08 ºC/min, respectively. For comparison, a silicalite-2 membrane (B/Si=0) was also synthesized on an α-Al2O3 tube which was pre-coated with silicalite-2 seeds with size of about 1 µm and 600 nm by a similar procedure to that

used for the B-ZSM-11 membranes. 2.4. Characterization The morphologies of zeolite crystals and membranes were observed by field emission scanning electron microscopy (FE-SEM, KYKY2800B) at an acceleration voltage of 15 kV after gold coating. X-ray diffraction (XRD, Philips Analytical X-ray diffractometer) were used to analyze the crystalline structure of the zeolite crystals and membranes using Cu Kα radiation. The spectra were scanned in the range of 2θ=5-50° at a scanning speed of 6°/min. The zeolite crystals was also characterized by Fourier transform infrared (FT-IR, Nicolet-20DXB, USA) spectroscopy using KBr pellets technology in the range of 4000-400 cm-1. The B/Si ratios of the as-synthesized membranes were measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 2000DV, USA). The surface hydrophobic/hydrophilic property of the membranes was measured by contact angle measurement system (DSA100, Krüss, Germany) at room temperature. The pervaporation properties of the as-synthesized membranes were evaluated by separating 5/95 wt% ethanol/water mixtures at 60 ºC using the homemade equipment [12]. A membrane tube was fixed on a stainless steel module connected by a vacuum pump. The module was immersed into feed solutions. The permeate vapor was condensed and collected in a cold trap cooled by liquid nitrogen for a certain time. The compositions

of the feed and permeate were measured by a gas chromatograph (GC 7890T). The total flux (F) and separation factor (S) were determined by the following equations: F=W/(A·S), S=(Yethanol/Ywater)/(Xethanol/Xwater), where W is the weight of the permeate (kg), A is effective membrane area (m2), t is collecting time (h), Y and X are weight fractions of species in the permeate and feed, respectively. 3. Results and discussion 3.1. Characterization of B-ZSM-11 seeds and seed layer The XRD patterns of B-ZSM-11 seeds prepared with different crystallization time together with silicalite-2 crystals are shown in Fig. 2. The XRD patterns represented the characteristic doublet peaks in the range of 2θ=23-25° and a single peak at 2θ of about 45° of MEL structure, indicating that pure MEL zeolites were formed without any other impurities [33]. From the enlarged view in the 2θ range of 6-10°, it could be clearly seen that the peaks of the B-ZSM-11 seeds (Fig. 2a, b) shifted towards higher 2θ position compared to those of silicalite-2 crystals (Fig. 2c), suggesting the contraction of the unit cell in the B-ZSM-11 according to Bragg equation (this will be discussed later). Such contraction of unit cell evidenced the successful incorporation of boron atom into MEL framework [3, 20, 34]. Moreover, the B-ZSM-11 seeds synthesized for 48 h showed sharper and narrower intensity of diffraction peaks at 6-10° than silicalite-2 crystals prepared with the same procedure (see inset in

Fig. 2), indicating that the seeds after the substitution of boron had higher crystallinity. Therefore, it could be inferred that the introduction of boron into zeolite structure promoted the formation of MEL structure. --------Fig.2-------Fig. 3 shows the SEM images of both B-ZSM-11 seeds and silicalite-2 crystals. It can be seen that the morphologies of the obtained crystals changed substantially after the incorporation of boron into zeolite structure. Both B-ZSM-11 seeds prepared at 150 ºC for 72 h and 48 h were uniform, rice-like crystals with size of 1 µm (Fig. 3a, 72 h) and 600 nm (Fig. 3b, 48 h), respectively. In contrast, silicalite-2 crystals were 10 µm agglomerates composed of small oval crystals (Fig. 3c). However, the exact mechanism of this change in crystal morphology after the introduction of boron was still not so clear. --------Fig.3-------FTIR analysis has been commonly used to determine the isomorphic incorporation of boron atom into zeolite structure [25, 34]. Fig. 4 shows IR spectra of 600 nm B-ZSM-11 seeds and silicalite-2 crystals prepared under the same condition. Typical adsorption bands at 1227, 1100, 796, 550 and 450 cm-1 from MEL zeolite can be seen in all spectra [33]. The B-ZSM-11 seeds exhibited a weak band near 961 cm-1 that can be assigned to B-O-Si symmetric stretching vibration [26], indicating the presence of framework B in the B-ZSM-11 zeolite. The relative

intensities of the 3461 and 1637 cm-1 bands in B-ZSM-11 spectrum, which are attributed to SiO-H stretching and bending modes [28], were weaker than those of in the silicalite-2 spectrum. It suggested that B-ZSM-11 seeds had less hydroxyl groups that arise from the presence of framework defect [35]. The lack of defects in a zeolite can have major effects on the hydrophobicity of the material [36]. Therefore, the less hydroxyl group in the B-ZSM-11 crystals evidenced that the incorporation of boron atom into MEL framework increased its hydrophobicity. --------Fig.4-------The SEM images of B-ZSM-11 seed layer prepared by the VTHDC method are shown in Fig. 5. A continuous and compact seed layer was formed on the support surface (Fig. 5a). The cross-section image showed that the thickness of seed layer was in the range of 2-4 µm due to the coarse support surface as marked with arrows in Fig. 5b. --------Fig.5-------3.2. Characterization of B-ZSM-11 membranes Fig. 6 shows XRD patterns of bare support and B-ZSM-11 membranes with B/Si ratios of 0-0.1 (M0, M2-M5). The XRD reflections of each membrane exhibited characteristic peaks of MEL structure in the range of 2θ=7-9° and 2θ=23-25°, confirming the formation of MEL membranes. Moreover, it could be seen that the reflections slightly moved towards

higher 2θ position with the increase of B/Si ratio in the synthesis solution, indicating the decrease of the unit cell volume in the B-ZSM-11. Table 1 shows the ICP analysis results, calculated crystal lattice parameters and unit cell volumes of B-ZSM-11 membranes with different B/Si ratios according to Bragg equation d=λ/sin θ, where d, λ and θ denote the spacing between the planes in the atomic lattice (a, b, c), wavelength of used Cu Kα radiations of 0.154006 nm and observed XRD diffraction angle, respectively. As the B/Si ratio in the synthesis solution increased, the decrease of each lattice of a, b and c was observed as shown in Table 1. As a result, the unit cell volumes also decreased, suggesting that the amount of boron introduced into the zeolite structure increased with increasing B/Si ratio in the synthesis solution as shown in previous reports [3, 20, 34], which was further confirmed by the ICP analysis results (Table 1). Therefore, the contraction of unit cell proved that boron atom was incorporated into zeolite structure and its amount increased with the increase of B/Si ratio. In addition, it was also noted that the intensity of zeolite peaks for the prepared membranes increased and then slightly decreased as shown in the magnified pattern in Fig. 6 when B/Si ratio increased from 0 to 0.1. The membrane M3 with B/Si ratio of 0.06 had the strongest zeolite peaks while the M0 with B/Si ratio of 0 showed quite low peaks. Such XRD observations are consistent with the SEM images for the M0-M5 as

shown in Fig.7. --------Fig.6---------------Table 1-------Fig. 7 shows SEM images of B-ZSM-11 membranes with B/Si ratios of 0.01-0.1 (M1-M5) and silicalite-2 membrane (M0). The top views of membranes M1-M5 in Fig. 7 a-e revealed that continuous and compact zeolite membranes composed of rice-like crystal grains were obtained on top surface of the supports. However, some intercrystalline defects in the membrane M0 were clearly observed in Fig. 7a0. As shown in Fig .7a0’-e’, the membrane thickness for M0-M3 slightly increased with thickness of about 3, 5, 6 and 7 µm for M0, M1, M2 and M3, respectively, then for M4-M5 slightly decreased with thickness of about 5 and 4 µm for M4 and M5, respectively. This increased thickness was consistent with the increased XRD peaks (Fig. 6), suggesting that isomorphous replacement of B for Si enhanced the crystallization rate of MEL membranes therefore the crystallinity, which was in good agreement with XRD results of B-ZSM-11 and silicalite-2 crystals (Fig. 2). Therefore, the prepared B-ZSM-11 membranes would be expected to exhibit better separation performance compared with silicalite-2 membrane. --------Fig.7-------It is well known that measurement of contact angle on the surface of a membrane can be used to determine its hydrophobic property [37]. Thus,

surface hydrophobic property of the silicalite-2 and B-ZSM-11 membranes were measured by water contact angle as shown in Fig. 8. The contact angles on membranes surface firstly increased from 11.2° (M0) to 52.9° (M3), and then decreased to 31.1° (M5) with the increase of B/Si ratio from 0 to 0.1. This means that the hydrophobicity of the prepared B-ZSM-11 membranes first increased, and then decreased with increasing B/Si ratio. The B-substituted membranes M3 and M5 had higher hydrophobicity than silicalite-2 membrane M0, which was consistent with the IR analysis (Fig. 4) and XRD (Fig. 6). They confirmed that their hydrophobicity increased after the insertion of boron into zeolite framework, which might be due to the improvement of their crystallinity. However, previous reports stated that boron atom as trivalent element made B-ZSM-11 more hydrophilic than silicalite-2 membrane [23-24, 28]. In a word, the incorporation of B into MEL framework plays two trade-off roles: improving the hydrophobicity of B-ZSM-11 by increasing the crystallinity and directly increasing the hydrophilicity of B-ZSM-11. When B/Si ratio was less than 0.6, the increase in crystallinity outweighed the increase in hydrophilicity, and thus hydrophobicity of the M0-M3 increased with increase of B/Si ratio from 0 to 0.06. When B/Si ratio was further increased more than 0.6, the two roles interacted oppositely, therefore, the hydrophobicity of the M3-M5 decreased with increasing B/Si ratio. This explains the change of water contact angle and

PV performance (will be discussed in the later) of the as-synthesized membranes with increasing B/Si ratio. --------Fig.8-------3.3. Pervaporation performance of the B-ZSM-11 membranes 3.3.1. Effect of B/Si ratio Table 2 shows pervaporation performance of the B-ZSM-11 membranes (M1-M5) with various B/Si ratios and silicalite-2 membrane (M0) for ethanol recovery at 60 ºC. The membrane M0 had a low separation factor of 3.7 and a flux of 5.78 kg·m-2·h-1 for ethanol/water mixtures, which was due to existence of some defects in membrane M0 as shown in Fig. 7a0. After the introduction of boron atom into zeolite structure, the as-synthesized membranes M1-M5 exhibited better pervaporation performance with total fluxes of 1.51-2.19 kg·m-2·h-1 and separation factors of 10.1-35.0. As shown in Table 2, the separation factors increased from 18.1 to 35.0 for the membranes M1-M3 with increasing B/Si ratio in the range of 0.01-0.06. When the B/Si ratio further increased to 0.1, the separation factors decreased to 10.1. From the above results, it can be seen that membrane M3 with B/Si ratio of 0.06 exhibited the highest separation factor of 35.0 with a total flux of 1.51 kg·m-2·h-1 for ethanol/water mixtures. The dependency of separation factor with B/Si ratio can be explained by the change in the hydrophobicity of the B-ZSM-11 membranes. As discussed in Section 3.2,

the hydrophobicity of the B-ZSM-11 membrane increased with B/Si ratio in the range of 0-0.06 while the hydrophobicity of the B-ZSM-11 membrane decreased with increase of B/Si ration from 0.06 to 0.1. This similar tendency was also observed by Zhou et al. [26]. They found that B-ZSM-5 membrane prepared with B/Si ratio of 0.03 showed the best PV performance for ethanol recovery. The optimized B/Si ratio was different, mainly due to the different zeolite structure and preparation procedure. --------Table 2-------3.3.2. The effect of feed temperature Membrane M3 prepared in the hydrothermal synthesis solution with B/Si ratio of 0.06 exhibited the highest separation factor for separating ethanol from water by pervaporation. Therefore, the effects of feed temperature and ethanol concentrations on separation performance of membrane M3 were further studied. Fig. 9 shows the pervaporation performance of membrane M3 in separating 5 wt% ethanol/water mixtures at various temperatures. As shown in Fig. 9, the separation factor firstly increased from 29.6 to 35.0 with increasing feed temperature from 30 to 60 ºC, and then slightly decreased to 31.9 as the temperatures increased to 75 ºC. The total flux monotonously increased from 0.25 to 1.61 kg·m-2·h-1 with feed temperature. The highest separation factor for membrane M3 was 35.0 with a total flux of 1.51 kg·m-2·h-1 at 60 ºC. The total fluxes increased with temperature because the diffusion rates of

ethanol and water increased with temperature. The influence of feed temperature on total fluxes and separation factors in the range of 30-60 ºC were similar to the results reported by Li et al. [30]. They claimed that the separation process was based on preferential adsorption of alcohol and differences in diffusion rates. When the feed temperature increased, the diffusion rates of ethanol and water increased, but the adsorption coverage of ethanol on the membrane decreased. If the diffusion rate dominated, the separation factor increased with temperature, otherwise decreased. Therefore, the separation factor increased with the increase of temperature from 30 to 60 ºC. When the adsorption mainly controlled separation process, the separation factor slightly decreased with further increase of temperature to 75 ºC. 3.3.3. The effect of feed concentration Fig. 10 shows the pervaporation performance of membrane M3 with different ethanol feed concentrations. The total flux firstly increased from 1.30 to 1.51 kg·m-2·h-1, and then decreased to 1.38 kg·m-2·h-1 with the increase of feed ethanol concentration in the range of 1-7 wt%. The separation factor obviously decreased from 42.9 to 20.0. According to Fig. 10, it can be seen that ethanol flux increased with increasing feed concentration in the range of less than 5 wt%. This is mainly due to that with the increase of feed ethanol concentration, the ethanol adsorption amount on membrane M3 increased. The high ethanol adsorption

coverage could block the water permeation, leading to the decrease of water flux. However, the increased ethanol flux was larger than the decreased water flux, thus showing that the total flux increased. When the ethanol feed concentration further increased to 7 wt%, the ethanol flux decreased. This may be due to that the chemical bonds formed between ethanol and Si-OH or B-OH of the B-ZSM-11 crystals. As suggested by Sun et al. [38] that the chemical bonds formed between ethanol and Si-OH or B-OH of the B-ZSM-11 crystals hindered the ethanol permeating through the membrane layer duo to the reduced intergrained void size, leading to the decrease of ethanol flux with the feed concentration increasing from 5wt% to 7wt%. This speculation was further proved by the fact that the ethanol flux was increased from 0.70 to 1.02 kg·m-2·h-1 for separation of 7 wt% ethanol/water mixtures at 60 ºC upon calcination for 6 h at 550 ºC after pervaporation measurements with different ethanol feed concentrations (the membrane presented a separation performance with a total flux of 1.66 kg·m-2·h-1 and separation factor of 32.66). The decreased water and ethanol flux both decreased the total flux. However, the influence of feed concentration on the flux of our membrane was different from that of B-ZSM-11 membrane prepared by Li et al. [30], in which the total flux was essentially unchanged. The differences probably resulted from the different microstructures of zeolite membranes prepared with different synthesis procedures. The separation

factor decreased with increasing feed concentration mainly because the adsorption coverage of ethanol on the membrane M3 did not increase proportionately with the feed ethanol concentration [30]. --------Fig.9---------------Fig.10-------3.4. Comparison with literature data Table 3 shows the pervaporation performance of the MEL membranes in this work and the previous reports for 5 wt% ethanol/water mixtures. Tuan et al. [30] first reported that B-ZSM-11 membranes prepared on the SS supports by the in situ synthesis showed separation factors of 42 and 24 for 5 wt% ethanol/water mixtures at 60 ºC, respectively, but their fluxes were low (about 1 kg·m-2·h-1) because membranes thickness were as high as 20-25 µm. Recently, Kosinov et al. [17] reported a high flux of 3.6 kg·m-2·h-1 for the high-silica MEL membrane synthesized on the α-Al2O3 hollow fiber by the secondary growth method and the support had low transport resistance due to its thin wall. The high flux was mainly due to formation of thin membrane with thickness of 4 µm and utilization of hollow fiber support. These results indicated that membrane with high flux could be obtained by decreasing membrane thickness and utilizing support with low transport resistance. In present work, thin B-ZSM-11 membranes with various B/Si ratios were prepared on the low-cost macroporous α-Al2O3 tubes by the

secondary growth synthesis. For example, a much higher flux of 2.19 kg·m-2·h-1 but with a slightly lower separation factor of 18.1 for a 5 wt% ethanol/water mixture at 60 ºC was achieved on the B-ZSM-11 membrane with the same B/Si ratio as that in the reference 30. Particularly, when the B/Si ratios of synthesis solutions increased to 0.06, the membrane flux was 1.51 kg·m-2·h-1 and the separation factor increased to 35.0. The pervaporation separation index (PSI) was 51.39, which is much higher than PSI values of the reported B-ZSM-11 membranes as shown in Table 3. Further efforts are needed to enhance PV performance of B-ZSM-11 membranes. --------Table 3-------4. Conclusions The isomorphously substituted B-ZSM-11 membranes were prepared on low-cost macroporous α-Al2O3 supports after coating different size B-ZSM-11 seeds by a VTHDC seeding method. The effects of B/Si ratio, temperature and concentration in the feed solution on separation performance of the obtained membranes were investigated. XRD and IR analysis confirmed the incorporation of boron into zeolite structure. WCA measurement revealed the introduction of boron into MEL structure enhanced hydrophobicity of the as-synthesized membranes, but excessive amounts of boron decreased their hydrophobicity. Thin and compact B-ZSM-11 membrane with B/Si ratio of 0.06 showed the highest

separation selectivity for ethanol recovery. Acknowledgements We gratefully acknowledge the financial supports of National Natural Science Foundation (Grant No.21376036) and Program New Century Excellent Talents in University (NCET-10-0286).

References [1] Y. Peng, Z.Y. Zhan, L.J. Shan, X.M. Li, Z.B. Wang, Y.S. Yan, Preparation of zeolite MFI membranes on defective macroporous alumina supports by a novel wetting-rubbing seeding method: Role of wetting agent, J. Membr. Sci. 444 (2013) 60-69. [2] M. Zhou, D. Korelskiy, P.C. Ye, M. Grahn, J. Hedlund, A uniformly oriented MFI membrane for improved CO2 separation, Angew. Chem. Int. Ed. 53 (2014) 34923495. [3] R.M. Mihályi, A. Patis, V. Nikolakis, M. Kollár, J. Valyon, [B]MFI membrane: Synthesis, physico-chemical properties and catalytic behavior in the double-bond isomerization of 1-butene, Sep. Purif. Technol. 118 (2013) 135-143. [4] H.Z. Li, J.Q. Wang, J. Xu, X.D. Meng, B. Xu, J.H. Yang, S.Y. Li, J.M. Lu, Y. Zhang, X.L. He, D.H. Yin, Synthesis of zeolite NaA membranes with high performance and high reproducibility on coarse macroporous supports, J. Membr. Sci. 444 (2013) 513-522. [5] Z.B. Wang, Q.Q. Ge, J.S. Gao, J. Shao, C.J. Liu, Y.S. Yan, High-performance zeolite membranes on inexpensive large-pore supports: highly reproducible

synthesis using a seed paste, Chem Sus Chem 4 (2011) 1570-1573. [6] X.R. Wang, Y.Y. Chen, C. Zhang, X.H. Gu, N.P. Xu, Preparation and characterization of high-flux T-type zeolite membranes supported on YSZ hollow fibers, J. Membr. Sci. 455 (2014) 294-304. [7] X.Q. Zou, P. Bazin, F. Zhang, G.S. Zhu, V. Valtchev, S. Mintova, Ethanol recovery from water using silicalite-1 membrane: an operando infrared spectroscopic study, Chem Plus Chem 77 (2012) 437-444. [8] L.J. Shan, J. Shao, Z.B. Wang, Y.S. Yan, Preparation of zeolite MFI membranes on alumina hollow fibers with high flux for pervaporation, J. Membr. Sci. 378 (2011) 319-329. [9] J.A. Stoeger, J. Choi, M. Tsapatsis, Rapid thermal processing and separation performance of columnar MFI membranes on porous stainless steel tubes, Energy Environ. Sci. 4 (2011) 3479-3486. [10] X.L. Zhang, M.H. Zhu, R.F. Zhou, X.S. Chen, H. Kita, Synthesis of a silicalite zeolite membrane in ultradilute solution and its highly selective separation of organic/water mixtures, Ind. Eng. Chem. Res. 51 (2012) 11499-11508. [11] X.J. Shu, X.R. Wang, Q.Q. Kong, X.H. Gu, N.P. Xu, High-flux MFI zeolite membrane supported on YSZ hollow fiber for separation of ethanol/water, Ind. Eng. Chem. Res. 51 (2012) 12073-12080. [12] Z. Chen, D.H. Yin, Y.H. Li, J.H. Yang, J.M. Lu, Y. Zhang, J.Q. Wang, Functional defect-patching of a zeolite membrane for the dehydration of acetic acid by pervaporation, J. Membr. Sci. 369 (2011) 506-513.

[13] F. Zhang, L.N. Xu, N. Hu, N. Bu, R.F. Zhou, X.S. Chen, Preparation of NaY zeolite membranes in fluoride media and their application in dehydration of bio-alcohols, Sep. Purif. Technol. 129 (2014) 9-17. [14] R.R Xu, W.Q. Pang, J.H. Yu, Q.S. Huo, J.S. Chen, Chemistry-zeolites and porous Materials, Science press, Beijing, 2004. [15] L. Song, L.V.C. Rees, Adsorpion and diffusion of cyclic hydrocarbon in MFI-type zeolites studied by gravimetric and frequency-response techniques, Micropor. Mesopor. Mater. 35-36 (2000) 301-314. [16] T.Q. Gardner, J.L. Falconer, R.D. Noble, Transient permeation of butanes through ZSM-5 and ZSM-11 zeolite membranes, AIChE J. 50 (2004) 2816-2834. [17] N. Kosinov, E. Hensen, Synthesis and separation properties of an α-alumina -supported high-silica MEL membrane, J. Membr. Sci. 447 (2013) 12-18. [18] H. Kalipcilar, J.L. Falconer, R.D. Noble, Preparation of B-ZSM-5 membranes on a monolith support, J. Membr. Sci. 194 (2001) 141-144. [19] T.C. Bowen, H. Kalipcilar, J.L. Falconer, R.D. Noble, Separation of C4 and C6 isomer mixtures and alcohol-water solutions by monolith supported B-ZSM-5 membranes, Desalination, 147 (2002) 331-332. [20] V.A. Tuan, J.L. Falconer, R.D. Noble, Isomorphous substitution of Al, Fe, B, and Ge into MFI-zeolite membranes, Micropor. Mesopor. Mater. 41 (2000) 269-280. [21] V.A. Tuan, R.D. Noble, J. L. Falconer, Boron-substituted ZSM-5 membranes: preparation and separation performance, AIChE J. 46 (2000) 1201-1208. [22] V. Sebastián, I. Kumakiri, R. Bredesen, M. Menéndez, Zeolite membrane for

CO2 removal: Operating at high pressure, J. Membr. Sci. 292 (2007) 92-97. [23] V.A. Tuan, S.G. Li, J.L. Falconer, R.D. Noble, Separating organics from water by pervaporation with isomorphously-substituted MFI zeolite membranes, J. Membr. Sci. 196 (2002) 111-123. [24] T.C. Bowen, H.Kalipcilar, J.L. Falconer, R.D. Noble, Pervaporation of organic/ water mixtures through B-ZSM-5 zeolite membranes on monolith supports, J. Membr. Sci. 215 (2003) 235-247. [25] F.H. Saboor, S.N. Ashrafizadeh, H. Kazemian, Synthesis of BZSM-5 membranes using nano-zeolitic seeds: characterization and separation performance, Chem. Eng. Technol. 35 (2012) 743-753. [26] R.F. Zhou, Y.X. Kong, M.H. Zhu, F. Zhang, X.L. Zhang, X.S. Chen, Studies on synthesis and pervaporation performance of boron-substituted MFI zeolite membranes, Chinese J. Inorg. Chem. 5 (2012) 942-948. [27] J. O’Brien-Abraham, Y.S. Lin, Effect of isomorphous metal substitution in zeolite framework on pervaporation xylene-separation performance of MFI-type zeolite membranes, Ind. Eng. Chem. Res. 49 (2010) 809-816. [28] Z. Deng, C.-H. Nicolas, Y. Guo, A. Giroir-Fendler, M. Pera-Titus, Synthesis and characterization of nanocomposite B-MFI-alumina hollow fibre membranes and application to xylene isomer separation, Micropor. Mesopor. Mater. 133 (2010) 18-26. [29] V.A. Tuan, S.G. Li, R.D. Noble, J.L. Falconer, Preparation and pervaporation properties of a MEL-type zeolite membrane, Chem. Commun. (2001) 583-584.

[30] S.G. Li, V.A. Tuan, R.D. Noble, J.L. Falconer, ZSM-11 membranes: characterization and pervaporation performance, AIChE J. 48 (2002) 269-278. [31] W. Xiao, Z. Chen, L. Zhou, J.H. Yang, J.M. Lu, J.Q. Wang, A simple seeding method for MFI zeolite membrane synthesis on macroporous support by microwave heating, Micropor. Mesopor. Mater. 142 (2011) 154-160. [32] X.X. Chen, J.Q. Wang, D.H. Yin, J.H. Yang, J.M. Lu, Y. Zhang, Z. Chen, Highperformance zeolite T membrane for dehydration of organics by a new varying temperature hot-dip coating method, AIChE J. 59 (2013) 936-947. [33] Y. Cheng, S.L. Pan, Preparation and characterization of nanosized silicalite-2 zeolites by steam-assisted dry gel conversion method, Mater. Lett. 100 (2013) 289-291. [34] M. Mehdipourghazi, A. Moheb, H. Kazemian, Incorporation of boron into nanosize MFI zeolite structure using a novel microwave-assisted two-stage varying temperatures hydrothermal synthesis, Micropor. Mesopor. Mater. 136 (2010) 18-24. [35] H. Koller, R.F. Lobo, S.L. Burkett, M.E. Davis, SiO-···HOSi Hydrogen Bonds in As-Synthesized High-Silica Zeolites, J. Phys. Chem. 99 (1995) 12588-12596. [36] L.A. Villaescusa, P.S. Wheatley, I. Bull, P. Lightfoot, R.E. Morris, The location and ordering of fluoride ions in pure silica zeolites with framework types IFR and STF; Implications for the mechanism of zeolite synthesis in fluoride media, J. Am. Chem. Soc. 123 (2001) 8797-8805. [37] G.P. Liu, F.J. Xiangli, W. Wei, S.N. Liu, W.Q. Jin, Improved performance of

PDMS/ceramic

composite

pervaporation

membranes

by

ZSM-5

homogeneously dispersed in PDMS via a surface graft/coating approach, Chem. Eng. J. 174 (2011) 495-503. [38] W.G. Sun, X.W. Wang, J.H. Yang, J.M. Lu, H.L. Han, Y. Zhang, J.Q. Wang, Pervaporation separation of acetic acid-water mixtures through Sn-substituted ZSM-5 zeolite membranes, J. Membr. Sci. 335 (2009) 83-88.

Table captions: Table 1 ICP analysis results, crystal lattice parameters and unit cell volumes of B-ZSM-11 membranes with different B/Si ratios calculated from XRD patterns.

Table 2 PV performance of B-ZSM-11 membranes M1-M5 with various B/Si ratios together with silicalite-2 membrane M0 for 5wt% ethanol/water mixtures at 60 ºC.

Table 3 PV performance of MEL membranes for 5 wt% ethanol/water mixtures at 60 ºC previously reported in the literature and in this work.

Figure captions: Fig.1 The pore structure of MEL zeolite. Fig.2 XRD patterns of B-ZSM-11 seeds with the size of 1 µm (a) and 600 nm (b) together with silicalite-2 crystals (c). Fig.3 SEM images of B-ZSM-11 seeds with the size of: (a) 1 µm and (b) 600 nm and silicalite-2 crystals (c). Fig.4 FT-IR spectra of 600 nm B-ZSM-11 seeds and silicalite-2 crystals. (I3461/I796 and I1637/I796 mean ratio of the intensity of the 3461 and 1637 cm-1 bands to the intensity of the 796 cm-1 band.) Fig.5 Surface (a) and cross-section (b) SEM images of the B-ZSM-11 seed layer. Fig.6 XRD patterns of bare support and B-ZSM-11 membranes with B/Si ratios of 0 (silicalite-2 membrane, M0), 0.03 (M2), 0.06 (M3), 0.08 (M4) and 0.1 (M5). Fig.7 Surface and cross-section SEM images of B-ZSM-11 membranes with different B/Si molar ratios. M0 (silicalite-2 membrane, a0 and a0’), M1 (a and a’), M2 (b and b’), M3 (c and c’), M4 (d and d’) and M5 (e and e’). Fig.8 Water contact angles of silicalite-2 and B-ZSM-11 membranes with B/Si ratios varying from 0.03 to 0.1. Fig.9 Feed temperature dependence of pervaporation performance for recovery of ethanol from 5 wt% EtOH/H2O mixtures through membrane M3. Fig.10 Feed concentration dependence of pervaporation performance for recovery of ethanol from aqueous solutions at 60 ºC through membrane M3.

Table 1 ICP analysis results, crystal lattice parameters and unit cell volumes of B-ZSM-11 membranes with different B/Si ratios calculated from XRD patterns. B/Si in synthesis

B/Si after a/Å

b/Å

c/Å

V/Å3

solution

synthesis

0

0

20.2030

20.2030

13.5085

5513.6813

0.03

0.011

20.1815

20.1815

13.4959

5496.8039

0.06

0.017

20.1612

20.1612

13.4732

5476.5000

0.08

0.033

20.1216

20.1216

13.4402

5444.0946

0.1

0.039

20.1158

20.1158

13.4111

5426.7548

Table 2 PV performance of B-ZSM-11 membranes M1-M5 with various B/Si ratios together with silicalite-2 membrane M0 for 5wt% ethanol/water mixtures at 60 ºC. Membrane no.

B/Si ratio

Flux (kg·m-2·h-1)

Separation factor

M0

0

5.78

3.7

M1

0.01

2.19

18.1

M2

0.03

2.03

19.6

M3

0.06

1.51

35.0

M4

0.08

2.17

12.7

M5

0.1

1.92

10.1

Table 3 PV performance of MEL membranes for 5 wt% ethanol/water mixtures at 60 ºC previously reported in the literature and in this work. Mmebrane Support

Flux

Separation

B/Si ratio -2

-1

PSI

Ref.

thickness/µm

(kg·m ·h )

factor

4

3.6

17.2

58.32

17

0.93

42

38.13

30

1.2

24

27.60

30

Al2O3 HF

0

SS tube

0.01

SS tube

0.01

Al2O3 tube

0.01

5

2.19

18.1

37.45

This

Al2O3 tube

0.06

7

1.51

35.0

51.39

work

20-25

Note: HF: hollow fiber, SS: stainless steel, PSI=Flux×(Separation factor-1).

Research highlights

·B-ZSM-11 membranes were synthesized by a secondary growth method. ·Cheap macroporous α-Al O tubes were used as supports. ·Boron atom was found to improve growth and hydrophobocity of zeolite MEL membrane. ·PV performance of the obtained membranes was improved. 2

3

Fig.1 The pore structure of MEL zeolite.

Fig.2 XRD patterns of B-ZSM-11 seeds with the size of 1 µm (a) and 600 nm (b) together with silicalite-2 crystals (c).

Fig.3 SEM images of B-ZSM-11 seeds with the size of: (a) 1 µm and (b) 600 nm and silicalite-2 crystals (c).

Fig.4 FT-IR spectra of 600 nm B-ZSM-11 seeds and silicalite-2 crystals. (I3461/I796 and I1637/I796 mean ratio of the intensity of the 3461 and 1637 cm-1 bands to the intensity of the 796 cm-1 band.)

Fig.5 Surface (a) and cross-section (b) SEM images of the B-ZSM-11 seed layer.

Fig.6 XRD patterns of bare support and B-ZSM-11 membranes with B/Si ratios of 0 (silicalite-2 membrane, M0), 0.03 (M2), 0.06 (M3), 0.08 (M4) and 0.1 (M5).

Fig.7 Surface and cross-section SEM images of B-ZSM-11 membranes with different B/Si molar ratios. M0 (silicalite-2 membrane, a0 and a0’), M1 (a and a’), M2 (b and b’), M3 (c and c’), M4 (d and d’) and M5 (e and e’).

Fig.8 Water contact angles of silicalite-2 and B-ZSM-11 membranes with B/Si ratios varying from 0.03 to 0.1.

Fig.9 Feed temperature dependence of pervaporation performance for recovery of ethanol from 5 wt% EtOH/H2O mixtures through membrane M3.

Fig.10 Feed concentration dependence of pervaporation performance for recovery of ethanol from aqueous solutions at 60 ºC through membrane M3.

Graphical Abstract (for review)