Seed-free synthesis of highly permeable zeolite NaA membranes through deposition of APTES-functionalized alumina particles on macroporous supports

Seed-free synthesis of highly permeable zeolite NaA membranes through deposition of APTES-functionalized alumina particles on macroporous supports

Journal of Membrane Science 471 (2014) 84–93 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.co...

4MB Sizes 2 Downloads 93 Views

Journal of Membrane Science 471 (2014) 84–93

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Seed-free synthesis of highly permeable zeolite NaA membranes through deposition of APTES-functionalized alumina particles on macroporous supports Huazheng Li, Jing Xu, Jinqu Wang, Jianhua Yang n, Ke Bai, Jinming Lu, Yan Zhang, Dehong Yin Institute of Adsorption and Inorganic Membrane, State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China

art ic l e i nf o

a b s t r a c t

Article history: Received 1 May 2014 Received in revised form 21 July 2014 Accepted 28 July 2014 Available online 4 August 2014

A new strategy through deposition of 3-aminopropyltriethoxysilane (APTES) functionalized Al2O3 particles on a coarse macrosporous tube with big defect holes was reported for seed-free synthesis of zeolite NaA membranes. Prior to the deposition of Al2O3 particles onto the macroporous support, APTES was covalently functionalized onto the surface of Al2O3 particles, aiming at providing heterogeneous sites for the growth of NaA zeolites on the support surface by serving as linker between the support and the silica and aluminum sources. Large APTES-Al2O3 particles were firstly deposited on the support surface to reduce the surface roughness and the effective pore sizes, subsequently the small APTES-Al2O3 particles were deposited on the support to provide plenty of heterogeneous nucleation sites for the growth of zeolite NaA crystals. In this way, a thin and continuous APTES-Al2O3 layer was formed on the support. After one single hydrothermal synthesis, a thin and compact zeolite NaA membrane with thickness of 2 μm was achieved. The synthesized NaA membranes showed high fluxes of 3.7– 3.9 kg m  2 h  1 with separation factors over 5000 in dehydrating the 90 wt% ethanol/10 wt% water mixtures at 75 1C. The strategy presented in this study was expected to extend for preparation of other types of thin zeolite membranes on such coarse symmetric macroporous supports. & 2014 Published by Elsevier B.V.

Keywords: Seed-free synthesis Zeolite NaA membrane Deposition of APTES-Al2O3 particle Coarse macroporous support Dehydration of ethanol

1. Introduction Zeolite membranes are intensively studied based on the application of sensors, reactors, separators and so on in the past two decades [1–8]. Zeolite NaA membranes, due to their hydrophilic framework and interesting pore size, are mostly used in dehydrating of organic solutions by pervaporation or vapor permeation [9–12]. After the Mitsui Engineering & Shipbuilding Co. Ltd. firstly industrialized the zeolite NaA membranes in 1999 [13], many industrial plants for the dehydration of solvents or biofuels now could be found around the world [14]. However, it is still far away from being broadly used in industrial applications due to the high cost of zeolite NaA membranes [15]. A thin and defect-free zeolite NaA membrane with high flux and separation performance supported on a low cost substrate is of great interest and preferable in industrial membrane application [16] since the high flux could reduce the required membrane

n

Corresponding author. Tel./fax: þ 86 411 84986147. E-mail address: [email protected] (J. Yang).

http://dx.doi.org/10.1016/j.memsci.2014.07.057 0376-7388/& 2014 Published by Elsevier B.V.

area and the low-cost substrates would largely reduce the required capital investment. Generally, the zeolite NaA membrane synthesis methods are classified into two categories, known as in-situ method and secondary growth method. The in-situ method was few performed to prepare zeolite membranes due to the insufficient understanding of nucleation and growth process in hydrothermal systems. And zeolite membranes are mostly synthesized through secondary growth in which the zeolite crystals were seeded onto the surface of the supports by a variety of seeding methods [17–20] before the hydrothermal synthesis. The pre-coated zeolite crystals acted as nuclei to provide sites for zeolite growth, therefore, the seed size, orientation, coverage are significant for the control of the membrane microstructure and the resulting separation performance [21]. Mostly, nano-sized crystals were used as seeds to prepare thin zeolite membranes [22–24] on supports with small pore size. Yoo et al. prepared 400–500 nm thick silicalite-1 membranes on porous alumina supports with pore size of 150–200 nm by coating polycrystalline spherical aggregates of zeolite seeds [23]. However, the cost of the preparation of the nano-sized seed crystals was high due to the low yield and the expensive structure-directing agent [25–27].

H. Li et al. / Journal of Membrane Science 471 (2014) 84–93

Recently, Huang et al. developed a seeding-free strategy by chemically modifying the supports with 3-aminopropyltriethoxysilane (denote as APTES hereafter) as a covalent linker to synthesize zeolite [28–31] and ZIF membranes [32,33]. The 3-aminopropylsilyl groups were used as highly efficient molecular linker to attract and anchor the zeolite or ZIF nutrients onto the support surface during the hydrothermal synthesis. The prepared zeolite membranes showed good separation performance in gas separation and iso-butanol dehydration [34]. Macroporous symmetric tubes could be cheaply purchased and had been successfully applied in industry for the water treatment. Also, commercial tubular membrane modules are available [13,35]. Compared with the asymmetric supports, the symmetric architecture of the supports would give higher fluxes due to the smaller mass transfer resistance. However, the poor surface property of the macroporous supports consisting of disorderly and irregular alumina particles, increased the risk of the formation of good zeolite membranes. In our previous work [36–38], the varying temperature hot dip-coating (VTHD) seeding method, gradually depositing large and small zeolite seeds, was developed to prepare continuous, dense and pinhole-free zeolite membranes on the macroporous symmetric supports. The surface roughness and large pores were largely decreased by the deposition of the large seeds, and then the density of the nucleation sites was largely improved by the deposition of the small seeds. Very recently, we proposed a “two-in-one” strategy [39], in which APTESfunctionalized alumina particles (denoted as APTES-Al2O3 below) was coated onto the supports to reduce the pore size and promote the heterogeneous nucleation sites, to prepare thin ZIF-8 membranes on coarse macroporous supports, and the as-synthesized ZIF-8 membranes showed a thickness of about 2 μm and exhibited remarkably high H2 permeance of 5.73  10  5 mol m  2 s  1 Pa  1 with H2/N2 ideal selectivity of 15.4. In this study, we present an extension of the “two-in-one” approach, in which the large and small APTES-Al2O3 particles were gradually deposited onto the support to form a functional alumina layer as shown in Fig. 1, to prepare thin zeolite NaA membranes on the low-cost macroporous symmetric tubular alumina supports. The deposited large APTES-Al2O3 particles serve as fillers to modify the large defect holes and reduce the support surface roughness, while the deposited small APTES-Al2O3 particles further reduce the pore size of the support and act as “tentacles” to provide more heterogeneous nucleation sites by serving as linker between the support and the Si and Al source. After the hydrothermal synthesis, the PV separation performance of the prepared zeolite NaA membranes was tested by dehydrating 90 wt% ethanol aqueous mixtures at 75 1C.

85

2. Experimental 2.1. Materials Coarse macroporous alumina tubes with an average pore size of 2–3 μm (OD: 13 mm, ID: 9 mm, length: 80 mm, porosity of 30–40%, Foshan Ceramics Research Institute, China) were used as supports. Sodium hydroxide (4 98 wt%, Tianjin Kermel Chemical Reagent Co., Ltd.), colloidal silica (SiO2: 25 wt%, Na2O: 0.3 wt%, Qingdao Haiyang Chemical Co., Ltd.), sodium aluminate (Al2O3: 41 wt%, Na2O: 24.92 wt%, Sinopharm Chemical Reagent Co., Ltd.), toluene (99.5 wt%, Tianjin Kermel Chemical Reagent Co., Ltd.), APTES (99 wt%, J&K Scientific Ltd.) were obtained commercially as reagent chemicals and used as received without purification. The deionized water was home-made. The supports were firstly polished by SiC sandpaper, and then washed in acid and alkaline solution and deionized water under ultrasonic bath to remove the possible inorganic particles retained in the support pores. Subsequently, the tubes were dried overnight in the air oven at 80 1C and then calcined in the air at 550 1C for 6 h with a rate of 1 1C min  1 for both heating and cooling to remove the excess organic compounds remained in the support pores. 2.2. APTES modification of the alumina particles Large (about 1–1.5 μm) and small α-Al2O3 particles (about 0.4 μm) were chemically functionalized with APTES according to the procedure as reported elsewhere [39]. Typically, 5 g of α-Al2O3 particles were added into 150 ml of toluene which were preheated to 110 1C under N2 conditions, then 5 ml of APTES was added dropwise into the vigorously stirring mixtures and reacted for 3–4 h. The APTES-Al2O3 particles were washed thoroughly with toluene and dried at 120 1C overnight to remove the toluene solvent. The APTES-Al2O3 particles were dispersed into ethanol to get 3 wt% or 0.6 wt% suspensions for alumina layer preparation. 2.3. Deposition of the alumina particles The varying temperature hot dip-coating (VTHD) method was used to prepare dense alumina layer on the surface of the macroporous supports. Typically, (1) the pre-heated (120 1C) tube with two ends sealed with Teflon plugs was dipped into welldispersed large APTES-Al2O3 suspension for about 20 s, and

Fig. 1. Schematic diagram of seed-free synthesis zeolite NaA membranes on the macroporous support by VTHD coating the APTES-functionalized alumina particles.

86

H. Li et al. / Journal of Membrane Science 471 (2014) 84–93

Table 1 Deposition and hydrothermal synthesis conditions of zeolite NaA membranes on the macroporous supports. Membrane Depositing conditions

Synthesis time (h)

Large particles (  1.5 μm) Small particles (  0.4 μm) M1 M2 M3 M4 M5 M6 M7 M8 M9 M10

a

– APTES-free-Al2O3 APTES-free-Al2O3 APTES-free-Al2O3 APTES-Al2O3 APTES-Al2O3 APTES-Al2O3 APTES-Al2O3 APTES-Al2O3 APTES-free-Al2O3

– APTES-free-Al2O3 APTES-free-Al2O3 APTES-free-Al2O3 APTES-Al2O3 APTES-Al2O3 APTES-Al2O3 APTES-Al2O3 APTES-Al2O3 APTES-free-Al2O3

24 18 21 24 18 21 24 24 24 24

a

Functionalize the pre-deposited support with APTES, then grow zeolite NaA membrane.

followed by drying at 50 1C for 3 h and then at 120 1C for 3 h; (2) the outer surface of the dried support was rubbed carefully with cotton to remove the superfluous alumina particles that loosely packed on the surface. Subsequently, the support was heated in an oven at 80 1C for 3 h; (3) the pre-heated support was then dipped into well-dispersed small APTES-Al2O3 particles suspension for about 20 s. For comparison, large and small APTES-free-Al2O3 particles were also deposited onto the support surface by the VTHD method to prepare alumina layers and then the alumina layers were treated with APTES. Then zeolite NaA membranes were synthesized on these supports. Besides, the insitu hydrothermal synthesis on the naked support was also performed. The coating details were all listed in Table 1.

Fig. 2. XRD patterns of alumina particles and APTES-functionalized alumina particles.

where X and Y are the weight fractions of the water components in the binary feed mixture and in the permeate, respectively. Subscripts a and b refer to water and ethanol components, respectively. 2.6. Characterization Scanning electron microscopy (SEM) images of the surfaces of supports, alumina layers and membranes were taken with KYKY2800B at an acceleration voltage of 20 kV after gold coating. X-ray diffraction (XRD) data were collected on a Philips Analytical X-ray diffractometer using Cu Kα radiation (30 mA and 40 KV). Elemental analysis of the alumina particles were conducted with 0.0001 mg/vario EL III (Elementar Analsen System GmbH, Germany). The FT-IR spectra were recorded as KBr pellets on JASCO FT/IR-430.

2.4. Hydrothermal synthesis of zeolite NaA membranes The membrane synthesis solution was prepared with the same as reported in [36], except that the aging time was increased from 6 h to 20 h at room temperature. The hydrothermal synthesis of zeolite NaA membranes was performed at 60 1C for 24 h [28–30]. After the hydrothermal growth, the prepared zeolite membranes were washed thoroughly with deionized water until the pH value of the washing water became neutral, and then were dried in air oven overnight at 50 1C. 2.5. PV separation The permeation properties of the zeolite NaA membranes were evaluated by pervaporation using ethanol (90 wt%)/water (10 wt%) mixture. PV experiments were performed on a home-made laboratory scale apparatus. The membrane within a membrane model was dipped into the feed alcohol/water mixture which was preheated to 75 1C. The inside of the membrane was evacuated with a vacuum pump. One cold trap with liquid N2 cooling was used to collect the permeate. The permeation flux (F, kg m  2 h  1) was measured by weighing the condensed permeate and calculated as F ¼ W=ðtAÞ

ð1Þ

where W is the weight of permeate, A refers to the permeating area of the membrane, and t is the permeating time. The concentrations of the permeate and the feed were measured by a gas chromatograph (Dalian Zhonghuida Scientific Instrument). The water/alcohol separation factor was calculated as α¼

Y a Xa = Y b Xb

ð2Þ

3. Results and discussion 3.1. Chemical functionalization of alumina particles with APTES Fig. 2 shows the XRD patterns of the α-Al2O3 particles before and after the modification. No other peaks were observed indicating that there was no crystalline damage on the particles during the chemical modification. Since the ethoxy groups of APTES can react with surface hydroxyl groups of alumina particles, bringing the terminal amino groups anchored onto the surface of the particles [40]. The presence of the amino groups was firstly proved by the formation of purple dye [41] (as shown in Fig. 3-2) by adding ninhydrin into the APTES-Al2O3 suspensions, whereas no obvious color changes were obtained for the APTES-free-Al2O3 suspensions (as shown in Fig. 3-1). The graft of amine groups on the surface of the alumina particles can be further qualitatively confirmed by the FT-IR spectra as shown in Fig. 4. Compared with the FT-IR spectrum of APTES-free-Al2O3 particles, the FT-IR spectrum of APTES-Al2O3 particles displayed additionally remarkable bands at 1035 cm  1, 1130 cm  1, 1490 cm  1, 1590 cm  1, 2935 cm  1. The peak at 1035 cm  1 can be contributed to Si–O–C stretching vibrations of alkoxy groups [42]. The peak at 1130 cm  1, which can be assigned to the C–N stretching vibration [43], is very intensive for the APTES-modified sample, and the N–H vibrations at 1490 cm  1 and 1585 cm  1, indicating the existence of R–NH2 groups. The 2935 cm  1 peak can be assigned to the C–H stretching vibrations [44]. The width of the broad –OH peak around 3440 cm  1 for APTES-Al2O3 sample was slightly larger than that of the APTESfree-Al2O3 sample. This widening probably results from the overlap of the symmetric stretching of N–H and –OH [43].

H. Li et al. / Journal of Membrane Science 471 (2014) 84–93

87

not be contained in the sample, probably resulted from the organic contamination remained originally on the surface of the particles. The detected H was assigned to the surface hydroxyl of the alumina particles and to the absorbed water in the sample. For the APTES-Al2O3 sample, reasonable amounts of both N and C were detected, and the increment of N was proportional to that of C. However, the experimental mol ratio of N:C (1:3.56) was slightly lower than the theoretical value (1:3). This could be attributed to those carbons from the ethoxy groups because of the incomplete hydrolysis of APTES and from the solvent toluene which was not removed thoroughly. The detachment of N, together with the color change and the FT-IR results, strongly proved that the amine groups were successfully attached onto the surface of the alumina particles in the chemical modification step.

Table 2 lists the chemical compositions of the APTES-free-Al2O3 and APTES-Al2O3 particles analyzed by elemental analysis under the CHN module. For the APTES-free-Al2O3 sample, no N element was detected. And the detected trace amount of C, which should

3.2. Surface modification of the support through deposition of APTES-Al2O3 particles After the successful modification of alumina particles by APTES, the particles were then deposited onto the support surface to prepare asymmetric alumina layer by the VTHD method. More specifically, it consisted of three sub-processes: (1) hot dip-coating of large APTES-Al2O3 particles at high temperature of 120 1C; (2) rubbing off the loose and superfluous alumina particles on the support surface; (3) hot dip-coating of small APTES-Al2O3 particles at low temperature of 70 1C. Fig. 5 shows the SEM images of the top surface and crosssection of the supports and the membranes prepared on the naked support with synthesis time of 24 h. It is obviously observed that the support surface was rather rough and discontinuous. Large pores with size of 2–3 μm and defect holes with size of 7–10 μm randomly distributed on the surface. The cross-section morphology (Fig. 5b) indicated that the supports were consisted of irregular alumina particles. Compared with the asymmetric supports, all of the poor properties are adverse for the preparation of thin and dense zeolite membranes. The zeolite NaA membrane (M1) prepared on the naked support grew along the support surface and showed a rough surface as shown in Fig. 5c and d. Cracks between the two adjacent poly-crystals could be obviously observed. Additionally, defect membrane pits were clearly observed in the support defect pores, resulting in a very discontinuous zeolite membrane as shown in Fig. 5c. Fig. 6 shows the SEM images of the support after coating the alumina particles. After coating the large alumina particles in step (1), it can be seen from Fig. 6a that the entire support surface was coated with randomly distributed alumina particles. Large defect pores with size up to 10 μm was eliminated. But defect holes with size of about 2 μm between the two APTES-Al2O3 particles could be observed, and the surface was still very rough. The thickness of the alumina layer was not uniform, of about 20 μm (Fig. 6b). After rubbing off the superfluous particles, as shown in Fig. 6c, the pores, especially the large defect pores were filled with large APTES-Al2O3 particles. The alumina particles randomly packed in the pits on the support surface resulting in a roughness-decreased surface. After step (3), the hot dip-coating of the small APTESAl2O3 particles, a compact, smooth and continuous alumina layer with thickness of about 2 μm was formed on the macroporous

Fig. 3. Optical photos of the small alumina particles dispersed in ethanol after the ninhydrin was dipped into the system. Bottle 1 was the APTES-free Al2O3 particles, and bottle 2 was the APTES-Al2O3 particles. The purple color in bottle 2 strongly indicated that the presence of amino groups. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. FT-IR spectra of the APTES-free-Al2O3 particles (black) and the APTES-Al2O3 particles (red). The arrows represent the specific peak positions of grafted APTES. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 2 Elemental analysis data of APTES-free-Al2O3 and APTES-Al2O3 particles. Particles

APTES-free-Al2O3 APTES-Al2O3

N

C

H

(wt%)

 10  5 (mol/g)

(wt%)

 10  5 (mol/g)

(wt%)

 10  5 (mol/g)

0 0.115

0 8.21

0.075 0.425

6.24 35.38

0.186 0.247

184.54 245.06

88

H. Li et al. / Journal of Membrane Science 471 (2014) 84–93

Fig. 5. SEM images of the top surface and cross-section of the supports (a, b) and the membranes (c, d) prepared on the naked support with synthesis time of 24 h.

support surface (as shown in Fig. 6e and f). The deposited large APTES-Al2O3 particles in step (1) stopped the penetration of the small APTES-Al2O3 particles into the depth of the support pores in step (3), therefore, a clear border was clearly observed between the macroporous support and the external alumina layer. For comparison, the large and small APTES-free-Al2O3 particles were deposited onto the support surface by VTHD method to investigate the role of APTES. The SEM images (not shown in this study) of the APTES-free-Al2O3 layer showed no differences with these of the APTES-Al2O3 layer. 3.3. Hydrothermal synthesis of zeolite NaA membranes After the deposition of asymmetric alumina layer on the surface of the macroporous support, the hydrothermal crystallization for membrane growth was performed at 60 1C for 18–24 h. Fig. 7 shows the XRD patterns of the as-prepared zeolite NaA membranes. Typical LTA-type peaks could be seen besides the strong diffraction peaks of alumina support (2θ¼ 251, 34.81 and 37.41), confirming the formation of pure zeolite NaA layer on the support for each membrane. The LTA typical peaks were relatively weaker than those of the substrate due to the thin thickness of the NaA layer compared to the macroporous support layer. Fig. 8 shows the SEM images of the zeolite NaA membranes prepared on the supports coated with APTES-free-Al2O3 particles for different synthesis time. It is clearly seen that hemispherical crystals (Fig. 8a) were formed on the support surface after 18 h, and the crystals distributed loosely and randomly on the zeolite layer. The thickness of the zeolite layer was about 1.5 μm (Fig. 8b). After 21 h, the crystals came to the intermediate state between the hemispherical and cubic, and crystals changed from isolated ones to polycrystalline ones (Fig. 8c). However, the zeolite layer

was still uneven, inter-polycrystalline gaps could be obviously observed. The thickness of the zeolite layer was about 2 μm (Fig. 8d). When the synthesis time increased to 24 h, the zeolite crystal became more cubic showing a typical cubic shape (Fig. 8e). Cracks which probably developed from inter-polycrystalline gaps could be detected on the membrane surface (as marked by red circles in Fig. 8e). Some irregular big crystals grew out the membrane layer (which could be observed in Fig. 8f) and formed some defects which would affect the membrane separation performances. The thickness of the zeolite membranes was about 2 μm (Fig. 8f), showing no obvious increase in the membrane thickness. Fig. 9 shows the SEM images of the zeolite NaA membranes prepared on the supports coated with APTES-Al2O3 particles for different synthesis time. When the synthesis time was 18 h, wellshaped hemispherical crystals was formed, and the crystals distributed compactly and uniformly on the support surface (Fig. 9a) which was consistent with that reported by Huang et al. [29]. After 21 h, crystals with less cubic shape were obtained (Fig. 9c). Compared with the zeolite layer prepared on the support surface coated with APTES-free-Al2O3 particles, the zeolite layer prepared on the support surface coated with APTES-Al2O3 particles was smoother and denser. When the synthesis time increased to 24 h, a dense, continuous and smooth zeolite membrane was obtained. The zeolite membrane was consisted of well-shaped cubic crystals, and no obvious cracks, pinholes and other defects were visible (Fig. 9e). The thicknesses of the zeolite layers (Fig. 9b, d, f) showed no obvious differences with those of prepared on the APTES-freeAl2O3 surfaces (Fig. 8b, d, f). The zeolite NaA membrane (M4) hydrothermally synthesized on the support deposited with the APTES-free-Al2O3 particles showed an obvious improvement in membrane morphology

H. Li et al. / Journal of Membrane Science 471 (2014) 84–93

89

Fig. 6. SEM images of the supports after the deposition of large alumina particles (a, b) and rubbing off the superfluous large particles (c, d) and the deposition of small alumina particles (e, f).

compared to that prepared on the naked support (M1). The differences between the two membranes indicated that a smooth surface with small pore size would favor the formation of zeolite membranes with good membrane morphology. Compared with the membranes (M2–M4) prepared on the APTES-free-Al2O3 surfaces, the membranes prepared on the APTES-Al2O3 surfaces (M5–M7) showed better membrane morphology. The only reason could be contributed to the difference in chemical nature between the two support surfaces. As reported previously [28,34], the APTES could efficiently attract and anchor the LTA nutrients onto the alumina surface due to the silazane-based silylation reaction, and the deposited small APTES-Al2O3 particles in step (3) highly promoted the density of the heterogeneous nucleation sites due to the larger resultant surface area [39]. Compared with the APTESfree-Al2O3 surface, the APTES-Al2O3 surface offered more

nucleation sites, therefore, the nucleation and crystal growth could take place uniformly and simultaneously which enabled the formation of a dense, continuous and pinhole-free zeolite NaA membrane on the macroporous support. Considering the fact that the surfaces of alumina particles were changed from negatively charged to neutral when the alumina particles were functionalized with APTES [31], a better control experiment was performed to rule out the possible difference between support with deposited APTES-free-Al2O3 and support with deposited APTES-Al2O3. Before the support pre-deposited with APTES-free-Al2O3 particles was hydrothermally treated to grow zeolite NaA membrane, the APTES functionalization step was performed. The SEM images of the alumina layer after APTES functionalization and zeolite membrane (M10) after crystallization were shown in Fig. 10. However, after functionalization, the

90

H. Li et al. / Journal of Membrane Science 471 (2014) 84–93

Fig. 7. XRD patterns of the zeolite NaA membranes prepared on the macroporous supports.

alumina particles were not compactly and uniformly distributed on the support surface even with some support surface naked (Fig. 10a) which was quite different from that of before functionalization (Fig. 6e). The main reason is probably because some of the Al2O3 particles peeled off from the support surface during the functionalization course. When performing functionalization, vigorous stir operation was needed to ensure that the surface hydroxyl groups of alumina particles completely react with ethoxy groups of APTES. After that, the support surface was washed thoroughly with toluene to remove the residual APTES. During these operations, some smaller and lighter alumina particles were washed away from the compact alumina layer, leaving a less compact and uniform alumina layer. In the region of large defect holes, the washing effect was severer. Thus large defect pin-holes (as shown in Fig. 10c) were detected on the top surface of the prepared zeolite NaA membrane.

Fig. 8. SEM images of zeolite NaA membranes prepared on the supports coated with APTES-free-Al2O3 particles for different synthesis time: 18 h (a, b), 21 h (c, d) and 24 h (e, f). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

H. Li et al. / Journal of Membrane Science 471 (2014) 84–93

91

Fig. 9. SEM images of zeolite NaA membranes prepared on the supports coated with APTES-Al2O3 particles for different synthesis time: 18 h (a, b), 21 h (c, d) and 24 h (e, f).

Very recently, Huang et al. synthesized zeolite NaA membranes by directly modifying APTES onto the surface of the alumina tubular supports, and the prepared dense zeolite NaA membranes showed good butanol and water separation performance with separation factor of 3000 [34]. Similar approaches were also being performed on our macroporous supports. However, poor membrane morphology and low separation performances similar with those of prepared on the naked support sample were observed (not shown in this study). The main reason is probably due to the use of poorer macroporous supports with large defect pores with size of up to 10 μm in this study. The large defect pores have a depth of about 7 μm, while the membrane thickness prepared in this study was only about 2 μm. The sharp mismatch of the depth of the defect pores and the thickness of the zeolite NaA membranes led to a non-ignorable dislocation, which resulted in a poor membrane morphology and separation performance.

3.4. Pervaporation performances of the zeolite NaA membranes The PV performance of the prepared zeolite NaA membranes were tested on the home-made PV equipment as reported in [38]. The feed was a 90 wt% ethanol/10 wt% water mixture, and the temperature was fixed at 75 1C. The results obtained from the different membranes and from literatures were listed in Table 3. The membrane prepared on the naked support showed a flux of 5.24 kg m  2 h  1 but with poor selectivity of 35. The large membrane cracks observed in the SEM image (Fig. 5c) should be the reason of the low separation selectivity of the membrane. While for the membranes prepared by coating the APTES-free-Al2O3 (M4) and APTES-Al2O3 (M7) particles showed fluxes of 4.16 kg m  2 h  1 and 3.87 kg m  2 h  1, respectively, and the separation selectivities were 943 and 5132, respectively. The membrane (M10) prepared on the support which was firstly coated with APTES-free-Al2O3 particles

92

H. Li et al. / Journal of Membrane Science 471 (2014) 84–93

Fig. 10. SEM images of alumina layer after functionalization (a, b) and as-synthesized zeolite NaA membrane after crystallization (c, d).

Table 3 PV performances of zeolite NaA membranes in dehydrating 90 wt% ethanol/10 wt% water mixtures at 75 1C. Membrane

Flux (kg m  2 h  1)

Separation factor

Reference

M1 M4 M7 M8 M9 M10 M11 M12 M13 M14

5.24 4.16 3.87 3.73 3.94 4.59 2.85 3.5–4.0 2.15 2.2–2.66

35 943 5132 6239 5086 439 410,000 10,000 10,000 410,000

This This This This This This [36] [15] [45] [46]

a

a

study study study study study study

Pervaporation temperature was 70 1C.

then functionalized with APTES showed a flux of 4.59 with separation factor of 439. PV results indicated that the membrane prepared by coating the APTES-Al2O3 particles showed the best PV separation performance among the samples, which was consistent with the SEM observations as discussed above. Compared with the previously reported zeolite NaA membranes (M11) which were hydrothermally prepared on the same macroporous supports by secondary growth method through VTHD seeding [36], the membrane prepared in this study showed a competitively higher flux at the range of 3.7– 3.9 kg m  2 h  1. They also showed competitively higher flux compared to those of reported in the literatures [15,45,46]. One possible reason was that the difference in the membrane thicknesses. The thinner membrane thickness in this study leads to a smaller mass transfer resistance when the membrane was performed under the

same PV conditions. Although this procedure showed a little increase in cost and complexity compared to the secondary growth method, it is of great value due to the competitively higher fluxes. The above results indicated that the step-wise deposition of large and small APTES-Al2O3 particles by VTHD method onto the surface of the macroporous support was feasible and effective to prepare thin zeolite NaA membranes on the macroporous support with big defect pores. The firstly deposited large alumina particles effectively reduced the surface roughness and pore sizes, and then the small APTES-Al2O3 particles further smoothed the support surface and improved the density of the heterogeneous nucleation sites. Besides the thin zeolite NaA membranes, other type thin zeolite membranes are expected to be prepared without [30] or with [47] changing the molecular linker on such macroporous supports by this strategy. Moreover, the strategy reported in this study also seemed to extend the chemical modification method in assembling zeolite layer onto the macroporous supports, which was ultra-uneven and unfeasible compared to the common used glass plate [40,48,49].

4. Conclusions A seed-free strategy was firstly applied to synthesis zeolite NaA membranes on the coarse macroporous tubular support with large defect holes. An asymmetric intermediate APTES-Al2O3 layer was previously coated onto the support surface by VTHD method to eliminate the negative influences of the poor support. The large APTES-Al2O3 particles were firstly coated to reduce the surface roughness and pore size, the small APTES-Al2O3 particles were coated to form a compact, continuous and smooth layer which was

H. Li et al. / Journal of Membrane Science 471 (2014) 84–93

favorable for the formation of good zeolite membranes. Furthermore, the 3-aminopropylsilyl groups attached on the small alumina particles improved the density of the heterogeneous nucleation sites. After one single hydrothermal synthesis at 60 1C for 24 h, a dense and continuous zeolite NaA membrane was formed. The prepared zeolite NaA membranes were very thin (with a thickness of 2 μm) and showed high fluxes of 3.7– 3.9 kg m  2 h  1 with separation selectivities over 5000 in dehydrating 90 wt% ethanol/10 wt% water mixtures at 75 1C. This strategy opened a new way to prepare thin zeolite membranes on macroporous support with poor surface morphology.

Acknowledgments We are grateful to the financial support from the National Natural Science Foundation of China (No. 21376036) and Program for New Century Excellent Talents in University (NCET-10-0286). References [1] E.E. McLeary, J.C. Jansen, F. Kapteijn, Zeolite based films, membranes and membrane reactors: progress and prospects, Microporous Mesoporous Mater. 90 (2006) 198–220. [2] D. Casanave, A. Giroir-Fendler, J. Sanchez, R. Loutaty, J.A. Dalmon, Control of transport properties with a microporous membrane reactor to enhance yields in dehydrogenation reactions, Catal. Today 25 (1995) 309–314. [3] X. Gu, J. Dong, T.M. Nenoff, Synthesis of defect-free FAU-type zeolite membranes and separation for dry and moist CO2/N2 mixtures, Ind. Eng. Chem. Res. 44 (2005) 937–944. [4] K. Kusakabe, T. Kuroda, A. Murata, S. Morooka, Formation of a Y-type zeolite membrane on a porous α-alumina tube for gas separation, Ind. Eng. Chem. Res. 36 (1997) 649–655. [5] M.E. Davis, Ordered porous materials for emerging applications, Nature 417 (2002) 813–821. [6] F. Bonhomme, M.E. Welk, T.M. Nenoff, CO2 selectivity and lifetimes of high silica ZSM-5 membranes, Microporous Mesoporous Mater. 66 (2003) 181–188. [7] M.P. Bernal, E. Piera, J. Coronas, M. Menendez, J. Santamaria, Mordenite and ZSM-5 hydrophilic tubular membranes for the separation of gas phase mixtures, Catal. Today 56 (2000) 221–227. [8] M. Yu, J.L. Falconer, T.J. Amundsen, M. Hong, R.D. Noble, A controllable nanometer-sized valve, Adv. Mater. 19 (2007) 3032–3036. [9] V.V. Hoof, C. Dotremont, A. Buekenhoudt, Performance of Mitsui NaA type zeolite membranes for the dehydration of organic solvents in comparison with commercial polymeric pervaporation membranes, Sep. Purif. Technol. 48 (2006) 304–309. [10] H. Ahn, H. Lee, S.B. Lee, Y. Lee, Dehydration of TFEA/water mixture through hydrophilic zeolite membrane by pervaporation,, J. Membr. Sci. 291 (2007) 46–52. [11] C.H. Cho, K.Y. Oh, J.G. Yeo, S.K. Kim, Y.M. Lee, Synthesis, ethanol dehydration and thermal stability of NaA zeolite/alumina composite membranes with narrow non-zeolitic pores and thin intermediate layer, J. Membr. Sci. 364 (2010) 138–148. [12] K. Sato, T. Nakane, A high reproducible fabrication method for industrial production of high flux NaA zeolite membrane, J. Membr. Sci. 301 (2007) 151–161. [13] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Sep. Purif. Technol. 25 (2001) 251–260. [14] M. Tsapatsis, Toward high-throughput zeolite membranes, Science 334 (2011) 767–768. [15] Z. Wang, Q. Ge, J. Gao, J. Shao, C. Liu, Y. Yan, High-performance zeolite membranes on inexpensive large-pore supports: highly reproducible synthesis using a seed paste, ChemSusChem 4 (2011) 1570–1573. [16] J. Hedlund, B. Schoeman, J. Sterte, Ultrathin oriented zeolite LTA films, Chem. Commun. (1997) 1193–1194. [17] T.A. Kuzniatsova, M.L. Mottern, W.V. Chiu, Y. Kim, P.K. Dutta, H. Verweij, Synthesis of thin, oriented zeolite A membranes on a macroporous support, Adv. Funct. Mater. 18 (2008) 952–958. [18] W. Liu, J. Zhang, N. Canfiels, L. Saraf, Preparation of robust, thin zeolite membranes sheet for molecular separation, Ind. Eng. Chem. Res. 50 (2011) 11677–11689. [19] Y. Liu, Z. Yang, C. Yu, X. Gu, N. Xu, Effect of seeding methods on growth of NaA zeolite membranes, Microporous Mesoporous Mater. 143 (2011) 348–356. [20] A. Huang, W. Yang, J. Liu, Synthesis and pervaporation properties of NaA zeolite membranes prepared with vacuum-assisted method, Sep. Purif. Technol. 56 (2007) 158–167. [21] J. Choi, S. Ghosh, Z. Lai, M. Tsapatsis, Uniformly a-oriented MFI zeolite films by secondary growth, Angew. Chem. Int. Ed. 45 (2006) 1154–1158.

93

[22] X. Yin, G. Zhu, W. Yang, Y. Li, G. Zhu, R. Xu, J. Sun, S. Qiu, R. Xu, Stainless-steelnet-supported zeolite NaA membrane with high permeance and high permselectivity for oxygen over nitrogen, Adv. Mater. 17 (2005) 2006–2010. [23] W.C. Yoo, J.A. Stoeger, P.S. Lee, M. Tsapatsis, A. Stein, High-performance randomly oriented zeolite membrane using brittle seeds and rapid thermal processing, Angew. Chem. Int. Ed. 49 (2010) 8699–8703. [24] M.A. Snyder, M. Tsapatsis, Hierarchical nanomanufacturing: from shaped zeolite nanoparticles to high-performance separation membranes, Angew. Chem. Int. Ed. 46 (2007) 7560–7573. [25] L. Tosheva, V.P. Valtchev, Nanozeolites: synthesis, crystallization mechanism and applications, Chem. Mater. 17 (2005) 2494–2513. [26] W. Song, V.H. Grassian, S.C. Larsen, High yield method for nanocrystalline zeolite synthesis, Chem. Commu. 41 (2005) 2951. [27] Z. Chen, S. Li, Y. Yan, Synthesis of template-free zeolite nanocrystals by reverse microemulsion-microwave method, Chem. Mater. 17 (2005) 2262–2266. [28] A. Huang, F. Liang, F. Steinbach, J. Caro, Preparation and separation properties of LTA membranes by using 3-aminopropyltriethoxysilane as covalent linker, J. Membr. Sci. 350 (2010) 5–9. [29] A. Huang, N. Wang, J. Caro, Synthesis of multi-layer zeolite LTA membranes with enhanced gas separation performance by using 3aminopyltriethoxysilane as interlayer, Microporous Mesoporous Mater. 164 (2012) 294–301. [30] A. Huang, N. Wang, J. Caro, Stepwise synthesis of sandwich-structured composite zeolite membranes with enhanced separation selectivity, Chem. Commun. 48 (2012) 3542–3544. [31] A. Huang, N. Wang, J. Caro, Seeding-free synthesis of dense zeolite FAU membranes on 3-aminopropyltriethoxysilane-functionalized alumina supports, J. Membr. Sci. 389 (2012) 272–279. [32] A. Huang, H. Bux, F. Steinbach, J. Caro, Molecular-sieve membrane with hydrogen permselectivity: ZIF-22 in LTA topology prepared with 3aminopropyltriethoxysilane as covalent linker, Angew. Chem. Int. Ed. 49 (2010) (4958–4561). [33] A. Huang, W. Dou, J. Caro, Steam-stable zeolitic imidazolate framework ZIF-90 membrane with hydrogen selectivity through covalent functionalization, J. Am. Chem. Soc. 132 (2010) 15562–15564. [34] B. Huang, Q. Liu, J. Caro, A. Huang, Iso-butanol dehydration by pervaporation using zeolite LTA membranes prepared on 3-aminopropyltriethoxysilanemodifed alumina tubes, J. Membr, Sci. 455 (2014) 200–206. [35] J. Caro, M. Noack, P. Kolsch, Zeolite membranes: from the laboratory scale to technical applications, Adsorption 11 (2005) 215–227. [36] H. Li, J. Wang, J. Xu, X. Meng, B. Xu, J. Yang, S. Li, J. Lu, Y. Zhang, X. He, D. Yin, Synthesis of zeolite NaA membranes with high performance and high reproducibility on coarse macroporous supports, J. Membr. Sci. 444 (2013) 513–522. [37] 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, AlChE 59 (2013) 936–947. [38] 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, Microporous Mesoporous Mater. 142 (2011) 154–160. [39] Z. Xie, J. Yang, J. Wang, J. Bai, H. Yin, B. Yuan, J. Lu, Y. Zhang, L. Zhou, C. Duan, Deposition of chemically modified α-Al2O3 particles for high performance ZIF8 membrane on a macroporous tube, Chem. Commun. 48 (2012) 5977–5979. [40] A. Kulak, Y.J. Lee, Y.S. Park, K.B. Yoon, Orientation-controlled monolayer assembly of zeolite crystals on glass and mica by covalent linkage of surface-bound epoxide and amine groups, Angew. Chem. Int. Ed. 39 (2000) 950–953. [41] A. Szegedi, M. Popova, I. Goshev, S. Klebert, J. Mihaly, Controlled drug release on amine functionalized spherical MCM-41, J. Solid State Chem. 194 (2012) 257–263. [42] H. Sardon, L. Irusta, M.J. Fernandez-Berridi, M. Lansalot, E. Bourgeat-Lami, Synthesis of room temperature self-curable waterborne hybrid polyurethanes functionalized with (3-aminopropyl)triethoxysilane (APTES), Polymer 51 (2010) 5051–5057. [43] A.S. Maria Chong, X.S. Zhao, Functionalization of SBA-15 with APTES and characterization of functionalized materials, J. Phys. Chem. B 107 (2003) 12650–12657. [44] S. Oh, T. Kang, H. Kim, J. Moon, S. Hong, J. Yi, Preparation of novel ceramic membranes modified by mesoporous silica with 3aminopropyltriethoxysilane (APTES) and its application to Cu2 þ separation in the aqueous phase, J. Membr. Sci. 301 (2007) 118–125. [45] K. Okamoto, H. Kita, K. Horii, K. Tanaka, M. Knodo, Zeolite NaA membrane: preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures, Ind. Eng. Chem. Res. 40 (2001) 163–175. [46] Z. Yang, Y. Liu, C. Yu, X. Gu, N. Xu, Ball-milled NaA zeolite seeds with submicron size for growth of NaA zeolite membranes, J. Membr. Sci. 392– 393 (2012) 18–28. [47] A. Huang, J. Caro, Facile synthesis of LTA molecular sieve membranes on covalently functionalized supports by using diisocyanates as molecular linkers, J. Mater. Chem. 21 (2011) 11424–11429. [48] A. Kulak, Y.S. Park, Y.L. Lee, Y.S. Chun, K. Ha, K.B. Yoon, Polyamines as strong molecular linkers for monolayer assembly of zeolite crystals on flat and curved glass, J. Am. Chem. Soc. 122 (2000) 9308–9309. [49] K. Ha, Y.J. Lee, H.J. Lee, K.B. Yoon, Facile assembly of zeolite monolayers on glass, silica, alumina, and other zeolites using 3-halopropylsilyl reagents as covalent linkers, Adv. Mater. 12 (2000) 1114–1117.