Synthesis of a Silicalite Zeolite Membrane in Ultradilute Solution and

Aug 10, 2012 - E-mail: [email protected] (X.S.C.); [email protected] (H.K.). ... Moreover, the membrane prepared with pure TPABr template inste...
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Synthesis of a Silicalite Zeolite Membrane in Ultradilute Solution and Its Highly Selective Separation of Organic/Water Mixtures Xiao-Liang Zhang,† Mei-Hua Zhu,†,‡ Rong-Fei Zhou,† Xiang-Shu Chen,*,† and Hidetoshi Kita*,‡ †

Jiangxi Inorganic Membrane Materials Engineering Research Centre, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, P. R. China ‡ Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University, Ube, 755-8611, Japan S Supporting Information *

ABSTRACT: A continuous intergrown silicalite zeolite membrane with high pervaporation (PV) performance was successfully prepared on seeded tubular mullite supports in ultradilute solution with a H2O/SiO2 ratio of 800 and an inexpensive template of tetrapropylammonium bromide (TPABr) instead of tetrapropylammonium hydroxide (TPAOH). Several parameters were systematically investigated to evaluate their influence on crystallization and PV performance of the membranes, including the H2O/SiO2 ratio, template type, alkalinity, synthesis temperature, crystallization time, and silica source. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and PV tests were used to characterize the as-synthesized membranes. The crystal growth and separation quality of the silicalite membranes were very sensitive to the H2O/SiO2 ratio and alkalinity in the precursor solution and synthesis temperature. Under the optimized synthesis conditions, the outer surface of support was fully covered with well-intergrown silicalite zeolite layer to form the zeolite membrane. For silicalite membrane prepared with the typical molar composition of synthesis solutions of 1SiO2/0.1TPABr/0.2TPAOH/800H2O at 180 °C for 16 h, the flux and separation factor are achieved to 1.91 kg·m−2·h−1 and 66 for a 5 wt % ethanol/water mixture at 60 °C, respectively. Moreover, the membrane prepared with pure TPABr template instead of TPAOH in ultradilute solution also showed the high PV performance with the flux of 1.77 kg·m−2·h−1 and separation factor of 63 under the same tests conditions. Due to the utilization of ultradilute precursor and cheap organic template to prepare the silicalite membrane on cheap mullite supports with high PV performance, the present developed technique could reduce the chemical consumption and decrease the costs of membrane toward an organic/water mixture separation.

1. INTRODUCTION Over the past two decades, increasing attention has been paid to the development of zeolite membranes, especially MFI-type zeolite (silicalite-1 and ZSM-5) membranes, due to their potential applications in a wide range of industrially interesting applications including gas separation, pervaporation (PV), membrane reactors, and catalysis.1−7 Pervaporation is today considered as an effective technique for liquid mixture separation because of its efficiency in separating azeotropic, close-boiling mixtures or isomers, and heat-sensitive compounds and the advantages of low energy consumption, simple operation, and low environmental impact.1−3 With medium pore size of 0.53 nm × 0.56 nm, good thermal and structural stability and hydrophobic properties, MFI-type zeolite membranes have been extensively investigated recently for organics extraction from low concentration aqueous solutions by pervaporation.1−3 The commercial viability of MFI membrane-based separation processes, however, have been so far hindered mainly by the poor separation performance, especially the low permeation flux, and the prohibitive costs of membrane preparation.8−42 In 1994, Sano et al.8 prepared silicalite-1 membranes on porous stainless steel or alumina discs with an in situ crystallization method from a clear solution with H2O/SiO2 ratio of 80. The membrane showed a high separation factor of 60 for a 5 wt % aqueous ethanol solution at 30 °C. Some groups have synthesized MFI membranes by in situ and © 2012 American Chemical Society

secondary growth methods on porous tubular supports such as alumina, stainless steel, silica tubes, and multichannel monolith alumina supports.15,16,19,20,22 All those membranes were reported to have a separation factor of 10−70 together with a flux of 0.1−1.0 kg·m −2 ·h −1 for ethanol/water mixture separation. Recently, silicalite-1 membranes were prepared on alumina hollow fibers to improve the permeation flux.22,26,33 Up to now, for their commercial applications, the ultimate objective for preparing MFI membrane processing remains (a) to grow a thin, defect-free, and reproducible membrane with high performance and (b) to decrease the cost of membrane production. MFI membranes are relatively expensive, and their price is typically determined by the support and chemicals.31 Generally, MFI membranes are synthesized in the presence of organic templates such as tetrapropylammonium hydroxide (TPAOH). Since TPAOH is expensive, the preparation of MFI zeolite membranes without organic template or with cheap substitute template such as tetrapropylammonium bromide (TPABr) has been reported to reduce membrane cost and minimize the intercrystalline gaps for improving membrane selectivity.17,18,24,37,41 It was reported that the microstructure of zeolite Received: Revised: Accepted: Published: 11499

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(Nikkato Corp.) had an outer diameter of 12 mm, a thick wall of 1.5 mm, an average pore size of 1.0 μm, and a porosity of ∼43%. The mullite tubes were polished with SiC sandpaper, washed in deionized water in an ultrasonic bath, and then dried in an oven at 100 °C for 6 h. The porous stainless steel (PSS) tubes (Mott Corp.) had an outer diameter of 12 mm, a thick wall of 1 mm, and an average pore size of 1.8 μm. The PSS tubes were cleaned with a NaOH solution of 0.5 mol·L−1, then washed with deionized water three times by ultrasonic cleaner, and dried. Unless otherwise stated, the silicalite-1 membranes were synthesized on mullite tubes because of their much cheaper price as supports. The MFI-type seeds were prepared by hydrothermal synthesis. The synthesis gel of seeds with the molar ratio of 1SiO2/0.25TPABr/0.15Na2O/45H2O was prepared by mixing and stirring colloidal silica (TM-40, 40 wt % in water, Aldrich), TPABr (99 wt %, Tokyo Kasei), NaOH (Tianjin Hengxing Chemical), and deionized water at room temperature for 24 h. After crystallization at 130 °C for 30 h, the uniform MFI-type seeds were obtained from the suspensions by washing, drying, and then calcining at 550 °C for 6 h. Before membrane synthesis, the support tubes were seeded by dip-coating using an 8 g·L−1 aqueous above synthesized zeolite seeds suspension for an immersed period of 30 s, and then, the seeding procedure was repeated again. Different from the report in the literature, synthesis solution with the molar ratio of H2O/SiO2 more than 800 is referred to as an ultradilute solution in this study since it contains over 97 wt % water. Clear synthesis solutions for silicalite-1 membrane were obtained by mixing and stirring tetraethylorthosilicate (TEOS, 98.3 wt %, Aldrich), TPAOH (20−25 wt % in water, Tokyo Kasei), TPABr, NaOH, and deionized water at room temperature for 1 h. The molar composition of resultant synthesis solutions were 1SiO2/0−0.4TPAOH/0−0.4TPABr/ 0−0.15Na2O/120−1000H2O. Then, the seeded support tubes were vertically immersed in the above synthesis solution with a stainless steel autoclave, and the crystallization was carried out at the synthesis temperature from 160 to 190 °C for a given time. After crystallization, the membrane samples were taken out, washed carefully with hot deionized water, dried at 60 °C for 12 h, and then calcined to remove the organic templates at 500 °C for 15 h with a heating and cooling rate of 0.5 °C·min−1. For comparison, colloidal silica (TM-40, Aldrich), colloidal silica (HS-40, Aldrich), and colloidal silica (CS-30, 30 wt %, Qingdao Haiyang Chemical) instead of TEOS in the membrane synthesis solution were used to study the effect of silica source on PV performance. 2.2. Pervaporation Measurements. Pervaporation tests of membrane performance were essentially the same as those described in our previous publications.39−42 The inside of the membrane tube was evacuated by a vacuum pump. The permeate vapor was collected by a cold trap cooled with liquid nitrogen. The downstream pressure was kept under 20 Pa. The effective membrane areas were about 27 cm2. The compositions of feed and permeate were analyzed by a gas chromatograph (GC, GC-14C, SHIMADZU) equipped with 3 m column packed with Polarpack Q poly(ethylene glycol)-1000. The permeation flux is calculated by weighing the condensed permeate. On the basis of the experimental data, the PV performance of zeolite membrane can be characterized in terms of permeation flux (J, kg·m−2·h−1) and separation factor (α) shown below: J = m/At, αo/w= (yo/yw)/(xo/xw), where m is the mass of permeate collected over a period time (t), A is the

membranes prepared without template might have less or smaller intercrystalline gaps in comparison with that prepared with template, but the membrane showed lower H2 permeation performance than those reported in previous literature.24 Commonly, the more diluted synthesis solution with the molar ratio of H2O/SiO2 over 120 was not suitable for preparation of a continuous and intergrowth silicalite membrane.36 It is difficult to clearly understand the nucleation and crystallization processes of zeolite membrane and explore the suitable synthesis parameters in dilute solution. The silicalite-1 crystals have been prepared from a dilute clear sol with H2O/SiO2 ratio of 475 and organic template.43 Recently, silicalite-1 membranes have been prepared by a restricting growth method in the initial synthesis solution with H2O/SiO2 ratio of 800 and TPAOH as template.44 The membrane displayed preferable hydrogen permeance and permselectivity but lack of the membrane pervaporation performance toward organic/water systems.44 However, the uncompacted membranes with poor permeation behavior were obtained by the conventional secondary growth method under the same synthesis parameters.44 As far as we know, there are no reports on the crystallization process of MFI-type membranes in ultradilute solution with a H2O/SiO2 ratio over 500 and their applications for separating liquid mixtures such as organic/ water systems by pervaporation. In our previous work, MFI-type membranes with high pervaporation performance have been synthesized using TPAOH as template or inexpensive TPABr instead of TPAOH by in situ and secondary growth.35−42 However, the silicalite-1 membrane prepared from a dilute initial synthesis solution with H2O/SiO2 ratio over 200 by in situ hydrothermal treatment was leaking because it was too dilute for membrane synthesis under those conditions. Moreover, with TPABr as template instead of TPAOH to prepare the MFI membrane, additional base is necessary for dissolving the silica source, while the alkali metal ions from inorganic bases such as sodium hydroxide might weaken the catalytic activity of MFI zeolite or decrease the separation properties under a certain condition.45−48 However, there is still inadequate understanding of the growth process of MFI membranes during the crystallization stage, and no proven method exists for predicting or controlling zeolite crystal growth and morphology under the above synthesis conditions. Fundamental investigations of the synthesis parameters such as solution composition, zeolite morphology, crystal structure, and composition during the crystallization process may provide valuable insights for developing methods to control zeolite crystal growth and permeation performance. In this study, well-intergrown silicalite-1 membranes with high pervaporation performance were successfully prepared on cheaper seeded tubular mullite supports in ultradilute solution with a H2O/SiO2 ratio of 800 and an inexpensive template of TPABr instead of TPAOH. The influence of synthesis parameters on membrane growth and pervaporation performance of silicalite membranes was studied systematically. The assynthesized membranes displayed highly selective separation of organic/water mixtures.

2. EXPERIMENTAL SECTION 2.1. Membrane Synthesis. The silicalite-1 membranes were prepared by a seeded secondary growth method on the outer surface of porous tubular supports with 100 mm in length. Two types of supports were used. The mullite tubes 11500

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Figure 1. SEM images of synthesized seed crystals.

effective membrane area for permeation, and xw, xo, yw, and yo denote the mass fractions of components water and organics at the feed and permeate sides, respectively. Except as noted, the pervaporation tests were carried out toward ethanol/water (5/ 95 wt %) mixtures at 60 °C. 2.3. Characterization. The crystal structure of zeolite seeds and as-synthesized membranes were characterized by X-ray diffraction (XRD, Advance D8, Bruker) with Cu Kα radiation at 40 kV and 120 mA. The morphology and thickness of zeolite membranes were observed using a field emission scanning electron microscopy (SEM, Quanta 200F, FEI), equipped with an energy dispersive spectrometer (EDS) for membrane composition analysis. The composition of zeolite crystals was also detected by inductively coupled plasma-optical emission spectrometry (ICP-OES, Varian, 725-ES).

synthesis solutions were investigated in detail to study the influence of H2O/SiO2 ratio on silicalite growth and the PV performance of the as-synthesized membranes. Table 1 shows Table 1. Effect of H2O/SiO2 Ratio on the PV Performance of Synthesized Membranes

3. RESULTS AND DISCUSSION 3.1. Synthesized Seed Crystals. The zeolite seed crystals prepared by the hydrothermal synthesis were characterized by SEM and XRD. As seen in Figure 1, the zeolite seeds displayed uniform distribution without other impurities and the average diameter of ca. 4 μm. According to the XRD analysis as shown in Figure 2a, the seed particles were pure crystalline with typical MFI-type zeolite structure without other zeolite phases. The crystals in the membrane layer display significant preferred orientation in the (011) plane. 3.2. Effects of Synthesis Parameters on Membrane Growth and Pervaporation Performance. 3.2.1. Effect of H2O/SiO2 Ratio. Three series of template systems in the

no.

n(H2O)/n(SiO2)

time (h)

J (kg·m−2·h−1)

α

S1a S2a S3a S4a S5b S6b S7b S8b S9c S10c S11c S12c

120 500 800 1000 120 500 800 1000 120 500 800 1000

8 12 16 20 8 12 16 20 8 12 16 20

1.35 1.40 1.70

70 75 70

1.52 1.75 1.91

58 75 66

0.82 1.51 1.77

40 66 63

a

The molar composition of synthesis solution: 1SiO2/0.2TPAOH/ xH2O. bThe molar composition of synthesis solution: 1SiO2/ 0.2TPAOH/0.1TPABr/yH2O. cThe molar composition of synthesis solution: 1SiO2/0.3TPABr/0.05Na2O/zH2O.

the PV performance of silicalite-1 membranes prepared with different H2O/SiO2 ratios (from 120 to 1000) in the synthesized solution on the seeded mullite tubes. The synthesis of all the membrane samples was carried out at 180 °C for 8− 30 h. As shown in Table 1, despite using three different organic templates, the more concentrated (e.g., H2O/SiO2 ratio of 120) and the more diluted (e.g., H2O/SiO2 ratio of 1000) solutions were not suitable for preparing the silicalite membranes with high permeation flux and separation factor. When the H2O/ SiO2 ratio was 800 during the seeded secondary growth process, the as-synthesized membranes displayed the highest flux and separation factors, even if with three different series of templates. All the membranes prepared with a H2O/SiO2 ratio of 1000 were leaking by hydrothermal treatment for 20 h even prolonged to 30 h. It was reported that increasing the water concentration would result in a monotonic decrease of zeolite growth and crystallinity.12 In such diluted solution with H2O/ SiO2 ratio of 1000, zeolite crystal growth was negligible and some etching seed layer was observed, even though there was a prolonged crystallization of 30 h. The synthesis solution was too dilute, and there was inadequate nutriment (silica source and structure-directing agent) to provide for zeolite crystallization under the present preparation conditions. Thus, the

Figure 2. XRD patterns of seeded zeolite crystal (a), mullite support (b), and as-synthesized membranes S5 (c), S6 (d), and S7 (e). 11501

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investigated (Figure 4), which were similar to that of membranes S5−S7 prepared with TPABr and TPAOH as

solutions with higher H2O/SiO2 ratio of 1000 could not form a continuous zeolite layer on the support. On the other hand, the solutions with lower H2O/SiO2 ratio of 120 also would not result in a continuous dense crystal layer because of the formation of more defects by the potential change of crystal growth, such as crystal sizes during hydrothermal treatment.15,36 As shown in Table 1, the membranes prepared with inexpensive TPABr as template displayed high PV performance. Figure 2 shows the XRD patterns of pure silicalite seeded crystal, porous mullite support, and silicalite-1 membranes prepared in the synthesis solutions of 1SiO2/0.1TPABr/ 0.2TPAOH/120−800H2O. It can be found that the patterns of zeolite membrane S7 prepared under ultradilute solution with H2O/SiO2 ratio of 800 was consistent with the strong characteristic peaks of silicalite-1 seeds together with those of the mullite tube, confirming that the outer surface of mullite support was fully covered by the silicalite-1 layer preferentially along the (011) direction (Figure 2e). Figure 3 shows the

Figure 4. SEM images of surface and cross section for the membranes S9 (a, b), S10 (c, d), and S11 (e, f).

templates (Figure 3) from the apparent images. However, as shown in Figure 4a,b, there were some larger defects, and intercrystalline porosities existed in the zeolite layer for membrane S9 (H2O/SiO2 ratio of 120 in synthesis solution) even though the average thickness of this membrane was over 12 μm. The synthesis solution of this membrane was too concentrated, and thus, it would form heterogeneous and larger crystals on the seeded support surface during the secondary growth process. However, from the images of Figure 4c−f (H2O/SiO2 ratio over 500 in synthesis solution), the crystal particles of silicalite membranes S10 and S11 became more small and uniform with decreasing membrane thickness. It seems to be responsible for the ultradilute synthesis solution and prolonged time up to present, but further study of the influence of grain growth is necessary. For example, it is clear from Figure 4e,f that an about 5 μm thick and continuous compact layer of silicalite zeolite film (membrane S11) formed after the hydrothermal synthesis. The rectangular-shaped silicalite particles have a small and uniform size, and silicalite crystals with preponderant orientation are covered on the surface of support. It is clear that the thickness of membrane layers (from 12 to 5 μm) decreased with the increasing water content in the crystallization solution due to the ultradilute solution with lower content of silica source and structuredirecting agent, which could enhance the flexibility and transport resistance in the membrane channel.

Figure 3. SEM surface and cross section images for the membranes S5 (a, b), S6 (c, d), and S7 (e, f).

surface and cross sectional SEM images of silicalite membranes corresponding to those membranes in Figure 2. Obviously, as seen in Figure 3, the well-intergrown and typical coffin-shaped MFI crystals grown with TPA+ were covered on the supports, and the crystals of the top layer seem to have a nearly monodisperse size distribution. As seen in Table 1, the flux and separation factor of membrane S7 with defect-free structure are achieved to 1.91 kg·m−2·h−1 and 66 for the 5 wt % ethanol/ water mixture at 60 °C, respectively. The morphologies of silicalite membranes S9−S11 prepared with pure TPABr as template instead of TPAOH were also 11502

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Figure 5. EDS analysis of silicalite membranes prepared with pure TPABr template: membranes S9 (a) and S11 (b). EDS scanning areas are 100 μm × 100 μm.

Compared with the membranes S9 and S10, as seen in Table 1, membrane S11 exhibited the highest PV performance with permeation flux of 1.77 kg·m−2·h−1 and separation factor of 63. High PV performance of membrane S11 could mainly be attributed to the thinner zeolite layer of about 5 μm, as described above, and its hydrophobic property. Generally, there exists a competition between TPA+ and Na+ incorporated into the channel during the silicalite crystallization process. The incorporation of TPA+ into the zeolite structure is independent of concentration of [OH−], but the occlusion of Na+ into the zeolite structure increases with increasing concentration of NaOH in the reaction solution.41,45−48 At the highest concentration of NaOH, the largest amount of Na+ became incorporated into the zeolite, which could reduce the hydrophobicity and permeation performance of silicalite membrane.45−48 Table S1 in the Supporting Information listed the results of EDS and ICP analysis of membranes S9, S10, and S11. Combining the results from Table S1 (Supporting Information) and Figure 5a, for the membrane S9 in synthesis solution of H2O/SiO2 ratio of 120, it might supply the superabundant “nutrition” and high alkalinity, thus making Al leach from the support and Na+ incorporate into the silicalite structure, which shows the poor PV performance as listed in Table 1. However, in ultradilute solution, it supplies the limited but probably appropriate “nutrition” and low alkalinity to avoid Al leaching from the support (Figure 5b). Thus, the membrane S11 shows the higher PV performance. Moreover, the existence of Na+ in the channel of zeolite layers (see the ICP results of membrane S11 in Table S1, Supporting Information) is probably the reason that membrane S11 had lower flux than membrane S7 in this work. 3.2.2. Effects of Template and Alkalinity. Table 2 shows the influence of TPAOH and TPABr as template and alkalinity on the PV performance of synthesized membranes. TPAOH as template in this work served as structure directing agent for zeolite framework but also provided the alkalinity of the synthesis solution. From Table 2, it showed that the separation factor of silicalite membranes increased with the amount of TPAOH reaching a constant value (TPAOH/SiO2 ratio of 0.3). Membrane S14 showed the highest separation factor of 80. However, the membrane prepared with the TPAOH/SiO2 ratio of 0.4 exhibited leaking with poor separation, which is consistent with Shan’s results.33 Negishi et al.14 and we40 had studied silicalite-1 membranes prepared with TPABr and NaOH in the concentrated solution

Table 2. Effect of Template and Alkalinity on the PV Performance of Synthesized Membranesa no.

n(TPAOH)/ n(SiO2)

n(TPABr)/ n(SiO2)

n(Na2O)/ n(SiO2)

S13 S3 S14 S15 S16 S17 S11 S18 S19 S20 S21 S7

0.1 0.2 0.3 0.4 0 0 0 0 0 0 0.1 0.2

0 0 0 0 0.1 0.2 0.3 0.4 0.3 0.3 0.2 0.1

0 0 0 0 0.05 0.05 0.05 0.05 0.10 0.15 0 0

J (kg·m−2·h−1)

α

1.79 1.70 1.42

50 70 80

1.20 1.34 1.77 1.64 0.78 0.46 1.85 1.91

68 65 63 68 70 40 40 66

a

Synthesis conditions: 1SiO 2 :0−0.4TPAOH:0−0.4TPABr/0− 0.15Na2O/800H2O at 180 °C for 16 h. To be compared conveniently, the PV performances of membranes S3, S7, and S11 in Table 1 are listed in this table.

(H2O/SiO2 ratio less than 100), but synthesized membranes had low permeation flux and poor separation performance. As shown in Table 2, the PV performance of membranes prepared with pure TPABr as template instead of TPAOH in ultradilute solution was also investigated. When the NaOH/SiO2 ratio was fixed at 0.1, these membranes prepared with TPABr showed a steady separation factor of 66. The membrane S11 showed higher PV performance with a high flux of 1.77 kg·m−2·h−1 and a high separation factor of 63 for 5 wt % aqueous ethanol/H2O feed solution at 60 °C. To the best of our knowledge, it is the highest PV performance so far among the silicalite-1 membranes prepared with pure TPABr as template.14,40,41 Generally, the NaOH/SiO2 ratio in the synthesis solution is an important parameter for zeolite growth, because it would influence zeolite crystallinity and degree of the isomorphous substitution even to structure stability. From Table 2, when the TPABr/SiO2 ratio was fixed at 0.3, the permeation flux of synthesized membranes (S11, S19, and S20) decreased with the amount of NaOH (NaOH/SiO2 ratio from 0.10 to 0.30). As illustrated in the previous section 3.2.1., the competition between Na+ and TPA+ would reduce the quality of silicatlite membranes. Moreover, when the molar ratio of NaOH/SiO2 reduced to 0.05, the alkalinity in the initial solution was too low to dissolve the silica source; thus, the as-synthesized membrane 11503

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in closing a pore between the crystal grains and the membrane would become thicker than that at lower temperature thus decreasing the permeation flux.11,36 Table 3 also showed the PV performance of silicalite membranes prepared with the same synthesized solution at 180 °C as a function of crystallization time. After 4 h of hydrothermal treatment, as seen in Figure 6, there were weaker

was leaking before calcination in this study (not shown in Table 2). The influence of alkalinity on silicalite membrane growth was also investigated in ultradilute solutions containing a mixture of TPABr and TPAOH. The TPA+ ion content in the solution was kept constant with the TPA+/SiO2 ratio of 0.3 (TPAOH partly instead of TPABr without NaOH). Compared with the PV performance of membranes S21, S7, and S14, which corresponded to the increasing amount of [OH−] from TPAOH in synthesis solution, although membrane S14 had the highest separation factor of 80, the permeation flux decreased to 1.42 kg·m−2·h−1. It suggested that alkalinity had a stronger influence on silicalite growth and PV performance than [TPA+] at the range of concentrations used in this work. Wong et al.12 reported that higher alkalinity gave slower deposition and poorer crystallinity. On the basis of the above facts, the synthesis solution containing [OH−]/SiO2 ratio fixed at 0.2 and [TPA+]/SiO2 ratio fixed at 0.3 is possibly suitable to prepare high PV performance of silicalite membrane with such ultradilute solution in this work. Moreover, under the same synthesized parameters as membrane S7, a silicalite-1 membrane was successfully prepared on the porous stainless steel supports. It exhibited the highest flux of 2.33 kg·m−2·h−1 and separation factor of 70 (also listed in Table S2 in the Supporting Information), which can be attributed to a stainless steel tube with larger pore size and higher porosity than those of mullite supports. 3.2.3. Effects of Synthesis Temperature and Crystallization Time. Table 3 shows the PV performance of silicalite

Figure 6. XRD patterns of silicalite membranes prepared with the same synthesized solution (1SiO2/0.1TPABr/0.2TPAOH/800H2O) at 180 °C as a function of cryatallizaiton time.

characteristic peaks of MFI-type zeolite while showing the stronger characteristic peaks of mullite support. It suggested that silicalite crystals could not fully cover all the pores of the support, and thus, the samples S25 did not have a separation property. After 8 h, although the membrane showed high crystallinity of MFI structure (Figure 6), there were also stronger characteristic peaks of mullite support. It illustrated that there were some intercrystalline porosities and defects of the membrane S26, showing poor separation performance. Compared with the concentrated solution such as H2O/SiO2 ratio of 120, the crystallization time to form a continuous compact silicalite layer in such ultradilute solution with H2O/ SiO 2 ratio of 800 should become longer, because of comparatively less nutrition in the ultradilute solution.15 Wong et al.12 reported that increasing the water concentration resulted in a monotonic decrease in film growth and crystallinity. It seems that there is a need to extend the cryatallizaiton time under such an ultradilute solution. After 12 h, the silicalite membrane S27 had a high flux of 2.20 kg·m−2·h−1 and a low separation factor of 30 for ethanol over water. For further treatment, the separation factors increased and the flux decreased maybe because of increasing the membrane thickness and density. It was consonant with the change tendency of the crystallinity and crystal orientation of typical MFI structure as a function of crystallization time shown in Figure 6. As illustrated in Figure 6, a significant degree of (011) preferred orientation was perpendicular to the mullite surface. The result was consistent with the previous SEM images (e.g. Figure 3e,f). The above results indicated that the synthesis temperature or crystallization time is one of the most important variable factors, which strongly affects the membrane growth and PV performance. 3.2.4. Effect of Silica Source. Table 4 showed the PV performance of silicalite-1 membranes prepared with different silica sources. As mentioned in Table 4, all of the synthesis solution was prepared with the same molar chemical

Table 3. Effect of Synthesis Temperature and Crystallization Time on the Membrane PV Performancea no.

crystallization temp. (oC)

time (h)

S22 S23 S7 S24 S25 S26 S27 S28

160 170 180 190 180 180 180 180

16−20 16 16 12−16 4 8 12 20

J (kg·m−2·h−1)

α

2.10 1.91 1.30

50 66 70

2.20 1.50

30 70

a Composition of synthesized solution: 1SiO2/0.1TPABr/0.2TPAOH/ 800H2O. To be compared conveniently, the PV performance of membrane S7 in Table 1 is listed in this table.

membranes prepared with the same molar composition synthesis solution at different synthesis temperatures from 160 to 190 °C. As seen in Table 3, the growth rate of silicalite crystal was too low to form the compact crystal layer on the seeded support at 160 °C. Even though the crystallization time prolonged to 20 h, the sample was still leaking without separation performance before calcination. The permeation flux of these membranes decreased and separation factors increased with increasing synthesis temperature at the temperature ranges of 170−190 °C. At 170 °C, it showed the highest flux of about 2.10 kg·m−2·h−1 and the separation factor lower at about 50. When the synthesis temperature increased to 190 °C for 12−16 h during the crystallization process, the flux of the membrane decreased to 1.30 kg·m−2·h−1 and the separation factor was hardly unconverted (α = 70). It suggested that the crystal growth rate at higher temperature was faster and more effective 11504

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low-cost mullite tube in ultradilute solution with H2O/SiO2 ratio of 800 and partially or absolutely using the inexpensive template of TPABr, showing high PV performance. The excellent permeation flux and separation factor of the membrane S7 were 1.91 kg·m−2·h−1 and 66, respectively, toward a 5 wt % ethanol/water mixture by PV at 60 °C. Moreover, as shown in Table S2 (Supporting Information), the silicalite membranes prepared with PSS as supports as the same synthesis conditions in this work also showed the highest PV performance to date. In this work, the quantity of synthetic raw materials such as silica source and organic template consumedly decreased by increasing the molar ratio of H2O/SiO2 from 120 (in the literature) to 800 (in this work) in synthesis solution and partially or absolutely using inexpensive template of TPABr instead of the expensive TPAOH. Therefore, the as-synthesized silicalite membrane with such high PV performance could decrease the fabricated costs and be suitable for the development of large-scale pervaporation application toward the organic/water mixture separation, which is of the most importance from the viewpoint of industrial application. 3.3. PV Performance of Membrane for Liquid Separation. As shown in Table S2 (Supporting Information), silicalite membrane prepared in ultradilute solution under optimized conditions displayed the highest PV performance for the 5 wt % EtOH/H2O mixture in this work. Therefore, silicalite membrane prepared with the same optimized parameters as membrane S7 was used to separate the different organic/H2O mixtures by PV in the present work. Figure 7 presents the membrane PV performance with different EtOH concentrations in the EtOH/H2O feed mixtures

Table 4. Effect of Silica Source on the Membrane PV Performancea no.

silica source

J (kg·m−2·h−1)

α

S-7 S-29 S-30 S-31

TEOS TM-40 HS-40 CS-30

1.91 1.10 0.90

66 70 60

a

Synthesized condition: SiO2/0.1TPABr/0.2TPAOH/800H2O, 180 °C, 16 h. To be compared conveniently, the PV performance of membrane S7 in Table 1 is listed in this table.

composition, but the moles of SiO2 from these different Si sources in synthesis conditions are not equivalent due to their different polymerization degree.13,36 Thus, the membranes prepared with the different silica sources exhibited the different separation performance. The silicalite membranes S29 and S30 prepared with TM-40 and HS-40 showed lower permeation flux than those with TEOS. Only under strong alkalinity, polymeric Si sources (TM-40 and HS-40) could be completely dissolved, so that the good silicalite membranes were obtained.11,13,36,49 In this work, the alkaline source was TPAOH, which did not completely dissolve these polymeric Si sources, and the membrane S31 formed on the seeded mullite tube with silica gel (CS-30) was leaking without separation property. Before the initial stage of zeolite crystallization, the colloidal SiO2 should first depolymerize to reach the critical degree of dissolution and the duration of nucleation period should become longer. During the crystallization process, most of the crystals were seen precipitating at the bottom as result of the formation of the bulk powder in the solution instead of the support surface. Consequently, the crystal layer formed on the seeded tube was not intergrown and continuous, as evidenced by the observation of the bare surface of the mullite and thus showing no separation performance. As shown in Table 4, TEOS was the better source for making dense, well-intergrown silicalite membranes with high PV performance, which was consistent with the reports of Gora11 and Lin.36 3.2.5. Comparison of Pervaporation Performance with Literature Data. The PV performance toward 5 wt % ethanol/ water mixtures and the corresponding synthesis parameters of the silicalite membranes in this work and the previous reports are summarized for comparison in Table S2 in the Supporting Information. Sano et al.8 first reported that the silicalite membranes prepared on the PSS support showed the separation factor of 58 for ethanol/water mixtures, but the flux was too low for the membrane because of membrane thickness as high as 400−500 μm. In our previous publication, silicalite membranes were prepared by the in situ synthesis or seeded secondary growth methods on the α-Al2O3 and mullite tubular supports, showing the high flux of 1.80 kg·m−2·h−1 and 2.23 kg·m−2·h−1 for the corresponding supports, respectively.35,36 Recently, using the ceramic α-Al2O3 hollow fiber (HF) supports, the silicalite membranes were prepared and reported to show the highest permeation flux (2.9 kg·m−2·h−1) and separation factor (66) for an ethanol/water mixture separation.26,33 However, the issues of assembling of membrane prepared with capillary supports and fabricated costs of silicalite membranes such as using expensive template of TPAOH for large-scale application still are problems. Compared to the previous literature,8,19,20,26,33,35,36,38 the silicalite membranes in the present work were prepared on the

Figure 7. Feed concentration dependence of the PV performance of membrane for various compositional EtOH/H2O mixtures at 60 °C.

at 60 °C. The permeation fluxes increased as the ethanol content increased in the feed, from 0.80 kg·m−2·h−1 for 1 wt % EtOH/H2O feed to 2.67 kg·m−2·h−1 for 20 wt % EtOH/H2O feed mixtures. However, as seen in Figure 7, their corresponding selectivity decreased with the increasing EtOH concentrations. Moreover, the effect of pervaporation temperature on the performance of membrane for 5 wt % EtOH/H2O mixtures was also investigated from 30 to 75 °C. As seen in Figure 8, the permetiaon flux increased 8 times, while separation factor decreased 25% with the increasing pervaporation temperature by 30 °C. The permeation fluxes for ethanol separation from water increased significantly with temperature but with the sacrifice of slightly decreasing separation factors, which was similar to the reports.8,26 Sano et al.8 had studied the 11505

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methanol diffused faster than the other organics, as indicated by its much higher flux in Figure 9. The permeation flux of membrane for the MeOH/H2O mixture and the IPA/H2O mixture was 5.30 kg·m−2·h−1 and 0.70 kg·m−2·h−1, respectively, but the organic/water permselectivity was in the order: DMK > EtOH > IPA> MeOH. Generally, the transport mechanism of organic/water mixtures by pervaporation through zeolite membrane can be described by the adsorption−diffusion model.1−3,10 According to the model, the acetone and IPA molecules have similar kinetic diameters, but the acetone molecule has higher hydrophobicity and polarity. Hence, the membrane had a better PV performance for the DMK/H2O mixture than that for the IPA/H2O mixture as seen in Figure 9. The flux and separation factor of silicalite membrane toward DMK/H2O mixtures were 1.53 kg·m−2·h−1 and 92, respectively.

Figure 8. Effect of pervaporation temperature on the PV performance of membrane for 5 wt % EtOH/H2O mixtures.

4. CONCLUSIONS The well-intergrown silicalite zeolite membranes were successfully synthesized on seeded mullite support in ultradilute solution with H2O/SiO2 ratio of 800 and an inexpensive template of TPABr instead of TPAOH. The synthesis parameters such as H2O/SiO2 ratio and alkalinity in the precursor solution and synthesis temperature could seriously influence the crystals growth and membrane separation properties. During the crystallization process, Na+ in the precursor solution incorporated into the zeolite, which would probably reduce the hydrophobicity and permeation of silicalite membrane. Under optimum synthesis conditions, silicalite membranes with high flux and high separation factor were obtained for the separation of organics from water. The resulting membranes prepared in this work not only reduce the synthesis cost but also provide high PV performance, showing the potential industrial application of silicalite membranes as a promising separation material for preferential separation of organics from aqueous solutions.

effects of the temperature and the feed composition on the PV performance details. It showed that the separation factor slightly decreased with the temperature and greatly with the ethanol concentration (>3 wt %) and that the flux increased with the PV temperature and ethanol concentration in feed. As the feed temperature increases, the partial vapor pressures of the mixture of EtOH and H2O at the feed side also increases. Due to the strong dependency of vapor pressure on temperature, the resulting driving force for membrane permeation increases with the pervaporation temperature.1,50 Hence, the gradually increasing flux and windless varied separation factor with the incremental temperatures was generally observed.26,50 Figure 9 shows the separation performance of silicalite membrane for 5 wt % organic/water mixtures including

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ASSOCIATED CONTENT

* Supporting Information S

Tables of the results of EDS and ICP-OES analysis of membranes prepared with pure TPABr template and PV performance of typical silicalite membranes toward 5 wt % ethanol/water mixtures at 60 °C reported in the literature and in this work. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 9. The PV performance of silicalite membrane for 5 wt % organic/H2O mixtures at 60 °C (MeOH: methanol; EtOH: ethanol; DMK: acetone; IPA: isopropanol).

*Tel.: +86-791-8812-0533 (X.S.C.); +81-836-859661 (H.K.). Fax: +86-791-8812-0843 (X.S.C.); +81-836-859601 (H.K.). Email: [email protected] (X.S.C.); [email protected] (H.K.).

MeOH/H2O, EtOH/H2O, DMK/H2O, and IPA/H2O binary solutions at 60 °C. It is well-known that the kinetic diameters of these molecules increase in the following order: H2O (0.296 nm) < MeOH (0.380 nm) < EtOH (0.430 nm) < DMK (0.469 nm) ≈ IPA (0.470 nm).1,16 Although the kinetic diameters of these organic molecules are smaller than the pore size of MFI structure (0.51 nm × 0.55 and 0.53 nm × 0.56 nm), the assynthesized membrane has different PV performance. As shown in Figure 9, the sequence of permeation flux was MeOH > EtOH > DMK > IPA (>H2O), which was agreed with the corresponding order of their kinetic diameters. The small sized

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program of China (No. 2012AA03A609), the Ministry of Science and Technology of China (No. 2010DFB52950), National Natural Science Foundation of China (Nos. 20906042, 20966003, and 21106059) and Key Technology R&D Program of Jiangxi 11506

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