Preparation of Zeolite NaA Membranes on Macroporous Alumina

Article pubs.acs.org/IECR

Preparation of Zeolite NaA Membranes on Macroporous Alumina Supports by Secondary Growth of Gel Layers Jie Ma,† Jia Shao,† Zhengbao Wang,*,† and Yushan Yan†,‡ †

Department of Chemical and Biological Engineering, and MOE Engineering Research Center of Membrane and Water Treatment Technology, Zhejiang University, Hangzhou 310027, P. R. China ‡ Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: Zeolite NaA membranes are synthesized on tubular α-Al2O3 supports by the secondary growth of gel layers. The gel layer is prepared by wetting−rubbing hydrogel with a composition similar to the secondary synthesis solution xNa2O:2SiO2:Al2O3:150H2O. The hydrogel’s good uniformity makes it easier to gain a uniform gel layer. Zeolite NaA membranes are characterized by SEM and pervaporation separation of a 90 wt % ethanol/water mixture at 75 °C. The effects of the pretreatment time, pretreatment temperature, gel loading, and Na2O/Al2O3 ratio of the hydrogel on properties of zeolite NaA membranes are investigated. It is found that zeolite NaA membranes with high separation factors up to 10 000 can be obtained by secondary growth of gel layers. The optimal pretreatment conditions of the hydrogel are as follows: pretreatment temperature, 50 °C; pretreatment time, ≥18 h; and gel loading, 0.6−0.9 mg/cm2. The optimal composition of the hydrogel is 2.2Na2O:2SiO2:Al2O3:150H2O, which is the same as the synthesis solution. Like crystal seeds, the gel layer improves the zeolite membrane formation on the support surface, and the role of the gel layer becomes more significant with greater amounts of crystals.

1. INTRODUCTION Membrane separation technologies have been widely used in many industries, including chemical, water, food, and medicine. In particular, zeolite NaA membranes have been intensively investigated for gas permeation1 and pervaporation and especially in dewatering of bioethanol,2 due to their high hydrophilicity,3 high chemical and thermal stability, and molecular sieving capability.4−11 Several methods have been used to prepare zeolite membranes, including in situ hydrothermal synthesis,12−14 secondary growth,15 vapor phase transfer,16 and other methods.17,18 The secondary growth method using a seed layer has been investigated most intensively and is considered to be the most effective in preparing defect-free membranes with high separation performance, especially on the external surfaces of the tubular supports.19 To ensure the quality of membranes, various seeding methods have been used to obtain a seed layer, such as dip-coating−wiping,19 dip-coating,20 vacuum-coating,21 rubbing,22,23 and cross-flow filtration techniques.24 A perfect seed layer is of vital importance for obtaining a dense membrane; otherwise, domelike defects will form.25 However, the preparation of the seed layer involves a series of complicated steps, and the seed size and seed layer quality affect the membrane performance significantly.26 Zeolite membranes can be prepared from dry gel layers by the vapor phase transfer method. Dong et al.16 first reported the preparation of zeolite ZSM-5 and ZSM-35 membranes using a vapor-phase transfer method. A thin layer of wet-gel precursor precoated from a synthesis sol containing SiO2, tetrapropylammonium hydroxide (TPAOH), NaOH, and H2O was converted to an MFI zeolite film by vapor-phase treatment at elevated temperatures; templates and other additives can be © 2014 American Chemical Society

added to the gel or vapor phase to improve the zeolite membrane formation.27 Ma et al.28 reported that zeolite NaA membranes can be synthesized on a porous alumina disk by the vapor-phase transfer of dry gel. Although the permeances of the synthesized membranes are consistent with the literature values, the separation factors are quite low, with an n-butane/isobutane separation factor of 2.3. With the vapor-phase transfer method, differences in the loading of initial gel applied to the support as well as the dilution of the initial gel may be used to directly control the thickness of the membrane. However, domelike defects are easier to form as a result of the steam going into the gel layer. Additionally, limited nutrition makes it difficult for the gel layers to completely crystallize. It is reported that a uniform layer of zeolite crystals in pores of the support could be obtained from wet gel or dry gel.29,30 A FAU-type zeolite membrane was prepared by in situ seeding (achieved by infiltrating the synthesis mixture in the pores), followed by a two-step secondary growth.29 The membrane exhibits a CO2 permeance of 3.5 × 10−7 mol/m2·s·Pa and a separation selectivity of 12 in the separation of equal molar CO2 and N2 gas mixtures at 50 °C. An aluminosilicate gel barrier as a zeolite precursor is first incorporated in the pores of an alumina support tube and then subject to hydrothermal crystallization, leading to zeolite ZSM-5 membranes with high compactness at the molecular level.30 But the procedures used in refs 29 and 30 are too complex to be applied to industrial production. Received: Revised: Accepted: Published: 6121

December March 17, March 18, March 18,

29, 2013 2014 2014 2014

dx.doi.org/10.1021/ie404420j | Ind. Eng. Chem. Res. 2014, 53, 6121−6130

Industrial & Engineering Chemistry Research

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2.4. Characterization. The structure of the powders from hydrogels pretreated under different conditions was confirmed by X-ray diffraction. The hydrogels were washed several times with deionized water by centrifugation until pH ∼8 to obtain the gel powders. The morphology and thickness of the supports coated with gel layers and as-synthesized membranes were examined by scanning electron microscopy (SEM) (Hitachi TM-1000). The pervaporation performance test of zeolite membranes obtained was carried out using a homemade setup.30 The pervaporation (PV) properties were evaluated for dehydration of 90 wt % ethanol aqueous solution at 75 °C. The composition of the permeate collected for 10 min was analyzed by gas chromatography with an OV-1 capillary column. The total flux (J) and the separation factor (α) were calculated from the weight and the composition of the permeate.19

In this study, we demonstrate a new secondary growth method to synthesize zeolite membranes on the macroporous supports: hydrothermal secondary growth of a dry gel layer. This method combines the advantage of both the traditional secondary growth (from crystal seeds) method and vapor-phase transfer method. This new method is significantly different from our recently reported wetting−rubbing of seed paste on the surface of the defective supports,31 in that the seed paste was a physically blended mixture of preformed seed crystals, hydrogel (crystal/gel = 90/10), and water. More specifically, the crystals in the seed paste were synthesized separately. In this study, the seed hydrogel is synthesized in one step and then used to prepare the gel layer using the wetting−rubbing method. The effects of the pretreatment time, pretreatment temperature, Na2O/Al2O3 ratio of the hydrogel, and the gel loading on the properties of zeolite NaA membranes are investigated. Finally, the role of the gel layer is briefly discussed. Since the optimal hydrogel for making the gel layer has the same composition as the synthesis solution, this method is more convenient for industrial application.

3. RESULTS AND DISCUSSION 3.1. Characterization of Hydrogel. The crystallinity of hydrogels for preparation of gel layers has great effects on properties of zeolite membranes. Hydrogels pretreated at

2. EXPERIMENTAL SECTION 2.1. Materials. Chemicals used included sodium silicate solution (Wako Pure Chemical Industries Ltd., Japan) as SiO2 source, sodium aluminate (Wako) as Al2O3 source, sodium hydroxide (>96%, Sinopharm Chemical Reagent Co. Ltd.), and ethanol (>99.7%, Sinopharm). α-Al2O3 tubes [outer diameter (o.d.) 12 mm and inner diameter (i.d.) 8 mm] were cut into 5 cm long segments, calcined at 600 °C for 6 h, and ultrasonically washed three times for 1 min each prior to use. 2.2. Preparation of Gel Layer. The hydrogel with the composition of xNa2O:2SiO2:Al2O3:150H2O (x = 2.2, 3.4, and 7.5) was prepared as follows. The aluminate solution was prepared by dissolving sodium aluminate powder in deionized water at room temperature under vigorous stirring in a closed polypropylene bottle. The silicate solution was prepared by dissolving sodium silicate solution and sodium hydroxide under stirring in a 50 °C water bath in another closed polypropylene bottle. Both solutions were separately stirred for 15 min. Then the hydrogel was obtained by adding the aluminate solution dropwise into the silicate solution under vigorous stirring. The hydrogel was pretreated at different temperatures under stirring for various times, which was designated as gelx-T-t [x = Na2O/Al2O3 ratio, T = pretreatment temperature (°C), and t = pretreatment time (h)]. Prior to rubbing the hydrogel, alumina tubular supports were first wetted by dipping into deionized water. The supports were dried at room temperature for 1−2 min until no free water could be observed on the surface and then coated by rubbing the hydrogel with fingers in latex gloves. Subsequently, the supports coated with gel were dried in an oven at 60 °C for 3 h. 2.3. Membrane Synthesis. The synthesis mixture of molar ratio 2.2Na2O:2SiO2:Al2O3:150H2O was prepared using the same method as the preparation of the hydrogel. Then the support coated with a gel layer was immersed into the synthesis mixture vertically. The autoclave was put into a convection oven preheated to 100 °C. The synthesis of zeolite NaA membranes was carried out at 100 °C for 3−4 h. The supported zeolite membranes were taken out from the autoclave and dried at 60 °C after rinsing with deionized water several times. Similar to the gel, the membrane was designated as Mx-T-t [x = Na2O/Al2O3 ratio, T = pretreatment temperature (°C), and t = pretreatment time (h) of hydrogel].

Figure 1. XRD patterns of the powders from the hydrogel with the molar composition of 2.2Na2O:2SiO2:Al2O3:150H2O with different pretreatment times at different pretreatment temperatures: (a) 30 °C, (b) 50 °C, and (c) 80 °C. 6122



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(Na2O/Al2O3 ratio), which accelerates the crystallization rate. The effect of the Na2O/Al2O3 ratio of the hydrogel on the hydrogel crystallinity was also investigated in this study. The hydrogels in Figure 2a,b had the composition xNa2O:2SiO2:Al2O3:150H2O, x = 3.4 and 7.5, respectively and their pretreatment temperature was 50 °C. It can be seen from Figures 2a,2b and 1b that the higher the Na2O/Al2O3 ratio of the hydrogel, the less time it took for the typical XRD pattern of LTA-type zeolite to appear. When x = 3.4 (Figure 2a), the time needed was only 12 h, whereas it was 5 h when x = 7.5. They were both shorter than that when x = 2.2 (gel2.2-50-18, Figure 1b). 3.2. Effect of Pretreatment Time of Hydrogel. The average pore sizes of the α-alumina tubes were 1−3 μm, and their porosities were 30−40%. However, there were defective pores (larger than 30 μm), as shown in refs 31 and 33. To eliminate the effect of these defective pores, wetting−rubbing hydrogel was used to coat the gel layer in this study. The wetting−rubbing seed paste method was reported in our previous paper.31 Wetting the support with water can prevent the hydrogel from penetrating into the inside pores of the support. To investigate the effect of the crystallinity of the hydrogel on properties of zeolite NaA membranes, the hydrogels pretreated at 50 °C for different times (12, 18, 24 h), designated as gel2.2-50-12, gel2.2-50-18, and gel2.2-50-24, were used to prepare gel layers on the surfaces of α-alumina tubes (Table 1). As shown in Figure 1b, there were no XRD peaks for gel2.2-50-12, and the XRD peak intensities of gel2.2-50-24 were higher than those of gel2.2-50-18. Zeolite NaA membranes were obtained on the supports coated with gel layers after synthesis at 100 °C for 4 h. This synthesis condition has proven to be suitable for obtaining zeolite NaA membranes with separation factors higher than 10 000 when using 90 wt % crystals in the seed paste or 100% crystals in the seed layer.19,31 Table 1 shows the pervaporation results of the zeolite membranes from the hydrogels pretreated for 12, 18, and 24 h, respectively. The membrane performance was greatly affected by the crystallinity of the hydrogel, as shown in Table 1. Membrane M2.2-50-12 made from gel2.250-12 had a poor separation performance; the separation factor is only 32 with a flux of 4.87 kg/m2·h. The vacuum pressure of membrane M2.2-50-12 is 510 Pa, much higher than 180 Pa, the value for which membranes show no detectable defects in leak tests. There were some pinholes between large crystals in the SEM image, as shown in Figure 3a. This is consistent with the results from the leak tests. When increasing the pretreatment time to 18 and 24 h, membrane M2.2-50-18 and M2.2-50-24 made from gel2.2-50-18 and gel2.2-50-24 had good performance, with separation factors higher than 10 000, and their fluxes were 3.17 and 3.14 kg/m2·h, respectively. Figure 3c−f shows that membranes M2.2-50-18 and M2.2-50-24 were continuous and had no obvious defects on the top and crosssectional views. From Figure 3, it can be seen that the crystal

Figure 2. XRD patterns of the powders from the hydrogel with the molar composition of xNa2O:2SiO2:Al2O3:150H2O with different pretreatment times at 50 °C: (a) x = 3.4 and (b) x = 7.5.

different temperatures for various times and with different compositions were characterized by X-ray diffraction (XRD). Figure 1 shows the XRD patterns of the powders from the hydrogels pretreated at different temperatures for various times. The molar composition of the hydrogels in Figure 1 was 2.2Na2O:2SiO2:Al2O3:150H2O, which is the same as the synthesis mixture. Pretreatment temperatures in parts a, b, and c of Figure 1 were 30, 50, and 80 °C, respectively. It can be seen from Figure 1 that the crystallization rate of the hydrogel increased with the pretreatment temperature. When the pretreatment temperature was 30 °C, the typical LTA pattern appeared after pretreatment for 6 days (gel2.2-30-144). And it took more than 10 days to reach 100% crystallinity. However, when the pretreatment temperature of the hydrogel was increased to 80 °C, only 4.25 h was needed for the typical LTA crystal peaks to appear (gel2.2-80-4.25), and it only took 7 h to complete the crystallization. In the case of 50 °C, 18 h was needed for the typical XRD pattern of LTA-type zeolite to appear (gel2.2-50-18). According to the literature,32 the solubility of the silica source is enhanced by increasing the alkalinity of the hydrogel

Table 1. Effects of Pretreatment Time of the Hydrogel on Properties of the Membranes Synthesized on Al2O3 Supports at 100 °C for 4 ha membrane

pretreatment time (h)

gel loading (mg/cm2)

membrane weight (mg/cm2)

vacuum pressure (Pa)

α

flux (kg/m2·h)

M2.2-50-12 M2.2-50-18 M2.2-50-24

12 18 24

0.97 0.91 0.78

2.23 1.89 2.27

510 180 180

32 >10000 >10000

4.78 3.17 3.14

Hydrogel composition, 2.2Na2O:2SiO2:Al2O3:150H2O; pretreatment temperature, 50 °C. For membrane Mx-T-t, x = Na2O/Al2O3 ratio, T = pretreatment temperature (°C), and t = pretreatment time (h).

a

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Figure 3. SEM images of top view and cross-sectional view of zeolite NaA membranes on Al2O3 supports with gel layers of different pretreatment times: (a, b) M2.2-50-12, 12 h; (c, d) M2.2-50-18, 18 h; and (e, f) M2.2-50-24, 24 h. Hydrogel composition, 2.2Na2O:2SiO2:Al2O3:150H2O; pretreatment temperature, 50 °C; synthesis conditions, 100 °C and 4 h.

Figure 4. SEM top view images of zeolite NaA membranes synthesized at 100 °C for 4 h on Al2O3 supports with hydrogels of different pretreatment temperatures: (a) M2.2-30-144, 30 °C and (b) M2.2-80-4.25, 80 °C.

These results mentioned above show that it is difficult to obtain high-performance zeolite membranes if there are no detectable crystals by XRD in the hydrogel. The possible reason is that the gel

size became smaller as the pretreatment time of the hydrogel increased. This may be due to more crystals or nuclei being present in the hydrogel pretreated for longer times. 6124



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Figure 5. SEM top view images of zeolite NaA membranes synthesized at 100 °C for 3 h on Al2O3 supports with gel of different pretreatment temperatures: (a) M2.2-30-144a, 30 °C and (b) M2.2-50-18a, 50 °C.

Figure 6. SEM top-view images of Al2O3 supports after the hydrogel deposition but before the membrane synthesis with different loadings of hydrogel: (a) 0.0 mg/cm2, (b) 0.34 mg/cm2, (c) 0.50 mg/cm2, (d) 0.78 mg/cm2, (e) 0.99 mg/cm2, and (f) 1.15 mg/cm2. The molar composition of hydrogel, 2.2Na2O:2SiO2:Al2O3:150H2O; pretreatment conditions, 50 °C and 18 h.

significant effects on the crystallinity of the hydrogel. Therefore, the effects of pretreatment temperature of the hydrogel on properties of zeolite membranes were also investigated here. The XRD patterns of LTA-type zeolite started to appear in the

layer has to have certain crystallinity to promote the formation of zeolite NaA membranes and to avoid various pinholes and defects. 3.3. Effect of Pretreatment Temperature of Hydrogel. As shown in section 3.1, the pretreatment temperature had 6125



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Figure 7. SEM images of the top view of zeolite NaA membranes on Al2O3 supports with different hydrogel loadings: (a) M2.2-50-24a, 0.34 mg/cm2; (b) M2.2-50-24b, 0.52 mg/cm2; (c) M2.2-50-24c, 0.78 mg/cm2; (d) M2.2-50-24d, 0.99 mg/cm2; and (e) M2.2-50-24e, 1.15 mg/cm2. Synthesis conditions: 100 °C and 4 h.

separation factor of membrane M2.2-50-18a prepared from gel2.2-50-18 was only 2368 (Table S2, Supporting Information). The surface of membrane M2.2-30-144a (Figure 5a) appears more continuous than that of membrane M2.2-50-18a (Figure 5b). It is known that zeolite samples containing smaller crystals will be obtained at a lower crystallization temperature. The hydrogels gel2.2-30-144, gel2.2-50-18, and gel2.2-80-4.25 had almost the same crystallinity, while gel2.2-30-144 had the highest crystals number compared with other two hydrogels; thus, a certain amount of zeolite crystals/nuclei may be needed inside the hydrogel to obtain zeolite membranes with high separation performance. For gel2.2-80-4.25, it is possible that there were not enough crystals/nuclei available in the gel layer to form a high-quality membrane, so the preferable pretreatment temperature is 30 or 50 °C. However, preparing gel2.230-144 pretreated at 30 °C takes longer; thus, we prefer to choose the pretreatment temperature of 50 °C. 3.4. Effect of Gel Loading. According to the literature, the loading of crystal seeds coated on the support greatly influences the properties of zeolite membranes.26 The effects of the gel

hydrogels (gel2.2-30-144, gel2.2-50-18, and gel2.2-80-4.25) that were used to prepare gel layers on alumina tubes. The pervaporation (PV) results of zeolite NaA membranes prepared from these gel layers are listed in Tables 1 (M2.2-50-18) and S1 [Supporting Information (M2.2-30-144 and M2.2-80-4.25)]. The membranes synthesized from the hydrogels pretreated at different temperatures have different properties. Membrane M2.2-30-144 made from gel2.2-30-144 had a good PV performance with a separation factor of over 10 000 and a flux of 3.04 kg/m2·h. The membrane surface was continuous with well intergrown crystals (Figure 4a). The separation factor of membrane M2.2-50-18 prepared from gel2.2-50-18 was also >10 000 (Table 1). However, the separation factor of membrane M2.2-80-4.25 prepared from gel2.2-80-4.25 was only 1972. The vacuum pressure of membrane M2.2-80-4.25 was 200 Pa, slightly higher than the optimal vacuum pressure of 180 Pa, and some pinholes could be found on the membrane surface (Figure 4b). It is also found that if the synthesis time was reduced to 3 h, membrane M2.2-30-144a prepared from gel2.2-30-144 had a separation factor of 8769, whereas the 6126



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Figure 8. SEM images of top and cross-sectional views of zeolite NaA membranes synthesized at 100 °C for 4 h on Al2O3 supports with gel layers of different alkalinities: (a, b) M3.4-50-18, x = 3.4 and (c, d) M7.5-50-5, x = 7.5.

According to the above results, it is clear that when the gel loading is too high (e.g., >1.0 mg/cm2), the gel layer and membrane layer tend to crack and the thick gel layer might prevent the nutrition in the synthesis mixture from penetrating it, resulting in a defective membrane layer. When the gel loading is too low (e.g., 10 000, and 5392, respectively. However, when the gel loading was 1.15 mg/cm2, the separation factor was only 1440. The flux of zeolite membranes lightly increased with the gel loading, mainly due to the existence of defects in the membrane layer. The SEM images of top view and cross-sectional view of the membranes with different gel loadings are shown in Figure 7. The membrane made with a gel loading of 0.34 mg/cm2 had pinholes on the surface (Figure 7a), resulting in the lower PV performance. The membrane with a gel loading of 1.15 mg/cm2 had cracks on the surface (Figure 7e), which led to the decrease of the membrane PV performance. The dense membranes without pinholes and cracks were observed with gel loadings of 0.52−0.99 mg/cm2 (Figure 7b−d) 6127



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Figure 9. SEM top view images of zeolite NaA membranes synthesized at 100 °C for different synthesis times: (a1, a2) 1 h, (b1, b2) 2 h, (c1, c2) 3 h and (d1, d2) 4 h, from the different hydrogels pretreated at 30 °C for 0.5 h (a1−d1), and at 50 °C for 18 h (a2−d2). The composition of hydrogel was 2.2Na2O:2SiO2:Al2O3:150H2O.

pattern of LTA-type zeolite (data not shown), whereas the hydrogel of gel2.2-50-18 started to show such patterns (Figure 1b). As shown in Figure 9a1,a2, only the gel layer could be observed on the support surface by SEM for both the hydrogels after a synthesis time of 1 h. The difference between the two hydgogels could be observed after synthesis times of longer than 2 h. At 2 h synthesis time, many crystals could be seen on the support surface (Figure 9b1,b2). However, there were more crystals on the support for gel2.2-50-18 (Figure 9b2) than for gel2.2-30-0.5 (Figure 9b1), as a result of more crystals in gel2.250-18 than gel2.2-30-0.5. Some gel still existed on the support after 2 h of synthesis. As the synthesis time was increased to 3 h, the membrane from gel2.2-50-18 was almost dense, whereas large apertures could be seen in the membrane from

was clearly not dense when the Na2O/Al2O3 ratio was 7.5 (Figure 8c). The vacuum pressure of membrane M7.5-50-5 was much higher than 180 Pa, and the separation performance was not measured. The results here suggest that the hydrogels with excessive alkalinity are not suitable for obtaining zeolite membranes with high PV performance; the hydrogel with low alkalinity (x = 2.2) is preferable. 3.6. Discussion. To find out the process of membrane formation from the gel layer by the secondary growth method, the dependence of the membrane morphology on the synthesis time (1−4 h) from two typical hydrogels (gel2.2-30-0.5 and gel2.2-50-18) was investigated. The composition of these two hydrogels is 2.2Na2O:2SiO2:Al2O3:150H2O. The hydrogel of gel2.2-30-0.5 pretreated at 30 °C for 0.5 h showed no XRD 6128



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gel2.2-30-0.5 (Figure 9c1). After 4 h, the membrane from gel2.2-50-18 was continuous and dense (Figure 9d2) and also showed better PV performance than the membrane from gel2.230-0.5. There were some pinholes in the latter (Figure 9d1). As presented above, the gel layer supplies crystals, possibly nuclei, and nutrient for the membrane formation. Therefore, it seems that like crystal seeds, the gel layer can improve the zeolite membrane formation on the support surface, and the more crystals/nuclei in the gel layer, the more significant such improvements are. This is why the hydrogel for preparing a gel layer should be pretreated at a certain temperature for a certain time, e.g., at 50 °C for 18 h. During the secondary growth period, the gel layer will partially dissolve in the synthesis solution and promote the supersaturation of the synthesis solution near the support. The liquid retained in the support pores during the wetting− rubbing process can affect the synthesis environment and also supply nutrition for the crystals. The gel layer from the hydrogel with higher alkalinity creates a stronger alkaline environment for crystal growth, which improves the crystallization rate and renders larger crystals. However, it is difficult for larger crystals to become well-intergrown. When the Na2O/Al2O3 ratio (x) increased to 3.4 and 7.5, there were many defects in the membrane (Figure 8a,c). Low alkalinity (x = 2.2) is the optimal alkalinity condition. On the other hand, excessive gel loadings cause the gel layer to crack and some of the gel blocks to leave the support surface during the secondary growth period. That is why there is an optimum gel loading for the gel layer, e.g. 0.6−0.9 mg/cm2.

AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-571-8795-2391. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial supports provided by the National Natural Science Foundation of China (21236006), the National Basic Research Program (2013CB228104), Science and Technology Department of Zhejiang Province (2009R50020), and the Hengyi Foundation at Zhejiang University are gratefully acknowledged.

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REFERENCES

(1) Caro, J.; Noack, M. Zeolite MembranesRecent Developments and Progress. Microporous Mesoporous Mater. 2008, 115, 215−233. (2) Koros, W. J. Evolving beyond the Thermal Age of Separation Processes: Membranes Can Lead the Way. AIChE J. 2004, 50, 2326− 2334. (3) Bein, T. Synthesis and Applications of Molecular Sieve Layers and Membranes. Chem. Mater. 1996, 8, 1636−1653. (4) Choi, S.; Coronas, J.; Jordan, E.; Oh, W.; Nair, S.; Onorato, F.; Shantz, D. F.; Tsapatsis, M. Layered Silicates by Swelling of AMH-3 and Nanocomposite Membranes. Angew. Chem., Int. Ed. 2008, 47, 552−555. (5) Ruiz, A. Z.; Li, H.; Calzaferri, G. Organizing Supramolecular Functional Dye−Zeolite Crystals. Angew. Chem., Int. Ed. 2006, 45, 5282−5287. (6) Sato, K.; Nakane, T. A High Reproducible Fabrication Method for Industrial Production of High Flux NaA Zeolite Membrane. J. Membr. Sci. 2007, 301, 151−161. (7) Li, Z. J.; Lew, C. M.; Li, S.; Medina, D. I.; Yan, Y. S. Pure-SilicaZeolite MEL Low-k Films from Nanoparticle Suspensions. J. Phys. Chem. B. 2005, 109, 8652−8658. (8) Gallego-Lizon, T.; Edwards, E.; Lobiundo, G.; dos Santos, L. F. Dehydration of Water/t-Butanol Mixtures by Pervaporation: Comparative Study of Commercially Available Polymeric, Microporous Silica and Zeolite Membranes. J. Membr. Sci. 2002, 197, 309−319. (9) Shah, D.; Kissick, K.; Ghorpade, A.; Hannah, R.; Bhattacharyya, D. Pervaporation of Alcohol−Water and Dimethylformamide−Water Mixtures using Hydrophilic Zeolite NaA Membranes: Mechanisms and Experimental Results. J. Membr. Sci. 2000, 179, 185−205. (10) Wang, H. T.; Huang, L. M.; Holmberg, B. A.; Yan, Y. S. Nanostructured Zeolite 4A Molecular Sieving Air Separation Membranes. Chem. Commun. 2002, 16, 1708−1709. (11) Li, Y. S.; Zhou, H.; Zhu, G. Q.; Liu, J.; Yang, W. S. Hydrothermal Stability of LTA Zeolite Membranes in Pervaporation. J. Membr. Sci. 2007, 297, 10−15. (12) Zah, J.; Krieg, H. M.; Breytenbach, J. C. Pervaporation and Related Properties of Time-Dependent Growth Layers of Zeolite NaA on Structured Ceramic Supports. Microporous Mesoporous Mater. 2006, 284, 276−290. (13) Aoki, K.; Kusakabe, K.; Morooka, S. Gas Permeation Properties of A-Type Zeolite Membrane Formed on Porous Substrate by Hydrothermal Synthesis. J. Membr. Sci. 1998, 141, 197−205. (14) Li, Y. S.; Chen, H. L.; Liu, J.; Yang, W. S. Microwave Synthesis of LTA Zeolite Membranes Without Seeding. J. Membr. Sci. 2006, 277, 230−239. (15) Hedlund, J.; Schoeman, B.; Sterte, J. Ultrathin Oriented Zeolite LTA Films. Chem. Commun. 1997, 1, 1193−1994. (16) Dong, J. X.; Dou, T.; Zhao, X. G.; Gao, L. H. Synthesis of Membranes of Zeolites ZSM-5 and ZSM-35 by the Vapor-Phase Method. J. Chem. Soc., Chem. Commun. 1992, 28, 1056−1058. (17) Balkus, K. J.; Scott, A. S. Zeolite Coatings on ThreeDimensional Objects via Laser Ablation. Chem. Mater. 1999, 11, 189−191.

4. CONCLUSIONS High-performance membranes were prepared successfully on Al2O3 supports by the secondary growth of gel layers. The effects of pretreatment time, pretreatment temperature, and alkalinity (Na2O/Al2O3 ratio) of the hydrogel, as well as gel loading on the crystallinity of hydrogels and properties of zeolite NaA membranes were investigated in detail. It is found that the crystallinity of hydrogels increased with the extension of the pretreatment time and increase of the pretreatment temperature. The optimal pretreatment conditions of the hydrogel for preparing the gel layer on the support were as follows: pretreatment temperature, 50 °C; pretreatment time, ≥18 h. The optimal hydrogel composition is the same as the synthesis mixture (the Na2O/Al2O3 = 2.2). When the Na2O/ Al2O3 ratio was much higher, the crystals grew larger, rendering more defects in the membrane. If the gel loading is excessive, the gel layer may crack and some of the gel blocks might leave the support surface during the secondary growth. The optimal gel loading is 0.6−0.9 mg/cm2. Like crystal seed layers, the gel layers play an important role in improving the zeolite membrane formation on the support surface, and it appears that the more crystals/nuclei in the gel layer, the greater the role of the gel layer. This secondary growth method using the gel layer as a “promoter” may open a novel synthesis method for zeolite membranes.

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ASSOCIATED CONTENT

S Supporting Information *

Gel loadings, membrane layer weights, and pervaporation results (Tables S1−S4). This material is available free of charge via the Internet at http://pubs.acs.org/. 6129



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Preparation of Zeolite NaA Membranes on Macroporous Alumina

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