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Influence of synthesis gel composition on morphology, composition, and dehydration performance of CHA-type zeolite membranes
Journal of Membrane Science 363 (2010) 256–264
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Influence of synthesis gel composition on morphology, composition, and dehydration performance of CHA-type zeolite membranes Y. Hasegawa ∗ , C. Abe, M. Nishioka, K. Sato, T. Nagase, T. Hanaoka National Institute of Advanced Industrial Science and Technology (AIST), Research Center for Compact Chemical System, 4-2-1, Nigatake, Miyagino-ku, Sendai 983-8551, Japan
a r t i c l e
i n f o
Article history: Received 31 May 2010 Received in revised form 21 July 2010 Accepted 23 July 2010 Available online 30 July 2010 Keywords: Zeolite membrane Chabazite Dehydration Ethanol Ion-exchange
a b s t r a c t CHA-type zeolite membranes with excellent dehydration performance can be prepared on a porous ␣-Al2 O3 support tube by the secondary growth of seeds in a synthesis gel containing strontium. The permeation flux and separation factor of the membrane were 2.89 kg m−2 h−1 and >100,000, respectively, for a 50 mol% ethanol solution at 313 K. In this work, the CHA-type zeolite membranes were prepared using the gel with different compositions to discuss the influences of the synthesis gel composition on the morphology, composition, and dehydration performance of the CHA-type zeolite membrane. As a result, the CHA-type zeolite membrane could be prepared in the gel with the following compositions: SiO2 /Al2 O3 = 2–16; H2 O/SiO2 = 65–250; H2 O/(K2 O + SrO) = 100–400; SrO/(K2 O + SrO) = 0.10–0.85. Furthermore, the CHA-type zeolite membrane was ion-exchanged with lithium, sodium, rubidium, and cesium ions, and the dehydration performances of the ion-exchanged membranes were also determined. Based on these results, the effect of strontium for the membrane preparation was discussed in this paper. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Zeolites have regular channel structures with diameters comparable to those of inorganic gas and light hydrocarbon molecules. Hence, polycrystalline zeolite membranes are quite suitable for separating gaseous and liquid mixtures. In particular, NaA-type zeolite membranes show excellent dehydration performance for mixtures of water and alcohol. For example, the permeation flux Jt and separation factor ˛ were 8.8 kg m−2 h−1 and 10,000, respectively, for a 90 wt% ethanol solution at 348 K [1]. However, the acid stability of the NaA-type zeolite membrane is poor because of the large aluminum content in the framework (Si/Al = 1). Over the last decade, several acid resistant alternatives such as T-, MOR-, MER-, and PHI-type zeolite membranes have been developed [2–5], but their dehydration performances are lower (e.g., for a T-type zeolite membrane: Jt = 1.1 kg m−2 h−1 and ˛ = 900 [2]). Fig. 1 shows the dehydration performances of several zeolite membranes reported in previous studies [2–9]. The dehydration performance of the zeolite membrane is largely determined by the zeolite properties (channel volume and channel diameter), although other factors such as the pressure drop in a substrate and the thickness of the zeolite layer are also important. Furthermore, the acid stability of zeolite is determined by the aluminum content in the framework, since the aluminum atoms are removed from the
∗ Corresponding author. Tel.: +81 22 237 3098; fax: +81 22 237 7027. E-mail address: [email protected] (Y. Hasegawa). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.07.040
framework by acids [10]. The T-, MOR-, MER-, and PHI-type zeolite membranes with the Si/Al ratio of 2.5–5 are stable for acetic acid [2–5]. These suggest that the zeolite channel volume, channel size, and aluminum content greatly influence the permeation flux, separation factor, and acid stability of zeolite membranes, respectively. CHA-type zeolite has eight-membered-ring channels and large cavities, as does NaA-type zeolite. The channel volume and diameter of CHA-type zeolite are 0.23 ml g−1 and 0.38 nm, respectively [11]. Additionally, the Si/Al ratio of CHA-type zeolite can be varied in the range 1–5 [11]. These features of the CHA-type zeolite propose that CHA-type zeolite is one of the potential materials for dehydration membranes. The shape of zeolite crystals forming the polycrystalline layer is also important for the performance of zeolite membranes [12,13]. Matsukata et al. adjusted the synthesis gel composition and hydrothermal conditions to obtain a compact mordenite membrane [13], and they concluded that the block-shaped crystals are favorable to form a compact and thin zeolite layer with the high dehydration performance. However, it is difficult for CHA-type zeolite to form block-shaped crystals without the use of organic templates [14]. Nevertheless, in our previous study, a polycrystalline CHA-type zeolite membrane comprising block-shaped crystals was successfully formed in a synthesis gel containing strontium (organic template free) [15]. In this paper, the membranes were prepared in the synthesis gels with different compositions to discuss the influence of the gel composition on the morphology, composition, and dehydration performance of the CHA-type zeolite membrane.
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Fig. 1. Dehydration performance of zeolite membranes: (a) influence of channel volume of zeolite on the permeation flux of the membrane, (b) separation factor of water with respect to ethanol as a function of zeolite channel diameter. () OFF [2], (䊉) MOR [3], () MER [4], () PHI [5], () LTA [6], () DDR [7], () SOD [8], () FAU [9].
2. Experimental 2.1. Membrane preparation Polycrystalline CHA-type zeolite layers were formed on the outer surface of porous ␣-Al2 O3 tubes by the secondary growth of seed crystallites [15]. The seed particles were prepared by the procedure described in Ref. [14]. Potassium hydroxide (12.8 g, Wako Pure Chemicals Industry) was dissolved into distilled water (217.4 g), and 25.2 g of Y-type zeolite powder (Tosoh Corp., HSZ-330HUA, SiO2 /Al2 O3 = 6.3) was added to the solution. The mixture was stirred at room temperature for 1 h and then poured into a Teflon-lined stainless steel autoclave. The autoclave was heated in an oven at 368 K for 100 h to convert Y-type zeolite into CHA-type zeolite. The CHA-type zeolite particles were washed with excess amount of distilled water and dried in air at 333 K overnight to obtain the CHA-type zeolite seed particles (SiO2 /Al2 O3 = 4.7; crystal size = ca. 5–10 m). A synthesis gel for the secondary growth was prepared using colloidal silica (Catalysts and Chemicals Industry Corp. Ltd., Cataloid SI-30), aluminum nitrate (Wako Pure Chemicals Industry), strontium nitrate (Wako Pure Chemicals Industry), potassium hydroxide, and distilled water. Aluminum nitrate and potassium hydroxide were dissolved into distilled water in a beaker. In another beaker, strontium nitrate was dissolved into distilled water, and colloidal silica was added to the solution slowly. The aluminum source solution was added to the silicon source solution, and the slurry gel was stirred for 6 h at room temperature. The molar composition of the gel was as follows: SiO2 /Al2 O3 = 2–35, H2 O/SiO2 = 30–300, H2 O/(K2 O + SrO) = 50–500, and SrO/(K2 O + SrO) = 0–1. The gel also contained potassium nitrate. For example, the standard gel composition was 12SiO2 :1Al2 O3 :2K2 O:1SrO:8KNO3 :780H2 O in terms of the molar ratio. The porous ␣-Al2 O3 tube was used as the support [16], and the properties were as follows: outside diameter = 2.0 mm, inside diameter = 1.6 mm, length = 30 mm,
Fig. 2. (a) XRD pattern and (b) SEM image of CHA-type zeolite membrane prepared in the standard gel. The composition of the standard gel was 12SiO2 :1Al2 O3 :2K2 O:1SrO:780H2 O (SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, and SrO/(K2 O + SrO) = 0.33).
porosity = 0.39, and mean pore diameter = 0.15 m. The outer surface of the support tube was rubbed with the seed crystallites to implant the seeds for nucleation. Three support tubes were placed in a Teflon-lined autoclave vertically using a Teflon stand, and 20 g of the synthesis gel was added to the autoclave. A hydrothermal reaction was carried out in an oven at 413 K for 20 h in order to form a polycrystalline layer on the support. After cooling the autoclave in a water bath, the membrane was washed with distilled water and dried in air at room temperature overnight to obtain the CHA-type zeolite membrane. The crystal structures of the seed particles and membranes were determined by X-ray diffraction (XRD, Bruker M21X), and the morphology was observed by a scanning electron microscopy (SEM, Hitachi High Technologies, Miniscope TM-1000). The composition of the zeolite layer was analyzed using an energy dispersive X-ray analyzer (EDX, Hitachi High Technologies, Swift-ED) attached to the SEM. 2.2. Ion-exchange The potassium and strontium ions in the CHA-type zeolite membrane were exchanged with a 1.0 mol l−1 solution of either LiCl, NaCl, KCl, RbCl, or CsCl. The membrane prepared in the standard synthesis gel was added to 20 g of the each solution and heated in an oven at 333 K for 3 h. The membrane was then washed with distilled
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Fig. 3. Surface morphologies of the zeolite membranes prepared in synthesis gels with different H2 O/SiO2 ratios. The gel compositions were SiO2 /Al2 O3 = 12; H2 O/(K2 O + SrO) = 260; SrO/(K2 O + SrO) = 0.33; and H2 O/SiO2 = (a) 30, (b) 50, (c) 100, (d) 250, and (e) 300.
water and dried at ambient temperature overnight to obtain the ion-exchanged CHA-type zeolite membrane. The composition of the ion-exchanged membrane was analyzed by EDX, and the degree of ion-exchange was calculated by the content of the exchanged ion divided with the total content of ions. Note that the degree of ion-exchange by lithium was the content reduction of potassium and strontium ions in the membrane because lithium cannot be detected by EDX.
2.3. Pervaporation The dehydration performance of the CHA-type zeolite membrane was determined by pervaporation for 50 mol% ethanol solutions at 313 K [15,17,18]. One end of the membrane was connected to a stainless steel tube with an inorganic sealant (Varian,
Torr Seal), and the other end was sealed. The membrane was added to the solution, and the inside was evacuated by a rotary pump. Further, helium was introduced into the inner surface of the membrane as a standard gas at 1.0 ml min−1 , and the molar composition in the evacuated stream was analyzed using a mass spectrometer (Hiden Analytical, HAL-301RC). The mass spectrometer and standard gas allowed determination of the permeation properties without the condensation of permeated vapor in the error range below 3% [15]. The back permeation of helium is negligible, since the partial pressure difference across the membrane is marginal. Accordingly, the molar permeation flux of component i, Ji , is given by Ji =
NHe yi , · A yHe
(1)
where NHe , A, and yi are the molar flow rate of helium, effective membrane area for permeation, and mole fraction of component i
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in the evacuated stream. The permeation flux on a mass basis Jt can be calculated as follows: Jt = 3600(Ji Mi + Jj Mj ),
(2)
where Mi is the molecular weight of component i. The separation factor of component i with respect to component j, ˛(i/j), is defined as follows: ˛
i j
=
yi /yj xi /xj
,
(3)
where xi is the mole fraction of component i in the solution. Detailed information on the pervaporation test is described in Refs. [17,18]. The detection limit of ethanol was 0.002 wt% under the measurement condition. 3. Results and discussion 3.1. Standard conditions The CHA-type zeolite membrane was first prepared in the standard synthesis gel with the molar composition SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, and SrO/(K2 O + SrO) = 0.33. The recovery powder from the liquid phase was amorphous because of the lower alkali content and higher water content compared to those for powder synthesis. Fig. 2 shows a SEM image and XRD pattern of the membrane. The outer surface of the support tube was completely covered with a polycrystalline layer, which comprised block-shape crystals with a size of ca. 2 m. The XRD pattern was a combination of those for CHA-type zeolite and support tube. This indicates that the polycrystalline layer on the support consisted of CHA-type zeolite crystallites. The composition of the layer was 6.3SiO2 :1Al2 O3 :0.92K2 O:0.14SrO (SiO2 : 75.4 mol%, Al2 O3 : 11.9 mol%, K2 O: 11.0 mol%, SrO: 1.7 mol%) on a dry basis. Furthermore, the membrane showed excellent dehydration performance for an equimolar mixture of water and ethanol at 313 K (Jt = were 1.04 kg m−2 h−1 and ˛ > 100,000). These results are in agreement with those in our previous study [15]. 3.2. Influence of water content Fig. 3 shows SEM images of the top surface of the CHA-type zeolite membranes prepared in the synthesis gel with H2 O/SiO2 ratios of 30, 50, 100, 250, and 300. At H2 O/SiO2 ratio = 30, the polycrystalline layer was not formed on the support. The small CHA-type zeolite crystals could be formed at the ratio of 50. However, many rod-shaped MER-type zeolite crystals were also observed on the surface. At the ratio of 65 (Fig. 2), the seed crystals on the support grew largely with fewer rod-shaped MER-type crystals than at the ratio of 50, although the size of the rod-shaped MER-type zeolite crystals was larger. In the ratio range of 100–250, the surface of the support tube was covered with block-shaped CHA-type zeolite crystals. However, the crystal size was smaller than that at the ratio of 65. At still higher ratios, dish-shaped CHA-type zeolite crystals were observed on the surface. Thus, the favorable H2 O/SiO2 ratio is 65–250 for the preparation of the CHA-type zeolite membrane consisted of the block-shaped crystals. The H2 O/SiO2 ratio is the inverse of the material concentration for the zeolite synthesis. For the secondary growth, precursors for the zeolite synthesis are transferred via water. Therefore, these results imply that the precursor concentration dissolved into water is important in the preparation of a CHA-type zeolite membrane. Fig. 4 shows the influence of the gel composition on the composition of the CHA-type zeolite membrane. The Si/Al ratio of the zeolite membrane decreased as the H2 O/SiO2 ratio increased. When the gel composition is aSiO2 :1Al2 O3 :bK2 O:cSrO:dH2 O, the Si/Al
Fig. 4. Influence of the H2 O/SiO2 ratio in the synthesis gel on the membrane composition. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/(K2 O + SrO) = 260, SrO/(K2 O + SrO) = 0.33, and various H2 O/SiO2 . The line indicates the Si/Al ratio calculated by Eq. (5).
ratio in zeolite can be calculated as follows [14,19]: 0.5aB + b + c − 1 Si , = Al B+b+c−1
(4)
where B is a constant. Assuming that the H2 O/SiO2 ratio is w in this experiment, the parameters a, b, c, and d are 12, 0.31w, 0.15w, and 12w, respectively. Therefore, Eq. (4) can be converted into: 6B + 0.46x − 1 Si = . B + 0.46x − 1 Al
(5)
This equation means that the Si/Al ratio decreases with an increase in the H2 O/SiO2 ratio w in the gel. The experimental results followed this tendency. Fig. 5 shows the dehydration performances of the CHA-type zeolite membranes for an equimolar mixture of water and ethanol at 313 K as a function of the H2 O/SiO2 ratio in the synthesis gel. When the H2 O/SiO2 ratio was 65–250, the separation factor was more than 10,000. The suitable H2 O/SiO2 range was the same as that determined by morphology observation. However, the permeation flux indicated a minimum at the ratio of 150. As shown in Fig. 3, the H2 O/SiO2 ratio influenced the phase, shape, and size of the zeolite crystals. The minimum permeation flux may depend on the growth rate of the block-shaped crystals. 3.3. Influence of alkali content Fig. 6 shows the influence of the (K2 O + SrO)/H2 O ratio in the synthesis gel on the surface morphology of the CHA-type zeolite
Fig. 5. Influence of the H2 O/SiO2 ratio in the synthesis gel on the dehydration performance for an equimolar mixture of water and ethanol at 313 K. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/(K2 O + SrO) = 260, SrO/(K2 O + SrO) = 0.33, and various H2 O/SiO2 .
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Fig. 6. Surface morphologies of the membranes prepared in synthesis gels with different H2 O/(K2 O + SrO) ratios. The gel compositions were SiO2 /Al2 O3 = 12; H2 O/SiO2 = 65; SrO/(K2 O + SrO) = 0.33; and H2 O/(K2 O + SrO) = (a) 100, (b) 200, (c) 300, and (d) 400.
membranes. When the ratio was 100, the CHA-type zeolite crystals grew largely. However, many large MER-type zeolite crystals were also observed on the surface. The number of the MERtype zeolite crystal on the surface decreased with an increase in H2 O/(K2 O + SrO), although the size of the CHA-type zeolite crystal reduced. The H2 O/(K2 O + SrO) ratio is the inverse of the alkali concentration in the solution. The solubility of a precursor for zeolite formation is higher at higher alkalinity, and this leads the higher nucleation density and crystal growth rate, which lead to the production of many MER-type zeolite crystals as well. As a result, a lower alkalinity gel (higher H2 O/(K2 O + SrO) ratio) is preferable for the growth of a CHA-type zeolite layer. Fig. 7 shows the influence of the gel composition on the composition of the CHA-type zeolite membrane. The Si/Al ratio of the zeolite membrane increased with H2 O/(K2 O + SrO). When H2 O/(K2 O + SrO) = z, Eq. (4) can be converted into, Si 6B + (780/z) − 1 = . Al B + (780/z) − 1
the permeation flux at ratios of 300–400. The lower permeation flux may be attributed to the crystallinity of the CHA-type zeolite crystallites. 3.4. Influence of aluminum content Fig. 9 shows the influence of the SiO2 /Al2 O3 ratio in the synthesis gel on the surface morphology of the CHA-type zeolite membranes. The polycrystalline layer consisted of block-shaped crystals at ratios of 2–20, although rod-shaped MER-type zeolite crystals were observed at a ratio of 20. Moreover, the crystal size decreased
(6)
This equation shows that the Si/Al ratio of the zeolite membrane increases with H2 O/(K2 O + SrO) as also seen in the experimental results. Fig. 8 shows the dehydration performances of the CHA-type zeolite membranes for an equimolar mixture of water and ethanol at 313 K as a function of the H2 O/(K2 O + SrO) ratio in the synthesis gel. The separation factor was over 10,000 in the H2 O/(K2 O + SrO) range of 100–300, and the permeation flux also indicated a maximum at the ratio of around 200–300. The alkali concentration influences the crystal growth rate as discussed in Fig. 6. Therefore, the permeation flux decreased at smaller H2 O/SiO2 ratios (higher alkali content). However, the crystal growth rate cannot explain the decrease in
Fig. 7. Influence of the H2 O/(K2 O + SrO) ratio in the synthesis gel on the membrane composition. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, SrO/(K2 O + SrO) = 0.33, and various H2 O/(K2 O + SrO). The line indicates the Si/Al ratio calculated by Eq. (6).
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Fig. 8. Influence of the H2 O/(K2 O + SrO) ratio in the synthesis gel on the dehydration performance for an equimolar mixture of water and ethanol at 313 K. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, SrO/(K2 O + SrO) = 0.33, and various H2 O/(K2 O + SrO).
with an increase in the SiO2 /Al2 O3 ratio. The polycrystalline layer could not be formed on the support surface at SiO2 /Al2 O3 ratios greater than 35. It is important in zeolite synthesis to form a precursor concerned with aluminosilicate and water [14]. Therefore, this result implies that the SiO2 /Al2 O3 ratio of the synthesis gel significantly influences the formation of the zeolite precursor. Fig. 10 shows the influence of the gel composition on the composition of the CHA-type zeolite membrane. The Si/Al ratio of the zeolite membrane increased for gel SiO2 /Al2 O3 ratios less than 12. However, the Si/Al ratio in the membrane was almost constant at gel SiO2 /Al2 O3 ratios of 12–35. Assuming that the SiO2 /Al2 O3 ratio in the gel is s, Eq. (4) can be rewritten as, Si (Bs/2) + (65s/260) − 1 = . Al B + (65s/260) − 1
(7)
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Fig. 10. Influence of the SiO2 /Al2 O3 ratio in the synthesis gel on the membrane composition. The membranes were prepared in synthesis gels with the compositions H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, SrO/(K2 O + SrO) = 0.33, and various SiO2 /Al2 O3 . The line indicates the Si/Al ratio calculated by Eq. (7).
The equation suggests that the Si/Al ratio of zeolite increases with parameter s. The Si/Al ratio in the membrane was close to the calculated values at s < 16. However, the Si/Al ratio was smaller than the calculation line at higher gel SiO2 /Al2 O3 ratios. The lower Si/Al ratio at higher gel SiO2 /Al2 O3 ratios may be due to aluminum from the support. Fig. 11 shows the dehydration performances of the CHA-type zeolite membranes for an equimolar mixture of water and ethanol at 313 K as a function of the H2 O/SiO2 ratio in the synthesis gel. When the SiO2 /Al2 O3 ratio in the gel was 2–16, the membrane showed excellent separation ability. However, at the higher SiO2 /Al2 O3 ratios, the membrane could not selectively separate water. The permeation flux increased with the ratio. The crystal size reduced with increasing SiO2 /Al2 O3 ratio in the gel, as shown
Fig. 9. Surface morphologies of the membrane prepared in synthesis gels with different SiO2 /Al2 O3 ratios. The gel compositions were H2 O/SiO2 = 65; H2 O/(K2 O + SrO) = 260; SrO/(K2 O + SrO) = 0.33; and SiO2 /Al2 O3 = (a) 2, (b) 4, (c) 16, and (d) 20.
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Fig. 11. Influence of the SiO2 /Al2 O3 ratio in the synthesis gel on the dehydration performance for an equimolar mixture of water and ethanol at 313 K. The membranes were prepared in synthesis gels with the compositions H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, SrO/(K2 O + SrO) = 0.33, and various SiO2 /Al2 O3 .
in Fig. 9. Therefore, a higher ratio may produce a membrane with many pinholes. 3.5. Effect of alkali earth cation Preparation of the CHA-type zeolite membrane was also attempted using magnesium nitrate, calcium nitrate, and barium nitrate instead of strontium nitrate. Table 1 indicates the effect of these alkali earth species on the preparation of a CHA-type zeolite membrane. The seed crystals did not grow in the gel without alkali earth metals, as shown in Fig. 12(a), and the outer surface morphology was also identical to Fig. 12(a) for the synthesis gel containing alkali earth metals, except for strontium. This indicates that only strontium is particularly effective for the crystal growth of CHA-type zeolite.
It is plausible that alkali earth metals work as a pH adjusting material in the synthesis gel. Since the synthesis gel is very alkaline (pH < 13), the alkali earth metals exist as hydroxides in the gel. The solubilities of magnesium hydroxide, calcium hydroxide, strontium hydroxide, and barium hydroxide are 1.7 × 10−4 , 2.0 × 10−2 , 7.9 × 10−2 , and 0.27 mol l−1 , respectively [20], which covers a wide range. However, the alkali earth metals, except for strontium, were not effective for the membrane formation. On the contrary, the zeolite layer could be formed in the wide range of the strontium content, as shown in Fig. 12. This implies that solubility control of strontium is not important for the preparation of a CHA-type zeolite membrane. Fig. 12 shows the influence of the (K2 O + SrO)/H2 O ratio in the synthesis gel on the surface morphology of the CHA-type zeolite membranes. The polycrystalline zeolite layer could not be formed using a gel without strontium, as discussed in Table 1. When the ratio was 0.1, the CHA-type zeolite crystals grew on the support surface. However, the membrane also contained many MER-type zeolite crystals. At higher SrO/(K2 O + SrO) ratios, the crystal size of CHA-type zeolite increased, and the subgeneration of MER-type zeolite crystals was inhibited. This suggests that strontium plays an important role in the formation of the polycrystalline CHA-type zeolite layer. Fig. 13 shows the influence of the gel composition on the composition of the CHA-type zeolite membrane. Although the Si/Al ratio of the membrane decreased slightly at the SrO/(K2 O + SrO) ratio of 0.8–0.9, the membrane composition was independent of this ratio. This implies that a little strontium is sufficient to promote crystal growth. Fig. 14 shows the dehydration performance of the CHA-type zeolite membranes for an equimolar mixture of water and ethanol at 313 K as a function of the H2 O/SiO2 ratio in the synthesis gel. At ratios of 0.15–0.85, the separation factor was more than 10,000, and the permeation flux was approximately 1 kg m−2 h−1 . This sup-
Fig. 12. Surface morphologies of the membrane prepared in synthesis gels with different SrO/(K2 O + SrO) ratios. The gel compositions were SiO2 /Al2 O3 = 12; H2 O/SiO2 = 65; H2 O/(K2 O + SrO) = 260; and SrO/(K2 O + SrO) = (a) 0, (b) 0.10, (c) 0.67, and (d) 0.90.
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Table 1 Effect of alkali earth metal species on the preparation of CHA-type zeolite membranes. Alkali earth metal M
Gel composition
Surface morphology
– Mg Ca Sr Ba
12SiO2 :1Al2 O3 :2K2 O:6KNO3 :780H2 O 12SiO2 :1Al2 O3 :2K2 O:1MgO:8KNO3 :780H2 O 12SiO2 :1Al2 O3 :2K2 O:1CaO:8KNO3 :780H2 O 12SiO2 :1Al2 O3 :2K2 O:1SrO:8KNO3 :780H2 O 12SiO2 :1Al2 O3 :2K2 O:1BaO:8KNO3 :780H2 O
Amorphous (Fig. 12a) Amorphous Amorphous Polycrystalline CHA layer (Fig. 2a) Amorphous
Fig. 13. Influence of the SrO/(K2 O + SrO) ratio in the synthesis gel on the membrane composition. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, and various SrO/(K2 O + SrO). The line indicates the Si/Al ratio calculated by Eq. (4).
ports the conclusion that a small amount of strontium is sufficient for the growth of block-shaped CHA-type zeolite crystals. 3.6. Effect of ion-exchange Table 2 lists the effects of the ion-exchange treatment on the dehydration performance of the CHA-type zeolite membranes for
Fig. 14. Influence of the SrO/(K2 O + SrO) ratio in the synthesis gel on the dehydration performance for an equimolar mixture of water and ethanol at 313 K. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, and various SrO/(K2 O + SrO).
Fig. 15. The ion-exchange sites in the CHA-type zeolite channel.
an equimolar mixture of water and ethanol at 313 K. The ionexchange treatment did not affect the membrane morphology. The degrees of ion-exchange were 52%, 65%, 57%, and 67% for Li+ , Na+ , Rb+ , and Cs+ , respectively. The potassium content decreased greatly, while the strontium content was almost constant. There are three ion sites in CHA-type zeolite, as shown in Fig. 15. Site I is located in a double-six-membered ring (D6R), and sites II and III are located in an ellipsoidal cavity [21]. Since the diameter of the six-membered ring is 0.28 nm [11], it is difficult to replace the ions at site I [21]. Therefore, we conclude that the strontium ion are mainly located in site I. The ion-exchanged membrane also showed excellent separation properties for an equimolar mixture of water and ethanol at 313 K. However, the original membrane showed the highest permeation flux. Morooka and co-workers determined the influence of ion-exchange on the permeation properties of FAU-type zeolite membranes for the separation of carbon dioxide [22–24]. The channel size and adsorption behavior of the zeolite are changed by the ion-exchange. The channel diameter can be increased by the ionexchange with smaller ions such as lithium. However, molecules strongly adsorb onto the zeolite because of the higher charge density of the small ion. It is considerable that the lower permeation flux was due to the high charge density.
Table 2 Effect of ion-exchange treatment on the composition and dehydration performance of the CHA-type zeolite membranes. Exchange ion
– Li+ Na+ Rb+ Cs+
Membrane composition (mol%) SiO2
Al2 O3
Na2 O
K2 O
Rb2 O
Cs2 O
SrO
75.9 80.7 75.0 75.7 75.5
12.0 13.2 12.5 12.2 12.3
0.0 0.0 8.1 0.0 0.0
11.0 4.4 3.5 4.4 2.2
0.0 0.0 0.0 6.9 0.0
0.0 0.0 0.0 0.0 8.2
1.0 1.8 0.9 0.8 1.9
Degree of ion-exchange (%)
Permeation flux Jt (kg m−2 h−1 )
– 52 65 57 67
2.89 2.62 2.69 2.25 0.37
Separation factor ˛ (H2 O/EtOH) >100,000 >100,000 >100,000 >100,000 >100,000
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Clearly, strontium is a key material for the preparation of CHAtype zeolite membrane. It promotes the growth of CHA-type zeolite crystals and controls their shape, while inhibiting the growth of MER-type zeolite crystals. As shown in Fig. 15, the CHA-type zeolite possesses a framework structure, in which some D6R units connect with the ellipsoidal cavity. However, MER-type zeolite does not have a D6R unit in its framework. Therefore, the strontium ion is important for the formation of the D6R unit, which influences the growth of the block-shaped CHA-type zeolite crystals [14]. 4. Conclusions CHA-type zeolite membranes were prepared on porous alumina tubes in synthesis gels of various compositions. The favorable gel composition range was as follows: SiO2 /Al2 O3 = 2–16, H2 O/SiO2 = 65–250, H2 O/(K2 O + SrO) = 100–300, and SrO/(K2 O + SrO) = 0.15–0.85. In particular, the CHA-type zeolite membrane with the highest dehydration performance was obtained at SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, and SrO/(K2 O + SrO) = 0.33. Moreover, the CHA-type zeolite membranes were ion-exchanged with alkali cations in this work. However, the dehydration performance of the ion-exchanged membranes was lower than that of the original membrane. References [1] K. Sato, K. Sugimoto, T. Nakane, Preparation of higher flux NaA membrane on asymmetric porous support and permeation behavior at higher temperature up to 145 ◦ C in vapor permeation, J. Membr. Sci. 307 (2008) 181–195. [2] Y. Cui, H. Kita, K. Okamoto, Zeolite, T membrane: preparation, characterization, pervaporation of water/organic liquid mixtures and acid stability, J. Membr. Sci. 236 (2004) 17–27. [3] G. Li, E. Kikuchi, M. Matsukata, Separation of water–acetic acid mixtures by pervaporation using a thin mordenite membrane, Sep. Purif. Technol. 32 (2003) 199–206. [4] T. Nagase, Y. Kiyozumi, Y. Hasegawa, F. Mizukami, Synthesis and pervaporation performances of merlinoite and phillipsite membranes on mullite tube, Clay Sci. 12 (Suppl. 2) (2006) 100–105. [5] Y. Kiyozumi, Y. Nemoto, T. Nishide, T. Nagase, Y. Hasegawa, F. Mizukami, Synthesis of acid-resistant phillipsite (PHI) membrane and its pervaporation performance, Micropor. Mesopor. Mater. 116 (2008) 485–490.
[6] K. Okamoto, H. Kita, K. Hori, K. Tanaka, 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. [7] J. Kuhn, K. Yajima, T. Tomita, J. Gross, F. Kapteijn, Dehydration performance of a hydrophobic DD3R zeolite membrane, J. Membr. Sci. 321 (2008) 344–349. [8] S. Kajavi, J.C. Jansen, F. Kapteijn, Application of hydroxyl sodalite films as novel water selective membranes, J. Membr. Sci. 326 (2009) 153–160. [9] H. Kita, K. Fuchida, T. Horita, H. Asamura, K. Okamoto, Preparation of Faujasite membranes and their permeation properties, Sep. Purif. Technol. 25 (2001) 261–268. [10] Y. Hasegawa, T. Nagase, Y. Kiyozumi, T. Hanaoka, F. Mizukami, Influence of acid on the permeation properties through NaA-type zeolite membranes, J. Membr. Sci. 349 (2010) 189–194. [11] D.W. Breck, Zeolite Molecular Sieves, Wiley & Sons, New York, 1974. [12] M.P. Bernal, J. Coronas, M. Menendez, J. Santamaria, On the effect of morphological features on properties of MFI-type zeolite membranes, Micropor. Mesopor. Mater. 60 (2003) 99–110. [13] M. Matsukata, K. Sawamura, T. Shirai, M. Takada, Y. Sekine, E. Kikuchi, Controlled growth for synthesizing a compact mordenite membrane, J. Membr. Sci. 316 (2008) 18–27. [14] H. Robson (Ed.), Verified Synthesis of Zeolitic Materials, 2nd ed., Elsevier, Amsterdam, 2001. [15] Y. Hasegawa, H. Hotta, K. Sato, F. Mizukami, Preparation of novel chabazite (CHA)-type zeolite layer on porous ␣-Al2 O3 tube using template-free solution, J. Membr. Sci. 347 (2010) 193–196. [16] R. Kojima, K. Sato, T. Nagano, Y. Iwamoto, Development of fine porous alumina capillary by a dry–wet spinning method, J. Ceram. Soc. Jpn. 114 (2006) 929–933. [17] Y. Hasegawa, K. Kimura, Y. Nemoto, T. Nagase, Y. Kiyozumi, T. Nishide, F. Mizukami, Real-time monitoring of permeation properties through polycrystalline MFI-type zeolite membranes during pervaporation using massspectrometry, Sep. Purif. Technol. 58 (2008) 386–392. [18] Y. Hasegawa, T. Nagase, Y. Kiyozumi, F. Mizukami, Preparation, characterization, and dehydration performance of MER-type zeolite membranes, Sep, Purif. Technol. 73 (2010) 25–31. [19] H. Lechert, H. Kacirek, Investigations on the crystallization of X-type zeolites, Zeolites 11 (1991) 720–728. [20] The Chemical Society of Japan (Ed.), Kagaku Binran II, The Chemical Society of Japan, Maruzen, Tokyo, 2004, pp. 149–155. [21] A. Dyer, M. Zubair, Ion-exchange in chabazite, Micropor. Mesopor. Mater. 22 (1998) 135–150. [22] K. Kusakabe, T. Kuroda, K. Uchino, Y. Hasegawa, S. Morooka, Gas permeation properties of ion-exchanged faujasite-type zeolite membranes, AIChE J. 45 (1996) 1220–1226. [23] Y. Hasegawa, K. Watanabe, K. Kusakabe, S. Morooka, The separation of CO2 using Y-type zeolite membranes ion-exchanged with alkali metal cations, Sep. Purif. Technol. 22–23 (2001) 319–325. [24] Y. Hasegawa, K. Watanabe, K. Kusakabe, S. Morooka, Influence of alkali cations on permeation properties of Y-type zeolite membranes, J. Membr. Sci. 208 (2002) 415–418.
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Influence of synthesis gel composition on morphology, composition, and dehydration performance of CHA-type zeolite membranes Y. Hasegawa ∗ , C. Abe, M. Nishioka, K. Sato, T. Nagase, T. Hanaoka National Institute of Advanced Industrial Science and Technology (AIST), Research Center for Compact Chemical System, 4-2-1, Nigatake, Miyagino-ku, Sendai 983-8551, Japan
a r t i c l e
i n f o
Article history: Received 31 May 2010 Received in revised form 21 July 2010 Accepted 23 July 2010 Available online 30 July 2010 Keywords: Zeolite membrane Chabazite Dehydration Ethanol Ion-exchange
a b s t r a c t CHA-type zeolite membranes with excellent dehydration performance can be prepared on a porous ␣-Al2 O3 support tube by the secondary growth of seeds in a synthesis gel containing strontium. The permeation flux and separation factor of the membrane were 2.89 kg m−2 h−1 and >100,000, respectively, for a 50 mol% ethanol solution at 313 K. In this work, the CHA-type zeolite membranes were prepared using the gel with different compositions to discuss the influences of the synthesis gel composition on the morphology, composition, and dehydration performance of the CHA-type zeolite membrane. As a result, the CHA-type zeolite membrane could be prepared in the gel with the following compositions: SiO2 /Al2 O3 = 2–16; H2 O/SiO2 = 65–250; H2 O/(K2 O + SrO) = 100–400; SrO/(K2 O + SrO) = 0.10–0.85. Furthermore, the CHA-type zeolite membrane was ion-exchanged with lithium, sodium, rubidium, and cesium ions, and the dehydration performances of the ion-exchanged membranes were also determined. Based on these results, the effect of strontium for the membrane preparation was discussed in this paper. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Zeolites have regular channel structures with diameters comparable to those of inorganic gas and light hydrocarbon molecules. Hence, polycrystalline zeolite membranes are quite suitable for separating gaseous and liquid mixtures. In particular, NaA-type zeolite membranes show excellent dehydration performance for mixtures of water and alcohol. For example, the permeation flux Jt and separation factor ˛ were 8.8 kg m−2 h−1 and 10,000, respectively, for a 90 wt% ethanol solution at 348 K [1]. However, the acid stability of the NaA-type zeolite membrane is poor because of the large aluminum content in the framework (Si/Al = 1). Over the last decade, several acid resistant alternatives such as T-, MOR-, MER-, and PHI-type zeolite membranes have been developed [2–5], but their dehydration performances are lower (e.g., for a T-type zeolite membrane: Jt = 1.1 kg m−2 h−1 and ˛ = 900 [2]). Fig. 1 shows the dehydration performances of several zeolite membranes reported in previous studies [2–9]. The dehydration performance of the zeolite membrane is largely determined by the zeolite properties (channel volume and channel diameter), although other factors such as the pressure drop in a substrate and the thickness of the zeolite layer are also important. Furthermore, the acid stability of zeolite is determined by the aluminum content in the framework, since the aluminum atoms are removed from the
∗ Corresponding author. Tel.: +81 22 237 3098; fax: +81 22 237 7027. E-mail address: [email protected] (Y. Hasegawa). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.07.040
framework by acids [10]. The T-, MOR-, MER-, and PHI-type zeolite membranes with the Si/Al ratio of 2.5–5 are stable for acetic acid [2–5]. These suggest that the zeolite channel volume, channel size, and aluminum content greatly influence the permeation flux, separation factor, and acid stability of zeolite membranes, respectively. CHA-type zeolite has eight-membered-ring channels and large cavities, as does NaA-type zeolite. The channel volume and diameter of CHA-type zeolite are 0.23 ml g−1 and 0.38 nm, respectively [11]. Additionally, the Si/Al ratio of CHA-type zeolite can be varied in the range 1–5 [11]. These features of the CHA-type zeolite propose that CHA-type zeolite is one of the potential materials for dehydration membranes. The shape of zeolite crystals forming the polycrystalline layer is also important for the performance of zeolite membranes [12,13]. Matsukata et al. adjusted the synthesis gel composition and hydrothermal conditions to obtain a compact mordenite membrane [13], and they concluded that the block-shaped crystals are favorable to form a compact and thin zeolite layer with the high dehydration performance. However, it is difficult for CHA-type zeolite to form block-shaped crystals without the use of organic templates [14]. Nevertheless, in our previous study, a polycrystalline CHA-type zeolite membrane comprising block-shaped crystals was successfully formed in a synthesis gel containing strontium (organic template free) [15]. In this paper, the membranes were prepared in the synthesis gels with different compositions to discuss the influence of the gel composition on the morphology, composition, and dehydration performance of the CHA-type zeolite membrane.
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Fig. 1. Dehydration performance of zeolite membranes: (a) influence of channel volume of zeolite on the permeation flux of the membrane, (b) separation factor of water with respect to ethanol as a function of zeolite channel diameter. () OFF [2], (䊉) MOR [3], () MER [4], () PHI [5], () LTA [6], () DDR [7], () SOD [8], () FAU [9].
2. Experimental 2.1. Membrane preparation Polycrystalline CHA-type zeolite layers were formed on the outer surface of porous ␣-Al2 O3 tubes by the secondary growth of seed crystallites [15]. The seed particles were prepared by the procedure described in Ref. [14]. Potassium hydroxide (12.8 g, Wako Pure Chemicals Industry) was dissolved into distilled water (217.4 g), and 25.2 g of Y-type zeolite powder (Tosoh Corp., HSZ-330HUA, SiO2 /Al2 O3 = 6.3) was added to the solution. The mixture was stirred at room temperature for 1 h and then poured into a Teflon-lined stainless steel autoclave. The autoclave was heated in an oven at 368 K for 100 h to convert Y-type zeolite into CHA-type zeolite. The CHA-type zeolite particles were washed with excess amount of distilled water and dried in air at 333 K overnight to obtain the CHA-type zeolite seed particles (SiO2 /Al2 O3 = 4.7; crystal size = ca. 5–10 m). A synthesis gel for the secondary growth was prepared using colloidal silica (Catalysts and Chemicals Industry Corp. Ltd., Cataloid SI-30), aluminum nitrate (Wako Pure Chemicals Industry), strontium nitrate (Wako Pure Chemicals Industry), potassium hydroxide, and distilled water. Aluminum nitrate and potassium hydroxide were dissolved into distilled water in a beaker. In another beaker, strontium nitrate was dissolved into distilled water, and colloidal silica was added to the solution slowly. The aluminum source solution was added to the silicon source solution, and the slurry gel was stirred for 6 h at room temperature. The molar composition of the gel was as follows: SiO2 /Al2 O3 = 2–35, H2 O/SiO2 = 30–300, H2 O/(K2 O + SrO) = 50–500, and SrO/(K2 O + SrO) = 0–1. The gel also contained potassium nitrate. For example, the standard gel composition was 12SiO2 :1Al2 O3 :2K2 O:1SrO:8KNO3 :780H2 O in terms of the molar ratio. The porous ␣-Al2 O3 tube was used as the support [16], and the properties were as follows: outside diameter = 2.0 mm, inside diameter = 1.6 mm, length = 30 mm,
Fig. 2. (a) XRD pattern and (b) SEM image of CHA-type zeolite membrane prepared in the standard gel. The composition of the standard gel was 12SiO2 :1Al2 O3 :2K2 O:1SrO:780H2 O (SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, and SrO/(K2 O + SrO) = 0.33).
porosity = 0.39, and mean pore diameter = 0.15 m. The outer surface of the support tube was rubbed with the seed crystallites to implant the seeds for nucleation. Three support tubes were placed in a Teflon-lined autoclave vertically using a Teflon stand, and 20 g of the synthesis gel was added to the autoclave. A hydrothermal reaction was carried out in an oven at 413 K for 20 h in order to form a polycrystalline layer on the support. After cooling the autoclave in a water bath, the membrane was washed with distilled water and dried in air at room temperature overnight to obtain the CHA-type zeolite membrane. The crystal structures of the seed particles and membranes were determined by X-ray diffraction (XRD, Bruker M21X), and the morphology was observed by a scanning electron microscopy (SEM, Hitachi High Technologies, Miniscope TM-1000). The composition of the zeolite layer was analyzed using an energy dispersive X-ray analyzer (EDX, Hitachi High Technologies, Swift-ED) attached to the SEM. 2.2. Ion-exchange The potassium and strontium ions in the CHA-type zeolite membrane were exchanged with a 1.0 mol l−1 solution of either LiCl, NaCl, KCl, RbCl, or CsCl. The membrane prepared in the standard synthesis gel was added to 20 g of the each solution and heated in an oven at 333 K for 3 h. The membrane was then washed with distilled
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Fig. 3. Surface morphologies of the zeolite membranes prepared in synthesis gels with different H2 O/SiO2 ratios. The gel compositions were SiO2 /Al2 O3 = 12; H2 O/(K2 O + SrO) = 260; SrO/(K2 O + SrO) = 0.33; and H2 O/SiO2 = (a) 30, (b) 50, (c) 100, (d) 250, and (e) 300.
water and dried at ambient temperature overnight to obtain the ion-exchanged CHA-type zeolite membrane. The composition of the ion-exchanged membrane was analyzed by EDX, and the degree of ion-exchange was calculated by the content of the exchanged ion divided with the total content of ions. Note that the degree of ion-exchange by lithium was the content reduction of potassium and strontium ions in the membrane because lithium cannot be detected by EDX.
2.3. Pervaporation The dehydration performance of the CHA-type zeolite membrane was determined by pervaporation for 50 mol% ethanol solutions at 313 K [15,17,18]. One end of the membrane was connected to a stainless steel tube with an inorganic sealant (Varian,
Torr Seal), and the other end was sealed. The membrane was added to the solution, and the inside was evacuated by a rotary pump. Further, helium was introduced into the inner surface of the membrane as a standard gas at 1.0 ml min−1 , and the molar composition in the evacuated stream was analyzed using a mass spectrometer (Hiden Analytical, HAL-301RC). The mass spectrometer and standard gas allowed determination of the permeation properties without the condensation of permeated vapor in the error range below 3% [15]. The back permeation of helium is negligible, since the partial pressure difference across the membrane is marginal. Accordingly, the molar permeation flux of component i, Ji , is given by Ji =
NHe yi , · A yHe
(1)
where NHe , A, and yi are the molar flow rate of helium, effective membrane area for permeation, and mole fraction of component i
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in the evacuated stream. The permeation flux on a mass basis Jt can be calculated as follows: Jt = 3600(Ji Mi + Jj Mj ),
(2)
where Mi is the molecular weight of component i. The separation factor of component i with respect to component j, ˛(i/j), is defined as follows: ˛
i j
=
yi /yj xi /xj
,
(3)
where xi is the mole fraction of component i in the solution. Detailed information on the pervaporation test is described in Refs. [17,18]. The detection limit of ethanol was 0.002 wt% under the measurement condition. 3. Results and discussion 3.1. Standard conditions The CHA-type zeolite membrane was first prepared in the standard synthesis gel with the molar composition SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, and SrO/(K2 O + SrO) = 0.33. The recovery powder from the liquid phase was amorphous because of the lower alkali content and higher water content compared to those for powder synthesis. Fig. 2 shows a SEM image and XRD pattern of the membrane. The outer surface of the support tube was completely covered with a polycrystalline layer, which comprised block-shape crystals with a size of ca. 2 m. The XRD pattern was a combination of those for CHA-type zeolite and support tube. This indicates that the polycrystalline layer on the support consisted of CHA-type zeolite crystallites. The composition of the layer was 6.3SiO2 :1Al2 O3 :0.92K2 O:0.14SrO (SiO2 : 75.4 mol%, Al2 O3 : 11.9 mol%, K2 O: 11.0 mol%, SrO: 1.7 mol%) on a dry basis. Furthermore, the membrane showed excellent dehydration performance for an equimolar mixture of water and ethanol at 313 K (Jt = were 1.04 kg m−2 h−1 and ˛ > 100,000). These results are in agreement with those in our previous study [15]. 3.2. Influence of water content Fig. 3 shows SEM images of the top surface of the CHA-type zeolite membranes prepared in the synthesis gel with H2 O/SiO2 ratios of 30, 50, 100, 250, and 300. At H2 O/SiO2 ratio = 30, the polycrystalline layer was not formed on the support. The small CHA-type zeolite crystals could be formed at the ratio of 50. However, many rod-shaped MER-type zeolite crystals were also observed on the surface. At the ratio of 65 (Fig. 2), the seed crystals on the support grew largely with fewer rod-shaped MER-type crystals than at the ratio of 50, although the size of the rod-shaped MER-type zeolite crystals was larger. In the ratio range of 100–250, the surface of the support tube was covered with block-shaped CHA-type zeolite crystals. However, the crystal size was smaller than that at the ratio of 65. At still higher ratios, dish-shaped CHA-type zeolite crystals were observed on the surface. Thus, the favorable H2 O/SiO2 ratio is 65–250 for the preparation of the CHA-type zeolite membrane consisted of the block-shaped crystals. The H2 O/SiO2 ratio is the inverse of the material concentration for the zeolite synthesis. For the secondary growth, precursors for the zeolite synthesis are transferred via water. Therefore, these results imply that the precursor concentration dissolved into water is important in the preparation of a CHA-type zeolite membrane. Fig. 4 shows the influence of the gel composition on the composition of the CHA-type zeolite membrane. The Si/Al ratio of the zeolite membrane decreased as the H2 O/SiO2 ratio increased. When the gel composition is aSiO2 :1Al2 O3 :bK2 O:cSrO:dH2 O, the Si/Al
Fig. 4. Influence of the H2 O/SiO2 ratio in the synthesis gel on the membrane composition. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/(K2 O + SrO) = 260, SrO/(K2 O + SrO) = 0.33, and various H2 O/SiO2 . The line indicates the Si/Al ratio calculated by Eq. (5).
ratio in zeolite can be calculated as follows [14,19]: 0.5aB + b + c − 1 Si , = Al B+b+c−1
(4)
where B is a constant. Assuming that the H2 O/SiO2 ratio is w in this experiment, the parameters a, b, c, and d are 12, 0.31w, 0.15w, and 12w, respectively. Therefore, Eq. (4) can be converted into: 6B + 0.46x − 1 Si = . B + 0.46x − 1 Al
(5)
This equation means that the Si/Al ratio decreases with an increase in the H2 O/SiO2 ratio w in the gel. The experimental results followed this tendency. Fig. 5 shows the dehydration performances of the CHA-type zeolite membranes for an equimolar mixture of water and ethanol at 313 K as a function of the H2 O/SiO2 ratio in the synthesis gel. When the H2 O/SiO2 ratio was 65–250, the separation factor was more than 10,000. The suitable H2 O/SiO2 range was the same as that determined by morphology observation. However, the permeation flux indicated a minimum at the ratio of 150. As shown in Fig. 3, the H2 O/SiO2 ratio influenced the phase, shape, and size of the zeolite crystals. The minimum permeation flux may depend on the growth rate of the block-shaped crystals. 3.3. Influence of alkali content Fig. 6 shows the influence of the (K2 O + SrO)/H2 O ratio in the synthesis gel on the surface morphology of the CHA-type zeolite
Fig. 5. Influence of the H2 O/SiO2 ratio in the synthesis gel on the dehydration performance for an equimolar mixture of water and ethanol at 313 K. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/(K2 O + SrO) = 260, SrO/(K2 O + SrO) = 0.33, and various H2 O/SiO2 .
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Fig. 6. Surface morphologies of the membranes prepared in synthesis gels with different H2 O/(K2 O + SrO) ratios. The gel compositions were SiO2 /Al2 O3 = 12; H2 O/SiO2 = 65; SrO/(K2 O + SrO) = 0.33; and H2 O/(K2 O + SrO) = (a) 100, (b) 200, (c) 300, and (d) 400.
membranes. When the ratio was 100, the CHA-type zeolite crystals grew largely. However, many large MER-type zeolite crystals were also observed on the surface. The number of the MERtype zeolite crystal on the surface decreased with an increase in H2 O/(K2 O + SrO), although the size of the CHA-type zeolite crystal reduced. The H2 O/(K2 O + SrO) ratio is the inverse of the alkali concentration in the solution. The solubility of a precursor for zeolite formation is higher at higher alkalinity, and this leads the higher nucleation density and crystal growth rate, which lead to the production of many MER-type zeolite crystals as well. As a result, a lower alkalinity gel (higher H2 O/(K2 O + SrO) ratio) is preferable for the growth of a CHA-type zeolite layer. Fig. 7 shows the influence of the gel composition on the composition of the CHA-type zeolite membrane. The Si/Al ratio of the zeolite membrane increased with H2 O/(K2 O + SrO). When H2 O/(K2 O + SrO) = z, Eq. (4) can be converted into, Si 6B + (780/z) − 1 = . Al B + (780/z) − 1
the permeation flux at ratios of 300–400. The lower permeation flux may be attributed to the crystallinity of the CHA-type zeolite crystallites. 3.4. Influence of aluminum content Fig. 9 shows the influence of the SiO2 /Al2 O3 ratio in the synthesis gel on the surface morphology of the CHA-type zeolite membranes. The polycrystalline layer consisted of block-shaped crystals at ratios of 2–20, although rod-shaped MER-type zeolite crystals were observed at a ratio of 20. Moreover, the crystal size decreased
(6)
This equation shows that the Si/Al ratio of the zeolite membrane increases with H2 O/(K2 O + SrO) as also seen in the experimental results. Fig. 8 shows the dehydration performances of the CHA-type zeolite membranes for an equimolar mixture of water and ethanol at 313 K as a function of the H2 O/(K2 O + SrO) ratio in the synthesis gel. The separation factor was over 10,000 in the H2 O/(K2 O + SrO) range of 100–300, and the permeation flux also indicated a maximum at the ratio of around 200–300. The alkali concentration influences the crystal growth rate as discussed in Fig. 6. Therefore, the permeation flux decreased at smaller H2 O/SiO2 ratios (higher alkali content). However, the crystal growth rate cannot explain the decrease in
Fig. 7. Influence of the H2 O/(K2 O + SrO) ratio in the synthesis gel on the membrane composition. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, SrO/(K2 O + SrO) = 0.33, and various H2 O/(K2 O + SrO). The line indicates the Si/Al ratio calculated by Eq. (6).
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Fig. 8. Influence of the H2 O/(K2 O + SrO) ratio in the synthesis gel on the dehydration performance for an equimolar mixture of water and ethanol at 313 K. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, SrO/(K2 O + SrO) = 0.33, and various H2 O/(K2 O + SrO).
with an increase in the SiO2 /Al2 O3 ratio. The polycrystalline layer could not be formed on the support surface at SiO2 /Al2 O3 ratios greater than 35. It is important in zeolite synthesis to form a precursor concerned with aluminosilicate and water [14]. Therefore, this result implies that the SiO2 /Al2 O3 ratio of the synthesis gel significantly influences the formation of the zeolite precursor. Fig. 10 shows the influence of the gel composition on the composition of the CHA-type zeolite membrane. The Si/Al ratio of the zeolite membrane increased for gel SiO2 /Al2 O3 ratios less than 12. However, the Si/Al ratio in the membrane was almost constant at gel SiO2 /Al2 O3 ratios of 12–35. Assuming that the SiO2 /Al2 O3 ratio in the gel is s, Eq. (4) can be rewritten as, Si (Bs/2) + (65s/260) − 1 = . Al B + (65s/260) − 1
(7)
261
Fig. 10. Influence of the SiO2 /Al2 O3 ratio in the synthesis gel on the membrane composition. The membranes were prepared in synthesis gels with the compositions H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, SrO/(K2 O + SrO) = 0.33, and various SiO2 /Al2 O3 . The line indicates the Si/Al ratio calculated by Eq. (7).
The equation suggests that the Si/Al ratio of zeolite increases with parameter s. The Si/Al ratio in the membrane was close to the calculated values at s < 16. However, the Si/Al ratio was smaller than the calculation line at higher gel SiO2 /Al2 O3 ratios. The lower Si/Al ratio at higher gel SiO2 /Al2 O3 ratios may be due to aluminum from the support. Fig. 11 shows the dehydration performances of the CHA-type zeolite membranes for an equimolar mixture of water and ethanol at 313 K as a function of the H2 O/SiO2 ratio in the synthesis gel. When the SiO2 /Al2 O3 ratio in the gel was 2–16, the membrane showed excellent separation ability. However, at the higher SiO2 /Al2 O3 ratios, the membrane could not selectively separate water. The permeation flux increased with the ratio. The crystal size reduced with increasing SiO2 /Al2 O3 ratio in the gel, as shown
Fig. 9. Surface morphologies of the membrane prepared in synthesis gels with different SiO2 /Al2 O3 ratios. The gel compositions were H2 O/SiO2 = 65; H2 O/(K2 O + SrO) = 260; SrO/(K2 O + SrO) = 0.33; and SiO2 /Al2 O3 = (a) 2, (b) 4, (c) 16, and (d) 20.
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Fig. 11. Influence of the SiO2 /Al2 O3 ratio in the synthesis gel on the dehydration performance for an equimolar mixture of water and ethanol at 313 K. The membranes were prepared in synthesis gels with the compositions H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, SrO/(K2 O + SrO) = 0.33, and various SiO2 /Al2 O3 .
in Fig. 9. Therefore, a higher ratio may produce a membrane with many pinholes. 3.5. Effect of alkali earth cation Preparation of the CHA-type zeolite membrane was also attempted using magnesium nitrate, calcium nitrate, and barium nitrate instead of strontium nitrate. Table 1 indicates the effect of these alkali earth species on the preparation of a CHA-type zeolite membrane. The seed crystals did not grow in the gel without alkali earth metals, as shown in Fig. 12(a), and the outer surface morphology was also identical to Fig. 12(a) for the synthesis gel containing alkali earth metals, except for strontium. This indicates that only strontium is particularly effective for the crystal growth of CHA-type zeolite.
It is plausible that alkali earth metals work as a pH adjusting material in the synthesis gel. Since the synthesis gel is very alkaline (pH < 13), the alkali earth metals exist as hydroxides in the gel. The solubilities of magnesium hydroxide, calcium hydroxide, strontium hydroxide, and barium hydroxide are 1.7 × 10−4 , 2.0 × 10−2 , 7.9 × 10−2 , and 0.27 mol l−1 , respectively [20], which covers a wide range. However, the alkali earth metals, except for strontium, were not effective for the membrane formation. On the contrary, the zeolite layer could be formed in the wide range of the strontium content, as shown in Fig. 12. This implies that solubility control of strontium is not important for the preparation of a CHA-type zeolite membrane. Fig. 12 shows the influence of the (K2 O + SrO)/H2 O ratio in the synthesis gel on the surface morphology of the CHA-type zeolite membranes. The polycrystalline zeolite layer could not be formed using a gel without strontium, as discussed in Table 1. When the ratio was 0.1, the CHA-type zeolite crystals grew on the support surface. However, the membrane also contained many MER-type zeolite crystals. At higher SrO/(K2 O + SrO) ratios, the crystal size of CHA-type zeolite increased, and the subgeneration of MER-type zeolite crystals was inhibited. This suggests that strontium plays an important role in the formation of the polycrystalline CHA-type zeolite layer. Fig. 13 shows the influence of the gel composition on the composition of the CHA-type zeolite membrane. Although the Si/Al ratio of the membrane decreased slightly at the SrO/(K2 O + SrO) ratio of 0.8–0.9, the membrane composition was independent of this ratio. This implies that a little strontium is sufficient to promote crystal growth. Fig. 14 shows the dehydration performance of the CHA-type zeolite membranes for an equimolar mixture of water and ethanol at 313 K as a function of the H2 O/SiO2 ratio in the synthesis gel. At ratios of 0.15–0.85, the separation factor was more than 10,000, and the permeation flux was approximately 1 kg m−2 h−1 . This sup-
Fig. 12. Surface morphologies of the membrane prepared in synthesis gels with different SrO/(K2 O + SrO) ratios. The gel compositions were SiO2 /Al2 O3 = 12; H2 O/SiO2 = 65; H2 O/(K2 O + SrO) = 260; and SrO/(K2 O + SrO) = (a) 0, (b) 0.10, (c) 0.67, and (d) 0.90.
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263
Table 1 Effect of alkali earth metal species on the preparation of CHA-type zeolite membranes. Alkali earth metal M
Gel composition
Surface morphology
– Mg Ca Sr Ba
12SiO2 :1Al2 O3 :2K2 O:6KNO3 :780H2 O 12SiO2 :1Al2 O3 :2K2 O:1MgO:8KNO3 :780H2 O 12SiO2 :1Al2 O3 :2K2 O:1CaO:8KNO3 :780H2 O 12SiO2 :1Al2 O3 :2K2 O:1SrO:8KNO3 :780H2 O 12SiO2 :1Al2 O3 :2K2 O:1BaO:8KNO3 :780H2 O
Amorphous (Fig. 12a) Amorphous Amorphous Polycrystalline CHA layer (Fig. 2a) Amorphous
Fig. 13. Influence of the SrO/(K2 O + SrO) ratio in the synthesis gel on the membrane composition. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, and various SrO/(K2 O + SrO). The line indicates the Si/Al ratio calculated by Eq. (4).
ports the conclusion that a small amount of strontium is sufficient for the growth of block-shaped CHA-type zeolite crystals. 3.6. Effect of ion-exchange Table 2 lists the effects of the ion-exchange treatment on the dehydration performance of the CHA-type zeolite membranes for
Fig. 14. Influence of the SrO/(K2 O + SrO) ratio in the synthesis gel on the dehydration performance for an equimolar mixture of water and ethanol at 313 K. The membranes were prepared in synthesis gels with the compositions SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, and various SrO/(K2 O + SrO).
Fig. 15. The ion-exchange sites in the CHA-type zeolite channel.
an equimolar mixture of water and ethanol at 313 K. The ionexchange treatment did not affect the membrane morphology. The degrees of ion-exchange were 52%, 65%, 57%, and 67% for Li+ , Na+ , Rb+ , and Cs+ , respectively. The potassium content decreased greatly, while the strontium content was almost constant. There are three ion sites in CHA-type zeolite, as shown in Fig. 15. Site I is located in a double-six-membered ring (D6R), and sites II and III are located in an ellipsoidal cavity [21]. Since the diameter of the six-membered ring is 0.28 nm [11], it is difficult to replace the ions at site I [21]. Therefore, we conclude that the strontium ion are mainly located in site I. The ion-exchanged membrane also showed excellent separation properties for an equimolar mixture of water and ethanol at 313 K. However, the original membrane showed the highest permeation flux. Morooka and co-workers determined the influence of ion-exchange on the permeation properties of FAU-type zeolite membranes for the separation of carbon dioxide [22–24]. The channel size and adsorption behavior of the zeolite are changed by the ion-exchange. The channel diameter can be increased by the ionexchange with smaller ions such as lithium. However, molecules strongly adsorb onto the zeolite because of the higher charge density of the small ion. It is considerable that the lower permeation flux was due to the high charge density.
Table 2 Effect of ion-exchange treatment on the composition and dehydration performance of the CHA-type zeolite membranes. Exchange ion
– Li+ Na+ Rb+ Cs+
Membrane composition (mol%) SiO2
Al2 O3
Na2 O
K2 O
Rb2 O
Cs2 O
SrO
75.9 80.7 75.0 75.7 75.5
12.0 13.2 12.5 12.2 12.3
0.0 0.0 8.1 0.0 0.0
11.0 4.4 3.5 4.4 2.2
0.0 0.0 0.0 6.9 0.0
0.0 0.0 0.0 0.0 8.2
1.0 1.8 0.9 0.8 1.9
Degree of ion-exchange (%)
Permeation flux Jt (kg m−2 h−1 )
– 52 65 57 67
2.89 2.62 2.69 2.25 0.37
Separation factor ˛ (H2 O/EtOH) >100,000 >100,000 >100,000 >100,000 >100,000
264
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Clearly, strontium is a key material for the preparation of CHAtype zeolite membrane. It promotes the growth of CHA-type zeolite crystals and controls their shape, while inhibiting the growth of MER-type zeolite crystals. As shown in Fig. 15, the CHA-type zeolite possesses a framework structure, in which some D6R units connect with the ellipsoidal cavity. However, MER-type zeolite does not have a D6R unit in its framework. Therefore, the strontium ion is important for the formation of the D6R unit, which influences the growth of the block-shaped CHA-type zeolite crystals [14]. 4. Conclusions CHA-type zeolite membranes were prepared on porous alumina tubes in synthesis gels of various compositions. The favorable gel composition range was as follows: SiO2 /Al2 O3 = 2–16, H2 O/SiO2 = 65–250, H2 O/(K2 O + SrO) = 100–300, and SrO/(K2 O + SrO) = 0.15–0.85. In particular, the CHA-type zeolite membrane with the highest dehydration performance was obtained at SiO2 /Al2 O3 = 12, H2 O/SiO2 = 65, H2 O/(K2 O + SrO) = 260, and SrO/(K2 O + SrO) = 0.33. Moreover, the CHA-type zeolite membranes were ion-exchanged with alkali cations in this work. However, the dehydration performance of the ion-exchanged membranes was lower than that of the original membrane. References [1] K. Sato, K. Sugimoto, T. Nakane, Preparation of higher flux NaA membrane on asymmetric porous support and permeation behavior at higher temperature up to 145 ◦ C in vapor permeation, J. Membr. Sci. 307 (2008) 181–195. [2] Y. Cui, H. Kita, K. Okamoto, Zeolite, T membrane: preparation, characterization, pervaporation of water/organic liquid mixtures and acid stability, J. Membr. Sci. 236 (2004) 17–27. [3] G. Li, E. Kikuchi, M. Matsukata, Separation of water–acetic acid mixtures by pervaporation using a thin mordenite membrane, Sep. Purif. Technol. 32 (2003) 199–206. [4] T. Nagase, Y. Kiyozumi, Y. Hasegawa, F. Mizukami, Synthesis and pervaporation performances of merlinoite and phillipsite membranes on mullite tube, Clay Sci. 12 (Suppl. 2) (2006) 100–105. [5] Y. Kiyozumi, Y. Nemoto, T. Nishide, T. Nagase, Y. Hasegawa, F. Mizukami, Synthesis of acid-resistant phillipsite (PHI) membrane and its pervaporation performance, Micropor. Mesopor. Mater. 116 (2008) 485–490.
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