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Influence of acid on the permeation properties of NaA-type zeolite membranes
Journal of Membrane Science 349 (2010) 189–194
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Influence of acid on the permeation properties of NaA-type zeolite membranes Y. Hasegawa ∗ , T. Nagase, Y. Kiyozumi, T. Hanaoka, F. Mizukami National Institute of Advanced Industrial Science and Technology (AIST), Research Center for Compact Chemical Process (CCP), 4-2-1, Nigatake, Miyagino-ku, Sendai 983-8551, Japan
a r t i c l e
i n f o
Article history: Received 24 September 2009 Received in revised form 18 November 2009 Accepted 21 November 2009 Available online 27 November 2009 Keywords: Destruction Zeolite membrane NaA-type zeolite Pervaporation Dehydration
a b s t r a c t The acid stability of the NaA-type zeolite membrane was evaluated in this study. For the rapid evaluation of membrane stability, the change in the permeation fluxes of the membrane was monitored in real-time by using a mass spectrometer during pervaporation. When sulfuric acid was added to the feed solution, the permeation flux of water decreased dramatically immediately after the acid addition (step (I)), then decreased slightly (step (II)), and finally increased significantly (step (III)). The permeation flux of ethanol also increased significantly in step (III). As a result, the NaA-type zeolite membrane lost its separation ability for an equimolar mixture of water and ethanol at 313 K in 10 min after the addition of 1.0 mL sulfuric acid. Hydrolysis reaction and dissolution occurred in steps (I)–(III). Furthermore, we also discuss the influence of the amount of added acid and ethanol content of the feed solution on the destruction of the zeolite membrane. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Zeolites are crystalline aluminosilicate compounds with uniform sized micropores. Since the micropore diameter is similar to the molecular sizes of inorganic gases and light hydrocarbons, the polycrystalline membrane formed on a porous substrate can separate gaseous and liquid mixtures. In particular, an NaA-type zeolite membrane shows excellent dehydration performance for alcohol–water mixtures. For example, the permeation flux and separation factor of water with respect to ethanol were 8.5 kg m−2 h−1 and >10,000, respectively, for a 90 wt% ethanol solution at 348 K [1]. Although the NaA-type zeolite membrane shows excellent performance, the acid stability of the membrane is poor. In the last decade, T-, MOR-, MER-, and PHI-type zeolite membranes were investigated to overcome the poor acid stability of the NaA-type zeolite membrane [2–5], and the newly developed membranes show higher stability in acid solutions. However, the influence of acid on the permeation properties of the zeolite membranes has remained unclear. To clarify the influence of the acid, it is essential to recover the permeated vapor and analyze it in situ. However, thus far, such an operation or system has not been developed. Recently, measurement techniques for the real-time monitoring of permeation properties were investigated [6,7]. In these systems, a mass spectrometer was coupled to a conventional pervaporation
∗ Corresponding author. Tel.: +81 22 237 3098; fax: +81 22 237 7027. E-mail address: [email protected] (Y. Hasegawa). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.11.052
unit for composition analysis of the evacuated stream without the condensation of permeated vapor. In our previous studies [7,8], a real-time monitoring system was developed, and the practicality of this system was confirmed. In the present study, the real-time monitoring system was applied to the measurement of the permeation properties of the NaA-type zeolite membrane, and the influence of acid on the permeation properties was determined. In this study the permeation fluxes through the NaA-type zeolite membrane was monitored using the real-time monitoring system to discuss the influence of acid on the permeation properties. On the basis of the permeation data and morphology observation, we discussed the relation between the change in membrane structure and the reduction of the membrane performance.
2. Experimental 2.1. Membrane preparation Polycrystalline NaA-type zeolite layers were formed on porous ␣-Al2 O3 tubes by a hydrothermal process [9]. A synthesis solution was prepared by mixing water glass (Wako Pure Chemicals Industry), sodium aluminate (Wako Pure Chemicals Industry), and distilled water, and the mixture was stirred for 1 h at room temperature. Porous ␣-Al2 O3 tubes [10] were used as the support in this study, and the properties were as follows: outside diameter = 2.0 mm; inside diameter = 1.6 mm; length = 220 mm; porosity = 39%; and mean pore diameter = 150 nm. The outer surface of the support tube was rubbed with NaA-type zeolite particles to implant seeds for nucleation. The synthesis mixture was poured
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Fig. 1. Schematic illustration of the vessel for membrane preparation.
into an autoclave, and the support tube was added, as shown in Fig. 1. The autoclave was placed in an oven at 373 K, and the hydrothermal process was carried out for 4.5 h to form a polycrystalline NaA-type zeolite layer on the support. After the crystal growth, the autoclave was replaced in a water bath for 10 min to cool to room temperature. The membrane was recovered, washed with distilled water, and dried in air overnight to obtain the NaAtype zeolite membrane as shown in Fig. 2. The morphology of the membrane was observed using a scanning electron microscope (SEM, Hitachi S-800), and the composition was analyzed by an energy dispersive X-ray analyzer (EDX, Horiba Emax). 2.2. Pervaporation test and acid stability The NaA-type zeolite membrane of 220 mm length was cut into six 30-mm long pieces, and the cut membranes were used for the pervaporation test [8]. One end of the cut membrane was connected to a stainless steel tube with a sealing material (Varian, Torr Seal), and the other end was sealed. The permeation properties of the membrane were determined for mixtures of water and ethanol by a pervaporation unit equipped with a real-time monitoring system [7,8]. The experimental setup is identical to a conventional pervaporation unit, except for the following two points: (1) the introduction of helium into the inner surface of the membrane; and (2) the composition analysis in the evacuated stream using a mass spectrometer. The membrane was added to the solution, and the inside of the membrane (permeate side) was evacuated by a rotary pump. Furthermore, helium was introduced into the permeate side at 1.0 mL/min as the standard gas, and the molar composition on the permeate side was analyzed using a mass spectrometer. By using this system, the permeation properties can be determined without the condensation of permeated vapor, and the measurement error was less than 3% for membranes with a small membrane area for permeation. More detailed information about this system is reported in the literatures [7,8]. For the evaluation of acid stability, sulfuric acid (2.4 mol/L) was added to 400 g of the feed solution during the pervaporation test, and the time course in the permeation fluxes of water and ethanol was monitored. 3. Results and discussion
Fig. 2. SEM images of the NaA-type zeolite membrane: (a) top surface and (b) fractured section.
water was 6.3 because of dissolved carbon dioxide. The reduction of water flux was attributed to the marginal acidity of the solution.
3.1. Dehydration performance 3.2. Influence of sulfuric acid addition Fig. 3 shows the permeation fluxes of water and ethanol through the NaA-type zeolite membranes for ethanol solutions with different ethanol concentrations at 313 K. The permeation flux of ethanol was below 10−6 mol m−2 s−1 for all ethanol concentrations. The permeation flux of water was 1.9 × 10−3 mol m−2 s−1 for a 95 mol% ethanol solution, and the flux increased with a decrease in the ethanol concentration. As a result, the permeation flux of water was 1.9 × 10−2 mol m−2 s−1 at an ethanol concentration of 0 mol%. The concentration dependency of the dehydration performance is in agreement with previous reports [11,12]. However, the water flux at 0 mol% ethanol decreased with time and was 1.5 × 10−3 mol m−2 s−1 after 5 h as shown in Fig. 3(a). The pH of
Fig. 4 shows the time courses in the permeation fluxes of water and ethanol at the amount of 0.10–1.0 mL of sulfuric acid added. Equimolar mixtures of water and ethanol were used for the experiments, and the temperature was 313 K. The relative water flux (Jw /Jw0 ), which is the permeation flux of water normalized with the water flux at t = 0, is used to easily compare the change in the permeation properties. When 0.1 mL of sulfuric acid was added, the relative water flux decreased dramatically immediately after the acid addition. The flux then slightly decreased between 15 and 80 min and finally increased significantly. In contrast, the permeation flux of ethanol suddenly and significantly increased at
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Fig. 3. Permeation properties of the NaA-type zeolite membrane for aqueous solutions of ethanol at 313 K. (a) The time courses in the permeation flux of water for 0, 25, 50, 75, and 95 mol% ethanol solutions at 313 K. Ethanol concentration in the feed solution: (black) 0 mol%; (blue) 25 mol%; (red) 50 mol%; (green) 75 mol%; (orange) 95 mol%. (b) The influence of the ethanol concentration in the solution on the permeation flux of water at 313 K. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
t = 65 min. The same trends were observed in shorter periods for the addition of a larger amount of sulfuric acid. Fig. 5 shows the time courses in the permeation fluxes of water and ethanol at the ethanol concentration of 0–95 mol%. The temperature was 313 K, and the amount of added sulfuric acid was 1.0 mL in these experiments. The permeation fluxes show the same trends as those shown in Fig. 4. For the 95 mol% ethanol solution, the first large decrease in the water flux was observed for a few minutes immediately after the acid addition, then the flux slightly decreased between 10 and 100 min, and finally, a significant increase occurred after t = 110 min. Although the same permeation behaviors were observed for the lower ethanol concentrations, the periods became shorter. Fig. 6 shows the typical time courses in the permeation fluxes of water and ethanol after adding sulfuric acid. The change in the permeation properties can be divided into three steps as follows: Step (I): large decrease in the relative water flux. Step (II): slight decrease in the relative water flux. Step (III): large increase in both the water and ethanol fluxes. The times at Jw /Jw0 = 1 and Je = 10−5 mol m−2 s−1 in step (III) were assigned as t1 and t2 , respectively, in order to discuss the influence of the amount of added acid and concentration on the changes in the permeation properties. The inverse time 1/t means the overall destruction rate of the NaA-type zeolite membrane. Figs. 7 and 8
Fig. 4. The time courses in the permeation fluxes of water and ethanol at the amount of added sulfuric acid of 0.10–1.0 mL for 25 mol% ethanol solutions at 313 K. Amount of added sulfuric acid: (black) 0.1 mL; (blue) 0.3 mL; (red) 0.5 mL; (green) 0.7 mL; and (orange) 1.0 mL. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
represent the influence of the amount of added sulfuric acid on the overall destruction rates. The rates were proportional to the amount of added sulfuric acid. However, the rates exponentially increased with the water concentration in the feed solution. This implies that water plays an important role in the destruction of the zeolite membrane. Fig. 9(a), (b), and (c) shows the SEM images of the NaA-type zeolite membranes in steps (I), (II), and (III), respectively. As shown in Fig. 2, the top surface of the original NaA-type zeolite membrane was covered with the polycrystalline layer, and the membrane thickness was ca. 6 m. The morphology of the membrane treated for 5 min was identical to that of the untreated membrane. For the membrane treated for 50 min, the top surface of the membrane was covered with an amorphous-like material as shown in Fig. 10, although the thickness was identical to the original membrane (ca. 6 m). When the membrane was treated in the acid solution for 100 min, the alumina particles of the support tube were observed within large cracks on the surface. Furthermore, the membrane became thinner (ca. 2 m). The Si/Al and Na/Al ratios of the original membrane were unity, as indicated in Table 1.The Si/Al ratio was constant until 50 min, while the value decreased to 0.51 after the
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Fig. 7. The influence of the amount of added sulfuric acid on the destruction rates (a) 1/t1 and (b) 1/t2 at 313 K. Ethanol concentration in the feed solution: () 0 mol%; () 25 mol%; () 50 mol%; () 75 mol%; and (♦) 95 mol%.
Fig. 5. The time courses in the permeation fluxes of (a) water and (b) ethanol at the ethanol concentration of 0–95 mol% at 313 K. Ethanol concentration in the feed solution: (black) 0 mol%; (blue) 25 mol%; (red) 50 mol%; (green) 75 mol%; (orange) 95 mol%. The amount of added sulfuric acid = 1.0 mL. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 6. Typical time courses in the permeation fluxes of water and ethanol by the addition of sulfuric acid. Component: (blue) water; (red) ethanol. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 8. The influence of the water concentration in the feed solution on the destruction rates 1/t1 and 1/t2 at 313 K. Amount of added sulfuric acid: () 0.1 mL; () 0.3 mL; () 0.5 mL; () 0.7 mL; and (♦) 1.0 mL.
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Fig. 9. The SEM images of the NaA-type zeolite membranes in steps (I), (II), and (III). The membranes were treated with 25 mol% ethanol solutions containing sulfuric acid (0.1 mL) for (a) 5 min, (b) 50 min, and (c) 100 min.
Since the added acid functions as a catalyst of hydrolysis, Si–O and/or Al–O bonds in the zeolite framework are dissociated [13]. As a result, the framework structure of the zeolite is destroyed, and the zeolite layer is converted to an amorphous silica-alumina layer. Na+ ions in the NaA-type zeolite may be also exchanged with H+ ions. Since soluble silica and alumina compounds are formed by further progression of the hydrolysis reaction [13], the amorphous silica–alumina layer is dissolved into the solution. Therefore, the results of the morphology observation and composition analysis suggest the following points: (1) The zeolite framework is mainly destroyed in steps (I) and (II). (2) The soluble amorphous-like layer is dissolved into the feed solution in step (III).
Fig. 10. The XRD patterns of the NaA-type zeolite membranes. The membranes were treated with 25 mol% ethanol solutions containing sulfuric acid (0.1 mL) for (a) 0 min, (b) 5 min, (c) 50 min, and (d) 100 min. Symbols: () NaA-type zeolite, (×) support tube.
100-min treatment. The large reduction in the Si/Al ratio at 100 min is due to the extrinsic aluminum constituent of the support tube. On the other hand, the Na/Al ratio decreased with the treatment time, in particular, the largest decrease occurring in steps (I) and (II).
The results indicate that the main factors for the destruction of the zeolite membrane were hydrolysis reaction and dissolution, as discussed above. Since acids catalyze the hydrolysis reaction, the reaction rate obeys first-order kinetics for the acid concentration (amount of catalyst). This is in agreement with the results shown in Fig. 7. On the other hand, water is essential for progression of these processes. As shown in Fig. 8, therefore, the reaction order of water became higher. In this study, the permeation properties of the NaA-type zeolite membranes could be monitored in real-time during their destruction by the addition of sulfuric acid. Membrane separation is an energy-saving process, and in particular, inorganic membranes (such as zeolite and microporous silica) can be used under severe conditions. The technique used in this study can be applied to the
Table 1 The composition of the NaA-type zeolite membranes before and after the acid treatment. No.
1 2 3 4
Acid treatment time (min)
0 5 50 100
SEM image
Fig. 2 Fig. 9(a) Fig. 9(b) Fig. 9(c)
XRD pattern
Fig.10(a) Fig.10(b) Fig.10(c) Fig.10(d)
Composition (mol%)
Composition ratio
SiO2
Al2 O3
Na2 O
Si/Al
Na/Al
51.3 54.3 60.9 49.1
24.1 26.1 31.1 48.0
24.6 19.6 8.0 2.9
1.06 1.04 0.98 0.51
1.02 0.75 0.26 0.06
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rapid evaluation of membrane stabilities. Additionally, the membrane stability can be predicted if the destruction process describes using the properties of powdery samples in the future. 4. Conclusions In this study the destruction process of the NaA-type zeolite membrane by acid was monitored using a real-time monitoring system. In this process, the permeation fluxes of water showed a large reduction immediately after the acid addition (step (I)), followed by a slight reduction (step (II)), and finally a significant increase (step (III)). The permeation flux of ethanol also increased significantly in step (III), although ethanol concentration on the permeate side was below the detection limit (<10−6 mol m−2 s−1 ) in steps (I) and (II). As a result, the membrane lost its separation ability. The hydrolysis reaction mainly occurred in steps (I) and (II). The hydrolysis reaction converted NaA-type zeolite into amorphous compounds, and the amorphous material was dissolved into the ethanol solution in step (III). The destruction process occurred in shorter period in a solution of lower ethanol concentration and/or higher acid concentration. References [1] K. Sato, K. Sugimoto, T. Nakane, Preparation of higher flux NaA zeolite membrane on asymmetric porous support and permeation behavior at higher temperatures 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 mixtures and acid stability, J. Membr. Sci. 236 (2004) 17–27. [3] A. Navajas, R. Mallada, C. Tellez, J. Coronas, M. Menedez, J. Santamaria, Study on the reproducibility of mordenite tubular membranes used in the dehydration of ethanol, J. Membr. Sci. 299 (2007) 166–173. [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] G. Li, E. Kikuchi, M. Matsukata, Separation of water–acetic acid mixtures by pervaporation using a thin mordenite membrane, Sep. Purif. Technol. 32 (2003) 100–206. [7] 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. [8] Y. Hasegawa, T. Nagase, Y. Kiyozumi, F. Mizukami, Preparation, characterization, and dehydration of MER-type zeolite membranes, Sep. Purif. Technol., in press. [9] K. Aoki, K. Kusakabe, S. Morooka, Gas permeation properties of A-type zeolite membrane formed on porous substrate by hydrothermal synthesis, J. Membr. Sci. 141 (1998) 197–205. [10] R. Kojima, K. Sato, T. Nagano, Y. Iwamoto, Development of fine porous alumina capillaries by a dry–wet spinning method, J. Ceram. Soc. Jpn. 114 (2006) 929–933. [11] 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. [12] M. Kondo, M. Komori, H. Kita, K. Okamoto, Tubular-type pervaporation module with zeolite NaA membrane, J. Membr. Sci. 133 (1997) 133–141. [13] D.W. Breck, Zeolite Molecular Sieves, Wiley & Sons, New York, 1974.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Influence of acid on the permeation properties of NaA-type zeolite membranes Y. Hasegawa ∗ , T. Nagase, Y. Kiyozumi, T. Hanaoka, F. Mizukami National Institute of Advanced Industrial Science and Technology (AIST), Research Center for Compact Chemical Process (CCP), 4-2-1, Nigatake, Miyagino-ku, Sendai 983-8551, Japan
a r t i c l e
i n f o
Article history: Received 24 September 2009 Received in revised form 18 November 2009 Accepted 21 November 2009 Available online 27 November 2009 Keywords: Destruction Zeolite membrane NaA-type zeolite Pervaporation Dehydration
a b s t r a c t The acid stability of the NaA-type zeolite membrane was evaluated in this study. For the rapid evaluation of membrane stability, the change in the permeation fluxes of the membrane was monitored in real-time by using a mass spectrometer during pervaporation. When sulfuric acid was added to the feed solution, the permeation flux of water decreased dramatically immediately after the acid addition (step (I)), then decreased slightly (step (II)), and finally increased significantly (step (III)). The permeation flux of ethanol also increased significantly in step (III). As a result, the NaA-type zeolite membrane lost its separation ability for an equimolar mixture of water and ethanol at 313 K in 10 min after the addition of 1.0 mL sulfuric acid. Hydrolysis reaction and dissolution occurred in steps (I)–(III). Furthermore, we also discuss the influence of the amount of added acid and ethanol content of the feed solution on the destruction of the zeolite membrane. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Zeolites are crystalline aluminosilicate compounds with uniform sized micropores. Since the micropore diameter is similar to the molecular sizes of inorganic gases and light hydrocarbons, the polycrystalline membrane formed on a porous substrate can separate gaseous and liquid mixtures. In particular, an NaA-type zeolite membrane shows excellent dehydration performance for alcohol–water mixtures. For example, the permeation flux and separation factor of water with respect to ethanol were 8.5 kg m−2 h−1 and >10,000, respectively, for a 90 wt% ethanol solution at 348 K [1]. Although the NaA-type zeolite membrane shows excellent performance, the acid stability of the membrane is poor. In the last decade, T-, MOR-, MER-, and PHI-type zeolite membranes were investigated to overcome the poor acid stability of the NaA-type zeolite membrane [2–5], and the newly developed membranes show higher stability in acid solutions. However, the influence of acid on the permeation properties of the zeolite membranes has remained unclear. To clarify the influence of the acid, it is essential to recover the permeated vapor and analyze it in situ. However, thus far, such an operation or system has not been developed. Recently, measurement techniques for the real-time monitoring of permeation properties were investigated [6,7]. In these systems, a mass spectrometer was coupled to a conventional pervaporation
∗ Corresponding author. Tel.: +81 22 237 3098; fax: +81 22 237 7027. E-mail address: [email protected] (Y. Hasegawa). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.11.052
unit for composition analysis of the evacuated stream without the condensation of permeated vapor. In our previous studies [7,8], a real-time monitoring system was developed, and the practicality of this system was confirmed. In the present study, the real-time monitoring system was applied to the measurement of the permeation properties of the NaA-type zeolite membrane, and the influence of acid on the permeation properties was determined. In this study the permeation fluxes through the NaA-type zeolite membrane was monitored using the real-time monitoring system to discuss the influence of acid on the permeation properties. On the basis of the permeation data and morphology observation, we discussed the relation between the change in membrane structure and the reduction of the membrane performance.
2. Experimental 2.1. Membrane preparation Polycrystalline NaA-type zeolite layers were formed on porous ␣-Al2 O3 tubes by a hydrothermal process [9]. A synthesis solution was prepared by mixing water glass (Wako Pure Chemicals Industry), sodium aluminate (Wako Pure Chemicals Industry), and distilled water, and the mixture was stirred for 1 h at room temperature. Porous ␣-Al2 O3 tubes [10] were used as the support in this study, and the properties were as follows: outside diameter = 2.0 mm; inside diameter = 1.6 mm; length = 220 mm; porosity = 39%; and mean pore diameter = 150 nm. The outer surface of the support tube was rubbed with NaA-type zeolite particles to implant seeds for nucleation. The synthesis mixture was poured
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Fig. 1. Schematic illustration of the vessel for membrane preparation.
into an autoclave, and the support tube was added, as shown in Fig. 1. The autoclave was placed in an oven at 373 K, and the hydrothermal process was carried out for 4.5 h to form a polycrystalline NaA-type zeolite layer on the support. After the crystal growth, the autoclave was replaced in a water bath for 10 min to cool to room temperature. The membrane was recovered, washed with distilled water, and dried in air overnight to obtain the NaAtype zeolite membrane as shown in Fig. 2. The morphology of the membrane was observed using a scanning electron microscope (SEM, Hitachi S-800), and the composition was analyzed by an energy dispersive X-ray analyzer (EDX, Horiba Emax). 2.2. Pervaporation test and acid stability The NaA-type zeolite membrane of 220 mm length was cut into six 30-mm long pieces, and the cut membranes were used for the pervaporation test [8]. One end of the cut membrane was connected to a stainless steel tube with a sealing material (Varian, Torr Seal), and the other end was sealed. The permeation properties of the membrane were determined for mixtures of water and ethanol by a pervaporation unit equipped with a real-time monitoring system [7,8]. The experimental setup is identical to a conventional pervaporation unit, except for the following two points: (1) the introduction of helium into the inner surface of the membrane; and (2) the composition analysis in the evacuated stream using a mass spectrometer. The membrane was added to the solution, and the inside of the membrane (permeate side) was evacuated by a rotary pump. Furthermore, helium was introduced into the permeate side at 1.0 mL/min as the standard gas, and the molar composition on the permeate side was analyzed using a mass spectrometer. By using this system, the permeation properties can be determined without the condensation of permeated vapor, and the measurement error was less than 3% for membranes with a small membrane area for permeation. More detailed information about this system is reported in the literatures [7,8]. For the evaluation of acid stability, sulfuric acid (2.4 mol/L) was added to 400 g of the feed solution during the pervaporation test, and the time course in the permeation fluxes of water and ethanol was monitored. 3. Results and discussion
Fig. 2. SEM images of the NaA-type zeolite membrane: (a) top surface and (b) fractured section.
water was 6.3 because of dissolved carbon dioxide. The reduction of water flux was attributed to the marginal acidity of the solution.
3.1. Dehydration performance 3.2. Influence of sulfuric acid addition Fig. 3 shows the permeation fluxes of water and ethanol through the NaA-type zeolite membranes for ethanol solutions with different ethanol concentrations at 313 K. The permeation flux of ethanol was below 10−6 mol m−2 s−1 for all ethanol concentrations. The permeation flux of water was 1.9 × 10−3 mol m−2 s−1 for a 95 mol% ethanol solution, and the flux increased with a decrease in the ethanol concentration. As a result, the permeation flux of water was 1.9 × 10−2 mol m−2 s−1 at an ethanol concentration of 0 mol%. The concentration dependency of the dehydration performance is in agreement with previous reports [11,12]. However, the water flux at 0 mol% ethanol decreased with time and was 1.5 × 10−3 mol m−2 s−1 after 5 h as shown in Fig. 3(a). The pH of
Fig. 4 shows the time courses in the permeation fluxes of water and ethanol at the amount of 0.10–1.0 mL of sulfuric acid added. Equimolar mixtures of water and ethanol were used for the experiments, and the temperature was 313 K. The relative water flux (Jw /Jw0 ), which is the permeation flux of water normalized with the water flux at t = 0, is used to easily compare the change in the permeation properties. When 0.1 mL of sulfuric acid was added, the relative water flux decreased dramatically immediately after the acid addition. The flux then slightly decreased between 15 and 80 min and finally increased significantly. In contrast, the permeation flux of ethanol suddenly and significantly increased at
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Fig. 3. Permeation properties of the NaA-type zeolite membrane for aqueous solutions of ethanol at 313 K. (a) The time courses in the permeation flux of water for 0, 25, 50, 75, and 95 mol% ethanol solutions at 313 K. Ethanol concentration in the feed solution: (black) 0 mol%; (blue) 25 mol%; (red) 50 mol%; (green) 75 mol%; (orange) 95 mol%. (b) The influence of the ethanol concentration in the solution on the permeation flux of water at 313 K. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
t = 65 min. The same trends were observed in shorter periods for the addition of a larger amount of sulfuric acid. Fig. 5 shows the time courses in the permeation fluxes of water and ethanol at the ethanol concentration of 0–95 mol%. The temperature was 313 K, and the amount of added sulfuric acid was 1.0 mL in these experiments. The permeation fluxes show the same trends as those shown in Fig. 4. For the 95 mol% ethanol solution, the first large decrease in the water flux was observed for a few minutes immediately after the acid addition, then the flux slightly decreased between 10 and 100 min, and finally, a significant increase occurred after t = 110 min. Although the same permeation behaviors were observed for the lower ethanol concentrations, the periods became shorter. Fig. 6 shows the typical time courses in the permeation fluxes of water and ethanol after adding sulfuric acid. The change in the permeation properties can be divided into three steps as follows: Step (I): large decrease in the relative water flux. Step (II): slight decrease in the relative water flux. Step (III): large increase in both the water and ethanol fluxes. The times at Jw /Jw0 = 1 and Je = 10−5 mol m−2 s−1 in step (III) were assigned as t1 and t2 , respectively, in order to discuss the influence of the amount of added acid and concentration on the changes in the permeation properties. The inverse time 1/t means the overall destruction rate of the NaA-type zeolite membrane. Figs. 7 and 8
Fig. 4. The time courses in the permeation fluxes of water and ethanol at the amount of added sulfuric acid of 0.10–1.0 mL for 25 mol% ethanol solutions at 313 K. Amount of added sulfuric acid: (black) 0.1 mL; (blue) 0.3 mL; (red) 0.5 mL; (green) 0.7 mL; and (orange) 1.0 mL. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
represent the influence of the amount of added sulfuric acid on the overall destruction rates. The rates were proportional to the amount of added sulfuric acid. However, the rates exponentially increased with the water concentration in the feed solution. This implies that water plays an important role in the destruction of the zeolite membrane. Fig. 9(a), (b), and (c) shows the SEM images of the NaA-type zeolite membranes in steps (I), (II), and (III), respectively. As shown in Fig. 2, the top surface of the original NaA-type zeolite membrane was covered with the polycrystalline layer, and the membrane thickness was ca. 6 m. The morphology of the membrane treated for 5 min was identical to that of the untreated membrane. For the membrane treated for 50 min, the top surface of the membrane was covered with an amorphous-like material as shown in Fig. 10, although the thickness was identical to the original membrane (ca. 6 m). When the membrane was treated in the acid solution for 100 min, the alumina particles of the support tube were observed within large cracks on the surface. Furthermore, the membrane became thinner (ca. 2 m). The Si/Al and Na/Al ratios of the original membrane were unity, as indicated in Table 1.The Si/Al ratio was constant until 50 min, while the value decreased to 0.51 after the
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Fig. 7. The influence of the amount of added sulfuric acid on the destruction rates (a) 1/t1 and (b) 1/t2 at 313 K. Ethanol concentration in the feed solution: () 0 mol%; () 25 mol%; () 50 mol%; () 75 mol%; and (♦) 95 mol%.
Fig. 5. The time courses in the permeation fluxes of (a) water and (b) ethanol at the ethanol concentration of 0–95 mol% at 313 K. Ethanol concentration in the feed solution: (black) 0 mol%; (blue) 25 mol%; (red) 50 mol%; (green) 75 mol%; (orange) 95 mol%. The amount of added sulfuric acid = 1.0 mL. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 6. Typical time courses in the permeation fluxes of water and ethanol by the addition of sulfuric acid. Component: (blue) water; (red) ethanol. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 8. The influence of the water concentration in the feed solution on the destruction rates 1/t1 and 1/t2 at 313 K. Amount of added sulfuric acid: () 0.1 mL; () 0.3 mL; () 0.5 mL; () 0.7 mL; and (♦) 1.0 mL.
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Fig. 9. The SEM images of the NaA-type zeolite membranes in steps (I), (II), and (III). The membranes were treated with 25 mol% ethanol solutions containing sulfuric acid (0.1 mL) for (a) 5 min, (b) 50 min, and (c) 100 min.
Since the added acid functions as a catalyst of hydrolysis, Si–O and/or Al–O bonds in the zeolite framework are dissociated [13]. As a result, the framework structure of the zeolite is destroyed, and the zeolite layer is converted to an amorphous silica-alumina layer. Na+ ions in the NaA-type zeolite may be also exchanged with H+ ions. Since soluble silica and alumina compounds are formed by further progression of the hydrolysis reaction [13], the amorphous silica–alumina layer is dissolved into the solution. Therefore, the results of the morphology observation and composition analysis suggest the following points: (1) The zeolite framework is mainly destroyed in steps (I) and (II). (2) The soluble amorphous-like layer is dissolved into the feed solution in step (III).
Fig. 10. The XRD patterns of the NaA-type zeolite membranes. The membranes were treated with 25 mol% ethanol solutions containing sulfuric acid (0.1 mL) for (a) 0 min, (b) 5 min, (c) 50 min, and (d) 100 min. Symbols: () NaA-type zeolite, (×) support tube.
100-min treatment. The large reduction in the Si/Al ratio at 100 min is due to the extrinsic aluminum constituent of the support tube. On the other hand, the Na/Al ratio decreased with the treatment time, in particular, the largest decrease occurring in steps (I) and (II).
The results indicate that the main factors for the destruction of the zeolite membrane were hydrolysis reaction and dissolution, as discussed above. Since acids catalyze the hydrolysis reaction, the reaction rate obeys first-order kinetics for the acid concentration (amount of catalyst). This is in agreement with the results shown in Fig. 7. On the other hand, water is essential for progression of these processes. As shown in Fig. 8, therefore, the reaction order of water became higher. In this study, the permeation properties of the NaA-type zeolite membranes could be monitored in real-time during their destruction by the addition of sulfuric acid. Membrane separation is an energy-saving process, and in particular, inorganic membranes (such as zeolite and microporous silica) can be used under severe conditions. The technique used in this study can be applied to the
Table 1 The composition of the NaA-type zeolite membranes before and after the acid treatment. No.
1 2 3 4
Acid treatment time (min)
0 5 50 100
SEM image
Fig. 2 Fig. 9(a) Fig. 9(b) Fig. 9(c)
XRD pattern
Fig.10(a) Fig.10(b) Fig.10(c) Fig.10(d)
Composition (mol%)
Composition ratio
SiO2
Al2 O3
Na2 O
Si/Al
Na/Al
51.3 54.3 60.9 49.1
24.1 26.1 31.1 48.0
24.6 19.6 8.0 2.9
1.06 1.04 0.98 0.51
1.02 0.75 0.26 0.06
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rapid evaluation of membrane stabilities. Additionally, the membrane stability can be predicted if the destruction process describes using the properties of powdery samples in the future. 4. Conclusions In this study the destruction process of the NaA-type zeolite membrane by acid was monitored using a real-time monitoring system. In this process, the permeation fluxes of water showed a large reduction immediately after the acid addition (step (I)), followed by a slight reduction (step (II)), and finally a significant increase (step (III)). The permeation flux of ethanol also increased significantly in step (III), although ethanol concentration on the permeate side was below the detection limit (<10−6 mol m−2 s−1 ) in steps (I) and (II). As a result, the membrane lost its separation ability. The hydrolysis reaction mainly occurred in steps (I) and (II). The hydrolysis reaction converted NaA-type zeolite into amorphous compounds, and the amorphous material was dissolved into the ethanol solution in step (III). The destruction process occurred in shorter period in a solution of lower ethanol concentration and/or higher acid concentration. References [1] K. Sato, K. Sugimoto, T. Nakane, Preparation of higher flux NaA zeolite membrane on asymmetric porous support and permeation behavior at higher temperatures up to 145 ◦ C in vapor permeation, J. Membr. Sci. 307 (2008) 181–195.
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