acetic acid mixtures

Journal of Membrane Science 372 (2011) 269–276

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Synthesis of MCM-22 zeolite membranes and vapor permeation of water/acetic acid mixtures Kyohei Makita a , Yuichiro Hirota a , Yasuyuki Egashira a , Kaname Yoshida b , Yukichi Sasaki b , Norikazu Nishiyama a,∗ a b

Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan Nanostructure Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan

a r t i c l e

i n f o

Article history: Received 6 December 2010 Received in revised form 8 February 2011 Accepted 9 February 2011 Available online 15 February 2011 Keywords: Zeolite MCM-22 ITQ-2 Vapor permeation Water–acetic acid mixtures Secondary growth

a b s t r a c t MCM-22 membranes were prepared on a porous ␣-alumina tube using a secondary growth technique consisting of the deposition of seed crystals on a substrate followed by crystal growth under hydrothermal conditions. In this study, two types of seed crystals, MCM-22 and ITQ-2, were used. The MCM-22 membrane prepared using ITQ-2 seed crystals (MCM-22 (I) membrane) was composed of MCM-22 polycrystals with a thickness of 5 ␮m, which is much thinner than that of the MCM-22 membrane prepared using MCM-22 seed crystals (MCM-22 (M) membrane) (15–20 ␮m). Vapor permeation of water/acetic acid mixtures through the MCM-22 membranes was studied. The MCM-22 (I) membranes calcined at 400 ◦ C showed a high separation factor of 78 with a water permeance of 4 × 10−8 mol m−2 s−1 Pa−1 at 120 ◦ C. Elevating the calcination temperature to 500 ◦ C resulted in a lower separation factor. The MCM22 (I) membrane calcined at 400 ◦ C has a hydrophilic internal surface due to the presence of silanol groups within the framework that contribute to the high selectivity to water permeation. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Acetic acid is an important chemical intermediate in the synthesis of vinyl acetate, terephthalic acid, cellulose ester, and other esters. The current processes for production of acetic acid include the carbonylation of methanol, the liquid-phase oxidation of hydrocarbons, and the oxidation of acetaldehyde [1]. The separation of water from acetic acid is an important part of these processes. Azeotropic distillation and extractive distillation have been developed to separate water and acetic acid, but distillation is energy-intensive due to the small differences in the volatilities of water and acetic acid in dilute aqueous solution [2,3]. Vapor permeation and pervaporation using membranes are alternative energy-conserving separation techniques that are often used for the separation of azeotropic and/or close boiling point mixtures. A hybrid distillation-membrane process, in which the vapor mixture from the top of the distillation column is led to a membrane separator, has been proposed as an energy-efficient separation process [4]. Another potentially useful application of membranes is the pervaporation of liquid mixtures with close boiling points, such as aqueous solutions of acetic acid.

∗ Corresponding author. Tel.: +81 6 6850 6256, fax: +81 6 6850 6255. E-mail address: [email protected] (N. Nishiyama). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.02.007

Zeolites have attracted considerable research attention as membrane materials due to their superior thermal, mechanical, and chemical properties compared to organic polymers. So far, most studies on zeolite membranes have focused on zeolite A [5–9], FAU-type zeolites (X and Y) [10,11], mordenite [12,13], MFI-type zeolite (ZSM-5 and silicalite) [14–19] and SAPO-34 membranes [20,21]. Zeolite NaA membranes have been extensively developed, and have been reported to show very good performances for vapor permeation and pervaporation of water/organic liquids [8]. They are currently commercially available and have been employed for large-scale dehydration of organic solvents [9]. However, long-term use of A-type zeolite membranes in aqueous solutions, especially in acidic media, remains problematic because of dealumination from the zeolite framework. The hydrophilicity of zeolites increases with increased Al content in the framework, whereas the acid resistance simultaneously decreases because strong acids leach Al from the zeolite, resulting in the breakdown of its framework structure [22,23]. New types of membranes that show both hydrophilicity and stability in acidic media are therefore needed for dehydration of organic liquids containing an acid, such as water–acetic acid mixtures. Zeolite membranes with different pore structures and pore sizes, such as T-type [24], mordenite [25], ZSM-5 [26], and MER [27] membranes have been studied. However, the appropriate pore size, pore structure and Si/Al ratio for use in practical applications are still unclear from the standpoint of not only

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1 0.8

I/I0 [-]

MCM-22 0.6 0.4 NaA 0.2 0 0

20

40

60

80

2 days

100

120

Immersion ime [min]

Fig. 4. Time course of the relative peak intensity of the XRD pattern of zeolites (MCM-22 and NaA) after immersion in acetic acid at 120 ◦ C; (3 0 4) reflection for MCM-22 and (822) reflection for NaA.

Fig. 1. Schematic representation of the preparation of MCM-22 and ITQ-2 from MCM-22 (P) [28].

Intensity [a.u.]

(b)

(a)

0

5

10

15

20

25

30

2theta[degree] Fig. 5. XRD patterns of (a) MCM-22 and (b) ITQ-2 seed crystals. Fig. 2. Schematic illustration of the apparatus for the vapor permeation tests.

separation performance but also the feasibility of membrane preparation. In this study, a new type of hydrophilic membrane, made of zeolite MCM-22, was prepared on a porous ␣-alumina support by hydrothermal synthesis. MCM-22 is predicted to have high acidproof properties due to the high Si/Al ratio in its framework. The precursor for MCM-22 (MCM-22 (P)) is composed of layered aluminosilicate with a high content of silanol groups at the surfaces

of the thin layers [28,29], as shown in Fig. 1. The MCM-22 zeolite structure is formed by calcination of MCM-22 (P) through the condensation of silanol groups. However, we would expect to calcine MCM-22 at relatively low temperatures (300–450 ◦ C) to retain sufficient silanol groups, which show hydrophilic properties, on its internal surfaces.

Intensity [a.u.]

1

Mass remainder [-]

0.8

MCM-22 0.6 0.4

(b)

NaA

(a)

0.2 0 0

20

40

60

80

100

2 days

Immersion time [min] Fig. 3. Time course of the mass remainder of zeolites (MCM-22 and NaA) after immersion in acetic acid at 120 ◦ C.

0

5

10

15

20

25

30

2 theta [degree] Fig. 6. XRD patterns of (a) MCM-22 (M) and (b) MCM-22 (I) membranes. Seed density is 1 wt.% in ethanol.

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Fig. 7. SEM images of the surface and cross-section of the MCM-22 (M) membranes.

In this study, MCM-22 membranes were prepared on a porous ␣-alumina tube using a secondary growth technique consisting of the deposition of seed crystals on the substrate, followed by crystal growth under hydrothermal conditions. Here, two types of seed crystals (MCM-22 and ITQ-2) were used. The ITQ-2 zeolite is a monolayer of crystalline aluminosilicate that can be obtained by delamination of the layered precursor MCM-22 (P), as shown in Fig. 1. The effect of type of seed crystals on the formation of the MCM-22 membranes was studied, and the morphology of the MCM-22 membranes and their vapor permeation performance for water/acetic acid mixtures were elucidated.

amorphous fumed silica (Aerosil 200, Evonik Industries), 1.04 g of hexamethyleneimine (HMI, Wako Pure Chemical Industries, Ltd.) and 16.3 g of deionized water. The molar ratios in the solution were 0.075Na2 O:1.0SiO2 :0.028Al2 O3 :0.5HMI:44H2 O. The crystallization was carried out in a closed Teflon-lined stainless steel vessel under autogenous pressure at 180 ◦ C for 3 days. To stir the solution during the hydrothermal synthesis, the vessel was rotated at 2 rpm in an oven. The product was washed with deionized water and dried at 90 ◦ C for 24 h. The product is a precursor for MCM-22 and is designated as MCM-22 (P). MCM-22 was obtained by calcination of MCM-22 (P) at 500 ◦ C for 8 h in air.

2. Experimental 2.1. Preparation of seed crystals 2.1.1. MCM-22 The MCM-22 and ITQ-2 seed crystals were synthesized using the method described in the literature [28,30]. A starting solution was prepared by mixing 0.106 g of sodium aluminate (Al/Na2 O = 0.79, Wako Pure Chemical Industries, Ltd.), 0.416 g of sodium hydroxide solution (4 N, Wako Pure Chemical Industries, Ltd.), 1.26 of

2.1.2. ITQ-2 MCM-22 (P) powder (0.202 g), hexadecyltrimethylammonium bromide (1.139 g) and 10 wt.% tetrapropylammonium hydroxide aqueous solution (4.94 g) were mixed and heated to reflux at 80 ◦ C for 16 h. The solution was treated with ultrasound for 1 h. An HCl solution was added to the solution until the pH of the solution reached about 2. The product was separated by centrifugation. Finally, ITQ-2 powder was obtained by calcination at 540 ◦ C for 12 h in air.

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Fig. 8. SEM images of the surface and cross-section of the MCM-22 (I) membranes.

2.2. Membrane preparation MCM-22 membranes were prepared using the secondary growth technique with seed crystals. The MCM-22 membranes were synthesized on the outer surface of a porous asymmetric ␣-alumina tube with an average pore size of 100 nm (NGK Insulators). Prior to the synthesis, the outer surface of the alumina tube was seeded by dip coating using an ethanol solution containing 0.1–3 wt.% of MCM-22 or ITQ-2 seed crystals. Hereinafter, the MCM22 membranes prepared using the MCM-22 and ITQ-2 seed crystals are designated as the MCM-22 (M) and MCM-22 (I) membranes, respectively. The seeded tube was immersed vertically in a Teflon-lined stainless steel autoclave and filled with precursor solution.

The precursor solution was the same as described above (0.075Na2 O:1.0SiO2 :0.028Al2 O3 :0.5HMI:44H2 O). The autoclave was placed in an oven under hydrothermal conditions at 180 ◦ C for 1–5 days. The membrane was then washed with deionized water and dried at 90 ◦ C. Finally, the membrane was calcined at 300–500 ◦ C. 2.3. Characterization The products were characterized by X-ray diffraction (XRD), recorded on a Rigaku Miniflex using Cu-K␣ radiation. The MCM22 membranes were synthesized on a porous ␣-alumina plate with an average pore size of 100 nm (NGK Insulators) only for the XRD measurements. The morphology of the MCM-22 mem-

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branes was observed with a scanning electron microscope (SEM) (Hitachi S-2250). The crystalline structure of the MCM-22 membrane was observed using a transmission electron microscope (TEM, Hitachi H9500) at an accelerating voltage of 300 kV. To obtain cross-sectional TEM image of MCM-22 membranes, the specimen was thinned using a focused ion beam lithograph (Hitachi FB2100). 2.4. Vapor permeation test A simple schematic diagram of the vapor permeation test apparatus is shown in Fig. 2. A water/acetic acid (H2 O/AcOH = 77/23 mol/mol) mixture solution was fed to the membrane module at 120 ◦ C and the solution was vaporized in the module. Helium gas was used as the carrier gas. The flow rates of the He/H2 O/AcOH mixture were 0.12/0.17/0.05 mol/s at 120 ◦ C. The inside of the membrane tube was evacuated using an oil-sealed rotary vacuum pump. The vapor which permeated through the membrane was collected by a condenser with a liquid nitrogen trap. The collected solution was analyzed using TCD gas chromatography (GC-8A, Shimadzu Co.). The vapor permeation performance of the membrane was evaluated based on permeance [mol m−2 s−1 Pa−1 ] and separation factor ˛ [−]. The separation factor of water over acetic acid is defined as ˛W/A =

yW /yA xW /xA

where yW /yA is the mass ratio of water to acetic acid in the permeate, and xW /xA is that in the feed. 2.5. Acid-resistance tests The acid resistance of zeolite MCM-22 was evaluated as follows. MCM-22 powder was immersed in acetic acid in a closed vessel at 120 ◦ C for 2 days. The loss of mass and XRD peak intensity were measured after the treatment. 3. Results and discussion 3.1. Acid resistance of MCM-22 Fig. 3 shows the time course of the mass remainder of MCM-22 after immersion in acetic acid at 120 ◦ C. Data for NaA zeolite are shown in this figure for comparison. The mass of MCM-22 powder fell by 20% in the first 30 min, but then remained constant with elapse of time. On the other hand, the mass of NaA zeolite fell by 80% after 90 min. The relative peak intensity of NaA and MCM-22 after the acid treatments is plotted in Fig. 4. The (8 2 2) XRD peak intensity of NaA zeolite fell by 70% in the first 30 min. On the other hand, supporting the results of the mass analysis, the (3 0 4) XRD peak intensity of MCM-22 fell by only 20% and then leveled out after 30 min, suggesting that the structure of MCM-22 is much more stable than that of NaA due to the higher Si/Al ratio in the MCM-22 framework. 3.2. Membrane preparation and characterization Fig. 5 shows XRD patterns of MCM-22 and ITQ-2 seed crystals. Both the XRD patterns are similar to those for MCM-22 and ITQ2 reported in the literature [28]. ITQ-2 does not show (0 0 1) or (0 0 2) peaks at 2 = 3–7◦ , which is consistent with the proposed structures: that is, ITQ-2 does not have a regular array of layers, with its characteristic 2.5-nm periodicity, typical of MWW topology. This result suggests that the ITQ-2 particle is just a few nm in size, consistent with the fact that the particle size of ITQ-2 could not be determined by FE-SEM measurement at a resolution of about 10 nm.

Fig. 9. TEM images of the MCM-22 (I) membrane. (a) Cross-sectional view of the MCM-22 (I) membrane. (b) The (0 0 1) direction of MCM-22. (c) The (1 0 0) direction of MCM-22.

The XRD patterns of the MCM-22 (M) and MCM-22 (I) membranes are shown in Fig. 6. Relative peak intensity for both the XRD patterns is similar to that of the MCM-22 powder, suggesting that the MCM-22 crystals formed on the alumina support are randomly oriented.

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Fig. 10. SEM images of the surface and the cross-section of zeolite MCM-22 (I) membranes synthesized for 4 days. Density of ITQ-2 in seed solutions: (a) without seeding, (b) 0.1 wt.% and (c) 3 wt.%.

Figs. 7 and 8 show SEM images of the surface and cross-sections of the MCM-22 (M) and MCM-22 (I) membranes, respectively. Figs. 7(a) and 8(a) show SEM images of the surface of the alumina tube after seeding with MCM-22 and ITQ-2, respectively. The MCM-22 seed crystals used in this study are spherical and have a crystal size of 5 ␮m. Similar MCM-22 crystals are also observed at the ITQ-2-seeded surface, indicating that the delamination of the layered precursor MCM-22 (P) was incomplete and the seed solution contained a mixture of MCM-22 and ITQ-2 crystals. The MCM-22 (M) membrane has a rough surface with a thickness of 15–20 ␮m after 4 days of synthesis, which is much thicker than the MCM-22 (I) membrane. The particle size of the MCM-22 formed is 5– 10 ␮m for both membranes. The crystal size did not markedly increase during membrane formation. The thickness of the MCM-22 (I) membrane is very uniform and measures about 5 ␮m after 4 days. It appears that the MCM-22 crystals grow densely on the surface of the alumina support when the ITQ-2 crystals are used as a seed, possibly because the nucleation density on the ITQ-2-seeded surface is higher than that on the MCM-22-seeded surface. Fig 8(b) shows that an amorphous gel layer was observed after 1 day. The surface of the alumina support was completely covered with the amorphous layer after 2–3, days as shown in Fig. 8(c) and (d). On the other hand, no amorphous layer was formed when hydrothermal synthesis was performed using an unseeded support. The ITQ-2 seed crystals appear to have effectively induced the formation of the amorphous phase. After 3 days, the amorphous layer was transformed into the MCM-22 layered structure. After 4 days, a densely packed MCM-22 layer was formed with MCM-22 intergrown crystal aggregates 5–10 ␮m in size. After 5 days, however, new crystals appeared on the first MCM-22 layer, as shown in Fig. 8(f). TEM images of the MCM-22 (I) membrane are shown in Fig. 9. A cross section of the membrane reveals randomly oriented layered structures that have grown on the alumina support. This result is consistent with the results of the XRD pattern of the MCM-22 (I) membrane. Fig. 9(b) shows a TEM image observed in the direc-

tion perpendicular to the surface of the layered MCM-22. The parts showing light and dark contrast correspond to the pores and the MCM framework, respectively. The layered structure of the MCM22 was observed in the TEM image of the cross section of the layered MCM-22 (Fig. 9(c)). The distance from layer to layer was approximately 2.5 nm, which was in good agreement with the d(0 0 1) value calculated from the XRD pattern. The pore sizes determined from TEM images (b) and (c) are about 0.8 nm and 0.4 nm, respectively, which agrees with the reported values of 0.71 nm × 0.71 nm (0 0 1) and 0.41 nm × 0.51 nm (1 0 0) pores. 3.3. Effect of seed crystals and synthesis period The permeance and separation factors for a water/acetic acid mixture through the MCM-22 (M) and MCM-22 (I) membranes are listed in Table 1. Both the MCM-22 (M) and MCM-22 (I) membranes showed low selectivity (˛ is nearly one) when the synthetic period was 1–3 days, since large defects were present among the MCM22 crystals, which were visible on the surface in the SEM images (Figs. 7 and 8). The MCM-22 membranes synthesized for 4–5 days showed selective permeation of water rather than acetic acid, suggesting that these membranes do not possess large defects among the MCM-22 crystals. The permeance of water through the MCM-22 (M) was smaller than that through the MCM-22 (I) membrane. The MCM-22 (I) synthesized for 5 days shows a smaller water permeance than that synthesized for 4 days. These results are consistent with the thickness of the membranes observed in the SEM images. The MCM-22 (I) synthesized for 4 days shows a higher water permeance, possibly because it has a more uniform and thinner layer (5 ␮m) than the other two membranes. 3.4. Effect of ITQ-2 density in the seed solution The effect on membrane formation of the density of ITQ-2 seeds in the ethanol solution was studied. Fig. 10 shows SEM images of the MCM-22 (I) membranes synthesized for 4 days. The ITQ-2 densities in the seed solution were 0.1 wt.% and 3 wt.%. The SEM image of the

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Table 1 The results of vapor permeation of water/acetic acid mixture through the MCM-22 (M) and MCM-22 (I) membranes at 120 ◦ C. Membranes

Synthesis period [days]

MCM-22 (M) MCM-22 (I) MCM-22 (I)

4 4 5

Permeance [×10−8 mol m−2 s−1 Pa−1 ] Water

AcOH

0.85 4.3 2.1

<0.02a 0.06 <0.02a

˛ [−]

>46 78 >117

p(AcOH) = 15 kPa, p(water) = 50 kPa. The MCM-22 (M) and MCM-22 (I) membranes were prepared using 1 wt.% MCM-22 and 1 wt.% ITQ-2 ethanol solutions. Calcination temperature is 400 ◦ C. a Below the detection limit.

40

AcOH H 2O

0

20

0 0

1

2

5.0 60 4.0 40

H 2O

3.0 2.0

20 1.0

AcOH

3

Densityof ITQ-2 in seed solution [wt%] Fig. 11. Permeances and separation factors of water/acetic acid vapor at 120 ◦ C. Effect of density of ITQ-2 in seed solution on MCM-22 (I) membranes. The membranes were calcined at 400 ◦ C.

MCM-22 (I) membranes for 1 wt.% is already shown in Fig. 8(e). As shown in Fig. 10(a), no MCM-22 layer was formed if seeding was not performed. On the other hand, an MCM-22 layer was formed even when the density of ITQ-2 was only 0.1 wt.%. The MCM-22 layer observed in Fig. 10(b) is less densely packed than the MCM22 (I) membrane prepared using 1 wt.% seed solution (Fig. 8(e)). The MCM-22 membrane synthesized using 3 wt.% seed solution (Fig. 10(c)) was very thick (15–20 ␮m) and composed of a multiple layers of MCM-22 crystals. Fig. 11 shows the results of vapor permeation of water/acetic acid for the MCM-22 membranes prepared using seed solutions with different ITQ-2 densities. The calcination temperature was 400 ◦ C. The MCM-22 (I) membrane prepared using 0.2 and 0.5 wt.% seed solutions showed no selectivity to water. The surface of the support was not fully covered with the MCM-22 crystals when the ITQ-2 density in the seed solution was lower than 0.5 wt.%. The permeation results are consistent with the results of the SEM observations. The MCM-22 (I) membrane prepared using 3 wt.% seed solution must have large defects among the crystals, since the membrane was composed of a multilayer with a rough surface. The optimum density for the ITQ-2 seed solution, to obtain a defect-free thin layer of MCM-22 crystals, was thus 1–2 wt.%. 3.5. Effect of calcination temperature The results of vapor permeation of water/acetic acid for the MCM-22 (I) calcined at different temperatures are shown in Fig. 12. The ITQ-2 density in the seed solution was 1 wt.% and the synthesis period was 4 days. The permeance of water through the MCM-22 (I) membrane calcined at 300–350 ◦ C was small, although the water/acetic acid separation factor was above 50. The permeance of water increased with increasing calcination temperature. These results suggest that, at below 350 ◦ C, the pores of MCM-22 are partly blocked by the organic structuredirecting agent (OSDA). The MCM-22 (I) membranes calcined at

0.0 300

350

400

0 500

450

Calcination temperature [ºC ] Fig. 12. Effect of calcination temperature of MCM-22 (I) membranes on the permeances and separation factors of water/acetic acid vapor at 120 ◦ C. The ITQ-2 density in the seed solution was 1 wt.%. Synthesis period was 4 days.

400 ◦ C show a high maximum separation factor of 78 with a water permeance of 4 × 10−8 mol m−2 s−1 Pa−1 . The separation factor decreased at higher calcination temperatures (450–500 ◦ C) due to a significant increase in the permeance of acetic acid. Thermogravimetric analysis (TGA) curves for MCM-22 powder calcined at 300, 400, and 500 ◦ C are shown in Fig. 13. The mass loss at above 330 ◦ C observed for the MCM-22 calcined at 300 ◦ C can be explained by the decomposition of the OSDA. The decrease in mass of the MCM-22 calcined at 400 ◦ C observed above 430 ◦ C is possibly due to dehydration by condensation of silanol groups. The loss of mass in this temperature region was not observed in MCM-22 calcined at 500 ◦ C, implying that the MCM-22 (I) membrane calcined at 500 ◦ C has no silanol groups on its internal surfaces. These results suggest that the MCM-22 (I) membrane calcined at 400 ◦ C has hydrophilic internal sur-

100 Calcinad at 300ºC

Mass remainder [%]

5

10 -8 mol m -2 s-1 Pa-1]

10

Permeance [

60

Separation factor (H2O/AcOH) [-]

Permeance [ 10 -8 mol m-2 s -1 Pa -1]

80 15

80

6.0

Separation factor (H 2O/AcOH) [-]

100

20

90

Calcined at 400ºC

80 Calcined at 500ºC 70

60 0

100

200

300

400

500

600

Temperature [ºC ] Fig. 13. TGA curves for MCM-22 powder calcined at 300, 400, and 500 ◦ C.

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faces due to the presence of silanol groups in the framework structure, which contribute to the high selectivity to water permeation. 4. Conclusions The MCM-22 membranes were synthesized by secondary crystal growth using ITQ-2 and MCM-22 seed crystals. The surface of the alumina support was completely covered with the amorphous layer from the early stage of membrane formation. The amorphous layer was converted to the layered MCM-22 structure after 4 days. The MCM-22 (I) membrane showed a larger water permeance than the MCM (M) membrane, possibly because it has a more uniform and thinner layer (5 ␮m) than the MCM-22 (M) membrane. The MCM-22 (I) membranes calcined at 400 ◦ C showed a maximum separation factor of as high as 78, with a water permeance of 4 × 10−8 mol m−2 s−1 Pa−1 . Elevating the calcination temperature to 500 ◦ C resulted in a lower separation factor. The MCM-22 (I) membrane calcined at 400 ◦ C has a hydrophilic internal surface due to the presence of silanol groups in the framework structure that contribute to the high selectivity to water permeation. Acknowledgements This work is supported by NEDO’s “Development of fundamental technologies for Green and Sustainable chemical processes, Green and sustainable chemistry/fundamental development of ordered nanoporous membranes for highly refined separation technology” (FY2009–FY2011). We gratefully thank the GHAS Laboratory at Osaka University for the SEM measurements. References [1] C.J. King, Acetic Acid Extraction, Handbook of Solvent Extraction, 1983, p. 567. [2] R.Y.M. Huang, A. Moreira, R. Notarfonzo, Y.F. Xu, Pervaporation separation of acetic acid–water mixtures using modified membranes. I. Blended polyacrylic acid (PAA)-nylon 6 membranes, J. Appl. Polym. Sci. 35 (1988) 1191. [3] Li Shiguang, A. Vu Tuan, R.D. Noble, J.L. Falconer, A Ge-substituted ZSM-5 zeolite membrane for the separation of acetic acid from water, Ind. Eng. Chem. Res. 40 (2001) 6165. [4] Wolfgang Stephan, R.D. Noble, C.A. Koval, Design methodology for a membrane/distillation column hybrid process, J. Membr. Sci. 99 (1995) 259. [5] H. Kita, K. Horii, Y. Ohtoshi, K. Tanaka, K.I. Okamoto, Synthesis of a zeolite NaA membrane for pervaporation of water/organic liquid mixtures, J. Mater. Sci. Lett. 14 (1995) 206. [6] K.I. Okamoto, H. Kita, K. Horii, 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. [7] H. Kita, T. Inoue, H. Asamura, K. Tanaka, K.I. Okamoto, NaY zeolite membrane for the pervaporation separation of methanol–methyl tert-butyl ether mixtures, J. Chem. Soc., Chem. Commun. (1997) 45. [8] M. Kondo, M. Komori, H. Kita, K.I. Okamoto, Tubular-type pervaporation module with zeolite NaA membrane, J. Membr. Sci. 133 (1997) 133.

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