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A study on the pervaporation of water–acetic acid mixtures through ZSM-5 zeolite membranes
Journal of Membrane Science 218 (2003) 185–194
A study on the pervaporation of water–acetic acid mixtures through ZSM-5 zeolite membranes Gang Li, Eiichi Kikuchi, Masahiko Matsukata∗ Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Received 21 October 2002; accepted 3 April 2003
Abstract A study was carried out on the pervaporation of water from water–acetic acid mixtures through tubular ZSM-5 zeolite membranes. ZSM-5 zeolite membranes were prepared on the outer surface of a porous ␣-alumina tube by the seed-assisted crystallization method in the absence of organic templates. It was observed that water preferentially permeated with selectivities higher than the selectivity based on the vapor–liquid equilibrium in the entire range of acetic acid concentration. The separation mechanism of water through ZSM-5 zeolite membranes was discussed and compared with that through mordenite membranes. It was found that the permeability coefficients of both water and acetic acid decreased with increasing temperature, although the pervaporation fluxes of both water and acetic acid increased. As a result, it was concluded that the increase in the pervaporation flux with temperature was due to the increase in the driving force in terms of transmembrane partial vapor pressure difference with temperature. According to the mechanism proposed herein, a study was also directed to modify as-synthesized membranes for the improvement in their pervaporation performance. It was observed that both the water flux and the separation factor were increased through a membrane treated in alkaline solution under appropriate conditions. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; ZSM-5 zeolite membrane; Water–acetic acid mixtures; Modification; Alkali treatment
1. Introduction Pervaporation is one of the effective membranebased processes for separation of liquid mixtures such as azeotropic, close-boiling, and heat-sensitive compounds [1]. In recent years, use of zeolite membranes for pervaporation has attracted considerable attention. In general, hydrophilic zeolite membranes have been used for dehydration of organic solvents [2–11]. On the other hand, hydrophobic zeolite membranes have been ∗ Corresponding author. Tel.: +81-3-5286-3850; fax: +81-3-5286-3850. E-mail address: [email protected] (M. Matsukata).
used for separation of organics from water [12–20]. Besides, a few studies have been reported on the separation of organic compounds or multi-component mixtures [21–29]. Acetic acid is one of the most important intermediates in chemical industry. As the relative volatility of water to acetic acid is close to unity [30], much energy is required for distillation to separate acetic acid from aqueous mixtures. Consequently, pervaporation is regarded as one of the potential processes for separation of aqueous acetic acid mixtures. A few studies have been reported on the pervaporation of acetic acid from water using hydrophobic zeolite membranes [13,17]. However, there are few studies on the use of hydrophilic zeolite membranes for separation of water
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00172-8
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from acetic acid, which would be desirable for purification of acetic acid and also for application to membrane reactors like in esterification. Although NaA zeolite membranes are very effective to remove water from organic compounds [2], they cannot be applied for dehydration of acetic acid due to their low resistance to acid. As the chemical stability of zeolite increases with increasing Si/Al ratios in the framework while the hydrophilicity decreases, zeolites having medium Si/Al ratios would be preferred for separation of water from acetic acid. In a previous paper [31], we have reported the pervaporation of water from acetic acid using thin mordenite membranes. Here we report the pervaporation of water from acetic acid using thin ZSM-5 zeolite membranes and the comparison of these two types of membranes.
2. Experimental 2.1. Membrane preparation ZSM-5 zeolite membranes were prepared on the outer surface of a porous asymmetric ␣-Al2 O3 tube with an average pore size of 100 nm (NGK Insulators, Japan). Prior to crystallization, the outer surface of the tube was seeded by means of dip coating in a colloidal ZSM-5 crystal suspension (solid content = 3.8 g/l, pH = 7.4), which was prepared from commercially available ZSM-5 powder (Si/Al = 11.9, Tosoh Co.) according to a procedure described previously [7]. A given amount of ZSM-5 powder was first ground in an agate mortar. The ground powders were then mixed with an appropriate amount of water in a beaker to form a slurry. The slurry was treated in an ultrasonic bath. Finally the slurry was kept at room temperature for several days. Due to the gravitational effect, bigger particles were settled down to the bottom of the beaker while ultra-small particles were dispersed in the upper part of slurry resulting in a milky suspension. This suspension was used for seeding. The seeded tube was immersed vertically in a Teflon-lined stainless steel autoclave and filled with an organic template-free gel. The gel had a molar composition of 26.75Na2 O:Al2 O3 :100SiO2 :4600H2 O as prepared by mixing distilled water, sodium hydroxide (Kanto Chemical Co.), sodium aluminate (Kanto
Chemical Co.), and colloidal silica ST-S (Nissan Chemical Ind.). The gel was stirred at 323 K for 4 h prior to use. The autoclave was placed in a preheated oven for hydrothermal treatment at 453 K for a given period of time. Then, the membrane was boiled in deionized water and dried at ambient conditions prior to characterizations and pervaporation tests. 2.2. Characterization The crystalline phase of the zeolite layer formed on the support surface was determined using a Rigaku RINT 2000 X-ray diffractometer (XRD) equipped with a Cu K␣ radiation source at 40 kV and 20 mA at a scanning speed of 2◦ (2θ) min−1 . The nanosized particles in the colloidal suspension and the seeded support were characterized using a Hitachi S4500S field emission scanning electron microscope (FE-SEM) operated at 15 keV. The morphology of the zeolite layer was examined by means of a Hitachi S-2150 scanning electron microscope (SEM) operated at 15 keV. To characterize the quality of the prepared membrane, single-gas permeances were measured for H2 , n-C4 H10 , and i-C4 H10 by sealing the membrane in a stainless steel module with graphite cylindrical rings. Prior to the gas permeation tests, the prepared membrane was dried at 473 K for longer than 12 h. The permeances were measured using the pressure-drop method. The pressure difference across the membrane was kept at 100 kPa and the permeate side was kept at atmospheric pressure. The ratio of single-gas permeances is referred to as the ideal selectivity. 2.3. Pervaporation test All the pervaporation experiments were performed using a stainless steel tube module cell. The membrane tube was placed inside the module cell and sealed using graphite cylindrical rings at the both ends. The feed was circulated at a flow rate of 26 cm3 min−1 from a feed tank of 1000 cm3 through the outer side of the membrane tube, while the inside was evacuated by use of an oil rotary vacuum pump. The stainless steel module cell was heated with a ribbon heater. A thermocouple was inserted into the module cell and placed close to the outer surface of the membrane. The feed in the tank was heated to about 2 K higher than the membrane surface.
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where YW /YA is the weight ratio of water to acetic acid in permeate, and XW /XA is that in feed. 2.4. Membrane modification by alkali treatment The alkali treatment of ZSM-5 membranes was carried out in the pervaporation apparatus without taking the membrane out of the module. The outer surface of membrane was in contact with a flow of NaOH solution. Between alkali treatment and pervaporation test, the membrane and the line were first washed in a one-way flow of deionized water at room temperature and further rinsed in a circulation of deionized water at 353 K for 2–4 h. Fig. 1. The water and acetic acid fluxes as a function of pervaporation time for a 50 wt.% acetic acid aqueous solution at 353 K through a ZSM-5 zeolite membrane (sample M5).
6.28 cm2 .
The effective membrane area was The permeate was condensed in a cold trap cooled by liquid nitrogen. The permeate was weighed and analyzed every 1–3 h of pervaporation time until the permeation reached a steady state. The relative deviations in the water and acetic acid fluxes were less than ±10%. Fig. 1 shows a typical pervaporation time dependence of the water and acetic acid fluxes through a ZSM-5 zeolite membrane. It can be seen that the reproducibility is good. The compositions of feed and permeate were determined by means of a gas chromatograph (Shimadzu GC-8A) equipped with a 2 m long stainless steel column packed with Porapak QS. The separation factor, αW/A , was defined by αW/A =
YW /YA XW /XA
3. Results and discussion 3.1. Membrane preparation and characterization Fig. 2 shows the FE-SEM images for ZSM-5 crystal seeds and the surface of a seeded support. It can be seen that the seeds were located in the voids between the alumina particles and some were attached to the alumina particles. Fig. 3 shows the surface and the cross-sectional SEM images for a zeolite membrane grown on a seeded alumina support after hydrothermal treatment for 12 h at 453 K. Surface SEM reveals that the surface of the support was completely covered with zeolite crystals. These crystals were intergrown so highly that the shape and size of the crystal could hardly be identified. The thickness of the zeolite layer was about 2 m, as shown in the cross-sectional SEM image.
Fig. 2. FE-SEM images. (A) ZSM-5 crystal seeds; (B) top surface of a seeded support.
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Fig. 3. SEM images for a zeolite membrane grown on a seeded ␣-Al2 O3 support at 453 K for 12 h in a gel with the molar composition of 26.75Na2 O:100SiO2 :Al2 O3 :4600H2 O. (A) Top surface; (B) cross-section.
Fig. 4 shows the XRD patterns for the zeolite membrane and commercially available ZSM-5 zeolite powder. The diffraction pattern for the membrane was identical to that for the ZSM-5 powder, indicating that the crystals formed on the support surface have MFI-topology. Fig. 5 shows the single-gas permeances of H2 , n-C4 H10 and i-C4 H10 at 473 K as a function of the permeation time. Note that kinetic diameters of H2 (0.289 nm) and n-C4 H10 (0.43 nm) are smaller whereas kinetic diameter of i-C4 H10 (0.5 nm) is close to the pore size (0.5–0.6 nm) of ZSM-5 crystal. The steady single-gas permeances at 473 K were 5.5 × 10−8 mol s−1 m−2 Pa−1 for
H2 , 1.7 × 10−8 mol s−1 m−2 Pa−1 for n-C4 H10 , and 3.8 × 10−9 mol s−1 m−2 Pa−1 for i-C4 H10 . The ideal selectivities at 473 K are 14.5 for H2 /i-C4 H10 and 4.4 for n/i-C4 H10 . These selectivities are three to four times higher than the corresponding Knudsen selectivities (5.4 and 1, respectively), indicating that the membrane has a low concentration of interzeolitic pores of size larger than zeolitic pores. 3.2. Pervaporation of water–acetic acid mixtures Table 1 summaries the pervaporation results for a 50/50 (w/w) water–acetic acid mixture through ZSM-5 zeolite membranes at 343 K. It can be seen that
Fig. 4. XRD patterns for (a) ZSM-5 powder; (b) surface of a zeolite membrane grown on a seeded ␣-Al2 O3 support at 453 K for 12 h in a gel with the molar composition of 26.75Na2 O:100SiO2 :Al2 O3 :4600H2 O.
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Fig. 5. Single-gas permeances at 473 K for H2 , n-C4 H10 and i-C4 H10 through a ZSM-5 membrane prepared at 453 K for 12 h.
water molecules preferentially permeated through the ZSM-5 membranes. It is interesting to note that the membranes showed very high fluxes (5–7 kg m−2 h−1 ) for pure water. However, the water fluxes were quite low (0.1–0.3 kg m−2 h−1 ) for the mixture. 3.2.1. Effect of temperature Fig. 6 shows the effect of temperature on the pervaporation results. Both water and acetic acid fluxes increased with temperature, while the separation factor was almost independent of temperature. Pervaporation differs from other membrane processes such as reverse osmosis and gas permeation because it involves a phase change of the permeating species. The temperature dependence of the pervaporation flux generally exhibits an Arrhenius-
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Fig. 6. Effect of temperature on the pervaporation of a 50/50 (w/w) water–acetic acid mixture through M5 membrane.
type relation EJ J = J0 exp − RT
(1)
where EJ is the apparent activation energy characterizing the overall temperature dependence of the permeation flux [1]. If the pervaporation through a microporous membrane can be described by the sorption–diffusion model, the flux equation can be written as follows [32]: K K J= (pf − pp ) = p (2) l l where K is the permeability coefficient, l the membrane thickness, and pf and pp are the partial vapor pressures of the permeant in feed and permeate sides,
Table 1 Pervaporation results for pure water and ca. 50 wt.% acetic acid aqueous solution through ZSM-5 zeolite membranes at 343 K Membrane
M1 M2 M3 M4 M5 M6 M7 M8
Synthesis time (h)
Pure water flux (kg m−2 h−1 )
Feed H2 O (wt.%)
Water–acetic acid mixture Water flux (kg m−2 h−1 )
Acetic acid flux (kg m−2 h−1 )
αH2 O/CH3 COOH
6 6 12 12 12 12 19 19
>15 >15 7.7 5.2 5.7 5.5 6.2 7.3
– – 53.3 53.5 52.2 53.0 54.1 54.0
– – 0.28 0.26 0.32 0.08 0.28 0.61
– – 0.03 0.01 0.02 0.02 0.02 0.02
– – 8 15 14 5 9 23
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respectively. p is the transmembrane partial pressure difference and defined as the mass transport driving force. As the permeate side is under vacuum in our study, p can thus be considered to be close to the partial vapor pressure in the liquid feed side. Consequently, we can obtain K J= (3) pf l The temperature dependence of K and pf can be expressed as follows: EP K = K0 exp − (4) RT HV pf = A exp − (5) RT where EP is the activation energy of permeation, HV the molar enthalpy of vaporization, and both K0 and A are the pre-exponential factors. Combining Eqs. (3)–(5) yields K0 A EP + HV J= exp − (6) l RT Comparing Eq. (1) with Eq. (6), thus EJ = EP + HV
(7)
As EJ and HV can be evaluated from the slopes of the plots of ln J versus 1/T and ln pf versus 1/T, respectively, EP can thus be obtained using Eq. (7). The calculated EJ , HV and EP data are listed in Table 2, where HV values were obtained using the partial vapor pressure data in [30]. It is interesting to note that the values of EP are negative for both water and acetic acid. These values indicate that the permeability coefficients of both water and acetic acid through ZSM-5 membrane decrease with increasing temperature, although the pervaporation fluxes of water and acetic acid increase with temperature. As a result, we conclude that the increase
Fig. 7. Effect of feed composition on the pervaporation results at 353 K through M5 membrane.
in both water and acetic acid fluxes with increasing temperature is due to the increase in the driving force in terms of transmembrane partial vapor pressure difference. 3.2.2. Effect of the mixture composition Fig. 7 shows the effect of feed composition on the pervaporation results. As the concentration of acetic acid increased, not only the water flux but also the acetic acid flux decreased. The separation factor also decreased with acetic acid concentration. Fig. 8 shows that the water selectivities on the basis of pervaporation are higher than those on the basis of
Table 2 EJ , HV , and EP data for a 50/50 (w/w) water–acetic acid mixture through M5 membrane
Water Acetic acid
EJ (kJ mol−1 )
HV (kJ mol−1 )
EP (kJ mol−1 )
35 37
43 41
−8 −4
Fig. 8. The mole fraction of acetic acid in permeate as a function of that in feed.
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vapor–liquid equilibrium in the entire range of acetic acid concentration. Since both water (ca. 0.28 nm) and acetic acid (ca. 0.4 nm) molecules are smaller than the pore size of ZSM-5 zeolite (ca. 0.6 nm), molecular sieving effect would play a minor role in the pervaporation of water from acetic acid. Thus, the separation of water from acetic acid would be due to the difference either in the adsorption or in the diffusion properties between water and acetic acid molecules through the membrane. Water permeation was considerably affected by acetic acid molecules. As shown in Table 1, the water flux for 50 wt.% acetic acid aqueous solution is about 1/10 to 1/20 of that for pure water at the same pervaporation temperature. The significant decrease in the water flux might be due to the preferential adsorption of acetic acid molecules, resulting in a significant decrease in the number of the adsorbed water molecules. While the number of adsorbed water molecules would be less than that of adsorbed acetic acid molecules, adsorbed water molecules can diffuse faster than adsorbed acetic acid molecules due to their smaller size, leading to the separation of water from acetic acid. In addition, acetic acid molecules entering the zeolitic pores should hinder the diffusion of water molecules, possibly due to the single-file diffusion since the zeolitic pores of ZSM-5 are not large enough to allow water and acetic acid to pass each other. This might be an additional contribution to the significant decrease in the water flux in the presence of acetic acid. Fig. 7 shows that the separation factor decreased at higher concentrations because of a significant decrease in the water flux at high concentrations of acetic acid, whereas being almost constant at low acetic acid concentrations. This behavior of separation factor should not be attributed to the deterioration of the membrane since both the water and acetic acid fluxes decreased at higher acetic acid concentrations. This phenomenon is similar to that observed with mordenite membranes [31]. The significant decrease in the water flux at higher acetic acid concentrations might be related to the presence of the association equilibrium between water and acetic acid molecules. At low acetic acid concentrations, water molecules are in excess of acetic acid molecules. It can be expected that water molecules would still be predominantly present
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in the form of isolated water molecules. Therefore, the permeation of water is less influenced by the presence of the association equilibrium. At higher acetic acid concentrations, however, acetic acid molecules are in excess of water molecules. The decrease in the concentration of isolated water molecule might be responsible for the decrease in the water flux and thus the separation factor. With regard to the decrease in the acetic acid flux at higher concentrations, it might be due to the formation of acetic acid dimers. The larger size of dimer makes it more difficult to diffuse through the pores, leading to a decrease in the acetic acid flux. It is important to note that the influence of acetic acid on the permeation of water depends on the nature of the membrane material. The ratio (θ) of the observed water flux to the ideal one (the product of the weight fraction of water in the feed mixture and the pure water flux) was adopted here to characterize the influence of acetic acid molecules on the permeation of water through a zeolite membrane. The term of θ was first proposed by Huang and Yeom [33]. The deviation of θ from unity is considered as a measure of the effect of acetic acid on the permeation of water. Fig. 9 shows the dependence of θ as a function of the weight fraction of acetic acid in the feed for mordenite
Fig. 9. Dependence of θ on the weight fraction of acetic acid in feed for M5 membrane and a mordenite membrane (M1004). M1004 membrane was prepared on the outer surface of a seeded porous alumina tube in a clear solution containing 10Na2 O:0.15Al2 O3 :36SiO2 :440H2 O at 453 K for 4 h [31].
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membrane (M1004 sample in [31]) and for a ZSM-5 zeolite membrane (M5 sample in this study). The influence of acetic acid molecules on the permeation of water is clearly more pronounced for M5 membrane than for M1004 membrane. This might be explained that M1004 membrane is more hydrophilic than M5 membrane.
It can be seen from the results with M6 sample that the alkali treatment is practically effective to improve both the water flux and the separation factor. After the treatment in a 0.1 mol dm−3 NaOH solution at 298 K for 2 h, the water flux at 343 K increased from 84 to 284 g m−2 h−1 and the separation factor increased from 5 to 14 for a 50/50 (w/w) water–acetic acid mixture. After a prolonged treatment (17 h at 298 K), however, slight decreases of both the water flux and the separation factor were observed. An excess dissolution of crystals possibly produced small pinholes in the membrane. The effect of treatment temperature was also studied using M6-M sample. The water flux increased from 259 to 783 g m−2 h−1 and the separation factor increased from 31 to 381 for a 50/50 (w/w) water–acetic acid mixture after the treatment in a 0.1 mol dm−3 NaOH solution at 343 K for 2 h. Similarly to the case of M6 sample, both the water flux and the separation factor were reduced when the treatment time was longer than 2 h. Alkali treatment at higher temperatures accelerated the excess dissolution of crystals and the membrane was thus damaged more rapidly. ZSM-5 zeolite membrane was damaged much more rapidly when being treated in a concentrated NaOH solution, as suggested with M4-M sample in Table 3. In summary, the pervaporation performance of ZSM-5 zeolite membranes could be improved by alkali treatment under appropriate conditions. This is probably due to the improvement in the hydrophilicity of the membrane surface resulting from the selective
3.3. Modification of membrane surface for hydrophilicity improvement As discussed previously, the water flux and the separation factor for water–acetic acid mixtures through ZSM-5 zeolite membrane were considerably reduced by the preferential adsorption of acetic acid molecules. We assumed that the separation properties would be improved if the water–membrane interaction is strengthened and/or the acetic acid–membrane interaction is weakened. It has been reported that alkali treatment of zeolite crystals could selectively extract Si atoms in the framework without damage to the microstructure [34–37]. This means that the zeolite crystals would become more hydrophilic after alkali treatment. In this study, we investigated the influence of alkali treatment of as-synthesized ZSM-5 zeolite membranes on their pervaporation performance for water–acetic acid mixtures. Several ZSM-5 zeolite membranes were treated in an alkaline solution and their pervaporation properties before and after the alkali treatment are summarized in Table 3.
Table 3 Effect of alkali treatment of ZSM-5 zeolite membranes on their pervaporation properties for a 50/50 (w/w) water–acetic acid mixture at 343 K Sample
NaOH treatment Concentration
(mol dm−3 )
Temperature (K)
Time (h)
Water flux (g m−2 h−1 )
Acetic acid flux (g m−2 h−1 )
Separation factor
M6
– 0.1 0.1
– 298 298
– 2 17
84 284 243
16 17 24
5 14 10
M6-M
– 0.1 0.1 0.1
– 343 343 343
– 2 4 6
259 783 363 282
8 2 13 196
31 381 27 1.4
M4-M
– 0.2
– 343
– 1
286 1467
11 1332
25 1.1
M6-M and M4-M samples were obtained, respectively, after a second hydrothermal treatment of M6 and M4 samples at 453 K for 2 h.
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dissolution of Si atoms in the framework of the crystals in the alkaline solution.
4. Conclusions Thin ZSM-5 zeolite membranes were successfully prepared on the surface of a porous ␣-alumina tube by the seed-assisted crystallization method in the absence of organic templates. The water selectivities in the pervaporation of water–acetic acid mixture were higher than those by vapor–liquid equilibrium in the entire range of acetic acid concentration. The selective separation of water from aqueous acetic acid solution was attributed to the faster diffusion of water than acetic acid, while the water permeation was significantly influenced by the preferential adsorption of acetic acid. It was found that the permeation coefficients of both water and acetic acid decreased with temperature, although the fluxes of water and acetic acid increased with temperature. These observations led us to conclude that the increases in the pervaporation fluxes of water and acetic acid with temperature are due to the increase in the driving force in terms of transmembrane partial vapor pressure difference. A study was also carried out on the modification of ZSM-5 zeolite membrane by alkali treatment. Both the water flux and the separation factor were improved by alkali treatment under appropriate conditions. References [1] X. Feng, R.Y.M. Huang, Liquid separation by membrane pervaporation: a review, Ind. Eng. Chem. Res. 36 (1997) 1048. [2] Y. Kondo, Y.M. Morigami, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Sep. Purif. Technol. 25 (2001) 251. [3] S.G. Li, V.A. Tuan, R.D. Noble, J.L. Falconer, Pervaporation of water/THF mixtures using zeolite membranes, Ind. Eng. Chem. Res. 40 (2001) 4577. [4] K. Okamoto, H. Kita, K. Horii, K. Tanaka, M. Kondo, Zeolite NaA membrane: preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures, Ind. Eng. Chem. Res. 40 (2001) 163. [5] D. Shah, K. Kissick, A. Ghorpade, R. Hannah, D. Bhattacharyya, Pervaporation of alcohol–water and dimethylformamide–water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results, J. Membr. Sci. 179 (2000) 185.
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[30] J. Gmehling, U. Onken, W. Arlt, Vapor–liquid Equilibrium Data Collection, Dechema, Frankfurt/Main, 1981. [31] 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. [32] J.G. Wijmans, R.W. Baker, The solution–diffusion model: a review, J. Membr. Sci. 107 (1995) 1. [33] R.Y.M. Huang, C.K. Yeom, Pervaporation separation of aqueous mixtures using crosslinked polyvinyl alcohol membranes. Part III. Permeation of acetic acid–water mixtures, J. Membr. Sci. 58 (1991) 33. [34] R.M. Dessau, E.W. Valyocsik, N.H. Goeke, Aluminum zoning in ZSM-5 as revealed by selective silica removal, Zeolites 12 (1992) 776. [35] R.L.V. Mao, S. Xiao, A. Ramsaran, J. Yao, Selective removal of silicon from zeolite frameworks using sodium carbonate, J. Mater. Chem. 4 (1994) 605. [36] M. Ogura, S. Shinomiya, J. Tateno, Y. Nara, M. Nomura, E. Kikuchi, M. Matsukata, Alkali-treatment technique-new method for modification of structural and acid-catalytic properties of ZSM-5 zeolites, Appl. Catal. A: Gen. 219 (2001) 33. [37] T. Suzuki, T. Okuhara, Change in pore structure of MFI zeolite by treatment with NaOH aqueous solution, Microporous Mesoporous Mater. 43 (2001) 83.
A study on the pervaporation of water–acetic acid mixtures through ZSM-5 zeolite membranes Gang Li, Eiichi Kikuchi, Masahiko Matsukata∗ Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Received 21 October 2002; accepted 3 April 2003
Abstract A study was carried out on the pervaporation of water from water–acetic acid mixtures through tubular ZSM-5 zeolite membranes. ZSM-5 zeolite membranes were prepared on the outer surface of a porous ␣-alumina tube by the seed-assisted crystallization method in the absence of organic templates. It was observed that water preferentially permeated with selectivities higher than the selectivity based on the vapor–liquid equilibrium in the entire range of acetic acid concentration. The separation mechanism of water through ZSM-5 zeolite membranes was discussed and compared with that through mordenite membranes. It was found that the permeability coefficients of both water and acetic acid decreased with increasing temperature, although the pervaporation fluxes of both water and acetic acid increased. As a result, it was concluded that the increase in the pervaporation flux with temperature was due to the increase in the driving force in terms of transmembrane partial vapor pressure difference with temperature. According to the mechanism proposed herein, a study was also directed to modify as-synthesized membranes for the improvement in their pervaporation performance. It was observed that both the water flux and the separation factor were increased through a membrane treated in alkaline solution under appropriate conditions. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; ZSM-5 zeolite membrane; Water–acetic acid mixtures; Modification; Alkali treatment
1. Introduction Pervaporation is one of the effective membranebased processes for separation of liquid mixtures such as azeotropic, close-boiling, and heat-sensitive compounds [1]. In recent years, use of zeolite membranes for pervaporation has attracted considerable attention. In general, hydrophilic zeolite membranes have been used for dehydration of organic solvents [2–11]. On the other hand, hydrophobic zeolite membranes have been ∗ Corresponding author. Tel.: +81-3-5286-3850; fax: +81-3-5286-3850. E-mail address: [email protected] (M. Matsukata).
used for separation of organics from water [12–20]. Besides, a few studies have been reported on the separation of organic compounds or multi-component mixtures [21–29]. Acetic acid is one of the most important intermediates in chemical industry. As the relative volatility of water to acetic acid is close to unity [30], much energy is required for distillation to separate acetic acid from aqueous mixtures. Consequently, pervaporation is regarded as one of the potential processes for separation of aqueous acetic acid mixtures. A few studies have been reported on the pervaporation of acetic acid from water using hydrophobic zeolite membranes [13,17]. However, there are few studies on the use of hydrophilic zeolite membranes for separation of water
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00172-8
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from acetic acid, which would be desirable for purification of acetic acid and also for application to membrane reactors like in esterification. Although NaA zeolite membranes are very effective to remove water from organic compounds [2], they cannot be applied for dehydration of acetic acid due to their low resistance to acid. As the chemical stability of zeolite increases with increasing Si/Al ratios in the framework while the hydrophilicity decreases, zeolites having medium Si/Al ratios would be preferred for separation of water from acetic acid. In a previous paper [31], we have reported the pervaporation of water from acetic acid using thin mordenite membranes. Here we report the pervaporation of water from acetic acid using thin ZSM-5 zeolite membranes and the comparison of these two types of membranes.
2. Experimental 2.1. Membrane preparation ZSM-5 zeolite membranes were prepared on the outer surface of a porous asymmetric ␣-Al2 O3 tube with an average pore size of 100 nm (NGK Insulators, Japan). Prior to crystallization, the outer surface of the tube was seeded by means of dip coating in a colloidal ZSM-5 crystal suspension (solid content = 3.8 g/l, pH = 7.4), which was prepared from commercially available ZSM-5 powder (Si/Al = 11.9, Tosoh Co.) according to a procedure described previously [7]. A given amount of ZSM-5 powder was first ground in an agate mortar. The ground powders were then mixed with an appropriate amount of water in a beaker to form a slurry. The slurry was treated in an ultrasonic bath. Finally the slurry was kept at room temperature for several days. Due to the gravitational effect, bigger particles were settled down to the bottom of the beaker while ultra-small particles were dispersed in the upper part of slurry resulting in a milky suspension. This suspension was used for seeding. The seeded tube was immersed vertically in a Teflon-lined stainless steel autoclave and filled with an organic template-free gel. The gel had a molar composition of 26.75Na2 O:Al2 O3 :100SiO2 :4600H2 O as prepared by mixing distilled water, sodium hydroxide (Kanto Chemical Co.), sodium aluminate (Kanto
Chemical Co.), and colloidal silica ST-S (Nissan Chemical Ind.). The gel was stirred at 323 K for 4 h prior to use. The autoclave was placed in a preheated oven for hydrothermal treatment at 453 K for a given period of time. Then, the membrane was boiled in deionized water and dried at ambient conditions prior to characterizations and pervaporation tests. 2.2. Characterization The crystalline phase of the zeolite layer formed on the support surface was determined using a Rigaku RINT 2000 X-ray diffractometer (XRD) equipped with a Cu K␣ radiation source at 40 kV and 20 mA at a scanning speed of 2◦ (2θ) min−1 . The nanosized particles in the colloidal suspension and the seeded support were characterized using a Hitachi S4500S field emission scanning electron microscope (FE-SEM) operated at 15 keV. The morphology of the zeolite layer was examined by means of a Hitachi S-2150 scanning electron microscope (SEM) operated at 15 keV. To characterize the quality of the prepared membrane, single-gas permeances were measured for H2 , n-C4 H10 , and i-C4 H10 by sealing the membrane in a stainless steel module with graphite cylindrical rings. Prior to the gas permeation tests, the prepared membrane was dried at 473 K for longer than 12 h. The permeances were measured using the pressure-drop method. The pressure difference across the membrane was kept at 100 kPa and the permeate side was kept at atmospheric pressure. The ratio of single-gas permeances is referred to as the ideal selectivity. 2.3. Pervaporation test All the pervaporation experiments were performed using a stainless steel tube module cell. The membrane tube was placed inside the module cell and sealed using graphite cylindrical rings at the both ends. The feed was circulated at a flow rate of 26 cm3 min−1 from a feed tank of 1000 cm3 through the outer side of the membrane tube, while the inside was evacuated by use of an oil rotary vacuum pump. The stainless steel module cell was heated with a ribbon heater. A thermocouple was inserted into the module cell and placed close to the outer surface of the membrane. The feed in the tank was heated to about 2 K higher than the membrane surface.
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where YW /YA is the weight ratio of water to acetic acid in permeate, and XW /XA is that in feed. 2.4. Membrane modification by alkali treatment The alkali treatment of ZSM-5 membranes was carried out in the pervaporation apparatus without taking the membrane out of the module. The outer surface of membrane was in contact with a flow of NaOH solution. Between alkali treatment and pervaporation test, the membrane and the line were first washed in a one-way flow of deionized water at room temperature and further rinsed in a circulation of deionized water at 353 K for 2–4 h. Fig. 1. The water and acetic acid fluxes as a function of pervaporation time for a 50 wt.% acetic acid aqueous solution at 353 K through a ZSM-5 zeolite membrane (sample M5).
6.28 cm2 .
The effective membrane area was The permeate was condensed in a cold trap cooled by liquid nitrogen. The permeate was weighed and analyzed every 1–3 h of pervaporation time until the permeation reached a steady state. The relative deviations in the water and acetic acid fluxes were less than ±10%. Fig. 1 shows a typical pervaporation time dependence of the water and acetic acid fluxes through a ZSM-5 zeolite membrane. It can be seen that the reproducibility is good. The compositions of feed and permeate were determined by means of a gas chromatograph (Shimadzu GC-8A) equipped with a 2 m long stainless steel column packed with Porapak QS. The separation factor, αW/A , was defined by αW/A =
YW /YA XW /XA
3. Results and discussion 3.1. Membrane preparation and characterization Fig. 2 shows the FE-SEM images for ZSM-5 crystal seeds and the surface of a seeded support. It can be seen that the seeds were located in the voids between the alumina particles and some were attached to the alumina particles. Fig. 3 shows the surface and the cross-sectional SEM images for a zeolite membrane grown on a seeded alumina support after hydrothermal treatment for 12 h at 453 K. Surface SEM reveals that the surface of the support was completely covered with zeolite crystals. These crystals were intergrown so highly that the shape and size of the crystal could hardly be identified. The thickness of the zeolite layer was about 2 m, as shown in the cross-sectional SEM image.
Fig. 2. FE-SEM images. (A) ZSM-5 crystal seeds; (B) top surface of a seeded support.
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Fig. 3. SEM images for a zeolite membrane grown on a seeded ␣-Al2 O3 support at 453 K for 12 h in a gel with the molar composition of 26.75Na2 O:100SiO2 :Al2 O3 :4600H2 O. (A) Top surface; (B) cross-section.
Fig. 4 shows the XRD patterns for the zeolite membrane and commercially available ZSM-5 zeolite powder. The diffraction pattern for the membrane was identical to that for the ZSM-5 powder, indicating that the crystals formed on the support surface have MFI-topology. Fig. 5 shows the single-gas permeances of H2 , n-C4 H10 and i-C4 H10 at 473 K as a function of the permeation time. Note that kinetic diameters of H2 (0.289 nm) and n-C4 H10 (0.43 nm) are smaller whereas kinetic diameter of i-C4 H10 (0.5 nm) is close to the pore size (0.5–0.6 nm) of ZSM-5 crystal. The steady single-gas permeances at 473 K were 5.5 × 10−8 mol s−1 m−2 Pa−1 for
H2 , 1.7 × 10−8 mol s−1 m−2 Pa−1 for n-C4 H10 , and 3.8 × 10−9 mol s−1 m−2 Pa−1 for i-C4 H10 . The ideal selectivities at 473 K are 14.5 for H2 /i-C4 H10 and 4.4 for n/i-C4 H10 . These selectivities are three to four times higher than the corresponding Knudsen selectivities (5.4 and 1, respectively), indicating that the membrane has a low concentration of interzeolitic pores of size larger than zeolitic pores. 3.2. Pervaporation of water–acetic acid mixtures Table 1 summaries the pervaporation results for a 50/50 (w/w) water–acetic acid mixture through ZSM-5 zeolite membranes at 343 K. It can be seen that
Fig. 4. XRD patterns for (a) ZSM-5 powder; (b) surface of a zeolite membrane grown on a seeded ␣-Al2 O3 support at 453 K for 12 h in a gel with the molar composition of 26.75Na2 O:100SiO2 :Al2 O3 :4600H2 O.
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Fig. 5. Single-gas permeances at 473 K for H2 , n-C4 H10 and i-C4 H10 through a ZSM-5 membrane prepared at 453 K for 12 h.
water molecules preferentially permeated through the ZSM-5 membranes. It is interesting to note that the membranes showed very high fluxes (5–7 kg m−2 h−1 ) for pure water. However, the water fluxes were quite low (0.1–0.3 kg m−2 h−1 ) for the mixture. 3.2.1. Effect of temperature Fig. 6 shows the effect of temperature on the pervaporation results. Both water and acetic acid fluxes increased with temperature, while the separation factor was almost independent of temperature. Pervaporation differs from other membrane processes such as reverse osmosis and gas permeation because it involves a phase change of the permeating species. The temperature dependence of the pervaporation flux generally exhibits an Arrhenius-
189
Fig. 6. Effect of temperature on the pervaporation of a 50/50 (w/w) water–acetic acid mixture through M5 membrane.
type relation EJ J = J0 exp − RT
(1)
where EJ is the apparent activation energy characterizing the overall temperature dependence of the permeation flux [1]. If the pervaporation through a microporous membrane can be described by the sorption–diffusion model, the flux equation can be written as follows [32]: K K J= (pf − pp ) = p (2) l l where K is the permeability coefficient, l the membrane thickness, and pf and pp are the partial vapor pressures of the permeant in feed and permeate sides,
Table 1 Pervaporation results for pure water and ca. 50 wt.% acetic acid aqueous solution through ZSM-5 zeolite membranes at 343 K Membrane
M1 M2 M3 M4 M5 M6 M7 M8
Synthesis time (h)
Pure water flux (kg m−2 h−1 )
Feed H2 O (wt.%)
Water–acetic acid mixture Water flux (kg m−2 h−1 )
Acetic acid flux (kg m−2 h−1 )
αH2 O/CH3 COOH
6 6 12 12 12 12 19 19
>15 >15 7.7 5.2 5.7 5.5 6.2 7.3
– – 53.3 53.5 52.2 53.0 54.1 54.0
– – 0.28 0.26 0.32 0.08 0.28 0.61
– – 0.03 0.01 0.02 0.02 0.02 0.02
– – 8 15 14 5 9 23
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respectively. p is the transmembrane partial pressure difference and defined as the mass transport driving force. As the permeate side is under vacuum in our study, p can thus be considered to be close to the partial vapor pressure in the liquid feed side. Consequently, we can obtain K J= (3) pf l The temperature dependence of K and pf can be expressed as follows: EP K = K0 exp − (4) RT HV pf = A exp − (5) RT where EP is the activation energy of permeation, HV the molar enthalpy of vaporization, and both K0 and A are the pre-exponential factors. Combining Eqs. (3)–(5) yields K0 A EP + HV J= exp − (6) l RT Comparing Eq. (1) with Eq. (6), thus EJ = EP + HV
(7)
As EJ and HV can be evaluated from the slopes of the plots of ln J versus 1/T and ln pf versus 1/T, respectively, EP can thus be obtained using Eq. (7). The calculated EJ , HV and EP data are listed in Table 2, where HV values were obtained using the partial vapor pressure data in [30]. It is interesting to note that the values of EP are negative for both water and acetic acid. These values indicate that the permeability coefficients of both water and acetic acid through ZSM-5 membrane decrease with increasing temperature, although the pervaporation fluxes of water and acetic acid increase with temperature. As a result, we conclude that the increase
Fig. 7. Effect of feed composition on the pervaporation results at 353 K through M5 membrane.
in both water and acetic acid fluxes with increasing temperature is due to the increase in the driving force in terms of transmembrane partial vapor pressure difference. 3.2.2. Effect of the mixture composition Fig. 7 shows the effect of feed composition on the pervaporation results. As the concentration of acetic acid increased, not only the water flux but also the acetic acid flux decreased. The separation factor also decreased with acetic acid concentration. Fig. 8 shows that the water selectivities on the basis of pervaporation are higher than those on the basis of
Table 2 EJ , HV , and EP data for a 50/50 (w/w) water–acetic acid mixture through M5 membrane
Water Acetic acid
EJ (kJ mol−1 )
HV (kJ mol−1 )
EP (kJ mol−1 )
35 37
43 41
−8 −4
Fig. 8. The mole fraction of acetic acid in permeate as a function of that in feed.
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vapor–liquid equilibrium in the entire range of acetic acid concentration. Since both water (ca. 0.28 nm) and acetic acid (ca. 0.4 nm) molecules are smaller than the pore size of ZSM-5 zeolite (ca. 0.6 nm), molecular sieving effect would play a minor role in the pervaporation of water from acetic acid. Thus, the separation of water from acetic acid would be due to the difference either in the adsorption or in the diffusion properties between water and acetic acid molecules through the membrane. Water permeation was considerably affected by acetic acid molecules. As shown in Table 1, the water flux for 50 wt.% acetic acid aqueous solution is about 1/10 to 1/20 of that for pure water at the same pervaporation temperature. The significant decrease in the water flux might be due to the preferential adsorption of acetic acid molecules, resulting in a significant decrease in the number of the adsorbed water molecules. While the number of adsorbed water molecules would be less than that of adsorbed acetic acid molecules, adsorbed water molecules can diffuse faster than adsorbed acetic acid molecules due to their smaller size, leading to the separation of water from acetic acid. In addition, acetic acid molecules entering the zeolitic pores should hinder the diffusion of water molecules, possibly due to the single-file diffusion since the zeolitic pores of ZSM-5 are not large enough to allow water and acetic acid to pass each other. This might be an additional contribution to the significant decrease in the water flux in the presence of acetic acid. Fig. 7 shows that the separation factor decreased at higher concentrations because of a significant decrease in the water flux at high concentrations of acetic acid, whereas being almost constant at low acetic acid concentrations. This behavior of separation factor should not be attributed to the deterioration of the membrane since both the water and acetic acid fluxes decreased at higher acetic acid concentrations. This phenomenon is similar to that observed with mordenite membranes [31]. The significant decrease in the water flux at higher acetic acid concentrations might be related to the presence of the association equilibrium between water and acetic acid molecules. At low acetic acid concentrations, water molecules are in excess of acetic acid molecules. It can be expected that water molecules would still be predominantly present
191
in the form of isolated water molecules. Therefore, the permeation of water is less influenced by the presence of the association equilibrium. At higher acetic acid concentrations, however, acetic acid molecules are in excess of water molecules. The decrease in the concentration of isolated water molecule might be responsible for the decrease in the water flux and thus the separation factor. With regard to the decrease in the acetic acid flux at higher concentrations, it might be due to the formation of acetic acid dimers. The larger size of dimer makes it more difficult to diffuse through the pores, leading to a decrease in the acetic acid flux. It is important to note that the influence of acetic acid on the permeation of water depends on the nature of the membrane material. The ratio (θ) of the observed water flux to the ideal one (the product of the weight fraction of water in the feed mixture and the pure water flux) was adopted here to characterize the influence of acetic acid molecules on the permeation of water through a zeolite membrane. The term of θ was first proposed by Huang and Yeom [33]. The deviation of θ from unity is considered as a measure of the effect of acetic acid on the permeation of water. Fig. 9 shows the dependence of θ as a function of the weight fraction of acetic acid in the feed for mordenite
Fig. 9. Dependence of θ on the weight fraction of acetic acid in feed for M5 membrane and a mordenite membrane (M1004). M1004 membrane was prepared on the outer surface of a seeded porous alumina tube in a clear solution containing 10Na2 O:0.15Al2 O3 :36SiO2 :440H2 O at 453 K for 4 h [31].
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membrane (M1004 sample in [31]) and for a ZSM-5 zeolite membrane (M5 sample in this study). The influence of acetic acid molecules on the permeation of water is clearly more pronounced for M5 membrane than for M1004 membrane. This might be explained that M1004 membrane is more hydrophilic than M5 membrane.
It can be seen from the results with M6 sample that the alkali treatment is practically effective to improve both the water flux and the separation factor. After the treatment in a 0.1 mol dm−3 NaOH solution at 298 K for 2 h, the water flux at 343 K increased from 84 to 284 g m−2 h−1 and the separation factor increased from 5 to 14 for a 50/50 (w/w) water–acetic acid mixture. After a prolonged treatment (17 h at 298 K), however, slight decreases of both the water flux and the separation factor were observed. An excess dissolution of crystals possibly produced small pinholes in the membrane. The effect of treatment temperature was also studied using M6-M sample. The water flux increased from 259 to 783 g m−2 h−1 and the separation factor increased from 31 to 381 for a 50/50 (w/w) water–acetic acid mixture after the treatment in a 0.1 mol dm−3 NaOH solution at 343 K for 2 h. Similarly to the case of M6 sample, both the water flux and the separation factor were reduced when the treatment time was longer than 2 h. Alkali treatment at higher temperatures accelerated the excess dissolution of crystals and the membrane was thus damaged more rapidly. ZSM-5 zeolite membrane was damaged much more rapidly when being treated in a concentrated NaOH solution, as suggested with M4-M sample in Table 3. In summary, the pervaporation performance of ZSM-5 zeolite membranes could be improved by alkali treatment under appropriate conditions. This is probably due to the improvement in the hydrophilicity of the membrane surface resulting from the selective
3.3. Modification of membrane surface for hydrophilicity improvement As discussed previously, the water flux and the separation factor for water–acetic acid mixtures through ZSM-5 zeolite membrane were considerably reduced by the preferential adsorption of acetic acid molecules. We assumed that the separation properties would be improved if the water–membrane interaction is strengthened and/or the acetic acid–membrane interaction is weakened. It has been reported that alkali treatment of zeolite crystals could selectively extract Si atoms in the framework without damage to the microstructure [34–37]. This means that the zeolite crystals would become more hydrophilic after alkali treatment. In this study, we investigated the influence of alkali treatment of as-synthesized ZSM-5 zeolite membranes on their pervaporation performance for water–acetic acid mixtures. Several ZSM-5 zeolite membranes were treated in an alkaline solution and their pervaporation properties before and after the alkali treatment are summarized in Table 3.
Table 3 Effect of alkali treatment of ZSM-5 zeolite membranes on their pervaporation properties for a 50/50 (w/w) water–acetic acid mixture at 343 K Sample
NaOH treatment Concentration
(mol dm−3 )
Temperature (K)
Time (h)
Water flux (g m−2 h−1 )
Acetic acid flux (g m−2 h−1 )
Separation factor
M6
– 0.1 0.1
– 298 298
– 2 17
84 284 243
16 17 24
5 14 10
M6-M
– 0.1 0.1 0.1
– 343 343 343
– 2 4 6
259 783 363 282
8 2 13 196
31 381 27 1.4
M4-M
– 0.2
– 343
– 1
286 1467
11 1332
25 1.1
M6-M and M4-M samples were obtained, respectively, after a second hydrothermal treatment of M6 and M4 samples at 453 K for 2 h.
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dissolution of Si atoms in the framework of the crystals in the alkaline solution.
4. Conclusions Thin ZSM-5 zeolite membranes were successfully prepared on the surface of a porous ␣-alumina tube by the seed-assisted crystallization method in the absence of organic templates. The water selectivities in the pervaporation of water–acetic acid mixture were higher than those by vapor–liquid equilibrium in the entire range of acetic acid concentration. The selective separation of water from aqueous acetic acid solution was attributed to the faster diffusion of water than acetic acid, while the water permeation was significantly influenced by the preferential adsorption of acetic acid. It was found that the permeation coefficients of both water and acetic acid decreased with temperature, although the fluxes of water and acetic acid increased with temperature. These observations led us to conclude that the increases in the pervaporation fluxes of water and acetic acid with temperature are due to the increase in the driving force in terms of transmembrane partial vapor pressure difference. A study was also carried out on the modification of ZSM-5 zeolite membrane by alkali treatment. Both the water flux and the separation factor were improved by alkali treatment under appropriate conditions. References [1] X. Feng, R.Y.M. Huang, Liquid separation by membrane pervaporation: a review, Ind. Eng. Chem. Res. 36 (1997) 1048. [2] Y. Kondo, Y.M. Morigami, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Sep. Purif. Technol. 25 (2001) 251. [3] S.G. Li, V.A. Tuan, R.D. Noble, J.L. Falconer, Pervaporation of water/THF mixtures using zeolite membranes, Ind. Eng. Chem. Res. 40 (2001) 4577. [4] K. Okamoto, H. Kita, K. Horii, K. Tanaka, M. Kondo, Zeolite NaA membrane: preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures, Ind. Eng. Chem. Res. 40 (2001) 163. [5] D. Shah, K. Kissick, A. Ghorpade, R. Hannah, D. Bhattacharyya, Pervaporation of alcohol–water and dimethylformamide–water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results, J. Membr. Sci. 179 (2000) 185.
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