- Email: [email protected]
formaldehyde polymer
Journal of Membrane Science 379 (2011) 52–59
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Pervaporation dehydration performance of microporous carbon membranes prepared from resorcinol/formaldehyde polymer Shunsuke Tanaka a,b,∗ , Tomohisa Yasuda a , Yugo Katayama a , Yoshikazu Miyake a,b a Department of Chemical, Energy and Environmental Engineering, Faculty of Environmental and Urban Engineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680, Japan b High Technology Research Center, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680 Japan
a r t i c l e
i n f o
Article history: Received 10 February 2011 Received in revised form 11 April 2011 Accepted 22 May 2011 Available online 27 May 2011 Keywords: Carbon membrane Resorcinol/formaldehyde Pervaporation Acetic acid Sulfonation Durability
a b s t r a c t Microporous carbon membranes for pervaporation applications were prepared on a porous ␣-alumina support by a partially carbonization of a resorcinol/formaldehyde resin. The stability and dehydration performances of the carbon membranes were determined. The carbon membranes were used for the dehydration of several organic solvents (methanol, ethanol, i-propanol, and acetic acid) containing water; it was found that water was selectively permeated through the membrane and the separation factor increased with the molecular diameter of the organic solvents. The high selectivity to water can be explained by not only the hydrophilic nature of the pore surface but also the molecular sieving effect. Furthermore, the membranes showed high durability in the pervaporation of water/alcohol mixtures. On the other hand, the membranes were unstable in water/acetic acid mixture. However, the sulfonated carbon membranes were stable in pervaporation of water/acetic acid mixture and maintained their separation properties. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The global focus on green technologies requires the development of separation processes, which account for about 40% of the total energy consumption in chemical and petrochemical industries worldwide. Currently, the alcohol dehydration processes become increasingly important because of the advancements in the development of renewable biomass and biofuel technologies. Additionally, acetic acid with low water content is also needed in various chemical processes, such as production of vinyl acetate monomer, terephthalic acid, and acetate esters. Pervaporation is a promising technique and practical alternative to facilitate alcohol and acetic acid dehydration [1,2]. This membrane-based separation technique permits the separation azeotrope, close-boiling mixtures, and thermally degradable organic mixtures. In addition, pervaporation possesses many advantages: easy process design, high selectivity, low energy consumption, and moderate cost-toperformance ratio. Polymeric membranes are widely adopted as separation materials in the field of pervaporation for solvent dehydration and
∗ Corresponding author at: Department of Chemical, Energy and Environmental Engineering, Faculty of Environmental and Urban Engineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680, Japan. Tel.: +81 6 6368 0851; fax: +81 6 6388 8869. E-mail address: shun [email protected] (S. Tanaka). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.05.046
separation of organic mixtures. However, polymeric membranes are often unstable at high temperatures. The applications of polymeric membranes are further restricted by unfavorable swelling, which subsequently results in the decline of their separation properties with respect to time [3]. Moreover, they cannot be used in concentrated acetic acid mixtures due to the low acid tolerance. Inorganic microporous membranes such as silica [4–6] and zeolite [7–10] have been developed as promising materials to overcome the above chemical and thermal instabilities. In general, they possess stable separation performance at high feed or water concentrations and elevated temperatures. Therefore, the separation operation can be applied for a broad range of applications and over an extended time period. Nevertheless, there are drawbacks reported in the preparation and separation stability of the inorganic microporous membranes. In addition to the chemical instability of silica against water and alkaline solutions [11], membranes are thick to form defect-free zeolite membranes [12]. Although well-known zeolite LTA membranes possess a high separation performance for water/alcohol systems, they have a major drawback of acid-sensitivity. Microporous carbon membranes offer the best candidates for the development of new membrane technologies, because of their advantages, such as excellent permeation selectivity, high hydrothermal stability, and high corrosion resistance [13]. Furthermore, their microstructure and surface properties can be altered by post-treatment. Due to their highly porous structures with a sharp distribution of pore sizes ranging from 0.3 to 0.6 nm, the
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
carbon membranes are suitable for the gas separations such as H2 /N2 , O2 /N2 , CO2 /N2 , and CO2 /CH4 . Research efforts have been focused on the gas separation [13–20]. However, very few investigations have been reported on the application of the carbon membranes for pervaporation [20,21]. Furthermore, to the best of our knowledge, there are no investigations on the application of the carbon membranes for dehydration of acetic acid. Suitable carbon precursors for molecular sieve membrane production must not cause any crack formation after a pyrolysis step. A thermosetting phenolic resin can withstand relatively high temperatures and possess surface-modifiable phenolic hydroxyl groups. In this study, microporous carbon membranes for pervaporation applications were prepared on a porous ␣-alumina support by a partially carbonization of a resorcinol/formaldehyde resin. In addition, we attempt to chemically modify the carbon surface by sulfonation treatment. The separation performances of the carbon membranes were investigated for the pervaporation separation of water/alcohol mixtures. Furthermore, we use the membranes for pervaporation separation of water/acetic acid mixture and investigate the effect of sulfonation on the stability and dehydration performance.
composition and thickness of the membranes were measured by scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analysis on a VE-8800 microscope (Keyence). The field emission scanning electron microscope (FESEM) images were recorded on an S-5000L Hitachi microscope at an acceleration voltage of 22 kV. No coating was carried out for the samples before the SEM and FESEM measurements. Raman spectra were recorded with a NRS3100 spectrometer (JASCO) using a 532 nm laser as an excitation source. The amount of CO2 adsorption on carbon powdery samples was measured at 298 K using a BELSORP 28 instrument (Bel Japan, Inc.). The carbon powdery samples were prepared on a non-porous silicon substrate and scratched from the substrate. 2.4. Pervaporation Pervaporation experiments were performed using several organic solvents (methanol, ethanol, i-propanol, and acetic acid) containing water. The carbon membrane side faced the feed side. In the experimental apparatus, the downstream compartment was evacuated, and the permeate was collected in a vacuum trap condenser cooled using liquid nitrogen. The permeation flux, J, is defined by
2. Experimental 2.1. Materials Resorcinol, formaldehyde (36–38 wt%), 5 N NaOH, 2 N H2 SO4 and ethanol were purchased from Wako Pure Chemical Industries and used as received. ␣-Alumina porous tubular supports (outer diameter: 10 mm; inner diameter: 7 mm; length: 450 mm; average porosity: 35%; average pore size: 0.1 m) were purchased from Noritake Co. Ltd. and cut into 35 mm long pieces. 2.2. Preparation of carbon membranes and sulfonation treatment A resorcinol/formaldehyde resin layer was coated on the ␣alumina layer as follows. A coating solution was prepared from resorcinol, formaldehyde, ethanol, and NaOH. In a typical synthesis, resorcinol was completely dissolved in formaldehyde solution. The above resorcinol/formaldehyde solution was added to ethanol, NaOH solution was added, and the solution was heated at 80 ◦ C for 5 min. The solution was sonicated at room temperature for 3 min before coating. The final molar composition of the coating solution was 1 resorcinol:2 formaldehyde:2 × 10−3 NaOH:6.4 ethanol:5.1 water. Membranes were prepared by dip-coating the alumina supports in the coating solutions at a withdrawal rate of 1.6 mm s−1 using a DIPCOATER DC4200 (Aiden Co., Ltd.). The coating procedure was repeated two times. For polymerization of the resorcinol with formaldehyde, the as-deposited samples were pre-heated at 100 ◦ C for 30 min in air. The resultant brown deposition was carbonized under a nitrogen atmosphere at 400–800 ◦ C for 1 h at a heating rate of 1 ◦ C min−1 . The sulfonated carbon membranes were as follows: 2 N H2 SO4 was used as a sulfonation reagent. The carbonized carbon membranes were immersed in H2 SO4 at room temperature for 1 h. After sulfonation, the membranes were rinsed with deionized water and dried at 200 ◦ C. 2.3. Characterization Fourier-transform infrared spectroscopy (FTIR) spectra of the samples were recorded in the 500–3500 cm−1 range using a IRAffinity-1 spectrometer (Shimadzu) at 4 cm−1 resolution. Thermogravimetric analysis (TGA) was performed on a DTG-60H apparatus (Shimadzu) at a heating rate of 2 ◦ C min−1 . FTIR and TGA measurements were performed with a powdery sample. The
53
Jmass =
Q , At
(1)
where Q is the weight of the collected permeate during the experimental time interval, t, and A is the effective membrane surface area (11 cm2 ). The permeation fluxes on the mass basis can be converted to the molar permeation fluxes as follows Jmole =
Jmass , 3600 M
(2)
where M is the molecular weight of component. The feed and permeate concentrations were determined by Karl Fisher titration. The separation factor of component i with respect to component j, ˛(i/j) , is defined by ˛(i/j) =
yi /yj xi /xj
,
(3)
where yi and yj are the mole fractions of component i and j in the permeate, respectively, and xi and xj are their corresponding mole fractions in the feed. 3. Results and discussion 3.1. FESEM and SEM/EDX observations SEM and FESEM images of the cross-sectional views of the carbon membrane carbonized at 400 ◦ C on the alumina support are given in Fig. 1. The carbon membrane had a smooth surface. There existed no defect from the top view of the membrane (data not shown). From the FESEM image, the membrane had a carbon dense layer approximately 1 m thick. On the other hand, as can be seen from the photograph image shown as an inset in Fig. 1, the carbon was deposited deep into the alumina pores. The molar ratios of carbon and aluminum at the positions indicated by a–f in Fig. 1 are shown in Table 1. The SEM/EDX results suggest that the resorcinol/formaldehyde resin penetrated into the pores of the porous alumina support. An effective thickness of the carbon membrane is estimated to be about 2 m. 3.2. FTIR and CO2 adsorption measurements The pyrolysis was performed in a nitrogen atmosphere at 400–800 ◦ C for 1 h at a heating rate of 1 ◦ C min−1 . The chemical
54
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
Fig. 1. SEM (left) and FESEM (right) images of the cross-sectional views of the carbon membrane carbonized at 400 ◦ C on the alumina support. The inset in FESEM image shows the photograph image of the cross-section of the membrane.
Table 1 The molar ratios of carbon and aluminum at the positions indicated by a–f in Fig. 1. The carbon membrane was carbonized at 400 ◦ C. Positions
Distance from the top surface (m)
C (%)
Al (%)
a b c d e f
0.5 2 4 20 55 115
98 18 10 8 7 8
2 82 90 92 93 92
and pore structure of the carbonized membranes were investigated by FTIR and CO2 adsorption measurements. The pore structure was relatively compared using the data of CO2 adsorption, because the samples resulted in only a small amount of N2 adsorption. The major reactions between resorcinol and formaldehyde include an addition reaction to form hydroxymethyl derivatives (–CH2 OH), and then a condensation reaction of the hydroxymethyl derivatives to form methylene (–CH2 –) and methylene ether (–CH2 OCH2 –)-bridged compound [22]. Fig. 2 shows FTIR spectra of the resorcinol/formaldehyde membranes carbonized at various temperatures. The bands can be assigned to the C–O (at 1000–1100 cm−1 ), OH bending of phenolic group (at about 1250 cm−1 ), C C (aromatic stretching) (at about 1450 cm−1 ), free water (at about 1600 cm−1 ), and C–H of methylene group stretch (at about 2900 cm−1 ). The broad band at 3000–3800 cm−1 is ascribed to the symmetrical and antisymmetrical stretching vibrations of water molecules with hydrogen bonding. The intensity of a band for C–H of methylene group stretch decreased with increasing ther-
Fig. 2. FTIR spectra of the resorcinol/formaldehyde membranes carbonized at 400–800 ◦ C.
Fig. 3. TGA curve of resorcinol/formaldehyde resin. The heating rate was 2 ◦ C min−1 under nitrogen flow.
mal treatment temperature, indicating that the carbonization of the membrane proceeds at elevated temperatures. The methylene groups remain in the membrane even after calcination at 600 ◦ C. The carbon membranes carbonized at a temperature below 600 ◦ C are still composed of an intermediate between a polymer and carbon. The weight loss of the resorcinol/formaldehyde resin became larger with increasing temperature, and the percentages of carbonaceous residues were 76%, 65%, 53%, and 47% after pyrolysis at 400, 500, 600, and 800 ◦ C, respectively (Fig. 3). On the other hand, the adsorbed CO2 volume increased with increasing temperature, as shown in Fig. 4. The porous structure is generated inside the carbonaceous framework by solidification and gasification of the polymer with increasing temperature. In the wide-angle XRD pattern, the Bragg diffraction peaks at around 2 = 26◦ and 45◦ arise from diffraction of graphitic (0 0 2) and (1 0 1) planes, respectively (data not shown). There is no large difference in XRD patterns between the carbon membranes carbonized at 400, 600, and 800 ◦ C. Raman spectroscopy was used to determine the structure of the carbon membranes because Raman spectroscopy has also been a useful tool for obtaining information on the microstructure of carbonaceous materials. The spectra featured two major peaks centered at 1350 and 1580 cm−1 (Fig. 5), which are referred to as D and G bands, respectively, for sp2 bonded carbon materials. The G band can be attributed to the in-plane carbon stretching vibrations of ideal graphene sheets, whereas the D band can be attributed to structural imperfection of graphene sheets. The intensity of G band increased with increasing carbonization temperature, indicating that the framework of carbon membrane became a more graphitized carbon wall at elevated temperatures. However, the ratio of the intensity of the G band to that of the D band was relatively
Fig. 4. CO2 adsorption isotherms of the carbon membranes carbonized at 400–800 ◦ C.
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
55
Fig. 5. Raman spectra of resorcinol/formaldehyde resin and carbon membranes carbonized at 400–800 ◦ C.
low, indicating that the carbon membranes consist of imperfect graphenes of a very small size. A micropore is the space between the nanographenes. The carbonization seemed to be mostly complete after pyrolysis at 800 ◦ C. From FTIR spectra, no broad band was observed at 3000–3800 cm−1 , indicating that the concentration of adsorbed water is extremely low. However, the band for free water can be observed even after pyrolysis at 800 ◦ C. The OH groups and water molecules with hydrogen bonding remain in the membrane even after pyrolysis at 600 ◦ C. We speculate that the membranes carbonized at a temperature below 600 ◦ C are more hydrophilic than that carbonized at 800 ◦ C. The carbon membranes have a hydrophilic surface and favors adsorption of water molecules. 3.3. Water/alcohol pervaporation The top layer of carbon membranes was exposed to water/alcohol mixtures for 1 h at 50 ◦ C. Fig. 6 shows the fluxes for water and the organic components and the separation factor as functions of the molecular kinetic diameter [23] of the organic components. For the water/methanol system, the permeation fluxes for water and methanol were 1.8 × 10−3 and 1.4 × 10−4 mol m−2 s−1 , respectively, and separation factor of water/methanol was 65. The flux for the organic components decreased with increasing the molecular diameter, and those for ethanol and i-propanol were 2.5 × 10−5 and 1.2 × 10−6 mol m−2 s−1 , respectively. On the other hand, the flux for water increased with increasing the molecule diameter of organic components. The separation factors of water/ethanol and water/i-propanol through the carbon membrane at 50 ◦ C were 380 and 6750, respectively. The high separation
Fig. 7. Effect of temperature on pervaporation performances of the carbon membrane carbonized at 400 ◦ C for 10 wt% water/i-propanol (upper). Water and i-propanol fluxes as a function of 1/T (T: 303–343) (bottom).
factors indicate that the carbon membrane has very narrow pore size distributions. These results suggest that the average pore size of the carbon membrane is in between 0.4 and 0.5 nm. The large permeation flux for methanol corresponds to the low separation factor. The methanol molecules can penetrate into the micropores of carbon membrane. Even the methanol molecules inhibited the diffusion of water molecules in the micropores. Fig. 7 shows the fluxes for water and i-propanol through the carbon membrane as a function of temperature. The fluxes increased with increasing the pervaporation temperature, and those for water and i-propanol were 3.6 × 10−3 and 2.3 × 10−6 mol m−2 s−1 at 70 ◦ C, respectively. The separation factor decreased with increasing the pervaporation temperature, and separation factor was 4150 at 70 ◦ C, indicating that the apparent activation energy for the permeation of i-propanol through the carbon membrane is larger than that of water. The effect of temperature on the flux is given by Ji = Ji0 exp
Fig. 6. Effect of penetrant molecule size on the fluxes of water and alcohols, and the separation factor. Pervaporation experiments were conducted using 10 wt% water/alcohol mixtures at 50 ◦ C. The carbon membrane was carbonized at 400 ◦ C.
−E RT
where Ji0 and E are the pre-exponential factor and activation energy, respectively. The apparent activation energies of water and ipropanol were 17.2 and 34.8 kJ mol−1 , respectively. The feed concentration of water varied from 10 to 90 wt% in the water/i-propanol separations. Fig. 8 shows the effect of the feed concentration on the separation of the water/i-propanol mixtures. The fluxes of water increased with its concentration in the feed solution. However, the i-propanol fluxes slightly decreased with increasing the feed concentration of water, indicating that the amount of adsorbed i-propanol decreased at a high concentration of water. In addition, the fluxes of water for each water/alcohol mixture were significantly lower than the pure water flux (1.6 kg m−2 h−1 ). These results indicate that water permeation was greatly blocked by alcohols, while the alcohol permeation was only slightly affected by water.
56
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
Fig. 8. Effect of feed concentration on water/i-propanol separation performance with the carbon membrane at 70 ◦ C. The carbon membrane was carbonized at 400 ◦ C.
The flux stability of the carbon membrane was monitored in dehydrating water/i-propanol system at 70 ◦ C for 196 h. As shown in Fig. 9, the membrane performance remains stable during the tested period. Literature data of pervaporation performance for i-propanol dehydration are cited from a review paper [1]. Polymeric membranes are based on organic polymer chains that are cross-linked together and/or with cross-linker. The representatives of these are poly(vinyl alcohol)- [24,27,31], chitosan[25,26,30], alginate- [27,32], and polyimides- [28,29] based membranes. Fig. 10 shows the comparison of membrane performance in this study with literature values. A more extensive literature summary on various membrane performances can be found in the review paper. Unfortunately, the direct comparison is difficult because the other types of membranes were used for the separation at different separation conditions. On the whole it can be seen that the inorganic membranes exhibit higher permeation flux than polymer-based membranes. The carbon membranes in this study exhibit comparable or even better dehydration performance compared to other types of polymeric membranes. It is worthwhile to make any further research for development and pervaporation application of carbon membranes, while the permeation flux of the carbon membranes is still lower than that of other inorganic membranes.
Fig. 10. Summary of literature data from various studies on pervaporation performance of water/i-propanol. Pervaporation experiments with polymer-based membranes (upper) and inorganic membranes (bottom) were conducted using 5–20 wt% water/i-propanol mixtures at 30–80 ◦ C. Keys, diamond: in this study, a: Ref. [24], b: Ref. [27], c: Ref. [33], d: Ref. [34], e: Ref. [26], f: Ref. [35], g: Ref. [36], h: Ref. [37], i: Ref. [29], j: Ref. [28], k: Ref. [38], l: Ref. [39], m: Ref. [25], n: Ref. [30], o: Ref. [31], p: Ref. [32], q: Ref. [40], r: Ref. [41], s: Ref. [4], t: Ref. [42], u: Ref. [43], v: Ref. [8], w: Ref. [5], x: Ref. [6], y: Ref. [20].
3.4. Water/acetic acid pervaporation Acetic acid is an important compound frequently used in the chemical industry. It is formed as an aqueous by-product in a number of processes such as the production of aspirin. The production of acetic acid itself yields water as a by-product, requiring dehydration to purify the acetic acid. However, the dehydration of acetic acid is more difficult than the dehydration of alcohols due to the acidic nature of the compound. This acidity inhibits the use of membranes that have a low stability in acid such as
Fig. 9. Membrane stability in pervaporative dehydration of water/i-propanol system. Pervaporation experiments were conducted using 10 wt% water/i-propanol mixtures at 70 ◦ C.
LTA zeolite and also damage polymeric membrane over time, requiring more chemically inert materials to be used in acidic conditions. The effect of pyrolysis temperature of the membrane on the separation performance was investigated. Fig. 11 shows the pervaporation results for the water/acetic acid mixture through the carbon membranes at 30 ◦ C for 1 h. The permeation fluxes for water and acetic acid through the membrane carbonized at 400 ◦ C were 1.8 × 10−3 and 2.3 × 10−6 mol m−2 s−1 , respectively,
Fig. 11. Fluxes of water and acetic acid and separation factor of the carbon membranes carbonized at 400–800 ◦ C. Pervaporation experiments were conducted using 10 wt% water/acetic acid mixtures at 30 ◦ C.
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
Fig. 12. Changes of pervaporation characteristics with time. Pervaporation experiments were conducted using 10 wt% water/acetic acid mixtures at 30 ◦ C. The fresh carbon membranes carbonized at 400 (upper), 500 (middle), and 600 ◦ C (bottom) were used.
and separation factor of water/acetic acid was 2090. The fluxes of water and acetic acid slightly increased with increasing pyrolysis temperatures ranging from 400 to 600 ◦ C. On the other hand, the separation factor decreased with increasing pyrolysis temperature. When the pyrolysis temperature was further increased to 800 ◦ C, a steep increase in the flux of acetic acid was observed, while the flux of water remained constant at temperatures ranging from 600 to 800 ◦ C. Then the separation factor significantly decreased to 2. The membrane carbonized at 800 ◦ C was more hydrophobic than that carbonized at a temperature below 600 ◦ C. In addition, we speculate that the pinholes and/or cracks form in the membrane after pyrolysis at 800 ◦ C. Hence, in this study, we used the membranes carbonized at temperatures ranging from 400 to 600 ◦ C for the separation performance stability tests. Fig. 12 shows a long time dependency of the separation properties of water/acetic acid mixture with the fresh membranes carbonized at 400, 500, and 600 ◦ C. The dependency of water and acetic acid fluxes and the separation factor on the separation time at a constant water concentration of 10 wt% in feed and at 30 ◦ C. The fluxes of acetic acid through the membrane carbonized at 400 ◦ C gradually increased, while the flux of water slightly decreased. The separation factor dramatically decreased to 2, although that is more than 2000 at the beginning. Similarly, when the membranes carbonized at 500 and 600 ◦ C, the fluxes of acetic acid increased and
57
that of water decreased. However, although the fluxes of acetic acid are higher at the beginning, they increased to the steady-state values smaller than that of the membrane carbonized at 400 ◦ C. Consequently, of particular interest is that the carbon membranes carbonized at 600 ◦ C have the highest steady-state separation factor because of their high resistance against acidic solutions. As shown in Fig. 12, moreover, the separation properties of the membranes carbonized at 600 ◦ C can be recovered by thermal treatment at 200 ◦ C, while those of the membranes carbonized at 400 ◦ C is irreversible. These results indicate that the deterioration behaviors of the separation properties result from different causes. We speculate that the change of separation properties with time is due to the formation of defect caused by acid dissolution of carbonaceous component and/or the surface modification of membranes. The acetic acid tolerance test was carried out using powder samples. We performed quantitative analysis using bulk powder samples because the sample weight was small due to membrane morphology. The precursor solution was left overnight at room temperature and turned into a gel. The resultant brown gel was preheated at 100 ◦ C in air. The carbonization process was performed according to the procedure described in experimental section. The 12% decrease in weight of carbonaceous powder carbonized at 400 ◦ C after immersing the sample in 10 wt% water/acetic acid mixture at 30 ◦ C for 24 h. On the other hand, the change in weight of the powder carbonized at 600 ◦ C hardly occurred. The stability in acid was enhanced by the development of carbonization. The possible reason for a recoverable decrease in separation factor can be considered as follows: The adsorption of acetic acid molecules on the membrane and the probable reaction of acetic acid molecules with the phenolic hydroxyl groups on the pore wall will make the pore hydrophobic for further acetic acid adsorption and permeation. The separation properties of the membranes could be recovered by thermal treatment because the phenolic hydroxyl groups are reformed due to a desorption of acetic acid molecules. On the other hand, the defects form in the membrane with a further increase in the carbonization temperature above 600 ◦ C, as described above. To optimize the membrane performance, it is important to control a balance between defect-free membrane coating condition and degree of carbonization via pyrolysis temperature. At carbonization temperature of 600 ◦ C, the carbon membrane had the best separation performance in this study. On the basis of these data, subsequent surface sulfonation treatment was performed using the carbon membrane carbonized at 600 ◦ C for further improvement of separation properties. Fig. 13 shows FTIR spectra of the carbon membrane after immersing the membrane in 10 wt% water/acetic acid mixture and sulfonated carbon membrane. FTIR measurements were performed with a powdery sample, which was scratched from the substrate
Fig. 13. FTIR spectra of the carbon membranes carbonized at 600 ◦ C before and after pervaporation of 10 wt% water/acetic acid mixtures, and after sulfonation.
58
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
Fig. 14. CO2 adsorption isotherms of the carbon membranes carbonized at 600 ◦ C before and after sulfonation.
and thermally treated at 100 ◦ C. In the spectra of the membranes after immersing the membrane in 10 wt% water/acetic acid mixture, the band at about 1700 cm−1 is ascribed to the C O bending of acetyl group [44]. The increase in acetic acid flux and decrease in water flux are probably due to the adsorption and/or reaction of acetic acid molecules with the OH groups on the pore surface, resulting high affinity of membrane for acetic acid. The adsorption and/or reaction of acetic acid on the pore surface also make the pore hydrophobic for less water permeation. On the other hand, in the spectra of the membranes after immersing the membrane in H2 SO4 at room temperature for 1 h, the bands at about 1050 and 1400 cm−1 are ascribed to the SO3 stretching and O S O stretching in SO3 H, indicating that the resultant membrane possesses SO3 H groups [44]. Fig. 14 shows CO2 adsorption isotherms of the carbon membranes before and after sulfonation. The adsorbed volume decreased after sulfonation, suggesting that the effective pore volume undergoing the mass transfer decreased by pore surface modification. The flux stability of the sulfonated carbon membrane was monitored in dehydrating water/acetic acid system at 30 ◦ C for 196 h. As shown in Fig. 15, the membrane performance remains stable during the tested period. The fluxes for water and acetic acid were 1.7 × 10−3 and 6.2 × 10−5 mol m−2 s−1 , respectively, as a result, the separation factor was 70. The stability in acid was enhanced by the sulfonation treatment, although the flux is slightly smaller than for untreated carbon membrane at the beginning of pervaporation. The surface sulfonation is thought to inhibit the adsorption and reaction of acetic acid on the pore surface, while the SO3 H groups make the effective pore size small for less permeation. Fig. 16 summarizes the literature data of pervaporation performance for acetic acid dehydration, although the direct comparison is difficult due to their different separation conditions.
Fig. 16. Summary of literature data from various studies on pervaporation performance of water/acetic acid. Pervaporation experiments with inorganic membranes were conducted using 2–50 wt% water/acetic acid mixtures at 30–100 ◦ C. Keys, circle: untreated carbon membrane (400 ◦ C), square: untreated carbon membrane (600 ◦ C), diamond: sulfonated carbon membrane (600 ◦ C), a: Ref. [45], b: Ref. [9], c: Ref. [10], d: Ref. [46], e: Ref. [47], f: Ref. [7], g: Ref. [48], H: Ref. [49], I: Ref. [50]; capital and small letters indicate the performances of sodium alginate-based polymer and inorganic membranes, respectively. For untreated membranes, the deterioration behaviors presented in Fig. 12 were indicated by arrows.
Sodium alginate-based polymeric membranes have been extensively studied for the separation of water/acetic acid mixtures [49,50]. Polymeric membranes possess generally poor mechanical strength but offer promising pervaporation performance. Zeolites and silica membranes are often affected by highly acidic or alkaline conditions requiring care in selecting a material with high chemical stability. ZSM-5, silicalite-1, mordenite, T, silica–zirconia, and silica–titania membranes have been selected due to their high resistance in acid. As shown in Fig. 16, further improvements in separation performance are needed for practical applications. 4. Conclusions Microporous carbon membranes were prepared on porous ␣-alumina support tubes through the carbonization of resorcinol/formaldehyde resin at relatively low temperatures ranging from 400 to 600 ◦ C. Pervaporation separation for water/alcohol (methanol, ethanol, and i-propanol) and water/acetic acid mixtures were conducted to investigate the feasibility of using the microporous carbon membranes for pervaporation. The pore size of the carbon membranes is estimated to be about 0.4 and 0.5 nm based on the kinetic diameter of the penetrant alcohol molecules in the pervaporation. The permeation flux and separation factor were 0.23 kg m−2 h−1 and 4150, respectively, for 10 wt% water/ipropanol mixture at 70 ◦ C. The membrane performance remains stable in dehydrating water/i-propanol system. The sulfonation treatment was effective for improvement of membrane stability in acid, while the carbon membranes carbonized at relatively low temperatures have poor stability against acetic acid because of acid dissolution of carbonaceous component and/or the surface modification of membranes by acetyl groups. The permeation flux and separation factor were stable 0.12 kg m−2 h−1 and 70, respectively, for 10 wt% water/acetic acid mixture at 30 ◦ C for 196 h. Acknowledgment
Fig. 15. Membrane stability in pervaporative dehydration of water/acetic acid system. Pervaporation experiments were conducted using 10 wt% water/acetic acid mixtures at 30 ◦ C. The sulfonated carbon membrane carbonized at 600 ◦ C was used.
This work was supported by a research grant from Kurita Water and Environmental Foundation (KWEF #22026).
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
References [1] P.D. Chapman, T. Oliveira, A.G. Livingston, K. Li, Membranes for the dehydration of solvents by pervaporation, J. Membr. Sci. 318 (2008) 5–37. [2] S.L. Wee, C.T. Tye, S. Bhatia, Membrane separation process – pervaporation through zeolite membrane, Sep. Purif. Technol. 63 (2008) 500–516. [3] X.Y. Qiao, T.S. Chung, Fundamental characteristics of sorption, swelling, and permeation of P84 co-polyimide membranes for pervaporation dehydration of alcohols, Ind. Eng. Chem. Res. 44 (2005) 8938–8943. [4] R.W. van Gemert, F.P. Cuperus, Newly developed ceramic membranes for dehydration and separation of organic mixtures by pervaporation, J. Membr. Sci. 105 (1995) 287–291. [5] A.W. Verkerk, P. van Male, M.A.G. Vorstman, J.T.F. Keurentjes, Description of dehydration performance of amorphous silica pervaporation membranes, J. Membr. Sci. 193 (2001) 227–238. [6] H.M. van Veen, Y.C. van Delft, C.W.R. Engelen, P.P.A.C. Pex, Dewatering of organics by pervaporation with silica membranes, Sep. Purif. Technol. 22–23 (2001) 361–366. [7] K. Tanaka, R. Yoshikawa, C. Ying, H. Kita, K.I. Okamoto, Application of zeolite membranes to esterification reactions, Catal. Today 67 (2001) 121–125. [8] 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) 163–175. [9] G. Li, E. Kikuchi, M. Matsukata, A study on the pervaporation of water–acetic acid mixtures through ZSM-5 zeolite membranes, J. Membr. Sci. 218 (2003) 185–194. [10] G. Li, E. Kikuchi, M. Matsukaka, Separation of water–acetic acid mixtures by pervaporation using a thin mordenite membrane, Sep. Purif. Technol. 32 (2003) 199–206. [11] M. Asaeda, S. Yamasaki, Separation of inorganic/organic gas mixtures by porous silica membranes, Sep. Purif. Technol. 25 (2001) 151–159. [12] Y.G. Li, S. Yong, W.C. Jiang, Peng.F C.G., A novel route for synthesis of ZSM-5 zeolite membranes, Mater. Lett. 37 (1998) 221–226. [13] S.M. Saufi, A.F. Ismail, Fabrication of carbon membranes for gas separation – a review, Carbon 42 (2004) 241–259. [14] H. Suda, K. Haraya, Molecular sieving effect of carbonized Kapton polyimide membrane, J. Chem. Soc., Chem. Commun. (1995) 1179–1180. [15] J. Hayashi, H. Mizuta, M. Yamamoto, K. Kusakabe, S. Morooka, Pore size control of carbonized BPDA-pp’ODA polyimide membrane by chemical vapor deposition of carbon, J. Membr. Sci. 124 (1997) 243–251. [16] H. Kita, H. Maeda, K. Tanaka, K. Okamoto, Carbon molecular sieve membrane prepared from phenolic resin, Chem. Lett. 2 (1997) 179–180. [17] T.A. Centeno, A.B. Fuertes, Supported carbon molecular sieve membranes based on a phenolic resin, J. Membr. Sci. 160 (1999) 201–211. [18] N. Nishiyama, Y.R. Dong, T. Zheng, Y. Egashira, K. Ueyama, Tertiary aminemediated synthesis of microporous carbon membranes, J. Membr. Sci. 280 (2006) 603–609. [19] M. Yoshimune, K. Haraya, Flexible carbon hollow fiber membranes derived from sulfonated poly(phenylene oxide), Sep. Purif. Technol. 75 (2010) 193–197. [20] Y.R. Dong, M. Nakao, N. Nishiyama, Y. Egashira, K. Ueyama, Gas permeation and pervaporation of water/alcohols through the microporous carbon membranes prepared from resorcinol/formaldehyde/quaternary ammonium compounds, Sep. Purif. Technol. 73 (2010) 2–7. [21] P.S. Tin, H.Y. Lin, R.C. Ong, T.S. Chung, Carbon molecular sieve membranes for biofuel separation, Carbon 49 (2011) 369–375. [22] R.W. Pekala, Organic aerogels from the polycondensation of resorcinol with formaldehyde, J. Mater. Sci. 24 (1989) 3221–3227. [23] T.C. Bowen, S. Li, R.D. Noble, L. Falconer, Driving force for pervaporation through zeolite membranes, J. Membr. Sci. 225 (2003) 165–176. [24] A.A. Kittur, M.Y. Kariduraganavar, U.S. Toti, K. Ramesh, T.M. Aminabhavi, Pervaporation separation of water–isopropanol mixtures using ZSM-5 zeolite incorporated poly(vinyl alcohol) membranes, J. Appl. Polym. Sci. 90 (2003) 2441–2448. [25] Y.M. Lee, S.Y. Nam, D.J. Woo, Pervaporation of ionically surface crosslinked chitosan composite membranes for water–alcohol mixtures, J. Membr. Sci. 133 (1997) 103–110. [26] W. Zhang, G.W. Li, Y.J. Fang, X.P. Wang, Maleic anhydride surface-modification of crosslinked chitosan membrane and its pervaporation performance, J. Membr. Sci. 295 (2007) 130–138. [27] M.D. Kurkuri, U.S. Toti, T.M. Aminabhavi, Syntheses and characterization of blend membranes of sodium alginate and poly(vinyl alcohol) for the pervaporation separation of water + isopropanol mixtures, J. Appl. Polym. Sci. 86 (2002) 3642–3651.
59
[28] X.Y. Qiao, T.S. Chung, K.P. Pramoda, Fabrication and characterization of BTDATDI/MDI (P84) co-polyimide membranes for the pervaporation dehydration of isopropanol, J. Membr. Sci. 264 (2005) 176–189. [29] X.Y. Qiao, T.S. Chung, Diamine modification of P84 polyimide membranes for pervaporation dehydration of isopropanol, AIChE J. 52 (2006) 3462–3472. [30] D.A. Devi, B. Smitha, S. Sridhar, T.M. Aminabhavi, Pervaporation separation of isopropanol/water mixtures through crosslinked chitosan membranes, J. Membr. Sci. 262 (2005) 91–99. [31] B.V.K. Naidu, M. Sairam, K.V.S.N. Raju, T.M. Aminabhavi, Pervaporation separation of water + isopropanol mixtures using novel nanocomposite membranes of poly(vinyl alcohol) and polyaniline, J. Membr. Sci. 260 (2005) 142–155. [32] S.D. Bhat, B.V.K. Naidu, G.V. Shanbhag, S.B. Halligudi, M. Sairam, T.M. Aminabhavi, Mesoporous molecular sieve (MCM-41)-filled sodium alginate hybrid nanocomposite membranes for pervaporation separation of water–isopropanol mixtures, Sep. Purif. Technol. 49 (2006) 56–63. [33] A.A. Kittur, S.S. Kulkarni, M.I. Aralaguppi, M.Y. Kariduraganavar, Preparation and characterization of novel pervaporation membranes for the separation of water–isopropanol mixtures using chitosan and NaY zeolite, J. Membr. Sci. 247 (2005) 75–86. [34] K. Rao, M.C.S. Subha, M. Sairam, N.N. Mallikarjuna, T.M. Aminabhavi, Blend membranes of chitosan and poly(vinyl alcohol) in pervaporation dehydration of isopropanol and tetrahydrofuran, J. Appl. Polym. Sci. 103 (2007) 1918– 1926. [35] Y.M. Lee, S.Y. Nam, S.Y. Ha, Pervaporation of water/isopropanol mixtures through polyaniline membranes doped with poly(acrylic acid), J. Membr. Sci. 159 (1999) 41–46. [36] A. Svang-Ariyaskul, R.Y.M. Huang, P.L. Douglas, R. Pal, X. Feng, P. Chen, L. Liu, Blended chitosan and polyvinyl alcohol membranes for the pervaporation dehydration of isopropanol, J. Membr. Sci. 280 (2006) 815–823. [37] C.K. Yeom, J.G. Jegal, K.H. Lee, Characterization of relaxation phenomena and permeation behaviors in sodium alginate membrane during pervaporation separation of ethanol–water mixture, J. Appl. Polym. Sci. 62 (1996) 1561–1576. [38] R.Y.M. Huang, R. Pal, G.Y. Moon, Crosslinked chitosan composite membrane for the pervaporation dehydration of alcohol mixtures and enhancement of structural stability of chitosan/polysulfone composite membranes, J. Membr. Sci. 160 (1999) 17–30. [39] Z. Gao, Y. Yue, W. Li, Application of zeolite-filled pervaporation membrane, Zeolites 16 (1996) 70–74. [40] J. Sekuli, M.W.J. Luiten, J.E. ten Elshof, N.E. Benes, K. Keizer, Microporous silica and doped silica membrane for alcohol dehydration by pervaporation, Desalination 148 (2002) 19–23. [41] T. Gallego-Lizon, Y.S. Ho, L.F.D. Santos, Comparative study of commercially available polymeric and microporous silica membranes for the dehydration of IPA/water mixtures by pervaporation/vapour permeation, Desalination 149 (2002) 3–8. [42] F.P. Cuperus, R.W. van Gemert, Dehydration using ceramic silica pervaporation membranes – the influence of hydrodynamic conditions, Sep. Purif. Technol. 27 (2002) 225–229. [43] S. Sommer, T. Melin, Performance evaluation of microporous inorganic membranes in the dehydration of industrial solvents, Chem. Eng. Process 44 (2005) 1138–1156. [44] S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato, S. Hayashi, M. Hara, Hydrolysis of cellulose by amorphous carbon bearing SO3 H, COOH, and OH Groups, J. Am. Chem. Soc. 130 (2008) 12787–12793. [45] D. Gorri, A. Urtiaga, I. Ortiz, Pervaporative recovery of acetic acid from an acetylation industrial effluent using commercial membranes, Ind. Eng. Chem. Res. 44 (2005) 977–985. [46] T. Masuda, S. Otani, T. Tsuji, M. Kitamura, S.R. Mukai, Preparation of hydrophilic and acid-proof silicalite-1 zeolite membrane and its application to selective separation of water from water solutions of concentrated acetic acid by pervaporation, Sep. Purif. Technol. 32 (2003) 181–189. [47] T.C. Bowen, H. Kalipcilar, J.L. Falconer, R.D. Noble, Pervaporation of organic/water mixtures through B-ZSM-5 zeolite membranes on monolith supports, J. Membr. Sci. 215 (2003) 235–247. [48] M. Asaeda, M. Ishida, T. Waki, Pervaporation of aqueous organic acid solutions by porous ceramic membranes, J. Chem. Eng. Jpn. 38 (2005) 336–343. [49] U.S. Toti, T.M. Aminabhavi, Different viscosity grade sodium alginate and modified sodium alginate membranes in pervaporation separation of water + acetic acid and water + isopropanol mixtures, J. Membr. Sci. 228 (2004) 199–208. [50] S.B. Teli, G.S. Gokavi, M. Sairam, T.M. Aminabhavi, Highly water selective silicotungstic acid (H4 SiW12 O40 ) incorporated novel sodium alginate hybrid composite membranes for pervaporation dehydration of acetic acid, Sep. Purif. Technol. 54 (2007) 178–186.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Pervaporation dehydration performance of microporous carbon membranes prepared from resorcinol/formaldehyde polymer Shunsuke Tanaka a,b,∗ , Tomohisa Yasuda a , Yugo Katayama a , Yoshikazu Miyake a,b a Department of Chemical, Energy and Environmental Engineering, Faculty of Environmental and Urban Engineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680, Japan b High Technology Research Center, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680 Japan
a r t i c l e
i n f o
Article history: Received 10 February 2011 Received in revised form 11 April 2011 Accepted 22 May 2011 Available online 27 May 2011 Keywords: Carbon membrane Resorcinol/formaldehyde Pervaporation Acetic acid Sulfonation Durability
a b s t r a c t Microporous carbon membranes for pervaporation applications were prepared on a porous ␣-alumina support by a partially carbonization of a resorcinol/formaldehyde resin. The stability and dehydration performances of the carbon membranes were determined. The carbon membranes were used for the dehydration of several organic solvents (methanol, ethanol, i-propanol, and acetic acid) containing water; it was found that water was selectively permeated through the membrane and the separation factor increased with the molecular diameter of the organic solvents. The high selectivity to water can be explained by not only the hydrophilic nature of the pore surface but also the molecular sieving effect. Furthermore, the membranes showed high durability in the pervaporation of water/alcohol mixtures. On the other hand, the membranes were unstable in water/acetic acid mixture. However, the sulfonated carbon membranes were stable in pervaporation of water/acetic acid mixture and maintained their separation properties. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The global focus on green technologies requires the development of separation processes, which account for about 40% of the total energy consumption in chemical and petrochemical industries worldwide. Currently, the alcohol dehydration processes become increasingly important because of the advancements in the development of renewable biomass and biofuel technologies. Additionally, acetic acid with low water content is also needed in various chemical processes, such as production of vinyl acetate monomer, terephthalic acid, and acetate esters. Pervaporation is a promising technique and practical alternative to facilitate alcohol and acetic acid dehydration [1,2]. This membrane-based separation technique permits the separation azeotrope, close-boiling mixtures, and thermally degradable organic mixtures. In addition, pervaporation possesses many advantages: easy process design, high selectivity, low energy consumption, and moderate cost-toperformance ratio. Polymeric membranes are widely adopted as separation materials in the field of pervaporation for solvent dehydration and
∗ Corresponding author at: Department of Chemical, Energy and Environmental Engineering, Faculty of Environmental and Urban Engineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680, Japan. Tel.: +81 6 6368 0851; fax: +81 6 6388 8869. E-mail address: shun [email protected] (S. Tanaka). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.05.046
separation of organic mixtures. However, polymeric membranes are often unstable at high temperatures. The applications of polymeric membranes are further restricted by unfavorable swelling, which subsequently results in the decline of their separation properties with respect to time [3]. Moreover, they cannot be used in concentrated acetic acid mixtures due to the low acid tolerance. Inorganic microporous membranes such as silica [4–6] and zeolite [7–10] have been developed as promising materials to overcome the above chemical and thermal instabilities. In general, they possess stable separation performance at high feed or water concentrations and elevated temperatures. Therefore, the separation operation can be applied for a broad range of applications and over an extended time period. Nevertheless, there are drawbacks reported in the preparation and separation stability of the inorganic microporous membranes. In addition to the chemical instability of silica against water and alkaline solutions [11], membranes are thick to form defect-free zeolite membranes [12]. Although well-known zeolite LTA membranes possess a high separation performance for water/alcohol systems, they have a major drawback of acid-sensitivity. Microporous carbon membranes offer the best candidates for the development of new membrane technologies, because of their advantages, such as excellent permeation selectivity, high hydrothermal stability, and high corrosion resistance [13]. Furthermore, their microstructure and surface properties can be altered by post-treatment. Due to their highly porous structures with a sharp distribution of pore sizes ranging from 0.3 to 0.6 nm, the
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
carbon membranes are suitable for the gas separations such as H2 /N2 , O2 /N2 , CO2 /N2 , and CO2 /CH4 . Research efforts have been focused on the gas separation [13–20]. However, very few investigations have been reported on the application of the carbon membranes for pervaporation [20,21]. Furthermore, to the best of our knowledge, there are no investigations on the application of the carbon membranes for dehydration of acetic acid. Suitable carbon precursors for molecular sieve membrane production must not cause any crack formation after a pyrolysis step. A thermosetting phenolic resin can withstand relatively high temperatures and possess surface-modifiable phenolic hydroxyl groups. In this study, microporous carbon membranes for pervaporation applications were prepared on a porous ␣-alumina support by a partially carbonization of a resorcinol/formaldehyde resin. In addition, we attempt to chemically modify the carbon surface by sulfonation treatment. The separation performances of the carbon membranes were investigated for the pervaporation separation of water/alcohol mixtures. Furthermore, we use the membranes for pervaporation separation of water/acetic acid mixture and investigate the effect of sulfonation on the stability and dehydration performance.
composition and thickness of the membranes were measured by scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analysis on a VE-8800 microscope (Keyence). The field emission scanning electron microscope (FESEM) images were recorded on an S-5000L Hitachi microscope at an acceleration voltage of 22 kV. No coating was carried out for the samples before the SEM and FESEM measurements. Raman spectra were recorded with a NRS3100 spectrometer (JASCO) using a 532 nm laser as an excitation source. The amount of CO2 adsorption on carbon powdery samples was measured at 298 K using a BELSORP 28 instrument (Bel Japan, Inc.). The carbon powdery samples were prepared on a non-porous silicon substrate and scratched from the substrate. 2.4. Pervaporation Pervaporation experiments were performed using several organic solvents (methanol, ethanol, i-propanol, and acetic acid) containing water. The carbon membrane side faced the feed side. In the experimental apparatus, the downstream compartment was evacuated, and the permeate was collected in a vacuum trap condenser cooled using liquid nitrogen. The permeation flux, J, is defined by
2. Experimental 2.1. Materials Resorcinol, formaldehyde (36–38 wt%), 5 N NaOH, 2 N H2 SO4 and ethanol were purchased from Wako Pure Chemical Industries and used as received. ␣-Alumina porous tubular supports (outer diameter: 10 mm; inner diameter: 7 mm; length: 450 mm; average porosity: 35%; average pore size: 0.1 m) were purchased from Noritake Co. Ltd. and cut into 35 mm long pieces. 2.2. Preparation of carbon membranes and sulfonation treatment A resorcinol/formaldehyde resin layer was coated on the ␣alumina layer as follows. A coating solution was prepared from resorcinol, formaldehyde, ethanol, and NaOH. In a typical synthesis, resorcinol was completely dissolved in formaldehyde solution. The above resorcinol/formaldehyde solution was added to ethanol, NaOH solution was added, and the solution was heated at 80 ◦ C for 5 min. The solution was sonicated at room temperature for 3 min before coating. The final molar composition of the coating solution was 1 resorcinol:2 formaldehyde:2 × 10−3 NaOH:6.4 ethanol:5.1 water. Membranes were prepared by dip-coating the alumina supports in the coating solutions at a withdrawal rate of 1.6 mm s−1 using a DIPCOATER DC4200 (Aiden Co., Ltd.). The coating procedure was repeated two times. For polymerization of the resorcinol with formaldehyde, the as-deposited samples were pre-heated at 100 ◦ C for 30 min in air. The resultant brown deposition was carbonized under a nitrogen atmosphere at 400–800 ◦ C for 1 h at a heating rate of 1 ◦ C min−1 . The sulfonated carbon membranes were as follows: 2 N H2 SO4 was used as a sulfonation reagent. The carbonized carbon membranes were immersed in H2 SO4 at room temperature for 1 h. After sulfonation, the membranes were rinsed with deionized water and dried at 200 ◦ C. 2.3. Characterization Fourier-transform infrared spectroscopy (FTIR) spectra of the samples were recorded in the 500–3500 cm−1 range using a IRAffinity-1 spectrometer (Shimadzu) at 4 cm−1 resolution. Thermogravimetric analysis (TGA) was performed on a DTG-60H apparatus (Shimadzu) at a heating rate of 2 ◦ C min−1 . FTIR and TGA measurements were performed with a powdery sample. The
53
Jmass =
Q , At
(1)
where Q is the weight of the collected permeate during the experimental time interval, t, and A is the effective membrane surface area (11 cm2 ). The permeation fluxes on the mass basis can be converted to the molar permeation fluxes as follows Jmole =
Jmass , 3600 M
(2)
where M is the molecular weight of component. The feed and permeate concentrations were determined by Karl Fisher titration. The separation factor of component i with respect to component j, ˛(i/j) , is defined by ˛(i/j) =
yi /yj xi /xj
,
(3)
where yi and yj are the mole fractions of component i and j in the permeate, respectively, and xi and xj are their corresponding mole fractions in the feed. 3. Results and discussion 3.1. FESEM and SEM/EDX observations SEM and FESEM images of the cross-sectional views of the carbon membrane carbonized at 400 ◦ C on the alumina support are given in Fig. 1. The carbon membrane had a smooth surface. There existed no defect from the top view of the membrane (data not shown). From the FESEM image, the membrane had a carbon dense layer approximately 1 m thick. On the other hand, as can be seen from the photograph image shown as an inset in Fig. 1, the carbon was deposited deep into the alumina pores. The molar ratios of carbon and aluminum at the positions indicated by a–f in Fig. 1 are shown in Table 1. The SEM/EDX results suggest that the resorcinol/formaldehyde resin penetrated into the pores of the porous alumina support. An effective thickness of the carbon membrane is estimated to be about 2 m. 3.2. FTIR and CO2 adsorption measurements The pyrolysis was performed in a nitrogen atmosphere at 400–800 ◦ C for 1 h at a heating rate of 1 ◦ C min−1 . The chemical
54
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
Fig. 1. SEM (left) and FESEM (right) images of the cross-sectional views of the carbon membrane carbonized at 400 ◦ C on the alumina support. The inset in FESEM image shows the photograph image of the cross-section of the membrane.
Table 1 The molar ratios of carbon and aluminum at the positions indicated by a–f in Fig. 1. The carbon membrane was carbonized at 400 ◦ C. Positions
Distance from the top surface (m)
C (%)
Al (%)
a b c d e f
0.5 2 4 20 55 115
98 18 10 8 7 8
2 82 90 92 93 92
and pore structure of the carbonized membranes were investigated by FTIR and CO2 adsorption measurements. The pore structure was relatively compared using the data of CO2 adsorption, because the samples resulted in only a small amount of N2 adsorption. The major reactions between resorcinol and formaldehyde include an addition reaction to form hydroxymethyl derivatives (–CH2 OH), and then a condensation reaction of the hydroxymethyl derivatives to form methylene (–CH2 –) and methylene ether (–CH2 OCH2 –)-bridged compound [22]. Fig. 2 shows FTIR spectra of the resorcinol/formaldehyde membranes carbonized at various temperatures. The bands can be assigned to the C–O (at 1000–1100 cm−1 ), OH bending of phenolic group (at about 1250 cm−1 ), C C (aromatic stretching) (at about 1450 cm−1 ), free water (at about 1600 cm−1 ), and C–H of methylene group stretch (at about 2900 cm−1 ). The broad band at 3000–3800 cm−1 is ascribed to the symmetrical and antisymmetrical stretching vibrations of water molecules with hydrogen bonding. The intensity of a band for C–H of methylene group stretch decreased with increasing ther-
Fig. 2. FTIR spectra of the resorcinol/formaldehyde membranes carbonized at 400–800 ◦ C.
Fig. 3. TGA curve of resorcinol/formaldehyde resin. The heating rate was 2 ◦ C min−1 under nitrogen flow.
mal treatment temperature, indicating that the carbonization of the membrane proceeds at elevated temperatures. The methylene groups remain in the membrane even after calcination at 600 ◦ C. The carbon membranes carbonized at a temperature below 600 ◦ C are still composed of an intermediate between a polymer and carbon. The weight loss of the resorcinol/formaldehyde resin became larger with increasing temperature, and the percentages of carbonaceous residues were 76%, 65%, 53%, and 47% after pyrolysis at 400, 500, 600, and 800 ◦ C, respectively (Fig. 3). On the other hand, the adsorbed CO2 volume increased with increasing temperature, as shown in Fig. 4. The porous structure is generated inside the carbonaceous framework by solidification and gasification of the polymer with increasing temperature. In the wide-angle XRD pattern, the Bragg diffraction peaks at around 2 = 26◦ and 45◦ arise from diffraction of graphitic (0 0 2) and (1 0 1) planes, respectively (data not shown). There is no large difference in XRD patterns between the carbon membranes carbonized at 400, 600, and 800 ◦ C. Raman spectroscopy was used to determine the structure of the carbon membranes because Raman spectroscopy has also been a useful tool for obtaining information on the microstructure of carbonaceous materials. The spectra featured two major peaks centered at 1350 and 1580 cm−1 (Fig. 5), which are referred to as D and G bands, respectively, for sp2 bonded carbon materials. The G band can be attributed to the in-plane carbon stretching vibrations of ideal graphene sheets, whereas the D band can be attributed to structural imperfection of graphene sheets. The intensity of G band increased with increasing carbonization temperature, indicating that the framework of carbon membrane became a more graphitized carbon wall at elevated temperatures. However, the ratio of the intensity of the G band to that of the D band was relatively
Fig. 4. CO2 adsorption isotherms of the carbon membranes carbonized at 400–800 ◦ C.
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
55
Fig. 5. Raman spectra of resorcinol/formaldehyde resin and carbon membranes carbonized at 400–800 ◦ C.
low, indicating that the carbon membranes consist of imperfect graphenes of a very small size. A micropore is the space between the nanographenes. The carbonization seemed to be mostly complete after pyrolysis at 800 ◦ C. From FTIR spectra, no broad band was observed at 3000–3800 cm−1 , indicating that the concentration of adsorbed water is extremely low. However, the band for free water can be observed even after pyrolysis at 800 ◦ C. The OH groups and water molecules with hydrogen bonding remain in the membrane even after pyrolysis at 600 ◦ C. We speculate that the membranes carbonized at a temperature below 600 ◦ C are more hydrophilic than that carbonized at 800 ◦ C. The carbon membranes have a hydrophilic surface and favors adsorption of water molecules. 3.3. Water/alcohol pervaporation The top layer of carbon membranes was exposed to water/alcohol mixtures for 1 h at 50 ◦ C. Fig. 6 shows the fluxes for water and the organic components and the separation factor as functions of the molecular kinetic diameter [23] of the organic components. For the water/methanol system, the permeation fluxes for water and methanol were 1.8 × 10−3 and 1.4 × 10−4 mol m−2 s−1 , respectively, and separation factor of water/methanol was 65. The flux for the organic components decreased with increasing the molecular diameter, and those for ethanol and i-propanol were 2.5 × 10−5 and 1.2 × 10−6 mol m−2 s−1 , respectively. On the other hand, the flux for water increased with increasing the molecule diameter of organic components. The separation factors of water/ethanol and water/i-propanol through the carbon membrane at 50 ◦ C were 380 and 6750, respectively. The high separation
Fig. 7. Effect of temperature on pervaporation performances of the carbon membrane carbonized at 400 ◦ C for 10 wt% water/i-propanol (upper). Water and i-propanol fluxes as a function of 1/T (T: 303–343) (bottom).
factors indicate that the carbon membrane has very narrow pore size distributions. These results suggest that the average pore size of the carbon membrane is in between 0.4 and 0.5 nm. The large permeation flux for methanol corresponds to the low separation factor. The methanol molecules can penetrate into the micropores of carbon membrane. Even the methanol molecules inhibited the diffusion of water molecules in the micropores. Fig. 7 shows the fluxes for water and i-propanol through the carbon membrane as a function of temperature. The fluxes increased with increasing the pervaporation temperature, and those for water and i-propanol were 3.6 × 10−3 and 2.3 × 10−6 mol m−2 s−1 at 70 ◦ C, respectively. The separation factor decreased with increasing the pervaporation temperature, and separation factor was 4150 at 70 ◦ C, indicating that the apparent activation energy for the permeation of i-propanol through the carbon membrane is larger than that of water. The effect of temperature on the flux is given by Ji = Ji0 exp
Fig. 6. Effect of penetrant molecule size on the fluxes of water and alcohols, and the separation factor. Pervaporation experiments were conducted using 10 wt% water/alcohol mixtures at 50 ◦ C. The carbon membrane was carbonized at 400 ◦ C.
−E RT
where Ji0 and E are the pre-exponential factor and activation energy, respectively. The apparent activation energies of water and ipropanol were 17.2 and 34.8 kJ mol−1 , respectively. The feed concentration of water varied from 10 to 90 wt% in the water/i-propanol separations. Fig. 8 shows the effect of the feed concentration on the separation of the water/i-propanol mixtures. The fluxes of water increased with its concentration in the feed solution. However, the i-propanol fluxes slightly decreased with increasing the feed concentration of water, indicating that the amount of adsorbed i-propanol decreased at a high concentration of water. In addition, the fluxes of water for each water/alcohol mixture were significantly lower than the pure water flux (1.6 kg m−2 h−1 ). These results indicate that water permeation was greatly blocked by alcohols, while the alcohol permeation was only slightly affected by water.
56
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
Fig. 8. Effect of feed concentration on water/i-propanol separation performance with the carbon membrane at 70 ◦ C. The carbon membrane was carbonized at 400 ◦ C.
The flux stability of the carbon membrane was monitored in dehydrating water/i-propanol system at 70 ◦ C for 196 h. As shown in Fig. 9, the membrane performance remains stable during the tested period. Literature data of pervaporation performance for i-propanol dehydration are cited from a review paper [1]. Polymeric membranes are based on organic polymer chains that are cross-linked together and/or with cross-linker. The representatives of these are poly(vinyl alcohol)- [24,27,31], chitosan[25,26,30], alginate- [27,32], and polyimides- [28,29] based membranes. Fig. 10 shows the comparison of membrane performance in this study with literature values. A more extensive literature summary on various membrane performances can be found in the review paper. Unfortunately, the direct comparison is difficult because the other types of membranes were used for the separation at different separation conditions. On the whole it can be seen that the inorganic membranes exhibit higher permeation flux than polymer-based membranes. The carbon membranes in this study exhibit comparable or even better dehydration performance compared to other types of polymeric membranes. It is worthwhile to make any further research for development and pervaporation application of carbon membranes, while the permeation flux of the carbon membranes is still lower than that of other inorganic membranes.
Fig. 10. Summary of literature data from various studies on pervaporation performance of water/i-propanol. Pervaporation experiments with polymer-based membranes (upper) and inorganic membranes (bottom) were conducted using 5–20 wt% water/i-propanol mixtures at 30–80 ◦ C. Keys, diamond: in this study, a: Ref. [24], b: Ref. [27], c: Ref. [33], d: Ref. [34], e: Ref. [26], f: Ref. [35], g: Ref. [36], h: Ref. [37], i: Ref. [29], j: Ref. [28], k: Ref. [38], l: Ref. [39], m: Ref. [25], n: Ref. [30], o: Ref. [31], p: Ref. [32], q: Ref. [40], r: Ref. [41], s: Ref. [4], t: Ref. [42], u: Ref. [43], v: Ref. [8], w: Ref. [5], x: Ref. [6], y: Ref. [20].
3.4. Water/acetic acid pervaporation Acetic acid is an important compound frequently used in the chemical industry. It is formed as an aqueous by-product in a number of processes such as the production of aspirin. The production of acetic acid itself yields water as a by-product, requiring dehydration to purify the acetic acid. However, the dehydration of acetic acid is more difficult than the dehydration of alcohols due to the acidic nature of the compound. This acidity inhibits the use of membranes that have a low stability in acid such as
Fig. 9. Membrane stability in pervaporative dehydration of water/i-propanol system. Pervaporation experiments were conducted using 10 wt% water/i-propanol mixtures at 70 ◦ C.
LTA zeolite and also damage polymeric membrane over time, requiring more chemically inert materials to be used in acidic conditions. The effect of pyrolysis temperature of the membrane on the separation performance was investigated. Fig. 11 shows the pervaporation results for the water/acetic acid mixture through the carbon membranes at 30 ◦ C for 1 h. The permeation fluxes for water and acetic acid through the membrane carbonized at 400 ◦ C were 1.8 × 10−3 and 2.3 × 10−6 mol m−2 s−1 , respectively,
Fig. 11. Fluxes of water and acetic acid and separation factor of the carbon membranes carbonized at 400–800 ◦ C. Pervaporation experiments were conducted using 10 wt% water/acetic acid mixtures at 30 ◦ C.
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
Fig. 12. Changes of pervaporation characteristics with time. Pervaporation experiments were conducted using 10 wt% water/acetic acid mixtures at 30 ◦ C. The fresh carbon membranes carbonized at 400 (upper), 500 (middle), and 600 ◦ C (bottom) were used.
and separation factor of water/acetic acid was 2090. The fluxes of water and acetic acid slightly increased with increasing pyrolysis temperatures ranging from 400 to 600 ◦ C. On the other hand, the separation factor decreased with increasing pyrolysis temperature. When the pyrolysis temperature was further increased to 800 ◦ C, a steep increase in the flux of acetic acid was observed, while the flux of water remained constant at temperatures ranging from 600 to 800 ◦ C. Then the separation factor significantly decreased to 2. The membrane carbonized at 800 ◦ C was more hydrophobic than that carbonized at a temperature below 600 ◦ C. In addition, we speculate that the pinholes and/or cracks form in the membrane after pyrolysis at 800 ◦ C. Hence, in this study, we used the membranes carbonized at temperatures ranging from 400 to 600 ◦ C for the separation performance stability tests. Fig. 12 shows a long time dependency of the separation properties of water/acetic acid mixture with the fresh membranes carbonized at 400, 500, and 600 ◦ C. The dependency of water and acetic acid fluxes and the separation factor on the separation time at a constant water concentration of 10 wt% in feed and at 30 ◦ C. The fluxes of acetic acid through the membrane carbonized at 400 ◦ C gradually increased, while the flux of water slightly decreased. The separation factor dramatically decreased to 2, although that is more than 2000 at the beginning. Similarly, when the membranes carbonized at 500 and 600 ◦ C, the fluxes of acetic acid increased and
57
that of water decreased. However, although the fluxes of acetic acid are higher at the beginning, they increased to the steady-state values smaller than that of the membrane carbonized at 400 ◦ C. Consequently, of particular interest is that the carbon membranes carbonized at 600 ◦ C have the highest steady-state separation factor because of their high resistance against acidic solutions. As shown in Fig. 12, moreover, the separation properties of the membranes carbonized at 600 ◦ C can be recovered by thermal treatment at 200 ◦ C, while those of the membranes carbonized at 400 ◦ C is irreversible. These results indicate that the deterioration behaviors of the separation properties result from different causes. We speculate that the change of separation properties with time is due to the formation of defect caused by acid dissolution of carbonaceous component and/or the surface modification of membranes. The acetic acid tolerance test was carried out using powder samples. We performed quantitative analysis using bulk powder samples because the sample weight was small due to membrane morphology. The precursor solution was left overnight at room temperature and turned into a gel. The resultant brown gel was preheated at 100 ◦ C in air. The carbonization process was performed according to the procedure described in experimental section. The 12% decrease in weight of carbonaceous powder carbonized at 400 ◦ C after immersing the sample in 10 wt% water/acetic acid mixture at 30 ◦ C for 24 h. On the other hand, the change in weight of the powder carbonized at 600 ◦ C hardly occurred. The stability in acid was enhanced by the development of carbonization. The possible reason for a recoverable decrease in separation factor can be considered as follows: The adsorption of acetic acid molecules on the membrane and the probable reaction of acetic acid molecules with the phenolic hydroxyl groups on the pore wall will make the pore hydrophobic for further acetic acid adsorption and permeation. The separation properties of the membranes could be recovered by thermal treatment because the phenolic hydroxyl groups are reformed due to a desorption of acetic acid molecules. On the other hand, the defects form in the membrane with a further increase in the carbonization temperature above 600 ◦ C, as described above. To optimize the membrane performance, it is important to control a balance between defect-free membrane coating condition and degree of carbonization via pyrolysis temperature. At carbonization temperature of 600 ◦ C, the carbon membrane had the best separation performance in this study. On the basis of these data, subsequent surface sulfonation treatment was performed using the carbon membrane carbonized at 600 ◦ C for further improvement of separation properties. Fig. 13 shows FTIR spectra of the carbon membrane after immersing the membrane in 10 wt% water/acetic acid mixture and sulfonated carbon membrane. FTIR measurements were performed with a powdery sample, which was scratched from the substrate
Fig. 13. FTIR spectra of the carbon membranes carbonized at 600 ◦ C before and after pervaporation of 10 wt% water/acetic acid mixtures, and after sulfonation.
58
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
Fig. 14. CO2 adsorption isotherms of the carbon membranes carbonized at 600 ◦ C before and after sulfonation.
and thermally treated at 100 ◦ C. In the spectra of the membranes after immersing the membrane in 10 wt% water/acetic acid mixture, the band at about 1700 cm−1 is ascribed to the C O bending of acetyl group [44]. The increase in acetic acid flux and decrease in water flux are probably due to the adsorption and/or reaction of acetic acid molecules with the OH groups on the pore surface, resulting high affinity of membrane for acetic acid. The adsorption and/or reaction of acetic acid on the pore surface also make the pore hydrophobic for less water permeation. On the other hand, in the spectra of the membranes after immersing the membrane in H2 SO4 at room temperature for 1 h, the bands at about 1050 and 1400 cm−1 are ascribed to the SO3 stretching and O S O stretching in SO3 H, indicating that the resultant membrane possesses SO3 H groups [44]. Fig. 14 shows CO2 adsorption isotherms of the carbon membranes before and after sulfonation. The adsorbed volume decreased after sulfonation, suggesting that the effective pore volume undergoing the mass transfer decreased by pore surface modification. The flux stability of the sulfonated carbon membrane was monitored in dehydrating water/acetic acid system at 30 ◦ C for 196 h. As shown in Fig. 15, the membrane performance remains stable during the tested period. The fluxes for water and acetic acid were 1.7 × 10−3 and 6.2 × 10−5 mol m−2 s−1 , respectively, as a result, the separation factor was 70. The stability in acid was enhanced by the sulfonation treatment, although the flux is slightly smaller than for untreated carbon membrane at the beginning of pervaporation. The surface sulfonation is thought to inhibit the adsorption and reaction of acetic acid on the pore surface, while the SO3 H groups make the effective pore size small for less permeation. Fig. 16 summarizes the literature data of pervaporation performance for acetic acid dehydration, although the direct comparison is difficult due to their different separation conditions.
Fig. 16. Summary of literature data from various studies on pervaporation performance of water/acetic acid. Pervaporation experiments with inorganic membranes were conducted using 2–50 wt% water/acetic acid mixtures at 30–100 ◦ C. Keys, circle: untreated carbon membrane (400 ◦ C), square: untreated carbon membrane (600 ◦ C), diamond: sulfonated carbon membrane (600 ◦ C), a: Ref. [45], b: Ref. [9], c: Ref. [10], d: Ref. [46], e: Ref. [47], f: Ref. [7], g: Ref. [48], H: Ref. [49], I: Ref. [50]; capital and small letters indicate the performances of sodium alginate-based polymer and inorganic membranes, respectively. For untreated membranes, the deterioration behaviors presented in Fig. 12 were indicated by arrows.
Sodium alginate-based polymeric membranes have been extensively studied for the separation of water/acetic acid mixtures [49,50]. Polymeric membranes possess generally poor mechanical strength but offer promising pervaporation performance. Zeolites and silica membranes are often affected by highly acidic or alkaline conditions requiring care in selecting a material with high chemical stability. ZSM-5, silicalite-1, mordenite, T, silica–zirconia, and silica–titania membranes have been selected due to their high resistance in acid. As shown in Fig. 16, further improvements in separation performance are needed for practical applications. 4. Conclusions Microporous carbon membranes were prepared on porous ␣-alumina support tubes through the carbonization of resorcinol/formaldehyde resin at relatively low temperatures ranging from 400 to 600 ◦ C. Pervaporation separation for water/alcohol (methanol, ethanol, and i-propanol) and water/acetic acid mixtures were conducted to investigate the feasibility of using the microporous carbon membranes for pervaporation. The pore size of the carbon membranes is estimated to be about 0.4 and 0.5 nm based on the kinetic diameter of the penetrant alcohol molecules in the pervaporation. The permeation flux and separation factor were 0.23 kg m−2 h−1 and 4150, respectively, for 10 wt% water/ipropanol mixture at 70 ◦ C. The membrane performance remains stable in dehydrating water/i-propanol system. The sulfonation treatment was effective for improvement of membrane stability in acid, while the carbon membranes carbonized at relatively low temperatures have poor stability against acetic acid because of acid dissolution of carbonaceous component and/or the surface modification of membranes by acetyl groups. The permeation flux and separation factor were stable 0.12 kg m−2 h−1 and 70, respectively, for 10 wt% water/acetic acid mixture at 30 ◦ C for 196 h. Acknowledgment
Fig. 15. Membrane stability in pervaporative dehydration of water/acetic acid system. Pervaporation experiments were conducted using 10 wt% water/acetic acid mixtures at 30 ◦ C. The sulfonated carbon membrane carbonized at 600 ◦ C was used.
This work was supported by a research grant from Kurita Water and Environmental Foundation (KWEF #22026).
S. Tanaka et al. / Journal of Membrane Science 379 (2011) 52–59
References [1] P.D. Chapman, T. Oliveira, A.G. Livingston, K. Li, Membranes for the dehydration of solvents by pervaporation, J. Membr. Sci. 318 (2008) 5–37. [2] S.L. Wee, C.T. Tye, S. Bhatia, Membrane separation process – pervaporation through zeolite membrane, Sep. Purif. Technol. 63 (2008) 500–516. [3] X.Y. Qiao, T.S. Chung, Fundamental characteristics of sorption, swelling, and permeation of P84 co-polyimide membranes for pervaporation dehydration of alcohols, Ind. Eng. Chem. Res. 44 (2005) 8938–8943. [4] R.W. van Gemert, F.P. Cuperus, Newly developed ceramic membranes for dehydration and separation of organic mixtures by pervaporation, J. Membr. Sci. 105 (1995) 287–291. [5] A.W. Verkerk, P. van Male, M.A.G. Vorstman, J.T.F. Keurentjes, Description of dehydration performance of amorphous silica pervaporation membranes, J. Membr. Sci. 193 (2001) 227–238. [6] H.M. van Veen, Y.C. van Delft, C.W.R. Engelen, P.P.A.C. Pex, Dewatering of organics by pervaporation with silica membranes, Sep. Purif. Technol. 22–23 (2001) 361–366. [7] K. Tanaka, R. Yoshikawa, C. Ying, H. Kita, K.I. Okamoto, Application of zeolite membranes to esterification reactions, Catal. Today 67 (2001) 121–125. [8] 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) 163–175. [9] G. Li, E. Kikuchi, M. Matsukata, A study on the pervaporation of water–acetic acid mixtures through ZSM-5 zeolite membranes, J. Membr. Sci. 218 (2003) 185–194. [10] G. Li, E. Kikuchi, M. Matsukaka, Separation of water–acetic acid mixtures by pervaporation using a thin mordenite membrane, Sep. Purif. Technol. 32 (2003) 199–206. [11] M. Asaeda, S. Yamasaki, Separation of inorganic/organic gas mixtures by porous silica membranes, Sep. Purif. Technol. 25 (2001) 151–159. [12] Y.G. Li, S. Yong, W.C. Jiang, Peng.F C.G., A novel route for synthesis of ZSM-5 zeolite membranes, Mater. Lett. 37 (1998) 221–226. [13] S.M. Saufi, A.F. Ismail, Fabrication of carbon membranes for gas separation – a review, Carbon 42 (2004) 241–259. [14] H. Suda, K. Haraya, Molecular sieving effect of carbonized Kapton polyimide membrane, J. Chem. Soc., Chem. Commun. (1995) 1179–1180. [15] J. Hayashi, H. Mizuta, M. Yamamoto, K. Kusakabe, S. Morooka, Pore size control of carbonized BPDA-pp’ODA polyimide membrane by chemical vapor deposition of carbon, J. Membr. Sci. 124 (1997) 243–251. [16] H. Kita, H. Maeda, K. Tanaka, K. Okamoto, Carbon molecular sieve membrane prepared from phenolic resin, Chem. Lett. 2 (1997) 179–180. [17] T.A. Centeno, A.B. Fuertes, Supported carbon molecular sieve membranes based on a phenolic resin, J. Membr. Sci. 160 (1999) 201–211. [18] N. Nishiyama, Y.R. Dong, T. Zheng, Y. Egashira, K. Ueyama, Tertiary aminemediated synthesis of microporous carbon membranes, J. Membr. Sci. 280 (2006) 603–609. [19] M. Yoshimune, K. Haraya, Flexible carbon hollow fiber membranes derived from sulfonated poly(phenylene oxide), Sep. Purif. Technol. 75 (2010) 193–197. [20] Y.R. Dong, M. Nakao, N. Nishiyama, Y. Egashira, K. Ueyama, Gas permeation and pervaporation of water/alcohols through the microporous carbon membranes prepared from resorcinol/formaldehyde/quaternary ammonium compounds, Sep. Purif. Technol. 73 (2010) 2–7. [21] P.S. Tin, H.Y. Lin, R.C. Ong, T.S. Chung, Carbon molecular sieve membranes for biofuel separation, Carbon 49 (2011) 369–375. [22] R.W. Pekala, Organic aerogels from the polycondensation of resorcinol with formaldehyde, J. Mater. Sci. 24 (1989) 3221–3227. [23] T.C. Bowen, S. Li, R.D. Noble, L. Falconer, Driving force for pervaporation through zeolite membranes, J. Membr. Sci. 225 (2003) 165–176. [24] A.A. Kittur, M.Y. Kariduraganavar, U.S. Toti, K. Ramesh, T.M. Aminabhavi, Pervaporation separation of water–isopropanol mixtures using ZSM-5 zeolite incorporated poly(vinyl alcohol) membranes, J. Appl. Polym. Sci. 90 (2003) 2441–2448. [25] Y.M. Lee, S.Y. Nam, D.J. Woo, Pervaporation of ionically surface crosslinked chitosan composite membranes for water–alcohol mixtures, J. Membr. Sci. 133 (1997) 103–110. [26] W. Zhang, G.W. Li, Y.J. Fang, X.P. Wang, Maleic anhydride surface-modification of crosslinked chitosan membrane and its pervaporation performance, J. Membr. Sci. 295 (2007) 130–138. [27] M.D. Kurkuri, U.S. Toti, T.M. Aminabhavi, Syntheses and characterization of blend membranes of sodium alginate and poly(vinyl alcohol) for the pervaporation separation of water + isopropanol mixtures, J. Appl. Polym. Sci. 86 (2002) 3642–3651.
59
[28] X.Y. Qiao, T.S. Chung, K.P. Pramoda, Fabrication and characterization of BTDATDI/MDI (P84) co-polyimide membranes for the pervaporation dehydration of isopropanol, J. Membr. Sci. 264 (2005) 176–189. [29] X.Y. Qiao, T.S. Chung, Diamine modification of P84 polyimide membranes for pervaporation dehydration of isopropanol, AIChE J. 52 (2006) 3462–3472. [30] D.A. Devi, B. Smitha, S. Sridhar, T.M. Aminabhavi, Pervaporation separation of isopropanol/water mixtures through crosslinked chitosan membranes, J. Membr. Sci. 262 (2005) 91–99. [31] B.V.K. Naidu, M. Sairam, K.V.S.N. Raju, T.M. Aminabhavi, Pervaporation separation of water + isopropanol mixtures using novel nanocomposite membranes of poly(vinyl alcohol) and polyaniline, J. Membr. Sci. 260 (2005) 142–155. [32] S.D. Bhat, B.V.K. Naidu, G.V. Shanbhag, S.B. Halligudi, M. Sairam, T.M. Aminabhavi, Mesoporous molecular sieve (MCM-41)-filled sodium alginate hybrid nanocomposite membranes for pervaporation separation of water–isopropanol mixtures, Sep. Purif. Technol. 49 (2006) 56–63. [33] A.A. Kittur, S.S. Kulkarni, M.I. Aralaguppi, M.Y. Kariduraganavar, Preparation and characterization of novel pervaporation membranes for the separation of water–isopropanol mixtures using chitosan and NaY zeolite, J. Membr. Sci. 247 (2005) 75–86. [34] K. Rao, M.C.S. Subha, M. Sairam, N.N. Mallikarjuna, T.M. Aminabhavi, Blend membranes of chitosan and poly(vinyl alcohol) in pervaporation dehydration of isopropanol and tetrahydrofuran, J. Appl. Polym. Sci. 103 (2007) 1918– 1926. [35] Y.M. Lee, S.Y. Nam, S.Y. Ha, Pervaporation of water/isopropanol mixtures through polyaniline membranes doped with poly(acrylic acid), J. Membr. Sci. 159 (1999) 41–46. [36] A. Svang-Ariyaskul, R.Y.M. Huang, P.L. Douglas, R. Pal, X. Feng, P. Chen, L. Liu, Blended chitosan and polyvinyl alcohol membranes for the pervaporation dehydration of isopropanol, J. Membr. Sci. 280 (2006) 815–823. [37] C.K. Yeom, J.G. Jegal, K.H. Lee, Characterization of relaxation phenomena and permeation behaviors in sodium alginate membrane during pervaporation separation of ethanol–water mixture, J. Appl. Polym. Sci. 62 (1996) 1561–1576. [38] R.Y.M. Huang, R. Pal, G.Y. Moon, Crosslinked chitosan composite membrane for the pervaporation dehydration of alcohol mixtures and enhancement of structural stability of chitosan/polysulfone composite membranes, J. Membr. Sci. 160 (1999) 17–30. [39] Z. Gao, Y. Yue, W. Li, Application of zeolite-filled pervaporation membrane, Zeolites 16 (1996) 70–74. [40] J. Sekuli, M.W.J. Luiten, J.E. ten Elshof, N.E. Benes, K. Keizer, Microporous silica and doped silica membrane for alcohol dehydration by pervaporation, Desalination 148 (2002) 19–23. [41] T. Gallego-Lizon, Y.S. Ho, L.F.D. Santos, Comparative study of commercially available polymeric and microporous silica membranes for the dehydration of IPA/water mixtures by pervaporation/vapour permeation, Desalination 149 (2002) 3–8. [42] F.P. Cuperus, R.W. van Gemert, Dehydration using ceramic silica pervaporation membranes – the influence of hydrodynamic conditions, Sep. Purif. Technol. 27 (2002) 225–229. [43] S. Sommer, T. Melin, Performance evaluation of microporous inorganic membranes in the dehydration of industrial solvents, Chem. Eng. Process 44 (2005) 1138–1156. [44] S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato, S. Hayashi, M. Hara, Hydrolysis of cellulose by amorphous carbon bearing SO3 H, COOH, and OH Groups, J. Am. Chem. Soc. 130 (2008) 12787–12793. [45] D. Gorri, A. Urtiaga, I. Ortiz, Pervaporative recovery of acetic acid from an acetylation industrial effluent using commercial membranes, Ind. Eng. Chem. Res. 44 (2005) 977–985. [46] T. Masuda, S. Otani, T. Tsuji, M. Kitamura, S.R. Mukai, Preparation of hydrophilic and acid-proof silicalite-1 zeolite membrane and its application to selective separation of water from water solutions of concentrated acetic acid by pervaporation, Sep. Purif. Technol. 32 (2003) 181–189. [47] T.C. Bowen, H. Kalipcilar, J.L. Falconer, R.D. Noble, Pervaporation of organic/water mixtures through B-ZSM-5 zeolite membranes on monolith supports, J. Membr. Sci. 215 (2003) 235–247. [48] M. Asaeda, M. Ishida, T. Waki, Pervaporation of aqueous organic acid solutions by porous ceramic membranes, J. Chem. Eng. Jpn. 38 (2005) 336–343. [49] U.S. Toti, T.M. Aminabhavi, Different viscosity grade sodium alginate and modified sodium alginate membranes in pervaporation separation of water + acetic acid and water + isopropanol mixtures, J. Membr. Sci. 228 (2004) 199–208. [50] S.B. Teli, G.S. Gokavi, M. Sairam, T.M. Aminabhavi, Highly water selective silicotungstic acid (H4 SiW12 O40 ) incorporated novel sodium alginate hybrid composite membranes for pervaporation dehydration of acetic acid, Sep. Purif. Technol. 54 (2007) 178–186.