Sulfonated polybenzimidazole membranes for pervaporation dehydration of acetic acid

Journal of Membrane Science 415–416 (2012) 486–495

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Sulfonated polybenzimidazole membranes for pervaporation dehydration of acetic acid Yan Wang a, Tai Shung Chung a,n, Michael Gruender b a b

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576 PBI Performance Products, Inc., 9800-D Southern Pine Boulevard, Charlotte, NC 28273, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 January 2012 Received in revised form 5 May 2012 Accepted 18 May 2012 Available online 29 May 2012

In this study, a novel sulfonated polybenzimidazole (SPBI) membrane has been developed and investigated for pervaporation dehydration of acetic acid, via a two-step sulfonation modification technique—sulfonation with sulfuric acid followed by a thermal treatment at 450 1C. Both steps are found indispensible in order to produce a stable SPBI membrane with enhanced acid resistance and superior separation performance. Effects of the sulfuric acid concentration and thermal treatment duration have been investigated and found to have significant impact on the pervaporation performance of the resultant SPBI membranes. Various characterizations (FTIR, XPS, TGA and XRD) are employed to elucidate the physicochemical changes of membranes as a function of chemical and thermal modifications. In addition, effects of pervaporation temperature and feed composition are studied not only in terms of flux and separation factor, but also of membrane intrinsic permeance and selectivity. The best pervaporation performance of the SPBI membrane has a flux of 207 g/m2/h and a separation factor of 5461 for dehydration of a 50/50 wt% acetic acid/water feed solution at 60 1C, which not only outperforms the conventional distillation process, but also surpasses most other polymeric pervaporation membranes reported in literature. It is therefore believed that the novel developed SPBI membrane may have great potential for pervaporation dehydration of acidic organics, as well as other applications that demand acid-proof materials. & 2012 Elsevier B.V. All rights reserved.

Keywords: Pervaporation dehydration Polybenzimidazole Sulfonation Acid resistance Acetic acid Operation condition

1. Introduction Acetic acid is an important chemical reagent, and has extensive applications in chemical, pharmaceutical and food industries. Its global demand in 2010 was 7 million tons and is expected to increase to 11.3 million tons by 2015 [1]. Acetic acid can be produced in very low concentrations (less than 0.1 wt%) during fermentation, where ethanol, acetone, butanol etc are the main products [2]. In the pharmaceutical industry, aqueous acetic acid is also formed as a by-product during the manufacture of aspirin. Further separation of acetic acid/water mixture is therefore required to obtain acetic acid of higher purity or avoid environmental pollution.

Abbreviations: AA, acetic acid; AN, acrylonitrile; DMAc, n,n-dimethylacetimide; FTIR, Fourier transform infrared spectroscopy; GA, glutaraldehyde; HEMA, hydroxyl ethyl methacrylate; LiCl, lithium chloride; H2SO4, sulfuric acid; PAA, poly(acrylic acid); PAN, polyacrylonitrile; PBI, polybenzimidazole; PEK-C, cardo polyetherketone; PS, polysulfone; PVA, poly(vinyl alcohol); PVC, poly(vinyl chloride); SPBI, sulfonated polybenzimidazole; SPEK-C, sulfonated cardo polyetherketone; STA, silicotungstic acid; TEOS, tetraethylorthosilicate; TGA, thermogravimetric analysis; XPS, X-ray photoelectron spectrometer n Corresponding author. Tel.: þ65 6516 6645; fax: þ65 6779 1936. E-mail address: [email protected] (T. Shung Chung). 0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.05.035

In today’s chemical processing industry, acetic acid is dehydrated using normal binary distillation columns; however, despite that acetic acid and water do not form an azeotrope, it is very difficult to separate them by distillation alone because they have very close volatilities [3]. As the progressive distillation produces a solution with less and less water, each further distillation becomes less effective to remove the remaining water. As a result, more energy is required to achieve acetic acid with purity higher than 95 wt% due to the need for greater reflux and a larger distillation column with many stages. Distilling the solution to dry acetic acid is therefore economically impractical. Alternatively, pervaporation is an effective technology for the acetic acid/water separation for being highly selective, economical, energy efficient and environmentally benign. In recent 20 years, many research works have been reported on pervaporation dehydration of acetic acid. Pervaporation is one of the most promising technologies for molecular-scale liquid/liquid separations existing in biorefinery, petrochemical, pharmaceutical industries etc [2,4–7]. It has been used to remove trace substances in liquid mixtures, such as removing water from organic solvents to produce high-purity solvents. Significant attention has been given to highly hydrophilic polymers, such as poly(acrylic acid) (PAA), poly(vinyl

Y. Wang et al. / Journal of Membrane Science 415–416 (2012) 486–495

alcohol) (PVA), sodium alginate, and chitosan, etc as membrane materials for pervaporation dehydration in the earlier stage. However, these materials—even when cross-linked, generally lack mechanical strength and stability in aqueous solutions due to the excessive swelling, thereby leading to a drastic decrease in separation performance [5,7]. For acetic acid dehydration, hydrophilic PVA-based membranes are most widely used [8–15]. Other polymers, such as polysulfone (PS) [16], poly(vinyl chloride) (PVC) [17], sulfonated cardo polyetherketonehave (SPEK-C) [8,9,18], poly(4-methyl-1pentene) [19], have also been explored as pervaporation membrane materials for acetic acid dehydration. Compared to the above conventional polymeric materials, polybenzimidazole (PBI) is a more suitable candidate as membrane material for the pervaporation dehydration [20–24]. As a high performance aromatic polymeric material, PBI is suitable for aggressive environments because of its outstanding chemical resistance and thermal stability. It is a glassy polymer with a high Tg (417 1C) [25] and has stable mechanical properties up to 350 1C. In addition, PBI possesses both donor and acceptor hydrogen-bonding sites [26,27], which are capable of participating in specific interactions [28,29]. PBI is also known to absorb 15 wt% water at equilibrium and the water in PBI is mobile [30]. Water can preferentially permeate the PBI membrane due to its stronger affinity with PBI molecules and smaller molecular size relative to most organics. All the above characteristics make PBI a promising pervaporation membrane material for the dehydration of various organics. Despite the promising characteristics of PBI material, its unique tendency to form a PBI:acid complex confounds its application for pervapoartive dehydration of acidic solvents since the PBI membrane has great sorption selectivity for acid as well as water. The reaction mechanism is shown in Fig. 1. Therefore, some modification techniques have to be sought to solve this problem. Principally, PBI should be modified in such a way that it can resist the attack from acids, i.e., the modification which can decrease the affinity of PBI membrane with acid in order to improve its separation efficiency. Presently, there are many modification methods [31–33] for the PBI material, such as cross-linking, n-substitution, sulfonation, etc. Among them, PBI sulfonation or phosphonation may be an effective method. Via this modification, a sulfonate group or phosphonate group will be attached to the imidazole ring and the as-modified PBI molecule could probably resist the attack from acids if they are weaker than sulfuric acid or phosphoric acid. In this work, sulfonated PBI (SPBI) membranes are successfully developed for the pervaporation dehydration of acetic acid. With this sulfonation modification, excellent pervaporation performance of SPBI membranes is achieved in terms of both flux and separation factor. The effects of sulfonation parameters and operation conditions on pervaporation performance are investigated. Via this study, the sulfonation technique may potentially H N

N

N

N H

H N

H N

C

+ CH3COO-H+

C

C N

+

N H

+ C

+ CH3COO-

Fig. 1. The reaction mechanism of the formation of PBI-acid complex.

487

open new perspectives for future research and development of PBI membranes for purification and separation in the fields of energy production, environmental impact mitigation, and pharmaceutical synthesis. It may also facilitate the applications of PBI materials with an enhanced acid-resistance and stability in acidic environments.

2. Experimental 2.1. Materials The PBI polymer solution was provided by PBI Performance Products, Inc. with a composition of 26.2 wt% PBI, 72.3 wt% n,n-dimethylacetimide (DMAc), and 1.5 wt% lithium chloride (LiCl). The LiCl serves the function of preventing PBI from phasing out of the solution. DMAc, employed as the solvent for membrane preparation, were supplied by Merck with analytical grade and used as received. Concentrated sulfuric acid (H2SO4) of analytical grade, obtained from Merck was used to mix with deionized water to prepare the sulfonation solution with various concentrations. Glacial acetic acid of analytical grade, also from Merck, was used to mix with deionized water to prepare the binary feed solution. 2.2. The fabrication of SPBI membranes Flat-sheet PBI dense membranes were cast from a 15 wt% PBI/DMAc polymer solution, which was prepared by diluting the original supplied PBI solution. The diluted solution was allowed to degas overnight prior to casting onto a glass plate with a casting knife at a thickness of about 100 mm. The as-cast membrane was then placed on a hot plate preset at 75 1C for 15 h, to allow the solvent evaporated slowly. The resultant film was peeled off carefully from the glass plate, immersed in water overnight to remove LiCl inside membrane, and then dried in a vacuum oven between two wire meshes. The oven temperature was gradually increased to 250 1C at a rate of 0.6 1C/min and held there for 24 h to remove the residual solvents before cooling down naturally. The wire meshes not only prevented the membrane from sticking to the glass plate but also helped uniformly dry the membrane from both surfaces. As such, the as-prepared membrane with ‘‘quasidense’’ morphology is not exactly symmetric membrane, since the bottom side of the membrane contacted the glass plate and formed relative porous morphology as compared to the top side of the membrane [7,34]. However, the difference might be trivial and even cannot be differentiated easily via SEM observation. For pervaporation tests in this work, the top side of the membrane was always used to face feed in order to avoid any inconsistency. Sulfonation modification of the PBI membrane was carried out via the following steps: (1) immersing the as-fabricated PBI membrane in a sulfuric acid aqueous solution of a fixed concentration at 50 1C for 2 h; (2) blotting the membrane with filter papers to remove the superficial acid solution; followed by (3) subsequently thermally treating it in a furnace pre-set at 450 1C for certain duration in air environment (without vacuum). Thereafter, the samples were immersed in boiling water for 3 h to remove unreacted sulfuric acid and then dried between two wire meshes at 100 1C in the a vacuum oven. A Mitutoyo micrometer was then employed to measure the final membrane thickness, which is about 15–20 mm. 2.3. Pervaporation study A static pervaporation cell made according to Prof. Matsuura’s design [4] was used for the pervaporation test of the flat-sheet

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membrane. The design schematic is available in our previous publication [20]. A test membrane was placed in the stainless steel permeation cell with an effective surface area of 15.2 cm2. A binary feed solution of 50/50 wt% water/acetic acid was used unless stated elsewhere. The feed concentration was considered to be constant since the feed solution is of a large quantity of compared to the permeate sample and less than 0.5 wt% variation was detected during the entire experiment. The operation temperature was room temperature (2272 1C) unless stated elsewhere. The permeate pressure was maintained less than 3 mbar by a vacuum pump. Retentate and permeate samples were collected after the membrane being conditioned for about 2 h. The flux J was determined by the mass of permeate divided by the product of the interval time and membrane area. The mass of the permeate sample was weighed using a Mettler Toledo balance. The separation factor a is defined by the equation below:

a¼

yw,1 =yw,2 xw,1 =xw,2

ð1Þ

where subscripts 1 and 2 refer to water and acetic acid, respectively, yw and xw are the weight fractions of component in the permeate and feed, and were analyzed through a Hewlett– Packard GC 7890 A with a HP-INNOWAX column (packed with cross-linked polyethylene glycol) and a TCD detector. The flux and separation factor were converted to the permeability (or permeance) and selectivity according to the basic transport equation for pervaporation as below.   P p J i ¼ i Uðxn,i gi psat ð2Þ i yn,i p Þ l where Pi is the membrane permeability of the component i, a product of diffusivity and solubility coefficients, l is the membrane thickness, xn,i and yn,i are the mole fractions of the component i in the feed and permeate, respectively, gi is the activity coefficient calculated by the Wilson equation, psat is the i saturated vapor pressure determined by the Antoine equation, and pp is the permeate pressure. The term xi gi psat i is defined as the feed fugacity of the component i and calculated with the aid of the Aspen DISTILl software (version 2004.1) [21]. The term [Pi/l], known as permeance, is often employed for an anisotropic membrane with an unknown thickness of the dense selective layer. The total permeance is defined as the sum of the permeance of all individual components, instead of the total flux divided by the total pressure difference because each component has individual feed fugacity and permeate partial pressure. The ideal membrane selectivity b is the ratio of the permeability or the permeance of two components. 2.4. Membrane characterization The changes in the chemical structure of membranes were monitored by a Perkin–Elmer FTIR Spectrum 2000 with a resolution of 2 cm. Each membrane sample was purged with gaseous nitrogen for 20 min before the spectrum was obtained with an average of 16 scans. An X-ray photoelectron spectrometer (Kratos XPS System-AXIS His-165 Ultra) was used to measure the surface chemical compositions of original and sulfonated PBI membranes. The thermal decomposition behavior of the membranes were characterized with a TGA 2050 Themogravimetric Analyzer (TA Instruments), with a ramp of 5 1C/min at the temperature ranging from 50 to 800 1C, under nitrogen atmosphere. The contact angle measurements were performed by a Rame´ -Hart Contact Angle Goniometer (model 100-22) at room temperature. Deionised water droplets were introduced by a Gilmont microsyringe onto the membrane surface. Wide-angle X-ray diffraction (XRD) measurements of the membranes were carried out by a Shimadzu

XRD-6000 X-ray diffractometer using copper radiation (Ka) with a ˚ at 40 kV and 30 mA. The wavelength (l) of 1.54  1010 m (1.54 A) average intersegmental distance of polymer chains (d-space) was reflected by the broad peak center on each X-ray pattern. The d-space was calculated by the Bragg’s equation as follows: nl ¼ 2d siny

ð3Þ

where y is the X-ray diffraction angle of the peak. All membrane samples were dried in a vacuum oven for overnight before the above tests. Sorption tests were carried out by immersing the pre-weighed dry membrane strips in DI water and pure acetic acid solutions, respectively. The swollen samples were taken out at different time intervals, blotted between tissue papers, and then weighed in a closed container. It was repeated until the equilibrium of sorption was reached after about one week (when the membrane weight showed no significant change). The sorption was calculated from the difference between the wet weight Mwet after equilibrium sorption and the dry weight Mdry as follows: Sorption ðg=g membraneÞ ¼

M wet M dry M dry

ð4Þ

3. Results and discussion 3.1. Pervaporation performance of original and sulfonated PBI membranes There are mainly three ways to add sulfonate groups into the PBI polymer backbone, i.e., (1) direct sulfonation of PBI molecules [31,32], (2) chemical grafting of monomers that comprise sulfonate groups [35], and (3) sulfonation after the radiation grafting of monomer groups [36]. The first method of direct sulfonation is employed in this work, which results in grafting sulfonate groups on the PBI backbone. The two-step mechanism of this sulfonation modification is reported previously [31] as shown in Fig. 2. In the 1st step, the amidine cations are formed when PBI molecules contact with sulfuric acid molecules. The attached sulfonate groups form ionic bonds with highly electronegative nitrogen atoms in the imidazole groups of PBI. The 2nd step is the attachment of sulfonate groups to the aromatic rings via thermal treatment at high temperatures. The thermal treatment here is a critical step to convert the ionic bond between the sulfonate groups and PBI molecules to permanent covalent bonds [32], so that the sulfonate groups will be retained after washing by water. Table 1 demonstrates the pervaporation performance of the original, thermaltreated and sulfonated PBI membranes. The results show the pervaporation performance of the PBI membranes in its neat form is quite poor, where a separation factor of about 7 and a flux of 100 g/m2/h are obtained. This occurs because the imidazole rings in the PBI molecules tend to react with the acetic acid (pKa ¼4.75) in the feed solution and the formed PBI-acid complex (pKa ¼ 5.5) [37] (Fig. 1), is a thermally and chemically stable polyionomer. Therefore, acetic acid has a higher solubility with the PBI membrane than that of water, leading to the poor separation for acetic acid dehydration. To show the importance of both steps in the sulfonation modification, PBI membranes with only thermal treatment at high temperature or only sulfonation with sulfuric acid are investigated for acetic acid dehydration. Table 1 shows that without sulfonation, thermal treatment alone does not have a positive effect on the separation performance of the PBI membrane. Similarly, without post thermal treatment, sulfonation by sulfuric acid alone cannot effectively improve separation performance either. As we mentioned above, only the combination of a sulfonation step and then a high-temperature thermal treatment (Z450 1C), the sulfonate groups can be covalently bonded to the PBI benzene rings in a stable state. Successful attachment of sulfonate groups helps the SPBI membranes to lower the affinity towards acetic acid but increase the affinity with water simultaneously because of its improved hydrophilicity, resulting in the significantly enhanced separation performance of the SPBI membrane. The SPBI membrane sulfonated with a 2.5 wt% sulfuric acid solution followed by a thermal treatment at 450 1C for 30 s shows both an impressive separation factor of 6631 and a comparable flux of 168 g/m2/h. The successful sulfonation modification of PBI membranes is characterized by FTIR and some other characterizations, which will be discussed in details in the following sections. The SPBI membranes mentioned in the following sections refer to the membranes with the sulfonation modification of the two steps, if not stated otherwise. Water contact angle measurements and sorption tests are carried out in order to investigate the affinity of the original and sulfonated PBI membranes towards the feed components (i.e., water and acetic acid). However we noticed that all sulfonated PBI membranes exhibit higher water contact angles (80–891) than that of the original membrane (681). This is because the water contact angle on the

Y. Wang et al. / Journal of Membrane Science 415–416 (2012) 486–495

489

Thermal treatment at 450 ˚C

Fig. 2. Sulfonation mechanism of PBI material.

Table 1 Pervaporation performance of the original, thermal-treated, and sulfonated PBI membranes. Sulfonation modification

Post thermal treatment

Permeate (AA wt%)

J (g/m2/h)

Separation factor

N.A. N.A. 2.5 wt% H2SO4 2.5 wt% H2SO4

N.A. 450 1C, 30 s N.A. 450 1C, 30 s

13.60 14.91 3.247 0.015

100 49 96 168

7 7 30 6631

Membranes

Sorption solution

Degree of sorption (g/g membrane)

Original Sulfonateda

DI-water pure AA DI-water pure AA

0.63 0.80 0.44 0.41

a The sulfonated membrane was PBI membrane sulfonated with a 2.5 wt% sulfuric acid solution followed by a thermal treatment at 450 1C for 30 s.

AA: acetic acid. material surface is not only determined by its hydrophilicity, but also affected by the roughness. Thermal treatment leads to the decrease in surface roughness, while according to the Wenzel Equation [38] (Eq. (5)), a decrease in surface roughness will lead to an increase in water contact angle on the hydrophilic surface, but a decrease on the hydrophobic surface. cosyW ¼ rcosyY

Table 2 Degrees of sorption of the original and sulfonated PBI membranes.

ð5Þ

In Eq. (5), yW is the observed contact angle on a rough surface, yY is the contact angle on a smooth surface, r is the roughness ratio which compares the true surface area of a rough surface with the area of a comparably sized smooth surface, and it is always larger than one. Therefore, sorption tests are further carried out to elucidate the change of sorption ability of the sulfonated PBI membrane in water and acetic acid as compared to the original one. Table 2 shows that the original PBI membrane is of a higher degree of sorption in acetic acid than in water in spite of the smaller size of water molecules, while the sulfonated one shows similar degrees of sorption in both solutions, indicating an improved acid-resistance of the PBI membrane after sulfonation modification. The sulfonated PBI membranes are of significant lower degrees of sorption in both water and acetic acid solutions, which are believed to be ascribed to the densified membrane structure by high-temperature thermal treatment. 3.2. Effect of sulfuric acid concentration on the pervaporation performance of SPBI membranes It has been reported that PBI molecules react with sulfuric acid immediately once they are immersed in the sulfuric acid solution [31]. It is therefore expected that a low concentration of sulfuric acid solution is effective for the sulfonation modification of PBI membranes. Table 3 demonstrates the effect of sulfuric acid concentration on the pervaporation performance of sulfonated PBI membranes for a 50/50 wt% acetic acid/water feed solution. The post thermal treatment for all membranes is carried at 450 1C for 30 s. As expected, the results show that a low sulfuric acid concentration of 2.5 wt% is enough to enhance the pervaporation performance significantly. A further increase in sulfuric acid concentration to 5 wt% shows little improvement on separation factor, but causes a slight decrease in permeation flux. With an additional increase in sulfuric acid concentration to 10 wt%, both permeation flux and separation factor drop significantly. Several effects on the SPBI membrane result from an increase in sulfuric acid concentration: (1) The initial increase in sulfuric acid concentration enhances the attachment of sulfonate groups to the benzene rings of PBI molecules. It leads to the higher hydrophilicity, which contributes to both higher flux and separation factor. (2) At the same time, higher hydrophilicity also means a higher degree of swelling of the sulfonated PBI membrane. It reduces the separation efficiency and mechanical rigidity. (3) In addition, it was reported that a strong sulfonating agent, such as sulfuric acid, may damage the structure of an asymmetric membrane [31]. So it is suspected that a high sulfuric acid concentration may cause defects on the membrane surface and a drop in separation performance. The above three factors therefore explain the up-and–down trends of the flux and separation factor.

Table 3 Effect of sulfuric acid concentration on the pervaporation performance of sulfonated PBI membranes. Sulfonation modification

Post thermal treatment

Permeate (AA wt%)

J (g/m2/h)

Separation factor

N.A. 2.5 wt% H2SO4 5 wt% H2SO4 10 wt% H2SO4

450 1C, 450 1C, 450 1C, 450 1C,

14.91 0.015 0.014 3.5

49 168 138 99

7 6631 7156 27

30 s 30 s 30 s 30 s

AA: acetic acid. To confirm that the sulfonate group is covalently bonded to the PBI polymer chain after the sulfonation modification, FTIR characterizations of the original and sulfonated PBI membranes are carried out and the curves are shown in Fig. 3 as a function of sulfuric acid concentration. All membranes here are with a post thermal treatment at 450 1C for 30 s except the original one. With the addition of sulfonate groups attached to the SPBI membranes, new peaks of O ¼S¼ O stretching vibrations [35,39] at 1231, 1174, 1086 and 867 cm and C–S stretching vibration at 603 cm [40] are observed as compared with the original PBI membrane. Since electrophilic substitution on the phenyl ring is preferred, the possible attachment of the sulfonate group to the nitrogen on the imidazole ring can be ruled out, although both give similar absorption spectrums [40]. As the concentration of the sulfuric acid solution increases, it can be observed that the intensity of the new peaks also increases slightly, implying more sulfonate groups attached to the PBI chain. However, the increment for the concentration range of 2.5–10 wt% is small because the sites available for bonding are already largely occupied when the PBI membrane is sulfonated by 2.5 wt% sulfuric acid [32,41]. Another new peak at 1633 cm may be ascribed to the C¼ N and C ¼ C stretching [35,40,42]. It is associated with the protonation of the imide groups with the addition of HSO4 anions. This peak is especially visible between the curves of the original membrane and the one sulfonated with 2.5 wt% sulfuric acid. The other peak appearing in the region between 3000 and 2500 cm, a characteristic of N–H þ group absorption, further proves the protonation of imide groups [43]. In summary, the FTIR result confirms the covalent bonding of sulfonate groups to the PBI backbone, as the postulated sulfonation mechanism above. As the amidine cation can form on both sides of the benzimidazole group, the attachment of a second sulfonate group on the benzene ring of the PBI molecule can be expected with a higher concentration of the sulfuric acid. Theoretically, if the sulfonation modification is uniform across the whole membrane structure, the weight gain of the modified PBI membrane with one sulfonate group replaced is 26 wt%, while it could be 52 wt% if two groups are replaced, according to their chemical structures. However, since the sulfonation modification of the PBI membrane is through the membrane surface, so the weight gain should be much lower than the theoretical data. Fig. 4 shows the measured weight gain of SPBI membranes after sulfonation modification increases with an increase in sulfuric

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C=N C=C 1633

N-H+ 2750

10 wt.%

O=S=O C-S 603 1174 1231 1086 867

5 wt.% 2.5 wt.% 0 wt.% original

4000

3600

3200

2800

2400

2000

1600

1200

800

400

Wavenumber (cm-1)

30

4

25

Sulfur content (wt.%)

Membrane weight increase (wt.%)

Fig. 3. FTIR characterization of the original and sulfonated PBI membranes treated with sulfuric acid solutions of various concentrations.

20 15 10

3

2

1

5 0

0

0

2

4 6 8 H2SO4concentration (wt.%)

10

0

12

Fig. 4. Effect of sulfuric acid concentration on the weight gain of SPBI membranes. acid concentration. The weight gain is defined as the weight increase percentage of the dry membrane weight after sulfonation compared to the dry membrane weight before sulfonation. A weight gain of about 15–24 wt% is obtained for the SPBI membranes after sulfonation modification. To further verify the increase in the amount of sulfonate groups with a higher concentration of sulfuric acid solution, XPS is employed to investigate the elemental composition on the membrane surface. The theoretical mass concentration of sulfur on membrane surface is 8.5 wt% if only one sulfonate group is bonded to each PBI structural unit (C20N4H12). The XPS results of the sulfur content on the membrane surface are shown in Fig. 5. Interestingly, it indicates a relatively lower weight gain when compared with the results in Fig. 5. This deviation is probably because that the XPS test only measures the sulfur content on membrane surface while the weight gain is based on the membrane bulk mass. It also implies that the sulfonation actually occurs within the bulk of the membrane, and the sulfonate groups on membrane surface tend to be decomposed easily during post thermal treatment. However, both figures show that an increase in sulfuric acid concentration results in an increase in the amount of sulfonate groups attached to the SPBI membrane. They also follow a similar trend as the FTIR curves, where sulfur content increases sharply for the membranes treated with sulfuric acid solutions with a concentration between 0 to 2.5 wt%, and then remains relatively constant for higher sulfuric acid concentrations.

3.3. Effect of post thermal treatment duration on pervaporation performance of SPBI membranes The effect of post thermal treatment duration on the pervaporation performance of the resultant SPBI membranes was studied using a feed composition of 50/50 wt% acetic acid/water. All PBI membranes discussed in this section have

2

4 6 8 H2SO4 concentration (wt.%)

10

12

Fig. 5. Sulfur contents of SPBI membranes treated with sulfuric acid solutions of various concentrations.

Table 4 Effect of post thermal treatment duration on the pervaporation performance of sulfonated PBI membranes. Thermal treatment duration (s)

Permeate (AA wt%)

J (g/m2/h)

Separation factor

0 10 20 30 60 120

3.247 6.309 0.016 0.015 0.019 1.15

96 110 124 168 201 114

30 46 6348 6631 5341 86

AA: acetic acid. been sulfonated with a 2.5 wt% sulfuric acid solution before the thermal treatment. Table 4 shows that, with an increase in thermal treatment duration up to 30 s, both permeation flux and separation factor are improved, whereas a further increase in thermal treatment duration results in declines in both permeation flux and separation factor simultaneously. The optimal thermal treatment duration is about 30 s. As discussed above, the thermal treatment after sulfonation at high temperatures is an important step to attach sulfonate groups to the aromatic rings because it stabilizes the sulfonated structure and enhances separation performance. However, over exposure of the PBI membrane to high-temperature thermal treatment may also cause the degradation of the attached sulfonate groups since

Y. Wang et al. / Journal of Membrane Science 415–416 (2012) 486–495

Weight loss (%)

100

90

80

70 0

200

400 600 Temperature (°C)

800

Fig. 6. TGA characterization sulfonated PBI membranes with thermal treatment for (a) 0, (b) 10, (c) 30, and (d) 120 s.

Sulfur concentration (wt.%)

4

3

2

1

0 0

30 60 90 120 Thermal treatment time (Second)

150

Fig. 7. Sulfur contents of sulfonated PBI membranes thermally treated for various durations.

491

they are thermally labile. It may result in a decrease in separation performance. These two competing factors determine the attached amount of the sulfonate groups on the PBI membrane surface. In addition, thermal treatment at temperatures above 450 1C induces relaxation and re-packing of PBI polymeric chains as well as free-radical thermal cross-linking [22]. As a consequence, the thermallytreated membrane has a denser morphology with fewer defects with enhanced separation factor but reduced flux. However, a much longer thermal treatment also has the possibility to increase the transport resistance of the membrane, causing the decline in both flux and separation factor. To prove our hypotheses about the thermal stability of sulfonate groups, TGA characterizations of the original and sulfonated PBI membranes are shown in Fig. 6. The results show that the weight loss of the original PBI sample begins at about 600 1C, while there are 3 stages of weight loss in SPBI membranes. The first weight loss starting at about 100 1C is mainly due to the adsorbed moisture; the 2nd stage of weight loss from 350 1C to 450 1C is probably contributed by the degradation of the sulfonic groups [41]; and the weight loss at the 3rd stage at about 600 1C corresponds to the degradation of the PBI backbone [44,45]. Clearly, the addition of the sulfonate groups to the PBI polymer chain impairs the thermal stability of the PBI membrane. Additionally, we notice that the difference in weight loss of the SPBI membranes with various thermal treatment durations is very small and cannot be differentiated easily via TGA characterization. Therefore, XPS characterization of the sulfonated membranes is further explored to verify the sulfur content change on the membrane surface with the increase in thermal treatment duration as shown in Fig. 7. The sulfur content on membrane surface increases with increasing duration of thermal treatment from 10 to 30 s but then decreases with longer durations of thermal treatment. This phenomenon is consistent with the change of separation performance of the SPBI membranes as shown in Table 4. FTIR characterizations of the original and sulfonated PBI membranes with various thermal treatment durations are carried out as shown in Fig. 8. With the initial increase in thermal treatment duration, a significant rise in the intensity of the absorption peaks at 1174 cm and 1231 cm of O ¼S¼ O is observed, which correspond to the stretching vibration of the sulfonate groups. In addition, it is observed that the peaks at 1086 cm and 867 cm representing stretching vibrations of the sulfonate groups [41], as well as C–S stretching vibrations at 603 cm become more obvious. Also, the intensity of these peaks is reduced if the duration of thermal treatment is longer than 30 s. This suggests that prolonged thermal treatment could break the C–S bonds, leading to fewer sulfonate groups on PBI chains. These results exhibit a trend similar to the XPS results. The XRD characterization verifies the effect of thermal treatment on membrane morphology. As shown in Fig. 9, SPBI membranes thermally treated for short durations (10–60 s) exhibit peaks at higher 2y values than those of the original one and the one treated for 120 s, indicating lower d-space values. The smaller d-space for membranes under shorter durations of thermal treatment is ascribed to the denser morphology and thermal-induced cross-linking, while the larger d-space for the membrane under a longer duration of 120 s is believed due to the degradation of the sulfonate groups after a prolonged thermal treatment.

3.4. Effects of operation conditions on the pervaporation performance of SPBI membranes Based on the solution diffusion model, the permeability of the permeant across the pervaporation membrane is a product of its diffusivity and solubility, which is not only dependent on the characteristics of the membrane but also on

O=S=O C-S

1231

1174 1086

867

603

120 s 60 s 30 s 20 s 10 s 0s

original

1600

1400

1200

1000

800

600

400

Wavenumber (cm-1) Fig. 8. FTIR characterization of the original and sulfonated PBI membranes thermally treated for various durations.

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the operation conditions such as operation temperature, feed composition, permeate pressure, etc. As operational conditions change, the physicochemical properties of polymeric membranes such as membrane morphology, free volume and its distribution vary significantly, resulting in the varied diffusion rates of the components. In addition, operation conditions also affect the mutual interactions among permeates and membrane materials, thus changes the equilibrium solubility of the permeants. As a result, these effects alter mass transport coefficients of permeants across the membrane. In this section, the effects of operation temperature and feed composition on the pervaporation performance of the SPBI membranes are investigated. In this section, all PBI membranes have been sulfonated with a 2.5 wt% sulfuric acid solution and thermally treated at 450 1C for 30 s. Permeance and selectivity are employed here to elucidate the intrinsic separation performance of the pervaporation membranes by decoupling the external process parameters [7,21,46]. We aim to provide: (1) useful criteria for the selection of appropriate operation conditions for this SPBI based pervaporation system to ensure the highest performance and economic viability; and (2) some insights on the transport mechanism of this pervaporation process through the study of the influence of external operation conditions. Permeance instead of permeability is used in this work, since the sulfonated PBI membrane does not have exact uniform morphology and the sulfonation modification is not uniform across the membrane. The effect of operation temperature on the separation performance of the SPBI membranes is shown in Table 5 using a feed composition of 50/50 wt% acetic acid/ water. The results show that the total flux increases while the separation factor decreases with an increase in operation temperature. Table 5 also exhibits the separation performance in terms of permeance and selectivity. However, the total permeance as well as the individual permeance of water and acetic acid follow the opposite trends with that of flux. The increase in operation temperature has manifold influence on permeants’ permeabilities in a pervaporation process. First, it will bring about the increase in driving forces for the feed components to diffuse across the membrane because a high temperature increases the saturated vapor pressure in the feed side while the downstream permeate pressure remains the same (near zero). Second, the interactions among feed components and membrane materials become weaker at high temperatures, which include (1) the solubility of the feed components in the membrane, (2) the swelling effect on the membrane by water molecules, and (3) the coupling effect between water and acetic acid molecules. The decreasing permeance trends are mainly due to the reduction in the solubilities of both water and acetic acid molecules and the weakened swelling effect when the operation temperature increases. Similar phenomena have been reported in previous research works [21,24,34,47,48]. The flux increase is primarily contributed by

Intensity

120 s

60 s 30 s 10 s 0s original

5

7

9

11

13

2 (C) Fig. 9. XRD characterization of the original and sulfonated PBI membranes thermally treated for various durations.

the increase in the driving forces of the feed components, according to the relationship between the flux and permeance as described in Eq. (2). Some other effects have also been reported to explain the enhanced permeation flux with increasing operation temperature; namely: (1) the increase in thermal motion of polymer chains and free volume inside the polymeric membrane; and (2) the enhanced mass transfer coefficients of the permeating components. However, since the permeabilities exhibit decreasing trends in the current study, these two factors are considered minor compared to the negative impact of the reduced sorption and the positive impact of increased saturated vapor pressure with increasing temperature. Table 5 also shows that both separation factor and selectivity decrease with an increase in operation temperature, which is probably caused by the reduced sorption abilities of the membrane towards water molecules at higher temperatures. The relationship of flux or permeance of a penetrant across a membrane with operation temperature can be described by the Arrhenius equations as follows [49]: J ¼ J0 expðEJ =RTÞ

ð6Þ

P ¼ P 0 expðEP =RTÞ

ð7Þ

where J0 and P 0 are the pre-exponential factors of flux and permeance, respectively, R is the universal gas constant, T is the operation temperature, and EJ and EP are the apparent activation energies of flux and permeability, respectively. Good linearity exists between logarithmic flux and permeance vs. reciprocal temperature and the values of regression coefficients R2 are close to unity, indicating the experimental data fits the Arrhenius equation well. EJ and EP can be calculated from these figures using the least square method. Table 6 summarizes the calculated values of EJ and EP for water, acetic acid and total feed components. Compared to water, acetic acid has a larger kinetic diameter and a much lower driving force (i.e., lower vapor pressure) to transport through the membrane. Therefore, it has higher activation energies of flux and permeance than those of water molecule (i.e., EJ,AA 4EJ,w, EP,AA 4EP,w). Interestingly, we also notice that the activation energies of permeance for water and acetic acid are both negative. These results reconfirm our hypothesis that the decrease in sorption with increasing temperature overshadows the increase in thermal motion of polymer chains and mass transfer coefficients of the penetrants in this SPBI pervaporation system. The effect of feed composition on the pervaporation performance of the SPBI membranes tested at room temperature is shown in Table 7. All PBI membranes have been sulfonated with 2.5 wt% sulfuric acid solution and thermal treated at 450 1C for 30 s. The results show that flux decreases but separation factor increases with an increase in acetic acid feed concentration. On the other hand, water permeance shows an interesting increasing trend with the increase in acetic acid feed concentration, while the acetic acid permeance exhibit no obvious trend, so the total permeance and the selectivity also increase with an increase in acetic acid feed concentration. Therefore, the decrease in the permeation flux is mainly caused by the reduction in the driving force of the water component. At the same time, a higher acetic acid feed content also means less swelling induced by water on the hydrophilic SPBI membranes. Acetic acid molecules may also tend to form dimmers [47] with larger sizes at high concentrations, which make them difficult to transport through the membrane. Hence the selectivity and separation factor increase. Tables 5 and 7 show that acetic acid compositions in permeate are well below 0.05 wt% for all the operation conditions studied in this work. This high separation efficiency cannot possibly be achieved by means of conventional distillation technologies due to the close boiling points of acetic acid and water. Table 6 Apparent activation energies of flux and permeability for SPBI membranes in AA dehydration. Component

EJ (kJ/mol)

EP (kJ/mol)

Water Acetic acid Total

7.69 11.91 7.76

 38.33  32.48  37.96

Table 5 Effect of the operation temperature on the pervaporation performance of SPBI membranes. T (1C) Permeate (AA wt%) J (g/m2/h) Separation factor Water permeance (g/m2/h kPa) AA permeance (g/m2/h kPa) Total permeance (g/m2/h kPa) Selectivity 22 40 60 80

0.0151 0.0164 0.0183 0.0195

AA: acetic acid.

168 203 207 276

6631 6215 5461 5110

72 32 12 7

4.22 1.8 0.86 0.54

77 34 13 7

17 18 14 12

Y. Wang et al. / Journal of Membrane Science 415–416 (2012) 486–495

493

Table 7 Effect of the feed composition on the pervaporation performance of SPBI membranes. Feed (AA wt%)

Permeate (AA wt%)

J (g/m2/h)

Separation factor

Water permeance (g/m2/h kPa)

AA permeance (g/m2/h kPa)

Total permeance (g/m2/h  kPa)

Selectivity

50 60 70 80 90 95

0.015 0.022 0.027 0.03 0.036 0.04

168 151 142 121 98 86

6631 6692 8825 13,000 24,000 39,000

69 69 70 72 77 102

4.2 4.7 4.5 3.7 3.1 2.6

73 73 74 76 80 105

17 15 16 19 26 35

AA: acetic acid, Operation temperature: room temperature.

Table 8 A comparison of pervaporation performance of polymeric membranes for AA dehydration. Feed (AA wt%)

Membrane

Separation factor (water/AA)

Flux (g/m2/h)

T (1C)

Reference

50 50 50 50 50 70 70 70 70 80 80 80 80 80 90 90 90 90 90 90 90 90 90 95

SPEK-C/PVA blend membrane GA cross-linked STA filled SPEK-C/PVA blend membrane on PEK-C substrate PVA membrane grafted with AN & HEMA SPBI membrane SPBI membrane SPEK-C membrane SPEK-C membrane PVA membrane grafted with AN & HEMA SPBI membrane SPEK-C membrane PS hollow fiber membrane PVC/PAN composite membrane with porous PAN support layer PVC/PAN bi-layer membrane with dense PAN support layer SPBI membrane poly(4-methyl-1-pentene) membrane modified with 4-vinylpyridine PVA membrane grafted with AN PVA-silicone hybrid membrane cross-linked by TEOS using sol–gel method PVA cross-linked by malic acid SPEK-C/PVA blend membrane GA cross-linked STA filled SPEK-C/PVA blend membrane on PEK-C substrate PVA cross-linked by PAA PVA membrane cross-linked by amic acid based on m-phenylene diamine SPBI membrane SPBI membrane

40 60 4 6631 5461 144 56 4 8825 69 63.5 182-274 5027 13,000 807 14.6 1116 670 59.3 91.2 795 176 24,000 39,000

850 675 550 168 207 421 590 300 142 310 511 560-740 35 121 68 90 33.3 48 492 592 5.6 12 98 86

50 50 30 22 60 30 50 30 22 50 70 80 80 22 25 30 30 40 50 50 30 30 22 22

[8] [9] [10] This study

3.5. Benchmarking Pervaporation dehydration of acetic acid had been studied by various researchers in recent years. Usually, there is a trade-off between the flux and separation factor in pervaporation although the ‘‘up-bound’’ line is not welldefined yet. A high flux is often accompanied with a low separation factor, and vice versa. Relatively, membrane materials with high selectivity are generally preferred, as the disadvantage of low flux can be compensated theoretically by introducing asymmetry and suitable fabrication optimization. A literature search reveals that hydrophilic poly(vinyl alcohol)-based membranes are most widely used for acetic acid dehydration. Much research has been focused on new potential membrane materials as well as modification methods to improve membrane performance. Table 8 summarizes separation performances of some polymeric membranes reported in literature in the past 20 years [8–19], as compared to those of the SPBI membranes in this study, in terms of separation factor and permeation flux under similar operation conditions. It can be seen that the SPBI membranes present much higher separation factors than most other polymeric membranes. The PBI membrane sulfonated with 2.5 wt% sulphuric acid solution followed by a thermal treatment at 450 1C for 30 s shows a separation factor of 5461 and a flux of 207 g/m2/h for pervaporation dehydration of 50/50 wt% acetic acid/water feed solution at 60 1C. However, the permeation flux of this SPBI membrane is relatively low, which is probably due to its dense membrane form. Further efforts to enhance the permeation flux should be devoted by employing asymmetric or composite membrane morphologies.

4. Conclusions In this study, novel SPBI membranes have been successful developed and employed for pervaporation dehydration of acetic

[18] [18] [10] This study [18] [16] [17] [17] This study [19] [11] [12] [13] [8] [9] [14] [15] This study

acid with impressive separation performance. The following conclusions can be drawn based on our study: (1) Two steps in the sulfonation modifications, i.e., sulfonation with sulfuric acid solution and post thermal treatment at high temperatures are both indispensable for a successful sulfonation modification of stable SPBI membranes. The sulfonated membranes show great enhancement in acid-resistance and separation performance for pervaporation dehydration of acetic acid. (2) The concentration of the sulfuric acid aqueous solution for sulfonation modification plays an important role. A low sulfuric acid concentration (2.5–5 wt%) is required in order to achieve a SPBI membrane with superior separation performance. A further increase in the sulfuric acid concentration will deteriorate the separation performance of the SPBI membrane. (3) Post thermal treatment at high temperatures helps to stabilize membrane morphology and separation performance. The optimal thermal treatment duration is about 30 s. (4) An increase in operation temperature increases flux but decreases separation factor, which is a combined result of increasing components’ driving forces and membrane intrinsic properties. Both intrinsic permeability and membrane selectivity decrease with increasing temperature because of the reduced sorption, swelling and interactions among permeants. On the other hand, with an increase in acetic acid

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feed concentration, the separation factor increases while the flux decreases due to less swelling effect induced by water molecules. (5) By comparison with previous research work on pervaporation dehydration of acetic acid, SPBI membranes developed in this study show superior separation performance versus most other polymeric membranes. Further efforts for flux enhancement are needed in the future.

for funding this research. Special thanks are given to Mr. Alvin Yip and Ms. Andrea Li for their helps with experiments. Dr. Wang Yan would also like to thank World Future Foundation for awarding her PhD Prize in Environmental and Sustainability Research, 2010.

Nomenclature

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d EJ EP EJ,w EJ,AA EP,w EP,AA J J0 l n P0 Pi psat i pp ppcrit pKa R R2 Td Tg xn,i xw,i yn,i yw,i

a b

gi l y

d-space, average intersegmental distance of polymer chains A˚ apparent activation energies of flux kJ/mol apparent activation energies of permeability kJ/mol apparent activation energies of water flux kJ/mol apparent activation energies of AA flux kJ/mol apparent activation energies of water permeability kJ/mol apparent activation energies of AA permeability kJ/mol total flux g/m2/h pre-exponential factor for the component flux g/m2/h membrane thickness mm an integer in the Bragg’s equation pre-exponential factor for the component permeability g/mm/m2/h/kPa permeability of the component i g/mm/m2/h/kPa saturated vapor pressure of the component i kPa permeate pressure kPa critical permeate pressure kPa acid dissociation constant universal gas constant J/K/mol R-squared value of the trend line thermal decomposition temperature 1C glass transition temperature 1C mole fraction of the component i in the feed mol% weight fraction of component i in the feed wt% mole fraction of the component i in the permeate mol% weight fraction of component i in the permeate wt% separation factor selectivity activity coefficient of the component i Cu Ka radiation wavelength of wide-angle X-ray diffraction A˚ X-ray diffraction angle of the peak 1

Subscripts i component i in the feed w weight-based n mole-based

Superscripts p permeate side sat saturated vapor pressure

Acknowledgement The authors thank PBI Performances Products, Inc. (R-279000-279–597) and the Singapore National Research Foundation under its Competitive Research Program for the project entitled, ‘‘New Biotechnology for Processing Metropolitan Organic Wastes into Value-Added Products’’ (grant number: R-279-000-311-281)

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