Hydrophilically surface-modified and crosslinked polybenzimidazole membranes for pervaporation dehydration on tetrahydrofuran aqueous solutions

Hydrophilically surface-modified and crosslinked polybenzimidazole membranes for pervaporation dehydration on tetrahydrofuran aqueous solutions

Author's Accepted Manuscript Hydrophilically surface-modified and crosslinked polybenzoimidazole membranes for pervaporation dehydration on tetrahydr...

1MB Sizes 1 Downloads 128 Views

Author's Accepted Manuscript

Hydrophilically surface-modified and crosslinked polybenzoimidazole membranes for pervaporation dehydration on tetrahydrofuran aqueous solutions Yi-Jen Han, Wen-Chiung Su, Juin-Yih Lai, YingLing Liu

www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(14)00821-7 http://dx.doi.org/10.1016/j.memsci.2014.10.050 MEMSCI13281

To appear in:

Journal of Membrane Science

Received date: 1 July 2014 Revised date: 16 September 2014 Accepted date: 25 October 2014 Cite this article as: Yi-Jen Han, Wen-Chiung Su, Juin-Yih Lai, Ying-Ling Liu, Hydrophilically surface-modified and crosslinked polybenzoimidazole membranes for pervaporation dehydration on tetrahydrofuran aqueous solutions, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2014.10.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hydrophilically surface-modified and crosslinked polybenzoimidazole membranes for pervaporation dehydration on tetrahydrofuran aqueous solutions

Yi-Jen Han1, Wen-Chiung Su2, Juin-Yih Lai3, Ying-Ling Liu1* 1. Department of Chemical Engineering, National Tsing Hua University, #101, Sec. 2, Kuang-Fu Road, Hsinchu 30013, Taiwan 2. Chemistry Division, National Chung-San Institute of Science and Technology, Lungtan, Taoyuan 32517, Taiwan 3. R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan University, Chungli, Taoyuan 32023, Taiwan

*Corresponding author. Tel.: +886 3 5711450; fax: +886 3 5715408. E-mail address: [email protected] (Y.-L. Liu).

A manuscript submitted to the Journal of Membrane Science to be considered for publication.



1

Abstract Chemical modification has been carried out on polybenzoimidazole (PBI) membranes for pervaporation dehydration on tetrahydrofuran (THF)/water mixtures. The PBI membrane crosslinked with 30 wt% of a polybenzoxazine (CR-PBI-30) shows good stability and high separation performance for pervaporation dehydration on the THF aqueous solutions in a wide range of concentrations above 50 wt%. Incorporation of poly(styrene sulfonic acid) (PSSA) chains to CR-PBI-30 membrane surface increases the surface hydrophilicity and water dissolution/absorption of the resulting membrane (CR-PBI-30-PSSA), so as to increase its water permeability without sacrificing membrane selectivity. The CR-PBI-30-PSSA membrane shows extremely high separation factors (the water concentrations at the permeate side (Cw) > 99.99 wt%) and moderate permeation fluxes (about 180 g h-1m-2) for pervaporation dehydration on the THF aqueous solutions in concentrations above 80 wt%. The membrane also shows a Cw > 97.5 wt% and a water flux of 220 g h-1 m-2 for a 70 wt% THF aqueous solution. This work demonstrates an effective approach to prepare membranes for pervaporation dehydration on aqueous solutions of high polar solvents, especially for the feed solutions possessing high water contents.

Keyword: pervaporation; tetrahydrofuran; polybenzoimidazole; surface-modification



2

1. Introduction Although distillation has been widely used for separation of liquid-liquid mixtures basing on the difference in the boiling points of the components, it is an energy-exhausting process due to liquid-vapor phase change in the process. Moreover, distillation is not suitable for azeotropic mixtures, thermally-unstable and thermally-sensitive materials, and mixed components which have close boiling points. On the other hand, membrane-based pervaporation is a suitable alternative for the above-mentioned cases [1-4]. Combination of distillation and pervaporation units in separation processes have shown great convenience for process design, high efficiency in energy-saving, and improved separation performance [5,6]. The liquid-liquid separation in the membrane-based pervaporation process could be illustrated with the solution-diffusion model. One of the components in the feed mixture has a relatively high affinity to the membrane surface, so as to exhibit a solution selectivity for the components of the mixture. The different diffusion rates of the components in the membrane contribute to the diffusion selectivity. As a result, the selection of the membrane for pervaporation is highly dependent on the components of the feeding solution. For examples, -cyclodextrin has a molecular recognition for the xylene isomers and can increase the pervaporation separation efficiency of membranes for xylene isomeric mixtures [7]. Hydrophilic polymer membranes have been utilized for the pervaporation dehydration on aqueous solutions of organic solvents, as the membranes have relatively high water solubility and diffusivity [8-16]. Moreover, the membranes stability in the feeding solution is quite important for practical application. This issue is especially critical for polymeric membranes applied to the liquid mixtures containing high polar solvents. As a result, chemically-inert and solvent-resistant polymers, such as poly(tetrafluoroethylene)



3

[17-19] and polyimide [20,21], have been used for preparation of membranes for separation of polar solvents. Nevertheless, the membranes usually show relatively low water permeability in the dehydration operation due to their hydrophobicity. Tetrahydrofuran (THF) is a polar aprotic and suitable solvent for many organic compounds, ionic species, and organometallics. As a result, THF is widely utilized in specialty chemical and pharmaceutical industries [22]. THF forms an azeotrope with water at about 94.7 wt% and 63 oC. As mentioned above, simple distillation is not suitable for obtaining high-purity THF from dehydration purification of THF/water mixtures. This fact attracts the utilization of pervaporation processes for the dehydration on THF/water mixtures [23]. Some polymeric membranes, including polyacrylonitrile-polyvinylpyrolidone blend membranes [24], regenerated cellulose membranes [25], and poly(vinyl alcohol) (PVA) composite membranes [26], blended PVA [27,28], blended cellulous [29] membranes, and polyaniline membranes [30], have been utilized for pervaporation dehydration on THF aqueous solutions. The separation performance reported to the membranes was not high due to the relatively poor membrane stability in THF aqueous solutions. To improve the separation performance, Oikawa et al. prepared pyridine-containing poly(acylhydrazone)s which showed high water selectivity in pervaporation dehydration on THF/water mixtures [31]. Kurkuri et al. grafted polyacrylamide to sodium alginate to increase the stability of the corresponding membranes used for dehydration pervaporation on THF aqueous solutions [32]. The separation performance of the membranes was attractive, but preparation of the polymers involved complicated synthesis route. Formation of composite membranes, which possess stable porous substrates and thin selective layers, has been reported to exhibit improved membrane stability [33,34]. Commercially-available composite membranes from Celfa have demonstrated a high



4

flux (3.5 kg m-2 h-1) and moderate separation factor (about 1,900) on a 96 wt% THF aqueous solution [33]. Another effective approach to increase the membrane stability is

chemically

crosslinking

the

membranes

[35-37].

Nevertheless,

the

chemically-crosslinked membranes usually show relatively low permeation fluxes. Since THF forms an azeotrope with water at the water content about 5.3 wt%, most of the previous studies deal with the feed THF aqueous solutions in concentrations above 90 wt% for breaking the azeotropic limits. THF aqueous solutions in lower concentrations could be applied to conventional distillation and the distilled product could be further purified with the dehydration pervaporation process. As most of the dehydration membranes are made with hydrophilic or water-affinitive polymers, naturally the membranes are not stable in high-water-content solutions and not suitable for pervaporation dehydration on high-water-content feed solutions. Rao et al. [36] showed that PVA/poly(ethyleneimine) blend membranes lost their water selectivity while the feed THF aqueous solutions have water contents above 15 wt%. Nevertheless, the THF aqueous solutions to be recovered and purified could be in a wide-range of concentrations and from various sources. Not all the solutions are suitable for the distillation process prior to membrane pervaporation. A distillation-free membrane separation process could be attractive for some cases of applications, such as small quantity on-site solvent recovery and temperature-sensitive feeding solutions. Moreover, a membrane which is stable for high-water-content feed solutions could bear much concentration tolerance in operation. An early work reported the pervaporation dehydration on a 53/47 (w/w) THF/water mixture with a surface-modified Teflon membrane. The flux was about 770 g m-2 h-1, but the separation factor was quite low (about 3) [38]. As a result, it is still a critical issue to develop polymeric membranes with satisfied stability and separation performance for



5

dehydration pervaporation on THF aqueous solutions in a wide range of concentrations. In this work we report the first attempt to employ polybenzoimidazole (PBI) membranes in dehydration pervaporation on THF aqueous solutions. As PBI absorbs about 15 wt% water at equilibrium and is a high performance polymer, PBI-based membranes have been applied for dehydration pervaporation separations [39-45] on aqueous solutions of ethylene glycol, tetrafluoropropanol, acetone, acetic acid, and isopropanol. The previous studies have demonstrated the suitability of PBI-based membranes for dehydration pervaporation on aqueous solutions of polar solvents. Nevertheless, to our best knowledge, PBI-based pervaporation membranes for THF aqueous solutions have not been reported due to the less stability of PBI in THF solutions. To improve the stability of PBI-based membranes in THF aqueous solutions and their performance for dehydration pervaporation on the solutions, in the present work PBI-based membranes have been crosslinked with a reactive polymer of polybenzoxazine

(PBz)

[46-48]

and

surface

hydrophilically-modified

with

poly(styrene sulfonic acid) (PSSA) chains. As the neat PBI membrane does not show satisfied separation performance for pervaporation dehydration on THF aqueous solutions, the crosslinked and surface-modified PBI membranes has shown attractive membrane stability, permeation fluxes, and separation performance in the pervaporation dehydration on THF aqueous solutions. As most of the reported membranes were only effective for THF aqueous solutions in high concentrations (above 90 wt%), it is noteworthy that the crosslinked and surface-modified membranes are effective for feeding THF solutions in a wide THF concentrations (above 50 wt%). 2. Experimental 2.1. Materials PBI with an inherent viscosity of about 1.78 dL g-1 (measured with a solution of



6

PBI in N,N-dimethylacetamide (DMAc) at 25 oC) was prepared in our laboratory according to the conventional method [49]. The preparation and characterization of PBz have been reported in the previous paper [50]. Sodium styrenesulfonate (NaSS) was purchased from Aldrich Chemical Co. and used as received. Reagent grade solvents were used in the chemical reactions and membrane fabrication processes. 2.2. Instrumental characterization Fourier transform infrared (FTIR) spectra were recorded with a Perkin Elmer Spectrum Two FTIR. A multiple internal reflectance apparatus and a ZnSe prism were used as an internal reflection element for attenuated total reflectance method. X-ray photoelectron spectroscopy (XPS) analysis was conducted with a VG Microtech MT-500 ESCA (British) using an Al-K line as the radiation source. Differential scanning calorimetric (DSC) thermograms were recoded with a Thermal Analysis Diamond DSC instrument at a heating rate of 10 oC min-1 and under a nitrogen gas flow of 50 mL min-1. Stress-strain curves were recorded with an instron instrument (Instron model 5543 analyzer) with an elongation rate of 5 mm min-1 at ambient temperature. Water contact angles were measured with a First-Ten-Angstroms FTA-100 series instrument with pure water drops of about 5 L. The measurements of the water absorption behavior of the membranes were performed with the reported method [45]. A dry membrane in about 2.0*2.0 cm2 (with a weight of Wdry) was immersed in pure water. After a certain period of time the membrane was draw out, swept with a cleaning paper, and then weighted (Wwet,t). The water uptakes of the membrane at time t is determined by Wwet,t – Wdry. The average values and the associating error were obtained with 4 pieces of membranes. 2.3. Pervaporation separation tests The method for pervaporation dehydration experiments has been reported in the



7

previous paper [51]. The effective area of the tested membranes is 7.0 cm2 and the downstream (permeate side) pressure is about 667 to 1067 Pa. The system was operated for about 2 h to get in steady state. The data was taken in a period of 1 h. A gas chromatography (Thermo Scientific Co., Trace 1300 GC equipped with a Porapak Q column) was utilized for determination of water concentration at the permeate side (Cw). The separation factor of the membrane (water/THF) was calculated from the equation below:

D



yw,1 / yw,2

xw,1 / xw,2 where yw and xw are the weight concentrations of the permeate and feed solutions, subscripts 1 and 2 refer to water and THF, respectively. Permeation flux was determined by measuring the weight of permeate liquid through the membrane at given time. Data was obtained from the average of measuring results from four pieces of separate membranes. 2.4. Fabrication of PBz-crosslinked PBI membranes A certain amount of PBz was added to a PBI solution (3 wt% in DMAc) at 70 oC. The added amount of PBz for the 3 samples is calculated to be 10, 20, and 30 wt% of PBI, respectively. After being filtered with a cotton cloth filter and standing for one day for degassing, the solution was cast on an aluminum plate and then thermally treated under vacuum at 70 oC for 2 days for removal of solvent. The obtained materials were then thermally cured at 140 oC for 3 h, 170 oC for 2 h, and 220 oC for 0.5 h to give the PBz-crosslinked PBI membranes (CR-PBI-X, X=10, 20 and 30; thickness: 40 m). The gel fraction of the crosslinked membrane, which was measured with the insoluble weight fraction of the membranes in DMAc, is 23 %, 51 %, and 87 % for CR-PBI-10, CR-PBI-20, and CR-PBI-30, respectively. 2.5. Preparation of PSSA surface-modified crosslinked PBI membrane 

8

The CR-PBI-30 membrane was immersed in 150 mL deionized water. A continuous O3/O2 mixture stream (flow rate: 6 L min-1; O3 concentration: 28 g m-3), which was generated from an ozone generator (Ozone Group, Taiwan), was bubbled through the liquid at room temperature for about 15 min. After being purged with an argon flow for 15 min. to remove the residual O3 in the liquid, 0.5 g of NaSS was added to the water. Surface-initiated radical polymerization of NaSS was carried out at 80 oC for 3 h. The membrane was draw out, treated with a 1 N HCl(aq) to transform the PNaSS chains to be PSSA chains, washed with deionized water, and then dried at 50 oC for 24 h to give the product of CR-PBI-30-PSSA.

3.

Results and discussion

3.1. The effect of crosslinking reaction on the pervaporation performance of PBI membranes The neat PBI membrane gives a Cw of about 73 wt% (corresponding to a separation factor of about 24) for pervaporation dehydration on a 90 wt% THF aqueous solution. The result indicates that the neat PBI membrane is water-selective in the pervaporation separation process. Nevertheless, the separation factor of the membrane is not high enough for practical dehydration application. The poor selectivity of the PBI membrane could be due to its poor stability in the THF aqueous solution. In this work the PBI membranes have been chemically crosslinked to improve their stability and pervaporation separation efficiency for the THF aqueous solutions. ON the other hand, most polymers might become brittle and lose their flexibility after crosslinking reactions. To overcome this problem, in this work a polybenzoxazine (PBz) which possesses reactive benzoxazine groups in the main chains has been used as the crosslinking agent for PBI (Figure 1), as it has been reported that the crosslinked main-chain PBz samples have shown high toughness, flexibility, and film formability



9

[50,52]. As a result, the PBz-crosslinked PBI membranes (CR-PBI-X, X denotes to the weight percentage of the PBz crosslinker in the membrane composition) have shown high flexibility and film formability, which are critical for polymeric membranes. The crosslinking behavior of CR-PBI-30 sample has been traced with a DSC (Figure 2). Not an exothermic behavior is observed for the PBI membrane in the heating scan, as it does not possess any reactive chemical groups. Consequently, the exothermic peak appearing in the DSC curves of the mixture of PBz and PBI could be attributed to the performance of the ring-opening reaction of the benzoxazine groups of PBz. As the benzoxazine groups of PBz in the PBI/PBz mixture consume in the thermally crosslinking reaction, the resultant CR-PBI-30 sample does not show any exothermic peaks in its DSC thermogram. Moreover, the neat PBI membrane is readily soluble in organic solvent like DMAc. The thermally-cured CR-PBI-30 membrane has a high gel fraction of 87 wt% and almost loses its solubility in DMAc due to formation of crosslinked structure. The results also support to the successful preparation of crosslinked PBI membranes. Figure 3 collects the stress-strain curves of the prepared samples recorded with an instron machine. The neat PBI sample is somewhat tough to show an elongation at break of about 120%. Thermally-crosslinking the PBI membranes results in decreases in the elongation ability and increases in the modulus of the CR-PBI-X membranes. Compared to the values recorded with the neat PBI membrane, the CR-PBI-30 membrane exhibits an increase in modulus from 1060 to 1780 MPa and an increase in stress at break from 29.3 to 37.9 MPa. The crosslinked PBI membranes have relatively high mechanical strength. Figure 4 shows the pervaporation dehydration results of the crosslinked CR-PBI-X membranes on THF aqueous solutions of difference concentrations. As mentioned above, the separation performance of the neat PBI membrane is not good enough as the



10

Cw values were about 68-73 wt% for the feeding THF aqueous solutions of 70-90 wt%. Crosslinking the PBI membrane with the PBz crosslinker improves the membrane stability in the feeding THF solutions and significantly increases the membrane selectivity. For the case of 90 wt% THF solution, all three crosslinked CR-PBI-X membranes exhibit good membrane selectivity with Cw values above 99.99 wt%. Nevertheless, CR-PBI-10 and CR-PBI-20 membranes do not show satisfied separation performance for feeding solutions possessing high water contents. The Cw values fall in 70 to 80 wt% for these two membranes being applied to THF solutions of 70-80 wt%. The large variation in the Cw values recorded with the CR-PBI-10 and CR-PBI-20 membranes on the 70 wt% THF solution indicates that the membranes are not stable enough in the operation condition. As a result, a high fraction (30 wt%) of crosslinker (PBz) is needed for preparation of stable membranes (CR-PBI-30) for pervaporation dehydration on THF solutions with high water contents. This result is reasonable as the gel fraction of CR-PBI-30 is about 87 %, which is much higher than the values measured with CR-PBI-20 (51 %) and CR-PBI-10 (23 %). The crosslinked PBI membranes show similar results which the hydrophilic membranes exhibit in pervaporation dehydration operation. The higher water content in the feeding solution is, the lower water concentration is obtained at the permeate side. As the neat PBI membrane is not highly hydrophilic (with a water contact angle of 82o) and is stable in water, the decrease in the separation efficiency of the CRF-PBI-X membranes at high-water-content feeding solutions might not result from the membrane swollen in the THF aqueous solutions. In contrast the result could be attributed to that the increases in the THF diffusion rates in the water-swollen membranes. The separation factors of the CR-PBI-X membranes for the high-water-content THF/water mixtures increase with increasing the weight fractions



11

(X values) of the PBz crosslinker, i.e. the crosslinking density of the CR-PBI-X membranes. The Cw recorded with CR-PBI-30 membrane is 97.5 and > 99.99 wt% for the feeding THF aqueous solutions of 70 and 90 wt%, respectively. Moreover, the CR-PBz-30 membrane is effective for pervaporation dehydration on a 50 wt% THF aqueous solution, as in the case a Cw of 90 wt% is obtained. The result demonstrates that the CR-PBI-X membranes are effective for pervaporation dehydration on THF aqueous solutions in a wide range of concentrations (above 50 wt% of THF). Nevertheless, the permeation fluxes of the CR-PBI-X membrane are relatively low compared to the value recorded with the neat PBI membrane, due to their crosslinked and dense structures and low surface hydrophilicity (CR-PBI-30 exhibits a water contact angle of 97o). As hydrophilic surface modification is an effective approach to increase the water fluxes of the membranes for pervaporation dehydration applications [18,43,45], hydrophilic surface modification has been applied to the CR-PBI-30 membrane to increase its surface hydrophilicity and water permeation flux in pervaporation dehydration processes. The results are discussed below. 3.2. Hydrophilic surface modification of CR-PBI-30 membrane Hydrophilic surface modification of the CR-PBI-30 membrane has been carried out with tethering hydrophilic PSSA chains to the membrane surface by means of ozone-mediated surface-initiated radical polymerization of NaSS (Figure 1) [53]. The obtained membrane (CR-PBI-30-PSSA) has crosslinked PBI matrix and hydrophilic PSSA surface. Figure 5 collects the characterization spectra of the CR-PBI-30 and CR-PBI-30-PSSA membranes. In the ATR-FTIR spectra, both membranes exhibit the absorption peaks corresponding to benzoimidazole groups at 1535, 1444, and 1284 cm-1 and to the C=O groups of the PBz crosslinker at 1710 and 1776 cm-1. The tethered PSSA chains of CR-PBI-30-PSSA have been characterized with the absorption



12

peaks of the -SO3H groups at 1033 and 1623 cm-1, which are not observed with the spectrum of CR-PBI-30. Grafting PSSA chains to the membrane surface also results in the appearance of the sulfur signal in the XPS wide-scanning spectrum of CR-PBI-30-PSSA. As the signal is not observed with the spectrum of CR-PBI-30, it could be attributed to the sulfonic acid groups of PSSA chains tethered on the CR-PBI-30-PSSA membrane surface. As mentioned above, crosslinking the PBI membrane with PBz results in a decrease in the surface hydrophilicity of the membrane as the water contact angle increases from 82o (the neat PBI membrane ) to 97o (CR-PBI-30). Introduction of PSSA chains to the CR-PBI-30 membrane surface successfully increases its surface hydrophilicity, as the water contact angle of CR-PBI-30-PSSA is 64o (Figure 6). The hydrophilic surface of the CR-PBI-30-PSSA membrane could promote water dissolution into the membrane, so as to enhance the water permeation through the membrane in the pervaporation dehydration process basing on the solution-diffusion model. Figure 7 shows the time-dependent water absorption curves of the CR-PBI-30 and CR-PBI-30-PSSA membranes. The CR-PBI-30 membrane shows a typical first-order water absorption behavior with a saturated water uptake of about 6 wt%. After hydrophilic surface modification, the CR-PBI-30-PSSA membrane shows a relatively high water absorption rate due to the increase in the surface hydrophilicity of the membrane. The result is also coincident with the water fluxes recorded with the CR-PBI-30 and CR-PBI-30-PSSA membranes. The hydrophilic PSSA layer promotes the water dissolution into the membrane and results in the rapid water absorption at the top layer of the membrane. A semi-saturated condition appears for CR-PBI-30-PSSA after the first stage of fast water absorption. This period corresponds to the saturated water absorption of the hydrophilic surface layer, due to the relatively high diffusion



13

barrier from the hydrophilic PSSA layer to the crosslinked PBI matrix. Water absorption starts again at the time which some water diffuses into the crosslinked PBI matrix. The above results further supports to that the hydrophilic surface-modification of the membrane effectively enhances the ability of CR-PBI-30 trap water from the feeding THF aqueous solution, so as to increase the water permeation flux of CR-PBI-30-PSSA in pervaporation dehydration operation. 3.3. Pervaporation dehydration on THF/water mixtures with the CR-PBI-30-PSSA membrane The CR-PBI-30-PSSA membrane, which is a chemically-crosslinked and hydrophically-modified PBI membrane, has been applied for pervaporation dehydration on THF aqueous solutions of various concentrations (Figure 8). The results indicate that surface modification with PSSA chains does not alter the separation factors of the membrane, as both CR-PBI-30 and CR-PBI-30-PSSA membranes shows similar Cw values. It is noteworthy that the hydrophilic surface modification results in an increase in the permeation fluxes of the membrane from 180 to 215 g h-1 m-2 for a 70 wt% THF aqueous solution at 25 oC. Similar results have also been obtained with feeding a 90 wt% THF aqueous solution. In the case the CR-PBI-30 and CR-PBI-30-PSSA membranes shows a water flux of 130 and 170 g h-1 m-2 at 25 oC, respectively. The permeation fluxes of CR-PBI-30-PSSA further increases to 185 g h-1 m-2 with increasing the operation temperature to 55 oC. As the data collected in Table 1, the CR-PBI-30-PSSA membrane shows extremely high separation performance (Cw > 99.99 wt%) and moderate permeation fluxes for pervaporation dehydration on the THF aqueous solutions above 80 wt%. Moreover, the membrane is also effective for feeding a 70 wt% THF aqueous solution to show a separation factor of about 90 (corresponding to a Cw of about 97.5 wt%) and a permeation flux of 220 g h-1 m-2. A membrane with



14

high separation factor could reduce the THF contents in the permeate solutions. For the cases that THF recovery and purification is required to be done on site, the PV systems should be produce waste water which contains too much THF. As a result, the membranes with high separation factors could be still attractive for pervaporation dehydration process. An attractive pervaporation membrane for THF dehydration application has been demonstrated. The selected data recorded with the membrane pervaporation dehydration THF aqueous solutions from this work and reported literature has been collected in Table 2. Most of the previous work focused on the low-water-content THF solutions. The high fluxes recorded with the commercial membranes of Celfa CMC-VP-31 [33] and Celfa CMC-CF-23 [54] are noteworthy based on their composite structures and thin-selective layers. For the case of 90 wt% THF aqueous solution, CR-PBI-30-PSSA shows a comparable flux and a much higher separation factor compared with other reported dense membranes. Nevertheless, only few examples have been reported for high-water-content THF solutions. Kurkuri et al [32] and Ortiz et al [38] reported some pervaporative separation data for 80 wt% and 53 wt% THF aqueous solutions, respectively. Nevertheless, the membrane performance of CR-PBI-30-PSSA is superior over the previous data for the THF solutions at low concentrations.

Conclusions This work reports the PBI-based membranes for pervaporation dehydration on THF aqueous solutions. Crosslinking reaction with a reactive polymer PBz and hydrophilic surface modification with PSSA chains have been applied to the PBI membranes to improve the membrane stability and to increase the membrane hydrophilicity, respectively. The resulting membrane is effective for pervaporation dehydration on THF aqueous solutions in a wide range of THF concentrations (above



15

50 wt%). Very high separation factors and moderate permeation fluxes have been recorded with the membranes. An effective approach to prepare membranes for pervaporation dehydration on aqueous solutions of high polar solvents in a wide-range of concentrations has been demonstrated.

Acknowledgements Financial supports from the Ministry of Economic Affairs, Taiwan (Grant 102-EC-17-A-09-S1-198) is highly appreciated.

References [1]

X. Feng, R. Y. M. Huang, Liquid separation by membrane pervaporation:

A

review, Ind. Eng. Chem. Res. 36 (1997) 1048-1066. [2]

B. Smitha, D. Suhanya, S. Sridhar, M. Ramakrishna, Separation of organic-organic mixtures by pervaporation - a review, J. Membr. Sci. 241 (2004) 1-21.

[3]

L. Y. Jiang, Y. Wang, T. S. Chung, X. Y. Qiao, J. Y. Lai, Polyimides membranes for pervaporation and biofuels separation, Prog. Polym. Sci. 34 (2009) 1135-1160.

[4]

P. Peng, B. Shi, Y. Lan, A review of membrane materials for ethanol recovery by pervaporation, Sep. Sci. Technol. 46 (2011) 234-246.

[5]

S. A. Ahmad, S. R. Lone, Hybrid process (pervaporation-distillation): a review, Int. J. Sci. Eng. Res. 3 (2012) 1-5.

[6]

J. B. Haelssig, A. Y. Tremblay, J. Thibault, A new hybrid membrane separation process for enhanced ethanol recovery: Process description and numerical studies, Chem. Eng. Sci. 68 (2012) 492-505.

[7]

L. Zhang, L. L. Li, N. J. Liu, H. L. Chen, Z. R. Pan, S. J. Lue, Pervaporation behavior of PVA membrane containing -cyclodextrin for separating xylene isomeric mixtures, AIChE J. 59 (2013) 604-612.



16

[8]

S. Xiao, R. Y. M. Huang, X. Feng, Preparation and properties of trimesoyl chloride crosslinked poly(vinyl alcohol) membranes for pervaporation dehydration of isopropanol, J. Membr. Sci. 286 (2006) 245-254.

[9]

W. Lin, Q. Li, T. Zhu, New konjac glucomannan-PVA composite membrane for application in pervaporation dehydration of caprolactam solution, Chem. Eng. Technol. 35 (2012) 1069-1076.

[10] E.J. Flynn, D. A. Keane, P. M. Tabari, M. A. Morris, Pervaporation performance

enhancement through the incorporation of mesoporous silica spheres into PVA membranes, Sep. Purifi. Sci. 118 (2013) 73-80. [11] Y. L. Liu, C. Y. Hsu, Y. H. Su, J. Y. Lai, Chitosan-silica complex membranes from

sulfuric acid functionalized silica nanoparticles for pervaporation dehydration of ethanol-water solutions, Biomacromolecules (2005) 368-373. [12] Y. L. Liu, C. H. Yu, L. C. Ma, G. C. Lin, H. A. Tsai, J. Y. Lai, The effects of surface

modifications on preparation and pervaporation dehydration performance of chitosan/polysulfone composite hollow-fiber membranes, J. Membr. Sci. 311 (2008) 243-250. [13] Q.

G. Zhang, W. W. Hu, A. M. Zhu, Q. L. Liu, UV-crosslinked

chitosan/polyvinylpyrrolidone blended membranes for pervaporation, RSC Adv. 3 (2013) 1855-1861. [14] A. K. Thakur, S. G., V. Kulshrestha, V. K. Shahi, Highly stable acid–base complex

membrane for ethanol dehydration by pervaporation separation, RSC Adv. 3 (2013) 22014-22022. [15] Tao Liu, Q. F. An, Q. Zhao, K. R. Lee, B. K. Zhu, J. W. Qian, C. J. Gao,

Preparation and characterization of polyelectrolyte complex membranes bearing alkyl side chains for the pervaporation dehydration of alcohols, J. Membr. Sci. 429



17

(2013) 181-189. [16] X. S. Wang, Q. F. An, T. Liu, Q. Zhao, W. S. Hung, K. R. Lee, C. J. Gao, Novel

polyelectrolyte complex membranes containing free sulfate groups with improved pervaporation dehydration of ethanol, J. Membr. Sci. 452 (2014) 73-81. [17] J. Neel, Q. T. Nguyen, R. Clement, L. Le Blanc, Fabrization of a binary liquid

mixture by continuous pervaporation, J. Membr. Sci. 15 (1983) 43-62. [18] C. Y. Tu, Y. L. Liu, K. R. Lee, J. Y. Lai, Hydrophilic surface-grafted

poly(tetrafluoroethylene) membranes using in pervaporation dehydration processes, J. Membr. Sci. 274 (2006) 47-55. [19] Y. L. Liu, C. H. Yu, J. Y. Lai, Poly(tetrafluoroethylene)/polyamide thin-film

composite membranes via interfacial polymerization for pervaporation dehydration on an isopropanol aqueous solution, J. Membr. Sci. 315 (2008) 106-115. [20] K. Vanherck, G. Koeckelberghs, I. F. J. Vankelecom, Crosslinking polyimides for

membrane applications: a review, Prog. Polym. Sci. 38 (2013) 874-896. [21] Y. K. Ong, H. Wang, T. S. Chung, A prospective study on the application of

thermally rearranged acetate-containing polyimide membranes in dehydration of biofuels via pervaporation, Chem. Eng Sc. 79 (2012) 41-53. [22] C. S. Slater, M. J. Savelski, T. M. Moroz, M. J. Raymond, Pervaporation as a green

drying process for tetrahydrofuran recovery in pharmaceutical synthesis, Green Chem. Lett. Rev. 5 (2012) 55-64. [23] C. A. McGinness, C. S. Slater, M. J. Savelski, Pervaporation study for the

dehydration of tetrahydrofuran-water mixtures by polymeric and ceramic membranes, J. Environ. Sci. Health Part A 43 (2008) 1673-1684. [24] Q. T. Nguyen, L. Le Blanc, J. Neel, Preparation of membranes from



18

polyacrylonitrile-polyvinylpyrolidone blends and the study of their behaviour in the pervaporation of water-organic liquid mixtures, J. Membr. Sci. 22 (1985) 245-255. [25]

J. Neel, Q. T. Nguyen, R. Clement, D. J. Lin, Influence of downstream pressure on the pervaporation of water-tetrahydrofuran mixtures through a regenerated cellulose membrane (Cuprophan), J. Membr. Sci. 27 (1986) 217-232.

[26] J. Mencarini Jr., R. Coppola, C. S. Slater, Separation of tetrahydrofuran from

aqueous mixtures by pervaporation, Sep. Sci. Technol. 29 (1994) 465-481. [27] T. M. Aminabhavi, B. V. K. Naidu, S. Sridhar, Computer simulation and

comparative study on the pervaporation separation characteristics of sodium alginate and its blend membranes with poly(vinyl alcohol) to separate aqueous mixture of 1,4-dioxane or tetrahydrofuran,

J. Appl. Polym. Sci. 94 (2004)

1827-1840. [28] P. S. Rao, S. Sridhar, M. Y. Wey, A. Krishnaiah, Pervaporation performance and

transport phenomenon of PVA blend membranes for the separation of THF/water azotropic mixtures, Polym. Bull. 59 (2007) 289-298. [29] B. V. K. Naidu, K. S. V. K. Rao, T. M. Aminabhavi, Pervaporation separation of

water + 1,4-dioxane and water + tetrahydrofuran mixtures using sodium aliginate and its blend membranes with hydroxyethylcellulose – a comparative study, J. Membr. Sci. 260 (2005) 131-141. [30] P. D. Chapman, X. X. Loh, A. G. Livingston, K. Li, T. A. C. Oliveira, Polyaniline

membranes for the dehydration of tetrahydrofuran by pervaporation, J. Membr. Sci. 309 (2008) 102-111. [31] E. Oikawa, S. Tamura, Y. Arai, T. Aoki, Synthesis of pyridine-moieties-containing

poly(acylhydrazone)s and solute separation through their membranes, J. Appl.



19

Polym. Sci. 58 (1995) 1205-1219. [32] M. D. Kurkuri, S. G. Kumar, T. M. Aminabhavi, Synthesis and characterization of

polyacrylamide-grafted sodium aliginate copolymeric membranes and their use in pervaporation separation of water and tetrahydrofuran mixtures, J. Appl. Polym. Sci. 86 (2002) 272-281. [33] P. D. Chapman, X. Tan, A. G. Livingston, K. Li, T. Oliveira, Dehydration of

tetrahydrofuran by pervaporation using a composite membrane, J. Membr. Sci. 268 (2006) 13-19. [34] M. B. Patil, R. S. Veerapur, S. D. Bhat, C. D. Madhusoodana, T. M. Aminabhavi,

Hybrid composite membranes of sodium alginate for pervaporation frhyfdration of 1,4-dioxane and tetrahydrofuran, Desalination Water Treat. 3 (2009) 11-20. [35] J. Lu, Q. Nguyen, J. Zhou, Z. H. Ping, Poly(vinyl alcohol)/poly(vinyl pyrrolidone)

interpenetrating polymer networks: synthesis and pervaporation properties, J. Appl. Polym. Sci. 89 (2003) 2808-2814. [36] P. S. Rao, S. Sridhar, A. Krishnaiah, Dehydration of tetrahydrofuran by

pervaporation using crosslinked PVA/PEI blend membranes, J. Appl. Polym. Sci. 102 (2006) 1152-1161. [37] S. Ray, S. K. Ray, Dehydration of tetrahydrofuran (THF) by pervaporation using

crosslinked copolymer membranes, Chem. Eng. Proc. 47 (2008) 1620-1630. [38] P. Aptel, J. Cuny, J. Jozefowicz, G. Morel, J. Neel, Liquid transport through

membranes prepared by grafting of polar monomers onto poly(tetrafluoroethylene) films. I. Some fractionations of liquid mixtures by pervaporation, J. Appl. Polym. Sci. 16 (1972) 1061-1076. [39] T. S. Chung, W. F. Guo, Y. Liu, Enhanced Matrimid membranes for pervaporation

by homogenous blends with polybenzimidazole (PBI), J. Membr. Sci. 271 (2006)



20

221-231. [40] Y. Wang, M. Gruender, T. S. Chung, Pervaporation dehydration of ethylene glycol

through polybenzimidazole (PBI)-based membranes. 1. Membrane fabrication, J. Membr. Sci. 363 (2010) 149-159. [41] Y. Wang, T. S. Chung, R. Rajagopalan, Dehydration of tetrafluoropropanol (TFP)

by pervaporation via novel PBI/BTDA-TDI/MDI co-polyimide (P84) dual-layer hollow fiber membranes, J. Membr. Sci. 287 (2007) 60-66. [42] G. M. Shi, Y. Wang, T. S. Chung, Dual-layer PBI/P84 hollow fibers for

pervaporation dehydration of acetone, AIChE J. 58 (2012) 1133-1145. [43] Y. Wang, T. S. Chung, M. Gruender, Sulfonated polybenzimidazole membranes

for pervaporation dehydration of acetic acid, J. Membr. Sci. 415-416 (2012) 486-495. [44] G. M. Shi, T. Yang, T. S. Chung, Polybenzimidazole (PBI)/zeolitic imidazolate

frameworks (ZIF-8) mixed matrix membranes for pervaporation dehydration of alcohols, J. Membr. Sci. 415-416 (2012) 577-586. [45] Y. J. Han, K. S. Wang, J. Y. Lai, Y. L. Liu, Hydrophilic chitosan-modified

polybenzoimidazole membranes for pervaporation dehydration of isopropanol aqueous solutions, J. Membr. Sci. 463 (2014) 17-23. [46] S. K. Kim, S. W. Choi, W. S. Jeon, J. O. Park, T. Ko, H. Chang, J. C. Lee,

Cross-linked benzoxazine-benzoimidazole copolymer electrolyte membranes for fuel cells at elevated temperature, Macromolecules 45 (2012) 1438-1446. [47] S. K. Kim, T. Ko, S. W. Choi, J. O. Park, K. H. Kim, C. Park, H. Chang, J. C. Lee,

Durable cross-linked copolymer membranes based on poly(benzoxazine) and poly(2,5-benzoimidazole) for use in fuel cells at elevated temperatures, J. Mater. Chem. 22 (2012) 7149-7205.



21

[48] H. Y. Li, Y. L. Liu, Polyelectrolyte composite membranes of polybenzimidazole

and crosslinked polybenzimidazole-polybenzoxazine electrospun nanofibers for proton exchange membrane fuel cells, J. Mater. Chem. A 1 (2013) 1171-1178. [49] Suryani, Y. L. Liu, Preparation and properties of nanocomposite membranes of

polybenzimidazole/sulfonated

silica

nanoparticles

for

proton

exchange

membranes, J. Membr. Sci. 332 (2009) 121-128. [50] C. I. Chou, Y. L. Liu, High performance thermosets from a curable Diels-Alder

polymer possessing benzoxazine groups in the main chain, J. Polym. Sci. Part A: Polym. Chem. 46 (2008) 6509-6517. [51] Y. L. Liu, W. H. Chen, Y. H. Chang, Preparation and properties of chitosan/carbon

nanotube nanocomposites using poly (styrene sulfonic acid)-modified CNTs, Carbohydrate Polym. 76 (2009) 232-238. [52] C. H. Lin, S. L. Chang, T. Y. Shen, Y. S. Shih, H. T. Lin, C. F. Wang, Flexible

polybenzoxazine thermosets with high glass transition temperatures and low surface free energies, Polym. Chem. 3 (2012) 935-945. [53] C.Y. Tu, Y.L. Liu, K.R. Lee, J.Y. Lai, Surface grafting polymerization and

modification on poly(tetrafluoroethylene) films by means of ozone treatment, Polymer 46 (2005) 6976-6985. [54] I. Ortiz, D. Gorri, C. Casado, A. Urtiaga, Modelling of the pervaporative flux

through hydrophilic membranes, J. Chem. Technol. Biotechnol. 80 (2005) 397-405.



22

Table 1. Pervaporation dehydration data for PBI-based membranes on THF aqueous solutions. Water concentration at the permeate side (wt%) 73.4±10.0

Neat PBI

25

THF concentration in the feeding solution (wt%) 90

252±27

30

CR-PBI-10

25

90

>99.99

124±13

>89,990

CR-PBI-20

25

90

>99.99

127±8

>89,990

CR-PBI-30

25

90

>99.99

130±6

>89,990

CR-PBI-10

25

70

79.7±19.1

273±21

9

CR-PBI-20

25

70

72.2±5.2

264±4

6

CR-PBI-30

25

70

96.8±5.1

180±5

71

CR-PBI-30-PSSA

25

90

>99.99

170±5

>89,990

CR-PBI-30-PSSA

55

90

>99.99

185±7

>89,990

CR-PBI-30-PSSA

25

80

>99.99

206±6

>40,000

CR-PBI-30-PSSA

25

70

97.5±4.2

217±12

108

CR-PBI-30-PSSA

25

50

92.5±1.1

220±8

12

Membrane

[a]



Temperature (oC)

Flux (g m-2 h-1)

separation factor is calculated with the averaged value of water concentration at the permeate side

23

Separation factor [a]

Table 2. Comparison of polymeric membrane performance on pervaporation dehydration on THF aqueous solutions.



THF/water ratio in the feeding solution (wt/wt)

Membrane type

Temperature (oC)

Flux (g m-2 h-1)

Separation factor

Reference

96/4

Celfa CMC-VP-31

25

3,500

1,900

[33]

93/7

Celfa CMC-CF-23

65

1,540

628

[54]

90/10

Celfa CMC-VP-31

25

600

191

[33]

90/10

Polyacryamide grafted Na-Alg

30

131

216

[32]

90/10

Na-Alg/HEC

30

183

1516

[29]

90/10

CR-PBI-30-PSSA

25

170

>89,990

This work

80/20

Polyacryamide grafted Na-Alg

30

70

21

[32]

80/20

CR-PBI-30-PSSA

25

206

>40,000

This work

70/30

CR-PBI-30-PSSA

25

217

108

This work

53/47

Teflon grafted 4-vinylpyrrolidone

20

770

3

[38]

50/50

CR-PBI-30-PSSA

25

220

12

This work

24

Figure 1. (a) Chemical structure of reagents and (b) chemical reactions for preparation of PBz-crosslinked PBI membranes; (c) chemical reaction of preparation of hydrophilically surface-modified membrane CR-PBI-30-PSSA.



25

Figurre 2. DSC thhermograms of the PBI-bbased memb branes for traace of the croosslinking reaction.



26

Figu ure 3. Stresss-strain curvees of the PBI-based mem mbranes recoorded with an instron maachine.



27

Figurre 4. The peerformance of o pervaporattion dehydraation on THF F aqueous soolutions at diffferent concenntrations recorded with the t neat PBI membrane aand PBz-cro osslinked PBI membranes. m



28

Fiigure 5. (a) FTIR and (b b) XPS specttra of CR-PB BI-30 and thee surface-moodified analogue CR R-PBI-30-PS SSA.



29

Figurre 6. Surfacee water contact angles off the PBI-based membraanes. Incrorpporation of thhe hydrophillic PSSA chaains significaantly reduce the surface water angle of the CR-PBI-30-P C PSSA memb brane.



30

Figure 7. Time-dependeent water abbsorption behhavior of thee CR-PBI-30 0 and C CR-PBI-30-P PSSA membranes.



31

Figurre 8. The peerformance of o pervaporattion dehydraation on THF F aqueous soolutions at d different con ncentrations recorded wiith the CR-P PBI-30 and C CR-PBI-30-P PSSA mem mbranes.



32

 Highlights • Polybenzoimidazole membranes are used for pervaporation dehydration on THF/water mixtures. • Membrane modification with chemical crosslinks and a hydrophilic surface layer is performed. • The modified membrane is effective for dehydration from THF/water mixtures above 70 wt%.



33

Graphical Abstract (for review)

Graphic abstract

PBI Cross-linking

hydrophilic surfacemodification For pervaporation dehydration on THF aqueous CR-PBI-30-PSSA solutions