Facile Synthesis of Dual-Layer Organic Solvent Nanofiltration (OSN

Letter pubs.acs.org/journal/ascecg

Facile Synthesis of Dual-Layer Organic Solvent Nanofiltration (OSN) Hollow Fiber Membranes Shi-Peng Sun,*,† Sui-Yung Chan,∥ Weihong Xing,† Yong Wang,† and Tai-Shung Chung*,‡,§ †

National Engineering Research Center for Special Separation Membrane, College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China ‡ Department of Chemical & Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore § NUS Environmental Research Institute, National University of Singapore, 5A Engineering Drive 1, #02-01, Singapore 117411, Singapore ∥ Department of Pharmacy, National University of Singapore, 18 Science Drive 4, Singapore 117543, Singapore S Supporting Information *

ABSTRACT: A dual-layer organic solvent nanofiltration (OSN) hollow fiber membrane was prepared by a single-step coextrusion process with polybenzimidazole as the outer selective layer and hyperbranched polyethylenimine cross-linked polyimide as the inner support layer. The OSN membrane shows a rejection of >99% against methylene blue (MW: 319.85 g mol−1) with good solvent fluxes in water, methanol, and acetonitrile. The newly invented fabrication technology may provide simple, cost-effective, scalable, and high-performance OSN membranes for organic solvent recovery.

KEYWORDS: Organic solvent nanofiltration (OSN), Hollow fiber membrane, Dual-layer, Polybenzimidazole, Polyimide, Cross-link

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Here we report for the first time a polybenzimidazole/ hyperbranched polyethylenimine (HPEI) cross-linked polyimide dual-layer hollow fiber OSN membrane by a one-step online cross-linking process in the dry-jet wet spinning process. The conceptual schematic is shown in Figure 1. The method utilizes a triple-orifice spinneret as shown in Figure S2 of the Supporting Information (SI). PBI and P84 polyimide are fed simultaneously into the outer and middle annulus of the spinneret, respectively. PBI was chosen as the outer selective layer because of its high solvent stability and controllable subnano-sized pores during the nonsolvent induced phase separation process. P84 polyimide was selected as the inner layer because the imide group can be easily cross-linked by the imine group of HPEI, as proposed in Figure S3 of the SI.21 10 wt % HPEI is premixed in the bore fluid with a ratio of nmethyl-2-pyrrolidone/water = 1:1 wt %. HPEI has a wide range of molecular weights available. In previous works, the HPEI cross-linked polyimide served as the barrier layer.20 Therefore, the HPEI molecular weight as high as 10 000 g mol−1 was used so as to increase the rejection. In contrast, in this work, a molecular weight of 2000 g mol−1 was chosen so that the HPEI

INTRODUCTION Organic solvent nanofiltration (OSN) has recently emerged as an important separation technology for solvent recovery in pharmaceutical syntheses and many other applications.1−6 Significant efforts have focused on improving membrane stability in harsh organic solvents by cross-linking polymers such as polyimide and polybenzimidazole (PBI).7−13 The crosslinked membranes then served as either integral asymmetric membranes for OSN or porous substrates for the fabrication of thin-film composite membranes.14−19 The cross-linking process consumes significant amounts of time, solvents, and posttreatments to deliver optimal OSN performance. To simplify the cross-linking process, several studies have proposed to cross-link the polyimide membranes during the phase inversion step. Vankelecom and his co-workers investigated the cross-linking of polyimide flat-sheet membranes with diamines premixed in the coagulation bath.15 The Stamatialis team studied the cross-linking of polyimide hollow fiber membranes during the hollow fiber spinning process through premixing the cross-linker in the bore fluid.20 These attempts demonstrated the possibility of simplifying the fabrication process of OSN membranes. However, both methods resulted in OSN membranes with insufficient rejections to solutes with a molecular weight lower than 1000 g mol−1, which is typical of pharmaceutical applications. © 2015 American Chemical Society

Received: June 11, 2015 Revised: November 9, 2015 Published: November 11, 2015 3019

DOI: 10.1021/acssuschemeng.5b01292 ACS Sustainable Chem. Eng. 2015, 3, 3019−3023

Letter

ACS Sustainable Chemistry & Engineering

addition of a sufficient amount of polyethylene glycol 400 (PEG 400) into the inner dope that brings the dope composition closer to the binodal composition.24 However, the macro-void-free structure of the outer layer is due to two coupling effects of the polyhedral oligomeric silsesquioxane (POSS) nanoparticles in the outer layer and the addition of HPEI in the bore fluid. As illustrated in Figure S4 of the SI, the outer layer of the fiber without adding HPEI in the bore fluid is almost sponge-like, with a few small macro-voids near the interface (Figure S4a). The sponge-like structure of the outer layer could be due to the hydrogen bonding interaction between the −NH group of the PBI and the hydroxyl group of POSS nanoparticles that hinders the water intrusion, as discussed in details in our previous work.23 The small macrovoids are totally suppressed when HPEI is added into the bore fluid (Figure S4b). This might be due to the fact that the presence of HPEI can reduce the release rate of PEG400 from the inner layer to the bore fluid and thus hinders the rapid intrusion of the nonsolvent from the outer coagulation bath.16,20 The inner porous structures are not affected by the addition of HPEI in the bore fluid, as evidenced in Figure S4 of the SI, suggesting that HPEI with a molecular weight of 2000 g mol−1 freely enters the polyimide substrate during the phase separation process. XPS results, as listed in Table 1, confirm the cross-link mechanism proposed in Figure 1 and Figure S3 of the SI. The

Figure 1. Schematic of the one-step fabrication of dual-layer hollow fiber OSN membranes.

molecules could freely enter the polyimide ultrafiltration substrate and cross-link with the whole polyimide substrate to maximize the cross-linking efficiency. Because the PBI selective layer has small pore sizes and is inert to HPEI, the leak of HPEI molecules to the coagulation bath is minimized.

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Table 1. XPS Analysis of the Dual-Layer Hollow Fiber Membranes with and without Cross-Link

RESULTS AND DISCUSSION Figure 2 shows the overall morphology of the cross-linked duallayer hollow fiber membrane. The membrane has an

atomic concentration (%) outer surface inner surface

cross-link

N1s

O1s

C1s

no yes no yes

3.81 4.07 6.74 11.85

15.62 15.26 13.97 13.12

80.57 80.67 79.29 75.03

content of N1s element is almost doubled on the inner surface of the cross-linked polyimide inner layer, compared to the plain substrate. However, the nitrogen content did not show a significant increase on the outer surface of the PBI outer layer after the cross-linking process. This suggests the HPEI molecules were effective in cross-linking the polyimide inner layer. In the meantime, it was prevented from entering the coagulation bath by the steric hindrance effect of the PBI outer layer. The cross-link mechanism is further confirmed by the pore structural parameters, as listed in Table 2 and pore size distribution graphs in Figure S5 of the SI, which were characterized by 200 ppm neutral organic solutes with progressively increased molecular weight, i.e., glucose, saccharose, raffinose, and α-cyclodextrin, at neutral pH according to the solute transport method elaborated in the SI. Table 2 shows that the membranes before and after cross-link had a similar

Figure 2. Morphology of the cross-linked dual-layer OSN hollow fiber membrane. (a) cross section; (b) outer layer; (c) outer surface; (d) inner surface.

asymmetric structure with a dense surface on the outer layer (∼1.5 μm thick) while a porous surface beneath the inner layer (∼90 μm thick). The membrane structure has two unique characteristics not found in conventional dual-layer hollow fiber membranes. First, the hydrogen-bonding interactions between carbonyl groups in P84 and −NH groups in PBI result in good attachment between the two layers.22 Such hydrogen-bonding induced miscibility eliminates the need for addition of polyvinylpyrrolidone (PVP), a bridging molecule, into the PBI spinning dope, which significantly lowers the water flux.23 Second, both the outer and inner layers are macrovoid-free. The macrovoid-free structure in the inner layer is due to the

Table 2. Pure Water Permeabiility, Mean Effective Pore Size, MWCO, and Collapse Pressure of the Dual-Layer Hollow Fiber Membranes with and without Cross-Link

3020

crosslink

PWP (l m−2 h−1 bar−1)

dp (nm)

MWCO (g mol−1)

collapse pressure (bar)

no yes

4.88 3.54

0.76 0.81

500 510

12.5 13.5

DOI: 10.1021/acssuschemeng.5b01292 ACS Sustainable Chem. Eng. 2015, 3, 3019−3023

Letter

ACS Sustainable Chemistry & Engineering pore size and MWCO while the difference of pore size distribution as shown in Figure S5 of the SI is also marginal. These imply the cross-link process did not have a significant impact on the pore structures of the PBI layer. In addition, the MWCO of the membrane is 510 g mol−1 whereas the molecular weight of HPEI is 2000 g mol−1. This indicates that the PBI layer restricts most HPEI molecules from entering the coagulation bath to prevent a significant loss of the crosslinker. However, there is a slight increase in pore size and the pore size distribution becomes broader. Therefore, there may be a trace amount of HPEI migrating into the coagulation bath. It can also be found that there was a 27% reduction of PWP as shown in Table 2, resulting mainly from the free volume reduction of the cross-linked polyimide substrate. The crosslinked fiber has a slight higher collapse pressure, which might be due to the fact that the cross-linked structure enhances fiber mechanical strength with a slight sacrifice of solvent permeability. The OSN performance of the cross-linked dual-layer hollow fiber membranes was tested with 25 ppm methylene blue (MB, C16H18N3SCl, 319.85 g mol−1) in water, methanol and acetonitrile as these solvents are commonly used in the pharmaceutical industry. The feed and permeate solutions collected at 1 bar were characterized by a UV−vis spectrometer (Pharo 300, Merck). The absorbance spectra at wavelengths between 400 and 800 nm are displayed in Figure 3. The UV

Figure 4. (a) Solvent permeability and (b) methylene blue rejection of the cross-linked dual-layer hollow fiber OSN membranes in various solvents.

that the fluxes of various solvents are linear under low pressure. However, membrane compaction under high pressure is inevitable. The rejections of molecules with various charge properties for the membranes with and without cross-linking are listed in Table S3 of the SI. Both membranes exhibit almost 100% rejections to methylene blue, which is positively charged. The rejections to negatively charged and neutral molecules of uncross-linked membranes are slightly higher than those of the cross-linked membranes. This phenomenon may be resulted from the coupling effects of the following reasons: The first is the more positive surface charge brought by the slightly increased N content, as implied by the XPS result. The other is the slightly larger surface pore size, which is evidenced in Table 2. The fouling and cleaning of the cross-linked dual-layer hollow fiber membranes were examined in the following sequence and the results are displayed in Figure 5. First, the

Figure 3. UV−vis absorbance spectra of the feed and permeate of methylene blue in (a) water, (b) methanol, and (c) acetonitrile.

absorbance for each MB/solvent mixture dropped to almost zero after the NF process, indicating superior rejections (Figure 4) of MB occurred in each solvent. The diagram on the top left of Figure 3 shows the blue color of the feed solutions totally disappears after the OSN process. Figure 4 also depicts the permeabilities of the pure solvents as water > methanol > acetonitrile. Such a trend is determined by the two physical properties listed in Table S2 of the SI. First, the solvent flux decreases with an increase in solvent molar volume because a large solvent size causes greater transport resistance through the membrane pores.25,26 Second, the methanol permeability is higher than that of acetonitrile primarily because the difference in solubility parameter between PBI and methanol (|δPBI − δMethanol| = 2.1 cal1/2 cm−3/2) is smaller than that between PBI and acetonitrile (|δPBI − δAcetonitrile| = 4.7 cal1/2 cm−3/2). Figure S6 of the SI illustrates the fluxes of various solvents through the cross-linked hollow fiber membranes as a function of pressure. The membranes were conditioned at 13 bar for 1 h prior to each test. It can be found

Figure 5. Solvent permeability of pure solvent, MB/solvent mixture, and water of the cross-linked dual-layer hollow fiber OSN membranes in various solvents.

membranes were tested with pure solvents to obtain the pure solvent permeability. After that, the MB/solvent mixtures were tested to get the solution permeability and rejections. After the testing, the membranes were rinsed by the individual pure solvent. Finally, the pure water permeability was obtained. From Figure 5, it can be observed that the flux of the MB/ 3021

DOI: 10.1021/acssuschemeng.5b01292 ACS Sustainable Chem. Eng. 2015, 3, 3019−3023

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ACS Sustainable Chemistry & Engineering

(5) Escobar, I. C.; Van der Bruggen, B. Modern Applications in Membrane Science and Technology; American Chemical Society Books Series; American Chemical Society: Washington, DC, 2011. (6) Cheng, X. Q.; Zhang, Y. L.; Wang, Z. X.; Guo, Z. H.; Bai, Y. P.; Shao, L. Recent advances in polymeric solvent-resistant nanofiltration membranes. Adv. Polym. Technol. 2014, 33, DOI: 10.1002/adv.21455. (7) Flanagan, M. F.; Escobar, I. C. Novel charged and hydrophilized polybenzimidazole (PBI) membranes for forward osmosis. J. Membr. Sci. 2013, 434, 85−92. (8) Vanherck, K.; Koeckelberghs, G.; Vankelecom, I. F. J. Crosslinking polyimides for membrane applications: A review. Prog. Polym. Sci. 2013, 38, 874−896. (9) Valtcheva, I. B.; Kumbharkar, S. C.; Kim, J. F.; Bhole, Y.; Livingston, A. G. Beyond polyimide: Crosslinked polybenzimidazole membranes for organic solvent nanofiltration (OSN) in harsh environments. J. Membr. Sci. 2014, 457, 62−72. (10) Xing, D. Y.; Chan, S. Y.; Chung, T.-S. The ionic liquid [EMIM]OAc as a solvent to fabricate stable polybenzimidazole membranes for organic solvent nanofiltration. Green Chem. 2014, 16, 1383−1392. (11) Chisca, S.; Duong, P. H. H.; Emwas, A. H.; Sougrat, R.; Nunes, S. P. Crosslinked copolyazoles with a zwitterionic structure for organic solvent resistant membranes. Polym. Chem. 2015, 6, 543−554. (12) Roy, S.; Ntim, S. A.; Mitra, S.; Sirkar, K. K. Facile fabrication of superior nanofiltration membranes from interfacially polymerized CNT-polymer composites. J. Membr. Sci. 2011, 375, 81−87. (13) Shi, G. M.; Wang, Y.; Chung, T.-S. Dual-layer PBI/P84 hollow fibers for pervaporation dehydration of acetone. AIChE J. 2012, 58, 1133−1145. (14) Solomon, M. F. J.; Bhole, Y.; Livingston, A. G. High flux hydrophobic membranes for organic solvent nanofiltration (OSN)Interfacial polymerization, surface modification and solvent activation. J. Membr. Sci. 2013, 434, 193−203. (15) Vanherck, K.; Cano-Odena, A.; Koeckelberghs, G.; Dedroog, T.; Vankelecom, I. A simplified diamine crosslinking method for PI nanofiltration membranes. J. Membr. Sci. 2010, 353, 135−143. (16) Kopeć, K. K.; Dutczak, S. M.; Wessling, M.; Stamatialis, D. F. Chemistry in a spinneretOn the interplay of crosslinking and phase inversion during spinning of novel hollow fiber membranes. J. Membr. Sci. 2011, 369, 308−318. (17) Sun, S. P.; Chung, T. S.; Lu, K. J.; Chan, S. Y. Enhancement of flux and solvent stability of Matrimid® thin-film composite membranes for organic solvent nanofiltration. AIChE J. 2014, 60, 3623−3633. (18) Sun, S. P.; Chan, S. Y.; Chung, T. S. A slow−fast phase separation (SFPS) process to fabricate dual-layer hollow fiber substrates for thin-film composite (TFC) organic solvent nanofiltration (OSN) membranes. Chem. Eng. Sci. 2015, 129, 232−242. (19) Shao, L.; Cheng, X.; Wang, Z.; Ma, J.; Guo, Z. Tuning the performance of polypyrrole-based solvent-resistant composite nanofiltration membranes by optimizing polymerization conditions and incorporating graphene oxide. J. Membr. Sci. 2014, 452, 82−89. (20) Dutczak, S. M.; Tanardi, C. R.; Kopec, K. K.; Wessling, M.; Stamatialis, D. ″Chemistry in a spinneret″ to fabricate hollow fibers for organic solvent filtration. Sep. Purif. Technol. 2012, 86, 183−189. (21) Ba, C. Y.; Langer, J.; Economy, J. Chemical modification of P84 copolyimide membranes by polyethylenimine for nanofiltration. J. Membr. Sci. 2009, 327, 49−58. (22) Wang, Y.; Goh, S. H.; Chung, T. S. Miscibility study of Torlon (R) polyamide-imide with Matrimid (R) 5218 polyimide and polybenzimidazole. Polymer 2007, 48, 2901−2909. (23) Fu, F. J.; Zhang, S.; Sun, S. P.; Wang, K. Y.; Chung, T. S. POSScontaining Delamination-free Dual-layer Hollow Fiber Membranes for Forward Osmosis and Osmotic Power Generation. J. Membr. Sci. 2013, 443, 144−155. (24) Liu, Y.; Koops, G. H.; Strathmann, H. Characterization of morphology controlled polyethersulfone hollow fiber membranes by the addition of polyethylene glycol to the dope and bore liquid solution. J. Membr. Sci. 2003, 223, 187−199.

mixture is about 15% to 20% lower than that of the pure solvent, which could be due to the increment of MB on the membrane surface. Because the membrane possesses a small pore size, the MB did not block the pores. Therefore, the pure water permeability recovered after a thorough rinse.

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CONCLUSION In summary, a dual-layer OSN hollow fiber membrane was synthesized via a facile but efficient one-step cross-linking process. The dual-layer OSN membrane also offers high rejections to low molecular weight compounds in various solvents. The newly invented method is much superior to the conventional hollow fiber technology for OSN membranes because it not only reduces solvent consumption during membrane formation but also eliminates tedious post-treatment steps for cross-linking reaction.13 Therefore, this study offers a potential platform for fabrication of simple, cost-effective, scalable, and high-performance OSN membranes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01292. Materials, membrane fabrication conditions, testing procedures, detailed FESEM images, and pore size distributions (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*S.-P. Sun. E-mail: [email protected]; [email protected]. *T.-S. Chung. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank GlaxoSmithKline-Economic Development Board (GSK-EDB) Trust Fund (R-706-000-019-592) and Singapore National Research Foundation under its Competitive Research Program for the project entitled, “Development of solvent resistant nanofiltration membranes for sustainable pharmaceutical and petrochemical manufacture” (NRF2014NRF-CRP002-006). The authors also thank National Natural Science Foundation of China (21506094) and a project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors also appreciate Mr. Feng-Jiang Fu for his valuable help.

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REFERENCES

(1) Vandezande, P.; Gevers, L. E. M.; Vankelecom, I. F. J. Solvent resistant nanofiltration: separating on a molecular level. Chem. Soc. Rev. 2008, 37, 365−405. (2) Marchetti, P.; Jimenez Solomon, M. F.; Szekely, G.; Livingston, A. G. Molecular Separation with Organic Solvent Nanofiltration: A Critical Review. Chem. Rev. 2014, 114, 10735−10806. (3) Kim, J. F.; Szekely, G.; Schaepertoens, M.; Valtcheva, I. B.; Jimenez-Solomon, M. F.; Livingston, A. G. In Situ Solvent Recovery by Organic Solvent Nanofiltration. ACS Sustainable Chem. Eng. 2014, 2, 2371−2379. (4) Zhu, J.; Zhang, Y.; Tian, M.; Liu, J. Fabrication of a Mixed Matrix Membrane with in Situ Synthesized Quaternized Polyethylenimine Nanoparticles for Dye Purification and Reuse. ACS Sustainable Chem. Eng. 2015, 3, 690−701. 3022

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ACS Sustainable Chemistry & Engineering (25) Bhanushali, D.; Kloos, S.; Kurth, C.; Bhattacharyya, D. Performance of solvent-resistant membranes for non-aqueous systems: solvent permeation results and modeling. J. Membr. Sci. 2001, 189, 1− 21. (26) Darvishmanesh, S.; Degreve, J.; Van der Bruggen, B. Physicochemical characterization of transport in nanosized membrane structures. ChemPhysChem 2010, 11, 404−11.

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DOI: 10.1021/acssuschemeng.5b01292 ACS Sustainable Chem. Eng. 2015, 3, 3019−3023