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Received 00th January 2014, Accepted 00th January 2014 DOI: 10.1039/x0xx00000x www.rsc.org/
Selective Interfacial Synthesis of Metal-Organic Frameworks on Polybenzimidazole Hollow Fiber Membrane for Gas Separation Bishnu P. Biswal,a,c Anand Bhaskar,b,c Rahul Banerjee*,a,c and Ulhas K. Kharul*,b,c Metal-organic frameworks (MOFs) have gained immense attention as the new age materials due to their tuneable properties and diverse applicability. Yet, efforts on developing promising materials for membrane based gas separation, and control over the crystal growth positions on polymeric hollow fiber membranes still remains a key challenge. In this investigation, a new, convenient and scalable room temperature interfacial MOF (ZIF-8 and CuBTC) growth on either outer or inner side of polybenzimidazole based hollow fiber membrane (PBI-BuI-HF) surface has been achieved in a controlled manner. This was made possible by appropriate selection of immiscible solvent pair and synthetic conditions. The growth of MOFs on PBIBuI-HF by interfacial method was continuous and showed appreciable gas separation performance, conveying promises towards their applicability.
Introduction Metal–organic frameworks (MOFs)1 are new class of crystalline porous materials with excellent properties like gas adsorption, catalysis, electronics, drug delivery etc.2 Although MOF chemistry has been studied extensively over last 15 years, the fabrication of MOF based membranes for gas/liquid separation has picked up attention only recently.3 There exist few reports wherein porous MOFs have grown on the surface of inorganic and organic supports such as alumina, silica, carbon materials, etc. for gases/liquids separation.4 However, such MOF@Support suffer from difficulties in scale-up due to hydrothermal synthetic approach, difficulties in continuous film fabrication, tedious synthesis, lack of support flexibility, etc.; and thus have limited practical applicability. These issues motivated researchers to grow MOFs on flexible porous polymeric membranes, wherein MOFs placed within the pores of the polymeric membrane will be responsible for separation and polymeric membrane backbone acts just as a mechanical support.5 Although there has been some reports on growth of MOFs in the polymer membranes, real benefits of MOF@polymer are yet to be realized. Moreover, MOF polymer composite membranes often result in sedimentation, agglomeration of crystals and high loading of MOF crystals, which results into brittleness of the membrane.6-8 Hence in order to design and fabricate a high performing, flexible, defect-free MOF-membrane composite, the development of both, support material as well as MOF-synthetic methodology are highly crucial to bring anticipated advantages of both these
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components into practice. On this line, polymeric hollow fiber membranes are advantageous over flat sheet and porous inorganic membrane structures due to their flexibility, ease in large scale production, high pressure stability and large membrane surface area available per unit volume.9 Notably, most of the work reported on the growth of MOFs on inorganic tubular/polymeric hollow fiber support employed hydrothermal synthesis method. Recently, only two exiting reports appeared based on microfluidic approach for processing MOF membranes on polymeric hollow fiber supports. The first one by Nair et al., wherein interfacial microfluidic membrane processing (IMMP) by two immiscible solvent (octanol/water) was demonstrated for the positional control of ZIF-8 formation on poly(amide-imide) hollow fibers installed within a stainless steel custom-made reactor.10a These membranes showed high permeance with good H2/C3H8 and C3H6/C3H8 separation factors (370 and 12, respectively). An another report by Coronas et al., in which ZIF-7 and ZIF-8 continuous membranes were synthesized on the inner surface of polysulfone hollow fiber using microfluidics.10b The resultant ZIFs/PSf membranes showed high CO2/N2 and CO2/CH4 separation factors (13.6 and 13.5, respectively). Although these reports demonstrating new routes towards the MOF@HF membrane fabrication, more simplistic methodology of MOF growth on the hollow fiber membrane surfaces that can easily be scaled up for practical applicability remains a challenging task. Herein, we demonstrate a facile, scalable and room temperature selective fabrication of MOFs (ZIF-8 and CuBTC)
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Figure 1. a) Representative digital photograph of a gas separation module [CuBTC growth on outer surface of PBI-BuI hollow fibers [(CuBTC@PBI-BuI-Out) is seen through the cut].; b) Schematic for interfacial synthesis approach of CuBTC@PBI-BuI-In and Out; c) Microscopic images of ZIF-8@PBI-BuI-In, ZIF-8@PBI-BuI-Out, CuBTC@PBI-BuI-In and CuBTC@PBI-BuI-Out composites synthesized.
on either inner or outer surfaces of polybenzimidazole based hollow fibers (PBI-BuI-HF) using interfacial11 synthesis method. High miscibility of organic ligands from volatile solvents (CHCl3 or isobutyl alcohol) into immiscible water phase containing metal salt can be a driving factor for obtaining selective MOF growth, based on the solution circulation preference on either side of the hollow fiber module. Such circulation approach could have advantages of forming continuous defect-free membranes rather than the static growth conditions that results in dense, non-continuous coatings of MOF crystals in the bore of the fiber, as nicely demonstrated by Nair et al.10a The selective MOF fabrication methodology described in this work is simpler than that of reported earlier10a and can be easily scalable, as it is developed using working membrane module of practical utility. We have chosen polybenzimidaole (PBI-BuI) hollow fibers for MOF growth because this polymer has excellent thermo-chemical stability (particularly stability in CHCl3, which is used as a solvent for the growth of ZIF-8 and CuBTC), outstanding mechanical properties at high temperatures12 and more importantly, good compatibility with MOFs, as evident from ZIF@PBI composite12a-c studied recently. Employing immiscible pair of low boiling solvents [CHCl3, isobutyl alcohol (IBA) and water] for MOF@membrane composite fabrication has advantage of easy removal of these solvents from the membrane and MOFs with mild activation process over other high boiling solvents
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such as octanol, which is commonly used for interfacial synthesis of MOFs.10 Moreover, these MOF@PBI-BuI-HF composite membranes were evaluated for gas permeation and found to offer appreciable performances in terms of permeance as well as selectivity.
Experimental Section Preparation of dope solution and fabrication of PBI-BuI hollow fibre (PBI-BuI-HF) membrane: The PBI-BuI was synthesized as per our earlier report.12d It was vacuum dried at 100 ºC for 24 hours prior to the preparation of dope solution (PBI-BuI:DMAc:LiCl = 11:83:6 on wt. basis). The spinning of PBI-BuI hollow fiber membranes was carried out using the dryjet wet spinning process. The dope solution and the bore fluid were passed through spinneret at the predefined extrusion rate to offer hollow fiber with internal diameter of 0.46 mm and outer diameter of 0.78 mm. Hollow fiber module preparation: The dry hollow fibers of PBI-BuI were used for the preparation of membrane module. A bunch of 10 fibers of 30 cm length were housed in a pipe of ½” diameter using epoxy resin. The photograph of obtained module is given in Figure 1a. The active length and active area of the membrane module is 26 cm and 63.6 cm2, respectively. The prepared module was used as such for the growth of ZIF-8 and CuBTC on either surface (inner or outer) of PBI-BuI hollow fibers present within the module. Fabrication of ZIF-8@PBI-BuI-In Composite: The Zn(NO3)2 (2.19 g; 7.34 mmol) dissolved in 150 mL of water was
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circulated through the tube side of fibers using a peristaltic pump. After 15 minutes, 150 mL CHCl3 solution of 2methylimidazole (2-mIm, 2.43 g; 29.63 mmol) was hold on to the shell side (outer side of the fibers), for 1 hour. During this period, circulation of aqueous Zn(NO3)2 solution was continued on the tube side. The solutions were then drained and this procedure was repeated twice. Finally, the membrane module was dried at 65 ºC under the vacuum for 12 hours and then used for further characterizations (Figure S2, ESI).
Paper DOI: 10.1039/C5NR00299K
Results and discussion The growth of CuBTC and ZIF-8 on PBI-BuI-HF was verified by Wide Angle X-ray Diffraction (WAXD) analysis (Figure 2). WAXD patterns of as-synthesized ZIF-8 and CuBTC in CHCl3/H2O and IBA/H2O matches well with their respective
Fabrication of ZIF-8@PBI-BuI-Out Composite: The 2methylimidazole (2-mIm, 2.43 g) dissolved in 150 mL of isobutyl alcohol (IBA) was circulated through the tube side of fibers using a peristaltic pump. After 15 minutes, aquous solution of Zn(NO3)2 (2.19 g in 150 mL) was hold on to the shell side (outer side of the fibers), for 2 hours. During this period, circulaton of 2-mIm in IBA was continued on the tube side. After that the solutions from both the sides were drained and the module was dried at 65 ºC under vacuum for 12 hours.
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Fabrication of CuBTC@PBI-BuI-In Composite: The Cu(NO3)2 (3.4 g; 14.07 mmol) dissolved in 150 mL of water was circulated through the tube side of fibers using peristaltic pump. After 15 minutes, 150 mL of CHCl3 solution of 1,3,5benzenetricarboxylic acid (BTC, 2.5 g; 11.9 mmol) and triethylamine (3.5 mL; 25 mmol) was hold on to the shell side (outer side of the fibers), for 1 hour. During this period, circulation of aqueous Cu(NO3)2 solution was continued on the tube side. The solutions were then drained and this procedure was repeated twice. Finally, the membrane module was dried at 65 ºC under the vacuum for 12 hours. Fabrication of CuBTC@PBI-BuI-Out Composite: The 1,3,5benzenetricarboxylic acid (BTC, 2.5 g) was dissolved in 150 mL of isobutyl alcohol (IBA) and triethylamine (3.5 mL), was circulated through the tube side of fibers. After 15 minutes, 150 mL aquous solution of Cu(NO3)2 (3.4 g) was hold on to the shell side (outer side of the fibers), for 2 hours. During this period, circulation of BTC/IBA/TEA solution was continued on the tube side. After that the solutions from both the sides were drained and the module was dried at 65 ºC under vacuum for 12 hours.
Characterizations The X-Ray Diffraction (XRD) analysis of MOFs and the composite membranes were carried on a Rigaku SmartLab Xray diffractometer in reflection mode using CuKα radiation (λ = 1.54 Å). The 2θ range from 5° to 40° was scanned with a scan rate of 3° min-1. Electron Microscopy (SEM) was performed on a FEI Quanta 200 3D ESEM (dual beam) instrument with a field emitter as an electron source and in FEI Nova NanoSEM 650 Scanning Electron Microscope. SEM images of membrane cross section were taken after freeze cut of membranes in LN2. Samples for SEM were gold sputtered before analyses. Fourier transform infrared (FT-IR) spectra were taken on a Bruker Optics ALPHA-E spectrometer with a universal Zn-Se ATR (attenuated total reflection) accessory in the 600-4000 cm–1 region or using a Diamond ATR (Golden Gate). Microscopy images of these hollow fibers were taken in Zeiss SteREO Discovery V20. Single-gas permeation experiments of He, N2, and C3H8 were performed at 35 °C using a variable volume method.12c Upstream pressure of range 15 psi was used while maintaining permeates side at the ambient.
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Figure 2. X-ray diffraction of a) ZIF-8 and b) CuBTC; grown on the inner and outer surface of PBI-BuI-HF using interfacial synthesis method in comparison with pristine PBI-BuI-HF, ZIF-8 (CHCl3/H2O and IBA/H2O) and CuBTC (CHCl3/H2O and IBA/H2O).
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Figure 3. SEM images showing cross section of hollow fiber, its cut along the length and zoomed view; a) and b): ZIF-8@PBI-BuI-In; c) and d): ZIF-8@PBI-BuI-Out; e) and f): CuBTC@PBI-BuI-In and g) and h): CuBTC@PBI-BuI-Out composite membranes respectively. [Top part of a-g represents the zoomed part with clear cut cross section showing the thickness of ZIF-8 and CuBTC crystals layer formed on inner and outer surface of PBI-BuI-HF; whereas bottom part of b-h showing the crystal morphology and continuous packed structures with different magnifications].
simulated patterns obtained from the single crystal X-ray data reported earlier.1e,f The WAXD profile of pristine PBI-BuI-HF membrane showed a broad amorphous hump in the 2θ range of 15°-30°, and a smaller hump at 2θ = 6°. The CuBTC@PBI-BuI and ZIF-8@PBI-BuI composite (inner and outer) membranes were analyzed by WAXD to ensure the phase purity of (CuBTC and ZIF-8) grown on the membrane surface via interfacial synthesis method. This confirms that phase pure MOF crystals have formed on the PBI-BuI-HF support. This analysis also conveys that such interfacial synthesis method with different solvent combinations does not affect the crystallinity and structural feature of MOFs. The FT-IR spectrum of pristine PBI-BuI-HF was characterized by the absorption in the range 1430-1650 cm-1 for benzimidazole units. The broad band at ~3145 cm-1 was
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ascribed to the N–H…N hydrogen bonding, while the peak at ~ 3410 cm-1 was due to the free non-hydrogen bonded N–H stretching. The peak at 2862 cm-1 in the IR spectrum could be ascribed to the presence of tert-butyl group of PBI-BuI-HF. For the CuBTC@PBI-BuI composite membrane, all major bands of CuBTC, e.g., 1647 (C=O symmetric), 1444 (C=C–Ar) and 1369 cm-1 (C–O) were appeared, which confirms the presence of CuBTC crystals within the membrane framework. Similarly the ZIF-8@PBI-BuI composite membranes showed the presence of weak bands at 3165, 2966 cm-1 (C–H bonds in the methyl group and the imidazole ring) which was merged with the strong band of pristine PBI-BuI-HF. While few medium bands at 994 cm-1 (C–H bonds in the imidazole ring) and 1107 cm-1 (C–N bonds in imidazole moiety) of ZIF-8 has been traced with other characteristic bands of PBI-BuI-HF host, which
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Nanoscale further confirms the presence of ZIF-8 crystals on the PBI-BuIHF matrix (Figure S3, ESI). Growth of CuBTC and ZIF-8 crystals on the inner and outer surfaces of PBI-BuI-HF support were further confirmed by SEM imaging (Figure 3a-h and Section S4, ESI). It is interesting to note that the crystal layers were actually stacked on one another and thus helps in covering the gaps, which almost look like a defect-free continuous sheet. Figure 3 shows the cross section at different magnifications of all the CuBTC@PBI-BuI-HF and ZIF-8@PBI-BuI-HF composites [inner (In) and outer (Out) MOF growth] membranes. This confirms that the surface of the hollow fiber membrane is covered by MOF crystals due to the diffusion of metal and ligand ions into the respective immiscible phases. As a result, MOF crystals were formed extensively at the interface of the two immiscible solvents. But the inner pore voids are not as densely populated as the top surface (pore openings). As the crystal layers increase on a surface, the chances of diffusion of the metal and ligand ions deeper inside the solution present in fiber pores decrease. This could be a reason for crystal growth predominantly seen at the respective hollow fiber surfaces. An exactly similar observation on crystal growth mechanism was observed for ZIF-8/Torlon membranes, as reported recently.10a The shapes of ZIF-8 crystals are dodecahedral and sizes ranging from ~4-8 µm for inner growth (ZIF@PBI-BuI-In). On the other hand, relatively small crystallites (~1-3 µm) with slightly different morphology were observed on the outer side of PBI-BuI-HF (ZIF-8@PBI-BuI-Out). Similarly, the crystal shapes of CuBTC are flake like having dimensions ~1 µm thickness/~5 µm length for inner growth and small (~1-2 µm) crystallites agglomerates seen for outer growth on the surfaces of PBI-BuI-HF similar to ZIF-8@PBI-BuI-Out. The exact reason for different morphologies of ZIF-8 and CuBTC crystals of both inner and outer grown hollow fibers are not fully understood to us. However, the sizes and shapes of ZIF-8 and CuBTC are expected to vary based on the nature of solvents and with the physical parameters like duration of crystallization in the mother liquor, space availability and the environment within the hollow fiber membrane. Further importantly, it has been noticed that the WAXD profiles of ZIF-8@PBI-BuIIn/Out and CuBTC@PBI-BuI-In/Out composite membranes are matching well with the WAXD profiles of synthesized ZIF-8 and CuBTC, irrespective of their crystal morphologies (Figure 2). To check the uniformity of MOF membrane formation on the hollow fibers, we obtained cross sections of the fiber and measured the membrane thickness at these locations (Figure 3ag, zoomed portion). The average thickness of the layers of CuBTC and ZIF-8 on the PBI-BuI membrane was approximately in the range of ~10-25 µm, respectively. This could also vary depending on the extent of growth of crystals with respect to time on the surface of PBI-BuI-HF membrane. EDAX elemental mapping confirms the presence of Cu and Zn atoms (yellow color) located exactly on the inner circular surface of MOF@PBI-BuI-HF (N atoms are designated in red color) composite membranes (Figure 4). We could successfully employ interfacial method for MOF growth on the inner and outer surface of the hollow fibers; without any need of critical seeding or hydrothermal growth, as demonstrated in the literature.5-8 Our ideology was, if we could grow the MOFs selectively on preferred surfaces (inner or outer) with some MOF-growth penetrating within the pores (Figure 4), the inherent limitation of MOF films being fragile can be conveniently tackled and effective molecular level
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Figure 4. a) and b) Elemental mapping of ZIF-8@PBI-BuI-In and CuBTC@PBI-BuI-In composite membranes [red corresponds to nitrogen signals from PBI-BuI-HF and yellow for Zn and Cu signals from ZIF-8 and CuBTC respectively grown selectively inside the hollow fiber membrane].
separation can be achieved through MOFs. In order to have the selective continuous growth of CuBTC and ZIF-8 crystals on the surface of PBI-BuI polymeric hollow fiber membranes, we have applied the interfacial synthesis approach, through a well thought circulation methodology and protocol, optimized via change of solvent pair and flow reversal approach for the successive growth of MOF films selectively on inner and outer surfaces as shown on Figure 1. Interfacial crystallization of ZIF-8 and CuBTC has been achieved by using immiscible low boiling solvent pairs such as CHCl3/water and IBA/water. These solvents were particularly chosen to achieve their effective removal from the MOF and membranes after the crystal growth, so that they would not interfere in the gas permeation. We have used an optimized reactants ratio for the formation of both ZIF-8 and CuBTC, the same fixed ratio used to fabricate the MOF on the surfaces of PBI-BuI hollow fiber in an interfacial manner. Nonetheless, we feel that the diffusion of ligands into aqueous phase through the interfacial boundary would control the growth of the crystal, than the reactant ratio present in respective solvent. Our focus herein is the easy MOF fabrication by using volatile solvents and circulation preferences for the selective MOF growth on either surfaces (inner and outer) of hollow fiber support. The porous nature of PBI-BuI-HF provides high surface area for the immiscible solvent interfaces to achieve effective diffusion of either the ligand or metal ion. This would provide not only the MOF nucleation possible at the interfaces, but also the effective growth because of the high surface area availability. We also believe that extremely low solubility of metal ions in organic solvents (CHCl3 and IBA) and the high solubility of ligands (2mIm and BTC) in water are governing the successive growth of MOF on the preferred surface. The affinity of ligand and metal ion to fast recombine could be another key factor for successive growth of MOF on preferred surface of PBI-BuI hollow fibers. It has been noticed that the MOF nucleation and growth occurs at the preferred surfaces wherein aqueous solution containing metal ions (Zn+2 or Cu+2) were present. These important observations are in consistence with the previous report10a on IMMP of ZIF-8.
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Journal Name DOI: 10.1039/C5NR00299K from ~4480 GPU to ~119 GPU; Table S2, ESI).13 Such a low permeance of MOF@PBI-BuI-HF composite membranes arises due to the PBI-BuI-HF support used, which itself has a very low He permeance of 6 GPU. It may be noted that the porosity of the base support can be easily increased by lowering the concentration of polymer in the dope solution.14 Further, the selective permeance of gases is indicative of continuous MOF layer formation on the top of flexible and scalable polymeric hollow fiber membranes. The chemical nature of PBI-BuI (presence of imidazole group) would additionally provide good interactions with Cu/Zn ions and thus good adhesion of MOF on the surfaces of PBI-BuI hollow fibers. In other words, as growth of MOFs on porous PBI-BuI-HF support leads to substantial improvement in the selectivity of PBI-BuI-HF. We hope that by further optimization and choosing hollow fibers support that possess higher porosity, the present methodology can give rise to better performance for practical utility. All these fundamental findings encourages enough to demonstrate the interfacial synthesis approach using volatile solvents to grow MOFs on preferred surfaces of hollow fiber membranes to serve as a separation membrane. More prominently, this methodology does not need any external seeding of MOFs, as usually done for such MOF growth on the porous substrates.4d,7
Figure 5. He permeance and its selectivity over N2 and C3H8 for pristine PBIBuI-HF and MOF@PBI-BuI-HF composite membranes. Permeance (P) expressed in GPU, (1 GPU = 1x10-6 cm.s-1.cmHg-1).
The permeance study was done using a variable volume method12c for three gases chosen based on their kinetic diameter He (2.89 Å), N2 (3.64 Å) and C3H8 (4.4 Å). Table S1 (Section S5, ESI) shows the permeance data of the pristine PBI-BuI-HF membrane, ZIF-8@PBI-BuI and CuBTC@PBI-BuI composite membranes, respectively. Due to the high porosity of pristine PBI-BuI-HF membrane, it evidenced high gas permeance (Figure 5) for all the three gases. However, when the gas permeation study was done for the CuBTC@PBI-BuI and ZIF8@PBI-BuI composite membranes (in and out), the permeance was reduced to ~1/3rd as compared to the pristine PBI-BuI-HF. These new class of MOF@PBI-BuI-HF membranes retains the flexibility while allowing direct contact of the feed gas with MOF so that the pore opening of the MOF can be utilized as channels for the gases to flow through. Further, between ZIF8@PBI-BuI and CuBTC@PBI-BuI composites, prior one has higher permeance because of the slightly lower coating thickness as confirmed from SEM images (Figure 3). This difference in thickness could be attributed to the moderate reactivity (fast nucleation) of Zn+2 ions with 2-mIm ions at the ambient condition leading to a small amount of crystals formation on the support membrane compared to that of CuBTC reactivity and the amount of crystals formation, which is very high. The pronounced decrease in permeance in case of CuBTC@PBI-BuI led to increase in the selectivity of different gas pairs than in the case of pristine hollow fiber membranes. As an example, the selectivity of He/N2 was increased from 1 to 12 and He/C3H8 from 1.1 to 17 for CuBTC@PBI-BuI-Out composite membrane. However, for CuBTC@PBI-BuI-In composite membrane, the He/N2 and He/C3H8 selectivity was found to be 8 and 8.7, respectively (Figure 5). On the other hand, in case of ZIF-8@PBI-BuI-In the selectivity of He/N2 and He/C3H8 was 3.7 and 8 respectively. However, ZIF8@PBI-BuI-Out showed low selectivity of He/N2 and He/C3H8 was 4.2 and 4.6, respectively. Although, CuBTC@PBI-BuI-Out showed an appreciable separation factor of 12 (He/N2), the He permeance value is rather low (1.3 GPU), as compared to other CuBTC@membranes reported earlier (H2 permeance ranges
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Conclusions In summary, we reveal a methodology for selective MOF growth on preferred surfaces (inner or outer) of a porous polymeric (PBI-BuIHF) substrate via interfacial synthesis approach using volatile solvents. The demonstrated methodology has its own merits owing to ease of MOF synthesis without any need of pre-seeding. Moreover, the interfacial MOF growth approach adopted employing volatile solvents (CHCl3, IBA and H2O) can be easily removed from the MOF-membrane system. Moreover, this work is performed using membrane module of practical applicability that can be easily scalable. This work also provided an understanding to draw benefits of flexible polymeric hollow fiber membrane porosity and its material (PBI-BuI) properties; as well as peculiarities of ligand/metal ion partitioning in immiscible solvent pairs to grow MOFs in a preferred manner. The separation performance of these MOF-HF composite membranes towards He, N2 and C3H8 were demonstrated that MOFs took part in molecular discrimination. Among all composite membranes synthesized, CuBTC@PBI-BuI-Out displayed an appreciable permeance and selectivity of 12 (He/N2) and 17 (He/C3H8). This work opens up an easy methodology of using volatile solvents for MOF fabrication on HF working modules. The understanding generated would help to develop composite membranes for wider range of applications of gases/liquids separation area in near future.
Acknowledgements B. P. B and A. B acknowledge UGC and CSIR, New Delhi, India, for fellowship. U. K and R. B acknowledge CSIR’s Five Year Plan (CSC0122 and CSC0115) for funding.
Notes and references a
Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. HomiBhabha Road, Pune-411008, India. Fax: (+) 9120-25902636, Tel: + 91-20-25902535. E-mail: [email protected]
b
Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India. Fax: (+) 9120-25902618, Tel: + 91-20-25902180. E-mail: [email protected]
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Academy of Scientific and Innovative Research (AcSIR), New Delhi, India
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Paper DOI: 10.1039/C5NR00299K 10. (a) A. J. Brown, N. A. Brunelli, K. Eum, F. Rashidi, J. R. Johnson, W. J. Koros, C. W. Jones, S. Nair, Science 2014, 345, 72; (b) F. CachoBailoa, S. Catalána, M. Etxeberría-Benavidesb, O. Karvanb, V. Sebastiána, C. Télleza, J. Coronasa, J. Membr. Sci. 2015, 476, 277. 11. R. Ameloot, F. Vermoortele, W. Vanhove, M. B. J. Roeffaers, B. F. Sels and D. E. De Vos, Nat. Chem. 2011, 3, 382. 12. (a) T. Yang, Y. Xiao and T. -S. Chung, Energy Environ. Sci. 2011, 4, 4171; (b) S. C. Kumbharkar, K. Li, J. Membr. Sci. 2012, 415, 793; (c) A. Bhaskar, R. Banerjee and U. K. Kharul, J. Mater. Chem. A 2014, 2, 12962; (d) S. C. Kumbharkar, P. B. Karadkar, U. K. Kharul, J. Membr. Sci. 2006, 286, 161. 13. (a) S. Qiu, M. Xue and G. Zhu, Chem. Soc. Rev., 2014, 43, 6116; (b) S. Zhou, X. Zou, F. Sun, F. Zhang, S. Fan, H. Zhao, T. Schiestel and G., J. Mater. Chem., 2012, 22, 10322; (c) J. Nan, X. Dong, W. Wang, W. Jin and N. Xu, Langmuir, 2011, 27, 4309; (d) T. Ben, C. Lu, C. Pei, S. Xu and S. Qiu, Chem. – Eur. J., 2012, 18, 10250. 14. H. Lohokare, Y. Bhole, S. Taralkar, U. Kharul, Desalination, 2011, 282, 46.
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Received 00th January 2014, Accepted 00th January 2014 DOI: 10.1039/x0xx00000x www.rsc.org/
Selective Interfacial Synthesis of Metal-Organic Frameworks on Polybenzimidazole Hollow Fiber Membrane for Gas Separation Bishnu P. Biswal,a,c Anand Bhaskar,b,c Rahul Banerjee*,a,c and Ulhas K. Kharul*,b,c Metal-organic frameworks (MOFs) have gained immense attention as the new age materials due to their tuneable properties and diverse applicability. Yet, efforts on developing promising materials for membrane based gas separation, and control over the crystal growth positions on polymeric hollow fiber membranes still remains a key challenge. In this investigation, a new, convenient and scalable room temperature interfacial MOF (ZIF-8 and CuBTC) growth on either outer or inner side of polybenzimidazole based hollow fiber membrane (PBI-BuI-HF) surface has been achieved in a controlled manner. This was made possible by appropriate selection of immiscible solvent pair and synthetic conditions. The growth of MOFs on PBIBuI-HF by interfacial method was continuous and showed appreciable gas separation performance, conveying promises towards their applicability.
Introduction Metal–organic frameworks (MOFs)1 are new class of crystalline porous materials with excellent properties like gas adsorption, catalysis, electronics, drug delivery etc.2 Although MOF chemistry has been studied extensively over last 15 years, the fabrication of MOF based membranes for gas/liquid separation has picked up attention only recently.3 There exist few reports wherein porous MOFs have grown on the surface of inorganic and organic supports such as alumina, silica, carbon materials, etc. for gases/liquids separation.4 However, such MOF@Support suffer from difficulties in scale-up due to hydrothermal synthetic approach, difficulties in continuous film fabrication, tedious synthesis, lack of support flexibility, etc.; and thus have limited practical applicability. These issues motivated researchers to grow MOFs on flexible porous polymeric membranes, wherein MOFs placed within the pores of the polymeric membrane will be responsible for separation and polymeric membrane backbone acts just as a mechanical support.5 Although there has been some reports on growth of MOFs in the polymer membranes, real benefits of MOF@polymer are yet to be realized. Moreover, MOF polymer composite membranes often result in sedimentation, agglomeration of crystals and high loading of MOF crystals, which results into brittleness of the membrane.6-8 Hence in order to design and fabricate a high performing, flexible, defect-free MOF-membrane composite, the development of both, support material as well as MOF-synthetic methodology are highly crucial to bring anticipated advantages of both these
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components into practice. On this line, polymeric hollow fiber membranes are advantageous over flat sheet and porous inorganic membrane structures due to their flexibility, ease in large scale production, high pressure stability and large membrane surface area available per unit volume.9 Notably, most of the work reported on the growth of MOFs on inorganic tubular/polymeric hollow fiber support employed hydrothermal synthesis method. Recently, only two exiting reports appeared based on microfluidic approach for processing MOF membranes on polymeric hollow fiber supports. The first one by Nair et al., wherein interfacial microfluidic membrane processing (IMMP) by two immiscible solvent (octanol/water) was demonstrated for the positional control of ZIF-8 formation on poly(amide-imide) hollow fibers installed within a stainless steel custom-made reactor.10a These membranes showed high permeance with good H2/C3H8 and C3H6/C3H8 separation factors (370 and 12, respectively). An another report by Coronas et al., in which ZIF-7 and ZIF-8 continuous membranes were synthesized on the inner surface of polysulfone hollow fiber using microfluidics.10b The resultant ZIFs/PSf membranes showed high CO2/N2 and CO2/CH4 separation factors (13.6 and 13.5, respectively). Although these reports demonstrating new routes towards the MOF@HF membrane fabrication, more simplistic methodology of MOF growth on the hollow fiber membrane surfaces that can easily be scaled up for practical applicability remains a challenging task. Herein, we demonstrate a facile, scalable and room temperature selective fabrication of MOFs (ZIF-8 and CuBTC)
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Figure 1. a) Representative digital photograph of a gas separation module [CuBTC growth on outer surface of PBI-BuI hollow fibers [(CuBTC@PBI-BuI-Out) is seen through the cut].; b) Schematic for interfacial synthesis approach of CuBTC@PBI-BuI-In and Out; c) Microscopic images of ZIF-8@PBI-BuI-In, ZIF-8@PBI-BuI-Out, CuBTC@PBI-BuI-In and CuBTC@PBI-BuI-Out composites synthesized.
on either inner or outer surfaces of polybenzimidazole based hollow fibers (PBI-BuI-HF) using interfacial11 synthesis method. High miscibility of organic ligands from volatile solvents (CHCl3 or isobutyl alcohol) into immiscible water phase containing metal salt can be a driving factor for obtaining selective MOF growth, based on the solution circulation preference on either side of the hollow fiber module. Such circulation approach could have advantages of forming continuous defect-free membranes rather than the static growth conditions that results in dense, non-continuous coatings of MOF crystals in the bore of the fiber, as nicely demonstrated by Nair et al.10a The selective MOF fabrication methodology described in this work is simpler than that of reported earlier10a and can be easily scalable, as it is developed using working membrane module of practical utility. We have chosen polybenzimidaole (PBI-BuI) hollow fibers for MOF growth because this polymer has excellent thermo-chemical stability (particularly stability in CHCl3, which is used as a solvent for the growth of ZIF-8 and CuBTC), outstanding mechanical properties at high temperatures12 and more importantly, good compatibility with MOFs, as evident from ZIF@PBI composite12a-c studied recently. Employing immiscible pair of low boiling solvents [CHCl3, isobutyl alcohol (IBA) and water] for MOF@membrane composite fabrication has advantage of easy removal of these solvents from the membrane and MOFs with mild activation process over other high boiling solvents
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such as octanol, which is commonly used for interfacial synthesis of MOFs.10 Moreover, these MOF@PBI-BuI-HF composite membranes were evaluated for gas permeation and found to offer appreciable performances in terms of permeance as well as selectivity.
Experimental Section Preparation of dope solution and fabrication of PBI-BuI hollow fibre (PBI-BuI-HF) membrane: The PBI-BuI was synthesized as per our earlier report.12d It was vacuum dried at 100 ºC for 24 hours prior to the preparation of dope solution (PBI-BuI:DMAc:LiCl = 11:83:6 on wt. basis). The spinning of PBI-BuI hollow fiber membranes was carried out using the dryjet wet spinning process. The dope solution and the bore fluid were passed through spinneret at the predefined extrusion rate to offer hollow fiber with internal diameter of 0.46 mm and outer diameter of 0.78 mm. Hollow fiber module preparation: The dry hollow fibers of PBI-BuI were used for the preparation of membrane module. A bunch of 10 fibers of 30 cm length were housed in a pipe of ½” diameter using epoxy resin. The photograph of obtained module is given in Figure 1a. The active length and active area of the membrane module is 26 cm and 63.6 cm2, respectively. The prepared module was used as such for the growth of ZIF-8 and CuBTC on either surface (inner or outer) of PBI-BuI hollow fibers present within the module. Fabrication of ZIF-8@PBI-BuI-In Composite: The Zn(NO3)2 (2.19 g; 7.34 mmol) dissolved in 150 mL of water was
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circulated through the tube side of fibers using a peristaltic pump. After 15 minutes, 150 mL CHCl3 solution of 2methylimidazole (2-mIm, 2.43 g; 29.63 mmol) was hold on to the shell side (outer side of the fibers), for 1 hour. During this period, circulation of aqueous Zn(NO3)2 solution was continued on the tube side. The solutions were then drained and this procedure was repeated twice. Finally, the membrane module was dried at 65 ºC under the vacuum for 12 hours and then used for further characterizations (Figure S2, ESI).
Paper DOI: 10.1039/C5NR00299K
Results and discussion The growth of CuBTC and ZIF-8 on PBI-BuI-HF was verified by Wide Angle X-ray Diffraction (WAXD) analysis (Figure 2). WAXD patterns of as-synthesized ZIF-8 and CuBTC in CHCl3/H2O and IBA/H2O matches well with their respective
Fabrication of ZIF-8@PBI-BuI-Out Composite: The 2methylimidazole (2-mIm, 2.43 g) dissolved in 150 mL of isobutyl alcohol (IBA) was circulated through the tube side of fibers using a peristaltic pump. After 15 minutes, aquous solution of Zn(NO3)2 (2.19 g in 150 mL) was hold on to the shell side (outer side of the fibers), for 2 hours. During this period, circulaton of 2-mIm in IBA was continued on the tube side. After that the solutions from both the sides were drained and the module was dried at 65 ºC under vacuum for 12 hours.
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Fabrication of CuBTC@PBI-BuI-In Composite: The Cu(NO3)2 (3.4 g; 14.07 mmol) dissolved in 150 mL of water was circulated through the tube side of fibers using peristaltic pump. After 15 minutes, 150 mL of CHCl3 solution of 1,3,5benzenetricarboxylic acid (BTC, 2.5 g; 11.9 mmol) and triethylamine (3.5 mL; 25 mmol) was hold on to the shell side (outer side of the fibers), for 1 hour. During this period, circulation of aqueous Cu(NO3)2 solution was continued on the tube side. The solutions were then drained and this procedure was repeated twice. Finally, the membrane module was dried at 65 ºC under the vacuum for 12 hours. Fabrication of CuBTC@PBI-BuI-Out Composite: The 1,3,5benzenetricarboxylic acid (BTC, 2.5 g) was dissolved in 150 mL of isobutyl alcohol (IBA) and triethylamine (3.5 mL), was circulated through the tube side of fibers. After 15 minutes, 150 mL aquous solution of Cu(NO3)2 (3.4 g) was hold on to the shell side (outer side of the fibers), for 2 hours. During this period, circulation of BTC/IBA/TEA solution was continued on the tube side. After that the solutions from both the sides were drained and the module was dried at 65 ºC under vacuum for 12 hours.
Characterizations The X-Ray Diffraction (XRD) analysis of MOFs and the composite membranes were carried on a Rigaku SmartLab Xray diffractometer in reflection mode using CuKα radiation (λ = 1.54 Å). The 2θ range from 5° to 40° was scanned with a scan rate of 3° min-1. Electron Microscopy (SEM) was performed on a FEI Quanta 200 3D ESEM (dual beam) instrument with a field emitter as an electron source and in FEI Nova NanoSEM 650 Scanning Electron Microscope. SEM images of membrane cross section were taken after freeze cut of membranes in LN2. Samples for SEM were gold sputtered before analyses. Fourier transform infrared (FT-IR) spectra were taken on a Bruker Optics ALPHA-E spectrometer with a universal Zn-Se ATR (attenuated total reflection) accessory in the 600-4000 cm–1 region or using a Diamond ATR (Golden Gate). Microscopy images of these hollow fibers were taken in Zeiss SteREO Discovery V20. Single-gas permeation experiments of He, N2, and C3H8 were performed at 35 °C using a variable volume method.12c Upstream pressure of range 15 psi was used while maintaining permeates side at the ambient.
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Figure 2. X-ray diffraction of a) ZIF-8 and b) CuBTC; grown on the inner and outer surface of PBI-BuI-HF using interfacial synthesis method in comparison with pristine PBI-BuI-HF, ZIF-8 (CHCl3/H2O and IBA/H2O) and CuBTC (CHCl3/H2O and IBA/H2O).
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Figure 3. SEM images showing cross section of hollow fiber, its cut along the length and zoomed view; a) and b): ZIF-8@PBI-BuI-In; c) and d): ZIF-8@PBI-BuI-Out; e) and f): CuBTC@PBI-BuI-In and g) and h): CuBTC@PBI-BuI-Out composite membranes respectively. [Top part of a-g represents the zoomed part with clear cut cross section showing the thickness of ZIF-8 and CuBTC crystals layer formed on inner and outer surface of PBI-BuI-HF; whereas bottom part of b-h showing the crystal morphology and continuous packed structures with different magnifications].
simulated patterns obtained from the single crystal X-ray data reported earlier.1e,f The WAXD profile of pristine PBI-BuI-HF membrane showed a broad amorphous hump in the 2θ range of 15°-30°, and a smaller hump at 2θ = 6°. The CuBTC@PBI-BuI and ZIF-8@PBI-BuI composite (inner and outer) membranes were analyzed by WAXD to ensure the phase purity of (CuBTC and ZIF-8) grown on the membrane surface via interfacial synthesis method. This confirms that phase pure MOF crystals have formed on the PBI-BuI-HF support. This analysis also conveys that such interfacial synthesis method with different solvent combinations does not affect the crystallinity and structural feature of MOFs. The FT-IR spectrum of pristine PBI-BuI-HF was characterized by the absorption in the range 1430-1650 cm-1 for benzimidazole units. The broad band at ~3145 cm-1 was
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ascribed to the N–H…N hydrogen bonding, while the peak at ~ 3410 cm-1 was due to the free non-hydrogen bonded N–H stretching. The peak at 2862 cm-1 in the IR spectrum could be ascribed to the presence of tert-butyl group of PBI-BuI-HF. For the CuBTC@PBI-BuI composite membrane, all major bands of CuBTC, e.g., 1647 (C=O symmetric), 1444 (C=C–Ar) and 1369 cm-1 (C–O) were appeared, which confirms the presence of CuBTC crystals within the membrane framework. Similarly the ZIF-8@PBI-BuI composite membranes showed the presence of weak bands at 3165, 2966 cm-1 (C–H bonds in the methyl group and the imidazole ring) which was merged with the strong band of pristine PBI-BuI-HF. While few medium bands at 994 cm-1 (C–H bonds in the imidazole ring) and 1107 cm-1 (C–N bonds in imidazole moiety) of ZIF-8 has been traced with other characteristic bands of PBI-BuI-HF host, which
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Nanoscale further confirms the presence of ZIF-8 crystals on the PBI-BuIHF matrix (Figure S3, ESI). Growth of CuBTC and ZIF-8 crystals on the inner and outer surfaces of PBI-BuI-HF support were further confirmed by SEM imaging (Figure 3a-h and Section S4, ESI). It is interesting to note that the crystal layers were actually stacked on one another and thus helps in covering the gaps, which almost look like a defect-free continuous sheet. Figure 3 shows the cross section at different magnifications of all the CuBTC@PBI-BuI-HF and ZIF-8@PBI-BuI-HF composites [inner (In) and outer (Out) MOF growth] membranes. This confirms that the surface of the hollow fiber membrane is covered by MOF crystals due to the diffusion of metal and ligand ions into the respective immiscible phases. As a result, MOF crystals were formed extensively at the interface of the two immiscible solvents. But the inner pore voids are not as densely populated as the top surface (pore openings). As the crystal layers increase on a surface, the chances of diffusion of the metal and ligand ions deeper inside the solution present in fiber pores decrease. This could be a reason for crystal growth predominantly seen at the respective hollow fiber surfaces. An exactly similar observation on crystal growth mechanism was observed for ZIF-8/Torlon membranes, as reported recently.10a The shapes of ZIF-8 crystals are dodecahedral and sizes ranging from ~4-8 µm for inner growth (ZIF@PBI-BuI-In). On the other hand, relatively small crystallites (~1-3 µm) with slightly different morphology were observed on the outer side of PBI-BuI-HF (ZIF-8@PBI-BuI-Out). Similarly, the crystal shapes of CuBTC are flake like having dimensions ~1 µm thickness/~5 µm length for inner growth and small (~1-2 µm) crystallites agglomerates seen for outer growth on the surfaces of PBI-BuI-HF similar to ZIF-8@PBI-BuI-Out. The exact reason for different morphologies of ZIF-8 and CuBTC crystals of both inner and outer grown hollow fibers are not fully understood to us. However, the sizes and shapes of ZIF-8 and CuBTC are expected to vary based on the nature of solvents and with the physical parameters like duration of crystallization in the mother liquor, space availability and the environment within the hollow fiber membrane. Further importantly, it has been noticed that the WAXD profiles of ZIF-8@PBI-BuIIn/Out and CuBTC@PBI-BuI-In/Out composite membranes are matching well with the WAXD profiles of synthesized ZIF-8 and CuBTC, irrespective of their crystal morphologies (Figure 2). To check the uniformity of MOF membrane formation on the hollow fibers, we obtained cross sections of the fiber and measured the membrane thickness at these locations (Figure 3ag, zoomed portion). The average thickness of the layers of CuBTC and ZIF-8 on the PBI-BuI membrane was approximately in the range of ~10-25 µm, respectively. This could also vary depending on the extent of growth of crystals with respect to time on the surface of PBI-BuI-HF membrane. EDAX elemental mapping confirms the presence of Cu and Zn atoms (yellow color) located exactly on the inner circular surface of MOF@PBI-BuI-HF (N atoms are designated in red color) composite membranes (Figure 4). We could successfully employ interfacial method for MOF growth on the inner and outer surface of the hollow fibers; without any need of critical seeding or hydrothermal growth, as demonstrated in the literature.5-8 Our ideology was, if we could grow the MOFs selectively on preferred surfaces (inner or outer) with some MOF-growth penetrating within the pores (Figure 4), the inherent limitation of MOF films being fragile can be conveniently tackled and effective molecular level
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Figure 4. a) and b) Elemental mapping of ZIF-8@PBI-BuI-In and CuBTC@PBI-BuI-In composite membranes [red corresponds to nitrogen signals from PBI-BuI-HF and yellow for Zn and Cu signals from ZIF-8 and CuBTC respectively grown selectively inside the hollow fiber membrane].
separation can be achieved through MOFs. In order to have the selective continuous growth of CuBTC and ZIF-8 crystals on the surface of PBI-BuI polymeric hollow fiber membranes, we have applied the interfacial synthesis approach, through a well thought circulation methodology and protocol, optimized via change of solvent pair and flow reversal approach for the successive growth of MOF films selectively on inner and outer surfaces as shown on Figure 1. Interfacial crystallization of ZIF-8 and CuBTC has been achieved by using immiscible low boiling solvent pairs such as CHCl3/water and IBA/water. These solvents were particularly chosen to achieve their effective removal from the MOF and membranes after the crystal growth, so that they would not interfere in the gas permeation. We have used an optimized reactants ratio for the formation of both ZIF-8 and CuBTC, the same fixed ratio used to fabricate the MOF on the surfaces of PBI-BuI hollow fiber in an interfacial manner. Nonetheless, we feel that the diffusion of ligands into aqueous phase through the interfacial boundary would control the growth of the crystal, than the reactant ratio present in respective solvent. Our focus herein is the easy MOF fabrication by using volatile solvents and circulation preferences for the selective MOF growth on either surfaces (inner and outer) of hollow fiber support. The porous nature of PBI-BuI-HF provides high surface area for the immiscible solvent interfaces to achieve effective diffusion of either the ligand or metal ion. This would provide not only the MOF nucleation possible at the interfaces, but also the effective growth because of the high surface area availability. We also believe that extremely low solubility of metal ions in organic solvents (CHCl3 and IBA) and the high solubility of ligands (2mIm and BTC) in water are governing the successive growth of MOF on the preferred surface. The affinity of ligand and metal ion to fast recombine could be another key factor for successive growth of MOF on preferred surface of PBI-BuI hollow fibers. It has been noticed that the MOF nucleation and growth occurs at the preferred surfaces wherein aqueous solution containing metal ions (Zn+2 or Cu+2) were present. These important observations are in consistence with the previous report10a on IMMP of ZIF-8.
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Journal Name DOI: 10.1039/C5NR00299K from ~4480 GPU to ~119 GPU; Table S2, ESI).13 Such a low permeance of MOF@PBI-BuI-HF composite membranes arises due to the PBI-BuI-HF support used, which itself has a very low He permeance of 6 GPU. It may be noted that the porosity of the base support can be easily increased by lowering the concentration of polymer in the dope solution.14 Further, the selective permeance of gases is indicative of continuous MOF layer formation on the top of flexible and scalable polymeric hollow fiber membranes. The chemical nature of PBI-BuI (presence of imidazole group) would additionally provide good interactions with Cu/Zn ions and thus good adhesion of MOF on the surfaces of PBI-BuI hollow fibers. In other words, as growth of MOFs on porous PBI-BuI-HF support leads to substantial improvement in the selectivity of PBI-BuI-HF. We hope that by further optimization and choosing hollow fibers support that possess higher porosity, the present methodology can give rise to better performance for practical utility. All these fundamental findings encourages enough to demonstrate the interfacial synthesis approach using volatile solvents to grow MOFs on preferred surfaces of hollow fiber membranes to serve as a separation membrane. More prominently, this methodology does not need any external seeding of MOFs, as usually done for such MOF growth on the porous substrates.4d,7
Figure 5. He permeance and its selectivity over N2 and C3H8 for pristine PBIBuI-HF and MOF@PBI-BuI-HF composite membranes. Permeance (P) expressed in GPU, (1 GPU = 1x10-6 cm.s-1.cmHg-1).
The permeance study was done using a variable volume method12c for three gases chosen based on their kinetic diameter He (2.89 Å), N2 (3.64 Å) and C3H8 (4.4 Å). Table S1 (Section S5, ESI) shows the permeance data of the pristine PBI-BuI-HF membrane, ZIF-8@PBI-BuI and CuBTC@PBI-BuI composite membranes, respectively. Due to the high porosity of pristine PBI-BuI-HF membrane, it evidenced high gas permeance (Figure 5) for all the three gases. However, when the gas permeation study was done for the CuBTC@PBI-BuI and ZIF8@PBI-BuI composite membranes (in and out), the permeance was reduced to ~1/3rd as compared to the pristine PBI-BuI-HF. These new class of MOF@PBI-BuI-HF membranes retains the flexibility while allowing direct contact of the feed gas with MOF so that the pore opening of the MOF can be utilized as channels for the gases to flow through. Further, between ZIF8@PBI-BuI and CuBTC@PBI-BuI composites, prior one has higher permeance because of the slightly lower coating thickness as confirmed from SEM images (Figure 3). This difference in thickness could be attributed to the moderate reactivity (fast nucleation) of Zn+2 ions with 2-mIm ions at the ambient condition leading to a small amount of crystals formation on the support membrane compared to that of CuBTC reactivity and the amount of crystals formation, which is very high. The pronounced decrease in permeance in case of CuBTC@PBI-BuI led to increase in the selectivity of different gas pairs than in the case of pristine hollow fiber membranes. As an example, the selectivity of He/N2 was increased from 1 to 12 and He/C3H8 from 1.1 to 17 for CuBTC@PBI-BuI-Out composite membrane. However, for CuBTC@PBI-BuI-In composite membrane, the He/N2 and He/C3H8 selectivity was found to be 8 and 8.7, respectively (Figure 5). On the other hand, in case of ZIF-8@PBI-BuI-In the selectivity of He/N2 and He/C3H8 was 3.7 and 8 respectively. However, ZIF8@PBI-BuI-Out showed low selectivity of He/N2 and He/C3H8 was 4.2 and 4.6, respectively. Although, CuBTC@PBI-BuI-Out showed an appreciable separation factor of 12 (He/N2), the He permeance value is rather low (1.3 GPU), as compared to other CuBTC@membranes reported earlier (H2 permeance ranges
6 | J. Name., 2012, 00, 1-3
Conclusions In summary, we reveal a methodology for selective MOF growth on preferred surfaces (inner or outer) of a porous polymeric (PBI-BuIHF) substrate via interfacial synthesis approach using volatile solvents. The demonstrated methodology has its own merits owing to ease of MOF synthesis without any need of pre-seeding. Moreover, the interfacial MOF growth approach adopted employing volatile solvents (CHCl3, IBA and H2O) can be easily removed from the MOF-membrane system. Moreover, this work is performed using membrane module of practical applicability that can be easily scalable. This work also provided an understanding to draw benefits of flexible polymeric hollow fiber membrane porosity and its material (PBI-BuI) properties; as well as peculiarities of ligand/metal ion partitioning in immiscible solvent pairs to grow MOFs in a preferred manner. The separation performance of these MOF-HF composite membranes towards He, N2 and C3H8 were demonstrated that MOFs took part in molecular discrimination. Among all composite membranes synthesized, CuBTC@PBI-BuI-Out displayed an appreciable permeance and selectivity of 12 (He/N2) and 17 (He/C3H8). This work opens up an easy methodology of using volatile solvents for MOF fabrication on HF working modules. The understanding generated would help to develop composite membranes for wider range of applications of gases/liquids separation area in near future.
Acknowledgements B. P. B and A. B acknowledge UGC and CSIR, New Delhi, India, for fellowship. U. K and R. B acknowledge CSIR’s Five Year Plan (CSC0122 and CSC0115) for funding.
Notes and references a
Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. HomiBhabha Road, Pune-411008, India. Fax: (+) 9120-25902636, Tel: + 91-20-25902535. E-mail: [email protected]
b
Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India. Fax: (+) 9120-25902618, Tel: + 91-20-25902180. E-mail: [email protected]
This journal is © The Royal Society of Chemistry 2012
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