Zeolitic Imidazolate Framework-Enabled Membranes: Challenges and

Perspective pubs.acs.org/JPCL

Zeolitic Imidazolate Framework-Enabled Membranes: Challenges and Opportunities Chen Zhang and William J. Koros*

Downloaded by CENTRAL MICHIGAN UNIV on September 15, 2015 | http://pubs.acs.org Publication Date (Web): September 15, 2015 | doi: 10.1021/acs.jpclett.5b01602

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100, United States ABSTRACT: ZIFs are a unique class of porous solids topologically associated with zeolites, but dramatically different in components. Current research on ZIF-enabled membranes is highly imbalanced. Despite a large selection of available ZIF materials, seven out of ten published papers discuss ZIF-8-based membranes. This is partially due to insufficient knowledge on ZIFs’ structure-transport property relationships as well as lack of capability to tailor their transport properties for particular separations. This Perspective will provide an account of recent progress in ZIF-enabled membranes and analyze the barriers that must be overcome to advance ZIF-enabled membranes beyond fundamental characterizations.

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membrane was enabled by Caro and co-workers, who synthesized a continuous polycrystalline ZIF-8 layer on a porous titania support with H2/CH4 separation factor ∼ 11.15 Since then, the ZIF membrane literature has expanded rapidly.16,17 Several ZIF materials (ZIF-7,18,19 ZIF-8,15,20−25 ZIF-22,26 ZIF-69,27 ZIF-78,28 ZIF-90,29,30 ZIF-93,31 ZIF-95,32 and ZIF-10033) have been processed into supported ZIF membranes or mixed-matrix ZIF/polymer membranes. The membranes were characterized for an extensive spectrum of gas and vapor separation applications, including H2 separations (e.g., H2/N2, H2/CO2, H2/CH4, H2/C2H6, H2/C3H8), natural gas/biogas purification (CO2/CH4), postcombustion carbon capture (CO2/N2), and olefin/paraffin separations (e.g., ethylene/ethane, propylene/propane). In this Perspective, we will provide an account of recent progress in ZIF-enabled membranes and a personal point of view of challenges that must be overcome to make ZIF-enabled membranes commercially viable. ZIF-8 (Zn(MeIM)2 , MeIM = 2-methylimidazole) is undoubtedly the most heavily studied ZIF and possibly one of the most extensively investigated MOFs. As shown by Figure 1a, ZIF-8-based membranes account for over 70% of total published work on the topic of ZIF-enabled membranes. ZIF-8 shares the same topology (SOD) with sodalite zeolite. Numerous experimental34−36 and computational37,38 studies have suggested that its framework is flexible, owing to thermal rotation of MeIM linkers. While the crystallographic size of its six-ring aperture is believed to be 3.4 Å, ZIF-8 is able to uptake guest molecules as large as 7.6 Å (1,2,4-trimethylbenzene).36

xtending polymeric membranes to challenging gas separations (e.g., purification of highly contaminated natural gas, separation of gaseous hydrocarbon mixtures) is difficult due to unattractive separation performance and compromised stability of conventional polymers. The membrane community has made substantial efforts to develop advanced membranes beyond conventional polymers; examples include zeolite membranes,1−3 carbon molecular sieve (CMS) membranes, 4−6 and hybrid mixed-matrix membranes (MMMs).7−10 Although some of these advanced membranes are capable of delivering attractive and stabilized permeance/ selectivity, none has been commercialized for gas separations due to insufficient scalability. Today, pure polymers remain the only membrane materials used for large-scale gas separations, and the topics addressed by contemporary commercial gas separation membranes are, in fact, not significantly different from what they were a decade ago. Zeolitic imidazolate framework (ZIF)-enabled membranes appeared in the late 2000s. ZIFs are a subfamily of metal− organic frameworks (MOFs). They share the same topology (e.g., SOD, LTA, RHO) with zeolites but are constructed by tetrahedrally coordinated transition metal (usually zinc or cobalt) cations bridged by imidazole-based ligands.11 Over one hundred ZIF materials have been synthesized, many of which are ultramicroporous (window size < 7Å) and therefore potentially attractive for molecular sieving-based membrane gas and vapor separations.12−14 ZIFs are a unique class of molecular sieves-their flexible framework enables the possibility of simultaneous high permeability and attractive selectivity. While zeolite membrane formation usually involves high temperature fabrication and subsequent template removal, ZIF membranes can be conveniently formed at room temperature. The first demonstration of good quality ZIF © XXXX American Chemical Society

Received: July 25, 2015 Accepted: September 10, 2015

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DOI: 10.1021/acs.jpclett.5b01602 J. Phys. Chem. Lett. 2015, 6, 3841−3849

The Journal of Physical Chemistry Letters

Perspective


Figure 1. Percentage breakdown of total number of publications on (a) ZIF membrane materials and (b) type of gas separations studied for ZIFenabled membranes. Data from Science Citation Index, July 2015.

Figure 2. (a) Electron microscope images of ZIF-8 crystals used for kinetic sorption measurements. (b) Kinetic uptake curves (35 °C) of C3 and C4 hydrocarbons in ZIF-8 crystals shown in (a). (c) Corrected diffusivities in ZIF-8 at 35 °C versus molecular diameter of probe molecules. (Solid blue squares: diffusivities estimated from mixed matrix membrane permeation. Hollow red circles: diffusivities calculated from kinetic uptake rate measurements. Blue line: XRD derived aperture size of ZIF-8. Magenta region: effective aperture size range of ZIF-8).35 Copyright 2012, American Chemical Society.

The “gate opening” phenomena and lack of molecular sieving cutoff were explained by the density functional theory (DFT). Because of its flexible aperture, ZIF-8 is not particularly selective toward small molecules (d < 4.0 Å). Its intrinsic selectivities for H2/N2 (∼10), H2/CH4 (∼10), CO2/N2 (∼5), and CO2/CH4 (∼5) are not competitive with state-of-the-art glassy polyimides.39−41 As shown by Figure 1b, half of ZIFenabled membranes reported in the literature are characterized for H2 separations. It should be noted, however, that H2 selectivities may not be good indicators of membrane quality. H2 molecule is much smaller and permeates much faster than other molecules, and H2 selectivity may be good even if the

membrane is moderately defective. Additionally, it is unlikely that the industry will consider ZIF membranes for separating H2/N2 or H2/hydrocarbons. Membrane-based processes are already commercialized by much less expensive polymeric membranes, and there is very limited commercial driving force for developing advanced membrane materials.42 A comprehensive diffusion study35 (Figure 2) published in The Journal of Physical Chemistry Letters by the same authors of this Perspective suggested that ZIF-8’s effective aperture size is 4.0−4.2 Å. This was evidenced by sharp molecular sieving (diffusion selectivity = 140) between propylene (4.0 Å) and propane (4.2 Å) molecules. In fact, propylene/propane 3842




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increased, n-butane diffusivity was enhanced, whereas n-butane/ iso-butane diffusion selectivity was reduced. The increase in diffusivity was believed to be due to aperture enlargement as a result of replacing MeIM linkers with slightly smaller OHC-IM linkers. In fact, it is possible to enlarge the aperture by controlled thermal treatments instead of the more complex mixed-linker ZIF synthesis. A recent thermogravimetric analysis-mass spectrometry (TGA-MS) study50 showed that methyl groups dissociate as ZIF-8 is heated to ∼ 350 °C and above, with crystal structure essentially unchanged up to 525 °C. The thermally treated ZIF showed increased CO2 uptake and CO2/N2 sorption selectivity due to creation of highly active local defects. We further speculate that the aperture was enlarged by dissociating methyl groups and therefore, such postsynthetic thermal modification (PSTM) can be potentially used as a facile technique to tune ZIF’s transport properties for membrane applications. Clearly, it will be important to develop ZIFs that are highly selective toward small molecules, that are, CO2/N2/CH4. This can be realized by either reducing the aperture size or suppressing the framework’s flexibility. While substitution with smaller OHC-IM linkers enlarges the ZIF’s aperture, replacing ZIF-8’s MeIM linkers with bulkier benzimidazole (BzIM) linkers was believed to be able to suppress the “gateopening” phenomena and reduce framework flexibility.48 Additionally, two recent studies show that ZIF-8’s molecular sieving capability can be improved when surrounded by polymers. One study51 showed that H2/CH4 selectivity of ZIF-8/Matrimid mixed-matrix membranes was substantially higher than the value predicted based on singe-phase ZIF-8 and Matrimid permeation. It was hypothesized that increased selectivity was due to hindered MeIM linker distortion and consequently reduced framework flexibility. Similar polymerinduced flexibility suppression was observed by advanced diffusion characterizations using pulse field gradient-nuclear magnetic resonance (PFG-NMR) on ZIF-8/6FDA-DAM mixed-matrix membranes.52 Ethylene diffusivities in ZIF-8 particles dispersed inside the MMM were found to be slightly lower than free-standing ZIF particles. More extensive characterizations are needed to support the hindered linker distortion hypothesis. If it can be verified, it will certainly add another dimension to tailor ZIF membrane’s molecular sieving properties. Zeolite-like metal−organic frameworks (ZMOFs)53 are a more general subclass of MOFs topologically related with zeolites. Similar to aluminosilicates,54 anionic ZMOFs can be exchanged with extra-framework cations, allowing the opportunity to enhance ZMOFs’ sorption selectivity by tuning sorbate−sorbent interaction toward specific sorbate molecules.55 A Monte Carlo simulation study56 predicted 10-fold increase in propylene/propane sorption selectivity for completely K+-exchanged sod-ZMOF. If such high sorption selectivity can be combined with attractive diffusion selectivity realized by fine aperture size tuning, ZMOFs propylene/ propane selectivity may exceed ZIF-8. Eddaoudi and coworkers55 reported synthesis of polycrystalline ZMOF membranes grown on alumina supports. The sod-ZMOF-1/ alumina membrane showed generally lower (H2/CH4, CO2/N2, and CO2/CH4) selectivities than well-grown ZIF-8/alumina membranes, which was possibly due to the ZMOF’s larger aperture size and lower diffusion selectivity. From H2 to C2H4, diffusivity drops by more than 2 orders of magnitude for ZIF8,35 and only 1 order of magnitude for sod-ZMOF-1.

separation has been the only industrially important separation for which ZIF membranes have shown commercially attractive permeability and selectivity that both well exceed the polymer “trade-off” upper bound.43,44 Since the first successful demonstration by Zhang and co-workers24 in 2012, numerous studies have been published on propylene/propane separation using ZIF-8-based membranes. ZIF-8 delivers even more attractive diffusion selectivities for separating larger C4 hydrocarbon mixtures (e.g., 1-C4H8/iso-C4H8, n-C4H10/isoC4H10); however, permeabilities were commercially unattractive. ZIF-8 is an ideal candidate for membrane-based propylene/ propane separation in terms of transport properties. It is highly propylene permeable, while its propylene/propane selectivity is sufficiently attractive to debottleneck conventional thermally driven separations. Our original 2012 work35 published on The Journal of Physical Chemistry Letters was intended to motivate more studies on ZIF’s structure-transport property relationship in general. We expect that by tuning their effective aperture size, ZIFs can be structurally engineered toward a particular separation. Three years have passed; however, ZIF-8 remains the most extensively studied ZIF and a generalized understanding of relationships between topology, dimension of imidazole linker, framework flexibility, and molecular sieving properties still have not been well-established. Extending ZIF-

Extending ZIF-enabled membranes to a broad spectrum of separations beyond propylene/ propane separation requires an understanding of ZIF’s structuretransport property relationships and capability to tune ZIF’s transport properties through rational synthesis or postsynthetic modifications. enabled membranes to a broad spectrum of separations beyond propylene/propane separation requires an understanding of ZIF’s structure-transport property relationships and capability to tune ZIF’s transport properties through rational synthesis or postsynthetic modifications. Several studies have reported structural tuning by postsynthetic modifications and direct synthesis. These approaches include solvent-assisted linker exchange,45 ligand covalent functionalization,29,46 metal ion exchange,47 and mixed-linker synthesis.48 Although many of them studied equilibrium sorption, very few reported diffusion data, which are crucial to evaluate the material’s potential for molecular sieving-based membrane separations. Indeed, some functionalized ZIFs have displayed moderate equilibrium sorption selectivities. Nevertheless, sorption selectivities based on physical sorbate-sorbent interactions are usually insufficient to be solely relied on for membrane gas separations. Instead, they are supposed to be complementary to diffusion selectivity. The only work systematically studied diffusion in structurally tuned ZIFs was enabled by Nair and co-workers,49 who measured n-butane/iso-butane diffusivities and diffusion selectivity in a mixed-linker ZIF (ZIF-8-90). As percentage of the ZIF-90 linker (carboxyaldehyde-2-imidazole, OHC-IM) 3843



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solution dissolving the zinc source and 2-methyimidazole. The reaction can be carried out at ambient temperature or hydrothermally. In the counter diffusion-based in situ method, the support is first saturated with zinc source solution followed by soaking in a ligand mixture containing 2-methylimidazole and sodium formate.61 A continuous ZIF-8 layer was grown by heterogeneous nucleation and crystal growth as a result of counterdiffusion of metal ions and ligand molecules at the interface. It was found that optimizing sodium formate to ligand ratio and choice of zinc source were crucial to formation of desirable membrane microstructures and, consequently, improved separation performance.23 ZIF-8 membranes with propylene permeance of 268 × 10−10 mol/m2 s Pa (∼80 GPU) and propylene/propane separation factor of ∼70 were obtained using the counter diffusion-based in situ method, which are among the most propylene-selective ZIF-8 membranes reported to date. Figure 3 shows schematic illustration of the counter diffusion-based in situ growth method (a), membrane SEM images (b), and membrane propylene/propane separation performance as a function of growth time (c) and permeation temperature (d). Another in situ growth method, reported by Caro and co-workers,62 involves coating the support with a layered double hydroxide (LDH) “buffer” layer prior to onestep growth of ZIF-8 membrane by soaking the support in the

Commercialization of ZIF-enabled membranes requires the capability to economically fabricate high quality, defect-free ZIF


Commercialization of ZIF-enabled membranes requires the capability to economically fabricate high quality, defect-free ZIF membranes in scalable membrane geometry. membranes in scalable membrane geometry. High-quality ZIF8/alumina membranes have been synthesized by secondary (seeded) growth57−60 or in situ growth methods.23,40,61 Both methods are able to prepare ZIF-8 membranes showing attractive propylene/propane selectivity. In the secondary growth method, a ZIF-8 seed layer is first deposited on the porous alumina support (disc or tube) by bringing in contact with a ZIF-8 seed suspension or rubbing with dry ZIF-8 seed particles. Kwon and co-workers58 suggested that microwave irradiation was helpful to rapid formation of uniform and strongly adhered ZIF-8 seeds on the support. Following deposition of ZIF-8 seeds on the support, a dense ZIF-8 layer is then formed by immersing the support in a reaction

Figure 3. (a) Schematic illustration of counter diffusion-based in situ growth method.61 (b) Electron microscope images of ZIF-8/alumina membrane synthesized by the counter diffusion-based in situ growth method.23 Reprinted with permission from Elsevier, copyright 2015. (c) Propylene/propane separation performance of ZIF-8/alumina membranes as a function of growth time.61 (d) Propylene/propane separation performance of ZIF-8/alumina membranes as a function of permeation temperature.61 Copyright 2014, American Chemical Society. 3844




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with excellent scalability. Their structural similarities are highlighted in Figure 4. Hollow fiber is the most practical membrane geometry in terms of membrane packing efficiency and is one of the few membrane geometry successfully commercialized for large-scale gas separations. Layered ZIF/ polymer hollow fiber membranes are composite membranes formed by growing a continuous ZIF layer on the shell or bore side of porous polymer hollow fibers.21,31 The layered ZIF/ polymer hollow fiber platform potentially combines excellent scalability of hollow fiber devices with pure ZIF membrane’s extraordinary separation performance. An example of layered ZIF-8/polymer hollow fiber membrane was given by Brown and co-workers.21 A thick ZIF-8 layer (∼8.8 μm) was in situ grown at the polyimide-amide Torlon hollow fiber bore side by flowing a dilute zinc salt solution in fiber bore and a concentrated ligand solution at fiber shell. Mixture permeation shows a propylene/propane separation factor of 12, which was substantially lower than previously discussed ZIF-8/alumina membranes (α ∼ 30−70), suggesting that the ZIF-8 layer was of poor quality. While the work demonstrates the possibility of layered ZIF/polymer hollow fiber membranes, substantial improvements must be made to membrane synthesis techniques. The layered ZIF-8/polymer hollow fiber was only 5 cm in length, and it remains unclear whether ZIF-8 layers with consistent quality can be formed when scaled up. Additionally, the membrane synthesis involves a rather complex “intermittent bore solution flow”, whose scalability is questionable. Mixed-matrix ZIF/polymer hollow fiber membrane is another promising option for ZIF-enabled membranes. It is interesting to compare its structure with layered ZIF/polymer hollow fiber membranes. While the hollow fiber support employed for layered ZIF/polymer membranes are preferably to be highly porous to minimize mass transfer resistance,

reaction solution containing all reactants (zinc source, 2methylimidazole, and sodium formate). The coated LDH layer was believed to possess high affinity with ZIF-8 and be crucial to formation of a continuous well-grown ZIF-8 layer. Despite attractive propylene/propane selectivity, all pure ZIF-8 membranes discussed above are synthesized on top of small alumina discs or tubes with active membrane less than 10 cm2. It is unclear whether consistently attractive separation performance can be realized when scaled up. Additionally, since large-scale gas separations usually require at least thousands of square meters of membrane contact area, the high cost of alumina supports and low packing efficiency of tubular membrane modules put the scalability of ZIF-8/alumina platform into question. Nevertheless, separation performance of ZIF/alumina membranes is expected to be further improved with emerging advanced ZIFs and continuing optimization of ZIF membrane synthesis techniques that can create lessdefective membranes with thinner separation layer. In case that permeance/selectivity of ZIF/alumina membranes can be sufficiently attractive to justify their high costs, the possibility should not be excluded that ZIF/alumina membranes may become economically viable. Layered ZIF/polymer hollow fiber membranes and mixedmatrix ZIF/polymer hollow fiber membranes are two platforms

Layered ZIF/polymer hollow fiber membranes and mixed-matrix ZIF/polymer hollow fiber membranes are two platforms with excellent scalability.

Figure 4. Schematic illustration of (a) layered ZIF/polymer hollow fiber with ZIF layer grown at the fiber bore side and (b) layered ZIF/polymer hollow fiber with ZIF layer grown at the fiber shell side. (c) Ternary phase diagram showing hollow fiber formation.63 Reprinted with permission from Elsevier, copyright 2015. (d) Mixed-matrix ZIF/polymer hollow fiber with ZIF particles dispersed in the fiber outside layer. Orange-colored region indicate the ZIF phase, gray-colored region indicate porous polymer hollow fiber support, blue-colored region indicates dense polymer matrix. 3845



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Table 1. Propylene/Propane Separation Performance, Measured at 22−35 °C, of ZIF-8 Membranes Realized on Different Platforms membrane platform ZIF/alumina

fabrication details

C3H6 permeance (× 10−10 mol/m2 s Pa)

secondary growth

278.2 ± 99.8 207.9 ± 6.5 110 268.5 ± 12.0 25 90.4 ± 10.0 7.4 1.0

counter diffusion


layered ZIF/polymer hollow fiber mixed-matrix ZIF/polymer hollow fiber

counter diffusion 17 wt % loading 30 wt % loading

mixed-matrix ZIF/polymer hollow fiber comprises a dense separation layer (i.e., fiber skin layer) where the ZIF phase (usually in the form of particles) is dispersed inside the continuous polymer matrix. ZIFs are ideal molecular sieving materials for mixed-matrix membranes. Unlike conventional molecular sieves such as zeolites or carbon molecular sieves, ZIFs are organic−inorganic hybrids with excellent compatibility with polymer matrix, which was shown to be critical for successful synthesis of mixed-matrix membranes with desirable transport properties.64 Numerous studies have been published on ZIF/polymer mixed-matrix membranes; however, only a very limited amount of work was extended to the scalable hollow fiber geometry. Li and co-workers19 reported synthesis of ZIF-7/Pebax mixedmatrix composite films with ZIF-7 loading up to 34 wt %. The membranes showed excellent enhancements in CO2/CH4 and CO2/N2 selectivities. However, as the employed matrix was a rubbery polymer, the mixed-matrix was not able to be spun into hollow fibers. An example of mixed-matrix ZIF/polymer membrane using spinnable polymers was demonstrated by Zhang and co-workers, who reported ZIF-8/6FDA-DAM polyimide mixed-matrix dense film membranes with ZIF-8 particle loading up to 48 wt %.24 Mixture permeation showed attractive propylene permeability (up to 58 Barrer) and propylene/propane separation factor (up to 22) that are both substantially enhanced over the polyimide matrix. The same ZIF-8/6FDA-DAM mixed-matrix material was successfully extended to the mixed-matrix ZIF/polymer hollow fiber platform shown in Figure 4d.25 The mixed-matrix hollow fiber comprises two layers with nanosized ZIF-8 particles only dispersed in the outside (sheath) layer. At 17 wt % ZIF-8 loading, the polydimethylsiloxane (PDMS)-coated mixedmatrix hollow fiber showed a propylene/propane separation factor above 17, which is higher than layered ZIF/polymer hollow fiber discussed previously. The mixed-matrix hollow fiber became increasingly defective as the ZIF-8 loading increased to 30 wt %. While coating the fiber increased propylene/propane separation factor to a level (27.5) that is even comparable with ZIF-8/alumina membranes, the coating introduced substantial resistance to propylene permeation and reduced propylene permeance to a level (0.27 GPU) that is economically unattractive. Mixed-matrix ZIF/polymer hollow fiber is by far the most scalable platform for ZIF-enabled membranes. They can be easily scaled up by slightly modifying existing manufacture protocols of pure polymer hollow fiber membranes. On the other hand, a continuous membrane fabrication process is challenging for ZIF/alumina tubes and layered ZIF/polymer hollow fibers. It should be noted that separation performance of

C3H6/C3H8 separation factor 34.6 40.4 30.1 70.6 59 12.0 17.7 27.5

± 10.4 ± 8.5 ± 11.1 ± 3.0

reference Pan and co-workers57 Kwon and co-workers58 Liu and co-workers60 Kwon and co-workers23 Hara and co-workers40 Brown and co-workers21 Zhang and co-workers25

mixed-matrix membranes is determined by both dispersed ZIFs and the continuous polymer matrix, which is usually less permeable and less selective than the pure ZIF phase. As a result, unless the ZIF loading is sufficiently high, it is difficult to achieve permeance and selectivity in mixed-matrix ZIF/ polymer hollow fibers that approach those of high quality, well-grown supported ZIF membranes. Table 1 summarizes propylene/propane separation performance of ZIF-8/alumina membranes, layered ZIF-8/polymer hollow fiber membranes, and mixed-matrix ZIF-8/polymer hollow fiber membranes.

Stability of ZIF-enabled membranes under realistic feed conditions needs to be verified. Commercial membranes are expected to have a lifetime of at least three years.42 To our knowledge, no studies reported longterm stability of ZIF-enabled membranes under realistic feed conditions. It should be noted that testing under realistic feed conditions does not simply indicate binary mixture permeation. Instead, it suggests permeation using an actual or simulated feed mixture containing all relevant impurities at the temperature, pressure, and humidity that are characteristic of realistic plant operations. While single-gas or binary mixture permeation is convenient to evaluate membrane quality, a good knowledge of performance stability under realistic feed conditions is critical to advance ZIF-enabled membranes beyond fundamental characterizations. ZIF-8 is basic and may degrade if the feed mixture contains highly acidic impurities. For example, H2S impurity exists in most olefin/paraffin feeds.65 While H2S concentration is usually on the parts per million level, long-term exposure may lead to gradual membrane structural change and selectivity loss. H2Sinduced membrane deactivation may be more pronounced for separating raw natural gas, which can be contaminated with up to 20 mol % H2S.66 While pretreatments may reduce feed H2S level prior to contact with ZIF membrane modules, the addition will possibly increase the overall separation cost. Another potential problem associated with feed impurities is membrane performance loss as a result of heavier feed contaminants. For example, raw natural gas usually contains trace amount of heavier hydrocarbons (e.g., heptane, toluene). Because ZIF’s aperture is flexible, we speculate that these heavier and more condensable components may diffuse (albeit slowly) and get strongly adsorbed inside ZIF’s ultramicropores, thereby reduce both membrane permeance and selectivity. It is worthwhile to note that the same problem usually does not exist for zeolite membranes with more rigid crystal structures, 3846




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good knowledge of ZIF’s structure-transport property relationships is fundamental to screen potential candidates from the existing ZIF pool and to understand how to tune ZIF’s transport properties through rational synthesis and postsynthetic modifications. Such an understanding is critical to extend ZIF-enabled membranes to a broader spectrum of gas and vapor separations beyond propylene/propane separation. Although both layered ZIF/polymer hollow fibers and mixed-matrix ZIF/ polymer hollow fibers possess excellent scalability, they have yet to demonstrate separation performance comparable to the less scalable ZIF/alumina platform. With emerging ZIF materials possessing tailorable transport properties and continued improvements in ZIF membrane synthesis techniques, ZIFenabled membranes may become a strong contender for largescale gas and vapor separations.

which would totally exclude adsorption of larger impurity molecules. ZIFs were believed to be more thermally and chemically stable when compared with isoreticular MOFs (IRMOFs).11,67 Several recent studies50,68,69 have, in fact, indicated that ZIFs may not be as robust as usually interpreted. ZIF-8 is thought to be thermally stable up to 500 °C as evidenced by negligible weight loss in TGA measurements.11,35 Nevertheless, as discussed previously, more sensitive TGA-MS characterizations suggested that MeIM methyl groups start to dissociate at temperature as low as ∼ 350 °C.50 Such compositional change will possibly affect the ZIF’s transport properties. While ZIF-8based membranes are usually not expected to function at such high temperature, caution should be given to avoid thermal activation at temperatures close to or above 350 °C. Similar to thermal stability, ZIF-8 was believed to possess excellent hydrothermal stability as well. Park and co-workers showed unchanged powder X-ray diffraction (PXRD) patterns for ZIF8 powder samples soaked in boiling water.11 However, a more recent study68 has noted otherwise, that ZIF-8’s hydrothermal stability is related to the ZIF-to-water ratio. Comprehensive characterizations (water pevaporation, electron microscope imaging, and PXRD) of ZIF-8 powder and ZIF-8/alumina membrane samples showed irreversible dissolution and morphological change under static water soaking (with low ZIF-to-water ratio) or continuous water permeation. It should be noted that the hydrothermal stability issue is less of a concern for ZIF-8 membranes used in gas separation applications, in which feed moisture level is minimal in most cases. Most realistic feeds are pressurized. Propylene plant feeds are usually over 20 bar. Raw natural gas could be as high as 340 bar in pressure.70 In most cases, it is economically unattractive to depressurize the feed and repressurize the retentate product. As a result, commercial gas separation membranes are expected to possess excellent mechanical robustness under high transmembrane pressure. Most reported permeation data of ZIF/ alumina and layered ZIF/polymer hollow fibers are measured at ∼ 1 bar upstream pressure using the “Wicke-Kallenbach” technique with permeates collected by sweep gases (e.g., helium, nitrogen, argon). It is unclear whether these membranes will deliver consistent and stable selectivity at > 20 bar trans-membrane pressure. Liu and co-workers60 studied the effects of trans-membrane pressure (up to ∼ 3 bar) on propylene/propane permeation in ZIF-8/alumina membranes. As feed pressure was increased, permeabilities of both components were reduced with slight drop in ideal selectivity, which was believed to be simply due to shape of the Langmuir isotherms. Shekhah and co-workers71 measured permeation on ZIF-8/alumina membranes at ∼ 2 bar transmembrane pressure using the “constant volume” permeation technique. Compared with other data40 measured at 1 bar, the study showed consistent selectivity for small molecules (H2/N2, H2/CH4, CO2/CH4) but significantly lower selectivity over C3H8 (H2/ C3H8, C3H6/C3H8). The difference was possibly due to small amount of defects that strongly affect permeability of the slowly permeating molecule (C3H8).25 However, it is unclear whether the defects were caused by higher trans-membrane pressure or simply associated with membrane quality. To summarize, in this Perspective we have taken a critical look at the challenges and opportunities of ZIF-enabled membranes. The field has attracted increasing interest since 2009 and is expected to continue flourishing in the future. A

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest. Biographies Chen Zhang is a research engineer working at Georgia Institute of Technology, where he earned Ph.D. degree in Chemical Engineering in 2014. He has worked under the supervision of Professor William J. Koros since 2009. Dr. Zhang’s research is focused on design and development of nanoporous material (metal−organic frameworks and carbon molecular sieves)-based membranes and adsorbents for advanced gas and vapor separations. William J. Koros is the Roberto C. Goizueta Chair and Georgia Research Alliance Eminent Scholar in Membranes at the Georgia Institute of Technology. Professor Koros served as the Editor-in-Chief of the Journal of Membrane Science from 1991−2008. He was elected to the National Academy of Engineering in 2000 for his work on advanced separations technology. He has over 375 refereed publications and 15 U.S. patents focused on the topic of membrane and sorbent science and technology. More information of Professor Koros and his research group can be found at http://koros.chbe. gatech.edu/index.php?do=home.

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ACKNOWLEDGMENTS W.J.K. acknowledges funding from King Abdullah University of Science and Technology (Award No. KUS-I1-011-21) and Office of Basic Energy Science of U.S. Department of Energy (DE-FG02-04ER15510).

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REFERENCES

(1) Caro, J.; Noack, M. Zeolite membranes-recent developments and progress. Microporous Mesoporous Mater. 2008, 115, 215−233. (2) Poshusta, J. C.; Tuan, V. A.; Pape, E. A.; Noble, R. D.; Falconer, J. L. Separation of light gas mixtures using SAPO-34 membranes. AIChE J. 2000, 46, 779−789. (3) Yuan, W.; Lin, Y. S.; Yang, W. Molecular sieving MFI-type zeolite membranes for pervaporation separation of xylene isomers. J. Am. Chem. Soc. 2004, 126, 4776−4777. (4) Kiyono, M.; Williams, P. J.; Koros, W. J. Effect of pyrolysis atmosphere on separation performance of carbon molecular sieve membranes. J. Membr. Sci. 2010, 359, 2−10. (5) Rungta, M.; Zhang, C.; Koros, W. J.; Xu, L. Membrane-based ethylene/ethane separation: The upper bound and beyond. AIChE J. 2013, 59, 3475−3489.

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