An Unconventional Rapid Synthesis of High Performance Metal

May 30, 2013 - James W. Maina , Jürg A. Schütz , Luke Grundy , Elise Des Ligneris , Zhifeng Yi , Lingxue Kong , Cristina Pozo-Gonzalo , Mihail Iones...
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An Unconventional Rapid Synthesis of High Performance Metal− Organic Framework Membranes Miral N. Shah,† Mariel A. Gonzalez,† Michael C. McCarthy,† and Hae-Kwon Jeong*,†,‡ †

Artie McFerrin Department of Chemical Engineering, Texas A&M University, 3122 TAMU, College Station, TX, United States Materials Science and Engineering Program, Texas A&M University, 3003 TAMU, College Station, TX, United States

‡

S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) are attractive for gas separation membrane applications due to their microporous channels with tunable pore shape, size, and functionality. Conventional MOF membrane fabrication techniques, namely in situ and secondary growth, pose challenges for their wider commercial applications. These challenges include reproducility, scalability, and high manufacturing cost. Recognizing that the coordination chemistry of MOFs is fundamentally different from the covalent chemistry of zeolites, we developed a radically different strategy for MOF membrane synthesis. Using this new technique, we were able to produce continuous well-intergrown membranes of prototypical MOFs, HKUST-1 and ZIF-8, in a relatively short period of time (tens of min). With a minimal consumption of precursors and a greatly simplified synthesis protocol, our new technique provides potential for a continuous, scalable, reproducible, and easily commercializable route for the rapid synthesis of MOF membranes. RTD-prepared MOF membranes show greatly improved gas separation performances as compared to those prepared by conventional solvothermal methods, indicating improved membrane microstructure.

1. INTRODUCTION

Apart from improving membrane microstructure, substantial reduction in the cost of membrane manufacturing is also critically important when it comes to potential commercial applications of MOF membranes.20 Since most of the synthesis protocols reported are generally derived from those developed for zeolite membrane synthesis, one can expect MOF membranes will face similar challenges that zeolite membranes have faced for their commercialization: batch crystallization, reproducibility, scalability, and manufacturing cost issues.21 Typically for in situ growth, macroporous supports such as porous α-alumina are modified14,22,23 to promote heterogeneous nucleation followed by crystal growth under solvothermal conditions. On the other hand, secondary growth involves seeding the support13,15,16,24−26 as an additional step. These additional steps limit the scalability and reproducibility of the membrane synthesis process. Furthermore, MOF membranes are expected to be even more costly than zeolite membranes, considering their additional challenges: expensive ligands, a large consumption of precursors due to unavoidable homogeneous nucleation, and substantial use of organic solvents. Therefore, it is highly desirable to develop new synthesis protocols for MOF membranes that are fundamentally different from the conventional ones. To substantially bring down the

Metal−organic frameworks (MOFs) are a class of hybrid porous crystalline materials comprising of metal centers coordinated to organic linkers. With a judicious choice of organic linkers, MOFs offer unprecedented opportunities for gas separations due to tunable pore sizes, ranging from microto mesoscale and controllable surface functionalities.1−4 These unique features of MOFs make them desirable candidates for membrane-based gas separation applications.5−9 However, it is not straightforward to form polycrystalline MOF films and membranes on porous supports mainly due to the fact that heterogeneous nucleation of MOF crystals is not always favored.5,10 Diverse synthesis protocols have been developed for polycrystalline MOF membranes. Apart from the step-by-step liquid-phase deposition,11 many of these protocols can be broadly categorized into two techniques, namely in situ12 and secondary growth.7,13 Despite early success in the formation of MOF membranes,5,10 most of MOF membranes reported to date have not shown as impressive of separation of gas molecules as expected,6−8,11,14−19 indicating a poor membrane microstructure (i.e., grain boundary structure) (and/or improper choice of MOF structures). Since membrane microstructure can greatly be affected by synthesis protocols, this lack of MOF membranes capable of decent gas separation strongly suggests the need for the development of new synthesis protocols. © 2013 American Chemical Society

Received: April 19, 2013 Revised: May 30, 2013 Published: May 30, 2013 7896

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Figure 1. Schematic illustration of rapid thermal deposition (RTD) technique. with the polished side facing up on a Pylex Petri dish. After 15 min, the oven was turned off and the samples were allowed to cool down naturally to room temperature in the oven. The membranes were then removed, rinsed with methanol, and solvent exchanged for 24 h in methanol. Membranes were dried under ambient conditions for 12 h thereafter. 2.3. Synthesis of ZIF-8 Membranes by RTD. A metal solution and a ligand solution were prepared by dissolving 1.32 g of zinc acetate dihydrate (Zn(OAc)2·2H2O) and 1.00 g of 2-methylimidazole (m-Im) in 15 mL solvent, respectively. The solvent used here was a mixture of DMA and DI water with the ratio 2:1 (v/v). The ligand solution was added dropwise to the metal salt solution and stirred for 1 min. This precursor solution with a molar ratio of Zn:m-Im:DMA/DI = 1:2:128 was immediately used for slip coating on an α-alumina support, in a similar manner as described above. The slip-coated supports were placed in the oven at 200 °C (RTD temperature) for 15 min (RTD time). After 15 min, membranes were slowly cooled down to room temperature, similar to the HKUST-1 membrane. ZIF-8 membranes were rinsed with DMA, followed by ethanol rinsing and solvent exchange in ethanol for 3 days. After solvent exchange, the membranes were dried at 85 °C for 12 h. 2.4. Characterization. Scanning electron micrographs were taken with a Quanta 600 field emission scanning electron microscope operating at 30 keV (5 keV for high magnification) acceleration voltage. Energy dispersive X-ray spectroscopy (EDS) was carried out using an Oxford EDS X-ray mapping and digital imaging accessory attached to the Quanta SEM. Prior to the measurements, membrane samples were thoroughly rinsed in methanol for 2 days to remove residual precursors that may be present inside the supports. X-ray diffraction (XRD) patterns were collected with a Rigaku Miniflex II powder X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). Thermogravimetric analysis (TGA) data was collected using a Netzsch thermo microbalance (TG 209 C Iris) under 10 mL/min N2 flow and a temperature ramping rate of 5 K/min. Single gas permeation measurements were carried out for HKUST-1 and ZIF-8 membranes for gases such as H2, N2, CH4, CO2, C2H4, C2H6, C3H6, C3H8, and SF6, using a time-lag method at room temperature with a feed pressure of 1 bar. Prior to the measurements, as-prepared HKUST-1 membranes were exchanged in methanol for 24 h and left on a shelf for at least 12 h prior to gas permeation measurements. Due to its open metal sites, HKUST-1 is hydrophilic (i.e., water molecules can easily coordinate with the open metal sites). To ensure complete removal of the coordinated water molecules, the membranes were placed in the gas permeation cell and flushed with helium on the feed side and vacuum on the permeate side for 24 h. Similarly ZIF-8 membranes were activated by exchanging solvents with methanol for 3 days and subsequent drying at RT. Single gas permeation measurements were conducted for these activated samples by a time-lag method. Binary C3H6/C3H8 (50/50) measurements were performed for ZIF-8 membranes by the Wicke−Kallenbach technique. Argon was used as a sweep gas on the permeate side. The feed gas mixture and permeate sweep gas were set to a flow rate of 100 mL/min, maintaining atmospheric pressure conditions on both sides. The permeate gas composition was analyzed using a Hiden QGA mass spectrometer.

cost of membrane manufacturing, new synthesis protocols should offer the following features: (1) scalable (i.e., rapid and continuous) with existing infrastructure, (2) reproducible (i.e., simple), (3) economical, and (4) environmentally friendly (minimal use of precursor chemicals and solvents). In this work, we have developed such a new synthesis protocol by recognizing that the coordination chemistry of MOFs is fundamentally different from the covalent chemistry of zeolites. Our new synthesis protocol, called “rapid thermal deposition” (hereafter, RTD), is based on the concept of evaporation-induced crystallization as reported by Ameloot et al.27 but performed at an elevated temperature. It is worth mentioning here that RTD is conceptually more similar to the evaporation-induced self-assembly28 than to the conventional solvothermal synthesis. In the rapid thermal deposition technique, porous supports are soaked with a MOF precursor solution and subjected to elevated temperature. Rapid solvent evaporation from the supports drives the flow of the precursor solution from inside the supports to outside. Simultaneously, crystallization occurs inside and outside the supports. The RTD technique was applied to synthesize well-intergrown membranes of two prototypical MOFs, HKUST-1 and ZIF-8. Microstructure of these RTD-prepared membranes was found to be distinct as compared to that of conventionally prepared counterparts. The gas permeation measurements present much enhanced gas separation performance of the membranes, strongly indicating their improved grain boundary structure.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All the chemicals were used as supplied. Zinc acetate dihydrate [Zn(OAc)2·2H2O > 97%, Alfa Aesar] and copper nitrate hemi (pentahydrate) [Cu(NO3)2·2.5H2O > 98%, Alfa Aesar] were used as copper and zinc sources. 2-Methylimidazole (m-Im) (C4H6N2 > 99%, Sigma-Aldrich) and 1,3,5-benzene tricarboxylic acid (BTC) (C9H6O6 > 98%, Alfa Aesar) were used as ligands for ZIF-8 and HKUST-1, respectively. N,N-dimethyl acetamide (DMA) (99%), N,N-dimethyl formamide (DMF) (99.8%), and methanol (99.8%) were obtained from Alfa Aesar. Ethanol (99.5%) was obtained from Sigma-Aldrich. 2.2. Synthesis of HKUST-1 Membranes by RTD. A metal solution and a ligand solution were prepared by dissolving 2.5 g of copper nitrate hemi(pentahydrate) [Cu(NO3)2·2.5H2O] and 1.25 g of 1,3,5-benzene tricarboxylic acid (BTC) in 10 mL of DMF, respectively. Both solutions were stirred for 10 min. The ligand solution was added to the metal solution dropwise and the mixture was stirred for 10 min until a clear solution was obtained. This precursor solution with a molar ratio of Cu:BTC:DMF = 1.8:1:43.4 was used for RTD processing. Home-made porous α-alumina disks (porosity = 46%, diameter = 22 mm, and thickness = 2 mm) were used as supports and obtained using a previously reported method.29 The supports were polished on one side. The supports were slip-coated with the precursor solution (the supports were held horizontally with the polished side facing down, and the precursor solution was brought up in contact with the supports for 30 s and then slid away and held vertically). After quickly wicking off the excess precursor solution from the polished side using Kimwipes, the slip-coated supports were placed in a preheated oven at 180 °C (RTD temperature) for 15 min (RTD time)

3. RESULTS AND DISCUSSION Figure 1 illustrates the concept of the rapid thermal deposition (RTD) technique for MOF membrane synthesis. The key step is to soak porous supports with a precursor solution and subsequently to subject the precursor-soaked supports to 7897

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Figure 2. XRD spectra of HKUST-1 membranes synthesized for different time durations (2, 5, 8, 12, 15, and 30 min) along with a simulated HKUST-1 pattern. Intensities of HKUST-1 membranes are multiplied by a factor of 2 for clarity.

solvent, and improved attachment of crystals.31 Below 100 °C, no substantial crystal formation was observed, while above 200 °C, brownish spots were formed on the membrane, possibly due to the decomposition of ligand molecules. Figure 2 presents the XRD patterns of the membrane samples synthesized by the RTD process at 180 °C, for varying deposition times. HKUST-1 phase formed on the support within just 2 min. It appears that the membranes exhibit preferred out-of-plane orientation along the ⟨222⟩ crystallographic direction. Figure 3 shows the top-view and cross-

elevated temperature (in an oven). There are several kinetic processes happening during the RTD process: (1) evaporation of solvents, (2) flow of precursor solutions (from inside of the supports to outside), (3) formation of crystals (both heterogeneous and homogeneous), and (4) drying (partial activation) of crystals. The most important processes to be controlled are the evaporation of solvents and the formation of crystals. Obviously these two kinetic processes are not completely independent of each other. But, the relative timescales (or rates) have to be balanced. For instance, if the evaporation timescale (te) is too short as compared to the crystallization timescale (tc), there will be no crystals forming inside or on the supports. On the other hand, when te is much greater than tc, it may take too long for crystals to form and those crystals formed may be distributed throughout the supports (i.e., unlikely to form well-intergrown films). Therefore, the relative timescale of the crystal formation and solvent evaporation must be optimized for the synthesis of continuous and well-intergrown MOF membranes. The parameters studied to control crystallization and evaporation rates are as follows: molar composition and concentration of the precursor solutions, RTD temperature, presence of deprotonators (i.e., catalysts) to increase reaction rate, and nature of solvents and vapor pressures. HKUST-1 (also known as Cu-BTC),30 a prototypical MOF, was chosen to demonstrate the RTD technique because of its robust synthesis protocol. HKUST-1 can be synthesized using several different solvents (such as water/alcohol, DMF, DEF, and DMSO) over a wide range of temperatures (from RT to 120 °C or possibly higher). With careful consideration of the relative timescale of the crystal formation and solvent evaporation, DMF was selected as the solvents: the precursor solutions prepared in both of these solvents remain clear under ambient conditions throughout the duration of the deposition process while enabling HKUST-1 crystals to form on supports at relatively high temperature. We then determined the deposition temperature by conducting a series of experiments at varying deposition temperatures (not shown here). Deposition temperature of 180 °C was found to be optimal for rapid HKUST-1 crystallization, fast evaporation of DMF

Figure 3. Scanning electron micrographs of HKUST-1 membrane synthesized by RTD: (a, b) top view and (c, d) cross-sectional view.

sectional view of the RTD-prepared HKUST-1 membrane treated at 180 °C for 15 min. As can be seen, the morphology of the HKUST-1 membrane appears very different from those of conventionally prepared HKUST-1 membranes.6,31,32 Grains are much smaller and do not display well-defined facets, possibly resulting from the rapid solvent evaporation and subsequent crystallization, which promotes nucleation while inhibiting growth of crystals and kinetically freezing the surface structure. The cross-sectional SEM images (see Figure 3, panels 7898

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those for conventionally prepared HKUST-1 membranes (hereafter, conventional membranes) that were synthesized using our previously reported technique.31 The permeances of all gases except CO2 through the RTD membranes are substantially lower than those through conventional membranes. The ideal selectivities (Table S1 of the Supporting Information) deviate from previously reported values.6,31,32 As compared to conventional membranes, RTD membranes show very high (∼ 600) H2/SF6 ideal selectivity. The lower permeances and enhanced ideal selectivities indicate that the RTD-HKUST-1 membranes have a much better microstructure (i.e., grain boundary structure) given the similar membrane thickness (∼ 20−25 μm). With better grain boundary structure, nonselective intracrystalline diffusion can be suppressed, resulting in high H2/SF6 selectivity. Polycrystalline membranes have two diffusion pathways: selective intercrystalline diffusion through grains and nonselective intracrystalline diffusion through grain boundaries. With better grain boundary structure, nonselective intracrystalline diffusion can be suppressed, resulting in high H2/SF6 selectivity. Furthermore, gas transport through membranes is influenced by both thermodynamic (i.e., affinity) and kinetic (i.e., diffusivity) factors. Keskin et al.35 predicted that CO2 is expected to be more selective than H2 for defect-free (i.e., single-crystal-like) HKUST-1 membranes due to its high affinity to the open metal sites of HKUST-1. Due to the presence of the grain boundary, even though substantially improved as compared to conventional membranes, CO2 is not selective over H2 for the RTD membranes. The RTD technique was then applied to synthesize membranes of zeolitic−imidazolate frameworks (ZIFs), a subclass of MOFs with high thermal and chemical stability. ZIF-8 is of particular interest due to its potential for propylene/ propane separations.8,16,36,37 After a series of experiments, the following parameters for RTD processing of ZIF-8 membranes were determined: 2:1 (by volume) mixture of DMA and DI water, RTD temperature of 200 °C, and RTD time of 15 min (Figure 5). Furthermore, it was also found that zinc acetate works better than other zinc salts such as zinc chloride and zinc nitrate, possibly due to the catalytic (i.e., deprotonating) property of acetate, promoting the ZIF crystallization.23,38 As observed in the RTD HKUST-1 membranes, the membrane microstructure (Figure 6) is very different from previously reported membranes:7,23 nanosized grains, ill-defined facets, and film grown inside and outside the support. On the basis of the cross-sectional electron micrograph and the EDS line map (see Figure S3 of the Supporting Information), the thickness of the membranes can be estimated in the range of 5−20 μm. Single gas permeation data of RTD-prepared ZIF-8 membranes (Figure S4 of the Supporting Information) shows that ZIF-8 membranes exhibit the molecular sieving behavior, favoring the smaller molecules with a sharp molecular cutoff between ethylene and propane. This observation is consistent with the previous report by Lai and his co-workers15,24 on their ZIF-8 membranes synthesized by secondary growth. Binary gas permeation properties of RTD ZIF-8 membranes were examined with a mixture of propylene/propane (50/50). Three membranes were tested and the results are presented in Table S2 of the Supporting Information. As compared to the membranes by Lai and co-workers8 and our earlier work,16 the average propylene permeance is about 4 times lower with a comparable propylene/propane selectivity of ∼30. The comparable high propylene/propane selectivity indicates that the grain boundary structure of the RTD membranes is as good

c and d) reveal a substantial amount of HKUST-1 crystals formed inside the support as well. The interface between the HKUST-1 film and the support is not clearly distinguishable, making it difficult to determine the membrane thickness. To gain insight on the membrane thickness, energy-dispersive Xray spectroscopy (EDS) line mapping of aluminum and copper were taken (see Figure S1 of the Supporting Information). Copper was detected up to approximately 15 μm underneath the membrane surface. A substantial fraction of the total copper detected is present inside the support, confirming the presence of HKUST-1 crystals inside the support. On the basis of the intensity of the EDS line map, the effective thickness of the membrane can be estimated in the range of 10−20 μm. RTD-prepared HKUST-1 membranes, in which a substantial fraction of crystals are grown inside supports, are expected to be mechanically strong as well as to be protected from further mechanical failure. For example, selective zeolite layers were synthesized within porous supports as a way of improving mechanical stability of the membranes (termed as pore plugging). 33,34 Pan et al. 8 reported ZIF-8 membranes synthesized by the secondary growth method, wherein nanosized ZIF-8 seeds were deposited into the pores of the support. The ZIF-8 nanocrystals inside the supports led to the growth of ZIF-8 crystals inside the support, making the membranes mechanically robust. Furthermore, when compared with MOF membranes grown predominantly outside the support, Betard et al.11 reported that MOF membranes formed inside the support show the non-Knuden gas separation, indicating the improved microstructure by growing MOF crystal layers inside the supports. When MOFs are synthesized in bulky organic solvents such as DMF, it is necessary to remove the solvent molecules occluded in MOFs (i.e., activation) for practical applications. Since the membranes are synthesized at elevated temperatures under ambient pressure where the solvent is allowed to evaporate, it is expected that RTD-prepared HKUST-1 membranes may be partially activated. Indeed, the RTDprepared HKUST-1 powder contains much less DMF molecules, confirming the partial activation during the RTD process (Figure S2 of the Supporting Information). Gas permeation properties of the RTD-prepared HKUST-1 membranes were measured using single-component gas permeation measurements. Figure 4 presents the single gas permeances of various small gases for RTD-prepared HKUST-1 membranes (hereafter, RTD membranes) in comparison with

Figure 4. Comparison of single gas permeances of HKUST-1 membranes by RTD and secondary growth (ref 31). 7899

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Figure 5. XRD patterns of ZIF-8 membranes synthesized for different time durations (2, 5, 8, 12, 15, and 30 min) along with simulated ZIF-8 pattern. Intensities of ZIF-8 membranes are multiplied by a factor of 2 for clarity.

Figure 7. C3H6 permeability and C3H6/C3H8 selectivity of RTD ZIF-8 membrane in comparison to polymer membranes (ref 39), carbon membranes (ref 40), ZIF-8 6FDA-DAM mixed-matrix membrane (ref 37), and ZIF-8 membrane by secondary growth (ref 8). Figure 6. Scanning electron micrographs of ZIF-8 membrane synthesized by RTD: (a, b) top view and (c, d) cross-sectional view.

industrial scale of production, this drastic reduction in expensive exotic ligands and organic solvents consumption gives the RTD technique a very strong economic and greener edge over conventional solvothermal methods. More importantly, unlike the conventional batch solvothermal methods, RTD is carried out under atmospheric pressure in a relatively short period of time. This unique feature of the RTD technique may enable the production of high-quality MOF membranes in a continuous manner, using existing infrastructure such as coating processes. For example, while supports are continuously brought on a conveyer belt, they may be coated with precursor solutions followed by rapid heating, leading to continuous production of MOF membranes on a commercial scale.

as those by Lai and co-workers. The lower permeance is due to the fact that the RTD membranes are thicker than the membranes by Lai and co-workers. Figure 7 depicts C3H6/ C3H8 permeation and separation performance of RTD ZIF-8 membranes in comparison to other membranes, including polymer membranes,39 carbon membranes,40 the ZIF-8/6FDADAM-mixed matrix membrane,37 and the ZIF-8 polycrystalline membrane synthesized by a secondary growth technique.8 Since the exact thickness of the membranes is not known, it is estimated that the permeability of the RTD ZIF-8 membranes ranges from 250 to 600 Barrer with an average selectivity of ∼30. Permeability for the RTD membrane lies between the end points of the line segment, due to uncertainty in thickness (5− 20 μm). Lastly, it is worth mentioning that our RTD technique offers a number of unique advantages over conventional solvothermal methods. First, since a fraction of the precursor solution is need for synthesis, one can synthesize several tens of RTD membranes from a precursor solution required for the synthesis of one membrane, using conventional techniques. At an

4. CONCLUSIONS In conclusion, we reported a novel strategy, rapid thermal deposition (RTD), for the rapid synthesis of high-quality MOF membranes. RTD technique was applied to rapidly synthesize high-quality membranes of HKUST-1 and ZIF-8, two important prototypical MOFs. The crystal morphology and membrane microstructure of these RTD membranes are quite different from their conventional counterparts. Both HKUST-1 7900

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(7) Bux, H.; Liang, F. Y.; Li, Y. S.; Cravillon, J.; Wiebcke, M.; Caro, J. Zeolitic imidazolate framework membrane with molecular sieving properties by microwave-assisted solvothermal synthesis. J. Am. Chem. Soc. 2009, 131, 16000−16001. (8) Pan, Y.; Li, T.; Lestari, G.; Lai, Z. Effective separation of propylene/propane binary mixtures by ZIF-8 membranes. J. Membr. Sci. 2012, 390−391, 93−98. (9) Aguado, S.; Canivet, J.; Farrusseng, D. Engineering structured MOF at nano and macroscales for catalysis and separation. J. Mater. Chem. 2011, 21, 7582−7588. (10) Shekhah, O.; Liu, J.; Fischer, R. A.; Woll, C. MOF thin films: Existing and future applications. Part of the themed issue on hybrid materials. Chem. Soc. Rev. 2011, 40, 1081−1106. (11) Bétard, A.; Bux, H.; Henke, S.; Zacher, D.; Caro, J.; Fischer, R. A. Fabrication of a CO2-selective membrane by stepwise liquid-phase deposition of an alkylether functionalized pillared-layered metalorganic framework [Cu2L2P]n on a macroporous support. Microporous Mesoporous Mater. 2012, 150, 76−82. (12) Liu, Y. Y.; Ng, Z. F.; Khan, E. A.; Jeong, H. K.; Ching, C. B.; Lai, Z. P. Synthesis of continuous MOF-5 membranes on porous alphaalumina substrates. Microporous Mesoporous Mater. 2009, 118, 296− 301. (13) Yoo, Y.; Lai, Z. P.; Jeong, H. K. Fabrication of MOF-5 membranes using microwave-induced rapid seeding and solvothermal secondary growth. Microporous Mesoporous Mater. 2009, 123, 100− 106. (14) Huang, A.; Bux, H.; Steinbach, F.; Caro, J. Molecular-sieve membrane with hydrogen permselectivity: ZIF-22 in LTA topology prepared with 3-aminopropyltriethoxysilane as covalent linker. Angew. Chem., Int. Ed. 2010, 49, 4958−4961. (15) Pan, Y.; Lai, Z. Sharp separation of C2/C3 hydrocarbon mixtures by zeolitic imidazolate framework-8 (ZIF-8) membranes synthesized in aqueous solutions. Chem. Commun. (Cambridge, U.K.) 2011, 47, 10275−10277. (16) Kwon, H. T.; Jeong, H.-K. Highly propylene-selective supported zeolite-imidazolate framework (ZIF-8) membranes synthesized by rapid microwave-assisted seeding and secondary growth. Chem. Commun. (Cambridge, U.K.) 2013, 49, 3854−3856. (17) Aguado, S.; Nicolas, C.-H.; Moizan-Basle, V.; Nieto, C.; Amrouche, H.; Bats, N.; Audebrand, N.; Farrusseng, D. Facile synthesis of an ultramicroporous MOF tubular membrane with selectivity towards CO2. New J. Chem. 2011, 35, 41−44. (18) Venna, S. R.; Carreon, M. A. Highly permeable zeolite imidazolate framework-8 membranes for CO2/CH4 separation. J. Am. Chem. Soc. 2009, 132, 76−78. (19) Ranjan, R.; Tsapatsis, M. Microporous metal organic framework membrane on porous support using the seeded growth method. Chem. Mater. 2010, 21, 4920−4924. (20) Gascon, J.; Kapteijn, F. Metal-organic framework membraneshigh potential, bright future? Angew. Chem., Int. Ed. 2010, 49, 1530− 1532. (21) Caro, J.; Noack, M. Zeolite membranes: Recent developments and progress. Microporous Mesoporous Mater. 2008, 115, 215−233. (22) Huang, A.; Dou, W.; Caro, J. Steam-stable zeolitic imidazolate framework ZIF-90 membrane with hydrogen selectivity through covalent functionalization. J. Am. Chem. Soc. 2010, 132, 15562−15564. (23) McCarthy, M. C.; Guerrero, V. V.; Barnett, G.; Jeong, H. K. Synthesis of zeolitic imidazolate framework films and membranes with controlled microstructure. Langmuir 2010, 26, 14636−14641. (24) Bux, H.; Feldhoff, A.; Cravillon, J.; Wiebcke, M.; Li, Y.-S.; Caro, J. Oriented zeolitic imidazolate framework-8 membrane with sharp H2/C3H8 molecular sieve separation. Chem. Mater. 2011, 23, 2262− 2269. (25) Stoeger, J. A.; Choi, J.; Tsapatsis, M. Rapid thermal processing and separation performance of columnar MFI membranes on porous stainless steel tubes. Energy Environ. Sci. 2011, 4, 3479−3486. (26) Yoo, Y.; Jeong, H. K. Rapid fabrication of metal organic framework thin films using microwave-induced thermal deposition. Chem. Commun. (Cambridge, Univ.) 2008, 2441−2443.

and ZIF-8 membranes have shown excellent gas separation properties, strongly indicating greatly enhanced membrane microstructure (in particular reduced grain boundary defects). Combined with the minimal consumption of expensive precursors, RTD offers unique opportunities for the commercial applications of MOF membranes by drastically reducing the cost of membrane manufacturing through scalable continuous membrane production, using existing commercial infrastructure.

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ASSOCIATED CONTENT

S Supporting Information *

Energy dispersive X-ray spectra of RTD-prepared HKUST-1 and ZIF-8 membranes (Figures S1 and S3 respectively), thermogravimetric data of HKUST-1 membranes (Figure S2), single gas permeation data of ZIF-8 membranes (Figure S4), tabulated single gas permeation data (Table S1) and propylene/propane binary gas permeation data (Table S2) of ZIF-8 membranes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1 979 862 4850. Fax: +1 979 845 6446. Author Contributions

These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (Grants CBET-0930079 and 1132157) and the Department of Energy Advanced Research Project Agency-Energy (Grant DEAR0000073). The authors are grateful to Mr. H. Kwon for taking some of the high magnification SEM images. The FESEM acquisition was supported by the National Science Foundation under Grant DBI-0116835, the VP for Research Office, and the Texas A&M Engineering Experiment Station.

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REFERENCES

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dx.doi.org/10.1021/la4014637 | Langmuir 2013, 29, 7896−7902