Isoreticular Metal−Organic Frameworks and Their Membranes with

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Isoreticular Metal-Organic Frameworks and Their Membranes with Enhanced Crack Resistance and Moisture Stability by Surfactant-Assisted Drying Yeonshick Yoo,† Victor Varela-Guerrero,‡ and Hae-Kwon Jeong*,†,‡ †

Artie McFerrin Department of Chemical Engineering and ‡Materials Science and Engineering Program, Texas A&M University, College Station, Texas 77843-3122, United States

bS Supporting Information ABSTRACT: Here we report a new strategy that can not only prevent the formation of cracks and fractures in the crystals and films of metal-organic frameworks (MOFs) but also substantially enhance their stability with respect to moisture. It involves the addition of surfactants during a drying process. Surfactants reduce interfacial tension, thereby repressing the formation of fractures and cracks during the final drying process. It was found that, once dried, surfactants adsorbed on the crystal surface render the surface hydrophobic, leading to the enhancement in the stability toward moisture. Using this new strategy, the first crack-free IRMOF-3 membrane was successfully prepared, and its gas permeation performance was tested. IRMOF-3 membranes are found to favor CO2 over C3H8 mainly due to the affinity of CO2 to the amine groups in the structure. In addition, crack-free IRMOF-3 membranes were postsynthetically modified with heptanoic anhydride, thereby changing the effective pore size and surface property of the MOF. Once modified with the anhydride, the membranes favor C3H8 over CO2 due to the increased solubility of C3H8 in the presence of the hydrocarbon moiety.

’ INTRODUCTION Nanoporous metal-organic frameworks (MOFs) have gained significant research interest primarily due to their potential applications in gas storage,1 gas separation,2 and catalysis.3 With judicious choice of organic linkers, structural traits of MOFs such as pore size, shape, and chemical functionality can be finely tuned.4 This unique structural feature offers unprecedented opportunities in gas separation2 as well as in catalysis.3 Isoreticular MOFs (IRMOFs) have been extensively studied due to their simple synthesis and their potential applications.4 IRMOFs show similar crystal structures consisting of zinc-based metal oxide clusters and benzenecarboxylate-based organic linkers. IRMOF3 is of particular interest due to the presence of amine groups in the linkers. These amine groups can serve as a base for catalytic applications5 as well as can interact with CO2 favorably for CO2 separation/storage.6 More interestingly, the amine groups can be functionalized by postsynthetic modifications (PSM) so that both the pore volume (thereby pore size) and surface property of the MOF can be engineered.7-12 Despite their potentials, there has been a limited success in the practical applications of MOFs. One of the most significant drawbacks is their mechanical and chemical instability. Particularly zinc/carboxylate-based MOFs including IRMOFs are prone to the formation of cracks and fractures as well as extremely sensitive to moisture.13 This instability stems from the nature of the bonding (coordination bonding) between zinc atoms and carboxylate ligands. It is well-known that coordination bonds are r 2011 American Chemical Society

kinetically not as strong as covalent bonds though thermodynamically as strong as covalent bonds.14 MOFs as membranes and thin films are of particular interest for continuous membrane-based separations, membrane reactors, and other advanced applications such as optical, electronic, and magnetic applications.15-19 Despite a great deal of research in MOFs as thin films and membranes,20-24 there still exist the same challenges mentioned above not only to fabricate continuous crack-free MOF membranes but also to find their practical applications: the formation of fractures and cracks during the synthesis and/or the activation process and the moisture instability mainly due to the nature of the bonds in MOFs as mentioned above. For instance, mistmatch in the thermal expansion coefficients between MOFs and supports results in tensile stress during cooling process.25 Capillary stress can develop during the activation (i.e., drying) process when removing solvents occluded in the pores. These stresses can easily lead to the formation of fractures and cracks during synthesis of MOF films and membranes.25,26 There are several works reported to address these issues, i.e., crack formation and moisture stability. Our previous studies22,25-27 showed that slow cooling upon the completion of the reaction as well as slow solvent evaporation under near saturated environment could prevent (or at least alleviate) the crack formation of Received: November 30, 2010 Revised: December 23, 2010 Published: February 07, 2011 2652

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Langmuir several MOF films and membranes including IRMOF-1 (also known as MOF-5),22 IRMOF-3,26 HKUST-1,25 and ZIF-8 membranes.27 Nyuyen et al.13 reported that moisture stability of IRMOF-3 was enhanced by introducing hydrocarbons using postsynthetic modification (PSM). Most recently, Wu et al.28 improved the moisture stability of MOFs by incorporating hydrophobic functional groups in the linkers. However, these approaches, whether synthetic28 or postsynthetic,13 cannot be applicable to a broad spectrum of MOFs. For instance, the syntheic approach with “prefunctionalized” ligands might not result in the formation of desired MOFs while the PSM approach requires labile groups such as amines in the structure that can be covalently modified. Therefore, it is of great interest to develop strategies that are rather general when it comes to preventing the crack formation and enhancing moisture stability of MOFs. Here we demonstrate that a simple surfactant-assisted drying process can not only prevent the formation of fractures and cracks but also enhance the stability of IRMOFs with respect to moisture. This method enables the fabrication of well intergrown crack-free IRMOF membranes with exceptional moisture stability. In addition, the pore size and surface property of IRMOF-3 membranes were tuned by postsynthetically modifying amine groups with anhydrides.

’ EXPERIMENTAL SECTION Synthesis and Activation of IRMOF-3 Crystals and Membranes. IRMOF-3 was synthesized by following a procedure reported earlier.7,26 In a typical synthesis, 0.018 M of zinc nitrate hexahydrate (98%, reagent grade, Sigma-Aldrich) in N,N-dimethylformamide (99%, Sigma-Aldrich, hereafter DMF) (solution A) and 0.006 M of 2-aminoterephthalic acid (99%, Sigma-Aldrich, hereafter ABDC) in DMF (solution B) were prepared separately. After vigorously stirring the solutions for 1 h, 10 g of the solution A was mixed with 10 g of the solution B in a glass vial. The final molar ratio of the precursor solution was 2.82 Zn:1 ABDC:310 DMF. After being sonicated for 5 min, the mixture was then treated solvothermally in a convective oven at 105 °C for 24 h. The vial was kept in the oven until the temperature naturally reached at room temperature. IRMOF-3 crystals were then rinsed with fresh DMF for three times and decanted. To exchange DMF occluded in IRMOF-3 pores with more volatile chloroform (99.8%, stabilized, Acros Organics), the crystals were kept in chloroform for 3 days. The chloroform was replenished daily. IRMOF-3 crystals were kept in chloroform until the next experiments. IRMOF-3 membranes were prepared as described in our earlier work26 using heteroepitaxial growth of IRMOF-3 on IRMOF-1 seeds. In short, IRMOF-1 crystals were seeded on a graphite-coated R-alumina support using our microwave-induced thermal deposition technique.22,29 The IRMOF-1 seeded support was then placed vertically in a precursor solution of IRMOF-3 and subjected to the solvothermal growth at 105 °C for 4 h. The IRMOF-3 membrane was left in the oven until the temperature naturally decreased to room temperature. The membrane was then washed with a copious amount of fresh DMF and solvent-exchanged using the procedure described above. The membrane was kept in chloroform until next experiments.

Postsynthetic Modification of IRMOF-3 Crystals and Membranes with Heptanoic Anhydride. IRMOF-3 crystals and membranes were postsynthetically modified by following a procedure reported by Cohen and co-workers.7,8 In a typical synthesis, heptanoic anhydride (99%, Acros Organics, hereafter AM6) was added into a vial containing solvent-exchanged IRMOF-3 crystals in 10 g of chloroform. The molar ratio was 1 -NH2: 2 AM6.7,8 The average amount of solventexchanged IRMOF-3 crystals was about 150 mg (note that 150 mg of IRMOF-3 contains 0.56 mmol of -NH2). The mixture of AM6 and

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IRMOF-3 in chloroform was kept at room temperature for 1 day under static conditions. Upon the completion of the modification reaction of IRMOF-3 with AM6, the sample (denoted as IRMOF-3-AM6) was rinsed with fresh chloroform three times and kept in the solvent. IRMOF-3 membranes were also modified with AM6 in the same manner. An IRMOF-3 membrane was dipped into a Falcon tube (30 mL) containing 0.27 g of AM6 (0.75 mmol) in 10 g of chloroform. The average weight of IRMOF-3 of membrane samples was estimated about 100 mg, which contains about 0.373 mmol of -NH2. After postsynthetic modification, the IRMOF-3-AM6 membrane was rinsed with fresh chloroform three times and kept in chloroform.

Surfactant-Assisted Activation of IRMOF-3 and IRMOF-3AM6 Membranes. Span 80 (Sorbitan oleate, C24H44O6, SigmaAldrich) and Pluronic P123 (HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H, BASF) were used as received. In a typical drying process, a sample (membrane or power) exchanged with chloroform described above was immersed into a surfactant solution. The surfactant solution was prepared by dissolving 1 g of a surfactant (either Span 80 or P123) in 10 g of chloroform. After keeping the sample in the surfactant solution at room temperature for 1 day under static conditions, the sample was gently washed with fresh chloroform and kept in chloroform for about 1 day. To dry the sample, it was placed in a glass chamber with a beaker of fresh chloroform. The chamber was covered with a glass cover with a small opening to create nearly saturated condition with chloroform. The sample was slowly dried in the nearly saturated chamber at RT for 3 days. Single Gas Permeation Measurements. The permeation of small gas molecules (H2, He, CO2, N2, CH4, and C3H8) through membranes was measured in a custom-made permeation cell using a time-lag method.30 The feed side was maintained at a pressure of 1 bar. The permeate side was initially under vacuum. The pressure on the permeate side was recorded as a function of time to estimate the permeance of gas molecules. Characterizations. Scanning electron micrographs were taken with a JEOL JSM-6400 electron microscope operating at 15 keV and a JEOL JSM-7500F field emission scanning electron microscope operating at 3 keV. Thermal gravimetric analysis (TGA, Netzsch TG 209C) was performed on powder samples of IRMOF-3 and IRMOF-3-AM6. Prior to the TGA measurement, the samples were fully evacuated at 100 °C under vacuum for overnight. Each sample was heated from 30 to 700 °C under nitrogen at the heating rate of 10 °C/min. X-ray diffraction (XRD) patterns of the IRMOF-3 powder and membrane samples were taken with a powder X-ray diffractometer (MiniFlex II, Rigaku) with Cu KR radiation (λ = 1.540 56 Å). Nitrogen adsorption isotherms at 77 K were obtained using a surface area and pore size analyzer (ASAP 2010, Micromeritics).

’ RESULTS AND DISCUSSION Figure 1 illustrates a procedure to prepare crack-free MOF (IRMOF-3 and IRMOF-3-AM6) membranes using our new surfactant-assisted drying process. As illustrated, IRMOF-1 seed layers were first prepared on a graphite-coated R-Al2O3 by our microwave-assisted seeding method as described in our previous report.29 IRMOF-3 membranes were then prepared by heteroepitaxially growing the IRMOF-1 seed layers.26 IRMOF-3-AM6 membranes were prepared by postsynthetically modifying IRMOF-3 membranes with AM6. In order to activate the pores, as-exchanged membranes need to be dried.31 When dried either at room temperature or at elevated temperature even under nearly saturated condition, cracks and fractures were formed in the IRMOF-3 membranes as described in our earlier report.26 Because of the nature of bonding (coordination bonds), metal-organic frameworks are 2653

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Figure 1. Schematic illustration of a procedure for the fabrication of crack-free MOF membranes (IRMOF-3 and IRMOF-3-AM6 membranes) using a surfactant-assisted drying process. The structures of IRMOF-3 and IRMOF-3-AM6 are illustrated on the right-hand side.

more prone to the formation of cracks and fractures than zeolites.14 It is speculated that during the drying process the IRMOF-3 films were subjected to capillary stress stemming from the evaporation of chloroform.25,32 The magnitude of the capillary stress in the films is proportional to the rate of evaporation.32 By slowly evaporating chloroform at RT under nearly saturated conditions, the formation of cracks was significantly improved.26 Nonetheless, slow evaporation alone could not prevent the formation of cracks completely (see Figure 2a). In order to further eliminate the formation of cracks, a surfactant was added prior to the final drying process (see Experimental Section for details). It is well-known in the ceramic communities that surfactants, when added during the drying step, can prevent crack formation by reducing the interfacial energy, thereby decreasing the capillary stress.32 However, this strategy has never been applied to MOF films. We have studied two different nonionic surfactants, Span 80 and P123. With P123 added to the solution, cracks are substantially reduced (Figure 2b) as compared to the sample that is not treated with surfactant (Figure 2a). With Span 80 added during the drying process, the sample exhibits no macroscopic cracks (Figure 2c). The thickness of the membranes was about 10 μm thickness (see Figure 2d). Another important observation is that electrons appear charging those samples dried with surfactants more than that dried without surfactants. This suggests that nonconducting surfactants are adsorbed on the sample surfaces, therefore reducing the conduction of electrons even with Au/Pd coats. It is also worthy of mentioning that the crystallinity of IRMOF-3 membranes remained intact after surfactant treatment as confirmed by X-ray diffraction. At this point, it is not clear why Span 80 works better than P123. Though further studies are needed, one possible explanation could be that smaller hydrophilic head groups of Span 80 can interact more favorably with the surface of IRMOF-3 as compared with the bulkier hydrophilic groups of P123. It is observed that the specific surface area (BET) of the sample was reduced after activated in the presence of the surfactant (see Figure 3) likely due to the surfactant molecules partially blocking the access of nitrogen molecules. In order to estimate the amount of the surfactant (Span 80) on IRMOF-3 after drying, thermal gravimetric analysis (TGA) experiments were performed and compared with IRMOF-3 dried without the surfactant. For the TGA measurements, IRMOF-3 power samples were prepared in an identical way as IRMOF-3

Figure 2. SEM images of IRMOF-3 membranes after drying (a) without surfactant, (b) with a triblock copolymer, P-123, and (c) Span 80. The cross-sectional view (d) is from the membrane dried in the presence of Span 80. The thickness of the membrane is ∼10 μm.

membrane samples. As can be seen in Figure 4a, the sample dried without Span 80 shows a stiff weight loss at about 400 °C, indicating decomposition of organic linker. On the other hand, the sample dried with Span 80 shows a gradual weight loss starting at ∼280 °C likely due to the decomposition of the surfactant followed by a stiff weight loss. The surfactant-modified IRMOF-3 exhibits about 3% more in weight loss as compared to unmodified IRMOF-3. This discrepancy between these two samples is attributed to the presence of the surfactant. Considering the formula weight of IRMOF-3 and the molecular weight of the surfactant (Span 80), the surfactant adsorbed on the external surface of IRMOF-3 was estimated about 3 wt %. On the basis of SEM and TGA observations and considering the fact that IRMOF-3 is hydrophilic,13 it is expected that IRMOF-3 becomes hydrophobic after dried in the presence of Span 80. To verify this, simple water contact experiments were performed. Optical micrographs presented in Figure 5 clearly show that with surfactant-assisted drying IRMOF-3 switched from hydrophilic to hydrophobic. Note that the membrane dried without surfactant (Figure 5a) appears white (slight brownish) while the membrane dried with surfactant (Figure 5b) looks black. The white color is due to the fact that the cracks and fractures formed on the membrane scatter light. In contrast, the black color of the surfactant-modified IRMOF-3 membrane is due to the graphite coating underneath on the alumina support, indicating the membrane is transparent. This strongly suggests that the membrane is free of cracks and fractures, consistent with SEM results. It is surmised that the surfactant added during drying step reduces capillary stress, thereby preventing crack formation. As discussed earlier, the instability of MOFs with respect to moisture is due to the fact that water molecules can easily displace carboxylic groups, thereby degrading the structure. It was hypothesized that the hydrophobic MOFs can have better stability with respect to moisture since water molecules will be repelled.13,28 Figure 6 shows time-resolved X-ray diffraction patterns of IRMOF-3 membranes exposed to ambient air. As can be seen in the figure, IRMOF-3 membranes dried in the absence of surfactant exhibit drastic changes in the diffraction patterns after 6 h (Figure 6a). This substantial change in the peak intensities 2654

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Figure 3. N2 adsorption isotherms of IRMOF-3 powder samples dried (a) without Span 80 and (b) with Span 80.

Figure 5. Water contact experiments of IRMOF-3 membranes dried (a) without Span 80 and (b) with Span 80. Figure 4. TGA of IRMOF-3 powder samples dried (a) without Span 80 and (b) with Span 80.

and positions implies degradation of the crystal structure. It should be noted that the appearance of a new peak (marked with an asterisk) at 2θ ∼ 8.9° is possibly due to a partial transformation to a second phase, nonporous ZnBDC 3 xH2O.33,34 In contrast, IRMOF-3 membranes dried in the presence of surfactant show no substantial change in the diffraction patterns even after 1 month (Figure 5b) even though there was a partial phase transformation. At this moment, it is not unreasonable to assume that further improvement in the moisture stability can be done by optimizing parameters such nature of surfactants and concentrations. This dramatically enhanced stability is attributed to the hydrophobic surface of surfactant-modified IRMOF-3 membranes. The same strategy was found to be applicable to IRMOF-1 membranes. After being dried in the presence of surfactant (Span 80), IROMF-1 membranes show significantly enhanced stability with respect to moisture (see Figure S1). In order to change its pore size and property, IRMOF-3 samples (both power and membranes) were postsynthetically modified by following a procedure developed by Cohen and coworkers,12 i.e., reacting the amine groups in the structure with heptanoic anhydride (see Experimental Section for details). The crystallinity of postsynthetically modified IRMOF-3 samples

remained intact judging from X-ray diffraction. Note that the postsynthetically modified powder sample was activated and dried in the absence of surfactant. The N2 isotherm and TGA of a power sample (see Figures S2 and S3) are consistent with the previous report,12 confirming successful modification of IRMOF-3 with the anhydride (IRMOF-3-AM6). Not surprisingly the surface area and the pore volume (thereby effective pore size) were decreased after modified with AM6 as shown in Figure S2. It was estimated that about 91% of the amine groups converted to amides based on the TGA measurement, which is in good agreement with the previously reported value (∼90%).12 After being dried in the presence of surfactant, crack-free IRMOF-3-AM6 membranes were obtained. As expected, the IRMOF-3-AM6 membranes show stability with respect to moisture (see Figure S4). It was reported that once postsynthetically modified with hydrophobic anhydrides, the stability of IRMOF-3 with respect to moisture was drastically enhanced.13 It is reasonable to speculate that surfactant-assisted drying and postsynthetic modification can work synergistically toward the improvement of the moisture stability of IRMOF-3. Finally, single gas permeation of small gas molecules through crack-free IRMOF-3 and IRMOF-3-AM6 membranes was tested for the first time using a time-lag method.22 Prior to the permeation measurements, the absence of macroscopic cracks in 2655

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Figure 6. Time-resolved X-ray diffraction patterns of IRMOF-3 membranes after exposed to ambient air at room temperature: (a) dried without Span 80 and (b) dried with Span 80. Note that the peak marked with an asterisk at 2θ ∼ 8.9° is possibly due to a partial transformation to a second phase.

Figure 7. Single gas permeation of IRMOF-3 membranes (dried in the presence of Span 80) showing Knudsen diffusion for small gases. Note that the permeance of CO2 is substantially deviating from Knudsen diffusion due to its high affinity with amine groups in IRMOF-3.

both membranes was confirmed by performing H2 permeation as a function of transmembrane pressures (see Figure S5). Note that IRMOF-3 and IRMOF-3-AM6 have functional groups in their structures, hydrophilic amine and hydrophobic hydrocarbon groups, respectively. It should be pointed out that the pores of IRMOF-3 and IRMOF-3-AM6 are too large for any kinetic separations of small gas molecules tested in this study. However, one can expect solubility-based separation.35 As shown in Figure7, not surprisingly, IRMOF-3 membranes show a Knudsen-type transport of small gases such as H2, He, and N2 similar to IRMOF-1 membranes22,36 despite a small deviation in the permeances of the hydrocarbons (CH4 and C3H8) from Knudsen diffusion. This deviation can be attributed most likely to the enhanced solubility of the hydrocarbons in the thin surfactant layer covering the membrane external surface. Unlike IRMOF-1 membranes,22 however, the permeance of CO2 through IRMOF3 membranes is almost twice as high as the predicted Knudsen

Figure 8. Single gas permeation of IRMOF-3-AM6 membranes (dried in the presence of Span 80) showing Knudsen diffusion for small gases. Note that the permeances of hydrocarbons, CH 4 and C3H8, are considerably deviating from Knudsen diffusion due to their high affinity with hydrocarbon groups in IRMOF-3-AM6.

permeance. This enhanced CO2 permeance can be ascribed to the enhanced adsorption of CO2 on IRMOF-3 owing to the presence of amine groups.6 In contrast, IRMOF-3-AM6 membranes exhibit the permeance of the hydrocarbons (CH4 and C3H8) notably deviating from their Knudsen diffusion while other small gases show Knudsen type transports as shown in Figure 8. IRMOF-3-AM6 membranes are selective toward C3H8 over CO2, which is opposite to IRMOF-3 membranes. It is surmised that, in addition to the surfactant, AM6 as a hydrocarbon moiety increases the solubilities of hydrocarbons, thereby increasing its permeance. It is noted that the permeabilities of these small gases through IRMOF-3 and IRMOF-3-AM6 membranes modified with surfactants are on the same order of magnitude as those through previously reported IRMOF-1 membranes.22,36 This implies that 2656

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Langmuir the surfactant molecules adsorbed on the solid surfaces do not prohibit the transport of small gas molecules substantially.

’ CONCLUSION Here we have demonstrated that surfactant-assisted drying process can not only prevent the formation of fractures and cracks in IRMOFs and their films but also increase the stability of IRMOFs with respect to moisture. Surfactants added at the drying step reduce capillary stress by lowering interfacial tension, thereby repressing the formation of fracture and cracks. Once dried, surfactants adsorbed on the crystal surface render the MOF surface hydrophobic, thereby significantly enhancing the stability toward moisture. Using this new strategy, crack-free IRMOF-3 membranes were successfully prepared and their gas permeation performance was tested, favoring CO2 over C3H8. This is mainly due to the strong affinity of CO2 to the amine groups in the structure. When postsynthetically modified with heptanoic anhydride (AM6), however, the membranes (IRMOF-3-AM6) favor C3H8 over CO2 due to the increased solubility of C3H8. It is expected that the surfactant-assisted drying method reported here can offer new opportunities for the synthesis of fracture-free MOF crystals as well as crack-free MOF membranes with exceptional moisture stability, thereby moving forward in addressing some of the challenges to the practical applications of these exciting new materials. ’ ASSOCIATED CONTENT

bS

Supporting Information. Time-resolved XRD patterns of IRMOF-1 membranes (Figure S1), N2 adsorption isotherms and pore size distribution of IRMOF-3 and IRMOF-3-AM6 powder sample (Figure S2), TGA of IRMOF-3 and IRMOF-3AM6 samples, and time-resolved XRD patterns of IRMOF-3AM6 membranes (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]; Tel þ1 979 862 4850; Fax þ1 979 845 6446.

’ ACKNOWLEDGMENT H.-K.J. acknowledges the financial support from the National Science Foundation (CBET-0930079) and from the Artie McFerrin Department of Chemical Engineering at Texas A&M University and Texas Engineering Experiment Station through a new faculty startup. V.V.G. thanks CONACYT for the financial support. We thank BASF research center in New Jersey to supply P123 surfactant for this study. ’ REFERENCES

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