Gas Adsorption and Storage in Metal−Organic Framework MOF-177

Langmuir 2007, 23, 12937-12944

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Gas Adsorption and Storage in Metal-Organic Framework MOF-177 Yingwei Li and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed August 10, 2007. In Final Form: October 1, 2007 Gas adsorption experiments have been carried out on a zinc benzenetribenzoate metal-organic framework material, MOF-177. Hydrogen adsorption on MOF-177 at 298 K and 10 MPa gives an adsorption capacity of ∼0.62 wt %, which is among the highest hydrogen storage capacities reported in porous materials at ambient temperatures. The heats of adsorption for H2 on MOF-177 were -11.3 to -5.8 kJ/mol. By adding a H2 dissociating catalyst and using our bridge building technique to build carbon bridges for hydrogen spillover, the hydrogen adsorption capacity in MOF-177 was enhanced by a factor of ∼2.5, to 1.5 wt % at 298 K and 10 MPa, and the adsorption was reversible. N2 and O2 adsorption measurements showed that O2 was adsorbed more favorably than N2 on MOF-177 with a selectivity of ∼1.8 at 1 atm and 298 K, which makes MOF-177 a promising candidate for air separation. The isotherm was linear for O2 while being concave for N2. Water vapor adsorption studies indicated that MOF-177 adsorbed up to ∼10 wt % H2O at 298 K. The framework structure of MOF-177 was not stable upon H2O adsorption, which decomposed after exposure to ambient air in 3 days. All the results suggested that MOF-177 could be a potentially promising material for gas separation and storage applications at ambient temperature (under dry conditions or with predrying).

1. Introduction The development of a safe and efficient hydrogen storage system is urgently needed for the realization of hydrogen as a clean energy carrier for automobiles.1,2 Recently, a new class of porous materials assembled with metal ions and organic linkers, known as metal-organic frameworks (MOFs), was developed as potential candidates for hydrogen storage.3-34 Because of their low densities and unusually high surface areas, these new * Corresponding author. Fax: [email protected].

(743) 764-7453. E-mail address:

(1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353-358. (2) Dillon, A. C.; Heben, M. J. Appl. Phys. A 2001, 72, 133-142. (3) Yaghi, O. M.; O’Keefee, M.; Ockwig, N. W.; Chae, H. K.; Eddaoui, M.; Kim, J. Science 2003, 423, 705-714. (4) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keefee, M.; Yaghi, O. M. Science 2003, 300, 1127-1129. (5) Snurr, R. Q.; Hupp, J. T.; Nguyen, S. T. AIChE J. 2004, 50, 1090-1095. (6) Dinca, M.; Dailly, A.; Liu, Y.; Brown, C. M.; Neumann, D. A.; Long, J. R. J. Am. Chem. Soc. 2006, 128, 16876-16883. (7) Kaye, S.; Long, J. R. J. Am. Chem. Soc. 2005, 127, 6506-6507. (8) Fe´rey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surble´, S.; Margiolaki, I. Science 2005, 309, 2040-2042. (9) Suh, M. P.; Ko, J. W.; Choi, H. J. J. Am. Chem. Soc. 2002, 124, 1097610977. (10) Frost, H.; Duren, T.; Snurr, R. Q. J. Phys. Chem. B 2006, 110, 95659570. (11) Pan, L.; Sander, M. B.; Huang, X. Y.; Li, J.; Smith, M.; Bittner, E.; Bockrath, B.; Johnson, J. K. J. Am. Chem. Soc. 2004, 126, 1308-1309. (12) Dinca, M.; Long, J. R. J. Am. Chem. Soc. 2005, 127, 9376-9377. (13) Zhao, X. B.; Xiao, B.; Fletcher, A. J.; Thomas, K. M.; Bradshaw, D.; Rosseinsky, M. J. Science 2004, 306, 1012-1015. (14) Rowsell, J. L. C.; Millward, A. R.; Park, K. S.; Yaghi, O. M. J. Am. Chem. Soc. 2004, 126, 5666-5667. (15) Dybtsev, D. N.; Chun, H.; Yoon, S. H.; Kim, D.; Kim, K. J. Am. Chem. Soc. 2004, 126, 32-33. (16) Kesanli, B.; Cui, Y.; Smith, M. R.; Bittner, E. W.; Bockrath, B. C.; Lin, W. Angew. Chem., Int. Ed. 2005, 44, 72-75. (17) Kubota, Y.; Takata, M.; Matsuda, R.; Kitaura, R.; Kitagawa, S.; Kato, K.; Sakata, M.; Kobayashi, T. C. Angew. Chem., Int. Ed. 2005, 44, 920-923. (18) Lin, X.; Jia, J. H.; Zhao, X. B.; Thomas, K. M.; Blake, A. J.; Walker, G. S.; Champness, N. R.; Hubberstey, P.; Schroder, M. Angew. Chem., Int. Ed. 2006, 45, 1-7. (19) Chen, B.; Ockwig, N. W.; Millward, A. R.; Contreras, D. S.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4745-4749. (20) Lee, J. Y.; Li, J.; Jagiello, J. J. Solid State Chem. 2005, 178, 2527-2532. (21) Lee, J. Y.; Pan, L.; Kelly, S. R.; Jagiello, J.; Emge, T. J.; Li, J. AdV. Mater. 2005, 17, 2703-2706. (22) Wong-Foy, A. G.; Matzger, A. J.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 3494-3495.

materials exhibited exceptional H2 storage capacities by mass at 77 K. For example, a microporous metal-organic framework composed of metal Mn and 1,3,5-benzenetristetrazolate (BTT3-) was reported to adsorb up to 6.9 wt % H2 at 77 K and 90 bar.6 Ferey et al. reported a high H2 uptake of 6.1 wt % at 77 K and 8 MPa on MIL-101 that was built up from benzene-1,4dicarboxylate and trimetric chromium(III) octahedral cluster.26 To date, the highest H2 adsorption capacity of 7.5 wt % was achieved at 77 K and 70 bar on MOF-177, a framework consisting of tetrahedral [Zn4O]6+ clusters linked by the tritopic linker BTB (1,3,5-benzenetribenzoate).22 However, the hydrogen adsorption in MOFs is mostly by weak van der Waals interactions with a low heat of adsorption (normally 4-7 kJ/mol).26,29,31 Therefore, cryogenic temperatures (typically 77 K) are needed to obtain high H2 storage capacities on MOFs. It has been shown that no significant amounts of hydrogen were adsorbed on the MOFs at room temperature.26,30 The structure stability of MOFs upon water adsorption is an important issue for potential applications of MOFs for gas adsorption and storage materials because H2O is very difficult to fully remove from industrial gas resources. However, this aspect has received little attention in the literature. Recently, the Panella Hirscher and Huang et al. groups both observed a different (23) Dinca, M.; Yu, A. F.; Long, J. R. J. Am. Chem. Soc. 2006, 128, 89048913. (24) Sun, D.; Ma, S.; Ke, Y.; Collins, D. J.; Zhou, H. C. J. Am. Chem. Soc. 2006, 128, 3896-3897. (25) Rowsell, J. L. C.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 1304-1315. (26) Latroche, M.; Surble, S.; Serre, C.; Mellot-Draznieks, C.; Llewellyn, P. L.; Lee, J. H.; Chang, J. S.; Jhung, S. H.; Ferey, G. Angew. Chem., Int. Ed. 2006, 45, 8227-8231. (27) Chen, B. L.; Ma, S. Q.; Zapata, F.; Lobkovsky, E. B.; Yang, J. Inorg. Chem. 2006, 45, 5718-5720. (28) Dailly, A.; Vajo, J. J.; Ahn, C. C. J. Phys. Chem. B 2006, 110, 10991101. (29) Panella, B.; Hirscher, M.; Putter, H.; Muller, U. AdV. Funct. Mater. 2006, 16, 520-524. (30) Panella, B.; Hirscher, M. AdV. Mater. 2005, 17, 538-541. (31) Surble, S.; Serre, C.; Millange, F.; Duren, T.; Latroche, M.; Ferey, G. J. Am. Chem. Soc. 2006, 128, 14889-14896. (32) Krungleviciute, V.; Lask, K.; Heroux, L.; Migone, A. D.; Lee, J. Y.; Li, J.; Skoulidas, A. Langmuir 2007, 23, 3106-3109. (33) Jiang, J. W.; Sandler, S. I. Langmuir 2006, 22, 5702-5707. (34) Seayad, A. M.; Antonelli, D. M. AdV. Mater. 2004, 16, 765-777.

10.1021/la702466d CCC: $37.00 © 2007 American Chemical Society Published on Web 11/22/2007

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X-ray diffraction pattern after exposing MOF-5 (i.e., IRMOF-1) in air for several weeks.30,35 The results indicated that the moisture in the air was adsorbed on MOF-5 that caused decomposition of the structure of MOF-5. The decomposition of the structure could also be observed on other MOFs because of their structures similar to MOF-5. Herein we report experimental data on the gas adsorption and storage in the metal-organic framework MOF-177. MOF-177 has been shown as an excellent sorbent for CO2 and H2 storage.22,36 At 77 K, MOF-177 exhibited the highest hydrogen storage capacity among the known MOF materials. However, there is still no report on the hydrogen adsorption nature on MOF-177 at room temperature. In this work, we synthesized the MOF-177 sample and examined its hydrogen adsorption isotherms at 298 K up to 100 atm. Hydrogen storage by spillover at room temperature on this sample was also examined, as recent reports have shown that the hydrogen storage capacities at room temperature in nanostructured materials including MOFs, carbon nanostructures, and zeolites could be enhanced significantly by hydrogen spillover.37-43 In addition, the adsorption isotherms of other gases, such as H2O vapor, N2, and O2, were investigated on the MOF-177 sample. The structure stability of MOF-177 upon exposure to indoor ambient air has been studied in detail in this paper. It was found that the MOF-177 crystals were unstable in indoor ambient air and decomposed gradually to form an amorphous material. N2 and O2 adsorption measurements showed that O2 was adsorbed more favorably than N2 on MOF-177 at 298 K. By bridged spillover, the hydrogen adsorption capacity in MOF-177 was enhanced by a factor of ∼2.5, to 1.5 wt % at 298 K and 10 MPa. In addition, the adsorption was reversible at room temperature. 2. Experimental Methods 2.1. Preparation of 1,3,5-Tris(4-carboxyphenyl)benzene. 1,3,5Tris(4-carboxyphenyl)benzene (H3BTB) is the organic linker for constructing the MOF-177 framework. At present, it is not commercially available. In this study, H3BTB was synthesized in our laboratory from 1,3,5-tris(4-bromophenyl)benzene.44 First, 2 g of 1,3,5-tris(4-bromophenyl)benzene (Aldrich, 97%) was added in 70 mL of dry tetrahydrofuran (THF; Aldrich, g99.9%) under an atmosphere of helium. The stirred solution was cooled in a bath containing dry ice in acetone. An 8 mL aliquot of BunLi in n-hexane (Aldrich, 1.6 M) was added dropwise under vigorous stirring and an atmosphere of He. Then, gaseous CO2 was passed into the solution to yield a white precipitate. The mixture was acidified with glacial acetic acid (Aldrich, g99.99%) at room temperature, and copious water was added to give a white precipitate. The precipitate was collected by repeated filtering, thorough washing with water, and recrystallized from glacial acetic acid. The solid was dried in an oven at 60 °C for at least 24 h. 2.2. Synthesis of MOF-177. MOF-177 was synthesized according to the reported procedures.45 The difference between the reported synthesis and ours was that we used a hydrothermal bomb (300 mL), (35) Huang, L. M.; Wang, H. T.; Chen, J. X.; Wang, Z. B.; Sun, J. Y.; Zhao, D. Y.; Yan, Y. S. Microporous Mesoporous Mater. 2003, 58, 105-114. (36) Millward, A. R.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 1799817999. (37) Li, Y. W.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 726-727. (38) Li, Y. W.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 8136-8137. (39) Li, Y. W.; Yang, F. H.; Yang, R. T. J. Phys. Chem. C 2007, 111, 34053411. (40) Lueking, A.; Yang, R. T. J. Catal. 2002, 206, 165-168. (41) Lachawiec, A. J.; Qi, G. S.; Yang, R. T. Langmuir 2005, 21, 1141811424. (42) Li, Y. W.; Yang, R. T. J. Phys. Chem. B 2006, 110, 17175-17181. (43) Li, Y. W.; Yang, R. T. J. Phys. Chem. C 2007, 111, 11086-11094. (44) Weber, E.; Hecker, M.; Keopp, E.; Orlia, W. J. Chem. Soc., Perkin Trans. 1988, 1251-1257. (45) Chae, H. K.; Siberio-Perez, D. Y.; Kim, J.; Go, Y.; Eddaoudi, M.; Matzger, A. J.; O’keeffe, M.; Yaghi, O. M. Science 2004, 427, 523-527.

Li and Yang while a Pyrex tube was used by the former. And because the volume of the bomb we used (300 mL) was much larger than that in the reported synthesis; 40× more starting materials were used in this study. In a typical synthesis, the prepared H3BTB (0.2 g, 0.46 mmol) and Zn(NO3)2‚6H2O (0.8 g, 2.68 mmol; Aldrich, 99.999%) were dissolved in 80 mL of N,N-diethylformamide (DEF; Acros, 99%). The mixture was introduced in a 300 mL hydrothermal bomb that was heated to 100 °C at a heating rate of 2 °C/min. The temperature was held at 100 °C for 23 h and then cooled to 20 °C at a rate of 0.2 °C/min. The solution was aged in the bomb for almost 30 days, yielding block-shaped large crystals. The product was isolated by filtration and washed with DEF and chlorobenzene and then immersed in chlorobenzene (35 mL) for 2 h. Then the solvent was decanted and replenished. The crystals were immersed in the solvent for another 24 h, filtered, and washed again with chlorobenzene. The sample was dried in a desiccator overnight and then degassed at room temperature. The solvent was fully removed by degassing in vacuum (∼10-4 Torr) at 150 °C for 8 h, yielding the porous material. 2.3. Preparation of Bridged Samples. An effective bridge building technique has been explored and described in detail in our previous publications.38,41,42 A catalyst containing 5 wt % platinum supported on active carbon (Pt/AC, Strem Chemicals) was used as the source for hydrogen dissociation. Active carbon can be considered as the primary receptor for hydrogen atoms. Here MOF-177 was the secondary spillover receptor. Carbon bridges between the source and receptor were formed by carbonization of sucrose that was previously introduced into a physical mixture of the two components. The receptor/precursor/source ratio was fixed at 8:1:1 on the basis of the complete carbonization of the precursor (into carbon). The ternary mixture was ground together for 1 h and then subjected to the heating treatment procedures as described in the previous paper for preparing the bridged IRMOF-8 sample.38 The formation of carbon bridges by using the bridge building technique has been confirmed by high-resolution transmission electron microscopy (HRTEM).42 2.4. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Miniflex diffractometer operating at 30 kV, 15 mA for Cu KR (λ ) 0.1543 nm) radiation, with a scan speed of 2°/min and a step size of 0.02° in 2θ. Infrared spectra (IR) were recorded on a Nicolet Impact 400 FT-IR spectrometer with a TGS detector. Scanning electron microscopy (SEM) images were obtained on a Philips XL 30 FEG SEM instrument equipped with UTW SiLi solid-state X-ray detector (XEDS) using a 15 kV accelerating voltage. Gravimetric analysis was performed on a TGA-50 thermogravimetric analyzer (Shimadzu). Brunauer-Emmett-Teller (BET) surface areas and pore volumes were measured on a Micromeritics ASAP 2020 sorptometer using nitrogen adsorption at 77 K. 2.5. Hydrogen Isotherm Measurements. Low-pressure H2, N2, or O2 adsorption isotherms at 77 or 298 K were measured with a standard static volumetric technique (Micromeritics ASAP 2020). Approximately 200 mg of sample was used for each measurement. Samples were degassed in vacuum (∼10-4 Torr) at 150 °C for at least 12 h prior to measurements to remove any residual guest molecules in order to obtain the highest gas adsorption capacity. Hydrogen adsorption at 298 K and pressures greater than 0.1 MPa and up to 10 MPa was measured using a static volumetric technique with a specially designed Sievert’s apparatus. In the apparatus, valves with high-pressure bellows seals were employed. Pressures at various points were monitored and automatically recorded with pressure transducers, which were later used for calculations on hydrogen uptake by our computer code. More details were given earlier.41 The apparatus was previously tested to prove to be leakfree and proven for accuracy through calibration by using LaNi5, AX-21, zeolites, and IRMOFs at 298 K. All isotherms matched the known values. Typically, approximately 200 mg of sample was used for each high-pressure isotherm measurement. Prior to measurements, the samples were degassed in vacuum (∼10-2 Torr) at 150 °C for at least 12 h.

Gas Adsorption and Storage in MOF-177

Figure 1. X-ray diffraction pattern of the MOF-177 (as synthesized).

Figure 2. Infrared spectra of MOF-177 (as synthesized), the synthesized linker H3BTB (1,3,5-tris(4-carboxyphenyl)benzene), and the (purchased) raw material (1,3,5-tris(4-bromophenyl)benzene).

3. Results and Discussion 3.1. Characterization Results. Powder X-ray diffraction pattern for the synthesized MOF-177 sample (washed with DEF and then dried in a desiccator) is shown in Figure 1. The extremely high intensities of the diffraction peaks indicated the good crystallinity of the MOF-177 crystals. The strongest peak at 2θ ) 5.2 and the peak at 2θ ) 10.8 matched well with the already published XRD pattern on the wet as-synthesized MOF-177 material (sample I).45 The small new peak at 2θ around 7.0 in Figure 1 in comparison with the reported XRD pattern on sample I can be attributed to the extent of the removal of guest molecules in the frameworks of the synthesized sample, because it has been reported by Chae et al.45 that some new peaks (including the new peak at 2θ ) 7.0 in Figure 1) could be observed upon sample drying. The sample prepared by Chae et al. was dried in air for only 1 min, while our sample was dried in a desiccator overnight. The difference in the drying time for the wet samples could result in various contents of guest molecules in the frameworks. Figure 2 shows the IR spectrum of the as-synthesized MOF177 sample. For comparison, the IR spectra of the commercial raw material 1,3,5-tris-(4-bromophenyl)benzene and the prepared H3BTB linker are also presented in Figure 2. The observation of the strong vibrational bands around 1710 and 1300 cm-1 and the strong broad bands between 2500 and 3300 cm-1 confirmed the presence of carboxyl groups in the synthesized H3BTB from

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1,3,5-tris-(4-bromophenyl)benzene. The presence of the bands characteristic of the framework -(O-C-O)- groups around 1550 and 1416 cm-1 indicated the presence of the dicarboxylate within the MOF-177 sample.8 The strong peak at 1660 cm-1 was the characteristic peak for DEF (CdO) solvent that was not totally removed after drying in a desiccator.45 The N2 adsorption isotherm at 77 K was measured on the MOF-177 sample in which the pores were fully evacuated. As seen in Figure 3, the N2 adsorption on the sample showed a reversible type I isotherm characteristic of a microporous material. No hysteresis was observed upon desorption of gas from the pores, indicating the stability of the pores. The BET surface area was about 3100 m2/g. By assuming a monolayer coverage of N2 and applying the Langmuir model, the Langmuir surface area could be calculated. The reported Langmuir surface areas on MOF-177 by Yaghi et al. varied quite substantially. They reported a value of ∼4500 m2/g in their two earlier papers with a much higher one of 5640 m2/g in a later paper.22,36,45 But the three MOF-177 samples were prepared by the same method under the same conditions. In this work, the Langmuir surface area of the synthesized MOF-177 sample was ∼4300 m2/g, which was very close to the value reported in Yaghi’s earlier papers.36,45 We synthesized the MOF-177 samples several times using the same procedures as Yaghi, yet each time we obtained nearly the same result on the Langmuir surface area. None of the samples we measured showed the high Langmuir surface area of 5,640 m2/g reported by Yaghi et al.22 The pore volume and the pore diameter of the MOF-177 sample was 1.58 cm3/g, and 10.6 Å, respectively. 3.2. H2O Adsorption Studies. The studies on the H2O adsorption/desorption on the materials for future hydrogen storage applications are very important because the adsorption of H2O on the material will decrease the hydrogen capacity when refilling. Moreover, the structures of some materials such as MOFs could be destroyed upon adsorption of water. Unfortunately, the reported promising porous materials for hydrogen adsorption all adsorb significant amounts of H2O. For example, zeolites can adsorb ∼25 wt % of H2O, while a superactivated carbon (AX-21) can adsorb up to 65 wt % of moistures at room temperature. In addition, the adsorbed H2O cannot be easily desorbed from the materials at room temperature. Thus, the hydrogen uptakes of the materials will decrease gradually with refilling times. The materials can be regenerated by heating to release the adsorbed H2O. However, if the material is not stable upon the adsorption of H2O, the original highest hydrogen uptakes cannot be regenerated. In this case, additional special procedures to remove any trace of H2O existing in the hydrogen sources would be needed to meet the U.S. Department of Energy (DOE) durability criteria for hydrogen storage applications for transportation.46 H2O vapor adsorption isotherm at 298 K on the MOF-177 sample after the removal of guest molecules is presented in Figure 4. It was observed that MOF-177 adsorbed up to ∼10 wt % H2O at 298 K at P/P0 ) 0.1 (P0 is the saturation pressure of H2O at 298 K). The H2O uptake in the MOF-177 sample was relatively low with such a high surface area and pore volume. Previous thermogravimetric analysis (TGA) revealed that a metal-organic framework MIL-101 with similar surface area and pore volume as MOF-177 could adsorb up to 40 wt % H2O.26 The much lower H2O adsorption capacity in MOF-177 than MIL-101 can be attributed to the difference in the number of hydrophobic benzene rings in the frameworks. There are four benzene rings in the linker for MOF-177, while only one for MIL-101. The adsorption (46) U.S. Department of Energy, Energy Efficiency and Renewable Energy (EERE), Hydrogen, Fuel cells & Infrastructure Technologies Program, MultiYear RD&D Plan, 2005. http://www.eere.energy.gov/hydrogenandfuelcells/mypp/ pdfs/storage.pdf (accessed June 2005).

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Li and Yang

Figure 3. N2 isotherm of MOF-177 at 77 K: circles, fresh sample; triangles, after exposure to ambient air (RH ∼ 40%) at 298 K for 3 days; open symbols, adsorption; filled symbols, desorption.

Figure 4. H2O isotherm at 298 K for MOF-177: O, adsorption; 2, desorption. P/P0 is the ratio of H2O vapor pressure (P) to saturation pressure of H2O at 298 K (P0).

rate of H2O at P/P0 ) 0.1 and 298 K was studied on a fresh MOF-177 sample and was shown in Figure 5. It can be seen that ∼90% of the saturation point (∼9 wt %) was reached in 4 h. From the kinetics data in Figure 5, the diffusion time constant (D/R2) can be approximated by using the following equation when Mt/M∞ < 0.3:

()

Mt 4 Dt ) M∞ xπ R2

1/2

(1)

where D is diffusivity, R is diffusion distance or radius, and Mt and M∞ denote the total amounts of H2O at time t and at equilibrium, respectively.47 The value of D/R2 was calculated as 5.5 × 10-6 s-1. Thus, a saturation adsorption of H2O would take about 50 h at P/P0 ) 0.1 and 298 K. The slow uptake rates can (47) Crank, J. The Mathematics of Diffusion; Clarendon: Oxford, U.K., 1979; pp 89-92.

Figure 5. H2O vapor adsorption rates on the MOF-177 sample at 298 K and P/P0 ) 0.1.

be attributed to the large crystal sizes of the MOF-177 sample. The crystal size reported by Yaghi et al. was about 0.3 mm.45 The crystal sizes of the synthesized MOF-177 sample in this study were even larger. Figure 6A shows the SEM image of the MOF-177 sample after the removal of guest molecules. It is seen that the crystal sizes of the dried sample were in the range of 0.5-1.0 mm. The desorption rates were much faster than adsorption; however, ∼6.5 wt % H2O did not desorb from the MOF-177 sample at 298 K and in an atmosphere of helium, as can be seen in Figure 5. The results indicated that the adsorption of H2O on MOF-177 was not reversible at 298 K. To study the stability of MOF-177 upon H2O adsorption, the MOF-177 sample after evacuating the guest molecules was exposed to indoor ambient air at room temperature, at a nominal relative humidity of 40% (air-conditioned). The XRD patterns of the samples after exposing to air for 1 and 3 days are compared in Figure 7 with the as-synthesized MOF-177 sample. The XRD pattern of the sample changed significantly after exposure to air for 1 day. The intensity of the peak at 2θ ) 5.2° decreased

Gas Adsorption and Storage in MOF-177

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Figure 6. SEM images of the fresh as-prepared MOF-177 after the removal of guest molecules (A, B), and the MOF-177 sample after exposure to ambient air (RH ∼ 40%) at 298 K for 3 days (C, D).

Figure 7. XRD patterns the fresh MOF-177 (as synthesized), MOF177 after removing guests and exposing to ambient air (RH ∼ 40%) at 298 K for 1 and 3 days.

substantially with several new peaks appearing at 2θ from 5 to 15°. This revealed that the crystals of the MOF-177 sample were decomposing, but some crystallinity still remained. It has been reported that some new peaks at 5-15° could also be observed when the MOF-177 was heated to 325 °C, which was very close to the decomposing temperature of MOF-177 (350 °C).45 After 3 days, the sample mainly existed as an amorphous phase, as shown in Figure 7. The results indicated that the framework structure of MOF-177 was degraded by H2O after exposure to indoor ambient air for 3 days. The SEM images of the MOF-177 sample in air for 3 days are also shown in Figure 6. It can be seen that the particle sizes of the decomposed sample were much smaller than that of the fresh sample. This indicated the breakup of the frameworks in the MOF-177 sample. The destruction of the frameworks can be

further verified by observing the samples at a higher resolution. As shown in Figure 6B, the fresh synthesized MOF-177 sample showed a latticelike morphology. However, the sample after exposure to air for 3 days exhibited a strawlike morphology. The framework structures observed on the freshly synthesized sample vanished. The destruction of the frameworks resulted in a significant decrease in the gas adsorption capacities. As shown in Figure 3, the N2 adsorption amounts at 77 K were much lower than that on the freshly synthesized sample. The BET surface area and pore volume for the sample after exposure to air for 3 days were 30 m2/g and 0.1 cm3/g, respectively. The results indicated that the MOF-177 sample could be decomposed significantly upon exposure to air for a short period of time. Greathouse and Allendorf reported the simulation results on the interaction of water with MOF-5 by molecular dynamics simulation.48 It was suggested that the relatively weak bonds between Zn ions and O atoms in MOF-5 were easily attached by water molecules. A possible decomposition mechanism might be

Zn4O(BTB)2 + 4H2O f [(Zn4O)(H2O)4(BTB)]3+ + BTB3(2) Another possible mechanism for decomposition was that proposed by Huang et al.,35 where the MOF undergoes hydrolysis (i.e., the reverse reaction of the synthesis) to form Zn ions and acid (H3BTB, in this case). They proposed that this reverse reaction would proceed under acid conditions. It can be calculated that 10.1 wt % H2O will fully decompose the structure of MOF-177. The humidity level in indoor air (with air conditioning) is normally 30-40% (i.e., P/P0 ) 0.3-0.4). The H2O uptake rates in air would be much faster than that at P/P0 ) 0.1 shown in Figure 4. Therefore, during the synthesis and gas storage applications, (48) Greathouse, J. A.; Allendorf, M. D. J. Am. Chem. Soc. 2006, 128, 1067810679.

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Figure 8. N2, O2 adsorption isotherms on MOF-177 at 298 K and pressures up to 1 atm.

exposing the MOF-177 sample to humid air should be avoided to obtain the highest storage capacities. For application to hydrogen storage, an additional drying would be required before hydrogen is refilled to the storage system containing MOFs with structures similar to that of MOF-177 as the storage materials. 3.3. N2, and O2 Adsorption Studies. Air separation is of importance for producing commercial N2 or O2 with high purities. Commercial sorption-based air separation is usually done using nitrogen selective zeolites in pressure swing adsorption (PSA) systems.49-51 But separation of air by adsorption of the less abundant oxygen is more desirable because less work would be done to accomplish the same separation. The low-pressure N2, and O2 adsorption isotherms at 298 K on the MOF-177 sample are shown in Figure 8. It can be seen that O2 was adsorbed more favorably than N2 on the MOF-177 sample with a pure-component selectivity of ∼1.8 at 1 atm. The equilibrium capacities for O2 and N2 were 0.18 and 0.10 mmol/g at 1 atm, respectively. The equilibrium adsorption capacity for O2 at 1 atm is comparable to that obtained on AgBr/SiO2 (0.243 mmol/g) with a high selectivity of 2.87 for O2 at 295 K.49 Although the selectivity for O2 over N2 was not very high at low pressure, it is interesting to note that the adsorption isotherm was linear for O2 while being strongly concave for N2, as shown in Figure 8. This behavior is unique among all known sorbents for air separation.50,51 It can be expected that higher selectivities could be obtained at higher pressures on the MOF-177 sample. Thus MOF-177 could be a promising sorbent for high-pressure air separation, particularly by PSA.50,51 A similar linear increase with pressure was also found on the MOF-177 material for CO2 storage.36 The O2 isotherm on MOF-177 is comparable to that on activated carbon and zeolites.50,51 The O2/N2 selectivity is caused by the higher magnetic susceptibility of O2 over that of N2. The low N2 isotherm indicates a lack of electric charges on the surfaces of MOF-177; otherwise there would be strong interactions between the quadrupole moment of N2 and the charges. The strongly concave shape of the N2 isotherm on MOF-177 would indicate early saturation. However, some reports have shown that MOFs could exhibit unusual isotherm shapes. For example, Bourrelly et al. observed a step increase with pressure in the adsorption (49) Jayaraman, A.; Yang, R. T. Chem. Eng. Sci. 2005, 60, 625-634. (50) Yang, R. T. Gas Separation by Adsorption Processes; Butterworth: London, 1987; Chapter 7. (51) Yang, R. T. Adsorbents: Fundamentals and Applications; Wiley: New York, 2003; Chapter 10.

Li and Yang

Figure 9. Hydrogen adsorption isotherm for MOF-177 at 77 K. Filled circles indicate the desorption branch.

isotherm of CO2 on MIL-53.52 And several mechanisms, such as molecular gate, structural disorganization/organization, and breathing type mechanism, for the steps in adsorption isotherms on MOFs have been proposed.13,52,53 Therefore, additional highpressure N2 adsorption experiments on MOF-177 would be needed to predict the saturation adsorption amount of N2 in MOF-177. 3.4. H2 Adsorption Studies. The hydrogen adsorption isotherm of the MOF-177 sample at 77 K and at pressures up to 1 atm is shown in Figure 9. The adsorption of H2 on MOF-177 was reversible, and no significant hysteresis was observed upon desorption. The highest H2 uptake at 1 atm and 77 K was ∼1.5 wt % on the MOF-177 sample synthesized in this study. Yaghi et al. reported a hydrogen capacity of 1.25 wt % at 77 K and 1 atm on a MOF-177 sample with a Langmuir surface area of ∼4500 m2/g.14 However, in a later report a lower hydrogen adsorption capacity of ∼1 wt % at 0.9 bar was reported on a MOF-177 sample with a higher Langmuir surface area of ∼5600 m2/g.22 The H2 uptakes reported on MOF-177 were relatively low for a material with the highest surface area because similar amounts of hydrogen can be adsorbed on other MOFs with much lower surface areas. For example, a 1.5 wt % H2 uptake can be obtained on IRMOF-8 with a Langmuir surface area of ∼1500 m2/g.14 It is interesting to note that the isotherm on MOF-177 was nearly linear at pressures > 100 Torr, as can be seen from Figure 9. Therefore, a high H2 capacity can be obtained on MOF177 at high pressures. At 70 bar and 77 K, the H2 uptake on the MOF-177 sample was measured as 7.5 wt %, which is the highest hydrogen storage capacity reported at 77 K up to date.22 The interaction of H2 with the adsorption material is very important to achieve a high capacity for H2. To date, there is no report on measuring the heats of adsorption of H2 on MOF-177 to the best of our knowledge. In this study, the heats of adsorption on MOF-177 were calculated using the Clausius-Clapeyron equation from the H2 adsorption isotherms at three different temperatures, as shown in Figure 10. The isosteric heats of adsorption were determined by evaluating the slopes of the plot of ln(P) vs 1/T at the same adsorption amounts. Such plots are given in Figure 11. The heats of adsorption were calculated as -11.3 kJ/mol (at 0.32 cm3/g), -10.1 kJ/mol (at 0.49 cm3/g), -7.5 kJ/mol (at 0.90 cm3/g), and -5.8 kJ/mol (at 1.50 cm3/g). (52) Bourrelly, S.; Llewellyn, P. L.; Serre, C.; Millange, F.; Loiseau, T.; Ferey, G. J. Am. Chem. Soc. 2005, 127, 13519-13521. (53) Sudik, A. C.; Millward, A. R.; Ockwig, N. W.; Cote, A. P.; Kim, J.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 7110-7111.

Gas Adsorption and Storage in MOF-177

Figure 10. Low-pressure H2 equilibrium adsorption isotherms on MOF-177 at 273 (9), 298 (]), and 323 K (4).

Figure 11. Plots of ln(P) vs T-1 at different H2 adsorption amounts at low pressures (