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Clathrate Formation Mechanism of Supercritical Hydrogen Adsorption on Copper(II) Benzoate Pyrazine Masashi Goto,*,† Masahiro Furukawa,† Junichi Miyamoto,‡ Hirofumi Kanoh,‡ and Katsumi Kaneko‡ Materials Research Laboratory, NGK Insulators, Ltd., 2-56 Suda-cho, Mizuho, Nagoya 467-8530, Japan, and Graduate School of Science and Technology, Chiba UniVersity, 1-33 Yayoi, Inage, Chiba 263-8522, Japan ReceiVed December 4, 2006. In Final Form: March 13, 2007 The adsorption isotherms of supercritical hydrogen on [Cu2(bz)4(pyz)]n were measured at 77 K up to 10 MPa. The amount of supercritical hydrogen adsorbed on [Cu2(bz)4(pyz)]n at 77 K was 1.4 wt % at 10 MPa. The adsorption isotherms of supercritical hydrogen on [Cu2(bz)4(pyz)]n showed a stepwise adsorption that suggests clathrate formation between [Cu2(bz)4(pyz)]n and hydrogen molecules.
Introduction Recently, organic-inorganic hybrid complexes have received much attention owing to their potential applications in catalysis, separation, and gas storage.1-12 Clathrate-formation-mediated adsorption is of great interest.13-27 Kaneko et al. found an unusual * Corresponding author. E-mail:
[email protected]. Tel: +81-52-8727732. Fax: +81-52-872-7537. † NGK Insulators, Ltd. ‡ Chiba University. (1) Li, D.; Kaneko, K. J. Phys. Chem. B 2000, 104, 8940-8945. (2) Kondo, M.; Yoshitomi, T.; Seki, K.; Matsuzaka, H.; Kitagawa, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 1725-1727. (3) Noro, S.; Kitaura, R.; Kondo, M.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Yamashita, M. Angew. Chem., Int. Ed. 2000, 39, 2082-2084. (4) Uemura, K.; Kitagawa, S.; Fukui, K.; Saito, K. J. Am. Chem. Soc. 2004, 126, 3817-28. (5) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276-279. (6) Eddaoudi, M.; Kim, J.; Posi, N.; Vodak, D.; Watchter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469-472. (7) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127-1129. (8) Tabares, L. C.; Navarro, J. A. R.; Salas, J. M. J. Am. Chem. Soc. 2001, 123, 383-387. (9) Barea, E.; Navarro, J. A. R.; Salas, J. M.; Masciocchi, N.; Galli, S.; Sironi, A. J. Am. Chem. Soc. 2004, 126, 3014-3015. (10) Pan, L.; Liu, H.; Lei, X.; Huang, X.; Olson, D. H.; Turrro, N. J.; Li, J. Angew. Chem., Int. Ed. 2003, 42, 542-546. (11) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim, K. Nature 2000, 404, 982-986. (12) Halder, G. J.; Kepert, C. J.; Moubaraki, B.; Murray, K. S.; Cashion, J. D. Science 2002, 298, 1762-1765. (13) Zhao, X.; Xiao, B.; Fletcher, A. J.; Thomas, K. M.; Bradshaw, D.; Rosseinsky, M. J. Science 2004, 306, 1012. (14) Bourrelly, S.; Llewellyn, P. L.; Serre, C.; Millange, F.; Loiseau, T.; Fe’rey, G. J. Am. Chem. Soc. 2005, 127, 13519-13521. (15) Dybtsev, D. N.; Chun, H.; Kim, K. Angew. Chem., Int. Ed. 2004, 43, 5033-5036. (16) Li, D.; Kaneko, K. Chem. Phys. Lett. 2001, 335, 50-56. (17) Onishi, S.; Ohmori, T.; Ohkubo, T.; Noguchi, H.; Di, L.; Hanzawa, Y.; Kanoh, H.; Kaneko, K. Appl. Surf. Sci. 2002, 196, 81-88. (18) Kondo, A.; Noguchi, H.; Ohnishi, S.; Kajiro, H.; Tohdoh, A.; Hattori, Y.; Xu, W.; Tanaka, H.; Kanoh, H.; Kaneko, K. Nano Lett. 2006, 6, 2581-2584. (19) Noguchi, H.; Kondoh, A.; Hattori, Y.; Kanoh, H.; Kajiro, H.; Kaneko, K J. Phys. Chem. B 2005, 109, 13851-13853. (20) Takamizawa, S.; Nakata, E.; Saito, T. Inorg. Chem. Commun. 2003, 6, 763-765. (21) Takamizawa, S.; Nakata, E.; Saito, T. Inorg. Chem. Commun. 2004, 7, 1-3. (22) Takamizawa, S.; Nakata, E.; Saito, T.; Akatsuka, T.; Kojima, K. CrystEngComm 2004, 6, 197-199. (23) Saito, T.; Nakata, E.; Akatsuka, T.; Takamizawa, S. 54th Japan Society of Coordination Chemistry Symposium, 2004, p 361. (24) Takamizawa, S.; Saito, T.; Akatsuka, T.; Nakata, E. Inorg. Chem. 2005, 44, 1421-1424. (25) Takamizawa, S.; Nakata, E. CrystEngComm 2005, 7, 476-479. (26) Seki, K. Phys. Chem. Chem. Phys. 2002, 4, 1968-1971.
vertical adsorption jump in the CO2 adsorption isotherms of latent porous crystals (LPCs), showing a remarkable expansion of the crystal lattice.16-18 This vertical adsorption jump pressure was recently associated with the clathrate formation for methane.19 Some organic-inorganic hybrid complexes were verified with X-ray crystallography to adsorb gas molecules by forming inherent clathrates with adsorbate molecules.18,20-27 Consequently, clathrate formation upon gas adsorption on organicinorganic hybrid crystals is a key factor in controlling their adsorptive property. [Cu2(bz)4(pyz)]n (bz ) benzoic acid, pyz ) pyrazine) (1) is one of the organic-inorganic hybrid complexes that adsorb gas molecules through clathrate formation.20-24 Nukada et al. first reported the synthesis and characterization of 1 in 1999.28 Takamizawa et al. have actively studied clathrate-formationmediated adsorption properties of 1 by using crystallographic methods.20-24 The crystal structure of 1 was formed by infinite coordination polymer 1-D chains where the carboxylate dimer units are linked to each other with pyrazine molecules.20 Onedimensional chains assemble through the weak intermoleculer interaction of π-π stacking to form a high-density packing structure. The 1 crystal has micropores whose cross-sectional geometry is evaluated to be 0.2 × 0.3 nm from the crystallographic data.20 The micropore size seems to be too small for the adsorption of gas molecules. However, it was found that carbon dioxide, ethanol, acetonitrile, and oxygen molecules can be adsorbed on 1 through the host framework transformation through clathrate formation.20-24 The clathrate-formation-mediated adsorption properties of 1 were shown only in the case of vapor adsorption. Because the organic-inorganic hybrid complexes are expected to be used for clean-energy storage materials, studies on the adsorption of supercritical hydrogen and methane have been requested. Also, the clathrate-formation mechanism of supercritical gas adsorption on organic-inorganic hybrid crystals was validated through thermodynamic and crystallographic evidence.19,25 It is hoped that the pore geometry of 1 is conducive to the adsorption of hydrogen molecules. Therefore, a study on supercritical hydrogen adsorption on 1 is required. In this letter, we report the stepwise adsorption isotherm of supercritical hydrogen on 1 in a high-pressure region that indicates clathrate formation of 1 with hydrogen molecules. (27) Kitaura, R.; Seki, K.; Akiyama, G.; Kitagawa, S. Angew. Chem., Int. Ed. 2003, 42, 428-431. (28) Nukada, R.; Mori, W.; Takamizawa, S.; Mikuriya, M.; Handa, M.; Naono, H. Chem. Lett. 1999, 367-368.
10.1021/la063497k CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007
Letters
Langmuir, Vol. 23, No. 10, 2007 5265
Figure 1. Powder X-ray diffraction pattern of prepared crystals of 1 and the simulated X-ray diffraction pattern from previously reported crystal data (vertical bars).20
Experimental Section 1 was synthesized by using a previously reported method.28 Copper(II) acetate hydrate, benzoic acid, and pyrazine were dissolved in a mixed acetonitrile/water solvent. After several weeks, green columnar crystals of 1‚2nCH3CN had precipitated. The precipitates were collected by filtration and dried under reduced pressure (P < 10-5 Pa) for 17 h at room temperature to remove acetonitrile molecules included as the crystal solvent. Finally, a blue-green microcrystal powder of 1 was obtained. The resulting microcrystals were examined by powder X-ray diffraction. Powder X-ray diffraction data was recorded on a Rigaku RAD-IB diffractometer at 35 kV, 20 mA for Cu KR ) 0.1543 nm. The adsorption isotherm of nitrogen at 77 K up to 100 kPa and that of supercritical hydrogen at 77 K up to 100 kPa were measured using an automatic volumetric adsorption apparatus (ASAP2020, Micromeritics). High-purity nitrogen and hydrogen gases (N2 >99.9995%, H2 >99.99999%, Taiyo Nissan) were used. The samples were outgassed under vacuum (P < 10-5 Pa) for 17 h at room temperature prior to adsorption isotherm measurements. The highpressure adsorption isotherms of supercritical hydrogen were measured at 77 K up to 10 MPa by use of a gravimetric method. The system is equipped with a magnetic suspension balance (Rubotherm) and high-pressure transducers (MKS Baratron). Highpurity hydrogen gas (99.99999%, Nippon Sanso) was also used after purification with a liquid-nitrogen trap. Pretreatment prior to adsorption measurements was performed under vacuum (P < 10-5 Pa) for 17 h at room temperature. In the gravimetric method, the buoyancy effect must be carefully corrected. To correct this buoyancy effect, it is necessary to determine the particle density. The particle density of the crystals was determined by the high-pressure helium buoyancy method at 303 K at up to 10 MPa of helium.29 SEM images of 1 microcrystals were taken before and after high-pressure supercritical hydrogen adsorption measurements with the aid of JEOL JSM-5410 operated at 15 kV.
Figure 2. Adsorption isotherms of nitrogen at 77 K on 1. The data was taken from Nukada et al. (0) and the present work (adsorption, O; desorption, b).28 The inset shows the amount of adsorbed nitrogen at 77 K on 1 against the logarithm of relative nitrogen pressure.
Stepwise Adsorption of Nitrogen at 77 K up to 100 kPa. Identification of prepared 1 was confirmed by X-ray diffraction analysis. Figure 1 compares the experimental and simulated powder X-ray diffraction patterns. The simulated X-ray diffraction pattern is represented by vertical bars in which the height is proportional to the diffraction intensity. The simulation was based on the supplemental crystal data for 1.20 Prepared 1 shows peak positions that are nearly identical to the theoretical powder diffraction pattern. The nitrogen adsorption isotherm measured at 77 K on 1 is given in Figure 2 along with the previous data of Nukada et al.28 The measured nitrogen adsorption isotherm was of type I, agreeing with the literature data. The amount of
adsorbed nitrogen on 1 is saturated near P/P0 ) 0, as shown in Figure 2. The saturated amount of adsorbed nitrogen is 96 cm3 (STP)/g, coinciding with the previous data of Nukada et al.28 Kaneko et al. showed that there are often several steps in the low-pressure region below P/P0 ) 10-2 in the case of adsorption in micropores.30 Accordingly, the nitrogen adsorption isotherm in the low-relative-pressure region (P/P0 < 10-2) was measured more precisely than in the preceding study.28 The inset of Figure 2 shows the amount of adsorbed nitrogen at 77 K on 1 against the logarithm of relative nitrogen pressure. This isotherm shows a clear stepwise adsorption isotherm in which two inflection points are observed at relative pressures of around P/P0 ) 1 × 10-6 and 2 × 10-4. The amounts of adsorbed nitrogen at two inflection points are 32 and 65 cm3(STP)/g, which correspond to one and two nitrogen molecule(s) per Cu2 unit, respectively. The saturated amount of adsorbed nitrogen, 96 cm3(STP)/g, also corresponds to three nitrogen molecules per Cu2 unit. This stepwise adsorption feature demonstrates the stepwise clathrate formation of 1 with nitrogen and the accompanying formation of clathrates of 1‚nN2, 1‚2nN2, and 1‚3nN2 with the increase in relative pressure. Takamizawa et al. previously reported clathrate formations of 1 with carbon dioxide and oxygen with crystal structures of 1‚3nCO2 and 1‚3nO2, respectively, in which three adsorbate molecules per Cu2 unit are included in a linear arrangement along the b axis.21,22 In a similar way, the above three clathrate compounds including N2 molecules are formed. Stepwise Adsorption of Hydrogen at 77 K up to 10 MPa. The low-pressure (
