NANO LETTERS
Novel Expansion/Shrinkage Modulation of 2D Layered MOF Triggered by Clathrate Formation with CO2 Molecules
2006 Vol. 6, No. 11 2581-2584
Atsushi Kondo,† Hiroshi Noguchi,† Shunsuke Ohnishi,† Hiroshi Kajiro,*,§ Aya Tohdoh,‡ Yoshiyuki Hattori,† Wei-Chun Xu,‡ Hideki Tanaka,† Hirofumi Kanoh,*,† and Katsumi Kaneko† Graduate School of Science and Technology, Chiba UniVersity, Yayoi, Inage, Chiba 263-8522, Nippon Steel Corporation, Shintomi, Futtsu, Chiba 293-8511, and Institute of Research and InnoVation, Takada, Kashiwa, Chiba 277-0861 Received August 29, 2006; Revised Manuscript Received October 2, 2006
ABSTRACT Crystal-to-crystal transformation from a 3D interpenetrated-type MOF {[Cu(BF4)2(bpy)(H2O)2] (bpy)} (1) to a 2D square-grid-type [Cu(BF4)2(bpy)2] (2) (bpy ) 4,4′-bipyridine) was observed. It was derived from dehydration and confirmed by in situ FT-IR, TG, and elemental analysis. Moreover, we elucidate the novel expansion/shrinkage dynamic modulation of 2 triggered by clathrate formation with gas molecules.
Porous metal-organic frameworks (MOFs) have attracted much attention for the potential application to gas storage, gas separation, catalysis, and nanospace engineering.1 Incorporation of weak interactions such as hydrogen bonding and π-π interaction in the architecture of porous materials induces flexibility in MOFs.2 The response of these materials to external stimuli results in structural transformation and, therefore, this unique function has the robust possibility of realizing new functions such as switching, sensing, and information transduction, which can hardly be realized by traditional “hard” coordination polymers.3 Zhao et al. reported hydrogen adsorption isotherms of long tail hysteresis on a flexible MOF.4 The hysteretic gas adsorption/desorption behavior was able to decrease the pressure of gas storage. An S-shaped adsorption hysteresis, expected by the structural change caused by the sliding motion of π-π interaction, was also reported by Kitaura et al.5 Biradha et al. showed that the sliding motion of a square grid 2D layered structure was caused by an exchange between aromatic compounds in the liquid phase, which results from their interaction with a long aromatic ligand.6 Matsuda et al. emphasized that MOF can respond sensitively to external stimuli with three types of structural change.7 * Corresponding authors. Dr. H. Kanoh Tel: (+81) 43-290-2770; fax: (+81) 43-290-2788; e-mail:
[email protected]. Dr. H. Kajiro Tel: (+81) 439-80-2709; fax: (+81) 439-80-2746; e-mail: hkajiro@ re.nsc.co.jp. † Graduate School of Science and Technology, Chiba University. ‡ Institute of Research and Innovation. § Nippon Steel Corporation. 10.1021/nl062032b CCC: $33.50 Published on Web 10/25/2006
© 2006 American Chemical Society
Thus, there should be great potential for novel structural changes in MOF systems for gas adsorption. We found that a Cu complex has a unique adsorption isotherm of carbon dioxide at 273 K, which has the vertical adsorption uptake accompanied by a predominantly rectangular-shaped hysteresis (Figure 1), which cannot be categorized by IUPAC classification,8 and named “gate adsorption.”9 Such a vertical adsorption jump is improbable if the adsorbent is a traditional hard nanoporous material. Although we attributed this phenomenon to the participation of weak interactions such as hydrogen bonding and π-π interaction, the details have not yet been clarified. In this paper, we report dehydration causing crystal-to-crystal transformation from a 3D interpenetrated-type MOF {[Cu(bpy)(H2O)2(BF4)2] (bpy)} (1) to a 2D square-grid-type [Cu(BF4)2(bpy)2] (2) (bpy ) 4,4′-bipyridine) and, moreover, elucidate the novel expansion/shrinkage dynamic modulation of 2 triggered by clathrate formation with gas molecules. Compound 1 was synthesized by the method described in the literature.10 The resulting crystals were examined by X-ray powder diffraction (XRPD) with an angle-dispersive diffractometer and monochromated Cu KR radiation at 40 kV. The dehydration of 1 by vacuum heating treatment (P < 10-1 Pa, for 2 h at 373 K) was confirmed by in situ Fourier transform infrared (FT-IR), thermogravimetric (TG) analysis, and elemental analysis. Structure modeling of 2 was performed on the basis of the results of extended X-ray absorption fine structure (EXAFS) analysis and X-ray diffraction pattern fitting. A synchrotron powder pattern was
Figure 1. Schematic representation of the gate adsorption phenomenon of CO2 associated with structural transformation and clathrate formation (a) and the high successive reproducibility over four cycles (b). Table 1. Results of Elemental Analysis of 2 element
Cu
B
F
C
N
H
experimental (%) 11.3 4.0 28.1 45.6 10.6 3.1 stoichiometric proportion (%) 11.6 3.9 27.7 43.7 10.2 2.9
measured at 273 K for structural determination of 2 adsorbing CO2 gas (becoming 2a), and the radiation wavelength is 0.1002 nm. The structural analysis of 2a was performed by a direct method using software EXPO2004.11 The observation of apparent volume change of 2 was measured at 273 K with CO2 gas adsorption. TG analysis of 1 showed the release of lattice water molecules with increasing temperature up to 420 K. The weight loss was 6.01% (calcd 5.98%). No further weight loss was observed between 420 and 450 K, giving rise to the stable phase. In situ FT-IR spectra also showed dehydration by vacuum heating treatment. Although an FT-IR spectrum of 1 showed an O-H stretching band at the range from 3600 to 3300 cm-1, that of 2 did not have a band in the region (see the Supporting Information). The result of elemental analysis of 2 was shown in Table 1. The experimental weight percentages of all elements agree with the stoichiometric proportion. From these results, 1 easily releases water molecules and becomes 2. The built model of 2 consists of the quasi-square grid 2D layered stacking structure. The stacking mode was checked by comparison between experimental and simulated XRPD patterns (see the Supporting Information). The square grid framework has dimensions of 1.13 × 1.15 nm2, and the distance between neighboring layers is 0.46 nm. Twodimensional layers stack upon each other with a shift of a half period to block the interlayer voids, thus leaving no open pores for CO2 gas adsorption. Compound 2 shows the characteristic gate adsorption/desorption behavior. 2582
Figure 2. Fundamental framework structure of 2a (a), 2D sheet structure (b), stacking structure (c), and side view of 2D sheets (d) (orange, Cu; gray, C; blue, N; pink, B; yellow green, F; white, H).
The crystal structure of 2a belongs to the monoclinic space group A12/n1 (no. 15) a ) 18.731, b ) 11.072, c ) 13.701 Å, β ) 95.583°, V ) 2828 Å3, Z ) 4, M ) 549.5, and F ) 1.290 g/cm3 (Figure 2) (Rp ) 7.536, Rwp ) 13.447). A Nano Lett., Vol. 6, No. 11, 2006
Figure 3. Change of volume of compound 2 before CO2 gas adsorption (a) and after CO2 gas adsorption at 273K (b, 6.66 kPa; c, 13.3 kPa; d, 26.7 kPa; e, 34.7 kPa; f, 45.3 kPa; g, 101 kPa).
fundamental framework structure formed with the Cu(II) ion and bpy is composed of a 2D square grid sheet of 1.07 × 1.08 nm2, and BF4 anions are located on axial positions. The effective pore size is 0.77 × 0.77 nm2, and this size is the same as that of 2; the interlayer distance between the neighboring sheets is 0.68 nm. A noteworthy point is interlayer distance. The distance changes from 0.46 nm (2) to 0.68 nm (2a) with CO2 gas adsorption, indicating an expansion of the interlayer structure. This change corresponds to a 49% increase of the interlayer distance.12 The expansion of the 2D sheet structure concomitantly induces the formation of many open pores, and thus CO2 molecules (minimum dimension: 0.28 nm)13 can accommodate in the micropores of 2a. In recent years, Kitagawa et al.5 and Fujita et al.6 have reported dynamic structural change of MOFs that induce the formation of straight channels into which guest molecules are incorporated. Compound 2 changes its structure with the large expansion of the interlayer structure, which weakens the interaction between 2D sheets. However, the structural transformation to a more stable compound is most likely produced because of the strong guest-host interaction caused by clathrate formation. In porous carbon and zeolite materials, it is well known that an interaction between the pore wall and a molecule is enhanced by the overlapping of interaction potentials from different walls.14 Molecules adsorbed in nanopores thereby form a specific intermolecular structure enabling greater accommodation.15 Thus, molecules are assumed to interact strongly with nanopores of coordination polymers to vary the flexible framework. It has been reported recently, with thermodynamic evidence, that compound 2 indicates a clathrate formation-mediated adsorption of methane molecules.16 The expansion/shrinkage motion of compound 2 is presumed to come from guest-MOF interaction to induce clathrate formation. We observed the volume change in 2 accompanying the CO2 gas adsorption. Volume change occurs slightly below the gate pressure (34.7 kPa) as shown in Figure 3, whereas an abrupt volume increase is seen at this pressure; The incremental volume change is 6.6% at 101 kPa. This apparent volume change corresponds exactly to CO2 gas adsorption (see the Supporting Information). In summary, we have found a dynamic structural change triggered by gas adsorption, a novel expansion/shrinkage reversible modulation of a 2D layered stacking compound. This structural change of its flexible framework caused by Nano Lett., Vol. 6, No. 11, 2006
the interaction between the pore wall and gas molecules is triggered by the clathrate formation and is responsible for the novel adsorption behavior. The gate adsorption isotherm on 2, which has a predominantly rectangular shaped hysteresis, is quite unique, although a shape of adsorption isotherms on microporous materials is generally type I not only in carbon materials but also in MOFs. Acknowledgment. The EXAFS measurements using synchrotron radiation were performed under the approval of the Photon Factory Program Advisory Committee (Proposal no. 2004G094) of the Institute of Materials Structure Science, KEK. We acknowledge Drs. K. Kato and K. Osaka (JASRI) for their support for the following experiment. The synchrotron radiation experiments were performed at SPring-8 with the approval of Japan Synchrotron Radiation Research Institute (JASRI) as Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science and Technology (Proposal No. 2006A1659/BL02B2). We also thank Dr. T. Ikeda of AIST for the fruitful discussion about crystallographic analysis. This work was supported by the Grantin-Aid for Scientific Research (Chemistry of Coordination Space) (No. 18033008) by the Japan Society for the Promotion of Science. A.K. is partially supported by the 21 COE program: Frontiers of Super-Functionality of Organic Devices. Supporting Information Available: Spectral analysis of dehydration, structural information, crystallographic information file, and experimental detail of apparent volume change. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) (a) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334-2375. (b) Kitaura, R.; Kitagawa, S.; Kubota, Y.; Kobayashi, T. C.; Kindo, K.; Mita, Y.; Matsuo, A.; Kobayashi, M.; Chang, H.C.; Ozawa, T. C.; Suzuki, M.; Sakata, M.; Takata, M. Science 2002, 298, 2358-2361. (c) Janiak, C. Dalton Trans. 2003, 2781-2804. (d) Carlucci, L.; Cozzi, N.; Ciani, G.; Moret, M.; Proserpio, D. M.; Rizzato, S. Chem. Commun. 2002, 1354-1355. (e) Pan, L.; Liu, H.; Lei, X.; Huang, X.; Olson, D. H.; Turrro, N. J.; Li, J. Angew. Chem., Int. Ed. 2003, 42, 542-546. (f) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim, K. Nature 2000, 404, 982-986. (g) Evans, O. R.; Ngo, H. L.; Lin, W. J. Am. Chem. Soc. 2001, 123, 10395-10396. (h) Purse, B. W.; Gissot, A.; Rebek, J., Jr. J. Am. Chem. Soc. 2005, 127, 11222-11223. (i) Yoshizawa, M.; Ono, K.; Kumazawa, K.; Kato, T.; Fujita, M. J. Am. Chem. Soc. 2005, 127, 10800-10801. (j) Stone, M. T.; Moore, J. S. J. Am. Chem. Soc. 2005, 127, 5928-5935. 2583
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NL062032B
Nano Lett., Vol. 6, No. 11, 2006
