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Hydrogen bond-regulated microporous nature of copper complex-assembled microcrystals
16 February 2001
Chemical Physics Letters 335 (2001) 50±56
www.elsevier.nl/locate/cplett
Hydrogen bond-regulated microporous nature of copper complex-assembled microcrystals Di Li a, Katsumi Kaneko b,* a b
New Energy and Industrial Technology Development Organization (NEDO), 3-1-1 Higashi Ikebukuro, Toshima-ku, Tokyo 170-6028, Japan Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-Shi 263-8522, Japan Received 20 June 2000; in ®nal form 2 November 2000
Abstract A Cu complex-assembled compound Cu bpy BF4 2 H2 O2 bpyn (bpy 4,40 -bipyridine) with non-interpenetrated structure was synthesized and its adsorption properties for N2 , Ar, and CO2 were studied. The adsorption of gas on this complex suddenly begins at a de®nite relative pressure (`gate pressure') regardless of almost nil adsorption below the gate pressure. Such a unique adsorption phenomenon is attributed to the hydrogen bond-regulated microporous nature of this complex; hydrogen bonds in pores block adsorption below the gate pressure. Additionally, the gate pressure depends on both the adsorbate and the pretreatment temperature of the Cu complex solid. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Adsorption is of great technological importance. This is not only because various conventional and novel adsorbents such as carbon materials and zeolites have been widely used on a large scale in industries, but also because the adsorption techniques are helpful to characterize the surface properties and pore structures of solids [1]. Recently, substantial advances in new porous materials have been made toward supramolecular architecture [2,3]. A typical example is the successful construction of some metal complex frameworks with two- or three-dimensional microporosity [4±10]. The coming of `open season for *
Corresponding author. Fax: +81-43-290-2788. E-mail address: [email protected] (K. Kaneko).
solid framework' [11] in this ®eld should contribute to the solutions of the next two problems. One is the inhibition of the interpenetrating phenomenon, which can aect the formation of pores within the assembled structure, and another one is the maintenance of microporous structure after the removal of the included guest molecules [12,13]. Newly designed micropores within these metal complex frameworks possess a variety of sizes and shapes which are yet unobserved in conventional porous solids such as zeolites and activated carbons [12]. Therefore, microporous metal-organic frameworks provide molecular-scale voids that may be expected to impact many technologies such as adsorption and separations [14,15], shape-selective catalysis [16], and microelectronics [17,18], in addition to the fundamental science. So far, a lot of metal-organic frameworks have been synthesized successfully and XRD data
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 4 1 9 - 6
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
also reveal the presence of micropores [19±21]. However, their microporous structures were not fully con®rmed by the adsorption techniques, leaving a vital question regarding the nature of porosity in this class of materials [22]. In this work, a Cu complex-assembled compound Cu bpy BF4 2 H2 O2 bpyn (bpy 4,40 bipyridine) with non-interpenetrated structure was synthesized and its adsorption properties for N2 , Ar, and CO2 were studied. Consequently, this complex shows a unique adsorption behavior which has not been found for conventional adsorbents. A reproducible adsorption jump in a high pressure range was observed in its adsorption isotherms. 2. Experimental section 2.1. Synthesis of Cu bpy BF4 2 H2 O2 bpyn complex Addition of an aqueous solution (50 ml) of hydrated copper(II) tetra¯uoroborate (0.04 M) to a re¯uxing acetonitrile solution (50 ml) of bpy (0.08 M) gave a solution with little gray precipitate, which shows no microporosity by N2 adsorption at 77 K. After ®ltration, diethyl ether vapor was diused into the mother-liquor by nitrogen ¯ow until the volume of 150 ml, then, the mother-liquor was kept at room temperature for several days. The gray deposition was obtained by ®ltration and dried under reduced pressure P < 10 3 Pa for 4 h. 2.2. X-ray diraction (XRD) and thermogravimetric analysis (TGA) The powder X-ray diraction (XRD) was measured using an angle-dispersive diractometer (MXP3 system, MAC Science) with monochro radiation at 15 kV. mated Mo-Ka k 0:7093 A The thermogravimetric analysis (TGA) was performed on a Thermal Analyzer System 001 (MAC Science) under a ¯ow of argon gas. The sample was heated from room temperature to 773 K at 5 K/min.
51
2.3. Adsorption measurements The adsorption isotherms of N2 at 77 K, Ar at 77 K and CO2 at 273 K were carried out volumetrically on an Autosorb-1, Quantachrom. N2 , Ar and CO2 gases of high purity (99.99%) were used. Prior to the adsorption isotherm measurements, the samples (around 50 mg) were outgassed under vacuum P < 10 4 Pa) for 2 h at dierent temperatures of 348±423 K.
3. Results and discussion 3.1. Crystal structure and thermal stability of Cu bpy BF4 2 H2 O2 bpyn complex The XRD pattern of the Cu complex synthesized by us agrees with the CCDC registered crystallographic data. Here we used the CCDC data for structure analysis. Cu bpy BF4 2 H2 O2 bpyn complex microcrystals belong to the monoclinic space group C2/c with a 16:228, b 11:078, c 13:985 A, b 114:43°; V 3 , Z 4 and q 1:710 g=cm3 [23]. This 2287:9 A unique feature comes from the dierent roles of the two bpy molecules; one coordinates Cu atoms directly by two nitrogen atoms, while the other bridges Cu atoms via intermediate water molecules by hydrogen-bonding. A two-dimensional sheet structure is shown in Fig. 1a, exhibiting the presence of a rectangular framework with the di 14:95 A. The eective mensions of 11:078 A cross-sectional area is given by subtracting the van der Waals radius of an aromatic ring (ca. 1.7 A) from each face of the framework, providing a void that should hold molecules with a cross-sectional 11:6 A. However, from Fig. 1b±d area of 7:7 A which gives a Cu complex-assembled cubic structure with ®ve two-dimensional sheets, we cannot ®nd a micropore space that can accommodate adsorbate molecules from any direction. It is because each sheet is displaced in a way that the Cu atoms are located close to the centers of the rectangular micropores of adjacent sheets thus enabling the BF4 anions to ®t into these pores. This arrangement is facilitated by the alternate parallel
52
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
Fig. 1. Space-®lling views of the Cu complex crystal structure (Cu, green; N, blue; O, red; C, gray; B, yellow; F, pale green spheres). The hydrogen atoms are omitted for clarity. (a) View of the xy plane for one sheet; (b), (c), (d) views of xy, yz, and xz plane for a cubic structure containing ®ve sheets.
p±p stacking of the bpy molecules. Therefore, the open microporosity of the as-synthesized Cu complex-assembled solid cannot be observed by XRD analysis. The TGA determination indicates a weight loss of 5.98% between 433 and 463 K. This is equivalent to the loss of two water molecules. The complex releases a bpy molecule and a BF3 molecule at 503 K, corresponding to a weight loss of 38.0%. Then the other bpy and the BF3 molecule are released at 603 K. This result con®rms the presence of two types of bpy molecules in this structure. The ®nal decompositional product (17.2 wt.%) of the complex is CuF2 .
The composition of the Cu complex after pretreatment with dierent temperatures was con®rmed by elemental microanalysis prior to adsorption studies and is shown in Table 1. No obvious changes in C, H, and N contents were observed below 423 K. However, the C and N contents decrease slightly above 423 K as the pretreatment temperature increases, indicating that a part of the bpy ligand molecules are removed from the Cu complex-assembled microcrystals after water molecules are released. However, the fundamental structure of the Cu complex-assembled microcrystals after the pretreating even above 423 K is preserved.
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
53
Table 1 Elemental microanalysis of the Cu complex after heating pretreatment under vacuum Pretreatment conditions
C (%)
H (%)
N (%)
Calculated for C20 H20 CuN4 B2 F8 O2 298 K; 10 3 Pa 373 K; 10 6 Pa 398 K; 10 6 Pa 423 K; 10 6 Pa 448 K; 10 6 Pa 473 K; 10 6 Pa
41.02 41.12 41.24 41.17 41.29 40.68 40.71
3.42 3.53 3.50 3.49 3.56 3.55 3.48
9.57 9.48 9.59 9.59 9.59 9.41 9.32
3.2. N2 adsorption The N2 adsorption isotherms measured at 77 K on the Cu complex are given in Fig. 2. These N2 adsorption isotherms cannot be classi®ed into the representative six types recommended by IUPAC. The adsorption performance of the Cu complex is quite sensitive to the pretreatment temperature; a maximum adsorption capacity is observed at the pretreatment temperature of 398 K. The adsorption suddenly begins at a de®nite relative pressure, which will be called the `gate pressure' in this Letter. This gate pressure depends on the pretreatment temperature. It is worth emphasizing that the amount adsorbed is almost zero before the gate pressure. Such a unique adsorption
Amount adsorbed / mg g
400
398 K 373 K
300
423 K
0
348 K
0.0
0.2
0.4
0.6
0.8
2
ln W ln W0 A=bE0 ;
1.0
P/P0 Fig. 2. Adsorption behavior of nitrogen at 77 K on the Cu complex pretreated at dierent temperatures.
A RT ln P0 =P ;
1
where W and W0 are the amount of adsorption at P =P0 and the micropore volume, A is the adsorption potential, b and E0 are the anity coecient and characteristic adsorption energy, respectively. All DR plots are almost linear in the higher P =P0 region, giving the micropore volume and bE0 . Furthermore, bE0 leads to the isosteric heat of adsorption qst;/1=e at the fractional ®lling of 1=e by the equation: qst;/1=e DHv bE0 ;
200 100
phenomenon should not be attributed to the pore blocking eect which can be found for microporous AlPO4 samples or molecular sieve carbons [24±26], because no micropores were found in the as-synthesized Cu complex-assembled solid by XRD analysis, as mentioned above. Also such a sharp adsorption jump cannot be observed in the case of pore blocking by the preadsorbed molecules. The micropore ®lling of vapors is well described by the Dudinin±Radushkevich (DR) equation:
2
where DHv is the heat of vaporization of the bulk liquid. The results of the DR analysis are listed in Table 2. Micropore parameters vary with the pretreatment temperature of the sample; the pretreatment at 398 K provides the greatest W0 value. The qst;/1=e values are much less than those of activated carbon ®bers with the slit-shaped micropores by about 4 kJ/mol, although the Cu-complex should have rectangular-shaped micropores [27]. Additionally, it is noteworthy that the surface area of the Cu complex exceeds 1400 m2 /g if the N2 molecules adsorbed in a monolayer.
54
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
Table 2 Micropore parameters of the Cu complex by N2 adsorption isotherms Pretreatment temperature (K)
W0 (mg/g)
Surface area m2 =g
Vm (ml/g)
bE0 (kJ/mol)
qst;/1=e (kJ/mol)
373 398 423
375.0 409.4 305.8
1306 1426 1065
0.46 0.51 0.38
2.1 2.6 1.4
8.5 9.0 7.8
3.3. CO2 and Ar adsorption The CO2 adsorption isotherms measured at 273 K on the Cu complex are shown in Fig. 3. They exhibit a steep adsorption similar to the N2 adsorption isotherms (Fig. 2). Distinct dierences, however, are observed. The maximum amount of adsorbed CO2 changes only slightly as the pretreatment temperature increases from 373 to 448 K. Also the gate pressure shifts slightly. The DR analysis shows that the maximum amount of adsorbed CO2 is about 200.5 mg/g which corresponds to 0.20 ml/g of the pore volume from the CO2 adsorbed phase density of 1.023 g/ml. On the contrary, the maximum amount of adsorbed N2 is 409.4 mg/g which equals 0.51 ml/g of the pore volume from the N2 adsorbed phase density of 0.808 g/ml (Table 2). The fractional ®lling ratio of the micropores by CO2 is only 40% of that by N2 ,
although the critical dimensions of the CO2 molecule (0.28 nm) are smaller than those of the N2 molecule (0.30 nm) [28]. Therefore, the micropores produced with the increase of adsorbate relative pressure should be favored by N2 molecules rather than CO2 molecules. The adsorption isotherm of Ar on the Cu complex also depends on the pretreatment temperatures, as shown in Fig. 4. However, the dependence is dierent from those of N2 and CO2 , The Ar adsorption isotherms of the Cu-complex pretreated above 398 K have no adsorption jump, but a linear increase with the Ar relative pressure. Ar molecules must penetrate the Cu complex structure gradually when the sample was pretreated above 398 K. The DR analysis shows that the maximum amount of adsorbed Ar is 681 mg/g which corresponds to the pore volume of 0.47 ml/g from the Ar adsorbed phase density of 1.451 g/ml
Amount adsorbed / mg g-1
700
500 400 300 200
423 K
100 0
Fig. 3. Adsorption behavior of carbon dioxide at 273 K on the Cu complex pretreated at dierent temperatures.
398 K
600
373 K 348 K
0.0
0.2
0.4 0.6 P/P0
0.8
1.0
Fig. 4. Adsorption behavior of argon at 77 K on the Cu complex pretreated at dierent temperatures.
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
at 77 K, which is estimated from observed values at 40 K (1.650 g/ml) and 85 K (1.4079 g/ml). Thus, the ®lling by Ar is similar to that by N2 . 3.4. Possibility of hydrogen bond-dissociation induced micropores The adsorption behavior must be associated with the formation of accessible micropores by both the structure change on pretreatment and an additional structures change at gate pressure. Such an interpretation should be reasonable, since the sheet structures mutually stack by the hydrogenbonding between an F atom of a BF4 anion from a sheet and an H atom of a water molecule from the second neighbor sheets. The hydrogen bonds in the Cu complex are shown in Fig. 5. When the chemical potential or relative pressure of the adsorbate reaches a certain value (gate pressure), the hydrogen-bonding among sheets breaks and the intrinsic rectangular frameworks rearrange in a
55
line to produce a one-dimensional channel which can accept adsorbate molecules. If the above hypothesis is correct, the Cu complex has a micropore volume of 0.52 ml/g from the XRD structural data, agreeing with the observed value (0.51 ml/g) from N2 adsorption as the sample was pretreated at 398 K. However, the eectiveness of these hydrogen bond-regulated micropores should sensitively depend on the adsorbate molecule-pore wall interaction. In this case, the N2 or Ar molecule just ®ts for adsorption by these micropores. As the CO2 molecule has a greater quadrupole moment than the N2 molecule, CO2 molecules should be more strongly adsorbed near the BF4 anion of a high surface electric ®eld, blocking further adsorption. On the other hand, the smallest spherical Ar molecules having no quadrupole moment can slip through the hydrogen-bonding barrier, showing a linear increase region in its adsorption isotherms. However, we need additional evidence for the above mechanism.
Fig. 5. Hydrogen bonds existing in the Cu complex. The hydrogen atoms in water molecules are omitted for clarity.
56
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
This study demonstrates that the Cu complexassembled solid exhibits a unique adsorption performance which is sensitive to the microenvironment. The practical utilization of this material, such as high pressure storage, sensor, and separation of gas, is of further interest. Acknowledgements This study was supported by the ProposalBased New Industry Creative Type Technology R&D Promotion Program (99E10-009-1) from the New Energy and Industrial Technology Development Organization of Japan (NEDO). References [1] F. Rouquerol, J. Rouquerol, K. King, Adsorption by Powders and Porous Solids, Academic Press, Great Britain, 1999. [2] P.J. Hagrman, D. Hagrman, J. Zubieta, Angew. Chem. Int. Ed. 38 (1999) 2638. [3] S. Kitagawa, M. Kondo, Bull. Chem. Soc. Jpn. 71 (1998) 1739. [4] H. Li, M. Eddaoudi, M. O'Keee, O.M. Yaghi, Nature 402 (1999) 276. [5] H. Li, C.E. Davis, T.L. Groy, D.G. Kelley, O.M. Yaghi, J. Am. Chem. Soc. 120 (1998) 2186. [6] T.M. Reineke, M. Eddaoudi, M. Fehr, D. Kelley, O.M. Yaghi, J. Am. Chem. Soc. 121 (1999) 1651. [7] T.M. Reineke, M. Eddaoudi, M. O'Keee, O.M. Yaghi, Angew. Chem. Int. Ed. 38 (1999) 2590. [8] M. Kondo, T. Okubo, A. Asami, S. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka, K. Seki, Angew. Chem. Int. Ed. 38 (1999) 140.
[9] M. Kondo, T. Yoshitomi, K. Seki, H. Matsuzaka, S. Kitagawa, Angew. Chem. Int. Ed. 36 (1997) 1725. [10] D. Li, K. Kaneko, J. Phys. Chem. B 104 (2000) 8940. [11] M.J. Zaworotko, Nature 402 (1999) 242. [12] O.M. Yaghi, C.E. Davis, G. Li, H. Li, J. Am. Chem. Soc. 119 (1997) 2861. [13] D. Venkataraman, G.B. Gardner, S. Lee, J.S. Moore, J. Am. Chem. Soc. 117 (1995) 11600. [14] D. Ramprasad, G.P. Pez, B.H. Toby, T.J. Markley, R.M. Pearlstein, J. Am. Chem. Soc. 117 (1995) 10694. [15] O.M. Yaghi, G. Li, H. Li, Nature 378 (1995) 703. [16] M. Fujita, Y.J. Kwon, S. Washizu, K. Ogula, J. Am. Chem. Soc. 116 (1994) 1151. [17] H. Kobayashi, H. Tomita, T. Naito, A. Kobayashi, F. Sakai, T. Watanaba, P. Cassoux, J. Am. Chem. Soc. 118 (1996) 368. [18] J.A. Real, E. Andres, M.C. Mu~ noz, M. Julve, T. Granier, A. Bousseksou, F. Varret, Science 268 (1995) 265. [19] D.M.L. Goodgame, D.A. Grachvogel, D.J. Williams, Angew. Chem. Int. Ed. 38 (1999) 153. [20] P. Losier, M.J. Zaworotko, Angew. Chem. Int. Ed. 35 (1996) 2779. [21] S. Subramanian, M.J. Zaworotko, Angew. Chem. Int. Ed. 34 (1995) 2127. [22] H. Li, M. Eddaoudi, T.L. Groy;, O.M. Yaghi, J. Am. Chem. Soc. 120 (1998) 8571. [23] A.J. Blake, S.J. Hill, P. Hubberstey, W. Li, J. Chem. Soc., Dalton Trans. (1997) 913. [24] C. Martin, N. Tosi-Pellenq, J. Patarin, J.P. Coulomb, Langmuir 14 (1998) 1774. [25] C. Lastoskie, K.E. Gubbins, N. Quirke, J. Phys. Chem. 97 (1993) 4786. [26] B. McEnaney, T.J. Mays, X. Chen, Fuel 77 (6) (1998) 557. [27] K. Kaneko, K. Suzuki, T. Suzuki, J. Chem. Phys. 97 (1992) 8705. [28] J. Carrido, A. Linares-Solano, J.M. Martõn-Martõnez, M. Molina-Sabio, F. Rodrõguez-Reinoso, R. Torregrosa, Langmuir 3 (1987) 76.
Chemical Physics Letters 335 (2001) 50±56
www.elsevier.nl/locate/cplett
Hydrogen bond-regulated microporous nature of copper complex-assembled microcrystals Di Li a, Katsumi Kaneko b,* a b
New Energy and Industrial Technology Development Organization (NEDO), 3-1-1 Higashi Ikebukuro, Toshima-ku, Tokyo 170-6028, Japan Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-Shi 263-8522, Japan Received 20 June 2000; in ®nal form 2 November 2000
Abstract A Cu complex-assembled compound Cu bpy BF4 2 H2 O2 bpyn (bpy 4,40 -bipyridine) with non-interpenetrated structure was synthesized and its adsorption properties for N2 , Ar, and CO2 were studied. The adsorption of gas on this complex suddenly begins at a de®nite relative pressure (`gate pressure') regardless of almost nil adsorption below the gate pressure. Such a unique adsorption phenomenon is attributed to the hydrogen bond-regulated microporous nature of this complex; hydrogen bonds in pores block adsorption below the gate pressure. Additionally, the gate pressure depends on both the adsorbate and the pretreatment temperature of the Cu complex solid. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Adsorption is of great technological importance. This is not only because various conventional and novel adsorbents such as carbon materials and zeolites have been widely used on a large scale in industries, but also because the adsorption techniques are helpful to characterize the surface properties and pore structures of solids [1]. Recently, substantial advances in new porous materials have been made toward supramolecular architecture [2,3]. A typical example is the successful construction of some metal complex frameworks with two- or three-dimensional microporosity [4±10]. The coming of `open season for *
Corresponding author. Fax: +81-43-290-2788. E-mail address: [email protected] (K. Kaneko).
solid framework' [11] in this ®eld should contribute to the solutions of the next two problems. One is the inhibition of the interpenetrating phenomenon, which can aect the formation of pores within the assembled structure, and another one is the maintenance of microporous structure after the removal of the included guest molecules [12,13]. Newly designed micropores within these metal complex frameworks possess a variety of sizes and shapes which are yet unobserved in conventional porous solids such as zeolites and activated carbons [12]. Therefore, microporous metal-organic frameworks provide molecular-scale voids that may be expected to impact many technologies such as adsorption and separations [14,15], shape-selective catalysis [16], and microelectronics [17,18], in addition to the fundamental science. So far, a lot of metal-organic frameworks have been synthesized successfully and XRD data
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 4 1 9 - 6
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
also reveal the presence of micropores [19±21]. However, their microporous structures were not fully con®rmed by the adsorption techniques, leaving a vital question regarding the nature of porosity in this class of materials [22]. In this work, a Cu complex-assembled compound Cu bpy BF4 2 H2 O2 bpyn (bpy 4,40 bipyridine) with non-interpenetrated structure was synthesized and its adsorption properties for N2 , Ar, and CO2 were studied. Consequently, this complex shows a unique adsorption behavior which has not been found for conventional adsorbents. A reproducible adsorption jump in a high pressure range was observed in its adsorption isotherms. 2. Experimental section 2.1. Synthesis of Cu bpy BF4 2 H2 O2 bpyn complex Addition of an aqueous solution (50 ml) of hydrated copper(II) tetra¯uoroborate (0.04 M) to a re¯uxing acetonitrile solution (50 ml) of bpy (0.08 M) gave a solution with little gray precipitate, which shows no microporosity by N2 adsorption at 77 K. After ®ltration, diethyl ether vapor was diused into the mother-liquor by nitrogen ¯ow until the volume of 150 ml, then, the mother-liquor was kept at room temperature for several days. The gray deposition was obtained by ®ltration and dried under reduced pressure P < 10 3 Pa for 4 h. 2.2. X-ray diraction (XRD) and thermogravimetric analysis (TGA) The powder X-ray diraction (XRD) was measured using an angle-dispersive diractometer (MXP3 system, MAC Science) with monochro radiation at 15 kV. mated Mo-Ka k 0:7093 A The thermogravimetric analysis (TGA) was performed on a Thermal Analyzer System 001 (MAC Science) under a ¯ow of argon gas. The sample was heated from room temperature to 773 K at 5 K/min.
51
2.3. Adsorption measurements The adsorption isotherms of N2 at 77 K, Ar at 77 K and CO2 at 273 K were carried out volumetrically on an Autosorb-1, Quantachrom. N2 , Ar and CO2 gases of high purity (99.99%) were used. Prior to the adsorption isotherm measurements, the samples (around 50 mg) were outgassed under vacuum P < 10 4 Pa) for 2 h at dierent temperatures of 348±423 K.
3. Results and discussion 3.1. Crystal structure and thermal stability of Cu bpy BF4 2 H2 O2 bpyn complex The XRD pattern of the Cu complex synthesized by us agrees with the CCDC registered crystallographic data. Here we used the CCDC data for structure analysis. Cu bpy BF4 2 H2 O2 bpyn complex microcrystals belong to the monoclinic space group C2/c with a 16:228, b 11:078, c 13:985 A, b 114:43°; V 3 , Z 4 and q 1:710 g=cm3 [23]. This 2287:9 A unique feature comes from the dierent roles of the two bpy molecules; one coordinates Cu atoms directly by two nitrogen atoms, while the other bridges Cu atoms via intermediate water molecules by hydrogen-bonding. A two-dimensional sheet structure is shown in Fig. 1a, exhibiting the presence of a rectangular framework with the di 14:95 A. The eective mensions of 11:078 A cross-sectional area is given by subtracting the van der Waals radius of an aromatic ring (ca. 1.7 A) from each face of the framework, providing a void that should hold molecules with a cross-sectional 11:6 A. However, from Fig. 1b±d area of 7:7 A which gives a Cu complex-assembled cubic structure with ®ve two-dimensional sheets, we cannot ®nd a micropore space that can accommodate adsorbate molecules from any direction. It is because each sheet is displaced in a way that the Cu atoms are located close to the centers of the rectangular micropores of adjacent sheets thus enabling the BF4 anions to ®t into these pores. This arrangement is facilitated by the alternate parallel
52
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
Fig. 1. Space-®lling views of the Cu complex crystal structure (Cu, green; N, blue; O, red; C, gray; B, yellow; F, pale green spheres). The hydrogen atoms are omitted for clarity. (a) View of the xy plane for one sheet; (b), (c), (d) views of xy, yz, and xz plane for a cubic structure containing ®ve sheets.
p±p stacking of the bpy molecules. Therefore, the open microporosity of the as-synthesized Cu complex-assembled solid cannot be observed by XRD analysis. The TGA determination indicates a weight loss of 5.98% between 433 and 463 K. This is equivalent to the loss of two water molecules. The complex releases a bpy molecule and a BF3 molecule at 503 K, corresponding to a weight loss of 38.0%. Then the other bpy and the BF3 molecule are released at 603 K. This result con®rms the presence of two types of bpy molecules in this structure. The ®nal decompositional product (17.2 wt.%) of the complex is CuF2 .
The composition of the Cu complex after pretreatment with dierent temperatures was con®rmed by elemental microanalysis prior to adsorption studies and is shown in Table 1. No obvious changes in C, H, and N contents were observed below 423 K. However, the C and N contents decrease slightly above 423 K as the pretreatment temperature increases, indicating that a part of the bpy ligand molecules are removed from the Cu complex-assembled microcrystals after water molecules are released. However, the fundamental structure of the Cu complex-assembled microcrystals after the pretreating even above 423 K is preserved.
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
53
Table 1 Elemental microanalysis of the Cu complex after heating pretreatment under vacuum Pretreatment conditions
C (%)
H (%)
N (%)
Calculated for C20 H20 CuN4 B2 F8 O2 298 K; 10 3 Pa 373 K; 10 6 Pa 398 K; 10 6 Pa 423 K; 10 6 Pa 448 K; 10 6 Pa 473 K; 10 6 Pa
41.02 41.12 41.24 41.17 41.29 40.68 40.71
3.42 3.53 3.50 3.49 3.56 3.55 3.48
9.57 9.48 9.59 9.59 9.59 9.41 9.32
3.2. N2 adsorption The N2 adsorption isotherms measured at 77 K on the Cu complex are given in Fig. 2. These N2 adsorption isotherms cannot be classi®ed into the representative six types recommended by IUPAC. The adsorption performance of the Cu complex is quite sensitive to the pretreatment temperature; a maximum adsorption capacity is observed at the pretreatment temperature of 398 K. The adsorption suddenly begins at a de®nite relative pressure, which will be called the `gate pressure' in this Letter. This gate pressure depends on the pretreatment temperature. It is worth emphasizing that the amount adsorbed is almost zero before the gate pressure. Such a unique adsorption
Amount adsorbed / mg g
400
398 K 373 K
300
423 K
0
348 K
0.0
0.2
0.4
0.6
0.8
2
ln W ln W0 A=bE0 ;
1.0
P/P0 Fig. 2. Adsorption behavior of nitrogen at 77 K on the Cu complex pretreated at dierent temperatures.
A RT ln P0 =P ;
1
where W and W0 are the amount of adsorption at P =P0 and the micropore volume, A is the adsorption potential, b and E0 are the anity coecient and characteristic adsorption energy, respectively. All DR plots are almost linear in the higher P =P0 region, giving the micropore volume and bE0 . Furthermore, bE0 leads to the isosteric heat of adsorption qst;/1=e at the fractional ®lling of 1=e by the equation: qst;/1=e DHv bE0 ;
200 100
phenomenon should not be attributed to the pore blocking eect which can be found for microporous AlPO4 samples or molecular sieve carbons [24±26], because no micropores were found in the as-synthesized Cu complex-assembled solid by XRD analysis, as mentioned above. Also such a sharp adsorption jump cannot be observed in the case of pore blocking by the preadsorbed molecules. The micropore ®lling of vapors is well described by the Dudinin±Radushkevich (DR) equation:
2
where DHv is the heat of vaporization of the bulk liquid. The results of the DR analysis are listed in Table 2. Micropore parameters vary with the pretreatment temperature of the sample; the pretreatment at 398 K provides the greatest W0 value. The qst;/1=e values are much less than those of activated carbon ®bers with the slit-shaped micropores by about 4 kJ/mol, although the Cu-complex should have rectangular-shaped micropores [27]. Additionally, it is noteworthy that the surface area of the Cu complex exceeds 1400 m2 /g if the N2 molecules adsorbed in a monolayer.
54
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
Table 2 Micropore parameters of the Cu complex by N2 adsorption isotherms Pretreatment temperature (K)
W0 (mg/g)
Surface area m2 =g
Vm (ml/g)
bE0 (kJ/mol)
qst;/1=e (kJ/mol)
373 398 423
375.0 409.4 305.8
1306 1426 1065
0.46 0.51 0.38
2.1 2.6 1.4
8.5 9.0 7.8
3.3. CO2 and Ar adsorption The CO2 adsorption isotherms measured at 273 K on the Cu complex are shown in Fig. 3. They exhibit a steep adsorption similar to the N2 adsorption isotherms (Fig. 2). Distinct dierences, however, are observed. The maximum amount of adsorbed CO2 changes only slightly as the pretreatment temperature increases from 373 to 448 K. Also the gate pressure shifts slightly. The DR analysis shows that the maximum amount of adsorbed CO2 is about 200.5 mg/g which corresponds to 0.20 ml/g of the pore volume from the CO2 adsorbed phase density of 1.023 g/ml. On the contrary, the maximum amount of adsorbed N2 is 409.4 mg/g which equals 0.51 ml/g of the pore volume from the N2 adsorbed phase density of 0.808 g/ml (Table 2). The fractional ®lling ratio of the micropores by CO2 is only 40% of that by N2 ,
although the critical dimensions of the CO2 molecule (0.28 nm) are smaller than those of the N2 molecule (0.30 nm) [28]. Therefore, the micropores produced with the increase of adsorbate relative pressure should be favored by N2 molecules rather than CO2 molecules. The adsorption isotherm of Ar on the Cu complex also depends on the pretreatment temperatures, as shown in Fig. 4. However, the dependence is dierent from those of N2 and CO2 , The Ar adsorption isotherms of the Cu-complex pretreated above 398 K have no adsorption jump, but a linear increase with the Ar relative pressure. Ar molecules must penetrate the Cu complex structure gradually when the sample was pretreated above 398 K. The DR analysis shows that the maximum amount of adsorbed Ar is 681 mg/g which corresponds to the pore volume of 0.47 ml/g from the Ar adsorbed phase density of 1.451 g/ml
Amount adsorbed / mg g-1
700
500 400 300 200
423 K
100 0
Fig. 3. Adsorption behavior of carbon dioxide at 273 K on the Cu complex pretreated at dierent temperatures.
398 K
600
373 K 348 K
0.0
0.2
0.4 0.6 P/P0
0.8
1.0
Fig. 4. Adsorption behavior of argon at 77 K on the Cu complex pretreated at dierent temperatures.
Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
at 77 K, which is estimated from observed values at 40 K (1.650 g/ml) and 85 K (1.4079 g/ml). Thus, the ®lling by Ar is similar to that by N2 . 3.4. Possibility of hydrogen bond-dissociation induced micropores The adsorption behavior must be associated with the formation of accessible micropores by both the structure change on pretreatment and an additional structures change at gate pressure. Such an interpretation should be reasonable, since the sheet structures mutually stack by the hydrogenbonding between an F atom of a BF4 anion from a sheet and an H atom of a water molecule from the second neighbor sheets. The hydrogen bonds in the Cu complex are shown in Fig. 5. When the chemical potential or relative pressure of the adsorbate reaches a certain value (gate pressure), the hydrogen-bonding among sheets breaks and the intrinsic rectangular frameworks rearrange in a
55
line to produce a one-dimensional channel which can accept adsorbate molecules. If the above hypothesis is correct, the Cu complex has a micropore volume of 0.52 ml/g from the XRD structural data, agreeing with the observed value (0.51 ml/g) from N2 adsorption as the sample was pretreated at 398 K. However, the eectiveness of these hydrogen bond-regulated micropores should sensitively depend on the adsorbate molecule-pore wall interaction. In this case, the N2 or Ar molecule just ®ts for adsorption by these micropores. As the CO2 molecule has a greater quadrupole moment than the N2 molecule, CO2 molecules should be more strongly adsorbed near the BF4 anion of a high surface electric ®eld, blocking further adsorption. On the other hand, the smallest spherical Ar molecules having no quadrupole moment can slip through the hydrogen-bonding barrier, showing a linear increase region in its adsorption isotherms. However, we need additional evidence for the above mechanism.
Fig. 5. Hydrogen bonds existing in the Cu complex. The hydrogen atoms in water molecules are omitted for clarity.
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Li, K. Kaneko / Chemical Physics Letters 335 (2001) 50±56
This study demonstrates that the Cu complexassembled solid exhibits a unique adsorption performance which is sensitive to the microenvironment. The practical utilization of this material, such as high pressure storage, sensor, and separation of gas, is of further interest. Acknowledgements This study was supported by the ProposalBased New Industry Creative Type Technology R&D Promotion Program (99E10-009-1) from the New Energy and Industrial Technology Development Organization of Japan (NEDO). References [1] F. Rouquerol, J. Rouquerol, K. King, Adsorption by Powders and Porous Solids, Academic Press, Great Britain, 1999. [2] P.J. Hagrman, D. Hagrman, J. Zubieta, Angew. Chem. Int. Ed. 38 (1999) 2638. [3] S. Kitagawa, M. Kondo, Bull. Chem. Soc. Jpn. 71 (1998) 1739. [4] H. Li, M. Eddaoudi, M. O'Keee, O.M. Yaghi, Nature 402 (1999) 276. [5] H. Li, C.E. Davis, T.L. Groy, D.G. Kelley, O.M. Yaghi, J. Am. Chem. Soc. 120 (1998) 2186. [6] T.M. Reineke, M. Eddaoudi, M. Fehr, D. Kelley, O.M. Yaghi, J. Am. Chem. Soc. 121 (1999) 1651. [7] T.M. Reineke, M. Eddaoudi, M. O'Keee, O.M. Yaghi, Angew. Chem. Int. Ed. 38 (1999) 2590. [8] M. Kondo, T. Okubo, A. Asami, S. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka, K. Seki, Angew. Chem. Int. Ed. 38 (1999) 140.
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