Langmuir 2002, 18, 2441-2443
Surface Area Evaluation of Coordination Polymers Having Rectangular Micropores K. Seki Department of Research and Development, Osaka Gas Company, Ltd., 6-19-9 Torishima, konohana-ku, Osaka 554-0051, Japan Received September 21, 2001. In Final Form: December 25, 2001
Introduction The Brunauer-Emmett-Teller (BET) method1 is now generally accepted as a standard procedure for evaluating the porosities of a variety of adsorbents such as activated carbon, zeolites, and silica gel.2 It has been argued, however, that BET surface area values are overestimates compared to the true values.3,4 The accuracy of any BET surface area estimation is unknown since a standard adsorbent possessing a truly well-defined surface area is unavailable. Recently, a great deal of attention has been directed toward the use of coordination polymers in the design and synthesis of new porous materials.5-32 Compared with conventional porous materials such as zeolites or activated carbons, these coordination polymers have uniform structures.5-7,11-12 These pore structures have been clarified by X-ray and simulation analysis. It seems that the accuracy of the BET method could be evaluated when coordination polymers are used as the model compound. This paper describes the results of surface area evaluations of two previously synthesized coordination polymers.6,12 These model compounds consist of uniformly porous rectangular three-dimensional structures. Experimental Section Synthesis. Cu(p-OOC-Ph-COO)‚1/2TEDA (1; TEDA ) triethylenediamine) and Cu(OOC-Ph-CHdCH-COO)‚1/2TEDA (2) were obtained using previously published procedures.6,12,32 Physical Measurements. X-ray powder diffraction data were measured, and the simulated powder patterns were calculated using previously published procedures.32 (1) Brunaner, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (2) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. (3) Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Carbon 1992, 30, 1075. (4) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem 1985, 57, 603. (5) Seki, K.; Takamizawa, S.; Mori, W. Chem. Lett. 2001, 122. (6) Seki, K.; Takamizawa, S.; Mori, W. Chem. Lett. 2001, 332. (7) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276. (8) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 1148. (9) Li, H.; Eddaoudi, M.; Groy, T. L.; Yaghi, O. M. J. Am. Chem. Soc. 1998, 120, 8571. (10) Kondo, M.; Okubo, T.; Asami, A.; Noro, S.; Yoshitomi, T.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Seki, K. Angew. Chem., Int. Ed. Engl. 1999, 111, 140. (11) Noro, S.; Kitagawa, S.; Kondo, M.; Seki, K. Angew. Chem., Int. Ed. Engl. 2000, 39, 2081. (12) Seki, K. Chem. Commun. 2001, 1496. (13) Eddaoudi, M.; Li, H.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 1391. (14) Zaworotko, M. J. Angew. Chem., Int. Ed. 2000, 39, 141. (15) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319. (16) Chen, B.; Eddaoudi, M.; Reineke, T. M.; Kampf, J. W.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 11559.
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Ar Adsorption Measurement. Adsorption isotherms were measured using ASAP 2000M volumetric adsorption equipment from Micromeritics. Samples were degassed under vacuum at 373 K before adsorption measurements were carried out. Adsorption isotherms were measured at a relative pressure ranging from 10-6 to 1.
Results and Discussion Structure of Compounds 1 and 2. Magnetic susceptibility, elemental analysis, and X-ray powder diffraction (XRPD) analysis confirmed that structures 1 and 2 are identical to the coordination polymers reported previously.6,12 The two-dimensional layers bridging the copper(II) ions with the dicarboxylate ions are linked with TEDA as pillar ligands to yield the three-dimensional structure as shown in Figure 1. Optimized structures, as determined by molecular mechanic and molecular dynamics using Cerius2, are shown in Figure 2. Space-filling representations of 1 and 2 reveal the presence of a single channel along the c axis in each compound of dimensions 7.4 × 7.4 Å and 9.4 × 9.4 Å, respectively, as determined from optimized structures and van der Waals radii. The pores in the ac and bc planes can be regarded as closed due to their smaller size compared with that of the adsorbates. The unit cell dimensions of 1 and 2, as determined from the crystallographic lattice spacing, are 10.8 × 10.8 × 10.2 Å and 13.2 × 13.2 × 10.2 Å, respectively (Figure 3). Evaluation of BET Surface Areas. The BET surface areas1 of 1 and 2 were determined by analyzing the respective high-resolution adsorption isotherms of Ar at 87.3 K (Figure 4). BET plots of the Ar adsorption isotherms of 1 and 2 displayed linearity in the pressure range used (Table 1). The surface area increased as the pressure range decreased. The theoretically predicted surface areas for 1 and 2 as determined from their geometrical structures are 3205 and 3729 m2/g, respectively. In general, however, the predicted surface area values are smaller than the true values. This is probably due to the presence of uneven walls within the adsorbents. In an attempt to arrive at a predicted surface area value that more closely resembles the true value, pore widths (17) Chen, B.; Eddaoudi, M.; Hyde, S. T.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Science 2001, 291, 1021. (18) Kitagawa, S.; Kondo, M. Bull. Chem. Soc. Jpn. 1998, 71, 144. (19) Tabares, L. C.; Navarro, J. A. R.; Salas, M. J. Am. Chem. Soc. 2001, 123, 383. (20) Min, K. S.; Suh, M. P. J. Am. Chem. Soc. 2000, 122, 6834. (21) Nossov, A. V.; Soldatov, D. V.; Ripmeester, J. A. J. Am. Chem. Soc. 2001, 123, 3563. (22) Manaov, A. Y.; Soldatov, D. V.; Ripmeester, J. A.; Lipkowski, J. J. Phys. Chem. B 2000, 104, 12111. (23) Soldatov, D. V.; Ripmeester, J. A.; Shergina, S. I.; Sokolov, I. E.; Zanina, A. S.; Gromilov, S. A.; Dyadin, Y. A. J. Am. Chem. Soc. 1999, 121, 4179. (24) Beauvais, L. G.; Shores, M. P.; Long, J. R. J. Am. Chem. Soc. 2000, 122, 2763. (25) Jung, O.; Kim, Y. J.; Lee, Y.; Park, J. K.; Chae, H. K. J. Am. Chem. Soc. 2000, 122, 9921. (26) Min, K. S.; Suh, M. P. Chem.sEur. J. 2001, 7, 303. (27) Min, K. S.; Suh, M. P. J. Am. Chem. Soc. 2000, 122, 6834. (28) Soldatov, D. V.; Henegouwen, A. T.; Enright, G. D.; Ratcliffe, C. I.; Ripmeester, J. A. Inorg. Chem. 2001, 40, 1626. (29) Reineke, T. M.; Eddaoudi, M.; Moler, D.; O’Keefe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 4843. (30) Li, D.; Kaneko, K. Chem. Phys. Lett. 2001, 335, 50. (31) Li, D.; Kaneko, K. J. Phys. Chem. B 2001, 104, 8940. (32) Seki, K.; Mori, W. J. Phys. Chem. B 2001, in press.
10.1021/la015588h CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002
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Langmuir, Vol. 18, No. 6, 2002
Notes Table 1. BET Surface Areas of Coordination Polymers in the Various Relative Pressure Ranges surface area/m2 g-1 compound
0.1-0.3a
0.02-0.1a
1 2
1464 2196
1883 2883
0.01-0.05a 0.008-0.01a 0.001-0.01a 1947 3013
2009 3396
2017 b
a The relative pressure range for the BET plot. b The BET plot has no linearity.
Figure 1. The three-dimensional structures of coordination polymers 1 and 2.
Figure 5. Ar filling states of micropores 1 and 2.
Figure 2. The space-filling illustration of the optimized plausible structures for 1 and 2.
These predicted surface area values of 1 and 2 by the geometrical method were much greater than those calculated by the BET method. The BET values calculated in the relative pressure range of 0.01-0.05, a pressure range recommended for the evaluation of BET surface areas of microporous materials,3 are even lower. In an attempt to interpret these results, an understanding of the filling states of Ar molecules into the micropore structure is needed. DR analysis2 was used to determine the saturated amount of adsorption. The DR equation used was as follows:
In W ) In W0 - (A/βE0)2
Figure 3. Unit cell dimensions for 1 and 2.
Figure 4. Ar adsorption isotherms at 87.3 K of 1 and 2.
derived from the geometrical structure and micropore volumes calculated using the Dubinin-Radushkevich (DR) method2 were used. The values thus obtained were 3849 and 4596 m2/g for 1 and 2, respectively. Both of these values are more than 20% higher than those derived only from the geometrical structures but are expected to be closer to the true surface area because the uneven shape of the pore surface is taken into account.
where W and W0 represent the amount of adsorption at P/P0 and the saturated amount of adsorption, respectively. E0 is the characteristic adsorption energy, A represents Polanyi’s adsorption potential defined by A ) RT ln(P0/ P), and β represents the affinity coefficient of the adsorbate-adsorbent interaction. DR plots for 1 and 2 were almost linear in the higher P/P0 region, yielding W0 values of 556 and 845 cm3 g-1 at STP, respectively. Filling states were determined from W0 assuming a square pore geometry and spherical Ar molecules with a 3.4 Å pore diameter. Figure 5 depicts the Ar filling states into the micropore structures 1 and 2. The spherical Ar molecules are depicted as filling the square micropores regularly with high packing density. Taking into account the rectangular pore structure of coordination polymers and the filling state of adsorbate molecules, it is not reasonable that as the value of the cross-sectional area of molecules (am) adsorbed we use the common value for the adsorption at the corners of rectangular pores because this value is calculated as the necessary area to cover the flat plane with one adsorbate molecule.2,33 On the other hand, if the cross-sectional area of molecules adsorbed at the corners of a rectangular pore is represented by 2am, the BET surface areas calculated in the relative pressure range of 0.01-0.05 for 1 and 2 are 3894 and 4520 m2 g-1, respectively. That is, in the case of 1, 2am is used as the value of the cross-sectional area of molecules adsorbed for all adsorbed molecules, and in (33) McClellan, A. L.; Harnsberger, H. F. J. Colloid Interface Sci. 1967, 23, 577.
Notes
Langmuir, Vol. 18, No. 6, 2002 2443 Table 2. Surface Areas Calculated in Different Ways cell/Å2
predicted area per unit (A) unit cell formula weight/g mol-1 (B) predicted surface area/m2 g-1 (A × 6.022 × 103/B) Dubinin-Radushkevich total pore volume/cm3 g-1 a surface area derived from DR pore volume/m2 g-1 experimental surface area weighting Ar on corners double/m2 g-1 a
compound 1
compound 2
4 × 10.2 × 7.4 ) 301.92 Cu2C22H20N2O8 ) 567.50 3204 556/cm3 g-1 (STP) × 0.00128 ) 0.712 4 × 0.712/7.4 × 104 ) 3849 1947 × 2 ) 3894
4 × 10.2 × 9.4 ) 383.52 Cu2C26H24N2O8 ) 619.577 3728 845/cm3 g-1 (STP) × 0.00128 ) 1.08 4 × 1.08/9.4 × 104 ) 4596 3013 × 1.5 ) 4520
1 cm3 of argon at STP is equivalent to 0.00128 cm3 of liquid argon at its boiling point.
the case of 2, 2am is used as the value of the cross-sectional area of molecules adsorbed for half of the totally adsorbed molecules as shown in Figure 5. These values are almost the same as those calculated from the geometrical structure (pore width and DR micropore volume), which are closer to the true surface area. In the other relative pressure ranges, the calculated values do not agree quantitatively with those values calculated from the geometrical structure. Judging from isotherms, the relative pressures at which the monolayer is completed on the pore surface as shown in Figure 5 are ca. 0.001 and 0.01 for 1 and 2, respectively. These results reflect the ideal range where the BET equation has its greatest validity, namely, in the region where the monolayer is completed on the pore surface. Table 2 summarizes the results of various surface area calculations. The surface areas arrived at using the improved BET method are the greatest of any previously reported adsorbents. Additionally, the values reported in this paper are greater than the theoretical surface area upper limit reported for carbonaceous materials calculated from infinite single graphite layers.3,34 (34) Everett, D. H.; Powl, J. C. J Chem. Soc., Faraday Trans. 1976, 172, 619.
Conclusion Surface areas for coordination polymers possessing various rectangular pores were evaluated by the conventional BET method. The BET values were compared with the values calculated from the geometrical structure. The values obtained by the conventional BET method are relatively smaller than those calculated from the geometrical structures. On the other hand, when 2am is used as a cross-sectional area of molecules for the adsorption at the corner of a pore, the BET surface area is almost the same as the calculated values geometrically. These results demonstrated that in calculating the BET surface areas for adsorbents having rectangular pores, reasonable results are obtained by using 2am in the case of the adsorption at the corner of a pore, especially for the BET value calculated in the relative pressure range of 0.010.05 where the monolayer is completed. Furthermore, the surface area calculated by this improved BET method for the coordination polymers tested, which have been reported to possess high porosity and high methane adsorption capacity, is more than 4000 m2 g-1. LA015588H
