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Langmuir 2007, 23, 3106-3109
Argon Adsorption on Cu3(Benzene-1,3,5-tricarboxylate)2(H2O)3 Metal-Organic Framework V. Krungleviciute,† K. Lask,† L. Heroux,† A. D. Migone,*,† J.-Y. Lee,‡ J. Li,‡ and A. Skoulidas§ Department of Physics, Southern Illinois UniVersity, Neckers 483 A, Carbondale, Illinois 62901, Department of Chemistry and Chemical Biology, Rutgers UniVersity, 610 Taylor Road, Piscataway, New Jersey 08854, and ExxonMobil Research and Engineering, 3225 Gallows Road, Fairfax, Virginia 22037 ReceiVed June 28, 2006. In Final Form: October 17, 2006 Using volumetric adsorption techniques, we have measured the adsorption of argon on Cu3(BTC)2(H2O)3, (BTC ) benzene-1,3,5-tricarboxylate), a microporous metal-organic framework structure, at temperatures between 66 and 143 K. In addition to the experiments, we have used Grand Canonical Monte Carlo simulations to calculate the adsorption isotherm of argon at 87 K. Our experimental and theoretical results are compared to those of previous studies. The experiments were performed using a high density of points, allowing us to obtain, in detail, the isosteric heat’s coverage dependence. Our values from the simulations are in reasonable agreement with those obtained in the experiments.
Introduction Metal-organic frameworks (MOFs) consist of metal centers and/or metal clusters connected by organic linkers, forming 3-D porous structures with 1-D, 2-D, or 3-D channel systems.1-8 Through a careful selection of linkers and metallic clusters, the MOFs can be designed so that they have a porous structure of predetermined porosity, with very well-determined and uniform pore diameters (both of which are controlled by the structure of the material).1-20 MOFs have attracted a great deal of interest * Corresponding author. Fax: (618) 453-1056. Phone: (618) 453-1053. E-mail:
[email protected]. † Southern Illinois University. ‡ Rutgers University. § ExxonMobil Research and Engineering. (1) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705-714. (2) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469-472. (3) Fletcher, A. J.; Thomas, K. M.; Rosseinsky, M. J. J. Solid State Chem. 2005, 178, 2491-2510. (4) Pan, L.; Liu, H.; Lei, X.; Huang, X.; Olson, D. H.; Turro, N. J.; Li, J. Angew. Chem., Int. Ed. 2003, 42, 542-542. (5) Pan, L.; Sander, M. B.; Huang, X.; Li, J.; Smith, M.; Bittner, E.; Bockrath, B.; Johnson, J. K. J. Am. Chem. Soc. 2004, 126, 1308-1309. (6) Lee, J. Y.; Li, J.; Jagiello, J. J. Solid State Chem. 2005, 178, 2527-2532. (7) Du¨ren, T.; Sarkisov, L.; Yaghi, O. M.; Snurr, R. Q. Langmuir 2004, 20, 2683-2689. (8) Eddaoudi, M.; Li, H. L.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 1391-1397. (9) Chui, S. S.-Y; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148-1150. (10) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 46704679. (11) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276-279. (12) Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre´, J. J. Mater. Chem. 2006, 16, 626-636. (13) Yang, Q.; Zhong, C. J. Phys. Chem. B 2005, 109, 11862-11864. (14) Chen, B.; Ockwig, N. W.; Millward, A. R.; Contreras, D. S.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4745-4749. (15) Chen, B.; Eddaouadi, M.; Hyde, S. T.; O’Keeffe, M.; Yaghi, O. M. Science 2001, 291, 1021-1023. (16) Chae, H. K.; Siberio-Pe´rez, D. Y.; Kim, J.; Go, Y. B.; Eddaoudi, M.; Matzger, A. J.; O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523-527. (17) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127-1129. (18) Rowsell, J. L. C.; Eckert, J.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 14904-14910. (19) Vishnyakov, A.; Ravikovitch, P. I.; Neimark, A. V.; Bulow, M.; Wang, Q. M. Nano Lett. 2003, 3, 713-718.
from researchers working in theory, experiment, simulation, and synthesis1-31 because they are among the most promising candidates currently being explored for their potential as storage materials for alternative fuels (e.g., hydrogen and methane).2,5-7,10,12-14,17,18,23,26,28 Because of the enormous societal and economic impact of a successful development in this area, it is important to thoroughly and rigorously test the adsorptive properties of these materials. Cu3(BTC)2(H2O)3 (benzene-1,3,5-tricarboxylate), see Figure 1, was one of the earliest studied metal-organic frameworks. Chui et al.9 first reported information on this MOF in 1999. In the framework of this material, two octahedrally coordinated Cu atoms are connected to eight oxygen atoms of tetra-carboxylate units to form a dimeric Cu paddle wheel. Each BTC ligand holds three dimeric Cu paddle wheels to form a microporous open framework with face-centered cubic symmetry. Cu-BTC has a 3-D channel structure connecting a system of tetrahedral-shaped cages accessible through small windows (∼3.5 Å in diameter). The large cavities are connected through squareshaped windows with a diameter of ∼9 Å. At moderate temperatures and pressures, the adsorption of molecules in the central cavity of the unit cell in Cu-BTC is localized close to the internal surface. Most of the central cavity is filled only at low temperatures or high pressures.21 (20) Eddaoudi, M.; Moler, D. B.; Li, H. L.; Chen, B.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319-330. (21) Skoulidas, A. I. J. Am. Chem. Soc. 2004, 126, 1356-1357. (22) Pan, L.; Liu, H.; Kelly, S. P.; Huang, X.; Olson, D. H.; Li, J. Chem. Commun. 2003, 7, 854-855. (23) Lee, J.-Y.; Pan, L.; Kelly, S. P.; Jagiello, J.; Emge, T. J.; Li, J. AdV. Mater. 2005, 17, 2703-2706. (24) Wang, Q. M.; Shen, D.; Bu¨low, M.; Lau, M. L.; Deng, S.; Fitch, F. R.; Lemcoff, N. O.; Semanscin, J. Microporous Mesoporous Mater. 2002, 55, 217230. (25) Rowsell, J. L. C.; Yaghi, O. M. Microporous Mesoporous Mater. 2004, 73, 3-14. (26) Garberoglio, G.; Skoulidas, A. I.; Johnson, J. K. J. Phys. Chem. B 2005, 109, 13094-13103. (27) Skoulidas, A. I.; Sholl, D. S. J. Phys. Chem. B 2005, 109, 15760-15768. (28) Sagara, T.; Klassen, J.; Ganz, E. J. Chem. Phys. 2004, 121, 1254312547. (29) Sarkisov, L.; Du¨ren, T.; Snurr, R. Q. Mol. Phys. 2004, 102, 211-221. (30) Skoulidas, A. I.; Sholl, D. S. J. Phys. Chem. A 2003, 107, 10132-10141. (31) Centrone, A.; Siberio-Pe´rez, D. Y.; Millward, A. R.; Yaghi, O. M.; Matzger, A. J.; Zerbi, G. Chem. Phys. Lett. 2005, 411, 516-519.
10.1021/la061871a CCC: $37.00 © 2007 American Chemical Society Published on Web 02/06/2007
Argon Adsorption on Cu3(BTC)2(H2O)3
Langmuir, Vol. 23, No. 6, 2007 3107 Table 1. Parameters of the Lennard-Jones Potential Ar-C Ar-Cu Ar-H Ar-O Ar-Ar
Figure 1. Cu3(BTC)2(H2O)3 (BTC ) benzene-1,3,5-tricarboxylate) metal-organic framework. In the figure, Cu is shown in green, O is shown in red, C is shown in gray, and H is omitted for clarity.
In the present work, we describe the results of experiments and computer simulations of Ar adsorption on Cu-BTC. We have analyzed the detailed features of the isotherms as a function of the amount of argon adsorbed and the temperature. We have also obtained experimental values for the isosteric heat of adsorption. These experimental data are compared with computer simulations. Synthesis The crystal structure of Cu3(BTC)2(H2O)3 was first reported by Chui et al.9 However, their reactions at 453 K under hydrothermal conditions also generated a Cu2O impurity phase as a byproduct. In our synthesis, the conditions were modified to produce a pure phase of Cu3(BTC)2(H2O)3. First, Cu(NO3)2‚3H2O (0.435 g or 1.8 mmol) was dissolved in 6 mL of deionized water, and BTC (0.110 g or 0.5 mmol) was dissolved in 6 mL of ethanol. The two solutions were then mixed in a 100 mL beaker and stirred for 30 min. The solution mixture was transferred into a Teflon-lined autoclave and heated in an oven at 403 K for 3 h. After cooling the autoclave down to room temperature naturally, the solution was filtered, and the product was washed with ethanol (10 mL × 3 times). Shiny blue crystals of Cu-BTC were collected. Adsorption Isotherm Measurements. We performed isotherm measurements on an in-house designed and built volumetric adsorption apparatus. Pressures were measured using three different room-temperature pressure gauges (of maximum ranges 1, 100, and 1000 Torr, respectively). Gas was dosed into the dosing volume in small steps through computer-controlled valves until a predetermined dosing amount was reached. The valve to the cell was then opened to allow the gas into the experimental chamber that contained the Cu-BTC. The temperature of the experimental cell was controlled in two stages. The temperature stability of the sample cell was (0.010 K. The verification that equilibrium conditions were reached, and the recording of data was done through the use of a data acquisition program that we developed using LabView. Prior to the start of the measurements, the Cu-BTC sample was placed into the stainless steel cell in which the measurements were to be conducted, and it was placed in an oven and outgassed under vacuum at 100 °C for 6 h. The cell was then transported, without breaching vacuum integrity, to the adsorption setup, attached to the closed-cycle refrigerator, and connected to the gas handling system. The initial sample weight was 102.1 mg. Our TGA measurements indicated that after 4 h of heating at 100 °C, the sample lost about 8.9% of its weight due to the removal of water molecules. The weight of the sample after outgassing was estimated to be 93 mg based on this consideration. Adsorption Isotherm Simulations. Cu-BTC was modeled as a rigid framework, with the atoms held fixed in their experimentally determined crystallographic positions.19,21 Cu-BTC has a cubic lattice with a unit cell dimension of 26.343 Å. The structure of Cu-BTC was taken from Chui et al.9 The crystal structure of Chui
(K)
σ (Å)
70.27 15.315 45.49 53.12 119.8
3.44 3.28 3 3.28 3.4
et al. includes axial oxygen atoms bonded to the Cu atoms, which correspond to water ligands. We simulated dry Cu-BTC with these oxygen atoms removed.19,21 To model Ar, we used a spherical Lennard-Jones potential; the values of the parameters used (which are the same as those used previously)30 are listed in Table 1. Interactions between adsorbed Ar molecules and Cu-BTC were modeled using pairwise interactions between adsorbates and each atom in the metal-organic framework. Only Lennard-Jones interactions were considered for these interactions. The parameters were taken from the UFF potential.32 Mixedatom interactions were defined using the Lorenz-Berthelot mixing rules. In the simulations, a cutoff distance of 17 Å was used for the Lennard-Jones interactions. Long-range corrections were included in the adsorbate-MOF interactions by assuming that the MOF was isotropic at distances beyond the cutoff. This application of longrange corrections inside what is an intrinsically structured material is of course an approximation, a point discussed carefully by Macedonia and Maginn.33 The interactions with Cu-BTC were pretabulated on a 0.2 Å grid. During the simulations, a 3-D cubic Hermite polynomial interpolation scheme was used to calculate the potential at each point in space.30 Adsorption isotherms of Ar were computed using GCMC simulations.34,35 The chemical potential of the bulk gas was related to its pressure by the virial equation of state, which was fitted to experimental data by NIST.36 A minimum 2 × 2 × 2 unit cell simulation box was used. The adequacy of the super cell size was tested, and a 2 × 2 × 2 super cell was found to be sufficient for the whole pressure range considered in this study. At the lowest densities, the size of the simulation volume was increased so as to contain at least 50 particles during the simulations. At least 2 million equilibration and 5 million production steps were used for each loading. At the highest loadings, as many as 10 million equilibration and 25 million production steps were used. In addition to calculating the equilibrium pore loading at each set of bulk phase conditions we considered, we also computed isosteric heats of adsorption, Qst. Assuming ideal behavior in the bulk phase, the isosteric heat can be determined in GCMC simulations in terms of fluctuations in the internal energy and number of adsorbed molecules.37,38
Results and Discussion In one of the earlier studies of Cu-BTC,19 Ar isotherm experiments were conducted at 87 K, and the GCMC technique was used to simulate several possible adsorption isotherms employing four different force fields to describe adsorbent and adsorbate interactions. Three different substeps were present in the simulated data. The first, low coverage, substep was (32) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024-10035. (33) Macedonia, M. D.; Maginn, E. J. Mol. Phys. 1999, 96, 1375-1390. (34) Frenkel, D.; Smit, B. Understanding Molecular Simulation: From Algorithms to Applications; Academic Press: London, 1996. (35) Heuchel, M.; Snurr, R. Q.; Buss, E. Langmuir 1997, 13, 6795-6804. (36) Lemmon, E. W.; McLinden, M. O.; Friend, D. G. Thermophysical Properties of Fluid Systems. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, 2005; p 20899 (http:// webbook.nist.gov). (37) Nicholson, D.; Parsonage, G. Computer Simulation and the Statistical Mechanics of Adsorption; Academic Press: New York, 1982. (38) Karavias, F.; Myers, A. L. Langmuir 1991, 7, 3118-3126.
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Figure 2. Experimental argon adsorption isotherms (black symbols) on the Cu-BTC sample and simulated adsorptin isotherm (red symbols) for the same sample at 87 K.
attributed to adsorption in the tetrahedral side pockets, the second substep to adsorption in the main channels, and the third one to the solidification of argon inside the main pores. The experimental isotherm measured on Cu-BTC only displayed the first two substeps, not the third. We measured Ar isotherms at eight different temperatures, from 66 up to 143 K. Unlike the previous experiments, we were able to observe all three substeps in the experimental data (see Figure 2). The first substep (A in Figure 2) occurs at around 24 cm3 STP/g. This step is attributed to adsorption in the tetrahedral side pockets of the Cu-BTC metal-organic framework. These pockets have ∼3.5 Å diameter windows that are large enough to let the argon atoms through and be adsorbed inside. This feature was also observed in previous theoretical and experimental work done on this system.19 The second substep (B in Figure 2) starts at around 55 cm3 STP/g and ends at around 180 cm3 STP/g. It is attributed to adsorption in the main channels. The third substep (C in Figure 2) occurs around 215 cm3 STP/g. This feature was attributed in the previous computer simulation studies19 to the solidification of argon in the pores of the metalorganic framework. While a feature that corresponds to this behavior was observed in our data (see C in Figure 2), we note that no such feature was observed in the previous experimental measurements.19 Our simulations show that between the second and the third substeps, the majority of the empty space is filled with argon molecules. It is possible that an ordering of the absorbed molecules or a complete saturation of the center regions of the large cages lead to the third observed step. However, the order parameter has not been calculated, and the origin of the third substep was not analyzed in terms of molecular configuration. We have computed the effective specific surface area (ESSA) for this sample from the isotherm measured at 78.5 K. We obtained 886 m2/g for this quantity; this value is 59% of the 1500 m2/g measured by Vishnyakov et al. using argon19 but is in good agreement with the value of 917.6 m2/g determined by Chui et al.9 using nitrogen. At least a portion of the difference in the ESSA values can be attributed to the different values used for the cross-sectional area of argon. We also obtained the total pore volume for this sample. We used a density of 1378.5 kg/m3 for Ar at 90.03 K.36 We obtained a total pore volume of 0.32 cm3/g. This is value is 86% of the 0.37 cm3/g reported by Vishnyakov et al.,19 80% of the 0.40
KrungleViciute et al.
Figure 3. Logarithm of pressure vs inverse of the isotherm temperature for different amounts of argon adsorbed on the substrate. The slope of the line is proportional to the isosteric heat of adsorption for each amount adsorbed.
Figure 4. Experimental and theoretical isosteric heat data. Arrows that point at three peaks correspond to the steps A-C, also pointed to by the arrows, in Figure 2.
cm3/g reported by Lee et al.,6 and in excellent agreement with the 0.333 cm3/g determined by Chui et al.9 using nitrogen isotherms. We note that all of the previous quoted results, including our own, are significantly lower than the total pore volume found by Wang et al., who reported a volume of 0.658 cm3/g for this material.24 (They note that the level of microporosity depends on the activation conditions of the sample.) We carried out GCMC simulations for argon adsorbed on Cu-BTC as described previously. The results from the calculations are shown as the red curve in Figure 2. There are three substeps present in the isotherm simulated at 87 K. The first one occurs at around 4.5 × 10-5 atm, the second at 8.1 × 10-4 atm, and the third at around 5 × 10-3 atm. All of these pressure values are in excellent quantitative agreement with the experimental adsorption isotherms we measured at 90 K. The amount adsorbed in the simulation data was scaled down by a factor of 1.6 to match our experimental isotherm (no adjustment was applied to the pressure values). The origin of this difference is unclear at this time. We note that this factor is within the range of the ratio of experimentally reported values for the pore volume.6,9,19,24 One of the important quantities that can be obtained from adsorption isotherm results is the isosteric heat of adsorption
Argon Adsorption on Cu3(BTC)2(H2O)3
Langmuir, Vol. 23, No. 6, 2007 3109
that, in terms of isotherm data, is given by eq 1 (ref 39)
ln P (∂∂1/T )
qst ) -kB
n
(1)
We have determined the isosteric heat of adsorption for argon adsorbed on Cu-BTC using this expression. In Figure 3, we present a plot of the logarithm of the pressure versus the inverse of the temperature. Each line represents data taken at the same amount adsorbed on the substrate, for the various temperatures measured. The slope of this line is proportional to the isosteric heat of adsorption corresponding to that value of the amount adsorbed (see eq 1). In Figure 4, we show both computer simulated and experimental results for the isosteric heat’s dependence on the total amount of argon adsorbed (the simulated coverages have been scaled down, as they were in Figure 2). The isosteric heat decreases significantly as the amount adsorbed increases. At the lower coverages, where the isotherms indicate that there is adsorption in the side pockets, the experimental qst value is around 145 meV. At coverages corresponding to the second substep, the experimental value of qst is around 112 meV. Finally, for coverages corresponding to the third substep, which in the theoretical calculations was attributed to the solidification of argon, we see a sharp peak in the isosteric heat. This peak is consistent with capillary condensation occurring in the pores (our simulations do not allow us to definitively identify this feature with a solidification transition, as was done in previous simulations).19 From the GCMC simulations, we found Ar isosteric heat values of 136 and 110 meV for the tetrahedral side pockets and main pores, respectively. Using the GCMC technique, Vishnyakov et al.19 obtained a value of 150.8 meV for the depth of the potential on the side pockets (corresponding to the first substep). The simulation values are very close to the values of 145 and 112 meV that we obtained from our experimental measurements. (39) Dash, J. G. Films on Solid Surfaces; Academic Press: New York, 1975.
To obtain an idea of the strength of the binding of argon on Cu-BTC, we calculated the binding energy of argon on the Cu-BTC sample using eq 2 (ref 40)
) qst - 2kT
(2)
We obtained a value of 124 meV for the binding energy. As a comparison, the value of for argon on the grooves of single walled carbon nanotubes (i.e., on the highest binding energy sites of closed-ended nanotubes) is 163 meV,41 and the value of for Ar on planar graphite is 99 meV.42 Since the value of the binding energy of Ar on Cu-BTC is smaller than that of Ar on close-ended nanotubes, and since both a large adsorptive capacity and a high binding energy sites are needed for an effective gas storage material, it is unlikely that Cu-BTC will be able to fulfill the practical requirements that an effective storage material must meet.
Conclusion We have studied the adsorption of argon on a Cu-BTC metalorganic framework using experimental and theoretical techniques. We found three substeps present in the sorption isotherms. The first two correspond to the adsorption in the tetrahedron side pockets and main channels. The third substep present in the experimental data (that was not found by Vishnyakov et al.) and also in GCMC simulations corresponds to capillary condensation. The values obtained in the simulations for the isosteric heat of adsorption and its dependence on adsorbent loading are in very good agreement with the experimental values we measure. Acknowledgment. J.Y.L. and J.L. acknowledge the donors of The Petroleum Research Fund administrated by the ACS for the partial support of this work. LA061871A (40) Wilson, T.; Tyburski, A.; DePies, M. R.; Vilches, O. E.; Becquet, D.; Bienfait, M. J. Low Temp. Phys. 2002, 126, 403-408. (41) Krungleviciute, V.; Heroux, L.; Migone, A. D.; Kingston, C. T.; Simard, B. J. Phys. Chem. B 2005, 109, 9317-9320. (42) Vidali, G.; Ihm, G.; Kim, H.-Y.; Cole, M. W. Surf. Sci. Rep. 1991, 12, 133-181.
