22987
2006, 110, 22987-22990 Published on Web 10/31/2006
Grand Canonical Monte Carlo Simulation Study on the Catenation Effect on Hydrogen Adsorption onto the Interpenetrating Metal-Organic Frameworks Dong Hyun Jung,† Daejin Kim,† Tae Bum Lee,† Sang Beom Choi,‡ Ji Hye Yoon,‡ Jaheon Kim,‡ Kihang Choi,§ and Seung-Hoon Choi*,† Insilicotech Co. Ltd., A-1101 Kolontripolis, 210, Geumgok-Dong, Bundang-Gu, Seongnam-Shi, 463-943, Republic of Korea, Department of Chemistry, Soongsil UniVersity, 1-1, Sangdo-5-Dong, Dongjak-Gu, Seoul 156-743, Republic of Korea, and Department of Chemistry, Korea UniVersity, 1, Anam-dong 5-Ga, Seongbuk-Gu, Seoul, 136-701, Republic of Korea ReceiVed: September 7, 2006; In Final Form: October 9, 2006
Among recently synthesized isoreticular metal-organic frameworks (IRMOFs), interpenetrating IRMOFs show high hydrogen adsorption capacities at low temperature and under ambient pressure. However, little is known about the molecular basis of their hydrogen binding properties. In this work, we performed grand canonical Monte Carlo (GCMC) simulations to investigate the effect of catenation on the interactions between hydrogen molecules and IRMOFs. We identified the adsorption sites and analyzed the adsorption energy distributions. The simulation results show that the small pores generated by catenation can play a role to confine the hydrogen molecules more densely, so that the capacity of the interpenetrating IRMOFs could be higher than that of the non-interpenetrating IRMOFs.
Hydrogen is widely recognized as one of the most promising fuels of the future.1 There are, however, many problems to be solved before hydrogen can serve as a main energy carrier, and these include its difficulties in storage and transportation. Metal-organic frameworks (MOFs) have attracted many researchers’ interest as potential carriers of hydrogen because this new class of materials has surface areas and gas adsorption capacities much larger than other microporous materials such as zeolites and carbon nanotubes.2 Since MOFs consist of threedimensional networks of metal clusters and organic linkers, simple variations of the linker as well as the metal parts can provide a series of new materials with modified structures. Among the isoreticular (having the same underlying topology) MOFs (IRMOFs) synthesized and tested to date,3-7 interpenetrating IRMOF-11 and -13 have the highest affinities for molecular hydrogen indicated by the largest values of the initial slope of the low pressure adsorption isotherm.3-5 Interpenetrating IRMOFs are composed of two IRMOF frameworks mutually interpenetrated as shown in Figure 1 and, unlike noninterpenetrating ones, the dimension of free volume is considerably restricted and additional pores with various sizes are generated between the frameworks. Although many theoretical studies have been carried out to understand the interactions between hydrogen and MOFs in detail, most of these efforts have been directed toward non-interpenetrating IRMOFs and little attention has been paid to the interpenetrating structure. The previous calculation studies,8-16 for instance, have shown that both the zinc oxo clusters and the organic linker sites are * Corresponding author. E-mail:
[email protected]. Telephone: +82-31-728-0441. Fax: +82-31-728-0444. † Insilicotech Co. Ltd. ‡ Soongsil University. § Korea University.
10.1021/jp065819z CCC: $33.50
Figure 1. Framework of interpenetrating IRMOFs shown schematically with two catenated cubes only for clarity. Secondary building units are drawn as octahedra defined by six carboxylate carbon atoms (gray balls) of the organic linkers.
the important positions of hydrogen adsorption in noninterpenetrating IRMOFs, but little is known about the adsorption sites and the binding energies of the interpenetrating structure. In this work, we would like to report the first molecular simulation studies assessing the effects of catenation on the hydrogen adsorption properties of IRMOFs. For our simulation study, interpenetrating IRMOF-9, -11, and -13 have been selected so that their properties are compared with those of the non-interpenetrating counterparts, IRMOF10, -12, and -14, respectively. All the GCMC simulations were performed by the Sorption program in the MS modeling 4.0 package.17 The initial framework structures of the IRMOFs were taken from the reported crystal structures2-3 and then fixed during the simulations. The cutoff radius was chosen as 12.5 Å for the Lennard-Jones 6-12 potential, and the spline method was applied to make the potential function smoothly converge to zero at the cutoff radius. The initial 2 × 106 steps were used to equilibrate the systems and the following 2 × 106 steps were used for the product simulation. All the simulations were carried © 2006 American Chemical Society
22988 J. Phys. Chem. B, Vol. 110, No. 46, 2006 out at temperature 77 K and in the pressure range of 0∼100 kPa. For the hydrogen molecules, we adopted the two-site Lennard-Jones potential model suggested by Yang and Zhong.11 For the modeling of the IRMOFs, we used a set of modified universal forcefield (UFF)18 parameters which had been developed for the study of IRMOF-1 and -18, and then successfully used to explain the adsorption isotherms of IRMOF-3.16As Figures 2a and 2b show, the simulation results are in reasonable agreement with the experimental data for IRMOF-11 and -13.3-4 For IRMOF-9, however, the calculated and actual isotherms showed significant deviation, which is presumably due to the loss of crystallinity of this material upon evacuation.4 Thus, it seemed reasonable to assume that our GCMC simulations can reproduce the experimental data of structurally intact IRMOFs. Nonetheless, it should be pointed out that, although our simulations reproduced the experimental results reasonably well in this work, quadrupole moment or polarizability of hydrogen molecules should be taken into account in the force field in order to manipulate the interactions between H2 and IRMOFs more accurately. Now we can compare the adsorption capacities of the interpenetrating IRMOFs to those of the non-interpenetrating counterparts with the isotherm curves shown in Figures 2a and 2b. Figure 2b reveals that all the interpenetrating IRMOFs considered here have the larger number of adsorbed hydrogen molecules per formula unit, and so the gravimetric capacities are increased as shown in Figure 2a, although the density of the interpenetrating IRMOF is as twice as that of the noninterpenetrating counterpart. In order to explain these changes caused by the catenation, we investigated the catenation effect on the adsorption sites and the binding energies in more detail. Further studies were focused on the IRMOF-11 and IRMOF12 because the experimental results of IRMOF-11 were most exactly reproduced with our force field. In Figure 2c, distributions of the hydrogen binding energy are compared between IRMOF-11 and IRMOF-12. There are two main peaks observed in the distribution curve for IRMOF12 and the interaction energies of these peaks coincide well with the previously reported binding energies.9,16 On the other hand, for IRMOF-11, there are three main peaks; two small peaks are centered around -2.5 and -1.7 kcal/mol and the large broad peak around -1.2 kcal/mol represents the sites where a large amount of hydrogen molecules are adsorbed. These observations indicate that new adsorption sites with the higher binding energies are formed in IRMOF-11 by the catenation. In order to investigate the relationship between the interpenetrating IRMOF structure and the binding energy, we examined the binding sites corresponding to the peaks of energy distribution in Figure 2c. The adsorption sites with the largest binding energy of about 2.5 kcal/mol are located in the very small volume surrounded by two zinc oxo clusters, so we call them the “metal-metal sites”. There have been experimental and theoretical results which prove that the zinc oxo clusters are the most favorable adsorption sites for gases,19-21 so it is not surprising that no other sites have the larger binding energy than the metal-metal sites created by overlap of two zinc oxo clusters. Though the binding energy of these sites is largest among the adsorption sites, the volume is so small (Figure 3a) that few hydrogen molecules can be adsorbed on these sites. Another small region, the “metal-linker sites”, confined by a zinc oxo cluster of one chain and an organic linker of the other framework, corresponds to the adsorption sites with the binding energy ranging 1.6∼2.0 kcal/mol (Figure 3b), and the number of hydrogen molecules adsorbed at these sites accounts
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Figure 2. Adsorption isotherms for IRMOF-9, -10, -11, -12, -13, -14 at 77 K over 0∼100 kPa, in gravimetric units (a) and normalized per Zn4OL3 formula unit (b). The experimental data are drawn in the line style and the simulation data in the dotted line style. The open marks mean the interpenetrating IRMOFs and the filled marks the noninterpenetrating IRMOFs. (c) The interaction energy distribution of IRMOF-11, -12 at 77 K and 100 kPa. The y-axis of the interaction energy distribution means the average number of H2 molecules with each binding energy.
for about 10% of total number of adsorbed molecules. These sites are also thought to be insufficient to adsorb a considerable amount of hydrogen molecules, so the increase of the capacity contributed by these sites may be limited to a small quantity. The new sites with the largest impact on the increase of capacity are made by two linkers which are shown in Figure
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J. Phys. Chem. B, Vol. 110, No. 46, 2006 22989
Figure 3. Interaction energy, Eint field for IRMOF-11 at 77 K and 100 kPa shown as colored dots for (a) Eint < -2.0 kcal/mol, (b) Eint < -1.6 kcal/mol, and (c) Eint < -1.2 kcal/mol. Zn atoms are shown as tetrahedra, and C and O atoms as sticks.
Figure 5. Snapshots of the structures of IRMOF-11 with adsorbed hydrogen molecules at (a) 0.5 kPa, (b) 5 kPa, (c) 50 kPa, and (d) 100 kPa. Zn atoms are shown as tetrahedra, C and O atoms as sticks, and hydrogen atoms of H2 as white balls.
Figure 4. (a) Schematic description of the linker-linker sites in IRMOF-11. Tetrahydropyrene moieties are drawn as space-filling models. (b) Slit model potential for the linker-linker sites. The dotted lines are the van der Waals potentials for two van der Waals plates and the solid line is the “slit” potential.
3c. These “linker-linker sites” have the binding energy of about 1.2 kcal/mol and Figure 2c shows this binding energy is very close to that of the zinc oxo clusters of the non-interpenetrating IRMOF. The relatively large binding energy of the linker-linker sites can be explained using the potential diagrams shown in Figure 4, where the linker-linker sites are depicted as a slit confined by two van der Waals plates. As the two plates are moved closer to each other, the potential well becomes deeper. The adsorption characteristics of the micropores with slit-like structure have been studied using van der Waals potential walls and atoms, and the enhancement of adsorption energy could be achieved when the slit width was sufficiently reduced to result in the overlap of two van der Waals potentials.22 Because of this effect, the binding energy is enlarged at the linker-linker sites. However, the peak around 1.2 kcal/mol is very broad and many different adsorption sites contribute to the peak, including the cup sites, ZnO3 sites, ZnO2 sites, and hex sites observed in the non-interpenetrating IRMOFs.20 In order to investigate the sequence of adsorption, snapshots are taken at various pressures as shown in Figure 5. Hydrogen molecules are first adsorbed on the metal-metal sites and the metal-linker sites at very low pressure. We did not observe the preference of the metal-metal sites to the metal-linker sites at the first stage of adsorption, and it may be because, as we mentioned above, the metal-metal sites are too narrow to accommodate many hydrogen molecules. As the pressure increases, hydrogen molecules begin to be adsorbed on the linker-linker sites and the zinc oxo clusters, and finally on the
hex sites as well. This sequence of adsorption confirms the binding features of the new adsorption sites discussed above. In conclusion, we performed GCMC simulation of hydrogen adsorption on the interpenetrating and non-interpenetrating IRMOFs to investigate the catenation effect on the hydrogen capacities of MOFs. Although the metal-metal sites are associated with the largest binding energy, the linker-linker sites play the most important role of increasing the hydrogen capacity of the interpenetrating IRMOFs at low temperature (77 K) and pressure (
