Molecular Hydrogen Interaction with IRMOF-1: A Multiscale

Aug 22, 2007 - C 111, 36, 13635-13640 .... Hydrogen Storage in Novel Li-Doped Corrole Metal-Organic ... The Journal of Physical Chemistry C 0 (proofin...
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J. Phys. Chem. C 2007, 111, 13635-13640

13635

Molecular Hydrogen Interaction with IRMOF-1: A Multiscale Theoretical Study E. Klontzas,‡ A. Mavrandonakis,‡,† G.E Froudakis,*,‡ Y. Carissan,§,| and W. Klopper*,†,§ Department of Chemistry, UniVersity of Crete, P.O. Box 2208, 71003 Heraklion, Crete, Greece, Institut fu¨r Nanotechnologie, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany, Lehrstuhl fu¨r Theoretische Chemie, Institut fu¨r Physikalische Chemie, UniVersita¨t Karlsruhe (TH), 76128 Karlsruhe, Germany, and Chirotechnologies UMR 6180 CNRS/UIII Campus St. Je´ roˆ me Case A 62, 13397 Marseille Cedex 20, France ReceiVed: July 11, 2007; In Final Form: July 17, 2007

By means of ab initio quantum chemical techniques, the interaction of molecular hydrogen with the first member of the IRMOF family is explored. Many different models and computational schemes, ranging from second-order Møller-Plesset perturbation theory (MP2) to density functional theory (DFT), have been applied in order to find the best model that can describe the IRMOF-1 cell in an accurate manner against moderate computational cost. The results show that the interaction energies of dihydrogen with the inorganic part of the IRMOF-1 are between 0.13 and 0.74 kcal/mol and can be attributed to dipole-induced dipole forces. Basis-set superposition errors are corrected for by the function counterpoise method. The effect of the corrections is large, almost 50% of the uncorrected interaction energy. Furthermore, the correction may shift the minimum of the potential energy curve toward larger distances. The computational approaches used in this study, allow us to find the true minimum of the potential hypersurface. As a conclusion, both organic and inorganic linkers contribute equally to dihydrogen physisorption.

Introduction Finding alternative energy resources to substitute petroleum has been an important issue for all of humanity during the past decade. The target is to find a fully renewable energy source that is environmental friendly and suitable for automotive applications. One of the most promising solutions that have been proposed is the use of dihydrogen as fuel. Although significant progress has been achieved in this direction, some problems have not yet been solved. The most important is the choice of the tank system, which will store adequate quantities of gas with maximum safety and relatively low cost. The U.S. Department of Energy (DOE) has established a series of hydrogen-storage targets for automotive applications.1 These targets include gravimetric and volumetric densities, refueling rate, system cost, cycle life, and so forth. The 2010 targets for system gravimetric and volumetric densities are 6 wt % and 45 kg H2 m-3. The ultimate 2015 targets are more-demanding at 9 wt % and 81 kg H2 m-3. The reversible adsorption/desorption of H2 should happen in a temperature range from -20 to 50 °C and moderate pressures (max. 100 atm). It should be noted that the system density target includes all of the needed hardware for fuel storage so that the amount of H2 on a sorbent material must be significantly higher than 6 wt % to meet the DOE target. The importance of hydrogen storage has led to an intensive research on the storage abilities of many different classes of materials and nanostructures. These classes include carbon-based materials2 in the form of fullerenes, nanotubes, and other carbon materials, in pure form or doped with heteroatoms or structurally * Correspondingauthors: G.E.F: Fax: +30-2810545001;e-mail: frudakis@ chemistry.uoc.gr. W.K: Fax: +49-721-6083319; e-mail: [email protected]. † Forschungszentrum Karlsruhe. ‡ University of Crete, Heraklion. § Universita ¨ t Karlsruhe (TH). | Campus St. Je ´ roˆme, Marseille.

defected. Another class are the zeolites with different structural types and exchanged metal cations and other zeolitic materials.3 Lately there have been studies on inorganic materials such as boron-nitride4 (BN) and silicon-carbide5 (SiC) nanotubes and some carbides.6 Also, the use of metal hydrides, chemical hydrides, and Prussian Blue7 analogues has been reported for the same purpose.8 Carbon-based materials and zeolites seem to have limited ability to store large amounts of dihydrogen. In contrast, doped carbon-based materials9 seem to have better abilities for hydrogen storage than pure ones. Hydrides can store even larger amounts of hydrogen, but some problems exist concerning the reversibility, reusability, and thermal management issues of these materials.10 A hydrogen molecule can interact with a material in three ways, by physisorption, chemisorption, or dissociative chemisorption.10,11 In the first case, the hydrogen molecule interacts weakly with the surface of the material with binding energies of a few kcal/mol. The interactions are attributed to dispersive and electrostatic forces. To this class, we can assign all of the porous materials that have been studied for hydrogen storage. In the other two cases, the hydrogen molecule interacts strongly with the material with interaction energies of the order of many kcal/mol. The hydrogen molecule binds strongly either in its molecular form or in many cases being dissociated in atomic hydrogen, which forms covalent/ionic bonds with the material. The last is the case for the interaction of dihydrogen with the hydrides. The release of the hydrogen in this case involves the breaking of strong bonds, which requires increased temperatures and might not release all of the chemisorbed hydrogen. As mentioned above, many studies for hydrogen storage in porous materials seem to be very promising. It has been suggested that these materials must incorporate some specific characteristics in their structure. Some of those are high surface area, low weight, accessible pores with diameters of a few angstroms, high interaction energies, fast kinetics for adsorption/

10.1021/jp075420q CCC: $37.00 © 2007 American Chemical Society Published on Web 08/22/2007

13636 J. Phys. Chem. C, Vol. 111, No. 36, 2007 desorption, and full reversibility. Recently, a new family of hybrid inorganic-organic nanoporous materials,12 which belongs to the category of coordination polymer materials, has been proposed for hydrogen-storage applications.13 These new materials are called metal-organic frameworks (MOFs) and the first member of the family that was synthesized by Yaghi et al. has the name IRMOF-1 (or MOF-5). It consists of a threedimensional periodic network with rock salt topology constructed from a primary and secondary building unit (noted as pbu and sbu, respectively). The pbus represent the inorganic part of the framework, and each pbu is connected to six sbus. The pbu consists of [Zn4O]6+, where the O atom (or rather O2anion) is tetrahedrally coordinated to four Zn2+ cations. Generally, the sbus (or organic linkers) represent the organic part of the framework and are built from a dicarboxylate acid. In IRMOF-1, the sbu is the benzenedicarboxylate (BDC) dianion, where each oxygen atom of the carboxylate group is coordinated to different Zn atoms of the same pbu. In this way, every Zn atom of a pbu becomes tetrahedrally coordinated to four O atoms, with a total number of four ZnO4 tetrahedra per pbu. The other members of the IRMOF family arise from the substitution of the benzene ring with larger aromatic molecules or from the substitution of the C and H atoms of the sbu with heteroatoms and functional groups, respectively. H2 storage studies in IRMOF-1 from many scientific groups have shown that the loading of H2 is not greater than 0.5 wt % at room temperature and shows a maximum uptake of ∼5 wt % at 77 K and pressures of 50-60 atm. XRD, neutron scattering, and infrared spectroscopy studies have been conducted in order to determine the strength of the interactions of the hydrogen with the primary and secondary building units of the nanostructure.13-15 As a result of these studies, it has first been proposed that the hydrogen uptake can be attributed mainly to the organic linker (sbu). Later, more specific investigations showed that both building units play an important role. Although binding at the metal corner is stronger, these sites are saturated very quickly, and at higher temperatures and pressures there is a broad range of binding sites around the organic linker and the metal corners.16 Three independent primary binding sites on the pbu and two on the sbu have been proposed, denoted as R, β, γ , δ, and  sites, respectively. The first three are located above: (i) the triangular face of the octahedron defined by the carboxylate (C) atoms, (ii) the face of a ZnO4 tetrahedron, and (iii) the site above the edge of a ZnO4 tetrahedron as presented in Figures 1-3. The other two are located (iv) above the C6H4 phenylene face and (v) on the CH2 phenylene edge, respectively. The latter have not been studied here because they have been considered extensively in previous works.16,19 Computational Details Many computational studies at different levels of theory have been performed in order to elucidate the interaction of dihydrogen with the IRMOF-1 framework.16,17 Most of these focused on molecular dynamics and Monte Carlo simulations.18 However, when applying DFT methodology, many different model systems were chosen each time. At the beginning, most of the works concentrated on the organic linker.16 That was the reason that most of the ab initio and DFT studies reported so far have focused on the interaction of dihydrogen with the organic linkers of the MOFs. It was found19 that termination of the sbu with Li+ cations gives a good representation of the effect of the metal unit on the organic sbu. However, this approximation can be

Klontzas et al.

Figure 1. Models of IRMOF-1 used in this study. (gray atoms ) C, red ) O, white ) O, blue ) Zn, and purple ) Li).

Figure 2. Model systems used in riMP2 calculations. (a) Bare metal corner of the IRMOF-1 cluster. (b) H2 in parallel orientation in R site with respect to the plane that is defined by the three nearest Zn atoms. (c) H2 in perpendicular orientation in R site. (d) H2 in perpendicular orientation in the β site with respect to the plane that is defined by the three O atoms. (e) H2 in perpendicular orientation in γ site (with respect to the center of the O-O edge of the ZnO2 triangle).

applied only to the δ and  binding sites. Hence, a proper model system must be developed, in which all five binding sites are included. To determine the kind of interactions of dihydrogen mainly with the pbu, we have performed DFT and MP2 (within the resolution-of-identity approximation, RI) quantum-chemical calculations on the first member of the IRMOF family, IRMOF1, using the TURBOMOLE program package.20 Our main goal was to evaluate possible model systems and to find the best and most-economical one, which can describe the MOF environment. The possible cases were constructed in a top-down approach: (a) In a first approximation, the whole MOF cube is studied. The corner, at which the hydrogen molecule is adsorbed, is saturated with three more organic linkers, thus resulting in a structure of 358 atoms as presented in Figure 1a. Cs symmetry restrictions are applied in order to reduce the computational cost. (b) In a second approximation, one metal cluster along with six organic linkers is studied. To take into account the effect of the remote carboxylate groups, we connected Li+ ions to the

Molecular Hydrogen Interaction with IRMOF-1

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13637 weakly adsorbed on all sites. The interactions between the MOF and the H2 molecule are very weak and dominated by van der Waals forces. DFT is known to lack in describing the dispersion forces and only functionals such as B3LYP, PBE, TPSS, and TPSSh can account for some portion of the dispersion energy. High-level wave-function-based theory such as MP2 is required to obtain reliable results. Then, large basis sets (such as TZVPP) and counterpoise calculations are obligatory, so as to avoid overestimation of the interaction energies. Because of computational costs, the model system for the MP2 calculations was restricted to the corner of the IRMOF-1 cluster, that is, a molecule of Zn4O(HCO2)6 stoichiometry (model e, Figure 1e). All possible orientations of the H2 molecule (either parallel or perpendicular) on all binding sites were considered. Only the R and β sites have been studied with MP2 until now.16a,17c The model system used is presented in Figure 2. To reduce the computational cost, instead of geometry optimizations, the interaction energies of the H2 with all binding sites were calculated by scanning the potential energy surface of the H2 approach toward all binding sites. This scheme has been tested at the DFT level, and the results in both cases follow the same trends. The triple-ζ valence basis supplemented with double polarization functions was applied to all atoms, as well as the appropriate auxiliary-TZVPP basis for the RI approximation. Results and Discussion 1. DFT Calculations. Two different orientations of the hydrogen molecule approaching each binding site have been studied. The two different orientations are denoted as parallel and perpendicular and are depicted in Figures 2 and 3 for each site separately. The binding energy, BE (i.e., the negative of the interaction energy), of the hydrogen molecule is obtained from the equation

BE ) (Estructure + EH2) - E(structure + H2) Figure 3. DFT optimized structures of a H2 molecule adsorbed over the R, β, and γ sites in perpendicular and parallel approach (model b). For clarity, the metal atoms are shown in polyhedral models. * denotes that there is a change in the relative orientation of the hydrogen molecule during the geometry optimization. The initial parallel configuration ended up being perpendicular.

oxygen atoms of the dicarboxylate linker, as presented in Figure 1b. (c) In a third approximation, a metal cluster connected to six carboxylate linkers is studied (Figure 1c). (d) In the next step, a metal cluster and three organic linkers are treated individually (Figure 1d). Furthermore, in this case a QM/MM approach was applied as implemented in the ChemShell program package.21 The QM part comprised of the structure of model D and the MM part of a whole cube cell. (e) Finally in the last model, a bare metal cluster with no linkers (Figure 1e) was considered. In all cases, RI-DFT using the PBE exchange-correlation functional along with the TZVPP basis set is applied (because of computational restrictions, the smaller TZVP basis set is used only in model a). In the QM/MM approach, the QM energies were computed from DFT and the MM energies were computed by applying the universal force field (UFF).22 The interaction energies of the H2 molecule with the MOF cell were computed by performing a potential energy surface scan with respect to various distances and orientations. All interaction energies were corrected by the counterpoise (CP) method for the basis-set superposition error (BSSE). We have examined molecular hydrogen adsorption in every binding site using the TURBOMOLE program package. As already reported above, H2 is very

We have applied the counterpoise correction in all calculated interaction energies to correct for the basis-set superposition error (BSSE). The results of our DFT calculations are presented in Table 1 and Figure 3. The table includes interaction energies before and after (bold numbers) the CP correction in conjunction with the distance of the hydrogen molecule from a selected MOF atom for each binding site (tetrahedrally coordinated O atom for the R site, nearest Zn atom for the β and γ sites). As can be seen in Table 1, model a, which is the most expensive, predicts that the β and γ metal sites are almost equivalent. The R site is less favored by only 0.05 kcal/mol. For the less-expensive computational model, b, the counterpoisecorrected results agree in almost all cases. The only exception is the parallel approach to the γ site. The two models give different binding energies because in model b the initial parallel approach ends up in a perpendicular configuration. The binding of H2 at the R site is almost repulsive for the perpendicular approach, and the adsorption is preferable in the parallel orientation. The main reason for this could be that the minimum of the potential energy surface (PES) may be significantly affected by the counterpoise correction. Indeed, this was verified by performing a scan of the H2 approach in models d and e. The results showed that the minimum was shifted by 0.3 (QM/ MM) and 0.6 (riMP2) Å in these two models. The reason that no binding energy is calculated is due to the fact that the minimum of the uncorrected PES could be in the repulsive region of the corrected PES. Only by performing a counterpoise corrected optimization could one obtain the true minimum of the PES.

13638 J. Phys. Chem. C, Vol. 111, No. 36, 2007

Klontzas et al.

Figure 4. Interaction energies (with and without CP correction) for H2 in the R site (structures b and c of Figure 2).

In the case of model c, we make the same observations as in the previous case, b. Two differences are that the β site has the same binding energy for both H2 orientations, and in the γ site the initial parallel orientation did not change to perpendicular. In the next model (Figure 1d), higher binding energies are computed in all cases. However, after the counterpoise correction, they reduce to 50% of the initial values. The mostpreferable site is the R site with a parallel orientation of H2, followed by the β and γ binding sites with H2 oriented perpendicularly. This change can be addressed to the H termination of the three CO2- groups of this molecular model, which fails to represent the effect of the sbu. The model is incorrect because it is highly polarized but it should not be. This is the reason that large binding energies are computed. We can see that the BE values for the β and γ sites are the same as those calculated for the previous molecular models (0.45-0.48 kcal/mol, CP-corrected). However, model system c cannot be used to describe adsorption in the R site. This problem could perhaps be solved by applying a QM/MM methodology. In that case, energies comparable to the mostexpensive model, a, are computed. This is attributed to the fact that long-range dispersion interactions are included in the Lennard-Jones potential energy terms of the MM part. 2. MP2 Results. All possible orientations of the H2 molecule (either parallel or perpendicular) on all binding sites have been

Figure 5. Interaction energies (with and without CP correction) for H2 in the β site (structure d of Figure 2). The CP correction to the perpendicular approximation has not been calculated in view of the small interaction energy.

taken into account. Only the R and β sites had been studied with MP2 before.16a,17c The model system used is presented in Figure 2 (see also Figure 1e). The results for each binding site are presented in Figures 4-6 and summarized in Table 1. The riMP2 results for the metal corner are in reasonable agreement with the RI-DFT results for the metal corner connected to three organic linkers (model d). At both levels of theory, the R site (cup site) is predicted to be the most-favorable position for H2 adsorption. The BE for the parallel configuration of the H2 molecule is estimated to be 0.7 kcal/mol according to both PBE and MP2. For comparison reasons, an riMP2 geometry optimization has been carried out for the adsorption of H2 on the R site. The interaction energies as predicted from the scans and the geometry optimization are exactly the same. The results are in good agreement with the work of Bordiga et al.,17c but we could not reproduce the value reported in ref 16a. The next-favorable position is the perpendicular orientation of the H2 molecule at the γ site. The BE is estimated to be 0.43 kcal/mol for both levels of theory. In all other cases, the binding energies are below 0.4 kcal/mol. The reason that MP2 predicts the R site to be energetically more favorable is the limitation of the model used. As in model d, the highly negative charge

TABLE 1: Binding Energies (from DFT and MP2 Calculations) of Dihydrogen Interacting with the Metal Sites in Various Binding Sites and Orientationsa model a model b model c model d model d (PES - QM/MM) model e (PES - riMP2)

R site-per

R site-par

β site-per

β site-par

γ site-per

γ site-par

BE/BSSE dist BE/BSSE dist BE/BSSE dist BE/BSSE dist BE/BSSE

0.24/0.05 3.9 0.27/-3.6 0.27/0.07 3.6 0.79/0.06 3.8 0.30/0.19

0.48/0.40 4.3 0.58/0.40 4.2 0.49/0.27 3.9 1.08/0.69 4.1 0.51/0.42

0.54/0.47 3.7 0.58/0.46 3.5 0.61/0.50 3.5 0.86/0.48 3.5 0.55/0.46

0.32/0.26 3.7 0.39/0.27 3.5 0.60/0.51 3.5* 0.93/0.47 3.5* 0.38/0.29

0.53/0.45 3.8 0.60/0.45 3.8 0.57/0.45 3.8 0.88/0.46 4.2 0.60/0.44

0.33/0.29 4.4 0.61/0.45 3.8* 0.31/0.23 4.2 0.95/0.23 3.8* 0.38/0.24

dist BE/BSSE

4.1/4.4 0.91/0.36

4.3/4.5 1.21/0.74

3.6/3.7 0.38 / 0.25

3.7/3.8 0.55/0.32

3.9 / 3.9 0.65/0.43

4.3 / 4.5 0.32/0.13

dist

3.4/4.0

3.6/4.0

3.6/3.8

3.4/3.4

3.8 / 3.8

4.0/4.2

Binding energies are in kcal/mol, distances in Å (with respect to the tetrahedrally coordinated O atom for the R site and the nearest Zn atom for the β and γ sites; values for the parallel orientation are with respect to the center of mass of H2 and toward the closest hydrogen atom for the perpendicular orientation). Values given in boldface denote that they are CP-corrected. * denotes that there is a change in the relative orientation of the hydrogen molecule during the geometry optimization. The initial parallel configuration ended up being perpendicular. a

Molecular Hydrogen Interaction with IRMOF-1

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13639 of the pbu in which the Zn atoms are well-protected and shielded by the surrounding O atoms.

Figure 6. Interaction energies (with and without CP correction) for H2 in the γ site (structure e of Figure 2). The CP correction to the parallel approximation has not been calculated in view of the small interaction energy.

cannot be delocalized over the organic linker and remains on the inorganic part. Conclusions The main conclusions drawn from the scans are that BSSE must always be taken into account and cannot be estimated, as has been proposed previously.16 Counterpoise corrections can reach up to 50% of the binding energy; that is, from an initial value of 0.55 kcal/mol for the parallel orientation at the β site, we get a corrected BE of 0.32 kcal/mol (riMP2 result). Furthermore, the minimum of the potential energy hypersurface may change significantly by virtue of the CP correction. This is most obvious at the R site, in which the minimum is shifted toward longer distances by 0.6 and 0.4 Å for the perpendicular and parallel approaches, respectively. Furthermore, based on the MP2 results, the metal cluster does not play the major role in hydrogen adsorption. This is attributed to the fact that the metal atoms are protected well by the carboxylate groups and hydrogen atoms cannot interact directly with the metal atoms. Better physisorption energies can be obtained in the case of MOFs with open metal sites18f in which hydrogen atoms could approach the metal atoms more efficiently. In addition, both inorganic metal clusters and organic linkers contribute equally to hydrogen physisorption. For the bare metal cluster, the BE in the present work is calculated to be 0.7 kcal/mol at the most, whereas in the work of Hu¨bner et al.19 the interaction of H2 with benzene is estimated to be 1.1 kcal/mol. We conclude from our study that the interaction between H2 and IRMOF-1 can be described satisfactorily by the models represented in Figure 1a-c, and also by a QM/MM approach. Substitution of sbu with hydrogen in order to reduce computational cost should be considered carefully in order to describe the interactions correctly. Independent of the molecular model used, the values of the interaction energies demonstrate that the H2 is weakly physisorbed in the pores of the MOF material. The β and γ binding sites are the most-preferable to be occupied followed by the R site. The interaction of H2 with the binding sites is more likely to be due to dipole-induced dipole interactions between the charged O atoms of the framework and H2 rather than a direct interaction of H2 with the Zn atoms. This is a reasonable result if we take into account the geometry

Acknowledgment. The present study is part of a GreekGerman collaborative linkage grant ‘‘DAAD Programm Griechenland IKYDA 2006”. Financial support for A.M. by the Greek Ministry of Education and European Union through the grant IRAKLITOS is gratefully acknowledged. G.E.F and M.K thank the Ministry of Development (General Secretariat-GSRT) of the Greek government for financial support of this research (IIENA∆ 03E∆ 548). Partial funding by the European Commission DG RTD (FP6 Integrated Project NESSHY, Contract SES6-518271) is gratefully acknowledged by G.E.F. The research of W.K. has been supported by the Deutsche Forschungsgemeinschaft through the Center for Functional Nanostructures (CFN, Project No. C3.3). It has been further supported by a grant from the Ministry of Science, Research and the Arts of Baden-Wurttemberg (Az: 7713.14-300). References and Notes (1) See U.S DOE website, http://www.eere.energy.gov. (2) (a) Liu, C.; Cheng, H. M. J. Phys. D: Appl. Phys. 2005, 38, R231R252. (b) Schimmel, H. G.; Kearley, G. J.; Nijkamp, M. G.; Visser, C. T.; de Jong, K. P.; Mulder, F. M. Chem.sEur. J. 2003, 9, 4764-4770. (c) Hirscher, M.; Panella, B. J. Alloys Compd. 2005, 404-406, 399-401. (3) (a) Weitkamp, J.; Fritz, M.; Ernst, S. Int. J. Hydrogen Energy 1995, 20, 967-970. (b) Langmi, H. W.; Book, D.; Walton, A.; Johnson, S. R.; Al-Mamouri, M. M.; Speight, J. D.; Edwards, P. P.; Harris, I. R.; Anderson, P. A. J. Alloys Compd. 2005, 404-406, 637-642. (c) Torres, F. J.; Civallery, B.; Terentyev, A.; Ugliengo, P.; Pisani, C. J. Phys. Chem. C 2007, 111, 1871-1873. (d) Arean, C. O.; Palomino, G. T.; Garrone, E.; Nachtigallova, D.; Nachtigall, P. J. Phys. Chem. B 2006, 110, 395-402. (e) Nachtigall, P.; Garrone, E.; Palomino, G. T.; Delgado, M. R.; Nachtigallova, D.; Arean, C. O. Phys. Chem. Chem. Phys. 2006, 8, 22862292. (f) Palomino, G. T.; Carayol, M. R. L.; Arean, C. O. J. Mater. Chem. 2006, 16, 2884-2885. (g) Li, Y.; Yang, R. T. J. Phys. Chem. B 2006, 110, 17175-17181. (h) Jhung, S. H.; Kim, H.-K.; Yoon, J. W.; Chang, J.-S. J. Phys. Chem. B 2006, 19, 9371-9374. (i) Torres, F. J.; Vitillo, J. G.; Civallery, B.; Ricchiardi, G.; Zecchina, A. J. Phys. Chem. C 2007, 111, 2505-2513. (4) (a) Sun, Q.; Wang, Q.; Jena, P. Nano Lett. 2005, 5, 1273. (b) Zhou, Z.; Zhao, J.; Chen, Z.; Gao, X.; Yan, T.; Wen, B.; Schleyer, P. R. J. Phys. Chem. B 2006, 110, 13363. (c) Ma, R.; Bando, Y.; Zhu, H.; Sato, T.; Xu C.; Wu, D. J. Am. Chem. Soc. 2002, 124, 7672. (d) Mpourmpakis, G.; Froudakis, G. E. Catal. Today 2007, 120, 341. (5) Mpourmpakis, G.; Froudakis, G. E.; Lithoxoos, G. P.; Samios, J. Nano Lett. 2006, 6, 1581-1583. (6) (a) Gogotsi, Y.; Dash, R. K.; Yushin, G.; Yildirim, T.; Laudisio, G.; Fischer, J. E. J. Am. Chem. Soc. 2005, 127, 16006-16007. (b) Yushin, G.; Dash, R.; Jagiello, J.; Fischer, J. E.; Gogotsi, Y. AdV. Funct. Mater. 2006, 16, 2288-2293. (c) Laudisio, G.; Dash, R. K.; Singer, J. P.; Yushin, G.; Gogotsi, Y.; Fischer, J. E. Langmuir 2006, 22, 8945-8950. (d) Zhao, Y.; Dillon, A. C.; Kim, Y.-H.; Heben, M. J.; Zhang, S. B. Chem. Phys. Lett. 2006, 425, 273-277. (7) (a) Chapman, K.; Southon, P. D.; Weeks, C. L.; Kepert, C. J. Chem. Commun. 2005, 3322-3324. (b) Chapman, K. W.; Chupas, P. J.; Maxey, E. R.; Richardson, J. W. Chem. Commun. 2006, 4013-4015. (c) Kaye, S. S.; Long, J. R. J. Am. Chem. Soc. 2005, 127, 6506-6507. (d) Dinca, M.; Daily, A.; Liu, Y.; Brown, C. M.; Neumann, D. A.; Long, J. R. J. Am. Chem. Soc. 2006, 128, 16876-16883. (e) Culp, J. T.; Matranga, C.; Smith, M.; Bittner, E. W.; Bockrath, B. J. Phys. Chem. B 2006, 110, 8325-8328. (8) (a) Bogdanovic, B.; Brand, R. A.; Marjanovic, A.; Schwickardi, M.; Tolle, J. J. Alloys Compd. 2000, 302, 36. (b) Schuth, F.; Bogdanovic, B.; Felderhoff, M. Chem. Commun. 2004, 2249. (9) (a) Kiran, B.; Kandalam, A. K.; Jena, P. J. Chem. Phys. 2006, 124, 224703. (b) Dag, S.; Ozturk, Y.; Ciraci, S.; Yildirim, T. Phys. ReV. B 2005, 72, 155404. (c) Yildirim, T.; Ciraci, S. Phys. ReV. Lett. 2005, 94, 175501. (d) Zhao, Y.; Kim, Y.-H.; Dilon, A. C.; Heben, M. J.; Zhang, S. B. Phys. ReV. Lett. 2005, 94, 155504. (e) Yildirim, T.; Iniguez, J.; Ciraci, S. Phys. ReV. B 2005, 72, 153403. (f) Kim, Y.-H.; Zhao, Y.; Williamson, A.; Heben, M. J.; Zhang, S. B. Phys. ReV. Lett. 2006, 96, 016102. (g) Lachawiec, A. J., Jr.; Qi, G.; Yang, R. T. Langmuir 2005, 21, 11418-11424. (i) Deng, W.-Q.; Xu, X.; Goddard, W. A. Phys. ReV. Lett. 2004, 92, 166103. (10) Fichtner, M. AdV. Eng. Mater. 2005, 7, 443-455. (11) Lochan, R. C.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2006, 8, 1357-1370.

13640 J. Phys. Chem. C, Vol. 111, No. 36, 2007 (12) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127-1129. (13) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4670-4679. (14) (a) Rowsell, J. L. C.; Spencer, E. C.; Eckert, J.; Howard, J. A. K.; Yaghi, O. M. Science 2005, 309, 1350-1354. (b) Rowsell, J. L. C.; Eckert, J.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 14904. (c) Yildirim, T.; Hartman, M. R. Phys. ReV. Lett. 2005, 95, 215504. (15) Spencer, E. C.; Howard, J. A. K.; McIntyre, G. J.; Rowsell, J. L. C.; Yaghi, O. M. Chem. Commun. 2006, 278-280. (16) (a) Sagara, T.; Klassen, J.; Ganz, E. J. Chem. Phys. 2004, 121, 12543. (b) Sagara, T.; Klassen, J.; Ortony, J.; Ganz, E. J. Chem. Phys. 2005, 123, 014701. (c) Sagara, T.; Klassen, J.; Ortony, J.; Ganz, E. J. Chem. Phys. 2005, 123, 214707. (17) (a) Mulder, F. M.; Dingemans, T. J.; Wagemaker, M.; Kearley, G. J. Chem. Phys. 2005, 317, 113. (b) Mueller, T.; Ceder, G. J. Phys. Chem. B 2005, 109, 17974. (c) Bordiga, S.; Vitillo, J.; Ricchiardi, G.; Regli, L.; Cocina, D.; Zecchina, A.; Arstad, B.; Bjorgen, M.; Hafizovic, J.; Lillerud, K. P. J. Phys. Chem. B 2005, 109, 18237-18242. (d) Buda, C.; Dunietz, B. D. J. Phys. Chem. B 2006, 110, 10479. (e) Samanta, A.; Furuta, T.; Li, J. J. Chem. Phys. 2006, 125, 084714.

Klontzas et al. (18) (a) Vishnyakov, A.; Ravikovitch, P. I.; Neimark, A. V.; Bullow, M.; Wang, Q. M. Nano Lett. 2003, 3, 713. (b) Skoulidas, A. I. J. Am. Chem. Soc. 2004, 126, 1356. (c) Garberoglio, G.; Skoulidas, A. I.; Johnson, J. K. J. Phys. Chem. B 2005, 109, 13094. (d) Yang, Q.; Zhong, C. J. Phys. Chem. B 2005, 109, 11862. (e) Skoulidas, A. I.; Sholl, D. S. J. Phys. Chem. B 2005, 109, 15760. (f) Yang, Q.; Zhong, C. J. Phys. Chem. B 2006, 110, 655. (g) Frost, H.; Du¨ren, T.; Snurr, R. Q. J. Phys. Chem. B 2006, 110, 9565. (h) Jung, D. H.; Kim, D.; Lee, T. B.; Choi, S. B.; Yoon, J. H.; Kim, J.; Choi, K.; Choi, S-H. J. Phys. Chem. B 2006, 110, 22987. (19) Hu¨bner, O.; Glo¨ss, A.; Fichtner, M.; Klopper, W. J. Phys. Chem. A 2004, 108, 3019-3023. (20) Turbomole version 5.8, http://www.turbomole.com. (21) (a) ChemShell, a Computational Chemistry Shell, see www.chemshell.org. (b) Sherwood, P.; de Vries, A. H.; Guest, M. F.; Schreckenbach, G.; Catlow, C. R. A.; French, S. A.; Sokol, A. A.; Bromley, S. T.; Thiel, W.; Turner, A. J.; Billeter, S.; Terstegen, F.; Thiel, S.; Kendrick, J.; Rogers, S. C.; Casci, J.; Watson, M.; King, F.; Karlsen, E.; Sjøvoll, M.; Fahmi, A.; Scha¨fer, A.; Lennartz, Ch. J. Mol. Struct.: THEOCHEM 2003, 632, 1. (22) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024.