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Why Li Doping in MOFs Enhances H2 Storage Capacity? A Multi-scale Theoretical Study A. Mavrandonakis,†,# E. Tylianakis,‡ A. K. Stubos,§ and G. E. Froudakis*,† Department of Chemistry, UniVersity of Crete, P.O. Box 2208, 71003 Heraklion, Crete, Greece, Materials Science and Technology Department, UniVersity of Crete, P.O. Box 2208, 71409 Heraklion, Crete, Greece, and NCSR Demokritos 15310 Aghia ParaskeVi Attikis, Greece ReceiVed: October 22, 2007; In Final Form: February 1, 2008
By means of Density Functional Theory (DFT) and Grand Canonical Monte Carlo (GCMC) computational techniques, the effect of Li doping on the hydrogen storage capability of Metal Organic Frameworks (MOFs) is explored. The Li atom is preferably located over the organic linker. It is found that the Li atoms significantly increase the interaction energy between the hydrogen molecules and the Li-doped organic linker of the MOF, compared to the undoped case. As a result, the GCMC simulations show that the presence of the Li atoms significantly enhances the hydrogen storage capacity, especially under intermediate pressure conditions.
Hydrogen has been recognized as an ideal energy carrier, suitable for substituting the fossil fuels. It is a fully renewable energy source, environmentally friendly and suitable as an automobile fuel. The U.S. Department of Energy (DOE) has set a series of targets for use in automotive applications, such as a 6.0 wt % reversible hydrogen storage by 2010.1 Considerable scientific work is concentrated on metal hydrides,2 carbonbased materials,3 and metal organic frameworks (MOFs).4 Currently, no material can fulfill these targets at ambient temperature and pressure conditions. Metal hydrides can theoretically store sufficient amounts of hydrogen, but their main disadvantages are the irreversible hydrogen release and the need of higher temperatures for the hydrogen unloading.2 A hydrogen molecule interacts with metal hydrides via a chemisorption mechanism.5 The dihydrogen molecule is dissociated in atomic hydrogen and is bonded to the hydride surface. The other possible interaction mechanism is via physisorption.5,6 The hydrogen molecule interacts weakly with the surface of the storage material. The interaction energies lay in the order of few kcal/mol, which is less than in the chemisorption case. The interactions can be attributed to dispersive and electrostatic forces. Although fast loading and unloading of the hydrogen is achieved in physisorption, the main disadvantage is the limited H2 coverage at room temperature. Carbon-based materials, such as fullerenes and nanotubes (CNTs), and MOFs belong to that class of hydrogen storage materials. Particularly, MOF materials are attractive due their simple and economic synthesis, high thermal stability, and significant hydrogen uptake at 77 K.7 The optimal interaction energies should be in the range between physisorption and chemisorption.6 In this case, the hydrogen complexes with the host material could likely survive at room temperature. Furthermore, the desorption could be done with no or with a very small energy barrier. In order to increase the interaction energies, a possible route is the incorporation of * To whom correspondence should be addressed. E-mail: frudakis@ chemistry.uoc.gr. † Department of Chemistry, University of Crete. ‡ Materials Science and Technology Department, University of Crete. § NCSR Demokritos. # Present address: Institut fu ¨ r Nanotechnologie, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany.
TABLE 1: Binding Energies and Average Distances of Li versus the Center of Mass of the Dihydrogen for 1, 2, and 3 Molecules Interacting with Atomic or Li, Doped in the Organic Linker of IRMOF-14 atomic Li 1 H2 2 H2 3 H2
B.E per H2 (kcal/mol)
R (Li - c.om. H2, Å)
-3.22 -3.06 -2.33
2.03 2.13 2.21
light weight metal atoms, such as Li. Following this direction, many theoretical works on Li doping have been published.8-13 Additionally, a recent experimental report states that chemical reduction of a mixed-ligand MOF with Li cations, significantly improves the hydrogen capacity.14 After the chemical reduction, the storage capacity is reported to be 1.63 wt % from an initial value of 0.93 wt % for the pure material. However, no explanation or possible mechanism has been given for this enhancement. A Li cation can bind strongly up to six H2 molecules, with a mean binding energy of -4.77 kcal/mol per hydrogen molecule.8 On the basis of these results, Deng et al. suggested a new alkalidoped pillared carbon graphite or nanotubes storage material.9 They suggest a chemical modification of the graphite or nanotube surface with an organic molecule, so that the interlayer spacing would be enlarged. In this way, Li and H2 molecules can be inserted and adsorbed inside the interlayer space. Similarly, in our previous work, it was proposed that the enlargement of the interlayer spacing of carbon nanoscrolls and the subsequent insertion of Li atoms will increase the storage capacity.10 In a similar context, Sun and co-workers suggested that Li-coated fullerenes could yield an approximate capacity of 9 wt % in H2.11 They showed that 12 Li atoms, which are placed over the 12 pentagonal faces of the buckyball, could totally absorb up to 60 H2 molecules. A similar methodology on CNTs has been proposed by our team.12 An alkali-doped CNT may interact strongly with three dihydrogens, which are absorbed over the alkali atom. Finally, in a recent work by Han et al., it was shown that dispersion of Li atoms over the organic linker of various MOFs can reach the DOE target of 6.0 wt % at nearly room temperature.13
10.1021/jp7102098 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/16/2008
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Figure 1. Optimized geometries of the model system, (A) doped with Li and (B, C, D) with 1, 2 and 3 H2 molecules adsorbed over the Li atom, respectively.
However, a more detailed examination of the effect of Li doping on various organic linkers of the MOFs has not been performed yet. In this work, we examine single and multiple Li doping on various sized organic linkers, in order to determine the maximum amount of Li doping with respect to the number of carbon atoms of the linker. We have studied various linkers, ranging from 6 carbon atoms in the benzene di-carboxylate (BDC), to 16 carbon atoms in the pyrene di-carboxylate (PDC). On top of this, we have calculated the interaction energies of single and multiple adsorbed hydrogen molecules in the Lidoped IRMOF-14, in which the organic linker is composed by PDC. Furthermore, Grand Canonical Monte Carlo simulations were performed in order to get an estimate for the hydrogen storage of this Li-doped-MOF. Computational Details Density Functional Theory (DFT) along with Grand Canonical Monte Carlo calculations have been employed in this study in order to show the effect of the Li presence on the hydrogen storage ability of the doped MOF materials. DFT in the Resolution of Identities (RI) approximation is applied in our systems. The PBE exchange-correlation functional along with the def2-TZVPP basis set are used in the calculations, as well as the corresponding auxiliary basis sets for the RI approximation. All structures are optimized without any symmetry constraints. Furthermore, counterpoise corrections are applied in all calculations, so as to take into account for the basis set superposition error (BSSE). All calculations are performed with the TURBOMOLE program package.15 Due to the large size of the MOFs cell, the model system must be decreased in size, in order not to end to prohibitively large ab initio computations. The MOF cell may be decreased by separating the organic linker, saturating the carboxylate groups with Li ions, and treating this as an individual system. This is a commonly used approximation, which can describe appropriately the effect of the metal cluster over the organic linker.16,17 Grand canonical Monte Carlo (GCMC), (Constant chemical potential, µ, cell volume, V, and temperature, T) simulations were conducted at room temperature and various pressures. The chemical potentials needed in the calculations were calculated from NPT ensemble Monte Carlo simulation using the testparticle insertion method. Periodic boundary conditions were applied in all three dimensions. For each state point, GCMC
simulation consisted of 5 × 106 steps to guarantee equilibration followed by 3 × 106 steps to sample the desired thermodynamic properties. Each simulation cell included 125 MOF unit cells, resulting to a system of total 21 000 atoms. Hydrogen molecules were treated as a diatomic molecule modeled by a LennardJones (LJ) core located at the center of mass and three partial charges with two located at hydrogen atoms and one at the center between two hydrogen atoms. Nonbonded interactions were treated using Lennard-Jones potential, where partial charges of MOFs were taken into consideration by using the quadrupole moment and induced dipole interaction of hydrogen with MOF atoms. The framework was assumed to be rigid in all simulations. A cutoff radius of 15 Å was applied to the LJ interactions. Calculations were carried out assuming room-temperature and various pressures, ranging from 1 to 100 bar for Li doped MOF. For the sake of comparison, similar simulations were conducted for undoped MOF at the same thermodynamic states. Results In the first part of this section, the effect of Li is explored by ab initio quantum mechanical techniques. The maximum degree of Li doping with respect to the number of carbon atoms of the organic linker is studied. As a next step, the interaction energies of one, two, and three H2 molecules with the Li doped MOFs are computed. In the second part, the GCMC simulations provide a clear trend of how the presence of Li atoms affects the hydrogen storage ability under various pressures. First, the structure of the model systems is optimized. The optimized structures of the benzene di-carboxylate (BDC), naphthalene di-carboxylate (NDC), and pyrene di-carboxylate (PDC) are presented in Figure S1 of the Supporting Information. These organic linkers are composed by one, two, and four fused aromatic rings, respectively. Then a single Li atom was placed over the center of the hexagonal aromatic rings. After the geometry optimizations, the Li-doped structures were strongly distorted, except for the PDC case. The distorted structures can be seen in Figure S2 of the Supporting Information. The main reason for these strong distortions is the charge transfer occurring from the Li atom to the organic molecule. As revealed from a Mulliken and natural population analysis, the lithium always gets a positive charge of +1.0 |e|, upon interacting with the organic molecules. The charge-transfer per carbon atom is inversely proportional to the size of the organic linker. That is, an additional charge accumulation of 0.16 |e| per C atom for
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Figure 2. Snapshots from GCMC calculations for Li doped MOF and adsorbed hydrogens at ambient temperature and pressure of 10 MPa.
benzene, 0.08 |e| per C atom for naphthalene and 0.06 |e| per C for pyrene. Additionally, the further doping of the pyrene molecule with one and two more Li atoms has been studied. It was found that double and triple Li doping induces strong distortions on the molecules and consequently only single doping may occur. The reason again in this case is the charge transfer from two/three Li atoms to the organic linker. A charge transfer of 0.12 |e| per C atom has been calculated for two/ three doped Li atoms. On the basis of the above results, we have decided to study the singly doped IRMOF-14 linker. As a second step, the favorable positions for the doped Li are explored. All possible sites have been investigated. Among them, it was found that the most favorable position was on top of the center of the hexagonal ring, as presented in Figure 1. The distance of the Li atom with respect to the center of the hexagonal ring is 1.7 and 1.8 Å for the neutral and charged systems, respectively. The binding energy (BE) for the Li atom over the first hexagon (denoted as 1 in Figure 1A) was calculated to be -17.4 kcal/mol, whereas -16.4 for the second one (denoted as 2). In all cases, the Li atom is positively charged by almost +1 |e|, as revealed after performing a natural population analysis. As a next step, one, two, and three hydrogen molecules were placed over the organic linker. We have chosen to study the linker doped in hexagon (2) since there is more free space for 3 H2 molecules to be adsorbed. Their geometries are optimized without any symmetry constraints and presented in Figure 1. After the optimization, the hydrogen molecules move away from the center of the phenyl rings and come close to the Li. Furthermore, the distance of the H-H bond in the hydrogen
molecules was elongated, from 0.750 to 0.757 Å. This suggests that strong interactions between the dihydrogen and the Li atom exist. The elongation can be explained by the formation of a dative bond between the electrons of the H2 σ bond and the empty Li 2s orbital.18,19 In the presence of a Li cation next to the dihydrogen, electron density is transferred from the H2 molecule. As a result of the loss of electron charge the H-H bond is elongated. The interactions are due to charge-induced dipole and charge-quadrupole moment between the positively charged Li atom and the H2 molecule. In the case of the three adsorbed hydrogen molecules, it has been found that a total of 0.1 |e| has been transferred from the H2 molecules to the Li atom. The binding energy of the first H2 is -3.2 kcal/mol (Table 1). The distance of the Li atom with respect to the center of mass of the dihydrogen is at 2.03 Å. When a second and third dihydrogen are adsorbed, the binding energies per H2 are -3.1 and -2.3 kcal/mol, respectively. After performing the GCMC simulations, the gravimetric hydrogen uptake of Li-doped IRMOF-14 is presented in Figure 3, under different thermodynamic conditions. For the sake of comparison, the corresponding results for the undoped IRMOF14 are presented too. It can be clearly seen that at every thermodynamic state, the Li-doped MOF provides increased hydrogen uptake compared with the undoped. This enhancement is more pronounced at lower pressures, i.e., 1 bar where Li doping increases the adsorption by a factor of greater than 7.5 in relation to the undoped MOF. This can be attributed to the high electronic density of Li ion acting in this way as positive core that attracts hydrogen molecules. Figure 2 presents a snapshot from these calculations, where hydrogen molecules
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Figure 3. Gravimetric Hydrogen uptake of pure (diamonds) and Li doped (squares) IRMOF-14 at ambient temperature and 77 K as a function of pressure. Lines have been added to guide the eye.
are shown as white spheres. From this figure, it seems that hydrogens tend to be adsorbed at positions close to Li sites. Summarizing, Li doping may significantly enhance hydrogen storage in MOFs, as three hydrogen molecules interact very strongly with the Li atom. Li is positively charged by almost +1 |e|. Upon interacting with the H2 molecules, strong polarization effects are observed. Charge distribution of approximately +|0.1|e is transferred from the 3 H2 molecules to the Li atom. This induces very strong dipoles and is the reason for the strong binding. This trend in the interaction energies is also reflected in the hydrogen storage capacity, as computed by GCMC simulations. The Li effect is larger at smaller pressures, in which the storage capacity is 7.5 times larger than the pure IRMOF14 material. As a conclusion, this study suggests that Li doping is a possible way for making MOFs effective as hydrogen storage materials. Acknowledgment. The present research study has been supported by the Ministry of Development (General SecretariatGSRT) (ΠΕΝΕ∆ 2003-03Ε∆ 548). Partial funding by the European Commission DG RTD (FP6 Integrated Project NESSHY, Contract SES6-518271) and Interreg IIIA Gr-Cy K2301.004 is gratefully acknowledged.
Supporting Information Available: The optimized structures of the benzene di-carboxylate (BDC), naphthalene dicarboxylate (NDC), and pyrene di-carboxylate (PDC) (Figure S1), and the distorted structures ( Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) See U.S. DOE website, http://www.eere.energy.gov. (2) Gross, K. J.; Spatz, P.; Zuttel, A.; Schlapbach, L. J. Alloys Comp. 1996, 240, 206. (3) Froudakis, G. E. J. Phys.: Condens. Matter 2002, 14, 453. (4) (a) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keefee, M.; Yaghi, O. M. Science 2003, 300, 1127-1129. (b) Rowsell, J. L. C.; Yaghi, O.M. Angew. Chem., Int. Ed. 2005, 44, 4670-4679. (5) Fichtner, M. AdV. Eng. Mater. 2005, 7, 443-445. (6) Lochan, R. C.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2006, 8, 1357-1370. (7) Rowsell, J. L. C.; Millward, A. R.; Park, K. S.; Yaghi, O. M. J. Am. Chem. Soc. 2004, 126, 5666-5667. (8) Barbatti, M.; Jalbert, G.; Nascimento, M. A. C. J. Chem. Phys. 2001, 114, 2213-2218. (9) Deng, W.-Q.; Xu, X.; Goddard, W. A. Phys. ReV. Lett. 2004, 92, 166103. (10) Mpourmpakis, G.; Tylianakis, E.; Froudakis, G. E. Nano Lett. 2007, 7, 1893-1897.
7294 J. Phys. Chem. C, Vol. 112, No. 18, 2008 (11) Sun, Q.; Jena, P.; Wang, Q.; Marquez, M. J. Am. Chem. Soc. 2006, 128, 9741-9745. (12) Froudakis, G. E. Nano Lett. 2001, 1, 531-533. (13) Han, S. S.; Goddard, W. A. J. Am. Chem. Soc. 2007, 129, 84228423. (14) Mulfort, K. L.; Hupp, J. T. J. Am. Chem. Soc. 2007, 129, 96049605 (15) TURBOMOLE program version 5.8, http://www.turbomole.com
Mavrandonakis et al. (16) Hubner, O.; Gloss, A.; Fichtner, M.; Klopper, W. J. Phys. Chem. A 2004, 108, 3019-3023. (17) Sagara, T.; Klassen, J.; Ortony, J.; Ganz, E. J. Chem. Phys. 2005, 123, 014701. (18) Vitillo, J. G.; Damin, A.; Zecchina, A.; Ricchiardi, G. J. Chem. Phys. 2005, 122, 114311. (19) Davy, R.; Skoumbourdis, E.; Kompachenko, T. Mol. Phys. 1999, 97, 1263-1271.
