Hydrogen Adsorption Sites in Zeolite Imidazolate Frameworks ZIF-8

Jun 10, 2010 - However, storage of hydrogen is still one of the main bottlenecks for the realization of an energy economy based on hydrogen. Many mate...
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J. Phys. Chem. C 2010, 114, 13381–13384

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Hydrogen Adsorption Sites in Zeolite Imidazolate Frameworks ZIF-8 and ZIF-11 Bassem Assfour, Stefano Leoni, and Gotthard Seifert* Physikalische Chemie, Bergstrasse 66b, D-01062 Dresden, Germany ReceiVed: March 4, 2010; ReVised Manuscript ReceiVed: April 22, 2010

The primary adsorption sites in two representative zeolite imidazolate frameworks (ZIF), ZIF-8 and ZIF-11, have been identified by molecular dynamics (MD) techniques. MD simulations reveal two symmetryindependent adsorption sites in ZIF-8. The first adsorption site is located above the imidazolate ring, in proximity of the CdC bond. The second one is in the pore channel. In ZIF-11, an additional adsorption site located on top of the benzene ring is identified. Introduction Hydrogen is an appealing energy carrier for clean energy use. However, storage of hydrogen is still one of the main bottlenecks for the realization of an energy economy based on hydrogen. Many materials have been synthesized and studied, with the aim to store practical amounts of hydrogen.1 Nonetheless, none of them could meet the U.S. DOE target values. Recently, zeolitic imidazolate frameworks (ZIF) have been synthesized.2 The ZIF crystal structures are based on aluminosilicate zeolite nets, whereby the tetrahedral Si(Al) sites are replaced by transition metals M (M ) Zn, Co, In) tetrahedrally coordinated by imidazolate ligands (Scheme 1). Like metal-organic framework (MOF) materials, ZIFs exhibit high porosity and chemical functionality, with the advantages of an exceptional chemical stability and a large structural diversity.3 The combination of these features makes ZIFs promising candidates for hydrogen storage applications. Unlike MOFs, only few experimental studies concerning the hydrogen storage in ZIFs have been reported.2,4-6 Yaghi et al.2 found that both ZIF-8 and ZIF-11 show reversible hydrogen sorption behavior. The hydrogen uptake of ZIF-8 (12.9 mg/g) is similar to that of ZIF-11(13.7 mg/g) at 77 K and 1 atm. Yildirim et al.5 studied the adsorption of hydrogen in ZIF-8 using neutron powder diffraction. In their work, the imidazolate organic linker represents the primary adsorption site for hydrogen. ZIF-8 was found to hold up to 4.2 wt % of hydrogen at low temperature. In order to provide guidelines not only to improve the performance of known materials but also to design novel materials with superior hydrogen storage capacities, an understanding of the H2 adsorption in ZIF in terms of energetics, mechanisms, and specificities of hydrogen absorption is mandatory. The number of adsorption sites as well as the strength of adsorption are essential prerequisites for hydrogen storage in porous materials because they determine the storage capacity and the operational conditions. Therefore, detailed information regarding the sites at which H2 can be adsorbed and on the necessary interaction energies is needed. Wang et al.7 used a refined OPLS all-atom (OPLS-AA)8 force field model for grand canonical Monte Carlo simulations to investigate the adsorption sites using computer tomography (mCT) techniques. Therein, a combination of the Dreiding9 and the OPLS-AA empirical * To whom correspondence should [email protected].

be

addressed.

force fields was used to obtain terms for zinc and ligands to H2 interactions in ZIF-8. In this paper, we investigate the H2 adsorption sites in ZIFs using molecular dynamics (MD) simulations. In order to achieve a better understanding of the adsorption properties of hydrogen in such nanoscale materials, we performed calculations using a density functional theory based method (DFTB).10 For proper consideration of the weak adsorption interactions, the dispersioncorrected extension of this method was used.11 This approach consists of the a posteriori addition of a van der Waals term to standard DFTB. This, for example, provides a much better description of the interaction between H2 and polyaromatic compounds, comparable with MP2 and higher-level ab initio methods, but at a much lower computational effort. In contrast to empirical force fields, a quantum mechanical treatment allows an unbiased description of ZIF framework structures. Additionally, the dispersion correction is crucial for a proper treatment of the interaction of hydrogen with ZIF networks. To investigate the effect of varying the organic linkers on the adsorption properties of ZIFs, we consider as representatives two materials ZIF-8 and ZIF-11 (Figure 1), which have the same metal (Zn) but a different organic linker around which hydrogen adsorption sites may be populated. The adsorption energies of the resolved adsorption sites were calculated and compared to non-ZIF MOFs (see section Comparison of ZIFs with MOFs). In the following, non-ZIF MOFs will be referred to as MOFs, for simplicity. Computational Methodology Molecular dynamics (MD) is an ideal method to harvest adsorption sites of guest molecules inside of host materials.12-14 ZIF-8 and ZIF-11 are relatively large systems, their unit cells containing ∼300 and ∼1400 atoms, respectively. Performing MD simulation for such large systems using standard DFT methods for long enough simulation times is prohibitively expensive. The density functional based tight binding method SCHEME 1: Imidazolate-Type Linkers in ZIFs

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10.1021/jp101958p  2010 American Chemical Society Published on Web 06/10/2010

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Figure 1. Crystal structures of the ZIFs considered in the study, (a) ZIF-8 and (b) ZIF-11. The topology of the framework is made explicit by a P minimal surface (pink/gray surface). Zinc, carbon, nitrogen, and hydrogen atoms are shown in pink, gray, blue, and white colors, respectively.

Figure 2. The main different adsorption sites of H2 on fragments of ZIF identified by our MD simulations. (a) Relative position of two adsorbed H2 molecules (site 1 and site 2) on a ZIF-8 fragment. (b) Distances from site 1 to the selected ZIF atoms, side view. (c) Distances from site 2 to selected ZIF framework atoms, top view. The H2 molecule is indicated with HH. Zinc, carbon, nitrogen, and hydrogen atoms are shown in pink, gray, blue, and white colors, respectively.

(DFTB) is known as a fast and effective quantum mechanical method10,11 which allows performing molecular dynamics simulations of systems with up to several thousands of atoms in the picosecond range, without requirement of exceedingly long computational time and significantly large computing resources.15-18 We performed molecular dynamics simulations (MD) using the dispersion-corrected density functional based tight binding theory (DC-DFTB),10,11 as implemented in the deMon program package.19 After optimizing the initial ZIF structures, up to 20 and 100 H2 (corresponding to 1.45 and 1.38 wt %) molecules were added randomly to the unit cells of ZIF-8 and ZIF-11, respectively. Initially, each structure was equilibrated for 4 ps at 100 K. Subsequently, the equations of motion were integrated for 16 ps using a 0.25 fs time step. Starting at 100 K, the temperature was gradually decreased by simulated annealing (24 K/4 ps) down to 5 K. The MD trajectories were collected in the NVT ensemble using the Berendsen thermostat. Such a procedure allows determination of the preferred adsorption sites in a rather unbiased manner. The H2 molecules move freely between different sites, until, upon decreasing temperature, they are trapped at the strongest adsorption sites. In a subsequent step, the orientations of the H2 molecules at their adsorption sites were optimized. The H2 adsorption energy at each of the resolved adsorption sites was calculated from the energy difference between the relaxed structure with adsorbed hydrogen (E(ZIF + H2)) and

the relaxed empty structure (E(ZIF)) corrected by the energy of the free hydrogen molecule for the same volume (E(H2)). That is, the adsorption energy was calculated using the formula

EB ) E(ZIF + H2) - E(ZIF) - E(H2)

(1)

Results and Discussion Adsorption Sites of H2 in ZIF-8 and ZIF-11 and Interaction Energies. The unit cells of ZIF-8 and ZIF-11 were fully relaxed in all positional and lattice parameters using a conjugated gradient scheme. The optimized structure of empty ZIF-8 is cubic, as reported in the literature.2 The calculated lattice parameter (a ) 16.97 Å) compares very well with that reported from experiment (a ) 16.999 Å). As for ZIF-8, the lattice parameter of the optimized structure of ZIF-11 (a ) 28.83 Å) is in good agreement with that reported from the experiment (a ) 28.7595 Å).2 Both ZIF-8 and ZIF-11 show negligible structural changes resulting from gas adsorption. The results of the MD simulation of ZIF-8 loaded with molecular hydrogen are shown in Figure 2. Two adsorption sites with the highest adsorption energy were identified. The first adsorption site is located on top of the imidazolate ring (organic linker) over the CdC bond (IM site) with an adsorption energy of 8.6 kJ/mol. The second adsorption site is located at the center of the channel of the Zn hexagon

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Figure 3. Four principal H2 adsorption sites at ZIF-11 identified by MD simulations, (a) on top of the IM ring, (b) on top of the benzene ring, and at the center of the channel of the (c) Zn pentagon and (d) Zn hexagon. The colors are like those in Figure 2.

(channel site) with an adsorption energy of 6.2 kJ/mol. The average adsorption energy for 20 H2 molecules amounts to 6.9 kJ/mol. Comparing our findings with the favorable adsorption positions found by neutron diffraction reported by Yildirim et al.,5 as well as that found by the mCT technique reported by Wang et al.,7 yields an excellent agreement. Both of them reported that IM and channel sites are the preferential adsorption sites for H2. Yildirim et al.5 considered a primitive unit cell of ZIF-8 together with the local density approximation (LDA) to DFT to calculate the adsorption energies of the obtained adsorption sites from neutron powder diffraction. They report adsorption energies for H2 in IM and channel sites of 16.40 and 14.18 kJ/ mol, respectively. Our calculated average adsorption energy of 6.9 kJ/mol is considerably closer to the value of 4.5 kJ/mol estimated from experimental data,6 thanks to the explicit inclusion of weak interactions within the DC-DFTB scheme. For ZIF-11, our results demonstrate that H2 molecules are preferentially adsorbed at four different adsorption sites (Figure 3a-d). Two of these are sites on the benzimidazolate (PhIM) ring (the organic linker). The first one is located on the imidazolate part of the PhIM ring (Figure 3a) with an adsorption energy of 13.07 kJ/mol. The other site (Figure 3b) is located on top of the benzene ring with an adsorption energy of 9.86 kJ/mol. The remaining two adsorption sites (Figure 3c,d) are at the center of the channel of the Zn pentagon and Zn hexagon (channel sites), with adsorption energies of 13.03 and 5.93 kJ/ mol, respectively. The average adsorption energy for 100 H2 molecules was found to be 10.23 kJ/mol. We note that the H2 adsorption energies found in ZIF-11 are higher than those in ZIF-8. This is in line with the experimental finding of Yaghi et al.,2 where the initial H2 uptake of ZIF-11 was much higher than that of ZIF-8 since the hydrogen uptake correlates mainly with the adsorption energy at low loading.20,21 The cavities available for H2 adsorption are better at fitting the shape of the hydrogen molecules, resulting in shorter distances from the framework (Figures 2 and 3), an enhanced number of neighbors, and thus higher adsorption energies. An estimation of the hydrogen adsorption uptake at very low pressure and temperature can be obtained by assuming full

occupation of all found adsorption sites together with the symmetry multiplicity of each adsorption site. Following this, we found that the maximum hydrogen adsorption uptakes amounts to 4.69 and 1.81 wt % for ZIF-8 and ZIF-11, respectively. Yaghi et al.2 reported ∼3.1 wt %, an excess adsorption capacity for ZIF-8 at 77 K and 80 bar, and 1.35 wt % for ZIF-11 at 77 K and 1 atm. In refs 5 and 6, the maximum total adsorption capacity of ZIF-8 is measured to be 4.2-4.4 wt % at low temperature and pressure. These values compare well with our results. The reason for the slightly higher maximum capacity obtained from the calculations is the assumption that each site is fully occupied. Since simultaneous occupation is expected to be hinderead by steric effects, our results represent upper hydrogen uptake estimates. Comparison of ZIFs with MOFs. A subset of the thousands of MOF structures which have been synthesized and studied in recent years shows remarkable H2 storage properties, such as MOF-177, where a saturation H2 uptake of 7.5 wt % is reached at 80 bar.22 With respect to ZIFs, MOFs show better H2 uptake at high pressure. However, in contrast to MOFs, where the hydrogen molecules first fill the adsorption sites near the metal cluster (ZnO), then the adsorption sites over the organic linkers, and finally the adsorption sites in the pore channels, no adsorption site near the ZnN clusters in ZIFs was observed. Instead, the preferential adsorption sites are around the organic linkers. This indicates that the metal in ZIF-type materials plays a minor role in determining the H2 storage capacity. An improvement of H2 capacity of ZIF materials can be achieved by substituting the metal in the metal cluster with another, lighter element (such as B or Li)23-25 while keeping all of the important properties such as high surface area, low density, and rigidity of the structure. The results also suggest changing the organic linker, around which more favorable hydrogen sorption sites may be generated, with longer organic linkers such as hexahydroxytriphenylene (HHTP). Apart from comparing the H2 capacities, the calculated adsorption energies of ZIFs are relatively high compared with those of the other MOFs.26,27 Moreover, theoretical studies suggested that the adsorption energy near the organic linker in MOFs can be enhanced by substitution of nitrogen in benzene

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(pyrazine).28 Therefore, one may also think of introducing aromatic rings where more nitrogen atoms are present such as purine or porphine. This is also observed experimentally by Yaghi et al.29 when studying the H2 adsorption in ZIF-20. The initial uptake of ZIF-20 was much higher than that of ZIF-8 and is comparable to that of ZIF-11, and by analyzing the isosteric heat of adsorption, a higher value compared with other porous materials is found. Conclusions Molecular dynamics simulations have been performed to investigate the hydrogen adsorption sites in ZIF-8 and, for the first time, in ZIF-11. Our results demonstrate that the uptake of hydrogen in ZIF-8 and ZIF-11 at low pressure and temperature results in the occupation of two and four different adsorption sites. In contrast to MOFs, the adsorption sites near the organic linkers are the primary adsorption sites. The gravimetric uptakes of ZIFs are found to be smaller than those of some recently synthesize MOFs (MOF-177). However, ZIFs show higher adsorption energies than MOFs. For further increase of the adsorption capacity, we suggest substituting the metal atom in the ZIF structure with a lighter element (such as B, Li), which leads to a lower framework density and consequently to an increase in the gravimetric uptake. The modification of the organic linker, around which more favorable hydrogen sorption sites may be generated, is also expected to improve the storage capacity. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft via the priority program “MetalOrganic Framework”. We are also thankful for the computational support of ZIH Dresden. The authors would like also to thank Dr. Igor Baburin and Dr. Augusto F. Oliveira for useful discussion. References and Notes (1) Zuttel, A. Naturwissenschaften 2004, 91, 157–172. (2) Park, K. S.; Ni, Z.; Cote, A. P.; Choi, J. Y.; Huang, R. D.; UribeRomo, F. J.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10186–10191. (3) Huang, X.-C.; Lin, Y.-Y.; Zhang, J.-P.; Chen, X.-M. Angew. Chem., Int. Ed. 2006, 45, 1557–1559. (4) Hayashi, H.; Cote, A. P.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Nat. Mater. 2007, 6, 501–506.

Assfour et al. (5) Wu, H.; Zhou, W.; Yildirim, T. J. Am. Chem. Soc. 2007, 129, 5314– 5315. (6) Zhou, W.; Wu, H.; Hartman, M. R.; Yildirim, T. J. Phys. Chem. C 2007, 111, 16131–16137. (7) Zhou, M.; Wang, Q.; Zhang, L.; Liu, Y.-C.; Kang, Y. J. Phys. Chem. B 2009, 113, 11049–11053. (8) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem. Soc. 1996, 118, 11225–11236. (9) Mayo, S. L.; Olafson, B. D.; Goddard, W. A. J. Phys. Chem. 1990, 94, 8897–8909. (10) Frauenheim, T.; Seifert, G.; Elstner, M.; Hajnal, Z.; Jungnickel, G.; Porezag, D.; Suhai, S.; Scholz, R. Phys. Status Solidi B 2000, 217, 41–62. (11) Zhechkov, L.; Heine, T.; Patchkovskii, S.; Seifert, G.; Duarte, H. A. J. Chem. Theory Comput. 2005, 1, 841–847. (12) Allen, M. P.; Tildesley, D. T. Computer Simulations of Liquids; Oxford: New York, 1987. (13) Mueller, T.; Ceder, G. J. Phys. Chem. B 2005, 109, 17974–17983. (14) Mulder, F. M.; Dingemans, T. J.; Wagemaker, M.; Kearley, G. J. Chem. Phys. 2005, 317, 113–118. (15) Porezag, D.; Jungnickel, G.; Frauenheim, T.; Seifert, G.; Ayuela, A.; Pederson, M. R. Appl. Phys. A 1997, 64, 321–326. (16) Mandumpal, J.; Gemming, S.; Seifert, G. Chem. Phys. Lett. 2007, 447, 115–120. (17) Porezag, D.; Frauenheim, T.; Kohler, T.; Seifert, G.; Kaschner, R. Phys. ReV. B 1995, 51, 12947–12957. (18) Albertazzi, E.; Domene, C.; Fowler, P. W.; Heine, T.; Seifert, G.; Van Alsenoy, C.; Zerbetto, F. Phys. Chem. Chem. Phys. 1999, 1, 2913– 2918. (19) Ko¨ster, A. M.; Calaminici, P.; Casida, M. E.; Flores-Moreno, R. ; Geudtner, G.; Goursot, A.; Heine, T.; Ipatov, A.; Janetzko, F. J.; del Campo, J. M.; Patchkovskii, S.; Reveles, J. U.; Salahub, D.; Vela, A. deMon2k; Vela, deMon developers, 2006. (20) Assfour, B.; Seifert, G. Int. J. Hydrogen Energy 2009, 34, 8135– 8143. (21) Frost, H.; Duren, T.; Snurr, R. Q. J. Phys. Chem. B 2006, 110, 9565–9570. (22) Wong-Foy, A. G.; Matzger, A. J.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 3494–3495. (23) Wu, T.; Zhang, J.; Zhou, C.; Wang, L.; Bu, X. H.; Feng, P. Y. J. Am. Chem. Soc. 2009, 131, 6111–6113. (24) Wu, T.; Zhang, J.; Bu, X. H.; Feng, P. Y. Chem. Mater. 2009, 21, 3830–3837. (25) Chen, S. M.; Zhang, J.; Wu, T.; Feng, P. Y.; Bu, X. H. Dalton Trans. 2010, 39, 697–699. (26) Dinca, M.; Yu, A. F.; Long, J. R. J. Am. Chem. Soc. 2006, 128, 8904–8913. (27) Rowsell, J. L. C.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 1304– 1315. (28) Samanta, A.; Furuta, T.; Li, J. J. Chem. Phys. 2006, 125, 08471/408471/8. (29) Hayashi, H.; Cote, A. P.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Nat. Mater. 2007, 6, 501–506.

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