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Adsorption Sites of Hydrogen in Zeolitic Imidazolate Frameworks Min Zhou, Qi Wang,* Li Zhang, Ying-Chun Liu, and Yu Kang Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, P. R. China ReceiVed: May 5, 2009; ReVised Manuscript ReceiVed: June 19, 2009
Grand canonical Monte Carlo simulations were employed to investigate hydrogen adsorption in zeolitic imidazolate frameworks [ZIFs, a new category of metal-organic frameworks (MOFs), here ZIF-8 used as an example] at 77 K and pressures increasing from 10 to 8000 kPa. A modified OPLS-AA force field was applied for the imidazolate frameworks and Buch’s model for hydrogen molecules. It shows good performance compared with the experimental measurements. The adsorption sites in the ZIF-8 materials were explored by a previously proposed technique named “computer tomography for materials (mCT)”. The mCT images suggest that the first adsorption site locates at both sides of the imidazolate ring and it is close to the imidazolate CdC bond. The hydrogen molecules then are adsorbed in the pore channel which forms the secondary adsorption site. The difference between the ZIF’s adsorption sites and the MOF’s was analyzed, and it may be helpful to design new ZIF materials with improved hydrogen adsorption capability. Hydrogen is one of the most important energy carriers in tomorrow’s world owing to its portability, environmental cleanness, high burning calories, and reproducibility. However, many problems should be solved before hydrogen can be applied in vehicles and in portable electronics as an energy carrier, including difficulties in storage and transportation.1 Although many materials have been studied to deposit hydrogen, unfortunately, none of them could reach the U.S. DOE targets in cost and performance.2,3 Therefore, it is urgent to explore new materials to meet the requirements at moderate temperatures and pressures. In the circumstances, adsorption on large-surfacearea microporous materials is a good approach to solve this storage problem, and metal-organic framework (MOF) materials have been considered as promising alternatives to zeolites and other nanoporous materials for gas adsorption. MOFs, which consist of metal oxide/nitride cluster connected by a variety of possible organic linkers, can be designed or modified to produce different pore sizes and functionalities by comparing with zeolites and activated carbons.4,5 Zeolitic imidazolate frameworks (ZIFs), a novel subfamily of MOFs which were synthesized by Yaghi’s group,6-9 are composed of a tetrahedral cluster of MN4 units (M ) Zn, Cu, Co, etc.) covalently joined by bridging simple imidazolate ligands.6 Like MOFs, ZIFs also possess a remarkably low density, potentially a large surface area, and many sites available for gas adsorption, in addition to the chemical and thermal stability and rich structural diversity of zeolites. Thus, ZIFs are considered as great candidates for hydrogen storage application. However, in contrast to many studies for other MOFs,10 few investigations related to hydrogen-ZIF interaction,11,12 and there are hardly any computations concerning the model in which hydrogen gas is adsorbed. Many experiments illustrate that MOFs and ZIFs would be of large uptake amount for gas adsorption, for example, * Corresponding author. Fax: +86-571-87951895. E-mail: qiwang@ zju.edu.cn.
10.1021/jp904170s CCC: $40.75
hydrogen, and they might be a category of promising materials for hydrogen storage. However, the mechanisms and details of hydrogen adsorption cannot be well understood just through the experimental studies. Thus, molecular simulation provides us an effective way to reveal the essence of hydrogen adsorption in MOFs and/or ZIFs. Most outcomes from molecular simulations based on the generalized force field (for example, UFF,13 Dreiding,14 and refined OPLSS-AA15-17) can predict the behavior of the gas adsorbed in materials at any temperature and pressure. Therefore, this technique has been widely used to optimize the structure of the sorbents to absorb more or particular gases. For example, Gaberoglio et al.18 reported the adsorption behavior of light gas in various MOFs with a standard force field. Frost et al.19 employed grand canonical Monte Carlo (GCMC) simulation to investigate the effects of surface area, free volume, and the heat of adsorption on hydrogen uptakes. Zhong et al.15-17 studied the adsorption and diffusion of pure and mixed gases in MOFs based on the refitted OPLS-AA force field. It is of great importance to understand the mechanism of hydrogen adsorption and the distribution in ZIFs to design or modify new ZIF materials for hydrogen storage. Yildirim et al.11 studied the adsorption of hydrogen in ZIF-8. They found that the imidazolate organic linker is the primary adsorption site by using neutron powder diffraction, and ZIF-8 was found to be able to hold hydrogen molecules as self- assembled nanostructures by using density functional theory (DFT). Recently, they studied the quantum methyl rotation in ZIF-8,20 using neutron inelastic scattering and diffraction. Leoni et al.21 enumerated more than 20 ZIFs, which are not yet synthesized, by a topological and DFT approach. Although different experiments and calculations were employed to investigate the ZIF structure and adsorption sites in ZIFs, the most important factor that influences the hydrogen adsorption in ZIFs is still not clear. It is limited about the interaction between the adsorbed hydrogen molecules and the frameworks, as well as the relationship 2009 American Chemical Society
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TABLE 1: Potential Parameters for the Atoms in the Framework of ZIF-8
Figure 1. Comparison of the simulated adsorption isotherm for hydrogen in ZIF-8 with the experimental one at 77 K and pressures from 0 to 100 kPa.
between the structure and the adsorption sites. However, it is of great importance to design new ZIFs with high hydrogen storage capacity. In this work, a computational study was performed to achieve a better understanding of the interaction characteristics of ZIFs. For this purpose, GCMC simulation based on the refined OPLSAA force field was employed to study the hydrogen adsorption in ZIFs. Furthermore, the hydrogen adsorption sites in ZIFs were discussed by the computer tomography for materials (mCT) technique. Grand canonical Monte Carlo simulations were employed to compute the adsorption of hydrogen gas in ZIFs. In order to compare the simulation results with the experimental data, ZIF-8 was first selected as an example. The structure data of ZIF-8 were obtained from the Cambridge Crystallographic Data Centre (CCDC) with ref 6. The interactions between hydrogen molecules themselves, as well as between hydrogen and frameworks, were modeled with a Lennard-Jones potential
u(rij) ) 4εij
[( ) ( ) ] σij rij
12
-
σij rij
6
where rij is the distance between two molecules i and j. The parameters for the hydrogen molecule were taken from Buch’s model,22 εH2/k ) 34.2 K (k is the Boltzmann constant) and σH2) 0.296 nm with a bond length of 0.074 nm, and those for the framework atoms were from the all-atom OPLS (OPLS-AA)23 and the Dreiding force fields.24 As the OPLS-AA force field distinguishes the types of the ZIFs’ nonmetallic atoms, it was ever used to model the behavior of argon adsorption in MOFs by Vishnyakov et al.,25 hydrogen, methane, and carbon dioxide in MOFs by Zhong et al.15-17 In addition, the Dreiding force field can be used to describe the metal atoms in ZIFs. The force field parameters used in this work were listed in Table 1. The cross interaction parameters between the different atoms were calculated by the Lorentz-Berthelot mixing rules,26 εij )(εii + εjj)/2 and σij ) (σiiσjj)1/2. The cutoff radius was set to be 1.28 nm. In the simulations, the simulation box consists of 2 × 2 × 2 unit cells (equivalent to 3.40 × 3.40 × 3.40 nm3) of ZIF-8 and the frameworks were assumed to be rigid. The periodic boundary conditions were applied in order to minimize the surface effects and guarantee the simulation accuracy. The probabilities of the trial moves (moving, inserting, and removing) were set to be 45, 27.5, and 27.5%, respectively. At 77 K, for each state point, the first 0.5 × 106 GCMC simulation steps
Figure 2. Adsorption isotherm for hydrogen in ZIF-8 at 77 K and pressures from 0 to 8000 kPa. The experimental data were also plotted for comparison.
were used to equilibrate the system, and the further 0.5 × 106 GCMC steps were used for ensemble average. Molecular simulation predicts the absolute amount adsorbed within the frameworks at given gas phase conditions, whereas the experimental measurements yield the excess amount adsorbed. The excess adsorption is defined as the amount of adsorbate within the adsorbent above and beyond what is found in the ambient gas phase. In order to compare the simulation results with the experimental data, the excess adsorption (in the adsorption phase) is required and it can be calculated from the absolute adsorption loadings by the equation nex ) nabs - VgFg using the free volume of the adsorbent. That is, the excess adsorption, nex, is determined from the absolute adsorption, nabs, by subtracting VgFg, the amount of gas in the pores, where the free volume, Vg, is 35% of the unit cell for ZIF-8 which is calibrated by Zhou using helium gas,11 and the density of the gas phase, Fg, at high pressures for hydrogen was calculated using the Peng-Robinson equation of state.27 Adsorption Isotherm The adsorption isotherm for hydrogen adsorbed in ZIF-8 at 77 K was calculated with GCMC simulation. The adsorption isotherms at pressures up to 10 and 8000 kPa were presented in Figures 1 and 2, respectively. So far, the adsorption isotherms of hydrogen in ZIFs have not yet been calculated with the simulation method. In order to verify the model and parameters used in this work, both the calculated and experimental data6 of Yaghi et al. at low pressures were plotted in Figure 1 for
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Figure 3. mCT images of the adsorption sites (the red-yellow-cyan region) superimposed with the Zn-hexagon opening of the ZIF-8 structure [(111) view]: (a) in section A, ∼0.33 nm above plane O; (b) in section B, ∼0.33 nm below plane O; (c) in section C, crossing the three CdC bonds which are not in plane O; (d) in section D, ∼0.1 nm below plane O, showing the channel site. (e) Schematic drawing of the cutting position of the Zn-hexagon opening (side view).
comparison. It is found that the simulation results are reasonably in good agreement with the experimental data considering the accuracy of the measurements, which demonstrates that the model and the force field parameters used in this work are able to describe the adsorption behavior of hydrogen in ZIFs. An excess adsorption isotherm for hydrogen adsorbed in ZIF-8 at higher pressures is presented in Figure 2 in terms of volumetric units. It is observed that the excess adsorption of hydrogen rapidly increases almost linearly at low pressure, and with the pressure increase, the adsorption amount reaches a maximum and then starts to decrease slightly. The maximum hydrogen adsorption in ZIF-8 at 77 K occurs around 3000 kPa, and it is about 380 cm3 · g-1 (at the standard temperature and pressure, STP). The adsorption isotherm shows that the amount adsorbed undergoes a tremendous step upward from 10 to 2000 kPa, corresponding to the condensation of hydrogen. At pressures around 3000 kPa, the density of hydrogen gas in the pore and that of the hydrogen adsorbed are both increasing at an equivalent rate with respect to pressure. At higher pressures, the amount adsorbed of hydrogen tends to be saturated, while the density of hydrogen gas in the pore still keeps increasing,
which eventually results in the decrease of excess adsorption. It agrees with the excess adsorption equation mentioned above. As shown in Figure 2, the simulated adsorption isotherm in our work is close to Yildirim’s data in the pressure range of their experiments, and then approaches Yaghi’s above 6000 kPa. The isotherm of Yildirim’s group is about 10% higher than that of Yaghi’s around 1200 kPa. However, the Yildirim’s group claimed that the error bar of their data is less than (2% and the reproducibility is within 0.5%. Comparison between the simulated isotherm and the experimental data shows that the simulations overestimate the measured data by about 6.7% at saturation pressures. The discrepancy mainly originates from two factors: the first is that the ZIF-8 framework undergoes a structural change under high loadings, while a rigid framework is assumed in our model, and the second is that the ZIF-8 may have kinetically inaccessible regions, but the GCMC simulation assumes that any points within the simulation cell can be accessed. The presence of intransitable bottlenecks would result in an experimental saturation adsorption lower than the simulations.
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Adsorption Sites In order to investigate the adsorption sites inside the adsorbent materials, a technique named “computer tomography for materials (mCT)” was proposed in our previous work.28,29 During the simulation process, the entire configurations of the hydrogen molecules in the simulation system were recorded every 1000 Monte Carlo steps, and then the overall averaged distribution of the hydrogen molecules adsorbed in the three-dimensional (3D) space was calculated statistically on the basis of all of these configurations. The mCT technique cuts the material (here, it is ZIF-8) at any position and any angle and obtains the mCT images. The mCT images are based on the statistical results and thus more reliable to confirm the adsorption sites. Moreover, it is very easy to get information regarding the adsorption sites of hydrogen molecules in any cross section inside the materials of ZIFs. Here, we used the mCT technique to investigate the adsorption sites of hydrogen near the Zn-hexagon opening which forms the unit cell of ZIF-8 with certain rules. The mCT images were employed to clearly exhibit the adsorption sites if some regions (red-yellow-cyan region) adsorb hydrogen more excessively than others do. The red region means the strongest hydrogen adsorbing ability, the yellow region represents strong interaction with hydrogen molecules, and the blue region suggests the least hydrogen distribution density. The density here represents the probability of the center of the hydrogen molecules on the distribution diagram. The adsorption sites of hydrogen in ZIF-8 at 77 K and the mCT images [(111) view] of different positions were plotted in Figure 3. First of all, the mCT image in Figure 3a (section A) shows the most important adsorption site in ZIF8, which is above the three 2-methylimidazolate rings (plane O), around the 2-methylimidazolate linkers and partial to the CdC bond. It is in good agreement with the neutron diffraction result reported by Yildirim et al.,10 in which they claimed that the hydrogen molecules tend to be adsorbed on the top of the 2-methylimidazolate linker and close to the CdC bond, that is, the “IM site”. It confirms that the force field parameters used in this simulation are reasonable and they are suited to be used to investigate the adsorption of hydrogen in ZIF materials. In addition, it was found that the other equivalent adsorption site locates at the other side of the CdC bond in section B, as shown in Figure 3b, they are symmetric about plane O and apart by 0.65 nm. Figure 3a and b illustrates that six adsorption sites belong to three imidazolate rings, which parallel plane O. Furthermore, we used another section, section C, to cut the three CdC bonds of the other three 2-methylimidazolate rings which do not parallel plane O. We obtained the images shown in Figure 3c. It shows six similar adsorption sites locating at both sides of the three imidazolate rings and close to the CdC bond. We considered that these adsorption sites are the “IM site”, the same as those in Figure 3a and b. Another type of hydrogen adsorption site locates at the center of the Zn-hexagon opening of the ZIF-8, donated as “channel sites”. One site, for the moment named “channel site I”, occurs in section D which is approximately 0.1 nm below plane O, as illustrated in Figure 3d. Another one, named “channel site II”, is also at the center of the Zn-hexagon opening, whereas it locates at the other side of plane O, as shown in Figure 3a. It is found that the adsorption amount in “channel site II” is obviously less than that in “channel site I” because the existence of a methyl group takes up some space where hydrogen molecules are supposed to be loaded. In summary, hydrogen molecules are adsorbed at two kinds of locations. One is around
Figure 4. Schematic description of the steric effect in ZIF-8 (a, b) and in MOF-5 (c): (a) larger free space for hydrogen molecule adsorption around the CdC bond; (b) steric hindrance for hydrogen molecule adsorption around the nitrogen atoms; (c) free space at the center of the bowl where the first adsorption site locates.
both sides of the imidazolate linker and close to the CdC bond, and the other type of adsorption site is at the center of the pore in ZIF-8. Comparison with MOFs As one knows, metal-organic frameworks (MOFs) are considered as good candidates for hydrogen storage. As a new subfamily of MOFs featuring a tetrahedral cluster of MN4 (M ) Zn, Co, Cu, etc.) and linked by imidazolate ligands, ZIFs have many characteristics similar to MOFs, for example, the tunable pore size7 and chemical functionality. However, it is surprising that the locations of hydrogen adsorption sites in ZIFs are greatly different from those in MOFs. In our previous works,29 the most important hydrogen adsorption site of classic MOFs is surrounded by the oxygen atoms where three COO groups are joined like a bowl, the secondary adsorption sites are around the organic linkers, and the tertiary adsorption sites are in the pore channels. However, in this study of ZIFs, as a contrast, the primary hydrogen adsorption site locates around the organic linkers and close to the CdC bonds, and interestingly, no adsorption site near the ZnN4 clusters was observed. It seems that the CdC bond interacts strongly with the hydrogen molecules in ZIFs, just as the oxygen atoms of MOFs interact with the hydrogen molecules. It was pointed out that the electronegativity of oxygen atoms plays an important role in hydrogen adsorption, and more electronegative atoms in the MOF frameworks are more favorable to hydrogen adsorption. Similarly, the hydrogen adsorption sites are supposed to be close to the nitrogen atoms in ZIFs at the same temperature and pressure. However, similar results were not observed despite the fact that the electronegativity of the nitrogen atom is approximate with that of the oxygen atom (3.04 for nitrogen and 3.44 for oxygen using the Pauling scale). There must be a more important factor that influences the primary hydrogen adsorption site in ZIFs. In order to explore this change of adsorption sites in ZIF-8, we investigated the steric effect on the adsorption sites in details. The steric effect is considered to be another factor affecting the hydrogen adsorption behavior in ZIFs. In Figure 4, it shows the free spherical space around the nitrogen atom and the CdC bond in ZIF-8, and the free space around the bowl sites in MOF-5. The diameter of the hydrogen molecule is 0.23 nm, whereas the diameter of the free spherical space around the nitrogen atom and the CdC bond is 0.227 and 0.342 nm, respectively. Apparently, the hydrogen molecules could be adsorbed to the CdC bond easily, whereas the entrance of hydrogen molecules to the free space around the nitrogen atom should be quite difficult. In contrast, since there are no geometric hindrance effects around the oxygen atoms in MOF-5, the most important adsorption site is the bowl site surrounded by six oxygen atoms, as shown in Figure 4c.
Letters On the other hand, the electron affinity of carbon atoms is expected to increase as its hybridization state changes from sp3 to sp2 and sp. The electronegativity of CH2sCH3, CHdCH2, and CtCH is 2.44, 2.63, and 2.92, respectively. Therefore, it is considered that the CdC bond interacts more strongly with hydrogen than carbon atoms with an sp3 hybridization state. In a word, the first hydrogen adsorption site in ZIF-8 close to the CdC bond is attributed to its larger free space without steric hindrance for hydrogen molecules and its higher electron affinity compared with carbon atoms with an sp3 hybridization state. This difference of hydrogen adsorption sites between ZIFs and MOFs might originate from the element component and the structure of ZIFs. In summary, GCMC simulations of hydrogen adsorption on ZIF-8 were carried out to investigate the adsorption isotherms and adsorption sites in ZIFs. First, the performance of the models and parameters applied in the simulations was evaluated by comparing the calculated results with the experimental data for the adsorption in the case of hydrogen. The results show an approximate 6.7% difference in the adsorbed amount of hydrogen near saturation at 77 K, which was considered to be in reasonable agreement with the experimental data. The hydrogen adsorption sites in ZIF-8 were investigated by the “computer tomography for materials” technique. The first adsorption site in ZIF-8 is near the 2-methylimidazolate organic linker and close to the imidazolate CdC bond, which agrees well with the neutron diffraction experiments. What we found new was the first adsorption sites existed at both sides of the imidazolate organic linkers. The secondary hydrogen adsorption sites donated as “channel sites” are located at the center of the Zn-hexagon opening of ZIF-8. When we compared the features of the hydrogen adsorption sites of the ZIFs with the MOFs, some significant differences were found. The steric hindrance effect around the nitrogen atoms and the higher electron affinity of sp2-hybrid carbon atoms in ZIF-8 lead to hydrogen molecule adsorption close to the CdC bond. It suggests that, probably, it is another strategy to design new hydrogen storage materials with tailored properties to store a higher amount of hydrogen at moderate temperatures and pressures. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant No. 20876132). References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353–358.
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