Cation Characterization and CO2 Capture in Li+-Exchanged Metal

Apr 8, 2010 - The hydration of cations in Li+-MOF leads to a reduced free volume and ... The Journal of Physical Chemistry C 2013 117 (1), 71-77 ... o...
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Cation Characterization and CO2 Capture in Li+-Exchanged Metal-Organic Frameworks: From First-Principles Modeling to Molecular Simulation† R. Babarao and J. W. Jiang* Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 117576 Singapore

We report a computational study for cation characterization and CO2 capture in Li+-exchanged metal-organic frameworks (Li+-MOFs). Density functional theory is adopted to optimize cation locations and evaluate atomic charges, and molecular simulation is subsequently used to examine the separation of CO2/H2 and CO2/N2 mixtures for pre- and post-combustion CO2 capture. The cations are observed to locate near the carboxylic O-donors of metal clusters. Specifically, H+ ions in dehydrated Li+-MOF form covalent bonds with the O-donors, and H3O+ ions in hydrated Li+-MOF form hydrogen bonds with the O-donors. CO2 is overwhelmingly adsorbed over H2 and N2 in both dehydrated and hydrated Li+-MOFs. Adsorption occurs preferentially near the cations and metal clusters, which possess strong electrostatic potentials, and then in the square channels. At ambient condition, the selectivity is approximately 550 for CO2/H2 mixture and 60 for CO2/N2 mixture, higher than that in nonionic MOFs and other nanoporous adsorbents. The charges of framework and cations have a significant effect on the selectivity, which is found to decrease by 1 order of magnitude by switching off the charges. The hydration of cations in Li+-MOF leads to a reduced free volume and consequently a lower extent of adsorption. 1. Introduction Metal-organic frameworks (MOFs) have emerged as a new class of hybrid nanoporous materials.1,2 Composed of readily tunable metal clusters and organic linkers, MOFs have extremely large surface areas and well-defined pores. They are regarded as promising candidates for storage, separation, catalysis, and other emerging applications.3,4 In the past few years, a large number of experimental and theoretical studies have been reported for gas adsorption and separation. For example, H2 capacity was measured in a series of MOFs and good correlation was found between surface area and saturation capacity.5 Adsorption of various light gases in MOFs were simulated and compared with available experimental data to test model potentials.6,7 In highly hydrophobic paddle-wheel Zn(BDC)(TED)0.5, the adsorption of H2 and hydrocarbons was studied.8,9 One, two and three-dimensional covalent-organic frameworks (COFs) were examined for CO2 storage and exceptionally high capacity of up to 100 mmol/g was predicted in COF-105 and COF-108.10 Separation of the CO2/CH4 mixture was examined in mixed-ligand MOFs, where mixture adsorption was predicted from single-component adsorption and subsequently verified by simulation.11 The robust reticular strategy to synthesize MOFs using molecular building blocks has paved the way for systematical development of novel porous materials. Recently, a new subfamily of MOFs have been produced, i.e., charged MOFs with charge-balancing ions.12-16 The presence of extraframework ions enhances the interactions with guest molecules, as reflected in the large isosteric heat of adsorption14-16 and the highly selective adsorption of gas mixtures.17-19 In this regard, the introduction of light metal ions such as Li+ into a charged MOF is particularly interesting. Yang et al. synthesized an anionic framework 1 [(CH3)2NH2]+[In(C16H6O8)]-.20 The charge-balancing [(CH3)2NH2]+ ions were generated via the † This work is dedicated to Prof. Stan Sandler on the occasion of his 70th birthday. * To whom correspondence should be addressed. Tel.: +6565165083. Fax: +65-67791936. E-mail: [email protected].

decomposition of dimethylformamide (DMF) solvent. By ion exchange, Li+-exchanged framework 1-Li+ [Li0.5(H3O)0.5]+[In(C16H6O8)]- was obtained. The structure of framework 1 remained intact after the exchange of Li+ ions as no new peaks or shifting of peaks were experimentally observed in X-ray diffraction. In 1-Li+, half of the cations are Li+ and the other half are H3O+. The dehydrated structure of 1-Li (1a-Li+) could be obtained by degassing 1-Li+ for 24 h at 180 °C and 10-10 bar. The locations of cations in 1a-Li+ and 1-Li+ were not available in the experiment. Nevertheless, Yang et al. underlined that precisely identifying the locations of cations would be very useful for further investigation.20 In this work, a hierarchical method was used to characterize cations and gas adsorption in 1a-Li+ and 1-Li+. Specifically, first-principles modeling was adopted to identify cation locations and calculate atomic charges. The fundamental insight into the location of cations can provide microscopic pictures that otherwise are experimentally inaccessible or difficult to obtain. Such information in charged MOFs is crucial for their applications in storage, catalysis, and ion-exchange. Subsequently, classical molecular simulation was carried out to examine the adsorption of CO2/H2 and CO2/N2 mixtures in order to explore the capability of Li+-MOFs for pre- and post-combustion CO2 capture. 2. Models and Methods Figure 1a shows the three-dimensional PtS-type structure of framework 1 [(CH3)2NH2]+[In(C16H6O8)]-, but the extraframework ions are not shown for clarity. The framework 1 consists of In(O2CR)4 nodes linked by tetracarboxylate ligands. Each In atom adopts an eight-coordinated geometry connecting with four carboxylate ligands and each ligand binds to four In atoms. The framework has a tetragonal space group and lattice constants a ) b ) 19.659 Å and c ) 36.149 Å.20 The squareshaped channels exist in the framework with approximate diameter of 7.2 Å. These channels are interconnected by square windows, which consist of four In centers and four ligands. Li+exchanged framework 1-Li+ [Li0.5(H3O)0.5]+[In(C16H6O8)]-

10.1021/ie100214a  2011 American Chemical Society Published on Web 04/08/2010

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Figure 1. (a) 2 × 2 unit cells of framework 1. Color code: In, cyan; C, gray; O, red; H, white. (b) Cluster of framework 1 used in the first-principles modeling. The cations are not shown for clarity. Figure 3. Electrostatic potentials around the cluster and cations of 1a-Li+ from DFT calculation. Color code: Li+, green; H+, yellow; In, cyan; C, gray; O, red; H, white.

Figure 2. Locations of Li+ and H+ cations near the cluster of 1a-Li+ from DFT optimization. The distances are in angstroms. Color code: Li+, green; H+, yellow; In, cyan; C, gray; O, red; H, white. The dangling bonds indicated by circles were saturated by methyl groups.

could be produced by ion exchange. Eight Li+ and eight H3O+ ions are present in a unit cell of 1-Li+. The IR spectroscopic analysis showed that the coordinated water molecules in 1-Li+ could be removed, which leads to eight Li+ and eight H+ ions in dehydrated 1a-Li+ [Li0.5H0.5]+[In(C16H6O8)]-.20 The framework contains a large number of atoms, which is computationally prohibited for first-principles modeling. As a consequence, a cleaved cluster as shown in Figure 1b was used. To maintain the original hybridization, the dandling bonds in the cluster were saturated by methyl groups. The cluster carried a charge of -4e, and hence, four cations (two H+ and Li+ ions in 1a-Li+ or two H3O+ and Li+ ions in 1-Li+) were added with each ion near the In(O2CR)4 node. The locations of ions were subject to geometry optimization using density functional theory (DFT) in DMol3.21 The PW91 functional was used along with the DNP basis set, which is comparable to the 6-31 g(d,p) Gaussian-type basis set. The DNP basis set incorporates d-type polarization into heavier atoms and p-type polarization into hydrogen atoms. Figure 2 shows the locations of Li+ and H+ ions in dehydrated framework 1a-Li+ from DFT optimization. H+ ion appears to form a covalent bond with one carboxylic O atom, while Li+ ion is located between two carboxylic O atoms. This confirms the experimental speculation by Yang et al.20 The distance is approximately 0.98 Å between the H+ ion and O atom, and 1.87 Å between the Li+ ion and O atom. We therefore infer that both Li+ and H+ ions are strongly coordinated to the cluster. The DFT calculations also gave the electrostatic potentials around the cluster and cations. As shown in Figure 3, there are large electric fields near the cations, particularly

Figure 4. Atomic charges of the cluster and cations in 1a-Li+ from DFT calculation. Color code: Li+, green; H+, yellow; In, cyan; C, gray; O, red; H, white. The dangling bonds indicated by circles were saturated by methyl groups.

the Li+ ions. As we will see below, the cations act as preferential adsorption sites. The atomic charges of the cluster and cations were fitted to the electrostatic potentials using the Merz-Kollman scheme.22,23 Figure 4 shows the estimated atomic charges. The locations of Li+ and H3O+ ions in hydrated framework 1-Li+ were also optimized. Figure S1 of the Supporting Information indicates that Li+ and H3O+ ions are also located proximally to the carboxylic O atoms, similar to Li+ and H+ ions in 1a-Li+. The distance from Li+ ion and to carboxylic O atom is around 1.86 Å, close to that observed in 1a-Li+. The distance between H atom of H3O+ and carboxylic O atom ranges from 1.55 to 1.59 Å, implying the formation of a hydrogen bond. The electrostatic potentials and atomic charges of the cluster and cations are shown in Figure S2 and S3. Similar to Figure 3, the strong potentials are observed near the cations. For gas adsorption in Li+-MOFs, the interactions of gasadsorbent and gas-gas were modeled as a combination of pairwise site-site Lennard-Jones (LJ) and Coulombic potentials uij(r) )

∑

R∈i β∈j

{ [( ) ( ) ] 4εRβ

σRβ rRβ

12

-

σRβ rRβ

6

+

qRqβ 4πε0rRβ

}

(1)

where ε0 ) 8.8542 × 10-12 C2 N-1 m-2 is the permittivity of the vacuum and σRβ and εRβ are the collision diameter and well

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Table 1. Potential Parameters of Adsorbates CO2, H2, and N2a adsorbates CO2 H2 N2

C O H COM N COM

σ (Å)

ε/kB (K)

Z (e)

2.789 3.011 0 2.958 3.320 0

29.66 82.96 0 36.7 36.4 0

+0.576 -0.288 +0.468 -0.936 -0.482 +0.964

a COM indicates the center-of mass in the three-site model for H2 and N2.

depth, respectively. Due to the strong coordination with the framework, the cations were considered to be immobile. The LJ potential parameters of the framework atoms and cations were adopted from the Universal Force Field (UFF).24 A number of simulation studies have revealed that UFF can accurately predict gas adsorption in MOFs. For example, the simulated adsorption isotherm of Ar adsorption in Cu-BTC matches well with experiment.25 Adsorption of CO2 and CH4 in IRMOF-1 from simulation is in good agreement with experimental data.26 UFF has also been used to study the separation and diffusion of gases in various MOFs.7,27-30 The three adsorbates CO2, N2, and H2 were mimicked as three-site model to account for the quadrupole moment. The C-O bond length in CO2 was 1.18 Å, and the bond angle ∠OCO was 180°. The charges on C and O atoms were +0.576e and -0.288e (e ) 1.6022 × 10-19 C, the elementary charge), resulting in a quadrupole moment of -1.29 × 10-39 C m2. The model reproduced the isosteric heat and isotherm of CO2 adsorption in slilicate.31 The H-H bond length in H2 was 0.74 Å, and the charge on the H atom was +0.468e. The center-ofmass was a LJ core with a charge of -0.936e. This model gave a quadrupole moment of -2.05 × 10-40 C m2 for H2 and has been widely used for H2 adsorption.32 N2 had N-N bond length of 1.10 Å, a charge of -0.482e on N atom, and a charge of +0.964 at the center-of-mass, which were fitted to the experimental bulk properties of N2.33 On the basis of this model, the quadrupole moment of N2 was -4.67 × 10-40 C m2. Table 1 lists the LJ parameters and atomic charges of the adsorbates.31-33 The cross LJ parameters were evaluated by the Lorentz-Berthelot combining rules. The adsorption of CO2/H2 and CO2/N2 mixtures at 298 K was simulated by the grand canonical Monte Carlo (GCMC) method. Because the chemical potentials of adsorbate in adsorbed and bulk phases are identical at thermodynamic equilibrium, GCMC simulation allows one to relate the chemical potentials of adsorbate in both phases and has been widely used for the simulation of adsorption. Both CO2/H2 and CO2/N2 mixtures were assumed to have a bulk composition of 15:85, which is found practically in chemical industry. The framework atoms together with the cations were frozen during simulation. This is because adsorption involves low-energy equilibrium configurations and the flexibility of framework has a marginal effect, particularly on the adsorption of small gases. The LJ interactions were evaluated with a spherical cutoff equal to half of the simulation box with long-range corrections added; the Coulombic interactions were calculated using the Ewald sum method. The number of trial moves in a typical GCMC simulation was 2 × 107, though additional trial moves were used at high loadings. The first 107 moves were used for equilibration and the subsequent 107 moves for ensemble averages. Six types of trial moves were attempted in GCMC simulation, namely, displacement, rotation, and partial regrowth at a neighboring position, entire regrowth at a new position, swap with reservoir, and exchange of molecular identity.

3. Results and Discussion 3.1. Pre-combustion CO2 Capture. The adsorption isotherm for CO2/H2 mixture (bulk composition 15:85) in 1a-Li+ is shown in Figure 5a. Over the entire range of pressure considered, CO2 is more predominantly adsorbed than H2 as attributed to two reasons. First, the temperature 298 K considered is subcritical for CO2 (Tc ) 304.4 K), but supercritical for H2 (Tc ) 33.2 K); that is, CO2 has a larger condensability than H2 at 298 K. Second, the ionic framework and presence of cations exert strong electrostatic interactions for CO2, which has a higher quadrupole moment than H2, and hence enhances CO2 adsorption. To examine the effect of charges, additional simulations were performed in a neutral structure but switching off the charges on framework and cations. As seen in Figure 5a, CO2 adsorption in the neutral structure is reduced significantly; however, H2 adsorption is increased slightly. The separation factor of gas mixture was quantified by the selectivity Si/j ) (xi/xj)(yj/yi), where xi and yi are the mole fractions of component i in adsorbed and bulk phases, respectively. Figure 5b shows the selectivity in 1a-Li+ and neutral structure, respectively. As a function of pressure, three regimes are observed in both structures. First, the selectivity in 1a-Li+ drops as pressure increases. This is because the adsorption sites are inhomogeneous and adsorption occurs on less favorable sites with increasing pressure. Second, at moderate pressures, the selectivity starts to increase as a result of cooperative attractions between adsorbed CO2 molecules. Finally, at high pressures, the packing effect becomes dominant because H2 molecule is smaller in size than CO2 and can pack into the partially filled channels more effectively; consequently, the selectivity drops within the pressure range in this study. At a typical operating condition (298 K and 1 bar) in pressure-swing adsorption process, the selectivity is approximately 550 in 1a-Li+. In the neutral structure with the charges on framework and cations switched off, the selectivity remains nearly constant at low pressures and increases marginally with increasing pressure due to the cooperative interactions. Finally, the selectivity drops due to packing effect. The selectivity in the neutral structure is almost one-order of magnitude smaller than in the charged 1aLi+. This implies that the charges have a significant effect on adsorption and selectivity. The adsorption selectivity of CO2 over H2 in 1a-Li+ is exceptionally high. It is indeed higher than that in most porous adsorbents. For example, the selectivity in an activated carbon ranges from 60 to 90 for CO2/H2 mixtures with different mole fractions.34 The selectivity is 10 in IRMOF-1 and 90 in CuBTC at 298 K and 1 atm.35 In zeolite Na-4A, the selectivity is 70 for a mixture with 98.6% H2 and 1.4% CO2.36 In MCM-41, the CO2 over H2 selectivity for a 20:80 mixture is around 30 at low pressures and 63 at 45 bar.37 The selectivity is about 5 in ZSM-5 and 3.5 in ETS-10 for an equimolar CO2/H2 mixture.38 Figure S4 (of the Supporting Information) shows the adsorption isotherm and selectivity of CO2/H2 mixture in hydrated 1-Li+. Compared to dehydrated 1a-Li+, CO2 adsorption and CO2/H2 selectivity in 1-Li+ decrease slightly. This is due to the fact that H3O+ has a larger size than H+ and the free volume for adsorption is thus smaller in 1-Li+. Interestingly, the selectivity behavior of CO2/H2 in Li+-MOFs is different from that in other charged MOFs such as soc-MOF and rho-ZMOF.18,19 In soc-MOF, the selectivity increases with pressure, reaches a maximum, and finally decreases as a function of pressure. The increase is caused by the strong interactions of CO2 molecules with multiple binding sites and by the cooperative interactions of CO2 molecules; the decrease is

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Figure 5. (a) Adsorption isotherm and (b) selectivity for CO2/H2 mixture in 1a-Li+. The dashed lines indicate in a neutral structure by switching off the charges on framework and cations.

Figure 6. Radial distribution functions between adsorbates and Li+ cations for CO2/H2 mixture in 1a-Li+ at 10, 100, and 1000 kPa.

attributed to the entropy effect. In rho-ZMOF, the selectivity decreases monotonically with increasing pressure, as a consequence of the significant reduction in the electrostatic interactions between CO2 and rho-ZMOF. However, as explained earlier, the selectivity in 1a-Li+ decreases initially, then increases as a function of pressure and finally decreases. The difference of the selectivity behavior is attributed to different framework topologies and charge densities in the charged MOFs. Specifically, soc-MOF contains carcerand-like capsules and narrow channels approximately 10 Å, rho-ZMOF has a very openframework with extra-large cavity of 18.2 Å connected by windows of 5.5 Å, and 1a-Li+ has one-dimensional square channels of 7.2 Å. A unit cell of soc-MOF contains 8 NO3ions, corresponding to a charge density of 7.06 × 10-4 e/Å3. In rho-ZMOF, 48 Na+ ions are present per unit cell and the charge density is 1.60 × 10-3 e/Å3. The 1a-Li+ consists of 16 cations including Li+ and H+ ions, corresponding to a charge density of 1.14 × 10-3 e/Å3. Therefore, the geometry constrains and charge densities differ in the three charged MOFs, which lead to the different selectivity behavior as a function of pressure. The selectivity is reasoned to decrease with increasing channel dimension and to increase with increasing charge density. The rho-ZMOF has the highest charge density and exhibits the largest selectivity, particularly at low pressures. This reveals that charged density plays a more dominant role than channel dimension in determining the selectivity. Figure 6 shows the radial distribution functions g(r) between adsorbates and Li+ ions for CO2/H2 mixture in 1a-Li+. At 10, 100, and 1000 kPa, a pronounced peak in g(r) is observed for CO2-Li+ at r ) 3.3 Å; however, no such peak exists for H2-Li+. This confirms that CO2 interacts with Li+ ions more

strongly than H2 because of higher quadrupole moment and stronger dispersion interaction. With increasing pressure from 10 to 100 and then to 1000 kPa, the peak height of g(r) for CO2-Li+ decreases. This is due to two factors as pressure increases. First, the average density of CO2 adsorbed in the structure increases; second, the location of CO2 moves away from the metal clusters and cations to the square channels. Though not shown, the coordination number of CO2 molecules around Li+ becomes larger with increasing pressure. 3.1. Post-combustion CO2 Capture. Figure 7 shows the adsorption isotherm and selectivity for CO2/N2 mixture (bulk composition 15:85) in 1a-Li+. Similar to the CO2/H2 mixture discussed above, CO2 is more preferentially adsorbed than N2. However, the selectivity shows a different trend compared to the CO2/H2 mixture. Here, the selectivity decreases first, then starts to increase, and finally tends to reach a plateau. The reason for the decrease and increase is similar to what was discussed earlier for the CO2/H2 mixture. Unlike CO2 and H2, however, the molecular sizes of CO2 and N2 are comparable and therefore the entropy effect is not pronounced at high pressures. In the neutral structure with the charges on framework and cations switched off, the selectivity increases with increasing pressure due to the cooperative attractions and finally reaches a plateau. The selectivity in the neutral structure is 1 order of magnitude smaller compared to that in the charged framework. Figure S5 (of the Supporting Information) shows the adsorption isotherm and selectivity for CO2/N2 mixture in hydrated 1-Li+. As a function of pressure, they behave largely similar to Figure 7 in dehydrated 1a-Li+, but with a smaller magnitude.

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Figure 7. (a) Adsorption isotherm and (b) selectivity for CO2/N2 mixture in 1a-Li+. The dashed lines indicate in a neutral structure by switching off the charges on framework and cations.

Figure 8. Density contours of CO2 for CO2/N2 mixture in 1a-Li+ at 10, 100, and 1000 kPa, respectively.

CO2/N2 separation has been investigated in other nanoporous materials for post-combustion CO2 capture. The selectivity is approximately 18.8 in zeolites Na-4A,36 15.3 in activated carbon Norit R1,39 30 in silicalite,40 100 in ITQ-3,40 14 in MFI,41 20 in FAU,42 35 in Cu-BTC,43,44 25-45 in cavity modified MOFs,45 35 and 50 in ZIF-82 and ZIF-78, respectively, at infinite dilution,46 18-20 in ZIF-95 and 25-27 in ZIF-100,47 98 in SNU-M10,48 75 in bio-MOF-11,49 and 3-6 in MOF508b.50 The simulated selectivity of CO2/N2 in 1a-Li+ is approximately 60 at ambient condition and reaches 110 at high pressures, greater than in most of the nanostructures. Nevertheless, it is smaller than in rho-ZMOF because the latter has a higher charge density. Figure 8 shows the density contours of CO2 for CO2/N2 mixture in 1a-Li+ at 10, 100, and 1000 kPa. As N2 uptake is low, its contours are not plotted. At 10 kPa, CO2 is adsorbed proximally to the cations and metal clusters, attributed to the strong electrostatic potentials as demonstrated in Figure 3. Nevertheless, these regions are small and get saturated rapidly. As a consequence, when pressure rises to 100 kPa, CO2 starts to occupy the square channels. At a high pressure of 1000 kPa, adsorption occurs mostly in the square channels. 4. Conclusions The cation locations and separation of CO2/H2 and CO2/N2 mixtures in Li+-exchanged MOFs have been investigated using a hierarchical approach combining first-principles modeling and molecular simulation. Each cation surrounds one metal cluster in Li+-MOFs. In dehydrated Li+-MOF, H+ ions form covalent bonds with the carboxylic O-donors at a distance of 0.98 Å, and Li+ ions are coordinated to carboxylic O atoms at a distance of 1.86-1.87 Å. Similarly, H3O+ ions in hydrated Li+-MOF

are also located near the metal clusters. The distance from the H atom of H3O+ to the carboxylic O atom is 1.55-1.59 Å, within the range of hydrogen bond. The regions proximal to the ions have strong electrostatic potentials and are the preferential sites for gas adsorption. For CO2/H2 and CO2/N2 mixtures, CO2 is predominantly adsorbed against H2 and N2 because CO2 possesses a larger quadrupole moment and a stronger dispersion interaction. As a counterbalance of energetic and entropic effects, the selectivity of CO2/H2 initially decreases, then increases due to cooperative interactions, and finally decreases at high pressures as a consequence of packing effect. However, the selectivity of CO2/ N2 exhibits a different trend with increasing pressure; it decreases and then increases at low and moderate pressures, and finally approaches a plateau at high pressures. At room temperature and 1 atm, the predicted selectivity of CO2 over H2 and N2 is 550 and 60, respectively. Li+-MOFs exhibit selectivity higher than nonionic MOFs and most other porous adsorbents. The charges on framework and cations have a substantial effect. In a neutral structure with the charges switched off, the selectivity drops almost 1 order of magnitude. The adsorption and selectivity in hydrated Li+-MOF are slightly decreased upon hydration because of the reduced free volume. The microscopic understanding from this study is useful in the rational design of new charged MOFs for pre-combustion and post-combustion CO2 capture and other separation processes. Acknowledgment The authors are grateful for the support from the National University of Singapore (R-279-000-198-112/133 and R-279000-297-112).

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Supporting Information Available: Locations of Li and H3O+ cations, electrostatic potentials, and atomic charges in 1-Li+; the adsorption isotherms and selectivities for CO2/H2 and CO2/N2 mixtures in 1-Li+. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keefe, M.; Yaghi, O. M. Systematic design of pore size and functionality in isoreticular metal-organic frameworks and their application in methane storage. Science 2002, 295, 469–472. (2) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Reticular synthesis and the design of new materials. Nature 2003, 423, 705–714. (3) Ferey, G. Hybrid porous solids: Past, present, and future. Chem. Soc. ReV. 2008, 37, 191–214. (4) Morris, R. E.; Wheatley, P. S. Gas storage in nanoporous materials. Ang. Chem. Int. Ed. 2008, 47, 2–20. (5) Wong-Foy, A. G.; Matzger, A. J.; Yaghi, O. M. Exceptional H2 saturation uptake in microporous metal-organic frameworks. J. Am. Chem. Soc. 2006, 128, 3494–3495. (6) Garberoglio, G.; Skoulidas, A. I.; Johnson, J. K. Adsorption of gases in metal organic materials: Comparison of simulations and experiments. J. Phys. Chem. B 2005, 109, 13094–13103. (7) Skoulidas, A. I.; Sholl, D. S. Self-diffusion and transport diffusion of light gases in metal-organic framework materials assessed using molecular dynamics simulations. J. Phys. Chem. B 2005, 109, 15760–15768. (8) Lee, J. Y.; Olson, D. H.; Pan, L.; Emge, T. J.; Li, J. Microporous metal-organic frameworks with high gas sorption and separation capacity. AdV. Funct. Mater. 2007, 17, 1255–1262. (9) Liu, J.; Lee, J. Y.; Pan, L.; Obermyer, R. T.; Simizu, S.; Zande, B.; Li, J.; Sankar, S. G.; Johnson, J. K. Adsorption and diffusion of hydrogen in a new metal-organic framework material: Zn(bdc)(ted)0.5. J. Phys. Chem. C 2008, 112, 2911–2917. (10) Babarao, R.; Jiang, J. W. Exceptionally high CO2 storage in covalent-organic frameworks: Atomistic simulation study. Energy EnViron. Sci 2008, 1, 139–143. (11) Bae, Y. S.; Mulfort, L. K.; Frost, H.; Ryan, P.; Punnathanam, S.; Broadbelt, L. J.; Hupp, J. T.; Snurr, R. Q. Separation of CO2 from CH4 using mixed-ligand metal-organic frameworks. Langmuir 2008, 24, 8592– 8598. (12) Fang, Q. R.; Zhu, G. S.; Xue, M.; Sun, J. Y.; Wei, Y.; Qiu, S. L.; Xu, R. R. A metal-organic framework with the zeolite mtn topology containing large cages of volume 2.5 nm3. Angew. Chem., Int. Ed. 2005, 44, 3845–3848. (13) Liu, Y. L.; Kravtsov, V. C.; Larsen, R.; Eddaoudi, M. Molecular building blocks approach to the assembly of zeolite-like metal-organic frameworks with extra-large cavities. Chem. Commun. 2006, 14, 1488– 1490. (14) Liu, Y. L.; Eubank, J. F.; Cairns, A. J.; Eckert, J.; Kravtsov, V. C.; Luebke, R.; Eddaoudi, M. Assembly of metal-organic frameworks based on indium-trimer building blocks: A porous metal-organic framework with soc topology and high hydrogen storage. Angew. Chem., Int. Ed. 2007, 46, 3278–3283. (15) Dinca, M.; Dailly, A.; Liu, Y.; Brown, C. M.; Neumann, D. A.; Long, J. R. Hydrogen storage in a microporous metal-organic framework with exposed Mn2+ coordination sites. J. Am. Chem. Soc. 2006, 128, 16876– 16883. (16) Dinca, M.; Han, W. S.; Liu, Y.; Dailly, A.; Brown, C. M.; Long, J. R. Observation of Cu2+-H2 interactions in a fully desolvated sodalitetype metal-organic framework. Angew. Chem., Int. Ed. 2007, 46, 1419– 1422. (17) Babarao, R.; Jiang, J. W.; Sandler, S. I. Adsorptive separation of CO2/CH4 mixture in metal-exposed, catenated and charged metal-organic frameworks: Insight from molecular simulation. Langmuir 2009, 25, 5239– 5247. (18) Jiang, J. W. Charged soc metal-organic framework for high-efficacy H2 adsorption and syngas purification: Atomistic simulation study. AIChE J. 2009, 55, 2422–2432. (19) Babarao, R.; Jiang, J. W. Unprecedentedly high selective adsorption of gas mixtures in rho zeolite-like metal-organic framework: A molecular simulation study. J. Am. Chem. Soc. 2009, 131, 11417–11425. (20) Yang, S. H.; Lin, X.; Blake, A. J.; Thomas, K. M.; Hubberstey, P.; Champnes, N. R.; Schroder, M. Enhancement of H2 adsorption in Li+exchanged Coordination framework materials. Chem. Commun. 2008, 6108– 6110.

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(21) Materials Studio 4.3; Accelrys: San Diego, 2008. (22) Singh, U. C.; Kollman, P. A. Atomic charges derived from smiempirical methods. J. Comput. Chem. 1984, 5, 129. (23) Besler, B. H.; Merz, K. M.; Kollman, P. A. Atomic charges derived from semiempirical methods. J. Comput. Chem. 1990, 11, 431–439. (24) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. UFF: a full periodic table force field for molecular mechanics and molecular dynamics simulations. J. Am. Chem. Soc. 1992, 114, 10024– 10035. (25) Vishnyakov, A.; Ravikovitch, P. I.; Neimark, A. V.; Bulow, M.; Wang, Q. M. Nanopore structure and sorption properties of Cu-BTC metalorganic framework. Nano Lett. 2003, 3, 713–718. (26) Babarao, R.; Hu, Z. Q.; Jiang, J. W.; Chempath, S.; Sandler, S. I. Storage and separation of CO2 and CH4 in silicalite, C168 schwarzite, and IRMOF-1: A comparative study from Monte Carlo simulation. Langmuir 2007, 23, 659–666. (27) Duren, T.; Sarkisov, L.; Yaghi, O. M.; Snurr, R. Q. Design of new materials for methane storage. Langmuir 2004, 20, 2683–2689. (28) Castillo, J. M.; Vlugt, T. J. H.; Calero, S. Understanding water adsorption in Cu-BTC metal organic frameworks. J. Phys. Chem. C 2008, 112, 15934–15939. (29) Chmelik, C.; Karger, J.; Wiebckw, M.; Caro, J.; van Baten, J. M.; Krishna, R. Adsorption and diffusion of alkanes in Cu-BTC crystals investigated using infra-red microscopy and molecular simulations. Microporous Mesoporous Mater. 2009, 117, 22–32. (30) Salles, F.; Kolokolov, D. I.; Jobic, H.; Maurin, G.; Llewellyn, P.; Devic, T.; Serre, C.; Ferey, G. Adsorption and diffusion of H2 in the MOF type systems MIL-47(v) and MIL-53(Cr). J. Phys. Chem. C 2009, 113, 7802–7812. (31) Hirotani, A.; Mizukami, K.; Miura, R.; Takaba, H.; Miya, T.; Fahmi, A.; Stirling, A.; Kubo, M.; Miyamoto, A. Grand canonical Monte Carlo simulation of the adsorption of CO2 on silicalite and Na-ZSM-5. Appl. Surf. Sci. 1997, 120, 81–84. (32) Darkrim, F.; Levesque, D. Monte Carlo simulations of hydrogen adsorption in single-walled carbon nanotubes. J. Chem. Phys. 1998, 109, 4981–4988. (33) Murthy, C. S.; Singer, K.; Klein, M. L.; McDonald, I. R. Pairwise additive effective potentials for nitrogen. Mol. Phys. 1980, 41, 1387–1399. (34) Cao, D. P.; Wu, J. Z. Modeling the selectivity of activated carbons for efficient separation of hydrogen and carbon dioxide. Carbon 2005, 43, 1364–1370. (35) Yang, Q. Y.; Zhong, C. L. Molecular simulation of carbon dioxide/ methane/hydrogen mixture adsorption in metal-organic frameworks. J. Phys. Chem. B 2006, 110, 17776–17783. (36) Akten, E. D.; Siriwardane, R.; Sholl, D. S. Monte Carlo simulation of single- and binary-component adsorption of CO2, N2, and H2 in zeolite Na-4A. Energy Fuels 2003, 17, 977–983. (37) Belmabkhout, Y.; Sayari, A. Adsorption of CO2 from dry gases on MCM-41 silica at ambient temperature and high pressure. Adsorption of CO2/N2, CO2/CH4 and CO2/H2 binary mixtures. Chem. Eng. Sci. 2009, 64, 3729–3735. (38) Gallo, M.; Nenoff, T. M.; Mitchell, M. C. Selectivities for binary mixtures of hydrogen/methane and hydrogen/carbon dioxide in silicalite and ETS-10 by grand canonical Monte Carlo techniques. Fluid Phase Equilib. 2006, 247, 135–142. (39) Dreisbach, F.; Staudt, R.; Keller, J. U. High pressure adsorption data of methane, nitrogen, carbon dioxide and their binary and ternary mixtures on activated carbon. Adsorption 1999, 5, 215–227. (40) Goj, A.; Sholl, D. S.; Akten, E. D.; Kohen, D. Atomistic simulations of CO2 and N2 adsorption in silica zeolites: The impact of pore size and shape. J. Phys. Chem. B 2002, 106, 8367–8375. (41) Bernal, M. P.; Coronas, J.; Menendez, M.; Santamaria, J. Separation of CO2/N2 mixtures using MFI type zeolite membranes. AIChE J. 2004, 50, 127–135. (42) Seike, T.; Matsuda, M.; Miyake, M. Preparation of FAU type zeolite membranes by electrophoretic deposition and their separation properties. J. Mater. Chem. 2002, 12, 366–368. (43) Yang, Q. Y.; Xue, C. Y.; Zhong, C. L.; Chen, J. F. Molecular simulation of separation of CO2 from flue gases in Cu-BTC metal-organic framework. AIChE J. 2007, 53, 2832–2840. (44) Martin-Calvo, A.; Garcia-Perez, E.; Castillo, J. M.; Calero, S. Molecular simulations for adsorption and separation of natural gas in IRMOF-1 and Cu-BTC. Phys. Chem. Chem. Phys. 2008, 10, 7085–7091. (45) Bae, Y. S.; Farha, O. K.; Hupp, J. T.; Snurr, R. Q. Enhancement of CO2/N2 selectivity in a metal-organic framework by cavity modification. J. Mater. Chem. 2009, 19 (15), 2131–2134. (46) Banerjee, R.; Furukawa, H.; Britt, D.; Knobler, C.; O’Keeffe, M.; Yaghi, O. M. Control of pore size and functionality in isoreticular zeolitic

68

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imidazolate frameworks and their carbon dioxide selective capture properties. J. Am. Chem. Soc. 2009, 131, 3875–3877. (47) Wang, B.; Cote, A. P.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Colossal cages in zeolitic imidazolate frameworks as selective carbon dioxide reservoirs. Nature 2008, 453, 207–211. (48) Choi, H. S.; Suh, M. P. Highly selective CO2 capture in flexible 3D coordination polymer networks. Ang. Chem. Int. Ed. 2009, 48, 6865– 6869. (49) An, J.; Geib, S. J.; Rosi, N. High and selective CO2 uptake in a cobalt adeninate metal-organic framework exhibiting pyrimidine- and aminodecorated pores. J. Am. Chem. Soc. 2010, 132, 38–39.

(50) Bastin, L.; Barcia, P. S.; Hurtado, E. J.; Silva, J. A. C.; Rodrigues, A. E.; Chen, B. A microporous metal-organic framework for separation of CO2/N2 and CO2/CH4 by fixed-bed adsorption. J. Phys. Chem. C 2008, 112 (5), 1575–1581.

ReceiVed for reView January 29, 2010 ReVised manuscript receiVed March 19, 2010 Accepted March 23, 2010 IE100214A