Computational Study on the Influences of Framework Charges on CO2

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Ind. Eng. Chem. Res. 2009, 48, 10479–10484

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Computational Study on the Influences of Framework Charges on CO2 Uptake in Metal-Organic Frameworks Chengcheng Zheng, Dahuan Liu, Qingyuan Yang, Chongli Zhong,* and Jianguo Mi Laboratory of Computational Chemistry, Department of Chemical Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China

This work involved a computational study to investigate the influences of framework charges on CO2 uptake in metal-organic frameworks (MOFs), in which a total of 20 MOFs with different topologies, pore sizes, and chemical characteristics were examined. The results showed that, at atmospheric pressure, the contribution of the framework charges is generally large, and a linear relationship with pore size was found, showing that, when the pore size is larger than 3.3 nm, the contribution becomes smaller than 10%. On the other hand, the framework charge contribution was found to decrease rapidly with increasing pressure and to become less than 10% at pressures higher than 2.0 MPa. This work shows that the framework charge contribution in MOFs cannot be ignored in computational screening of MOF materials for CO2 capture under low-pressure conditions, whereas at moderate operating pressures, the contribution can be ignored in large-scale prescreening such as in the natural gas upgrading process. 1. Introduction Carbon dioxide capture is of great economic and environmental importance,1,2 as this gas plays a significant role in global warming,3-5 natural gas upgrading,6 hydrogen purification,7 and so on. To achieve this purpose, exploring a high-performance adsorbent is crucial. Recently, metal-organic frameworks (MOFs)8-20 have emerged as a new class of porous materials and shown great promise for CO2 storage1 and separation applications,6,7 and efforts have been made to design and select MOFs with improved performance.21 In this respect, computational chemistry is a powerful method for obtaining microscopiclevel insights into the phenomena studied, and it can serve as a large-scale prescreening tool. Normally, the electrostatic interactions between the framework and CO2 have to be considered; however, the framework charges of MOFs can be obtained only by using time-consuming quantum mechanical calculations, which becomes a key obstacle to screening MOFs at a large scale through computational methods. Therefore, in this work, a series of typical MOFs was examined to determine the extent to which the framework charges influence CO2 uptake in MOFs, which could provide useful information for large-scale computational screening of MOFs for a given application and thus stimulate the industrial applications of MOFs in processes such as flue gas purification and natural gas upgrading. 2. Models and Simulation Method 2.1. MOF Structure. A total of 20 typical MOFs were chosen, including eight isoreticular MOFs (IRMOF-1, -8, -9, -12, -13, -14, -15, and -16),8 four porous coordination networks (PCN-6,9 -6′,10 -10,11 and -1312), three zeolitic imidazolate frameworks (ZIF-3, -10,13 and -6914), two MILs (Materiaux Institut Lavoisier) [MIL-47(V)15 and -53(Al)16], copper(II) benzene-1,3,5-tricarboxylate (Cu-BTC),18 MOF-177,19 and MOF505.20 These MOFs have different topologies, pore sizes, and chemical characteristics to ensure the reliability of the results, and the structural properties are shown in Table 1. 2.2. Force Fields. Because of the large molecular quadrupole moment, CO2 was modeled as a rigid linear triatomic molecule * To whom correspondence should be addressed. Tel.: +86-1064419862. E-mail: [email protected].

with three charged Lennard-Jones (LJ) interaction sites and with a C-O bond length of l ) 0.116 nm. The intrinsic quadrupole moment is approximately described by partial point charges centered at each LJ site (qo ) -0.350 e and qc ) 0.700 e). Similarly to the approach employed for CO2, an atomistic representation was used to model all of the MOFs. A combination of the site-site LJ and Coulombic potential was used to calculate the interactions between adsorbate molecules and adsorbents. The potential parameters for the framework atoms in MOFs were taken from the Dreiding force field,23 as listed in Table 2. In our simulations, all of the LJ cross-interaction parameters were determined using the Lorentz-Berthelot mixing rules. 2.3. Grand Canonical Monte Carlo Simulation Details. In this study, a conventional grand canonical Monte Carlo (GCMC) simulation was performed to calculate the adsorption of CO2 in the MOFs. The simulation boxes representing MIL53(Al) and MIL-47(V) contained 36 (6 × 2 × 3) unit cells, whereas 8 (2 × 2 × 2) unit cells were employed for the other MOFs. No finite-size effects existed, as verified by checking the simulations with larger boxes. As in previous works,4 all of the MOFs were treated as rigid with atoms frozen at their crystallographic positions during simulations. The cutoff radius was set to 12.8 Å for the LJ interactions, and long-range electrostatic interactions were handled using the Ewald summation technique with tinfoil boundary conditions. For each state point, the GCMC simulation consisted of 1.0 × 107 steps to guarantee equilibration, followed by 1.0 × 107 steps to sample the desired thermodynamics properties. The isosteric heat of adsorption, qst, was calculated from qst ) RT -

〈UffN〉 - 〈Uff〉〈N〉 〈N 〉 - 〈N〉〈N〉 2

-

〈UsfN〉 - 〈Usf〉〈N〉 〈N2〉 - 〈N〉〈N〉

(1)

where R is the gas constant, N is the number of molecules adsorbed, and 〈〉 indicates the ensemble average. The first and second terms on the right-hand side are the contributions from the molecular thermal energy and adsorbate-adsorbate interaction energy, Uff, respectively. The third term is the contribution from the adsorbent-adsorbate interaction energy, Usf. A detailed

10.1021/ie901000x CCC: $40.75  2009 American Chemical Society Published on Web 10/08/2009

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Table 1. Structures and Properties of the MOFs Studied in This Work material

space groupa

pore shapea

dporea (nm)

Fcrysta (g/cm3)

Vfreeb (cm3/g)

Saccb (m2/g)

IRMOF-1 IRMOF-8 IRMOF-9 IRMOF-12 IRMOF-13 IRMOF-14 IRMOF-15 IRMOF-16 ZIF-3 ZIF-10 ZIF-69 PCN-6 PCN-6′ PCN-10 PCN-13 Cu-BTC MOF-177 MOF-505 MIL-47(V) MIL-53(Al)

Fm3jm Fm3jm Pnnm Fm3jm R3jm Fm3jm Im3jm Pm3jm Pm j nm Im j mm Pm j mc R3jm Fm3jm R3j I4j3d Fm3jm P3j1c R3jm Pnjma Im j ma

cubic cubic cubic/catenation cubic cubic/catenation cubic cubic/catenation cubic pore/channel pore/channel pore/channel pore/catenation pocket/channel pore/channel pore/channel pocket/channel pore/channel pore/channel channel channel

1.09/1.43 1.25/1.71 0.45/0.63/0.81/1.07 1.30/2.45 0.42/0.47/0.61/0.73/1.11 1.47/2.01 2.33 0.46/0.60 0.82/1.21 0.44/0.78 1.52/3.03 0.81 0.35 0.50/0.90 1.08/1.18 1.01 1.05 1.14

0.59 0.45 0.66 0.38 0.75 0.37 0.41 0.21 0.88 0.70 1.21 0.56 0.28 0.77 1.25 0.88 0.43 0.93 1.00 0.98

1.36 1.87 1.14 2.23 0.95 2.30 2.01 4.46 0.77 1.04 0.57 1.39 3.04 1.04 0.44 0.82 1.96 0.74 0.63 0.70

3748 4360 3563 5240 2830 4800 5915 5882 2263 2734 1250 3912 4859 2794 1149 2368 4689 2455 1684 1652

a

Obtained from the XRD crystal data. b Calculated with the Materials Studio package.22

Table 2. LJ Potential Parameters for CO2 and the MOFs Used in This Work atom

σ (nm)

ε/νB (K)

CO2_C CO2_O MOF_Zn MOF_Cu MOF_Al MOF_C MOF_H MOF_O MOF_N MOF_Cl MOF_V

0.280a 0.305a 0.404b 0.309b 0.391b 0.347b 0.285b 0.303b 0.326b 0.352b 0.280c

27.00a 79.00a 27.68b 25.16b 156.00b 47.86b 7.65b 48.16b 38.95b 142.57b 8.05c

a Taken from the TraPPE force field.24 b Taken from the Dreiding force field.23 c Taken from the all-atom UFF force field25 (missing in the Dreiding force field).

description of the simulation methods can be found in ref 26 and our previous work.4 3. Results and Discussion 3.1. Calculation of Framework Charges in MOFs. In all of the simulations, atomic partial charges in the frameworks were required as input parameters. Those for the IRMOFs, CuBTC, MOF-177, ZIF-69, MIL-47(V), MIL-53(Al), and MOF505 were taken from the literature,4,5,27,28 whereas for ZIF-3, ZIF-10, and the PCNs (-6, -6′, -10, and -13), the atomic partial charges were calculated in this work using density functional theory (DFT) on the basis of the fragment clusters with the

Figure 1. Model clusters and partial charges of the MOFs obtained in this work. (Zn, yellow; Cu, pink; O, red; C, gray; H, white; N, blue).

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Figure 2. Comparison of simulated and experimental adsorption isotherms of CO2: (a) in Cu-BTC, IRMOF-1, MOF-177, and MOF-505 at 298 K1 and in MIL-47(V)32 at 304 K and (b) in ZIF-6914 at 273 K.

Figure 3. Contribution of framework charges to CO2 uptake at 298 K: (a-c) in MOFs without catenation and (d) in MOFs with catenation.

Gaussian 03 package.29 Based on the ChelpG method,30 DFT calculations using the unrestricted B3LYP function were carried out to compute the atomic partial charges, and two kinds of basis sets were employed: For heavy atoms, such as Zn and Cu, the LANL2DZ basis set was used, and 6-31+G* was employed for the rest of the atoms. These methods have been widely used for framework charge calculations in MOFs.4,28,31 For the cleaved clusters of ZIF-3 and ZIF-10, the terminations are connected with metal Zn atoms in the real system, and thus, they were saturated by light metal Li atoms, whereas for the cleaved clusters of PCNs (-6, -6′, -10, -13), the terminations are all connected with organic linkers, and therefore, they were saturated with -CH3 groups, as was done in our previous work.4 The atomic partial charges and the corresponding model clusters as shown in Figure 1. 3.2. Validation of the Method. To confirm the reliability of the force fields and the atomic partial charges employed in

this work, the adsorption isotherms of CO2 were simulated and compared with available experimental data, as shown in Figure 2. As can be seen from the figure, the simulated isotherms are in reasonable agreement with the corresponding experimental results.1,14,32,33 Considering the experimental uncertainties, it can be concluded that the force fields used are applicable to the MOFs considered in this work. 3.3. Influences of Framework Charges. To investigate the effects of framework charges on CO2 uptake in MOFs, additional simulations were performed by switching off the electrostatic interactions between the CO2 molecules and the frameworks atoms. We define the contribution of the framework charges to the total uptake as (Nwith - Nwithout)/Nwith, where Nwith and Nwithout denote the absolute amounts adsorbed with and without the CO2-MOF electrostatic interactions, respectively. Because the calculation of the contribution of framework charges is valid and accurate only until pore

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Figure 4. Probability distribution plots of the COM of CO2 in the XY plane at 0.1 MPa (Zn, yellow; Cu, pink; O, red; C, gray; H, white; N, blue; Cl, green).

saturation, only the calculated results up to a pressure slightly higher than the saturation pressure of each MOF are given, as shown in Figure 3. 3.3.1. Low Pressures. From Figure 3, it can be seen that, at low pressures, the contribution of framework charges to CO2 uptake is large in all of the MOFs studied in this work. The value can reach about 40% at very low pressure, such as for ZIF-69 and Cu-BTC. To understand this observation, the centerof-mass (COM) probability distributions of CO2 in different MOFs at 0.1 MPa were examined in terms of all of the configurations recorded during the GCMC simulations. Figure 4 shows the distributions in ZIF-69, IRMOF-1, and PCN-6′ as examples. As can be seen from the figure, the strongest adsorption for CO2 molecules occurs around the metal cluster regions at 0.1 MPa; this indicates that the electrostatic interaction between the framework and CO2 plays a dominant role in the adsorption capacity, as the absolute values of the charges in the metal clusters are highest in the frameworks. From Figure 3, it is interesting to note that, for materials with the same primitive topologies, the electrostatic contribution increases with decreasing pore size at 0.1 MPa (Figure 3a for IRMOFs and Figure 3b for ZIFs and PCNs). Thus, the relationship between the contribution of the framework charges and the pore size at 0.1 MPa was analyzed in this work. First, the specific pores in which the CO2 molecules are adsorbed should be determined because there are pores of different sizes in some MOFs. For ZIF-69, CO2 molecules are mainly adsorbed in the small pores, as shown in Figure 4a. Similar situations are found in ZIF-3 and ZIF-10, which are not shown in the figure. For the IRMOFs, the CO2 molecules first occupy the corner regions of the larger pores, as shown in Figure 4b. From Figure 4c, it can be seen that pockets rather than channels are the adsorption sites at 0.1 MPa in PCN-6′. For the MOFs with

Figure 5. Framework charge contribution to CO2 uptake vs pore size at 0.1 MPa.

catenation, it is difficult both to measure the pore size and to accurately determine the pores of adsorption; thus, these MOFs were not considered. The calculated framework charge contribution to CO2 uptake versus the size of the pores in which adsorption occurs at 0.1 MPa is shown in Figure 5. Obviously, there is a good linear relationship in each series with similar structure and topology, whereas for all the MOFs considered, an approximate linear relationship also exists. The framework charge contribution decreases with increasing pore size, and a linear relationship was obtained as given in eq 2, with a correlation coefficient of 0.911 Y ) 36.4 - 0.8X

(2)

where Y is the contribution of framework charges (%) and X is the pore size (Å). An extrapolation of eq 2 shows that, when

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Figure 6. Probability distribution plots of the COM of CO2 in the XY plane in IRMOF-14 (Zn, yellow; O, red; C, gray; H, white).

4. Conclusions

Figure 7. Contribution of framework charges to CO2 uptake at 298 K in three zeolites.

the pore size is larger than 3.3 nm, the contribution of the framework charges is less than 10% at 0.1 MPa; this suggests that it might be acceptable to ignore the effects of the framework charges in MOFs with large pores for the initial screening of MOF materials for CO2 capture applied at atmospheric or higher pressures, such as in the flue gas purification process. 3.3.2. Moderate Pressures. From Figure 3, it can be seen that the framework charge contribution decreases rapidly with increasing pressure. The COM probability distributions of CO2 at different pressures were examined, and Figure 6 shows, as an example, the distributions in IRMOF-14. With increasing pressure, the CO2 molecules begin to adsorb mainly around the organic units. It is known that the charges in the organic linkers are smaller than those in the metal clusters. Furthermore, the CO2 molecules tend to gather in the center of the pore with a further increase in pressure (2.0 MPa). The above observations explain the quick decrease of the contribution of framework charges with increasing pressure. Interestingly, Figure 3 shows that, when the pressure is above 2.0 MPa, the contribution of the framework charges to CO2 uptake is less than of approximately equal to 10% for all of the MOFs considered; this indicates that, in applications of CO2 capture operated at moderate or high pressures, the contribution of the framework charges can be ignored for large-scale initial computational screening of MOF materials without introducing large errors, such as in the natural gas upgrading process. To make a comparison with zeolites, three typical zeolites, DDR, MFI, and LTA, were considered with the framework charges taken from the literature.34 The results shown in Figure 7 demonstrate that the contribution of the zeolite framework charges is larger than that in MOFs; however, because the methodologies for calculating framework charges in MOFs and zeolites are different, a definite conclusion remains open and requires further investigation.

The simulation results reported herein show that the contribution of framework charges to CO2 uptake depends largely on pressure. Generally speaking, the contribution is more significant at low pressures, and it decreases rapidly with increasing pressure. For applications around atmospheric pressure, such as flue gas purification, the effect of framework charges cannot be ignored unless the pores are large, normally larger than 3.3 nm. The results of this work show that an approximate linear relationship between framework charge contribution and pore size exists at atmospheric pressure. On the other hand, for applications operated at moderate or high pressures, such as in the natural gas purification process, the framework contribution becomes less important and is usually less than 10%. In this case, it is reasonable to neglect the framework charge contribution in the initial material screening, which makes it possible to pursue a large-scale computational screening of MOF materials for applications operated at moderate or high pressures. Acknowledgment Financial support from the NSFC (Nos. 20725622, 20876006, 20821004) is greatly appreciated. Literature Cited (1) Millward, A. R.; Yaghi, O. M. Metal-Organic Frameworks with Exceptionally High Capacity for Storage of Carbon Dioxide at Room Temperature. J. Am. Chem. Soc. 2005, 127, 17998. (2) Ramsahye, N. A.; Maurin, G.; Bourrelly, S.; Llewellyn, P. L.; Serre, C.; Loiseau, T.; Devic, T.; Fe´rey, G. Probing the Adsorption Sites for CO2 in Metal Organic Frameworks Materials MIL-53 (Al, Cr) and MIL-47(V) by Density Functional Theory. J. Phys. Chem. C 2008, 112, 514. (3) Keskin, S.; Liu, J.; Rankin, R. B.; Johnson, J. K.; Sholl, D. S. Progress, Opportunities, and Challenges for Applying Atomically Detailed Modeling to Molecular Adsorption and Transport in Metal-Organic Framework Materials. Ind. Eng. Chem. Res. 2009, 48, 2355. (4) Yang, Q.; Zhong, C.; Chen, J.-F. Computational Study of CO2 Storage in Metal-Organic Frameworks. J. Phys. Chem. C 2008, 112, 1562. (5) Liu, D.; Zheng, C.; Yang, Q.; Zhong, C. Understanding the Adsorption and Diffusion of Carbon Dioxide in Zeolitic Imidazolate Frameworks: A Molecular Simulation Study. J. Phys. Chem. C 2009, 113, 5004. (6) Yang, Q.; Zhong, C. Electrostatic-Field-Induced Enhancement of Gas Mixture Separation in Metal-Organic Frameworks: A computational Study. ChemPhysChem 2006, 7, 1417. (7) Yang, Q.; Zhong, C. Molecular Simulation of Carbon Dioxide/ Methane/Hydrogen Mixture Adsorption in Metal-Organic Frameworks. J. Phys. Chem. B 2006, 110, 17776. (8) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage. Science 2002, 295, 469. (9) Sun, D.; Ma, S.; Ke, Y.; Collins, D. J.; Zhou, H.-C. An Interweaving MOF with High Hydrogen Uptake. J. Am. Chem. Soc. 2006, 128, 3896.

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ReceiVed for reView June 19, 2009 ReVised manuscript receiVed September 18, 2009 Accepted September 30, 2009 IE901000X