Energy & Fuels 2007, 21, 953-956
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Comparative Molecular Simulation Study of Methane Adsorption in Metal-Organic Frameworks Sanyue Wang* Department of Chemical Engineering, The Key Lab of Bioprocess of Beijing, Beijing UniVersity of Chemical Technology, Beijing 100029, China ReceiVed NoVember 16, 2006. ReVised Manuscript ReceiVed January 26, 2007
A systematic Monte Carlo simulation has been performed on the adsorption of CH4 in a series of 10 metalorganic frameworks (MOFs) to confirm the desired characteristics of an optimal adsorbent for methane storage. The simulation shows that high isosteric heat of adsorption, specific accessible area, free volume, and low density of framework are all desirable properties for a material with high adsorption capacity. However, not all these properties are compatible, and there exists a complex interplay of these that influence the uptake of methane. We conclude that the accessible surface area and free volume play a main role in determining methane uptake at 298 K and 3.5 MPa from this simulation.
1. Introduction Recently, the increasing demand for energy has led to an acceleration of efforts to utilize new technologies based on alternate sources such as hydrogen and natural gas. Natural gas, which consists mainly of methane, is widely available in many countries. However, its application has been impeded by the absence of safety and economical techniques for storage compared as compressed natural gas (CNG). An attractive alternative to CNG is adsorbed natural gas (ANG), which is usually stored in carbon materials, zeolite, and other porous materials.1-3 As a new class of microporous materials, metal-organic frameworks (MOFs), has emerged as promising materials for gas storage, separation, catalysis, etc.3-7 To date, a variety of MOFs have been synthesized, and some of them show high capacity for methane storage.3,8-10 On the other hand, molecular simulations have been used to study the adsorption of various gases in MOFs.11-20 Sarkisov et al.12 has used molecular dynamics (MD) to examine the self-diffusion of methane, n-pentane, n-hexane, n-heptane, and cyclohexane in MOF-5 (also known as IRMOF-1), each at low loadings. This type of calculation described the transport rate of individual, isolated adsorbates. Skoulidas and Sholl14 used MD to probe the self* To whom correspondence should be addressed. Tel.: +86-1064431705. E-mail:
[email protected]. (1) Celzard, A.; Fierro, V. Energy Fuels 2005, 19, 573. (2) Bhatia, S. K.; Myers, A. L. Langmuir 2006, 22, 1688. (3) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keefe, M.;Yaghi, O. M. Science 2002, 295, 469. (4) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334. (5) Snurr, R. Q.; Hupp, J. T.; Nguyen, S. T. AIChE J. 2004, 50, 1090. (6) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4670. (7) Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre, J. J. Mater. Chem. 2006, 16, 626. (8) Kondo, M.; Okubo, T.; Asami, A.; Noro, S.; Yoshitomi, T.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Seki, K. Angew. Chem., Int. Ed. 1999, 38, 140. (9) Noro, S.; Kitagawa, S.; Kondo, M.; Seki, K. Angew. Chem., Int. Ed. 2000, 39, 2081. (10) Noro, S.; Kitaura, R.; Kondo, M.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Yamashita, M. J. Am. Chem. Soc. 2002, 124, 2568.
diffusion and transport diffusion of a number of small gas species in several MOFs as a function of pore loading at room temperature. They concluded that the diffusion of Ar in MOF2, MOF-3, and Cu-BTC has been assessed in a similar manner. Particularly, Snurr and co-workers18 investigated the adsorption of methane in a series of isoreticular metal-organic frameworks (IRMOFs) synthesized by Yaghi and co-workers3 and compared them with other porous materials, such as zeolites and carbon nanotubes, using grand canonical Monte Carlo (GCMC) simulations. More recently, they19 examined the hydrogen adsorption in 10 IRMOFs by GCMC simulations and revealed the relationships between hydrogen adsorption capacity and the characteristics of IRMOFs. They found the existence of three adsorption regimes: at low pressure (loading), hydrogen uptake correlated with the heat of adsorption; at intermediate pressure, uptake correlated with the surface area; and at the highest pressures, uptake correlated with the free volume. In this work, a systematic simulation study was performed on the adsorption of methane in various MOFs with different topologies, including five IRMOFs (IRMOF-1, 6, 8, 10, and 14),3 Cu-BTC,21 two CPL MOFs (CPL-28 and CPL-522), and Cu(AF6)(bpy)2 (A ) Si and Ge).9,10 Compared with the work of Snurr and co-workers,18 the topologic effects can be considered, leading to more information for understanding the adsorption of methane in other MOFs. (11) Vishnyakov, A.; Ravikovitch, P. I.; Neimark, A. V.; Bulow, M.; Wang, Q. Nano Lett. 2003, 3, 713. (12) Sarkisov, L.; Du¨ren, T.; Snurr, R. Q. Mol. Phys. 2004, 102, 211. (13) Garberoglio, G.; Skoulidas, A. I.; Johnson, J. K. J. Phys. Chem. B 2005, 109, 13094. (14) Skoulidas, A. I.; Sholl, D. S. J. Phys. Chem. B 2005, 109, 15760. (15) Skoulidas, A. I. J. Am. Chem. Soc. 2004, 126, 1356. (16) Yang, Q.; Zhong, C. J. Phys. Chem. B 2005, 109, 11862. (17) Mulder, F. M.; Dingemans, T. J.; Wagemaker, M.; Kearley, G. J. Chem. Phys. 2005, 317, 113. (18) Du¨ren, T.; Sarkisov, L.; Yaghi, O. M.; Snurr, R. Q. Langmuir 2004, 20, 2683. (19) Frost, H.; Du¨ren, T.; Snurr, R. Q. J. Phys. Chem. B 2006, 110, 9565. (20) Jiang, J.; Sandler, S. I. Langmuir 2006, 22, 5702. (21) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148. (22) Uemura, T.; Kitaura, R.; Ohta, Y.; Nagaoka, M.; Kitagawa, S. Angew. Chem., Int. Ed. 2006, 45, 4112.
10.1021/ef060578f CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007
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Table 1. Properties of MOFs Investigated in This Worka MOFs material
space group
Cu-BTC CPL-2 CPL-5 Cu(SiF6)(bpy)2 Cu(GeF6)(bpy)2 IRMOF-1 IRMOF-6 IRMOF-8 IRMOF-10 IRMOF-14
Fm/3m P21/c P21/c P4/mmm P4/mmm Fm/3m Fm/3m Fm/3m Fm/3m Fm/3m
a
porous structures pockets/pores channel channel cuboid cuboid cubic cubic cubic cubic cubic
Obtained from the Materials Studio package.23
b
dporeb (Å)
q∞st c (kJ/mol)
Fcrys (g/cm3)
Sacc (m2/g)
Vfree (cm3/g)
5.0/9.0 8.2 10.3 8.0 8.0 10.9/14.3 9.1/14.5 12.5/17.1 16.7/20.2 14.7/20.1
18.7 16.1 15.5 12.4 12.3 9.8 11.4 9.2 8.0 9.1
0.88 1.42 1.29 0.86 0.92 0.59 0.65 0.45 0.33 0.37
1909.28 378.54 594.03 1566.90 1555.03 3414.80 2862.49 4182.59 4775.39 4713.07
0.82 0.31 0.38 0.76 0.71 1.36 1.18 1.87 2.66 2.34
Obtained from the XRD data.3,8-10,21,22
2. Model and Methods MOFs Model. The structural models of MOFs: Cu-BTC, CPL2, CPL-5, Cu(AF6)(bpy)2, and IRMOFs using in the simulation were all guest-free microporous frameworks and were constructed from the X-ray diffraction (XRD) data3,8-10,21,22 using Materials Studio Visualizer.23 The types of pores in CPL-2 and CPL-5 were channel and Cu-BTC, Cu(AF6)(bpy)2, and IRMOFs have cubic pores. The available surface area (Sacc) and free volume (Vfree) were calculated for MOFs by using Materials Studio Visualizer.23 The surface area was defined by “rolling” a probe molecule with a diameter equal to the Lennard-Jones σ parameter for CH4 (σCH4 ) 0.373 nm) over the framework surface. The free volume was calculated by using a similar method of trial insertions within the entire volume of the unit cell. The details of properties for all MOFs are shown in Table 1. Simulation Details. In the present work, a single Lennard-Jones (LJ) interaction site model was used to describe a methane molecule. The potential parameters for methane (σCH4 ) 0.373 nm, CH4/kB ) 148.0 K) was taken from the TraPPE forced field developed by Martin and Siepmann.24 The above potential models have been successfully used to model the adsorptions of methane, ethane and carbon dioxide in MOFs.18,25 For MOFs materials, an all-atom representation was adopted. The all-atom OPLS (OPLS-AA) force field28 was adopted as the force field to calculate the interactions between the methane molecule and the atoms in the framework of the MOFs materials. The standard grand canonical Monte Carlo simulation (GCMC) was employed to calculate the adsorption of methane in all MOFs. Details on the method are given elsewhere.29 It is notable that there are simulation techniques related to GCMC that give global information on the adsorption isotherm more rapidly than GCMC.30 The number of the unit cells of MOFs adopted in the simulations is from 1 × 1 × 1 to 3 × 3 × 4 so that enough molecules are accommodated to guarantee the simulation accuracy. All frameworks are assumed to be rigid in simulations. A cutoff radius of 13.5 Å is applied to the LJ interactions. Periodic boundary conditions are applied in all three dimensions. For each state point, GCMC simulation consisted of 1 × 107 steps to guarantee equilibration followed by 1 × 107 steps to sample the desired thermodynamic properties. To estimate the statistical uncertainty, the production phase of each state point is divided into 10 blocks and the standard deviation of the block average was calculated. The uncertainties on the final results, including the ensemble averages of the number of adsorbate molecules in the simulation cell and the total potential energy, are estimated to be within (2%. (23) Accelrys, Inc. Materials Studio, 3.1 V; Accelrys Inc: San Diego, CA, 2003. (24) Martin, M. G.; Siepmann, J. I. J. Phys. Chem. B 1998, 102, 2569. (25) Yang, Q.; Zhong, C. Chem. Phys. Chem. 2006, 7, 1417. (26) Yang, Q.; Zhong, C. J. Phys. Chem. B 2006, 110, 17776. (27) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (28) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem. Soc. 1996, 118, 11225. (29) Frenkel, D.; Smit, B. Understanding Molecular Simulation: From Algorithms to Applications; Academic Press: San Diego, 2002. (30) Chen, H.; Sholl, D. S. Langmuir 2006, 22, 709.
c
Obtained from the simulations in this work.
Table 2. Potential Parameters for the Atoms in the Framework of Cu(AF6)(bpy)2 (A ) Si, Ge) atom
Cu
Si/Ge
F
Cbpy
Hbpy
N
σ (nm) /k (K)
0.311a 2.52a
0.383/0.381a 202.29/190.72a
0.295c 17.34b
0.355c 28.18b
0.242c 15.10c
0.325c 42.78b
a Taken from the all-atom UFF force field (they are missed in the OPLSAA force field) of Rappe et al.27 b Obtained in this work. c Taken from the OPLS-AA force field of Jorgensen et al.28
Table 3. Potential Parameters for the Atoms in the Framework of CPL-2 atom
Cu
O
Cpyz
Hpyz
N
σ (nm) /k (K)
0.311a
0.296c
CCarboxyl 0.375c
0.355c
0.242c
2.52a
52.9b
36.98b
35.23c
15.10c
0.325c 42.78b
a Taken from the all-atom UFF force field (they are missed in the OPLSAA force field) of Rappe et al.27 b Obtained in this work. c Taken from the OPLS-AA force field of Jorgensen et al.28
In addition, the chemical potentials needed in the GCMC simulations were calculated from NPT ensemble Monte Carlo simulation using the test-particle insertion method.31 On the basis of the simulated chemical potentials at various pressures, relationships between pressure and chemical potential were established to convert pressures to chemical potentials, and vice versa.
3. Results and Discussion Refinement of Part Force Field Parameters. Since the parameters of the OPLS-AA force field were developed for liquids with the standard combining rules of σij ) (σiσj)1/2 and ij ) (ij)1/2, the existing parameters may not properly represent the interactions of the atoms of the solid MOF materials with the adsorbate molecules. Therefore, parts of the OPLS-AA force field parameters were adjusted to yield better agreement with the experimental single-component adsorption data. The parameters of Cu-BTC and IRMOFs were obtained from the work of Yang and Zhong,25,26 while those for the other MOFs were obtained in this work: the energy parameters of fluorin, carbon, and nitrogen in Cu(SiF6)(bpy)2 and those of oxygen, carboxyl carbon, and nitrogen in CPL-2 were adjusted to give better representation of the corresponding experimental adsorption isotherms of methane,8-10 as shown in Tables 2 and 3, respectively. Figure 1b shows that the parameters obtained for Cu(SiF6)(bpy)2 as shown in Figure 1a could predict the experimental adsorption isotherm of methane in Cu(GeF6)(bpy)2 quite well; thus, the same parameters were adopted for Cu(GeF6)(bpy)2 with no refinements. On the basis of this observation, the parameters obtained for CPL-2 were adopted for CPL-5 since no experimental adsorption data of methane are available in CPL-5. Prediction of Methane Adsorption Isotherms up to 10 MPa. With the refined parameters, the methane adsorption (31) Shing, K. S.; Chung, S. T. J. Phys. Chem. 1987, 91, 1674.
Methane Adsorption in MOFs
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Figure 1. Comparison of simulated and experimental adsorption isotherms of CH4 (a) for Cu(SiF6)(bpy)2, (b) for Cu(GeF6)(bpy)2, and (c) for CPL-2 at 298 K.
Figure 2. Methane adsorption isotherms in terms of (a) volume and (b) mass at 298 K in various MOFs predicted from simulation.
isotherms in the ten MOFs at 298 K were predicted as a function of pressure up to 10 MPa, as shown in Figure 2 in which the results are both in cubic centimeters (STP) per cubic centimeter (Figure 2a) and cubic centimeters (STP) per gram (Figure 2b) were given. Figure 2 shows that the highest uptake of methane at low pressures does not mean the largest uptake at higher pressures as well. Also, the sequence for adsorption capacity in cubic centimeters (STP) per cubic centimeter may be different with that in cubic centimeters (STP) per gram. This can be attributed to the interplay of various factors, such as topology, accessible surface, framework density, free volume, and adsorbate/ adsorbent interactions, etc. Therefore, a systematic study on the relationships between the methane adsorption capacity and the various influence factors is necessary to guide future design of MOFs with improved methane storage capacity. Relation of Accessible Surface Area and Adsorption at Modest Pressure. On the basis of other adsorption litera-
ture,1,2,18,19,32 the following properties of absorbents are important factors in the adsorption capacity: accessible surface (Sacc), adsorbent framework density (Fcrys), free volume (Vfree), and adsorbent/guest affinity. Usually, the last factor can be characterized by the isosteric heat of adsorption at infinite dilution (q∞st ). Although the properties of a complicated adsorbent listed above all are not compatible, there should be some main factors influencing the adsorption storage at some conditions. The amount adsorbed per mass at 3.5 MPa is a primary target for methane storage in practical applications, so I examined the properties of the adsorbent influencing the adsorption at 298 K and 3.5 MPa for all MOFs first. I examined the relations between all properties of the absorbent listed above and the amount adsorbed at 298 K and 3.5 MPa and found that the amount of methane adsorbed can be plotted as a near linear function of the accessible surface area per mass as shown in Figure 3a. I also plotted the amount adsorbed against the isosteric heat of adsorption at infinite dilution and the amount adsorbed against the free volume of the frameworks. The results, as presented in Figure 3b and c, show that there are not excellent correlations in both cases. Thus, it can be concluded that the accessible surface area would be more important than others for methane adsorbed at room temperature and moderate pressure. Pores Sizes and Heat of Adsorption Change on Adsorption at Low Pressure. As described above, at low loading (pressure) materials with the strongest enthalpy interactions with sorbed molecules show the highest levels of adsorption capacity. It is natural to expect that enthalpy interactions play a main role in methane adsorption at low loading and can correlate with the amounts adsorbed. To make this idea more quantitative, I plotted the amount adsorbed at low pressure (0.05 MPa) against the isosteric heat of adsorption at infinite dilution. The results, as presented in Figure 4a, show that there are some excellent correlations in this case. I also examined the amount adsorbed against the accessible surface area and the amount adsorbed against the free volume of the frameworks. But, there are not evident correlations in both cases and the figures are omitted for clarity. Considering that there are some correlations between the q∞st and the sizes of pores, I examined their relations as shown in Figure 4b for all MOFs. We can know from the Figure 4b that the q∞st values of all MOFs decrease with the sizes of pores increasing in general. Some MOFs materials (CPL-2, 5) disobey the trends just for the different framework topology, constituent, atom types, and wall chemistry. To explain this, I plotted the q∞st of the same series: IRMOFs against the sizes of pores (32) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; VCH: New York, 1984.
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Figure 3. Methane amount adsorbed at 3.5 MPa vs (a) accessible surface area, (b) isosteric heat of adsorption at infinite dilution, and (c) free volume.
Figure 4. (a) Methane amount adsorbed at 0.05 MPa vs isosteric heat of adsorption at infinite dilution and heat of adsorption vs the sizes of pores in (b) various MOFs and (c) IRMOFs.
alone as shown in Figure 4c. We can obtain a single linear correlation of the heat of adsorption with the sizes of pores in IRMOFs. Free Volume of Adsorbent Framework vs Amount Adsorbed at High Pressure. High free volume and low framework density are related features for an efficient adsorbent material. I examined the amount adsorbed vs free volume, heat of adsorption, and accessible surface area at high pressures like Frost et al.19 As shown in Figure 5a and b, the free volume and surface area can be plotted in linear relations with the amount adsorbed. There are not evident correlations between the capacity of gas storage and heat of adsorption; thus, the figure is omitted. At higher pressures (loading), the packing effects are important and become the leading factor influencing the amount adsorbed because more molecules adsorbed are far away from the preferential sites. On the other hand, adsorbents should afford more room to accommodate new molecules when the pressures (loadings) increase. Therefore, high free volume, large accessible surface area, and low framework density are expected for the adsorbent so that there are still plenty of open spaces in the middle of the pores or the surface area at high pressures (loadings). 4. Conclusions Natural gas mainly consisting of methane is a valuable alternative to more conventional fuels; however, its very low intrinsic density constitutes a major disadvantage in practical application. To synthesize a new desirable material for methane storage, an adsorbent with sufficiently high isosteric heat of adsorption, specific accessible area, free volume, and low density of framework should be considered. By comparing methane uptake in various MOFs in simulations, we were able to confirm the complex interplay of factors influencing adsorption. Although the desirable properties listed above all are not neces-
Figure 5. Methane amount adsorbed at 10 MPa vs free volume (a) and accessible surface (b).
sarily compatible, the accessible surface area and free volume play main roles in methane uptake at 298 k and 3.5 MPa by GCMC in this work. It shows again that GCMC simulation is an effective tool to predict methane adsorption and guide the design of new materials for gas storage. EF060578F
