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Hydrogen and methane storage in ultrahigh surface area Metal–Organic Frameworks
Microporous and Mesoporous Materials 182 (2013) 185–190
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Hydrogen and methane storage in ultrahigh surface area Metal–Organic Frameworks Lifeng Ding a,1, A. Ozgur Yazaydin a,b,⇑ a b
Department of Chemical Engineering, University of Surrey, Guildford GU2 7XH, United Kingdom Department of Chemistry, Michigan State University, East Lansing, MI 48824, United States
a r t i c l e
i n f o
Article history: Received 8 May 2013 Received in revised form 23 August 2013 Accepted 27 August 2013 Available online 6 September 2013 Keywords: Surface area Metal–organic frameworks Gas storage Molecular simulations
a b s t r a c t In this work we computationally studied eight Metal–Organic Frameworks (MOFs) which showed or is expected to have ultrahigh surface areas (NU-100, NU-108, NU-109, NU-110, MOF-180, MOF-200, MOF-210 and MOF-399). Successful activation for some of these MOFs have not been possible since their synthesis, and for most of them experimental surface area, pore volume and hydrogen and methane adsorption data do not exist. Geometric surface areas and pore volumes of these eight MOFs were calculated, and in order to assess their hydrogen and methane storage capacities adsorption isotherms were computed using grand canonical Monte Carlo simulations. Our results reveal that if it can be successfully activated MOF-399 will have the highest gravimetric surface area and pore volume (exceeding 7100 m2/g and 7.55 cm3/g) among all MOFs synthesized until now. Thanks to its substantially large pore volume MOF-399 is predicted to store much more hydrogen and methane in gravimetric terms compared to other ultrahigh surface area MOFs. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The environmental problems caused by the overuse of fossil fuels have driven the global research community to search for cleaner fuels [1]. Hydrogen and methane are currently recognized as two promising energy carriers. Both of them have higher gravimetric heats of combustion than gasoline (120 MJ/kg for H2, 55.7 MJ/kg for CH4 vs. 44.5 MJ/kg for gasoline) [2,3]. More importantly, methane has lower carbon emission compared to gasoline and hydrogen has zero carbon emission. However, the main bottle neck of commercializing hydrogen and methane as alternative fuel is the cost-efficient and safe storage of these two gases. For this purpose, the US Department of Energy (DOE) set targets for on-board storage of hydrogen [4] and methane [5]. The newly revised hydrogen storage DOE target for 2015 is 0.055 kg/kg and 40 mg/cm3 in gravimetric and volumetric terms, respectively. The volumetric methane storage DOE target is 180 v(STP)/v at 35 bar and room temperature. Hydrogen and methane storage relying on high pressure or cryogenic conditions have not found wide use due to the additional costs associated with manufacturing reinforced storage ⇑ Corresponding author. Present address: Department of Chemical Engineering, University College London, London WC1E 7JE, United Kingdom. Tel.: +44 1483 686555. E-mail addresses: [email protected], [email protected] (A.O. Yazaydin). 1 Present address: Department of Chemistry, Xi’an Jiaotong-Liverpool University, 111 Ren’ai Road, Dushu Lake, Higher Education Town, Suzhou, Jiangsu Province, China. 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.08.048
cylinders and energy requirements for maintaining cryogenic conditions. An alternative way of storing gases is via adsorptive means by using porous solid materials. A relatively recent addition to the family of porous solids is MOFs which are crystalline inorganic– organic hybrid materials formed by the self-assembly of metal ions and organic linker molecules. It is possible to tune the chemical functionality, surface area, pore volume and topology of MOFs by judiciously choosing a combination of metal ions and organic linker molecules. Compared to their traditional porous counterparts, such as active carbons and zeolites, MOFs can have much higher surface areas and specific pore volumes [6–12]. There are a number of factors which affect the gas adsorption performance of MOFs, or any sorbent, such as the heat of adsorption, surface area and pore volume. In general, the gas uptake correlates with the heat of adsorption at low loadings, with the surface area at intermediate loadings, and with the free volume at high loadings [13]. This has motivated the research community to develop new MOFs with ultrahigh surface areas which also generally yield large free volumes. Such efforts have recently focused on designing MOFs with expanded linkers in non-interpenetrating topologies [14,15]. In this paper we will refer a MOF with a specific surface area near or above 6000 m2/g (whether geometrically predicted or experimentally determined Brunaauer–Emmett–Teller (BET) surface area) as an ultrahigh surface area MOF. These are MOF-180 [14], MOF-200 [14], MOF-210 [14], MOF-399 [16], NU100 [15], NU-108 [17], NU-109 [18], and NU-110 [18]. MOFs with
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L. Ding, A.O. Yazaydin / Microporous and Mesoporous Materials 182 (2013) 185–190
ultrahigh surface areas usually have large pores and these are susceptible to collapsing during the activation process. Despite the development of advanced techniques for the removal of solvents from the pores, such as the method based on the use of supercritical CO2[19,20], it has not been possible to activate some of the ultrahigh surface area MOFs fully or even partially, such as MOF180, MOF-200, MOF-399 and NU-108. Besides, although reported to have been successfully activated [18], no hydrogen or methane uptake data were reported for some of these MOFs (e.g. NU-109 and NU-100). Molecular simulations have been widely used in predicting adsorption capacities of MOFs, and for the characterization of their specific surface areas and pores volumes based on guest and defect free structures [21–24]. Furthermore, they can be utilized to draw structure–property relationships for the adsorption of gases [25– 27]. In this work using molecular simulations we studied eight MOFs with ultrahigh surface areas in order to assess and compare their hydrogen and methane storage capacities. 2. Computational details Atomistic Grand canonical Monte Carlo (GCMC) simulations were performed to estimate the adsorption isotherms of H2 and CH4 in MOFs. Interaction energies between the non-bonded atoms were computed through the Lennard–Jones (LJ) potential.
U ij ¼ 4eij
"
rij
12
r ij
rij
6 #
"
rij
12
r ij
Nabsolute ¼ Nexcess þ qgas V p where qgas is the bulk density of the gas [31] at simulation conditions, and Vp is the pore volume computed by the helium insertion method as described elsewhere [13]. The geometric surface areas of the MOFs were calculated as explained in the work of Düren et al. [22] by using a probe with a diameter equal to the van der Walls diameter of N2 (3.72 Å). 3. Results and discussion 3.1. Structural characteristics of the ultrahigh surface area MOFs
rij
where i and j are interacting atoms, and rij is the distance between atoms i and j. eij and rij are LJ well depth and diameter, respectively. LJ parameters between different types of sites were calculated using the Lorentz–Berthelot mixing rules. The positions of the MOF atoms were taken from experimental crystallographic data and kept fixed during the simulations. The guest-free crystal structures of these MOFs are given in Supporting Information Figs. S1–S8. LJ parameters for MOFs atoms were taken from UFF force field (Tables S1 and S2). Both H2 and CH4 molecules were represented by single site united atom models. The LJ parameters for H2 and CH4 were taken from Garberoglio et al. work [28] and TraPPE force field [29], respectively. Due to the importance of quantum diffraction effects for H2 adsorption at cryogenic temperatures, H2 adsorption at 77 K was simulated with the quasiclassical Feynman–Hibbs (FH) corrected potential. [30]
U FH ¼ 4eij
and T is the absolute temperature. For comparison, we also ran H2 adsorption at 77 K without FH correction. All GCMC simulations included a 1.0 105 cycle equilibration period followed by a 1.0 105 cycle production run. Each cycle is N steps where N is the number of molecules in the system (which fluctuates during a GCMC simulation). During each step, random insertion, deletion, and translation moves were sampled with equal probabilities. Cutoff distance was 12.8 Å in all simulations. Fugacities needed to run the GCMC simulations were calculated using the Peng–Robinson equation of state. H2 adsorption in all MOFs was predicted up to 200 bar at 77 K and 500 bar at 298 K. CH4 adsorption was predicted up to 200 bar at 298 K. GCMC simulations report absolute adsorption data which were then used to compute the excess adsorption data for comparison with experimental data using the following relation
rij
6 #
r ij
þ
! ! 2 132r12 30r6ij h ij 4ij 24lkT r14 r8ij ij
where UFH is the FH corrected potential, h is Planck’s constant divided by 2p, l is the reduced mass, k is the Boltzmann constant,
In order to judge whether the activation of the selected ultrahigh surface area MOFs were successful or not, the computed pore volume, void fractions and surface areas are compared in Table 1 with experimental data where available. For NU-108, MOF-180 and MOF-399, activation has not been possible due to the difficulty of removing the solvents from their pores and thus no experimental data gas sorption was reported. For these three MOFs our calculations provide a prediction of their solvent-free properties. For NU-100, the experimentally determined BET surface is slightly larger than the geometric surface area which suggests that the activation was most probably complete. However, there is a significant difference between the experimental and computed pore volumes indicating that there might still be some solvent left in the pores. For NU-109 and NU-110 experimental BET surface areas are larger than geometric surface areas, and computed and measured void fractions match perfectly, and there is good agreement between the computed and experimental pore volumes. In fact, for NU110 the experimental pore volume is larger than the computed value. Thus, all indicators point out that activation of NU-109 and NU-110 were mostly complete. For MOF-200, all experimental values are smaller than the computed ones which clearly show incomplete activation of MOF-200. Finally, for MOF-210 the experimental BET surface area is greater than the geometric surface area,
Table 1 Structural properties of ultrahigh surface area MOFs.
a b c
MOFa
Framework density (g/cm3)
Pore volumeb (cm3/g)
Pore volumec (cm3/g)
Void fractionb
Geometric SAb (m2/cm3)
Geometric SAb (m2/g)
BET SAc (m2/g)
NU-100 (rht) NU-108 (rht) NU-109 (rht) NU-110 (rht) MOF-180 (qom) MOF-200 (qom) MOF-210 (toz) MOF-399 (tbo)
0.279 0.266 0.237 0.222 0.254 0.215 0.247 0.126
3.23 3.43 3.89 4.17 3.55 4.31 3.70 7.55
2.82 – 3.75 4.40 – 3.59 3.60 –
0.903 0.912 0.923 0.927 0.901 0.927 0.914 0.955
1657 1581 1506 1434 1612 1377 1445 905
5938 5943 6350 6456 6082 6407 5846 7157
6143 – 7010 7140 – 4530 6240 –
Topologies are given in parenthesis. Calculated. Experimental, SA = Surface Area.
[15] [18] [18] [14] [14]
[15] [18] [18] [14] [14]
L. Ding, A.O. Yazaydin / Microporous and Mesoporous Materials 182 (2013) 185–190 Table 2 Simulated maximum excess H2 and CH4 uptake.
a
MOF
H2 uptake at 77 K (mg/g)/pressure data obtained at (bar)
H2 uptake at 298 K (mg/g)/pressure data obtained at (bar)
CH4 uptake at 298 K (mg/g)/pressure data obtained at (bar)
NU-100 NU-108 NU-109 NU-110 MOF-180 MOF-200 MOF-210 MOF-399
81.7/45 87.6/40 83.0/45 88.2/45 85.1/50 91.3 (73.6) a/45 (60) 80.4 (85.9) a/45 (60) 90.2/45
5.2/200 5.7/200 5.1/180 5.7/180 5.0/200 5.7/180 4.5/160 4.6/140
304.9/100 355.9/95 330.6/100 350.5/100 329.3/110 375.5/110 304.7/100 365.4/110
Values in parenthesis are from experiments [14].
187
and the experimental pore volume and void fraction are slightly smaller than the computed values. Thus, it can be suggested that the activation of MOF-210 was mostly successful. One interesting results from our calculations is that MOF-399, if fully activated, will have the highest surface area among all the ultrahigh surface area MOFs. Its geometric surface area in gravimetric terms (m2/g) is about 10% larger than NU-110 and MOF200. MOF-399 has also the highest calculated pore volume and void fraction. In particular, its predicted pore volume is substantially larger than the others. Our calculations also demonstrate that the geometric surface area of MOF-200 is comparable to that of NU-109 and NU-110. In other words, if MOF-200 could be fully activated then it should have a BET surface area comparable to NU-109 and NU-110. 3.2. H2 adsorption
Fig. 1. Experimental and simulated excess H2 adsorption at 77 K in MOF-200, MOF-210 and NU-100. (EXP = experimental results, LJ = simulated results from Leonard Jones potential and FH = simulated results from FH corrected Leonard Jones potential).
A comparison of the predicted maximum excess H2 uptake in ultrahigh surface area MOFs are given in Table 2. Among all the ultrahigh surface area MOFs considered here experimental H2 adsorption data are only available for MOF-210, MOF-200 and NU-100, and they are compared with our GCMC simulated adsorption isotherms in Fig. 1. As explained above, MOF-210 and NU-100 are considered to be successfully activated. In these two MOFs H2 isotherms obtained with the uncorrected potential overestimates the experimental data. On the other hand, when the FH corrected potential is used, the resulting isotherms underestimate the experimental data. A similar trend was observed by Liu et al. [30] while elsewhere [32] FH corrected potential resulted in a better match with experimental H2 adsorption data. However, both models capture the rank that the excess H2 adsorption is larger in NU-100 compared to MOF-210. In MOF-200, both the FH corrected and uncorrected models overestimate the experimental H2 adsorption isotherm thanks to the incomplete activation of MOF-200. Because of the incomplete activation MOF-200 reported to have adsorbed less H2 than MOF-210 in the experiments [14]. However, our simulations demonstrate that if properly activated MOF-200 would show higher excess gravimetric adsorption of H2 compared to MOF-210 (Figs. 1, S9a and S1a).
Fig. 2. Simulated absolute (a) gravimetric, and (b) volumetric H2 adsorption at 77 K in the ultrahigh surface area MOFs.
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Fig. 3. Simulated absolute (a) gravimetric, and (b) volumetric H2 adsorption at 298 K in ultrahigh surface area MOFs.
Comparing the absolute H2 uptake in ultrahigh surface area MOFs is more relevant to the practical gas storage application (Figs. 2 and 3). At 77 K all of them exceed the gravimetric target for H2 storage under about 10 bar (Fig. 2a). One remarkable finding is the significantly larger gravimetric H2 storage capacity of MOF-399 compared to other MOFs, thanks to its substantially large pore volume. For instance, at around 200 bar the H2 uptake in all ultrahigh surface area MOFs except MOF-399 start plateauing, whereas, the uptake in MOF-399 remains to increase almost linearly around the same pressure. However, when it comes to H2 storage in volumetric terms, MOF-399’s high pore volume (7.55 cm3/g) becomes a disadvantage (Fig. 2b). The very low density of MOF-399 (0.126 g/cm3) drastically reduces its volumetric geometric surface area (surface area per volume) (Table 1). Therefore, at 77 K MOF-399 has the lowest predicted volumetric H2 storage capacity and at 200 bar the amount of H2 stored in MOF-399 is about the same with the density of H2. The volumetric surface areas of other MOFs are 50–80% higher than that of for MOF-399 and because of this they provide improved H2 storage in comparison to the density of H2 within the pressure range investigated (Fig. 2b). At 298 K gravimetric H2 storage target is quickly met by MOF-399 at about 80 bar, again, thanks to its pore volume (Fig. 3a). Other ultrahigh surface area MOFs meet the same target between 120 and 200 bar. This reveals the importance of developing materials with large specific pore volumes for high H2 storage capacities at room temperature in gravimetric terms. On the other hand, in volumetric terms the amount of H2 stored in the ultrahigh surface area MOFs are roughly equal to each other (Fig. 3b). This is because at this temperature the H2 uptake is far from saturation in these MOFs; therefore, their ultrahigh surface areas matter little for storage purposes. Due to the very small heats of adsorption (around 1–2 kJ/mol, Table S3) the excess H2 uptake are rather minor (smaller than 6 mg/g, Fig. S10 and the inset in Fig. 3b) and the adsorptive storage becomes meaningless. At both 77 K and 298 K MOF-200, which was partially activated, is predicted to store more hydrogen than MOF-210, which was fully activated. Simulated H2 uptake in MOF-108 and NU-108 show that if these MOFs can be activated the amount of H2 stored will be comparable to those predicted for other ultrahigh surface area MOFs.
3.3. CH4 adsorption A comparison of the predicted maximum excess CH4 uptake in ultrahigh surface area MOFs are given in Table 2. To the best of our knowledge no experimental data exist in the literature for the ultrahigh surface area MOFs except for MOF-200 and MOF-210. As can be seen from Fig. 4, the agreement between the simulated and the experimental CH4 excess adsorption in MOF-210 is good. On the other hand, the experimental CH4 adsorption isotherm of MOF-200 is largely overestimated by the simulated isotherm obviously due to the incomplete activation as discussed previously. The absolute gravimetric CH4 uptake in ultrahigh surface area MOFs are compared in Fig. 5a. Similar to the H2 adsorption, MOF-399 largely outperforms all other MOFs in absolute gravimetric adsorption of CH4 due to its high pore volume. From the volumetric aspect, NU-108 shows the highest uptake. However, all ultrahigh
Fig. 4. Experimental and simulated excess CH4 adsorption at 298 K in MOF-200 and MOF-210.
L. Ding, A.O. Yazaydin / Microporous and Mesoporous Materials 182 (2013) 185–190
189
Fig. 5. Simulated absolute (a) gravimetric, and (b) volumetric CH4 adsorption at 298 K in ultrahigh surface area MOFs.
surface area MOFs are predicted not to achieve the DOE CH4 storage target at 35 bar (Fig. 5b). It was suggested that the CH4 heat of adsorption must be high in order to achieve the DOE target. The presence of small pockets and a high density of open-metal sites per available pore volume were identified as the two properties required for obtaining high heats of adsorption [33]. These two properties are obviously not possible in ultrahigh surface area MOFs due to the presence of large pore volumes. As a result the CH4 heats of adsorption in ultrahigh surface area MOFs are relatively low (Table S3). Although NU-100 has a higher volumetric surface area and a higher CH4 heat of adsorption compared to NU-108, its volumetric CH4 adsorption is lower than that of for NU-108. The structural differences in the ligands used in these two MOFs can be the reason for this. The ligand used in NU-108 is relatively more planar in comparison to the ligand used in NU-100 (Figs. S1 and S2). The planarity of the ligand in NU-108 can help to accommodate more guest molecules. Such ligand–CH4 interaction is particularly important in the intermediate pressure region where guest molecules start occupying the surface once all the preferred adsorption sites (e.g. open metal sites) have been taken. 4. Conclusion According to our study if MOF-399 can be successfully activated then it will have the highest surface area and pore volume of any porous materials synthesized to date. Based on the simulated isotherms, MOF-399 largely outperforms other highly porous MOFs for the storage of H2 and CH4 in gravimetric terms. MOF-200, which was reported to have been partially activated, is predicted to store significantly higher H2 and CH4 than the experimentally reported values. The amount of H2 and CH4 stored in MOF-180 and NU-108, whose activation were unsuccessful after their synthesis, are predicted to be comparable to those predicted for other ultrahigh surface area MOFs. Acknowledgements The authors acknowledge financial support from EC MarieCurie International Reintegration Grant (2010-277124), computational resources from UK National Supercomputing Service
(HECToR) and from Center for Nanoscale Materials at Argonne National Laboratory (Carbon). Use of the Center for Nanoscale Materials was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2013. 08.048. References [1] [2] [3] [4]
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Hydrogen and methane storage in ultrahigh surface area Metal–Organic Frameworks Lifeng Ding a,1, A. Ozgur Yazaydin a,b,⇑ a b
Department of Chemical Engineering, University of Surrey, Guildford GU2 7XH, United Kingdom Department of Chemistry, Michigan State University, East Lansing, MI 48824, United States
a r t i c l e
i n f o
Article history: Received 8 May 2013 Received in revised form 23 August 2013 Accepted 27 August 2013 Available online 6 September 2013 Keywords: Surface area Metal–organic frameworks Gas storage Molecular simulations
a b s t r a c t In this work we computationally studied eight Metal–Organic Frameworks (MOFs) which showed or is expected to have ultrahigh surface areas (NU-100, NU-108, NU-109, NU-110, MOF-180, MOF-200, MOF-210 and MOF-399). Successful activation for some of these MOFs have not been possible since their synthesis, and for most of them experimental surface area, pore volume and hydrogen and methane adsorption data do not exist. Geometric surface areas and pore volumes of these eight MOFs were calculated, and in order to assess their hydrogen and methane storage capacities adsorption isotherms were computed using grand canonical Monte Carlo simulations. Our results reveal that if it can be successfully activated MOF-399 will have the highest gravimetric surface area and pore volume (exceeding 7100 m2/g and 7.55 cm3/g) among all MOFs synthesized until now. Thanks to its substantially large pore volume MOF-399 is predicted to store much more hydrogen and methane in gravimetric terms compared to other ultrahigh surface area MOFs. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The environmental problems caused by the overuse of fossil fuels have driven the global research community to search for cleaner fuels [1]. Hydrogen and methane are currently recognized as two promising energy carriers. Both of them have higher gravimetric heats of combustion than gasoline (120 MJ/kg for H2, 55.7 MJ/kg for CH4 vs. 44.5 MJ/kg for gasoline) [2,3]. More importantly, methane has lower carbon emission compared to gasoline and hydrogen has zero carbon emission. However, the main bottle neck of commercializing hydrogen and methane as alternative fuel is the cost-efficient and safe storage of these two gases. For this purpose, the US Department of Energy (DOE) set targets for on-board storage of hydrogen [4] and methane [5]. The newly revised hydrogen storage DOE target for 2015 is 0.055 kg/kg and 40 mg/cm3 in gravimetric and volumetric terms, respectively. The volumetric methane storage DOE target is 180 v(STP)/v at 35 bar and room temperature. Hydrogen and methane storage relying on high pressure or cryogenic conditions have not found wide use due to the additional costs associated with manufacturing reinforced storage ⇑ Corresponding author. Present address: Department of Chemical Engineering, University College London, London WC1E 7JE, United Kingdom. Tel.: +44 1483 686555. E-mail addresses: [email protected], [email protected] (A.O. Yazaydin). 1 Present address: Department of Chemistry, Xi’an Jiaotong-Liverpool University, 111 Ren’ai Road, Dushu Lake, Higher Education Town, Suzhou, Jiangsu Province, China. 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.08.048
cylinders and energy requirements for maintaining cryogenic conditions. An alternative way of storing gases is via adsorptive means by using porous solid materials. A relatively recent addition to the family of porous solids is MOFs which are crystalline inorganic– organic hybrid materials formed by the self-assembly of metal ions and organic linker molecules. It is possible to tune the chemical functionality, surface area, pore volume and topology of MOFs by judiciously choosing a combination of metal ions and organic linker molecules. Compared to their traditional porous counterparts, such as active carbons and zeolites, MOFs can have much higher surface areas and specific pore volumes [6–12]. There are a number of factors which affect the gas adsorption performance of MOFs, or any sorbent, such as the heat of adsorption, surface area and pore volume. In general, the gas uptake correlates with the heat of adsorption at low loadings, with the surface area at intermediate loadings, and with the free volume at high loadings [13]. This has motivated the research community to develop new MOFs with ultrahigh surface areas which also generally yield large free volumes. Such efforts have recently focused on designing MOFs with expanded linkers in non-interpenetrating topologies [14,15]. In this paper we will refer a MOF with a specific surface area near or above 6000 m2/g (whether geometrically predicted or experimentally determined Brunaauer–Emmett–Teller (BET) surface area) as an ultrahigh surface area MOF. These are MOF-180 [14], MOF-200 [14], MOF-210 [14], MOF-399 [16], NU100 [15], NU-108 [17], NU-109 [18], and NU-110 [18]. MOFs with
186
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ultrahigh surface areas usually have large pores and these are susceptible to collapsing during the activation process. Despite the development of advanced techniques for the removal of solvents from the pores, such as the method based on the use of supercritical CO2[19,20], it has not been possible to activate some of the ultrahigh surface area MOFs fully or even partially, such as MOF180, MOF-200, MOF-399 and NU-108. Besides, although reported to have been successfully activated [18], no hydrogen or methane uptake data were reported for some of these MOFs (e.g. NU-109 and NU-100). Molecular simulations have been widely used in predicting adsorption capacities of MOFs, and for the characterization of their specific surface areas and pores volumes based on guest and defect free structures [21–24]. Furthermore, they can be utilized to draw structure–property relationships for the adsorption of gases [25– 27]. In this work using molecular simulations we studied eight MOFs with ultrahigh surface areas in order to assess and compare their hydrogen and methane storage capacities. 2. Computational details Atomistic Grand canonical Monte Carlo (GCMC) simulations were performed to estimate the adsorption isotherms of H2 and CH4 in MOFs. Interaction energies between the non-bonded atoms were computed through the Lennard–Jones (LJ) potential.
U ij ¼ 4eij
"
rij
12
r ij
rij
6 #
"
rij
12
r ij
Nabsolute ¼ Nexcess þ qgas V p where qgas is the bulk density of the gas [31] at simulation conditions, and Vp is the pore volume computed by the helium insertion method as described elsewhere [13]. The geometric surface areas of the MOFs were calculated as explained in the work of Düren et al. [22] by using a probe with a diameter equal to the van der Walls diameter of N2 (3.72 Å). 3. Results and discussion 3.1. Structural characteristics of the ultrahigh surface area MOFs
rij
where i and j are interacting atoms, and rij is the distance between atoms i and j. eij and rij are LJ well depth and diameter, respectively. LJ parameters between different types of sites were calculated using the Lorentz–Berthelot mixing rules. The positions of the MOF atoms were taken from experimental crystallographic data and kept fixed during the simulations. The guest-free crystal structures of these MOFs are given in Supporting Information Figs. S1–S8. LJ parameters for MOFs atoms were taken from UFF force field (Tables S1 and S2). Both H2 and CH4 molecules were represented by single site united atom models. The LJ parameters for H2 and CH4 were taken from Garberoglio et al. work [28] and TraPPE force field [29], respectively. Due to the importance of quantum diffraction effects for H2 adsorption at cryogenic temperatures, H2 adsorption at 77 K was simulated with the quasiclassical Feynman–Hibbs (FH) corrected potential. [30]
U FH ¼ 4eij
and T is the absolute temperature. For comparison, we also ran H2 adsorption at 77 K without FH correction. All GCMC simulations included a 1.0 105 cycle equilibration period followed by a 1.0 105 cycle production run. Each cycle is N steps where N is the number of molecules in the system (which fluctuates during a GCMC simulation). During each step, random insertion, deletion, and translation moves were sampled with equal probabilities. Cutoff distance was 12.8 Å in all simulations. Fugacities needed to run the GCMC simulations were calculated using the Peng–Robinson equation of state. H2 adsorption in all MOFs was predicted up to 200 bar at 77 K and 500 bar at 298 K. CH4 adsorption was predicted up to 200 bar at 298 K. GCMC simulations report absolute adsorption data which were then used to compute the excess adsorption data for comparison with experimental data using the following relation
rij
6 #
r ij
þ
! ! 2 132r12 30r6ij h ij 4ij 24lkT r14 r8ij ij
where UFH is the FH corrected potential, h is Planck’s constant divided by 2p, l is the reduced mass, k is the Boltzmann constant,
In order to judge whether the activation of the selected ultrahigh surface area MOFs were successful or not, the computed pore volume, void fractions and surface areas are compared in Table 1 with experimental data where available. For NU-108, MOF-180 and MOF-399, activation has not been possible due to the difficulty of removing the solvents from their pores and thus no experimental data gas sorption was reported. For these three MOFs our calculations provide a prediction of their solvent-free properties. For NU-100, the experimentally determined BET surface is slightly larger than the geometric surface area which suggests that the activation was most probably complete. However, there is a significant difference between the experimental and computed pore volumes indicating that there might still be some solvent left in the pores. For NU-109 and NU-110 experimental BET surface areas are larger than geometric surface areas, and computed and measured void fractions match perfectly, and there is good agreement between the computed and experimental pore volumes. In fact, for NU110 the experimental pore volume is larger than the computed value. Thus, all indicators point out that activation of NU-109 and NU-110 were mostly complete. For MOF-200, all experimental values are smaller than the computed ones which clearly show incomplete activation of MOF-200. Finally, for MOF-210 the experimental BET surface area is greater than the geometric surface area,
Table 1 Structural properties of ultrahigh surface area MOFs.
a b c
MOFa
Framework density (g/cm3)
Pore volumeb (cm3/g)
Pore volumec (cm3/g)
Void fractionb
Geometric SAb (m2/cm3)
Geometric SAb (m2/g)
BET SAc (m2/g)
NU-100 (rht) NU-108 (rht) NU-109 (rht) NU-110 (rht) MOF-180 (qom) MOF-200 (qom) MOF-210 (toz) MOF-399 (tbo)
0.279 0.266 0.237 0.222 0.254 0.215 0.247 0.126
3.23 3.43 3.89 4.17 3.55 4.31 3.70 7.55
2.82 – 3.75 4.40 – 3.59 3.60 –
0.903 0.912 0.923 0.927 0.901 0.927 0.914 0.955
1657 1581 1506 1434 1612 1377 1445 905
5938 5943 6350 6456 6082 6407 5846 7157
6143 – 7010 7140 – 4530 6240 –
Topologies are given in parenthesis. Calculated. Experimental, SA = Surface Area.
[15] [18] [18] [14] [14]
[15] [18] [18] [14] [14]
L. Ding, A.O. Yazaydin / Microporous and Mesoporous Materials 182 (2013) 185–190 Table 2 Simulated maximum excess H2 and CH4 uptake.
a
MOF
H2 uptake at 77 K (mg/g)/pressure data obtained at (bar)
H2 uptake at 298 K (mg/g)/pressure data obtained at (bar)
CH4 uptake at 298 K (mg/g)/pressure data obtained at (bar)
NU-100 NU-108 NU-109 NU-110 MOF-180 MOF-200 MOF-210 MOF-399
81.7/45 87.6/40 83.0/45 88.2/45 85.1/50 91.3 (73.6) a/45 (60) 80.4 (85.9) a/45 (60) 90.2/45
5.2/200 5.7/200 5.1/180 5.7/180 5.0/200 5.7/180 4.5/160 4.6/140
304.9/100 355.9/95 330.6/100 350.5/100 329.3/110 375.5/110 304.7/100 365.4/110
Values in parenthesis are from experiments [14].
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and the experimental pore volume and void fraction are slightly smaller than the computed values. Thus, it can be suggested that the activation of MOF-210 was mostly successful. One interesting results from our calculations is that MOF-399, if fully activated, will have the highest surface area among all the ultrahigh surface area MOFs. Its geometric surface area in gravimetric terms (m2/g) is about 10% larger than NU-110 and MOF200. MOF-399 has also the highest calculated pore volume and void fraction. In particular, its predicted pore volume is substantially larger than the others. Our calculations also demonstrate that the geometric surface area of MOF-200 is comparable to that of NU-109 and NU-110. In other words, if MOF-200 could be fully activated then it should have a BET surface area comparable to NU-109 and NU-110. 3.2. H2 adsorption
Fig. 1. Experimental and simulated excess H2 adsorption at 77 K in MOF-200, MOF-210 and NU-100. (EXP = experimental results, LJ = simulated results from Leonard Jones potential and FH = simulated results from FH corrected Leonard Jones potential).
A comparison of the predicted maximum excess H2 uptake in ultrahigh surface area MOFs are given in Table 2. Among all the ultrahigh surface area MOFs considered here experimental H2 adsorption data are only available for MOF-210, MOF-200 and NU-100, and they are compared with our GCMC simulated adsorption isotherms in Fig. 1. As explained above, MOF-210 and NU-100 are considered to be successfully activated. In these two MOFs H2 isotherms obtained with the uncorrected potential overestimates the experimental data. On the other hand, when the FH corrected potential is used, the resulting isotherms underestimate the experimental data. A similar trend was observed by Liu et al. [30] while elsewhere [32] FH corrected potential resulted in a better match with experimental H2 adsorption data. However, both models capture the rank that the excess H2 adsorption is larger in NU-100 compared to MOF-210. In MOF-200, both the FH corrected and uncorrected models overestimate the experimental H2 adsorption isotherm thanks to the incomplete activation of MOF-200. Because of the incomplete activation MOF-200 reported to have adsorbed less H2 than MOF-210 in the experiments [14]. However, our simulations demonstrate that if properly activated MOF-200 would show higher excess gravimetric adsorption of H2 compared to MOF-210 (Figs. 1, S9a and S1a).
Fig. 2. Simulated absolute (a) gravimetric, and (b) volumetric H2 adsorption at 77 K in the ultrahigh surface area MOFs.
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Fig. 3. Simulated absolute (a) gravimetric, and (b) volumetric H2 adsorption at 298 K in ultrahigh surface area MOFs.
Comparing the absolute H2 uptake in ultrahigh surface area MOFs is more relevant to the practical gas storage application (Figs. 2 and 3). At 77 K all of them exceed the gravimetric target for H2 storage under about 10 bar (Fig. 2a). One remarkable finding is the significantly larger gravimetric H2 storage capacity of MOF-399 compared to other MOFs, thanks to its substantially large pore volume. For instance, at around 200 bar the H2 uptake in all ultrahigh surface area MOFs except MOF-399 start plateauing, whereas, the uptake in MOF-399 remains to increase almost linearly around the same pressure. However, when it comes to H2 storage in volumetric terms, MOF-399’s high pore volume (7.55 cm3/g) becomes a disadvantage (Fig. 2b). The very low density of MOF-399 (0.126 g/cm3) drastically reduces its volumetric geometric surface area (surface area per volume) (Table 1). Therefore, at 77 K MOF-399 has the lowest predicted volumetric H2 storage capacity and at 200 bar the amount of H2 stored in MOF-399 is about the same with the density of H2. The volumetric surface areas of other MOFs are 50–80% higher than that of for MOF-399 and because of this they provide improved H2 storage in comparison to the density of H2 within the pressure range investigated (Fig. 2b). At 298 K gravimetric H2 storage target is quickly met by MOF-399 at about 80 bar, again, thanks to its pore volume (Fig. 3a). Other ultrahigh surface area MOFs meet the same target between 120 and 200 bar. This reveals the importance of developing materials with large specific pore volumes for high H2 storage capacities at room temperature in gravimetric terms. On the other hand, in volumetric terms the amount of H2 stored in the ultrahigh surface area MOFs are roughly equal to each other (Fig. 3b). This is because at this temperature the H2 uptake is far from saturation in these MOFs; therefore, their ultrahigh surface areas matter little for storage purposes. Due to the very small heats of adsorption (around 1–2 kJ/mol, Table S3) the excess H2 uptake are rather minor (smaller than 6 mg/g, Fig. S10 and the inset in Fig. 3b) and the adsorptive storage becomes meaningless. At both 77 K and 298 K MOF-200, which was partially activated, is predicted to store more hydrogen than MOF-210, which was fully activated. Simulated H2 uptake in MOF-108 and NU-108 show that if these MOFs can be activated the amount of H2 stored will be comparable to those predicted for other ultrahigh surface area MOFs.
3.3. CH4 adsorption A comparison of the predicted maximum excess CH4 uptake in ultrahigh surface area MOFs are given in Table 2. To the best of our knowledge no experimental data exist in the literature for the ultrahigh surface area MOFs except for MOF-200 and MOF-210. As can be seen from Fig. 4, the agreement between the simulated and the experimental CH4 excess adsorption in MOF-210 is good. On the other hand, the experimental CH4 adsorption isotherm of MOF-200 is largely overestimated by the simulated isotherm obviously due to the incomplete activation as discussed previously. The absolute gravimetric CH4 uptake in ultrahigh surface area MOFs are compared in Fig. 5a. Similar to the H2 adsorption, MOF-399 largely outperforms all other MOFs in absolute gravimetric adsorption of CH4 due to its high pore volume. From the volumetric aspect, NU-108 shows the highest uptake. However, all ultrahigh
Fig. 4. Experimental and simulated excess CH4 adsorption at 298 K in MOF-200 and MOF-210.
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Fig. 5. Simulated absolute (a) gravimetric, and (b) volumetric CH4 adsorption at 298 K in ultrahigh surface area MOFs.
surface area MOFs are predicted not to achieve the DOE CH4 storage target at 35 bar (Fig. 5b). It was suggested that the CH4 heat of adsorption must be high in order to achieve the DOE target. The presence of small pockets and a high density of open-metal sites per available pore volume were identified as the two properties required for obtaining high heats of adsorption [33]. These two properties are obviously not possible in ultrahigh surface area MOFs due to the presence of large pore volumes. As a result the CH4 heats of adsorption in ultrahigh surface area MOFs are relatively low (Table S3). Although NU-100 has a higher volumetric surface area and a higher CH4 heat of adsorption compared to NU-108, its volumetric CH4 adsorption is lower than that of for NU-108. The structural differences in the ligands used in these two MOFs can be the reason for this. The ligand used in NU-108 is relatively more planar in comparison to the ligand used in NU-100 (Figs. S1 and S2). The planarity of the ligand in NU-108 can help to accommodate more guest molecules. Such ligand–CH4 interaction is particularly important in the intermediate pressure region where guest molecules start occupying the surface once all the preferred adsorption sites (e.g. open metal sites) have been taken. 4. Conclusion According to our study if MOF-399 can be successfully activated then it will have the highest surface area and pore volume of any porous materials synthesized to date. Based on the simulated isotherms, MOF-399 largely outperforms other highly porous MOFs for the storage of H2 and CH4 in gravimetric terms. MOF-200, which was reported to have been partially activated, is predicted to store significantly higher H2 and CH4 than the experimentally reported values. The amount of H2 and CH4 stored in MOF-180 and NU-108, whose activation were unsuccessful after their synthesis, are predicted to be comparable to those predicted for other ultrahigh surface area MOFs. Acknowledgements The authors acknowledge financial support from EC MarieCurie International Reintegration Grant (2010-277124), computational resources from UK National Supercomputing Service
(HECToR) and from Center for Nanoscale Materials at Argonne National Laboratory (Carbon). Use of the Center for Nanoscale Materials was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
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