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Design strategies for metal alkoxide functionalized metal–organic frameworks for ambient temperature hydrogen storage
Microporous and Mesoporous Materials 171 (2013) 103–109
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Design strategies for metal alkoxide functionalized metal–organic frameworks for ambient temperature hydrogen storage Stephen K. Brand 1, Yamil J. Colón 1, Rachel B. Getman 2, Randall Q. Snurr ⇑ Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, United States
a r t i c l e
i n f o
Article history: Received 8 October 2012 Received in revised form 8 December 2012 Accepted 10 December 2012 Available online 20 December 2012 Keywords: Gas adsorption Open metal sites IRMOF-16 NU-100 UiO-68
a b s t r a c t Grand canonical Monte Carlo simulations were used to calculate hydrogen adsorption in IRMOF-16, NU100, and UiO-68 functionalized with Mg or Fe catecholates on the linkers. We examined how altering the number of metal catecholate groups affects H2 uptake and deliverable capacity near ambient temperature. We find that large free volume and an isosteric heat of adsorption (Qst) of 20 kJ mol1 at low loading will maximize gravimetric deliverable capacity while a small pore diameter will maximize volumetric deliverable capacity. This suggests a trade-off between the properties that lead to maximal gravimetric and volumetric capacities. For example, our calculations suggest that NU-100 functionalized with six Fe catecholate groups per linker takes up 5.5 wt.% deliverable H2 at 243 K and 100 bar, but only 24.2 g L1 deliverable H2. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Hydrogen is a potential replacement for fossil fuels in automotive applications. However, hydrogen gas has a very small volumetric density, 0.0899 kg m3 at standard temperature and pressure (STP), which makes storage difficult. It has been estimated that 4 kg of H2 are necessary to fuel a modestly sized car over a range of 400 km [1]. Thus 44 m3 (44,000 L) of gaseous H2 at STP would be necessary to provide enough energy to fuel a typical automobile. A major challenge in materials research is to design hydrogen storage systems that effectively and safely store H2 in a more realistic volume. To motivate research and development of hydrogen storage materials, the US Department of Energy (DOE) has established targets of 2.5 kWh kg1 and 2.3 kWh L1 [2]. These translate to 7.0 wt.% and 70 g L1 of H2 for the entire storage system. Short term targets are 5.2 wt.% and 40 g L1 by 2015. The minimum storage temperature is 243 K, and the maximum storage pressure is 100 bar. To be effective, hydrogen storage materials must release H2 for use at approximately 2 bar. A variety of storage systems are under development, including high pressure gas and cryogenic liquid tanks [3]. In addition, materials that adsorb H2 either dissociatively, such as metal and chemical hydrides [4], or molecularly, such as metal–organic ⇑ Corresponding author. Tel.: +1 (847) 467 2977; fax: +1 (847) 491 3728. E-mail address: [email protected] (R.Q. Snurr). These authors contributed equally. Present address: Department of Chemical and Biomolecular Engineering, Clemson University, Clemson, SC 29634, United States. 1 2
1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.12.020
frameworks [5,6], are promising for hydrogen storage. However, to date, no system that meets the DOE targets has been developed. In general, chemical and metal hydrides bind H2 very strongly and take up significant amounts of H2. H2 storage capacities of over 10 wt.% are possible [7]. However, due to the strong H2 binding energies as well as prohibitive kinetics for H–H recombination, these materials do not release H2 for use at ambient temperature [7]. In direct contrast, carbon-based molecular adsorbents tend to bind H2 much more weakly. Therefore, they take up significantly less H2 but are much more effective at releasing it. Solving the H2 storage problem relies in part on developing materials with intermediate properties. Metal–organic frameworks (MOFs) are arguably the most promising class of storage materials for molecular hydrogen. MOFs are porous, crystalline materials comprised of metal or metal oxide nodes connected by organic ‘‘linker’’ compounds. They have extremely large surface areas. For example, recently synthesized NU-100 [8] and MOF-210 [9] have surface areas over 6000 m2 g1. MOFs are highly tunable, as different nodes and linkers can be combined to create different materials with different properties. In addition, the linkers contain all the functionality of organic chemistry. For example, MOFs with carboxylic acid [10], amine [11], and alcohol [12,13] groups on the linkers have been synthesized. Because of the large surface areas, MOFs exhibit significant H2 storage at cryogenic temperatures [8,14,15]. However the H2 adsorption enthalpies are typically only around 5 kJ mol1 [5], which is too weak to counteract the thermal effects of motion at ambient temperatures. In order for MOFs to be effective for H2 storage, the heats of adsorption must be increased. One method
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of doing this that has been successfully demonstrated in experiments is metal alkoxide functionalization [12,13]. In this strategy, a MOF with alcohol groups on the linkers is modified post synthesis to exchange hydroxyl protons for metal cations. The metal sites exhibit strong positive charges that interact favorably with the H2 quadrupole [16–19]. MOFs with lithium and magnesium alkoxide groups have been synthesized and shown to increase both the H2 heat of adsorption and uptake at cryogenic temperatures and low pressures [12,13,20]. Simulations of H2 adsorption in metal alkoxide functionalized MOFs suggest that this strategy is promising at ambient temperatures as well [17,21–23]. A recent report from our group suggests that Mg alkoxides, which exhibit H2 binding energies of 22 kJ mol1, are effective at both storing and releasing H2 [17]. In fact, our results suggest that MOFs with large surface areas and low crystal densities that incorporate Mg alkoxide groups can meet the DOE gravimetric target for 2015. In our prior work on this subject, we examined lithium, magnesium, manganese, nickel, and copper alkoxide functionalization in IRMOF-1, IRMOF-10, IRMOF-16, UiO-68 and UMCM-150 [17]. In this work, we extend our analysis to include beryllium and the remaining divalent 3d transition metals, iron and zinc. (Cobalt can also exist in the divalent state, but it binds H2 too strongly to release it for use at 2 bar.) We find that Mg and Fe alkoxides store the most in terms of deliverable H2, so we focus further analysis on those metals. We analyze Mg and Fe alkoxide functionalization in three MOFs, IRMOF-16 and NU-100, which have surface areas of 6000 m2 g1, and UiO-68, which has a surface area of 4000 m2 g1 and provides an interesting contrast. We compute H2 adsorption in these materials functionalized with one, three, and six alkoxide groups per linker and propose design strategies for meeting the DOE gravimetric and volumetric targets.
2. Computational details The methods used in this study are the same as in our prior report on metal alkoxide functionalization [17]. Quantum chemical calculations were performed with the Gaussian 09 software [24] to explore the potential energy surfaces (PESs) for H2 adsorption at the metal sites on metal alkoxide benzenes. H2 binding energies at Be and Zn alkoxide sites were calculated at the MP2/6-311+G⁄⁄ level of theory, using the counterpoise method [25] to offset basis set superposition error (BSSE). Binding energies at Fe alkoxide sites were calculated using the M06 [26] implementation of density functional theory along with the 6-311++G⁄⁄ basis set without correcting for BSSE. We find this method gives binding energies that are similar to those calculated with MP2/6-311+G⁄⁄ using counterpoise corrections (Table S1, Supporting Information), and it is more computationally efficient. To study metal alkoxide functionalization in MOFs, we added metal alkoxide sites to the experimentally determined structures of IRMOF-16 [27], NU-100 [8], and UiO-68 [28]. Unfunctionalized structures are shown in Fig. 1, and structural details are provided in Table 1. Functionalized versions were modeled by adding one, three, or six metal alkoxide groups to each linker. Note that for alkoxides of divalent cations, which require two oxygen anions, six is the maximum number of alkoxide groups per linker in IRMOF-16, NU-100, and UiO-68 (Fig. S6). We calculated the pore size distributions (PSDs) of the unfunctionalized MOFs in order to relate H2 uptake with crystallographic properties. PSDs were calculated using the method of Gelb and Gubbins [31]. In this method, points within the structures are randomly selected and archived by the radius of the largest sphere that can be drawn which encloses the point without overlapping the van der Waals surface of the framework. The PSD is the derivative of the pore volume covered by spheres of radius r with
Fig. 1. Structures of the metal–organic frameworks used in this work. C = gray, H = white, O = red, Zn = cyan, Zr = blue, Cu = orange. The purple spheres illustrate the cavities. (For interpretation of reference to color in this figure legend, the reader is referred to the web version of this article.)
respect to r, which provides the pore volume covered by spheres of radius r but not r + dr.
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S.K. Brand et al. / Microporous and Mesoporous Materials 171 (2013) 103–109 Table 1 Calculated surface area (SA), free volume (V), and diameter of the largest pore (dpore) for the MOFs examined in this work.
a b c
MOF
Refs.
Linkersa
Corners
SAb (m2 g1)
V (cm3 g1)
dpore (Å)
IRMOF-16 NU-100 UiO-68
[27] [8] [28]
TPDC hexatopic ligand TPDC
Zn4O Cu2 paddlewheel Zr6O8
6055 [29] 5813c 4082 [17]
4.413 [29] 2.82 [8] 1.541 [17]
23.6 [30] 27.6 [8] 15.8c
linker abbreviations: TPDC = terphenyl dicarboxylate. See Fig. S6 or Ref. [8] for the structure of the hexatopic ligand. H2 probe. This work.
H2 adsorption isotherms were calculated in a single unit cell with grand canonical Monte Carlo simulations using our in-house code RASPA [32]. At each pressure, 200,000 total Monte Carlo cycles were performed, 100,000 to equilibrate the system and 100,000 to compute ensemble averages. In each cycle, an average of N moves was performed, where N is the number of H2 molecules in the system. Translation, rotation, insertion, deletion, and reinsertion moves were employed. H2/framework interactions were calculated in the following manner: H2 interactions with atoms in the metal alkoxide groups (oxygen and the metal) and the immediate neighboring carbons were calculated with modified-Morse and Coulomb potentials. Modified-Morse parameters were obtained from fitting to the PES for H2 adsorption at the metal sites in metal alkoxide benzenes (obtained from quantum chemical calculations. See Table S3 and Ref. [17].) Coulomb interactions were calculated using calculated partial charges on alkoxide group atoms (Table S3) and the Darkrim-Levesque model [33] for H2. H2 interactions with all other framework atoms and other H2 molecules were calculated using Lennard-Jones (LJ) potentials. LJ parameters for framework atoms were taken from the DREIDING force field [34] where available and the Universal Force Field [35] otherwise. LJ parameters for H2 were taken from an empirical model [36]. Cross-terms were calculated using Lorentz-Berthelot mixing rules. All LJ parameters are provided in Table S4. All other computational details are identical to those reported in Ref. [17].
3. Results 3.1. Hydrogen adsorption in IRMOF-16 with different metals We calculated adsorption isotherms (Fig. 2, left) in variants of IRMOF-16 functionalized with one metal alkoxide group per linker in order to screen additional metals. In agreement with our prior work [17] we find that metal alkoxide functionalization generally enhances gravimetric H2 uptake at 100 bar. Exceptions are Mn and Zn alkoxides. Mn and Zn bind H2 too weakly to enhance ambient temperature H2 storage. Though the binding energies for a single H2 molecule are 20 and 29 kJ mol1, respectively, they drop off significantly as additional H2 molecules adsorb. For example, differential binding energies:
H2 þ ½ðn 1ÞH2 MO2 C6 H4 ! ½nH2 MO2 C6 H4
ð1Þ
for the second H2 molecule are 6 and 5 kJ mol1 for Mn and Zn, respectively. (Binding energies are provided in Fig. S2 and in Ref. [17].) Of the metals analyzed, Fe alkoxides exhibit the strongest binding energies for up to 5 H2, and they also exhibit the largest uptake for PH2 > 10 bar.
Deliverable capacities, defined as the amount of useable H2 a material can store [37], for the different metals are shown in the righthand side of Fig. 2:
D wt% ðPH2 Þ ¼ wt% ðPH2 Þ wt% ð2 barÞ
ð2Þ
As previously reported, Ni and Cu alkoxides bind the first H2 molecule too strongly and subsequent H2 molecules too weakly and thus exhibit poor deliverable capacities [17]. We find the same for Be alkoxides. We previously reported that Mg alkoxides exhibit good binding energies, do not dissociate the hydrogen molecules, and perform well in terms of deliverable capacity [17]. Fe alkoxide functionalized IRMOF-16 exhibits an even better deliverable capacity than its Mg counterpart. 3.2. Hydrogen adsorption in metal–organic frameworks with multiple metal alkoxide groups per linker Our analysis shows that, of the metal alkoxide groups studied, Mg and Fe perform the best in terms of deliverable gravimetric capacities. We thus focus our analysis on these functional groups, exploring now how increasing the number of alkoxide groups per MOF linker can improve H2 adsorption uptake. Total and deliverable capacities for IRMOF-16, NU-100, and UiO-68 functionalized with one, three, and six Mg or Fe alkoxide groups per linker are shown in Figs. 3 and 4, respectively. In general, considering MOFs with the same number of functional groups per linker, the deliverable capacities follow the order IRMOF-16 > NU-100 > UiO-68, suggesting a possible correlation with surface area or free volume, which we discuss below. From here on out, we refer to the functionalized MOFs using the abbreviation metal_MOF_n, where metal is either Mg or Fe; MOF is either IRMOF-16, NU-100, or UiO68; and n is either one, three, or six and describes the number of alkoxide groups per MOF linker. The following MOFs meet or exceed the DOE gravimetric targets for 2015 based on total amount adsorbed (Fig. 3): Mg_IRMOF-16_1 (5.4 wt.%), Mg_IRMOF-16_3 (6.0 wt.%), Mg_IRMOF-16_6 (6.7 wt.%), Fe_IRMOF16_1 (6.4 wt.%), Fe_IRMOF-16_3 (8.0 wt.%), Fe_IRMOF-16_6 (8.7 wt.%), Fe_NU-100_3 (5.5 wt.%), Fe_NU-100_6 (6.9 wt.%), Fe_UiO-68_3 (5.6 wt.%), and Fe_UiO-68_6 (5.7 wt.%). However, when looking at the deliverable capacities (Fig. 4) only the following structures meet or exceed the target: Mg_IRMOF-16_1 (5.2 wt.%), Mg_IRMOF-16_3 (5.4 wt.%), Mg_IRMOF-16_6 (5.5 wt.%), Fe_IRMOF-16_1 (5.6 wt.%), Fe_IRMOF-16_3 (6.2 wt.%), Fe_IRMOF-16_6 (5.4 wt.%), and Fe_NU-100_6 (5.5 wt.%). Fig. 4 shows that, in general, increasing the number of functional groups per linker enhances deliverable capacity at 100 bar. Exceptions are Mg_UiO-68_6, Fe_IRMOF-16_6 and Fe_UiO-68_6. Additionally, Mg_UiO-68_3 and Fe_UiO-68_3 exhibit the same deliverable capacities at 100 bar as Mg_UiO-68_1 and Fe_UiO68_1, respectively. Comparing with the absolute adsorption isotherms in Fig. 3 shows that MOFs that exhibit lower deliverable capacities than structures with fewer functional groups per linker take up significant amounts of H2 at low pressures and then saturate, leading to poor deliverable capacities. Interestingly, IRMOF16 and UiO-68 exhibit this behavior, but NU-100 does not. The
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Fig. 2. H2 absolute adsorption isotherms (left) and deliverable capacities, D wt.% = wt.% (PH2) wt.% (2 bar ), (right) for metal alkoxide functionalized versions of IRMOF-16 at 243 K. Functionalized versions have one metal alkoxide group per organic linker.
Fig. 3. H2 adsorption isotherms for Mg and Fe alkoxide functionalized IRMOF-16 (top), NU-100 (middle), and UiO-68 (bottom) at 243 K. Functionalized versions have one, three, or six alkoxide groups per linker. Lines without symbols represent the unfunctionalized structures.
Fig. 4. H2 deliverable capacities [D wt.% = wt.% (PH2) wt.% (2 bar )] for Mg and Fe alkoxide functionalized IRMOF-16 (top), NU-100 (middle), and UiO-68 (bottom) at 243 K. Functionalized versions have one, three, or six alkoxide groups per linker. Lines without symbols represent the unfunctionalized structures.
tendency to saturate or not can be attributed to differences in the pore sizes in these MOFs. We have previously reported that NU100 has three types of pores with diameters of 27.4, 15.4 and 13.4 Å, respectively [8]. At very low pressure, H2 molecules adsorb at the metal alkoxide sites, regardless of pore size. As the pressure increases H2 molecules then start to adsorb on the rest of the material. These molecules adsorb in the smaller pores, which have more alkoxide groups per unit volume. When the smaller pores are
saturated, molecules adsorb in the large pores. The large pores are not saturated at 100 bar, and thus alkoxide sites are available out to large pressures. In contrast, IRMOF-16 and UiO-68 effectively have one size of pore each, with diameters of 23.6 and 15.8 Å, respectively. Saturation is most pronounced in IRMOF-16 and UiO-68 variants functionalized with six alkoxide groups per linker and with Fe alkoxides, because they take up more H2 at low pressure. Saturation is more dramatic in UiO-68 than in
S.K. Brand et al. / Microporous and Mesoporous Materials 171 (2013) 103–109
Fig. 5. Simulated isosteric heats of adsorption (Qst) for Mg alkoxide functionalized UiO-68 at low pressure (top left) and high pressure (bottom left) and Fe alkoxide functionalized IRMOF-16 (top right) and UiO-68 (bottom right) at high pressure. Qst in Fe alkoxide functionalized MOFs exceed 90 kJ mol1 at very low pressure, but we do not include these values in the graphs in order to show more clearly the high pressure behavior. Results were obtained at 243 K. The line closest to the x-axis corresponds to the unfunctionalized structure.
IRMOF-16 because it has smaller pores. Pore size analysis in UiO68 and snapshots of H2 adsorption in IRMOF-16 and NU-100 are provided in Figs. S7–S9 in the Supporting information. Isosteric heats of adsorption (Qst) were obtained from the GCMC simulations [38] and are shown in Fig. 5.
Q st ¼ RT
@hVi @hNi
ð3Þ T;V
In general, MOFs with more alkoxide groups per linker have higher Qst out to higher pressures, leading to larger deliverable capacities. Considering MOFs with the same number of alkoxide groups per linker, Qst values are larger in UiO-68 than in IRMOF-16 and NU100. UiO-68 has a smaller free volume, and thus the density of alkoxide sites is larger, allowing more effective interactions with H2 [17]. In fact, at 100 bar the highest calculated Qst is 14 kJ mol1 in Fe_UiO-68_6. MOFs that exhibit larger Qst at high pressure do not necessarily perform well in terms of deliverable capacity. We find that MOFs with larger Qst at high pressure also exhibit larger Qst at 2 bar (and lower pressures). This drives larger uptake at low pressure and can lead to lower deliverable capacities. We discuss a quadratic relationship between deliverable capacity and Qst at low loading in the next section. As in our previous work, Qst values are relatively large initially but drop off as pressure (and thus loading) increases [17]. Remarkably, UiO-68 variants functionalized with three and six Mg or Fe alkoxide groups per linker exhibit very strong low loading Qst of 45 and 90 kJ mol1, respectively, more than twice the H2 binding energies to a single alkoxide for these metals [17]. We find that these large interaction energies are due to H2 adsorbing in sites near the corners amidst clusters of metal alkoxide sites. Molecules adsorbed in these sites can interact with multiple metal alkoxide groups simultaneously at close range, increasing the H2 interaction energies (Fig. S10). However, these corner sites saturate at
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Fig. 6. H2 deliverable capacities [D g L1 = g L1 (PH2) g L1 (2 bar )] for Mg and Fe alkoxide functionalized IRMOF-16 (top), NU-100 (middle), and UiO-68 (bottom) at 243 K. Functionalized versions have one, three, or six alkoxide groups per linker. Lines without symbols represent the unfunctionalized structures.
pressures lower than 0.1 bar, and the Qst values drop significantly. Therefore, their contribution to Qst decreases significantly as pressure (and thus H2 loading) increases. Volumetric deliverable capacities
Dðg L1 Þ ¼ g L1 ðPH2 Þ g L1 ð2 barÞ
ð4Þ
are shown in Fig. 6. As with gravimetric deliverable capacities, we find that increasing the number of functional groups per linker increases the volumetric deliverable capacites. The one exception, Fe_UiO-68_6, is due to saturation. Considering all other MOFs, volumetric deliverable capacities follow the order UiO-68 > IRMOF-16 > NU-100 for MOFs with the same number of alkoxide groups per linker, suggesting a possible correlation with pore size (discussed below). Absolute volumetric capacities are shown in Fig. S12. MOFs analyzed in this work that meet the DOE volumetric target, considering total amount adsorbed, for 2015 are: Fe_IRMOF16_6 (44.3 g L1), Fe_UiO-68_3 (45.8 g L1), and Fe_UiO-68_6 (64.0 g L1). However, in terms of deliverable volumetric capacity, none meet the target and only the following exceed deliverable H2 storage capacities of 20 g L1 at 100 bar: Mg_IRMOF-16_6 (21.9 g L1), Fe_IRMOF-16_3 (21.9 g L1), Fe_IRMOF-16_6 (26.6 g L1), Fe_NU-100_6 (24.2 g L1), Mg_UiO-68_3 (20.2 g L1), Mg_UiO-68_6 (21.8 g L1), Fe_UiO-68_1 (20.9 g L1), Fe_UiO-68_3 (28.5 g L1) and Fe_UiO-68_6 (20.9 g L1). 4. Discussion The results presented in this work suggest three relationships between MOF properties and deliverable capacity (Fig. 7):
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Gravimetric deliverable capacity is related to Qst. This relationship is quadratic, in agreement with recent work from our group that sought to optimize Qst in order to maximize deliverable capacity [37]. Deliverable capacities are plotted against Qst for the Mg and Fe alkoxide functionalized variants of the MOFs considered in this work in Fig. 7, middle. The Qst values are taken at average loadings of 1–2 H2 per metal alkoxide group, low enough that Qst is dominated by H2 interacting with metal alkoxide sites but high enough that contributions from H2 interacting with metal alkoxide clusters near the corners are obscured. From the least squares fit curve, the optimum Qst is 21.4 kJ mol1, which is smaller than our previous estimate of 28 kJ mol1 based on MOFs with a single alkoxide group per linker [17]. However it is in very good agreement with the optimal heat of adsorption calculated by Bae and Snurr [37]. In their analysis, Bae and Snurr considered MOFs with surfaces that were effectively uniform, and they systematically increased the H2 Qst by increasing the Lennard-Jones parameters. They found distinct maxima in their trends for deliverable gravimetric capacity vs. Qst and reported an optimal Qst of 20 kJ mol1. It is interesting that we find a similar optimal value in this analysis. As the MOF linkers become saturated with alkoxide groups, their surfaces become more uniform. Taken together, this work, along with that of Bae and Snurr, indicates that the optimal Qst for MOFs with effectively uniform surfaces is 20 kJ mol1. Volumetric deliverable capacity for MOFs with the same number of functional groups per linker is inversely correlated to the size of the largest pore. MOFs with smaller pore sizes enclose more metal alkoxide sites in smaller volumes. Since H2 molecules prefer adsorption at metal alkoxide sites over other sites in the MOFs, MOFs that have more alkoxide sites per unit volume also adsorb more hydrogen per unit volume.
5. Conclusions
Fig. 7. Top: H2 gravimetric deliverable capacities taken at 243 K and 100 bar in MOFs with three Mg alkoxide groups per linker vs. MOF free volume. Middle: H2 gravimetric deliverable capacities taken at 243 K and 100 bar vs. isosteric heats of adsorption (Qst) taken at loadings of 1–2 H2 per metal alkoxide site. Bottom: H2 volumetric deliverable capacities taken at 243 K and 100 bar vs. the diameter of the largest pore in the MOF for MOFs with three Mg alkoxide groups per linker. Lines are least squares fits.
In this work, we assessed H2 adsorption in Li, Be, Mg, Mn, Fe, Ni, Cu and Zn alkoxide functionalized MOFs. Of these metals, Mg and Fe alkoxide functionalized MOFs exhibit the best H2 storage properties. We examined H2 storage in IRMOF-16, NU-100 and UiO-68 functionalized with multiple Mg or Fe alkoxide groups per linker to develop design criteria for meeting the DOE gravimetric and volumetric targets. We found H2 uptake is correlated to MOF free volume, pore size, and heat of adsorption at low loading. Our analysis indicates that MOFs that have large free volume and a low loading Qst of 20 kJ mol1 will maximize the gravimetric deliverable capacity, while small pore size will maximize the volumetric deliverable capacity. This suggests a trade-off between the properties that maximize gravimetric and volumetric deliverable capacities. We have shown that optimizing Qst can be done by functionalizing with multiple alkoxide groups per linker. For example, IRMOF-16 functionalized with three Fe alkoxide groups per linker exhibits a Qst of 23 kJ mol1 at low loading. It takes up greater than 6 wt.% deliverable H2 and 20 g deliverable H2 L1 at 243 K and 100 bar.
Acknowledgments Gravimetric deliverable capacity for MOFs with the same number of functional groups per linker is directly correlated to MOF free volume. This correlation is in good agreement with a prior report from our group that showed that H2 uptake at high loading is correlated to MOF free volume [29]. In agreement with another recent report from our group [37], we find gravimetric deliverable capacity is also correlated to the surface area, but the correlation is slightly weaker (Fig. S11).
This work was funded by the Department of Energy’s Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Program under Grant No. DE-FC36–08G018137. Some of the calculations were performed on Northwestern University’s Quest cluster and the National Energy Research Scientific Computing Center’s Carver cluster. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-0824162.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Design strategies for metal alkoxide functionalized metal–organic frameworks for ambient temperature hydrogen storage Stephen K. Brand 1, Yamil J. Colón 1, Rachel B. Getman 2, Randall Q. Snurr ⇑ Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, United States
a r t i c l e
i n f o
Article history: Received 8 October 2012 Received in revised form 8 December 2012 Accepted 10 December 2012 Available online 20 December 2012 Keywords: Gas adsorption Open metal sites IRMOF-16 NU-100 UiO-68
a b s t r a c t Grand canonical Monte Carlo simulations were used to calculate hydrogen adsorption in IRMOF-16, NU100, and UiO-68 functionalized with Mg or Fe catecholates on the linkers. We examined how altering the number of metal catecholate groups affects H2 uptake and deliverable capacity near ambient temperature. We find that large free volume and an isosteric heat of adsorption (Qst) of 20 kJ mol1 at low loading will maximize gravimetric deliverable capacity while a small pore diameter will maximize volumetric deliverable capacity. This suggests a trade-off between the properties that lead to maximal gravimetric and volumetric capacities. For example, our calculations suggest that NU-100 functionalized with six Fe catecholate groups per linker takes up 5.5 wt.% deliverable H2 at 243 K and 100 bar, but only 24.2 g L1 deliverable H2. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Hydrogen is a potential replacement for fossil fuels in automotive applications. However, hydrogen gas has a very small volumetric density, 0.0899 kg m3 at standard temperature and pressure (STP), which makes storage difficult. It has been estimated that 4 kg of H2 are necessary to fuel a modestly sized car over a range of 400 km [1]. Thus 44 m3 (44,000 L) of gaseous H2 at STP would be necessary to provide enough energy to fuel a typical automobile. A major challenge in materials research is to design hydrogen storage systems that effectively and safely store H2 in a more realistic volume. To motivate research and development of hydrogen storage materials, the US Department of Energy (DOE) has established targets of 2.5 kWh kg1 and 2.3 kWh L1 [2]. These translate to 7.0 wt.% and 70 g L1 of H2 for the entire storage system. Short term targets are 5.2 wt.% and 40 g L1 by 2015. The minimum storage temperature is 243 K, and the maximum storage pressure is 100 bar. To be effective, hydrogen storage materials must release H2 for use at approximately 2 bar. A variety of storage systems are under development, including high pressure gas and cryogenic liquid tanks [3]. In addition, materials that adsorb H2 either dissociatively, such as metal and chemical hydrides [4], or molecularly, such as metal–organic ⇑ Corresponding author. Tel.: +1 (847) 467 2977; fax: +1 (847) 491 3728. E-mail address: [email protected] (R.Q. Snurr). These authors contributed equally. Present address: Department of Chemical and Biomolecular Engineering, Clemson University, Clemson, SC 29634, United States. 1 2
1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.12.020
frameworks [5,6], are promising for hydrogen storage. However, to date, no system that meets the DOE targets has been developed. In general, chemical and metal hydrides bind H2 very strongly and take up significant amounts of H2. H2 storage capacities of over 10 wt.% are possible [7]. However, due to the strong H2 binding energies as well as prohibitive kinetics for H–H recombination, these materials do not release H2 for use at ambient temperature [7]. In direct contrast, carbon-based molecular adsorbents tend to bind H2 much more weakly. Therefore, they take up significantly less H2 but are much more effective at releasing it. Solving the H2 storage problem relies in part on developing materials with intermediate properties. Metal–organic frameworks (MOFs) are arguably the most promising class of storage materials for molecular hydrogen. MOFs are porous, crystalline materials comprised of metal or metal oxide nodes connected by organic ‘‘linker’’ compounds. They have extremely large surface areas. For example, recently synthesized NU-100 [8] and MOF-210 [9] have surface areas over 6000 m2 g1. MOFs are highly tunable, as different nodes and linkers can be combined to create different materials with different properties. In addition, the linkers contain all the functionality of organic chemistry. For example, MOFs with carboxylic acid [10], amine [11], and alcohol [12,13] groups on the linkers have been synthesized. Because of the large surface areas, MOFs exhibit significant H2 storage at cryogenic temperatures [8,14,15]. However the H2 adsorption enthalpies are typically only around 5 kJ mol1 [5], which is too weak to counteract the thermal effects of motion at ambient temperatures. In order for MOFs to be effective for H2 storage, the heats of adsorption must be increased. One method
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of doing this that has been successfully demonstrated in experiments is metal alkoxide functionalization [12,13]. In this strategy, a MOF with alcohol groups on the linkers is modified post synthesis to exchange hydroxyl protons for metal cations. The metal sites exhibit strong positive charges that interact favorably with the H2 quadrupole [16–19]. MOFs with lithium and magnesium alkoxide groups have been synthesized and shown to increase both the H2 heat of adsorption and uptake at cryogenic temperatures and low pressures [12,13,20]. Simulations of H2 adsorption in metal alkoxide functionalized MOFs suggest that this strategy is promising at ambient temperatures as well [17,21–23]. A recent report from our group suggests that Mg alkoxides, which exhibit H2 binding energies of 22 kJ mol1, are effective at both storing and releasing H2 [17]. In fact, our results suggest that MOFs with large surface areas and low crystal densities that incorporate Mg alkoxide groups can meet the DOE gravimetric target for 2015. In our prior work on this subject, we examined lithium, magnesium, manganese, nickel, and copper alkoxide functionalization in IRMOF-1, IRMOF-10, IRMOF-16, UiO-68 and UMCM-150 [17]. In this work, we extend our analysis to include beryllium and the remaining divalent 3d transition metals, iron and zinc. (Cobalt can also exist in the divalent state, but it binds H2 too strongly to release it for use at 2 bar.) We find that Mg and Fe alkoxides store the most in terms of deliverable H2, so we focus further analysis on those metals. We analyze Mg and Fe alkoxide functionalization in three MOFs, IRMOF-16 and NU-100, which have surface areas of 6000 m2 g1, and UiO-68, which has a surface area of 4000 m2 g1 and provides an interesting contrast. We compute H2 adsorption in these materials functionalized with one, three, and six alkoxide groups per linker and propose design strategies for meeting the DOE gravimetric and volumetric targets.
2. Computational details The methods used in this study are the same as in our prior report on metal alkoxide functionalization [17]. Quantum chemical calculations were performed with the Gaussian 09 software [24] to explore the potential energy surfaces (PESs) for H2 adsorption at the metal sites on metal alkoxide benzenes. H2 binding energies at Be and Zn alkoxide sites were calculated at the MP2/6-311+G⁄⁄ level of theory, using the counterpoise method [25] to offset basis set superposition error (BSSE). Binding energies at Fe alkoxide sites were calculated using the M06 [26] implementation of density functional theory along with the 6-311++G⁄⁄ basis set without correcting for BSSE. We find this method gives binding energies that are similar to those calculated with MP2/6-311+G⁄⁄ using counterpoise corrections (Table S1, Supporting Information), and it is more computationally efficient. To study metal alkoxide functionalization in MOFs, we added metal alkoxide sites to the experimentally determined structures of IRMOF-16 [27], NU-100 [8], and UiO-68 [28]. Unfunctionalized structures are shown in Fig. 1, and structural details are provided in Table 1. Functionalized versions were modeled by adding one, three, or six metal alkoxide groups to each linker. Note that for alkoxides of divalent cations, which require two oxygen anions, six is the maximum number of alkoxide groups per linker in IRMOF-16, NU-100, and UiO-68 (Fig. S6). We calculated the pore size distributions (PSDs) of the unfunctionalized MOFs in order to relate H2 uptake with crystallographic properties. PSDs were calculated using the method of Gelb and Gubbins [31]. In this method, points within the structures are randomly selected and archived by the radius of the largest sphere that can be drawn which encloses the point without overlapping the van der Waals surface of the framework. The PSD is the derivative of the pore volume covered by spheres of radius r with
Fig. 1. Structures of the metal–organic frameworks used in this work. C = gray, H = white, O = red, Zn = cyan, Zr = blue, Cu = orange. The purple spheres illustrate the cavities. (For interpretation of reference to color in this figure legend, the reader is referred to the web version of this article.)
respect to r, which provides the pore volume covered by spheres of radius r but not r + dr.
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S.K. Brand et al. / Microporous and Mesoporous Materials 171 (2013) 103–109 Table 1 Calculated surface area (SA), free volume (V), and diameter of the largest pore (dpore) for the MOFs examined in this work.
a b c
MOF
Refs.
Linkersa
Corners
SAb (m2 g1)
V (cm3 g1)
dpore (Å)
IRMOF-16 NU-100 UiO-68
[27] [8] [28]
TPDC hexatopic ligand TPDC
Zn4O Cu2 paddlewheel Zr6O8
6055 [29] 5813c 4082 [17]
4.413 [29] 2.82 [8] 1.541 [17]
23.6 [30] 27.6 [8] 15.8c
linker abbreviations: TPDC = terphenyl dicarboxylate. See Fig. S6 or Ref. [8] for the structure of the hexatopic ligand. H2 probe. This work.
H2 adsorption isotherms were calculated in a single unit cell with grand canonical Monte Carlo simulations using our in-house code RASPA [32]. At each pressure, 200,000 total Monte Carlo cycles were performed, 100,000 to equilibrate the system and 100,000 to compute ensemble averages. In each cycle, an average of N moves was performed, where N is the number of H2 molecules in the system. Translation, rotation, insertion, deletion, and reinsertion moves were employed. H2/framework interactions were calculated in the following manner: H2 interactions with atoms in the metal alkoxide groups (oxygen and the metal) and the immediate neighboring carbons were calculated with modified-Morse and Coulomb potentials. Modified-Morse parameters were obtained from fitting to the PES for H2 adsorption at the metal sites in metal alkoxide benzenes (obtained from quantum chemical calculations. See Table S3 and Ref. [17].) Coulomb interactions were calculated using calculated partial charges on alkoxide group atoms (Table S3) and the Darkrim-Levesque model [33] for H2. H2 interactions with all other framework atoms and other H2 molecules were calculated using Lennard-Jones (LJ) potentials. LJ parameters for framework atoms were taken from the DREIDING force field [34] where available and the Universal Force Field [35] otherwise. LJ parameters for H2 were taken from an empirical model [36]. Cross-terms were calculated using Lorentz-Berthelot mixing rules. All LJ parameters are provided in Table S4. All other computational details are identical to those reported in Ref. [17].
3. Results 3.1. Hydrogen adsorption in IRMOF-16 with different metals We calculated adsorption isotherms (Fig. 2, left) in variants of IRMOF-16 functionalized with one metal alkoxide group per linker in order to screen additional metals. In agreement with our prior work [17] we find that metal alkoxide functionalization generally enhances gravimetric H2 uptake at 100 bar. Exceptions are Mn and Zn alkoxides. Mn and Zn bind H2 too weakly to enhance ambient temperature H2 storage. Though the binding energies for a single H2 molecule are 20 and 29 kJ mol1, respectively, they drop off significantly as additional H2 molecules adsorb. For example, differential binding energies:
H2 þ ½ðn 1ÞH2 MO2 C6 H4 ! ½nH2 MO2 C6 H4
ð1Þ
for the second H2 molecule are 6 and 5 kJ mol1 for Mn and Zn, respectively. (Binding energies are provided in Fig. S2 and in Ref. [17].) Of the metals analyzed, Fe alkoxides exhibit the strongest binding energies for up to 5 H2, and they also exhibit the largest uptake for PH2 > 10 bar.
Deliverable capacities, defined as the amount of useable H2 a material can store [37], for the different metals are shown in the righthand side of Fig. 2:
D wt% ðPH2 Þ ¼ wt% ðPH2 Þ wt% ð2 barÞ
ð2Þ
As previously reported, Ni and Cu alkoxides bind the first H2 molecule too strongly and subsequent H2 molecules too weakly and thus exhibit poor deliverable capacities [17]. We find the same for Be alkoxides. We previously reported that Mg alkoxides exhibit good binding energies, do not dissociate the hydrogen molecules, and perform well in terms of deliverable capacity [17]. Fe alkoxide functionalized IRMOF-16 exhibits an even better deliverable capacity than its Mg counterpart. 3.2. Hydrogen adsorption in metal–organic frameworks with multiple metal alkoxide groups per linker Our analysis shows that, of the metal alkoxide groups studied, Mg and Fe perform the best in terms of deliverable gravimetric capacities. We thus focus our analysis on these functional groups, exploring now how increasing the number of alkoxide groups per MOF linker can improve H2 adsorption uptake. Total and deliverable capacities for IRMOF-16, NU-100, and UiO-68 functionalized with one, three, and six Mg or Fe alkoxide groups per linker are shown in Figs. 3 and 4, respectively. In general, considering MOFs with the same number of functional groups per linker, the deliverable capacities follow the order IRMOF-16 > NU-100 > UiO-68, suggesting a possible correlation with surface area or free volume, which we discuss below. From here on out, we refer to the functionalized MOFs using the abbreviation metal_MOF_n, where metal is either Mg or Fe; MOF is either IRMOF-16, NU-100, or UiO68; and n is either one, three, or six and describes the number of alkoxide groups per MOF linker. The following MOFs meet or exceed the DOE gravimetric targets for 2015 based on total amount adsorbed (Fig. 3): Mg_IRMOF-16_1 (5.4 wt.%), Mg_IRMOF-16_3 (6.0 wt.%), Mg_IRMOF-16_6 (6.7 wt.%), Fe_IRMOF16_1 (6.4 wt.%), Fe_IRMOF-16_3 (8.0 wt.%), Fe_IRMOF-16_6 (8.7 wt.%), Fe_NU-100_3 (5.5 wt.%), Fe_NU-100_6 (6.9 wt.%), Fe_UiO-68_3 (5.6 wt.%), and Fe_UiO-68_6 (5.7 wt.%). However, when looking at the deliverable capacities (Fig. 4) only the following structures meet or exceed the target: Mg_IRMOF-16_1 (5.2 wt.%), Mg_IRMOF-16_3 (5.4 wt.%), Mg_IRMOF-16_6 (5.5 wt.%), Fe_IRMOF-16_1 (5.6 wt.%), Fe_IRMOF-16_3 (6.2 wt.%), Fe_IRMOF-16_6 (5.4 wt.%), and Fe_NU-100_6 (5.5 wt.%). Fig. 4 shows that, in general, increasing the number of functional groups per linker enhances deliverable capacity at 100 bar. Exceptions are Mg_UiO-68_6, Fe_IRMOF-16_6 and Fe_UiO-68_6. Additionally, Mg_UiO-68_3 and Fe_UiO-68_3 exhibit the same deliverable capacities at 100 bar as Mg_UiO-68_1 and Fe_UiO68_1, respectively. Comparing with the absolute adsorption isotherms in Fig. 3 shows that MOFs that exhibit lower deliverable capacities than structures with fewer functional groups per linker take up significant amounts of H2 at low pressures and then saturate, leading to poor deliverable capacities. Interestingly, IRMOF16 and UiO-68 exhibit this behavior, but NU-100 does not. The
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Fig. 2. H2 absolute adsorption isotherms (left) and deliverable capacities, D wt.% = wt.% (PH2) wt.% (2 bar ), (right) for metal alkoxide functionalized versions of IRMOF-16 at 243 K. Functionalized versions have one metal alkoxide group per organic linker.
Fig. 3. H2 adsorption isotherms for Mg and Fe alkoxide functionalized IRMOF-16 (top), NU-100 (middle), and UiO-68 (bottom) at 243 K. Functionalized versions have one, three, or six alkoxide groups per linker. Lines without symbols represent the unfunctionalized structures.
Fig. 4. H2 deliverable capacities [D wt.% = wt.% (PH2) wt.% (2 bar )] for Mg and Fe alkoxide functionalized IRMOF-16 (top), NU-100 (middle), and UiO-68 (bottom) at 243 K. Functionalized versions have one, three, or six alkoxide groups per linker. Lines without symbols represent the unfunctionalized structures.
tendency to saturate or not can be attributed to differences in the pore sizes in these MOFs. We have previously reported that NU100 has three types of pores with diameters of 27.4, 15.4 and 13.4 Å, respectively [8]. At very low pressure, H2 molecules adsorb at the metal alkoxide sites, regardless of pore size. As the pressure increases H2 molecules then start to adsorb on the rest of the material. These molecules adsorb in the smaller pores, which have more alkoxide groups per unit volume. When the smaller pores are
saturated, molecules adsorb in the large pores. The large pores are not saturated at 100 bar, and thus alkoxide sites are available out to large pressures. In contrast, IRMOF-16 and UiO-68 effectively have one size of pore each, with diameters of 23.6 and 15.8 Å, respectively. Saturation is most pronounced in IRMOF-16 and UiO-68 variants functionalized with six alkoxide groups per linker and with Fe alkoxides, because they take up more H2 at low pressure. Saturation is more dramatic in UiO-68 than in
S.K. Brand et al. / Microporous and Mesoporous Materials 171 (2013) 103–109
Fig. 5. Simulated isosteric heats of adsorption (Qst) for Mg alkoxide functionalized UiO-68 at low pressure (top left) and high pressure (bottom left) and Fe alkoxide functionalized IRMOF-16 (top right) and UiO-68 (bottom right) at high pressure. Qst in Fe alkoxide functionalized MOFs exceed 90 kJ mol1 at very low pressure, but we do not include these values in the graphs in order to show more clearly the high pressure behavior. Results were obtained at 243 K. The line closest to the x-axis corresponds to the unfunctionalized structure.
IRMOF-16 because it has smaller pores. Pore size analysis in UiO68 and snapshots of H2 adsorption in IRMOF-16 and NU-100 are provided in Figs. S7–S9 in the Supporting information. Isosteric heats of adsorption (Qst) were obtained from the GCMC simulations [38] and are shown in Fig. 5.
Q st ¼ RT
@hVi @hNi
ð3Þ T;V
In general, MOFs with more alkoxide groups per linker have higher Qst out to higher pressures, leading to larger deliverable capacities. Considering MOFs with the same number of alkoxide groups per linker, Qst values are larger in UiO-68 than in IRMOF-16 and NU100. UiO-68 has a smaller free volume, and thus the density of alkoxide sites is larger, allowing more effective interactions with H2 [17]. In fact, at 100 bar the highest calculated Qst is 14 kJ mol1 in Fe_UiO-68_6. MOFs that exhibit larger Qst at high pressure do not necessarily perform well in terms of deliverable capacity. We find that MOFs with larger Qst at high pressure also exhibit larger Qst at 2 bar (and lower pressures). This drives larger uptake at low pressure and can lead to lower deliverable capacities. We discuss a quadratic relationship between deliverable capacity and Qst at low loading in the next section. As in our previous work, Qst values are relatively large initially but drop off as pressure (and thus loading) increases [17]. Remarkably, UiO-68 variants functionalized with three and six Mg or Fe alkoxide groups per linker exhibit very strong low loading Qst of 45 and 90 kJ mol1, respectively, more than twice the H2 binding energies to a single alkoxide for these metals [17]. We find that these large interaction energies are due to H2 adsorbing in sites near the corners amidst clusters of metal alkoxide sites. Molecules adsorbed in these sites can interact with multiple metal alkoxide groups simultaneously at close range, increasing the H2 interaction energies (Fig. S10). However, these corner sites saturate at
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Fig. 6. H2 deliverable capacities [D g L1 = g L1 (PH2) g L1 (2 bar )] for Mg and Fe alkoxide functionalized IRMOF-16 (top), NU-100 (middle), and UiO-68 (bottom) at 243 K. Functionalized versions have one, three, or six alkoxide groups per linker. Lines without symbols represent the unfunctionalized structures.
pressures lower than 0.1 bar, and the Qst values drop significantly. Therefore, their contribution to Qst decreases significantly as pressure (and thus H2 loading) increases. Volumetric deliverable capacities
Dðg L1 Þ ¼ g L1 ðPH2 Þ g L1 ð2 barÞ
ð4Þ
are shown in Fig. 6. As with gravimetric deliverable capacities, we find that increasing the number of functional groups per linker increases the volumetric deliverable capacites. The one exception, Fe_UiO-68_6, is due to saturation. Considering all other MOFs, volumetric deliverable capacities follow the order UiO-68 > IRMOF-16 > NU-100 for MOFs with the same number of alkoxide groups per linker, suggesting a possible correlation with pore size (discussed below). Absolute volumetric capacities are shown in Fig. S12. MOFs analyzed in this work that meet the DOE volumetric target, considering total amount adsorbed, for 2015 are: Fe_IRMOF16_6 (44.3 g L1), Fe_UiO-68_3 (45.8 g L1), and Fe_UiO-68_6 (64.0 g L1). However, in terms of deliverable volumetric capacity, none meet the target and only the following exceed deliverable H2 storage capacities of 20 g L1 at 100 bar: Mg_IRMOF-16_6 (21.9 g L1), Fe_IRMOF-16_3 (21.9 g L1), Fe_IRMOF-16_6 (26.6 g L1), Fe_NU-100_6 (24.2 g L1), Mg_UiO-68_3 (20.2 g L1), Mg_UiO-68_6 (21.8 g L1), Fe_UiO-68_1 (20.9 g L1), Fe_UiO-68_3 (28.5 g L1) and Fe_UiO-68_6 (20.9 g L1). 4. Discussion The results presented in this work suggest three relationships between MOF properties and deliverable capacity (Fig. 7):
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Gravimetric deliverable capacity is related to Qst. This relationship is quadratic, in agreement with recent work from our group that sought to optimize Qst in order to maximize deliverable capacity [37]. Deliverable capacities are plotted against Qst for the Mg and Fe alkoxide functionalized variants of the MOFs considered in this work in Fig. 7, middle. The Qst values are taken at average loadings of 1–2 H2 per metal alkoxide group, low enough that Qst is dominated by H2 interacting with metal alkoxide sites but high enough that contributions from H2 interacting with metal alkoxide clusters near the corners are obscured. From the least squares fit curve, the optimum Qst is 21.4 kJ mol1, which is smaller than our previous estimate of 28 kJ mol1 based on MOFs with a single alkoxide group per linker [17]. However it is in very good agreement with the optimal heat of adsorption calculated by Bae and Snurr [37]. In their analysis, Bae and Snurr considered MOFs with surfaces that were effectively uniform, and they systematically increased the H2 Qst by increasing the Lennard-Jones parameters. They found distinct maxima in their trends for deliverable gravimetric capacity vs. Qst and reported an optimal Qst of 20 kJ mol1. It is interesting that we find a similar optimal value in this analysis. As the MOF linkers become saturated with alkoxide groups, their surfaces become more uniform. Taken together, this work, along with that of Bae and Snurr, indicates that the optimal Qst for MOFs with effectively uniform surfaces is 20 kJ mol1. Volumetric deliverable capacity for MOFs with the same number of functional groups per linker is inversely correlated to the size of the largest pore. MOFs with smaller pore sizes enclose more metal alkoxide sites in smaller volumes. Since H2 molecules prefer adsorption at metal alkoxide sites over other sites in the MOFs, MOFs that have more alkoxide sites per unit volume also adsorb more hydrogen per unit volume.
5. Conclusions
Fig. 7. Top: H2 gravimetric deliverable capacities taken at 243 K and 100 bar in MOFs with three Mg alkoxide groups per linker vs. MOF free volume. Middle: H2 gravimetric deliverable capacities taken at 243 K and 100 bar vs. isosteric heats of adsorption (Qst) taken at loadings of 1–2 H2 per metal alkoxide site. Bottom: H2 volumetric deliverable capacities taken at 243 K and 100 bar vs. the diameter of the largest pore in the MOF for MOFs with three Mg alkoxide groups per linker. Lines are least squares fits.
In this work, we assessed H2 adsorption in Li, Be, Mg, Mn, Fe, Ni, Cu and Zn alkoxide functionalized MOFs. Of these metals, Mg and Fe alkoxide functionalized MOFs exhibit the best H2 storage properties. We examined H2 storage in IRMOF-16, NU-100 and UiO-68 functionalized with multiple Mg or Fe alkoxide groups per linker to develop design criteria for meeting the DOE gravimetric and volumetric targets. We found H2 uptake is correlated to MOF free volume, pore size, and heat of adsorption at low loading. Our analysis indicates that MOFs that have large free volume and a low loading Qst of 20 kJ mol1 will maximize the gravimetric deliverable capacity, while small pore size will maximize the volumetric deliverable capacity. This suggests a trade-off between the properties that maximize gravimetric and volumetric deliverable capacities. We have shown that optimizing Qst can be done by functionalizing with multiple alkoxide groups per linker. For example, IRMOF-16 functionalized with three Fe alkoxide groups per linker exhibits a Qst of 23 kJ mol1 at low loading. It takes up greater than 6 wt.% deliverable H2 and 20 g deliverable H2 L1 at 243 K and 100 bar.
Acknowledgments Gravimetric deliverable capacity for MOFs with the same number of functional groups per linker is directly correlated to MOF free volume. This correlation is in good agreement with a prior report from our group that showed that H2 uptake at high loading is correlated to MOF free volume [29]. In agreement with another recent report from our group [37], we find gravimetric deliverable capacity is also correlated to the surface area, but the correlation is slightly weaker (Fig. S11).
This work was funded by the Department of Energy’s Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Program under Grant No. DE-FC36–08G018137. Some of the calculations were performed on Northwestern University’s Quest cluster and the National Energy Research Scientific Computing Center’s Carver cluster. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-0824162.
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