Isoreticular Series of (3,24)-Connected Metal–Organic Frameworks

Feb 6, 2014 - A Metal-Organic Framework with Optimized Porosity and Functional Sites for High Gravimetric and Volumetric Methane Storage Working Capac...
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Isoreticular Series of (3,24)-Connected Metal−Organic Frameworks: Facile Synthesis and High Methane Uptake Properties Gokhan Barin,† Vaiva Krungleviciute,‡,§ Diego A. Gomez-Gualdron,¶ Amy A. Sarjeant,† Randall Q. Snurr,¶ Joseph T. Hupp,† Taner Yildirim,*,‡,§ and Omar K. Farha*,†,∥ †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA § Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA ¶ Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA ∥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: We have successfully used a highly efficient copper-catalyzed “click” reaction for the synthesis of a new series of hexacarboxylic acid linkers with varying sizes for the construction of isoreticular (3,24)-connected metal−organic frameworks (MOFs)namely, NU-138, NU-139, and NU140. One of these MOFs, NU-140, exhibits a gravimetric methane uptake of 0.34 g/g at 65 bar and 298 K, corresponding to almost 70% of the DOE target (0.5 g/g), and has a working capacity (deliverable amount between 65 and 5 bar) of 0.29 g/g, which translates into a volumetric working capacity of 170 cc(STP)/cc. These values demonstrate that NU-140 performs well for methane storage purposes, from both a gravimetric and a volumetric point of view. Adsorption of CO2 and H2 along with simulated isotherms are also reported.

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INTRODUCTION Natural gas has long been considered as a possible alternative to current transportation fuels, mainly gasoline and diesel. It is abundant and cheap. In addition, it is clean-burning and emits less carbon than other fossil fuels, making it an ideal candidate as a replacement for petroleum. However, the widespread use of natural gas in vehicular applications depends on the development of strategies to efficiently and safely store and deliver it under ambient temperatures. Currently, natural gas powered vehicles utilize compressed natural gas (CNG) tanks, in which very high pressures (∼250 bar) are required to store a sufficient amount of natural gas to drive a reasonable distance. The tank size restricts the applicability of CNG mainly to largesize vehicles, since small-size and midsize passenger vehicles have limited storage space to accommodate the tanks. Recently, the U.S. Department of Energy (DOE) started1 a new program to promote research to identify a practical material for methane (the primary component of natural gas) storage in passenger cars. Such a material should have a gravimetric capacity of 0.5 g(CH4 )/g(sorbent) and a volumetric capacity of ρ = 0.188 g/cm3 (11.74 mmol/cc) to achieve the corresponding density of CNG at 250 bar and 298 K. This volumetric target translates into (assuming no packing loss) 263 cc(STP: 273.15 K, 1 atm)/cc, which is considerably higher than the previous target (180 cc(STP)/cc) at 35 bar. Both gravimetric and volumetric targets are clearly challenging and require the development of new materials with higher methane uptake capacities. © 2014 American Chemical Society

Adsorption of methane in porous materials is a promising strategy2−22 to achieve these targets under ambient temperature and moderate pressures (35−65 bar). Metal−organic frameworks (MOFs) have emerged23−25 as a unique class of porous materials that are highly porous and offer26 exceptionally high surface areas. These unusual properties of MOFs, which often surpass traditional porous materials such as zeolites and activated carbons, have been extensively exploited27−41 in applications such as gas storage and separations. The modular chemistry of MOFsi.e., a large selection of building blocks including organic linkers and metal ions/clustersallows one to easily tune the surface areas, pore sizes, and chemical functionalities of these materials, thereby enabling their rational design for a desired application.42,43 In the context of methane storage, there has been a growing interest in the investigation of MOFs with higher methane storage capacities during the past decade.6−20,44,45 The interplay between the surface area, pore size, and density of open metal sites has been demonstrated46,47 to determine the performance (total volumetric and gravimetric uptake) of MOFs for methane storage. Although achieving high methane uptake is essential in these materials, an equally important factor to consider is the working capacity of MOFs, which can be defined as the deliverable amount of methane between 65 Received: December 19, 2013 Revised: February 6, 2014 Published: February 6, 2014 1912

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Scheme 1. (a) General Scheme for the Syntheses of the Hexacarboxylic Acid Linkers L1−L4; (b) Corresponding MOFsNU125, NU-138, NU-139, and NU-140Prepared under Solvothermal Conditionsa

(a) CuSO4·5H2O and sodium ascorbate were used as the catalyst for the “click” reaction between a trisalkyne precursor and dimethyl 5azidoisophthalate. The saponification of hexaester products afforded the final hexacarboxylic acid linkers. (b) The crystal structures of the MOFs, reflecting each hexacarboxylate linker connecting six copper-paddlewheel units, are depicted. The synthesis of L1 and the structure of NU-125 were reported45 previously. a

subsequently used in the preparation of NU-125 (Scheme 1b) in gram scales. Encouraged by this previous foray, we sought to expand the “click” chemistry strategy in the syntheses of a series of linkers and then prepare an isoreticular MOF series in a facile manner. Here, we present (i) the preparation of three new hexacarboxylic acid linkers (L2−L4), (ii) the construction of corresponding (3,24)-connected MOFs (NU138, NU-139, NU-140) incorporating copper-paddlewheel units, and (iii) the gas uptake properties.

bar (upper storage pressure) and 5 bar (lower pressure limit at which the natural gas engine operates). In a recent report by Peng et al.,47 in which six top performers to date were reevaluated for high-pressure methane uptake, second-generation MOFsnamely, NU-11148,49 and NU-12545with high surface areas and pore volumes have been shown to exhibit volumetric working capacities as high as 179 cc/cc and 183 cc/ cc, respectively. These values are quite comparable to HKUST1,47,50 which has a working capacity of 190 cc/cc at 298 K. However, HKUST-1 performs rather poorly for gravimetric uptake while NU-111 demonstrates remarkably high total gravimetric capacity (0.36 g/g at 65 bar), corresponding to almost 75% of the DOE’s gravimetric target, and a working capacity of 0.31 g/g. We should also note that DUT-49, reported51 by the Kaskel group, has so far the highest gravimetric methane uptake (0.41 g/g) at 65 bar with a working capacity of 0.36 g/g. The impressive gravimetric uptake of these materials can be attributed to their high surface areas (>5000 m2/g) and pore volumes (>2 cm3/g); a linear correlation has been shown47,52 between the total gravimetric uptake and the surface areas or pore volumes of the materials tested. Although it is possible to reach ultrahigh surface areas with MOFs using longer linkers,26,29 they usually require iterative palladium-catalyzed coupling reactions and overall yields become lower, thus limiting the preparation and evaluation of these materials in larger scales. It is also important to prepare these materials in a cost-effective manner for their actual commercialization. Therefore, it becomes crucial to identify facile synthetic routes that utilize cheaper catalysts and involve high-yielding transformations to obtain high surface area materials with high methane uptake capacities. We and others have recently employed45,53 the highly efficient copper-catalyzed azide−alkyne cycloaddition reactionso-called “click” chemistry54in the synthesis of hexacarboxylic acid linker L1 (Scheme 1a), which was

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RESULTS AND DISCUSSION The syntheses of all hexacarboxylic acid linkers were achieved (Scheme 1a) in good yields via copper(I)-catalyzed 1,3cycloaddition of a trisalkyne-incorporating precursor and dimethyl 5-azidoisophthalate in the presence of CuSO4·5H2O and sodium ascorbate, which was then followed by saponification of the hexaester products to afford the linkers L2−L4. Accordingly, L2 was obtained using commercially available precursor tripropargylamine, and L3 and L4 were synthesized using tris(4-ethynylphenyl)amine and 1,3,5-tris(4ethynylphenyl)benzene, respectively, both of which can be prepared in one step from commercially available precursors. Syntheses and characterization of L2−L4 are described in the Supporting Information. Corresponding MOFs were synthesized (Scheme 1b) from a mixture of a copper(II) salt and the linker. Solvothermal reaction of L2 with Cu(NO3)2·2.5H2O in a mixture of N,N-dimethylformamide (DMF) and water (in the presence of HCl) at 80 °C for 24 h afforded teal-colored crystals of NU-138 with a framework formula of [Cu4(L)1.3(H2O)3(DMF)1]n. Similarly, the reaction of L3 or L4 and Cu(NO3)2·2.5H2O in DMF/HBF4 at 80 °C for 24 h yielded truncated cuboctahedra shaped crystals of NU-139 (dark green-colored) and NU-140 (teal-colored), respectively, with a framework formula [Cu3(L)(H2O)3]n. 1913

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packing of three different types of cages.57 The first cage is a cuboctahedral one (Figure 1b, c) and formed by 24 isophthalate units and 12 (CuII)2 paddlewheel nodes. Because all linkers carry the same isophthalate moeity, the cuboctahedral cage is essentially identical in NU-138/139/140. The triangular windows in the cuboctahedral cage are shared (Figure 1f) with the second cage, which can be defined as a truncated tetrahedron (Figure 1d) and formed by isophthalate groups from four linkers and 12 (CuII)2 paddlewheel nodes. As expected, the size of the truncated tetrahedral cage is dependent on the linker size, and it increases progressively from NU-138 to NU-140. We should also note that the flexibility of the core (−N(CH2)3) in linker L2 changes the geometry of the cage noticeably from a tetrahedron to almost a sphere (Figure S7 of the Supporting Information). The rigidity of the core component in L3 and L4, however, preserves the geometry of the truncated tetrahedral cage in NU-139 and NU140. The third and largest cage is described as a truncated cuboctahedron (Figure 1e) constructed from eight distinct linkers and 24 (CuII)2 paddlewheel nodes. The rectangular windows of a cuboctahedral cage are shared (Figure 1f) with truncated cuboctahedral cages. Similar to the truncated tetrahedral cage, NU-140 has the largest truncated cuboctahedral cage as a result of expansion in linker size for L4. Although the phase purity of bulk samples of NU-138 and NU-139 was confirmed (Figures S8−S11 of the Supporting Information) using powder X-ray diffraction (PXRD) measurements, removal of the guest solvent molecules to obtain activated materials resulted in the collapse of frameworks, at least in our hands, despite numerous attempts.58 However, we were able to activate NU-140 without the collapse of the framework, in which the PXRD (Figure S12) of the bulk material was in excellent agreement with the simulated pattern. Thermogravimetric analysis (TGA) of NU-140 revealed a mass loss at ∼150 °C (Figure S13), assigned to a loss of solvent molecules (DMF), and confirmed that the framework is thermally stable up to 260 °C. An as-synthesized NU-140 sample was soaked in acetone for 4 days prior to drying and then degassed under dynamic vacuum at 110 °C for 18 h, which resulted in the activated NU-140 sample. The N2 sorption isotherm (Figure 2) measured at 77 K on the activated NU-140 showed a maximum N2 uptake of 1275 cm3(STP)/g, corresponding to a pore volume of 1.97 cm3/g. The Brunauer−Emmett−Teller (BET) surface area of NU-140 is determined to be 4300 m2/g. These values are in good agreement with the calculated60 ones: Vp = 1.90 cm3/g (81.5% porosity) and SBET = 4820 m2/g. Recently, we have shown47 that the gravimetric methane uptake in MOFs is linearly proportional to the surface area, roughly 0.10 g/g adsorption for each 1500 m2/g surface area. This suggests that NU-140 could be an excellent methane sorbent with a high gravimetric uptake capacity around 0.30 g/g. Motivated by this, we have studied the adsorption properties of methane and other gases in NU-140. The temperature-dependent, high-pressure adsorption measurements were performed using a custom-built, computer-controlled Sievert apparatus, details of which have been published61 elsewhere. Figure 3 summarizes our results. NU-140 exhibits a maximum methane uptake of ∼0.80 g/g at 125 K, which effectively corresponds to the upper limit for the amount of methane that can be adsorbed under very large external pressures at ambient temperature. Using the liquid methane density at 125 K, the observed maximum methane uptake corresponds to a pore volume of 1.97 cm3/g, which is in

Single-crystal X-ray analysis of NU-138/139/140 revealed that all three frameworks are noncatenated structures and display rht topology, which was pioneered55 by Eddaoudi and further studied by us29 and other18,56 groups. Each framework node consists of (CuII)2 units coordinated by four carboxylates from different linkers in a paddlewheel fashion with water molecules occupying the axial positions. NU-138 crystallizes in a cubic space group Fd3̅c with a large unit cell (a = b = c = 81.799 Å). Both NU-139 and NU-140 have tetragonal space group I4/m with unit-cell dimensions a = b = 34.685 Å and c = 50.692 Å and a = b = 36.788 Å and c = 54.618 Å, respectively. The previously reported45 isoreticular MOF NU-125 with the same tetragonal space group has the unit-cell dimensions of a = b = 31.311 Å and c = 44.807 Å. The expansion in linker size (L1 < L3 < L4) clearly results in an increase in the unit-cell dimensions from NU-125 to NU-140. The structural features of NU-140 are illustrated in Figure 1 as a representative of the isoreticular series (see the Supporting Information for NU-138 and NU-139). The isoreticular MOFs NU-138/139/140 with rht topology can be visualized as the

Figure 1. (a) Structural formula of linker L4 used to make NU-140. The single-crystal X-ray structure of NU-140 showing (b) three cuboctahedral building blocks connected by a hexacarboxylate unit, (c) a single cuboctahedral cage, (d) a truncated tetrahedral cage, (e) a truncated cuboctahedral cage, and (f) a packing of these three different cages. Hydrogens and disordered solvent molecules are omitted for clarity. Carbon, gray; oxygen, red; nitrogen, blue; copper, teal. The largest spheres that the cavity of each cage can accommodate are represented in purple. 1914

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This suggests that the larger size of CO2 and higher temperature lead to a lower packing efficiency of CO2 when compared to smaller gases, such as N2 and CH4. NU-140 exhibits rather high CO2 adsorption, 1.52 g/g at 298 K and 30 bar, which is one of the largest uptake values reported so far for a MOF (i.e., 1.45 g/g for MOF-177,64 1.45 g/g for PCN-68,18 1.68 g/g for NU-111,48 1.70 g/g for MOF-210,65 1.77 g/g for MOF-200,65 and 1.76 g/g for NU-10029,66). NU-140 also exhibits significant hydrogen adsorption at low temperatures as shown in Figure 3. The excess isotherm at 77 K (Figure S15 of Supporting Information) shows a maximum near 45 bar with 51 mg/g hydrogen uptake. The total uptake at 77 K does not saturate over the pressure range examined and reaches 90 mg/g hydrogen uptake at 65 bar. This number is quite comparable to benchmark MOFs such as NOTT-11256 (100 mg/g, 70 bar) and PCN-6818 (115 mg/g, 65 bar) and slightly lower than that for NU-11148 (120 mg/g, 65 bar). Figure 2. N2 isotherm of NU-140 at 77 K. The black and red lines show the N2 isotherms before and after CO2/CH4/H2 adsorption measurements that were done at various temperatures, indicating no sample degradation over adsorption cycles. Closed and open symbols represent adsorption and desorption branches, respectively. The insets show the consistency plot and the BET fitting (see ref 59).

excellent agreement with the nitrogen pore volume. Figure 3 shows that, with increasing temperature, the methane uptake values go down rapidly; at 270 K and 65 bar, the total uptake is reduced to 0.42 g/g, slightly lower than the DOE’s target of 0.5 g/g. At 300 K and 65 bar, the total methane uptake is 0.34 g/g, which is among the highest values reported in the literature (i.e., 0.30 g/g for NOTT-119,62 0.35 g/g for PCN-68,18,63 0.36 g/g for NU-111,48 and 0.41 g/g for DUT-4951). As discussed above, the working capacity is more important than the total uptake. The methane gravimetric working capacity, defined here as the difference in uptake between the pressures of 65 and 5 bar, is 0.29 g/g, ∼60% of the DOE’s target of 0.5 g/g. Using the crystal density of the activated NU140 structure (ρ = 0.426 g/cm3) and ignoring the packing loss, the total volumetric uptake value and working capacity are about 200 cc(STP)/cc and 170 cc(STP)/cc, respectively. These numbers are comparable to those of HKUST-1, a current record holder for volumetric methane uptake (working capacity =190 cc/cc). However, NU-140 exhibits almost twice the gravimetric working capacity of HKUST-1 as a result of the lower crystal density of NU-140. Moreover, NU-140 performs volumetrically better than DUT-49, which has a corresponding total uptake of 177 cc/cc at 65 bar and a working capacity of 157 cc/cc. The results reported here indicate that NU-140 is comparable to NU-111, which has been shown47 to exhibit simultaneously high gravimetric and volumetric methane uptakes. The advantage of NU-140 over NU-111 is that the linker synthesis does not rely on palladium-catalyzed reactions, therefore enabling the synthesis of NU-140 in large scale easily and efficiently, which we believe is an important step forward to reaching high surface areas and pore volumes using extended linkers. We have also studied CO2 and H2 uptake properties of NU140 over wide temperature and pressure ranges as shown in Figure 3. The CO2 isotherm at 220 K was collected up to the saturation pressure. The maximum adsorption at the saturation pressure (2.16 g/g) yields a pore volume of 1.87 cm3/g, which is slightly lower than the nitrogen and methane pore volumes.

Figure 3. Total gravimetric gas uptake (g(gas)/g(MOF)) at various temperatures. The lines with filled circles are experimental data whereas the dashed lines are simulated isotherms calculated using Dreiding force field. The pressure axis for the 125 K CH4 isotherm is scaled by a factor of 10 for clarity.

Figure 3 also shows the calculated isotherms from molecular simulations, which are in good agreement with those obtained from experiments for all gases and in all pressure−temperature ranges.67 This gives us further confidence in the results reported here. To get better insight into the nature of adsorption sites and interactions in NU-140, we extracted isosteric heats of adsorption (Qst) from the temperaturedependent total adsorption isotherms shown in Figure 3 (see the Supporting Information for details). Similarly, the Qst values are calculated from the simulations and compared with experiment in Figure 4. The experiment and theory are in good agreement; the biggest discrepancy is near the initial loading where the gas−copper interactions are determinant and 1915

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metal centers such as zinc and cobalt, as well as other topologies.

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ASSOCIATED CONTENT

S Supporting Information *

Details of linker and MOF synthesis, various characterization measurements, total and excess isotherms, and details of computer simulations (pdf, encifer). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 4. Isosteric heats of adsorptions (Qst) as a function of gas loading in mmol/g. The red lines are from virial fitting whereas the black lines are from original isotherm data with a spline (without any fitting). Blue scattered points represent the simulated data obtained using Dreiding force field.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.Y. acknowledges the support by the U.S. Department of Energy through BES Grant no. DE-FG02-08ER46522. The Northwestern group gratefully acknowledges DOE ARPA-E and the Stanford Global Climate and Energy Project for support of work relevant to CH4 and CO2, respectively.

may not be captured effectively in the simulations. The Qst starts at high values at initial loadings, which is attributed to stronger interaction between the copper sites and the adsorbed molecules. Then it decreases rapidly with increasing loading. Although the Qst for H2 keeps decreasing, the gas−gas interactions become important at higher loadings for CO2 and CH4, and the Qst starts to increase. In the case of CH4 adsorption, the initial Qst for NU-140 is almost 14 kJ/mol, which is similar to those observed in NU-111 (14.2 kJ/mol) and NU-125 (15.1 kJ/mol). The large pore volumes and high surface areas account for the relatively lower Qst in these materials when compared47 to those with higher density of open-metal sites such as Ni-MOF-74 (21.4 kJ/mol) and with smaller pores such as PCN-14 (18.7 kJ/mol) and UTSA-20 (18.2 kJ/mol). The lower Qst also decreases the uptake at lower pressures, therefore resulting in a higher working capacity (65− 5 bar). Although the total uptake of NU-140 at 65 bar is lower than that for Ni-MOF-74, PCN-14, and UTSA-20,47 its volumetric working capacity (170 cc/cc) surpasses most of these MOFs (129, 157, and 170 cc/cc, respectively).

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REFERENCES

(1) See DOE MOVE program at https://arpa-e-foa.energy.gov/. (2) Makal, T. A.; Li, J.-R.; Lu, W.; Zhou, H.-C. Chem. Soc. Rev. 2012, 41, 7761. (3) He, Y.; Zhou, W.; Krishna, R.; Chen, B. Chem. Commun. 2012, 48, 11813. (4) Konstas, K.; Osl, T.; Yang, Y.; Batten, M.; Burke, N.; Hill, A. J.; Hill, M. R. J. Mater. Chem. 2012, 22, 16698. (5) Mason, J. A.; Veenstra, M.; Long, J. R. Chem. Sci. 2014, 5, 32. (6) Menon, V. C.; Komarneni, S. J. Porous Mater. 1998, 5, 43. (7) Seki, K. Chem. Commun. 2001, 1496. (8) Bourrelly, S.; Llewellyn, P. L.; Serre, C.; Millange, F.; Loiseau, T.; Férey, G. J. Am. Chem. Soc. 2005, 127, 13519. (9) Rodríguez-Reinoso, F.; Almansa, C.; Molina-Sabio, M. J. Phys. Chem. B 2005, 109, 20227. (10) Llewellyn, P. L.; Bourrelly, S.; Serre, C.; Vimont, A.; Daturi, M.; Hamon, L.; De Weireld, G.; Chang, J.-S.; Hong, D.-Y.; Kyu Hwang, Y.; Hwa Jhung, S.; Férey, G. Langmuir 2008, 24, 7245. (11) Senkovska, I.; Kaskel, S. Microporous Mesoporous Mater. 2008, 112, 108. (12) Alcañiz-Monge, J.; Lozano-Castelló, D.; Cazorla-Amorós, D.; Linares-Solano, A. Microporous Mesoporous Mater. 2009, 124, 110. (13) Dietzel, P. D. C.; Besikiotis, V.; Blom, R. J. Mater. Chem. 2009, 19, 7362. (14) Kim, H.; Samsonenko, D. G.; Das, S.; Kim, G.-H.; Lee, H.-S.; Dybtsev, D. N.; Berdonosova, E. A.; Kim, K. Chem.Asian J. 2009, 4, 886. (15) Klein, N.; Senkovska, I.; Gedrich, K.; Stoeck, U.; Henschel, A.; Mueller, U.; Kaskel, S. Angew. Chem., Int. Ed. 2009, 48, 9954. (16) Gedrich, K.; Senkovska, I.; Klein, N.; Stoeck, U.; Henschel, A.; Lohe, M. R.; Baburin, I. A.; Mueller, U.; Kaskel, S. Angew. Chem., Int. Ed. 2010, 49, 8489. (17) Wu, H.; Simmons, J. M.; Liu, Y.; Brown, C. M.; Wang, X.-S.; Ma, S.; Peterson, V. K.; Southon, P. D.; Kepert, C. J.; Zhou, H.-C.; Yildirim, T.; Zhou, W. Chem.Eur. J. 2010, 16, 5205. (18) Yuan, D.; Zhao, D.; Sun, D.; Zhou, H.-C. Angew. Chem., Int. Ed. 2010, 49, 5357. (19) Moellmer, J.; Moeller, A.; Dreisbach, F.; Glaeser, R.; Staudt, R. Microporous Mesoporous Mater. 2011, 138, 140. (20) Sahoo, P. K.; John, M.; Newalkar, B. L.; Choudhary, N. V.; Ayappa, K. G. Ind. Eng. Chem. Res. 2011, 50, 13000.

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CONCLUSIONS In summary, we have successfully used highly efficient “click” chemistryin the form of copper-catalyzed azide−alkyne cycloadditionfor the preparation of a new series of hexacarboxylic acid linkers with varying sizes in high yields. The corresponding isoreticular (3,24)-connected MOFs (NU138, NU-139, and NU-140) were constructed under solvothermal conditions. Among these isoreticular MOFs, NU-140 maintains its structure after thermal activation, resulting in a high BET surface area of 4300 m2/g and pore volume of 1.97 cm3/g, and exhibits a high gravimetric methane uptake (0.34 g/g, 65 bar, 298 K) and a working capacity (65−5 bar) of 0.29 g/gvalues that are among the highest in the literature. The volumetric working capacity (170 cc(STP)/cc) is also comparable to the current best material HKUST-1. Additionally, it shows very high CO2 uptake at high pressures. NU-140 is an important material for practical methane storage applications on account of its facile and high-yielding synthesis and its simultaneously high gravimetric and volumetric methane working capacities. We expect that the new hexacarboxylic acid linkers reported herein will be also invaluable in the construction of other MOFs with different 1916

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Chemistry of Materials

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(55) Nouar, F.; Eubank, J. F.; Bousquet, T.; Wojtas, L.; Zaworotko, M. J.; Eddaoudi, M. J. Am. Chem. Soc. 2008, 130, 1833. (56) Yan, Y.; Lin, X.; Yang, S. H.; Blake, A. J.; Dailly, A.; Champness, N. R.; Hubberstey, P.; Schröder, M. Chem. Commun. 2009, 1025. (57) When the curvature of the ligand is taken into account, an additional fourth cage can be observed that consists of two copperpaddlewheel nodes and four different linkers, each of which contributes with two carboxylate units. See ref 49 for a detailed explanation. (58) Given the presence of a noncatenated framework built from relatively flexible nitrogen-centered linkers, it is likely that the evacuation of guest solvent molecules causes the (partial) collapse of the frameworks, which results in lower surface areas than expected. (59) Walton, K. S.; Snurr, R. Q. J. Am. Chem. Soc. 2007, 129, 8552. (60) Pore volume was calculated using PLATON (C) 1980−2011. A. L.Spek, Utrecht University, Padualaan 8, 3584 CH, Utrecht, The Netherlands. The van der Waals radii used in the analysis C:1.70, H:1.2, Cu:1.4, N:1.5, O:1.52. Surface area was calculated using nonorthoSA. See: Düren, T.; Millange, F.; Férey, G.; Walton, K. S.; Snurr, R. Q. J. Phys. Chem. C 2007, 111, 15350. (61) Zhou, W.; Wu, H.; Hartman, M. R.; Yildirim, T. J. Phys. Chem. C 2007, 111, 16131. (62) Yan, Y.; Yang, S. H.; Blake, A. J.; Lewis, W.; Poirier, E.; Barnett, S. A.; Champness, N. R.; Schröder, M. Chem. Commun. 2011, 47, 9995. (63) Yan, Y.; Telepeni, I.; Yang, S. H.; Lin, X.; Kockelmann, W.; Dailly, A.; Blake, A. J.; Lewis, W.; Walker, G. S.; Allan, D. R.; Barnett, S. A.; Champness, N. R.; Schröder, M. J. Am. Chem. Soc. 2010, 132, 4092. (64) Millward, A. R.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 17998. (65) Furukawa, H.; Ko, N.; Go, Y. B.; Aratani, N.; Choi, S. B.; Choi, E.; Yazaydin, A. O.; Snurr, R. Q.; O’Keeffe, M.; Kim, J.; Yaghi, O. M. Science 2010, 329, 424. (66) Shimizu, G. K. H. Nat. Chem. 2010, 2, 909. (67) The simulated isotherms for H2 agree well with experimental data at temperatures above 77 K. At 50 K, the disagreement is probably due to the quantum nature of H2 molecules, which may not be captured well in the simulations.

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dx.doi.org/10.1021/cm404155s | Chem. Mater. 2014, 26, 1912−1917