Exceptional Mechanical Stability of Highly Porous Zirconium Metal

Mar 5, 2013 - Energy Storage during Compression of Metal–Organic Frameworks ..... First-principles study of elastic mechanical responses to applied ...
0 downloads 1 Views 2MB Size
Download PDF
Letter pubs.acs.org/JPCL

Exceptional Mechanical Stability of Highly Porous Zirconium Metal− Organic Framework UiO-66 and Its Important Implications Hui Wu,†,‡ Taner Yildirim,†,§ and Wei Zhou*,†,‡ †

NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States ‡ Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States § Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) with high porosity usually exhibit weak mechanical stabilities, in particular, rather low stabilities against shear stress. This limitation remains one of the bottlenecks for certain applications of porous MOFs, such as gas storage or separation that requires dense packing of the MOF powders under mechanical compression without collapsing the pores. We found that UiO-66, a prototypical Zr-MOF with high porosity, exhibits unusually high shear stability. Its minimal shear modulus (Gmin = 13.7 GPa) is an order of magnitude higher than those of other benchmark highly porous MOFs (e.g., MOF-5, ZIF-8, HKUST-1), approaching that of zeolites. Our analysis clearly shows that the exceptional mechanical stability of UiO-66 is due to its high framework connections (i.e., the high degree of coordination of Zr−O metal centers to the organic linkers). Our work thus provides important guidelines for developing new porous MOFs targeting at high mechanical stabilities. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

P

On the energy landscape, porous phases correspond to local energy minima and are intrinsically metastable. At low mechanical loading (within the elastic limit), the MOF deformation could be small and reversible. With increasing loading, the structural distortion becomes excessive (exceeding the elastic limit); the MOF may then transform to less porous phases with lower system energies and eventually fully collapse, losing its porosity. The plasticity of MOFs is limited because of their directional bonding,16 and plastic deformation is not desired anyway because it means permanent pore shape distortion and pore volume reduction. Therefore, MOFs with higher elastic moduli (i.e., exhibiting smaller lattice deformation under equal mechanical loading) are desired because they would have a higher chance of staying within the elastic limit and exhibit better mechanical stability. In typical industrial processing and applications, hydrostatic compressions and shear forces are the two most common mechanical loadings that would be encountered by MOF powders, and thus the corresponding elastic properties, bulk modulus K (inverse of compressibility), and shear modulus G (also called rigidity modulus) are the most relevant mechanical properties. Previous studies on benchmark porous MOFs (MOF-5 and ZIF-8) show that their K values range from 7 to 20 GPa, whereas the G values are exceedingly low, ∼1 GPa, an order of magnitude

orous metal−organic frameworks (MOFs) are a relatively new family of functional materials that are currently under intensive investigation.1−4 Thousands of MOFs have been synthesized and reported, and many potential applications utilizing their versatile pore structures have been proposed and demonstrated.5−15 Thus far, relatively little attention has been paid to the mechanical properties of MOFs.16 We emphasize that for practical applications the mechanical stabilities of MOFs, especially those with high porosity (with pore fraction >40%), must be considered. MOFs could experience mechanical loadings in both processing and applications. For example, because MOFs mainly exist in microcrystalline powder forms, pressurizations are often needed to assemble the loose powders into more compact forms (such as membranes or pellets) to facilitate applications. Furthermore, in typical high-pressure gas storage or separation applications, MOFs would go through many cycles of hydrostatic compressions. Clearly, to maintain their useful material functionality, MOFs must be mechanically stable enough to retain their crystallinity and porosity during these processes. Unfortunately, MOFs are significantly less mechanically stable than their inorganic counterparts, zeolites. Many MOFs were found to suffer partial pore collapse, or even amorphization, under modest mechanical loadings.16−18 This limitation remains a bottleneck for large-scale applications of MOFs and warrants immediate addressing. The fact that porous materials have lower mechanical stabilities than their corresponding dense phases is not unusual. © XXXX American Chemical Society

Received: January 31, 2013 Accepted: March 5, 2013

925

dx.doi.org/10.1021/jz4002345 | J. Phys. Chem. Lett. 2013, 4, 925−930

The Journal of Physical Chemistry Letters

Letter

Figure 1. (a) Schematic of the Zr−O cluster in Zr-MOF. Each Zr−O metal center is connected to 12 organic linkers. Zr, cyan; C, gray; O, red. (b) Crystal structure of UiO-66, consisting of Zr−O metal centers connected by benzene-1,4-dicarboxylate (BDC) linkers. The large yellow and pink spheres represent the two types of pores in this MOF, located at the octahedral sites and the tetrahedral sites, respectively. H atoms are omitted for clarity.

Figure 2. Simple 2D lattices of nodes connected by springs under shear stresses τ. The lattice shear stability (or rigidity) improves significantly with increasing network connections.

smaller than the K values.16,19−22 Clearly, MOFs are most susceptible to distortion under shear loading, and shear modulus is the most critical parameter to consider when evaluating their mechanical stabilities. In seeking for MOFs with higher elastic moduli, particularly higher G, we noticed that the recently developed zirconiumbased metal−organic frameworks23−28 (Zr-MOFs) are potentially interesting. Despite their high porosities, Zr-MOFs possess exceptional thermal and chemical stabilities.23,24 They are stable up to 500 °C in air and stable in most chemical solvents. Although MOFs with high thermal/chemical stability do not necessarily have high mechanical stability (e.g., ZIF-8 has excellent thermal/chemical stability but rather low shear stability), the high inorganic−organic coordination number (12) of Zr-MOF (Figure 1a) suggest that this type of MOF could have better mechanical stability than other MOFs and particularly better resistance to shear stress because the framework connection of the inorganic−organic nodes is the highest possible. Empirically, the shear stability of a network is expected to improve with increasing network connections. This can be easily understood using a simple 2D lattice as an example. (See Figure 2.) We hypothesize that this simple concept might also apply to MOF networks. The best way to test our hypothesis is to perform direct, experimental mechanical measurements. This is, however, rather difficult. At present, Zr-MOF samples synthesized using known recipes are all fine crystalline powders, and a single crystal large enough for traditional type of measurement

is difficult to prepare, which is also the case for the majority of other MOFs. Thus far, there is only one report on the experimental determination of the shear modulus of a MOF, which was done on a single-crystal ZIF-8 sample using Brillouin scattering.22 Because of this experimental difficulty, here we adopt a theoretical approach (details available in the Supporting Information) to evaluate the mechanical stabilities of Zr-MOFs. The previous experimental study on ZIF-8 has proved that calculations based on first-principles density functional theory (DFT) can provide quite reliable results for MOFs.22 We first focus our effort on UiO-66,23 the prototypical ZrMOF (with a porosity of ∼53%). It has a face-centered cubic crystal structure, as schematically shown in Figure 1b. The Zr ion is eight-coordinated by O in the as-synthesized material, and six of them cluster together, forming the Zr6O4(OH)4 metal center. Each metal center is linked to 12 BDC linkers to form the 3D framework. Upon full activation at high temperature (∼250 °C) in vacuum, each Zr−O cluster loses two H2O molecules, reducing the Zr−O coordination to 7. These two MOF forms were traditionally referred to as the hydroxylated and dehydroxylated UiO-66, respectively.23 Upon exposure to water vapor in air, the fully dehydroxylated MOF form can readily convert back to the hydroxylated form. We start by considering the bulk moduli of both forms of UiO-66. The lattice constants and atomic coordinates of the MOF structures were fully optimized. The optimized primitive cell volumes for the two are 2271 and 2250 Å3, corresponding to a cubic cell constant of 20.866 and 20.802 Å, respectively, in 926

dx.doi.org/10.1021/jz4002345 | J. Phys. Chem. Lett. 2013, 4, 925−930

The Journal of Physical Chemistry Letters

927

68.7% 4 5 BTC Cu2O8

BDC: benzene-1,4-dicarboxylate; BPDC: biphenyl-4,4′-dicarboxylate; TPDC: terphenyl dicarboxylate; MeIM: 2-methylimidazole; BTC: benzene-1,3,5-tricarboxylate. bPorosities were calculated based on the DFT-optimized crystal structures using PLATON.34

HKUST-1

MeIM ZnN4

a

0.17 0.69 0.73 4.24 1.4 0.94 0.97 1.04 8.05 1.36 1.32 1.04 1.4 0.94 0.97 4.41 13.1 8.33 6.86 23.84

source

ZIF-8

4

4

45.8%

29.2 11.04 9.52 25.92

18.5 9.23 7.75 24.53

1.28 1.27 1.50 1.48 1.76 0.14 13.75 14.17 12.58 5.69 4.18 1.16 13.75 14.17 12.58 5.69 4.18 8.43 17.63 18.03 18.91 8.44 7.35 1.16 41.01 39.49 42.07 17.15 14.40 18.20 31.85 30.04 33.68 13.35 11.61 12.56 59.35 58.38 58.84 24.77 19.97 29.42 52.6% 53.0% 48.6% 67.7% 77.8% 76.5% 12 12 12 12 12 6 BDC BDC BDC BPDC TPDC BDC Zr6O4(OH)4 Hf6O4(OH)4 Ti6O4(OH)4 Zr6O4(OH)4 Zr6O4(OH)4 Zn4O13 UiO-66 Hf-UiO-66 Ti-UiO-66 UiO-67 UiO-68 MOF-5

8 8 8 8 8 4

Zener ratio A Gmin (GPa) shear modulus μ′ (GPa) shear modulus c44 (GPa) bulk modulus K (GPa) c12 (GPa) c11 (GPa) porosityb metal-center-linker coordination no. metal−O/N coordination no. organic linkera metal center MOF

Table 1. Summary of the Structure Characteristics and the Elastic Moduli of the MOFs Studied in This Work and Several Benchmark MOFs Reported in the Literature

reasonable agreement with the experimental values (20.7551 and 20.7004 Å).23 Bulk moduli were then calculated by applying hydrostatic strains to the cells and fitting the total energy versus volume data to the Murnaghan equation of state,29 as shown in Figure S1 of the Supporting Information. We obtained K = 41.0 and 42.5 GPa for the two UiO-66 forms, respectively. As expected, the two structures have very similar bulk elasticity because dehydroxylation does not change the framework topology and the metal−organic coordination. In the following, we focus on the hydroxylated form of UiO-66, which not only is the technically more important, nonair sensitive form but also has higher crystal symmetry and is thus easier to deal with computationally. We next directly evaluate all three elastic moduli (c11, c12, and c44) of hydroxylated UiO-66, in particular, to obtain the shear moduli. This was done by applying two additional types of strains (a volume-conserving tetragonal strain along the z axis and a volume-conserving monoclinic shear strain) to the crystal and fitting the “total energy E versus strain parameter γ” data.30,31 The data fits are shown in Figure S2 of the Supporting Information. The derived elastic moduli are: c11 = 59.35 GPa; c12 = 31.85 GPa; and c44 = 17.63 GPa. For a cubic crystal, the shear modulus is not isotropic. The dilatational {110} shear modulus μ′ [μ′ = (c11 − c12)/2)] and the {100} shear modulus c44 represent two extreme shear coefficients.32 The Zener ratio, A = c44/μ′, quantifies the degree of anisotropy.33 The smaller of c44 and μ′ is also called the minimal shear modulus (Gmin), an ultimate parameter for evaluating the shear stability. For UiO-66, μ′ = 13.75 GPa (= Gmin), slightly lower than c44, meaning that this MOF is relatively isotropic in terms of shear resistance, with a moderate A value (1.28) close to 1. In Table 1, we summarized the structural characteristics and the calculated elastic moduli of UiO-66, along with those of the two benchmark MOFs (MOF-5 and ZIF-8) reported previously by us and others in the literature,19−22 for the purpose of comparison. MOF-5 and ZIF-8 are well-known representations of the two large families of Zn4O13 and ZnN4 metal-centers-based MOFs. To enrich our comparison, we also included data for HKUST-1, the prototype of a large number of Cu2O8 paddlewheel-based MOFs, the elastic constants of which were calculated in this work. (See Figure S3 of the Supporting Information for details.) A large group of MOFs not considered here are highly flexible MOFs35 because they are intrinsically much “softer” than the common rigid MOFs, with Gmin as low as 0.08 GPa.36 From the Table, it is clear that the elastic constants of UiO66 are significantly higher than those of other prototypical MOFs. The bulk modulus of UiO-66 is higher by almost a factor of two. Most interestingly, the minimal shear modulus is an order of magnitude higher than those of other MOFs. To put the values in context, the elastic constants of UiO-66 are approaching those of pure inorganic porous frameworks, such as zeolites (typically in the range of ∼20 to 120 GPa37,22). The high shear modulus of UiO-66 is exceptional for highly porous MOFs. To elucidate the mechanism, we need to understand why common MOFs show very little resistance to shear forces. Our DFT-relaxed crystal structures of MOFs under shear strains provide a very useful clue. Figure 3a,b shows as examples the MOF-5 and HKUST-1 structures under shear strains that yield Gmin for them, respectively. In MOF-5, the shear deformation is dominated by the cooperative bending of the O−Zn−O bond angle (i.e., distortion of the ZnO4

this work this work this work this work this work our previous work19 cal., ref 20 cal., ref 22 exp., ref 22 this work

Letter

dx.doi.org/10.1021/jz4002345 | J. Phys. Chem. Lett. 2013, 4, 925−930

The Journal of Physical Chemistry Letters

Letter

Figure 3. (a) DFT-relaxed MOF-5 crystal structure under monoclinic shear strain that yields Gmin. [001] view. (b) DFT-relaxed HKUST-1 crystal structure under tetragonal strain that yields Gmin. [110] view. (c,d) DFT-relaxed UiO-66 structures under monoclinic shear strain and tetragonal strain, respectively. Zn, blue; Cu, orange; Zr, cyan; C, gray; O, red. H atoms are omitted for clarity.

angles, and the surrounding porosity can easily accommodate such bond-angle adjustments to generate the needed lattice deformation. Significant bond-length changes, which are much more energy costly than the coordination bond-angle changes, are effectively avoided. (In contrast, under hydrostatic pressure, the MOF deformation always involves significant bond length changes, and thus all MOFs exhibit much higher resistance to hydrostatic pressure than to shear stress.) Overall, the MOF deformations under shear stress bear very little energy penalty. This is the common scenario for MOFs because the majority of them have moderate metal−organic coordination numbers of four, five, or six. In UiO-66, the high Zr−O coordination (8), and the high metal center to organic unit coordination (12) effectively restrict the freedom of the coordination bond angles, making the overall framework much less flexible. Most importantly, as shown in Figure 3c,d, significant bond stretching and compressing are inevitable for UiO-66 under shear strains, in particular, in those structural units that lay in the directions highlighted by green arrows in the Figure. The maximal bond elongations or compressions in UiO-66 take place on the Zr−O bonds and the Ccarboxylate−Cphenyl bridging bonds (in the highlighted directions). Under the same shear strain, the magnitude of maximal bond length change in UiO-66 is ∼5−10 times larger than those found in the other three MOFs. (See Figure 5.) Clearly, the much enhanced shear resistance in UiO66 is a direct consequence of the bond stretching/compression resulting from the high network connections. This mechanism is indeed analogous to the simple concept illustrated in Figure 2. Because high metal−organic coordination plays an important role, one would guess that other metal-based MOFs isostructural to UiO-66 would have similar elastic properties.

Figure 4. Schematic showing the metal−organic coordination angles in the three benchmark MOFs, which are relatively flexible. In contrast, the phenyl rings and the C3N2 rings are much more rigid in terms of both bond lengths and bond angles.

tetrahedral), the Zn−O−Ccarboxylate bond angle, and the O2− Ccarboxylate−Cphynel angle. (See Figure 4.) In ZIF-8, it is mainly the bending of the N−Zn−N bond angle (i.e., distortion of the ZnN4 tetrahedral) and the Zn−MeIM−Zn angle that provides the needed shear deformation, in agreement with what was previously found.22 In HKUST-1, the deformation is mainly on the O2−Cu2−O2 angle (the interfacial angle between the two paddlewheel planes) and the O2−Ccarboxylate−Cphynel angle. (Also see Figure 4.) One common feature for all three MOFs is that they have modest network connections, with metal−organic coordination number of four or six. The metal−organic coordinations are thus relatively flexible in terms of bond 928

dx.doi.org/10.1021/jz4002345 | J. Phys. Chem. Lett. 2013, 4, 925−930

The Journal of Physical Chemistry Letters

Letter

Figure 5. Lengths of the bonds exhibiting the largest elongations/compressions in MOF structures under monoclinic shear strain (left column) and tetragonal strain (right column). Note that under the same shear strain the magnitude of maximal bond length change in UiO-66 is ∼5−10 times larger than those found in the other three MOFs.

We thus performed calculations (Figure S4 of the Supporting Information) on two isostructural MOFs: a recently reported Hf-UiO-6638 and a hypothetical Ti-UiO-66. (Ti, Zr, and Hf are all group IVB elements.) The results are included in Table 1. As expected, the elastic moduli of the three MOFs are indeed very similar despite the difference in ion properties. Last, we evaluate to what extent increasing linker length may affect the mechanical stability. Usually longer linker leads to lower crystal density, higher porosity, and weaker mechanical stability, as shown by previous studies on MOF-5 analogues.21 We performed calculations on UiO-67 and UiO-68 (Figure S5 of the Supporting Information), whose linkers are two and three phenyl ring dicarboxylate, respectively (in comparison with the BDC linker of UiO-66, containing one phenyl ring). As expected, both K and G values decrease with increasing linker length. (See Table 1.) Importantly, the minimal shear moduli of the two are still several times higher than the other three Zn- and Cu-based benchmark MOFs. This further confirms that Zr-MOFs with high metal−organic coordination are indeed a special class. In summary, we have the following two important findings. (1) High porosity and mechanical stability are intrinsically two competing factors for MOFs. Whether highly porous MOFs can have reasonable mechanical stability to survive typical industrial powder processing and certain commercial applications has been a daunting question. Using DFT calculations, we have shown that a highly porous MOF, UiO-66, possesses higher bulk modulus and exceptionally higher shear modulus than other common MOFs (with comparable high porosity) and consequently is much more mechanically stable. UiO-66 analogue MOFs show similarly high elastic moduli, approaching that of zeolites. This excellent material property, in addition to their well-known chemical and thermal stabilities, makes ZrMOFs, especially UiO-66-type MOFs, truly outstanding candidates for practical application consideration. Future experimental studies (when single crystals become available) will be valuable to quantitatively confirm our predictions. (2). The general principles outlined for the first time in this work

for MOFs should make it straightforward to qualitatively evaluate the shear stabilities of other types of existing MOF compounds not covered here. This can be done by examining the MOF crystal structure and network connection and empirically judging whether shear deformation might lead to significant bond length changes. When developing new porous MOFs targeting at high mechanical stability, high metal− organic coordination is likely a prerequisite to be considered.

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed description of the computational method; detailed elastic moduli data plots for UiO-66, HKUST-1, Hf-UiO-66, Ti-UiO-66, UiO-67, and UiO-68; and cif files of the DFTrelaxed, unstrained UiO-66 structure, and structures under shear strains. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.Z.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Dr. John J. Rush for his critical reading of this manuscript. T.Y. acknowledges partial support from the DOE BES grant no. DE-FG02-08 ER 46522.

■

REFERENCES

(1) Kitagawa, S.; Kitaura, R.; Noro, S.-i. Functional Porous Coordination Polymers. Angew. Chem., Int. Ed. 2004, 43, 2334−2375. (2) O’Keeffe, M.; Peskov, M. A.; Ramsden, S. J.; Yaghi, O. M. The Reticular Chemistry Structure Resource (RCSR) Database of, and Symbols for, Crystal Nets. Acc. Chem. Res. 2008, 41, 1782−1789. (3) Férey, G. Hybrid Porous Solids: Past, Present, Future. Chem. Soc. Rev. 2008, 37, 191−214. (4) Zhou, H.-C.; Long, J. R.; Yaghi, O. M. Introduction to Metal− Organic Frameworks. Chem. Rev. 2012, 112, 673−674. 929

dx.doi.org/10.1021/jz4002345 | J. Phys. Chem. Lett. 2013, 4, 925−930

The Journal of Physical Chemistry Letters

Letter

(5) Murray, L. J.; Dincă, M.; Long, J. R. Hydrogen Storage in Metal− organic Frameworks. Chem. Soc. Rev. 2009, 38, 1294−1314. (6) Sculley, J.; Yuan, D.; Zhou, H.-C. The Current Status of Hydrogen Storage in Metal−Organic FrameworksUpdated. Energy Environ. Sci. 2011, 4, 2721−2735. (7) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C. Selective Gas Adsorption and Separation in Metal−Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1477−1504. (8) Zhou, W. Methane Storage in Porous Metal−Organic Frameworks: Current Records and Future Perspectives. Chem. Rec. 2010, 10, 200−204. (9) Makal, T. A.; Li, J.-R.; Lu, W.; Zhou, H.-C. Methane Storage in Advanced Porous Materials. Chem. Soc. Rev. 2012, 41, 7761−7779. (10) He, Y.; Zhou, W.; Krishna, R.; Chen, B. Microporous Metal− Organic Frameworks for Storage and Separation of Small Hydrocarbons. Chem. Commun. 2012, 48, 11813−11831. (11) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal−Organic Framework Materials as Catalysts. Chem. Soc. Rev. 2009, 38, 1450−1459. (12) Ma, L.; Abney, C.; Lin, W. Enantioselective Catalysis with Homochiral Metal−Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1248−1256. (13) Chen, B.; Xiang, S.; Qian, G. Metal−Organic Frameworks with Functional Pores for Recognition of Small Molecules. Acc. Chem. Res. 2010, 43, 1115−1124. (14) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal−Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112, 1105−1125. (15) Horcajada, P.; Gref, R.; Baati, T.; Allan, P. K.; Maurin, G.; Couvreur, P.; Férey, G.; Morris, R. E.; Serre, C. Metal−Organic Frameworks in Biomedicine. Chem. Rev. 2012, 112, 1232−1268. (16) Tan, J. C.; Cheetham, A. K. Mechanical Properties of Hybrid Inorganic−Organic Framework Materials: Establishing Fundamental Structure−Property Relationships. Chem. Soc. Rev. 2011, 40, 1059− 1080. (17) Chapman, K. W.; Halder, G. J.; Chupas, P. J. Pressure-Induced Amorphization and Porosity Modification in a Metal−Organic Framework. J. Am. Chem. Soc. 2009, 131, 17546−17547. (18) Hu, Y. H.; Zhang, L. Amorphization of Metal−Organic Framework MOF-5 at Unusually Low Applied Pressure. Phys. Rev. B 2010, 81, 174103. (19) Zhou, W.; Yildirim, T. Lattice Dynamics of Metal−Organic Frameworks: Neutron Inelastic Scattering and First-Principles Calculations. Phys. Rev. B 2006, 74, 180301. (20) Samanta, A.; Furuta, T.; Li, J. Theoretical Assessment of the Elastic Constants and Hydrogen Storage Capacity of Some Metal− Organic Framework Materials. J. Chem. Phys. 2006, 125, 084714. (21) Han, S. S.; Goddard, W. A. Metal−Organic Frameworks Provide Large Negative Thermal Expansion Behavior. J. Phys. Chem. C 2007, 111, 15185−15191. (22) Tan, J. C.; Civalleri, B.; Lin, C. C.; Valenzano, L.; Galvelis, R.; Chen, P. F.; Bennett, T. D.; Mellot-Draznieks, C.; Zicovich-Wilson, C. M.; Cheetham, A. K. Exceptionally Low Shear Modulus in a Prototypical Imidazole-Based Metal−Organic Framework. Phys. Rev. Lett. 2012, 108, 095502. (23) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. A New Zirconium Inorganic Building Brick Forming Metal Organic Frameworks with Exceptional Stability. J. Am. Chem. Soc. 2008, 130, 13850−13851. (24) Kandiah, M.; Nilsen, M. H.; Usseglio, S.; Jakobsen, S.; Olsbye, U.; Tilset, M.; Larabi, C.; Quadrelli, E. A.; Bonino, F.; Lillerud, K. P. Synthesis and Stability of Tagged UiO-66 Zr-MOFs. Chem. Mater. 2010, 22, 6632−6640. (25) Garibay, S. J.; Cohen, S. M. Isoreticular Synthesis and Modification of Frameworks with the UiO-66 Topology. Chem. Commun. 2010, 46, 7700−7702. (26) Morris, W.; Doonan, C. J.; Yaghi, O. M. Postsynthetic Modification of a Metal−Organic Framework for Stabilization of a Hemiaminal and Ammonia Uptake. Inorg Chem. 2011, 50, 6853−6855.

(27) Jiang, H.-L.; Feng, D.; Liu, T.-F.; Li, J.-R.; Zhou, H.-C. Pore Surface Engineering with Controlled Loadings of Functional Groups via Click Chemistry in Highly Stable Metal−Organic Frameworks. J. Am. Chem. Soc. 2012, 134, 14690−14693. (28) Foo, M. L.; Horike, S.; Fukushima, T.; Hijikata, Y.; Kubota, Y.; Takata, M.; Kitagawa, S. Ligand-Based Solid Solution Approach to Stabilisation of Sulphonic Acid Groups in Porous Coordination Polymer Zr6O4(OH)4(BDC)6 (UiO-66). Dalton Trans. 2012, 41, 13791−13794. (29) Murnaghan, F. D. The Compressibility of Media under Extreme Pressures. Proc. Natl. Acad. Sci. U.S.A. 1944, 30, 244−247. (30) Le Page, Y.; Saxe, Pa. Symmetry-General Least-Squares Extraction of Elastic Coefficients from Ab Initio Total Energy Calculations. Phys. Rev. B 2001, 63, 174103. (31) Zhou, W.; Wu, H.; Yildirim, T. Structural Stability and Elastic Properties of Prototypical Covalent Organic Frameworks. Chem. Phys. Lett. 2010, 499, 103−107. (32) Walpole, L. J. The Elastic Shear Moduli of a Cubic Crystal. J. Phys. D: Appl. Phys. 1986, 19, 457−462. (33) Zener, C. Elasticity and Anelasticity of Metals; University of Chicago Press: Chicago, IL, 1948. (34) Spek, A. L. Single-Crystal Structure Validation with the Program PLATON. J. Appl. Crystallogr 2003, 36, 7−13. (35) Horike, S.; Shimomura, S.; Kitagawa, S. Soft Porous Crystals. Nat. Chem. 2009, 1, 695−704. (36) Ortiz, A. U.; Boutin, A.; Fuchs, A. H.; Coudert, F.-X. Anisotropic Elastic Properties of Flexible Metal−Organic Frameworks: How Soft are Soft Porous Crystals? Phys. Rev. Lett. 2012, 109, 195502. (37) Auerbach, S. M.; Carrrado, K. A.; Dutta, P. K. Handbook of Zeolite Science and Technology; CRC Press: New York; 2003. (38) Jakobsen, S.; Gianolio, D.; Wragg, D. S.; Nilsen, M. H.; Emerich, H.; Bordiga, S.; Lamberti, C.; Olsbye, U.; Tilset, M.; Lillerud, K. P. Structural Determination of a Highly Stable Metal−Organic Framework with Possible Application to Interim Radioactive Waste Scavenging: Hf-UiO-66. Phys. Rev. B 2012, 86, 125429.

930

dx.doi.org/10.1021/jz4002345 | J. Phys. Chem. Lett. 2013, 4, 925−930