Microporous Metal–Organic Framework Based on Mixing Nanosized

Article pubs.acs.org/crystal

Microporous Metal−Organic Framework Based on Mixing Nanosized Tris((4-carboxyl)-phenylduryl)amine and 4,4′-Bipyridine Ligands for Gas Storage and Separation Yan-Xi Tan, Yan-Ping He, and Jian Zhang* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *

ABSTRACT: The assembly of mixing nanosized tris((4carboxyl)phenylduryl)amine ligand and 4,4′-bipyridine ligand with Zn2+ ion leads to a new microporous framework FIR-2 having pillared-layer structure and unusual (3,4,6)-connected topology, which exhibits large surface area and gas adsorption selectivity for the adsorption of CO2 over N2 or CH4. The results reveal the potential application of the long tris((4carboxyl)phenylduryl)amine ligand on the construction of functional microporous metal−organic frameworks with interesting structural topologies for gas storage and separation.

■

■

INTRODUCTION Porous coordination polymers or metal−organic frameworks (MOFs) with both inorganic and organic building blocks are of great interest during the past decade because they have attractive structural topologies and potential applications in gas storage/separation and catalysis.1−3 Through careful assembly of multifunctional organic carboxylate ligands with metal ions, a lot of functional MOFs can be synthesized.4−7 For example, the well-known HKUST-1 is constructed from 1,3,5-benzenetricarboxylate ligand,5a and simple replacement of this ligand with a longer 1,3,5-tris(4-carboxyphenyl)benzene ligand leads to another 2-fold interpenetrating framework MOF-14.5b Another typical feature in both HKUST-1 and MOF-14 is the presence of exposed (or unsaturated) metal sites coming from paddlewheel Cu2(COO)4 building units. It has been recognized that coordinatively unsaturated metal centers are desirable for enhancing gas-storage capacity and for promoting catalytic activity.1 Although many porous MOFs with exposed metal sites have been reported to date, syntheses of such materials from nanosized organic ligands remain rarely explored.7 In this work, we present a nanosized trigonal ligand tris((4carboxyl)phenylduryl)amine (L) and report its structural assembly with Zn2+ ion to form an interesting microporous framework, namely, [Zn3L2(4,4′-bipy)2(H2O)2]·x(solvent) (FIR-2; 4,4′-bipy =4,4′-bipyridine, FIR denotes Fujian Institute of Research). FIR-2 is synthesized under solvothermal condition and structurally characterized by single-crystal X-ray diffraction. It exhibits an organically pillared-layer architecture, being deemed to a MOF possessing topology with vertex symbol of (4·62)2(42·68·83·102)(64·82). Remarkably, gas sorption measures indicate FIR-2 has permanent porosity with a large BET surface area of 1356.2 m2/g and shows selectivity for the adsorption of CO2 over N2 or CH4 at 0 °C and 1 bar. © 2012 American Chemical Society

EXPERIMENTAL SECTION

General Procedures. All reagents were purchased commercially and used without further purification. All Powder X-ray diffraction (PXRD) analyses were recorded on a Rigaku Dmax2500 diffractometer with Cu Kα radiation (λ = 1.54056 Å) with a step size of 0.05°. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer with a heating rate of 10 °C/min under an air atmosphere. Gas adsorption measurement was performed in the ASAP (Accelerated Surface Area and Porosimetry) 2020 System. Synthesis of [Zn3L2(4,4′-bipy)2(H2O)2]·x(solvent) (FIR-2). H3L (60 mg, 0.1 mmol), 4,4′-bipy (40 mg, 0.25 mmol), and Zn(NO3)2·6H2O (150 mg, 0.5 mmol) were dissolved in DMF/EtOH/ H2O (4:2:1, v/v), which were placed in a small vial. The mixture was heated at 60 °C for 72 h and then cooled to room temperature. Yellow rodlike crystals of the product were formed and collected by filtration. Yield: 0.080 g (66% based on L). X-ray Crystallographic Study. The diffraction data for FIR-2 was collected on an Oxford Xcalibur diffractometer equipped with a graphite-monochromatized Mo−Kα radiation (λ = 0.71073 Å) at 293(2) K. Crystal data for FIR-2: C98H68N6O14Zn3; M = 1749.75; triclinic; a = 9.4710(11) Å, b = 17.3579(17) Å, c = 19.6831(19) Å; α = 76.227(3)°, β = 83.232(3)°, γ = 88.407(3)°; V = 3120.9(6) Å3; T = 293(2) K, space group P1̅; Z = 1; 15 223 reflections measured; 9703 independent reflections (Rint = 0.0216). The final R1 value was 0.0564 (I > 2σ(I)). The final wR(F2) value was 0.2069 (I > 2σ(I)). The goodness of fit on F2 was 1.063. The structures were solved by the direct method and refined by the full-matrix least-squares on F2 using the SHELXTL-97 program. The guest molecules are highly disordered and can not be identified from X-ray diffraction. CCDC-846688 (FIR2) contain the supplementary crystallographic data for this article. Received: January 23, 2012 Revised: March 13, 2012 Published: March 23, 2012 2468

dx.doi.org/10.1021/cg3001009 | Cryst. Growth Des. 2012, 12, 2468−2471

Crystal Growth & Design

Article

12 Å2 is occupied by the structural disordered solvent molecules. To our knowledge, only two MOFs (SNU-77 and FIR-1) based on this nanosized L ligand have been reported to date, and both of them are simple Zn-L frameworks without exposed metal sites.7 FIR-2 presents the first Zn-L framework with auxiliary ligands and potential exposed metal sites. From the viewpoint of topology, each simple framework in FIR-2 can be topologically represented as a (3,4,6)-connected net by reducing each paddle-wheel unit as a 6-connected node, each octahedral coordinated Zn2 atom as a distorted 4connected node, and each L ligand as a planar 3-connected node (Figure 2c).9 Notably, this topology with a vertex symbol of (4·62)2(42·68·83·102)(64·82) is only known in another two previously reported nonporous coordination polymers.10 In order to examine the stability of the framework, thermal gravimetric analysis (TGA), X-ray powder diffraction (XRPD), and desolvation experiments were carried out. The TGA curve of FIR-2 shows the release of guest molecules between 25 and 250 °C, leading to a weight loss of 26.47% (Figure S4, Supporting Information). Such high evacuated temperature goes against the activation of the sample for gas adsorption. To attain a more complete evacuation and protect the structural integrality, DMF was exchanged with methanol by soaking crystals of FIR-2 in methanol and heated at 60 °C for 24 h. While other organic solvents can be exchanged inside the framework, methanol was chosen for its relatively low boiling point and ease of potential removal via evacuation. As indicated by TG analysis, the methanol activated sample (FIR-2a) showed a weight loss step below 110 °C (Figure S4, Supporting Information). Thus, FIR-2a becomes very easy to be desolvated, and a hollow phase FIR-2-ht can be obtained by putting FIR-2a under high vacuum (10 −8 mbar) at room temperature overnight. Further powder XRD experiments are carried out to verify the framework stability of FIR-2a (Figure S5, Supporting Information). The good agreement of the peaks in all diagrams demonstrates that the framework structure of FIR-2a can retain upon a complete removal of the guest molecules. The permanent porosity of FIR-2-ht was established by reversible gas sorption experiments using N2 and Ar at 77 and 87 K, respectively, which all show a typical type I behavior characterized by a plateau reached at low relative pressure indicating the presence of permanent micropores in FIR-2-ht (Figure 3). The maximum N2 and Ar uptakes at 1 bar for FIR2-ht are 349 cm3/g and 379 cm3/g, respectively. N2 adsorption data gives a BET surface area of 1356.2 m2/g and a Langmuir surface area of 1491.3 m2/g. In the adsorption data of FIR-2-ht,

These data can be obtained free of charge via www.ccdc.cam.ac.uk/ conts/retrieving.html.

■

RESULTS AND DISCUSSION FIR-2 with pillared-layer structure is prepared through the selfassembly of Zn(NO3)2·6H2O with H3L and 4,4′-bipy in DMF/ EtOH/H2O (v/v/v = 4:2:1) solvent at 60 °C for 3 days. In the structure of FIR-2, there are two independent Zn centers, and they have different coordination geometry. One Zn center (Zn1) is five-coordinate with four carboxylate O atoms and one pyridine N atom, showing square pyramidal geometry (Figure 1). Such two Zn centers are bridged by four carboxylates to

Figure 1. Coordination environment in FIR-2.

form a paddle-wheel Zn2(COO)4 unit with Zn···Zn distance being 2.9385(8) Å. Another independent Zn center (Zn2) has octahedral geometry and is coordinated by two carboxylate O atoms, two pyridine N atoms, and two water molecules (Figure 1). Once these coordinated water molecules are removed from the Zn2 sites, it is possible to generate exposed metal sites. Each L ligand links to two paddle-wheel Zn2(COO)4 units in a bidentate fashion and one Zn2 atom in a monodentate fashion, forming a 2D layer with two kinds of rings (i.e., a rhombic ring with a border length of about 12.5 Å and a hexahydric ring with two different border lengths of about 12.5 Å and 24.7 Å) (Figure 1 and 2a). Linear 4,4′-bipy ligands

Figure 2. (a) Layer constructed from L ligands and Zn ions in FIR-2; (b) the 3-fold interpenetrating pillared-layer structure showing 1D channels; (c) the (3,4,6)-connected net derived from the structure of FIR-2.

occupy the axial sites of the Zn(II) atoms and connect adjacent layers to generate a 3D open framework with 1D channels possessing a window size of 11 × 20 Å2. Such large cavities induce 3-fold interpenetration of the network, whose interpenetration vector runs along the c axis (Figure 2b). After the interpenetration, the structure of FIR-2 still shows large 1D channels along the a axis, and the solvent accessible volume is estimated bythe PLATON program to be about 47.2% of the total crystal volume.8 Each channel with a window size of 6.5 ×

Figure 3. N2 (black) and Ar (blue) isotherms recorded at 77 K for FIR-2-ht. Inset: pore size distribution using incremental surface area. 2469

dx.doi.org/10.1021/cg3001009 | Cryst. Growth Des. 2012, 12, 2468−2471

Crystal Growth & Design

Article

CO2/N2 selectivity is 39.5 at 273 K and 65 at 298 K. To our knowledge, MOFs with such high CO 2/N2 adsorption selectivity at ambient conditions were rarely reported. Interestingly, the CO2 uptake of FIR-2-ht at 273 K is much higher than that at 298 K, but the CO2/N2 selectivity at 298 K is much higher than that at 273 K. That reveals unusual temperature-dependent CO2/N2 selectivity. From the consideration of the structural feature of FIR-2-ht, such gasadsorption selectivity is closely associated with the combination of the cage-based structure with relative pore sizes and exposed Zn2+ coordination sites. Meanwhile, the high quadrupole moment and polarizability of CO2 (3.3−3.4 × 10−26 e.s.u.) induce better interaction with the accessible Zn2+ centers of the framework. In addition, CH4 adsorption was also measured at 273 and 298 K under 1 bar. The maximal CH4 uptakes for FIR2-ht are 15.7 cm3/g at 273 K and 11.3 cm3/g at 298 K, which indicate the obvious gas adsorption selectivity of CO2 over CH4. Low-pressure H2 adsorption, isotherms collected for the sample of FIR-2-ht indicated that the framework had a strong affinity for binding H2 (Figure 5). At 77 K and 1.06 bar (800

a single data point at relative pressure 0.01 gives a micropore volume of 0.45 cm3/g by the Horvath−Kawazoe equation. A pore size distribution analysis by DFT methods utilizing N2 gas at 77 K shows that there is a narrow distribution of micropores at 0.6−1.1 nm. Obviously, the 1.1 nm cages take advantage of numbers in FIR-2-ht as shown in the Figure 3 inset. To further explore its potential properties on CO2/N2 and CO2/CH4 gas separation under ambient conditions, the adsorption isotherms of CO2, CH4, and N2 for FIR-2-ht at 273 and 298 K were measured, respectively (Figure 4). The

Figure 4. Sorption isotherms for FIR-2-ht: (a) CO2 recorded at 273, (b) CO2 recorded at 298 K, (c) CH4 recorded at 273 K, (d) CH4 recorded at 298 K, (e) N2 recorded at 273 K, and (f) N2 recorded at 298 K.

uptake value of CO2 at 273 K was 71 cm3/g (3.2 mmol/g and 139.5 mg/g), which is comparable to the well-known MOF-5 and larger than many other reported compounds.1 To better understand these observation, the CO2 adsorptive capability of FIR-2-ht was measured under 298 K, giving an uptake of 35.1 cm3/g. It is notable that obvious hysteresis loops are observed in the CO2 sorption isotherms of FIR-2-ht at 273 and 298 K. The mechanism for hysteresis in the adsorption/desorption isotherms for some coordination networks may usually be attributed to guest-directed framework rearrangements or the hindrance of some residual solvent molecules and metal hydroxide fragments at the pores for the diffusion of the gas molecules. It is unlikely that this mechanism operates in the hysteresis described here since FIR-2-ht keeps its robust framework up after removeal of all of the guest by subsequent drying under vacuum at 0 °C for 24 h. We speculate that such behavior is a result of the increased sorbate−sorbent interactions as the gas molecules access the pore region around the unsaturated metal sites. The enthalpy of CO2 adsorption for FIR-2-ht was estimated from the sorption isotherms at 273 and 298 K using the virial equation to understand the strong affinity toward CO2 (Figures S6 and S7, Supporting Information). At zero coverage, the enthalpy of CO2 adsorption is 25.6 kJ/mol for FIR-2-ht, which is comparable to those of MOFs with organic ammonium ions in the pores for strong CO2 binding.6c Although the CO2 uptake ability for FIR-2-ht is at a moderate level compared to some currently reported MOFs, it shows high CO 2/N2 adsorption selectivity at ambient conditions. In contrast, N2 was hardly adsorbed by FIR-2-ht under the conditions of 273 and 298 K (just 1.78 and 0.56 cm3/ g). The maximal uptakes of the CO2 and N2 at 273 and 298 K and 1 bar for FIR-2-ht were used to estimate the adsorption selectivity for CO2 over N2. From these data, the calculated

Figure 5. H2 isotherms recorded at 77 and 87 K for FIR-2-ht.

Torr), it has a fully reversible uptake of 160.8 mL/g H2, a value surpassing that of the most favorable zeolite ZSM-5 (0.7 wt %) and closing to those of recently reported ZIFs.1c As a further test, a second H2 adsorption isotherm was measured at 87 K, and two data sets were used to determine the isosteric heat of H2 adsorption. The enthalpy of H2 adsorption for FIR-2-ht was estimated from the sorption isotherms at 77 and 87 K using the virial equation to understand the affinity of FIR-2-ht toward H2 (Figures S8 and S9, Supporting Information). The isosteric heat of H2 adsorption for FIR-2-ht reach the values of 7.2 kJ/ mol. What is more, the fraction of the pore volume filled by liquid H2 (ρH2 = 0.0708 g/cm3) at 1 atm and 77 K for FIR-2-ht is 45.1%, which is much larger than those of many famous porous materials, such as IRMOFs and MOF-5.5

■

CONCLUSIONS In summary, by employing a nanosized tris((4-carboxyl)phenylduryl)amine ligand to assemble a Zn2+ ion with the presence of 4,4′-bipyridine ligand, a new microporous framework FIR-2 with exposed metal sites is successfully synthesized. FIR-2 features a 3-fold interpenetrating pillared-layer structure and has unusual (3,4,6)-connected topology. Moreover, it exhibits large surface area and gas adsorption selectivity for the adsorption of CO2 over N2 or CH4. The results reveal the potential application of the long tris((4-carboxyl)phenylduryl)amine ligand on the construction of functional microporous 2470

dx.doi.org/10.1021/cg3001009 | Cryst. Growth Des. 2012, 12, 2468−2471

Crystal Growth & Design

Article

(9) (a) Blatov, V. A.; Carlucci, L.; Ciani, G.; Proserpio, D. M. CrystEngComm 2004, 6, 377. (b) Alexandrov, E. V.; Blatov, V. A.; Kochetkov, A. V.; Proserpio, D. M. CrystEngComm 2011, 12, 3947. (10) (a) Zhao, W.; Fan, J.; Song, Y.; Kawaguchi, H.; Okamura, T.; Sun, W.-Y.; Ueyama, N. Dalton Trans. 2005, 1509. (b) Dinnebier, R. E.; Nuss, H.; Jansen, M. Z. Kristallogr. 2005, 220, 954.

metal−organic frameworks with interesting structural topologies for gas storage and separation.

■

ASSOCIATED CONTENT

* Supporting Information S

Additional figures, TGA, powder X-ray diffraction patterns, sorption isotherms, and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the support of this work by National Basic Research Program of China (973 Programs 2011CB932504 and 2012CB821705), NSFC (21073191), NSF of Fujian Province (2011J06005), and the Innovation Program of CAS (KJCX2YW-H21).

■

REFERENCES

(1) (a) Férey, G. Chem. Soc. Rev. 2008, 37, 191. (b) Cheon, Y. E.; Park, J.; Suh, P. Chem. Commun. 2009, 5436. (c) Zhang, J.; Wu, T.; Zhou, C.; Chen, S.; Feng, P.; Bu, X. Angew. Chem., Int. Ed. 2009, 48, 2542. (d) Liu, Y.; Boey, F.; Lao, L. L.; Zhang, H.; Liu, X.; Zhang, Q. Chem Asian J. 2011, 6, 1004. (2) (a) D’Alessandro, D. M.; Smit, B.; Long, J. R. Angew. Chem., Int. Ed. 2010, 49, 6058. (b) Lin, X.; Jia, J.; Zhao, X.; Thomas, K. M.; Blake, A. J.; Walker, G. S.; Champness, N. R.; Hubberstey, P.; Schröder, M. Angew. Chem., Int. Ed. 2006, 45, 7358. (c) Guo, Z.; Wu, H.; Gadipelli, S.; Liao, T.; Zhou, Y.; Xiang, S.; Chen, Z.; Yang, Y.; Zhou, W.; O’Keeffe, M.; Chen, B. Angew. Chem., Int. Ed. 2011, 50, 3178. (d) Chen, B.; Xiang, S.; Qian, G. Acc. Chem. Res. 2010, 43, 1115. (e) Jiang, H. L.; Tatsu, Y.; Lu, Z. H.; Xu, Q. J. Am. Chem. Soc. 2010, 132, 5586. (3) (a) Lin, Q.; Wu, T.; Zheng, S.; Bu, X.; Feng, P. Chem. Commun. 2011, 47, 11852. (b) Jiang, G.; Wu, T.; Zheng, S.-T.; Zhao, X.; Lin, Q.; Bu, X.; Feng, P. Cryst. Growth Des. 2011, 11, 3713. (c) Zheng, S.; Bu, J. J.; Wu, T.; Chou, C.; Feng, P.; Bu, X. Angew. Chem., Int. Ed. 2011, 50, 8858. (d) Chen, L.; Bu, X. Chem. Mater. 2006, 18, 1857. (4) (a) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C. Chem. Soc. Rev. 2009, 38, 1477. (b) Zeng, M.-H.; Wang, Q.-X.; Tan, Y.-X.; Hu, S.; Zhao, H.-X.; Long, L.-S. J. Am. Chem. Soc. 2010, 132, 2561. (5) (a) Chui, S. S. Y.; Lo, S. M. F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148. (b) Chen, B.; Eddaoudi, M.; Hyde, S. T.; O'Keeffe, M.; Yaghi, O. M. Science 2001, 291, 1021. (c) Ma, L.; Mihalcik, D. J.; Lin, W. J. Am. Chem. Soc. 2009, 131, 4610. (d) Ma, S.; Yuan, D.; Chang, J.-S.; Zhou, H.-C. Inorg. Chem. 2009, 48, 5398. (e) Chen, B.; Ma, S.; Zapata, F.; Fronczek, F. R.; Lobkovsky, E. B.; Zhou, H.-C. Inorg. Chem. 2007, 46, 1233. (f) Chen, B.; Ma, S.; Hurtado, E. J.; Lobkovsky, E. B.; Liang, C.; Zhu, H.; Dai, S. Inorg. Chem. 2007, 46, 8705. (6) (a) Caskey, S. R.; Wong-Foy, A. G.; Matzger, A. J. Am. Chem. Soc. 2008, 130, 10870. (b) Bae, Y. S.; Farha, O. K.; Hupp, J. T.; Snurr, R. Q. J. Mater. Chem. 2009, 19, 2131. (c) An, J.; Rosi, N. L. J. Am. Chem. Soc. 2010, 132, 5578. (d) Vaidhyanathan, R.; Iremonger, S. S.; Shimizu, G. K. H.; Boyd, P. G.; Alavi, S.; Woo, T. K. Science 2010, 330, 650. (e) Lee, J. Y.; Pan, L.; Huang, X.; Emge, T. J.; Li, J. Adv. Funct. Mater. 2011, 21, 993. (7) (a) Park, H. J.; Lim, D.-W.; Yang, W. S.; Oh, T.-R.; Suh, M. P. Chem.Eur. J. 2011, 17, 7251. (b) He, Y. P.; Tan, Y. X.; Wang, F.; Zhang, J. Inorg. Chem. 2012, 51, 1995. (8) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7. 2471

dx.doi.org/10.1021/cg3001009 | Cryst. Growth Des. 2012, 12, 2468−2471