Water Stable Metal–Organic Framework Evolutionally Formed from a

Communication pubs.acs.org/crystal

Water Stable Metal−Organic Framework Evolutionally Formed from a Flexible Multidentate Ligand with Acylamide Groups for Selective CO2 Adsorption Zhiyong Lu, Hang Xing, Ran Sun, Junfeng Bai,* Baishu Zheng, and Yizhi Li State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: A water stable porous metal−organic framework has been synthesized through an evolution approach in terms of both structures and properties. This interesting pillar-layered MOF ({[Cu2(TCMBT)(bpp)(μ3-OH)]·6H2O}n (1)) constructed from a flexible ligand possesses a BET surface area of 808.5 m2 g−1 and exhibits good selectivities of CO2/N2 (20.1:1) and CO2/CH4(4.0:1), which are comparable to those of the ZIF materials reported by Yaghi’s group. The excellent water stability of complex 1 would be quite preferable in practical application.

(bpp = 1,3-bis(4-pyridyl)propane), from a flexible ligand, N,N′,N″-tris(carboxymethyl)-1,3,5-benzenetricarboxamide (TCMBT). The evolution steps were carefully controlled by increasing the complexity of both inorganic and organic building blocks, and four complexes, {[Cu(TCMBT)(H 2 O) 3 ] 2 [Cu(H 2 O) 6 ]·3H 2 O} n (4) with simple layers, {[Cu3(TCMBT)2(H2O)3]·2H2O}n (3) with advanced layers, {[Cu2(TCMBT)(py)2(μ3-OH)]·2H2O}n (2) (py = pyridine) with augment layers, and porous MOF {[Cu2(TCMBT)(bpp)(μ3-OH)]·6H2O}n (1) with a pillared-layers structure, were formed (Figure 1). Quite interestingly, 1 is quite stable in water for 2 months and exhibits a CO2/N2 selectivity of 20.1:1 and a CO2/CH4 selectivity of 4.0:1, which is comparable to the water stable ZIFs reported by Yaghi’s group.11 Initially, a hydrothermal reaction using CuCl2·2H2O and TCMBT in H2O/DMF led to complex 4, in which every pair of neighboring TCMBT ligands links six mononuclear copper ions. Complex 4 exhibits the initial simple layers with a

T

he past decade has witnessed the rapid development of metal−organic frameworks (MOFs),1 due to their fascinating topologies and potential applications, particularly in gas storage and separation.2 The rational design and assembly of porous MOFs is a long-standing research topic.3 In the early 1990s, Robson4a initially introduced the “node and spacer” approach described by Wells4b,c into the field of coordination chemistry for the construction of high dimensional structures. Later on, another breakthrough was made by O’Keeffe and Yaghi5a by using inorganic clusters as the augments of vertices in a given net to construct porous MOFs, and reticular chemistry5b,c was formed.6 Zaworotko and Yaghi further developed the idea of the metal−organic polyhedras (MOPs), as supermolecular building blocks (SBBs) to extend MOFs.7 However, for flexible ligands, the limits of isoreticular chemistry have been revealed by Férey’s group;8 thus, rational synthesis of porous MOFs from them is still challenging. In addition, water stable MOFs are quite crucial for future practical applications in gas storage and separation.9 We are interested in constructing novel porous MOFs from flexible ligands.10a−e Herein, we present an evolution approach for constructing a water stable pillar-layered porous MOF, {[Cu2(TCMBT)(bpp)(μ3-OH)]·6H2O}n (1) © 2012 American Chemical Society

Received: December 2, 2011 Revised: January 12, 2012 Published: January 18, 2012 1081

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Figure 1. Schematic illustrations of the evolution tree from complex 4 to 1. With the evolution of both the inorganic and organic parts, the coordinative simple layer can evolve to an advanced layer and then to an augment layer and finally to a porous MOF with a pillared-layers structure.

honeycomb (6, 3) network. In order to form more condensed inorganic clusters with a high degree of connectivity, complex 3 and complex 2 were formed respectively by gradual increase of the concentration of pyridine in the reaction system. Complex 3 shows an advanced layer structure with every TCMBT ligand linking to three trinuclear inorganic building blocks. In complex 2, pyridine molecules serve as not only the organic base but also neutral monoconnected ligands stabilizing the novel {[Cu 4 ( OH ) 2 ] 6 + } inorganic building block. E ach {[Cu4(OH)2]6+} cluster is linked by six TCMBT ligands to generate a 2-dimensional CdCl2 network in the bc plane. Most importantly, considering the potential covalent bonding sites of pyridine molecules, this 2-dimensional structure can be regarded as an augment layer which can be pillared to a porous MOF by linking every pair of pyridine molecules together. Regarding the augment vertices of complex 2, we introduced bpp ligand into the reaction system to construct complex 1,12,13 a well-evolved porous MOF. The structure of 1 is constructed from the 2-dimensional hybrid sheets in complex 2 pillared by linear exobidentate bpp ligands. In complex 1, every pair of TCMBT ligands are nearly parallel and adopt a face-to-face orientation to connect four {[Cu4(OH)2]6+} inorganic building blocks to form the augment 2-dimensional layer. The bpp ligands act as pillars linking inorganic clusters in neighboring layers to form a porous MOF with unusual (46, 614, 88)(43)2 topology. Between every two layers, there exist two kinds of open channels with channel size of 6.0 × 3.4 Å2 and 4.6 × 3.0 Å2 along the a and b axes, respectively (defined by the diameters of the inserted interior contact columns, Figure 2). Interestingly, we use a new concept of evolution approach to construct porous complex 1. This evolution approach may be a good strategy to somehow solve the problem of the limits of isoreticular chemistry. To confirm the permanent porosity of complex 1, the sample was soaked in methanol for 3 days to remove the water guest molecules. The solvent was refreshed every 8 h. Then the sample was degassed under high vacuum at 120 °C for 10 h to obtain activated sample 1a (see Supporting Information, Figure

Figure 2. Side view of complex 1. Yellow (b axis) and pale orange (a axis) columns indicate the cavities of two kinds of channels between neighboring layers. Gray polyhedra represent the pairs of TCMBT ligands in face-to-face orientation.

S7). The N2 adsorption for 1a at 77 K exhibits a typical type I curve, which indicates the permanent microporosity of the activated sample, as a potential candidate for gas storage and separation. The BET surface area of 1a was calculated to be 808.5 m2 g−1. On the basis of the N2 adsorption isotherm, the total porous volume of 1a was determined to be 0.32 cm3 g−1. Considering future practical applications, a good gas storage (or separation) material must be stable toward moisture. Thus, prior to further adsorption measurement for other gases, we examined the water stability of complex 1. The water stability of complex 1 was performed by room temperature water and boiling water for 2 months. Complex 1 retains its structure by the evidence of PXRD (Figure 3). The activated sample 1a also shows no structure transformation after treatment in water for 2 months (see Figure S7). The results demonstrate that complex 1 is a water stable MOF. This excellent stability is likely attributed to the factor that the four-member copper cluster is partially and strongly coordinated to N atoms from coligands (bpp) instead of being purely coordinated to O 1082

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Table 1. Comparison of CO2 Adsorption and Selectivity of Complex 1a with Those of Some ZIF Materials at 298 K MOF 1a ZIF68a ZIF69a ZIF70a ZIF78a ZIF79a ZIF81a ZIF82a ZIF95b ZIF100b,c

Figure 3. Powder X-ray diffraction patterns for complex 1 after treatment in room temperature water and boiling water for various durations.

BET surface area (m2 g−1)

CO2 uptake at 1 bar (cm3 g−1)

CO2/N2

CO2/CH4

808.5 1090

44.8 (2.0 mmol g−1) 37.6

20.1 18.7

4.0 5.0

950

40.6

19.9

5.1

1730

55.0

17.3

5.2

620

51.5

50.1

10.6

810

33.5

23.2

5.4

760

38.2

23.8

5.7

1300

52.7

35.3

9.6

1050

19.7

18 ± 1.7

4.3 ± 0.4

595

32.6

24 ± 2.4

5.9 ± 0.4

a

Reported in ref 11a. bReported in ref 11b. cAdsorption data measured at 273 K.

Although the separation ratio of CO2/N2 of complex 1a is moderate when being contrasted to those MOFs with record making CO2 selectivities (e.g., Zn2(bpdc)2bpe (99:1 at 0.16 bar),15a SNU-M10 (98:1 at 1 bar),15b MOF74-Mg (49:1 at 0.16 bar)15a,c), it is still comparable to those of HKUST-1 (about 25:1),14a MOF-5 (17.5:1), MOF-177 (17.7:1),14b H3[(Cu4Cl3)(BTTri)8] (10:1 at 0.09 bar), and en-H3[(Cu4Cl3)(BTTri)8] (13:1 at 0.1 bar).14c These results indicate that complex 1a has a good CO2 selectivity at lower pressure (< or =0.1 bar, a typical partial pressure of CO2 in industrial flue gas). The adsorption enthalpies at zero coverage for CO2, CH4, and N2 are calculated to be 26.7 kJ mol−1, 19.1 kJ mol−1, and 16.0 kJ mol−1, respectively. Obviously, the adsorption enthalpy for CO2 is much larger than those for CH4 and N2, and is quite comparable to our group’s previous work on MOFs with amide group decoration.10g Considering the absence of open metal sites in complex 1a, the CO2 adsorption enthalpy of 1a is still higher than the values of the well-known pillar-layered MOFs, M2(bdc)2(dabco) (M = Cu, 10.1 kJ mol−1;16a Zn, 11.5 kJ mol−1 (ref 16a)/21.6 kJ mol−1;16b Ni, 20.4 kJ mol−1 (ref 16b)), which possess no open metal sites either. From the comparison of their structural differences, the higher CO2 adsorption enthalpy in complex 1a can be mainly attributed to incorporated bridging acylamide groups10g along the small channels.17 Because of a much larger quadrupole moment of CO2 (13.4 × 10−40 C m2) than those of N2 (4.7 × 10−40 C m2) and CH4 (nonpolar),18 the large dipole moment of the bridging acylamide groups along the small channels could facilitate the dipole−quadrupole interactions with CO2, leading to the selectivity of CO2 over CH4 and N2.10g Thus, complex 1 is one of the few MOFs which exhibit not only good CO2 selectivity but excellent water stability. In summary, for the flexible multidentate ligands, an evolution approach in terms of both structures and properties toward MOFs was initially presented. The resulted pillarlayered porous MOF 1 constructed from bbp (pillar), TCMBT, and a four-member copper cluster (layer) exhibits a good CO2 selectivity similar to that of ZIF materials. More importantly, complex 1 possesses excellent water stability, which is preferred

atoms of carboxylate, which is believed to be fragile when encountering water molecules.14c,9a,b In the context of global warming, large amounts of work have been devoted to the discovery of new MOFs for trapping carbon dioxide. Recently, our group has made some progress in CO2 adsorption and separation.10f,g As a continuous work, we evaluated the capability of complex 1a for CO2 storage and selectivity. CO2, CH4, and N2 adsorption were measured from 0 to 20 bar at 298 K. As shown in Figure 4, 1a can take a larger

Figure 4. Adsorption isotherms of CO2, CH4, and N2 for complex 1a at 298 K.

amount of CO2 (2.0 mmol g−1 at 1 bar and 5.8 mmol g−1 at 20 bar) than CH4 (0.4 mmol g−1 at 1 bar and 2.5 mmol g−1 at 20 bar) and N2 (0.1 mmol g−1 at 1 bar and 1.0 mmol g−1 at 20 bar). In spite of possessing a relatively low BET surface area, the CO2 uptake of complex 1a at 1 bar outperforms those of most of the ZIF materials11 listed in Table 1. From the ratio of the initial slope,14 the separation ratios of CO2/N2 and CO2/ CH4 at 298 K were calculated to be 20.1:1 and 4.0:1 (Figure S10), which are similar to those of ZIF materials (Table 1). 1083

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(8) Devic, T.; David, O.; Valls, M.; Marrot, J.; Couty, F.; Férey, G. J. Am. Chem. Soc. 2007, 129, 12614. (9) (a) Choi, H. J.; Dinca, M.; Dailly, A.; Long, J. R. Energy Environ. Sci. 2010, 3, 117. (b) Cychosz, K. A.; Matzger, A. J. Langmuir 2010, 26, 17198. (c) Yang, J.; Grzech, A.; Mulder, F. M.; Dingemans, T. J. Chem. Commun. 2011, 47, 5244. (d) Wu, T. J.; Shen, L. J.; Luebbers, M.; Hu, C. H.; Chen, Q. M.; Ni, Z.; Masel, R. I. Chem. Commun. 2010, 46, 6120. (e) Yoo, Y.; Varela-Guerrero, V.; Jeong, H. K. Langmuir 2011, 27, 2652. (10) (a) Sun, R.; Wang, S. N.; Xing, H.; Bai, J. F.; Li, Y. Z.; Pan, Y.; You, X. Z. Inorg. Chem. 2007, 46, 8451. (b) Wang, S. N.; Xing, H.; Li, Y. Z.; Bai, J. F.; Scheer, M.; Pan, Y.; You, X. Z. Chem. Commun. 2007, 2293. (c) Zheng, B.; Dong, H.; Bai, J. F.; Li, Y. Z.; Li, S. H.; Scheer, M. J. Am. Chem. Soc. 2008, 130, 7778. (d) Min, T. Y.; Zheng, B.; Bai, J. F.; Sun, R.; Li, Y. Z.; Zhang, Z. X. CrystEngComm 2010, 12, 70. (e) Duan, J. G.; Bai, J. F.; Zheng, B. S.; Li, Y. Z.; Ren, W. C. Chem. Commun. 2011, 47, 2556. (f) Shen, Y. M.; Bai, J. F. Chem. Commun. 2010, 46, 1308. (g) Zheng, B. S.; Bai, J. F.; Duan, J. G.; Wojtas, L.; Zaworotko, M. J. J. Am. Chem. Soc. 2011, 133, 748. (11) (a) Banerjee, R.; Furukawa, H.; Britt, D.; Knobler, C.; Keeffe, M. O.; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131, 3875. (b) Wang, B.; Cote, A. P.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Nature 2008, 453, 207. (12) {[Cu2(TCMBT)(bpp)(μ3-OH)]·6H2O}n (1) was prepared by mixing CuCl2·2H2O (0.1 mmol, 17.0 mg), TCMBT (0.1 mmol, 38.1 mg), bpp (0.2 mmol, 39.7 mg), and H2O/MeOH (10 mL, v/v = 1:1) . Then the mixture was stirred for ca. 30 min in air and then transferred and sealed in a 20 mL Teflon-lined autoclave, which was heated at 80 °C for 48 h. After slowly cooling to room temperature, blue block crystals appeared. After filtration, the crystals were washed with a little water and dried in a vacuum desiccator to give 47.5 mg (57% based on TCMBT). The similarity between the powder X-ray diffraction pattern of the bulk product and the simulated pattern from single-crystal analysis demonstrates the phase purity of the product (Figure S7). Anal. Calcd for C28H39N5O16Cu2: C, 40.58; H, 4.74; N, 8.45. Found: C, 40.91; H, 4.84; N, 8.42. IR (KBr, pellet): ν = 3385(m), 1618(w), 1527(s), 1421(w), 1392(w), 1289(m), 1226(s), 1025(s), 810(s), 707(w), 591(w) cm−1. (13) Crystal data for 1: C28H39N5O16Cu2, Fw = 828.72, triclinic, space group P1̅, a = 11.575 (2) Å, b = 12.634 (3) Å, c = 14.335 (3) Å, α = 101.3690 (10)°, β = 102.935 (3)°, γ = 96.457 (2)°, V = 1976.0 (6) Å3, Z = 2, Dc = 1.393 g cm−3, F000 = 856, Mo Kα radiation, λ = 0.71073 Å, T = 291 (2) K, 2θ = 52.00°, 10911 reflections collected, 7624 independent reflections which were used in all the calculations. Final residuals for 478 parameters were R1 = 0.0558, wR2 = 0.1196 for [I > 2σ(I)]. CCDC-735374 (4), CCDC-735375 (3), CCDC-735376 (2), and CCDC-735377 (1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif. (14) (a) Liang, Z. J.; Marshall, M.; Chaffee, A. L. Energy Fuels 2009, 23, 2785. (b) Saha, D.; Bao, Z. B.; Jia, F.; Deng, S. G. Environ. Sci. Technol. 2010, 44, 1820. (c) Demessence, A.; D’Alessandro, D. M.; Foo, M. L.; Long, J. R. J. Am. Chem. Soc. 2009, 131, 8784. (15) (a) Wu, H. H.; Reali, R. S.; Smith, D. A.; Trachtenberg, M. C.; Li, J. Chem.Eur. J. 2010, 16, 13951. (b) Choi, H. S.; Suh, M. P. Angew. Chem., Int. Ed. 2009, 48, 6865. (c) Caskey, S. R.; Wong-Foy, A. G.; Matzger, A. J. Am. Chem. Soc. 2008, 130, 10870. (16) (a) Tanaka, D.; Higuchi, M.; Horike, S.; Matsuda, R.; Kinoshita, Y.; Yanai, N.; Kitagawa, S. Chem. Asian J. 2008, 3, 1343. (b) Liang, Z. J.; Marshall, M.; Chaffee, A. L. Microporous Mesoporous Mater. 2010, 132, 305. (17) An, J.; Geib, S. J.; Rosi, N. L. J. Am. Chem. Soc. 2010, 132, 38. (18) (a) Bae, Y.-S.; Lee, C.-H. Carbon 2005, 43, 95. (b) Reid, C. R.; Thomas, K. M. Langmuir 1999, 15, 3206. (c) Rutherford, S. W.; Do, D. D. Langmuir 2000, 16, 7245.

in practical applications. We believe that this evolution approach will facilitate, to some extent, rational synthesis of more interesting MOFs with amazing properties from flexible ligands, and our continuous work is currently underway.

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ASSOCIATED CONTENT

* Supporting Information S

X-ray crystallographic information files (CIF), additional structural illustrations of complexes 1−4, Figure S7 and Figure S10, supplementary synthesis information, and adsorption data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Manfred Scheer and Michael J. Zaworotko for their helpful suggestions. This work was supported by the Major State Basic Research Development Programs (2011CB808704), the NSFC (20931004), the Science Foundation of Innovative Research Team of NSFC (20721002), the Fundamental Research Funds for the Central Universities (1114020501), and the Specialized Research Fund for the Doctoral Program of the Ministry of Education of China (200802840011).

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REFERENCES

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