Temperature dependent selective gas sorption of the microporous ...

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Temperature dependent selective gas sorption of the microporous metal-imidazolate framework [Cu(L)] [H2L=1,4-di(1H-imidazol-4-yl)benzene]w Shui-Sheng Chen,ac Min Chen,a Satoshi Takamizawa,b Man-Sheng Chen,a Zhi Sua and Wei-Yin Sun*a

Downloaded by NANJING UNIVERSITY on 13 October 2011 Published on 11 November 2010 on http://pubs.rsc.org | doi:10.1039/C0CC04085A

Received 26th September 2010, Accepted 8th October 2010 DOI: 10.1039/c0cc04085a A highly stable copper(II) microporous framework with cylindrical channels constructed from 1,4-di(1H-imidazol-4-yl)benzene (H2L) and CuCl22H2O is composed of Cu(II)-imidazolate tubes interconnected by the 1,4-phenylene group of L2, and shows temperature dependent selective gas sorption properties. Microporous metal–organic frameworks (MMOFs) have attracted great attention in recent years owning to their aesthetic framework structures and exceptional properties such as gas storage/ separation and catalysis.1 Apparently, the desired porosity and stability are important for MMOFs considering their application as tested for MOF-5 as sorbent for determination of atmospheric formaldehyde.1g The well-known zeolite structures are composed of tetrahedral Si(Al)O4 units covalently joined by bridging oxygen atoms to produce varied important porous inorganic materials,2 and recent reports show a shift in the exploratory synthesis from porous zeolite materials to zeolitic imidazolate frameworks (ZIFs) in the construction of MMOFs due to their structure similarity.3,4 Now, ZIFs as a new class of porous materials have potential application and advantages over inorganic zeolites due to their high porosity, organic functionality and designability. The reported ZIFs are mostly based on simple imidazole and its derivatives,3,4 and recently we designed a rigid 4-imidazolecontaining ligand, 1,4-di(1H-imidazol-4-yl)benzene (H2L),5 on the basis of our previous systematic studies on 1H-imidazol-1yl-containing organic ligands.6 1H-imidazol-1-yl-containing organic ligands, for example 1,4-di(1-imidazolyl)benzene (bib), can only act as neutral ligands and their metal complexes must have an anionic component to balance the positive charge of the metal ions which may occupy the vacancies of the porous MOFs.6a,7,8 By contrast, H2L can serve not only as a neutral bidentate ligand,5 but also as an anionic ligand when the 1H-imidazol-4-yl groups are deprotonated to generate an imidazolate anion. Also, there are differences from simple imidazolate and its derivatives such as benzimidazolate,

2-methylimidazolate etc. which just act as two connectors to ligate M2+,3 and each M2+ in turns links four Im ligands, forming uninodal 4-connected ZIFs while the deprotonated L2 can serve as a four connector to ligate four M2+, and each M2+ in turns connect four Im treated as a 4-connector, as a result, the eventual structure of the complex can be a binodal (4,4) connected net. With the coordination chemistry of multi1H-imidazol-4-ylimidazole ligands still largely unexplored, we report herein a microporous copper(II) framework with L2 and its temperature dependent selective gas sorption properties. [Cu(L)] (1) was prepared by hydrothermal reaction of CuCl22H2O and H2L in aqueous NH3 solution.z X-Ray crystallographic analysis revealed that 1 crystallizes in trigonal space group P 3c1.y Each Cu(II) atom has distorted squareplanar coordination geometry with four nitrogen atoms from four different L2 (Fig. S1, ESIw) with Cu–N distances of 1.985(3) and 1.963(3) A˚ and N–Cu–N bond angles varying from 90.46(11) to 168.01(15)1. Interestingly, the Cu(II) atoms are bridged by deprotonated imidazolate moieties to form an infinite 1D cylindrical tube (Fig. 1a), which is further interconnected by 1,4-phenylene groups of L2 in six different directions to form a non-interpenetrating extended 3D framework (Fig. 1b and c). It is clear that the resulting structure of 1 is a binodal (4,4)-connected net with a Schla¨fli symbol (6510) (Fig. S2, ESIw). There are two different types of channels with almost the same size of 3.4 A˚ in diameter with considering van der Waals radii: one formed by the above-mentioned Cu(II)-imidazolate groups (yellow balls in Fig. 1c), and the other generated by 1,4-phenylene and Cu(II)-imidazolate groups (violet balls in Fig. 1c). PLATON9 calculations show that the porous structure of 1 is composed of voids of 411.9 A˚3

a

Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, China. E-mail: [email protected]; Fax: +86 25 8331 4502 b Graduate School of Nanobioscience, Yokohama City University, Kanazawa-ku, Yokohama, Kanagawa 236-0027, Japan c School of Chemistry and Chemical Engineering, Fuyang Teachers College, Fuyang 236041, China w Electronic supplementary information (ESI) available: experimental and additional structures for 1; DHads calculations. CCDC 786083. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cc04085a

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Fig. 1 (a) 1D Cu(II)-imidazolate cylindrical tube. (b) Side (up) and top (down) views of neighboring cylindrical tubes interconnected by the 1,4-phenylene group of L2. (c) 3D porous framework of 1 with two different channels.

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that represents 23.1% per unit cell volume [1782.3(3) A˚3]. It is noticeable that the cylindrical Cu(II)-imidazolate tubes as secondary building units (SBUs) are interconnected by 1,4-phenylene linkers to form the novel 3D porous framework of 1, while the reported cadmium carboxylate SBUs are linked by 4,4 0 -biphenylene groups.10,11 The thermal and chemical stability of 1 was examined and the powder X-ray diffraction (PXRD) patterns under different conditions are shown in Fig. 2. The results imply that complex 1 is stable up to 290 1C as evidenced by PXRD patterns (Fig. 2a). The high thermal stability is known for the ZIFs as reported previously.3f Especially, 1 exhibits high chemical stability in boiling benzene, methanol, water or even 1 M NaOH, as indicated by PXRD shown in Fig. 2b. Such high stability of 1 is originated from the rigid metal-imidazolate framework as described above and provides an opportunity for probing gas adsorption property. The permanent porosity of 1 is verified by gas adsorption capabilities measured for CO2, CH4, N2 and H2, and adsorption isotherms of these gases display a steep rise at the relative low pressure region which can be categorized as reversible type-I, exhibiting a typical permanent microporosity (Fig. 3). Complex 1 shows comparatively high adsorption amount of CO2 at 195 K and 1 atm (80.88 cm3 g1 at STP, 15.9 wt%) corresponding to 1 CO2 molecule per formula unit (Fig. 3a). The CO2 uptake is nearly 1.5 times higher than that of CH4 at 195 K, and 2 times higher than that of N2 at 77 K. The Langmuir and BET surface areas estimated from the CO2 adsorption isotherm are 579 and 435 m2 g1, respectively. The adsorption value is comparable to those of tris-o-phenylenedioxycyclotriphosphazene containing 1D channels (60.5 cm3 g1 at STP, 12 wt%) and of SNU-15 0 (78.02 cm3 g1 at STP, 15.3 wt%).12 The high affinity for CO2 may be due to the unsaturated metal sites as revealed in other MOF materials,1e,13 e.g. a Cu(II) framework with 1,3,5-tris(4-carboxyphenyl)benzene.13c 1 adsorbs a certain amount of CH4 gas up to 4.04 wt% (56.33 cm3 g1 at STP) at 195 K and 1 atm, which may also be related to the unsaturated metal sites as discussed for M2(dhtp) (M = Mg, Mn, Co, Ni, Zn; dhtp = 2,5dihydroxyterephthalate) studied by neutron diffraction.14 However, 1 takes up only 5.01 wt% (40.05 cm3 g1 at STP) of N2 at 77 K and 0.95 atm, lower than the adsorption amount of CH4 which is attributed to the lower polarizability of N2 (17.6 1025 cm3) vs. CH4 (26 1025 cm3).15 More interestingly, the CO2 uptake of 7.57 wt% at 273 K and 4.88 wt% at 298 K, is comparable to that for reported ZIFs

Fig. 2 (a) The XPRD patterns of 1 at varied temperature. (b) The XPRD patterns of 1 measured after treating for up to 7 days. (1) simulated; (2) in boiling water; (3) in 1 M refluxing aqueous sodium hydroxide; (4) in refluxing methanol; (5) in refluxing benzene.

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Fig. 3 (a) Gas adsorption isotherms of 1: CO2 (rectangles), CH4 (diamonds) at 195 K and N2 (triangles) at 77 K. (b) Gas adsorption isotherms of 1 at 273 and 298 K: CO2 (rectangles) and N2 (triangles). (c) H2 adsorption isotherms of 1: at 77 K (circles) and 87 K (triangles). In (a)–(c) filled symbols: adsorption; open symbols: desorption.

under the same conditions, but almost no N2 adsorption was observed for 1 (Fig. 3b) as reported for ZIF-95 and -100.3b Therefore, the distinct difference of adsorption capacity between CO2 and N2 prompts us to study the enthalpy of CO2 adsorption and estimate the adsorption selectivity for CO2 over N2. Isosteric adsorption enthalpies as a function of the quantity of gases adsorbed were calculated using a variant of the Clausius–Clapeyron equation.16 At the onset of adsorption, the CO2 adsorption enthalpy of 39.06 kJ mol1 (Fig. S3, ESIw), further supporting the high CO2 binding affinity associated with the pore structure, is higher than the values for siliceous zeolite (27 kJ mol1) and activated carbons (less than 26 kJ mol1),17 but slightly lower than for amine-functionalized MMOFs.18 The ratios of these initial slopes of the CO2 and N2 adsorption isotherms were used to estimate the adsorption selectivity for CO2 over N2 (see ESIw). From the calculated CO2/N2 selectivity of 35 at 273 K and 22 at 298 K, we conclude that 1 may have potential application in the separation of CO2/N2 mixtures. Given the favorable CO2 capturing property of 1, we further studied its storage capacity for the attractive energy carrier gas hydrogen. The hydrogen gas adsorption isotherms measured at 77 and 87 K are both steep at the low pressure region exhibiting the strong interaction between relatively small pores and H2 molecules. It takes up a moderate amount of hydrogen 0.57 wt% at 77 K and 0.55 wt% at 87 K shown in Fig. 3c, and these values are comparable to those for reported mesoporous materials such as MCM-41 (0.57 wt%) and Mirkin’s amorphous infinite coordination polymer (0.57 wt%) under the same conditions.19 The enthalpies of H2 adsorption are also applied to evaluate the H2 binding affinity. At low coverage, 1 exhibits an H2 adsorption enthalpy of 8.6 kJ mol1 (Fig. S4, ESIw), and the adsorption value is close to that of ZIF-20 (8.5 kJ mol1) known as a potential H2 storage material,3d and higher than those for typical van der Waals type interactions (4–6 kJ mol1) seen in most reported Chem. Commun., 2011, 47, 752–754

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MOFs,20 but is lower than that of IRMOF-11 (9.1 kJ mol1), PCN-9 (10.1 kJ mol1) and CPO-27 (13.5 kJ mol1).21 The comparatively high adsorption enthalpy can be mainly attributed to the Coulomb interactions between the coordination unsaturated Cu(II) sites and H2, and the strengthened interactions of H2 molecules with narrow pore walls constructed from the electron-rich conjugated p system of deprotonated imidazole.22 In conclusion, we have successfully prepared a novel porous 3D framework based on Cu(II)–Im moieties with high thermal and chemical stability. The imidazolate-bridged framework exhibits gas sorption capacities for CO2, CH4, N2 and H2. Furthermore, the complex exhibits selective gas adsorption property for CO2 over N2 around room temperature. The results not only provide progress in imidazolate chemistry and confirm the advantage of ZIFs, but also provide useful guidance for further improvements on design of MMOFs for gases storage by virtue of the designability of incorporating 1H-imidazol-4-yl imidazole ligands. This work was financially supported by the National Natural Science Foundation of China (Grant nos. 20731004 and 20721002) and the National Basic Research Program of China (Grant nos. 2007CB925103 and 2010CB923303).

Notes and references z Synthesis: a mixture of CuCl22H2O (0.171 g, 1.0 mmol), H2L (0.210 g, 1.0 mmol), aqueous ammonia (25%, 2 mL) and H2O (10 mL) was stirred for 1 h in air, then transferred to and sealed in a 25 mL Teflon-lined reactor, and heated in an oven to 100 1C for 72 h. The resulting black needle crystals were collected by filtration in a yield of 72% and washed with water and ethanol several times. Elemental analysis: calc. (%) for C12H8N4Cu 1: C, 52.99; H, 2.94; N, 20.61%; found: C, 52.62; H, 3.31; N, 20.42%. IR (KBr pellet, cm1): 3441 (m), 1637 (s), 1467 (m), 1384 (s), 1131 (s), 1085 (s), 963 (w), 834 (w), 795 (w), 655 (w), 536 (w). y Crystal data for 1: C12H8N4Cu, Mr = 271.76, trigonal, space group P 3c1, a = 15.1399(10), b = 15.1399(10), c = 8.9784(12) A˚, a = 90, b = 90, g = 1201, V = 1782.3(3) A˚3, Z = 6, Dc = 1.2519 g cm3, F(000) = 822, m = 1.818 mm, 8537 reflections measured, 1080 unique (Rint = 0.0608), final R1 = 0.0355, wR2 = 0.0800, GOF = 1.089, R indices based on 1080 reflections with I > 2s(I) (refinement on F2). 1 (a) N. L. Rosi, J. Kim, M. Eddaoudi, B. L. Chen, M. O’Keeffe and O. M. Yaghi, J. Am. Chem. Soc., 2005, 127, 1504; (b) G. Fe´rey, Chem. Soc. Rev., 2008, 37, 191; (c) J. J. P. IV, J. A. Perman and M. J. Zaworotko, Chem. Soc. Rev., 2009, 38, 1400; (d) Y. E. Cheon and M. P. Suh, Angew. Chem., Int. Ed., 2009, 48, 2899; (e) J. R. Li, R. J. Kuppler and H. C. Zhou, Chem. Soc. Rev., 2009, 38, 1477; (f) S. Hasegawa, S. Horike, R. Matsuda, S. Furukawa, K. Mochizuki, Y. Kinoshita and S. Kitagawa, J. Am. Chem. Soc., 2007, 129, 2607; (g) Z. Y. Gu, G. Wang and X. P. Yan, Anal. Chem., 2010, 82, 1365. 2 J. Zhang, S. M. Chen and X. H. Bu, Dalton Trans., 2010, 39, 3080. 3 (a) R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O’Keeffe and O. M. Yaghi, Science, 2008, 319, 939; (b) B. Wang, P. Coˆte´, H. Furukawa, M. O’Keeffe and O. M. Yaghi, Nature, 2008, 453, 207; (c) X. C. Huang, Y. Y. Lin, J. P. Zhang and X. M. Chen, Angew. Chem., Int. Ed., 2006, 45, 1557; (d) H. Hayashi, A. P. Coˆte´, H. Furukawa, M. O’Keeffe and O. M. Yaghi, Nat. Mater., 2007, 6, 501; (e) Y. Q. Tian, C. X. Cai, Y. Ji, X. Z. You, S. M. Peng and G. S. Lee, Angew. Chem., Int. Ed., 2002, 41, 1384; (f) K. S. Park, Z. Ni, A. P. Coˆte´, J. Y. Choi, R. Huang, F. J. Uribe-Romo,

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