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A microporous Zn(II)–MOF with open metal sites: structure and selective adsorption properties† Xiaofang Zheng,a Yumei Huang,a Jingui Duan,b,c Chenggang Wang,a Lili Wen,*a Jinbo Zhaoa and Dongfeng Li*a A three-dimensional microporous framework, Zn(II)–MOF [Zn(HPyImDC)(DMA)]n (1) (H3PyImDC = 2-( pyridine-4-yl)-1H-4,5-imidazoledicarboxylic, DMA = N,N’-dimethylacetamide), with open metal sites and small-sized pores, exhibits excellent selective capture of CO2 over N2 and CH4 at 273 K, as well as alcohols from water. The excellent CO2 adsorption selectivity of 1 allows its potential use in the capture of
Received 28th January 2014, Accepted 15th March 2014
CO2 from industrial flue gas or the removal of CO2 from natural gas. More interestingly, compound 1 rep-
DOI: 10.1039/c4dt00307a
resents the rare case of porous materials separating propanol isomers, which may be caused by the relative flexibility of the linear n-propanol considering that both n-propanol and i-propanol have similar
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kinetic diameters.
Introduction Microporous metal–organic frameworks (MOFs) have emerged as a new type of functional adsorbent materials and have undergone very rapid progress over the past two decades.1 Compared with other traditional porous materials (zeolites, activated carbon and silica), MOFs possess many outstanding features, including but not limited to large surface area, high crystallinity with well-defined pore properties, easily tunable and tailorable structures and chemical functionality.2 A number of investigations on MOFs have indicated their promising potential for CO2 capture and separation.3 Nevertheless, designing MOFs with exceptionally high capacity and selectivity toward CO2 adsorption remains significantly challenging due to the fact that most of the MOFs not only take up CO2 strongly but also adsorb other small gases, such as N2, CH4, CO and O2. In this regard, there is an urgent need to be able to tailor the pore metrics and functionality of MOFs specifically and selectively for CO2. Remarkably, one of the effective strategies to enhance the CO2 binding affinity and gas
a Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan, 430079, P. R. China. E-mail: [email protected], [email protected]; Fax: +86 27 67867232; Tel: +86 27 67862900 b Institute for Integrated Cell-Material Sciences, (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan c School of Chemistry and Chemical Engineering, Xuzhou Institute of Technology, Xuzhou, 221011, P. R. China † Electronic supplementary information (ESI) available: X-ray crystallographic file (CIF), additional crystal figures, Qst and CO2 selectivity calculation details for complex 1. CCDC 980157. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00307a
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selectivity in MOFs is to build structures bearing open metal sites (OMSs) on the pore surface,4 which serve as charge-dense binding sites, interacting more strongly with CO2 over other components of gas mixtures owing to the propensity for CO2 to be polarized to a higher extent. A rarely explored tripodal ligand, 2-( pyridine-4-yl)-1H-4,5imidazoledicarboxylic acid5 (H3PyImDC, Scheme S1†), was chosen as a particularly effective building block because (1) the robust rigid organic moiety facilitates the preparation of stable porous materials; (2) the deprotonated HnPyImDC (n = 0, 1, 2) species could be formed by controlling the pH carefully, and therefore, the potential multiple coordination donors allow the formation of versatile structures; (3) the pyridyl ring and the imidazole ring in H3PyImDC can rotate around the C–C bond; in addition, carboxyl and the imidazole ring are not in the same plane, favoring the acentric framework, which is an essential requirement for the nonlinear-optical (NLO) effect; (4) the relatively large conjugated π-system in H3PyImDC may enhance the fluorescence of the resulting architecture. Herein, we report a three-dimensional (3D) microporous framework, [Zn(HPyImDC)(DMA)]n (1) (DMA = N,N′-dimethylacetamide), possessing exposed Zn(II) cation sites following activation. The open Zn(II) metal sites and small-sized pores enable desolvated 1 to exhibit excellent selective capture of CO2 over N2 and CH4 at 273 K, as well as alcohols from water. The excellent CO2 adsorption selectivity of 1 allows its potential use in the capture of CO2 from industrial flue gas or the removal of CO2 from natural gas. More interestingly, compound 1 represents the rare case of porous materials separating propanol isomers, which may be caused by the relative flexibility of the linear n-propanol considering that both n-propanol and i-propanol have similar kinetic diameters.
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Experimental section
Dalton Transactions Table 1
Crystal data and refinement information for complex 1
1
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Materials and methods All chemicals and solvents were commercially available and used as received. The H3PyImDC ligand and template bimb were prepared by literature methods.6,7 Elemental microanalyses (C, H and N) were carried out on a Perkin-Elmer 240 elemental analyzer. IR spectra were recorded on a Bruker Vector 22 spectrometer as dry KBr discs in the 400–4000 cm−1 region. Thermogravimetric analyses (TGA) were performed on a Netzsch STA449C apparatus under a N2 stream (heating rate of 10 °C min−1). Powder X-ray diffraction patterns were collected on a Bruker D8 Advance diffractometer with a Cu anode (λ = 1.5406 Å). A pulsed Q-switched Nd:YAG laser at a wavelength of 1064 nm was used to generate an SHG signal from the sample. The backward scattered SHG light was collected using a spherical concave mirror and passed through a filter which transmits only 532 nm radiation. Solid-state fluorescence spectra were recorded on an Edinburgh Analytical instrument FLS920. Synthesis of [Zn(HPyImDC)(DMA)]n (1). A mixture of Zn(NO3)2·6H2O (29.7 mg, 0.10 mmol), H3PyImDC (23.3 mg, 0.10 mmol) and bimb (28.6 mg, 0.10 mmol), N,N′-dimethylacetamide (DMA, 2 mL) and H2O (1 mL) was placed in a Parr Teflon-lined stainless steel vessel (25 cm3), and then the vessel was sealed and heated at 110 °C for 3 days. After slowly cooling to room temperature, colorless crystals of 1 were collected (yield: 45% based on Zn). Anal. Calcd for C14H14N4O5Zn: C, 43.83; H, 3.68; N, 14.60%; found: C, 43.87; H, 3.72; N, 14.58%. IR spectrum (cm−1): 3445w, 1713s, 1617s, 1583s, 1483s, 1393w, 1286w, 1221w, 1127m, 1025m, 843m, 791w, 752w, 729m. X-Ray crystallography study The crystallographic data for 1 were collected on a Bruker Smart Apex CCD area-detector diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) at 293 K using the ω-scan technique. The diffraction data were integrated by using the SAINT program,8 which was also used for the intensity corrections for the Lorentz and polarization effects. Semi-empirical absorption correction was applied using the SADABS program.9 The structure was solved by direct methods, and all non-hydrogen atoms were refined anisotropically on F2 by the full-matrix least-squares technique using the SHELXL-97 crystallographic software package.10 Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre: CCDC 980157 for compound 1. Details of the crystal parameters, data collection, and refinements for the complex are summarized in Table 1, and selected bond lengths and angles are listed in Table 2. Sorption measurements UHP-grade (99.999% purity) gases and HPLC-grade alcohols were used throughout the sorption measurements. Lowpressure nitrogen (N2), carbon dioxide (CO2), methane (CH4), water and alcohol sorption experiments (up to 1 bar) were
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Formula Formula weight Crystal system Space group Flack parameter a/Å b/Å c/Å V/Å3 Z ρcalcd/g cm−3 μ/mm−1 Collected reflections Unique reflections R1 [I > 2σ (I)] wR2 (all data)
Table 2
C14H14N4O5Zn 383.68 Orthorhombic P212121 0.003(13) 8.902(2) 11.887(3) 14.757(4) 1561.6(7) 4 1.632 1.606 11 359 2896 0.0234 0.0851
Selected bond distances (Å) and angles (°) for complex 1a
Zn1–O1 Zn1–N3 Zn1–N4#1 O1–Zn1–O4#1 O5–Zn1–N3
2.154(2) 2.194(3) 2.218(3) 170.34(8) 163.26(10)
Zn1–O5 Zn1–O4#1 Zn1–N5#2 N4#1–Zn1–N5#2
2.120(3) 2.092(2) 2.124(3) 163.84(10)
Symmetry codes for 1: #1 −1/2 + x, 1/2 − y, 1 − z; #2 1 − x, −1/2 + y, 3/2 − z. a
measured on a Quantachrome IQ2 system. The as-made sample was soaked in anhydrous methanol (50 mL) for 8 h. The supernatant was decanted and replenished four times over three days. A completely desolvated sample of about 100 mg was obtained by heating the solvent-exchanged bulk at 393 K under dynamic high vacuum overnight. Before the gas/ vapor measurement, the samples were evacuated again by using the “degas” function of the surface area analyzer for 10 h at 393 K.
Results and discussion Description of the crystal structure Interestingly, crystallographic analysis revealed that compound 1 crystallizes in the chiral space group P212121. The asymmetric unit of 1 includes one Zn(II) atom, one doubly deprotonated HPyImDC− anion and one coordinated DMA molecule (Fig. S1a†). As shown in Fig. 1a, the Zn1 atom is coordinated by three nitrogen atoms from three individual HPyImDC− moieties, and three oxygen atoms from two different HPyImDC− ligands and one coordinated DMA molecule (O5) to furnish a distorted octahedral geometry. The Zn1–N bond lengths range from 2.124(3) to 2.218(3) Å, and the Zn1–O ones are in the range of 2.092(2)–2.154(2) Å, which are comparable to previously reported values.5b In each ligand HPyImDC−, two carboxylate groups are essentially each coplanar with respect to the imidazole ring with dihedral angles varying from 4.4 to 7.6°, while the imidazole ring and the pyrazyl motif have a
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Synthesis, TG, PXRD and adsorption properties of complex 1
Fig. 1 (a) The coordination environment of the Zn1 atom with hydrogen atoms for clarity of 1 with ellipsoids drawn at the 30% probability level. (b) 3D structure of 1 with microporous channels. (c) Schematic representation of the uninodal 3-connected SrSi2 3D network of 1.
dihedral angle of ca. 35.4°. The distortion of the HPyImDC− ligand may lead to the crystallization of 1 in the chiral space group. In 1, on the one hand, each HPyImDC2− ligand acts as a µ3-bridge: the pyridyl nitrogen atom coordinates to one zinc atom and the imidazole dicarboxylic groups chelate two zinc atoms in a bis-bidentate fashion via one oxygen of the carboxylic moiety and one nitrogen atom of the imidazole ring. On the other hand, each Zn(II) connects three HPyImDC− fragments by turn (Fig. S1b†), giving a non-interpenetrated threedimensional (3D) porous structure (Fig. 1b). From the topological analysis,11 the overall structure of 1 can be reduced to a uninodal 3-connected SrSi2 net with a Schäfli symbol of 103 (Fig. 1c). Viewed along the a-axis in 1, rhombic microporous channels with an effective aperture size of 4.8 × 4.0 Å2 are present, taking into account van der Waals distances (Fig. 1b). The total solvent accessible volume per unit cell of 1 determined by PLATON is close to 42.0% (656 Å3) without the ligated DMA molecules.12 Worthy of mention is the exposed Zn(II) cation sites on the pore surface of desolvated 1, which may enhance the adsorption enthalpy for CO2.
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It is worth emphasizing that template-assisted synthesis has been demonstrated to be a powerful method for the construction of novel MOFs.13 Complex 1 was obtained using Zn(NO3)2·6H2O and H3PyImDC, employing bimb as a template. When the reactions were carried out in the absence of a templating agent, only undetermined polycrystalline samples were attained. This fact indicates that the templating agent was crucial for the formation of the compound. Thermogravimetric analyses (TGA) were carried out for complex 1 and activated 1, as shown in Fig. S2.† No obvious weight loss was found for complex 1 below 453 K. Further weight loss of 22.20% occurs between 453 and 545 K, ascribed to the loss of coordination DMA molecules (calcd 22.70%) before the decomposition of the framework starts. The phase purity of the as-synthesized samples was confirmed by the evident similarity between the calculated and experimental powder X-ray diffraction (PXRD) patterns. The similar PXRD patterns of as-synthesized and activated 1 indicates that the guest-free sample remains intact as that of 1. In addition, complex 1 is stable in open air even after months (Fig. 2). This excellent stability is likely ascribed to being strongly coordinated to N atoms from HPyImDC− instead of being purely coordinated to O atoms of carboxylate, which is believed to be fragile when encountering moisture.14 CO2 adsorption of activated 1 performed at 195 K and 1 bar exhibits type-I adsorption isotherm behavior with an uptake of 68.9 cm3 g−1, characteristic of the microporous material15 (Fig. 3). On the basis of the CO2 adsorption isotherm, the Brunauer–Emmett–Teller (BET) surface area was calculated to be 185.6 m2 g−1. The pore-size distributions derived from the CO2 isotherms by the Horvath–Kawazoe (HK) method suggest that the average pore size for desolvated 1 is 4.425 Å in diameter (Fig. S3†), which is consistent with the value expected from the crystallographic data (ca. 4.4 Å). Interestingly, desolvated 1 takes up a maximum CO2 uptake of 19.5 cm3 g−1 at 273 K/1 bar (Fig. 4a). Comparatively, the very limited CH4 and N2 adsorption amounts of 1.2 and 0.532 cm3 g−1 were detected under the same conditions. The adsorption capacity of CO2 for 1 is 16.2 and 36.6 times higher than those for CH4 and N2 at 273 K/1 bar, highlighting that 1 is a promis-
Fig. 2 PXRD profiles for complex 1. The simulated spectrum was calculated from the single crystal data.
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Fig. 3 CO2 (195 K) sorption isotherms for desolvated 1, where filled shape and open shape represent adsorption and desorption, respectively.
Fig. 4 (a) Sorption isotherms for CO2, CH4, and N2 at 273 K of desolvated 1 (adsorption and desorption branches are shown with filled and empty shape, respectively). (b) Evaluation of the initial slope in the Henry region of the sorption isotherms of CO2 (triangle), CH4 (square), and N2 (circle) at 273 K. The ratios of the initial slopes allowed an estimation of the sorption selectivity.
ing material for the highly selective separation of CO2/CH4 and CO2/N2 at room temperature. In order to gain a more complete understanding of the separation phenomena, the gas selectivities have been analyzed using three different methods. The selectivity calculation for CO2 over N2 is best performed using the adsorption capacities at pressures of approximately 0.15 bar for CO2 and 0.75 bar for N2. Since the total pressure of a flue gas is approximately 1 bar, selectivity calculations based upon the quantity of both CO2 and N2 adsorbed at 1 bar drastically overestimate the fraction of CO2 in post-combustion flue gas and the total pressure of the gas.16 At 273 K, the uptake amount of CO2 is 9.08 cm3 g−1 at 0.15 bar and that of N2 is just
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0.4 cm3 g−1 at 0.75 bar. Therefore, the selectivity of CO2/N2 is 68 at 273 K, which is calculated from the pure-component isotherms according to eqn (S1) (ESI†). The value is lower than that of HPIP@ZnPc-2 (CO2/N2, 94),17 but it still matches with those of several most promising MOFs with high selectivity of CO2/N2,18 apparently indicating that 1 may be an excellent candidate for the post-combustion capture of CO2. Also, the initial slopes of the CO2, N2 and CH4 adsorption isotherms were calculated, and the ratios of these slopes were also used to estimate the adsorption selectivity for CO2 over N2 and CH4 (Fig. 4b).19 From these data, the calculated CO2/N2 and CO2/CH4 selectivities are 297 : 1 and 132 : 1 at 273 K, respectively, which obviously exceed the superior performances of MOFs with significant sorption affinity for CO2 over N2 and CH4, such as CAU-119 (CO2/N2, 101; CO2/CH4, 28) and NJU-Bai83a (CO2/N2, 111; CO2/CH4, 40.8). Additionally, the selectivity for equimolar mixtures of CO2 and CH4 at 273 K on 1 was analyzed using the ideal adsorbed solution theory (IAST)20 for reference (Fig. S5†). The isothermal parameters were well fitted by the Langmuir–Freundlich method from the experimental pure adsorption isotherms of CO2 and CH4 at 273 K.21 Because CO2 has greater quadrupole moment (CO2, 13.4 × 10−40 C m2; N2, 4.7 × 10−40 C m2) and polarizability (CO2, 29.1 × 10−25 cm−3; CH4, 25.9 × 10−25 cm−3; N2, 17.4 × 10−25 cm−3) compared with CH4 and N2, it interacts more strongly with the open metal cation sites (Zn2+) in compound 1, resulting in significant CO2 sorption selectivity over CH4 and N2 under ambient conditions. In addition, the distinct adsorption capacity of CH4 and N2 for 1 can be related to their different polarizability, leading to the selectivity of CH4 over N2. The excellent CO2 adsorption selectivity of 1 allows its potential use in the capture of CO2 from industrial flue gas or the removal of CO2 from natural gas. The extent of adsorbent–adsorbate interactions was estimated by the enthalpy of CO2 adsorption (Qst), which was calculated using the virial equation22 from the adsorption isotherms recorded at 273 and 298 K (Fig. S6†). 1 exhibits a strong binding affinity for CO2 (around 43.1 kJ mol−1) at zero coverage (Fig. 5), which is quite comparable to those of famous compounds with open metal sites such as Mg2(dobdc)23 and MIL-10124 (47 and 44 kJ mol−1, respectively). The high initial Qst can be mainly ascribed to the strong interactions between the exposed Lewis acidic Zn2+ sites in the
Fig. 5
The isosteric heat of CO2 adsorption (Qst) for desolvated 1.
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channel and CO2 molecules. It is noteworthy that the isosteric heat of adsorption decreases to 23.6 kJ mol−1 (approximately the heat of adsorption for the weak sites) at maximum coverage, which has significant implications for regeneration in different industrial applications. Inspired by selective gas adsorption behaviors for complex 1, we also studied the vapor sorption properties at 298 K with respect to water (kinetic diameter, 2.64–2.9 Å) and a series of light alcohols (methanol: 3.626–4.0 Å; ethanol: 4.3–4.53 Å; n-propanol & i-propanol: 4.7 Å). For 1, the adsorption isotherms of vapor molecules show obvious hysteresis, indicating that desorption of guest molecules from the microporous channels is difficult, which may be ascribed to the strong adsorbate–adsorbent and (or) adsorbate–adsorbate interactions, such as hydrogen bonding between the vapor molecules and the framework or between the absorbed molecules. The adsorption profiles also illustrate that the saturated uptake amounts vary inversely with the size of the adsorbed molecules. As shown in Fig. 6, for 1, the final uptake volume of water was around 95 cm3 g−1, 60 cm3 g−1 for methanol, 23 cm3 g−1 for ethanol, 16 cm3 g−1 for n-propanol, and 2.6 cm3 g−1 for i-propanol, which suggests the potential selective separation of alcohols from water. Comparing n-propanol and i-propanol, despite both having similar kinetic diameters (4.7 Å), the isotherm of i-propanol is a characteristic type-II curve,25 which indicates that i-propanol shows weak interaction with 1 and only surface adsorption has occurred. In contrast, in the low P/P0 region, the adsorption of n-propanol reached saturation quickly (typical type-I curve), thus indicating strong host–guest interactions. Therefore, 1 has the potential ability to separate the isomers of propanol. To the best of our knowledge, this is the rare case of the separation of propanol isomers by using porous materials.26 This selective sorption behavior may be caused by the relative flexibility of the linear n-propanol. These results suggest that not only the size of guest molecules but also the shape can be crucial factors in the selective separation. Nonlinear-optical and fluorescence properties Since 1 crystallizes in a chiral space group, it is associated with a second harmonic generation (SHG) response. Preliminary
Fig. 6 Water and alcohol vapor adsorption–desorption isotherms of the desolvated 1: water, methanol, ethanol, n-propanol and i-propanol at 298 K, where filled and open shape represent adsorption and desorption, respectively.
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Fig. 7
The solid-state fluorescent spectrum of 1 at room temperature.
measurements on a powdered sample of 1 at ambient temperature suggest that 1 is approximately 0.4 times as SHG active as urea.27 In addition, the fluorescence properties of compound 1 and free ligand H3PyImDC were evaluated in the solid state at room temperature. The free H3PyImDC ligand exhibits a weak broad emission at 455 nm under the excitation at 335 nm (Fig. S7†), which is due presumably to the π–π* transition of the intraligands. Upon single-wavelength photo-excitation at 195 nm, compound 1 exhibits significantly enhanced multiple emissions with bands in the 300–800 nm region (Fig. 7), and the λmax values of the emission bands are 422, 458, 491, 530, 606, 699, 727 and 785 nm. Compared with the free organic linker, the large enhancement of emission intensity in 1 may be attributed to metal-to-ligand charge transfer (MLCT) and the coordination effect of the ligand to Zn(II) cations, which increases the ligand conformational rigidity and reduces the non-radiative decay.5c
Conclusions We have synthesized and structurally characterized a 3D microporous framework, [Zn(HPyImDC)(DMA)]n (1), possessing exposed Zn(II) cation sites following activation. Owing to the open Zn(II) metal sites and small-sized pores, the obtained MOF shows ultrahigh selective sorption of CO2 over N2 and CH4, suggesting possible applications in capturing CO2 from flue gases or upgrading natural gases by CO2/CH4 separations. More interestingly, compound 1 represents the rare case of porous materials separating propanol isomers, which may be caused by the relative flexibility of the linear n-propanol considering that both n-propanol and i-propanol have similar kinetic diameters.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (no. 21171062, 21371065, 21172084
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and 21301148), Program for Chenguang Young Scientists of Wuhan (2013070104010020).
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18 (a) J. An, S. J. Geib and N. L. Rosi, J. Am. Chem. Soc., 2010, 132, 38; (b) J. A. Mason, K. Sumida, Z. R. Herm, R. Krishna and J. R. Long, Energy Environ. Sci., 2011, 4, 3030; (c) T. M. McDonald, D. M. D’Alessandro, R. Krishna and J. R. Long, Chem. Sci., 2011, 2, 2022; (d) P. Aprea, D. Caputo, N. Gargiulo, F. Iucolano and F. J. Pepe, Chem. Eng. Data, 2010, 55, 3655; (e) R. R. Yun, J. G. Duan, J. F. Bai and Y. Z. Li, Cryst. Growth Des., 2013, 13, 24. 19 (a) X. L. Si, C. L. Jiao, F. Li, J. Zhang, S. Wang, S. Liu, Z. B. Li, L. X. Sun, F. Xu, Z. Gabelica and C. Schick, Energy Environ. Sci., 2011, 4, 4522; (b) F. P. Zhai, Q. S. Zheng, Z. X. Chen, Y. Ling, X. F. Liu, L. H. Weng and Y. M. Zhou, CrystEngComm, 2013, 15, 2040. 20 A. L. Myers and J. M. Prausnitz, AICHE J., 1965, 11, 121. 21 (a) R. Babarao, Z. Q. Hu, J. W. Jiang, S. Chempath and S. I. Sandler, Langmuir, 2007, 23, 659; (b) V. Goetz, O. Pupier and A. Guillot, Adsorption, 2006, 12, 55.
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A microporous Zn(II)–MOF with open metal sites: structure and selective adsorption properties† Xiaofang Zheng,a Yumei Huang,a Jingui Duan,b,c Chenggang Wang,a Lili Wen,*a Jinbo Zhaoa and Dongfeng Li*a A three-dimensional microporous framework, Zn(II)–MOF [Zn(HPyImDC)(DMA)]n (1) (H3PyImDC = 2-( pyridine-4-yl)-1H-4,5-imidazoledicarboxylic, DMA = N,N’-dimethylacetamide), with open metal sites and small-sized pores, exhibits excellent selective capture of CO2 over N2 and CH4 at 273 K, as well as alcohols from water. The excellent CO2 adsorption selectivity of 1 allows its potential use in the capture of
Received 28th January 2014, Accepted 15th March 2014
CO2 from industrial flue gas or the removal of CO2 from natural gas. More interestingly, compound 1 rep-
DOI: 10.1039/c4dt00307a
resents the rare case of porous materials separating propanol isomers, which may be caused by the relative flexibility of the linear n-propanol considering that both n-propanol and i-propanol have similar
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kinetic diameters.
Introduction Microporous metal–organic frameworks (MOFs) have emerged as a new type of functional adsorbent materials and have undergone very rapid progress over the past two decades.1 Compared with other traditional porous materials (zeolites, activated carbon and silica), MOFs possess many outstanding features, including but not limited to large surface area, high crystallinity with well-defined pore properties, easily tunable and tailorable structures and chemical functionality.2 A number of investigations on MOFs have indicated their promising potential for CO2 capture and separation.3 Nevertheless, designing MOFs with exceptionally high capacity and selectivity toward CO2 adsorption remains significantly challenging due to the fact that most of the MOFs not only take up CO2 strongly but also adsorb other small gases, such as N2, CH4, CO and O2. In this regard, there is an urgent need to be able to tailor the pore metrics and functionality of MOFs specifically and selectively for CO2. Remarkably, one of the effective strategies to enhance the CO2 binding affinity and gas
a Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan, 430079, P. R. China. E-mail: [email protected], [email protected]; Fax: +86 27 67867232; Tel: +86 27 67862900 b Institute for Integrated Cell-Material Sciences, (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan c School of Chemistry and Chemical Engineering, Xuzhou Institute of Technology, Xuzhou, 221011, P. R. China † Electronic supplementary information (ESI) available: X-ray crystallographic file (CIF), additional crystal figures, Qst and CO2 selectivity calculation details for complex 1. CCDC 980157. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00307a
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selectivity in MOFs is to build structures bearing open metal sites (OMSs) on the pore surface,4 which serve as charge-dense binding sites, interacting more strongly with CO2 over other components of gas mixtures owing to the propensity for CO2 to be polarized to a higher extent. A rarely explored tripodal ligand, 2-( pyridine-4-yl)-1H-4,5imidazoledicarboxylic acid5 (H3PyImDC, Scheme S1†), was chosen as a particularly effective building block because (1) the robust rigid organic moiety facilitates the preparation of stable porous materials; (2) the deprotonated HnPyImDC (n = 0, 1, 2) species could be formed by controlling the pH carefully, and therefore, the potential multiple coordination donors allow the formation of versatile structures; (3) the pyridyl ring and the imidazole ring in H3PyImDC can rotate around the C–C bond; in addition, carboxyl and the imidazole ring are not in the same plane, favoring the acentric framework, which is an essential requirement for the nonlinear-optical (NLO) effect; (4) the relatively large conjugated π-system in H3PyImDC may enhance the fluorescence of the resulting architecture. Herein, we report a three-dimensional (3D) microporous framework, [Zn(HPyImDC)(DMA)]n (1) (DMA = N,N′-dimethylacetamide), possessing exposed Zn(II) cation sites following activation. The open Zn(II) metal sites and small-sized pores enable desolvated 1 to exhibit excellent selective capture of CO2 over N2 and CH4 at 273 K, as well as alcohols from water. The excellent CO2 adsorption selectivity of 1 allows its potential use in the capture of CO2 from industrial flue gas or the removal of CO2 from natural gas. More interestingly, compound 1 represents the rare case of porous materials separating propanol isomers, which may be caused by the relative flexibility of the linear n-propanol considering that both n-propanol and i-propanol have similar kinetic diameters.
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Experimental section
Dalton Transactions Table 1
Crystal data and refinement information for complex 1
1
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Materials and methods All chemicals and solvents were commercially available and used as received. The H3PyImDC ligand and template bimb were prepared by literature methods.6,7 Elemental microanalyses (C, H and N) were carried out on a Perkin-Elmer 240 elemental analyzer. IR spectra were recorded on a Bruker Vector 22 spectrometer as dry KBr discs in the 400–4000 cm−1 region. Thermogravimetric analyses (TGA) were performed on a Netzsch STA449C apparatus under a N2 stream (heating rate of 10 °C min−1). Powder X-ray diffraction patterns were collected on a Bruker D8 Advance diffractometer with a Cu anode (λ = 1.5406 Å). A pulsed Q-switched Nd:YAG laser at a wavelength of 1064 nm was used to generate an SHG signal from the sample. The backward scattered SHG light was collected using a spherical concave mirror and passed through a filter which transmits only 532 nm radiation. Solid-state fluorescence spectra were recorded on an Edinburgh Analytical instrument FLS920. Synthesis of [Zn(HPyImDC)(DMA)]n (1). A mixture of Zn(NO3)2·6H2O (29.7 mg, 0.10 mmol), H3PyImDC (23.3 mg, 0.10 mmol) and bimb (28.6 mg, 0.10 mmol), N,N′-dimethylacetamide (DMA, 2 mL) and H2O (1 mL) was placed in a Parr Teflon-lined stainless steel vessel (25 cm3), and then the vessel was sealed and heated at 110 °C for 3 days. After slowly cooling to room temperature, colorless crystals of 1 were collected (yield: 45% based on Zn). Anal. Calcd for C14H14N4O5Zn: C, 43.83; H, 3.68; N, 14.60%; found: C, 43.87; H, 3.72; N, 14.58%. IR spectrum (cm−1): 3445w, 1713s, 1617s, 1583s, 1483s, 1393w, 1286w, 1221w, 1127m, 1025m, 843m, 791w, 752w, 729m. X-Ray crystallography study The crystallographic data for 1 were collected on a Bruker Smart Apex CCD area-detector diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) at 293 K using the ω-scan technique. The diffraction data were integrated by using the SAINT program,8 which was also used for the intensity corrections for the Lorentz and polarization effects. Semi-empirical absorption correction was applied using the SADABS program.9 The structure was solved by direct methods, and all non-hydrogen atoms were refined anisotropically on F2 by the full-matrix least-squares technique using the SHELXL-97 crystallographic software package.10 Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre: CCDC 980157 for compound 1. Details of the crystal parameters, data collection, and refinements for the complex are summarized in Table 1, and selected bond lengths and angles are listed in Table 2. Sorption measurements UHP-grade (99.999% purity) gases and HPLC-grade alcohols were used throughout the sorption measurements. Lowpressure nitrogen (N2), carbon dioxide (CO2), methane (CH4), water and alcohol sorption experiments (up to 1 bar) were
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Formula Formula weight Crystal system Space group Flack parameter a/Å b/Å c/Å V/Å3 Z ρcalcd/g cm−3 μ/mm−1 Collected reflections Unique reflections R1 [I > 2σ (I)] wR2 (all data)
Table 2
C14H14N4O5Zn 383.68 Orthorhombic P212121 0.003(13) 8.902(2) 11.887(3) 14.757(4) 1561.6(7) 4 1.632 1.606 11 359 2896 0.0234 0.0851
Selected bond distances (Å) and angles (°) for complex 1a
Zn1–O1 Zn1–N3 Zn1–N4#1 O1–Zn1–O4#1 O5–Zn1–N3
2.154(2) 2.194(3) 2.218(3) 170.34(8) 163.26(10)
Zn1–O5 Zn1–O4#1 Zn1–N5#2 N4#1–Zn1–N5#2
2.120(3) 2.092(2) 2.124(3) 163.84(10)
Symmetry codes for 1: #1 −1/2 + x, 1/2 − y, 1 − z; #2 1 − x, −1/2 + y, 3/2 − z. a
measured on a Quantachrome IQ2 system. The as-made sample was soaked in anhydrous methanol (50 mL) for 8 h. The supernatant was decanted and replenished four times over three days. A completely desolvated sample of about 100 mg was obtained by heating the solvent-exchanged bulk at 393 K under dynamic high vacuum overnight. Before the gas/ vapor measurement, the samples were evacuated again by using the “degas” function of the surface area analyzer for 10 h at 393 K.
Results and discussion Description of the crystal structure Interestingly, crystallographic analysis revealed that compound 1 crystallizes in the chiral space group P212121. The asymmetric unit of 1 includes one Zn(II) atom, one doubly deprotonated HPyImDC− anion and one coordinated DMA molecule (Fig. S1a†). As shown in Fig. 1a, the Zn1 atom is coordinated by three nitrogen atoms from three individual HPyImDC− moieties, and three oxygen atoms from two different HPyImDC− ligands and one coordinated DMA molecule (O5) to furnish a distorted octahedral geometry. The Zn1–N bond lengths range from 2.124(3) to 2.218(3) Å, and the Zn1–O ones are in the range of 2.092(2)–2.154(2) Å, which are comparable to previously reported values.5b In each ligand HPyImDC−, two carboxylate groups are essentially each coplanar with respect to the imidazole ring with dihedral angles varying from 4.4 to 7.6°, while the imidazole ring and the pyrazyl motif have a
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Synthesis, TG, PXRD and adsorption properties of complex 1
Fig. 1 (a) The coordination environment of the Zn1 atom with hydrogen atoms for clarity of 1 with ellipsoids drawn at the 30% probability level. (b) 3D structure of 1 with microporous channels. (c) Schematic representation of the uninodal 3-connected SrSi2 3D network of 1.
dihedral angle of ca. 35.4°. The distortion of the HPyImDC− ligand may lead to the crystallization of 1 in the chiral space group. In 1, on the one hand, each HPyImDC2− ligand acts as a µ3-bridge: the pyridyl nitrogen atom coordinates to one zinc atom and the imidazole dicarboxylic groups chelate two zinc atoms in a bis-bidentate fashion via one oxygen of the carboxylic moiety and one nitrogen atom of the imidazole ring. On the other hand, each Zn(II) connects three HPyImDC− fragments by turn (Fig. S1b†), giving a non-interpenetrated threedimensional (3D) porous structure (Fig. 1b). From the topological analysis,11 the overall structure of 1 can be reduced to a uninodal 3-connected SrSi2 net with a Schäfli symbol of 103 (Fig. 1c). Viewed along the a-axis in 1, rhombic microporous channels with an effective aperture size of 4.8 × 4.0 Å2 are present, taking into account van der Waals distances (Fig. 1b). The total solvent accessible volume per unit cell of 1 determined by PLATON is close to 42.0% (656 Å3) without the ligated DMA molecules.12 Worthy of mention is the exposed Zn(II) cation sites on the pore surface of desolvated 1, which may enhance the adsorption enthalpy for CO2.
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It is worth emphasizing that template-assisted synthesis has been demonstrated to be a powerful method for the construction of novel MOFs.13 Complex 1 was obtained using Zn(NO3)2·6H2O and H3PyImDC, employing bimb as a template. When the reactions were carried out in the absence of a templating agent, only undetermined polycrystalline samples were attained. This fact indicates that the templating agent was crucial for the formation of the compound. Thermogravimetric analyses (TGA) were carried out for complex 1 and activated 1, as shown in Fig. S2.† No obvious weight loss was found for complex 1 below 453 K. Further weight loss of 22.20% occurs between 453 and 545 K, ascribed to the loss of coordination DMA molecules (calcd 22.70%) before the decomposition of the framework starts. The phase purity of the as-synthesized samples was confirmed by the evident similarity between the calculated and experimental powder X-ray diffraction (PXRD) patterns. The similar PXRD patterns of as-synthesized and activated 1 indicates that the guest-free sample remains intact as that of 1. In addition, complex 1 is stable in open air even after months (Fig. 2). This excellent stability is likely ascribed to being strongly coordinated to N atoms from HPyImDC− instead of being purely coordinated to O atoms of carboxylate, which is believed to be fragile when encountering moisture.14 CO2 adsorption of activated 1 performed at 195 K and 1 bar exhibits type-I adsorption isotherm behavior with an uptake of 68.9 cm3 g−1, characteristic of the microporous material15 (Fig. 3). On the basis of the CO2 adsorption isotherm, the Brunauer–Emmett–Teller (BET) surface area was calculated to be 185.6 m2 g−1. The pore-size distributions derived from the CO2 isotherms by the Horvath–Kawazoe (HK) method suggest that the average pore size for desolvated 1 is 4.425 Å in diameter (Fig. S3†), which is consistent with the value expected from the crystallographic data (ca. 4.4 Å). Interestingly, desolvated 1 takes up a maximum CO2 uptake of 19.5 cm3 g−1 at 273 K/1 bar (Fig. 4a). Comparatively, the very limited CH4 and N2 adsorption amounts of 1.2 and 0.532 cm3 g−1 were detected under the same conditions. The adsorption capacity of CO2 for 1 is 16.2 and 36.6 times higher than those for CH4 and N2 at 273 K/1 bar, highlighting that 1 is a promis-
Fig. 2 PXRD profiles for complex 1. The simulated spectrum was calculated from the single crystal data.
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Fig. 3 CO2 (195 K) sorption isotherms for desolvated 1, where filled shape and open shape represent adsorption and desorption, respectively.
Fig. 4 (a) Sorption isotherms for CO2, CH4, and N2 at 273 K of desolvated 1 (adsorption and desorption branches are shown with filled and empty shape, respectively). (b) Evaluation of the initial slope in the Henry region of the sorption isotherms of CO2 (triangle), CH4 (square), and N2 (circle) at 273 K. The ratios of the initial slopes allowed an estimation of the sorption selectivity.
ing material for the highly selective separation of CO2/CH4 and CO2/N2 at room temperature. In order to gain a more complete understanding of the separation phenomena, the gas selectivities have been analyzed using three different methods. The selectivity calculation for CO2 over N2 is best performed using the adsorption capacities at pressures of approximately 0.15 bar for CO2 and 0.75 bar for N2. Since the total pressure of a flue gas is approximately 1 bar, selectivity calculations based upon the quantity of both CO2 and N2 adsorbed at 1 bar drastically overestimate the fraction of CO2 in post-combustion flue gas and the total pressure of the gas.16 At 273 K, the uptake amount of CO2 is 9.08 cm3 g−1 at 0.15 bar and that of N2 is just
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Dalton Transactions
0.4 cm3 g−1 at 0.75 bar. Therefore, the selectivity of CO2/N2 is 68 at 273 K, which is calculated from the pure-component isotherms according to eqn (S1) (ESI†). The value is lower than that of HPIP@ZnPc-2 (CO2/N2, 94),17 but it still matches with those of several most promising MOFs with high selectivity of CO2/N2,18 apparently indicating that 1 may be an excellent candidate for the post-combustion capture of CO2. Also, the initial slopes of the CO2, N2 and CH4 adsorption isotherms were calculated, and the ratios of these slopes were also used to estimate the adsorption selectivity for CO2 over N2 and CH4 (Fig. 4b).19 From these data, the calculated CO2/N2 and CO2/CH4 selectivities are 297 : 1 and 132 : 1 at 273 K, respectively, which obviously exceed the superior performances of MOFs with significant sorption affinity for CO2 over N2 and CH4, such as CAU-119 (CO2/N2, 101; CO2/CH4, 28) and NJU-Bai83a (CO2/N2, 111; CO2/CH4, 40.8). Additionally, the selectivity for equimolar mixtures of CO2 and CH4 at 273 K on 1 was analyzed using the ideal adsorbed solution theory (IAST)20 for reference (Fig. S5†). The isothermal parameters were well fitted by the Langmuir–Freundlich method from the experimental pure adsorption isotherms of CO2 and CH4 at 273 K.21 Because CO2 has greater quadrupole moment (CO2, 13.4 × 10−40 C m2; N2, 4.7 × 10−40 C m2) and polarizability (CO2, 29.1 × 10−25 cm−3; CH4, 25.9 × 10−25 cm−3; N2, 17.4 × 10−25 cm−3) compared with CH4 and N2, it interacts more strongly with the open metal cation sites (Zn2+) in compound 1, resulting in significant CO2 sorption selectivity over CH4 and N2 under ambient conditions. In addition, the distinct adsorption capacity of CH4 and N2 for 1 can be related to their different polarizability, leading to the selectivity of CH4 over N2. The excellent CO2 adsorption selectivity of 1 allows its potential use in the capture of CO2 from industrial flue gas or the removal of CO2 from natural gas. The extent of adsorbent–adsorbate interactions was estimated by the enthalpy of CO2 adsorption (Qst), which was calculated using the virial equation22 from the adsorption isotherms recorded at 273 and 298 K (Fig. S6†). 1 exhibits a strong binding affinity for CO2 (around 43.1 kJ mol−1) at zero coverage (Fig. 5), which is quite comparable to those of famous compounds with open metal sites such as Mg2(dobdc)23 and MIL-10124 (47 and 44 kJ mol−1, respectively). The high initial Qst can be mainly ascribed to the strong interactions between the exposed Lewis acidic Zn2+ sites in the
Fig. 5
The isosteric heat of CO2 adsorption (Qst) for desolvated 1.
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channel and CO2 molecules. It is noteworthy that the isosteric heat of adsorption decreases to 23.6 kJ mol−1 (approximately the heat of adsorption for the weak sites) at maximum coverage, which has significant implications for regeneration in different industrial applications. Inspired by selective gas adsorption behaviors for complex 1, we also studied the vapor sorption properties at 298 K with respect to water (kinetic diameter, 2.64–2.9 Å) and a series of light alcohols (methanol: 3.626–4.0 Å; ethanol: 4.3–4.53 Å; n-propanol & i-propanol: 4.7 Å). For 1, the adsorption isotherms of vapor molecules show obvious hysteresis, indicating that desorption of guest molecules from the microporous channels is difficult, which may be ascribed to the strong adsorbate–adsorbent and (or) adsorbate–adsorbate interactions, such as hydrogen bonding between the vapor molecules and the framework or between the absorbed molecules. The adsorption profiles also illustrate that the saturated uptake amounts vary inversely with the size of the adsorbed molecules. As shown in Fig. 6, for 1, the final uptake volume of water was around 95 cm3 g−1, 60 cm3 g−1 for methanol, 23 cm3 g−1 for ethanol, 16 cm3 g−1 for n-propanol, and 2.6 cm3 g−1 for i-propanol, which suggests the potential selective separation of alcohols from water. Comparing n-propanol and i-propanol, despite both having similar kinetic diameters (4.7 Å), the isotherm of i-propanol is a characteristic type-II curve,25 which indicates that i-propanol shows weak interaction with 1 and only surface adsorption has occurred. In contrast, in the low P/P0 region, the adsorption of n-propanol reached saturation quickly (typical type-I curve), thus indicating strong host–guest interactions. Therefore, 1 has the potential ability to separate the isomers of propanol. To the best of our knowledge, this is the rare case of the separation of propanol isomers by using porous materials.26 This selective sorption behavior may be caused by the relative flexibility of the linear n-propanol. These results suggest that not only the size of guest molecules but also the shape can be crucial factors in the selective separation. Nonlinear-optical and fluorescence properties Since 1 crystallizes in a chiral space group, it is associated with a second harmonic generation (SHG) response. Preliminary
Fig. 6 Water and alcohol vapor adsorption–desorption isotherms of the desolvated 1: water, methanol, ethanol, n-propanol and i-propanol at 298 K, where filled and open shape represent adsorption and desorption, respectively.
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Fig. 7
The solid-state fluorescent spectrum of 1 at room temperature.
measurements on a powdered sample of 1 at ambient temperature suggest that 1 is approximately 0.4 times as SHG active as urea.27 In addition, the fluorescence properties of compound 1 and free ligand H3PyImDC were evaluated in the solid state at room temperature. The free H3PyImDC ligand exhibits a weak broad emission at 455 nm under the excitation at 335 nm (Fig. S7†), which is due presumably to the π–π* transition of the intraligands. Upon single-wavelength photo-excitation at 195 nm, compound 1 exhibits significantly enhanced multiple emissions with bands in the 300–800 nm region (Fig. 7), and the λmax values of the emission bands are 422, 458, 491, 530, 606, 699, 727 and 785 nm. Compared with the free organic linker, the large enhancement of emission intensity in 1 may be attributed to metal-to-ligand charge transfer (MLCT) and the coordination effect of the ligand to Zn(II) cations, which increases the ligand conformational rigidity and reduces the non-radiative decay.5c
Conclusions We have synthesized and structurally characterized a 3D microporous framework, [Zn(HPyImDC)(DMA)]n (1), possessing exposed Zn(II) cation sites following activation. Owing to the open Zn(II) metal sites and small-sized pores, the obtained MOF shows ultrahigh selective sorption of CO2 over N2 and CH4, suggesting possible applications in capturing CO2 from flue gases or upgrading natural gases by CO2/CH4 separations. More interestingly, compound 1 represents the rare case of porous materials separating propanol isomers, which may be caused by the relative flexibility of the linear n-propanol considering that both n-propanol and i-propanol have similar kinetic diameters.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (no. 21171062, 21371065, 21172084
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