Communication pubs.acs.org/IC
First Cationic Uranyl−Organic Framework with Anion-Exchange Capabilities Zhuanling Bai,†,‡,§ Yanlong Wang,†,‡,§ Yuxiang Li,†,‡ Wei Liu,†,‡ Lanhua Chen,†,‡ Daopeng Sheng,†,‡ Juan Diwu,†,‡ Zhifang Chai,†,‡ Thomas E. Albrecht-Schmitt,⊥ and Shuao Wang*,†,‡ †
School for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University, Jiangsu 215123, China Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Jiangsu 215123, China ⊥ Department of Chemistry and Biochemistry, Florida State University, 95 Chieftain Way, Tallahassee, Florida 32306, United States ‡
S Supporting Information *
the environment.7 It should be noted that purely inorganic cationic framework compounds are quite scarce and their synthesis often lacks the mean of rational design.8 In recent years, a series of cationic MOFs with transition metals and lanthanides have been designed and synthesized using several different strategies.9 One way is to react metal cations with neutral nitrogen-donor ligands to afford [ML]+ frameworks along with unbound or weakly coordinated anions. For instance, Wang et al. reported the cationic MOF synthesized by combining AgClO4 and a neutral ligand, and this material shows fast exchange kinetics, high trapping capacity, and superior selectivity for the capture of Cr2O72− in water.10 Similarly, Oliver and co-workers constructed SLUG-21 exhibiting highly efficient uptake of permanganate and perrhenate anions used as surrogates for the key radioactive anionic contaminant pertechnetate.11 The other strategy involves postsynthetic modification, which can directly convert a neutral framework into a positively charged one. One recent example is ZJU-101, constructed by Qian et al. and developed for the efficient removal of Cr2O72−.7,9e In addition, Bu et al. reported the systematic synthesis of cationic P-MOFs using a family of [In3O(COO)6]+ showing useful applications in organic dye separation.9f Herein, we utilized a feasible strategy in which zwitterionicbased ligands (H3TTTPCBr3)12 containing three positively charged nitrogen atoms are selected to construct the cationic M O F 1 , 1 ′ , 1″ - (2 , 4 , 6 -t r i m e t h y l b e nz e n e - 1 , 3 , 5 - t r iy l ) trimethylenetris(4-carboxypyridinium) tribromide) (H3TTTPCBr3). In the self-assembly process, the formal charge of the ligand would be changed to be associated with deprotonation and coordination. If other negatively charged ligands such as oxide/hydroxide are absent, a cationic framework can be expected. The solvothermal reaction of H3TTTPCBr3 with UO2(NO3)2·6H2O in DMF and water afforded a neutral framework of SCU-6 due to hydrolysis of uranyl. Indeed, by reducing the extent of hydrolysis, the first reported cationic uranyl−organic framework SCU-7 was successfully obtained. Single-crystal X-ray diffraction analysis reveals that SCU-6 crystallizes in the triclinic space group P1̅ and displays a twodimensional (2D)-layered structure. As shown in Figure 1a, the asymmetric unit of SCU-6 consists of two uranyl ions, one ligand, one hydroxyl group, one oxygen, and one formate group. Both
ABSTRACT: By controlling the extent of hydrolysis during the self-assembly process of a zwitterionic-based ligand with uranyl cations, we observed a structural evolution from the neutral uranyl−organic framework [(UO 2 ) 2 (TTTPC)(OH)O(COOH)]·1.5DMF·7H 2 O (SCU-6) to the first cationic uranyl−organic framework with the formula of [(UO 2 )(HTTTPC)(OH)]Br· 1.5DMF·4H2O (SCU-7). The crystal structures of SCU6 and SCU-7 are layers built with tetranuclear and dinuclear uranyl clusters, respectively. Exchangeable halide anions are present in the interlaminar spaces balancing the positive charge of layers in SCU-7. Therefore, SCU-7 is able to effectively remove perrhenate anions from aqueous solution. Meanwhile, the H2PO4−-exchanged SCU-7 material exhibits a moderate proton conductivity of 8.70 × 10−5 S cm−1 at 50 °C and 90% relative humidity, representing nearly 80 times enhancement compared to the original material.
Metal−organic frameworks (MOFs) have aroused vast amounts of interest because of their advantages in terms of high surface area, tunable pore size, intriguing and uniform architecture, and ligand functionalizability.1 More specifically, actinide-based hybrid inorganic−organic solid-state compounds are an emerging class of materials under exponential growth in recent years that have a wide range of potential applications in magnetism, photoluminescence, and separations in addition to their roles in the nuclear fuel cycle.2 However, actinide−organic frameworks are relatively rare owing to their radioactive nature compared with MOFs constructed by transition metals and lanthanides.3 Uranyl−organic frameworks play a dominant role in AnOFs owing to their diverse coordination, high reactivity, and relatively low radioactivity in contrast to other actinides.4 We have recently documented the first case of a uranyl polycatenated framework based on aromatic polycarboxylates with outstanding radiation resistance and high performance to selectively remove cesium ions from aqueous solutions.5 Materials with cationic extended structures are extremely rare in nature and are mostly represented by the hydrotalcite clays, also known as layered double hydroxides.6 These materials possess exchangeable anions in the interlaminar spaces and therefore find applications for removing key anionic pollutants in © XXXX American Chemical Society
Received: April 14, 2016
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DOI: 10.1021/acs.inorgchem.6b00930 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
ligands, and two μ2-bridging oxygen atoms from the hydroxyl group. All of the carboxylic acids of the ligands adopt the trans conformation and bind to the uranyl ions with bridging oxygen atoms. In contrast with SCU-6, only two carboxylic acids of the ligands are deprotonated. Combined with the reduced amount of oxide/hydroxide, a cationic layered structure is therefore achieved. The presence of a charge-balancing bromide anion can be confirmed by energy-dispersive X-ray (EDX) measurement (Figure S8). However, we are unable to identify Br− in the electron density map, implying that Br− is highly disordered and present in the interlayer spaces. This indicates the anionexchange potential of SCU-7 at the first place. Likewise, SCU-7 is also a 3,6-connected 2D network (Figure 2c) and also stacks through hydrogen bonds, forming a pseudo-3D structure (Figure S1b). Powder X-ray diffraction (PXRD) analysis was conducted to confirm the phase purity of both compounds (Figure S2). Thermogravimetric analysis measurements performed under a nitrogen atmosphere show both structures are stable up to ca. 250 °C. The hydrolytic stability of these two compounds was also tested by soaking the crystals in aqueous solutions at different pH values. As shown in Figure S4, SCU-6 and SCU-7 can well maintain their crystallinity in aqueous solutions over an extremely wide pH range from 2 to 12. Similar hydrolytic stability had also been observed in other cases of uranyl−organic frameworks,5 which contrasts sharply with the majority of divalent metal carboxylate based MOFs that often exhibit poor hydrolytic stabilities.16 Because SCU-7 is a cationic 2D framework with unbound anions located between the layers and void spaces, an anionexchange experiment was therefore conducted to initially evaluate its removal capacity for anionic environmental pollutants. Among these, the pertechnetate anion TcO4− is quite problematic because its fission yield (Tc-99), chemical stability, aqueous solubility, and environmental mobility are extremely high.17 Here, ReO4− was selected as a surrogate for TcO4− owing to their almost identical charge densities. ReO4− uptake experiments were conducted by soaking crystals of SCU6 and SCU-7 in aqueous solutions containing ReO4− of 1 ppm. After being separated by filtration, inductively coupled plasma mass spectrometry measurements were used to detect the concentration of ReO4− during the exchange process. The results show that SCU-7 is able to rapidly and efficiently remove ReO4− from aqueous solutions (Figure 3). Specifically, more than 75%
Figure 1. (a) Asymmetric unit of SCU-6. (b) Tetranuclear SBU of U4O20 in SCU-6. (c) 2D-layered structure of SCU-6.
the U1 and U2 ions are seven-coordinated, showing distorted pentagonal-pyramidal configurations. U1 is connected by two μ3oxo and two μ2-oxo atoms from the ligand, as well as one formic oxo atom in the equatorial plane. However, U2 is bound to one μ3-oxo (O5), two μ2-oxo, and two oxygen atoms from two ligands. The carboxylate groups of the ligand are all deprotonated and bound to uranyl as bridging oxygen atoms, affording a neutral layer. Uranyl has been reported to possess a tendency to form a large coordination structure,13 which forces the ligand to adopt the trans conformation in order to reduce the steric hindrance.14 As shown in Figure 1b, four uranyl pentagonalpyramidal units are connected through a μ3-oxo bridge to compose the secondary building unit (SBU) of a [(UO2)4(OH)O(COOH)6] tetramer as a result of hydrolysis. This type of SBU is not common in uranyl compounds in general.15 Each SBU is surrounded by six H3TTTPCBr3 ligands, which can be considered as a 6-connected node. Each H3TTTPCBr3 ligand links three SBU and acts as a 3-connected node. Therefore, the whole structure of SCU-6 can be simplified as a 3,6-connected 2D network (Figure 1c). The neighboring layers are connected through hydrogen bonds to constitute a pseudo-three-dimensional framework (Figure S1a). SCU-7 is obtained through slightly reducing the water content and consequently the extent of uranyl hydrolysis compared to that of SCU-6. Single-crystal X-ray diffraction analysis reveals that SCU-7 also crystallizes in the space group P1̅ and displays a similar 2D-layered structure with a μ2-oxo-bridged dinuclear unit formed by uranyl ions (Figure 2a). As shown in Figure 2b, the asymmetric unit of SCU-7 consists of one uranyl ion and one μ2bridging oxygen atom from the hydroxyl group. U1 and U1A share two μ2-oxo atoms, resulting in a dimer composed of the uranium−oxygen subunit of [U2O12] in SCU-7. Thus, the cluster comprises two uranyl ions, six carboxylic oxo atoms from six
Figure 3. ReO4− exchange kinetic curves for reactions containing 50 mL of an aqueous solution of 1 ppm of ReO4− and 150 mg of SCU-6 or SCU-7.
Figure 2. (a) Asymmetric unit of SCU-7. (b) Dinuclear SBU of U2O12 in SCU-7. (c) 2D-layered structure of SCU-7. B
DOI: 10.1021/acs.inorgchem.6b00930 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry of ReO4− was removed in less than 1 h, and exchange equilibrium was reached at a removal rate of nearly 90% after 2.5 h. In comparison, SCU-6 with a neutral layered structure can only remove up to 14% of ReO4− under the same ion-exchange conditions (Figure 3). We recently documented a new anion-exchange strategy for making proton-conducting materials, which shows a clear advance over all of the acid-impregnated materials as practical solid-state fuel-cell electrolytes in terms of hydrolytic stability.18 Considering the anion-exchangeable nature of SCU-7, protonconducting experiments can be executed through the exchange of H2PO4− anions into the structure of SCU-7, which is confirmed by EDS analysis (Figure S9). The PXRD data (Figure S7) reveal that the structure of SCU-7 retains its integrity throughout the exchange process. To evaluate the proton conductivity, alternating-current (ac) impedance analysis was carried out on SCU-6, SCU-7, SCU-6P, and SCU-7P (where SCU-6P and SCU-7P denote treated samples of SCU-6 and SCU-7 after H2PO4− exchange, respectively). Figure S11 shows typical Nyquist plots measured at 50 °C and 90% relative humidity. The result shows that SCU-6 and SCU-6P exhibit similar proton conductivities at 7.66 × 10−7 and 1.63 × 10−6 S cm−1, respectively. Impressively, SCU-7P improves its proton conductivity by nearly 80 times compared to the original sample, from 1.15 × 10−6 to 8.77 × 10−5 S cm−1. In conclusion, the combination of crystallography, elemental analysis, ReO4− removal experiment, and proton-conductivity measurements on the anion-exchanged samples demonstrates that SCU-7 is truly the first reported uranyl-based cationic framework material with decent anion-exchange capabilities. More importantly, the SBUs formed by hydrolysis of uranyl units may indeed by utilized for the construction of a variety of functional materials with controlled topologies and structures, which may find useful applications for dealing with mobile fission products during waste partitioning and contamination remediation processes.
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for financial support of this work. Support for T.E.A.-S. was provided by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, Heavy Elements Chemistry Program, U.S. Department of Energy, under Grant DE-FG02-13ER16414.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00930. Experimental methods, PXRD, scanning electron microscopy−EDX, bond distances and angles, and ac impedance analysis (PDF) X-ray crystallographic file (CIF)
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Author Contributions §
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the National Science Foundation of China (Grants 91326112, 21422704, 21471107, and U1532259), the Science Foundation of Jiangsu Province (Grants BK20140303, BK20140007, and 1501156B), “Young Thousand Talented Program”, and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions C
DOI: 10.1021/acs.inorgchem.6b00930 Inorg. Chem. XXXX, XXX, XXX−XXX
