Communication pubs.acs.org/IC
Hydrolytically Stable Nanoporous Thorium Mixed Phosphite and Pyrophosphate Framework Generated from Redox-Active Ionothermal Reactions Daxiang Gui,†,‡,∥ Tao Zheng,†,‡,∥ Lanhua Chen,†,‡ Yanlong Wang,†,‡ Yuxiang Li,†,‡ 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 *
and phosphate groups,6 providing important clues for safe storage of transuranium wastes. Meanwhile, phosphite-containing trivalent phosphorus also can strongly bind to actinides but is easily oxidized. Actinide phosphite compounds such as Sr[(UO2)(HPO3)2]·2H2O and actinide mixed phosphate/phosphite compounds such as Cs4[(UO2)8(HPO4)5(HPO3)5]·4H2O can be obtained via traditional hydrothermal reactions with phosphoric acid as the starting reagent.7 These reactions sometimes yield actinide phosphate compounds, such as Th(HPO4)2(H2O), etc.8 In some cases, phosphoric acid is used as a reducing agent to afford actinide compounds with low valence states, such as PuIII2(HPO3)3(H2O).9 Herein we report the first open-framework compound of thorium(IV) mixed phosphite/pyrophosphate, [BMMim]2[Th3(PO3)4(H2P2O7)3] (ThP-1) (BMMim = 1-butyl-2,3dimethylimidazolium), generated from the ionothermal reaction of thorium nitrate and phosphoric acid. Pyrophosphate was formed in situ and took part in coordination and crystallization. The formation of pyrophosphate was achieved by a combination of partial oxidation of phosphoric acid to afford phosphates and thermal dehydration under the high-temperature and lowhumidity conditions.10 Single-crystal X-ray diffraction (XRD) analysis revealed that ThP-1 crystallizes in the centrosymmetric cubic space group Pm3̅m. The asymmetric unit of ThP-1 consists of a thorium cation coordinated by a phosphite group and a pyrophosphate group. The thorium center is eight-coordinated, adopting a coordination geometry of a standard D4d square antiprism. As shown in Figure 1c, within the coordination sphere, four oxygen atoms (O1, O1A, O1B, and O1C) are provided by four different phosphite ligands while the other four oxygen atoms (O2, O2B, O2D, and O2E) come from two separated chelating pyrophosphate ligands. Each phosphate ligand bridges three adjacent thorium centers with an umbellately distorted triangle configuration (Figure 1a), further leading to a unique Th6P8 cluster with an overall octahedral geometry (Figure 1d), contrasting sharply with the traditional hexanuclear thorium cluster formed by hydrolysis and connected by μ3-OH, μ2-O, or μ3-O groups.11 The pyrophosphate ligands bridge and chelate
ABSTRACT: The first thorium framework compound with mixed-valent phosphorus-based (phosphite and pyrophosphate) ligands, [BMMim] 2 [Th 3 (PO 3 ) 4 (H2P2O7)3] (ThP-1), was synthesized by ionothermal reactions concurrent with the partial oxidation of phosphoric acid. The overall structural topology of ThP1 highly resembles that of MOF-5, containing only one type of three-dimensional channels with a window size of 11.32 Å × 11.32 Å. ThP-1 has a free void volume of 50.8%, making it one of the most porous purely inorganic actinide-based framework materials. More importantly, ThP-1 is highly stable in aqueous solutions over an extremely wide pH range from 1 to 14 and thus may find potential applications in selective ion exchange and catalysis.
O
pen-framework compounds are an important research area for their applications in molecular sieves, catalysis, gas adsorption, and ion exchange.1 The majority of recent research on open-framework compounds has mainly been focused on the preparation and functionalization of hybrid metal−organic frameworks (MOFs) and their organic analogues, covalent organic frameworks (COFs) and porous aromatic frameworks (PAFs).2 Although substantial progress has been made in these fields during the past two decades, two factors are still the main hurdles for their further industrial applications: long-term stability and synthetic cost. Purely inorganic open-framework compounds, mostly represented by zeolites or zeolite analogue materials such as transition-metal phosphates, silicates, sulfates, selenites, and borates, possess clear advantages in terms of stability and cost but at the sacrifice of functional group tunability.1a,b,3 Compared with the number of transition-metal and lanthanide open-framework compounds reported,3b actinide open-framework compounds are rather scarce, despite the fact that they may have applications in selective ion exchange for nuclear waste reprocessing and remediation.4 Because of their low solubility and high thermal stability, actinide phosphate compounds possess potential applications in geological repositories of nuclear wastes.5 Large amounts of geologically stable actinide phosphate minerals has been discovered, originated from the high affinity of actinide cations © 2016 American Chemical Society
Received: February 3, 2016 Published: March 25, 2016 3721
DOI: 10.1021/acs.inorgchem.6b00293 Inorg. Chem. 2016, 55, 3721−3723
Communication
Inorganic Chemistry
(XPS), and the P 2p spectrum and the fitting data are shown in Figure S3 and Table S4. The XPS spectrum was fitted with two components for P(V) and P(III), for which the P 2p3/2 bond energies (BEs) are 131.12 and 133.20 eV, respectively. These can be assigned to pyrophosphate (−P2O7) (132.9 ± 0.4 eV) and phosphite (−PO3) (134.3 ± 0.3 eV), respectively, indicating that both the P(V) and P(III) valence states are present in ThP-1.15 In addition, the P(V)/P(III) 2p3/2 peak area ratio is 1.4:1, consistent with the ratio of 3:2 in the molecular formula determined by X-ray crystallography. Typical antisymmetric and symmetric stretch vibration peaks of PO3 and PO4 groups were observed in the range of 1409−995 cm−1 in the FTIR spectrum (Figure S2), while the P−O−P vibrations of the P2O7 group are found at 943 and 741 cm−1.10b Powder XRD (PXRD) was performed on the samples separated manually, and the pattern was consistent with that calculated from the single-crystal structure (Figure 2). Hydro-
Figure 1. (a−c) Coordination modes of (a) phosphate, (b) pyrophosphate, and (c) thorium cation. (d) Hexanuclear clusters achieved by phosphite ligands. (e) 3D framework structure achieved by pyrophosphate ligands. (f) Overall framework structure of ThP-1. Color scheme: Th, green; O, purple; P, orange). (g) Simplified topology containing Th6P8 clusters as the nodes (shown as red balls) and pyrophosphate groups as the linkers (shown as black lines).
two thorium centers as vertexes from two neighboring Th6P8 clusters via four oxygen atoms (O2, O2B, O2D, and O2E), as shown in Figure 1b. Every two adjacent Th6P8 clusters are bridged by two parallel pyrophosphate ligands. As a result, each Th6P8 cluster is coordinated by 12 bridging pyrophosphate ligands at six vertexes, further leading to a nanoporous threedimensional network (Figure 1e,f). The overall framework is negatively charged, with [BMMim]+ cations from the ionic liquid filling the open space to balance the charge. However, these cations could not be located in the electron density map because of the severe disorder, indicating their exchange capabilities. From the topological point of view, the Th6P8 clusters in ThP1 act as six-connected nodes and are further linked by twoconnected pyrophosphate ligands, resulting in a structural topology identical to that of MOF-5, one of the earliest and most investigated MOF structures.12 Th6P8 clusters show the same connection as the Zn4O units in MOF-5 as six-connected nodes, while the pyrophosphate ligands play the same role as terephthalic acid in MOF-5 in bridging the nodes to achieve the 3D framework structure. The windows of the channels along the three axes are identical, with dimensions of 11.32 Å × 11.32 Å (excluding the van der Waals radii of oxygen atoms) in ThP-1, which are smaller than those in MOF-5 (15.1 Å × 11.0 Å). Therefore, the porosity of ThP-1 (50.8%) is slightly lower than that of MOF-5 (55%), as calculated using PLATON.13 Compared with other reported actinide open-framework compounds,14 the window size of ThP-1 is the largest and the porosity is the highest among all except [HNC4H9]2[(UO2)6(SO4)7(H2O)2] (see Table S8). Elemental analysis was performed using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), and the measured Th:P atomic ratio was 1:2.98, highly consistent with the crystallographic results. The carbon and nitrogen peaks in the EDS spectrum demonstrate the presence of [BMMim]+ cations in the structure (Figure S4). Meanwhile, the valence state of phosphorus was studied by X-ray photoelectron spectroscopy
Figure 2. Simulated and experimental PXRD patterns for ThP-1 and for ThP-1 soaked in aqueous solutions at pH 1, 7, and 14 for 72 h.
lytic stability tests of ThP-1 were also carried out in aqueous solutions with pH ranging from 1 to 14. PXRD patterns measured on soaked samples demonstrate that ThP-1 maintains its crystallinity at all pH values tested (Figure 2), showing a clear advance over MOF-5, which is not stable even in the moisture. This is surprising given that both pyrophosphate and phosphite are unstable at ambient conditions because of their intrinsic tendencies to be hydrolyzed and oxidized, respectively. In view of the 3D channels that pierce the whole structure, the highly disordered [BMMim]+ cations within the channels, and the decent hydrolytic stability over an extremely wide pH range, ThP-1 is well-suited as an ion-exchange material for remediation of other heavy-metal cations such as uranium and its fission products, including cesium, in nuclear wastes. As an initial study, upon soaking of ThP-1 crystals in an aqueous solution with a moderate Cs+ concentration (500 ppm), 100% of [BMMim]+ originally present in the structure was rapidly substituted with Cs+, as demonstrated by the Th:Cs atomic ratio of ∼1.2 in the Cs-exchanged crystals as measured by EDS analysis (Figure S5), indicating a high exchange capacity toward cesium. Similar ionexchange experiments were also carried out for UO22+, and the EDS results showed a Th:U atomic ratio of ∼3 in the exchanged crystals, confirming the complete ion-exchange process. Impressively, the uranyl-exchanged crystals of ThP-1 exhibited an intense greenish emission stemming from the intrinsic HOMO−LUMO charge transfer of the uranyl cation upon excitation with 365 nm light (Figure 3). The emission feature is highly vibration-coupled. The exchanged crystals could be cut, and the interior showed the same emission color as the surface, 3722
DOI: 10.1021/acs.inorgchem.6b00293 Inorg. Chem. 2016, 55, 3721−3723
Communication
Inorganic Chemistry
T.E.A.-S. was supported 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 (DE-FG02-13ER16414).
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Figure 3. Fluorescence spectra and microscopic photos of ThP-1 before and after exchange with uranyl ions and subsequent displacement by Cs cations excited under 365 nm light.
confirming the replacement of [BMMim]+ with UO22+ through the whole structure. In addition, when these uranyl-exchanged crystals were soaked in a concentrated Cs+ solution, the uranyl cations were reversibly exchanged out, as confirmed by emission (Figure 3) and EDS (Table S7) measurements. In conclusion, the first thorium mixed phosphite/pyrophosphate compound, with a highly open framework structure and the largest pore size among inorganic actinide porous compounds, has been obtained. ThP-1 is a rare example of an inorganic structural analogue of MOF-5, and its hydrolytic stability shows a clear advance, as it maintains its crystallinity over an extremely wide pH range from 1 to 14. Initial investigations indicated a superior ion-exchange capability of ThP-1, and other functions such as size-selective catalysis can also be expected. Finally, the synthetic strategy of redox-active ionothermal reactions is also expected to yield more functional materials with interesting structures and properties for their combined capabilities to generate new in situ ligands, direct the structure using uncoordinated cations or anions as templates, and effectively avoid hydrolysis/solvation of hard metal cations.
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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.6b00293. Methods and additional data (PDF) Crystallographic data for ThP-1 (CIF)
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AUTHOR INFORMATION
Corresponding Authors
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D.G. and T.Z. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (91326112, 21422704, and 21471107), the Science Foundation of Jiangsu Province (BK20140303 and BK20140007), “Young Thousand Talented Program”, and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for financial support of this work. 3723
DOI: 10.1021/acs.inorgchem.6b00293 Inorg. Chem. 2016, 55, 3721−3723
