Immobilization of Alkali Metal Fluorides via Recrystallization in a

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Immobilization of Alkali Metal Fluorides via Recrystallization in a Cationic Lamellar Material, [Th(MoO4)(H2O)4Cl]Cl·H2O Jian Lin,*,† Hongliang Bao,† Meiying Qie,†,‡ Mark A. Silver,§ Zenghui Yue,†,‡ Xiaoyun Li,† Lin Zhu,§ Xiaomei Wang,†,‡ Linjuan Zhang,† and Jian-Qiang Wang*,† †

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 2019 Jia Luo Road, Shanghai 201800, China University of Chinese Academy of Sciences, 2019 Jia Luo Road, Shanghai 201800, China § School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, 199 Ren’ai Road, Suzhou 215123, China ‡

S Supporting Information *

intriguing due to their structural diversity and stability under environmentally relevant conditions. Tetravalent metal ions display coordination numbers that approach or exceed 12, commonly observed for Th4+, and are capable of utilizing various frontier orbitals in bonding.20,21 Hyperpolarizable oxyanions can adopt multiple geometries and polymerize, forming more polarizable anions and tunable structures, thereby making them appealing structure-directing agents for synthesizing cationic materials.22 This is exemplified by the thorium borate [ThB5O6(OH)6][BO(OH)2]·2.5H2O (NDTB1), which can selectively remove TcO4− from nuclear waste streams.1 Furthermore, our research efforts when using tellurite as the ligand have successfully given rise to cationic frameworks [Ce2Te7O17]X2 (X = Cl, Br)23 and cationic layered structures [M2Te4O11]X2 (M = Pu, Ce, Zr; X = Cl, Br).24 Unfortunately, the halides in neither of these structures can be exchanged with other anions, due to the electrostatic locking effect generated from the nonbonded electron pairs on Te4+. A similar effect and resulting unexchangeable nature of Cl− have been observed in the 2D cationic network [Th3O2(IO3)5(OH)2]Cl, indicating that multiple strong halogen−halogen interactions actually inhibit ion exchange.25 Aiming to develop cationic extended structures with ion-exchange capacities, we chose molybdate as a ligand because the oxo-anion lacks nonbonding electron pairs and is capable of forming an open-framework, e.g., Ce3Mo6O24(H2O)4.26 Hydrothermal reactions between Th(NO3)4 and MoO3 with concentrated HCl at 230 °C resulted in the formation of a cationic lamellar material, [Th(MoO4)(H2O)4Cl]Cl·H2O (TMC). Single crystal X-ray diffraction reveals that TMC crystallizes in the space group P21/m (cf. Table S1) and adopts a cationic lamellar topology as shown in Figure 1a. The structure consists of zigzagging [Th(MoO4)(H2O)4Cl]+ layers that propagate parallel to the ab plane (Figure 1b) with dissociated Cl− residing within the interlamellar spacing as charge-balancing anions, in addition to hydrating water molecules. The interlayer distance between cationic layers is approximately 4.7 Å, and Cl(2)− ions inhabit the pockets of each layer. There is no covalent bond between Cl(2)− and any atoms that form the layers. The [Th(MoO4)(H2O)4Cl]+ layers

ABSTRACT: Searching for cationic extended materials with a capacity for anion exchange resulted in a unique thorium molybdate chloride (TMC) with the formula of [Th(MoO4)(H2O)4Cl]Cl·H2O. The structure of TMC is composed of zigzagging cationic layers [Th(MoO4)(H2O)4Cl]+ with Cl− as interlamellar charge-balancing anions. Instead of performing ion exchange, alkali thorium fluorides were formed after soaking TMC in AF (A = Na, K, and Cs) solutions. The mechanism of AF immobilization is elucidated by the combination of SEM-EDS, PXRD, FTIR, and EXAFS spectroscopy. It was observed that four water molecules coordinating with the Th4+ center in TMC are vulnerable to competition with F−, due to the formation of more favorable Th−F bonds compared to Th−OH2. This leads to a single crystal-to-polycrystalline transformation via a pathway of recrystallization to form alkali thorium fluorides.

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ationic frameworks or lamellar materials are of interest owing to their potential applications in ion exchange to trap anionic pollutants, including small oxo-anions such as chromate, dichromate, permanganate, and pertechnetate, as well as large anionic carcinogenic dyes.1−7 However, most materials in nature are composed of either neutral or anionic extended structures, and the charges of the latter are balanced by cations, like alkali-metal or quaternary ammonium cations.8 As a result, great effort has been investigated to prepare cationic materials with extended structures.9,10 One of the most wellknown families of such materials is layered double hydroxides (LDHs). A number of LDHs with various interlamellar anions, including chloride,11 nitrate,12 carbonate,13 sulfate,14,15 disulfonate,4−7 etc., have been prepared, and they are capable of sequestrating multiple oxo-anions from solutions. Cationic metal−organic frameworks (MOFs) represent another family of materials that exhibit anion exchange, and Wang et al. have recently achieved great success when trapping TcO4− using MOFs.16−19 Our and many other research groups have noted that the combination of large tetravalent metal ions with hyperpolarizable oxo-anions often yields materials with inorganic cationic extended structures. This series of compounds is © XXXX American Chemical Society

Received: April 9, 2018

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DOI: 10.1021/acs.inorgchem.8b00954 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 2. SEM images and EDS elemental maps of (a) a TMC crystal and (b) a TMC crystal after being immersed in 0.1 M CsF solution for 15 h.

detected, and single crystals of TMC transformed into numerous crystallites, despite retaining their original crystal habit (Figure 2, SEM images). Such results suggest that TMC undergoes a single crystal-to-polycrystalline transformation via a pathway of recrystallization. The phase transformations can be further confirmed by both PXRD and FTIR measurements as shown in Figure 3a and b.

Figure 1. (a) Polyhedral representations of TMC viewed down the aaxis revealing stacked [Th(MoO4)(H2O)4Cl]+ layers. (b) A single layer extending along the ab plane. (c) Coordination of Th4+ in TMC. Color code: Th 4+ polyhedral/atoms shown in gray, MoO 4 2− tetrahedral/atoms in light blue, coordinating Cl− in light green, dissociated Cl− in dark green, O in red, and H in pink. Hydration water molecules were omitted for clarity.

are composed of one crystallographically independent Th4+, one MoO42− anion, one Cl−, and two coordination water molecules. The MoO42− anion provides all four O atoms and serve as bridges to connect adjacent Th polyhedra. The coordination environment of Th4+ is composed of four O atoms donated from MoO42−, four O atoms donated from H2O molecules, and one Cl− ion, forming a capped square antiprismatic geometry as shown in Figure 1c. The bond distances of Th−OMo range from 2.378(10) to 2.396(7) Å, and those of Th−OW range from 2.495(7) to 2.520(7) Å, respectively, all of which agree well with the literature.27−29 The presence of water molecules in TMC can be further confirmed by the thermogravimetric analysis, showing that TMC exhibits a weight loss of 16.6% from 100 to 209 °C, which corresponds to the loss of four coordinating and one hydrating H2O molecules (calcd. 16.3%; Figure S1). The bonded Cl(1) ions do not coordinate with Th4+ at first glance; however the distance of Th−Cl(1) is 3.007(4)Å, which is longer than the 2.808(1) Å of Th−Cl in CsTh(MoO4)2Cl and other actinide-Cl bonds.30−33 Bond valence sum (BVS) of Th calculated with solely Th−O bonds is 3.815, and BVS equals 4.106 when both Th−O and Th−Cl(1) bonds are included, suggesting that weak bonding is involved between Th and Cl(1).34 The MoO42− adopts a nearly ideal tetrahedral geometry with an average Mo−O bond distance of 1.753(4) Å. Selected bond distances are given in Table S2. The cationic structure of TMC suggested the possibility of possessing anion-exchange behavior. Anion-exchange experiments were conducted by soaking TMC crystals in 0.1 M AF (A = Na, K, or Cs) solutions for 15 h. Surprisingly, SEM-EDS demonstrated the presence of not just the anion F− but rather the alkali metal as well in the AF-soaked TMC crystals (Figure 2 and Figure S2). Neither MoO42− nor Cl− could be further

Figure 3. (a) PXRD patterns and (b) FTIR spectra of TMC before and after immersion in 0.1 M AF (A = Na, K, and Cs) solutions for 15 h. Peaks from TMC are labeled with ∗ in the pattern of CsF-soaked TMC sample.

The experimental PXRD pattern of TMC is consistent with the calculated one and is obviously different from those of TMC samples after being immersed in 0.1 M AF (A = Na, K, and Cs) solutions. The PXRD patterns of the AF-soaked samples were compared with the theoretical patterns of all published alkali thorium fluoride compounds in the inorganic crystal structure database (ICSD), within which K7Th6F31 (ICSD-2711) has a good agreement with the KF-soaked TMC sample.35 The PXRD patterns of NaF- and CsF-soaked TMC samples do not agree with any published sodium and potassium thorium fluorides, respectively, suggesting that they are either new complexes or mixtures of multiple fluoride species. The phase difference between NaF-, KF-, and CsF-soaked TMC products could be attributed to the variety of ionic radii of the alkali metals, which influence the overall packing of the crystal structures. A similar effect has been observed in a large family of alkali metal−thorium−nitrato molecular complexes.36 The CsF-soaked TMC sample partially retained the characteristic B

DOI: 10.1021/acs.inorgchem.8b00954 Inorg. Chem. XXXX, XXX, XXX−XXX

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Å).39−45 The second coordination shells primarily originate from Th−A or Th−F paths. The presence of K7Th6F31 in the KF-soaked sample can be further confirmed by their similar EXAFS spectra and fitting results (Figure S3 and Table S3). The coordination number of Th−F is nine, and the Th−F bond distance ranges from 2.30 ± 0.02 Å to 2.39 ± 0.02 Å for K7Th6F31, both of which are highly consistent with values for KF-soaked TMC (2.29 ± 0.02 Å to 2.40 ± 0.02 Å). In conclusion, we have introduced a novel cationic thorium molybdate chloride lamellar material by combining large Th4+ ions with hyperpolarizable molybdates. TMC is capable of immobilizing alkali metals from solutions, and the mechanism of uptake was illustrated by the combination of SEM-EDS, PXRD, FTIR, and EXAFS studies. The Th−H2O connectivity from the Th4+ center of TMC is vulnerable to replacement by F−, due to the formation of more favorable Th−F bonds, resulting in a single crystal-to-polycrystalline transformation via recrystallization. By forming alkali metal thorium fluoride species, this mechanism may provide an alternative route of dealing with the highly radioactive fission product, 137Cs. Although TMC is not reusable compared to other ion-exchange materials, trapping such a highly problematic radionuclide permanently without a regeneration process would in fact be desirable.

peaks of TMC, though the formation of cesium thorium fluoride onto the crystal surface may have inhibited further reaction between TMC and CsF. The presence of water molecules in TMC can be confirmed by the stretching and bending vibration bands of O−H at 3143 and 1597 cm−1, respectively.37 The vibrational spectroscopy of MoO42− shows bands appearing at 928 cm−1 (ν1 stretching), 768 and 710 cm−1 (ν3 stretching), and 557 cm−1 (ν4 bending mode).38 The NaFand KF-soaked TMC materials did not show obvious characteristic bands of water and MoO42−, and new peaks corresponding to the fluoride species occur at ∼800 and ∼840 cm−1. Water peaks can be observed in CsF-soaked TMC due to the presence of unreacted TMC as confirmed by PXRD. The uptake mechanism was further studied by extended Xray absorption fine structure (EXAFS) spectroscopy, of which spectra were collected in transmission mode for TMC and AFsoaked TMC samples. Spectra of K7Th6F31 were collected as well for comparison with KF-soaked TMC. As shown in Figure 4, differences in the radial distribution function between TMC

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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.8b00954. Synthesis, crystallographic studies, SEM images and EDS spectra, EXAFS analysis, TGA and DSC analysis, EXAFS spectra of K7Th6F31, crystallographic data, selected bond distances, and structural parameters obtained from the EXAFS spectra (PDF) Accession Codes

CCDC 1835814 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Figure 4. (a) The k3-weighted Th L3-edge experimental χ(k) data and (b) the corresponding Fourier transforms of TMC and AF-sorbed TMC samples.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

and AF-soaked TMC samples can be clearly observed. The Fourier transformed data of TMC show two main peaks developing at 2.3−2.6 Å and 3.8−4.2 Å, respectively, indicating the existence of two coordination shells around Th (Figure 4b). Theoretical fittings indicate that the first shell is contributed from eight coordinating O atoms, while the second shell could be attributed to the Th−Cl/Mo paths, both of which agree well with the bonding distances from the SCXRD measurement (Table S3). For all AF-soaked TMC samples, the intensities of the first coordination shells are significantly higher than those of second coordination shells. These strong peaks for NaF-, KF-, and CsF-soaked samples can be well fitted to nine F atoms with Th−F bonding distances ranging from 2.29(2) to 2.48(2)

Jian Lin: 0000-0002-3536-220X Mark A. Silver: 0000-0002-2285-3616 Jian-Qiang Wang: 0000-0003-4123-7592 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the National Natural Science Foundation of China (U1532259, 21701184), the Shanghai Pujiang Program (17PJ1410600), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA02040600, XDA21080200). C

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