Postsynthesis Modification of a Metallosalen-Containing Metal

Sep 29, 2017 - This work was supported by the “Hundreds Talents Program”, the National Natural Science Foundation of China (21571179), and the Sci...
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Postsynthesis Modification of a Metallosalen-Containing Metal− Organic Framework for Selective Th(IV)/Ln(III) Separation Xiang-Guang Guo,†,‡ Sen Qiu,†,‡ Xiuting Chen,§,∥ Yu Gong,§ and Xiaoqi Sun*,†,‡ †

Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China Xiamen Institute of Rare Earth Materials, Haixi Institute, Chinese Academy of Sciences, Xiamen 361021, P. R. China § Department of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, P. R. China ∥ University of Chinese Academy of Sciences, Beijing 100049, P. R. China ‡

S Supporting Information *

ABSTRACT: An uncoordinated salen-containing metal− organic framework (MOF) obtained through postsynthesis removal of Mn(III) ions from a metallosalen-containing MOF material has been used for selective separation of Th(IV) ion from Ln(III) ions in methanol solutions for the first time. This material exhibited an adsorption capacity of 46.345 mg of Th/ g. The separation factors (β) of Th(IV)/La(III), Th(IV)/ Eu(III), and Th(IV)/Lu(III) were 10.7, 16.4, and 10.3, respectively.

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safety concerns, there is a tremendous need for the development of a highly efficient method for separating and recovering thorium.4 Thorium is difficult to separate from RE elements because its properties are similar to those of RE elements. Solvent extraction is a widely used method for the industrial separation of Th from RE elements. Various extractants such as organophosphorus molecules were studied for the extraction and separation of Th and RE elements.5 For this purpose, the disadvantages of liquid to liquid extraction were in the use of large amounts of volatile organic compounds in the preconcentration and separation of low-level radioactive thorium. Hence, the development of a high-performance material for the separation of Th is significant. Solid phase materials for extraction may be the most probable candidates for solving their trace analysis problem. Various materials, including metal−organic frameworks (MOFs), imprinted polymers, and zeolites, were employed to separate, remove, and recover the radionuclides.6−8 MOFs make up a class of hybrid materials composed of organic ligands coordinated to metal ions or metal oxygen clusters, showing potential applications in selective gas and metal ion adsorption, separation, etc.9−11 Compared to traditional inorganic materials, MOFs, which are easily structurally elucidated by diffraction methods and have a tunable pore size and controllable functionalities, can facilitate their rational design and the explanation of structure−property relationships.12 To date, there have been several studies of the application of the MOFs in uranium adsorption, and Sun and

atural thorium is a very important element for nuclear energy fuel as thorium is transformed into 233U when it

Scheme 1. Synthesis of Mn-MOF and Demetalation of Mn(III) Ions

absorbs slow neutrons.1 On the other hand, thorium is a toxic, heavy, hazardous metal that is radioactive, causing environmental problems.2 Thorium and rare-earth (RE) elements often coexist in their minerals, even in wastewater. Thorium in RE waste residues should be recovered to prevent radioactive pollution and meet the radioactive limitation of the waste residues.3 Because of its usage in nuclear energy and health and © XXXX American Chemical Society

Received: July 19, 2017

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DOI: 10.1021/acs.inorgchem.7b01835 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Photographs of (a) Mn-MOF, (b) dMn-MOF, and (c) dMn-MOF remetalated with Cu(II) ions.

dMn-MOF showed peaks at 7.76 and 7.29 corresponding to the tetratopic ligand, indictaing that the acid ligand also exists in the dMn-MOF.

co-workers reported a magnetic nanocomposite for selective UO22+/Ln3+ separation.13−16 Very recently, Wang and coworkers reported several studies of nuclear waste partitioning and radioactive remediation using MOFs.17−23 However, studies of MOFs with respect to the selective separation of thorium from lanthanide ions have rarely been reported. To date, there have been several studies about incorporation of a chiral Mn−salen catalyst into MOFs for asymmetric organic transformations.24,25 To the best of our knowledge, this is the first time that a salen MOF was used in selective Th(IV)/ Ln(III) separation. Schiff bases such as N,N′-bis(salicylidene) ethylenediamine (H2salen) have been used to separate actinicles from lantanides, because of soft-donor nitrogen atoms with preferable coordination to softer actinicles [according to hard and soft acid base theory (HSAB)].26,27 During the synthesis of MOFs, the Schiff base was always coordinated to metals. MOFs with noncoordination sites to metal ions were not easily obtained through direct hydrothermal or solvothermal methods. Therefore, to obtain the MOFs with a potential metal coordination site, postmodification was usually adopted. Postsynthesis modification has recently emerged as a useful strategy for preparing a variety of MOFs that would be difficult to produce by other methods.28 Hupp and co-workers reported the “de-manganation” of MnIIISO-MOF crystals that was achieved by soaking them in methanol and then treating them with aqueous H2O2 overnight in a water/methanol solution to afford almost complete demetalation of Mn(III) ions.29,30 Along with this idea, uncoordinated salen-containing MOF was obtained through postsynthesis removal of Mn(III) ion from a metallosalencontaining MOF, and this material has been used for selective separation of Th(IV) ion from Ln(III) ions for the first time. A representation for the postsynthetic process and selective adsorption is shown in Scheme 1. In this Article, Mn-MOF was obtained from a combination of chiral Mn(salen) struts and the tetratopic ligand 1,4-dibromo2,3,5,6-tetrakis(4-carboxyphenyl)benzene, which features a structure similar to that reported for MnSO-MOF, with only 1,4-dibromo-2,3,5,6-tetrakis(4-carboxyphenyl)-benzene used instead of 1,2,4,5-tetrakis(4-carboxyphenyl)benzene. Singlecrystal diffraction analysis revealed that the obtained brown crystals had unit cell parameters (a = 11.20 Å, b = 15.77 Å, c = 26.62 Å, and α = β = γ = 90°) similar to the data reported by Hupp et al. Zn(II) ions were bridged by DBTCB4− to obtain a two-dimensional layer structure and further by the Mn(salen) ligand to obtain a three-dimensional structure. Brown color indicates the presence of Mn(III) ions, upon treatment with H2O2, a dramatic color change from dark to bright yellow in the crystals, suggesting retention of the salen molecules. On the other hand, nuclear magnetic resonance (NMR) analysis of

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REMOVAL OF Mn(III) IONS A mixture of hydrogen peroxide and methanol [1/1 (v/v)] was used to remove Mn(III) ions from MOFs, and the mixture was placed in a mechanical shaker for 12 h at 298 K. The depletion of the Mn of the MOF was accompanied by a change in the color of the crystals from brown to bright yellow, suggesting retention of the salen ligands despite the loss of Mn(III) ions. After being decanted and rinsed with methanol, the crystals were then washed with methanol for 24 h via Soxhlet extraction. After treatment of dMn-MOF with a stock solution of anhydrous copper chloride in a methanol solution, the color of the crystals changed from light yellow to green, featuring the color of Cu(II) ions. The obvious change in color could be observed as shown in Figure 1. The color of the crystals changed from brown (Figure 1b) to yellow and green after they had been soaked in copper chloride solutions, as reported for MnSO-MOF by Hupp et al.30

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SEPARATION OF Th(IV) ION FROM A RARE-EARTH SOLUTION qe =

CV mMOF

(1)

Kd =

Cad Ce

(2)

31

β=

Kd(Th) Kd(Re)

(3)

where qe (milligrams per gram) is the amount of metal ion adsorbed on the adsorbent, C (milligrams per liter) is the metal ion concentration of the H2SO4/H2O2 solution dissolved with the absorbed MOF, V is the volume of the solution, mMOF is the mass of dMn-MOF, Kd is the equilibrium constant, Cad (milligrams per liter) is the adsorbed concentration of metal ion, Ce (milligrams per liter) is the equilibrium concentration of metal ion in solution, and β is separation factor between Th(IV) and RE ion(III). The dMn-MOFs were used for selective adsorption of Th(IV) from a mixed rare-earth methanol solution. To study the selectivity between Th(IV) and Ln(III) ions, we chose one each of the light, middle, and heavy lanthanides, i.e., La(III), Eu(III), and Lu(III), respectively, as the representatives. Therefore, the competitive adsorption of Th(IV)/La(III), Th(IV)/Eu(III), and Th(IV)/Lu(III) from their coupled B

DOI: 10.1021/acs.inorgchem.7b01835 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Th(IV), La(III), Eu(III), and Lu(III) ions, placed on a shaker for ∼18 h, an ICP test was conducted when the absorbed MOF was dissolved in the H2SO4/H2O2 solution after being rinsed several times with CH3OH. As shown in Figure 2, the qe of Th(IV) is as high as 46.3 mg/g, while the qe values of La(III), Eu(III), and Lu(III) are merely 3.54, 2.54, and 0.84 mg/g, respectively. Besides, the separation factors (β) of Th(IV)/ La(III), Th(IV)/Eu(III), and Th(IV)/Lu(III) could be 10.7, 16.4, and 10.3, respectively. The qe and β values were higher than those of most of the other adsorbents such as functionalized mesoporous silica (19.2 mg/g),32 fiber-reinforced polymer (24.43 mg/g),33 impregnating resin XAD-7 (1.7 mg/g),34 ethylamine-bridged cyclodextrin (10 mg/g),35 and aluminum ores (0.98 mg/g).36 The thermodynamic parameters were calculated according to eqs 4 and 5, and the results are listed in Table 1. Those thermodynamic parameters were acquired at different temperature (301, 308, and 318 K).

Figure 2. Adsorption capacities of Mn-MOF for Th(IV), La(III), Eu(III), and Lu(III). The inset shows separation factors between Th and RE ion.

ΔG° = −RT ln Kd

Table 1. Estimation of Thermodynamic Parameters for the Adsorption of Th4+ ions (Cini = 0.0015 mol/L) T (K)

Kad

ΔG° (kJ/mol)

ΔH° (kJ/mol)

ΔS° (J mol−1 K−1)

301 308 318

0.572 0.443 0.231

1.398 2.084 3.882

−43.2

−147.9

ln Kd =

(4)

ΔS° ΔH ° − R RT

(5) −1

−1

where Kd is the equilibrium constant, R (8.314 J mol K ) is the gas constant, and T (kelvin) is the temperature. ΔH° and ΔS° were obtained from the slope and intercept of the van’t Hoff plot of ln Kd versus 1/T. The enthalpy is negative, which suggests that the adsorption of Th4+ to MOF was an exothermic sorption process. Hence, if we want to improve the adsorbing capacity, a low temperature

mixture (25 mL) was investigated. When a certain amount of dMn-MOF was added to the methanol solution of equimolar

Figure 3. ESI-MS of dMn-MOF after adsorption with Th(IV) and Ln(III) ions. C

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would be an alternative. The negative value of entropy shows the decreased degree of disorder of the adsorption system, which is caused by the range of thorium ion movement being restricted by the adsorbent solid surface after the adsorption.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Hundreds Talents Program”, the National Natural Science Foundation of China (21571179), and the Science and Technology Service Network Initiative of the Chinese Academy of Sciences (y7f6061gg-xt).

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ELECTROSPRAY IONIZATION STUDIES OF THE EXTRACTION COMPLEX OF Th(IV) ION Electrospray ionization (ESI) is a very useful tool for the analysis of metal−organic complexes that allows the sample molecules to be transferred directly from the solution to the gas phase through the generation of charged ions without the disruption of metal−ligand bonds. As shown in Figure 3, the m/z 409.17 peak represents the complex of one Th(IV) ion and one salen ligand, [Th(salen)]2+. To supplement the studies of Th(IV) and a salen molecule by Hill and Rickard,37,38 we postulated that the absorbed Th(IV) ions were coordinated to the salen ligand via the N, O chelating method when involved in the adsorption.

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ENERGY DISPERSIVE SPECTROSCOPY RESULTS The surface morphology and chemical composition of the MOF sample material after adsorption of Th(IV) ions were characterized by scanning electron microscopy and energy dispersive spectroscopy (EDS). As shown in Figure S8, the crystals of as-synthesized samples erode somewhat after adsorbing Th(IV) ions. The contents given by the EDS spectra of Th(IV) and Zn(II) are 1.144 and 1.548%, respectively, are higher than the ion ratio of Mn(III) and Zn(II) in the MOF structure. In summary, the MnSO-MOF analogue with Zn2 paddle wheel units and a chiral salen ligand as pillars was synthesized with a tetratopic ligand and Mn(III) salen struts. The selective demetalation of Mn(III) was achieved by treating the crystals with H2O2. Salen-containing MOF material was first studied for the separation of Th(IV) from Ln(III) ions. This material exhibited an adsorption capacity of 46.345 mg of Th/g, and the separation factors (β) of Th(IV)/La(III), Th(IV)/Eu(III), and Th(IV)/Lu(III) reached 10.7, 16.4, and 10.3, respectively, which are higher than those of many of the reported absorbents for Th(IV). EDS results revealed the adsorption of Th(IV) ions, and ESI-MS indicated that Th(IV) ion was coordinated with salen ligand via N, O chelating mode. This work expanded the application of MOFs in the separation of thorium from rare-earth elements.

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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.7b01835. Synthesis for the ligands, Mn-MOF, NMR of ligands, the experimental procedure for separation, TGA and PXRD patterns of dMn-MOF and after adsorption of Th(IV) ions, and EDS results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yu Gong: 0000-0002-8847-1047 Xiaoqi Sun: 0000-0003-3490-3673 D

DOI: 10.1021/acs.inorgchem.7b01835 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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