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Water-stable MOF Material with Uncoordinated Terpyridine Site for Selective Th(IV)/Ln(III) Separation Ying Xiong, Yun Gao, Xiangguang Guo, Yanliang Wang, Xiang Su, and Xiaoqi Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04875 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on January 4, 2019

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ACS Sustainable Chemistry & Engineering

Water-stable MOF Material with Uncoordinated Terpyridine Site for Selective Th(IV)/Ln(III) Separation

Ying Xionga, Yun Gaoa,b, Xiangguang Guob, Yanliang Wangb, Xiang Sub, Xiaoqi Sunb,c*

aCollege

of Chemistry, Liaoning University, No. 66 Chongshan Middle Road, Huanggu District, Shenyang, Liaoning 110036, P. R. China bState

Key Laboratory of Design and Assembly of Functional Nanostructures and Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, No. 155 Yangqiao West Road, Gulou District, Fuzhou, Fujian 350002, P. R. China. cGanzhou

Rare Earth Group Co., Ltd., No. 1 Ganxian Road, Ganxian District, Ganzhou, Jiangxi 341000, P. R. China.

*Corresponding author: X.Q. Sun. Tel.: +86-592-3594019. E-mail: [email protected]

1

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ABSTRACT Here in this article, [Ln(μ2–OH)(dtp)(H2O)][Ln=Ho(III) and Tb(III)] is prepared using a solvothermal method to assemble the rare earth (RE) ion and the H2dtp ligand (H2dtp = 4’-(3, 5-dicarboxyphenyl)-4,2’:6’4’’-terpyridine). The obtained metal-organic framework (MOF) materials were characterized with X-ray diffraction (XRD), infred spectrum (IR), thermogravimetric analysis (TGA). The prepared two MOF exhibits 2D layer structures, respectively. To our knowledge, the Ln-MOF material with uncoordinated terpyridine site has never been developed for Thorium (Th) separation. Therefore, we attempted to use these MOF materials for the purpose of separating Th(IV) from REs in the aqueous phase. The separation factors of Th/Y, Th/La, Th/Ce, Th/Eu, Th/Lu reach 21, 19.2, 18.9, 7.5, 6.2, respectively. Moreover, the MOF materials could remain stable until 500 °C, which exhibit good selectivities and stabilities for the Th(IV)/REs(III) separation.

KEYWORDS: Metal-organic framework; Crystal structure; Water stability; Lanthanide separation

INTRODUCTION The main pollutants in nuclear waste and industrial effluents which include radioactive metal ions, toxic heavy metal cations and oxyanions have created a serious threat to humans and other species. Unlike the other pollutants, radioactive metal ions are hard to be degraded into eco-friendly substances. The metal ions accumulate in living organisms, and they are proverbially highly toxic or carcinogenic.1-5 Thorium (Th) is a typical radioactive element. Th is considered as a nuclear fuel for the reason that it could be converted to uranium-233. Th is always coexisted with REs in monazite, bastnasite, even in waste water.6-9 Although the purification of Th is of the essence for dealing with 2


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waste products generated from nuclear energy, the purification is difficult owing to the similar property between Th and REs. Due to the applications of Th in energy of nucleus, health and security problem, there is an urgent requirement to develop effective methods to separate and recover of Th. Over the past few decades, solvent extraction method and ion-exchange method have been broadly used for separating Th in industry. The particular amino-bearing extractants have more preferential affinity for Th than REs.10-12 However, concerns associated with extraction are not only the large amount of used solvent but also the liquid waste contaminated with Th, the waste disposal is still a problem. Ion-exchange method has some disadvantages as well, although a higher separation factor can be obtained, it is difficult to achieve large scale and continuous operation. The above mentioned methods are unsuitable for the separation of Th with trace low concentration. In view of these problems, some other approaches have been developed to separate Th from REs. Solid adsorption material maybe a sustainable candidate for solving these problems. For the purposes of separating radioactive elements, some functional materials have been prepared, i.e., metal organic frameworks, imprinted polymers, magnetic composite particles and zeolites.13-15 For one thing, the solid phase materials are better separated from solution by simple filtration. For another thing, it reveals some advantages, such as high separation efficiency, low equipment requirement and large adsorption capacity. Among the solid phase materials, MOF materials have attracted widely attention because of their structural regularity, ease of design and structural characterization. Metal organic frameworks (MOFs) are well-organized crystalline polymers, comprising of metal ions and organic ligands connected via coordination bonds linked into one-, two-, or three-dimensional networks. MOFs is well established as one of the most widely investigated materials due to their various structures, adjustable cavities, feasible modifications, well 3



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stabilities16-19 and can be employed in gas separation and storage,20-24 heterogeneous catalysis,25, 26 adsorption,27 drug delivery28 and sensing application.29,30 Organic linkers, such as amines, carboxylates, pyridines, sulfonates and phosphates, are key components for the syntheses of MOFs. There are several studies of MOFs in the adsorption processes for the selective UO22+/Ln3+ separation.31-36 The study in regard that the MOF materials can be applied to separate Th from REs has seldom been accounted.7,

12

In the recent decades, the studies on tripyridine compounds containing

carboxylic acid groups have been extensively carried out. Terpyridines served as immaculate bifunctional ligands could assemble neoteric composites with intriguing architectures. The terpyridine characterizes by -conjugated substituents, which can coordinate with most metals. In the past research, terpyridines were applied to grow MOF through N-metal key connection (such as Zn(II), Ni(II) and Fe(III)).37-40 As a part of our researches on various structures and functions, a rigid ligand of 4’-(3,5-dicarboxyphenyl)-4,2’6,4’’-terpyridine (H2dtp) has been concentrated: (i) it is a symmetric multidentate ligand, the position of two carboxylic groups is favorable for the generation of crystalline polymers; (2) H2dtp as a triangular rigid member can produce porous crystalline polymers.41 The Ln-MOF can be considered as a notable adsorbent, due to its high stability, high sensitivity, strong selectivity and good reproducibility.36 Ln(III) and oxygen belong to terpyridine ligand coordinate while the N is not participating in coordination. On the basis of H2dtp, the new Ln-MOF is synthesized and structurally characterized for separating Th from REs. Two MOFs featured by isomorphism, [Ln(μ2-OH)(dtp)(H2O)][Ln3+ = Ho3+(1) and Tb3+(2)], are generated using a solvothermal synthetic method. And it turns out that, the Ln-MOF have a good adsorption performance to Th. Compared with the studies,41-43 only O atoms are participated in the coordination 4


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while the N atoms in terpyridine ligands are not involved. As far as we know, this is the first time that terpyridine complexes has been applied to recover of Th(IV) from RE(III).

EXPERIMENTAL SECTION Materials and Methods NaOH (>96%), KOH (>96%), HNO3 (69%), NH4OH (30%), Ethanol (>99.5%), DMF (>99.5%) were provided from Sinopharm Chemical Reagent Co., Ltd. Ho(NO3)3·5H2O and Tb(NO3)3·6H2O and 3,5-dimethylbenzaldehyde, 2-acetylpyridine were purchased from Saan Chemical Technology Co., Ltd. The above mentioned chemicals were used without further purification. RE solutions including the Y, La, Ce, Eu and Lu were obtained by dissolving the rare earth oxides (REO) with the muriatic acid. The above mentioned REO were provided by the Fujian Changting Golden Dragon Rare-Earth Company, Ltd. Thin layer chromatography (TLC) means had been used for supervising the process of the reaction. The concentrations of metal ions before and after adsorption were measured by ICP-OES. 1HNMR

of H2dtp were received by dissolving the H2dtp in the DMSO-d6 (C2D6OS). IR spectra were

obtained by a Nicolet iS50 FT-IR spectrometer. The pH values were measured by the pHS-3C digital pH meter. The morphology and compound of prepared materials were measured by scanning electron microscope / energy dispersive spectrometer (SEM-EDS). The weight losses of MOFs were obtained by the TGA under N2 atmosphere.

X-ray Crystallography A Bruker D8 venture single-crystal (a graphite-monochromated Mo Kα source; λ=0.71073Å) 5



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was applied to gather the crystal data. The crystal structure was worked and refined by OLEX-2 package. The PXRD pattern was measured by Bruker D8 Advance X-ray diffraction. Synthesis and Characterization of H2dtp H2dtp was synthesized according to the published procedures and made some improvements.44 The 3,5- dimethylbenzaldehyde (0.818g, 6.1 mmol) and 2-acetylpyridine (1.37 mL, 12.2 mmol) were dissolved into the ethanol (25 mL). Then sodium hydroxide (0.488 g, 12.2 mmol) was added into the solution with a small quantity of water. After stirred 30 minutes, 30% aqueous ammonia (1 mL) was slowly poured. At ambient temperature, the solution was stirred for 17 hr, and the obtained yellow solid was filtered and washed with ethanol. And an amount of 0.87g solid, the 8mL deionized water and 2mL nitric acid were placed in teflon lined vessel which is stirred for 30 minutes. Then the vessel was sealed and heated at 160 ℃ for 24 h. The resulting light yellow solid was collected by filtration. The 1HNMR was seen in Figure S1. 1HNMR (d6-DMSO): δ 7.83 (dd, 2H), δ 8.38 (m, 2H), δ 8.61 (t.1H), δ 8.71 (d.2H), δ 8.93 (d, 4H), δ 9.00 (d, 2H). Syntheses of Compounds 1-2 For the preparation of 1, Ho(NO3)2·6H2O (44mg, 0.1 mmol), H2dtp (39.7 mg, 0.1 mmol), KOH (50.4 mg, 0.9 mmol) were placed to a stainless steel Teflon lined reactor which contain a mixture solution of DMF (3 mL) and H2O (7 mL), and then stirred for 1 hr. The vessel was sealed and heated at 120 °C for 2 days, accompanied by gradually (4 °C/h) cooling to room temperature. Yellow crystals (MOF 1) were obtained by the separation, and then dried at the room temperature. Other materials were obtained according to the similar method using Tb nitrates (Ln(NO3)3·6H2O, 0.1mmol) instead of Ho(III) nitrate. The FT-IR was shown in Figure S2. 6


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[Ho(μ2–OH)(dtp)(H2O)] (1) IR (KBr, cm−1): 3581 (w), 1607 (m), 1534 (s), 1426 (s), 1369 (s), 1077 (m), 985(w), 874 (m), 777 (s), 642 (s). [Tb (μ2–OH) (dtp)(H2O)] (2) IR (KBr, cm−1): 3572 (w), 1607 (m), 1531 (s), 1426 (s), 1366 (s), 1077 (m), 985(w), 874 (m), 774 (s), 642 (s). Separation of Th(IV) from RE Solution To investigate the selective adsorption between Th (IV) and RE (III), the REs of yttrium (III), lanthanum (III), cerium (III), europium (III), lutetium (III) were selected as the representatives. The separation experiments were carried out at ambient temperature for 24 hrs by shaking mixed solutions (10 mL) with the Ho-MOF in a vibrating mixer. The mixed solution contained the equal molar of Th (IV) and RE ions (each 1.0×10-3 mol/L in aqueous phase). ICP was used to measure the concentration of REs and Th before and after the adsorption. 𝑞𝑒 =

(𝐶𝑎𝑑 ― 𝐶𝑒)𝑉 𝑚𝑀𝑂𝐹

𝐾𝑑 =

𝐶𝑎𝑑

(1) (2)

𝐶𝑒

𝐾𝑑(𝑇ℎ)

(3)

β = 𝐾𝑑(𝑅𝑒)

As defined in Equation (1–3), qe (mg/g) is the quantity of Th or RE recovered on the MOFs, V represents the volume, Cad and Ce (molar per liter) are the concentrations of Th or RE before and after adsorption. Kd andβare defined as the equilibrium and separated coefficients from Th (IV) and REs (III), respectively. RESULTS AND DISCUSSION Structural Characterization MOF 1 and MOF 2 are dramatically isostructural (as shown in Table 1). The PXRD patterns of 1−2 (as shown in Figure 1) have similarity in nature and are matched with those simulation from 7



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corresponding CIF files, suggesting their similarity and isostructural essence of these MOFs. The MOF 1 (Ho-MOF) will be discussed as a representative. Single X-ray analysis demonstrates that complexes 1-2 are isostructural featured by the same space group of P21/c. Crystal data and structure refinements for Ho-MOF (CCDC 1864636) and Tb-MOF (CCDC 1864637) are available (Supplementary material, CIF Ho and Tb). The structure of 1 (as seen in Figure 2) feature [Ho2(OH)2(H2O)2(dtp)2] is highly symmetrical which attributes to the organic ligands that comprise a pair of identical Ho(III) centers, each Ho(III) center that is eight coordination (Figure 2a) resides in a asymmetric 12-facets (Figure 2c) with basal consisting of 4 carboxylate O atoms from three Hdtp-, while the remaining position O atoms from two coordinating H2O and two O atoms from two –OH, respectively. A pair of Ho(III) ions are bridged by two carboxylate groups and two –OH (Figure 2b). As a consequence, that compound 1 consists of a pair of μ4-dtp blocks, two μ2 –OH and two H2O ligands (Figure 2b) which the cluster is extended by benzene spacer to arrange into a 2D metal-framework (Figure 2d).

8


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Simulated Ho Ho Simulated Tb Tb

Relative Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

20

30

40

2/degree Figure 1. PXRD patterns of compounds 1-2.

9


50


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Figure 2. (a) Ho centers coordination condition. (b) Asymmetric units [Ho(μ2–OH)(dtp)(H2O)] spacer. (c) Simplified representation of the coordination environments. (d) 2D metal−organic framework.

10


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Table 1 Crystal Data and Structure Refinements for 1−2 Compound

1

2

formula

C23H15HoN3O6

C23H15 Tb N3O6

formula weight

594.29

588.30

crystal system

Monoclinic

Monoclinic

space group

P21/c

P21/c

a (Å)

21.6918(14)

21.607(2)

b (Å)

4.3874(3)

4.4095(5)

c (Å)

21.0954(13)

21.169(2)

α(deg)

90.00

90.00

β (deg)

96.726(2)

96.626(4)

γ (deg)

90.00

90.00

Z

4

4

V (Å3 )

1993.8(2)

2003.5(4)

T (K)

199.98

100.01

Dc (g cm−3 )

1.973

1.950

μ(Mo Kα) (mm−1 )

4.018

3.579

refines collected

22064

22524

independent refines

4573

4524

Rint

0.0310

0.0525

R1,a wR2b[I≥2σ(I)]

0.0264, 0.0693

0.0670, 0.1630

GOF on F2

1.130

1.190

aR 1

= Σ||F0|-|Fc||/Σ|F0|; bwR2 = Σ[w(F02-Fc2)2]/Σ[w(F02)2]1/2

Thermogravimetric Analyses Thermal analyses obtained from MOFs 1−2 (corresponding to Ho-MOF and Tb-MOF) have been measured in N2 and suggested similarity attributed to their isostructural essence. As shown in Figure S3, MOF 1 shows a little mass loss that the materials could remain stable until 500 °C. The 11



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release of two coordinated water molecules on heating to 500°C, there are a weight loss of 2.81% (calcd. 3%). And after that, it begins to collapse with the loss of organic ligands, which revealing its high thermal stability. Compound 2 (as seen in Figure S3) shows the similar behavior with the removal of water ligands at 400 ℃, respectively. Stability in Aqueous Solution The prerequisite of the porous materials is the water stability and the good water stability is vital for its successful application in the removal of radionuclides in water. Their PXRD pattern and FT-IR of the water-stable MOFs should be maintained after dealing with the water. The bonds coordinated between metal ion and organic ligand that can be assumed the ‘weakest’ point in the whole MOFs are associated with the stabilities of MOFs. The MOFs stabilities can be viewed as the mutual effect between H2O and MOFs, which is the competitive reactions of metal ions/metal modes coordinated by organic ligands or water. If the bond between metal ion and organic ligand is strong enough, it is difficult for water molecules to replace the coordination bonds in existence. Hence, the materials exhibit the perfect water stabilities. Also, the thermodynamic stabilities of MOFs can be demonstrated by the strengths of the metal–ligand bonds.45, 46 More importantly, Ho-MOF exhibits outstanding stability toward aqueous solution, providing a substantial prerequisite for the isolation of Th(IV) from REs(III). The stability is confirmed by PXRD (Figure 3) and FT-IR spectra (Figure 4). According to the Figure 3 and Figure 4, we can achieve a conclusion that the peak of Ho-MOF does not shift before and after soaked in water. Very impressively, the Ho-MOF stability remains unaffected by the aqueous solution, even after steeping with 10 days of 50 mL of water. As a consequence, the materials have been indicated with a good hydrolytic stability. 12


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ho-MOF(soaked in water ) Ho-MOF

Relative Intensity/a.u.

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20

40

60

2/degree Figure 3. PXRD patterns of Ho-MOF before and after being soaked in water

Ho-MOF (soaked in water ) Ho-MOF

4000

3500

3000

2500

2000

1500

Wave number (cm-1) Figure 4. IR spectra of Ho-MOF

13


1000

500


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Separation of Th (IV) from REs To study the adsorption of Th(IV) and RE (III) into the Ho-MOF, the adsorption experiments were carried out in a mixed solution containing typical REs(III) (mentioned above) and Th(IV), respectively. Ho-MOF was placed in a centrifuge tube, then 10 mL solution with equal molar concentration of metal ion was added. After being placed in a shaker for 24h, the content of Th (IV) and REs (III) after adsorption was measured by ICP. As revealed in Figure 5, the separation factors of Th(IV)/Y(III), Th(IV)/La(III), Th(IV)/Ce(III), Th(IV)/Eu(III), Th(IV)/Lu(III) reach 21, 19.2, 18.9, 7.5, 6.2, respectively. In the previous studies, some other adsorption materials were applied to separate Th (IV) including functionalized mesoporous silica (19.2 mg/g),47 fiber-rein-forced polymer (24.43 mg/g),48 UiO-66-COOH (236 mg/g),49 magnetic chitosan composite particle (312.50 mg/g),50 GA-TODGA (66.8 mg/g),51 ox-MWCNTs (62.11 mg/g).52 Compared with the materials, the prepared Ln-MOF materials show attracting adsorption and separation abilities.

Figure 5. qe of Ho-MOF for adsorbing Th(IV) and REs(III). The embedded reveals β of Th (IV) and REs (III) 14


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As compared in Figure S4b and S4c, the intensities of 1530 cm-1 and 1358 cm-1 which attribute to the stretching vibrations of the pyridine have changed significantly that prove the adsorption of Th. Moreover, the recovery of Th(IV) process can be confirmed by SEM-EDS (Figure 6). In order to comprehend perfectly the recovering process of Th(IV) on the Ho-MOF, the adsorption thermodynamic at 303, 308 and 313K were studied. The evaluation of ΔG0, ΔH0 and ΔS0, which relevant with the process of adsorbing the metal ions, are gained by the following formula (4–6): ∆𝐺0 = ―𝑅𝑇ln 𝐾𝑑

(4)

∆G0 = ∆𝐻𝑂 ―𝑇∆𝑆0

(5)

ln 𝐾𝑑 =

∆𝑆0 𝑅

―

∆𝐻0 𝑅𝑇

(6)

where R represents the ideal gas constant, 8.314 Jmol−1 K−1, and T(K) is the temperature. On the grounds of calculated results, △H0 of the adsorption process is −61.8 kJmol−1, which demonstrate that the adsorption of Th(IV) to MOF is an exothermic procedure. △S0 of the adsorption process is -216 Jmol-1K-1, the negative value of entropy indicates a reduction in the degree of disorder, which is resulted from the Th(IV) motion scope restricted by the adsorption solid surface. SEM-EDS Results The SEM-EDS results reveal the surface morphology and formation after adsorption. As can be seen in Figure 6, the Ho-MOF remains almost unchanged after the adsorption of Th(IV). The elemental distribution maps show that the C, O, N, Th and Ho are uniformly distributed (as shown in Figure 6). The concentrations are 46.87% of C, 25.66% of O, 7.05% of O, 7.22% of Th, and 13.20% of Ho.

15



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Figure 6. EDS result of Ho-MOF after adsorption with Th (IV) ions.

According to the FTIR, the 1530 cm-1 and the 1368 cm-1 can be considered as the pyridine stretching vibrations. After adsorption, the C=N peaks shift from 1368 cm-1 to 1358 cm-1, also the intensity of 1530 cm-1 is changed. Moreover, EDS results reveal the adsorbed Th (IV). Accordingly, the adsorption mechanism can be attributed to chemical adsorption, i.e., the adsorption of Th(IV) ions have an coordination on the MOF via the N chelating mode.

CONCLUSIONS In summary, two novel lanthanide (III) (Ln=Ho(III) and Tb(III)) metal-organic frameworks with a general formula of [Ln(μ2–OH)(dtp)(H2O)](Ln = holmium (1) and terbium (2)) were prepared, their 2D metal-frameworks were assembled under the same solvent systems and completely characterized. Complexes fit to measure by XRD were grown by solvothermal method at 120 ℃ . The structures and component contents were acquired by single crystal XRD. The PXRD and 16


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single-crystal XRD demonstrate that the Ho-MOF and Tb-MOF are isostructural with a P21/c space group. In addition, the Ln-MOF shows good thermodynamics and water stability which can be proved by TGA analysis. PXRD and FT-IR indicate the MOF material is remain stable until 500 ℃. Water-stable MOFs are sustainable materials that exhibit strong adsorption abilities to recover the target elements. In this article, the materials were performed to separate Th(IV) from RE(III) ions, which can be characterized by EDS. With good selectivity and high stability, the MOF materials with uncoordinated terpyridine sites are attractive. This work has provided a novel method to recover the Th (IV) and REs (III). AUTHOR INFORMATION Corresponding Author (X.Q. Sun) E-mail addresses: [email protected]. Tel.: +86 592 3594019. ACKNOWLEDGMENTS This work was supported by National Key R&D Program of China (2017YFE0106900), Science and Technology Major Project of Ganzhou (2018), ‘Hundreds of Talents Program’ and Science and Technology Service Network Initiative from Chinese Academy of Sciences. NOTES The authors declare no competing financial interest. CCDC 1864636−1864637 contain the supporting crystallographic data. Appendix A. Supplementary data Crystallographic data for 1−2 (CIF Ho and Tb) REFERENCES [1] Li, J.; Wang, X.; Zhao, G.; Chen, C.; Chai, Z.; Alsaedi, A.; Hayat, T.; Wang, X. Metal-organic 17



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For Table of Contents Use Only

A brief ： Ln-MOF material with uncoordinated terpyridine site has exhibited good selectivity and stability, which maybe a sustainable candidate for the separation.

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Water-Stable MetalâOrganic Framework Material ... - ACS Publications

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Water-Stable MetalâOrganic Framework Material ... - ACS Publications