A Uranyl-Organic Framework Featuring Two-Dimensional Graphene

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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A Uranyl-Organic Framework Featuring Two-Dimensional Graphene-like Layered Topology for Efficient Iodine and Dyes Capture Na Zhang, Yong-Heng Xing,* and Feng-Ying Bai*

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 05/09/19. For personal use only.

College of Chemistry and Chemical Engineering, Liaoning Normal University, Huanghe Road 850#, Dalian 116029, P.R. China S Supporting Information *

ABSTRACT: A novel uranyl−organic framework [(CH3)2NH2][UO2(TATAB)]·2DMF·4H2O (U-TATAB) was assembled through the solvothermal synthesis of UO2(CH3COO)2·2H2O and triazine tricarboxylate linker H3TATAB (H3TATAB = 4,4′,4″-s-triazine-1,3,5-triyltri-paminobenzoate). It was characterized by single-crystal X-ray diffraction, elemental analysis, IR, powder X-ray diffraction, and thermogravimetric microanalysis. It is worth mentioning that U-TATAB exhibits an original arrangement of structures with interesting topology. Structural analysis confirmed that U-TATAB displays an interesting two-dimensional graphenelike layered topology. In order to broaden its functional characteristics, we first investigated the functional adsorbent U-TATAB for iodine capture in cyclohexane solution and for the removal of dye in aqueous solution. Large porosity, unique structure character, and the π-electron walls made of TATAB make the material a promising functional absorbent exhibiting excellent adsorption performance for iodine and dye molecules. Most notably, this is the first case of a uranyl−organic framework built by secondary building unit (SBU) [UO2(RCOO)3]− and extended trigonal triazine tricarboxylate ligand H3TATAB with amino functional groups.

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INTRODUCTION Dimension is one of the most decisive material parameters; the same compound can show significantly different properties regardless of whether its crystal structure is arranged as zerodimensional, one-dimensional, two-dimensional, or threedimensional.1 Since single-layer graphene was discovered in 2004, various types of two-dimensional (2D) materials represented by graphene have drawn extensive attention because of their unique electronic structure and charming physical and chemical performances.2 In the past decade, 2D layered materials have developed rapidly and become one of the most valuable research fields.3−5 Under the efforts of scientists, various 2D layered graphene-like materials have been developed, like graphitic carbon nitride (g-C3N4),6 black phosphorus (BP),7 hexagonal boron nitride (h-BN),8 transition-metal dichalcogenides (TMDs)9 and some emerging metal−organic frameworks,10,11 covalent-organic frameworks,12,13 and so forth, which exhibit broad application prospects in the fields of electronic devices, optoelectronic devices, catalysis, and energy.14 Uranium is one of the most studied actinides due to its importance in nuclear fuel. Uranyl−organic frameworks (UOFs) have drawn great attention owing to their intricate and varied topological structures, which have important application values in many fields, such as luminescence, photoelectric conversion, adsorption, and sensing.15−19 It is very significant to develop © XXXX American Chemical Society

uranyl−organic framework with novel topologies for understanding actinides chemistry and management and disposal of radionuclides.20−26 Usually, linear UO22+ is the most common form of presence of uranium, and a bond angle of 180° in theory was formed by the coordination of the uranium atom and two terminal oxygen atoms on the axis.27−29 On the equatorial plane, uranyl ions can coordinate with multiple −N, −O receptors to form tetragonal bipyramid (4 + 2), pentagonal bipyramid (5 + 2), and hexagonal (6 + 2) bipyramid configurations.30−32 It is because of such structural characteristics that uranyl-containing metal−organic frameworks usually have two-dimensional layered structure. As we all know, it is a common strategy to use a secondary building unit to construct a specific topology. The regular triangular secondary building unit (SBU) [UO2(RCOO)3]−, which is chelated by tricarboxylic acid ligands, can be used to form a two-dimensional layered network with graphene-like topology.33 Some conjugated trigonal tricarboxylate ligands have been utilized to assemble uranyl−organic framework (Scheme 1.)33−37 Wang et al. constructed two rare 3D uranyl organic framework [(CH 3 ) 2 NH 2 ][UO 2 (L1)]·DMF·6.5H 2 O and [(CH3)2NH2][UO2(L2)]·0.5DMF·15H2O using two conjuReceived: February 2, 2019

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DOI: 10.1021/acs.inorgchem.9b00317 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Some Molecular Structures of Trigonal Tricarboxylic Acid Ligands

gated tricarboxylate ligands (Scheme 1a,b), which are constructed by multiple links of three groups of graphene layers and displays radioresistance and the stability of chemistry in water solutions.34 Thuery et al. prepared 12 novel homo- and heterometallic uranyl complexes by applying UO2(NO3)2·6H2O and 1,3,5-benzenetriacetic acid (H3BTA) (Scheme 1c) and reported the synthesis and structure of these crystals, and fluorescence spectrum.35 Then, Wang et al. obtained the first cationic uranium-based metal−organic framework [(UO 2 )(HTTTPC)(OH)]Br·1.5DMF·4H 2 O (SCU-7) by employing UO2(NO3)2·6H2O and trigonal planar ligand H3TTTPCBr3 (Scheme 1d).36 Shi et al. constructed five isomeric uranyl complexes with 2D honeycomb topology with a formula [(CH3)2NH2]UO2(BTPCA) by employing semirigid tricarboxylate ligand H3BTPCA (Scheme 1e).33 Recently, they synthesized four new uranyl−organic complexes by applying tri(biphenyl)amine tricarboxylic acid or triphenylamine tricarboxylic acid ligands (Scheme 1f,g), which provide guidance for the mechanism of solvent/ligand dependence regulation of uranyl complexes.37

In the design of organic ligands, we consider triazine tricarboxylate ligand H3TATAB containing amino functional groups (Scheme 1h). On the one hand, it possesses an uncommon aromatic moiety which can form anion-p interactions and p−p stacking.38−40 On the other hand, this ring consists of three nitrogen atoms which can form a rich and varied network structure.41 Many transition metal compounds and rare earth compounds constructed by TATAB ligands have been reported, which show excellent performance in luminescent sensing and adsorption.42−46 As we know, the crystal structure constructed by tripodal ligand of H3TATAB in the family of uranyl complex has not been reported yet. Generally, for the triazine tricarboxylic acid ligand, because the coordination mode of the carboxyl group is diverse and there is free rotation of the CN bond, the molecular packing structure formed after coordination with the metal is mostly 3D or multiple interpenetrating structures, and it is difficult to form a 2D network. Here, we constructed a novel 2D MOF UTATAB with graphene-like layered topology for the first time by employing linear uranyl cation (UO2)2+ and triazine B


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Inorganic Chemistry

3400, 1689, 1594, 1486, 1417, 1244, 1175, 1119, 768. 1HNMR (d6DMSO, 400 MHz): δ = 7.88 (d, 6H), 7.97 (d, 6H), 9.84 (s, 3H), 12.64 (s, br, 3H). The 1H NMR spectrum of the ligand H3TATAB is shown in Figure S1. Synthesis of [(CH3) 2NH2 ][UO2 (TATAB)]·2DMF·4H2O (UTATAB). UO2(CH3COO)2·2H2O (10.6 mg, 0.025 mmol) and H3TATAB (12 mg, 0.025 mmol) were dissolved 4 mL of DMF and 1 mL of H2O. Upon adjusting to pH ≈ 3 with HNO3 (4 mol/L), the mixture was placed in 20 mL Teflon-lined stainless steel autoclave and reacted at 160 °C for 7 days. After naturally cooling at room temperature, the yellow crystals were obtained. The product with was washed with DMF and dried naturally providing a yield of 63.3% (11.25 mg, based on metal). The formula of the crystal is [(CH3)2NH2][UO2(TATAB)]·2DMF·4H2O (U-TATAB), which was evidenced from single crystal X-ray diffraction analysis, elemental analysis (calc/found: C 37.70/37.96, H 4.40/4.28, N 12.40/12.65). IR data (KBr, cm−1): 3400, 1628, 1584, 1486, 1383, 1244, 1175, 1119, 910, 768. X-ray Crystal Structure Determination. Crystal with suitable size for U-TATAB was selected and mounted on glass fibers for the Xray structure determinations. The reflection points were collected at room temperature on a Bruker AXS SMART APEX II CCD diffractometer with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å). A semiempirical absorption correction was used by the program SADABS.50 The direct method was employed to solve the crystal structure and the SHELX-2016 program was applied to refine it by full-matrix least-squares on F2.51,52 Non-hydrogen atoms were all refined with anisotropic displacement parameters. The riding model was applied to fix and refine the positions of hydrogen atoms on skeleton carbon atoms. Some voids in the lattice indicate that the other, unresolved solvent molecules may be exist.53,54 PLATON/ SQUEEZE was used to calculate the contributions of guest molecules [(CH3)2NH2]+, DMF, and H2O in the channels to the scattering and given a set of guest-free diffraction intensities. Employing the generated data refined the crystal structure again.55 The crystallographic data are listed in Table S1. The selected bond distances (Å) and angles (deg) are presented in Table S2.

Figure 1. (a) Coordination environment of (UO2)2+ in U-TATAB. (b) The hexagonal (6 + 2) bipyramid geometry. (c) The coordination model of TATAB3− ligand. (d) Two-dimensional graphene-like layered structure of U-TATAB. (e) The 2D graphene-like (6,3) net topology of U-TATAB. (f) Four-layered π−π stacking in U-TATAB. Atom color codes: uranium, red; oxygen, turquoise; carbon, pink; nitrogen, yellow. Uranium coordination polyhedron are displayed in turquoise. Hydrogens are omitted for clarity.

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tricarboxylic acid ligand H3TATAB. Because of its planar structure and abundant sorption sites of nitrogen-rich triazine ring, plentiful phenyl rings, as well as N of NH−, it is very advantageous to interact with specific molecules such as iodine and dyes, thus promoting their adsorption.11,47,48 Moreover, U-TATAB is an anionic 2D framework, whereas ionic MOFs are a branch of the porous materials, which can incorporate different charged substances into the confined nanospace through ion exchange.19 Driven by the above ideas, we first explored the adsorption properties of anionic 2D MOF UTATAB for iodine and dye molecules.

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RESULTS AND DISCUSSION

Crystal Structures. Single-crystal X-ray diffraction analysis confirmed that U-TATAB displays two-dimensional graphenelike layered structure and crystallizes in a monoclinic system, space group C2/c. The molecular structure of U-TATAB includes one crystallographically independent UO22+, one TATAB3−, one [(CH3)2NH2]+, two free DMF, and four free H2O. The uranium atom coordinates with six carboxyl oxygen atoms from three individual TATAB3− ligands on the equatorial plane, which can be considered as a [UO2(RCOO)3]− secondary building unit (Figure 1a) to form a hexagonal (6 + 2) bipyramid geometry (Figure 1b). The bond lengths of the equatorial UO are 2.424 Å (U1 O3) and 2.450 Å (U1O4), and the bond lengths of the axial UO are 1.769 and 1.788 Å, which are comparable to those of uranyl−organic frameworks reported in the literature.56,57 Furthermore, the adjacent U···U contact is approximately 18 Å. The completely deprotonated ligand TATAB3− connects three UVI centers (Figure 1c) through chelating bidentate carboxylate moieties (μ3-η1η1η1η1η1η1). Each UO22+ is chelated by three TATAB3− and each TATAB3− connects with three [UO2(COO)3]− SBUs, which further extended into the anticipated 2D graphene-like (6,3) net topology (Figure 1e). As shown in Figure 1d, the effective size of the nanoscale sixmembered ring windows is about 18.54 Å (distance between the parallel edges) × 21.37 Å (distance between the vertices), which is larger than many other 2D graphene-like layered compounds containing uranyl.33−35 The total accessible

EXPERIMENTAL SECTION

Caution! Uranium element is chemically toxic and radioactive. Even the UO2(CH3COO)2·2H2O used in the our work contained depleted uranium, the precautions for handling radioactive materials must be strictly observed. Synthesis of 4,4′,4″-s-Triazine-1,3,5-triyltri-p-aminobenzoate (H3TATAB). Ligand H3TATAB was prepared with reference to the literature43,49 with minor modifications. 4-Aminobenzoic acid (9 g, 66 mmol), NaOH (3 g, 75 mmol), and NaHCO3 (4.5 g, 55 mmol) were added in H2O of 100 mL. The mixed solution was stirred for 30 min at 0 °C. Cyanuric chloride (3 g, 16.5 mmol) was dissolved in 1,4-dioxane (30 mL) and then dripped to the above mixed solution. Then the above mixed solution was refluxed for 24 h in an oil bath at 115 °C. The resulting solution was acidified to pH ≈ 3 with 20% hydrochloric acid. The light yellow solid product was filtered and collected, washed with deionized water several times to neutral, and dried to get H3TATAB (9.0 g, yield: 80%). IR data (KBr, cm−1): C


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Inorganic Chemistry

Figure 2. UV−vis spectra of iodine solution when the different amount of adsorbents is added (5, 10, 20, and 40 mg). The inset shows the difference of solution color before and after the adsorption of iodine by U-TATAB.

volume of U-TATAB is approximately 45.7% calculated by PLATON,55 which is regarded as a porous material. PXRD Analysis. The powder X-ray diffraction data of the compound U-TATAB was obtained and compared with the simulated single-crystal diffraction data (Figure S2). It could be considered that the corresponding compound U-TATAB is pure phase, because position of the peaks is basically consistent. The difference of the intensity is probably the result of the preferred orientation from the powder sample. Thermogravimetric Analysis. TG analysis (Figure S4) suggests that the framework of U-TATAB is stable before 450 °C. Before 450 °C, the weight loss of the guest molecule is about 26%, which is attributed to the elimination of [(CH3)2NH2]+, DMF and H2O moieties in the pores of the MOF. From 450 up to 530 °C, the collapse of the structure is observed and after 530 °C, the finally relative plateau corresponds to uranium oxide (U3O8.). Infrared Spectroscopy. The IR spectrum of the synthesized ligand H3TATAB and uranium compound UTATAB is distinctly revealed in Figure S5. For U-TATAB, the absorption peaks appearing around 1628 and 1584 cm−1 correspond to the νas(COO−) asymmetric stretching vibrations. The νCC and νCN bands in plane of benzene ring and triazine are observed at about 1486 cm−1. The strong peak occurring at 1383 cm−1 is correspond to the νs(COO−)

symmetric stretching vibrations. The peaks appearing near 2929 and 2856 cm−1 are assigned to the νC−H stretching vibration modes of the −CH3 group. A broad absorption peak appeared at approximately 3400 cm−1 and was attributed to νO−H stretching vibration originating from H2O. Compared with the infrared spectrum of the ligand H3TATAB, the νas(COO−) asymmetric and νs(COO−) symmetric stretching vibrations of U-TATAB shifted from 1689 and 1417 cm−1 to 1628 and 1383 cm−1, which red-shifted by 61 and 34 cm−1, respectively. It can be inferred that the uranium center coordinated with the carboxyl oxygen atoms of the ligand H3TATAB. The Solid-State UV−vis Absorption Spectra. The absorbance spectra for ligand H3TATAB and compound UTATAB were collected. As shown in Figure S8, for ligand H3TATAB, the three absorption peaks appearing at about 224, 260, and 341 nm is belonged to the π → π* transition of the H3TATAB ligand; for U-TATAB, the absence of the bands about 221 and 284 nm are associated with the π → π* transition of the H3TATAB ligand, while absorption peaks near 348 nm correspond to the ligand-to-metal transition (LMCT). By comparing the UV−vis spectra of H3TATAB ligand with that of the compound U-TATAB, it was found that the peak of the compound U-TATAB broadened obviously. D


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Inorganic Chemistry

Figure 3. (a) The illustration of pseudo-second-order kinetics for the adsorption process of iodine by U-TATAB in cyclohexane solution. (b) The adsorption amount of iodine in cyclohexane solution when the different amount of U-TATAB is respectively added (5, 10, 20, and 40 mg). (c) Removal efficiency of iodine in cyclohexane solution when the different amounts of U-TATAB is added (5, 10, 20, and 40 mg). (d) The influence of initial concentrations on the adsorption of iodine by U-TATAB.

L−1, while removal efficiency was achieved at 58.6%, 96.9% and 98.8%, when adsorbents are added of 5, 10, 20, and 40 mg, respectively (Figure 3b,c). The experimental results show that 40 mg is the optimal dose. It can be clearly seen from Figure 3d that the maximum amount of iodine adsorbed by UTATAB is about 146 mg/g. Furthermore, linear fit was performed using pseudo-second-order equation to investigate the impact of contact time on adsorption performance of UTATAB toward iodine (Figure 3a). Obviously, the fitting degree of the pseudo-second-order kinetic equation is better, which can better demonstrate the kinetic process of iodine adsorption by U-TATAB. The formula of pseudo-second-order equation is as follows64

Photoluminescence Properties. The luminescent property of U-TATAB has been investigated at room temperature under excitation at 345 nm. As is revealed in Figure S11, the main maxima at 470, 496, 518, 553, 566, and 618 nm in the spectrum were observed, which correspond to the S11 → S00, S10 → S0v (v = 0−4) electronic transitions and are similar to the vibronic progression with the emission spectra of most uranyl-organic skeletons possessing six equatorial donors.58,59 The solid-state photoluminescence quantum yields (PLQYs) of U-TATAB were measured to be 7.24%, which is comparable to that of common uranyl complexes.60,61

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CAPTURE AND RELEASE OF IODINE Capture of Iodine in Cyclohexane Solution. U-TATAB possessed the large porosity, unique 2D graphene-like layered topology and the π-electron walls made of ligands TATAB, which prompted us to explored the adsorption properties of UTATAB to iodine in cyclohexane solution, because of the importance of iodine capture and storage in the nuclear energy.48,62,63 U-TATAB (40, 20, 10, and 5 mg) were soaked in cyclohexane solution of I2 (200 mg L−1, 5 mL) at room temperature (no stirring). The determination of the concentration of iodine solution was conducted by UV−vis spectrophotometer (Figure 2). The solution of iodine turns pale or even colorless (Figure 2 insets) after U-TATAB was immersed in I2 cyclohexane solution. After 72 h, the adsorption amount of U-TATAB toward iodine was achieved (123.8, 57.5, 47.6, and 24.2 mg) for iodine cyclohexane solution of 200 mg

t 1 t = 2 + Qt Q Q e k2 e

where Qe (mg/g) is the equilibrium adsorption amount, Qt (mg/g) is the adsorption amount of the adsorbent at any time t, and k2 represents the pseudo-second-order rate constant (g mg−1 min−1). The PXRD of I2@U-TATAB are basically consistent with that of U-TATAB, inferring that after adsorbing the iodine the host framework is retained (Figure S2) but the absorption peaks of I2@U-TATAB are significantly broadened. When comparing the infrared spectra of U-TATAB, it can be seen that the iodine-loaded networks exhibits significantly enhanced benzene and triazinering vibrations (Figure S6). Compared with the UV−vis spectrum of the complex, the absorption E


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Inorganic Chemistry

Figure 4. Kinetic studies of iodine release from I2@U-TATAB monitored by ultraviolet−visible spectrum in ethanol.

Figure 5. Photograph of U-TATAB loading and releasing iodine.

equilibrium and almost completely released (Figures 4b and 5). It can be inferred that the adsorption process of iodine by U-TATAB is reversible. In ethanol, the delivery of iodine increased linearly along with time, demonstrating that such release behaviors are managed by host−guest interactions.72,73 Dye Adsorption. Selective Adsorption of Cationic Dyes in U-TATAB. Dye contaminants have become one of the main sources of water pollution. The removal of dye contaminants from industrial wastewater has become an important issue. Compared with 3D MOFs,18,19,74,75 the removal of dye molecules by 1D and 2D MOFs as adsorbents is relatively rare.76 MOFs with 2D layered structures generally have more accessible active sites, which may enhance host−guest interactions and promote adsorption of specific molecules.11,76 Herein, we carried out a cation exchange experiment to preliminarily evaluate the adsorption capacity of the anionic 2D framework U-TATAB with a graphene-like layered topology on cationic dye molecules. Five dye molecules with different sizes and shapes positively charged methylene blue (MB) and Safranine T, electrically neutral alizarin, and negatively charged methyl orange (MO) and Eosin Y (Figure S15) were selected to investigate the adsorption performance of U-TATAB. First, the U-TATAB crystals were soaked in 5 mL of aqueous solution containing the above five dyes. The maximum absorbance change of dye solution with time was determined by UV−vis spectroscopy. After 10 h, in single solution containing the cationic dyes, about 95.4% of methylene blue was adsorbed by U-TATAB and 94.1% of Safranine T was adsorbed, whereas neutral dye alizarin and anionic dyes methyl orange and Eosin Y show almost no adsorption behavior (Figure 6). Dye solutions containing methylene blue and Safranine T change from blue and red to colorless, and the color of the crystals change from yellow to green and red, respectively (Figure 6 insets). As evidenced by the adsorption investigation, the cationic dye can be adsorbed by U-TATAB, whereas the anionic and neutral dyes are denied access to the interior of the MOF. Subsequently, MOF U-

band of I2@U-TATAB is significantly broadened (Figure S9). Moreover, the absorption band appearing near 350 nm is characteristic peak of a charge transfer complex relating iodine and the same observation has been reported by Sension et al.65 The affinity of U-TATAB for I2 can be ascribed to high porosity, the π-electron walls, and π−π stacking in U-TATAB networks with 2D graphene-like layered topology,66 chargetransfer interactions, as well as effective sorption sites, which can be explained as follows. First, the strong interactions exist between the I2 molecule and the π-electron walls made of ligand TATAB.67 Strong π-electron-iodine and host−guest interactions would promote iodine uptake.68 Moreover, the pore surface of U-TATAB has a secondary-amine groups (amines are excellent electron donors), which is very advantageous for forming a strong charge transfer complex with iodine. This explanation is based on researches in solutions that have been achieved, but there are no reports related amino-based solids.69,70 In our work the adsorption capacity of U-TATAB to iodine in cyclohexane solution is much better than that of MOF without functionalized amino group, such as MIL53.70 Finally, the nitrogen-rich triazine ring, aromatic rings, and N of NH− in U-TATAB networks become more accessible sorption sites, because charge transfer (CT) complex is formed between U-TATAB networks and polyiodide anions.47 Release of Iodine in Ethanol Solution. The I2 capture reversibility has been also investigated for the loaded iodine sample U-TATAB. Twenty milliggrams of iodine-loaded complexes U-TATAB were immersed in 10 mL of fresh ethanol. The delivery of iodine from I2@U-TATAB composite with time was recorded at room temperature by UV−vis spectra (Figure 4a). The ultraviolet−visible absorption spectrum of all extracts displayed maximum absorbance around 290 and 360 nm, which are attributed to polyiodide anions (Figure S13).71,72 The release process of iodine is relatively rapid. After 2 h, the removal efficiency of iodine reached 75% and after 4 h reached 90% (Figure 4b). After 6 h, it approached F


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Inorganic Chemistry

Figure 6. UV−vis spectra of water solutions containing single dye. (a) Methylene blue, (b) Safranine T, (c) alizarin, (d) methyl orange, and (e) Eosin Y in the presence of U-TATAB monitored with time. The inset shows the color difference of the dye solution before and after adsorption.

the mixed solution of methylene blue and Eosin Y changed from violet to orange, almost completely showing the color of the Eosin Y solution. This observation indicates that even though the existence of anionic dyes, U-TATAB can practically completely adsorb the cationic dye methylene blue from the aqueous solution containing two different charge dyes. In addition, PXRD confirmed that the framework of the MOF UTATAB collapsed partly after adsorbing dye (Figure S2). The solid-state ultraviolet−visible absorption spectrum revealed that the absorption band of MB@U-TATAB is broadened

TATAB was immersed in aqueous solution containing two different charged dyes (methylene blue and methyl orange, methylene blue and Eosin Y) for further dye selective adsorption studies. As is obviously revealed in Figure 7, the removal efficiency for methylene blue can reach 95% and 90%, respectively, after 8 h, when exposed to the binary mixture of methylene blue/methyl orange and methylene blue/Eosin Y. The color of the mixed solution of methylene blue and methyl orange changes from green to yellow, almost completely showing the color of the methyl orange solution. The color of G


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Inorganic Chemistry

Figure 7. UV−vis spectra of mixed dye solution. (a) Methylene blue and methyl orange; (b) methylene blue and Eosin Y. The inset shows the color difference of the dye solution before and after adsorption.

work provides a new strategic idea for expanding the practical application of 2D MOF in the adsorption and separation of environmental pollutants and opening up the unique properties and potential applications of uranyl−organic framework.

compared with that of the complex (Figure S10). It can be seen from the infrared spectra the benzene and triazine ring vibrations of MB@U-TATAB composite is significantly enhanced and slightly broadened (Figure S7). U-TATAB can selectively adsorb cationic dyes, which can be explained as follows. First, because U-TATAB is an anionic framework with 2D graphene-like layered structure, there are uncoordinated cations between the layers and the voids, which can be exchanged with cationic dyes by ion exchange.36 Moreover, we conclude that TATAB ligands may play a significant role in selective adsorption behavior, probably due to secondaryamine groups on its pore surfaces.74 According to previous studies, MOFs modified with an amino group exhibit superior selective adsorption properties for cationic dyes.77,78 A similar situation has been reported in other frameworks constructed by TATAB ligands.74 It should be noted that although a variety of experimental methods were performed to study the dye release process, no release behavior was observed. We conclude that the unique host−guest interaction promotes dye adsorption but also impairs the desorption of dye molecules from the adsorbent.

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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.9b00317. Experimental and characterization data; supporting Figures S1−S15, supporting Tables S1−S2) (PDF) Accession Codes

CCDC 1884547 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 [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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CONCLUSIONS In summary, a novel uranium-based MOF U-TATAB featuring 2D graphene-like layered structure was successfully assembled by using conjugated π-electron ligand H3TATAB and UO2(CH3COO)2·2H2O under conditions of solvothermal synthesis. The experimental results show that U-TATAB can be employed as a potential and outstanding functional adsorbent for iodine capture and dye pollutant removal. Although the adsorption of iodine and dye by threedimensional porous MOFs has been developed, the application of two-dimensional MOF, especially the two-dimensional uranyl−organic framework, is relatively rare. This present

AUTHOR INFORMATION

Corresponding Authors

*(Y.-H.X.) E-mail: [email protected]. *(F.-Y.B.) E-mail: [email protected]. ORCID

Yong-Heng Xing: 0000-0002-7550-2262 Feng-Ying Bai: 0000-0001-6202-054X Author Contributions †

N. Zhang, Y.-H. Xing, and F.Y. Bai contributed equally.

Notes

The authors declare no competing financial interest. H


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Inorganic Chemistry

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ACKNOWLEDGMENTS This work was supported by the grants of the National Natural Science Foundation of China (No. 21571091) and Commonweal Research Foundation of Liaoning province in China (No. 20170055) for financial assistance.

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