Design, Synthesis, and Characterization of a Bifunctional Chelator

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Design, Synthesis, and Characterization of a Bifunctional Chelator with Ultrahigh Capacity for Uranium Uptake from Seawater Simulant Marek Piechowicz, Carter W. Abney, Xin Zhou, Nathan C Thacker, Zhong Li, and Wenbin Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03304 • Publication Date (Web): 19 Nov 2015 Downloaded from http://pubs.acs.org on November 23, 2015

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Industrial & Engineering Chemistry Research

Design, Synthesis, and Characterization of a Bifunctional Chelator with Ultrahigh Capacity for Uranium Uptake from Seawater Simulant Marek Piechowicz,†,a Carter W. Abney,†,a Xin Zhou,†,‡,a Nathan C. Thacker,† Zhong Li,‡ and Wenbin Lin†, * †

Department of Chemistry, University of Chicago, 929 E. 57th Street, Chicago, IL 60637, United States

‡

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, P.R. China Bifunctional chelator, uranium extraction, sorption, XAFS, DFT

ABSTRACT: Informed by density functional theory (DFT) calculations, a novel bifunctional chelator (BFC), (Z)-2-(2-(N'hydroxycarbamimidoyl)phenoxy) benzoic acid, was designed and synthesized for ultrahigh uranium uptake from seawater. Investigation of the ligand for uranium sorption was conducted in artificial seawater (pH = 8.2). An exceptional uranium uptake of 553 mg uranium per gram of sorbent was obtained with a theoretical saturation capacity of 710 mg per gram obtained by fitting isotherm data with the Langmuir-Freundlich model. The resulting yellow precipitate was characterized via x-ray absorption fine structure (XAFS) at the uranium LIII-edge, with the extended XAFS (EXAFS) spectra best fitted by a model where uranyl is coordinated by monodentate amidoxime, one chelating carboxylic acid, and two water molecules. These results are consistent with the formation of a uranium coordination polymer. The ultrahigh uranium uptake capacity obtained by the bifunctional chelating ligand makes it a promising candidate for deployment as uranium adsorbent.

1. Introduction

much—albeit at a uniformly low concentration of 3.3 ppb.10 Thus, substantial effort in recent years has been devoted to developing new materials for extracting uranium from seawater.

Climate change is one of the greatest crises the global community faces in the 21st century. Without swift and sweeping changes to climate policy, the alarming exponential increase in CO2 levels will have catastrophic consequences. Indeed, we are likely to see food supplies diminish,1 sea levels rise,2 and severe weather events increase in frequency and severity.3 Besides such physical calamities, climate change will also have negative spillover effects on the global economy4 and human behavior.

Various methods have been investigated to accomplish this task since the 1960s when Great Britain sought to secure a pelagic supply of uranium after World War II.11,12 Adsorption of uranium was favored on account of its reproducibility and environmental compatibility compared to other methods such as precipitation or liquid-liquid extraction. Early efforts focusing on titanium hydroxide impregnated bulk materials exhibited poor sorption capacity and inefficiency.11 The amidoxime moiety (RNH2NOH) rose to popularity in the 1980s due to its high affinity for the uranyl cation (UO22+) and facile incorporation into organic ligands.13–19

Nuclear energy remains the only mature, scalable technology capable of continuous base-load power generation with ultralow carbon dioxide emissions. On average, nuclear energy outputs less than 1.5% the carbon dioxide emissions per gigawatt hour of energy relative to coal, an estimate comparable to onshore wind power.5 However, in order for nuclear energy to be considered a viable option, a stable supply of uranium must be secured. Current estimates suggest there is less than 100 years’ worth of uranium in terrestrial ores (6.3 million tons) if current consumption levels remain unchanged.6 Although a portion of this uranium can be recovered and recycled if used as part of the fast reactor fuel cycle,7–9 the lack of available infrastructure limits this approach in the immediate future. On the other hand, earth’s oceans contain over 4.5 billion tons of uranium (U),10 almost 1000 times as

Recently, Noriaki and coworkers fabricated a nonwoven fabric supporting amidoxime-functionalized sorbent which harvested > 1 kg uranium yellow cake after contact with seawater for 240 days.20 While proving that seawater uranyl extraction is technically possible, the demonstrated sorption capacity of 1.5 mg U per g sorbent needs much improvement before this technique becomes economically feasible.21,22

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To improve upon the sorption capacity for uranyl extraction, fine tuning of the amidoxime functionality remains an area of active research. Simple modifications, such as alkylation of the adjacent amine have been proposed to increase uranyl affinity,23and recent DFT-guided computational studies reveal a correlation between uranyl affinity and pKa of the oxime.24 Design of entirely new ligand systems are also actively being pursued. For instance, a de novo structure-based strategy was employed to design a bis-amidoxime uranophile,25 for which a synthesis was subsequently developed.26 While such ligands may display impressive thermodynamics for uranyl binding, slow sorption kinetics and lengthy synthetic processes limit their applicability. Kinetics for uranyl binding is known to be a reactionlimited process27 due to speciation of uranium as the highly stable calcium uranyl tris(carbonato) complex.28 Computational and spectroscopic work suggest carboxylic acids adjacent to amidoxime may facilitate carbonate dissociation, resulting in enhanced capacities and kinetics.12,29 This has been observed experimentally, as incorporation of methacrylic and itaconic acid comonomers are known to improve sorption capacities for amidoxime-functionalized polymers.30,31 Recent publications report sorbents with highlyengineered, well-designed binding pockets that display remarkable affinity for uranyl.32–34 However, the stochastic distribution of binding functionalities resulting from traditional methods of sorbent preparation impedes any control of polymer tacticity.35 The objective of the work reported herein is to design, synthesize, and characterize a well-designed bifunctional chelating ligand, where a carboxylic acid and an amidoxime are located in an orientation for optimal uranyl binding. Following prescreening by density functional theory (DFT) calculations, (Z)-2-(2-(N'-hydroxycarbamimidoyl)phenoxy) benzoic acid was chosen as a representative ligand for preliminary study on account of its favorable thermodynamics and relatively facile synthesis. This chelator was prepared, characterized, and tested for uranyl sorption in seawater simulant. The recovered chelator was subsequently collected and analyzed by X-ray Absorption Fine Structure (XAFS) Spectroscopy to investigate the environment of the bound uranium. 2. Experimental 2.1. General Experiment Details All chemicals of reagent-grade quality were obtained from commercial sources and used as received. All NMR data were recorded on a Bruker 500 MHz NMR spectrometer. Chemical shifts were determined via reference to solvent resonance (CDCl3, δ 7.26 ppm; DMSO, δ 2.50 ppm). Thermogravimetric analysis (TGA) was performed under air at a heating rate of 5 ℃ min-1 using a Shimadzu TGA50H equipped with a platinum pan. Liquid chromatography–mass spectrometry was performed using an Agilent 6130 LCMS instrument. Single crystal X-ray diffraction data of the bifunctional chelator (BFC, VII) were collected with a Bruker APEX II CCD-based detector at ChemMat-

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CARS (Sector 15), Advanced Photon Source (APS), Argonne National Laboratory (ANL). ICP-MS was carried out using an Agilent 7700x instrument, and samples were diluted with 2% HNO3 matrix in the range from 0.1 ppb to 500 ppb with correlation coefficient > 0.9997. 2.2. Density Functional Theory Methods All calculations were performed on the Plutonium or Midway computational clusters at the University of Chicago using the Gaussian 09c01 software package36 with DFT at the B3LYP level of theory.37,38 Light atoms (e.g. carbon, nitrogen, oxygen, hydrogen) were modeled by the 6-311+G* basis set.39,40 Uranium was modeled by the Stuttgart RSC 1997 relativistic effective core potential.41 Such specifications have been shown to give accurate bond lengths, angles, and thermodynamic values for complexes containing uranyl.23,42–45 All structures were computed separately and summed in order to satisfy equations (1) and (2): UO2 •5H2O + L → UO2(L)x(H2O)5-x + xH2O (1) ΔGreaction = Σ (ΔGproducts) - Σ (ΔGreactants) (2) All structures were first optimized geometrically, and then subjected to frequency calculations with the same basis set and at the same level of theory. Solvation was modeled throughout using the polarizable conductor calculation model (CPCM).46 Binding enthalpies (ΔH) and Gibbs free energy values (ΔG) were calculated using zeropoint energy (ZPE) and thermal corrections. This computational method has been used extensively in the literature and is known to yield accurate geometries and energetics for actinide complexes.23 2.2. Ligand Synthesis See Supporting Information section 3 for full spectroscopic details. The bifunctional chelator was prepared according to Scheme 1. Synthesis of 1-iodo-2-(2-methylphenoxy)benzene (II) 2-(2-methylphenoxy)aniline (I) (5.0 g, 25 mmol) was added to a solution of HCl (27.0 g, 750 mmol) in a 3:1 (v/v) solution of water and acetone (24 mL). The solution was cooled to 0 °C and an aqueous solution of NaNO2 (3.45 g, 50.0 mmol) was added dropwise followed by solid KI (8.33 g, 50.0 mmol). The reaction was allowed to stir for three hours at 0 °C and then heated to 42 °C overnight. The resulting reaction mixture was cooled to room temperature, extracted with chloroform (40 mL) five times and washed with water (50 mL) twice. Organic extracts were dried with Na2SO4, filtered, and concentrated yielding a dark purple oil which was further purified by flash column chromatography on silica gel affording II as a red, viscous oil. Yield (6.27 g, 81%) 1H NMR (500 MHz, CDCl3) δ ppm: 7.86 (d, 1H, J = 9 Hz), 7.23 (t, 1H, J = 8.5 Hz), 7.17 (t, 1H, J = 7.5 Hz), 6.83 (q, 2H, J = 7 Hz), 6.68 (d, 1H, J = 7 Hz), 2.26 (s, 4H). 13C NMR (125 MHz, CDCl3) δ ppm: 156.82, 154.26, 139.87, 131.62, 129.68, 127.24, 124.46, 119.18, 117.16, 87.59, 16.41. LRMS (EI) calculated for C13H11IO: 310.0 m/z expected, found: 310.0 m/z

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Industrial & Engineering Chemistry Research Scheme 1. The complete synthesis of the bifunctional chelator 1. HCl 2. NaNO2 H2O

KMnO4

3. KI

pyridine, H2O

I

II

MeOH, ∆ cat. H2SO4

CuCN, L-Proline

LiOH

∆, 18 hrs.

Methanol VI

III

V

IV

NH2OH ∆, 48 hrs.

VII

Synthesis of 2-(2-iodophenoxy)benzoic acid (III)

Synthesis of methyl 2-(2-cyanophenoxy)benzoate (V)

Molecule II (6.0 g, 19.4 mmol) was added to a 3:1 (v/v) solution of pyridine and water (24 mL). The resulting solution was heated to 90 °C and solid KMnO4 (18.4 g, 116 mmol) was added. After three hours at reflux, a second portion of solid KMnO4 (18.4 g, 116 mmol) was added and the reaction was allowed to reflux for an additional three hours. The mixture was filtered through a plug of celite with lukewarm H2O, acidified to a pH of 1 with 0.1 M HCl, extracted with ethyl acetate (25 mL) three times and washed with H2O (25 mL) and brine (25 mL) before being dried over Na2SO4 and concentrated under vacuum affording III as a pale yellow solid. Yield (4.96 g, 75%).1H NMR (500 MHz, CDCl3) δ ppm: 8.26 (d, 1H, J = 6 Hz), 7.94 (d, 1H, J = 6 Hz), 7.42 (t, 1H, J = 7.5 Hz), 7.10 (d, 1H, J = 6 Hz), 7.05 (t, 1H, J = 6.5 Hz). 13C NMR (125 MHz, CDCl3) δ ppm: 156.34, 140.44, 134.86, 133.84, 130.16, 127.39, 123.88, 121.23, 116.70, 89.60. LC-MS (ES-API) calculated for C13H9IO3: 340 m/z, found 340 m/z

Molecule IV (4.6 g, 13 mmol) in DMF (15 mL) was added to a mixture of CuCN (2.35 g, 26 mmol) and L-proline (1.50 g, 13 mmol). The resulting solution was heated to 115 °C for 18 hours. After cooling to room temperature, 4 equivalents of NH4Br (10.2 g, 104 mmol) in 20 mL distilled water were added and the solution was mixed well. Ethyl acetate (10 mL) was added and the reaction mixture was filtered with a Büchner funnel. Extraction with ethyl acetate (50 mL) five times and concentration under vacuum afforded the crude product as a pale brown oil. Purification by flash column chromatography on silica gel affords V as a pale yellow solid. Yield (1.38 g, 42%) 1H NMR (500 MHz, CDCl3) δ ppm: 8.03 (d, 1H, J = 7 Hz), 7.66 (d, 1H, J = 7.5 Hz), 7.59 (t, 1H, J= 4 Hz), 7.42 (t, 1H, J = 4.5 Hz), 7.35 (t, 1H, J = 7.5 Hz), 7.16 (d, 1H, J = 9 Hz), 7.09 (t, 1H, J = 7.5 Hz), 6.63 (d, 1H, J = 8 Hz), 3.76 (s, 3H). 13C NMR (500 MHz, CDCl3) δ ppm: 160.45, 153.57, 134.17, 132.57, 125.75, 123.99, 123.05, 122.38, 118.20, 115.29, 52.32. LRMS (EI) calculated for C15H11NO3: 253.1 m/z expected, found: 253.1 m/z

Synthesis of methyl 2-(2-iodophenoxy)benzoate (IV) Molecule III (4.85 g, 14.4 mmol) was dissolved in anhydrous methanol (22 mL). To this solution was added a catalytic amount of 18 M H2SO4 (80.00 μL, 1.44 mmol). The solution was heated to 65 °C for 24 hours. The solution was cooled to room temperature and concentrated under vacuum before a saturated solution of NaHCO3 (20 mL) was added. The resulting mixture was extracted with ethyl acetate (25 mL) three times and brine (25 mL) before being dried over Na2SO4 and concentrated under vacuum to afford IV as a yellow, viscous oil. Yield (4.65 g, 92%) 1H NMR (500 MHz, CDCl3) δ ppm: 7.98 (d, 1H, J = 7 Hz), 7.88(d, 1H, J = 6.5 Hz), 7.50 (t, 1H, J = 8 Hz), 6.96 (d, 1H, J = 8 Hz), 6.86 (t, 1H, J = 4 Hz), 6.73 (d, 1H, J = 6 Hz), 3.83 (s, 3H). 13C NMR (125 MHz, CDCl3) δ ppm: 166.00, 157.01, 155.36, 139.89, 133.73, 129.74, 124.89, 124.02, 123.05, 120.66, 117.62, 87.72, 52.28. LRMS (EI) calculated for C14H11IO3: 354.0 m/z expected, found: 354.0 m/z

Synthesis of 2-(2-cyanophenoxy)benzoic acid (VI) Molecule V (1.38 g, 5.45 mmol) was added to a 1:1 v/v solution of methanol and tetrahydrofuran (14 mL). To this solution was added a 1.5 M aqueous solution of LiOH (314 mg in 5 mL water) dropwise. The solution was allowed to stir for five hours. Concentration under vacuum and acidification with 0.1 M HCl gave a white precipitate. The resulting mixture was extracted with ethyl acetate (20 mL) three times before being concentrated under vacuum to afford VI as a white solid. Yield (1.25 g, 96%) 1H NMR (500 MHz, CDCl3) δ ppm: 8.17 (d, 1H, J = 7 Hz), 7.70 (d, 1H, J =8 Hz), 7.60 (t, 1H, J= 10 Hz), 7.50 (t, 1H, J= 8 Hz), 7.36 (t, 1H, J =7 Hz), 7.19 (t, 1H, J = 7.5 Hz), 7.07 (t, 1H, J=7 Hz), 6.80 (d, 1H, J= 8 Hz). 13C NMR (125 MHz, CDCl3) δ ppm: 166.11, 160.42, 153.24, 135.52, 134.80, 134.39, 132.55, 126.52, 125.08, 123.40, 123.16, 115.68, 102.14



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2+

Figure 1: DFT optimized structures of chelator-bound UO2 . Functionalization at the ortho position is displayed in the top panel, while the analogous meta-functionalized ligand is displayed in the bottom panel. Red, white, gray, dark blue, and light blue spheres represent O, H, C, N, and U, respectively. Insets display the chemical structure of the unbound bifunctional chelating ligands.

Synthesis of 2-[2-[(Z)-N'-hydroxycarbamimidoyl]phenoxy] benzoic acid (VII, BFC) Molecule VI (992 mg, 4.1 mmol) was dissolved in a 10:1 v/v solution of ethanol:water (11 mL). To this solution was added 2 equivalent of aqueous NH2OH (274 μL, 8.3 mmol). The solution heated to reflux at 80 °C for 24 hours. An additional equivalent of NH2OH was added (137 μL, 4.1 mmol) and the solution was heated at 80 °C for an additional 24 hours. Concentration under vacuum and subsequent acidification with 0.1 M HCl gave a pale orange precipitate. The resulting solid was collected via centrifugation. Subsequent washing with ethanol (20 mL) three times and centrifugation afforded a white solid. White, needle-like crystals suitable for x-ray diffraction analysis were grown in methanol from a saturated solution over three days. Yield (530 mg, 44%)

Table 1: Composition of seawater simulant Concentration (mg L )

UO2(NO3)2•6H2O

various

NaCl

25.6

NaHCO3

194

Bifunctional chelator VII (1 mg) was added to 1 mL seawater simulant and agitated on a plate shaker at 300 rpm for 18 hours to extract the uranyl species. The uranium concentration before and after agitation was determined by ICP-MS to evaluate uranium uptake by the chelator. The quantity of uranium adsorbed by the chelator at equilibrium (Qe) was calculated with equation (3).

1

H NMR (500 MHz, DMSO) δ ppm: 9.56 (s, 1H), 7.89(d, 1H, J = 6 Hz), 7.58 (m, 2H), 7.33 (t, 1H, J = 8 Hz), 7.28 (t, 1H, J = 7.5 Hz), 7.12 (t, 1H, J = 7.5 Hz), 7.01 (d, 1H, J = 8 Hz), 6.68 (d, 1H, J = 8 Hz), 5.91 (s, 2H).13C NMR (125 MHz, DMSO) δ ppm: 166.96, 155.07, 154.89, 150.71, 134.34, 132.26, 130.89, 130.59, 124.63, 124.53, 123.50, 121.07, 117.54. LC-MS (ES-API) calculated for C14H13N2O4: 273 m/z, found 273 m/z 2.3. Uranium Sorption Experiments A series of simulated seawater solutions with various uranium concentrations was prepared by dissolving UO2(NO3)2·6H2O in seawater simulant (pH = 8.2, detailed composition27,47 Table 1).

-1

Species

∗ 3

where is the uranium adsorbed at equilibrium, is the initial uranium concentration, is the uranium concentration of seawater simulant at equilibrium, V is the volume of the seawater simulant, and m is the mass of the bifunctional chelator. 2.4. Crystallography General crystallographic data are listed in the Supporting Information. Systematic absences were consistent with the orthorhombic space group Pna21, as determined via XPREP. Systematically absent reflections were removed and symmetry equivalent reflections were averaged to yield the set of unique data. The resulting 4369 reflections were used in the least squares refinement. The structure


was solved using direct methods made accessible by the SHELXTL software package. The correct positions for the nitrogen and oxygen atoms were deduced from the electron-density map. Subsequent least-squares refinement cycles (SHELXL) revealed the positions of the remaining non-hydrogen atoms of the benzene rings. Non-hydrogen atoms were refined with independent anisotropic displacement parameters. Hydrogen atoms were placed geometrically in ideal positions and refined independently. An isotropic extinction parameter was not necessary. The weighting parameter was 0.051. Successful convergence was indicated by the maximum shift/error of 0.000 for the last cycle of least squares refinement. The largest peak in the residual electron density map (0.35 eÅ-3) was located 1.4 Å away from the amine of the amidoxime. 2.5. XAFS Following sorption studies, the uranyl saturated adsorbent was collected by centrifugation for XAFS investigation. The resulting yellow solid was washed with water and dried in vacuo before being ground into a fine powder with a mortar and pestle. Samples were diluted with dextrose to prevent self-absorption phenomena and pressed into the center of a Nylon washer. The filled washer was then placed into a small, enclosed pouch made of multiple layers of Kapton film and Kapton tape to comply with radiological safety considerations (See Supporting Information). XAFS measurements were performed on beamline 10BM-B48 at the Advanced Photon Source of Argonne National Laboratory. Data were collected at the uranium LIII-edge (17166 eV) using fluorescence detection, with simultaneous measurement of a Y foil calibration standard in transmission mode. Six spectra were aligned to the yttrium reference foil and averaged in μ(E) prior to normalization. Analysis was conducted using the Athena and Artemis programs of the Demeter software suite of the IFEFFIT package based on FEFF 6.49,50 Degeneracy from the amidoxime and carboxylic acid was varied in quadrature by scaling respective amplitude reduction factors (S02) during the fitting process. The degeneracy of the axial oxygen on uranyl was not considered to vary and was fixed at 2. Parameters varied for each single scattering path include the change in scattering halfpath length (ΔR) and the mean squared relative disorder of the scatterer (σ2). A single global energy shift parameter (ΔE) was also varied. Multiple scattering paths were constructed from constituent single scattering paths. 3. Results and Discussion

bridging oxygen, and both options were investigated computationally. The upper panel shows the orthofunctionalized species whereas the lower panel shows the meta-functionalized species. Note that in either case, uranyl is five-coordinate in the equatorial sphere—a geometry that has been shown to be favored in aqueous media.51 Furthermore, each structure exhibits monodentate coordination through the carboxylate moiety and η2 bonding by the amidoxime, in agreement with previous studies.52–54 After optimization of the geometry of each ligand and the ligand/UO22+ complex, frequency calculations were conducted to ascertain the energetics of each system. The results are shown in Figure 2. The bifunctional chelator-uranyl complex shows an impressive Gibbs free energy of -68.44 kcal mol-1 and a binding enthalpy of -54.19 kcal mol-1. Increasing the alkyl chain length at the amidoxime and carboxylate positions makes the complex more thermodynamically favorable by allowing the binding groups to envelop the uranyl cation and achieve more optimal molecular orbital interactions. Accommodating this optimized binding geometry can improve bond strength by as much as 6 kcal mol-1. Although these results are intriguing, we chose to focus our efforts on the single-carbon analog in an attempt to establish a baseline for uranium sorption. -∆H -∆G

70 60

65

55

50

60 O

O

O – –

O

O

N O

N H2

–

O N

–

O

N H2 O

O–

N

O– O

O

O –

O

–

N O

3.1. DFT-based Ligand Design

N H2 O

O

N H2 –

–

O

O

N

N H2

N H2 O

O N

O–

O

–

2+

Density functional theory (DFT) calculations were used to rapidly screen several ligand derivatives for favorable uranyl binding prior to the commencement of synthesis. All structures were based on the diaryl ether framework due to the inherent flexibility of the –COC– linkage and the relative ease with which adjacent sites could be functionalized. Figure 1 displays the optimized structures of the uranyl bound complexes and corresponding chemical diagrams of the unbound ligands. The diaryl ether backbone can be functionalized either ortho or meta to the

Figure 2: Thermodynamic values of various UO2 /Ligand structures. Ligands used shown below.

Diaryl amine bridged chelators were also considered, instead of diaryl ethers, as it was speculated the hydrogen may participate in hydrogen bonding with the uranyl oxygen. However, this was not observed computationally, and the calculated bond strength of uranyl binding was comparable with the diaryl ether ligands. These results are described in further detail in the Supporting Information section 1.2.


−∆G

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−∆ H

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Industrial & Engineering Chemistry Research 3.2. Materials Characterization

C14-O3

The proton NMR spectrum confirmed the successful synthesis of the bifunctional chelator by the appearance of a sharp singlet at 5.91 ppm and a broad singlet at 9.56 ppm corresponding to the –(H)N-H and –COO-H respectively. Integration confirms two protons for –NH2 and one proton for –COOH. LC-MS run in positive mode found the molecular ion corresponding to the desired product; (M+H) were found at 273.1 m/z. Thermogravimetric analysis (TGA) of the bifunctional chelator is shown in the Supporting Information section 3.3. No obvious weight loss is observed until 170 °C, which suggests chelator stability in environments appropriate for uranyl extraction, or even harsher temperature swing adsorption-desorption cycles.

O2-O3

2.238

2.220

0.018

O1-O2

2.879

2.702

0.177

O1-N2

2.749

2.702

0.047

N2-O2

2.661

2.713

0.052

The crystal structure of the bifunctional chelating ligand is shown in Figure 3. A comparison of several bond lengths and angles between the crystal structure and the DFT calculated structure using the CPCM solvation model with methanol reveals the data are in good agreement (Table 2). The average error in both lengths and angles was 0.042 Å and 1.65° respectively, consistent with those resulting from crystal-packing interactions. Such small deviations suggest that the optimized structure calculated by DFT does an adequate job at modeling geometries, and its thermodynamic values can be expected to be reasonable.

1.246

1.268

Average Error

0.022

0.042

Angles Bifunctional Chelator

Calc.

Exp.

|Δ|

O3-C14-O2

125.48

122.76

2.72

C8-O1-C7

120.64

116.89

3.75

H2-N2-O4

117.67

116.84

0.83

N2-C1-N1

120.82

120.78

0.04

N2-C1-C2

118.69

118.88

0.19

O4-N2-C1

117.58

119.94

2.36

Average Error

1.65

3.3. Uranium Sorption The uranium sorption data obtained at room temperature are plotted in Figure 4. Due to possessing no appreciable solubility in water, the chelator can be added directly without need of any supporting structure. This results in extremely rapid sorption kinetics and facile isolation by centrifugation. The sorption data were fitted using the LangmuirFreundlich model55 as shown in equation (4),

(4)

Where Qe is the uranium uptake (mg g-1) at equilibrium, Qmax is the uranium capacity (710 mg g-1) of the accessible sorption site at saturation, ce is the uranium concentration (ppm) in seawater simulant, b is the affinity coefficients (7.3*10-4), n is the ideal homogeneous surface deviation (1.12). 600

-1

Uranium Uptake (mg g )

Figure 3: ORTEP structure of the bifunctional chelator (VII) at 50% probability ellipsoids.

Table 2. Selected calculated and experimental bond lengths and angles of the bifunctional chelator

400 60 -1

Uranium Uptake (mg g )

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200

40

20

0

0

0

10 20 Uranium Concentration (ppm)

Bond Lengths Bifunctional Chelator

Calc.

Exp.

|Δ|

C1-N1

1.327

1.316

0.011

C1-N2

1.314

1.307

0.007

N2-O4

1.388

1.384

0.004

0

4000

8000

12000

Uranium Concentration (ppm) Figure 4. Uranium isotherm of the bifunctional chelator at room temperature. Data points are black squares, with the fit obtained by the Langmuir-Freundlich model displayed as a 2 red line, R = 98.2%. Inset: the linearized data and fit at dilute uranium concentration.


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Table 3. Scattering paths, path lengths, and mean-square disorder of path distance for the best XAFS fit

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

2

Scattering Path

R (Å)

Error

σ (Å2)

Error

Coordination Number

Error

U→Oyl

1.78

< 0.01

0.002

< 0.001

2

-

U→O1

2.33

0.02

0.006

0.002

2.8

0.3

U→O2

2.49

0.02

0.006

0.002

3.2

0.3

U→C1

2.87

0.02

0.002

0.002

1.4

0.4

U→Nam

3.41

0.03

0.003

0.002

1.1

0.6

U→Oyl1→Oyl2

3.56

0.04

0.004

< 0.001

2

-

U→Oyl1→U→Oyl2

3.56

0.04

0.004

< 0.001

2

-

U→Oyl1→U→Oyl1

3.56

0.04

0.004

< 0.001

2

-

U→C1→C2

4.35

0.02

0.003

0.002

1.4

0.4

E0 = -0.9 ± 1.0 eV, R-factor = 0.9 %

While a saturation capacity of 710 mg g-1 was obtained by fitting the isotherm data, saturation capacity of 874 mg g-1 sorbent would be achieved if a 1:1 uranium-ligand complex is assumed to form. These results indicate more than 80% of binding sites can be occupied under artificial seawater conditions. However, a portion of the BFC sorption sites are not readily accessible due to the tendency of the free ligand to agglomerate. The BFC exhibits respectable sorption capacity even at low uranium concentration—56 mg uranium g-1 chelator at 22 ppm—which is higher than previously reported phosphonate ligand modified mesoporous silica56 or mesoporous carbon,57 and is comparable with diethoxyphosphorylurea functionalized UiO-68 metal-organic frameworks.34 3.4. Uranyl-Chelator Coordination Numerous attempts to grow single crystals of uranyl chelated BFC were unsuccessful. Addition of chelator to solvated uranyl resulted in instantaneous formation of an amorphous yellow precipitate, regardless of efforts to retard the rate of complexation. The yellow precipitate obtained during all crystal growth experiments suggests a distinct lack of strong binding. Strong ligand backbonding interactions, weakening the axial uranium-oxygen bonds, would likely result in a red-colored complex.53,58 As crystallographic investigation of the uranyl-chelator structure was not feasible, we applied XAFS analysis to examine the uranium coordination environment after extraction from seawater simulant. In addition to the coordination environment investigated by DFT, binding of two different uranyl molecules by the amidoxime and carboxylic acid functionalities would result in the formation of a coordination polymer. Models representative of both motifs were investigated through fitting of the extended XAFS (EXAFS) region. The structure model for the uranyl coordination environment was constructed in a bottom-up fashion, including sequential shells of coordinating elements. All data were fit with multiple k-weights (k=0.5, 1, 2, 3) in R space. The best fitted coordination environment consists of 2 axial uranyl oxygen atoms (Oyl) tightly bound at 1.78 Å, 2.8 ± 0.3 oxygen or nitrogen atoms from amidoxime moieties (O1) at 2.33 Å, 3.2 ± 0.3 equatorial oxygen atoms from

Figure 5. (a) The proposed uranyl coordination environment based on XAFS analysis. (b) XAFS data and fits in R space. The top plot (black) displays the magnitude of the Fourier transform. The bottom plot (red) displays the real component and is offset for the sake of clarity. Data are not phase shift corrected. (c) XAFS data and fit displayed in k space. XAFS data were fitted simultaneously with k-weighting of 1, 2, and 3. Data and fit with k-weighting of 3 are displayed.



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

chelating carboxylate or water (O2) at 2.49 Å, 1.4 ± 0.4 carbon atoms from carboxylate or amidoxime moieties (C1) at 2.87 Å, and 1.1 ± 0.6 nitrogen atoms from the amidoxime group (Nam) at 3.41 Å. The multiple scattering paths U→Oyl and U→C1→C2, where C2 is the carbon atom adjacent to C1 on carboxylic acid, contributed significantly to the quadrature of the spectrum and are also included in the fitting. Final fits displayed in R-space and k-space are shown in Figure 5(b) and (c). Table 3 lists the parameters of this fit. Interpretation of the fit to assign the average coordination environment was made by comparing XAFS-determined bond lengths with those of representative crystal structures (See Supporting Information section 7).19,52 The best-fit model is representative of uranyl coordination by one monodentate-bound amidoxime, one or two bidentate chelating carboxylic acids, and the remainder of the equatorial plane filled with coordinating water molecules as shown in Figure 5(a). Uranyl species with five or six equatorially-coordinating molecules have been shown to be stable and prevalent in seawater.28,51 The fit is consistent with empirical observations and suggests that strong binding via an η2 motif is not occurring, despite previous small molecule and computational investigations.52,54 The yellow precipitate observed is likely due to weak coordination by chelating carboxylates as well as monodentate binding by amidoxime functionalities. The rapid uranium adsorption displayed by the bifunctional chelator precludes the formation of the slower, more thermodynamically stable η2 binding motif suggested by DFT calculations, instead favoring the formation of a coordination polymer.

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Finally, it should be noted that the bifunctional chelator represents a baseline technology for uranium sorption. More work is needed to exploit the “binding pocket effect” in order to create deployable materials with an even higher capacity for uranium.

ASSOCIATED CONTENT Supporting Information DFT-calculated Cartesian coordinates, graph of energies; 1 H and 13C NMR peak positions, splitting and integration; MS data; crystallographic data; comparison of bond lengths between proposed EXAFS and crystal structures; comparison of various uranium chelating materials; DFTcalculated ligand structures.This material is available free of charge via the Internet at http://pubs.acs.org. Crystal structure available from the CCDC, No. 1421017.

AUTHOR INFORMATION Corresponding Author *Fax: 773-702-0805. Tel.: 773-834-7163. E-mail: [email protected] a denotes these authors contributed equally to this work.

Present Addresses §Oak Ridge National Laboratory, P.O. Box 2008, MS-6201, Oak Ridge, Tennessee 37831-6181, United States

Author Contributions The manuscript was written with the contributions of all authors. All authors have given approval to the final version of the manuscript.

4. Conclusions A new material was synthesized that shows great potential in aqueous uranium sequestration for applications to the nuclear fuel cycle. Using density-functional theory as a high throughput tool to quickly and efficiently screen several ligand types, a bifunctional chelator based on a flexible diaryl-ether framework, was chosen as the model system to study. Although uranyl binding did not occur through cooperative interactions by adjacent functionalities as expected, uranium sorption was nevertheless achieved. Indeed, a uranium saturation capacity of 553 mg U g-1 sorbent was observed in seawater simulant, which surpasses the current state of the art materials by several-fold (SI section 8) EXAFS analysis suggests the formation of a coordination polymer with the carboxylate and amidoxime moieties bonding distinct uranyl groups. While efforts to immobilize the bifunctional chelator onto a solid support are currently underway, this ligand may have direct application for deployment in liquid-liquid extractions. Otherwise, the ligand may be a good candidate for incorporation into previously described hard shell enclosures for uranium uptake.59 However, further engineering is necessary before the BFC’s high uranium capacity and rapid kinetics are fully exploited for uranium sequestration.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the DOE Office of Nuclear Energy’s Nuclear Energy University Program (SubContract-20 No. 120427, Project No. 3151). Mr. Xin Zhou acknowledges the financial support from the Oversea Study Program of Guangzhou Elite Project (JY201326). MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357. The authors also thank Zekai Lin and Chris Poon for mass spectrometry assistance. We acknowledge the University of Chicago Research Computing Center, the Nuclear Magnetic Resonance Facility, and NSF instrumentation grant CHE-1048528 in conjunction with the Mass Spectrometry Facility for support of this work.


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ABBREVIATIONS BFC = bifunctional chelator (18)

DFT = density functional theory EXAFS = extended x-ray absorption fine structure ICP-MS = inductively coupled plasma mass spectrometry

(19)

MS = mass spectrometry NMR = nuclear magnetic resonance PPB = parts per billion

(20)

PPM = parts per million SXRD = single-crystal x-ray diffraction (21)

TGA = thermogravimetric analysis Uranyl = uranium dioxide (+2) cation XAFS = x-ray Absorption Fine Structure

(22)

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Design, Synthesis, and Characterization of a Bifunctional Chelator

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