Sulfate Separation from Aqueous Alkaline Solutions by Selective

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Sulfate Separation from Aqueous Alkaline Solutions by Selective Crystallization of Alkali Metal Coordination Capsules Arbin Rajbanshi, Bruce A. Moyer, and Radu Custelcean* Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

bS Supporting Information ABSTRACT: Self-assembly of a tris(urea) anion receptor with Na2SO4 or K2SO4 yields crystalline capsules held together by coordinating Naþ or Kþ cations and hydrogen-bonding water bridges, with the sulfate anions encapsulated inside urea-lined cavities. The sodium-based capsules can be selectively crystallized in excellent yield from highly competitive aqueous alkaline solutions (∼6 M Naþ, pH 14), thereby providing for the first time a viable approach to sulfate separation from nuclear wastes.

esearch in the area of supramolecular chemistry of anions1 started mainly as an academic endeavor aimed at understanding the underlying principles of anion coordination, but more recently it has been increasingly driven by the need to find solutions to utilitarian problems, generally related to health, environment, or energy. While the fundamental aspects of anion recognition are most often investigated in organic solvent media, real-world problems typically involve much more competitive aqueous environments.2 One such problem of special interest to us is sulfate separation from radioactive wastes. Sulfate is a problematic component of legacy nuclear wastes, particularly those in the U.S. Department of Energy (DOE) complex, as it interferes with the vitrification process selected for waste disposal, increases the volume of waste forms that must be produced and stored, and reduces their geologic performance.3 Sulfate removal from aqueous solutions is already challenging enough due to the very strong hydration of this anion (ΔG°h =
1080 kJ/mol).4 The extreme ionic strength (>6 M) and alkalinity (pH 14) of the waste, as well as the high concentrations of competing anions (mainly NO3
, NO2
, OH
, and CO32
),5 further increase the complexity of the problem. Though examples of sulfate selective receptors have been previously reported,6 none of them has been demonstrated to work in a viable binding-release cycle under the extremely demanding conditions found in nuclear wastes. A promising approach to effective sulfate recognition and separation from competitive aqueous environments is to take inspiration from Nature’s sulfate-binding protein7 and completely isolate the anion from the surrounding solvent by encapsulation inside structurally constrained cavities functionalized with complementary binding groups. Though such cryptand-like architectures8 are often difficult to assemble via traditional organic synthesis, a

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more practical approach to cage receptors for anions via selfassembly from relatively simple building units has been recently demonstrated.9 Alternatively, self-assembly of crystalline solids that selectively include targeted anions upon crystallization can be effectively employed for anion separation.10,11 A distinct advantage of such crystalline “hosts” is that the stiffer environment inside crystals may prevent the structural distortion of the anion-binding cavities and accommodation of competing anions, resulting in superior selectivity. We have recently reported that two molecules of the tripodal ligand L1, consisting of a urea-functionalized tren scaffold,12 selfassemble with Mg(H2O)62þ cations and encapsulate sulfate upon crystallization from competitive aqueous solutions.13 The rigid and highly complementary binding cavities of these crystalline capsules, comprising 12 urea hydrogen bonds to the sulfate, ensured exceptional sulfate selectivity based on shape, size, and charge discrimination. However, in spite of their excellent anion recognition abilities, these capsules have limited utility for sulfate separation from alkaline nuclear wastes, as they do not form under basic conditions (pH > 10) due to Mg(OH)2 precipitation. We subsequently made the argument that if similar capsules could be selectively crystallized with alkali metal cations instead,14 which are tolerant to highly basic conditions, it would potentially provide a viable solution to the problem of sulfate separation from nuclear waste. A Na-based system, in particular, could take advantage of the abundance of sodium ions in the waste, which not only would circumvent the need for adding external ionic components to the waste but also would significantly decrease the solubility of the Received: April 22, 2011 Published: June 01, 2011 2702

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capsules through the common ion effect. We now report sulfate encapsulation in two new crystalline systems self-assembled from L1 and M2SO4 (M = Na, K). The sodium coordination capsules were selectively crystallized from highly competitive aqueous alkaline solutions (∼6 M Naþ, pH 14), which could provide the basis for the much-needed technology for sulfate separation from nuclear wastes.

Slow evaporation of water/methanol (1:1) solutions containing stoichiometric amounts of L1 and Na2SO4 or K2SO4, afforded crystals with the composition Na2SO4(L1)2(H2O)4 (1) or K2SO4(L1)2(H2O)2 (2), respectively, as determined by single-crystal X-ray diffraction15 and elemental analysis. The solvent composition appears to be critical for the formation of these complexes. When the solutions containing the crystallized 1 or 2 were left open to allow the methanol solvent to almost completely evaporate, the initial crystals slowly dissolved, and crystals of L1 3 (H2O)2, with lower water solubility than 1 and 2, started to form. The crystal structure of 1 (Figure 1) comprises anionic SO4(L1)2 2
and cationic Na2(H 2O)4 2þ secondary building units (SBUs) interconnected by coordination and hydrogen bonds into a three-dimensional framework with NaCl-type topology. The anionic SBU consists of an SO42
anion encapsulated by two embracing L1 molecules (Figure 1a). This results in 12 complementary hydrogen bonds between the six chelating urea groups and SO4 2
, which represents the preferred coordination number of sulfate according to electronic-structure calculations. 16 However, as in other sulfateencapsulating structures of L1,13,14 the orientation of the urea groups deviates from the ideal tetrahedral arrangement requiring each of the six ureas to chelate an O
S
O edge of the sulfate. Instead, the two L1 ligands are symmetryrelated by an inversion center, resulting in distorted hydrogen-bonding geometries for three of the urea groups (lower half in Figure 1a), with two of them chelating O vertices rather than O
S
O edges from sulfate. Another consequence of this centrosymmetric arrangement is that sulfate, which lacks an inversion center, is rotationally disordered over two positions to emulate the symmetry of the capsule. Though not ideal, this anion-binding geometry provides good shape and size recognition for sulfate, which had previously led to remarkable sulfate separation selectivity in competitive crystallizations.13,14 The SO4(L1)22
SBU is externally functionalized with six electron-donating pyridyl and CdO (urea) pairs, defining six chelating groups arranged in a pseudooctahedral geometry around the sulfate capsule (Figure 1b). These groups can engage in metal coordination or hydrogen bonding, thereby forming three-dimensional frameworks in which the SO4(L1)22
capsules act as 6-connecting nodes. We initially found that the

Figure 1. Crystal structure of 1. (a) Sulfate encapsulation by two molecules of L1, with the formation of 12 hydrogen bonds from six urea groups. (b) Anionic SO4(L1)22
SBU showing the pseudooctahedral arrangement of the six pyridyl
CdO(urea) chelating groups externally functionalizing the capsule. The green octahedron was drawn by connecting the six centroids defined by the N(py) and O(urea) atom pairs. (c) Cationic Na2(H2O)42þ SBU and its interactions with the six pyridyl
urea chelating groups. (d) NaCl-type framework formed by octahedral connectivity of the anionic and cationic SBUs (SO42
and Na2(H2O)42þ are shown in magenta and green; L1 is omitted for clarity).

octahedral Mg(H2O)62þ cationic clusters served as complementary SBUs to the SO4(L1)22
capsules by providing six chelating water hydrogen-bond donors, and forming a three-dimensional framework with octahedral connectivity.13 In the case of 1, the sulfate capsules are interconnected into a similar NaCl-type framework by Na2(H2O)42þ cationic SBUs. As depicted in Figure 1c, these SBUs are cyclic chair-shaped hexanuclear clusters consisting of two Naþ cations linked by two hydrogenbonded water dimers. The clusters act as pseudooctahedral 6-connecting nodes linking six pyridyl
urea chelating groups from different anionic capsules via Na coordination (py1
u1, py10
u10 ), water hydrogen bonding (py2
u2, py20
u20 ), or a combination of the two interactions (py3
u3, py30
u30 ). The crystal structure of 2 consists of similar anionic SO4(L1)22
cages interconnected into a NaCl-type framework by K2(H2O)22þ cationic clusters (Figure 2). The sulfate anions are encapsulated via 12 hydrogen bonds from the 6 urea groups lining the internal cavities of the capsules (Figure 2a). Once again, due to the inversion symmetry of the capsules, three of the urea groups (lower half in Figure 2a) deviate from the tetrahedral arrangement required for optimal sulfate binding, either displaying distorted hydrogen bonding parameters, or switching from an edge binding to a vertex binding mode. The outer surface of the capsules is decorated with six chelating pyridyl
CdO (urea) groups oriented in a pseudooctahedral symmetry. This geometry is evidently quite persistent, having been identified now in a number of different sulfate-binding capsules made from L1 in the presence of various external cations. In the present case, the cations consist of K2(H2O)22þ clusters acting as pseudooctahedral 6-connecting nodes. The cationic clusters link the sulfate capsules via K coordination by pyridyl and urea (py1
u1, py10
u10 ), K coordination by pyridyl and water hydrogen bonding to urea (py2
u2, py20
u20 ), 2703

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Figure 2. Crystal structure of 2. (a) Sulfate encapsulation by two molecules of L1, with the formation of 12 hydrogen bonds from six urea groups. (b) Pseudooctahedral capsule held together by K coordination and water hydrogen bonding. (c) K2(H2O)22þ cluster and its interactions with the six pyridyl
urea chelating groups.

Figure 3. (a) Powder X-ray diffraction: simulated pattern from the single-crystal X-ray of 1 (black), experimental pattern from 1 (red), and experimental pattern from the crystalline solid isolated from the waste simulant (blue); (b) FTIR spectra of 1 (blue) and the crystalline solid isolated from the waste simulant (red).

or K coordination by urea and water hydrogen bonding to pyridyl (py3
u3, py30
u30 ). While both 1 and 2 are made from alkali metal cations, and thus are tolerant to highly basic conditions, crystallization of 1 appeared particularly promising for sulfate separation from nuclear wastes, as it could benefit from the abundance of sodium cations in the waste. There are some clear advantages for employing a Na-based crystallization system. First, no external ionic components need to be added to the waste, thereby minimizing its volume. Moreover, the increment of sodium

removal from the waste is itself mildly beneficial in reducing the waste’s volume. Second, the high sodium concentration in the waste is expected to decrease the solubility of 1 through the common ion effect, which would increase the sulfate separation efficacy. We therefore set out to investigate the possibility of employing crystallization of 1 for sulfate separation under conditions similar to those found in nuclear waste. For this purpose, we prepared a simple aqueous solution containing 5 M NaNO3, 1.25 M NaOH, and 0.044 M Na2SO4, which simulates the tank waste in terms of the SO42
, OH
, and Naþ concentrations, as well as the high alkalinity (pH 14).5 The very high nitrate to sulfate molar ratio of 114 found in this simulant is also in line with the highly competitive conditions found in the actual waste. In order to assess the efficacy of sulfate separation by selective crystallization from an aqueous alkaline environment, a stoichiometric amount of solid L1 (2:1 relative to SO42
) was added to a solution of waste simulant, and the resulting suspension was magnetically stirred at room temperature, and the reaction progress was monitoring by powder X-ray diffraction (PXRD). The PXRD pattern of 1 started to emerge in 1
2 days, while the intensities of the peaks corresponding to L1 slowly decreased, completely disappearing after 4 days (Figure S1, Supporting Information). The final crystalline solid was isolated in 90% yield, and its PXRD and Fourier transform infrared (FTIR) spectra corresponded to those of pure 1 (Figure 3). The relatively long reaction time required to convert L1 to 1 is not surprising, considering the three-phase process at work, involving slow dissolution of L1 and its recrystallization into 1. While L1 is less soluble than 1 in pure water, the large excess of sodium cations in the waste simulant apparently reversed this order, favoring the crystallization of 1, presumably through the common ion effect. It follows that the ligand could be easily recovered in almost quantitative yield by stirring a suspension of 1 in fresh water at room temperature for 24 h, which yielded crystalline L1 and aqueous Na2SO4 (Supporting Information). The overall separation process is summarized in Scheme 1. Future investigations will address more complex waste simulant mixtures and seek a detailed understanding of thermodynamics and kinetics of crystallization, with the goal of optimizing the sulfate separation process under realistic nuclear waste conditions. We have demonstrated here for the first time that sulfate could be effectively separated from highly competitive aqueous alkaline solutions by selective crystallization of Na-coordination capsules. Functionalization of the anion-binding sites with six urea groups ensured efficient and selective sulfate encapsulation, through the formation of 12 complementary hydrogen bonds. The major breakthrough came from the employment of hydrated sodium 2704

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Crystal Growth & Design Scheme 1. Sulfate Separation from Aqueous Alkaline Solutions by Selective Crystallization of 1

cations to hold the capsules together in the solid state, which provided enhanced stabilities in aqueous solutions with high sodium concentrations and strongly basic pH. As these conditions are very similar to those found in nuclear wastes, the results reported here promise to provide in the near future the basis for a viable technology for sulfate separation from such wastes.

’ ASSOCIATED CONTENT

bS

Supporting Information. Crystallographic data in CIF format, details of the X-ray structural determinations, synthetic procedures, sulfate separation and ligand recovery experiments. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: (865) 574-4939. Phone: (865) 574-5018. E-mail: custelceanr@ ornl.gov.

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1. Crystal data for 2: C24H32KN10O6S0.50, M = 611.73, colorless prism, 0.23  0.12  0.06 mm3, monoclinic, space group P21/n (No. 14), a = 11.9118(7), b = 18.5604(11), c = 12.8360(7) Å, β= 91.6400(10)°, V = 2836.7(3) Å3, Z = 4, Dc = 1.432 g/cm3, F000 = 1284, T = 173(2) K, 2θmax = 56.7°, 16432 reflections collected, 7020 unique (Rint = 0.0361). Final GOF = 1.019, R1 = 0.0471, wR2 = 0.1086, R indices based on 5186 reflections with I > 2σ(I), 402 parameters, 0 restraints. Lp and absorption corrections applied, µ = 0.282 mm
1. (16) Hay, B. P.; Firman, T. K.; Moyer, B. A. J. Am. Chem. Soc. 2005, 127, 1810.

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