Binding Mechanism of the Amidoxime Functional Group on Chelating

Article pubs.acs.org/IECR

Binding Mechanism of the Amidoxime Functional Group on Chelating Resins toward Gallium(III) in Bayer Liquor Hongming Long,† Zhuo Zhao,*,† Yanquan Chai,† Xiaohang Li,† Zhongsheng Hua,† Yanping Xiao,‡ and Yongxiang Yang‡ †

School of Metallurgical Engineering, Anhui University of Technology, 243002 Maanshan, Anhui, PR China Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD, Delft, The Netherlands

Ind. Eng. Chem. Res. 2015.54:8025-8030. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 09/03/15. For personal use only.

‡

ABSTRACT: Amidoxime is of broad interest as a functional group for extraction and separation of different metals from aqueous solution due to its high affinity for a wide range of metal ions. Resins which contain an amidoxime group have exhibited good extractive properties for gallium and are adopted as the most efficient adsorbent for the recovery of gallium from Bayer liquor. However, the coordination mechanism between the amidoxime functional group of the resin and Ga(III) in Bayer liquor is unclear so far. Here, to identify the preferred action and the binding motif when amidoxime binds with Ga(III), we performed density functional theory (DFT) calculations and subsequent FT-IR spectra analysis for the amidoxime resin before and after adsorption of Ga(III) in Bayer liquor. Single-crystal X-ray diffraction was also used to validate the prediction. The fully relaxed structure and binding energy calculations of a series of complexes containing acetamidoximate (AO−) and gallium species, [Ga(OH)4‑x(AO−)x]−, showed that, of the three general binding motifs of the amidoxime group toward metal ions, complexes with oxygen binding motif I are more stable than complexes with chelate binding motif II or η2 binding motif III; and Ga(III) in Bayer liquor prefers to coordinate with only one amidoximate rather than two or more amidoximates. The FT-IR spectrum of the resin after adsorption confirmed the coordination between Ga and oximido by the shifting of N−O and O−H stretching vibrations to higher wavenumber and the appearance of Ga−O stretching vibrations. No characteristic stretching band of Ga−N was found in the FT-IR spectra, indicating binding motifs II and III unlikely happened. Structural analysis of the single-crystal Xray diffraction date revaled that the amidoxime ligand is deprotonated at a single oxime oxygen atom and coordinates to one gallium atom only through this oxygen, confirming the oxygen binding motif. Accordingly oxygen binding mode I might be the main binding motifs of amidoxime toward Ga(III) in Bayer liquor, which validated the prediction of DFT calculations. were able to reduce the absorbed Au3+ to metallic gold.6,7 Simultaneously the amidoxime group was oxidized into the carboxyl group. The amidoxime group also has potential applications in the preparation of gold9 or silver10,11 nanoparticles which exhibited good catalysis properties. Moreover the amidoxime functionalized materials also have promising applications in CO2 capture and storage.12 Gallium is a metallic element which has been widely used in advanced semiconductors because when it combined with elements of group 15 it displayed excellent semiconducting properties, such as gallium arsenide (GaAs) and gallium nitride (GaN).13,14 In nature, gallium is a very widespread trace element which usually occurs in combination with bauxite and sphalerite (ZnS).15 Generally, gallium is recovered as a byproduct or at best a coproduct from mineral resources. It was estimated that about 90% of the world’s primary gallium is produced from Bayer liquor, which is obtained during alumina production from bauxite.16 Among the methods for extraction of gallium from Bayer liquor, ion exchange has been accepted as the most efficient method and is now widely applied to industrial practice.17 Resins functionalized with the amidoxime group such as ES-346 have exhibited good extractive properties

1. INTRODUCTION The amidoxime group contains both an oximido (=N−OH) and an amino (−NH2) at the same carbon atom. Under basic conditions the H in the hydroxyl of oximido would be released, leading to high affinity for metal cations. While in an acidic solution the amino would be protonated to −NH3+, which has the ability to coordinate with metal complex anions. Therefore, the amidoxime group is capable of forming strong complexes with a wide range of metal ions in aqueous solution,1 and amidoxime-containing resins or fibers have been of extensive application for the extraction and separation of different metals. For example, polymeric adsorbents that contain amidoxime have been shown to be one of the few materials that are able to sequester the uranyl ion, UO22+, at the slightly alkaline pH of seawater (8.0−8.3);2 and amidoxime chelating resins are adopted as effective adsorbents for removing heavy toxic metals from wastewater since they exhibited high affinity toward Pd(II) and Cu(II) and to a much lesser extent for other metal ions such as Zn(II), Cd(II), Co(II), etc.1,3,4 According to the Hard−Soft Acid Base (HSAB) theory,5 functional groups containing N donor atoms interact strongly with the soft acids like precious metals. Consequently, fibers or resins with the functional group of amidoxime which contains two N atoms have been used for the recovery of Au(III) and Ag(I) from wastewater.6−8 It is worth noting that in the solution containing Au(III), Cu(II), Zn(II), and Cr(III) the amidoxime chelating fibers not only presented high selectivity to Au(III) but also © 2015 American Chemical Society

Received: Revised: Accepted: Published: 8025

May 18, 2015 August 6, 2015 August 6, 2015 August 6, 2015 DOI: 10.1021/acs.iecr.5b01835 Ind. Eng. Chem. Res. 2015, 54, 8025−8030

Article

Industrial & Engineering Chemistry Research 4AO + Ga(OH)4 − = Ga(AO)4 − + 4H 2O


for gallium in terms of the loading capacity, kinetics, and selectivity. However, the coordination mechanism between the amidoxime group of the resin and Ga(III) in Bayer liquor remains unclear. Generally the binding motifs of the amidoxime group in isolation toward metal ions undergoes a hydroxyl proton dissociation4 and might be in three different modes2,18 (Figure 1). In the first motif, metal ion is bound to the oxygen atom.

In the present work we have performed density functional theory (DFT) calculations to identify the preferred action and the binding motif when amidoxime binds with Ga(OH)4−. Structures of the fully relaxed geometry and binding energy of possible binding motifs in a series of complexes containing acetamidoximate (AO − ) and gallium species, [Ga(OH)4‑x(AO−)x]−, are presented. To verify the prediction of theoretical results, a chelating resin containing the amidoxime group was used to experimentally investigate the adsorption of Ga(III) in Bayer liquor and single-crystal X-ray diffraction of the complex prepared from the solution of NaGa(OH)4 with MeOH/CH3NO2 and benzamidoxime. In-depth understanding of the binding mechanism between amidoxime and Ga(III) will help us to construct a more efficient process for the recovery gallium from Bayer liquor.

2. METHODOLOGY 2.1. Modeling. Calculations were performed with the commercial software Material Studio 5.0 (Accelrys, San Diego, CA, USA) using density functional theory (DFT). The geometry of the molecule was fully optimized in the framework of a generalized-gradient approximation (GGA) hybrid exchange-correlation functional and a double numeric plus polarization (DNP) basis set. No constraint was applied to bond distances, bond angles, or dihedral angles in the calculations, and all atoms were free to optimize. Frequency calculations were performed to verify that geometries were minima. Binding energies (BE) were calculated as follows20

Figure 1. Three possible bonding motifs between amidoxime and metal ions: I, oxygen; II, chelate; III, η2.

Kelley et al.18 demonstrated that when a 4,5-di(amidoxime)functionalized imidazole ligand reacted with VO2+, the ligand was deprotonated at a single oxime oxygen atom and coordinated to the vanadium atom only through this oxygen. A second motif involves the formation of a five-membered chelate ring with oxygen and nitrogen donor atoms. Li et al.19 showed that during the absorption process of polyacrylonitrile amidoxime nanofibers toward Cu(II), the H in the hydroxyl was released by the forming of an ion, and the bonding electrons came from the O atom and the Cu ion, respectively, and form the bond. At the same time, the lone pair of valence electrons from the N atom of the nitrile group entered the empty valence shell of Cu to form the coordination band. A five-membered ring chelate was finally formed. Finally, the third motif exhibits η2 binding with the N−O bond of oximido. This binding motif is almost exclusively observed in oximate complexes with UO22+.2,18 Bayer liquor is a strong basic solution with NaOH concentration more than 200 g/L, wherein Ga(III) is usually in the form of Ga(OH)4−. When Ga(OH)4− is adsorbed by the amidoxime chelating resin, the reactions were triggered by the release of H+ on hydroxyl. Subsequently the remaining O atom with electrons forms a bond with Ga. At the same time, the released H+ will take OH− from Ga(OH)4− to form H2O. The reactions between amidoxime (AO) and Ga(OH)4− might be described as eqs 1−4. Unfortunately, during these reactions the exact nature of the binding motif of Ga(OH)4− in the amidoxime-based resin remains obscure. AO + Ga(OH)4 − = [AO− ‐Ga(OH)3 ]− + H 2O

(1)

2AO + Ga(OH)4 − = [2AO− ‐Ga(OH)2 ]− + 2H 2O

(2)

3AO + Ga(OH)4 − = [3AO− ‐GaOH]− + 3H 2O

(3)

(4)

BE = Ex AO‐Ga(OH)

4‐ x

+ xE H2O − (xEAO + EGa(OH)4−)

(5)

where ExAO‑Ga(OH)4‑x, EH2O, ExAO, and EGa(OH)4− refer to the energy of the xAO-Ga(OH)4‑x, H2O, AO, and Ga(OH)4−, respectively. 2.2. Adsorption. To validate computed results, adsorption experiments were conducted with an amidoxime chelating resin. The resin is obtained from the Henan branch of Chalco., beige-colored beads, having a particle size range of 0.2−0.6 mm and a nitrogen capacity of 12.1%. 1 g of an amidoxime chelating resin was added to 200 mL of a real Bayer liquor which is from the same company and contains 210 mg/L Ga in a 250 mL glass bottle with a stopper. After being shaken at 298 K for 24 h, the solution was separated from the resin. The loaded resin was first washed to pH = 7 with deionized water and then dried for FT-IR analysis. The spectra were recorded on a Bruker TENSOR 27 at a resolution of 4 cm−1. 2.3. Crystallization. Single-crystal X-ray diffraction was also used to test the prediction of the calculation. Crystals suitable for X-ray diffraction were prepared by slowly evaporating a solution of NaGa(OH)4 with five molecular equivalents of MeOH/CH3NO2 and benzamidoxime, in the presence of triethylamine. Single-crystal X-ray patterns were recorded on a Bruker SMART APEX II diffractometer with Mo Kα radiation (λ = 0.7107 Å). The structures were solved by direct methods and refined with the computer program SHELX.

3. RESULTS AND DISCUSSION 3.1. Structure and Binding Energies. DFT calculations were performed to evaluate the geometries and relative stabilities of different amidoximate binding motifs with Ga(OH)4−. Given its small size, acetamidoximate (Figure 1, 8026

DOI: 10.1021/acs.iecr.5b01835 Ind. Eng. Chem. Res. 2015, 54, 8025−8030

Article


Industrial & Engineering Chemistry Research

Figure 2. Fully relaxed structures of [Ga(OH)4‑x(AO−)x]− complexes.

R = CH3), referred to as AO−, was chosen as the representative ligand for these calculations. The relative stabilities of motifs I, II, and III were extensively investigated in a series of [Ga(OH)4‑x(AO−)x]− complexes. This resulted in four categories of anionic complexes with 1−4 AO− ligands. For each stoichiometry, fully relaxed structures are given in Figure 2. Table 1 summarizes the binding energies and geometric parameters of these structural isomers. Table 1 demonstrates that, among these 12 structures of four binding motifs, complexes with binding motif I are energetically more stable than complexes with binding motifs II or III. For motifs II, their binding energies are not much lower than motifs I especially when binding with two and three AO. However, when Ga(OH)4− combines with two or more AO, the binding energies of motifs III are much lower. Based on these calculated binding energies, it could be concluded that the oxygen binding mode I and chelate mode II are the preferred motifs for the complexation of Ga(OH)4− by the amidoximate ligand. Furthermore, the conclusion can also be made from Table 1 that Ga(OH)4− prefers to coordinate with one AO. According to Figure 2 and the geometric parameters in Table 1, the Ga−O bonds are very short (1.88−1.98 Å). The O−N bonds (1.38− 1.43 Å) are comparatively unchanged relative to the length of

the protonated oxime (1.40 Å21 vs 1.40 Å22), and the Ga−O− N bond angle is near tetrahedral from 106.44° to 122.85°.23 All these parameters suggest that the oxime O−Ga bond is similar in strength and covalency to an O−H bond. The intramolecular hydrogen bond in the ligand is preserved during the complexation, which is consistent with the complexation of uranyl by AO.18 The Ga−N bonds in motifs II and III are 3.30−3.59 Å and 2.66−2.79 Å, respectively, which are much longer than the U−N bonds in the UO22+-AO complexes indicating the weak interaction between Ga and N atoms. This might be explained by the HSAB theory5 that N-containing ligands belong to the borderline base while Ga(OH)4− is a hard acid, so the coordination force between them is weak. UO22+ is also a hard acid, but due to the large radius of U(VI), UO22+ is relatively softer than Ga(OH)4−. Consequently the interaction between U and N is relatively strong, resulting in the η2 motif. Furthermore, when AO coordinates with metal ions such as Cu2+ which belong to the borderline acid, a five-membered chelate ring could be formed due to the very strong interaction between N and metal ions. This has been observed during the complexation of AO with Cu(II).19 Accordingly the quite long Ga−N bonds in motifs II suggest that motifs II are unstable, although their binding energies are relatively high. In summary, 8027


Article 111.59/111.78/112.23/114.77 115.28/118.46/112.75/107.79 108.04/106.44/108.60113.91 1.38/1.39/1.39/1.40 1.39/1.39/1.39/1.40 1.42/1.42/1.42/1.42 1.30/1.30/1.30/1.30 1.30/1.30/1.30/1.30 1.29/1.29/1.29/1.29 3.59/3.43/3.47/3.37 2.69/2.66/2.70/2.78 −16.572 −13.34 −0.887 I/I/I/I II/II/II/II III/III/III/III

1.90/1.91/1.90/1.88 1.89/1.91/1.92/1.89 1.90/1.90/1.90/1.89

1.42/1.40/1.42/1.43 1.43/1.41/1.43/1.43 1.40/1.40/1.41/1.41

111.98/111.33/108.88 119.19/114.00/119.03 108.55/109.87/107.43 1.40/1.40/1.40 1.38/1.39/1.39 1.42/1.42/1.43 1.30/1.30/1.30 1.30/1.30/1.30 1.29/1.29/1.29 3.30/3.31/3.35 2.71/2.72/2.73 −18.307 −17.471 −4.383 I/I/I II/II/II III/III/III

1.90/1.93/1.91 1.91/1.91/1.92 1.91/1.93/1.90

1.41/1.40/1.41 1.43/1.42/1.42 1.39/1.39/1.39

109.28/111.13 115.77122.85 109.86/110.24 1.40/1.41 1.39/1.39 1.43/1.43 1.30/1.30 1.30/1.30 1.29/1.29 3.39/3.38 2.73/2.75 −21.175 −20.349 −11.401 I/I II/II III/III

1.91/1.94 1.90/1.93 1.91/1.95

1.40/1.40 1.42/1.42 1.39/1.39

110.99 119.63 111.27 1.41 1.39 1.43 1.30 1.30 1.29 3.40 2.79 −23.879 −20.819 −18.764 I II III

1.96 1.90 1.98

1.39 1.42 1.38

CN Ga−N Ga−O

O−N

the binding motifs between AO and Ga(OH)4− might be dominated by oxygen binding mode I. 3.2. FT-IR Analysis. To experimentally validate the prediction that the oxygen motif is the preferred Ga(OH)4− binding motif for amidoximate ligands, the FT-IR spectra of the amidoxime chelating resin before and after absorption of Ga(III) in Bayer liquor were recorded and shown in Figure 3.

Figure 3. FT-IR spectra of the amidoxime chelating resin before and after adsorption: (a) whole range, (b) range between 500 and 2000 cm−1.

The FT-IR spectrum of the resin before adsorption exhibited the characteristic stretching bands of N−O (935 cm−1), CN (1646 cm−1), −CH3 (1384 cm−1), and −CH2 (2929 cm−1).24 The bands at 3100−3700 cm−1 (peak position at 3399 cm−1) are assigned to O−H and −NH2 stretching vibrations.9 These bands confirm the amidoxime group in the resin. In the FT-IR spectrum of the resin after adsorption, the characteristic stretching bands of CN, −CH3, and −CH2 remained unchanged. However, the bands for N−O and O−H stretching vibrations shifted to 950 cm−1 and 3200−3700 cm−1 (peak position at 3421 cm−1), respectively. Simultaneously a new band appeared at 592 cm−1 which corresponds to the stretching vibrations of Ga−O.25 Therefore, the coordination between Ga and N−O−H indeed happened during the adsorption. It is worth noting that the characteristic stretching band of Ga−N, which is at around 800−850 cm−125, was not found in the FTIR spectrum after adsorption, indicating the coordination

[Ga(OH)3(AO−)]− 1 2 3 [Ga(OH)2(AO−)2]− 4 5 6 [Ga(OH) (AO−)3]− 7 8 9 [Ga(AO−)4]− 10 11 12

bending energy (kcal/mol) AO motif stoichiometry

bond length (Å)

Table 1. Binding Energy and Geometric Parameters of [Ga(OH)4‑x(AO−)x]− Complexes


C−N

Ga −O−N angle


8028



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between Ga and N unlikely happened. Consequently the conclusion could be made that the oxygen binding mode I might be the main binding motif of AO toward Ga(OH)4− in Bayer liquor, which confirms the prediction of the DFT calculation. 3.3. Single-Crystal X-ray Diffraction. Structural analysis of the crystals prepared for single-crystal X-ray diffraction revealed the formation of [Ga(OH)3AOMeOH]. The complex consists of one Ga3+ ion linked by one bridging amidoxime ligand (Figure 4). The amidoxime ligand is deprotonated at a

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86 138 5550 5238. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51104002, 51274005, and 51204002).

■

REFERENCES

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Figure 4. Views of [Ga(OH)3AOMeOH] structure obtained by singlecrystal X-ray diffraction.

single oxime oxygen atom and coordinates to one gallium atom only through this oxygen. The structure of this complex confirms that amidoximate ligands bind Ga(OH)4− with the oxygen binding motif.

4. CONCLUSIONS Binding energy calculations showed that oxygen binding motif I and chelate binding motif II are the preferred binding modes when the amidoximate anion binds with Ga(OH)4− in Bayer liquor. However, by observing the optimized structures of [Ga(OH)4‑x(AO−)x]− complexes, we noticed that the distances of Ga−N bonds in motif II are irregularly long, suggesting a very weak interaction between Ga and N atoms. According to the HSAB theory, as the affinity of N-containing ligands which belong to the borderline base toward hard acid Ga(OH)4− is very weak, the coordination between Ga and N in the AO group unlikely happens. Therefore, the binding motifs between AO and Ga(OH)4− might be dominated by oxygen binding mode I. Furthermore, the very short Ga−O bonds, the comparatively unchanged length of the O−N bond relative to the length of the protonated oxime, and the near tetrahedral bond angle centered at O suggest that the oxime O−Ga bond is similar in strength and covalency to an O−H bond. The FT-IR spectrum indicated that the bands for N−O and O−H stretching vibrations of the amidoxime chelating resin after adsorption shifted to higher wavenumbers. Simultaneously a new band corresponding to the stretching vibrations of Ga−O appeared. These confirmed the coordination between Ga and N−O−H. At the same time, the characteristic stretching band corresponding to Ga−N was not found from the FT-IR spectrum, revealing that there might be no coordination between Ga and N atoms. According to the refined structure of the complex based on single-crystal X-ray diffraction data, amidoximate ligands also coordinated with Ga(OH)4− in the form of an oxygen binding motif. Consequently when amidoxime binds with Ga(OH)4− the oxygen binding mode I might be the main binding motif, which is consistent with the results of DFT calculations. 8029


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Binding Mechanism of the Amidoxime Functional Group on Chelating

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