Molybdenum(VI) Coordination in Tributyl ... - ACS Publications

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Molybdenum(VI) Coordination in Tributyl Phosphate-Chloride based system Peter Tkac, Md Abdul Momen, Andrew T Breshears, M. Alex Brown, and George F. Vandegrift Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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SYNOPSIS

2 MoO2Cl2

1.5

absorbance

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

Industrial & Engineering Chemistry Research

1 0.5 0

4M HCl

2M HCl

1.5M HCl 1M HCl

220

240

260

280 300 wavelegth, nm

320

340

In TBP, there is a striking resemblance of spectral features of solid MoO2Cl2 crystals dissolved in 30% TBP/n-dodecane and spectra of Mo in TBP/n-dodecane extracted from 4 M HCl; variations in the extinction coefficients of Mo extracts from 1-4 M HCl at 220-280nm are within 3%. Spectra show that in this range of HCl conditions, extracted Mo species in TBP are MoO2Cl2 with two peak maxima at 227.5nm and 253.5nm.

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Molybdenum(VI) Coordination in Tributyl Phosphate-Chloride Based System Peter Tkac*, Md Abdul Momen, Andrew T. Breshears, M. Alex Brown, George F. Vandegrift Argonne National Laboratory, Nuclear Engineering Division, Argonne, IL, 60439 United States KEYWORDS: Molybdenum, speciation, solvent extraction, tri-n-butyl phosphate, spectroscopy

ABSTRACT

Fundamental coordination chemistry of Mo(VI) at its macro concentrations in solvent extraction systems is of great importance for industrial processes that require the purification or recovery of large concentrations of Mo. The coordination of Mo(VI) in tri-n-butyl phosphate (TBP) from solutions of hydrochloric acid and up to 0.3 M Mo was investigated using UV, FTIR, and 31P NMR spectroscopies, as well as EXAFS. From these techniques we resolved near-neighbor atoms, speciation, structural information on the coordination environment, as well as thermodynamic parameters affiliated with the solvent extraction of Mo(VI) and HCl. The solvated extracted form of Mo(VI) as MoO2Cl2·2TBP was identified. High extraction yield of Mo at >5 M HCl concentration is driven by replacing HCl in the organic phase by Mo. The existence of additional organic Mo adducts is also discussed with the aid of density functional theory, however no evidence of dimeric or polymeric Mo species was found to be present in TBP.


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INTRODUCTION The radioactive isotope,

99

Mo, is a parent of the most widely used diagnostic radioisotope –

99m

Tc – which is used to diagnose millions of cases of heart disease and cancer every year. As

alternatives to the fission-based production technologies for

99

Mo, there has been a noteworthy

interest in use of enriched 100Mo or 98Mo for its production using 100Mo(γ, n)99Mo by accelerator [1-3], by neutron capture via

98

Mo(n, γ)99Mo [4-6], or direct production of

99m

Tc via

100

Mo(p,

2n)99mTc using cyclotrons [7, 8]. Due to low specific activity (LSA) 99Mo produced in (γ, n) and (n, γ) production pathways, these technologies require to use different generator system compared to generators currently used for fission made Mo. Several options for LSA Mo/Tc generator systems have been recently discussed [9] Moreover, high costs of the enriched Mo material ($850– $3000 per gram of >95% enriched

100

Mo [10]) creates a demand for efficient purification and

recycling methods to process enriched Mo material [7, 9-12]. To design an effective recovery process for handling kilogram quantities of Mo, the solvent extraction based MOEX (molybdenum extraction) process was developed [9] under the sponsorships of the National Nuclear Security Administration Material Management and Minimization Program’s (NNSA M3) scope to fulfill domestic 99Mo demands without highly enriched uranium (HEU). Consequently, our aims are to scale this technology to be implemented on gram- to kilo-gram quantities of recovery and to probe and understand the aqueous/organic coordination environments of Mo(VI) with the intent of accurately modelling the extraction separations.


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Molybdenum has a rich chemistry with oxidation states of 0, II, III, IV, V, and the most stable VI. Under alkaline conditions, molybdate ion MoO42- with four terminal oxygens is the primary Mo species. Cruywagen and Heyns [13] determined that under strong acidic conditions (>0.5 M HClO4) and [Mo]≤2.5 mM) H2MoO4, H3MoO4+, MoO22+, Mo2O4(OH)3+ and Mo2O52+ are the predominant species. In the presence of 1-11 M HCl, additional species such as H3MoO4Cl, MoO2Cl2 and MoO2Cl3- exist [14]. Given the complicated Mo chemistry at its higher concentrations involving dimeric and possibly polymeric oxo cations and anions, most of the publications focuses on speciation of Mo at low concentration (≤0.1 mM Mo), at which point the polymeric species may be negligible [15-18]. Additionally, UV spectroscopy served as the primary means to determine the speciation of Mo, and, due to very high extinction coefficients for Mo species under acidic condition (>103 cm-1mol-1L in 200-250nm), many experiments were limited to low Mo concentrations. Yet metallurgical practices, or processes to recover valuable enriched 98/100

Mo for

99

Mo/99mTc production, require processes that can accommodate high Mo

concentrations of up to ~0.4 M Mo. Solvent extraction is an industry standard practice considering its high metal loading capability and the ability to recycle the solvent in order to minimize its footprint. At low concentration of non-complexing acid (pH=1.7 HNO3) and very low Mo concentrations (0.1mM), the main species extracted by TBP reported is molybdic acid H2MoO4, which might be solvated with HNO3 [19, 20]. Goletskii et al [20], claim that at higher Mo concentration (>3 mM) Mo oligomers are present in the TBP phase. Similar conclusions were also made for other extraction systems [18, 21-24]. In the case of Mo extraction from HCl, literature data agree that at low Mo concentration MoO2Cl2 is the primary component that is extracted [15, 25]. Xia et al [26] proposed extraction of dimeric Mo species by 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (P507) from HCl. Nelidov


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and Diamond [15] further proposed that at high HCl concentrations where MoO2Cl3- species in aqueous phase are significant, extraction of higher order chloro-complexes could be possible. Although extraction of the complex acid H[MoO2Cl3] in diethyl ether has been reported [27], and [H3O(H2O)3nACP]+[MoO2Cl3(H2O)]- solvate by acetophenone (ACP)[28], to the best of our knowledge, evidence of tri-chloro Mo species in TBP has not been experimentally determined. Here we report our experimental results on the extraction of molybdenum and hydrochloric acid into tri-n-butyl phosphate diluted by tetrachloroethylene (TCE), which is the basis for the MOEX process [9], and present spectroscopic evidence of Mo speciation in the organic phases at macro Mo concentration. Spectroscopic data are supported by DFT calculations, and both suggest that MoO2Cl22TBP species is a major species present in a wide range of experimental conditions.

EXPERIMENTAL Reagents. TBP (99%+, Acros Organics) was further purified by washing it three times with 0.5 M sodium carbonate solution and water. Tetrachloroethylene was used as received, and was chosen to prevent formation of the formation of 3rd phase in TBP, which occurred when n-dodecane was used. Hydrochloric acid was trace metal grade. All other chemical reagents used were of analyticalreagent-grade purity and were used without further purification. All aqueous solutions were prepared with deionized water with a resistivity ≥ 18 MΩ·cm.

Solvent extraction. Samples were vigorously agitated in extraction vials using a vortex mixer for two minutes with an equal aqueous and organic volume under ambient temperature conditions (20°C). After agitation, the phases were separated using centrifugation, and aliquots from both organic and aqueous phases were taken to measure equilibrium concentrations of the metal ion


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based on the activity of 99Mo using a NaI detector (Wizard with RiaCalc WIZ program 3.6 software with energy discrimination of 700–900 keV) or a HPGe (Ortec) detector. The 99Mo was provided from a TechneLite generator (Lantheus) that had been eluted with 10 mL of 1 M NH4OH ammonium hydroxide. Concentration of Mo was determined using ICP-MS. The data obtained by ICP-MS are reported with 10% uncertainty.

UV spectroscopy. UV-Vis absorption spectra were collected using quartz cuvette and a Varian Carry 5000 spectrophotometer with a resolution of 0.25 nm. All spectra were taken against the blank solution, which was identical to the measured sample except for the component of interest. For Mo concentrations above 1×10-4 M Mo, 1mm, 0.1mm and 0.01mm quartz Starna cells were used. Due to a strong absorbance of TCE in UV region, n-dodecane was used as a diluent for UV spectra of Mo samples in 30% TBP (1.1 M) extracted from [HCl] 5.5 M were taken using neat TBP due to interference of TCE and formation of 3rd phase if dodecane was used as a diluent.

NMR spectroscopy.

31

P NMR spectroscopy was obtained on a 500 MHz ESB Bruker

spectrometer. 31P NMR spectra were referenced externally to 85% H3PO4 in D2O at 0.00 ppm. The pulse program used for all samples was zgig30, meaning that the pulse width was a 30o pulse width and that the proton decoupling was performed via inverse-gating. The pulse delay time (D1) was set to 10 seconds to allow for 31P spin relaxation. Number of scans (NS) was set to 320 and line broadening (LB) was set to 5.0 Hz.


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Infrared spectroscopy. A Nicolet 6700 Fourier Transform Infrared Spectrometer equipped with a Smart iTR (Attenuated Total Reflectance) sampling accessory with a diamond crystal was used to collect the spectra. A few microliters of the organic phase were deposited on the diamond disk for every measurement and scanned immediately after. Thirty-two scans with a resolution of 4 cm1

in the range of 650-4000 cm-1 were taken for each sample. A scan of the empty diamond iTR

accessory was used to collect background spectrum.

EXAFS. Samples were irradiated in a spectrophotometric cuvette that contained a Kaptoncovered window. EXAFS measurements were made on the insertion device beam line of the Materials Research Collaborative Access Team (MRCAT) at the Advanced Photon Source, Argonne National Laboratory.[29] The X-ray energy was selected with a liquid nitrogen cooled, double-crystal Si(111) monochromator, and a platinum-coated mirror was used for harmonic rejection. The monochromator was calibrated using a Mo foil (K-edge = 20,000 eV). Data were collected in step-scan transmission mode using an Ar-filled ion chamber. The scan mode was continuous scanning with undulator tracking. The average shift within a series of scans for each sample was less than 0.02 eV. The data were truncated to Δk = 4―16 Å-1 and ΔR = 1—3 Å, and the post-edge spline function and data fits were generated using Athena and Artemis software, respectively.[30, 31] The E0 was given a value of 20013 eV due to the smaller electronic transitions located just past the 20,000 eV K-edge. The amplitude reduction factor, S02, was fixed at 0.9.[32] Multiple scatter pathways were ignored and coordination numbers were fixed.

DFT calculations. Initial geometries were first optimized using the MM2 calculations found in Chem 3D of the Chem Office 15.1 suite [33]. Each of the MM2 geometries was input into the


6


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Gaussian09 suite of software for examining their electronic structure [34]. Density functional theory was performed at the B3LYP (Becke-3 exchange and Lee-Yang-Parr correlation functional) level of theory [35-37]. Full geometry optimizations were performed, and stationary points were determined to be global minima using analytical frequency calculations. The Stuttgart/Dresden triple zeta basis set was used to model molybdenum [38-39]. The Pople double-ζ quality basis set, 6-31G(d,p), was used for all non-metal atoms [40-41]. Solvation model was calculated using the default oniom option within the Gaussian09 software. Solvent parameters were those defined within the program by using the keywords Solvent = Tetrachloroethylene or Water, depending on the geometry and calculation being interrogated. All final geometries of the optimized molecules are included in the Supporting Information as xyz coordinate files.

Potentiometric titration. Potentiometric titrations were performed using Metrohm 836 Titrando at constant ionic strength and at 25°C. For maintaining constant ionic strength 0.1 M NaCl solution was added to the titration vessel. Titrations were conducted on aqueous samples of Mo solutions in HCl before and after extraction to calculate the organic proton concentration. For every titration in the presence of Mo, concentration of Mo was determined by ICP-MS. For samples containing Mo, the concentration of proton was calculated upon subtracting the proton released by MoO22+ species according to 2H2O + MoO22+=MoO42- + 4H+.

Determination of organic chloride concentration. The concentration of chloride in TBP was determined using silver chloride precipitation technique. 1 M AgNO3 stock solution was used. The aqueous samples of Mo solutions in HCl were combined with excess AgNO3. The amount of silver nitrate used depends on concentration of chloride and Mo, and was in excess to make sure all


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chloride was precipitated. It should be noted that Mo also interacts with AgNO3 (molybdate species interact with Ag to form silver molybdate Ag2MoO4) and its precipitate was removed by repeated washes using 0.1 M HNO3, H2O and 1 M NaOH. Determination of chloride concentration in the presence of Mo was checked by standard addition using standard 1 M HCl. AgCl precipitate was removed using 0.22µm PVDF Steriflip® filter, and chloride content was determined gravimetrically.

RESULTS AND DISCUSSION Extraction of HCl by TBP Extraction of HCl by TBP can be described as: .

∙

(eq. 1)

and its extraction profile is shown in Figure 1. Extraction constant KH for HCl in TBP/TCE system was determined using the MS Excel Solver plugin. The Solver was set to find optimal value of extraction constant with minimum differences between the calculated and experimentally observed organic HCl concentrations. Extraction constant determined is KH=2.88×10-4. Based on the extraction of HCl by TBP/TCE (Figure 1), one would expect to see a maximum in Mo distribution ratio and decreased Mo extraction at high HCl due to saturation of TBP with HCl and lack of free TBP for solvating the metal in the organic phase. However, it was determined that upon extraction of Mo into TBP, Mo replaces HCl from the organic phase. Data in Table 1 show the concentration of HCl in 30% TBP in TCE after extraction with and without presence of Mo.


8


1 0.8 [HCl]org

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

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0.6 0.4 0.2 0 2

4

6 [HCl]aq

8

10

Figure 1. Experimental concentration of HCl in 30% TBP in TCE (circles), and calculated concentration of HCl in 30% TBP in TCE (line) calculated according to eq. 1

In the absence of Mo and after extraction from ~8.8 M HCl, about 80% of TBP is associated with HCl. However, in the presence of ~0.3M Mo, the concentration of HCl in the TBP is much lower (~0.08 M HCl in TBP after extraction from 9.2 M HCl, which corresponds to 2 indicating presence of some HCl or tri-chloro molybdenum species.


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Table 1. Concentrations of Mo, H+ and Cl- in aqueous and 30% TBP/TCE phases before and after contact at a 1/1 volume ratio. Aqueous concentrations [Mo], M

[H+], M

initial

initial

Organic concentrations [Cl-], M initial

[H+], M after

[Cl-], M

[Mo], M

after

after

[Mo]org, M

[H+]org, M

[Cl-]org, M

Cl:Mo ratio

4.74

4.74

N/A

N/A

0.046a

5.41

5.41

N/A

N/A

0.093a

5.73

5.73

N/A

N/A

0.142a

6.35

6.35

N/A

N/A

0.246a

6.69

6.69

N/A

N/A

0.316a

7.74

7.74

N/A

N/A

0.586a

8.37

8.37

N/A

N/A

0.763a

8.78

8.78

N/A

N/A

0.860a

0.292

2.51

2.64

2.32

2.37

0.166

0.126

0.19b

0.27

2.14

0.289

3.38

3.65

3.19

3.21

0.06

0.229

0.19b

0.44

1.92

0.301

4.16

4.70

4.10

4.12

0.002

0.299

0.05b

0.58

1.94

0.293

5.15

5.79

5.14

5.18

0.009

0.284

0.01b

0.61

2.15

0.270

6.78

7.35

6.77

6.79

0.001

0.269

0.01b

0.56

2.08

0.268

8.31

8.9

8.26

8.28

0.001

0.267

0.05b

0.62

2.32

0.273

9.20

9.75

9.11

9.13

-

0.273

0.08b

0.62

2.27

a- value corresponds to the concentration determined by titrating organic phase, b- value corresponds to the concentration determined as follow: [H+]org=[H+]-4[Mo]org , where [H+] value was determined by potentiometric titration of aqueous phase before and after titration. When MoO22+ is titrated from acidic to neutral pH, four protons are released according to: 2H2O + MoO22+=MoO42- + 4H+, which do not correspond to H+ from HCl. Concentration of Mo in the organic phase was determined by mass balance of aqueous Mo concentration before and after extraction.


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Mo-TBP species 1-5M HCl To understand the extraction of Mo from HCl media by TBP, it is essential to determine the solvation number of TBP. For this purpose, infrared spectroscopy was employed. Extraction of Mo into TBP causes a shift in P=O absorption band from 1280cm-1 to ~1200cm-1 (Figure 2a). Decrease in absorbance at 1280cm-1 is directly related to the composition of the complex between Mo and TBP, and therefore the slope obtained for a plot of decreasing free TBP concentration(TBP non-associated with Mo) against the increase of Mo in TBP gives a ratio between TBP:Mo. Figure 2b shows the slope of two dependences between free TBP and [Mo]org for Mo extracted from 2 M HCl and 10.5 M HCl, and agrees with the data obtained for MoO2Cl2 salt dissolved in 30% TBP/TCE. Spectra were normalized to give a constant intensity in the absorption band due to CH3 and CH2 deformation vibration at 1466 cm-1.


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0.2 a) absorbance

0.35M Mo →

0.1

0 1500

no Mo →

1400

1300 1200 wavenumber, cm‐1

1

1100 b)

0.8 [TBP]free

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


0.6 0.4 0.2 0 0

0.1

0.2 [Mo]org

0.3

0.4

Figure 2. a) Infrared spectra of Mo extracts into 30% TBP in TCE from 10.45 M HCl for 0.0180.35 M Mo, b) slope (m = -2) of TBPfree (TBP non-associated with Mo) vs. organic Mo concentration for MoO2Cl2 solid dissolved in TBP and Mo extracted from 2.5 and 10.45 M HCl: MoO2Cl2 (triangles); extracts from 2.5M HCl (circles); extracts from 10.45M HCl (squares)

In TBP, there is a striking resemblance of spectral features of solid MoO2Cl2 crystals dissolved in 30% TBP/n-dodecane and spectra of Mo in TBP/n-dodecane extracted from 4 M HCl (Figure 3); variations in the extinction coefficients of Mo extracts from 1-4 M HCl at 220-280nm are within 3%. Spectra show that in this range of HCl conditions, extracted Mo species in TBP are MoO2Cl2 with two peak maxima at 227.5nm and 253.5nm. Even from 1 M HCl, when concentration of MoO2Cl2 in aqueous phase is very low (based on equilibrium constants


12


determined for Mo in HCl system [14]), the extracted Mo species very closely resembles that of the species extracted from 4M HCl.

2 MoO2Cl2

1.5 absorbance

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

Page 14 of 38

1 0.5 0

4M HCl

2M HCl

1.5M HCl 1M HCl

220

240

260 280 300 wavelength, nm

320

340

Figure 3. UV spectra of MoO2Cl2 dissolved in 30% TBP in n-dodecane (dashed line) and spectra of Mo in 30% TBP in n-dodecane after extraction from 1, 1.5, 2 and 4 M HCl.

EXAFS was used to resolve the local coordination environments of Mo and to accurately portray the neighboring elements. We identified the components of the predicted organic complex, namely the axial oxygens of molybdenyl, outer-sphere adducts such as water and organic solvents, and molybdenyl-chloride interactions. The Mo-TBP complex in question was expected to exhibit all three or more of these entities. First, the spectrum of Mo in 5M HCl (Figure 4) was obtained to resolve Mo(VI) in a highchloride environment and the most distinguishable component of the spectrum was the peak at approximately 1.3 Å (uncorrected for phase shift). This peak reflects the first shell oxido atoms of molybdenyl that results in the bond distance of 1.68 Å at a fixed coordination number of two; the EXAFS metrics are listed in Table 2. Further out from the molybdenyl signal is another


13

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distinguishable feature of the spectrum: chloride atoms 2.38 Å from the Mo center. Our approach initially fixed two chloride anions in the model and the results were mediocre (R-factor = 3.3%). Increasing the NCl to a fixed value of three improved the fit remarkably to our reported value below (1.1%). Nonetheless, the uncertainties derived from coordination numbers in EXAFS are significant and the value of NCl could lie between 2-4. These scattering paths and results are in good agreement with previous x-ray studies that probed Mo in a wider range of HCl concentrations [42]. Resolving the complexation equilibria of Mo(VI) with chloride has been the subject of many previous investigations but the results are generally in disagreement with one another, particularly on the cationic form of Mo(VI) in acidic solution [43]. There is agreement, however, in the fact that chloride anions have a strong influence on the electronic spectra and anion exchange [44] behavior of Mo(VI) and thus, exhibits relatively strong complexation constants. Yokoi et al [45] reported EXAFS spectra for dimeric Mo species, obtained for 0.4 M Mo in 2 M HClO4 solution with Mo-Mo bond length of 3.37 Å and Mo-O-Mo angle of 125°. The presence of dimeric species in non-complexing acid such as HClO4 is not surprising. No such feature was identified using EXAFS for Mo extracts in TBP and 0.1 M Mo aqueous samples in 5 M HCl, and presence of only monomeric Mo species was confirmed.


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Table 2: K-edge EXAFS fit results of Mo in 5 M HCl with R-factor of 1.1%. The uncertainties are reported in 1-σ notation; E0 = 20013 eV; values without uncertainty were fixed during the refinement. N

R (Å)

σ2×10-3 (Å2)

Mo 5M HCl

ΔE0 -3 ± 1

Mo-O1

2

1.679 ± 0.005

0.7 ± 0.4

Mo-Cl

3

2.385 ± 0.009

4.8 ± 0.4

Table 3: K-edge EXAFS fit results of Mo in 30% TBP in n-dodecane) and MoO2Cl2 in TBP with R-factors of 0.9% and 0.9%, respectively. The uncertainties are reported in 1-σ notation; E0 = 20013 eV; values without uncertainty were fixed during the refinement. N

R (Å)

σ2×10-3 (Å2)

Mo extract into TBP

ΔE 2±2

Mo-O1

2

1.692 ± 0.007

0.9 ± 0.4

Mo-O2

2

2.20 ± 0.02

3±1

Mo-Cl

2

2.41 ± 0.01

2.6 ± 0.9

MoO2Cl2 TBP

-0.1 ± 2.4

Mo-O1

2

1.690 ± 0.008

0.9 ± 0.4

Mo-O2

2

2.20 ± 0.03

3±1

Mo-Cl

2

2.41 ± 0.01

2.5 ± 1.0

By sufficiently characterizing molybdenyl, outer-sphere oxygens, and chloride ligands by EXAFS, we approached the Mo-TBP spectrum as a neutral solvated species in accordance with the adduct nature of TBP. The di-oxo ligands reprised themselves at 1.69 Å, which validate that


15

Page 17 of 38

the hexavalent oxidation state is retained during the extraction. Two additional oxygen backscatter pathways were calculated at 2.20 Å, which likely represent the coordinated TBP ligands. The chloride interaction was refined with a coordination number of 2; the bond distance is similar to the chloride scattering path in 5 M HCl. Finally, the data and fit of MoO2Cl2 dissolved in TBP were almost identical to the Mo extracted by TBP – including the two-coordinate chloride fit. (Figure 5, Table 3).

12 8

Mo(VI) 5M HCl

k3χ(k)

FT Magnitude

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


8

4

4 0 -4 -8

0 0

1

2

3

4

5

4

6

8

10

12

14

k (Å-1)

R+Δ (Å)

Figure 4. k3-Weighted K edge Mo EXAFS R- and k-space of 0.1 M Mo(VI) in 5M HCl. The points represent experimental data, the solid lines represent the model fit.


16


12

Mo(VI) 30%TBP

8 4

8

0 4

-4

0 12 0

1

2

3

4

5

MoO2Cl2-TBP

k3χ(k)

FT Magnitude

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

Page 18 of 38

-8 4

6

8

10

12

14

4

6

8

10 -1

12

14

8 4

8

0 4

-4 -8

0 0

1

2

3

4

5

k (Å )

R+Δ (Å)

Figure 5. k3-Weighted K edge Mo EXAFS R- and k-space of 0.1 M Mo(VI) extracted into 30% TBP n-dodecane from 5 M HCl (top row) and MoO2Cl2 dissolved in neat TBP (bottom row). The points represent experimental data, the solid lines represent the model fit.

The Debye-Waller factors are in good agreement with previous studies that also calculated lower values for the first shell axial oxygen atoms (~ 1×10-3 Å2) than the second coordination shell components (~ 4‒7×10-3 Å2).[32]. These differences likely reflect more vibrational contributions between the first and second coordination spheres. The ΔE ‒ which represent shifts in the absorption edge ‒ range from ‒3 to 2 eV across the three species. The values are reasonably low and good for the interpretations of atomic distances since ΔE shifts of around ±3 eV can alter bond distances by only ± 0.01 Å [46]. The coordination numbers presented here can be interpreted with roughly 50% uncertainty [31]. Nonetheless, the existence of the same Mo-O and Mo-Cl potentials


17

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in all EXAFS spectra (aqueous and organic) directed our conclusion to the conventional interpretation of neutral solvated species found in TBP – specifically MoO2Cl22TBP when extracted from ≤ 5M HCl. Results acquired by EXAFS are in great agreement with the data obtained by UV and FTIR spectroscopy that confirmed the main Mo species in TBP is MoO2Cl22TBP.

>5M HCl Speciation UV studies of Mo in TBP after extraction from >5 M HCl were performed by using neat TBP, due to formation of third phase when dodecane is used as a diluent, and interference of TCE in UV region. Although changes in UV spectra of Mo extracts into neat TBP from 5-9 M HCl are small, as the aqueous HCl concentration increases, noticeable increase in absorbance at ~300-310nm was detected (Figure 6). This could potentially indicate the presence of another Mo species in the organic phase at relatively low concentration. It should also be noted that the presence of water in the equilibrated organic phase and consequently the Mo coordination sphere could affect the Mo-UV spectra which was recently observed for trivalent lanthanides and actinides with organophosphorus extractants [47]. Another explanation for changes in spectra could be due to some presence of a tri-chloro Mo species. In aqueous spectra, MoO2Cl3- species appear to have a peak at ~227nm with shoulder at ~290-300nm [14]. Likewise, in the organic phase, increasing the aqueous HCl concentration results in Mo spectra with shoulder at ~300nm that is absent in the organic spectrum of MoO2Cl2. Based on this observation, some presence of another Mo species in TBP such as HMoO2Cl3xTBP or MoO2Cl2xTBP(HCl) from high HCl concentration is very likely. The main driving force for the extraction of tri-chloro Mo species at high HCl concentration could be the decrease of water activity. According to Bromley’s method


18


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of activity coefficient calculations [48], activity of Cl- at 11 M HCl is {Cl-}=390, while {H2O}=0.278. At dilute acid, hydronium ion can be solvated by three waters in the first coordination shell (other ions are hydrated as well). As the concentration of HCl is increased, more water is coordinated with the hydronium ion until even a primary hydration shell of all ions in the aqueous phase cannot be satisfied [15]. This lack of solvation satisfaction in the aqueous phase forces some of the ions into the organic phase. In this case MoO2Cl3- species partition into TBP with H+ and forms a neutral species solvated with TBP. Nevertheless, due to a small change in the UV spectra, the concentration of tri-chloro Mo species in TBP is expected to be low. Furthermore, 31

P NMR spectra of Mo extracts from ~11 M HCl also points towards existence of additional Mo

species besides MoO2Cl2. When investigating the region of 10 to 30 ppm, in which the MoO2Cl22TBP resonance is present at ~27 ppm, another resonance at 15.7 ppm was detected, which denotes a possible second Mo-TBP species in which the Mo-TBP is more electronegative. Based on the peak area at 15.7ppm, less than ~10% of second Mo species, likely tri-chloro Mo species, is expected. It should be noted that the 31P NMR spectra for the HCl pre-equilibrated 30% TBP in TCE were also recorded with changes in the region of -2 to 1 ppm. Samples were also examined using 95Mo NMR spectroscopy, however, no resonances were observed. This is most likely from TBP exchange causing the 95Mo resonance to broaden out, as seen previously for a molybdenyl di-nitrate complex extracted into an organophosphorus containing organic phase [49].


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Figure 6. a) UV spectra of Mo in neat TBP after extraction of 0.04 M Mo from 5-9 M HCl. Expanded view shows increase in absorbance at ~310nm with increased aqueous HCl concentration. B) 31P NMR spectra of Mo in 30% TBP/TCE after extraction from 5 and 11 M HCl.

Both UV and NMR spectra suggest some presence of other than MoO2Cl2 species in TBP extracts from higher HCl concentrations that based on similarity with aqueous spectra of MoO2Cl3could be assigned to some form of HMoO2Cl3xTBP. As previously mentioned, extraction of trichloro molybdenum species by diethyl ether [27], and acetophenone (ACP)[28] have been reported, the presence of tri-chloro Mo species in TBP has not been previously experimentally


20


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determined. Complex acids in the system containing TBP and nitrate, however, have been reported for U(IV) [50] and Np(IV) [51]. General formula for these complexes can be written as: (TBP)n2H5O2(H2O)p+[An(NO3)6]2- where An=U(IV) and Np(IV). While the chemistry of U(IV) and Np(IV) is very different from Mo(VI) these data suggest that presence of complex acids in neutral extractant such as TBP is possible. We further explored the extraction of MoO2Cl2 species by DFT calculations. As already proven experimentally, once the MoO2Cl2 species is formed, its extraction into TBP is highly favored. The change in the free energy for the formation of such Mo adduct with TBP is -246.46 kcal/mol, via B3LYP/SD-3Z/6-31g** DFT frequency calculations with an oniom solvent model. As the formation enthalpy is rather large for this level of theory, it is only indicative of favorable formation of the species and is not meant as a quantitative value. The nature of DFT calculations and DFT limitations has been examined extensively by other groups [52-55]. The major source of potential error for these solvation calculations is dative bonding between the metal species and the TBP in B3LYP, which has been documented for other dative bond interactions [56-57]. The work to achieve quantitative values of solvation was outside the scope of this study and thus the qualitative study sufficed. The high affinity for Mo-TBP adducts follows with the abundance of literature on molybdenum coordination and extraction by phosphate extraction ligands [58, 59]. Figure 7 illustrates the coordination environment around molybdenum which was found to be the minimum energy structures via DFT for both aqueous and organic MoO2Cl2 species. The bond distances are listed in Table 4. It is noteworthy that the bond distance for MoO2Cl2TBP2 is longer than that of the EXAFS fitting. This can be reasoned as the solvation model used for the DFT calculations used TCE rather than n-dodecane which was used for the EXAFS experiment. The formation of the polar environment, as opposed to a non-polar environment from n-dodecane,


21

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around the molecule using oniom calculations would affect the length of the dative bond to the molybdenum. The slight lengthening of the dative bond can also be an effect of the level of theory used.

Figure 7. Aqueous phase speciation of the molybenyl dichloride coordinated to water (left), which looks identical to the TBP coordinated molybdenyl dichloride, optimized in the organic phase (right). Butyl groups have been omitted for clarity.

While the major Mo species determined to be extracted from HCl by TBP is MoO2Cl2, UV and NMR spectra (Figure 6) indicate some presence of another Mo species in the organic phase. Due to similarity of UV spectra of Mo extracts in TBP from high HCl to MoO2Cl3- species in the aqueous phase, we investigated a possible extraction of tri-chloro Mo species by DFT.


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Table 4: Bond distances from Mo to other close heavy atoms in the DFT optimized di-, and trichloro species in both water and TBP-TCE. If two different distances are given, then it denotes that there are two different distances due to the trans effect. Distance (Å) Bonding atoms (type of atom MoO2Cl2X bond) X= 2H2O

X= 2TBP

Mo-O (oxo)

1.70 (oxo)

1.71 (oxo)

Mo-Cl

2.38

2.43

Mo-O (H2O)

2.37

-

Mo-O (TBP)

-

2.30

Mo-P

-

3.59

For extraction of tri-chloro molybdenyl species, it was determined that the minimum energy structure was one in which the HCl protonated one of the oxo bonds, effectively making it a hydroxide, and the chloride addition occurs trans to the protonated oxo. As seen with all other trans-effect transition metals, the neutral donor ligand then bonds trans to the unaffected oxo. This same environment found for the aqueous species was also the minimum energy structure to the isoelectronic TBP complex in TCE where Mo is coordinated with one TBP (Figure 8).


23

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Figure 8. Aqueous phase speciation of the molybenyl trichloride coordinated to water (left), which looks identical to the TBP coordinated molybdenyl trichloride, optimized in the organic phase (right). Butyl groups have been omitted for clarity

Interestingly, when investigating the Gibb’s Free Energy results of the frequency calculation, it was found that the protonation and addition of chloride to the molybdenum metal center to form the tri-chloride species is moderately un-favored by 1.7 kcal per mole. This slightly un-favored product could explain low formation of tri-chloro species without high HCl concentrations. This is also intuitive as the protonation is affecting one of the strongly bound oxo bonds for the addition of a chloride. While slightly un-favored, according to LeChatlier’s principle, the high HCl concentration could cause a shift in equilibrium towards production of a tri-chloro species in TBP. Further investigation via frequency calculations showed that upon possible extraction of the trichloro species into TBP, there is a favorable back reaction converting the tri-chloro species back to the di-chloro species and HCl. This is facilitated by the loss of HCl and the coordination of a second moiety of TBP. Overall, this back reaction was favored by -11.6 kcal per mole. This was


24


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reasoned as not being an immediate reaction and was kinetically hindered by the loss of HCl and coordination of a second TBP. It should also be noted that possible extraction of H3MoO4Cl species by TBP/TCE at low HCl concentrations was also probed by DFT, but it was found that its extraction is thermodynamically unfavorable by 14.4 kcal per mole. DFT calculations discussed are supporting our conclusion made based on experimental results. Although the presence of tri-chloro Mo species in TBP is possible, it is not expected to be stable and predominant species even at extraction from very high HCl concentration. CONCLUSION Extraction studies carried with up to 0.3 M Mo and a wide range of HCl concentrations by TBP showed very high extraction yields even at high HCl concentrations >9 M HCl. Spectroscopic techniques such as UV, NMR, EXAFS and DFT calculations showed that major Mo species extracted is MoO2Cl22TBP. Extraction of Mo at higher HCl concentrations is driven by replacing HCl in the organic phase by Mo. Additionally, UV and NMR spectroscopy suggested that at very high HCl concentration there is the possibility of another Mo species. Although the extraction of tri-chloro Mo species was found to be thermodynamically favorable, the stability of such a complex in TBP was low and would most likely exchange to form the di-chloro species. No dimeric Mo species were observed to be present in the TBP phase.

Corresponding Author Peter Tkac, email: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. Funding Sources Work supported by the U.S. Department of Energy’s National Nuclear Security Administration Office of Material Management and Minimization, under Contract DE-AC02-06CH11357.

ACKNOWLEDGMENT The authors would like to thank Yifen Tsai for performing ICP-MS analyses, and Dr. R. Jeremy Kropf for assistance with EXAFS at MRCAT-Sector 10. 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. DEAC02- 06CH11357. Argonne National Laboratory is operated for the U.S. Department of Energy by Chicago Argonne, LLC. This manuscript was also partially prepared under award #NRC-HQ15-G-0036, from the Office of the Chief Human Capital Officer of the Nuclear Regulatory Commission. Any statements, findings, conclusions, and recommendations are those of the author(s) and do not necessarily reflect the view of the Outreach and Recruitment Branch or the US Nuclear Regulatory Commission.

Supporting Information Available: xyz coordinates for DFT calculations.


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and

B−N

Bond

Dissociation

Energies

of

Amine−Boranes

(X3C)mH3-

mB−N(CH3)nH3-n (X = H, F; m = 0−3; n = 0−3): Poor Performance of the B3LYP Approach for Dative B−N Bonds. The Journal of Physical Chemistry A 2004, 108 (13), 2550-2554. [57] Holme, T. A.; Truong, T. N., A test of density functional theory for dative bonding systems. Chemical Physics Letters 1993, 215 (1), 53-57.

[58] Goletskii, N. D.; Zilberman, B. Y.; Fedorov, Y. S.; Puzikov, E. A.; Lumpov, A. A.; Mashirov, L. G.; Sidorenko, G. V. A spectroscopic study of molybdenum extracts in organic


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solutions of dibutyl hydrogen phosphate in equilibrium with aqueous nitric acid solutions. Radiochem. 2011, 53, 619-632. [59] Tkac, P.; Paulenova, A., Speciation of molybdenum (VI) in aqueous and organic phases of selected extraction systems. Sep. Sci. Techn. 2008, 43, 2641-2657.


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Graphical Abstract/Synopsis

2 MoO2Cl2

1.5 absorbance

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


1 0.5 0

4M HCl

2M HCl

1.5M HCl 1M HCl

220

240

260 280 300 wavelength, nm

320

340

In TBP, there is a striking resemblance of spectral features of solid MoO2Cl2 crystals dissolved in 30% TBP/n-dodecane and spectra of Mo in TBP/n-dodecane extracted from 4 M HCl; variations in the extinction coefficients of Mo extracts from 1-4 M HCl at 220-280nm are within 3%. Spectra show that in this range of HCl conditions, extracted Mo species in TBP are MoO2Cl2 with two peak maxima at 227.5nm and 253.5nm.


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Molybdenum(VI) Coordination in Tributyl ... - ACS Publications

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