Polymerisation effects in the extraction of Co(II) into polymer inclusion membranes containing Cyanex 272. Structural studies of the Cyanex 272–Co(II) complex

Journal of Membrane Science 497 (2016) 377–386

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Polymerisation effects in the extraction of Co(II) into polymer inclusion membranes containing Cyanex 272. Structural studies of the Cyanex 272–Co(II) complex Stephen P. Best n, Spas D. Kolev, June R.P. Gabriel, Robert W. Cattrall School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia

art ic l e i nf o

a b s t r a c t

Article history: Received 29 June 2015 Received in revised form 10 September 2015 Accepted 19 September 2015 Available online 21 September 2015

The instability and poor reproducibility of polymer inclusion membranes (PIMs) containing Cyanex 272 (di(2,4,4-trimethylpentyl)phosphinic acid, HCyanex) for the extraction of cobalt(II) have been investigated using chemical and spectroscopic means. The differing transport properties of the cobalt: Cyanex adducts, and the formation of deep blue oils exuded from the surface of the PIM under particular extraction conditions are explained in terms of oligomerisation of the cobalt centres with the Cyanex anion acting as a bridging ligand, {Co:Cyanex2}n. Oligomers present within the polymer matrix are not available for back-extraction and if formed on the surface of the membrane are unable to diffuse into the membrane. The X-ray absorption spectra of cobalt(II)-loaded PIMs and of the {Co:Cyanex2}n oils confirm tetrahedral coordination geometry about the cobalt(II) centre. Analysis of the X-ray absorption fine structure (XAFS) of the cobalt(II)-loaded Cyanex PIMs and {Co:Cyanex2}n oils both give a local structure with unidentate phosphinate ligands bridging cobalt centres in an extended structure closely related to polymeric cobalt(II) phosphinate forms previously characterised by X-ray crystallography. These results show clearly the impact of oligomerisation on the migration properties of metal:extractant adducts within PIMs which can be responsible for substantial differences in the performance of extractants in solvent extraction and PIM applications. Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.

Keywords: Polymer inclusion membrane (PIM) Cyanex 272 Cobalt(II) Membrane transport X-ray absorption spectroscopy (XAS)

1. Introduction In the past decade, there have been many papers published on the application of polymer inclusion membranes (PIMs) for the separation and transport of numerous metallic cations, complex metal anions and simple anions [1–7]. PIMs are cast from a homogeneous solution which always contains a base polymer and an extractant (often referred to as carrier), which usually exhibits plasticizing properties. In many cases use has been made of commercial solvent extraction reagents as carriers and the separation systems normally mimic the extraction properties of the analogous solvent extraction processes. Of course, this is also true for supported liquid membranes (SLMs), however PIMs have been shown to have superior stability in terms of leaching of the organic Abbreviations: PIM, polymer inclusion membrane; CTA, cellulose triacetate; PVC, poly(vinyl chloride); SLM, supported liquid membrane; NPOE, 2-nitrophenyl octyl ether; TBP, tri-n-butyl phosphate; THF, tetrahydrofuran; XAS, X-ray absorption spectra; XANES, X-ray absorption near edge spectra; XAFS, X-ray absorption fine structure; CSD, Cambridge Structural Database n Corresponding author. E-mail address: [email protected] (S.P. Best). http://dx.doi.org/10.1016/j.memsci.2015.09.046 0376-7388/Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.

phase to the aqueous phase [2,8–10]. An important feature of a successful PIM is that the carrier and the extracted species bound to the carrier are compatible with the base-polymer. In this sense, the membrane must be visually homogeneous and have sufficient flexibility and mechanical strength to be self-supporting not only after casting but also after application to a separation problem. In some cases an additional plasticizer or modifier needs to be added to the PIM composition to achieve this. The majority of PIMs described in the literature use PVC (polyvinylchloride) or CTA (cellulose triacetate) as the base polymer. We previously discussed the evaluation of several PIMs based on PVC and CTA that contained a number of commercial solvent extraction reagents as carriers [11]. The four properties of the extractant and presumably the extracted adduct found to be important in order to form a successful PIM are its lipophilicity, hydrogen bonding capacity, ability to participate in dipole–dipole interactions and propensity to form a homogeneous membrane liquid phase [11]. Consequently, the extractants with a strong ability to form hydrogen bonds will form more successful PIMs with CTA and less polar and more lipophilic extractants will be

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more compatible with PVC. One of the extractants studied was Cyanex 272 (di(2,4,4-trimethylpentyl)phosphinic acid, HCyanex) which has a high ability to form hydrogen bonds, is highly lipophilic and can also participate in dipole–dipole interactions and so formed successful PIMs with both CTA and PVC. HCyanex is a potentially important extractant since it has long been known for its ability to separate Co(II) from Ni(II) using conventional solvent extraction [12–15] or supported liquid membranes (SLM) [12,16]. While PVC-based PIMs containing Aliquat 336 as carrier have been used for the selective extraction of Co(II) in the presence of Ni(II) [17,18] high chloride concentration in the feed aqueous phase was required to ensure the formation of the tetrahedral [CoCl4]
complex. Despite the promise of separation of Co(II) and Ni(II) present in low chloride concentration by HCyanex PIMs, to the best of our knowledge, this separation has not been reported. While satisfying the four properties required for formation of successful PIMs, it has been observed that HCyanex PIMs when used for the extraction of Co(II) suffer from instability and poorly reproducible behaviour which suggests that the four properties mentioned above, while necessary, are not always sufficient for ensuring successful PIM–base separation. In this investigation we establish the molecular basis for the varying extraction properties of HCyanex PIMs with the aim of better understanding what other properties of the extractant maybe be critical for the formation of successful PIMs.

2. Experimental 2.1. Chemicals and reagents HCyanex (Cytec), high molecular weight PVC (Fluka), CTA (Fluka), 1-dodecanol (Aldrich), 2-nitrophenyl octyl ether (NPOE) (Aldrich), tri-n-butyl phosphate (TBP) (Chem-Supply), dichloromethane (Chem-Supply), tetrahydrofuran (THF) (HPLC grade, Merck), 1,8-Bis(dimethylamino)naphthalene, N,N,N′,N′-Tetramethyl-1,8-naphthalenediamine (proton sponge, Aldrich), cobalt (II) perchlorate hydrate (Merck), cobalt(II) chloride hydrate (Aldrich), anhydrous cobalt(II) chloride (Aldrich), tetraethylammonium tetrachlorocobaltate(II) (Aldrich) and cobalt(II) sulphate hydrate (Aldrich) were used as received. Samples of potassium tris(oxalato)cobaltate(II), potassium tris(oxalato)cobaltate (III), tris(phenanthroline)cobalt(II) bromide and tris(phenanthroline)cobalt(III) tetrafluoroborate were prepared using published procedures [19,20]. It should be noted that commercial HCyanex contains approximately 87% mono-acid, 1% di-acid and 12% trialkyl phosphine oxide [17]. Cobalt perchlorate was used to prepare solutions of Co(II) in non-aqueous solvents. Solutions used in the extraction experiments were prepared from CoSO4  7H2O and Co (II) standards were prepared from a 1000 mg L
1 Co(II) standard solution for atomic absorption spectrometry, AAS, (Ajax Finechem, Australia) diluted with1% (v/v) HNO3 (Chem-supply, Australia) to obtain Co(II) standards in the required concentration range. Deionized water (Millipore, Synergy185, 18.2 MΩ cm) was used in the preparation of all aqueous solutions. Dilute sodium hydroxide (NaOH, Merck) and sulphuric acid (H2SO4, Chem-Supply) solutions were used when necessary to adjust the pH of the Co(II) solution for the extraction experiments. 2.2. Instrumentation Aqueous Co(II) solution concentrations from the extraction and back-extraction experiments were determined by AAS (Z-2000 Series tandem type atomic absorption spectrometer, Hitachi). An Ionode-44 pH electrode connected to a smartCHEM analyser was used to measure the pH. The thickness of the membranes

was measured using a stereomicroscope (SMZ-140, Motic) with 60 times magnification combined with a microscope camera (MotiCam 1000, Motic). X-ray absorption measurements were conducted using the bending magnet source of Beamline 20B at the KEK Photon Factory (Tsukuba, Japan). A channel cut Si(111) monochromator with energy resolution (ΔE/E) of ca. 2.4  10
4 provided the source of monochromatic radiation where higher order harmonics at the selected wavelength were rejected by detuning the monochromator by a factor of one half. Samples, maintained at temperatures near 10 K using a Displex cryostat (Air Products), were measured in transmission mode using ion chamber detection. Reference materials were prepared as finely-ground mixtures of the sample with cellulose (Aldrich) pressed into 13 mm discs. The ratio of sample to cellulose was adjusted so as to give a transmittance at the cobalt edge of at least 20% with a total mass of sample plus cellulose of ca. 50 mg. The XCOM Photon Cross Sections Database [21] was used to calculate of the attenuation of the sample. Cobalt–Cyanex 272 samples were measured either as cobalt-loaded PIMs with either PVC or CTA as the base polymer or as the cobalt-rich oils exuded from cobalt-loaded Cyanex PIMs. In the former case the PIM had a thickness of 10–20 μm and it was necessary to cut the PIM into 3 mm wide sheets and 3–8 sheets of membrane were stacked so as to give a sample with a transmittance at the cobalt edge of 20–30%. The oils were examined in a slotted aluminium plate with Kapton tape forming the front and back windows. Data analysis was conducted using the XFIT suite of programs [22,23] which incorporates FEFF ver. 6.01 [24]. Details of the procedures followed for the analysis of related systems have been described previously [25]. 2.3. Membrane preparation HCyanex, PVC and in some cases a plasticizer (1-dodecanol, TBP or NPOE) with a combined mass of 800 710 mg were weighed into a glass beaker and 8 mL of THF was added. The amount of each component was calculated based on the desired mass percentage in the PIM and the corresponding concentrations are expressed throughout this paper in mass percentages. The mixture was stirred with a magnetic stirrer until all components were thoroughly mixed and dissolved. The resulting solution was poured into a glass ring (internal diameter 65 mm) placed on top of a flat glass plate. A filter paper was used to cover the glass ring to ensure slow evaporation of the solvent. The solvent was allowed to evaporate for at least 24 h and the prepared PIM was peeled off the glass plate. Successful membranes for use in the extraction and back-extraction experiments were transparent, homogeneous and flexible. Circular segments measuring 45 mm in diameter were cut from the centre of the successful membranes and used in the extraction and back-extraction experiments. The membrane thickness measurements were carried out by obtaining a digital image of the membrane cross-section taken through the optical microscope used in this study and comparing it against a calibration slide with markings of known dimensions. Similar procedures were followed to prepare CTA-based PIMs containing HCyanex and PVC and CTA-based PIMs containing Aliquat 336. For the CTA-based PIMs dichloromethane was used as the solvent. Templated membranes were also prepared in which Co(II) was already embedded in the membrane by including Co(ClO4)2  6H2O with stoichiometric quantities of proton sponge (Section 2.1) in the casting solution. In the THF casting solution the proton sponge is needed to stabilise the proton released from HCyanex following coordination of cobalt. The resulting membranes were homogeneous and deep blue in colour.

S.P. Best et al. / Journal of Membrane Science 497 (2016) 377–386

2.4. Membrane extraction experiments Circular membranes were immersed in glass jars containing 100 mL of 100 mg L
1 Co(II) solution previously adjusted to pH 6 by the addition of dilute H2SO4. The solutions were shaken using an orbital platform shaker (OM06, Ratek) at a speed of 150 rpm. Samples of the Co(II) solution (0.2 mL) were taken at regular time intervals throughout the experiment, diluted with 1% (v/v) HNO3 and the Co(II) concentration was determined by AAS. All extraction experiments were carried out in triplicate. The extraction properties of the membranes were not significantly altered by the choice of anion (e.g. chloride).

379

Table 1 Membrane compositions containing 30% HCyanex, PVC and a plasticizer/modifier. Plasticizer/modifier (% concentration)

PVC Remarks

1-dodecanol (5) 1-dodecanol (10) 1-dodecanol (20) TBPa (5) TBP (10) TBP (20) 2-NPOE (5) 2-NPOE (10)

65 60 50 65 60 50 65 60

2-NPOE (20)

50

a

Oily membrane Oily membrane Oily membrane Poor homogeneity Good homogeneity. Extracts 7% Co(II) Oily membrane Poor homogeneity Flexible, transparent, slightly oily. Extracts Co (II). Blue solid formed after 24 h Oily membrane

PIMs containing only PVC and TBP do not extract Co(II).

3. Results and discussion 3.1. Membrane composition and extraction studies

3.2. Structural studies

PIMs containing HCyanex:PVC compositions between 25:75 and 45:55 were prepared. It was found that PIMs with concentrations of HCyanex greater than 35% had oily surfaces suggesting some incompatibility between the PVC and HCyanex. On the other hand, PIMs with 25% and 30% HCyanex were visually homogeneous, flexible and with oil-free surfaces. In order to obtain maximum extraction of Co(II), the PIM containing 30% HCyanex was used for the Co(II) extraction studies. The extraction studies were carried out at pH 6 since this provided efficient extraction of Co(II) in traditional solvent extraction systems with HCyanex [26]. Initial results were encouraging with an extraction percentage of ∼45% and the formation of a deep blue colour for the PIM. However, after extraction a deep blue coloured oil was observed on the membrane surface with some drops of the oil suspended in the aqueous phase. Of course, such behaviour suggests an incompatibility between the extracted Cyanex/Co(II) complex and the membrane phase. This result was unexpected since our previous work [11] predicted good compatibility between HCyanex and PVC and any extracted complexes and suggests that the structure of the complex extracted in the PIM is more complicated than simply a monomeric complex consisting of two bidentate Cyanex anions bound tetrahedrally to a Co(II) ion. There is some evidence for the above suggestion in a paper reporting on the solvent extraction of Co(II) using HCyanex where the organic phase was found to become more viscous at high Co(II) loadings and the reason for this was attributed to polymerisation of the extracted complex in the organic phase [27]. Another important observation in the present study is that backextraction from the PIM of any extracted Co(II) was not achievable using hydrochloric or sulphuric acid solutions whereas in solvent extraction systems with HCyanex back-extraction was easily achieved using sulphuric acid solutions [12,13]. In a number of cases in order to improve the compatibility of the components in a PIM, it is useful to add a modifier or plasticizer to the membrane composition [1,28]. Thus, a plasticizer/ modifier (i.e. 1-dodecanol, TBP or NPOE) was added to the membrane compositions studied in order to see if improved compatibility could be obtained between the membrane and the extracted Cyanex/Co(II) complex. The amount of HCyanex in the PIM was kept constant at 30% and the results are summarised in Table 1. Of the three plasticizers/modifiers tested, only the PIM with 10% TBP, 30% Cyanex 272 and 60% PVC produced a homogeneous PIM after extraction of Co(II) although the extraction efficiency was extremely poor at around 7% and difficulty was again encountered in attempts to back-extract the Co(II).

The difficulties described above in the membrane extraction and back-extraction studies can be summarised by the following points. (1) The extraction properties of a PIM containing only PVC and HCyanex were not reproducible even though initially 45% extraction was obtained giving a blue PIM. (2) Any extracted Co(II) could not be back-extracted using 2 M H2SO4. (3) Although the PIMs after preparation were clear, transparent and homogeneous, after extraction of Co(II) most had a blue oil on the surface and blue droplets were observed in the aqueous solution. (4) The addition of the plasticizer TBP to the PIM composition gave blue membranes on extraction of Co(II) but only 7% extraction was achieved and Co(II) could not be back-extracted with 2 M H2SO4. (5) Co(II) in templated membranes, where Co(II) perchlorate was included in the PIM composition, could not be back-extracted with 2 M H2SO4. As mentioned in Section 3.1, it was considered that these difficulties could be related to the complex structure of the extracted Cyanex/Co(II) complex which was not only incompatible with the polymer medium, but also sufficiently stable that it could not be broken down using 2 M H2SO4. Thus, a detailed study of the structure of this complex was carried out using X-Ray Absorption Spectroscopy (XAS) [29,30]. Qualitative and quantitative insight into the coordination environment about the cobalt centre can be obtained from XAS recorded at the metal K edge. The weak absorption and scattering from the sample matrix by the high energy X-radiation simplifies sampling and allows high-quality measurements to be made directly from the cobalt-loaded PIMs. The XAS measurements are intended to answer the following questions: (1) what is the predominant coordination geometry and oxidation state of cobalt in the PIM systems under investigation? (2) Does cobalt exist in a single predominant form? (3) Are there differences in the coordination of cobalt in oil and in PVC and CTA-based PIMs containing the extracted Cyanex/Co(II) complex? (4) What are the molecular details of the Cyanex/Co(II) complex and can these be related to the extraction performance of the PIM? The first three of these questions can be answered by considering the X-ray absorption near-edge region (XANES) while the last requires analysis of the fine structure on the absorption at energies well above the K edge (XAFS).

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[NEt4][CoCl4] CoCl2(aniline)2 CoCl2(anhyd) K4[Co(ox)3] CoCl2(OH2)4 [Co(OH2)6]SO4·H2O

Normalised Intensity

1.5

1.0

100

80

0.5

x10

60

40

20

0 7698

7700

7702

7704

7706

7708

7710

7712

0.0 7700

7710

7720

7730 Energy/eV

7740

7750

7760

Fig. 1. X-Ray absorption spectra of the Co(II) reference compounds. The inset highlights the pre-edge, 1s-3d transitions.

3.2.1. XANES results The XANES region includes energies close to the ionisation energy and includes pre-edge features due to transitions to bound electronic states and, immediately above the edge, to interference effects due to longer-range backscattering interactions. The highly structured spectra can be used to fingerprint individual species or to assess the likelihood of a change in structure near to the absorbing atom. For first-row transition metal ions the lowest energy bound transitions near the K edge are 1s-3d in character and are Laporte forbidden for the free ion. Static (e.g. tetrahedral complexes) or dynamic (i.e. coupling to asymmetric vibrations) removal of the centre of symmetry of the complex lifts the Laporte restriction and accounts for the observed intensity. Importantly non-centrosymmetric tetrahedral complexes give substantially more intense 1s-3d transitions than distorted octahedral or square planar complexes. Similar considerations apply to the d–d transitions of metal complexes and the extinction coefficients of approximately tetrahedral complexes are 5–10 times that of 6-coordinate complexes. The pre-edge and XANES spectra of a series of Co(II) reference compounds are shown in Fig. 1. Within this group of compounds [NEt4][CoCl4] and CoCl2(aniline)2 are 4-coordinate tetrahedral and in both cases give distinctive preedge features near 7.710 keV with half-widths of ca. 2 eV. The remaining compounds give much less intense bands in this region. It is important to note that whereas anhydrous CoCl2 has a distinct blue colour, as is normally associated with tetrahedral Co(II), the solid has a layer structure with hexagonally close-packed chloride ions with Co(II) occupying the octahedral interstices (analogous to CdI2). After normalisation of the K edge absorption, the relative intensities of the 1s-3d pre-edge features of octahedral Co(II) centres are approximately 10% of those of the tetrahedral complexes. The half-widths and structure of these absorption bands reflects the relative energies of the different excited states where the methods for identifying the ligand-field excited states have been outlined previously by Solomon and co-workers [31]. Aside from the broadening normally associated with the d–d transitions,

the short lifetime of the (1s)1 hole leads to a half width of approximately 1 eV. Notwithstanding the large half-widths for the individual transitions, differences in the energies and intensities of the 1s-3d bands allow clear distinction between tetrahedral and octahedral complexes and give some information on the donor atoms bound to the metal [31,32]. The XANES region of the Co(II) compounds (Fig. 1) are highly structured and contain intense transitions to bound states, which are predominantly metal 1s-4p in character, and the absorption edge due to photoionisation of the 1s electron. Among the group of Co(II) complexes studied (Fig. 1) those with at least one chloride ligand in the primary coordination sphere have the major absorption increase near 7.712 keV while the remaining two complexes have solely oxygen or nitrogen donor atoms and the absorption edge is shifted ca. 3 eV to higher energy. Additionally, the spectra are sensitive to the coordination number where 4-coordinate compounds with a similar ligand set have an absorption edge energy ca. 0.5 eV lower in energy than is observed for 6-coordinate complexes. The influence of the oxidation state of the metal is well illustrated by the spectra obtained for the cobalt complexes with oxalate and phenanthroline ligands (Fig. 2) where a 2.5–3 eV increase in the energy of the onset of strong absorption accompanies the increase in oxidation state from II to III. While the shifts of the edge energy broadly follow the expected trends, it is noted that the interpretation of the spectra is complicated by overlap of intense electric-dipole allowed transitions to bound excited states (e.g. 1s-4p) and photoionisation. At energies above the absorption edge interference due to backscattering by the ejected photoelectron gives a complicated absorption profile that at low photoelectron energies is sensitive to the longer-range structure of the complex. Accordingly, the XANES are sensitive to changes in both the primary and secondary coordination spheres of the absorbing atom and can be used qualitatively to indicate whether the coordination of a complex is retained as the complex is placed in different environments. More quantitative analysis of the XANES spectra presents significant

S.P. Best et al. / Journal of Membrane Science 497 (2016) 377–386

381

1.6

4-

1.4

[Co(ox)3]

3-

[Co(ox)3]

2+

[Co(phen)3]

1.2

1.0

0.8 x10

Normalised Intensity

3+

[Co(phen)3]

80

0.6 60

0.4

40

20

0.2 0 7698

7700

7702

7704

7706

7708

7710

7712

0.0 7700

7710

7720

7730

7740

7750

7760

Energy/eV Fig. 2. Oxidation state dependence of the XANES of Co complexes.

theoretical and computational challenges and is outside the scope of this investigation. The pre-edge and XANES spectra for the cobalt complexes formed following extraction of Co(II) into Cyanex 272 and Aliquat 336 PIMs are shown in Fig. 3. The spectra, while very distinct for the Aliquat 336 and HCyanex extractants, have a highly conserved spectral profile for the different sample forms. These results show

clearly that; (a) cobalt is in the þ2 oxidation state, (b) there is tetrahedral coordination of the Co(II) centre in all of the studied forms of the Cyanex 272 and Alquat 336 PIM-extracted complex and (c) the local coordination geometry of the complex is independent of the identity of the polymer (CTA or PVC). The pattern of oscillations in the XANES region is similar for [NEt4]2[CoCl4] and the Aliquat 336 samples suggesting a common [CoCl4]2
complex

Fig. 3. XAS of complexes obtained following extraction of Co(II) cation into Cyanex 272 or Aliquat 336 PIMs. (a) Oil exuded from Aliquat-based PIMs, (b) Aliquat/PVC PIM, (c) Aliquat/CTA PIM, (d) oil exuded from Cyanex-based PIMs, (e) Cyanex/PVC PIM and (e) Cyanex/CTA PIM.

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ion. In keeping with the expected weak interaction between the [CoCl4]2
anion and the surrounding environment there is little difference between the XANES of the Co(II):Aliquat 336 samples measured as an oil or bound within a polymer matrix. The Cyanex/Co(II) complexes in each of the three sample environments have strong 1s-3d pre-edge features indicative of approximate Td coordination. The 2 eV shift of the absorption edge to higher energy relative to the Aliquat 336 samples is consistent with a change in the primary coordination sphere of the cobalt from chloride ligands to harder O or N donor ligands with retention of the þ2 oxidation state of cobalt. Phosphinates such as monovalent Cyanex anions could conceivably act as either unidentate or bidentate O-donor ligands to a single cobalt atom or could bridge two cobalt centres. In the absence of suitable reference compounds XANES cannot distinguish between these alternatives. The more general similarity of the XANES for the three sample forms of the Cyanex/Co(II) complex (i.e. PVC and CTA PIMs and oil) is surprising and indicates that there is either a single predominant form of the Co(II) species in the three samples or that the same distribution of species is present in each of them. The small differences in the spectra for the Cyanex/Co(II) oil and the two PIM samples between 7720 and 7740 eV are reproducible and may be related to the conformation of the phosphinate, however the details of any structural changes requires a more quantitative analysis of the spectra as considered in the following section.

In summary, the XANES results show clearly that for both the Cyanex 272 and Aliquat336 membranes the geometry about the Co is tetrahedral and the oxidation state is 2 þ. The inability to backextract Co from the Cyanex 272 membranes is not due to any difference in oxidation state or coordination number of the metal. 3.2.2. XAFS results The details of the Cyanex/Co(II) complex are most straightforwardly explored by analysis of the oscillations in the absorption spectrum at energies well above the absorption edge (ca. 34– 747 eV, kE 3–14 Å). The XAFS oscillations (plotted in terms of the photoelectron momentum, k) and Fourier transform (distance or R space) can be used for structure analysis based on calculation of the corresponding functions using theoretical models embedded in the FEFF codes. Evaluation of the derived structures is based on the statistical agreement between the observed and calculated spectra both in k and R space. The extent of the structural detail able to be extracted from the XAFS is, however, restricted both by the inverse square dependence on the absorber–scatterer distance and the information content of the spectra (2δkδR/π þ2) with the latter based on the range of data used in the analysis. Since the paper focuses on the extraction properties of the HCyanex containing PIMs, the XAFS analysis will be restricted to the Cyanex/Co(II) complexes either in CTA or PVC PIMs or Cyanex/ Co(II) oils.

Scheme 1. Models used for structural analysis of the XAFS of Co(II):Cyanex complexes.

S.P. Best et al. / Journal of Membrane Science 497 (2016) 377–386

The extent to which useful structural information can be obtained from the longer range interactions can be assessed by comparison of the fits able to be obtained from progressively complex models. The simplest approach is based on a distorted tetrahedral arrangement of nearest neighbours, M1 (Scheme 1). Consistent with this approach, the nearest Co–O distances are well defined, although the difference in pairs of Co–O distances is not significant (ΔRo π/2Δk). Additional information is inferred by the well-defined longer-range ( 42 Å) features in the Fourier transform (FT) of the XAFS. The alternate models to describe the structures would be based on either bidentate binding of the phosphinate ligands or the phosphinate groups bridging adjacent Co(II) centres. The alternate models would have either 2 or 4 P atoms as next nearest neighbours. Satisfactory fits could not be obtained with models based on bidentate coordination of phosphinate but good fits were obtained with simple models based on bridging coordination modes (M2, Scheme 1). These could be elaborated by the addition of shells of scattering atoms involving oxygen and cobalt (M3, Scheme 1). While the simple model does not have the flexibility to accurately represent all the multiple scattering interactions, the approach allows the main backscattering interactions to be calculated while minimising the number of parameters needed in the refinement. In this way the ratio of information content to refined parameters is maintained well above 1. Additionally, the non-crystalline nature of the samples makes it likely that there is a mixture of conformers with there being less ordering of the scattering atoms with greater distance from the Co(II) centre. Excellent fits and refinement statistics can be obtained using M3 (Figs. 4 and 5 and Tables 2 and 3) with an improvement in the quality of fit observed with the addition of each successive shell of scattering atoms. The X-ray structures of some tetrahedrally-bound Co(II) alkylphosphinate complexes reported in the literature are shown in Fig. 6 where the labels refer to the Cambridge structure database (CSD) structure codes. The alkylphosphinate anions are similar to the anion associated with HCyanex. A sphere of enclosure of 4.8 Å is used for each of the structures where this is drawn so as to show all the atoms that would contribute single scattering paths in the Co:cyanex solution 4 0 -4 PVC:cyanex PIM 4 0 -4 CTA:cyanex PIM 4 0 -4 2

4

6

8 k /Å

10

12

14

383

range used for the XAFS analysis. In each of the examples in Fig. 6 the phosphinate ligand is bound as a unidentate ligand. Bidentate phosphinate binding to metals is known, but this is associated with higher coordination number complexes. For IHAJUX [33] (Fig. 6) the phosphinate ligands have a geometry that gives a neutral compound by protonation of the pairs of oxygen atoms which are positioned to form a O–H–O hydrogen bond (H not shown). Similar structures are obtained for both JIRNON [34] and WACRIB [35] (Fig. 6) where the protonation site is replaced by a Co (II) centre. Extension of the structure gives infinite chains of tetrahedral Co(II) centres which differ in terms of the conformation of the chains which lead to differences in the longer range contacts. The last structure type considered, WACROH [35] (Fig. 6), has three phosphinate groups bridging the trigonal faces of two CoO4 tetrahedra with the fourth phosphinate ligand bridging the apices two CoO4 tetrahedra. This structure gives short and long Co–Co interactions. 3.2.3. Relationship between the XAFS structural data for the Cyanex/ Co(II) complex and the X-ray structures Despite the differences in the R groups of the phosphinates (Scheme 1), the possible structural forms of the Cyanex/Co(II) species are well illustrated by the four compounds shown in Fig. 6 and these provide a good framework for interpretation of the XAFS results. First, while M3 does not provide for any backscattering from the carbon and hydrogen atoms that will certainly be within the R window used in the analysis, the positions of those atoms are likely to be disordered for a non-crystalline sample and the inclusion of these atoms is unlikely to improve the information able to be extracted from the analysis. Second, in the case where unsaturated alkyl groups are incorporated in the phosphinato ligand the Co–O distances give Co–O bond distances in good agreement with those obtained from refinement of the XAFS. It is noted, however, that the argument for constraining the Co–O distances to two short and two long is not strongly supported by the structural data. Third, the Co–P distances refine to values in excellent agreement with those obtained by X-ray crystallography. Once again a range of Co–P distances are obtained and an argument can be mounted for refining 4 different Co–P distances, however the impact of different local environments for the cobalt centres within the non-crystalline sample argues more forcefully for a less highly parameterised model. Fourth, while a wide range of longer range Co–O distances are obtained from the crystal structures, the average of those distances agrees well with the oxygen scatter included in M3. Last, while weak, the Co–Co scattering is of greatest significance in terms of Cyanex/Co(II) speciation within the membrane. This interaction is diagnostic of whether the Cyanex/Co(II) species are present as the limiting monomeric or polymeric forms. Even within the polymeric forms the Co–Co distance varies according to whether there is edge or face bridging of the CoO4 tetrahedra or even the conformation of the chains for the edge-bridged forms. For the Cyanex/Co(II) in oil and in the PVC and CTA membranes, the long range scattering interaction gave optimised fits with a cobalt population of 2 and with a Co–Co separation of 4.31 Å. This fits closely with the longer range structure represented by JIRNON (Fig. 6). Since changes in both a mix of structural forms and conformations of the chains will give a distribution of Co–Co distances, it is surprising that such a feature is so clearly reproduced in the XAFS of the different Cyanex/Co(II) samples and this suggests a common ordered structure for the Cyanex/Co(II) chains within the samples. 4. Conclusions

-1

Fig. 4. XAFS refinement to M3. Calculated and observed k3χ.

X-ray spectroscopy has been demonstrated to provide clear insights into the oxidation state and coordination geometry of

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12

Co:cyanex solution

8

Fourier Transform Intensity

4 0

16 PVC:cyanex PIM

12 8 4 0

12 CTA:cyanex PIM 8 4 0 1

2

3

4

R /Å Fig. 5. XAFS refinement to M3. Calculated and observed plots of the FT of the XAFS.

Table 2 XAFS refinement parameters.

k (Å
1) R (Å) Refined parameters Nidp/parameters χ2(XAFS) R(XAFS) E0

S02 x(1) s2(1) x(2) s2(2) x(5) s2(5) x(6) s2(6) z(9) s2(9) x(10) s2(10) a

Table 3 Derived XAFS and X-ray crystallographic structural parameters. PVC

CTA

Oil

Distances

PVCa

CTAa

Oila

IHAJUX

JIRNON

WACRIB

WACROH

2.8–14 1.2–3.8 14 1.5 2.92 14.08
1.5 (21) 0.97(23)

2.8–13 1.2–3.8 14 1.3 3.86 18.3
1.7 (24) 0.98a

2.8–14 1.2–3.8 14 1.5 1.79 12.8
2.8 (22) 0.96 (15)

Co–O (Å)

1.92

1.89

1.90

1.96

1.98

1.98

3.19

3.20

3.18

3.37

3.38

3.35

1.151 (78) 0.001a 1.130 (69) 0.003 (14) 2.980 (105) 0.006 (7) 2.788 (58) 0.002 (3) 3.675 (80) 0.008 (11) 4.32 (21) 0.017 (29)

1.165 (17) 0.001a 1.113 (9) 0.002 (3) 2.978 (40) 0.002 (3) 2.805 (29) 0.001a 3.692 (78) 0.007 (9) 4.31 (19) 0.017 (26)

1.164 (8) 0.001a 1.114 (7) 0.001a 2.950 (87) 0.007 (8) 2.784 (37) 0.003 (3) 3.671 (136) 0.011 (17) 4.32 (19) 0.016 (26)

3.70 4.32

3.71 4.31

3.69 4.32

1.944 1.949 1.968 1.974 3.263 3.286 3.294 3.309 3.860

1.951 1.951 1.951 1.963 3.090 3.156 3.297 3.323 3.738 4.329 4.363

1.907 1.930 1.930 1.933 3.255 3.308 3.309 3.333 3.977 4.704 4.704

1.930 1.943 1.954 1.958 3.185 3.219 3.252 3.336 3.727 3.892

Constrained during the refinement.

metal–extractant complexes within a PIM matrix. In addition, specific structural information including longer range interactions, as may indicate oligomerisation, can be obtained from more detailed analysis of the XAFS. For the Co(II)-loaded PIMs examined in this investigation both Aliquat 336 and Cyanex 272 extractants give cobalt(II) complexes with tetrahedral coordination. The mobile species within the membrane is therefore most likely [NR4]2[CoCl4] and [Co(Cyanex–HCyanex)2], respectively. Cyanex is able to act as a bridging ligand between metal centres to give

P (Å)

Ob (Å) Co (Å)

a b

Derived from XAFS analysis. Average of 4 distances to the next nearest oxygen neighbours.

oligomeric {Co(Cyanex)2}n species and these may be formed at the surface of the PIM to give the deep blue oils exuded from the PIM or may be trapped in the polymer scaffold leading to Cyanex/Co(II) forms which are unable to be back-extracted in acid. XAFS of the {Co(Cyanex)2}n oils and of Co-loaded Cyanex PIMs provide evidence for similar longer range Co–Co interactions consistent with oligomeric forms with a structure most closely related to the Co(II) phosphinate chains of the JIRNON structure (Fig. 6). The study provides the first demonstration of the impact of oligomerisation on the performance and extraction properties of PIMs and the first application of X-ray absorption spectroscopy to the characterisation of the oxidation state, geometry and oligomeric form of metal complexes in a PIM membrane. The conclusion that oligomerisation impacts significantly on the chemistry of the Cyanex/Co(II) complex is supported by the following three observations: (1) previous solvent extraction

S.P. Best et al. / Journal of Membrane Science 497 (2016) 377–386

IHAJUX

JIRNON

WACRIB

WACROH

385

Fig. 6. X-ray structures of Co(II) phosphinate complexes. All atoms within a sphere of radius 4.8 Å are shown for each structure.

studies report that at high Co(II) loadings in the organic phase the viscosity sharply increases [27], an observation explained in terms of polymerisation of the Cyanex/Co(II) complex. The structure of the polymer was not investigated. (2) Cobalt-loaded Cyanex PIMs prepared by addition of Co(II) perchlorate and non-coordinating proton base to the PIM formulation are homogeneous and flexible and have a high cobalt loading but do not release the Co(II) under back-extraction with concentrated acids. Under basic conditions oligomeric Cyanex/Co(II) complexes will be most favoured and these will not be mobile within the polymer framework of the PIM. (3) The addition of TBP as a plasticizer/modifier to the membrane composition yields membranes of reproducible, though modest, performance. While weakly coordinating, TBP is able to act as a chain breaking ligand. Disruption of the {Co(Cyanex)2}n oligomers will allow an equilibrium with lower nuclearity mobile species. Clearly, oligomerisation of the metal/extractant complexes will lead to significantly different outcomes for processes involving solvent extraction and PIM separation. However the impact of oligomerisation has not previously been considered when optimising the formulation of PIMs. This study illustrates the impact of oligomerisation on the performance of PIM formulations including Cyanex as extractant. The four requirements of an extractant for successful PIM design [11] needs to be extended to five; that is the extractant should not give oligomeric forms under the conditions used for extraction or back-extraction of the metal ion.

Acknowledgements The XAFS experiments were performed at the Australian National Beamline Facility with support from the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. Dr. Garry Foran is thanked for expert assistance with the XAFS experiments. We are grateful to the Australian Research Council for financial support through Discovery grant to SK, DP0557575.

References [1] L.D. Nghiem, P. Mornane, I.D. Potter, J.M. Perera, R.W. Cattrall, S.D. Kolev, Extraction and transport of metal ions and small organic compounds using polymer inclusion membranes (PIMs), J. Membr. Sci. 281 (2006) 7–41. [2] M. Almeida, R.W. Cattrall, S.D. Kolev, Recent trends in extraction and transport of metal ions using polymer inclusion membranes (PIMs), J. Membr. Sci. 415 (2012) 9–23. [3] M. Ulewicz, E. Radzyminska-Lenarcik, Application of polymer inclusion membranes doped with 1-hexyl-4-methylimidazole for pertraction of Zinc(Ii) and other transition metal ions, Physicochem. Probl. Miner. Process. 51 (2015) 447–460. [4] A. Garcia-Rodriguez, V. Matamoros, S.D. Kolev, C. Fontas, Development of a polymer inclusion membrane (PIM) for the preconcentration of antibiotics in environmental water samples, J. Membr. Sci. 492 (2015) 32–39. [5] M.I.G.S. Almeida, A.M.L. Silva, R.W. Cattrall, S.D. Kolev, A study of the ammonium ion extraction properties of polymer inclusion membranes containing commercial dinonylnaphthalene sulfonic acid, J. Membr. Sci. 478 (2015) 155–162. [6] T. Ohshima, S. Kagaya, M. Gemmei-Ide, R.W. Cattrall, S.D. Kolev, The use of a polymer inclusion membrane as a sorbent for online preconcentration in the flow injection determination of thiocyanate impurity in ammonium sulfate fertilizer, Talanta 129 (2014) 560–564. [7] C. Fontas, R. Vera, A. Batalla, S.D. Kolev, E. Antico, A novel low-cost detection method for screening of arsenic in groundwater, Environ. Sci. Pollut. Res. Int. 21 (2014) 11682–11688 , Epub 2014/05/07. [8] N. Bayou, O. Arous, M. Amara, H. Kerdjoudj, Elaboration and characterisation of a plasticized cellulose triacetate membrane containing trioctylphosphine oxyde (TOPO): application to the transport of uranium and molybdenum ions, C. R. Chim. 13 (2010) 1370–1376. [9] C.A. Kozlowski, W. Walkowiak, Applicability of liquid membranes in chromium(VI) transport with amines as ion carriers, J. Membr. Sci. 266 (2005) 143–150. [10] M.I. Vazquez, V. Romero, C. Fontas, E. Antico, J. Benavente, Polymer inclusion membranes (PIMs) with the ionic liquid (IL) Aliquat 336 as extractant: effect of base polymer and IL concentration on their physical–chemical and elastic characteristics, J. Membr. Sci. 455 (2014) 312–319. [11] N. Pereira, A. John St, R.W. Cattrall, J.M. Perera, S.D. Kolev, Influence of the composition of polymer inclusion membranes on their homogeneity and flexibility, Desalination 236 (2009) 327–333. [12] P.R. Danesi, L. Reichley-Yinger, C. Cianetti, P.G. Rickert, Separation of cobalt and nickel by liquid-liquid extraction and supported liquid membranes with bis (2,4,4-trimethylpentyl)phosphinic acid [CYANEX 272], Solv. Extr. Ion Exch. 2 (1984) 781–814. [13] M.N. Gandhi, N.V. Deorkar, S.M. Khopkar, Solvent extraction separation of cobalt(II) from nickel and other metals with Cyanex 272, Talanta 40 (1993) 1535–1539. [14] B.K. Tait, Cobalt-nickel separation: the extraction of cobalt(II) and nickel(II) by Cyanex 301, Cyanex 302 and Cyanex 272, Hydrometallurgy 32 (1993) 365–372.

386

S.P. Best et al. / Journal of Membrane Science 497 (2016) 377–386

[15] N.B. Devi, K.C. Nathsarma, V. Chakravortty, Sodium salts of D2EHPA, PC-88A and Cyanex-272 and their mixtures as extractants for cobalt(II), Hydrometallurgy 34 (1994) 331–342. [16] B. Swain, J. Jeong, J.-C. Lee, G.-H. Lee, Separation of Co(II) and Li(I) by supported liquid membrane using Cyanex 272 as mobile carrier, J. Membr. Sci. 297 (2007) 253–261. [17] A.H. Blitz-Raith, R. Paimin, R.W. Cattrall, S.D. Kolev, Separation of cobalt(II) from nickel(II) by solid-phase extraction into Aliquat 336 chloride immobilized in poly(vinyl chloride), Talanta 71 (2007) 419–423. [18] S. Kagaya, R.W. Cattrall, S.D. Kolev, Solid-phase extraction of cobalt(II) from lithium chloride solutions using a poly(vinyl chloride)-based polymer inclusion membrane with Aliquat 336 as the carrier, Anal. Sci. 27 (2011) 653–657. [19] N.C. Thomas, K. Pringle, G.B. Deacon, Cobalt(II) and cobalt(III) coordination compounds, J. Chem. Educ. 66 (1989) 516–517. [20] M.A. Malati, Experimental Inorganic/Physical Chemistry, 1st ed., Horwood Publishing Limited, Chichester, England, 1999 1999. [21] M.J. Berger, J.H. Hubbell, S.M. Seltzer, J. Chang, J.S. Coursey, R. Sukumar, et al., XCOM: Photon Cross Sections Database, NIST Standard Reference Database 8 (XGAM). Available from: 〈http://www.nist.gov/pml/data/xcom/〉. [22] P.J. Ellis, H.C. Freeman, XFIT - an interactive EXAFS analysis program. J. Synchrotron Radiat. 2 (1995) 190-195. [23] P.J. Ellis, Xfit for Windows 95, Australian Synchrotron Research Program, Sydney, 1996. [24] S.I. Zabinsky, J.J. Rehr, A. Ankudinov, R.C. Albers, M.J. Eller, Multiple-scattering calculations of x-ray-absorption spectra, Phys. Rev. B – Condens. Matter Phys. 52 (1995) 2995–3009. [25] M.I. Bondin, S.J. Borg, M.H. Cheah, G. Foran, S.P. Best, Integration of EXAFS, spectroscopic, and DFT techniques for elucidation of the structure of reactive Diiron compounds, Aust. J. Chem. 59 (2006) 263–272.

[26] Z. Hu, Y. Pan, W. Ma, X. Fu, Purification of organophosphorus acid extractants, Solv. Extr. Ion Exch. 13 (1995) 965–976. [27] F. Xun, J.A. Golding, Solvent extraction of cobalt and nickel in bis(2,4,4-trimethylpentyl)phosphinic acid, “Cyanex-272”, Solv. Extr. Ion Exch. 5 (1987) 205–226. [28] Y. Cho, C.L. Xu, R.W. Cattrall, S.D. Kolev, A polymer inclusion membrane for extracting thiocyanate from weakly alkaline solutions, J. Membr. Sci. 367 (2011) 85–90. [29] S. Calvin, K.E. Furst, XAFS for Everyone, CRC Press, Boca Raton, 2013. [30] G. Bunker, Introduction to XAFS: A Practical Guide to X-ray Absorption Fine Structure Spectroscopy, Cambridge University Press, Cambridge, UK; New York, 2010. [31] T.E. Westre, P. Kennepohl, J.G. DeWitt, B. Hedman, K.O. Hodgson, E.I. Solomon, A multiplet analysis of Fe K-Edge 1s-3d pre-edge features of iron complexes, J. Am. Chem. Soc. 119 (1997) 6297–6314. [32] T. Yamamoto, Assignment of pre-edge peaks in K-edge x-ray absorption spectra of 3d transition metal compounds: electric dipole or quadrupole? XRay Spectrom. 37 (2008) 572–584. [33] J. Beckmann, A. Duthie, R. Ruettinger, T. Schwich, Hydrothermal synthesis of chiral metal(II) phosphinates derived from camphor, Z. Anorg. Allg. Chem. 635 (2009) 1412–1419. [34] J.L. Du, S.J. Rettig, R.C. Thompson, J. Trotter, Synthesis, structure, and magnetic properties of diphenylphosphinates of cobalt(II) and manganese(II). The crystal and molecular structures of the γ forms of poly-bis(μ-diphenylphosphinato)cobalt(II) and manganese(II), Can. J. Chem. 69 (1991) 277–285. [35] S.J. Liu, R.J. Staples, J.P. Fackler Jr., The structure and characterization of isomeric cobalt(II) diphenylphosphinate polymers, Polyhedron 11 (1992) 2427–2430.