Homogeneous liquid-liquid extraction of metal ions with non-fluorinated bis(2-ethylhexyl)phosphate ionic liquids having a lower critical solution temperature in combination with water

Article abstract of DOI:10.1039/c5cc05649g

Ionic liquids with an ether-functionalised cation and the bis(2-ethylhexyl)phosphate anion show thermomorphic behaviour in water, with a lower critical solution temperature. These ionic liquids are useful for homogeneous liquid-liquid extraction of first-row (3d) transition metals.

Full text of DOI:10.1039/c5cc05649g

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DOI: 10.1039/C5CC05649G
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Homogeneous liquid-liquid extraction of metal ions with non-
fluorinated bis(2-ethylhexyl)phosphate ionic liquids having a
lower critical solution temperature in combination with water
Received 00th January 20xx,
Accepted 00th January 20xx
Daphne Depuydt,a Liwang Liu,b Christ Glorieux,b Wim Dehaen,a and Koen Binnemansa*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Ionic liquids with an ether-functionalised cation and the bis(2- °C.18 It has been used in the homogeneous liquid-liquid extraction
ethylhexyl)phosphate anion show thermomorphic behaviour in water, (HLLE) of metal ions.19-21
he binary mixture of choline
T
with a lower critical solution temperature. These ionic liquids are useful bis(trifluoromethylsulfonyl)imide ([Chol][Tf2N]) and water shows
for homogeneous liquid-liquid extraction of first period (3d) transition temperature-dependent phase behaviour with an upper critical
metals.
solution temperature of 72 °C and has been used for the HLLE of
Nd(III).22
A
series of ILs based on Girard’s reagents have
Ionic liquids (ILs) are solvents that consist entirely of ions.1-3 They
are distinguished from molten salts by their low melting
temperature, arbitrarily set at 100 °C. ILs are often considered as
environmentally friendly alternatives for molecular solvents in
solvent extraction since they have a negligible vapour pressure.4
The physical properties of ILs depend on their structure. Depending
on its composition, an ionic liquid is miscible or immiscible with
water, leading to a classification into hydrophilic and hydrophobic
ionic liquids. Yet, this classification is ambiguous, since the
miscibility of some ILs with water is strongly temperature-
dependent. In upper critical solution temperature (UCST) systems,
the solubility of the IL in water increases with increasing
temperature and a homogeneous system is found above a certain
point in the cloud point curve, namely above the upper critical
solution temperature, the components are miscible in all
concentrations. The opposite happens for the lower critical solution
temperature (LCST) systems, in which the homogeneous phase is
formed with a decrease of temperature. It has been reported that
ionic liquids have temperature-dependent miscibility with different
molecular solvents.5,6,7,8 Ohno and co-workers made seminal
contributions to LCST phase changes in ionic liquids.9-16 They have
prepared ionic liquids derived from amino acids that show LCST-
type phase changes with water.17 IL-water biphasic mixtures are of
interest for the separation of molecules by solvent extraction. In
2006, we reported on the IL betainium bis(trifluoromethylsulfonyl)
imide ([Hbet][Tf2N]) which, mixed with water, shows an UCST at 55
tunable thermomorphism of the UCST type and were used for
HLLE of transition metals.23 In the literature, the LCST behaviour
of both glycol ether–water mixtures,24 and functionalised polymers
with glycol chains25 were described. The ionic liquid interpretation
of the LCST phase behaviour can be comprehended based on the
knowledge from polymer studies. Although Ohno and co-workers
reported the HLLE of LCST ionic liquids for the extraction of
proteins,26,27 the use of such a LCST system was, until now, never
executed for the extraction of metal ions.
In this Communication, ionic liquids having the bis(2-ethylhexyl)
phosphate as anion are presented that show lower critical solution
temperature phase behaviour.
A proof-of-principle extraction of
first row transition metals by the ionic liquid [P444E3][DEHP] is the
first example of homogeneous liquid-liquid extraction of metal
ions with an LCST thermomorphic system. The designed ILs are
depicted in Figure 1. The synthesis of the ether-functionalised ionic
liquids was based on literature procedures.28 After tri-n-
butylphosphine was reacted with
a chloride oligo ethylene
precursor, a metathesis reaction was performed in which the
chloride anion of the ether-functionalised ILs was exchanged to the
bis(2-ethylhexyl)phosphate (DEHP) (Experimental Section).
O
O
O
P
P
O
O
1
P
O
P
O
O
O
O
a KU Leuven, Department of Chemistry, Celestijnenlaan 200F, P.O Box 2404, B-
3001 Heverlee (Belgium). E-mail: Koen.Binnemans@chem.kuleuven.be
b KU Leuven, Department of Physics and Astronomy, Celestijnenlaan 200D P.O Box
2416, B-3001 Heverlee (Belgium).
3
2
Figure 1. Structures of investigated ILs. Anion: bis(2-ethylhexyl)phosphate ([DEHP]-).
Cations: 1: tri-n-butyl-2-methoxyethylphosphonium ([P444E1]+), 2: tri-n-butyl[2-(2-
methoxyethoxy)ethyl]phosphonium
methoxyethoxy)ethoxy]ethyl}phosphonium [P444E3]+).
([P444E2]+),
3:
tri-n-butyl-{2-[2-(2-
† Electronic Supplementary Information (ESI) available: Experimental details of
synthesis and characterisation of ionic liquids, temperature dependence of
transmitted intensity, ATR-FTIR, extraction details. See DOI: 10.1039/x0xx00000x
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Table 1. Viscosity, water content and phase behaviour of ether-functionalised ILs.
View Article Online
DOI: 10.1039/C5CC05649G
Entry
Viscosity (cP)
at 50 °C
Water content
(ppm)
Phase
behaviour
with water
miscibleb
miscibleb
miscibleb
LCST
1a
2a
3a
1b
2b
3b
[P444E1]Cl
[P444E2]Cl
[P444E3]Cl
[P444E1][DEHP]
[P444E2][DEHP]
[P444E3][DEHP]
364
321
172
94
50
86
n.d.a
1261
1259
811
858
790
LCST
LCST
a n.d.: not determined. Solid at room temperature, melting point: 31 °C. b Fully
miscible with water in the temperature range 0-100 °C.
The physical properties of the DEHP ILs are listed in Table 1. A
decrease in viscosity with longer oligo ethylene chain is seen for the
chloride ILs (see ESI). Introduction of the bis(2-ethylhexyl)
phosphate anion lowers the viscosity. Regarding the phase
behaviour of the ionic liquids with water, the structure of the ionic
liquid plays a crucial role. Most importantly, the anion drastically
changes the miscibility of the ionic liquid with water. All synthesised
chloride ionic liquids are fully miscible with water, whilst all the
synthesised DEHP ionic liquids show LCST phase behaviour. Also,
the role of the cation was investigated. The cloud point
temperature was determined for the three different ether-
functionalised ILs in a 1:1 wt/wt mixture with water. Complete
phase diagrams were constructed via visual observation of the
cloud point upon heating the homogeneous mixtures of different
compositions (Figure 2). In addition, more accurate but slow
transmission measurements were carried out for some of the
compounds to verify the visual observations. In this technique, the
cloud point temperature was defined as the temperature at which
50% of the initial transmission is reached. Temperatures found are:
34 °C, 38 °C and 44 °C for 1:1 wt/wt binary mixtures with water of
[P444E1][DEHP], [P444E2][DEHP] and [P444E3][DEHP], respectively
(Figure S1). These temperatures are practical for extraction
applications, since increasing the temperature by 10 to 20 °C above
room temperature is sufficient for complete phase separation. The
cloud point temperature increases with increasing oligo ethylene
chains. This can be explained as follows. Mixing of the IL and water
will occur when the Gibbs free energy of mixing ΔGmix is negative:
Figure 2. Phase diagrams of the binary mixtures of three ether-functionalised ILs with
water .
Figure 3. Comparison of cloud point temperatures between functionalised
([P444E3][DEHP]) and non-functionalised (P4444][DEHP]) ionic liquid.
Bis(2-ethylhexyl)phosphoric acid is a well-known extractant in
hydrometallurgical processes for the separation of divalent
transition metals.29-31 Therefore, the bis(2-ethylhexyl)
phosphate ILs were tested for homogeneous liquid-liquid
extraction of 3d transition metals. The binary mixture of the
ionic liquid [P444E3][DEHP] and water was used for the
extraction of cobalt(II), copper(II), nickel(II) and zinc(II). The
original cloud point of the 1:1 wt/wt mixture was 44 °C. Yet, the
presence of salts in the extraction system influences this cloud
point temperature. It is known that the alkaline-earth chloride salts
induce a lowering of the lower critical solution temperature.32 In
this case, with the divalent transition metal ions, the cloud point
temperature is reduced significantly. The cloud point temperatures
are decreased to approximately 20 °C for contacting the IL with an
aqueous solution of approximately 5000 ppm of the metal. The
reduction in temperature is beneficial for homogeneous liquid-
liquid extraction on a laboratory scale, since the homogeneous
region is formed by cooling the mixtures down in an ice bath.
However, this lowering of the cloud point temperature can also be
disadvantageous since it was observed that no metal
concentrations higher than 8000 ppm could be used. The high salt
concentration induces a lowering of the cloud point temperature
outside the window of cooling by ice water (<0 °C).
ΔGmix = ΔHmix – TΔSmix
(1)
In case of LCST phase behaviour, ΔSmix is negative. Below the LCST,
the two components are fully miscible due to the formed hydrogen
bonds between water and the IL, resulting in a hydration shell
around the ions. With increasing temperature, this hydrogen
bonding is more and more lost until eventually all water molecules
from the hydration shell are expelled into the bulk water. A longer
oligo ethylene chain in the ionic liquid has more possibilities for
hydrogen bonds with water; hence a higher temperature is needed
to break them up. The lower cloud point temperature of
[P4444][DEHP]:water in comparison with [P444E3][DEHP]:water
can be explained in a similar way: [P4444][DEHP] allows for less
possible interactions with water, since this IL lacks functional
groups (Figure 3).
2 | J. Name., 2015, 00, 1-3
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Prior to the homogeneous liquid-liquid extraction experiments, the where cILbs and cILas are the metal concentration of the ionic liquid
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IL was presaturated with water to minimize phase changes. A 1:1 phase before and after stripping, respectDivOeIl:y1.0.T1h0i3s9/pCr5eCcCip0i5ta6t4i9oGn
wt/wt binary mixture of the IL and water was made. After stripping technique is advantageous since the metal is removed in
homogenisation and subsequent settling of the two phases, it was one single step. Using oxalic acid as stripping agent, almost all Co(II)
found that 18 wt% of water dissolved in the IL phase (Karl Fischer and Ni(II) was precipitated as their respective oxalates. The
titration) and 6 wt% of IL was lost into the water layer (quantitative stripping with oxalic acid was still very efficient for Zn(II), more than
NMR - see ESI). The influence of the extraction time at 0 °C below 90% of the metal was recovered as oxalate. For Cu(II), the IL
LCST on the percentage extraction (%E) was investigated first solution was still very blue coloured and only 58% of the metal ion
(Figure S2). After 5 min, no significant changes in the %E were was stripped from the organic phase (Figure S4). Apparently, the
observed, so this cooling time was chosen. In order to determine most extracted ion, Cu(II), is also the most difficult to remove from
the distribution ratios, four different aqueous solutions of its complex with the addition of oxalic acid. Instead, the stronger
approximately 5000 ppm of the metal (CoCl2, CuCl2, NiCl2 and ZnCl2) acid H2SO4 was used to strip Cu(II). 500 mg of the loaded IL phase
were mixed with the ionic liquid [P444E3][DEHP] in the homogeneous was contacted with 500 mg of a 1M H2SO4 solution. A light blue
phase. After settling at room temperature for 10 min to form the precipitate was formed and a percentage stripping of 99% was
biphasic system again, the mixtures were centrifuged to ensure full obtained (Table 2).
phase separation. Then, the aqueous phases were separated and It must be noted that the IL cannot be used for a new extraction
analysed for their metal content by total reflection X-ray step directly after stripping, since the anion is now protonated and
fluorescence (TXRF). The distribution ratios were calculated as:
the chloride IL reformed. Simply by a washing step with an alkaline
solution, the DEHP IL is fully restored and ready for another
extraction/stripping cycle.
Table 2. Extraction of 1st row transition metals (Co(II), Ni(II), Cu(II), Zn(II)) with
[P444E3][DEHP], initial metal concentrations in the aqueous phase (ci) and metal
concentration in the aqueous phase after extraction (caq), the initial pH of the solutions,
their distribution ratios D and the percentage stripping %S when contacted with solid
oxalic acid (for Cu(II), also with H2SO4).
(2)
where cin and caq are the metal concentrations in the initial
aqueous phase and the aqueous phase after extraction,
respectively. The distribution ratios listed in Table 2 were
measured for chloride solutions of the respective metals with
initial pH around 3.5. In Figure 4, the three stages of the
extraction are presented for copper(II).
Metal
ion
ci
pHin
caq
D
%S
Co(II)
Ni(II)
Cu(II)
Zn(II)
6866
5578
4480
7466
3.2
3.4
3.9
3.0
1278
286
128
285
4.4
19
34
25
99
99
58/99
91
a
b
c
The coordination of the metal ions by the IL was studied by
FTIR spectroscopy. By comparing ATR-FTIR spectra for
presaturated IL and the IL loaded with Co(II), an obvious shift
to lower wavenumbers was observed for two peaks (Figure
S3). The first one: around 1200 cm-1, a small shift in the peak
corresponding to a P=O stretch, and around 1600 cm-1, a larger
shift in the peak corresponding to the P-O- stretch.34 As
expected, this indicates the interaction between this
functional group and the metal cation in the extraction.
Also, the selectivity towards other metals was investigated.
The extraction of CsCl has a distribution ratio of 4.1, even
lower than the worst divalent metal ion tested (Co(II)). Taking
into account that indium and some rare earths are listed as
critical metals in the EU Report because of their high supply
risk,35 and therefore, their recycling demand will increase,
these elements were chosen for the study of the selectivity.
Unfortunately, these elements form a precipitate when
contacted with the ionic liquid. This suggests strong
complexation with the bis(2-ethylhexyl)phosphate anion. It is
known that the di(2-ethylhexyl)phosphoric acid is an excellent
extractant for rare-earth elements and indium.36,37 Yet,
introducing it as an anion in the ionic liquid leads to formation
of an insoluble complex, as was confirmed by TXRF
measurements.
Figure 4. Homogeneous liquid-liquid extraction of Cu(II) with [P444E3][DEHP]/ H2O
1:1 wt/wt mixture (concentration 6866 ppm) a: initial stage, b: homogeneous
phase after 5 min in ice bath, c: after settling to room temperature. The upper
layer is the aqueous phase; the ionic liquid phase is the bottom layer.
In the Irving-Williams series, the general stability sequence of high
spin octahedral metal complexes for the replacement of water by
other ligands is given as Co(II) < Ni(II) < Cu(II) > Zn(II).33 In case of
the extraction with the bis(2-ethylhexyl)phosphate anion, the
general order is followed, Co(II) is extracted the least, Zn(II) is
extracted better and Cu(II) is the metal ion with the highest
distribution ratio (Table 2).
Stripping was performed by adding a 1:1 molar ratio of solid oxalic
acid to the isolated IL phase. After shaking at 40 °C for 30 min at
3000 rpm, a sample of the IL phase was analysed with TXRF for its
metal content (Table 2). The percentage stripping %S is defined as
follows:
(3)
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In conclusion, we have shown that the introduction of bis(2-
ethylhexyl)phosphate as anion in oligo ethylene functionalised
phosphonium ionic liquids gives ionic liquids with LCST phase
behaviour. The cloud point temperature is tuneable by making
modifications in the structure of the ionic liquid. Longer oligo
ethylene chains induce higher cloud points. Addition of a metal
chloride salt significantly lowers the cloud point temperature. The
ionic liquid [P444E3][DEHP] was used in this first example of
homogeneous liquid-liquid extraction of metal ions with an LCST
thermomorphic system. The extraction capacity towards DEHP ionic
liquid is excellent for divalent metal ions.
7
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dried in a vacuum oven at 60 °C for 2h. The phosphate salt (1 mol
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water and stirred overnight. Dichloromethane was added and the
product was extracted to the organic phase and washed with water
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evaporated to yield pure product. Full characterisation can be
found in the ESI. In a typical extraction experiment, 500 mg of the
presaturated IL is contacted with 500 mg of an aqueous metal
solution. The mixtures were cooled in an ice bath for 5 min to reach
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room temperature for 10 min. A sample was taken from the
aqueous layer and analysed for its metal content by TXRF. For a
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Acknowledgements
This research was financially supported by IWT-Flanders (PhD
fellowship to DD), the FWO Flanders (research project G.0900.13)
and the KU Leuven (projects GOA/13/008 and IOF-KP RARE3). The
authors also wish to thank Karel Duerinckx for NMR measurements
and Dirk Henot for performing CHN analyses.
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