Functionalized phosphonium based ionic liquids: Properties and application in metal extraction

Article abstract of DOI:10.1039/c4ra04723k

The extraction of Cu(ii), Zn(ii), Co(ii), Ni(ii) and Pb(ii) from model aqueous solutions using phosphonium based ionic liquids as sole extraction agents has been explored. The hydrophobic trioctyl(4-vinylbenzyl) phosphonium chloride, [P_888(4-VB)

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Functionalized phosphonium based ionic liquids: properties and
application in metal extraction
Alessandra Caterina Barsanti, Cinzia Chiappe,* Tiziana Ghilardi and Christian Silvio Pomelli
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The extraction of Cu(II), Zn(II), Co(II), Ni(II) and Pb(II) from model aqueous solutions using phosphonium based
ionic liquids as sole extraction agents has been explored. The hydrophobic trioctyl(4ꢀvinylbenzyl) phosphonium
chloride, [P888(4ꢀVB)]Cl, allows almost complete removal of Pb(II), Cu(II), Zn(II) without addition of HCl, while the
extraction of Co(II) and Ni(II) was almost null. An efficient extraction (ca.100%) of Pb(II), Cu(II) and Zn(II) was
10 obtained using [P888(4ꢀVB)][NO2].
might be exploited not only in catalysis but also for other peculiar
applications, such as extraction of metals and water remediation
from industrial wastes and effluents. Liquidꢀliquid extraction is
50 one of the most important techniques employed for metal ion
separation. The possibility to operate in continuous mode, using
relatively simple equipment and employing only small amounts
of the reagents are surely some of the advantages.7 The principal
drawback of classical liquidꢀliquid extraction8 is however the use
55 of flammable, volatile or toxic waterꢀimmiscible solvents. The
employ of ILs as waterꢀimmiscible extractants has been
considered as an important alternative approach to improve the
sustainability of the process.9 However, as stated by Binnemans
et al.,7 the application of ILs as solvents for the extraction of
60 metal ions has only partially met the high expectations. Due to
the high hydrophilicity of hydrated metal ions often other
extractants have to be added. Moreover, extraction of metal ions
from water solution to IL generally occur via an ion exchange
mechanism: thus, IL cations are lost during extraction of a metal
65 ion with a neutral extractant, whereas ILs anions are lost when
anionic metal complexes are extracted.10 Finally, problems can
arise for the application of ILs in water recovery if a significant
dissolution of the IL in the aqueous phase occurs during the
extraction process. Although these losses can be reduced
70 modifying the IL’s structure through the introduction of long
alkyl or fluorinated chains, thus increasing IL hydrophobicity, the
same structural alterations often have negative effects on
extraction efficiency. Unfortunately, hydrophobic ILs generally
contain fluorinated anions which can release HF after prolonged
75 contact with water (in particular, [BF4]ꢀ and [PF6]ꢀ) or are quite
expensive. Phosphonium salts are actually an exception: they
give hydrophobic salts also with the hydrophilic, not expensive
and ecofriendly chloride as counteranion. Tetralkylphosphonium
chlorides in toluene or kerosene have been used in extraction of
80 metal ions (from aqueous solutions).11 More recently, they are
been used as undiluted solvents for separation of nickel by cobalt
from 8M HCl aqueous solutions.7
Introduction
Phosphonium salts constitute an important subꢀclass of ionic
liquids (ILs) which are known to have important properties,
15 sometimes superior in comparison with those of the widely
investigated nitrogenꢀbased ILs, in particular dialkylimidazolium
salts. Tetraalkylphosphonium ILs generally present greater
inertness under basic conditions1 and increased thermal and
electrochemical stabilities.2 Their potential applications include
20 the use as electrolytes for batteries and dyeꢀsensitized solar cells,
as corrosion inhibitors, lubricants, reaction media and extraction
agents. However, this class of ILs has been less systematically
studied than nitrogenꢀbased ILs and some detailed studies about
the
physicochemical
and
thermal
properties
of
25 tetralkylphosphonium chlorides have been only recently
published.3
The principal drawbacks limiting their use are the high viscosity
and the physical state (solid) at room temperature. However,
analogously with nitrogen based ILs, a careful selection of anion
30 and substituents on cation can minimize these problems. For
example, it has been shown4 that trialkyl benzyl phosphonium
cations combined with bis(trifluoromethanesulfonyl)imide anion
can effort lowꢀmelting salts with high thermal stability and
conductivity. On the other hand, hydrophobic and polar ILs,
35 exhibiting stable phase separation and high hydrogen bonding
ability after mixing with water, have been obtained coupling
tetraalkylphosphonium
cations
(in
particular,
tetraꢀnꢀ
hexylphosphonium and triꢀnꢀhexylꢀnꢀoctylphosphonium) with
ethylphosphonate anion.5 It is noteworthy that phosphonium
40 cations can give hydrophobic ILs also when two hydroxyl groups
are
introduced
on
the
alkyl
chains:
trioctyl(2,3ꢀ
dihydroxypropyl)phosphonium chloride is only partially soluble
in water and the analogous bis(trifluoromethanesulfonyl)imide
salt is completely insoluble.6
45 It has been previously suggested6 that the ability of these
hydrophobic dihydroxyl functionalized ILs to complex metals
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Considering the potential binding ability of the vicꢀdiol moiety
towards metal cations, we decided to investigate the possibility to
apply trioctyl(2,3ꢀdihydroxypropyl)phosphonium salts with
(upper layer), the solvent was removed at reduced pressure. The
isolated IL was dried under vacuum (2x10ꢀ3 mm Hg, 80°C, 6h)
and analyzed by NMR.
1H NMR (DMSOꢀd6, δ ppm): 6.00 (s, 1H, OH), 5.23 (s, 1H, OH),
50 3.90 (m, 1H, CHOH), 3.38 (m, 2H, CH2OH), 2.41 (m, 8H,
P+CH2), 1.25 (m, 36H, CH2), 0.86 (m, 9H, CH3); 13C NMR
chloride,
hexafluorophosphate,
5
bis(trifluoromethanesulfonyl)imide and nitrite counteranion (1) in
the extraction of several common heavy metal pollutants (Cu(II),
Zn(II), Ni(II) and Pb(II)) and Co(II) from water, avoiding the
addition of HCl to the aqueous phase.
(DMSOꢀd6, δ ppm): 66.6 (m, CH OH, CHOH), 31.3, 30.2 (d, J =
2
15 Hz), 28.4, 28.2, 23.3 (d, J = 47 Hz, P+CH2CH(OH)CH2OH),
22.5, 21.2 (d, J = 4.5 Hz), 19.1 (d, 47Hz P+CH2CH2CH2CH2–),
The extraction efficiency of these ILs was furthermore compared
10 with that of another class of hydrophobic phosphonium salts
based on trioctyl(4ꢀvinylbenzyl) phosphonium cation (2). The
polymerizable nature of the vinyl group might be useful to
develop immobilized ILs for metal separation from water.
55 14.2.
Trioctyl(4-vinylbenzyl)phosphonium chloride, [P8,8,8(4-VB)]Cl
Under an argon atmosphere, trioctylphosphine (16,7 g, 45 mmol)
was added to a solution of 1ꢀ(chloromethyl)ꢀ4ꢀvinylbenzene (6,87
60 g, 45 mmol) in toluene (20 ml), previously accurately deareated
and containing
a
small amount of 2,6ꢀdiꢀtertꢀbutylꢀ4ꢀ
methylphenol as radical inhibitor. The resulting mixture was
stirred for 48 h at room temperature, then the solvent was
removed at reduced pressure and the resulting white solid (99%
65 yield) was washed with ethyl acetate, dried under vacuum and
analyzed by NMR. The 1H NMR spectrum exactly matched
literature data.12
1H NMR (CDCl3, δ ppm): 7.32 (m, 2H, aromatic CH), 7.18 (m,
2H, aromatic CH), 6.65 (dd, J = 17, 11 Hz, 1H, CH=), 5.70 (d, J =
70 17 Hz, 1H, =CH2), 5.25 (d, J = 11 Hz, 1H, =CH2), 4.30 (d, J = 15
Hz, 2H, P+CH2Ph), 2.33 (m, 6H, P+CH2Ph), 1.18 (m, 36H, CH2),
0.83 (m, 9H, CH3). 13C NMR (CDCl3, δ ppm): 137.6, 135.9,
130.4, 129.0, 128.2, 127.1, 114.9, 31.7, 30.9 (d, J = 15 Hz), 28.4,
28.2, 26.5 (d, J = 47.5 Hz, P+CH2Ph), 22.6, (d, J = 4.5 Hz), 19.1
75 (d, 47 Hz, P+CH2CH2CH2CH2), 14.2.
15
The main physicochemical (viscosity, conductivity and polarity)
properties of these phosphonium salts were determined. The
effect of IL structure on metal extraction was evaluated
20 determining extraction percentages by ICPꢀMS.
Experimental
Trioctyl(4-vinylbenzyl)phosphonium
[P8,8,8(4-VB)][PF6]
hexafluorophosphate,
NMR spectra were recorded at room temperature using a Bruker
instrument at 250 MHz (1H) and 75.7 MHz (13C), in DMSOꢀd6,
25 CDCl3 or D2O. ICPꢀMS analyses were carried out using Agilent
7500 ce instrument.
To a solution of [P8,8,8(4ꢀVB)]Cl (12 g, 23 mmol) in waterꢀethanol
80 (2:1, 20 ml) HPF6 (2.23 ml, 25 mmol) was added and the
resulting reaction mixture was stirred at room temperature for 24
h. The resulting biphasic system was separated and the nonꢀ
aqueous phase was diluted with dichloromethane and repeatedly
washed with water (3 x 10 ml). The organic phase was dried
85 (MgSO4) and concentrated at reduced pressure to give 13.8 g of
[P8,8,8(4ꢀVB)][PF6] as a colourless liquid.
1H NMR (CDCl3, δ ppm): 7.38 (m, 2H, aromatic CH), 7.26 (m,
2H, aromatic CH), 6.65 (dd, J = 17, 11 Hz, 1H, CH=), 5.70 (d, J =
17 Hz, 1H, =CH2), 5.30 (d, J = 11 Hz, 1H, =CH2), 3.80 (d, J = 15
90 Hz, 2H, P+CH2Ph), 2.13 (m, 6H, P+CH2Ph), 1.23 (m, 36H, CH2),
0.87 (m, 9H, CH3). 13C NMR (CDCl3, δ ppm): 137.6, 135.9,
130.4, 129.0, 128.2, 127.1, 114.9, 31.7, 30.9 (d, J = 15 Hz), 28.4,
28.2, 26.5 (d, J = 47.5 Hz, P+CH2Ph), 22.6, (d, J = 4.5 Hz), 19.1
(d, 47 Hz, P+CH2CH2CH2CH2), 14.2.
Trioctyl(2,3ꢀdihydroxypropyl)phosphonium
[P8,8,8(gly)]Cl, hexafluorophosphate, [P8,8,8(gly)][PF6]
bis(trifluromethanesulfonyl)imide, [P8,8,8(gly)][Tf2N]
chloride,
and
were
30 prepared as previously reported.6 CuCl2·2H2O (>99.0%), ZnCl2
(>98%), NiCl2·6H2O (> 99.9 %), CoCl2·6H2O (>98%), ZnCl2
(>98%) were purchased from Aldrich.
Trioctyl(2,3-dihydroxypropyl)phosphonium nitrite,
35 [P8,8,8(gly)][NO2]
To
a solution of trioctyl(2,3ꢀdihydroxypropyl)phosphonium
chloride, [P8,8,8(gly)]Cl, (16 g, 33 mmol) in acetonitrile (10 ml) an
equimolar amount of AgNO2 (yellow solid, freshly prepared from
AgNO3 and NaNO2 in water) was added. The mixture was stirred
40 at room temperature for 24h, then the precipitate was filtered off
and the resulting aqueous solution was evaporated at reduced
pressure. The residue liquid was dissolved in anhydrous acetone
and the solution was cooled at ꢀ40°C for 48 h. After filtration on
glass septa (porosity 4) containing two different powdered layers
45 of 1 cm each of celite (lower layer) and decolorizing carbon
95
Trioctyl(4-vinylbenzyl)phosphonium
bis(trifluoromethane)sulfonimide, [P8,8,8(4-VB)][Tf2N]
A round bottom flask was charged with [P8,8,8(4ꢀVB)]Cl (12.00 g,
23 mmol) and a mixture of ethanol and water (2 : 1) (20.0 mL)
100 then Li(Tf2N) (7.1 g, 25.2 mmol) was added in one portion under
stirring. The resulting reaction mixture was stirred at 60 °C
2
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for18h and diluted with CH Cl2 and water. The organic phase was
2
mixed for several times before the measurements. To prevent dye
aggregation, the dye concentration was chosen low enough but
sufficient to allow an absorbance value of about 0.2.
repeatedly washed with water (3 x10 ml), concentrated at reduced
pressure and dried at 70 °C for 18 h under high vacuum to give
1
16.7 g (95%) of [P8,8,8(4ꢀVB)][Tf2N] as a colorless liquid. The H
60 Extraction experiments
5
NMR spectrum exactly matched literature data.11
1H NMR (CDCl3, δ ppm): 7.41 (m, 2H, aromatic CH), 7.21 (m,
2H, aromatic CH), 6.65 (dd, J = 17, 12 Hz, 1H, CH=), 5.70 (d, J =
17 Hz, 1H, =CH2), 5.30 (d, J = 12 Hz, 1H, =CH2), 3.60 (d, J = 14
Hz, 2H, P+CH2Ph), 2.06 (m, 6H, P+CH2Ph), 1.23 (m, 36H, CH2),
10 0.87 (m, 9H, CH3). 13C NMR (CDCl3, δ ppm): 137.6, 135.9,
129.2, 129.0, 128.2, 127.1, 114.9, 31.7, 30.9 (d, J = 15 Hz), 28.9,
28.7, 26.5 (d, J = 47.5 Hz, P+CH2Ph), 22.6, (d, J = 4.5 Hz), 19.1
(d, 47 Hz, P+CH2CH2CH2CH2), 14.2.
Extraction of metal ions from water was performed as follows.
Aqueous stock solutions of metal salts were prepared by addition
of the proper metal chloride to 1 L of highly purified water
(MilliꢀQ). Each solution was diluted 1:10 to obtain
65 concentration of metal ions of 100 ppm.
a
Procedure 1: 0.5 g of dried IL were added to 1.5 g of the proper
aqueous metal stock solution (metal concentration 100 ppm). The
resulting two phases were mixed using an ultra high vibration
shaker (Vortex 3) for 10 min, then the mixture was centrifuged at
70 3000 rpm for 10 min to favour phases separation. After that, the
metal concentration in the aqueous phase was determined by ICPꢀ
MS.
15 Trioctyl(4-vinylbenzyl)phosphonium nitrite, [P8,8,8(4-VB)][NO2]
To a solution of trioctyl(4ꢀvinylbenzyl)phosphonium chloride,
[P8,8,8(4ꢀVB)]Cl, (16 g, 30.6 mmol) in acetonitrile (10 ml) an
equimolar amount of AgNO2 (yellow solid, freshly prepared from
AgNO3 and NaNO2 in water) was added. The mixture was stirred
20 at room temperature for 24h, then the precipitate was filtered off
and the resulting aqueous solution was evaporated at reduced
pressure. The residue liquid was dissolved in anhydrous acetone
and the solution was cooled at ꢀ40°C for 48 h. After filtration on
glass septa (porosity 4) containing two different powdered layers
25 of 1 cm each of celite (lower layer) and decolorizing carbon
(upper layer), the solvent was removed at reduced pressure. The
isolated IL was dried under vacuum (2x10ꢀ3 mm Hg, 80°C, 6h)
and analyzed by NMR.
1H NMR (CDCl3, δ ppm): 7.35 (m, 2H, aromatic CH), 7.26 (m,
30 2H, aromatic CH), 6.65 (dd, J = 17, 11 Hz, 1H, CH=), 5.74 (d, J =
17 Hz, 1H, =CH2), 5.27 (d, J = 11 Hz, 1H, =CH2), 4.07 (d, J = 15
Hz, 2H, P+CH2Ph), 2.27 (m, 6H, P+CH2Ph), 1.21 (m, 36H, CH2),
0.85 (m, 9H, CH3). 13C NMR (CDCl3, δ ppm): 137.6, 135.9,
129.2, 129.0, 128.2, 127.1, 114.9, 31.7, 30.9 (d, J = 15 Hz), 28.4,
35 28.2, 26.5 (d, J = 47.5 Hz, P+CH2Ph); 22.6, (d, J = 4.5), 19.1 (d,
47 Hz, P+CH2CH2CH2CH2), 14.2.
Procedure 2: 0.5 g of dried IL were added to 1.5 g of the proper
aqueous metal stock solution (metal concentration 100 ppm). The
75 resulting two phases were mixed using an ultra high vibration
shaker (Vortex 3) for 10 min, then the resulting mixture was
centrifuged at 3000 rpm for 10 min. After that, the organic phase
was reused and another extraction was performed, as described in
procedure 1 (see above). Subsequently, the metal concentration in
80 the two aqueous phases were determined by ICPꢀMS.
Procedure 3: 0.5 g of dried IL were added to 1.5 g of the proper
aqueous metal stock solution (metal concentration 100 ppm). The
resulting two phases were mixed using an ultra high vibration
shaker (Vortex 3) for 10 min, then the resulting mixture was
85 centrifuged at 3000 rpm for 10 min. After that, the aqueous phase
was reꢀextracted with fresh IL. The procedure was repeated two
times. Finally, after three subsequent extractions, the metal
concentration in the aqueous phase was determined by ICPꢀMS.
Experimental results, collected at least in duplicate, agree within
90 5%.
Extraction percentage is calculated as follows:
Measurements of Physical Properties
The viscosity of each liquid compound was measured using
Brookfield DVꢀII + Pro instrument equipped with a Brookfield
40 TCꢀ502 water bath. The viscometer was calibrated over the
temperature range 20 – 100 °C using reference oils and ionic
liquids. Conductance measurements were performed using a
CON 510 bench meter supplied with conductivity/TDS electrode.
This electrode has a stainless steel ring and a cell constant of
45 K=1.0. It also has an inbuilt temperature sensor for automatic
temperature compensation. UV−vis absorption spectra of
solvatochromic dyes (Reichardt’s betaine dye, N,Nꢀdiethylꢀ4ꢀ
nitroaniline and 4ꢀnitroaniline) dissolved in ILs recorded with a
Cary 2200 spectrophotometer employing 1 mm path length quartz
50 cell. Individual stock solutions of Reichardt’s betaine dye, N,Nꢀ
diethylꢀ4ꢀnitroaniline and 4ꢀnitroaniline were prepared in
dichloromethane. In order to prepare a given dye/IL solution, the
appropriate amount of the dye stock solution was transferred into
a quartz cuvette. Residual dichloromethane was evaporated under
55 gentle stream of argon gas in dry box. The IL was then added to
95 Where [M]aq,i is the initial metal concentration in the aqueous
phase before extraction, [M]aq is the metal concentration in the
aqueous phase after extraction.
Results and Discussion
The synthesis of phosphonium salts was performed by reacting
100 commercial trioctylphosphine with 3ꢀchloropropaneꢀ1,2ꢀdiol or
1ꢀ(chloromethyl)ꢀ4ꢀvinylbenzene under an inert atmosphere (see
experimental) to prevent oxidation of trialkylphosphine.
Phosphonium chlorides were then converted into the
corresponding
hexafluorophosphates,
tetrafluoroborates,
105 bis(trifluoromethane)sulfonylamides and nitrites through
metathesis reactions. As expected, ILs 1-2 appeared as viscous
liquids and although the factors that determine ILs viscosity are
still not fully understood, a dependence on ionic structure could
be immediately evidenced. Table 1 reports some fundamental
the cuvette. The cuvette was sealed and the sample was hand 110 properties (viscosity, conductivity) of ILs 1 and 2 at 20 °C.
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Viscosities and conductivities were measured at several
temperatures between 20 and 80 °C.
Table 2. Arrhenius and VFT viscosity parameters
R
B
K
T0
K
R2
Eη
KJ
lnη∞
mPa
s
η0
mPa
.s
IL
molꢀ1
27.2
Table 1. Physicoꢀchemical properties of phosphonium based ILs
at 20 °C.
[P888(gly)]Cl
[P888(gly)][PF6]
ꢀ13.4
0.985
0.995
0.981
0.989
0.994
21.0
209
476
312
933
216
262
239
242
184
254
0.999
0.999
0.999
0.999
0.999
IL
status
50.0
35.4
38.0
28.8
ꢀ16.1
ꢀ14.3
ꢀ12.1
ꢀ9.2
6.4
11.0
0.3
σ
η
Pa.s
ꢀS/cm
18.5a
5.0
11.5
29.0
24.8 a
1.6
[P888(gly)][BF4]
[P888(gly)][Tf2N]
[P888(gly)][NO2]
[P888(4ꢀVB)][PF6]
[P888(4ꢀVB)][BF4]
[P888(4ꢀVB)] [Tf2N]
[P888(4ꢀVB)] [NO2]
[P888(gly)]Cl
Waxy
Liquid
Liquid
Liquid
Waxy
Liquid
Liquid
Liquid
Waxy
1.24 a
11.62
4.95
[P888(gly)] [PF6]
[P888(gly)] [BF4]
[P888(gly)][Tf2N]
[P888(gly)][NO2]
[P888(4ꢀVB)][PF6]
[P888(4ꢀVB)][BF4]
[P888(4ꢀVB)] [Tf2N]
[P888(4ꢀVB)] [NO2]
22.0
1.53
81.4
47.9
32.7
39.0
ꢀ47.8
ꢀ17.8
ꢀ12.2
ꢀ18.2
0.975
0.988
0.986
0.972
0.91 a
35.20
15.26
1.61
1.1
4.3
729
369
175
216
230
274
0.999
0.999
0.999
2.9
29.9
18.5
11.6 a
2.87a
45
5
aMeasured at 40 °C
Data have been therefore fitted using the more appropriate VTF
equation 4, where A (ꢀS cmꢀ1), B (K) and T0 (K) are fitting
parameters.
For these salts, a striking decrease of viscosity on increasing
temperature was generally observed and, in agreement with the
behaviour characterizing most ILs, the temperature dependency
only approximately follows the Arrhenius equation 1, where Eη is
50
σ = A e -B/(TꢀT )
0
(4)
10 the activation energy for viscous flows, and lnη∞ is the viscosity
at infinite temperature, over the examined temperature range (20ꢀ
80 °C).
The optimized parameters and related correlation coefficients are
reported in Table 3, together with the Arrhenius parameters.
55
lnη = lnη∞ + Eη / RT
(1)
Table 3. Arrhenius and VFT conductivity parameters
R
B
K
T0
(K)
R2
Eσ
KJ
Α
lnσ∞
(ꢀs/cm)
Anion
(ꢀs/cm)
The Arrhenius plots (here not reported) show indeed a slight
15 upward curvature. In contrast, the VogelꢀFulcherꢀTammann
(VFT) equation 2 (where η0 (cP), B (K), and T0 (K) are fitting
parameters) with few exceptions is able to model the temperature
effect:
molꢀ1
34.2
42.8
35.6
35.2
31.7
51.5
47.1
38.2
34.1
[P888(gly)]Cl
[P888(gly)][PF6]
13.5
17.0
15.8
15.2
13.3
18.2
17.8
15.8
15.9
0.997
0.998
0.998
0.989
0.988
0.996
0.998
0.989
0.999
1701
3981
101278
4033
6138
1016
6098
2913
1291
429
548
1544
360
678
352
680
332
1018
218
213
123
221
182
243
206
230
169
0.999
0.999
0.999
0.999
0.998
0.995
0.999
0.996
0.999
[P888(gly)][BF4]
[P888(gly)][Tf2N]
[P888(gly)][NO2]
[P888(4ꢀVB)][PF6]
[P888(4ꢀVB)][BF4]
[P888(4ꢀVB)] [Tf2N]
[P888(4ꢀVB)] [NO2]
20
η = η0 eB/(TꢀT )
0
(2)
The optimized parameters and related correlation coefficients are
reported in Table 2 together with the Arrhenius parameters. The
calculated viscosities of the investigated ILs display a good
25 agreement with the corresponding experimental viscosity.
It is quite surprising that glyceryl substituted phosphonium salts 1
are generally characterized by a lower viscosity and higher
conductivity than the corresponding styryl substituted salts 2,
despite the presence of two hydroxyl groups.
30 As expected, the anion chemical structure exerts a significant
effect also on conductivity and the less viscous ILs, with [Tf2N]ꢀ
as anion, show the highest conductivities. Conductivity increases
on increasing temperature but, in agreement with viscosity, the
temperature dependence follows only “approximately” the
35 Arrhenius equation 3:
Subsequently, we examined the solvent properties of these ILs
determining the solvatochromic KamletꢀTaft parameters, π*
60 (dipolarity/polarizability), β (hydrogen bonding basicity), α
(hydrogen bonding acidity) and the Reichardt parameter ET(30)
.
The solvatochromic parameters have been determined exclusively
for phosphonium salts 1, since the salts 2 have an exceedingly
high absorption in the UV region of interest. Table 4 reports the
65 KamletꢀTaft parameters of the investigated ILs. Values related to
two unfunctionalized tetraalkylphosphonium salts have been
added for comparison.
As expected, ILs 1 are generally characterized by a significantly
higher hydrogen bonding acidity when compared with the
70 unfunctionalized tetraalkylphosphonium salts: as previously
observed13 for other classes of ILs, the introduction of two
hydroxyl groups on cation gives ILs with α values comparable to
that of water (1.13), with the exception of chloride salts.
lnσ = lnσ∞ ꢀ Eσ / RT
(3)
where Eσ is the activation energy for conductivity.
75
40
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Table 4. ET(30) and KamletꢀTaft parameters determined at 20 °C.
IL
ET(30)
π∗
α
β
Co
Ni
E %
1.00
[P888(gly)]Cla
48.93
54.88
55.65
56.97
0.84
0.54
0.86
0.95
0.97
0.37
0.25
100
80
60
40
20
0
[P888(gly)] [PF6]
[P888(gly)] [BF4]
[P888(gly)][Tf2N]
[P666(14)][Tf2N] b
[P8888][(EtO)HPO2] c
0.93
0.88
0.96
0.83
0.85
0.55
0.61
0.35
0.27
1.38
Zn
Pb
Cu
aDetermined at 50 °C. bFrom ref. 14. cFrom ref. 5.
Furthermore, also for these ILs the hydrogen bonding basicity
5
(β ) depends mainly on the anion although an inverse relationship
Figure 1. Effect of IL anion on extraction percentage (E%) of metals
between
α and β parameters can be envisaged: basic anions (such
cations from aqueous solutions (procedure 1 and 3) using [P888(gly)]+ based
ILs.
as chloride) reduce the cation hydrogen bond donor ability (α
decreases from 0.97 to 0.54 going from [Tf2N]ꢀ to Clꢀ, Table 4).
The strong proton accepting nature of chloride anion, probably
10 through the formation of hydrogen bonds with the hydroxyl
groups of the glyceryl moiety,6 increases the networking inside
the IL and reduces the ability of IL cation to interact with the
probe dyes.
[P888(gly)]Cl and [P888(gly)] [BF4] have been not employed for
15 metal extraction. Preliminary experiments showed indeed a cleanꢀ
cut decrease in the volume of the organic phase, attributable to a
partial dissolution in water, when exactly measured amounts of
these ILs were added to water and the phases were vigorously
mixed and subsequently separated by centrifugation. Moreover,
20 we avoided the use of [P888(4ꢀVB)][PF6] owing to its surfactant
properties that drastically reduced the phase separation efficiency
after mixing of organic and aqueous phase.
35 To be sure that the equilibrium conditions were reached, the
extraction efficiencies obtained after 10 min stirring were
compared with the efficiencies obtained after stirring for 30 min.
Since the values were practically identical, a stirring time of 10
min was selected for all experiments. It is noteworthy that the
40 reuse of the organic phase for extraction of fresh metal ion
solutions gave only a noꢀsignificative increase in extraction
efficiency: for these ILs the preꢀconditioning phase appears not
important.
For each metal, the extraction efficiency was therefore evaluated
45 after one or three consecutive extractions using fresh IL (3 x 0.5
gr), values are depicted in Figure 1. As expected an increase in E
(extraction efficiency) was always observed on increased the
number of extractions (Figure 1): in particular, three extractions
assured a practically complete recover of Pb(II) and Cu(II)
.
The selected ILs 1 and 2 have been therefore tested as metal ions
extractants from water in biphasic separations. The extraction of
25 the individual metal ions Cu(II), Zn(II), Co(II), Ni(II) and Pb(II)
was performed by mixing 0.5 g of IL with 1.5 g of water
containing a metal ion concentration of 100 ppm. Chloride salts
were used as Cu(II), Zn(II), Co(II), Ni(II) and Pb(II) sources.
50 Unfortunately, despite the wellꢀknown chelation ability of two
vicinal hydroxyl groups, the association of the dihydroxyl
+
functionalized phosphonium cation [P888(gly)
]
to hydrophobic
anions (such as [PF6]ꢀ or [Tf2N]ꢀ) gave ILs having only a
moderate metal extraction ability. The high extraction efficiency
55 towards Pb(II) and Cu(II), observed in [P888(gly)][NO2], is most
likely attributable to the anion properties, i.e. its coordinating
ability or reducing power. The formation of metallic particles in
the organic phase was indeed detected in the samples containing
Pb(II). The subsequent, ICPꢀMS analysis of the recovered
60 precipitate after dissolution in HNO3 (65%) confirmed the
formation of Pb(0) in nitrite based ILs.
Table 5. Extraction efficiency for metal extraction (procedure 3)
30 using [P888(gly)]
+ based ILs.
[P888(gly)][PF6]
[P888(gly)][Tf2N] [P888(gly)][NO2]
M2+
E%
E%
24.1
14.8
8.2
ꢀ
E%
99.9
99.6
37.6
27.2
25.2
Pb2+
Cu2+
Zn2+
Co2+
Ni2+
40.1
40.2
ꢀ
ꢀ
ꢀ
Since the hydroxyl groups on cation appeared unable to affect
significantly extraction efficiency we have decided to test four
trioctyl(4ꢀvinylbenzyl)phosphonium salts having as counteranion
65 [Tf2N]ꢀ, [NO2]ꢀ and Clꢀ. It is noteworthy that the highly
hydrophobic nature of this aryl substituted phosphonium cation
makes it possible the association with the coordinating chloride
anion: the corresponding IL is indeed water insoluble.
ꢀ
70
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Table 6. Extraction efficiency for metal extraction (procedure 3)
Therefore, taking into account the coordination number of the
different anions, we propose the following pathway (eq 5) for
extraction of Cu(II) and Zn(II),
using [P888(4ꢀVB)]
+ based ILs.
[P888(4ꢀVB)]Cla
[P888(4ꢀVB)]Cl [P888(4ꢀVB)][Tf2N] [P888(4ꢀVB)][NO2]
M2+
E%
92.1
25.1
84.2
ꢀ
E%
99.9
99.4
99.9
ꢀ
E%
28.9
36.3
75.6
ꢀ
E%
99.9
99.7
97.8
39.3
36.5
Pb2+
Cu2+
Zn2+
Co2+
Ni2+
50
ꢀ
ꢀ
ꢀ
based on the formation of anionic tetrahedral metal chloride
complexes, which become the “new” IL counteranions. A similar
pathway can be hypothesized also for Pb(II),18 even if the very
55 low concentration of the metal salt could favour the formation of
an octahedral complex [PbCl6]4ꢀ arising from an analogous
process:
aProcedure 1.
100
80
60
40
20
0
5
Pb
Cu
Zn
Co
Ni
E %
60 Although the ionic nature of ILs can allow a variety of
mechanisms, including solvent ionꢀpair extraction (eq 5 and 6),
ion exchange (metalꢀIL cation) and simultaneous combination of
them,19 the inability to detect the IL cation ([P888(4ꢀVB)]+) in water
phase after extraction of concentrated metal solutions (0.1M) by
65 NMR suggests a moderate (if any) involvement of the metalꢀIL
cation exchange mechanism, with a very low IL leaching. It is
noteworthy that only very small quantities of other
tetralkylsubstituted phosphonium based ILs have been measured
by Binnemans et al. in the aqueous phase after solvent
70 extraction.7
Probably, an analogous mechanism can be hypothesized also for
bis(trifluoromethane)sulfonylamide based ILs, although the
chelating ability of this anion should give complexes of different
speciation and geometry.
75 Finally, it is to mention the chelating and reducing properties of
nitrite anion which favour also in the case of the styryl
functionalized cation, [P888(4ꢀVB)]+, extraction and deposition of
reducible metals, such as Pb(II).
10
Clꢀ1 extr
Cl
Tf2N
NO2
Figure 2. Effect of IL anion on extraction percentage (E%) of metals
cations from aqueous solutions using [P888(4ꢀVB)]+ based ILs.
15 Extraction efficiency values (Table 6 and Figure 2) show that
[P888(4ꢀVB)][Tf2N], analogously to the corresponding IL of the
other series, has no affinity for Co(II) and Ni(II). However, it
shows a fairly good extraction ability towards Zn(II) (E% = 76).
On the other hand, [P888(4ꢀVB)][NO2] presents an excellent
20 extraction ability for Pb(II), Cu(II) and Zn(II). These metal
cations can be however efficiently extracted also by [P888(4ꢀVB)]Cl,
which is able to remove 92% of Pb(II) and 84% Zn(II) after a
single extraction.
The coordination ability of the IL anion appears therefore
25 fundamental in metal extraction from water. The hydrophobic
[P888(4ꢀVB)]Cl is indeed able to perform an efficient transfer of
metal ions from water to organic phase. The formation of
polychlorometallate anions, as key species able to positively
affect the transfer process, could be hypothesized considering
30 their stability in ILs. Halometallate ILs have been widely applied
as Lewis acids and solvents in synthesis or in electrochemistry, 15ꢀ
16 and their involvement for the cobalt/nickel separation from 8 M
HCl solutions has been recently reported.7
Conclusions
80
In this paper we report the successful use of phosphonium
based ILs for the extraction of some common metal cations from
water using two series of functionalized phosphonium based ILs.
IL anion strongly affects the efficiency of the extraction process
which is however affected also by cation functionalization.
However, considering the different working conditions (in our
35 experiments the pH of the aqueous medium was in the range 4.5ꢀ
5) to verify their actual formation, UVꢀvis measurements have
been carried out on the organic phase in the case of Cu(II).
Extraction of a blue colored (ca. 0.1 M) aqueous solution of
CuCl2 (the blue color is indicative for the fully hydrated Cu(II)
40 ion) with [P888(4ꢀVB)]Cl gave a yellow organic phase attributable to
the formation of [CuCl4]2− ion. This species was identified by
UVꢀvis spectroscopy, on the basis of its typical absorption around
408 nm the other two maxima (at 291 nm and 231 nm,
respectively) being completely covered by the strong absorption
45 of the IL below 300 nm.17
85 Differences in extraction are observed between the ILs possessing
diolꢀsubstituted (hydrogen bond donor) and styrylꢀsubstituted
(nonꢀ hydrogen bond donor) cations. In this context, [P888(4ꢀVB)]Cl
allows a nearly complete extraction of Pb(II), Cu(II), Zn(II),
while the extraction of Co(II) and Ni(II) was negligible.
90
Reduction and possible chelation of metal ion species by
nitriteꢀbased ILs distinguish these solvents from the other
investigated ILs. On the other hand, the chelation ability of
chloride anion appears to be the driving force in the extraction of
metal ion species able to give stable polychlorometallate anions.
95 Pb(II), Cu(II), Zn(II) can be extracted avoiding the addition of
HCl (8 M) to the aqueous phase.
6
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Acknowledgements
This research was partially supported by the Fondazione Cassa di
Risparmio di Pisa (grant: POLOPTEL). The authors wish to
thank Mr Franco Castellani (ARPAT, PISA) for his technical
assistance in ICPꢀMS analysis.
5
Notes and references
Dipartimento di Farmacia, via Bonanno 33, 56126 Pisa, Italy. Fax: 39
050 2216990; Tel: 39 050 2216996; E-mail: cinziac@farm.unipi.it
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